Dyeing practices including finishing / scouring / bleaching

1. ASSIGNMENT
RMIT
MithilaEranda
StudentID- 3570101
[DYEING PRACTICES]
MithilaErandaLiyanaarachchi

2. CONTENTS
1.0 DYEINGPRACTICES (Including finishing) ............................................................................ 2
1.1 Laboratory dyeing and color matching of textile fabrics ......................................................... 2
1.2 Dyeing of cotton fabrics.......................................................................................................6
1.3 Dyeing synthetic fabrics.......................................................................................................9
1.4 Finishing of textile fabrics.................................................................................................. 12
2.0 SCOURING AND BLEACHING .......................................................................................... 15
2.1 Scouring and milling of woolen fabrics............................................................................... 16

3. 1.0 DYEING PRACTICES (Includes Finishing)
1.1 Laboratory dyeing and color matching of textile fabrics
Definition of dyeing is “the application of one single color uniformly to the textile substrate”.
The process of dyeing consists of three major actions. They are migration, diffusion and
fixation. The most essential and necessary areas in Dyeing process are,
 Selection of the suitable dye type
 Preparation of the dye bath
 Selection of the textile materials
 Maintaining the appropriate conditions
It is necessary that when selecting dyes, we should know that all dye types are not
compatible with all types of materials. There are specific dye types for specific fabric
type or material.
Therefore it is required to identify the most suitable dye type and the material in the process
of dying. The type of the dye depends on various factors .Not only the material type, when a
garment is produced, we have to focus on the end look of that garment as well. So it also
affects the selection of dye type.
In next stage after selecting the dye type, Preparation of material and dye solution is done. In
the testing laboratory the process of dyeing is not done in mass scale. Process is carried out
using beakers (small), or tall straight vessels which is about 500ml in capacity with the use of
5gm of fabric material. For the laboratory experiments 100oC or near, gas fired hot plates or
electric heating are being used. When testing above 100oC, Pressure cookers (household) will
be used if there are no special designs in materials. Furthermore there are special equipments
for dyeing experiments.
 Spectrophotometer
 Tristimulus colour meter
 pH meter

4. Preparing the dye solution
The preparation of dye solution is done as this. First should place 0.5 grams of dry dye
powder in a basin or a beaker.Then should add a little amount of cold water in to the dye
powder and paste-up well.Then should bring up the total volume till 250 ml. After that should
heat the solution until it boils. After boiling it for few minutes then should leave it to become
cool. After it becomes cool it makes up to a final volume of 500 ml. This is how a dye
solution with 1gram per liter strength is prepared.
And in dyeing specially, quantities of dye being added are basically based on how much the
textile material weighs. To find the stock solution quantities, we can use an equation as
follows which is very sound and clear.
No:of ml stock solutionrequired = WP
C
W = sample ‘weight’ to be dyed in grams
P = dye amount to be used as a ‘percentage’ on weight of the fabric
C = ‘concentration’ of thr stock solution as a percentage
Note: Sometimes quantities will be based on the ‘volume of the bath’ instead of the
‘weight of the textile material’.
How to prepare the material
It has to be scoured and bleached when selecting a material for dyeing. And prior to dyeing,
there are certain things that you should pay much attention on. The yarn type and weave
specification are few among them. Furthermore, the material should be weight out accurately
(Between 2% of accuracy). The material should be pre-wetted prior to dyeing (immerse in
water at 50oC).
Following conditions should be maintained in a proper dye bath.
 Mn+2 < 0.01ppm
 Fe+2 < 0.01ppm
 Turbidity less
 Colorless
 pH – 6-8
C

