J. Phys. Chem. C 2010, 114, 18693–18697
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Effect of Thermal Modification on Wood Cell Structures Observed by Pulsed-Field-Gradient Stimulated-Echo NMR Pa¨ivi M. Kekkonen, Ville-Veikko Telkki,* and Jukka Jokisaari Department of Physics, UniVersity of Oulu, P.O. Box 3000, FI-90014, Finland ReceiVed: June 30, 2010; ReVised Manuscript ReceiVed: September 8, 2010
Thermal modification is an environment friendly method for increasing the lifetime and usability of timber products. In our previous work (J. Phys. Chem. B 2009, 113, 1080), we introduced a pulsed-field-gradient stimulated-echo (PGSTE) NMR based method that enables determining the highly anisotropic size distribution of voids (pores) inside wood cell structures in three orthogonal directions. Here, we demonstrate that the method can be used to quantify the effect of thermal modification on the pore dimensions in Pinus sylVestris pine wood. The results show that the modification decreases the dimensions of lumens inside tracheid cells both in the longitudinal and two transverse directions. However, the relative decrease becomes smaller at the highest modification temperature, implying partial destruction of the cell wall structure. The decrease is larger in the radial direction than in the tangential direction at all the modification temperatures. Introduction Thermally modified timber products have been paid attention already for one century,1,2 and several treatment processes with different treatment time, temperature, atmosphere, and so forth have been developed.3 However, larger scale commercialization has arisen only during the past decade after development of the ThermoWood process.4 It is a three phase process in which the intensive modification takes place at a temperature of around 200 °C for a few hours in the presence of steam.3 Steam acts as a protective gas reducing the rate of oxidation processes, and it also accelerates the desired chemical changes in the wood. The main wood components respond differently to the modification as cellulose and lignin degrade slower and at higher temperatures than the hemicelluloses. As a result of the modification, swelling and shrinkage due to moisture decrease, biological durability improves, color darkens, equilibrium moisture content decreases, and thermal insulation properties improve.5 ThermoWood consumption quadrupled from 2003 to 2008, and it was about 80 000 m3 in 2008. From the total consumption, 80% is used in exterior cladding and decking, internal wall and ceiling panels, and internal flooring.6 Pine is the most used ThermoWood species. Pinus sylVestris consists of three main cell structures (Figure 1a): Boxlike, long and narrow longitudinal tracheid cells constitute the major part (93%) of the wood volume. Their average length and width are about 3 mm and 30 µm, respectively. The hollow interior of tracheid cells is called lumen. Other cell structures in wood are radial rays (6%) and longitudinal resin canals (1%).7 The structure of wood has been investigated by several NMR techniques9 including relaxation measurements,10 magnetic resonance imaging (MRI),11,12 position exchange spectroscopy (POXSY),13 NMR cryoporometry,14 and high resolution magic angle spinning (HRMAS).15 Chemical changes in wood at the molecular level and changes in the connectivity of wood cells caused by thermal modification have been studied using solid state NMR9,16-18 and remote detection NMR,19 respectively. * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
Figure 1. (a) Main cell structures of Pinus sylVestris (modified after refs 7 and 8). (b) FESEM image of REF180 sample. White doubleheaded arrows illustrate how the dimensions of lumens in different directions were determined from the FESEM images.
