Applied Surface Science

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1 Applied Surface Science 254 (2008) Contents lists available at ScienceDirect Applied Surface Science journal homepage: Comparative study of the photo-discoloration of moso bamboo (Phyllostachys pubescens Mazel) and two wood species Xiaoqing Wang *, Haiqing Ren The Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing , China ARTICLE INFO ABSTRACT Article history: Received 25 February 2008 Received in revised form 21 April 2008 Accepted 7 May 2008 Available online 13 May 2008 PACS: Ty Ba Hp Keywords: Moso bamboo Photo-discoloration Photo-degradation Color changes FTIR Lignin Bamboo or bamboo products undergo surface degradation during outdoor exposure resulting in lower quality in service. In this study, the effect of UV vis light irradiation on changes in color and surface chemistry of moso bamboo (Phyllostachys pubescens Mazel) was investigated. For comparison purpose, two wood species (a soft and a hardwood) were also studied to present their differences in degradation performance. Color characterization was performed by measuring CIELab parameters (L*, a*, b* and DE*), and Fourier transform infrared (FTIR) spectroscopy was used to analyze the chemical changes induced by irradiation. The results showed that the surface color of bamboo changed rapidly during the irradiation process. Compared with the wood species, bamboo was less influenced by photo-irradiation. Chemical analysis indicated that irradiation altered the chemical structures of bamboo surfaces. Lignin was the most sensitive component to photo-degradation and the intensities of its characteristics bands decreased significantly during the irradiation process. This was accompanied by formation of new carbonyl groups at 1735 cm 1. The rate of lignin degradation and carbonyl formation in bamboo was relatively lower compared with the wood species. The color changes (DE*) was well correlated with lignin degradation and carbonyl formation regardless bamboo or the wood species. ß 2008 Elsevier B.V. All rights reserved. 1. Introduction Bamboo, a highly fibrous grass, is one of the most important forest resources growing abundantly in many tropical and subtropical countries. Its fast growing rate, high strength and stiffness, easy workability, and local availability make it an excellent material used for furniture, construction, and handicrafts [1]. Bamboo fascinates people by its vivid color and fine grain structure. However, when exposed to an outdoor environment, the surface of bamboo culms is susceptible to degradation, primarily resulting in disappearance of its attractive fresh color and gloss. The extent of degradation depends on a combination of weathering factors such as solar irradiation, moisture, and temperature in the outdoor environment. Among these factors, solar irradiation contributes most to the degradation process. The photo-discoloration process involves very complex physical and chemical reactions. There have been many studies attempted to clarify the mechanism of wood weathering [2 5]. It has been shown that the degradation process is triggered by the * Corresponding author. Tel.: ; fax: address: wxqlily@yahoo.com.cn (X. Wang). formation of free radicals by UV irradiation [6]. The wood constituent polymers show different capacity in absorbing UV light to form radicals. Lignin is extremely susceptible to UV irradiation, leading to formation of aromatic free radicals (phenoxy radicals). These free radicals further react with oxygen to produce carbonyl and carboxyl groups, resulting in photo-degradation of wood surface [7]. The wood discoloration is thought to be associated with the formation of colored unsaturated carbonyl compounds (quinones) due to the photochemical reactions of lignin. The photosensitivity of lignin is also demonstrated by microscopic studies of UV-irradiated wood surface, showing that cell corner and middle lamella were preferentially degraded due to their high-lignin content [8,9]. The presence of extractives may act as antioxidants exerting protective effects on photo-degradation of wood [10]. Despite many studies carried out several decades, the precise mechanism involved in the photo-degradation of wood is not well defined. The photo-degradation process largely depends on the surface properties of woody material considered. Bamboo is different from wood in many aspects. For example, the anatomical structure of bamboo is characterized by vascular bundles embedded in the parenchymatous ground tissue without radial tissue like the rays in wood, while wood consists of various types of functional cells /$ see front matter ß 2008 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 7030 X. Wang, H. Ren / Applied Surface Science 254 (2008) such as tracheids, fibers and parenchyma cells depending on species. These anatomical differences make bamboo and wood quite different in surface structure, roughness and density. The chemical components of bamboo, particularly