Design of Hygroscopic Bioplastic Products Stable in Varying Humidities

Abstract

Hygroscopic biopolymers like proteins and polysaccharides suffer from humidity‐dependent mechanical properties. Because humidity can vary significantly over the year, or even within a day, these polymers will not generally have stable properties during their lifetimes. On wheat gluten, a model highly hygroscopic biopolymer material, it is observed that larger/thicker samples can be significantly more mechanically stable than thinner samples. It is shown here that this is due to slow water diffusion, which, in turn, is due to the rigid polymer structure caused by the double‐bond character of the peptide bond, the many bulky peptide side groups, and the hydrogen bond network. More than a year is required to reach complete moisture saturation (≈10 wt.%) in a 1 cm thick plate of glycerol‐plasticized wheat gluten, whereas this process takes only one day for a 0.5 mm thick plate. The overall moisture uptake is also retarded by swelling‐induced mechanical effects. Hence, hygroscopic biopolymers are better suited for larger/thicker products, where the moisture‐induced changes in mechanical properties are smeared out over time, to the extent that the product remains sufficiently tough over climate changes, for example, throughout the course of a year.

Discrepancy of Effective Water Diffusivities Determined from Dynamic Vapor Sorption Measurements with Different Relative Humidity Step Sizes: Observations from Cereal Materials

“…Water diffusivity varied with initial water concentration and RH of drying air because these factors alter the material properties such as microstructure [ 41 ] and size [ 31 ] simultaneously as they cause water transfer. Thickness–dependence of water diffusivity is well documented in the literature for synthetic polymer [ 43 ] and biopolymer [ 44 ] materials, which can be explained in terms of polymer confinement [ 41 ] or a two-region model [ 45 ]. Therefore, the thickness–dependence of water diffusivity is understandable, as all these proposed explanations point to property modification accompanied by thickness variation.…”