Murugs Textile Notes: TEXTILE WET PROCESSING

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:

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:

Limited to synthetics.

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.

Murugs Textile Notes

Thursday, 22 May 2025

TEXTILE WET PROCESSING

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