Effects of moisture on the properties of epoxies and carbon-epoxy composite laminates
The effects of water on uncured epoxies and epoxy composites are studied. Experiments show that water causes an increase in the cure rate of epoxy materials at low degrees of cure and a decrease in cure rate and total cure at high degrees of cure. Molecular modeling is used to provide insight into these observed behaviors. Molecular images show that water fits into the free volume of uncured epoxy, but that when epoxy cures and density increases, the water no longer fits in the free volume and creates nanophase separation which prevents epoxy cure and decreases glass transition temperature. The molecular model predicts an increase in the self-diffusion of epoxy molecules in the presence of water; it is hypothesized that this is the cause of the increased cure rate at low degrees of cure. An autoclave simulation is presented which predicts temperature, cure, and moisture concentration profiles during an autoclave cure cycle. Water bubble sizes and shapes are modeled based on these profiles and compared to observed bubbles. Experimental observations show that there is a threshold bubble size, depending on surrounding local fiber volume fraction, below which the water in the bubble will re-dissolve into the epoxy and create a resin-filled void and above which local fiber volume fraction becomes so high that there is no path for the water to exit the bubble. It is also shown that the observed unfilled void content is very sensitive to the timing of the applied pressure and possibly the rate at which pressure is increased in the autoclave. Fracture toughness and shear strength of samples with different amounts of water are measured and it is found that a small amount of absorbed water (0.5%) causes an increase in fracture toughness due to plasticization, but a larger amount of absorbed water (2%) causes a reduction in fracture toughness of 20%. Shear strength is reduced in both water conditions with 0.5% absorption of water causing a reduction in shear strength of 20% and 2% absorption of water causing a reduction in shear strength of 40%. Based on simulation results and experimental observations, it is proposed that the effects of water can affect composite parts in ways that fit with observed behavior of kissing bonds or zero-volume defects. Water is also shown to have a significant impact on residual stress in composites.
Adams, Purdue University.
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