Effects of partial extraction of amorphous wood polymers combined with densification on the surface characteristics of birch veneers

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 698; p. 134582
• Main Authors: Jakob, Matthias, Halbauer, Peter, Gindl-Altmutter, Wolfgang, Müller, Ulrich
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.10.2024
• Subjects: acid insoluble lignin; Alkaline treatment; Betula; Contact angle; Densification; Fourier transform infrared spectroscopy; Gibbs free energy; hemicellulose; Surface energy; Surface roughness; Wettability; wood; Alkaline treatment; Wettability; Surface roughness; Densification; Contact angle; Surface energy
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2024.134582

The densification of wood for modern applications is currently the subject of intensive research. However, despite the extensive research on the mechanical properties of densified wood, there is scarcity of research on the characterization of surface properties, especially for densified wood that has been chemically treated in a first step. Since surface properties have a major role in bonding and thus in the production of larger composites, the influence of partial extraction of amorphous wood polymers and densification of wood on its surface chemistry, morphology/haptic, surface free energy and wettability was evaluated. Therefore, birch veneers were pretreated with an alkaline solution for various durations (2, 4 and 6 h) and subsequently densified via hot pressing. The changes in surface chemistry were studied by means of Klason lignin analysis and Fourier transform infrared spectroscopy (FTIR). Morphology and haptic were studied by means of surface roughness measurements. The surface free energy was determined according to the acid base theory using contact angle measurements of diiodomethane, formamide and deionized water. Additionally, the water contact angle data were further used for wettability measurements. In general, it can be stated, that the two-step densification process has an influence on the surface properties of birch veneers. However, regarding pretreatment durations, almost no differences were observed within densified veneers. The results, based on the FTIR spectra, revealed, that the two-step densification process caused distinct changes within the hemicelluloses bands (1735 cm−1) on the veneers’ surfaces. Simultaneously it was shown, that the Klason lignin content slightly increased from ∼20 % to ∼21.8–23.8 % after pretreatment, what in turn indicated a decrease in hemicelluloses. The average surface roughness was reduced by the factor ∼5 and ∼1.8 parallel and perpendicular to the fiber direction, respectively, after applying the two-step densification process. No distinct differences of the total surface free energy were observed between untreated (∼41 mJm−2) and densified veneers (∼37.4–40.6 mJm−2). However, notable differences were observed between aged and sanded surfaces, latter exhibiting with values between 56.6 and 58.5 mJm−2 distinct higher total surface free energy, regardless of pretreatment and pretreatment duration. [Display omitted]