5. After completing the dyeing process, material should be removed carefully and should rinse
well in warm water if any other solution is not specially pointed out. And dry it in hot air.
If dyeing experiment done number of times, the rate of dye uptake should be measure. This
process can do in two types.
1. Dry the sample and remove portion of the material at regular intervals and measure
the dye amount that absorb.
2. Measure the quantity that exhaust of the dye bath using spectrophotometer.
Visual color matching
If the color matching or color assessments done visually, the sample and the standard should
be compared side by side carefully. For the color assessments there is a standard light
condition that must use. Traditionally northern facing daylight was used. But it depends on
where the laboratory is. If the laboratory is in Northern Hampshire, northern facing daylight
is use and if the laboratory is in Southern Hampshire, southern facing daylight light is use.
But those light conditions are variable. So now in industry, color cabinets are used. Those
color cabinets are fitted with reproducible light conditions that can select easily. Because of
that, using color cabinets are available at any working time (even at night). Artificial daylight
is also included in color cabinet lamps. According to CIE define, standard light sources are
mainly named as A, B and C.
Source A – Reddish tungsten filament lamp – Color temperature is 2854K
Source B – Filtered tungsten; noon sunlight – 4800K
Source C – Filtered tungsten; overcast-sky daylight – 6500K
Using grey scales in color cabinets
Grey scale use for assess the changes in colors. Place original and tested pieces of it side by
side in the same plane oriented in the same direction. Use two or more layers of original piece
both under two pieces to avoid effects of the backing on the appearance of the textile. Then
place grey scale nearby in the same plane. Illuminate the surface with source with an
illumination of 600lx or more.
Light should be indicated upon the surface at approximately 45o angle and direction of
viewing approximately perpendicular to the plane of the surface. Then compare the visual
difference between two pieces with the difference represented the grey scale.

6. 5 step grey scale rating:
Grade Quality
5
4
3
2
1
Excellent
Very good
Average
Poor
Very poor
Instrumental color matching
Principle of this process is throwing a certain light on the samples what we call illuminate
and measure the reflectance of light from the sample. This reflectance data mathematically
measured and calculated as per the CIE standard observer response function.
L – 100 – White
(-L) – 0 – Black
(-a) – Green (+b) – Yellow
(-b) – Blue (+a) - Red

7. 1.2 Dyeing of cotton fabrics
“Cotton” is considered as the king of fibers. It is one of the major natural fibers which
consists a mass amount of cellulose. It is said to be the eldest and the most important fiber
among the textile fibers.
The most commonly used methods for imparting color to cotton can be shown as follows.
Even though it is not mentioned clear in the above diagram, the most common methods are
yarn and piece dyeing. In piece dyeing, which is used primarily for fabrics that are to be a
solid color, a continuous length of dry cloth is passed full-width through a trough of hot dye
solution. The cloth then goes between padded rollers that squeeze in the color evenly and
removes the excess liquid. In one variation of this basic method, the fabric, in a rope-like coil,
is processed on a reel that passes in and out of a dye beck or vat.
And in yarn dyeing, which occurs before the cloth is woven or knitted, is used to produce
with special effects. Blue dyed warp yarns, for example, are combined with white filling
yarns in denim construction.

8. For all the cellulose fibers like cotton, we use the following types of dyes,
 Direct dye
 Acid dye
 Reactive dye
 Vat dye
 Sulphur-dye
Considering the above dye types, the most compatible and suitable dye for cotton is
‘reactive dye’. In the first place it is important to identify the structural
characteristics of reactive dyes. The process flow of reactive dye on cotton is shown
in the following diagram.
The 4 main groups that ‘Reactive dyes’ consist
1. Reactive groups
2. Chromophoric group
3. Bridging groups
4. Solubilizing group
Note: “The group Chromophor is attached to the reactive group via a – NH – group”

9. Reactive dye chemical structure
Dyeing Mechanism/ Mechanism of dyeing
In cotton fabric, dyeing mechanism with reactive dyes can divide mainly in to 3 parts.
1. Exhaustion (of dye)
2. Fixation
3. Wash off the unfixed dye
Next the second step is ‘fixation’. It is the reaction occur in between the reactive group of the
dye molecule and the terminal – OH group of fiber via a strong covalent bond. This step is
very essential and should have to maintain the pH of the bath by adding sufficient alkali.
NH (bridging part)
C
C C
N
NH
N N
Cl
N N

