Effects of different types of bleaching

School of Science and Engineering, Department of Textile Engineering

SOUTHEAST UNIVERSITY

Effects of different types of bleaching

Name : MAZADUL HASAN SHESHIRID: 2010000400008Batch: 13th Batch (Session 2009-2013)Department: Wet Processing Technology Email: [email protected]: www. Textilelab.blogspot.com Southeast UniversityDepartment of Textile Engineering

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Prof . Engr. Dr. Ayub Nabi Khan Bleaching 3

BleachingThe process of decolorize the natural coloring matter present in the cloth treating with some oxidizing agent and reducing agent and ensure the permanent whiteness is called bleaching.

ObjectsA high uniform absorbency of fabric to water and dye stuffs.Uniform degree of whiteness.Fabric should not damage and DP should remain high.Destruction of natural coloring matters from the fabric.To ensure a level dyeing properties.To make the textile materials suitable for subsequent processing. (Dyeing, printing, etc.)



Types of bleaching agentsOxidative bleaching agent

Hydrogen peroxide bleaching.Sodium chlorite bleaching.Hypochlorite bleaching.Percretic acidOzoneBleaching powder

Reductive bleachingZn dustStaneous chloriteFerous sulphateSulpher dioxideSodium hydrosulphate





Reducing bleaching is a temporary bleaching process because after bleaching there is possibility that the oxygen in the air may reoxidize and it will return to its original state. But oxidizing bleaching is permanent because it gives almost invariably a more permanent white.

Bleaching can be done in various process:1. Hypochlorite bleaching for 100% cotton (less used)2. Hydrogen peroxide bleaching for 100% cotton (more used)3. Sodium Chlorite bleaching only used for special sector and only for polyester and cotton blend.4. Peracetic acid bleaching can also be done for the better improvement of the process and this is not still used in our country commercially.



Hypochlorite bleaching

Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching 6




In textile hypochlorite bleaching sodium hypochlorite [NaOCl] or calcium hypochlorite [Ca(OCl)2] may be used as hypochlorite bleaching agent.

Differences between Ca(OCl)2 and NaOCl bleaching

Ca(OCl)2 NaOCl

1.It is unstable 1.It is stable

2.It produces CaCO3 precipitate 2. It doesn’t produce any precipitate

3.It makes harsh feeling on the fabric 3.It doesn’t make harsh feeling on

the fabric

4.Comperatively cheaper than

NaOCl bleaching

4.Higher cost than Ca(OCl)2

bleaching



Sequence of cotton bleachingNaOCl Antichlore treatment Cold washCa(OCl)2 souring Antichlore treantmnet Cold was

It is seen that souring is used only for Ca(OCl)2 but antichlore treatment is used for

both. Cause when Ca(OCl)2 is used, it reacts with atmospheric CO2 to give CaCO3

as white precipitate. CaCO3 deposited on the fabric causing harsh handling and

uneven dyeing, hence it has to separate and souring (acid treatment) is done to

remove it.

Ca(OCl)2 + CO2 + HO2 CaCO3 + HOCl




Souring

The treatment by which the fabrics after processing with alkali (scouring/bleaching) are treated

with HCl or dilute H2SO4 for removing alkali or neutralization for alkali is called scouring.

This process is necessary in case of Ca(OCl)2 bleaching.

CaCO3 + 2HCl = CaCl2 + CO2 + H2 O

CaCO3 + H2SO4 = CaSO4 + CO2 + H2 O

Antichlore treatment

In case of hypochlorite bleaching (OCl -) hypochlorous ion is produced, which will react with

residual protein to form chloramines > NCl which is corrosive and toxic. Cl2 is also produced

which also unhygienic and irritate to skin. Moreover >NCl reacts with moisture and gradually

cotton becomes yellowish due to forming of HCl.




OCl - + resedual protein >NaCl

>NaCl + H2 O HCl




Treatment procedure:In an experiment the hypochlorite bleaching scoured fabric samples (10 g. each) were treated with sodium hypochlorite solutions (0.5, 1.0, 1.5 g/liter available chlorine) at different pH values (11, 12, and 13) for 15 min., keeping a liquor ratio of 30:1 at 30º, 45º, 60º C. the bleached samples were washed, scoured with 0.1% hydrochloric acid and washed free from the acid.For the other purpose of comparison two pieces of scoured cloth were bleached with sodium hypochlorite solution (3 g/liter available chlorine) at pH 11 and 30ºC for 1 hr and scoured as above.




