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,z ! ԅ 8 RV3 ~ j ą ê 0 ̅ ~ ԅ ԅ " T l r t l v $ /w r r r r r r ,z ,z ,z ,z r r r r r r r r r J : Changes in Wettability of Heat-Treated Wood due to Artificial Weathering
Xianai Huang1, Duygu Kocaefe*1, Yasar Kocaefe1, Yaman Boluk2, Andre Pichette1
1Universit du Qubec Chicoutimi, Canada
555, boul. de lUniversit, Chicoutimi Qubec Canada G7H 2B1
2University of Alberta, Canada3-142 Markin/CNRL Natural Resources Engineering Facility Edmonton, Alberta, Canada T6G 2W2
Corresponding author:
Duygu Kocaefe
E-mail: HYPERLINK "mailto:dkocaefe@uqac.ca" dkocaefe@uqac.ca
Phone: 418-545 5011ext.5215
Fax: 418-545-5012
Journal Name: Wood Science and Technology
Abstract
Effect of artificial weathering on the wettability of three heat-treated North American species (jack pine, aspen, and birch) is studied from the point of view of the structural and chemical changes taking place on the wood surface. Weathering increases wettability of all three heat-treated woods by water. Changes in wettability during artificial weathering differ according to heat treatment procedure and wood species, and are likely due to combination of structural and chemical changes of the surfaces. SEM analysis indicates that cracks form due to degradation taking place during weathering. As a result, water has easier entry into the cell wall, which consequently increases wettability. IR spectra suggest that the OH/CH2 ratio for heat-treated specimens is inversely proportional to the contact angle regardless of the type of wood species. The presence of cellulose-rich layer on wood surface and increasing amount of amorphous ce l l u l o s e t r a n s f o r m e d f r o m c r y s t a l l i z e d c e l l u l o s e d u e t o w e a t h e r i n g r e s u l t i n i n c r e a s e o f h y d r o x y l , c o n s e q u e n t l y , i t i n c r e a s e s h e a t - t r e a t e d w o o d w e t t a b i l i t y .
I n t r o d u c t i o n a n d b a c k g r o u n d
H e a t - t r e a t e d w o o d i s n a t u r a l w o o d h e a t e d t o t e m p e r a t u r e s o f 1 6 0 2 3 0 C , u s u a l l y a b o v e 2 0 0 C , d e p e n d i n g o n t h e s p e c i e s u s e d a n d t h e d e s i r e d m a t e r i a l p r o p e r t i e s A D D I N E N . C I T E A D D I N E N . C I T E . D A T A ( H Y P E R L I N K \ l " _ E N R E F _ 3 2 " \ o " K o c a e f e , 2 0 0 8 # 1 1 " K o c a e f e e t a l . 2 0 0 8 b ) . H e a t - t r e a t e d w o o d s h a v e b e e n u s e d f o r o u t d o o r p u r poses because of the new properties such as reduced hygroscopy, improved dimensional stability, better resistance to degradation by insects and micro-organisms, and attractive dark color. Wood heat treatment has widely spread in the last few years as an industrial process which is used to improve wood properties ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_10" \o "Esteves, 2009 #60" Esteves and Pereira 2009). Decrease in hygroscopicity, improvement in dimensional stability and loss of mechanical properties of wood treated at high temperature are the focus of many early research work reported in the literature ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_59" \o "Weiland, 2003 #23" Weiland and Guyonnet 2003; HYPERLINK \l "_ENREF_9" \o "Duchez, 2001 #24" Duchez et al. 2001; HYPERLINK \l "_ENREF_33" \o "Kocaefe, 2008 #26" Kocaefe et al. 2008c). Numerous investigations have been carried out on the resistance against fungal degradation ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_59" \o "Weiland, 2003 #23" Weiland and Guyonnet 2003; HYPERLINK \l "_ENREF_9" \o "Duchez, 2001 #24" Duchez et al. 2001; HYPERLINK \l "_ENREF_33" \o "Kocaefe, 2008 #26" Kocaefe et al. 2008c). Heat-treatment process does not require any chemical addition to modify wood. A number of studies have examined extensively the chemical transformations of high temperature treated wood ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_60" \o "Windeisen, 2007 #52" Windeisen et al. 2007; HYPERLINK \l "_ENREF_48" \o "Salmn, 2008 #53" Salmn et al. 2008; HYPERLINK \l "_ENREF_1" \o "Aydemir, 2011 #54" Aydemir et al. 2011; HYPERLINK \l "_ENREF_4" \o "Brosse, 2010 #55" Brosse et al. 2010; HYPERLINK \l "_ENREF_56" \o "Tumen, 2010 #56" Tumen et al. 2010). The properties and characteristics of heat-treated wood surfaces differ strongly from those of untreated wood as a result of chemical changes during heat treatment. The heat-treated wood surface properties, such as wettability and wood color, have been extensively studied ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_46" \o "Pavlo, 2003 #28" Pavlo and Niemz 2003; HYPERLINK \l "_ENREF_41" \o "Nuopponen, 2004 #57" Nuopponen et al. 2004; HYPERLINK \l "_ENREF_49" \o "Shi, 2011 #58" Shi et al. 2011; HYPERLINK \l "_ENREF_34" \o "Li, 2011 #59" Li et al. 2011; HYPERLINK \l "_ENREF_16" \o "Hakkou, 2005 #3" Hakkou et al. 2005a; HYPERLINK \l "_ENREF_17" \o "Hakkou, 2005 #10" Hakkou et al. 2005b; HYPERLINK \l "_ENREF_32" \o "Kocaefe, 2008 #11" Kocaefe et al. 2008b; HYPERLINK \l "_ENREF_57" \o "Wang, 2007 #12" Wang et al. 2007; HYPERLINK \l "_ENREF_47" \o "Ptrissans, 2003 #15" Ptrissans et al. 2003).
