When rubber is cured with an organic peroxide, the share of the peroxide that actually turns into crosslinks is measured by the crosslinking efficiency E. It is a basic parameter of peroxide cure systems and is especially relevant for silicone rubber.

Definition of E

E is the ratio of the network density v of the rubber to the concentration 2[RO] of free radicals generated by the peroxide: E = v / (2[RO]).

• v: network density per unit volume of rubber, in mol/mL.
• 2[RO]: concentration of free radicals produced by peroxide decomposition per unit volume of rubber, in mol/mL.

For the same number of radicals, the more network that forms, the higher E is, meaning the peroxide is being used more effectively.

E in silicone rubbers

On heating, the peroxide decomposes into radicals. A radical abstracts hydrogen from a side group or adds to a double bond, which links neighboring chains into a three-dimensional network.

Plain methyl silicone rubber has no double bonds in its chain, and its E is about 1. In a methyl vinyl silicone rubber the double bonds make it much easier for radicals to take part in crosslinking, and E rises to about 10. A vinyl-containing gum can therefore reach the same crosslink density with less peroxide.

When specifying a peroxide cure for silicone rubber, first confirm whether the gum contains vinyl groups, and then set the peroxide level accordingly.