The Manufacturing of Composite Materials in the Matrix of Modi ed Phenol-Formaldehyde Resins

The work develops the method of obtaining new composite materials based on novolac phenol-formaldehyde resin with improved adhesion properties and heat resistance. The composites were obtained by chemical modifi cation of phenol-formaldehyde resin by epoxy resin and polyvinylpyrrolidone during the hardening of the composition. The mechanism of resins modifi cation and hardening has been determined. It is shown that due to the modifi cation, the heat resistance and adhesive strength of the obtained enamels and coatings have signifi cantly increased. In particular, with the polyvinylpyrrolidone content from 0.5 to 1 wt% and epoxy resin from 25 to 50 wt%, the adhesive strength of the compositions increases 5–7 times.


INTRODUCTION
Given the extensive application of phenolformaldehyde resins (PFR) in industry and private life, it is now necessary to develop new phenoplasts with high physic mechanical characteristics and a universal complex of properties for operation under various conditions. The universal applicability of these materials is connected with the broad temperature range of their hardening and the possibility of getting various operating characteristics depending on the fi eld of their application [2,15].
The phenol-formaldehyde polymers are used as glue materials, anti-corrosion coatings, and binders to produce molding powders [9]. The development of the fi eld of polymeric glues and protective coatings requires creating new polymeric materials with given combinations of properties and, in the fi rst place, with elevated adhesion strength and high water, chemical, and thermal resistances.
Parallel with signifi cant advantages (accessibility of raw materials, low-cost production, easiness of preparation of the lacquer, good dielectric properties, and high chemical stability), glue compositions based on the novolac PFR also have serious shortcomings: high brittleness of the fi lled PFR caused by their porosity and insuffi cient adhesion to metals and glass [1,13]. Therefore, the problem of developing new polymeric glue compositions and modifying the existing compositions used for gluing metals with the glass always remains quite urgent.

ANALYSIS OF RECENT RESEARCH
The modification of phenol-formaldehyde compositions performed by using epoxy-diane resins and polymers of the N-vinyl series guarantees the possibility of complex improvement of the properties of "cross-linked" phenoplasts via the formation of additional three-dimensional networks in the resite and owing to the presence of polar functional groups in the macrochain.
At different ratios of epoxy and phenol-formaldehyde resins, coatings with particularly high protective mechanical properties are obtained [10,12,16]. At the same time, it is shown that the highest chemical resistance of the coating is achieved due to the reaction of phenolic resin with diane epoxy resins of high molecular weight [10].
In work [4], bio-phenolic/epoxy polymer blends were fabricated using the hand lay-up method at different loading of bio-phenolic (5 wt%, 10 wt%, 15 wt%, 20 wt%, and 25 wt%) in the epoxy matrix whereas neat bio-phenolic and epoxy samples were also fabricated for comparison. Results indicated that mechanical properties were improved for bio-phenolic/epoxy polymer blends compared to neat epoxy and phenolic. In addition, there is no sign of phase separation in polymer blends. The highest tensile, flexural, and impact strength was shown by P-20(biophenolic-20 wt% and Epoxy-80 wt%) whereas P-25 (biophenolic-25 wt% and Epoxy-75 wt%) has the highest tensile and flexural modulus.
The authors of [18] showed that changing the crosslinking reaction temperature of epoxy-phenolic novolac resins makes it possible to adjust the porous morphology of the formed microcapsules, shell thickness, mechanical strength, and release behavior. The microcapsules surface varied from nonporous to dense small pores with increasing temperature from 40 °C to 70 °C, but they showed decreasing tendency at higher temperature (> 70 °C).
One of the little-studied directions of modification of PFR is using polymers based on N-vinyl compounds. Among the polymers of the N-vinyl series of great practical interest is polyvinylpyrrolidone (PVP). It is known that PVP has an initiating effect in the polymerization of hydroxyalkylene methacrylates as an active complexing agent [3,7] and also participates in the formation of new polymer matrices of liquid-structured type [14].
In work [17], authors proposed to use treated PVP fibers as reinforcing fillers of phenol-formaldehyde compositions. It is shown that the increase in adhesion and shear strength between phases, the increase in the modulus of elasticity during bending of composites occurs due to the receipt of a layer of PVP between phases.
Based on PVP, as a film-forming agent, compositions for protective coatings are obtained [8], in which from 2 to 10 wt% phenol-formaldehyde oligomer is introduced to increase thermal stability and water resistance. This composition is also characterized by good storage stability and forms a coating with high adhesion.
Prepolymers based on phenol-formaldehyde resins in a mixture with vinylpyrrolidone latex [6] are used for gluing all types of cord: viscose, polyamide, polyester, fiberglass, aramid, and metal cord. These compositions are stable under normal conditions and cured at 145 °C.
Thus, PVP in phenol-formaldehyde compositions can be used to improve adhesion or as a rheological additive. PFR in PVP-based compositions improve thermal and water resistance and the hardness of the coating.
This work aimed to develop new composite materials based on novolac phenol-formaldehyde resin with improved adhesion properties and heat resistance. The chemical modification of PFR by epoxy resin and PVP during the hardening of the composition solves this problem.

