Crosslinking

Crosslinking Comparison

Crosslinking with peroxides

Crosslinking with sulfur

A peroxide crosslink is a carbon-to-carbon bond between adjacent polymer chains Simple May be used for most  polymers Confers an excellent thermal stability and a compression set Little sensitive to reversion Difficult to balance between scorch times and crosslinking speed Formulation limit: interference with the air oxygen (tacking surface), interference with antioxidants, certain plasticizers and additives

The sulfur crosslink is a bridge formed by one or more sulfur atoms between adjacent polymer chains Used for unsaturated rubbers only Excellent mechanical characteristic (tear strength, elongation at break) Sensitivity to reversion More complicated system but no limitation of formulations Linked to the stocking sometimes problematic Problems of surface stains and bad odor

Bond Energy

Higher crosslinking bond energy means generally better resistance to heat modifications. That's the reason why organic peroxide crosslinked articles resist better to high temperature aging and compression set. Sulfur cured articles are more elastic, as they contain longer sulfur bridges. This means better values of tensile strength and elongation at break. But at elevated temperatures those bridges break easily. They are also more sensitive to chemical aggression and ozone oxidation.

Bond energy of sulfur and C-C bridges:

		Bond type	Bond energy (Kcal/Mole)
Improved heat resistance	Conventional sulfur peroxide donor	- C - Sn - C - - C - S - S - C - - C - S - C - - C - C -	< 64 64 68 84