10.  Reaction of dye with the cellulose in cotton
Dye – X – Cl + Cellulose – OH Dye – X – O – Cellulose + HCl
Dye – X – Cl + H – OH Dye – X – O – H + HCl
After this reaction between dye and water occurs, it deactivates the dye. That is because the
active group will be removed with this reaction. Even though if we add some alkali to the
medium, then it will stop making ‘Hydrochloric Acid’ and provides extra OH into the bath.
Factors which affects the efficiency of fixation
 Substantivity ratio
 Reactivity ratio
 Liquor ratio
 Diffusion coefficient
 Surface area of substrate which is available for the absorption of dye
1.3 Dyeing ofsynthetic fabrics
By joining monomers into polymers by the polymerization process, synthetic fabrics are
made out from synthetic fibers. There are some chemicals used to make synthetic fibers.
They are,
 Sodium hydroxide
 Carbon disulphide
Stretchy fabric materials and water proof fabric materials are made out with synthetic
fibers. These are some examples for synthetic fibers,
1. Nylon
2. Polyester
3. Spandex
4. Acrylic
5. Acetate
Due to some reasons and properties of these synthetic fibers, we cannot use every
dye type for dyeing. One reason is synthetic fibers are hydrophobic. So it doesn’t
attract to water soluble dyes.

11. In the following table I have mentioned some of the synthetic fibers and the dye types which
are compatible with them
Type of the fabric/fiber Compatible dye type
Nylon Acid / Disperse dyes
Polyester Disperse dye
Spandex Disperse dye
Acrylic Disperse dye
Rayon Acetate rayon dyes
Acetate Requires cross dyeing
Dyeing of polyester fiber/fabric (using disperse dye)
As mentioned earlier only disperse dye is compatible with polyester fiber/fabric. Disperse
dye have relatively high molecular weight and low diffusion rate. Due to that, it is hard to
gain a full deep shade in polyester.
But, we can deeper the shade by
 Using carriers (class B&C dyes)
 Dyeing in high temperature
Nylon fiber/fabric dyeing (using acid dyes)
Nylon is a weird fabric type which is very easy to dye and at the same time very difficult. We
know that disperse dye has good leveling properties but it gives pale shades on nylon due to
the poor wet fastness properties with these dyes. All other dye classes except for the disperse
and acid dyes are limited in use.
As an e.g.: basic dyes for basic dye-able nylon and few Sulphur and vat dyes for nylon/cotton
blends.

12. Structure of Nylon fiber is more crystalline and it is having a lesser number of amino groups
in its structure. Fibers that consist of free amino groups (- NH2) are dye-able with acid dyes.
Bonds which form between fiber and dye are ionic bonds.
Fiber – NH + H+ Fiber – NH3
+
Fiber – NH3 + DSO3 Fiber – NH3
- + SO3D-
Due to the limited no: of amino groups, nylon becomes saturated with anionic dyes in
relatively low depths. Therefore, if full depths are created through mixing more dyes in
different sulphonation degrees together, preferentially one dye will be taken .Then the dye
will donate more to the final hue color (of the material). This is known as blocking. Because
of these reasons, we should thoroughly understand regarding the dye types that we are willing
to use for nylon.
Acid Dyes
Mainly there are 2 types in acid dye;
ACID DYES
Non - Metalised
acid dyes
Self levelling
acid dyes
Moderate
levelling acid
dyes
Poor levelling
acid dyes
Metal complex
acid dyes
1:1 metal
complex acid
dyes
1:2 metal
complex acid
dyes

