Hygrothermal resistance of large tow carbon fiber and its reinforced polymer composites

Large tow carbon fiber reinforced polymer (CFRP) has been used to reinforce structures serving in hygrothermal conditions, which can be greatly affected by hygrothermal conditions. In this study, the hygrothermal resistance of 48K and 12K carbon fibers and their CFRPs immersed in deionized water at 25, 40, and 60°C were analyzed and compared. The sizing agent on the carbon fiber surface was deboned or hydrolyzed after immersion, resulting in a reduction in the tensile strength, an increase in dispersion, and a decrease in surface activity for carbon fibers. The retention rate of tensile strength of 48K CFRP was higher than 89.7% but was less than that of 12K CFRP due to the higher dispersion of 48K fiber filaments. The interlaminar shear strength (ILSS) and Mode I interlaminar fracture toughness of 48K CFRP was greater than 12K CFRP due to the higher fiber surface activity of 48K carbon fibers and the stronger obstructing effect of 48K fiber tows on the diffusion of water. The degrees of post-curing and hydrolysis of CFRP were less than 10%. The degradation of the fiber-matrix interface and a decrease in the strength and elastic modulus of the matrix led to the degradation of the tensile properties, ILSS, and Mode I interlaminar fracture toughness of CFRP. Highlights: Debonding of sizing agent reduced the strength of the carbon fiber filaments. Degrees of hydrolysis and post-curing of the matrix caused by immersion were low. Retentions of ILSS and fracture toughness of 48K CFRP were higher than 12K CFRP. 48K fiber tow possessed a stronger obstructing effect on water diffusion.