TEXTILE
WET
PROCESSING
UNIT I
Water Hardness
Water Hardness:
Water
hardness is caused primarily by the presence of calcium (Ca²⁺) and magnesium
(Mg²⁺) ions. Hard water can cause scaling in pipes, boilers, and equipment,
which can hinder industrial processes, especially in textile dyeing, washing,
and finishing.
Types of Water Hardness:
- Temporary Hardness:
- Caused by the presence of calcium
bicarbonate (Ca(HCO₃)₂) and magnesium bicarbonate (Mg(HCO₃)₂).
- Properties:
- Can
be removed by boiling, as boiling causes the bicarbonates to
decompose into insoluble carbonates, which precipitate out of the
water.
- When
heated, the hardness-causing minerals form solid deposits, often called
"scale."
- Permanent Hardness:
- Caused by calcium sulfate
(CaSO₄), calcium chloride (CaCl₂), magnesium sulfate
(MgSO₄), and magnesium chloride (MgCl₂).
- Properties:
- Cannot
be removed by boiling because the salts are non-bicarbonate and remain
in the dissolved form.
- Requires
treatment methods like ion exchange, lime softening, or reverse
osmosis.
- Total Hardness:
- The sum of both temporary and
permanent hardness.
- Expressed as ppm (parts
per million) or mg/L of calcium carbonate (CaCO₃)
equivalents.
Softening of Water:
- Ion Exchange Method:
- Involves the exchange of
calcium and magnesium ions in the water for sodium (Na⁺) ions from a
resin bed.
- The process is reversible, and
the resin can be regenerated by flushing it with a concentrated salt
(NaCl) solution.
- Uses: Widely used in industries
(including textile) where soft water is needed to prevent scale formation
on equipment and ensure better dyeing, washing, and finishing.
- Sequestering Agent Method:
- Sequestering agents, like phosphates
(e.g., sodium tripolyphosphate) or citric acid, bind to
hardness-causing minerals, keeping them in solution and preventing them
from reacting with soap or detergents.
- Example: EDTA
(ethylenediaminetetraacetic acid) is commonly used in this method to
form stable, soluble complexes with calcium and magnesium ions.
- Uses: Helps in controlling the
hardness in washing and dyeing processes in textiles by keeping minerals in
their dissolved form.
Textile Industry Terminology & Terms:
- pH:
- A measure of hydrogen ion
concentration in a solution, indicating its acidity or alkalinity.
- pH Scale: 0 to 14; neutral = 7; acidic
< 7; alkaline > 7.
- In the textile industry, pH
affects dye uptake, fabric structure, and the effectiveness of chemical
treatments.
- pH in
Dyeing:
Acidic pH is used for acid dyes, while an alkaline pH is needed
for reactive dyes.
- Acid:
- A substance that releases
hydrogen ions (H⁺) in solution.
- Common acids in textiles: acetic acid
(used in acid dyeing), sulfuric acid, and formic acid.
- Uses in Textiles: Acidic conditions are often
used for acid dyeing, delustering, and finishing to
give fabrics certain colors or properties (like softness).
- Base (Alkaline):
- A substance that releases
hydroxide ions (OH⁻) in solution.
- Common bases in textiles: sodium
hydroxide (NaOH), ammonia (NH₃), sodium carbonate (Na₂CO₃).
- Uses in Textiles: Alkaline conditions are used
for processes such as scouring (removal of natural oils and
impurities from fibers) and bleaching (especially with hydrogen
peroxide).
- Oxidation:
- The process of losing
electrons or increasing the oxidation state of a substance.
- In textiles, oxidizing
agents (like hydrogen peroxide or sodium hypochlorite)
are used to bleach fabrics or remove colors.
- Example: Bleaching cotton with
hydrogen peroxide in the presence of an alkali.
- Reduction:
- The process of gaining
electrons or reducing the oxidation state of a substance.
- Reductive bleaching is used in some fabric
treatments (e.g., in indigo dyeing, the dye is reduced to a
soluble form, applied to the fabric, then oxidized to give the
characteristic blue color).
- Example: The reduction clearing
process, used in dyeing, removes residual dye from the fabric.
Role of Textile Auxiliaries:
Textile
auxiliaries are chemical agents used to enhance the performance of fabrics or
to aid in the production processes. They can affect wetting, softness,
dyeing, and finishing.
- Surfactants:
- Lower the surface tension
of water, allowing for better penetration and spreading of liquids on
textiles.
- Used in wetting agents,
detergents, emulsifiers, and dispersing agents.
- Emulsifiers:
- Help in mixing water-insoluble
substances (e.g., oils) with water, creating stable emulsions.
- Example: Used in the preparation of dye
solutions to disperse pigments evenly.
- Wetters:
- Improve wetting and
help water penetrate fibers, ensuring uniform treatment of the fabric
during washing, dyeing, or finishing.
- Softening Agents:
- Added to fabric to make it
softer and more pliable.
- Types: Silicone-based softeners,
fatty acid derivatives, polymeric softeners.
- Also help improve the handle
(feel) of fabrics after finishing.
- Antifoaming Agents:
- Used to reduce foam
formation during processes like dyeing or washing, where foam could
interfere with operations or cause overflow.
