Adsorption properties of direct dyes on viscose/chitin bicomponent fiber: evaluation and comparison with viscose fiber

Abstract

Two direct dyes were applied for dyeing the viscose/chitin bicomponent fiber, whose chitin component can provide functionalities and enhanced dyeing properties. Dyeing rates, adsorption isotherms and mechanisms, electrolyte and dye concentration dependence on dye adsorption, as well as dye desorption from dyed fiber, were investigated in comparison with dyeing system of regular viscose fiber. Higher adsorption and lower desorption of dyes occurred for viscose/chitin fiber. The viscose/chitin fiber showed great dye adsorption capability even at a low salt dosage providing environmental benefits for dyeing process. The rates of dye uptake by viscose/chitin fiber were faster and followed the pseudo-second-order kinetic model. Dye adsorption isotherms were closely correlated to the dual Langmuir–Nernst model consisting of site-specific and non-site-specific interactions between dyes and fibers; moreover, the Langmuir adsorption was predominant in total adsorption, and this predominance was more obvious for viscose/chitin fiber. The characteristics of above dye adsorption on viscose/chitin fiber resulted mainly from its partially deacetylated chitin component, which can reduce the negative charge density of the fiber and provide more site-specific dyeing sites.

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1. Introduction

Most of the world's man-made cellulose fibers are produced by the classical viscose process based on obtaining cellulose with carbon disulfide.¹ These fibers are called rayons. Although some of the problems related with the viscose process, such as relatively high production costs and environmental pollution, need to be reduced and alternative production routes need to be stimulated, the production of viscose fiber has an accepted importance within the fiber family and not only among the standard fibers but also among specialty fibers such as micro-fibers, spun-dyed fibers, functional fibers, and fibers for nonwovens.¹ The viscose fiber has no inherent functionality other than wear comfort and moisture management, but functional additives incorporated during the spinning process can endow it with diverse functionalities such as flame retardancy, antibacterial activity, ultraviolet protection capability, water repellency, and combined antibacterial activity and enhanced dyeability.^1–5

Cellulose is the most abundant natural polymer in the world, followed by chitin. Chitin and its deacetylated product (chitosan) have been extensively applied in many fields because of their outstanding properties such as biocompatibility, non-toxicity, biodegradability and antimicrobial activity.^6,7 In the fiber industry, chitin, chitosan and their blends with other polymers can be applied to manufacture functional fibers by wet spinning process.^6,8 Nowadays, chitosan fiber and viscose/chitin or chitosan bicomponent fiber are commercially produced. The manufacturing of viscose/chitin or chitosan bicomponent fiber by wet spinning can be mainly divided into the following two approaches: one is the use of cellulose viscose in which a fine chitosan powder is dispersed, and the other is the use of a mixed solution of cellulose viscose and chitin viscose. A typical product prepared by the former technique is Chitopoly fiber (Fuji Spinning Co., Japan), and products prepared by the latter technique are Crabyon (Omikenshi Co., Japan) and Chitcel (CHTC Helon Co. Ltd., China) fibers.^9,10 The viscose/chitin bicomponent (viscose/CH) fiber is a new kind of modified viscose fiber that has the advantages of both viscose fiber and chitin polymer. Its merits include no allergenic reaction, low antigenicity, high safety, organism compatibility, gentleness to the body, antibacterial and deodorizing effects, and moisture-retaining capability.^8–10

