Evaluation of Glass Fibre/Epoxy Prepreg Quality during Storage

This paper reports the characterisation methods used in evaluating the chemical changes in glass fibre/epoxy prepregs during storage in an environment chamber. The specimens were exposed to an environment with a relative humidity of 50% at 25°C for up to 196 days. Various physicochemical analysis methods were used to determine the extent of the changes to the prepregs during storage. These methods include infrared spectroscopy (IR), thermal analysis (TA) and other chemical methods to measure the volatile and insoluble components content. During storage, the chemical reaction occurs between some epoxide groups, which is known as precuring. Results indicate that precuring in the epoxy resin system consumed a considerable number of epoxide groups during the exposure. The epoxide index obtained from IR analysis gives a straight measure of the number of epoxide groups remaining in the prepregs at different stages of the storage. A parameter known as the “precured degree”, defined from the epoxide index, can be used to measure the prepregs’ quality. A set of laminates made from prepregs that had been degraded to different degrees were subjected to flexural and tensile testing. The flexural and tensile properties were significantly reduced as a consequence of the increase in the precured degree for the prepregs. The results show that it is important to control the precuring reaction in epoxy-based prepregs during storage before they are transferred into composite products.