A Numerical Model Based on Response Surface ... - Springer Link

Fibers and Polymers 2015, Vol.16, No.6, 1281-1288 DOI 10.1007/s12221-015-1281-5

ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version)

Ultrasound-Assisted Mercerizing Process of Cotton Fabric: A Numerical Model Based on Response Surface Methodology (RSM) Ramin Khajavi*, Amirhosein Berendjchi1, Mohammad Bameni Moghaddam2, and Mahshid Akhani Department of Textile Engineering, South Tehran Branch, Islamic Azad University, Tehran 1777613551, Iran Department of Textile Engineering, Tehran Science and Research Branch, Islamic Azad University, Tehran 1477893855, Iran 2 Department of Statics, Allameh Tabatabai University, Tehran 1513615411, Iran (Received February 4, 2015; Revised April 5, 2015; Accepted May 12, 2015)

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Abstract: The current study investigates the use of ultrasonic energy to provide a more efficient and eco-friendly mercerizing process. For both conventional and ultrasound-assisted techniques, the process variables, including sodium hydroxide concentration, reaction time, temperature, tensile strength and degree of mercerization, were modeled and compared by using the response surface methodology (RSM). The results revealed that the relationship between explanatory and response variables was similar in both techniques. However, there were greater changes for the ultrasonic-assisted treatment of samples. The optimum process parameters showed a lower alkali concentration in the mercerizing bath (≈17 %) and a higher tenacity (≈23 %) during the ultrasonic-assisted process, while the degree of mercerization was also higher in the same fibers. The study concluded that the ultrasound-assisted mercerization could be introduced as a more efficient alternative to the conventional mercerization process. Keywords: Ultrasound-assisted process, Ultrasonic energy, Response surface methodology, Tensile strength, Barium activity number, Mercerization

bleaching of linen fabrics using a combined laccase-hydrogen peroxide process with and without ultrasonic waves. The authors reported improvements in lightness, color strength, and dye uptake of samples. Kamel et al. [20] investigated ultrasonic-assisted dyeing of acrylic fabrics and the result indicated higher color strength and dye ability of the samples. Sivakumar and Rao [12] observed an increase in dye exhaustion and a decrease in dyeing period on leathers. Khajavi et al. [21] studied the effect of ultrasonic on the denim fabric worn out process. The results of measurements showed increased values between 50 to 98 % of brightness, color difference, whiteness degree and color absorption, as well as a significant decline in the back staining for the samples treated in an ultrasonic method. Mercerization is one of the most important wet processes of cotton textiles. This preparatory treatment is necessary to modify the structure of cotton and, thereby, to achieve enhanced physical (dye uptake and luster) and mechanical (tenacity, elongation and work of rupture) properties [2428]. The process was developed by John Mercer in 1844, when he observed noticeable swelling of cellulose fibers in strong alkali solutions, especially sodium hydroxide [29]. Sodium hydroxide results in improvement in the mechanical properties of cellulosic fabrics, through the reorientation of polymer chains, altering fiber helix angle, hardening weaknesses along the length of fiber, and the elimination of cellulose fibers with lower degree of polymerization [30,31]. During mercerizing, the cellulose crystalline structure is changed from cellulose I to cellulose II, which can be examined by the X-ray diffraction technique. There are a wide range of detailed studies in this field [32,33]. As the mercerization consumes large amounts of alkali, it

Introduction Ultrasonic energy shows a high potency to improve a number of chemical processes, especially in aqueous systems such as ultrasonic bath and textile wet treatments like ultrasonic-assisted cleaning process [1-16]. The phenomena of formation and subsequent collapse of bubbles (cavitation) can be regarded as the main factor for rate acceleration in chemical reactions (sonochemistry) and efficiency improvement through cleaning [15,17]. For many years, the application of ultrasound has been investigated in textile wet processing [2,5-7,9-14,18-23]. Studying the effect of ultrasonic waves in the scouring of raw wool fibers, Bahtiyari and Duran [2] reported that the use of ultrasonic waves represents an energy efficient procedure and leads to improvement in the cleaning. Khajavi and Azari [5] applied the same process for improving the wet chlorination process of wool. The desirable mechanical and chemical properties were obtained in the presence of ultrasonic waves. Şahinbaşkan and Kahraman [11] achieved a significant increase in the removal of starch from cotton fibers under the conditions of ultrasonic enzymatic processing. Parvinzadeh et al. [9] showed that the treatment of cotton fabrics with a cationic softener in the ultrasonic-assisted process can lead to enhanced efficiency and physical properties. This procedure has also been used by McNeil and Mc Call [23] in reducing the environmental impact on dyeing and finishing of wool fibers, and by Ferrero and Periolatto [19] for dyeing wool fibers with leveling acid dyes at 60-80 oC temperatures. Abou-Okeil et al. [18] assessed *Corresponding author: [email protected] 1281

