Sand Blasting

Sand blasting is a mechanical process in which localised abrasion or colour change on the denim garment is created. The process involves blasting an abrasive material in granular, powdered form at a very high speed and pressure through a nozzle, on certain areas of the garment such as knees and elbows. The treated surface shows distressed/abraded/used look.

Acid Washing

Pumice stones are first pre-soaked in a solution of a strong oxidising agent (either sodium hypochlorite (5–10%) or potassium permanganate (3–6%)) and are then applied to the garments by means of dry tumbling. This results in a localised washing effect with clear blue/white contrast. This technique is also termed ‘acid washing’, ‘snow washing’ or ‘ice washing’.

Enzyme Washing

In enzyme washing, cellulase enzymes are used. Hydrolysis of the cellulose, which is catalysed by cellulase, causes the surface fibres to become weakened and later they get removed when there is either fabric-to-fabric abrasion or fabric-to-stone abrasion during washing. The temperature and the pH used are specific to the type of cellulase employed. Usually neutral cellulases are applied at pH 6–7, while acid cellulases are applied at pH 4.5–5.5. However, the latter result in a greater extent of back staining, being more penetrative. An enzyme dose of 2–4 g/L is normally enough, provided that the enzyme activity is not impaired. In general, the colour of the enzyme washed goods is more uniform, particularly when stone is not added. Because cellulases are reactive only on cellulose, any size materials or other impurities must be removed before the cellulase treatment.

Stone Washing

In this Process of denim washing volcanic rocks or pumice stones are added during washing as abradants. The color fading is more apparent but less uniform. The degree of color fading depends on the washing time (30–120 min), stone ratio (weight of stones relative to weight of the garment) (0.5: 1–3:1), size of stones (diameter: 1–7 cm), liquor ratio (∼10:1) and garment load. The world’s major pumice stone supplying countries include the USA, Turkey, Italy, Germany, Iceland, New Zealand, Japan, Indonesia and Philippines.

Advantages and disadvantages of different physical and chemical methods used for discoloration of denim

Denim garments are elegant fashionable textile products with very high level of market all over the World. It has been well-known that unfinished denim textiles are uncomfortable to wear due to their heavy and rigid structures, therefore, it is necessary to achieve a soft handle, an enhanced comfort as well as a desirable worn-out look by finishing. Great efforts have been hailed to explore different chemical and physical finishing procedures for discoloration and improving the handle of denim. They include oxidizing-bleaching treatment, stonewashing, electrochemical reduction, bio-washing, plasma treatment, laser irradiation and combination of nanoparticles with enzymes. Oxidizing agents and stone washing with pumice were first introduced in garment industry for aged-look denim finishing. However, several negative impacts were observed on finished products and washing machines by these two methods. To overcome their drawbacks, electrochemical reduction technique has been later adopted to bleach denim fabrics by in-situ producing reactive radical scavengers, but it has not yet been a simple scale-up procedure. Enzymatic treatment of textiles has also been demonstrated as an environmentally friendly method for textile finishing. In this regard, great efforts have been devoted for denim finishing by pure laccase, mixture of laccase/cellulase, pure acid cellulase, pure neutral cellulase, mixture of amylase/cellulase/laccase, mixture of cellulase/methacrylate copolymer and mixture of liquid ammonia/cellulase. It is worth mentioning that plasma and laser etching methods are recently introduced as environmentally friendly physical approaches to reduce the chemical agents and water consumption usage in textile industry.

Several research groups have found that stone-washing technique can be replaced by low temperature plasmas of O², air, argon and helium gases for removing the oxidized indigo from denim surface. Laser has also been known as another environmentally friendly approach for discoloration of denim in garment industry. However, considerations should be undertaken to control laser procedure parameters in order to avoid damaging in denim structure and preventing reduction in the tensile strength. The advantages and disadvantages of different methods are described in table below:

