Effects of alternating temperatures and humidity on the moisture absorption and mechanical properties of ramie fiber reinforced phenolic plates

In this article, ramie fiber reinforced phenolic (RFRP) plates were prepared with compression molding process, and the plates were subjected to 98% humidity environment and alternating temperatures (from 25°C to 55°C in 24 h for a cycle) for 4 weeks. The resulted moisture absorption and the variation of the mechanical properties of RFRPs were studied. As found, compared to constant exposure temperatures (25°C or 60°C), alternating temperatures brought in higher moisture uptake and more serious degradation in the flexural strength, flexural modulus and short beam shear strength of the RFRP samples under the same humidity condition. The deteriorated effects of alternating temperatures is attributed to more remarkable degradation of the bonding between the fiber and resin, due to the moisture uptake and the internal cyclic stress around the ramie fibers with alternating temperatures. The flexural modulus of RFRP plates was much more susceptible to the moisture uptake than the flexural strength. After fully drying, the mechanical properties of the RFRP samples were recovered to some extent, but still less than the original values, indicating permanent damages occurred. Fiber Bragg grating sensors embedded in the RFRP plate was applied to monitor the variation of the internal strain during the exposure. As indicated, the moisture absorption and alternating temperatures bring in relaxation of the internal tension stress formed during compressing process, and decrease in the coefficient of thermal expansion of the RFRP samples. POLYM. COMPOS., 36:1590–1596, 2015. © 2014 Society of Plastics Engineers