ultrasound-assisted bleaching of linen fabrics and its influence on dyeing efficiency

Ultrasonics Sonochemistry 17 (2010) 383–390

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Ecofriendly laccase–hydrogen peroxide/ultrasound-assisted bleaching of linen fabrics and its influence on dyeing efficiency A. Abou-Okeil, A. El-Shafie, M.M. El Zawahry * National Research Centre, Textile Research Division, El-Behoos Street, Dokki, P.O. Box 12622, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 1 February 2009 Received in revised form 29 July 2009 Accepted 19 August 2009 Available online 23 August 2009 Keywords: Laccase Bleaching Ultrasound Linen Dyeing Kinetic

a b s t r a c t This study evaluates the bleaching efficiency of enzymatically scoured linen fabrics using a combined laccase–hydrogen peroxide bleaching process with and without ultrasonic energy, with the goal of obtaining fabrics with high whiteness levels, well preserved tensile strength and higher dye uptake. The effect of the laccase enzyme and the combined laccase–hydrogen peroxide bleaching process with and without ultrasound has been investigated with regard to whiteness value, tensile strength, dyeing efficiency and dyeing kinetics using both reactive and cationic dyes. The bleached linen fabrics were characterized using X-ray diffraction and by measuring tensile strength and lightness. The dyeing efficiency and kinetics were characterized by measuring dye uptake and colour fastness. The results indicated that ultrasound was an effective technique in the combined laccase–hydrogen peroxide bleaching process of linen fabrics. The whiteness values expressed as lightness of linen fabrics is enhanced by using ultrasonic energy. The measured colour strength values were found to be slightly better for combined laccase– hydrogen peroxide/ultrasound-assisted bleached fabrics than for combined laccase–hydrogen peroxide for both reactive and cationic dyes. The fastness properties of the fabrics dyed with reactive dye were better than those obtained when using cationic dye. The time/dye uptake isotherms were also enhanced when using combined laccase–hydrogen peroxide/ultrasound-assisted bleached fabric, which confirms the efficiency of ultrasound in the combined oxidative bleaching process. The dyeing rate constant, half-time of dyeing and dyeing efficiency have been calculated and discussed. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Use of enzymes in the textile industry is becoming increasingly popular, in large part due to advances in biochemistry that have led to the introduction of a variety of highly specific enzymes. Laccases are multi-copper enzymes that catalyze the oxidation of a wide range of inorganic and organic substances by using oxygen as an electron acceptor [1]. The laccase molecule contains four copper atoms per monomer, bound to three redox sites (T1, T2 and T3). The mechanism of laccase catalysis involves binding of the reducing substrate to the T1 active site, and a reduction of the T1–Cu(II) to Cu(I). The oxidation of a reduced substrate typically involves the formation of free cation radicals after the transfer of a single electron to laccase [2]. Laccases have found various biotechnical and environmental applications. Of these colours removal from bath liquors and materials (bleaching) is of particular interest in the current discussion [3].

* Corresponding author. Tel.: +20 233371718; fax: +20 233370931. E-mail addresses: [email protected], [email protected] (M.M. El Zawahry). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.08.007

During preparation of cellulose textile fibres, the bleaching process not only brightens the fibres and removes the natural colouring materials, (e.g. fats, waxes, pectins, proteins and pigments), but it is also directly related to the success of subsequent wet processing operations such as dyeing, printing and finishing. Enzymatic treatment of cotton textiles, like any wet processing approach, involves transfer of mass (enzyme molecules) from the solution onto the surface of the textile substrate. Although there are numerous advantages associated with conventional enzymatic bio-processing of cotton, there are also several shortcomings that greatly impede broader acceptance by the textile industry. These include expensive processing costs, relatively slow reaction rates and fabric damage. Ultrasound-based approaches have long been studied as an alternative to the conventional methods to accelerate mass transfer during some textile processing steps such as desizing, scouring, bleaching, mercerizing and dyeing of cotton fabrics [4–9]. Ultrasonic waves are vibrations with frequencies above 17 kHz, beyond the audible range of humans. They require a medium with elastic properties for propagation. Formation and collapse of bubbles formed by ultrasonic waves (known as cavitation) are generally considered responsible for most of ultrasound’s physical and chemical effects in solid/liquid or liquid/liquid

