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..

Stability of Carboxylated SBR Latex

Manufacturer: How do we stabilize the carboxylated SBR latex compared to normal SBR latex? We do use high loading of filler (e.g. calcium carbonate).

John Woon (Senior Latex Consultant): Compared to standard SBR latex, carboxylated SBR latex generally offers better chemical and mechanical stability due to the carboxylation.

Hence, no extra stabilizer is required to be added at your end unless it is to be blended with other latices with different pH range. Please note that some soaps (stabilizers) are most efficient in the pH range of 8.0 - 9.5 while others are workable only outside this range.
You need to check with your supplier to confirm this.

It is advisable to premix your filler with a chelating agent or similar phosphate type of dispersing agent before use.

How to Cure Carboxlylated SBR Latex?

Manufacturer: We have been advised to use carboxylated SBR latex as an alternative latex for our products. Please advise us the best curing system for this.

John Woon (Senior Latex Consultant): Carboxylated SBR latex can sometimes be used without any cross-linking. However to further enhance its physical properties, you can cure it with sulphur curing system or ZnO. Sodium aluminate with urea or melamine can also be added for cross-linking. A combination of all these possibilities have been successfully used commercially.