Synthesis And Curing Kinetics Of A Novel Heat Resistant Epoxy Resin Curing Agent Containing Biphenyl And Maleimide Moieties

In this paper, the progress of heat resistant epoxy resins was reviewed. Introduction of some heat resistant functional groups into the epoxy resins and increasing the functionality of the epoxy resins are the two most effective methods to improve the heat resistant of the cured epoxy resins. Based on the literature research, herein, we designed and synthesized a new macromolecular curing agent for epoxy resins containing both maleimide and biphenyl moieties.The novel macromolecular epoxy resin curing agent (named as MIBP) was synthesized by the condensation polymerization of 4,4'-bismethoxymethylbiphenyl and N-(4-hydroxyphenyl)maleimide catalyzed in the presence of strong Lewis acid. After the product was purified by washing with ethanol for several times, the structure of the MIBP was confirmed by FTIR and the molecular weight of the new curing agent was determined by GPC. The GPC spectra showed that the number-average molecular weight of MIBP was 1100 and the weight-average molecular weight was 1548, with polydispersity index (PDI) value of 1.41. This results indicated that the majority of the polymers possesses 2 and 3 repeat units (n=2 and 3) with relative low polydispersity.DSC measurement and TG analysis were employed to systematically evaluate the thermal properties of the O-cresol-formaldehyde epoxy resin (CNE) cured with MIBP (CNE/MIBP), which was compared with other three systems, they are CNE cured with commercial curing agents (CNE/DDM, CNE/DDS and CNE/PN). The results showed that cured CNE/MIBP polymer possessed the relatively higher thermal stability compared with the other three controls. T10% of the cured CNE/MIBP polymer were 408 and 400°C under N2 and air, respectively, which were 20～30°C higher than the controlled ones. The char residue of the cured polymer at 800°C was 44.5% under nitrogen which was more than 15% higher than the controls. The high glass transition temperature of 155°C was obtained evaluated by the DSC measurement. The comprehensive good thermal properties indicated that the introduction of the stable imide structure and rigid biphenyl groups into the cured epoxy resins drastically improved the thermal properties of the cured polymer, especially, the introduction of maleimide which underwent the self-addition and increased the cross-linking density.The curing behavior of CNE/MIBP was investigated by differential scanning calorimetry (DSC) and the exotherm exhibited two partially overlapping exothermic peaks during curing process. FTIR was used to verify the reactions of each curing process and the results indicated that the first peak at lower curing temperature (around 125°C) in the DSC curve was attributed to the self-addition of maleimide and the second one at higher temperature was responsible for the reaction of epoxy groups/hydroxyl groups of MIBP. These two kinds of reactions occurred during curing process agreed with the two exothermic peaks exhibited in DSC curves.In the end, the kinetics analysis of the DSC curves for CNE/MIBP mixture was carried out by separating of the two overlapping exothermic peaks using Gaussian distribution function from PeakFit v4.12 program and analyzing the distinct characteristic of each exothermic peak. Three methods which are Kissinger, Ozawa and Flynn-Wall-Ozawa method were used to calculate the activation energy of the CNE/MIBP system, and the results showed that the Eas calculated from the three methods were very close, about 90 kJ/mol for reaction one and 79 kJ/mol for the second reaction. Flynn-Wall-Ozawa method was also employed to investigate the relationship between the Ea and the curing degree. At last, Friedman method was used to demonstrate that the two reactions of the curing process of CNE/MIBP system were both n-th mechanism, and the calculated n value of the n-th reaction was 1.42 for reaction one and 1.11 for reaction two. The two independently reactions as well as the whole curing process of the CNE/MIBP system were simulated and the calculated DSC curves fit relatively well with the experimental data using the obtained models.