polyoses and lignin are also quite different from those derived from wood [11]. It has been reported that the polyoses structures of bamboo are described as intermediate between those of softwood and hardwoods, and lignin of bamboo is characterized as a G-S-lignin with a high proportion of syringyl residues [11,12]. Besides, the minor constituents (starch, sugar, protein, fat, etc.) of bamboo may also influence the degradation process though they contribute only a few percent to the total bamboo mass. Although the fundamental anatomical, physical and mechanical properties of bamboo have been investigated extensively, relevant studies on photo-discoloration of bamboo are scarcely found in the literature. However, this information is of great importance for applying measures to protect bamboo products against weathering. The objective of the present study was to investigate the photodiscoloration and photo-degradation of moso bamboo (Phyllostachys pubescens Mazel), which is the most widely used bamboo species in China. For comparison purpose, a softwood species (Cunninghamia lanceolata) and a hardwood species (Populus tomentosa Carr.) were also studies to present their differences in degradation performance. The degradation was monitored by measuring color and chemical changes due to UV vis irradiation. Fourier transform infrared (FTIR) spectroscopy was used to characterize chemical and structural changes occurring in bamboo/wood components. Since the color changes of bamboo/ wood surfaces are induced by the photo-oxidation reactions of their chemical constituents, the relationship between color changes and changes in chemical composition during the degradation process was established to gain a better understanding of the mechanism of photo-discoloration in bamboo and wood species. 2. Materials and methods 2.1. Samples and irradiation treatment Five-year-old moso bamboo (Phyllostachys pubescens Mazel) culms were obtained from an experimental forest located in Zhejiang Province, China. Bamboo culms were cut into strips with dimensions of 100 mm (longitudinal) 20 mm (tangential) 3 mm (radial) to be used as specimens. The wood specimens from sapwood of C. lanceolata (softwood speices) and P. tomentosa Carr. (hardwood species) with the same dimensions were also prepared. The moisture content of all the specimens was kept in a range of 8 10%. The tangential surfaces of the specimens were exposed to artificial sunlight from a xenon lamp at temperature of 63 8C (black panel) and 50% relative humidity in a commercial chamber (X25F, Suga Test Instruments Co. Ltd., Japan). The irradiation was interrupted every 5, 10, 20, 40, 80, 120, and 160 h of treatment, respectively. After each exposure period, six irradiated specimens were removed from the chamber for further analysis Color measurements The surface color changes of specimens due to irradiation were measured by a portable chromatic aberration meter (CR-300, Minolta) fitted with a D 65 light source with a test-window diameter of 8 mm. The CIE L*, a* and b* color parameters (according to the International Commission on Illumination) of the specimens were obtained directly from the color meter and were used for color evaluation. L* represents the lightness ranging from 0 (black) to 100 (white), and a* and b* are the chromaticity parameters (+a* for red, a* for green, +b* for yellow, and b* for blue). The overall color change (DE*) of the specimens before and after irradiation was calculated according to the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DE ¼ ðdl Þ 2 þðda Þ 2 þðdb Þ 2 (1) where DL*, Da* and Db* represent the changes in L*, a* and b* of specimens after irradiation, respectively. A lower DE* means lower color change. For each treatment, six replicates were used and the average value was calculated for color analysis Measurement of FTIR spectra The FTIR spectra were measured by direct transmittance method using a Nicolet Impact 410 spectrometer. The powder samples were obtained by removing the top layer of the irradiated surface using a sharp razor blade, and then were mixed with KBr in a weight ratio of about 1:100 to form a pellet. Spectra were collected from 4000 to 400 cm 1 and 64 scans were recorded for each sample at a resolution of 8 cm 1. Peak heights of absorption bands were measured using OMNIC software (Version 7.3, Nicolet Instruments Corporation, USA) according to the previously published methods [13]. 