In order to correctly understand what is being discussed in this article, you first need to correctly define the phrase - thermoplastic composite materials (T.K.M.), and in no case be confused with a compound, since we are talking about completely different materials. So what is a thermoplastic composite material (composite)? is a heterogeneous multiphase material of two or more components with a clear interface between them and qualitatively new properties while maintaining the chemical individuality of each component. It consists of a plastic base (matrix), which serves as a binder, and inclusions of various components in the form of powders, fibers, etc. (filler). The matrix ensures the solidity of the material, the transfer and distribution of stresses between the filler, determines the tightness, heat, moisture, fire and chemical resistance of the composite, its technological, as well as thermophysical, electrical and radiotechnical properties. The optimal combination of operational and technological properties is directed to regulate the properties and content of the matrix and filler, interacting between them at the interface, the orientation of the filler. The use of several matrices (polymatrix composites) or fillers of different nature (hybrid composites) expands the possibilities of regulating the properties of composites. Basic grades of polymers are used as a matrix of thermoplastic composite materials. The modern assortment of basic thermoplastic polymers, depending on the level of their elastic strength properties and deformation heat resistance, is conventionally divided into three groups.

According to their molecular structure, thermoplastics are divided into two groups - amorphous and crystalline. Due to the structural features, the greatest interest for manufacturers is polymers of the second group, which can offer a higher level of physical mechanical properties and great chemical resistance.

The volume of world production of thermoplastics (in 1990 - 86 million tons, in 2000 - 150 million tons, in 2010, according to forecasts - 258 million tons) significantly exceed the volume of world production of thermosetting plastics. Solid fillers in the form of powders, fibers of various lengths, woven and non-woven structures formed from fibers of various chemical nature can be used as fillers. Depending on the functions performed, the fillers are divided into three groups:

Inert- barite, dolomite, natural chalk, marble, etc. Their use is due to the desire to reduce the cost of the final product, when some deterioration in the properties of the material is permissible;

Active- mainly based on natural silicates - wollastonite, kaolin, mica, talc. Their improved technological properties are determined by “natural factors: the shape of the particles, the level of their anisotropy, the chemistry of the surface of the particles in relation to polymers;

Functionalized or superficially modified. It is known that functional modification of the filler surface with organic and / or inorganic compounds is of great importance in order to improve the quality and competitiveness of composite materials, which make it possible to impart additional properties to the filler that improve or optimize important parameters of the thermoplastic. It is the third group of fillers that is most promising for the production of thermoplastic composite materials.

In connection with the above, the filler becomes a carrier of special properties, which makes it possible to supplement, replace or save the corresponding technological additives. The use of fillers in polymers allows you to control the properties of products in the widest range of applications.

Thermoplastic composite materials can be conditionally divided into the following groups depending on the required qualities for the final product and scope of application:

Filled - have increased strength characteristics due to the introduction of mineral fillers - rigidity, strength, shrinkage resistance;

Flame retardant - have increased fire resistance and do not support combustion without an external source of flame due to the introduction of special additives - fire retardants;

Adhesive - have increased adhesive properties in polymer-polymer, polymer-metal, etc. systems. by modifying such copolymers as: ethylene vinyl acetate copolymer, ethylene ethyl acrylate copolymer;

Frost-resistant - have increased resistance to low temperatures due to the introduction of mineral fillers and elastomers;

Crosslinked - have increased heat resistance, strength and rigidity due to radiation or chemical crosslinking of the polymer;

Polymatrix - have additional properties that are different from the base grades due to the mixing of different grades of polymers;

Hybrid - have expanded possibilities of regulating the properties of the composite due to the introduction of fillers of various nature.

One of the leading problems of modern materials science is the creation of a new generation of thermoplastic composite materials that would satisfy the rather contradictory requirements of manufacturers and consumers.

Dictionary.

Plastics (plastics, plastics)- structural materials containing a polymer, which is in a viscous-flowing state during the formation of a product, and in a glassy state during its operation. Depending on the reason for the transition from a viscous-flowing to a glassy state, which occurs during the molding of products, plastics are divided into thermosets and thermoplastics.

Polymers- high molecular weight compounds, the molecules of which (macromolecules) consist of a large number of repeating groups, or monomer units, linked by chemical bonds.

Thermoplastics- polymeric materials that allow multiple transitions to a viscous-flow state when heated.

Reaktoplastics, thermosetting plastics- polymeric materials, which, when heated or under the action of hardeners, turn into an infusible and insoluble state.

Elastomers- polymers and materials based on them. Possessing highly elastic properties in a wide range of operating temperatures. Typical elastomers are rubbers and rubbers.

Polymer compounds- compositions based on thermosetting oligomers (epoxy and polyester resins, liquid organosilicon rubbers) or monomers (methacrylates, precursors for the synthesis of polyurethanes), intended for the isolation of conductive circuits and parts in electrical, radio engineering and electronic equipment. Basic requirements for compounds: no volatile matter; sufficiently large vitality; low viscosity.

Heat resistance of polymers- the ability to maintain hardness (that is, not to soften) when the temperature rises. The quantitative indicator of heat resistance in these cases is the temperature at which the deformation of the sample under conditions of constant load does not exceed a certain value.

dx.doi.org/ 10.18577 / 2307-6046-2015-0-6-9-9

UDC 541.6: 539.25

METHODOLOGICAL ISSUES OF ANALYSIS OF PHASE MORPHOLOGY OF MATERIALS BASED ON SYNTHETIC RESINS MODIFIED BY THERMOPLASTES (review)

An effective way to increase the fracture toughness of polymer composite materials (PCM) is the modification of synthetic resins with thermoplastics. Structural formation in such systems is accompanied by microphase separation with the formation of a characteristic phase morphology. Is being considered state of the art electron microscopic studies of the phase morphology of the thermoset-thermoplastic systems and PCMs based on them. The following methodological issues in the study of phase morphology are considered: the level of information content of the research method, the efficiency of contrasting characteristic elements of the microstructure, the rationale for the choice of key morphological parameters and methods of their measurement.

Introduction

Improving the service properties of thermosetting plastics when they are modified with thermoplastics is an important area in polymer materials science. The main goal of such modification is to increase the fracture toughness of thermosetting plastic and composite materials based on it. Increasing impact and crack resistance is especially important for materials used in aircraft construction.

Many modern scientific works emphasize the need to apply the "composition-technology-structure-properties" approach in the development of new materials. This approach is also effective in the development of polymer composite materials (PCMs) with increased fracture toughness. Controlling the physicochemical properties of the components and the composition of the mixture of thermoplastic with synthetic resin makes it possible to create new structural and functional materials with a predetermined set of properties. One of the key parameters through which it is possible to regulate and control the properties of a material based on the "thermoset-thermoplastic" system is its phase morphology. At present, the influence of the structural-phase state of PCMs on their properties is the subject of intensive research. Integral part scientific works to improve the dissipative properties of polymer matrices of PCM is the study of phase morphology and its influence on the service properties of the material.

The thermoset-thermoplastic systems differ significantly in their phase morphology. Depending on the concentration and thermodynamic compatibility of the components, the temperature of the onset of the chemical reaction of curing and a number of other factors, a structure is formed with different phase morphology and interfacial adhesion. If the initial reaction mixture was a homogeneous solution of a thermoplastic in a synthetic resin, then as the curing reaction proceeds, the solubility of the thermoplastic decreases due to an increase in the molecular weight of the resin. Another important factor affecting the thermodynamic compatibility of components during the curing reaction is the change in the chemical structure of the synthetic resin during the transformation of functional groups into reaction products. In the majority of thermoset-thermoplastic systems, which are interesting from the point of view of practical application, a further increase in conversion leads to microphase separation. The primary morphology is formed predominantly before gelation in the α-phase (the phase enriched in thermosetting plastic). The formation of a secondary phase morphology can be observed in the β-phase (the phase enriched in thermoplastic) after gelation in the α-phase. The parameters of the secondary phase morphology are sensitive to the post-curing temperature of the thermoset-thermoplastic system. Depending on the properties of the "synthetic resin-thermoplastic" system and the parameters of the curing regime, phase decomposition can proceed by the mechanism of nucleation and growth, by the mechanism of spinodal separation, or by a mixed type. The mechanism of phase decomposition determines such morphological parameters as the size, spatial distribution and size distribution of particles of the dispersed phase.

The concentration of thermoplastic in the initial reaction mixture is one of the main parameters that determine the phase morphology of the cured material. With an increase in the concentration of thermoplastic, the phase morphology changes from dispersed morphology, first to co-continuous, and then to morphology with phase reversal (Fig. 1). It is much more difficult to generalize the effect of the curing temperature on the morphological parameters of the microstructure, because it changes the ratio of the rates of phase separation and the chemical reactions of curing. Analysis of scientific and technical literature shows that in the development of materials based on synthetic resins modified with thermoplastics, the main attention in microstructural studies is paid to the influence of the concentration, chemical structure and molecular weight of the thermoplastic and the temperature regime of curing on the phase morphology of the material. Currently, active research is underway aimed at regulating phase morphology and interfacial adhesion using compabilizers (substances that reduce interfacial surface tension and increase interfacial adhesion at the polymer-polymer interface).

Rice. 1. Type of phase morphology:

a - dispersed; b - sleep-continuous; c - with phase reversal; d - its relationship with the concentration of thermoplastic

Electron microscopy in combination with specialized methods of sample preparation is an informative method for studying the phase morphology of polymer mixtures. The main methodological issues of the electron microscopic study of phase morphology are: the level of information content of the research method, the efficiency of contrasting characteristic elements of the microstructure, substantiation of the choice of key morphological parameters and methods of their measurement. The solution of these issues in combination with a deep understanding of the physicochemical processes of the formation of the structure of the studied polymer material will contribute to the development of electron microscopy as one of the methods providing information on the relationship "composition-technology-structure-properties" in materials based on the systems "thermosetting plastic-thermoplastic" ... This article discusses the current state of electron microscopic studies of the phase morphology of materials based on synthetic resins modified with thermoplastics, and the application of the results of these studies. All micrographs presented in the work were obtained by the authors of the article during electron microscopic studies of the thermoset-thermoplastic systems (since the article deals with general methodological issues, information on specific brands of materials is not provided).

Informative value of the study of the structure of materials

based on the systems "thermosetting-thermoplastic" by the method of electron microscopy

The main information provided by the electron microscopic study of thermoset-thermoplastic systems is the type of phase morphology, the geometric characteristics of the phases and their spatial distribution. The primary phase morphology (decay into α- and β-phases) is investigated by scanning electron microscopy (SEM). It is for this level of organization of the structure of materials that some correlation dependences of properties on the parameters of phase morphology are known. An interesting feature The structure formation predicted on the basis of thermodynamic analysis of phase separation using the Flory-Huggins mean field model is the formation of a secondary phase morphology during the decay of the β-phase. During the decomposition of the β-phase, a dispersion of thermoset-rich domains (γ-phase) is formed in the continuous phase, enriched with thermoplastic (δ-phase). Secondary phase morphology is investigated using transmission electron microscopy (TEM) on sections of submicron thickness prepared on a microtome. In the scientific literature, there are only a few works devoted to the study of this level of organization of the structure; therefore, there is no information on the influence of the parameters of the secondary phase morphology on the properties of materials. In fig. 2 shows photomicrographs of the phase morphology of an epoxy thermosetting plastic modified with polysulfone.

Rice. 2. Primary ( a) and secondary ( b) phase morphology of the "thermosetting-thermoplastic" system

The information content of electron microscopic studies has now significantly increased due to the use of analytical electron microscopy, which is a set of methods united by a common task - to obtain information about the elemental composition and chemical structure of phases. The use of X-ray spectral microanalysis makes it possible to reveal the spatial distribution of a polymer in a mixture if it contains contrasting atoms. For example, if the thermoplastic component of the thermosetting-thermoplastic system is polysulfone (contains sulfur atoms), then the researcher will be able to draw a conclusion about the distribution of polysulfone from the intensity of the characteristic X-ray radiation of sulfur atoms. An example of constructing the concentration profile of sulfur in an epoxy thermosetting resin modified with polysulfone by the method of analytical transmission microscopy is shown in Fig. 3. It is shown that the characteristic phase formations correspond to the change in the sulfur concentration along the coordinate, which makes it possible to determine the nature of the identified structural elements. The spatial resolution during the elemental microanalysis of the "thermoset-thermoplastic" system is significantly increased when using the methods of transmission analytical electron microscopy. Scanning analytical electron microscopy is more versatile and provides information on the elemental composition not only for microstructural studies, but also for fractographic studies.

Rice. 3. Concentration profile of sulfur in epoxy thermosetting plastic modified with polysulfone

The limited applicability of X-ray spectral microanalysis for studying polymeric materials is due to the low sensitivity of this method to elements with low atomic numbers (C, O, N, etc.), low electrical conductivity, and insufficient radiation-thermal stability of most polymers. Another disadvantage of this method is that it only provides information on the elemental composition. A promising method, devoid of many of the above-described disadvantages, is the formation of an electron microscopic image based on spectroscopy data of characteristic energy losses by electrons (EELE). The application of this method provides information on the chemical structure of phases, makes it possible, without special contrasting sample preparation, to reveal phase formations in polymer mixtures consisting only of elements with low atomic numbers, and also significantly increases the accuracy of quantitative elemental analysis of such systems. In this work, using this method, signs of microphase separation in the "bis (vinylfinyl) ethane-polyphenylene oxide" system were revealed and maps of oxygen distribution (and, consequently, of the phase enriched in polyphenylene oxide) were constructed with a spatial resolution of up to 10 nm.

Special methods of sample preparation

for electron microscopic examination

The main task of sample preparation is to achieve the best contrast between the studied inhomogeneities of the material microstructure. Depending on the method of electron microscopy and the required information on the structural-phase state of the system, different methods contrasting. Samples for TEM studies are prepared using microtomy. The most effective means of contrast staining of microtome sections are osmium tetroxide OsO 4 and ruthenium tetroxide RuO 4. Osmium tetroxide is used to color phases containing components with unsaturated bonds. For contrasting the phase morphology of the thermosetting-thermoplastic systems, RuO 4 is more effective, since it intensively colors the components containing ether, alcohol, amine and aromatic groups.

The SEM depth of field allows this method to be used to study samples with a developed surface relief. In this regard, to study the phase morphology by the SEM method, chips of the polymer matrix are made at a temperature liquid nitrogen... The obtained samples are suitable for rough estimation of interfacial adhesion and particle size distribution of the dispersed phase. In many works on the quantitative analysis of phase morphology, selective etching with solvents is used. Etching with organic solvents leads to the complete removal of the thermoplastic phase and allows obtaining an electron microscopic image suitable for direct stereometric quantitative analysis. Another popular method of sample preparation for SEM is the manufacture of thin sections. In this case, as in microtomation, the study of phase morphology is carried out on a two-dimensional section of the material, and in order to determine the true spatial morphological parameters, it is necessary to carry out a certain mathematical processing of the data.

Parameters determined by qualitative and quantitative analysis

phase morphology, and their relationship with the macroscopic properties of the material

A qualitative parameter of phase morphology, on which the properties of the "synthetic resin-thermoplastic" system and the curing parameters of this system have the greatest influence, is the type of phase morphology. This parameter provides important information about the dissipative properties of the material. It is shown that, in the general case, the fracture toughness increases with the transition from dispersed morphology to morphology with phase inversion. At the same time, the data on the optimal type of phase morphology, at which a significant increase in fracture toughness is simultaneously achieved and the valuable properties of thermosets (high modulus, heat resistance, resistance to organic solvents, etc.) are preserved. The work indicates that the optimal combination of properties is achieved when a dispersed morphology is formed with the maximum possible volume fraction of thermoplastic, while the work indicates that the most effective morphology is sleep-continuous. These studies indicate the need to control such a quantitative morphological parameter as the volume fraction of the dispersed phase of a thermoplastic. The determination of this parameter using the SEM method is most correctly carried out on a thin section. According to the first basic stereometric relationship, the volume fraction of the phase in the material is equal to the fraction occupied by the cross-sections of the phase in the area of ​​the thin section.

Other important quantitative morphological parameters are the size and particle size distribution of the phases. Direct measurement of these parameters is carried out by low-temperature cleavages of the polymer matrix. More accurate values ​​of these parameters can be obtained through special mathematical processing of data obtained in the study of thin sections or microtome sections. The mathematical processing algorithm and the model on the basis of which the mathematical processing is carried out are described in the work. It is indicated in the work that the optimal combination of the properties of the modified thermosetting plastic is achieved if the size of the dispersed phase of the thermoplastic is in the range from 0.1 to 10 μm. The particle size of the dispersed phase of the thermoplastic depends on the concentration of the thermoplastic, the temperature regime of curing, the use of compabilizers, and a number of other factors. With the formation of dispersed morphology, the particle size of the thermoplastic phase increases with an increase in the concentration of the thermoplastic. Increasing the initial cure temperature can lead to opposite particle size trends. The scientific literature describes both an increase and a decrease in the particle size of the thermoplastic phase with an increase in the initial curing temperature. This is due to the fact that an increase in temperature leads to an increase in the rate of the curing chemical reaction and to an increase in the rate of phase separation. These processes affect the particle size of the dispersed phase of the thermoplastic in the opposite way and which process will intensify to a greater extent with increasing temperature and determine the phase morphology of the cured polymer matrix. A number of works indicate that the formation of morphology with a bi- or polymodal size distribution of thermoplastic particles leads to an additional increase in the dissipative properties of the material. Phase morphology with such a particle size distribution can be formed upon joint modification of a synthetic resin with thermoplastics of different chemical structures or at a high rate of the curing reaction.

Determination of the phase morphology parameters provides important information for fractographic studies of thermoset-thermoplastic systems. At present, the qualitative mechanisms of increasing the dissipative properties of polymer matrices are described by dispersed thermoplastic particles and quantitative models of hardening are proposed. The main mechanisms of hardening in thermosetting plastics modified with thermoplastics include overlapping a crack with thermoplastic particles, bending around thermoplastic particles by a crack, and the formation of shear bands and microcracks in the matrix. The mechanism that most effectively increases the dissipative properties of the polymer matrix is ​​considered to be crack bridging by particles of the dispersed phase of the thermoplastic, which is accompanied by plastic stretching and rupture of these particles. This mechanism is realized with high interfacial adhesion and nanosized particles of the thermoplastic phase. In fig. 4 shows the fracture surfaces of the polymer matrix of an epoxy thermosetting resin modified with polysulfone with a co-continuous phase morphology. In the area of ​​dispersed morphology, a characteristic element of the structure is thermoplastic particles destroyed as a result of plastic deformation. The area of ​​phase-inversion morphology is characterized by a complex fracture surface relief, which is caused by the bending of a growing crack around hard particles of epoxy thermosetting plastic and plastic deformation of the continuous phase of the thermoplastic.

In view of the fact that thermoset-thermoplastic systems are used as polymer matrices of modern PCMs, an important issue is the change in phase morphology in the presence of a reinforcing filler. A number of scientific research works are devoted to a systematic study of the influence of the chemical nature of reinforcing filler fibers and the state of their surface on the qualitative and quantitative parameters of phase morphology. It is shown in the work that a layer enriched with epoxy thermosetting plastic is formed around glass fibers, which negatively affects the dissipative properties of PCM. No such layer was found around carbon and aramid fibers. The paper reports an increase in the average particle size of the dispersed phase of the thermoplastic near the fibers of the reinforcing filler. In the works, a quantitative parameter of the change in the phase morphology in the presence of a reinforcing filler is proposed: the number of particles of the dispersed phase of the thermoplastic per unit area at a certain distance from the fiber. It is also shown that the concentration of dispersed thermoplastic particles near the fiber increases upon activation of its surface and depends on the chemical structure of the thermoplastic. It should be noted that, despite the research work carried out in this direction, a unified idea of ​​the effect of the filler on the formation of phase morphology has not been formulated at present.

Rice. 4. Phase morphology of epoxy thermosetting resin modified with polysulfone ( a), and the fracture surface in the area of ​​dispersed morphology ( b) and phase-reversed morphology ( v)

The presented work reflects the role of electron microscopic studies in the development of polymer matrices based on thermoset-thermoplastic systems for PCMs with high impact and crack resistance. Since the optimal combination of the properties of such materials is achieved during the formation of a microstructure formed as a result of microphase separation, the most important issues are the control and monitoring of phase morphology. This paper provides examples of information on the structural-phase state of the system, which is provided by electron microscopic examination. It is shown that at present, electron microscopy allows not only to carry out studies of phase morphology at various hierarchical levels of system organization, but also to determine the elemental composition and chemical structure of phase formations with high spatial resolution. The presently available concepts of the control of morphological parameters in the development of materials based on synthetic resins modified with thermoplastics are described. Methodological approaches for measuring such parameters as the volume fraction of the dispersed phase of the thermoplastic, the average particle size and the particle size distribution are outlined. Information on the influence of the qualitative and quantitative parameters of phase morphology on the properties of the material is given. World and domestic experience in applying the results of studies of phase morphology to control the properties of PCM proves the effectiveness of electron microscopy as one of the methods providing information on the relationship "composition-technology-structure-properties" in materials based on thermoset-thermoplastic systems.

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Modeling the deformation of composites

Introduction

The presence of a large number of cement plants in Ukraine makes reinforced concrete and its modifications one of the main building materials for hydraulic structures.

Reinforced concrete with a frequent regular arrangement of steel rods in two or three directions can be considered as a composite reinforced material with anisotropy, that is, the dependence of mechanical properties on the direction of action of forces, which is due to reinforcement and nonlinearity of deformation associated with cracking, plastic properties of concrete and steel. In hydraulic engineering, concentrated placement of reinforcement in a stretched zone is often used, therefore, structures of this type will be the subject of further study.

