Hydroxypropyl Sulfonated Lignin as Dye Dispersant: Effect of Average

Research Article pubs.acs.org/journal/ascecg

Hydroxypropyl Sulfonated Lignin as Dye Dispersant: Effect of Average Molecular Weight Yanlin Qin,† Dongjie Yang,*,†,‡ and Xueqing Qiu*,†,‡ †

School of Chemistry and Chemical Engineering, ‡State Key Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China ABSTRACT: Sulfonated lignin obtained from pulping waste liquor is a nontoxic and renewable polymer that can be used as a dispersant in the dyeing industry. In order to reveal the effect of the lignin dispersant’s molecular weights on disperse dye, three hydroxypropyl sulfonated alkali lignin (HSL) samples with different molecular weights were obtained by controlling the dosage of etherification to cross-link lignin molecules. The molecular weight of HSL can be adjusted from 8,100 to 14,830 Da. More than 80% of phenolic hydroxyl groups of HSL samples were blocked by etherification compared to that of AL and which decrease with increasing molecular weight. The increasing molecular weight of HSL causes a considerable reduction in the staining effect of HSL on fiber since the adsorption amount of HSL on the fiber decreases by reducing the phenolic hydroxyl group. HSL with Mw of 11,020 Da contains 2.10 mmol·g−1 of the sulfonic group and as low as 0.46 mmol·g−1 of the phenolic hydroxyl group (compared to 2.32 mmol·g−1 of AL), providing excellent dispersive ability and high temperature stability on dye. More importantly, the dye uptake with added HSL with Mw of 11,020 Da is the highest of 85.17% among all dispersants here. Therefore, the etherification modification is a promising approach to increase the molecular weight of lignin and for widespread applications of lignin as a highly efficient dye dispersant. KEYWORDS: Hydroxypropyl sulfonated alkali lignin, Dye dispersant, Molecular weight, Dispersive ability, Fiber staining effect, Dye uptake

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INTRODUCTION

The investigations on and application of lignin-based polymers in the dyestuff industry have recently attracted more and more attention. Sulfonated lignin polymers as anionic surfactants can be helpful for breaking agglomeration and keeping particles stable by steric hindrance repulsive forces and electrostatic repulsive forces via adsorption onto the surface of dispersed particles,8 while the steric hindrance has been greatly affected by molecular weight.9 Researchers suggested that the main factor of sulfonated lignin impacting dispersive ability is its molecular weight.6,10,11 Being derived from the natural polymer, sulfonated lignin usually has a wide molecular weight distribution varying from several hundreds to tens of thousands, which probably makes it very difficult to further increase its molecular weight. An ultrafiltration method was usually used to gain the different molecular weights of lignosulfonates, and the performance affected by molecular weight was investigated in pesticide,10 chalcopyrite,12 gypsum paste areas,13 and coal water slurry.4 These results declared that sulfonated lignin with high molecular weight usually displayed a good dispersive ability on particles due to its higher adsorption amount.

As natural virtues of renewable bioresources, lignin-based polymers have attracted considerable attention in many application fields. Industrial lignin-based polymers mainly contain lignosulfonates, alkali lignin (AL), and enzymatic hydrolysis lignin, which are mainly byproducts from pulping, papermaking, and the biorefinery industry.1−3 The sulfonated lignin obtained by introducing sulfonic acid groups into lignin molecules refers to sulfomethylated lignin from the kraft pulping processes and lignosulfonates from the sulfite pulping process. Because of the favorable wettability, adsorptivity, and dispersive ability, sulfonated lignin may be widely used to considerable advantage as dispersants in many various application areas.4−7 Disperse dyes have extremely low solubility in aqueous solutions, and it is generally preferred to add 75%−200% dispersants base on the weight of dye to get small particles and increase dispersion stability. The dye dispersants mainly include sulfonated lignin (lignosulfonates and sulfomethylated lignin), naphthalene-sulfonated formaldehyde condensate and acid− phenol−formaldehyde condensate. Among these dispersants, sulfonated lignin is the only one from environmentally friendly and renewable polymers. More importantly, sulfonated lignin dispersants have an obvious advantage of good high temperature stability compared to that of other types of dispersants. © 2015 American Chemical Society

Received: August 5, 2015 Revised: October 21, 2015 Published: October 27, 2015 3239

DOI: 10.1021/acssuschemeng.5b00821 ACS Sustainable Chem. Eng. 2015, 3, 3239−3244

Research Article

ACS Sustainable Chemistry & Engineering

Three different molecular weight samples, HSL1, HSL2, and HSL3, were synthesized by controlling the mass concentration of epichlorohydrin as 12 g·L−1, 24 g·L−1, and 36 g·L−1, respectively. The reaction equation20 is as follows:

