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Treating wood with water-soluble resins is a well-known and effective method to improve the durability of wood. However, there has been no systematic work to date related to the influence of average molecular size of phenol-formaldehyde (PF) resin on the decay resistance of wood, especially of hardwoods. Therefore, the goal of this study was to investigate the effect of average molecular size of PF resin treatment on the resistance of beech wood against brown- and white-rot fungi. Four different average molecular weights ( M  w ) of resol type resin oligomers (297, 421, 655 and 854 g/mol) were examined. Different weight percent gains (WPGs) in European beech ( Fagus sylvatica ) wood blocks (15 × 20 × 50 mm 3 ) were attained through vacuum impregnation using various concentrations of aqueous-PF solutions. Afterwards treated wood blocks passed the leaching and were exposed to brown-rot fungi ( Gloeophyllum trabeum ;  Coniophora puteana ) and white-rot fungi ( Trametes versicolor ) for 16 weeks. No effect of oligomer size on the resistance against  G. trabeum  decay of wood blocks was observed, resulting in resin loadings of 7–8%. The required WPG for resistance to brown-rot decay by  C. puteana  increased slightly with increasing oligomer molecular size: 6, 7, 10 and 11% for wood treated with 297, 421, 655 and 854 g/mol, respectively. The extent of white-rot fungal decay resistance of treated wood was affected by the molecular size of oligomers. Resin loadings of 8% and of 17% against  T. versicolor  were required to attain similar durability levels for beech wood treated with  M  w  = 297 and 854 g/mol, respectively.

Effect of phenol-formaldehyde (PF) resin oligomer size on the decay resistance of beech wood