Environmental Engineering and Management Journal August 2011, Vol.10, No. 8, 1149-1154

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

ECOLOGIC MODIFICATION OF WOOD USING

ALKYLIMIDAZOLIUM-BASED IONIC LIQUIDS

Catalin Croitoru1, Silvia Patachia1, Attila Porzsolt1, Christian Friedrich2

1“Transilvania” University of Brasov, Chemistry Department, 29 Eroilor Blvd., 500036 Brasov, Romania 2Albert-Ludwigs-Universität Freiburg, 28 Stefan Meier Street, Freiburg, Germany

Abstract In this paper, the influence of two types of imidazolium-based ionic liquids (ILs) on the properties of sycamore maple (Acer pseudoplatanus) veneers, at 25 0C, 60 0C and 80 0C has been studied by using contact angle measurements and Fourier transform infrared spectroscopy analysis. The measurements showed that wood wettabillity is increased by IL treatment. It has been determined that the ILs decrease the crystallinity and improve the flexibility of the cellulose matrix. Also, it has been determined that at 60 0C and 80 0C delignification of wood occurred, thus making the studied ionic liquids useful alternatives to traditional toxic and expensive reagents used in wood industry. Key words: FTIR spectroscopy, ionic liquids, surface energy, wood Received: April, 2011; Revised final: August, 2011; Accepted: August, 2011

Author to whom all correspondence should be addressed: e-mail: c.croitoru@unitbv.ro; Phone: +40748126598

1. Introduction Wood is one of the most abundant

lignocellulosic resources on the planet, and it has long been used as a raw material for constructions or as fuel. During the last four decades, modification of wood has been intensively studied with the aim of improving or modifying its compatibility with thermoplastic polymers, dimensional stability and resistance to decay (Klemm et al., 1998; Grāvitis et al., 2010; Timar et al., 2009).

These modifications were always achieved at low efficiency due to the insolubility or limited swelling of the lignocellulosic materials in the most common used reagents for wood modification.

Furthermore, during wood modification (e.g. for plastifying, impregnation or dimensional stabilization) environmentally toxic reagents (such as volatile organic solvents, acid chlorides or anhydrides) are used, which could be released in the atmosphere.

Also, leaching of residual volatile compounds during wood products use could represent an important health hazard (Hill, 2006).

Ionic liquids are new organic salts that exist as liquids at relatively low temperature (<100 0C). They have many attractive properties, such as good chemical and thermal stability, non-flammability and low vapor pressure (Patachia et al., 2011; Pernak, 2000; Pinkert et al., 2010).

Alkylimidazolium chlorides and acetates have been extensively used in cellulose or lignocellulose dissolution or functionalization studies (Pinkert et al., 2010).

There are very few studies regarding wood modification with the help of ionic liquids that are not reported as cellulose or lignin solvents (Pinkert et al., 2010). The use of this type of ionic liquids could be also useful, because they could promote swelling of the wood material and impart wood some useful properties, such as higher compatibility with polar adhesives, plasticity and so forth.

Croitoru et al./Environmental Engineering and Management Journal 10 (2011), 8, 1149-1154

1150

2. Objectives One of the main objectives of this paper was to

extend our previous results concerning ionic liquids influence on wood properties (Croitoru et al., 2011).

Another objective was to enlarge the current database concerning ILs effect on different wood species (Zakzeski et al., 2010).

In this paper, the properties of sycamore maple (Acer pseudoplatanus) wood treated at different temperatures ranging from 25 to 80 0C with two types of imidazolium-based ionic liquids, namely 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate, which, due to the nature of their anions do not dissolve lignocellulose, have been determined by Fourier transform infrared spectroscopy (FTIR) and contact angle measurements, in comparison with the untreated wood.

Sycamore maple is widely used in wood industry, for furniture, floors, musical instruments and decorative veneers manufacturing, and its processing with ionic liquids could offer some useful properties and lower the toxic emissions that occur when using traditional processes and reagents.

For example, traditional wood finishes (based on shellac or rosin) use toluene as solvent (Hill, 2006), which has a vapor pressure of 28.44 mmHg at 250C (Haynes, 2011), or ethanol (Hill, 2006), which has a vapor pressure of 43.7 mmHg at 250C (Haynes, 2011), comparing to ionic liquids which have vapor pressures of 10-3 to 10-2 mmHg at the same temperature (Zhang et al., 2009). Sycamore maple wood has not been used before in relation with ionic liquids.

