About this previous year paper
Course: B.Tech (Textile Engineering)
Subject Code: EC402
Subject Title: Technology of Bleaching and Mercerization
Exam Type: Theory
Duration: 3 Hours
Maximum Marks: 100
SECTION – A (10 × 2 = 20 Marks)
Short, concept-based questions covering fundamental definitions and process principles
| No. | Question | Concept Summary |
|---|---|---|
| (a) | Natural and Added Impurities in Cotton Fabric | Natural: waxes, pectins, proteins, minerals. Added: sizing agents (starch, PVA), dirt, oil, and lubricants. |
| (b) | Object of Singeing | Removes protruding fibers and fuzz from fabric surface to give a smooth, clean finish and prevent pilling. |
| (c) | Purpose of Bleaching | Destroys natural coloring matter to make fabric white, absorbent, and suitable for dyeing or printing. |
| (d) | Soft Water | Free from calcium and magnesium salts; prevents soap scum and uneven processing in wet treatments. |
| (e) | Enzyme Used in Desizing | Amylase — breaks down starch used in warp sizing of cotton. |
| (f) | Effect of Alkali on Wool | Alkali damages keratin protein → causes felting, loss of strength, and yellowing. |
| (g) | Application of Carbonizing Process in Wool | Removes vegetable impurities (seeds, burrs) using dilute sulfuric acid. |
| (h) | Scouring vs Souring | Scouring: removes fats, waxes, pectins (alkaline). Souring: removes residual alkali using mild acid wash. |
| (i) | Slack Mercerizing | Mercerization under low tension to enhance absorbency rather than luster. |
| (j) | Examples of Natural & Synthetic Fibers | Natural: Cotton, Wool, Silk. Synthetic: Nylon, Polyester, Acrylic. |
SECTION – B (5 × 10 = 50 Marks)
Detailed descriptive and process-based questions
(a) Desizing Process of Cotton
Purpose: Remove size materials (starch, gums, resins) applied to warp yarns.
Methods:
Enzymatic: Uses amylase to hydrolyze starch (eco-friendly).
Acid Desizing: Uses dilute HCl or H₂SO₄ to dissolve starch (less preferred).
Oxidative Desizing: Uses sodium bromite or hypochlorite for synthetic sizes.
Test for Completion: Iodine test – absence of blue color indicates full desizing.
(b) Scouring of Cotton Fabric
Objective: Remove waxes, fats, pectins, and dirt to increase absorbency.
Carried Out Using:
NaOH, soda ash, wetting agent, sequestering agent.
Temperature: 100–130°C for 2–3 hours in kier or j-box.
Check: Drop test (water drop spreads quickly = well scoured).
(c) Bleaching of Cotton Using Hydrogen Peroxide
Chemical Used: H₂O₂ + NaOH + stabilizer (sodium silicate).
Mechanism:
- H2O2→H2O+[O]H_2O_2 → H_2O + [O]H2O2→H2O+[O]
The nascent oxygen oxidizes coloring matter.
Advantages: Eco-friendly, no chlorine damage, good whiteness.
(d) Scouring in Kier (with Diagram)
Kier: Closed, pressurized vessel (vertical or horizontal).
Steps:
Load fabric in rope form.
Circulate hot alkali solution.
Maintain 120–130°C for 6–8 hours.
Wash and neutralize.
Outcome: Uniform scouring with minimal fiber damage.
(e) Bleaching in J-Box (with Diagram)
Used For: Continuous bleaching of cotton.
Process:
Impregnate fabric with H₂O₂ bleaching liquor.
Pass through J-box for dwell time (1–2 hours).
Rinse and neutralize.
Advantages: Continuous, energy efficient, uniform bleaching.
(f) Physical & Chemical Changes in Cotton During Mercerization
| Property | Before | After Mercerization |
|---|---|---|
| Strength | Normal | Increases by ~25% |
| Luster | Dull | Silky sheen |
| Absorbency | Low | High (due to fiber swelling) |
| Dye Affinity | Moderate | High |
| Cross-section | Bean-shaped | Circular |
Reason: Swelling of cellulose fibers in strong NaOH (18–25%) solution under tension.
(g) Impurities in Silk & Bio-Polishing
Impurities: Sericin (gum), wax, mineral salts.
Degumming: Treating silk with soap solution or enzyme to remove sericin.
Bio-Polishing: Enzymatic treatment using cellulase/protease to improve softness and smoothness.
SECTION – C (2 × 15 = 30 Marks)
Analytical and conceptual long-answer questions
Q3. Cotton Degradation Tests
Copper Number Test: Measures reducing substances formed after scouring.
Methylene Blue Absorption: Determines available reactive groups.
Cuprammonium Fluidity: Measures fiber degradation by viscosity comparison.
Q4. Wool Processing and Decatizing
Impurities in Wool: Grease, suint (perspiration salts), burrs, dust.
Processing:
Scouring (detergent + alkali)
Carbonizing (acid treatment for vegetable matter)
Neutralizing & Drying
Decatizing Process:
Post-dyeing heat–steam treatment.
Improves dimensional stability, hand feel, and appearance.
Q5. Singeing Technologies
| Type | Description | Limitation |
|---|---|---|
| Plate Singeing | Fabric passes over heated copper plate. | Uneven for heavy fabrics. |
| Roller Singeing | Fabric moves across gas-heated roller. | Fire risk, uneven burning. |
| Gas Singeing | Exposes fabric to controlled gas flame. | Needs precision in flame control. |
| Infrared Singeing | Uses infrared radiation. | Costly, maintenance-heavy. |
Summary
| Unit | Key Topics |
|---|---|
| I | Fiber impurities, desizing, scouring |
| II | Bleaching (chemical and continuous) |
| III | Mercerization and fiber transformation |
| IV | Wool, silk processing, and carbonizing |
| V | Singeing and finishing technologies |