Gelcoat, also called Gel Coat, is a material used to provide a high-quality finish on the visible surface of a fibre-reinforced composite. The most common gelcoats are

thermosetting polymer In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...

s based on

epoxy Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...

or unsaturated

polyester resin Polyester resins are synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Maleic anhydride is a commonly used raw material with diacid functionality in unsaturated polyester resins. Unsaturated polyester res ...

chemistry. Gelcoats are modified resins which are applied to moulds in the liquid state. They are cured to form crosslinked

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s and are subsequently backed with thermoset polymer matrix composites which are often mixtures of polyester resin and

fiberglass Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...

, or epoxy resin which is most commonly used with carbon fibre for higher specific strength. The manufactured component, when sufficiently cured and removed from the mould, presents the gelcoated surface. This is usually

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...

ed to provide a coloured, glossy surface which improves the aesthetic appearance of the article, such as the surface of a boat hull. Many marine craft and some aircraft are manufactured using composite materials with an outer layer of gelcoat, typically 0.5 mm to 0.8 mm (0.02 in to 0.03 in) thick. Gelcoats are designed to be durable, providing resistance to

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

degradation and

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

. Specialized gelcoats can be used to manufacture the moulds which in turn are used to manufacture components. These require very high levels of durability to overcome the mechanical and thermal stresses encountered during the curing and demoulding processes. Suitable resin chemistries for the manufacture of gelcoats vary, but the most commonly encountered are unsaturated polyesters or epoxies. Within each of these categories, the resin chemistries are further subdivided. In addition to any pigment, if necessary a gelcoat will contain a

thixotropic Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed (ti ...

additive to assist its tenacity to vertical portions of the mould whilst it cures.

Sources

* * {{cite book, title=GRP technology, url=https://books.google.com/books?id=2IlPAQAAIAAJ, year=1966, last1 = Penn, first1 = W. S. Composite materials