The plastic is an organic substance and it consists of natural or synthetic binders or resins with or without moulding compounds. The plastic is manufactured by the polymerization.
A polymer consists of thousands of monomers joined together.
The simplest substance consisting of one primary chemical are known as the monomer.
Monomers are to be combined to form polymers by the process known as polymerization. The polymer molecule is also called a macromolecule.
A polymeric material consists of a large number of these long chain molecules.
The properties such as strength, rigidity and elasticity are considerably improved by the polymerization and it further leads to the manufacture of plastics in an economy way.
CLASSIFICATION OF PLASTICS
The classification of plastics can be made by considering various aspects and for the purpose of discussion, they can be classified according to their
1. Behaviour with respect to heating
2. Structure and
3. Physical and chemical properties
As case-1 is the topic of our discussion we will concentrate on that.
1. Behaviour with respect to heating
According to this classification the plastics are divided into two groups:
The above classification is based on the inherent characteristics of each group. These two groups can further be divided into several distinct sub-divisions. These sub-divisions are based on the raw materials from which plastics are prepared. It is interesting to note that each of above group contains several hundred different products and with the advance of plastic industry, the number of sub-divisions under each category is constantly increasing.
(i) Thermoplastic polymer
The thermo-plastic or heat nonconvertible group is the general term applied to the plastics which become soft when heated and hard when cooled. The process of softening and hardening may be repeated for an indefinite time. Provided the temperature during heat is not so high as to cause chemical decomposition. It is thus possible to shape and reshape these plastics by means of heat and pressure. One important advantage of this variety of plastics is that the scrap obtained from old and warn-out articles can be effectively used again.
(ii) Thermosetting polymer
The thermosetting or heat convertible group is the general term applied to the plastics which become rigid when moulded at suitable pressure and temperature. When they are heated in temperature range of 1270C to 1770C, they set permanently and further application of heat does not altered their form or soften them. But at temperature of about 3430C, the charring occurs. This charring is a peculiar characteristic of the organic substances.
Properties of Thermosetting polymer
The thermo setting plastics are soluble in alcohol and certain organic solvents when they are in thermo-plastic stage. This property is utilised for making paints and varnishes from these plastics.
These plastics are durable, strong and hard. They are available in a variety of beautiful colours. They are mainly used in engineering application of plastics.
Properties of plastics
1. Appearance :
2. Chemical resistance:
The plastics offer great resistance to moisture, chemicals and solvents, excellent corrosion resistance.
3. Dimensional stability.
The plastic lacks ductility. Hence its members may fail without warning.
The plastics are quite durable, if they possess sufficient surface hardness.
6. Electric insulation:
They are far superior to ordinary electric insulators.
Any surface treatment may be given to the plastics.
8. Fire resistance:
All plastics are combustible.
Can be easily fixed in position.
PVC plastics offer great resistance to the moisture.
It is easy to maintain plastic surfaces. They do not require any protective coat of paints.
12. Melting point:
Most of the plastics have low melting point and MP of some plastics is only about 500 degree celcius.
13. Optical property:
Several types of plastics are transparent and translucent.
It does not give a serious problem to pollution as generated by a host of other industries. The plastics used for soft drink bottle, milk and juice bottles, bread bags, syrup bottles, coffee cups, plastic utensils etc can be conveniently recycles into carpets, detergent bottles, drainage pipes, fencings, handrails, grocery bags, car battery cases pencil holders, benches, picnic tables, roadside posts etc.
15. Sound absorption:
The acoustical boards are prepared by impregnating fibre-glasses with phenolic resins. This material has absorption co-efficient of about 0.67.
The tensile members are generally made of plastics as their strength to weight ratio in tension very nearly approaches to that of metals.
17. Thermal property:
The thermal conductivity of plastics is low and it can be compared with that of wood.
18. Weather resistance:
Certain plastics are seriously affected by sun light, but other plastic can resist weather which as prepared from phenolic resins.
The plastics, whether thermo-plastic or thermo-setting have low specific gravity being 1.30 to 1.40.
Applications of Thermosetting polymer
The typical use of plastics in building are as follows :
1. Bath and sink units.
2. Cistern ball floats.
3. Corrugated and plain sheets.
4. Decorative laminates and mouldings.
5. Electrical conduits.
6. Electrical insulators.
7. Floor tiles.
8. Foams for thermal insulation.
9. Joint less flooring.
10. Lighting fixtures.
11. Overhead water tanks.
12. Paints and varnishes.
13. Pipes to carry cold water.
14. Roof lights.
15. Safety glass.
16. Wall tiles.
|Thermo-Plastic and Thermo-Setting plastic
Properties of Elastomers
These plastics are soft and elastic materials with a low modulus of elasticity. They deform considerably under load at room temperature and return to their original shape, when the load is released. The extensions can range up to ten times their original dimensions.