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Properties of plastics

Er. Parbhakar Dwivedi
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To appreciate the importance of plastics as an engineering material, it will be interesting to study its some of the outstanding general properties. It may however be remembered that each plastic material has its own peculiar properties to suit its particular uses. The success of plastic as an engineering material for a particular purpose will depend upon the correct choice of the variety of plastic.

Following are the propertis of plastics:

(1) Appearance: Some plastics are completely transparent in appearance. With the addition of suitable pigments, the plastics can be made to have appearance of variety of attractive, opaque, stable and translucent colours.

(2) Chemical resistance: The plastics offer great resistance to moisture. Chemicals and solvents. The degree of chemical resistance depends on the chemical composition of plastics Many plastics are found to possess excellent corrosion resistance. Hence they are used to convey chemicals.

(3) Dimensional stability: This property of plastic favours quite satisfactorily with that of other common engineering materials.

(4) Durability: The plastic lacks ductility. Hence its members may fail without warning.

(5) Durability: The plastics are quit durable, if they possess sufficient surface hardness. The plastics, especially thermo- plastic varieties, are likely to be attacked by termites and rodents. But the danger of such an attack is not very serious due to the fact that the plastics have no nutritional value.

(6) Electric insulations: The plastics possess excellent electric insulating property. They are far superior to ordinary electric insulators.

(7) Finishing: Any surface treatment may be given to the plastics. It is also easy to have technical control during its manufacture. It results in mass production of plastic articles with uniformity of surface finish.

(8) Fire-resistance: The plastics are organic in nature and hence all plastics are combustible. But, depending upon the structure, the resistance to high temperature and fire varies considerably among various varieties of plastics. The cellulose acetate plastics burn slowly. The polyvinyl chloride plastics are non- inflammable. The phenol formaldehyde and urea formaldehyde resist fire and they are used as fire-proofing materials.

(9) Fixing: The plastics can be easily fixed in position. They can be bolted, clamped, drilled, glued, screw-threaded or simply push-fitted in position.

(10) Humidity: The properties of plastics are governed to some extent by humidity. The strength of plastics containing water attracting groups such as cellulosic plastics, is considerably affected by the presence of moisture. On the other hand, the plastics which do not contain water attracting groups such as polyvinyl chloride plastics, offer great resistance to the moisture.

(11) Maintenance: It is easy to maintain plastic surfaces. They do not require any protective coat of paints.

(12) Melting point: Most of the plastics have low melting point and the melting point of some plastics is only about 50°C. They cannot therefore be used in positions having high temperatures or to convey boiling water. In general, it can be said that the coefficient of thermal expansion of plastics is about ten tines than that of steel. The thermo-setting varieties of plastics are less susceptible to heat than thermo-plastic varieties of plastics. To improve the resistance of plastic to heat, the glass fibre reinforcement may be added in the structure of plastics.

(13) Optical property: Several types of plastics are transparent and translucent.

(14) Recycling: The most environmentally aware people condemn the use of plastics for the amount of pollution caused by them in disposal. However this is not a serious problem in comparison to the waste and pollution generated by a host of other industries. The plastics used for soft-drink bottles, milk and juice bottles, bread bags, syrup bottles, coffee cups, plastic utensils, etc. can be conveniently recycled into carpets, detergent bottles, drainage pipes, fencing, handrails, grocery bags, car battery cases, pencil holders, benches, picnic tables, roadside posts, etc.

(15) Sound absorption: The acoustical boards are prepared by impregnating fibre-glass with phenolic resins. This material has absorption coefficient of about.

(16) Strength: An ideal section of plastics for structural member has yet not been designed. The plastics are reasonably strong. The strength of plastics may be increased by reinforcing with various fibrous materials. The plastic members can be used as tensile members as their strength to weight ratio in tension very nearly approaches to that of metals. But the following considerations are responsible to discourage the use of plastics as the structural material:

(i) The plastics are costly.

(ii) The plastics are subject to creep under constant heavy loads.

(iii) The behaviour of plastics is very sensitive to the changes in temperature.

(iv) The stiffness of plastics is very poor.

(17) Thermal property: The thermal conductivity of plastics is low and it can be compared with that of wood. The foamed or expanded plastics are among the leading thermal insulators. (18) Weather resistance: Only limited varieties of plastics can be exposed to weather. The important group of plastics which can resist weather effects is one prepared from phenolic resins. The certain plastics are seriously affected by ultraviolet light in the presence of sunlight. The resistance to sunlight of such plastics can be improved by incorporating fillers and pigments which absorb or reflect the ultraviolet light at the surface. Thus the interior of plastics is protected.

(18) Weather resistance: Only limited varieties of plastics can be exposed to weather. The important group of plastics which can resist weather effects is one prepared from phenolic resins. The certain plastics are seriously affected by ultraviolet light in the presence of sunlight. The resistance to sunlight of such plastics can be improved by incorporating fillers and pigments which absorb or reflect the ultraviolet light at the surface. Thus the interior of plastics is protected.

(19) Weight: The plastics, whether thermo-plastic or thermo-setting, have low specific gravity, the average being 1.30 to 1.40. The light weight of plastics reduces the transport costs and facilitates fixing.

The Indian Petrochemicals Corporation Ltd. (IPCL), near Baroda, has put a commercial brand of plastics, known as the ‘Koylene’. It is the lightest of all commercial known plastics and it is available in a wide range of grades. It is tailor-made to suit various applications such as automotive and scooter parts, box strappings, industrial woven fabrics, ball pen refills, drinking straws, etc. This material possesses rigidity, good gloss and ability to withstand temperatures upto 100°C and easy processibility.

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