Wednesday, December 16, 2009

Bleaching With Peroxy Compounds

Of the peroxy type bleaches, sodium perborate (NaBO34H2O) is the least expensive. Organic peroxides such as dibenzoyl peroxide, are used in the bleaching of flour.Hydrogen peroxide is generally made from nonrenewable resources, but decomposes into hydrogen and oxygen
Bleaching with peroxides.
Hydrogen peroxide
 (H2O2), a colourless liquid usually produced as aqueous solutions of various strengths, used principally for bleaching cotton and other textiles.Hydrogen peroxide is a simple chemical compound. It's water with an extra atom of oxygen attached to it, H2O2. Hydrogen peroxide is valuable as an oxidizing agent (like ozone, or bleach for example) because it can release that single oxygen atom in the presence of another reactive substance. This reaction is called oxidation or bleaching.
Bleaching Process with Hydrogen Peroxide
Hydrogen peroxide bleaching can be done by
1.Batch wise, 
2.Continuous 
3.Semi continuous method.
a.Quantity of peroxide required in Bleaching
The quantity of hydrogen peroxide is used at 4- 6% on the weight of goods , but must be optimized to get the desired ground whiteness of the fabric.
b.Temperature
Cotton and Bast fibers are bleached at 80 - 95°C in bath processes, while blends of cotton and regenerated cellulose fibers are bleached at 75 - 80°C. The bleaching time is generally between 2 and 5 hours. In a pressurized high temperature (HT) apparatus cotton can also be bleached at temperatures of 110 - 130°C in only 1 to 2 hours.
c.Time
During the impregnation processes the temperature and as well the retention time varies widely. During a cold bleach process a dwell time of 18 to 24 hours is necessary. In the pad steam process under atmospheric pressure the bleaching time is generally between 1 to 3 hours. The above mentioned processes describe batch processes. Today a lot of continuously, intelligent finishing equipment exists in which the bleaching step is only one of some other treatments and the reaction time of the impregnated material in such steamer is only between 7 to 20 minutes. In general these bleaching process correspond to a preliminary bleach.
d.pH:-The pH value depend on the fibres to be bleached and pre-treatment.
NaOH is used in case of H2O2 bleaching. This is used to bring the PH upto 9-10 because H2O2 become active at this pH or oxidation is start at this pH.
For the bast fibres, such as linen, weaker alkaline or soda alkaline baths are used in order to avoid a cottonizing. Regenerated cellulose fibres are more sensitive. Therefore, they are only bleached in weak alkaline baths.
Alkali sensitive animal fibers must be bleached in very weak alkaline solutions. Phosphates and ammonia are most widely used as alkalization source. With tetrasodium pyrophosphate simultaneously a stabilization of the bleaching liquor can be attained.

e.Water Quality
Soft Water free of iron and copper impurities is recommended for peroxide bleach treatment.
f.Peroxide Stabilizers
High pH and temperature lead to the faster decomposition of peroxide bleaching liquor and degradation of cellulose.The role of the stabilizer is simply to control or regulate these effects the
act as buffers, sequestrates and in special cases, enhancing performance of the surfactant used in the bleach bath.
 For caustic alkaline bleach sodium silicate, organic stabilizers or the combination of both are suitable. In weak alkaline baths the addition of tetrasodium pyrophosphates can be used alone or together with an organic stabiliser.
Advantages of Peroxide Bleaching:
1.Among the oxidizing bleaching agents, only hydrogen peroxide provides a high bleaching effect     at reasonable costs, especially if modern short-term bleaching processes are used with only a        few minutes bleaching time.
2.Peroxide bleaching keeps the fibre quality intact.
3.Cotton can be bleached with peroxide in a single stage. Other processes require two or three
   bleaching stages,(desize with scour, scour with bleach and desize with scour and bleach).
4.No separate pre treatment is necessary because hot, alkaline bleaching has not only a bleaching but also a cleaning effect, it therefore combines  the advantages of an alkaline extraction with the bleaching treatment.
5.Animal fibres can only be bleached with peroxide to a high and stable degree of whiteness.
- Corrosion of stainless steel equipment does not occur during peroxide bleaching.
6.The spent peroxide baths still contain residuals of hydrogen peroxide which fever the                  degradation of the organic impurities in the effluent, and this helps to decrease the chemical        oxygen demand (COD).
Bleaching with sodium perborate
Sodium Perborate monohydrate (Synonyms: sodium peroxyborate, sodium peroxoborate, PB1) is similar to sodium percarbonate as a famous oxygen bleach agent. It has been described as a stable, solid form of hydrogen peroxide allowing its introduction into the wash at the same time as the detergent. Sodium perborate is a gentler bleach than sodium hypochlorite, causing less damage to fabrics and dyes, but by itself is only effective at high (>60ºC) temperatures. Although solid chlorine bleaches exist, they are rarely used in laundry detergents.
 The oxidative power of sodium perborate improves the cleaning, bleaching, stain removal and deodorizing performance of powder detergent formulations, all fabric dry bleaches, denture cleaners, automatic dishwasher detergents and various institutional and industrial laundry products.
Bleaching with peracetic acid.(CH3COOOH)
Peracetic acid is a colorless liquid with a pungent, sour smell and is included in the chemical group of hydroperoxides. It is highly oxidant and corrosive and can explode at temperatures exceeding 110ºC. It is more fat-soluble than hydrogen peroxide.
Peracetic acid is produced by the chemical reaction of acetic acid and hydrogen peroxide. It works in a very narrow pH range of 7 to 8. Below pH 7.0 the bleaching is not proper and above pH 9.0 fiber degradation takes place. Peracetic acid is used a bleaching agent for nylon, and acetate where hydrogen peroxide can not be used. It can be used for viscose rayon and even cotton also.
Per-carbonates
Sodium percarbonate (Synonyms: sodium carbonate peroxyhydrate, sodium carbonate peroxide, PCS) is a free-flowing powder with a common name of solid hydrogen peroxide, it is an addition compound of sodium carbonate and hydrogen peroxide. Percarbonate has an active available oxygen content which is equivalent to 27.5% H2O2. It breaks down to oxygen, water and sodium carbonate upon decomposition. It's an environmentally safe bleaching agent.

Per-carbonates have increased in use as bleaching agents due to their ‘kind nature’. Per-carbonate behave quite similar to the per-borates by creating active oxygen and they also needs stabilisers for storage, but the active effects starts at 400C and hence no activator is needed. The reaction products – carbonates – are substances naturally found in the environment.
Percarbonate is used as an activated oxygen component in detergents, bleaches and cleaning agents.
Persulfates
Ammonium and sodium persulfates are used in the desizing and bleaching of textiles and the development of dyestuffs.

Molecular Formula : Na2S2O8
Molecular weight : 238.13

Sodium persulfate is a white,crystalline,odourless salt.It is used as initiator for the Polymerization of monomers and as a strong oxidizing agent in many applications.It has the particular advantage of being almost non - hygroscopic,of having a particularly good storage stability as a result of its extremely high purity and of being easy and safe to handle. 

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