Gel Coats in composites

Gel coats are considered resins but have a very special purpose. A gel coat is a specially formulated polyester resin incorporating thixotropic agents to increase the gel coat’s viscosity and non-sag properties, fillers for flow properties, pigments to give the desired color, and additives for specific application properties, such as gel time and cure. Gel coats are primarily used for contact molding (hand or spray lay-up). The gel coat, usually pigmented, provides a molded-in finished surface that is weather and wear resistant. The gel coat helps in hiding the glass reinforcement pattern that may show through from the inherent resin shrinkage around the glass fibers. Considerations used for the proper selection of a gel coat are compatibility of the underlying FRP materials to ensure good adhesion of the gel coat, as well as the operating environment.

The most common current usage of gel coats in “in-mold applications.” That is, the gel coat is sprayed into the mold and the laminate is applied behind it. Adhesion of the laminating resin to the gel coat is a critical issue. Thickness of the gel coat can vary depending on the intended performance of the composite product. Gel coats are typically applied by spray application to approximately 16-20 mils wet film thickness. While gel coats do not add any structural strength to the FRP part, gel coats should be resilient. Gel coats should be able to bend without cracking. They should be resistant to thermal cracking (cracking that may occur with dramatic changes in temperature). The primary measurements of resilience are flexural modulus and elongation. Gel coats should be UV stable and pigmented sufficiently to provide good opacity.

Gel coats are used to improve weathering, filter out ultraviolet radiation, add flame retardancy, provide a thermal barrier, improve chemical resistance, improve abrasion resistance, and provide a moisture barrier. Gel coats are used to improve the product appearance such as the surface of a boat hull or golf cart. A unique benefit of gel coats is that they are supplied in many colors by the incorporation of pigments per the specification of the engineer.