Bleaching Processes in Denim Washing Industries and Problem Solution

Bleach in Denim Wash

Definition: Bleach refers to the chemical (typically sodium hypochlorite or hydrogen peroxide) used to lighten the color of denim fabric by removing dye.

?Purpose: achieve lighter shades of denim (from mid-blue to almost white).

To create a base for further processes like stone washing, acid washing, or tie-dye effects.

?Process:

1.Denim is soaked or treated with bleach, either directly or diluted with water.

2.The duration and concentration of bleach determine the lightness achieved.

3.The process is stopped by neutralizing the bleach with anti-chlor agents (like sodium metabisulfite) to prevent fabric damage.

?Effect on Fabric: Excessive using bleach can weaken the fibers, making the denim softer but potentially less durable.

Bleach Wash in Denim

?Definition: Bleach wash is a denim washing technique that uses bleach in controlled conditions to achieve specific aesthetics and textures.

?Purpose:

To create unique, uneven fading or marbled effects.

To soften the denim and enhance its comfort.

?Variations:

Light Bleach Wash: Slightly lightening for a subtle faded look.

.Heavy Bleach Wash: Intense lightening, often leading to very pale or white denim.

The dyeing and finishing of readymade jeans has gained increasing prominence in recent years. Due to their comfort and fashion, jeans have gained an ever-increasing importance in the clothing market. Designers have learned to make jeans attractive to an ever-increasing number of customers through innovative modifications and styling.

Purpose of Bleaching:

Bleaching denim, of the garment washed, are stone-washed and bleached with different chemicals and treatments. Bleaching is a treatment of textiles in order to lighten the fabric for the final shades. The purposes of doing Bleaching are:

?To achieve the fading effect on the garments.

?To remove the starch present on the garments.

?To remove the size material from the garments.

?To achieve a soft effect on the garments.

?To increase colorfastness.

?To increase rubbing fastness.

Bleach is the generic name for any chemical product that is used industrially or domestically to remove the color (whitening) from fabric or fiber or to clean or remove stains in a process called bleaching. It often refers specifically to a dilute solution of Calcium Hypochlorite and Sodium Hypochlorite, also called "liquid bleach".

Bleaches work by reacting with many colored organic compounds, such as natural pigments, and turning them into colorless ones. While most bleaches are Oxidizing Agents (chemicals that can remove electrons from other molecules), some are Reducing Agents (that donate electrons).

?Chlorine is a powerful oxidizer and is an active agent in many household bleaches. Since pure Chlorine is a toxic corrosive gas, these products usually contain Hypochlorite, which releases chlorine. "Bleaching powder" usually refers to a formulation containing Calcium Hypochlorite.

?Oxidizing bleaching agents that do not contain chlorine are usually based on Peroxides, such as Hydrogen Peroxide, Sodium Per-Carbonate and Sodium Per-borate. These bleaches are called "Non-chlorine bleach", “Oxygen Bleach ‘’or ‘’Color-Safe Bleach’’.

?Reducing bleaches has niche uses, such as Sulfur Dioxide, which is used to bleach wool, either as gas or from solutions of Sodium Dithionite and Sodium Borohydride.

Bleaches generally react with many other organic substances besides the intended colored pigments, so they can weaken or damage natural materials like fibers, cloth, and leather, and intentionally applied dyes, such as the Indigo of Denim. For the same reason, ingestion of the products, breathing of the fumes, or contact with skin or eyes can cause bodily harm and damage health.

?Oxidizing bleaching agents that do not contain chlorine are usually based on Peroxides, such as Hydrogen Peroxide, Sodium Per-Carbonate and Sodium Per-borate. These bleaches are called "Non-chlorine bleach", “Oxygen Bleach ‘’or ‘’Color-Safe Bleach’’.

?Reducing bleaches have niche uses, such as Sulfur Dioxide, which is used to bleach wool, either as gas or from solutions of Sodium Dithionite and Sodium Borohydride.

Bleaches generally react with many other organic substances besides the intended colored pigments, so they can weaken or damage natural materials like fibers, cloth, and leather, and intentionally applied dyes, such as the Indigo of Denim. For the same reason, ingestion of the products, breathing of the fumes, or contact with skin or eyes can cause bodily harm and damage health.

