Synthesis of [2-(3,4-epoxycyclohexyl) ethyl] triphenylsilane and study on its amine curing properties

Different reactant ratios result in varying ECETPS yields. As shown in Table 1, the yield increases with the TPS: ECV molar ratio. The optimal yield is achieved when the TPS:ECV molar ratio is 1:1.2. Beyond a molar ratio of 1:1.2, the increase in yield is not significant. The excess of 1,2-epoxy-4-vinylcyclohexane is due to the difficulty of distilling out the Si-H bonds from the triphenylsilane during the hydrosilylation process. Therefore, a slight excess of 1,2-epoxy-4-vinylcyclohexane allows triphenylsilane to participate in the reaction as much as possible.

The yield of ECETPS is the ratio of the actual value to the theoretical value, where the actual value is the mass of the product at the end of the experiment, and the theoretical value is the value calculated according to the chemical equation.1 mol of triphenylsilane with 1 mol of 1,2-epoxy-4-vinylcyclohexane to produce 1 mol of ECETPS.

The yield of ECEDS can be calculated using the following formula (Eq. 1): 1 γ=mM×min(n1,n2) \[\gamma =\frac{\text{m}}{\text{M}\times \min \left( {{\text{n}}_{1}},{{\text{n}}_{2}} \right)}\] formulas:

γ—Yield

This experiment explores the influence of two catalysts, the Karstedt catalyst and the Wilkinson catalyst, on the synthesis of ECETPS. From the graphs, it can be observed that as the catalyst concentration increases, the yield of ECETPS rises.

It’s evident from Figure 2 that using the Wilkinson catalyst yields better results compared to the Karstedt catalyst. With the Wilkinson catalyst at 0.1%, a yield of 59.43% is achieved, whereas the Karstedt catalyst at the same concentration only reaches 45.23%. As the catalyst dosage increases, the yield with the Wilkinson catalyst reaches 95.31% at 0.5%, while the Karstedt catalyst only achieves 73.49% at the same concentration. The yield with the Wilkinson catalyst is higher than that with the Karstedt catalyst. This is attributed to the higher catalytic activity of the Wilkinson catalyst in the hydrosilylation reaction, facilitating the progress of the chemical reaction. The optimal catalytic reaction dosage is reached when the Wilkinson catalyst dosage is 0.4%.

Fig. 2.

In the literature, the Wilkinson catalyst [24, 25] in organosilicon hydrogenation reactions is considered a valuable alternative to Karstedt catalysts. Compared to Karstedt catalysts, the advantages of rhodium catalysts like Wilkinson’s are that their cross-linking is more uniform and their reactivity is better [26]. Additionally, Wilkinson catalysts are relatively inexpensive, which can help to appropriately reduce production costs.

Different reaction temperatures and durations have a significant impact on the synthesis of ECETPS. It’s evident from Figure 3 that under constant temperature, the yield increases over time. After 6 hours, the yield stabilizes. Before 3 hours, the yield increases with temperature. However, after 3 hours, the yield peaks at 90° C, reaching up to 95.21%. As the temperature exceeds 90°C, the yield decreases, possibly due to the exothermic nature of the hydrosilylation reaction. At 95°C, the reaction becomes intense, leading to polymerization of unsaturated double bonds in the reactants and a decrease in the yield. Therefore, the optimal synthesis conditions for ECETPS are a reaction temperature of 90° C and a reaction duration of 6 hours.

Fig. 3.

After placing EVC, TPS, and ECETPS into a Fourier Transform Infrared Spectrometer (FTIR) and conducting a test within the wavenumber range of 400–4000 cm⁻¹, we obtained the Fourier Infrared Absorption Spectrum depicted in Figure 4.

Fig. 4.

Upon examining the FT-IR (Fourier Transform Infrared Spectroscopy) spectra of the reactants EVC, TPS, and the synthesized compound ECETPS, we can observe several distinctive characteristic peaks. For EVC, the peak at 1642 cm⁻¹ represents the stretching vibration of the characteristic C=C backbone [27], which is based on literature data [28]. At 2935 cm⁻¹ and 908 cm⁻¹, these peaks correspond to the symmetric stretching vibration of C=C methylene and the out-of-plane bending vibration of C=C, respectively. The peak at 994 cm⁻¹ arises from the asymmetric C-O-C stretching in the epoxy group. The peak at 1434 cm⁻¹ is attributed to the deformation vibration of saturated C-H bonds. Observing the FT-IR spectrum of TPS, the characteristic peak at 2120 cm⁻¹ clearly originates from the stretching vibration of Si-H bonds. The range from 1425 cm⁻¹ to 1113 cm⁻¹, and the peak at 1695 cm⁻¹ reflects the characteristic peaks of the benzene ring. Meanwhile, the absorption peak at 800 cm⁻¹ represents the stretching vibration of Si-C (on the benzene ring).

