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The effects of thermal modification on crystalline structure of cellulose in soft and hardwood
ARTICLE IN PRESS
Building and Environment 42 (2007) 62–67 www.elsevier.com/locate/buildenv
The effects of thermal modification on crystalline structure of cellulose in soft and hardwood Sibel Yildiz, Esat Gu¨mu¨s-kaya The Department of Forest Industry Engineering, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey Received 14 April 2005; received in revised form 1 July 2005; accepted 11 July 2005
Abstract In this study the crystallinity and estimations of relative triclinic (Ia) and monoclinic (Ib) structure content of cellulose isolated from heated spruce (Picea orientalis) and beech (Fagus orientalis) wood samples were determined by using Fourier Transform Infrared (FT-IR) spectrometry. Heat treatment was applied on the test samples in an oven at three different temperatures (150, 180 and 200 1C) and two different durations (6 and 10 h) under atmospheric pressure. It was determined that crystallinity of cellulose in wood samples increased with thermal modification. The results indicate that the changes in crystallinity of cellulose in wood samples related to not only temperature but also time during thermal modification. I a =I b ratio of cellulose in spruce and beech wood samples changed with thermal modification, but it was established that monoclinic structure was dominant in cellulose crystalline structure. It was designated that the crystalline structure of cellulose in spruce wood samples affected from thermal modification more than in beech wood samples. r 2005 Elsevier Ltd. All rights reserved. Keywords: Heat treatment; Fagus orientalis; Picea orientalis; Crystallinity; Cellulose
1. Introduction Wood, as a biopolymer composite, has been known and recognized as construction material for a long time. Advantages of this material are appreciated very widely. However, considerable possibilities still remain associated with the improvement of wood natural quality characteristics such as dimensional stability or resistance to bio-corrosion, which might prolong the service life of wood products. In order to achieve these objectives, wood modification was developed as a method of equipping this material with new properties [1]. Heat treatment is one of the processes used to modify the properties of wood. Heat-treated wood has been considered as an ecological alternative to impregnated Corresponding author. Tel.: +90 462 377 28 62, +90 462 377 34 99; fax: +90 462 325 74 99. E-mail addresses: [email protected] (S. Yildiz), [email protected] (E. Gu¨mu¨s-kaya).
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.07.009
wood materials. Recent efforts on thermal treatment of wood lead to the development of several processes introduced to the European market during the last few years [2]. Nowadays, work on the heat treatment process of timber to prepare what is currently called torrefied wood in France; plato wood in the Netherlands and retified wood in the USA has shown that such types of processes can improve the performance of timber in several aspects [3]. The chemical modifications in wood structure occurring at high temperature are accompanied by several favorable changes in physical structure: reduced shrinkage and swelling, low equilibrium moisture content, enhanced weather resistance and decorative, dark color, better decay resistance. Thermally modified wood finds use in outdoor furniture, claddings on wooden buildings, floor material, musical instruments and a diversity of other outdoor and indoor applications. Unfortunately, the mechanical properties, e.g. strength, hardness and stiffness are reduced at the same time [4].
ARTICLE IN PRESS S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
The decrease in strength depends on the method of the thermal modification, the wood species and its characteristic properties, the initial moisture content of wood, the surrounding atmosphere, treatment time and temperature [5]. All the European heat processes have in common the treatment of sawn wood at elevated temperatures in the range between 160 and 260 1C [2]. The maximum temperature during the heat treatment is varied from 180 to 280 1C and from 15 min to 24 h depending on the process, wood species, sample size, moisture content of the wood, the desirable mechanical properties, resistance to biological attack and the dimensional stability of the final product. The presence of air or oxidant during the heat treatment may accelerate the degradation of wood components. Inert or reducing atmosphere is reported to facilitate the heat treatment. A nitrogen atmosphere is normally used. The rate of thermal degradation of wood hemicelluloses in air is greater than in an inert environment. The chemical degradation of wood occurs in the order of hemicellulose, cellulose and lignin. A limited decomposition of lignin is observed at as low as 220 1C with the presence of phenolic substances such as vanillin, coniferaldehyde and syringyl aldehyde [6,7]. Wood is a complex composite material, which consists mainly cellulose, hemicellulose and lignin. Cellulose represents the crystalline parts of wood, while the structure of hemicellulose and lignin are amorphous. The main mechanical function of hemicelluloses and lignin is to buttress the cellulose fibrils [4]. The strength properties of cellulosic materials are intrinsically related to the interactions between the cellulose molecules, and an understanding of the molecular structure of cellulose is therefore extremely important [8]. The fibre texture and the complex chemical composition of wood complicate the crystallinity determination. Furthermore, the separation of amorphous background from the diffraction pattern of cellulose crystallites is difficult because the cellulose crystallinity of wood cannot be determined accurately [9]. Most of the studies have aimed at determining the relative crystallinity of wood [10–15]. The aim of this study was to investigate the crystallinity and estimations of relative monoclinic (I b ) and triclinic structure (I a ) content of cellulose isolated from heated spruce (Picea orientalis) and beech (Fagus orientalis) wood samples grown in Turkey. It was investigated relationship between chemical, crystalline structure and mechanical properties of spruce and beech woods.
