Some applications of a polymer

In many cases the marketability of a polymer is due to the rheologically complex behaviour 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 rheology 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 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.

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.

3. Make up the plan of the text.

4. Give a brief summary of the text.

Text D

1. Read the text and choose the most suitable title out of the given once:

a) The “Big Six”.

b) Six Polymers.

c) Plastics.

In spite of polyethylene’s range of properties and many uses, it cannot fill all the roles we assign to plastics. It melts at a low temperature, it is permeable to gases, it swells in the presence of oil or organic solvents, it is very transparent, and it is very expensive to make polyethylene crystalline enough to be exceptionally rigid and strong.

Today, more than 60,000 plastics are known. Most have been developed for special purposes ranging from fry pan coatings to resins for restoring antiques. Yet, the two types of polyethylene (LDPE and HOPE) and four other polymers make up the bulk of the plastics you regular encounter. Approximately 20 million tons of these six polymers are made annually in the United States, and they account for about 63 % of all plastics used in this country. In addition to polyethylene, the other four plastics are polypropylene, polystyrene, polyvinyl chloride (PVC), and polyethylene terephthalate (PET). All are ultimately based on petroleum.

No discussion of polymers can be complete without including one of the most important classes of natural polymers and the synthetic substitute that brilliantly duplicates some of the properties of the natural material. The naturally occurring polymer is protein. Actually there is a wide variety of these biological macromolecules that make up our skin, hair, muscle and enzymes. All proteins are polyamides, which are polymers of amino acids. As the name suggests, molecules of amino acids contain both amine groups (-NH₂) and acidic groups (-COOH). A general formula for an amino acid is given below. The amine and acid groups are both attached to the same carbon atom. In addition, a hydrogen atom and another group (represented by an R) are bonded to the same carbon.

The 20 amino acids found in most proteins differ in the identity of the R group. In some amino acids, R consists of carbon and hydrogen atoms, as in alanine, where R is a methyl group. In others, R also includes oxygen, nitrogen, or sulfur atoms. Some R groups are acidic and some are basic.

2. Write out:

a) key worlds and terms;

b) the sentences expressing the main idea of each paragraph.

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