A Novel Low Temperature Approach for Simultaneous Scouring and

May 17, 2014 - Department of Textiles, Merchandising and Interiors, The University of Georgia, Athens, Georgia 30602, United States. Ind. Eng. Chem...
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A Novel Low Temperature Approach for Simultaneous Scouring and Bleaching of Knitted Cotton Fabric at 60 °C Shenxi Wang,† Shiqi Li,† Quan Zhu,*,‡ and Charles Q. Yang§ †

Dymatic Chemicals, Inc., Shunde, Foshan, Guangdong 528305, China College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China § Department of Textiles, Merchandising and Interiors, The University of Georgia, Athens, Georgia 30602, United States ‡

ABSTRACT: Exceedingly high temperatures (normally ∼98 °C) are used to perform hydrogen peroxide (H2O2) bleaching of cotton fabrics in textile industrial practice. Such harsh conditions lead to high energy consumptions and high fabric mass loss. In recent years, the industry and academic communities have conducted extensive research to reduce the temperature for industrial cotton bleaching. In this research, we developed a new H2O2 activator based on amino nitriles. All of the data demonstrated that in the presence of the new H2O2 activator, the combined scouring/bleaching of the knitted cotton fabric could be performed at 60 °C. The cotton knit fabric treated using the low temperature procedure also has lower fabric weight loss; some of the hydrophobic substances were retained on the fiber surface after the process. We found that the residual hydrophobic substances had little effect on the whiteness of the treated fabrics after application of optical brighteners, and had little effect on the shadedepth and colorfastness to washing and rubbing of the treated fabrics after dyeing. The higher was the weight retention, the more value of the treated cotton knitted fabrics was increased. The removal of less substance from cotton also resulted in lower chemical oxygen demand in the wastewater, thus providing additional environmental benefits. Moreover, the low temperature scouring/bleaching procedure had significantly lower energy consumption than the traditional procedure. The possibility and effectiveness of this new technology has been confirmed in our industrial scale trials. The disadvantage of this method was that it required higher quantity of chemicals for the treatment.

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bleaching at 60 °C was also achieved in a two-step desizing/ scouring and bleaching process when N-[4-(triethylammoniomethyl)-benzoyl] caprolactam chloride was used as a cationic activator for H2O2 bleaching of cotton.7 Five cationic activators containing lactam-based leaving groups of varying sizes were reported for achieving H2O2 bleaching temperature of cotton at 70 °C.8 New cationic bleaching activators, exemplified by N-[4(triethylammoniomethyl)benzoyl] butyrolactam chloride, were also reported being used to remove the yellowish colors on regenerated cellulose fibers.9 It was reported that peracetic acid formed by the reactions of H2O2 and acetic acid in the presence of concentrated sulfuric acid could reduce cotton bleaching temperature to 70 °C.10 Precationized cotton using 3-cholor-2hydroxy propyltrimethylammonium chloride was bleached with H2O2 at low temperature, and the cationic site on cotton functioned as both a built-in catalyst and a powerful alkali site for activation of H2O2 bleaching.11 Enzymes were used in the bleaching and scouring of cotton. Hydrogen peroxidegenerating enzyme covalently bound to alumina was used for cotton bleaching.12 Cellulase and pectinase were used in combination with H2O2, TAED, and sodium perborate in simultaneous beaching and scouring of cotton.13 Dicyandiamine was reported as a peroxide bleach activator to shorten the bleaching time of kraft paper at 80−90 °C.14

INTRODUCTION In cotton wet preparation, bleaching is generally required to remove colored substances using hydrogen peroxide (H2O2) as the bleaching agent so that the treated cotton fabrics can have a desirable white appearance.1 During a bleaching process, those colored substances on cotton are either chemically converted to water-soluble molecules to be removed, or their conjugated chromophores are destroyed by oxidation. The chemical reactions, catalysts, and cellulose degradation associated with bleaching processes were thoroughly reviewed previously.2−4 In industrial practice, cotton bleaching requires both exceedingly high temperatures (normally ∼98 °C) and high alkalinity to achieve satisfactory performance for cotton fabrics. Such harsh conditions lead to high energy consumptions, high fabric weight loss, high chemical oxygen demand (COD) discharge, and large quantity of wastewater generated by beaching as well as subsequent rinsing processes. Therefore, it becomes a critical task for the industry to develop new cotton bleaching processes for reduction of the temperature and emissions. Extensive efforts have been made to develop new H2O2 activator to reduce the bleaching temperature and alkalinity in recent years. Tetraacetylethylenediamine (TAED) was the most commonly used activator for H2O2 to reduce the temperature for industrial bleaching of cotton. TAED-based activator systems were reported to achieve the optimum performance under near neutral treatment conditions.5 When cotton was modified with a triazine derivative containing multicationic groups, the bleaching temperature for the H2O2/ TAED system could be lowered to 60 °C.6 Low temperature © 2014 American Chemical Society

