Bleaching with Peroxides
The bleaching bath is composed of hydrogen peroxide (35% or 50% by wt.) as the bleaching agent, an activator (usually alkali) and stabilizers.
Bleaching Process with Hydrogen Peroxide
Hydrogen peroxide bleaching can be done by
1.Batch wise,
2.Continuous
3.Semi continuous method.
Factors of Peroxide Bleaching:
a.Quantity of peroxide required in Bleaching
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 of Wool with Hydrogen Peroxide
After scouring, wool may be bleached by immersion or pad and dry techniques, using alkaline or acid solutions.
Bleaching of Silk with Hydrogen Peroxide
Prior to bleaching, silk is usually degummed. Hydrogen Peroxide addition assists this process and it is universally used as the bleaching agent for natural silk, usually in an alkaline solution.
Bleaching of synthetic fibres Hydrogen Peroxide
When used alone, synthetic fibers do not normally require bleaching. However, blends of synthetic fibers with natural or regenerated fibers, e.g. cotton-polyester are frequently bleached. The most popular bleaching agent is Hydrogen Peroxide and it is used in both batch and continuous processes.
Advantages and disadvantages of peroxide over hypochlorite bleaching.
Bleaching with Sodium Perborate
Sodium perborate (PBS, NaBO3.nH2O where n=1 or 4) can readily be incorporated. 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.
Bleaching with Sodium Chlorite.(NaClO2 )
The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.
a. bleaching mechanism
b. effect of pH.
c. effect of temperature.
d. effect of metals
Bleaching with Peracetic Acid
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.
Reductive Bleaching Systems
Reductive bleaches work by reducing colored impurities into colorless forms.
1.sodium hydrosulphite.
Sodium hydrosulphite is available as free flowing powder and a strong reducing agent. This is explosive in nature when come into contact with water. It is available in different purity ranges.
2.sodium sulphide.
Sodium sulphide is also a strong reducing agent
3.sulphur dioxide
Sulphur dioxide was used as a bleaching agent in early 20th century for bleaching of wool.
The bleaching bath is composed of hydrogen peroxide (35% or 50% by wt.) as the bleaching agent, an activator (usually alkali) and stabilizers.
Bleaching Process with Hydrogen Peroxide
Hydrogen peroxide bleaching can be done by
1.Batch wise,
2.Continuous
3.Semi continuous method.
Factors of Peroxide Bleaching:
a.Quantity of peroxide required in Bleaching
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 of Wool with Hydrogen Peroxide
After scouring, wool may be bleached by immersion or pad and dry techniques, using alkaline or acid solutions.
Bleaching of Silk with Hydrogen Peroxide
Prior to bleaching, silk is usually degummed. Hydrogen Peroxide addition assists this process and it is universally used as the bleaching agent for natural silk, usually in an alkaline solution.
Bleaching of synthetic fibres Hydrogen Peroxide
When used alone, synthetic fibers do not normally require bleaching. However, blends of synthetic fibers with natural or regenerated fibers, e.g. cotton-polyester are frequently bleached. The most popular bleaching agent is Hydrogen Peroxide and it is used in both batch and continuous processes.
Advantages and disadvantages of peroxide over hypochlorite bleaching.
Bleaching with Sodium Perborate
Sodium perborate (PBS, NaBO3.nH2O where n=1 or 4) can readily be incorporated. 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.
Bleaching with Sodium Chlorite.(NaClO2 )
The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.
a. bleaching mechanism
b. effect of pH.
c. effect of temperature.
d. effect of metals
Bleaching with Peracetic Acid
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.
Reductive Bleaching Systems
Reductive bleaches work by reducing colored impurities into colorless forms.
1.sodium hydrosulphite.
Sodium hydrosulphite is available as free flowing powder and a strong reducing agent. This is explosive in nature when come into contact with water. It is available in different purity ranges.
2.sodium sulphide.
Sodium sulphide is also a strong reducing agent
3.sulphur dioxide
Sulphur dioxide was used as a bleaching agent in early 20th century for bleaching of wool.
Reductive Bleaching of silk
Commonly Sodium hydrosulphite, Sulfurdioxide and sodium sulphoxylates are the reductive bleaching agents which are used for silk.
Reductive bleaching of nylon
Oxidative bleaching isn’t suitable for polyamides as H2O2 attacks the polymer, instead reductive dyeing using sodium hydrosulphite is used.
Commonly Sodium hydrosulphite, Sulfurdioxide and sodium sulphoxylates are the reductive bleaching agents which are used for silk.
Reductive bleaching of nylon
Oxidative bleaching isn’t suitable for polyamides as H2O2 attacks the polymer, instead reductive dyeing using sodium hydrosulphite is used.
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