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Effects of two-year weather exposure on thermally modified Picea abies, Pinus sylvestris, and Fraxinus excelsior wood

Publication: Canadian Journal of Forest Research
26 May 2020

Abstract

The use of thermally modified timber (TMT) in outdoor applications is well established, but its performance against the stresses of outdoor conditions is not comprehensively understood. This study investigates the changes in density, surface chemical composition, color, equilibrium moisture content (EMC), checking, hardness, and cupping of thermally modified boards of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst.), and European ash (Fraxinus excelsior L.) during a two-year weather exposure test in Eastern Finland. Unmodified pine, spruce, and ash and copper salt impregnated pine boards were used as controls. The results show that a two-year weather exposure period causes degradation of hydrophobic lignin, and the leaching of the degraded compounds changed the color, increased the EMC, and decreased the hardness of specimens. Although the EMC of TMT specimens was lower compared with unmodified and impregnated ones during weathering, the changes were more obvious in TMT than in the controls. More checks occurred in the TMT specimens than in unmodified ones after the exposure. On the other hand, the TMT specimens had a lower degree of cupping. These findings also indicate that an increase in modification temperature induces more checks but improves the durability of all studied species by reducing the chemical degradation, color change, cupping, and hygroscopicity during weather exposure.

Résumé

L’utilisation du bois modifié thermiquement (BMT) pour des usages extérieurs est bien établie mais sa performance face aux stress dus aux conditions extérieures n’est pas complètement connue. Cette étude examine les changements dans la densité, la composition chimique en surface, la couleur, le degré d’humidité d’équilibre (EMC), le fendillement, l’indice de dureté et le voilement de planches de pin sylvestre (Pinus sylvestris L.), d’épicéa commun (Picea abies (L.) H. Karst.) et de frêne commun (Fraxinus excelsior L.) modifiées thermiquement durant un test d’exposition aux intempéries d’une durée de deux ans dans l’est de la Finlande. Des planches non modifiées de pin, d’épicéa et de frêne ainsi que des planches de pin imprégnées de sel de cuivre ont été utilisées comme témoins. Les résultats montrent qu’une période d’exposition aux intempéries de deux ans a causé une dégradation de la lignine hydrophobe et que le lessivage des composés engendrés par la dégradation a changé la couleur, augmenté l’EMC et diminué l’indice de dureté des spécimens. Bien que l’EMC des spécimens de BMT ait été plus faible comparativement à ceux qui ont été imprégnés ou qui n’ont pas été modifiés lors du vieillissement aux intempéries, les changements étaient plus évidents chez le BMT que chez les témoins. Davantage de fendillement est survenu chez les spécimens de BMT que chez les spécimens non modifiés suivant l’exposition aux intempéries. Par contre, les spécimens de BMT avaient un plus faible degré de voilement. Ces résultats indiquent aussi qu’une augmentation de la température du traitement thermique entraîne davantage de fendillement mais améliore la durabilité de toutes les espèces étudiées en réduisant la dégradation chimique, le changement de couleur, le voilement et l’hygroscopicité durant l’exposition aux intempéries. [Traduit par la Rédaction]

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Information & Authors

Information

Published In

cover image Canadian Journal of Forest Research
Canadian Journal of Forest Research
Volume 50Number 11November 2020
Pages: 1160 - 1171

History

Received: 19 December 2019
Accepted: 22 May 2020
Accepted manuscript online: 26 May 2020
Version of record online: 26 May 2020

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Key Words

  1. thermally modified wood
  2. weathering
  3. FTIR
  4. checking
  5. cupping

Mots-clés

  1. bois modifié thermiquement
  2. vieillissement aux intempéries
  3. FTIR
  4. fendillement
  5. voilement

Authors

Affiliations

Chenyang Cai [email protected]
School of Forest Sciences, University of Eastern Finland, Joensuu, Finland.
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing, China.
Antti Haapala
School of Forest Sciences, University of Eastern Finland, Joensuu, Finland.
Mohammad Habibur Rahman
School of Forest Sciences, University of Eastern Finland, Joensuu, Finland.
Markku Tiitta
Puumit Oy, Kuopio, Finland.
Valtteri Tiitta
Puumit Oy, Kuopio, Finland.
Laura Tomppo
Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
Reijo Lappalainen
Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
Henrik Heräjärvi
Natural Resources Institute Finland, Joensuu, Finland.

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