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

Abstract

A B S T R A C TBlack 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 oilheat 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.

Thermal treatment of wood using vegetable oils: A review

“…Bak et al [70] treated pannonia poplar in sunflower oil and found that the wood treated at the highest temperature and longest duration underwent the least total colour changes after 1-year exposure. Treated in sunflower oil, black locust and poplar wood exposed to short-term UV radiation exhibited better photostability compared to untreated samples [62]. Apart from a more stable colour, oil heat treatment was also found to be able to prevent cracking, warping or twisting of the treated wood [69].…”

“…The colour of natural wood is decided by the chromophores that exist in the lignin and extractives. Degradation of hemicelluloses during heat treatment produced some additional chromophores to the wood and lead to changes in colour [62]. Darkening of wood after heat treatment is a common observation where the extent of darkening is a function of increasing temperature.…”

“…Treated using bi-oleothermal process (160°C/0.5 h followed by room temperature/0.5 h), the wood turned almost black with formation of char clearly visible on the surface of wood [67]. Nemeth et al [62] reported that the extent of darkening is highly dependent on the extractive content of the wood. The effect of addition darkening effect due to oxidation process is more pronounced in the wood with higher extractive content.…”

Effects of heat treatment systems on the physical properties of coated Scots pine (Pinus sylvestris L.) and poplar (Populus euramericana)

“…While lower temperatures (below 140 °C) cause no major changes in the properties of wood, at higher temperatures the structure of the wood is severely degraded, which causes changes to the physical and chemical properties of the wood (Yildiz et al 2006;Candelier et al 2013;Yang et al 2015). An important physical property that changes at high temperatures is the color of the wood (Nemeth et al 2016). The lignins and the chromophores contained in the extractives are responsible for the color of wood.…”

“…The lignins and the chromophores contained in the extractives are responsible for the color of wood. The degradation of the hemicelluloses found in the wood during the heat treatment leads to the formation of new chromophores, which as a consequence, causes the color of the wood to change (Kučerová et al 2016;Nemeth et al 2016). The darkening of the wood surface increases with the increase in the temperature of the heat treatment.…”