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Establishing a rapid pad-steam process for bleaching of cotton fabric with an activated peroxide system Minghua Peng, Shouying Wu, Jinmei Du, Chang Sun, Change Zhou, Changhai Xu, and Xiaolin Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00912 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018
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Establishing a rapid pad-steam process for bleaching of cotton fabric with an activated peroxide system Minghua Peng1‡, Shouying Wu2‡, Jinmei Du1, Chang Sun1, Change Zhou1, Changhai Xu1*, Xiaolin Hu2
1
Key Laboratory of Eco-textiles, Ministry of Education, College of Textiles and Clothing, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, China
2
School of Textile and Clothing, Nantong University, 9 Seyuan Road, Nantong, Jiangsu 226019, China
*Corresponding Author:
[email protected].
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ABSTRACT: Bleaching of cotton fabric with hydrogen peroxide (H2O2) is conventionally carried out under high temperature and strongly alkaline conditions, for which the pad-steam process can be employed to present the benefits in lowering water and chemical consumptions. A notable drawback observed with the pad-steam process for bleaching of cotton fabric with H2O2 is that although cotton fabric is bleached in an acceptable whiteness, it requires to dwell in steam for almost an hour, causing tremendous energy consumption as well as extensive damage to cotton fibers. It has been found that an activated peroxide system can be formed with the addition of N-[4-(triethylammoniomethyl)benzoyl]caprolactam chloride (TBCC)
to H2O2
solution to generate 4-(triethylammoniomethyl)perbenzoic acid (TPA), allowing to establish a rapid pad-steam process. Sodium citrate was selected as the desired alkali for use in the rapid pad-steam process to neutralize 4-(triethylammoniomethyl)benzoic acid (TBA) released from the TBCC-activated peroxide system. By shortening the steaming time to 4 minutes or less, the rapid pad-steam process afforded enormous advantages over the conventional pad-steam process in saving energy and increasing production efficiency, and also the bleached cotton fabric underwent no apparent chemical damage and met the requirements for post dyeing.
KEYWORDS: cotton, pad-steam process, bleaching, activated peroxide system, degree of whiteness
INTRODUCTION Cotton is an important fiber resource for garment, taking almost 50% share in total fiber
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consumption. Proper preparation of cotton fabric is required for production of an acceptable dyed and finished cotton-based garment because raw cotton contains natural impurities such as protein, pectin, fats, waxes, colorants, minerals, etc..1,2 However, cotton preparation methods that are conventionally developed on the basis of energy, water and chemical consumptions are facing tremendous challenges of sustainable development. Bleaching is a key step in the preparation of cotton fabric, which serves to decolorize the natural colorants present in cotton fibers and ensures a white substrate for post dyeing and finishing. Hydrogen peroxide (H2O2) is most commonly used in the industrial bleaching of cotton fabric. Chemically, H2O2 plays the role of bleaching by releasing perhydroxyl anions (HOO-) which would be the active species to destroy the natural colorants in cotton.3 However, H2O2 hardly dissociates in water because of its extremely low dissociation constant (Kd = 1.78 × 10−12, 20 ºC). The most common approach to increasing the amount of HOO- in the bleaching bath is to add a strong alkali which can effectively promote the H2O2 dissociation (Scheme 1).4 In practice, cotton fabric can be bleached in an alkaline H2O2 solution by the hot exhaust process which is often operated at a temperature higher than 95 ºC for a period of at least 30 minutes by using a liquor-to-goods ratio of greater than 10:1. Hence, tremendous amounts of energy are utilized in the hot exhaust process to heat and maintain the bulk solution at such a high temperature, and also the utilization rates of chemicals are relatively low due to invalid decomposition of H2O2. Alternatively, the cold pad-batch and pad-steam processes are utilized for bleaching of cotton fabric with H2O2, partly overcoming the drawbacks of the hot exhaust
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process. Bleaching with the cold pad-batch process is carried out by padding cotton fabric in an impregnating solution for a wet pickup of no more than 100% and dwelling it at room temperature for nearly 24 hours, presenting great advantages over the hot exhaust process in saving energy, water and chemicals. In the cold pad-batch process, the long dwell time of cotton fabric significantly limits the production efficiency. Instead of dwelling at the room temperature, cotton fabric is steamed in the pad-steam process for no more than an hour. Although the padsteam process has a higher production efficiency than the cold pad-batch process, the energy consumption in the pad-steam process remains unsolved.
