Rheological characterization of carbon black/varnish suspensions

Abstract

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) ∝ ω n , was observed at a critical gel concentration ( ϕ crit). 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 ϕ crit depended strongly on the aggregate structure although it hardly depended on the primary particle size. Both the n and ϕ crit values changed with T. The n values increased from 0.72 at 30 °C to 0.83 at 60 °C and the correspondent ϕ crit 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.