Lixuan
Yu
a,
Jue
Yu
a,
Wenjie
Mo
a,
Yanlin
Qin
a,
Dongjie
Yang
*ab and
Xueqing
Qiu
*ab
aSchool of Chemistry and Chemical Engineering, South China University and Technology, Guangzhou, 516640, China. E-mail: cedjyang@scut.edu.cn; xueqingqiu66@163.com
bKey Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China
First published on 11th July 2016
Lignosulfonates (SLs) are widely used as dye dispersants. However, they have some disadvantages, including poor high temperature dispersibility and severe fiber staining resulting from the abundant phenolic hydroxyl content in the SL molecules. In this work, etherified lignosulfonates (ESLs) were obtained by using epichlorohydrin to reduce the content of phenolic hydroxyl while increasing the molecular weight. ESLs with lighter color can reduce fiber staining rate by 52% due to whitening by the epichlorohydrin process. The lower adsorption capacity of ESLs onto the surface of fibers can reduce the fiber staining effect, owing to the lower phenolic hydroxyl content. The ESLs also exhibit superior high temperature stability to SL because of their higher adsorption capacity and more rigid adsorption films.
Currently, anionic surfactants play a major role in dye dispersants, especially lignosulfonates (SL) and naphthalene sulfonates (NS) from the petroleum industry.8 NS, derived from fossil resources, has less fiber staining and good dispersibility in dyes; however, it is expensive and toxic. However, SLs, which are the main component in waste liquid from paper pulping, are abundant, renewable, economical and environmentally friendly. Therefore, the dosage of SL used in dye dispersants is increasing every year. Meanwhile, there are a number of disadvantages in employing SLs as dye dispersants, such as fiber staining and poor dispersive ability;9,10 these limit the application of SLs in the dye industry, especially when they are employed in the dyeing process, where superior performance is required. Numerous studies have revealed that the fiber staining rate of SL is greatly dependent on its phenolic hydroxyl groups,11,12 which can form hydrogen bonds with electronegative groups in fibers.13 Therefore, an adequate modification method is necessary to reduce the phenolic hydroxyl content of SL.
At present, the methods to reduce phenolic hydroxyl content include oxidation, chelation with divalent metal salts, and other chemical reactions. Falkehag14 treated SL with 2-chloroethanol, and the product showed decreased fiber staining. A two-step process was also introduced to block the phenolic hydroxyl groups in lignin followed by oxidation with chlorine dioxide, and a light-colour lignin azo dye dispersant was obtained.15 By reacting divalent metal salts with lignin through chelation and an ester formation mechanism, the amount of dihydroxyl groups in lignin was reduced; therefore, the fiber staining was weakened.16
Etherification by epichlorohydrin is also a useful method to block the phenolic hydroxyl group in lignin molecules. Recently, by epoxidation with epichlorohydrin followed by etherification with polyethylene glycol, alkaline lignin was converted into water-soluble lignin derivates which can enhance the efficiency of enzymatic hydrolysis and ethanol fermentation. However, the molecular weight of the product was only slightly changed.17 Therefore, in our present study, epichlorohydrin was used as a crosslinking agent via a ring-opening reaction in aqueous solution to decrease the content of phenolic hydroxyl groups as well as to increase the molecular weight of the SL molecules.
It has been reported that decreasing the phenolic content may decrease the water solubility of lignin18 and thus decrease its dispersive ability and high temperature stability.19 Compared with NS, SLs have better high temperature stability, probably due to the phenolic hydroxyl group content in the molecules. Therefore, there is a contradiction between the influence of the phenolic hydroxyl group on dispersive ability and on fiber staining. If the phenolic hydroxyl content is decreased in order to reduce fiber staining, the dispersive properties may deteriorate. Therefore, it is highly necessary to develop a method to not only reduce the amount of phenolic hydroxyl groups in lignin but also to prevent the decrease of its heat stability. It has been reported that the high molecular weight of SL makes a great contribution to the dispersive ability and stability of particles;20,21 therefore, increasing the molecular weight and reducing the phenolic hydroxyl content simultaneously become the key factors for improving the dispersive ability while weakening the fiber staining. Fiber staining and the dispersive ability of the dye suspension are also related to the adsorption characteristics of SL on fibers and dyes. Quartz crystal microbalance with dissipation (QCM-D) is a new technique to detect the adsorption characteristics of dispersants at a solid/liquid interface by measuring the change of the third overtone of the frequency shift (Δf) and the dissipation shift (ΔD). The larger the value of |Δf|, the higher the adsorption amount of the dispersant on a quartz chip. ΔD is related to the viscoelastic property of the film on the quartz chip. The smaller the value of ΔD, the stronger the density and rigidity of the film.22 The absolute value of the slope of |ΔD/Δf| reflects the firmness of the adsorption structure. The adsorption structure becomes solid with increasing |ΔD/Δf| value. However, the application of QCM-D in revealing the mechanism of absorption of SL in the dye process has not yet been widely reported. Therefore, in previous work, we developed a method using QCM-D and atomic force microscopy (AFM) to study the absorption characteristics of SL on fiber and dye surfaces.23
In this paper, SL was modified with epichlorohydrin by etherification to reduce the content of phenolic hydroxyl groups while increasing the molecular weight. ESLs with different phenolic hydroxyl contents were obtained by adjusting the pH and adding different amounts of epichlorohydrin. The influences of the phenolic hydroxyl content and the molecular weight on the properties of dye liquor were investigated by determining the fiber staining rate and the particle size of the dye. The absorption characteristics of ESLs on fiber and dye particle surfaces were further examined by QCM and AFM.
