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Saturday, January 24, 2015

What is bleaching? Bleaching process of Cotton

Definition of Bleaching

A process of whitening fibres or fabrics or depriving a colored material. This is brought about by using various bleaching-agents. Generally chlorine is used for cotton and other vegetable fibres and peroxide and sulphur dioxide for animal fibres.

Bleaching of cotton

Cotton is bleached in the raw state (as yarns) or in piece. The impurities present are cotton wax, fatty acids, peptic substances, coloring matters and albuminoids (another mineral matter). Although these are do not amount to more than 5% of the weight.
Bleaching of cotton is comparatively a simple process, the main operations being:
  • Boiling with alkali.
  • Bleaching the organic matters by some bleaching agents.
  • Souring treatment with dilute H2SO4 or HCl.

Loose cotton

In this process, the necessary operations are:
  • Steeping in warm solution of soda for some hours.
  • Washing.
  • Bleaching powder treatment or sodium hypochlorite.
  • Washing.
  • Souring with dilute acids.
  • Washing free of acids.

Cotton yarns

In principle the process employed for yarns is the same as for loose cotton, but this requires different machinery:
  • Boiling in alkaline lye. This is carried out in kier-boil (6 to 8 hours) in alkaline lye (3% soda ash or 2% caustic soda on weight of cotton).
  • Washing.
  • Washing.
  • Washing.
  • Bleaching powder solution 2% Tw treatment contained in stone cistern which is provided with a false bottom.
  • Washing.
  • Souring with dilute acids HCl about 2% Tw.
  • Washing to remove traces of acids.
  • Optical bluing.
The bleaching of cotton yarns is a very straight-forward process and with reasonable care, is almost foolproof.


By far the largest bulk of cotton-goods are bleached in the piece as this happens to be the most convenient form, the principle is similar to yarn-blending but here some alter-actions and additional operations have to be performed. These are necessary to remove factors like sizing and other chemical substances which are generally added in sizing or weaving operations. The sizing materials are starches, farinas with other substances like tallow, ZnCl2, soaps, MgCl2, etc., and sometimes some mineral oils. All these materials have to be completely removed if perfect results are desired. So two steps viz. singeing, and desizing become necessary.
The surface of the fabric, coming from weaving dept has a downy appearance as it is covered with nap and loose fibres. A clean surface of the fabric is absolutely necessary for further operations. The napping must be removed first, and this is achieved by singeing.
The cotton-pieces are run at full width through the singeing machines, over a non-luminous gas flame.
The operation simply consists of running pieces through ordinary washing machines.
In order to wet these out on leaving the machines, these are piled in heaps and left as such overnight when fermentation sets in which results in hydrolysis of starch and renders it soluble in water.

What is bleaching

Treatment with alkaline-charge (NaOH + Na2CO3)
In this operation the pieces are first run through alkaline of strength of about 4% of the weight of charge. They are run into kier.
After this the pieces are run through continuous washing machine with plentiful supply of water.
In this operation, the goods are run through a washing machine containing HCl of strength of 20 Tw. Object of this is to remove the lime completely. The goods are then well washed.
The pieces are then washed free of alkali and the bleaching proper or chemicking follows. This operation is effected in various ways. The most efficient or effective being to run the goods in a washing machine through bleaching powder solution (10 Tw), and allow them to lie loosely piled up overnight. The goods are then washed and run through very dilute solutions of acids (at 20 Tw), called white-sour and again washed thoroughly to free them completely from acids as otherwise this results in tendering of fibres.
The next operation of bleaching is scotching or opening the cloth out from rope form to its full width. After scotching the opened cloth is dried on drying machine.

Thursday, January 1, 2015

Textile Bleaching

What is Textile Bleaching?

Bleaching is the 3rd step of wet processing. The process by which the natural color of a fibre can be removed and make the textile material pure white and bright is called bleaching.

Objects of bleaching

There are many objects as follows –
·        To remove the natural color.
·        To make the fabric permanent white and bright.
·        To increase the absorbency power of the cloth.
·        To increase dye affinity.
·        To ensure level dyeing property.
·        To make the fabric suitable for the next subsequent process.

Types of Textile Bleaching agent

They are in two types as follows –
·        Oxidizing and
·        Reducing
Oxidizing Bleaching Agent
The chemical agents which remove coloring material from textile materials by oxidizing reaction and make the material pure white and bright and to increase absorbency power is called oxidizing bleaching agent. Ex: O3, Ca(OCl)Cl, H2O2, NaOCl, Ca(OCl)2, K2Cr2O7 etc.
Generally natural vegetable fibres are bleached by these bleaching agents.
Reducing Bleaching Agent
The chemicals which removes coloring materials from textile materials by reduction reaction and make the material pure white and bright and increase absorbency power is called reducing bleaching agent. Ex: H2S, FeSO4, NaHSO4, Na2SO4 etc.
Generally protein fibre i.e. silk, wool, etc. are bleached by these agents.

Factors/Conditions of bleaching process

Bleaching process depends on some important conditions. Some important conditions are given below –
·        Concentration of the chemical.
·        M:L ratio.
·        PH of the liquor.
·        Temperature of the liquor.
·        Time of treatment.
·        Catalyst.

Machine Chemicking bleaching process

Machine chemicking process is one of the discontinuous or, batching processes. This process takes more time.
            HOCl – 3-4Tw0
            Temperature – Below 400C
            Time – 8-12 hrs
            PH – 9.2-11
Bleaching solution is to be prepared according to the above recipe. Here, material and liquor ratio is less. First the fabric is openly passed through the guide roller and immersed into the bleaching solution by immersion roller.

