Degradation mechanism of carbon fiber-reinforced thermoplastics exposed to hot steam studied by chemical and structural analyses of nylon 6 matrix

Degradation testing of nylon 6-based carbon fiber-reinforced thermoplastic (CFRTP) was performed by exposure to hot steam in a closed vessel. The degraded structure was investigated to understand the degradation mechanism. Aging-induced micro-cracking in the nylon 6 matrix of CFRTP indicated that internal stress was generated by changes in its local density, along with a decrease in the molecular weight due to hydrolysis. Aging process of nylon 6 increased the free volume hole size in the amorphous part and also induced crystallization, suggesting that the density of the amorphous part decreases with increasing crystallinity in the matrix. Therefore, the degradation mechanism of the CFRTP was proposed to begin with a change in the local density of nylon 6 owing to crystallization, followed by water infiltration and hydrolysis of the matrix. Stress due to density changes combined with reduced molecular weight forms micro-cracks, decreasing the mechanical strength of CFRTP.