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is likely inevitable to present a more cost-effective method for the mercerization due to the increasing cost of chemicals, labor, and effluent control. To the best of our knowledge, however, there is no research on the use of ultrasonic energy during the mercerizing process of cotton fabrics. Our previous study [22] dealt only with the effect of ultrasonic energy on the mercerization treatment of cotton yarns under both slack and tension conditions. The results demonstrated that the amounts of cellulose II cotton were increased up to 71 % and 61 % under the slack and tension conditions, respectively, and the tensile properties of these composites were improved. The present paper is the continuation of the previous published research which, as stated above, was focused just on the ultrasonic-assisted mercerization of cotton yarns in slack and under tension conditions [22]. However this procedure here is expanded for mercerizing cotton in fabric form which is more practicable. Furthermore, a numerical method is presented by using the RSM for modeling the relationships between process variables and selected responses (such as tensile properties) for both conventional and ultrasonic assisted conditions. Also, the optimum processing parameters are obtained and compared for each condition.

Experimental Samples Preparation The cotton fabric (Plain weave, 138 g/m2) was cut into 30×10 cm2 pieces and desized by using enzymatic treatment (Novozymes company Denmark) (4 g/l) at 60 oC for 60 min, at pH=4.5-5.5 and L:R=30:1. Design of Experiments Since the significant input variables are well known for the mercerization process [22,31,34], a central composite design (CCD) can be implemented to estimate a second order polynominal model and used to optimize key factors from both conventional and ultrasonic-assisted procedures. Three input variables, namely time (A), temperature (B), and sodium hydroxide concentration (C) were selected based on previous studies [22,31,35,36] and also, tenacity (T), elongation (E), work of rupture (WR), and degree of mercerization (the barium activity number (BN) were considered as the output variables. Mercerization A number of sodium hydroxide solutions was prepared in different concentrations in a range of 10 % to 30 % wt/wt (owf) (AATCC 89-2003). For all samples, the mercerization process was performed using the alkali prepared solutions with an ultrasonic bath (2.5 l capacity, 100 W power and 40 kHz frequency) in the presence and absence of ultrasound waves (L:R=40:1). Then, the samples were neutralized with 1.1 % (v/v) acetic acid for 5 min, followed by rising in distilled water at 70 oC.

Ramin Khajavi et al.

Characterization The output variables, i.e. tenacity (T), elongation (E), work of rupture (WR), and degree of mercerization (the barium activity number (BN)), were determined on fifteen runs during both conventional and ultrasonic assisted processes (Table 3). The tensile mechanical properties of the samples were determined by a constant rater of elongation type tensile testing machine (Mesdan, Brescia - Italy) according to ASTM D5034-09 (2005) [37]. The crystalline structure of the raw and mercerized cotton fabrics were analyzed by an X-ray diffractometer (Model X Perdmpd, Phillips, Netherlands). The X-ray diffraction (XRD) patterns were recorded using Ni-filtered CuKα radiation (λ=1.54 Å) at an excitation voltage of 40 kV at a current of 40 mA. X'Pert High Score Plus software was used to measure the crystalline parameters. The crystal size, t (nm), was determined by applying the Scherrer equation (perpendicular to the (002) crystal planes for both cellulose I and cellulose II). The crystallinity index (CI, %) was obtained based on the method of Jayme and Knolle (equation (1)). CI = 1 − ham/hcr = 1 − ham/((htot − ham))

(1)

