Washing Techniques for Denim Jeans

Factors affecting washing effect

Method of desizing

The Kubelka–Munk theory can be used for measuring the colour yield of denim fabrics washed with different desizing methods. The Kubelka–Munk theory states that colour yield (K/S) can be measured by K/S = (1 − R)2/2R where K is the absorption coefficient at a specific wavelength; S is the scattering coefficient; and R is the reflectance at a specific Kan (unpublished data). wavelength. K/S values of denim fabrics can be measured at 660 nm (Hong KongApparel Product Development and Marketing Research Center, 2002). Table 1.3 shows the results of bleach washed denim fabrics pretreated with different desizing methods (Kan, unpublished data).

 

Desizing method	K/S value after bleach washing
Unbleached normal denim	1.96
Amylase enzyme desizing	4.41
Detergent	4.49
Hot water	5.59

Table 1.3 Effects of desizing methods on the washing effects

 

The results indicate that the amylase enzyme desizing can gener- ate the lowest K/S value which indicates that the colour yield of the bleached fabric pretreated with amylase enzyme desizing is the lowest, implying that bleached fabric pretreated with amylase enzyme desizing has the palest colour when compared with others. This indicates that amylase enzyme desizing is the most effective method to remove sizing agents from denim fabrics. On the other hand, undesized denim fabric has the highest K/S value which means the darkest colour. It is because the sizing agent covers the yarn surface which prevents the chemical reaction between enzyme and the sizing agents.

Bleach washing

Increased concentration of sodium hypochlorite can effectively accelerate the colour fading effect. Generally speaking, the active chlorine content in sodium hypochlorite has an optimum value to achieve the best colour fading effect. If the concentration of sodium hypochlorite is higher than this optimum value, no further improvement in colour fading is obtained. The recommended active chlo- rine content is 12% when concentration of sodium hypochlorite is about 40 g/L. In addition, increase in temperature increases the bleaching reaction to fade the colour from denim garments. The optimum temperature is 70 °C; temperature higher than 70 °C does not enhance colour fading effect (Hong Kong Apparel Product Development and Marketing Research Center, 2002). The pH value used for bleach washing also affects colour fading. When pH = 7 or pH < 7, the bleach- ing reaction is very fast and the degree of colour fading is very difficult to control. When pH = 9–10 (adjusted by adding alkali), the bleaching effect can be controlled by the bleaching time (Hong Kong Apparel Product Development and Marketing Research Center, 2002).

Stone washing

Stone ratio

Stone ratio is defined as the ratio of the quantity of stones used to the quantity of goods being treated. For example: a stone ratio = 2:1 means a 2 kg stone is used when 1 kg of goods are washed. Generally speaking, the higher the stone ratio, the better will be the colour fading effect because more stones abrade the denim garment surface to have good colour fading effect. However, too high a stone ratio may prevent rotation of denim garments inside the washing machine and hence abrasion gets decreased. As a result, the colour fading effect is not good (Hong Kong Apparel Product Development and Marketing Research Center, 2002).

Size of stones

Generally speaking, stones of smaller sizes have better colour fading effects. How- ever, because of the small stone size, abrasion on the denim garments is more uniform (than with stones of larger sizes) which means the colour contrast produced by small size stones is not good (Hong Kong Apparel Product Development and Marketing Research Center, 2002).

Stone washing time

The washing time affects colour fading effect in the stone washing process. It is recommended that after 90 min washing time, the colour fading effect is not further enhanced (Hong Kong Apparel Product Development and Marketing Research Center, 2002). Too long a stone washing time can lead to fibre damage also.

Enzyme washing

Choice of cellulase enzyme

Acid and neutral cellulases are commonly used for washing. A total of 1/5 to 1/10 the amount of acid cellulase can achieve similar colour fading effect as neutral cellulase. However, acid cellulase may cause backstaining and cause a certain degree of fabric dam- age (Hong Kong Apparel Product Development and Marketing Research Center, 2002).

pH value

In enzyme washing, the pH should be adjusted to suitable values for specific enzymes; neutral cellulases are applied at pH 6–7, whereas acid cellulases are applied at pH 4.5–5.5. Also the temperature of the washing liquor should be controlled to prevent change in pH value during enzyme washing.

Concentration of enzyme

Generally speaking, enzyme concentration between 0.5 and 2 g/L gives the best colour fading effect provided the enzyme used for washing has sufficient enzyme activity. The enzyme activity decreases upon storage and thus the shelf time of enzyme should be noted before preparing enzyme washing liquor. Some manufacturers provide enzyme in powder and granule forms which may contain a stabiliser and buffer to maintain the enzyme activity for a longer time (Hong Kong Apparel Product Development and Marketing Research Center, 2002).

