M. Dolz

A time-dependent expression for thixotropic areas. Application to Aerosil 200 hydrogels

Experimental determination of the areas enclosed under the up curve, sigma = sigma(;gamma), corresponding to time zero, and the down curve, which would be obtained following complete rheodestruction of the system structure, is not an easy matter. In the present study a semi-empirical procedure for calculating the hypothetical values of these parameters is proposed. With these parameters it is possible to obtain a time-dependent expression for the thixotropic areas, S(T). This procedure has been applied to the study of the thixotropic behavior of Aerosil 200 hydrogels at different concentrations. The comparative analysis of these systems has been realized by studying the relative thixotropic areas and relative thixotropic rates that are obtained from S(T).

Rheological characterization of easy-to-disperse (ETD) Carbopol hydrogels

Experimental determinations of apparent viscosity were used to investigate the rheological behaviour of easy-to-disperse (ETD) Carbopol 2001, 2020 and 2050 hydrogels, involving different polymer concentrations, degrees of neutralization, agitation times and shear rates. Mathematical expressions were obtained, based on the power law, to allow calculation of the viscosity of each system, for the different variables studied. These equations proved valid within the 5.5-7.5 pH range, i.e., the pH interval of greatest interest in pharmaceutics and cosmetics. The rheological behaviours of the three types of ETD hydrogels were investigated comparatively and with respect to Carbopol 940, i.e., one of the most widely utilized hydrogels.

Thixotropic Behavior of Salad Dressings Stabilized with Modified Starch, Pectin, and Gellan Gum. Influence of Temperature

The thixotropy of low‐oil salad dressing has been analyzed at different temperatures (8–26°C). The usual formulation containing modified starch (4%) was compared with others in which part of the starch had been substituted by pectin (0.5%) or different concentrations of gellan (0.1 and 0.5%). Up and down flow curves were measured, showing in all cases shear thinning behavior and fitting the Herschel‐Bulkley model. Thixotropic areas enclosed by the up curve and the corresponding different down curves, STh, were obtained. With the aim of establishing comparative results, and since the viscosities were quite different, the parameter considered was relative thixotropic area, SR (%)=100 (STh/Sup). These areas increased with agitation time and depended on temperature. Their tendencies were reproduced by equations SR=SRmax {1‐exp [‐f(t)]}, with f(t)=βt0.5, where the maximum relative thixotropic area, SRmax , increases with increasing temperature while β decreases. The parameter β has different values for each formulation, and indicates the time dependence. The emulsion formulated with pectin was found to be the most stable with temperature and agitation time, while the greatest thixotropy corresponded to the emulsion formulated with starch only. The thixotropic behavior of the emulsions formulated with gellan gum was seen to be intermediate between the two above formulations—the emulsion formulated with the largest percentage of gellan gum being the least thixotropic of both.

Pseudoplasticity and thixotropy of different types of starch hydrogels prepared with microcrystalline cellulose-sodium carboxymethyl cellulose

ABSTRACT Based on experimental measurements of apparent viscosity and employing the Ostwald-de Waele model, a number of mathematical functions were obtained representing the pseudoplastic thixotropic behaviour of hydrogels prepared with microcrystalline cellulose-sodium carboxymethyl cellulose (MCC-NaCMC) and four types of starch of different origin (corn, potato, rice and wheat). The results obtained corresponding to each parameter of the model and for each gel were analyzed as a function of starch concentration and agitation time.

THE INFLUENCE OF STARCH ON THE RHEOLOGICAL BEHAVIOUR OF MICROCRYSTALLINE CELLULOSE HYDROGELS

ABSTRACT A study has been made of the influence of physical factors such as temperature, agitation time and shear rate upon the rheological behaviour of hydrogels composed of 2·5% microcrystalline cellulose -sodium carboxymethyl cellulose (Avicel RC 591(r), American Viscose Div., FMC Corp.) and different percentages of corn starch (0-3% by weight, w/w).