Wood hydrolysates- New concepts in renewable material design

Abstract: Wood hydrolysates (WHs) are biomasses obtained in processes involving hydrothermal treatments of wood. WHs are rich in hemicelluloses and lignin. Instead of complicated extraction processes, such as precipitation, or extensive purification, this work utilizes the crude WHs. These WHs were successfully developed into oxygen-barrier films and coatings and a conceptual model was established to predict biomass matrix performance from molecular structures and interactions.Free standing films and coatings from blends of WHs and either chitosan or carboxylmethyl cellulose (CMC) as a co-component, were produced. The films had an excellent oxygen-barrier performance which was even better than the corresponding films prepared from highly purified hemicelluloses. From a fundamental point of view, the Hansen solubility parameter (HSP) theory revealed the interactions between molecules in WHs, as well as in WH-based blends, which shed light on the great barrier performance of WH-based films. The hypothesis was that these strong interactions increased the mutual molecular affinity in the matrix, which led to a denser molecular packing and hence a good oxygen-barrier performance. The positron annihilation lifetime spectroscopy (PALS) measurements quantified the free volumes in the WH-based matrices. The HSP results, free volume size and distribution and thermal analyses supported the hypothesis well.The HSP model was then used as a tool for designing oxygen-barrier coatings from WH-based blends. Using the HSP model, the interactions between different WHs and CMC were calculated and quantified. According to our hypothesis, the oxygen-barrier performance could thus be predicted. The free volume from PALS, oxygen permeability (OP) results were generally in consistence with the prediction from HSP model, which shows the potential of the HSP model for designing formulations of WHs for oxygen-barrier coatings for food packaging. To realize an industrially feasible and efficient process for WH-based barrier coating layers, some new approaches were attempted. Previously, coatings were prepared manually from WH-based blends. Now, a creative spray drying (SPD) coating technique was developed for a reproducible, efficient coating process of WHs with no additives or second components. Also, glyoxal crosslinking improved the ductility of the WH-based coatings which in turn improved the quality and oxygen-barrier performance of the coatings. Finally, scanning electron microscopy (SEM) integrated with a micro-tensile test device made it possible the real-time observation of coating surfaces during the deformation process. This can successfully analyze the mechanical performance of the coatings without being influenced by substrate. In summary, the WHs present a viable and very promising resource for green barrier design.