Inspections of the changes in the microstructure of wood caused by thermal modification have been mainly restricted to studies carried out with microscopic techniques, such as SEM,20
10.1021/jp1060304 2010 American Chemical Society Published on Web 10/07/2010
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that provide information only about the surface of a sample. Pulsed-field-gradient spin-echo (PGSE) NMR21 based diffusion experiments can provide structural information about the whole wood sample.22-25 Usually, liquids (in most cases water) are used as probe fluids in PGSE NMR investigations, in which case the maximum observable pore size is relatively small because of the small diffusion coefficient of liquids. However, gases (large diffusion coefficient) enable one to study much larger dimensions.26 In our previous work,27 we demonstrated that the combined use of liquids and gases expands the scale of the dimensions observable by pulsed-field-gradient stimulatedecho (PGSTE) NMR28 (variant of PGSE NMR) to over four orders of magnitude, covering all the interesting length scales in the wood cell structures. Here, we demonstrate that the method can be used to quantify the effect of thermal modification on the microstructure of Pinus sylVestris pine wood. Theoretical Background The solid matrix restricts the self-diffusion of a fluid absorbed into the wood, and this is reflected in the echo amplitudes measured by PGSTE NMR.29 If the probe fluid molecules are confined within a rectangular box (like a tracheid cell) with perfectly reflecting walls, and diffusion is studied in the direction parallel to one side of the box of length a, the amplitude of the echo observed in the PGSTE experiment is30 E(q, a) )
2[1 - cos(2πqa)] + (2πqa)2 ∞
4(2πqa)2
(
2 2
∑ exp - n πa D∆ n)1
2
)
1 - (-1)ncos(2πqa) [(2πqa)2 - (nπ)2]2
(1)
Here, q ) γδg/2π, where γ is the gyromagnetic ratio of the nucleus and δ and g are the length and amplitude of the gradient pulse, respectively. D is the molecular self-diffusion coefficient, and ∆ is the diffusion delay. If the sample contains a large amount of rectangular pores of various sizes, and if the pore size distribution is the sum of n Gaussian functions, the observed echo amplitude is n
Eobs )
∑ pi σ √12π ∫0 i)1
∞
i
(
exp
-(a - ai)2 2σi2
)
E(q, a)da
(2)
where pi, ai, and σi are the portion, mean value, and standard deviation of the ith component, respectively. Hence, the pore size distribution can be determined by measuring the echo amplitude as a function of q and fitting eq 2 to the data points. The observable pore size range depends on the D of the probe fluid, the ∆ used in the experiment, the narrow gradient pulse approximation, and the maximum q value.27 In this work, water and methane were used as probe fluids. With the parameters used in the experiments, the pore size range water can probe is about 10-300 µm, whereas it is from 300 µm to 10 mm for methane.27 Hence, water is an optimal probe for measuring the transverse wood cell structure dimensions whereas methane is best suited for measurements in the longitudinal direction. Experimental Methods The effect of thermal modification on the wood cell structures was investigated by comparing the pore size distributions measured from thermally modified and unmodified reference
samples. The sample preparation process was the following: First, a wood plank kiln-dried at 70 °C was split in the longitudinal direction into two pieces from which one was thermally modified. Then, a cylindrical sample with axis parallel to the longitudinal direction was drilled from both modified and reference pieces from the locations next to each other in the original wood plank (see Figure 1 in ref 18) in order to ensure the comparability of the results. The pieces were large (length 25 mm, diameter 8 mm) as compared with the microstructure of the wood, and they contained several annual rings. About half a million tracheid cells were estimated to be inside the sample volume studied by PGSTE NMR. Therefore, a representative statistical average of the properties of wood was observed in the experiments. One set of the sample pieces was immersed in water for two weeks before the experiments in order to get sufficient amount of water to be absorbed in the wood. Another set of the (dry) samples was in 1.7 atm methane atmosphere inside a flamesealed sample tube. Wood samples modified at three different temperatures (180, 200, and 240 °C) as well as their reference samples were