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ASHISH KUMAR ANNEPU
ROLL NO.-6
DFT SEM 5PREPARATORY PROCESSESSubject :- Dyeing and Printing
FABRIC PREPERATION - INTRODUCTION
Textile materials posses various kinds of impurities which may be natural or
inherited in nature.
These impurities may be added purposely for better spinability or weaveability.
These impurities need to be removed from the fabric before dyeing it or printing
it.
The steps by which these impurities are removed are known as
PREPARATORY PROCESSES.
As the impurities present may differ from fiber to fiber the preparatory process
may differ and the conditions too may differ considering the fiber properties.
Preparatory processes may be classified into two main classes as follows:-
a. Cleaning processes where the impurities are removedby physical or
chemical means.
b. Whitening process ,in which trace colouring materials are destroyed
and the whiteness of the materials is improved optically.
1.A Objectives of preparatory operations
The main objectives of the preparatory processes of textile materials are as follows:
Remove the impurities from the fibers ,both natural as well as added impurities
as it may interfere in subsequent processes of dyeing or finishing applications.
Improve the capability of the fibers to absorb water solutions of dyes and
chemicals.
Impart proper required brightness or whiteness to the fibers according to
need ,especially when brilliant or pastel shades are desired.
Impart dimensional stability to thermoplastic textile materials.
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1.B Preparatory processes
The preparatory processes can be generalized into the following categories:-
Removal or loose fibers or yarn projections from the fabric surface. In
Singeing the fibers are burnt away by flames, while in Shearing and
Cropping the projecting fibers are cut by a extruder type blade.
The process of removal of sizing material applied upon the yarn before
weaving is known as Desizing.
The most important of all processes is of Scouring in which most of the
water soluble and water insoluble impurities are removed.
The last process is of destroying the colouring materials by the process of
Bleaching, this helps in increasing the optical brightness of the fabric.
Synthetics are processed by the an addition preparatory process known
as Heat setting.
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2. CELLULOSIC MATERIAL
2.1 Cotton and cotton blends
Cotton fibers on the average consist of 90%-96% cellulose when bone-dry. The
approximate amounts of other materials present in the fiber are: 1.1-1.9%
proteins, 0.7-1.2% pectin, 0.4-1.0%waxes, 0.7-6% ash, and 0.5-1.0% other
impurities.
These materials are located mainly in the primary walls of the fiber that is at or
near the fiber's surface. Because of their hydrophobic nature it is difficult to wet-
out unsecured cotton.
In the scouring process these impurities are removed to the extent that the fibers
will become hydrophilic and will easily wet-out.
In addition to the natural impurities, cotton fibers contain other foreign materials.
The preparation of fabrics made of cotton or polyester/cotton blends may include
some or all of the following operations:
o Singeing
o Desizing
o Scouring,
o Bleaching
o Mercerizing.
Increasing demands for savings in chemicals, energy, and water, certain
treatments are often combined. An example of a combined application is the so
called 'oxidative desizing' where desizing and scouring take place at the same
time.
4
2.1.a Singeing
Singeing is an important part of pretreatment. This is the burning off of
protruding fiber ends from the surface of the fabric.
If not done properly, unclear print patterns, mottled fabric surfaces, and
pilling results.
Singeing of textile materials is necessary for te following reasons:
o Cotton materials are valued for their smooth appearance. After the
formation of fabric it has a fuzzy or hairy appearance due to
projecting fibers, thus affecting the luster and smoothness cotton is
known for.
o Unsigned fabrics are soiled easily
o The protruding fibers obstruct the subsequent dyeing and printing
process
o Goods which are to be mercerized are signed to maximize the
luster
o In fabrics of polyester and cellulosic fiber blends singeing is the
best method to control pilling, sometimes double singeing is done
to minimize the pilling.
Some of the precautions to be taken in considerations during singeing:-
o The fabric to be singed should be dry as wet fabric tend to scorch
more readily than dry.
o Uneven singeing may cause streaks on fabric or bubbles when the
fabric is finished.
o Improper singeing may lead to loss of 75 % loss in tensile strength
loss in warp direction.
o The fabric should not contain any acid releasing salt,which may
release acid on heating and tender the fabric.5
o Stopping the machines may cause bars on the fabrics.
o Singeing may cause hardening of the size thus leading to difficulty
in its removal.
o Possibility of thermal damage to temperature sensitive fabrics.
o The burning characteristics of fibers must be taken into account
when this process is applied, as heat-sensitive fibers melt, forming
tiny balls on the surface of the fabric. These balls interfere with dye
absorption, so that, as a general rule, heat sensitive fibers would
be singed after dyeing or printing.
Singeing process is as follows:
o To produce a smooth surface finish on fabrics made from staple
fibers first the fabric surfaces are brushed lightly to raise the
unwanted fiber ends.
o Then the fabric is singed with or passed over heated copper plates
or open gas flames. The fiber ends burn off.
o The fabric is moved very rapidly, and only the fiber ends are
destroyed.
o As soon as the fabric leaves the singeing area, it enters a water
bath or desizing bath. This stops any singeing afterglow or sparks
that might damage the cloth.
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A gas flame type singeing machine
2.1.b Desizing
Desizing is a process which removes the chemical stiffener (a starch or other
stiffener) applied to warp yarns to make them easier to weave.
This is done to ensure that subsequent dyeing and finishing materials can be
absorbed as evenly as possible.
Desizing also softens and removes any trash particles and seed-coat fragments.
The desizing bath can be a hot water bath or a bath of enzymes depending on
the sizing material that were used.
