Rubber latex

Latex, before the advent of synthetic rubber, was a term applied solely to the milk of tropical plants and rubber trees which are known to yield a rubber latex, a natural emulsion of rubber particles.

Today, latex is a term also used to refer to any rubber-like polymer in emulsified form.

The rubber molecule is in the form of a very long hydrocarbon chain.

It may have a molecular weight as high as 250,000. In natural rubber the molecules are interwined and held together by van der Waals forces to give “drops” of rubber about 3 μ in diameter. These drops are suspended in water, when first obtained from the plant, the system resembling an emulsion. Proteins, fats, soaps and other substances are present in the milky fluid, which is called “latex”.

The usual process applied to rubber latex is to add acid, usually formic, which renders the emulsion unstable and coagulates the rubber. A spongy mass is produced which is passed through rolls to squeeze out excess latex and to form a sheet of rubber. The sheet is usually “smoked” with the fumes from burning green wood, which helps to preserve the rubber. This compound is called “crepe rubber” and is the raw material for most rubber manufacture,

It is necessary, for certain purposes, to make use of the latex itself as a raw material. This necessitates two modifications to the natural latex. Firstly a preservative must be added to suppress bacterial action, which would eventually lead to coagulation. This is achieved by the addition of ammonia. This prevents the development of acidity, which is the means of causing coagulation. The second necessity is to concentrate the latex. The natural product contains only 35 per cent rubber suspended in about 60 per cent of water. Concentration may be effected by using a centrifuge, in a manner similar to the centrifuging of to give cream. It has recently been shown that the addition of konnyaku meal, which is a complex carbohydrate soluble in water, to latex is very effective in producing a rich cream of high rubber content. Rubber is lighter than water and hence forms a cream and not a sediment.

The creaming is done in a tank. The initial effect of the creaming agent is to cause a clustering of the rubber particles. The resulting large effective size of the units reduces their Brownian motion and increases the speed with which they rise through the liquid. Clustering differs from coagulation in that it is reversible, suitable changes being able to redisperse the particles. This is of importance since coagulation would prevent the use of latex for special methods of manufacture. Soon after the addition of the creaming agent to the tank, a very deep cream layer is formed, which is built of clusters linked together, with water filling the spaces. Water then passes out of the cream layer, the lower boundary line gradually ascending. After about four days the liquid may be run off from valve A and the latex cream from valve B. Proteins and swollen meal particles may then be centrifuged off, and the cream is stabilized with ammonia for shipment in drums.

The colloidal behaviour of latex rubber is largely the result of the protective layer of protein with which the particles are surrounded. Clustering is influenced by the action of the creaming agent on this surface layer. Coagulation occurs when the acidity is such as to render the adsorbed protein molecules electrically neutral. These facts are closely related to the protective action of proteins on gold sols.

3. Divide the text into some logical parts and entitle them.

4. Translate the following sentences into English using lexical material of the text:

1. В настоящее время синтетические каучуки широко используются как для замены натурального каучука, так и для изготовления изделий со специальными свойствами.

2. Известно, что латекс, полученный из гевеи, представляет собой дисперсию частиц каучука в воде.

3. Действие агента сливкоотделения на первой стадии заключается в том, что он вызывает агрегацию каучуковых частиц.

4. Агрегация, как известно, представляет собой коагуляцию, которая может быть обратима, а именно: при соответствующих условиях агрегаты глобул могут распадаться.

5. Установлено, что коагуляция по своему существу является не химическим, а физическим процессом.

6. Нагрев, замораживание, интенсивное перемешивание и т.д. относятся к внешним воздействиям, обусловливающим коагуляцию.

Text C

1. Pay attention to the following words and word combinations:

crude rubber – необработанный каучук;

sulphur – сера;

melting point – температура плавления;

raw rubber – сырой каучук;

extremes of temperature – резкие изменения температуры;

resistance to flow under stress – сопротивляемость текучести при нагрузке;

abrasion resistance – сопротивление истиранию;

resilience – эластичность;

impermeability to fluids – непроницаемость для жидкости;

linear molecules – линейные молекулы;

slippage under stress – скольжение под нагрузкой;

compounding – зд. наполнение (приготовление смеси);

internal mixer – закрытый резиносмеситель;

accelerator – ускоритель;

zinc oxide – окись цинка;

fatty acid – жирная кислота;

carbon black – сажа;

filler – наполнитель;

antioxidant – противоокислитель;

to extrude – шприцевать;

to calender – каландровать, каландрировать;

heat and solvent resistant rubbers – каучуки, устойчивые к перемене температуры и растворителям;

peroxide – перекись;

modified phenols – модифицированные фенолы;

Дата добавления: 2014-10-17