In outdoor applications, wood is subjected to various environmental conditions, resulting in various modifications such as surface roughening and color changes in varying degrees ADDIN EN.CITE Kishino20041(Kishino and Nakano 2004a)1117Kishino, M.Nakano, T.Hokkaido Forest Prod. Res. Institute, Hokkaido, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, Shimane, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, JapanArtificial weathering of tropical woods. Part 1: Changes in wettabilityHolzforschungHolzforschung552-557585Artificial weatheringContact angleDRIFT spectroscopyStereoscopic micrographWettabilityWood surfaceAdsorptionDensity (specific gravity)Infrared radiationSurface roughnessWettingWoodDensityInfrared SpectraPhotomicrographsAcaciaAcacia auriculiformisDipterocarpusEucalyptusEucalyptus marginataEucalyptus robustaShoreaTabebuia200400183830 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-4444264595&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a), consequently, surface degradation. Therefore, the changes in surface properties of heat-treated wood during weathering are of significant practical concern. If heat-treated woods are to have a long service life, the weathering process must be understood and treatments to retard this degradation should be developed. Chemical and physical changes of untreated wood during weathering has been extensively studied and reported ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_20" \o "Hon, 1985 #31" Hon 1985; HYPERLINK \l "_ENREF_13" \o "Feist, 1984 #29" Feist and Hon 1984; HYPERLINK \l "_ENREF_22" \o "Hon, 1986 #30" Hon and Feist 1986; HYPERLINK \l "_ENREF_18" \o "Hon, 1981 #18" Hon 1981; HYPERLINK \l "_ENREF_21" \o "Hon, 1984 #32" Hon and Chang 1984). The photo degradation of wood is a surface phenomenon. It was reported that the UV radiation penetrates only 75m below the surface whereas visible light penetrates 200 m ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_23" \o "Hon, 1978 #17" Hon and Ifju 1978; HYPERLINK \l "_ENREF_18" \o "Hon, 1981 #18" Hon 1981). More recent research has shown that the degradation depth is exceeds the limit of 75 m and chemical changes take place up to the depth of 900 m ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_23" \o "Hon, 1978 #17" Hon and Ifju 1978; HYPERLINK \l "_ENREF_18" \o "Hon, 1981 #18" Hon 1981; HYPERLINK \l "_ENREF_25" \o "Horn, 1992 #19" Horn et al. 1992; HYPERLINK \l "_ENREF_28" \o "Kataoka, 2001 #20" Kataoka and Kiguchi 2001; HYPERLINK \l "_ENREF_44" \o "Park, 1996 #21" Park et al. 1996; HYPERLINK \l "_ENREF_58" \o "Wang, 1991 #22" Wang and Lin 1991). Considerable research has been carried out especially on the changes of untreated woods surface properties such as discoloration and structure degradation during weathering by Hon and his coworkers ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_24" \o "Hon, 1991 #34" Hon and Minemura 1991; HYPERLINK \l "_ENREF_22" \o "Hon, 1986 #30" Hon and Feist 1986; HYPERLINK \l "_ENREF_13" \o "Feist, 1984 #29" Feist and Hon 1984). Changes in wettability and color of tropic (untreated) woods due to artificial weathering were also reported ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_30" \o "Kishino, 2004 #35" Kishino and Nakano 2004b, HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" a).
Wetting properties of wood is one of the surface properties that is of considerable practical and economic significance for understanding chemical and physical property changes occurring during weathering. This information also gives an idea on the different adhesion or coating characteristics required in order to retard the degradation. A number of investigations were carried out on the wettability changes of wood during heat treatment. Wettability and chemical composition of four heat-treated European wood species (pine, spruce, beech, and poplar) performed at 240C were studied ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_47" \o "Ptrissans, 2003 #15" Ptrissans et al. 2003; HYPERLINK \l "_ENREF_16" \o "Hakkou, 2005 #3" Hakkou et al. 2005a; HYPERLINK \l "_ENREF_17" \o "Hakkou, 2005 #10" Hakkou et al. 2005b). Effect of drying method on the surface wettability of wood strands was reported ADDIN EN.CITE Wang200712(Wang et al. 2007)121217Wang, S.Zhang, Y.Xing, C.Tennessee Forest Products Center, University of Tennessee, 2506 Jacob Drive, Knoxville, TN 37996-4570, United States
Nanjing Forestry University, Nanjing 210037, ChinaEffect of drying method on the surface wettability of wood strandsEinfluss des Trocknungsverfahrens auf die Oberflchenbenetzbarkeit von OSB-spnenEinfluss des Trocknungsverfahrens auf die Oberflchenbenetzbarkeit von OSB-spnen437-442656Contact angleDryingWoodLiquid sorption capacitySurface contact angleWood strandsWettingWettability200700183768 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-35748984930&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_57" \o "Wang, 2007 #12" Wang et al. 2007). Kocaefe et al. (2008) studied the effect of heat treatment on the wettability of white ash and soft maple by water. There also exists some information in the literature which discusses the changes in untreated wood wettability during weathering. The effects of aging on extracted and unextracted polar and dispersion components of the surface free energy of redwood and Douglas-fir were investigated ADDIN EN.CITE Nguyen19798(Nguyen and Johns 1979)8817Nguyen, T.Johns, W. E.Forest Products Laboratory, University of California, Richmond, California, United States
