Effects of a multifunctional additive on bound rubber in carbon black and silica filled natural rubbers

The effects of a multifunctional additive (MFA), i.e. a diamine salt of a carboxylic acid, on the bound rubber in carbon black and silica filled natural rubber compounds in o-xylene, have been studied. For the carbon black filled compound, the bound rubber initially decreases with the MFA concentration and then does not change significantly above an MFA concentration which approximately corresponds to the monolayer surface coverage of the carbon black. This phenomenon has been explained in terms of the release of immobilised rubber within the carbon black agglomerates. Bound rubber measured at high temperatures indicated that the MFA did not improve the chemical interaction between the carbon black and the rubber in an unvulcanised rubber mix. The MFA has no pronounced effects on the bound rubber for silica. This may be caused by the combined effects of improved filler dispersion and silica-rubber interfacial interaction. The ozone oxidation of the carbon black significantly increases its surface oxygen concentration but has little effect on its specific surface area. The oxidation has been found to decrease the bound rubber.