Production of Composites Based on High-Temperature Thermoset Plastics

As noted above, the production of composite materials, by mixing the matrix and filler in solution or melt, is compatible with existing industrial technologies, and therefore, is well known and well developed. However, these methods are unsuitable for the production of composites of high-temperature polymers or tliennoset composite. In turn, methods of producing composite materials based on such matrices are not as detailed in the literature as those obtained from solutions or melts. In this regard, we would like to focus on obtaining polymer composites by the in situ method using the example of a polyimide matrix, which is a typical example of a high-temperature polymer, since most polyimide matrices are insoluble in organic solvents, and their softening temperature exceeds 300°C.

Polyimides are produced by polycondensation of carboxylic acid dianliydrides with diamines. There are single-stage and two-stage polymerization reactions. In the one-step method, the stages of acylation and cyclization proceed simultaneously in a high-boiling solvent at 180-200°C. In the case of a two-stage process, the reaction proceeds with the formation of an intermediate product of polyamic acid, and in the second stage, cyclodehydration (imidization) is carried out with the formation of polyimide. However, in both cases, the initial monomers are dissolved in a suitable solvent, and at this stage an inorganic filler may be added to the reaction mixture. Depending on the method of synthesis, the target polyimide composites can be obtained in the form of powders or films.

4.1 Composite Materials Based on Polyimides

One of the interesting fillers for polyimides is single-walled (SWCNT) or multi- walled (MCNT) carbon nauotubes. The combination of CNTs and polyimides will play an important role in the development of new highly efficient nauocomposites [47]. There are two general methods for producing such composites. One is to mix the CNT with the polymer matrix in a molten form to produce a composite. Another is to disperse the CNT in the polymer solution, solidify the resulting solution, and remove the solvent.

Park and his colleagues reported on the method of efficiently dispersing SWCNTs in a polyimide matrix [48]. The resulting SWCNT polyimide films are electrically conductive and optically transparent. A shaip increase in conductivity was observed between 0.02% and 0.1% vol. SWCNT, and during this process, the nanocomposite was transformed from a capacitor to a conductor. The introduction of 0.1% vol. SWCNTs increased conductivity by 10 orders of magnitude, which exceeds the antistatic criterion for thin films for use in astronautics (1 * 10-s S cm-1). Polyimide film containing 1.0% vol. The SWCNT still transmitted 32% of the visible light at 500 mil, while the film produced by direct blending passed less than 1%. Dynamic mechanical test showed that by the addition of 1.0% vol., the SWCNT increases the elastic modulus by 60%, and the thermal stability of the polyimide is improved in the presence of the SWCNT.

Connell et al. [49] reported on the synthesis of alkoxysilane polyamic acids, and SWCNTs were added to a previously prepared polyamic acid solution. When loading 0.05% wt., the SWCNT achieved a percolation barrier, which can be seen from the sharp decrease in the surface resistance of the material. Surface resistance and volume resistance indicates that the SWCNT polyimide composite is conductive. However, the presence of SWCNT in polyimide has very little effect on the temperature at which the fracture stalls (Tg) and the tensile strength of the polymer. An increase in the ionic strength of the polyimide matrix by the addition of an inorganic salt (CuS04) led to the formation of a SWNT network sufficient for conductivity, for example, adding 0.014% wt. CuS04 in a composite containing 0.03% wt.

The SWCNT led to films reduced by 4 orders of magnitude by surface and volume resistance [50, 51]. An increased electrical conductivity of nanocomposite films is observed; however, electrical percolation occurs at larger loads than those commonly used in SWCNT polyimide nanocomposites. The modulus of the films slightly increases with increasing content of single-walled carbon nauotubes. Electrospun fibers were obtained from the same SWCNT polyamide suspensions used to make films. High resolution scanning electron microscopy images have shown that the SWCNT is located inside the fibers and may have a direction parallel to the fiber axis [52].

San and his colleagues [53] reported on the production of functionalized CNTs using polyimides with pendant hydroxyl groups. It was found that the resulting polyimide-functionalized CNTs are soluble in the same solvents as the original polyimide. A significant advantage of this method is that these functionalized nanotubes can be used directly to produce polyimide-CNT composites with a relatively high content of nano tubes.