3. Experimental

3.1. Materials

The two ionic liquids, 1-butyl-3-

methylimidazolium tetrafluoroborate (BMIMBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) have been purchased from IoLiTec Ionic Liquids Technologies GmbH, Germany. Both are transparent viscous liquids at room temperature.

The purity of the purchased ionic liquids was 99.5%. Romanian sycamore maple wood technical veneers with 2.5 mm thickness have been used.

3.2. Ionic liquid treatment of wood veneers

Tangential sycamore maple wood veneers

have been cut in 20 mm x 20 mm x 2.5 mm rectangular test pieces and before their immersion in ionic liquids were conditioned at 55% relative humidity and 220C for a week, until they reached equilibrium moisture content (EMC) of 8%.

The wood samples were soaked in 5 mL of the BMIMBF4 and BMIMPF6, for 1 hour at 25 0C, 60 0C and 80 0C.

3.3. Tests and analysis 3.3.1. Gravimetric determination of the absorbed amount of ionic liquids

After the immersion in ionic liquids, the samples (with initial mass m0) have been dried at the surface with filter paper, introduced in a drying stove at 1050C for 24 h, and then reweighed (the corresponding mass = md).

The mass of absorbed ionic liquid reported to 1g of dry wood has been calculated using Eq. (1):

1100

100)/(

0

EMCm

mwooddryggm d

IL

(1)

Five parallel determinations were used in order

to calculate the mass of absorbed ionic liquid.

3.3.2. Contact angle measurements and surface energy calculation

The ionic liquid treated samples have been re-equilibrated in an atmosphere of 55% relative humidity for 24 days for the contact angle measurements. Contact angle measurements of the untreated and ionic liquid treated specimens using distilled water, glycerol and 1-bromonaphtalene as reference liquids were performed with an OCA System 20 goniometer, provided by Data Physics Co., Ltd. at 25 ◦C.

Five different single drops of test liquid with 4 μL volume were deposited on the surface of the same specimen and the initial contact angle θ0, at the beginning of the wetting process (for t = 0) has been recorded.

It is widely recognized that the wettability and surface free energy of wood are useful parameters which provide information on the interaction between adhesives or coating materials and the wood surface (Mohammed-Ziegler et al., 2004; Rudawska and Zajchowski, 2007). In this article the surface energy of the wood veneers is determined by the means of contact angle measurements, using the sessile drop technique. The surface energy of the wood samples has been calculated using the Lifshitz-van der Waals and Lewis Acid-Base (LW/AB) approach, with the help of the instrument’s software.

According to this approach, the surface energy (γ) is decomposed into an Lifshitz-van der Waals (γLW) dispersive component as well as into a polar component - γp- with Lewis acid (γp+) and Lewis base (γp-) contributions respectively (Eq. 2) (Mohammed-Ziegler et al., 2004):

ppLWpLW 2 (2)

The initial contact angle θ0 was used in the

calculation of surface energy. The dispersive and polar components of the surface tension of the test liquids have been obtained from the reference literature (Rudawska and Zajchowski, 2007).

Ecologic modification of wood using alkylimidazolium-based ionic liquids

1151

3.3.3. FTIR spectroscopy measurements The FTIR spectra analysis of the untreated and

ionic liquid treated wood were recorded with an Perkin-Elmer BXII Fourier transform infrared spectrometer equipped with an attenuated total reflectance (ATR) device with a resolution of 2 cm-1 in the 4000-600 cm-1 interval.

Also, FTIR spectra of the ionic liquids in which the wood samples have been immersed for 1 hour at 25 0C, 60 0C and 80 0C have been recorded in transmittance mode, using a liquid cell with ZnSe windows. The scanning resolution and interval was the same as for the solid samples.

3.3.4. UV-VIS spectroscopy measurements

UV-VIS spectra of the maple extractives obtained in ionic liquids at different temperatures have been analyzed by using a Perkin-Elmer Lambda 25 spectrophotometer, in the 200-700 nm intervals, with a resolution of 5 nm.

4. Results and discussion

The amounts of absorbed ionic liquids,

reported to 1g of dry wood are given in Table 1.

Table 1. Amounts of ionic liquids absorbed in wood

mIL (g)/g dry wood Ionic liquids

250C 600C 800C BMIMBF4 0.2387 0.3798 0.4765 BMIMPF6 0.2116 0.3124 0.4211

It can be seen that generally, the absorption of

ILs in wood is influenced by the nature of the anion, its interactions with the lignocellulose matrix and its molecular weight. Also, the temperature increase favors the absorption of ionic liquid in wood, probably due to the fact that at higher temperatures the viscosity of the IL is lower and it could penetrate better into wood. Also, at higher temperatures, a stronger interaction with cellulose or lignin could be promoted.

