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

Abstract

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

Conservation of Waterlogged Wood—Past, Present and Future Perspectives

“…The untreated samples of Vasa wood had slightly higher indentation hardness, stiffness and lower creep deformation compared with the sound wood reference samples. The reason for this was not explained, but is likely to be due to the increased relative lignin content and loss of the plasticising effect of the hemicelluloses, similar to thermal degradation [155]. The observed cell wall softening effect indicates that the PEG is able to plasticise the cell wall both through direct interaction and in the presence of sorbed water, showing that there is no discernable restraining effect of the PEG when located in the cell wall nano-pore space.…”

“…It was stated by the author that the reduction in hygroscopicity was due to the blocking of the wood hydroxyl groups [195]. However, since sucrose contains many OH groups, this would not provide an explanation; moreover, the role that OH group concentration has to play in determining sorption properties in wood is not fully understood [155]. Rather, the reduction in hygroscopicity can be attributed to the extra weight due to the added sucrose plus the bulking effect of the sucrose in the pore space of the wood, which is then no longer available for water molecules.…”

Enhancing Weathering Resistance of Wood—A Review

“…Thermal modification is the controlled process of heating the wood at high temperatures between 180 • C and 240 • C under an oxygen free atmosphere, involving either steam, nitrogen or oil [196]. Today, it is today a commercialized technique for increasing the durability and dimensional stability of wood [197]. Thermal modification induces a darker colour of wood which is not stable against light exposure [172,[198][199][200][201][202][203].…”

Effect of the post-heat treatment on the properties of medium density particleboard of Eucalyptus sp.

“…While Sivonen et al (2002) explain that heat treatment increases the degree of entanglement within the lignin matrix and that this fact may be associated with the reduced hygroscopicity of thermally treated wood, thus improving the dimensional stability of the material. In the case of chemical modification, the hemicelluloses are initially broken down into monomeric structures, which are then dehydrated in aldehydes and form furfural units when derived from the pentoses and hydroxymethylfurfural units when derived from the hexoses, thus rendering a material less hygroscopic (Tjeerdsma & Militz, 2005;Yildiz & Gumuskaya, 2007;Winandy & Krzysik, 2007;Uimonen et al, 2020;Hill et al, 2021).…”

“…Vilela AP, Surdi PG, Sabino TPF, Metzker SLO, Coelho NPF of panels, i.e., in the particles (Ayrilmis et al, 2011;Mendes et al, 2013a;Mendes et al, 2013b;Ribeiro et al, 2020), and (ii) the post-treatment performed in the panels after being consolidated and stabilized (Mendes et al, 2013b;Carvalho et al, 2015;Oliveira et al, 2017;Hill et al, 2021). Therefore, regardless of how it is applied, the heat treatment consists of the application of temperature between 100 and 250 °C in the wood or wood products (Kocaefe et al, 2010;Silva et al, 2018;Wang et al, 2018).…”