Plastics

By surya teja

• Plastics are made from resin with or without fillers, plasticisers and pigments. These are organic materials of high molecular weight which can be moulded to any desired form when subjected to heat and pressure in the presence of a catalyst. ex:-acrylic,cellulose acetate,cellulose nitrate ,polyethylene,polysterane,poly vinyl chloride,polyvinyl acetate

Thermo plastic

• The thermoplastic variety softens on heating and hardens on cooling, i.e., their hardness is a temporary property subjected to change with rise or fall of temperature and can be brought again to plastic stage on heating. These are formed by addition polymerisation and have long chain molecular structure. They can be remoulded, for use, as many times as required. Examples are material resins— rosin, kopal, amber, shellac; cellulose derivatives— cellulose acetate, cellulose nitrate, nitrocellulose or celluloid, cellulose acetate-butyrate; polythenic or vinyl resin—polyethylene

• Thermosetting plastic cannot be reused. They require great pressure and momentary heat during moulding and finally get hardened on cooling. The chemical reaction in this porcess cannot be reversed. Once solidified they cannot be softened. The thermosetting plastics acquire three-dimensional cross-linked structure with predominantly strong covalent bonds during polymerisation retaining strength even on heating; under prolonged heating they fail by charring. Compared to thermoplastics, they are hard, strong and more brittle. The important thermosetting resins are phenolic resins or phenoplasts (bakelite), amino resins, polyester resins, epoxy resins and silicon resins. The principal uses are in electrical equipments, plugs, sockets, switches, ash trays, knobs, handles, etc.

Thermo setting plastics

Properties of plastics

• 1. Can be moulded to any desired shape or size and have high tensile and compressive strengths.

• 2. Easy to work upon. 3. Light in weight and a few varieties are glossy like glass. 4. Not attacked by insects and fungi. 5. Available in desired colour and texture. 6. Require a little maintenance. 7. Good electrical insulators and have low thermal conductivity. 8. Shock absorbing material. 9. Can be sawn, drilled and punched and welded easily.

• 10. High strength to weight ratio.

Properties of plastic

• 11. High resistance to weathering conditions. 12. Corrosion resistance. 13. Decorative surface effect—painting or polishing of the surface is not required. 14. High refractive index. 15. Some varieties are as hard as steel. 16. Withstands moisture, oil and grease well. 17. Inflammable. 18. High coefficient of thermal expansion (about ten times of steel). 19. Deterioration under prolonged exposure to sun’s ultra violet rays. 20. Low manufacturing cost, hence cheap.

Constituents of plastic

• Resin:-Synthetic resin, short for resin, is artificial synthesized high molecular polymer. Therefore, different types of plastic can be called after the name of the synthetic resin it is made from.

• Synthetic resin, the basic raw material of plastic, takes up 30%-60% or more of its composition. It has the function of agglutination, not only bindingitself together, but also the other materials firmly together. As the type, property, and amount of synthetic resin change, the physical and mechanical properties of plastic also change. Therefore, the main properties of plastic depend on the synthetic resin it is made from.

Constituents of plastic

• (1) Filling Material• It is filler for short. By adding filling material to the synthetic

resin, the flow between the chains of macromolecular compound can be slowed down; the strength, stiffness, and heat resistance of the synthetic resin can be improved; and the cost of plastic can be lowered. The common inorganic filling materials are talcum powder, diatomite, mica, limestone powder, and glass fiber, etc. The common organic filling materials are wood flour and wastepaper, etc..

• (2) Plasticizer• The purpose of adding plasticizer is to improve the plasticity

and flexibility of plastic as well as to lessen the brittleness.

Constituents of plastic

• Plasticizer is either liquid with high boiling point and poor volatilization or solid with low melting point. Plasticizer has the disadvantage of reducing the mechanical properties and heat resistance of plastics. The common plasticizers are dibutyl phthalate (DBP), dioctyl phthalate (DOP), and camphor, etc. .

• 3.)Lubricant :- is used to make the moulding of plastic easier to prevent sticking of materials to the mould for a flawless finish. The examples are stearates, oleates and soaps.