For any new kind of fiber to stand a chance of success on the market, its performance is a primary factor. In addition, a suitable dyeing and finishing process cannot be neglected. The viscose/CH fiber contains chitin component, which is partially deacetylated in the course of mixing and spinning of the chitin viscose and cellulose viscose because of their strong alkalinity. The deacetylation degree of the chitin in Crabyon fiber is about 55%.¹⁰ The inclusion of partially deacetylated chitin in the viscose/CH fiber delivers different dyeing and finishing characteristics from regular viscose fiber. Therefore, research on the wet processing properties of viscose/CH fiber is of great significance both in theory and in practice. Several investigations have been undertaken on the dyeing and functional modification of viscose/CH fiber. Nakajima et al. measured the dyeing rate curves of C.I. Acid Orange 7 with a small molecular size and C.I. Direct Red 28 with a large molecular size for viscose/CH fibers under conditions of pH 4, at 30 to 50 °C.¹¹ They found that the dyeing rates of the two dyes for viscose/CH fibers were higher than those for silk and viscose fibers, and increased with increasing chitin content. Shimizu et al. observed the sigmoid type adsorption of C.I. Acid Orange 7 on viscose/CH fibers is different from the Langmuir-type adsorption on silk and wool fibers at pH 4 and temperatures below 50 °C.¹⁰ Shimizu et al. also studied the effect of pH on the fixation of four reactive dyes on viscose/CH fibers in the absence of neutral electrolyte.¹² They noticed that C.I. Reactive Blue 5 with a monochlorotriazinyl group in a weakly acidic solution and C.I. Reactive Blue 19 with a sulfatoethylsulfonyl group in a weakly acidic to weakly alkaline solutions showed high fixation on viscose/CH fibers, whereas viscose fiber could not react entirely with Blue 19 in the same pH region. Recently, Kokol and co-workers utilized polyphenol oxidases to graft natural flavonoids onto viscose/CH fiber with the aim to significantly upgrade the antioxidant activity of this fiber.^13,14 In addition, Vasile et al. carried out the modification of viscose/CH fiber with a variety of monomers under plasma conditions.^9,15

The viscose/CH fiber consists of viscose and partially deacetylated chitin, and its most predominant component is viscose. Therefore, the dyes (widely used reactive and direct dyes) suitable for cellulosic fibers can also be applied for the dyeing of the fiber. Although previous research discussed the adsorption mechanisms and dyeing properties of some dyes on viscose/CH fiber, further studies need to be carried out in order to better understand the adsorption mechanism of various dyes, to know the dyeing properties of dyes under conditions close to those used in practical dyeing process, and to provide help for the dyers to control the dyeing process and optimize the application of dyes. In this work, two commercial direct dyes bearing four sulfonate groups were employed to dye viscose/CH fiber, and the rates, equilibrium isotherms and mechanisms of dye adsorption were investigated. The dual Langmuir–Nernst model consisting of site-specific and non-site-specific interactions between dyes and fibers was used to describe the adsorption isotherms of dyes with the aim of providing an insight into the mechanism of the direct dyeing process of viscose/CH fiber. Moreover, the effect of neutral electrolyte on dye adsorption, the building-up properties of dyes, and the desorption properties of dyes from the dyed fiber were also discussed. In these studies, regular viscose fiber was selected as the reference sample.

2. Experimental

2.1 Materials

1.67 dtex × 38 mm viscose fiber was supplied by Hebei Jigao Chemical Fiber Co., Ltd., China. 1.67 dtex × 38 mm viscose/chitin bicomponent (abbreviated as viscose/CH) fiber under the trade name of Chitcel was purchased from CHTC Helon Co. Ltd., China. According to a previous report, the viscose/CH fiber contains 9–11 wt% chitin.⁹ To remove finish oils added to the fibers during post-spinning treatments, fiber samples were treated in a scouring bath containing 0.5 g L⁻¹ Levelling Agent O and 1 g L⁻¹ sodium carbonate at 80 °C for 60 min using a 30 : 1 liquor ratio. The scoured fibers were then rinsed thoroughly under tap water, allowed to dry in open air and maintained in a desiccator with silica gel.

Everdirect Supra Yellow RL (C.I. Direct Yellow 86) and Everdirect Blue BRR (C.I. Direct Blue 71) were purchased from Everlight Chemical Industrial Co. and used as received. The dye structures are shown in Fig. 1. Acid Orange II (C.I. Acid Orange 7, a sulfonated monoazo dye) was a commercial product and purified by repeatedly dissolving in N,N-dimethylformamide followed by precipitating in acetone. Sodium carbonate, sodium sulfate, glacial acetic acid, sodium acetate, N,N-dimethylformamide and acetone were of analytical grade. Levelling Agent O (a nonionic polyoxyethylene ether surfactant) was provided by Jiangsu Hai'an Petrochemical Plant, China, and used as received.

Fig. 1 The chemical structures of direct dyes.

