Effects of dry heat and saturated steam on the physical properties of chitosan

Abstract

Heat may be employed to facilitate the processing of chitosan and to confer sterility on chitosan-based medical products. In this study, changes were analyzed of the physical properties of purified chitosan heated at 60 to 160°C under specified conditions for periods ranging from 0.5 to 4 h. Two forms of heat were used for processing: dry heat generated by a convection oven and saturated steam generated by an autoclave. Dry heat at ≤80°C resulted in less rigid chains with lower glass transition temperature, improved aqueous solubility, and slightly higher [η]. At higher temperatures, dry heat produced chromophores, which may be related to interchain crosslink formation involving the NH2 groups. The [η] and aqueous solubility of the samples decreased with temperatures ≥ 120°C. The coloration of the samples intensified from yellow to brown with increasing temperatures and duration of heat exposure. Chitosan heated at 160°C for ≥2 h was insoluble in the 0.2 M acetic acid/0.1 M sodium acetate solvent. The rate and extent of the thermal reactions were increased in the presence of saturated steam; the autoclaved samples became insoluble after 2 h of heating at 115°C and after 1 h at ≥120°C. On the other hand, the physical changes induced by dry heat at ≤120°C were not affected significantly by heating the chitosan samples under anoxic conditions.