Plastics are synthetic materials based on carbon compounds derived from petroleum and to a small extent from coal. All plastics materials are polymers (long chains of molecules loosely tangled together), the lengths and characters of which can easily be adjusted in manufacture, which explains the immense variety of plastics.
All plastics materials can be classified as either thermoplastic or thermosetting:
• Thermoplastics soften on heating without undergoing chemical change (if not overheated) and harden again on cooling.
• Thermosets undergo an irreversible chemical change during moulding, so that they do not soften on heating and thus cannot be recycled.
While some developing countries have their own plastics industries, many others have to import the raw materials or finished products, which consequently are expensive. This is not always a disadvantage in building construction, as plastics are not essential materials, but if available, they have numerous applications in building, either to substitute or protect other materials, or to improve comfort conditions.
• Rigid plastics for various uses in water supply and sanitation; transparent, translucent or opaque sheets for non-loadbearing wall and roofing elements, glazing, facing, etc.; extruded profiles for window frames, furniture elements, etc.; fibre reinforced plastics (eg with glass, jute or sisal fibres) for (double curved or folded) self-supporting wall and roof elements (complete building systems).
• Plastic films and membranes for damp-proof courses; covering for concrete curing; temporary rain and wind protection of openings; tent structures. Thicker varieties and tubes for electrical insulation.
• Synthetic fibres for high strength ropes and fabrics, and as alkali resistant reinforcements (eg in fibre concrete roofing elements).
• Foamed plastics mainly as thermal insulation materials, lightweight ceiling panels, or as aggregate in lightweight concrete components.
• Synthetic resins and adhesives for production of various composite materials, such as particle board, plywood, all kinds of laminated and sandwich panels.
• Emulsion paints, distempers, enamel paints, varnishes.
• Sealants for movement joints, weather and waterproof joints.
• Impermeability and resistance to most chemicals, hence no corrosion.
• Good strength: weight ratios of most plastics materials; lightness in weight makes handling and transportation easier and cheaper; no heavy supporting structure is required.
• Capability to take on a wide variety of forms, colours and other physical properties; imitation and substitution of scarce and expensive materials.
• Generally good resistance to biological hazards.
• Excellent electrical insulation.
• High costs and limited availability in many developing countries.
• Flammability of most plastics, with development of noxious fumes and dense smoke.
• High thermal expansion, up to ten times that of steel, and rapid decline of mechanical properties at elevated temperatures.
• Deterioration of most plastics due to prolonged exposure to the sun's ultra-violet rays.
• Use of plastics only for special purposes, eg for waterproofing, thermal and electrical insulation, easier and cheaper transports or for use in earthquake prone areas.
• Avoidance of combustible materials installed close to plastics, and provision of sufficient ventilation openings to remove smoke and fumes in case of fire.
• Provision of sufficient movement joints for plastics components.
• Avoidance of uses of plastics exposed to sunlight.
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