Laszlo Persze 1 and Laszlo Tolvaj 2
MATERIALS AND METHODS
During the research we examined 2 hardwood and 2 softwood species, which were ash (Fraximus excelsior L.) and poplar (P.Xeuramericana Pannonia), Scots pine (Pinus silvestris L.) and spruce (Picea abies Karst.).
Two pieces of samples were cut 100x30x10 (mm) from the same wood board, and ten places were marked for the colour measurement on all of the samples.
Because we made 10 measures in each samples of species, we have got 20 results per species.
As mentioned two different air temperatures - 30°C and 80°C - were used in the irradiation chamber. To irradiate our samples we used a mercury vapour lamp which emits strong UV light. We have put the samples 64 cm from the lamp, and the total electric power what we used were 800 W. Total irradiation time was 200 hours. The exposures were interrupted after 8; 20;
40 and 90 hours because we need to measure the change of the colours. We also measured the colour change before and after the irradiation.
Measurements were carried out with a colorimeter (Konica-Minolta 2600d).
The L*, a*, b* colour co-ordinates were calculated based on the D65
illuminant and 10° standard observer with a test-window diameter of 8 mm.
A series of samples were treated in the same chamber set for 80°C but without light irradiation. This way we were able to observe that the effect of pure thermal degradation was determined.
RESULTS AND DISCUSSION
During the investigation we realised that in the first 20 hours of light irradiation the lightness decreased really fast. After this period, the lightness change was moderate and almost linear.
Figure 1: The lightness change of poplar and ash samples at 30°C and at 80°C temperatures
During the first quarter of the irradiation we concluded that the irradiation at 80°C caused slightly greater lightness decrease than at 30°C. The only exception was the poplar, which showed a considerably greater lightness decrease at the higher temperature.
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Fig. 2 represent the lightness change of pine and sruce samples caused by photo-irradiation at 30°C and at 80°C temperatures.
Figure 2: The lightness change of pine and spruce samples at 30°C and at 80°C temperatures
The greatest difference between the two types of irradiation was observed in red colour change, which we can see on the figure 3 and 4.
Figure 3: Red colour change in case of pine and spruce samples
The red colour change increased during the process continuously. At the very beginning of the investigation (in the first 8 hours) we found that the increase was more intensive for hardwood than for softwood.
We also saw clear that the light irradiation at 80°C produced considerably greater redness than the light irradiation at 30°C in all cases. The pure thermal treatment at 80°C in total darkness produced negligible redness increase, as it is presented in Figure 3 and figure 4.
Figure 4: Red colour change in case of ash and poplar samples
It is clear, that the redness increase of light irradiation at 80°C is not the superposition of the effect of the light irradiation at 30°C and the effect of thermal treatment at 80°C in total darkness. The elevated temperature amplifies the consequence of light irradiation. Pine samples showed 57%
higher redness change at 80°C than at 30°C during the 200 hours light exposure. The same percentages for spruce, ash and poplar are 33%, 40%
and 15%, respectively. The chromophoric groups in wood are located in the lignin, in the extractives and in their derivatives. Lignin derivatives play main role mostly in yellowing. In the case of poplar wood the low extractive content could be the reason for the low thermal effect during light irradiation. This result shows the importance of extractives in the redness change. Based on experiences the indoor wooden constructions change their colour towards brown over the years. Probably, the amplification effect at elevated temperature is a contributing factor to the photodegradation, that responsible for that kind of colour changes. The thermal effects are exponentially proportional to the temperature. That is why the redness change needs long time period at room temperature.
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Yellowing is the main colour alteration effected by the photodegradation.
Yellowing is produced mainly by the degradation of lignin. The change of the yellow colour co-ordinate is presented in figure 5 and 6.
Figure 5: Yellow colour change in case of poplar and ash samples
Figure 6: Yellow colour change in case of poplar and ash samples
We can also see on the figures, rapid yellowing happens during the first 20 hours of treatments; followed by a moderate but continuous yellowing.
Conifers presented a little greater yellowing at 80°C than at 30°C up to 90 hours treatment. In contrast, hardwood suffered greater yellowing at 30°C
than at 80°C (ash showed only in the second part of irradiation). It is also really interesting that the yellowing was not altered by the thermal effect.
After all, we have to assume that we need further investigations regarding to the chemical background of the yellowing.
CONCLUSIONS
In this study we found out that the temperature has a considerable effect on the photodegradation of the wood.
We were able to prove that the higher temperature (80°C) has greater effect on the redness increase that the lower (30°C) temperature. The most considerable changes were made in pine samples – this sample showed 57%
higher redness change at 80°C during the 200 hours than in 30°C –. Also the other samples showed higher redness changes on the higher temperature (spruce, ash and poplar were 33%, 40% and 15%,)
As we supposed the extractive content has a significant role in thermal discolouration during photodegradation.
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