Moisture absorption in low level porosity, thermoplastic toughened epoxy composites

• Main Author: Fellows, Linda Mary Patricia
• Format: Dissertation
• Language: English
• Published: University of Southampton 1999
• Subjects: Plastics

In this report, a new polyethersulphone toughened epoxy resin system, developed by ICI Materials Research Centre, is studied. The moisture absorption characteristics are determined over a range of test temperatures for nine varieties of the blend - each containing a different amount of thermoplastic. Results reveal that the addition of thermoplastic causes a decrease in saturation level, an increase in diffusivity and a decrease in activation energy. Equations are developed to fully describe the relationship between the absorption characteristics and thermoplastic content - enabling values to be extrapolated for any thermoplastic content blend at any temperature below glass transition temperature. The changes in absorption characteristics are attributed to a decrease in polarity on copolymerisation of the constituents, in addition to an initial increase in free volume. The absorption is shown to be initially Fickian in all blends, with non-Fickian anomalies occurring at the latter stages of testing in both neat resin and composite samples. The existing equation for edge diffusion in three-dimensional samples was found to be inaccurate at low aspect ratios. A new equation is developed for use with all sample geometries, which gives results accurate to ± 4%. Modification of the diffusion equations for uni-directional composites has been made (including a new for anisotropic edge effects). When fitted to our data the model shows no enhanced diffusion at the fibre matrix interface, but supports the theory that differences in diffusion parallel and perpendicular fibre can be attributed to geometrical blocking effects alone. Diffusion coefficients and saturation levels are shown to be sensitive to void contents even below 1% volume fraction. This correlation between absorption properties and sample quality is shown even more closely when using ultrasonic attenuation rather than void content analysis.