Sorption behavior and hydroxyl accessibility of wood treated with different cyclic N -methylol compounds
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Sorption behavior and hydroxyl accessibility of wood treated with different cyclic N-methylol compounds Lukas Emmerich1,* 1 2
, Michael Altgen2, Lauri Rautkari2, and Holger Militz1
University of Göttingen, Wood Biology and Wood Products, Büsgenweg 4, 37077 Göttingen, Germany Department of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, 00076 Aalto, Finland
Received: 28 July 2020
ABSTRACT
Accepted: 30 August 2020
Cyclic N-methylol compounds such as 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) have been used to modify wood and prevent negative effects related to the uptake of moisture. However, the changes in the sorption behavior of wood by treatments with DMDHEU and its derivatives are not fully understood. In the present study, wood blocks were treated with DMDHEU, ether-modified DMDHEU and diethyleneglycolated DMDHEU in order to study the factors that control the changes in moisture uptake in the hygroscopic range (0–95% RH). Dimensional changes of wood blocks during water soaking cycles suggested that the treatments caused a permanent cell wall bulking, whereas the swelling restraint by cross-linking of adjacent cell wall polymers was not permanent. However, the changes in water vapor sorption were not only a result of the cell wall bulking effect that reduced the space in the cell wall to accommodate water. The N-methylol compounds within the wood also provided additional sorption sites, but there was no correlation between absorbed water and accessible OH groups. It was speculated that the co-condensation of the N-methylol compounds with wood polymers had a significant effect on the sorption of the treated wood. At elevated RH, pure resins that were formed by self-condensation took up large quantities of moisture. However, when the N-methylol compounds were heat-cured within the hierarchical structure of wood, the moisture uptake of the treated wood at elevated RH was even lower compared to unmodified wood. Furthermore, the covalent bond formation between wood and resin prolonged the attainment of an equilibrium moisture content.
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The Author(s) 2020
Handling Editor: Stephen Eichhorn.
Address correspondence to E-mail: [email protected]
https://doi.org/10.1007/s10853-020-05224-y
J Mater Sci
Introduction Cyclic N-methylol compounds such as 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) can be used to modify wood and improve its service life in exterior applications [1, 2]. Compared to untreated wood, DMDHEU-treated wood has an improved dimensional stability and decay resistance [3, 4]. The treatment with N-methylol compounds also affects the water vapor sorption of wood in the hygroscopic range [5, 6]. However, the underlying modes of action are not fully understood, because the water sorption of the treated wood is influenced by a number of factors, such as the location of the compounds within the hierarchical structure of wood or the additional moisture uptake of the resin. Originally, cyclic N-methylol compounds like DMDHEU were developed and used for finishi
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