Characterization of suberinic acids from birch outer bark as bio-based adhesive in wood composites

Huge amounts of synthetic binders and polymers are used for the production of wood-based composites which are toxic to humans during the production and exploitation of products because of the risk of harmful vapor emissions. The bio-based society requires new value chains for the production of bio-based materials. Suberinic acids (SA) have a high potential to replace synthetic binders and polymers due to the unique structure. In this study, hydrolytic depolymerization of birch outer bark in alkaline environment was carried out. To evaluate the adhesive properties, the chemical composition and structure were analyzed using chemical and instrumental methods. To set independent variables for the design of experiments (DOE), the following preliminary parameters were investigated: filtered and unfiltered SA; impact of the depolymerization agent; the fractional composition of the raw material. Based on the obtained results, DOE with three independent variables (depolymerization time, pH after acidification, alkali concentration) was used for the selection of optimal depolymerization parameters. It has been concluded that, with increasing concentration of KOH in the depolymerization solution and the reaction time, suberin is depolymerized to a higher extent and thus there is a higher amount of monomeric SA, monosaccharides and polyphenolic compounds in the resulting adhesive, which positively affects the physico-mechanical properties of the obtained particleboards. These substances together form cross-linked structures that provides not only the mechanical strength but also water resistance of the resulting particleboards. The optimal SA-based adhesive was improved by increasing the KOH concentration to 4%, acidifying to the pH 3 and reducing the reaction time to 0.5 h. The obtained particleboards' properties (modulus of rupture – 19.0 N mm−2, modulus of elasticity – 2604 N mm−2, thickness swelling – 14.3% and internal bonding – 2.25 N mm−2) meet the requirements of the particleboard standard EN 312 type P3. Still, this is an experimental work and the hot-pressing parameters must be decreased to consider the possibility to introduce the adhesive in the large-scale industry.