The most commonly used sizing materials are starches or polyvinyl alcohol.
The cloth continues through additional baths containing more enzymes and
detergents that loosen the sizing present and prepare the fabric for the scouring
and bleaching.
Typical types of starches are as follows :-
o Natural starches(vegetable starches)
o Chemical modified starches(ethers /esters)
o Organic
polymers(polyacrylates,carboxymethylcellulosemethylcellulose,polyesters)
o Solvent soluble materials(co-polymers of methyl- methacrylate).
There are two methods of desizing:
(a) Desizing with hot water: The greige cloth is washed with hot water to
remove PVA.
(b) Desizing with enzymes: The cloth is steeped in baths containing enzymes
(1% solution by volume) or malt and kept for 4 to 8 hours at 55° to 80° C.
7
The enzymes hydrolyze starch and convert it to readily soluble substances.
Cloth moves through the singeing and desizing process at a rate of up to 300
yards per minute.
The classical desizing process fundamentally involves:
o Swelling of highly polymerized size
o Dissolving
o Washing of the swollen polymer
Solubility of starches are promoted by action of acids, oxidizers and
alkalis,if proper care is not taken then the acid and oxidizers not only attack
starch but cellulose also,leading to degradation of fabric and making it
weak.
Various other methods of desizing are:
Rot Steeping: The fabrics are impregnated with hot water
squeezed and then stored for 24 hours in the pits. Later,
given hot and cold wash.
Acid Desizing: It is a fast process done by using mineral
like Sulphuric acid or Hydrochloric acid.As it is a fast process
may cause damage to cotton.
Alkali Desizing: Usually done by Sodium Hydroxide NaOH –
2to 3% (owf) at about 50oC and stored for 4-6 hrs before
washing with hot water. High Chemical cost. Danger of Oxy
cellulose formation because of local drying. Suitable for fabrics
sized with PVA and/or acrylics.
Enzymatic Desizing:
Most acceptable, economic and safe, Quicker and
better than rot steeping.Cellulose does not get
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tendered.Care is needed for temperature of 60oC and pH
of 5.5 to 6.5.
The fabric is padded with
bacterial enzyme (1.5 -2g/l) +
Common salt (1-2g/l)+
Wetting agent (1-2g.l)+
Acetic acid to adjust the pH.
and stored for about 8hr, followed by hot and cold wash.
Oxidative Desizing:
By Sodium Hypochlorite or Sodium Bromite
Process: 1. Fabric + NaOCl (2g/l available chlorine) at
Room Temperature and stored for 90-120 min.
2. Hydrogen Peroxide (50%) -1%
Sodium Silicate-1%
Sodium Carbonate- 0.5%
Steamed for 30-35min and then washed
3. Sodium Bromite of 0.1-3% at 60oC
and dwell for 15 min and Scouriing
Chemical Process involved :-
Starch(insoluble) dextrin(insoluble) dextrin(soluble)
alpha-glucose(soluble) maltose(soluble)
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2.1.c Scouring
This cleaning treatment is also referred to as alkali boil-off.
Scouring removes waxes and destroys vegetable matter residues in cotton
and cotton blend fabrics.
The success of the scouring process is judged by the improvement in wet
ability of the scoured material.
The process essentially consists of treatment with a soap or detergent with or
without a alkali.
Caustic souring
o An alkaline solution of sodium hydroxide (5-10 g/l) or a mixture of
sodium hydroxide and sodium carbonate of a similar alkalinity, close to
or above the boil, for 1-2 hours,is used to saponify the fats on the
fibers.
o The soap that is formed then serves to emulsify the remaining waxes
and wash away any dirt or other impurities.
o Pectin is converted into their sodium salts which are water soluble,
and proteins undergo basic hydrolysis to form water soluble amino
acids.
A sequestering agent is added to the scouring bath to prevent ca++ and mg+
+ ions combiing with soap molecules.
Organic sequestering agents stable under hot alkali conditions are:-
o EDTA
o NTA
o HEDTA10
There are two types of scouring:
(a) Kier- boiling: Fabrics are heated under pressure using steam in steel
stainless steel pressure vessels (kiers) of 2 to 3m Diameter and 3 to 4 m
height. The kiering liquor is an alkaline solution containing caustic soda (1
to 3% of fabric wt) (major component), soda ash, sodium silicate and
sodium peroxide with small amounts of detergents. The boiling is carried
out for several hours (1 to 12 hrs).
(b) Continuous Scour: In this method, the desized fabric is passed through a
caustic solution (3 to 6% caustic soda, surfactants and sodium phosphate)
and after saturation, passed on to J box where the fabric is heated with live
steam at a temperature of 100°C for 1 hour.
The changes caused by treating with alkaliare summarized as follows:
o Saponifiable oils and fats are converted into soaps
o Unsaponifiable sopas melt at scouring temperature and are
emulsified by the soap formed during saponification
o Pectins and pectoses are converted into soluble salts of pectic acids
and metapectic acids.
o Water soluble mineral substances are dissolved
o Insoluble dirt is removed and retained as suspension
o Sizing and other impurities are broken into soluble products.
o Proteins are hydrolyzed by formation of soluble sodium salts of
amino acids or ammonia.
After scouring, the cloth is rinsed well with water to remove the excess
chemicals. The final rinse may include a small amount of acetic acid if the fabric
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has to be neutralized.
2.1.D Bleaching
Bleaching is required to obtain pure whiteness since fibers are seldom pure white
in their
Cotton fabrics are naturally off-white to tan in color, depending on the amount of
pigmentation in the fiber natural state.