investigated. The modified samples are referred to as MODX, where X is the modification temperature in Celsius, and the corresponding reference sample referred to as REFX. 1 H NMR experiments were carried out on a Bruker Avance DSX300 spectrometer equipped with a microimaging unit with x, y, and z gradients. The tangential, radial, and longitudinal directions of wood samples were oriented parallel with the x, y, and z gradients, respectively. In the PGSTE experiments, the echo amplitude was measured as a function of gradient amplitude keeping the diffusion delay constant. In the case of the methane samples, the gradient amplitude was increased from 0 to 50 G/cm using 64 steps. The gradient pulse length and the diffusion delay were 200 µs and 100 ms, respectively. In the case of the water samples, the gradient amplitude was increased from 0 to 71 G/cm using 16 gradient steps. The gradient pulse length and the diffusion delay were 1 and 200 ms, respectively. Least-squares adjustments of eq 2 to the data points were performed numerically using the Microsoft Office Excel program. The pore size distributions were assumed to be composed of either one (n ) 1 in eq 2) or two (n ) 2) components. When calculating E(q,a) (eq 1), only 10 first terms were taken into account from the infinite summation, because the other terms appeared negligible. Absorption of probe fluids in wood was observed by magnetic resonance imaging (MRI). Axial 1H spin-echo MRI images were measured using the Bruker Paravision program. Slice thickness in the z direction was 2 cm, and the resolution in both x and y directions was 125 and 141 µm for the methane and water samples, respectively. Echo time, TE, and repetition time, TR, were 2.0 and 500 ms, respectively. Signal of protons in solid matrix was decayed before observing the echo due to short relaxation time, and hence, the images reflect the amount of probe fluid absorbed in different parts of wood sample. For comparison, the tracheid cell lumen dimensions in the transverse directions were determined also from field emission scanning electron microscopy (FESEM) images (Figure 1b). Rectangular wood pieces whose area and thickness were about 1 cm2 and 0.5 cm, respectively, were cut for FESEM experiments. The surface of the pieces was first polished and then coated with platinum. Only early wood lumens were seen in the images as the late wood lumens were blocked by dust in cutting. The average dimensions of 7-19 lumens close to each
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Figure 2. Results of MRI and PGSTE measurements. (a) Methane samples. (b) Water samples. Along with MRI images, PGSTE echo amplitudes (EA) and the pore size distributions (PSD) in the longitudinal (lon), tangential (tan), and radial (rad) directions, resulting from the fits of eq 2 to the data points, for each sample are shown. In the case of the methane samples, the theoretical echo amplitude and PSD is a sum of two components l and r that are also presented in the plots.
TABLE 1: Results in the Longitudinal (lon) Directiona
REF180 MOD180 REF200 MOD200 REF240 MOD240
lon lon lon lon lon lon
al σl pl [mm] [mm] [%]
ar [mm]
σr pr [mm] [%]
3.53 2.91 3.76 2.76 2.88 2.46
0.27 0.17 0.27 0.29 0.093
0.21 0.14 0.32 0.13 0.18
0.71 0.61 0.71 0.83 0.78 0.76
52 98 49 100 87 98
48 2 51 13 2
R2 0.9992 0.9999 0.9990 0.9991 0.9999 0.9997
a The parameters resulting from the least-squares adjustments of eq 2 (n ) 2) to the PGSTE echo amplitudes measured from the methane samples and the coefficient of determination, R2. The subscript l refers to the lumen component, and subscript r refers to the second fitted component. The results weight lumen dimensions in early wood.
other were measured. Therefore, the results represent local average lumen sizes in early wood on the surface of the sample pieces. Results and Discussion MRI images measured from the samples reveal distinct absorption of the probe fluids in wood as methane is mainly absorbed in early wood whereas most of water is absorbed in late wood (see Figure 2). Consequently, the pore size distributions measured by PGSTE NMR using methane probe weight the dimensions in early wood, whereas water probe weights the late wood dimensions. However, after a longer immersion period, water is absorbed also in early wood (see Figure 3 in ref 27). The bright patches in the images of methane REF180 and REF200 samples may arise from residual moisture or extractives in the wood structure, possibly in resin canals.