13. One or more sulphonate groups are present and water soluble in non – metalized acid dyes.
Therefore those acid dyes belong to chemical classes of Azo and antraquinone. This group is
classified according to leveling, compatibility and built up.
And for the process of dyeing, dye bath assistance is directly affecting. The acid is influenced
to the rate of dyeing and to the total amount of dye exhausted. As for leveling agents for most
of dyes Electrolytes are used. And also the temperature is highly influenced for the rate of
absorption. When there’s a high temperature, the rate of absorption is also high. Acid dyes
are easily dissolved; even though undissolved dyes may deposit on the material. Filtering
process is required to prevent that.
1.4 Finishing of textile fabrics
Finishing is done after dyeing process in order to improve the performance, hand feel and
look of a finished garment. So to gain those things in a garment the fabric should go under
different chemical and physical treatments.
In other words, finishing is the general term for a huge no: of processes and treatments which
a fabric may undergo after it has been made and coloured. It is the final processing of the
cloth and its purpose is to make the fabric suitable for its intended end use. For example,
making the fabric water proof, shrink proof, softer, stiffer, water repellent, crease resistant or
a combination of these properties.
‘Ann M Collier’ in 1970 has given a definition to textile finishing in ‘Handbook of textiles’.
It is as follows…
“Finishing refers to the processes that convert the woven or knitted cloth into a usable
material and more specifically to any process performed after dyeing”
As an e.g.: A swimsuit should undergo a waterproof treatment before use.
‘Physical class’ and ‘chemical class’ are the 2 main classes of finishing process. So within the
words the definitions are implied that those are physical and chemical.
Process of finishing has 3 stages technically;
1. Washing and drying
2. Stabilising
3. Pressing and aesthetics

14. Mechanical finishing treatments
These are also called dry finishing treatments. Under this category, Principle physical
applications like friction, tension, pressure and temperature are included.
Sanforizing
Sanforizing is done for shrinkage control in the fabric. There is an equipment called
sanforizer and through that the fabric is passing. The rollers in sanforizer contain hot steam.
By this mechanical forces and water vapor an optimum dimensional stability could be
achieved in the fabric.
Furthermore some other mechanical finishing treatments are also done to the fabric to
achieve the best characteristics. Then the fabric will come in a perfect manner.
Calendaring
Just like Sanforizing in calendaring also there are rollers. When a garment is finished, to give
an after effect to that calendaring is used. In other words, to create a variety of textures and
effects in a fabric. E.g.; flat, glazed compact, smooth
Fabric will be passed between rollers normally under carefully controlled pressure and heat.
In this process there are 2 calendars or rollers are involved. Roller no:1 is highly polished,
and usually heated steel rotate in a very high speed and the roller no:2 is softer. During this
process of calendaring, Yarns within the fabric are smashed and squashed in to a flatten
ellipse shape and the intersections are made to close-up between yarns. So the fabric surface
will become more flat and compact due to that.
Compacting
The compacting machine is also called compactor or felt compactor. This machine is
generally equipped with two steam chambers and two felt units. It pre-shrinks and stabilizes
the garments in order to avoid too much shrinkage after washing.
Simply we can say compacting is done In order to have best durability and also to produce a
bulky texture.

15. Chemical finishing
Mechanical and chemical finishings overlap at a place. Some mechanical finishes need
chemicals, for example milling agents for the fulling process or reductive and fixation agents
for the decatering of wool fabrics. On the other hand chemical finishing is impossible without
mechanical assistance, such as fabric transport and product application. Simply, to achieve
properties which cannot achieve through mechanical finishing processes, Chemical finishing
is done.
Anti-mildew
Mildew is a thin whitish coating consisting of minute fungal hyphae, growing on plants or
damp organic material such as paper or leather. But In textiles also this thing happens.
Cellulose fibers can be damaged due to humid conditions and heat. By depolymerizing these
damages may occur and also it may occur because of a certain microorganism/mildew. They
(mildew) react with starch finishing agents on the surface of the fabric. Damages caused by
them can be prevented by the use of bacteria controlling products and antiseptics containing
phenol derivatives and quaternary ammonium salts.
Softening
This is done by considering the fiber composition of the substrate and its properties.
Softening process is carried out only when the softening characteristic of a fabric/garment
need to be improved or developed. Softening agents facilitate the fiber sliding within the
fabric structure, thus granting easier deformation and creasing of the fabric. In most cases, the
duration of the effect is limited since the products applied during the treatment are eliminated
by subsequent washing. For this reason they must be applied in the final stage of the
treatment
These are the most common types of softeners;
1. Non-ionic Softener
2. Anionic Softener
3. Cationic Surfactants
4. Silicone-Based Softeners
5. Reactive Softeners