The 002 reflection (hcr) at 2θ of 22.5 o for cellulose I and the 101 reflection at 2θ of 20.1 o for cellulose II were considered as the crystal height peaks. Also, ham represented the height of the amorphous reflection at 2θ of 18 o for cellulose I and 16 o for cellulose II, respectively. The barium activity number (BN) was used according to AATCC 89-2003 to evaluate completeness of the reaction between the cotton fabrics and the mercerization bath. The ratio of the amount of barium hydroxide (Ba(OH)2) absorbed by the mercerized fabric sample to that absorbed by the unmercerized sample is expressed as the barium activity number. Separate specimens of 2 g of mercerized and unmercerized cotton fabrics were immersed in separate flasks containing 30 ml of 0.25 N barium hydroxide solutions and kept in overnight. 10 ml aliquots of the solution from each flask were transferred and then, titrated with 0.1 N hydrochloric acid, using phenolphthalein as the indicator. A blank titration was carried out in 10 ml of 0.25 N barium hydroxide solution with 0.1 N hydrochloric acid, using phenolphthalein as the indicator as well. The barium activity number was calculated based on the following equation (equation (2)): Barium activity number = (b − s)/(b − u) × 100

(2)

where, b=ml required for blank test, s=ml required for mercerized cotton, and u=ml required for unmercerized cotton.

Results and Discussion Tensile Mechanical Properties Table 1 shows the statistical (ANOVA analysis) summary

Ultrasonic Assisted Cotton Fabric Mercerizing


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Table 1. ANOVA for response surface reduced quadratic model TCO

ECO WRCO BNCO F P-value F P-value F P-value F P-value value Prob>F value Prob>F value Prob>F value Prob>F Mean vs Total Model 17.05 0.00 Model 3.00 0.08 Model 3.71 0.07 Linear vs Mean 0.42 0.74 A-Time 0.41 0.54 A-Time 0.24 0.63 C-Con 3.71 0.07 2FI vs Linear 0.56 0.65 B-Temp 1.18 0.31 B-Temp 0.086 0.77 Residual Quadratic vs 2FI 0.41 0.75 C-Con 16.49 0.00 C-Con 0.15 0.71 Lack of fit 0.19 0.98 Cubic vs Quadratic 1.69 0.26 AC 4.87 0.06 AB 1.78 0.22 Pure error BC 10.06 0.01 AC 1.64 0.24 Cor total B2 17.28 0.00 BC 2.49 0.15 40.48 0.00 C2 3.34 0.11 C2 Lack of fit 2.04 0.25 Lack of fit 0.26 0.85 TUA EUA WRUA BNUA Model 4.58 0.02 Model 5.62 0.01 Model 4.56 0.02 Mean vs Total A-Time 1.469E-006 0.99 B-Temp 2.06 0.18 A-Time 0.08 0.77 Linear vs Mean 0.46 0.71 B-Temp 0.56 0.47 C-Con 11.70 0.00 B-Temp 1.02 0.34 2FI vs Linear 0.71 0.57 C-Con 0.25 0.62 BC 4.71 0.05 C-Con 7.96 0.02 Quadratic vs 2FI 0.34 0.79 AC 4.94 0.05 C2 4.04 0.07 BC 5.55 0.04 Cubic vs Quadratic 0.39 0.56 2 B 14.42 0.00 Residual A2 5.43 0.04 Residual 6.84 0.03 Lack of fit 1.36 0.40 C2 3.21 0.11 Total C2 Lack of fit 0.63 Cor total Lack of fit 0.45 0.77 CO: Conventional mercerizing, UA: ultrasonic-assisted mercerization, T: tensile strength, E: elongation, WR: work of rupture, BN: barium number.

results of treated samples. The tenacity values of the conventional mercerized samples do not fit any models and the independent variables (time, temperature, concentration) do not show a significant effect within the selected range of tenacity (α=0.05). For the ultrasonic-assisted process, the model F-value of 4.58 implies that the model is significant and that there is only a 2.6 % chance this could occur due to noise. B2 and C2 (0.0053 and 0.0308, respectively) show a significant effect on tenacity values. The Lack of Fit F-value of 0.63 implies the Lack of Fit is not significant relative to the pure error. As seen from Table 4 (Rows 1 to 3), increasing sodium hydroxide concentration from 10 wt% to 20 wt% improves the mechanical properties. However, at higher alkali content (30 %), the properties drop. The degree of swelling depends directly on the size of NaOH molecules hydrated by the water molecules. It is reported that the rate of penetration for alkali solution is controlled by its concentration, since the size of the NaOH hydrates decreases with increasing the alkali concentration. The NaOH hydrates should be small enough to penetrate into the regions of higher lateral order [8,33,39]. On the other hand, this can be related to low concentration of NaOH in the solution for disrupting the cellulose crystals and form a new physical network. At lower concentrations of sodium hydroxide, there are many

