About this previous year paper
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
| No. | Question | Concept Explanation |
|---|---|---|
| (a) | Difference between Dyes and Pigments | Dyes are soluble and chemically bond with fibers; pigments are insoluble and require a binder or resin to adhere. |
| (b) | Chromophores & Auxochromes | Chromophore: Color-producing group (–N=N–, –NO₂, –C=O). Auxochrome: Intensifies color and helps dye attach to fiber (–OH, –NH₂). |
| (c) | Vinyl Sulphone Dyes | Reactive dyes containing –CH=CH₂–SO₂– group that reacts with cellulose under alkaline conditions to form covalent bonds. |
| (d) | Chrome Mordant Dyes | Dyes that require chromium salt (Cr³⁺) as a mordant to fix on wool/silk fibers, forming coordination complexes. |
| (e) | Reduction Clearing Process | Removes unfixed disperse dyes from polyester using alkaline hydrosulphite, ensuring brightness and fastness. |
| (f) | 1:1 vs 1:2 Metal Complex Dyes | 1:1: One metal atom combines with one dye molecule; 1:2: One metal atom complexes with two dye molecules — giving better fastness. |
| (g) | Dyes for Acrylic Fabric | Basic (cationic) dyes — acrylic has anionic sites, forming ionic bonds with basic dyes. |
| (h) | Moiré Effect | Wavy appearance or watermark effect on fabric caused by interference of two fabric layers with differing weave. |
| (i) | pH for Reactive Dyes | Optimal pH ≈ 10–11 (alkaline medium) using sodium carbonate or bicarbonate for covalent bonding with cellulose. |
| (j) | Sequence of Vat Dyeing | (1) Vatting → (2) Impregnation → (3) Oxidation → (4) Soaping → (5) Rinsing → (6) Drying. |
SECTION – B (5 × 10 = 50 Marks)
Descriptive and application-based questions covering processes, mechanisms, and classifications
(a) Classification of Dyes (Based on Application)
| Class | Fiber | Example | Dyeing Medium |
|---|---|---|---|
| Direct | Cotton | Congo Red | Aqueous salt bath |
| Acid | Wool, Silk | Acid Blue 9 | Acidic solution |
| Basic | Acrylic | Rhodamine B | Neutral/acid medium |
| Vat | Cotton | Indigo | Alkaline reducing medium |
| Reactive | Cotton | Procion Red | Alkaline fixation |
| Disperse | Polyester | Disperse Orange 3 | Non-aqueous or carrier method |
| Sulphur | Cotton | Sulphur Black | Alkaline reducing bath |
(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
| Fibre Type | Interaction Type | Example |
|---|---|---|
| Cotton | Covalent (Reactive dyes) | Procion, Remazol |
| Wool/Silk | Ionic (Acid dyes) | Acid Blue 113 |
| Polyester | Dispersion (Disperse dyes) | Disperse Red 60 |
| Acrylic | Ionic (Basic dyes) | Basic Violet 10 |
(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
| Process | Fibre | Dye Class | Key Conditions |
|---|---|---|---|
| Direct | Cotton | Direct | NaCl, neutral pH |
| Reactive | Cotton | Reactive | Na₂CO₃, 60°C |
| Acid | Wool/Silk | Acid | pH 4–5 |
| Basic | Acrylic | Basic | pH 4–5 |
| Disperse | Polyester | Disperse | 130°C (HTHP) |
| Vat | Cotton | Vat | Alkaline reducing |
| Sulphur | Cotton | Sulphur | Na₂S bath |