Importance of foamed plastics material in plastic industry:
Versatility of foamed material:
1. Foamed materials are versatile in many forms in which the materials are used to produce the foams including the production of various shapes by casting, extruding, injection moulding, thermoforming and RIM.
2. Any plastic material, whether thermoplastic or a thermoset, may be produced in a cellular form (foam form).
3. The solid parts made by using foam in place of plastic and /or metal, the part may be improved and/or made more cheaply, simply because of savings in the material quantity and weight.
4. The parts made by using foam in place of a solid plastics part may be lighter and/or stiffer.
General production methods:
Plastics foams are principally made by following different methods:
1. Incorporating a chemical blowing agent (CBA or foaming agent) into the polymer to form a gas by decomposition at a given elevated temperature:
a) Finely powdered CBA is either evenly dispersed in a liquid resin or is mixed with moulding pellets.
b) From azo compounds (organic material), usually nitrogen gas is liberated.
c) CBAs decompose at temperatures ranging from 1100—2800C
d) The CBA should match the melting point or the processing temperature of the polymer.
e) Typical CBA is azobisformamide (ABFA), also called azodicarbonamide.
2. A gas usually N2 is injected into the molten or partially cured resin either in the barrel of an extruder or injection press or into a large mass in an autoclave. When the pressure is decreased the gas is expanded to form the cellular structure.
3. A bifunctional material (isocyanate) may be combined with polyester or other liquid polymer. During the polymerisation reaction to form a solid polymer, these bifunctional materials also reacts to liberate gas which forms the cells.
4. Volatilization of a low boiling liquid by applying heat. This heat required for the volatilization may be externally supplied or may be liberated by an exothermic reaction. Fluorocarbons (Freons) are commonly used liquids. PUR foams are usually made by this method.
5. Whipping air into a colloidal resin suspension and then gelling the porous mass. Foamed latex rubber is made by this method.
6. Incorporating a non chemical, gas-liberating agent into the resin mix which releases a gas when heated. Gas adsorbed on finely divided carbon can be used for the purpose.
7. Expanding the small beads of thermoplastic resin by heating an internally contained blowing agent. Expansion of Polystyrene beads used to make cups, packaging, mannequin heads etc is made by this method.
Applications of foamed plastics:
In very soft cushioning material used in upholstery, clothing inter layer, automobile seats, vibration absorbers etc, as electrical, thermal and acoustical insulator.
Semi rigid foams:
For floatation devices, marine bumpers, special electrical insulation on TV cables,packaging etc.
In production of airplane parts, boats, electronic encapsulation and in many furniture applications where wood was formerly used.