Bean and colleagues [54] obtained polyimide-CNT composites by carrying out in situ polymerization in the presence of MCNT. The percolation barrier of the electrical conductivity of the resulting PIMUNT composites is about 0.15% by volume. The electrical conductivity increases by more than 11 orders of magnitude to 10-J S cm-1 when the percolation barrier is reached, and subsequently increases to 10-1 S cm-1 with an increase hi the concentration of MCNT to 3.7% by volume.

Nakasliima [47] reported on the synthesis of frilly aromatic polyimides containing disulfonic acid triethylammonium salts (Figure 6.9). The polyimides obtained have an increased ability to dissolve MWCNTs in themselves. The mam driving force for solubilization of MWCNTs are the л-л-interactions between the condensed aromatic part of the polyimide and the MWCNT surface. A high concentration of MWCNTs

Polyimides containing disulfonic acid salts [43]

Figure 6.9. Polyimides containing disulfonic acid salts [43].

in polyimide solutions leads to the formation of gels consisting of individually dissolved MWCNTs.

Ando and his colleagues [55] obtained new nano-ZnO/hyperbranched polyimide hybrid films by in situ sol-gel polymerization. Films obtained from colorless, fluorinated polyimides and homogeneously dispersed ZuO nauoparticles show good optical transparency. Later on, two types of model compounds (with and without ZnO) and a liyperbranched polyimide (HBPI) film with ZnO microparticles were obtained. These materials were used to study the mechanism of fluorescence of the original HBPI and in situ hybrid films. In in situ hybrid films, an effective energy transfer from ZnO nanoparticles to the main HBPI chain was observed, while in ordinary HBPI films only energy transfer from local excited states was observed. This shows that HBPI are terminally associated with ZnO particles through a monoethanolamine group, which is an effective way to transfer energy, which leads to fluorescence.

Liu and his colleagues [56] obtained hybrid optical films based on Pl-nanocrystalline titanium with a relatively high titanium content and a large thickness of soluble polyimides containing hydr oxyl groups (Figure 6.10). Two types of new soluble polyimides were synthesized from hydroxy-substituted diamines and various commercially available tetracarboxylic dianhydrides. The hydroxyl groups in the main chain of the polyimide bind the organic and inorganic parts and also control the molar ratio of titanium butyloxide to hydroxyl groups. This leads to

Hybrids based on PI nauocrystallme titanium [56]

Figure 6.10. Hybrids based on PI nauocrystallme titanium [56].

homogeneous hybrid solutions. Flexible hybrid films were obtained, and analysis showed that these films have relatively good surface flatness, thermal stability, variable refractive index, and high optical transparency. Three-layer anti-reflection coatings based on these hybrid films were obtained, and the reflectivity was less than 0.5% in the visible region. These characteristics suggest that these films can be applied in optics.

Polyimide conductive composites are made from appropriate polyimides and conductive fillers, such as carbon nanotubes, graphite, and acetylene black. The polyimide precursor (polyamic acid) was synthesized from 3,4,3’,4'biphenyl tetracarboxylic dianhydride and 4,4’diaminodiphenyl ether with the help of intensive mechanical mixing at -5°C. Experimental results showed that the electrically conductive composite based on carbon nanofiibes and polyimide possess better electrical, mechanical, and adhesive properties than the other two composites [57].

A new highly porous composite based on polyimide and silicon with high flexibility, mechanical strength, and heat resistance has been developed. The composite material was prepared using a new process consisting of phase separation of a mixture of a polyimide precursor (polyamic acid), solvent, and alkoxide of silicon using C02 at high pressure (40°C, 20 MPa), the formation of silicate by sol-gel reaction, and solvent extraction with supercritical C02. The composite has a bimodal porous structure with micropores of 10-30 pm and nanopores of 50 nm. Silicon nanoparticles with a diameter of less than 100 nm are dispersed in the polyimide matrix. The porosity of this composite is 78%, which is higher than the porosity of the polyimide produced by the foaming method.

The relative dielectric constant of the composite is below 1.4 at 1 MHz. The porous sheet of a polyimide silicon composite material proved to be quite flexible, and does not collapse even when bent. It should be noted that the Young’s modulus (0.80 GPa) and the decomposition temperature (600°C) of this composite are higher than those of ordinary porous polyimide with the same porosity. These properties make a composite material based on polyimide and silicon suitable for use as a flexible thermally insulating material [58].

