Issue of Monomers and Oligomers Left in Rubbers

Usually a small amount of monomer and oligomer are left unreacted at the end of a polymerisation reaction (such as styrene, acrylonitrile and isocyanate), concentrations of which can be critical to know - because their molecular weights are small and the matrices (rubber) are flexible, letting them migrate to the surface easily. Especially for some toxic monomers like acrylonitrile, this can be of utmost importance and permissible limits of such species must be defined and followed carefully for food contact applications (in NR, the concentration of free monomer is set at 1 mg/kg as the maximum). Similarly, oligomers that can exist in the system after completion of the polymerisation can pose similar problems. 

3 Issue of Vulcanisation Agents (and Cure Products) Left in Rubbers 

Vulcanisation (cure) reaction of rubbers is usually complex, and a number of different reaction products can be produced. These new (unwanted) products plus the unreacted part of the accelerators (like thiourams, certain thiazoles, sulfenamide and dithiocarbamates) are potential dangers for migration in food contact applications. Especially for rubbers contacting aqueous media, breakdown product migration is more common. 

However, accelerators can also be very important to consider in all cases for rubber certain cure accelerators (such as thiouram) can lead to the production of a (suspected) carcinogen nitrosamines. 

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