Table 2. Models proposed for different variables in both conventional and ultrasonic assisted processes No Model 1 TUA =2218.44−0.063*A+55.17*B+25.71*C+229.14*A*C− 201.83*B2 −134.14*C2 2 ECO =+56.93−1.13*A+1.92*B+4.97*C−5.47* A*C+7.88* B*C−5.31*B2 −7.85*C2 3 EUA =+59.53+2.97* B+6.99* C+6.31*B*C−4.14*C2 4 WRCO =+748.66−49.07*A+29.07*B+37.24*C− 183.93*A*B−178.40*A*C+220.36*B*C−126.74*C2 5 WRUA=+821.85+24.23*A+58.05*B+159.94*C+270.83*B *C+138.75*A2−102.28*C2 6 BNCO =+329.29+35.55*C A: time (min), B: temperature (oC) and C: sodium hydroxide concentration % by weight. CO: conventional mercerizing, UA: ultrasonic assisted mercerizing.

more water molecules and the hydration of NaOH molecules produce large diameter hydrate ions. Such ions are too big to penetrate into the macromolecular structure of cotton cellulose to disrupt the cellulose lattice. As the inter-fibrillar regions are likely to be less rigid, it is only possible to rearrangement of fibers. By further increasing the NaOH concentration, the crystalline structure of the cotton fibers converts into a

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Table 3. CCD experimental design withtarget parameters values for each conventional and ultrasonic-assisted conditions Processing variables Output (Independent) variables WRUA Conc. TCO TUA ECO EUA WRCO B BNUA BNCO A (min) (cN·cm) (%) (g/den) (g/den) (mm) (mm) (cN·cm) (oC) 1 9.3 52.0 13 1559.5 1686.0 33.7 44.6 343.8 440.3 294.1 400 2 10.0 30.5 20 1356.9 1981.7 42.1 53.1 645.3 913.6 356.7 493.8 3 9.3 9.0 27 1167.7 1951.0 32.4 55.7 371.2 776.0 358.8 470.3 4 7.5 30.5 20 1418.3 1987.0 51.0 58.0 648.9 919.7 335.6 462.1 5 7.5 30.5 20 1410.4 1981.6 52.5 58.9 655.4 926.5 331.2 459.8 6 5.7 52.0 27 1623.0 1715.5 65.2 70.0 558.7 671.8 323.5 417.6 7 7.5 30.5 20 1446.9 1995.1 52.0 59.2 651.7 946.1 332.7 465.6 8 7.5 30.5 10 1359.9 1521.1 40.9 46.3 431.9 521.3 241.2 286.5 9 5.7 9.0 13 1528.3 2146.5 37.0 55.7 467.7 855.7 352.9 505.9 10 7.5 30.5 20 1459.6 2006.2 53.5 58.8 658.6 952.6 330.2 453.0 11 7.5 30.5 20 1465.0 2010.8 53.0 59.6 691.4 934.7 327.5 459.6 12 7.5 60.0 20 1496.0 2010.3 57.3 67.4 662.6 1024.5 342.5 475.5 13 5.0 30.5 20 1615.1 2172.3 55.6 64.9 889.6 1058.0 298.6 406.5 14 7.5 30.5 30 1391.2 1942.6 45.2 52.2 538.3 786.0 367.8 503.0 15 7.5 0.0 20 1321.4 1741.5 42.2 45.4 585.6 690.5 315.4 435.2 A: Process time in the range of 5.7 to 9.30 min, B: temperature range from 9.0 to 52.0 oC and sodium hydroxide concentration within the range of 13 to 27 % (by weight). CO: conventional mercerization, UA: ultrasonic assisted mercerization, T: tensile tenacity, E: elongation, WR: work of rupture, BN: barium number. Run no.

