Fillers, reinforcement elastomers surface chemistry

Surface Treatment. Carbon black remains the particulate filler of choice for rubber articles since the inherent reinforcing effect of the nonblack fillers in hydrocarbon elastomers is not comparable. This is primarily due to the nonbonded interactions established between the particulate filler and polymer functionality (28). Surface chemistry plays an important role in the interaction of the nonblack fillers and the polymer with contributions ranging from electrostatic interactions to covalent bonding to the polymer backbone. However, surface chemistry also strongly affects the interaction of the nonblack filler with other chemicals in the rubber compound, particularly active metal oxides, curatives, and antidegradants. 

Wolff, S. Wang, J. (1992). Filler— Elastomer Interactions. Part IV. The Effect of the Surface Energies of Fillers on Elastomer Reinforcement. Rubber Chemistry and Technology, 65, 329-342. 

In contrast with carbon black, silica has particular surface properties that bring a number of problems when using such materials as reinforcing fillers, particularly in hydrocarbon elastomers. Indeed the surface of silica, either fumed or precipitated, is strongly polar and hydrophilic, owing to its polysi-loxane structure with numerous silanol groups. This particular surface chemistry of silica has several immediate consequences ... 

Compared to morphology, filler chemistry has been only slightly studied, partly because of the difficulty of such characterizations and more probably because since the 1970s reinforcement is broadly considered as a physical interaction between elastomer and filler. So carbon black chemical characterizations mainly date from the 1960s, and few new technical methods have been applied to carbon black surface characterization since then. The situation is somewhat different for silicas, because silica reinforcement is the consequence of a chemical reaction of silane with silica surface. Few studies have been published in the elastomer reinforcement area, probably because silica surface was already well characterized for other applications. 

The phenomenon of bound rubber has played a crucial role in the chemistry of elastomer reinforcement. Very simply, if vulcanized rubber is masterbatched with a reinforcing filler, a certain fraction of the rubber is found to become insoluble, and to remain as a gel. This gel, a swollen mass of rubber and dispersed carbon particles, is called bound rubber. The percentage of bound rubber depends, of course, on the quantity of filler employed, and also on mixing conditions and choice of solvent (typically benzene). Working under standardized conditions, the percentage of bound rubber was found to correlate with the specific surface area of the filler as shown in Figure 10.12. 

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