Wtent in kraft pulps, kappa numbers

Transcription

1 PEER-REVIEWED PULP TESTING FT-Raman spectroscopy for direct measurement of lignin concentrations in kraft pulps UMESH P. AGARWAL, IRAA.WEINSTOCK, AND RAJAI H. ATALLA ABSTRACT:This study used near-infrared (IR) Fourier transform (FT) Raman spectroscopy was used to quantify residual lignin in unbleached and partially bleached pine kraft pulps. The pulps used in this study were bleached using CEDED and polyoxometalate (POM) bleaching sequences. The intensity of lignin's 1600 cm -1 Raman band, which represents one of the phenyl ring vibrations, was used as a measure of the total amount of lignin in pulp samples. When the 1600 cm -1 band intensities were compared with pulp kappa numbers, the band intensities were found to be linearly related to the kappa numbers. This linear relationship holds for both CEDED- and POM-bleached pulps, indicating that the 1600 cm -1 band intensity is mostly independent of bleaching chemistry and is dependent upon the number of phenyl groups in pulps. Consequently, FT-Raman spectroscopy has the potential for determining lignin concentrations in unbleached and partially bleached kraft pulps. The Raman technique is attractive because it is rapid, is not labor intensive, and does not result in the destruction of pulp samples. Application: A new method for measuring kappa numbers, near-ir FT-Raman spectroscopy is capable of determining lignin concentration in pulps. idely used as indices of lignin con- Wtent in kraft pulps, kappa numbers are obtained by measuring amounts of permanganate consumption by pulp slurries under standardized conditions. However, the presence of hexenuronic acids in pulps can lead to abnormally high kappa numbers, as shown by recent research [1, 2]. This finding is especially important for hardwood kraft pulps, for which up to 55% of the pulp kappa number can be attributed to hexenuronic acids in some cases [2]. In softwood pulps, the problem is less severe because the concentration of the hexenuronic acids produced is not that high [2]. Besides being inaccurate, the kappa measurement provides little insight about the chemical aspects of kraft lignin. In many cases, such as in the investigation of new bleaching technologies, it would be helpful to obtain more direct information concerning the degradation or removal of residual lignin phenyl groups from treated pulps. (The phenyl groups represent lignin monomer units, the basic building blocks of lignin polymer.) Kappa number determinations are also time consuming. In both research and production contexts, it would be preferable to assess lignin content more rapidly. Near-IR FT-Raman spectroscopy is a rapid technique for obtaining Raman spectra of chemical substances. Unlike IR or conventional Raman techniques, FT- Raman spectroscopy is capable of providing a rapid, direct, and accurate measure 22 TAPPI JOURNAL JANUARY 2003 of the concentration of phenyl groups within a kraft pulp sample. Results of FT- Raman investigations of pulps removed after each stage of CEDED [3] and polyoxometalate (POM) [4] bleaching sequences are presented here and compared with traditional kappa number values. This research had two goals: to provide more detailed information concerning the chemical significance of kappa number values and to assess the applicability ofthe FT-Raman technique to lignin quantification. Raman and IR spectroscopies provide complementary information. Vibrational modes that are weakly active in one technique are usually detected as strong bands in the other. However, the conventional Raman and IR techniques are difficult to apply to the study of kraft pulps [5, 6]. (In conventional Raman techniques, visible laser light is used to excite the sample.) In the case of diffuse reflectance infrared Fourier transform (DRIFT), which is an IR spectroscopy technique often used to study pulps and woods, quantitative information is generally difficult to obtain because of the heterogeneous nature of pulp and wood samples. Reflectance of pulp samples varies not only with the coefficients of absorption and scattering but also with sample thickness. Consequently, light is absorbed in an irreproducible manner, resulting in unpredictable baseline fluctuations. This difficulty is particularly troublesome in the quantification