20°Bé Caustic Soda Mercerisation Fabric Tension and Temperature Window

Continuous cotton mercerisation in 20°Bé sodium hydroxide is not controlled by alkali concentration alone but by a coupled thermomechanical window involving liquor density, fabric dwell time, longitudinal and transverse restraint, and the exothermic peak temperature generated inside the fibre bundle. A 20°Bé NaOH solution has a measured specific gravity of 1.16 at 20°C, corresponding to approximately 15% NaOH by mass, which is high enough to disrupt the native hydrogen-bond network of cellulose I and form alkali celluloses but not so high that fibre dissolution becomes the dominant competing reaction. On production-scale chain mercerisers processing cotton fabric between 100 g/m² and 400 g/m² at machine speeds from 20 m/min to 60 m/min, the fabric enters a two-roll or three-roll saturator in which the caustic liquor is cooled through plate heat exchangers to a setpoint between 15°C and 20°C. The narrow temperature band exists because the caustic sorption exotherm can raise fabric surface temperature by 8°C to 12°C inside the dwell section when heat removal is inadequate, producing a measurable lateral difference in barium activity number under AATCC TM 89. The washing sequence after tensioning must remove alkali in a counterflow configuration while the selvedges remain restrained, otherwise the degree of mercerisation collapses unevenly and dimensional stability measured according to ISO 3759:2011 deteriorates.

What Temperature Window Prevents Non-Uniform Swelling in 20°Bé NaOH?

Uniform swelling during immersion in 20°Bé NaOH is achieved when the saturator trough, the fabric core and the dwell zone remain within 15°C to 20°C. Below 15°C, the dynamic viscosity of the caustic liquor increases substantially, restricting penetration into tightly woven constructions and increasing the risk of surface-only mercerisation; above 25°C, the equilibrium degree of swelling declines because the alkali–cellulose complex is less stable and hydrolytic side reactions begin to consume available alkali. The practical upper control limit on many continuous mercerising lines is 22°C at the second wash-box entry, because fabric above that threshold shows reduced lustre uniformity after drying. To maintain the window, the saturator bath is recirculated through a plate heat exchanger at 20 m³/h to 40 m³/h using chilled water at 8°C to 10°C. A mill should not rely on a single trough sensor; three Class A PT100 probes with an accuracy of ±0.15°C are placed at the delivery-end liquor inlet, the centre trough and the fabric exit side, and the temperature deviation across these probes should not exceed 1.0°C. The fabric itself is monitored with a non-contact infrared pyrometer at the downstream clip entry, where the emissivity is calibrated against a contact thermocouple because water films and caustic residues alter radiative readings. The same temperature restriction applies to the first stretch zone: if the clip chain enters the hot-water washing stage before the alkali has been sufficiently exchanged, the residual swelling potential is lost, and the final barium activity number cannot be restored by later tension adjustment.

Transverse and longitudinal restraint in a chain merceriser is applied by clip chains that grip the selvedges after the saturator and hold the fabric at a controlled width while driven roller speed differentials impose a defined length change. The tension state is normally expressed as percentage width or length change relative to grey fabric dimensions, because direct measurement of local fibre tension during caustic swelling is not possible with standard load cells. Production settings for tension mercerisation of medium-weight cotton poplin and sheeting usually maintain the tenter frame width at 98% to 102% of grey width, while the length is controlled to a residual extension of 2% to 4% through differential speeds between the saturator, the clip chain and the washing compartments. Overstretching beyond 5% residual extension raises the tensile strength at break measured under ISO 13934-1:2013 but depresses elongation at break to values below 8%, which compromises later garment sewing and wear resistance. The exact limit depends on yarn twist multiplier, fabric construction and pre-treatment history. Tension mercerisation at this caustic strength typically increases tensile strength because the paracrystalline rearrangement of cellulose II permits better stress distribution in the fibre longitudinal direction, but the corresponding loss of extensibility is irreversible and must be considered when the same fabric is intended for stretch-sensitive finishing.

Fabric Tension Control Limits During Chain Mercerisation

Tension control in chain mercerisation operates through two interacting loops: a width loop that adjusts clip chain positions based on laser edge sensors, and a length loop that adjusts driven roller differential speed. The width sensor resolution should be ±1 mm at clip entry, and the clip chain is generally equipped with load cells rated between 10 kN and 25 kN per side to record selvedge force trends. For 20°Bé treatment of cotton broadcloth, the target residual length extension is 2% to 4%, while the fabric width is held within ±1% of the specified grey width to minimise bow and skew. The first wash stage after the chain must not relax the fabric until the residual alkali content drops below 0.1% on fabric mass; otherwise the structure collapses before cellulose II is fully fixed. Bow and skew values after mercerisation are measured with a laser grid or pin ruler against a tolerance of ±2% for printed goods, and the test commonly cited is ISO 16322-1:2014 for skew and bow in woven and knitted fabrics after washing. Fabric entering the chain with uneven moisture content from the padder will exhibit variable tension across the width, producing a visible banding defect that cannot be corrected later by heat setting.