The bleaches are chemical agent or compounds that react with the color
compounds in the fiber, oxidize them and render them colorless.
Most bleaches used by cotton industry are either chlorine bleaches (10%) or per
oxygen bleaches.
The per oxygen bleaches, and particularly hydrogen peroxide bleaches(90%),
are used most frequently in commercial bleaching of cotton greige (untreated)
goods,
Hydrogen peroxide bleaching is carried out by exhaust, semi continuous and
continuous methods. Of these, the preferred and most widely used is the
continuous bleaching in the open-width. In particular, the open-width method is
preferred when bleaching blends· of cotton with heat-sensitive fibers, such as
polyester, in order to avoid crease marks and other defects caused by treating
fabrics in the rope form.
In typical batch treatments of cotton fabrics with hydrogen peroxide in
kiers, the bleaching bath is prepared as follows:
Hydrogen peroxide (35%) - 4-8% (o.w.f.) ~
Sodium hydroxide - 0.5-1% (o.w.f.)
Sodium silicate - 2-4% (o.w.f.)
Wetting agent or detergent - when needed
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The bleaching is then carried out near the boil or at temperatures above the boil,
under pressure, for an hour or more. After bleaching, the goods are thoroughly
rinsed with a slightly basic solution to avoid the formation of insoluble silicates.
In a typical continuous bleaching in the open-width the padding bath
contains the following:
Hydrogen peroxide - (50%) 1.5-2.5%
Sodium hydroxide - 0.2-0.6%
Sodium silicate -1-1 .5%
Wetting agent - 0.1-0.2 %
After padding the fabric is passed through a steamer. Steaming time may vary
from as little as a few minutes to one hour or more, depending on the type of
steam and steamer used. Needless to say that the optical brighteners are
also employed in bleaching.
2.1.E MERCERIZATION
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Singeing, desizing, scouring, and bleaching are routine finishing processes,
whereas mercerization is an optional step in the finishing of cotton and cotton-
blend fabrics.
the treatment of cotton with a strong solution of sodium hydroxide altered the
strength, absorbency, and appearance of the fabric.
Fabric treated in this way shrunk as much as 25 percent of its length, the finish
was not applied commercially untiit was discovered that applying the finish under
tension not only minimized shrinkage but also increased luster
In mercerization, as this finish is called, the cotton fabric is immersed under ten-
sion in a strong solution of sodium hydroxide for a short, controlled period of
time (usually 4 minutes or less), the alkali is washed off, and any excess alkali is
neutralized. The sodium ions in the solution displace the hydrogen on the
cellulose-OH groups, pushing the polymeric chains farther apart and swelling
the fiber.
Mercerized cotton fabrics have greatly increased luster. During mercerization
the fiber swells, the natural convolutions of cotton are largely lost, and the fiber
retains a fuller, rounded diameter. This smooth surface reflects more light than
does the untreated, flatter fiber.
The strength of the fiber is increased as much as 20 percent. The cotton
becomes more absorbent and has a greater affinity for moisture and for
dyestuffs.
Mercerized fabrics are also more reactive. So, they are more easily damaged by
14
acids and oxidizing agents, but mercerized fabrics are more receptive to resin
finishes.
Mercerization can be applied to either yarns or fabrics
o Slack mercerization:
mercerization of fabrics that are not held under tension, can be
used to produce stretch fabrics. During slack mercerization yarns
shrink and develop a good degree of elasticity. The finished fabric
can be stretched, and when the tension is removed, the goods will
return to their original length. Yarns that have been slack
mercerized do not have the high luster of yarns mercerized under
tension. This process is not widely used.
Factors affecting the degree of mercerization must be carefully
controlled; otherwise the uneven application will lead to unlevel
dyeing. In particular, the amount of tension applied, and the
concentration and temperature of the sodium hydroxide-bath
should be the same through out the entire application.
o Continuous mercerization
The fabric is padded with about 20-25% sodium hydroxide solution
containing a wetting agent, and then passed over several cans to
allow a dwelling time of approximately one minute during which the
caustic solution will penetrate the fibers and react with them
properly.
At this stage the tension is applied only in the length direction.
The fabric is then placed on a tenter frame (tension now is applied
in both the warp and the filling directions) and is pulled to its
original or desired dimensions. While on the tenter frame, the fabric
is washed by spraying water until the amount of sodium hydroxide
on the fabric is reduced to only a few percent.
15
The fabric is removed from the tenter frame, and the remaining
alkali is removed by passing, the fabric through several washers,
one of which contains a dilute solution of sulfuric or acetic acid.
Mercerization is both inexpensive and permanent, and for these reasons it is widely
used on cotton goods
2.1.F TENTERING
Tentering is the mechanical straightening and drying of fabrics to secure
uniform width and to keep the filling yarns on-grain.
Fabric goes through a lot of stress during preparation, dyeing, printing, and
finishing
This often causes the warp and filling threads to be off-grain.
Tentering establishes the alignment of these warps and filling yarns.
Properly tentered fabrics will be straight. This means that the warp and filling
yarns are at 90° angles to one another.
A tenter frame holds the fabric between two parallel chains, with either clips
(tenters) or pins, while adjusting the side-to-side alignment of the cloth.
The chains spread apart to the desired fabric width, move with the fabric
through drying units, and release the fabric to the next process.
This process is used after several of the finishing operations.
If the fabric is fed to the chains so that the yarns are perpendicular, the
fabric stays on-grain. If not, a bow or skew situation develops.
3. PROTENEOUS MATERIAL
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Animal or protein fibres are very irregular in composition. They contain a
significant amount of non-fibrous impurities-about 50% in case of wool and about
15-30% in case of silk.