Wood Technology Section College of Engineering, Washington State University, Pullman, 99164, WA, United StatesThe effects of aging and extraction on the surface free energy of Douglas fir and redwoodWood Science and TechnologyWood Science and Technology29-401311979Springer-Verlag00437719 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-0001881441&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_40" \o "Nguyen, 1979 #8" Nguyen and Johns 1979). It was suggested that loss of surface free energy with aging is related to environmental rather than wood factors. Wettability of Western red cedar panels exposed to outdoor weathering were studied from the standpoint of wood compositional change induced by weathering ADDIN EN.CITE Kalnins199337(Kalnins and Feist 1993)373717Kalnins, Martins AFeist, William CIncrease in wettability of wood with weatheringForest Products JournalForest Products Journal554321993http://proquest.umi.com/pqdweb?index=0&did=1093118&SrchMode=1&sid=1&Fmt=3&VInst=PROD&VType=PQD&RQT=309&VName=PQD&TS=1301584560&clientId=13817( HYPERLINK \l "_ENREF_26" \o "Kalnins, 1993 #37" Kalnins and Feist 1993). Surface aging is a significant variable affecting the wettability and adhesion of coating on wood surface ADDIN EN.CITE Gindl200413(Gindl et al. 2004)131317Gindl, M.Reiterer, A.Sinn, G.Stanzl-Tschegg, S. E.Institute of Physics and Materials Science, Christian-Doppler-Laboratory for Fundamentals of Wood Machining, BOKU-University of Natural Resources and Applied Life Sciences Vienna, Tu?rkenschanzstr. 18, 1180 Vienna, Austria
Austrian Industrial Research Promotion Fund (FFF), Vienna, AustriaEffects of surface ageing on wettability, surface chemistry, and adhesion of woodHolz als Roh - und WerkstoffHolz als Roh - und Werkstoff273-280624Aging of materialsFree energyParameter estimationSurface chemistryWettingX ray photoelectron spectroscopyAcid base theorySurface ageingWood adhesionWood coatingWood preservationAgingChemistryEscaParametersSurface PropertiesX Ray Spectroscopy200400183768 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-21044437690&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_14" \o "Gindl, 2004 #13" Gindl et al. 2004). It was also reported that the effects of ultraviolet light exposure on the wetting properties of spruce and teak wood, to assess the viability of ultraviolet light irradiation as a surface pretreatment technique to activate surfaces for coating adhesion ADDIN EN.CITE Gindl20066(Gindl et al. 2006)6617Gindl, M.Sinn, G.Stanzl-Tschegg, S. E.Department of Material Sciences and Process Engineering, Institute of Physics and Materials Science, BOKU - University of Natural Resources and Applied Life Sciences, Peter Jordan Strasse 82, A-1190 Vienna, Austria
Austrian Science Fund, Weyringerstrasse 35, A-1040 Vienna, AustriaThe effects of ultraviolet light exposure on the wetting properties of woodJournal of Adhesion Science and TechnologyJournal of Adhesion Science and Technology817-828208Acid-base approachDynamic contact angle (DCA)SEMUV-activationContact angleScanning electron microscopyUltraviolet radiationSurface pretreatmentWood200601694243 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-33746529802&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_15" \o "Gindl, 2006 #6" Gindl et al. 2006). They proposed UV irradiation provided cleaning of the wood surface, consequently, the wettability and surface free energy increased significantly after a specific exposure period to UV light. Changes in wettability of tropic woods due to artificial weathering were reported ADDIN EN.CITE Kishino20041(Kishino and Nakano 2004a)1117Kishino, M.Nakano, T.Hokkaido Forest Prod. Res. Institute, Hokkaido, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, Shimane, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, JapanArtificial weathering of tropical woods. Part 1: Changes in wettabilityHolzforschungHolzforschung552-557585Artificial weatheringContact angleDRIFT spectroscopyStereoscopic micrographWettabilityWood surfaceAdsorptionDensity (specific gravity)Infrared radiationSurface roughnessWettingWoodDensityInfrared SpectraPhotomicrographsAcaciaAcacia auriculiformisDipterocarpusEucalyptusEucalyptus marginataEucalyptus robustaShoreaTabebuia200400183830 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-4444264595&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a).
As it is explained above, various studies were carried out on the wettability of untreated and heat-treated wood, and weathering of untreated wood. To our knowledge, however, the published literature on the change in heat-treated wood wettability during weathering is still lacking.
The aim of this study is to investigate the evolution in wettability of heat-treated North American jack pine (Pinus banksiana), aspen (Populus tremuloides), and birch (Betule papyrifera) during artificial weathering.
Materials and methods
Testing materials
The following three species, one softwood and two hardwood, which are commonly used for outdoor applications in North America, were studied: jack pine (Pinus banksiana), aspen (Populus tremuloides), and birch (Betule papyrifera). Wood boards of approximately 6500 200 30 mm were heat-treated in a prototype furnace of UQAC. Table 1 shows the conditions used during the heat treatment. Then, they were subjected to artificial weathering. Untreated wood boards, kiln dried with the final moisture content of about 12%, were also exposed to artificial weathering along with specimens heat-treated at high temperatures for comparison purposes. Specimens were arbitrarily selected for a complete statistical randomization. They were stored in a room at 20C and 40% relative humidity (RH) until they were exposed to the artificial weathering and the characterization tests described below.
Specimens of 70 65 mm cross-section on tangential surface ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_1" \o "Aydemir, 2011 #54" Aydemir et al.) and 20 mm long were cut from sapwood of heat-treated and untreated wood, and then planed to smooth surfaces. The prepared specimens of 70 65 20 mm were used in artificial weathering tests. All analysis tests (contact angle test, FTIR analysis and SEM evaluation) were carried out on the tangential surface ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_1" \o "Aydemir, 2011 #54" Aydemir et al.) of the wood. Specimens for surface wettability tests (contact angle measurement) were cut to 20 20 60 mm in the radial, tangential, and longitudinal directions, respectively. Those for FTIR analysis and SEM evaluation were cut to 1020 20 mm in the radial, tangential, and longitudinal directions, respectively.