Composite materials are widely used in various industries modern technology. Further progress in the development of many areas of construction is largely associated with an increase in the share of the use of such materials, and when creating new and special equipment, their role becomes decisive. The requirements for optimal design, reduction in time and material costs for experimental development have determined a significant interest in improving methods for predicting the deformation and strength properties of composites.

On the other hand, the development of the mechanics of a deformable solid follows the path of increasing the complexity of the models under study and the formulation of problems. Based on the model concepts of mechanics, a composite material can be defined as an inhomogeneous medium described by rapidly oscillating material functions discontinuous in coordinates, which, as a rule, are considered either periodic or random homogeneous. The need to develop methods for solving differential equations with such coefficients has led to the emergence of a relatively new area of ​​mathematical research - the theory of averaging of partial differential operators, which makes it possible to obtain a solution to the original problem using simpler differential equations called averaged.

The problem of calculating the coefficients of averaged equations, known in the mechanics of composites as the problem of predicting effective characteristics, is one of the central ones, since it opens up the possibility of synthesizing materials with a predetermined set of properties that best suit specific operating conditions. Thus, each inhomogeneous medium is associated with a certain anisotropic medium with effective properties, for which it is convenient to carry out calculations of structures and parts made of composite materials using well-known mathematical methods of solid mechanics.

At the same time, the study of the mechanical behavior of structural elements, taking into account the concentration of stress and strain fields inhomogeneous within each of them, allows not only to directly determine the effective properties, but also provides extensive information on the nature and characteristics of deformation and fracture of materials, depending on the real structure of composites. and their components.

In this work, special attention was paid to the analysis of the results of theoretical and experimental studies of dissipative processes of inelastic deformation and fracture of anisotropic structurally inhomogeneous bodies. Much attention is paid to the study of the regularities of the supercritical stage of deformation, during the implementation of which the material loses its bearing capacity not immediately, but gradually, which is reflected in the deformation diagram in the form of a falling branch.

This mechanical phenomenon (previously known) was discovered when solving the problems of mechanics of elastoplastic deformation of composite materials, taking into account structural failure.

However, the desire for an adequate description of the behavior of structures and optimal design of the structure of the created composite materials from the standpoint of fracture resistance led to the need to take into account the softening stage (at the stage of setting the problem) in the constitutive relationships and study the conditions of supercritical deformation of structure elements in the composition of the composite.

The paper considers an approach in which the destruction of inhomogeneous bodies is considered as a result of the loss of stability of deformation processes at the supercritical stage, accompanied by structural destruction. New mathematical models make it possible to describe in a natural way the stages of dispersed damage accumulation, fracture localization, as well as the merging of destroyed zones, taking into account plastic deformations in inhomogeneous anisotropic media using special functions of the state of the material, the transition to an unstable stage can be modeled using stability criteria for damage accumulation, and the energy relations of fracture mechanics should be written using the parameters of the falling branches of the complete deformation diagrams. The paper investigates the concept of a loading system and its influence on the stability of dissipative processes. Some questions of the theory of stable supercritical deformation are presented. The problem of averaging, traditional for the mechanics of composites, is considered in new aspects related to the expansion of the physical base of the mathematical models used.

1. Modeling the deformation and fracture of composite materials

The methods for predicting the effective elastic properties of modern composites are well developed. The results achieved in the linear theory of elasticity on predicting effective properties and the accompanying results on determining the fields of microstresses and microstrains are a good basis for studying the elastoplastic and strength properties of microinhomogeneous materials. The striving for a more complete use of the bearing capacity of critical structures inevitably leads to the need for comprehensive studies preceding the construction of complex models of deformation and fracture of real materials under a complex stress state and nonlinear properties of structural elements.

1.1 Inelastic deformation of composites and its structural failure

Mechanical micro- and macroscopic processes in inhomogeneous materials have been studied in sufficient detail within the framework of deterministic and statistical models of composite mechanics. The advantage of statistical models is that they naturally take into account such important factor the real structure of composites, as the randomness of the mutual arrangement of elements and the statistical scatter of their properties. However, in the statistical mechanics of composites, the question of more complete, in comparison with the one-point approximations, taking into account the many-particle interaction of the components remains open. Therefore, in the overwhelming majority of works in this direction, the analysis of the stress-strain state of composites is limited to calculating the deformation fields averaged over the components. The calculation of other statistical characteristics of deformation fields for cases of anisotropic and combined loading, as well as the construction of solutions of nonlinear boundary value problems for the processes of accumulation of plastic deformations and damage in the components of composites, taking into account the inhomogeneity of the deformation fields, is of particular importance in the problems of predicting strength properties.

It is characteristic that the properties of composite materials can fundamentally differ from the properties of the constituent components. For example, the absence of plastic changes in the volume of structural elements can be accompanied by a plastic change in the volume of the composite, a hardening material can be created from ideally plastic components, a highly hardening material can be created from weakly hardened components, etc. This indicates the complexity and diversity of the phenomenon under consideration, the theoretical description of which requires the development of special approaches and mathematical models.

the physical phenomenon of the elastic-plastic behavior of composite materials and, most importantly, the need to study it were discovered long before the creation of the corresponding mathematical theory. Therefore, many researchers in the mid-sixties turned to the analysis of the behavior of materials using simple models. A model in the form of a set of parallel constituent elements was used to approximate the inelastic deformation of a unidirectional composite under tension across the fibers. Some scientists used the model of coaxial cylinders, assuming the simplest stress state of the matrix material. The approximation of a real material by an infinite medium with a single reinforcing element located in it was used. Many techniques still in use are based on the mixture rule, which assumes homogeneity of either the stress field or the strain field. Various modifications of this rule make it possible to achieve agreement with experimental data.

To date, thanks to the use of numerical methods of mechanics of a deformable solid and some new approaches developed directly for structurally inhomogeneous bodies, solutions have been obtained for a number of problems of inelastic deformation, taking into account the complex nature of the distribution of stresses and strains in structural elements. Composite materials considered as homogeneous with effective properties, depending on the structure, can be either isotropic or anisotropic, even if they consist only of isotropic components. When formulating the problems of determining the effective characteristics of anisotropic composite materials, it becomes necessary to choose the theory of plasticity of an anisotropic body, which makes it possible to adequately describe the behavior of an equivalent homogeneous medium.

Many different versions of the deformation theory of plasticity and the theory of flow have been proposed. Much attention is paid to determining the number and structure of independent invariants of a given set of tensors. The issue under consideration seems to be very important for the mechanics of composites, however, the extremely limited number of works on the experimental study of the regularities of deformation of anisotropic materials under conditions of a complex stress state does not allow us to fully assess the reliability and generality of one or another version of the theory of plasticity of anisotropic media.

The study of the elastoplastic behavior of anisotropic composites, such as fibrous unidirectional and spatially reinforced, layered with homogeneous and inhomogeneous layers, is a rather difficult problem. The solution of problems in the mechanics of composites for these materials is carried out mainly in some of the simplest cases of the stress state, which, of course, is a definite scientific achievement. However, such solutions usually do not allow constructing all material functions describing the behavior of a composite in an arbitrary complex stress-strain state within the framework of the chosen theory of plasticity of an anisotropic body.

The inelastic deformation of layered composites under uniaxial tension along the layers was investigated under tension across the layers. the behavior of the composite in a plane stress state is considered, when the forces stretching in two directions lie in a plane parallel to the layers. It should be noted that a significant part of the results were obtained without taking into account interlayer interactions. It is clear that such a simplification in some cases may turn out to be too rough. This is confirmed by the fact that the destruction of laminated structures often occurs by delamination.

The nonlinear nature of the relationship between stresses and deformations of composite materials can be a consequence of not only plastic deformation and take place even in the case of linearly elastic components. This is due to the fact that the complete (macroscopic) destruction of composite products is preceded by a complex process of destruction of individual structural elements. The study of this process is important not only for analyzing the conditions for the formation of a macroscopic crack, but also for studying the behavior of a material under load.

Each act of structural destruction is accompanied by a redistribution of stresses in the elements of the composite, leading either to the continuation or termination of destruction at a given level of external load.

The construction of models of inelastic deformation of composite materials taking into account these processes raises as the main questions of choosing criteria for structural destruction and describing the residual deformation and strength properties of elements of an inhomogeneous medium after meeting certain conditions of their destruction. In this case, it is important that an element of the composite structure can be destroyed by various mechanisms. For example, in the case of a reinforced monolayer, cracking or delamination of the matrix, splitting, breaking or pulling of fibers, etc. is possible. These and other mechanisms for changing the bearing capacity of a structural element are identified with one or another scheme for changing its stiffness properties.

As already noted, in the study of composite materials, it becomes necessary to use probabilistic concepts and the apparatus of the theory of random functions, due to the random nature of the properties, the mutual arrangement of structural elements and, as a consequence, the stochastic process of their destruction.

Thus, among other problems in the mechanics of composite materials, the development of nonlinear models of the behavior of composites taking into account the destruction of structural elements and the development of methods for solving problems of inelastic deformation for various cases of a complex stress-strain state are urgent.

1.2 Phenomenological models of fracture mechanics

There are two approaches to the construction of theories in natural and applied sciences - phenomenological and structural. Phenomenological models are built on the basis of empirical data on the behavior of an object. In this case, the task of explaining or fully describing the essence of phenomena is not posed. The structural approach consists in developing models that allow describing and explaining phenomena based on the internal structure of the objects under consideration. These approaches are closely related and should mutually enrich each other. The construction of nonlinear models of the behavior of a medium with effective properties for describing the deformation of the composite, accompanied by the destruction of structural elements, corresponds to the methodology of phenomenological description.

The necessity and usefulness of phenomenological theories was substantiated by V.V. Novozhilov. In this case, it is permissible to establish different levels phenomenological description. For example, damage accumulation can be modeled by considering a system of disk cracks or pores in a continuous medium. L.M. Kachanov and Yu.N. Rabotnov introduced the parameter of damage (or the opposite - continuity), determined by the area of ​​cracks per unit area cross section body. At the same time, this parameter may not be identified with any characteristic of specific defects and damages, if it is included in the relations connecting the averaged values. This is natural when, when determining the material functions of a model, one can do without direct microstructural studies, for example, measuring the area of ​​discontinuities.

The phenomenological approach to modeling the damage of materials consists in describing the formation of internal discontinuities using some functions of the state of the material. This idea was reflected in famous works A.A. Ilyushin, V.V. Bolotin, V.P. Tamuzha and A. Zh. Lagedinsha. It was developed thanks to the efforts of many other researchers and was the basis for the creation of the mechanics of a damaged continuous medium, within which damage to a material is defined as any microstructural change leading to any change in mechanical properties.

Currently, a significant number of scalar and tensor characteristics of damage are known. The main provisions of the three-dimensional theory of anisotropic damage and the corresponding tensor models are substantiated.

The process of destruction of structurally inhomogeneous media has a multistage character. The most pronounced stage is volumetric, or diffuse, destruction, which is associated with the volumetric accumulation of stable microcracks and, when the threshold concentration is reached, passes through enlargement and merging to the next scaled level. In addition, it is shown that the effective deformation characteristics depend on the correlation radius of a random set of defects. It is natural to assume that the nature of the interaction of microdamages also determines the conditions for macrofracture of an inhomogeneous medium and, consequently, its strength properties.

The multilevel nature of the formation of the reaction of the material to external mechanical stress predetermines the possibility of a multilevel phenomenological description. Each structural level is associated with a certain system of heterogeneity elements (natural or caused by damage). Analysis of the stresses and strains introduced at the structural level as averaged quantities serves as a means of studying the mechanical behavior of a material within the framework of the corresponding level of phenomenology. A two-level consideration of the processes of deformation and fracture is the basis of the Davidenkov-Fridman classification and the structural-phenomenological approach in the mechanics of composites.

The problem of describing the transition from micro- to macrofracture is very important for the mechanics of composites. At the same time, there are many different initial prerequisites and methods for assessing strength from the standpoint of structural mechanics. In the present work, an approach is developed according to which macrofracture is considered as a result of the loss of stability of the deformation process associated with the accumulation of damage. The process of loading an elastoplastic system becomes unstable if the catastrophic development of displacements and deformations corresponds to an arbitrarily small continuation of this process. The decisive role of a special kind of nonlinearity (a descending branch in the deformation diagram) in stability issues related to the problem of fracture was noted in the work of A.A. Ilyushin. All physical processes occurring in a material under loading are reflected in full deformation diagrams, with the falling sections of these diagrams corresponding to individual stages of destruction.

The possibility of the appearance of a falling section on the diagram due to the process of cracking and damage was noted in S.D. Volkov. This character of material behavior at the final stage of material deformation is in many cases associated with the formation or development of a macrodefect. In this regard, along with an explicit description of a crack in a deformable body, the phenomenological direction of fracture mechanics, which describes the behavior of a material at the stage of formation and growth of a macrocrack, seems promising. This direction was initiated by S.D. Volkov. The use of this approach is associated with the assumption that the mechanical behavior of an arbitrarily small volume of a material in the presence of discontinuities commensurate with its dimensions is similar to the behavior of a macrosample at the final stage of deformation. This, to a certain extent, reflects the self-similarity of the destruction process.

According to the hypothesis of macrophysical determinability of A.A. Ilyushin, each point of the medium can be assigned a macrosample in the form of a body of finite dimensions, which is in a uniform stress-strain state and on which, in principle, all processes occurring at the depicted point of the medium can be studied.

The indicated correspondence can be established as follows: the displacements of the boundaries of the working zone of an imaginary ideal homogeneous specimen of ee material filling an elementary deformable volume, under conditions of a uniform stress state under the same loads, must coincide with the displacements of the boundaries of the working zone of the experimental specimen at all stages of deformation, including the stage formation and growth of macrocracks. On the basis of these assumptions, the phenomenological equations and criteria adopted in the mechanics of a deformable solid can be used.

There is a certain analogy and commonality between the approaches of crack propagation mechanics and phenomenological fracture mechanics. In particular, within the framework of the first theory, subcritical fracture diagrams are considered, which are the dependences between the average tensile stress in the undamaged section of the specimen and the length of the crack at different initial values. The locus of the critical (corresponding to the dynamic growth of cracks) points of the individual curves is called the critical fracture diagram. Naturally, when testing smooth specimens, the critical point corresponds to the ultimate strength.

Without explicitly considering cracks and ruptures and describing the behavior of the material using the descending branch of the deformation diagram, we can conclude that it, in fact, is also a critical diagram, since it is the locus of critical points for samples with different degrees of damage, obtaining as a result of equilibrium deformation to one degree or another and subsequent elastic unloading.

When describing the subcritical growth of a defect, the approach of J.R. Irwin is also used, which consists in considering the dependence of the fracture work R on the crack length as a characteristic of resistance to crack growth. If, within the framework of the phenomenological approach, the work of destruction is understood as energy dissipation associated with the process of damage accumulation, then it can be calculated using the deformation diagram at any deformation interval. The resulting graphical dependence of the fracture work on deformation has a character similar to the R-curves known in fracture mechanics.

The phenomenological approach makes it possible not to encounter the problems of modeling the complex geometry of real cracks and ruptures in damaged structurally inhomogeneous media and determining the area of ​​the fracture surface, which is complicated by its unlimited increase with more detailed consideration. At the same time, it makes it possible to describe all stages of damage, including the transition to an unstable stage, by functions of the state of the material and to use the energy relations of fracture mechanics and complete diagrams of material deformation.

1.3 Supercritical stage of material deformation

Supercritical deformation of structurally inhomogeneous media prone to destruction of various nature at mechanical stress, is one of the important mechanical processes requiring special research. The critical stress-strain state corresponds to the moment of reaching the maximum stress values ​​for a given material under the given conditions, and the post-critical stage is characterized by a decrease in the stress level with progressive deformations. The noted feature of mechanical behavior is inherent in metals, both for the connection of conditional and true stresses and strains, geological, ceramic, polymer and composite], as well as other materials.

The material at the supercritical stage of deformation does not satisfy Drucker's postulate and is classified as rheologically unstable. However, many real materials are adequately described by the models of rheologically unstable materials. In this case, instead of the requirement for rheological stability, the principle of stability for the body as a whole is put forward: the state of the material is realizable if in this state it is part of a stable mechanical system

Improvement of material models in order to describe the accumulation of damage at the supercritical stage of deformation is an important task in the mechanics of composites. A refined design of structures using complete diagrams requires, in addition, the development of methods for solving boundary value problems, taking into account the softening of the material and obtaining conditions for the stability of postcritical deformation in weakened zones.

Naturally, this should be based on effective experimental methods for constructing equilibrium deformation diagrams.

theoretically substantiated the feasibility of the states of the material corresponding to the descending branch of the deformation diagram. Based on the theorems of Hadamard and Van Hoof, which give local necessary and sufficient conditions for stability for elastic bodies, and their generalizations to the case of elastoplastic bodies, it is shown that even in the presence of a "falling" diagram, a body fixed at the boundary with a sufficient (not even necessarily very large) rigidity, can be stable. There are no fundamental obstacles to the registration of such states in experiment, in particular, under uniaxial tension or shear (in the deviatorial sense) deformation, and the interpretation of the corresponding experimental data in terms of the softening property inherent in the material.

It has been experimentally confirmed that the fracture resistance is determined not only by the strength constants of the material, but also depends on the rigidity of the loading system, which includes the loading device (testing machine, transferring loads, power and kinematic elements of structures, working fluid and gas) and the deformable body itself, surrounding area of ​​damage. Under "soft" loading, when forces that do not depend on its resistance are applied to a body in a uniform stressed state, destruction occurs when the maximum stresses are reached.

In another limiting case, when the specified displacements of the boundary points ("hard" loading) are provided, as well as with a finite, as already noted, but sufficient rigidity of the loading system, an equilibrium process of damage accumulation is possible, which is reflected in the deformation diagram in the form of a falling branch.

Depending on the loading conditions, each point on the descending branch of the deformation diagram can correspond to the moment of failure. Deformation of this kind is feasible only for a local object as part of a mechanical system with the required properties. Otherwise, there is a nonequilibrium accumulation of damage and macrofracture as a result of the loss of stability of the deformation process at the supercritical stage. In the area of ​​softening, the occurrence of localization of deformation in the form of shear bands is also possible. A falling branch is observed when there are mechanisms and conditions for the gradual dissipation of elastic energy. Thus, the states of the material under consideration can be called conditionally realizable.

It may be appropriate to use a somewhat abstract analogy for illustration. Deformation of a softening medium is stable approximately to the same extent as a more or less viscous liquid in a certain vessel is stable. Loss of stability occurs if the walls of the vessel do not have sufficient rigidity. In this case, the role of the vessel is similar to that of the loading system. The main difficulty in the experimental construction of complete diagrams is to create a sufficient rigidity of the loading system for a material element. For this purpose, devices for increasing the rigidity of standard machines, special samples, and testing machines with high-speed feedback have been developed.

The falling branch of the graph of the deformation dependence during testing of metal samples is, for the most part, a reflection of the equilibrium growth of the main crack. In some cases, this is also true for composites. At the same time, if the strength and deformation properties of the structural elements of an inhomogeneous medium differ significantly, which is typical for most composite materials, then the formation of a pronounced macrocrack may not occur. However, the developed discrete scattered destruction of weak elements in this case also leads to a drop in the diagram. The randomness of inclusions ensures the sequence of the occurrence of fracture zones in parts of a heterogeneous medium that are distant from each other, which creates an obstacle to localizing deformations and allows using probabilistic approaches to determine the relationship between the average stress and average deformation. A certain structural heterogeneity provides a predominant type of deformation, different from localized. In particular, for bodies of a fibrous structure, the falling portion of the diagram arises as a result of successive breaking of unequal fibers. The nature of the process of destruction of inhomogeneous media significantly depends on the randomness in the arrangement and the degree of scatter of the properties of structural elements, therefore, the statistical characteristics of the strength of these elements largely predetermine the parameters of the descending branch, in particular, its slope, which reflects the tendency of the material to brittle fracture.

The relationship between the type of falling sections of the diagram and micromechanisms and stages of destruction is noted in the works. S. D. Volkov put forward the idea that the nature of the stress distribution at the crack tip, in principle, repeats the descending portion of the curve in the complete diagram of material deformation obtained by testing a smooth specimen. In this case, the problem of the singularity of the problem is solved automatically due to the decrease to zero of the resistance of the material at the singular point (crack tip), where the deformation is maximum and is equal to the limiting one for a completely equilibrium state. The rigidity of the loading system for a material element at the crack tip can be finite and sufficient for stable supercritical deformation in this zone, which explains the possibility of the existence of equilibrium cracks.

There is a connection between the deformation diagram and the energy consumption of the fracture process. The area under the falling branch of the complete diagram determines, at the same time, the performance of the material at the stage of macrocrack formation. S. D. Volkov suggested a relationship between this value and the characteristics of the fracture toughness of materials. To date, A.A. Lebedev and N.G. Chausov developed and experimentally substantiated an express method for assessing the fracture toughness of plastic materials using the parameters of the falling sections of complete deformation diagrams.

It is necessary to take into account the close relationship between the compliance of the loading system and the kinetics and localization of the fracture process. For example, in engineering practice, a significant difference has been noted in the nature of the destruction of hydraulic and pneumatic pressure vessels and pipelines. From the point of view of traditional formulations of boundary value problems, these cases are equivalent. In this regard, the boundary conditions that do not take into account changes in external loads associated with a change in the configuration of the body during deformation and damage do not fully correspond to the real operating conditions of structural elements and the tests performed.

From this point of view, for a more adequate description of the processes of deformation, accumulation of damage and destruction, it is advisable to use boundary conditions of the third kind, which make it possible to expand the physical base of the available models of the mechanics of structurally inhomogeneous media, to clarify the strength estimates, to determine the reserves of the bearing capacity and to predict the catastrophic failure of structures.

