The dyeing and finishing of read-made jeans garments has gained increasing prominence in recent years. It has gained a position of ever-increasing importance in the clothing market, due to wearing comfort and fashion. Designers learned to make jeans attractive to an ever increasing number of customers through innovative modifications and styling.
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.
Let us now discuss the different bleaching processes, their benefits and features, and also the problems of the various treatments. Most bleaching processes are made on open-pocket washing machines or barrel machines (sometimes in Asia), in which many denim products can be treated together. The difference between these machine types is the diameter of the drum and the speed of rotation. The recipes and running times in this report are based on recipes for open-pocket machines. But there are also some mechanical and technical treatments, which are carried out for each single garment.
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).
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 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 Clothings" 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 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 as of today they are not yet used in jeans bleaching process.
The electrochemical bleaching system (ECB) of indigo dyed jeans garments was developed at the institute for Textile Chemistry and Textile Physics, Dornbirn, 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 remove the decomposition product from the surface and a dispersing agent hold it the bleaching bath.
This article was originally published in the August issue of magazine, "New Cloth Market - The complete textile magazine from textile technologists."