Fibre Swelling Control by Caustic Concentration Thresholds in Cotton Mercerization

In continuous cotton mercerization ranges, the swelling response of the fibre is not a linear function of caustic soda concentration but a threshold-driven transformation governed by the formation of alkali cellulose complexes and the subsequent disruption of the native cellulose I crystalline lattice. Production experience on Benninger Dimensa and Goller chainless mercerizing machines has demonstrated that when caustic concentrations fall below 18 % (w/w) NaOH at ambient liquor temperatures of 18–22 °C, only interfibrillar swelling occurs, leaving the crystallinity index measured by X‑ray diffraction essentially unchanged from untreated cotton at 0.68–0.72 as per the Segal method (peak height at 2θ ≈ 22.5°). This partial wetting induces an irregular increase in fibre diameter of 8–12 %, but the lack of intracrystalline penetration prevents the characteristic conversion of cellulose I to cellulose II, and the resulting fabric fails to meet the barium activity number threshold of 150 specified in AATCC Test Method 89‑2014 for full mercerization. At the opposite extreme, caustic concentrations exceeding approximately 30 % NaOH (w/w) can generate excessive osmotic swelling pressures that exceed the tensile integrity of the primary wall, particularly in immature fibres with low micronaire values (3.2 µg/inch), causing localized fibre rupture and a decline in lea strength of up to 12 % when tested according to ASTM D2256‑21. In the intermediate window, the mercerization effect becomes detectable at 15–16 % NaOH, where the formation of Na‑cellulose I begins, but the critical threshold for uniform intracrystalline swelling and subsequent regeneration into cellulose II lies between 21 % and 24 % NaOH at 20 °C, a range validated by wide‑angle X‑ray diffraction patterns that show the progressive disappearance of the (1 1 0) and (1 1 0̅) reflections of cellulose I and emergence of the characteristic (1 1 0) and (0 2 0) peaks of cellulose II. In practice, maintaining a caustic bath concentration within ±0.8 °Bé (equivalent to ±0.6 % NaOH w/w at 28 °Bé) across the dwell zone is mandatory to prevent stripe formation, and this requires in‑line refractometric control with automatic PID‑regulated dosing of 50 % caustic stock, a configuration standard on modern Kuesters calender‑type mercerizing units equipped with vacuum impregnation slots achieving a wet pickup of 95–105 % on dry mass.

If tension is applied during swelling, concentration fluctuations become catastrophic

In tension mercerization carried out on a clip‑type or chainless machine where the fabric is stretched in the warp direction between the swelling zone and the stabilizing wash box, the interplay between caustic concentration and applied tensile stress creates a processing window narrower than ±1 % NaOH (w/w). Experimental data obtained on a Brückner chain mercerizer processing 100 % ring‑spun cotton poplin of 120 g/m² revealed that at a stretch ratio of 1.03–1.05 (imparted through a differential speed between the lower and upper clips), a drop in caustic concentration from 25 % to 23 % NaOH while maintaining a fixed tension of 40 N/cm warp‑wise led to a reduction in the degree of crystallinity from 46 % to 38 % (by X‑ray integration) and a corresponding loss of luster measured as a decrease in specular gloss from 48 GU to 34 GU at 60° incident angle under ISO 2813:2014. The mechanism responsible is the concentration‑dependent plasticity of the swollen fibre: at 25 % NaOH the cellulose chains are sufficiently mobilized to allow plastic deformation and alignment under tension, locking in the parallel chain configuration that gives high lustre and strength, while at 23 % the plastification is incomplete, and the applied stress instead induces micro‑compressive buckling of fibrils as the swelling is constrained, resulting in a permanently crinkled morphology. A further operational hazard arises from the temperature rise caused by exothermic dilution when make‑up caustic is added improperly; a temperature excursion of just 5 °C above the set point of 18 °C shifts the effective critical concentration for full plastification upward by approximately 0.8 % NaOH, making the bath simultaneously hotter and effectively leaner—a dual deviation that has caused entire production batches to be downgraded for uneven dye uptake when tested with Chlorantine Fast Blue 2GL dyeing according to the Sandoz differential dyeing test referenced in AATCC 89. For this reason, tension mercerizing systems incorporate plate heat exchangers with a cooling capacity of 0.4 kW per kg/h of fabric throughput and use absolute filtration to 5 µm to prevent undissolved carbonate flocs from nucleating non‑uniform swelling.

Continuous monitoring of the swelling front is achieved by measuring the width of the fabric web exiting the caustic saturator relative to the grey width. On a Benninger Dimensa range running at 80 m/min, the width after the caustic padder, prior to the tension zone, is typically 82–85 % of the entry width for a 28 °Bé caustic liquor, a contraction driven by the initial rapid osmotic swelling. If the observed width contraction deviates by more than ±1.5 percentage points from the target, this immediately signals a caustic strength drift, and the in‑line process refractometer (e.g., a K‑Patents PR‑23‑W with a measurement accuracy of ±0.0002 R.I.) triggers a cascade adjustment of the metering pump stroke to restore the set point within 45 seconds, a control lag that must be shorter than the bath turnover time of 90 seconds to avoid producing off‑spec fabric in the transition zone. In addition to refractometry, the bath’s carbonate content—formed by NaOH reaction with atmospheric CO₂—is maintained below 1.5 g/L Na₂CO₃ because carbonate ions reduce the effective activity of hydroxide in swelling by buffering the system and can precipitate as crystalline deposits on fibre surfaces, interfering with the subsequent acid neutralization in the wash boxes. Routine titration of bath alkalinity (total vs. hydroxide alkalinity by double indicator method per ASTM E291‑18) on grab samples taken every 30 minutes provides the audit trail necessary for ISO 9001:2015 batch release documentation. Published data for the precise correlation between continuous refractometer readings and wet chemical analysis in the presence of wetting agents is limited, but plant records indicate that a properly conditioned sensor achieves agreement within ±0.15 % NaOH across 24‑hour runs without recalibration when the wetting agent concentration is held constant at 5–7 g/L of a sulfated ethoxylate with a cloud point above 80 °C.