2. Materials and method 2.1. Thermal modification Raw material beech and spruce woods were obtained from Black Sea Region in Turkey. The woods were cut
63
in parallel to grain directions and sawn into specimens measuring 2 2 3 (tangential radial longitudinal) cm long. Conditioning to 12% moisture content was performed at 21 1C and 65% relative humidity. Heat treatment was then applied on the test samples in an oven controlling the temperature 71 1C sensitivity, at three different temperatures (150, 180 and 200 1C) and two different durations (6 and 10 h) under atmospheric pressure and in the presence of air. Before the chemical analyses heat-treated and untreated (control) wood samples were ground in a Wiley mill to a homogeneous meal. Cellulose analyses were made according to Kurschner-Hoffner’s nitric acid method [16]. 2.2. FT-IR spectroscopy The cellulose powders obtained from spruce and beech wood samples treated thermal modification were used for Fourier Transform Infrared (FT-IR) spectroscopy measurement. The dried samples were embedded in potassium bromide (KBr) pellets, and were analyzed by using a Perkin Elmer Spectrum One FT-IR spectroscopy model 2000. They were recorded in the absorption mode in the range 4000–400 cm 1 with an accumulation of 32 scans, resolution of 2 cm 1 and normalized C–O–C band at 2900 cm 1.
3. Results and discussion In this study, we investigated how the crystalline structure of cellulose in spruce and beech wood samples changed with thermal modification at 150, 180 and 200 1C. For this reason, cellulose was isolated from spruce and beech wood samples treated by thermal modification. We used different methods for determination of crystallinity of cellulose in wood samples. The first approach for determination of crystallinity of cellulose in wood samples was the ratio of the peaks areas at 1370 and 670 cm 1 (A1370 =A670 ) proposed by Richter 1991 [17] in FT-IR spectra of wood samples. This method actually uses the ratio of the combined areas of the three peaks at 1370, 1335 and 1315 cm 1, which have called the 1370 cm 1 peak (CH bending), to that of the peak at 670 cm 1 (C–OH out of plane bending mode). Spectra were recorded in absorbance modes using KBr discs containing a constant amount of the samples [18]. According to this method, crystallinity of cellulose in spruce wood samples increased with thermal modification in 6 h at all temperatures. As can be seen in Table 1 and 2, when crystallinity of cellulose in spruce wood samples increased at 150 and 200 1C in 10 h, it was felled at 180 1C. Crystallinity of cellulose in beech wood samples was mounted at 180 1C in 6 h and decreased at 200 1C in 10 h.
ARTICLE IN PRESS S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
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Table 1 Crystallinity of cellulose and estimations of relative content I a =I b in heated spruce wood Sample
Cellulose in spruce
Temperatures (1C)
Control 150 180 200
a
Time (h)
— 6 10 6 10 6 10
Crystallinity A1370 =A670
H 1429 =H 897
H 1374 =H 2900
6.9 8.9 8.6 8.8 7.5 8.4 8.5
0.67 0.85 0.76 0.71 0.65 0.89 0.80
0.56 0.84 0.64 0.86 0.64 0.75 0.71
I a a (%)
I b b (%)
39 46 46 42 47 50 47
61 54 54 58 53 50 53
I a a (%)
I b b (%)
37 44 42 48 45 47 46
63 56 58 52 55 53 54
One-chain triclinic unit cell. Two-chain monoclinic unit cell.
b
Table 2 Crystallinity of cellulose and estimations of relative content I a =I b in heated beech wood Sample
Cellulose in beech
Temperatures (1C)
Control 150 180 200
a
Time (h)
— 6 10 6 10 6 10
Crystallinity A1370 =A670
H 1429 =H 897
H 1372 =H 2900
7.2 7.3 7.2 8.4 7.5 7.8 6.7
0.78 0.91 0.97 0.81 0.83 0.94 0.91
0.45 0.63 0.58 0.51 0.55 0.73 0.64
One-chain triclinic unit cell. Two-chain monoclinic unit cell.