- Stabilizers:
- Hydrogen peroxide stabilizers are used to keep bleach
solutions effective.
- Chelating agents like EDTA prevent the
precipitation of metal salts during processing.
- Finishing Agents:
- Chemicals applied to fabric to
give it desirable properties such as:
- Wrinkle
resistance
(e.g., permanent press finishes).
- Water
repellency
(e.g., fluorocarbon-based finishes).
- Flame
retardancy
(e.g., phosphorus compounds).
Preparatory Process Sequence for Woven & Knitted
Fabrics
Woven Fabrics:
The
preparatory processes for woven fabrics aim to remove impurities, prepare the
fabric for dyeing, and improve fabric properties.
- Desizing:
- Purpose: Removal of the sizing material
(often starch or synthetic gums) applied to yarn during weaving to
improve strength and smoothness.
- Methods:
- Enzymatic
desizing
(using amylase to break down starch).
- Acid
or alkaline desizing.
- Scouring:
- Purpose: To remove natural impurities
like wax, pectin, proteins, and dirt from the fabric.
- Common Agents: Sodium hydroxide (for
cotton) or detergents (for wool or synthetics).
- Bleaching:
- Purpose: To whiten the fabric by
removing natural color (e.g., in cotton, which has a natural beige tint).
- Common Agents: Hydrogen peroxide
(for cotton), chlorine (for synthetic fibers).
- Mercerizing:
- Purpose: To improve the strength,
luster, and dye affinity of cotton fabrics by treating them with a
concentrated solution of sodium hydroxide.
- Effect: Increases the fabric’s lustrous
appearance, dye uptake, and strength.
- Dyeing:
- The fabric is dyed using
various techniques (e.g., pad dyeing, exhaust dyeing, piece
dyeing), depending on the fiber type and desired outcome.
- Finishing:
- The final step in which
chemical agents are applied to enhance the fabric’s properties (e.g., softening,
wrinkle resistance, flame retardancy).
Knitted Fabrics:
Knitted
fabrics have different characteristics, such as stretchability and porosity,
which require slightly different treatments.
- Scouring:
- Like woven fabrics, knitted
fabrics are scoured to remove natural oils, dirt, and impurities.
- Desizing:
- If sized during knitting,
desizing will remove any residual starch or other sizing agents.
- Dyeing:
- Knitted fabrics are dyed using
techniques that accommodate their stretchiness and loop structure, such
as padding or exhaust dyeing.
- Finishing:
- Similar to woven fabrics but
may include additional treatments to maintain the fabric's elasticity,
softness, or shape retention.
UNIT II
1. Singeing
Objectives of Singeing:
- Singeing is a process used to remove
protruding fibers from the surface of fabrics, making the fabric smooth
and clean. The primary goals of singeing are:
- Remove fuzz or projecting
fibers:
These fibers can cause pilling and affect the fabric’s appearance and
feel.
- Improve the fabric's finish: It enhances the fabric’s
smoothness and luster.
- Prevent defects in later
processes:
Helps avoid issues in dyeing and printing by providing a uniform surface for
better chemical uptake.
Types of Singeing:
- Flame Singeing:
- Method: The fabric is passed over a
flame or heated plates, which burns off the loose fibers without damaging
the fabric.
- Uses: Common in cotton and synthetic
fibers.
- Advantages: Gives the fabric a smooth
finish with minimal damage.
- Hot Plate Singeing:
- Method: The fabric passes over a
heated surface (often metal plates), which burns off loose fibers.
- Uses: Often used for wool
or silk fabrics.
- Advantages: Provides a more controlled
singeing effect compared to flames.
- Gas Singeing:
- Method: Similar to flame singeing,
but instead of a flame, a gas burner is used to control the burning
process.
- Advantages: More energy-efficient and
controlled.
2. Desizing
Objectives of Desizing:
- Desizing is the process of removing the
sizing agents (such as starch or synthetic gums) applied to the
fabric during the weaving process to improve the strength and smoothness
of yarns. The primary objectives of desizing are:
- Remove sizing agents: These agents are typically
water-insoluble and need to be removed before further processing (dyeing,
printing).
- Prepare the fabric for
subsequent treatments: Ensure the fabric is ready for scouring,
bleaching, and dyeing by making it clean and receptive to these chemicals.
Methods of Desizing:
- Enzymatic Desizing:
- Method: Uses amylase enzymes
to break down starch sizing agents into water-soluble sugars.
- Advantages: It is environmentally
friendly and less harsh on the fabric compared to chemical desizing
methods.
- Chemical Desizing:
- Method: Involves the use of alkaline
solutions, typically sodium hydroxide (NaOH) or potassium
hydroxide (KOH), to dissolve the sizing agents.
- Advantages: More aggressive and can
handle a broader range of sizing materials, though it may be harsher on
the fabric.
- Oxidative Desizing:
- Method: Uses agents like hydrogen
peroxide or sodium hypochlorite to oxidize and degrade sizing
agents.
- Advantages: Effective for certain
synthetic sizing agents.
- Acidic Desizing:
- Method: Involves using weak acidic
solutions to solubilize the sizing agents.