Received: Revised: Accepted: Published: 9985

January 6, 2014 May 4, 2014 May 17, 2014 May 17, 2014 dx.doi.org/10.1021/ie500062f | Ind. Eng. Chem. Res. 2014, 53, 9985−9991

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for 60 min, and its H2O2 concentration and chemical oxygen demand (COD) value were analyzed right before it was drained. The knitted fabric was washed at 60 °C for 10 min and was subjected to one cycle of cold water rinsing. The bath was drained, and the treated fabric was ready for dyeing or optical brightener treatment. For the traditional procedure, the initial H2O2 concentration and pH of the solution were first tested. The system was heated to 98 °C at a rate of 2 °C/min. The system was kept at 98 °C for 45 min, and then was cooled to 80 °C and drained after the solution sample was collected from the bath for testing its H2O2 concentration and COD value. The treated fabric was washed at 90 °C for 10 min, and followed by one cycle of cold water rinsing. The bath was then drained, and the fabric was ready for dyeing or optical brightener treatment. The formulas and treatment conditions of the knitted cotton fabric are summarized in Table 1.

Bleach activators are generally used in combination of hydrogen peroxide to generate peracids for the purpose to accelerate the rate of oxidative reaction and consequently to reduce the bleaching temperature.15 Under alkaline conditions, hydrogen peroxide dissociates to form perhydroxyl anion, which reacts with the activator (R−CO−L) to form a peracid as shown in Scheme 1. Peracids are considered to be kinetically Scheme 1. Formation of a Peracid as the Starting Material for Bleaching

potent bleaching chemicals and consequently are more effective bleaching agents.15 The formation of a peracid as the starting material drastically reduced the temperature for bleaching. In this research, we developed a novel H2O2 activator based on amino nitrile, which made it possible to reduce the cotton bleaching temperature to 60 °C. The reaction of hydrogen peroxide with amino nitrile generates peroxycarboximidic acid (Scheme 2), which has a molecular structure similar to that of

Table 1. Formulas and Treatment Conditions for the Low Temperature and the Traditional Scouring/Bleaching Procedures scouring/bleaching treatment method low temp procedure

Scheme 2. Mechanism of Hydroperoxide Activation by Amino Nitrile

treatment formula

peracid shown in Scheme 1. Peroxycarboximidic acid is responsible for reducing the bleaching temperature.14 We have successfully applied this new technology to knitted cotton fabrics in industrial scales. The experiment data are discussed here.

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scouring/bleaching conditions rinsing conditions

EXPERIMENTAL SECTION Materials. The cotton fabric used was a combed doublefaced knitted fabric weighing 180 g/M2 supplied by Meimin Textiles, Guangdong, China. The scouring agent was a commercial product (DM-1361) based on a nonionic poly(alkyl oxide) and was provided by Dymatic Chemicals, Guangdong, China. The H2O2 was a reagent grade chemical (30% active ingredient) supplied by Shilung Chemicals, Guangdong, China. The H2O2 stabilizer was a commercial product (DM-1404) based on organophosphorus chelates. The antistaining detergent was a commercial product (DM-1572) based on a maleic anhydride-acrylic acid copolymer; both products were supplied by Dymatic Chemicals, Guangdong, China. The optical brightener was a commercial product (DM2622) supplied by Dymatic Chemicals. Sodium hydroxide, sodium carbonate (anhydrous), and sodium sulfate (anhydrous) were reagent-grade chemicals supplied by Shilung Chemicals, Guangdong, China. The two dyestuffs (“C. I. Reactive Blue 21” and “C. I. Reactive Blue 19”) were supplied by Corrett Chemical Materials, Hebei, China. Methods. Traditional and New Scouring/Bleaching Procedures. The knitted fabric (1 kg) was loaded into a lab jet machine before the scouring/bleaching solution was added. The initial H2O2 concentration and pH of the solution were tested after the system was heated to 30 °C and then kept at the temperature for 5 min. For the new low temperature scouring/bleaching procedure, the bath was continually heated to 60 °C at a rate of 2 °C/min. It was kept at that temperature

