Reducing Defectiveness of the Interphase Layer in Polymer Composite Materials after Exposure to a Microwave Electromagnetic Field

. The microstructure in the area of the interfacial layer (IFL) of structural carbon and glass-reinforced plastics after exposure to a microwave electromagnetic field in the cured state has been studied. It was found that the IFL of control and experimental samples is characterized by the presence of structural formations of complex shape with granular morphology, the element sizes of which are 10-20 nm. The IFL of control samples is characterized by increased defectiveness, expressed in numerous microcracks of 100-500 nm in size and more, and matrix delaminations from the fiber up to several hundred nanometers in size. In experimental samples, such defects are practically not detected. In the modes of microwave exposure, corresponding to the temperature of the temporary restoration of the plasticity of the matrix, the formation of waviness is noted. The research results allow us to conclude that one of the reasons for the increase in the physical and mechanical properties of cured polymer composite materials as a result of microwave exposure is a significant reduction in defectiveness in the IFL.


Introduction
Development and implementation of perspective promising systems require creation of new high-strength and lightweight materials -composite materials on polymer matrix. It is also necessary to improve the technologies of their shaping. [1][2][3]. According to the analytical company Markets&Markets, the global carbon fiber market will reach $8 billion by 2026, the annual increase in PCM production is currently 8-10 % annually, and in two years from 2022 to 2024, PCM production will increase from 13 to 14 million tons. Various thermosoftening and thermosetting material are used as binders. Mainly (more than 60% of production volume) PCM are formed on thermosetting binders with good fluidity and better wetting properties in comparison with thermoplastic ones [4]. At the same time, increased requirements are put forward for the mechanical and thermal strength of structural elements of products that have complex shapes, which strongly affect the distribution of mechanical and thermal stresses [5].
Due to a significant difference in the mechanical and thermophysical properties of the components of polymer composite materials (PCM), during their curing, defects associated with the shrinkage of the binder occur. It leads to a decrease in physical and mechanical properties. This causes the need for additional strengthening of the structure in some dangerous areas, which leads to an increase in weight [6]. Therefore, various technological methods are more promising, for example, physical modification [7]. An analysis of studies performed in Russia and abroad shows that one of the promising areas for the physical modification of PCM products is finishing in a microwave electromagnetic field [8][9][10][11][12][13]. Nevertheless, the literature contains insufficient information about the mechanism of the action of a microwave electromagnetic field on the cured structures of materials consisting of components with sharply different thermal and electrical properties.

Formulation of the problem
The authors are developing a hypothesis [14] that the main mechanism of the influence of a microwave electromagnetic field on the physical and mechanical properties of cured PCMs is the combined effect of thermal and wave factors that contribute to a temporary increase in the plasticity of the binder when heated to temperatures of (40-80) °C, which increases the likelihood of conformational rotations links of macromolecules and the fluidity of the material. In this case, an increased number of points of contact interaction "matrix-fiber" and, accordingly, the area of the contact surface are formed, which contributes to a more uniform and complete redistribution of external loads between the reinforcing fibers and an increase in the strength characteristics of the product.
The authors' study of the interlayer microstructure of PCM after microwave exposure confirms its change. It consists in the increase of the contact surface. This is determined by the energy parameters of microwave radiation. The phase composition of control and test samples of PCM was studied by DSC. Based on the analysis of the DSC results, it was found that the effect of the microwave electromagnetic field increases the content of the crystalline phase in the IFL of cured carbon fiber reinforced plastic by 42 %. This fact confirms the increase in the orderliness of the structure and indirectly proves an increase in the number of supramolecular formations and, accordingly, the number of areas of contact interaction "matrix -fiber" [14].
Thus, the study of the features of the PCM microstructure by various methods revealed an increase in its homogeneity (orderliness) and density, which confirms our assumptions about the effects of structure changes occurring under the exposure to an electromagnetic field.
It is known that PCM are characterized by various structural defects. Their causes may be different. Due to the difference in the properties of the components and the complexity of the molding process, one of the main types of defects are technological ones, which can have a significant negative impact on the physical and mechanical properties of products, which actualizes their elimination or minimization [15,16]. It is obvious, taking into account the above mentioned, that microwave exposure can affect microstructure defects in the form of microcracks and delaminations, due to the influence of internal stresses that arise during the shrinkage of the binder.
The aim of the work was to study changes in the structure of the interfacial layer (IFL) of PCM as a result of exposure to a microwave electromagnetic field.

Methodology and equipment
The microstructure of carbon fiber and glass-fiber reinforced plastics samples was studied after delamination tests in accordance with GOST R 56815-2015 "Polymer composites. Method for determining the specific fracture work under G1C separation conditions. We used PCM produced by Evrokomplekt LLC, Kaluga, obtained by pressing. For testing, samples were made in the form of a short beam with dimensions of 25x10x5 mm. After testing, the samples were completely split along the delamination plane. The delamination surface was studied using a MIRA II LMU Tescan scanning electronic microscope. Microwave processing of the samples was carried out on an experimental setup based on the Zhuk-2-02 microwave emitter manufactured by LLC NPP AgroEcoTech, Obninsk, Kaluga Region. at a frequency of 2450 MHz, energy flux density (EFD) and time equal for carbon fiber reinforced plastic (17-18)x10 4 μW/cm 2 and 2 minutes, for glass-fiber reinforced plastics -(45-50)x10 4 μW/cm 2 and 5 minutes. In these modes, heating temperatures in the range of (65-70) °С are typical for carbon fiber reinforced plastic, for glass-fiber reinforced plastics (35-40) °С, and the maximum efficiency of microwave exposure to increase the limiting stresses of threepoint bending and interlaminar shear is established. Thus, for carbon fiber reinforced, an increase in the limiting stresses of three-point bending by (25-42) % on average, interlaminar shear -by (14-16) %, stretching -by (7-8) % is noted [14]. For glass-fiber reinforced plastics, the effect is less pronounced and does not exceed (9-12) %. It can be assumed that the revealed features of the structure of control samples and samples subjected to microwave exposure will satisfactorily correlate with the indicated change in physical and mechanical properties.

