Synthesis of starch acetate: Statistical designed experiments to optimize the reaction conditions

The synthesis conditions for Durabond A corn starch acetate (SA) were varied by using statistical designed experiments with the aim to optimize the yield and solubility, and to minimize the color. The thirty experiments in the design vary the: (1) Percentage water in the starch (0, 3, 6); (2) catalyst concentration (0.25, 0.60, 0.95 wt%); (3) total amount of acetic anhydride (1.0, 1.25, 1.5 mol times the number of hydroxyl groups on the starch); (4) starch concentration (15, 25, 35 wt%); (5) reaction time (1, 2.5, or 4 h); and (6) reaction temperature (60, 80 or 100°C). The major factors controlling the acetyl value (AV), yield and the solubility are the reaction temperature (T) and the T squared. Some two factor interactions that effect the process are: (1) water percentage with T, (2) the reaction time with T, and (3) the catalyst concentration with starch concentration. The optimal conditions are: (1) a T range from 84°C to 92°C; (2) a sulfuric acid catalyst concentration of 0.25 wt% with respect to the starch concentration, (3) an acetic anhydride ratio of 1.0; (4) a 35 wt% starch concentration; (5) a 4 h reaction time, and (6) a percentage water in the starch of (0–6%). There is a strong correlation between the 10% and 20% SA solution solubility in acetone:water (96:4) with the AV. If the AV is > 55 then > 90% of the SA at a concentration of 10% is soluble in acetone:water (96:4).