Saturated rubbers have no double bonds, so sulfur cannot crosslink them. Peroxides can. A peroxide breaks down when heated into reactive radicals. The radicals pull hydrogen atoms off the polymer chains, leaving polymer radicals that join to form carbon-carbon crosslinks. Carbon-carbon bonds are more stable at heat than the sulfur bridges of sulfur cure, which is one reason peroxide-cured materials give better heat ageing and compression set.
Why a peroxide alone is not enough
For many polymers, a peroxide on its own wastes much of its radical output on chain scission and side reactions. A coagent is a reactive molecule, such as triallyl isocyanurate (TAIC), triallyl cyanurate or a multifunctional methacrylate, that takes part in the radical reaction and builds a more efficient network.
- In FKM and FEPM, TAIC is the standard coagent, and the network will not form well without it.
- In HNBR, coagents can be used to raise crosslink density. Metal acrylates such as zinc diacrylate and zinc dimethacrylate give very high hardness and strength by forming ionic crosslinks too.
Coagent level affects hardness, modulus, tear, compression set and scorch. More coagent gives a tighter network, but too much can make the compound brittle and may scorch.
Choosing the peroxide
Peroxides differ in how fast they decompose at a given temperature, described by half-life. The cure temperature, scorch safety and mould time all depend on this choice. Commonly used types include dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and bis(tert-butylperoxyisopropyl)benzene. Peroxides are supplied as pure material or on an inert carrier at 40% to 50% active, so the phr to add depends on the active content.
Things that interfere
- Acidic ingredients and fillers can break down peroxide early or consume radicals. Acid acceptors or basic fillers help.
- Antioxidants are designed to scavenge radicals, so they can reduce cure efficiency. Pick antioxidants that are known to work in peroxide systems.
- Oxygen at the surface. Cure in contact with air can leave tacky or under-cured skin. Closed-mould pressure cure avoids it.
- Moisture can cause porosity. Dry ingredients and an oxide such as calcium oxide help in some compounds.
- Overheating. Mixing temperatures that start the cure cause scorch in the mixer.
Post-cure
Most peroxide-cured fluoro-rubbers are post-cured to complete cure and remove peroxide by-products, which lowers compression set and improves heat ageing. Some grades are designed to cure well enough that post-cure can be shortened, but the claim should be proved with compression-set data on your own parts.
Safety
Organic peroxides are hazardous materials. Store them cool and away from acids, amines and reducing agents, and follow the supplier's safety data sheet.
Sources and further reading
- P. A. Ciullo and N. Hewitt, The Rubber Formulary, 1999, for the general chemistry of peroxide cure.