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Electron beam irradiation of wood: an experimental parameter study.

ABSTRACT

Surface modification by electron beam irradiation is one of the most advanced industrial and scientific techniques for improving surface properties of a polymer. This modification process may beneficially influence various functional and structural properties of wood.

In this study, different wood species and process parameters (e.g. irradiation dosage) were used. The changes physical properties (e.g. discoloration and weathering behavior) were analyzed.

The results of colour measurement show different effects of the various wood species and irradiation dosages during an artificial weathering test. For specific applications, this wood surface modification process can contribute to improving the functional and structural properties of wood. These results provide a helpful basis for further fundamental research.

Key words: artificial weathering, color measurement, fir, larch, Norway spruce, wood modification

INTRODUCTION

The modification of wood and wood surface properties has been a long tradition and has focused on ion irradiations (e.g. plasma treatment) to develop new modification processes (Hill 2006, Militz 2012, Petric 2013). Surface modification is one of the most advanced scientific techniques that may lead to the goal of very precise applied surfaces and can enhance polymer properties on the surface. Polymer modification of materials can be described here creating beneficial changes of the properties, whereas two interesting technologies can be used. On the one hand a reactive, excited-state or partly ionizing gas (e.g. plasma treatment) alters the surfaces (Ellinghorst 2005, Sheton and Stevens 2001, Wolkenhauer et al. 2008). On the other hand electron beam irradiation (ionizing radiation) can be applied to modify polymers (Jarovic 1990). Also, the electron beam irradiation process may beneficially influence various functional and structural properties of wood.

Wood is susceptible to degradation by UV-light and in exterior application undergoes various changes in properties during its life time. The ageing mechanisms are very complex and are influenced by many factors (e.g. rain, solar radiation and temperature) (Evans et al. 1992, Feist and Hon 1983, Hon 2001). These studies show that chemical bonds break and the lignin content of wood decreases as a consequence of photo-degradation. New chromophores form as exposure time is increased (George et al. 2005, Muller et al. 2003, Pandey 2005, Tolvaj and Faix 1995). Furthermore, the surface color of wood varies rapidly when it is exposed to light (Hon and Minemura 2001). If the surface is wetted by water (e.g. rain) then the degradation products from wood components (e.g. lignin) can be leached out (Evans et al. 1992, Kishino and Nakano 2004).

To improve the resistance to weathering ion irradiation is one possible method to reach this objective; however, a systematic study of the effects of high energy electron beam radiation on wood surfaces to influence the weathering behaviour is not available.

Fengel and Wegener (2003) give a literature overview of the effects on wood materials of ionizing rays (gamma-rays) at high dose levels resulting in changes of the properties of wood in structural and chemical behaviour. High energy irradiation of cellulose causes depolymerisation, a reduction in crystallinity and extensive decomposition with increasing dose levels (Charlesby 1955, Saeman et al. 1952). Fischer et al. (1985) described a pre-processing of pulp with ionizing radiation for the production of regenerated cellulose. In dependence on the dose level (10 to 100 kGy) the degree of polymerization changes to lower values, simultaneously the number of functional groups increases in the cellulose. Also the side chains of xylan (hemicellulose) are subject to cleavages resulting from oxidative reactions (Seifert 1964). Irradiation of lignin (MWL) does not change the aromatic hydroxyl groups and conjugated carbonyls; however changes take place on the aliphatic hydroxyl groups and guajacyl residues, which were decreased (Fischer and Goldberg 1987). They have assumed that condensation reactions between the aromatic nucleus and the side chain occur during the irradiation.

This proposed condensation reaction may improve the stability of wood against weathering effects. Therefore, the objectives of the current study are the analysis of the effect of electron irradiation doses on weathering behaviour of larch, fir and Norway spruce wood samples.

2 MATERIAL AND METHODS

2.1 Wood species

In this study, three samples from fir, larch and Norway spruce wood were cut to dimension of 150 x 75 x 20 [mm.sup.3] according to EN 927-6 (2006). The untreated and treated samples were stored in a climatic chamber (20[degrees]C and 65 % relative humidity) until the weathering test.

2.2 Electron beam irradiation (EBI)

For the E-beam treatment at doses of 25, 50, 100 and 200 kGy an Electrocurtain[R] LAB Unit CB 175/15/10L electron accelerator was used with radiation energy of 150 keV. Five spruce samples were irradiated under nitrogen atmosphere for each EBI doses. Higher electron beam doses than 50 kGy were reached by repeating the process with 50 kGy. Moreover, three larch and three fir samples were only irradiated with 50 kGy.

2.3 Artificial weathering

The artificial weathering procedure was applied according to EN 927-6 (2006) with test cycles of water condensation (40 [degrees]C temperature), light irradiation with UVA-340 bulbs (0.89 W/[m.sup.2]/nm and 60 [degrees]C temperature) and water spray (6-7 litres/min) with an exposure period of 950 hours. For the determination of color and chemical changes before and after the weathering test the samples were always collected after a 2 hours light irradiation cycle and cooled down in an ambient climate room (temperature 20[degrees]C and relative humidity 65 %).

