On 16-01-2012


Preliminar : High Temperature cure Elastomers (rubbers, silicones) are polymerized ( “vulcanized” that means a chemical change under temperature + pressure combined influence ) polymers. A catalyst is necessary to realize this reticulation; this catalyst (various types are existing) is in higher quantity than strictly needed in order to fulfill reticulation and after vulcanization, gas residues are still “jailed” in “vulcanized” matrix.
One ‘effective’ extraction way (economically /qualitatively)
- other extraction ways will be necessary “to purify” more the vulcanized rubber by using “solvents” 'extraction' ( pharmaceutical applications )
of these residues of vulcanization consists in placing the semifinished products after vulcanization in the mould ( for example in a ventilated oven ) at a “high” temperature constraint . The substrate is then thermally expanded and the “volatiles" are partly extracted. The substrate will have a higher density (less “alveolar”).

Post curing is generally mentioned on elastomer technical data sheets. Post curing temperature and time are determined to get “volatiles” into products correctly eliminated.
In case of thick parts, these must be gradually brought up to the requested post curing temperature. This is mainly the case for fluorocarbon parts (which are very gasproof). Otherwise, cracks could appear.

To reach the lowest possible Compression Set for a rubber formulation, can lead to post cure after molding operation, in order to allow maximum decking between molecular chains.
Curing time is continued in order to let into the molding part, the minimum of free reactive sites, deteriorating the aggregate compactness.

1- for food and medical applications the post curing is oftenly necessary to “purify” moulded products
2- products “specifications in chemical behaviour & resistance will require a post curing ( a better resistance to the various chemical aggressions (oils, solvents, acids. ) will be got by getting a more compact elastomer)
3- a low Compression Set specification on products (Remanent Deformation after Compression: see one of our previous information letter Nr 90) will also require a post cure post treatment

Elastomers / rubbers usually employed requiring a post curing are : silicones, fluorosilicones, fluorocarbons, polyacrylates.

Of course, this operation is not suggested for lubrificated elastomers ( see our information letter Nr 23 ).

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