However, sulfonated lignin used as dye dispersant has some disadvantages, such as low and wide molecular weight distribution results in poor dispersive ability,11 especially the fiber staining effect resulting from its dark color,14 which limits its application in the dyeing process of light colored dyes. The fiber staining effect of lignin is affected by its molecular weight, sulfonation degree, color, phenolic hydroxyl content, and so on. It is considered that the fouling of lignin onto fiber is mainly produced by its darkened color and the hydrogen bonding force between phenolic hydroxyl and carboxyl of lignin and the amide bond of the fiber.15 According to our earlier research,16 lignosulfonates with low molecular weight contain small amounts of sugar sour and other organic compounds which have more carboxyl and phenolic hydroxyl group content than the higher molecular weight lignosulfonates. Therefore, lignosulfonates with lower molecular weight may cause a severe staining of the fiber. Ultrafiltration separations are usually used to improve the purity and molecular weight of lignin by removing lower molecular weight components, but it is time-consuming and has a lower yield (less than 20% generally). Unfortunately, lignosulfonates with high molecular weight obtained by ultrafiltration have less sulfonic groups compared with unpurified lignosulfonates,16 which significantly lowers its dispersive ability on dye.17 Diligent study and efforts have continued to increase the sulfonation degree and molecular weight of sulfonated lignin. It is regrettable that it was not really effective in increasing the molecular weight of sulfomethylated AL by formaldehyde condensation. This is because the sulfonation and polycondensation of AL are more likely to occur on the ortho position of the phenolic hydroxyl group, and the competition between these reactions mitigated the effects.18,19 In our work, AL was first sulfonated efficiently by the graftedsulfonation method with 3-chloro-2-hydroxy propyl sodium sulfonic acid to prepare a hydroxypropyl sulfonated lignin (HSL) with higher sulfonation degree.20 The etherification reaction was followed to increase the molecular weight of HSL, meanwhile reducing the content of the phenolic hydroxyl and carboxyl groups. The molecular weight of products was adjusted by controlling the dosage of epichlorohydrin. The structure characteristics and performances of hydroxypropyl sulfonated lignin with different average molecular weights as dye dispersant were further investigated.

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Sulfonic Group Content Measurement. The sulfonic group content was determined by the potentiometric titration in an automatic potentiometric titrator (809 Titrando, Metrohm Corporation, Switzerland) with 0.05−0.10 mol·L−1 NaOH as titrant at 25 °C.5 The samples were ion-exchanged through the anion and cation exchange resins to remove salts and other impurities before titration. The titration end point was determined by the peak of the first-order derivative of the titration curve. The sulfonic group content was calculated by eq 1: SO3H content (mmol ·g −1) = (mL of NaOH × mmol · mL−1 of NaOH/mL of SO3H) /mass of dry sample

(1)

Phenolic Hydroxyl Content Measurement. The OHphen content was measured used the FC-reagent method through a duplicate UV−vis measurement at 760 nm.23 The procedure was run in duplicate. The average data and the experimental errors are less than 0.01 mmol·g−1. A calibration curve was done with the above procedure using standard vanillin solutions with different concentrations to replace the dispersant. Determination of the Molecular Weight. The molecular weight distribution of dispersants was determined by employing aqueous gel-permeation chromatography (GPC) with the Waters 2487 UV absorbance detector (Waters Crop., Milford, MA, USA), and the polystyrenesulfonate was used as the standard substance, and 0.1 mol/L NaNO3 solution was used as the mobile phase. Preparation of the Dye Bath. Dried dispersant, dried dye filter cake (1.5:1 w/w), and agate beads were mixed in an agate jar of planetary ball mill (QM-3SP2, Nanjing University Instrument Co., Nangjing, China). The suspension was kept at 30%, and the pH was adjusted to 5.5 with HAc/NaAc. Then, the dye bath was prepared by diluting the dye suspension after milling for 4 h and diluted to 0.5% with distilled water and adjusted pH to 5.5 with HAc/NaAc solution. Dyeing Process and Dye Uptake. Dyeing was carried out in a closed stainless steel vessel on high-temperature computer controlled dyeing equipment (GRY-12, Wuxi Quanrun Machinery Co. Ltd., Wuxi, China). The liquid/solid ratio was 100:1 (w/w), and 2.0 g of polyester fiber (pretreated as described above) was added to the 200 mL dye bath. The sample was initially heated to 20 °C, and the temperature raised to 130 °C (2 °C min−1) and kept at 130 °C for 45 min, then cooled to 80 °C (4 °C min−1). The dye uptake was measured by UV spectroscopy (UV-2450, Shimadzu Corp.,Tokyo, Japan) at 580 nm (the absorbance of C.I. Disperse Blue 79). The concentration of the initial and final dye bath was diluted to 0.02 g·L−1 with acetone. The dye uptake was determined by eq 2:

