Equilibrium Isotherms of a Biodegradable Polymer Polyhydroxybutyrate (PHB): Heat of Desorption Determination

Polyhydroxybutyrate (PHB) belongs to a group of bacterial polyesters that may be considered as "green plastics", because of their biodegradable nature. The knowledge of the relationship between moisture content and water activity is essential for drying and storage. The thermodynamic equilibrium is characterized by equilibrium isotherms, and its determination is essential for better understanding of mechanisms in drying operations. Results on desorption isotherms of PHB are not available in the literature. Thus, the aim of this study was to analyze the equilibrium isotherms of PHB using GAB (Guggenhein-Anderson-de Boer) and GDW (generalized D'Arcy and Watt) models, and to determine the heat of desorption using Clausius-Clapeyron equation. The moisture equilibrium data for desorption isotherms of PHB were investigated at 40, 50 and 60 degrees C using the gravimetric static method. The monolayer moisture contents (from 0.16 to 0.23 % d.b.) decreased with increasing temperature. The Clausius-Clapeyron equation was considered adequate to forecast the heat of desorption, which was estimated as a function of the moisture content.