16. Crease resistance
Crease resistance is resistance to wrinkling. So for that we use finishes. After this finishing is
done the garment will remain un-creased when subjected to wear or use.
Especially for the cellulose fibers which easily get wrinkles, these finishing applications are
done. Permanent pressing on fabrics could resist creasing and wrinkling and also can
maintain pleats throughout cleaning and wearing.
Enzyme finishing
Bio finishing is another name for Enzyme finishing. Bio-finishing can be simply defined as a
Biological way of giving wet treatment to the textiles.
This is applied by using enzymes to the cellulose fabrics to produce effects (Permanent)
.Enzyme finish removes slubs and protruding fibers from fabrics and also it reduces piling
and even provides a smooth appearance to the fabric.

17. 2.0 SCOURING AND BLEACHING
2.1 Scouring and milling ofwoolenfabrics
What is Scouring? it is a wet process that applies to textile materials. Removing of waxes,
oils, fats, soluble impurities and solid dirt attached to the fibers is the main purpose of
scouring a fabric. And also to improve the absorbency of the fabric is also a major purpose
of scouring. Scouring is done using detergents with or without adding alkali.
And also we can say scouring is the process by which all natural and additive impurities are
removed to produce hydrophilic and clean textile material. It is one of the vital processes of
wet processing.
At the end of the process an absorbency test is carried out in order to evaluate the efficiency
of scouring.
Wool (Raw) contains of 40% - 50% of impurities. But even when the scouring is done, still
those impurities and waxes may add during fabric manufacturing process. So that produced
fabrics may contain 5% of dirt together with oil from the machines or from somewhere else.
Therefore we can say scouring is an important and difficult process that is necessary to done
for wool fabrics. Usually wool fabrics are scoured by using mild alkali such as soda ash
(Na2CO3) at 60oC.
 In the first place remove excess acid carbonized pieces which are usually running in
cold water for 15 to 30 minutes. Through this acid amount will be reduced normally
to like 3%.
 Normally saponification scouring is carried out for about 20 minutes adding some
soda ash
 Emulsion scouring with alkali and detergent are carried out at 40oC.
 For the completion of process, hot and cold water rinse can be carried out.
Milling
To improve so many characteristics of woolen fabrics milling is done. Through milling,
following characteristics could be improved
1. To enhance surface appearance
2. To improve fabric strength
3. To improve the handle
4. To increase the density of the fabric

18. In presence or absence of detergent, milling is carried out under alkaline (pH 10) or acid
(pH 2) conditions. Outcome, the soap from scouring Saponification is capable of improving
the effectiveness of the process of milling.
Three main milling processes
1. Acid milling
2. Grace milling
3. Soap milling
Acid milling
A potential processing problem exists whenever the fabric pH is changed from alkaline to
acid; is twice as likely to occur in soap milling as in acid milling. Because of some special
reasons acid is preferred as a milling medium.
Grace milling
Greasy fabric is run through the liquor containing 20 – 25 g/l soda ash and then squeezed to a
liquor ratio of 1:1 or 3:2 in this process. Then the fabric is milled to the required cover and
dimensions before washing. The fabric is milled in presence of the dirt and loose color during
the scouring process. For medium and good quality woolen fabrics this method is rarely used.
Soap milling
The most soft and attractive handle is given by this soap milling. To give a 5% solution in hot
water sodium or potassium soaps are dissolved. Solution needs to be applied warm to prevent
soap solidifies. Should neutralize, if the fabric has been carbonized. Soap act as a lubricant in
soap milling, and give more rapid shrinkage, by decreasing the inter fiber frictions.