Machine design parameters

Rotation speed

Speed of the washing machine affects the washing and abrasion effect. Generally speaking, high rotation speed increases the contact between garment/garment or garment/pumice stone.

Drum diameter

When the drum diameter of the washing machine is larger, more space is available inside the machine. During washing, the contact time between garment/garment or garment/pumice stone increases, leading to a better washing and abrasion effect.

Machine load

Machine load refers to the quantity of denim garments in a batch. Generally speaking, about 30–50% of space inside the washing machine is occupied by garments during the washing process. If the loading is too high, rotation of the garments inside the washing machine decreases and hence the washing and abrasion effect gets reduced. Moreover, if the garments are entangled together during washing, streak marks are formed.

Height of baffles

Baffles are the intrusions from the surface of the inner drum in a washing machine (Figure 1.17). The baffles help lift the garments from the bottom of the drum to the top before falling back to the bottom and into the washing liquor. If the height of the baf- fles is too low, they cannot carry the garments. The result may be a reduction in the total washing and abrasion effect in the final garments. If the height is increased, they can carry more garments and the washing and abrasion effects are improved.

Steaming

Steaming provides energy for the washing process and it helps better control of the temperature of washing instead of direct heating. Steaming is very important to control the temperature of enzyme washing. If the temperature is too high, the enzyme gets deactivated but when the temperature is too low, the enzyme cannot function well.

Liquor to goods ratio

Liquor ratio refers to the amount of washing liquor used based on the weight of denim garments to be washed. Generally speaking, if a large liquor ratio is used, garment movement inside the washing machine increases, leading to better washing and abra- sion effect. However, usage of chemicals, water and energy also increase. On the other hand, if the liquor ratio is too short, usage of chemicals, water and energy decrease but movement of denim garments inside the washing machine is reduced and washing and abrasion effect becomes poorer. If the denim garments are entangled together, streak marks appear in the finished garments. Normally a liquor ratio of 10:1–15:1 is recom- mended but the ratio can be adjusted according to the desired effect.

Troubleshooting in denim washing

As with other textile dyeing and finishing processes, the quality of the final garments plays an important role in buyers’ final decision of acceptance or rejection. There are a number of problems associated with denim washing (Li and Liu, 2013; Mei, 2009; Lin et al., 2012).

Washing marks

Causes

• The fabric has synthetic During washing, the fabric becomes stiff. As a result, the stiff fabric abrades the washing machine, leading to washing marks.
• The fabric itself has creases formed during production or The existing creases are not removed before washing. Thus during washing, washing marks will form.
• The fabric was not wetted completely and the fabric was not evenly placed in the washing machine before starting the washing machine.
• The loose and long threads in the denim garments were not removed Then during the washing process, the loose and long threads cause entanglement of the denim garments, leading to washing marks.
• The amount of water used for washing is too much or too little
• Insufficient hydroextraction before drying or too much fabric being dried at the same time.
• Before drying, the denim garments were not properly placed in the dryer with too long a drying time and/or too high a drying temperature.

Remedial actions

• Before washing, fabric composition should be checked properly for synthetic In addition, existing crease marks should be found out.
• If possible, before washing, the garments should be completely wetted and placed evenly in the washing machine for washing.
• All loose and long threads must be removed before The maximum length of loose threads should be less than 2 cm to prevent fabric entanglement during washing.
• The amount of water used for washing should be checked during washing. The washing machine may be stopped regularly to see whether there is entanglement during washing.
• The denim garments should be properly hydroextracted and placed evenly in the dryer for drying. The drying time and temperature should be carefully controlled.

