Selection of Carbon Black Fillers for Natural Rubber Springs

From the observations made above, it is apparent that the modulus of a filled rubber plays a considerable role in determining other properties. Nevertheless, when allowances are made for variation in modulus, considerable differences between rubbers filled with different types of carbon black still exist. The black appears to fall into three main categories. The small-particle abrasion grade furnace blacks (N100–N300 series) gave rubbers with the highest strength properties, but the highest low-strain stiffness and highest loss angles. The larger particle size semireinforcing grades of furnace blacks (N500– N700 series) gave lower values of these properties, while medium thermal (N990) black gave no advantages over the semireinforcing grades in dynamic properties but gave considerably weaker rubbers. Within these categories, variations in structure of the blacks had a second-order effect on loss angle, the lower loadings of high structure blacks resulting in somewhat lower values than low structure blacks. The effects of structure on strength properties, if any, were small. The influence of filler type on the processing characteristics studied here was small, the only significant difference being the effect of semireinforcing grades on vulcanization rates. The Mooney viscosity of the rubber was related only to the moderate strain modulus of the vulcanized rubber, and the scorch delay to the Mooney viscosity. These conclusions were reached by comparing the properties of rubbers of equal modulus. If comparisons had been made at equal filler loadings, different conclusions would have been reached. For example, the low structure fillers would appear to give lower loss angles and higher tensile strengths than high structure blacks, and at higher loadings the thermal and low structure semireinforcing blacks would give tensile strengths similar to the abrasion grades. This illustrates the subjective nature of the assessment of the influence of carbon blacks. The results obtained here suggest that many of the reported differences in the properties of rubbers filled with different grades of carbon black are due simply to differences in modulus of the rubbers. Adjustment of the black loadings to give equal moduli removes most of the differences between various grades of small particle blacks and between semireinforcing grades. Considerable differences in properties are obtained, however, between rubbers filled with abrasion grades (N100–N300) of black and those filled with semireinforcing grades (N500 to N700). The choice of a filler to give required physical properties, then, is between a small particle size black or a semireinforcing furnace black. Medium thermal offers no advantage in dynamic properties to compensate for poor strength properties. There being little difference between different grades within abrasion and semireinforcing types of furnace blacks, in terms of physical properties, final selection of a filler is likely to take cost considerations into account. The relative volume costs of blacks and rubbers at present favors the use of low structure blacks to minimize compound costs, but the factory processing behavior obtained with different blacks may also be relevant. The latter cannot be objectively assessed in a laboratory exercise.