plastic

Examples of thermosoftening plastics, their basic monomer origins, polymer names, and everyday uses.

Unlike thermoplastics, thermosets remain rigid when set and do not soften when heated.

Any of the stable synthetic materials that are fluid at some stage in their manufacture, when they can be shaped, and that later set to rigid or semi-rigid solids. Plastics today are chiefly derived from petroleum. Most are polymers, made up of long chains of identical molecules.

Environmental influence

Most plastics cannot be broken down by micro-organisms, so cannot easily be disposed of. Incineration leads to the release of toxic fumes, unless carried out at very high temperatures.

Processed by extrusion, injection-moulding, vacuum-forming, and compression, plastics emerge in consistencies ranging from hard and inflexible to soft and rubbery. They replace an increasing number of natural substances, being lightweight, easy to clean, durable, and capable of being rendered very strong - for example, by the addition of carbon fibres - for building aircraft and other engineering projects.

Thermoplastics Thermoplastics soften when warmed, then re-harden as they cool. Examples of thermoplastics include polystyrene, a clear plastic used in kitchen utensils or (when expanded into a ‘foam’ by gas injection) in insulation and ceiling tiles; polyethylene (polythene), used for containers and wrapping; and polyvinyl chloride (PVC), used for drainpipes, floor tiles, audio discs, shoes, and handbags.

Thermosets Thermosets remain rigid once set, and do not soften when warmed. They include Bakelite, used in electrical insulation and telephone receivers; epoxy resins, used in paints and varnishes, to laminate wood, and as adhesives; polyesters, used in synthetic textile fibres and, with fibreglass reinforcement, in car bodies and boat hulls; and polyurethane, prepared in liquid form as a paint or varnish, and in foam form for upholstery and in lining materials (where it may be a fire hazard). One group of plastics, the silicones, are chemically inert, have good electrical properties, and repel water. Silicones find use in silicone rubber, paints, electrical insulation materials, laminates, waterproofing for walls, stain-resistant textiles, and cosmetics.

Polyamides Polyamides are widely used for the production of film, sheet, and injection-moulded articles. Nylon, the first polyamide, was synthesized in 1935 by Wallace Carothers at the du Pont laboratories in the USA and was intended to have many of the properties possessed by natural silk. Although it does have other applications, nylon is known principally for its applications in the textile field. Nylon yarn, once it has been stretched during the filament-forming process, has a combination of properties unique among textile fibres. One of the most notable is remarkable tensile strength, combined with lightness in weight and a high degree of resilience.

Shape-memory polymers Shape-memory polymers are plastics that can be crumpled or flattened and will resume their original shape when heated. They include trans-polyisoprene and polynorbornene. The initial shape is determined by heating the polymer to over 35°C/95°F and pouring it into a metal mould. The shape can be altered with boiling water and the substance solidifies again when its temperature falls below 35°C/95°F.

Biodegradable plastics Biodegradable plastics are increasingly in demand: Biopol was developed in 1990. Soil micro-organisms are used to build the plastic in their cells from carbon dioxide and water (it constitutes 80% of their cell tissue). The unused parts of the micro-organism are dissolved away by heating in water. The discarded plastic can be placed in landfill sites where it breaks back down into carbon dioxide and water. It costs three to five times as much as ordinary plastics to produce. Another plastic digested by soil micro-organisms is polyhydroxybutyrate (PHB), which is made from sugar.