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    European Coatings Journal

    04/2007

    106

    Darker shades of wood

    Thermally modified timber (TMT) as a new material for parquet floorings. Wood panels can be treated thermally to produce novel materials with enhanced properties for use in floorings. They show many advantages, for example in the currently-fashionable darker colours. Other physical properties, which have a bearing on their coatability, are, however, also changed. Rico Emmler, Wolfram Scheiding.* * Corresponding Author. Contact: Dr. Wolfram Scheiding, Institut für Holztechnologie gGmbH, Zellescher Weg 24, D-01217 Dresden, Germany, Tel. +49 351 4662-280, scheiding@ihd-dresden.de Thermally modified timber (TMT) is a material, where the composition of the cell wall material and physical properties have been modified by exposure to a temperature higher than 160°C under conditions of reduced oxygen. The changes in the material are different from those due to impregnation with preservatives and the wood is altered such that at least some of its properties are changed permanently. Furthermore, this occurs throughout the material. Wood modification can be considered to be one of the most important innovations in the forestry industry in recent years. Although chemical modification (wood cross linking) is also coming onto the market (e.g. acetylation, furfurylation, the Belmadur process), thermal modification remains the most important technology. TMT is to be regarded as a group of new wood materials, with properties varying for different wood types, as well as with the modification process and treatment level. The desired changes due to thermal modification, when compared to untreated wood, are improved dimensional stability, increased durability against wood-attacking fungi and darker colours. The equilibrium moisture content of timber thus treated is normally reduced. Since the strength properties are altered compared to untreated wood, for the use of TMT in constructions for load-bearing applications (structural uses) the relevant requirements of European construction standards and national requirements need to be taken into consideration. The properties of TMT and the extent to which the properties have changed depend on the wood used, the type of technology and the process parameters, particularly the treatment temperature.

    Finnish production dominates The manufacturing technologies of thermally modified timber are mainly characterized by the way the oxygen concentration is reduced in the treatment chamber/kiln. The current industrial-scale processes use atmospheres of heated air and steam, heated nitrogen or a bath of heated oil. Table 1 lists some important European producers. TMT was first developed in Finland and has been produced on an industrial scale since the middle of the nineties. Currently, most TMT still comes from Finland. European production has been increased from approximately 35,000 m ³ in 2002 to 50,000 m³ in 2005, of which 31.146 m³ came from Finland. 55 % of this is exported and 85 % of the material comes from spruce and pine. Because of limited production capacities for TMT in Central Europe and, because some varieties, particularly hardwoods, are not native to Finland, much timber from Central Europe is treated in Finland. This incurs much transportation. The increasing market need for TMT, however, has led to the installation of new kilns and to the

    recent expansion of production capacity.

    Standardisation has been initiated Initially, thermal treatment was developed for use with cheap softwoods and for exterior applications such as cladding or decking. Meanwhile, dark colours have become of interest for interior applications, particularly for floorings. Thus, hardwood species such as beech, ash, birch maple and, surprisingly, tropical woods like rubber wood or teak (sapwood), are being treated. In reaction to the increasing need for regulation and standardisation, CEN TC 175 initiated the development of a European Technical Specification for TMT (CEN/WI 175-108), under the responsibility of CEN/TC175/WG3/TG6 "Thermally Modified Timber". The publication of this CEN/TS is expected in mid 2007. At the same time, some quality-assessment systems have been established on national levels: - Finland: Quality control system of the Finnish Thermowood Association, - Netherlands: KOMO product certificate for modified timber, basing on a national assessment directive, - Germany: Quality label TMT, awarded by the "Entwicklungs- und Prüflabor Holztechnologie GmbH (EPH)" Dresden.

