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High-value utilization of eucalyptus kraft lignin: Preparation and characterization as efficient dye dispersant
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High-value utilization of eucalyptus kraft lignin: Preparation and characterization as efficient dye dispersant Hui Zhang, Boming Yu, Wanpeng Zhou, Xinxin Liu, Fangeng Chen ∗ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
a r t i c l e
i n f o
Article history: Received 27 September 2017 Received in revised form 11 November 2017 Accepted 18 November 2017 Available online xxx Keywords: Eucalyptus lignin Sulfonation Dispersant Color reduction Phenolic hydroxyl blocking Sodium borohydride
a b s t r a c t The dark color of industrial lignin is the main obstacle for their high value-added use in areas such as dyestuff dispersants. A kind of light-colored lignosulfonate with favorable dispersibility and remarkable stain resistance is prepared using fractionated eucalyptus kraft lignin. The fractionated lignins named as D (insoluble part) and X (soluble part) and sulfonated lignin fractions named as SD and SX are characterized by FTIR spectroscopy, 1 H NMR spectroscopy, GPC and brightness test. The results reveal that fraction X presents a lower molecular weight but a higher hydroxyl content than that of fraction D, which lead to the differences on the SO3 H content, dispersibility and color performance of SD and SX. The sulfonated fractions perform a similar molecular weight to that of unsulfonated lignins and show light color due to the phenolic hydroxyl blocking of 1,4–BS (1,4–butane sultone) and the postprocessing of sodium borohydride. The SX that performs the best of all exhibits obvious decrease on phenolic hydroxyl groups and increase on brightness value which is improved by 85.8% compared with control sample. The SX reaches the highest level (grade 5) in the dispersibility test and presents remarkable stain resistance on different textiles, especially on the dacron and cotton. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Fossil resources supplied over 95% of organic chemicals, however fossil reserves are limited and large consumption of fossil resources caused the greenhouse effect [1]. Hence, nonfossil-based lignocellulose that can be used as alternatives to fossil resources are becoming more and more important [2–4]. Lignin derived from lignocellulose is a renewable resource with the potential for producing bio-based materials and chemicals [5]. Making full use of lignin can not only alleviate the petroleum crisis, but also relieve the solid waste disposal problem, as most of them from the pulp and paper industry are underutilized feedstocks [6,7]. Currently, the pulp and paper industry liberates approximately 50 million tons of kraft lignin annually, and only approximately 2% of this lignin is used commercially [8,9]. Although significant advances have been made in recent research, the selective conversion of lignin into value-added products is still a challenge [10–13]. There are numerous researches about lignin-based dispersant [14–17], but on the account of dark color, lignin was not good enough to be used as dye dispersant or to be used in some lightcolor requested conditions. At present, most of the color reduction processes refer to the bleaching of pulp or color reduction of pulp-
∗ Corresponding author. E-mail address: [email protected] (F. Chen).
ing effluent and degradation of technical lignin [18–20]. But those methods aimed to remove or destroy lignin molecules as much as possible, thus the products were not suitable for the following use as dispersant. In recent research [21], lignin was whitened through a self-assembly method. But the lignin needs to be acetylated initially. As we all know, the acetylated lignin is hydrophobic and cannot dissolve in water, therefore it is inappropriate for the utilization as dispersant. UV irradiation was used to fade the color of lignin and obtained considerable achievement [22], but the work was processed under low concentration and cost too much tetrahydrofuran because of the lower penetrability of UV, besides it took a long time to reduce the color and part of the lignin was depolymerized at last. The effect of average molecular weight on the staining of lignin dispersant was investigated [23,24]. Different dosage of cross-linker was used to adjust the molecular weight of lignin dispersant. The on-fiber adsorption amount of dispersant was decreased with the increase of molecular weight, therefore the lignin based dispersant which was with higher molecular weight presented lower fiber staining. Although the results have guiding significance for the dispersant design, the staining still exists due to the dark color of lignin. Lignin is almost colorless in wood, while technical lignins, e.g., kraft lignin is in dark color because a variety of chromophores are introduced into the lignin structure via the isolation procedure and pulping process. Among all the chromophores, quinones play the most important roles [22]. As seen in Fig. 1a, lignin contains many
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Fig. 1. Coloring mechanism of lignin (a) and color reduction mechanism of lignin (b).
methoxy group ( OCH3 ) which is easy to transform into phenolic OH in the kraft pulping process. The phenolic OH further transforms into quinoid structure and finally results in a dark color. Obviously, hard wood lignin that contains more OCH3 present darker color after the kraft pulping process. Based on the mechanism of Fig. 1a, we are inspired that the OH blocking may lead to a way to reduce the color of lignin, because the transformation between phenolic OH and quinoid can be prevented. We do obtained positive results by using OH blocking method [25] and the decoloration is further enhanced by the treatment of sodium borohydride to eliminate the remaining chromophores. Furthermore, in the follow-up works we find the methanol fractionated kraft lignin shows a polarization on color performance and the soluble fraction presents a lighter color. It is worthy to detect whether the light-colored fraction could make more contribution to the stain resistance of lignin-based dispersant after the sulfonation. In this work, fractionated eucalyptus kraft lignin was used to prepare dye dispersant. 1,4–butane sultone (1,4–BS) was applied as sulfonating agent and hydroxyl blocking agent (Fig. 1b). The lignin based dispersant was characterized by potentiometric titration, FTIR spectroscopy, 1 H NMR spectroscopy, GPC and brightness test. The reasons that led to the color difference of samples were illustrated. Furthermore, the lignin based dispersants were evaluated according to the Chinese national standards and the dispersant exhibited favorable dispersibility and stain resistance. For the fact that sulfonation and color reduction can be processed simultaneously under mild condition, therefore, more energy can be saved in the preparation process. In addition, the solvents used in fractionation and purification are recyclable methanol and ethanol, therefore the process is kind of green and effective.