MATERIALS AND METHODS

AL was a byproduct of alkaline pulping of pinewood from Tumen Papermaking Co. Ltd. (Jilin, China). The product consists of 90% lignin, 7% reductive substances and low molecular weight organic components, and 3% other impurities such as inorganic salts, which was analyzed by a two-step acid hydrolysis procedure.21 Sodium naphthalene sulfonic acid formaldehyde condensation (SNF) is a commercial dispersant used for dye dispersion, which was supplied by Shangyu Wencai Co. Ltd. (Zhejiang, China). Preparation of Hydroxypropyl Sulfonated AL20 HSL. Sodium 3-chloro-2-hydroxy-propanesulfonate was prepared22 first. Next, 100 g of AL and 100 mL of water were mixed together, and pH was adjusted to 10 with 2 mol·L−1 NaOH solution at 90 °C, and then 35 g of the obtained sodium 3-chloro-2-hydroxy-propanesulfonate was added dropwise. Sulfonation was carried out for 2 h at 90 °C. Then, epichlorohydrin was fed into the reactor by a dropping funnel to cross-link lignin molecules, and the reaction time was 1 h. The pH value was kept constant in the range of 9−10 with 2 mol·L−1 NaOH solution.

θ % = 100(1 − A t /A 0)

(2)

where θ is dye uptake, A0 and At are the absorbance of the initial and final dye baths, respectively (both measured at λ580). Measurement of the Particle Size of the Dye Bath and High Temperature Stability. Two hundred milliliters of the dye bath was at 25 °C. Another 200 mL of the dye bath was placed into a closed stainless steel vessel. Dyeing was carried out without polyester fiber using the above dyeing procedure. Both dye baths at 25 and 130 °C were diluted to 0.1% using distilled water. The particle sizes were determined by a laser particle analyzer (EyeTech-Laser, Ankersmid Corp., Nijverdal, The Netherlands). 3240

DOI: 10.1021/acssuschemeng.5b00821 ACS Sustainable Chem. Eng. 2015, 3, 3239−3244

Research Article

ACS Sustainable Chemistry & Engineering The stability was measured by Turbiscan Lab Expert Analyzer (Formulaction Co., L′Union, France).24 The precipitate thickness of the dye bath was calculated by the analyzer’s software.25 Measurement of Fiber Staining and Adsorption Amount. The fiber staining of dispersant was evaluated by measuring a K/S value of the stained polyester fiber on a Datacolor 110 photometer (Datacolor Corp., Kennesaw, GA, USA). The dispersant solutions with different concentrations of 0.2 to 2.6 g·L−1 were prepared with distilled water. All dispersant solutions were pH adjusted to 5.5 with HAc/NaAc. Two grams of pretreated polyester fiber was submerged in 200 mL of the dispersant solution and was treated analogously to the dyeing process. No dispersant solution experiment was carried out as a control sample. After the dyeing process, the fiber was used to test K/S value, and residual liquid was used for testing the adsorption amount. Kubelka-Mtmk equation is shown in eq 3, where K is the absorption coefficient; S is the scattering coefficient; and R is reflectivity.

K /S = (1 − R )2 /(2R )

The molecular weights of HSL samples increase from 8,750 Da to 14,830 Da with the increase of epichlorohydrin concentration. The Mw of HSL samples is all much more than those of AL and SNF. The SO3H group of HSL is approximately 2.10 mmol·g−1, which is little lower than that of SNF. The OHphen group of HSL declined essentially compared with 2.32 mmol·g−1 of AL, and it decreased with increasing Mw. This suggests that the molecules of lignin were linked in the OHphen position. The polydispersibility index of HSL decreased with increasing molecular weight, indicating the smaller molecular weight lignin was easier to link by epichlorohydrin. This is due to the fact that the lower molecular weight lignin has more phenolic hydroxyl so as to possess higher reactivity.4 Dispersive Performance and Dye Uptake. A dye bath with smaller particle size exhibits excellent dispersive ability and high temperature stability during the dyeing procedure so as to lead to good dye uptake and dying quality.27 The particle size of the dye bath at 25 and 130 °C was measured, and the results are shown in Figure 2. The particle size of the dye baths with

(3) 26

Then, the stain rate is calculated using eq 4.