2.2 Adsorption and dyeing methods

All adsorption and dyeing experiments were carried out in sealed, conical flasks immersed in a XW-ZDR low-noise oscillated dyeing machine (Jiangsu Jingjiang Xingwang Dyeing and Finishing Machinery Factory, China). All the dye solutions were prepared with direct dyes, sodium sulfate and Levelling Agent O (0.3 g L⁻¹). Liquor-to-goods ratio was kept at 60 : 1. At the end of dyeing, the dyed fibers were rinsed thoroughly in distilled water and allowed to dry in open air.

2.2.1 Effect of sodium sulfate dosage on the uptake of dyes. To assess the dependence of the uptake of two dyes on sodium sulfate concentration, fibers were dyed in solutions including 3% owf (on the weight of fiber) dye and 0–20 g L⁻¹ sodium sulfate. The dyeing was started at 30 °C, and then the temperature was raised at a rate of 2 °C min⁻¹ up to 90 °C, and at this temperature the dyeing was continued for 60 min.

2.2.2 Adsorption rates of dyes. Fibers were dyed in the dye solution containing 3% owf dye and 10 g L⁻¹ sodium sulfate for different times at 90 °C. The fibers were not placed in the dye-bath until the dye-bath reached the dyeing temperature.

2.2.3 Equilibrium adsorption isotherms of dyes. The adsorption isotherms were measured for a series of dye solutions of various concentrations (0.5–5% owf), which contained 10 g L⁻¹ sodium sulfate. The fibers were dyed at 90 °C for 3 h in order to achieve equilibrium adsorption.

2.2.4 Building-up properties of dyes. The building-up properties of dyes on fibers were measured for the dye solutions, which were composed of 0.5–5% owf dye and 10 g L⁻¹ sodium sulfate. The fibers were immersed in the solutions at 30 °C, and subsequently the solutions were heated to 90 °C at a rate of 2 °C min⁻¹ and the dyeing was continued for 80 min.

2.3 Measurements

2.3.1 Dye uptake. Absorption spectra and the absorbance at the maximum absorption wavelength of dye solutions were measured using the Shimadzu UV-1800 UV-vis spectrophotometer (Shimadzu Co., Japan). The quantity of dye in the solution was calculated using a previously established absorbance–concentration relationship, and the percentage of exhaustion (%E) was determined using eqn (1), where C₀ and C₁ are the quantities of dyes in solution before and after dyeing, respectively. The quantity of dyes on fibers was calculated by taking into account the initial and final quantity of dyes in solution and the weight of fibers.

2.3.2 Desorption of dyes from dyed fibers. The desorption of Yellow RL and Blue BRR from the dyed fibers in water was tested at 60 °C using a liquor ratio of 100 : 1. Prior to desorption experiments, the viscose/CH and viscose fibers were dyed using the method described in Section 2.2.4. For the desorption of Yellow RL, both the viscose/CH fiber and the viscose fiber were dyed with 2% owf dye, whereas for desorption of Blue BRR, the viscose/CH and viscose fibers were dyed with 2.5% and 4% owf dye, respectively. For both Yellow RL and Blue BRR, the corresponding dye dosages resulted in nearly the same quantity of dye adsorption by the viscose/CH and viscose fibers according to the previous evaluation in Section 2.2.4. After dyeing, the fibers were completely rinsed in distilled water, washed at 40 °C for 5 min, and finally dried in open air. The desorption rate at different time intervals was calculated by the difference in the concentrations of the dye on the fibers before and after desorption.

2.3.3 Determination of amino groups in viscose/CH fiber. The number of amino groups in viscose/CH fiber was evaluated by using adsorption of C.I. Acid Dye 7 according to a previously reported method.¹³ For this adsorption experiment, 0.25 g of the fiber sample was immersed in an acidic dye solution of 4 × 10⁻⁴ mol L⁻¹; the pH value of the dye solution was adjusted to 3.6 with acetic acid/sodium acetate buffer; and the adsorption was conducted at 25 °C for 6 h in order to attain equilibrium. After adsorption, the residual dye solution was analyzed spectrophotometrically, and the amount of the adsorption of C.I. Acid Dye 7 was calculated. This procedure was found to result in 100% stoichiometric reaction at anionic dye sites with protonated amino groups along the partially deacetylated chitin chains integrated into a viscose fiber skeleton.¹³ The average results of two repetitions were presented as the amount of amino groups per kg of fiber.

