Thermal Decomposition of Sodium Carboxymethyl Starch.

Abstract

Potato starch (PS) was carboxymethylated with monochloroacetic acid to prepare sodium carboxymethyl starches (Na-CMSs) with different degrees of substitution (DS) at the OH moieties, and their first thermal decomposition was invesitigated at 200 to 300°C. Chemical analyses suggested that Na-CMSs (1), (2) and (3) having DS≤0.3 were the compounds substituted with carboxymethyl groups at the primary OH moieties and Na-CMSs (4), (5) and (6) having 0.3≤DS<1.1 at both primary and secondary OH moieties. Remarkable differences in weight decrease curves and onset temperatures of thermal decomposition were observed between the two groups, Na-CMSs (1)-(3) and Na-CMSs (4)-(6). Upon heating, the C=O bonds based on carboxymethyl groups of Na-CMSs (1)-(3) were partially decomposed, although their structures were unclear. In contrast, the decomposition of C-O bonds based on intra- and inter- molecular ether linkages or hydroxyl groups in the glucose units was mainly observed for Na-CMSs (4)-(6). The increase of DS accompanied the increase of activation energy, but there was no significant difference in the activation energy among Na-CMSs (4), (5) and (6).