Article pubs.acs.org/IECR
Lignin Modification with Carboxylic Acids and Butyrolactone under Cold Plasma Conditions Oana Chirila,† Marian Totolin,† Georgeta Cazacu,† Marius Dobromir,‡ and Cornelia Vasile*,† †
“P. Poni” Institute of Macomolecular Chemistry, Department of Physical Chemistry of Polymers, 41A Gr. Ghica Voda Alley, Ro 700487 Iasi, Romania ‡ Faculty of Physics, “Alexandru Ioan Cuza” University, 11 Carol I Boulevard, Ro 700487 Iasi, Romania S Supporting Information *
ABSTRACT: The modification of organosolv lignin powder (ALCELL) with different carboxylic acids such as oleic, lactic, and butyric acids and butyrolactone under cold plasma discharge has been performed. The X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR-FTIR), and 1H NMR spectroscopy, scanning electron microscopy (SEM), and thermal methods (differential scanning calorimetry (DSC) and thermogravimetry (TG)) proved that the lignin modification took place. The structure, morphology and thermal properties are specific for each kind of product obtained. The modification degree determined on the basis of XPS data varies from 4 to 13% depending on reagent used. The significant changes in the thermal properties indicate that the modification by cold plasma affected also some bulk properties. This is the case mainly for modification with butyric acid and butyrolactone. It was concluded that the cold plasma modification is an efficient and eco-friendly technique able to produce new valuable products from lignin, widening its compatibility with various polymers and also its applications.
1. INTRODUCTION Lignin is a natural polymer found in vegetables and is one of the most abundant biomacromolecules, available in large quantities at low cost. Lignin, a readily available form of biomass, is a potential source of renewable aromatic chemicals. Every year, the pulp and paper industry generates over 45 million metric tons of lignin as a byproduct of chemical wood pulps and uses about 10 million metric tons of lignin as a component of mechanical wood pulps. The majority of the byproduct lignin is being used internally as a low-grade fuel for the chemical pulping operation, while the lignin-rich mechanical wood pulps are being used mainly to make short-life paper products such as newsprint and telephone directories because of the light-instability of lignin. There is a tremendous economic incentive to find better uses of lignin and to expand the markets of mechanical wood pulps. Chemical modification of lignin is an area of significant scientific work.1,2 Chemical modification of lignin is aimed at the photostabilization and upgrading of lignin and also to obtain new products. The lignin has been subjected to the modification reaction for introducing functional groups into its structure, thus assuring a more complete exploitation of this natural aromatic polymer. Lignin and especially modified lignins can be successfully used in blends and composites with various synthetic polymers because it offers some advantages such as biocompatibility, exhibits good adhesion and adsorption characteristics, and has a good compatibility with a relatively large number of solvents and small molecular compounds.3,4 The hydroxymethylation reaction induces the modification of lignin functionality and polymolecularity.5 Further Kraft lignins from hardwood and softwood were esterified with several anhydrides to alter their solubility behavior in nonpolar © 2013 American Chemical Society