2. Materials and methods 2.1. Materials The eucalyptus kraft lignin (KL) was acid-precipitated from eucalyptus wood pulping black liquor supplied by a pulp mill in south China. Sulfuric acid and sodium hydroxide were produced by Guangzhou Chemical Reagent Factory. Methanol and ethanol absolute were purchased from Guangzhou Jinhuada Chemical Reagent Co., Ltd. 1,4–butane sultone and sodium borohydride were produced by Shanghai Dibai Chemical technology Co., Ltd. and Tianjin
Zhiyuan Chemical Reagent Co., Ltd. All the chemical reagents mentioned above were of analytical grade and used without further purification. Strong acid cation exchange resin (Type 732) and strong basic anion exchange resin (Type 717) were obtained from Guangzhou No. 2 Chemical Reagent Factory. 2.2. Fractionation of lignin The eucalyptus kraft lignin was slowly added in 80% methanol/water (V/V) solution under stirring condition and then was filtrated. Two fractions, the soluble and the insoluble, were obtained. The insoluble fraction which was labeled as D was directly oven-dried at 60 ◦ C and the soluble part which was labeled as X was vacuum rotary evaporated to solid powder at 60 ◦ C. 2.3. Sulfonation of lignin Lignin was added in water and adjusted to pH 12 with 1.5 mol/L NaOH aqueous solution. When the lignin was dissolved, 1,4–BS was added. The mixture was stirred for 3 h at 70 ◦ C to proceed the sulfonation and then sodium borohydride was added and kept stirring for another 2 h at room temperature. At last, the solution was precipitated in ethanol absolute and oven-dried to solid powder sample. The sulfonated fraction D and fraction X were named as SD and SX, respectively. 2.4. Sulfonic group ( SO3 H) content detection About 0.15 g lignosulfonate was added into 50 mL deionized water and treated by ion-exchange resin to remove the excessive starting materials and salt residues, and to transform lignosulfonate into lignosulfonic acid. Then 100 mL deionized water was used to wash the resin for the lignosulfonic acid elution. The SO3 H content of lignosulfonate samples was detected by a potentiometric titrator with the titrant NaOH standard solution (about 0.005 mol/L). The SO3 H content was calculated according to the following equation: S = C NaOH V NaOH /m Where S was the SO3 H content (mmol/g), CNaOH was the molar concentration of NaOH (mmol/L), VNaOH was the volume of used NaOH solution (L) and m was the weight of lignosulfonate sample (g).
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2.5. Gel permeation chromatography (GPC) analysis
2.9. Investigation of staining of lignosulfonate
The molecular weights of different fractions were determined by Agilent GPC equipped with PL1110-6300 and PL1110-6530 columns. All unsulfonated lignins were acetylated and tetrahydrofuran was used as mobile phase. The molecular weights of the sulfonated lignins were determined by GPC equipped with Waters 2414 refractive index detector (RID). The TSK-GEL columns in series were G-5000 PWXL and G-3000PWXL. The column was eluted with 0.02 mol/L KH2 PO4 aqueous solution at a flow rate of 0.6 mL/min. The tests were calibrated by polystyrene and polystyrene sulfonate respectively with the molecular weight range from 2 to 100 k. The temperature of the columns was kept at 40 ± 0.1 ◦ C. The volume of injection was 20 L in each run.
The staining test was proceeded according to the Chinese standard sodium lignie sulphonate dispersing agent (HG/T 3507-2008 part 5.10). Lignosulfonate aqueous solution with a concentration of 4 g/L and pH 5.5 which was adjusted by acetic acid glacial was prepared. Three kinds of fabrics i.e. chinlon, dacron and cotton cloth were used in the investigation and all the textiles were pretreated based on the requirement of standard. 10 mL of lignosulfonate solution and 150 mL of distilled water were mixed and heated to 95 ◦ C. Then 2 g of chinlon was added and stirred for 15 min. At last the chinlon was washed by distilled water and oven dried. Dacron and cotton were respectively added in the mixture which consisted of 50 mL of lignosulfonate solution and 110 mL of distilled water. Afterwards they were heated to 130 ◦ C using a high temperature dyeing autoclave. After stirred for 1 h, the dacron and cotton were washed and oven dried. The samples obtained from the staining test were evaluated by the AATCC standard card which was provided by a dyeing factory in south China. The AATCC is short for American Association of Textile Chemists and Colorists and the AATCC standard card is a commonly used reference substance for the evaluation of staining of textiles.
2.6. Fourier transform infrared (FTIR) spectroscopy analysis The FTIR spectroscopies of lignin and lignosulfonate samples were obtained by Vector 33 spectrophotometer (Bruker Corp., Germany) within the frequency range of 4000–400 cm−1 . The measurement method was a potassium bromide pressed-disk technique. Pellets were prepared by mixing 200 mg of KBr with 2 mg of samples in an agate mortar. Then the squashes were tableted and tested for infrared spectrum analysis. 2.7.
1H
nuclear magnetic resonance (NMR) spectra analysis
The 1 H NMR spectra of acetylated lignin fractions were recorded with 30 mg of each sample dissolved in 1 mL of dimethyl sulfoxided6 with an internal standard of Tetramethylsilane (TMS). The sulfonated lignin fractions were recorded with 30 mg of each sample dissolved in 1 mL of deuterium oxide at room temperature with an internal standard of 3-(trimethylsilyl)-1-propane sulfonic acid sodium salt (DSS). The samples mentioned above were detected by an Avance-3-HD 400 spectrometer (400 MHz 1 H NMR frequency, Bruker Co., Ettlingen, Germany). The group content was calculated by an area integral method using MestReNova software. 2.8. Brightness test The color degree of lignosulfonate samples were measured on an L&W brightness tester (Elrepho 070, Sweden). It was carried out under the light of 457 nm according to a modified brightness test method (standard: BS ISO 2470-1999) which was commonly used in the pulp and paper industry. Lignosulfonates aqueous solutions were made with a same concentration. Pipettes were used to drop 0.1 mL solutions on the center of the rapid qualitative filter papers which had been placed on watch glasses. After the solutions stopped diffusing on the filter papers, the filter paper samples were oven-dried, and were used for the following brightness test. Brightness value was obtained by the reflected light ratio of tested sample and standard sample. Light colored sample achieved high brightness value, therefore, the color degree of lignosulfonates could be expressed and evaluated. Each sample was tested ten times to get an average.
