Polymerization of epoxy resins can be carried out using a wide range of curing agents, such as acid anhydrides, amines, polyamides, polysulfides, phenol formaldehydes, and imidazoles [1, 2]. Imidazoles have attracted attention because of their high reactivity in the chain (ionic) polymerization of epoxy resins. Polymers synthesized by the mechanism of anionic polymerization with imidazoles are characterized by increased thermal stability and resistance to the action of acids and alkalis, which is determined by the formation of simple thermostable ether bonds –C–O–C [3, 4]. With a sufficiently large number of reports on the kinetics of polymerization of epoxy compounds under the action of imidazoles of various structures, there is not enough information on the effect of the initiator amount and polymerization regimes on the final properties of polymers [5, 6]. For example, in [7], when studying the anionic polymerization of bisphenol A diglycidyl ether under the action of imidazoles in an amount of 2 wt %, polymers with a glass transition temperature of 160–162°С were obtained. The authors of [4] present polymers synthesized based bisphenol F diglycidyl ether (YDF 170) under the action of the same imidazoles in an amount of 10 wt % with lower glass transition temperatures, from 98 to 134°C. The used ethers are similar in structure and differ only in the bridging group in the bisphenol. The significant difference in the values ​​of the glass transition temperature is apparently associated with the concentration of the initiator in the reaction system.

This work is aimed at determining the amount of an imidazole initiator in the reaction of anionic polymerization of epoxy resin to synthesize polymers and composites with a high glass transition temperature (heat resistance).

EXPERIMENTAL

We used epoxy resins of different functionality: bifunctional epoxy resin ED-22—diphenylolpropane diglycidyl ether (mass fraction of epoxy groups 23%, dynamic viscosity at 25°C 9.8 Pa s, JSC Himex Limited):

trifunctional epoxy resin UP-643— polyglycidyl ether of novolac epoxy resin (epoxy group content 23.4%, dynamic viscosity at 50°C 55 Pa s, Himex Limited JSC):

four-functional epoxy resin EHD [State Specification (TU 2225-607-11131395–03)], content of epoxy groups 28.3%, dynamic viscosity at 50°C 15 Pa∙s, JSC Himex Limited):

Various imidazoles were used as anionic polymerization initiators: 1H-imidazole 99% (Alfa Aesar, cat. no. A10221), 1-(n-butyl)imidazole 99% (Alfa Aesar, cat. no. L07793), 1-methylimidazole 99% (Alfa Aesar, cat. no. A12575), 1-vinylimidazole 99% (Alfa Aesar, cat. no. L16174), 2-ethyl-4-methylimidazole 96% (Alfa Aesar, cat. no. A15798), N,N'-carbonyldiimidazole 97 % (Alfa Aesar, cat. no. A14688):

Anionic polymerization of epoxy resin ED-22 under the action of imidazoles was studied by differential scanning calorimetry on a DSC 822e calorimeter (METTLER TOLEDO) in dynamic mode at a heating rate of 5 deg min–1 in the temperature range of 25–300°C.

IR absorption spectra were recorded on a VERTEX-80v Fourier spectrometer (Bruker) in the range 4000–400 cm–1 with a spectral resolution of 2 cm–1. The spectra were processed using the standard procedure of the OPUS 6.5 software. The polymerization reaction of the epoxy resin in the presence of imidazoles was controlled by the peak in the region of deformation vibrations of 916 cm–1, which is characteristic of the epoxy group.

The rheological properties of the epoxy–imidazole compositions were evaluated by the change in viscosity on a rotational viscometer Rheotest 2.1 (Rheotest Manufactory GmbH) with a cone–plate assembly at a constant shear rate of 180 s–1 at a temperature of 60 ± 0.5°C.

Samples for physico-mechanical testing were prepared by the following procedures. The reaction mixtures were obtained by mixing the resin with initiators at a temperature of 40°C under vacuum for 10–15 min. The compositions were poured into slot-type metal molds and heated in a heating cabinet according to specified modes. After polymerization of the compositions, the molds were kept at room temperature for 24 h. The tensile physical and mechanical characteristics of the cured polymers (strength and relative critical strain) were registered on an Instron 3565 tensile testing machine (Instron Corporation) at a tensile rate of 0.28 s–1 at different temperatures according to the method (State Standard GOST ISO 37-2013. Interstate standard. Rubber or thermoplastic. Determination of elastic and strength properties in tension). The parameters of the composite material under compression were determined on samples in the form of bars with a electromechanical universal machine FS100CT (Testometric Company Ltd.) with a climatic chamber T48150E at a test rate of 0.016 s–1 at different temperatures according to the procedure [GOST 25.602–80. Calculations and strength tests. Methods of mechanical testing of composite materials with a polymer matrix (composites). Compression test method at normal, elevated and low temperatures].