Scheme 1. Dissociation of hydrogen peroxide in alkaline bleaching bath.
Much effort has been made to develop an approach towards sustainable process for bleaching of cotton fabric. Catalytic bleaching, which employs transition metal complexes as bleach catalysts to accelerate the conversion of H2O2 into active oxygen species,5,6 has been considered as a promising method for exhaust bleaching of cotton fabric at low temperatures.7,8 The catalytic bleaching can significantly cut down the energy consumption by lowering the temperature of the hot exhaust process. However, the low utilization rates of chemicals such as H2O2 and alkali are inextricable in the exhaust process. Enzymatic bleaching has been also suggested as a sustainable method for bleaching of cotton fabric, which can be achieved by two basic strategies depending on the enzymes used. One strategy is to use glucose oxidase which
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catalyzes the oxidation of glucose to simultaneously generate H2O2.9 However, the enzymatically generated H2O2 must be activated by a strong alkali or other auxiliaries for effective bleaching of cotton fabric.10,11 The other strategy is to use an arylesterase which catalyzes the perhydrolysis of propylene glycol diacetate to simultaneously generate peracetic acid.12 It has been confirmed that peracetic acid is a powerful bleaching agent which can establish an exhaust process for bleaching of cotton fabric at low temperatures (e.g. 60 - 70 ºC).13 The enzymatic bleaching with arylesterase has seen industrial interest, but to date has not been widely adopted. Bleaching of cotton fabric with an activated peroxide system has been attracting more attention from the textile industry due to its advantages of reducing energy and water consumptions. By the reaction of a so-called bleach activator (BA) with H2O2 (Scheme 2), the activated peroxide system can be established to produce a peracid (PA) which allows cotton fabric to be bleached under milder conditions.14 The bleaching performance of the activated peroxide system depends greatly on the bleach activator that could be one of the organic compound such as tetraacetylethylenediamine (TAED),15 sodium nonanoyloxybenzene sulfonate (NOBS),16 N-[4(triethylammoniomethyl)benzoyl]lactam chlorides (TBLCs),17 guanidine derivatives,18 and cyanamide.19 Among these reported bleach activators, TBLCs exhibit a strong competitive advantage over others because the activated peroxide systems they establish (Scheme 3) can be applied for exhaust bleaching of cotton fabric under near-neutral pH conditions at a much lower temperature (e.g. 50 ºC).20,21 Due to their good water solubility and low-temperature effect, TBLCs have ever been
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considered for use in a cold pad-batch process for bleaching of cotton fabric for the purpose to shorten the dwell time to no more than 10 hours.22 However, it is found that an increased concentration of TBLC in the impregnating solution, which is necessarily required for the cold pad-batch process, causes a massive reduction in the bleaching performance. It is hypothesized that the interaction between the active bleaching species and cotton fibers should be greatly inhibited in such a high concentration of TBLC, especially at room temperature. In our preliminary exploration, an attempt was made to design a pad-steam process for bleaching of cotton fabric with the TBLC-activated peroxide system. With an acceptable bleaching performance, the steaming time of cotton fabric in the pad-steam process was surprisingly shortened to 4 minutes or less. The purpose of this work is to present the rapid pad-steam process as a promising approach for sustainable bleaching of cotton fabric.
Scheme 2. Generation of peracid by the reaction of bleach activator with H2O2.
Scheme 3. TBLC-activated peroxide system for bleaching of cotton fabric (n = 1 – 5) in which TPA
refers
to
4-(triethylammoniomethyl)perbenzoic
acid
and
TBA
refers
to
4-
(triethylammoniomethyl)benzoic acid.