Table 4. Changes in responses independent variable changes, while two others are constant Output (Independent) variables TUA ECO WRCO WRUA EUA TCO (g/den) (g/den) (mm) (CN·cm) (CN·cm) (mm) 10 1359* 1 1821 40 46 431 521 Conc. 2 20 1440 1996 50 58 650 935 (%) 3 30 1391 1942 45 52 538 849 5 1615 2172 55 64 889 1058 4 Time 5 7.5 1440 1996 50 58 650 935 (min) 6 10 1356 1981 42 53 645 913 7 0 1321 1741 42 45 585 812 Temp. 8 30.5 1440 1996 50 58 650 935 (oC) 9 60 1496 2010 57 67 691 1024 * As the decimal values could be ignored, all data in this table have been rounded. No

Processing variables

swollen state. Because the amount of water molecules is much less at higher concentrations of sodium hydroxide solution, so the produced hydrated ions have smaller diameters with capability to penetrate into macromolecular structure of cotton and cause the swelling of fibers. On the other hand, at higher concentrations of alkali and because of the high viscosity of the solution, the rate of penetration into the interior of the fibers is very low. So, an optimum concentration of alkali is needed in order to achieving optimal mechanical properties. For example, Goswami et al. [36] reported decreases

BNCO

BNUA

241 331 367 298 331 356 315 331 342

286 458 503 406 458 493 435 458 475

in the degree of orientation, crystallinity and tensile strength of regenerated lyocell fibers at higher concentrations of the sodium hydroxide solution. As the authors suggested, the greatest change in fiber properties occurs between 3.0 and 5.0 mol dm-3. Wagaw and RB [31] obtained the optimum values of sodium hydroxide concentrations (14 to 16 % W/V) for improved color yield. Liu and Wang [30] reported the optimum concentration of alkali solution (250 g/l) during the mercerization process of blended cellulosic yarns (Kapok/ Cotton, 70/30).


Furthermore, for the samples at the ultrasonic assisted system, the maximum values are obtained at 20 % of sodium hydroxide concentration. Increases in the tenacity under the conventional and ultrasonic assisted methods are 6 % and 10 %, respectively. At 20 % sodium hydroxide concentration, the tenacity of the ultrasonic assisted process is 4 % higher than that of the conventional one. This can be related to a better mass transfer during wet processing due to the application of ultrasound energy which is recently well documented [38]. Powerful shock wave generation as a result of the cavitation phenomenon leads to improved swelling and penetration of sodium hydroxide inside cotton fibers and fabrics. However, ultrasonic vibrations in liquid can enhance the mobility of the polymer chains, and thereby improving their orientation along the fiber axis. This theory is discussed in another paper [22]. So, the tenacity properties of mercerized samples could be further increased. The relationship between process parameter and tenacity was obtained and modeled (Table 2, No1). According to Table 4 (Rows 4 to 6), when the temperature and concentration were hold constant in the mercerization process, the tenacity of samples decreased by increasing the process time. Such effect was also observed for tenacity in the proposed model. It can be seen that the coefficient of time is a negative factor (a negative coefficient of A; Table 2). This may be attributed to the degradation of cellulose chains during mercerization [38]. It can be seen that for ultrasound process at a constant time such as five minutes, the value of obtained tenacity is 25 % greater in compare with obtained value for conventional process. By increasing the temperature (Table 4; rows 7 to 9), values of tenacity are being improved for conventional and ultrasonic assisted processes (13 % and 15 %, respectively). The reaction of sodium hydroxide with cotton and the formation of alkali-cellulose are exothermic processes. Hence, the alkali solution is conventionally cooled to a certain degree, because the heat emerged during the reaction prevents the swelling of cotton fibers. However, a decreased viscosity of the alkali solution promotes the impregnation rate of fibers with sodium hydroxide. Relying on this effect, a large number of studies have introduced hot mercerizing process. Gemci [34] claimed that the swelling speed of fibers is slow at lower temperatures and this is also reported by Lawson and Hertel [41], presenting that the warm solution of sodium hydroxide (30-40 oC) can penetrate the fiber much more rapidly. In the previous study improvement of cellulose I transformation to the cellulose II lattice for ultrasonic assisted mercerized cotton yarns at 40 oC was reported [22]. Besides, the lower amount of wetting agents is among other possible advantage achieved by the use of higher temperatures. At 60 oC, the tenacity value obtained from the ultrasonicassisted process is higher than that from the conventional process by 34 %. High temperatures in an ultrasound bath