traditional procedure

scouring agent 1.0 g/L H2O2 (30%) 15.0 g/L H2O2 activator 3.0 g/L NaOH 5.0 g/L fabric/liquor ratio 1:10 solution pH 12.1 60 °C/60 min

scouring agent 0.3 g/L

60 °C/10 min

90 °C/10 min

H2O2 (30%) 5.0 g/L the H2O2 stabilizer 1.0 g/L NaOH 2.0 g/L fabric/liquor ratio 1:10 solution pH 12.0 98 °C/45 min

Application of the Optical Brightener. 5 L of optical brightener solution (0.5% on weight of fabric abbreviated as owf) was added to the lab jet, where the bleached and scoured wet knitted fabric (dry weight 1 kg) had been loaded. Additional water was added to the system until the total bath value reached 10 L. The temperature of the system was increased to 100 °C at a rate of 2 °C/min, and the solution was kept at that temperature for 30 min. The treated fabric was finally rinsed and dried. Dyeing Procedure. The following two dyes were used in this study: (1) C. I. Reactive Blue 21 and (2) C. I. Reactive Blue 19. All of the dyeing solutions contained 4.0 g/L dyestuff, 100.0 g/ L Na2SO4, and 40.0 g/L Na2CO3. The bleached and scoured wet knitted fabric (dry weight 1 kg) was first loaded into a lab jet machine before 5 L of the dyeing solution was added. Additional water was added to the system until the total bath value reached 10 L. The concentrations of dyestuff, Na2SO4, and Na2CO3 in the dye bath were 2.0, 50.0, and 20.0 g/L, respectively. The temperature of the system was increased to 60 °C at a rate of 2 °C/min. After keeping that temperature for 60 min, the dye solution was drained, and the dyed fabric was subjected to one cycle of cold water rinse followed by one cycle of washing at 95 °C for 20 min using a 1.0 g/L antistaining detergent (DM-1572). The fabric was finally washed with cold water and dried. 9986

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Testing and Analysis Methods. Whiteness index (WI) of the cotton fabrics was measured using a spectrophotometer according to the standard Chinese testing method (GB/T 8424.2-2001, “tests for colorfastness-instrumental assessment of relative whiteness”). H2O2 concentration was measured according to the Chinese standard method (GB-1616-2003, industrial hydrogen peroxide). Burst strength of the knitted fabrics was measured according to the Chinese standard method (GB/T 14800-2010, geosynthetics-static puncture test). The capillary effect of fabrics was measured according to the standard Chinese method (FZ/T 01071-2008, capillary effect test method for textiles). The K/S and ΔE values (CIE LAB) were measured according to the Chinese standard method (GB/T 8424.1-2001, tests for colorfastness-general principle for measurement of surface color). The fabric colorfastness to washing was measured according to the Chinese standard method (GB/T 3921.3-1997, “tests for colorfastness-colorfastness to washing: Test 1”). Fabric colorfastness to rubbing (dry and wet) was determined according to the Chinese standard method (GB/T 3920-1997, test for colorfastness-colorfastness to rubbing”). The COD of the treatment solutions were measured according to the Chinese standard method (GB 11914-89, “water quality-determination of the chemical oxygen demand-dichromate method”). Percent fabric weight loss was determined by formula 1 shown below, where M1 and M2 were the weights of a fabric specimen before and after the scouring/bleaching process, respectively. Both specimens were conditioned at 20 °C and 65% relative humidity for 24 h prior to testing.