b
Heating of wood modifies the cell wall components. Several modifications are reported to result from high temperature and steam. As a result of these modifications; it was concluded that hemicelluloses and less ordered cellulose deteriorate and as a consequence, the degree of cellulose crystallinity increases [4]. In a previous study, it was observed an increase of crystallinity in thermally treated cellulose up to temperatures of 120–160 1C followed by a decrease [19]. According to the researchers; crystalline structure of cellulose is not changed [20,21] or even can improve up to a certain temperature, which may be as high as 200 1C depending on the conditions involved. The crystallinity of the alkali-resistant cellulose from thermally treated spruce wood increased up to a temperature of 200 1C because of a preferred degradation of the less ordered molecules. Several authors have noted that cellulose undergoes changes in crystallinity upon chemical and physical treatments, particularly heating. It was reported that cotton linters underwent moderate increases in crystallinity upon mere exposure to humid atmospheres. It was demonstrated that suspension of cellulose in water and subsequent drying resulted in variable increases in
relative crystallinity that were more dramatic at lower initial relative crystallinity [22]. Crystallinity of cellulose and estimations of relative Ia/Ib content in heated spruce and beech wood were shown in Tables 1 and 2, respectively. FT-IR spectra were in Fig. 1 given for spruce wood samples and in Fig. 2 for beech wood samples. The ratio of peak heights at 1429 and 897 cm 1 (H 1429 =H 897 ), and at 1372 and 2900 cm 1 (H 1372 =H 2900 ) in FT-IR spectra of wood samples were used for determination of crystallinity of cellulose in wood samples [23]. In this study, The H 1429 =H 897 ratio for cellulose in spruce wood samples ranged from 0.67 to 0.89, whereas the same IR ratio for cellulose in beech wood samples varied from 0.78 to 0.97. According to these results, it was concluded that crystalline structure of cellulose was affected from thermal modification in beech wood samples more than in spruce wood samples. In the literature [24,25], it is stated that the value of index proposed as crystallinity varied from 2.80 for untreated cotton down to 0.11 for ball-milled cellulose. The H 1429 =H 897 and H 1372 =H 2900 ratios for cellulose in spruce increased with thermal modification according to control samples. It was determined that H 1429 =H 897 and
ARTICLE IN PRESS 65
1430 1280 Absorbance
Absorbance
S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
1280 1635 1430
1740
1635
1740
Control
Control
6h
6h 150°C
150°C
10h 10h 6h
6h
180°C
180°C 10h
10h 6h
6h
200°C 10h 200°C
10h
Wavelength (cm–1)
Wavelength (cm–1)
Fig. 1. FT-IR spectra of spruce wood samples.
Fig. 2. FT-IR spectra of beech wood samples.
H 1372 =H 2900 ratios for cellulose in spruce wood samples increased at 6 h more than at 10 h during thermal modification at all temperatures, in this study. As can be seen in Table 2, the H 1429 =H 897 and H 1372 =H 2900 ratios for beech wood samples were influenced from thermal modification at 150 and 200 1C more than at 180 1C. In course of cellulose isolation from spruce and beech wood samples, less ordered and amorphous parts formed during thermal modification remove. For this reason, it is possible to say that crystallinity of cellulose in wood samples increased with thermal degradation, relatively. In a previous study, it was designated no change of X-ray diffraction up to 220 1C during the heating of pinewood. Beyond this temperature the supramolecular structure was destroyed and completely amorphous state was reached about 270 1C. A similar result was obtained with sulfite pulp from slash pine. At 240 1C the crystalline structure of cellulose obviously breaks down as the degree of polymerization (DP) decreases within 2 h below 200 1C and within 8 h below 100 1C [19].
In this study, it was determined that monoclinic structure was dominant in beech and spruce wood samples. I a =I b ratio established 39/61 for spruce and 37/63 for beech wood samples, respectively, before thermal modification. It was designated that monoclinic structure ratio of cellulose in wood samples decreased, when triclinic structure ratio of cellulose in wood samples increased with thermal modification, as can be seen in Tables 1 and 2. As can be seen in Fig. 1 (FT-IR spectra for spruce wood samples), peak shoulder at 1635 cm 1 (adsorbed water) for spruce wood samples decreased by rising thermal modification temperature and time. Band at 1635 cm 1 came back to same level band at 1740 cm 1 (C QO valance vibration of COOH) at 200 1C. It was determined that bands at 1280 cm 1 (CH deformation) and 1430 cm 1 (CH2 scissoring) decreased with thermal modification. In Fig. 2 (FT-IR spectra for beech wood sample), it can be seen that band at 1635 cm 1 (adsorbed water) for beech wood samples came back to same level at band
ARTICLE IN PRESS 66
S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