Measured moisture content in the samples was approximately the same as in REF240 sample (∼6.5%), and hence, the latter option is more probable. The PGSTE echo amplitudes measured from the methane samples in the longitudinal direction as well as the least-squares adjustments of eq 2 to the data points are shown in Figure 2a. The pore size distributions were assumed to be composed of two components (n ) 2 in eq 2). The resulting values of the fit parameters are shown in Table 1, and the pore size distributions are shown in Figure 2a. The major component (subscript l) was interpreted to represent the lengths of lumens in tracheid cells as al is close to the average length of tracheid cells cited above, whereas the minor component (subscript r) was assumed to originate from methane in rays and residual moisture or extractives in the wood structures. The fitting parameters of the minor component does not provide reliable information about wood structure because the diameter of rays is smaller than the smallest observable dimension of the methane probe and the diffusion coefficient of the moisture or extractive molecules is different from that of methane. The component is largest for REF180 and REF200 samples probably because the residual moisture/extractive content reflected as patches in the MRI images is largest in these samples. Results of corresponding PGSTE experiments measured from the water samples in the tangential and radial directions are shown in Figure 2b and Table 2. In this case, one component fits gave satisfactory results, and the component reflects mainly the width distribution of lumens. PGSTE experiments show that the dimensions of lumens decrease in all the three orthogonal directions in thermal modification (see Figure 3 and Table 3). The cell wall micropores collapse when the wood is dried,3 and thermal modification may
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TABLE 2: Results in the Tangential (tan) and Radial (rad) Directionsa
REF180 MOD180 REF200 MOD200 REF240 MOD240 REF180 MOD180 REF200 MOD200 REF240 MOD240
tan tan tan tan tan tan rad rad rad rad rad rad
al FESEM [um]
σl FESEM [um]
al PGSTE [um]
σl PGSTE [um]
R2 PGSTE
20.2 28.3 22.9 21.4 22.1 15.5 39.6 30.2 35.1 30.1 32.3 29.1
6.3 3.6 3.8 6.3 6.5 3.0 5.6 4.2 6.3 4.2 5.1 7.1
28.1 22.8 27.4 20.3 22.5 22.3 31.9 24.4 28.2 19.6 27.4 24.8
11.6 9.5 13.8 8.4 7.8 12.1 15.4 12.5 19.2 11.1 10.5 17.8
0.995 0.999 0.992 0.999 0.997 0.991 0.997 0.998 0.996 0.998 0.999 0.994
a FESEM results and the parameters resulting from the least-squares adjustments of eq 2 (n ) 1) to the PGSTE echo amplitudes measured from the water samples as well as the coefficient of determination, R2. The FESEM dimensions were measured from early wood whereas the PGSTE results weight lumen dimensions in late wood.
close the pores permanently preventing the wood swelling even when it is exposed to moisture. This may cause the observed decrease in lumen dimensions. The relative decrease is larger in the radial direction than in the tangential direction at all the modification temperatures (see Figure 3b). Another interesting discovery is that the relative decrease is largest for the sample modified at intermediate temperature (200 °C) in all the directions (see Figure 3b), implying that, at first, the lumen dimensions decrease with increasing modification temperature but then, after a critical temperature, they begin to increase. The increase may indicate that the tracheid cell walls begin to be destroyed around 200 °C, because when the probe molecules can diffuse through the broken cell walls, the apparent lumen dimensions observed by PGSTE NMR increase. The decomposition of the cell walls at high temperatures was observed also by SEM.20 In the transverse directions, the standard deviation representing the width of the lumen size distribution
Figure 3. (a) Mean lumen size al and standard deviation σl representing the width of the lumen size distribution for each sample in the three orthogonal directions. (b) Percentage change of the mean lumen size in thermal modification. Gray columns show the FESEM results. Note, that the column of the positive change (+39.6%) measured by FESEM from MOD180 sample in the tangential direction was omitted in order to save plot space.
Effects on Wood Cells Observed by PGSTE NMR TABLE 3: Percentage Changes in the Lumen Dimensions (∆al) and Standard Deviations (∆σl) Representing the Width of the Pore Size Distribution in Thermal Modification Measured by PGSTE NMR and FESEMa 180 200 240 180 200 240 180 200 240
∆al FESEM [%]
∆al PGSTE [%]
∆σl PGSTE [%]
39.60 -6.34 -29.77 -23.73 -14.29 -10.00
-17.56 -26.60 -14.58 -18.71 -26.08 -1.02 -23.30 -30.55 -9.60
-2.83 3.19 -0.69 -7.38 -19.59 18.95 -9.20 -28.86 26.69
lon lon lon tan tan tan rad rad rad
a The values were calculated by the following equations: ∆al ) {[al(MOD) - al(REF)]/al(REF)}×100% and ∆σl ) {[σl(MOD) σl(REF)]/σl(REF)}×100%.