Figure 1. Fabric whiteness of the knitted cotton treated with the low temperature procedure versus H2O2 concentration.

comparison, the cotton fabric was also treated with the scouring agent and H2O2 in the absence of the activator. The WI data of that treated fabric were also included in Figure 1. It is obvious that in the presence of the new activator, the whiteness of the knitted cotton fabric subjected to the scouring/bleaching procedure at 60 °C was significantly higher than that treated without the activator (Figure 1). When the H2O2 concentration was 4.5 g/L, the cotton fabric treated without the activator had a WI of 61.4%. It became 65.3% when the activator was present. The WI of the scoured and bleached cotton was significantly higher in the presence of the activator than that treated without the activator in the entire H2O2 concentration range. It could be seen that the WI of the treated cotton fabric was increased as H2O2 concentration was increased, but it was not a linear relationship for both curves in Figure 1. The slopes of both curves decreased as the H2O2 concentration was increased. The cotton fabric was also treated with the scouring/ bleaching system when the H2O2 concentration was 4.5 g/L and the activator concentration ranged from 0.0 to 6.0 g/L. Presented in Figure 2 is the WI of the treated cotton fabric as a function of the activator concentrations. Without the presence of the activator, the WI of the treated cotton was 61.4%. The WI of the cotton fabric was drastically increased to 63.5% when 1.0 g/L activator was present. As the activator concentration was increased further to 3.0 g/L, the WI of the fabric was increased continually to 65.3%. Further rising in the activator concentration to 6.0 g/L only resulted in a marginal increase of WI of the fabric to 66.4% as shown in Figure 2. We also studied the relationship between the percentages of the H2O2 decomposed at the end of the 60 min scouring/ bleaching process at 60 °C and the activator concentrations (Figure 3). Without the activator, only 19.4% of H2O2 was decomposed. When 1.0 g/L of the activator was present, the amount of H2O2 decomposed increased to 31.3%. The percentage of H2O2 decomposed increased almost linearly with the increase of the activator concentration as shown in Figure 3. 76.4% of the H2O2 in the system was decomposed when 6.0 g/L activator was used. The WI of the treated fabric reached 65.3% when the activator concentration was 3.0 g/L, and the H2O2 decomposed was 52.7% as shown in Figures 2 and 3. When the activator concentration was increased from 3.0 to 6.0 g/L, the H2O2

percent fabric weight loss (%) = (M1 − M 2)/M1 × 100% (1)

Calculation of the Theoretical Energy Consumption for Heating a Treatment Solution. The theoretical energy consumption (kJ) for heating a scouring/bleaching solution, which contained the cotton fabric being treated, was calculated using formula 2 shown below, where ΔQ was the energy (kJ) required to heat the scouring/bleaching solution from T1 to T2 , the starting and final temperatures (°C), respectively. The solution contained 1 kg of cotton fabric and 10 kg of solution. Ccotton was the specific heat capacity of cotton (1. 2975 kJ/[kg °C]), and CH2O was the specific heat capacity of water (4.186 kJ/[kg °C]).12,13 Here, we used the specific heat capacity of water instead of the scouring/bleaching solution for the calculation, because the concentrations of the chemicals in the solution were very low; therefore, the difference between the specific heat capacity of water and that of the scouring/ bleaching solution is very small. This was a simplified method to estimate the amount energy required to raise the temperature of the scouring/bleaching system based on 1 kg of cotton.

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ΔQ = (Ccotton + 10·C H2O) ·(T2 − T1)

(2)

RESULTS AND DISCUSSION Effectiveness of the New H2O2 Activator. The knitted cotton fabric was treated with the scouring/bleaching bath containing 1.0 g/L scouring agent, 3.0 g/L H2O2 activator, H2O2 with concentration ranging from 1.5 to 8.4 g/L, and 5.0 g/L NaOH under the condition described previously. The bath was heated to 60 °C and then kept at that temperature for 60 min. The WI of this treated cotton fabric is presented against the H2O2 concentration in Figure 1. For the purpose of 9987

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bleaching and scouring system discussed here was developed for such applications. Cotton textiles with modest WI of about 60% also have the advantage of lower strength loss after bleaching. This effectiveness of the new low temperature scouring and bleaching procedure was conformed in our industrial trials. It should also be pointed out that to achieve the desirable scouring/bleaching performance at 60 °C, the quantity of chemicals required by this new method was much higher than what is used by the traditional high temperature method as shown in Table 1. It must be considered as a disadvantage for this new method. Performance of the Knitted Cotton Fabric Treated with the Low Temperature Scouring/Bleaching Procedure. For the comparison of performances, the knitted cotton fabrics were treated either by the new low temperature scouring/bleaching procedure at 60 °C or by the traditional procedure at 98 °C. Their treatment conditions, the WI, the WI with the treatments of the optical brightener, and the burst strengths of the treated cotton fabrics are shown in Table 2. For

Figure 2. New H2O2 activator concentration versus the fabric whiteness of the knitted cotton treated with the low temperature procedure.