- Advantages: Less aggressive and safer on
delicate fibers like wool and silk.
3. Scouring
Objectives of Scouring:
- Scouring is the process of cleaning the
fabric by removing natural oils, waxes, dirt, and
other impurities from the fiber surface, making it more receptive to
dyeing and finishing. The objectives of scouring are:
- Remove impurities: It eliminates oils, waxes,
proteins, and natural dirt that are present in the fiber (especially in
cotton, wool, and silk).
- Improve dyeing quality: Clean fabric ensures uniform
uptake of dyes and finishes.
- Enhance fabric’s feel: Scouring improves the hand
of the fabric by making it cleaner and smoother.
Methods of Scouring:
- Alkaline Scouring:
- Method: Involves treating the fabric
with an alkaline solution, typically sodium hydroxide (NaOH)
or sodium carbonate (Na₂CO₃).
- Uses: Common in cotton
scouring.
- Advantages: Effective for removing
pectin, waxes, and proteins from plant fibers.
- Enzymatic Scouring:
- Method: Uses enzymes like pectinase
or lipase to break down specific natural impurities such as pectin
or lipids.
- Advantages: More environmentally
friendly and less damaging to the fabric.
- Uses: Often used for delicate
fabrics like wool or silk.
- Solvent Scouring:
- Method: Uses organic solvents (like toluene
or petroleum-based solvents) to remove oils and dirt from
synthetic fibers.
- Advantages: Can be effective for fibers
that are difficult to scour using water-based methods.
4. Bleaching
Objectives of Bleaching:
- Bleaching is the process of whitening
fabrics by removing or lightening natural colorants, impurities, or
residues. The main objectives of bleaching are:
- Whiten the fabric: Remove natural color and
impurities, especially in natural fibers like cotton, linen, and silk.
- Improve dyeability: Provide a clean, uniform
base for subsequent dyeing processes.
- Enhance fabric brightness: Give the fabric a bright and
even appearance.
Types of Bleaching Agents:
- Chlorine-based Bleaching Agents:
- Common agents: Sodium hypochlorite
or chlorine gas.
- Uses: Effective in removing color
from cotton, linen, and certain synthetics.
- Disadvantages: Chlorine can be harsh,
leading to fabric damage if not controlled properly.
- Hydrogen Peroxide (H₂O₂):
- Common agent: Hydrogen peroxide is
used for bleaching, especially in cotton and other cellulosic fibers.
- Advantages: Safer, environmentally
friendly, and less aggressive than chlorine. It also provides a whiter
finish.
- Peracetic Acid:
- Common agent: Peracetic acid is a
powerful oxidizing agent that can be used to bleach fabrics, especially
when an extra level of whitening is needed.
- Uses: Often used for delicate
fibers or fabrics that are sensitive to harsher bleaches.
- Optical Whiteners:
- Purpose: Used to enhance the whiteness
of fabrics after bleaching. These agents absorb ultraviolet (UV) light
and re-emit it as visible blue light, making the fabric appear whiter
than it actually is.
- Common agents: Fluorescent whitening
agents (FWAs), such as stilbene derivatives.
- Uses: Common in cotton and synthetic
fabrics.
- Advantages: Improve fabric brightness
without affecting the chemical structure of the fiber.
Advantages of Peroxide Bleaching:
- Environmentally friendly: It is non-toxic and does not
release harmful chlorine byproducts.
- Fabric stability: Causes less damage to fibers
(compared to chlorine bleaching), making it suitable for delicate fabrics.
- Optical whiting effect: Hydrogen peroxide leaves the
fabric with a clean, bright, and white appearance, especially when
used with optical whiteners.
- Safer for workers: Unlike chlorine bleach,
hydrogen peroxide is safer to handle and less harmful to health.
5. Mercerizing
Objectives of Mercerizing:
- Mercerizing is a process specifically used
for cotton to improve its properties, such as strength, luster, and
dye affinity. The main objectives of mercerizing are:
- Increase the strength of
cotton: It
increases the tensile strength of cotton fibers by causing the
fibers to swell.
- Improve the luster: The fabric becomes more
lustrous and shiny due to the effect of the treatment on the cellulose
structure.
- Increase dye affinity: Mercerized cotton can absorb
more dye, leading to brighter and more vibrant colors.
- Reduce shrinkage: The process helps to
stabilize the fabric and reduce shrinkage.
Types of Mercerizing:
- Cold Mercerizing:
- Method: The cotton fabric is treated
with a concentrated solution of sodium hydroxide (NaOH) while kept
under tension at room temperature.
- Advantages: Provides the fabric with
increased strength and smoothness.
- Hot Mercerizing:
- Method: Similar to cold mercerizing,
but the fabric is exposed to a higher temperature (heated sodium
hydroxide solution).
- Advantages: May provide even greater
luster and dye uptake.
- Continuous Mercerizing:
- Method: Involves the continuous
passage of fabric through the mercerizing solution and subsequent washing
and neutralizing steps in a continuous processing machine.
- Advantages: More efficient for
large-scale production.
- Batch Mercerizing:
- Method: Involves treating fabric in
batches, where the cotton is immersed in sodium hydroxide solution for a
specific time and then neutralized.
