NaOH Mercerizing in Textile Dyeing — Alkaline Treatment Process for Cotton Fiber

TL;DR: NaOH (sodium hydroxide, CAS 1310-73-2) is the main mercerizing agent in textile dyeing, treating cotton fiber with 20–25% NaOH solution at 15–20°C under tension, producing uniform fiber swelling, increased luster, 20–30% tensile strength improvement, and enhanced reactive dye uptake. Typical consumption is 200–300 g NaOH per kg of fiber. Loc Thien supplies liquid NaOH 20–50% and NaOH flakes 99% to textile dyeing plants, with batch COA/MSDS, delivered to industrial parks nationwide.

What Is NaOH Mercerizing in Textile Dyeing?

Mercerizing is a chemical treatment process for cotton fiber or fabric using concentrated NaOH solution (18–25% w/w) under mechanical tension, aimed at altering the morphological and supramolecular structure of cellulose fibers. The process was discovered by John Mercer in 1844 and perfected by Horace Lowe in 1889–1890 with the addition of the stretching step to create the characteristic luster.

When cotton fiber is immersed in concentrated NaOH at low temperature (15–20°C), OH⁻ ions penetrate the crystalline regions of cellulose, breaking intramolecular hydrogen bonds and causing the fiber to swell. The fiber cross-section changes from kidney-shaped to round, enabling more uniform light reflection — this is the source of the luster. Simultaneously, the cellulose structure transitions from Cellulose I to Cellulose II, increasing free hydroxyl (-OH) groups by approximately 25%, thereby improving dye absorption and moisture regain.

Typical operating parameters:

The BAN index (AATCC TM89) is the standard method for evaluating mercerizing quality: untreated fiber yields BAN 100–105, fully mercerized fiber gives BAN above 130.

Mechanism of NaOH Action on Cotton Fiber

NaOH acts as a swelling agent in mercerizing. The mechanism proceeds through three stages:

Stage 1 — Penetration and Swelling: The 20–25% NaOH solution penetrates the cellulose fiber, breaking hydrogen bonds between cellulose molecular chains. Water carries OH⁻ and Na⁺ ions into the crystalline regions, increasing the spacing between chains. At this concentration, cotton swells maximally, with fiber diameter increasing by 10–15%.

Stage 2 — Restructuring Under Tension: Once fully swollen, mechanical tension holds the fiber in a straight state, preventing shrinkage. The cellulose chains realign parallel to the fiber axis, forming a more stable Cellulose II structure. The fiber cross-section transitions from kidney-shaped to round, creating a smooth surface that reflects light well.

Stage 3 — Neutralization and Fixation: The fiber is rinsed with hot water (60–70°C) to remove residual NaOH, then neutralized with dilute acetic acid or 0.5–1% HCl to pH 6–7. The rinsing and neutralization must be performed quickly to preserve the new fiber morphology. If rinsing is slow or uneven, the fiber may shrink back and lose its luster.

Results after mercerizing:

• Luster: markedly increased, fiber has a silky sheen
• Tensile strength: increased 20–30% compared to untreated fiber
• Dye uptake: increased 25–50%, achieving the same depth with less dye
• Moisture regain: increased due to expanded amorphous regions
• Dimensional stability: improved, reduced shrinkage during washing

Cold Mercerizing vs Hot Mercerizing

Cold Mercerizing

This is the traditional and most common method in the Vietnamese textile dyeing industry.

• NaOH concentration: 20–25% (21–24°Be)
• Temperature: 15–20°C
• Dwell time: 30–60 seconds (up to 180 seconds for thick fabric)
• Wetting agent: Required, alkali-stable type, helps NaOH penetrate quickly due to high solution viscosity at low temperature

Advantages: Highest luster, maximum tensile strength increase, best swelling effect due to rapid swelling at low temperature.

Disadvantages: Requires alkali cooling system, uses more wetting agent, longer dwell time, lower productivity compared to hot mercerizing.

Hot Mercerizing

This method was developed later and is common in European and Chinese knitting mills.

• NaOH concentration: 25–30% (approx. 30°Be)
• Temperature: 55–70°C
• Dwell time: 20–50 seconds (30–50% faster than cold)
• Wetting agent: Not required — low solution viscosity at high temperature enables rapid NaOH penetration

Advantages: Shorter processing time → 30–50% higher productivity, no wetting agent cost, higher uniformity due to rapid deep NaOH penetration, shrinkage half that of cold mercerizing, suitable for knitted fabrics.

