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XPS and IGC characterization of steam treated triticale straw
Applied Surface Science 257 (2010) 180–185
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XPS and IGC characterization of steam treated triticale straw Liyan Zhao ∗ , Yaman Boluk 1 Cellulose and Hemicellulose Program, Forest Products, Alberta Innovates–Technology Futures, 250 Karl Clark Road, Edmonton, AB T6N 1E4 Canada
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 31 May 2010 Accepted 21 June 2010 Available online 30 June 2010 Keywords: Triticale straw Steam treatment X-ray photoelectron spectroscopy Inverse gas chromatography (IGC)
a b s t r a c t The surface chemical composition and surface energy of native and steam treated triticale straws have been investigated by X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC) to reveal the effect of steam treatment temperature and time. The XPS results show that the contents of C elements and C–C group on the exterior surface of native triticale straw are much higher than those on the interior surface, indicating that there was a high quantity of wax on the exterior surface of the native triticale straw. Upon steam treatment, both carbon levels and C–C groups reduce with increasing steam temperature and treatment time of the exterior surfaces. However, the effect of steam treatment on the interior surface is very limited. In terms of the surface acid and base properties, the steam treated samples exhibited higher acid and base properties than the native sample, indicating a more polar surface of the steam treated sample. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Triticale, the first man-made hybrid crop of wheat and rye, has attracted a lot of attention in Canada recently. In 2005, the Canadian Triticale Biorefinery Initiative (CTBI) was set up. The mission of CTBI is to develop triticale as a dedicated industrial biorefining crop for Canada. In fact, triticale provides many attributes that the emerging bioindustrial sector is looking for. Firstly, with a special designed breeding, triticale grain has higher protein content than wheat and good disease and environmental tolerance like rye. Secondly, potential exists to produce high yield triticale varieties which make it better feedstock for chemicals and fuels production than corn. Thirdly, the triticale straw has higher fiber content than other agricultural straws such as barley and oat straws. This makes it a good raw material for the production of pulp, fiber and composite board, etc. As a new bioindustrial raw material, the characterization of triticale straw is very scarce in the literature. Since triticale is the hybrid of wheat and rye, it is expected that the chemical compositions of triticale straw are similar to those of wheat straw. The major chemical components of wheat straw include cellulose, hemicellulose, lignin and secondary metabolites. It has been found that the main difference between wheat straw and wood is wheat straw’s high wax content, concentrated primarily on the surface. This surface layer reduces the moisture absorbance of straw from water and affects its wettability. Without treatment, it limits the utiliza-
∗ Corresponding author. Tel.: +1 780 450 5226; fax: +1 780 450 5397. E-mail address: [email protected] (L. Zhao). 1 Present address: Nanofibre Chair in Forest Products, Department of Civil & Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2W2. 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.060
tion of wheat straw in some applications such as composite board because the wax layer reduces the adhesion of water-based resin to the straw surface [1–4]. It has been reported that steam treatment improved the wettability and bonding property of wheat and reed straw with water-based resin because the steam treatment destroyed the wax layer of the straw [4]. Therefore, in order to develop applications for triticale straw, it is necessary to characterize its surface composition and surface property. This is the objective of this work. In particular, the exterior and interior of triticale straws will be characterized separately to determine their differences. Moreover, the triticale straws will be treated by steam at different temperatures and duration of treatment. The effect of steam treatment on the surface composition and surface property is also studied. Two analytical techniques were adopted in this work: X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC). XPS is particularly suitable for determination of surface chemical composition of different material surfaces. Numerous studies have been reported to study the surface chemistry analysis of wood, wool, pulp and other lignocellulosic materials [5–9]. For example, Bradley and Mathieson [5] employed XPS to investigate the surface oxygen levels of native and UV-exposed wool fiber surfaces. Inari et al. [6] studied the effect of heat treatment on the chemical composition of wood surface using XPS. Inverse gas chromatography (IGC) has been proved to be a very powerful tool to evaluate the wetting, adhesion and bonding potential of fiber surfaces in different polymeric matrices [10]. In particular, this technique has been widely used to investigate the surface properties of different lignocellulosic fibers and pulps [11–16]. Different from XPS, which has been used to analyze the chemical composition, IGC characterizes the surface energy including surface dispersive force and Lewis acid and base forces by
L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
181
Table 1 Properties of the probes used in IGC experiment. Probe
a (Å)2
LD (mJ/m2 )
AN* (kJ/mol)
DN (kJ/mol)
Specific characteristic
n-Hexane n-Heptane n-Octane n-Nonane Ethyl acetate Acetone Methylene chloride Chloroform THF
51.5 57.0 62.8 68.9 48.0 42.5 31.5 44.0 45.0
18.4 20.3 21.3 22.7 19.6 16.5 27.6 25.9 22.5
0 0 0 0 6.3 10.5 22.6 2.1 16.3
0 0 0 0 71.6 71.2 0 0 84
Non-polar Non-polar Non-polar Non-polar Amphoteric Amphoteric Acidic Acidic Basic
measuring the interaction between well defined probe molecules and test surfaces. For example, Liu et al. [17], studied the relationship of surface energy of wood meal with surface composition due to the various treatments. Tshabalala and Han [13] investigated the effect of solvent extraction on surface energy of kenaf powder and found that the acceptor number of kenaf extracted with toluene/ethanol, followed by ethanol and water, was significantly increased compared to that of unextracted kenaf. 2. Materials and methods 2.1. Materials The feedstock material is the industrial triticale straw. The moisture content was determined to be 6.44%. Stock material was dried in oven at 105 ◦ C before use. The non-polar probe liquids used for IGC were n-hexane, n-heptane, n-octane and n-nonane. The polar probes including ethyl acetate, acetone, methylene chloride, chloroform and tetrahydrofuran (THF) were selected based on their electron pair acceptor and donor characteristics. They were purchased from Sigma–Aldrich. Since these solvents were analytical grade solvents, they were used as received without further purification. The physical properties of the probe solvents are listed in Table 1 [18–21]. 2.2. Surface treatments of straws Ten grams of triticale straw were treated with steam using an autoclave. Four one-stage treatments and one two-stage treatment have been carried out. For the one-stage treatment, two steam pressures have been adopted, which were 0.6 and 1.3 MPa. The treated times for 0.6 MPa were 5 and 10 min, and 2.5 and 5 min for 1.3 MPa. For the two stage, the triticale straw was treated at 6 kPa for 30 min and then 1.3 MPa for 2.5 min. In two-stage treatment, the straw sample was first cleaned in the first stage, and was treated with steam in the second stage.
