Moisture absorption and hygrothermal aging in a bismaleimide resin and its carbon fiber composites.

Abstract

Moisture absorption and hygrothermal aging in a commercial bismaleimide (BMI) resin (Cytec 5250-4 RTM) and its carbon fiber composites were systematically studied in this work. The neat resin displays a distinct two-stage absorption behavior, with an initial fast diffusion followed by a gradual increase in weight gain over an extended timescale. The true equilibrium uptake was not reached after two years. The second-stage diffusion is associated with moisture enhanced structural relaxation in the resin. Since water is a good plasticizer, it significantly decreases the glass transition temperature and enhances segmental mobility. Hence, the resin network can slowly relax in response to the swelling stresses created by absorbed moisture, resulting in additional uptake. As the resin is deep in the glassy state, the relaxation process is much slower than diffusion. Therefore, the first stage and second stage are diffusion and relaxation controlled, respectively. A simple mathematical model has been developed to describe this absorption behavior. Though a phenomenological model, this two-stage model can be quantitatively correlated with relaxation theories in the glassy stage. The general application of this model has also been demonstrated. Cyclic absorption experiments indicate that the structural relaxation is irreversible upon desorption. The material response under cyclic absorption conditions is solely determined by the absorption cycles, not the desorption cycles. This phenomenon is consistent with the plasticizing effect of water. Two types of reinforcements are used in the composites, namely, uni-weave and woven fabrics. The diffusivity and water uptake level of the composites are compared with those of the flash in an attempt to deduce the interface effect. In the case of the uni-weave composite, which is essentially a uni-directional fiber composite, the composite diffusivity and uptake level can be predicted from those of the neat resin and the fiber volume fraction. It seems that in a properly manufactured composite, the presence of fiber has little effect on moisture diffusion in the composite. On the other hand, anomalous diffusion behavior was observed in the woven composite. A dual-diffusivity model that can successfully describe the observed uptake curves has been developed. The fast diffusivity in the dual-diffusivity model is related to voids and defects in the composite. The bi-directional fiber arrangement in the woven composite may induce large residual stresses and the formation of voids. Therefore, the fiber architecture has important effect on moisture diffusion in a composite. Although moisture absorption at relatively low temperatures is dominated by structural relaxation of the resin, chemical degradation occurs at a considerable rate at elevated temperatures. At 90°C, hydrolytic ring opening of the imide groups takes place, which eventually results in chain scission and depolymerization.