Cell wall bulking and distribution of different chemicals in pine, Pinus Sylvestris

Sammanfattning: The interaction between wood, Pinus sylvestris, (60% RH) and polyethyleneglycol (PEG) of different molecular weights (PEG 200 and PEG 1500), pentaerythritol and glycerol, impregnated into the wood, has been investigated using Scanning Electron Microscopy, EDS-analysis, dynamical mechanical techniques (DMTA), X-ray diffraction (WAXS) and macroscopic dimensional measurement. Reduced dimensional changes after impregnation when exposed to environments with changing moisture content, showed that the stabilization effect of glycerol and PEG 200 impregnation is very good. The other chemicals used, especially pentaerythritol, were not as effective as glycerol and PEG 200. Cell wall measurements using SEM show that an increase in cell wall thickness gives a corresponding increase in stabilization effect. DMTA-measurements showed that interaction between wood molecules and the chemicals used differs. In general, a higher degree of cell wall penetration of the chemicals gives a better stabilization effect. WAXS- investigations showed free crystalline pentaerythritol, PEG 1500, glycerol and PEG 200. Examination using SEM/EDS-linescan of potassium stained chemicals confirmed the results from the cell wall thickness measurements. However, the better resolution obtained in the STEM/EDS-linescan showed an inhomogenous distribution of the chemical in the cell wall. High temperature dried and green wood of Swedish pine was impregnated with glycerate and silver nitrate. Silver was precipitated in the cell wall by a new method. A significant difference in the distribution, size and location of the silver was observed. The silver particles are small and dispersed in the green wood samples but clustered and irregulary spaced in the dried specimens. The inhomogenous distribution in the dried specimens is believed to be the result of damage inside the wood cell walls due to drying stresses, which in turn will negatively affect the dimensional stabilizing result. The strength and energy to fracture is increased by a factor of two if wood not is allowed to be dried before cell wall bulking. This is because the impregnating chemical (glycerol in the present case) in the cell wall substitutes some of the moisture and therefore limits drying stresses. The glycerol is believed to be soluble inbetween the hemicellulose. The average distance between the silver particles in the impregnated green wood indicates that the impregnant is distributed in the cell wall at the microfibrilar level. Experimental results showed that the fastest diffusion path into the cell wall is from the lumen over the pit membrane through the compound middle lamell and not from the lumen through the secondary wall layer S3. The darker compound middle lamella seen in TEM is believed to be an artefact, as it is possible that the ML is thicker than the other layers in the wood cell wall after microtoming.