solvents, such as styrene-containing thermoset resins. The main goal was to obtain a styrene soluble Kraft lignin that could be used in unsaturated polyesters and vinyl esters, and it was achieved with fully butyrated Kraft lignin and a butyrated/ methacrylated Kraft lignin. The solubility of the latter is governed by the butyrate/methacrylate ratio.6 Unfortunately, all chemical modifications use organic solvents, which pollute environment and increase toxicological problems. Cold plasma (low temperature and low pressure) chemistry is a useful technique for in situ polymer synthesis and surface modification of both synthetic and natural polymers.7−9 Plasma-chemistry technology offers an alternative and efficient way also for lignin functionalization. Plasma modification approaches have a number of advantages that include environmental benefits because the technique involves dry chemistry, the active species (electrons, ions, atoms and molecules, in excited and ground) states only penetrate about 10 nm deep, so it does not alter the base structure and even the most inert surface can be functionalized.10 By applying relative intense electric fields, it can be produced a plasma discharge without the risk of damaging of bulk properties of the solid material. The process does not require large amounts of organic chemicals (often toxic), that will affect the environment. The energy levels of the plasma species are comparable with the common bond energies and, consequently, even the most inert material surfaces can be conveniently modified. Silicon chloride plasma functionalized lignin has been successfully grafted with polydimethylsiloxane in a postplasma Received: Revised: Accepted: Published: 13264
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procedure using oxygen, argon, and silicon chloride plasma gases and three different cold-plasma installations such as a parallel-plate diode configuration 30 kHz radio frequency (RF)reactor: a rotating 13.56 MHz, RF-plasma reactor, and a dense medium (liquid) plasma (DMP) reactor.10−12 Modified lignins used as binder yield low-cost composite materials that have a reasonable wet strength. A modified lignin has been added to formaldehyde-based binder systems such as phenol formaldehyde (PF), ureaformaldehyde (UF), melamine formaldehyde (MF), resorcinol formaldehyde (RF), and/or tannin formaldehyde resins and may be used for panel boards such as plywood, hard board, medium density fiberboard, or particleboards.13 The incorporation of lignin into polymeric systems has been demonstrated, and this depends on solubility and reactivity characteristics. Several industrial utilization examples for sulfur-free, water-insoluble lignins were presented such as materials for automotive brakes, wood panel products, biodispersants, polyurethane foams, and epoxy resins for printed circuit boards.14 In our previous paper, the lignin from furfural lignocellulosic obtained by acid hydrolysis has been modified with methylmethacrylate and acrylonitrile under the RF plasma conditions, and modified lignin with good properties have been obtained.15 This paper deals with the modification of the Alcell lignin powder with different carboxylic acids such as oleic, butyric, and lactic and with butyrolactone under RF cold plasma discharge, to increase its applicability and compatibility with other polymers. The modified lignins have been comparatively investigated with unmodified one by spectroscopic and microscopic methods.
Figure 1. Laboratory experimental setup for RF plasma device for the powdery materials.
ethyl ether and deposited on plasma reactor. After treatment, the product was removed from reactor, and the unreacted reagent was extracted for 8 h in a Soxhlet extractor with ethyl ether, and then, the modified samples were dried and analyzed. In a previous paper,18 the weight average molecular weights have been determined by static light scattering measurements. The weight average molecular weight of the Alcell lignin (Mw = 15.6 KDa) is not influenced too much by the modification with lactic acid (LA) and oleic acid (OA) (Mw= 13.5 KDa), but it is drastically reduced by modification with butyric acid (Mw = 7.2 KDa) and butyrolactone (Mw = 2.3 KDa). The hydrodynamic volume of the particles (Z-values) ranges from 800 to 6500 nm for Alcell lignin and decreased to 500−700 nm after plasma modification. 2.3. Investigation Techniques. 