The document discusses the chemical processing and pre-treatment of cotton fabrics, detailing the objectives and importance of fabric preparation to remove impurities for effective dye uptake and improved appearance. It outlines various preparatory operations such as singeing, desizing, scouring, and bleaching, including the chemicals involved and their mechanisms. The document also covers quality control measures and the effects of bleaching on fabric properties, emphasizing the balance between desired whiteness and strength retention.

1. Chemical processing of cotton
fabrics
(Pre treatment)
BY:OLIYAD EBBA
ETHIOPIAN TEXTILE INDUSTRY
DEVELOPMENT INSTITUTE

2. Cotton textiles
• Fibers - Loose Stock
• Yarn - Hank, Package (Cone /
Cheese)
• Fabric - Open width, Rope form.

3. Chemical Processing of Textile
Materials
• Preparatory/pre-treatment Process
• Dyeing / Printing
• Finishing

4. Preparatory/pre-treatment
process
• What is the objective of
fabric preparation in textile
processing?

5. Preparation:
• To remove the natural and added impurities
from fibers.
• To achieve high and uniform dye uptake
• To get required brightness of shade in the
fabric after dyeing
• To impart good hydrophilic properties
combined with high absorbency
• To produce acceptable degree of whiteness

6. • What will happen if the impurities are not
removed properly ?

7. Preparatory Process
• If the impurities are not removed, the
material will not be receptive to the
subsequent processes – dyeing / printing /
finishing.
• Impurities are also to be removed to
improve the appearance

8. What are these impurities?
• In cotton

9. Impurities in cotton
• In Fibers – natural impurities such as wax,
oils, pectin's, pentose's, vegetable matter
and COLOURING MATTER (Yellowish /
Brownish)
• In Yarns – All natural impurities plus traces
of lubricants - like oils & greases, rust, dirt,
etc.,

10. Cont…..
• In Fabrics
Woven: all impurities that are present in fibers
and yarns plus sizing materials (if yarn is sized)
such as starches, PVA etc.,
Knits: for knitting yarn is not sized. Therefore
knits are likely to have all impurities that
woven fabric have except the sizing materials.

11. Natural constituents of raw
cotton
% by wt. of raw cotton
Cellulose 88.0 – 96.0
Nitrogenous matter 1.0 – 1.9
Waxes 0.3 – 1.0
Pectins 0.7 – 1.2
Ash 0.7 – 1.6
Malic, citric and other
0.5 – 1.0
organic matter
Total sugars 0.1 – 0.3
Others 0.8 – 0.9
Moisture regaiñ~ 8%

12. what are the steps to be taken before
going to wet processing?

13. Following are the steps to be taken:
Grey inspection
Stitching
Shearing and cropping

14. GREY FABRIC INSPECTION
• Fabric is inspected for defects before it
is dyed i.e. grey stage.
• Longer yarns can be cut
• A record of the defects in a fabric length
is kept to determine its quality .
• The process of giving points and
assigning a quality level is called fabric
grading.

15. Fabric Stitching
• Most of the machines in preparatory section
are huge capacity machines. Therefore to
utilize them to its full capacity it is
necessary to feed them with suitable lots
prepared by stitching together of fabrics
from different job orders but of similar
quality and same preparatory sequence
requirement, thus enabling us to process
them in a single batch

16. Shearing and cropping
• This is used for removing loose yarns of
length up to 8cm from the surface of the
fabric

17. What are the various
operations in preparatory
process?

18. The various operations in
preparatory process are:
Singeing
Desizing
Scouring
Bio polishing
Bleaching
Mercerization

19. • Typically a woven cotton fabric
would be prepared by sequence of
process as shown above
• In case of knitting sizing step is not
involved

20. Singeing
• Burning of protruding fibers from the
surface of fabric or yarn - A mechanical
process
What it do?
• Makes fabrics smooth
• Prevents pilling
• Improves luster

22. What should control during
singeing operation?
• Intensity and uniformity of flame
• Working speed of the fabric to be singed
• Effectiveness of singeing : should not add
harshness in the fabric
• Should not damage the synthetic filament
yarn in the fabric
• More care should be taken while singeing
blended fabrics,

