Ultrasonic scouring as a pretreatment of wool and its application in low-temperature dyeing

Abstract

Ultrasonic technology has shown the potential to reduce the cost and environmental impact of textile wet processing. This work investigates the effects of ultrasonic irradiation as a pretreatment on wool and its application in low-temperature dyeing. A significant increase in dye uptake and color strength was observed on the fabric ultrasonically pretreated at 40 kHz, followed by that at 80 kHz and the conventionally treated sample, in both acid dyeing and reactive dyeing. This could be due to the changes of the fiber surface structure and modification of the chemical structure in the cell membrane complex as a result of ultrasonic pretreatment. In acid dyeing, a 20% increase in dye uptake was achieved at 70℃ upon applying ultrasonic pretreatment at 40 kHz. With the assistance of a leveling agent, 80% dye uptake of the fabric treated with ultrasonics at 40 kHz was measured at 70℃ in reactive dyeing. Ultrasonic pretreatment can be applied in raw wool scouring and fabric scouring to achieve an efficient dye uptake, and these are also discussed in this paper.

Keywords

ultrasonics wool pretreatment low-temperature dyeing scouring

The advantage of using ultrasonic irradiation for supplying mechanical energy in wet textile processing has been reported as far back as the early 1950s.¹ In recent decades, research on the adoption of ultrasonics on wet wool processing, such as scouring^2–9 and dyeing,^10–17 has shown the potential to improve product quality and reduce the use of water, energy and auxiliary chemicals.

Wool scoured with ultrasonics shows few changes to the fiber mechanical properties^5,18 and has less fiber felting, which can result in an improved subsequent top making process and product yield.¹⁹ Studies on wool dyeing demonstrated that with the assistance of ultrasonics, a better fiber dye uptake can be achieved with a variety of dyes in a shorter dyeing time period, at a lowered liquor ratio or at a lower temperature.^10–17 Ultrasound can improve laundering efficiency by achieving good stain removal whilst maintaining the quality of delicate fabric, which may be disrupted by mechanical washing.^20,21 In bleaching, it was shown that ultrasonic pretreatment of wool can increase the effectiveness of the oxidative-reductive process.²²

Acoustic cavitation is considered responsible for most of the ultrasound physical and chemical effects observed in solid/liquid or liquid/liquid systems. Ultrasonic cavitation performs in two ways: micro-jetting caused by cavitation bubble implosion and micro-streaming resulting from cavitation bubble oscillation.^23,24 Cavitation within an ultrasonically irradiated liquid occurs when microscopic bubbles of about 10–100 µm form,²⁵ grow and implode to produce hydraulic pressure of over hundreds of megapascals, fluid jets and shock waves.^24,26 Oscillation of the bubble size during growth produces micro-streaming in the vicinity of the bubble. The thickness of the surface boundary layer, which blocks the access of chemical agents (such as detergent) to the fiber surface, is believed to be reduced by the ultrasonic irradiation,^27,28 and this also assists cleaning processing.

When used in textile dyeing, ultrasonic irradiation is believed to be able to disintegrate agglomerated particles, cause milling, mixing and disintegration of cells, and provide more pathways for dyes to diffuse into the fibers, hence leading to a quick and efficient fiber dye uptake.^29–31

Unlike cotton, silk and synthetic fibers, wool is a keratin fiber with a complex cellular morphology. The fibers have closely packed cortical cells surrounded by single or multiple layers of cuticle cells.³² Wool is composed mainly of the protein ɑ-keratin, which is a crystalline polypeptide with adjacent polypeptide chains crosslinked with disulfide bonds from the amino acid cystine. These crosslinks make keratin relatively hard and insoluble, and hence it is difficult for dye molecules to enter into the fibers.³³ Conventional wool dyeing is carried out at a temperature of around 100℃. Dyeing of wool at close to boiling temperature may cause damages to the physical and chemical structure of fibers, which can result in yellowing and post defects in the subsequent processes. When maintained at temperatures near 100℃ for a period of time, especially in acidic conditions, the structure of fibers is gradually disrupted by the hydrolysis of peptide bonds. These damages can be minimized by reducing the operating time or by reducing the dyeing temperature.

