Types of Plastics

TYPES OF PLASTICS

Plastics are broadly classified into the following two main groups:

(i) Thermosetting plastics

(ii) Thermoplastics.

1. Thermosetting Plastics

The plastics which are hardened by heat effecting a non-reversible chemical change are called thermosetting plastics.

Thermosetting plastics do not soften on reheating and cannot be reworked. Thermosetting molecules are formed by condensation polymerization. The molecules of such type of plastics have three dimensional network and very strong binding force between molecules. The raw material for thermosetting plastics is in the form of liquid or solid. These types of plastics are polymerized when moulded or formed. It consumes more time for formation. The various types of thermosetting resin are as follows:

1. Phenol formaldehyde:

It is also named as bakelite. Phenol formaldehyde or phenolic resins are synthetic polymers obtained by the reaction of phenol with formaldehyde. It is generally produced in dark colour and it has high strength, stability and rigidity. It can be easily cast or laminated.

Uses: Plugs, knobs, pulleys, bottle caps, tooling and forming dies.

2. Polyester resin:

It has low moisture, good electrical resistance and variety of colours. It is used in paper mat, TV parts and car bodies. The main drawback of the polyster is high cost.

3. Melamines:

It has excellent electrical resistance and heat resistance. It has good stability and low moisture absorption. The melamines are available under various names of Melmac, Catlin, Melantine and Plaskon. It is widely used for moulded parts.

Uses: Telephone sets, circuit breakers, switch panels and lighting fixtures.

4. Phenol furfural:

It has good flowability at low moulding temperature and sets quickly at correct temperature. The phenol furfural has good resistance to moisture and electricity.

Examples: Brake linings, electrical parts and instrument cabinets.

Uses: It is used as a binder in resinoid abrasive wheels, laminating varnishes and adhesives.

5. Epoxy resins:

The most popular variety of epoxy resins is Araldite. It has good chemical resistance and electrical resistance. It is mostly available in the form of liquid. They also have good resistance to wear and impact. But they are quite expensive.

Uses: Tools and dies, jigs and fixtures, housings for electrical parts and enamels.

6. Silicones:

Silicones have high resistance to high temperature up to 260°C and possess excellent dielectric strength at high temperatures. In liquid form, they are used as water repellants. They can be compressed and reinforced.

Uses: It is used in coatings, laminates, foam products and induction heating apparatus.

In rubber form, it is used in gaskets for providing high heat resistance.

7. Urea formaldehyde (Amino resin):

It is obtained by the condensation of urea and aqueous formaldehyde. It cannot be cast. But, it can withstand temperature up to 77°C only. It is widely used as an adhesive and binding material.

Uses: It is used in toilet seats, table ware, buttons, clock cases, electric switches and plugs.

8. Alkyds:

It is also known as oil-modified polyesters. Alkyds are used in synthetic enamels and varnishes. It is used in solid form where high electrical resistance and heat resistance are required.

Example: Automobile ignition parts.

9. Polyurethanes:

It is mainly used for cushions in transportation seats for insulation and electronic equipment as a packing material.

2. Thermoplastics

The thermoplastics have separate long and large size molecules arranged side by side. It does not have any cross linking in their molecular structure. Some of the thermoplastic structure is amorphous in nature and others are crystalline structure in nature. It is formed by addition polymerization process. When thermoplastics are heated, it bemes very soft and rehardens on cooling. During heating, the linear bonding links between molecules breakup and molecules are separated: Relinking takes place on cooling and retains their hardness. It is easily remoulded or extruded to any shape. These plastics do not have a definite melting temperature. Thermoplastics are classified into the following types:

1. Cellulose derivatives

2. Synthetic resins.

1. Cellulose derivatives:

(i) Cellulose nitrate:

It is obtained by treating the cellulose with a mixture of nitric and sulphuric acid. It has high toughness, good resistance to moisture and highly inflammable.-

Uses: Spectacle frames, toilet articles, pen bodies and table tennis balls.

(ii) Cellulose acetate:

It is obtained by treating the cellulose with acetic acid. It can be injected and compressed in the mould for obtaining better stability and high mechanical strength. It is lighter than cellulose and tendency to absorb moisture.

Uses: Photographic films, buttons, radio panels, toys, extruded sheets, tubes and rods.

(iii) Ethyl cellulose:

The ethyl cellulose is the lightest of all cellulose derivatives. It has good electrical properties, chemical resistance, surface hardness and strength.

Uses: Jigs, fixtures, forming dies, hose nozzles and moulded articles.

(iv) Cellulose acetate butyrate:

It is obtained by treating cellulose with acetic acid and butoric acid. It has good stability against light and has heat and moisture absorption tendency. It can also be injection moulded and extruded.

Uses: Radio cabinets, pipes and tubing, steering wheels, insulating tapes, handles and coatings.

(v) Cellophane:

It is available in an extruded form. It has attractive appearance and good resistance to moisture, fire and solvents.

Uses: Curtains, drapers wrapping and packaging.

(vi) Cellulose propionate:

It has low tendency for moisture absorption and can easily be moulded. The cellulose propionate can withstand temperature up to 93°C.

Uses: Fountain-pens, telephones and flash light cases.

2. Synthetic resins:

(i) Polyethylenes:

It has very high resistance to acids, solvents. It can be made flexible, tough and good insulators. It has low water also are softened at 93°C.

Uses: Fabrics, trays, pipes and corrosion resistant coatings.

(ii) Polystyrenes:

It has dimensional stabilities and strain resistance. It is easily mouldable and has a tendency to crack under load. The polystyrenes are easily joined by cementing. It can be produced in any form and colours.

Uses: Battery boxes, radio parts, tableware, toys and high frequency insulation parts.

(iii) Acrylic resins:

It has high transparency tendency. It can be made in any colour. It has dielectric properties, resistance to moisture, good strength and excellent light transmitting capcacity. It can also be cast, injection moulded, extruded and stretch-formed into sheets.

Uses: Tubes, plates, coatings, adhesives, laminates, display cases, lenses and helmets.

(iv) Vinyles:

Its trade name is polyvinyl chloride (PVC). Vinyl plastics are made in the form of flexible or rigid. It has good electrical and weather resistance. The vinyls are water resistant and can be produced in various colours.

Uses: Tarpaulin, water roofing, raincoats, tubes, pipes and insulation.

(v) Polytetrafluoroethylene:

Its trade name is Teflon. It has maximum chemical resistance, can withstand temperatures up to 288°C, and cannot be dissolved in any solvent. It has high electrical resistance, low friction and very low adhesion to other substances. It is available in forms such as rods, sheets and tubes.

Uses: Gaskets, greaseless bearing, electrical insulators and chemical containers.

(vi) Polyamide:

It is popularly known by its trade name Nylon. It has high strength, toughness and elasticity. It can be moulded and extruded into rods. The power metallurgy methods can also be used for this type of plastics. It is a good insulator and has good wear resistance.

Uses: Yarn for cloth, bearings and coupling, gears, wire insulation and combs.

(vii) Methyl methacrylate:

It trade name is Lucite and plexiglass. It can be formed easily at temperature around 120°C. It is marked by its clear colour and high light transmission capability.

Uses: Aircraft parts, transparent bowls, contact lenses and various surgical instruments.

3. Difference between Thermoplastics and Thermosetting Plastics