THEORY EXAMINATION (SEM–IV) 2016-17 TECHNOLOGY OF BLEACHING AND MERCERIZATION

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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