Since the end of last century, much attention has been paid to the investigation of low-temperature dyeing of wool. The processes proposed include introducing specific auxiliary agents,^34–37 pretreatment with enzyme or plasma^38–42 and ultrasound assisted dyeing of wool. Auxiliary agents promote dissolution and dispersion of dyes by combining dye molecules with that of auxiliaries through hydrogen bonding and/or Van der Waals’ forces, which accelerates the entering speed of dyes into fibers. Pretreatment uses physical, chemical and/or biological methods to modify wool structures/properties. Rapid diffusion of dyes can be achieved by increasing functional groups on the fiber surface to promote swelling, or by changing the cell membrane complex (CMC) in the wool structure to provide increased pathways for suitably sized molecules, such as water and dyestuff molecules, to diffuse into fibers. Auxiliary agents and pretreatment of wool can reduce the dyeing temperature to 80–85℃.

One of the reasons that sonication improves the migration of dye molecules from the dye bath to the fiber is through the modification of the fiber surface structure. Recent studies have shown that when scoured with non-ionic detergent and soda ash, the scale structure of wool is subjected to some minor changes, in the form of micro cracks and scale peeling, under ultrasonic irradiation.^18,19 While these fiber surface modifications are dependent on the intensity of the sonication applied, they can result in a quicker fiber dye uptake in the early stage of low-temperature dyeing than that of conventionally scoured wool.¹⁸ In addition to the visual observation of the fiber surface structure obtained by scanning electron microscopy (SEM) images, it was shown that ultrasonically treated wool has a better water absorbency due to the damage of internal waxy lipids within the fibers, as measured by Fourier transform infrared spectroscopy (FTIR).⁴³ Waxy lipids reside in the CMC that hold together the cortical cells, extending throughout the whole fiber. It is expected that disruption of lipids as a result of ultrasonic treatment can make an additional contribution to fiber swelling as well as quick diffusion of dyes.

Most research in the area of ultrasonic dyeing of wool has focused on applying sonication directly in the dyeing process. Little has been reported on the investigation into the effect of ultrasonic pretreatment of wool on the low-temperature dyeing. Scouring is a process prior to dyeing. Because fiber surface modification has already been completed during ultrasonic scouring (pretreatment), the kinetics of dyestuff diffusion within the dye bath is expected to be more effective in the subsequent dyeing than that treated with the conventional process.

This study extends previous work by investigating cavitation intensity and its effect on the fiber surface disruption. Ultrasonic frequencies of 40 and 80 kHz were used to investigate the effects of the modification of wool surface structure and the subsequent changes in fiber dyeing ability. Variations such as temperature, treatment time and pH were considered in both scouring and dyeing processes. Commercially available acid dye and reactive dye (with and without a leveling agent) were used in this study to measure any changes to the dyeing ability of wool under sonic irradiation at various cavitation intensities. The applications of the ultrasonic pretreatment process were discussed.

The outcome of this study is expected to provide implications to the wool wet processing, especially to the wool scouring and dyeing industry, where the ultrasonic technology is replacing the traditional process to enable a green and friendly working environment.

Materials and experimental details

Materials

The materials used in this work are listed in Table 1.

Table 1.

Materials used in this work

Material		Supplier
Fabric	Pure-wool fabric	CSIRO, Australia
Dyes	Acid brilliant green 3GM	Anpai Import & Export Co., Ltd, China.
	Reactive blue19	Maclean Biochemical Technology Co., Ltd, China
Leveling agent	Fatty Amine Ethoxylates	Jiuling Interfacal Agent Co., Ltd, China
Auxiliaries	sodium carbonate, sodium sulfate, Sulfuric acid, acetic acid, sodium acetate	Taixin Chemical Co., Ltd, China

Experimental details

Fabric sample preparation

Loomstate, undyed, 2/1 twill pure-wool fabric samples taken from ordinary mill production (supplied by CSIRO, Australia), with fabric weight of 157 g/m² and ends and picks of 33 × 27/cm, were prepared for investigation. Before treatment, wool fabric samples were wet-relaxed in water at room temperature for 24 hours and then dried in oven at 105℃ for 1 hour.