Poor hand feel

Causes

• Washing time: insufficient washing time may cause insufficient abrasion between denim garments. Generally speaking, longer the washing time, softer is the hand feel achieved.
• Washing method: different washing methods give different hand For example, desized fabrics for stone washing have softer hand feel than nondesized fabrics for stone washing. However, if the washing process is not properly controlled, poor hand feel is the result.
• Amount of softening agent used for washing: there are many types of softening agents used for washing process, for example, nonionic type and cationic Generally speaking, cationic type softening agents are used in washing. However, silicone based softeners can also be used for washing. Nonsilicone based softening agents give good softness, whereas silicone based softening agents give good smoothness. Moreover, the amount of a softening agent used for washing should be carefully controlled. Although the more the softening agent is used for wash- ing, the better the hand feel is, too much softening agent leads to wastage without improvement of hand feel.
• Softening process: softening treatment time and liquor to goods ratio affect the hand feel. Under normal circumstances, the softening process is carried out as an individual If rinsing and softening processes are carried out at the same time in the same bath, this leads to uneven rinsing and backstaining or insufficient hand feel. The softening process is generally carried out at room temperature (about 40 °C–60 °C) for 5–8 min. It should be noted that at too large a liquor to goods ratio, the amount of softening agent required is increased and the hand feel is not good. On the other hand, with too short a liquor to goods ratio, the softening agent may not be completely dissolved in water. The undissolved softening agent may cause ‘oily’ marks on the garment leading to ‘seconds’.
• Drying time: the drying time affects the hand Generally speaking, after washing, denim garments are hydroextracted to remove 80% of water content before drying. During dry- ing, the amount of denim garments in the dryer should not be too great. Too many denim garments in the dryer need a longer drying time which increases the contact between the garments leading to backstaining.

Remedial action

• Washing time: proper control of washing time and checking the hand feel of the garments, if possible, during the washing process.
• Washing method: do preliminary trials to select the suitable washing method before the actual washing.
• Amount of softening agent used for washing: suitable softening agents should be selected, for example normal emulsion or microemulsion.
• Softening process: properly control the softening treatment
• Drying time: the drying time should be carefully Normally, about 95% water is removed in the drying process. After that, the denim fabric is cooled. The cooling time affects the hand feel. Too long a cooling time increases the contact between garments, leading to staining.

Backstaining in the pocket

Causes

• No antistaining detergent was added during the washing An example of backstain- ing is shown in Figure 1.18.
• The quality of washing chemicals is not good or the washing chemicals have lost functioning
• Washing temperature is too high or too low in the case of enzyme washing, leading to deactivation or inactivation of enzyme.
• Hydroextraction was not properly conducted (water content in denim garments remains high) and the stacking time of hydroextracted fabric was too long. In addition, drying too many denim garments at the same time and at too high a temperature also cause backstaining.
• Insufficient rinsing and no rotation of washing machine during rinsing may cause back staining

Remedial action

• During washing, add antistaining detergent or increase the number of rinses and rotations in the washing machine during rinsing.
• In case of enzyme washing, the quality of the enzyme should be checked, for example for shelf time. In addition, washing time and temperature should be properly controlled.
• The hydroextraction and drying process should be carefully controlled

Loss of elasticity after washing

Causes

• Spandex is a commonly used fibre in denim garments that increases the elasticity of the garment. At high temperatures, spandex decomposes, whereas with a strong alkali, it gets hydrolysed. Decomposition and hydrolysis damage the spandex, leading to loss of Chlorine also damages spandex.

Remedial action

• In desizing process, use enzyme instead of alkali for desizing
• Generally speaking, the loss of elasticity is caused by a high water temperature in the washing machine during the washing process. Normally, the washing temperature should be kept at around 55 °C and should not exceed 60 °C. Otherwise, it produces irregular watermarks
• In the case of bleach washing, the amount of sodium hypochlorite used should be properly controlled

Excess abrasion of background colour

Causes

• These situations are generally found in bleach washed, snow washed and enzyme-stone washed denim garments. In the washing process, if the washing time is not properly con- trolled, it causes excess abrasion of background Once the background colour is over- abraded in the washing stage, the following rinsing time should be shortened to reduce any further abrasion.

Remedial action

• To prevent excess abrasion, colour should be checked with the standard regularly during
• Before actual production, washing trial should be conducted to find the best

Dimensional change after washing

Causes

• If the denim fabrics do not have proper shrinkage prevention treatment before being made into denim garments and jeans, dimensional change may occur after washing.

Remedial action

• If the size of the garment increases after washing, it may be put in the washing machine and washed with water only for gentle This helps achieve a slight shrinkage in the denim garments. However, this gentle washing must not lead to colour change.
• Compressive shrinkage treatment should be conducted on the denim fabric. If the size of denim garment decreases after washing, it can be steamed inside the washing machine (with- out adding water) to improve the shrinkage problem.

Yellowing

Causes

• Yellowing of denim garments after washing is generally caused by excessive usage of sodium hypochlorite in bleach washing. If neutralisation is not carried out properly, the residual chlorine leads to yellowing.
• The unremoved impurities or metallic/mineral contents in pumice stones can possibly con- tribute to yellowing. Yellowing can also be caused from the softening agents. In addition, atmospheric contaminants/gases can accelerate the yellowing of washed denim on storage (Paul and Naik, 1997d).
• If the optical brightening process is not carried out properly, it causes yellowing
• If the water used for washing contains a high amount of calcium or magnesium ion, it also causes yellowing.