Many authors note the attractiveness of the implementation of the supercritical stage of deformation in the elements of structures or structures, which leads to the use of their strength reserves and an increase in their safety. The completeness of the implementation of the bearing capacity of the material is determined by the degree of supercritical deformation. In addition, it should be noted the importance of the previously unexplored task of determining the conditions for stable supercritical deformation of structural elements in the composition composite material as a basis for creating materials with improved mechanical characteristics.

Optimal (from the point of view of damage processes in an equilibrium mode) design requires a mathematical description of supercritical deformation, which is not reduced only to the approximation of diagrams with falling sections. The issues of substantiating continuous models of softening media and determining the area of ​​their applicability have not lost their relevance. A number of mathematical problems arise, connected, first of all, with the analysis of the stability of the deformation process, the uniqueness of the solution of the boundary value problem and a possible change in the type of differential equations, as well as the need to take into account the properties of the loading system, the development of constitutive relations (even for isotropic materials), the development of numerical methods and the creation of effective iterative procedures for solving this kind of nonlinear problems.

2. Structural-phenomenological model of the mechanics of micro-inhomogeneous media

In the previous section, we noted the existence of two approaches to constructing models of mechanics - phenomenological and structural. In the works of a number of scientists, an approach developed in relation to the mechanics of composites and called structural-phenomenological has become widespread. It consists in the fact that the phenomenological equations and criteria generally accepted in the mechanics of a deformable solid are considered at several, in particular, two levels: microscopic (structural), associated with the elements of the composite structure, and macroscopic, reflecting the behavior of the composite material as homogeneous with effective properties. The relationship between physical quantities, established within the framework of this approach, determines the structural-phenomenological model. In this section, the main provisions of the theoretical study of the deformation and fracture of composite materials under quasi-static loads undertaken in this work are formulated in the framework of an approach associated with the formulation and solution of a hierarchical sequence of boundary value problems. The involvement of probabilistic representations and the apparatus of the theory of random functions allows one to study models that simultaneously take into account the random nature of the properties and the mutual arrangement of structural elements.

2.1 Models of random and periodic piecewise homogeneous media

In mathematical modeling of the processes of deformation and fracture of composites, the development of studies in which the material is considered as a micro-inhomogeneous medium is relevant.

Let a domain V with boundary S contain the set of disjoint domains uk bounded by surfaces Sk. For two-component composites, part of the V1 = Ushk region is filled with a homogeneous material with properties (first phase) within the range, and the remaining part of the region V2 = V - V1 is filled with a homogeneous material with properties. Multiply connected surface S12 = УSk is an interface separating the structural elements of the composite. Part S (1) of the surface S passes through the first phase, and the other part S (2) = S - S (1) passes through the second.

If complete information is known about the nature of the mutual arrangement of the regions uk and phenomenological models of phases are given, then they say that the post-Roena model of a piecewise homogeneous (compositional) medium.

Let us accept the following definition. A subdomain Vl with a characteristic size l is called the representative volume of the region V (with a characteristic size L >> l) for a function g (r) that is continuous throughout the phases V1 and V2, if there exists and is bounded the averaged quantity

and if, for any positive, arbitrarily small number g, there is a positive number g depending only on g such that

Obviously, in order for this definition to be valid, and the representative volume Vl at the physical level of rigor to have the meaning of an elementary macro-volume of a micro-inhomogeneous medium, it must be assumed that

L >> l >> lsh (where lsh is the characteristic size of the regions uk). When condition (2.2) is satisfied, the influence of the averaging scale on the value of the averaged quantity can be neglected.

The model of the mechanics of a micro-inhomogeneous medium, considered in what follows for composites, is based on the assumption that the characteristic size of the regions uk is much larger than the molecular-kinetic dimensions and much less than the distances at which the averaged or macroscopic values ​​change significantly. Then phenomenological equations and relations of mechanics remain valid for structural elements, i.e. elementary microvolumes dV, constituting structural elements of composites and having a size dl (dl<

This assumption provides an opportunity, on the one hand, to single out studies of the behavior of individual inhomogeneities and processes around them (for the material as a whole, these are microprocesses), carrying them out independently using models and methods of solid mechanics. On the other hand, it allows one to describe macroscopic processes in a medium as homogeneous, while the results of the study of microprocesses will be used in continual equations with the help of some averaged parameters reflecting, in particular, the interaction of structural elements.

Let for each of the components of the composite filling the volume V, the stress and strain tensors are connected using the operators

where are the material functions of the constitutive equations of the i-th component (phase). By a component of a composite material we mean a set of all structural elements with the same physical and mechanical properties.

We introduce the indicator functions of the composite structure

where Vi- is the area occupied by the i-th component, f is the number of composite components. Let us construct piecewise continuous functions of structural properties

Now the constitutive relations of a micro-inhomogeneous medium

are presented as equations with rapidly oscillating coefficients. In this case, one of the general cases is a model of a composite with a random structure, when there are random homogeneous functions, and a contain random variables, i.e. the statistical spread of properties of structural elements is taken into account. For random indicator functions, sets of one- and multi-point probability densities or moment functions that are invariant with respect to a parallel translation of the coordinate system should be known:

where r "is an arbitrary radius vector.

The relationship between moment functions and probability densities is

In the particular case, for δ = 1, we arrive at the concept of an averaging operator for random fields, which, under the conditions of statistical homogeneity and ergodicity, is equivalent to the statistical averaging operator. For the mathematical expectation of the functions, we have

and, replacing the improper integral in (2.7) by the integral over the elementary macrovolume Vl at uniform densities satisfying the normalization condition Vldr = 1, we obtain

The transition for calculating the moment functions of higher orders of random homogeneous fields is carried out in a similar way. For composites with a periodic structure, the indicator functions are periodic.

where b is a constant translation vector, n are arbitrary integers.

The periodic structure of composites can be considered as a possible realization of a random homogeneous structure.

2.2 Boundary value problems of the mechanics of composites

Let the stresses in the region V in the absence of mass forces satisfy the equilibrium equations

уij, j = 0. (2.9)

and small deformations are associated with displacements by the Cauchy relations

еi, j = (ui, j + uj, i). (2.10)

In the constitutive relations (2.6) for the composite material filling the region V, the material functions akl (r), in accordance with (2.5), form random homogeneous fields, the statistical characteristics of which are assumed to be known.

Let us assume that linear boundary conditions of the contact type are given on the part S (q) of the surface S of the region V:

where, are some positive definite tensors, pi is the vector of the unit normal to the surface S, is the vector of contact forces.

From conditions (2.11), as special cases, the boundary conditions for the region V in stresses, in displacements (at Ni = kui °, when k is a dimensional constant, ui ° is a displacement vector specified on the boundary) and of a mixed type follow.

Equations (2.9), (2.6), and (2.10) together with the boundary conditions (2.11) constitute the boundary value problem for the region V.

Accordingly, the quasi-static boundary value problem in displacements consists in solving the equations obtained by successive substitution of (2.10), (2.6) in (2.9), of the form

subject to the boundary conditions

When solving the boundary value problem (2.12), (2.13) for composites, due to the discontinuity of the material functions bi, j (r) of the operator F, it is necessary to seek the so-called generalized solution.

We multiply equation (2.12) by an arbitrary sufficiently smooth function wi (r) and use the formula for integration by parts:

Boundary conditions (2.13) are transformed by multiplying their left and right sides by the tensor t (q) inverse to the tensor c (q), that is, such that:

Then by a generalized solution to the boundary value problem (2.12), (2.13) we mean a continuous vector field u (r) that satisfies the identity

for arbitrary vector functions w (r).

For a composite material, an equivalent concept of a generalized solution can also be given. The corresponding problem must be solved inside each structural element of the region V, the material functions aij of the constitutive relations (2.6) of which are continuous (i.e., find a classical solution), and on the interface S12, the conditions of ideal contact must be fulfilled:

In what follows, speaking about the solution of boundary value problems for composites, we will understand the construction of precisely generalized solutions.

To simulate the processes of destruction of structural elements of composites, we assume that when the condition

where P (i), respectively, is the operator of the strength criterion and the strength characteristics of the ith component, at some point in the region V there is a partial or complete loss of the material's ability to resist the action of internal forces, which is reflected in a change in the defining relations of the form (2.3) for a given points.

It is usually impossible to directly obtain the solution of boundary value problems in the mechanics of deformation and fracture for systems of equations (2.9), (2.6), (2.10) or (2.12) taking into account condition (2.15), since these solutions, as well as the coefficients of the equations, are rapidly oscillating functions of coordinates. Therefore, an approach has become widespread when the system of equations of the structural-phenomenological model is put in correspondence with the system of equations for averaged stresses, deformations and displacements, which are called macroscopic.

For example, in a boundary value problem for elastic composites of the form

you can go to the averaged values ​​as follows.

Let the coefficients of equations (2.16) be rapidly oscillating (random homogeneous or periodic) piecewise homogeneous functions, and at all points of the domain V the condition of uniform ellipticity is satisfied:

where k0, K0 are positive scalar values.

Then the solution to the boundary value problem (2.16) exists and is unique. Asymptotic expansion of this solution in a small parameter

is such that the first term of series (2.17) is a solution to the boundary value problem

moreover, the operator of the boundary value problem (2.18) is uniformly elliptic

From the expansion (2.17), as well as due to the existence and uniqueness of the solution to the boundary value problem (2.18), it follows

The convergence condition (2.20), in which the quantities ui * (r) have the meaning of averaged (or macroscopic) displacements in the norms of various spaces for random homogeneous, quasiperiodic, and periodic operators, is shown in the works of various authors.

In the mechanics of micro-inhomogeneous media, from the fields of equations (2.6), (2.9), (2.10), called the fields of micro- or structural displacements, deformations and stresses, one can go over to averaged fields using the concept of the elementary macrovolume Vl.

The stressed state of elementary macrovolumes is characterized by a macrostress tensor with components, and the deformed state is characterized by a macrostrain tensor with components. The resistance of elementary macrovolumes to deformation determines the relationship between macrostresses and macrodeformations:

If the operator is invariant with respect to the parallel transfer of coordinates, then the micro-inhomogeneous medium is macro-homogeneous. The condition of macrohomogeneity is satisfied, in particular, by environments whose material functions are either random homogeneous or periodic.

For a micro-inhomogeneous region V with a random structure of the medium in boundary-value problems with boundary conditions of a particular form in displacements

or in voltages

where, are symmetric tensors-constants, strain fields еij (r) and stresses уij (r) are random homogeneous (and for media with a periodic structure - periodic) everywhere, except for a small neighborhood adjacent to the boundary S. Under the boundary conditions general form (2.11), these conditions are not met and the averaged components and fields of deformations and stresses are functions of coordinates. In this case, under the assumption of sufficient smoothness of the averaged fields (r) and (r), an approximate approach is valid, according to which the deformation fields еij (r) and уij (r) in the elementary macrovolume are equivalent to those found from the solution of problems for the domain V in displacements at given macrostrains = (see (2.22)) and in stresses at a given

given macrostresses = (see (2.23)).

Then the averaged (macroscopic) deformations and stresses at each point of the region V are determined by averaging over the elementary macro-volume Vl selected around this point:

For random homogeneous fields, this averaging coincides with the averaging operator introduced by expressions (2.7), (2.8).

Postulating the following properties of statistically averaged fields under the accepted conditions of ideal contact between the components of the medium:

we obtain from (2.9) the macroscopic equilibrium equations, and from (2.10)

Geometric Equations:

Now a closed system of equations for macroscopic physical quantities has been obtained (i.e., a macroscopic model of the composite has been built), and the main task is to find the form of the operator and determine its material functions. Macroscopic material functions can be found from testing samples or calculated by solving boundary value problems of structural-phenomenological models of composites. These functions can be found approximately by solving problems for the domain V with boundary conditions of a particular form (2.22) or (2.23).

Thus, the boundary value problem of the mechanics of composites within the framework of the structural-phenomenological model:

a boundary value problem for a homogeneous region with effective properties is put into correspondence:

and from the solution of the latter, the averaged components of the fields are found

deformation.

If, along with the deformation processes of the composite, the processes of destruction of its components are also modeled, then strength criteria of the form (2.15) are included in the boundary-value problem (2.27), and the physical equations of system (2.28) reflect not only the deformation properties of the structure elements, but their destruction during loading. In this case, the macroscopic model (2.28) can be supplemented by the criterion relations of strength

whose operator is P * and macroscopic material quantities

can be calculated.

The two-stage hierarchy of composite material models allows us to divide the solution of the initial boundary value problem of the mechanics of deformation and fracture (2.6), (2.9) - (2.11), (2.15) into a number of successive stages associated with the construction of macroscopic constitutive relations, the solution of the boundary value problem for a domain with effective properties , by finding structural deformation fields in elementary macro-volumes, by describing the processes of destruction of structural elements, by assessing the probability of destruction of elementary macro-volumes (i.e., the probability of macro-destruction).

As a rule, when using nonlinear constitutive relations of the components of the composite and taking into account the processes of structural destruction, it becomes necessary to organize iterative computational procedures for solving nonlinear problems of each of the stages, on the one hand, and agreeing the stages in a general sequence, on the other. In this case, in the process of deformation, the initially macro-uniform region V becomes macro-inhomogeneous, since the elementary macro-volumes selected around different points are not equally loaded.

This approximate approach made it possible to obtain solutions to a number of applied problems related to the analysis of mechanical behavior and prediction of the bearing capacity of structural elements made of composite materials: special-purpose pressure cylinders wound from glass and organoplastic tapes, aircraft engine casings obtained by laying out layers of glass, organo- and carbon fabrics, carbon-carbon elements of engine nozzle blocks, large-size sockets with a heat-shielding layer and others.

2.3 The principle of locality

The initial information about the structure of a micro-inhomogeneous medium, as already noted in Section 2.1, can be specified by a set of moment functions of material tensor or scalar quantities. These moment functions are constructed, as a rule, experimentally on real samples or with the help of computer simulation of random structures. The studies carried out in this area show that the moment functions of the second and higher orders of composites with random statistically homogeneous structures are local, and the size of the region of statistical dependence for two-component matrix-type composites is approximately equal to half the average distance between inclusions.

If the moment functions of the structural properties of a micro-inhomogeneous medium rapidly decay, then it is said that the arrangement of the structural elements is short-range order.

When solving stochastic problems in the theory of elasticity of composites with a random structure, the property of locality of moment functions was usually postulated along with the condition of statistical homogeneity. The hypothesis of the limiting locality of moment functions is also known, which allows one to obtain one-point approximations of stochastic boundary value problems and to avoid the difficulties associated with calculating integrals over regions of statistical dependence, whose integrands include moment functions.

After the property of locality of moment functions of material properties of composites with a random structure has been confirmed by numerous studies, there is a basis for its deeper use in mechanics.

It should be noted the same property of locality, but already characterizing the interaction of structure elements.

For example, the interaction of inclusions in the matrix composite, which causes distortions in the elastic field of the matrix, can be replaced by the interaction of point multipoles, the power and order of which depend on the shape and properties of the structural elements. It is proposed to select a limited volume containing a finite number of multipoles, in the matrix of which an elastic field is generated that is adequate to the elastic field of the periodic problem for a matrix with an infinite number of inclusions.

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"Definitions and classification of polymer composites Composite materials are materials obtained from two or more components and ..."

-- [ Page 1 ] --

TOPIC 1. DEFINITIONS AND CLASSIFICATION OF POLYMER

COMPOSITES. MECHANISM OF INTERACTION OF COMPONENTS

The modern era can be called the age of polymers and composite materials.

Definitions and classification of polymer composites

Composite materials are materials obtained from two or more components and

consisting of two or more phases. One component (matrix) forms a continuous

phase, the other is filler. Composite materials are heterogeneous systems and can be divided into three main classes:

1. Matrix systems consisting of a continuous phase (matrix) and a dispersed phase (discrete particles).

2. Compositions with fibrous fillers.

3. Compositions having an interpenetrating structure of two or more continuous phases.

Advantages of heterogeneous polymer compositions over homogeneous polymers:

1. increased rigidity, strength, dimensional stability.

2. Increased work of destruction and impact resistance.

3. increased heat resistance.

4. lowered gas and vapor permeability.

5. adjustable electrical properties.

6. reduced cost.

It is impossible to achieve a combination of all these properties in one composition. In addition, the achievement of advantages is often accompanied by the appearance of undesirable properties (obstruction of the flow, hence shaping, deterioration of some physical and mechanical properties).

A wide variation in the properties of the compositions can be achieved only by changing the morphology and adhesion strength between the phases.

For the uniform transfer of external influence through the matrix and its distribution to all filler particles, a strong adhesion is required at the matrix-filler interface, achieved through adsorption or chemical interaction.

The existence of such adhesion between mismatched components in heterogeneous plastics distinguishes them from mechanical mixtures.

The matrix can be metal, ceramic, carbon. The filler is presented in the form of particles and fibers, which have significantly higher physical and mechanical properties than the matrix.

The particles are usually called a dispersed filler; they have an indefinite, cubic, spherical or flaky shape with sizes from fractions of a mm to micron and nanoscale values.

The inert filler practically does not change the properties of the composition.

The active filler significantly changes the properties of the composition. For example, fibers have elastic strength characteristics which are two orders of magnitude higher than those of the matrix. They can be continuous or short. The diameter of thin fibers is 5-15 microns, thick (boric or silicon carbide) - 60-100 microns. The length of the short fibers is from 1-2 to 20-50 mm.

The name of the composites corresponds to the nature of the fibers: glass-, carbon-, organo-, boron plastics, etc. For hybrid versions - fiberglass, organoboroplastics, etc.

Fiber orientation determines the transition from filled plastics to reinforced plastics. It is a system of oriented fibers held together by a polymer matrix. Plastics include materials, an indispensable component of which is any polymer, which is in a plastic or viscous-flow state during the molding period, and in a glassy or crystalline state during operation. Plastics can be homogeneous or heterogeneous. Plastics are divided into thermoplastics and thermosets.

Classification of composites:

1. By the nature of the matrix:

thermosetting thermoplastic.

hybrid.

Thermosetting matrix is ​​a matrix obtained by curing epoxy, ether, imide, organosilicon and other oligomers in the process of making composites.

Thermoplastic matrix - a matrix that is melted to impregnate the filler and then cooled. These are PE, PP, polyarylene sulfones, sulfides, ketones.

The hybrid matrix can combine thermosetting and thermoplastic components.

2. By the nature and form of the filler.

Organic and inorganic substances of natural or artificial origin. The elastic modulus of the filler can be lower or higher than the elastic modulus of the binder. Low modulus fillers, which are usually elastomers, without lowering the heat resistance and hardness of the polymer, give the material increased resistance to alternating and shock loads, but increase its coefficient of thermal expansion and reduce deformation resistance. The higher the elastic modulus of the filler and the degree of filling, the greater the deformation resistance of the material.

Dispersed - filled composites, Materials based on short and continuous fibers.

The chemical nature of the particles is diverse: chalk, mica, metal oxides, glass spheres, carbon in the form of soot or fullerenes, aerosil, glass or clay flakes, rubber-like inclusions, etc.

Reinforcing fibers - glass, organic, carbon, etc. There are also well-known high-temperature boron and silicon carbide fibers, which are often used for the reinforcement of metals.

3. By the structure of polymer composites Matrix - for materials based on dispersed and short fibrous particles, Layered (two-dimensional) and volumetric for reinforced plastics based on woven and nonwoven materials.

Gradient materials with variable structure.

4. According to the degree of orientation of the filler, anisotropy of the material:

Composites with a random arrangement of particles and fibers, with an isotropic structure, composites with a unidirectional fiber orientation, with a pronounced anisotropy, 90o), composites with a cross, orthotropic orientation (0, with a given anisotropy, composites with an oblique fiber orientation at angles different from 90 , composites with a fan-shaped structure consisting of layers with different fiber orientations.

5. By methods of manufacturing materials and products:

one-stage methods - extrusion and "wet" winding, pultrusion (broaching), vacuum forming, two-stage methods of preliminary preparation of non-oriented (premixes) or oriented (prepregs) fibrous materials (semi-finished products) impregnated with a binder, followed by forming the material (laminate) by means of "dry" winding , pressing, autoclave molding.

6. By the number of components:

two-component, three-component PCMs combining dispersed particles and short fibers, polyfiber hybrid PCMs combining fibers with similar (glass-fiber-reinforced plastic) or significantly different (glass-carbon-fiber reinforced plastic) deformability, polymatrix structures, for example, based on a combination of thermosetting and thermoplastic binders.

7. By volume of filler content:

with non-oriented structure - filler content 30-40% -, with oriented structure - 50-75%, highly and extremely filled organofibers - 75-95% -.

8. By functionality:

single-functional (structural), multifunctional, capable of self-diagnosis (smart), multifunctional, capable of self-diagnosis and self-adaptation (intelligent).

When designing composite plastics, there are two stages (see table):

1 - computational - analytical, 2 - experimental - technological.

1 - includes: analysis of specified loading conditions and determination of a method for designing a plastic with the required properties. The concepts and formulas taken from the mechanics of composite materials are used:

a) the phenomenological approach is based on the application of the equations of the theory of elasticity, creep, etc. for anisotropic materials, b) - establishing the dependences of the mechanical characteristics of the composition on the size of the filler particles, the mechanical properties of the components, their volumetric content, etc. These dependences are analyzed at the microscopic, macroscopic and intermediate levels. Microlevel - the level of structural heterogeneity, commensurate with the transverse dimensions of the filler elements - the diameter of the filler particles or the thickness of the reinforcing layer.

Table Required mechanical characteristics of the composite plastic Selection of components and their selection of the scheme of reinforcement ratio in the composition

- & nbsp– & nbsp–

Shape Size ratio Mechanism of interaction of PCM components Let us consider the mechanism of stress transfer from the matrix to the filler, depending on its configuration.

