Because the free radicals generated by peroxide decomposition will then take over the hydrogen atoms on the elastomer molecular chain to generate peroxide decomposition products, while forming polymer radicals, which lay the foundation for the creation of polymer cross-linking bonds.
Since the peroxide radicals prefer to take over hydrogen atoms with smaller dissociation energies, i.e., relatively reactive hydrogen atoms, the most important side reaction at this stage is the interference of the additives with the crosslinking.
As a side reaction, the peroxide radical can also react with the hydrogen atom of the auxiliary, which consumes the peroxide radical and thus reduces the cross-linking efficiency. Therefore, additives with a large number of active hydrogen atoms should be avoided, and these additives can interfere significantly with peroxide vulcanization. For example, ordinary aromatic oil contains a large number of active hydrogen and unsaturated bonds, which will seriously affect the cross-linking efficiency, and even difficult to vulcanize, so the peroxide formulation system should be eliminated the use of aromatic oil, and priority should be given to the use of refined paraffin oil and ester plasticizers, etc.; and for example, 6PPD, BHT and other antioxidants will obviously consume peroxide radicals, should also be used with caution, in contrast, antioxidant RD (or TMQ), etc., is often recommended for peroxide systems because of the small interference with peroxide crosslinking.
At the same time, this is also one of the reasons for the different cross-linking efficiency of peroxides in different elastomers. A high number of active hydrogens in the main chain of elastomers results in high cross-linking efficiency and vice versa. For example, in NBR, BR, etc., there are a large number of double bonds and active allyl hydrogen on the main chain, which are easier to form polymer radicals and usually have higher cross-linking efficiency, while in rubber such as EPM, the cross-linking efficiency is significantly lower.