4.2 Composites with Nanostructured Silicon Carbide

Over the past decade, many research papers have been devoted to combining polymers with nauoparticles to obtain materials with increased rigidity, toughness, and tribological properties [59]. By adding nanoscale fillers to the polymer matrix, the material acquires new chemical and physical properties. This is due to the influence of the unique nature of the nanoscale filler on the bulk properties of polymer-based nanocomposites [60, 61]. Polymer nauocomposites are intensively used in various fields, due to then ease of processing, low production cost, good adhesion to the substrate, and unique physicochemical properties. Dispersion of inorganic materials in a polyimide matrix is a complex task and a key factor influencing the final properties of hybrid materials. Adding a crosslinking agent is a solution to a number of difficulties associated with dispersing. By adding a crosslinking agent, organic and inorganic materials can be covalently bonded, and the compatibility between these two phases is improved [62, 63].

Silicon carbide nanoparticles (SiC) are chosen for their unique physical properties, such as excellent chemical resistance, heat resistance, high electron mobility, excellent thermal conductivity, and outstanding mechanical properties. They are used to produce highly efficient composites [64-67], and are used hr electronics [68, 69]. These properties make SiC nanoparticles a suitable material for the production of polymer nauoconrposites with an enhanced structure [70].

The properties of nauocomposite films obtained from the new polyimide and silicon carbide nanoparticles SiC using two simple methods are reported. In the fir st, SiC nanoparticles were first functionalized with epoxy (y-glycidoxypropyltriethoxy silane) terminal groups (mSiC), and then the solution was mixed with a polytriazolimide. A homogeneous solution for the preparation of a polytriazolinride/ mSiC film was heated under vacuum. In the second method, a new diamine containing the 1,2,4-triazole cycle - 4,4,-(4-(2,3-diplrenylphenoxal-6-yl)-4H-l,2,3-triazole-3,5- diyl), and a commercially available dianhydride (4,4’-(hexafluoroisopropylideue) diphthal diauhydride) react in situ in the presence of SiC nanoparticles to form a homogeneous mixture of polyanric acid and silicon carbide (PAA/SiC), which is then transformed into a polytriazolimide in a vacuum in a high-temperature process/ SiC film. The results of the study showed that strong chemical bonding between SiC nanoparticles and the polymer matrix leads to an increase in glass transition temperature Tg from 300°C to more than 350°C, tensile strength from 108 MPa to 165 MPa, and a temperature of 5% mass loss (T5%) from 380°C to 500°C. The photolunrinescence intensity has increased, and the spectrum shows a shift to the blue region with increasing content of SiC [71].

A highly efficient composite material based on silicon carbide (SiC) and bismaleimide modified with allyl novolac for abrasive tools and wear-resistant elements was developed and characterized. The results showed that the residual strength at 440°C for 1 hour decreased to 64% of the strength without heat treatment, and the thermal-oxidative stability is better than for SiC/polyimide composites made in a similar way. The proportion of polymer in the composition of the composite affects the mechanical properties. The flexural strength of a composite increases with an increase in the proportion of bismaleimide, but its excess leads to the formation of bubbles in the composite. The best composite with a bending strength of 82.4 MPa was obtained using 13% wt. bismaleimide. After treatment at 280°C for 1 hour, the bending strength increased by 34% due to the further crosslinking of the polymer at a higher temperature [72].

It is expected that the combination of polyimides and other organic/inorganic compounds will play an important role in the development of innovative high- performance nauocomposites for various applications.

4.3 Preparation of Powder Polyimide Composites

The process of obtaining a powder composite material can be divided into four stages:

  • • modification of the surface of the inorganic filler (if necessary);
  • • dispersion of inorganic filler in a solution of high-boiling solvent and diamine;
  • • carrying out polymerization and imidization reactions;
  • • isolation of the resulting composite material in the form of a powder.
  • 4.3.1 Modification of the Surface of the Inorganic Filler

Modification of the surface of the inorganic component before introducing into the composite, on the one hand, allows an increase in the affinity between the organic and inorganic phases of the material, on the other hand, makes it possible to vary the properties of the materials obtained. For example, it is well known that doping of carbon nanotubes leads to a sharp increase in the conductivity of nanotubes, due to a change in then electronic structures caused by charge transfer [73-76]. The authors [77-79] reported on the production and electrical properties of air-stable polymer composites filled with CNTs (doped with iodine). It is reported that conductivity increases 2-5 tunes as compared to composites with unalloyed CNTs.