is smaller in the thermally modified samples than in the reference samples at the two lowest modification temperatures, but it becomes larger at the highest temperature (Figure 3a). This may also be a consequence of the decomposition of the cell walls as the diffusion of probe fluid through the cell walls increases the apparent width of the size distribution. However, the trend is not as clear in the longitudinal direction. The lumen dimensions measured from FESEM images are on average close to the values measured by PGSTE NMR (see Figure 3a) confirming that PGSTE results are reliable, even though in the analysis it was assumed that the walls of the pores are perfectly reflecting and the effect of wall relaxation31 was ignored (see discussion about the assumptions in ref 27). Some values measured from individual samples differ quite much because FESEM measures only the local average pore size on the surface of the sample piece whereas PGSTE NMR reveals the pore size distribution of the whole sample volume inside the NMR coil. The FESEM lumen widths in the radial direction are systematically larger than the PGSTE widths because FESEM measures dimensions in early wood whereas PGSTE weights the dimensions in late wood, and as illustrated in Figure 1a, the lumen widths are much larger in early wood than in late wood in the radial direction. Apart from one exception, FESEM results support the conclusion that thermal modification decreases the lumen sizes (Figure 3b). However, no clear trend in the lumen sizes as a function of modification temperature can be seen because of the low statistical accuracy in FESEM measurements. Conclusions This work demonstrates that the combined use of liquids and gases makes PGSTE NMR a powerful tool in material research and specifically in determining the effect of thermal modification on the structure of wood. There are various NMR methods, such as NMR relaxometry,32 NMR cryoporometry,33 xenon NMR,34 and xenon porometry,35-39 that can be used for determining pore sizes. However, they cannot probe as large of a range of dimensions (over four orders of magnitude) and provide as detailed anisotropic information about pore geometry as the method used in this work. The measurements reveal that the lumens become smaller in thermal modification. In addition, they indicate that, above a critical temperature, which was determined to be around 200 °C in the case of Pinus sylVestris, the size begins to increase because of the decomposition of the cell walls. Hence, this method can be used for improving the ThermoWood process by determining an optimal modification temperature that is as high as possible in order to maximize the dimensional stability and biological durability of wood but just
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18697 below the critical temperature in which the wood structure begins to destroy for different wood species. Acknowledgment. This work was supported by the Academy of Finland (grants no. 116824 and 123847) and Tauno To¨nning Foundation. References and Notes (1) Tiemann, H. D. Lumber World ReV. 1915, 28, 19–20. (2) Stamm, A. J.; Seborg, R. M. Ind. Eng. Chem. 1937, 29, 831–833. (3) Hill, C. A. S. Wood Modification; Wiley: Chichester, U.K., 2006. (4) Viitaniemi, P.; Ja¨msa¨, S.; Ek, P.; Viitanen, H. Method for improving biodegradation resistance and dimensional stability