2.10. Investigation of dispersibility The dispersibility of sulfonated lignin was tested according to Chinese standard HG/T 3399-2001. The dark blue S-3BG (requested in the standard) and sulfonated lignin were mixed at solid state and then it was slowly added into distilled water with a total volume of 100 mL under a magnetic stirring condition. 0.2 mL of the homogeneously dispersed slurry was dropwise transferred onto the center of rapid qualitative filter papers which had been placed on watch glasses. The well dispersed dye will diffuse with the water and form a uniform circle in which there is no obvious edge separation between water and dye. On the contrary, an edge separation means the dye was not well dispersed and wider separation stands for worse dispersibility of lignosulfonate. The evaluation of dispersibility refers to the Appendix A of HG/T 3399-2001. There are five grades (from 1 to 5) in the card and higher grade represent better dispersibility. 3. Results and discussion 3.1. Molecular weight analysis The molecular weights of samples were listed in Table 1. The fractions D and X reached the Mw of 14870 and 5143, which was attributed to the effect of methanol fractionation. In the original kraft lignin, there are a lot of condensed structures [26] which are with higher molecular weight and are not easy to dissolve in the methanol/water solution. As a consequence, the high molecular part could be separated by methanol fractionation. Furthermore, the condensed structures create stronger steric hindrance which leads to a decrease on the reactivity of lignin. This is one of the reasons why SD got a lower SO3 H content than SX (Table 1). In the fractionation, the D fraction aggregated and formed small particles
Table 1 Molecular weight and group content of samples. Sample
Phenolic
D X SD SX
1.60 2.41 0.29 0.26
OH (mmol/g)
Aliphatic 2.27 2.33 – –
OH (mmol/g)
SO3 H (mmol/g) – – 1.32 1.63
Mw (Da)
Mn (Da)
PDI
14870 5143 13980 4860
6501 2800 7428 3175
2.287 1.837 1.882 1.531
– Not tested.
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1,4–BS. Besides, the intensity of SX is stronger, which means that the SX contains more sulfonic groups. It is in accordance with the SO3 H content which is shown in Table 1, where the SX reached 1.63 mmol/g and the SD reached 1.32 mmol/g. All samples show broad bands at 3400–3550 cm−1 , attributed to the hydroxyl groups in phenolic and aliphatic structures, and the bands centered around 2943 and 2844 cm−1 , predominantly arise from C H stretching in aromatic methoxyl groups and in methyl and methylene groups of side chains. In the carbonyl/carboxyl region, bands are found at 1680–1740 cm−1 which is originated from unconjugated carbonyl/carboxyl stretching, although the intensity of the bands may differ. Aromatic skeleton vibrations at 1605, 1510 and 1425 cm−1 and the C H deformation combined with aromatic ring vibration at 1460 cm−1 are common for all samples (Fig. 2b) [29]. But bands intensities of sulfonated samples (SD and SX) at 1510 and 1460 cm−1 show obvious decrease and increase respectively compared with the unsulfonated lignins. Besides, 1117 cm−1 associated with C H in-plane vibration of syringyl (S) ring and guaiacyl (G) ring and 1040 cm−1 caused by the C H vibration in G structure are significantly enhanced in the spectrums of SD and SX. Band of 879 cm−1 , which can be observed in SD and SX, is caused by the C H out-of-plane vibration of S ring and G ring [30,31]. It is mainly attribute to the hydrogen donating, i.e., redox effect of sodium borohydride. In addition, vibration at 1220 cm−1 that can be associated with C C plus C O stretching present a decrease with SD and SX, this may attribute to the hydrogen reduction of C O [32]. Meanwhile, 1185 cm−1 that refers to the stretching of C O in staurted alcohol shows more obvious signal in the sulfonated samples. Based on the FTIR analysis, we can infer that some of the conjugated structure was eliminated and the decrease of conjugated structure has great relationship with the light color performance of lignin.
3.3.
1H
NMR spectra analysis
Fig. 2. FTIR spectra of samples (a) and partially magnified spectra (b).
with diameters around 0.5 mm, some soluble part was locked in the particle and was isolated from the methanol, therefore, the soluble part was not totally dissolved in the methanol/water, which lead to a high PDI value of the fraction D. The sulfonated SD and SX reached the Mw of 13980 and 4860 respectively and showed slight decrease on molecular weight when compared to D and X, which is in accordance with our previous work [25]. The PDI values of SD and SX were decreased compared with the unsulfonated fractions, it seems that the slight degradation in the sulfonation is helpful in building a narrower molecular weight distribution. For some lignosulfonate, it is synthesized under severe reaction condition which is easy to cause the degradation of lignin and create more molecular fragments. The fragment contain more carboxyl and phenolic hydroxyl group and is easier to attach on fiber surface or in fiber gaps, which lead to a more serious staining of textile [15,23]. The SD and SX were prepared under mild conditions which could avoid the degradation and fragments to some extent. Thus, the SD and SX that are with similar molecular weight to D and X should be advantageous in the reduction of staining theoretically. 3.2. FTIR spectra analysis As seen in Fig. 2a, the kraft lignin (D and X) has a weak band at about 630 cm−1 , which arises from C–S bonds, due to the presence of sodium sulfide in kraft pulping process [27]. However, the sharp bands at 613 cm−1 and 522 cm−1 are characteristics of lignosulfonate, which arise from the vibration of C SO3 [28]. It is an evidence of SD and SX for the reaction between lignin and
1 H NMR spectra with acetylated lignin fractions in DMSO-d 6 are presented in Fig. 3a. The two fractions show corresponding signals although the intensity of the signals may differ. The signals at 2.45–2.1 ppm and 2.1–1.5 ppm are related to the protons of phenolic and aliphatic acetyl, respectively [33]. After the acetylating, the hydroxyl was replaced by acetyl, thus, the hydroxyl can be determined by the proton of acetyl [34,35]. The integral results with an internal standard of TMS are show in Table 1. Fraction D and X get a similar result at aliphatic hydroxyl content, but, the D shows an obvious decrease at phenolic hydroxyl content. Lower hydroxyl content led to a lower reactivity which should be another reason that leads to the lower sulfonic group content of SD than that of SX. The sulfonated lignins are water-soluble and cannot be acetylated, therefore, the D2 O was used as solvent in the 1 H NMR with an internal standard of DSS [36]. As seen in Fig. 3b, the peaks at 0.6, 1.8 and 2.9 ppm are associated with the methylene of DSS. The signal of aliphatic hydroxyl should appear at the range of 0.5–5 ppm, but it is easy to be influenced by temperature and concentration [37], beside, there are many disturbances in the area of 0.5–5 ppm. As a result, the calculation of aliphatic hydroxyl content of sulfonated lignin was not proceeded here. The phenolic hydroxyl appears at 8.48 ppm [38], but the sulfonated lignin SD and SX present weak peaks. It is mainly attributed to the hydroxyl blocking effect of 1,4–BS which consumed numerous phenolic hydroxyl. The phenolic hydroxyl content integrals of SD and SX were listed in Table 1. The two samples get similar results and show obvious decrease compared with the unsulfonated samples. The pesks at 1.2 and 3.4 ppm are related to the coupling of C4 H8 SO3 H which derive from 1,4–BS [14]. Meanwhile, the signal intensity of SX is much stronger, which means that more 1,4–BS have grafted on the SX
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Fig. 4. Brightness value of samples.
