Micro-crack behavior of carbon fiber reinforced thermoplastic modified epoxy composites for cryogenic applications

Three different types of thermoplastics, poly(ether imide) (PEI), polycarbonate (PC), and poly(butylene terephthalate) (PBT) were used to modify epoxy for cryogenic applications. Carbon fiber reinforced thermoplastic modified epoxy composites were also prepared through vacuum-assisted resin transfer molding (RTM). Dynamic mechanical analysis (DMA) shows that the storage moduli of PEI, PC, and PBT modified epoxies are 30%, 21%, and 17% higher than that of the neat epoxy respectively. The impact strength of the modified epoxies at cryogenic temperature increases with increasing thermoplastic content up to 1.5 wt.% and then decreases for further loading (2.0 wt.%). The coefficient of thermal expansion (CTE) values of the PBT, PEI, and PC modified epoxies also decreased by 17.76%, 25.42%, and 30.15%, respectively, as compared with that of the neat epoxy. Optical microscopy image analysis suggests that the presence of PEI and PC in the carbon fiber reinforced epoxy composites can prevent the formation of micro-cracks. Therefore, both the PEI and PC were very effective in preventing micro-crack formation in the composites during thermal cycles at cryogenic condition due to their low CTE values and high impact strength.