- Advantages: More controlled for
small quantities or high-end fabrics.
Summary of Preparatory Processes:
|
Process
|
Objectives
|
Common Methods
|
Examples of Chemicals
|
|
Singeing
|
Remove
fuzz and protruding fibers, smoothen fabric
|
Flame
singeing, hot plate singeing
|
Gas,
flame, or electric heat sources
|
|
Desizing
|
Remove
sizing agents like starch or gums
|
Enzymatic
desizing, chemical desizing
|
Amylase,
NaOH, H₂O₂
|
|
Scouring
|
Remove
natural oils, waxes, proteins, and dirt
|
Alkaline
scouring, enzymatic scouring
|
NaOH,
pectinase, lipase
|
|
Bleaching
|
Whiten the
fabric, remove natural color
|
Hydrogen
peroxide, chlorine, optical whitening
|
H₂O₂,
NaOCl, FWAs
|
|
Mercerizing
|
Increase
strength, luster, dye affinity, reduce shrinkage
|
Cold
mercerizing, hot mercerizing
|
NaOH
|
UNIT III
Dyeing in Textiles
Dyeing is a
process in which a textile material (usually fabric or yarn) is treated with
colorants (dyes or pigments) to give it a desired color. There are different
types of dyes and dyeing methods, each suited to specific fibers and desired
effects.
Classification of Dyes
Dyes are
classified based on various criteria such as their chemical composition, the
type of fibers they are used on, and their application methods. Below is the
classification based on their chemical nature and affinity to fibers:
- Azo Dyes:
- Contain an azo group
(–N=N–) and are widely used in textile dyeing.
- Examples: Direct dyes, Reactive
dyes.
- Anthraquinone Dyes:
- Anthraquinone-based dyes are used for dyeing cotton,
wool, and nylon.
- Examples: Vat dyes, Disperse
dyes.
- Natural Dyes:
- Derived from plants, insects,
or minerals.
- Examples: Indigo, Logwood,
Cochineal.
Principles of Different Types of Dyes
1. Direct Dyes
- Principle: Direct dyes are water-soluble
and can be applied directly to the fiber without the need for a mordant
(binding agent). They have an affinity for cellulose fibers such as cotton.
- Application: Used for cotton, viscose,
linen, and other plant fibers.
- Mechanism: The dye molecules interact
with the hydroxyl groups on the cellulose fibers, forming weak bonds
(hydrogen bonding).
- Advantages: Simple application, no need
for additional chemicals.
- Disadvantages: Limited color range, poor
wash fastness, and light fastness.
2. Reactive Dyes
- Principle: Reactive dyes form covalent
bonds with the fiber, usually through the reaction of the dye molecule
with the fiber’s hydroxyl or amino groups. This results in more permanent
coloring.
- Application: Mainly used on cotton
and other cellulose fibers.
- Mechanism: The dye has a reactive group
(e.g., chloro-s-triazine or vinyl sulfone), which reacts
with the fiber's functional groups.
- Advantages: Excellent color fastness,
vibrant colors, wide color range.
- Disadvantages: More complex application
process, requires alkaline conditions.
3. Vat Dyes
- Principle: Vat dyes are insoluble
in water and are reduced into a soluble form using an alkaline
solution. Once applied to the fabric, they are oxidized back to
their insoluble state, forming strong bonds with the fiber.
- Application: Primarily used for cellulose
fibers like cotton.
- Mechanism: The dye is reduced to a leuco
form (soluble) and absorbed by the fiber. After oxidation, it turns
back into its insoluble form.
- Advantages: Excellent light fastness and
wash fastness.
- Disadvantages: Requires a complex
application process, expensive, time-consuming.
4. Disperse Dyes
- Principle: Disperse dyes are insoluble
in water and are applied to synthetic fibers (such as
polyester, nylon, and acetate) in a dispersed form in water.
- Application: Mainly used for synthetic
fibers like polyester and nylon.
- Mechanism: The dye is dispersed in water
as very fine particles and absorbed by the fiber.
- Advantages: Good fastness properties for
synthetic fibers, wide color range.
- Disadvantages: Poor fastness on natural
fibers, requires high temperatures.
Principles of Dyeing of Synthetic & Blended
Textile Materials
- Synthetic Fibers (e.g.,
Polyester, Nylon):
- Synthetic fibers generally
require disperse dyes because they are non-porous and don’t
readily absorb water-soluble dyes.
- Polyester and Nylon require
higher dyeing temperatures (up to 130°C) and pressure to allow the
dye to penetrate the fiber.
- Blends (e.g., polyester-cotton) are
dyed using a combination of reactive dyes for cotton and disperse
dyes for polyester.
- Blended Materials:
- In blends, the dyeing process
involves using two or more dyes that work on different fibers
within the blend.
- For example, a polyester-cotton
blend can be dyed using disperse dyes for polyester and reactive
dyes for cotton. The challenge is achieving uniform color on
different fibers.
