Thermoplastics: characteristics and uses
Thermoplastics have linear to branched long-chain molecules which are not interconnected Since their molecular chains are not properly bonded together, they have high inherent plasticity which increases with rise in temperature. The structure of thermoplastics is shown in Fig.(a) with random type attractions and hit-or-miss kissing (secondary bond) among polymer chains (weak cross-links), thus giving limited strength. These secondary
bonds are easily broken by heat. Besides heat, thermoplastics are softened by chemical solvents. However, they do not undergo chemical changes when heated and cooled and their plasticity is retained intact since the structure remains unchanged on alternate heating and cooling.
Various types of thermoplastics can be categorized as below:
(b) Cellulosic derivatives
(c) Synthetic resins
(a) Natural resins:
These are made from naturally occurring materials such as shellac, asphaltic and bituminous materials mixed with mica and asbestos as filler. Natural resins are very hard and hence used as bonding material for grinding wheels. Shellac compounds have good scratch hardness and resistance.
(b) Cellulosic derivatives:
These are the earliest polymers made of natural organic materials derived from animals and vegetable products. Cellulosic materials are wood pulp, saw dust, cotton, etc. Various cellulosic plastics are given in the following.
(ii) Cellulose acetate
(iii) Cellulose acetate-butyrate
(iv) Ethyl cellulose
(v) Cellulose propionate.
(c) Synthetic resins or synthetic organic polymers:
The earliest polymers were made of natural organic materials from animals and vegetables. With the development of plastic technology, polymers were extracted from coal and petroleum products. The earliest synthetic (man-made) polymer was phenol-formaldehyde (bakelite). Then came up a large variety of thermoplastics (or commercial synthetic organic polymers) as listed below.
(ii) Acrylics (polymethylmethacrylate, PMMA)
(iii) Acrylonitrile-butadiene-styrene (ABS)
(iv) Fluorocarbons (teflon)
(v) Polyamides (nylons and aramids).
(vii) Polyethylenes [low density (LDPE) and high density (HDPE)]
(viii) Polypropylenes (ix) Polystyrenes
(xi) Polyvinyl chloride (PVC) or vinyl resins
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