engleski

Use of oils in finishing of thermally modified wood for outdoor applications.

Thermally modified wood is increasingly used in outdoor conditions, but its surface also undergoes to degradation by weathering as well as unmodified wood. Oils are largely used for protection of wood surface in outdoor conditions because they are easy to maintain and recoat, and they enhance the appearance of wood surface. The change of colour and gloss and surface cracking of thermally modified and unmodified wood samples (oak (Quercus robur L.), ash (Fraxinus excelsior L.) and beech (Fagus silvatica L.).) were measured, treated with three types of oils during the accelerated weathering in QUV device. The use of 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. Colour change of thermally modified samples finished with teak oil and with thermowood oil was smaller during entire exposure than colour change of unmodified samples and samples treated with universal oil. 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. 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. Oil treatment enhance gloss of wood surface and highest gloss values were measured on thermally modified beech wood surfaces, and the smallest were measured on ash wood surfaces thermally modified at 200°C. 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 permeability of liquid water and water-vapour were also measured according to EN 927-5 and EN 927-4. 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 showed the value of liquid water permeability lower than 175 g/m2 which is according to EN 927-2 limit value for coatings intended to use for dimensionally stable wood products. Water-vapour absorption does not show such great deviations from values of unfinished wood surfaces 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. 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. It can be concluded that colour changes of thermally modified wood samples of all three wood species are reduced by teak oil and thermowood oil treatment during accelerated weathering. Gloss changes are the most pronounced at the beginning of the exposure, and later they are very small compared to the colour changes, for all wood species. All three species of thermally modified wood samples showed a lower permeability of liquid water than the unmodified wood samples. The lowest permeability of liquid water exhibited the 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 species cracked less than untreated thermally modified samples during accelerated weathering, and oil finishing of thermally modified samples decreased cracking. For a pleasant appearance of oiled surface during outdoor use the regular maintenance according to the manufacturer’s instructions is essential.

Thermally modified wood; wood finishing oils; accelerated weathering; general appearance of the surface; water permeability

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