Artificial weathering tests
Artificial weathering tests were conducted at the Laval University in collaboration with FPInnovation. The samples are exposed to UV light using a commercial chamber, Atlas Material Testing Technology LLC (USA) Ci65/Ci65A Xenon Weather-Ometer. A controlled irradiance water-cooled xenon arc with a CIRA inner filter and a Soda outer filter was used as the source of radiation to simulate sunlight. Tests were performed according to Cycle 1 of Standard ASTM G155: 102 min Xenon light, 18 min light and water spray (air temperature is not controlled) without dark cycle to simulate rain in natural weathering. The black panel temperature was set up to 6 3 3 C a n d t h e i r r a d i a n c e l e v e l w a s 0 . 3 5 W / m 2 a t 3 4 0 n m . H e a t - t r e a t e d s a m p l e s a n d u n t r e a t e d c o n t r o l s a m p l e s o f e a c h s p e c i e s w e r e e x p o s e d t o U V l i g h t . T h e i r r a d i a t i o n w a s i n t e r r u p t e d a f t e r 7 2 , 1 6 8 , 3 3 6 , 6 7 2 , 1 0 0 8 , a n d 1 5 1 2 h o u r s o f e x p o s u r e a n d t w o s a m p l e s f or each set of experimental conditions were taken out for evaluation of properties. Thus, the values of different properties evaluated in our experiment were the average measurements of two samples.
Surface wettability tests
Wetting parameters obtained with water are significantly linked to coating adhesion ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_7" \o "de Moura, 2005 #16" de Moura and Hernndez 2005, HYPERLINK \l "_ENREF_8" \o "De Moura, 2006 #2" 2006). Surface wettability e x p e r i m e n t s w e r e p e r f o r m e d u s i n g d i s t i l l e d w a t e r . M e a s u r e m e n t o f c o n t a c t a n g l e w a s p e r f o r m e d a t r o o m c o n d i t i o n s o f 2 0 C a n d 4 0 % R H . T h e c o n t a c t a n g l e s b e t w e e n w a t e r a n d w o o d s p e c i m e n s u r f a c e s w e r e d e t e r m i n e d u s i n g a s e s s i l e - d r o p s y s t e m ( F i r s t T e n A n g s t r o m s FTA200 equipped with CCD camera and image analysis software). This system uses video image processing which allows the faster determination of dynamic contact angles compared to the conventional contact angle goniometry. The initial period after trigger was 0.033s and the post-trigger period multiplier was set to 1.1. A drop of test liquid with a volume of 15l was dosed automatically by an auto-syringe and picked up by the specimen (20 20 60 mm) placed on a movable sample table. Measurement of contact angle was carried out by the sessile drop method with a view across the grain. Therefore, the wetting process parallel to the grain was investigated. Six to twelve tests were performed on two different samples for each set of experimental conditions in order to account for the non-homogeneous nature of wood. The contact angles between each droplet and specimen surface were measured both on the left side and the right side of the droplet and the mean contact angles were automatically calculated. Images of the drop in contact with the substrate were continuously captured at full video speed. The dynamic contact angles were determined as a function of time from the images. The mean initial contact angles and the total wetting time by water were recorded to assess wood surface wettability.
Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) analysis was used to investigate the microscopic structural changes occurring on wood surface during weathering. Small wood blocks measuring 20 20 mm on the weathered tangential face were cut from heat-treated and untreated boards after exposure to UV irradiation for different times (0, 336, 672, and 1512 hours). For subsurface cell degradation analysis, same blocks measuring 20 10 mm on the transverse face and radial face were used. The specimens were immerged in water for 30 minutes and then cut with a razor blade mounted onto a microtome by carefully cutting one of the end-grain surfaces and one radial surface. A new razor blade should be used for each final cut. Another method is to split these surfaces; however, they are rough and usually they do not allow observation into the cell lumen. The specimens were washed in distilled water and then air-dried at room temperature more than two nights and desiccated with phosphorus pentoxide for ten days. Finally all blocks were sputter coated with a palladium/gold layer (20 nm) and then mounted onto standard aluminum stubs using electrically conducting paste. The samples were scanned in a Jeol scanning electron m i c r o s c o p e ( J S M 6 4 8 0 L V ) w i t h m a g n i f i c a t i o n u p t o 3 0 0 0 0 0 a t 1 0 k V o f a c c e l e r a t i n g v o l t a g e . T h e d i s t a n c e b e t w e e n s a m p l e a n d e l e c t r o n m i c r o s c o p e h e a d w a s 2 0 - 2 5 m m w i t h s p o t s i z e o f 3 5 . T h e s p e c i m e n t e m p e r a t u r e w a s a p p r o x i m a t e l y 2 0 C a n d t h e c o l u m n v a c u u m w a s 6 .66 104 Pa. Digital images were transferred to a personal computer and saved as image files. To improve image quality, resolution, contrast and brightness were corrected digitally on the computer. Electron micrographs were taken for UV irradiated longitudinal tangential surface for different artificial weathering times. SEM micrographs of longitudinal radial surface were also taken in order to observe the cell damage in radial direction. The maximum depth of damage was measured from each transverse SEM image.
