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 SHESHIR
ID: 2010000400008
Batch: 13th Batch (Session 2009-2013)
Department: Wet Processing Technology
Email: mazadulhasan@yahoo.com
Blog: www. Textilelab.blogspot.com
Southeast University
Department of Textile Engineering
PREPARED BY ©right

Prof . Engr. Dr. Ayub Nabi Khan Bleaching 3
Bleaching
The 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.
Objects
A 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 agents
Oxidative bleaching agent
Hydrogen peroxide bleaching.
Sodium chlorite bleaching.
Hypochlorite bleaching.
Percretic acid
Ozone
Bleaching powder
Reductive bleaching
Zn dust
Staneous chlorite
Ferous sulphate
Sulpher dioxide
Sodium 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 bleaching
NaOCl Antichlore treatment Cold wash
Ca(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 + H2O 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

Conc. Of NaOCl g/liter
avail. Cl2
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

avail. Cl2
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 White

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.
avail. Cl2
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 - -
80
2 10 32.5 85.0
10 0 66.0 72
60
2 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 Bleaching
The 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 Bleaching
Two 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 Bleaching
The 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 them
properties 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.
•Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching 37

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 procedure
A 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

Effects of different types of bleaching

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