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Text c.

Plastics have established themselves as active competitors to the older materials of construction, even on the score of price, and the rapid growth of the plastic industry is attributable in part to the uti­lization of this group of materials for industrial construction. Of the vinyl resins, polyvinyl chloride is perhaps most widely used and is fabricated in sheets and drawn in tubes by welding. Sharp-tipped tools and high machine speeds produce the best results with this class of material. However, where dimensional stability is of paramount importance the filled phenolics still lead the field. This material is used as the material of construction not as a lining, even to very large units of equipment. By selecting the filler this may be produced as a thermal and electrical insulator or conductor. The material is fabricated by moulding, so that roughly speaking, any detail that can be cast in iron can Ъе formed from a filled phenolic, but where flatsided vessels are involved, plates are fabricated by join­ing with chemically accelerated cements, and this can be done in situ.

Exercise 18 .Discussion points :

1.The origin of polymers .

2.The main properties of polymers .

3. The application of polymers.

Supplementary reading Text d .The polymer industry

The unusual flow properties of polymer melts and solutions, to­gether with the desirable attributes of many polymeric solids, have resulted in development of the huge worldwide industry of polymer processing. We have already referred to the manufacture of synthetic fiber from polymer melts. In a typical installation, fiber is made by forcing a molten polymer, such as nylon, through a die containing perhaps a few hundred holes, each with a diameter of approximately 0.01 in. As individual filaments of molten polymer are drawn away from the die, they are cooled by the surrounding air and are simul­taneously stretched to a smaller diameter. Following cooling and so­lidification the filaments are weld together to form a composite fila­ment on a bobbin or take-up reel. Filaments speeds of 5000 ft/min are not uncommon.

Large parts of automobiles and domestic appliances are often formed by injection molding. This is a highly unsteady process in which a molten polymer is forced into a mold and then allowed to solidify. Often the whole process is repeated by a machine at intervals of only a few seconds.

Text e .Plastic glasses

Sunglasses with plastic lenses weigh less than those made of glass, but they scratch easily. The SGL Homalite division of SGL Industries, Inc., Wilmington, Del., has a new plastic lens material. It is scratch resistant and half the weight of tempered glass. The ther­mosetting co-polymer is a sheet material labeled H-91M, and it con­tains additives that permit it to be easily dyed and formed into a lens shape. The material can be coloured with a permanent surface dye and can accept almost any colour and transmission level includ­ing colour gradations. Available in sheets up to 37 by 63 inches the lens blanks are pre-heated and formed in standard doming machines.