BMIMPF6 ionic liquid, being dominating hydrophobic, is absorbed in a lower amount, probably due to the lower affinity of the anion towards the lignocellulose matrix, by comparing with BMIMBF4. The surface energies of the untreated and ionic liquid treated samples are presented in Table 2:

As shown in Table 1, the total surface energy of untreated and treated wood veneers ranges within 40-60 mN/m, and it is mainly contributed to the LW component, which agrees well with the results obtained for wood reported in the literature (Croitoru et al., 2011). BMIMPF6 ionic liquid, which is dominating hydrophobic imparts lower surface energy to wood, while the treatment of the veneers with BMIMBF4 seems to improve their surface wettabillity, as remarked by higher surface energies and lower initial contact angles for water, by comparing to the untreated sample.

Also, it could be noted that the dominating contribution to the polar component of the surface energy is the Lewis base one, which means that the treated veneers surfaces have electron-donor properties. As the BF4

- and PF6- anions are known to

present electron-donor ability (Pitula and Mudring, 2010), it is suggested that these ions remain on the surface of the wood during treatment, while the imidazolium cations interact with the cellulose-lignin matrix.

The higher the temperature, more ionic liquid is absorbed into wood, which determines a more pronounced decrease in surface energy, in the case of BMIMPF6 and an increase in the case of BMIMBF4.

The FTIR spectra for the ionic liquids, untreated and ionic liquid treated wood veneers are presented in Figs. 1 and 2 and the FTIR spectra for the ionic liquids in which the wood was immersed 1 hour at 25 0C, 60 0C and 80 0C are presented in Figs. 3 and 4.

The main bands in the FTIR spectra of the untreated wood are assigned to asymmetric methoxyl C-H stretching at 2921 cm-1, absorbed water at 1640 cm-1, bands assigned to different lignin groups, such as those at 1770, 1740, 1590, 1510, 1270, and 1140 cm-1; bands common to lignin and cellulose such as those at 1460, 1420, 1375, 1330, 1230, 1160, 1110, and 1030 cm-1; and bands assigned to cellulose such as those at 1315, 1280, 1180, and 1060 cm-1 (Croitoru et al., 2011; He et al., 2007; Moore and Owen, 2001; Popescu et al., 2007).

For the ionic liquid treated wood at 250C, it could be generally observed that the bands specific to cellulose and lignin tend to shift to higher wavenumbers, which could be attributed to a flexibilization effect of the ionic liquids on the lignocellulosic matrix.

Table 2. Surface energies of the ionic liquid treated samples

Ionic liquid treatment

Temperature of treatment

(0C)

θ0 water

(degree)

γ (mN/m)

γLW

(mN/m) γp

(mN/m) γp+

(mN/m) γp-

(mN/m)

Blank 25 43.85 47.66 41.64 6.02 1.21 7.48 25 39.34 50.97 40.06 6.91 0.39 59.83 60 30.43 54.99 37.67 17.32 1.25 119.99 BMIMBF4 80 25.76 78.34 46.67 31.67 3.89 128.91 25 57.75 40.51 29.51 11 1.02 59.31 60 58.55 40.20 29.20 11 2.55 23.72 BMIMPF6 80 69.64 39.33 29.12 10.21 4.66 11.18

Croitoru et al./Environmental Engineering and Management Journal 10 (2011), 8, 1149-1154

1152

Fig. 1. FTIR spectra of BMIMBF4 (1); wood (2) and BMIMBF4 treated wood, at 250C (3); 600C (4) and 800C (5)

Fig. 2. FTIR spectra of BMIMPF6 (1); wood (2) and BMIMPF6 treated wood, at 250C (3); 600C (4) and 800C (5)

Fig. 3. FTIR spectra of BMIMBF4 after 1 hour of wood immersion at 250C (1); 600C (2) and 800C (3)

Fig. 4. FTIR spectra of BMIMPF6 after 1 hour of wood immersion at 25 0C (1); 60 0C (2)

and 80 0C (3)

No supplementary bands from that of wood and IL could be noted in the ionic liquid treated wood spectra, so it is suggested that the interaction between ILs and wood is of physical nature. As the alkyl-substituted imidazolium cation enters the lignocellulosic matrix it is expected to decrease the crystallinity index of cellulose.

