Crosslinking

Crosslinking of Rubber

For crosslinking of rubber, peroxides provide the following advantages:

• Exceptional heat and ageing stability
• Excellent compression set at higher temperatures
• Simple formulations
• Cures also in saturated rubbers
• Nitrosamine-free-system
• Good compound shelf life

The crosslinking of rubber occurs only if the macroradicals generated by hydrogen abstraction are stable. This is not always the case. For example, polypropylene generates unstable ternary macroradicals that restabilize by a reaction called beta-scission, leading to chain degradation.

This process is advantageously used to control the rheology of polypropylene. In PVC, radicals abstract labile chlorines leading to unstable structures and chain degradation.

The table below summarizes the list of rubbers crosslinkable and not crosslinkable by peroxide.

Crosslinkable polymers	Non crosslinkable polymers
ABS EPDM EPR CPE EBA/EEA HNBR Natural rubber NBR Fluoropolymers SBR Silicone rubber Polyisoprene EVA Chlorosulfonyl rubber	PP PVC Polyisobutene Butyl rubber polyacrylate rubber

Co-agents or crosslinking activators are used to increase the crosslinking efficiency of the peroxide. Among the coagent commercially available Arkema provides:

• Triallylcyanurate (TAC)
• Triallylisocyanurate (TAIC)
• Trimethylolpropanetrimethacrylate (TRIM or TMPTM)
• Ethyleneglycoldimethacrylate (EDMA)
• m-phenylene dimaleimide
• 1,2-cis-polubutadiene
• Sulfur (do not exceed 0.3 phr)

The use of these co-agents, generally introduced at levels between 1and 3 phr, typically lead to an increase of hardness and tensile strength, a decrease of elongation at break, and an improvement of the compression set (except with sulfur)

As mentioned previously the use of peroxide leads to simple formulations. The table below compares the ingredients of two EPDM formulations cured by peroxides and sulfur vulcanization.

Peroxide cured	Sulfur cured
EPDM Filler EPDM Filler Antioxydant Coagent (TAC) Paraffinic oil Luperox® F	Stearic Acid ZnO Aromatic oil Sulfur MTB MBTS TMTM TDEC ZDBC

If you need assistance for the crosslinking of your rubber of if you need an alternative crosslinking system please contact us and an expert in this field will answer you shortly, or consult our product specifier.