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systems. Cavitation can be initiated at moderate intensities, so less power is required than at high frequencies [10]. Chlorine and oxygen containing oxidizing agents are used during conventional bleaching process of cellulosic fibres. When a higher whiteness is needed, it is necessary to perform multiple oxidizing treatments. Rapid bleaching with laccase–hydrogen peroxide enhances the whiteness of cotton fabrics and significantly reduces the amount of hydrogen peroxide required during subsequent chemical bleaching processes [3,11]. The enhanced bleaching efficiency associated with a combination of ultrasound technology, laccase and hydrogen peroxide provides or causes less fibre damage and greater uniformity of the treatment [12]. The objective of this work is to study the influence of ultrasonic power during bleaching of linen fabrics by a combined laccase– hydrogen peroxide process as an alternative to both combined laccase–hydrogen peroxide and the conventional bleaching processes. The end goal of this research is to find milder conditions that effectively and efficiently bleach linen fabric. Various factors that may affect the bleaching process were thoroughly investigated using both conventional and combined bleaching processes in the presence and absence of ultrasound. Comparative results demonstrating the dyeing kinetics of bleached linen fabrics using conventional and combined laccase–hydrogen peroxide approaches in the presence and absence of ultrasound are also presented. The effect of the three bleaching processes on the fine structure of linen fabric was also investigated using an X-ray diffraction technique. 2. Materials and methods 2.1. Linen fabrics Grey 100% linen fabric (plain weave, 270 g/m2, 38 yarns/inch in wrap and 30 yarns/inch in weft) was supplied from Textile Industries Egyptian Co., Dintex, Cairo, Egypt. 2.2. Enzyme Laccase enzyme was kindly supplied by Jeans Care Company under the commercial name Ecolite II. Alkaline Pectinase (Bio Prep 3000 L) with an activity of 3000 APSU/g was supplied by Novo Nordisk. 2.3. Dyestuffs and chemicals The reactive dye used was Cibacron Brilliant Red 3B-A (C.I. Reactive Red 4) and was supplied from Ciba Geigy Co. (Egypt). The basic dye used was Astrazon Red 5BL (C.I. Basic Red 24) and was obtained from DyStar Co. (Egypt). The two dyes were used as received. Sodium hydroxide, hydrogen peroxide, sodium sulphate, sodium carbonate, sodium silicate and Triton X-100 were all laboratory grade reagents. 2.4. Ultrasound equipment A model 300 series NEY ULTRA sonikä, Cleaner Controller ultrasonic bench top cleaner bath with a 6 L tank was used. The experimental set up was composed of an electrical generator with a frequency of 26 kHz and a maximum power 300 W. The output power levels ranged from 60 to 300 W, and were supplied by transducers at the bottom of the industrial grade tank. Adjustable power burst and degassing period with heat and cleaning cycle timer also were used in this model. The internal dimensions of the tank were 270  200  150 mm.

2.5. Bio-scouring of linen fabrics The linen fabrics were subjected to treatment with Bio Prep enzyme (0.2–4% of the weight of fabric, owf) at a liquor ratio of 1:20 and non-ionic detergent Triton X-100 (1 g/L) was added, pH 8.5 at 60 °C for 45 min. After that step (1 g/L) chelating agent was added portion-wise and the enzyme was inactivated by raising the temperature to 95 °C for 15 min. The fabric was thoroughly washed with water, neutralized with (2 g/L) acetic acid and air dried. 2.6. Bleaching of linen fabric 2.6.1. Conventional bleaching Bio-scoured linen fabrics were treated with an aqueous solution containing hydrogen peroxide (5 g/L of 45% hydrogen peroxide) and sodium silicate (3 g/L). The pH was adjusted to 10.5 using aqueous sodium hydroxide solution and the bleaching process was carried out at a liquor ratio 1:20 and a temperature of 95 °C for 30 min. The fabric was washed several times with boiling water, then with cold water and finally dried at room temperature. 2.6.2. Laccase and combined laccase–hydrogen peroxide bleaching of linen fabric The bio-bleaching process was carried out by immersing a known weight of linen fabric in a bath containing 1–3% owf laccase enzyme at a liquor ratio of 1:20, pH 5 using buffer solution at 60 °C for 30 min. After that, pH was raised to 10 using a sodium hydroxide solution, and the temperature was raised to 90 °C for 3 min to deactivate the enzymes. The fabric was then washed with water and non-ionic detergent and finally dried at room temperature. In the second experiment, the above bio-bleaching process was repeated using 2% owf laccase under the same bleaching condition. After raising the temperature and pH, different concentrations of hydrogen peroxide were added (1–4 g/L). Then, the conventional bleaching process was carried out under the same conditions as described in Section 2.6.1. 2.6.3. Combined laccase–hydrogen peroxide/ultrasound-assisted bleaching of linen fabric A known weight of linen fabric was immersed in a bath containing laccase at a concentration of 2% owf, a liquor ratio of 1:20 at pH 5 in presence of buffer solution and at a temperature of 60 °C for 30 min. After that the temperature was raised to 80 °C, and the pH was increased to pH 10.5; then (3 g/L) 45% hydrogen peroxide and (3 g/L) sodium silicate were added. The bath was sonicated for 45 min using sonic powers ranging from 60–300 W. Finally, the samples were washed several times with tap water and nonionic detergent and air dried. 2.7. Dyeing procedures 2.7.1. Reactive dyeing Dyeing of linen fabrics (1 g) was carried out in a dye bath containing 1% owf shade and 40 g/L sodium sulphate which was added portion-wise to a liquor ratio of 1:40. Linen fabric was immersed in the dye bath at 40 °C for 30 min. Then, 20 g/L sodium carbonate was added portion-wise and the temperature was raised to 60 °C for 45 min. The dyed samples were rinsed with cold water and washed in a bath with a liquor ratio of 1:50 using 2 g/L non-ionic detergent at 60 °C for 30 min. Finally, the samples were rinsed and dried at room temperature. 2.7.2. Basic dyeing In this case the dyeing process was carried out in a dye bath containing 1% owf shade, 1 g/L wetting agent and a liquor ratio of 1:20. The pH was adjusted to 5 with acetic acid and the temper-