In the simplest version, when the polymer is reinforced with unidirectional continuous fibers and is stretched in the direction of their orientation, the deformation of the components is the same and the stresses arising in them are proportional to the elastic modulus of the fibers and matrix. If, in the same model, the fibers are discrete, then the stress distribution turns out to be inhomogeneous along the fiber length. There is no stress at the ends of the fiber, but shear stresses arise at the interface of the fiber matrix, which gradually draws the fiber into work. The growth of tensile stresses in the fiber continues until they reach the average level of the stresses observed in the continuous fiber. Accordingly, the length at which this occurs is called "ineffective". With increasing deformation, the "ineffective" length grows and reaches its maximum value at a stress corresponding to the strength of the fiber. In this case, the “ineffective” length is called “critical” I. It is an important characteristic of the interaction of composites and can be calculated by the Kelly formula lcr / dwave = fiber / 2mat (1) where dwave and wave are the fiber diameter and strength; mat - the yield strength of the matrix or the adhesive strength of the system.

Depending on the strength of the fibers and the type of polymer matrix, the ratio lcr / dvol can vary from 10 to 200; at dwave 10 microns, lcr = 0.15-2.0 mm.

From the above reasoning, it follows that in the transition from continuous fibers to discrete, part of the length of each fiber will not perceive the full load. The shorter the reinforcing fiber, the less effective it is. At l lcr, the matrix under no circumstances can transfer the voltage to the fiber, sufficient for its destruction. It follows from this that the reinforcing ability of short fibers (an increase in the elastic strength characteristics of the polymer) is very low. Especially when you consider the orientation of the fibers, which is never ideal in such materials.

The structure of materials based on short fibers is rather chaotic. The advantage of short-fiber fillers is determined by the possibility of high-speed processing of materials into products. However, in the process of casting or extrusion, additional destruction of the fibers occurs, the length of which is usually reduced to 0.1-1 mm.

On going over to a dispersed powder filler, the possibility of stress transfer from the matrix to the filler is so reduced that its contribution to the increase in the strength of the composite begins to compete with the decrease in the strength of the matrix due to the arising non-uniformity of stresses and the development of defects. Because of this, the strength of such a composite usually does not increase in comparison with the strength of the matrix (sometimes it even slightly decreases).

When viscous thermoplastics are filled with hard fillers in an amount of more than 20%, a transition from plastic flow to brittle fracture is observed. In this case, there is a significant decrease in impact toughness, work of destruction. The modulus of elasticity increases with an increase in the amount of filler, but at the same time the size and number of cracks, “pseudopores” that arise during loading during the exfoliation of the matrix from dispersed particles at the moment of reaching stresses corresponding to the adhesive strength of the system, increase. Theoretical and experimental studies show that by reducing the size of the filler particles and the spread of their diameters, it is possible to significantly reduce the likelihood of the appearance of large defects.

The main reason for hardening is a change in the direction of crack growth when they come into contact with solid filler particles. The most probable direction of crack growth is perpendicular to the direction of the applied force. If a filler particle is located in this direction, then the crack should change its direction tangentially to the particle surface. Therefore, if the particles are in the form of fibers and are elongated in the direction of the acting force. Crack propagation along the filler particles is excluded.

When using a monolithic fiber of a circular cross-section, the indicators of mechanical properties usually reach a maximum at 2 = 0.65 - 0.7. When using precision methods of laying profiled fibers, it is possible to increase 2 to 0.85, after which the strength of the composites begins to depend more on the adhesion strength at the fiber-binder interface than on the strength of the fiber.

With the same degree of filling (2 = 0.7) and the ratio of elastic moduli (E2 / E1 = 21), the rigidity of plastic with fibers of a triangular cross section in the transverse direction exceeds the rigidity of plastic with fibers of circular cross section by a factor of 1.5.

Replacing a monolithic fiber with a hollow fiber makes it possible to sharply increase the specific values ​​of the strength and stiffness of products in compression and bending, since the moment of inertia increases with the same mass of fibers.

It is ineffective to use hollow fibers in tensile compositions due to the low strength of the profile fibers. When shearing, it is better to use profiled fibers.

Another direction in the creation of dispersed-filled polymers is their modification with rubber particles to reduce fragility and increase impact resistance.

Positive results were obtained for high impact polystyrene, epoxy and other matrices. The hardening mechanism of materials is apparently very complex, but the main role is assigned to the inhibition of crack propagation by rubber particles. Many authors point to the advisability of creating a transition layer with high adhesion to the matrix polymer and the rubber phase in order to increase the strength.

Let's return to a unidirectional composite based on continuous fibers and consider micromechanical models of its destruction. Elementary fibers have very high strength characteristics, tens of times higher than the strength of bulk samples. For example, the strength of bulk glass is 50-70 MPa, and in the form of fibers - 2.5-3.0 GPa; a similar picture is observed for organic and carbon fibers, the strength of which reaches 4-6 GPa. This difference is explained either by the influence of the scale factor (the size of the fiber surface determines the size of the possible defect), or by the orientational effect, which is very characteristic of organic fibers.

When testing filaments, a large scatter of experimental strength values ​​is observed. Therefore, usually at least 50 samples are tested, the average value and its variance are found.

Based on the hypothesis of the weak link, Weibull obtained the following equation for the fracture probability P () of the sample at stress and sample length L:

Р () = 1 - exp (–L), (2)

the constants of which are determined from the experimentally obtained distribution of the strength of the elementary fibers. Parameter P characterizes the defectiveness of the samples.

Coefficient values ​​range from 3-5 for normal glass fibers and up to 10-12 for “undamaged” glass fibers.

In reality, one is rarely dealing with a single fiber, usually with a bundle consisting of many fibers. According to the theoretical concepts of Daniels, the decrease in the strength of a bundle of unbonded fibers in comparison with the average strength of waves is determined by the dispersion of their strength. During loading, when the ultimate strength of a fiber is reached, it breaks and no longer participates in the work.

The force is redistributed to the whole fibers, the process continues until the moment of avalanche destruction of most, and then all fibers in the thread (bundle). At = 10, the strength of the filament n is approximately 80% of the average strength of the filament.

Analysis of the thread loading diagram - makes it possible to trace the entire process of gradual fiber breaking. It also allows you to identify some defects in the thread, in particular, the difference in length (uneven tension) of the fibers, which enhances the non-simultaneity of their destruction. The interaction (connectivity) of the fibers, due to twisting or partial adhesion, is manifested in the nature of the diagrams

- that become more linear. The Weibull coefficient for an unbonded bundle of fibers should remain the same as for filaments: In the case of bundle, it tends to increase.

The polymer matrix that binds the bundle into a single whole - microplastic - leads to an increase in its strength. In this case, the strength is practically independent of the length of the sample (= 30-50), which indicates a change in the fracture mechanism. The fact is that a fiber torn in some place does not cease to perceive the load, as in a thread, but continues to work at the same stress level as in adjacent fibers. This occurs at a distance lcr from the fracture site in accordance with the mechanism considered above for materials based on short fibers.

According to the statistical theory of strength developed by Gurland and Rosen, the destruction of a unidirectional composite under tension occurs through the accumulation of breaks, crushing of fibers in the polymer matrix. In this case, the theoretical strength of the fibers tr in the composite is equal to the strength of the unbonded fiber bundle of the "critical" length lcr.

tr = (lkre) –1 / In practice, the process of crushing the fibers cannot be completed. Usually, it is interrupted by the occurrence and development of a main crack due to overstress in the section, where the greatest number of defects accumulates, or delamination at the fiber-binder interface. This mechanism allows one to obtain the highest strength values, since it is associated with energy dissipation for the formation of large free surfaces. Based on this, when considering the implementation of the fiber strength in the composite, it is advisable to compare the experimental values ​​of waves with the strength tr, which could be in the implementation of the fiber crushing mechanism:

Kp = vol / tr, where Kp is the coefficient of strength realization.

Its real values ​​reach 60-80% for unidirectional glass, organic and carbon fiber-reinforced plastics based on super-strong fibers.

A similar approach has also been proposed to study the realization of the strength of fiberglass in longitudinal compression.

Currently, two main variants of the destruction mechanisms are being considered:

Fracture due to loss of stability of fibers on an elastic base;

Delamination of the material from the effect of shear stresses.

The main dependence arising from the consideration of the first fracture model relates the strength of the material in compression hcw with the shear modulus of the matrix Gm and its volumetric content m:

tszh = Gm / Vm Calculations carried out according to this formula give very high theoretical values ​​of tszh. For example, with a shear modulus Gm = 1-1.5 GPa, which is typical for epoxy resins, and m = 30%, the compressive strength of hfc could be 3-5 GPa, while for real materials it does not exceed 1.5 GPa ...

It can be argued that, in all cases, there is a proportionality between the strength of fiberglass plastics in compression and shear shear:

tszh = K shift, which suggests that the second mechanism is prevalent. This can be explained by structural defects of the samples and the inhomogeneous stress field arising during testing. Special methods for the preparation and study of unidirectional fiberglass made it possible to increase the hcf up to 2-3 GPa, that is, to a large extent, it was possible to implement the mechanism of fiber stability loss, increasing the coefficient of strength realization from 30-40 to 60-70%.

When compressed organoplastics, destruction occurs along the shear plane oriented at an angle of 45 ° to the fiber axis, which is typical for plastic fibers.

A similar mechanism, apparently, takes place for CFRPs, although in this case it is combined with a shear element.

The variety of mechanisms of destruction of composites makes it possible to raise the question of optimizing the properties of the binder. For example, to increase the tensile strength of the material along the fibers, it is necessary to reduce the "critical" length, which is achieved by increasing the rigidity of the matrix. On the other hand, this leads to an increase in stress concentration and growth of the main crack. The competition between these mechanisms is observed in the form of an extreme dependence of the strength of the composite on the yield point of the binder, which is varied by changing the temperature, test rate, or the introduction of plasticizing additives.

In each case, the optimum is different:

it depends on the nature of the fibers, the presence of existing technological stresses and defects. The inconsistency of the requirements for the binder is aggravated when taking into account its manufacturability, heat resistance, ability to absorb dynamic effects (impact strength), etc. The weakest point of composite materials is their low strength and shear deformability. Therefore, technological and operational stresses often lead to cracking of the material.

It is customary to characterize the fracture toughness of a composite by its specific fracture toughness Gc - the energy dissipated during the formation of a new surface. The higher the specific fracture toughness, the higher the resistance of the composite to delamination. Interlaminar viscosity increases with increasing matrix deformability, fiber-to-matrix adhesion, and fiber-to-fiber binder (VCB) interlayer thickness.

Modification of epoxy matrices with rubbers did not lead to a significant improvement in the properties of materials. Perhaps this is due to the fact that the plastic zone in the composite is limited by the size of the interfiber space. A much greater effect is observed when using thermoplastic matrices, for example, polyarylene sulfone PSF, the deformability of which reaches 80-100%. In this case, the Gc values ​​increase by almost an order of magnitude.

Micromechanical models of polymer composites make it possible to reveal analytical dependences showing the effect of the properties of fibers, matrix, their adhesive interaction, material structure and fracture mechanisms on the macroscopic elastic strength characteristics of a unidirectional layer. They describe the ultimate modulus of elasticity and tensile strength of a composite most successfully. In the case when the deformations of the fibers and the matrix are the same, the following additive ratios take place, which show the contribution of each component in proportion to its volumetric content Ek = Evv + Em

- & nbsp– & nbsp–

These equations are called the "mixture rule".

Since the contribution of the polymer matrix usually does not exceed 2-5%, it can be ignored:

Ek () = Evv and k () = cv The modulus of elasticity E () can be calculated by the formula 1 / Eк () = w / Ev + m / Em It should be taken into account that the modulus of elasticity of the fibers themselves in the transverse direction coincides with the modulus of elasticity in the longitudinal direction only for isotropic glass and boron fibers. For carbon and organic fibers, the transverse modulus is significantly lower than the longitudinal one. A similar dependence takes place for the shear modulus of a unidirectional composite "in the plane" of the fibers.

The strength of composites under transverse tension-compression and shear depends on many factors, primarily on the properties of the matrix, adhesive interaction, material structure - the presence of pores and other defects. Analytical dependences in this case can only be of a correlation nature. It is generally accepted that the reinforcement reduces the strength of the composite in the transverse (transverse) direction by about 2 times in comparison with the strength of the homogeneous matrix.

Elastic strength properties of composites Strength and stiffness are the most important characteristics of any material. When a specimen is loaded by tension or compression, normal stresses and corresponding deformations arise in it, which grow until the material is destroyed.

Ultimate (maximum) stress is called its strength. For linear elastic materials, there is a direct proportionality between stress and deformation Hooke's law = E. The proportionality coefficient characterizes the rigidity of the material and is denoted as the elastic modulus, or Young's modulus E.

This law is also fulfilled when the sample is loaded with shear (tangential) stresses and deformations, arising, for example, during torsion.

The proportionality coefficient in this case is called the shear modulus G: = .G.

When the material is stretched, simultaneously with elongation, its transverse dimensions decrease, which is characterized by Poisson's ratio, which establishes a relationship between deformations along x and across y of the sample: x = µ y.

The elastic properties of isotropic materials are well described by two constants E and G, the relationship between which corresponds to the equation G = E / 2 (l + µ).

The above relations describe well isotropic materials, the properties of which are the same in all directions. These include dispersed-filled polymers, as well as composites based on short or continuous fibers of a chaotic structure. (For fibrous materials, there is always a certain degree of orientation, determined by the influence of technological factors.) When a structure is loaded, the stress-strain state of the material most often becomes inhomogeneous. This provides an opportunity to identify the main (maximum) stresses that can cause its destruction. For example, in the case of a pipe under internal or external pressure, circumferential stresses are twice the axial stresses, that is, half of the thickness of an isotropic material is ineffective in terms of axial stresses. The inhomogeneity of the stress field can be significantly higher. For casings with an open outlet (guns, grenade launcher barrels), the ratio of radial and axial stresses reaches 8-10 or more. In these cases, one should take advantage of the remarkable ability of the fibrous materials, which can be oriented in the matrix according to the distribution of the main operational stresses.

Let's look at an example of a unidirectional layer. The unidirectional layer is isotropic in the direction perpendicular to the fiber orientation axis x Typical values ​​of the elastic constants of unidirectional composites are given in table. one.

- & nbsp– & nbsp–

The tensile strength of a unidirectional layer along the grain can range from 1.0 to 2.5 GPa, depending on the strength level of the fibers, the type and content of the binder. In this case, the strength in the transverse direction does not exceed 50-80 MPa, i.e. the anisotropy coefficient is 20-30.

A slight deviation of the direction of action of the load from the direction of orientation of the fibers has practically no effect on the tensile strength of the composite. Therefore, some misorientation of the fibers (3-5 °) is allowed, created by a special spreader or by increasing the winding pitch in order to increase the transverse strength of the material. In the case of compression, this is unacceptable, since it promotes the development of shear stresses, which determine the strength of the material in compression.

The unidirectional composite is the basis of a complex structure that is created by combining individual layers in accordance with the performance requirements of the structural element. Manufacturing methods: vacuum or autoclave forming, pressing, winding.

Let us further consider theoretical models for describing the processes of deformation and fracture of layered composites of complex structure. Conventionally, two main approaches can be distinguished in the development of calculation methods: phenomenological and structural. In the phenomenological approach, the composite material is considered as a homogeneous anisotropic medium, the model of which is based on experimentally obtained data. The selected strength criterion applies to the entire material as a whole. The advantage of phenomenological models is the ease of computation. However, for materials with a complex reinforcement scheme, it is required to determine many empirical coefficients, which requires a lot of experiments. In addition, phenomenological models do not take into account structural processes during fracture: cracking, micro-bulging, etc.

Determination of the optimal size of filler particles The stress arising at different areas of the particle surface (micro flakes or microfibers) depends on the distance r from the corresponding area of ​​the surface = - o (1 -) / 2r, where is Poisson's ratio.

Strength with an increase in the specific surface area of ​​a highly dispersed filler increases to a certain maximum, depending on the nature of the components of the composition.

The optimal diameter d of continuous fibers in stretchable orthotropic plastic at a given distance between the fibers is determined by the equation d (1/2 - 1), where 1, 2 are the elongations at break of the binder and filler fibers, respectively.

The choice of the shape of the filler particles The shape of the particles affects the mechanism of destruction of the plastic. The size and shape of products, processing technology are taken into account.

In the case of products of small thickness and complex configuration, preference is given to highly dispersed fillers (powders), since they are easily distributed in the binder, maintaining the original distribution during the molding of the product.

The use of highly dispersed fillers reduces the likelihood of destruction, delamination of products during subsequent mechanical processing.

Solid inclusions in a stretched sample reduce the stress in the contact zone of the binder with the filler, but in the spherical particle itself, the stress exceeds

1.5 times the voltage in the binder zones remote from it, i.e. the filler takes up the bulk of the load.

The influence of the filler increases if the particles are ellipsoidal and oriented in the direction of the deformation axis.

Selection of components with an optimal balance of mechanical characteristics Conditions: adhesive interaction is greater than the cohesion of the binder, both components work together until destruction, ideally elastic behavior of the filler material and the binder.

Determination of the optimum degree of filling Even reinforcing fibers do not always have a reinforcing effect on plastics. If the ratio of the deformation characteristics of the binder and reinforcing in unidirectional plastic satisfies the condition c c, then up to the critical volumetric content of fibers (c, cr), even a linear decrease in tensile strength = c (1 - c) is observed.

Due to the slight deformation of the binder at break equal to c, the stress taken by the fibers is too low to compensate for the decrease in the strength of the polymer matrix. Only starting from b, cr, the total strength of the reinforced fiber can compensate for the decrease in the strength of the matrix, and the strength of the plastic begins to increase.

Each plastic is characterized by its own b, cr, which for the selected polymer binder is less, the stronger the reinforcing fibers, and for the selected type of fibers it grows with increasing strength of the binder c.

The maximum degree of filling in, max ideally corresponds to such a packing density of the fibers at which they touch each other along the generatrix of the cylindrical surfaces. The ultimate packing density is achieved at different degrees of filling.

LLC в, max = 0.785, hexagonal LLC в, max = 0.907 Tetragonal LLC LLC If fibers of different diameters are used, then it is possible to achieve в, max = 0.924.

The optimal degree is less than the maximum in, opt 0.846 / (1 + min / D) 2, where min is the minimum possible distance between the fibers.

Features of the structure and properties of polymer composite materials (PCM).

PCM with a high fiber content. The physical and mechanical properties of composites essentially depend on the relative content of the components. According to the "rule of mixture", the higher the content of fibers, the higher the density of their packing, the higher (other things being equal) should be the modulus of elasticity and strength of the composites. The calculation of the mass content of fibers in the material is based on their amount in the sample, which is determined from technological considerations (linear density, the number of layers of fabric or winding parameters). For fiberglass, you can use the binder burnout method. There is a ratio ox + sv = 1.

Theoretically, the maximum possible content of fibers of one diameter with the densest hexagonal packing is 90.8% by volume. Taking into account the real dispersion of fiber diameters (10%), this value decreases to about 83%. In many works, the optimal fiber content is w = 0.65. This value, apparently, characterizes not the thickness of the binder films (they are different), but the fibrous framework formed during the formation of the material by one method or another. The influence of force factors (tension during winding and pressing pressure) in this case is ineffective, since it will only lead to the destruction of the fibers.

The real way to increase the elastic strength properties of composites by increasing the content of fibers is to compact their packing in the prepreg until their position in the composite structure is fixed. By reducing the viscosity of the binder and increasing the effect of force factors, it was possible to increase the content of glass and organic fibers in the unidirectional composite to 78% by volume. At the same time, its elastic-strength characteristics increased accordingly. Theoretically, the content of fibers does not depend on their diameter, but in practice this is of great importance. In the case of carbon fibers, which have a diameter half as much as glass or organic fibers, it was possible to increase their content in CFRP only up to 65%, since it is more difficult to overcome friction in such a system and remove an excess of binder.

When using organic fibers of SVM, it is possible to obtain highly reinforced organoplastics with a fiber content of up to 90-95%. This is achieved due to the irreversible thermal deformation of the fibers in the direction perpendicular to their axis, leading to a change in the fiber cross-section from round to an arbitrary cross-section due to contact with adjacent fibers. Interaction between CBM fibers is achieved either through the thinnest layers of a binder, which is probably partially located inside the fibers, or through an autohesion bond formed during the mutual diffusion of fiber components.

The modulus of elasticity and strength of ring-shaped specimens vary linearly practically in the entire range of increase in the volumetric content of fibers, which indicates the fulfillment of the “rule of mixtures”.

The effect of increasing the elastic strength characteristics of the composite (20-40%) is so significant that it significantly overlaps the observed in some cases a decrease in the shear and transverse properties of materials, as well as an increase in their water absorption.

Highly and ultra-reinforced composites should be used in non-shear components. To improve the weather resistance, the outer layers of the structure can be made of composites with a normal or increased binder content.

HYBRID AND GRADIENT REINFORCED PLASTICS (HAP) C

ADJUSTABLE MECHANICAL PROPERTIES

The creation of hybrid polymer composite materials combining two or more types of fibers - glass, organic, carbon and boron, is a promising direction in the development of modern technology, since it makes it possible to expand the possibility of creating materials with desired properties. The most significant factor affecting the nature of the mechanical behavior of HAP, especially during tension, is the value of the ultimate deformations of the fibers reinforcing the material. Among the HAPs, which combine fibers that have similar deformation characteristics, are glass-fiber-reinforced plastics and carbon-boroplastics.

The mechanical behavior of such materials in tension, compression, bending and shear basically corresponds to the principle of additivity, that is, the “rule of mixtures”.

