Washing Techniques for Denim Jeans
Denim Jeans Washing Methods
Washing can be considered 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 (Wang, 1995).
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 colour 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 leisurewear. These effects are difficult to achieve through other processing techniques.
The results obtained from denim washing represent a combined effect of colour dissolution, destruction of the dye and mechanical abrasion, which sometimes causes the removal of surface fibres from the materials. Thus surface dyed (ring dyed effect) colours in denim garments are more easily washed down during the washing process.
According to textile terms and definition, denim is defined as ‘Traditionally a 3/1 warp-faced twill fabric made from the yarn-dyed warp and undyed weft yarn. Typical construction of the fabric is 32 × 19; 45 × 54 tex; 310 g/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 × 21 tex cotton. Although by definition, 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 dyeing effect.
As a result, the washing technique and effect described in this chapter are based on indigo dyed fabric.
Evolution of denim garment washing
Denim garments in the market are originally stiff and uncomfortable when they were first purchased because of the finishing system used for denim fabrics. Generally speaking, 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 them once or several times before the first wearing.
Swicegood (1994) reviewed the evolution of denim garment washing. 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 colour 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 colour 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 pre-soaked 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. Actually, the term ‘acid wash’ is a misleading term because mineral acids are not used for this process.
Other than bleaching agents, the use of enzyme (cellulase) treatment to obtain the colour fading effect similar to the 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 labour 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 and laser treatments have been introduced as an alternative to conventional wet processing.
Washing is the final process of denim garments
Washing can be considered 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 colouration form the base of colour in denim garments (Wang, 1995). However, the washing process is the key to creating the style in denim garments which is now becoming an art of creating fashion trends (Zheng, 2009).
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
- Removing sizing agents and unfixed dyes to remove contaminants added during the manufacturing process so as to generate ‘clean’ denim Also, the washing can increase the surface lustre 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, ground 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 colour yield, dimensional stability, colour fastness or improper surface treatment.
Classification of 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. Only the basic treatment conditions are addressed in this chapter; the number of variations is very large and the evolution of chemical and mechanical techniques is continuing. There often are some secretive and proprietary methods. Regardless of the specific look and name chosen, the following are the process steps normally used to attain the desired results.
Desizing → Rinsing → Washing (abrasion) → Rinsing → Softening → Drying → Packing.
In denim garments, preparation consists primarily of desizing, which enables sub- sequent chemical and mechanical treatments (washing/abrasion) by removing the previously applied warp size. Untreated denim garments are extremely rigid due to the size applied at the yarn stage for increasing weaving efficiency, by coating the yarn with a protective outer layer that retards yarn breakage.
The sizing remains in the yarn after the fabric is woven and, therefore, provides the fabric stiffness necessary for more efficient sewing of garments. Sizing is typically made up of starch (or polyvinyl alcohol/starch), binders and waxes and lubricants (The Hong Kong Cotton Spinners Association, 2007). Additionally, finishes containing starch and/or polyvinyl alcohol are applied to denim as a topical finish before the fabric is shipped to the sewing plant. After desizing, denim garments are rinsed to remove all size materials before washing/ abrasion.
Different washing/abrasion techniques are introduced in the following sections. Again after the washing/abrasion processes, denim garments are rinsed to remove unwanted materials from the fabric surface. Then a softening process enhances the hand feel and softness of the denim garments. Finally, the denim garments are dried and packed for delivery.
The most popular method of removing starch from denim garments is to use the amylase enzyme. This product can break down the long starch molecular chains (water-insoluble) into smaller molecules (water-soluble) which can be more easily washed away. The removal of starch from the fabric being desized can also usually release some quantities of indigo into the bath. Therefore, a neutral pH nonionic surfactant is used for suspending loose dye in the water, to prevent redeposition onto the garments as well as to aid penetration of the desizing liquor into the interior of the fibres. It is also important to follow the desizing bath with a hot water rinse. The introduction of cold water onto the denim garments at this point can resolidify the fats and waxes and tends to redeposit the gelatinous components unevenly on the surface.
Regular washing is the simplest and most commonly used washing method for denim garments. The degree of colour 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. Generally speaking, detergent is used for regular washing for about 15 min at temperatures between 60 °C and 90 °C. Softening is applied after the washing process. Regular washing can improve the softness and comfort properties of denim garments as well as enhance the aesthetic property. Depend- ing on the time and amount of chemicals used, regular washing can be classified into
- light washing (washing time about 5 min);
- normal washing (washing time about 15 min);
- heavy washing (washing time about 30 min).
