Abstract
The development of an eco-friendly anti-felting treatment based on a calcium hypochlorite and hydrogen peroxide (Ca(ClO)2 + H2O2) system, the so-called MEFT (Mori Eco-friendly Treatment) process, is reported in this paper. This treatment is not new in the processing of wool; however, controlling the temperature and time gives rise to further effectiveness of the system, resulting in excellent anti-felting property, fabric handle, color intensity and whiteness. The relationship of the anti-felting and scale structure of Merino and Corriedale wool fabrics treated with the present system is discussed. Although the anti-felting effect was slightly lower than that of the chlorination treatment based on dichloroisocyanuric acid and NARS (improved new method of the Agricultural Research Service) treatment, the excellent handle characteristics of wool fabrics were retained. The anti-felting property, based on various other oxidative and non-oxidative treatments, was compared and the rank was obtained approximately in the following order: chlorination > NARS = MEFT > plasma > Caroat.
Wool is a high-quality fiber material with characteristic properties that are excellent in many respects as compared to synthetic fibers. 1 In particular, the surface property is a potentially important factor that determines the superiority. However, undesirable felt-shrinkage generating by home washing is mainly associated with the surface properties of wool. There have been many shrink-proofing methods developed by applying a variety of surface modification techniques, and shrink-resistant wool materials have also been produced and commercialized widely. However, the production has encountered not only environmental problems, but also deterioration of mechanical and surface properties of wool fibers, such as fabric handle, dyeability and so on.
There are many processes available for imparting shrink resistance to wool, and the procedures can mainly be divided into three categories: (i) resin processes; (ii) Kroy–Hercosett processes 2 of polymer application after chlorination treatments; and (iii) oxidation processes. At present, the majority of the world’s production of machine washable wool is wool treated with resin by applying polyurethane to garments. Although resin treatments are effective, extensional elasticity, softness, fabric handle and other properties characteristic of wool are lost. In Kroy–Hercosett continuous processes for tops, wool is damaged by a lot of chlorine. Strong acid is used in this process, which necessitates a neutralization/anti-chlorination procedure to remove residual chlorine, which causes AOXs (Absorbable Organic Halogens) and pollution of wastewater.
The oxidative treatment system is a chlorine exhaustion method developed by Bereck and Reincke. 3 This is a two-step process comprising Basolan DC, which is a commercial dichloroisocyanuric acid (DCCA) method of BASF Co. in Germany, and subsequent hydrogen peroxide treatments. This is an interesting process performing a full exhaustion of chlorine within the texture of wool fabrics and results in little or no harmful active chlorine in wastewater, since peroxide as the anti-chlor agent acts with chlorine and thus the chlorine used can be significantly reduced and wool damage can also be reduced to give a softer handle by removing scales on the fiber surface. This process can be carried out easily by using a conventional batch-type small-scale chlorination vessel at a lower temperature for a long time, and thus temperature control is sometimes difficult due to a change in climate, which leads to unevenness in processing.
Recently, Cardamone and Yao 4 reported a novel process, the so-called ARS (Agricultural Research Service, USA) method for wool bleaching and shrink-proofing treatment using alkaline H2O2 systems and followed by enzyme treatment at near-room temperature conditions. Since this method was dangerous to fabric handle because of using a strong alkali, the NARS (improved new method of the ARS by Mori) 5 method was adopted as a modified method. The effectiveness of this treatment will be described in the latter section of this paper.
In the present research for a MEFT (Mori Eco-friendly Treatment) process, which involves the reactions of Ca(ClO)2 with H2O2, the most adequate treatment conditions of time, temperature and pH were selected, and anti-felting performance, fabric handle and color intensity were investigated. These properties imparted to the treated fibers and fabrics were compared with the results obtained from various other oxidative and non-oxidative treatment systems. MEFT is a modified method of the ARS method and was developed by using the methods of Bereck and Reincke. 3 Improvement of both environmental pollution and AOX control was aimed at in this method. By using calcium hypochlorite, which is easy to decompose compared to DCCA, the aim is achieved and the yellowing problem brought about by chlorination is also solved. If we compare the amount of chlorine used in the MEFT process with DCCA, it decreases from 3.0% to 2.4%, which means the reduction of 20% of effective chlorine. This value contributes a lot to eco-friendly treatment of wool fabrics.
Experimental details
Materials
Two kinds of fabric samples M and C were prepared. Sample M was 2/2-twill fabric made of 1/40 (250 dtex) Merino wool (100%) yarn in warp and weft, weighing 94 g/m2. Sample C was plain weave made of 1/30 (333 dtex) New Zealand Corriedale wool (100%) yarn in warp and weft, weighing 134 g/m2. A standard clear finishing was done without milling and raising. For the measurement of the breaking strength of yarns, yarns were treated in the yarn state by the same method as fabrics and used for experiment.