23. What is Bio polishing?
It is an enzyme treatment designed to
improve fabric quality and provide
following advantages:
• Improve pilling resistance
• A clear, lint and fuzz-free surface structure
• Improved drape and softness
• The effect are durable

24. When it should be carried out?
• It can be carried out at any time, but after
bleaching is most popular
• After dyeing shade may be affected

25. Process
• Cellulase enzyme is used
• Enzyme dosage 1-2% owf
• pH 4.5-5.5
• Temperature 40-55oC
• Time 30-60 min.
It is similar to singeing as it also provide
similar effect on the fabric

26. DESIZING
• To remove size material from the warp yarn
• Size material: Starch, PVA, Synthetic ingredients
• If not desized?
Problems in dyeing like patchy or uneven dyeing
What is sizing?
This is a process of coating warp yarn with sizing
material to make it strong so it can withstand
during weaving operation . In other words it will
not break during weaving operation.

27. Desizing methods
• Rot steeping
• Acid steeping
• Enzymatic Desizing
• Oxidative Desizing

28. Mechanism of Desizing
• Starch Amylopectin &Amylose(insoluble)
Dextrin(soluble) Dextrin(insoluble)
Maltose(soluble) Alpha glucose(soluble)

29. Enzymatic Desizing: Enzyme Desizing:
• Enzyme solution in water with 0.5% to 2.0% on
weight of fabric enzyme
• Require quantity of Common salt is also required
• Neutral pH
• Overnight Padding fabric with solution
• Other method fabric is run continuously in
machine having enzyme solution.
• Enzyme: Amino acid based catalyst.
• pH: Indicates acid, neutral and base character of
the material.

30. QUALITY CONTROL
CHECKS
• Iodine test

31. SCOURING
• It is process to remove all undesirable
impurities except COLOURING MATTER. After
scouring, oils and waxes are removed and hence
the material becomes receptive to water, dyes and
chemicals. In other words, the absorption capacity
of the material is improves.
• Natural Impurities: Based on the
composition of natural material like cotton,
wool, silk etc.

32. Mechanism of Scouring
• Saponifiable oils & free fatty acids are converted
into soap and glycerol
• Pectic substances(pectin, pectose and
pectates)converted into soluble salts of pectic or
meta pectic acids
• Proteins are degraded to simple soluble amino
acids

33. Cond…
• Mineral matter is dissolved
• Unsaponifiable oils & waxes are emulsified by
soaps
• Adventitious dirt is removed and retained in
suspension by the soap
• Other added impurities are broken down into
soluble products

34. Scouring of Cotton
• Sodium hydroxide and Soda
ash are used for scouring of
cotton at boiling
temperature: called alkali
scouring
• Now enzymatic scouring is
also used
• Solvent scouring was also
used few year back by using
solvent by using
trichloroethylene, which
dissolve wax-But now it is
not used

35. BLEACHING
• It is a process of whitening-fibers, yarns, or
fabrics having natural color.
It is carried out by using oxidizing agents like:
Sodium hypochlorite (NaOCl)
 Hydrogen peroxide (H2O2)
 Sodium Chlorite (NaClO2)
• Among these hydrogen peroxide is widely
used.

36. Styles of bleaching
• Full Bleach - Full white / Snow white
• Half Bleach - to dye light / pale /
medium shade.
• No Bleach - to dye dark shades.

37. Hydrogen Peroxide for cotton
• Most popular and environmentally accepted
process
• But require higher temperature near boiling
• Hydrogen peroxide is used along with NaOH (to
maintain pH 10-11)
Drawback: Poor stability for small amount of Iron
and copper present in the textiles or the
processing water. Cause catalytically
decomposition of the peroxide, This loss of H2O2
mean less available bleaching agent. To prevent it
Sodium silicate is used as stabilizer along EDTA.

38. Mechanism of peroxide
bleaching
• H2O2 is a weak acid and ionizes in water to form:
(active bleaching
agent)
HOO- (unstable) OH + O* (active or nascent oxygen)
O* + X X-O, X- oxidizable substance
• H2O2 decomposition is catalyzed by metal ions e.g. Cu++, Fe+
+ ...undesired rxn: no bleaching effect and causes fiber
damage.

