Thermally Modified Wood Is Increasingly Used in Outdoor Conditions, but Its Surface Also

OILS USAGE IN FINISHING OF THERMALLY MODIFIED WOOD IN OUTDOOR APPLICATIONS

Josip Miklečić, BSc

Prof. Vlatka Jirouš-Rajković, PhD

Assoc. Professor Stjepan Pervan, PhD

Siniša Grujić, student

University of Zagreb

Faculty of Forestry

Croatia

SUMMARY

Thermally modified wood is increasingly used in outdoor conditions, but its surface also undergoes to degradationby weatheringas well as unmodified wood. Oils are largely used for protection of wood surface in outdoor conditions because they are easyto maintain and recoat, and theyenhance the appearance of wood surface. In this study, we measured the change of colour and gloss and surface cracking of thermallymodified and unmodified wood samples treated with three types of oils during the acceleratedweatheringin QUV device. We also measured the permeability of liquid water and water-vapour according to HRN EN 927-5 and HRN EN 927-4. It was found that unmodified untreated samples had less surface cracks than the thermally modified untreated samples during acceleratedweathering. Oils treatment reduced the cracking of the thermally modified samples. Teak oil and oil for thermally modified wood offered similar protection to exposed thermally modified wood surfaces from the effects of water and UV radiation, while the universal oil did not offer any protection from the colour change of modified wood during exposure. Since the oil coating is also susceptible to photochemical degradation, for a pleasant appearance of wood surfaces during outdoor use it is very important the timely refinishing in accordance with the manufacturer's instructions.

Key words: thermally modified wood, wood finishing oils, accelerated weathering, general appearance of the surface, water permeability

1 INTRODUCTION

Due to the enhanced dimensional stability, improved biological durability, reduced permeability and good appearance thermally modified or heat treated wood is increasingly used in many outdoor applications such as exterior cladding, decking, terraces and garden furniture. The resistance of heat treated wood against weathering (UV-light, wetting) is not changed largely when compared to untreated wood, making a surface treatment with coatings necessary (Militz, 2002; Hill, 2009). There are many products on the market available for outdoor wood finishing which are commonly classified into film-forming finishes, as these form a film on the wood surface, and penetrating finishes, as these penetrate into the wood without forming a film. The horizontal surface, pooling of water and full exposure to sun and rain make deck finishing more demanding than other wood finishing (e.g. exterior claddings). For such surfaces penetrating finishes are recommended because they don’t form a film and give better overall performance and have the easiest maintenance and refinishing (Williams and Feist, 1993, Feist, 2006, Gibson, 2007). Oils fall into the category of penetrating finish which enhance the natural wood grain and appearance (Bulian and Graystone, 2009).With oil treatment wood surface absorption is reduced and ingress of liquids and dirt is more difficult (Hägele, 2003). There are many oils on the market intended for outdoor use and to look „natural“, designed specifically for decks and garden furniture and formulated to resist weathering. In this paper we wanted to examine the efficacy of such oils in protecting the surface of thermally modified and unmodified wood during accelerated weathering.

2 MATERIALS AND METHODS

Unmodified and thermally modified samples of three wood species were used in this study: oak (Quercus robur L.), ash (Fraxinus excelsior L.) and beech (Fagus silvatica L.). Oak and beech wood were represented by two types of samples: unmodified samples (marks: H, B), thermally modified oak wood samples at 180 ° C (mark: H180) and thermally modified beech wood samples at 190 ° C (mark: B190), and ash wood samples were represented by three types of samples: unmodified samples (mark: J), thermally modified samples at 190 ° C (mark: J190) and thermally modified samples at 200 ° C (mark: J200). The process of thermal modification of the samples was carried out 68 hours in seven phases. Selected samples were radial texture, free from defects and even colour. Surface treated and untreated samples of dimensions 150 x 75 x 18 mm (L x R x T)were used for acceleratedweathering, and samples of dimensions 150 x 70 x 20 mm (L x R x T)were usedfor assessment of theliquid water and water-vapour permeability. Samples were previously conditioned at 20 °C and 65% relative humidity.

Samples were surface treated with three types of oils: universal oil (mark: 110), thermowood oil (mark: 113), and teak oil (mark: 118). All three types of oils are formulated from natural oils with the addition of aliphatic hydrocarbon solvents andadditives. Teak oil and thermowood oil contain UV-resistant mineral and organic pigments. Oils were applied manually by brush in two layers in the amount of 45 g/m2 per layer, and the drying time between layers was 24 hours.

