Rheology of concentrated suspensions of carbon black in low molecular weight vehicles

Abstract

The rheology of suspensions of carbon black in low molecular weight liquid vehicles can be described by a double logarithmic flow curve; i.e., two straight lines on a log-log plot of shear stress vs. shear rate. At low loadings the slope of the low shear rate line is less than that of the high shear rate line; i.e., the log-log flow curve is concave upward. At high loadings the relative slopes are reversed; i.e., the curve is concave downward. Qualitatively the flow curves may be regarded as “windows” on a complete Ostwald flow curve. Over a given range of experimentally accessible shear rates, high viscosity pastes are, in effect, “viewed” at an early part of their Ostwald flow curves; while low viscosity pastes are “viewed” at an effectively higher shear rate region of their Ostwald flow curves. A similar interpretation is made for the effect of temperature. Decreased carbon black-vehicle interaction raises the level of the flow curves; i.e., makes a stiffer paste, due to formation of a stronger carbon black network. The apparent viscosity can be calculated as the ratio of shear stress to shear rate at the wall. Empirically we have found that the apparent viscosity of a suspension in a given vehicle at a given shear rate,η, is a function of the surface area of the carbon black, its volume loading and its structure. The contribution of the last two terms is expressed in terms of the effective volume fraction,V, defined as in previous work as $$V = \left[ {1 + F\frac{{\left( {0.02139 \overline {DBPA} - 0.46} \right)}}{{1.46}}} \right]$$ whereF is an effectiveness factor. The best relation is thatη is a power-law function of (ρS)1/2 V 2, withF = 0.5. For several sets of data, such functions gave correlation coefficients of 0.91–0.95.