Characterisation of Epoxy Powders for Processing Thick-Section Composite Structures

Epoxy powders were investigated as a processing route for fast, low-cost manufacturing of thick-section fibre reinforced polymer parts. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and parallel-plate rheometry were used to characterise the material for realistic processing conditions. The epoxy powders contained heat-activated curing agents and exhibited good thermal stability at and above typical processing temperatures (160-180°C). The exothermic heat produced during curing was found to be small when compared to some conventional epoxies. Similarly, it was shown that epoxy powders can be melted between 45 and 120°C to achieve low viscosities for fibre tow impregnation, without inducing significant cure. Semi-empirical cure kinetics and chemorheological models were presented, which can be used to predict the epoxy’s behaviour during part consolidation and curing. Modifications were made to an existing cure kinetics model to better represent the behaviour of the epoxy at lower temperatures. The relationship between glass transition temperature and the degree-of-cure was described using the DiBenedetto equation and was implemented in an existing chemorheological model. The chemorheological model was applied to a standard process cycle to assess the accuracy of the model and the effectiveness of the process cycle.