These fibres are very sensitive to alkali and are damaged quickly on wet
treatment at high temperature (especially wool). Hence, these fabrics are
scoured with large amount of soap or detergent, with little or without alkali at low
temperature.
3.1 SILK
3.1.A DEGUMMING OF SILK
The silk filament (Chemically a mono filament of protein) extruded by the
silkworm called a bave consists of two brins.
These two brins are stuck together by sericin and become a single continuous
filament. Thus the silk bave is of two brins held together by sericin.
The composition of mulberry raw silk is as follows:
Fibroin - 70-80%
Sericin - 20-305
Waxy matter - 0.4-0.8%
Carbohydrates - 1.2-1.65
Inorganic matter - 0.7%
Pigment - 0.2%
The process of eliminating “Gum (sericin)” from raw silk is known as degumming
of silk.
Degumming of silk involves mainly the removal of sericin from the fibroin. Sericin
is insoluble in water.
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It is comparatively easily hydrolyzed, whereby the long protein molecule of
sericin, is broken down into smaller fractions, which are easily dispersed or
solubillsed in hot water.
Hydrolysis of proteins can be carried out by treatment with acids, alkalis and
enzymes. Acids are non-specific and tend to attack vigorously.
Alkalis also attack both, sericin and fibroin. However, the variation in the rate of
hydrolysis is large enough to control the reaction.
The degumming with soaps in the presence of mild alkalis like soda ash is in
practice.
Degumming with alkalis is a function of pH and temperature and duration of
treatment. pH. The pH should be kept at the leve1 of 9.5 to 10.5.
If the level is below 9.5, then the process of removing sericin will be slow. If the
pH is over 10.5, the degumming loss will be greatly increased.
Process:- The degumming of raw silk is generally carried out using 4-5 g/l
soap and I g/l soda ash at boiling temperature for 5-60 minutes, maintaining the
liquor ratio at 30:1.the treated material is given a hot wash for 10-15 minutes and
finally it is washed in cold water.
The degumming process which removes the silk gum is also called as boiling-of
process and this is accomplished by the use of soap and soda.
The degumming loss in this process in 20-25%.
In certain cases, entire silk gum is not removed, but only sufficient amount is
removed to make the silk soft and lustrous and workable in dyeing and
bleaching.
This is known as “Soupling “in which only 10% to 15 % of the gum is removed.
Process of scoupling :
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o raw silk is soaked in lukewarm dilute soap bath for several hours and
rinsed in fresh water.
o After soupling the silk may be bleached or dyed.
o After bleaching the soupled silk is frequently given a treatment with hot
(85oC) solution of tartar.
o This causes the silk gum remaining on the fibre to soften and remain in
that condition permanently.
In addition to removing the soil or additives used while weaving silk, scouring
removes any sericin (gum) that remains on the silk. Often a quantity of the
natural gum has been allowed to remain on the silk fiber to give it additional body
and to make it easier to handle in spinning and weaving.
Although for raw silk fabrics the gum is retained purposely to provide body or
produce a different texture, most silk fabrics are degummed as a part of the
finishing process.
The resultant fabric has a much softer hand and a whiter appearance. Silk is
usually bleached with dilute solutions of hydrogen peroxide.
Raw silk is sometimes given a very mild scouring for the purpose of softening the
fibre. This is called as "ECRU SILK” in which only 2 to 5% in weight of silk gum is
removed.
ECRU SILK can be prepared by simply washing the raw silk in lukewarm or hot
water without the use of soap. This is used mainly for warp; hence the gum is left
purposely.
The degumming of silk fabrics is generally carried out batch-wise in small lots
using equipments
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a) Open beck: Most common used equipment for degumming. The fabric is evenly
piled in the open vessel, filled with the degumming liquor and treated from the
bottom. In case of fabrics containing small quantities of sericin the treatment in
open beck may be quite effective.
b) Star machine: Used for the degumming of delicate fabrics and cases where the
primary consideration is perfect production quality, especially for goods which are
to be plain dyed.
The Assessment of effectiveness of degumming is done by the extent of removal
of sericin by calculation the of weight loss of fabric after degumming, scanning
through electro microscope or by chemical staining method.
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3.1.Bleaching of silk
The silk being spun by silk worm contains natural colouring matter tinted with
yellow, yellow -green and brown pigments.
During degumming the removal of sericin from the silk results in dull white to
lightly tinted material.
Since some of the sericin is closely held by fibroin, complete elimination of the
colour by degumming is not possible.
During bleaching these natural colouring matters are decolorized/removed to
produce pure white material. An efficient bleaching process must ensure pure
and permanent whiteness, level dyeing properties and non degradation of the
material.
The bleaching of silk is based on the use of either reducing agents or oxidizing
agents.
Some of the important reducing agents used for bleaching are:
a) Sodium Hydrosulphite (Hydrose)
b) Sulphur Dioxide.
c) Sodium/Zinc Sulphoxylate Formaldehyde.
The above reducing agents at time tend to reoxidise original colour may be restored in
the bleached material.
The popular oxidizing agents used for bleaching of silk are:
a) Hydrogen Peroxide
b) Potassium Permanganate.
c) Sodium Per borate
d) Sodium Peroxide.
21
The chlorine based agents such as bleaching powder, Sodium Hypochlorite are not
generally used as they tend to chlorinate the silk fibroin. Of the above, Hydrogen
Peroxide is mostly preferred. .