What concentration window prevents transition to cellulose II without fibre degradation?

From a structural standpoint, the lower boundary for complete conversion of cellulose I to cellulose II in cotton at 20 °C has been established by X‑ray diffractometry at 11–12 % NaOH (w/w) for intracrystalline swelling and Na‑cellulose I formation, yet the fibre remains stiff and the mercerizing effect is not realized because the amorphous regions are insufficiently plasticized to permit lattice transformation upon subsequent washing and neutralization. Literature data compiled by Warwicker and later confirmed by high‑resolution ¹³C CP/MAS NMR show that a fully mercerized cellulose II crystallite structure with a degree of conversion above 90 % requires a minimum NaOH concentration of 18–19 % at 20 °C. The upper boundary is dictated by a different transition: at concentrations exceeding 35 % NaOH, the Na‑cellulose III structure begins to form, which upon washing yields a mixture of cellulose II and amorphous cellulose, resulting in a progressive loss of tensile strength that cannot be recovered by tension. This is evidenced by viscosity measurements of the fibre dissolved in cuprammonium hydroxide (ISO 5351:2010) showing a drop in limiting viscosity number from 1 100 mL/g for 25 % NaOH mercerized cotton to 720 mL/g for 38 % NaOH treated samples, indicating chain scission through alkaline hydrolysis. The safe operating bandwidth for commercial mercerization thus lies between 20 % and 30 % NaOH (w/w), with an optimal target of 24–26 % for slack mercerization and 23–24 % for tension mercerization, the latter requiring a slightly leaner bath to avoid over‑plasticization that would lead to excessive width‑wise shrinkage during the critical first seconds of stretching. This optimal range is codified in the European standard EN 20105‑A03:1994 (equivalent to ISO 105‑A03:1993) for assessing the colour fastness of mercerized dyeings, which implicitly assumes that the cotton substrate has been treated within the range where dye sorption is maximized without morphological damage.

The inclusion of wetting agents and penetrants shifts the effective swelling thresholds downward by approximately 1–2 % NaOH, because the reduced surface tension (typically 28–32 mN/m at 0.5 % active agent) accelerates the diffusion of alkali into the fibre lumen and allows the reaction front to reach the interior of the secondary wall before the outer layers gelatinize and seal the pathway. In plant trials on a Goller mercerizing range processing 100 % compact cotton woven fabric at 60 m/min, replacing a conventional cresol‑based penetrant (cloud point 45 °C) with an alcohol ethoxylate having a cloud point of 68 °C enabled the caustic concentration to be reduced from 30 °Bé to 28 °Bé while maintaining identical barium activity numbers and a 10‑minute kettle‑dyeing shade consistency with C.I. Reactive Blue 19 under the dyeing profile of EN ISO 105‑C06:2010. The choice of wetting agent is constrained by the requirement that it must remain soluble and surface‑active in 25 % NaOH without salting out; sulfated short‑chain alcohols meet this criterion, but their tendency to coalesce into gel phases at temperatures below 15 °C imposes a minimum bath temperature of 18 °C, effectively narrowing the permissible temperature window of the mercerizing liquor to 18–25 °C. In practice, achieving a uniform temperature across the entire width of a 2.4‑metre padder trough requires circulation of the caustic through external heat exchangers at a rate of 5–7 turnovers per hour, and a distributive spray bar above the fabric path to ensure temperature deviation no greater than ±0.5 °C from left to centre to right, a condition verified by an array of three immersion‑type Pt100 sensors logged at 1 Hz.

NaOH concentration profiling across the mercerization stapling zone

The swelling is not a homogeneous event across the staple length of the cotton fibre; the tip, base, and convoluted regions exhibit different sensitivity to caustic concentration owing to variations in cuticle thickness and microfibril packing. Optical microscopy of cross‑sections taken at the 3‑, 15‑, and 25‑mm positions from the fibre base after treatment with 23 % NaOH slack mercerization revealed that the tip region, where the cuticle is thinnest (0.12–0.18 µm), swelled to a lumen‑to‑wall area ratio of 0.32, while the middle region swelled to only 0.25, implying a gradient in the degree of mercerization that can be averaged out during yarn spinning but may reappear as barré in sensitive dyeings if the yarn is assembled from fibres of a narrow length distribution. This length‑dependent swelling variance can be mitigated by subjecting the fibre to a pre‑wetting step with water at 70–80 °C prior to caustic impregnation, which plasticizes the cuticle and reduces the differential, a technique embodied in so‑called “hot‑wet mercerization” processes where the fabric is first saturated with hot water and then run through a cooled caustic bath (10–15 °C) to achieve a temperature shock that drives caustic diffusion before the cuticle can re‑seal; the associated control parameter—the thermal gradient from the hot‑water padder to the cold caustic saturator—must be maintained such that the fabric surface temperature just before the first caustic dip is 55–60 °C, a value monitored with a contacting thermocouple roller and used to automatically adjust the steam injection into the pre‑wetting bath. Standards like ASTM D5861‑07(2017) provide a laboratory‑scale procedure for measuring the degree of mercerization by caustic swelling using a microscope and image analysis, a method that is routinely applied to retained samples from every mercerizing lot in mills certified to OEKO‑TEX® Standard 100 to confirm that the within‑bale coefficient of variation of the swelling index is below 8 %.