Establishing an ultrasound-assisted activated peroxide system for efficient and sustainable scouring-bleaching of cotton/spandex fabric

Highlights

• A low-temperature and high-efficient ultrasonic bleaching process was proposed.

• Both peracetic acid and hydroxyl radical played significant roles during bleaching.

• The whiteness was enhanced by 43% after ultrasonic bleaching at 40 °C for 40 min.

• The bleached fabric displayed a marginal mechanical loss owing to the mild condition.

Abstract

This study presents a high-efficient and cost-effective ultrasound-assisted strategy for one-bath one-step scouring and bleaching of cotton/spandex fabric using sodium percarbonate (SPC) and tetraacetylenediamine (TAED) couple. SPC plays both roles of pH regulator and H₂O₂ donor to initiate the peracetic acid (PAA) release from TAED. The significance and interaction effects of operating parameters (TAED concentration, temperature and time) on the WI (Whiteness Index) of fabrics were investigated through a central composite design. The bleaching mechanism was studied by exploring the relationship between WI and PAA and hydroxyl radical (HO·) concentrations. The mechanical and dyeing performances of treated fabrics were also evaluated. Results show that temperature exerted a significant impact on WI followed by TAED concentration and time. The PAA concentration decreased and HO· concentration increased upon the temperature rise. Both PAA and HO· were significant to upgrade WI and ultrasound was effective in enhancing their bleaching efficiency. The fabric treated only with 15 mmol/L TAED and 10 mmol/L SPC at 40 °C for 40 min under ultrasound could achieve a WI of 68.6 (43% higher than greige fabric), which was almost equivalent to that of the fabric treated at 60 °C without ultrasound. This verifies the contribution of ultrasound technology in reducing bleaching temperature for energy-saving purpose. Moreover, the treated fabric displayed less than 5% tensile strength loss, having a marginal impact on the apparel performance. The wettability of fabric was greatly improved leading to a good dyeing performance. Encouraging results demonstrate the high efficiency of the ultrasound-assisted pre-treatment process of cotton/spandex fabric, which contributes to the sustainable production of textiles.

Introduction

Cotton/spandex fabric that integrates the merits of cotton and spandex displaying hygroscopicity, comfortableness, elasticity, etc., has aroused great interest and attention among textile industry and market in recent years [1], [2], [3], [4], [5]. To date, a broad spectrum of textile products such as apparel, hosiery, jeans, etc. are fabricated using cotton/spandex fabrics [3], [6]. Nowadays, a rapid increase in the use of cotton/spandex stretch fabrics for apparel has occurred greatly. However, conventional textile pre-treatment process which is labelled as a high energy consuming and massive effluent discharge process still dominates the textile manufacture, and it hardly meets the sustainability criteria. In addition, most current studies are mainly focused on pure cotton fabric, thus to design a customized pre-treatment process for cotton/spandex fabric is in great demand.

Scouring and bleaching of cotton/spandex fabrics are two essential pre-treatment steps to provide a qualified semi-finished product for the subsequent dyeing, printing and finishing. A majority of impurities such as protein, wax, ash and pectin as well as spinning oil on cotton/spandex [7], [8], [9] can be removed during scouring stage, while natural yellowish impurities like flavones on cotton are decolorized by bleaching using certain oxidants [9], [10]. In view of the analogous treatment requirements, the alkaline scouring and peroxide bleaching procedures can be incorporated into one step for higher efficiency and lower cost. Conventional one-bath scouring and bleaching process is usually carried out at high temperature (>90 °C) in an alkaline medium (c.a. pH 10) for a long time (>60 min) [11]. Such processing condition demands high energy consumption to maintain a high temperature and a plenty of water to neutralize the alkalinity after bleaching. More significantly, the harsh condition inevitably impairs the mechanical property of cotton due to the reduction of polymerization degree of cellulose induced by oxidation [12]. Besides, the urethane linkages (-O-CO-NH-) of spandex are vulnerable to alkaline hydrolysis and hydrogen peroxide (H₂O₂) oxidation at high temperature [13], which is often neglected in most studies. Those defects motivate us to explore a more efficient pre-treatment process for cotton/spandex fabrics under a mild condition.