Fig. 3.
1
H NMR spectra of lignin fractions (a) and sulfonated lignin fractions (b).
and the SX contains more SO3 H. It is in accordance with FTIR and SO3 H content results. 3.4. Brightness evaluation It is easy and effective to compare the color degree with aqueous solutions of same concentration, but the performances can not be quantified, and it is easy to be influenced, when the observation condition changed. Besides, in our previous works, we found that the UV spectrum was not a good way to evaluating the color degree of lignosulfonate. In view of these shortages, we determined to evaluate the color degree with brightness method, where samples were dissolved with same concentration and treated at same conditions, thus the difference was obvious and could be quantified. In the test, M is stand for the lignosulfonate which is sulfonated by original kraft lignin without fractionation and SML, which was sulfonated using sulfomethylation method in our previous work [25], is regarded as a control sample for the color comparison. The brightness of SML, SD, M, SX, and blank filter paper (labeled as Blank in Fig. 4) reached 34.28, 53.15, 57.92, 63.68 and 90.37% ISO, respectively. SX performed the best among all the samples and SD showed a darker color than that of SX and M. The brightness result indicates that the fractionation of lignin does play a role in the color reduction of lignosulfonate, i.e., the part with darker color was separated and the part left was improved on the color performance. In the previous work [25], the color degree of SML and M were compared. We found that the hydroxyl-blocking sulfonation reduced the color obviously. Therefore, in this work, the color performance
Fig. 5. Dispersibility of lignosulfonate.
was further enhanced combined with the fractionation of lignin. The brightness value of SX was improved by 9.9% compared with M and was improved by 85.8% compared with the control sample SML. 3.5. Investigation of staining of lignosulfonate Lignosulfonate is always with deep color, especially the followup sulfonated kraft or alkali lignins that have already been through severe reaction condition in the pulping process contain more chromophores and show darker color [21]. The staining of lignosulfonate gives rise to the color distortion in the dyeing process and is hard to be avoided. The chromophores in the lignosulfonate cannot be totally eliminated, therefore, people just try their best to reduce the staining to a minimum [24]. In this work, the staining of lignosulfonate was evaluated by the AATCC standard card which was divided into 9 grades with an increment of 0.5 from grade 1 to grade 5 and a higher grade stands for a lighter staining performance. As seen in Fig. 5, SX performs very well in all of the three tests and shows no obvious staining when compared with the original textiles which are labeled as Blank. According to the AATCC card, the test result of X is definitely above the grade of 4.5 and is very close to the grade of 5, which means the staining test of SX is remarkable
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Fig. 6. Staining performance of lignosulfonate.
and beyond the reach of some other lignin-based dye dispersants. The SD present similar result in the dacron test, which reaches the grade of 4.5 and is close to grade 5. But in the tests of cotton and chinlon, SD performs slightly worse especially in the chinlon where there looks like a yellow shadow covered on the surface of chinlon. Although the result of SD performed not as well as that of SX, it is still beyond the grade of 4, at which most of the restraint of staining could be satisfied.
all reached the highest brightness value of 63.68% ISO which was improved by 85.8% compared with control sample, and showed remarkable stain resistance on different textiles. Furthermore, the SX is with favorable dispersibility which reached the highest level i.e. grade 5 in the dispersibility investigation based on the standard method. The application scope of kraft lignins is expanded by using this simple but efficient color reduction method, which makes it possible to be used in a high-end area as dye dispersant.
3.6. Investigation of dispersibility
References
There are lots of hydrophobic groups on the surface of dye particle, therefore, the dye particles are easier to aggregate and precipitate in water. On this condition, dispersant is necessary to keep the dye particles separated and to form a homogenous suspension [39]. As seen in Fig. 6, the circles of dye suspension without any lignin dispersant (labeled as Blank) show obvious edge separation where the distance between dye edge and water edge is almost 6 mm. According to the standard card, the dye suspension reaches the grade 3 indicating that it is difficult to be used in the dyeing process because the aggregated dye particles will result in a dyeing unevenness. The suspension treated with SD shows a better dispersibility where the edge distance decreases to 2 mm and reaches the grade of 4, which means that it can be used in most dyeing conditions. Obviously, the SX performs the best and presents excellent dispersibility. As seen in Fig. 6, the dye suspension prepared with SX is well dispersed and shows no edge separation in the test. The satisfactory result may attribute to specific structure of lignosulfonate, i.e., it contains lots of aliphatic groups which make it easy to attach on the surface of dye participles. Besides, the sulfonic groups, introduced by sulfonation, make the surface of dye particle negatively charged which build the electrostatic effect on the particle surface thus prevent the aggregation of particles and make them uniform suspensions. The SX is with higher SO3 H content than that of SD, which means that the SX contains more negative charges and creates a stronger electrostatic effect. This may be one of the reasons why SX performs better on the dispersibility.
