Vulcanization

High energy radiation – радиация с высокой энергией.

2. Read the text and be ready to say if the following sentences are true or false. Use the following phrases:

As far as I know …

It seems to be wrong (right) …

I can’t agree with you …

I’m afraid you are mistaken …

On the contrary …

That’s right …

1. Charles Goodyear tried to improve the properties of rubber.

2. Vulcanization is a process of making raw rubber useful.

3. Vulcanization makes weak bonds between the long linear molecules of the rubber polymer.

4. Selenium is the most used vulcanizing agent.

5. Antioxidants are used for final product protection against the effects of light, heat and oxygen.

Starting in 1831, an American Charles Goodyear carried out a great many experiments in attempts to improve the properties of rubber. He tried to discover a ‘drier’, which would prevent manufactured goods from becoming sticky. In 1839 he made an observation, which was to revolutionize the industry he found that if crude rubber is treated with sulphur above the melting point of the latter, it undergoes a change, which produces a marked improvement in its mechanical properties and also in its ability to withstand changes in temperature.

Vulcanization is a process used to make raw rubber useful; it decreases sensitivity to extremes of temperature, confers resistance to flow under stress, and stabilizes rubber’s valuable properties; e. g. abrasion resistance, resilience, and impermeability to fluids. Chemically, vulcanization creates strong bonds between the long linear molecules of the rubbery polymer, efficiently fixing the molecules’ position with respect to one another, and preventing their slippage under stress. Solid rubber is prepared for vulcanization by a process known as compounding, i. e. mixing with other ingredients in internal mixers which uniformly blend the rubber and the added ingredients. Sulphur is the most used vulcanizing agent. Other rnaterials commonly added before vulcanization include: accelerators to speed vulcanization, zinc oxide and fatty acid to improve properties, carbon black or other fillers to strengthen the final product, and antioxidants to protect against effects of light, heat, and oxygen.

The fully compounded rubber is extruded or calendered (depending on the properties wanted) to the final form, and then vulcanized. This usually is done in a mould with the aid of heat, although some mixtures permit cure at room temperature.

Vulcanizing agents besides sulphur are often used, particularly for some heat and solvent resistant rubbers, including peroxides, modified phenols and metallic oxides. Some types of rubber have been vulcanized experimentally by high energy radiation.

3. Make up the plan to the text.

4. Give a brief summery of the text.

Text D

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

a) Storage Hardening.

b) Hardening.

c) Rubber hardness.

The presence of microgel particles in natural rubber latex has a directly practical consequence in the phenomenon of storage hardening. Rubber, either in the form of latex or solid sheet, when stored for considerable periods develops an increased hardness (as measured by Mooney viscosity or Williams plasticity), the change in solid sheet being generally greater the lower the initial hardness.

The changes are greatly influenced by the relative humidity of storage. In dry air the rate at which hardening occurs increases with decreasing sample thickness or with increasing temperature. The removal of acetone-soluble and nitrogenous components does not prevent the hardness increase.

One might expect that the increase in hardness which occurs when ammoniated latex is stored is a result of intra-particle crosslinking and microgel formation. With solid rubber the crosslinking process is no longer confined to within the original latex particles, but there is no reason to suppose that the mechanism of the crosslinking reaction differs in the two cases.

Although the nature of the spontaneous hardening reaction remains obscure considerable information about the functional groups involved and on methods of control have recently been forthcoming. Rubber hydrocarbon interacts with oxygen to form hydroperoxidic compounds and the decomposition of such compounds could result either in degradation or crosslinking of the rubber molecules, according to conditions. However, crosslinking via hydroperoxide decomposition does not appear to be a major cause of storage hardening. Since hardening can be almost completely suppressed by the addition to latex of monofunctional carbonyl reagents (especially dimedone which is a specific reagent for aldehyde groups) it has been inferred that the crosslinking reaction involves carbonyl groups in the rubber. The number of such carbonyl groups per rubber molecule can be estimated by measuring the concentration of hydroxylamine required to inhibit completely storage hardening; values of 9-29 were found for a number of clonal rubbers. Difunctional amines, such as benzidine, in contrast to their monofunctional analogues, promote crosslinking by coupling reactions.

If it is accepted that storage hardening results from the presence of carbonyl groups in very small proportions, it is then desirable to determine how these groups originate. It has been suggested that they are incorporated at intervals along the “hydrocarbon chain”, but it has still to be shown that they are an integral feature of natural rubber molecules. Biologically induced oxidation could proceed in the vicinity of the tapping cut or in the latex vessels. The generation of oxygenated groups in secondary reactions is consistent with our present knowledge of polyisoprene biosynthesis.

2. Write out:

a) the key word and terms;

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

Дата добавления: 2014-10-17