of residual lignin in kraft pulps, where the concentration of absorbing species is low and the prominent lignin aromatic ring bands at 1600 cm -1 and 1510 cm -1 are therefore weak. In addition, these IR bands are partly obscured by the contributions of adsorbed water at 1640 cm -1 (H 2O bending mode) and of a neighboring polysaccharide band that rises sharply at 1460 cm -1 (CH 2 bending mode). In near-ir spectroscopy, overtones and combination bands of fundamental vibrational modes are observed. Considering that there is significant overlap among the contributions of lignin, water, cellulose, and hemicelluloses, this approach to analyzing kraft pulps has proven to be indirect and unreliable for lignin quantification [6]. In Raman spectroscopy, optical heterogeneity of the lignocellulosics (including pulps) and the presence of adsorbed water do not present a problem. For example, conventional Raman spectroscopy techniques have been used effectively in studies of plant cell walls [7], lignin orientation in native woody tissue [8], and mechanical pulps [9]. In some of the work, samples were even immersed in water. However, when applied to unbleached and partially bleached kraft pulps, conventional Raman spectroscopy has not been very successful [10].The residual lignins in the pulps contain chromophores that highly absorb light in the visible region. When excited with nm or nm laser

2 PULP TESTING 1. FT-Raman spectra of CEDED-bleached kraft pulps in the 2. FT-Raman spectra of polyoxometalate (POMI-bleached region of cm -1 (C, chlorine treated. CE, chlorine kraft pulps in the region of cm -1 (EV, extracted treated and extracted. CED, chlorine treated, extracted, and POM treated. EVV, EV pulp after second-stage POM and chlorine dioxide treated. CEDE, CED pulp after extrac- treatment. EVVV, EVV pulp after third-stage POM treattion stage. CEDED, CEDE pulp after dioxide stage). ment. EVVVE, EVVV pulp after extraction stage). light, these samples gave rise to fluores- bers of pulps were determined using the nally present ClO 2 had been consumed cence that overwhelmed the Raman sig- TAPPI useful method UM-246. (iodometric method). nal. Moreover, various agents used to quench the fluorescence were not effective. CEDED bleaching Approximately 14 g of unbleached kraft pulp (never dried) was initially treated. In the second extraction stage, performed at 60 C, 1.2% NaOH on pulp was used. The pulp was treated for 60 min at A new Raman technique, near-ir FT- After each treatment step, the pulp was 10% consistency. The procedure was the Raman, was developed in 1986 [11]. In thoroughly washed with reverse osmosis same as in the first E stage. this technique, Raman scattering is gen- water, and approximately 2 g of pulp was In the second ClO 2 stage, 0.6% ClO 2 erated by laser excitation in the near-ir removed for chemical analyses. Prior to (on pulp) was used. The pulp consistenregion. For excitation in the near-ir, a chlorination (C stage), the pulp was dis- cy was 3.2%, and the ph was maintained Nd:YAG laser with a wavelength of 1064 persed in 1 L of water and stirred with a between 4 and 5. The treatment was carnm is most often used. Because most mixer. The pulp was filtered and washed ried out for 1 h and 45 min at 70 C. The materials do not absorb at near-ir wave- with 2 L of additional water. treatment procedure was the same as in lengths, fluorescence is significantly Chlorine solution (1.45% Cl 2 on pulp) the first ClO 2 stage. reduced or completely eliminated. was added to the pulp slurry (1.8% con- Polyoxometalate bleaching Although the Raman signals resulting sistency), and the slurry was constantly The unbleached pulp was pretreated from near-ir excitation are weaker than stirred for the length of the reaction time with alkali in nitrogen atmosphere, desthose observed in conventional Raman (45 min). At the end of the reaction, the ignated as E stage, to remove easily solspectroscopy, this drawback is more than chlorine consumption was found to be ublized lignin. The POM used in this compensated for by the use of Fourier 89% of the original amount. The iodomet- study was a-keggin-k 5 [SiVW 11O 40] [13]. transform techniques. In fact, the time ric method was used to determine the The total POM treatment (V) was carried needed to acquire an FT-Raman