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Evaluating the quality of surface carbonized woods modified with a contact charring or a gas flame charring technique
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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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19
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Wood Science and Technology, Volume 57, issue 6, pp. 1299-1317
Abstract
Surface carbonization, or charring, of wooden exterior cladding boards is a modification method that creates a fully organic barrier layer in resemblance to a coating. The process effectively degrades the wood and transforms it into a carbonaceous residue that protects the underlying unmodified wood from environmental stresses. The surface quality of wood modified in this manner is a combination of several factors and depends on the manufacturing method and wood species. To assess the quality of spruce and birch modified with contact and flame charring techniques, several experiments were set up from the nanoscale to macroscopic evaluation of surface resistance to different stresses. The changes in elemental composition are scaled with the modification severity with little differences between wood species. The carbon structures analyzed by high-resolution transmission electron microscopy (HR-TEM) were found to be amorphous, but the electron energy-loss spectroscopy (EELS) revealed higher ordering with what is assumed to be random graphitic stacking of carbon sheets. These carbon–carbon bonds are stable, so a higher ordering is hypothesized to induce improved resistance to exterior stresses. The scanning electron microscopy (SEM) revealed a clear difference between contact-charred and flame-charred woods. The selected contact charring temperature was not high enough to induce the transformation of cell walls from anisotropic into an isotropic material but provided other benefits such as a relatively crack-free, smooth and scratch resistant surface. Surface roughness was able to adequately predict the surface quality of the contact-charred samples, and scratch tests were found to be suitable for evaluating the mechanical stress resistance of the surface instead of abrasion. In terms of overall quality, birch instead of spruce was concluded to better respond to both charring methods, although contact charring eliminates some species-specific characteristics, resulting in more homogeneous surfaces.
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Funding Information: Support from the Academy of Finland granted for project CHARFACE (project number 13315408) is highly appreciated by lead author. The authors acknowledge the help of research laboratory assistant Mikael Hytti. J.D. and V. Š. are grateful for the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 952314. We also acknowledge the OtaNano Nanomicroscopy Center (NMC) for provision of equipment and sincerely thank Senior scientist Jiang Hua for his expertise in transmission electron microscopy and EELS. Publisher Copyright: © 2023, The Author(s).
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Kymäläinen, M, Dömény, J, Schwarzkopf, M, Šeda, V & Rautkari, L 2023, 'Evaluating the quality of surface carbonized woods modified with a contact charring or a gas flame charring technique', Wood Science and Technology, vol. 57, no. 6, pp. 1299-1317. https://doi.org/10.1007/s00226-023-01488-0