Preparation of various kinds of shrink-resistant treatment samples
Chlorination treatment
Fabric samples were dipped in water at 20–25°C with a liquor ratio of 1:20. An amount corresponding to 0.05% non-ionic surfactant was added as a penetrating agent, and then the bath was rotated until the samples were soaked. Then, 3.0% (on the weight of fabric = o.w.f. as active chlorine) of DCCA (dichloroisocyanuric acid) and 5% (o.w.f.) of sodium pyrophosphate (Na4P2O7·10H2O) were added, followed by acetic acid to adjust the pH within the range of 5.5–6.0; the temperature was maintained for 50 min. The solution temperature was then raised to 35°C in 10 min and maintained for 20 min, and the amount corresponding to 6% (o.w.f.) of sodium hydrogen sulfite (NaHSO3) was further added and maintained for 10 min after the temperature was raised to 60°C. The treated fabrics were rinsed and air dried. The chlorination-treated samples thus obtained were subjected to property measurements.
Mori Eco-friendly Treatment
Fabric samples were well soaked as described above. Then, 4% (o.w.f.) Nisso Hyclon (calcium hypochlorite: Ca(ClO)2·3H2O, which contains 70% as active chlorine) was added to the solution and, after standing for 5 min, 12 ml/l of H2O2 (35%) with 2 g/l of sodium pyrophosphate as a peroxide stabilizer and 1 g/l of ethylenediaminetetraacetic acid (EDTA) as a sequestering agent were further added under gentle stirring. The mixed solution (pH = 5.0) thus obtained was maintained at 40°C for 20 min to complete the reaction of hypochlorite ions with peroxide and, subsequently, 6% (o.w.f.) sodium sulfite and 1 ml/l of formic acid (80%) were added to the solution and adjusted to pH = 3.0, and the treatment was further continued at 50°C for 10 min. The hydrogen peroxide species remaining in solution were completely reduced with sulfite ions in the acidic environment. Finally, after a gradual cooling, the samples picked up from the solution were rinsed in water and air dried. The treated samples thus obtained by applying the so-called MEFT were subjected to property measurement.
Caroat treatment
Fabric samples were thoroughly soaked as described above. Then, the amount corresponding to 6.0% (o.w.f.) Caro’s acid (potassium salt of peroxysulfuric acid: KHSO5) was added, and the solution pH was adjusted to 3.5–4.0 with acetic acid and maintained at 35°C for 30 min, before the solution was drained out from the bath. Again, the samples were dipped in water at 20–25°C with liquor ratio 1:20 and 5% (o.w.f.) sodium sulfite was further added; the pH was adjusted to 8.0 by using a 2% solution of sodium carbonate, and the temperature was raised to 60°C, which was maintained for 10 min, then the solution was drained out from the bath and, finally, the treated samples were rinsed in water and then air dried. The KHSO5-treated samples thus obtained were subjected to property measurements. This process is called the Caroat method.
NARS treatment
The treatment was carried out according to the NARS (improved new method of ARS) 5 standard process. Compared to the ARS 4 method, the strength of wool fibers and the anti-felting effect of wool fabrics are improved by this method.
Method of dyeing
Treated and untreated wool fabric samples were dyed by using a dyeing machine, Colorpet 12LMP-E (Nissen Co.). The samples were immersed for 10 min in 0.1% surfactant solution as a penetrating agent with a liquor ratio 1:20. Dye was added into the surfactant solution at the temperature of 50°C and then the temperature was raised to 100°C by 1.5°C/min and maintained for 20 min under a gentle stirring; the dyeing was further continued for 40 min at 100°C in the presence of 2.0% (o.w.f.) acetic acid (99%) for adjusting to pH 4.0–4.5. The samples were thoroughly washed and dried. Dye stuffs used were 1.0% (o.w.f.) Irgalan grey BL (C.I. Acid black 58) and 4.0% (o.w.f.) C.I. Acid blue 185.
Plasma treatment
The plasma reactor used was type DSSO-422 (Daia Sinku Co., Ltd). The discharge frequency was fixed at 13.56 MHz. Plasma treatment was carried out by the following procedure. The fiber sample was placed in the reactor, and evacuated at 0.001 Torr (0.13 Pa), and then argon (Ar) gas was fed into it at a flow rate of 10 ml/min. The electric power for plasma discharge is fixed at 100 W. The internal pressure of the reactor was maintained at 0.1 Torr (13.3 Pa) during the plasma treatment time of 300 s. The treatment samples were finally rinsed in water and air dried, and the plasma-treated samples were subjected to property measurements.