Methods	Advantages	Disadvantages
Oxidizing-Bleaching Treatment	Strong Discoloration power Simple Application	Relatively high cost Possibility to reduce mechanical properties Yellowing effect Not environmentally friendly Toxic chemical usage Corrosion of equipment Unpleasant odor in working environment
Stone Washing	Desired vintage effect Desired Softness Desired worn-out look	Corrosion of equipment Time consumed stone dust removing Possible to damage denim Not environment friendly
Electrochemical Reduction	Versatility and high-energy efficiency Environmentally friendly	Not available in scale-up Relatively high cost
Bio-washing	Mild processing conditions Environment friendly Low risk in denim damage Recyclability of enzymes	Necessity of enzyme neutralization Chances of back staining in enzyme treatment Inability to create different designs
Plasma Treatment	Environment friendly Fast discoloration No solid waste and air pollution Dry operation No negative effect on bulk properties	Necessity to have operational skills Necessity to optimize process parameters for effective discoloration Technical challenges in scale-up Yellowing effect
Laser Irradiation	Environment friendly Fast discoloration Dar operation Repeatability of designs	Necessity to control laser parameters Necessity to have operational skills Yellowing effect Possibility to reduce the mechanical properties
Combination of Nana-particles with enzymes	Additional functionality on denim depending on nanoparticle type No negative impact on mechanical properties	Yellowing effect

 

Bleach Washing

Bleach washing is normally carried out with a strong oxidative bleaching agent such as sodium hypochlorite (NaOCl) or potassium permanganate (KMnO4). Bleach washing may be carried out with or without the addition of stone. The bleach washing effect and discoloration usually depend on strength of the bleach liquor, liquor quantity, temperature and treatment time. The bleached fabric materials should be properly antichlored or after washed with peroxide to reduce the subsequent yellowing or tendering of the bleached denim fabric.

Regular Washing/Rinse Washing

Rinse washing is the simplest and most commonly used washing method for denim garments. The degree of color fading using regular washing is comparatively slight, but it provides uniformity, depending on whether it is deeply dyed classic denim or only moderately dyed with poor penetration.

Classification of Denim Washing Techniques

General finishing sequence of denim

There are almost countless variations of processing techniques used by designers and textile chemists to achieve fashionable looks that are distinctive and desirable. The number of variations is very large and the evolution of chemical and mechanical techniques is continuing. There often are some secret and proprietary methods. Regardless of the specific look and name chosen, the following are the process steps normally used to attain the desired results.

What is Denim Washing?

The process of giving denim special washed/vintage looks and color effects after stitching it into jeans or other garments. Over the past few decades, different denim washing techniques have been developed and used on different materials to create a large variety of designs for trendy denim garments and jeans. Special color effects and washed/vintage looks are often achieved in denim garments. The hand feel of the washed goods is relatively superior, which makes them suitable for leisure wear. These effects are difficult to achieve through other processing techniques. The results obtained from denim washing represent a combined effect of color dissolution, destruction of the dye and mechanical abrasion, which sometimes causes the removal of surface fibers from the materials. Thus, surface dyed colors in denim garments are more easily washed down during the washing processes.

According to textile terms and definition, denim is defined as ‘Traditionally a 3/1 warp faced twill fabric made from yarn dyed warp and undyed weft yarn. Typical construction of the fabric is 32×19; 45× 54tex; 310g/m². More recently, other weaves have been used with lighter constructions. Jeans are defined as ‘A 2/1 or 3/1 warp faced twill fabric used chiefly for overalls or casual wear with a typical construction of 35× 24; 32×21tex cotton’. Although, denim and jeans refer to different things, they now mean the same thing in the market. Conventionally, warp yarn in denim fabric is dyed with indigo with a ring dyed effect.

Evolution of denim garment washing

Denim garments in the market are originally stiff and uncomfortable when first purchased because of the finishing system used for denim fabrics. After weaving, the heavily sized fabric is subjected to desizing and compressive shrinkage treatments. After the treatments, the softness of the denim fabric is seriously affected. In the past, many consumers used to take a newly purchased pair of jeans home and soften it by washing once or several times before the first wearing.

