To lighten the jeans, preferably stonewashed goods, there are different bleaching agents possible with slightly different results refer to the color cast, some are more blueish some are more greyish. The most common once are described below.
The bleaching with Potassium Permanganate is a bit easier then with Hypo because it is a powder and doesn’t lose strength as long as you keep the container closed. Due to this you are able to obtain continuously the same outcome, as long as you keep the same conditions in terms of load weight, water ratio, temperature, run time, pH and grams of Potassium Permanganate. During the bleaching process do not arise acids, the goods are not so much stressed then with Hypo. But the blue cast is very grey and the garments become very flat. As a result of the chemical reaction Manganese Dioxide is formed which needs to be eliminated by neutralization with f.i. Na-Bisulfit, after the bleaching process is finished. Usually bleaching is done with the water ratio at 1:8 to 1:10, the pH slightly acid at 5 – 6, the temperature not higher than 40 °C. Latest after 15 min the process is finished and neutralization should be done for about 10 min.
A rather weak bleaching agent, however with good reproducibility and good contrast is Sodium Persulphate. It is necessary to work at high temperature (80 °C) and with high amounts (at least 6 g/l) to see a lighten effect. In many cases you need to repeat the procedure 2 or 3 times. The cast is bit greyish but not as much as with Potassium Permanganate. Water ration should be 1:7.
A stronger bleaching effect than with Sodium Persulphate you obtain with Glucose. It has also a quit good reproducibility and gives a grey cast, less than with Potassium Permanganate but more than with Sodium Persulphate. For good results you need about 10 – 15 g/l Glucose and to run at high temperature (80 °C) and with a high amount of alkali, at least 15 ml/l NaOH. Water ratio 1:7 and for ca. 30 min, then rinse warm and cold.
The glucose bleaching process is a reductive bleaching with glucose as reducing agent. Glucose converts at strong alkaline pH values and temperatures of at least 80 °C (mostly carried out at 90–95 °C) 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.
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 (chemical oxygen demand).
A good bleaching effect is also possible if you use Laccase. Sometimes the process must be repeated for 2 or 3 times but it is easy to handle. As it is an enzyme it keeps it strength as long as you keep the container closed and in not too hot or too humid conditions. It has a very good ecology and no waste water charge. It doesn’t damage the fiber and is very well suited for elastane fibers. As it bleaches just the Indigo pigment and no other dyestuff, the resulting cast is more grey-blue color. The Laccase oxidizes Indigo to soluble compounds. Usually the purchasable Laccases contain a buffer system to keep always the correct pH. The water ratio should be 1:4 to 1:6, Temperature 50 – 70 °C, pH 4 – 6 and the run time 20 – 30 min, perhaps several times.
In many above described cases the responsible bleaching atom is the active oxygen. So, why not to use it directly, f.i. out of Ozon. Ozon is a very reactive gas which gives one of its three oxygen atoms very easy free for further reactions. This free and high reactive oxygen atom “bleaches” the Indigo. Usually you need a generator to produce out of f.i. compressed air the Ozon gas. This gas needs to be discharged into a washer with jeans. The goods can be wet or dry. If the garments are wet the bleaching effect is stronger, faster. The obtained color cast is also a bit more greyish than bleached by Hypo. Hypo bleaches everything: Indigo, other dyestuffs and the white yarn as well. That’s the difference!
Bleaching with Peroxide is especially used on black Denim to obtain a nice clear and bright grey color. For low temperature bleaching with Peroxide like 50 °C and pH 9 – 10 use complex builder to soft water and remove any foreign ions such as Fe. Use Peroxide activator from any supplier and work in high water ratio like 1:10 with slow rotation to avoid bleach escapes.
For high temperature bleach at 90 – 95 °C and pH 12 – 13 you need same but instead of activator you need stabilizer! If you do so the oxygen will not escape too fast but working on the garment. The best and cheap stabilizer is after my experience Na2SiO3 (Waterglass). TAED (Tetra-Acetyl-Ethylene-Diamine) is also a very good stabilizer. It is usually used along with Detergent and can be obtained in granulated form. It can enable bleaching right down to ~ 70 °C. Or use any organic stabilizer from supplier.
Some jeans have a bottoming with Sulfur dyes, especially with Sulfur Black. This is the most important dyestuff for black denim and also used for topping or bottoming of dark indigo blue dyeing. But not all bleaching agents have the same effect in bleaching Sulfur dyes. If the bleaching process only affects the indigo dye, black or grey shades will be seen.
Hydrogen peroxide is a chemical compound, which reacts as an oxidizing agent. Bleaching with hydrogen peroxide is typically chosen for denim articles whenever a high whiteness is requested. Hydrogen peroxide dissociates only slightly in an aqueous medium. The free acid has only a very weak bleaching action. For this reason, hydrogen peroxide bleaching is carried out exclusively in alkaline medium. If the hydrogen peroxide solution is activated with alkali, then the dissociation equilibrium is moved to the right and the concentration of perhydroxy anions is increased:
Stabilization of the hydrogen peroxide is of fundamental importance to achieve a uniform bleaching effect. Moreover, for bleaching in hard water the use of a complexing agent is required. Otherwise catalytic damages may cause small holes in the garment.
The bleaching process can be stopped by 1-2 rinse baths and a neutralization of the alkali process with acetic acid. But residues of peroxide can cause catalytic damages. By using catalase the processor is on the safe side. Catalase is a common enzyme, which catalyzes the decomposition of two molecules of hydrogen peroxide to water and oxygen.
Bleaching with Hypochlorite is the most cheap way of lighten blue jeans and it is the general way how to do it. You can find it all over the world and it gives a pretty “sky blue” cast. To neutralize the arising acids during the bleaching process, it is useful to work with NaOH and a pH around 12. Otherwise the tear strength is going down too much. 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, it is more save to run the bleaching at lower temperature (40 – 45 °C). After obtaining the desired bleach effect, a neutralization of the Hypo rest is necessary. This is done usually in an oxidative way (with Peroxide) plus a reductive way (with Na- Bisulfit). The water ratio for bleach bathes should be 1:7 or 1:8, the temperature not higher than 50 °C and the amount of Hypo chosen referring to the desired look. All over a bath should not run longer than 20 min. When the desired look is achieved the first neutralization of the rest Hypo should be done with Peroxide adding it into the same bath and let the machine run for another 3 – 5 min. Then rinse cold and start the second neutralization with cold water and Na-Bisulfit for another 5 – 10 min, rinse again. The disadvantage of Hypo is, that when you open the container it loses strength during the time, especially in summer time (hot and bright). That makes it for the Laundry Manager difficult to choose always the correct amount of Hypo. He must check the bleaching continuously by watching the garments.
The actual bleaching agent in bleaching liquors is not sodium hypochlorite (NaOCl) itself, but the hypochlorous acid (HOCl) which is formed from it in a pH sensitive equilibrium. In order to ensure mild bleaching conditions, a pH range between 10 - 12 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 between 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 10 – 12, at pH 5 – 7 the activity is the lowest.
Even 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.
But the “too grey” garments can be made again a bit more blueish with following recipe:
1.0g/l NaOH + 3.0-5.0 g/l H2O2 + 1.0 g/l Dispergent
Water ratio 1:7 | Temperature 70 – 80 °C | run time 5 – 15 min
Rinse several times
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