separately by X-ray photoelectron spectroscopy (XPS) using a spectrometer (Model AXIS 165, Kratos Analytical, Japan) with a dual Mg and Al X-ray source. In XPS measurements, the sample is irradiated with soft X-ray photons and chemical compositions and functional groups about the top 2–20 atomic layers on the sample surface can be obtained. For composition analysis, a survey spectrum was measured with pass energies of 100 eV, while for high resolution scans, a pass energy of 10 eV was used. Chemical compositions were calculated from the area of the corresponding photoelectron peak for each element. The compositions of chemical bond were obtained by deconvolution of the respective spectra. In the experiment, the straw sample was mounted on a sample holder and during the measurement, the pressure inside the analysis chamber was maintained below 5 × 10−6 Pa. 2.3.2. Surface dispersive and acid/base properties Inverse gas chromatography was used to characterize the dispersive and acid/base properties of native and treated straw samples. The GC column was prepared by the standard procedures [14]. The column dimensions and detailed operation conditions were summarized in Table 2. The IGC experiments were carried out using a Varian 3380 Gas Chromatograph equipped with a flame ionization detector (FID). Pre-purified helium was used as the carrier gas. The flow rate was measured at the corresponding experimental temperatures with the use of a soap bubble flow-meter. The inlet and outlet pressures of the column were monitored with pressure gauges during the experiments. For each solvent, three injections of 1 L of its vapor were made to measure the retention time with a reproducibility of within 3%. The net retention times were determined using methane as the marker. Three injections were made for each probe at each of the following three temperatures: 40 ◦ C, 50 ◦ C, 60 ◦ C and 70 ◦ C. Methane was used as noninteracting marker to determine the column gas holdup. 3. Results and discussion 3.1. Surface chemical composition
2.3. Surface characterization 2.3.1. Surface chemical composition Since triticale straw has a tubular structure, the exterior and interior surfaces of native and treated straws were characterized
From the XPS survey spectra, the atomic composition of the investigated sample surface was calculated based on the peak area of each species. The atomic compositions of exterior and interior surfaces of native and treated triticale straw samples were summa-
Table 2 Dimensions and operating conditions of the GC columns packed with different samples. Native triticale straw
Steam treated 160 ◦ C 5 min
Steam treated 160 ◦ C 10 min
Steam treated 190 ◦ C 2.5 min
Steam treated 190 ◦ C 5 min
Steam treated two stages
Weight of sample (g) Column pressure (kPa) Flow rate (mL/min)
2.17 184 20.01
2.74 170 18.01
2.26 149 21.40
1.96 152 20.03
1.96 163 20.70
1.34 135 21.43
Column length Column temperature Carrier gas
0.5 m 40 ◦ C, 50 ◦ C , 60 ◦ C and 70 ◦ C Pre-purified helium
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L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
Table 3 Surface composition (at.%) of the (a) exterior surfaces and (b) interior surfaces of native and treated wheat straw samples. Sample (a) Native triticale straw—exterior surface Steam treated triticale straw (160 ◦ C 5 min)—exterior surface Steam treated triticale straw (160 ◦ C 10 min)—exterior surface Steam treated triticale straw (190 ◦ C 2.5 min)—exterior surface Steam treated triticale straw (190 ◦ C 5 min)—exterior surface Steam treated triticale straw (two stages)—exterior surface (b) Native triticale straw—interior surface Steam treated triticale straw (160 ◦ C 5 min)—interior surface Steam treated triticale straw (160 ◦ C 10 min)—interior surface Steam treated triticale straw (190 ◦ C 2.5 min)—interior surface Steam treated triticale straw (190 ◦ C 5 min)—interior surface Steam treated triticale straw (two stages)—interior surface
C
O
N
Ca
S
Si
O/C
94.28 85.15 76.25 78.12 73.91 70.58
5.66 12.61 18.45 18.56 23.93 27.06
0.02 1.18 1.15 0.8 0.98 0.46
0 0.65 2.62 1.28 0.63 1.33
0 0.10 0.44 0.33 0.33 0.14
0.04 0.31 0.73 0.91 0.22 0.42
0.06 0.15 0.24 0.24 0.32 0.38
63.11 64.59 65.45 68.75 62.43 66.26
35.89 33.56 32.02 30.17 36.09 31.26
0.98 1.04 1.06 0.96 1.38 0.60
0.01 0.55 0.62 0 0.04 1.09
0 0.19 0.07 0.06 0.06 0.32
0 0.08 0.24 0.06 0 0.47
0.57 0.52 0.49 0.44 0.58 0.47
rized in Table 3(a) and (b). It can be seen that for all samples carbon and oxygen are the predominant species along with traces of nitrogen, silicon and sulfur. The exterior surface of native triticale straw is composed of approximately 94% carbon and 5% oxygen, while the interior surface is composed of 63% carbon and 36% oxygen. Steam treatment of native straw reduced the carbon content of exterior surfaces to 85–70% and increased the oxygen content to 12–27%. Similar trend was not observed on the interior surfaces of steam treated samples. There is no obvious change in carbon and oxygen contents compared to the native sample. The O/C ratio, which was determined from the peak area of oxygen and carbon, was also listed in Table 3(a) and (b). It has been reported that the theoretical O/C ratio of cellulose and hemicellulose is 0.83 and that of lignin is 0.33 [21,22]. The O/C ratio of exterior surface of the native triticale straw is only 0.06, which is much lower than the O/C ratios of cellulose, hemicellulose and lignin. This result indicates that the major components of the exterior surface of the native straw are not cellulose, hemicellulose or lignin. The surface is covered with a highly aliphatic layer, which is also called wax layer. On the other hand, the O/C ratio of the interior surface of the native straw is 0.57, which is the average of the O/C ratios of cellulose/hemicellulose and lignin. Obviously this result reflects absence of wax layer on the interior surface. The O/C ratio of mechanical processed pulp of wheat straw was reported to be 0.24, which is about average of O/C values of the exterior and interior surfaces of triticale straw [23]. This is because the pulp used in the literature was a mechanically defibrillated fibers and the pulp surface is a mixture of exterior and interior surfaces. The O/C ratios of exterior surfaces of steam treated samples are four to six times higher than that of the native sample. The values of O/C ratio increase with increasing treatment temperature and time (Fig. 1). The increase in O/C ratio following steam treatment is due to the removal of carbon-rich wax layer. However, the O/C ratios of interior surfaces of steam treated samples are slightly lower than that of the native sample. This result indicates that steam treatment has little effect on the wax-free surface. Detailed XPS C1s core energy spectra of exterior and interior surfaces of native straw are shown in Fig. 2(a) and (b). C1s can be deconvoluted into four Gaussian peaks, which were found to be at about 284.8 eV, 286.3 eV, 287.9 eV and 289.0 eV, corresponding to C–C/C–H, C–O, C O and COOH groups, respectively [7]. The C1s spectra of surfaces of steam treated straws were also deconvoluted into four peaks and the graphs were omitted for clarity. The relative area of each peak was calculated and listed in Table 4(a) and (b). It can be found that the composition of chemical groups of exterior surface is very different from the interior surface for both native and steam treated samples. For exterior surfaces, the content of C–C/C–H group is much higher than C–O group. However, the content of C–C/C–H group of interior surfaces is comparable to
the content of C–O group. These results indicate that the exterior surfaces are more non-polar than the interior surfaces. Moreover, the interior surfaces exhibit a higher content of −COOH group than the exterior surfaces, suggesting that the interior surfaces are more polar than the exterior surfaces. From Table 4(a), it is obvious that the contents of C–C/C–H group on the exterior surfaces of the steam treated samples are lower than that of the native sample, while the content of C–O group is higher. These results suggested that steam treatment is an effective method to remove the wax layer on the exterior surface of triticale straw. Furthermore, increasing treatment temperature and extending treatment time made the steam treatment more efficient to reduce the content of C–C/C–H group and increase the content of C–O group. As for the effect of steam treatment on interior surface, it can be seen from Table 4(b) that both C–C/C–H group and C–O group show little difference between treated and native samples. This means that the effect of steam treatment on interior surface is very limited. 3.2. Surface dispersive and acid/base properties Since the theory of IGC has been well developed and reviewed in the literature [24], the detailed derivation and calculation of surface dispersive force and acid and base numbers are omitted and only the critical equations are described here. The specific retention volume, Vg0 , is the direct data obtained from IGC experiment [10]: Vg0 =
273.15tn FJ wT
(1)
Fig. 1. Effects of time and temperature of steam treatment on O/C ratio of exterior surface.
