Humidity and immersion based hygrothermal aging of pultruded GFRP composites: Moisture uptake and diffusion

Pultruded glass fiber reinforced polymer (GFRP) composites are used extensively in civil, marine, and offshore infrastructure applications. These components can be exposed to long periods of humidity or immersion with long-term performance characteristics being influenced by the rate and level of moisture uptake and its consequent effects. This paper reports on the moisture uptake response and kinetics of a pultruded GFRP composite exposed to a range of relative humidity levels and immersion over a range of temperatures below the glass transition. Four models for diffusion are used to assess the range of uptake responses. It is seen that humidity and temperatures have a significant effect on both rate of uptake and maximum apparent uptake level with there being a significant difference between effects of 99% RH and immersion emphasizing that immersion, as is often used in durability testing, cannot be directly used as a substitute for accelerating effects of humidity even at high levels. The two-stage structural modification model which considers both an initial diffusion dominated regime and a second relaxation/deterioration dominated regime is seen to best model the range of uptake regimes and characteristics while the simple Fickian model is shown to be deficient in modeling most of the regimes accurately.