To give desired properties to the finished plastic articles, certain moulding compounds are to be added to the plastics.
Following are such moulding compounds:
(1) Catalysts: These compounds are added to assist and accelerate the hardening of resin. For instance, the ester acts as catalyst for urea formaldehyde. They are used for quick and complete polymerization.
(2) Fillers: The fillers are inert materials and they impart strength, hardness and other properties to the plastics. The choice of a filler should be carefully made. It should be confirmed that the addition of a filler does not have detrimental effect on other properties of plastics. The fillers may be used in the following forms:
(i) Fibrous fillers: They increase thermal resistance and impact strength of plastics. They also increase strength and reduce brittleness of plastics. They are widely used in the manufacture of plastics. The commonly used fibrous fillers are asbestos, wood and glass Fibres.
(ii) Laminated fillers: They make the plastics very strong. The commonly used laminated fillers are papers, wood veneers, asbestos cardboards, cotton, etc.
(iii) Powder fillers: They provide valuable properties to plastics such as acid- resistance, water-resistance, etc. They also lead to increase in durability, improvement in hardness and reduction in cost. The commonly used powder fillers are quartz powder, chalk, wood flour, etc.
(3) Hardeners: These compounds are added to increase the hardness of resin. For instance, the hexamethylene tetramine acts as hardener for phenol formaldehyde.
(4) Lubricants: The lubricants are applied on the surface of moulds so that the articles of plastics do not stick to the moulds. The application of lubricants on surface of moulds allows easy removal of articles of plastics from the moulds. The commonly used lubricants are graphite, paraffin, wax, etc.
(5) Pigments: The addition of dyes and pigments helps in two ways, namely they act as fillers and they impart desired colour to the plastics. They should be durable and adequately fast to light. The commonly used pigments are zinc oxides, barytes, etc. The selection of pigments should be done in such a way that their addition does not alter or affect the other properties of plastics.
(6) Plasticizers: The plasticizers are the organic compounds which are oily in nature and of low molecular weight. They are used to separate the polymer chain by à¸ª greater distance to make i e crystallization difficult. A non-crystalline solid is thereby produced from a polymer that normally crystallizes. These compounds are added to improve plasticity and to impart softness to the plastics. They give flexibility to the material and act as lubricants. They should be chemically inert, poorly volatile and non-toxic.
The addition of plasticizers facilitates the moulding process of plastic articles. The commonly used plasticizers are camphor, triacetin, tributyl phosphate, etc. The commonly used plasticizers are camphor, triacetin, tributyl phosphate, etc. The proportion of plasticizers in pasties should not exceed 10 per cent. Otherwise the strength of pasties will decrease.
(7) Solvents: These compounds are added to dissolve the plasticizer. For instance, the alcohol is added in cellulose nitrate plastics to dissolve camphor.
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