In the earliest evolution, the garments were laundered (prewashing) by the manufacturer before selling. These ‘prewashed’ denim garments had a slightly faded appearance and a softer hand that felt comfortable. These prewashed garments generated a trend of fashion and consumers were willing to pay the extra cost involved in this additional processing. As the popularity of prewashed garments grew, the idea of using abrasive stones to accelerate the color fading process was developed and ‘stone washing’ became the second step in the evolution. Pumice stones were included in the washing process or tumbled with the damp garments to wear down the stiffest portions, for example, belt areas, cuffs and pockets. The third development was the use of chlorine (e.g. sodium hypochlorite as a bleaching agent) in the washing process. A new and color lightened blue denim garments category was the result. With the use of chlorine bleaching, in 1987–1989, ‘ice washing’ was developed, in which the pumice stones were first presoaked in the bleaching agent and then tumbled with dry or slightly damp garments.

In the industry, ‘ice washing’ has alternative names such as ‘acid wash’, ‘snow wash’, ‘white wash’ and frosting, etc. The term ‘acid wash’ is a misleading term because mineral acids are not used for this process. Other than bleaching agent, the use of enzyme (cellulase) treatment to obtain the color fading effect like stone washing effect in denim garments has attracted considerable interest over the past several years. The primary attraction is to reduce or eliminate the need for stones or to reduce the time needed to obtain the desired abrasion effect. Moreover, using pumice stones to abrade denim garments is destructive to equipment. In addition, the pumice stones, after washing, get entrapped in pockets of denim garments which must be removed by hand, leading to increased labor and production cost. Also, stone particles and grit play havoc in the effluent. For these reasons, the use of cellulase was promoted with the promise of eliminating stone as the abrasive agent for achieving the ‘stone wash’ look.

However, because of the increased time and other considerations, the trend today is to use combinations of stone and cellulase to achieve the worn and faded look in denim garments. With the increasing awareness about and concern for environmental issues, such as large amounts of effluents produced and high consumption of water and energy, wet processes related to denim washing are considered as not environmentally friendly. To address the environmental concerns, dry finishing techniques such as plasma treatments have been introduced as an alternative to the conventional wet processing.

Washing as final process of denim garments

Washing can be considered as the final process in denim production and is the core of denim finishing. The washing of denim is directly related to the aesthetic, quality and value of denim garments. In processing, sizing and coloration form the base of color in denim garments. However, the washing process is the key to create the style in denim garments which is now becoming an art of creating fashion trends. The three-dimensional (3D) effect and worn look can be achieved through different types of finishing and washing processes. Under the influence of different chemicals, washing conditions and washing equipment used, different final effects can be achieved in denim garments. As a result, the washing of denim and jeans is aimed at:

• Preshrinking for good dimensional stability during selling and use.
• Removing sizing agent and unfixed dyes to remove contaminants added during the manufacturing process to generate ‘clean’ denim garments. Also, the washing can increase the surface luster and lightness of the fabric.
• Improving the hand feel through various finishing processes such as softening, stiffening or polishing to enhance the comfort of denim garments.
• Improving aesthetic properties through fading, bleaching or tinting processes. After these treatments, cloudy, frosted, wrinkle, grinded or peach skin effects are achieved which finally affect aesthetic properties.
• Improving the functional properties such as wrinkle free, anti-soil, water repellence, oil repellence or antistatic, etc.
• Improving the quality in cases of poor color yield, dimensional stability, color fastness or improper surface treatment.

Indigo Properties

Indigo, also known as indigo blue and indigotin (C.I. Vat Blue 1, C I 7300, CAS number: 82/582-89-3, IUPAC name 3H-indol-3-1, 2-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro-, chemical formula C16H10N2O2), is present at ambient temperature and normal pressure as dark blue-violet needles or prisms with a distinct coppery luster, melting point 300°C. Indigo absorbs light in the orange part of the spectrum (λmax=613nm). The compound owes its deep color to the conjugation of the double bonds. Indigo is insoluble in water and poorly soluble in most of the common solvents. It is more soluble in polar organic solvents than non-polar ones. The poor solubility is most likely due to the strong inter- and intramolecular hydrogen bonds that are formed in indigo crystals. The hydrogen bonding also explains indigo’s relatively high melting point (∼300°C).

The color of indigo is dependent on its environment. In the gas phase, where indigo is in its monomeric form, it is red, and in non-polar solvents it is violet, but in solid form and in polar solvents as well as when it is applied to textiles as a vat dye, it is blue. Indigo is non-biodegradable, has a low mammalian toxicity and there is no indication of sensitization in humans after repeated skin applications. Indigo is classified as a vat dye, although its properties are not typical of the vat dyes.