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ature was raised to 95 °C for 60 min. After dyeing, the fabrics were rinsed with cold water and washed in a bath of liquor ratio 1:50 using 2 g/L non-ionic detergent at 60 °C for 30 min. Finally, the samples were rinsed and dried at room temperature.

2.8.7. Colour fastness properties The dyed samples were tested according to ISO standard methods. The specific tests were ISO 105-X12 (1987), colour fastness to rubbing; ISO 105-C02 (1989), colour fastness to washing and ISO 105-E04 (1989), colour fastness to perspiration.

2.8. Measurements and analysis 3. Results and discussion 2.8.1. Bleaching efficiency and colour strength The bleaching efficiency of the treatments was examined as a function of CIE lab parameter lightness (L*). Colourimetric analysis of the dyed fabrics was measured using a Hunter Lab Ultra ScanÒ PRO Spectrophotometer (USA). 2.8.2. Tensile strength Tensile strength (TS) was determined according to the ASTM Standard Test Method [13]. 2.8.3. Wettability Wettability (wet) of the fabrics was measured by means of the drop test before and after the scouring process using AATCC test method 39-1980 (evaluation of the wettability). The time period (in seconds) between the contact of the water drop with the fabric and the disappearance of the water drop into the fabric was defined as the wettability time. 2.8.4. Weight loss Fabric weight loss (WL%) was measured as dried sample weight loss. The samples were dried at 105 °C for 1 h and were then weighed after cooling in closed weighing bottles in a desiccator. The following equation was applied to calculate the WL%.

WL% ¼

W1  W2  100 W1

ð1Þ

where W1 and W2 are the weights of the sample before and after treatment respectively. 2.8.5. X-ray diffraction An X-ray diffraction analysis was performed at room temperature for the conventional bleaching process, the combined laccase–hydrogen peroxide and the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process using a Philips X0 Pert MPP X-ray diffractometer with a type PW 3050/10 goniometer. The diffractometer was controlled by a PC computer with P Rofit software and used a Mo Ka source with wavelength 0.70930 Å for excitation. The Mo-tube power and current were at 50 kV and 40 mA respectively. The scans covered the 2h range from 2° to 50° at a step size of 0.03, with measurements performed using reflection geometry. 2.8.6. UV/Vis absorption spectra The visible absorption spectra of the reactive and cationic dye solutions were measured using a Shimadzu UV/Vis spectrophotometer. The quantity of dye uptake was estimated using the following equation:

Q¼

ðC 0  C f Þ V  100 W

ð2Þ

where, Q is the quantity of dye uptake (g/100 g fabric), C0 and Cf are the initial and the final concentration of dye in solution (mg/L), respectively, V is the volume of the dye bath in litres and W is the weight of fabric (g). The concentrations of the dye solutions were determined based on absorption measurements and calibration curves constructed for each dye using the Beer–Lambert law.

