Shrinkage reduction of thermoset fibre composites during hardening using microwaves irradiation for curing

Composites made from vinylester resins by Fibre Composite Design and Development (FCDD) group, University of Southern Queensland (USQ) suffer considerable shrinkage during hardening. This shrinkage is particularly serious if the fibre composite components are large. It can be more than ten percent, which is much higher than claimed by some researchers and resins’ manufacturers (Clarke, 1996; Matthews and Rawlings, 1994). The main drawbacks of this shrinkage in a composite component are the stresses set up internally. These stresses are usually tensile in the core of the component and compressive on the surface (Ossward and Menges, 1995). When these stresses act together with the applied loads during service they may cause premature failure of the composite components. Currently, FCDD group solves the shrinkage problem by breaking a large composite component into smaller composite parts because smaller parts tend to have less shrinkage. These smaller parts are then joined together to form the overall structure. This is shown in Figure 1. The individual item of the component shown in Figure 1 is manufactured by casting liquid form, uncured 44 percent by volume or 33% by weight flyash particulate reinforced vinlyesters [VE/FLYASH (33%)] into moulds. By doing this, the manufacturing lead-time and costs of a composite component is significantly increased. Now, industrial microwave technology for processing polymers and polymer based composites is increasingly being applied because of its deep penetration into the material without overheating the surface (Metaxas and Meredith, 1983; Siores, 1994; Ku et al., 1997). This project attempts to reduce the shrinkage of the components by shortening the curing time of the resins using penetrating microwave energy. In addition, some of the shrinkage will be compensated for by low profile additives, which are dissolved in a solvent and fused into the composites by microwave irradiation. The amount of initiator used to initiate polymerisation, the input power of microwave energy and the amount of the low profile additives are varied to obtain an optimum combination of these parameters for minimum shrinkage of the composite components. If the project is successful, a large composite component can be made in one part with minimum shrinkage and hence minimum internal stresses. Quality of the composite component is thus improved. On top of it, building a larger microwave cavity for larger components has been considered. Its design and detailed calculations are clearly described in a book (Fuller, 1990).