L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
183
1/2
Fig. 3. Plots of −RT ln Vg0 versus 2Na LD
at four temperatures for native triticale
straw.
The interactions of polar probes with solid surface include both dispersive and acid/base contributions, GA0 = GAD + GAS , where GAD and GAS are the dispersive and Lewis acid/base contributions to the total Gibbs free energy of adsorption. The dispersive component can be calculated from Eq. (2) with molecular area and dispersive energy of polar probes. GAS can be written as follows:
GAS = −RT ln Vg0 − 2NaP SD
= −RT ln
+ C = 2Na
1/2 SD
1/2 LD
1/2
(3)
GAS = HAS − TSAS HAS
(4)
SAS
Here, and are enthalpy and entropy of adsorption corresponding to the acid/base interaction, respectively. The plot of GAS /T versus 1/T should yield a straight with the slope of HAS . According to Papirer’s approach, the Lewis acid (KA ) and base (KD ) numbers, which are used to describe the acidic and basic characteristics of the solid surface, are calculated using the following equation [26]:
where tn is the net retention time, which is the difference of the retention times between a selected solvent and the marker; F is the flow rate of the carrier gas measured at the experimental temperature T; w is the mass of the test material in the column; and J is the James–Martin correction factor that is used to correct for the pressure gradient across the column [25]. From Vg0 , the dispersive component (SD ) of the solid surface energy with non-polar solvent can be calculated from the following equation: Vg0
D L,P
D is the diswhere aP is the surface area of one polar molecule; L,P persive energy of polar probe. Thermodynamically, the Gibbs free energy of adsorption corresponding to the acid/base interaction is related to the enthalpy of adsorption:
Fig. 2. XPS C1s spectra with peak deconvolution of native wheat straw (a) exterior surface, (b) interior surface.
GAD
1/2
−HAS = KA DN + KD AN∗
(5)
where AN* and DN are the acceptor and donor numbers of the polar probe, respectively. The plot of −HAS /AN∗ versus DN/AN* gives a straight line with a slope of KA and an intercept of KD [27].
(2)
where GAD is the dispersive contribution to the total Gibbs free energy of adsorption; SD and LD are the dispersive energies of the solid and liquid surfaces, respectively; N is the Avogadro number; a is the surface area covered by one alkane molecule.
3.2.1. Dispersive force of triticale straw
1/2
of native Fig. 3 shows the plot of −RT ln Vg0 versus 2Na LD triticale straw for four n-alkane probes at four different tempera-
Table 4 Surface bond compositions of C1s of the (a) exterior surfaces and (b) interior surfaces of native and treated wheat straw samples. Sample (a) Native triticale straw—exterior surface Steam treated triticale straw (160 ◦ C 5 min)—exterior surface Steam treated triticale straw (160 ◦ C 10 min)—exterior surface Steam treated triticale straw (190 ◦ C 2.5 min)—exterior surface Steam treated triticale straw (190 ◦ C 5 min)—exterior surface Steam treated triticale straw (two stages)—exterior surface (b) Native triticale straw—interior surface Steam treated triticale straw (160 ◦ C 5 min)—interior surface Steam treated triticale straw (160 ◦ C 10 min)—interior surface Steam treated triticale straw (190 ◦ C 2.5 min)—interior surface Steam treated triticale straw (190 ◦ C 5 min)—interior surface Steam treated triticale straw (two stages)—interior surface
C–C/C–H (%)
C–O (%)
C O (%)
COOH (%)
85.01 80.78 76.48 74.69 69.09 60.54
8.74 14.44 17.80 20.46 24.74 32.14
5.12 3.34 3.02 3.34 3.68 4.83
1.12 1.44 2.69 1.53 2.49 2.48
39.89 42.88 47.18 50.96 44.37 43.69
45.18 44.21 41.05 38.93 43.65 44.61
14.09 10.35 9.67 8.55 10.43 6.85
0.02 2.56 2.01 1.55 1.55 4.86
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L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
Table 5 Dispersive force of triticale straws, SD (mJ/m2 ).
4. Conclusions
Sample
40 ◦ C
50 ◦ C
60 ◦ C
70 ◦ C
Native triticale straw Steam treated triticale straw (160 ◦ C 5 min) Steam treated triticale straw (160 ◦ C 10 min) Steam treated triticale straw (190 ◦ C 2.5 min) Steam treated triticale straw (190 ◦ C 5 min) Steam treated triticale straw (two stages)
38.29 38.63 39.78 37.07 37.82 38.31
34.63 36.08 35.11 35.25 36.21 35.63
33.32 31.34 33.31 33.73 33.69 33.2
30.11 30.06 29.79 30.82 29.59 27.4
tures. These curves show linear regression lines with a least-square correlation coefficient of at least 0.99 in all cases. Similar plots for steam treated samples were also obtained and omitted here. The dispersive components of surface energy, which were calculated from the slope of these curves, are summarized in Table 5. As shown in Table 5, the values of dispersive energy of native triticale straw ranged from 38 mJ/m2 at 40 ◦ C to 30 mJ/m2 at 70 ◦ C, which are comparable to the results for reef and kenaf and slightly higher than wheat straw in the literature. Mills et al. [28] reported the dispersive energy for reef and wheat straw being 37.2 mJ/m2 and 35 mJ/m2 at 40 ◦ C, respectively and Tshabalala reports a value of 40 mJ/m2 (36.9 mJ/m2 ) at 40 ◦ C for Kenaf [13,14]. The dispersive energy deceased with the increase in temperature for both native and steam treated samples. It can be seen that steam treatment did not change the values of SD significantly at all temperatures. 3.2.2. Acid/base characteristics In terms of acid–base property, the KA and KD values of native and steam treated samples were listed in Table 6. It can be seen that the KA number is around 0.03 and the KD number is in the range from 0.18 to 0.33. The KA number is lower than the KD number for all the samples, which means that all samples show strong basicity and weak acidity. This property is attributed to the strong basicity and weak acidity of cellulose and hemicellulose. It has been reported that wheat straw and reed also showed the same acid/base properties [28]. Moreover, KA and KD values of steam treated samples are slightly higher than those of the native sample, indicating that the removal of relatively hydrophobic wax layer from wheat straw can increase the free energy of its surface. It seems that increasing steam temperature can increase both KA and KD values. However, there is no such trend for increasing treatment time. The reason for this result is not clear. These findings are consistent with our chemical composition results obtained from XPS. It is worth pointing out that in IGC measurement the exterior surface and the interior surface cannot be examined separately. The KA and KD values are the combinational results of both surfaces. From XPS results we can find that the steam treatment only affected the chemical composition of the exterior surface. This explains why steam treatment did not change KA and KD values dramatically. According to the theory developed by Fowkes [29], the increase in KA and KD values of steam treated straw indicates that the steam treated straw surface is more polar than the native straw. Consequently, the adhesion of the treated straw with resin would be stronger than that between the native straw and resin.
