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

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. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 111–116, 1999