After Treatments
After
dyeing, textiles often undergo post-treatment processes to enhance the appearance,
durability, or performance of the dye:
- Fixation:
- The dyeing process is followed
by a fixation step, particularly with reactive dyes. The
dye must be fixed or chemically bonded to the fiber to ensure it doesn’t
fade or wash out.
- Soaping:
- Used to remove any excess dye
or chemicals, especially for reactive dyes and direct dyes.
This helps improve the color fastness.
- Rinsing:
- Fabric is thoroughly rinsed to
remove any leftover dye and chemicals.
- Finishing:
- Additional finishing
treatments (e.g., softening, stiffening, waterproofing)
can be applied after dyeing to improve the fabric’s properties.
Types and Principles of Different Dyeing Machines
The type of
dyeing machine used depends on the type of material, dyeing method, and desired
effects. Below are some common dyeing machines:
1. Winch Dyeing Machine
- Principle: The fabric is wound around a winch
(a rotating drum) and immersed in a dye bath. The winch is then rotated to
move the fabric through the dyeing liquor, allowing it to be dyed evenly.
- Uses: Primarily used for woven
fabrics (especially cotton).
- Merits: Simple, cost-effective for
small to medium batches.
- Demerits: Limited to certain types of
fabric and dyeing processes, slower than other machines.
2. Soft Flow Dyeing Machine
- Principle: The fabric is passed through
the dyeing liquor under gentle flow conditions, where the fabric is
not subject to harsh mechanical action.
- Uses: Used for delicate fabrics,
such as synthetics or blended fabrics.
- Merits: Gentler on fabrics,
uniform dyeing, suitable for synthetics and delicate fibers.
- Demerits: Higher cost, slower
processing.
3. Cabinet Dyeing Machine
- Principle: This machine consists of vertical
chambers or cabinets where the fabric is placed in bags and
dyed by spraying or immersing the fabric in a dye bath.
- Uses: Mostly used for yarn
dyeing.
- Merits: Low liquor ratio, good
for small batches and high-quality dyeing.
- Demerits: Limited capacity, more
labor-intensive.
4. Cheese Dyeing Machine
- Principle: The yarn is wound onto a cheese
(a cylindrical package) and dyed by immersing the yarn in a dye bath.
- Uses: Yarn dyeing for
various fibers (usually wool, silk, cotton).
- Merits: Suitable for small or medium
batches, good for dyeing yarn in tubes.
- Demerits: Lower productivity compared
to other machines.
5. High Temperature High Pressure (HTHP) Dyeing
Machine
- Principle: The fabric is dyed under high
pressure and high temperature, typically around 130°C
and 5-8 bar pressure. This is particularly useful for polyester
and blended fabrics.
- Uses: Mostly used for synthetic
fibers like polyester, nylon, and blends.
- Merits: High efficiency, good
for large-scale production.
- Demerits: Expensive, energy-intensive,
requires careful control.
Comparison of Dyeing Machines
|
Dyeing Machine
|
Principle
|
Uses
|
Merits
|
Demerits
|
|
Winch
Dyeing Machine
|
Fabric is
wound around a winch and immersed in dye bath.
|
Woven
fabrics, cotton.
|
Simple,
cost-effective.
|
Limited to
certain fabrics, slower process.
|
|
Soft Flow
Dyeing Machine
|
Fabric
flows gently through the dye bath.
|
Delicate
fabrics, synthetics.
|
Gentle,
uniform dyeing, good for delicate fabrics.
|
Higher
cost, slower processing.
|
|
Cabinet
Dyeing Machine
|
Fabric
placed in bags in vertical chambers and dyed.
|
|
|
|
UNIT IV
Printing in Textiles
Textile
printing refers to
the process of applying color (usually in the form of dyes or pigments) to
fabric in a predetermined pattern or design. Unlike dyeing, where the entire
fabric is uniformly colored, printing is localized, affecting only specific
areas of the fabric, allowing for the creation of patterns or designs. Printing
can be done on a variety of fabrics, including cotton, silk, polyester,
nylon, and blends. The process involves the use of various techniques,
each offering different aesthetic and functional qualities.
Methods of Textile Printing
Textile
printing methods can be broadly classified into traditional and modern
methods. Below are the most commonly used printing methods:
1. Screen Printing (Block Printing)
- Principle: Screen printing uses a
mesh screen (or stencil) to transfer ink onto fabric. The screen,
typically made of silk, nylon, or polyester mesh, is
coated with a light-sensitive emulsion. The areas where the ink needs to
pass through are left open, while other areas are blocked off.
- Process:
- Screen Preparation: The design is transferred
onto the screen using photographic exposure or a manual stencil
process.
- Printing: Ink is applied to the screen
and pushed through using a squeegee.
- The fabric is then cured
(heat-set) to fix the ink.
- Uses: Used for both cotton
and synthetic fibers and is commonly used in bulk printing
for items such as T-shirts, bedding, and curtains.
- Merits:
o Suitable for large runs.
o Produces vibrant colors with high
opacity.
Demerits:
o Requires high setup cost.
o Limited in the number of colors
(each color needs a separate screen).
2. Rotary Screen Printing
- Principle: Similar to screen printing, rotary
screen printing uses a cylindrical screen (instead of flat
screens) to transfer ink onto fabric. The screen rotates around a cylinder
while fabric is passed underneath it.