However, there is no significant distinction between light, normal and heavy regular washing as it depends on the actual washing conditions (Chong, 1994; Zhao, 2008; Li and Liu, 2013). The General regular washing process is as follows:
Wetting → Desizing → Regular washing → Softening.
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 decolouration usually depend on strength of the bleach liquor, liquor quantity, temperature and treatment time. The bleached fabric materials should be properly anchored or washed with peroxide to reduce the subsequent yellowing or tendering of the bleached denim fabric. The basic steps of denim bleach washing are as follows (Chong, 1994; Zhao, 2008; Pal, 2010):
Garment loading with or without stone → Desizing (10–15 min, at 55–60 °C) → Rinsing → Bleaching (15–30 min) → Rinsing →Bleaching with cold water → Optical brightening → Softening (Chong, 1994).
At any process stage, if sodium hypochlorite is used, the characteristics of the chemical must be taken into account to assure minimum fabric degradation while achieving the desired colour fading effect. In storage, the active chlorine content of sodium hypo- chlorite solution loses daily, especially in hot weather. Therefore, concentration should be checked before dosing the washer. The pH range used in sodium hypochlorite bleach washing is critical because of the formation of hypochlorous acid.
It is often not possible to use pH paper for measuring sodium hypochlorite solutions because many indicators get bleached during the measurement. However, when a slight excess of hydrogen peroxide is added to the test solution, the hypochlorite is destroyed without altering the pH. There are many indicators such as universal indicators, phenolphthalein and thymol blue which are not sensitive to traces of hydrogen peroxide. There is greater danger in pH when used in bleaching than is the case with sodium hypochlorite because of carbonate formations (from atmospheric carbon dioxide).
Sodium carbonate is formed in the solution which can help buffer the pH. However, an alkaline buffer is recommended for sodium hypochlorite baths. In the sodium hypochlorite process, after treatment, all chlorine must be cleared from the fabric by use of an antichlor, such as hydrogen peroxide, sodium bisulphite or sodium thiosulphate. Hydrogen peroxide is often chosen as it not only neutralises chlorine but also offers the advantage of additional whitening during the process. On the other hand, bisulphite or thiosulphate are used for the antichlor process.
The normal pH use range for sodium hypochlorite bleaching is 9–1.5, where bleaching species of the chemical are most stable. The stability decreases rapidly above 55–60 °C. Therefore, these variables must be controlled to ensure that the cellulose substrate is not damaged. If damage is occurring, lower temperature or concentration or a shorter cycle should be considered. Buffers should also be added to the sodium hypochlorite bath to aid in controlling the pH.
Some surfactants perform this role, acting as dual agents to provide alkalinity and aiding in wetting and penetration of the fabric by the chemical bath. Another negative property of sodium hypochlorite is the ability to form chloramines with protein impurities in cotton, which tend toward yellowing and ageing. This source of yellowing is a function of impurities in cotton itself.
In the case of potassium permanganate, manganese oxide (MnO2), a brown and water-insoluble compound is produced as a by-product. Some of the preventive methods involve high-temperature water rinsing, use of chelating agents, good detergent and mechanical agitation, and especially a sufficient number of reduction stages, scouring and rinses to completely remove the residual materials. However, the most effective way to remove MnO2 is by treating the fabric with oxalic acid (Li and Liu, 2013). Limitations of bleach washing (Pal, 2010):
- The same level of bleaching is very difficult to achieve in repeated
- Bleaching treatment sometimes damages cellulose, resulting in strength loss and or pinholes at the seam, pocket, etc.
- Bleaching liquor is harmful to human health. This may also cause corrosion to the machine
- Bleaching treatment needs antichlor treatment to eliminate the subsequent yellowness in the
- Chlorinated substances occur as abundant products in bleaching, and pass into the effluent where they cause severe environmental pollution.
Volcanic rocks or pumice stones (Figure 1.1) are added during washing as abradants. The colour fading is more apparent but less uniform. The degree of colour fading depends on the washing time (60–120 min), stone ratio (weight of stones relative to the weight of the garment) (0.5: 1–3:1), size of stones (diameter: 1–7 cm), liquor ratio (∼10:1) and garment load (Chong, 1994; Pal, 2010; Li and Liu, 2013). The world’s major pumice stone supplying countries include the USA, Turkey, Italy, Germany, Iceland, New Zealand, Japan, Indonesia and the Philippines. The basic steps of denim stone washing are as follows (Chong, 1994):
Desizing (10–15 min) → Rinsing → Stone washing → Rinsing (with perborate and optical brightener if necessary) → Softening.