Results and discussion
The electronic micrographs of the IFL and the isolated fragments are shown in Figs. 1 and 2. The following features of the structure of the binder formations in the IFL of control and test samples of the investigated PCMs are noted.
In the control samples of carbon fiber reinforced plastic, matrix fragments are represented by rounded randomly located formations with a granular structure (sizes 10-20 nm) or formations with a relatively even surface, which has the character of chips with delaminations from the fiber surface. Separate cracks with dimensions of about 100 nm and cavities 20x200 and 100x100 nm were revealed (Fig. 1a). This objectively reduces the strength characteristics of the matrix. The main type of defects are delaminations of the binder from the surface of the fiber and the void (Fig. 1a, photo on the right). This indicates an incomplete "matrixfiber" contact and a decrease in the ability of the matrix to transfer the load to the fibers. In this regard, the physical and mechanical properties of carbon fiber reinforced plastic are reduced. In experimental samples, there is an almost complete absence of cracks in the matrix fragments. Part of the matrix fragments is represented by rounded chaotically located formations or has elongated structures (Fig. 1b). On average, there is a significantly smaller surface of delamination of matrix fragments from the fiber surface. The changes contribute to an increase in the uniformity of the distribution of the load between the fibers through a denser matrix and may be one of the reasons for the increase in the strength characteristics of carbon fiber reinforced plastic's experimental samples. a b Fig. 2. Typical elements of the IFL of the control (a) and test (b) samples of glass-fiber reinforced plastics EFD = (45-50)x10 4 µW/cm 2 , time 5 minutes.
Fragments of the binder in the control samples of glass-fiber reinforced plastics are significantly more defective compared to similar samples of carbon fiber reinforced plastic.
The surface of the formations is also granular, but more embossed. Fragments of the matrix show cracks 200-300 nm in size, cracks over 500 nm long, and wide zigzag cracks about 200 nm in size (Fig. 2a, photo on the left). The contact areas of a small area "like the roots of trees" are characteristic (Fig. 2a, photo on the right). On these fragments, multiple cracks 50-100 nm in size and delamination from the fiber surface were revealed. The noted features of glass-fiber reinforced plastics IFL can characterize its strength. It is low compared to the strength of carbon fiber. This is confirmed by the results of bending and interlayer shear tests. As a result of testing the control samples, the ultimate stresses during three-point bending and interlaminar shear were 100-110 and 40-43 MPa for carbon fiber reinforced plastic, and 60-70 and 25-28 MPa for glass-fiber reinforced plastics.
For test samples of glass-fiber reinforced plastics, the almost complete absence of cracks in the IFL is characteristic, there are individual cracks with a length of not more than 200 nm. The surface of the fragments is smooth with a fine-grained morphology (Fig. 2b, photo on the left). In approximately 50 % of cases, wave-like formations with a step of about 150-200 nm were recorded on the surface of matrix fragments (Fig. 2b, photo on the right).
It is obvious that cracks and delaminations in the control samples of PCM are formed due to shrinkage of the binder [15,16]. During its curing, due to a significant difference in the coefficients of linear thermal expansion of epoxy, carbon fiber and glass-fiber reinforced plastics, respectively: 55*10 -6 1/°С, (-1.5...-0.7)х10 -6 1/°С and (5…6)х10 -6 1/°С, (2.5…-2.9)х10 -6 1/°С [17,18]. This causes significant deformations under conditions of temperature difference. It has been established that during the microwave treatment of the samples, the matrix and the reinforcing component are heated. Under such conditions, according to the data of [19], the matrix passes into a highly elastic state. This makes possible the conformational rotations of macromolecular units, the "healing" of microdefects in the IFL, and the relaxation of residual stresses. Since the difference between the temperature of microwave heating and the final temperature of the composite after cooling is not as large as during the formation process, then, accordingly, there is less thermal deformation of the fibers and the binder, less stress, and the formation of defects while re-curing does not occur, or they are small. The virtual absence of cracks in the matrix fragments and its slight exfoliation (Figs. 1b and 2b) confirm this assumption.

Conclusions
The presented results of the defects study in the IFL of control and test samples of carbon fiber reinforced plastic and glass-fiber reinforced plastics allow us to conclude that, in addition to increasing the area of contact interaction and interlayer adhesion, another manifestation of the mechanism of the microwave electromagnetic field influence on the increase in the physical and mechanical properties of cured PCMs is a decrease in the number and size of defects in IFL because of their closure during the temporary transition of the matrix to a plastic state.