Bleaching of Black Pigmented Karakul Wool Fibers ...

Fibers and Polymers 2014, Vol.15, No.11, 2297-2306

DOI 10.1007/s12221-014-2297-y

Bleaching of Black Pigmented Karakul Wool Fibers Using Copper Sulfate as Catalyst Sayed Majid Mortazavi, Somayeh Safi*, Meghdad Kamali Moghadam, and Maede Zamani Textile Engineering Department, Isfahan University of Technology, Isfahan 84156-83111, Iran (Received August 7, 2013; Revised May 10, 2014; Accepted June 8, 2014) Abstract: The best chance for an efficient bleaching of highly pigmented wool with minimum fiber damage is provided by the use of metal catalysts in mordanting step preceding peroxide bleaching. This study evaluates the catalytic effect of copper sulfate (CuSO ) in the bleaching process of pigmented wool fibers under the used condition. The effects of CuSO and Na P O (as a stabilizer) concentration, bleaching time and rinsing time after mordanting on yellowness index, optical and mechanical properties were investigated and the optimum conditions for each step was reported. The results showed that an excellent depigmentation with minimum fiber damage is provided by using 1 %w/v CuSO and subsequent rinsing for 60 min, as well as bleaching with 60 ml/l H O and 7 %w/v Na P O for 15 min. The optical properties of fibers were improved after bleaching under optimum conditions compared with raw samples. The color indices revealed that the black wool fibers have turned into a pale light brown shade. The morphology and structure of wool fibers, before and after bleaching, were characterized by using optical microscopy, scanning electron microscopy (SEM), and EDAX test method. 4

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Keywords: Copper sulfate, Depigmentation, Bleaching, Pigmented wool, Karakul wool

recent research on the electrical properties of eumelanin has indicated that it may consist of more basic oligomers adhering to one another by some other mechanism. Thus, the precise nature of eumelanin’s molecular structure is once again the object of study [5,6]. The generally accepted chemical structures of eumelanin and pheomelanin pigments are presented in Figure 1 [5-7]. For light pastel shade textile articles, it is essential to use white material, and this can be obtained by the depigmentation of colored fibers [1,8,9]. Bleaching is a potential solution to ‘lighten’ the color so that bright colored textile articles can be produced from brown/black wool fibers. An efficient pigment bleaching with minimum fiber damage is provided by the use of metal catalysts in mordanting step preceding peroxide bleaching. A comparison of different metal salts that were candidates for bleaching catalysts showed that only iron (II), iron (III), and copper (II) ions had a significant catalytic effect under the conditions used [2]. Because the electron density of native melanin is higher than that of keratin, the metal cations are preferably absorbed by the melanin. After mordanting, the excess un-complexed metal ions are thoroughly rinsed from the wool. In the second stage, metal cations bound to the melanin catalytically decompose hydrogen peroxide (H2O2) to produce highly aggressive hydroxyl free radicals. This selectively attack and bleach the melanin, while un-pigmented regions experience a normal peroxide bleach [1,10,11]. The basic principles of this process are illustrated in Figure 2. A review of the literature reveals that many studies on pigmented fiber bleaching are concerned with improving the whiteness and mechanical properties of bleached fibers [2,8,11-17]. The method of depigmentation commonly used in industrial practice consists of a mordanting bath containing iron (II) ions which precedes a bleaching process using

Introduction The native color of wool fibers is closely related to the character of the environment in which sheep live. In nature, wool fibers are usually found in various shades of white, yellow, and in some case brown or black, due to the natural pigment, melanin [1,2]. Melanins are biological polymeric pigments formed from the sequential oxidation of tyrosine and located inside the cortex of wool fiber. Melanin produced in follicular melanocytes is the major basis for pigmentation of hair and wool in mammals [3]. There are two types of melanin: eumelanin is found in wool (or hair and skin), and colors wool gray, black, yellow, and brown; pheomelanin imparts a pink to red hue and, thus, is found in particularly large quantities in red wool [3,4]. Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid polymers. However,

Figure 1. Chemical structure of pheomelanin (A) and eumelanin (B). *Corresponding author: [email protected] 2297

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Fibers and Polymers 2014, Vol.15, No.11

Sayed Majid Mortazavi et al.

Figure 2. Principles of pigmented fiber bleaching; (a) pigmented fiber, (b) mordanted fiber, (c) mordanted/rinsed fiber, and (d) bleached fiber.