Indigo has moderate to very high light fastness depending on the substrate it is on or whether it is a pigment or a dye. The light mostly affects the oxidative degradation of indigo to the degradation products such as isatin, isatoic anhydride and anthranilic acid. There are synthetic dyes, especially vat dyes, with better fastness properties particularly to light, washing and chlorine bleaching, than indigo, but it is this fading of color that is so characteristic of indigo that has kept it so popular with jeans-wearing people.

Microbial Production of Indigo

While synthetic indigo has enjoyed a virtual monopoly for nearly a century, an environmentally friendly microbial production of indigo is under development. The microbial production of indigo has been known since the 1920s. Indigo production with hydrocarbon degrading bacteria expressing mono-oxygenases or dioxygenases has also been investigated in search of a possible alternative for the chemical synthesis of indigo. In 2002 Berry et al. and his companions developed a fermentation process where indigo was produced from glucose with recombinant Escherichia coli that had been modified with Pseudomonas putida genes. However, the method produced also indirubin, which gave an undesirable red hue to the dyeing.

Several bacteria, most notably Pseudomonas species, can use a variety of organic compounds such as naphthalene, toluene, xylene and phenol as their sole carbon source. In many instances, the genes encoding the enzymes for the degradation of these organic compounds are located on large, naturally occurring plasmids. For example, pseudomonads that contain NAH7 plasmid could grow on naphthalene as a sole carbon source. The clone bank was then introduced into E. coli cells. During the characterization of one of the transformants that could convert naphthalene to salicylic acid, it was observed that when the growth medium contained tryptophan, it turned blue. A thorough analysis of the blue color revealed that the transformed E. coli cells were synthesizing the dye indigo. This synthesis is achieved in four steps:

• Conversion of tryptophan in the growth medium to indole by the enzyme tryptophanase, which is produced by the E. coli host cell.
• Oxidation of indole to cis-indole-2,3-dihydrodiol by naphthalene dioxygenase, which is encoded by the DNA that was cloned from NAH7 plasmid.
• Spontaneous elimination of water.
• Air oxidation to form indigo.

In addition, introduction of the gene for enzyme xylene oxidase, which is encoded in the TOL plasmid, can convert tryptophan to indoxyl, which then spontaneously oxidizes to indigo. In pathway A, the naphthalene dioxygenase is derived from the NAH plasmid. In pathway B, the xylene oxidase is from the TOL plasmid. E. coli transformants that synthesize indigo contain either pathway A or B. The conditions for large scale growth of an E. coli strain capable of synthesizing indigo, including temperature, pH and the amount of tryptophan that must be added to the medium to give maximum yields, are being tested.

Although this system has not yet been commercialized, a microbial process for the synthesis of indigo might include a bioreactor in which the recombinant E. coli is chemically immobilized to a solid matrix (e.g. cellulose or silica gel). The unit could be run continuously by adding tryptophan to one end and removing indigo at the other. Genencor International, of Rochester, New York, is experimenting on a process to produce indigo using biotechnology. However, at this stage the technology is expensive and production costs might be prohibitive.

The research and development efforts made in the field of microbial synthesis of indigo from 1927 onwards has been critically reviewed. The highlights of this critical review indicated that biosynthesis of indigo could be divided into three periods: biosynthesis by wild microbes, whole cell catalysis by engineering bacteria and biotransformation regulated by metabolic engineering. Most aromatic degrading microbes and their relevant enzymes possess the ability to convert indole to indigo.

New technologies such as directed evolution, metagenome and two-phase reaction systems could facilitate in-depth investigations of the enzyme resources, and they will play a crucial role in indigo biosynthesis research. Meanwhile, hydroxyl-indoles and indigo derivatives produced in the process are promising pharmaceutical and chemical precursors with great research interest. However, the transformation interactions between intermediates and by-products are still unclear. Besides, low indigo yield and efficiency with high cost have hampered practical production. Therefore, it is essential to combine the molecular biology and metabolic engineering technologies to investigate the mechanisms and industrial application of indigo biosynthesis in the future.