Colour stability of oil-heat treated black locust and poplar wood during short-term UV radiation

• Wood extractives determined mainly the colour change.
• Thermal treatments reduced the lightness change effect of photodegradation.
• The redness of oil-heat treated black locust showed photostability.
• The thermal treatment was not able to protect the lignin of wood during UV radiation.

Black locust (Robinia pseudoacacia L) samples with high extractive content and poplar (Populus × euramericana cv. Pannónia) samples with low extractive content were chosen for the test. The specimens were thermally treated at 160 and 200 °C in sunflower oil for 2, 4, and 6 h then irradiated by a strong UV emitter mercury lamp up to 36 h. The effects of thermal treatment and UV radiation were monitored by colour measurement. The results indicated that the extractives play an important role in the photodegradation of oil-heat treated wood. Thermal treatments reduced the lightness change effect of photodegradation. The oil-heat treated black locust samples showed similar photodegradation properties independently on the thermal pre-treatment time and temperature. The redness of oil-heat treated black locust samples hardly changed during UV radiation proving the photostability of the thermally modified extractives. The redness change of poplar samples caused by UV radiation was partly determined by the temperature of thermal treatment (darkening and lightening also happened). The yellow colour change of the investigated samples showed that the lignin of thermally modified wood undergoes similar photodegradation as that of the untreated natural wood.