RESEARCH METHODS
The polymeric compositions were prepared in the following way. Phenol-formaldehyde lacquer (PFL) was obtained via dissolution at 40-50 °С of a given mass of novolac phenol-formaldehyde resin in isopropyl alcohol. PVP was dried for 4 h at 60 °С. The corresponding ass of PVP was dissolved in isopropyl alcohol and thoroughly mixed with PFL and epoxy resin ED-20. Then a hardening catalyst N,N-dimethylaniline (DMA) was added.
Thermal analysis was performed using a Q-1500D derivatograph of Paulik-Paulik-Erdey system [11]. The equipment makes it possible to simultaneously determine the mass loss (thermogravimetry -TG), the rate of mass loss (differential thermogravimetry -DTG), and thermal effects (DTA). The study was carried out in a dynamic mode in the air with a heating rate of 3 °С min −1 . For the research samples weighing 0.2 g in powdered form were used. The reference substance was aluminum oxide. Derivatographic studies of the samples were performed in the temperature range of 20-900 °С.
The fine powder samples were investigated by IR spectroscopy in the medium of vaseline oil using SPECORD M-80 spectrometer (Carl Zeiss Jena, Germany), equipped with a singlehorizontal Golden Gate ATR cell. The spectra were taken after 10 scans, at a resolution of 4 cm −1 , in the range of 4000-400 cm −1 [5]. Powdered composite samples were ground in an agate mortar in vaseline oil. Then they were placed between two plates with KBr, which is transparent in this area of the IR spectrum.
To study the process of structuring compositions, prepared the specimens of compositions in the form of films in molds of polytetrafluoroethylene (GOST 10007-80) with 55 mm in diameter. To separate the film from the substrate, it was subjected to slight heating. The content of gel fraction was found by the extraction method of preliminarily crushed films with ethanol in a Soxhlet device.
The adhesion strength of the glue line at uniform tearing (GOST 14760-69) and shearing (GOST 14759-69) were performed on a TiraTest 2200 tensile-testing machine (Germany) with an extension rate of 50 mm/min.

