Crystallization

A key distinguishing feature of the raw polymers is their tendency to crystallize. While virtually all liquids – water, for example – have a clearly defined temperature at which they undergo spontaneous crystallization on cooling (solidification point), some high molecular- weight products, such as natural rubber and a number of synthetic rubber grades, do not have a clearly defined crystallization point. Instead, they harden slowly as the temperature falls. With these rubber grades, this is caused by a partial orientation of the macromolecules (crystallization). This hardening overlaps the increase in hardness that results from a drop in temperature (thermoplasticity). Hardening due to crystallization is a fully reversible process that can be performed by subjecting the crystallized material to heat or dynamic stress. Unvulcanized polymers and compounds produced from them display the most pronounced tendency to crystallize. Vulcanization, however, considerably slows down the extent of hardening resulting from crystallization in rubber articles. The tendency of the polymers to crystallize can be influenced by appropriately controlled production processes. For this reason, rubber articles made from Baypren^® grades with a low tendency to crystallize display virtually no increase in hardness due to crystallization even after exposure to low temperatures for long periods. In contrast, polymers with a particularly pronounced tendency to crystallize display a clear increase in hardness even after a relatively short time.