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EXPERIMENTAL SECTION Materials. The desized and scoured woven cotton fabric (100%) was obtained from Hongdou Group Textiles Co., Ltd., China. N-[4-(triethylammoniomethyl)benzoyl]caprolactam chloride (TBCC) was used as a prototype of TBLC (n = 3), and synthesized according to the previously reported method.25 H2O2 (30%, w/w), sodium citrate, sodium bicarbonate, sodium hydroxide, sodium chloride, sodium carbonate and soap flakes were all obtained from Sinopharm Chemical Reagent Co., Ltd., China. A polyacrylate-based peroxide stabilizer (DM-1403) and a nonionic wetting agent (Penetrant JFC) were obtained from Dynamic chemicals, Inc., China. The Megafix B trichromatic reactive dyes (RML, YML, BML) were obtained from Matex Chemicals Co., Ltd., China. Cupriethylenediamine (1.0 M) was obtained from Sigma-Aldrich, USA. Methods. Bleaching. The impregnating solution was prepared with the addition of a 1:1.2:1.4 molar ratio of TBCC, H2O2 and an alkali (sodium citrate, sodium bicarbonate, sodium carbonate or sodium hydroxide) , in which the excessive amounts of H2O2 and alkali were used to drive the reaction to completion. Peroxide stabilizer (3 g/L) and wetting agent (3 g/L) were also added to the impregnating solution as auxiliaries to ensure an optimal bleaching performance. A sample of cotton fabric (around 10 g) was impregnated with the prepared solution and padded for a wet pick up of 100%. The impregnated cotton fabric was steamed in a steamer (102 ±2 ºC) for a period of time. When the pad-steam process was completed, the bleached cotton fabric was rinsed adequately with fresh water and dried under ambient conditions. For comparison purpose, cotton fabric was bleached with a conventional pad-steam bleaching
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recipe.24 The impregnating solution was prepared with the addition of H2O2 (30%, w/w) and sodium hydroxide in a mass ratio of 2.1:1. Peroxide stabilizer (3 g/L) and wetting agent (3 g/L) were also added to the impregnating solution as auxiliaries to ensure an optimal bleaching performance. The pad-steam process was operated same as the above except that impregnated cotton fabric was steamed for an extended period of time (e.g. 30 and 50 minutes). Dyeing of bleached cotton fabric. The bleached cotton fabrics were dyed into grey shades with the Megafix B trichromatic reactive dyes (RML, YML and BML) to evaluate their quality for post dyeing. Table 1 shows the amounts of dyes based on the weight of fabric (o.w.f.). A total of 10 g of cotton fabric consisting of one half bleached with the rapid pad-steam process and the other half bleached with the conventional pad-steam process was added to the dyeing bath with a liquor-to-goods ratio of 15:1. The dyeing bath was heated to 60 ºC and held for 15 minutes. With the addition of sodium chloride (40 g/L), the dyeing was continued for another 15 minutes. As the dyeing bath was heated to 90 ºC, sodium carbonate (10 g/L) was added for dye fixation at this temperature for 30 minutes. The dyed cotton fabric was soaped at 95 ºC in a soaping bath containing 2 g/L soap flakes and 2 g/L sodium carbonate, and then rinsed thoroughly with fresh water. Table 1. The dye amounts used for dyeing of bleached cotton fabrics. Grey shade
Total (o.w.f.)
RML (o.w.f.)
YML (o.w.f.)
BML (o.w.f.)
