What are plastics??
The term plastics, as it is commonly used today, refers to a large group of synthetic materials which are made from a number of common substances such as coal, salt, oil, natural gas, cotton, wood, and water .Simple chemicals are pulled from the material to produce monomers. Which are capable of reacting with one another.
Monomers are then built up in a
chainlike molecules called polymers. This is called plastics.
Classification of Plastics:
Plastics can be classified as follows:
1. Thermoplastics :
Thermoplastics become soft when heated and hard when cooled,regardless of the number of times the process isrepeated. This is due to the fact that linear molecularchains can move together when heated and cooled. Their hardness is a temporary property subjected tochange with rise or fall of temperature and can be brought again to plastic stage.These are formed by addition of polymerization which have long chainmolecular structure. At low temperature these plastics are generallybrittle in their natural state and hence plasticizers are generallycombined in varying proportions to improve the quality during theirmanufacturing stage.
The plasticizers may be granular or fibrous materials. Thermoplastics aregenerally comprising of Acrylic, Poly vinyl chloride (PVC), poly ethylene,poly vinyl acetate, cellulose acetate, poly propylene etc.
These are also chainlike in molecular structure but the curing and hardening process involves the formation of cross-links between molecules in adjacent chains. Resulting in complex chains that are no longer free to move.
These cross-linked structure have with predominantly strong covalent bonds during polymerisation retaining strength even on heating while under prolonged heating they fail by charring. Compared to thermoplastics, they are hard, strong and more brittle.
Thermosetting plastic cannot be reused. They require greatpressure and momentary heat during moulding and finallyget hardened on cooling. The chemical reaction in this process cannot be reversed. Once solidified they cannot be softened.
Thermosetting plastics comprise of Polyster, Vulcanized rubber, Polyimides, Epoxy resin, Melamine resin, Bakelite etc.