Stone washing of denim fabric gives a ‘used’ or ‘vintage’ look to the garments. This is due to the varying degree of abrasion in the garment. Traditionally, stone washing of denim garments is carried out with pumice stones to achieve a soft hand and desirable look. The pumice stones having an oval and round shape with a rough surface work as an abradant in the washing cycle. The variations in shape, composition, hardness and porosity result in different washing effects in the denim fabric.
During washing, these stones scrape off dye particles from the surface of the yarn of the denim fabric which shows a faded, worn out and brilliance effect in the denim fabric. From ring dyeing of denim fabric and heavy abrasion during stone washing, the fading is more apparent but less uniform. Stone washing makes the denim garments more supple so that they fit comfortably. To get the desired washed effect, the stone should be of proper hardness, shape and size. For heavyweight denim fabric, large and hard stones are suitable and also last longer. Similarly, smaller and softer stones are suitable for lightweight denim fabrics (Pal, 2010; Paul and Naik, 199c).
Effect of stone ratio
Figure 1.2 shows that with a stone ratio below 0.5:1, no significant colour fading effect in the denim garments is noted. When the stone ratio is high, the colour fading effect becomes enhanced. However, too high a stone ratio may cause severe abrasion leading to fabric damage.
Effect of stone size
Figure 1.3 shows the effect of stone size on the colour fading effect. It is noted that the smaller the stone size, the better is the colour fading effect. In addition,
Small stones can provide an even and uniform abrasion effect, but the colour contrast produced is not good when compared with large stones (Kan, unpublished data).
Effect of stone washing time
Figure 1.4 shows the colour fading effect under the influence of stone washing time. It is shown that after 90 min, there is no further increase in the colour fading in denim garments (Kan, unpublished data). Stone washing of denim fabric with pumice stones has some disadvantages and limitations, such as (Pal, 2010):
- Stones may cause wear and tear of the fabric and may damage the washing machine from abrasion of the stone with fabric or machinery parts.
- It may also create the problem of environmental disposition of waste of the grit produced by the stones.
- Increase the labour cost required to remove stone dust from finished Denim garments are required to be washed several times for complete removal of the stones.
- The stone washing process may cause back staining and redeposition
- The process is nonselective
- Metal buttons and rivets in the garments, as well as the drum of the washing machine sometimes, get abraded which substantially reduces the quality of the garment and the life of the equipment
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 sufficient, provided that the enzyme activity is not impaired.
In general, the colour of the enzyme washed goods is more uniform, particularly when the stone is not added. Because cellulases are reactive only on cellulose, any size materials or other impurities must be removed before the cellulase treatment. The basic steps of denim enzyme washing are shown as follows (Chong, 1994; Zhao, 2008):
Desizing (10–15 min) → Rinsing → Enzyme washing, e.g. 30–60 min at 50–60 °C with pH 4.5–5.5 → Hot rinsing, e.g. 80 °C → Softening (Chong, 1994).
Temperature is the most important factor in enzyme treatment. Generally speaking, enzyme reaction increases with temperature but it is only activated within a temperature range in which the enzyme structure remains stable and unchanged. Beyond this optimum range, the enzyme activity decreases sharply as the protein structure of the enzyme is tangled through thermal agitation.
There is a trend of increase of colour fading effect when temperature increases from 50 °C to 60 °C (Kan, unpublished data). This is because the enzyme reaction is activated by higher temperatures within the optimum temperature range and thus more surface fibres are hydrolysed by cellulase (Aly et al., 2004) and the weakened fibres are removed by abrasion of fabrics and mechanical agitation (Dhurai and Natarajan, 2007). The indigo dye particles are also removed along with the cotton fibres.
The mechanical action can provide a colour fading effect of denim garments in enzyme washing. The specimen treated without any agitation has a better colour fading effect than that treated with mild and vigorous agitation because the fibres are weakened after treating with enzyme alone and are not well removed by mechanical agitation (Vishnu Vardhini and Selvakumar, 2006; Kan and Wong, 201). The surface of fabric becomes hairier and a layer of fuzz gets formed on the surface (Cavaco-Paulo, 1998). For specimens treated under vigorous agitation, a biopolishing effect is obtained. The weakened fibres are well removed by strong mechanical agitation, and thus a cleaner and smoother surface are produced and the fabric has a lighter shade as dye particles are also removed with cellulose fibres (Kan, unpublished data).