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4. Conclusions In this work, we find a new way to improve the utilization value of kraft lignin as dye dispersant. The eucalyptus kraft lignin was fractionated using 80% methanol/water as solvent and two fractions (D and X) were obtained. The X is with lower molecular weight and is with higher hydroxyl content compared with D. The post-sulfonated fractions named as SD and SX show no obvious decrease on molecular weight, which mainly owe to the mild reaction conditions that was processed under the temperature of 70 ◦ C. In addition, the sulfonated fractions present high brightness value due to the phenolic hydroxyl blocking of 1,4–BS and the postprocessing of sodium borohydride. The SX that performed the best of
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High-value utilization of eucalyptus kraft lignin: Preparation and characterization as efficient dye dispersant Hui Zhang, Boming Yu, Wanpeng Zhou, Xinxin Liu, Fangeng Chen ∗ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
a r t i c l e
i n f o
Article history: Received 27 September 2017 Received in revised form 11 November 2017 Accepted 18 November 2017 Available online xxx Keywords: Eucalyptus lignin Sulfonation Dispersant Color reduction Phenolic hydroxyl blocking Sodium borohydride
a b s t r a c t The dark color of industrial lignin is the main obstacle for their high value-added use in areas such as dyestuff dispersants. A kind of light-colored lignosulfonate with favorable dispersibility and remarkable stain resistance is prepared using fractionated eucalyptus kraft lignin. The fractionated lignins named as D (insoluble part) and X (soluble part) and sulfonated lignin fractions named as SD and SX are characterized by FTIR spectroscopy, 1 H NMR spectroscopy, GPC and brightness test. The results reveal that fraction X presents a lower molecular weight but a higher hydroxyl content than that of fraction D, which lead to the differences on the SO3 H content, dispersibility and color performance of SD and SX. The sulfonated fractions perform a similar molecular weight to that of unsulfonated lignins and show light color due to the phenolic hydroxyl blocking of 1,4–BS (1,4–butane sultone) and the postprocessing of sodium borohydride. The SX that performs the best of all exhibits obvious decrease on phenolic hydroxyl groups and increase on brightness value which is improved by 85.8% compared with control sample. The SX reaches the highest level (grade 5) in the dispersibility test and presents remarkable stain resistance on different textiles, especially on the dacron and cotton. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Fossil resources supplied over 95% of organic chemicals, however fossil reserves are limited and large consumption of fossil resources caused the greenhouse effect [1]. Hence, nonfossil-based lignocellulose that can be used as alternatives to fossil resources are becoming more and more important [2–4]. Lignin derived from lignocellulose is a renewable resource with the potential for producing bio-based materials and chemicals [5]. Making full use of lignin can not only alleviate the petroleum crisis, but also relieve the solid waste disposal problem, as most of them from the pulp and paper industry are underutilized feedstocks [6,7]. Currently, the pulp and paper industry liberates approximately 50 million tons of kraft lignin annually, and only approximately 2% of this lignin is used commercially [8,9]. Although significant advances have been made in recent research, the selective conversion of lignin into value-added products is still a challenge [10–13]. There are numerous researches about lignin-based dispersant [14–17], but on the account of dark color, lignin was not good enough to be used as dye dispersant or to be used in some lightcolor requested conditions. At present, most of the color reduction processes refer to the bleaching of pulp or color reduction of pulp-
∗ Corresponding author. E-mail address: [email protected] (F. Chen).
ing effluent and degradation of technical lignin [18–20]. But those methods aimed to remove or destroy lignin molecules as much as possible, thus the products were not suitable for the following use as dispersant. In recent research [21], lignin was whitened through a self-assembly method. But the lignin needs to be acetylated initially. As we all know, the acetylated lignin is hydrophobic and cannot dissolve in water, therefore it is inappropriate for the utilization as dispersant. UV irradiation was used to fade the color of lignin and obtained considerable achievement [22], but the work was processed under low concentration and cost too much tetrahydrofuran because of the lower penetrability of UV, besides it took a long time to reduce the color and part of the lignin was depolymerized at last. The effect of average molecular weight on the staining of lignin dispersant was investigated [23,24]. Different dosage of cross-linker was used to adjust the molecular weight of lignin dispersant. The on-fiber adsorption amount of dispersant was decreased with the increase of molecular weight, therefore the lignin based dispersant which was with higher molecular weight presented lower fiber staining. Although the results have guiding significance for the dispersant design, the staining still exists due to the dark color of lignin. Lignin is almost colorless in wood, while technical lignins, e.g., kraft lignin is in dark color because a variety of chromophores are introduced into the lignin structure via the isolation procedure and pulping process. Among all the chromophores, quinones play the most important roles [22]. As seen in Fig. 1a, lignin contains many
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Fig. 1. Coloring mechanism of lignin (a) and color reduction mechanism of lignin (b).
methoxy group ( OCH3 ) which is easy to transform into phenolic OH in the kraft pulping process. The phenolic OH further transforms into quinoid structure and finally results in a dark color. Obviously, hard wood lignin that contains more OCH3 present darker color after the kraft pulping process. Based on the mechanism of Fig. 1a, we are inspired that the OH blocking may lead to a way to reduce the color of lignin, because the transformation between phenolic OH and quinoid can be prevented. We do obtained positive results by using OH blocking method [25] and the decoloration is further enhanced by the treatment of sodium borohydride to eliminate the remaining chromophores. Furthermore, in the follow-up works we find the methanol fractionated kraft lignin shows a polarization on color performance and the soluble fraction presents a lighter color. It is worthy to detect whether the light-colored fraction could make more contribution to the stain resistance of lignin-based dispersant after the sulfonation. In this work, fractionated eucalyptus kraft lignin was used to prepare dye dispersant. 1,4–butane sultone (1,4–BS) was applied as sulfonating agent and hydroxyl blocking agent (Fig. 1b). The lignin based dispersant was characterized by potentiometric titration, FTIR spectroscopy, 1 H NMR spectroscopy, GPC and brightness test. The reasons that led to the color difference of samples were illustrated. Furthermore, the lignin based dispersants were evaluated according to the Chinese national standards and the dispersant exhibited favorable dispersibility and stain resistance. For the fact that sulfonation and color reduction can be processed simultaneously under mild condition, therefore, more energy can be saved in the preparation process. In addition, the solvents used in fractionation and purification are recyclable methanol and ethanol, therefore the process is kind of green and effective.