Stain rate = [(R 0 − R i)/R 0] × 100%

(4)

where R0 is the reflectance of the polyester fiber of blank control, and Ri is the reflectance of the polyester fiber with dispersant. The amount of dispersants adsorbed onto the fiber was measured by the residual mass fraction method. The content of the dispersant in residual liquid was measured by a UV spectrophotometer (UV-2450, Shimadzu Corp., Tokyo, Japan) at 280 nm. The amount of dispersant adsorbed was determined through calibration.

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RESULTS AND DISCUSSION Molecular Weight and the Functional Group. The molecular weight distribution of HSL1-HSL3, AL, and SNF is presented in Figure 1. The mass average of molecular weight

Figure 2. Effect of dispersant on the particles size of the dye bath at 25 and 130 °C.

HSL and SNF are slightly different, all approximately 0.90 μm at 25 °C. However, the particle size of all samples increased sharply when temperature was increased to 130 °C. The particle sizes with HSL samples are much smaller than those of SNF; in addition, the dispersant in the dye bath with the smallest particles size is HSL2, which indicates that hydroxypropyl sulfonated AL, especially HSL2 with a Mw of 11,020 Da, has better heat stability than sodium naphthalene formaldehyde condensation. In order to further characterize the dispersive ability and stability, the sediment thicknesses of dye baths with different dispersants at 25 and 130 °C dyeing processes at different times

Figure 1. Molecular weight distribution of AL and dispersants.

(Mw), the numerical average of molecular weight (Mn), and polydispersibility (Mw/Mn) as well as functional group content are given in Table 1.

Table 1. Molecular Weight Data and Functional Groups of AL and HSL1-HSL3 samples AL HSL1 HSL2 HSL3 SNF

SO3H (mmol·g−1)

OHphen (mmol·g−1)

Mw (Da)

Mn (Da)

Mw/Mn

2.12 2.10 2.09 2.31

2.32 0.51 0.46 0.42 0.42

5,660 8,750 11,020 14,830 8,050

2,000 3,610 5,570 8,440 3,180

2.83 2.42 1.98 1.76 2.65

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Figure 3. Sediment thicknesses of the dye bath with time at (A) 25 °C and (B) 130 °C.

were tested by Turbiscan LabExpert. The results are given in Figure 3. The earlier appearing sediment and high sediment thicknesses value indicate worse stability and poorer dispersive ability. The sediment thicknesses of dye baths with all HSL samples are thinner than SNF both at 25 and 130 °C, and it confirms that SNF has poor dispersive ability and stability, especially at a high temperature of 130 °C. From Figure 3A, HSL2 has little sediment thickness than others, exhibiting a good dispersibility and grinding effect on disperse dye. The sediment thicknesses of all samples are thicker at 130 °C compared to that at 25 °C (Figure 3B), which indicates the high temperature would lead the dye particles to aggregate. The dye bath with HSL2 exhibits the best stability, whose sediment does not appear until after 40 min. The dye dispersant is a solid−liquid surfactant, which adsorbs on the dye particles to facilitate the breaking of the agglomeration of dye particles and to keep them as the fine grain form by steric hindrance and electrostatic repulsion. HSL with high Mw can provide a stronger steric hindrance. The OHphen groups are etherified in molecules of HSL while OHaliph groups are introduced, which contributes to H-bond formation between dispersant and dye.28 The higher sulfonation degree of longer hydroxypropyl chains can provide a stronger electrostatic repulsion,29 which makes the dye bath with HSL more stable in aqueous environments than SNF. HSL2 with medium Mw has better dispersibility than that of HSL1 and HSL3. This is because HSL with higher Mw usually displays larger steric hindrance than the one with lower Mw, but the lower Mw dispersant would be easier to adsorb onto the dye particles.4 HSL1 with small Mw can adsorb on dye particles easily, but the less weak steric hindrance provided by HSL1 than HSL2 may make the dye aggregate easily too. HSL3 with too large of a Mw would produce stronger steric hindrance between dye particles and HSL3 when it adsorbs on the dye particle, which makes HSL3 differently continue to adsorb on dye particles. What’s more, HSL3 with higher Mw makes its hydrophile−lipophile balance value (HLB) more small, which can break the balance and lead to poor disperse performance30 than HSL2. In summary, HSL with the proper Mw of 11,020 Da can display good dispersive performance. The dye uptake of C.I. disperse blue 79 added to different dispersants is presented in Figure 4. The dye particle size at 130 °C versus dye uptake was plotted in Figure 4. It can be seen clearly that the dye uptake decreased significantly with increasing dye particles size at high temperature by fitting the curve.

