The peroxide vulcanisation occurs by free radical mechanism and competes with the reaction of the peroxide with the other additives containing labile hydrogen atoms. The antioxidants, depending on their category, react in different ways:
- The phenolic and amine types neutralize alkoxy and peroxy radicals before they attack the polymer
- A phosphite type decomposes hydroperoxides and stops the propagation of the radicals. Consequently, there is a competition between the antioxidant and the peroxide (see fig 1 and table 1) and some antioxidants can inhibit peroxide curing. The peroxide is partially destroyed before crosslinking and the antioxidant is partially consumed during the vulcanisation and is not completely effective for the ageing protection.
Generally speaking, it is expected from the antioxidant that:
* A negligible effect on the cure level corresponds to a good ageing protection because the antioxidant is not consumed during the crosslinking.
* A big impact on the cure level leads to a poor protection.