Scots pine (Pinus sylvestris L.) sapwood was impregnated with aqueous solutions of phenol formaldehyde and methylated melamine formaldehyde resins and subsequently cured in an oven. One set of specimens was cured in plastic bags to avoid drying (wet curing) while another set of samples was heated and water was allowed to freely evaporate (dry curing). Macroscopic resin distribution was investigated using X-ray densitometry and infrared spectroscopy (FTIR-ATR). During dry curing, the resins migrated to the wood surface resulting in a gradient. Wet curing resulted in even distribution of the resins because it was immobilized due to condensation and precipitation in the wood. Neither densitometry nor FTIR-ATR was found to be generally applicable for investigating resin distribution in modified wood. Wet curing resulted in low cell wall bulking as compared to dry curing, probably because resin precipitated before drying. Storing impregnated wood prior to curing under non-drying conditions (''diffusion phase'') also reduced cell wall penetration and bulking.

Wood modification improves the properties of wood as a building material by altering the wood structure on a cellular level. This study investigated how dimensional changes of wood on a macroscopic scale are related to the cellular level chemical changes on the micron level after impregnation modification with melamine formaldehyde (MF) resin under different heat curing conditions. Our results showed that the curing conditions affected the polycondensation reactions and the morphological structure of the MF resin within the cell lumen. The diffusion of the resin into the cell wall was estimated based on the triazine ring vibration of melamine in the Raman spectrum at 950-990 cm −1. Thereby, it was shown that macroscopic changes in wood dimensions do not provide a reliable estimate for the cell wall diffusion of the resin. The removal of cell wall constituents during the modification, which was facilitated by the alkaline pH of the impregnation solution, counterbalanced the cell wall bulking effect of the resin. This was particularly evident for wet cured samples, where diffusion of MF resin into the cell wall was observed by confocal Raman microscopy, despite a reduction in macroscopic wood dimensions.

Impregnation modification of wood with melamine formaldehyde resin reduces the adverse effects caused by moisture uptake, but the underlying modes of action are not fully understood. The present study showed that it is crucial to understand the sorption behavior of the pure resin when interpreting the behavior of resin-modified wood. Furthermore, the applied heat-curing conditions had a significant effect on the moisture uptake of resin-modified wood. At the same resin loads, dry curing conditions were more effective in causing a cell wall bulking effect than wet curing conditions. This reduced the water-accessible cell wall pore volume in dry cured wood and counterbalanced the moisture uptake by the resin. Deuterium exchange measurements suggested that the occupancy of cell wall pores reduced the number of simultaneously active sorption sites. However, there was no evidence that a swelling restraint or reduced mechanical relaxation affected the water sorption of resin-modified wood significantly.

Scots pine micro-veneers were subjected to hydrolysis with sulphuric acid or delignifi cation with acidic sodium chlorite and a combination of both treatments. The tensile strength of untreated and treated veneers was determined at fi nite span (f-strength) and zero span (z-strength) under both dry (20 ° C, 65 % relative humidity) and water-saturated conditions. Acidic hydrolysis resulted in signifi cant strength losses in both testing modes and both moisture conditions, with the greatest strength reduction found for f-strength tested dry. After delignifi cation, only f-strength under wet conditions was substantially reduced; dry f-strength and both dry and wet z-strength hardly changed. A combined treatment of prehydrolysis and delignifi cation resulted in disintegration of the veneers, which made strength determination impossible. It was concluded that, in addition to cellulose, the hemicelluloses determine the f-strength under dry conditions, while lignin confers wet strength but appears not to contribute to interfi bre adhesion and f-strength under dry conditions.

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