Result:


Conc. Of NaOCl g/liter avail. Cl2

Temp. ºC Whiteness

pH 11

0.5 30 Yellowish white

0.5 45 Dull white

0.5 60 White


1.0 45 White

1.0 60 White

1.5 45 White

1.5 30 Dull white




Temp. ºC Whiteness

pH 12

0.5 30 Good white

0.5 45 Slightly white

0.5 60 Dull white


1.0 45 White

1.0 60 White


1.5 45 Dull white

1.5 60 White





Temp. ºC Whiteness

pH 13

0.5 30 Dull white

0.5 45 White

0.5 60 White

1.0 30 Dull white

1.0 45 Cream white

1.0 60 White

1.5 30 Slightly white

1.5 45 White

1.5 60 WhiteProf . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching 14



At a given hypochlorite concentration, the bleaching efficiency increases with an

increase in either the temperature or in the pH. Good white effects are obtained when

bleached for 15 min. at 60ºC with 1.5 g/liter available chlorine. In case of 1 hr process

with 3 g/liter available chlorine at 30ºCwhite effects are obtained.


Temp. ºC Whiteness

pH 11

3.0 30 (1hr) White

3.0 30 (1hr) White




Advantages– Cheapest– Variety of machine can be used– Large batches fabric can be processed– Simple machine can be used

Disadvantages– There is a danger of damaging the fabric due to accidental lowering of pH.– The process is slow, it is carried out at low temperature and it is therefore

difficult to integrate into rapid continuous operation.– There is danger of yellowish upon storage.– The relatively high salt loads in the process are undesirable for ecological

reason




Hydrogen peroxide bleaching

Prof . Engr. Dr. Ayub Nabi Khan 17



Hydrogen peroxide is called universal bleaching agent and most widely used for bleaching and over 90% of all fibre can be bleached with H2O2. It can be used for vegetable fibre as well as protein fibre although the cost of H2O2 is greater then hypochlorite.

Properties of hydrogen peroxide:(a) H2O2 is a clear colorless liquid.

(b) Strength of H2O2 is 35% - 70%.

(c) Should store away from light.(d) In presence of heavy metal like gold, silver, platinum it decomposes with the (e) liberation of oxygen.(f) This material is irritant for skin(g) This is non flammable.

Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching 18



Bleaching methods with hydrogen peroxide:There are two chief methods for bleaching of cotton goods with H2O2. They are:

(a) Bleaching in Kier (discontinuous).(b) Bleaching in J-Box.

Treatment procedure:In continuous bleaching the cloth was first desized with desizing agent (3 g/liter) at Ph 7 for 2 hrs at 70˚ - 75˚C. This was followed by scouring by boiling in a stainless steel vessel with sodium hydroxide (3 g/liter), sodium carbonate 2(g/liter) and soap 2(g/liter) for 4 hrs. The cloth was then mercerized under tension for 10 min. with sodium hydroxide solution (50˚ Tw) containing wetting agent (15 ml/liter). It was washed , scoured with oxalic acid (10 g/liter) solution.the liquor ratio was 20:1.




Temperature ºC

Time, hr. Sodium silicate, g 100ºTw

solution/liter

Peroxide decomposition

%

Reflectance

95

2 5 36.0 84

2 40 95.0 88

½ 5 - -

½ 40 - -

802 10 32.5 85.0

10 0 66.0 72

602 40 67.0 86.5

10 5 - 78.5




It is seen that bleaching at 95˚ C for 2 hrs with 40 G/liter Tw solution of sodium silicate gave the best result with the reflectance of 88.0, but 95% peroxide has been decomposed. With the increase in time and temperature improved the bleaching rate but a marked decrease in peroxide decomposition has been observed.




Advantages– H2O2 doesn’t react with residual protein of fiber and no need of antichlore

treatment.– Permanent white cotton is obtained and the bleached fabrics are highly

hydrophobic since the waxes are solublised and removed by hot alkaline solution.

– Its reaction products are relatively non toxic and it decomposes to oxygen and thus reducing greatly the effluent pollution of the bleaching plant..

– Weight of fabric after H2O2 bleaching is higher than that of hypochlorite bleaching.

– Bleaching and dyeing can be sometimes combined in a single operation.Disadvantages

– It is irritant to skin.– Stabilizer should be used in order to hold the strength of H2O2

– It decomposes in the presence of heavy metal like gold, silver, platinum with the liberation of oxygen.

– Higher energy is needed to bleach the fabric





Peracetic acid bleaching



PAA (CH3COOOH) is a strong oxidizing agent with excellent antimicrobial and bleaching properties. Though it is not widely used in textile sector but has some advantages in comparison to HP bleaching.

Recipe for exhaustion bleaching with PAA:

Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching 24

Persan S15 (PAA) 6 to 240 ml/L

Temperature 60 C

Time 40 min

pH 7.5

M:L 1:20



Exhaustion BleachingThe cotton fabric samples were bleached with different concentrations of Persan S15, ranging from 6 to 240 ml/l at 60 °C, and at pH 7.5 for 40 minutes, in closed beakers in the Launder-Ometer apparatus at a cotton:liquor ratio of 1:20. The temperature of bleaching baths was raised from 25 °C to 60 °C within 15 minutes.

