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III. Control text

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The human body, all animal and plant tissues, and most build­ing substances in organic nature, such as proteins, wood, consist of polymeric or macromoiccular materials. Many minerals, such as silica and feldspar are inorganic polymers, and numerous products of an­cient and modern industry such as porcelain, glass, textiles, paper, rubber and plastics are either entirely or substantially polymeric. All these substances possess one essential common feature, they consist of very large molecules.

A polymer is a substance consisting of molecules which are multiples of low-molecular-weight units. The low-molecular-weight unit is the monomer.

Isomeric polymers are polymers which have essentially the same percentage composition, but differ with regard to the arrangement of the individual atoms or atom groups in the molecules.

A homopolymer consists of macromolecules which are formed either by a single type of unit or by two (or more) chemically dif­ferent types in regular sequence.

Copolymers are macromolecules containing two or more chemi­cally different monomeric units in a more or less irregular sequence.

Further examples can be given almost without limit. We note that most foodstuffs are non-newtonian. This is important in respect both to food processing and to the preparation of acceptable natural food substitutes.

In many cases the marketability of a polymer is due to the rhe­ologically complex behavior of the material. An example can be found in the compounding of materials for coating of surfaces. The non-newtonian behaviour of paints is an important factor in deter­mining the "brushability" of a paint. Interesting tests have been de­vised to measure this quality in the laboratory. One wishes to have a paint that will not show brush marks after drying; on the other hand, if the paint is too thin it will not adequately cover a surface. Similarly, the art of paper coating is highly dependent on the rheol-ogy of the coating material. In conventional coating applications the coating "colour", as it is called, is subjected to extreme variations of high and low shearing.

The petroleum industry uses large quantities of "drilling muds" to lubricate the drill bit and to carry rock chips out of the hole during drilling of oil wells. It is important to have muds which ex­hibit low viscosity under sharing but which are very thick at rest, thus preventing rapid settling of chips when the drilling unit is not in operation.

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 be 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.

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