Methods of bleaching:
1. Bleaching with sodium hydrosulphite (hydros)
a. The degummed silk goods are treated in a bath containing 4 grams per
litre of sodium hydrosulphite at 50o C for 4-6 hours.
b. The entire lot must be completely immersed and the bleaching liquor is
stirred sufficiently to ensure uniform distribution.
c. Finally the material is thoroughly washed. It is to be noted that the
solution should be made up only when required as its reducing power
deteriorates rapidly on storage
2. Bleaching with sulphur dioxide of sulphurous acid-
a. The degummed silk goods are hung in a suitable chamber where they are
exposed to sulphur dioxide gas or immersed in sodium bisulpite in water for
4-6 hours.
b. Approximately 5 kg of sulphur is required to bleach 100 kg of silk. This
method of bleaching is not generally practiced as it causes air pollution.
3. Bleaching with sulphoxylate-
a. Sodium sulphoxylate formaldehyde is a stable reducing agent marketed as
rongolite, sofolite, etc.
b. A bath containing 2-4% (o.w.m) sodium sulphoxylate formaldehyde and 1-2.5%
(o.w.m), formic acid (85%) is prepared.
c. the goods are treated at boil for 20-30 minutes. Finally the material is thoroughly
22
washed.
d. Zinc sulphoxylate formaldehyde also known a decolin,safolin can also be used
as bleaching agent.
4. Bleaching with sodium peroxide-
a. In this process bleaching is done with the following typical recipe:
Sodium peroxide 2-4% (o.w.m)
Magnesium sulphate 0.5% (o.w.m)
Sodium silicate 3% (o.w.m)
Sulphuric acid (96%) 2-4% (o.w.m)
Sodium bicarbonate 0.5% (o.w.m)
Material:water 1:30
Time 4-5 hrs at 60o C
The oxygen yielded by the decomposition of sodium peroxide oxidizes the chromo
gens of silk so that it becomes colorless.
5. Bleaching with hydrogen peroxide-
a. Hydrogen peroxide is the most preferred bleaching agent which is sold as 35-
50% aqueous solution. It is quite stable under acidic conditions.
b. The typical recipe of peroxide bleaching process is given below:
Silk goods - x kg
Material: water - 1:30
H2O2 - 6 g/l
Sodium silicate -1.5 g/l
Soda ash - 0.5 g/l
Temperature - 80-85o C
Time - 60-90o C
23
c. In this process the liberation of perhydroxyions acts as a true bleaching
agent.
d. Here the sodium silicate acts as a stabilizing agent and helps to maintain the
speed of the process slow and steady.
e. The hydrogen peroxide is used as an important agent for silk as the white
colour generated lasts long and further it possesses good stability on storage.
6. Optical bleaching method –
1. In order to achieve the required degree of whiteness of a fabric, colour
complementaries are introduced to achieve desired luster.
2. To obtain this the material is treated with optical brightening agents.
3. The mechanism behind this is that these agents absorb invisible UV light and
emit in the visible range of the spectrum, some of the commercial available
agents are Ranipal WHN, Leucopher PAT.
A typical recipe:
5% (o.w.m) Ranipal WHN
2 g/l sodium hydrosulphite
Material: water= 1:30
The material is immersed in the above bath at 60o C and treated for 30-60 minutes.
Finally, the goods are washed and dried.
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3.2 WOOL
3.2.A Carbonizing
Wool fabrics that have some vegetable matter clinging to the woven or knitted
yarns must be carbonized.
Carbonizing is accomplished by the immersion of wool in sulfuric acid or
hydrochloric acid. Because strong acids readily attack the cellulose of the
vegetable matter and do not immediately harm protein fibers like wool, the burrs,
sticks, leaves, and the like that remain in the wool are destroyed.
The treatment is carried out under carefully controlled conditions so that the wool
is not damaged, and the fabric is given a careful scouring afterward to remove or
neutralize all the acid that remains.
3.2.B SOURING
The scouring of wool is done in solutions of less concentration and at lower tem-
peratures than the scouring of other fibers because the alkalinity of most
scouring solutions damages wool.
Mild detergents and sodium carbonate are frequently used. Wool can be scoured
with a detergent solution at a pH of upto11 (e.g.O.4% Na2C03), when the
scouring-bath temperature is kept below 125o F.
a typical scouring procedure of raw wool the fibers are treated at temperatures
below 600C (l40oF), with an anionic or a non-ionic detergent and a small amount
of a weak base such as sodium bicarbonate or ammonia
25
3.2.C FULLING
Wool fabrics are fulled, or milled, to give the fabric a more compact structure. In a
type of preshrinking, fabrics are subjected to moisture, heat, soap, and pressure.
Fulling involves two processes—scouring and milling (thickening). These are
followed by stretching the cloth on great frames known as tenters and held onto
those frames by tenterhooks. It is from this process that we derive the phrase
being on tenterhooks as meaning to be held in suspense.
In the milling machine, the wet fabric is pounded with hammers or is alternately
stretched and compressed for mechanical action.
Fulling causes the yarns to shrink and to lie closer together and gives the fabric a
denser structure.
Wool cloth may be given more or less fulling, depending on the desired
characteristics of the resultant fabrics.
3.2.D BLEACHING
Although wool fabrics often are finished in the natural color or dyed without
bleaching, sometimes bleaching may be necessary.
Traditionally, wools were bleached by process called stoving, which exposed the
fabric to sulfur dioxide.
Current Processes use hydrogen peroxide and mild temperatures.
26
4. SYNTHETICS
Synthetic fibres are man made.
The most prominent fibres are rayon, acetate, nylon, acrylic and polyester.
Rayon is regenerated cellulose.
Acetate fibre is cellulose acetate.