Table 6 Acid–base numbers of different triticale straws. Sample
KA
KD
Native triticale straw Steam treated triticale straw (160 ◦ C 5 min) Steam treated triticale straw (160 ◦ C 10 min) Steam treated triticale straw (190 ◦ C 2.5 min) Steam treated triticale straw (190 ◦ C 5 min) Steam treated triticale straw (two stages)
0.030 0.035 0.035 0.038 0.036 0.037
0.18 0.26 0.23 0.31 0.33 0.28
In this work, XPS and IGC have been successfully used to characterize the surface chemical composition and surface energy of steam treated triticale straw. The results presented here illustrate that there was a high quantity of wax on the exterior surface of the native triticale straw. Steam treatment can effectively remove the wax layer, which makes it possible to bond with water-based resin such as urea–formaldehyde resin instead of the expensive isocyanate resin. Increasing treatment temperature and time can effectively reduce the amount of wax on the exterior surface. However, the steam treatment has little effect on the composition of the interior surface of triticale straw. In terms of the surface acid and base properties, the steam treated samples exhibited higher acid and base properties than the native sample, indicating a more polar surface of the steam treated sample. Our results indicate that steam treated triticale straw shows a good potential in straw composite board applications. Acknowledgements The authors thank Ms. Heather Lorenz for performing the IGC measurements. The financial support from the Alberta Forestry Research Institute is gratefully acknowledged. References [1] G. Han, C. Zhang, D. Zhang, K. Umemura, S. Kawai, Upgrading of UF-bonded reed and wheat straw particleboard using silane coupling agents, J. Wood Sci. 44 (1998) 282–286. [2] G. Han, K. Umemura, S. Kawai, H. Kajita, Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments, J. Wood Sci. 45 (1999) 299–305. [3] G. Han, K. Umemura, M. Zhang, T. Honda, S. Kawai, Development of highperformance UF-bonded reed and wheat straw medium-density fiberboard, J. Wood Sci. 47 (2001) 350–355. [4] J.M. Lawther, R. Sun, W.B. Banks, Effect of steam treatment on the chemical composition of wheat straw, Holzforschung 50 (1996) 365–371. [5] R.H. Bradley, I. Mathieson, Chemical interactions of ultraviolet light with wool fiber surfaces, J. Colloid Interf. Sci. 194 (1997) 338–343. [6] G.N. Inari, M. Petrissans, J. Lambert, J.J. Ehrhardt, P. Gerardin, XPS characterization of wood chemical composition after heat-treatment, Surf. Interf. Anal. 38 (2006) 1336–1342. [7] G. Sinn, G.S. Reiterer, S.E. Stanzl-Tschegg, Surface analysis of different wood species using X-ray photoelectron spectroscopy (XPS), J. Mater. Sci. 36 (2001) 4673–4680. [8] L.S. Johansson, J.M. Campbell, K. Koljonen, P. Stenius, Evaluation of surface lignin on cellulose fibers with XPS, Appl. Surf. Sci. 145 (1999) 92–95. [9] K. Li, D.W. Reeve, Sample contamination in analysis of wood pulp fibers with X-ray photoelectron spectroscopy, J. Wood Chem. Technol. 24 (2004) 183–200. [10] J.R. Conder, C.L. Young, Physicochemical Measurement by Gas Chromatography, Wiley-Interscience, New York, 1979. [11] D.P. Kamdem, S.K. Bose, P. Luner, Inverse gas chromatography characterization of birch wood meal, Langmuir 9 (1993) 3039–3044. [12] H.F. Wang, B. Li, B.L. Shi, Preparation and surface acid–base properties of porous cellulose, Bioresources 3 (2008) 3–12. [13] M.A. Tshabalala, J.S. Han, Effect of solvent extraction on surface energy of kenaf powder, in: Kenaf Properties, Processing and Products, 1999, pp. 121–131. [14] M.A. Tshabalala, Determination of the acid–base characteristics of lignocellulosic surfaces by inverse gas chromatography, J. Appl. Polym. Sci. 65 (1997) 1013–1020. [15] W.T. Tze, D.J. Gardner, Contact angle and IGC measurements for probing surface–chemical changes in recycling of wood pulp fibers, J. Adhes. Sci. Technol. 15 (2001) 223–241. [16] W.Y. Tze, M.E.P. Walinder, D.J. Gardner, Inverse gas chromatography for studying interaction of materials used for cellulose fiber/polymer composites, J. Adhes. Sci. Technol. 20 (2006) 743–759. [17] F.P. Liu, T.G. Rials, J. Simonsen, Relationship of wood surface energy to surface composition, Langmuir 14 (1998) 536–541. [18] H. Chtourou, B. Riedl, B.V. Kokta, Surface characterizations of modified polyethylene pulp and wood pulps fibers using XPS and inverse gaschromatography, J. Adhes. Sci. Technol. 9 (1995) 551–574. [19] Y. Marcus, The properties of organic liquids that are relevant to their use as solvating solvents, Chem. Soc. Rev. 22 (1993) 409–416. [20] F.L. Riddle, F.M. Fowkes, Spectral shifts in acid–base chemistry. 1. Van der Waals contributions to acceptor numbers, J. Am. Chem. Soc. 112 (1990) 3259–3264. [21] G.M. Dorris, D.G. Grey, The surface analysis of paper and wood fibers by ESCA I. Application to cellulosics and lignin, Cellul. Chem. Technol. 12 (1978) 9–23.
L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185 [22] G.M. Dorris, D.G. Grey, The surface analysis of paper and wood fibers by ESCA II. Surface composition of mechanical pulp, Cellul. Chem. Technol. 12 (1978) 721–734. [23] M. Sain, S. Panthapulakkal, Bioprocess preparation of wheat straw fibers and their characterization, Ind. Crops. Prod. 23 (2006) 1–8. [24] J. Schultz, L. Lavielle, C. Martin, The role of the interface in carbon-fiber epoxy composites, J. Adhes. 23 (1987) 45–50. [25] A.T. James, A.J.P. Martin, Gas–liquid partition chromatography—the separation and micro-estimation of volatile fatty acids from formic acid to dodecanoic acid, J. Biochem. 50 (1952) 679–690. [26] J. Schultz, L. Lavielle, C. Martin, Surface-properties of carbon-fibers determined by inverse gas-chromatography, J. Chim. Phys. Physio-Chimie Biol. 84 (1987) 231–237.
185
[27] C. Saint Flour, E. Papirer, Gas solid chromatography—a method of measuring surface free-energy characteristics of short glass-fibers 2 through retention volumes measured near zero surface coverage, End. Eng. Chem. Prod. Res. Dev. 21 (1982) 666–669. [28] R.H. Mills, D.J. Gardner, R. Wimmer, Inverse gas chromatography for determining the dispersive surface free energy and acid–base interactions of sheet molding compound-part II 14 ligno-cellulosic fiber types for possible composite reinforcement, J. Appl. Polym. Sci. 110 (2008) 3880–3888. [29] F.M. Fowkes, Quantitative characterization of the acid–base properties of solvents, polymers, and inorganic surfaces, J. Adhes. Sci. Technol. 4 (1990) 669–691.