spec- amount of chlorine. out in three stages. The V stage was cartrum (10-15 min) is much shorter than The chlorine-treated pulp was washed ried out by heating mixtures of pulp, the time needed in conventional Raman and stored in a ziplock plastic bag. An water, inorganic buffer, and POM in a spectroscopy (4-8 h). Taken together, alkaline solution (2% NaOH on pulp) was stirred, high-pressure Parr reaction vesthese advantages make the near-ir added to this pulp. The bag was kneaded sel (Parr Instrument Co., Moline, IL). FT-Raman technique well suited to lig- (by hand) to disperse the alkali solution Details of the bleaching treatment are nocellulosic research [ 12, 13]. throughout the pulp. The plastic bag con- provided elsewhere [4]. All three V taining the pulp was immersed in a water stages were performed under identical EXPERIMENTAL bath at 60 C for 60 min. This alkali treat- conditions. After each V stage, the pulp Pulps ment constituted the first extraction was washed thoroughly. After a third V Unbleached pine kraft pulps were stage (E). stage, the second alkali extraction stage obtained from Consolidated Papers, Inc. For the chlorine dioxide stage (D), was performed. After various stages of (now Stora Enso North America, water was preheated to 70 C. Pulp pulp treatment, a small amount of pulp Wisconsin Rapids, Wisconsin). Two different batches of kraft pulp were used in and ClO 2 was added at 1.2% on pulp. The removed after alkali extraction, after the obtained after the first E stage was added, was removed for analyses. Pulp samples bleaching. Kraft pulp of kappa no ph of the mixture was maintained first V stage, after the second V stage, was used for the CEDED sequence, between 4 and 5 (using 1M NaOH) during the course of the reaction. At the end alkali extraction are respectively desig- after the third V stage, and after second whereas pulp used in POM system had a kappa number of Microkappa num- of the reaction (3 h), 95.5% of the originated as E,EV,EVV, EVVV, and EVVVE. VOL. 2: NO. 1 TAPPIJOURNAL 23

3 3. Linear regression of lignin's 1600 cm -1 Raman intensities 4. Linear regression of lignin's 1600 cm -1 Raman intensities (peak heights) on microkappa numbers for the CEDED- (peak areas) on microkappa numbers for the CEDEDbleached kraft pulps (R 2 = Data points read right to bleached kraft pulps (R 2 = Data points read right to left. US, unbleached. C, chlorine treated. E, alkali extract- left. US, unbleached. C, chlorine treated. E, alkali extracted. D, chlorine dioxide treated). ed. D, chlorine dioxide treated). Raman analyses Raman spectra of pulps obtained in the CEDED sequence were obtained using a Bruker RFS 100 instrument (Bruker Instruments, Inc., Billerica, MA). This inhouse spectrometer is equipped with a 1000 mw Nd:YAG diode laser for sample excitation. The POM pulps were analyzed using a Bruker IFS 66/FRA 106 system (spectra were recorded at Bruker Instruments), equipped with a 350-mW Nd:YAG laser. Pulp samples (50 mg) were compressed in a KBr pellet press. The pulp pellets were sampled with 100 mw of laser power at a spectral resolution of 4 cm -1. With the use of a double-sided, forward-backward scanning mode, 600 scans were accumulated. The pellets were sampled by keeping a front-surfacecoated mirror behind the samples to enhance the signal-to-noise ratio. The spectral acquisition time per pulp sample was about 15 min. The spectra between 850 cm -1 and 1850 cm -1 were corrected for the background contribution. Although a large part of the background was removed, a residual amount was still present. Intensities of the bands at 1098 cm -1 and 1600 cm -1 were calculated with the baseline method. For this purpose, a baseline was drawn between 1216 and 1010 cm -1 for the 1098 cm -1 band and another between 1671 and 1545 cm -1 for the 1600 cm -1 band. Peak-height and bandarea intensities were calculated for each band. The relative intensities of the 1600 cm -1 band were calculated by dividing its peak-height intensity and its area Intensi 1600 cm -1 INTENSITY 1600 cm -1 INTENSITY BLEACHING KAPPA 1098 cm -1 INTENSITY 1098 cm -1 INTENSITY STAGE NO. (area ratio) (peak-height ratio) CEDED sequence Unbleached C CE CED CEDE CEDED Polyoxometalate sequence Unbleached E EV EVV EVVV EVVVE Kappa numbers and Raman intensity data for kraft pulps. ty