- Organic peroxides as curing agent.
- Especially for saturated rubbers which do not contain any reactive group capable of forming x-links.
- Peroxide does not enter into the polymer chains but produces radicals which form C-to-C linkages with adjacent polymer chains.
- Eg. dicumyl peroxide, zinc peroxide, benzoyl peroxide, 2,4-chlorobenzoyl peroxide & 2,5-bis(t-butylperoxy)-2,5-dimethylhexane.
- Applicable to both diene (NR) and saturated (silicone, urethane, ethylene-propylene, etc.) rubbers.
- The mechanism of crosslinking using peroxides is a homolytical one. At the beginning of the vulcanization process, the organic peroxide splits into 2 radicals, according to the equation :
RO:OR → 2RO•
- The free radicals formed as a consequence of the decomposition of the peroxide, abstract hydrogen atoms from the elastomer macromolecules, converting them into macroradicals.
~CH2C(CH3)=CHCH2 + RO• → ROH + ~CH2C(CH3)=CHHC•~
- The resulting macroradicals react each other by forming carbon – carbon intermolecular bridges.
- Simultaneously with these crosslinking processes, side reactions, which reduce vulcanization (crosslinking) yield, occur. Thus, peroxides can react with the components of the compound, i.e., antioxidants, plasticizers, extenders, etc., and can be deactivated
- Other side reaction can take place to the radical centers formed on the elastomer backbone. These radicals can disproportionate leading to a saturated molecule and an unsaturated one.
Mechanism of Peroxide Vulcanisation
- The mechanism of vulcanization (crosslinking) with peroxides is exemplified below involving natural rubber and benzoyl peroxide :
Curing Agent & Vulcanizate Properties
- Dicumyl peroxide performs cross-linking of NR, SBR, nitrile rubber, resulting in vulcanizates with good cold and aging resistance.
- Higher tensile strength of the vulcanisates obtained with peroxides as vulcanizing agents, can be obtained by the addition of small amounts of sulfur, amines or unsaturated compounds.
- By comparison with the vulcanization with sulphur and accelerators, peroxides produce a lower reaction rate and the resulting vulcanizates have a lower tensile strength, scorching tendency and unpleasant smell.
- Most peroxides are available as:
- liquid (90% – 98% active),
- powders (40% – 50% active), or
- pastes made from silicone fluids and gums (20% – 80% active) to facilitate handling and dispersion
Comparison with other x-link systems
- C-C crosslinks, peroxide initiated.
- The material has a tensile strength about 40% that of the Sx network. Again, this is probably due to the immobility of the carbon-carbon bond.
- Advantages of Peroxide Cure:
- Applicable to both saturated & unsaturated rubber.
- Transparent products due to the absent of sulphur that causes staining of the vulcanisates.
- C-C x-link is more stable and thus has good ageing resistance.
- Excellent resistance to compression set at high temperatures (70o – 100oC) due to the absence of ZnO and Stearic acid (activators in S-vulcanization).
- No x-link reversion.
- Disadvantages of Peroxide Cure
- Scorch rather easily (Scorchy).
- Slow cure rate with no delayed-action.
- Long cure times for completion to obtain the best heat resistance.
- Poor hot tear strength.
- Incompatibility with chemical antiozonants.
- Exposure of rubber compound to the air (oxygen) gives a sticky rubber surface.