2. Materials and methods 2.1. Materials The eucalyptus kraft lignin (KL) was acid-precipitated from eucalyptus wood pulping black liquor supplied by a pulp mill in south China. Sulfuric acid and sodium hydroxide were produced by Guangzhou Chemical Reagent Factory. Methanol and ethanol absolute were purchased from Guangzhou Jinhuada Chemical Reagent Co., Ltd. 1,4–butane sultone and sodium borohydride were produced by Shanghai Dibai Chemical technology Co., Ltd. and Tianjin
Zhiyuan Chemical Reagent Co., Ltd. All the chemical reagents mentioned above were of analytical grade and used without further purification. Strong acid cation exchange resin (Type 732) and strong basic anion exchange resin (Type 717) were obtained from Guangzhou No. 2 Chemical Reagent Factory. 2.2. Fractionation of lignin The eucalyptus kraft lignin was slowly added in 80% methanol/water (V/V) solution under stirring condition and then was filtrated. Two fractions, the soluble and the insoluble, were obtained. The insoluble fraction which was labeled as D was directly oven-dried at 60 ◦ C and the soluble part which was labeled as X was vacuum rotary evaporated to solid powder at 60 ◦ C. 2.3. Sulfonation of lignin Lignin was added in water and adjusted to pH 12 with 1.5 mol/L NaOH aqueous solution. When the lignin was dissolved, 1,4–BS was added. The mixture was stirred for 3 h at 70 ◦ C to proceed the sulfonation and then sodium borohydride was added and kept stirring for another 2 h at room temperature. At last, the solution was precipitated in ethanol absolute and oven-dried to solid powder sample. The sulfonated fraction D and fraction X were named as SD and SX, respectively. 2.4. Sulfonic group ( SO3 H) content detection About 0.15 g lignosulfonate was added into 50 mL deionized water and treated by ion-exchange resin to remove the excessive starting materials and salt residues, and to transform lignosulfonate into lignosulfonic acid. Then 100 mL deionized water was used to wash the resin for the lignosulfonic acid elution. The SO3 H content of lignosulfonate samples was detected by a potentiometric titrator with the titrant NaOH standard solution (about 0.005 mol/L). The SO3 H content was calculated according to the following equation: S = C NaOH V NaOH /m Where S was the SO3 H content (mmol/g), CNaOH was the molar concentration of NaOH (mmol/L), VNaOH was the volume of used NaOH solution (L) and m was the weight of lignosulfonate sample (g).
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2.5. Gel permeation chromatography (GPC) analysis
2.9. Investigation of staining of lignosulfonate
The molecular weights of different fractions were determined by Agilent GPC equipped with PL1110-6300 and PL1110-6530 columns. All unsulfonated lignins were acetylated and tetrahydrofuran was used as mobile phase. The molecular weights of the sulfonated lignins were determined by GPC equipped with Waters 2414 refractive index detector (RID). The TSK-GEL columns in series were G-5000 PWXL and G-3000PWXL. The column was eluted with 0.02 mol/L KH2 PO4 aqueous solution at a flow rate of 0.6 mL/min. The tests were calibrated by polystyrene and polystyrene sulfonate respectively with the molecular weight range from 2 to 100 k. The temperature of the columns was kept at 40 ± 0.1 ◦ C. The volume of injection was 20 L in each run.
The staining test was proceeded according to the Chinese standard sodium lignie sulphonate dispersing agent (HG/T 3507-2008 part 5.10). Lignosulfonate aqueous solution with a concentration of 4 g/L and pH 5.5 which was adjusted by acetic acid glacial was prepared. Three kinds of fabrics i.e. chinlon, dacron and cotton cloth were used in the investigation and all the textiles were pretreated based on the requirement of standard. 10 mL of lignosulfonate solution and 150 mL of distilled water were mixed and heated to 95 ◦ C. Then 2 g of chinlon was added and stirred for 15 min. At last the chinlon was washed by distilled water and oven dried. Dacron and cotton were respectively added in the mixture which consisted of 50 mL of lignosulfonate solution and 110 mL of distilled water. Afterwards they were heated to 130 ◦ C using a high temperature dyeing autoclave. After stirred for 1 h, the dacron and cotton were washed and oven dried. The samples obtained from the staining test were evaluated by the AATCC standard card which was provided by a dyeing factory in south China. The AATCC is short for American Association of Textile Chemists and Colorists and the AATCC standard card is a commonly used reference substance for the evaluation of staining of textiles.
2.6. Fourier transform infrared (FTIR) spectroscopy analysis The FTIR spectroscopies of lignin and lignosulfonate samples were obtained by Vector 33 spectrophotometer (Bruker Corp., Germany) within the frequency range of 4000–400 cm−1 . The measurement method was a potassium bromide pressed-disk technique. Pellets were prepared by mixing 200 mg of KBr with 2 mg of samples in an agate mortar. Then the squashes were tableted and tested for infrared spectrum analysis. 2.7.
1H
nuclear magnetic resonance (NMR) spectra analysis
The 1 H NMR spectra of acetylated lignin fractions were recorded with 30 mg of each sample dissolved in 1 mL of dimethyl sulfoxided6 with an internal standard of Tetramethylsilane (TMS). The sulfonated lignin fractions were recorded with 30 mg of each sample dissolved in 1 mL of deuterium oxide at room temperature with an internal standard of 3-(trimethylsilyl)-1-propane sulfonic acid sodium salt (DSS). The samples mentioned above were detected by an Avance-3-HD 400 spectrometer (400 MHz 1 H NMR frequency, Bruker Co., Ettlingen, Germany). The group content was calculated by an area integral method using MestReNova software. 2.8. Brightness test The color degree of lignosulfonate samples were measured on an L&W brightness tester (Elrepho 070, Sweden). It was carried out under the light of 457 nm according to a modified brightness test method (standard: BS ISO 2470-1999) which was commonly used in the pulp and paper industry. Lignosulfonates aqueous solutions were made with a same concentration. Pipettes were used to drop 0.1 mL solutions on the center of the rapid qualitative filter papers which had been placed on watch glasses. After the solutions stopped diffusing on the filter papers, the filter paper samples were oven-dried, and were used for the following brightness test. Brightness value was obtained by the reflected light ratio of tested sample and standard sample. Light colored sample achieved high brightness value, therefore, the color degree of lignosulfonates could be expressed and evaluated. Each sample was tested ten times to get an average.
