Cure Terms Glossary
Vulcanisation is the term specifically applied to the cure of (solid) rubbers. The usage of the term follows Goodyear's discovery of the sulphur vulcanisation of natural rubber (polyisoprene) and reflects the combination of both sulphur and heat (Vulcan was the Roman god of fire and volcanoes).
The unsaturation in natural and other diene rubbers provides allylic hydrogens which are the sites of crosslinking. The high molecular weights (100k and above) of these raw rubbers result in starting materials of exceptionally high functionality, and premature vulcanisation (scorch) is a problem if the cure profile is not suitably optimised. Compound design is very much an empirical process with the role of sulphur (the curing agent) complemented by accelerators, activators - or, in some cases, retarders.
The numerous permutations of ingredients do not allow for any simple mechanistic interpretation and perhaps point to numerous reactions in sequence or parallel. Possibly the majority of these may fall into two generic types: SS/SS interchange and the reaction of SS with allylic H.
The resulting crosslinks
include both CS and SS bonds, the latter containing varying
numbers of sulphur atoms, e.g.,
The relative proportions of each depend on the formulation of the vulcanisation mix - especially the level of accelerators in relation to the level of sulphur used.
The relative proportions of each crosslink type can also depend on the age of the vulcanisate, as the network can mature with heat or time to see a gradual reduction in sulphur rank in the crosslinks. Sulphur vulcanisates intended for relatively high temperature service may require an oven post-cure to provide a more stable network at the outset.
The numerous permutations of ingredients for accelerated-sulphur vulcanisation make for a highly versatile system - no doubt aided by a measure of scorch control from the chemistry required to create the active intermediates for cure. Scorch control is more problematic with alternative curing systems based on simpler chemistry.
Peroxide vulcanisation is possible is possible for solid rubbers containing C=C bonds, but tends to be restricted to low C=C rubbers, such as the vinyl silicones (i.e. siloxane copolymers containing some SiMe(CH=CH2)O in the backbone) or ethylene acrylic elastomers. An oven postcure may be necessary to remove some of the more volatile breakdown products of peroxide action. Saturated polymers are amenable to peroxide cure, notably when tertiary hydrogens are present (as in ethylene-propylene rubbers), but co-agents containing C=C may also be required. Given the ingenuity of the process chemist, a host of reactions may be employed in vulcanisation.
Examples of this diverse chemistry include: salt formation (e.g. on chlorosulphonated polyethylene), nucleophilic addition (e.g. following HF elimination in fluoroelastomers) or nucleophilic sunstitution (e.g. eliminating methanol from ethylene methyl acrylate copolymers). All are considered vulcanisations - irrespective the curing chemistry. The term is specific to the rubber industry and can be applied to any rubber type, whether commodity or speciality.