Wet Processing Engineering-1

Preparatory Process

Dr. Abu Bakr Siddique, WPT-1

Flow chart of pretreatment processes
with basic objects

Dr. Abu Bakr Siddique, WPT-2

Inspection of grey fabric:

Fig.6.1: Online Inspection

The fabric found from loom or knitting machine is

passed through the inspection table to find any kind of
fault such as end breakage, picks, holes, neps etc. &
then fault free fabric is selected for subsequent
processing. It may be manually or online Dr.done.
Dr.Abu
AbuBakr
BakrSiddique,
Siddique,TRM2-3
WPT-3
II) Stitching:
Sometimes the woven fabric from loom is not of
enough length as required & in such cases the
fabrics are joined to make it larger through
stitching.

III) Shearing & Cropping:

The excess warp or weft yarn on fabric edges
are removed by cutting with blade or scissors,
this process is called searing & cropping

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Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-4
WPT-4
Shearing & Cropping
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-5
WPT-5
 IV) Singeing:
A singe is a slight scorching, burn or treatment
with flame. The process of removing loose,
hairy & projecting fibers from fabric surface by
burning is called singeing and that uses gas
flame or hot plate in order to get rid of small
fibers protruding (hairs) on the fabrics (these
fibers normally come from spun yarns).

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-6
WPT-6
It has the following objectives:
1. To get rid of the small fibers
2. To make the fabrics smoother than before
3. To help the printing ink or dyestuff to make
clearer marks on the fabrics
4. To improve rubbing fastness and washing
fastness of the dyed or printed fabrics

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-7
WPT-7
Fig.6.3: Gas flame singeing

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-8
WPT-8
 V) De-sizing:
Sizing materials are added to increase the
strength of warp yarn. It needs only weaving
process. For knitting as well as Non-woven
process, no need the sizes on the fabrics thus no
need to desizing. During wet processing, various
chemicals are applied & before starting these
processes we must be sure of maximum
absorption of these chemicals. For this, the
sizing materials are removed by de-sizing. If the
sizing materials cover the warp yarns, it will
lead to uneven dyeing, printing or finishing.
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-9
WPT-9
 V) De-sizing:
If not taken out completely, it will affect
washing and rubbing fastness of the fabrics.
Desizing depends upon the sizing material,
desizing agents, temperature, washing,
penetration depth of sizing agents, adhesions
between sizing materials and fabric materials,
allowing the desizing agent to degrade or
solubilise the size material, and finally to wash
out the degradation products. The major

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-10
WPT-10
 Methods of De-sizing:

Hydrolytic method: Enzymatic -, Washing with

hot water -, Acid - desizing
Oxidative desizing: Chlorine -, Chlorite -,
Bromite - & Peroxy de-sizing

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-11
WPT-11
 Enzymatic de-sizing
These enzymes, as vary through their source,
they also vary according to their application
condition.
Enzyme Temperature (°C) Concentration (g/L) pH value

Pancreatic 50 – 55 1.0 – 5.0 7.0 – 7.5

Malt 55 – 60 3.0 – 20.0 5.5 – 6.5

extract
Bacterial 60 – 65 0.5 – 3.0 5.5 – 7.5

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-12
WPT-12
Enzymatic desizing is the classical desizing
process of degrading starch size on cotton fabrics
using enzymes. Enzymes are complex organic,
soluble bio-catalysts, formed by living organisms,
which catalyze chemical reaction in biological
processes. Enzymes are quite specific in their
action on a particular substance. A small quantity
of enzyme is able to decompose large polymer
molecules. Amylases are the enzymes that
hydrolyse and reduce the molecular weight of
amylose and amylosepectin molecules in starch,
rendering it water soluble enough to be washed off
the fabric. Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-13
WPT-13
 Effective enzymatic desizing
Effective enzymatic desizing requires strict
control of pH, temperature, water hardness,
electrolyte addition and choice of surfactant.
Starch is decomposed by the enzyme of the
malt (amylase) and by maltase into individual
molecules of α-glucose.
Name of some Enzymes
Pancreatic: Lipase
Malt extract: Diastase, Deglatol, Gabalit etc.
Bacterial: E. coli, Biolase, Rapidase etc.
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-14
WPT-14
Enzymatic desizing

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-15
WPT-15
 The oxidative de-sizing:
In oxidative desizing, the risk of damage to the
cellulose fiber is very high, and its use for
desizing is increasingly rare. Oxidative desizing
uses potassium or sodium persulfate or sodium
bromite as an oxidizing agent.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-16
WPT-16
 WATER-SOLUBLE SIZES
De-sizing of water-soluble sizes using washing
procedure water molecules opened the water
soluble groups –OH or –COOCH3.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-17
WPT-17
 VI) Scouring:
This is a process in which the fabric is treated
with alkali at room temperature or at suitable
higher temperatures & by this process oil, fat,
wax & other natural & added impurities are
removed. Absorbency of the fabric also
increases a greater extent in this process i.e.
makes the fabrics hydrophilic.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-18
WPT-18
 Scouring reaction:
Raw cotton is hydrophobic because, fiber
surface is protected with cuticle and cuticle
consists of oil, fat, wax. These are high
molecular fatty acid i.e. steeric acid
(CnH2n+1COOH, where n: 17-35). If NaOH
(Alkali) reacts with steeric acid, cuticle is
removed from fiber surface and regular cellulose
chains are opened and the fiber became
hydrophilic.
CnH2n+1COOH+NaOH= CnH2n+1COONa +H2O
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-19
WPT-19
Impurities of Raw – Cotton
Raw cotton contains a wide range of organic &
mineral impurities in such amount that vary
according to the origin of the cotton.
Pectins 0.7 – 1.2 %
Waxes 0.4 – 1 %
Proteins 1.1 – 1.9 %
Inorganic components 0.7 – 1.6 %
Other organic compounds 0.5 – 1.0 %

