8 - The welding of plastics

Summary

Plastics

Plastics are replacing steel and other metals used in construction. They are now used in the home, in gas engineering, farming, sports equipment, and in the automobile industry etc. Equipment made of plastic is lighter than that made of steel, and since welding and fabrication is relatively easy and the cost of repair lower, it will be well to consider some of the processes involved.

Plastics are derived mainly from oil, coal and wood. Polystyrene and polyvinylchloride (PVC) are examples of those derived from oil and coal while the cellulose types (such as cellulose acetate and ethylcellulose) are produced from wood. Plastics consist of larger molecules built up into chains arranged randomly, from single molecules (monomers) by a process of polymerization, assisted by a catalyst. (A catalyst is a substance that accelerates a chemical reaction but undergoes no permanent change during the reaction.) If differing monomers are used, copolymers are produced from which the characteristics of the original monomers cannot be obtained.

There are two main types of plastic: (1) thermo-setting and (2) thermoplastic. A thermo-setting plastic is one that softens when first heated and is then moulded into the required form. Upon cooling it sets hard and gains its full strength. Upon reheating it does not again soften and thus cannot be reshaped. An example of a thermo-setting plastic is that made from phenol and formaldehyde, which with an acid catalyst gives a soluble and fusible resin used in varnishes, and with an alkaline catalyst gives an infusible and insoluble resin (Bakelite).