The height ratio of the bands at 1371 and 2900 cm-1 (H1371/H2900) (He et al., 2007; Moharram and Mahmoud, 2008) was used to determine the crystallinity index (CrI) of cellulose material and the advantage is that it can be applied to both cellulose I and II (Table 3). Also, all of the ILs that were used for wood treatment are more or less hygroscopic, so it is expected that the surface of the samples will absorb water.

The ratio of the band areas at 1640 and 2900 cm-1 can be used for a moisture content index (MI) calculation (Table 3) (Croitoru et al., 2011; Popescu et al., 2010).

At higher temperatures (60 0C and more significant at 80 0C), it could be observed that changes occur in the intensity of the absorptions centered at 1507 cm-1 and 1600 cm-1 (aromatic skeletal vibrations) which caused by lignin (Dawson et al., 2008). It is suggested that delignification of wood could occur starting from 60 0C in BMIMBF4 and BMIMPF6, in agreement with UV spectroscopy results, that evidenced an increase in the absorbance maxima at 278 nm (Lee et al., 2009) of the wood extractives, with the increase of temperature.

In Fig. 5 and Fig. 6 are presented the FTIR spectra of pure BMIMBF4 and BMIMPF6 ionic liquids, at 25 0C and heated for 1 hour at 60 0C and 80 0C.

It could be seen from Figs. 5 and 6, no degradation of the ionic liquid at higher temperatures occurred, but instead possible solubilization of lignin and/or small molecular compounds (wood extractives) from the wood occurred, as it can be seen from Fig. 3 and Fig. 4.

The influence of temperature treatment on wood delignification has been assessed by calculating the lignin index (LI), based on the ratio of the band at 1507 cm-1 (specific to lignin) and the band centered at 1730-1 cm-1 (C=O stretch in non-conjugated ketones, carbonyls and in ester groups specific to holocellulose) (Table 3) (Dawson et al., 2008; Popescu et al., 2010).

The lower crystallinity indexes for the cellulose from the treated wood could be explained by the ability of the ionic liquids to break the inter- and intramolecular –OH bonds from the cellulose matrix. Also, due to the ability of the ILs to mobilize the bonded water from the wood’s structure, the MIs of the IL treated wood are lower.

The BMIMPF6 ionic liquid, due to its higher viscosity comparing to BMIMBF4 could remain at the wood’s surface and determines a higher MI by avoiding the water release.

Ecologic modification of wood using alkylimidazolium-based ionic liquids

1153

Generally, at higher temperatures, lower CrI and MI have been obtained for wood, due to the stronger interactions of lignocellulose with the ionic liquid.

Also, lower lignin indexes have been obtained for the treated wood at 60 0C and 80 0C. Bands characteristic to lignin (1510 cm-1 and 1600cm-1) have been found in the FTIR spectra of ionic liquids in which the wood has been immersed for 1 hour at 60 0C and 80 0C, as it can be seen from Fig. 3 and 4, which confirm that the wood has been delignified.

Fig. 5. FTIR spectra of BMIMBF4 at 250C (1); 600C (2) and 800C (3)

Fig. 6. FTIR spectra of BMIMPF6 at 250C (1); 600C (2) and 800C (3)

Table 3. CrI, MI and LI regarding the treated wood

specimens

Ionic liquid treatment

Temperature of treatment

(0C)

CrI

MI

LI

Blank 25 0.91 0.98 0.8 25 0.44 0.20 0.8 60 0.38 0.18 0.72 BMIMBF4 80 0.29 0.08 0.68 25 0.53 0.60 0.8 60 0.50 0.50 0.68 BMIMPF6 80 0.36 0.20 0.28

5. Conclusions Sycamore maple wood have been treated with

two types of alkylimidazolium-based ionic liquids: namely 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate. The treated wood evidenced some useful properties, such as improved wetability or higher compatibility with polar adhesives, in comparison with untreated wood.

Also it has been demonstrated by FTIR spectroscopy that the ionic liquids break the H-bonds between the cellulose macromolecules and decreases its crystallinity.

The treatment of sycamore maple wood veneers with 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquids could be useful in improving the workability of wood by decreasing its rigidity, and in preventing the build-up of static electric charges on the surface of the wood during finishing.

These results are in agreement with our former studies concerning the influence of the same ILs on poplar wood (Croitoru et al., 2011), giving a general view of the influence of these ILs on different wood species. Also, by comparison with other types of wood (maple, pine) (Lee et al., 2009), it seems that 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquids are able to extract a higher quantity of lignin from sycamore maple.