Figure 4. Particle size of dye at 130 °C versus dye uptake.

The uptake rate of the dye bath with HSL2 reached 85.17%, which is superior to that of the others. The dye bath added HSL2 has the best dispersion and high temperature stability, and this is because HSL2 has suitable Mw and higher sulfonic group content and makes the dye adsorb on fiber more efficaciously. The dye uptake added SNF is lower than HSL samples, and this is due to its poor high temperature stability. Fiber Staining of Dispersants. The dark color of dispersants especially lignin dispersants may have undesirable effects as a textile dye dispersant in union or light color dyeing processes due to fiber staining. The K/S value (apparent depth of a color) of dispersant was measured by a Datacolor photometer instrument (Figure 5A). A higher K/S value expresses more serious fiber staining.31 The K/S values of all samples increased with increasing concentration of dispersants. They all reached a platform approximately at the equilibrium concentration of 2.2 g·L−1. In order to display the staining effect intuitively, the K/S value of dispersant at the concentration of 1.8 g·L−1 just before the plateau was chosen to calculate the staining rate, and the results are shown in Figure 5B. HSL have a much lower staining rate than AL, and the staining rate of HSL decreases as Mw increases, which should be related to the phenolic hydroxyl group content. Because free phenol hydroxyls (pyrocatechol) in lignin molecules can be easily oxidized to quinoid structure during heat conditions, and the quinoid structure will result in darker colors of lignin.15 From Table 1, more than 80% of OHphen groups in HSL are etherified, which prevents the formation of quinones and at some time point whitens the lignin color to reduce fiber staining.20 To further explore the effect of molecular weight on fiber staining, the adsorption amount of dispersants with different dosage on fiber was carried out in Figure 6. The adsorption amount of all dispersants and AL increase sharply, then slowly with increasing dosage. AL and HSL1 have the highest adsorption amount. Higher OHphen contents of AL and HSL1 increase the H-bonding possibilities between lignin and fiber. In addition, the fiber was expanded, and there appeared many voids during dyeing. The smaller molecules AL or HSL1 or dispersant are more easily embedded into fiber voids, which will increase the adsorption amount. HSL3 with higher Mw has higher adsorption amount than HSL2, indicating 3242

DOI: 10.1021/acssuschemeng.5b00821 ACS Sustainable Chem. Eng. 2015, 3, 3239−3244

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ACS Sustainable Chemistry & Engineering

Figure 5. K/S values of fiber treated with different dispersants and fiber staining rate.

hydrophobic and H-bonding interaction between fiber and dispersant. According to the above description, the adsorption model of dispersants on fiber was drawn in Figure 7. AL has small Mw and high OHphen groups content, and its molecules can embed into the fiber interspace easily especially during the higher temperature dyeing process when the fibers have more pores.32 So AL has a very serious fiber staining effect. On the contrary, HSL2 has higher molecular weight according cross-linking by epichlorohydrin, and most of its molecules just adsorb on the dye surface. SNF shows low fiber staining because of its light brown color and no OHphen group content; the molecular structure of SNF is a linear structure with the main chain of aromatic ring,33 which adsorbs on the fiber by the lying form.

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Figure 6. Adsorption amount of dispersant with different dosages on fiber.

CONCLUSIONS The hydroxypropyl sulfonated AL dye dispersants with different molecular weights were prepared by using different dosagea of epichlorohydrin to cross-link lignin molecules. HSL with suitable molecular weight of 11,020 Da has good dispersive ability and good high temperature stability and high dye uptake compared with those of the other HSL and SNF investigated here. HSL with higher molecular weight has less phenolic hydroxyl group content, which whitens the color and reduces hydrogen bonding adsorption amount on fiber. The results produce lower fiber staining.

the hydrophobic interaction adsorption between the lignin of the fiber. Although HSL3 has a higher adsorption amount, the lower OHphen content makes it have lighter color than HSL2.20 This is why HSL3 has a higher adsorption amount but has lower fiber staining rate than HSL2. SNF has minimum adsorption amount, due to it is a linear molecular structure with the main chain of the aromatic ring, which is difficult to get into fiber voids. Besides, SNF with lower Mw and small amounts of OHphen group compared to HSL provided a weaker

Figure 7. Adsorption model of dispersants on fiber. 3243

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AUTHOR INFORMATION

Corresponding Authors

*(D.Y.) Tel: +862087114722. Fax: +862087114721. E-mail: [email protected]. *(X.Q.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of International S&T Cooperation Program of China (2013DFA-41670), National Natural Science Foundation of China (21436004) and the Fundamental Research Funds for the Central Universities (2014ZP0003).

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