Shelf-life of Natural Rubber Latex Compounds without Zinc Oxide

Manufacturer: For some of our latex products that require maximum degree of transparency, we have developed some recipes without the addition of ZnO using both zinc and sodium salts of dithiocarbamates as the accelerators. We found that the shelf-life (by measuring precure) of the compounds having Zinc dithiocarbamates had shorter shelf-life compared to the sodium salts. Why is this so?

John Woon (Senior Latex Consultant): The reason is obvious. Zinc dithiocarbamates, although in the absence of ZnO, do release sufficient amount of zinc ion to activate the vulcanization. Also, the soluble sodium dithiocarbamates are weaker accelerators comparatively.

Do be aware that some of the water soluble accelerators would be lost during the leaching process. Therefore you have to adjust their level of addition accordingly. Also, I suggest you add some Zinc carbonate otherwise you might not get sufficient cure.