It is now a common practice to use sulfur in conjunction with several other additives. First amongst them are accelerators, which are of many types. In the absence of an accelerator, about 8-10 parts of sulfur is required and the vulcanization can prolong for hours; much of the sulfur is consumed in intramolecular rather than crosslinking reactions. Use of about 1 phr accelerator with 2-3 phr sulfur, enables effective vulcanization to occur not only in shorter time (which in extreme cases may be seconds rather than minutes), but also gives vulcanizates with much better properties.
Though accelerating effect of aniline on sulfur vulcanization was discovered by Oenslager in 1906, its reaction product with carbon disulfide, thiocarbanilide, was first introduced as an accelerator in 1907. Further development led to guanidine accelerators. Thiurams and dithiocarbamates were first used as accelerators in 1919. Though these are the most active accelerators in respect of both crosslinking rate and extent, they give little or no scorch resistance and therefore cannot be used in many commercial processes. Thiurams, dithiocarbamates and xanthates find practical applications in latex technology where problems of scroching are less likely to occur .
It is important that the vulcanizing system should give not only a rapid and effective crosslinking at the desired vulcanizing temperatures but also that it should resist premature vulcanizing (scorching) at the somewhat lower temperature that may be required to mix, extrude, calender or subject the rubber to other shaping operation before crosslinking.