Acceleratedweathering was conducted in a QUV device (Q-Panel Company) equipped with UVA-340 fluorescent lamps. Samples were exposed 672 hours (4 cycles): two identical thermally modified wood samplesfor each oil; one unmodified wood sample for each oil and one surface untreated wood sample.

The parameters of the cycle of accelerated weathering:

total duration of one cycle: 168 h (1 week)

24 h condensation at (45 ± 3) °C

2.5 h UV lamps at (60 ± 3)°C and 0.77 W/(m2nm) radiation

0.5 h spraying water 6-7 l/min with no light and no temperature control

the period of UV lamps and spraying was repeated 48 times

Assessmentof the liquid water and water-vapour was carried out according to standard HRN EN 927-5 and HRN EN 927-4.

During accelerated weathering the surface of samples was visually checked for any cracking visible to the naked eye or visible under microscope magnification of 10 times. At the end of the exposure the general appearance of the surface of samples (discoloration and cracking) was assessed using a scale from 0-5 (0-no changes, 5-greatest changes).

Before and after 26,5; 115; 168; 336; 504 and 672 hours of acceleratedweatheringchange in colour and gloss, and the appearance of surface cracks was measured on the samples. The measurement of colour change was made with a portable spectrophotometer Microflash 100d produced by Datacolor (d/8° measuring geometry, 10° standard observer, D65 standard illuminate, xenon flash lamp source) always on the same eight marked locations on sample. The overall colour change (ΔE*) was calculated according to the CIEL*a*b* colour measuring system where, L* describes brightness, a* and b* describe chromatic coordinates on the red-green and yellow-blue axis. ΔE* is the colour difference between the initial colour of the sample and the colour of the sample after exposure and it was calculated using the following formula:

E* = [(L*)2 + (a*)2 + (b*)2]1/2 (1)

where L* describes the difference in brightness (+ light, - dark), a* and b* describes differences in chromaticity coordinates ((a*: + redder, - greener; b*: + yellower, - bluer). The measurement of gloss was made with a portable glossmeter Glosmaster Erichsen 507 with 65° measuring geometry on the three locations on sample.

3 RESULTS AND DISCUSSION

Colour changes (E*) of wood samples during QUV exposure are presented in Figures 1 and 2. After only 26,5 hours of exposure (24 h condensation and 2,5 hours UV lamps) all wood samples changed the colour, but unfinished samples changed more.

Figure 1 Colour change of unmodified oak wood (a), unmodified ash wood (b) and unmodified beech wood during accelerated weathering

It can be seen from Fig. 1a and 1b that oils reduced colour change of unmodified oak wood samples only in first hours of exposure (universal oil and teak oil up to 120 hours, and thermowood oil up to 180 hours) and for unmodified ash wood oils reduced colour changes during the most of the exposure with the exception of ash samples finished with universal oil. On unmodified beech wood samples universal oil reduced colour change up to 360 hours of exposure and teak oil and thermowood oil during entire exposure. It should be mention that teak oil and thermowood oil change the colour of unmodified wood to some degree because they contain pigments. Among three wood species of unmodified and finished samples ash wood samples showed the most prominent change of colour after QUV exposure.

Figure 2 Colour change of thermally modified oak wood at 180°C (a), thermally modified ash wood at 190°C (b), thermally modified beech wood at 190°C (c) and thermally modified ash wood at 200°C (d) during accelerated weathering

Colour change of thermally modified samples finished with teak oil (118) and with thermowood oil (113) was smaller during entire exposure than colour change of unmodified samples and samples treated with universal oil (110) (Fig 2). The colour change of thermally modified wood samples finished with universal oil was lower compared to unfinished samples: on oak wood during the first 500 hours of exposure, on beech wood during the 500 hours, on ash wood thermally modified at 190°Cduring 630 hours and on ash wood modified at 200°C during 650 hours of exposure. At the end of accelerated exposure total discoloration (E*) of the thermally modified samples finished with teak oil (oil 118) and with thermowood oil (oil 113) was 3,5 times and 3 times smaller, respectively for oak wood, 3 times and 2 times smaller respectively for beech wood, 5 and 4 times smaller, respectively for ash wood thermally treated at 190°C and 2,5 and 2,2 times smaller, respectively for ash wood thermally treated at 190°C than total discoloration of the unfinished samples (Fig.2). The most prominent trend of colour change can be seen for thermally modified ash wood (at 190°C and 200°C) samples unfinished or finished with universal oil (Fig. 2b and Fig. 2d). Among unfinished thermally modified wood samples the smaller colour change could be seen for oak and beech wood samples. At the end of exposure the colour change of these samples was 1,7 smaller than colour change of ash wood samples (Fig.2).