Table 2. WI, WI with the Treatments of the Optical Brightener, and Burst Strength of the Cotton Knitted Fabric Subjected to the Two Procedures treatment method

initial H2O2 concentration (g/L) H2O2 decompositiona (%) temp (°C) WI after scouring/bleachingb (%) WI after further treatment of optical brighteners (%) burst strengthb (N)

low temp procedure

traditional procedure

4.5 53 60 65 154

1.5 74 98 68 156

522

536

a

The percentage of total H2O2 decomposed at the end of the scouring/bleaching procedures. bThe WI and burst strength of the untreated fabric are 3% and 513 N, respectively.

the new low temperature procedure, the initial concentration of H2O2 (4.5 g/L) was 3 times that for the traditional procedure (1.5 g/L). Table 2 showed that 53% of H2O2 in the bath of the low temperature procedure was decomposed at the end of the scouring/bleaching procedure, thus indicating that in the low temperature treatment bath, the total quantity of H2O2 decomposed to form oxidative species was 2.39 g/L, whereas it was 1.11 g/L in the traditional treatment bath. Because the oxidative bleaching process took place at a temperature 38 °C lower than that of the traditional bleaching process, a much higher H2O2 concentration became necessary to increase the rate of the oxidative reactions at the low temperature procedure. Table 2 also showed that the cotton fabric treated with the low temperature procedure had a WI of 65%, which is modestly lower than that of the fabric treated with the traditional procedure (68%). When an optical brightener was applied to these treated cotton fabrics, their WI could reach 154% and 156% on the fabrics previously treated with the low temperature procedure and the traditional procedure, respectively. The burst strengths of the cotton fabrics treated with the two procedures were also shown in Table 2. The data indicated that the burst strength of the fabric treated with the low temperature procedure (522 N) was only 2.6% lower than that of the fabric

Figure 3. Percentage of H2O2 decomposed versus the new H2O2 activator concentration for the low temperature procedure at 60 °C.

decomposed was increased from 52.7% to 76.4% (Figure 3), whereas the fabric WI increased only 1.1% from 65.3% to 66.4% (Figure 2). It was obvious that most of the extra H2O2 decomposed (23.7% of total H2O2 in the system) at the activator concentration of 6.0 g/L was ineffective in the bleaching process. We were still unable to explain this phenomenon and will study it further. In summary, the data presented in Figures 1−3 demonstrate that the amino nitrile derivative was effective as a H2O2 activator in reducing the scouring/bleaching temperature to 60 °C for the cotton knitted fabrics. The formula using 4.5 g/L H2O2 and 3.0 g/L activator appeared to have the optimum or near-optimum effectiveness. The WI at 65% for knitted cotton fabrics after scouring/bleaching is sufficient for many end uses, such as the knitted cotton fabrics subjected to dyeing or printing processes. It should be pointed out that bleaching with H2O2 can achieve WI as high as 85%. However, for many end uses of cotton textiles, particularly those knitted cotton fabrics to be dyed and printed, such high fabric whiteness is not necessary, and WI around 65% is adequate. This new low temperature 9988

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treated with the traditional procedure (536 N). The concentration of the H2O2 decomposed during the scouring/ bleaching procedures for the low temperature procedure (2.39 g/L) was twice higher than that for the traditional procedure (1.11 g/L). The higher concentration of the oxidative species, which were formed by the decomposition of H2O2, probably caused more cellulose degradation. Nevertheless, the burst strength of the fabric treated with the low temperature procedure was still very close to that treated with the traditional method, indicating the high consumption of H2O2 did not significantly affect the strength of the fabric treated with the low temperature procedure. We also investigated the capillary effect of the cotton fabric treated with the two different scouring/bleaching methods. The one through the low temperature procedure had a capillary effect of 4.8 cm, whereas the other treated with the traditional method had a capillary effect of 10.9 cm (Table 3). Evidently,

