darker shades of wood - european coatings · darker shades of wood thermally modified timber (tmt)...

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European Coatings Journal

04/2007

106

Darker shades of wood

Thermally modified timber (TMT) as a new material forparquet floorings.Wood panels can be treated thermally to produce novelmaterials with enhanced properties for use in floorings. Theyshow many advantages, for example in thecurrently-fashionable darker colours. Other physicalproperties, 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,[email protected] modified timber (TMT) is a material, where thecomposition of the cell wall material and physical propertieshave been modified by exposure to a temperature higherthan 160°C under conditions of reduced oxygen. Thechanges in the material are different from those due toimpregnation with preservatives and the wood is alteredsuch that at least some of its properties are changedpermanently. Furthermore, this occurs throughout thematerial.Wood modification can be considered to be one of the mostimportant innovations in the forestry industry in recent years.Although chemical modification (wood cross linking) is alsocoming onto the market (e.g. acetylation, furfurylation, theBelmadur process), thermal modification remains the mostimportant technology.TMT is to be regarded as a group of new wood materials,with properties varying for different wood types, as well aswith the modification process and treatment level.The desired changes due to thermal modification, whencompared to untreated wood, are improved dimensionalstability, increased durability against wood-attacking fungiand darker colours. The equilibrium moisture content oftimber thus treated is normally reduced. Since the strengthproperties are altered compared to untreated wood, for theuse of TMT in constructions for load-bearing applications(structural uses) the relevant requirements of Europeanconstruction standards and national requirements need tobe taken into consideration. The properties of TMT and theextent to which the properties have changed depend on thewood used, the type of technology and the processparameters, particularly the treatment temperature.

Finnish production dominatesThe manufacturing technologies of thermally modified timberare mainly characterized by the way the oxygenconcentration is reduced in the treatment chamber/kiln. Thecurrent industrial-scale processes use atmospheres ofheated air and steam, heated nitrogen or a bath of heatedoil. Table 1 lists some important European producers.TMT was first developed in Finland and has been producedon an industrial scale since the middle of the nineties.Currently, most TMT still comes from Finland. Europeanproduction has been increased from approximately 35,000 m³ in 2002 to 50,000 m³ in 2005, of which 31.146 m³ camefrom Finland. 55 % of this is exported and 85 % of thematerial comes from spruce and pine.Because of limited production capacities for TMT in CentralEurope and, because some varieties, particularlyhardwoods, are not native to Finland, much timber fromCentral Europe is treated in Finland. This incurs muchtransportation. 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 initiatedInitially, thermal treatment was developed for use with cheapsoftwoods and for exterior applications such as cladding ordecking. Meanwhile, dark colours have become of interestfor 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 andstandardisation, CEN TC 175 initiated the development of aEuropean Technical Specification for TMT (CEN/WI175-108), under the responsibility of CEN/TC175/WG3/TG6"Thermally Modified Timber". The publication of this CEN/TSis expected in mid 2007. At the same time, somequality-assessment systems have been established onnational levels:- Finland: Quality control system of the Finnish ThermowoodAssociation,- 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 woodThe use of TMT for parquet floorings is mainly of interestbecause wood materials of various dark colours can bemade in this way. Such colours became popular some yearsago. Furthermore, TMT may be used as a substitutematerial for dark, tropical woods. Also, its better dimensionstability under changing climatic conditions recommends theuse of TMT for parquet floorings.Thermal treatment affects the mechanical and hygroscopicproperties of wood. Tables 2 and 3 provide typical figures forthe density, Brinell hardness and EMC, at different standardclimates, using TMT beech and ash (two different productsin each case). The samples were produced by twomanufacturers using the "Finnish" process.