3.1. Scouring process Linen fibres contain 30% non-cellulosic impurities that negatively influence the dyeing and finishing process. Conventionally, an alkaline scouring process must be carried out to remove some of these non-cellulosic impurities. Today, alkaline scouring has been replaced with the more environmentally acceptable alkaline pectinase scouring process, which is performed at a lower temperature and produce biodegradable waste water [14,15]. Table 1 shows the effect of different concentrations of alkaline pectinase on the WL%, TS and wet of linen fabric. It is clear from Table 1 that an increase in the wettability occurs if the pectinase concentration is increased. The optimum wettability of the bioscoured fabric was <2 s using 0.2% owf alkaline pectinase compared to the untreated linen fabric (205 s). Further increase of the pectinase concentration gave almost the same results, but the WL% increased with increasing pectinase concentration. As a result, all our bleaching experiments were carried out using fabric scoured with 0.2% owf alkaline pectinase. 3.2. Bleaching processes 3.2.1. Effect of laccase concentration Table 2 shows the effect of laccase concentration on the lightness (L*) and TS of scoured linen fabric. It is clear from Table 2 that increasing laccase concentration from 0–3% owf leads to a slight increase in L* values. This increase is only 1–2 units, which means that laccase alone was insufficient to obtain linen fabric with good appearance (whiteness). It is believed that pigments present in the unbleached fabrics are normally removed by oxidative bleaching. In addition, the lignin substances present in the scoured fabric might be substrates for the laccase as well. Table 2 shows that bleaching using only laccase had almost no effect on TS. A concentration of 2% owf laccase was selected for subsequent studies. 3.2.2. Combined laccase–hydrogen peroxide bleaching process of linen fabric 3.2.2.1. Effect of hydrogen peroxide concentration. Table 3 shows the effect of hydrogen peroxide (H2O2) concentration (1–4 g/L) on linen fabric pretreated with 2% owf laccase in terms of L* and TS. Table 3 indicates that pretreatment of linen fabric with laccase followed by bleaching with hydrogen peroxide has an important ef-

Table 1 Effect of alkaline pectinase concentration on the weight loss percentage, tensile strength and wettability of the pretreated linen fabrics. Concentration of pectinase (% owf)

WL (%)

TS (kgf)

Wet (s)

0 0.2 0.4 0.6 0.8 1.0 2.0 4.0

0 7.71 7.85 7.98 8.16 8.20 8.30 8.57

83.85 80.00 81.00 81.06 81.50 64.00 60.00 60.00

205 <2 <2 <2 <2 <2 <2 <2

Scouring condition: L.R. 1:20, Triton X-100 1 g/L, pH 8.5, at 60 °C for 45 min.

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Table 2 Effect of laccase concentration on lightness and tensile strength of the scoured linen fabrics. Concentration of laccase (% owf)

L*

TS (kgf)

0 1 1.5 2 2.5 3

67.70 67.50 68.16 69.01 69.00 69.00

80.0 76.0 75.0 75.0 73.0 72.0

Laccase bleaching condition: L.R. 1:20, pH 5, at 60 °C for 30 min.

Table 3 Effect of hydrogen peroxide concentration on the lightness and tensile strength of linen fabric treated using the combined laccase–hydrogen peroxide bleaching process. Concentration of H2O2 (g/L)

L*

TS (kgf)

0 1 2 3 4

90.50a 87.85 89.40 89.50 90.42

64.00 75.00 70.50 68.00 66.50

a

Reference sample (conventional bleaching process using (5 g/L) hydrogen peroxide. Combined laccase–hydrogen peroxide bleaching conditions: 2% owf laccase, L.R. 1:20, 3 g/L sodium silicate, pH 10.5 at 95 °C for 30 min.

fect on both the bleaching efficiency and the quantity of hydrogen peroxide needed to give a good whiteness to the linen fabric. The results showed that 2–3 g/L hydrogen peroxide combined with 2% owf laccase pretreated linen fabric was almost enough to give the same value of L* as the conventional bleaching method using (5 g/L) hydrogen peroxide (i.e. the reference sample). If we compare the results of Tables 2 and 3, we note that the values of L* were higher for all hydrogen peroxide concentrations when laccase and hydrogen peroxide were combined than for the scoured linen fabric (unbleached). This may be attributed to the ability of laccase to transform the colouring materials into a form that is more easily removed during the oxidative bleaching process. This decrease in the amount of hydrogen peroxide needed to improve the whiteness of linen fabric indeed has the advantage of decreasing adverse effects on the physical properties of linen fabric. This capability is clear from the TS results as shown in Tables 2 and 3. The TS value of scoured and bleached flax fabric with 5 g/L hydrogen peroxide is much lower than that of scoured laccase pretreated flax fabric and bleached with 2–3 g/L hydrogen peroxide. The value of L* was almost the same.