H2O2. However, as far as the author is aware, there is not a comprehensive study on copper (II) ions as a mordant agent in the bleaching process of pigmented wool. The use of copper ion as a bleaching catalyst was developed some advantages compared with the iron mordanting method [2]: i. No reducing agents during mordanting are needed. ii. Air need not be excluded, and therefore there is no limitation in the choice of bleaching machinery. iii. Un-pigmented wool is not discolored. iv. Treatment times are much shorter. This study evaluates the catalytic effect of copper sulfate (CuSO4) in the bleaching process of pigmented wool fibers under the used condition. Investigation is concerning the effects of copper sulfate and tetra sodium pyrophosphate (Na4P2O7; as a stabilizer) concentration, bleaching and rinsing time after mordanting on yellowness degree, surface morphology, optical and mechanical properties. Moreover, the optimum conditions for each step is reported.

Experimental Materials Naturally pigmented black karakul wool fiber (60±22 µm fineness and 80-170 mm length) was used. The properties of this fiber are summarized in Table 1. Table 1. Yellowness index (Y.I.), CIE Lab indices, and strength of pigmented wool fiber CIELab a b L 16.268 1.119 0.267

Fiber

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Karakul wool

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Y.I

Strength (cN)

22.66

6.98

The hydrogen peroxide (H2O2) used was a 30 % (w/w) aqueous solution. The non-ionic detergent (Ultravon GP, Ciba) and Na2CO3 (Merck Co., Germany) was used for scouring process of wool fiber. Copper sulfate (CuSO4) in reagent grade was purchased from Merck Co. Tetra sodium pyrophosphate (Na4P2O7) was obtained from Aldrich Chemicals Company. All other chemicals used were supplied by Merck Co., Germany. Pigment Bleaching of Wool Fibers Scouring Process Samples of karakul wool fiber were scoured in three steps before using for the bleaching process experience. Firstly, raw wool fibers were rinsed with warm water, and then treated in a bath containing 0.1 %w/v Na2CO3 and 0.3 %w/v non-ionic detergent (Ultravon GP, Ciba) at 40 oC for 15 min. Finally, these fibers were rinsed with cold water. Depigmentation Process The depigmentation process consisted of mordanting step followed by a bleaching step and was carried out at the experimental conditions (shown in Table 2). The sample was depigmented (mordanting, rinsing, bleaching, and rinsing) at a fiber liquor ratio of 1:40. Different concentrations of Na4P2O7 (2.5-10 %w/v) (as a stabilizer in bleaching bath) and CuSO4 (0.4-1.2 %w/v) were tested. The H2O2 concentration of 60 ml/l was used. Also, the effect of bleaching and rinsing time after mordanting treatment were studied. After the bleaching step, each sample was rinsed with pure water, centrifuged and dried at room temperature. Evaluation of Bleaching Effects Color Measurement Bleached product quality can be defined by its whiteness

Table 2. Experimental conditions of depigmentation process for different fiber samples Mordanting CuSO : 0.4-1.2 %w/v L.R.: 1:40 pH (HCO H): 3.5 Temperature: 80 C Time: 15 min 4

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Rinsing Cold water Neutralization with NH OH pH: 11 Temperature: 45 C Time: 15-75 min 4

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Bleaching Na P O : 2.5-10 %w/v H O 30 %: 60 ml/l L.R.: 1:40 pH: 8.5 Temperature: 50 C Time: 15-180 min 4

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Rinsing Pure water Temperature: 50 C Time: 5 min o

Bleaching Karakul Wool Using Copper Sulfate as Catalyst


or yellowness. Yellowness index (Y.I) measurements were performed on a Datacolor Texflash, a reflectance spectrophotometer. The color of bleached pigmented wool fibers was calculated on the basis of CIELAB color value. L* is the position on the axis dark/light (0 for black and 100 for white), a* is the position on the axis red (+a*)/ green (−a*), and b* is the position on the axis yellow (+b*)/ blue (−b*). Mechanical Properties of Fibers The tensile properties of wool fibers, before and after bleaching treatment, was determined by Zwick universal testing machine (Model 1446-60, Germany) according to ASTM D3822-01 (at an elongation rate of 20 mm/min and a length of 25 mm). For mechanical properties, 300 measures were realized for each sample. Alkaline Solubility The extent of fiber damage caused by bleaching was determined by the alkaline solubility test. Alkaline solubility is the weight loss of wool yarns after treatment with alkali solution and was determined using the following equation: Alkaline solubility % = (----------------------------------------------------------------------------------Initial weight – Secondary weight )× 100 Initial weight