Nylon is a polyamide fibre manufactured by reaction of hexamethylene
diamine and adipic acid.
Acrylic fibres are formed from wet or dry spinning of co-polymers containing
at least 85% by weight of acrylonitrile units.
Polyester fibre is manufactured by esterification of ethylene glycol with
terephthalic acid.
These fabrics require no processing for the removal of natural impurities as
they are man made.
They do require some treatments such as removal of size, antistat and
lubricating oils used in weaving operations.
Due to the low moisture uptake of synthetic fibres in comparison to cotton
27
and wool, static electricity problems are encountered in yam preparation
and weaving operations. Hence antistatic oils and lubricants are applied to
the fibre before weaving. The commonly employed sizes, antistats and
lubricants are (i) polyvinyl alcohol (ii) styrene -base resins (iii) polyalkylene
glycols (iv) gelath, and (v) polyvinyl acetate.
The preparation processes are generally similar for all these fabrics and
include:
o Heat setting
o Scouring and
o Rinsing (for the removal of process chemicals used in weaving
operations .
o Bleaching, and rinsing.
The above processes are carried out in the same way as for cotton textiles. The
fibres and clothes are readily processed on the conventional machineries used for
cotton.
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4.1. DESIZING OF SYNTHETICS ng of synthetics and blends
Synthetic fibres are stronger; hence increasing strength by sizing is not the
aim. The adhesion of sizing material to the yarn is also difficult.
Sizing of man made fibres with starch creates a lot of problems.
The main synthetic sizes comprise of polyvinyl alcohol (PVA), polyacrylic
acid, CMC, plasticized acidic vinyl acetates and acidic methyl acrylate
polymers.
When water soluble PVA is used as a sizing agent in weaving, no desizing
is necessary. light scouring with 3-5 g/l nonionic detergent at 95-100% for
20-30 minutes is enough to remove the size material. However, high
molecular weight (>1,00,000) PVA is difficult to remove
. Polyester sizing agents are not removed by simple washing treatment.
They can be removed by treating with a solution containing 2-3 g/l nonionic
detergent and1-2g/l caustic soda or soda-ash(with or without 1-2g/l sodium
tripolyphosphate)at 80-90oC. the use of soft water or sequestering agent is
recommended. polyacrylic acid is soluble in mildly acidic or alkaline
solution.
The desizing of polyester/cotton blends depends on the nature of size
used. Acrylic copolymer is removed by simple mild alkaline scour.PVA and
CMC are removed by hot wash. a mixture of starch and emulsifying wax is
removed by means of enzymatic desizing followed by alkaline scour.
When acrylic sizes are mixed with starch, their ph of about 9.5 will
inactivate enzymes. In such cases, the fabric may be pre-acidified or acetic
acid may be added to the desizing bath to bring ph at 8. PVA can be easily
29
removed using hydrogen peroxide in the presence of a metal catalyst
under neutral condition
4.2 HEAT SETTING
Unlike natural and regenerated fabrics, the synthetic fabrics require a
special preparatory operation called heat-setting.
During this operation, the material is subjected to thermal treatment in
tensioned condition in hot air, steam or hot water medium.
Unless this treatment is done these materials will shrink when treated with
aqueous solution and as a result the linear dimensions and the shape of
ready articles change.
Setting process releases the strains imposed during drawing, knitting or
weaving processes, minimizes creasing during wet processing and imparts
high degree of dimensional stability.
In order to achieve a satisfactory degree of set in synthetic thermoplastic
fibres it is necessary to supply sufficient energy to the fibre to weaken or to
break inter-chain bonds and allow new bonds to be formed whilst the
material is held to optimized dimensions.
Thus the material must be heated to a temperature close to the
thermoplastic point of the fibre and some degree of width ways or
lengthways tensions are normally imposed on the fabric.
The setting process consists of two stages:
Heating of the material to a specific temperature depending on the
fibre type, ensuring breakage of intermolecular bonds and
equalizing stresses in chains.
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Cooling of the material resulting in the restoration of intermolecular
bonds, the fibres being in new state, free from internal stress. The
quicker the cooling and the lower the temperature, the better is the
stabilization effect.
Main techniques of heat-setting are:
1. Dry-heat setting (or simply heat setting /thermo setting)-using indirect oil or electrical
heating in pin or clip stentering machine
2. Steam-setting using high pressure steam
3. Hydro-setting using hot water
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4.3 SOURING OF SYNTHETICS:
The scouring procedures for synthetic fibres are relatively simple since the
fibres contain fewer impurities, most of which have at least some degree
of water solubility, the most important being sizes and lubricants.
The major sizes used are polyvinyl alcohol, carboxylmethyl cellulose and
polyacrylic acid, all of which are completely or partially water-soluble.
4.4.1 Scouring of acetate:
Secondary acetate or triacetate can be scoured with soap or synthetic detergent
usually at 60\70 C, this being sufficient to remove soil, oil, coloring impurities and
antistatic agents.
Anionic synthetic detergents such as polyoxyethylene sulphates are preferred for
all the synthetic fibres that are to be subsequently dyed with disperse dyes, since
nonionic detergents with low cloud point may hamper stability of disperse dyes at
high temperature.
Addition of sequestering agent will be helpful for hard water. Relaxation of
knitting or weaving tensions occurs during scouring under minimum tension at
high temperature.
4.4.2 Scouring of nylon
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Polyamide or polyester fabrics are adequately scoured using an alkyl
polyoxyethylene sulphate and sodium carbonate.With polyamides or nylons,
nonionic synthetic detergents (1.5-2 g/l) with the addition of an alkali (0.5-1.5 g/l
sodium carbonate or trisodium phosphate) are suitable.