2.4 Colour Measurement

Wood colour was measured with a Mercury 2000 spectrophotometer (Datacolor) and the selected diameter for measurement was 11 mm. Colour is expressed according to the Commission International de l'Eclairage (CIE) L*a*b* colour space (abbreviation CIELAB) with a standard illuminant [D.sub.65] and a 10[degrees] standard observer. The L* parameter represents lightness where the values L vary from 0 (black) to 100 (white). The a* and b* parameters describe the chromatic coordinates on green-red (a*) and blue-yellow (b*) axes. The [DELTA]L*, [DELTA]a* and [DELTA]b* values of the colour differences from measurements were taken before, during and after artificial weathering. In this study, the positive values of [DELTA]a* and [DELTA]b* indicate an increased in intensity of red and yellow colour tone, while the negative values of [DELTA]a* and [DELTA]b* reflect a desaturation of red and yellow colour tone. The mean of three measurements per sample was used for colour analysis.

3 RESULTS AND DISCUSSIONS

The results of the color measurements before and after 24 hours of the electron beam irradiations of all used wood samples did not show a statistical significant difference. Therefore, it could be assumed that the irradiated has not visible effect of the wood color.

However, it is a fact that the wood color changes during the exterior applications, while varying effects of weather show different consequences on the wood surfaces (Anderson et al. 1991). Therefore, it is not surprising that the tendency of change in colour of the wood samples used in this study was similar to other investigations.

The [DELTA]L*, [DELTA]a* and [DELTA]b* values show differences between the unirradiated and electron beam irradiated Norway spruce samples (Figure 1). In particular the [DELTA]L* and [DELTA]b* values were varied after the first 320 and 530 hours. All irradiated samples show lower changes in lightness ([DELTA]L*), however, higher shifts in yellow color tone ([DELTA]b*). After 740 hours the influence of the electron beam irradiations became less important for the Norway spruce samples.

With regard to all three parameters L*, a* and b*, the color of the different sample groups became uniform to the end of the weathering test at 950 hours.

[FIGURE 1 OMITTED]

The [DELTA]L*, [DELTA]a* and [DELTA]b* values of the unirradiated and irradiated fir samples changes in equal tendency, however, the fir samples after the electron beam irradiation with 50 kGy show lower discoloration after the first 320 hours of the weathering test (Figure 2). Then the [DELTA]L* values of the irradiated samples changed without a trend, the values were stable between -11 and -8. Whereas the [DELTA]L* of the untreated fir samples decreased until to the 740 exposure hours. This behavior of a very slight increase up to the end of the artificial weathering test was also found by Temiz et al. (2005). They concluded that this phenomena may generated by removing and loosening fibers.

The [DELTA]a* and [DELTA]b* values of the EBI treated samples showed a faster decrease in intensity of the color. The red and yellow color tone faded. This behaviour is consistent with the investigation of discoloration of wood surfaces due to natural and artificial weathering test by Hon (2001), Kishino and Nakano (2004) and Schnabel et al. (2009).

[FIGURE 2 OMITTED]

The [DELTA]a* and [DELTA]b* values show the highest differences between unirradiated and electron beam irradiated samples (Figure 3), while the changes in [DELTA]L* values (lightness) do not differentiate significantly between those samples.

[FIGURE 3 OMITTED]

The [DELTA]a* values of untreated samples decreased after the first 320 hours considerable until the end of the weathering test, while the [DELTA]b* values of untreated samples decreased until the 950 hours of the weathering test, during this period the color intensity was faded.

The larch samples treated with electron beam irradiation using 50 kGy irradiation doses show a slighter increase in [DELTA]a* values during the first 530 hours compared to the untreated sample. Afterwards the decrease in [DELTA]a* can further be observed until the 950 hours of the weathering test. Moreover, the changes in the mean of [DELTA]b* values of the irradiated samples behave different compared to the [DELTA]a* values. The intensity did not change significantly in the first 530 hours. Then the [DELTA]b* values decreased until the end of the weathering test. Also the intensity of the color faded. However, with regard to all three parameters L*, a* and b*, the color of the different sample groups were different at the end of the weathering test.

The increase in the b* values is influenced on the photo-yellowing of the photo-degradation of phenolic products (Kishino and Nakano 2004, Muller et al. 2003, Pandey 2005), and the decrease in the b* values is estimated by the leaching effect of photo-degradation products (Kishino and Nakano 2004). Changes in wood colour attend with chemical changes in components due to photo-degradation (Kishino and Nakano 2004, Muller et al. 2003, Pandey 2005, Tolvaj and Faix 1995). Therefore, it could be assumed that different discoloration processes reflect varied photo-chemical reactions on the wood surfaces during the weathering test. The electron beam irradiation affects not only the color changes during the artificial weathering but also the photo-degradation of wood components (Schnabel etal. 2013, Schnabel and Huber 2014).

4 CONCLUSIONS

The unirradiated and irradiated samples showed different intensities in discolorations during the artificial weathering test. The electron beam irradiations influence differently the weathering behavior of the various wood species. This result may give some indication of condensation reactions of different wood components and improvement the colour stability of larch wood samples in the first 530 hours of the weathering test. For Norway spruce and fir wood samples the differences in discoloration between unirradiated and irradiated samples were smaller compared to the larch samples.

These results show that the electron beam processing modified the weathering behaviour of larch wood samples. Even the improvement of the colour stability may serve to extend the service life performance of wood products. However, further studies on the effect of low radiation dose on wood materials were helpful to prevent undesirable reactions of only wood component cleavage.

Hermann HUBER

Salzburg University of Applied Science, Department of Forest Products Technology & Timber Constructions Markt 136a, 5431 Kuchl, AUSTRIA

Thomas SCHNABEL

Salzburg University of Applied Science, Department of Forest Products Technology & Timber Constructions Markt 136a, 5431 Kuchl, AUSTRIA

hermann.huber@fh-salzburg.ac.at

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Author:Huber, Hermann; Schnabel, Thomas
Publication:Forest Products Journal
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2015
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