1740 cm 1 (C QO valance vibration of COOH) at 200 1C within 6 h, but it moved down to band 1740 cm 1. Bands at 1240 cm 1 (–OH plane deformation of COOH) and 1430 cm 1 (CH2 scissoring) for beech wood samples were decreased by thermal modification. Yıldız [26] found that the elasticity modulus and compression strength values of same wood samples decreased by thermal modification, however, the water repellence efficiency of the same wood samples (spruce and beech) were increased heated in the same temperature and duration. It was determined that elasticity modulus of wood polymers decreased, when crystallinity of cellulose in wood samples increased by thermal modification. Since amorphous region of cellulose in wood samples decreased, absorbed water by wood samples diminished by increasing in crystalline region. All wood constituent polymers are responsible for most of the physical and chemical properties exhibited by wood and wood products. Because of its high degree of polymerization and crystallinity, cellulose responsible for strength in wood fiber [27]. Stamm et al. reported [28] that when wood is heated at elevated temperatures, the reduction in hygroscopicity and in swelling and shrinkage are due to the formation of ether linkage by the splitting of two adjacent hydroxyl groups. An appreciable increase in decay resistance and significant losses in strength was obtained at a temperature above 270 1C which is close to the temperature at which exothermic decomposition of wood becomes appreciable, and where the hemicellulose and lignin are attacked and the crystallinity of the cellulose modified. The changes in the mechanical and physical properties of heated wood are generally attributed to the thermal degradation of wood substance [5]. According to the thermogravimetric analysis the first change observed in the heating of cellulose, which occurs at about 100 1C, is the elimination of adsorbed water [20]. By raising the temperature above 200 1C the thermal degradation of cellulose and formation of volatile products proceeds rapidly [19,29]. Due to structural heterogeneity of hemicelluloses, it is a complex matter to reveal their thermal behavior. The lower thermal stability of hemicelluloses compared to cellulose is usually explained by the lack of crystallinity [20]. It was reported that cellulosic materials usually form crystal structures in part, and water cannot penetrate inside the crystalline domains at room temperature [30].
4. Conclusion It was concluded that crystallinity of cellulose in spruce and beech wood samples increased with thermal modification. It was designated that changes
in crystallinity related to not only temperature but also time. I a =I b ratio of cellulose in spruce and beech wood samples changed with thermal modification, but it was established that monoclinic structure was dominant in cellulose crystalline structure. According to these results, it can be seen that crystalline structure of cellulose in spruce wood samples affected from thermal modification more than in beech wood samples.
Acknowledgements The authors would like to thank Karadeniz Technical University, Research Fund for support. References [1] Mazela B, Zakrzewski R, Grzes´ kowiak W, Cofta G, Bartkowiak M. Resistance of thermally modified wood to basidiomycetes. Wood Technology 2004;7(1):253–62. [2] Militz H. Thermal treatment of wood: European processes and their background. IRG/WP 02-40241. 33rd Annual Meeting, 12–17 May, Cardiff-Wales 2002;4:1–17. [3] Kamdem DP, Pizzi A, Triboulot MC. Heat treated timber: potentially toxic byproducts presence and extent of wood cell wall degradation. Holz als Roh-und Werkstoff 2000;58:253–7. [4] Wikberg H, Maunu SL. Characterization of thermally modified hard- and softwoods by 13C CPMAS NMR. Carbohydrate Polymers 2004;58:461–6. [5] Mitchell PH. Irreversible property changes of small loblolly pine specimens heated in air, nitrogen, or oxygen. Wood and Fiber Science 1988;20(3):320–55. [6] Kamdem DP, Pizzi A, Jermannaud A. Durability of heat-treated wood. Holz als Roh-und Werkstoff 2002;60:1–6. [7] Sanderman W, Augustin H. Chemical investigation on the thermal decomposition of wood—Part III: chemical investigation on the course of decomposition. Holz als Roh-und Werkstoff 1963;22(10):377–86. [8] Hinterstoiser B, Salmen L. Two-dimensional step-scan FTIR: a tool to unravel the OH-valency-range of the spectrum of Cellulose I. Cellulose 1999;6:251–63. [9] Andersson S, Serimaa R, Paakkari T, Saranpaa P, Pesonen E. Crystallinity of wood and the size of cellulose crystallites in Norway spruce (Picea abies). Journal of Wood Science 2003;49:531–7. [10] Andersson S, Wikberg H, Pesonen E, Maunu SL, Serimaa R. Studies of crystallinity of Scots pine and Norway spruce cellulose. Journal of Wood Science 2004;18:346–53. [11] Nomura T, Yamada T. Structural observation on wood and bamboo by X-ray. Wood Research 1972;52:1–12. [12] Lofty M, El-osta M, Kellogg RM, Foschi RO, Butters RG. A mechanistic approach to crystallite length as related to cell-wall structure. Wood Fiber 1974;6:36–45. [13] Tanaka F, Koshijima T, Okamura K. Characterization of cellulose in compression and opposite woods of a pinus densiflora tree grown under the influence of strong wind. Wood Science Technology 1981;15:265–73. [14] Diwanto W, Tanaka F, Inoue M, Norimoto M. Crystallinity changes of wood by heat or steam treatment. Wood Research 1996;83:47–9. [15] Kubojima Y, Okano T, Ohta M. Effect of annual ring widths on structural and vibrational properties of wood. Mokuzai Gakkaishi 1997;43:634–41. [16] TAPPI test methods. Atlanta: Tappi Press; 2002–2003.
