35. Moisture Sorption Isotherm and Isosteric Heat of Sorption Characteristics of PVP-CMC Hydrogel Film: A Useful Food Packaging Material

Hydrogels are polymeric materials possessing a three-dimensional network structure and can absorb a large quantity of liquid water. Recently, hydrogel-based polymeric materials are being focused and encouraged as they are breathable and maintain the shelf life of fresh fruit and vegetables. A hydrogel film was prepared using synthetic polymer, polyvinylpyrrolidone (PVP), and biopolymer, carboxymethyl cellulose (CMC) and agar, along with polyethylene glycol and glycerol as plasticizer to create a breathable and biodegradable film termed as “PVP-CMC” hydrogel film. In general, hydrogel film provides poor but composition- and structure-dependent moisture resistance in normal atmosphere. Further, interaction among the plasticized ingredients, which are dependent on its ultimate moisture content, controls the physical and mechanical properties of the film. Therefore, it is important to know moisture sorption characteristics of each hydrogel film. The “PVP-CMC” hydrogel film exhibited a tendency to adsorb/desorb moisture depending on environmental relative humidity and temperature during storage. Hence, the general features and the equilibrium relationship between moisture contents of “PVP-CMC” hydrogel film at different temperatures (25, 35, 45 and 55 °C) and relative humidities (≈ 10–90%) of the environment in which the film generally resides, i.e., moisture sorption isotherm (MSI), will be discussed in this chapter. The isosteric heat of sorption at different moisture contents of “PVP-CMC” hydrogel film will also be discussed.