Property measurements
Evaluation of felt shrinkage of wool fibers
The Aachen felt-ball method, IWTO-20-69 (E), was employed for evaluating the felt shrinkage of wool fibers. The testing machine – equipped with a cylindrical vessel, which contains 1 g of wool sample and 50 ml of distilled water – was rotated three-dimensionally at a speed of 150 rpm. After 60 min had passed, the sample was taken out from the vessel, dried on the filter paper in the ambient air for 48 h, and the diameter of the felt ball was measured. The degree of felt shrinkage, S(%), is defined by
Evaluation of fabric handle
In this paper, the Kawabata Evaluation System (KES) system was used for quantitative evaluation of fabric handle. Changes of mechanical properties and fabric handle were inspected by objective evaluation method using the KES for untreated and treated wool fabrics. Tensile, bending, shearing and surface properties were measured for both the direction of warp and weft yarns, and the compressional property was measured for the fabric together with thickness and weight. Fabric size was exactly 20 cm × 20 cm and samples were kept for more than 24 h under the conditions of 20°C and 65% relative humidity (RH).
Observation of surface morphology
A scanning electron microscope (SEM; JSM-670F) was used for observation of the wool surface at direct magnification ranging from 300 to 3000 at 3 kV, 20 A, after coating the surface with evaporated Pt for 7 min. The samples were taken from treated fabrics. Samples treated in the state of yarn were also tested.
Breaking strength and elongation of single yarn
According to the method of JIS-L-1095, the breaking strength of single yarn was measured by using a tensile tester Shimazu AG-10HNIS. The sample length between clamps was 200 mm, and extension speed was 100 mm/min. Fifty samples were tested and the average value for breaking load (N) was measured. Here, a control sample was prepared by immersing the untreated Merino and Corriedale wool fibers in distilled water at 25°C for 30 min, following which they were dehydrated and air dried, and the effect of contraction of yarn in an aqueous medium was removed. Samples were treated in the state or yarn in this experiment.
Color measurement
The color of samples was measured by spectro-colorimetry using a minolta CM-3600D. The power source was D65, the visual angle was 10 degrees and the sample size was 5 cm × 5 cm. The dyeing yield was measured from the reflection ratio at the maximum absorption wavelength and K/S was calculated using the Kubelka–Munk equation. L*, a*, b* on the CIELAB system were obtained for dyed samples and L* was used for the reference value of dyeing yield. Reflectance of treated wool was measured and L* was calculated according to the Kubelka–Munk equation. The whiteness was obtained from the change to the untreated wool fiber.
Hydrophilic property
The hydrophilic property was measured according to the Byreck method of the JIS-L-1096 water-absorbency test. A fabric sample of 10 mm in width and 200 mm in length was dipped in 2% C.I. Acid Red 116 solution, and the absorbed height of water, h (mm), was measured after 10 min.
Water absorbency, H(%), was calculated from the following equation:
Results and discussion
Anti-felting effect of variously treated wool fabrics
Figure 1 shows the comparison of the degree of shrinkage, S(%) defined by Equation (1), for the untreated Merino and Corriedale wool and various shrink-resistant treated wool fabrics. It can be shown that there are no significant differences between the values for the two kinds of wool, but there are considerable differences among the treated fabrics, and the effectiveness of chlorine processes is more prominent than that of the other oxidative and plasma treatments. The shrinkage is under 10% for both Merino and Corriedale wool fabrics in the case of chlorination, NARS (improved new method of ARS), MEFT and plasma treatment. The application of the present MEFT process is clearly shown to be acceptable for effective shrink-proofing. The level is similar to that of chlorination. This suggests that the effect of uniform degradation of the wool surface is due to the reactions of hypochlorite ions and peroxide, as shown in Equation (3):
6
Degree of felting shrinkage, S(%), for various shrink-resistance treatments of wool fabrics. MEFT: Mori Eco-friendly Treatment.
Fabric handle
Differences of mechanical parameters measured by the KES between each treatment were quite small except 2HG5, which is shearing hysteresis at a shear angle of 5 degrees. The calculated primary hand values, such as Koshi, Numeri and Fukurami, were also similar between each treatment. Therefore, 2HG5 (N/m) was adopted as the representative parameter of fabric handle; the results are shown in Figure 2 and discussed here. Compared with the value of untreated wool, all the fabrics treated with any different processes became less recoverable at large shear deformation; in particular, fine Merino wool had a higher value than that of Corriedale. However, there were considerable differences among the treatments, namely, a somewhat higher level of chlorination, which was relatively lower in NARS (improved new method of ARS), MEFT, plasma and Caroat treatments. The value of 2HG5 is sufficiently small with the MEFT compared to that of Caroat and NARS (improved new method of ARS), and it is concluded that the MEFT process is effective in the point of the fabric handle.