Textile bleaching

Then the fabric is squeezed by a pair of squeezing roller and pilling in a definite box. This fabric is kept in this box for a certain period for bleaching action. Then the fabric is treated with 0.5% HCl or H2SO4.

Faults in bleaching and their remedies

Several faults of Bleaching are as follows –
·        Incomplete and uneven bleaching
·        Over bleaching
·        Weaken the fabric.
·        Roughness of the textile material.
·        Yellowish effect over the textile material.
Incomplete and uneven bleaching
·        For using insufficient bleaching chemicals.
·        Uncontrolled temperature.
·        Sufficient chemicals should be used.
·        Temperature should be controlled.
Over Bleaching
·        For using excess/over chemicals.
·        Sufficient chemicals should be used according to the weight of the textile material.
Weaken the fabric
·        Prolong stay in the sunlight causes weaken the fabric.
·        Incorrect concentration of the solution.
·        We should protect the bleaching process from the sunlight.
·        Concentration of the solution should be correct.
Roughness of the textile materials
When we use calcium hypochlorite for bleaching purpose, hypochlorite is reacted with the atmospheric carbon-dioxide and thus produces calcium carbonate which is precipitated. Calcium carbonate is insoluble in water and if attached to the fabric surface that makes fabric rough.
Ca(OCl)Cl + CO2 → CaCO3↓ + Cl2
Fabric should be scoured for removing calcium carbonate.

What are the points to be considered before bleaching?

The points that are considered are given below –
·        In case of proteinous fibre reducing type of bleaching agent is used.
·        If proper bleaching is not done, it will not give proper result.
·        Jute, Hemp, Flax type fibres required two types of bleaching chemicals, Firstly, oxidative is used and finally the reducing type agent is used.
·        Most of the bleachings are performed in the presence of ideal bleaching agent. H2O2 may be termed as ideal bleaching agent because, it is very cheap. It has no side effect.

Description of the bleaching of cotton by hydrogen per-oxide (H2O2)

Thenard produced H2O2 from BaO2 and dilute HCl in 1818. He named this as oxygenated water.
BaO2 + 2HCl BaCl2 + H2O2
H2O2 (35% S) – 0.75 litre/kg (According to the wt. of the material)
            NaOH – 0.05 kg
            Na2CO3 – 0.02 kg
            Wetting agent – 1%
            NaSiO2 – 0.07 kg
            H2O2 (50%) - 0.5 l/kg
            H2O – 100 l/kg
H2O2 is an oxidizing agent which is used largely for cellulose fibre bleaching. H2O2 is also used for protein fibre such as wool, silk-bleaching etc. In H2O2 bleaching, H2O2 releases hydrogen ion (H+) and per hydroxyl ion (HO2-). Here, PH level is maintained 10.7 – 10.9.
Per hydroxyl ion is released for alkalinity of hydrogen per-oxide solution. This per ion bleached the textile materials. H+ ion has no bleaching action. It maintains PH level.
(HO2-) + Colored materials = Bleached materials + H+
H2O2 is normally unstable, moreover Cu, Ni, Fe, etc. catalyst decomposes H2O2 and produces H2O and O2.
2H2O2 + Catalyst 2H2O + O2
The produced O2 has no bleaching action. So, bleaching ability can be reduced if water contain Cu, Ni, Fe, etc ion, then H2O2 catalytically decomposes and reduces bleaching ability. So, all the above matters should be considered in H2O2 bleaching.

Wednesday, February 26, 2014

Singeing with objects | Gas flame singeing machine

What is singeing | Objects | Gas flame singeing m/c | Advantages and disadvantages

Singeing is carried out rarely on knitted fabrics and frequently on yarns and woven fabrics. Instead of the traditional singeing process, it is possible to apply an enzymatic treatment (for cotton and lyocell fabrics), for example with cellulose, which uses chemical agents to corrode the fibre surface and remove the fuzz from the fabric.

Singeing is also done at yarn stage. For the yarn singeing it is an operation carried out in order to eliminate yarn hairiness. The singeing system consists of a package to package winder and a gas burner. The yarn is passed through the flame, which singes the protruding fibres that cause the hairiness. It runs at a rate of 400 to 1000 m/min. The machine must, in order to obtain even singeing, maintain a constant machine speed and an even flame.
The process by which the projecting or floating or hairy fibres stand out on the fabric surface are burnt off, is called singeing.
                   a)    To remove hairy fibres projecting on the surface of cloth and to smooth surface.
                   b)   Optical levelness of the dyeing and clean out lines of a printing design. 
                   c)    To increase luster in the finished fabric.
                   d)   To prepare the fabric for next process.
Gas flame singeing m/c:
This is the most commonly used process for woven fabric. In gas singeing machine both side of the fabric can be singed in same time. Normally two burner are used for both side singed. In this machine the fabrics are passed through some guide roller over the gas burner in open width as shown in the figure. The length of the burner can be adjusted according to the width of the fabric. The fabric passes over the burner at the speed of 200-400 yard/min. When the fabric passes over the burner, it burns the projecting fibre from the surface of the fabric.
This is the most commonly used process for woven fabric. In gas singeing machine both side of the fabric can be singed in same time.

Saturday, December 7, 2013

Definition of pretreatment and Utilities

What is pretreatment?|Utilities of pretreatment

Pretreatment : The process which is done to make the textile materials suitable for dyeing &printing .Such as singeing ,desizing ,scouring, bleaching etc.

Why we do pretreatment/utilities of pretreatment:
1. High absorbency
2. Low ash contain
3. High degree of whiteness
4. Optimum swelling of fibre
5. Good dimensional stability
6. Minimum loss of degree of polymer (D.P)
7. Shorter reaction time
8. Less energy consumption
9. Less Labour time