Cure kinetics, morphology and miscibility of modified DGEBA-based epoxy resin - Effects of a liquid rubber inclusion

Kinetics of the curic reaction and morphology of a diglycidyl ether of bisphenol-A based epoxy resin (DGEBA), using an anhydride hardener (nadic methyl anhydride) at different weight contents of carboxyl-terminated copolymer of butadiene and acrylonitrile liquid rubber (CTBN) was investigated using a differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and scanning electron microscopy (SEM). The aim of the work is to understand the effects of inclusion of the liquid rubber phase in the transition phenomena that occur during the curing reaction. The curic reaction at three different curic temperatures and at varying rubber contents in the range of 5-20 wt% has been studied. The reaction rate and conversions that occurred at the curic temperatures were analyzed. The increase in the rate with the curic temperature showed this as a thermally catalyzed reaction. The rate of the reaction was found to decrease in liquid rubber-modified epoxies due to the effect of dilution and viscosity increase as obtained from the gelation times. The experimental data showed an autocatalytic behavior of the reaction, which is explained by the model predicted by Kamal. This model includes two reaction constants k1 and k2 and two reaction orders m and n. The order of the overall reaction was found to be approximately 2. The activation energies Ea1 and Ea2 were estimated at all curic temperatures for neat and all modified epoxies. The results obtained from the DSC data were also applied to diffusion controlled kinetic models. A schematic model to represent the curic reaction and phase separation was introduced and the molecular mechanism of this curing reaction was discussed. During the curic reaction, phase separation of the liquid rubber from the epoxy matrix took place and the modified epoxies showed phase separated morphology. The dispersed phase showed a homogenous particle size distribution. The size of the phase separated domains increased with increasing concentration of the CTBN and decreased with rise in curing temperature. The glass transition temperature (Tg) of the modified epoxies decreased with increase in curic temperature as studied from dynamic mechanical thermal analysis. Addition of the liquid rubber lowered the Tg of the network. This became prominent in the modification of the matrix with 15 and 20 wt% of the elastomer. This is attributed to flexibilization of the matrix. The dissolved rubber plasticizes the epoxy network. The Tg of the neat rubber in the low-temperature region was shifted to higher temperature upon addition of the elastomer. A higher shift was noted for 15 and 20 phr inclusion. This was due to dissolved epoxy in the rubber-rich phase that increased the modulus of the rubbery phase. The inclusion of a large wt% of carboxyl-terminated butadiene-co-acrylonitrile (CTBN) decreased the cross-linking density of the thermoset matrix.