U N I T 3 NON METALS This Unit is about the most widely used non-metallic materials. It gives information on the general features and main applications of plastics, ceramics and composites. A. POLYMERS Polymers are both natural (e.g. proteins, cellulose, rubber) and synthetic substances: they are carbon-based chemical compounds characterized by very large molecules, formed of long chains of smaller molecules (monomers) linked together. Synthetic polymers are generally derived from oil and are usually referred to as plastic materials. Easily formed into shape, stiff, light and durable, plastics are also poor conductors of heat and electricity, with low density and a low melting point. They have many different applications: from optical components to telecommunications equipment, from food storage to adhesives, from architectural elements to car parts, clothing, ski boots and safety helmets. They are so widely used that some of their difficult names sound even familiar: abbreviations or trademarks such as PET (polyethylene terephthalate), PVC (polyvinyl chloride) or Plexiglas (polymath methacryalate PMMA) are commonly known today. According to their chemical and structural bonds, plastics are usually classified as thermoplastics or thermosettings. The former soften or melt when heated and harden when cooled: in particular, they can undergo reheating processes in order to be reshaped into a new form. The latter, on the contrary, cannot undergo reheating processes: they become permanently hard and rigid when heated. Elastomers are polymers whose main quality is that of returning to their original shape after deformation: most of them are synthetic and play a fundamental role in the production of tyres. 66