Crosslinked Borate HPG Equilibria and Rheological Characterization

Abstract

Abstract Viscoelastic properties of hydroxypropyl guar (HPG) crosslinked by borate ions were studied using steady shear and dynamic oscillatory shear experiments. Chemical equilibria involving boric acid, borate ions, and borate ions associated with cis-diol sites on the polysaccharide chains determine the number of crosslinks. These equilibria are functions of temperature and pH, and thus the rheology of the borate gels depend on temperature and pH. Dynamic oscillatory measurements on polymer solutions crosslinked by borax were performed over the temperature range from 10°C to 65°C, and the pH range from 6.35 to 9.5. The storage and loss moduli obey time-temperature superposition so that master curves can be constructed over 6 decades in frequency. Leibler, et al. have recently proposed a theory for the rheology of associating polymer systems. Following this theory, we have subtracted the solution moduli from the gel moduli and the resulting reduced moduli data follow a single time constant Maxwell model. Steady shear experiments were performed in capillary tubes at two different diameters to determine wall slip. The shear rates studied were such that the chains do not have sufficient time to relax completely. By analysing the data from the two tubes, the gel was found to slip at the wall. Three different regions were seen in the stress – shear rate behavior of these gels. At low rates, the stress increases rapidly with shear rate. At intermediate shear rates, the stress remains constant and at high shear rates, the stress increases slowly with rate and the gel is found to slip in this region. The actual stress – shear rate behavior after accounting for slip and slip velocities were determined.