Rheology of carbon black suspensions. IV. Effect of suspending media on the sol–gel transition behavior

Linear viscoelastic properties of carbon black (CB) suspensions with various CB concentrations (c CB ) in two suspending media, mixtures of a rosin-modified phenol resin-type varnish (Varnish-1)/an alkyd resin-type varnish (Varnish-2) and Varnish-1/petroleum solvent (AF) were investigated. The former medium has higher affinity and the latter has poorer affinity toward CB particles than Varnishi-1, although these two media have almost the same viscoelastic properties. Both CB/(Varnish-1/Varnish-2 = 60/40) and CB/(Varnish-1/AF = 80/20) suspensions exhibited a sol–gel transition on an increase in c CB , as similar to the behavior of CB/Varnish-1 suspensions. But, for the CB/(Varnish-1/Varnish-2 = 60/40) suspensions the critical gel concentration (c gel ) was lower and the critical relaxation exponent ( n ) was higher than that of CB/Varnish-1 suspensions. In contrast, for the CB/(Varnish-1/AF = 80/20) suspensions c gel and n, respectively, were almost the same value as those of CB/Varnish-1 suspensions. These results strongly suggest that c gel and n are attributable to the medium affinity, but not to the medium viscosity, and the CB fractal structure changes from plane-like to straight-line structure with increasing medium affinity.