FTIR analysis
The effect of weathering on cellulose crystallinity and the chemical compositions of both cellulose and lignin on wood surface were studied using Fourier transform infrared spectroscopy. The air-dried specimens (102020 mm) were analyzed using Jasco FT/IR 4200 equipped with a diamond micro-ATR crystal. IR spectra were recorded in the wave number range of 5504000cm "1 a t 4 c m "1 r e s o l u t i o n f o r 2 0 s c a n s p r i o r t o t h e F o u r i e r t r a n s f o r m a t i o n . A s i t w a s s t a t e d i n i n t r o d u c t i o n , t h e w e a t h e r i n g d e g r a d a t i o n a f f e c t s a d e p t h o f 7 5 m t o 9 0 0 m o f w o o d s u r f a c e . T h e s a m p l i n g d e p t h o f i n f r a r e d r a d i a t i o n w a s o f 0 . 2 5 m d e p e n d i n g o n t h e w a v e n u m b e r w i t h t h e m i c r o - A T R c r y s t a l i n c i d e n t a n g l e o f 4 7 . T h i s e n s u r e d t h a t t h e r e c o r d e d I R s p e c t r a o f w o o d s u r f a c e s w e r e s u f f i c i e n t l y s u r f a c e s e n s i t i v e . T h u s , c h a n g e s i n I R s p e c t r a l f e a t u r e s w e r e s o l e l y c a u s e d b y c h a n g e s i n s u r f a c e c h e m i s t r y d u r i n g weathering duration, and there was no change in bulk chemistry of the interior part of wood specimen. The aperture diameter was 7.1mm. All spectra were analyzed using Jasco spectra manager software. The IR spectra for each treatment and artificial weathering time were transformed into absorbance spectra. All relative intensity ratios were normalized relative to the peak of the band at 2900 cm-1 which is C-H stretching in methyl and methylene groups ADDIN EN.CITE Kishino20041(Kishino and Nakano 2004a)1117Kishino, M.Nakano, T.Hokkaido Forest Prod. Res. Institute, Hokkaido, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, Shimane, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, JapanArtificial weathering of tropical woods. Part 1: Changes in wettabilityHolzforschungHolzforschung552-557585Artificial weatheringContact angleDRIFT spectroscopyStereoscopic micrographWettabilityWood surfaceAdsorptionDensity (specific gravity)Infrared radiationSurface roughnessWettingWoodDensityInfrared SpectraPhotomicrographsAcaciaAcacia auriculiformisDipterocarpusEucalyptusEucalyptus marginataEucalyptus robustaShoreaTabebuia200400183830 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-4444264595&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a). In FTIR, it is very important to use a spectral band that does not change during the treatment process if quantitative analysis is to be performed. And it is difficult to identify a reference spectral band that remains completely invariable throughout the whole treatment. The band of 2900 cm-1 is one of the bands that change less than C-OH, C-OC, R-COO-R OR Ar-OCH3 bonds during the treatment. This band was assumed to be invariable during experiment. In reality, it does not remain fully unchanged because it is present in volatile components, such as hydrocarbons, fatty acids, steroids, lactones, furans terpenes, etc. These volatiles leave wood surface and are partially replaced by those migrating toward surface from the interior of wood substance during treatment ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_31" \o "Kocaefe, 2008 #48" Kocaefe et al. 2008a). But the chosen band is one of the bands that changes least during the treatment. The quantitative FTIR values that are used here serve only for qualitative comparison in the discussion of the results.
Results and discussion
Surface wettability changes
The dynamic wettability of wood samples which are exposed to artificial weathering for different times is recorded. This information is useful in understanding the weathering mechanism of heat-treated wood. During this study, the effect of the weathering on dynamic contact angle, initial contact angle and total wetting time with water, consequently, on the wetting properties of three heat-treated woods is investigated. Furthermore, the effects of heat treatment and type of wood species on the wettability were also studied.
Figures 1 presents dynamic contact angle of water as a function of time for heat-treated jack pine, aspen, and birch tangential surfaces, respectively. In these figures, the contact angle evolution with time is given for an unweathered specimen (0 h) as well as for specimens after artificial weathering for different times (72 h, 168 h, 336h, 672 h, 1008 h, and 1512 h). As can be seen in the all three figures, the weathering reduces the hydrophobic behavior of these three heat-treated woods; consequently, all the contact angles of weathered heat-treated wood are lower than those of unweathered wood of the same species (0 h). This shows that the artificial weathering increases the wettability of wood by water. The contact angles decrease significantly after weathering of 72 h for all the three species at different extents depending on the species. As shown in Figure 1 (a), the contact angles of heat-treated jack pine do not seem to differ significantly after weathering for 72 h and 168 h, whereas at longer times they continue to decrease with the increasing weathering times. The trends observed for both hardwood species (aspen and birch) studied are found to be very similar (see Fig. 1 (b) and (c)). Contact angles of heat-treated aspen and birch after weathering reduce with increasing weathering time.
Figure 2 shows the variation in initial contact angle with the weathering time for each species with and without heat treatment. The initial contact angles of all heat-treated and untreated specimens with the exception of untreated aspen and birch decrease with increasing exposure time during weathering. Those of untreated aspen and birch increase with artificial weathering time up to 72 h, above this time they again decrease. This result is similar to that reported in literature ADDIN EN.CITE Kishino20041(Kishino and Nakano 2004a)1117Kishino, M.Nakano, T.Hokkaido Forest Prod. Res. Institute, Hokkaido, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, Shimane, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, JapanArtificial weathering of tropical woods. Part 1: Changes in wettabilityHolzforschungHolzforschung552-557585Artificial weatheringContact angleDRIFT spectroscopyStereoscopic micrographWettabilityWood surfaceAdsorptionDensity (specific gravity)Infrared radiationSurface roughnessWettingWoodDensityInfrared SpectraPhotomicrographsAcaciaAcacia auriculiformisDipterocarpusEucalyptusEucalyptus marginataEucalyptus robustaShoreaTabebuia200400183830 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-4444264595&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a). The initial contact angles of heat-treated specimens before artificial weathering are higher than those of untreated wood for all the three species which is in agreement with literature ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_32" \o "Kocaefe, 2008 #11" Kocaefe et al. 2008b; HYPERLINK \l "_ENREF_47" \o "Ptrissans, 2003 #15" Ptrissans et al. 2003; HYPERLINK \l "_ENREF_16" \o "Hakkou, 2005 #3" Hakkou et al. 2005a). However, after artificial weathering for 1008 h, the initial contact angle of heat-treated jack pine becomes smaller than that of untreated jack pine, while same trend is observed for aspen and birch only after 72 h of weathering. After artificial weathering for 1512 h, the difference between the initial contact angles of untreated and heat-treated specimens is largest for jack pine. Untreated jack pine has a contact angle of 40.3 , w h i l e o t h e r s p e c i m e n s e x h i b i t a c o n t a c t a n g l e o f l e s s t h a n 2 0 . 9 f o r t h e s a m e w e a t h e r i n g t i m e . T h e e f f e c t o f h e a t t r e a t m e n t o n i n i t i a l c o n t a c t a n g l e a f t e r w e a t h e r i n g f o r 7 2 h t o 1 0 0 8 h i s t h e l a r g e s t f o r a s p e n . T h e s e r e s u l t s s h o w t h a t t h e c h a n g e s i n w e t tability during artificial weathering differ according to heat treatment and type of wood species.