Impregnation BleachingTwo bleaching baths were prepared with 15 and 60 ml/l of Persan S15. The samples of cotton fabric were impregnated with the bleaching baths on a Werner Mathis AG (CH) two-roll padder with a wet pick-up of 100 %. Pad-batch BleachingThe samples impregnated with bleaching solutions were wrapped in a plastic sheet, sealed, and stored at room temperature (23 °C) or at 60 °C for 0.5 to 24 hours.




Pad-steam Bleaching

The samples impregnated with bleaching solutions were steamed for 10 minutes at

100 °C in a 100 % steam atmosphere in a Werner Mathis AG (CH) laboratory

steamer. Steaming was performed:

(a) Immediately after impregnation(b) After 1 hour of storage at room temperature(c) After 24 hours of storage at room temperature(d) After 1 hour of storage at 60 °C




Treatment process

Bleaching baths were prepared with demineralized water. First, a corresponding

amount of Persan S15 was added into water at a temperature of 25 °C, and

secondly, the pH was adjusted to 7.5 by the addition of Na2CO3 at 0.5 M. After

completing all bleaching processes, the samples were washed once with hot (80 °C)

and twice with cold (40 °C) demineralized water, and finally dried in ambient

conditions.




Result


Person S15

(ml/l)

w L* C* h

6.0 70.3 94.3 3.4 90.8

7.5 74.2 94.5 2.7 89.4

10 74.5 94.5 2.6 89.9

15 79.2 95.3 2.0 89.5

24 82.0 94.9 1.2 80.1

30 83.7 95.0 0.9 73.1

40 85.3 95.1 0.7 52.3

60 88.2 96.3 0.6 58.8

120 92.5 96.5 0.4 154.2

240 94.4 96.7 0.8 301.3



The results for the degree of whiteness and the color coordinates in Table 1 reveal

that the concentration of the bleaching agent is very important in exhaustion

bleaching of cotton fabric with PAA. As expected, with increasing concentration of

PAA, the degree of whiteness was also increased. The highest whiteness was

obtained with 240 ml/l of Persan S15 (W = 94.7) and the lowest at 6 ml/l of Persan

S15 (W = 70.3). Along with the increase of the degree of whiteness, the lightness

(L*) was also increasing and chroma (C*) was decreasing. The hue (h) value shows

that most samples had a yellow hue, and only the samples bleached with the highest

concentration of PAA had a bluish hue. At lower concentrations of Persan S15 (up to

60 ml/l), the whiteness was lower (up to W = 88.2), but still high enough to allow the

dyeing in light, medium and dark shades.




Exhaustion bleaching with PAA at 60 °C and an initial pH of 7.5 for 40 minutes

gave whiteness values from W = 70 to W = 95, depending on the concentration of

the bleaching agent. The remarkable drop in DP occurred only at very high PAA

concentrations. In the cases where preparations with 15 % PAA (Persan S15) were

used, the use of 15 to 25 ml/l of bleaching agent can be recommended to obtain a

whiteness value of around W = 80 without any damage to the cotton fibers. With

60 ml/l of the bleaching agent, values near W = 90 could be achieved, but with a

small degree of damage to the fibers (DP ≈2070). Higher concentrations are not

recommended.




Advantages– There is no need of stabilizers, acetic acid, caustic soda, sequestrants and

dispersants.– Bleaching takes place at the temperature between 40 °C to 80 °C and at pH 7

to 8– Neutralization is not required.– Low energy comsumption.

Disadvantages– This is not used in commercially.– More experiments are needed.– Not available.




Sodium chlorite bleaching




Chlorine dioxide is a non-chlorinating reagent used to obtain a high degree of whiteness on cotton as well as synthetic fibers without degrading themproperties of sodium chlorite:(a) At temperature of up to 150ºC sodium does not decompose.(b) When heated to 200ºC for 30 min. less than 50% of sodium chlorite decomposed with the formation of sodium chlorate and chloride; less than 5% of the chlorine decomposed into sodium chloride and gaseous oxygen.3NaClO2 2NaClO3 + NaCl

NaClO2 NaCl + O2

(C) Sodium chlorite is fairly highly soluble in water.(d) Sodium chloride exists in two forms – NaClO2 (anhydrous) and NaClO2. 3H2O (trihydrate).

(e) Solid sodium chlorite has been found to be extremely stable. It decomposes in the temperature range 180º – 220ºC.

Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching 33



(f) Dilute solution of sodium chlorite containing 0.5% sodium hydroxide are stable for several days if protected from light of the laboratory.

