Here is a list of raw materials used in rubber industry.
1. Thermoplastic Polymers:
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A further broad classification into those which are used in conjunction with other polymers and chemicals and those which are used in a relatively pure state gives a further guide to recycling.
Polymers which are used on their own i.e. Polyethylene, Polypropylene, Polystyrene, etc., are relatively easy to recycle and recycling may be repeated a number of times without undue deterioration.
Blended and compounded polymers are less stable and more liable to deterioration during recycling, i.e. P.V.C. and A.B.S. Additions may be made to polymers to improve their stability but are expensive and hence are used very sparingly.
2. Thermoset Polymers:
As their name implies thermoset plastics are distinguished by the fact that during processing they are ‘cured’ and form a 3-dimensional network. Fig. 23.2 Whereas thermoplastics may be described as long thin molecules laid in a random manner but having no firm bond between the molecules, the curing process transforms thermoset plastics from a state similar to that of thermoplastics into a state in which permanent bonds are made at numerous points in the chain with other molecules.
3. Rubbers:
These are regarded as particular class of Thermosets in many respects. Until they are vulcanised, the material is largely thermoplastic in that it may be physically reshaped without major loss of properties. Vulcanising is similarly explained as a molecular cross linking but the process of de-vulcanisation has not yet been discovered.
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Because of the particular peculiarities of the rubber industry, considerable attention has been given to the problem of recycling. Two forms of product are marketed-Crumb and Reclaim. Nevertheless, the problem of de-vulcanisation remains unsolved.
4. Natural Rubber:
This material is obtained from trees and thus is to some extent, a regenerating raw material.
5. Synthetic Rubbers:
These are obtained from oils in various forms and are dependent upon reserves of oil. They are ‘tailored’ to provide particular properties such as oil resistance, flexibility at extremes of temperature, resistance to ozone, etc., and therefore cannot be regarded as capable of replacement by natural rubbers although in some respects synthetics can be used in place of natural.
6. Compounding Ingredients:
Particular optimum properties of many polymers are only obtained by compounding both with other polymers and with mineral fillers. A very wide range of properties are thus obtained by the incorporation of a large variety of compounding ingredients. This is particularly true of thermoset plastics, including rubbers and of thermoplastics such as PVC.
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P.V.C. is thus possible to be made, either as flexible sheet for clothing and packaging or, as a rigid material for building applications such as drainage piping, guttering, etc.
Thermosets are often compounded with paper, mica, clays, marble flour, etc. The quantity of polymer is then changed to meet the service conditions. Often it is reduced to a level which is just adequate to act as a binder. Price is greatly affected since most of the fillers are also cheaper than the polymer.
Rubber compounds are even more complex. Additional to the polymer and filler there are added small percentages of chemicals, often expensive ones, designed to assist or control the vulcanisation process and to provide resistance from attack (i.e., ozone).
With all these materials visual identification of the compounded ingredients in a product is impossible and difficult even with the aid of analytical techniques.