Wood-Water Relations Affected by Anhydride and Formaldehyde Modification of Wood.

The moisture uptake of wood is influenced by accessible hydroxyl groups acting as sorption sites and the water-available cell wall space. To what extent do these mechanisms control the moisture uptake in wood needs to be addressed. For this purpose, we modified sorption site density and cell wall space by wood treatments with acetic anhydride or formaldehyde and investigated their effects on moisture uptake. Chemical changes at the cell wall level caused by the treatments were first determined by confocal Raman imaging. Following this, the deuterium exchange method was used to gravimetrically measure the hydroxyl accessibility, while the moisture uptake and the consequent swelling of the wood were determined by dynamic measurements of mass and dimensions within the hygroscopic range. The results showed that the effectiveness in reducing the moisture content of untreated wood across the hygroscopic range differed between the anhydride- and formaldehyde-modified wood. We also observed a poor correlation of accessible hydroxyl concentration in formaldehyde-modified wood with weight percentage gain and water uptake. Moreover, the dynamic mass and dimension analysis indicated that the reduction in swelling in formalized wood was affected by an unidentified mechanism in addition to reduced moisture content.

Effect of Moisture on Polymer Deconstruction in HCl Gas Hydrolysis of Wood.

The HCl gas system previously used to produce cellulose nanocrystals was applied on Scots pine wood, aiming at a controlled deconstruction of its macrostructure while understanding the effect on its microstructure. The HCl gas treatments resulted in a well-preserved cellular structure of the wood. Differences in wood initial moisture content (iMC) prior to HCl gas treatment played a key role in hydrolysis rather than the studied range of exposure time to the acidic gas. Higher iMCs were correlated with a higher degradation of hemicellulose, while crystalline cellulose microfibrils were not largely affected by the treatments. Remarkably, the hydrogen-deuterium exchange technique showed an increase in accessible OH group concentration at higher iMCs, despite the additional loss in hemicelluloses. Unrelated to changes in the accessible OH group concentration, the HCl gas treatment reduced the concentration of absorbed D2O molecules.

Extractive concentrations and cellular-level distributions change radially from outer to inner heartwood in Scots pine.

The heartwood of many wood species is rich in extractives, which improve the wood material's resistance to biological attack. Their concentration is generally higher in outer than inner heartwood, but the exact radial changes in aging heartwood remain poorly characterized. This investigation studied these radial changes in detail in Scots pine (Pinus sylvestris L.), using radial sample sequences prepared from three different trees. Stilbene and resin acid contents were first measured from bulk samples, after which the extractive contents of individual heartwood annual rings were investigated using Raman spectroscopy and fluorescence microscopy. Raman imaging and fluorescence microscopy were also used to study the cellular-level distributions of extractives in different annual rings. Although there were substantial differences between the trees, the content and distribution of stilbenes seemed to follow a general radial trend. The results suggest that stilbenes are absorbed into heartwood tracheid cell walls from small stilbene-rich extractive deposits over several years and then eventually transform into non-extractable compounds in aging heartwood. Resin acids followed no consistent radial trends, but their content was strongly connected to the frequency of large extractive deposits in latewood tracheid lumens. The results highlight the variability of heartwood extractives: their content and distribution vary not only between trees but also between and even within the annual rings of a single tree. This high variability is likely to have important effects on the properties of heartwood and the utilization of heartwood timber.

Bundling of cellulose microfibrils in native and polyethylene glycol-containing wood cell walls revealed by small-angle neutron scattering.

Wood and other plant-based resources provide abundant, renewable raw materials for a variety of applications. Nevertheless, their utilization would greatly benefit from more efficient and accurate methods to characterize the detailed nanoscale architecture of plant cell walls. Non-invasive techniques such as neutron and X-ray scattering hold a promise for elucidating the hierarchical cell wall structure and any changes in its morphology, but their use is hindered by challenges in interpreting the experimental data. We used small-angle neutron scattering in combination with contrast variation by poly(ethylene glycol) (PEG) to identify the scattering contribution from cellulose microfibril bundles in native wood cell walls. Using this method, mean diameters for the microfibril bundles from 12 to 19 nm were determined, without the necessity of cutting, drying or freezing the cell wall. The packing distance of the individual microfibrils inside the bundles can be obtained from the same data. This finding opens up possibilities for further utilization of small-angle scattering in characterizing the plant cell wall nanostructure and its response to chemical, physical and biological modifications or even in situ treatments. Moreover, our results give new insights into the interaction between PEG and the wood nanostructure, which may be helpful for preservation of archaeological woods.

Distribution and curing reactions of melamine formaldehyde resin in cells of impregnation-modified wood.

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.

Hyperspectral Near-Infrared Image Assessment of Surface-Acetylated Solid Wood.

Visualization of acetic anhydride flow and its heterogeneity within the wood block necessitates the development of a reliable and robust analytical method. Hyperspectral imaging has the potential to acquire a continuous spectrum of chemical analytes at different spectral channels in terms of pixels. The large set of chemical data (3-dimensional) can be expanded into relevant information in a multivariate fashion. We quantified gradients in acetylation degree over cross sections of Scots pine sapwood caused by a one-sided flow of acetic anhydride into wood blocks using near-infrared hyperspectral imaging. A principal component analysis (PCA) model was used to decompose the high-dimensional data into orthogonal components. Moreover, a partial least-squares (PLS) hyperspectral image regression model was developed to quantify heterogeneity in acetylation degree that was affected by the flow of acetic anhydride through wood blocks and into the tracheid cell walls. The model was validated and optimized with an external test data set and a prediction map using the root-mean-squared error of an individual predicted pixel. The model performance parameters are well suited, and prediction of the acetylation degree at the image level was complemented with confocal Raman imaging of selected areas on the microlevel. NIR image regression showed that the acetylation degree was determined not only by the time-dependent flow of the acetic anhydride through the wood macropores but also by the diffusion of the anhydride into the wood cell walls. Thereby, thin-walled earlywood sections were acetylated faster than the thick-walled latewood sections. Our results demonstrate the suitability of near-infrared imaging as a tool for quality control and process optimization at the industrial scale.

Cellular level chemical changes in Scots pine heartwood during incipient brown rot decay.

The heartwoods of many wood species have natural resistance to wood decay due to the accumulation of antifungal heartwood extractives. The natural durability of heartwoods has been extensively investigated, yet very little information is available on the initiation of heartwood decay. This experiment examined the onset of Rhodonia placenta brown rot decay in Scots pine heartwood in order to identify the key changes leading to heartwood decay. An imaging approach based on Raman imaging and multivariate image analysis revealed that the degradation of heartwood began in the innermost cell wall layers and then spread into the remaining cell walls and the middle lamella. Pinosylvins were extensively degraded in the cell walls, middle lamella and extractive deposits, while unidentified material most likely consisting of hemicelluloses and/or lipophilic extractives was removed from the inner cell wall layers. Changes similar to inner cell wall degradation were seen in the remaining cell walls in more advanced decay. The results indicate that the key change in incipient heartwood decay is the degradation of antifungal heartwood extractives. The inner cell wall degradation seen in this experiment may serve a nutritive purpose or facilitate the penetration of degradative agents into the cell walls and middle lamella.