Mechanism of bleaching action:

Color in most dyes and pigments is produced by molecules, such as beta carotene, which contain Chromophores. Chemical bleaches work in one of two ways:

?An oxidizing bleach works by breaking the chemical bonds that make up the Chromophore. This changes the molecule into a different substance that either does not contain a Chromophore, or contains a Chromophore that does not absorb visible light. This is the mechanism of bleaches based on Chlorine but also of oxygen-anions which react through the initial Nucleophilic Attack.

?A reducing bleach works by converting double bonds in the Chromophore into single bonds. This eliminates the ability of the Chromophore to absorb visible light. This is the mechanism of bleaches based on Sulfur Dioxide.

Sunlight acts as a bleach through a process leading to similar results: high energy Photons of light, often in the Violet or Ultraviolet range, can disrupt the bonds in the Chromophore, rendering the resulting substance colorless. Extended exposure often leads to massive discoloration usually reducing the colors to a white and typically very faded blue.

Bleaching with Calcium Hypochlorite (CaOCl2):

Bleaching powder is a mixture of calcium hypochlorite, CaOCl2 and calcium chloride CaCl2. Thus, its chemical composition is CaOCl2 + CaCl2 or can be written as CaOCl2 or Ca(OCl)Cl (Calcium Chloro Hypochlorite).

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime at 400C.

Ca(OH)2 + Cl2 ← CaOCl2 + H2O

It is a yellowish white powder with strong smell of chlorine and is soluble in water. Aqueous solution of bleaching powder contains chloride and hypochlorite ions.

CaOCl2 ? Ca2+ + Cl- + ClO-

In presence of small quantities of dilute acids, it liberates nascent oxygen. Thus, it acts as oxidizing agent.

2CaOCl2 + H2SO4 → CaCl2 + CaSO4 + 2HClO

HClO ← HCl + [O]

When bleaching powder is treated with excess of dilute acid or CO2, whole of the chlorine present in the molecules is evolved. The amount chlorine so liberated is called available chlorine. A good sample of bleaching powder contains 35-38% available chlorine.

CaCl2 + 2HCl → CaCl2 + H2O + Cl2

CaOCl2 + CO2 → CaCO3+ Cl2

Bleaching with Sodium Hypochlorite (NaOCl):

Bleaching with sodium hypochlorite is by far the most common bleaching process. It is used when the indigo's color needs to be lightened as required by fashion, as well as clean up the pocket linen and the inner face of the jeans (weft). This particular bleaching process is preferably used whenever a strong contrast between blue and white fibers is desired, since it gives the bluest cast of all bleaching processes. Moreover, this process is very cost-effective, however, a major disadvantage is that the AOX content, a measure of organic halogen compounds, often exceeds' the permitted effluent pollution.

Sodium hypochlorite is a powerful oxidizing agent with a corresponding high redox potential. Hypochlorite consequently reacts relatively unselective; it reacts even with the fibers. The danger of fiber damage with this bleaching agent is much greater compared to other bleaching processes. Due to its high oxidation agent potential, hypochlorite bleaching is carried out at low temperatures (approx. 40C).

The actual bleaching agent in bleaching liquors is not sodium hypochlorite (NaOCI) itself, but the Hypochlorous acid (HOCI) which is formed from it in a pH sensitive equilibrium. In order to ensure mild bleaching conditions, a pH range between 9.0-11.5 has been found as optimum range for effective bleaching. Since, under practical conditions, pH levels below 10 are difficult to keep constant, an initial pH range in 11.0-12.5 is selected, as a rule to take into account the consumption of soda ash or caustic soda.

The control of the pH value is very important. The strongest bleaching activity is at pH 9.5 - 12.5; at pH 5 - 7 the activity is the lowest. After reaching the required bleaching effect, the bleaching process has to be stopped in the same bath by using hydrogen peroxide or sodium bisulfite.

Another disadvantage of hypochlorite is that the solution loses strength over time, especially at higher temperatures in hot regions. This makes the reproducibility much more complicated, because the amount of hypochlorite should be adjusted by the strength of the solution. But mostly it is only adjusted by continuously checking the bleaching progress.

Bleaching with Potassium Permanganate:

Potassium permanganate is a very strong oxidative agent. It's a deep red-violet metallic shiny crystalline solid. It can be dissolved in water to form a dark violet solution which is well suitable to bleach denim. Potassium permanganate can be used for local bleaching by spraying application or for complete treatments in drum washing machines. The potassium permanganate solution must be mixed very well with the washing liquor, because of dissolved permanganate can cause different shades in only one bath.

After reaching the desired bleaching effect the rest of unconsumed potassium permanganate and the formed brown manganese oxide hydrate have to be removed thoroughly. After 1-2 rinse bathes the removal of the remaining manganese oxide, is usually carried out with sodium bisulfate, respectively S02.