Light
0.25
0.0357
0.0952
0.1191
Medium
1.0
0.1429
0.3809
0.4762
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Dark
2.0
0.2857
0.7619
0.9524
Evaluation. The degree of whiteness of the bleached cotton fabric was measured by the CIE whiteness index (WI) in terms of the AATCC Test Method 110-2015. The color appearance and color difference (△E) of the dyed cotton fabric were measured by the CIE 1976 L*a*b* Color Space in terms of the AATCC Evaluation Procedure 6-2016. Both the whiteness and color measurements were performed on a Datacolor 650 spectrophotometer with the settings of the CIE 1964 Standard Observer and the CIE Illuminant D65. The degree of polymerization (DP) was used as an indicator of chemical damage that cotton fabric underwent during the bleaching process, and is determined by eqn.1, (1) where F is the fluidity of dispersion of cellulose from the bleached cotton fabric in a cupriethylenediamine solution that is prepared and measured according to the AATCC Test Method 82-2016. A lower DP value indicates a more serious chemical damage to cotton fabric. RESULTS AND DISCUSSION As demonstrated in Scheme 3, during the bleaching process of cotton fabric with the TBCCactivated
peroxide
system,
TBCC
is
initially
converted
into
4-
(triethylammoniomethyl)perbenzoic acid (TPA) which is responsible for bleaching of cotton fabric and converted into 4-(triethylammoniomethyl)benzoic acid (TBA) as bleaching proceeds. Therefore, it is necessary to add an alkali to neutralize the produced TBA so as to maintain a desired pH at which the bleaching performance is optimized. It has been found that the TBCC-
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activated peroxide system prefers a near-neutral pH for exhaust bleaching of cotton fabric at a low temperature (e.g. 50 ºC), for which sodium bicarbonate is desired.20 However, sodium carbonate readily decomposes under steaming conditions, which may cause significant impact on the bleaching performance of the pad-steam process. Hence, an initial investigation was carried out to examine the performance of various alkalis for use in the pad-steam process. As shown in Figure 1, when sodium bicarbonate, sodium carbonate, sodium hydroxide and sodium citrate were respectively used in the pad-steam process, the cotton fabric was bleached to various degrees of whiteness. Sodium citrate exhibited the best performance for the optimal bleaching performance. As can be expected, sodium citrate is converted to disodium citrate by neutralizing TBA, and during the process of bleaching sodium citrate and disodium citrate form a conjugate base-acid pair which conducts a buffer system to maintain a pH range of 6 – 8 in which the TBCC-activated peroxide system is most effective.20 However, sodium carbonate and sodium hydroxide are strong alkalis that can catalyze the bimolecular decomposition of TPA,25 consequently resulting in a severe reduction in the bleaching performance. Although sodium bicarbonate is a weak alkali capable of maintaining near-neutral pH for the TBCC-activated peroxide system at low temperatures, its decomposition at the steaming temperature (102 ±2 ºC) produces sodium carbonate which conversely causes an abrupt rise of pH so as to reduce the bleaching performance.
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Figure 1. Impact of alkalis in the impregnating solution on the degree of whiteness of cotton fabric by steaming for 6 minutes. The concentrations of TBCC, H2O2 and alkali used in the impregnating solution are determined by using a molar ratio of 1:1.2:1.4 on the basis of 50 g/L TBCC.
Cotton fabric was impregnated with TBCC, H2O2 and sodium citrate at a molar ratio of 1:1.2:1.4 by padding to a wet pickup (e.g. 100%). Hence, the concentration of TBCC in the impregnating solution is of great importance because it determines the amount of active species produced for bleaching of cotton fabric in the pad-steam process. Figure 2 shows the impact of TBCC in the impregnating solution on the degree of whiteness of cotton fabric. As can be seen, the addition of TBCC to the solution of H2O2 and sodium citrate could significantly enhance the degree of whiteness of cotton fabric. As the concentration of TBCC in the impregnating solution increases, the degree of whiteness of cotton fabric is effectively improved and achieves an almost constant value with a concentration of TBCC of greater than 10 g/L. Compared to the
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reported exhaust process that was optimized to bleach cotton fabric in a degree of whiteness of 78.94 by using a concentration of TBCC of 9.2 g/L at a liquor-to-goods ratio of 15:1,21 the padsteam process provided cotton fabric with an equivalent degree of whiteness (78.85) but had great advantages in saving water and chemicals because it used the same concentration of TBCC in the impregnating solution at a small wet pick up of 100% that equals to a liquor-to-goods ratio of 1:1 used in an exhaust process.
Figure 2. Impact of concentration of TBCC in the impregnating solution on the degree of whiteness of cotton fabric by steaming for 6 minutes. The concentrations of TBCC, H2O2 and sodium citrate used in the impregnating solution are determined by using a molar ratio of 1:1.2:1.4.
In a conventional pad-steam process, cotton fabric is generally steamed for a period of nearly an hour.24 The teaming time can be significantly shortened by using the TBCC-activated peroxide system. As shown in Figure 3, cotton fabric was bleached to an optimal degree of
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whiteness by steaming for 2 minutes. Apparently, a rapid pad-steam process was actualized for bleaching of cotton fabric by using the TBCC-activated peroxide system.
Figure 3. Impact of steaming time on the degree of whiteness of cotton fabric. The concentrations of TBCC, H2O2 and sodium citrate used in the impregnating solution are determined by using a molar ratio of 1:1.2:1.4 on the basis of 25 g/L TBCC.