2.3.4 FTIR analysis. The infrared spectra of viscose/CH and viscose fibers were recorded with the Nicolet 5700 FT-IR spectrometer (Thermo Fisher Scientific Inc., USA) using potassium bromide pellets. The fibers were cut into powder with a pair of scissors and the fiber powders were then used as samples. All the IR data were collected from 32 scans with a resolution of 4.0 cm⁻¹.

2.3.5 X-ray diffraction characterization. The wide angle X-ray diffraction (WAXD) measurements of fiber powders were carried out on the X'Pert-Pro MPD X-ray diffractometer (PANalytical B.V., NL) equipped using copper K alpha radiation of wavelength 0.15418 nm at room temperature. The scattering angle range was 5–45°, and scans were performed at 40 kV and 35 mA. Crystallinity degree was determined by corresponding peak separation analysis.

3 Results and discussion

3.1 Characterization of viscose/CH fiber

The FTIR spectra in the 2000–400 cm⁻¹ region of viscose/CH and viscose fibers are shown in Fig. 2. This region contains the largest number of spectral differences that allows for the identification of structural changes in cellulosic fibers.¹⁶ The spectrum of the viscose fiber showed characteristic peaks of cellulose II at 1419 (CH₂ symmetric bending), 1375 (C–H bending), 1317 (CH₂ wagging), and 895 cm⁻¹ (γ[COC] in plane, symmetric stretching), in agreement with the literature.¹⁶ Because of the inclusion of partially deacetylated chitin, the viscose/CH fiber displayed an IR spectrum slightly different from that of viscose fiber. For the viscose/CH fiber, the weak bands at 1660 and 1557 cm⁻¹, attributed to the C=O stretching of amide I and the N–H bending of amide II from the partially deacetylated chitin component, respectively,^17,18 were found, and these characteristic absorptions became good evidence for the presence of nitrogen atoms on the surface of this fiber.

Fig. 2 FTIR spectra in the range of 2000 to 400 cm⁻¹ of viscose/CH and viscose fibers.

The number of amino groups as an indication of the partially deacetylated chitin component present in viscose/CH fiber was detected using the Acid Orange 7 adsorption method.¹³ Corresponding values for the viscose/CH and viscose fibers were 68.19 and 2.82 mmol kg⁻¹, respectively. Actually, there is no amino group in viscose fiber. An insignificant amount was detected in viscose fiber because of very weak and non-ionic interaction between the acid dye and this fiber. The presence of amino groups in viscose/CH fiber may affect the adsorption of anionic dyes.

The dyeing properties of a fiber are usually affected by its crystalline structure. Thus, the crystalline structure of viscose/CH and viscose fibers was studied by WAXD. Fig. 3 shows X-ray diffraction traces for these two fibers. Three diffraction peaks were observed at angular positions (2θ) of approximately 12.4°, 20.4° and 21.8°, indicating the crystalline structure of cellulose II.^19,20 The inclusion of partially deacetylated chitin into the viscose/CH fiber had no obvious impact on the crystalline structure of viscose fiber. Crystallinity degrees of the viscose/CH and viscose fibers determined by a peak separation analysis were 41.5% and 34.8%, respectively. The difference of crystallinity degree between these two fibers possibly resulted from the different spinning parameters employed by the corresponding manufacturers. In general, regenerated cellulosic fibers with low crystallinity degree would possess great capacity for the adsorption of dyes if their other physical and chemical characteristics were the same.

Fig. 3 X-ray diffraction patterns of viscose/CH and viscose fibers.

