The primary functions of the resin are to transfer stress between the reinforcing fibers, act as a glue to hold the fibers together, and protect the fibers from mechanical and environmental damage. Resins used in reinforced polymer composites are either thermoplastic or thermoset.

Many materials are capable of reinforcing polymers, but glass fibers account for more than 90 percent because they are inexpensive to produce and have relatively good strength-to weight characteristics. The type, amount and orientation of fibers or other reinforcement materials are selected to provide strength and stiffness in a particular direction.

Core materials are thicker lightweight materials “sandwiched” between fiber reinforced laminate skins. From balsa to foam or honeycomb structures, core materials allow the creation of strong, stiff, light and highly durable structures that otherwise would not be practical.

The most important additives are the catalysts used to initiate, speed and slow the chemical reaction that allows composites to bond and cure. A range of additives also modify materials’ properties and tailor laminate performance. Fillers are also used to modify some properties of the resin. Fillers are commonly called “extenders” because they are a lower cost ingredient that reduce the overall cost of the composite material.

Surface finishes are mainly used for UV protection, corrosion resistance and aesthetics. Gel coat is the most common and is molded in process. Other finishes are secondarily applied as coatings and support the long-term appearance of composite products.