Scouring of fabric

Scouring was carried out in aqueous detergent solutions containing 0.5 g/l non-ionic surfactant (Jiuling Interfacal Agent Co., Ltd, China) and 0.2 g/l sodium carbonate at 50℃ for 15 and 30 min separately. Wool samples were conditioned at 20 ± 2℃ and 65 ± 2% relative humidity for 24 hours and weighed before addition to the scouring bath. A scouring bath of liquor-to-goods ratio of 100:1 and both ultrasonic irradiation and a conventional heating bath (HH-S Electrical water bath pot, Jinyi Instrument Technology Co., Ltd, China) were applied. Scouring was carried out at a low temperature to minimize the possibility of a permanent set being introduced into the fabric. After completion of scouring, the wool fabric samples were washed twice with tap water and then dried in oven at 105℃ for 1 hour.

Ultrasonic equipment

Sweeping digital control ultrasonic cleaning units of 40/60 and 80/100 kHz frequency (Haoshun Ultrasonic Instrument Co., Ltd, Shenzhen, China) were used for ultrasonic scouring. Both ultrasonic baths have the capacity of 20 l, with a maximum power of 130 W/l, and can be adjusted from 40% to 100% power. Both have adjustable temperature control between 20℃ and 90℃ and are capable of frequency sweeping around the working frequency. Temperature control and frequency sweeping were turned off for all experiments.

For bath degassing, ultrasonic energy was applied for at least 10 min in each bath prior to use in order to avoid the cushion effect caused by excess dissolved gas in the treatment liquor, which may reduce the cleaning efficiency.

Fabrics were placed horizontally in the bath around 5 cm from the bottom in each measurement. Samples were clamped unstrained to minimize the possibility of permanent set being introduced into the fabrics. All the experiments were carried out at room temperature of 20℃. Temperatures in the baths increased gradually from ambient to 42–45℃ (depending on the ultrasonic frequency applied) within 30 min, as a result of the increasing thermal energy produced by the micro-jetting resulting from ultrasonic cavitation bubble implosion.^23,24 The temperature in the conventional heating bath was set from ambient to 45℃ within 30 min, accordingly.

Fiber surface analysis

Fiber surface morphology was examined using a scanning electron microscope (Model JSM-6100, JEOL Corporation, China). Measurements were conducted with an Extra High Tension (EHT) of 5 kV. All the fabric samples were coated with gold before SEM testing to prevent from charging.

Conventional dyeing

Fabric was conditioned in an environment at 20 ± 2℃ and 65 ± 2% relative humidity for 24 hours, and then cut into samples of size 6 cm × 9 cm (approximately 1 g). Dyeing was conducted with three samples (3 g) in each beaker in an HH-S Electrical water bath pot (Jinyi Instrument Technology Co., Ltd, China) to allow samples of the dyeing liquor to be collected for the measurement of dye exhaustion. Aluminum foil was used to cover the beakers to avoid liquor evaporating. The dyeing recipes shown in Table 2 are commercially recommended. For comparison purposes, reactive dyeing with and without 1% leveling agent were examined.

Table 2.

Fabric dyeing recipes

Method	Acid dyeing	Reactive dyeing
Recipe (o.w.f.)	1% C. I. Acid brilliant green 3GM 10% sodium sulfate 4% sulfuric acid	5% C. I. Reactive blue19 1% acetic acid 1.5% sodium acetate (1% leveling agent AN)

Fabrics were equilibrated for 15 min at 45℃ at a liquor-to-goods ratio of 50:1 bath containing 10% sodium sulfate and 4% sulfuric acid for acid dyeing, and 1.5% sodium acetate and 1% acetic acid for reactive dyeing. The pre-dissolved dye was then added to the dye liquors and the pH adjusted to 4.5–4.8. The temperature was held at 45℃ for five min before increasing at 0.5℃ per minute to a maximum of 95℃ and maintained at 95℃ for 30 min. The dyeing time was recorded from the moment the dye was added to the dye bath. After a predetermined dyeing time, the fabric samples were immediately removed from the beakers, rinsed twice in water at 60℃ and then dried at 105℃ for 1 hour. The time–temperature regime is shown in Figure 1.