3.2 Effect of sodium sulfate dosage on the uptake of dyes

Fig. 4 shows the effect of sodium sulfate dosage on the uptake of two direct dyes, i.e. Yellow RL and Blue BRR. The uptake of two dyes by the viscose/CH and viscose fibers increased with increasing salt concentration until a plateau was reached, which is when the salt concentration was higher than a certain value. It was found that two dyes exhibited very high uptake by viscose/CH fiber even at a very low dosage of salt. The dye exhaustion of the viscose/CH fiber exceeded 90% at a sodium sulfate dosage of 5 g L⁻¹, whereas 15–20 g L⁻¹ sodium sulfate was required for viscose fiber to acquire the same dye exhaustion. Taking into consideration that the viscose/CH fiber had a slightly higher crystallinity degree than the viscose fiber and possessed amino groups as discussed in Section 3.1, the higher dye exhaustion of viscose/CH fiber at a low dosage of salt is preliminarily considered to be the result of the reduced negative charge density of the fiber, caused by the partially protonated amino groups of the partially deacetylated chitin component.¹⁰ Generally, neutral salt is applied in the dyeing of cellulosic fiber to overcome repulsive forces between direct dyes and fibers, and its dosage should be increased as a function of dye concentration. The great dyeability of the viscose/CH fiber at a low dosage of salt suggests that the dosage of salt can be saved for the direct dyeing of the viscose/CH fiber in comparison with the dyeing of regular viscose fiber, thus providing an eco-friendly benefit.

Fig. 4 Effect of sodium sulfate dosage on the uptake of direct dyes by viscose/CH and viscose fibers.

3.3 Adsorption kinetics of dyes

The adsorption rates of two direct dyes for the viscose/CH and viscose fibers are depicted in Fig. 5. As shown in Fig. 5, the adsorption quantity of two dyes increased dramatically in the first 15 min, and then reached equilibrium at 30–45 min. The two dyes exhibited higher adsorption rates for the viscose/CH fiber than for the viscose fiber at the initial stage of dyeing. Furthermore, the quantity of dye adsorption at equilibrium on the viscose/CH fiber was higher than that on the viscose fiber.

Fig. 5 Adsorption rates of direct dyes for viscose/CH and viscose fibers.

To obtain the kinetic parameters of the dyeing of the viscose/CH and viscose fibers, the following pseudo-second-order equation (eqn (2))²¹ was used to analyze the experimental data:

where k (g mg⁻¹ min⁻¹) is the equilibrium rate constant of pseudo-second-order adsorption, and C_∞ and C_t refer to the amount of dye (mg g⁻¹) at equilibrium and at t time (min), respectively.

It was possible to calculate the rate constant (k) and dye adsorption quantity at equilibrium (C_∞) from the slope and intercept of the linear plot of t/C_t versus t (Fig. 5), and the values are listed in Table 1. The correlation coefficients (R²) for the linear plots were very close to 1 for all the experimental data. C_∞ values obtained also agreed with the experimental data. These results suggest that the pseudo-second-order kinetic model can be used to describe the adsorption process of direct dyes onto the viscose/CH and viscose fibers.

Table 1 Kinetic parameters of the adsorption of direct dyes

Dyes	Fibers	K (g/[mg min])	hi (mg/[g min])	t1/2 (min)	C∞ (mg g^−1)	R^2
Yellow RL	Viscose/CH	0.0998	78.75	0.36	28.09	1.0000
	Viscose	0.0875	60.29	0.44	26.25	1.0000
Blue BRR	Viscose/CH	0.0363	31.04	0.94	29.24	0.9995
	Viscose	0.0271	12.54	1.71	21.51	0.9996

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On the basis of the pseudo-second-order kinetic model, the initial adsorption rate (h_i) and the half-adsorption time (t_1/2) were calculated using the following eqn (3) and (4), respectively.^22,23 Half-adsorption time is defined as time required for adsorbents to uptake half the amount adsorbed at equilibrium, and it is often considered as a measure of the adsorption rate.

h_i = kC_∞² (3)

As can be seen from Table 1, the kinetic parameters for the adsorption of direct dyes varied according to the fiber and dye categories. For viscose/CH fiber, the higher rate constant and initial rate of the dye adsorption were found with the shorter half-adsorption time. The higher dye adsorption or the faster dyeing rate of the viscose/CH fiber should be ascribed to additional positive adsorbing sites and reduced negative charge density, which are caused by the partially protonated amino groups of the partially deacetylated chitin in this fiber.

3.4 Adsorption isotherms of dyes

In general, equilibrium adsorption will be reached in longer times when dye concentrations are high. Therefore, isotherms were determined on the basis of adsorption for 3 h. Isotherms of the adsorption of direct dyes on viscose/CH and viscose fibers are depicted in Fig. 6.

Fig. 6 Adsorption isotherms of direct dyes on viscose/CH and viscose fibers (solid lines present the Langmuir–Nernst plots).