The study of the thermomechanical properties of polymers was carried out on a NETZSCH DMA 242C unit, which makes it possible to found the dynamic modulus of elasticity E and the loss tan E'/E, which is equal to the ratio of the loss modulus E' to the elastic modulus E. These indicators characterize the viscoelastic properties and heat resistance, which was evaluated by the maximum tan E'/E at a load application frequency of 0.5 Hz (glass transition temperature value). The experiment was carried out in the temperature range of 30–280°C at a heating rate of 2 deg min–1. Loading was carried out according to the three-point bending test, the deflection of a given value was applied to the average section of the sample. The sample bending amplitude was 50–60 μm.

RESULTS AND DISCUSSION

In studying the anionic polymerization of epoxy resins under the action of imidazoles, along with those previously employed, we chose imidazoles substituted in position 1—1-(n-butyl)imidazole, 1-vinylimidazole, and N,N '-carbonyldiimidazole. The maximum change in the intensity of the bending vibration peak of the epoxy group at 916 cm–1 in the IR spectra of the reaction mixture of the ED-22 resin with the initiator 2-ethyl-4-methylimidazole in an amount of 3 wt % at temperatures of 130–190°C occurs during the first 30 min, further changes in intensity are insignificant. A some amount of epoxy groups does not react (Table 1). At 100°С, the consumption rate of epoxy groups is noticeably lower; the conversion is 87.9%. With a larger amount of initiator (10 wt %) the reaction proceeds somewhat faster, but also does not lead to the complete consumption of epoxy groups (Fig. 1). The conversion of epoxy groups is 96.1%.

Table 1. Intensity of the bending vibration peak of the epoxy group at 916 cm–1 in the reaction mixture ED-22–2-ethyl-4-methylimidazole (initiator content 3 wt %) at different temperatures
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Fig. 1.
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Fragments of IR absorption spectra in the range of vibrations of the epoxy group of the reaction mixture of ED 22–2-ethyl-4-methylimidazole (initiator content 10 wt %). Spectrum: initial (1) and after keeping at 130°С 30 (2), 90 (3), 150 min (4).

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On the differential scanning calorimetry thermograms of anionic polymerization of ED-22 resin (initiator content 3 wt %), two peaks were recorded in the case of imidazoles unsubstituted in the 1-position - 1H-imidazole, 2-ethyl-4-methylimidazole (Fig. 2a). The first peak at a lower temperature characterizes the formation of adducts with epoxy resin, the second peak characterizes anionic polymerization under the action of these adducts, which corresponds to the mechanism of anionic polymerization with imidazoles [6, 8]. In the presence of substituted imidazoles [1-(n-butyl)-imidazole, 1-methylimidazole, 1-vinylimidazole], the thermograms show one peak corresponding to anionic polymerization (Fig. 2b). Some features of resin polymerization with substituted N,N'-carbonyldiimidazole should be noted. On the thermogram, by analogy with unsubstituted imidazole, there are two peaks. This is due to the fact that N,N'-carbonyldiimidazole is an unstable chemical compound and is easily hydrolyzed with the release of two imidazole molecules according to the following scheme [9]:

Fig. 2.
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Differential scanning calorimetry thermograms of ED-22 resin anionic polymerization under the action of imidazoles (a) not substituted in position 1: 1Н-imidazole (1), 2-ethyl-4-methylimidazole (3), and N,Nʹ-carbonyldiimidazole (2) and (b) substituted: 1-methylimidazole (4), 1-(n-butyl)imidazole (5), 1-vinylimidazole (6). The content of the initiator 3 wt %.

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Evaluating the reactivity of the mentioned imidazoles by the peak on the thermograms (Table 2) they can be arranged in the following order: 1Н-imidazole, N,N '-carbonyldiimidazole, 1-methylimidazole, 2-ethyl-4-methylimidazole, 1-(n-butyl)-imidazole, 1-vinylimidazole. The latter is the least reactive. 1-Vinylimidazole contains a double bond, and when it is used as an initiator, in addition to anionic polymerization, radical polymerization of 1-vinylimidazole itself can occur. In this case, the reaction can proceed according to the mechanism of interpenetrating polymer networks [10].

Table 2. Parameters of the polymerization reaction of epoxy resin ED-22 under the action of imidazoles in an amount of 3 wt %
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The temperature range of polymerization changes insignificantly with an increase in the amount of initiator by a factor of 10 (from 1 to 10 wt %) (Fig. 3). In both cases (2-ethyl-4-methylimidazole and N,N '-carbonyldiimidazole), the maximum heat release is shifted by 15°, the reaction start temperature remains practically unchanged.

Fig. 3.
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Differential scanning calorimetry thermograms of ED-22 resin anionic polymerization under the action of (a) N,N '-carbonyldiimidazole and (b) 2-ethyl-4-methylimidazole. Content of initiator (wt %): (1) 10, (2) 5, (3) 3, (4) 1.

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Thus, the anionic polymerization of epoxy resin ED-22 under the action of imidazoles can be carried out at low concentrations of the latter in the reaction system (1–3 wt %), and the temperature-time modes can be chosen according to the maximum degree of completion of the reaction.

