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This study investigated the effect of phenol-formaldehyde (PF) resin treatment on the dimensional stability of birch solid wood and plywood. Therefore, three different low molecular weight PF resins with average molecular weights (M w ) of 292, 528, and 884 g/mol were synthesized and used for silver birch (Betula pendula) solid wood (20 × 20 × 20 mm3) and veneer (100 × 100 × 1.4 and 300 × 300 × 1.4 mm3) impregnation to produce plywood. The weight percent gain (WPG) and bulking after curing of resin treated wood specimens were determined. The leaching was performed to evaluate the PF resin fixation stability. All tested resins at all concentrations were similarly fixed in wood blocks after the leaching (1.5–2.0% WPG loss). The dimensional stability of birch wood after treatment with all tested PF resins was significantly improved. The anti-swelling efficiency (ASE) of birch wood blocks treated with PF resins after seven water soaking-drying cycles was in the range of 39–53%, 46–65% and 49–65% using 10, 15 and 20% solution concentrations, respectively. Whereas, the ASE of plywood obtained from veneers treated with 10% of PF solution was improved by 6–20%. The equilibrium moisture content (EMC) and volumetric swelling (VS) of PF treated plywood at 98% of relative humidity (RH) decreased significantly. All PF resin treated plywood surfaces were more hydrophobic compared to untreated plywood.

The effect of thermo-hydro treatment (THT) on the properties of birch (Betula spp.) wood veneers has been studied. THT was carried out in a multi-functional pilot scale wood modification device of wood treatment technology (WTT, Latvia) under elevated water vapor pressure conditions at four combinations of temperature and treatment time (°C/min): 150/10; 150/50; 160/10 and 160/50. After THT, the following veneer properties were examined: mass loss (ML), chemical composition, bending strength (BS), tensile strength (TS), equilibrium moisture content (EMC), resistance to decay by mould and blue stain fungi, and surface contact angle (CA). The chemical components were changed by THT. Increased THT temperature and time resulted in hydrophobization of veneers as indicated by decreasing EMC and increasing CA data. All THT were effective against wood discoloring fungi, although insufficient decay resistance was observed. The mechanical strength properties of THT veneers were also deteriorated.

Thermal modification (TM) of wood has occupied a relatively narrow but stable niche as an alternative for chemical wood protection. There are different technological solutions for TM and not all details of their effects on wood tissue have been understood. The one-stage hydrothermal modification (HTM) at elevated vapour pressure essentially changes the wood’s composition and structure. In the present paper, the changes in three hardwood lignins (alder, aspen, and birch) were observed within the cell wall by means of cellular UV microspectrophotometry. The lignin absorbances in the compound middle lamella (CML) of unmodified wood are 1.7- to 2.0-fold higher than those in the fibre S2 layer. The woods were modified in the temperature range from 140 to 180°C, while in the lower temperature range (140°C/1 h), the UV absorbances are little affected. Essential changes occur in the range of 160–180°C and the UV data reflect these by absorbtion changes, while the absorbances at 278 nm rise with factors around 2 more in the S2 layer than in the CML. The absorbance increments are interpreted as polycondensation reactions with furfural and other degradation products of hemicelluloses with the lignin moiety of the cell wall.

The white-rot fungus Trametes versicolor and brown-rot fungi Coniophora puteana, Gloeophyllum trabeum, and Postia placenta have a potential to produce a range of hydrolytic enzymes participating in the degradation of lignocellulosic materials. In the colonization of untreated and thermally modified wood blocks wheat bran-containing medium (WBA) greatly favored the secretion of xylanase by C. puteana and both endoglucanase and xylanase expression by T. versicolor, although rather higher carboxymethyl cellulase activity of C. puteana was observed on the malt extract-based medium (malt extract-containing agar (MEA). Coniophora puteana and T. versicolor are able to actively colonize and degrade the untreated pine and birch wood blocks. The mycelium applied on blocks in the form of pellets was more aggressive in wood degradation in comparison with the colonized mycelium. WBA favored the dry matter loss during the initial stages (10–20 days) of pine wood colonization; however, MEA further promoted more rapid wood degradation. The main finding was that the thermal treatment of birch wood was accompanied by a low degree of fungal colonization and increased durability against the fungal attack in spite of the high cellulase and xylanase activities. Thermal modification of wood was not toxic to the fungi because it did not disturb the secretion of the hydrolytic enzymes involved in wood degradation. The thermally modified hollocellulose is probably less susceptible to the enzymes involved in the degradation of polysaccharides or less accessible to hydrolytic enzymes due to the change of the cell wall structure.

Hot water extraction of pine wood was carried out after 10 days of exposure to brown-rot fungi (Gloeophyllum trabeum, Coniophora puteana and Poria placenta) and a white-rot fungus (Coriolus versicolor). The microstructure of the wood cell wall was analysed by the water vapour sorption method. The content and composition of monosaccharides in the extracts were determined by high performance liquid chromatography. Despite the absence of essential mass losses at the initial stage of the contact with the fungi, slight changes in the cell wall microstructure can be observed, namely, the hydrophobisation and the change in the character of porosity in the region of 1–10 nm pore sizes. After hot water treatment, the mass decreases considerably, i.e., to 25% for the control and brown-rot treated samples and more than 30% for the white-rot treated sample. The sorption-desorption isotherms for the washed control sample and brown-rot samples were not changed practically, but the isotherm was changed clearly in the case of the white-rot treated sample. The microstructure of the white-rot sample changed after hot water washing radically and was accompanied with the formation of wide mesopores in the range of 5–9 nm width.

Brown rotted Scots pine (Pinus sylvestris) sapwood was studied using scanning UV microspectrophotometry. Wood blocks were exposed to the fungus Coniophora puteana (Schum.: Fr.) Karst. (BAM Ebw.15) for 6, 8, 10, 30, and 50 days. No wood weight loss was detected in the initial decay periods. On the other hand, point analyses of lignin distribution in wood cells revealed higher absorbance after 6–10 days of decay, which we interpret as an increase in the absorption coefficient of lignin due to its oxidative modification by the fungus. Uneven wood degradation occurred in the later periods (30 and 50 days), with both significantly decayed and visually sound cells observed. The decayed cells showed a higher absorbance at 280 nm, although the apparently sound cells were also degraded to a lower extent. Degradation of lignin-rich compounds in middle lamellae and cell corners was not observed during fungal attack.

Pinewood was modified by vacuum impregnation with various aqueous lignin solutions of low concentration (0.5– 1.0%) and its decay resistance was tested by the standard procedure EN 113. Five lignin types were tested against three brown rot and one white rot fungi. The bio-durability of wood was considerably increased by the treatment. The highest effect of modification was for alkali, kraft, hydrolysis lignins and industrial lignosulfonate, when mass losses of wood for brown-rot fungi were negligible. The effect of the modification with certified lignosulfonates was insignificant. Chemical analysis revealed that phenols, which are leached from lignin and are adsorbed by wood in the impregnation process, could act as a biocide. The hydrophilic properties of wood either did not change (certified lignins) or were enhanced because of some change in the pore structure (industrial lignins). Further tests are needed to verify the positive effects of this technologically simple and environmentally friendly treatment.