To help us better understand the interactions between the direct dyes and viscose/CH fibers, three isothermal models, namely, Langmuir, Freundlich, and Langmuir–Nernst (the dual adsorption equation consisting of the partition and Langmuir models) were selected to compare the equilibrium adsorption data.

The expression for the widely used Langmuir isotherm is as follows:²⁴

where C_f (mg g⁻¹) and C_s (mg L⁻¹) are the concentrations of dyes on fibers and in solution at equilibrium, respectively; S (mg g⁻¹) is the saturation concentration of dyes on fibers; and K_L (L mg⁻¹) is the Langmuir affinity constant.

The empirical Freundlich model equation can be written as follows:²⁵

C_f = K_FC_s^1/n (6)

where K_F (mg^1−1/n L^1/n g⁻¹) is the Freundlich constant, and 1/n stands for heterogeneous factor.

The Langmuir–Nernst model can be described by the expression:^26,27

where C_P and C_L are the concentrations of dyes on fibers by Nernst-type partitioning and Langmuir adsorption, respectively; S is the saturation concentration of dyes on fibers by Langmuir adsorption; and K_P (L g⁻¹) and K_L (L mg⁻¹) are the partition coefficient and the Langmuir affinity constant, respectively.

All the experimental adsorption isotherms in Fig. 6 were fitted to the three isotherm models using the nonlinear least-squares fitting procedure; thus, the parameters in eqn (5)–(7) were obtained. The adsorption parameters for the Langmuir–Nernst model are summarized in Table 2 (data for other models not shown). For the purpose of assessing the fitting results, normalized deviations (ND) of the experimental values used to estimate the extent of fitting were calculated according to eqn (8):

where C_f,exp,i and C_f,calc,i are the experimental and calculated values (the amount of adsorbed dyes on fibers), respectively. The latter is a result of the calculation using eqn (5)–(7) on the basis of the parameters, which were obtained by the fitting procedure. Note that index i refers to sequence number of adsorption data; N is the total number of data sets.

Table 2 Parameters in the Langmuir–Nernst equation

Dyes	Fibers	KL (L mg^−1)	S (mg g^−1)	KP (L g^−1)
Yellow RL	Viscose/CH	0.026	60.25	0.081
	Viscose	0.024	33.76	0.099
Blue BRR	Viscose/CH	0.443	30.31	0.144
	Viscose	0.055	18.78	0.027

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ND values for the three adsorption models are presented in Table 3. Evidently, the Langmuir–Nernst model nearly gave the lowest ND values, and thereby showed the highest fitting extent. The Langmuir and Freundlich equations did not give a good fit to the experimental data. Thus, the Langmuir–Nernst isotherm is the most appropriate model to describe adsorption behaviors of direct dyes. Fig. 6 also shows the plots of the fitting of the Langmuir–Nernst model to the experimental data. The fitting curves almost passed through all the experimental data almost exactly, further suggesting the validity of the proposed adsorption mechanism.

Table 3 Normalized deviations (%) of the isotherm models

Models	Yellow RL		Blue BRR
	Viscose/CH	Viscose	Viscose/CH	Viscose
Langmuir	4.17	3.78	16.20	10.37
Freundlich	11.91	7.25	13.11	5.26
Langmuir–Nernst	4.00	0.34	7.13	3.76

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To clearly demonstrate the effect of initial dye concentrations on adsorption, the percentage of the contribution of Langmuir or Nernst adsorption to total adsorption^26,27 was calculated using the parameters in Table 2 and the known C_s according to eqn (7) at each initial dye concentration and depicted in Fig. 7. As shown in Fig. 7, the contribution of Langmuir adsorption to total adsorption decreased with increasing initial dye concentration, while the contribution of Nernst adsorption increased. Moreover, the contribution of Langmuir adsorption was always considerably higher than that of Nernst adsorption; in other words, Langmuir adsorption was predominant in total adsorption. For the viscose/CH fiber, this predominance was more obvious.

Fig. 7 Contribution of Langmuir and Nernst adsorption to total dye adsorption on viscose/CH and viscose fibers: (a) Yellow RL and (b) Blue BRR.