3.2.2.2. Effect of sodium silicate concentration. Table 4 shows the effect of the sodium silicate concentration (0–3 g/L) in terms of L* and TS on linen fabric pretreated with 2% owf laccase and then bleached with (3 g/L) hydrogen peroxide. As shown in Table 4,

Table 4 Effect of sodium silicate concentration on the lightness and tensile strength of the linen fabric treated using the combined laccase–hydrogen peroxide bleaching process. Concentration of sodium silicate (g/L)

L*

TS (kgf)

0 0.5 1 2 3

78.00 87.00 88.20 89.70 89.50

70.00 68.00 68.00 68.00 68.00

Combined laccase–hydrogen peroxide bleaching conditions: 2% owf laccase, L.R. 1:20, 3 g/L hydrogen peroxide, pH 10.5 at 90 °C for 30 min.

increasing the sodium silicate concentration had only a marginal effect on the bleaching efficiency as measured by L* and TS. As such, a sodium silicate concentration of 2 g/L was selected for subsequent studies. 3.2.2.3. Effect of sodium hydroxide concentration. Table 5 shows the effect of the sodium hydroxide concentration in terms of L* and TS on linen fabric pretreated with 2% owf laccase. It is clear that as the concentration of sodium hydroxide increases from 0.04–0.1 M, the value of L* also increases as in the hydrogen peroxide bleaching process. The data in Table 5 also indicate that the concentration of sodium hydroxide has only a marginal effect on TS over the studied range. A sodium hydroxide concentration of (0.1 M) was selected for further studies. The optimum conditions for the combined laccase–hydrogen peroxide bleaching process were as follows: pretreatment of linen fabric with 2% owf laccase at pH 5 and 60 °C for 30 min at a liquor ratio of 1:20. This pretreatment process is followed by bleaching with 3 g/L hydrogen peroxide, 2 g/L sodium silicate and 0.1 M sodium hydroxide at 90 °C for 30 min. 3.2.3. Combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process of linen fabric The optimised conditions of the combined laccase–hydrogen peroxide bleaching process (Section 3.2.2.3) were applied during these studies to examine the use of ultrasonic energy to enhance the bleaching efficiency of the combined process for linen fabric. The effect of the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process on the L* and TS values of linen fabric was examined at different power levels (60–300 W) using an ultrasonic frequency 26 kHz. As shown in Fig. 1, the L* value increases with increasing ultrasonic power up to 180 W. Further increase in the ultrasound power above 180 W had no apparent effect on the L* values, but a marginal decrease in the TS values was observed. This result indicates the effectiveness of ultrasound in enhancing the combined laccase–hydrogen peroxide bleaching process, which may be due to cavitation and the heating effect. Cavitation in an aqueous medium (growth and explosive collapse of microscopic bubbles) can generate ‘‘hot spots” [16,17] i.e. localized high temperature and shock waves producing high pressure capable of breaking chemical bonds. If the collapse occurs in the vicinity of linen fabric surface, the bubbles deform. This deformation, results in formation of a high velocity microjet directed towards the fabric surface, with stirring of the adjacent border layer of the liquid. Powerful agitation of this border layer of the liquid due to sonication should substantially improve the transport of bulky enzyme macromolecules, and it should also increase the efficiency of the hydrogen peroxide bleaching process at low concentrations from the liquor to the linen fabric surface as a result of the over all reaction rates increase up to 180 W. Generally, ultrasonic waves increase the oxidative bleaching efficiency of the combined laccase–hydrogen peroxide bleaching process which could produce a linen fabric with a slight high whiteness level and good mechanical properties. Above 180 W, the combined laccase–

Table 5 Effect of sodium hydroxide concentration on the lightness and tensile strength of the linen fabric treated using the combined laccase–hydrogen peroxide bleaching process. Concentration of sodium hydroxide (M)

L*

TS (kgf)

0.04 0.08 0.10

85.90 85.40 89.70

69.9 68.9 68

Combined laccase–hydrogen peroxide bleaching conditions: 2% owf laccase, L.R. 1:20, 3 g/L hydrogen peroxide, 2 g/L sodium silicate, pH 10.5 at 90 °C for 30 min.

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100 L* value TS

90 80

L* & TS

70 60 50 40 30 20 10 0 60

90

120

150

180

210

240

270

300

Power level (W) Fig. 1. Effect of ultrasound power on the lightness and tensile strength of linen fabric treated using the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process (combined conditions: 2% owf laccase, L.R. 1:20, 3 g/L hydrogen peroxide, 2 g/L sodium silicate, pH 10.5 at 80 °C for 30 min).

