Design of Hygroscopic Bioplastic Products Stable in Varying Humidities

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 (approximate to 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.

Automated summary

Hygroscopic biopolymers like proteins and polysaccharides have unstable mechanical properties due to humidity variations. Thicker samples of wheat gluten, a highly hygroscopic biopolymer, exhibit greater mechanical stability than thinner samples, which is attributed to slow water diffusion caused by the rigid polymer structure. Moisture uptake in glycerol-plasticized wheat gluten is significantly slower in thicker plates, taking more than a year to reach saturation compared to one day in thinner plates. The moisture-induced changes in mechanical properties are also affected by swelling-induced mechanical effects.