Rheology of carbon black suspensions. I. Three types of viscoelastic behavior

Dynamic viscoelastic properties were investigated for carbon black (CB) suspensions in three suspending media, an alkyd resin (AR), a rosin-modified phenol resin (PR), and a polystyrene/dibutyl phthalate solution (PS/DBP). The affinity of the medium toward the CB particles decreased in this order, and different types of rheological behavior were observed accordingly. In AR having a high affinity, the CB particles were well dispersed to form no agglomerates. These particles exhibited a slow relaxation process attributable to their diffusion. In PR having a moderate affinity, the suspension showed a sol-gel transition with increasing CB concentration, and the critical gel behavior characterized with a power-law relationship between the modulus and frequency (ω), G′ = G″/tan(nπ/2)∝ωn with n=0.71, was observed at a critical concentration. This behavior suggested formation of a self-similar, fractal agglomerate of the CB particles in PR. These two suspensions in AR and PR exhibited a moderate nonlinearity. In contrast, in PS/DBP, the suspension exhibited a strong nonlinearity attributable to strain-induced disruption of a fully developed three-dimensional (3D) network structure of the CB particles therein. Thus, the structure and rheology of the CB particles changed with the affinity of the suspending medium. This result in turn posed a caution to a conventional structural interpretation that the CB particles in the actually used ink/paint (always) form the fully developed 3D agglomerates.