Figure 1.

The time–temperature regime of the dyeing process.

Low-temperature dyeing

A HWX-12 Infrared universal color dyeing machine (Shaoxing Yuan More Electromechanical Devices Corporation Co, Ltd, China) was using for fabric low-temperature dyeing. Wool samples of the size described in the Conventional dyeing section were prepared after being conditioned in an environment at 20 ± 2℃ and 65 ± 2% relative humidity for 24 hours. Distilled water and a liquor-to-goods ratio of 50:1 were used. The temperature was increased at 1℃ per minute to a maximum of 50℃ and maintained for 40 min, or to a maximum of 70℃ and maintained for 30 min. Both acid dyeing and reactive dyeing were carried out using the same recipe shown in Table 1 and a pH of 4.5–4.8 adjusted.

Measurement of dye bath exhaustion

Dye liquors were measured for color absorbance using a UV-5900PC spectrophotometer (METASH) before dyeing and during dyeing (liquor samples were taken out and measured every 10 min). Absorbance was recorded at 630 nm for acid dye and 589 nm for reactive dye. The exhaustion was calculated using Equation (1)

DE (%) = \frac{Ab - Aa}{Ab} \times 100

(1)

where DE is dye bath exhaustion expressed as a percentage, Ab is absorbance of the dye bath before the dyeing and Aa is absorbance of the dye bath after dyeing.

Measurement of color strength

A Datacolor650 spectrophotometer (SGS Incorporated, USA) was used to measure the color strength of dyed samples, expressed as the K/S value calculated using the Kubelka–Munk equation given in Equation (2). At least 10 measurements were carried out for each set of samples and the results were averaged

K / S = \frac{(1 - R)^{2}}{2 R}

(2)

where K is the adsorption coefficient, S is the scattering coefficient and R is the reflectance at the maximum wavelength.⁴⁴

Results and discussion

Fiber surface modification

In this work, fabric samples were placed horizontally in the bath around 5 cm from the bottom (transducers) in each treatment in order to receive homogeneous irradiation and a more efficient cavitation effect.⁴⁵ SEM images of fabric/fiber surface morphologies are shown in Figure 2.

Figure 2.

Scanning electron microscopy images of wool after scouring: (a) an enlarged overview image of the fabric after ultrasonic scouring; (b) scoured with the conventional method (control); and (c)–(e) scoured with ultrasonic irradiation.

It is seen from Figure 2(a) that fibers maintained their regular alignment in the fabric after ultrasonic scouring. For the conventionally scoured wool, a small number of scale flakes are seen attached on the fiber surface (Figure 2(b)). These scale flakes could be generated during the spinning and weaving processes, where fibers had been subject to some mechanical movement, such as stretching and abrasion. In addition to scale flakes, fibers with ultrasonic pretreatment show various cuticle disruptions, seen in Figures 2(c)–(e). Figure 2(c) sees scales starting to peel along the fibers by opening up the scale angles, which is Consistent with those observed in a previous study.¹⁸ Cuticle cracking (Figures 2(d) and (e)) is common for fibers treated under ultrasonic irradiation, especially in the alkaline condition. It is believed that scale cracking happened mostly in the area where the fibers received strong ultrasonic irradiation. Upon strong cavitation implosion, cracking not only occurred along the weak line of the edges of the scale (Figure 2(d)), but also appeared on the cuticle surface at a random basis, as pointed out by the arrows shown in Figure 2(e).

It was noticed in this work that the fiber surface damages in the form of peeling and cracking shown in Figures 2(c)–(e) were found in all the sonically scoured samples at both 40 and 80 kHz applied in this work; however, the ultrasonically irradiated samples at a lower frequency (40 kHz) and with a relatively longer time (30 min) tend to have more surface disruptions than those treated at a higher frequency (80 kHz) and with a shorter time period (15 min).