Sodium naphthalene sulfonic acid formaldehyde condensation (SNF) (Shangyu Wencai Co., Zhengjiang, China) is a commercial dispersant used for dye dispersion; its purity was greater than 90%.
Ultrazine Na (UNA) (Borregaard Co., Sarpsborg, Norway) is a widely used dispersant lignosulfonate; it was purified by ultrafiltration, and its purity was 95%, with a small amount of inorganic salts.
C.I. disperse blue 79, an azo dye, was supplied by Runtu Co. Ltd., Zhejiang, China. Folin–Ciocalteu phenol reagent (2 mol L−1), vanillin and poly(diallyldimethylammonium chloride) (PDAC, Mw 200000 to 350
000, 20% solution) were supplied by Sigma-Aldrich (Xuhui District, Shanghai, China).
K/S = (1 − R2)/2R |
Then, the staining rate was calculated using the following equation:
Fiber staining rate/% = [(R0 − Ri)]/R0 × 100 |
A series of ESLs were prepared by modification with epichlorohydrin. The structural characteristics of the ESLs depended on the amount of ECH added and the pH value. By adjusting the amounts of epichlorohydrin added at pH 10.8 and 12.0, ESLs with different molecular weights were prepared. The Ph-OH content and the mass average of the molecular weight (Mw) of the ESLs are shown in Fig. 1. With increasing amount of epichlorohydrin and increasing pH, the Ph-OH content decreased and the Mw increased gradually. At pH 12.0, the Ph-OH content and Mw changed more significantly as increasing amounts of epichlorohydrin were added, from 0.5 to 2.5 mmol g−1. The degree of change is described by the degree of etherification and polymerization and can be calculated as follows:
The fitting relationships between the degrees of etherification and polymerization are shown in Fig. 2, where y is the degree of polymerization and x is the etherification degree; the slope reflects the degree of change. At pH 10.8, the equation is y = 0.0203x + 1.0085, R2 = 0.9939, and at pH 12.0, the equation is y = 0.0336x + 0.9314, R2 = 0.9626. With increasing etherification rate, the degree of polymerization increases more obviously at pH 12.0 than at pH 10.8, which indicates that under high pH conditions, SL molecules could be more readily linked together by epichlorohydrin.
It is suspected that two routes exist in the etherification reaction between lignin and epichlorohydrin, as shown in Fig. 3. Therefore, it is supposed that when a relatively small amount of epichlorohydrin is added or the pH is lowered, product (2) is mainly obtained, and the molecular weight changes only slightly after the reaction; when the amount of epichlorohydrin added is increased or the pH is higher, product (1) is predominant and the molecular weight increases obviously.
The molecular weight distributions are shown in Fig. 4. The Mw, the numerical average of molecular weight (Mn), and the polydispersibility index (Mw/Mn) as well as the functional group contents are given in Table 1. As the etherification degree increased from 15% to 45%, the molecular weight increased from 13285 Da to 19
261 Da and was much higher than that of SL. The polydispersibility index decreased with increasing etherification degree, indicating that the smaller molecular weight lignin was linked together by epichlorohydrin. It was known to us that the solubility and hydrophilicity of ESL-81 would be too weak for it to serve as a dye dispersant when the molecular weight of lignosulfonate was too high;27 therefore, ESL-81 will not be discussed later.
Sample | M w (Da) | M n (Da) | M w/Mn | Ph-OH content (mmol g−1) | Surface charge (eq. kg−1) |
---|---|---|---|---|---|
SL | 10![]() |
5264 | 1.90 | 1.90 | 1.07 |
ESL-15 | 13![]() |
8252 | 1.61 | 1.61 | 1.02 |
ESL-36 | 19![]() |
16![]() |
1.20 | 1.20 | 0.95 |
ESL-45 | 19![]() |
18![]() |
1.04 | 1.04 | 0.89 |
UNA | 10![]() |
4780 | 2.14 | 0.71 | — |
SNF | 8050 | 3180 | 2.53 | 0.42 | — |
![]() | ||
Fig. 6 Effects of the dispersants on fiber staining (a) and particle size of the dye bath at 25 and 130 °C (b). |
It is worth noting that the fiber staining of SL depends on the color of the SL and the adsorption characteristics of SL on the fiber surface and inside the fibers, whilst the dispersive ability and heat stability of a dye are related to the adsorption characteristics of SL on the dye.
