Resistivity and Surface Roughness Analyses for Evaluating Carbon Black Dispersion in Rubber

A comparison of the three techniques shows that while all can discriminate dispersions, their useful ranges are different. The electrical resistivity method is relatively insensitive at the early stages of mixing but has a high sensitivity at the intermediate and latter stages. This method is directly influenced by the dispersed black. There are limitations, however, to its utilization. The method works only within a narrow range of volume loadings (35–75 phr) and can only be used for small and intermediate particle size carbon blacks. One advantage that the resistivity technique has is that uncured rubber is tested. Elimination of the vulcanization step makes the resistivity technique an attractive quality control test. The Phillips rating and surface roughness are not limited by the volume loading of filler or the nature of the filler. Each technique works quite well at intermediate dispersions and only loses sensitivity at very good dispersions. These methods differ from the resistivity in that they are sensitive to the undispersed filler. The surface analysis is a more objective and quantitative technique than the Phillips rating. The surface analysis technique can have a microcomputer interfaced to store data and calculate results. This reduces the test time to a matter of minutes. The Phillips dispersion rating is made on vulcanized rubber so that there is added sample preparation which leads to a longer analysis time. The surface analysis has been reported for both vulcanized and unvulcanized rubber; however, the unvulcanized samples present special problems in cutting and do not show as strong a correlation with other dispersion ratings.