The times observed for complete surface wetting of all three species (heat-treated and untreated) by water before and after 1512h of artificial weathering are presented in Table 2. The specimens before weathering (0 h) exhibit different total wetting times depending on the species and whether they are heat-treated or not, ranging from 235.43 s for untreated aspen to 3003.73 s for heat-treated jack pine. The total wetting times of all specimens after weathering of 1512 h are less than 3.39s. This result suggests that weathering accelerates significantly absorption and penetration of water on both heat-treated and untreated wood surfaces; consequently, it reduces significantly total wetting time.
As it was stated before, weathering changes wood structural properties ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_22" \o "Hon, 1986 #30" Hon and Feist 1986; HYPERLINK \l "_ENREF_36" \o "Miniutti, 1964 #55" Miniutti 1964, HYPERLINK \l "_ENREF_37" \o "Miniutti, 1967 #47" 1967; HYPERLINK \l "_ENREF_38" \o "Miniutti, 1973 #46" Miniutti 1973; HYPERLINK \l "_ENREF_19" \o "Hon, 1984 #60" Hon 1984; HYPERLINK \l "_ENREF_11" \o "Evans, 1989 #41" Evans 1989; HYPERLINK \l "_ENREF_42" \o "Paajanen, 1994 #7" Paajanen 1994; HYPERLINK \l "_ENREF_12" \o "Evans, 1996 #62" Evans et al. 1996). The difference in wood surface structure can cause wettability differences of wood surfaces ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a; HYPERLINK \l "_ENREF_45" \o "Patton, 1970 #38" Patton 1970). Weathering induces changes not only in physical properties of a wood surface but also in its chemical properties ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_28" \o "Kataoka, 2001 #20" Kataoka and Kiguchi 2001; HYPERLINK \l "_ENREF_25" \o "Horn, 1992 #19" Horn et al. 1992). The changes in wettability during weathering can also be related to changes in the chemical properties of a wood surface ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_26" \o "Kalnins, 1993 #37" Kalnins and Feist 1993; HYPERLINK \l "_ENREF_14" \o "Gindl, 2004 #13" Gindl et al. 2004; HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a). As described in the introduction, heat treatment can cause chemical changes such as hemicelluloses degradation on wood surface, consequently, result in an increase in its wettability ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_32" \o "Kocaefe, 2008 #11" Kocaefe et al. 2008b).
Surface structural changes (SEM observations)
SEM analysis of untreated and heat-treated specimens indicates that anatomical structure of wood is only slightly affected during heat treatment. Fibers and rays are still obvious after heat treatment. Previous research reported that the main differences were the presence of important quantities of extractives deposited in the resins channels, which disappeared after thermal treatment ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_35" \o "Mburu, 2007 #51" Mburu et al. 2007). This implies that the structural factors do not play an important role on wettability during heat treatment. The comparison of heat-treated wood surfaces before weathering reveals the differences in structure for all three species (see Fig. 3 (b), (e) and (h)). The structure of jack pine (softwood) is relatively simple. The axial system is composed mostly of axial tracheids and the radial system is composed mostly of ray parenchyma cells. The structures of aspen and birch, which are hardwoods, are much more complicated than that of jack pine. Their axial systems are composed of vessel elements, fibers and axial parenchyma cells in different patterns and abundance. The presence of vessel elements is the unique feature that separates hardwoods from softwoods, which are for water conduction. Fibers in hardwoods function solely as support. Similar to jack pine, the radial system of aspen and birch is composed of ray parenchyma cells, but unlike softwoods, their rays are much more diverse in size and shape. On the longitudinal tangential section, vessels appear as large cracks. Water in contact with wood surfaces is able to penetrate into the wood substance in three ways: as liquid water flow into cell lumena by capillarity; as water vapour by diffusion into cell lumena; as bound water by diffusion within the cell wall ADDIN EN.CITE Banks197340(Banks 1973)404017Banks, W. B.Building Research Establishment, Princes Risborough Laboratory, Aylesbury, United KingdomWater uptake by scots pine sapwood, and its restriction by the use of water repellentsWood Science and TechnologyWood Science and Technology271-284741973Springer-Verlag00437719 (ISSN)http://www.scopus.com/inward/record.url?eid=2-s2.0-0001369949&partnerID=40&md5=8999c8142cbe7b0f91870caa682c829010.1007/bf00351073( HYPERLINK \l "_ENREF_2" \o "Banks, 1973 #40" Banks 1973). Therefore, structural differences in surfaces could exert an influence on the water entrance into wood specimens before weathering. The existence of large cracks of vessels on the surface can be observed in aspen and birch which results in lower contact angles than those of jack pine (see Fig. 1). This result is in agreement with a previous study ADDIN EN.CITE Kishino20041(Kishino and Nakano 2004a)1117Kishino, M.Nakano, T.Hokkaido Forest Prod. Res. Institute, Hokkaido, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, Shimane, Japan
Dept. of Nat. Rsrc. Proc. Eng., Interdisc. Fac. of Sci. and Eng., Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, JapanArtificial weathering of tropical woods. Part 1: Changes in wettabilityHolzforschungHolzforschung552-557585Artificial weatheringContact angleDRIFT spectroscopyStereoscopic micrographWettabilityWood surfaceAdsorptionDensity (specific gravity)Infrared radiationSurface roughnessWettingWoodDensityInfrared SpectraPhotomicrographsAcaciaAcacia auriculiformisDipterocarpusEucalyptusEucalyptus marginataEucalyptus robustaShoreaTabebuia200400183830 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-4444264595&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_29" \o "Kishino, 2004 #1" Kishino and Nakano 2004a). The structural comparison of both hardwood surfaces, aspen and birch, reveals the reason for difference in their wettability. The main cells (fibers) of aspen are thinner than those of birch, which results in larger lumen volume and decrease in specific gravity. Consequently, water enters the cell wall of aspen at a faster rate; therefore, aspen has smaller contact angles (less wettable and slower water penetration) than birch (shown in Fig. 1 (b) and (c)).
