The document discusses the steps involved in preparing fabric for printing, including singeing, desizing, scouring, bleaching, and mercerization. It then explains several chemicals and agents used in the printing process, such as wetting agents, hygroscopic agents, dispersing agents, oxidizing agents, reducing agents, and discharging agents. Wetting agents lower surface tension to help liquids spread evenly. Hygroscopic agents help fabrics absorb moisture. Dispersing agents prevent dye aggregation. Oxidizing agents develop color. Reducing agents are used in processes like vatting and discharge printing.

1. UNIT-2
(Technology of Printing-I (NTT-508)
TC 3rd Year)

2. Preparation of fabric for Printing
• Steps of preparation of fabric for printing is
almost same as fabric preparation for dyeing.
• Following are the main steps:
Singeing
Desizing
Scouring
Bleaching
Mercerizing (For cotton fabric)

3. Singeing
• It is a process of removal of protruding fibres
from the surface of fabric using thermal and
mechanical energy. Singeing is mostly carried
out using gas singeing machine.
• Advantages:
1. Fabric surface become smooth, so the
printing will be sharp.
2. Pilling problem of fabric is reduced.

4. Desizing
• Desizing is a process of removing sizing chemicals from the
fabric. There are four methods for desizing. These are:
• Rot steeping ( Treating fabric with hot water and kept for
overnight)
• Oxidative desizing (Sodium Hypochlorite: 2.0-5.0 gm/lit. at
room temperature at pH 7.0 or Hydrogen peroxide:3.0-6.0
gm/lit of H2O2 and 7.0-15.0 gm/lit Sodium Hydroxide, 40oC for
12-16 hours)
• Acid steeping (Treating fabric with dilute acids-Sulphuric or
Hydrochloric acids)
• Enzymatic desizing (Most common process, Amylase enzyme
is used for desizing purpose, Enzyme solution in water with
0.5% to 2.0% on weight of fabric enzyme, Require quantity of
Common salt is also added at Neutral pH)

5. Scouring
• It is process to remove all undesirable
impurities (Natural: like dirt, vegetable
matter, grease wax etc and Added: like
remaining size material, stains etc.)
• Natural Impurities: Based on the
composition of natural material like
cotton, wool, silk etc.

6. Bleaching
• This is a process of whitening-fibers,
yarns, or fabrics having natural colour.
• It is carried out by using oxidising agents
like
• Sodiumhypochlorite (NaOCl)
• Hydrogen peroxide (H2O2)
• Sodium Chlorite (NaClO2)

7. Bleaching
• Oxidative bleaches oxidize color bodies
into colorless compounds. For example,
double bonds are known to be oxidize into
epoxides which easily hydrolyze into
diols.
• The major bleaching agents used in textile
preparation are sodium hypochlorite,
hydrogen peroxide and sodium chlorite.

8. MERCERIZATION
• Treatment of cotton yarn or fabric with concentrated
sodium hydroxide for a minute or less is called
mercerization.
• The process of mercerization is named after the John
Mercer, who was the first to discover in 1844, that the
treatment of cotton with concentrated sodium hydroxide
solution brought about permanent changes in the fibre's
properties.
• He claimed that the treatment resulted in
significant increase in the tensile strength, water
absorbency and dyeability of the fibres. It was also
observed by him that treated material underwent
considerable shrinkage.

9. • The excessive shrinkage in the cotton yarn and
fabric make this process nonviable in the
industry thus it was not commercialized.
• It took more than forty years until it was
found by H.A.Lowe that shrinkage could be
prevented if the yarns or fabrics were kept
under tension during mercerization. When
mercerizing is done under tension, the fibre
also acquire a silk-like luster.
MERCERIZATION

10. • This is the process applicable only on
cellulosic fibres especially cotton.
• The main purpose of mercerization is to
alter the chemical and physical properties
of the fibre.
MERCERIZATION

11. What changes mercerizing bring to
cotton?
• Improved luster
• Increased ability to absorb dye
• Improved reactions with a variety of chemicals
• Improved stability of form
• Improved strength/elongation
• Improved smoothness
• Improved hand

12. Wetting Agents
• A substance is referred to as a wetting agent if it
lowers the surface tension of a liquid and thus
allows it to spread more easily.
• There are two types of intermolecular forces:
• Cohesive Forces: The forces exerted between
molecules holding them together. If cohesive
forces are strong, a liquid tends to form droplets
on a surface.
• Adhesive Forces: The forces between liquid
molecules and a surface. If adhesive forces are
strong, a liquid tends to spread across a surface.

