A plastic in which the polymer molecules are cross linked (chemically bonded) with another set of molecules to form a "net like" or "ladder-like" structure is a thermoset. Once cross linking has occurred, a thermoset plastic does not soften, melt, or flow when heated. However, if the cross linking occurs within a mold, the shape of the mold will be formed. Typical thermoset plastics are: unsaturated polyester (UP) – used for bowling balls and boats; epoxy– used for adhesives and coatings; and polyurethanes (PURs) – used in foams and coatings.
In addition to these basic characteristics, polymers provide the FRP composite designer with a myriad of characteristics that can be selected, depending on the application. Combined with reinforcement of the polymer matrix, a vast range of characteristics are available for FRP composites.