Viscosity Stability of Aqueous Polysaccharide Solutions

Abstract

Abstract The viscosity stability of carbohydrate polymers under thermal-oxidative, mechanical and acid-catalyzed hydrolysis is examined and compared with the performance of a synthetic water-soluble polymer, hydrolyzed (30%) poly(acrylamide) (HPAM). Under thermal-oxidative conditions HPAM is the most stable polymer and the carbohydrate polymers prepared by fermentation systhesis (i.e., Xanthan gum (XCPS) or Sclerotium glucanicum polysaccharide, (SGPS) are more stable than non-fermentation polysaccharides (i.e., guar gum, hydroxyethyl cellulose, cellulose sulfate ester, etc.). In acid catalyzed hydrolysis the same differential performance between fermentation and non-fermentation carbohydrate polymers is observed. This difference is correlated with an ability to form helical aggregates which protects the repeating acetal linkage in carbohydrate polymers from oxygen and proton attack. Additives that disrupt hydrogen bonding among pyranose units and therefore disrupt helical aggregate structures in fermentation polymers decrease their solution viscosity stabilities. A molecular weight dependence is not observed under either thermaloxidative or acid catalyzed degradation conditions, but a dependence is observed in mechanical degradation. Lower molecular weight polymers are more stable. For mechanical stability, a rigid-rod conformation is sufficient for stability; some non-fermentation polymers exhibit stability equivalent to fermentation polymers. Less mechanically stable carbohydrate polymer solutions (i.e., hydroxyethyl cellulose, xanthan gum in a 4M urea or SGPS in alkaline solutons) are more stable than conformationally and segmentally flexible synthetic water-soluble polymers (i.e., HPAM or polyethylene oxide). The influence of oxygen as an antagonist to polymer stability is investigated in the presence of two stabilizing additives, magnesium oxide (MgO) and tetraethylenepentamine (TEPA). In oxygen saturated solutions thickened with non-fermentation carbohydrate polymers, both function effectively, primarily by inhibiting the formation of hydroperoxide groups. In aqueous systems containing oxygen scavengers, MgO is ineffective; TEPA continues to be effective due to its ability to chelate transition metals, modify the scavenger's activity and intercede in the synergistic interaction of these activators with oxygen. The use of either of these additives in field applications can result in well bore problems.