Comparison of shearing hysteresis, 2HG5, for untreated and treated fabrics. MEFT: Mori Eco-friendly Treatment.
Observation of surface morphology
Figure 3 shows the surface appearances of the untreated and treated wool fibers. Well-defined contour lines of scale edge can be seen on the surface of the untreated wool fibers. It is shown that no perceptible change is recognized on the surface of plasma-treated fibers. However, descaling and smoothing occurs more or less through oxidative treatments and the surface degradation increases in the following order: chlorination > MEFT > NARS (improved new method of ARS) > Caroat = plasma = untreated. This order is approximately similar order in the degree of anti-felting, that is, chlorination > MEFT = NARS (improved new method of ARS) > plasma > Caroat > untreated. Except for plasma treatment, these relations accord well with many of the theories of felting-shrinkage explained on the basis of the directional frictional effect being responsible for the unidirectional movement of wool fibers during machine washing.
7–10
Plasma treatment may lead to a non-uniform surface modification within the fiber or fiber to fiber, resulting in difficulty evaluating the felting shrinkage.
Surface morphology of untreated and treated Merino wool fibers: (a) untreated; (b) chlorination; (c) Caroat; (d) NARS; (e) Mori Eco-friendly Treatment (MEFT); (f) plasma.
Breaking strength of the treated yarn
The breaking strengths of single wool yarns for untreated control and treated samples are shown in Figure 4. All of the chemical treatments tend to lower the breaking strength of yarn. On the contrary, plasma treatment shows almost no change in strength as compared with that of the control, suggesting that oxidation and etching reactions occurring during plasma irradiation may be limited within or in near-scale layers. However, oxidative treatments have considerable effects on the cortex region, which is the strength-bearing component. Figure 4 shows that the breaking strength is decreased in the following order: untreated control = plasma > MEFT > Caroat > chlorination > NARS (improved new method of ARS). The present MEFT process well preserves mechanical properties under the controlled reaction conditions of the calcium hypochlorite ions and H2O2 followed by complete remove of the peroxide species from the fabrics using the reaction with sulfite ions.
Comparison of breaking strength for untreated and treated fabrics. MEFT: Mori Eco-friendly Treatment.
Hydrophilic property
The result of hydrophilicity is shown in Figure 5. It is noted that there is an approximate parallel relationship between the K/S (Figure 6) value and hydrophilicity, which is particularly evident for Merino wool. An extraordinary high level of the hydrophilicity of chlorination suggests that surface damage is considerable. It is of interest that dyeing depends much more on hydrophilicity than yellowing due to chlorination.
Comparison of hydrophilicity for untreated and treated fabrics. MEFT: Mori Eco-friendly Treatment. Comparison of K/S values for untreated and treated fabrics. MEFT: Mori Eco-friendly Treatment.
Color measurement
Figure 7 shows the whiteness for untreated and treated fabrics. The value of whiteness is significantly improved in NARS (improved new method of ARS) and the same levels in MEFT and Caroat, but is greatly reduced in chlorination. The effect of bleaching by MEFT seems to be due to the controlled reactions of Equation (3).
Comparis on of whiteness values for untreated and treated fabrics. MEFT: Mori Eco-friendly Treatment.
Figure 6 shows the K/S values indexing the color intensity of the dyed sample for untreated and treated fabrics. It is noteworthy that the NARS (improved new method of ARS) and MEFT processes make a remarkable improvement to the dyeability of the fabrics, that is, they result in a double or more increase as compared to that of the untreated sample. It should be further emphasized that, for the present MEFT process, tippy dyeing observed in practical fabrics was also significantly reduced.
Conclusions
By applying the present MEFT process, excellent properties in anti-felting and fabric handle were imparted to wool fabrics without perceptible change in the breaking strength, whiteness and dyeability. Other properties, such as hydrophilicity, descaliness and softness, are comparable with the other processes, such as chlorination, NARS (improved new method of ARS) and Caroat. In the NARS (improved new method of ARS) process, whiteness and dyeability were found to be excellent. The lowest breaking strength and elasticity property of shearing may be due to the presence of sodium hydroxide in the treating solution at relatively high pH values.
Footnotes
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Acknowledgements
The authors thank Dr Kozo Arai, Director of the Keratin Research Association, Wool Research Laboratory, Kiryu City, Gunma Prefecture 376, Japan, for his kind discussion and critical reading of the manuscript.
A part of this paper was presented at the 40th Textile Research Symposium in Kyoto, 2011.