Viscosity Profiles of Printing Thickeners at High Shear Rates and Their Use in Predicting Paste Flow in Screen Printing

Abstract

Apparent viscosities of various natural and synthetic printing thickeners were measured at shear rates from 0.05 to 7000 s-1. Log viscosity-log shear rate plots (viscosity profiles) were curves, not straight lines as generally believed. Nonlinear regression equations were computed and required significant second or third order terms or both to fit the data. The slopes and shapes (or curvature) of the profiles were distinctly different for synthetic copolymers, for nonionic cellulose or starch ethers, and for anionic polysaccharides (either carboxylmethyl cellulose or sodium alginate). The interaction efforts on both apparent viscosity and slope of viscosity profiles of such variables as pH, temperature, concentration of thickener and added electrolyte, thickener composition, and molecular weight were estimated by analysis of variance. The appearance of prints made with different thickeners was compared visually, photometrically, and microscopically. Prints with optimum sharpness of mark, levelness, and penetrations were obtained from pastes whose profiles, even with widely different shapes, all intersected at the same shear rate and apparent viscosity. The concentrations of each thickener required for optimum paste spread and appearance could be calculated from the slope of the viscosity profiles and from the estimated change in viscosity with concentration.