Plastic | Definition, Classification, Composition, Properties

Plastic | Definition, Classification, Composition, Properties

Plastics are one of the recent engineering materials which have appeared in the market all over the world. Some varieties of naturally occurring thermo-plastics were known to Egyptians and Romans who extracted and used these plastics for various purposes. A number of scientists carried out experiments for plastics and as a result of long research, the birth of the plastic industry took place in the nineteenth century.

Composition of Plastics:-

    Plastic is an organic substance and it consists of natural or synthetic binders of resins with or without moulding compounds. The various types of resins and commonly used moulding compounds.

Plastic is a compound of carbon with other elements such as hydrogen, nitrogen and oxygen. The carbon combines with itself and other elements and forms more complicated compounds.
     The finished product of plastic is rigid and stable at normal temperature. It is, therefore, improper to associate the term plastic to such substance in the ordinary sense of the term.

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:- Thermoplastics and thermosetting plastics.
  2. Structure:- Homogeneous and heterogeneous plastics.
  3. Physical and mechanical property:- Rigid, soft, semi-rigid and elastomers.
  • Thermo-Plastic:-
         The thermoplastic or heat non-convertible 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 the 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.

Plastic Bottle

  • Thermosetting:-
       .The thermosetting or heat convertible group is the general term applied to the plastics which become rigid when moulded at suitable pressure and temperature. This type of plastics passes originally through the thermo-plastic stage. When they are heated in the temperature range of 127 degrees centigrade to 177-degree centigrade, they set permanently and further application of heat does not alter their form or soften them. But at a temperature of about 343 degrees centigrade, the charring occurs. The charring is a peculiar characteristic of the organic substance.
            The thermo-setting plastic is soluble in alcohol and certain organic solvents when they are in the thermo-plastic stage.  This property is utilized for making paints and varnishes from these plastics.

Electric Plugs

  • Homogeneous Plastic:-
       This variety of plastic contains a carbon chain i.e. the plastics of this group are composed only of carbon atoms and they exhibit homogeneous structure.

  • Heterogeneous Plastic:-
       This variety of plastic is composed of a chain containing carbon and oxygen, nitrogen and other elements and they exhibit heterogeneous structures.

  • Rigid plastics:-
          These plastics have a high modulus of elasticity and they retain their shape under exterior stresses applied at normal or moderately increased temperatures.

  • Semi-rigid plastics:-
        These plastics have a medium modulus of elasticity and the elongation under pressure completely disappears when pressure is removed.

  • Soft plastics:-
       These plastics have a low modulus of elasticity and the elongation under pressure disappears slowly When pressure is removed.

  • Elastomers:-
       These plastics are soft and elastic materials with a low modulus of elasticity. They deform considerably under load at room temperature at a return to their original shape, when the load is released. The extensions can range up to ten times their original dimensions.

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