Anionic detergents like fatty alcohol sulphonates behave like acid dyes and are
adsorbed by the fibre.
Their presence on the fibre will slow down dyeing and may reduce the
exhaustion of the dye bath. If the material is to be dyed with acid dyes, a nonionic
detergent is to be used.
The temperature of scouring varies according to whether the material is already
heat-set or not, being 70-80o C for the unset and 95-100o C for heat-set. High
temperature may cause deterioration of setting.
After scouring the nylon fabric should be washed thoroughly before drying. The
fabric should not be dried unevenly as this could lead to irregular results in
dyeing, particularly with anionic product.
4.4.3 Souring of polyester
Polyester products which bear low quantities of processing aids compatible with
disperse dyes can simply be washed with water or may even be allowed to
remain in the dye bath..
If sizing agents are not removed easily, an additional step of enzymatic desizing
is to be carried out.
Normally scouring bath should have 1-2 g/l synthetic detergent or soap (alone or
admixture with nonionic surfactant) and 2 g/l soda ash. For light weight materials
the temperature should be below 60 o C. for other materials, scouring may be
done at boil for 20-30 minutes.
33
A soil suspending agent like sodium tripolyphosphate may be added.
The same condition may be maintained for polyester viscose blends. For blends
with cotton and linen the concentration of alkali should be doubled. For polyester-
wool worsted fabrics, the scouring may be done with 3-4% (o.w.m) soap and 20-
30g/l soda ash at room temperature for 20-30 minutes, followed by rinsing at
40oC.
4.4.4 Scouring of acrylic fibres
For acrylic fibres, anionic surfactants should be avoided, because they may
restrain the uptake of basic dyes.
These fibres are scoured with an ethoxylated alcohol, either alone or with a mild
alkali such as sodium carbonate or phosphate.
. The best method depends on the type and quantity of substances to be
removed. Sometimes a short cold rinse is sufficient.
Usually a weal alkaline treatment with nonionic detergent (0.5-1% o.w.m) and
ammonia (0.5-1% o.w.m) may be carried out at 80oC.phosphates can be used as
an alkaline agent. Addition of a solvent based detergent helps removal of oil and
grease, if present.
After scouring a thorough rinse at 50-60o C is necessary to remove any residual
alkali.
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4.5 BLEACHING
In synthetic fabric, discolouration results from the by products of manufacture,
which include processing auxiliaries, spinning oils in yarn making, and
formulations for yarn sizing in weaving
.Synthetic fibres are generally manufactured with high levels of whiteness except
when the conditions for fibre formation cause discolouration. In these cases,
bleaching can include fluorescent whitening agents.
Formulations are composed for safe temperatures and pH ranges that minimize
acid and alkali degradation of the fibre. As an example, polyester fibres have
been bleached with di- and tri- chloroisocyanuric acid at PH 3-9, at 60-65o C, for
30-45 minutes.
4.5.A Nylon
Heat setting temperatures for the various forms of nylon may cause yellowing
from oxidation. Heat sensitivity is a special consideration for bleaching the nylon
thermoplasts.
Hydrogen peroxide is not used because the bleach bath processes at too high a
temperature. Recommended bleaches are non chlorinated types: sodium
perborate, percarbonate, and peracetic acid. a peracetic acid bleach bath is
processed with sodium pyrophosphate at pH 6-7.
The material is introduced cold and the temperature is raised to 80-85 c over 30
min with subsequent bleaching at this temperature for 30 minutes. Where
possible, lower temperatures and pH are recommended.
35
\\
4.5.B Bleaching of fibre blends
In 50/50 blends of natural and synthetic fibres, bleaching procedures are based
on the sensitivity of the natural fibre component, provided that temperature and
pH are not damaging to the synthetic fibre.
When the natural fibre component represents a small percentage of the
composition, it may withstand harsher conditions than would normally be
prescribed for bleaching it alone.
When a textile contains a fibre blend with fibres of equal sensitivity but unequal
percentages, bleaching formulations are tailored to the safe requirements of the
fibres present in the larger amount.
4.5.C Polyester
Polyester may be bleached using acidified sodium chlorite.
There is no damage to the fier but a slight increase in crystallization.
A standard recipe for making 100 litrs solution for bleaching is:
o Sodium chlorite-2-4 kgs
o Chlorite stabilize
o Formic acid –to pH 3.5 to 33.8
o Sodium nitrate-1.5-2 kgs
o Wetting and dispersing agents.
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4.5.D Acrylic
Bleaching is always accompanied with fluorescent whitening
Acrylic fibers when boiled in a solution of Ph >7 neiher hydrogen peroxide nor
hydrogen hypochlorite are applicable.
Bleaching is done mostly by sodium chlorite for 1 hr at 90 degrees
If a fluorescent agent is preset then it is preferable to bleach near boil for 35-40
mins.
An alternative is to bleach acrylics with sodium chlorite or hypochlorite at pH
below 3 .
37
Preparation
Of
Cotton fabric
(muslin)
With
Practical samples
38
Experiment 11. AIM OF EXPERIMENT :- To Desize a untreated sample of muslin fabric as a
preparatory process for dyeing process.
2. THEORY OF THE EXPERIMENT
a. Desizing is a process which removes the chemical stiffener (a starch or
other stiffener) applied to warp yarns to make them easier to weave.
b. The desizing bath can be a hot water bath or a bath of enzymes
depending on the sizing material that were used.
c. The cloth continues through additional baths containing more enzymes
and detergents that loosen the sizing present and prepare the fabric for
the scouring and bleaching.
