Effect of Starch Loading on the Properties of Natural Rubber Compounds

The effect of starch addition on the processing and properties of a natural rubber compound was investigated. The starch loading used was varied from 36 p.h.r. to 66 p.h.r. loading. The compound is based on a simple engine mount compound filled with 49 p.h.r. carbon black which corresponds to the same volume loading as 36 p.h.r. of starch. The extent of the adhesion between filler and rubber under different deformation conditions was determined using several simple models such as the Einstein(1), Guth-Gold(2) and Sato-Furukawa(2) models. In general, the study shows relatively poor reinforcement of natural rubber by starch, resulting in low stiffness and strength compared with a compound filled to a similar volume fraction with carbon black. The poor reinforcement is due to its different polarity and large particle size. In spite of that, it is observed that good adhesion can be obtained at strains below 50% elongation, agreeing with the Einstein model of perfect adhesion. However, at a higher strain, poor adhesion leads to debonding of starch from the rubber, resulting in lower stiffness and strength.