Colour stability of thermally modified black locust and poplar wood during short-term photodegradation

Thermal treatment of wood has a long history, and different methods have been continuously modified and developed not only in Europe but worldwide. Main advantages of thermal treatment of wood material include reduced hygroscopicity, improved dimensional stability, and better resistance to degradation due to insects and micro-organisms. Improved properties facilitate outdoor application of thermally modified wood. Despite the extensive researches, behaviour of thermally modified wood during UV light exposure is still not clearly understood. In this study, black locust (Robinia pseudoacacia L.) and Pannonia poplar (Populus x euramericana cv. Pannonia) wood samples were subjected to dry thermal treatment in a laboratory scale drying chamber at 160 degrees C and 200 degrees C in the presence of air. The effective treatment duration was 2, 4, and 6 hours. To induce fast photodegradation, heat treated samples along with untreated control samples were then irradiated with a strong UV light emitter mercury vapour lamp. The total irradiation time was 36 hours. To monitor the colour stability, colour measurements were carried out with a Konica-Minolta 2600d type colorimeter. Objective colour measurements revealed considerable colour change of the samples due to thermal treatments (Fig. 1). The first section of the lines (continuous lines) represents the effect of thermal treatment while the second part (dotted lines) shows the effect of UV treatment. The red colour increased for all species during thermal treatment, and the order of change followed the intensity of the treatment. The poplar proved to be highly sensitive to the temperature of the thermal treatment, but not sensitive to the treatment time. The yellow colour of poplar increased following the tendency of red colour change. The colour of black locust was less sensitive to the temperature than that of poplar. Its originally strongly yellowish colour decreased due to the thermal treatment following the same tendency as the lightness change produced. The lightness decreased for both species. The magnitude of the change depends on the intensity of the thermal treatment. The selected thermal treatment parameters resulted in a large variety of lightness.