MODES OF ACTION OF A MULTIFUNCTIONAL ADDITIVE IN A CARBON-BLACK FILLED NATURAL-RUBBER COMPOUND

The modes of action of a diamine salt of fatty acid with a general str ucture of RNH2+(CH2)3NH3+(R'COO-)2, referred to as a multifunctional a dditive (MFA) in carbon black filled natural rubber compound have been studied. The addition of MFA to a natural rubber compound with 50 phr of N330 carbon black is shown to increase certain physical properties up to a level of 2 phr (identified at the critical amount to coat the carbon black with a monolayer of MFA) after which there is a gradual deterioration of properties. The MFA exerts its greatest effect at the elastomer-carbon black interface as gum compounds show minimal change s in physical properties with MFA concentration. Swelling test results show very little change in the solvent uptake with increasing MFA, in dicating the effect on crosslink density is small. In contrast, carbon black dispersion, measured by computer-aided image analysis and scann ing electron microscopy (SEM) showed a substantial improvement with in creasing MFA concentration. Bound rubber was found to decrease with th e addition of MFA and a limiting bound rubber value was obtained at th e MFA loading which corresponds to a monolayer coverage of the carbon black. This is attributed to a reduction of the amount of rubber immob ilised in carbon black agglomerates due to the improved carbon black d ispersion.