2.3.1. XPS Spectra. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a 5000 VersaProbe spectrometer, equipped with a monochromatic Al Kα X-ray source (hν = 1486.7 eV). During measurements, the pressure in the analysis chamber was maintained at 5.9 × 10−8 Pa, and the photoelectron takeoff angle relative to the surface was 45°. Survey spectra for each sample over a binding energy range from 0 to 1150 eV were the average of three scans acquired at a pass energy of 160 eV and resolution of 1 eV/step. High-resolution spectra of C 1s, and O 1s were the average of five scans acquired with constant pass energy of 20 and 0.05 eV/step resolution. Quantitative analysis of the spectral data (surface chemical compositions, expressed as relative atomic percentage concentration (at. %)) was obtained from the peak-areas ratios corrected with the experimentally determined sensitivity coefficients for the most intense spectral line for each elemental species. The estimated uncertainty is ±1% for C and ±2% for O. The CasaXPS software was used for background subtraction (Shirley-type), peak integration, fitting, and quantitative chemical analysis. All binding energies were referenced to the C 1s (C−C) peak at 285 eV. The resolution for measurements of binding energy is about 0.2 eV. The high resolution spectra were curve-fitted using a mixed Gaussian−Lorentzian (70:30) function to input the required component contributions. Data from three replicates of each sample type were recorded, and at least three separate areas on each individual sample were analyzed. The XPS data were analyzed using PHI Summit software for deconvolution. 2.3.2. ATR-FTIR Spectroscopy. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra have been recorded with 64 scans at 4 cm−1 resolution by means of a spectrometer Bruker VERTEX 70, in absorbance mode, by the ATR technique with a 45° ZnSe crystal with refraction index of 2.4. Penetration thickness was about 100 μm. For each sample, the evaluations were made on the average spectrum obtained from three recordings. Background and sample spectra were obtained in the 500 to 4000 cm−1
2. EXPERIMENTAL SECTION 2.1. Materials. Organosolv lignin (Alcell), fine brown powder, was delivered by a commercial-scale demonstration plant in New Brunswick, Canada. It is produced in tonnage quantities, and it shows a strong promise as a partial replacement on an equal weight basis for PF resins in wafer board, oriented strand board, and other wood composites. (Alcell) (L) is soluble in ethyl alcohol and ethyl ether, and chloroform is a byproduct of an organosolv pulping process called the Alcell process.16 Organosolv lignin (Alcell lignin) is highly hydrophobic and insoluble in neutral or acidic aqueous media, but it is soluble in moderate to strong alkaline solutions and certain organic solvents. It has a Tg of ∼89 °C. Reagents used for the lignin modification were butyric acid (BA) (Merck, purity 99%); oleic acid (OA) (Aldrich, 90% purity) and lactic acid (2-hydroxypropionic acid) (LA) (Aldrich, 90% purity) and 4-butyrolactone (4-hydroxybutiric acid lactone) (BL) (Merck, purity >99%). 2.2. Plasma Treatment Procedure. The modification of lignin has been carried out in an RF cold plasma reactor specially designed for treatments on powdery materials under atmospheric pressure.17 The experimental setup is presented in (Figure 1). The main advantage of this plasma reactor (20 × 20 cm) is that the whole electrode system (4 m long copper two wires cable) is embedded in polymeric material. This system is coupled to a plasma generator (500 Hz; 50 W). The discharge time was 60 min. Before treatment, the ALCELL lignin (L) powder was impregnated with 5 wt % reagent solutions of oleic acid (OA), lactic acid (LA), butyric acids (BA), or butyrolactone (BL) in 13265
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Figure 2. XPS survey spectra of (a) pristine lignin powder and modified lignin with OA.
