Plastics

PLASTICS

Introduction:
The plastics are synthetic organic materials which can be moulded into any desired shape when subjected to heat and pressure. These materials are prepared from natural or synthetic resins with or with out the moulding compounds. These moulding compounds are mixed with the granular resins before moulding.
The plastics, are mostly used in the manufacture of aeroplane and automobile parts. They are also used for making safety glasses, laminated gears, pulleys, self-lubricating bearings.
1. Raw Material for Plastics
In general, the plastics are compounds of carbon with other elements such as oxygen, hydrogen and nitrogen. The carbon combines with itself and other elements and forms more complicated compounds.
The raw material for plastic compounds are various agricultural products and numerous minerals and organic materials including petroleum, coal, gas, limestone, silica and sulphur. The natural resins are wax, shellac, pitch, bitumen resin, rubber etc. The synthetic resins, for the plastic materials, are produced by linking together a large number of small molecules of synthetic resin (known as monomers or monoliths) which forms large or long-chain molecules known as polymers (poly means
many and mers means units or parts). The process by which monomers are combined to form polymers is known as polymerization. The following two methods are used to achieve polymerization.
1. Addition polymerization, and
2. Condensation polymerization.
In addition polymerization, two or more similar monomers directly react to form polymers. The condensation polymerization takes two or more dissimilar monomers that react to form polymers plus the by-product water.
2. General Properties of Plastics
The following are the general properties of plastics due to which it is widely used in engineering practice.
1. Lightness in weight. The specific gravity varies from 1 to 2.4.
2. Easy workability. The plastics can be easily casted, moulded, drilled, machined etc.
3. Highly resistant to corrosion.
4. Highly resistant to abrasion.
5. Good thermal and electrical insulators.
6. Good strength and rigidity.
7. Absorbent of vibrations and sound.
8. Good resistant to most of the chemicals.
9. Impermeable to water.
10. Low fabrication cost.
11. Good dimensional stability.
12. Can be made transparent or coloured.
3. Type of Plastics
The plastics are broadly classified in the following two groups:
1. Thermoplastic materials, and
2. Thermosetting plastics.
1. Thermoplastic materials: The thermoplastic materials (also known as cold-setting materials) are those plastics which do not become hard with the application of heat and pressure and no chemical change occurs. They remain soft at elevated temperatures and become hard on cooling. They can be remelted repeatedly by the successive application of heat. These
plastics are better than thermosetting plastics because they can be used again and again.
The scrap obtained from the moulding operations and rejected articles can be shaped into granules and reused to make first quality products. These plastics are used for makings toys and other ordinary purposes.