3. Results and discussion 3.1. Color changes due to irradiation Fig. 1 shows the color changes (L*, a*, b* and DE*) due to photoirradiation on surfaces of moso bamboo and two wood species C. lanceolata and P. tomentosa. Both bamboo and wood species were susceptible to photo-irradiation and the surface color changed obviously from light to dark as indicated by a decrease in lightness (L*) with exposure time. Besides, the irradiated surfaces showed a tendency to turn reddish and yellowish by increases in chromaticity parameters a* and b*. The two wood species displayed a similar pattern of discoloration. The surface color changed rapidly at short exposure time up to about 40 h, above which the rate of change became relatively small. It also can be observed clearly that the overall color change (DE*) values in C. lanceolata were consistently higher than those in P. tomentosa throughout the entire irradiation process, indicating that the softwood was more susceptible to photo-discoloration. By contrast, the overall color change (DE*) of bamboo surfaces increased significantly at the initial stage of irradiation and attained almost a constant value up to 20 h, above which the DE* value progressed steadily. After 160 h of irradiation, the DE* value arrived at 15.85, which is significantly lower than the corresponding values of C. lanceolata (22.84) and P. tomentosa (21.20), indicating that the bamboo surfaces were less influenced by photo-irradiation than the wood species. Color changes in bamboo and wood surfaces reflect chemical changes of certain components during the process of irradiation. The different discoloration behavior of bamboo and wood also mirrors their differences in photochemical reactions occurring during photodegradation FTIR analysis Untreated bamboo and wood species (controls) FTIR spectroscopy is a very useful technique for analyzing the structure of wood components and the chemical changes in wood induced by light irradiation. FTIR spectra of untreated moso bamboo and two wood species showed the same basic structure

3 X. Wang, H. Ren / Applied Surface Science 254 (2008) Fig. 1. Color changes (L*, a*, b* and DE*) as a function of irradiation time for moso bamboo (Phyllostachys pubescens Mazel) (*) and two wood species (Cunninghamia lanceolata) (*) and (Populus tomentosa) (&). (Fig. 2). There was a strong broad O H stretching absorption around 3425 cm 1 and a prominent C H stretching absorption around 2931 cm 1. Since these bands have contributions from both carbohydrates and lignin, they present limited use in analyzing the chemical changes of bamboo/wood due to irradiation. In the fingerprint region between 800 and 1800 cm 1, there were many well-defined peaks providing abundant information on various functional groups present in bamboo/wood constituents (Table 1). For example, the non-conjugated C O stretch (in hemicellulose) was observed at 1735 cm 1, and the aromatic skeletal vibration in lignin appeared at 1508/1512 and 1596/ 1604 cm 1. The absorption peaks at 1462, 1238/1242, and 1269 cm 1 had significant contributions from lignin, while the peaks around 1157, 1057, and 898 cm 1 mainly arose from carbohydrates. The relative higher intensities of peaks at 1512 and 1269 cm 1 in C. lanceolata indicated a higher lignin content in the softwood, while the stronger absorption at 1735, 1330, and 1238 cm 1 in P. tomentosa reflected a higher holocellulose (cellulose and hemicellulose) content present in the hardwood. The spectrum of moso bamboo showed similar patterns with that of the hardwood. There was also a stronger carbonyl band at 1735 cm 1, indicating a relatively high-xylan content present in bamboo. It has been reported that over 90% of the bamboo hemicelluloses consist of a xylan with a unique structure characterized as an intermediate between hardwood and softwood xylans [14,15]. The intensity of absorption peak at 1512 cm 1 in bamboo was comparable to that of 1508 cm 1 in P. tomentosa, indicating a similar lignin content present in them. However, the lignin structure of bamboo is different from that of wood. Bamboo presents a typical Gramineae lignin composed of three phenylpropane units, i.e. p-coumaryl, coniferyl and sinapyl alcohols, with a high proportion of syringyl residues [11,16]. The strong absorption peaks at 1242 and 1330 cm 1 demonstrated the presence of high amount of syringyl units in bamboo lignin, which was significantly different from that Table 1 Assignment of absorption IR spectra peaks in moso bamboo (Phyllostachys pubescens Mazel) and two wood species (Cunninghamia lanceolata and Populus tomentosa) Wave number (cm 1 ) Assignments and remarks Fig. 2. FTIR spectra of untreated moso bamboo (Phyllostachys pubescens Mazel) and two wood species (C. lanceolata and P. tomentosa) (1) O H stretching 2931 (2) C H stretching 1735 (3) Non-conjugated C O in hemicellulose (xylans) 1639/1654 (4) Conjugated C O in lignin 1596/1604 (5) Aromatic skeletal vibration in lignin 1508/1512 (6) Aromatic skeletal vibration in lignin 1462 (7) C H deformation in lignin and carbohydrates (8) C H deformation in lignin and carbohydrates 1330 (9) C H vibration in cellulose; C O vibration in syringyl derivatives 1238/1242 (10) Syringyl ring and C O stretch in lignin and xylan 1269 (11) Guaiacyl ring breathing with C O stretch 1227 (12) Syringyl ring and C O stretch in lignin and xylan 1157 (13) C O C vibration in cellulose and hemicellulose (14) C O stretch in cellulose and hemicellulose 898 (15) C H deformation in cellulose Numbers in parentheses correspond to those in Fig. 2.