Table 2 shows a comparison of the rapid and conventional pad-steam processes for bleaching of cotton fabric. As can be seen, by using the same concentration of H2O2 (i.e. 8.5 g/L), the rapid pad-steam process provided cotton fabric with a degree of whiteness much higher than the conventional pad-steam process, but spent only 4 minutes or less on steaming of cotton fabric. The rapid pad-steam process that was established with 8.5 g/L of H2O2 (30%, w/w) for a 4minute steaming period could afford the same degree of whiteness as the conventional pad-steam process that was established with 30 g/L of H2O2 (30%, w/w) for a 50-minute steaming period. The extensively shortened steaming time suggests that the rapid pad-steam process could bring enormous potentials for saving energy and improving production efficiency. In addition, the
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degree of polymerization of cotton fabric bleached with the rapid pad-steam process was found to be almost same as that of the unbleached cotton fabric but much greater than that of the cotton fabric bleached with the conventional pad-steam process. This indicates that the rapid pad-steam process causes no apparent chemical damage to cotton fabric. Table 2. Performance comparison of the rapid and conventional pad-steam process for bleaching of cotton fabric. Impregnating solution Pad-steam
Control
TBCC (g/L)
H2O2, 30% w/w (g/L)
Sodium hydroxide (g/L)
Sodium citrate (g/L)
Steaming time (min)
CIE WI
DP
N/A
N/A
N/A
N/A
N/A
36.30
4212
5.0
1.84
N/A
4.9
2
71.79
4207
5.0
1.84
N/A
4.9
4
73.24
4181
22.9
8.5
N/A
22.6
2
75.13
4193
22.9
8.5
N/A
22.6
4
78.65
4165
N/A
8.5
4
N/A
30
60.84
3741
N/A
8.5
4
N/A
50
63.56
3629
N/A
30
14
N/A
30
74.28
2982
N/A
30
14
N/A
50
78.93
2797
Rapid
Conventional
The two cotton fabrics bleached with rapid and conventional pad-steam processes were dyed in a gray shade in one bath, of which the color difference (△E) was used as a sensitive indicator to test whether the cotton fabric met the requirement for post dyeing. As shown in Table 3, all the color differences of the three pairs of cotton fabrics dyed in gray shades were found to be below
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a tolerance of 1, which are technically insignificant. Therefore, the rapid pad-steam process can be applied for bleaching of cotton fabric with good quality for post dyeing. Table 3. Color differences of the dyed cotton fabrics. Pad-steam process
Gray shade
Rapid
L*
a*
b*
71.79
1.69
5.21
71.58
1.74
5.14
54.95
1.26
4.31
54.39
1.49
4.19
46.35
1.49
3.77
46.57
1.38
3.86
Light Conventional Rapid
0.23
Medium Conventional Rapid
0.62
Dark Conventional
△E
0.26
CONCLUSIONS A rapid pad-steam process was established for bleaching of cotton fabric with the TBCCactivated peroxide system by using a 1:1.2:1.4 molar ratio of TBCC, H2O2 and an alkali. Sodium citrate was selected as the desired alkali for use in the rapid pad-steam process. Compared to the low-temperature exhaust process, the rapid pad-steam process afforded cotton fabric with an equivalent degree of whiteness but had great advantages in saving water and chemicals. By shortening the steaming time to 4 minutes or less, the rapid pad-steam process brought enormous potentials for saving energy and improving production efficiency in bleaching of cotton fabric. The cotton fabric bleached with the rapid pad-steam process underwent no apparent chemical damage and met the requirements for post dyeing. Hence, the rapid pad-steam process established with the TBCC-activated peroxide system provided an alternative approach to
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sustainable bleaching of cotton fabric in the textile industry. ASSOCIATED CONTENT There is no supporting information. AUTHOR INFORMATION Corresponding Author. E-mail:
[email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was partially supported by the National First-class Discipline Program of Light Industry Technology and Engineering under Grant LITE2018-21, the National Natural Science Foundation of China under Grant 21276106, and the Science and Technology Support Program of Jiangsu Province under Grant BE201596.
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Rapid pad-steam process based on the TBCC-activated peroxide system affords tremendous energy and water saving benefits.
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