- Process: The ink is fed into the
screen, and as the screen rotates, the ink is applied to the fabric.
- Uses: This is commonly used for
continuous printing, especially for long runs on woven and knitted
fabrics.
- Merits:
- Faster than flat screen
printing, making it cost-effective for large-scale production.
- Allows for multicolor
printing in a single run.
- Demerits:
- Higher initial setup costs.
- Complex for short-run or
small-scale printing.
3. Heat Transfer Printing
- Principle: In heat transfer printing, a
design is printed onto a paper or film using specialized
inks. The design is then transferred onto fabric using heat and
pressure.
- Process:
- The design is printed on paper
using sublimation inks or plastisol inks.
- The paper is placed against
the fabric, and heat is applied.
- The heat causes the ink to bond
with the fabric.
- Uses: Common in T-shirt printing,
sportswear, and fashion garments.
- Merits:
o Can reproduce highly detailed and
complex designs.
o Suitable for both synthetic
and natural fibers.
Demerits:
o Expensive for large-scale
production.
o Limited to synthetic fabrics
when using sublimation inks.
4. Digital Printing
- Principle: Digital printing involves
directly applying digital designs onto fabric using inkjet or laser
printers. The ink is sprayed onto the fabric in droplets to create the
design.
- Process: The design is created
digitally and then printed directly onto the fabric using a
computer-controlled printing machine.
- Uses: Custom prints, short
runs, and highly detailed patterns.
- Merits:
- No need for screens or
stencils.
- Fast turnaround for small
runs or custom designs.
- High-quality prints with fine
details.
- Demerits:
- Costly for large-scale
production.
- Can be slow for large runs,
especially on natural fibers.
Styles of Textile Printing
Various
styles of printing can be employed depending on the desired effect, fabric, and
application:
1. Direct Printing
- Principle: In direct printing,
the dye or pigment is applied directly onto the fabric, creating a sharp,
vibrant pattern.
- Process: The design is printed
directly on the fabric, and the printed fabric is then set using
heat or other techniques.
- Uses: Suitable for cotton, polyester,
and other fibers.
- Merits:
- Quick and easy to apply.
- High color vibrancy.
- Demerits:
- Limited wash fastness (may
fade after several washes).
2. Resist Printing
- Principle: Resist printing
involves applying a resist paste (a substance that prevents dye
absorption) onto the fabric, and then dyeing the fabric. The resist areas
remain undyed, creating the pattern.
- Process: A resist paste (e.g., wax,
starch, gum or synthetic resins) is applied to the
fabric where you do not want the dye to penetrate. The fabric is then dyed,
and the resist is removed, leaving a design.
- Uses: Used for batik printing
(with wax) or shibori dyeing methods.
- Merits:
- Allows for intricate, unique
designs.
- Textured patterns with a handmade
look.
- Demerits:
- Labor-intensive.
- Limited to certain types of
fabric.
3. Discharge Printing
- Principle: Discharge printing
removes color from the fabric using a discharge agent (usually a
bleaching agent or chemical). A design is created by printing the
discharge paste onto the fabric.
- Process: The design is printed with a discharge
paste containing a bleach or reducing agent, which removes the dye
from the fabric in the pattern.
- Uses: Common on dark-colored
fabrics, especially cotton.
- Merits:
- Allows for the creation of light-colored
patterns on dark fabrics.
- Produces a soft, smooth
finish.
- Demerits:
- Limited to cellulosic
fibers (mainly cotton).
- Risk of uneven discharge
if not carefully controlled.
4. Transfer Printing
- Principle: In transfer printing,
the design is first printed onto a special paper and then
transferred onto fabric using heat and pressure.
- Process: The design is printed on
paper, and heat is applied to transfer the ink from the paper to the
fabric.
- Uses: Commonly used for polyester
and other synthetic fibers.
- Merits:
- High-quality prints.
- Suitable for synthetic
fabrics and garment production.
- Demerits:
Print Paste Ingredients
Print paste
is a mixture of various ingredients used in the printing process. Its purpose
is to ensure that the dye or pigment is transferred onto the fabric in the
desired pattern and remains stable throughout the process.
Common Ingredients:
- Water: Acts as the medium for the
dye or pigment.
- Thickeners: Control the viscosity of the
paste and allow for accurate pattern formation. Common thickeners include carboxymethyl
cellulose and sodium alginate.
- Dyes/Pigments: The colorants responsible for
the design.
- Urea: Helps in solubilizing the dye
to ensure better penetration into the fabric.
- Acid or Alkali: Adjusts the pH to ensure
proper reaction between the dye and fabric (especially for reactive
dyes).
- Fixing Agents: Help the dye adhere to the
fabric, improving color fastness.
- Sodium Chloride: Used to increase the
solubility of some dyes, particularly direct dyes.
After Treatments in Printing
After
printing, fabrics often undergo post-treatment processes to ensure that the
prints are durable, bright, and fast. Common after treatments include:
- Curing/Heating: Heat is applied to set the
print (usually at 140-180°C) to ensure that the print is fixed to
the fabric.
- Washing: To remove excess printing
paste, chemicals, and residues.