When comparing the FT-IR spectra of TPS and EVC with the spectrum of ECETPS, several notable changes can be observed. In the spectrum of ECETPS, the Si-H bond stretching vibration peak at 2120 cm⁻¹ and the C=C bond stretching vibration peak at 908 cm⁻¹ have disappeared. This is due to the addition reaction between the carbon-carbon double bond in EVC and the siliconhydrogen bond in TPS, resulting in the formation of a new characteristic Si-C bond stretching vibration peak at 809 cm⁻¹. Furthermore, the absorption peaks observed at 2924 cm⁻¹, 1432 cm⁻¹, and 1108 cm⁻¹ in the FT-IR spectrum of ECETPS further confirm the presence of aromatic rings and epoxy groups in the synthesized compound.

Using CDCl3 as the solvent and ECV, TPS, and ECETPS as internal references, the synthesized 1H-NMR spectrum was recorded on a 400MR DD2 Nu-81 high-resolution nuclear magnetic resonance spectrometer operating at 600 MHz, from Agilent Technologies (USA).

From Figure 5, it can be observed that the carbon-carbon double bonds in 1,2-epoxy-4-vinylcyclohexane have completely disappeared, and the silicon-hydrogen bonds in triphenylsilane have reduced to almost none. This is consistent with the previous Fourier-transform infrared spectroscopy (FT-IR) data, indicating the successful synthesis.

Fig. 5.

The 1H-NMR hydrogen spectrum of the whole synthesized compound has six major peaks, as can be seen in Figure 6, and the area ratio of the fronts is placed in Table 2, from which it can be learned that the peaks have an area ratio of 2:6:1:2:2:6:9. This is consistent with the number of species of hydrogen atoms on the molecular structural formula of ECETPS and their number ratios. This indirectly verifies the successful synthesis of ECETPS by TPS and EVC.

Fig. 6.

As shown in Figure 9, the tensile strength increases as the curing temperature rises before the curing temperature is 35°C. When the curing temperature is above 35 °C and the curing time is greater than 8 hours, the tensile strength will decrease.

Fig. 9.

When the temperature is constant, the tensile strength will increase with the curing time, and when the curing time is less than 4 hours, the cured material is gelatinous and cannot be tested. The curing time is between 4 and 10 hours, and the tensile strength increases with the curing time. However, beyond 10 hours, the tensile strength did not further improve, indicating that the resin was fully cured. Further extending the curing time has no noticeable effect. Therefore, it can be concluded that the optimal curing temperature is 35°C, and the curing time is 10 hours.

Tensile and bending samples were prepared with [2-(3,4-epoxycyclohexyl)ethyl] triphenylsilane, bisphenol A epoxy resin E-51 and 593 amine, in accordance with the standards of GB/T 1040-92 “Test Methods for Plastic Tensile Properties” and the standards of GB/T 9341-2008 “Determination of Plastic Bending Properties”.

The mechanical properties were tested according to the different ratios in Table 3, provided that the cured samples were cured at a temperature of 35°C for 10 hours, and the results of the experiments are shown in Tables 4 and 5, as well as in Figure 10.

Fig. 10.

It can be observed from Tables 4 and 5 that the mechanical properties of the resin system can be changed by changing the addition content of ECETPS. With the increase in ECETPS content, the tensile strength and flexural strength of the spline increased first and then decreased, and both reached the maximum value when the ECETPS was 10%(ETE-1). The reason is that ECETPS contains Si-C bonds and phenyl groups that can be strengthened, and the introduction of Si-C bonds and phenyl groups in the resin system can strengthen the interaction of epoxy resin segments, promote the cross-linking of molecular chains, and improve the tensile strength.

As can be seen from the left panel of Figure 10, the best flexural strength is achieved when the ECETPS content is added at 10%(ETE-1), indicating that the stronger the bending resistance of the sample, the higher its structural stiffness. As can be seen from the right panel of Figure 10, the addition of ECETPS can increase the strength of the sample, and when the ECETPS content reaches more than 20%, the sample begins to produce yield strength. At the same time, a significant increase in elongation at break can be observed. The results indicated that the addition of ECETPS also helped to improve the toughness of bisphenol A epoxy resin curing.

Thermogravimetric analysis (TGA) has been performed with TGA 4000 Thermogravimetric Analyzer (American PE Company), in a range of 50 to 6600° C at a heating rate of 10°C/min in a stream of nitrogen (60 mL/min). The thermogravimetric curves of the amine-cured samples tested are shown in Figure 11. The left graph shows the amine-cured TG and DTG curves without the addition of ECETPS, and the right graph shows the amine-cured TG and DTG curves with the addition of ECETPS.

Fig. 11.

When the sample weight loss reaches 5wt%, the temperature corresponding to epoxy resin samples with added ECETPS (197.58°C) is higher than that of epoxy resin without ECETPS (181.20°C). This is mainly due to the increased cross-linking density in the system caused by the addition of ECETPS, resulting in improved thermal stability. At 50wt% weight loss, the corresponding temperatures for samples with added ECETPS are 374°C, while the temperature for amine-cured samples without ECETPS is 361°C. Additionally, it can be observed that between 250°C and 450°C, the mass loss of the uncured sample without ECETPS reaches 71.96%, while the mass loss of the sample with added ECETPS is 60.20%.

This rapid mass loss is attributed to the oxidative decomposition of the main chain carbons in the cured resin structure. It is evident that the mass loss of cured samples with added ECETPS is relatively lower, indicating improved thermal resistance of the cured samples with ECETPS at 450°C.