Rubber properties : Freezing

The effects of temperature so far described take place instantaneously with any change of temperature. There is , however at least with certain types of rubber including natural rubber, another effect which is produced by long exposure to moderately low temperature. This is a gradual hardening, often referred to as ‘freezing’ , due to the tendency of part of neighbouring molecules to line up in parallel ; this permits stronger intermolecular attractions which bind the parallel molecule segments into a more or less rigid ‘crytallite’ , to use a familiar, though not very accurate, name. This process is quite gradual and proceeds over periods of many days; it is most rapid at a particular temperature which in the case of natural rubber about –25 *C . Vulcanisation reduces, though it does not eliminate, this technically undesirable effect, which must be noted especially in connection with the storage of rubber articles. The change from rubber-like to glass-like properties occurs at a temperature called the glass transition temperarture. Certain other properties also change suddenly at this temperature, for instance, the coefficient of thermal expansion is less below the glass transition temperature, but rubber is never used in practice below this temperature. Another inportant consequence of the influence of temperature on deformation rate is that, broadly speaking, the behaviour of rubber is change in the same way either by increasing the rate of deformation or by lowering the temperature; conversely, reducing the deformation rate produces the same effect as raising the temperature. There is, in fact, a fundamental relationship between temperature and strain-rate effect.

Cellular rubber (Foams)

There are two processes for the manufacture of foams from natural and synthetic latices. The introduction of air or gas into the latex to create foam is common to both processes. This foam is then gelled, cast into moulds, vulcanised and dried. With latex beating the latex mix is suspended in soap or gelatine and beaten from a 7- to 14-fold vulcanisable volume. For this machine wire beater with a selected steplessly variable beater speed is used. Gelling with sodium silicate fluoride keeps the foam for an extended time in a castable condition. The foam is cast into a mould and vulcanised at approximately 373 K . The cured foam rubber is removed from the mould washed free of clinging chemicals and dried.

With blown latex, hydrogen peroxide is added together with blowing agents to a vulcanisable latex mix which is decomposed. The created oxygen foams up the volume of the latex from 8 to 14 fold, the foam consisting of small regularly spaced cells. It is frozen at 258 to 263 K and carbon dioxide is passed through the rigid foam. After thawing out, the foam becomes liquid. It is then vulcanised, washed and dried in the usual way.

The manufacture of plastic foams is basically different from natural and synthetic latex foams. Polystyrene foam contains a blowing agent which expands by heating the mass to over 350 K and produces a cell structure. With polyether or polyester respectively the foam structure is built by a chemical reaction which releases carbon dioxide; the mass rises and sets simultaneously.

Rubber properties : Endurance limit

The strength of rubber under a continuous oscillating stress is of particular significance because such an application occurs so frequently in practice. By endurance strength is meant that stress which a rubber can sustain indefinitely under an oscillating load without damage. It is experimentally ascertained with help of suitable fatigue-tensile machines through a stress-frequency curve. Modern rubber testing machines permit the rubber to be subjected to a loading which corresponds to the one occurring in practice. Thus a good insight is achieved into the elastic and thermal relationship, especially into the fatigue life.