Mankind has used natural polymeric materialssuch as wood, leather and wool since the beginning of history, but synthetic polymers became possible only after the development of rubber technology in the 1800s. The rst synthetic polymer material, celluloid, was invented by John Wesley Hyatt in 1869, from cellulose nitrate and camphor. A major breakthrough in synthetic polymers was the invention of Bakelite by Leo Hendrik Baekeland in 1907. Hermann Staudinger’s work in the 1920s clearly demonstratedthe macromolecular nature of long chains of repeating units.1 The word ‘polymer’ comes from Greek, and it means ‘many parts’. The rapid growth of the polymer industry started shortly before the Second World War, with the development of acrylic polymers, polystyrene, nylon, polyurethanes and the subsequent introduction of polyethylene, polyethylene terephthalate, polypropylene and other polymers in the 1940s and 1950s. While only about 1 million tons were produced in 1945, the production of plastics in volume surpassed that of steel in 1981, and the gap has been continuously growing ever since.
Pure polymers are seldom processed on their own. They are compounded with other materials, typically by mechanical blending or melt state mixing to produce pellets, powders or akes to be used in subsequent processing operations.2 Such compoundedproductsare referredto as‘plastics’, which means‘pliables’ in Greek. The compounds may involve llers (to reduce cost), reinforcements, other polymers, colourants, ame retardants, stabilisers (to prevent deterioration from light, heat or other environmental factors) and various processing aids.
Synthetic polymers can be classi ed in two categories. Thermoplastics (by far the largest volume) can be melted by heating, solidi ed by cooling and remelted repeatedly. Major types are polyethylene (PE), polypropylene(PP), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) and polyamide (PA, nylon). Thermosets are hardened by the application of heat and pressure, owing to crosslinking, i.e. the creation of permanent threedimensional networks. They cannot be softened by heating for reprocessing. Bakelite, epoxies and most polyurethanes are thermosets.
The present overview is exclusively devoted to the processing of thermoplastics. Commercial thermoplastics are classi ed according to their performance as ‘commodity’ (low performance,such as PE, PP, PS and PVC), ‘Engineering’ (such as PC, nylon and PET) or ‘advanced’ (highest performance, such as liquid crystal polymers (LCPs), polyphenylene sulphide (PPS) and polyetheretherketone (PEEK)). The anticipated explosive growth in engineering and advanced polymers did not materialise. The use of plastics has been continuously growing throughout the past three decades, but mainly in the commodity category. Currently, commodity polymers amount to ~88% of the volume produced,3 engineering plastics ~12% and advanced less than 1%. Although the prices of advanced polymers per kilogram are much higher than those for commodity polymers, their global value to the economy is still very small.
Commodity plastics have low strengths and stiffnesses when compared with metals or ceramics, and they tend to exhibit creep under an applied force. They also have temperature limitations in their use as solids (most melt in the range 100– 250°C). The tensile moduli of commodity plastics are ~1 GPa (compared with 210 GPa for steel). Signi- cant improvement can be achieved by alignment of the polymer chains. Actually, the carbon– carbon bonds are very strong, and single lament polyethylenes have been produced with modulus values exceeding that of steel. High orientation can be achieved by special processing techniques, for example extrusion and subsequent drawing at low temperatures. At low temperaturesthe polymer chains have limited mobility, and the orientation remains after stretching. Recent discoveries and developments of single site, metallocene based catalysts have resulted in new grades of commodity polymers having controlled molecular architecture with improved properties.
The world production of polymers increased3 from 27 million tons in 1975 to ~200 million tons per year in 2000 and is still growing. According to a recent report,4 shipments of plastics products in the USA in 2000 amounted to $330 billion, and upstream supplying industries had sales of $90 billion, bringing the annual total to $420 billion. Total employment was estimated to be 2 .4 million – about 2% of the US workforce. The growth of the polymer industry is a result of the unique combination of properties of plastic products, which include easy shaping and fabrication, low densities, resistance to corrosion, electrical and thermal insulation, and often favourable rigidity and tough- ness per unit weight.
Post time: Feb-04-2018