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XPS and IGC characterization of steam treated triticale straw Liyan Zhao ∗ , Yaman Boluk 1 Cellulose and Hemicellulose Program, Forest Products, Alberta Innovates–Technology Futures, 250 Karl Clark Road, Edmonton, AB T6N 1E4 Canada
a r t i c l e
i n f o
Article history: Received 12 March 2010 Received in revised form 31 May 2010 Accepted 21 June 2010 Available online 30 June 2010 Keywords: Triticale straw Steam treatment X-ray photoelectron spectroscopy Inverse gas chromatography (IGC)
a b s t r a c t The surface chemical composition and surface energy of native and steam treated triticale straws have been investigated by X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC) to reveal the effect of steam treatment temperature and time. The XPS results show that the contents of C elements and C–C group on the exterior surface of native triticale straw are much higher than those on the interior surface, indicating that there was a high quantity of wax on the exterior surface of the native triticale straw. Upon steam treatment, both carbon levels and C–C groups reduce with increasing steam temperature and treatment time of the exterior surfaces. However, the effect of steam treatment on the interior surface is very limited. In terms of the surface acid and base properties, the steam treated samples exhibited higher acid and base properties than the native sample, indicating a more polar surface of the steam treated sample. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Triticale, the first man-made hybrid crop of wheat and rye, has attracted a lot of attention in Canada recently. In 2005, the Canadian Triticale Biorefinery Initiative (CTBI) was set up. The mission of CTBI is to develop triticale as a dedicated industrial biorefining crop for Canada. In fact, triticale provides many attributes that the emerging bioindustrial sector is looking for. Firstly, with a special designed breeding, triticale grain has higher protein content than wheat and good disease and environmental tolerance like rye. Secondly, potential exists to produce high yield triticale varieties which make it better feedstock for chemicals and fuels production than corn. Thirdly, the triticale straw has higher fiber content than other agricultural straws such as barley and oat straws. This makes it a good raw material for the production of pulp, fiber and composite board, etc. As a new bioindustrial raw material, the characterization of triticale straw is very scarce in the literature. Since triticale is the hybrid of wheat and rye, it is expected that the chemical compositions of triticale straw are similar to those of wheat straw. The major chemical components of wheat straw include cellulose, hemicellulose, lignin and secondary metabolites. It has been found that the main difference between wheat straw and wood is wheat straw’s high wax content, concentrated primarily on the surface. This surface layer reduces the moisture absorbance of straw from water and affects its wettability. Without treatment, it limits the utiliza-
∗ Corresponding author. Tel.: +1 780 450 5226; fax: +1 780 450 5397. E-mail address: [email protected] (L. Zhao). 1 Present address: Nanofibre Chair in Forest Products, Department of Civil & Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2W2. 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.060
tion of wheat straw in some applications such as composite board because the wax layer reduces the adhesion of water-based resin to the straw surface [1–4]. It has been reported that steam treatment improved the wettability and bonding property of wheat and reed straw with water-based resin because the steam treatment destroyed the wax layer of the straw [4]. Therefore, in order to develop applications for triticale straw, it is necessary to characterize its surface composition and surface property. This is the objective of this work. In particular, the exterior and interior of triticale straws will be characterized separately to determine their differences. Moreover, the triticale straws will be treated by steam at different temperatures and duration of treatment. The effect of steam treatment on the surface composition and surface property is also studied. Two analytical techniques were adopted in this work: X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC). XPS is particularly suitable for determination of surface chemical composition of different material surfaces. Numerous studies have been reported to study the surface chemistry analysis of wood, wool, pulp and other lignocellulosic materials [5–9]. For example, Bradley and Mathieson [5] employed XPS to investigate the surface oxygen levels of native and UV-exposed wool fiber surfaces. Inari et al. [6] studied the effect of heat treatment on the chemical composition of wood surface using XPS. Inverse gas chromatography (IGC) has been proved to be a very powerful tool to evaluate the wetting, adhesion and bonding potential of fiber surfaces in different polymeric matrices [10]. In particular, this technique has been widely used to investigate the surface properties of different lignocellulosic fibers and pulps [11–16]. Different from XPS, which has been used to analyze the chemical composition, IGC characterizes the surface energy including surface dispersive force and Lewis acid and base forces by
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181
Table 1 Properties of the probes used in IGC experiment. Probe
a (Å)2
LD (mJ/m2 )
AN* (kJ/mol)
DN (kJ/mol)
Specific characteristic
n-Hexane n-Heptane n-Octane n-Nonane Ethyl acetate Acetone Methylene chloride Chloroform THF
51.5 57.0 62.8 68.9 48.0 42.5 31.5 44.0 45.0
18.4 20.3 21.3 22.7 19.6 16.5 27.6 25.9 22.5
0 0 0 0 6.3 10.5 22.6 2.1 16.3
0 0 0 0 71.6 71.2 0 0 84
Non-polar Non-polar Non-polar Non-polar Amphoteric Amphoteric Acidic Acidic Basic
measuring the interaction between well defined probe molecules and test surfaces. For example, Liu et al. [17], studied the relationship of surface energy of wood meal with surface composition due to the various treatments. Tshabalala and Han [13] investigated the effect of solvent extraction on surface energy of kenaf powder and found that the acceptor number of kenaf extracted with toluene/ethanol, followed by ethanol and water, was significantly increased compared to that of unextracted kenaf. 2. Materials and methods 2.1. Materials The feedstock material is the industrial triticale straw. The moisture content was determined to be 6.44%. Stock material was dried in oven at 105 ◦ C before use. The non-polar probe liquids used for IGC were n-hexane, n-heptane, n-octane and n-nonane. The polar probes including ethyl acetate, acetone, methylene chloride, chloroform and tetrahydrofuran (THF) were selected based on their electron pair acceptor and donor characteristics. They were purchased from Sigma–Aldrich. Since these solvents were analytical grade solvents, they were used as received without further purification. The physical properties of the probe solvents are listed in Table 1 [18–21]. 2.2. Surface treatments of straws Ten grams of triticale straw were treated with steam using an autoclave. Four one-stage treatments and one two-stage treatment have been carried out. For the one-stage treatment, two steam pressures have been adopted, which were 0.6 and 1.3 MPa. The treated times for 0.6 MPa were 5 and 10 min, and 2.5 and 5 min for 1.3 MPa. For the two stage, the triticale straw was treated at 6 kPa for 30 min and then 1.3 MPa for 2.5 min. In two-stage treatment, the straw sample was first cleaned in the first stage, and was treated with steam in the second stage.