The bleaching process can be used even for every pale shade, because potassium permanganate can reduce indigo as well as sulphur dyes. When a longer bleaching process is applied, the jeans processed accordingly can even be completely decolorized.

The reproducibility of the bleaching effect is very good, as long as the same conditions are kept in terms of load weight, liquor ratio, temperature, run time, pH and amount of potassium permanganate. The fabric is not as stressed as with hypochlorite. However, the blue cast is very grey and the garments become very flat.

Hydrogen Peroxide Bleaching:

Hydrogen peroxide bleaching has gained an importance in view of the effluent problems caused by hypochlorite bleaching. Hydrogen peroxide is a chemical compound, which reacts as and oxidizing agent. Bleaching with hydrogen peroxide is typically chosen for denim articles whenever a high whiteness is requested. But also black denims are usually bleached with hydrogen peroxide, because it avoids a yellowish tone, as it is obtained in case of hypochlorite bleaching.

Bleaching with Organic Peroxide:

The bleaching with organic peroxide is comparable to bleaching with hydrogen peroxide. Both are compounds of the peroxide family. The bonding between the both oxygen atoms is very instable and can be easily split at mild conditions. One of the most important organic peroxides in terms of applications is dibenzoyl peroxide.

In the textile industry this product is used in lower concentrations (approx. 20-30% in mixtures). Benzoyl peroxide is a very aggressive and corrosive product. The most common mixtures are based on inert material like calcium chloride.

In the jeans bleaching has the product very versatile and dynamic properties. On different material it can have different effects. It is therefore not possible to give standard recommendations. The bleaching effect increases proportionally to the amount of organic peroxide. But this product can also be used for Jeans containing elastic fibers. The bleaching effect can be stopped by adding caustic soda to the bleaching bath. Blue denims get after the bleaching a grey cast.

Bleaching with Laccase:

Laccase are useful enzymes for industrial applications. They would be used in the beginning for the waste water treatment. The advantage of enzyme products is the specific treatment. This product preferentially breaks down the indigo molecule, without affecting sulphur, reactive or direct dyes. This process is not useful, if the fabric contains special bottoming or top- Comparison of denim treated with cellulose and pings which are typically made with said other dyestuff classes.There is a tendency to have a grayish or antique shade, as opposed to the light blue cast normally associated with sodium hypochlorite bleaching.

Laccase does not affect harmfully on the strength of the fabric and is also suitable for denim containing elastic yarns. It also does not damage leather labels or metal trims. Laccases have relative low redox potential, which can be increased though a mediator-involved reaction mechanism.

Bleaching with Ozone:

Ozone is a very reactive gas which can easily release one of his three oxygen atoms for further reactions. This free and high reactive oxygen atom bleaches the indigo molecule. Usually a generator is needed to produce ozone gas from compressed air. This gas needs to be injected into a washing machine loaded with jeans. The garment can be wet or dry. If the fabric is wet the bleaching effect is stronger and faster. The obtained color cast is also a bit grayish than bleached with hypochlorite.

Bleaching with Glucose:

The glucose bleaching process is a reductive bleaching. This bleaching process is an inverse dyeing process with glucose as reducing agent. Glucose converts at strong alkaline pH values and temperatures of at least 80C (mostly carried out at 9Q-95C) the indigo pigment into the water soluble leuco indigo, which can strip easily from the fiber.

In the bleaching bath a strong dispersing agent is needed, so that the leuco indigo does not dye the white weft yarn. Due to the oxygen in the drum washing machine the soluble leuco form converts back to the insoluble indigo pigment. This pigment form is drained with the bleaching bath, followed by 1-2 rinse bathes.

The bleaching process with glucose was promoted for long time as the preferred ecological form of bleaching, however, this process requires much energy due to high temperature, and the waste water has a high pH value and a very high COD load.

Sandblasting:

Sandblasting is a mechanical method to lighten Jeans. In this process the sand is blown with high pressure to the surface until the blue shade gets lighter and the vintage look arise. The abrasion on the surface gives a look "like used yet'. Starting end of the 90ies the vintage look has been a high fashion look with a great demand.The biggest problems of the sandblasting process with high pressure are the working conditions. Most workers do not wear neither protective masks nor workwear. Due to the high pressure quartzous dust particles are formed. If this mineral gets in the lung, composite nettings results in cicatrization of the lung. At the end the worker suffers from silicosis, which is known in the metal and mining industry.

