This phenomenon was also attributed to the sulphur content, since CB reacts with sulphur during the curing process and forms CB sulphur bonds that link the rubber chains and tie the filler onto the rubber [25]. This is rubber-filler crosslinking which is considered as another type of crosslink to the rubber system, and defined as coupling bonds [25,26]. The higher the sulphur content, the higher the solubility of CB in the sulphur phase, and hence the higher the degree of CB dispersion.