The relation between macro- and micro-dispersion of silica filler in rubber matrix

The dispersion of fillers in elastomers can be discriminated into two categories based on the dimensions of the filler clusters: macro- (2 to 100 μm) and micro- (smaller than 2 μm) dispersion. Both dispersion types are determining the final properties of the composite, but the micro-dispersion of fillers is crucial for good reinforcement of the rubber. Two basic dispersion mechanisms are proposed in literature for carbon black: agglomerate rupture and onion peel model. These filler dispersion mechanisms imply that micro-dispersion occurs only after agglomerate fragmentation of fillers, and that therefore these processes are strongly related. For silica fillers, Blume and Uhrlandt proposed a specific micro-dispersion mechanism: The polymer firstly penetrates into the voids of the agglomerates, followed by polymer chains pulling off fragments from the larger agglomerates under shearing conditions. This mechanism suggests that micro- and macro-dispersion of silica are independent.
In this study, the relation between micro- and macro-dispersion of silica in a rubber matrix was investigated. Different degrees of dispersion were achieved by variation of the mixing parameters (silanization temperature, time) and the amount of coupling agent. The degree of micro- and macro-dispersion was analyzed by measuring the Payne effect and using microscopic methods, respectively. No correlation was found between micro- and macro-dispersion. The results of this study confirm indeed that these filler dispersion processes happen simultaneously, but independently. Additionally, the factors affecting micro- and macro-dispersion will be discussed.