Dynamic vapour sorption of wood and holocellulose modified with thermosetting resins

Micro-veneers of wood and holocellulose (HC) were modified with the thermosetting resins phenol formaldehyde and melamine formaldehyde. The dynamic water vapour sorption of the modified and untreated veneers was studied in a dynamic vapour sorption apparatus to assess the effects of resin modification. The adsorption of wood and HC as well as the desorption of wood was little affected by the modification in the low relative humidity (RH) range but decreased in the RH range above 60-70 %. The desorption isotherm of HC, however, was increased in the RH range of 15-80 % due to modification. Resin modification gradually decreased the EMC ratio of wood and HC and also influenced the moisture increment, equilibrium time and sorption rate in RH range above 50-60 % RH for wood and above 70-80 % RH for HC. HC exhibited a clearly lower hysteresis than wood. Modification of wood slightly reduced hysteresis compared to untreated wood, but modification of HC clearly increased hysteresis about to the same extent as that of wood. This indicates that the stiffening effect of lignin and thermosetting resins reduces the flexibility of the HC matrix, which results in increased hysteresis.