Fabric dye uptake

Acid dyeing

Fabric samples treated with conventional (control) and ultrasonic methods (40 and 80 kHz) for different time periods (15 and 30 min) were examined for dye uptake. The selected treatment time (15 and 30 min) was based on the actual scouring process where the length of sonic irradiation is applicable.

Figure 3 shows the results of fabric dye uptake rate, expressed as dye bath exhaustion. It can be seen that for a treatment time of 15 min (Figure 3(a)), and at the beginning of the dyeing stage, the dye exhausted more rapidly for the ultrasonically treated sample at 40 kHz than for that treated at 80 kHz and the control sample. A dyeing time of around 45 min, with the corresponding temperature of 60–65℃ (Figure 1), was recorded for the control sample to achieve the same exhaustion rate as the ultrasonically treated samples. The three testing samples performed almost the same, within experimental error, as the temperature increased further to the equilibrium exhaustion at the boil. At a prolonged treatment time of 30 min (Figure 3(b)), the delay in dye exhaustion for the control sample becomes relatively even through the dyeing process, with a 20% increase in dye uptake observed from the control sample to the ultrasonic treated at 40 kHz at the dyeing time of 50 min (corresponding temperature of 70℃ in Figure 1), and a dyeing time of around 70 min (80℃) observed as a point for the three samples to reach the equilibrium exhaustion, within experimental error.

Figure 3.

Dye bath exhaustion of acid dyeing. Wool fabric samples pretreated for (a) 15 min and (b) 30 min, with the conventional method (control) and ultrasonic irradiation (40 and 80 kHz).

It is inferred from Figure 3(a) that fabrics pretreated for 15 min have less difference in color strength when the dyeing temperature is set at around 60℃ or higher. Figure 4 presents the color strength of fabric samples (pretreated for 30 min), expressed as K/S values, as a result of low-temperature dyeing at 50℃ for 40 min and 70℃ for 30 min. Consistent with the results of fabric dye uptake shown in Figure 3(b), it is seen in Figure 4 that the fabric ultrasonically treated at 40 kHz shows the best color strength, followed by the sample treated at 80 kHz and the control sample. Also proved in Figure 4 is that thermal energy (temperature) plays a significant role in increasing the diffusion of dye, with fabrics dyed at 70℃ for 30 min showing much higher color strengths than their counterparts dyed at 50℃ for 40 min.

Figure 4.

Color strength of fabric samples (K/S) pretreated for 30 min, dyed with acid dyeing at 50℃ for 40 min and 70℃ for 30 min.

Reactive dyeing

Shown in Figure 5 are the dye exhaustion curves of reactive dyeing for fabric samples that received different treatments. It can be seen that when using a leveling agent as a dyeing assistant (method a), the dye exhausted much more rapidly and the equilibrium exhaustion at the boil was much higher than when the dye was applied without the leveling agent (method b). It is a well-known feature of the leveling agent to have the ability to promote level exhaustion of dye at a low temperature. Similar results have been published previously.⁴⁶

Figure 5.

Dye bath exhaustion of reactive dyeing: (a) with leveling agent; and (b) without leveling agent. Wool fabric samples pretreated with the conventional method (control) and ultrasonic irradiation (40 and 80 kHz).

While the uptake of dye was more rapid than in the absence of the leveling agent, both methods a and b saw the highest dye exhaustion for the samples pretreated with ultrasonics at 40 kHz, followed by those treated at 80 kHz. It is important to notice that with the assistance of the leveling agent (method a), at 50 min (corresponding temperature of around 70℃), the dye uptake of the sample treated with ultrasonics at 40 kHz has already reached 80%, which is close to the equilibrium exhaustion at the boil of around 87%. For dyeing without the leveling agent (method b), the difference in dye uptake of the samples tends to be more evenly placed throughout the dyeing process, with the sample that received the conventional treatment (control) exhibiting the least efficient dyeing ability.

