Evaluation of Sustainable Zein-Based Barrier Coatings for Cellulose Materials

Sammanfattning

In recent years, there has been a surge in attention towards biodegradable polymer films, driven by the growing environmental awareness concerning the harmful effects of petroleumbased plastics. The maize protein zein, a by-product from wet milling processes, has shown potential to work as hydrophobic coating. The aim of this study has therefore been to find the most promising process conditions for zein-based liquid barrier coatings, including dissolution, application method, film formation and drying conditions, to achieve maximum barrier efficiency. Different concentrations of zein were dissolved in various concentrations of ethanol. In some solutions, a plasticizer, either oleic acid (OA) or linoleic acid (LA) was added. The solutions were then coated, using a blade coater, onto printing paper. Several parameters were explored in this study, including the solution´s application temperature, the number of coated layers,the coating´s drying temperature and drying time. Additionally, the significance of the solution´s storage time and the conditioning time (23°C and 50% Relative humidity) for the finished coatings were investigated. The most promising liquid barrier properties were found for the coating consisting of 10% (g/g EtOH) zein, 30% (g/g zein) OA, and 96% (v/v) EtOH, with the process conditions of; 3 layers with 1h air-drying in between each layer and where the solution has been stored for 8 days before reheated and applied onto the substrate. The coating received a 47% decrease in water uptake compared to reference/ uncoated paper. However, the coatings with 10% (g/gEtOH) zein does not appear to be particularly robust as some of them experience water leakage, suggesting that 20% zein should be used instead for future attempts. SEM imaging revealed that coatings without a plasticizer displayed sharp cracks, indicating that zein becomes brittle in the absence of a plasticizer. In contrast, coatings with OA exhibited smooth and even surfaces on top of the paper fibers and without any distinct pits. In this study it became evident that the liquid barrier properties of the coatings were significantly improved by the addition of plasticizer, where coatings with oleic acid yielded lower water uptake than coatings with linoleic acid. It was further found that the ethanol concentration affects both the solution´s rheology and the final coating´s barrier properties. An ethanol concentration of 96% proved to yield the most effective barrier. Rheological changes were also observed when the solution was cooled down to room temperature. However, the storage time of the solution as well as the conditioning time of the coating showed to have no significant effect on the barrier properties. Lastly, air drying yielded a better barrier than drying the coating in oven at 60°C. From this study it has become evident that the biopolymer zein shows great potential to work as a liquid barrier within a cellulose material. Continued research of these zein-based polymer coatings is highly recommended.