Penetration depth of phenol-formaldehyde (PF) resin into beech wood studied by light microscopy

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 (M_w) 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.