separately by X-ray photoelectron spectroscopy (XPS) using a spectrometer (Model AXIS 165, Kratos Analytical, Japan) with a dual Mg and Al X-ray source. In XPS measurements, the sample is irradiated with soft X-ray photons and chemical compositions and functional groups about the top 2–20 atomic layers on the sample surface can be obtained. For composition analysis, a survey spectrum was measured with pass energies of 100 eV, while for high resolution scans, a pass energy of 10 eV was used. Chemical compositions were calculated from the area of the corresponding photoelectron peak for each element. The compositions of chemical bond were obtained by deconvolution of the respective spectra. In the experiment, the straw sample was mounted on a sample holder and during the measurement, the pressure inside the analysis chamber was maintained below 5 × 10−6 Pa. 2.3.2. Surface dispersive and acid/base properties Inverse gas chromatography was used to characterize the dispersive and acid/base properties of native and treated straw samples. The GC column was prepared by the standard procedures [14]. The column dimensions and detailed operation conditions were summarized in Table 2. The IGC experiments were carried out using a Varian 3380 Gas Chromatograph equipped with a flame ionization detector (FID). Pre-purified helium was used as the carrier gas. The flow rate was measured at the corresponding experimental temperatures with the use of a soap bubble flow-meter. The inlet and outlet pressures of the column were monitored with pressure gauges during the experiments. For each solvent, three injections of 1 L of its vapor were made to measure the retention time with a reproducibility of within 3%. The net retention times were determined using methane as the marker. Three injections were made for each probe at each of the following three temperatures: 40 ◦ C, 50 ◦ C, 60 ◦ C and 70 ◦ C. Methane was used as noninteracting marker to determine the column gas holdup. 3. Results and discussion 3.1. Surface chemical composition
2.3. Surface characterization 2.3.1. Surface chemical composition Since triticale straw has a tubular structure, the exterior and interior surfaces of native and treated straws were characterized
From the XPS survey spectra, the atomic composition of the investigated sample surface was calculated based on the peak area of each species. The atomic compositions of exterior and interior surfaces of native and treated triticale straw samples were summa-
Table 2 Dimensions and operating conditions of the GC columns packed with different samples. Native triticale straw
Steam treated 160 ◦ C 5 min
Steam treated 160 ◦ C 10 min
Steam treated 190 ◦ C 2.5 min
Steam treated 190 ◦ C 5 min
Steam treated two stages
Weight of sample (g) Column pressure (kPa) Flow rate (mL/min)
2.17 184 20.01
2.74 170 18.01
2.26 149 21.40
1.96 152 20.03
1.96 163 20.70
1.34 135 21.43
Column length Column temperature Carrier gas
0.5 m 40 ◦ C, 50 ◦ C , 60 ◦ C and 70 ◦ C Pre-purified helium
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Table 3 Surface composition (at.%) of the (a) exterior surfaces and (b) interior surfaces of native and treated wheat straw samples. Sample (a) Native triticale straw—exterior surface Steam treated triticale straw (160 ◦ C 5 min)—exterior surface Steam treated triticale straw (160 ◦ C 10 min)—exterior surface Steam treated triticale straw (190 ◦ C 2.5 min)—exterior surface Steam treated triticale straw (190 ◦ C 5 min)—exterior surface Steam treated triticale straw (two stages)—exterior surface (b) Native triticale straw—interior surface Steam treated triticale straw (160 ◦ C 5 min)—interior surface Steam treated triticale straw (160 ◦ C 10 min)—interior surface Steam treated triticale straw (190 ◦ C 2.5 min)—interior surface Steam treated triticale straw (190 ◦ C 5 min)—interior surface Steam treated triticale straw (two stages)—interior surface
C
O
N
Ca
S
Si
O/C
94.28 85.15 76.25 78.12 73.91 70.58
5.66 12.61 18.45 18.56 23.93 27.06
0.02 1.18 1.15 0.8 0.98 0.46
0 0.65 2.62 1.28 0.63 1.33
0 0.10 0.44 0.33 0.33 0.14
0.04 0.31 0.73 0.91 0.22 0.42
0.06 0.15 0.24 0.24 0.32 0.38
63.11 64.59 65.45 68.75 62.43 66.26
35.89 33.56 32.02 30.17 36.09 31.26
0.98 1.04 1.06 0.96 1.38 0.60
0.01 0.55 0.62 0 0.04 1.09
0 0.19 0.07 0.06 0.06 0.32
0 0.08 0.24 0.06 0 0.47
0.57 0.52 0.49 0.44 0.58 0.47
rized in Table 3(a) and (b). It can be seen that for all samples carbon and oxygen are the predominant species along with traces of nitrogen, silicon and sulfur. The exterior surface of native triticale straw is composed of approximately 94% carbon and 5% oxygen, while the interior surface is composed of 63% carbon and 36% oxygen. Steam treatment of native straw reduced the carbon content of exterior surfaces to 85–70% and increased the oxygen content to 12–27%. Similar trend was not observed on the interior surfaces of steam treated samples. There is no obvious change in carbon and oxygen contents compared to the native sample. The O/C ratio, which was determined from the peak area of oxygen and carbon, was also listed in Table 3(a) and (b). It has been reported that the theoretical O/C ratio of cellulose and hemicellulose is 0.83 and that of lignin is 0.33 [21,22]. The O/C ratio of exterior surface of the native triticale straw is only 0.06, which is much lower than the O/C ratios of cellulose, hemicellulose and lignin. This result indicates that the major components of the exterior surface of the native straw are not cellulose, hemicellulose or lignin. The surface is covered with a highly aliphatic layer, which is also called wax layer. On the other hand, the O/C ratio of the interior surface of the native straw is 0.57, which is the average of the O/C ratios of cellulose/hemicellulose and lignin. Obviously this result reflects absence of wax layer on the interior surface. The O/C ratio of mechanical processed pulp of wheat straw was reported to be 0.24, which is about average of O/C values of the exterior and interior surfaces of triticale straw [23]. This is because the pulp used in the literature was a mechanically defibrillated fibers and the pulp surface is a mixture of exterior and interior surfaces. The O/C ratios of exterior surfaces of steam treated samples are four to six times higher than that of the native sample. The values of O/C ratio increase with increasing treatment temperature and time (Fig. 1). The increase in O/C ratio following steam treatment is due to the removal of carbon-rich wax layer. However, the O/C ratios of interior surfaces of steam treated samples are slightly lower than that of the native sample. This result indicates that steam treatment has little effect on the wax-free surface. Detailed XPS C1s core energy spectra of exterior and interior surfaces of native straw are shown in Fig. 2(a) and (b). C1s can be deconvoluted into four Gaussian peaks, which were found to be at about 284.8 eV, 286.3 eV, 287.9 eV and 289.0 eV, corresponding to C–C/C–H, C–O, C O and COOH groups, respectively [7]. The C1s spectra of surfaces of steam treated straws were also deconvoluted into four peaks and the graphs were omitted for clarity. The relative area of each peak was calculated and listed in Table 4(a) and (b). It can be found that the composition of chemical groups of exterior surface is very different from the interior surface for both native and steam treated samples. For exterior surfaces, the content of C–C/C–H group is much higher than C–O group. However, the content of C–C/C–H group of interior surfaces is comparable to
the content of C–O group. These results indicate that the exterior surfaces are more non-polar than the interior surfaces. Moreover, the interior surfaces exhibit a higher content of −COOH group than the exterior surfaces, suggesting that the interior surfaces are more polar than the exterior surfaces. From Table 4(a), it is obvious that the contents of C–C/C–H group on the exterior surfaces of the steam treated samples are lower than that of the native sample, while the content of C–O group is higher. These results suggested that steam treatment is an effective method to remove the wax layer on the exterior surface of triticale straw. Furthermore, increasing treatment temperature and extending treatment time made the steam treatment more efficient to reduce the content of C–C/C–H group and increase the content of C–O group. As for the effect of steam treatment on interior surface, it can be seen from Table 4(b) that both C–C/C–H group and C–O group show little difference between treated and native samples. This means that the effect of steam treatment on interior surface is very limited. 3.2. Surface dispersive and acid/base properties Since the theory of IGC has been well developed and reviewed in the literature [24], the detailed derivation and calculation of surface dispersive force and acid and base numbers are omitted and only the critical equations are described here. The specific retention volume, Vg0 , is the direct data obtained from IGC experiment [10]: Vg0 =
273.15tn FJ wT
(1)
Fig. 1. Effects of time and temperature of steam treatment on O/C ratio of exterior surface.
