The alterations which may take place in rubber as a result of keeping are of two distinct types ; the first is a relatively unimportant effect known as " freezing," which develops at low temperatures and disappears on warming, and the second a fundamental change in the physical and chemical properties of the material which sometimes occurs as a result of atmospheric oxidation. " Freezing" however, is essentially a characteristic of unvulcanised rubber, and the second phenomenon is the only one which is of interest to the user of rubber goods. When rubber oxidises or " perishes," its strength and elastic properties rapidly deteriorate, a sour acid odour often develops, and finally it becomes converted into a hard, brittle resin." Perishing " was one of the first of the many technical difficulties encountered by the two great pioneers, Thomas Hancock and Charles Goodyear, whose inventions brought the rubber industry into being, and if the mechanism of the change is not yet fully understood, the manufacturing precautions which have to be observed to ensure a durable product are, at least, common knowledge as a result of the practical experience of more than a century.

While the manufacturer is alive to what may occur it his products are of unsuitable composition, or over-vulcanised, the user of rubber goods is probably less fortunately placed, and may unwittingly subject his property to unsatisfactory treatment. A more general knowledge of the possibilities and limitations of rubber in its various forms must therefore be all to the good, not only in widening the field of its applications, but in helping the consumer to select suitable qualities and employ these to best advantage. It is for this reason that the present notes have been written.

Apart from over-vulcanisation, the two most potent factors in bringing about deterioration are exposure to light and high temperatures. The first consideration in selecting a store for rubber goods, therefore, should be the absence of direct sunlight and a relatively low and uniform temperature. It has been found, moreover, that the presence of moisture exerts a decidedly beneficial effect on the keeping properties of the material, so that an atmosphere possessing a high relative humidity is decidedly advantageous. Under such conditions the storage problem becomes a comparatively simple matter.

There are, however, some other points which it is well to keep in mind in this connection. For some reason-as yet imperfectly understood- -compounds of copper exert a most deleterious effect on the stability of rubber, and direct contact with this metal or its alloys is therefore highly undesirable. The same probably applies in a lesser degree to the effect of certain oils, and even if the influence of these on ageing is slight, the small amount absorbed by the surface of the rubber will somewhat impair its physical strength and render it less resistant to abrasion. Turpentine, on the other hand, undoubtedly promotes oxidation, and for this reason it is quite unsuitable for the purpose of cleaning or polishing rubber surfaces-for example, printers' blankets or rubber flooring.

Finallv, attention may be drawn to the fact that rubber articles age more rapidly when maintained under tension-an effect which is more physical than chemical in nature. Goods such as garments or motor tubes should not be kept stacked for prolonged periods in a folded Condition, as cracking is likely to develop at the sharply curved surface. To sum up, the general precautions which must be observed to keep rubber articles in good condition over prolonged periods may be taken to be as follows :-

(1) ;The goods must be kept at a relatively low temperature, and protected from exposure to bright light.

(2) ;The relative humidity of the atmosphere should be kept high.

(3) ;Contact should be avoided with substances such as copper compounds and turpentine, which exert a deleterious effect.

(4) ;Sharp folds or local tension should be absent.

No hard and fast rule can be laid down, however, as to what constitutes satisfactory storage or satisfactory conditions of use, in view of the wide variation which is possible in the character of rubber products to meet special requirements. For example, rubber balloon fabric can be obtained which will resist exposure to tropical sunlight for long periods ; rubber hose is widelv used for the transmission of steam and hot liquids ; while printers' " offset " blankets will withstand the effect of oil in the ink for months without showing any sign of deterioration.

But a material which is eminently suitable for one purpose may fail completely when put to another use, and the quality of rubber just mentioned, which gives such excellent service in balloon fabric, would deteriorate rapidly if subjected to the action of printers' ink. Those who desire to employ rubber to best advantage must remember that the name denotes not one substance but a multitude of products from which must be selected that best adapted to the specific requirement. (The Rubber Growers' Association.)

Some Terms Used In The Rubber

Industry

Accelerators - - Organic and inorganic substances which are added in small amounts to rubber mixings to reduce the time occupied in vulcanisation or permit the process being carried out at a lower temperature, and also to improve the properties of the finished product.

Acetone Extraction

A test fre quently used in the examination of vulcanised rubber goods. The extract contains, in addition to the natural resins of the rubber, free sulphur, softeners, and certain other organic compounding ingredients.

Ageing

The gradual changes in physical and mechanical proper-lies which occur in vulcanised rubber on keeping.

Ageing Teste

A variety of methods have been employed as rapid tests for the durability of rubber goods, those involving acceleration of the ageing process by heating in air at 70° C. or in oxygen under pressure being the best known. The results, however, are not always reliable as an index of behaviour under normal conditions of use.