Combination of internal and external plasticization of hydroxypropylated birch xylan tailors the properties of sustainable barrier films

To develop functional and sustainable products from future wood biorefineries, birch kraft pulp xylan was alkali-extracted and modified in aqueous solvent with hydroxypropylation to enhance its processability to films. For the first time, the roles of internal vs. external plasticization in the mechanical, thermal, and barrier performance of xylan films were systematically evaluated as a function of the degree of substitution (DS) of hydroxypropyl (HP) groups (internal plasticizer) and the added sorbitol (external plasticizer). In addition, the moisture uptake and the degree of crystallinity of hydroxypropyl xylan (HPX) films were characterized. Internal plasticization of xylan with HP groups enabled cohesive film formation, but did not form barrier materials alone. The addition of sorbitol as external plasticizer was necessary to obtain gas barrier films. The combinations of internal and external plasticization at selected ratios enabled the tailoring of film properties. Low to medium DS of HP and the addition of 10% or 20% sorbitol resulted in promising barrier properties. The tensile strength of the HPX films was rather high between 27 and 43 MPa, depending on the level and type of plasticization. The thermal stability of HPXs increased and the degree of crystallinity of xylan in the films decreased as the internal plasticization increased. The glass transition temperature of HPXs and HPX films decreased more with external than with internal plasticization. The studied concept would benefit the sustainable exploitation of natural resources. The promising material properties, high degree of biobased raw materials, and non-food origin make HPX-based films an interesting option for multilayer paperboard coating for food packaging, for example.