Residual alkali removal after 20°Bé treatment is governed not only by wash-water temperature but by the tension applied during the first wash stage. If the fabric is relaxed too early, the swollen cellulose structure collapses before the sodium ions are fully exchanged, producing a loss of lustre that cannot be recovered by subsequent stenter framing. The first wash box after the clip chain is therefore operated at 80°C to 90°C with a counterflow water supply, and the liquor is directed to caustic recovery rather than sent to drain. The selvedge restraint must be maintained until the fabric exits the second post-wash nip, at which point inline pH electrodes and conductivity meters confirm that the extracted liquor no longer exceeds a pH of 8.5. For certification of residual alkali on fabric, extraction with distilled water of grade 3 according to ISO 3696 followed by titration with 0.05 mol/L hydrochloric acid is used; the result is expressed as percentage NaOH on bone-dry fibre mass and should remain below 0.1% for apparel-grade cotton.

When 20°Bé Caustic Soda Is Applied Below 15°C

Operation below 15°C is sometimes requested to increase equilibrium swelling and lustre because the swelling of cellulose in NaOH is thermodynamically favoured by lower temperature. However, the viscosity of 20°Bé NaOH at 10°C is approximately 3.5 mPa·s to 4.5 mPa·s, compared with roughly 2.5 mPa·s at 20°C; this higher viscosity reduces the rate of penetration into dense fabric structures. Reducing the saturator temperature below 15°C without increasing the dwell time therefore leads to a higher proportion of surface mercerisation and lower core swelling, which is detectable as a radial dyeing difference in fibre cross-section and as a lower barium activity number in the fabric interior. Published data for this specific configuration with highly twisted yarns is limited. A low-temperature operation below 15°C is viable only when immersion time is extended by 20% to 30% and the squeeze nip pressure is reduced to avoid excessive liquid film thickness at the fabric surface, otherwise the interior remains partially unmercerised.

Changes in fabric tension at low temperature interact with the elastic modulus of the caustic-swollen fibre, which is lower than that of dry fibre. At 10°C, the swollen fibre is more plastic and can be drawn more easily, so a fixed tenter width setting may produce a higher internal stress after washing than the same setting at 20°C. A mill transferring a recipe from a conventional cold mercerising line to a low-temperature line must not assume that the same clip width and differential speed settings will reproduce the same dimensional stability. The load cells on a clip chain, typically rated 10 kN to 25 kN per side, often show larger fluctuations at 10°C because the fabric has not fully relaxed in the saturator and responds more unevenly to the transverse restraint. For this reason, low-temperature mercerising recipes are normally qualified with on-site trials that measure width and length change after three ISO 6330:2021 washing cycles, and the tension settings are adjusted until the dimensional change falls within ±1% in both directions.

The Saturator Pad Requires Active Cooling Because Caustic Sorption Releases Heat

The pad trough and dwell zone in continuous mercerising are thermally open systems. Heat enters through the incoming greige fabric, the exothermic dilution of recovered caustic and the mechanical energy of the squeeze rolls; heat leaves through the plate heat exchanger, evaporation from the fabric surface and losses through the trough walls. The cooling duty required for a 2000 L trough running at 60 m/min on 300 g/m² cotton is often specified by machine suppliers as 350 kW to 500 kW; without this duty the trough temperature climbs above 24°C within 10 min. Return caustic from the first wash stage is fed to a recovery tank where evaporation concentrates the spent liquor for reuse; this loop must be isolated from the saturator by a temperature-controlled mixing valve. A failure in the mixing valve can introduce hot recovered caustic at 60°C to 70°C into the saturator, causing a sudden rise in fabric temperature and a measurable drop in lustre after neutralisation. Process interlocks therefore shut down the padder motor and activate a cold-water bypass when the saturator temperature exceeds 22°C for more than 30 s.

Fabric construction influences the temperature and tension window in ways that are not fully captured by a single standard. Dense twill or sateen constructions with short yarn floats resist caustic penetration and require a dwell time at the upper end of the 35 s to 60 s range, while open percale constructions can be processed at lower dwell but demand more delicate tension control because their low bending stiffness makes them susceptible to clip marks and torn selvedges. The presence of cotton seed husk fragments, starch sizing residues or metallic contaminants also changes local caustic consumption and heat release. When a production batch contains mixed cotton varieties, the mill should measure the barium activity number at no fewer than five positions across the fabric width using AATCC TM 89, because hot spots or incomplete penetration frequently appear first at the selvedges where heat transfer differs from the centre. Published data for specific high-density constructions in 20°Bé at the lower temperature boundary is limited; therefore, any claim of a universal temperature–tension combination without reference to fabric mass per unit area, yarn twist multiplier and machine speed is not technically reliable.