Longer enzyme treatment time prolongs enzymatic degradation of cellulose and the time for further abrasion. It is shown that the colour fading effect increases with increased treatment time. The increase in colour fading effect is mainly due to desorption of dye particles that are weakly adsorbed on fabric. In addition, the increased colour fading effect with more time is also due to the fuzziness of fabric caused by the prolongation of the enzyme treatment. With a longer treatment time, cellulase effectively hydrolyses fragments of cotton fibrils (Aly et al., 2004; Kan and Wong, 201) and thus the fabric is less fuzzy than the original (Kan and Wong, 201). Usage of cellulase has some distinct advantages, such as (Pal, 2010):
- Cellulase is economical and environmentally friendly as compared with stone washing using pumice stones. The percentage of fabric damages is reduced with cellulase treatment. The enzymatic treatment of denim fabric ensures the same result but consumes less water and time, resulting in less waste and damage to The pollution, quality variability and imperfections are also reduced in enzymatic treatment.
- Enzymes can be
- The productivity of washing is increased due to the space formerly taken up by the pumice stones; the same washing machines can handle more jeans.
- The time consumption for removing stone fragments from the denim garments is eliminated in the case of cellulase treatment.
- The duration or number of rinse washing after enzyme treatment is less than a pumice stone
- A small quantity of enzyme can replace several kilograms of pumice stones during washing, which ultimately leads to less damage to garments and machines.
- Washed garments with a softer feel and better appearance is achieved in cellulase
Cellulase treatment of denim fabric has some disadvantages, too. There may be chances of back staining in cellulase treatment. To remove the back staining, the garments are rigorously washed. However, this adds to the usage of water for washing (Pal, 2010; Kan and Wong, 201).
Pumice stones are first presoaked 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’ (Chong, 1994; ; Pal, 2010).
After the stones are soaked in a strong oxidising agent such as sodium hypochlorite or potassium permanganate, the excess liquor is removed. This can be accomplished by placing stones in the net or mesh fabric before soaking. Then the stones can be removed and the excess liquor drained off. Another alternative is to place the stones in a rotary tumble machine along with ‘waste’ fabric and tumbling for several minutes to remove the excess solution.
Another alternative is to use presoaked stones. There are many shapes, varying levels of chemicals and other additives which produce different effects. After soaking the stones, stones and denim garments are put into the machine for tumbling for 10–30 min or until the desired effects are achieved. Results are dependent upon dyestuff, fabric, the concentration of chemicals, stones and equipment. In some cases, the stones can be reused for another load before resoaking, depending upon their porosity. It is advantageous to remove the denim garments from the machine and transfer them to another machine for washing to minimise the number of machines used for acid washing.
After getting the desired effect, the machine is filled with water for rinsing for 5 min to remove the stone dust. If potassium permanganate is used, manganese dioxide gets formed (a brown/orange colour) and must be removed by rinsing with oxalic acid. In the case of sodium hypochlorite, a reduction process, antichlor with a reducing agent (e.g. sodium bisulphite) is used for removing the residual chlorine (Li and Liu, 2013).
In acid washing, the addition of water is not required. The colour contrast after tumbling can be further enhanced by a subsequent optical brightening process. Soft stones that dust off easily are more suitable for this process. The basic steps of denim acid washing are shown as follows (Chong, 1994):
Desizing (10–15 min) → Soaking stones with strong oxidising agent → Draining excess liquor → Dry tumbling (10–30 min) → Rinsing to remove stone-dust → Rinsing with oxalic acid (in case of potassium permanganate)/antichlor with reducing agent (in case of sodium hypochlorite) → Optical brightening → Softening.
Sandblasting is a mechanical process in which localised abrasion or colour change on the denim garment is created (Pal, 2010; Zhao, 2008). 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 as shown in Figure 1.5 (Pal, 2010). The treated surface shows a distressed/abraded/used look. The common blasting materials used are sand and metal granules.
During the sand blasting process, denim garments are first subjected to stone washing to the desired degree of washing and are then sandblasted. A solution of sodium hypochlorite or potassium permanganate is often sprayed on the desired area of the garment in order to obtain the same look. The garment is then neutralised, rinsed, softened and dried. Sandblasting is a water-free process and, therefore, no drying is required.
Sandblasting removes the dark indigo from a denim garment giving it a popular pre-worn look. This process involves smoothing, shaping and cleaning a hard surface by forcing abrasive particles across that surface at high speeds using special types of sands.
These are sprayed onto the selected parts of the denim garments at high pressure through air compressors to remove the colour from those areas to create the desired design.
