Influence of nano-clay compounding on thermo-oxidative stability and mechanical properties of a thermoset polymer system

This paper investigates the effect of Cloisite 20A (C20A) nano-clay compounding on the thermo-oxidative degradation and the residual stresses due to thermal oxidation for a thermoset polymer. A variant of bismaleimide (BMI) is considered for this work, which could potentially be used in high temperature airframe applications. To examine the influence of nano-clay compounding on the thermo-oxidative behavior of BMI, isothermal aging experiments were conducted at 250 °C on nano-clay modified BMI with 3 wt% C20A and baseline BMI in different oxygen environments (0%, 20% and 60% O2) for more than 1000 h. Comparing the weight loss data for neat BMI and C20A modified BMI; it was observed that nano-clay enhances the thermal stability of thermoset polymer BMI by reducing oxygen diffusion and mitigating oxidative degradation during isothermal aging. Reaction–diffusion modeling parameters involved in the modeling of thermal-oxidation of BMI were obtained from weight loss data. To study the effect of nano-clay compounding on the mechanical properties, nano-indentation experiments were conducted on neat as well as nano-clay modified BMI. To determine the long term shrinkage strains caused by thermal oxidation at elevated temperatures, shrinkage experiments were conducted at 250 °C. For these experiments, digital image correlation (DIC) technique was used to capture the strain field. Using data from these experiments as input, a three-dimensional (3-D) micro-mechanics based finite element analysis (FEA) was carried out to investigate the residual stresses due to thermal oxidation using an in-house FEA code (NOVA-3D). Based on the FEA analysis, the presence of elevated radial stresses at the fiber–matrix interface near the free edge during the early stages of oxidation makes the fiber end a likely site for debond initiation, both for neat resin and for C20A modified resin. Similarly, elevated hoop stresses, responsible for possible matrix cracking were also observed, and are corroborated by experimental observations. The micromechanical FEA model predicted a significant (∼25%) reduction in the residual radial and hoop stresses for the 3 wt% C20A modified BMI, because of the ameliorating effect of dispersed nano-clay on resin shrinkage.