SEM analysis suggests that the changes occurring due to weathering in the wettability of heat-treated woods tested in this study might be attributed to the structural changes of wood surface. Large longitudinal and horizontal cracks present on all three heat-treated species longitudinal tangential surfaces after artificial weathering for 1512 h allow easier entrance of water into cell wall, which consequently decreases contact angles (increases wood wettability). The structural differences in surfaces can also influence the contact angles of specimens after weathering. The longitudinal cracks of heat-treated jack pine, originated from radial ray parenchyma cells during artificial weathering produce slightly higher contact angles (see Fig. 1). In contrast, the large cracks observed on aspen and birch surfaces, originated from vessel elements due to weathering, result in lower contact angles after artificial weathering for all times. However, the contact angles of the three heat-treated woods after weathering for 1512 h are almost the same, which indicates the structural differences on different heat-treated species surfaces at this weathering stage does not have any significant effect on wettability.
Figure 3 shows the surface micrographs on transverse surface of untreated woods before artificial weathering, heat-treated woods before and after artificial weathering for 1512 h. Figure 4 shows the influence of maximum damage depth on the contact angles of the specimens before and after artificial weathering for 72 h, 672 h, and 1512 h. Comparing Figures 3(a) and (b), it can be seen that the cracks on middle lamella and slight thinning of cell wall take place on jack pine transverse surface after thermal treatment. These checks appear to be a result of a stress caused by differential shrinkage due to heat treatment. Heat-treated jack pine wood looks more brittle than its untreated counterpart. However, structural changes due to heat treatment are not distinct, and it is likely that plasticization of cell wall material occurs only to a limited degree during heat treatment. This is in agreement with the result of Kollmann and Sachs who found comparable features in spruce after thermal treatment between 190C and 240C ADDIN EN.CITE Boonstra200633(Boonstra et al. 2006)333317Boonstra, M. J.Rijsdijk, J. F.Sander, C.Kegel, E.Tjeerdsma, B.Militz, H.Van Acker, J.Stevens, M.Plato International BV, PO Box 2159, NL-6802 CD Arnhem, NetherlandsMicrostructural and physical aspects of heat treated wood. Part 1. SoftwoodsMaderas: Ciencia y TecnologiaMaderas: Ciencia y Tecnologia193-20883Heat treatmentMicroscopySoftwoodWood modificationPicea abiesPseudotsugaPseudotsuga menziesiiRadiata200607173644 (ISSN)http://www.scopus.com/scopus/inward/record.url?eid=2-s2.0-33845402677&partnerID=40&rel=R8.0.0( HYPERLINK \l "_ENREF_3" \o "Boonstra, 2006 #33" Boonstra et al. 2006). Compared to jack pine wood, aspen wood displays less structural changes due to heat treatment as presented in Figure 3 (e), showing presence of smaller cracks on cell wall and slighter thinning of cell wall width. Similar to jack pine wood, the presence of small cracks on middle lamella, slight thinning cell wall width as well as plasticization for birch takes place after heat treatment (Fig. 3(h)). The structural changes, such as cracks on cell wall and middle lamella and the thinning of cell wall should boost wood wettability. However, as stated above, the wettabilty of heat-treated wood reduces due to heat treatment for all the three species. This suggests that the chemical changes of wood surfaces have more significant effect on the wettability changes than that of structural changes during heat treatment. This supports the idea described in the previous section. Figures 3 (c), (f) and (i) show the microstructural changes of cell occurring after artificial weathering for 1512 h on transverse surfaces of heat-treated jack pine, aspen, and birch, respectively. The development of cracks on middles lamella and thinning of cell wall width for all of the specimens are observed (Fig. 3); however, their magnitude which is different for different species is difficult to differentiate after the weathering of 1512 h. The differences in contact angles of different species are small after weathering for 1512 h. This can be due to the fact that the differences in microstructure between different species are no longer significant at this stage of weathering. Because the lignin concentration is higher in the middle lamella than in the cell wall, the weather degradation occurs preferentially in this area of wood surface. This is noticeable in Figures 3 (c), (f), and (i). The loss of lignin makes the surface more hydrophilic; that is, contact angles decrease as shown in Figures 1and 2.
The damaged wood layer has different physical and chemical characteristics than those of wood bulk. As shown in Figure 4, the contact angles of the specimens clearly decrease with increasing maximum damage depth for all the heat-treated wood species and their decreasing rates differ according to species type during artificial weathering. Thus, the maximum damage depth seems to play an important role in wettability of the species during artificial weathering. After weathering for the same time, the maximum damage depth on heat-treated aspen is the highest. However, the maximum damage depth effect is more significant on the initial contact angle of heat-treated aspen than those of heat-treated jack pine and birch. This result means that the change of heat-treated wood surface structure is just one of the reasons responsible for the changes in wettabiltiy due to weathering.
Surface chemical changes (FTIR analysis)
The most representative bands studied within the spectral range of 4000-550 cm-1 are summarized in Table 3. Figure 5 shows the FTIR spectra between the spectral region of 1800-800 cm-1 on heat-treated and untreated jack pine, aspen, and birch before weathering, respectively. Differences due to species and heat treatment can be clearly seen in the infrared spectra in the band shapes.