4 7032 X. Wang, H. Ren / Applied Surface Science 254 (2008) of C. lanceolata characterized by high quantity of guaiacyl units indicated by strong absorption at 1269 cm Photo-degradation of bamboo and wood species The FTIR spectra of photo-irradiated moso bamboo and wood species for different exposure time are show in Fig. 3. Significant changes can be observed in the fingerprint region of the IR spectra in all species. In the case of moso bamboo, the intensities of absorption bands related to lignin at 1604, 1512, 1462, 1330, and 1242 cm 1 decreased rapidly with exposure time, with those at 1604 and 1330 cm 1 almost disappeared after 160 h of irradiation. Fig. 3. FTIR spectra of moso bamboo (Phyllostachys pubescens Mazel) (a) and two wood species C. lanceolata (b) and P. tomentosa (c) after various periods of photoirradiation: (a) 0 h, (b) 5 h, (c) 10 h, (d) 20 h, (e) 40 h, (f) 80 h, (g) 120 h, and (h) 160 h. Since the absorption peak at 1512 cm 1 arising from aromatic skeletal vibration of benzene ring is characteristic for lignin, the decrease in the intensity of this peak indicated degradation of lignin during the irradiation process. Lignin is susceptible to photodegradation, and depolymerization of lignin in wood surface due to photo-irradiation has been extensively demonstrated [4,6,17]. In addition, the lignin degradation was accompanied by a significant increase in the intensity of band at 1735 cm 1, indicating formation of new non-conjugated carbonyl compounds during photo-degradation. This also reflected that photo-oxidation occurred in bamboo surfaces. Bamboo consists of about 50 70% holocellulose, 30% pentosans and 20 25% lignin with some minor constituents [14]. Similar to wood, bamboo is also a good light absorber and is susceptible to photo-oxidation to produce carbonyl groups, which is especially true for the lignin component of bamboo due to its photosensitivity nature. Previous studies have clearly shown that both lignin and cellulose components can participate in the photochemical reactions in wood surface exposed to UV irradiation, and the resulting increase in the carbonyl absorption is attributable to the oxidation of cellulose and lignin [3,6]. By contrast, intensities of peaks at 1157, 1049, and 898 cm 1 associated with carbohydrates underwent no significant changes with exposure time. However, the intensities of absorption bands at 1380 cm 1 increased significantly after 160 h of irradiation. This may be attributed to an increase in the content of carbohydrate components relative to lignin during the degradation process. In the cases of C. lanceolata and P. tomentosa, similar changes in IR spectra of irradiated wood surfaces can be observed. For the softwood, the intensities of lignin associated bands at 1512, 1462, 1269, and 1227 cm 1 decreased rapidly during photo-degradation, whereas significant decrease in absorption of lignin related peaks in the hardwood was observed at 1508, 1462, 1330, and 1238 cm 1. This variation arises from different lignin structures present in soft and hardwoods. It has been reported that the guaiacyl type (softwood lignin) absorbs around 1270 and 1230 cm 1 and the syringyl type (the major type of hardwood lignin) absorbs only around 1230 cm 1 [18]. This is the case for the present study, where the absorption bands at 1269 and 1227 cm 1 indicate the presence of guaiacyl unit in the softwood lignin and the presence of syringyl unit in the hardwood lignin is evident from the absorption at 1238 and Moreover, significant increase in the intensity of peak at 1735 cm 1 was also observed for both wood species, indicating the photo-oxidation of wood surfaces. The absorption peaks at 1157, 1057, 1030, and 898 cm 1 were not significantly influence by irradiation. Although bamboo and the wood species show similar pattern of photo-degradation in general, there may be variation in the extent of degradation owing to their different chemical structures present. In order to compare the rate of lignin decay and carbonyl groups formation during the irradiation process, the ratios of intensities of lignin characteristic peak at 1512/1508 cm 1 and carbonyl peak at 1735 cm 1 against carbohydrate reference peak at 898 cm 1 was calculated respectively, since the 898 cm 1 band is not significantly influenced by irradiation [6,7]. The relative changes in the ratios of lignin/carbohydrate (I 1512(1508) /I 898 ) and carbonyl/carbohydrate (I 1735 /I 898 ) at different exposure time for moso bamboo and the wood species are compared in Fig. 4. It can be observed that the lignin/carbohydrate ratio decreased with increasing exposure time in all the cases. The rate of lignin degradation was very high at the initial stage for the softwood C. lanceolata, while moso bamboo and P. tomentosa presented a steady decrease in lignin decay during the exposure periods. After 80 h of irradiation the relative intensity of lignin peak in C. lanceolata decreased to 42% of its initial value, followed in turn by

