Effect Of Molecular Mass Concentration And Temperature On The Rheological Properties Of Non Newtonian Aqueous Polymeric Solutions

An experimental study has been conducted to understand the rheological behavior of several non-newtonian polymeric liquids. A constant stress/strain rheometer with cone and cylinder geometry is used to determine the variation of shear stress (and hence apparent viscosity) with shear rate. A number of non-newtonian polymeric liquids that consist of carefully prepared aqueous solutions of Hydroxyethylcellulose (HEC), Polyethyleneoxide (PEO), Carboxymethylcellulose (CMC), Xanthan gum, and Polyacrylic acid (Carbopol) are used in this study. In order to ascertain the effect of molecular mass of the polymers on the apparent viscosity variation with shear rate, different grades of each polymer are used. The tests are done for varying concentrations and temperatures (23-80 deg C) to determine their requisite effects on rheology. Also, intrinsic viscosity of each polymeric liquid is obtained by using an Ubbelohde capillary tube viscometer. This characterizes the inherent capacity of each polymer to alter its solutions viscosity. Depending upon the polymer and its concentration, the results show both shear-thinning behavior with shear rate as well as time-dependent (thixotropy) effects. Time-dependent effects, which are found to be a strong function of concentration and temperature, are evaluated by measuring viscosity by first increasing and successively decreasing the shear rates. The zero-shear viscosity and shear-thinning effects are seen to decrease with concentration. In general for most polymers, high temperatures decrease the viscosity of the solution, with the exception of Carbopol for which the viscosity is seen to increase slightly with temperature and then decrease at even higher temperatures. Carbopol polymers also show thixotropic effects at higher temperatures which are significant at 80 deg C. They further exhibit yield stress at higher concentrations and no yield stress is noticed at lower concentrations. A number of models described in the literature are used to fit the rheological data. It is noticed that the modified power-law model, Sisko model and Herschel Bulkley model are insufficient approximations of the asymptotic forms and hence, could not be used to describe the rheological behavior of all the polymers. The appropriately modified and redefined asymptotic forms of all these models are used to correctly fit the data for various polymers. The variation of the different model parameters with changes in concentration and temperature are quantified, and a detailed comparative study on the effects of molecular mass, concentration, and temperature on rheological properties is presented.