Commodity Polymers

A class of polymeric materials which are applied in large amounts for various applications. The structure is specified by the conformation and the configuration. The chain configuration is formed during polymerisation. Thermplastic polymers consist of long, uncrosslinked chain molecules. During processing these chain molecules are easily oriented, which leads to anisotrophy of the mechanical and other properties. Polymers with regular chains (in particular PE, but also PP and nylons) can crystallize, although crystallinity is never complete; these materials are semi-crystalline. Polymers with irrigular, bulky chains are amorphous. They are commonly used below their glass-transition temperature Tg (for most comm. pol. around 100 C). Amorphous thermoplastics are glassy, transparent materials with a youngs modulus (E) of typically 3000 MPa. Some are brittle (PS), others are tough (PC). Above Tg a rubbery state is reached where E is 1000 to 10,000 times lower. In this region forming methods such as vacuum forming are possible. Above the flow temperature (Tf) a fluid state occurs which enables processing methods such as extrusion and injection moulding. Most semi-crystalline polymers have a Tg below room temperature. Their utillity range lies between Tg an the melting temperature, Tm. In this region the modulus is strongly temperature dependent and the tendency to creep is larger than in the glassy state. Semi-crystalline thermoplastics are opaque due to the presence of many small crystallites.