by the 1098 cm -1 band intensities (ratio of 1600 cm -1 /1098 cm -1 ). The calculated intensities were called peak and area intensities. The division by the 1098 cm -1 band intensity (mostly attributable to cellulose [13]) amounts to normalizing the Raman intensities between spectra and is needed to compare the 1600 cm -1 band intensities among various pulp spectra. RESULTS AND DISCUSSION Figures 1 and 2 show Raman spectra for CEDED- and POM-bleached kraft pulps, respectively. In the 850 cm -1 to 1850 cm -1 region, the main lignin feature is seen at 1600 cm -1 [12, 13]. Most other bands are attributable to cellulose. Cellulose's band at 1098 cm -1 was used for internal reference purposes. With this band, relative intensities of the 1600 cm -1 band were calculated. Relative intensities, both area and peak height, are listed in Table I. Table I also lists microkappa numbers for CEDED- and POM-bleached pulps. The results of FT-Raman lignin quantification are shown in the remaining figures. For the CEDED sequence, when the 1600 cm -1 peak-height intensity ratio was plotted against kappa numbers, a linear relationship was observed (Fig. 3). The linear regression line of Raman intensity (peak height) on kappa number had a coefficient of correlation (R 2 ) of The value of R 2 was slightly lower (0.93) when peak areas were used as a measure of Raman intensities (Fig. 4). In both 24 TAPPI JOURNAL JANUARY 2003

4 PULP TESTING 5. Linear regression of lignin's 1600 cm -1 Raman band 6. Linear regression of lignin's 1600 cm -1 Raman band intensities (peak heights) on microkappa numbers for the intensities (peak areas) on microkappa numbers for the EVVVE-bleached kraft pulps (R 2 = Data points read EVVVE-bleached kraft pulps (R 2 = Data points read right to left. UB, unbleached. E, alkali extracted. V, polyox- right to left. UB, unbleached. E, alkali extracted. V, polyoxometalate treated). ometalate treated). BLEACHING AVERAGE OF STANDARD RELATIVE STANDARD STAGE FIVE ANALYSES DEVIATION DEVIATION, % Unbleached C CE CED CEDE CEDED II. Reliability of 1600 cm -1 /1098 cm -1 intensity data (area ratio). cases, the results indicated that if the 1600 cm -1 band intensity of a kraft pulp is known, its kappa number (and therefore lignin concentration) can be estimated with a fair degree of accuracy. As can be seen in Figs. 3 and 4, the regression line has a negative intercept of the y axis, rather than being zero. One possible reason for this offset is that the kappa numbers are overestimating the amount of lignin in pulps, a discrepancy that may have been caused by the presence of hexenuronic acid groups [1, 2]. It has been reported that in presence of hexenuronic acids, a pulp gives a higher kappa number that needs to be corrected for a contribution from this acid. In the case of POM bleaching, where the sequence used was EVVVE, the linear regression was somewhat better (Figs. 5 and 6). The use of peak areas produced a better fit = 0.98, Fig. 6) compared with the regression based on peak heights (R 2 = 0.96, Fig. 5). Moreover, between the POM and the CEDED sequences, the linear fit was slightly better in the former case (R 2 EVVVE = 0.98, R 2 CEDED = 0.95). In both cases, the linear regression was useful to accurately estimate lignin content from the intensity of the pulp's 1600 cm -1 Raman band. In the case of certain POM-oxidized kraft pulps, it has been found that the kappa number measurement tends to underestimate the amount of lignin [15]. It has been speculated that a portion of lignin could be present already in the oxidized form and would therefore remain undetected because permanganate would not oxidize it any further. However, with the specific POM used in this study, this occurrence does not seem to be the case because the y intercept is close to zero (Figs. 5 and 6). In Figs. 3-6, plots of Raman intensity vs. kappa number are less linear at higher kappa numbers. There could be several reasons for this outcome. First, at higher kappa numbers, the pulps have a higher residual background in the spectra, which in turn can affect the intensity of the lignin band at 1600 cm -1. (Even when spectra are background corrected, there is some residual background.) Second, assuming that in pulps of high kappa number the kraft pulp chromophores have Raman contributions in