Moisture Sorption Isotherms and Thermodynamic Properties of Biodegradable Polymers for Application in Food Packaging Industry
This work aims to evaluate the influence of two starch-based materials (B16 and B20) on the moisture sorption isotherms, determined at 30, 40, and 50 &deg;C, where B16 contains 5% (w/w) more starch than B20. Thermodynamic functions (differential enthalpy (&#8710;Hdif), differential entropy (&#8710;Sdif), integral enthalpy (&Delta;hint), integral entropy (&Delta;Sint), free Gibbs energy (&#8710;G), and spreading pressure (&phi;)) were used to understand the water-binding behaviors and the energy requirements to remove the moisture content from the surface of these materials. The moisture sorption isotherms exhibited type III behavior, and the Guggenheim&ndash;Anderson&ndash;de Boer (GAB) model was the most suitable to fit the experimental moisture adsorption data. The adsorption isotherms of microparticles were enthalpy-controlled, with isokinetic temperature values of 221.45 and 279.77 K for B16 and B20, respectively, being higher than the harmonic mean temperature (312.94 K). The values of &#8710;G were positive (45.274 and 44.307 kJmol&minus;1 for B16 and B20, respectively), indicating a non-spontaneous process. The spreading pressure values increased with increasing water activity (aw) for all isotherms. Higher values of &#8710;Hdif and &#8710;Sdif obtained from B16 confirmed its higher number of sorption sites available for binding with water molecules when compared to B20, making it less suitable for application in the food packaging industry.