ARTICLE IN PRESS S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67 [17] Ritcher U. Struktura¨nderungen von Cellulose durch Alkali Behandlung: physikalissche und chemische Beurteilungsmethoden. Doctoral thesis, 1991. [18] Evans R, Newman RH, Roick U, Suckling ID, Wallis AFA. Changes in cellulose crystallinity during kraft pulping. Comparison of infrared, X-ray diffraction and solid state NMR results. Holzforschung 1995;49:498–504. [19] Fengel D, Wegener G. Wood chemistry, ultrastructure, reactions. Berlin: Walter de gruytr Publications; 1989. [20] Kotilainen R. Chemical changes in wood during heating at 150–260 1C. PhD Thesis, Jyva¨skyla¨ University, Research report 80, Finland, Press; 2000. [21] Runkel ROH, Wilke KD. Chemical composition and properties of wood heated at 140 to 200 1C in a closed system without free space. Part II. Holz Roh-u. Werkstoff 1951;9:260–70. [22] Weimer PJ, Hackney JM, French AD. Effects of chemical treatments and heating on the crystallinity of celluloses and their implications for evaluating the effect of crystallinity on cellulose biodegradation. Biotechnology and Bioengineering 1995;48:169–78. [23] Akerholm M, Hinterstoisser B, Salmen L. Characterization of the crystalline structure of cellulose using static and dynamic FT-IR spectroscopy. Carbohydrate Research 2004;339:569–78.
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[24] Tripp VW. Mesurement of crystallinity. In: Bikales M, Segal L, editors. Cellulose and cellulose derivatives. New York: Wiley Interscience; 1971. p. 305–23. [25] Roberts GAF. Accesibility of Cellulose. In: Roberts JC, editor. Paper chemistry. London: Blackie Academic and Professional; 1992. p. 9–22. [26] Yıldız S. Isıl is-lem uygulanan dog˘u kayını ve dog˘u ladini odunlarının fiziksel, mekanik, teknolojik ve kimyasal o¨zellikleri, KTU¨, Fen Bilimleri Enstitu¨su¨, Doktora tezi, 2002; p. 263, Trabzon. [27] Pandey KK. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. Journal of Applied polymer Science 1999;71:1969–75. [28] Stamm AJ. Thermal degradation of wood and cellulose. Industrial and Engineering Chemistry 1956;48(3):413–7. [29] Manninen AM, Pasanen P, Holopainen JK. Comparing the VOC emissions between air-dried and heat-treated Scots pine wood. Atmospheric Environment 2002;36(11):1763–8. [30] Meshitsuka G, Isogai A. Chemical structures of cellulose, hemicelluloses, and lignin. In: David N, Hon S, editors. Chemical modification of lignocellulosic materials. New York: Marcel Dekker Ltd.; 1996. p. 370.
Building and Environment 42 (2007) 62–67 www.elsevier.com/locate/buildenv
The effects of thermal modification on crystalline structure of cellulose in soft and hardwood Sibel Yildiz, Esat Gu¨mu¨s-kaya The Department of Forest Industry Engineering, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey Received 14 April 2005; received in revised form 1 July 2005; accepted 11 July 2005
Abstract In this study the crystallinity and estimations of relative triclinic (Ia) and monoclinic (Ib) structure content of cellulose isolated from heated spruce (Picea orientalis) and beech (Fagus orientalis) wood samples were determined by using Fourier Transform Infrared (FT-IR) spectrometry. Heat treatment was applied on the test samples in an oven at three different temperatures (150, 180 and 200 1C) and two different durations (6 and 10 h) under atmospheric pressure. It was determined that crystallinity of cellulose in wood samples increased with thermal modification. The results indicate that the changes in crystallinity of cellulose in wood samples related to not only temperature but also time during thermal modification. I a =I b ratio of cellulose in spruce and beech wood samples changed with thermal modification, but it was established that monoclinic structure was dominant in cellulose crystalline structure. It was designated that the crystalline structure of cellulose in spruce wood samples affected from thermal modification more than in beech wood samples. r 2005 Elsevier Ltd. All rights reserved. Keywords: Heat treatment; Fagus orientalis; Picea orientalis; Crystallinity; Cellulose
1. Introduction Wood, as a biopolymer composite, has been known and recognized as construction material for a long time. Advantages of this material are appreciated very widely. However, considerable possibilities still remain associated with the improvement of wood natural quality characteristics such as dimensional stability or resistance to bio-corrosion, which might prolong the service life of wood products. In order to achieve these objectives, wood modification was developed as a method of equipping this material with new properties [1]. Heat treatment is one of the processes used to modify the properties of wood. Heat-treated wood has been considered as an ecological alternative to impregnated Corresponding author. Tel.: +90 462 377 28 62, +90 462 377 34 99; fax: +90 462 325 74 99. E-mail addresses: [email protected] (S. Yildiz), [email protected] (E. Gu¨mu¨s-kaya).