39. calculation of active oxygen
content in H2O2-solutions
• For the determination of the active oxygen
content the bimolecular decomposition
reaction of hydrogen peroxide is taken as
basis:
34.0146 g/mol 18.0152 g/mol + ½ · 31.9988 g/mol

40. • Out of 34.0146g H2O2 100 % result from
½ · 31.9988g =15.9994g of so-called
active oxygen.
• The conversion factor of % by weight of
H2O2-solution in active oxygen is the
following:
active oxygen [g/kg] = % in weight H2O2
solution * 4.704

41. Examples:
1.How much active oxygen in g/kg does a
H2O2-solution of 50% contain ?
2. How much active oxygen does a bleaching
bath with 10ml/l H2O2 50% (density ρ20 =
1.132g/cm3) contain?

42. solutions
#1.Active oxygen = 50 · 4.704
Active oxygen = 235 g/kg
Active oxygen = 23.5 %
•#2. 10 ml H2O2 50 % contain
10 ⋅ 1.195 = 11.95 g H2O2 50 %
or 11.95 g H2O2 50% /1000 ml
x=11.95*50*4.7 1000 =2.8 g/l

43. i. Effect of pH
H2O2 is an extremely weak acid.
Caustic neutralizes the proton and shifts the
reaction to the right.
• pH <10, H2O2 is the major species so no
bleaching.
• pH = 10 -11, moderate conc. of per hydroxyl ions.
• pH = 10.2-10.7 ( with NaOH) is optimum.
• pH >11, rapid generation of per hydroxyl ion.
• pH 11.8 all H2O2 is converted to HOO- and rxn is
out of control.

44. ii. Effect of Time and
Temperature
• Stabilized H2O2 does not decompose at high
temperature therefore faster and better
bleaching occurs at 95 to 100 0C… ideal for
continuous operations.
• Temp - rate of bleaching … but solution
becomes unstable and degradation of cotton
increases.
• Below 80 0C the evolution of per hydroxyl
ion is very slow so also the rate of
bleaching.

45. Effect of concentration of liquor
and time
iii. Effect of concentration of liquor
• Batch process = 2-4% (o.w.f.) H2O2 MLR of
10:1 to 20:1.
• In the continuous process =1-2% (o.w.f.)
H2O2 Very high concentration may damage
the fiber.
iv. Effect of time
• The time depends on temp, class of fiber
and equipment used for bleaching.
• temp. of bleaching bath time of bleaching

46. Auxiliaries for Bleaching With
H2O2
1. Stabilizers
• To control the decomposition of H2O2 .
Provide buffering action to control the pH
and to complex with trace metals which
catalyze the degradation of the fibers.
Sodium silicate, organic compounds and
phosphates

47. Cont.
2.Activator: provide alkalinity
 NaOH, Na2CO3, Na3PO4,etc.
3. Sequestering agent: organic stabilizers or
separate
 EDTA, Sodium hexa meta phosphate.
4. Wetting agent – to provide wetting and
detergency

48. Usability of bleaching agents for
various types of fibers
Hydrogen
peroxide
Sodium
chlorite
Sodium
hypochlorite
Persulphate Sodium
dithionite
Cotton CO ++ ++ ++ ++ +
Wool WO ++ -- -- -- ++
Silk SE ++ -- -- -- ++
Viscose CV ++ ++ + + +
Acetate AC Δ ++ 0 -- ++
Polyester PE
S
+/0 ++ +/0 0 +/0
Polyacryl (EL + / 0 -- -- Δ + / 0
++ important process,+ suitable,0 ineffective,-- harmful
Δ suitable under special conditions

49. scour/ bleach
Scour bleaching recipe on jet.
Material to liquor ration:1:6
Wetting agent : 1 gpl
Sequestering agent:1 gpl
Stabilizer :0.2 gpl
Lubricant : 1 gpl
Caustic soda : 2gpl
Hydrogen peroxide:2 gpl
Acetic acid: 0.5 gpl
Peroxide killer:0.5 gpl

50. Process diagram of scour
bleaching

51. Common tests should be done
after pretreatment
PH : for reactive dyeing the dye start ph should
be in the range of 5.5-6.5,electronic ph meter of
litmus paper can be used.
Peroxide residue: there shouldn’t be any residual
peroxide for it result in unlevel dyeing. Peroxide
stick can be used.
Hydrophility : water or methyl blue reagent can be
used to under go water drop test. The water
should be absorbed in three seconds time to assure
the fabric for dyeing.

