Rheological Factors for Attapulgite Suspended in Water

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Abstract
Stress‐strain curves and stress‐relaxation curves were determined at various temperatures (1° to 70°C) and concentrations (16 to 22 wt%) for attapulgite suspended in water. Stress‐relaxation experiments were conducted under different conditions on samples which were relaxed for various durations of time between successive cyclic deformations. The flow properties are given in terms of rheopexy and thixotropy. The flow curve (for shear rates 0 to 700 per second versus shear stress) shows, first, a rheopectic hysteresis loop which changes on repeated shearing to a reversible and reproducible non‐Newtonian flow curve. After resting the system for about 12 hours, the resulting flow curve shows a thixotropic loop. Viscosity as a function of pH shows an inversion at pH ⋍ 6.3. A model for explaining the observed facts is proposed. The script S type of looped‐over flow curves showing both rheopectic and thixotropic characteristics is explained in terms of breakage of “logjammed” and crystalline bundles. A plot of the yield‐point viscosities versus the reciprocal of temperature for the rheopectic and thixotropic curves shows activation energies of about 1 and 2 kcal respectively. An autographic rotational viscom‐eter was designed and utilized to obtain the data reported so that the objectionable effects of frictional heating in the sample were minimized. The calibration response of the viscometer to a glycerol standard shows good agreement with published viscosity data.

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