3.2.3.1. Effect of temperature. Fig. 2 shows the effect of ultrasound temperature on the L* value and TS of linen fabric during the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process. As shown in Fig. 2, increasing the temperature from 50 °C to 80 °C was accompanied by an increase in L* values and a decrease in TS values. This result may be due to fibre swelling effects, which increase as the temperature increases from 50 °C to 80 °C and hence enhance diffusion during the combined laccase– hydrogen peroxide process. Also, ultrasound provides the additional effect of de-aggregation of enzyme molecules, which could increase enzyme activity and lead to a faster motion and stirring effects in the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process. Although ultrasound leads to further enhancement in the diffusion of enzyme and a decrease in the concentration of hydrogen peroxide required to increase L* values and decrease TS, it is still higher than both the conventional and combined laccase–hydrogen peroxide bleaching process. Thus, 80 °C was chosen as the optimum temperature [6,18]. 3.3. X-ray analysis

100 90

L* & TS (Kgf)

387

L* value

TS

80 70 60 50 40 30 20 10 0 50

60 70 o Temperature ( C)

80

X-ray diffraction patterns for linen fabrics bleached using the conventional process, combined laccase–hydrogen peroxide process and combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process are shown in Fig. 3. The two characteristic main peaks of bleached linen fabrics are clearly shown as one intense peak at 2h ffi 23° and a second weaker peak at 2h ffi 16.83° [19]. After evaluating the area of the sharp and broad peaks in Fig. 3, the crystallinity percentage of the three bleached samples was calculated and estimated according to the following equation [7,20]:

Cx% ¼ Fig. 2. Effect of ultrasound temperature on the lightness and tensile strength of linen fabric treated using the combined laccase–hydrogen peroxide/ultrasoundassisted bleaching process (combined conditions: 2% owf laccase, L.R. 1:20, 3 g/L hydrogen peroxide, 2 g/L sodium silicate, pH 10.5, 180 W for 30 min).

hydrogen peroxide bleaching process showed a slight decrease in the TS values, which may be due to increased formation of hydroxyl radicals (OH) of both ultrasound generated and hydrogen peroxide oxidation with ultrasound-assisted in situ.

Ic  100 Ic þ Ia

ð3Þ

where, Cx% is the crystallinity percentage and Ic and Ia are the X-ray diffraction intensities of the crystalline and amorphous components, respectively. It is clear from Table 6 that the crystallinity for the linen fabrics bleached either by the combined laccase–hydrogen peroxide process or the combined laccase–hydrogen peroxide/ultrasound process was higher than that of linen treated using the conventional bleaching process. On the other hand, the crystallinity for the fabric bleached using combined laccase–hydrogen peroxide

Fig. 3. X-ray diffraction patterns of linen fabrics bleached with (1) the combined laccase–hydrogen peroxide process, (2) the combined laccase–hydrogen peroxide/ ultrasound-assisted process and (3) the conventional hydrogen peroxide process.

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Table 6 Effect of the conventional, combined laccase–hydrogen peroxide and combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process on crystallinity, crystallite size, 2h angle and d-spacing of bleached linen fabric. Bleaching process

Crystallinity (%)

2h (°)

d-spacing (Å)

Crystallite size (nm)

Conventional hydrogen peroxide (reference sample) Combined laccase–hydrogen peroxide/ultrasound-assisted Combined laccase–hydrogen peroxide

60.58 63.24 65.00

23.00 22.83 22.50

3.92 3.98 3.95

5.30 5.16 4.92

in the presence of ultrasound was lower than that of linen bleached by combined laccase–hydrogen peroxide in the absence of ultrasound as a consequence of the cavitation effect [6,12]. However, the crystallites size for linen bleached using the combined laccase–hydrogen peroxide/ultrasound-assisted process was higher than that using the combined laccase–hydrogen peroxide only, which is close to that of the conventional bleaching process. This may be due to the enhancement of the ultrasonic waves in the combined bleaching process in removing the natural colouring matter from the scoured linen fabric [21,22]. The minor change in d-spacing indicates that the three bleaching processes have mainly affected the amorphous region following the order:Conventionally bleaching process > combined laccase–hydrogen peroxide process/ultrasound-assisted bleaching process > combined laccase–hydrogen peroxide bleaching process. 3.4. Dyeing of the bleached linen fabric To better examine the effects of the conventional bleaching method, combined laccase–hydrogen peroxide bleaching method and combined laccase–hydrogen peroxide/ultrasound-assisted bleaching method on linen fabric, the kinetics of dyeing were investigated by comparing the dyeability of bleached linen fabrics with reactive and cationic dyes. The conventional bleaching process was used as a control sample (L.R.1:20, 5 g/L hydrogen peroxide, 3 g/L sodium silicate, pH 10.5 at 95 °C for 30 min) and the optimised conditions were used for the combined laccase–hydrogen peroxide bleaching process in both the presence of (2% owf laccase, L.R.1:20, 3 g/L hydrogen peroxide, 2 g/L sodium silicate, pH 10.5, 180 W at 80 °C for 30 min) and absence of ultrasonic waves (2% owf laccase, L.R.1:20, 3 g/L hydrogen peroxide, 2 g/L sodium silicate, pH 10.5 at 90 °C for 30 min). 3.4.1. Kinetics of dyeing Time–dye uptake isotherms of conventional bleached, combined laccase–hydrogen peroxide bleached and combined laccase–hydrogen peroxide/ultrasound-assisted bleached process dyed with both reactive and cationic dyes are shown in Figs. 4

