Glass Reinforced Plastic is chosen because it provides a good strength to weight ratio over other materials. Strength is determined by compression load, tensile load and stiffness. GRP gains its compression load from the resin used and the stiffness and tensile load from the fibres.
The resin used in GRP is a thermoset, which basically means setting under heat. Once the liquid resin is set it is not possible to reheat it and use it again. To initiate the setting of the resin three basic chemicals are required, the resin, a catalyst and an accelerator. The catalyst is the initiator and the accelerator, as the name suggests accelerates the reaction. Usually the accelerator is premixed into the resin as the direct mixing of accelerator and catalyst causes an explosive reaction! The main type of resin used in the GRP industry is polyester resin. This resin has good all-round properties for most moulding applications. Other resins such as phenolic, which are used for their non-flammable properties. Vinyl esters, which are used for their high laminate strength and resistance to water impregnation and epoxy resin which is used for its high strength.
Glass fibres are the main reinforcements used in the GRP industry, though they come in many forms. Chopped Strand Mat is the most commonly used glass reinforcement. It is constructed of strands of glass about 45mm long held together in bundles of about 200 fibres called strands. The strands are randomly distributed and held together with a binder to form a mat which is supplied in rolls. Another form of glass reinforcement used are Woven Rovings, these are long strands of glass woven into a cloth. Depending on which direction the required tensile strength is needed the cloth can be woven at many different angles. It can also be combined with the Chopped Strand Mat to produce a product called Combination Mat.
Usually the outer finish of a GRP component is the Gelcoat. Gelcoat is a modified resin, usually coloured. The gelcoat gives the GRP good abrasion and weathering resistance. It can be applied by spray or brush to the mould surface. It can be colour matched to a customer's specific requirement, more common are the BS and RAL ranges of colours. Metallic and mirror flake are effects that can be achieved with the gelcoat. It is also used as a waterproof coating to the inside of a GRP component.