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exert two contradictory effects: decreasing the cavitation phenomena and lowering the alkalinity of the solution. Bahtiyari and Duran [2] reported limited effects of ultrasound at high temperatures due to the lowering of intensity of bubble collapse and cavitation phenomenon. However, at higher temperatures, the effect of mass transfer acceleration leads to obtain improved results. The application of high-frequency ultrasonic waves can improve segmental motion, conformational change, vibration and translational energy interchange. Moreover, the pressure applied through this treatment increases with the use of ultrasonic energy. All the parameters mentioned can promote the penetration of sodium hydroxide, reorientation of polymeric chains, transformation from cellulose 1 to cellulose 2, and degree of mercerization [22]. Elongation values of mercerized samples under both conditions show significant difference in response to independent variables (Table 1). For the samples conventionally mercerized, the Model F-value of 17.05 (Table 1) implies that the model is significant and that there is only a 0.07 % chance that a Model F-Value this large could occur due to noise. In this case, C, BC, B2, and C2 are significant terms of the model. The Lack of Fit F-value of 2.04 indicates the Lack of Fit is not significant relative to the pure error. For the samples by ultrasonic-assisted processing, the Model F-value of 5.62 (Table 1) reveals that the model is significant. There is only a 1.23 % chance that a Model F-Value this large could occur due to noise. In this case, just C (Prob> F=0.0065) is a significant model term. It means that using ultrasonic bath can fade the effects of time and temperature. The Lack of Fit F-value of 1.36 implies the Lack of Fit is not significant relative to the pure error. According to Table 4 (Rows 1 to 3), there is no significant difference in the values of elongation between both methods at different concentrations of sodium hydroxide. However, at 20% concentration of sodium hydroxide, the elongation for ultrasonic assisted process is 16 % higher than the conventional one. These can be attributed to the facilitate transformation of Cellulose I to Cellulose II. Because in Cellulose II structure, rate of amorphous regions are more than Cellulose I, elongation of fibers enhances [22]. The process duration has a negative effect on elongation for all mercerized samples (Table 4, rows 4 to 6). Decreases in the elongation are 23 % and 17 % for the conventional and ultrasonic assisted methods, respectively. At a constant process time (5 minutes), the elongation for ultrasonic-assisted process is 16 % higher than the conventional mercerization. By increasing in temperature (Table 4, rows 7 to 9), the elongation for both methods enhances (35 % and 49 % for the conventional and ultrasonic assisted processes, respectively). At 60 oC, the elongation for ultrasonic assisted process is 17 % higher than conventional one. For work of rupture in both conditions (Table 1). It would be revealed that there are only significant model terms including C, BC, and A2 for ultrasonic assisted condition

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which can be influenced by tenacity as it is a function of tenacity and elongation. The Model F-value of 4.56 implies that the model is significant. There is only a 2.64 % chance that a Model F-Value this large could occur due to noise. The Lack of Fit F-value of 0.45 reveals the Lack of Fit is not significant relative to the pure error. Work of rupture enhances by increasing in sodium hydroxide concentration up to 20 % for both methods (51 % and 79 % for conventional and ultrasonic assisted respectively (Table 4, rows 1 to 3). At 20 % alkali concentration, it is about 44 % greater than obtained work of rupture for conventional process. Obtained models are depicted in Table 2 (Rows 4, 5). X-ray Diffraction Analysis The X-ray diffraction pattern of unmercerized cotton fabric showed the characteristic peaks at 2θ=14.6 o (101), 16.4 o (101-), and 22.6 o (002), representing the crystalline structure of cellulose I (Figure 1 – Sample 1) [22]. A significant variation in diffraction patterns was observed for all mercerized samples (Figure 1 – Samples 2-4). The formation of the crystal lattice of cellulose II was observable in the mercerized samples due to splitting of the crystalline peaks

Figure 1. X-ray diffraction patterns of cotton fabrics: Sample 1: unmercerized, Sample 2: conventional mercerized, Samples 3: ultrasonic assisted mercerized (similar time but lower NaOH concentration in compare with Sample 2), Sample 4: optimized ultrasonic assisted mercerized by numerical modeling (a=101, b= 101−, c=021, d, e=002 planes).