- Soaping: Used to remove non-reactive
dye or any unwanted residues and improve color fastness.
- Finishing: Treatments like softening,
stiffening, or waterproofing can be applied to enhance the
tactile feel or functionality of the printed fabric.
Pigment Printing
Pigment
printing involves the use of pigments (insoluble colorants) that are
bonded to the fabric using a binding agent. Unlike dyeing, pigments do
not dissolve in water.
- Advantages:
- Wide range of color options.
- Easy handling, with low cost.
- Can be used on all types of
fabrics, including synthetics.
- Disadvantages:
- The color may not penetrate
deeply, so the prints may not be as durable as those made with
dyes.
- Poor wash fastness compared to other printing
methods.
Garment Printing
Garment
printing involves printing directly onto finished garments rather than just
fabric. Different types of garment printing include:
1. Flock Printing
- Principle: Flock printing involves
applying a glue to the fabric in the desired pattern, followed by
the application of tiny fibers (flock), creating a velvet-like texture.
- Uses: Adds a luxurious, textured
effect to garments, commonly used in T-shirts and jackets.
2. Hi-Density Printing
- Principle: High-density printing
involves creating raised, textured prints using specialized inks that expand
when heat is applied.
- Uses: Commonly used for fashion
garments and graphic T-shirts.
3. Foil Printing
- Principle: Metallic foil is
applied to fabric by printing with an adhesive and then bonding the foil
to the adhesive.
- Uses: Provides a shiny, metallic
effect, commonly used for fashion and novelty garments.
4. Plastisol Printing
- Principle: Plastisol ink is a polymer-based
ink that sits on top of the fabric, providing vibrant colors and durability.
- Uses: Commonly used for T-shirts
and sportswear.
5. Foam Printing
- Principle: Involves printing a foam-like
ink that expands to create a puffy, 3D texture.
- Uses: Common for novelty
garments.
6. Khadi Printing
- Principle: Khadi fabric is
printed with natural dyes or pigments, maintaining its
rustic texture and appearance.
- Uses: Popular for ethnic wear
and organic cotton garments.
7. Burnout Printing
- Principle: Involves printing a discharge
paste onto fabric made from a blended fiber (usually cotton and
polyester). The paste removes the synthetic fiber, leaving behind only the
cotton part of the fabric, which creates a sheer, transparent
effect.
- Uses: Typically used for fashion
and evening wear.
UNIT V
Finishing in Textiles
Finishing is
a crucial step in textile processing, and it involves a range of treatments
applied to fabric after weaving or knitting to enhance the fabric’s appearance,
performance, and handle. The main objective of finishing is to improve the
aesthetic qualities of the fabric (such as its texture, color,
and surface), as well as to enhance its functional properties
(such as durability, comfort, water resistance, etc.).
Objectives of Finishing
The main
objectives of finishing are as follows:
- Improved Appearance: To enhance the visual
appeal of fabrics by altering the texture, color, or surface
characteristics.
- Improved Durability: To increase the lifespan
of fabrics and prevent deterioration due to wear or environmental
factors.
- Enhanced Comfort: To improve the handle,
softness, and breathability of fabrics, making them more comfortable to
wear.
- Functional Properties: To impart specific
characteristics like water resistance, stain resistance, wrinkle
resistance, and antimicrobial properties.
- Increased Value: Adding special finishes can increase
the value of the fabric, making it suitable for higher-end products.
Types of Finishing
Finishes can
be broadly classified into two categories: temporary finishes and permanent
finishes.
1. Temporary Finishes
- Definition: These finishes last only for
a short period and typically wear off with time or after laundering.
- Examples:
- Stiffening finishes for fabrics like denim
or organza.
- Water-repellent finishes (temporary) for rainwear.
- Crease-resistant finishes (temporary), where the fabric
resists creasing for a limited number of washes.
2. Permanent Finishes
- Definition: These finishes last for the
lifetime of the fabric and do not wear off after washing or wear.
- Examples:
- Wrinkle-free finishes (chemical finishes that keep
fabrics wrinkle-free permanently).
- Waterproof finishes (applied to fabrics for raincoats
or outdoor wear).
- Flame retardant finishes that prevent fabrics from
catching fire.
Chemical Finishes
Chemical
finishes are applied using chemical agents that change the physical or
chemical properties of the fabric. Common chemical finishes include:
1. Wrinkle-Free Finish
- Objective: To prevent the fabric from
wrinkling and maintain its appearance.
- Chemicals Used: Resins like dimethylol
dihydroxyethylene urea (DMDHEU), urea formaldehyde, or glyoxal
are applied to cellulosic fabrics (especially cotton) to create cross-links
between cellulose fibers, making them less prone to wrinkling.
- Process: After applying the resin, the
fabric is cured at high temperature.
- Merits: Reduces the need for ironing
and maintains a crisp look.
2. Softeners
- Objective: To soften the fabric
and make it more comfortable to wear.
- Types:
- Cationic softeners: Provide a soft, silky
feel and improve the handle of fabrics.
- Non-ionic softeners: Used for natural fibers
(such as cotton).
- Silicone softeners: Used for both synthetic
and natural fibers to provide a smooth and soft finish.
