Abstract

Graft copolymers of starch/amylose and acrylamide have been synthesized using ceric-ion-initiated solution polymerization technique. These polymers have immense prospects for improved oil recovery operations. The intrinsic viscosity, biodegradability and the rheological properties of aqueous solutions of these polymers have been measured. Elemental analysis, XRD and thermal analysis have also been carried out for the above graft copolymers. It is observed that when the graft copolymers contain longer grafts, their intrinsic viscosities become higher. These polymers are shear-stable and controllable biodegradation resistant. The presence of nitrogen is found to be the highest in the starch-g-acrylamide of longer branches. The polymers are amorphous and aqueous solutions of the graft copolymers are non-Newtonian and pseudoplastic in nature.

Introduction

Synthetic polymers like polyacrylamides are widely used as viscosity improvers during improved oil recovery operations. Shear degradation and loss of viscosity under the reservoir conditions are commonly encountered during such operations. Although natural polymers like starch and guar gum have been used in polymer flooding for improved oil recovery, these polymers are highly biodegradable.

It is envisaged that acrylamide grafted polysaccharides would offer shear resistance and controlled biodegradation resistance polymers for such operations. In the present investigation, ceric-ion initiated solution polymerization technique in an inert atmosphere has been followed for the synthesis of these graft co-polymers. Scaling up of starch-g-acrylamide up to 10 litre batch reactor level has also been carried out. Synthesis and characterization of these copolymers have been discussed in detail to find out the possible use of these graft copolymers in improved oil recovery operations.

Experimental

In the present investigation, ceric ammonium nitrate initiated redox polymerization technique and scale up procedure as mentioned by Ungeheuer et al. have been followed. Five different graft copolymers have been synthesized using various polysaccharide backbones, of which two are based on starch and three are based on amylose backbones respectively. Soluble potato starch (GR grade) from Loba Chemie, India, amylose from corn (Practical grade) from Sigma Chemical Co., USA, acrylamide from Merck-Schuchardt, Germany, ceric ammonium nitrate (Reagent grade) from Loba Chemie, India, nitric acid (Analar grade) and acetone (Analar grade) from BDH, India have been used. The detailed synthesis procedure has been described elsewhere. The details of synthesis materials is given in Table 1. The synthesized graft copolymers of starch/amylose have been characterized by the following techniques.

Intrinsic viscosity measurements

The viscosities of the dilute aqueous solutions of the graft copolymers have been measured by Ubbelohde viscometer (CS/S:0.01). The intrinsic viscosities are obtained by extrapolation of the data to zero concentration. The polymer solutions have been prepared by dissolving the graft copolymer in deionized distilled water and viscosity measurements have been carried out at constant temperature 30 0.1 C. The intrinsic viscosities have been determined by using Schultze-Blaschke equation:

(1)

where, is the specific viscosity, is the intrinsic viscosity, QSB is the Schultze-Blaschke constant and c is the concentration of the polymer solution.

Biodegradation studies

For studying the biodegradation of starch, amylose and their graft copolymers, their solutions in deionized distilled water have been prepared. For starch and amylose, 10,000 ppm and for all solutions of the graft copolymers, the solutions of 1,000 ppm concentration have been prepared. The loss of viscosity of the polymer solution under test primarily indicates the onset of biodegradation. Monitoring the viscosity of the polymer solution over a period of time has been used as the measure for biodegradation. The viscosity measurements have been performed with Ubbelohde viscometer (CS/S:0.01) following the procedure as mentioned above for three weeks at 30 0.1 C. The results have been plotted as relative viscosity () vs. time.

Rheological studies

The polymer solutions in deionized distilled water have been prepared as mentioned earlier. The apparent shear viscosity of polymer solutions of 0.25% wt have been measured at various shear rates from 1.5 s-1 to 1312 s-1 using Rotational Viscometer (Rheotest-II, Germany). A commercial linear polyacrylamide (PAM-AP-45) was used for the comparison of the results obtained.

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