The solvent resistance of aromatic polymer composites

The diffusion rate of a large range of solvents into carbon fibre reinforced PEEK (APC-2) has been measured to discover the effect of the physical characteristics of the solvent. Three dimensional graphs have been plotted which correlate four parameters (solvent size, shape, hydrogen bonding capacity and solubility parameter) to solvent uptake. In the composite the effects of: thickness, lay-up, background water content and strain level on solvent diffusion have been assessed. The effect of composite thickness can be predicted using the film thickness scaling law provided the diffusion is Fickian. The effect of background water content is small, tending to make the diffusion profile two-stage. The effect of lay-up has been shown to have a major Affect on diffusion rate, unidirectional lay-ups having a much slower diffusion rate. Several theories have been postulated to explain this behaviour. The effects of stress on diffusion rate can be predicted by free volume models, provided that the stress/strain is kept below a certain critical level. It has been shown that the damage caused by a solvent, provided the stress does not exceed a critical value, is dependent on the amount of solvent in the matrix. This is due to plasticisation effects. Attempts to model this behaviour using free volume models have proved successful. Stress has been shown to enhance environmental attack. With certain solvents, above a critical stress or strain, environmental stress cracking occurs, leading to a considerable reduction in mechanical properties. Photographic evidence shows that cracking is initiated at stress concentrators within the matrix. Crack propagation is entirely matrix related and independent of spherulite boundaries. Overall, APC-2 has been shown to possess excellent environmental resistance when used in aerospace applications.