A different nature of the regularities is observed in the study of HAP, combining fibers with different deformability. When stretching carbon-glass, carbon-organic, boron-glass and boron-organoplastics, the destruction of fibers does not occur simultaneously.

The ultimate deformation of the composite is determined in this case mainly by the deformation of those fibers, the volumetric content of which prevails.

Let us denote high-modulus fibers by the index "1", and the low-modulus fibers by the index "2".

At a high content of fibers with a high modulus of elasticity (and a small value of the ultimate deformation 1), the strength of the composite is calculated by the formula k1 = 1 (ECBf + E11 + E22) At a high content of fibers with a low modulus of elasticity, the strength of the composite k is calculated by the formula k2 = 2 (ECBf + E22) The mechanism of destruction of three-component materials changes when a certain critical ratio of multi-modulus fibers µcr is reached, at which the destruction of fibers with different breaking elongations is equally probable, i.e. k1 =.

k2. Neglecting the strength of the matrix, we obtain the ratio 1 Е11 + 1Е22 = 2 Е22 after the transformation of which we have:

1/2 = k = Е2 (2 - 1) / 1 Е1 Since 2 = 1 - 1, then µкр2 = k / (1 + k).

For carbon fiber reinforced plastics, we can take E1 = 250 GPa, E2 = 95 GPa, 1 = 0.8%, 2 = 3.5%, then k = 0.3; µcr1 = 23% or µcr2 = 77%.

The concept of critical volume also applies to composites based on one fiber type. It characterizes the transition from the destruction of the binder to the destruction of the fibers.

Due to the large difference in their elastic characteristics, µcr is very small and amounts to 0.1-0.5% of the fibers.

Let us consider the deformation curves of carbon fiber reinforced plastics with different content of different-modulus wires. In the initial section I, the deformation curves are linear, carbon and glass fibers are deformed together, the elastic modulus is composed of two components and corresponds to additive representations. Samples containing more than a critical amount of carbon fibers are destroyed at a deformation of 0.7-0.9%. Nonlinear section II on the deformation curves - of carbon fiber reinforced plastics, in which the content of carbon fibers is less than the critical one, can be considered as a "pseudoplasticity" section due to the gradual crushing of carbon fibers in the fiberglass matrix, which ensures the integrity of the material. Non-linear section II ends at a deformation of about 2%. Further, an almost linear section III is observed, in which the elastic modulus corresponds to the fraction of glass fibers in the composite, and the ultimate deformation

- limiting deformation of glass fibers 2 3-3.5%.

Upon repeated loading of the sample, the diagram is completely linear and corresponds to the third section of the original curve. At the same time, the fragmentation of the fibers, apparently, occurs during another two or three cycles of loading - unloading, since only after this a constant correlation dependence of the electrical resistance on the deformation of the sample is established.

The dependence of the tensile strength of HAP on the ratio of multimodal fibers is characterized by a curve with a minimum corresponding to the critical ratio of fibers.

For materials tested in compression, diagrams - and strength dependences are almost linear. Low-strength (in compression) organic and carbon fibers, being in a glass or boroplastic matrix, may not lose stability during deformation and, therefore, at stresses 2-3 times higher than in conventional organic and carbon fiber reinforced plastics. These effects, as well as an increase in the deformability of carbon fibers in a fiberglass matrix under tension, are called synergistic by many authors.

Fibers of different types are mixed within the same layer or alternate layers.

Below are some examples of the most rational combination of multi-modulus fibers in HAP:

the combination of glass and organic fibers makes it possible to obtain materials, on the one hand, with a higher compressive and shear strength (compared to organoplastics), on the other hand, to increase the specific characteristics of the hybrid system under tension (compared to fiberglass);

HAP based on a combination of glass and carbon fibers have a higher modulus of elasticity compared to fiberglass, while maintaining the specific characteristics of the strength of materials in compression and slightly decrease in tension; the work of destruction of samples increases;

the addition of boron fibers to fiberglass can significantly increase their modulus of elasticity, while maintaining (or increasing) the strength of materials in compression.

One of the varieties of HAP is gradient PCM, the structure and properties of which are spatially inhomogeneous. A smooth, controlled change in the elastic strength properties of PCM in a number of cases makes it possible to create a uniform stress field. For example, when homogeneous PCM shells are loaded with internal or external pressure with an increase in the thickness of the structure, a significant decrease in their effective elastic strength characteristics is observed. Only the layers adjacent to the pressure medium are fully loaded. Beginning with a certain thickness, the PCM practically ceases to perceive additional load, and it makes no sense to increase the shell thickness. Theoretically, this phenomenon can be avoided by using a PCM with a variable (increasing in thickness) modulus of elasticity.

At the same time, the weight and size characteristics of the material will be improved by 1.5-2 times.

In practice, this option can be realized, for example, by winding a PCM shell layer by layer, gradually (in accordance with the calculation) increasing the amount of carbon fibers in relation to glass. Similar problems (and their solution) are also encountered when creating super flywheels or rotor tires rotating at high speed. Varying the position of layers with different fiber content increases the shear, vibration and fatigue strength, water and weather resistance of materials.

Gradient-structural composites significantly expand the capabilities of PCM.

Almost all "natural structures" have such a structure (trunks and stems of plants, protective needles of plants and animals, beaks and feathers of birds, and many other examples). It is obvious that in this matter there is a strong lag behind nature and there is a huge reserve for improving the performance characteristics of artificially created products.

"Intelligent" composites At the end of the XX century. a new term has appeared in materials science - "intellectual"

materials. The accepted concept of "intelligent" material defines it as a structural material capable of self-diagnosis and self-adaptation. These materials should be able to recognize the emerging situation (sensory function), analyze it and make a decision (processor function), as well as excite and carry out the necessary response (executive function).

Currently, there are no composites that would meet all of the listed requirements. However, these tasks can be partially (step by step) solved, first of all, the tasks of creating materials that inform about their condition, about the approach of operational loads to the maximum permissible, about cracking, chemical corrosion, water absorption, etc.

The main requirement for the sensor elements of such composites is sensitivity to mechanical stress and the ability to be distributed throughout the volume. An ideal sensor must convert deformation into electrical signals. In this sense, conductive fibers are promising, which can be incorporated into composites during their formation. These include constantan or nichrome wire, conductive carbon or boron fibers, polyvinylidene fluoride piezoelectric films, etc.

The control of the viscoelastic properties of polymer composites (flaw detection) is carried out using acoustic methods, fixing the relationship between the speed of sound and the coefficient of its absorption. When using the magneto-dielectric properties of polymers for PCM diagnostics, it is recommended to add dispersed (colloidal) particles of magnetic and electrically conductive materials, including ultradispersed powders of iron, copper, nickel, carbon nanoparticles (fullerenes and nanotubes).

The operating principle of executive (adaptive) mechanisms is deformation resulting from any phenomena - heating, supply of an electric signal, etc. The piezoelectric effect, electro- and magnetostriction and shape memory effect are most acceptable for material activation. These mechanisms ensure the conversion of an electrical signal into a triggered deformation. The greatest effect is observed for shape memory metals. An alloy of titanium and nickel provides up to 2% deformation. Another important indicator of the actuator is its modulus of elasticity, which determines the possibility of creating a given stress-strain state. It is usually comparable to the modulus of elasticity of the base material.

The manufacturing process for "smart" composites is basically the same as for making a product from a base material. In this case, it is necessary to introduce information and executive elements into the material, minimally violating its structure. It is also necessary to pay attention to the complexity of the micromechanical processes occurring during the curing of the binder.

"Intelligent" composites are, of course, the material of the future, but already abroad (in the USA, Japan, Great Britain, Canada) intensive scientific and technical work is being carried out to create such materials for modern technology, primarily aviation, rocket and space, etc. etc., as well as for the mass media. Examples of designs using "smart" materials include the leading edge of the wing of the F-15 aircraft, the segment reflector and actuators of the slewing structure for spacecraft, aircraft with reduced noise and vibration. German companies that create modern wind power generators monitor the condition of blades with a diameter of up to 100 m and more. The optical fibers placed inside the material allow the structural integrity of the material to be monitored and the loads acting on the blades to be automatically maintained at an optimal level. The possibility of delamination of the material, for example, due to a lightning strike, is also controlled.

Dependence of the properties of composite plastics on the interaction of components The mutual influence of components in the interfacial zone is determined by the composition of the composition and the conditions of its formation. In rare cases, it is possible to establish a functional relationship between mechanical performance and interaction.

When sizing increases the adhesion strength, there is a correlation between adhesion strength and tensile breaking stress.

The choice of the fiber arrangement is made on the basis of data on the distribution of the force field and the nature of the loading.

Residual stresses in products made of composite materials affect the performance properties. Residual stress (mechanical, thermal, shrinkage, diffusion, etc.) is understood as stresses that are mutually balanced in the volume of the product, appeared in it as a result of the influence of external force, thermal and other fields and exist in the product after the cessation of the action of the field and the disappearance of temporary stress. Temporary temperature, shrinkage, diffusion stresses disappear as soon as the temperature, the depth of curing, the degree of crystallinity or the amount of absorbed substance are the same throughout the volume of the material. Mechanical temporary stresses disappear after the cessation of the action of the external field.

Residual stresses arise in a molded product only when the maximum temporary stresses in some part of the volume of the product exceed the yield point of the material and irreversible deformations (plastic and highly elastic) at ordinary temperatures appear in it, or due to an unequal degree of transformation (curing, crystallization ) separate areas of the material volume acquire different thermoelastic properties. The difference in thermoelastic properties of the polymer matrix and filler also leads to the appearance of residual stresses.

The molding process is carried out at elevated temperatures and pressures.

Consequently, temperature gradients arise, which are further increased, since the curing is usually exothermic.

During cooling, significant thermal stresses arise in the surface layers, which can lead to the appearance of additional irreversible deformations and cause an increase in residual stresses in finished products.

Residual stress determination method. Solvent method.

The sample is treated with a solvent that penetrates into the polymer and increases the tension of the surface layer. When the surface stress exceeds the breaking stress of the swelling layer, a network of small cracks will appear in it. In this case, lg = lgm + nlgres, where rest is the residual stress (kg / cm2), m and n are constant values.

Voltage at the interface between the binder and the filler.

The main reason is the shrinkage of the polymer matrix during curing and cooling, which significantly differs from the temperature shrinkage of the filler associated with the matrix by an adhesive bond. The pressure of the cured resin on the filler can be calculated using the equation (1 2) TE 2 P =, (1 + 1) + (1 + 2) (E1 / E 2) where 1 and 2 are the coefficients of thermal expansion, T is the difference between the curing temperatures and cooling, 1 and 2 - Poisson's ratios, E1 and E2 - deformation moduli (1 - binder, 2 - filler).

If the stresses in the material are not symmetrical, they can cause distortion in the shape.

TOPIC 2. UNSATURATED POLYESTER RESINS

Unsaturated oligoesters are oligomeric esters obtained using unsaturated monomers containing a vinyl group. Such oligomers are widely used in the production of reinforced plastics and other composite materials. In this case, two types of unsaturated oligoesters are used: oligoester maleinates and oligoester acrylates.

The idea of ​​combining reactive polymers and monomers was proposed by K. Ellis in the 1930s, who discovered that unsaturated polyester resins obtained by reacting glycols with maleic anhydride cure into an insoluble solid material when a peroxide initiator is added. Ellis patented this discovery in 1936.

Oligoester maleinates are obtained by the interaction of maleic anhydride with dihydric alcohols (ethylene glycol, diethylene glycol, 1,2-propylene glycol), while other dicarboxylic acids (adipic acid, isophthalic acid, isophtric acid, phthalic anhydride, etc.). It should be noted that during the synthesis of oligomers, which is carried out when heated from 50 to 230 ° C, partial or almost complete isomerization of maleate units into fumarate occurs: Fumarate double bonds are 20-60 times more active than maleate in curing reactions and contribute to the production of a cured polymer more High Quality.

Ellis later discovered that more valuable products could be obtained by reacting an unsaturated polyester alkyd resin with monomers such as vinyl acetate or styrene. The addition of monomers significantly reduces the viscosity of the resin, which facilitates the addition of the initiator to the system and allows for a more vigorous and fuller curing process. In this case, the polymerization of the mixture is faster than that of each component separately.

Since the curing proceeds by a radical mechanism, initiators are introduced into the mixture during curing, which serve as a source of free radicals and initiate the polymerization chain reaction. Free radicals can be formed from peroxides or other unstable compounds such as azoedinenine. To increase the rate of their decomposition, activators (promoters) are additionally introduced into the composition.Typical initiators of curing are benzoyl hieroxide and cumene hydroperoxide. acids. Co naphthenate is usually used to cure polymeinate styrene binders at 20 - 60 ° C. At 80 - 160 ° C - benzoyl and dicumyl peroxide.

Oxygen is an inhibitor. Therefore, waxy substances are introduced. Having a low softening point and being a surfactant, they cover the surface of the binder and protect it from oxygen.

Sometimes, to increase the fire resistance, fire retardants are introduced into polymaleinate binders: Sb2O3, chlorine and phosphorus-containing organic compounds.

Styrene-free polyester compositions are obtained by replacing styrene with less volatile (styrene is volatile and toxic) monomers such as divinylbenzene, vinyltoluene, diallyl phthalate.

Instead of styrene, triethylenglycol dimethacrylate (TGM-3) is successfully used as an active diluent:

At room temperature, liquid resins are stable for many months or even years, but with the addition of a peroxide initiator, they solidify in a few minutes. Curing occurs as a result of the "addition reaction and transformation of double bonds into simple ones; this does not generate any by-products. Styrene is most often used as the addition monomer. It interacts with reactive double bonds of polymer chains, stitching them together into a strong three-dimensional structure. The curing reaction takes place with the release of heat, which in turn contributes to a more complete course of the process. It has been found that usually about 90% of the double bonds in the polymer are reacted when the resin is cured.

Oligoester acrylates are obtained by polycondensation of polyhydric alcohols, saturated aliphatic dicarboxylic acids and unsaturated aliphatic acids of the acrylic series. For the synthesis of these oligomers, dihydric alcohols (glycols) are usually used. Oligoester acrylates are liquid or low-melting substances with a molecular weight of 300-5000. Polymerizing in the presence of radical polymerization initiators, they transform into infusible and insoluble polymers of three-dimensional structure, which, depending on the chemical structure of the initial oligomer, are solid glassy or elastic materials. Oligoester acrylates are capable of copolymerization with various monomers (styrene, methyl methacrylate, etc.), as well as with polyester maleinates.

Oligo ether acrylates have a definite advantage over oligo ether maleinates: they are capable of homopolymerization, which makes it possible to prepare varnishes and other compositions based on them without the use of highly volatile and toxic unsaturated monomers.

In the art, oligoester acrylates are cured by radical polymerization or copolymerization; volumetric shrinkage during curing is 4-10%.

Curing at 50-120 ° C (hot curing) is initiated by the peroxides of benzoyl, dicumil, etc. For curing at room temperature (cold curing), binary systems are used (for example, benzoyl peroxide + dimethylaniline; cumene hydroperoxide + naphthenate or cobalt linoleate).

The curing of oligoester acrylates can also be initiated by light, high energy radiation (γ-rays, fast electrons) and ionic polymerization catalysts.

Epoxy acrylate oligomers can be considered as a type of oligoester acrylates. Obtained by the interaction of oligomers containing terminal epoxy groups with methacrylic or acrylic acids.

Allyl alcohol ester prepolymers are prepared by polymerization of allyl alcohol esters and phthalic or isophthalic acids. Less commonly used diallyl maleinate, diethylene glycol bis-allyl carbonate or triallyl cyanurate.

Polymerization is carried out in a monomer medium, precipitating a prepolymer with methanol, or in a thin monomer layer with distillation of its excess at a given reaction stage in a vacuum.

The reaction is stopped before the onset of gelation, i. E. before conversion of 25% of all double bonds in the monomer. Molecular weight 6000, softening temperature ~ 60 ° C.

The prepolymers have a long shelf life under normal conditions. and a high cure rate at 135-160 ° C in the presence of dicumyl peroxide or tert-butyl perbenzoate. Prepolymers are more often used in the production of prepregs and premixes that have a lower viscosity and fill forms at low pressure.

Polyester resins are used in a wide variety of products, including boats, building panels, automobile and aircraft parts, fishing rods and golf clubs. About 80% of polyester resins produced in the USA are used with reinforcing fillers, mainly glass fibers.

Unreinforced polyester resins are used in the manufacture of buttons, furniture, artificial marble and body putty.

Unlike most other plastics, which are composed of a single ingredient, polyester resins often contain multiple components (resin, initiator, filler, and activator). The chemical nature and the ratio of the components can be varied, which makes it possible to obtain a large number of different types of polyester resins.

Maleic anhydride is used as a source of reactive double bonds for a large number of unsaturated polyester resins. When it interacts with glycols (usually propylene glycol is used), linear polyester chains with a molecular weight of 1000 ... 3000 are formed. Despite the lower cost of ethylene glycol compared to the cost of propylene glycol, the former is used only to obtain several special resins. This is due to the poor compatibility of ethylene glycol-based polyesters with styrene. In the process of esterification, the cis-configuration of maleic anhydride transforms into the fumaric trans-structure. This turns out to be useful in connection with the greater reactivity of the double bonds of the fumaric moiety in the reaction with styrene. Thus, a high degree of isomerization to the trans structure is an important factor in the preparation of reactive polyester resins. Despite the high degree of isomerization of maleic anhydride, which reaches more than 90%, more expensive fumaric acid is used to obtain polyester resins with increased reactivity.

Other biaxial acids or anhydrides, such as adipic and isophthalic acids or phthalic anhydride, are often added to the base reagent to modify the final properties of the resin and control the number of double bonds.

A typical structure of a polyester resin is shown below (where R is an alkyl or aryl group of a modifying diacid or anhydride):

О О СН3 О О СН3 II II I II.11 I Н [О-С-R-С-О-СН-СН2-О-С-СН = СН-С-О-СН-CH2] nOH Due to various properties and low cost polyester resins are widely used for various products.

Types of unsaturated polyester resins The wide variety of properties of polyester resins makes them suitable for use in a variety of applications. Below are brief characteristics of seven specific types of unsaturated polyester resins.

- & nbsp– & nbsp–

This type of polyester resin is prepared by esterification of propylene glycol with a mixture of phthalic and maleic anhydrides. The ratio of phthalic to maleic anhydrides can range from 2: 1 to 1: 2. The resulting polyester alkyd resin is mixed with styrene in a 2: 1 ratio. Resins of this type have a wide range of applications: they are used to make pallets, boats, shower parts, racks, swimming pools and water tanks.

2. Elastic polyester resin

If linear dibasic acids (for example, adipic or sebacic) are used instead of phthalic anhydride, a much more elastic and softer unsaturated polyester resin is formed. The diethylene or dipropylene glycols used instead of propylene glycol also give the resins their elasticity.

The addition of such polyester resins to tough general purpose resins reduces brittleness and simplifies processing. Elastic resins can also be obtained by replacing part of the phthalic anhydride with tall oil monobasic acids, which create flexible groups at the ends of polymer chains. Such resins are often used for decorative molding in the furniture industry and in the manufacture of picture frames. To do this, cellulose fillers (for example, crushed nutshells) are introduced into elastic resins and molded into silicone rubber molds. Excellent reproduction of wood carvings can be achieved by using silicone rubber molds cast directly from the original carving.

3. Resilient Polyester Resins This type of polyester resins are intermediate between rigid general purpose resins and elastic resins. They are used to make products that are shock-resistant, such as playing balls, safety helmets, fencing, car and airplane parts. To obtain such resins, isophthalic acid is used instead of phthalic anhydride. First, by reacting isophthalic acid with glycol, a low acid number polyester resin is obtained. Maleic anhydride is then added and the esterification is continued. As a result, polyester chains are obtained with a predominant arrangement of unsaturated fragments at the ends of molecules or between blocks consisting of a glycol-isophthalic polymer. In this type of esterification, phthalic anhydride is much less effective than isophthalic acid, since the resulting phthalic acid monoester tends to reverse into the anhydride at those high temperatures that are used to obtain high molecular weight polyester resins.

4. Low shrinkage polyester resins

When fiberglass-reinforced polyester is molded, the difference in shrinkage between resin and fiberglass results in pits on the surface of the article. The use of low-shrinkage polyester resins mitigates this effect, and the thus obtained cast products do not require additional sanding prior to painting, which is an advantage in the manufacture of parts for automobiles and household appliances.

Low shrinkage polyester resins include thermoplastic components (polystyrene or polymethyl methacrylate) that only partially dissolve in the original composition. During curing, accompanied by a change in the phase state of the system, microvoids are formed, which compensate for the usual shrinkage of the polymer resin.

5. Polyester resin, weather resistant

This type of polyester resins should not turn yellow when exposed to sunlight, for which ultraviolet (UV) radiation absorbers are introduced into its composition. Styrene can be replaced by methyl methacrylate, but only partially, because methyl methacrylate interacts poorly with the double bonds of fumaric acid, which is part of the polyester resin. Resins of this type are used in the manufacture of coverings, exterior panels and skylights.

6. Chemically resistant polyester resins Ester groups are easily hydrolyzed by alkalis, as a result of which the instability of polyester resins to alkalis is their fundamental disadvantage.

An increase in the carbon skeleton of the starting glycol leads to a decrease in the proportion of ether bonds in the resin. Thus, resins containing "bisglycol" (the product of the interaction of bisphenol A with propylene oxide) or hydrogenated bisphenol A have significantly fewer ester bonds than the corresponding general purpose resin. Such resins are used in the production of parts for chemical equipment: exhaust hoods or cabinets, chemical reactor vessels and tanks, as well as pipelines.