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-20
WPT-20
Objectives of scouring:
To remove natural fat, wax, and oil materials
containing in the fabrics without damaging the
fibers
To accelerate dye and chemical absorption of the
fabrics
To improve the handle of the goods (softer)

Chemical Used: Caustic soda and soda ash are

mainly used for scouring of cotton fabric.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-21
WPT-21
 Some Scouring recipe
Cotton Jute Polyester

Wetting agent : 0.5 – 1.0 g/L Wetting agent : 1.0 – 2.0 g/L Wetting agent: 0.5 – 1.0 g/L
Detergent : 1.0 – 2.0 g/L Detergent : 0.5 – 1.0 g/L Detergent :1.0 – 2.0 g/L
Sequestering agent 1.0 – 3.0 g/L Sequestering agent 1.0 – 2.0 g/L Sequestering agent: 1.0 – 2.0 g/L
Caustic soda (36°Be) 2.0 – 4.0 g/L Caustic soda : 0.5 – 1.0 g/L Soda ash : 2.0 – 3.0g/L
Temperature : 95 - 100°C Temperature : 90 – 95°C Caustic soda : 0.5 – 1.0 g/L
Time : 15 – 45 minutes Time : 45 – 60 minutes Temperature : 80 – 90°C
pH : 10.5 – 11.0 pH : 10.5 ±0.5 Time : 15 – 30 minutes
M:L - 1: 10 M: L - 1:10 – 1:20 pH : 9 – 10
M: L - 1:10

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-22
WPT-22
VII) Bleaching:
Natural coloring matters and remaining trace
impurities are removed from the grey fabric in
this process. So, the fabric gets a permanent
white color which is very important for the
following dyeing or printing process. The degree
of bleaching necessary is determined by the
required whiteness and absorbency.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-23
WPT-23
Objectives of bleaching:
• To whiten the goods
• To make the goods to be suitable for dyeing
and printing with pale or bright shade
• To be follow with adding optical brightening
agent process (super white)

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-24
WPT-24
Types of Bleaching Agents

Bleaching process can be performed by two

types of bleaching agents.
1) Oxidizing agents
2) Reducing agents

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-25
WPT-25
Oxidizing Bleaching: There are several
oxidizing bleaching agents. Such as
 Cl2,
 O3, Ca(OCl)Cl,
 H2O2,
 NaOCl,
 Ca(OCl)2,
 K2Cr2O7,
 Na2Cr2O7,
 KMnO4,
 KClO3,
 Na2O2 etc.
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-26
WPT-26
Why natural cellulosic fibers are bleached by
oxidizing agents?
Oils, waxes & other coloring matters are present
in the natural fibres as impurities. Natural
coloring matters are mostly nitrogenous
compounds. To remove these impurities
oxidizing agents are used. Because if we use
reducing agents for bleaching, afterwards they
can react with oxygen of the air & can make
yellowish or reddish color on the material. In
case of oxidizing agent there is no such chance
of yellowish color formation
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-27
WPT-27
Reducing agents
The reducing agents which are used for
bleaching action are H2, C, CO, H2S, SO2,
FeSO4, SnCl2, TiCl2, Zn, Na2SO4 etc.

Why protein fibres are bleached by reducing

agents?
Most of the oxidizing bleaching agents contain
chlorine (Cl) in their composition. This chlorine
can react directly with the protein of fibre & can
damage the fibre. Thus, protein fibres are
bleached with reducing agents (mainly SO2).
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-28
WPT-28
Bleaching Recipe for cotton:
Peroxide(H2O2) Bleaching Sodium Hypochlorite (NaOCl)
Wetting agent 0.5 – 1.0 g/L Wetting agent 0.3 – 0.5 g/L
Sequestering agent as required Caustic soda (36° Be) 0.5 – 1.0 g/L
Detergent 1.0 – 2.0 g/L Soda ash 2.0 – 4.0 g/L
Stabilizer 0.3 – 0.8 g/L NaOCl solution 5.0 – 15.0 ml/L
Caustic soda (36°Be) 1.0 – 3.0 g/L Temperature 25 – 60°C
H2O2 (35%) 4.0 – 8.0 g/L Time 30 – 60 minutes
Temperature 90 – 95°C pH 10.5±0.5
Time 20 – 45 minutes M: L 1: 10
pH 10 – 11
M: L 1: 10

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-29
WPT-29
Combined Scouring & Bleaching