Results of the color strength of fabric samples dyed with reactive dyeing (with a leveling agent), shown in Figure 6, are very similar to that of acid dyeing shown in Figure 4. A significant increase in K/S value is seen on the fabric samples ultrasonically treated at 40 kHz, when compared with the control samples.

Figure 6.

Color strength of fabric samples (K/S) pretreated for 30 min, dyed with reactive dyeing (with a leveling agent) at 50℃ for 40 min and 70℃ for 30 min.

Ultrasound induced rapid dye uptake of wool and its industrial applications

Ultrasonic irradiation creates intensive cavitation bubble implosions, resulting in local hot spots of 4000–5000℃.⁴⁷ Modification of the fiber surface structure, in the form of fiber cuticle cracking and scale peeling (Figures 2(c)–(e)), had completed in the ultrasonic pretreatment process. The fiber surface damages therefore easily provided increased migration paths for dye to diffuse into the fibers in the subsequent dyeing process. This effect becomes more evident especially at the low-temperature dyeing stage (prior to 70℃⁴⁸), and for the samples treated at a lower frequency where the fiber surface disruptions were found to be more severe (as 40 kHz applied in this work).

On the other hand, the CMC determines the process of dye uptake when dye molecules first enter the wool fiber.⁴⁹ During dyeing process, the improved pathway resulting from the removal of fiber internal lipids⁴³ makes both water and dyes diffuse with ease into the swollen regions of the CMC. This also contributes to the increase in dye uptake for ultrasonically pretreated wool, as observed in both acid dyeing and reactive dyeing at a low temperature in this work.

There are two steps in the wool processing pipeline in which the ultrasonic pretreatment could be applied.

The first is raw wool scouring, determined to remove contaminants such as wool grease, suint and dirt before wool can be further processed into yarns and fabrics. It has been shown in various studies^2–9 that by adapting ultrasonics, a reduction in the usage of water, detergent and heating is possible in making scouring of wool a cleaner and greener process. Also by adapting ultrasonics, an improvement in processing performance and in product quality can be achieved.^5,7,19 An industrial trial on ultrasonic wool scouring has been successfully completed in the recent years.⁷

Fabric scouring, as a step before dyeing, seems to have received little attention in applying ultrasonics when compared with raw fleece scouring. Fabric scouring normally uses a detergent and a pH of about 9 to enable cleaning of spinning lubricants, oil stains, dirt and other contamination that fibers pick up during processing.³³ The multiple cycles of impregnation and removal of scouring and rinse liquors can be employed with the presence of ultrasonics. It is expected that when fabrics pass open width with ultrasonics in the baths at each step, in addition to the modification of the fiber surface structure, a cleaner fabric with a shorter scouring time can be achieved.

Conclusions

This study investigated fabric dyeing abilities at low temperatures in both acid dyeing and reactive dyeing. In both dyeing methods used in this work, a significant increase in dye uptake and fabric color strength was seen on the fabric with ultrasonic pretreatment at 40 kHz, followed by that at 80 kHz and the control sample. This could be due to the changes of fiber surface structure and modification of the chemical structure in the CMC as a result of ultrasonic pretreatment. In acid dyeing, a 20% increase in dye uptake was observed at the dyeing time of 50 min (corresponding temperature of 70℃) from the control sample to the fabric ultrasonically pretreated at 40 kHz for 30 min. With the assistance of a leveling agent, 80% dye uptake of the fabric treated with ultrasonics at 40 kHz was measured at 70℃ in reactive dyeing.

It is proved in this work that ultrasonic pretreatment enables a completion of modification of the fiber structure (both mechanical and chemical) prior to dyeing, and hence helps to improve the fiber dye uptake in the subsequent dyeing process, especially at the low-temperature dyeing stage. Ultrasonics can be applied in the industrial raw wool scouring and fabric scouring in order to achieve a greener and more effective wool process.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (Project ID: XDJK2017D041) and the China Southwest University Grant Scheme (Project ID: SWU116040).

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