Constituents of plastic

• 4.)Catalyst :-is added only in the case of thermosetting plastics to accelerate the polymerisation of fusible resin during moulding operation into cross-linked infusible form.

• 5.)Blowing Agent:- Sodium bicarbonate and ammonium carbonate are sometimes added to plastics to produce porous articles.

Fabrication of Plastics.

extrusion

• Extrusion:• • Using this processing it is possible to produce a wide range of different forms of

plastic,• such as tubes, sheets and films, structural parts, etc. 1º Step: Plastic pellets are

placed in• a feed hopper which feeds into the system. 2º Step. A turning screw pushes the

plastic• into the barrel where heaters increase the temperature and a melted polymer is

obtained.• 3º Step. The melted plastic is forced through a shaping die. Depending on the

particular• shape of this element, a continuous shape is formed and pulled out of the

extrusion• machine. 4º Steep. Solidification by cooling

Injection molding

Injection molding

• Plastics pellets flows, due to gravity, from the feed hopper

onto a turning screw. It is converted into a melted plastic by the action of heaters situated along the barrel. The screw moves the molten plastic forward, forcing the plastic through a gate into the cooled mould. The mould is opened once the plastic has solidified and the piece is pushed from the mould by automatic ejector pins. After we get the manufactured piece,the mould is closed and clamped and the process begins again.

Compression molding

Compression molding

• This is a similar process to extrusion. In compression moulding, plastics pellets,

• sometimes called moulding powder, are placed in the feed hopper and pushed to the

• gate by the action of the turning screw. It is heated and compressed while it passes

• through the barrel. After the gate, the molten charge is quickly transferred to a press

• where it is moulded while still hot. The part is removed after sufficient cooling.

Blow molding

Blow moulding

• Plastic grocery bags, bottles and similar items are made using this processing.

• As in compression processing, plastic pellets are melted and the plastic is forced

• through a gate into the blow pin camera. The plastic substance is expanded and cooled

• by being made to flow around a massive air bubble. After a few seconds, the mould is

• opened and the manufactured product is ready.

Blow moulding

Polymerization and condensation

• Polymerization is the process by which small molecules are joined together to form very long chains. Condensation: process by which a covalent bond forms between two or more different types of molecules and a by-product, such as water, is released; significantly alters the properties of the components.

Poly vinyl chloride

• Polyvinyl chloride plastic is a kind of common building plastic made fiom the polymerized vinyl chloride monomer. Polyvinyl chloride plastic has better chemical stability and aging resistance, but poor heat resistance. It may decompose and metamorphose if the temperature exceeds 100°C. Usually, it should be used at temperature of below 60-80°C. By adding different amounts of plasticizer, hard and soft polyvinyl chloride plastic can be made.pvc is used in wall coverings,gutters,floorings,cables and wiring,windows etc;

Reinforced plastics

• Glass Fiber Reinforced Plastics It has the properties of light weight, water resistance, high strength, corrosion resistance, and decorativeness. It is suitable to be used as lighting and decorative sheet.

• Synthetic fiber reinforced film It is the main raw material used in building aerated house. It has the properties of light weight, airtight, heat insulation, and easiness to be transported. It is suitable for the exhibition hall, gymnasium, agricultural greenhouse, temporary granary, and other kinds of temporary buildings.

Plastic laminate

• Plastic laminate is a composite building material made from kraft paper, resin, and adhesives. Multiple layers of kraft paper are soaked in resin, then pressed together at high levels of temperature and pressure to form a single sheet. Plastic laminate adhesive or resin is then used to add a decorative paper or melamine top layer. Finally, a protective urethane top coat, or wear coat is added to enhance strength and durability. Sheets of plastic laminate may be bonded to a wood or composite base using plastic laminate glue, adhesives, or resin. The laminate-topped base is then used as a countertop, floor finish, or wall paneling. The sheets of laminate themselves can also be used as a decorative finish material is a wide variety of applications.

Polythene sheets

• Polyethylene (abbreviated PE) or polythene is the most common plastic.its primary use is in packaging.PE is usually a mixture of organic compounds (C2H4)nH2 ,they only differ in the value of n.