Aggregate breakdown

Stmcture is usually measured by a void volume test such as the absorption of dibutyl phthalate (DBPA) (15), or by bulk density measurements of the carbon black under compression. In order to eliminate the effects of pelletizing conditions the DBPA test has been modified to use a sample that has been precompressed at a pressure of 165 MPa (24,000 psi) and then broken up four successive times (24M4) (16). This procedure causes some aggregate breakdown and is claimed to more closely approximate the actual breakdown that occurs duting mbber mixing. 

The muller is useful for mixing problems requiring certain types of aggregate breakdown, fricdional anchorage of particles to one another, and densification of the final mix. Materials which are excessively fluid or sticky should be avoided. The muller mixer is generally used for batch operations (Fig. 19-9g), although Fig. 19-9/j shows a continuous muller. 

Polymer compounding will cause its own degree of agglomerate and aggregate breakdown, which in turn can vary from system to system, and the aim of morphology determination should be to use measuring conditions that give a similar answer. This is not usually attempted and caution should be used with published values, especially for the finer synthetic fillers, where the problem tends to be most acute. 

In the literature it has been found that during mixing aggregate breakdown occurs for a number of carbon blacks in highly viscous rubbers [115— 118]. Recently, the aggregate breakdown was also attributed to classes of specific shapes of individual carbon blacks [108]. The opinion about the mechanical consequences of this process is quite different. On the one side, no obvious relationship to reinforcement is conjectured [116]. On the other side, improvements of the mechanical performance, due to the creation of new, active carbon surface, is assumed, which participates in formation of a strong filler-rubber coupling [118]. 

At low levels of addition, aggregates breakdown with simple shaking (before measurement) and later reagglomerate as they settle. These data show the effect of... 

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