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Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents

Propriétés mécaniques de bois résineux modifiés par traitement thermique en relation avec la constitution en polymères ligneux structuraux

Annals of Forest Science volume 64, pages 679–690 (2007)Cite this article

Abstract

Thermal modification at relatively high temperatures (ranging from 150 to 260 °C) is an effective method to improve the dimensional stability and resistance against fungal attack. This study was performed to investigate the impact of heat treatment on the mechanical properties of wood. An industrially-used two-stage heat treatment method under relative mild conditions (< 200 °C) was used to treat the boards. Heat treatment revealed a clear effect on the mechanical properties of softwood species. The tensile strength parallel to the grain showed a rather large decrease, whereas the compressive strength parallel to the fibre increased after heat treatment. The bending strength, which is a combination of the tensile stress, compressive stress and shear stress, was lower after heat treatment. This decrease was less than the decrease of only the tensile strength. The impact strength showed a rather large decrease after heat treatment. An increase of the modulus of elasticity during the bending test has been noticed after heat treatment. Changes and/or modifications of the main wood components appear to be involved in the effects of heat treatment on the mechanical properties. The possible effect of degradation and modification of hemicelluloses, degradation and/or crystallization of amorphous cellulose, and polycondensation reactions of lignin on the mechanical properties of heat treated wood have been discussed. The effect of natural defects, such as knots, resin pockets, abnormal slope of grain and reaction wood, on the strength properties of wood appeared to be affected by heat treatment. Nevertheless, heat treated timber shows potential for use in constructions, but it is important to carefully consider the stresses that occur in a construction and some practical consequences when heat treated timber is used.

Résumé

La modification thermique du bois à des températures relativement élevées (entre 150 et 260 °C) présente une méthode efficace pour améliorer la stabilité dimensionnelle et la résistance aux attaques de champignons. Ce travail porte sur les effets du traitement thermique sur les propriétés mécaniques du bois. Les planches ont été soumises à un traitement thermique à des températures relativement modérées (< 200 °C) selon un procédé industriel en deux phases. Il s’est avéré qu’un tel traitement influe nettement sur les propriétés mécaniques des bois résineux. La résistance à la traction dans la direction parallèle au fil du bois est diminuée de manière assez importante, tandis que, dans la même direction, la résistance à la compression est augmentée. La résistance au fléchissement, qui intègre la résistance aux efforts de traction, de compression et de cisaillement, était plus réduite après le traitement thermique. Cette diminution était moins importante que celle de la résistance à la traction considérée seule. La résistance aux efforts de choc a diminué de manière importante. Les tests de flexion ont permis de constater également une augmentation du module d’élasticité à la suite du traitement thermique. Des changements et/ou des modifications au niveau des principaux composants du bois semblent être impliqués dans les effets d’un traitement thermique sur les propriétés mécaniques. Les effets potentiels de dégradation et de modification d’hémicelluloses, de dégradation et/ou de cristallisation de cellulose amorphe, et de réactions de polycondensation de lignine sur les propriétés mécaniques de bois traité thermiquement ont été discutés. Le traitement thermique influait également sur l’effet de défauts naturels, tels que nœuds, poches de résine, déviations anormales du fil du bois et bois de réaction, sur les propriétés mécaniques du bois. Le bois ayant subi un traitement thermique présente malgré tout un bon potentiel en applications en structure. Néanmoins, il reste important de prendre en compte soigneusement les efforts mécaniques ainsi que quelques implications pratiques qui jouent un rôle lors de l’utilisation de bois traité thermiquement dans une construction.

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Authors and Affiliations

  1. Plato International BV, PO Box 2159, NL-6802, CD Arnhem, The Netherlands

    Michiel J. Boonstra & Edo V. Kegel

  2. Laboratory of Wood Technology, Ghent University, Coupure links 653, 9000, Ghent, Belgium

    Joris Van Acker

  3. SHR Hout Research, PO Box 497, 6700, AL Wageningen, The Netherlands

    Bôke F. Tjeerdsma

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  1. Michiel J. Boonstra
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  2. Joris Van Acker
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  3. Bôke F. Tjeerdsma
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  4. Edo V. Kegel
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Correspondence to Michiel J. Boonstra.

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Boonstra, M.J., Van Acker, J., Tjeerdsma, B.F. et al. Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents. Ann. For. Sci. 64, 679–690 (2007). https://doi.org/10.1051/forest:2007048

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Annals of Forest Science

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