poly(propylene terephthalate) (PTT, Sorona), etc are high-melting semicrystalline materials (m. p. 160–280 °C) that and have found use as engineering thermoplastics, fibers and films. Wholly aromatic linear copolyesters present superior mechanical properties and heat resistance and are used in a number of high-performance applications. Unsaturated polyesters are produced from multifunctional alcohols and unsaturated dibasic acids and are cross-linked thereafter; they are used as matrices in composite materials. Alkyd resins are made from polyfunctional alcohols and fatty acids and are used widely in the coating and composite industries as they can be cross-linked in the presence of oxygen. Also rubber-like polyesters exist, called thermoplastic polyester elastomers (ester TPEs). Unsaturated polyesters (UPR) are thermosetting resins. They are used in the liquid state as casting materials, in sheet molding compounds, as fiberglass laminating resins and in non-metallic auto-body fillers. They are also used as the thermoset polymer matrix in pre-pregs. Fiberglass-reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars. Depending on the chemical structure, polyester can be a thermoplastic or thermoset. There are also polyester resins cured by hardeners; however, the most common polyesters are thermoplastics. The OH group is reacted with an Isocyanate functional compound in a 2 component system producing coatings which may optionally be pigmented. Polyesters as thermoplastics may change shape after the application of heat. While combustible at high temperatures, polyesters tend to shrink away from flames and self-extinguish upon ignition. Polyester fibers have high tenacity and E-modulus as well as low water absorption and minimal shrinkage in comparison with other industrial fibers. Increasing the aromatic parts of polyesters increases their glass transition temperature, melting temperature, thermostability, chemical stability, and solvent resistance. Polyesters can also be telechelic oligomers like the polycaprolactone diol (PCL) and the polyethylene adipate diol (PEA). They are then used as prepolymers. Aliphatic vs. aromatic polymers Thermally stable polymers, which generally have a high proportion of aromatic structures, are also called high-performance plastics. This application-oriented classification compares such polymers with engineering plastics and commodity plastics. The continuous service temperature of high-performance plastics is generally stated as being higher than 150 °C, whereas engineering plastics (such as polyamide or polycarbonate) are often defined as thermoplastics that retain their properties above 100 °C.⁠ Commodity plastics (such as polyethylene or polypropylene) have in this respect even greater limitations, but they are manufactured in great amounts at low cost. Poly(ester imides) contain an