In this study, we examined the distribution and penetration depth of phenol-formaldehyde (PF) resin into wood specimens by light microscopy (LM). Two PF resins from different producers, of concentrations 9, 18 and 27 wt % in water, were vacuum impregnated into European beech wood (Fagus sylvatica) blocks of 15x25x50 mm³. The presence of chemical agent in the wood was carried out by way of their identification on the specimen cross-section as result of different intensity of safranin stains. The amount of the adsorbed dye on the cross-section of beech wood was reduced by an increased amount of incorporated resin in the cell walls, which is corresponding to less free microvoids available for adsorption of safranin. Less intensity of dye was observed from cross-sections of treated samples compared to untreated. The lowest amount of safranin was found for wood treated with oligomers of resins A, whereas wood treated with oligomers of B in comparison differed. Measurements of the increase in cell wall thickness (bulking) showed that oligomers A resulted in greater cell wall bulking (approx. up to 25-30%) compared to oligomers B. This relationship indicates that the latter resins are more uniformly distributed along the specimens, and mainly located in the fiber lumens. The current studies show that the dyeing with safranin and visual evaluation by light microscopy (LM) is a simple and reliable method to determine the PF resin location in the treated wood.

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