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.07.009
wood materials. Recent efforts on thermal treatment of wood lead to the development of several processes introduced to the European market during the last few years [2]. Nowadays, work on the heat treatment process of timber to prepare what is currently called torrefied wood in France; plato wood in the Netherlands and retified wood in the USA has shown that such types of processes can improve the performance of timber in several aspects [3]. The chemical modifications in wood structure occurring at high temperature are accompanied by several favorable changes in physical structure: reduced shrinkage and swelling, low equilibrium moisture content, enhanced weather resistance and decorative, dark color, better decay resistance. Thermally modified wood finds use in outdoor furniture, claddings on wooden buildings, floor material, musical instruments and a diversity of other outdoor and indoor applications. Unfortunately, the mechanical properties, e.g. strength, hardness and stiffness are reduced at the same time [4].
ARTICLE IN PRESS S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
The decrease in strength depends on the method of the thermal modification, the wood species and its characteristic properties, the initial moisture content of wood, the surrounding atmosphere, treatment time and temperature [5]. All the European heat processes have in common the treatment of sawn wood at elevated temperatures in the range between 160 and 260 1C [2]. The maximum temperature during the heat treatment is varied from 180 to 280 1C and from 15 min to 24 h depending on the process, wood species, sample size, moisture content of the wood, the desirable mechanical properties, resistance to biological attack and the dimensional stability of the final product. The presence of air or oxidant during the heat treatment may accelerate the degradation of wood components. Inert or reducing atmosphere is reported to facilitate the heat treatment. A nitrogen atmosphere is normally used. The rate of thermal degradation of wood hemicelluloses in air is greater than in an inert environment. The chemical degradation of wood occurs in the order of hemicellulose, cellulose and lignin. A limited decomposition of lignin is observed at as low as 220 1C with the presence of phenolic substances such as vanillin, coniferaldehyde and syringyl aldehyde [6,7]. Wood is a complex composite material, which consists mainly cellulose, hemicellulose and lignin. Cellulose represents the crystalline parts of wood, while the structure of hemicellulose and lignin are amorphous. The main mechanical function of hemicelluloses and lignin is to buttress the cellulose fibrils [4]. The strength properties of cellulosic materials are intrinsically related to the interactions between the cellulose molecules, and an understanding of the molecular structure of cellulose is therefore extremely important [8]. The fibre texture and the complex chemical composition of wood complicate the crystallinity determination. Furthermore, the separation of amorphous background from the diffraction pattern of cellulose crystallites is difficult because the cellulose crystallinity of wood cannot be determined accurately [9]. Most of the studies have aimed at determining the relative crystallinity of wood [10–15]. The aim of this study was to investigate the crystallinity and estimations of relative monoclinic (I b ) and triclinic structure (I a ) content of cellulose isolated from heated spruce (Picea orientalis) and beech (Fagus orientalis) wood samples grown in Turkey. It was investigated relationship between chemical, crystalline structure and mechanical properties of spruce and beech woods.
2. Materials and method 2.1. Thermal modification Raw material beech and spruce woods were obtained from Black Sea Region in Turkey. The woods were cut
63
in parallel to grain directions and sawn into specimens measuring 2 2 3 (tangential radial longitudinal) cm long. Conditioning to 12% moisture content was performed at 21 1C and 65% relative humidity. Heat treatment was then applied on the test samples in an oven controlling the temperature 71 1C sensitivity, at three different temperatures (150, 180 and 200 1C) and two different durations (6 and 10 h) under atmospheric pressure and in the presence of air. Before the chemical analyses heat-treated and untreated (control) wood samples were ground in a Wiley mill to a homogeneous meal. Cellulose analyses were made according to Kurschner-Hoffner’s nitric acid method [16]. 2.2. FT-IR spectroscopy The cellulose powders obtained from spruce and beech wood samples treated thermal modification were used for Fourier Transform Infrared (FT-IR) spectroscopy measurement. The dried samples were embedded in potassium bromide (KBr) pellets, and were analyzed by using a Perkin Elmer Spectrum One FT-IR spectroscopy model 2000. They were recorded in the absorption mode in the range 4000–400 cm 1 with an accumulation of 32 scans, resolution of 2 cm 1 and normalized C–O–C band at 2900 cm 1.