2.10. Investigation of dispersibility The dispersibility of sulfonated lignin was tested according to Chinese standard HG/T 3399-2001. The dark blue S-3BG (requested in the standard) and sulfonated lignin were mixed at solid state and then it was slowly added into distilled water with a total volume of 100 mL under a magnetic stirring condition. 0.2 mL of the homogeneously dispersed slurry was dropwise transferred onto the center of rapid qualitative filter papers which had been placed on watch glasses. The well dispersed dye will diffuse with the water and form a uniform circle in which there is no obvious edge separation between water and dye. On the contrary, an edge separation means the dye was not well dispersed and wider separation stands for worse dispersibility of lignosulfonate. The evaluation of dispersibility refers to the Appendix A of HG/T 3399-2001. There are five grades (from 1 to 5) in the card and higher grade represent better dispersibility. 3. Results and discussion 3.1. Molecular weight analysis The molecular weights of samples were listed in Table 1. The fractions D and X reached the Mw of 14870 and 5143, which was attributed to the effect of methanol fractionation. In the original kraft lignin, there are a lot of condensed structures [26] which are with higher molecular weight and are not easy to dissolve in the methanol/water solution. As a consequence, the high molecular part could be separated by methanol fractionation. Furthermore, the condensed structures create stronger steric hindrance which leads to a decrease on the reactivity of lignin. This is one of the reasons why SD got a lower SO3 H content than SX (Table 1). In the fractionation, the D fraction aggregated and formed small particles
Table 1 Molecular weight and group content of samples. Sample
Phenolic
D X SD SX
1.60 2.41 0.29 0.26
OH (mmol/g)
Aliphatic 2.27 2.33 – –
OH (mmol/g)
SO3 H (mmol/g) – – 1.32 1.63
Mw (Da)
Mn (Da)
PDI
14870 5143 13980 4860
6501 2800 7428 3175
2.287 1.837 1.882 1.531
– Not tested.
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1,4–BS. Besides, the intensity of SX is stronger, which means that the SX contains more sulfonic groups. It is in accordance with the SO3 H content which is shown in Table 1, where the SX reached 1.63 mmol/g and the SD reached 1.32 mmol/g. All samples show broad bands at 3400–3550 cm−1 , attributed to the hydroxyl groups in phenolic and aliphatic structures, and the bands centered around 2943 and 2844 cm−1 , predominantly arise from C H stretching in aromatic methoxyl groups and in methyl and methylene groups of side chains. In the carbonyl/carboxyl region, bands are found at 1680–1740 cm−1 which is originated from unconjugated carbonyl/carboxyl stretching, although the intensity of the bands may differ. Aromatic skeleton vibrations at 1605, 1510 and 1425 cm−1 and the C H deformation combined with aromatic ring vibration at 1460 cm−1 are common for all samples (Fig. 2b) [29]. But bands intensities of sulfonated samples (SD and SX) at 1510 and 1460 cm−1 show obvious decrease and increase respectively compared with the unsulfonated lignins. Besides, 1117 cm−1 associated with C H in-plane vibration of syringyl (S) ring and guaiacyl (G) ring and 1040 cm−1 caused by the C H vibration in G structure are significantly enhanced in the spectrums of SD and SX. Band of 879 cm−1 , which can be observed in SD and SX, is caused by the C H out-of-plane vibration of S ring and G ring [30,31]. It is mainly attribute to the hydrogen donating, i.e., redox effect of sodium borohydride. In addition, vibration at 1220 cm−1 that can be associated with C C plus C O stretching present a decrease with SD and SX, this may attribute to the hydrogen reduction of C O [32]. Meanwhile, 1185 cm−1 that refers to the stretching of C O in staurted alcohol shows more obvious signal in the sulfonated samples. Based on the FTIR analysis, we can infer that some of the conjugated structure was eliminated and the decrease of conjugated structure has great relationship with the light color performance of lignin.
3.3.
1H
NMR spectra analysis
Fig. 2. FTIR spectra of samples (a) and partially magnified spectra (b).
with diameters around 0.5 mm, some soluble part was locked in the particle and was isolated from the methanol, therefore, the soluble part was not totally dissolved in the methanol/water, which lead to a high PDI value of the fraction D. The sulfonated SD and SX reached the Mw of 13980 and 4860 respectively and showed slight decrease on molecular weight when compared to D and X, which is in accordance with our previous work [25]. The PDI values of SD and SX were decreased compared with the unsulfonated fractions, it seems that the slight degradation in the sulfonation is helpful in building a narrower molecular weight distribution. For some lignosulfonate, it is synthesized under severe reaction condition which is easy to cause the degradation of lignin and create more molecular fragments. The fragment contain more carboxyl and phenolic hydroxyl group and is easier to attach on fiber surface or in fiber gaps, which lead to a more serious staining of textile [15,23]. The SD and SX were prepared under mild conditions which could avoid the degradation and fragments to some extent. Thus, the SD and SX that are with similar molecular weight to D and X should be advantageous in the reduction of staining theoretically. 3.2. FTIR spectra analysis As seen in Fig. 2a, the kraft lignin (D and X) has a weak band at about 630 cm−1 , which arises from C–S bonds, due to the presence of sodium sulfide in kraft pulping process [27]. However, the sharp bands at 613 cm−1 and 522 cm−1 are characteristics of lignosulfonate, which arise from the vibration of C SO3 [28]. It is an evidence of SD and SX for the reaction between lignin and
1 H NMR spectra with acetylated lignin fractions in DMSO-d 6 are presented in Fig. 3a. The two fractions show corresponding signals although the intensity of the signals may differ. The signals at 2.45–2.1 ppm and 2.1–1.5 ppm are related to the protons of phenolic and aliphatic acetyl, respectively [33]. After the acetylating, the hydroxyl was replaced by acetyl, thus, the hydroxyl can be determined by the proton of acetyl [34,35]. The integral results with an internal standard of TMS are show in Table 1. Fraction D and X get a similar result at aliphatic hydroxyl content, but, the D shows an obvious decrease at phenolic hydroxyl content. Lower hydroxyl content led to a lower reactivity which should be another reason that leads to the lower sulfonic group content of SD than that of SX. The sulfonated lignins are water-soluble and cannot be acetylated, therefore, the D2 O was used as solvent in the 1 H NMR with an internal standard of DSS [36]. As seen in Fig. 3b, the peaks at 0.6, 1.8 and 2.9 ppm are associated with the methylene of DSS. The signal of aliphatic hydroxyl should appear at the range of 0.5–5 ppm, but it is easy to be influenced by temperature and concentration [37], beside, there are many disturbances in the area of 0.5–5 ppm. As a result, the calculation of aliphatic hydroxyl content of sulfonated lignin was not proceeded here. The phenolic hydroxyl appears at 8.48 ppm [38], but the sulfonated lignin SD and SX present weak peaks. It is mainly attributed to the hydroxyl blocking effect of 1,4–BS which consumed numerous phenolic hydroxyl. The phenolic hydroxyl content integrals of SD and SX were listed in Table 1. The two samples get similar results and show obvious decrease compared with the unsulfonated samples. The pesks at 1.2 and 3.4 ppm are related to the coupling of C4 H8 SO3 H which derive from 1,4–BS [14]. Meanwhile, the signal intensity of SX is much stronger, which means that more 1,4–BS have grafted on the SX
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Fig. 4. Brightness value of samples.
