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This study reports on a new approach for the detection of phenol-formaldehyde (PF) resin distribution on the strands surface after air-spray application in a glue drum using reflecting light microscopy (LM). The method allows visualization and quantification of PF adhesive distribution on wood strands by using either waterproof paper or 200 µm thick microtomed kiri (Paulownia tomentosa) veneer slices, which were clipped with staples on conventional strands typically used for the production of oriented strand boards (OSB). These strands were mixed with other conventional OSB strands and two types of molecular weight PF prepolymers were sprayed on the strand mixture. While the total area of adhesive coverage was similar for both adhesives, the low molecular weight (LMW) PF prepolymer formed a higher number of adhesive droplets than the high molecular weight (HMW) PF prepolymer. The study offers a new, easy applicable approach for a qualitative investigation of adhesive distribution on the strand surface during gluing on a laboratory and pilot plant scale. [ABSTRACT FROM AUTHOR]

The objective of this study was to determine the effects of four phenol–formaldehyde (PF) resin treatments with different molecular weights at four different concentrations (5, 10, 15, and 20%) in treated beech wood. The mechanical properties of untreated and treated beech wood were evaluated. After impregnation with PF resin, all modified beech wood at all PF resin concentrations exhibited an increase in weight percent gain compared with that in untreated beech samples. PF resins with lower molecular weights more easily penetrate the wood cell wall, leading to increased bulking of the wood structure, which in turn improves the dimensional stability of the wood. The PF resin treatment with a molecular weight of 305 g/mol showed better impregnation ability than that of the other PF resins. The impact bending strength of PF-treated wood was considerably reduced because PF-cured resins formed inside the wood and are rigid and brittle. Additionally, PF resin treatments at all concentrations decreased the modulus of elasticity of the wood. Scanning electron microscopy and light microscopy revealed that the PF resins were comparatively well fixed in the wood samples. The results indicate that the large molecular weight PF resins are more uniformly distributed in the fiber lumens. [ABSTRACT FROM AUTHOR]

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 (Mw) 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. [ABSTRACT FROM AUTHOR]

Using light microscopy (LM), the distribution and penetration depth of phenol-formaldehyde (PF) resin into wood specimens were examined after the seventh dry-wet cycle following the anti-swelling efficiency method. PF resins with the average molecular weights (Mw) of 297, 421, 655 and 854 g/mol at concentrations of 9, 18 and 27 wt% in water were vacuum-impregnated into beech wood (Fagus sylvatica) blocks (25 × 25 × 10 mm3). The measurements of the increase in cell wall thickness (bulking) showed that the 297 and 421 g/mol oligomers resulted in greater cell wall bulking (14.9 and 15.4%, respectively) than the 655 and 854 g/mol oligomers (11 and 9.9% bulking, respectively). The presence of chemical agents in the wood was detected based on the safranin staining intensity of the specimen's transverse sections. The amount of safranin adsorbed on the beech wood sections decreased as the amount of resin incorporated into the cell walls increased, apparently due to a decrease in the number of free microvoids available for safranin adsorption. Less intense dye staining was observed in treated sample sections compared with untreated transverse sections. The lowest amount of safranin was found in wood that was treated with phenol oligomers of 297 and 421 g/mol; wood treated with oligomers of 655 and 854 g/mol adsorbed greater amounts of safranin. Thus, high-molecular-weight PF resins were more uniformly distributed in the specimen and mainly located in the fiber lumens. The results show that safranin staining and visual evaluation by LM is a simple and reliable method to determine the location of PF resin in treated wood. [ABSTRACT FROM AUTHOR]

This study investigated the effect of phenol-formaldehyde (PF) resin treatment on the weathering stability and biological durability of birch plywood. Silver birch (Betula pendula) veneers were vacuum-pressure impregnated with four different PF resins with average molecular weights (Mw) of 292 (resin A), 528 (resin B), 703 (resin C), and 884 g/mol (resin D). The aging properties of PF resin modified birch plywood were analyzed using artificial weathering with ultraviolet (UV) light, UV and water spray, and weathering under outdoor conditions. The same combinations of PF-treated plywood specimens were then tested in soil-bed tests to determine their resistance against soft-rot wood decay. It was not possible to compare weathering processes under artificial conditions to processes under outdoor conditions. However, the weathering stability of birch plywood treated with PF resins A, B, and C, scored better than plywood treated with commercial resin D (regardless of solid content concentration [%]). Results from unsterile soil bed tests showed improvements in resistance to soft-rot wood decay compared to untreated plywood and solid wood. Mass loss [%] was lowest for birch plywood specimens treated with resin of highest solid content concentration (resin D, 20%). Provisional durability ratings delivered durability class (DC) ratings of 2–3, considerably improved over untreated solid wood and untreated birch plywood (DC 5). [ABSTRACT FROM AUTHOR]