Flow properties of carbon black (CB) dispersed printing inks were studied. In oscillatory rheological measurements, printing inks showed pseudo sol-gel transition behavior, and critical gel concentration (W
gel) was determined. W
gel was applied as an index to express fluidity of the printing inks, since W
gel was found to show fairly good correlation with BET surface area that is known as a major factor of the fluidity. Moreover, W
gel showed better correlation with CB's external surface area that was determined from electron microscopy and that from mercury porosimetry. Furthermore, W
gel showed the best correlation with CB's specific surface area of the effective volume fraction calculated from relative viscosity in steady state rheological measurements by using a semi empirical model. These results showed that the network structure at W
gel is constructed of complex particle made up of CB and polymer-adsorbed layer on it, not from CB's primary particle as understood generally.
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