A comparison between the survey XPS spectra presented in Figure 2 reveals that C and O are the predominant species (usually found at virgin lignin surface). Plasma exposure led to very small weight loss, and changes in the chemical composition on the polymer surface reflected in the decrease of the C1s percentage for all modified lignins from 80.2 at. % for pristine polymer to 75.0 at. % for lignin modified with oleic acid, which showed the highest decrease in carbon percentage in respect with all samples. Correspondingly, this decrease in carbon percentage is accompanied by an increase in oxygen percentage from 19.8 at. % for unmodified lignin to 25 at % for sample modified with OA. There is also a shakeup satellite at approximately 291 eV, suggesting the existence of the aromatic functional groups.23 Guo et al. analyzed lignins by XPS and assigned the binding energy of C1s to 283.52 eV (CH or C−C), 284.58−285.72 eV (COR or COH), 286.10−286.44 eV (CO or HO COR), 287.65−287.72 eV (OCO), and O1s binding energy was to 530.31 (OH), 531.45−531.72 (RCO), 532.73−533.74 (OCO) eV, respectively.24 The O/C ratio of 0.25 is characteristic to lignin, and it presented an increase for samples modified with OA and BA, which show probably a high degree of modification. Oxygen−carbon ratios of 0.25, 0.38, and 0.39 were also reported by Ahmed et al.25 for Iotech lignin, thiolignin, and milled wood lignin, respectively. Hon (1984)26 observed an oxygen−carbon ratio of 0.43 for milled wood lignin. Oxygen−carbon ratios in the range of 0.31 ± 0.36 were found by Dorris and Gray (1978)27 for dioxane lignin. High resolution scans of the XPS spectra of C 1s can be curve-fitted with four peak components at around 285, 286.5, 288, and 289 eV, from the chemically nonequivalent carbon atoms: resulting from C1 (carbon atoms only linked to carbon or hydrogen atom (CC, CH), C2 (carbon atoms bonded to single oxygen (COR or COH), C3 (carbon atoms bonded to two noncarbonyl oxygen atoms or to a single carbonyl oxygen atom (CO, HOCOR), and C4 (carbon atoms bonded to a carbonyl and a noncarbonyl oxygen (OCO), respectively.27−29 C1 amount decreases after cold plasma modification with all reagents used, while C2, C3, and C4 amount increase from 24 at. % to 27 at. %, from 9.4 at. % to 19.4 at. % or from 1.3 at. % to 2.96 at. %, respectively. O1s can be curve-fitted with three peak components assigned: O1/531.07 to OH, O2/532.24 to RCO, and O3/533.73 to OCO. Their amount increase or decrease in respect with the value of unmodified lignin specifically for each modifying reagent. All XPS results demonstrate that the
wavenumber range. The processing of spectra was achieved using a SPECVIEW program. The following spectral characteristics of the unmodified and modified lignin samples have been evaluated (additional spectral data available in Supporting Information): The energy of the H-bonds has been calculated using eq 1:19 E H (kJ) = 1/k[(vo − v)/vo]
(1)
where νo is the standard frequency corresponding to free −OH groups (3650 cm−1); ν is the frequency of the bonded −OH groups; k is a constant of 4 × 10−3 kJ−1. The enthalpy of H-bond formation has been evaluated using eq 2:20 ΔH (kJ/mol) = 0.0672ΔνOH + 2.646
(2) −1
where Δ νOH is the OH wavenumber shift (cm ). The H-bonding distance (R) (Å) is obtained by the Sederholm eq 3:21 Δν (cm−1) = 4.43 × 103(2.84 − R )
(3)
where Δν = νo − ν; νo is OH monomeric stretching frequency of 3600 cm−1; and ν is OH stretching frequency in the IR spectrum of the sample.22 1 H NMR spectra have been recorded using a 1H NMR Bruker Avance DRX 400, solvent DMSO-D6. 2.3.3. SEM. Scanning electron microscopy (SEM) images have been taken by means of an electronic microscope, BALEAJ (ESEM) Quanta type 200; samples were analyzed in powder form, and the examination mode was ESEM/EDAX. 2.3.4. Differential Scanning Calorimetry (DSC). DSC measurements were conducted on a DSC 200 F3Maia device (Netzsch, Germany). A mass of 10 mg of each sample was heated in pressed and pierced aluminum crucibles at a heating rate of 10 °C min−1. Nitrogen was used as inert atmosphere at a flow rate of 50 mL min−1. 2.3.5. Thermogravimetry (TG). TG measurements were performed on a STA 449 F1 Jupiter device (Netzsch, Germany). About 10 mg of each sample was weighed and heated in alumina crucibles. Nitrogen was purged as inert atmosphere at a flow rate of 40 mL min−1. Samples were heated in the temperature range from 30 to 700 °C at a heating rate of 10 °C min−1.