Fig. 3.
1
H NMR spectra of lignin fractions (a) and sulfonated lignin fractions (b).
and the SX contains more SO3 H. It is in accordance with FTIR and SO3 H content results. 3.4. Brightness evaluation It is easy and effective to compare the color degree with aqueous solutions of same concentration, but the performances can not be quantified, and it is easy to be influenced, when the observation condition changed. Besides, in our previous works, we found that the UV spectrum was not a good way to evaluating the color degree of lignosulfonate. In view of these shortages, we determined to evaluate the color degree with brightness method, where samples were dissolved with same concentration and treated at same conditions, thus the difference was obvious and could be quantified. In the test, M is stand for the lignosulfonate which is sulfonated by original kraft lignin without fractionation and SML, which was sulfonated using sulfomethylation method in our previous work [25], is regarded as a control sample for the color comparison. The brightness of SML, SD, M, SX, and blank filter paper (labeled as Blank in Fig. 4) reached 34.28, 53.15, 57.92, 63.68 and 90.37% ISO, respectively. SX performed the best among all the samples and SD showed a darker color than that of SX and M. The brightness result indicates that the fractionation of lignin does play a role in the color reduction of lignosulfonate, i.e., the part with darker color was separated and the part left was improved on the color performance. In the previous work [25], the color degree of SML and M were compared. We found that the hydroxyl-blocking sulfonation reduced the color obviously. Therefore, in this work, the color performance
Fig. 5. Dispersibility of lignosulfonate.
was further enhanced combined with the fractionation of lignin. The brightness value of SX was improved by 9.9% compared with M and was improved by 85.8% compared with the control sample SML. 3.5. Investigation of staining of lignosulfonate Lignosulfonate is always with deep color, especially the followup sulfonated kraft or alkali lignins that have already been through severe reaction condition in the pulping process contain more chromophores and show darker color [21]. The staining of lignosulfonate gives rise to the color distortion in the dyeing process and is hard to be avoided. The chromophores in the lignosulfonate cannot be totally eliminated, therefore, people just try their best to reduce the staining to a minimum [24]. In this work, the staining of lignosulfonate was evaluated by the AATCC standard card which was divided into 9 grades with an increment of 0.5 from grade 1 to grade 5 and a higher grade stands for a lighter staining performance. As seen in Fig. 5, SX performs very well in all of the three tests and shows no obvious staining when compared with the original textiles which are labeled as Blank. According to the AATCC card, the test result of X is definitely above the grade of 4.5 and is very close to the grade of 5, which means the staining test of SX is remarkable
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Fig. 6. Staining performance of lignosulfonate.
and beyond the reach of some other lignin-based dye dispersants. The SD present similar result in the dacron test, which reaches the grade of 4.5 and is close to grade 5. But in the tests of cotton and chinlon, SD performs slightly worse especially in the chinlon where there looks like a yellow shadow covered on the surface of chinlon. Although the result of SD performed not as well as that of SX, it is still beyond the grade of 4, at which most of the restraint of staining could be satisfied.
all reached the highest brightness value of 63.68% ISO which was improved by 85.8% compared with control sample, and showed remarkable stain resistance on different textiles. Furthermore, the SX is with favorable dispersibility which reached the highest level i.e. grade 5 in the dispersibility investigation based on the standard method. The application scope of kraft lignins is expanded by using this simple but efficient color reduction method, which makes it possible to be used in a high-end area as dye dispersant.
3.6. Investigation of dispersibility
References
There are lots of hydrophobic groups on the surface of dye particle, therefore, the dye particles are easier to aggregate and precipitate in water. On this condition, dispersant is necessary to keep the dye particles separated and to form a homogenous suspension [39]. As seen in Fig. 6, the circles of dye suspension without any lignin dispersant (labeled as Blank) show obvious edge separation where the distance between dye edge and water edge is almost 6 mm. According to the standard card, the dye suspension reaches the grade 3 indicating that it is difficult to be used in the dyeing process because the aggregated dye particles will result in a dyeing unevenness. The suspension treated with SD shows a better dispersibility where the edge distance decreases to 2 mm and reaches the grade of 4, which means that it can be used in most dyeing conditions. Obviously, the SX performs the best and presents excellent dispersibility. As seen in Fig. 6, the dye suspension prepared with SX is well dispersed and shows no edge separation in the test. The satisfactory result may attribute to specific structure of lignosulfonate, i.e., it contains lots of aliphatic groups which make it easy to attach on the surface of dye participles. Besides, the sulfonic groups, introduced by sulfonation, make the surface of dye particle negatively charged which build the electrostatic effect on the particle surface thus prevent the aggregation of particles and make them uniform suspensions. The SX is with higher SO3 H content than that of SD, which means that the SX contains more negative charges and creates a stronger electrostatic effect. This may be one of the reasons why SX performs better on the dispersibility.
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4. Conclusions In this work, we find a new way to improve the utilization value of kraft lignin as dye dispersant. The eucalyptus kraft lignin was fractionated using 80% methanol/water as solvent and two fractions (D and X) were obtained. The X is with lower molecular weight and is with higher hydroxyl content compared with D. The post-sulfonated fractions named as SD and SX show no obvious decrease on molecular weight, which mainly owe to the mild reaction conditions that was processed under the temperature of 70 ◦ C. In addition, the sulfonated fractions present high brightness value due to the phenolic hydroxyl blocking of 1,4–BS and the postprocessing of sodium borohydride. The SX that performed the best of
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