7. Flame retardant polyester resins

Glass fiber reinforced polyester resin moldings and laminates are combustible, but have a relatively low burning rate. An increase in the resistance of the resin to ignition and combustion is achieved by using instead of phthalic anhydride halogenated diacids such as tetrafluorophthalic, tetrabromophthalic and "chlorendic" (the addition product of hexachlorocyclopentadiene to maleic anhydride, also known as chaet acid). Dibromneopentyl glycol can also be used.

A further increase in fire resistance is achieved by introducing various combustion inhibitors into the resin, such as phosphoric acid esters and antimony oxide. Flame retardant polyester resins are used in fume hoods, electrical parts, building panels, and hulls for some types of naval vessels.

The seven types of unsaturated polyester resins described are the most commonly used in industry. However, there are also resins for special purposes. For example, the use of triallyl isocyanurate instead of styrene significantly improves the heat resistance of the resins. By replacing styrene with less volatile diallyl phthalate or vinyl toluene, monomer losses during polyester resin processing can be reduced. Special resins can be obtained by curing using UV radiation, for which photosensitive agents such as benzoin or its ethers are introduced into them.

Production of unsaturated polyester resins Typically, batch processes are used to produce unsaturated polyester resins. This is due to the variety of feedstocks required to produce the various resins, since the periodicity of the process allows for a quick and easy transition to the production of other resins. Continuous processes are commonly used for the large-scale production of general purpose resins.

The preferred material of construction for the manufacture of equipment is corrosion-resistant steel due to its chemical resistance to polymer resins and other reagents used in the manufacture of polyester resins.

Since iron and copper ions inhibit the free radical polymerization of polyester resins, these materials are not used for making reactors. Glass-lined reactors are preferred when using halogen-containing materials as feedstocks.

Typically, glycol is charged to the reactor and then phthalic and maleic anhydrides are added. Typically a 5-10% excess of glycol is used to compensate for losses caused by evaporation and side reactions. Before mixing and heating, the air in the reactor is displaced with an inert gas. The first stage of the reaction - the formation of "half-ester" - occurs spontaneously at a relatively low temperature, after which the reaction mass is heated to complete the formation of the ester. The flow rate of the inert gas through the reactor can be increased to distill off water from the condensation reaction. To more completely remove water from the glycol returned to the reactor, a steam-heated heat exchanger is often used.

During the last stage of esterification, the temperature of the reaction mixture rises to 190 - 220 ° C. A higher temperature favors the isomerization of maleates to fumarates, but at the same time causes side reactions at the double bonds. There is an optimum temperature at which the proportion of fumarate reaches its maximum. For general purpose resins, this occurs at 210 ° C.

To control the degree of esterification, the acidity and viscosity of the reaction mass are determined, and upon reaching the required values, the polyester is pumped into the final reactor.

This reactor already contains the required amount of styrene, and the polyester alkyd resin dissolves in it as it arrives. To exclude any polymerization processes that may occur upon contact of the hot alkyd resin with styrene, an inhibitor may additionally be added to the reaction mass at this stage. Sometimes, to maintain the required temperature, the reaction mass must be cooled. After the completion of the process, the conformity of the properties of the reaction mixture to the technical requirements is checked. A complete production cycle lasts 10 - 20 hours. The described method for the production of polyester resins is often implemented as a melting process. The reagent melt is heated until the conversion reaches the required level. Another method uses a small amount of a solvent (toluene or xylene) to remove the water released during the esterification as an azeotropic mixture.

The solvent is no more than 8% of the total reaction mass; it is separated from the water by decantation and returned to the reactor. After the end of the esterification process, the remaining solvent is distilled off from the reaction mixture, first at atmospheric pressure, and then, to completely remove it, under vacuum. During esterification, some side reactions can occur. For example, attachment of the hydroxyl group of the glycol to the double bond of the maleic or fumar moiety can occur to form a branched polymer. It was found that about 10 - 15% of the double bonds of the unsaturated polymer are consumed for side reactions.

The simplest continuous process for the production of unsaturated polyester resins is the reaction of a mixture of maleic and phthalic anhydrides with propylene oxide.

A small amount of glycol is required to initiate this chain reaction. Since the reaction of the interaction of anhydrides with epoxy groups occurs at relatively low temperatures, the double bonds of the maleate are not isomerized to the more active trans configuration. For this isomerization to take place, which is necessary for further interaction with styrene, the resulting polymer must be subjected to additional heating.

Continuous production of polyester resin from anhydrides and glycols can also be carried out in a series of heated stirred tank reactors, pumping the resin sequentially through reactors with different temperature conditions.

Curing of unsaturated polyester resins Unsaturated polyester resins are cured by the addition of initiators that serve as a source of free radicals and initiate the polymerization chain reaction.

Free radicals can form from peroxides or other unstable compounds such as azo compounds. These compounds can be split into radical fragments when heated or exposed to ultraviolet or other high energy radiation. Typically, the polyester resin contains an inhibitor that is essentially a free radical scavenger. The polymerization reaction with the introduction of initiators begins only after the action of the inhibitors has been overcome. This induction period makes it possible to mechanically mix the initiator-containing resin with the reinforcing agent and place it in the mold required for curing prior to the initiation of the polymerization reaction. Hydroquinone and its derivatives and quaternary ammonium halides are good inhibitors of polymerization.

Most of the peroxide initiators, when introduced into the polymer mass, decompose relatively slowly. Activators (promoters) are used to increase the rate of their decomposition. In fact, activators are catalysts for initiators.

Both the initiator and the activator are reactive compounds, the violent interaction of which is accompanied by ignition or even explosion. These compounds should be added to the resin separately, making sure that the first is completely dissolved before adding the second. Many resins contain a pre-added activator.

The curing behavior of the polyester resin is determined by the ratio of the effects of inhibitor, initiator and activator.

The substituents at the ethylene carbon atom can have two effects on the reactivity of the double bond. The spatial influence is due to the fact that bulky groups screen the double bond and reduce the ability of the second reactive group to take a favorable position for attack, thereby reducing the reactivity of the entire compound. Polarity is determined by the ability of a substituent group to attract or donate electrons. Electron donating groups (eg methyl, phenyl and halogen) render the double bond electronegative. This is precisely their effect manifested in styrene, vinyltoluene and chlorinated styrene.

Electron-withdrawing groups (such as vinyl or carbonyl) make the double bond electropositive. This occurs in the fumaric acid fragments in the polyester resin chains. The opposite polarity of the double bond in styrene and in the fumaric fragments of the alkyd resin promotes their interaction and the curing of polyester resins. Monomeric styrene, which is more mobile than the long polymer chains of unsaturated polyester, can homopolymerize. It has been experimentally established that the molar ratio of styrene and double bonds of polyester 2: 1 is optimal.

Initiators and activators

There is a wide variety of initiator-inhibitor-activator systems available for use in the manufacture of polyester resins. For example, a general purpose resin inhibited by hydroquinone can cure very quickly when an active peroxide initiator such as methyl ethyl ketone peroxide is used in combination with an activator such as naphthenate or cobalt octoate. In another case, a much more stable initiator, tert-butyl perbenzoate, is introduced to cure the polyester resin. This allows the polyester composition to be filled with calcium carbonate and ground glass fibers. Such an initiator-containing and molded compound is stable at room temperature for months, but can be cured within one minute by hot pressing at 140 to 160 ° C.

The selection of a suitable initiator and its amount depends on the type of resin and its cure temperature, on the required time for the entire process and on the gelation time. Since none of the available initiators usually satisfies all the necessary requirements by itself, various combinations of initiators and initiators with activators are used to obtain the best results.

When thermally curing polyester resins, the most commonly used initiator is benzoyl peroxide (BP), which is extremely effective and easy to use. It is easily soluble in styrene, can be stored for a long time without loss of activity, is stable at room temperature, and easily decomposes at elevated temperatures. In addition, BP causes a high exothermic temperature peak, which promotes complete curing of the resin. The amount of BP introduced into the resin ranges from 0.5 to 2%, depending on the type of resin and the monomer used. When using BP in the form of a paste (usually mixed with 50% tricresyl phosphate), the amount of the introduced initiator increases slightly (~ 1 - 3%).

Sometimes it is desirable (or even necessary) to carry out the curing of the resin from start to finish at low temperatures so that the heat released during polymerization is dissipated. This is especially important when wet-forming laminates where heating is difficult to use. In such cases, methyl ethyl ketone peroxide (MEK) is usually used as an initiator. Although the use of PMEC does not completely cure the resin at room temperature, the addition of an activator .. (eg cobalt naphthenate) leads to gelation and almost complete curing of the resin within a short period of time.

TOPIC 3. RESINS BASED ON COMPLEX DIESTERS

VINYL CARBONIC ACIDS

Resins based on complex diesters of vinyl carboxylic acids (DVA) are thermosetting polymers, the main chain of which is esterified at the terminal hydroxyl groups by the residue, R, acrylic (I: R = H) or methacrylic (II: R = CH3) acid: -O-C- C - R = CH2. The main chain of macromolecules of these resins is made up of epoxy, polyester, polyurethane or other segments, and practically valuable materials are obtained on the basis of epoxy resins.

Although various DCA have been obtained in laboratory quantities since the late 1950s, the commercial production of these resins was only established in 1965 by Shell Chemical under the trademark "epocrylic resins". These resins were identified as epoxy methacrylates and had excellent chemical resistance, superior to the best (at that time) polyester resins.

In 1966, Dau Chemical released the Derakan resin, which is a diester of vinyl carboxylic acids, and a number of similar resins intended for coatings. In 1977, the firms "Interplastic" and "Reichhold Chemical" began the production of DVK under the names "Koretsin" and "Corrolit"

respectively.

Resin characteristics

Resins can be used either neat (i.e. without diluent) or mixed with other ingredients. In the latter case, the resin may contain a reactive vinyl-containing comonomer (styrene, vinyl toluene, tri-methylolpropane triacrylate) or a non-reactive "diluent" (methyl ethyl ketone, toluene). Typically, methacrylic acid ester resins contain styrene and are used in the manufacture of chemically resistant glass fiber reinforced plastics (GRP). Resins - derivatives - of acrylic acid are supplied undiluted and the corresponding co-reagents are added directly to the preparation of UV-curable coatings and printing inks.

The physical properties and applications of DCA depend on the type of end groups (methacrylic or acrylic), on the amount and type of coreagents, as well as on the nature and molecular weight of the blocks that make up the main chain of resin macromolecules. As a result of curing, styrene-containing DVKM-II acquire high resistance to acids, bases and solvents. Acrylic acid derivatives are more sensitive to hydrolysis than methacrylic acid derivatives, and therefore they are usually not used in the manufacture of chemically resistant materials. Due to their high reactivity, these resins are preferably radiation cured.

Undiluted DCA are solid or waxy substances. Therefore, to provide the viscosity required for processing and to increase their reactivity, both reactive and inert diluents are introduced into the composition.

The main part of DVA macromolecules consists of epoxy oligomeric blocks of various molecular weights. The higher the molecular weight of such blocks, the higher the strength and elasticity of the resin, but the lower its heat resistance and resistance to solvents.

Compared to complex polyesters, DVKs are characterized by a lower content of ester groups and vinyl fragments. This leads to an increase in the resistance of these resins to hydrolysis, as well as a decrease in the temperature of the peak exotherm. Resin shrinkage during curing is reduced. Like polyesters, DVA have a limited shelf life, which is ensured by the introduction of polymerization inhibitors (free radical traps) during the resin production process.

Resin production

DCA is obtained by the interaction of methacrylic or acrylic acids with an oligomeric epoxy resin. The addition of an acid to an epoxide (esterification) is exothermic. As a result of this reaction, free hydroxyl groups are formed on the oligomeric block, but the formation of by-products does not occur (as, for example, in polyesterification, when water is formed). After completion of the reaction or during its course, suitable diluents or polymerization inhibitors are added to the reaction mixture.

Epoxy resins, which are used for the production of DVA, can be based on bisphenol A (in this case, general-purpose and heat-resistant DVA are obtained), on phenol-novolac fragments (heat-resistant DVK), as well as on a tetrabromine derivative of bisphenol A (fire-resistant DVK). When preparing DCA with acrylic groups at the ends, oligomeric epoxy blocks based on bisphenol A are usually used as the polymer of the main chain.

Curing

DVK, like unsaturated polyester resins, contain double bonds, which, when cured, react to form intermolecular crosslinks. This process takes place in the presence of free radicals, which are formed as a result of chemical, thermal or radiation transformations. The free-radical curing process includes the stages of initiation (induction period), growth and chain termination. Initiation is the rate-limiting stage of the process during which the initiator suppresses the action of the polymerization inhibitors. This leads to the occurrence of the reaction along the double bonds of the vinyl ether, which is part of the macromolecules, and its coreagent.

Molding Semi-finished products (prepregs) based on DVK for bulk molding or for sheet plastics are used for direct pressing of fittings for pipes, housings of household appliances, impellers, pumps and car parts. Typically, these prepregs contain approximately equal weight percentages of resin, crushed glass fibers and fillers. They also include: "hidden" initiator, pigments, release agent and thickeners.

TOPIC 4. POLYBUTADIENE RESINS

Polybutadiene resins are high molecular weight, hydrocarbon thermosetting resins. They have excellent electrical properties, significant chemical resistance, fairly high thermal stability, low moisture absorption, and are easily cured in the presence of peroxide initiators. They can be used for processing by direct and injection molding, injection molding, wet-laid-to-mold processing for laminates and prepregs. Due to the fact that there are many derivatives of polybutadiene, the scope of these polymers is wide: they are used as modifiers of other resins, for the manufacture of coatings, adhesives and electrical insulating potting compounds.

Polybutadiene resins were made around 1955 and used in Bud-type compounds in Injey laboratories. The resin that was used in these compounds consisted of a large amount of liquid 1,2-polybutadiene, some styrene butadiene copolymers and adducts of the two resins. Since then, similar products have been produced by Richardson and Lithium. In 1968, under the Gistil trademark, they began to produce polybutadiene with a high content of double bonds and a small amount of isocyanate groups at the ends of macromolecules. A certain amount of a peroxide initiator was introduced into it.

Now this resin is produced by the firms "Dianachem" and "Nippon Souda" under the trade name "Nisso-RV". This resin is a liquid atactic polybutadiene with a molecular weight of 1000-4000, about 90% of the double bonds of which are located in the side chains (vinyl groups).

There are three types of this resin:

type B does not contain terminal functional groups; type G contains hydroxyl groups and type C - carboxyl groups at both ends of the macromolecules. Other polybutadiene resins are now marketed under the Rickon name by Colorado Chemicals. Dienit resins are a mixture of 1,2- and 1,4-polybuta-denenes (Dienite PD-702, PD-503) or mixtures with monomers-co-reagents such as vinyl toluene (PM-520, PM-503 ) or styrene butadiene oligomer (PDPD-753).

Commercial types of polybutadiene resins are usually a mixture of low molecular weight 1,2- and 1,4-polybutadienes. These isomers differ in the position of the reaction center involved in polymerization. 1,2-polybutadiene, in which the double bonds are located in the side chains, is more reactive than 1,4-polymers, where the double bonds are in the main chain. Therefore, resins with a high content of 1,2-polybutadiene cure faster and easier, and resins with a significant proportion of 1,4-polymer are usually used to obtain highly elastic materials.

In order for the 1,2-polybutadiene (PBB) resin to be more conveniently processed into composite materials, it should be obtained with a high molecular weight and a narrow molecular weight (MW) distribution. To increase the reactivity of the resin during various chemical transformations, terminal functional groups (for example, hydroxyl, carboxyl, or isocyanate) are introduced into its macromolecules, and mixtures containing polybutadiene and reactive monomers such as styrene and vinyl toluene are prepared. Terminal hydroxyl groups allow reactions with polyurethanes, and carboxyl groups with epoxy groups. PBBs containing isocyanate end groups are used primarily for the preparation of electrically insulating potting compounds.

With a high content of vinyl groups (over 85%), polybutadiene resins are easily cured in the presence of peroxide initiators. Reactive terminal functional groups allow the molecular weight of the resin to be increased even before curing. An increase in MW leads to a decrease in the flowability of the resin before crosslinking, which causes gelatinization and the appearance of rigid polymer structures.

As a result, a more convenient technological time for processing the resin in the reactor is also achieved. The growth stage of the chain can be controlled (over time) so as to obtain polymers with various properties: from highly viscous liquids to solids with high MW. The ability to grow chain is the basis for the widespread use of polybutadiene resins in the preparation of press compositions, coatings, adhesives, electrical insulating potting compounds and thermosetting laminates. The polybutadiene derivatives listed below can be used both as modifiers for other resins and in the production of special laminated plastics.

- & nbsp– & nbsp–

Resin Cure The similarity of the cure process of polybutadiene resins to the cure of well-known polyester polymers using peroxide initiators makes them extremely useful for composite technology.

Polymer curing goes through three stages: low temperature gelation, high temperature curing and thermal cyclization. At low temperatures, an increase in the molecular weight and viscosity of the resin occurs.

This can cause gelation and initiation of curing. High-temperature curing begins at 121 ° C, with the prevalence of reactions at the double bonds of vinyl groups. During this stage of the process, solid products are formed. Thermal cyclization begins at a temperature of ~ 232 ° C, and the remaining unsaturated fragments of the polymer substrate react with the formation of a tightly cross-linked network.

Below are typical prepreg processing data:

Forming temperature, ° С

Pressure, MPa

Curing cycle at 77 ° С for laminated plastic with a thickness of 3.2 mm, min |

Time after curing ................... No Chemical structure and properties Polybutadiene resins have excellent electrical properties and chemical resistance. The high hydrocarbon content and the minimum aromatic content result in low dielectric constants and damping factors as well as excellent chemical resistance. The low content of aromatic fragments explains the high arc resistance, as well as resistance to the formation of conductive traces.

These properties of polybutadiene resins, similar to the behavior of polyethylene, are related to the resistance of these polymers to the formation of carbon during high voltage pyrolysis. The absence of ester bonds, which make polyesters vulnerable to acids and bases, explains the hydrophobicity, as well as the resistance of polybutadiene resins to acids and alkalis.

Application of PBB CMs Due to their unique combination of excellent electrical properties with chemical resistance, PBB CMs have been successfully applied in the design of radomes for onboard radar antennas. For operation in the frequency band exceeding the K-band (10.9 - 36.0 GHz), reinforced epoxy fiberglass was used, inadequately satisfying this purpose due to the high values ​​of the dielectric constant (4.5 - 5.0).

This becomes clear if we take into account that the radome wall thickness, as follows from the equality below, is a function of the dielectric constant and the operating wavelength:

n 0 D =, 2 (sin 2) 0.5 where d is the wall thickness of the antenna radome; n is an integer 0 (n = 0 for a thin wall; n - 1 for a wall with a thickness equal to the half-wave length); 0 - wavelength in free space; - the dielectric constant; - angle of incidence.

Since the radome wall thickness must be directly proportional to the effective wavelength, but inversely proportional to the dielectric constant, the combination that simultaneously increases the frequency and uses a composite material with a high dielectric constant creates the problem of wall thickness mismatch when using longer wavelengths.

Obviously, if the wavelength decreases simultaneously and the dielectric constant of the material increases, then it becomes possible to decrease the thickness of the fairing walls. However, the use of thin walls presents a problem of impact failure, which can be accelerated by severe surface erosion of thin laminates.

Another problem with materials with higher dielectric properties is the possibility of deviations in the wall thickness of the fairing, which leads to higher production costs or the use of additional materials to provide an accurate "electrical" thickness. When using antennas on airplanes and ships, additional requirements are imposed on the CM from which the fairings are made: they must have stable properties over a wide range of temperatures and in conditions of high humidity. The stringent material requirements associated with high operating frequencies and harsh environmental conditions are not easy to meet using conventional composites. However, these requirements can be more fully realized when using materials based on polybutadienes.

When preparing prepregs, the curing of the resins is carried out in the presence of a peroxide initiator. Despite the excellent processability of this CM and the ease of curing, which is completed in one stage in 2 hours at a temperature of 177 ° C, low mechanical properties in the transverse direction limit its use as a structural material. This disadvantage is possibly associated with the high density of intermolecular crosslinks, which leads not only to brittleness, but also to low adhesion of the binder to carbon fibers.

When obtaining polybutadiene laminated plastics for structural purposes, various reinforcing fibers are used: glass, quartz and aramid (Kevlar-49). Composites reinforced with Kevlar-49 fiber with a volume fraction of 60% are suitable for the manufacture of radars for radar antennas. In order to improve some of the mechanical properties of the material, especially the tensile strength in the transverse direction and in the interlayer shear, the adhesion properties and wettability of the Kevlar-49 fiber need to be improved.

An additional requirement when using these materials for the manufacture of radars for radar antennas is low moisture absorption.

Storage Polybutadiene resins do not require any special storage conditions compared to conventional ones associated with the use of volatile, flammable organic solvents such as heptane or toluene. When stored at 0, 20 or 35 ° C for 10 weeks, there is no noticeable change in viscosity or separation of the solution. However, longer storage at temperatures above 35 ° C should be avoided due to the tendency of the solution to gel formation.

EPOXY RESINS Epoxy resins are one of the best binders for a large number of fiber composites for the following reasons:

Good adhesion to a wide range of fillers, reinforcing components and substrates;

Variety of available epoxy resins and curing agents, allowing to obtain after curing materials with a wide combination of properties, meeting various technology requirements;

The absence of water or any volatile substances during the chemical process and small shrinkage phenomena during curing;

Chemical resistance and good electrical insulating properties.

The main component of epoxy binders is a mixture of oligomeric products with epoxy groups in the end links (epoxy resins).

