Bleaching is chemical treatment employed for the removal of natural coloring matter from the substrate. The source of natural color is organic compounds with conjugated double bonds , by doing chemical bleaching the discoloration takes place by the breaking the chromophore , most likely destroying the one or more double bonds with in this conjugated system. The material appears whiter after the bleaching.
Objects of Bleaching:
A bleaching agent is a substance that can whiten or decolorize other substances. Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing agents or reducing agents .
Bleaching agents are classified as:
b)Hydrogen peroxide bleaching
(ii) Reducing agents.
(i) Oxidizing agents:
Generally oxidative bleachings are carried out hypochloretes, sodium chlorite or peroxy compounds. Natural fibres like cotton, ramie, jute, wool, bamboo are all generally bleached with oxidative methods. The double bond breaks in the oxidative bleaching, which makes the substrate colorless.
Na-hypochlorite (NaOCl), Ca-Hypochlorite [Ca(OCl)],
Hydrogen peroxide (H2O2), Ozone (O3)
Reducing bleaching agents:
Reductive mothod of bleaching is done with sodium hydrosulphite sodium sulphide and sulfur dioxide are powerful reducing agents. Fibers like polyamide, Polyacrylics and Polyacetates can be bleached using reductive bleaching technology. Double bond is converted into single bond in case of reductive bleaching.
Carbon monoxide (CO), Hydrogen sulphide (H2S), Zinc dust (Zn)
Sulphur di-oxide (SO2), Sodium bi-Sulphate (NaHSO4)
Hypochlorite Bleaching process:
The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness required, types of machine and next operation.
Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it is necessary to optimize it on a possible lower value for safe bleaching process.
How to Test available chlorine content in a bleaching bathANALYSIS METHOD
Determination of available Chlorine.
Apparatus :Erlemeyer flask(100 ml) -I
Burette stand :I
Pipette(50 ml) :I
Measuring Cylinder :I
Micro burette(10ml.) :I
Measuring Cylinder. :I
Reagent :Sodium Thio Sulphate Solution(.1N)
Glacial acetic acid
Pottassium Iodide(KI crystal)
Starch indicator solution.
1) Collect sample solution ( Sodium hypo chlorite ) in an air-tight bottle.
a) Keep sample solution away from excessive light and do not agitate the sample solution.
b) Start chlorine determination IMMEDIATELY after sampling. ( i.e. do not store sample to be analysed for chlorine ).
2) Introduce 10ml of glacial acetic acid and about 0.5 g of Pottassium Iodide (Spatula full ) crystal into a 100ml erlemeyer flask.
3) Withdraw y ml of sample solution with bulb pipette and introduce into above Solution.
4) Swirl the flask to well mix the solution.
5) Titrate with 0.1N Sodium Thio Sulphate Solution from upto a light
6) Add 1 ml of starch indicator solution and continue to titrate until the last drop of titrant causes the solution to turn colourless.
% active chlorine = 3.545 x N x V/y (i;e 3.545 x N x V divided by y)
V = Volume of (N ) Na2S2O3
N = Normality of Na2S2O3
y = Sample amount
How to test caustic gpl in bleaching solution:
pH :General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth.The PH value is maintained by adding sod ash or by buffering agent.
Temperature:The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with increases of temperature.
Water Quality:-water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++.
Effect of metals:The bleaching equipment should be made of stainless steel , to avoid the catalytic degradation of the cellulose in the presence of copper and iron.
The substrate must be pre scoured in the presence of chelating agents , it should be free from rust spots and traces of metallic impurities when bleaching with hypochlorites.
Bleaching Time :The time factor depends upon the following consideration.
(b) PH value.
(c) Degree of Whiteness.
(d) The type of machine used in bleaching.
Roughly for normal machine the time is 2-3 hrs is required for completion of bleaching process.
antichlor and proper neutralization treatments should be followed by proper wash for removal of reagents after hypochlorite bleaching process.
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
3.Semi continuous method.
a.Quantity of peroxide required in Bleaching
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.
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.
Soft Water free of iron and copper impurities is recommended for peroxide bleach treatment.
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 with 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.