Page 32 - Science Class 08

monomers long. The end of the chain will have plas cs do not react with air, water and many
an addi onal atom of hydrogen, termina ng other chemicals, they keep food fresh and free
the polymer. from contamina on. We can easily mould them

Type of Polymers into desired shapes.

There are two types of polymers. These are: Plas cs can be classified into two groups,
depending on their reac on to heat:
v Natural Polymers
v Thermoplas cs, and
v Synthe c Polymers
v Thermose ng plas cs
v Natural Polymers : Naturally occurring
In thermoplas cs, the process of so ening by
polymers are called natural polymers.
hea ng and hardening on cooling can be
For example, cellulose, natural rubber, repeated again and again to so en the plas c
co on, wool, silk, proteins, etc. to get different shapes. However, thermo-

v Synthe c Polymers : Polymers obtained by se ng plas cs can be so ened by hea ng only
synthesising different raw materials once. Once, they are put into a mould and
(inorganic) are called synthe c polymers. allowed to harden on cooling, they cannot be
For example, nylon, terylene, thicket, so ened again on hea ng, e.g., bakelite and
neoprene, etc. melamine. They can maintain their shape and
size even at very high temperatures.
PLASTICS
Common Uses of Plas cs
A material is said to be ‘plas c’ if it can be
Today, plas cs play an important part in our
moulded or formed into different shapes. Thus,
lives. It is the versa lity of plas cs that is
dough and plas cine are plas cs; so is clay and
responsible for their many uses. Carry bags,
heated glass. However, the word plas cs is now
toys, so drink bo les, car parts, computer
commonly used for several human-made
parts, refrigerator parts, telephones and many
materials that can be moulded into desired
other things around us are made of plas cs.
shapes or drawn into fibres. All these materials
Such variety of uses is possible because plas cs
have one common property— they are made
can be manufactured to have a variety of
of polymers. Scien sts have created an
proper es. They can be thin and flexible as well
enormous range of human-made plas cs with
as thick and tough; transparent or coloured;
different proper es. The main source of these so or hard; smooth or rough. They are
plas cs is crude oil. inexpensive, lightweight, resistant to corrosion

Plas cs are similarly, widely used to make and ac on by bacteria and last much longer
containers, toys, pens, bags, cups, etc. Since, than any other material.

Man-made Plas c Type of Plas c Property Uses

1. Polythene thermoplas c strong but flexible; sheets of polythene are used to
(or polyethylene) can be rolled into pack liquids such as milk;
sheets or moulded polythene pipes are used to
into any shape; transport liquids such as oil or
water resistant water; polythene containers are

used to store liquids

32 Science-8