3. Results and discussion In this study, we investigated how the crystalline structure of cellulose in spruce and beech wood samples changed with thermal modification at 150, 180 and 200 1C. For this reason, cellulose was isolated from spruce and beech wood samples treated by thermal modification. We used different methods for determination of crystallinity of cellulose in wood samples. The first approach for determination of crystallinity of cellulose in wood samples was the ratio of the peaks areas at 1370 and 670 cm 1 (A1370 =A670 ) proposed by Richter 1991 [17] in FT-IR spectra of wood samples. This method actually uses the ratio of the combined areas of the three peaks at 1370, 1335 and 1315 cm 1, which have called the 1370 cm 1 peak (CH bending), to that of the peak at 670 cm 1 (C–OH out of plane bending mode). Spectra were recorded in absorbance modes using KBr discs containing a constant amount of the samples [18]. According to this method, crystallinity of cellulose in spruce wood samples increased with thermal modification in 6 h at all temperatures. As can be seen in Table 1 and 2, when crystallinity of cellulose in spruce wood samples increased at 150 and 200 1C in 10 h, it was felled at 180 1C. Crystallinity of cellulose in beech wood samples was mounted at 180 1C in 6 h and decreased at 200 1C in 10 h.
ARTICLE IN PRESS S. Yildiz, E. Gu¨mu¨s- kaya / Building and Environment 42 (2007) 62–67
64
Table 1 Crystallinity of cellulose and estimations of relative content I a =I b in heated spruce wood Sample
Cellulose in spruce
Temperatures (1C)
Control 150 180 200
a
Time (h)
— 6 10 6 10 6 10
Crystallinity A1370 =A670
H 1429 =H 897
H 1374 =H 2900
6.9 8.9 8.6 8.8 7.5 8.4 8.5
0.67 0.85 0.76 0.71 0.65 0.89 0.80
0.56 0.84 0.64 0.86 0.64 0.75 0.71
I a a (%)
I b b (%)
39 46 46 42 47 50 47
61 54 54 58 53 50 53
I a a (%)
I b b (%)
37 44 42 48 45 47 46
63 56 58 52 55 53 54
One-chain triclinic unit cell. Two-chain monoclinic unit cell.
b
Table 2 Crystallinity of cellulose and estimations of relative content I a =I b in heated beech wood Sample
Cellulose in beech
Temperatures (1C)
Control 150 180 200
a
Time (h)
— 6 10 6 10 6 10
Crystallinity A1370 =A670
H 1429 =H 897
H 1372 =H 2900
7.2 7.3 7.2 8.4 7.5 7.8 6.7
0.78 0.91 0.97 0.81 0.83 0.94 0.91
0.45 0.63 0.58 0.51 0.55 0.73 0.64
One-chain triclinic unit cell. Two-chain monoclinic unit cell.
b
Heating of wood modifies the cell wall components. Several modifications are reported to result from high temperature and steam. As a result of these modifications; it was concluded that hemicelluloses and less ordered cellulose deteriorate and as a consequence, the degree of cellulose crystallinity increases [4]. In a previous study, it was observed an increase of crystallinity in thermally treated cellulose up to temperatures of 120–160 1C followed by a decrease [19]. According to the researchers; crystalline structure of cellulose is not changed [20,21] or even can improve up to a certain temperature, which may be as high as 200 1C depending on the conditions involved. The crystallinity of the alkali-resistant cellulose from thermally treated spruce wood increased up to a temperature of 200 1C because of a preferred degradation of the less ordered molecules. Several authors have noted that cellulose undergoes changes in crystallinity upon chemical and physical treatments, particularly heating. It was reported that cotton linters underwent moderate increases in crystallinity upon mere exposure to humid atmospheres. It was demonstrated that suspension of cellulose in water and subsequent drying resulted in variable increases in
relative crystallinity that were more dramatic at lower initial relative crystallinity [22]. Crystallinity of cellulose and estimations of relative Ia/Ib content in heated spruce and beech wood were shown in Tables 1 and 2, respectively. FT-IR spectra were in Fig. 1 given for spruce wood samples and in Fig. 2 for beech wood samples. The ratio of peak heights at 1429 and 897 cm 1 (H 1429 =H 897 ), and at 1372 and 2900 cm 1 (H 1372 =H 2900 ) in FT-IR spectra of wood samples were used for determination of crystallinity of cellulose in wood samples [23]. In this study, The H 1429 =H 897 ratio for cellulose in spruce wood samples ranged from 0.67 to 0.89, whereas the same IR ratio for cellulose in beech wood samples varied from 0.78 to 0.97. According to these results, it was concluded that crystalline structure of cellulose was affected from thermal modification in beech wood samples more than in spruce wood samples. In the literature [24,25], it is stated that the value of index proposed as crystallinity varied from 2.80 for untreated cotton down to 0.11 for ball-milled cellulose. The H 1429 =H 897 and H 1372 =H 2900 ratios for cellulose in spruce increased with thermal modification according to control samples. It was determined that H 1429 =H 897 and
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1430 1280 Absorbance
Absorbance
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1280 1635 1430
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1740
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Control
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6h
180°C
180°C 10h
10h 6h
6h
200°C 10h 200°C
10h
Wavelength (cm–1)
Wavelength (cm–1)
Fig. 1. FT-IR spectra of spruce wood samples.