5 X. Wang, H. Ren / Applied Surface Science 254 (2008) Fig. 4. Decay of lignin reference peak at 1512 (1508) cm 1 (a) and formation of carbonyl groups at 1735 cm 1 (b) during photo-degradation of moso bamboo (Phyllostachys pubescens Mazel) (*) and two wood species (C. lanceolata) (*) and (P. tomentosa) (&). Fig. 5. Correlation between color changes (DE*) and decay of lignin reference peak at 1512 (1508) cm 1 (a), and formation of carbonyl groups at 1735 cm 1 (b) during photo-degradation of moso bamboo (Phyllostachys pubescens Mazel) (*) and two wood species (C. lanceolata) (*) and (P. tomentosa) (&). P. tomentosa for 50% and moso bamboo for 70%. The lignin decay was higher in the softwood compared with the hardwood and bamboo. This can be attributed to the higher lignin content in C. lanceolata (about 33%) [19] than those in P. tomentosa (about 21%) [20] and moso bamboo (about 27%) [21]. Compared with the wood species, bamboo was less influenced by lignin degradation. This may be linked to the higher amount of syringyl units present in bamboo lignin, since it has been demonstrated that the syringyl type of lignin degrades slower than the guaiacyl type of lignin [22]. In addition, the high-density of bamboo may provide more resistance against photo-irradiation, resulting in less degradation of bamboo surface [9]. During the process of lignin degradation, the concentration of carbonyl groups increased rapidly as shown in Fig. 4(b). In all the cases, the rate of increase is very high at the initial stage of irradiation. After 80 h of irradiation the relative intensity of the carbonyl peak (I 1735 /I 898 ) increased rapidly to nearly 3 times of its initial value in C. lanceolata, followed in turn by P. tomentosa for 1.7 times and moso bamboo for 1.6 times. This trend of carbonyl formation corresponded well with that of lignin degradation for the three species, indicating a close relationship between them. Following that, the rate of change in carbonyl intensity became small at longer exposure time. Comparing the rate of carbonyl formation and lignin decay clearly showed that the former is remarkably higher than the latter, indicating the formation of carbonyl bands at 1735 cm 1 probably resulted from not only lignin oxidation but also from other different reactions. The photooxidation of wood carbohydrates (cellulose and hemicellulose) has been demonstrated by many investigators [2,3,6] Correlation of discoloration with chemical changes The discoloration of bamboo and wood surfaces reflects chemical changes occurring during photo-degradation, resulting in formation of some colored substances. It has been reported that the carbonyl based chromophoric compounds (quinones) derived from photo-oxidation of lignin account primarily for the color changes of irradiated wood surfaces [5,23]. Therefore, we correlated the color changes (DE*) with lignin degradation and carbonyl formation during the irradiation process as shown in Fig. 5. For bamboo and the wood species, a negative linear correlation was observed between lignin degradation and DE* values with relative high-correlation coefficients. Besides, the formation of carbonyl groups was also closely correlated with the DE* values. This suggests that the color changes are related to lignin degradation and resulting carbonyl formation during the irradiation process, regardless of bamboo or wood. Similar results have also been observed by other investigators in study of the photo-degradation of wood species [4,6]. 4. Conclusions This study dealt with the photo-discoloration behavior of moso bamboo and two wood species as a result of UV vis light irradiation. Similar to the wood species, bamboo was susceptible to degradation resulting in rapid color changes during the exposure periods. However, bamboo surfaces were less influenced by photo-irradiation than the wood species. FTIR spectroscopy results showed that irradiation altered the chemical structures of bamboo surfaces.