13. • Wetting agents are substances that reduce the surface
tension of water to allow it to spread drops onto a
surface, increasing the spreading abilities of a liquid.
Lowering the surface tension lowers the energy
required to spread drops onto a film, thus weakening
the cohesive properties of the liquid and strengthening
its adhesive properties.
• One example of how wetting agents work is in the
formation of micelles. Micelles consist of hydrophilic
heads forming an outer layer around lipophilic tails.
When in water, the micelles' tails can surround an oil
droplet while the heads are attracted to the water.
Wetting Agents

14. • Dish soap is a great example of a wetting
agent. With all the food oils and such on the
plate cohesive forces make it difficult for the
water to spread and clean the plate. The soap
dissolves all theses unwanted particles,
exposing a clean surface. The soap also lowers
the surface tension of water, allowing it to
spread evenly across the entire surface.
Wetting Agents

15. Types of Wetting Agents
• There are four main types of wetting agents: anionic,
cationic, amphoteric, and nonionic.
• Anionic, cationic, and amphoteric wetting agents ionize
when mixed with water.
• Anions have a negative charge, while cations have a
positive charge.
• Amphoteric wetting agents can act as either anions or
cations, depending on the acidity of the solution.
• Nonionic wetting agents do not ionize in water. A
possible advantage for using a nonionic wetting agent
is that it does not react with other ions in the water,
which could lead to formation of a precipitate.

16. How to Tell if a Liquid Contains a
Wetting Agent
• One method of knowing whether or not a liquid has a
wetting agent in it is to spread the liquid on a surface
that is coated in grease.
• If the liquid does not contain a wetting agent, then its
cohesive forces would overpower adhesive forces,
causing the liquid to form droplets on the surface.
• If the liquid does contain a wetting agent, the grease
would be dissolved and the surface tension of the
liquid would be lowered, causing the adhesive forces to
overpower the cohesive forces. This would result in the
liquid spreading evenly along the surface.

17. More cohesive force
than adhesive
More adhesive force
than cohesive
Wetting Agents

18. • Another method is to place the liquid in a test tube and
observe the liquid's meniscus.
• If the liquid contains a wetting agent, its adhesive
forces are stronger than cohesive forces, which means
the liquid molecules are more inclined to stick to the
surface than other liquid molecules. This results in a
concave meniscus.
• If the liquid does not contain a wetting agent and is
naturally very cohesive, like mercury, it forms a convex
meniscus. This is caused by the fact the molecules of
the liquid have a stronger attraction to each other than
to the surface of the test tube.
Wetting Agents

19. Concave
Convex
More cohesion More Adhesion
Wetting Agents

20. Uses for Wetting Agents
•Detergents
•Fabric softener
•Paints
•Inks
•Anti-fogging
•Soil remediation
•Foaming agents
•Shampoo etc
Wetting Agents

21. Hygroscopic agents
• In printing fixation of dyestuff is carried out by either
steaming or dry heat. Since the amount of moisture in
the dried print is less, hygroscopic agents are often
added to enable the absorbing of moisture during
steaming.
• Optimum quantity of these agents should be used as
less quantities will result in poor transfer/fixation of
the colour due to improper dissolution, whereas large
quantities result in spreading of dye solution beyond
the boundaries.
• Examples: Glycerin, ethylene glycol, polyethylene
glycol, urea etc.

22. Dispersing agents
• In the printing paste, the dye/pigment has to be
dissolved/disperse in a very small quantity of water.
Hence, certain solvents or dispersing agents are used
to prevent aggregation of dyestuff molecules in the
printing paste.
• Solution aid or hydrotropic agent is added to facilitate
the dissolution of the dye in a small quantity of water.
• Commonly used solvents are acetin (triacetates of
glycerine), diethylene glycol for vat dyes in leuco and
solublised vat form and cellosolve (glycol monoethyl
ether) for printing of vat and azoic etc.
A hydrotrope is a compound that solubilises hydrophobic compounds in aqueous solutions.
Typically, hydrotropes consist of a hydrophilic part and a hydrophobic part (like surfactants)
but the hydrophobic part is generally too small to cause spontaneous self-aggregation.

23. Oxidising agents and oxygen carrier
• An oxidizing agent is generally added to the
printing paste of solublised vat, aniline black etc
for the final development of colour. The agent
should not be active in the printing paste itself so
as to prevent, development of colour in the
printing paste.
• Therefore, sodium chlorite, potassium chlorite,
sodium nitrite, sodium chromate, etc, which are
potential oxidizing agents are used in the printing
paste and after printing, fabric is treated with an
acid for the development of the colour.

24. • In certain cases, the dye (especially azo dye)
are susceptible to a reducing influence during
steaming, which may result in loss of colour
value. In such instances, sodium m-
nitrobenzene sulphonate (alkaline media) and
sodium chlorate (acidic media) are used for
reactive and disperse dye respectively.
Oxidising agents

25. Reducing/resisting/discharging
agents
• Reducing agents are used in the printing paste of
vatting of the vat pigments, fixation of vat
pigments onto cotton and resist and discharge
style of printing.
• Reducing agents are also used in the after
clearing of disperse dyes from the polyester
textiles.
• For vatting of vat dye and after clearing process
(Reduction clearance or RC), sodium hydrosulhite
is mainly used.

26. List of reducing agents used in resist
and discharge printing
• Sodium sulphoxylate formaldehyde
• Zinc sulphoxylate formaldehyde
• Thiourea dioxide
• Stannous chloride
• Benzoyl peroxide with sodium chlorite