Vulcanization retarders and inhibitors have been employed in rubber compounds for many years as a means of enhancing processing safety. A vulcanization retarder extends the scorch time, thus improving processing safety, but it also slows down the rate of vulcanization. However, this reduced rate of vulcanization is often an undesirable effect. On the other hand, true vulcanization inhibitors increase scorch safety without adversely affecting the rate of vulcanization.

Substances falling into the first category are acidic in nature, such as benzoic acid, salicylic acid, and phthalic anhydride. They prolong scorch time in thiazole-accelerated compounds, but their effect in sulfenamide-accelerated stocks is poor.

N-chlorosuccinimide, nitroparaffins, sulfenic acid, and sulfonic acid derivatives have been described as retarders, but they appear to have little practical significance.

N-nitrosodiphenylamine (NDPA) has a positive effect on the processing safety of compounds accelerated with sulfenamides. It prolongs the induction period regarding the onset of cure but does not reduce the rate of vulcanization. Hence, it can be classified as a vulcanization inhibitor. However, it is no longer in use due to toxicological concerns.

The major and most effective representative of the class of vulcanization inhibitors is N-(cyclohexylthio)phthalimide (CTP), often referred to as a pre-vulcanization inhibitor (PVI). It is effective with a wide range of polymers, accelerators, and other compounding ingredients. It neither affects vulcanizate properties nor causes staining or porosity. While it is most effective in sulfenamide-accelerated stocks, it is also used with both MBT and MBTS.

There is linearity in its response, allowing for easy determination of the dosage required to achieve a certain scorch resistance. In most applications, a dosage of 0.1-0.3 phr (parts per hundred parts of rubber) is used. The scorch resistance increases proportionally with the dosage, and an addition of 0.1 phr leads to a significant improvement. Furthermore, the addition of PVI enables processing at elevated temperatures, thereby increasing productivity.

While PVI is typically added during the mixing stage, it can also be introduced to compounds during processing when there is a risk of premature vulcanization. By doing so, additional processing safety is provided. However, PVI is not effective in thiuram-accelerated stocks or resin and metal oxide curing systems.

In butyl-based compounds, dosages should be adjusted upward from the given range, possibly up to 1 phr. There are other commercially available products, but they are less effective in terms of providing scorch resistance. Examples include N-isopropylthio-N-cyclohexyl-2-benzothiazolylsulphonamide and N,N’,N”-tri(thioisopropyl)-N,N’,N”-triphenylphosphorotriamide.

Typically, a dosage of 0.1-0.2 phr of CTP is generally sufficient. It is important to avoid excessive dosage of CTP as it can delay the cure time or result in an undercured product. The effect of CTP on scorch time shows a linear relationship with its dosage. CTP demonstrates its highest effectiveness with the fastest curing polymers, with the approximate order of response being NR > NBR > SBR > EPDM > IIR > CR.