In the last few years, the popularity of denim apparel washing has been increased and many researchers have investigated the effect of the washing for denim apparels. Some important works of various washes on denim apparels are presented below.
Enzymatic Treatment in Denim Apparel Washing
The study of enzymatic washing on denim apparel is important for physical, aesthetical and environmental point of view. Denim apparel manufactures have washed their apparels for many years with chemicals to achieve a soft-hand as well as desirable washing effects. Indigo-dyed denim apparel is the most popular for youth [27]. Therefore, the properties of denim apparels have been widely studied due to its fundamental importance and its many applications in current fashion trends. The existing literature in this domain has focused considerable attention with enzymatic washing for denim apparels. The use of environmentally friendly, nontoxic, fully biodegradable enzymes have been using in the modern textile wet process industries for decades. Enzymes are produced by living organism and one kind of protein that is obtained from fermentations method from naturally existing bacteria and fungi and attack to a specific molecular group. Structurally, enzyme is a biological polymer. Cellulases are enzymes and commonly used in textile industry. According to their amino acid sequences, it consists of either a catalytic domain (CD), or a cellulose-binding domain (CBD) or both domains [28]. Most of the cellulases used in the denim washing are fungal (with a CBD of family I, cellulose-binding domain) [29]. Cavaco- Paulo et al. [30] reported that the cellulases used in the denim washing industry have CBDs from family I (30-36 amino acids, i.e., fungal cellulases from Trichoderma ressei and
Humicola insolens), whereas CBDs of cellumonas fimi bacteria belong to family II (103-146
amino acids). Commercially, there are mainly two kinds of cellulase being used for denim washing, namely acid cellulose and neutral cellulase. Acid cellulases are more aggressive on cotton [31]. Cellulase hydrolyses the cellulose, yielding long chain cellulose polymer to a short-chain polysaccharides and glucose. The enzymatic action also loosens the indigo dye, which is more easily removed by the mechanical abrasion of rotating cylinder washing machine. Cellulases are inducible enzymes synthesized only in the presence of cellulosic materials or other appropriate inducers [32-36]. Today approximately 80% denim apparels are treated with cellulase enzymes [37]. Cavaco-Paulo [38] reported that desizing with amylases was the first applications of enzymes in textile industry [38]. Enzymatic treatment with amylase enzymes has replaced the harsh processes since the beginning of the last century [39]. Many commercial α-amylases are available now and it is estimated that approximately 15% of all commercial textile enzymes are used in desizing processes [40]. In order to prevent the yarn breaking during weaving, warp yarns are sized with starch and its derivatives. The starch is a natural, biodegradable, and a mixture of two polysaccharides, amylase and amylopectin consisting mainly of α-1, 4-linked glucose units [41].
Cavaco-Paulo et al. [30, 42] carried out a series of studies to investigate the washing effects of denim garments by cellulase treatment. From their studies reported that cellulases are most successful in producing the stone-washed look denim apparels with modified appearance. Aged/old looked denim with cellulase is the non-homogeneous removal of dye, giving the fashionable contrast of various blue shades. Cavaco-Paulo [43] reported that cellulases are always applied in washing processes where strong mechanical action on the fabric is provided. As a result, the weight and strength loss increased. Nevell [44] reported that, the primary wall of cotton contains waxes, proteins, lipids, pectins, organic acids and noncellulosic polysaccharides constituting up to 10% of the total fiber weight and by washing the fiber loss weight mostly. The secondary wall contains a mature fiber and consists almost entirely of fibrils of cellulose arranged spirally around the fiber axis [45] and by enzymatic washing the fibrils of cellulose in secondary wall is slightly disoriented and partly damaged and strength is lost and softens apparels are produced.
Cavaco-Paulo et al. [30] explained that the slow kinetics of enzymatic degradation of crystalline cellulose improves fabric and fiber properties (remove fuzz fibers) without excessive damage. Mori et al. [46] showed that cellulase treatment improves the handle of cotton fabric. They found that the primary wall of the cotton fiber is eliminated in the initial step of hydrolysis; as a result a reduction in the fineness of the cotton fibers takes place. They also suggested that enzymatic hydrolysis occurs in the secondary wall of the cotton fibers, even during the initial step of hydrolysis so that cotton fabric becomes soft and loses strength. Also, Walker and Wilson [47], Pedersen et al. [48], Duran and Marcela [5] studied cellulase on cotton and found that cellulase improves fabric hand and enhance aesthetic properties. Similar, many studies of cellulase applications on textiles and the properties of cotton fabrics were reported by Buschle-Diller et al. [49] and Radhakrishnaiah et al. [50]. Heikinheimo et al. [8] reported that cellulases are introduced to replace aggressive chlorine bleach in textile industry.
Pumice Stone in Denim Apparel Washing
The fundamental problem of enzyme in denim wash has received considerable attention from researchers. Such a problem is usually overcome by stone wash. A few but some important studies of the stone washes are given below.
Pumice stone is generally used on the denim apparel to achieve a soft handle as well as a desirable bleached-out character. In denim washing, pumice stones are mixed with enzymatic processes to obtain irregular, nice stone-wash look effects. The surface of pumice stone is rough, irregular, light weight and perforated and floats on water during washing in machine. The use of stone makes brushing action on the apparel surface; as a result irregular color fading effect is produced rapidly. But stone wash causes processing and equipment problems. The main disadvantages of stone washing are the difficulty of removing residual pumice from processed clothing items and the damage to the equipment by the overload of tumbling stones [51]. In spite of these disadvantages, pumice stone is still used in denim washing industries and researchers using certain researches with pumice stones [52]. Pumice stones combined with cellulases cause the desired fading and softening of the apparel [53]. They concluded that mechanical action by pumice stone opens the outermost layers in secondary cell of cellulosic crystals, thus increasing the part of the cellulose accessible to enzymes and enhancing enzymatic removal of the dye in presence of pumice stone. Again, pumice stone with cellulases reduces time in washing process [54].
High levels of mechanical friction with pumice stone will produce strong mechanical abrasion of yarn surfaces, releasing the indigo dye quickly and produced the stone-wash effect [30, 42]. Feki et al. [55] examined the effect of stone-washing on denim garments and evaluated compressibility, bending rigidity, shear rigidity and breaking work, but they did not worked on the other properties like water absorption, elongation at break, tensile strength and color fading.
1.5. Motivation
From the literature review it is clear that very little investigational study have been carried out on the effect of chemicals in denim apparel washing. The study of denim apparel washing with sustainable designs is important for the apparel designers and manufacturers and is the new challenge in the fast changing current trends. The consumer’s has interest now in eco-fashion. To apply a system as an effective wash method for denim with chemicals is important. Thus to produce specific washing effect, considering sustainability, the analysis of the effect of parameters in denim washing is necessary. Previously, majority of the studies on denim apparels were carried out with dry processes. Thus, so far, none have conducted studies involving the effect of chemical wash for producing sustainable denim apparels, although denim is very popular apparel. Therefore, from the buyer’s point of view, consumers are concern now on sustainable denim designs, which forms the basis of the motivation behind the present study.