Evaluation of the change in the viscosity of epoxy–imidazole compositions over time made it possible to evaluate the possibility of their use in the processing and production of polymers. The series of imidazoles corresponding to the rheological curves practically coincides with the series of their reactivity determined by the method of differential scanning calorimetry. The time to achieve a viscosity of ~50 Pa s determines the possibility of using the composition in the technological process of production of certain products and for the presented imidazoles is from 75 to 120 min at 60°C (Fig. 4). The viscosity of the composition with the less reactive initiator 1-vinylimidazole remains virtually unchanged for 5 h at 60°C. Being active at higher temperatures, 1-vinylimidazole can be a latent initiator of anionic polymerization of epoxy resins [11].

Fig. 4.
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The increase in dynamic viscosity at 60°C of reaction systems based on ED-22 and initiators—imidazoles in an amount of 3 wt %. (1) 1-Methylimidazole, (2) 1Н-imidazole, (3) N,Nʹ-carbonyldiimidazole, (4) 1-(n-butyl)imidazole, (5) 2-ethyl-4-methylimidazole, (6) 1- vinylimidazole.

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The final choice of the initiator concentration in the curing system was made based on the results of determining the glass transition temperature and the physicomechanical parameters of the polymers synthesized under the action of two imidazoles, 2-ethyl-4-methylimidazole and N,N '-carbonyldiimidazole (Table 3). Polymers based on reaction mixtures with a large amount of initiator (15–5 wt %) are characterized by high strength values ​​at room temperature, but at the same time low glass transition temperatures, and the strength at 150°С is close to zero. A decrease in the amount of initiator to 2–3 wt % during the polymerization of ED-22 resin leads to the formation of polymers with a significantly higher glass transition temperature and, accordingly, heat resistance. At the same time, their strength decreases by a factor of 1.5–2, but practically does not change during tests at 150°C, which indicates their thermal stability. The polymers formed with 1 wt % initiator had increased brittleness and were not tested. Thus, more heat-resistant polymers based on epoxy resins and imidazoles can be prepared at a minimum initiator concentration of 2–3 wt %.

Table 3. Physical and mechanical properties and glass transition temperature of anionic polymerization polymers based on reaction systems with different initiator content
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(–) Polymers have not been tested.

This conclusion is confirmed by the results of testing polymers by the method of dynamic mechanical analysis, which makes it possible to evaluate the behavior of materials under load in a wide temperature range (Fig. 5). The data show the advantage of the polymerization mode with a gradual rise in the curing temperature (100°C, 1 h + 130°C, 1 h + 150°C, 2 h) over the polymerization mode at a single temperature (150°C, 3 h) to obtain materials with higher modulus of elasticity and glass transition temperature, which is 171 and 159°C, respectively.

Fig. 5.
figure 5

Dynamic modulus of elasticity E and mechanical loss tangent tan Eʹ/E vs. temperature for polymers obtained by anionic polymerization of ED-22 resin in the presence of 1H-imidazole in an amount of 3 wt %. (1) Curing at 150°C, 3 h; (2) curing with a gradual rise in temperature (100°С, 1 h + 130°С, 1 h + 150°С, 2 h).

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The polymers were produced by the mechanism of anionic polymerization in the presence of initiators, imidazoles, in an amount of 3 wt % using a mode of gradual temperature rise. The resulting materials have a high bending modulus at 30°С (2670–3250 MPa) and high mechanical glass transition values (170–192°С). As expected, the performance of polymers based on three- and four-functional resins is higher than polymers based on two-functional resin (Table 4). The table shows the values ​​of the modulus of elasticity, the glass transition temperature and the temperature T1000, at which the material retains the values ​​of the modulus of elasticity equal to 1000 MPa and can be used as a structural material in various applications.

Table 4. Thermomechanical properties of anionic polymerization polymers
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The result of the research is the production and testing of a composite material based on a glass fiber filler and an epoxy–imidazole binder with an initiator 1-methylimidazole in an amount of 3 wt % (Table 5). It was obtained by pressing the MKT-4.2 fabric impregnated with a binder, followed by polymerization in a stepwise mode. In terms of the breaking compressive stress the composite produced based on the epoxy-imidazole binder is not inferior to the composite based on the well-known binder EDT-10, in which the polymerization initiator is triethanolamine titanate.

Table 5. Composite material properties in compression
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CONCLUSIONS

Anionic polymerization polymers based on epoxy resins of different functionality with high heat resistance (glass transition temperature 170–192°C) and stable physical and mechanical parameters in a wide temperature range (25–150°C) are formed at an initiator concentration of 2–3 wt % in the mode of the gradual temperature rise. The initiator 1-vinylimidazole exhibits latent properties during polymerization.

Composite samples based on the experimental epoxy-imidazole binder have parameters comparable to the those of samples based on the standard EDT-10 binder, and surpass it in heat resistance (the drop in compressive strength at 200°C is 69 and 81%, respectively).