3. RESULTS AND DISCUSSION 3.1. XPS Results. The XPS survey spectra of untreated and representative plasma-treated/modified sample are shown in Figure 2. 13266
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Figure 3. FTIR spectra of Alcell lignin unmodified (L) and modified lignins with LA, OA, BA, and BL: (a) total spectra, and in selected spectral regions (b) 2700−4000 cm−1; (c) 600−1800 cm−1.
modification reaction in RF cold plasma conditions took place both with carboxylic acids and also with butyrolactone. The average modification degree determined on the basis of XPS data varies in the order: LA (>4.3) < BL (11.7) < BA (13.3) < OA (17%) 3.2. ATR-FTIR Spectroscopy Results. ATR-FTIR spectra of the Alcell lignin unmodified and modified with carboxylic acids and butyrolactone are given in Figure 3.
The spectra of the plasma modified lignins and unmodified lignin are almost similar, Figure 3a, because many of the bands of components are overlapped and the modification degree is low. The assignment of the bands was done according to the literature data.30−33 Small differences appear in the 2800−3000 cm−1 region by presence of the band at 2970 cm−1 and also by slightly increase of the band intensities (Figure 3b). This band is assigned to methyl and methylene groups in carboxylic acid chains or in the structure of butyrolactone, which indicates the 13267
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Figure 4. 1H NMR spectra of L and cold plasma modified lignin.
Figure 5. Scanning electron micrographs for L and cold plasma modified lignins. Magnification: 5000×.
lignin samples. H-bonding distance (R) values do not show variation with sample nature; it ranges from 2.788 Ǻ to 2.795 Ǻ . In the “fingerprint” 1650−1800 cm−1 region, the deconvoluted bands are characteristic for each sample and also their spectral characteristics. This band corresponds to some possible new bonds which could appear between lignin substrate and reagents under plasma conditions. The 1650−1800 cm−1 band can be decomposed in three bands found at 1688−1693 cm−1 assigned to CO vibration in unsaturated structures, at 1703−1708 cm−1 assigned to CO vibration in aromatic structures, while the third at 1717−1743 cm−1 can be assigned to CO functionality in ester or ketone structures. Their proportion varies from one sample to the other. The lignin band centered at 1704 cm−1 is the most important, and it is accompanied by other two small bands. The deconvoluted bands of L/LA, LOA and L/BL have approximately the same heights and areas under curves, which should means that all three kinds of bonds are formed with the same probability; therefore, the bonding of acids and butyrolactone could take place both by ester groups, or some unsaturation is introduced in structure. By modification with butyrolactone the most important is ester bond. 3.2. 1H NMR Results. 1H NMR is able to quantify a number of important residual lignin structural features including carboxylic acid (δ 12.6−13.5 ppm), aldehyde (δ
attachment of these on lignin molecule. In the 3050−3650 cm−1 region assigned mainly to −OH groups, the lignin shows a band centered at 3420 cm−1, while for modified lignins − νOH absorption bands have lower intensity and is slightly shifted to higher wavenumbers. In the case of the L/BL and L/BA samples (Figure 3c), one can note the differences in intensities of absorption bands at 1720 cm −1 attributed to CO groups, at 1340 cm −1 deformation vibration of CH group, and 1270 cm −1 corresponding guaiacyl-type ring plus CO stretch, the last band also can belong to modifying agents. In the 1750−1200 cm−1 range, the spectra of L/LA and L/OA are similar to L spectrum. To evidence clearer the differences between samples some bands have been deconvoluted. The 3000−3700 cm−1 deconvoluted band shows three peak components whose peak positions and intensities are specific for each sample. The deconvoluted bands of L and L/BL are similar while for other three samples the shapes are different. The results of the deconvolutions and some spectral characteristics for this spectral region, which mainly are assigned to hydroxyl groups and hydrogen bonds, are summarized. For the main deconvoluted band, both enthalpy and energy of the hydrogen bonds are higher for modified lignin with LA, OA, and BA than those in L and L/BL, which should mean that hydrophilicity was increased for the first group of modified 13268
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Figure 6. EDAX spectra of the L (a) and L/OA (b) modified lignin.