    Access to dark colours without tropical wood The use of TMT for parquet floorings is mainly of interest because wood materials of various dark colours can be made in this way. Such colours became popular some years ago. Furthermore, TMT may be used as a substitute material for dark, tropical woods. Also, its better dimension stability under changing climatic conditions recommends the use of TMT for parquet floorings. Thermal treatment affects the mechanical and hygroscopic properties of wood. Tables 2 and 3 provide typical figures for the density, Brinell hardness and EMC, at different standard climates, using TMT beech and ash (two different products in each case). The samples were produced by two manufacturers using the "Finnish" process.

    Reduced moisture leads to better dimensional stability The influence of the thermal modification on the bending strength is shown in Table 4. In Figure 1, a comparison of moisture change due to normal climate change between tropical woods, TMT and European woods is demonstrated. Comparing the values of density, Brinell hardness and bending strength of TMT and untreated wood, it can be seen that the thermal modification leads to a decrease in these values. This must be taken into consideration if the flooring is to be used under situations that demand a high level of hardness or if there is a high bending load on the flooring. It can be clearly seen that the equilibrium moisture content decreases (Table 3) with, as a result, a reduction in the value of dimension changes. Additionally, the speed of moisture uptake is reduced (see Figure 1), but it is comparable to some tropical wood species. This leads also to better dimension stability.

    Brittleness causes measurement problems No adhesion problems were detected with the coating, but there can be difficulties with the measurement of adhesion by cross cut according to EN ISO 2409 [1]. This is due to the brittleness of the wood, which often leads to "bad" cross cut values. Thus, the reason is not an adhesion problem of the coating but damage to the brittle TMT during the cross-cut

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    European Coatings Journal

    04/2007

    106

    process. This behaviour complicates the assessment of comparative adhesion. The castor chair-resistance test was chosen to determine the mechanical resistance of different oiled TMT surfaces compared to different oiled untreated surfaces (Table 5). The test was carried out according to EN 425 over 25,000 cycles. It is obvious that the castor chair resistance depends on the Brinell hardness. To avoid damages during practical use, the use of protection mats is recommended. Another factor is that the brittleness of the TMT can lead to very significant fractures in the wood structure. This was not detected on untreated wood species with the same range of Brinell hardness (compare untreated oak and ash with TMT oak 1 - 3). To avoid damage during practical use, the application of TMT for situations with high mechanical loads should be checked.

    Inferior light fastness The main reason for the interior use of TMT is to create dark colours. The end user very often expects that this colour is stable over a long period. For this reason, the performance of five different oiled TMT parquets was compared to lacquered Wenge parquet - the dark colour of the latter was imitated by the TMT. The light fastness test was carried out according to EN 15187 (irradiation behind window glass, 55°C BST, 50 % relative humidity, exposure up to blue wool scale 6) in a Xenon test device. The change of colour was visually assessed by using of a grey scale according to EN ISO 20105-A02. A summary of the results is given in Table 6. The results show clearly that the light fastness of the tested samples of TMT was inferior to that of light woods. The results with dark woods, which they try to imitate, were even worse. Thus, coating producers need to develop materials offering light protection to these dark TMTs, with the aim of meeting consumer demand for a dark TMT with long-term stability.

    Emissions under the microscope It must be expected that in the near future all coated parquet in Germany will need a trading permit from the DIBT. This permit will be based on a VOC emission test and an assessment of the results according to the AgBB (Board for the sanitary evaluation of building material) scheme. Furthermore, it must be taken into consideration that the thermal treatment can cause emission of furfural in some cases. The reason is a mild pyrolysis of the wood cell wall. Initial work has shown that the potential emission level depends on the treatment temperature. A higher temperature leads to a lower emission potential [2]. Furfural has a low LCI-value (lowest concentration of interest) of 20 µg/m³, according to the AgBB scheme. The measured VOC emission (in µg/m³) of a substance is divided by its LCI value to give the so-called R-value. According to the requirement of AggB scheme, the total R-values of all measured substances must be lower than 1 or equal. As was shown in [2], furfural emission can lead t

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