- Process: The softeners are applied
during the washing or final finishing stage, and the fabric
is usually dried and heat-set.
3. Antimicrobial Finish
- Objective: To impart antibacterial
or antifungal properties to the fabric, preventing the growth of
microorganisms like bacteria or fungi.
- Chemicals Used: Silver nanoparticles, quaternary
ammonium compounds, and triclosan.
- Uses: Commonly applied to activewear,
underwear, bed linens, and medical textiles.
- Merits: Prevents odors, improves
hygiene, and extends the lifespan of textiles.
4. Fire Retardant Finish
- Objective: To reduce the flammability
of fabrics, making them more resistant to ignition and fire.
- Chemicals Used: Phosphorus-based, nitrogen-based,
or chlorine-based compounds.
- Uses: Applied to fabrics for
home textiles, workwear, and upholstery.
- Merits: Ensures safety in
certain environments by reducing fire hazards.
Mechanical Finishes
Mechanical
finishes alter the physical structure or texture of fabrics
through physical means like heat, pressure, or abrasion. Some common
mechanical finishes include:
1. Calendaring
- Objective: To give the fabric a smooth
and shiny appearance by passing it through heated rollers under
pressure.
- Process: Fabric is passed through a
pair of rollers that apply pressure and heat, giving the fabric a glossy
finish.
- Uses: Applied to cotton, polyester,
and blended fabrics for sheen and smoothness.
- Merits: Enhances aesthetic
appearance, adds a polished look.
2. Raising (Napping)
- Objective: To create a fuzzy surface
on the fabric by raising the fibers.
- Process: The fabric is brushed with wire
brushes or carding machines to pull up the fibers, creating a
soft, textured surface.
- Uses: Commonly used on wool,
cotton, and fleece fabrics to produce flannel and velvet.
- Merits: Softens the fabric, giving it
a plush texture and warmth.
3. Shearing
- Objective: To trim the surface fibers to
uniform length, creating a smooth and even surface.
- Process: Fabric is passed through cutting
blades or shearing machines that remove excess or uneven fibers.
- Uses: Often applied to pile
fabrics (like velvet or corduroy) to maintain a smooth, consistent
surface.
- Merits: Helps in producing uniform
finishes and improving fabric appearance.
4. Bio-Polishing
- Objective: To enhance the smoothness
and luster of cotton and other cellulose-based fabrics by removing
the excess fibrils (short fibers that protrude from the surface of
the fabric).
- Process: Enzymes, such as cellulases,
are used to selectively break down and remove the short fibers.
- Uses: Applied to cotton, denim,
and knitted fabrics to improve smoothness and finish.
- Merits: Enhances color brightness,
smoothness, and softness of the fabric.
5. Stone Washing
- Objective: To give denim fabrics
a faded, worn-out look and improve the fabric's softness.
- Process: The fabric is washed in a
machine with stones (usually pumice stones) to abrade the surface.
- Uses: Primarily applied to denim
fabric to achieve a distressed look for jeans and jackets.
- Merits: Provides a vintage,
worn-in appearance.
Application of Enzymes in Textile Processing
Enzymes are biological
catalysts that are used in a variety of textile processing operations to
improve fabric properties while being more environmentally friendly than
traditional chemical processes.
Types of Enzyme Applications:
- Bio-polishing: Using cellulase enzymes
to remove excess fibers from cotton, enhancing smoothness and luster.
- Desizing: Amylases are used to
remove starch from fabrics before dyeing or finishing.
- Stone Washing: Pectinase or cellulase
is used as an alternative to physical stone washing to achieve a distressed
effect on denim.
- Denim Softening: Cellulase can soften
denim without the need for harsh chemicals.
- Bleaching: Laccases and peroxidases
are used for environmentally friendly bleaching of textiles.
Pollution in Textile Industry
The textile
industry is known to be one of the largest polluters globally, causing
environmental damage through both chemical and water pollution.
Main Sources of Pollution:
- Water Pollution: Dyeing and finishing
processes generate toxic wastewater, which can contain harmful
chemicals like azo dyes, heavy metals, and surfactants.
- Air Pollution: The emission of volatile
organic compounds (VOCs) from printing, dyeing, and finishing
processes.
- Waste Generation: Large amounts of solid
waste, including textile scraps, scrap fibers, and used
chemicals.
Effluent Treatment Plants (ETP)
Effluent
Treatment Plants (ETP) are systems designed to treat the wastewater generated from
textile processing before it is discharged into the environment. The primary
objective is to reduce the pollutants in wastewater, making it safe for
disposal or reuse.
ETP Process:
- Preliminary Treatment: Physical processes like screening
and settling to remove larger debris.
- Primary Treatment: Involves coagulation
and flocculation to remove suspended solids and some dissolved
materials.
- Secondary Treatment: Biological treatment using activated
sludge or bacteria to decompose organic matter in the water.
- Tertiary Treatment: Chemical and physical
processes such as filtration, activated carbon adsorption,
or reverse osmosis to further clean the water.
- Sludge Treatment: The solid waste (sludge) is
treated and disposed of, often using methods like composting or incineration.
