PHOTORESISTANCE OF HEAT TREATED WOOD IN INTERIOR USE

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1 PHOTORESISTANCE OF HEAT TREATED WOOD IN INTERIOR USE Josip Miklečić, BSc Associate Professor Vlatka Jirouš-Rajković, PhD Siniša Čmarec University of Zagreb Faculty of Forestry Croatia ABSTRACT Wood is photochemical unstable substrate witch change colour even in interior use because of photooxidation of lignin. In this study, an attempt was made to analyze surface discolouration of heat treated wood when exposed to UV radiation filtered through window glass. The colour change was measured using a spectrophotometer on heat treated samples of beech and ash which were uncoated and coated with three different varnishes. The samples were exposed 32 days using QUV tester equipped with UVA-351 lamps. The experiment shows that the trend of discolouration could be observed on the all samples tested according to the overall changes in colour ( E*) after 32 days of exposure. The greatest discolouration, according to E* values were recorded for uncoated samples and samples coated with UV-cure water borne varnish. The 2k PU varnish had reduced discoloration of wood, and adding the UV-absorber to this varnish did not improve its colour stability. Key words: heat treated wood, QUV exposure, clear varnishes, and photo stabilisation 1 INTRODUCTION Heat treated wood has an increasing market in Europe (1). The main advantage of heat treatment is the improvement in a dimensional stability and resistance to decay (2). Heat treatments change the chemical and physical properties of wood and dimensional stability and hygroscopic are affected as a result of modification of wood cell components. However, it heat treatment may reduce the mechanical resistance, mainly of bending (2, 3). With heat treatment the colour of wood is modified acquiring a darker tonality which is often justified by the formation of coloured degradation product from hemicelluloses (4) and from the extractives that seem to participate in the colour formation of heat treated wood (5). Colour intensity of wood during heat treatment increases with treatment time and temperature (6). The formation of oxidation products such as quinones is also referred as the reason for colour change (6, 7). In species with pale coloured wood which are usually considered less appellative, the darkening would be an important advantage of the heat treatment giving the wood a tropical flavour that is valued in many countries (8).

2 Wood surface colour is a very important quality criterion in the utilization of wood, especially in indoor applications (9, 10). If we want to maintain the original characteristics of natural wood, the most preferable method is to use clear coatings as protection. Unfortunately clear finishes perform badly on wood during interior or exterior exposure because these types of coatings cannot absorb UV light which cause photodegradation of wood. It is generally known that ultraviolet light is one of the most effective environmental factors which cause severe changes in wood surface discoloration (11, 12). Some wood became bleached or grey, others turn yellow, red-orange or brown colours depending on the influence of the wood compositions (13, 14). Undesirable consequences of the photodegradation can be substantially limited by using UV-absorbers (15). For the colour stabilization of indoor clear coats, typically organic UVabsorbers are used in concentrations of 1-3% (calculated on binder), depending on the coating thickness (16). By measuring the colour change of the clear coated wood with different UV absorbers during artificial weathering is therefore possible to obtain information on their photostabilisation performances (17). The light induced degradation of wood is often investigated under artificial conditions, due to the fact that it needs less time and the conditions remain constant during exposure (9). Ayadi, N. et al. showed that heat treated wood change colour during artificial weathering but colour change is lower than colour change of non treated wood (18). In the present study, we have investigated the colour changes of uncoated and coated heat treated wood when exposed to UV radiation filtered through window glass. A detailed classification according to the CIE L* a* b* colour values was provide during 32 days of simulated indoor sunlight. 2 MATERIALS AND METHODS In this paper we have studied two wood species samples: beech (Fagus silvatica L.) and ash (Fraxinus excelsior L.), which were treated at 190 C and 212 C. The samples of 150 x 70 x 18 mm dimensions, free from defects, for the most part of radial texture were used. Four replicate samples were prepared in accordance with EN Three panels were used for exposure and the fourth was unexposed reference. Wood panels were coated with 2k solvent borne PU varnish provided by Chromos with or without UV absorbers and with clear UV-cured water-born coating without UV absorbers provided by Bona. Water-born coating was applied in factory in five layers in total amount of 60 g/m 2, and PU coating was applied by brush in tree layers, each approximately 120 g/m 2, with 24 hours drying time between coats.