19. 2.2 Pretreatments of synthetic materials
Natural fibers and synthetic fibers contain primary impurities that are contained naturally,
and secondary impurities that are added during spinning, knitting and weaving processes.
Pretreatment is the way of cleaning operations. All impurities which causes adverse effect
during dyeing and printing is removed during the pretreatment process
Pre-treatment processes include desizing, scouring, and bleaching which make subsequent
dyeing and softening processes easy. Uneven desizing, scouring, and bleaching in the
pretreatment processes might badly affect the qualities of products.
There are 2 types of Pre-treatments.
 Mechanical pretreatments - are done to remove imperfections
 Chemical pretreatments - are done to remove impurities
Firstly dry (mechanical) processes are done and afterwards the wet (chemical) processes are
done.
Pre-treatments are varied from one to another. As an example; pretreatments done for
polyester woven fabrics and pretreatments done for polyester knitted fabrics are different
from one another
Pretreatment Of Synthetic Textile Materials:
Although most of the synthetics do not need to be given a very strong pretreatment
however the possible steps in pretreatment of synthetics are
1. Desizing
2. Heat setting
3. Washing
4. Bleaching if necessary.
Singeing
To remove protruding fibre ends by heat or by using a live flames this process is carried out.
Technically, singeing refers to the burning-off of. Loose fibers not firmly bound into the yarn
and/or fabric structure. 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.
Loose yarns not firmly bound into the fabric structure;
Protruding fiber ends sticking out of the textile yarns.

20. Heat setting
In the process of Synthetic fabric manufacturing, heat setting is carried out in two different
times. That is before dyeing and after dyeing. But In pretreatment process heat setting carried
out before dyeing.
Stabilizing the material against shrinkage, Improve dye-ability and resistance to pilling are
the main purpose of heat setting. Quality heat settings resist the undesirable creasing and
wrinkling before dyeing. Therefore dyeing is more uniform. Eventhough variations in
temperature settings will lead to non-uniform dyeing. And if there is any residues in sizes or
stains in fabric while heat setting, It will be more difficult to remove at the end of process.
Desizing
Desizing is done in order to remove the size from the warp yarns of the woven fabrics. Warp
yarns are coated with sizing agents prior to weaving in order to reduce their frictional
properties, decrease yarn breakages on the loom and improve weaving productivity by
increasing weft insertion speeds. The sizing material present on the warp yarns can act as a
resist towards dyes and chemicals in textile wet processing. Therefore, It must be removed
before any subsequent wet processing of the fabric
General size mixture contains,
 Softeners
 Size
 Anti-mildew agents
 Hygroscopic agents
Using detergent in hot washing all other ingredients except starch can be removed. Starch is
water insoluble and therefore we need to convert them in to water soluble form to wash-off.
There are two types of polymers in Starch. They are natural and synthetic. Synthetic
polymers have a linear structure. Therefore it is easy to dissolve it in water. But natural
polymers have branches in its chemical structure. So that those branches should break to be
dissolved.

21. 2 main desizing methods
 Hydrolytic method Rot steep
Alkali steep
Acid steep
Enzymetic steep
 Oxidative method Chlorine
Chlorite
Bromite
Here enzymatic steeping method has many cons. Without damaging the fiber it completely
removes starch.
Testing for starch will be carried out after desizing . Drop of iodine solution spotting on the
fabric give,
o Dark blue – indicates presence of the starch
o Dark brown – indicates satisfactory removal of starch
o Yellowish – indicates complete removal of starch
Scouring
The term ‘scouring’ applies to the removal of impurities such as oils, was, gums, soluble
impurities and sold dirt commonly found in textile material and produce a hydrophilic and
clean cloth.
Objectives of Scouring:
 To remove natural as well as added impurities of essentially hydrophobic character as
completely as possible
 To increase absorbency of textile material
 To leave the fabric in a highly hydrophilic condition without undergoing chemical or
physical damage significantly.
Scouring process depends on:
 The type of cotton
 The color of cotton
 The cleanliness of cotton
 The twist and count of the yarn
 The construction of the fabric.

