Why Does Natural Rubber Products Become Soft On Ageing?

Manufacturer: We manufacture rubber products using both synthetic and natural rubber. We observe that those made from natural rubber behaves differently compared to those made from synthetic rubbers after ageing (i.e. oxidation).

Unlike synthetic rubber products, natural rubber products tend to become soft after ageing. Why?

John Woon (Senior Latex Consultant): Oxidation is a free radicals chain process initiated when the rubber (polymer) chains are broken and carbon radical formed as a result.

Oxidation of rubbers leads to either hardening or softening of the rubbers. The free radicals are highly reactive and unstable having the tendency to form more stable products. For some synthetic rubbers, these free radicals form cross-links and the rubbers become harder.

In the case of natural rubber, the hindrance caused by the bulky methyl side group prevent the formation of such cross-links and the end result is "chain cleavage" or "chain scission" which breaks down the polymer chain. This means a reduction in the molecular weight and hence the rubber becomes soft.