Course: B.Tech (Textile Engineering)Subject Code: EC403Subject Title: Technology of Dyeing Exam Type: TheoryDuration: 3 HoursMaximum 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:
(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