3 Table 1 Marks and description of samples Mark Description B190K heat treated beech on 190 C - uncoated B212K heat treated beech on 212 C - uncoated B190PU heat treated beech on 190 C - coated with 2k PU varnish B212PU heat treated beech on 212 C - coated with 2k PU varnish B190PUA heat treated beech on 190 C - coated with 2k PU varnish with UV absorbers B212PUA heat treated beech on 212 C - coated with 2k PU varnish with UV absorbers B190UV heat treated beech on 190 C - coated with UV-cured water-born varnish B212UV heat treated beech on 212 C - coated with UV-cured water-born varnish J190K heat treated ash on 190 C - uncoated J212K heat treated ash on 212 C - uncoated J190PU heat treated ash on 190 C - coated with 2k PU varnish J212PU heat treated ash on 212 C - coated with 2k PU varnish J190PUA heat treated ash on 190 C - coated with 2k PU varnish with UV absorbers J212PUA heat treated ash on 212 C - coated with 2k PU varnish with UV absorbers J190UV heat treated ash on 190 C - coated with UV-cured water-born varnish J212UV heat treated ash on 212 C - coated with UV-cured water-born varnish The coated samples were exposed 32 days (768 hours) using QUV weathering tester (Q Panel Company) equipped with UVA-351 lamps. A Figure 1 shows the UVA-351 fluorescent lamp compared to sunlight through ordinary window glass. The samples were exposed to lamps directly with temperature of the black panel of 60±3 C. The colour of the samples was measured using a portable spectrophotometer Microflash 100d produced by Datacolor (d/8 measuring geometry, 10 standard observer, D65 standard illuminate, xenon flash lamp source). The colour was measured before exposure and during exposure after 1, 2, 4, 8, 16, 32 days on each sample, always on the same six marked locations, and arithmetic mean of 16 measurements for each group of samples was calculated. The CIE L* a* b* colour system was used where changes in colour coordinates equate the following: L*: + lighter or darker; a*: + redder or greener; b*: + more yellow or bluer. E* is the colour difference between the initial colour of the sample and the colour of the sample after exposure and is calculated as follows: E*= 2 2 ( *) ( *) ( *) 2 L + a + b Note that E* states the extent of colour difference but not direction (4).

4 Figure 1 UVA-351 Fluorescent Lamp compared to sunlight through ordinary window glass. 3 RESULTS AND DISCUSSION In Figures 2, 4, 5 and 6 the results of colour changes are presented QUV exposure, days J212K J190K B212K B190K Figure 2 Colour changes of uncoated heat treated wood samples The E* values of uncoated heat treated wood were increased during the exposure time (Figure 2). Terziev and Boutelje (19) reported that the last colour difference E* possible for the human eye to distinguish corresponds to 1-3 units and according to this E* values of uncoated heat treated wood were visible already after first measurement (i.e. after 24 hours of exposure). Hon and Feist (20) state that the change of colour for over 3 E* units is in industrial use unacceptable. Figure 2 shows the higher colour changes resulting from UV light irradiation for samples treated at 212 C. If we compare results in Figure 2 with results in Figure 3 obtained

5 by Jirouš-Rajković et al. (21) for uncoated and untreated wood samples irradiated on same exposure conditions it can be seen that heat treated wood samples exhibit better colour stability during UV exposure than untreated wood samples. This is in agreement with research of Ayadi et al. (18) and Deka et al. (22) QUV exposure, days BEECH ASH Figure 3 Colour changes of untreated and uncoated wood (21) 1 J212PU J190PU B212PU B190PU QUV exposure, days Figure 4 Colour changes of heat treated wood samples coated with 2K PU varnish

6 1 J212PUA J190PUA B212PUA B190PUA QUV exposure, days Figure 5 Colour changes of heat treated wood samples coated with 2k PU varnish with UV absorbers The E* values of heat treated ash samples on 190 C coated with 2k PU varnish with or without UV absorbers were decreased until sixteen days of exposure when they started to rise, and the E* values of beech heat treated samples (B190PU, B190PUA) were decreased until eight days of exposure when they started to rise (Figures 4 and 5). The colour changes ( E*) on other samples were increased during the time of exposure. Figure 4 and 5 shows that the addition of UVabsorber to 2k PU varnish did not improve the colour stability of samples QUV exposure, daysi J212UV J190UV B212UV B190UV Figure 6 Colour changes of heat treated wood samples coated with UV-cured waterborn varnish The samples heat treated on 212 C showed stronger changes of colour than the samples heat treated on 190 C as can be seen in all Figures. It is interesting to note that the samples of heat treated beech on 212 C have greater E* values than the samples of ash heat treated on same temperature while the ash samples heat

