THEORY EXAMINATION (SEM–IV) 2016-17 TECHNOLOGY OF DYEING

Course: B.Tech (Textile Engineering)
Subject Code: EC403
Subject Title: Technology of Dyeing
Exam Type: Theory
Duration: 3 Hours
Maximum Marks: 100

SECTION – A (10 × 2 = 20 Marks)

Short, concept-based questions checking dyeing fundamentals

SECTION – B (5 × 10 = 50 Marks)

Descriptive and application-based questions covering processes, mechanisms, and classifications

(a) Classification of Dyes (Based on Application)

(b) Dyeing Methods

Batch-wise: Fabric dyed in fixed volume of dye liquor (jigger, winch).

Semi-continuous: Pad–batch or pad–roll process.

Continuous: Pad–steam or thermosol process.
Comparison:
Batch = Flexible but slow; Continuous = Fast, uniform, industrial scale.

(c) Cotton Dyeing with Direct Dye

Mechanism:

Adsorption → 2. Diffusion → 3. Fixation by H-bond & van der Waals forces.

Why Poor Fastness: Dyes held by weak physical forces, easily removed during washing.

Improvement: After-treatment with cationic fixing agents or metal salts.

(d) Wool Dyeing with Acid Dyes

Principle: Ionic attraction between –NH₃⁺ groups of wool and –SO₃⁻ groups of dye.

Conditions: pH 4–5, temperature 80–100°C.

Process: Dye bath with acid (acetic/formic) → heating → leveling → rinsing.

Result: Bright, strong shades with good leveling but moderate fastness.

(e) Dye–Fibre Interaction

(f) Carriers in Polyester Dyeing

Function: Increase fiber swelling and reduce Tg (glass transition temperature) for easier dye diffusion.

Examples: Phenol derivatives, o-phenyl phenol, benzyl benzoate.

Alternative: High-temperature dyeing (HTHP) or supercritical CO₂ dyeing.

(g) Cotton Dyeing with Sulphur Dyes

Mechanism: Insoluble sulphur dye → reduced with Na₂S → leuco form → absorbed → oxidized to insoluble form in fiber.

Bronziness: Metallic luster due to incomplete oxidation.

Tendering: Fiber degradation by residual alkali or sulphide.

(h) Acrylic Dyeing with Basic Dyes

Acrylic (polyacrylonitrile) has negatively charged groups (–COO⁻).

Cationic dyes (basic) form strong ionic bonds → bright shades, excellent fastness.

Bath pH 4–5; temperature 90–100°C.

SECTION – C (2 × 15 = 30 Marks)

Detailed reaction-based and analytical long-answer questions

Q3. Vat Dyeing

Vatting: Reduction of insoluble vat dye into soluble leuco form using NaOH (caustic soda) + Na₂S₂O₄ (hydros).

Reaction:

• Vat dye (oxidized)+2e−+2H+→Leuco dye (soluble)\text{Vat dye (oxidized)} + 2e^- + 2H^+ → \text{Leuco dye (soluble)}Vat dye (oxidized)+2e−+2H+→Leuco dye (soluble)

Classification (Based on Application):

IN (Indanthrene Normal): High temp. & long time.

IW (Indanthrene Warm): Medium temp.

IC (Indanthrene Cold): Room temp. process.

After Dyeing: Oxidation with air or hydrogen peroxide restores original insoluble color inside fiber.

Q4. Azoic (Ice) Dyeing

Why “Ice Colors”: Reaction carried out at low temperature (~0–5°C) to prevent premature coupling.

Steps:

Naphthol impregnation (coupling component).

Diazotization of base (e.g., aniline → diazonium salt).

Coupling reaction to form insoluble azo dye inside fiber.

Reaction:

• Ar–N₂⁺Cl⁻ + Ar’–OH → Ar–N=N–Ar’ + HCl\text{Ar–N₂⁺Cl⁻ + Ar'–OH → Ar–N=N–Ar' + HCl}Ar–N₂⁺Cl⁻ + Ar’–OH → Ar–N=N–Ar’ + HCl

Q5. Reactive Dyeing

Types:

Monochlorotriazine (MCT)

Dichlorotriazine (DCT)

Vinyl Sulphone

Mixed Bi-functional

Reaction with Cellulose:

• Cell–OH + Dye–Cl → Cell–O–Dye + HCl\text{Cell–OH + Dye–Cl → Cell–O–Dye + HCl}Cell–OH + Dye–Cl → Cell–O–Dye + HCl

Role of Salt: Promotes dye exhaustion (reduces repulsion).

Role of Alkali: Activates cellulose for covalent bond formation.

Summary Table