Recipe of the process
1. Add 5% mineral acid(diluted HCL)
3. MATERIAL REQUIRED
a. Large container/bath
b. Spoon
c. Glooves
d. Mineral acid
e. Water
f. Stirring rod
4. PROCEDURE
Add water into the container/bath and put for boiling
Boil the bath till the temperature reaches 100 degrees(boiling)
Add the 5% mineral acid to the bath
39
Stir the mixture using a stirring rod
Immerse the fabric into the bath and stir it .
Let the fabric be in the bath for the next 1 hr.
Remove the fabric after 1 hr and dry the fabric.
5. OBSERVATIONS
The colour of the fabric changes
The size of the fabric also shrinks .
The solution in the dye bath is of muddy color which confirms the removal of the
sizings.
6.RESULT
The fabric has been freed of various sizes applied to it during the process of
weaving basically sizes of soluble nature are removed in this process.
40
Experiment 21. AIM OF EXPERIMENT :- To scour a desized sample of muslin fabric as a
second preparatory process for dyeing process.
2. THEORY OF THE EXPERIMENT
a. This cleaning treatment is also referred to as alkali boil-off.
b. Scouring removes waxes and destroys vegetable matter residues in
cotton and cotton blend fabrics.
c. . In addition, caustic scouring swells the motes and opens them up.
Recipe of the process
1. NAOH-2%(owf)
2. Soap-1% (owf)
3. Soda ash-1%(owf)
4. Material:liquor ratio-1:20
3. MATERIAL REQUIRED
a. Large container/bath
b. Spoon
c. Glooves
d. Chemicals- NAOH,soap, soda ash
e. Water
f. Stirring rod
4. PROCEDURE
Add water into the container/bath and put for boiling
41
Heat the bath upto 60-65 degrees and add soap /detergent
Stir the solution and immerse fabric
Add the NAOH and Soda ash into the solution
Heat the bath upto 95 degrees for the scouring to start
Let the fabric be in the bath for the next 2-3 hr.
Keep stirring the solution and don’t keep the solution stagnant
Remove the fabric after 2-3 hrs and dry the fabric.
5. OBSERVATIONS
The colour of the fabric changes,the whiteness of the fabric as compared to
desized fabric is more.
The size of the fabric doesn’t change muh.
6.RESULT
The resultant fabric is Scoured with waxes removed and d vegetable matter
residues in the fabrics
42
Experiment 31. AIM OF EXPERIMENT :- To bleach a scoured sample of muslin fabric as a
second preparatory process for dyeing process.
2. THEORY OF THE EXPERIMENT
a. Bleaching is required to obtain pure whiteness since fibers are seldom
pure white in their natural state
b. The bleaches are chemical agent or compounds that react with the color
compounds in the fiber, oxidize them and render them colorless..
c. . If the fabric is not properly bleached during preparation, it may retain its
natural color, which would distort the color of the fabric when dyes or
pigments are added
Recipe of the process
a. H2O2- 2% -4 vol
b. Sodium silicate-4 gpl
c. pH-10.4-10.8
d. soda ash- as per pH
e. Material:liquor ratio-1:20
3. MATERIAL REQUIRED
a. Large container/bath
b. Spoon
c. Glooves
d. Chemicals- NAOH,soap, soda ash
e. Water
43
f. Stirring rod
4. PROCEDURE
Add water into the container/bath and put for boiling
Heat the bath upto 60-65 degrees and add soap /detergent
Stir the solution and immerse fabric
Add the NAOH and Soda ash into the solution
Heat the bath upto 95 degrees for the scouring to start
Let the fabric be in the bath for the next 2-3 hr.
Keep stirring the solution and don’t keep the solution stagnant
Remove the fabric after 2-3 hrs and dry the fabric.
5. OBSERVATIONS
The colour of the fabric changes,the whiteness of the fabric as compared to
desized fabric is more.
The size of the fabric doesn’t change muh.
6.RESULT
The resultant fabric is Scoured with waxes removed and d vegetable matter
residues in the fabrics
44
CONCLUSIONPretreatment process of fabric constitutes an important stage in the textile industry. The
colouring and the quality of the finished textile depends to a large extent on the
pretreatment processes. Thus, utmost care should be taken while selecting the proper
preparatory treatments for the fabric.
45
BIBLIOGRAPHY
WEBSITE/ EBOOKS:
1. Textile preparation and dyeing By Asim Kumar Roy Choudhury
http://books.google.co.in/books?
id=0TamObsaaPQC&pg=PA577&lpg=PA577&dq=importance+of+pr
eparation+process+in+dyeing&source=bl&ots=7C09ag7AmU&sig=l
5Y1NMHyYomoowAt8LyqlEUR-
sw&hl=en&ei=2j6ySpWaHsaRlAf04fGMDw&sa=X&oi=book_result&
ct=result&resnum=8#v=twopage&q=introduction%20to
%20preparatory%20process&f=false
2. Textile technologyBy Burkhard Wulfhorst, Thomas Gries, Dieter
Veit
http://books.google.co.in/books?id=vBAk4dY6-
zgC&pg=PA225&lpg=PA225&dq=FELTING/
MILLING+PROCESS&source=bl&ots=q9loeREgDO&sig=G5lPUYj0Y
FoS8SzZEdx3tdnWGrE&hl=en&ei=tdeySsjlB4G4swPTlaieDQ&sa=X
&oi=book_result&ct=result&resnum=7#v=onepage&q=&f=false .
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BOOKS
3. FIBER TO FABRIC- P.COBMANN
47
