Fillers for Latex Dipped Products - Tensile Strength

Manufacturer: We are attempting to improve the tensile strength of our latex-dipped products by adding various fillers such as carbon black into our latex compounds but no avail. The products become harder and in fact, the tensile has dropped. Any quick advice?

John Woon (Senior Latex Consultant): I'm not surprised with your findings at all. 

In reinforcing of rubber in the dry rubber technology, there is a process called "mastication" where the raw rubber is first softened using mechanical force (i.e., two-roll mills or internal mixers), by breaking down the rubber molecules into smaller units.

As a result, rubber-bound free radicals are formed that can interact with the reactive sites of the filler particles, hence reinforcing the rubber.

No mastication process is involved in latex technology. Therefore, generally, one cannot increase the tensile strength of latex products. Also, filler will increase the rubber modulus resulting in an increase in hardness.

However, if you want to increase the tear strength, you can try adding a small amount of fine-particle-size silica.

Having said that, some manufacturers do manage to increase the tensile strength marginally with a small amount of bentonite clay (at about 5 to 8phr), but such addition tends to increase the latex compound viscosity. Also, it is very much more expensive than kaolin clay and it is darker in colour.