Study of the hygrothermal aging of a phenolic matrix and glass fiber composite

The hygrothermal-oxidative aging of a phenolic/short glass fiber composite was studied in a humid environment (100% relative humidity) for over a year at three temperatures: 50°C, 70°C, and 90°C. Wet aging of this composite is rarely studied in the literature due to the nature of its matrix. Indeed, phenolic resins are usually used to resist thermal stress at very high temperatures. Infrared spectroscopy by ATR mode, dynamic thermomechanical analyses, water sorption measurements and optical microscopy were used in a complementary way to monitor the influence of humidity, oxygen and temperature on the composite structure. From the first aging times, resin plasticizing and post-curing of resin are associated to cracks occurring from the resin/fiber interface, due to the hydration of the fibers which would be treated by silane-type coupling agent. Sub-crosslinking would be related to the presence of intermediate products such as amides, imides, or phenol groups in excess, strongly hydrophilic. Consequently, along with time, hydrolysis is accelerated. Moreover, the oxidation products revealed by FTIR analyses will favor interactions with water. The cracks are then deviated inwards the resin matrix.