Sandblasting can be done manually or mechanically. The mechanical process encloses the sand and dust particles in blasting cabinets and is–if used correctly–therefore less hazardous for the operating workers. However, manual sandblasting is preferred by factories as it is cheaper because it does not require investment in advanced and expensive industrial equipment. Sandblasting also costs less than other fading methods (like hand sanding) which are more labour intensive. Whilst sandblasting to achieve a worn look on denim is a relatively new phenomenon in the clothing industry, similar methods have been widely used in mining and building industries for many decades and the link between the use of sandblasting and the risk of silicosis has long been acknowledged.
It was the high health risks associated with the manual sandblasting process that prompted regulation of the technique in Europe in the 1960s. Sandblasting can expose workers to extreme health hazards and can cause death within months or years of starting work as a sandblaster. Sandblasting using natural sand is especially problematic because workers inhale crystalline silica dust particles during production, causing serious damage to the respiratory passages. These particles are so tiny that they are invisible to the naked eye. The body is unable to expel the silica particles, causing diseases such as silicosis.
The particles penetrate the pulmonary alveoli and the connective tissue, gradually impairing lung capacity and the workers’ ability to oxygenate the blood. Symptoms include shortness of breath; as the disease develops, this is common even when resting. This puts additional strain on the heart, eventually leading to death. However, the progress of silicosis can be slowed if symptoms are diagnosed at an early stage (Riddselius, 2010).
Monkey wash is a description of a special colour fading effect normally at the thigh and buttocks of denim jeans. In this process, denim jeans are sprayed with a strong oxidising agent such as potassium permanganate (Lin, 2009; Zheng, 2009; Zhao, 2008). Therefore, monkey wash can be technically termed as ‘PP spray’ (with PP being potassium permanganate).
The denim garments should be laid flat or fixed in manikin properly (Figure 1.6) before PP spray. About 2–5% concentration of potassium permanganate is sprayed on the marked surface and the colour is then faded. After PP spraying, the brown colour of manganese oxide can be removed by oxalic acid or hydrogen peroxide. The basic steps of PP spray-on denim garments are as follows:
PP spraying → Washing → Treating with oxalic acid or hydrogen peroxide to remove brown manganese oxide.
Washing/abrasion → PP spraying → Treating with oxalic acid or hydrogen peroxide to remove brown manganese oxide.
PP spray is used for making a specific abraded area appear whiter than the background indigo colour shade. This can be applied by a spray gun or a towel dipped in PP solution and rubbed on the desired area, followed by neutralisation in a wet process. This process can be done right after doing hand scraping/sanding/blasting or in the middle of the washing. Doing this after enzyme washing or bleach washing cycle gives a more natural and white effect (Rai, 2009).
Potassium permanganate spray is best done in specific spray booths, where rubber dummies (manikins) are installed for holding denim garments. The garment is mounted on a dummy and air is filled so the garment is full-fit exposed. Specific dummies are used for different sizes and styles, such as for children, trousers, jackets and shirts.
The booths are fitted with a proper air exhaust system. This system has a treatment room where the chemicals and air are mixed and the material is usually passed through water showers. PP is dissolved in water and clean air is blown. Shower water is further treated with mild quantities of a neutraliser before adding it to the main drain. But if the PP spray is used in low concentrations, there is no need to treat the fabric with shower water. This mild PP mixed water is rather useful for water reservoirs to keep the water clean and germ-free.
PP spray concentrations range from 2% to 5% depending upon the required results and fabric types. Usually, indigo-dyed fabrics are treated with low concentrations of PP, whereas black sulphur fabric requires high concentrations. Sulphur is not much affected by PP and hence requires high concentrations and sometimes even multiple sprays. It is more effective to add PP brushing to aid in the spray effect. It is very important to equip the operator with gloves, a gas mask and goggles. Breathing in PP spray over a long period may cause health complications, so proper preventive measures need to be taken (Denim Help, 2010a).
Denim garments are mounted on air-filled rubber dummies or mannequins and chemicals are sprayed on blasted areas. The variables in the spray process are as follows (Denim Help, 2010a):
- Distance of spray gun to garment: less distance gives a more defined and sharp effect, whereas spray from a larger distance results in a more mild and merged effect. Distance ranges from 30.48 to 76.20
- Air to water ratio of gun: this is to be set very carefully. Low air pressure can result in PP drops on denim garments, causing bright white spots, whereas high pressure produces very few bright effects on areas where it is not required.
