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.