Unsaturated Polyester Resins (UPR) are thermoset polymers dissolved in a reactive diluent, used in combination with glass fibers or carbon fibers to make reinforced composite components. UP Resins are made up of at least two separate components; reinforced fiber and embedding matrix.  Other additives may be used to improve properties or characteristics. Carbon, aramid or most commonly glass fibers may be used yielding FRP (Fiber Reinforced Plastic) or specifically GRP (Glass Reinforced Plastics) in the case of glass.