Submitted to: Polymer Engineering & Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2003
Publication Date: 10/15/2003
Citation: Willett, J.L., Finkenstadt, V.L. 2003. PREPARATION OF STARCH-GRAFT-POLYACRYLAMIDE COPOLYMERS BY REACTIVE EXTRUSION. Polymer Engineering & Science. 43(10):1666-1674. Interpretive Summary: Starch, a renewable resource derived from corn and other grains, often requires chemical modification to provide specific properties desired for end-use applications. These processes often use large quantities of water and generate significant amounts of by-products. New processes for modifying starch which use less water and reduce environmental impact are desirable. One promising method, known as reactive extrusion, is a continuous process using equipment similar to that used for production of puffed food products. We have demonstrated that reactive extrusion can be used to modify starch and other natural materials in a rapid and continuous manner. This process can be used to prepare modified starches with a wide range of properties, while decreasing water use by half or more. Application of this technology may reduce the cost of producing modified starches and lead to new materials based on starch, in addition to reducing the environmental impact of starch processing.
Technical Abstract: Graft copolymers of starch and polyacrylamide (PAAm) were prepared by reactive extrusion using a co-rotating twin screw extruder and ammonium persulfate initiator. Feed rates were 109 g/min to 325 g/min (all components) at a moisture content of 50%, with screw speeds in the range 100 rpm to 300 rpm. Starch/acrylamide weight ratios ranged from 5:1 to 1.3:1. Conversions of acrylamide to PAAm were generally 80% or greater with residence times of 400 seconds or less. Conversion was independent of residence time, and increased with feed rate, suggesting that reaction efficiency was proportional to the degree of fill in the extruder. Grafting efficiencies were in the range of 50% to 80%. Grafting efficiencies were lower for waxy maize starch than for normal corn starch. Extractable fractions in 30/70 ethanol-water solvent were approximately 10% to 15%, of which 25% to 70% was polyacrylamide. Soluble PAAm increased with decreasing starch/acrylamide ratio. PAAm molecular weight increased with increasing acrylamide content, consistent with free radical polymerization kinetics. Extrusion temperature had no significant impact on acrylamide conversion, while PAAm molecular weights did not show the expected decrease with increasing temperature. Graft frequency, as measured by the number of anhydroglucose units per graft, were essentially constant over the starch:acrylamide ratio and temperature range studied. At a starch:acrylamide ratio of 5:1, graft frequency decreased with increasing screw speed, giving materials with fewer grafts of lower molecular weight. These results show that reactive extrusion offers the potential for rapid production of starch graft copolymers with unsaturated monomers.