and 5. The results in both figures show that dye uptake was generally higher for fabric bleached with a combination of ultrasound and laccase–hydrogen peroxide than for fabric bleached with a combination of laccase–hydrogen peroxide in the absence of ultrasound. Better dye uptake is seen for both reactive and cationic dyes when ultrasound is combined with laccase–hydrogen peroxide bleaching process than in the absence of ultrasound (combined laccase–hydrogen peroxide only). Fig. 4 shows that the dye uptake with reactive dye by fabric bleached using the combined laccase–hydrogen peroxide/ultrasound- assist was marginally higher than the combined laccase– hydrogen peroxide bleached fabric in the absence of ultrasound. Fig. 5 indicates that the dye uptake of cationic dye by combined laccase–hydrogen peroxide bleached fabrics in the presence of ultrasound was significantly higher than the combined laccase– hydrogen peroxide bleached fabric in the absence of ultrasound. This indicates that the cationic dye may better bind to the combined laccase–hydrogen peroxide bleached fabric in the presence of ultrasound. These results may be attributed to an additional ultrasonic oxidation effect (radical formation) of the hydrogen peroxide [23,24] and also the enhancing effect of ultrasonic power due to the cavitation effects. As discussed above, cavitation is thought to lead to [2,16]: improved dispersion (reduced aggregation of bulky enzyme molecules which could improve enzyme activity and enhanced transport of bulky laccase enzyme molecules toward the fibre surface), degassing (expulsion of dissolved or entrapped gas or air molecules from the fibre into the liquid and removal by cavitation, thus facilitating enzyme and hydrogen peroxide–fibre contact) and diffusion (accelerating the rate of enzyme molecule, diffusion toward the fibre surface through the border layer of the liquid). Together, these factors lead to an increase in the overall reaction rate by enhancing the kinetic and equilibrium stages of each dyeing system [5]. Table 7 provides values of the half dyeing time (t½), the specific dyeing rate constant (K) and the dye uptake calculated for bleached linen fabric using conventional bleached method and combined laccase–hydrogen peroxide in the presence and absence of ultrasound. The dyeing rate constant is calculated using Eq. (4) [25,26].

8

30 25 20 conventional hydrogen peroxide bleached

15

combined laccase-hydrogen peroxide bleached

10

combined laccase-hydrogen peroxide/ultrasound bleached

5

dye uptake (g/100g fabric)

dye uptake (g/100g fabric)

35

7 6 5 4 3 conventional hydrogen peroxide bleached

2

combined laccase-hydrogen peroxide bleached

1

combined laccase-hydrogen peroxide/ultrasound bleached

0

0 0

20

40

60

80

100 120 140 160 180 200 220 240 Time (min)

Fig. 4. Dyeing rate of linen fabric with reactive dye after bleaching with different techniques (Dyeing conditions: shade 1% owf, L.R. 1:40, sodium sulphate 40 g/L, sodium carbonate 20 g/L at 60 °C for 45 min).

0

20

40

60

80

100 120 140 160 180 200 220 240

Time (min) Fig. 5. Dyeing rate of linen fabric with cationic dye after bleaching with different techniques (Dyeing conditions: shade 1% owf, L.R. 1:20, wetting agent 1 g/L, pH 5 at 95 °C for 60 min).

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Table 7 Dyeing rate constant K, efficiency of dyeing DK, half dyeing times t½ and amount of final dye uptake of reactive and cationic dyes by linen fabrics bleached using different techniques. Bleaching process

Conventional hydrogen peroxide (reference sample) Combined laccase–hydrogen peroxide/ultrasound-assisted Combined laccase–hydrogen peroxide

C.I. Reactive Red 4

C.I. Basic Red 24

K (cm/min)

t½ (min)

DK (%)

Qf (g/100 g)

K  100 (cm/min)

t½

DK (%)

Qf (g/100 g)

14.99 14.50 13.50

3 5 10

– 6.90 –

30 28.55 26.57

3.99 2.88 2.26

19 30 36

– 21.53 –

7.55 7.00 5.50

K ¼ 0:5C a ðd=t 1=2 Þ1=2

treatment with the combined laccase–hydrogen peroxide bleaching process in the presence and absence of ultrasound and the conventional bleached fabrics (as a reference sample). The results indicate that K/S values for the combined laccase–hydrogen peroxide bleached fabrics in the presence of ultrasound and dyed with both reactive and cationic dyes were slightly better than those bleached in the absence of ultrasound. In addition, the washing, rubbing and perspiration fastness properties of combined laccase–hydrogen peroxide bleached fabric in the presence of ultrasound and dyed with reactive dyes generally give better results than in the absence of ultrasound if compared with the reference sample. The fastness improvement may be due to better dye penetration and better covalent fixation with linen fabrics. Although, the fastness properties of washing and perspiration of the dyed fabric are not altered in the case of the cationic dye, a decrease in rubbing fastness was observed for fabric bleached using the combined laccase–hydrogen peroxide bleached process in both the presence and the absence of ultrasound.