In this study, the adsorption equation giving the best correlation of the experimental data was the Langmuir–Nernst equation rather than the Freundlich equation. The traditional equation used to describe direct dye adsorption is the classical Freundlich equation. This was used because the cellulose substrate was considered to be heterogeneous in the accessible regions where the dye is absorbed without having obvious specific dye sites like wool or nylon fibers.²⁸ However, some researchers have reported that the classical Langmuir model offers a better interpretation of the adsorption of direct dyes^28–38 and hydrolyzed reactive dyes^37,38 on cellulose. Porter explained this adsorption phenomenon in terms of limited dye space or sites for dye adsorption²⁸ and considered that the interaction of the dyes with cellulose substrate is rather specific. Blackburn and co-workers tended to consider that the adsorption of hydrolyzed reactive dye with a large molecular size on caustic soda-treated cellulose II may occur by virtue of a combination of Langmuir (limited) and Freundlich (unlimited) adsorption mechanisms³⁸ and that adsorption in excess of the experimentally determined saturation value occurs as a result of non-site-specific interaction. Bae et al. reported that the saturation values of direct dyes on cellulose depend on the inner surface areas of the fibers, the size of dye molecules, and the intermolecular repulsive interaction between the dye anions adsorbed on the inner surface of cellulose (restricting further adsorption).³⁰ Despite the above findings, in theory, how the limited adsorption or the site-specific interaction occurs has been somewhat obscure.

According to previously reported investigations, it may be concluded that the dual Langmuir–Nernst mechanism of the adsorption of direct dyes on the viscose/CH and viscose fibers consists of site-specific and non-site-specific interactions. Because in viscose/CH fiber there exist the partially protonated amino groups of the partially deacetylated chitin, this fiber apparently possesses more specific dyeing sites for the adsorption of anionic dyes, which allow a higher contribution percent of dye adsorption by Langmuir mechanism. Consequently, the affinity of direct dyes to viscose/CH fiber also becomes higher as compared with their affinity to viscose fiber, leading directly to the higher Langmuir affinity constant of dye adsorption and the higher dyeing saturation of the fiber as expressed by the K_L and S values in Table 2. The higher contribution percent of Langmuir adsorption at lower initial dye concentrations shown in Fig. 7 may be explained by the fact that the adsorption is mainly controlled by the specific adsorbing sites in fibers. At higher initial dye concentrations, a number of adsorbed dyes on the exterior and interior surface areas of fibers give rise to an increased negative charge density of fibers and a decreased quantity of specific adsorbing sites, which accordingly reduce the contribution percent of Langmuir adsorption.

3.5 Building-up properties of dyes

Building-up properties of direct dyes on fibers could be related to factors such as fiber and dye structures, dye affinity to fibers, dye adsorption mechanisms, dyeing temperature, and additives used in dye solution. Dyes having good building-up performance are particularly suitable to dye fibers required for dark shades. The building-up properties are of great importance for practical application. The viscose/CH fiber contains partially deacetylated chitin and accordingly possesses positive dyeing sites capable of capturing anionic dyes. It stands to reason that direct dyes have better building-up properties on viscose/CH fiber than on viscose fiber. However, it is still worth exploring how the partially deacetylated chitin in the viscose/CH fiber affects the building-up properties of direct dyes.

In practical dyeing, processing time is not so long as that employed for thermodynamic studies. Therefore, building-up properties of direct dyes were determined in a temperature-rise process instead of a constant temperature process. The building-up properties expressed by C_f, as well as the exhaustion of dyes, are depicted in Fig. 8. The quantity of adsorption increased almost linearly with an increase in initial dye concentration, even at a high concentration of dye; moreover, the slopes of the curves of the adsorption quantity versus initial dye concentration for viscose/CH fiber were higher than those for viscose fiber, indicating the better building-up properties of direct dyes on viscose/CH fiber. This was particularly true for the dye Blue BRR. As applied dye concentration increased, the difference between the adsorption capability of the viscose/CH and viscose fibers became greater for Blue BRR. These observations indicate that direct dyes have better building-up properties on the viscose/CH fiber and higher affinity to viscose/CH fiber, an observation that is consistent with the higher K_L values for viscose/CH fiber as shown in Table 2.

Fig. 8 Building-up properties of direct dyes on viscose/CH and viscose fibers: (a) Yellow RL and (b) Blue BRR.