(g) Increase in the concentration of sodium chlorite does not further accelerate the chlorite decomposition reaction.Bleaching process:Semi-continuous bleaching Two semi-continuous processes are used for open-width bleaching of cotton fabrics. In the first, called Pad-Roll, the fabric is saturated with a pad (80-100% delivery rate), placed in a steamer at 95-98°C and wound in a box in which steam is injected at a temperature of 95- 98°C to keep the air inside moist and warm. The fabric is spun on its shaft for 1-3 hours. In the second, called Pad-Batch, the fabric is impregnated with a pad, wound around a shaft at ambient temperature, wrapped in a sheet of plastic to avoid evaporation and spun for 15-20 hours. The fabric then passes into the washer, where it is rinsed in hot then cold water and then dried. Before being dried, it may be neutralized with acetic acid.




Fig: Pad - Roll process




Fig: Pad – Batch process




Continuous bleaching

Continuous bleaching lines generally utilize the Pad-Steam process and include

continuous desizing and boiling steps. Continuous processes are suitable for large

amounts of fabric of the same, or at least similar, quality. The machines used are a

saturator (pad), a preheater and a steamer in the shape of a ‘U’ or ‘J’ (called a U-box

or J-Box). The fabric travels through these chambers for a relatively long period of

time (30 minutes on average) in folded (cuttled) rope form (open-width boxes may be

used, but they are expensive and bulky) in a 100-102°C steam atmosphere. Several

units may be placed side by side so that the fabric may be desized, boiled and

bleached once or twice without stopping. A washer is placed at the end to rinse and

neutralize the fabric. Hydrogen peroxide is particularly suited to this type of process.•




Batch bleaching

Batch processes are used for small amounts of fabric. The fabric is bleached in

machines called winches with a high liquor ratio (1:20) or jet or overflow machines

with a low liquor ratio (1:5 to 1:10). If the latter are used, non-foaming auxiliary

agents are necessary on account of the agitation. Jiggers are best for crease-

sensitive fabrics (heavy fabrics or those with a high thread /count). The liquor ratio

is around 1:5. Kiers and beam machines are used more for delicate fabrics such as

gauze and bunting as well as knitted fabrics. The liquor ratio is nearly 1:10.




Result


Sample

no.

Concentratio

n of

NaOCl2,

g/liter

Temp.

ºC

Time required for

complete

decomposition of

NaOCl2, min.

Remarks

1 10 50 30 Not completely white, motes and

shieves still present.

2 20 60 30 Not completely white, motes and

shieves still present.

3 30 70 35 Fairly good white, shieves still present.

4 40 80 40 Good white, almost free from shieves.

5 50 90 40 Good white, almost free from shieves.



The cloth is bleached with sodium chlorite solution (40 g/liter) at 50º C the yellowness

persisted and so also motes and shieves. But its feel was better than that of grey cloth.

When it was bleached at 60ºC there was slight improvement, but yellowness was not

completely removed. Yellowness was absent on cloth bleached at 70ºC but some motes

and shieves still remained. When bleached at 80ºC and 90ºC, the cloth acquired good

whiteness with less no. of shieves. The sample bleached at the boil had the maximum

whiteness and was almost free from motes and shieves.

When the bleaching process was repeated once again, samples bleached at all the

temperatures had very good whiteness and were completely free from motes and shieves.

The feel was further improved by the second bleaching process and improved gradually

as the temperature increased.




Advantages– Permanent whiteness.– High brightness degree (especially for acrylic fibers)– Negligible degradation of fibers (1-2% weight loss for cellulosic fibers and no attack

to the polymeric chains in the synthetic fibers)– Lower environmental impact of wastewaters (negligible level of COD).– Versatile bleaching agent for cellulosics, synthetics and blends, especially poly-

cotton blends. It is also safe for those synthetic fibers which are sensitive to alkali.– Non sensitive to metal ions such as iron and Water hardness under acidic conditions.

Disadvantages– It is expensive as compared to hydrogen peroxide.– It cannot be used for wool and silk.– Chlorine dioxide is highly corrosive to metals and toxic in nature.– Bleaching in neutral and acidic pH damage cotton.




Conclusion:In textile factory before dyeing, pretreatment i. e. singeing, desizing, scouring and bleaching play an important role for the convenience in dyeing procedureA bleaching agent can whiten or decolorize a substance by reacting with the chromophores that are responsible for the color of the substance. Depending on the nature of the chromophores, the bleaching agent will either be an oxidizing or reducing agent. That is, the chromophore is either oxidized or reduced to produce a colorless or whitened substance.As there is various types of bleaching agent, selecting the suitable one is the major task to conduct. What may be the bleaching agent there are some parameters which have to be maintained strictly, they are: time, temperature and pH, otherwise the whole procedure would be faulty.


Effects of different types of bleaching

Engineering

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