22. Impurities can contact with synthetic fibers during spinning and weaving/knitting processes.
To remove these impurities scouring is done in the presence of a strong alkali and a synthetic
detergent. .
If the material contains cellulose acetate, scouring should be carried out at 70oC for 20-30
minutes in liquor containing 1 to 1.5 parts of detergent and 1.5 parts of ammonia per
thousand. To avoid the risk of hydrolysis of acetyl groups, a mild alkali and the lower
temperature are specified.
For nylon 6-6, best conditions are 70oC and the liquor that contain 1.5 to 2 parts of detergent
and 0.5 to 1.5 parts of sodium carbonate per thousand. Scouring time is 1 to 1 ½ hours.
Bleaching
Bleaching is chemical treatment employed for the removal of natural coloring matter from
the substrate. The source of natural color is organic compounds with conjugated double
bonds , by doing chemical bleaching the discoloration takes place by the breaking the
chromophore , most likely destroying the one or more double bonds with in this conjugated
system. The material appears whiter after the bleaching.
For cellulosic manmadefibres best liquor condition is 1 to 5 g/l cold chlorine solution. pH
value should maintain between 10-11. To obtain best result, bleaching should carry out for
one hour.
For polyester bleaching sodium hypochlorite, hydrogen peroxide and sodium hydrosulphite
or its derivatives can use as the solution. Suitable concentration is 1.5 g/l and temperature
should maintain between 98oC-100oC.
The aims of bleaching can be described as following:
1. Removal of coloured impurities.
2. Removal of the seed coats.
3. Minimum tendering of fibre.
4. Technically reliable & simple mode of operation.
5. Low chemical & energy consumption.
6. Increasing the degree of whiteness.
Bleaching Agent
A bleaching agent is a substance that can whiten or decolorize other substances.Bleaching
agents essentially destroy chromophores (thereby removing the color), via the oxidation or
reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing
agents or reducing agents.

23. Type of Bleaching Agents
 a.Oxidative Bleaching Agents
 b.Reductive Bleaching Agents
 c.Enzymatic Bleaching Agents
Mercerizing
Mercerization is a process in which textiles (Usually in Cotton) are treated with a caustic
(NaOH) solution to improve properties such as fiber strength, shrink- age resistance, luster,
and dye affinity. The caustic actually rearranges the cellulose molecules in the fiber to
produce these changes.
Also mercerizing increases the crease recovery ability and wetting property. After
mercerizing, chemical structure will be the same but physical structure will change.
To achieve a best luster hot mercerizing should be carrried out at 60oC and to achieve best
strength liquid ammonia treat can be carry out. It gives relatively 40% of increase in strength
than mercerization.
Mercerizing completeness tests can carry out accessing by barium activity number test.
 100-150 – No mercerizing
 105-150 – Incomplete mercerizing
 >150 – Complete mercerizing
2.3 Peroxide bleaching for cotton materials
Hydrogen peroxide is virtually the only bleaching agent available for protein fibers and it is
also used very extensively for the cellulosic fibers. Hydrogen peroxide is a colorless liquid
soluble in water in all proportions. It is reasonably stable when the pH is below 7 but tends to
become unstable as the alkalinity increases.
Cotton is usually bleached in 1-volume liquor at the boil. The most important factor in
bleaching is to achieve the right degree of stability in the bleach liquor. If the pH were too
low no per hydroxyl ions are set free and bleaching does not take place
The bleaching liquor must be made alkaline, otherwise it would be too stable, but it is
virtually impossible to adjust to the optimum pH with alkali alone and there is a marked
tendency for the liquor to is too unstable, however carefully it has made alkaline. It is,
therefore, necessary to add a stabilizer, and of all the substances, which have been, tried
sodium silicate is the most effective.