The high dye adsorption capability of viscose/CH fiber in the case of the use of a high dye concentration indicates that this fiber has the advantage in terms of deep dyeing. At the same time, the dyeing system of the viscose/CH fiber exhibited higher dye exhaustion than that of the viscose fiber as confirmed by the results of Fig. 8. Even when a dye concentration of 5% was used, exhaustion of the Yellow RL and Blue BRR was able to reach to approximately 90%. High dye exhaustion allows a high utilization of dyes, a reduced cost of dye consumption in the dyeing process and a decrease of wastewater discharge, which becomes another advantage of the viscose/CH fiber with respect to dyeing.

3.6 Desorption of direct dyes from dyed fibers

Dyed fibers and textiles are subjected to frequent washing during their usage. Hence, the resistance of dyes on fibers to washing is very important. Washing resistance can be evaluated using color fastness to washing and also characterized using desorption of dyes from fibers in water. On the other hand, the adsorption of dyes onto fibers and the desorption of dyes from dyed fibers occur simultaneously during the dyeing process, and the desorbed dyes are likely to be again adsorbed by the fibers. Note that The adsorption, desorption and re-adsorption performance of dyes is defined as migration property. Good migration property of dyes allows fibers to obtain a good leveling effect. A certain degree of desorption of dyes from dyed fibers is prerequisite for the migration of dyes and even-dyeing of fibers. Low and slow desorption of dyes from fibers usually indicates their high affinity to fibers and poor migration property and easily leads to the poor dyeing levelness. Taking the above factors into consideration, the performance of the desorption of direct dyes from dyed viscose/CH and viscose fibers in water was investigated.

Fig. 9 displays the variation of dye desorption percentage with time. Desorption of dyes increased as a function of time and showed a trend towards equilibrium as the time was prolonged. As expected, dye desorption from dyed viscose/CH fiber was remarkably lower than that from dyed viscose fiber. The desorption test suggests that the washing resistance of the direct dyes on the viscose/CH fiber is very good, indicating that the dyed viscose/CH fiber has better washing color fastness as compared with the viscose fiber. Moreover, the very low and slow desorption of dyes from the dyed viscose/CH fiber indicates higher affinity and poor migration of direct dyes towards this fiber, which easily results in poor dyeing levelness. In fact, low desorption of dyes from the dyed viscose/CH fiber has a direct correlation with the rapid rate of dye uptake by the fiber as shown in Fig. 4 and Table 1 with the high Langmuir affinity constant of dyes as shown in Table 2.

Fig. 9 Desorption of direct dyes from dyed viscose/CH and viscose fibers in water.

4. Conclusions

Studies on the equilibrium isotherms of dye adsorption indicated that adsorption of direct dyes on viscose/CH bicomponent fiber occurred via a combination of Langmuir and Nernst models, i.e., site-specific and non-site-specific interactions between dyes and fibers and that the Langmuir adsorption was predominant in total adsorption. The rates of dye uptake by viscose/CH fiber followed the pseudo-second-order kinetic model. Viscose/CH fiber showed faster dye adsorption rate, as well as lower dye desorption and migration, as compared with regular viscose fiber, suggesting that some measures should be taken to improve dyeing levelness of the fiber in the industrial dyeing process. Viscose/CH fiber exhibited higher dye adsorption capability, higher dye exhaustion, and better washing resistance of adsorbed dyes than the viscose fiber, indicating that it possesses the advantages of deep dyeing performance, good color fastness to washing, and environmental benefits of the dyeing process. The above dye adsorption and dyeing characteristics of viscose/CH fiber may be primarily ascribed to its partially deacetylated chitin component.

The observed difference between dye adsorption and dyeing properties of viscose/CH and viscose fiber was related to dye categories. Comparison between adsorption properties of two dyes was not carried out in the present work due to the fact that the dyes used were commercial products and not purified dyes. Further studies on the difference between purified dyes in adsorption are merited in order to obtain information about the effect of dye structures on the dyeing of viscose/CH fiber and to achieve a deeper understanding of the fundamental physical chemistry involved in dye adsorption onto the viscose/CH fiber.

Acknowledgements

This study was funded by the Jiangsu Provincial Natural Science Foundation of China (BK20131178) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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