Figure 7. DSC curves of unmodified and modified lignins.
Figure 8. TG/DTG curves of lignin and modified lignins under cold plasma conditions.
9.4−10.0 ppm), phenolic hydroxyl (δ 8.0−9.4 ppm), β−5 phenolic hydroxyl (δ 8.99 ppm), syringyl C5 phenolic hydroxyl (δ 8.0−8.5 ppm), aromatic protons (δ 6.3−7.8 ppm) and corresponding aromatic structures, and aliphatic protons.34−37 The spectra of the grafted lignins evidence the structural modifications in lignin macromolecules by the presence of the signals with different intensities in the regions: 0.8−2 ppm due to protons from saturated aliphatic structures and 3.7−3.9 ppm methoxyl protons (Figure 4). 3.3. SEM Results. In the micrographs (Figure 5 at magnification 5000×), aggregates of lignin particles, which are
covered by modified polymer, can be seen. In the case of the samples modified with butyric acid and butyrolactone, the particles seem to be expanded probably due to of a high modification degree in accordance with ATR-FTIR results. As it was expected only carbon and oxygen were detected on the surface of the samples both untreated and plasma modified by EDAX advanced microanalysisFigure 6. The average composition is 78.2−78.4 wt % C or 82.8 at. % C and 21.6− 21.9 wt % O or 17.2−17.4 at % O for most of samples excepting sample L/LAwhich has a different composition of 79.98 wt % C (84.2 at. %) and 20.2 wt % O (15.82 at. %). This 13269
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The cold plasma modification is an eco-friendly technique able to produce new valuable products from lignin widening its applications.
change in the surface composition is a proof of the surface modification; in other cases probably modification can occur also in bulk, take in view of the powdered form of substrate sample. Thermal characterization is capable to give accurate information on molecular arrangement, phase transition, degradation temperature, heat capacity, enthalpy of transition, and interaction between lignin and low molecular weight compounds such as water, solvent traces, etc.38−40 Chemical modification of various kinds of lignins was achieved by esterification with succinic anhydride in aqueous solutions. FTIR spectroscopy clearly proved that succinoylation took place. The changes were reflected also by an increased thermal stability compared to the corresponding unmodified lignins.2 DSC curves recorded in the second run evidence the variation of the glass transition of lignin after modification under cold plasma conditionsFigure 7. The glass transition temperature was determined as the middle point of the transition. Alcell lignin shows a glass transition at 88.6 °C, while all cold plasma modified lignin have higher glass transition temperatures above 112 °C, with the highest value for lignin modified with butyrolactone of 137.1 °C. All values are close to those found in other papers.38−40 Thermogravimetric data are given in Figure 8 and in Supporting Information. All samples show a first step of mass loss at low temperature ranging from 40 to 150 °C with a mass loss of 1.5−3.2% assigned to water loss and other low molecular weight compounds. Unmodified lignin decomposes by a single step that occurs on a large temperature range from 161 to 551 °C with a shoulder at 279.4 °C. Similar behavior to the thermal decomposition shows lignin modified with oleic acid. In the DTG curve of the L/LA sample, a separate peak is present in the 155−262 °C region with a mass low of 11%, while lignin samples modified with butyric acid and butyrolactone are stable in this interval, their decomposition occurring in one step at higher temperatures from 220 to 439 °C. These last two samples seem to be the thermally stable in respect to other samples. The significant changes in the thermal properties indicate that the modification by cold plasma affected also some bulk properties. This is the case, mainly for modification with butyric acid and butyrolactone. The remaining mass at temperatures higher that 550 °C is higher for unmodified lignin (46.8 wt %) than for modified ones (41−44 wt %) that should means an advanced carbonization process for unmodified lignin sample.