(1)

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The wool fibers (1 g) were first dried in an oven at 110 oC for 1 h. After cooling for 10 min, the sample was weighed again and dipped in 100 ml of sodium hydroxide solution (0.1 N) at 65 oC for 1 h. Subsequently, the wool fibers were filtered and rinsed six times with distilled water. Then, the sample was neutralized with acetic acid (1 %) and rinsed again with distilled water for several. Finally, the fibers were dried at 110 oC for 1 h, cooled and re-weighed [18]. Weight Loss The weight loss percentage (wi) of bleached fibers was determined using following equation [11]: wpre – wafter - × 100 wi = -----------------------wpre

(2)

where wpre is the mass of fibers before treated (at 20±2 oC and 65±2 RH) and wafter is the mass of fiber after bleaching treatment. Surface Characterization The surface morphology of wool fibers, before and after bleaching, was observed using optical microscopy (Nikon, CSM, Japan) and scanning electron microscopy (SEM; model VEGA-TESCAN-LMU). Moreover, elemental analysis was conducted with EDAX spectra. A sputter coater was

Figure 3. Effect of copper sulfate concentration on; (a) yellowness index, (b) weight loss, (c) strength, and (d) alkaline solubility of bleached wool fibers (treated with 60 ml/l H O and 7 %w/v Na P O for 30 min). 2

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used to pre-coat conductive gold onto the surface before measuring the micro structures.

Results and Discussion Effect of Mordant Concentration In this work, copper sulfate (CuSO4) is used as a mordant and its catalytic effect under the used conditions was investigated. To determine the effect of mordant concentration on bleaching of black pigmented wool fibers, a series of trials were performed using five different CuSO4 concentrations (0.4, 0.6, 0.8, 1.0, and 1.2 %w/v). The yellowness index, weight loss percentage, strength, and alkaline solubility results of bleached wool fibers for different mordant concentrations are shown in Figure 3. The results of tensile test of 300 fiber specimen are illustrated in graph with error bar. When the results of the bleaching trials are compared (Figure 3), it is clear that bleached wool mordanted with higher CuSO4 concentration has a lower yellowness index, so that the minimum yellowness index (55.85) was obtained in the mordant concentration of 1.2 %w/v. In fact, the increasing of Cu (II) concentration caused more decomposition of H2O2 and led to an increase in the reaction of the oxidative agent with wool fibers. However, this caused the breaking of chemical bonds in fiber structure and reduction in fiber strength. This is reflected in weight loss percentage, tensile strength, and alkaline solubility results of bleached wool fibers (Figure 3(b)-(d)). The results show that the tensile strength decreased (from 4.68 to 4.44 cN) and the weight loss, increased (from 14 to 20 %) with CuSO4 concentration, respectively. The Figure 3(d) shows that the alkali solubility of treated fibers was higher than 78 %, whereas this parameter was 8.98 % for untreated wool. It is

Figure 4. Effect of copper sulfate concentration on optical properties of bleached wool fibers for 30 min with 60 ml/l H O and 7 %w/v Na P O . 2

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Figure 5. Effect of copper sulfate concentration (%w/v) on a -b of bleached wool fibers.

obvious that the alkali solubility increased with copper concentration. According to the results obtained from Figure 3, an excellent depigmentation with minimum fiber damage is provided by using CuSO4 concentration equal to 1 %w/v. The effect of the CuSO4 concentration on optical properties of the samples is shown in Figure 4. The curves show an increase and decrease of the lightness (L*) and redness (a*) of bleached fibers, respectively, with raising the CuSO4 concentration, so that the maximum value of L* was obtained at 1.2 %w/v CuSO4. The reason for this observation could be explained by the fact that copper can catalytically degrade H2O2 and cause better bleaching to produce larger L* and smaller a* value. The L*, a*, and b* values also increased in comparison with the raw sample. As shown in Figure 5, all bleached mordanted samples were in red-yellow quadrant of CIELab color space. The color indices revealed that the black wool fibers have turned into a pale light brown shade after bleaching process. Effect of Tetra Sodium Pyrophosphate Concentration Tetra sodium pyrophosphate (Na4P2O7) acts as a stabilizer in bleaching bath and enhances the stability of the bleaching samples and inhibits the breakdown of perhydroxy ion radicals. To investigate the effect of Na4P2O7 concentration on the bleaching efficiency of the black wool fibers, six different Na4P2O7 concentrations (2.5, 4.0, 5.5, 7.0, 8.5, and 10 %w/v) was used. The yellowness index, weight loss percentage, alkaline solubility and strength results of bleached wool fibers for different Na4P2O7 concentrations are shown in Figure 6. The curves show minimum yellowness index (50.15) was obtained at 5.5 %w/v Na4P2O7. As shown in Figure 6, the