The addition of peroxide activator into traditional pre-treatment bath has been confirmed to be a facile and effective strategy to achieve a low-temperature and near-neutral short process [14]. Tetraacetylenediamine (TAED) is a widespread peroxide activator showing much less toxic effect to aquatic life compared to other activators such as nonanoyloxybenzenesulphonate (NOBS), N-[4-(triethylammoniomethyl)benzoyl]lactam chlorides (TBLC) [15], [16]. In the presence of perhydroxyl anions (HO₂^–), TAED undergoes perhydrolysis and decomposes into triacetylethylenediamine (TriAED) and diacetylethylenediamine (DAED), and releases peracetic acid (PAA) consecutively [16]. PAA is an eco-safe bleaching alternative and able to function at a lower temperature with a shorter period of time due to its higher redox potential compared to H₂O₂ [17], [18], and its decomposition product hydroxyl radicals (HO·) is also regarded as an efficient bleaching agent [19]. In most previous studies, TAED was usually applied accompanied with H₂O₂ [20], [21]. However, the solid TAED powers and liquid H₂O₂ should be stored separately and used immediately after mixing, making their application inconvenient in the practical textile processing.

Sodium percarbonate (SPC), a nontoxic, cost-effective and biodegradable compound containing sodium carbonate and H₂O₂ parts, has been employed in various products, including laundry detergents, toothpaste, and deodorizer [22], [23]. The solid state of SPC makes it possible to be premixed with TAED and stored for future use. After dissolved in water, SPC can release HO₂^– which is able to initiate the perhydrolysis of TAED to generate PAA in situ. As yet, few studies adopted TAED/SPC couple for the bleaching of textiles. Additionally, it is reported that TAED are converted into acetic acid (AA) after bleaching, resulting in a drastic drop in pH that in turn decelerates or eventually terminates the perhydrolysis of TAED [5], [24]. The sodium carbonate released by SPC can neutralize the pH, thus no additional alkaline is needed. Therefore, it is interesting to explore the bleaching effectiveness of TAED/SPC system on cotton/spandex fabrics.

The mechanical agitation of fabric aiming to enhance its contact with chemical solution, is an unavoidable defect in the conventional pretreatment process, having a negative impact on the fabric mechanical property due to the deformation of fabric and friction between fabric surfaces. Ultrasound technique which provides an efficient none-mechanical mode of action to the fabric [25], has aroused growing interests in cotton processing such as scouring, bleaching, mercerizing, bleaching, dyeing and functionalization recently [11], [26], [27], [28]. Compressions and rarefactions produced during the propagation of ultrasonic waves (known as cavitation) are generally considered responsible for most of physical and chemical effects, which intensifies the diffusion and reaction of chemicals [26], [29]. Furthermore, less power is required to initiate ultrasonic cavitation, making it an energy and time-saving strategy for a sustainable production [26]. Assisted by ultrasonic technique, common oxidants (e.g. H₂O₂) [30], enzymes (e.g. laccase) [31], ozone [32], etc. have been used for cotton bleaching. However, few studies have been carried out to combine ultrasound with activated peroxide system for textile bleaching. With these regards, this work presents an efficient and cost effective one-bath one-step scouring and bleaching strategy for cotton/spandex fabric using TAED/SPC system assisted by ultrasonic technique. PAA and its decomposition products HO· play a major role of bleaching agent in the designed system (Fig. 1). Importantly, the generation of PAA and decomposition of pigments are expected to be promoted by ultrasound through accelerating the mass transfer [26], further reducing the bleaching temperature and shortening the duration of process. In addition, the powerful shock wave caused by the collapse of cavitation bubbles is conducive to remove other impurities such as wax, pectin and protein with less damage for fabric.

The present study investigates the significance and interactive effects of variables (TAED concentration, temperature and time) on hunter whiteness index (WI) of fabrics through the analysis of the variance (ANOVA) of central composite design (CCD) using Minitab 19 Software. The concentrations of PAA and HO· with or without ultrasound at different temperatures as well as their impact on the WI of fabrics were explored to interpret the bleaching mechanism. The scouring effect was detected by the wettability of fabric. The morphology of treated fabric was observed by Scanning Electronic Microscope (SEM). The mechanical property and dyeing performance of the treated fabric were also evaluated.