52. Cont.…
The liquor amount is very important. The pick up,
water used for dye stuff , soda dissolution should
be subtracted from the total liquor volume (fabric
weight*M:L ratio).
Whiteness /refractive index: to check for the
standard whiteness to be achieved before dyeing.

53. If brilliant white finish is required in textile, then
bleaching is not sufficient. For this purpose
Fluorescent brightening agents are used

54. Fluorescent brightening
agent(FBA)
• Fluorescence is the ability of a substance to
absorb radiation of light and re-radiate it at
a longer wavelength.

55. Mechanism
• OBAs absorb in the UV range (300–400 nm) and re-emit
into longer visible (400–500 nm) wavelengths.
• Optical brighteners increase the apparent reflectance
of the textile in the visible region by converting UV-radiation
in the visible light and so increase the
whiteness or brightness.

56. Recipe of full bleach
Wetting agent : 1 gpl
Stabilizer : 1gpl
Lubricant : 1 gpl
Hydrogen peroxide( 50 % concentration):4gpl
Caustic soda( flake ) :4 gpl
Optical brighter : 0.3%
Acetic acid :0.5 gpl
Material to liquor ratio: 1:6

57. Process diagram for Full white

58. Change in properties after bleaching
• Whiteness Increases
• Absorption Increases
• Drop in strength

59. Relation between Whiteness and
Strength
• Increase in whiteness is directly
proportional to loss in strength. That is
higher the degree of whiteness achieved,
higher will be the loss in strength.

60. Why loss in Strength after
Bleaching?
• The oxidizing agent not only oxidizes the
natural coloring matter present in cotton but
also oxidizes the cotton cellulose. Thus
cellulose is chemically damaged.
• The chemical damage is seen in terms of
drop in molecular chain length (DP) and
conversion of cellulose into oxy cellulose

61. What are the adverse effects of
Oxy Cellulose formation?
• Drop in reactive dye up take
• Drop in moisture absorption
• Drop in reactivity
These changes are due to conversion of
hydroxyl groups into carboxyl and aldehyde
groups.

62. Acceptable quality after bleaching
• Minimum loss in strength – 5 to 10 % depending
upon
– desired degree of whiteness &
– fabric construction (coarse, medium & fine)
• Permanent whiteness – (no yellowness on
exposure to sun light)
• High degree of absorption
• Fluidity less than 4 or 4.5 depending upon
whiteness required

63. Conversion of g/l into % or %
into g/l
• On discontinuous bleaching proceses (and
of course on dyeing and finishing
processes), the chemicals, auxiliaries,
optical brighteners, dyestuffs etc. are either
stated in percent (%) related to the fabric
weight or in g/l resp. ml/l.

64. Cont...
• Conversion of g/l into % or from % into g/l
with the given liquor ratio is as follows:
% =g/l * liquor ratio 10
g/l= %*10 liquor ratio

65. examples
• 3 g/l H2O2 50 % are in the bleaching
liquor. What is the concentration in % with
a liquor ratio of 1:8 ?

66. MERCERISATION
• This is the process applicable only on
cellulosic fibers especially cotton
• The main purpose of mercerization is
converting the inter fiber structure from
alpha-cellulose to a thermodynamically
more favorable beta-cellulose polymorph.
Mercerizing results in the swelling of the
cell wall of the cotton fiber. This causes
increase in the surface area and reflectance,
and gives the fiber a softer feel

67. Microscopical analysis of
mercerized cotton
Fiber cross-section of a
non-mercerized cotton
Fibre cross-section of a
mercerised cotton