Figure 3 shows gloss changes of thermally modified wood samples during accelerated weathering. It can be seen that oil treatment enhance gloss of wood surface. The highest gloss values were measured on thermally modified beech wood surfaces (Fig. 3c), and the smallest were measured on ash wood surfaces thermally modified at 200°C (Fig. 3d).

Figure 3 Gloss change of thermally modified oak wood at 180°C (a), thermally modified ash wood at 190°C (b), thermally modified beech wood at 190°C (c) and thermally modified ash wood at 200°C (d) during accelerated weathering

The gloss value of thermally modified oak wood surface finished with teak oil was 12 time higher than gloss of unfinished thermally modified oak wood; the gloss value of thermally modified ash wood surface (at 190°C ) finished with same oil was 13 time higher than gloss of unfinished thermally modified ash wood, the gloss of thermally treated beech wood treated with teak oil was 19 time higher than gloss of unfinished samples and the gloss of value of thermally modified ash wood surface (at 200°C ) finished with same oil was 7 time higher than gloss of unfinished thermally modified ash wood. A rapid decrease can also be seen in gloss of thermally modified oil finished oak wood and ash wood samples during the first 26,5 hours of QUV exposure followed by small gloss changes with continued exposure. During the first 26, 5 hours of QUV exposure marked gloss change of thermally modified beech wood samples is also evident followed by more intensive decrease in gloss values compared to thermally modified oak and ash wood samples. The unfinished thermally modified samples showed only small changes in gloss during QUV exposure.

The results of the liquid water absorption test show that wood samples finished with oils absorbed less water than unfinished samples: thermowood oil exhibited the lowest permeability, followed by teak oil, and the highest permeability exhibited universal oil (Fig. 4). It can also be seen that liquid water absorption of thermally modified wood samples is lower than liquid water absorption of unmodified samples and ash wood samples thermally modified at 200°C exhibited lower liquid water absorption than ash wood samples thermally modified at 190°C. Among all tested samples only thermally modified samples of oak and ash wood at 190°C, finished with thermowood oil and ash wood samples thermally modified at 200°C and finished with all three kinds of oils exhibited the value of liquid water permeability lower than 175 g/m2 which is according to HRN EN 927-2 limit value for coatings intended to use for dimensionally stable wood products (Fig. 4).

Water-vapour absorption does not show such great deviations from values of unfinished wood surfaces (Fig. 5) which means that the treated surfaces do not prevent water-vapour absorption and liquid water uptake in the exposures to the high humidity for longer intervals (e.g. in autumn and winter months). All beech samples exhibited the highest values of water vapour absorption with the exception of unmodified samples treated with teak oil.

Figure 4 Liquid water permeability according to HRN EN 927-5

Figure 5 Water-vapour absorption and desorption according to HRN EN 927-4

The surface cracks were detected by visual inspection after 168 hours (1 week) of QUV exposure in unfinished thermally modified wood whereas in unmodified unfinished samples cracks were detected after two weeks of exposure inoak and beech woodand after 3 weeks in ash wood. During accelerated QUV exposure unmodified, unfinished samples of all three wood species had less cracks than thermally modified wood samples. It can be seen from Table 1 that surface treatment with oil reduce surface cracking of thermally modified wood. The cracking of ash wood thermally modified at 200°C was less pronounced than cracking of ash wood samples thermally modified at 190°C. Among all tested thermally modified wood samples finished with oils the cracking of ash wood samples modified at 190°C was mostly pronounced (Table 1).

Table1 General appearance of the surface of samples after 672 hours accelerated weathering

* value 0-no changes, value 5-greatest changes

4 CONCLUSION

Colour changesof thermally modified wood samples of all three wood species are reducedby teak oil and thermowood oil treatment during accelerated weathering.

Gloss changes arethe most pronounced at the beginning of the exposure, and later theyare very small compared to the colour changes, for all wood species.

All three species of thermally modified wood samples exhibited a lower permeability ofliquid water thanthe unmodified wood samples.

The lowest permeabilityof liquid water exhibitedthe samples treated with thermowood oil, followed by teak oil, and the highest permeability exhibited universal oil for all three wood species.

Tested oils do not prevent the absorption of large amount of water in the exposure to the high humidity for longer intervals.

Untreated, unmodified samples of all three wood speciescracked less than untreated thermally modified samples during acceleratedweathering, andoil finishing of thermally modified samples decreased cracking.

For a pleasant appearanceof oiled surface during outdoor use the regular maintenance according to the manufacturer’s instructions is essential.

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