the range of cm -1, these contributions would affect the intensity measured at 1600 cm -1. Finally, in Raman spectroscopy, because the scattering contribution is usually dependent upon the extent of conjugation (which is expected to be higher in high kappa pulps), a change in the nature of a chromophore is expected to also impact the intensity at 1600 cm -1. Although some variation at higher kappa numbers is seen in both CEDED and POM systems, it does not seem to be of great concern, because the R 2 values are reasonably good. To test the reliability of the intensity data at the 1600 cm -1 band, we analyzed five samples from each stage of the CEDED sequence by FT-Raman spectroscopy. Only pulps bleached using the CEDED sequence were used in this test. Relative 1600 cm -1 band intensities (1600 cm -1 /1098 cm -1 area ratios) were calculated from the spectra. For this analysis, average, standard deviation, and relative standard deviation (coefficient of variability, or COV) were also calculated (Table II). Of the various stages of the CEDED bleaching sequence, stage C showed the least COV and stage CE showed the most COV, although all values fell within the COV range of 3%-8%. These results indicate that reliable measurements of lignin content can be made using the FT-Raman approach. CONCLUDING REMARKS FT-Raman spectroscopy was used to quantify lignin concentrations in VOL 2: NO. 1 TAPPI JOURNAL 25

5 PULPTESTING unbleached and partially bleached pine kraft pulps. Both CEDED- and POMbleached pulps were studied. Although we found some deviations from linearity in relative Raman intensity (1600 cm -1 ) compared with kappa number, good quantification of the lignin amount was obtained with either peak-height or area intensity measurements. Pulp sample heterogeniety posed no problem in Raman spectroscopy. Some fluorescence contribution was detected for high kappa number pulps, but it was not a significant problem. In the pulp spectra, the phenyl group contribution at 1600 cm -1 provided a direct measure of lignin content. This investigation showed that FT-Raman spectroscopy is apparently a useful method for quantifying lignin in kraft pulps. TJ Department of Agriculture of any product or service. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. LITERATURE CITED ACKNOWLEDGEMENTS The authors thank Jim Bond who was at Consolidated Papers Inc. (now Stora Enso North America, Wisconsin Rapids, Wisconsin) at the time of this study, for providing the pine kraft pulps. They also acknowledge help received from the following individuals: Jim McSweeny for performing the CEDED bleaching sequence, Rick Reiner for determining microkappa numbers, and Sally Ralph for acquiring FT-Raman spectra. DISCLAIMER The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. INSIGHTS FROM THE AUTHORS Studying kraft pulps by Raman spectroscopy has been-a problem for a long time because of sample fluorescence. Using near-infrared excitation at 1064 nm to clear this hurdle, we found that the technique can be used to determine the amount of residual lignin in kraft pulps. In fact, it is rather easy to do. One day, the method may even be used in the mills. In the meantime, our next task is to show that the kraft pulp can be analyzed successfully to determine its lignin concentration regardless of the bleaching process used. Received: May 11,2001 Revised: November 28,2001 Accepted: February 21,2002 This research was presented at the 1996 International Pulp Bleaching Conference, Washington, DC. This paper is also published on TAPPI's web site < and summarized in the January Solutions! for People, Processes and Paper magazine (Vol. 86 No. 1) Agarwal, Weinstock, and Atalla are scientists at the USDA Forest Service, Forest Products Laboratory, One Gifford Pinchot Drive, Madison, Wl ; Agarwal at uagarwal@fs.fed.us. Agarwal Weinstock Atalla Who was Raman? - FT-Raman spectroscopy is named after Chandrasekhara Venkata Raman, winner of the 1930 Nobel Prize in Physics. He was professor at the Indian Institute of Science at Bangalore ( ), and then served as director of the Raman Institute of Research at Bangalore. He collaborated on a series of investigations that led to the discovery of the "Raman"radiation effect, for which he was recognized with the Nobel Prize. He died in TAPPI JOURNAL JANUARY 2003