VISCOELASTIC PROPERTIES OF HIGH SOLIDS SOFTWOOD KRAFT BLACK LIQUORS

The linear viscoelastic functions of several softwood slash pine kraft black liquors from a two level, four variable factorially designed pu lping experiment were determined for solids concentrations from 65% to 81% and temperatures from 40 to 85 degrees C. At high solids and lowe r temperatures, black liquors behave like un-cross-linked polymers. Th e exact level of dynamic viscosity and storage modulus at any given co ndition is dependent upon the solids composition which will vary from liquor to liquor. The linear viscoelastic functions were described usi ng Cross and Carreau-Yasuda models. Superposition principles developed for polymer melts and concentrated polymer solutions were applied to obtain reduced correlations for dynamic viscosity and storage modulus. The data for dynamic viscosity were shifted over the whole range of t emperature, solids concentrations, and frequency, and a single curve f or dynamic viscosity behavior of every liquor was obtained. The data f or storage modulus did not superimpose into a single curve for the eff ects of solids concentration. The reduced correlations were used to es timate the viscoelasticity of the liquors near normal firing condition s and found that black Liquors will not have any problem in droplet fo rmation for concentrations up to 81% solids and temperatures above 120 degrees C. The viscometric and linear viscoelastic functions of black liquors were compared (Cox-Merz rule), and it was shown that at suffi ciently low shear rates and frequencies both shear viscosity and the m agnitude of the complex viscosity approach zero shear rate viscosity.