24. Hydrogen peroxide is a stable chemical under acidic conditions and needs the addition of an
alkali for activating it. Above pH 10, it is extremely unstable when it gets decomposed under
water and oxygen.
This liberated oxygen, however, has no bleaching action and the catalysts are therefore a
cause of loss of bleaching power. In fact, hydrogen peroxide is used bleaching under alkaline
conditions (pH 10) after stabilizing at this pH by adding sodium silicate, borax, phosphate
etc. Generally bleaching is done at 80ºC to 85ºC temperature.
Hydrogen peroxide solution at any concentration can be stable or unstable depending upon
the several factors listed below.
1. pH: Stable in acidic solution and unstable in alkaline baths.
2. Temperature: As temperature increases the solution becomes increasingly unstable.
3. Buffers: Silicates, Phosphates, Borax, Proteins and others tend to stabilize peroxide.
4. Metals: (a) Ca and Mg in the presence of silicates tend to stabilize baths; (b) other
metals, i.e., Cu, Fu, etc. tend to unstabilize bleach solutions.
5. Hard water: Depending upon the hardness of water and the metals making it hard,
peroxide is unstabilize.
It was at one time believed that the bleaching action of hydrogen peroxide was due to the
liberation of nascent oxygen but this explanation is no longer tenable. It is known that under
certain conditions, particularly with regard to pH, hydrogen peroxide will liberate hydrogen
and per hydroxyl ions in the following manner:

25. Hydrogen peroxide (H2O2) is a universal bleaching agent and is used extensively for the
bleaching of cotton materials. The advantages in its use are:
1. It can be employed for bleaching fibers like wool, silk and jute also.
2. It requires less manipulation of fabric and hence less labor.
3. The loss in weight in bleaching is less than that with hypochlorite bleaching
4. Less water is required with peroxide bleaching and there is no need for souring after
bleaching.
5. Peroxide bleached goods are more absorbent than hypochlorite bleached goods.
6. After – yellowing of white goods bleached with peroxide or less than with
hypochlorite bleached goods.
7. Peroxide bleaching is safer in regard to chemical degradation and
8. Continuous scouring and bleaching in one operation is possible by employing
peroxide.
2.4 Optical brightening agents for different types of fibers
There has been a considerable increase in the use of Optical brighteners. In addition the
number of fiber types and fiber mixtures has increased tremendously, which means that the
processor must now cope with a greater variety of application methods. There is hardly a
white textile, or a white paper or hardly a household detergent which does not contain a
brightener. The development, thus has led to continually increasing demand made on these
products both by processors and consumers. They are required to be used on a variety of
finishing processes and they should be compatible with practically all chemicals and
auxiliaries used at different stages. Furthermore, good all round fastness properties and a
good yield are also desired. In addition to this, different shades of whites are desired, as white
shades are subject to fashion trends.
Optical Brightening Agents (OBA) is added to many substrates to reduce yellowing, improve
whiteness and enhance the brightness of the product. Optical whiteners function by absorbing
ultraviolet radiation and re-emitting blue light. The emitted blue light will reduce the yellow
color of the substrate and impart a pleasing whiter-than-white appearance. Optical brighteners
give more intensity to some colors as well.
Different Types of Optical Brightening Agent find suitable application on different substrates
like Cellulose, Nylon, Silk, Wool, Paper, Detergents, Soaps, Plastics, Printing Inks, Coatings
and Polymers.

26. There are 3 different types of Optical Brightning Agents based on the sulfonic group.
1. Disulfonic / Divalent
It constitutes of two sulfonic acid groups and is good for hydrophobic fiber (nylon, silk &
wool) application in Acidic pH.
2. Tetrasulfonic / Tetravalent
It constitutes of four sulfonic groups and is having good solubility and is ideal for cellulosic
fiber and paper application at neutral or alkaline pH.
3. Hexasulfonic / Hexavalent
It constitutes of six sulfonic groups and has excellent solubility for surface coating
application like photographic paper