Table 4. Shade Depth (K/S) and Color Difference (ΔE) of the Cotton Knitted Fabric Treated with the Two Procedures dyes C. I. Reactive Blue 21 (600 nm)

a

4.8 4.13 6079

traditional

low temp

traditional

4.08 0.61

4.22 0.00a

15.02 0.97

15.05 0.00a

a

The fabrics subjected to the traditional scouring/bleaching procedure and dyed were used as the standards.

shown in Table 4. In industrial practice, the slightly lower shade depth of the dyed fabric treated with the low temperature procedure can be easily adjusted through dye formulation if necessary. Colorfastness to washing and colorfastness to dry and wet crocking of the cotton fabrics treated with the two different scouring/bleaching procedures are shown in Table 5. The data Table 5. Color Change and Colorfastness of the Knitted Cotton Fabric Treated with the Two Procedures

scouring/bleaching treatment method

capillary effecta (cm/30 min) fabric weight loss after scouring/ bleaching (%) COD of the residual bath (mg/L)

low temp K/S ΔE

Table 3. Capillary Effects and Fabric Weight Loss of the Cotton Knitted Fabric with the Two Procedures and COD of the Residual Baths after the Treatments

low temp procedure

C. I. Reactive Blue 19 (670 nm)

dyes

traditional procedure 10.9 5.29 colorfastness to washing

6619

The capillary effect of the untreated fabric was 0.0 cm/30 min. colorfastness to rubbing

the capillary effect data presented here demonstrated that the removal of the hydrophobic substances from cotton fiber surfaces was less complete for the low temperature procedure than that for the traditional procedure. The weight losses of cotton fabrics treated with the low temperature procedure and the traditional procedure were 4.13% and 5.29%, respectively. The lower weight loss for the fabric treated with the low temperature procedure was consistent with the lower capillary effect of the same treated fabric, indicating incomplete removal of the hydrophobic substances on the fabric through the low temperature procedure, which remarkably reduced the COD in the residue bath. Table 3 also showed that the COD of the treatment bath of the low temperature procedure (6079 mg/L) was 8.2% lower than that of the traditional procedure (6619 mg/L). Table 2 shows that the WI of the cotton fabrics treated with optical brightener after either the low temperature scouring/bleaching procedure or the traditional procedure were 154% and 156%, respectively The fact that the two WI values were very close indicated that the existence of the hydrophobic substances on the treated cotton fiber surfaces did not interfere with the absorption of the optical brighteners. To further evaluate the dyeing performance of the low temperature scouring/bleaching procedure, we dyed the knitted fabrics treated with the two procedures using two reactive dyes. The shade depth (K/S) and color difference (ΔE) values of the dyed cotton knitted fabrics treated with the two procedures are presented in Table 4. The dyed fabric from the low temperature procedure had a K/S value slightly lower than those treated with the traditional procedure. The color differences between these two dyed fabrics were very small (ΔE= 0.61 for C. I. Reactive Blue 21 and ΔE = 0.97 for C. I. Reactive Blue 19) as

color change stain to cotton dry wet

C. I. Reactive Blue 21

C. I. Reactive Blue 19

low temp

traditional

low temp

traditional

4−5

4−5

4−5

4−5

4−5

4−5

3−4

3−4

4 3−4

4 3−4

4−5 3−4

4−5 3−4

presented here clearly demonstrated that the colorfastness to washing and colorfastness to crocking were identical for the cotton fabrics with the two different procedures. Even though the treated cotton fabric with the low temperature procedure contained some hydrophobic substances on the surfaces, such hydrophobic substances did not have negative effects on the dyed fabric for shade depth, as well as for the colorfastness to washing and colorfastness to crocking. Energy Consumption Analysis for the Low Temperature and the Traditional Procedures. We calculated the energy required to raise the temperature of the fabric/treatment bath from 30 °C to the temperatures required by the two different scouring/bleaching procedures, and the energy required to raise the temperature for the subsequent washing procedure (Table 6). It is a highly simplified method for assessing the energy consumption for the two different Table 6. Energy Required To Raise the Temperature for Two Scouring/Bleaching Procedures and for Subsequent Washing treatment procedure low temp scouring/ bleaching subsequent washing total 9989

raising from 30 to 60 °C, 1295 kJ/(kg cotton) raising from 40 to 60 °C, 863 kJ/(kg cotton) 2158 kJ/(kg cotton)

traditional raising from 30 to 98 °C, 2935 kJ/(kg cotton) raising from 50 to 90 °C, 1726 kJ/(kg cotton) 4661 kJ/(kg cotton)