Ramin Khajavi et al. Table 5. Crystallinity parameters of unmercerized and mercerized cotton fabrics under different conditions (For 002 lattice plane) Crystallinity Crystal size d-spacing index (%) (nm) (nm) Unmercerized 78 7.4 3.96 a Conventional mercerized 67 9.5 4.12 Ultrasonic assisted 65 9.9 4.12 mercerizedb Optimum ultrasonic 64 9.9 4.12 assisted mercerized a,b Similar time but lower NaOH concentration for b sample. Sample

into two weak peaks at 2θ=20.1 o (101-) and 2θ=21.6 o (002) [40,42]. Table 5 represents the crystallinity parameters obtained from the XRD analysis of the samples. As mentioned before, at a specific alkali concentration, the small hydrated ions are formed with the ability to penetrate the internal crystals and disrupt crystalline regions, and thereby reducing the CI value. As it was expected, mercerization of cotton decreased the total contents of the crystallinity phase and the CI decreased from 78 % for the untreated sample to 67 % for the mercerized fabric. CI for samples mercerized in ultrasonic bath decreased up to 64 %. High-frequency ultrasonic waves facilitate penetration of hydrated hydroxide ions to the internal crystals and disruption of crystalline areas lead to decrease in CI value to some extent in compare with conventional mercerization. As a result of crystalline area disruption by alkali solutions during mercerization, new crystalline lattices are formed which cause the transformation of crystal lattices and convert cellulose I to cellulose II. According to previous studies, the structural changes in cotton, e.g. increases in the crystallite length, occurs after mercerization [33]. The findings of the present study were in accordance with the literature (Table 5). Crystal sizes for cotton fibers varied from 7.4 nm to 9.9 nm and they were increased by mercerization. Through using ultrasonic energy, further increase was observed in the size of crystals. However, no significant difference was noted in comparison with the conventional process. Also, ultrasonic duration had no effect on the size of crystals. Inter-planar spacing (d-spacing) between adjacent lattices showed a similar tendency. After mercerizing, the dspacing became larger for the samples treated. There was no difference in d-spacing between the samples treated with the conventional or ultrasonic-assisted mercerization. Barium Activity Number Barium activity number of conventional mercerized samples depends on the concentration (Prob>F=0.0764) of the alkali solution (Table 1). According to the F-value of 3.71, there is a 7.64 % chance that a Model F-value this large could occur due to noise. In the range of investigated concentration of sodium hydroxide, it implies that its



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Table 6. Optimal conditions for set goals (maximum values of tenacity, elongation, barium no., and work of rupture and time, temperature and concentration in range) Processing variables Output (Independent) variables Desirability Time Temp NaOH Tenacity Elongation Barium Work of rupture (min) (oC) conc. (%) (g/den) (mm) no. (CN·cm) Conventional 6 50.00 27.00 1702 65 364 1319 0.762 Ultrasound assisteda 10 50.00 27.00 2247 70 444 1313 0.784 6 45.00 23.00 2098 64 444 1065 0.635 Ultrasound assistedb a Optimized ultrasonic assisted mercerized by numerical modeling and bsimilar to conventional condition with considering time factor and lowering desirability. Mercerizing condition

influence is not statistically significant on mercerizing efficiency. So, no model can be presented for it. Contrary, the conventional mercerizing process efficiency in experienced range is dependent to NaOH concentration. The obtained model for the barium activity number of conventionally treated samples is shown in Table 2 (Row 6). Comparative results for two conventional and ultrasonicassisted processes in their optimum processing values (Table 6) show that there is no significant difference in elongation and work of rupture. However, there is a significant difference in tenacity and barium number activity. Considering longer process duration in ultrasonic assisted process, the values of responses are given at 6 minutes process duration as well (Table 6). At the same time, note that the use of ultrasonic energy in the mercerization process of cotton fabric leads to achieve samples of higher tenacity (≈23 %) at lower concentration of sodium hydroxide solution and process temperature.

Conclusion In this study, an efficient mercerization process was presented through the application of ultrasonic energy on cotton fabrics. However, the response surface methodology (RSM) was used for process parameters modeling and optimization of both the conventional and ultrasonic assisted processes. The ultrasonic-assisted mercerizing process led to a significant improvement in the treatment efficiency. The use of lower amounts of alkali and decreased time can provide the same results. The optimized parameters of the ultrasonic-assisted mercerization process yield higher average tenacity by 23 %, while the degree of mercerization was also higher.

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