Fig. 2. FT-IR spectra of beech wood samples.
H 1372 =H 2900 ratios for cellulose in spruce wood samples increased at 6 h more than at 10 h during thermal modification at all temperatures, in this study. As can be seen in Table 2, the H 1429 =H 897 and H 1372 =H 2900 ratios for beech wood samples were influenced from thermal modification at 150 and 200 1C more than at 180 1C. In course of cellulose isolation from spruce and beech wood samples, less ordered and amorphous parts formed during thermal modification remove. For this reason, it is possible to say that crystallinity of cellulose in wood samples increased with thermal degradation, relatively. In a previous study, it was designated no change of X-ray diffraction up to 220 1C during the heating of pinewood. Beyond this temperature the supramolecular structure was destroyed and completely amorphous state was reached about 270 1C. A similar result was obtained with sulfite pulp from slash pine. At 240 1C the crystalline structure of cellulose obviously breaks down as the degree of polymerization (DP) decreases within 2 h below 200 1C and within 8 h below 100 1C [19].
In this study, it was determined that monoclinic structure was dominant in beech and spruce wood samples. I a =I b ratio established 39/61 for spruce and 37/63 for beech wood samples, respectively, before thermal modification. It was designated that monoclinic structure ratio of cellulose in wood samples decreased, when triclinic structure ratio of cellulose in wood samples increased with thermal modification, as can be seen in Tables 1 and 2. As can be seen in Fig. 1 (FT-IR spectra for spruce wood samples), peak shoulder at 1635 cm 1 (adsorbed water) for spruce wood samples decreased by rising thermal modification temperature and time. Band at 1635 cm 1 came back to same level band at 1740 cm 1 (C QO valance vibration of COOH) at 200 1C. It was determined that bands at 1280 cm 1 (CH deformation) and 1430 cm 1 (CH2 scissoring) decreased with thermal modification. In Fig. 2 (FT-IR spectra for beech wood sample), it can be seen that band at 1635 cm 1 (adsorbed water) for beech wood samples came back to same level at band
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1740 cm 1 (C QO valance vibration of COOH) at 200 1C within 6 h, but it moved down to band 1740 cm 1. Bands at 1240 cm 1 (–OH plane deformation of COOH) and 1430 cm 1 (CH2 scissoring) for beech wood samples were decreased by thermal modification. Yıldız [26] found that the elasticity modulus and compression strength values of same wood samples decreased by thermal modification, however, the water repellence efficiency of the same wood samples (spruce and beech) were increased heated in the same temperature and duration. It was determined that elasticity modulus of wood polymers decreased, when crystallinity of cellulose in wood samples increased by thermal modification. Since amorphous region of cellulose in wood samples decreased, absorbed water by wood samples diminished by increasing in crystalline region. All wood constituent polymers are responsible for most of the physical and chemical properties exhibited by wood and wood products. Because of its high degree of polymerization and crystallinity, cellulose responsible for strength in wood fiber [27]. Stamm et al. reported [28] that when wood is heated at elevated temperatures, the reduction in hygroscopicity and in swelling and shrinkage are due to the formation of ether linkage by the splitting of two adjacent hydroxyl groups. An appreciable increase in decay resistance and significant losses in strength was obtained at a temperature above 270 1C which is close to the temperature at which exothermic decomposition of wood becomes appreciable, and where the hemicellulose and lignin are attacked and the crystallinity of the cellulose modified. The changes in the mechanical and physical properties of heated wood are generally attributed to the thermal degradation of wood substance [5]. According to the thermogravimetric analysis the first change observed in the heating of cellulose, which occurs at about 100 1C, is the elimination of adsorbed water [20]. By raising the temperature above 200 1C the thermal degradation of cellulose and formation of volatile products proceeds rapidly [19,29]. Due to structural heterogeneity of hemicelluloses, it is a complex matter to reveal their thermal behavior. The lower thermal stability of hemicelluloses compared to cellulose is usually explained by the lack of crystallinity [20]. It was reported that cellulosic materials usually form crystal structures in part, and water cannot penetrate inside the crystalline domains at room temperature [30].
4. Conclusion It was concluded that crystallinity of cellulose in spruce and beech wood samples increased with thermal modification. It was designated that changes
in crystallinity related to not only temperature but also time. I a =I b ratio of cellulose in spruce and beech wood samples changed with thermal modification, but it was established that monoclinic structure was dominant in cellulose crystalline structure. According to these results, it can be seen that crystalline structure of cellulose in spruce wood samples affected from thermal modification more than in beech wood samples.
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