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

Brittleness causes measurement problemsNo adhesion problems were detected with the coating, butthere can be difficulties with the measurement of adhesionby cross cut according to EN ISO 2409 [1]. This is due to thebrittleness of the wood, which often leads to "bad" cross cutvalues. Thus, the reason is not an adhesion problem of thecoating but damage to the brittle TMT during the cross-cut

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

Inferior light fastnessThe main reason for the interior use of TMT is to create darkcolours. The end user very often expects that this colour isstable over a long period. For this reason, the performanceof five different oiled TMT parquets was compared tolacquered Wenge parquet - the dark colour of the latter wasimitated by the TMT. The light fastness test was carried outaccording to EN 15187 (irradiation behind window glass,55°C BST, 50 % relative humidity, exposure up to blue woolscale 6) in a Xenon test device. The change of colour wasvisually assessed by using of a grey scale according to ENISO 20105-A02. A summary of the results is given in Table6.The results show clearly that the light fastness of the testedsamples of TMT was inferior to that of light woods. Theresults with dark woods, which they try to imitate, were evenworse. Thus, coating producers need to develop materialsoffering light protection to these dark TMTs, with the aim ofmeeting consumer demand for a dark TMT with long-termstability.

Emissions under the microscopeIt must be expected that in the near future all coated parquetin Germany will need a trading permit from the DIBT. Thispermit will be based on a VOC emission test and anassessment of the results according to the AgBB (Board forthe sanitary evaluation of building material) scheme.Furthermore, it must be taken into consideration that thethermal treatment can cause emission of furfural in somecases. The reason is a mild pyrolysis of the wood cell wall.Initial work has shown that the potential emission leveldepends on the treatment temperature. A highertemperature leads to a lower emission potential [2]. Furfuralhas a low LCI-value (lowest concentration of interest) of 20µg/m³, according to the AgBB scheme. The measured VOCemission (in µg/m³) of a substance is divided by its LCI valueto give the so-called R-value. According to the requirementof AggB scheme, the total R-values of all measuredsubstances must be lower than 1 or equal. As was shown in[2], furfural emission can lead to an R-value close to theupper limit of the AggB scheme. In one case this value waseven exceeded. Table 7 shows the results of selected, notrepresentative emission tests on TMT. A coating for a TMTmust therefore have very low emission, in order to obtain alow single R-value.

References[1] H. Engelking, Beschichtung und Oberflächenvergütungvon Thermoholz. Conference Proceedings of the 3.European Thermowood Congress, 2 - 3. Juni 2005, IHDDresden, Germany

[2] M. Broege, Emissionen aus Thermohölzern - einProblem für Innenräume? Conference Proceedings of the 3.European Thermowood Congress, 2 - 3. Juni 2005, IHDDresden, Germany[3] Holzlexikon, DRW Verlag

Results at a glance- TMT, thermally modified timber, has to be regarded as aseparate group of new materials made from wood, theproperties of which depend on the wood species, themodification process and treatment level.- The process was originally developed to improve thedurability of cheap woods for exterior applications.- Currently TMT is interesting for floorings, because of thedarker colours that can be produced and increaseddimensional stability.- The thermal treatment usually reduces the strength of thematerial and its Brinell hardness and it shows higherbrittleness compared to untreated wood.- The equilibrium moisture content is approximately 50 % ofthat of untreated wood. The moisture uptake speed is lowerthan that of the European woods. Both properties give riseto better dimension stability.- Like untreated wood, the colour of TMT is not stable tolight irradiation, even indoors. TMT must be protected fromdiscolouration and greying by coatings with very good lightprotection.-. In an emission test of six TMT materials, five met therequirements of the German AgBB scheme. Only in onecase was an unacceptably high furfural emission detected.- The varying property profile of TMT has to be taken intoconsideration for use as a material for parquet floors.

The authors:-> Dr.-Ing. Rico Emmler (born in 1967) studied wood andfibre technology at the Dresden University of Technology.Since 1993, he has been working as a scientist in surfacetesting at the Institute of Wood Technology Dresden. Since2005 he is heading the Department Surface of that institute.-> Dr. rer. silv. Wolfram Scheiding (born in 1965) studiedforestry at the Dresden University of Technology. From 1993until 1998, he was a scientific assistant at the TharandtInstitute of Forest Utilisation and Forest Technology,developing wood-fibre based insulation materials andcompleting his PhD thesis on this research. Since 1999, heis working at the Institute of Wood Technology Dresden, asHead of Department Biology/Wood Preservation.This paper was presented at the European CoatingsConference "Parquet Coatings IV", Berlin, 9/10 November2006.


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Figure 1: Moisture change as a function of climate change.


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