- PP concentration controls the extent to
Brushing/grinding (manual or mechanical) is used manually or mechanically for the worn-out effect, abraded look or used look. Some mechanical processes have been developed that are based on mechanical abrasion by which the indigo can be removed. Some of these processes are sueding, raising, immersing, peaching and brushing. Advantages of these processes (Zhao, 2008; Lin, 2009) are as follows:
- Control of the abrasion
- Different looks on the garment can be achieved
- All are dry process
- Economical, ecological and environmentally friendly
Brushing is generally being done in a rigid form of garments to get the distressed look. Locations can be front thigh and seat or it can be overall/global application as is standard. In the case of hand brushing, emery paper is used to brush the garments in particular places and designs. Emery paper comes in different numbers, generally starting from 40 to 600 and higher; the higher the number the finer is the emery paper (i.e. a lower is a more coarse paper). In the garment industry, 220, 320 and 400 paper numbers are the most popular and widely used (Rai, 2009).
The purpose of this process is to impart a used worn look to the garments. The most important factor of brushing is to select the right sanding material according to the fabric strength and the intensity needed. Figures 1.7–1.9 show the brushing process in the factory.
Grinding is done on pocket edges and bottom hems edges by rubbing them against an abrasive surface or stone to achieve a worn effect. Many different makes of machines and pen grinding tools are available in the market which run with pneumatic systems. Figures 1.10 and 1.1 show the grinding effect on pocket edges and bottom hems, respectively.
Whisker is one of the most important designs of used look denim garments and is taken from the worn out lines and impression patterns generated by natural wearing on hips and the front thigh area. In old jeans, several patterns can be found consequential to fabric, body shape of user or sitting posture (Denim Help, 2010b). There are two types of whiskers: two dimensional (2D) and three dimensional (3D) (Lin, 2009; Zhao, 2008).
Various methods are designed to create a 2D whisker impression on jeans. Mostly, rubber balloons (e.g. manikin) are available with different pattern designs. Garments are mounted on balloons and filled with air to get whisker impressions. The garment is scrubbed over the pattern carefully with sandpaper on engraved pattern lines. This operation requires highly skilled labour who can handle uniformity and sequence in whisker lines. Fabric may get damaged during rubbing if care is not taken. But this method is more successful in industries where similar articles are produced in large quantities. This method is famous for its high quality and cost-effectiveness (Lin, 2009; Zhao, 2008; Denim Help, 2010b).
Engraved patterns on thick boards like hard rubber sheets are widely used for imprinting whiskers. The idea is very simple, to draw lines on rubber sheets and engrave them with a blade. The garment is placed on a sheet and scraped on a specific area to draw this impression. Because of the low-cost patterns, it is most frequently used in small industries, especially where the production is not consistent with the style. In some workshops, it is done manually with the help of sharp edges rolled on fine wood sticks pasted on plastic material. Before starting, placement and pattern must be marked on the garment to help the operator execute the pattern correctly to match the aesthetics of the garment (Denim Help, 2010b). Figures 1.12–1.14 show the operation of producing 2D whisker manually. The basic steps of 2D whisker making in denim garments are as follows:
Sanding/grinding (manually or mechanically) → Desizing →Washing → Softening.
Three-dimensional (3D) whisker
The 3D whisker is an aesthetic 3D effect (Figure 1.15) where resin (formaldehyde-free) is used for achieving the 3D effect and a rigid look. This process can be done by spraying or dipping the garments into resin, catalyst, silicone and polyurethane in the right combination, according to the fabric strength and the desired effect (Rai, 2009; Hu and Fang, 2008).
After applying resin solution in the right proportion, a design is made manually or mechanically on the thigh, hip and behind the knee, area to get 3D effects such as crease or fold. Then it is manually dried with hot press or hairdryer and is cured in an oven at the right temperature, with time being as mentioned in resin product manuals. If the resin is not cured properly, the 3D effect is not permanent and can cause skin irritation/rashes to the wearer. Highly skilled operators are needed to execute this process in order to get consistency and uniformity (Rai, 2009).
The whole garment can have a crushed look after it is dipped in resin and crushed manually, followed by oven curing. Silicone plays an important role in getting a softer hand after oven curing. Special streaky effects and crackles are also achieved by applying resin on the desired area, followed by scraping and potassium permanganate spray and then neutralisation (Hu and Fang, 2008). The basic steps of 3D whisker making in denim garments are as follows:
Spraying/dipping with resin → Making 3D effect → Drying → Curing.