Instead of stepwise scouring & bleaching, now –

a – day’s combined process is getting popularity.
This is mainly because to simplify the process. It
may also reduce the cost, save energy, decrease
effluent & requires less time. Industrially
combined scouring & bleaching is popular
today.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-30
WPT-30
Typical combined Scouring and Bleaching recipe
For Cotton For Jute
Wetting agent : 0.5 – 1.0 g/L Wetting agent (TRO) 1.0 – 2.0 g/L
Detergent: 1.0 – 2.0 g/L Detergent 0.5 – 1.0 g/L
Peroxide stabilizer: 0.5 – 1.0 g/L Sequestering agent 1.0 – 2.0 g/L
Caustic soda (36° Be): 3.0 – 6.0 g/L Na-silicate 1.0 – 2.0 g/L
H2O2 (35%): 2.0 – 6.0 g/L Caustic soda 0.5 – 1.0 g/L
Temperature: 95 – 100°C Soda ash 4.0 – 6.0 g/L
Time: 40 – 60 minutes H2O2 (50%) 6.0 – 8.0 g/L
pH: 11±0.5 Temperature 85 – 90°C
M: L: 1: 10 Time 40 – 60 minutes
pH 10.5 – 11
M: L 1:10 – 1:20
Polyester
Wetting agent: 0.1 – 0.5 g/L
Detergent: 1.0 – 2.0 g/L
Peroxide stabilizer: 0.5 – 1.0 g/L
Caustic soda (36°Be): 0.5 – 1.0 g/L
Soda ash: 1.0 – 3.0 g/L
H2O2 (35%): 2.0 – 6.0 g/L
Temperature: 90 – 100°C
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-31
WPT-31
Estimation of Bleaching Effect
Two tests are performed for the assessment of
bleaching effect.
1) Degree of whiteness
2) Permanency of whiteness

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-32
WPT-32
Reflectance: When a fabric is bleached, the
capacity of reflecting incident light upon it
increases. Bleaching effect is measured by
reflectance. Reflectance can be measured by
“Spectrophotometer” or “Reflectometer”. Here,
reflectance 100 means a perfect white whereas 0
means perfect black.
For testing, the standard range of reflectance is
considered as follows. Reflectance 68 – 70
means a “Good”, 70 – 72 means a “Very Good”
and a reflectance value of 73 means an
“Outstanding” whiteness. Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-33
WPT-33
Permanency of whiteness:
Degree of whiteness of bleached goods is
measured. If the degree remains same for 7 days
– then the whiteness should be considered as
permanent.
Souring: Bleaching is done in the alkaline
medium. Since it is difficult to remove alkali from
the fabric this is followed to neutralize the alkali
with an acid, the excess of which can then be
easily washed out. This process is known as
“souring” in which the bleached material is treated
with dilute H2SO4 or HCl at room temperature.
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-34
WPT-34
Mercerizing is a chemical process to improve
the luster & increase strength. It also has some
other engineering advantages. In this process the
fabric is treated with specific concentration of
alkali (NaOH) under tension. The is to make the
cotton goods more luster than before using
sodium hydroxide solution in cold condition and
tension. This will lead the cotton fibers to be
swollen and increased strength. Also this will
improve dye absorption of the fibers. John
Mercer(1844) has invented this phenomenon in.
Therefore, the process was called “Mercerizing”
Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-35
WPT-35
Caustizing is the process nearly the same as
mercerizing but it uses lower concentration. To
make the goods more luster because raw cotton
fibers are kidney-shaped. After this process, the
fibers change to cylindrical shape resulting more
evenness reflected light from the goods. To
improve dyeability of the goods about 5-10%

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-36
WPT-36
 Effect of Mercerizing

Optical refraction before

mercerizing
After mercerizing

Dr. Abu Bakr Siddiue, WPT-1

REM view before mercerizing After mercerizing

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-38
WPT-38
 Effect of Mercerizing

Microscopic view before mercerizing After mercerizing

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-39
WPT-39
 Effect of Mercerizing

Structural view before After mercerizing

mercerizing (not oriented) (oriented)

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-40
WPT-40
X) Washing:
Normally after every wet process, the fabric is
washed with hot & cold water. Sometimes soap
solutions are also used for washing.
XI) Rinsing:
The process of washing something with clean
water to remove soap, detergent, dirt, or
impurities from fabric is called rinsing.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-41
WPT-41
XII) Drying:
Drying means the removal of excess water from
fabric. The wet fabric after various processing
stages is dried in different types of drying
machines.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-42
WPT-42
Chromophore, Chromogen & Auxochrome.
Chromophore:
The name is derived from greek word “chroma”
meaning “color’ & “phore” from protein meaning
“to bear”. The color bearing unsaturated group
of a dye molecule is called chromophore group.
Examples are

Dr. Abu Bakr Siddique, TRM2-

 Surfactants
Surfactants: are wetting agents that lower the
surface tension of a liquid, allowing easier
spreading, and lower the interfacial tension
between two liquids.

Dr.Abu
Dr. AbuBakr
BakrSiddique,
Siddique,TRM2-44
WPT-44