Remedial action

• Neutralisation should be carried out thoroughly to remove residual chlorine after using sodium hypochlorite for washing.
• The operational parameters of optical brightening process should be properly controlled
• The water used for washing should be free of hardness

Odour

Causes

• Odour is generally found in stone washing, bleach washing and enzyme The main causes of odour are
  • insufficient rinsing;
  • incomplete neutralisation with residual chlo- rine;
  • water quality used for rinsing is poor.

Remedial action

• After washing, denim garments should be properly neutralised and/or Also, the water quality should be checked for washing.

Size marks

Causes

• Size marks are generally found in stone washing, stone bleaching and stone abrasion pro- cesses. The cause of size mark is improper desizing which leads to incomplete desizing, that is uneven desizing or low desizing rate. Meanwhile, uneven sizing may also be a cause of size marks.

Remedial action

• The desizing process should be properly conducted and, after desizing, the residual size in the denim fabrics should be checked.

Damages

Causes

• During washing, sharp parts in the washing machine may come in contact with the denim garments, causing When using stones for washing, the stones may also get abraded along with the denim garment. Thus the quartz in the stone gets exposed and develops sharp points which may abrade on the garments leading to surface damage.

Remedial action

• When using stone, the hardness and size of the stone should be monitored to prevent excess abrasion which may cause surface damage.

Too light or dark final shade

Causes

• Chemical variation occurs during the washing process or chemical variation occurs between each batch of washing.
• Washing time and temperature are not properly controlled
• Variation in colour of each lot of denim fabric

Remedial action

• Check the concentration of chemical regularly to ensure that there is no loss of the function of chemical, for example enzyme activity of cellulase and active chlorine content in sodium
• Conduct trial washing to minimise the colour variation between each lot of denim fabric and find the suitable washing A trial run to find the variations of final shades in different lots of denim fabric after washing, as compared with the customer sample (black square) is shown in Figure 1.19.

Future trends in Denim Washing

The textile industry is striving to develop environmentally friendly wet processing techniques that can result in zero effluent discharge. Denim garment washing, being a type of textile wet processing treatment, cannot be separated from this trend for achieving sustainability. If the history of denim and jeans washing is reviewed, the washing depends much on the use of chemicals for achieving the colour fading effect. Even though enzymes have now replaced the oxidising agents to some extent, the process still involves water, and some chemicals may still be required to maintain the enzyme stability.

The dry treatments or nearly water free treatments are slowly becoming a sus- tainable trend for replacing traditional wet treatments in denim washing. Some of these techniques are promising and are commercially available such as laser treat- ment, ozone treatment and dry ice blasting. Laser treatment is a water free, colour fading treatment of denim and is an ecological and economical process. Laser can create local abrasion, fabric breaks and a used look effect with excellent reproduc- ibility and higher productivity. Being an automatic system, chances of human error are eliminated in laser treatment.

In ozone treatment, the ozone generated in the equipment can provide bleaching effect. Commercially available ozone equipment is operating like a washing machine, but without much use of water for the colour fading process. In dry ice blasting, the working principle is like sand blasting using abrasive materials. However, in this case, dry ice is used for the blasting. The advan- tage of using dry ice is that after blasting, the residual material is carbon dioxide gas without any secondary contaminants. In coming years, denim and jeans washings may involve only such dry treatments or water free treatments.

In the future, consumers may also desire to see washed denim garments having durable colours in diversified material types, and more environmentally friendly processes employed for denim garment washing. Thus the scope of the washing applications can be further broadened to cover more fibre types and colourants.

Conclusion

Denim fabric has first appeared in the late eighteenth century. It is a cotton twill fabric and the weft passes under two or more warp yarns. Denim was traditionally coloured blue with the indigo dye with ring dyed effect. Through the years, denim has evolved and has become one of the most fashionable items in the fashion and textile industry. It can provide durability and different appearance effects for the garment and fashion products.

Various washing effects have been introduced to make the denim garments and jeans attractive to consumers. Denim washing machines also play an important role and, depending on the market requirements, these machines can develop uniform colour fading or appearance effects in a short time. In the future, new sustainable types of denim washing techniques may be developed; however, some problems still need to be addressed, such as colour consistency, piece-to-batch reproducibility, yellowing and staining.