Plastic is an organic material prepared out of resin. It may or may not contain fillers, plasticisers and solvents. Plastic may be defined as a natural or synthetic organic material which are having the property of being plastic at some stage of their manufacture when they can be moulded to required size and shape.
Shellac and bitumen are the natural resins used as plastic for a long time. In 1907, Blackland produced synthetic resin from the reaction of phenol and formaldehyde. The resin was hardened under pressure and heat to produce useful plastic articles.
Types of Plastics
Primarily there are two types of plastics:
Thermosetting Plastics: It needs momentary heated condition and great pressure during shaping. When heated cross linkage is established between the molecules and chemical reaction takes place. During this stage shape can be changed with pressure. This change is not reversible. The scrap of such plastic is not reusable. Bakelite is an example of such plastic.
Thermoplastic: In this variety, the linkage between the molecules is very loose. They can be softened by heating repeatedly. This property helps for reuse of waste plastic. These plastic need time to cool down and harden. These plastics are to be kept in moulds till cooling takes place completely. Bitumen, cellulose and shellac are the examples of this variety of plastics.
Properties of Plastics
- Colour: Some plastics are completely transparent. Using pigments plastics of any attractive colour can be produced.
- Dimensional Stability: It is dimensionally stable to a great extent.
- Durability: Plastic offers great resistance to moisture and chemicals and hence more durable.
- Electrical Insulation: The plastics possess excellent electrical insulating property.
- Fire Resistance: The phenol-formaldehyde and urea-formaldehyde plastics resist fire to a great extent and hence they are used as fire proofing materials.
- Strength: The plastics are reasonably strong. Their strength may be increased by reinforcing with various fibrous materials. Attempts are being made to produce structurally sound plastics.
- Specific Gravity: The specific gravity of plastics is very low and hence convenient to handle.
- Ductility: The plastics are not ductile and hence they fail without giving warning.
- Fixing: Plastics can be bolted, drilled, glued, clamped or simply push fitted in position.
- Maintenance: There is no maintenance cost for plastic articles i.e., they do not need painting and polishing.
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