L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
183
1/2
Fig. 3. Plots of −RT ln Vg0 versus 2Na LD
at four temperatures for native triticale
straw.
The interactions of polar probes with solid surface include both dispersive and acid/base contributions, GA0 = GAD + GAS , where GAD and GAS are the dispersive and Lewis acid/base contributions to the total Gibbs free energy of adsorption. The dispersive component can be calculated from Eq. (2) with molecular area and dispersive energy of polar probes. GAS can be written as follows:
GAS = −RT ln Vg0 − 2NaP SD
= −RT ln
+ C = 2Na
1/2 SD
1/2 LD
1/2
(3)
GAS = HAS − TSAS HAS
(4)
SAS
Here, and are enthalpy and entropy of adsorption corresponding to the acid/base interaction, respectively. The plot of GAS /T versus 1/T should yield a straight with the slope of HAS . According to Papirer’s approach, the Lewis acid (KA ) and base (KD ) numbers, which are used to describe the acidic and basic characteristics of the solid surface, are calculated using the following equation [26]:
where tn is the net retention time, which is the difference of the retention times between a selected solvent and the marker; F is the flow rate of the carrier gas measured at the experimental temperature T; w is the mass of the test material in the column; and J is the James–Martin correction factor that is used to correct for the pressure gradient across the column [25]. From Vg0 , the dispersive component (SD ) of the solid surface energy with non-polar solvent can be calculated from the following equation: Vg0
D L,P
D is the diswhere aP is the surface area of one polar molecule; L,P persive energy of polar probe. Thermodynamically, the Gibbs free energy of adsorption corresponding to the acid/base interaction is related to the enthalpy of adsorption:
Fig. 2. XPS C1s spectra with peak deconvolution of native wheat straw (a) exterior surface, (b) interior surface.
GAD
1/2
−HAS = KA DN + KD AN∗
(5)
where AN* and DN are the acceptor and donor numbers of the polar probe, respectively. The plot of −HAS /AN∗ versus DN/AN* gives a straight line with a slope of KA and an intercept of KD [27].
(2)
where GAD is the dispersive contribution to the total Gibbs free energy of adsorption; SD and LD are the dispersive energies of the solid and liquid surfaces, respectively; N is the Avogadro number; a is the surface area covered by one alkane molecule.
3.2.1. Dispersive force of triticale straw
1/2
of native Fig. 3 shows the plot of −RT ln Vg0 versus 2Na LD triticale straw for four n-alkane probes at four different tempera-
Table 4 Surface bond compositions of C1s of the (a) exterior surfaces and (b) interior surfaces of native and treated wheat straw samples. Sample (a) Native triticale straw—exterior surface Steam treated triticale straw (160 ◦ C 5 min)—exterior surface Steam treated triticale straw (160 ◦ C 10 min)—exterior surface Steam treated triticale straw (190 ◦ C 2.5 min)—exterior surface Steam treated triticale straw (190 ◦ C 5 min)—exterior surface Steam treated triticale straw (two stages)—exterior surface (b) Native triticale straw—interior surface Steam treated triticale straw (160 ◦ C 5 min)—interior surface Steam treated triticale straw (160 ◦ C 10 min)—interior surface Steam treated triticale straw (190 ◦ C 2.5 min)—interior surface Steam treated triticale straw (190 ◦ C 5 min)—interior surface Steam treated triticale straw (two stages)—interior surface
C–C/C–H (%)
C–O (%)
C O (%)
COOH (%)
85.01 80.78 76.48 74.69 69.09 60.54
8.74 14.44 17.80 20.46 24.74 32.14
5.12 3.34 3.02 3.34 3.68 4.83
1.12 1.44 2.69 1.53 2.49 2.48
39.89 42.88 47.18 50.96 44.37 43.69
45.18 44.21 41.05 38.93 43.65 44.61
14.09 10.35 9.67 8.55 10.43 6.85
0.02 2.56 2.01 1.55 1.55 4.86
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L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185
Table 5 Dispersive force of triticale straws, SD (mJ/m2 ).
4. Conclusions
Sample
40 ◦ C
50 ◦ C
60 ◦ C
70 ◦ C
Native triticale straw Steam treated triticale straw (160 ◦ C 5 min) Steam treated triticale straw (160 ◦ C 10 min) Steam treated triticale straw (190 ◦ C 2.5 min) Steam treated triticale straw (190 ◦ C 5 min) Steam treated triticale straw (two stages)
38.29 38.63 39.78 37.07 37.82 38.31
34.63 36.08 35.11 35.25 36.21 35.63
33.32 31.34 33.31 33.73 33.69 33.2
30.11 30.06 29.79 30.82 29.59 27.4
tures. These curves show linear regression lines with a least-square correlation coefficient of at least 0.99 in all cases. Similar plots for steam treated samples were also obtained and omitted here. The dispersive components of surface energy, which were calculated from the slope of these curves, are summarized in Table 5. As shown in Table 5, the values of dispersive energy of native triticale straw ranged from 38 mJ/m2 at 40 ◦ C to 30 mJ/m2 at 70 ◦ C, which are comparable to the results for reef and kenaf and slightly higher than wheat straw in the literature. Mills et al. [28] reported the dispersive energy for reef and wheat straw being 37.2 mJ/m2 and 35 mJ/m2 at 40 ◦ C, respectively and Tshabalala reports a value of 40 mJ/m2 (36.9 mJ/m2 ) at 40 ◦ C for Kenaf [13,14]. The dispersive energy deceased with the increase in temperature for both native and steam treated samples. It can be seen that steam treatment did not change the values of SD significantly at all temperatures. 3.2.2. Acid/base characteristics In terms of acid–base property, the KA and KD values of native and steam treated samples were listed in Table 6. It can be seen that the KA number is around 0.03 and the KD number is in the range from 0.18 to 0.33. The KA number is lower than the KD number for all the samples, which means that all samples show strong basicity and weak acidity. This property is attributed to the strong basicity and weak acidity of cellulose and hemicellulose. It has been reported that wheat straw and reed also showed the same acid/base properties [28]. Moreover, KA and KD values of steam treated samples are slightly higher than those of the native sample, indicating that the removal of relatively hydrophobic wax layer from wheat straw can increase the free energy of its surface. It seems that increasing steam temperature can increase both KA and KD values. However, there is no such trend for increasing treatment time. The reason for this result is not clear. These findings are consistent with our chemical composition results obtained from XPS. It is worth pointing out that in IGC measurement the exterior surface and the interior surface cannot be examined separately. The KA and KD values are the combinational results of both surfaces. From XPS results we can find that the steam treatment only affected the chemical composition of the exterior surface. This explains why steam treatment did not change KA and KD values dramatically. According to the theory developed by Fowkes [29], the increase in KA and KD values of steam treated straw indicates that the steam treated straw surface is more polar than the native straw. Consequently, the adhesion of the treated straw with resin would be stronger than that between the native straw and resin.
