Rheological characterization of carbon black/varnish suspensions

Linear viscoelastic properties of carbon black (CB) suspensions in a rosin-modified phenol resin-type varnish (Varnish-1) were studied as a function of volume fraction (ϕ) of CB and temperature (T) to clarify the effects of the primary particle size and the aggregate structure of CB particles on the rheological properties. All the CB/Varnish-1 suspensions showed a typical sol–gel transition behavior with an increase in ϕ. The critical gel behavior characterized with a power-law relationship between the storage and loss moduli (G′ and G″) and frequency (ω), G′ = G″/tan(nπ/2) ∝ ωn, was observed at a critical gel concentration (ϕcrit). At 30 °C, the critical relaxation exponents (n) were estimated to be 0.72 ± 0.02, irrespective of the primary particle size and aggregate structure of the CB particles. In contrast, the ϕcrit depended strongly on the aggregate structure although it hardly depended on the primary particle size. Both the n and ϕcrit values changed with T. The n values increased from 0.72 at 30 °C to 0.83 at 60 °C and the correspondent ϕcrit values decreased slightly with elevating T for each CB suspension. The increase in n with T indicated that the CB aggregate structures changed from plane-like to more straight with increasing T.