6 7034 X. Wang, H. Ren / Applied Surface Science 254 (2008) Lignin was the most sensitive component to photo-degradation and the intensities of its characteristics bands decreased significantly during the process of irradiation. This was accompanied by formation of new carbonyl groups at 1735 cm 1. Compared with the wood species, the rate of lignin degradation and carbonyl formation was relatively lower in bamboo. In other words, bamboo was less influenced by chemical changes induced by irradiation, which could explain the lower color changes on its surfaces. Acknowledgements This study is funded by the sub-project of National Science & Technology Support Program of China (no. 2006BAD19B0403). The authors would like to acknowledge Mr. H.P. Yan for his suggestions, encouragements and supports during the FTIR experiments. The authors also express sincere gratitude to Mr. C.S. Li for helps in carrying out experiments. References [1] K.T. Lu, J. Wood Sci. 52 (2006) [2] D.N.S. Hon, J. Appl. Polym. Sci. 37 (1983) [3] D.N.S. Hon, S.T. Chang, J. Polym. Sci. Polym. Chem. 22 (1984) [4] K.K. Pandey, Polym. Degrad. Stabil. 90 (2005) [5] B. George, E. Suttie, A. Merlin, X. Deglise, Polym. Degrad. Stabil. 88 (2005) [6] U. Müller, M. Rätzsch, M. Schwanninger, M. Steiner, H. Zöbl, J. Photochem. Photobiol. B: Biol. 69 (2003) [7] K.K. Pandey, Polym. Degrad. Stabil. 87 (2005) [8] M.L. Kuo, N.H. Hu, Holzforschung 45 (5) (1991) [9] R.M. Rowell, Handbook of Wood Chemistry and Wood Composites, CRC Press, [10] P. Nzokou, D.P. Kamdem, Color Res. Appl. 31 (5) (2006) [11] D. Fengel, X. Shao, Wood Sci. Technol. 19 (1985) [12] M.D. Li, China Pulp Pap. (1990) (in Chinese). [13] K.K. Pandey, A.J. Pitman, Int. Biodeter. Biodegrad. 52 (2003) [14] W. Liese, Wood Sci. Technol. 21 (1987) [15] Z.H. Jiang, Bamboo and Rattan in the World, Liaoning Science and Technology Publishing House, China, [16] T. Higuchi, Wood Res. 48 (1969) [17] A. Temiz, N. Terziev, M. Eikenes, J. Hafren, Appl. Surf. Sci. 253 (2007) [18] K.K. Pandey, J. Appl. Polym. Sci. 71 (1999) [19] F.C. Bao, Z.H. Jiang, X.M. Jiang, X.X. Lu, X.Q. Luo, S.Y. Zhang, Wood Sci. Technol. 35 (2001) [20] D.M. Zhang, F.C. Bao, Z.Y. Zhang, R.F. Huang, Sci. Sil. Sin. 41 (4) (2005) (in Chinese). [21] Q.S. Zhang, M.J. Guan, W.L. Ji, J. Nanjing For. Univ. 26 (2) (2002) 7 10 (in Chinese). [22] H.T. Chang, T.F. Yeh, S.T. Chang, Polym. Degrad. Stabil. 77 (2002) [23] E.L. Anderson, Z. Pawlak, N.L. Owen, W.C. Feist, Appl. Spectrosc. 45 (4) (1991)