Organic Fillers for Latex Products

Manufacturer: We are familiar with inorganic fillers (i.e., calcium carbonate and clay), but what are organic fillers? What do they do? We are thinking of evaluating organic fillers if we have more knowledge of them.

John Woon (Senior Latex Consultant): Organic fillers normally used in the latex industry are polymeric in nature. Some examples are MMA grafted NR latex, and high-styrene SBR latex.

For some high-styrene SBR latex added in NR latex compounds, the tensile strength initially increases marginally before dropping drastically as the amount of SBR increases. Needless to say, the elongation at break drops while the modulus and hardness increase. The main reason for using high-styrene SBR is to improve the tear strength of the latex products.

MMA grafted NR latex behaves quite similarly as high-styrene SBR latex.

The other advantage of organic fillers is that one can increase the modulus and stiffness of latex products without having to add inorganic fillers such as clay and calcium carbonate, and hence the tensile strength could be easier to maintain.

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.