Susceptibility of Thermo-Mechanically Modified Scots Pine (Pinus Sylvestris L.) Sapwood and Heartwood to Colour Change under the Influence of Ultraviolet Radiation

The impact of ultraviolet radiation on the colour of thermo-mechanically modified sapwood and heartwood of pine (Pinus sylvestris L.) was examined in the study. Samples of wood were densified in a hydraulic press at unit pressure of 80 MPa, at three levels of temperature of the press platens: 100 °C, 150 °C and 200 °C, for 360 s. The parameters of wood colour were determined using the mathematical colour space models CIE L*a*b* and L*C*h. As a result of the modification of pine sapwood at the temperature of 200 °C, the change in wood colour ΔE was twice as substantial as the change in wood colour after modifications at lower temperatures. The total colour difference for pine heartwood, irrespective of the modification temperature, was at least twice as substantial as the change in the colour of pine sapwood. The most substantial changes in sapwood and heartwood lightness occurred after 20 h exposure to UV radiation. Changes in lightness occurring within 20 h, irrespective of the temperature of treatment, accounted for about 30 - 60 % of the total change. After 300 h of irradiation, the value of ΔE for sapwood densified at temperatures of 100 °C, 150 °C and 200 °C was, respectively, 15 %, 27 % and 34 % higher than the value of ΔE for heartwood densified at the same temperatures.