of cellulosic products. U.S. Patent 5678324. (5) ThermoWood Handbook. www.thermowood.fi, Finnish ThermoWood Association 2003. (6) Ala-Viikari, J.; Mayes, D. New Generation ThermoWood - How to Take ThermoWood to the Next LeVel; Proceedings European Conference on Wood Modification, 2009. (7) Kettunen, P. O. Structure and Properties of Wood; Trans Tech Publications Ltd: Switzerland, 2006. (8) Howard, E. T.; Manwiller, F. G. Wood Sci. 1969, 2, 77–86. (9) Maunu, S. L. Prog. Nucl. Mag. Res. Sp. 2002, 40, 151–174. (10) Cox, J.; McDonald, P. J.; Gardiner, B. A. Holzforschung 2010, 64, 259–266. (11) Oven, P.; Merela, M.; Mikac, U.; Sersˇa, I. Holzforschung 2008, 62, 322–328. (12) Eberhardt, T. L.; So, C.-L.; Protti, A.; So, P.-W. Holzforschung 2009, 63, 75–79. (13) Telkki, V.-V.; Jokisaari, J. Phys. Chem. Chem. Phys. 2009, 11, 1167–1172. (14) Viel, S.; Capitani, D.; Proietti, N.; Ziarelli, F.; Segre, A. L. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 357–361. (15) Koenig, A. B.; Sleighter, R. L.; Salmon, E.; Hatcher, P. G. J. Wood Chem. Technol. 2010, 30, 61–85. (16) Tjeerdsma, B.; Boonstra, M.; Pizzi, A.; Tekely, P.; Militz, H. Holz Roh. Werkst. 1998, 56, 149–153. (17) Kosikova´, B.; Hricovı´ni, M.; Cosentino, C. Wood Sci. Technol. 1999, 33, 373–380. (18) Sivonen, H.; Maunu, S. L.; Sundholm, F.; Ja¨msa¨, S.; Viitaniemi, P. Holzforschung 2002, 56, 648–654. (19) Telkki, V.-V.; Saunavaara, J.; Jokisaari, J. J. Magn. Reson. 2010, 202, 78–84. (20) Viitaniemi, P.; Ja¨msa¨, S. Modification of Wood with Heat Treatment; VTT Julkaisuja - Publikationer 814: Espoo, Finland, 1996. (21) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288–292. (22) Wycoff, W.; Pickup, S.; Cutter, B.; Miller, W.; Wong, T. C. Wood Fiber Sci. 2000, 32, 72–80. (23) Johannessen, E. H.; Hansen, E. W.; Rosenholm, J. B. J. Phys. Chem. B 2006, 110, 2427–2434. (24) Meder, R.; Codd, S. L.; Franich, R. A.; Callaghan, P. T.; Pope, J. M. Holz Roh. Werkst. 2003, 61, 251–256. (25) Hietala, S.; Maunu, S. L.; Sundholm, F.; Ja¨msa¨, S.; Viitaniemi, P. Holzforschung 2002, 56, 522–528. (26) Mair, R. W.; Wong, G. P.; Hoffman, D.; Hu¨rlimann, M. D.; Patz, S.; Schwartz, L. M.; Walsworth, R. L. Phys. ReV. Lett. 1999, 83, 3324–3327. (27) Kekkonen, P.; Telkki, V.-V.; Jokisaari, J. J. Phys. Chem. B 2009, 113, 1080–1084. (28) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523–2526. (29) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Clarendon Press: Oxford, U.K., 1991. (30) Tanner, J. E.; Stejskal, E. O. J. Chem. Phys. 1968, 49, 1768–1777. (31) Coy, A.; Callaghan, P. T. J. Chem. Phys. 1994, 101, 4599–4609. (32) Stallmach, F.; Ka¨rger, J. Adsorption 1999, 5, 117–133. (33) Petrov, O. V.; Furo´, I. Prog. Nucl. Magn. Reson. Spectrosc. 2009, 54, 97–122. (34) Terskikh, V. V.; Moudrakovski, I. L.; Breeze, S. R.; Lang, S.; Ratcliffe, C. I.; Ripmeester, J. A.; Sayari, A. Langmuir 2002, 18, 5653–5656. (35) Telkki, V.-V.; Lounila, J.; Jokisaari, J. J. Phys. Chem. B 2005, 109, 757–763. (36) Telkki, V.-V.; Lounila, J.; Jokisaari, J. J. Phys. Chem. B 2005, 109, 24343–24351. (37) Telkki, V.-V.; Lounila, J.; Jokisaari, J. J. Chem. Phys. 2006, 124, 034711. (38) Telkki, V.-V.; Lounila, J.; Jokisaari, J. Phys. Chem. Chem. Phys. 2006, 8, 2072–2076. (39) Telkki, V.-V.; Lounila, J.; Jokisaari, J. Magn. Reson. Imaging 2007, 25, 457–460.
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