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ASSOCIATED CONTENT
S Supporting Information *
Table 1: Surface atomic composition of the unmodified and modified lignin. Table 2: Assignment of the carbon and oxygen peak components, C 1s, and O 1s for samples. Table 3: Spectral characteristics of deconvoluted 3000−3700 cm−1 band. Table 4: Spectral characteristics of the 1650 − 1800 cm −1 deconvoluted band of the L and cold plasma modified samples. Table 5: Thermogravimetric data for unmodified and cold plasma modified lignins. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +40 232217454. Fax: +40 232211299. E-mail: cvasile@ icmpp.ro. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by Romanian National Authority for Scientific Research, CNCS−UEFISCDI, project 164/2012
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REFERENCES
(1) Chemical Modification. Properties, and Usage of Lignin; Hu, T. Q., Ed.; Springer: Berlin, 2002. (2) Xiao, B.; Sun, X. F.; Sun, R. C. The Chemical Modification of Lignins with Succinic Anhydride in Aqueous Systems. Polym. Degrad. Stab. 2001, 71 (2), 223−231. (3) Vasile, C.; Downey, M.; Wong, B.; Macoveanu, M. M.; Pascu, MC.; Choi, J.-H.; Sung, C.; Baker, W. Polyolefins/Lignosulfonates Blends. II. Isotactic Polypropylene/Epoxi-Modified Lignin. Cell. Chem. Technol. 1998, 32 (1−2), 61−88; Composite Sci. Technol. 1993, 48, 317−326. (4) Cazacu, G.; Pascu, M. C.; Profire, L.; Kowarski, A. I.; Vasile, C. Lignin Role in a Complex Polyolefin Blend. Ind. Crops Prod. 2004, 20 (204), 205−219. (5) Ungureanu, E.; Ungureanu, O.; Căpraru, A.-M.; Popa, V. I. Chemical Modification and Characterization of Straw Lignin. Cellulose Chem. Technol. 2009, 43 (7−8), 263−269. (6) Thielemans, W.; Wool, R. P. Lignin Esters for Use in Unsaturated Thermosets: Lignin Modification and Solubility Modeling. Biomacromolecules 2005, 6 (4), 1895−1905, DOI: 10.1021/bm0500345. (7) Simionescu, Cr. I.; Cazacu, G.; Macoveanu, M. M.; Totolin, M.; Rozmarin, Gh. Cellulose. Chem. Technol. 1983, 17, 487. (8) Roth, J. R. In Industrial Plasma Engineering, Vol. 2: Applications to Nonthermal Plasma Processing; IOP: Bristol and Philadelphia, 2001; pp 201−223. (9) Denes, F.; Young, R. A.; Sarmadi, M. Surface Functionalization of Polymers under Cold Plasma ConditionsA Mechanistic Approach. J. Photopolymer Sci. Technol. 1997, 10, 91−98. (10) Toriz, G.; Denes, F.; Young, R. A. Plasma Modification of Lignin Chapter 19 in Lignin: Historical, Biological, and Materials Perspectives. ACS Symp. Ser. 1999, 742, 367−389. (11) Toriz, G.; Ramos, J.; Young, R. A. Lignin−Polypropylene Composites. II. Plasma Modification of Kraft Lignin and Particulate Polypropylene. J. Appl. Polym. Sci. 2004, 91 (3), 1920−1926. (12) Toriz-Gonzalez, G. Use of Cold Plasmas for Lignin Modification and Improvement of Lignin−Polypropylene Composites. Dissertation Abstracts Int., Section B 2000, 61−05 (2712), 157p.
4. CONCLUSIONS Alcell lignin was modified with different organic acids and butyrolactone in cold plasma conditions. Data obtained by XPS, ATR-FTIR, and 1H NMR spectroscopy and SEM indicated that the modification took place. It has been established that the structure and the morphology of modified lignin depend on type of reagent used. The average modification degree determined on the basis of XPS data varies in the order: LA (>4.3)
