6 - Polyesters

Abstract

This chapter focuses on the polyesters that are formed by a condensation reaction in which a diacid and dialcohol are reacted to form the polyester with the elimination of water. While the actual commercial route for making the polyesters may be more involved, the end result is the same polymeric structure. The diacid is usually aromatic. Polyester resins can be formulated to be brittle and hard, tough and resilient, or soft and flexible. In combination with reinforcements such as glass fibers, they offer outstanding strength, a high strength-to-weight ratio, chemical resistance, and other excellent mechanical properties. The three dominant materials in this plastics family are polycarbonate (PC), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Thermoplastic polyesters are similar in properties to Nylon 6 and Nylon 66 but have lower water absorption and higher dimensional stability than the nylons. PBT is semicrystalline, white or off-white polyester similar in both composition and properties to PET. It has somewhat lower strength and stiffness than PET, is a little softer but has higher impact strength and similar chemical resistance. As it crystallizes more rapidly than PET, it tends to be preferred for industrial scale molding. Theoretically, PC is formed from the reaction of bis-phenol A and carbonic acid. PC performance properties include: very impact resistant and is virtually unbreakable and remains tough at low temperatures; “Clear as glass” clarity and high heat resistance.