Ozone typically acts as a mild bleaching agent as well as a sterilising agent. In this technique of denim washing, the garment is bleached with ozone dissolved in water in a washing machine. However, this technique can also be carried out in a closed chamber by using ozone gas. The advantages of this method are: (1) a minimum loss of strength and (2) it is a simple method that is environmentally friendly.
The ozonised water after laundering can easily be deozonised by ultraviolet radiation. Nowadays, ozone fading can also be achieved by plasma equipment (Jeanologia, 201; Cheung et al., 2013a,b). Under the influence of plasma treatment, high energy electrons are formed. Some of the high energy electrons react with moisture in the air and a mixture of radicals is generated (Zhang et al., 2008).
During the generation of ozone plasma, a combination of charged particles, free radicals and ultraviolet light is generated. The ultraviolet light, being the by-product of the plasma treatment process, also contributes to production of the •OH radical. Hydroxyl radical •OH is the most oxidative radical among radicals generated in the plasma process and is the main radical responsible for degradation of indigo dye in textile materials. The •OH can oxidise indigo dye molecules (RH) producing organic radicals R•, which are highly reactive and can be further oxidised (Khraisheh, 2003; Khan et al., 2010). As a result, the colour fading effect of the indigo-dyed textile is achieved.
The K/S (in which K is the absorption coefficient at a specific wavelength and S is the scattering coefficient) values of different treated denim fabrics are shown in Table 1.1 (Kan and Yuen, 2012). From the results, it is noted that the differently treated denim fabrics have lower K/S values than the untreated denim fabric. The K/S value is linearly related to the concentration of the colourant in the medium and it can be concluded that a paler shade is obtained after different treatments.
|Plasma induced ozone-treated||382.18|
|Plasma induced ozone-treated followed by cellulase treatment||365.32|
|Enzyme desized followed by cellulase treatment||378.90|
Table 1.1 Colour properties of different denim fabrics
Without the cellulase treatment, the plasma-induced ozone-treated denim fabric has a paler shade than the enzyme desized denim fabric because, during the plasma-induced ozone treatment, ozone oxidises indigo dyes on the denim fabric surface leading to a colour fading effect (Ghoranneviss et al., 2006;Kan andYuen, 2012). However, in the case of enzyme desizing, the enzyme only reacts with the sizing material at the fibre surface and no breakdown of indigo dyes molecules occurs. Therefore, no significant shade change takes place.
In the case of cellulase treatment, cellulase in the aqueous medium can penetrate effectively into the denim fabric. The enzymatic hydrolysis induced by cellulase in the plasma-induced ozone-treated denim fabric is more severe than the enzyme desized denim fabric. As a result, the cellulase treatment for plasma-induced ozone-treated denim fabric gives a paler shade than the enzyme desized denim fabric.
The CO2 laser treatment has been used in different areas of textile industry for sev- eral years because it allows short time surface designing of patterns with good precision, desirable effects, various sizes and intensity without causing much damage to the bulk properties of the textile materials (Hung et al., 201, 2014; Ondogan et al., 2005). The CO2 laser treatment considered a dry treatment, can be applied to textile materials as an alternative to conventional wet treatments such as stone washing, sand washing and bleaching for achieving the faded look and worn-out effects (Dascalu et al., 2000; Ozguney, 2007; Tarhan and Sariisik, 2009).
In the case of denim fabric, CO2 laser treatment is proved to be an effective method for fading the colour from denim fabric surface in a short time depending much on the laser process parameters (Kan et al., 2010; Kan, 2014). Figure 1.16 shows the colour fading effect on denim fabric under the influence of various combinations of resolution and pixel time (Kan, 2014). It is clearly shown that with an increase in resolution and pixel time, the colour fading effect in the denim fabric increases accordingly.
Hydrojet treatment has been developed for patterning and/or enhancing the surface finish, texture, durability and other characteristics of denim garments (Shalini, 2013).
Hydrojet treatment generally involves exposing one or both surfaces of the garment through hydrojet nozzles. The degree of colour washout, clarity of patterns and softness of the resulting fabric are related to the type of dye in the fabric and the amount and manner of fluid impact energy applied to the fabric. Particularly good results are obtained with blue indigo-dyed denim. As this process does not involve any chemicals, it is pollution-free. A water recycling system can make this a very economical and environmentally friendly way of denim processing. Colour washout of dye in the striped areas produces a faded effect without blurring, loss of fabric strength or durability, or excessive warp shrinkage.
In dip dyeing, dip dyeing machine is used to achieve special effects on denim gar- ments (AKDMKS, 2009). Direct dye is commonly used but the colour fastness is comparatively poor and therefore fixation is required to improve fastness. Pigments can also be used for dip dyeing. If a bleaching agent is used, the bleaching effect can be achieved on garments.
