Browsing by Author "Rautkari, Lauri, Assoc. Prof., Aalto University, Department of Bioproducts and Biosystems, Finland"
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- Hyperspectral imaging and chemometrics to investigate the chemical wood modification
School of Chemical Technology | Doctoral dissertation (article-based)(2023) Awais, MuhammadWood possesses an anisotropic hierarchical structure that causes a heterogeneous distribution of chemical reagents in modified wood at different spatial scales. Due to the heterogeneity in chemical distribution, localised regions of wood may remain susceptible to moisture uptake, dimensional instability, and fungal decay. The identification of regions with insufficient chemical uptake is necessary to develop efficient treatment processes, but standard gravimetric methods are insensitive to the location of chemical reagents within the wood. The primary objective of this thesis was to analyse the suitability of spectroscopic-based imaging methods to reveal the distribution of chemical reagents in modified wood at different length scales. The studies focused on the chemical modification of wood with acetic anhydride, paraformaldehyde, and thermosetting resins. The added chemical reagents are known to either react with cell wall polymers to create covalent bonds or to polymerize macromolecules within the cell wall space. To analyze the chemical changes caused by the modification agents on different spatial scales, the studies combined two chemical imaging techniques that differ in their lateral resolution to identify the process-dependent heterogeneity in modified wood. Near-infrared (NIR) hyperspectral imaging identified and quantified the distribution of chemical reagents and the corresponding moisture content at a macroscopic scale of a few millimeters. Chemometric analysis not only revealed the sample-to-sample variations in chemical uptake and the associated moisture content but also highlighted the localised variations, most notably earlywood and latewood differences. Confocal Raman imaging validated the differences between earlywood and latewood on the cellular level and visualised chemical differences between cell wall regions. Following this, the moisture uptake and the consequent swelling of the modified samples were determined by the dynamic measurements of mass and dimensions within the hygroscopic range. The results indicated the effectiveness of chemical modifications in reducing the moisture content of untreated wood. Overall, the results in this thesis demonstrated the ability of chemical imaging techniques to localise chemical reagents in small woodblocks and larger board sections. The findings provide a step forward in understanding the chemical changes caused by wood modification in different hierarchical structures in wood on different length scales. In the future, the methods may be used to characterise other treatments and processes that affect the wood composition. - Pressurized hot water extraction as a modification method for solid wood
School of Chemical Technology | Doctoral dissertation (article-based)(2023) Kyyrö, SuviPressurized hot water extraction (HWE) of the wood utilizes high-temperature water kept in liquid form with pressure. The simultaneous obtainment of hemicellulose-based carbohydrates and treatment of the solid wood is a notable advantage of HWE treatment, but it has only limited effects on the material properties compared to the thermal modification traditionally used to produce durable timber. Finding solutions to the limitations could enable the development of novel wood products. In this thesis, a broad range of HWE treatment conditions was applied to wood samples of varying sizes. Both the effects of increasing wood size and important material properties such as hygroscopicity and resistance against wood-inhabiting fungi were studied. The aim was to improve understanding of the properties of HWE treated wood in view of its potential in engineered products and to evaluate the underlying mechanisms. The studies showed that the HWE treatment efficiency differences between sample sizes were mainly caused by the higher amount of water and acetone soluble degradation products that remained in wood with larger dimensions. Furthermore, water soluble degradation products accumulated on the wood surface and were removable via leaching or surface planing. The removal of these degradation products with either approach was shown to improve the resistance of HWE treated wood against discolouration by mould fungi. Prior to first drying, HWE treatment did not considerably change the water-saturated dimensions of the wood as degraded components were replaced with water. The first drying and resoaking resulted in a shrinkage in the saturated dimensions. This restriction in the reswelling of the cell walls was enhanced by a longer treatment time. Hydroxyl accessibility of the wood was assessed by measuring the mass loss caused by the reprotonation of deuterated samples. The effects of first drying could not be assigned solely to the variations in the sorption site density. The results suggested that the cell wall space formed during HWE treatment partially closes due to first drying and does not fully reopen upon resoaking. Therefore, diffusion of modification agents and subsequent esterification was tested as an approach to improve the properties of HWE treated wood. Successful esterification and cell wall bulking were confirmed, but the method's efficiency in improving durability varied between tested wood-decaying fungi. The results of this thesis offer new insights into the characteristics of HWE treated wood and potential solutions to improve its dimensional stability and resistance against wood-inhabiting fungi. The thesis also provides the groundwork for future studies regarding the material properties of wood modified with HWE treatment.