Why Can't We reinforce Latex Products with Reinforcing Fillers?

Manufacturer: We are manufacturing rubber products using both natural rubber latex and dry rubber such as SMR 10 and SMR 20. To improve the strength of our products, we usually add reinforcing filler such as carbon black and silica. However we find that this only works with dry rubber compounds and not the latex compounds. Why?

John Woon (Senior Latex Consultant): In dry rubber compounding and mixing, one has to masticate the rubber either in a 2-roll mill or an internal mixer such as Banbury before other additives such as curatives could be added and homogeneously mixed. In the process of mastication, rubber molecules are broken down with the formation of free radicals which would initiate the chemical bonding with the reinforcing fillers especially carbon black. The following diagram shows how a dry rubber compound is being mixed in a two-roll mill:

The difference in the speed of the two rolls results in the required shear force for the mastication of rubber.

No such free radicals are produced in normal latex compounding and hence no significant reinforcing can be achieved with fillers as in the case of dry rubber compounding although in some cases, there could some marginal improvement in tear strength..