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Effect of grain orientation and surface wetting on vertical density profiles of thermally compressed fir and spruce

Einfluss der Jahrringstellung und der Oberflächenbefeuchtung auf das Rohdichteprofil von thermisch verdichtetem Tannen- und Fichtenholz

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Abstract

The effects of grain orientation and surface wetting on wood densification by compression in a hot press were evaluated for two commercial Canadian wood species, balsam fir (Abies balsamea) and black spruce Picea mariana. The vertical density profiles (VDP) of wood densified at 180 °C could be engineered to achieve different properties depending on press closing rate, wood permeability and annual ring orientation. The lower permeability of spruce caused it to split frequently during hot pressing. For balsam fir, at a press closing time of 2 min, the compressed wood with an original grain angle of 0° (radial compression) shows widened high density bands due to collapse of low density earlywood adjacent to the dense latewood. All grain orientations show higher density areas close to the wood surfaces similar to those of wood-based composites. However, when wood was preheated without pressure for 5 min followed by a press closing time of 2 min, water migrated to and plasticized the board centre causing it to be densified while the surface density remained low. Wood surface plasticizing with water or urea solution causes some localized surface densification, but the effect was not great.

Zusammenfassung

An zwei kanadischen Nutzhölzern, Balsam fir (Abies balsamea) und Black spruce ( Picea mariana) wurde untersucht, wie sich die Jahrringstellung und eine Oberflächenbefeuchtung auf die Verdichtung dieser Hölzer in einer Heisspresse auswirkt. Über die Querschnittshöhe kann ein unterschiedliches Rohdichteprofil erzeugt werden. In Abhängigkeit der Presszeit, der Permeabilität und der Jahrringstellung der Hölzer konnten bei einer Verdichtung bei 180 °C unterschiedliche Dichteprofile (VDP) erzeugt werden. Die geringe Permeabilität von Fichtenholz führte während des Heisspressens häufig zum Aufspalten. Bei Tannenholz, das mit einer Presszeit von 2 min. und einer Jahrringstellung von 0 °C (Druck in radialer Richtung) verdichtet wurde, bildeten sich durch den Kollaps des an Spätholz angrenzenden Frühholzes verbreiterte Bereiche hoher Dichte. Bei allen Jahrringstellungen bildete sich wie bei plattenförmigen Holzwerkstoffen ein Rohdichteprofil mit erhöhter Dichte an den Oberflächen aus. Wenn das Holz jedoch 5 min. lang ohne Druck vorgeheizt und dann bei einer Presszeit von 2 min. verdichtet wurde, diffundierte das Wasser nach innen. Der innere Bereich des Holzes plastifizierte und verdichtete sich dadurch, während sich die oberflächennahen Schichten weniger stark verdichteten. Eine Plastifizierung der Holzoberflächen mit Wasser oder Harnstofflösung ergab eine örtlich begrenzte, nicht stark ausgeprägte Oberflächenverdichtung.

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  1. Faculty of Forestry, University of Toronto, 33 Willcocks St., Toronto, Ontario, M5S 3B3, Canada

    J.Y. Wang & P.A. Cooper

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Wang, J., Cooper, P. Effect of grain orientation and surface wetting on vertical density profiles of thermally compressed fir and spruce. Holz Roh Werkst 63, 397–402 (2005). https://doi.org/10.1007/s00107-005-0034-3

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