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Thermo-mechanical densification combined with thermal modification of Norway spruce (Picea abies Karst) in industrial scale – Dimensional stability and durability aspects

Thermo-mechanische Verdichtung und thermische Modifikation von Fichtenholz (Picea abies Karst) im industriellen Maßstab – Betrachtung der Dimensionsstabilität und Dauerhaftigkeit

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Abstract

Heat-treatments of wood to improve selected wood properties, e.g. durability and dimensional stability, are well established industrial processes. However, the main drawbacks of thermally modified timber are the reduced strength properties. In a previous study, thermo-mechanically densified wood with increased initial strength was successfully applied to an oil-heat treatment (OHT) in laboratory scale to overcome the problem of reduced strength properties. Consequently, the up-scaling of processes to industrial scale was the objective of this study. Therefore, Norway spruce (Picea abies Karst.) was thermo-mechanically densified in laboratory scale at 140 °C, 160 °C, 180 °C, and 200 °C for 0.5 h, 1 h, 2 h, and 4 h and afterwards modified by a laboratory OHT-process at 180 °C, 200 °C, and 220 °C for 2 and 4 h. Swelling properties and biological properties were investigated on matched samples to identify suitable combinations of densification and OHT for use in outdoor application. Further on, the process-parameters assessed from laboratory scale were taken over for industrial scale production. The results show that compression-set recovery of densified and oil-heat treated spruce was almost completely eliminated by an OHT at temperatures above 200 °C, as demonstrated in laboratory tests and after 30 months natural weathering. Thus, with respect to the dimensional stability and improved durability, the industrially densified and oil-heat treated spruce timber appears to be suitable for weathered application.

Zusammenfassung

Hitzebehandlungen von Holz zur Verbesserung ausgewählter Eigenschaften, z.B. Dauerhaftigkeit und Dimensionsstabilität, sind industriell etablierte Verfahren. Der Hauptnachteil von hitzebehandeltem Holz besteht jedoch in den durch die thermische Modifikation reduzierten Festigkeitseigenschaften. Zur Überwindung dieses Nachteils wurde in einer früheren Studie thermo-mechanisch verdichtetes Holz mit durch die Verdichtung erhöhten Festigkeitseigenschaften erfolgreich einer Öl-Hitze Behandlung (OHT) im Labor unterzogen. Das Ziel der vorliegenden Arbeit war es, die im Labor erfolgreich erreichte Kombination von Verdichtung und OHT auf industriellen Maßstab zu übertragen. Dafür wurde Fichtenholz (Picea abies Karst.) bei Temperaturen von 140 °C, 160 °C, 180 °C, und 200 °C für 0.5 h, 1 h, 2 h, und 4 h im Labor verdichtet und nachfolgend bei 180 °C, 200 °C, und 220 °C für 2 und 4 h durch eine OHT im Labormaßstab vergütet, um geeignete Parameterkombinationen hinsichtlich Quelleigenschaften und Dauerhaftigkeit für einen Einsatz des Materials in bewitterter Außenanwendung zu ermitteln. Diese geeigneten Parameterkombinationen aus Optimierungsversuchen im Labor wurden für die industrielle Herstellung herangezogen. Ergebnisse aus Quellversuchen und nach 30monatiger natürlicher Bewitterung zeigen, dass das Rückerinnerungsvermögen des verdichteten Fichtenholzes durch eine OHT über 200 °C nahezu komplett unterbunden werden konnte. Ein Einsatz des industriell verdichteten und Öl-Hitze behandelten Materials im bewitterten Außeneinsatz erscheint deshalb im Hinblick auf die erreichte Dimensionsstabilisierung und die erhöhte Dauerhaftigkeit als möglich.

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Authors and Affiliations

  1. Institute for Wood Biology and Wood Protection, Federal Research Centre for Forestry and Forest Products (BFH), Leuschnerstr. 91, 21031, Hamburg, Germany

    C. R. Welzbacher & A. O. Rapp

  2. Institute of Steel and Timber Structures, Technical University Dresden (TUD), Eisenstuckstr. 33 NB, 01062, Dresden, Germany

    J. Wehsener & P. Haller

Authors
  1. C. R. Welzbacher
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  2. J. Wehsener
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  3. A. O. Rapp
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  4. P. Haller
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Correspondence to C. R. Welzbacher.

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Welzbacher, C.R., Wehsener, J., Rapp, A.O. et al. Thermo-mechanical densification combined with thermal modification of Norway spruce (Picea abies Karst) in industrial scale – Dimensional stability and durability aspects . Holz Roh Werkst 66, 39–49 (2008). https://doi.org/10.1007/s00107-007-0198-0

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