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CONCLUSIONS All of the data demonstrated that in the presence of the new H2O2 activator, the scouring/bleaching of the cotton knitted fabric could be performed at 60 °C with the fabric whiteness and strength comparable to that treated with the traditional procedure performed at 98 °C. We found that the fabric treated with the low temperature procedure has lower fabric weight loss. Consequently, the weight of the fabric subjected to the low temperature procedure was 1.2% higher than that treated with the traditional procedure. High weight retention of the treated fabric increased the value of the treated cotton knitted fabric. We also found that an increase in hydrophobicity of treated cotton had little effect on the whiteness of the treated fabric that an optical brightener was applied to. The increase in hydrophobicity also had little effect on the shade depth, colorfastness to washing, and colorfastness to crocking of the dyed fabrics. The removal of less substance from cotton also resulted in lower COD in the residual bath and higher soft property on the treated fabric, thus providing additional environmental benefits. Moreover, the low temperature scouring/bleaching procedure had significantly lower energy consumption than the traditional procedure.

scouring/bleaching procedures. The data in Table 6 indicated that the energy consumption for the traditional scouring/ bleaching procedure (4661 kJ/kg of cotton) was 2.2 times that for the low temperature procedure (2158 kJ/kg of cotton). The savings of energy for the low temperature procedure were very significant. It should be pointed out that the energy consumption for the two different scouring/bleaching procedures presented in Table 6 was only a portion of the total energy saving for the entire scouring/bleaching processes. It included neither the energy loss due to the heat transfer from the process equipment to the surrounding environment during the temperature rising and holding period, nor the heat transfer to the environment during the subsequent washing period. Therefore, the energy savings for entire scouring/bleaching process using the low temperature procedure should be much larger than what is presented in Table 6. The amount of water used for this new method is similar to that of the traditional method. However, the traditional scouring/bleaching method requires additional water for cooling the bath from 98 to ≤70 °C after the treatment procedure is complete. Therefore, the total quantity of water used for the new method is lower than that for the traditional method. This new method also saves the time required for the process. Considering the following facts, (1) raising the bath temperature from 60 to 98 °C requires 19 min at a 2 °C/min heating rate, and (2) an additional 10 min is needed for the bath to cool to 70 °C before the residue bath is drained at the end of this procedure, the time required for this new method (60 min) is actually 14 min shorter than that of the traditional method (74 min).Therefore, the processing time for the new low temperature method is shorter than that for the traditional method. Benefits from the Low Temperature Procedure. As compared to the traditional high temperature treatment, many benefits on cotton fabric treated with the low temperature (60 °C) bleaching and scouring have been recognized. First, the energy required is significantly lowered for raising the temperature of a bleaching and scouring bath to the processing temperature (60 °C instead of 98 °C) and maintaining the bath at such a temperature. Second, cotton fabrics lose significantly less mass during a low temperature bleaching and scouring process than does the traditional process. On the basis of the results of our recent plant trials on cotton knitted fabrics, the final weight of the fabric treated with this low temperature method was 1.2% higher than that treated with the traditional method. The third advantage is that the COD of the wastewater generated by the low temperature procedure is significantly lower than that generated by a traditional process. Reducing the mass loss directly causes decreasing COD excretion, which is an additional benefit the new method brings to the environment. The fourth benefit is the treated fabric hand can be efficiently improved with the low temperature process. As the existence of some hydrophobic substances on this treated cotton fiber surfaces, which include a certain amount of natural cotton wax, the treated fabric from the low temperature process shows much softer property than that from the traditional process. Therefore, the natural resources on cotton fiber can be much more efficiently used with the low temperature bleaching and scouring process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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