Pigment washing is generally used on pigment-dyed or printed garments by applying pigments. Pigment washing is required for the following reasons (Lin, 2009; Kiron, 2012a):
- To get fading effect/old-looking effect on garments and also the seam
- For a soft feel to wear the garment after
- To achieve the buyer’s washing
- To increase the colour and rubbing
The basic steps of pigment washing are as follows:
Immersing garment in pigment → Drying → Baking (130–150 °C) → Washing (with or without pumice stones/with or without enzyme, 30–90 min) → Washing twice with water at 70 °C → Removing stones → Softening → Drying.
After denim jeans have been faded, they are dyed with a very light colour (0.001% or 0.002% yellow or pink). This dyeing process is called tinting/overdyeing (Lin, 2009; Zhao, 2008; Kiron, 2012b) that is applied only on garments, not in fabric or yarn. The reasons of tinting are:
- For making new fashion
- To give the finished garments a better
- For inventing a new
A very small amount of dye is used in tinting (Kiron, 2012b). The process changes the hue/cast/tone of indigo. As soon as the tinting colour covers up the indigo, it reaches the level of dyeing. Tinting gives garments a used/vintage and muddy look. This process takes from 5 to 15 min and for better results it is followed by dye fixing and clean up of the superficial dye. Dyeing is done on very light shades of indigo, grey denim and is ready for dyeing denim. There are various types of dyes that can be used for tinting: (1) direct dyes; (2) reactive dyes; (3) pigment colours; and (4) sulphur dyes. The basic steps of tinting of denim garments are given in Table 1.2 (Lin and Liu, 2009; Kiron, 2012b).
Tinting without changing the indigo tone
Tinting is achieved by applying direct/reactive dyes. In the case of direct dyes, the second application is a dye-fixing agent for adequate fastness. In case of reactive dyes, the fixing chemical goes along with the dyestuff. After the application, the treated garment is dried in the air or in a tumble drier for adequate fastness.
If tinting is done by the immersion method, the tint is all over the garment. However, if a localised area is the target, a spray method is used. In either case, tinting, as the term signifies, is done very lightly to achieve a particular cast, either overall or localised. If indigo does not change tone, it looks like a print with a sharp boundary. When tone changes the fading gradient, the effect looks more aesthetically appealing and is considered desirable. The tone change should be gradual and not abrupt (Kiron, 2012b).
|Desizing → Bleach washing/enzyme washing → Tinting*,† → Rinsing (25 °C, 2 min) → Colour fixing → Rinsing (25 °C, 2 min) → Softening → Hydroextracting → Drying.|
|*Tinting with direct dye:|
|Direct dye||× g/L|
|†Tinting with sulphur dye:|
|Sulphur dye||× g/L|
|Sodium sulphide||0.85 × g/L|
Table 1.2 Tinting with direct and sulphur dyes
Tinting with white discharge effect
The standard route to blasting (scraping)/discharging/tinting is quite elaborate, considering three effects are imparted to the same garment. There have been shortcuts, which may be right/wrong/ingenious. If tinting is done on blasted/discharged areas, it obviously remains a spray method. However, if the tinting is overall, it follows the dyeing route. The process starts with grey denim garments being hand blasted (scraped), followed by desizing and enzyme washing. After drying the garment, a tinted spray is followed by air/machine drying and finishing. For tinting by dyeing method, the enzyme washed garment can straight away go for tinting wet-on-wet followed by finishing and drying (Kiron, 2012b).
In the case of white discharge tinting, the route starts with desizing-enzyme washing of grey garments. After drying the garment, a potassium permanganate spray is applied to areas where the white discharge is required. After air-drying for about 10–15 min, a neutralisation step consisting of providing acetic acid is applied, which is followed by hot and cold rinses. In the case of spray tinting, the garment has to be dried first, whereas, for overall tinting, dyeing can be followed wet-on-wet. Finally the finishing and drying complete the process (Kiron, 2012b; Lin and Liu, 2009).
In tie-dyeing, a rubber band or similar material is used to tie/bunch the denim garment to make different patterns (Zhao, 2008; Lin, 2009). Then when carrying out dyeing with a direct dye, the dye cannot enter the tied portion. After dyeing, patterns can be created and fixation is needed. Pigments can also be used in tie dyeing. If a bleaching agent is used, a bleaching effect can be obtained, which is called ‘tie bleach’ (Denim Trends, 2013).