The quinoid structure of SL is the key cause of its colour, as well as some chromophore and auxochrome groups, such as double bonds, carboxyl groups, carbonyl groups and hydroxyl groups.29 It has been stated that the quinoid structure and chromophore groups can account for the significant UV absorption at 450 nm (A450). All the A450 values of the ESLs are lower than that of SL, which is believed to be the result of the reduced Ph-OH content, except ESL-36. This is because under basic conditions, the catechol structure of SL will be oxidized to the quinone structure, resulting in a dark colour. With increasing etherification degree, more phenolic hydroxyl groups are blocked, which inhibits the formation of quinoid structures and results in a lighter colour. ESL-36 is obtained at pH 12.0, and the amount of ECH added is 0.5 mmol g−1 in the etherification process. Thus, the quinoid structure is produced extremely readily, leading to a dark colour. The A450 values of UNA and SNF are all lower than that of SL; thus, they exhibit less fiber staining than SL. After etherification, on the one hand, the colour of the lignin is lighter; on the other hand, the phenolic hydroxyl content declines significantly, from 1.90 mmol g−1 to 1.04 mmol g−1, which can distinctly lessen the hydrogen bond force between the dispersants and the fiber. Furthermore, the molecular weight also increased as the etherification degree increased. SL with low Mw is believed to embed into the fiber interspace more easily, especially during the high temperature dying process, because the fiber has more pores. Thus, the fiber staining rate decreased to a value close to that of UNA when the etherification degree was increased. Compared with SL, the particle size of the dye with ESLs decreases greatly at 130 °C, while it changes slightly at 25 °C. According to Table 1, with increasing etherification degree, the molecular weight increases from 10001 Da to 19
261 Da. It is believed that SL with high Mw provides sufficient steric hindrance to disrupt the agglomeration of dye particles and maintain its stability. The Mw of ESL-45 is much greater than those of UNA (Mw = 10
250) and SNF (Mw = 8050); therefore, it exhibits much better high temperature dispersive ability and stability, especially compared to SNF.
In summary, ESL-45 shows superior performance both in fiber staining and high temperature stability due to its desirable molecular weight and low amount of phenolic hydroxyl groups. As is proposed above, the performance of a dispersant mainly results from its adsorption characteristics on fibers and dyes; therefore, it is of great importance to find a visual method to measure the absorption characteristics to further verify the above results.
The adsorption characteristics of SL on internal fibers are mainly related to Mw. During the dying process, the fiber expands and many pores appear. SL with low Mw can more readily diffuse inside the fiber voids and cannot be washed away easily, while SL with high Mw cannot easily enter the fibers due to steric hindrance.26,30 After etherification, the Mw of SL increases as a result of crosslinking, which results in less fiber staining. In accordance with the above description, the adsorption model of dispersants on fiber is depicted in Fig. 8.
The surface charges of the dispersants are shown in Table 1. The surface charge decreases from 1.07 eq. kg−1 to 0.80 eq. kg−1 with increasing etherification degree, which indicates that the hydrophilic property of the ESLs weakens. This is because the Ph-OH content of ESL decreases after etherification. In addition, the ESL molecules adopt a curling conformation due to the increase of Mw, causing some charged groups, including sulfonic groups, carboxyl groups and Ph-OH, to be wrapped inside the molecule.32
Therefore, when the dye suspension is placed under high temperature conditions, the particles assemble readily. Hence, the dispersant with optimum molecular weight shows a favorable performance. In accordance with the above description, the adsorption model of SL and ESL-45 on dye particles is illustrated in Fig. 11. Compared with SL, ESL-45 has higher Mw and lower phenolic hydroxyl group content; therefore, it can strongly adsorb on the dye surface by hydrophobic effects. In addition, with higher Mw, the 3-D molecular structure of ESL-45 is more flexible and curly, which can disrupt the agglomeration of dye particles by steric hindrance.
With increasing etherification degree, the adsorption capacity of ESLs on the fiber surface decreases due to the gradual decrease in hydrogen bonding forces. Therefore, the fiber staining became weaker with increasing Mw of the ESLs. Also, the adsorption amount of ESLs on the dye increased and the adsorption structure became denser because of the enhanced hydrophobic interactions between the ESLs and the dye. Consequently, the high temperature stability improved significantly.
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