ð4Þ

where, Ca is the dye uptake by the sample at equilibrium conditions between the sample and the dye bath divided by the weight of the fabric and d is the fibre diameter in centimeters. The results in Table 7 indicate that the rate constant of dyeing linen fabrics bleached using combined laccase–hydrogen peroxide/ultrasound-assistance with reactive and cationic dyes was increased in comparison with combined laccase–hydrogen peroxide only. Also, the values of t½ of dyeing are clearly shorter in both dyeing systems (reactive and cationic dyes) for those linen fabrics bleached with the combined laccase–hydrogen peroxide/ ultrasound-assisted method in comparison with those bleached with only the combined laccase–hydrogen peroxide process. 3.4.2. Dyeing efficiency The effect of the combined laccase–hydrogen peroxide bleaching process on dyeing efficiency was evaluated in the presence of ultrasound using the following equation:

Dk% ¼

k1  k2  100 k2

5. Conclusions

ð5Þ

The use of power ultrasound (26 kHz, 180 W) appears to be an effective technique for improving the bleaching of linen fabric with a combined laccase–hydrogen peroxide bleaching process such that this process yields results similar to a conventional bleaching process. Examination of factors affecting the combined laccase– hydrogen peroxide bleaching process in the presence and absence of ultrasound have indicated that ultrasound is an effective technique in improving the whiteness of enzymatically scoured linen fabric with acceptable TS. X-ray diffraction studies proved that the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process renders fabric less crystalline and more hydrophilic than the combined laccase–hydrogen peroxide bleaching process. In addition, slightly better dye uptake of both reactive and cationic dyes was observed for fabric bleached using the combined laccase–hydrogen peroxide/ultrasound-assisted bleaching process versus a similar process in the absence of ultrasound. Additionally, the fastness properties of fabrics dyed with reactive dyes after bleaching using the combined laccase–hydrogen peroxide bleaching process in the presence of ultrasound were better than

where k1 and k2 are the rate constants of dyeing bleached linen fabric with the combined laccase–hydrogen peroxide process in the presence and absence of ultrasound, respectively. As shown in Table 7, the dyeing efficiency (Dk%) of linen fabric bleached with the combined laccase–hydrogen peroxide/ultrasound process is positive, indicating that ultrasound improved bleaching efficiency of the combined laccase–hydrogen peroxide bleaching process. Also, it was observed that the dyeing efficiency of combined laccase– hydrogen peroxide/ultrasound-assisted bleached fabric with cationic dye was three times greater than the reactive dye. This result may be attributed to the sensitivity of the cationic dye to oxidation processes associated with combined laccase–hydrogen peroxide/ ultrasound bleached linen fabric. 4. Fastness properties Table 8 compares the colour strength values and fastness properties of fabrics dyed with both reactive and cationic dyes after

Table 8 Comparative colour strength and fastness properties of dyed linen fabrics. Bleaching process

K/S

Washing

Perspiration

Rubbing

Acidic

C.I. Reactive Red 4 Conventional hydrogen peroxide Combined laccase–hydrogen peroxide/ultrasound-assisted Combined laccase–hydrogen peroxide C.I. Basic Red 24 Conventional hydrogen peroxide Combined laccase–hydrogen peroxide/ultrasound-assisted Combined laccase–hydrogen peroxide

Alkaline

Alt

C

L

W

Alt

C

L

W

Alt

C

L

W

Dry

Wet

4.60 4.45 4.26

4–5 4–5 4–5

4 4 3–4

4 4 3–4

5 5 4–5

5 5 5

4 4 4

4–5 4–5 4

4–5 4–5 4–5

4–5 4–5 4

4–5 4–5 4

4–5 4–5 4

4–5 4–5 4–5

4–5 4–5 4–5

4 4 3–4

7.89 7.56 7.13

4 4 3–4

3–4 3–4 3

3 3 2–3

3 3 2–3

3–4 3–4 3–4

3 3 3

3 3 3

3 3 2–3

3–4 3–4 3–4

3 3 3

3 3 2–3

3 2–3 3

3 2–3 3

2–3 2–3 2–3

390

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