7 treated on 190 C have greater E* values than beech samples heat treated on 190 C. It can also be seen that E* values of uncoated samples are similar to the values of samples coated with UV-cured water-born varnish (Figures 2 and 6). After 32 days of QUV exposure the limits of overall colour changes were in the range of minimum of E* = 2,21 (J190PUA) and maximum of E* =17,42 (B212UV). The highest discolouration for ash wood was observed for uncoated ash wood samples treated on 212 C (mark J212K, E*=13,77) and the lowest discolouration for ash wood was observed for ash wood samples heat treated on 190 C and coated with 2k PU varnish with absorber (mark J190PUA, E* = 2,21). In the case of beech wood samples the highest discolouration was found for samples heat treated on 212 C and coated with UV varnish ( mark B212UV, E* = 17,42), and the lowest for beech wood samples heat treated on 190 C and coated with 2kPU varnish (mark B190PU, E* = 4,39). J212K B212K J190K B190K J212PU B212PU J190PU B190PU J212PUA B212PUA J190PUA B190PUA J212UV B212UV J190UV B190UV Figure 7 L*, a* and b* values after 32 days of QUV test According to L*, a* and b* values in Figure 7 it can be seen that the most of tested samples showed brightening and yellowish behaviour, and the brightening behaviour was mostly obvious.

8 4 CONCLUSION The trend of discolouration could be observed on the all samples, tested according to the overall changes in colour ( E*) after 32 days of simulated indoor sunlight exposure. The samples of heat treated beech on 212 C have greater E* values than the samples of ash heat treated on same temperature while the ash samples heat treated on 190 C have greater E* values than beech samples heat treated on 190 C. The greatest discolouration, according to E* values were recorded for uncoated samples and samples coated with UV-cure water borne varnish. The 2k PU varnish had reduced discolouration of wood, and adding the UV-absorber to this varnish did not improve its colour stability. After 32 days of exposure the values of L* and b* in absolutely amount had mostly changed and the brightening of samples was mostly obvious. These observations can be useful for users of heat treated wood, who are in search of finishing systems able to protect the surface of wood against UV light. REFERENCES 1. Homann, W.J., Tjeerdsma, B. (2005): CONTROL SYSTEM, QUALITY ASSESSMENT AND CERTIFICATION OF MODIFIED WOOD FOR MARKET INTRODUCTION, In: Proceedings of the 2 nd European Conference on wood Modification, 6-7 October 2005, Göttingen. pp Jämsä, S., Viitaniemi, P.(2001): HEAT TREATMENT OF WOOD BETTER DURABILITY WITHOUT CHEMICALS, IN: REVIEW ON HEAT TREATMENTS OF WOOD, Proceedings of the special seminar on heat treatments, 9 February 2001, Antibes, France. Ed. Rapp, A.O. Office for Official Publications of the European Communities, Luxembourg. pp Esteves, B. et al. (2007): INFLUENCE OF STEAM HEATING ON THE PROPERTIES OF PINE (PINUS PINASTER) AND EUCALYPT (EUCALYPTUS GLOBULES) WOOD, Wood Sci Technol 41: Sehistedt-Persson, M. (2003): COLOUR RESPONSES TO HEAT TREATMENT OF EXTRACTIVES AND SAP FROM PINE AND SPRUCE, In: 8 th International IUFRO wood drying conference, Brasov, pp Sundqvist, B., Moren, T. (2002): THE INFLUENCE OF WOOD POLYMERS AND EXTRACTIVES ON WOOD COLOUR INDUCED BY HYDROTHERMAL TREATMENT, Holz Roh Werkst 60: Bekhta, P., Niemz, P. (2003): EFFECT OF HIGH TEMPERATURE ON THE CHANGES IN COLOR, DIMENSIONAL STABILITY AND MECHANICAL PROPERTIES OF SPRUCE WOOD, Holzforschung 57: Tjeerdsma, B. et al. (1998): CHARACTERISATION OF THERMALLY MODIFIED WOOD: MOLECULAR REASON FOR WOOD PERFORMANCE IMPROVEMENT, Holz Roh Werkst 56: Esteves, B. et al. (2008): HEAT-INDUCED COLOUR CHANGES OF PINE (PINUS PINASTER) AND EUCALYPT (EUCALYPTUS GLOBULUS) WOOD, Wood Sci Technol 42: Oltean, L. et al. (2008): WOOD SURFACE DISCOLOURATION DUE TO SIMULATED INDOOR SUNLIGHT EXPOSURE, Holz Roh Werkst 66:51-56

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