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Why do we use plastics?

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Plastic materials were first used at around2000BC.Plastics can occur naturally in trees or even milk;amber is an example (a resin from trees andinsects). Amber was used by early Egyptians tomake jewellery and is still used for this purposetoday.

Brief history

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Brief history The plastic is one of the recent engineering material which has

appeared in the market all over the world. The first stage of its development came when Scottish chemist

,Alexander parkes tried to duplicate the natural plastic bymixing camphor and alcohol.

The second stage of development came when a Belgianscientist prepared a product called bakelite , a thermo settingplastic which was found to be hard and strong.

The last stage includes present trend and aims at improvingthe old plastics ,where a substance transprent like glass and

unbreakable was prepared by an australian scientist Pollak,during the second world war.

A strong revolution in plastic industry came during the periodof second world war.

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Composition of plastic Plastics are compounds of carbon with other elements such as

hydrogen ,oxygen and nitrogen. It is basically a polymer(combination of more than one

primary chemical).

The carbon compound combines with itself and otherelements to form complicated compounds.

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Polymerization The process of converting a monomer (consisting of one

primary chemical) to a polymer is called as polymerization. The properties such as strength, rigidity , and elasticity are

considerably improved by polymerization.

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Methods of polymerization Addition polymerization in this method , similar or different

molecules join together due to opening of double bond . themolecular weight of the resulting polymer is the sum of themolecular weight of the reacting molecules. Polymers obtainedare polyethelene ,poly-propelene etc.

Condensation polymerization In this method the low molecular substance are removed from high molecular substance formedfrom a large number of identical or different molecules.compared to addition polymerization it yields industriallyimportant by products like h 2o , hcl etc. it takes a lot of time tocomplete.

Co polymerization co polymerization is additionpolymerization of two or more different Monomers. manymonomers does not polymerize with themselves but will co-polymerize with other compounds. Thus a wide variety of plasticscan be obtained by this process.

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Classification of plastics Thermo plastics Thermo setting plastics Elastomers

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Thermo plastics The thermo plastics or heat non-convertible group is the

general term applied to the plastics which become soft whenheated and hard when cooled.

The process of softening and hardening may be repeated

indefinite number of times, provided the heat given should notbe high enough to cause chemical decomposition. It is possible to shape and reshape these particles. The scrap obtained from the old of these types can be

effectively used.

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Thermo setting plastics Plastics which become rigid when molded at suitable pressure

and temperature are thermo setting plastics. They set permanently and further application of heat does not

soften or alter their form.

But at temperature above 343degree Celsius charring occurs. They are strong ,durable and hard.

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Elastomers These plastics are soft and elastic materials with a low modulus

of elasticity. They deform under load at room temperature and return back

to its original shape ,when load is released.

The extension can range up to ten times their originaldimensions.

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Humidity The strength of plastics containing water attracting

groups such as cellulosic plastics, is considerably affected bymoisture. The plastics which does not containing poly vinylchloride plastics offer great resistance to moisture.

Maintenance Is easy to maintain plastics. Melting point Most of the plastics have very low melting point

and melting point of some plastics is only 50 degree Celsius .To increase the heat resistance glass fiber reinforcement maybe added.

Recycling Plastics can be recycled and used again

Sound absorption

The acoustical boards are prepared byimpregnating fiber-glass with phenolic resins. This material hasabsorption coefficient of 0.67.

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What implications do you think this has

for the environment?

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