They are received by:

the interaction of epichlorohydrin with diatomic (less often, polyatomic) alcohols or phenols to form diglycidal oxyesters CH2-CH-CH2Cl + HO-R-OH CH2-CH-CH2-OR - (- O-CH2-CH (OH) -CH2-O- RO O) -O-CH2-CH-CH2 \ / O or CH2-CH-CH2Cl + H2N-C6H4-NH2 \ / O or CH2-CH-CH2Cl + HO-C6H4-C (CH3) 2-C6H4-OH bisphenol A \ / O The most common resins are derived from epichlorohydrin and diphenylolpropane (bisphenol A) (ED type resins) or from epichlorohydrin and methylolphenol polycondensation products (EP, EN epoxyphenolic resins). Recently, resins have been used from epichlorohydrin and aniline (EA resin), diaminodiphenylmethane (EMDA).

Application Epoxy resins are used in the production of various composite materials and structural parts. They are also used as encapsulating and sealing compounds, press powders and adhesives.

Epoxy resins are very resistant to acids, alkalis and moisture, do not deform when heated to high temperatures, have low shrinkage and high volume resistivity. Epoxy resins can be used not only to protect materials from the environment, but also to glue parts together. In the electronics industry, for example, epoxy resins are used for encapsulating welded modules, pouring windings of transformers and motors, and also for sealing electrical cable joints.

Since the Second World War, epoxy resins have been used to make tooling (for example, molds for sheet metal stamping or patterns for making parts). Particulate or fiber reinforcing fillers are readily incorporated into the resin, reducing cost and increasing dimensional stability. The possibility of replacing metals with epoxy resins is due to two factors: economy in production and speed (without large material costs) modification. In addition, these resins have good shape and size retention, high mechanical properties and low shrinkage, which allows them to be manufactured into parts with close tolerances.

Molding Epoxy molding compounds (powdered, partially cured mixtures of resin and hardener that become fluid when heated) are used to manufacture all kinds of structural parts. Fillers and reinforcements are easily incorporated into epoxy resins to form a molding compound. Epoxy resins provide low shrinkage, good adhesion to fillers and reinforcing agents, chemical stability, and good rheological properties.

Bonding Epoxy resins have the highest adhesive strength of all known polymeric materials. They are used to impregnate a variety of substrates while minimizing shrinkage. Therefore, these resins can be used to bond many dissimilar materials. In addition, they can be cured at different temperatures and at different rates, which is very important for industrial production of adhesives.

Fabrication of CMs by Winding Fiber and as Laminates One of the most important uses of epoxy or binder is in the production of laminates and fiber-wound composites for the manufacture of structural parts. Such parts are used in various industries, including aircraft construction, space and military technology. Laminates are also used in the electronics industry to make printed circuit boards. In the chemical and petrochemical industry, containers and pipes made of epoxy composites are widely used.

Epoxy resins can be used in a variety of processes: wet winding or wet forming of laminates, dry winding or pre-impregnated layering of strands of fiber, fabric or tape (as prepregs). In general, epoxy resins are more expensive than most other resins, but their excellent performance often makes them more profitable in the long run.

Curing of resins with amines The vast majority of epoxy oligomers are either viscous liquids or low-melting solids, readily soluble in ketones, ethers, and toluene.

Hardeners for epoxy oligomers are divided into two large groups according to their mechanism of action:

Crosslinking hardeners contain functional groups that chemically interact with functional groups of the epoxy oligomer;

Catalytic curing agents cause the formation of a spatial reticular structure through the polymerization of epoxy groups.

Crosslinking hardeners contain amino, carboxyl, anhydride, isocyanate, hydroxyl and other groups in their molecules.

Amine type hardeners are used for curing in the operating temperature range of 0-150 ° C. As aliphatic amines, 1,6-hexamethylenediamine and polyethylene polyamines of the general formula H2N (CH2CH2NH), CH2CH2NH2, where n = 1-4, are widely used, having high activity even at a temperature of 20 ° C.

The aromatic amines used are m-phenylenediamine, 4,4 "-diaminodiphenylmethane, 4,4" -diaminodiphenylsulfone. Aromatic amines are less active than aliphatic ones, and they cure at temperatures of 150 ° C and above.

Dicyandiamine is widely used as an amine-type hardener.

Dicyandiamine practically does not react with epoxy oligomers at room temperature, but quickly cures them at elevated temperatures (150 ° C and higher).

To carry out complete crosslinking of epoxy resin, the ratio between the number of hydrogen atoms in the amino groups of the hardener and the number of epoxy groups in the resin must be 1: 1. The reaction between aliphatic amines and epoxy groups takes place at room temperature. Heating is required when using harsh aromatic amines. The chemical bond between carbon and nitrogen atoms, which occurs when the resin is crosslinked with amines, is resistant to the action of most inorganic acids and alkalis. However, this bond is less stable to the action of organic acids than intermolecular bonds formed by hardeners of other classes. In addition, the electrical insulating properties of "amino-cured" epoxies are not as good as with other curing agents. This may be due to the polarity of the hydroxyl groups formed during curing.

Isocyanate hardeners readily react with the hydroxyl groups of epoxy oligomers even in cold conditions (= 20 ° C). At high curing temperatures (180-200 ° C), the reaction of the isocyanate group with the epoxy group is possible with the formation of the oxazolidone ring. As isocyanates, 2,4- and 2,6toluene diisocyanates, hexamethylene diisocyanate and prepolymers based on them with terminal isocyanate groups are used.

For the curing of epoxy oligomers, phenol-formaldehyde oligomers of both novolac and resole types are widely used. Novolaks cure epoxy oligomers by reacting phenolic hydroxyls with epoxy groups at 150-180 ° C, and in the presence of catalysts (tertiary amines) at 80C. In the case of resols, the hydroxymethyl groups of the resols react with the secondary OH groups of the epoxy oligomers and, in addition, can alkylate the aromatic rings of the epoxy oligomers.

Catalytic curing agents catalyze the cationic and anionic polymerization of epoxy groups.

Cationic polymerization is initiated by Lewis acids - BF3, BF30 (C2H5) 2, SnCl4, etc.

Anionic polymerization is initiated by alkali metal hydroxides and alcoholates, as well as tertiary amines such as triethanolamine and 2,4,6-tris (dimethylaminomethyl) phenol.

In anionic polymerization in the presence of tertiary amines, the active center is formed by the combined reaction of an amine, an epoxy center, and an alcohol according to the OH OH scheme. Aliphatic tertiary amines are usually cold curing agents. Recently, imidazoles (in particular, 2-ethyl-4-methylimidazole) have been successfully used as hardeners of the Lewis base type; However, they can irritate the skin in some people and therefore require careful handling.

Curing of resins with acid anhydrides Cyclic aldehydes of carboxylic acids, such as phthalic, maleic, and also trimellitic (TMA), pyromellitic (PMA), benzophenone tetracarboxylic acid anhydride (ABTC), are most commonly used as acid curing agents. 180 ° C.

Storage of these hardeners requires special care to prevent degradation by moisture in the air. To ensure complete curing, the reaction is carried out with heating. Often a small amount of accelerator is added to speed up the curing process, which is extremely slow. There are also anhydrite hardeners that react with resin when heated above 200 ° C. Acid anhydrides react with epoxy resins to form esters. For this reaction to occur, an anhydride ring opening is required. A small amount of proton-containing substances (for example, acids, alcohols, phenols and water) or Lewis bases contribute to the opening of the ring.

The ester group formed as a result of curing is resistant to the action of organic and some inorganic acids, but is destroyed by alkalis. The resulting materials have higher thermal stability and better electrical insulating properties than when using amine hardeners.

Catalytic curing with Lewis acids Boron trifluoride, one of the Lewis acids, is widely used as a curing agent for epoxy resins. When added in small amounts to pure epoxy resin, this curing agent acts as a catalyst for the cationic homopolymerization of the resin to form a polyether. Boron trifluoride causes a very rapid, exothermic polymerization within a few minutes. Therefore, when a large amount of resin is cured, to maintain room temperature in the mass, it must be blocked using a special technology. When combined with monoethylamine (MEA) to form the BF3-MEA complex, boron trifluoride is converted at room temperature into a latent curing agent. At temperatures above 90 ° C, it becomes active and causes rapid curing of the epoxy resin, accompanied by a controlled release of heat. When preparing prepregs, which are often stored for weeks before processing, the use of a latent hardener is absolutely essential.

Epoxy resins containing the BF3-MEA complex are widely used for sealing, tooling, laminates and winding products.

A certain limitation in this case is the detected instability of prepregs and curable compositions containing VG3MEA to moisture.

Accelerators Accelerators are added to mixtures of resin and hardener to accelerate the reaction between them. They are introduced in small non-stoichiometric amounts, which are selected empirically, guided by the properties of the resulting material. Some of the tertiary amines - curing catalysts - can also be accelerators for a number of systems. Most often they are used to increase the rate of curing of epoxy resins with acid anhydrides. For this purpose, tin octanate, which is a Lewis acid, is used. In some cases, it allows curing at room temperature.

Cured Epoxies Some generalizations can be made regarding the relationship between the chemical structure and properties of cured epoxies:

The more aromatic rings are included in the composition of the epoxy resin, the higher its thermal stability and chemical resistance;

When using hardeners of the aromatic series, more rigid and durable materials are formed than in the case of aliphatic agents, however, the increased rigidity of such systems reduces molecular mobility and thereby complicates the interaction between the reaction groups, and curing in this case is carried out at elevated temperatures;

A decrease in the density of intermolecular "crosslinks" can lead to an increase in the strength of the material, due to an increase in the elongation at break;

Reducing crosslink density can also lead to less resin shrinkage during cure;

An increase in the density of "crosslinks" leads to an increase in the chemical resistance of the cured material;

An increase in the density of "crosslinks" leads to an increase in the temperature of thermal destruction (and the glass transition temperature Tc), but the density of "crosslinks" is too high

reduces deformation of destruction (increased fragility);

When replacing aromatic fragments of molecules with aliphatic or cycloaliphatic ones, which is not accompanied by a change in the number of "crosslinks" in the system, the elasticity and elongation of the cured resin increase;

Acid anhydride cured epoxy resins perform better in an acidic environment than in an alkaline environment.

Due to the fact that epoxy resins are viscoelastic materials, their properties depend on both temperature and test duration (speed, frequency).

Properties of specially cured epoxy resins.

When using specifically curing epoxy systems, there are some limitations to consider. For example, in the case of manufacturing large parts that are inconvenient for heating, and thick-walled parts, where thermal stresses should be minimal, it is inappropriate to use systems that require high-temperature curing. In these cases, systems with low temperature hardeners are used. Such compositions include aliphatic amine curable epoxy resins. Curing such compositions at room temperature results in materials with excellent properties, further enhanced by low heat. Of course, these resins cannot be used at high temperatures.

Epoxy oligomers and polymers are used in various fields of technology due to the successful combination of simple processing technology with high physical and mechanical properties, heat resistance, adhesion to various materials, resistance to various environments, and the ability to cure at atmospheric pressure with low shrinkage. So, they are widely used in the production of high-strength structural materials, in rocket and space technology, aviation, shipbuilding, mechanical engineering, electrical engineering, radio electronics, instrument making.

Epoxy oligomers and polymers are widely used as matrices for producing carbon plastics characterized by a combination of high strength and stiffness with low density, low temperature coefficient of friction, high thermal and electrical conductivity, wear resistance, resistance to thermal and radiation effects. Coked and pyrocarbon epoxy carbon fiber reinforced plastics are resistant to thermal and thermal oxidative degradation, have high strength characteristics, and have good heat-shielding properties.

Epoxy resins are good matrices for making fiberglass. In addition to glass fibers and glass fabrics, quartz fibers and fabrics, boron carbon fibers, silicon carbide and other inorganic fibers are used.

In addition to inorganic fibers, fibers from organic polymers are used to obtain reinforced epoxy plastics, in particular, high-strength synthetic fibers from poly-and-phenylene terephthalamide and other aramids.

Due to good adhesion to glass, ceramics, wood, plastics, metals, epoxy oligomers and polymers are widely used in the production of adhesives, hot and cold curing compounds.

Epoxy oligomers are used to seal and encapsulate various parts for environmental protection.

In electrical engineering, epoxy oligomers are used to fill the windings of transformers and motors, to seal the joints of electrical cables, etc.

TOPIC 6. HEAT RESISTANT RESINS

Heat-resistant resins are linear or cross-linked heteroaromatic polymers with a high glass transition temperature and can withstand prolonged heating in air above 300 ° C without noticeable structural changes.

Despite the process of thermooxidative destruction, which inevitably occurs under these conditions, the decomposition of such polymers is relatively slow. In addition, it is assumed that the fragments into which these polymers decompose are relatively stable, which increases the "lifetime" of the material at elevated temperatures.

The key point in the preparation of heat-resistant resins is the synthesis of polymers containing a large number of heteroaromatic fragments. These fragments, containing the minimum number of hydrogen atoms capable of oxidation, can absorb thermal energy. Unfortunately, the same elements of the chemical structure that determine the thermal-oxidative stability of such resins lead to serious difficulties, and often even to the impossibility of processing them into the desired products.

In the 1960s, a number of heteroaromatic polymers were synthesized, which, according to thermogravimetric analysis (TGA), had good thermo-oxidative stability at elevated temperatures. However, attempts to use these polymers as binders for composite materials with improved properties have been either unsuccessful or economically disadvantageous.

Therefore, in the early 70s, the future of heat-resistant polymer binders looked very vague and uncertain. It seemed that this useful class of materials would remain a "laboratory curiosity". However, the development of the chemistry of polyimide polymers in 1972-74. not only revived interest in them and caused new developments in the field of heat-resistant binders, but also made it possible to practically realize many of the potential possibilities of these binders. Currently, polyimide fibrous composite materials are used as structural materials operating at a temperature of about 300 ° C. or three-dimensional (spatial mesh).

The main disadvantage of composite materials based on high molecular weight polyimides is their high porosity, which sharply limits the possibilities of effective practical application of these materials under conditions of simultaneous exposure to high mechanical loads, high temperatures and an oxidizing atmosphere.

Therefore, it seems more expedient to use the initial fusible oligomeric imides that can solidify by the polymerization reaction, since polymerization is not accompanied by the release of volatile by-products leading to high porosity of the resulting materials. Of greatest importance are polymerizable oligomeric imides containing maleicmide and engroups at the ends of the chains.

These requirements are largely satisfied by bismaleinimils obtained by the interaction of diamines of various structures and maleic acid anhydride. The double bond in bis-maleimides is electron-deficient due to the proximity to the carbonyl groups of the imide ring; therefore, bis-maleimides easily polymerize when heated above the melting point, forming three-dimensional polymers.

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2.5.9. Removing castings from molds and cores from castings

2.5.10. Finishing operations for processing castings

2.6. Manufacturing of castings in one-time thin-walled (shell) molds

2.7. Other casting methods for one-off patterns

2.8. Making castings in multiple molds

2.8.1. Manufacturing of castings in metal molds (chill molds)

2.8.2. Production of high-pressure castings in metal molds

2.8.3. Squeeze casting

2.8.4. Continuous casting

2.8.5. Electroslag casting

2.9. Variable pressure casting

2.10. Freeze casting

2.11. Centrifugal casting

2.12. Suspension casting

2.13. Casting alloys

2.13.1. The concept of casting alloys

2.13.2. Casting properties of alloys

2.13.3. Mechanical properties

2.13.4. Physical and chemical properties

2.13.5. Technological properties

2.13.6. Operational properties

13.7. Brief characteristics of casting alloys

2.13.8. Smelting of foundry alloys

2.14. Technological requirements for the design of the casting

2.14.1. General concept of manufacturability of casting

2.14.2. Some basic requirements for the design of the casting

2.15. Basics of designing a casting manufacturing technology

Section 3. Processing of metals by pressure

3.1. General information

3.1.1. Physical foundations of plastic deformation

3.1.2. Advantages of metal forming

3.1.3. Influence of pressure treatment on the structure and properties of metals and alloys

3.2. Heating metal before pressure treatment

3.2.1. The choice of temperature conditions for pressure treatment

3.2.2. Heating devices

3.3. Types of metal forming

3.3.1. Rolling production

3.3.2. Pressing

3.3.3. Drawing

3.3.4. Forging

3.3.5. Bulk stamping

3.3.6. Sheet stamping

3.3.7. Special methods of pressure treatment

Section 4. Technology of welding processes, soldering and gluing

4.1. Physical fundamentals of welding

4.1.1. The essence of the formation of a welded joint

4.1.2. General characteristics of welded joints

4.2. Fusion welding

4.2.1. THE ESSENCE OF THE ARC WELDING PROCESS

4.2.2. Electric arc

4.2.4. Manual arc welding

4.2.5. Automatic submerged arc welding

4.2.6. Gas shielded arc welding

4.2.7. Plasma welding

4.2.8. Electroslag welding

4.2.9. Electron beam welding

4.2.10. Laser welding

4.2.11. Gas welding

4.3. Pressure welding

4.3.1. The main methods of resistance welding

4.3.2. Resistance welding machines

4.3.3. Spot and seam welding technology

4.3.4. Butt welding technology

4.3.5. Condenser welding

4.3.6. Special types of pressure welding

4.4. Physicochemical fundamentals of weldability

4.5. Welding technology for structural materials

4.5.1. Features of welding carbon steels.

4.5.2. Features of alloy steel welding.

4.5.3. Features of welding cast iron

4.5.4. Features of welding non-ferrous alloys

4.6. Manufacturability of welded joints

4.7. Soldering and Bonding Materials

4.7.1. Soldering

4.7.2. Gluing

Section 5. Technology of production of products from powders, polymers, rubbers, composite and inorganic materials

5.1. Powder metallurgy

5.1.1. Technology Basics

5.1.2. Powder materials

5.2. Self-propagating high-temperature synthesis (SHS)

5.3. Polymers

5.3.1. The structure and properties of polymers

5.3.2. Manufacturing technologies

5.4. Composite materials (km)

5.4.1. Metal matrix composites

5.4.2. Polymer matrix composites

5.4.3. Methods of obtaining products from km

5.5. Rubber products

5.6. Inorganic materials

5.6.1. Inorganic glass

5.6.2. Ceramics

Section 6. Technological methods of processing machine parts

6.1 General information

6.1.1. Methods for processing blanks of machine parts

6.1.2. Precision and roughness of processing

6.2. Metal cutting fundamentals

6.2.1. Cutting movements and machining patterns

6.2.2. Cutting characteristics and geometry of the cut layer

6.2.3. Turning tool elements

6.2.4. Coordinate planes of incisors

6.2.5. Static cutter angles

6.2.6. The physical foundations of the cutting process

6.2.7. Choosing Cutting Conditions and Ways to Improve Productivity

6.3. Materials for making cutting tools

6.4. General information about metal cutting machines

6.4.1. Classification of metal cutting machine tools

6.4.2. Kinematic diagram of the machine

6.5. Lathe machining

6.5.1. Turning method

6.5.2. Screw-cutting lathe

6.5.3. Vertical turning lathes

6.5.4. Lathes - turret lathes

6.5.5. Automatic lathes and semi-automatic machines

6.6. Drilling and boring machines

6.6.1. Drilling and Hole Machining Tool

6.6.2. Drilling machine types

6.7. Processing on milling machines

6.7.1. Milling method and types of cutters

6.7.2. General purpose milling machines

6.7.3. Accessories for milling machines

6.8. Broaching

6.8.1. Types of machines and their purpose

6.8.2. Cutting tools and machining schemes

6.9. Gear cutting processes

6.9.1. Cogwheel Teeth Profiling Techniques

6.9.2. Gear cutting tool

6.9.3. Technological methods of cutting gears

6.10. Threading

6.10.1. Threading tool

6.10.2. Cutting threads with cutters and combs

6.10.3. Threading with milling cutters

6.10. 4. Tapping

6.10.5. Dice tapping

6.10.6. Tapping heads

6.10.7. Thread rolling

6.11. Abrasive processing

6.11.1. Abrasive tools

6.11.2. Grinding

6.11.3. Honing

6.11.4. Superfinishing

6.11.5. Polishing

6.11.6. Debugging

6.12. Electrical, chemical and combined processing methods

6.12.1. Ultrasonic cutting

6.12.2. Cutting with heating

6.12.3. Electro-discharge machining methods

6.12.4. Chemical processing methods

6.12.5. Beam processing methods

6.13. Manufacturability of the design of machines, mechanisms and parts

5.4.2. Polymer matrix composites

Composite materials with a polymer matrix are distinguished by low density (1200 ... 1900 kg / m 3), low sensitivity to notching, thermal and electrical conductivity, high fatigue and specific strength, manufacturability of processing, radio transparency (a number of materials), etc. As a polymer matrices for composites are used as thermosetting (mainly) and thermoplastic polymers, and fillers - any of the above.

Materials based on thermoplastic polymers with dispersed fillers of various nature (talc, graphite, metal oxides, layered solid lubricants, metal powders, discrete fiberglass, etc.) are used for the manufacture of low- and medium-loaded parts of machines and apparatus, body parts, gear wheels and sprockets, bearings and seals, drive belts, containers, etc.

Among thermoplastic composites, glass-filled materials are most widely used. As a filler, fibers with a diameter of 9 ... 13 μm are used from alkali-free aluminoborosilicate glass, short (0.1 ... 1 μm) and long (3 ... 12 mm long) with a filling degree of 10 ... 40% of polymer mass. Glass-filled plastics based on polyamides, polycarbonate, polypropylene and other thermoplastics are produced. Filling thermoplastics with glass fiber increases the strength characteristics of polymers and heat resistance, reduces creep by 1.5 ... 2 times, reduces thermal expansion by 2 ... 7 times, increases the endurance limit and wear resistance. The introduction of solid layered lubricants into composites, such as graphite, molybdenum disulfide, boron nitride, etc., reduces the coefficient of friction of polymers and increases their wear resistance.

The strength of composites based on thermoplastics reaches 150 ... 160 MPa at a sufficiently high impact strength (KCU = 8 ... 60 J / m 2).

