Carbon Black Structure

The occlusion of elastomer molecules described above as carbon black-elastomer interaction is a consequence of the carbon black structure. The main effects associated with filler permanent structure are illustrated in Table 5. The carbon blacks used here are of similar specific surface area (and particle size), but differ significantly in oil absorption and hence structure. They both dispersed equally well in the elastomer. Some properties are little affected by the changes in structure, notably tensile strength and abrasion loss, but others show significant effects. With respect to processing, compound viscosity and the incorporation time in the rubber matrix increase, while extrusion shrinkage decreases and extrusion smoothness increases, and loading capacity and scorch time decrease with increasing COAN; these effects are consistent with the concept of occluded rubber mentioned earlier. High extension modulus increases, while elongation decreases with increasing structure, again consistent with occluded rubber.

The carbon black structure is responsible for the interactions between the carbon aggregates in the elastomer compound. These interactions can be evidenced in the cyclic deformation of carbon black-filled vulcanized rubber in compression or shear at various amplitudes or frequencies discovered by Payne. The Payne effect describes the elastic dynamic modulus which at very small amplitudes is high but then drops very rapidly as the amplitudes increase, to level off at the highest amplitudes. The effect is reversible provided enough time is allowed for recovery. The effect is more pronounced at higher carbon black loading; as concentrations decrease, it becomes less noticeable. The higher modulus at low strains is caused by the elastic deformation of the network formed by the carbon black aggregates. In the case of fumed silica where aggregates are well known to form, due to the hydrogen bonds between the particles, particle networks with higher strength, the Payne effect is even more pronounced. The dynamic properties of filled elastomers are described below in more detail.

Table 5 The effect of carbon black structure on elastomer properties (adapted from Boonstra 1982)

Table 6 The direction and magnitude of the effects of the main filler properties on those of filled elastomers (adapted from Boonstra 1982)

Notes: + and ++ mean increase in that property and - and - mean a decrease. They do not mean that the change is beneficial or detrimental. This depends very much on the application. For instance, high hysteresis is good for sound damping, but bad where heat buildup has to be avoided

The effect of carbon black key particle properties on a number of important reinforcement properties of the elastomer composite is summarized in Table 6.

The main technical application where the reinforcement of elastomers is achieved with carbon black fillers is in tires. Table 7 lists the most common carbon black types used in tires, the ASTM designation, and key carbon black properties. Table 8 gives data for strength, abrasion, and wear for these carbon blacks in SBR composites.