Since 2008 several media published reports about the sandblasting process and the problem of workers affected by silicosis. The campaign "Clean Clothing" and the Swede Fair Trade Center tried to convince the brands to abstain from the sandblasting for jeans treatment. In the meantime most jeans brands have put sandblasting on their restricted lists. Turkey has restricted the use of sandblasting in 2009, but mostly this treatment is used in countries like Pakistan, China, Bangladesh and Egypt. In particular, sandblasting is used for plagiarism of the big brands.

Electrochemical Bleaching:

Electrochemical procedures are methods, in which electrons are transferred between chemicals via electrodes through electrical currents. In the production of reduction and oxidation agents electrochemical processes are well known, and frequently used in the production of various raw materials. But today they are not yet used in the jeans bleaching process.

The electrochemical bleaching system (ECB) of indigo-dyed jeans garments was developed at the Institute for Textile Chemistry and Textile Physics, Durbin, based on former studies in the institute about the waste water discoloration with an electrochemical treatment. Based on the comparison with other bleaching systems, it was shown that the total eco balance of the production of blue jeans based on pre-reduced indigo, and using the electrochemical bleaching process, is the optimum.

The ECB has economic and ecological advantages against the other bleaching processes. The process is easy to carry out, well repeatable, cost-saving and possibly with low waste water.

ECB can be used for oxidative or reductive processes. In both processes a mediator is needed -like in the laccase process.

In the oxidative ECB process, the mediator is oxidized at the anode. The activated mediator oxidizes the water-insoluble indigo on the fabric surface to a water-soluble decomposition product. The movement in the washing machine removes the decomposition product from the surface and a dispersing agent hold it in the bleaching bath.

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Some solution :

Common Problems and Solutions in Denim Bleaching with Chlorine

1. Uneven Bleaching

?Cause: Poor distribution of bleach or improper washing machine loading.

?Solution:

Use a proper liquor ratio to ensure even bleach distribution.

Pre-dissolve chlorine bleach in water before adding to the machine.

Use tumbling machines with optimized rotations for better agitation.

2. Fabric Strength Loss

?Cause: Overexposure to chlorine or inadequate neutralization after bleaching.

?Solution:

Use controlled bleach dosing and monitor the bleaching duration carefully.

Neutralize chlorine residues with sodium bisulfite, sodium metabisulfite, or hydrogen peroxide after bleaching.

3. Yellowing

?Cause: Overuse of bleach or improper neutralization.

?Solution:

Avoid over-bleaching. Monitor pH levels during and after the process (ideal pH: 6-7 for neutralization).

Use optical brighteners or anti-yellowing agents after bleaching.

4. Environmental Concerns

?Cause: Chlorine bleach generates harmful byproducts like dioxins and organochlorines.

?Solution:

Replace chlorine-based bleach with eco-friendlier alternatives, such as ozone bleaching, potassium permanganate, or enzyme-based processes.

Treat effluent water using advanced treatment methods like Reverse Osmosis or Activated Carbon Filtration.

5. Corrosion of Metal Parts in Machines

?Cause: Chlorine's corrosive nature.

?Solution:

Ensure regular machine maintenance and protective coatings.

Flush machines with neutralizers immediately after chlorine-based processes.

Alternatives to Chlorine-Based Bleaching

1.Ozone Treatment:

Environmentally friendly and effective in achieving a faded look.

Requires less water and chemicals.

2.Enzymatic Bleaching:

Uses enzymes like laccase to break down indigo dye.

Gentle on fabric and eco-friendly.

3.Potassium Permanganate Spraying:

Used for localized bleaching effects.

Requires careful handling due to potential residues.

4.Laser Technology:

Non-chemical method for achieving faded effects.

High precision and minimal environmental impact.

Best Practices for Chlorine Bleaching

?Conduct small-scale trials before full-scale production.

?Use high-quality chlorine bleach with low impurity levels.

?Train operators to control and monitor all process parameters (time, pH, temperature, and chemical concentration).

?Implement a robust effluent treatment plan to mitigate environmental damage.

By addressing the specific root cause of the problem and incorporating eco-friendly practices, you can minimize issues related to chlorine-based denim bleaching while improving quality and sustainability.

Written By

Syed Maruf Ahmed

MSc in Textile (KUET)

Assistant Technical Officer Ha-Meem Group

(Maximum Information collected from my dear sir Moniruzzaman sir AGM Technical Ha-Meem Group)