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Figure 6. Effect of tetra sodium pyrophosphate concentration on; (a) yellowness index, (b) weight loss percentage, (c) strength, and (d) alkaline solubility of wool fibers (mordanted with 1 %w/v CuSO and bleached with 60 ml/l H O for 30 min). 4

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Na4P2O7 concentration is 7 %w/v in testing conditions. The optical properties of bleached fibers presented in Figure 7, also revealed that a slight increase and decrease of lightness and a*-b* of wool fibers, respectively, at 5.5 %w/v Na4P2O7. However, the maximum whiteness degree which can be obtained by H2O2 bleaching is not considered optimal unless provided a slightly lower damage incurred by wool fibers.

Figure 7. Effect of tetra sodium pyrophosphate concentration on optical properties of bleached wool fibers (mordanted with 1 %w/v CuSO and bleached with 60 ml/l H O ). 4

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increasing of Na4P2O7 concentration from 5.5 to 7 %w/v provides maximum strength (4.62 cN) with minimum weight loss (16 %) and alkaline solubility (79.12 %). So, the optimum

Effect of Bleaching Time To study the effect of different bleaching times on the bleaching efficiency of karakul wool fibers, seven different bleaching times (15, 30, 45, 60, 90, 120, and 180 min) were applied. The yellowness index, weight loss percentage, alkaline solubility and strength results of bleached wool fibers for different bleaching times are shown in Figure 8. It can be seen from the figure that a minimum yellowness index (44.65) was obtained at bleaching time equal to 120 min. However, the use of high bleaching times should be avoided because they reduce the fiber strength [8,11,19]. The curves (Figure 8) show an increase and decrease evolution of weight loss percentage and strength of bleached wool fibers, respectively, according to the bleaching time.

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Figure 8. Effect of bleaching time on; (a) yellowness index, (b) weight loss percentage, (c) strength, and (d) alkaline solubility of bleached wool fibers (mordanted with 1 %w/v CuSO , bleached with 60 ml/l H O and 7 %w/v Na P O ). 4

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damage arising from H2O2 attack on amino acids in the keratin fiber due to the longer treatment in bleaching bath. Maximum strength value (4.66 cN) and minimum weight loss (14 %) and alkaline solubility (80.64 %) were observed at bleaching time equal to 15 min. Moreover, after 15 min, the yellowness index of beached fiber (46.2) is reasonable and hence bleaching time equal to 15 min was considered as the optimum condition. The effect of bleaching time on optical properties of the samples is shown in Figure 9. As bleaching time increased, the lightness value (L*) increased and a*, and b*slightly reduced in the samples. This could be related to the removal of pigments from wool fibers. The a* and b* values also increased in comparison with the raw sample.

Figure 9. Effect of bleaching time on optical properties of bleached wool fiber at optimum conditions.

The alkaline solubility of these fibers also increased with bleaching time. These are probably explained in term of

Effect of Rinsing Time after Mordanting Process Rinsing following the mordanting step proved to be critical with regard to fiber damage [2,11]. In this experiment, five different rinsing times (15, 30, 45, 60, and 75 min) after the mordanting step was tested. The curves in Figure 10 demonstrate the influence of the rinsing process on the properties of treated wool. It can be seen from the figure that a minimum yellowness index (26.6) was obtained at rinsing



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Figure 10. Effect of rinsing time after mordanting on; (a) yellowness index, (b) weight loss percentage, (c) strength, and (d) alkaline solubility of bleached wool fiber at optimum conditions.