4.3.2 Dispersion of the Inorganic Filler in a Solution of High-Boiling Solvent and Diamine

As noted earlier, the key stage in obtaining a composite with an inorganic nanosized filler is dispersion of the filler in solution. As high-boiling solvents, choose those that are capable of dissolving both the initial monomers and the polymerization product. In the case of precipitation of the product from the solution in the process of growth of the polymer chain, a product with a low molecular weight will be obtained. Suitable solvents can be, for example, high-boiling aprotic polar solvents, such as N, N-dimethylfonnamide, dimethyl sulfoxide, etc.

The processing time and frequency of acoustic waves during ultrasonic dispersion are the determining parameters of the process. The effect of dispersion time and ultrasound frequency on the quality of the distribution of carbon nanotubes and nanoscale boron carbide in the final polyimide composite is considered in [80]. It was shown that when using a frequency of 10-15 kHz for both fillers, agglomerates of nanoparticles are observed on the surface of the composite, regardless of the time of dispersion. Increasing the frequency to 20 kHz at small (5-10 minutes) exposure times leads to an uneven distribution of the filler in the final composite with agglomerates of 2-5 pm in size. And only with an increase in the dispersion time to 15-20 minutes, a uniform distribution of the inorganic filler over the surface and in the structure of the composite is observed.

After obtaining a uniform dispersion of the filler and the diamine in the solvent, the usual mechanical stirring can be used to further cany out the process. Dianliydride is added to the resulting dispersion with stilling, and then proceed to the next stage.

4.3.3 Polymerization and Imidization

The polymerization reaction is usually canied out at elevated temperatures for a long time. If necessary, a polymerization initiator, for example, benzoic acid, may be added to the reaction mixture. The parameters of this stage of the process will depend on the starting monomers. For polyimide composites, the process is canied out in steps: they hold the reaction mass at 90°C for 4 hours, and then, at a temperanire of 180-200°C for 12-16 hours.

4.3.4 Selection of the Resulting Powder Composite Material

Isolation of the composite is earned out by precipitating the polymer from the solvent of the reaction medium. To do this, the reaction mass is poured into a large volume of solvent that does not dissolve the final polymer. In the case of polyimides, ethyl alcohol is usually used as such a solvent. The precipitation is filtered, washed, and dried to constant weight.

4.3.5 Production of Film Polyimide Composites

The process of obtaining film polyimide composite material can be divided into five stages [81-84]:

  • 1. Modification of the surface of the nanoscale filler (if necessary);
  • 2. Dispersing inorganic filler in a solution of high-boiling solvent and diamine;
  • 3. Conducting the polymerization reaction to obtain a solution of the precursor
  • (polyamic acid);
  • 4. Applying the resulting solution of polyamic acid on a substrate;
  • 5. Drying the solvent followed by stepwise imidization in a vacuum oven.

The last stage is the drying of the solvent and stepwise imidization is carried out in a vacuum drying oven. An extremely important step is pre-drying, at a temperature of 70-80°C for 8-12 hours, allowing you to remove the solvent. It is important to avoid rapid heating of the plate in order to avoid the formation of bubbles on the surface of the film due to the rapid evaporation of the solvent. Further gradual increase in temperature provides the most complete and uniform imidization. Step polymerization begins at a temperature of 150°C, keeping the plate under these conditions for 20-30 minutes, then the temperature is increased by 50°C, and maintained for the same amount of time. Thus, the imidization temperature is increased hi increments of 50°C, and the imidization is completed at a temperature of 300°C.

The most important criterion in obtaining polymer composite films is uniformity and uniform thickness.

The simplest way to apply a polyamic acid solution to a substrate is to immerse the substrate in the solution or pom the solution onto the substrate. However, this method does not allow the control of the thickness of the resulting film. At present, for the manufacture of polyimide films, the method of spin-coating is used, which appeared relatively recently, but gained most popularity in the manufacture of polyimide films. Using this method, it is possible to adjust the thickness of the resulting film by changing the speed of rotation of the plate.

Samples of films can be obtained on various laboratory instruments designed to apply liquids to coatings. Concomitant evaporation of the solvent, which is induced by rapid rotation, leads to the formation of a semi-solid film. On the previously cleaned and dried plate of glass or metal, put the prepared polymer solution. Place the plate on the Spin Coater vacuum chuck and rotate at a speed ranging front 100 to 6000 revolutions per minute, depending on the desired film thickness.

The study was carried out with the financial support of the Russian Foundation for Basic Research, within the framework of the RFBR research project No. 18-29- 1808720.

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