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Wood thermal-modification at Luleå University of Technology
Luleå University of Technology.
Luleå University of Technology.
Luleå University of Technology.
2014 (English)In: Final Cost Action FP0904 Conference “Recent Advances in the Field of TH and THM Wood Treatment” May 19-21, 2014, Skellefteå, Sweden: Books of Abstracts / [ed] Mojgan Vaziri, Dick Sandberg, Skellefteå: Luleå tekniska universitet , 2014, 75-75 p.Conference paper, Abstract (Refereed)
Abstract [en]

The Wood Physics group at Luleå University of Technology (LTU) has the vision of transforming Swedish solid wood into the material of choice for the renewable economy of the future. To realize that vision, the group believes, stability and durability of local softwood species must be enhanced at a reasonable cost without jeopardizing the natural beauty of this environmentally friendly material. One of the methods for enhancing stability and durability of solid wood is thermal modification, and LTU's Wood Physics group has vast experience in developing and evaluating thermal modification process. In simple words, thermal modification involves exposing the wood to relatively high temperatures, between 160o C and 240o C depending on the products and technologies used, and in the absence of oxygen to avoid degradation of the wood by combustion. It has been proved that these relatively high temperatures modify the chemical structure of the wood polymers (cellulose, hemicellulose and lignin), and wood becomes less prone to absorb moisture from the environment and more resistance to biological degradation. There are a number of thermal modification methods that have been implemented in Europe at the commercial level, such as ThermoWood® and WTT thermo-treatment. ThermoWood® process is performed under normal atmospheric pressure with superheated steam containing as little oxygen as possible. The wood is first dried to almost 0% moisture content with steam temperatures up to 130°C, and then exposed to steam temperatures between 185°C to 212°C for a few hours. Afterward the vapor temperature is reduced to below 90°C to saturate the steam and allow the wood regain moisture. The WTT thermotreatment is performed with saturated steam under pressure up to 20 bars and temperatures between 160°C and 210°C, so the wood is not dried during the process. In the last years, LTU's Wood Physics group has performed several studies in collaboration with local wood producers interested in the evaluation and optimization of thermal modification processes. To study thermal modification in laboratory, LTU's Wood Physics has built pilot scale kiln/thermal-modification unit that fits through the field of view of a CT-scanner unit specially adapted for wood material studies. This combined equipment allowed measuring wood density profiles through entire thermal modification process, thus providing valuable information about the effect of the process conditions in the material. More recently, LTU's Wood Physics group became interested in the process of thermal modification by boiling in linseed oil for 2 to 4 hours. This technology is available in the market, but the novelty at LTU was the implementation of an additional oil impregnation cooling phase in which the wood is submerged in cool oil after thermal modification. This creates a sudden contraction of the gases inside the wood, which in turn draws considerable amounts of oil into the wood. The authors believe that this combined thermal-modification/oil-impregnation treatment offers a simple but effective methodology for simultaneously: 1) enhance the stability and durability of solid wood, 2) impregnate the wood surfaces with oil for increasing the repellency to moisture. This presentation includes an example of the combined thermal-modification/oil-impregnation treatment applied to common Swedish softwood and hardwood species. Both species were treated by using the WTT heat treatment technology and impregnated with different types of preservative oils. After impregnation, the samples were tested for water repellency, dimensional stability, and resistance to mould. Water repellency and dimensional stability were assessed for both liquid water and air relative humidity, and the resistance to oil leaching was determined by exposing the treated wood to cycles in which the samples absorbed water by immersion and then release the water under vacuum. As expected, the treatments showed a significant improvement in the water repellency and dimensional stability of the wood. Overall, untreated wood was more stable after thermal modification, and thermally modified wood was more stable after oil impregnation. The resistance to mould was evaluated by using an accelerated technique also developed by the Wood Physics group at Luleå University of Technology. The technique consists in placing the wood samples in the upper zone of a conditioning chamber in which there are other pieces of wood already infected by mould in the lower zone. Typically, the source of mould is pine sapwood infected with mould of aspergillus, rhizopus, penicillium genus along with other various species, and the test samples are exposed approximately 20 days to the infected environment. After incubation, the incidence of mould over the surfaces is graded in scale from 0 to 6 based on the visual assessment of two independent observers. The results of the study showed that some of the oil impregnation treatments did not significantly improved mould resistance, and it was still questionable whether the oil would not leach from the wood when the products are in service. Future research in wood modification would be certainly needed to find the right thermal-modification/oil-impregnation combination for the right application, as well as to realize the vision of transforming solid wood in the material of choice for the renewable economy of future.

Place, publisher, year, edition, pages
Skellefteå: Luleå tekniska universitet , 2014. 75-75 p.
Keyword [en]
wood Physics group, wood modification, thermal modification, oil impregnation
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
URN: urn:nbn:se:lnu:diva-57844ISBN: 9789174399378 ISBN: 9789174399387 (print)OAI: oai:DiVA.org:lnu-57844DiVA: diva2:1044937
Conference
Final Cost Action FP0904 Conference: “Recent Advances in the Field of TH and THM Wood Treatment” : May 19-21, 2014, Skellefteå, Sweden
Available from: 2016-11-07 Created: 2016-11-07 Last updated: 2016-12-16Bibliographically approved

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