Comparison of EMC and durability of heat treated wood from high versus low water vapour pressure reactor systems

Willems, Wim; Altgen, Michael; Militz, Holger

doi:10.1179/2042645314y.0000000083

Cited by 20 publications

(18 citation statements)

References 31 publications

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“…Heating at high temperatures (over 200°C) and low water activity, results in dry thermolysis, with the pressure making little contribution to the variables and the main controlling factor being the process temperature. Wet processes operate at lower temperatures (160-180°C) in saturated steam, with the necessary moist conditions ensured by control of temperature and pressure, although pressure is the most important parameter when determining mass loss (before and after post-treatment leaching) [22,23]. Saturated steam is in equilibrium with liquid water at the same temperature and pressure, whereas superheated steam is dry water vapour that has been heated above its saturation point.…”

Section: Description Of Process Conditionsmentioning

confidence: 99%

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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Section: Description Of Process Conditionsmentioning

confidence: 99%

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

2021

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“…Thermal modification of wood has often been performed at atmospheric pressure together with superheated steam where volatile degradation products are allowed to evaporate, and severe drying takes place (Militz and Altgen 2014). The modification process can also take place in a closed reactor system, which accelerates thermal degradation and enables the regulation of the water vapor pressure and the RH during the modification process (Willems et al 2015;Altgen et al 2016c). Altgen et al (2016c) showed for European beech (Fagus sylvatica L.) wood that the mass loss (ML) obtained in a closed reactor system is a function of the maximum vapor pressure, and less dependent on the peak temperature ( 180°C) applied.…”

Section: Introductionmentioning

confidence: 99%

Sorption and Surface Energy Properties of Thermally Modified Spruce Wood Components

Källbom

Altgen

Militz

et al. 2018

W&FS

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The objective of this work is to study the water vapor sorption and surface energy properties of thermally modified wood (TMW) components, ie wood processing residuals in the form of sawdust. The thermal modification was performed on spruce wood components using a steam-pressurized laboratoryscale reactor at two different temperature (T ) and relative humidity (RH) conditions, T ¼ 150°C and RH ¼ 100% (TMW150), and T ¼ 180°C and RH ¼ 46% (TMW180). A dynamic vapor sorption (DVS) technique was used to determine water vapor sorption isotherms of the samples for three adsorption-desorption cycles at varying RH between 0% and 95%. Inverse gas chromatography (IGC) was used to study the surface energy properties of the samples, including dispersive and polar characteristics. The DVS results showed that the EMC was reduced by 30-50% for the TMW samples compared with control samples of unmodified wood (UW) components. A lower reduction was, however, observed for the second and third adsorption cycles compared with that of the first cycle. Ratios between EMC of TMW and that of UW samples were lower for the TMW180 compared with the TMW150 samples, and an overall decrease in such EMC ratios was observed at higher RH for both TMW samples. The IGC results showed that the dispersive contribution * Corresponding author † SWST member to the surface energy was higher at lower surface coverages, ie representing the higher energy sites, for the TMW compared with the UW samples. In addition, an analysis of the acid-base properties indicated a higher K B than K A number, ie a higher basic than acidic contribution to the surface energy, for all the samples. A higher K B number was also observed for the TMW compared with the UW samples, suggested to relate to the presence of ether bonds from increased lignin and/or extractives content at the surface. The K B was lower for TMW180 compared with TMW150, as a result of higher modification temperature of the first, leading to cleavage of these ether bonds.

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“…Heat treatment of wood is generally performed at temperatures ranging from 160 to 260 °C. To avoid wood combustion, heat treatment is performed in a low-oxygen environment, such as under steam conditions, nitrogen, oil, and, more recently, in a vacuum (Hakkou et al 2005;Ding et al 2011;Allegretti et al 2012;Cao et al 2012a;Dubey et al 2012;Willems et al 2015). Heat treatment has also been performed under air conditions (Aydemir et al 2012;Jiang et al 2014).…”

Section: Introductionmentioning

confidence: 99%

Effect of Treatment Duration and Clamping on the Properties of Heat-Treated Okan Wood

Hidayat¹,

Qi²,

Jang³

et al. 2016

BioResources

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Effects of treatment duration and clamping during heat treatment were evaluated relative to the color and physical and mechanical properties of okan wood. Sapwood and heartwood boards from okan wood were treated with and without clamping for 1 to 4 h at 180 °C. Changes in color were mostly due to a reduction in lightness (L*) and yellow/blue chromaticity (b*). These values decreased more for longer treatment durations. Red/green chromaticity (a*) was not affected by the treatment duration. Weight loss and volume shrinkage increased with increased treatment duration. Density only slightly decreased because of a balanced reduction in weight and volume. Clamping during treatment prevented surfaces from having direct contact with the heated air, which resulted in lower weight loss and volume shrinkage than in the samples treated without clamping. Heat-treated wood absorbed less water than the control group, as suggested by the lower equilibrium moisture content and water absorption. Furthermore, the heartwood absorbed less water than the sapwood. An evaluation of the mechanical properties showed a reduction in both the modulus of rupture and modulus of elasticity after heat treatment. Clamping minimized strength reduction in both sapwood and heartwood, particularly for 1 and 2 h heat treatments.

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Comparison of EMC and durability of heat treated wood from high versus low water vapour pressure reactor systems

Cited by 20 publications

References 31 publications

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

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

Sorption and Surface Energy Properties of Thermally Modified Spruce Wood Components

Effect of Treatment Duration and Clamping on the Properties of Heat-Treated Okan Wood

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