Disadvantages: Lower luster than cold mercerizing, higher NaOH consumption (higher concentration), requires solution heating, less tensile strength increase.

Comparison Table

NaOH Consumption in Mercerizing

NaOH consumption in mercerizing depends on equipment type, solution concentration, and fabric-to-liquor ratio. Reference parameters:

• Standard consumption: 200–300 g pure NaOH per kg of fiber
• Using liquid NaOH 32%: approximately 625–940 g solution per kg of fiber
• Using liquid NaOH 45%: approximately 445–670 g solution per kg of fiber
• Fabric-to-liquor ratio: 1:4 to 1:8 depending on equipment

Example consumption calculation: A textile dyeing plant processing 1,000 kg of cotton fiber/day using NaOH 32%:

• Pure NaOH needed: 1,000 × 250 g = 250 kg NaOH
• NaOH 32% needed: 250 ÷ 0.32 = ~780 liters/day
• NaOH cost for mercerizing accounts for approximately 5–10% of total textile chemical costs

Standard Mercerizing Process for Cotton Fiber

Step 1: Prepare NaOH Solution

Prepare NaOH solution to 20–25% concentration (measured by hydrometer, equivalent to 21–24°Be or 55–65°Tw). The solution must be cooled to 15–20°C. With NaOH flakes 99%, dilute with soft water at approximately 1:3 (NaOH flakes:water). Note the strong exothermic reaction — add NaOH slowly to water with continuous stirring, never the reverse.

Step 2: Immerse Fiber in NaOH Solution

Fiber or fabric is passed through a mercerizing bath containing 20–25% NaOH solution at 15–20°C. Immersion time is 30–60 seconds. In the bath, the fiber is held under full tension (both lengthwise and widthwise). Mercerizing machines typically have three mangle rolls to control impregnation and remove excess solution.

Step 3: Stabilize Under Tension

After immersion, the fiber is held under tension for 30–90 seconds in the reaction zone. This stage determines the degree of cellulose structure conversion. Woven fabric requires weft tension to maintain fabric width; knitted fabric typically uses hot mercerizing to reduce tension requirements.

Step 4: Hot Rinse and Neutralization

The fiber is rinsed with hot water at 60–70°C to remove surface NaOH, then neutralized with 0.5–1% acetic acid or dilute HCl to pH 6–7. Check pH using indicator paper or a probe. A cold rinse follows to bring the fiber back to room temperature.

Step 5: Quality Inspection

Measure BAN according to AATCC TM89 — values ≥115 confirm satisfactory mercerizing. Check luster visually or with a luster meter. Evaluate dye uptake by trial dyeing with reactive dyes.

Effect of Mercerizing on Dyeing Quality

Mercerizing comprehensively improves the dyeability of cotton fiber:

• Increased color strength: Mercerized fiber gives K/S (color strength) values 25–50% higher than untreated fiber under the same dyeing conditions
• Reduced dye consumption: Can reduce reactive dye usage by 20–30% to achieve the same color depth
• Improved color uniformity: Uniform fiber swelling enables even dye penetration, reducing blotchiness or uneven color uptake
• Better colorfastness: Dyes bond with more -OH groups, forming more stable covalent bonds with cellulose fiber, improving rub and wash fastness

A 2024 study in Textile & Leather Review showed that hot-mercerized fabric (65°C) with 23% NaOH achieved a degree of fixation of 84.91% when wet-on-wet dyed with heterobifunctional reactive dyes — higher than traditional non-mercerized dyeing processes.

Downstream Connection: Loc Thien Chemical in the Mercerizing Process

NaOH is the primary chemical in the mercerizing process and a product Loc Thien supplies to textile dyeing plants in industrial parks. Besides NaOH, the textile dyeing process uses other chemicals that Loc Thien carries in its catalog:

Loc Thien supplies NaOH at various concentrations: liquid caustic 20%, 32%, 45%, 48%, 50% and caustic flakes/prills 99% — all with CAS 1310-73-2. Each batch includes COA (Certificate of Analysis) and MSDS upon request.

🛒 Need NaOH for mercerizing? Get a quote for liquid NaOH 20–50% — batch COA/MSDS, delivered to industrial parks nationwide. Hotline 0979 891 929.