Composite materials based on thermosetting plastics are created on the basis of polymers that cure when heated or under the action of hardeners to form three-dimensional polymer structures. Composites based on phenol-formaldehyde, urea and melamine-formaldehyde, organosilicon and other resins are among the cured when heated. The second type includes composites based on polysiloxanes, epoxy resins, and unsaturated polyesters.

Thermosetting plastics, in contrast to thermoplastics, are characterized by a complete absence of cold flow, have significantly higher heat resistance, are insoluble, and have insignificant swelling properties. They show stability of properties up to the temperature of heat resistance, the ability to withstand prolonged loads at temperatures from -60 to +200 ... 300 ° C, depending on the type of polymer, and have good dielectric properties. But these materials are less processable than thermoplastics.

Epoxy resins have the greatest adhesion to the filler. Cured epoxy resins are resistant to alkalis, oxidants and most organic acids. However, composites based on them have low mechanical properties, are heat resistant up to 200 ° C, and, moreover, these resins are toxic.

Composites based on silicon-organic and polyimide binders (up to 280 ... 350 ° C) have the highest heat resistance.

The use of epoxy resins and unsaturated polyesters makes it possible to obtain materials capable of curing at room temperature (cold curing), which is very important in the manufacture of large-sized products.

Composite materials with dispersed fillers which are used as powders of organic (wood flour, cellulose) and mineral (quartz, talc, mica, metal oxides, solid layered lubricants, including graphite, molybdenum disulfide, boron nitride) substances, have isotropic properties, low mechanical strength and toughness.

As fibrous reinforcing materials cotton fleece, cord thread, asbestos fiber, fiberglass are used. Accordingly, these materials are called fiberglass, cordofibre, asbestos fiber, fiberglass.

Fibers - plastics based on cotton tows impregnated with phenol-formaldehyde resin. The materials have a higher impact strength (up to 10 kJ / m2) compared to press powders, but they have a much lower fluidity, which does not allow the production of thin-walled parts. Fibers have low dielectric properties, are unstable to tropical climates, and have anisotropy properties. They are used for the manufacture of products for general technical purposes with increased resistance to vibrations and shock loads operating in bending and torsion, for example, belt pulleys, flanges, handles, covers, etc.

Asbestos fibers - composites containing a fibrous mineral - asbestos, which breaks down into thin fibers with a diameter of up to 0.5 microns. Phenol-formaldehyde and organosilicon resins are used as a binder. They have high impact strength and heat resistance up to 200 ° C, are resistant to acidic environments, and have good frictional properties. They are mainly used as materials for braking devices (brake pads, linings, clutch discs).

Asbestos fibers based on phenol-formaldehyde are used for the production of high-strength heat-resistant parts for electrical purposes (electrical panels, high- and low-voltage collectors), and based on organosilicon polymers - for parts that operate for a long time at temperatures up to 200 ° C (material K-41-5) and for arc suppression chambers of high power contactors, terminal blocks (KMK-218). The latest materials are tropical resistant. Faolite - asbestos fibers obtained by impregnating asbestos fibers with phenol-formaldehyde resin followed by rolling the mixture is used for the manufacture of acid-resistant pipes and containers.

Fiberglass are plastics containing fiberglass as filler. Glass fibers with a diameter of 5 ... 20 microns are used, high-strength with a temporary resistance B = 600 ... 3800 MPa and high-modulus (VM-1, VMP, M-11), having B = 3900 ... 4700 MPa and an elastic modulus at stretching up to 110 GPa. Fibers, threads, bundles of different lengths are used, which largely determines the impact strength of fiberglass. The thinner the fiber, the less defectiveness and the higher its strength.

The mechanical properties of fiberglass depend on the composition, quantity and length of fiberglass, the type of binder, physicochemical processes occurring at the fiberglass-binder interface, and the processing method. For example, replacing glass fiber from E glass (alkali-free aluminosilicate) with S (heat-resistant high-strength) glass fiber in an epoxy binder increases the strength of the composite by 40%.

In order to improve the wettability of glass fiber with the binder, reduce the stresses arising at the interface, increase the adhesion between the fiber and the binder, finishing (processing) of fibers with compounds containing various reactive groups (vinyl, methacryl, phenyl, amino and imino groups, etc.) is used. A decrease in stresses in the binder layer boundary with the fiber, a decrease in shrinkage and porosity, and an increase in heat resistance are facilitated by the introduction of powdered fillers, in particular, a cured binder powder, into the binder.

Fiberglass is subdivided into: entangled fibrous, granular and fine-dispersed press masses.

Tangled fiber glass fibers is obtained by impregnation of lengths of fibers 40 ... 70 mm, followed by fluffing and drying to remove the solvent (for example, AG-4V). The disadvantage of these materials is the uneven distribution of the binder, a greater scatter of mechanical properties and less fluidity compared to other fiberglass.

Granular fiberglass(premixes) is obtained by impregnation of untwisted glass yarns and glass yarns, followed by drying and cutting into granules with a length of 5, 10, 20 and 30 mm. The diameter of the granules is 0.5 ... 8 mm. The material has good flowability and fluidity, greater stability of mechanical properties. This category of materials includes dosed glass fibers DSV.

Finely dispersed fiberglass press compounds made by mixing crushed glass fibers up to 1.5 mm long with a binder followed by granulation (granules 3 ... 6 mm in size). Also produced is "glass crumb" with granules up to 10 ... 50 mm in length from impregnated glass cloth waste.

Granulated fiberglass with granules up to 6 mm in size is processed by injection molding. Finely dispersed fiberglass can be processed by injection molding, and in the manufacture of products with metal fittings - injection molding. Fiberglass with a granule length of 10 mm is processed by injection molding and direct pressing, and with a granule length of 20 and 30 mm - only by direct pressing.

Body parts, elements of shields, insulators, plug connectors, antenna fairings, etc. are made of fiberglass. Products operating at temperatures from -60 to +200 ° C are made on the basis of aniline-phenol-formaldehyde resins and alkali-free aluminoborosilicate glass fiber, and for the temperature range - 60 ... + 100 ° C on the basis of epoxy resins.

Fiberglass based on organosilicon resins are operated up to a temperature of 400 ° C, and with the use of silica or silica fibers for a short time and at higher temperatures. For heat-shielding parts, fiberglass based on silica fiber and phenol-formaldehyde resins are used.

Based on glass mats and unsaturated polyester resins, prepregs, which are used for the manufacture of large-sized parts (bodies, boats, body parts of devices, etc.). The use of oriented fibers makes it possible to obtain glass fibers with improved mechanical properties. For example, oriented glass fiber AG-4S has:  B = 200 ... 400 MPa, KCU = 100 kJ / m 2; while for AG-4V based on tangled fiber:  B = 80 MPa, KCU = 25 kJ / m 2.

Organofibers are composite materials based on polymer binders, in which organic polymer fibers (polyamide, lavsan, nitron, vinol, etc.) serve as a filler. Bundles, fabrics and mats made of these fibers are also used for reinforcement. Thermosetting resins (epoxy, phenol-formaldehyde, polyimide, etc.) are used as binders.

The use of polymer binders and fillers with similar thermophysical characteristics, as well as those capable of diffusion and chemical interaction between them, provide composites with stability of mechanical properties, high specific strength and impact strength, chemical resistance, resistance to thermal shock, tropical atmosphere, and abrasion. The permissible operating temperature of most organic fibers is 100 ... 150 ° C, and on the basis of a polyimide binder and heat-resistant fibers - up to 200 ... 300 ° C. The disadvantages of these materials include low compressive strength and creep.

To obtain high-strength composites, fibers based on aromatic polyamides (aramid fibers SVM, Terlon, Kevlar) are used, which have high mechanical properties, thermal stability in a wide temperature range, and good dielectric and fatigue properties. In terms of specific strength, these fibers are inferior only to boric and carbon fibers.

Borovoloknit - composite materials on a polymer matrix filled with boron fibers. They have good mechanical properties, low creep, high thermal and electrical conductivity, resistance to organic solvents, fuels and lubricants, radioactive radiation, and cyclic alternating loads.

Boron fibers are obtained by chemical deposition of boron from a gas mixture of BCl 3 + H 2 onto a tungsten filament at a temperature of ~ 1130 ° C. To increase the heat resistance, the fibers are coated with silicon carbide, which is also deposited from the vapor-gas phase in argon and hydrogen. Such fibers are called borsic. As a binder for boron fibers, modified epoxy resins and polyimides are used. Borovoloknits KMB-3, KMB-Zk ensure the performance of products at temperatures up to 100 ° C, KMB-1 and KMB-1k up to 200 ° C, and KMB-2k up to 300 ° C. In order to improve the manufacturability of processing, composites containing a mixture of boron fiber with glass fiber are used.

Borovoloknits are used in aviation and space technology for the manufacture of various profiles, panels, compressor parts, etc.

Carbofiber (carbon fiber reinforced plastics) - composite materials based on a polymer binder and carbon fibers. Carbon fibers are highly resistant to heat; specific strength, chemical and weather resistance, low coefficient of thermal linear expansion.

Two types of fibers are used: carbonized and graphite. Viscose or polyacrylonitrile (PAN) fibers, stone and petroleum pitches, which undergo special heat treatment, are used as the starting material. In the process of high-temperature treatment in a non-oxidizing environment, a transition from organic to carbon fibers occurs. Carbonization is carried out at a temperature of 900 ... 2000 ° С, and graphitization - at temperatures up to 3000 ° С. According to their mechanical properties, carbon fibers are divided into high modulus and high strength. Thermosetting polymers are used as binders: epoxy, phenol-formaldehyde, epoxy-phenolic resins, polyimides, etc., as well as carbon matrices.

Carbofibers have good mechanical properties, static and dynamic endurance, water and chemical resistance, etc.

Carbofibers based on epoxy-aniline-formaldehyde binder (KMU-3, KMU-Zl) are efficient at temperatures up to 100 ° C, on epoxy-phenolic (KMU-1l, KMU-ly) up to 200 ° C, on poly-lyimide (KMU- 2, KMU-2l) up to 300 ° C, on a carbon matrix up to 450 ° C in air and up to 2200 ° C in an inert atmosphere.

Carbon fiber is used for the manufacture of structural parts for aviation and rocket technology, antennas, ships, cars, sports equipment.

Laminated composite materials have sheet fillers (fabrics, paper, veneer, etc.), impregnated and fastened together with a polymer binder. These materials have anisotropic properties. Fabrics based on high-strength fibers of various natures are used as fibrous reinforcing elements: cotton, glass-asbestos fabric, organo-fabric, carbon-fiber, organo-glass, boro-organo-glass. Fabrics differ in the ratio of fibers in the warp and weft, in the type of weave, which affects their mechanical properties. Laminated composites are produced in the form of sheets, pipes, blanks.

Getinax - plastic based on modified phenolic, amino-formaldehyde and urea resins and various types of paper.

Organogetinax is made on the basis of synthetic fiber paper, most often from aromatic polyamides and polyvinyl alcohol. Polyimides, phenol-formaldehyde, epoxy resins and others are used as binders. Compared to getinaks, they have a higher resistance in aggressive environments and the stability of mechanical and dielectric properties at elevated temperatures.

Textolite - laminated plastic based on polymer binders and cotton fabrics. The material has high mechanical properties, vibration resistance. Depending on the main purpose, PCBs are divided into structural, electrical, graphite, flexible cushioning.

Structural textolite grades PTK, PT, PTM is used for the manufacture of gears, sliding bearings operating at temperatures in the friction zone no higher than 90 ° C, in rolling mills, turbines, pumps, etc. Produced in the form of sheets with a thickness of 0.5 to 8 mm and slabs with a thickness of 8 to 13 mm.

Electrotechnical textolite is used as an electrical insulating material in environments with operating temperatures from minus 65 to + 165 ° С and humidity up to 65%. It is produced in the form of sheets with a thickness of 0.5 to 50 mm grades A, B, D, VCh. Dielectric strength in transformer oil up to 8 kV / mm. Grade A - with increased electrical properties for operation in transformer oil and in air at an industrial frequency of 50 Hz. Grade B - with enhanced electrical properties for operation in air at a frequency of 50 Hz. Grade G - in terms of properties and area of ​​use, it is similar to grade A, but with extended tolerances for warpage and thickness. HF grade - for operation in air at high frequencies (up to 10 6 Hz).

Graphite textolite is used for the manufacture of rolling equipment bearings and is produced in the form of sheets 1 ... 50 mm thick, up to 1400 mm long and up to 1000 mm wide.

Flexible cushioning textolite is used for the production of sealing and insulating gaskets in machine units exposed to oils, kerosene, and gasoline. Produced in the form of sheets with a thickness of 0.2 ... 3.0 mm.

V asbestos laminates and asbogetinax as fillers, respectively, asbestos cloth or asbestos paper (up to 60%), and as a binder - phenol-formaldehyde and melamine-formaldehyde resins, silicon-organic polymers, which determine the permissible operating temperature.

Materials on a melamine-formaldehyde base allow the operation of products at temperatures up to 200 ° C, on phenol-formaldehyde up to 250 ° C and on organosilicon up to 300 ° C during long-term operation. For a short time, the temperature can reach 3000 ° C. Asbestos laminates are used mainly for the manufacture of brake pads, brake linings, as heat-insulating and heat-shielding materials.

Glass fiber laminates made on the basis of fiberglass and various polymer binders. On phenolic-formaldehyde resins (KAST, KAST-V, KAST-R) they are more heat-resistant than PTK textolite, but worse in vibration resistance. On organosilicon resins (STK, SK-9F, SK-9A) they have high heat and frost resistance, have high chemical resistance, do not cause corrosion of the metal in contact with it. Fiberglass laminates are used mainly for large-sized products for radio engineering purposes.

High impact strength KCU up to 600 kJ / m2, ultimate strength up to 1000 MPa have fiberglass anisotropic materials, reinforced with glass veneer (SVAM). In terms of specific stiffness, these materials are not inferior to metals, and in terms of specific strength they are 2 ... 3 times higher.

Gas-filled plastics can also be attributed to the class of composites, since their structure is a system consisting of solid and gaseous phases. They are divided into two groups: foams and cellular plastics. Styrofoam have a cellular structure, the pores in which are isolated from each other by a polymer layer. Poroplastics have an open porous system and the gaseous or liquid products present in them communicate with each other and the environment.

Styrofoam obtained on the basis of thermoplastic polymers (polystyrene, polyvinyl chloride, polyurethane) and thermosetting resins (phenol-formaldehyde, phenol-rubber, organic silicon, epoxy, urea). To obtain a porous structure, in most cases, gas-forming components are introduced into the polymer binder, called porophores. However, there are also self-foaming materials, for example, polyester urethane foam, polyepoxy foam. Foams based on thermoplastic resins are more processable and flexible, but the temperature range of their operation is from -60 to +60 ° C.

Poroplastics are obtained mainly by mechanical foaming of the compositions, for example, with compressed air or using special foaming agents. When the foamed mass solidifies, the solvent, being removed during the drying and curing process from the walls of the cells, destroys them. Through pores can be obtained by filling the compositions with water-soluble substances. After pressing and curing the product, it is immersed in heated water, in which soluble substances are washed out.

Foam plastics are used for the manufacture of shock absorbers, soft seats, sponges, filters, as vibration-damping and sound-insulating gaskets in ventilation units, mufflers, gaskets in helmets and helmets, etc. Their density is 25 ... 500 kg / m 3.

Metal-polymer frame materials are composite materials in which a three-dimensional metal mesh is the supporting base, and the interframe cavity is filled with a polymer composition containing various functional components (Figure 5.11).

Rice. 5.11. The structure of the metal-polymer frame material (a) and the MPK material (b):

1 - metal particles, 2 - polymer, 3 - solid lubricant, 4 - pyrolytic graphite

In mechanical engineering, metal-polymer self-lubricating materials based on a cermet framework and polymer binders containing various dry lubricants (graphite, molybdenum disulfide, cadmium iodide, etc.) have found application. Such materials are used for the manufacture of plain bearings, roller bearing cages, piston rings, etc.

To obtain a cermet frame, powders of tin bronze, stainless steel, glass ceramics are used. The interframe cavities are filled with PTFE-4D by impregnation with a 50% aqueous suspension of PTFE or a mixture of PTFE-4D with lead. Metal-ceramic antifriction material MPK, made on the basis of stainless steel powders, contains pyrographite and fluoroplastic-4.

The technology of its production is as follows: a framework with a porosity of 20 ... 70% is pressed and sintered from metal powders. Then, in a special chamber, a carbon-containing gas is passed through the pores at a temperature that ensures gas pyrolysis and deposition of graphite on the walls of the frame until about 3/4 of the pore volume is filled, after which the product is repeatedly vacuum impregnated with a suspension of fluoroplastic-4 with simultaneous heat treatment.

Self-lubricating materials of the given type are efficient at temperatures up to 250 ° C.

The use of self-lubricating tape frame materials is very promising; The pores of the frame are filled with compositions based on fluoroplastic-4 and solid lubricants.

Tape materials They are highly technological, allow the manufacture of plain bearings (rolled) and bushings of any size) allow operation without lubrication at temperatures up to 280 ° C at high pressures (up to 200 ... 300 MPa) and sliding speeds. The use of a metal base strip and a porous bronze frame ensures good heat removal from the friction zone, and PTFE-4 located in the pores and on the surface with solid lubricants has a low coefficient of friction and high wear resistance of friction pairs. Tape materials such as DU, DP, DQ are widely used abroad.

One of the disadvantages of frame tape materials is the small thickness of the surface running-in layer (10 ... 20 microns), which excludes the possibility of mechanical processing of bearings after their installation in the housing.

The use of self-lubricating frame materials is effective, the frame of which is sintered from metal fibers or meshes, and various polymer compositions are used as a matrix, as well as materials based on carbon-graphite and metallized carbon-graphite fabrics impregnated with polymer binders with solid lubricants.

Currently, they are widely used composite wood materials, which are reinforcing wood materials (fillers), combined in a matrix (usually polymeric) with the introduction of special additives. In some cases, they are called wood plastics, or KDPM (composite wood polymer materials).

Chipboards - large-sized products made by hot flat pressing of wood particles mixed with a binder. According to GOST 10632-89, slabs are produced in sizes 2440x1220; 2750x1500; 3500x1750; 3660x1830; 5500х2440 mm, thickness from 10 to 25 mm, polished and not polished. In accordance with the purpose, the slabs are divided into three grades: P-1 (P-1M multilayer and P-1T three-layer)- manufacture cases, panels and other parts in radio and instrument making, furniture and construction elements. Coated with films based on thermosetting and thermoplastic polymers, paints and varnishes; P-2 (P-2T and P-20 single-layer, subdivided into groups A and B) - manufacture of cases of devices, machines, containers and containers (except food), racks, elements of furniture and building structures. They are used with veneer, decorative paper - laminated plastics and without facing; P-3 (P-ET)- details of bodies of vans, partitions of cars, elements of building load-bearing structures. According to the surface quality, the slabs are subdivided into polished (grades 1 and II) and unpolished (grades I and II).

Fiber boards (GOST 4598-86), depending on the density, are subdivided into soft (M), semi-hard (PT), hard (T) and superhard (ST) and, depending on the ultimate strength in bending, into seven grades: M-4, M- 12, M-20, PT-100, T-350, T-400 and ST-500, where the numbers mean the minimum ultimate strength of the plates in bending in kgf / cm 2. Thickness of slabs 2, .5; 3.2; 4; 5; 6; 8:12; 16 and 25mm, width from 1220 to 1830 mm and length from 1200 to 5500 mm. Designed for use in products and structures protected from moisture.

Wood laminates (particle board) - hot-pressed multilayer veneer sheets of various types of wood impregnated with synthetic resins. Chipboards are characterized by high strength and wear resistance, low coefficient of friction and good running-in properties.

Chipboard with a thickness of 1 to 15 mm are made in the form of rectangular sheets, with a thickness of 15 to 60 mm - in the form of plates. Sheets and plates glued from whole veneer sheets along the length are called solid, and from several - composite (with slightly reduced properties). Solid sheets are produced with a width of 950 mm and a length of 700, 1150 and 1500 mm and 1200x1500 mm; composite 2400х950, 4800х1200, 5000х1200 mm; solid slabs: 750x750, 950x700 (1150, 1500); 1200x1200 (1500), composite plates are produced in the same sizes as composite sheets. In accordance with GOST 13913-78 and GOST 20366-75, chipboard is subdivided into 11 grades.

Among perspective assemblies and parts from KDPM can be attributed to:

belt conveyor rollers;

rolling bearing housings;

blind and walk-through covers, hatches;

central parts of wheels and rollers (wheel centers with rims made of steel);

blocks of cables for cranes, telphers, chain hoists, etc .;

pulleys, sprockets, gears fixed on shafts using keyless connections;

weights, counterweights, dampers, flywheels with an inner part of pressed metal shavings and an outer part of KDPM;

interior trim panels for cars, buses, wagons, cabins of various cars, etc .;

pistons of pneumatic and hydraulic cylinders;

window frames;

frames for parts made of polyurethane foam;

bent-glued profiles and veneer panels;

sandwich panels with outer sheets of plywood, fiberboard, chipboard, DSG1, chipboard or metal (steel, aluminum) and the central part of foamed plastics with wood fillers;

parts made of foamed plastics with wood fillers for structural and heat-insulating purposes (for example, parts for fastening the ceilings of cars, heat, noise and vibration insulation of cars, diesel locomotives, refrigerators and garage doors, heat insulation of pipes for channelless laying, etc.);

reservoirs (gas tanks, receivers, etc.).

plain bearings operating in selective transfer mode;

Of course, the considered promising areas of application of the CMRM do not claim to be complete, do not exhaust all possible areas of use and can be significantly expanded.