Rheology of carbon black suspensions. III. Sol-gel transition system

Linear viscoelastic properties of carbon black (CB) suspensions with various CB volume fractions () in a rosin-modified phenol resin type varnish (Varnish-1) were investigated at various temperatures (T). The CB/Varnish-1 suspensions exhibited a sol-gel transition on an increase in , and the _gel value at the gelation point decreased with increasing T. This T dependence of _gel, being opposite to the dependence seen for usual gelling systems, can be related to a phenol resin type polymeric component included in the Varnish-1. At low T, this polymeric component appeared to be rather well solvated in the Varnish-1 thereby allowing the gelation due to bare attraction between the CB particles at large . In contrast, at high T, the polymeric component appeared to have been less solvated, as evidence from a moderate failure of the time-temperature superposition of pure Varnish-1 and a decrease of its elasticity (in a shifted frequency scale) with increasing T. This less solvated polymeric component would have been adsorbed on the CB particles, thereby allowing the agglomeration of the particles at small _gel at high T.