Table 6 Acid–base numbers of different triticale straws. Sample
KA
KD
Native triticale straw Steam treated triticale straw (160 ◦ C 5 min) Steam treated triticale straw (160 ◦ C 10 min) Steam treated triticale straw (190 ◦ C 2.5 min) Steam treated triticale straw (190 ◦ C 5 min) Steam treated triticale straw (two stages)
0.030 0.035 0.035 0.038 0.036 0.037
0.18 0.26 0.23 0.31 0.33 0.28
In this work, XPS and IGC have been successfully used to characterize the surface chemical composition and surface energy of steam treated triticale straw. The results presented here illustrate that there was a high quantity of wax on the exterior surface of the native triticale straw. Steam treatment can effectively remove the wax layer, which makes it possible to bond with water-based resin such as urea–formaldehyde resin instead of the expensive isocyanate resin. Increasing treatment temperature and time can effectively reduce the amount of wax on the exterior surface. However, the steam treatment has little effect on the composition of the interior surface of triticale straw. In terms of the surface acid and base properties, the steam treated samples exhibited higher acid and base properties than the native sample, indicating a more polar surface of the steam treated sample. Our results indicate that steam treated triticale straw shows a good potential in straw composite board applications. Acknowledgements The authors thank Ms. Heather Lorenz for performing the IGC measurements. The financial support from the Alberta Forestry Research Institute is gratefully acknowledged. References [1] G. Han, C. Zhang, D. Zhang, K. Umemura, S. Kawai, Upgrading of UF-bonded reed and wheat straw particleboard using silane coupling agents, J. Wood Sci. 44 (1998) 282–286. [2] G. Han, K. Umemura, S. Kawai, H. Kajita, Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments, J. Wood Sci. 45 (1999) 299–305. [3] G. Han, K. Umemura, M. Zhang, T. Honda, S. Kawai, Development of highperformance UF-bonded reed and wheat straw medium-density fiberboard, J. Wood Sci. 47 (2001) 350–355. [4] J.M. Lawther, R. Sun, W.B. Banks, Effect of steam treatment on the chemical composition of wheat straw, Holzforschung 50 (1996) 365–371. [5] R.H. Bradley, I. Mathieson, Chemical interactions of ultraviolet light with wool fiber surfaces, J. Colloid Interf. Sci. 194 (1997) 338–343. [6] G.N. Inari, M. Petrissans, J. Lambert, J.J. Ehrhardt, P. Gerardin, XPS characterization of wood chemical composition after heat-treatment, Surf. Interf. Anal. 38 (2006) 1336–1342. [7] G. Sinn, G.S. Reiterer, S.E. Stanzl-Tschegg, Surface analysis of different wood species using X-ray photoelectron spectroscopy (XPS), J. Mater. Sci. 36 (2001) 4673–4680. [8] L.S. Johansson, J.M. Campbell, K. Koljonen, P. Stenius, Evaluation of surface lignin on cellulose fibers with XPS, Appl. Surf. Sci. 145 (1999) 92–95. [9] K. Li, D.W. Reeve, Sample contamination in analysis of wood pulp fibers with X-ray photoelectron spectroscopy, J. Wood Chem. Technol. 24 (2004) 183–200. [10] J.R. Conder, C.L. Young, Physicochemical Measurement by Gas Chromatography, Wiley-Interscience, New York, 1979. [11] D.P. Kamdem, S.K. Bose, P. Luner, Inverse gas chromatography characterization of birch wood meal, Langmuir 9 (1993) 3039–3044. [12] H.F. Wang, B. Li, B.L. Shi, Preparation and surface acid–base properties of porous cellulose, Bioresources 3 (2008) 3–12. [13] M.A. Tshabalala, J.S. Han, Effect of solvent extraction on surface energy of kenaf powder, in: Kenaf Properties, Processing and Products, 1999, pp. 121–131. [14] M.A. Tshabalala, Determination of the acid–base characteristics of lignocellulosic surfaces by inverse gas chromatography, J. Appl. Polym. Sci. 65 (1997) 1013–1020. [15] W.T. Tze, D.J. Gardner, Contact angle and IGC measurements for probing surface–chemical changes in recycling of wood pulp fibers, J. Adhes. Sci. Technol. 15 (2001) 223–241. [16] W.Y. Tze, M.E.P. Walinder, D.J. Gardner, Inverse gas chromatography for studying interaction of materials used for cellulose fiber/polymer composites, J. Adhes. Sci. Technol. 20 (2006) 743–759. [17] F.P. Liu, T.G. Rials, J. Simonsen, Relationship of wood surface energy to surface composition, Langmuir 14 (1998) 536–541. [18] H. Chtourou, B. Riedl, B.V. Kokta, Surface characterizations of modified polyethylene pulp and wood pulps fibers using XPS and inverse gaschromatography, J. Adhes. Sci. Technol. 9 (1995) 551–574. [19] Y. Marcus, The properties of organic liquids that are relevant to their use as solvating solvents, Chem. Soc. Rev. 22 (1993) 409–416. [20] F.L. Riddle, F.M. Fowkes, Spectral shifts in acid–base chemistry. 1. Van der Waals contributions to acceptor numbers, J. Am. Chem. Soc. 112 (1990) 3259–3264. [21] G.M. Dorris, D.G. Grey, The surface analysis of paper and wood fibers by ESCA I. Application to cellulosics and lignin, Cellul. Chem. Technol. 12 (1978) 9–23.
L. Zhao, Y. Boluk / Applied Surface Science 257 (2010) 180–185 [22] G.M. Dorris, D.G. Grey, The surface analysis of paper and wood fibers by ESCA II. Surface composition of mechanical pulp, Cellul. Chem. Technol. 12 (1978) 721–734. [23] M. Sain, S. Panthapulakkal, Bioprocess preparation of wheat straw fibers and their characterization, Ind. Crops. Prod. 23 (2006) 1–8. [24] J. Schultz, L. Lavielle, C. Martin, The role of the interface in carbon-fiber epoxy composites, J. Adhes. 23 (1987) 45–50. [25] A.T. James, A.J.P. Martin, Gas–liquid partition chromatography—the separation and micro-estimation of volatile fatty acids from formic acid to dodecanoic acid, J. Biochem. 50 (1952) 679–690. [26] J. Schultz, L. Lavielle, C. Martin, Surface-properties of carbon-fibers determined by inverse gas-chromatography, J. Chim. Phys. Physio-Chimie Biol. 84 (1987) 231–237.
185
[27] C. Saint Flour, E. Papirer, Gas solid chromatography—a method of measuring surface free-energy characteristics of short glass-fibers 2 through retention volumes measured near zero surface coverage, End. Eng. Chem. Prod. Res. Dev. 21 (1982) 666–669. [28] R.H. Mills, D.J. Gardner, R. Wimmer, Inverse gas chromatography for determining the dispersive surface free energy and acid–base interactions of sheet molding compound-part II 14 ligno-cellulosic fiber types for possible composite reinforcement, J. Appl. Polym. Sci. 110 (2008) 3880–3888. [29] F.M. Fowkes, Quantitative characterization of the acid–base properties of solvents, polymers, and inorganic surfaces, J. Adhes. Sci. Technol. 4 (1990) 669–691.