Below is a list of Fiberglass Reinforced Plastics manufacturers from West Mifflin, Pennsylvania. Fiberglass reinforced plastic is a composite material composed of a thermoplastic resin and a reinforcement layer of glass fibers. By adding glass thread mats to plastic, the material increases in strength, durability and chemical and heat resistance.The plastic resin material is always thermoplastic because in order to manufacture FRP products, heat treatments must take place. FRP products can handle temperatures up to nearly 300º F.