It is found that there are significant differences at 1260 cm-1 and 1230 cm-1 between hardwood (aspen and birch) and softwood (jack pine). A doublet can be detected at 12601230 cm-1 in heat-treated and untreated jack pine, while only one band at 1230 cm-1 can be found in the aspen and birch spectra. This is attributable to the difference in the guaiacyl content in lignin of jack pine (softwood), aspen and birch (hardwood). The lignin of softwood (jack pine) consists of guaiacyl nuclei, while those of hardwoods (aspen and birch) are components of guaiacyl and syringyl nuclei ADDIN EN.CITE Colom200346(Colom et al. 2003)464617Colom, X.Carrillo, F.Nogus, F.Garriga, P.Structural analysis of photodegraded wood by means of FTIR spectroscopyPolymer Degradation and StabilityPolymer Degradation and Stability543-549803LigninCelluloseFTIRCrystallinityWood component2003http://www.sciencedirect.com/science/article/B6TXS-48B5M71-1/1/3f1f1e399ec841d933d970168e94fe29( HYPERLINK \l "_ENREF_6" \o "Colom, 2003 #46" Colom et al. 2003). The band at 1260 cm-1 represents guaiacyl ring breathing with CO-stretching in lignin and hemicelluloses, is higher in jack pine than in aspen and birch. It can be observed that the intensity of the band at 1740 cm-1 is slightly higher in aspen and birch than in jack pine for both heat-treated and untreated woods. Colom et al. (2003) interpreted that this is probably caused by more acetyl groups of hardwood than softwood. The intensity at 1600 and 1510 cm-1 of aspen and birch are similar, while in jack pine spectra the band at 1510 cm-1 is stronger than at 1600 cm-1, which is attributable to a stronger guaiacyl element than syringyl at 1510cm-1. This result is similar to that reported by Colom et al. (2003).
The spectra differences between heat-treated and untreated woods have to be taken into consideration. Upon analysis of the spectra, it can be seen that the band at 1510 cm-1 which is assigned to lignin increases slightly after heat treatment for all species. This peak indicates splitting of the aliphatic side chains in lignin and cross-linking formation by condensation reactions of lignin, which can decrease the water absorption and consequently increase wood dimensional stability ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_31" \o "Kocaefe, 2008 #48" Kocaefe et al. 2008a). Another peak which has to be taken into consideration is the increase in C-O peak at 1103 cm-1 after heat treatment. This might suggest that the formation of new alcohols and esters, which can reduce the number of the free hydroxyl groups, increases hydrophobic character of wood ADDIN EN.CITE ADDIN EN.CITE.DATA ( HYPERLINK \l "_ENREF_31" \o "Kocaefe, 2008 #48" Kocaefe et al. 2008a; HYPERLINK \l "_ENREF_6" \o "Colom, 2003 #46" Colom et al. 2003).
Figures 6 (a-c) show the FTIR spectra in the spectral regions of heat-treated jack pine, aspen, and birch during artificial weathering, respectively. The overall IR spectrum of three heat-treated species indicate that a number of spectral features appear to be sensitive to weathering. As stated in Table 3, all the bands at 1600 cm-1, 1510 cm-1, 1465 cm-1, 1263 cm-1, and 1103 cm-1 represent lignin characteristics. As shown in Figures 6 (a-c), all these characteristic bands of lignin decrease significantly as a result of weathering process for all species. It indicates lignin is the component of heat-treated wood which is most degraded during weathering.
Out of the five bands mentioned above, the evolution of lignin loss is best explained by the peak at 1510 cm-1 of wood samples ADDIN EN.CITE Colom200346(Colom et al. 2003)464617Colom, X.Carrillo, F.Nogus, F.Garriga, P.Structural analysis of photodegraded wood by means of FTIR spectroscopyPolymer Degradation and StabilityPolymer Degradation and Stability543-549803LigninCelluloseFTIRCrystallinityWood component2003http://www.sciencedirect.com/science/article/B6TXS-48B5M71-1/1/3f1f1e399ec841d933d970168e94fe29( HYPERLINK \l "_ENREF_6" \o "Colom, 2003 #46" Colom et al. 2003). Figure 7 shows the lignin loss for heat-treated and untreated wood specimens for three species after artificial weathering at different times. Before weathering, heat-treated wood surfaces show higher lignin content than those of untreated wood surfaces for the same species. After artificial weathering of 72 h, lignin in all specimens becomes degraded although a large difference can be seen in the evolution of the degradation process. It is worth noting that the difference in lignin content between heat-treated and untreated wood reduces after weathering for 1512 h. All of the cell wall polymers such as cellulose, hemicelluloses, and lignin are hydroscopic. The order of hydroscopicity is: hemicellulose (HEMI) > cellulose > lignin (KLIG) ADDIN EN.CITE Skaar198442(Skaar 1984)424217Skaar, C.Wood-water relationshipsChemistry of Solid Wood. Adv. Chem.Chemistry of Solid Wood. Adv. Chem.127-1722071984( HYPERLINK \l "_ENREF_50" \o "Skaar, 1984 #42" Skaar 1984). Thus, the loss of lignin can increase the content of other components and consequently make the surface more hydrophilic. The same observation is reported by Kalnins and Feist (1993). They reported that contact angle measurements on weathered western red c e d a r d r o p p e d f r o m 7 7 t o 5 5 a f t e r f o u r w e e k s o f o u t d o o r w e a t h e r i n g . I t i s a l s o r e p o r t e d t h a t w e t t a b i l i t y f o r S i t k a s p r u c e i n c r e a s e d w h e n e x p o s e d t o x e n o n a r c r a d i a t i o n a n d w a t e r s p r a y A D D I N E N . C I T E <