ZDHC MRSL and Compliance Requirements for NaOH in Textile Dyeing

ZDHC (Zero Discharge of Hazardous Chemicals) MRSL is the list of substances prohibited from intentional use in textile, leather, and footwear manufacturing. For NaOH (CAS 1310-73-2), two points are worth noting:

• Gateway Level 1/2/3 applies to chemical products certified by a ZDHC Approved MRSL Certifier, not to factories.
• For commercial NaOH, brands typically review the SDS and the batch COA to control impurities.

Pure NaOH is not a prohibited substance under ZDHC MRSL, but impurities in commercial NaOH (mercury, arsenic, lead) must be controlled below permitted thresholds. Textile dyeing plants should require their NaOH suppliers to provide complete SDS and COA for each batch to meet ZDHC audit requirements.

Loc Thien provides batch-specific MSDS and COA for NaOH, and can supply documents on request for the audits of export-oriented textile dyeing customers. If a brand requires ZDHC records, contact us to confirm the documents currently available for each product. The QC team with inspectors Phan Cam Thuy and Vo Thi Nhu Hoa is responsible for quality verification of each batch before delivery.

Safety When Using NaOH in Mercerizing

NaOH is a strongly corrosive chemical, classified as hazardous under GHS. Mandatory safety measures when operating mercerizing baths:

• PPE: Chemical splash goggles, nitrile or neoprene gloves, rubber boots, PVC apron, splash mask when preparing solutions
• Dilution: Slowly add NaOH to cold water with continuous stirring — never the reverse (water into NaOH causes violent boiling and splashing)
• Temperature control: Strong exothermic reaction during preparation — cool the mixing tank when preparing from NaOH flakes 99%
• Spill response: Neutralize with dilute acetic acid or 1% HCl, then absorb with inert materials (sand, clay), collect as hazardous waste per regulations
• Storage: HDPE plastic tanks or 316L stainless steel, dry cool area, away from acids and strong oxidizers

Frequently Asked Questions

What NaOH concentration is optimal for cotton mercerizing?

Optimal concentration is 20–25% w/w (21–24°Be). Below 18% is insufficient for complete swelling, BAN below 115. Above 30% may damage fiber due to cellulose hydrolysis.

Which is better — hot or cold mercerizing?

Cold mercerizing (15–20°C) gives higher luster and greater tensile strength increase. Hot mercerizing (55–70°C) offers higher productivity, better uniformity, and is suitable for knitted fabrics. Choice depends on product type and plant equipment.

How to verify mercerizing quality?

Measure BAN (Barium Activity Number) per AATCC TM89. BAN ≥ 115 confirms effective mercerizing. Also check luster and moisture regain.

How much NaOH is consumed in mercerizing?

Approximately 200–300 g pure NaOH per kg of fiber. With liquid NaOH 32%, equivalent to 0.6–1 liter per kg of fiber. Actual consumption depends on equipment and fabric-to-liquor ratio.

Does NaOH used in mercerizing need to meet ZDHC MRSL?

Pure NaOH is not banned under ZDHC MRSL. However, impurities in commercial NaOH must be below thresholds. Require suppliers to provide SDS and COA for each batch.

What NaOH concentrations does Loc Thien supply for mercerizing?

Loc Thien supplies liquid NaOH 32%, 45%, 50% — plants can dilute to 20–25% as needed. Or use NaOH flakes 99% for direct preparation. See liquid NaOH and NaOH flake price list.

Where is NaOH delivered and what is the lead time?

Loc Thien delivers from 6 warehouses nationwide (Ba Ria – Vung Tau, Dong Nai, Binh Duong, Da Nang, Can Tho and Bac Ninh) to all industrial parks. Estimated delivery time: within 4 hours in inner-city Ho Chi Minh City, 24–48 hours to industrial parks in the South and Mekong Delta, 2–3 days to Northern and Central Vietnam.

Request a Quote for NaOH for Mercerizing

To receive a NaOH quote suitable for your plant’s mercerizing process, please provide:

• Desired NaOH concentration (32%, 45%, 50% or flakes 99%)
• Monthly quantity in kg/tons
• Delivery location (industrial park, province/city)
• COA/MSDS requirements

Hotline: 0979 891 929 QC contacts: Phan Cam Thuy — Vo Thi Nhu Hoa Email: [email protected] Warehouses: Ba Ria – Vung Tau, Dong Nai, Binh Duong, Da Nang, Can Tho and Bac Ninh