RESULTS AND DISCUSSION
The predicted change of physicomechanical properties, PFR adhesive activity, is caused by the controlled process of chemical interaction between PVP+PFR and epoxy resin ED-20 or PFR and ED-20 rather than physical effects due to the unique properties of PVP. Since the resin hardening takes place at high temperatures (150-180 °С), taking into account exoeffect, we may expect the reaction between free hydroxy groups of PFR and lactam cycles of PVP. These predictions are confirmed by a series of physical and chemical investigation methods -Differential Thermal Analysis (DTA), IR-spectroscopy, chemical titration, etc. DTA results are represented in Figure 1.
Two endothermal effects are observed at DTA curves within the temperature range of 50-175 °С. The first one is observed within the range of 50-120 °С and is typical for pure PFR. It corresponds to the process of resin melting. The presence of PVP in the composition causes the second endoeffect in the range of 100-125 °С. The increase of PVP amount deepens the effect and shifts it to the region of higher temperatures (Fig.  1, curve 2). The reason is that hydrogen bonds are formed between PFR macromolecules (methoxy end-groups take part) and carbonyl groups in PVP macromolecules. Such bonds cause the origin of the ordered fluctuated network, which is destroyed at high temperatures. According to IRspectroscopy data (Table 1)   The wide endoeffect appears for the PVP sample in the range of 50-175 °С (Fig. 1, curve 5). This effect is connected with moisture loss and polymer softening. The form of endoeffect does not correspond to DTA curves for the composition samples. The effect is observed in the wider range of temperatures and is less express compared with PFR+PVP mixtures.
The chemical reaction is grafting of PFR chains to PVP with the formation of grafted copoly(vinylpyrrolidone-gr-methylphenol) copolymer accordingly to the mechanism: The presence of methoxy groups in the PFR structure is confirmed by chemical titration and IRspectroscopy. The absorption bands in the region of 1040 cm −1 typical of methoxy groups are clearly observed in IR-spectra, as well as absorption bands at 1200 cm −1 typical of phenol hydroxyls ( Table 1).
The absence of chemical interaction between PVP and phenol hydroxy groups of PFR is confirmed by model experiments with phenol. The chemical titration demonstrates the constant amount of phenol hydroxyls while mixing phenol and PVP. The reason is a subacid reaction between PVP and phenol. The absorption bands of the pyrrolidone cycle and methoxy groups are absent in the spectra of the composition PFR:PVP = 147:1 heated at 175 °С for 1 hour. The maximal intensity of the absorption band of esteric bond indicates the end of chemical interaction between PFR and PVP in the mentioned temperature range.
DTA data (Figs. 2-3) of the samples hardening at 150-160 °С for 25-30 min indicates the proceeding of chemical interaction between PFR, PVP, and epoxy resin in the presence of hardening catalyst N,N-dimethylaniline (DMA), leading to the formation of a three-dimensional structure with high thermal stability compared with PFR hardening by urotropine. The results of the gel-fraction investigation reveal the mentioned fact (Table 2).
We studied before the interaction between phenol-formaldehyde and epoxy oligomers in the presence of PVP using model two-components systems. We did not observe the network formation at PVP introduction in the amount of 1-50 mass parts to novolac phenol-formaldehyde or epoxy resins at heating to 150-160 °С for 0.5-1 hour.
At the same time, the epoxy-novolac compositions are intensively crosslinked in DMA's presence as a catalyst (Table 2). It was established that the hardening degree depends upon both the ED-20 amount and the DMA catalyst amount. Therefore, at the epoxy resin content of 25 wt%, the hardening degree is 99.4% at a DMA content of 1 wt%. At ED-20 content of 50 wt%, the hardening degree is 99.9%. However, the most rational ratio is PFR:ED-20 = 3:1, at which the high hardening degree is observed at DMA content of 1 wt%, taking into account necessary thermal stability and coating strength.
It is expected that increase in hardening temperature increases gel-fraction yield of the compositions independently of their component structure; the rational hardening temperature is 150-160 °С.
The hardening degree also increases with the increase of PVP amount in the composition to 1 wt%. The gel-fraction yield decreases with the further increase of PVP amount because the dissolved part of PVP, which does not react with PFR, has washed out the resite. Thus, PVP plays The described factors essentially affect the physicomechanical and adhesive properties of the composite. The effect of PVP content on adhesion has an extreme character. The adhesion has a maximum value at PVP content up to 1.0 wt%, then the decrease of glued connection strength is observed ( Table 3). The values of adhesion strength are in good agreement with concentrations of internal stress in glue lines.

CONCLUSIONS
Studies confirm the possibility of chemical modification of novolac phenol-formaldehyde resins by polyvinylpyrrolidone and epoxy resin at temperatures higher than 120 °С. Using differential-thermal and IR-spectroscopical analyses and  The combined chemical network is formed, determining the high thermal stability of the coating. The modification of novolac PFR by polyvinylpyrrolidone and epoxy resin creates conditions for the composition hardening without urotropine.
Owing to this fact, we obtained thermostable nontoxic paintwork materials with high technological effectiveness and adhesion.