Plastics

Common organic materials are plastics and synthetic rubbers which are termed as organic polymers. Other examples of organic materials are wood, many types of waxes and petroleum derivatives. Organic polymers are prepared by polymerization reactions, in which simple molecules are chemically combined into long chain molecules or three-dimensional structures. Organic polymers are solids composed of long molecular chains. These materials have low specific gravity and good strength. The two important classes of organic polymers are as follows:

1. Thermoplastics: On heating, these materials become soft and hardened again upon cooling, e.g., nylon, polythene, etc.
2. Thermosetting plastics: These materials cannot be resoftened after polymerization, e.g., urea-formaldehyde, phenol-formaldehyde, etc. Due to cross-linking, these materials are hard, tough, non-swelling, and brittle. These materials are ideal for moulding and casting into components. They have good corrosion resistance. The excellent resistance to corrosion, ease of fabrication into desired shape and size, fine lustre, light weight, strength, and rigidity have established the polymeric materials and these materials are fast replacing many metallic components. PVC (polyvinyl chloride) and polycarbonate polymers are widely used for glazing, roofing, and cladding of buildings. Plastics are also used for reducing weight of mobile objects, e.g., cars, aircraft, and rockets. Polypropylene and polyethylene are used in pipes and manufacturing of tanks.

Thermo-plastic films are widely used as lining to avoid seepage of water in canals and lagoons. To protect metal structure from corrosion, plastics are used as surface coatings. Plastics are also used as main ingredients of adhesives. The lower hardness of plastic materials compared with other materials makes them subjective to attack by insects and rodents. Because of the presence of carbon, plastics are combustible. The maximum service temperature is of the order of 100°C. These materials are used as thermal insulators because of lower thermal conductivity. Plastic materials have low modulus of rigidity, which can be improved by addition of filters, e.g., glass fibres. Natural rubber, which is an organic material of biological origin, is a thermoplastic material. It is prepared from a fluid, provided by the rubber trees. Rubber materials are widely used for tyres of automobiles, insulation of metal components, toys, and other rubber products.

Timber: Timber is general name of wood. It is composite of cellulose and lignin. Cellulose fibres are strong in tension and are flexible. Lignin works as a binding material to bind the fibres and give them stiffness. It has applications in many engineering works and has been used as common construction materials. It has advantages over other engineering materials as easily available, strongest among cellular materials, easy processing, light weight, good surface finish, and inexpensive.