In silicone addition curing, inhibitors must be used to exclude the vinyl groups from the platinum at temperatures around room temperature to have enough pot life. At elevated temperatures, e.g. above 80°C, these inhibitors evaporate and dissociate from the platinum, or react with the SiH and become part of the cured matrix. In either case, they can no longer exclude the polymer vinyl groups, and cure takes place rapidly.
Such inhibitors include various acetylenic alcohols, materials containing double and triple carbon–carbon bonds, various unsaturated dicarboxylic acid esters, and some ketoximes, as shown in the following figure.
Addition cure mechanism and inhibitor examples. Sources: Technology of Pressure-Sensitive Adhesives and Products; page 428
Comparison between different types of inhibitors
It is also worthy to mention a “catalyst poisoning” phenomenon. Materials such as amines, suldes, phosphides, and the organotin salts used as catalysts in the condensation system form somewhat stronger bonds with the platinum and are generally referred to as poisons. Their presence makes it difficult, if not impossible, to cure an addition-curing coating. If both condensation cure or addition cure process are adopted to make silicone products in the manufacturing line, this poisoning could occur. Special care, e.g. thorough cleaning, is required when switching between different curing processes.