Figure 11. Effect of rinsing time on optical properties of bleached wool fiber at optimum conditions.

time equal to 60 min. However, the bleached fiber’s strength is significantly increased in all cases, particularly at the long rinsing time. In mordant bleaching, adsorption of an excessive amount

of copper during mordanting, and its retention after rinsing, may cause severe damage to the fiber. Rinsing following the mordanting is important to reduce the fiber damage. During rinsing, the copper ion not bound to the melanin pigments will be removed from the keratin fiber matrix. Existence of non-melanin bound copper in the fiber causes over bleaching and reduces the fiber strength. If absorbed copper is completely removed from the fiber keratin, the attack of radicals will be localized exclusively at the melanin pigment, while the fiber keratin undergoes only a simple peroxide bleaching [2,10]. Although the maximum strength value (4.53), minimum weight loss (18 %), and alkaline solubility (81.02 %) were obtained at rinsing time of 75 min, considering the yellowness index results, rinsing time of 60 min was selected as an optimum condition. The optical properties of bleached wool fiber were presented in Figure 11. As the rinsing time (after mordanting) increased, the lightness value (L*) decreased and a* and b*nearly remained unchanged in the samples. However, a slight improvement in the L* was observed at rinsing time equal to 60 min. The L* and b* values also increased in comparison with the raw sample, but a* has not changed significantly. Smaller lightness of bleached fibers after increasing of rinsing time could be related to the removal of

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Figure 12. Optical microscopic image of the wool fiber before (a) and after (b) of bleaching process.

Figure 13. SEM images of the wool fibers with two magnifications: raw pigmented wool (a: 250× and d: 1000×), mordanted fiber (b: 250× and e: 1000×), and bleached fiber (c: 250× and f: 100×).

some copper ions from melanin, in addition to fiber keratin. Microscopic Observations Figure 12 shows the longitudinal section of unbleached and bleached karakul wool fibers. It is clearly obvious that the black karakul wool fibers are white after the bleaching process. It is seen that the epicuticle remain unchanged after the bleaching and the bleaching process do not modify the surface of wool fibers. The SEM images of the surface morphology of wool

fibers are shown in Figure 13. The raw pigmented wool has some unknown material deposited on the cuticle surface (Figure 13(a)). As shown in Figure 13(b), the surface of the mordanted wool fibers was covered with copper particles. However, the bleached fibers (Figure 13(c)) have a much smoother surface than the mordanted one, so it appears lustrous. In the latter case, the scale structure remained almost intact without noticeable damage. The EDAX spectra obtained for the surface of samples shown in Figure 13, where indicated the characteristic peaks



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Figure 14. EDAX spectra of the wool fibers; (a) raw pigmented wool, (b) mordanted fiber, and (c) bleached fiber.

for different element in the functional layer of wool fibers (Figure 14). As expected, by mordanting of karakul wool fibers, more copper particles have been deposited on the surface of the fibers (Figure 14(b)). The EDAX analyses revealed that the surface of wool fibers contained significant concentrations of C, O, N, and much less S or Ca.

Conclusion In this study, the catalytic effects of copper sulfate in the bleaching process of pigmented karakul wool fibers were evaluated under the used condition. Investigation is concerning the effects of CuSO4 and Na4P2O7 (as a stabilizer) concentration, bleaching and rinsing time after mordanting on

yellowness degree, optical and mechanical properties. The results showed that an excellent depigmentation with minimum fiber damage is provided by using CuSO4 concentration equal to 1 %w/v. In mordant bleaching, adsorption of an excessive amount of copper during mordanting, and its retention after rinsing, may cause severe damage to the fiber. Therefore, rinsing following the mordanting is important to reduce the fiber damage. In this study, the rinsing time equal to 60 min was selected as an optimum condition. Optimum results for bleaching conditions were obtained when bleaching time was 15 min, H2O2 concentration was 60 ml/l, and using the 7 %w/v Na4P2O7 in bleaching bath. The advantages of using Cu (II) mordant (compare to iron (II) ions) are no need the use of any reducing agent in

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mordanting bath and lower limitations for the selected bleaching machine, due to the lack of sensitivity of copper mordant to oxygen of air during the bleaching process. In all samples, L*, a*, and b* increased after bleaching in comparing to raw samples; however, increasing copper sulfate concentration and bleaching time led to increasing L* while decreasing the values of a* and b*. As the rinsing time (after mordanting) increased, the lightness value (L*) decreased and the values of a* and b*remained unchanged. The color indices revealed that the black wool fibers have turned into a pale light brown shade. The microscopic images (SEM and optical) of fibers showed that the bleaching process led to an increase in fiber whiteness and did not cause an apparent change in the surface morphology of karakul wool fibers.

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