Functions of Higher Fatty Acid Soaps of Natural Rubber Latex

Manufacturer: What are other functions of fatty acid soaps we add to latex besides improving the mechanical stability of our latex compounds?

John Woon (Senior Latex Consultant): The fatty acid soaps or rather the Higher Fatty Acid (some times referred to as HFA) soaps do not come solely from those you add during compounding. In fact, it derives from the so-called non-rubbers of natural rubber latex of which about 50% is lipid material. The lipid materials contain phospholipids along with natural waxes, sterols and sterol esters. (HFA contains mainly palmitic, stearic, oleic and linoleic)

When we produce latex concentrates, ammonia is added as a preservative. With time, the phospholipid would be hydrolysed into fatty acid soaps, among other things, in the presence of ammonia. The whole process takes about 3 to 4 weeks. We therefore normally observe a gradual a rise in the mechanical stability of natural rubber latex (MST) from the day it is produced. Occasionally such rise is inadequate for good proccesibility hence the requirement of extra addition during compounding.

The crosslinking potential in a typical sulphur vulcanization system is also affected by the presence of HFAs since they form part of the activating system by solubilizing Zinc. It is therefore understandable that in the curative recipes for many dry rubber products, the addition of 1 to 2 parts stearic acid is very common.

Also, some quarters believe that fatty acid soaps lead to a reduction of the surface tension of the latex compounds resulting in better wetting, a prerequisite property required for good dipping process and more even spread of latex onto former surface.

Having said this, one should bear in mind the negative effects of such fatty acid soaps i.e. the increased foaming and webbing tendency.

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