A molecular model for reversible and irreversible hygroscopicity changes by thermal wood modification

Willems, Wim; Altgen, Michael; Rautkari, Lauri

doi:10.1515/hf-2019-0057

Cited by 16 publications

(14 citation statements)

References 25 publications

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“…Some studies explained that cross-linking reactions during Dry-HT enhance the cell wall matrix stiffness and restrict the expansion of the polymers to accommodate water molecules [30,31,43]. Other studies speculated that changes in the conformation of the matrix polymers hinder the relaxation of the cell wall polymers toward their thermodynamically most favorable arrangement [31,37,44]. Similar to the reduction in wood MC, the present study also showed differences in the change in tensile properties by the two HT techniques.…”

Section: Discussionsupporting

confidence: 68%

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

et al. 2020

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Heat treatments reduce the strength and ductility of wood, but the extent depends on the direction of load and the treatment conditions applied. The tensile behavior of wood is very sensitive to heat treatments, but there is a lack of understanding how this is related to different heat treatment conditions. In this study, we treated homogeneous micro-veneers under different time-, temperature-, and moisture-environments and compared the effect on the tensile behavior of the treated veneers based on their chemical composition changes. The results confirmed the adverse effect of the preferential hemicellulose removal on the strength and toughness of wood. However, chemical composition changes could not fully explain the tensile behavior of dry heat-treated wood, which showed an additional loss in maximum load and work in traction at the same residual hemicellulose content compared to wet heat-treated wood. The scission of cellulose chains as well as the enhanced cross-linking of the cell wall matrix under dry heat conditions and elevated temperatures was discussed as additional factors. The enhanced cross-linking of the cell wall matrix helped in preserving the tensile properties when testing the veneers in water-saturated state, but may have also promoted the formation of cracks that propagated across the cell wall during tensile loading.

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Section: Discussionsupporting

confidence: 68%

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

et al. 2020

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“…If linked to impermanent H-bonding networks, this suggests that measurements of accessible OH content using D 2 O would change before and after the water-soaking stage. But this has not been observed [135]. The loss of mobile cell wall components during thermal modification will result in a cell wall that is in a higher energy stressed state under HT dry conditions, which can then relax when subsequently exposed to higher levels of moisture.…”

Section: Changes In Physical Propertiesmentioning

confidence: 98%

“…There are still possible sources of error arising from the estimation of dry mass within the DVS [134], and although the HDX experiments using a modified protocol produced results with repeatable results, a higher drying temperature in the DVS in order to measure the dry weight was found to produce an increase in accessible OH content by ? 0.5 mmol OH groups per gram of dry wood mass [135]. Further development is apparently required to produce reliably accurate OH accessibility data using a DVS apparatus in HDX experiments, but this remains a very convenient method for determination of accessible OH groups once reliability can be established.…”

Section: Hydroxyl (Oh) Contentmentioning

confidence: 99%

“…When wood was modified under HT dry conditions, the initial configuration of the polymer network left unpaired OH groups, subsequent annealing allowed new pairs to be formed and consequently an increase in hygroscopicity. This idea was further developed by Willems et al [135] who measured the accessible OH content of the same HT dry sample before and after a post-treatment water saturation. They found that there was a slight decrease in OH content, but that the EMC increased significantly after the post-treatment water soaking experiment.…”

Section: Changes In Physical Propertiesmentioning

confidence: 99%

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Thermal modification of wood—a review: chemical changes and hygroscopicity

2021

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Thermal modification is a well-established commercial technology for improving the dimensional stability and durability of timber. Numerous reviews of thermally modified timber (TMT) are to be found in the scientific literature, but until now a review of the influence of cell wall moisture content during the modification process on the properties of TMT has been lacking. This paper reviews the current state of knowledge regarding the hygroscopic and dimensional behaviour of TMT modified under dry (cell wall at nearly zero moisture content) and wet (cell wall contains moisture) conditions. After an overview of the topic area, the review explores the literature on the thermal degradation of the polysaccharidic and lignin components of the cell wall, as well as the role of extractives. The properties of TMT modified under wet and dry conditions are compared including mass loss, hygroscopic behaviour and dimensional stability. The role of hydroxyl groups in determining the hygroscopicity is discussed, as well as the importance of considering the mobility of the cell wall polymers and crosslinking when interpreting sorption behaviour. TMT produced under wet processing conditions exhibits behaviour that changes when the wood is subjected to water leaching post-treatment, which includes further weight loss, changes in sorption behaviour and dimensional stability, but without any further change in accessible hydroxyl (OH) content. This raises serious questions regarding the role that OH groups play in sorption behaviour. Graphical abstract

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“…Several studies have shown that the amount of absorbed water can change independently from the OH accessibility (Stevens and Smith 1970;Rautkari et al 2013;Salmén and Stevanic 2018). There is evidence that additional factors have a strong impact on the amount of absorbed water without affecting the sorption site density, such as the degree of crosslinking in heat-treated wood (Altgen et al 2018;Willems et al 2020) or the spatial availability of wood cell walls in modified wood (Thybring et al 2020;Altgen et al 2020). Furthermore, Lindh et al (2016) showed that OH(3) groups in cellulose that are associated with the C(3) atom of the glucose units are unreactive to H-D exchange, but these OH groups may still form hydrogen bonds with water molecules.…”

Section: Introductionmentioning

confidence: 99%

Humidity-dependence of the hydroxyl accessibility in Norway spruce wood

Altgen

Rautkari

2020

Cellulose

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This study aimed at a better understanding of the wood-water interaction, in particular the role of the hydroxyl accessibility during the humidity-dependent change in moisture content. Thin sections (80 µm) of never-dried Norway spruce sapwood that contained early- and latewood were used for the experiments. Sorption isotherm measurements confirmed the humidity-dependent moisture content changes and the effect of the first drying of the wood sections. Changes in hydroxyl accessibility were then determined by deuteration of the sections using deuterium oxide, followed by their re-protonation in water (H2O) vapor at different relative humidity: 15, 55 or 95%. The deuteration and re-protonation of the wood sections were quantified by dry mass changes as well as by changes in the OH and OD stretching vibrations in the Fourier transform infrared spectra. The results showed that the deuterated sections could be almost completely re-protonated in H2O vapor, nearly irrespective of the applied relative humidity. Therefore, changes in hydroxyl accessibility were not the driving force for the humidity-dependent changes in moisture content. However, a slow re-protonation rate at low relative humidity had to be considered. Nonetheless, a small quantity of OD groups persisted the re-protonation in H2O vapor and liquid H2O, which was not related to the drying of the wood. Graphic abstract

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A molecular model for reversible and irreversible hygroscopicity changes by thermal wood modification

Cited by 16 publications

References 25 publications

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

Thermal modification of wood—a review: chemical changes and hygroscopicity

Humidity-dependence of the hydroxyl accessibility in Norway spruce wood

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