PLASTICS
Plastics are commonly known as synthetic resins or polymers. In Greek terminology, the term polymer comprises ‘poly’ means ‘many’ and ‘mers’ means ‘parts’. Thus, the term, polymer represents a substance built up of several repeating units, each unit being known as a monomer. Thousands of such units or monomers join together in a polymerization reaction to form a ‘polymer’. Some natural polymers like starch, resins, shellac, cellulose, proteins, etc are vary common in today’s use.
Synthetic polymers possess a number of large applications in engineering work. Therefore plastic materials are fairly hard and rigid and can be readily molded into different shapes by heating or pressure or both. Various useful articles can be produced from them rapidly, accurately and with very good surface quality. They can be easily produced in different colors or as transparent. They are recognized by their extreme lightness, good corrosion resistance and high dielectric strength.
Plastics are synthetic resins characterized as a group by plastic deformation under stress. These materials generally are organic high polymers (i.e. consisting of large chain like molecules containing carbon) which are formed in a plastic state either during or after their transition from a low molecular weight chemical to a high molecular weight solid material. These materials are very attractive organic engineering materials and find extensive applications in industrial and commercial work such as electrical appliances, automotive parts, communication products bodies (Telephone, Radio, TV), and those making household goods.
They possess a combination of properties which make them preferable to other materials existing in universe.
Properties of plastics
The properties of plastics are given as under.
1. Plastics are light in weight and at the same time they possess good toughness strength and rigidity.
2. They are less brittle than glass, yet they can be made equally transparent and smooth.
3. Their high dielectric strength makes them suitable for electric insulation.
4. They resist corrosion and the action of chemicals.
5. The ease with which they can be mass-produced contributes greatly to their popularity as wrappers and bags.
6. They possess the property of low moisture absorption.
7. They can be easily molded to desired shapes.
8. They can easily be made colored.
9. They are bad conductance of heat.
10. They are hard, rigid and heat resistance.
11. They possesses good deformability, good resiatance against weather conditions, good colorability, good damping characteristics and good resistance to peeling.
Plastics are broadly classified into thermo plastics and thermo-setting plastics.
Thermo Plastics
Those plastics which can be easily softened again and again by heating are called thermoplastic. They can be reprocessed safely. They retain their plasticity at high temperature, i.e. they preserve an ability to be repeatedly formed by heat and pressure. Therefore, they can be heated and reshaped by pressing many times. On cooling they become hard. They are some times also called as cold-setting plastics. They can be very easily shaped into tubes, sheets, films, and many other shapes as per the need.
Types of Thermo Plastics
(A) Amorphous
1. Polystyrene
2. Acrylonitrite-butadiene-styrene
3. Mrthacrylate
4. P.V.C (Polyvinyl chloride)
5. Polychloroacetal
6. Auorinated polymers,
7. Polycarbonate etc.
(B) Crystalline
1. Polyethylene
2. Polyamides
3. Polyacetal
4. Polypropylene
The reason for the re-softening of thermoplastic resins with heat is that they are composed of linear or long chain molecules. Application of heat weakens the intermolecular bonds by increasing thermal agitation of the molecules, and the material softens and thus plastic can be easily molded and remolded without damage
Thermo-Setting Plastics
Those plastics which are hardened by heat, effecting a non-reversible chemical change, are called thermo-setting. Alternatively these plastics materials acquire a permanent shape when heated and pressed and thus cannot be easily softened by reheating. They are commonly known as heat-setting or thermosets.
Thermosetting resins
(i) Phenol-formaldehyde resins
(ii) Urea-formaldehyde resins
(iii) Melamine-formaldehyde resins
(iv) Polyester resins
(v) Epoxy resins
(vi) Silicone resins
Other thermosetting compounds are phenol furfural, polysters, alkyds, and polyurethanes. The most common thermosetting compound is phenol formaldehyde which is discussed as under.
Phenol formaldehyde
Phenol formaldehyde is called as bakelite due to the name of its inventor Bakelite. It is the most commonly and widely used plastic. It is made by the reaction of phenol with formaldehyde. It has high strength, hardness, stability, rigidity and can be easily casted or laminated. It is highly resistant to heat, electricity and water. It is made in dark color shades. Its general uses are in making articles such as stereo cabinets, radio cabinets, plugs, knobs, dials, bottle cap, pulleys, wheels, telephones, switches and handles.
Thermosetting resins, when subjected (once only) to the heat and pressure required for forming, change into a hard and rigid substance. Once done so, they cannot be softened again by the application of heat. The reason for the above phenomenon is that the thermosetting plastics consist of linear, relatively low molecular weight thermoplastic polymer chains with cross-links which bond the chains together with primary valence bonds. Such three-dimensional
polymers, once cross-linked, will not soften when heated (but may decompose disintegrate at higher temperatures) because this process is an irreversible chemical reaction and the entire structure becomes essentially a single molecule. In contrast the thermoplastic resins can be re-softened and remolded by application of heat and pressure. They retain their plasticity at high temperature, i.e. they preserve an ability to be repeatedly formed by heat and pressure.
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