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.

Dyes used for denim and why does Indigo is so popular for Denim?

Dyes used for dyeing denim are:

• Indigo
• Vat
• Direct
• Sulphur
• Reactive
• Pigment Colors

Among all these dyes indigo dye is the most popular and widely used consisting of almost 67% of all the dyes used for denim production.

Indigo for Denim

The indigo color is the principal source of the almost magical appeal of denim. The dye imparts a brilliant blue hue to fabric. In fact, there is no other dye that creates such an intense blue color with so few carbon atoms in its molecule Indigo is unique in its ability to impart surface color due to partial penetration in cotton fibers. When cotton yarn dyed with indigo is untwisted, the inner layers remain uncolored. The dye also fades and abrades continually. This characteristic of indigo lets denim fabric have its final worn look with different types of washing and finishing applications. It enables denim fabric to respond to finishing applications that give a real life to the fabric.

Despite many other blue dyestuffs, indigo has kept its popularity for denim dyeing. This no doubt is achieved by the fact that indigo has several properties that have not yet been achieved by another single dyestuff. The unique feature of indigo dyed denim is the possibility of achieving wash down effects on repeated washing without losing the freshness of the color. Another important feature of indigo is that unlike many other dyes, indigo dyed denim does not pose health hazards. In fact, indigo is so safe for living things that it has long been used to color polyester medical sutures. Indigo is also used as food color and as a medical indicator applied intravenously.

Technical features of indigo include possibility of dyeing at room temperature, favored shades from black to navy to sky blue that are not too brilliant or not too dull, reduced indigo solution is not sensitive to water hardness and this allows the dyeing of greige or minimum pretreated cotton, does not strongly bond to the fiber, repeated washings slowly remove the dye without losing freshness and most importantly the competitive price. In fact, many synthetic blue dyes have superior qualities compared to indigo, and this would have ruined synthetic indigo as well if denim had not begun its invasion to Western culture after the Second World War.

Other Dyes Used for Denim

Since denim is made of 100% cotton, therefore in addition to indigo it can be dyed with dye classes such as direct, vat (other than indigo), reactive, sulphur dyes and pigment colors. In the last decade, denim was mainly dyed with indigo (67%), sulphur black (26%) and other sulphur colors (6%). The changing fashion trends have also led to vat, reactive and direct dyestuffs, as well as pigments being used to color denim.

Direct dyes are currently being applied in garment dyeing of jeans in light colors for the higher fashion market. Direct dyes can also be used for tinting denim fabrics by adding to finishing formulation. Reactive dyes are often used in garment dyeing. There have been attempts to apply reactive dyes on indigo machinery, but control of dyeing is nearly impossible because of the large dye boxes on most indigo equipment, much of the dye being hydrolyzed before it can react with cotton. Therefore, reactive dyes should be applied in a small box, then dried, chemical padded and steamed. The only practical alternative for most denim companies is to apply reactive dyes during sizing with special procedures. There are few vat dyes that can be mixed with indigo for casting; however, pure vat colors are best applied on denim with specially designed rope ranges with an additional set of drying cylinders, steamer and small pad boxes for dye pad, dry, chemical pad, steam methods. However, vat dyes produce shades with high fastness properties and therefore it is difficult to get the classic denim look.

Sulphur dyes are of low cost and can be applied on indigo machinery, fabric dyeing machinery, jiggers, pad-batch and garment dyeing machinery with properly designed methods. Sulphur dyes have an appearance that is more natural than reactives or directs, having a softer appearance and allowing versatile wash down effects in laundering. Therefore, among the different dye classes other than indigo, sulphur dyes are more popular and are used for bottoming or topping or over dyeing of finished indigo dyed garments to produce a variety of shades with fancy looks. Pigments are relatively easy to apply, and since they are a surface coloration, can produce a distressed look, but they present color fastness problems in darker shades and have a harsher feel than dyed garments.