Acknowledgements This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/89/1.5/S/59323. The purchasing of the ionic liquids has been funded by the National University Council from Romania (CNCSIS) through IDEI 839/2008 national grant.

References

Croitoru C., Patachia S., Cretu N., Boer A., Friedrich, C.,

(2011), Influence of ionic liquids on the surface properties of poplar veneers, Applied Surface Science, 257, 6220-6225.

Dawson B.S.W., Singh A.P., Kroese H.W., Schwitzer M.A., Gallagher S., Riddiough S.J., (2008), Enhancing exterior performance of clear coatings through photostabilization of wood. Part 2: coating and weathering performance, Journal of Coatings Technology and Research, 5, 207-219.

Grāvitis J., Ābolinš J., Tupčiauskas R., Vēveris A., (2010), Lignin from steam exploded wood as binder in wood composites, Journal of Environmental Engineering and Landscape Management, 18, 75-84.

Haynes W.M., (2011), CRC Handbook of Chemistry and Physics, 91st Edition, CRC Press, London, UK.

He J.X., Tang Y.Y., Wang S.Y., (2007), Differences in morphological characteristics of bamboo fibres and other natural cellulose fibres: Studies on X-ray diffraction, solid state C-13-CP/MAS NMR, and

Croitoru et al./Environmental Engineering and Management Journal 10 (2011), 8, 1149-1154

1154

second derivative FTIR spectroscopy data, Iranian Polymer Journal, 16, 807-818.

Hill C., (2006), Wood Modification: Chemical, Thermal and Other Processes, (Wiley Series in Renewable Resource), Wiley-Blackwell, West Sussex.

Klemm D., Philipp B., Heinze T., Heinze U., Wagenknecht W., (1998), Comprehensive Cellulose Chemistry Volume I, Fundamentals and Analytical Methods. Wiley-VCH, Weinheim.

Lee S.H., Doherty T., Linhardt R., Dordick J.S., (2009), Ionic liquid-mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis, Biotechnology and Bioengineering, 102, 1368-1376.

Mohammed-Ziegler I., Oszlanczi A., Somfai B., Horvolgyi Z., Paszli I., Holmgren A., Forsling W., (2004), Surface free energy of natural and surface-modified tropical and European wood species, Journal of Adhesion Science and Technology, 18, 687-713.

Moharram M.A., Mahmoud O.M., (2008), FTIR spectroscopic study of the effect of microwave heating on the transformation of cellulose I into cellulose II during mercerization, Journal of Applied Polymer Science, 107, 30-36.

Moore A.K., Owen N.L., (2001), Infrared spectroscopic studies of solid wood, Applied Spectroscopy Reviews, 36, 65-86.

Patachia S., Friedrich C., Florea C., Croitoru C., (2011), Study of the PVA hydrogel behaviour in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, Express Polymer Letters, 5, 197-207.

Pernak J., (2000), Ionic liquids - the solvent of the XXI century, Przemsyl Chemiczny, 79, 153-156.

Pinkert A., Marsh K.N., Pang S.S., (2010) Reflections on the solubility of cellulose, Industrial & Engineering Chemistry Research, 22, 11121-11130.

Pitula S., Mudring A.V., (2010), Optical basicity of ionic liquids, Physical Chemistry Chemical Physics, 12, 7056-7063.

Popescu C.M., Popescu M.C., Singurel G., Vasile C., Argyropoulos D.S., Willfor S., (2007), Spectral characterization of eucalyptus wood, Applied Spectroscopy, 61, 1168-1177.

Popescu C.M., Larsson P.T., Tibirna C.M., Vasile C., (2010), Characterization of Fungal-Degraded Lime Wood by X-ray Diffraction and Cross-Polarization Magic-Angle-Spinning C-13-Nuclear Magnetic Resonance Spectroscopy, Applied Spectroscopy, 64, 1054-1060.

Rudawska A., Zajchowski S., (2007), Surface free energy of polymer/wood composites, Polimery, 52, 453-455.

Timar M.C., Beldean E., Porojan M., Gurau L., (2009), Field testing and microscopy - important tools for a realistic long-term evaluation of wood improvement treatments, Environmental Engineering and Management Journal, 8, 669-678.

Zakzeski J., Bruijnincx P., Jongerius A., Weckhuysen B., (2010), The catalytic valorization of lignin for the production of renewable chemicals, Chemical Reviews, 110, 3552-3599.

Zhang S., Lu X., Zhou Q., Li X., Zhang X., Li S., (2009), Ionic Liquids - Physicochemical Properties, Elsevier Press, Amsterdam, The Netherlands.