Lye Concentration Profiles and Fabric Tensile Response in Mercerisation

How NaOH Concentration Gradients Affect Yarn-to-Yarn Tensile Uniformity

Commercial chain mercerising ranges operating at 60–80 m/min fabric speed expose a moving cotton web to a counter-current cascade of lye baths, typically maintained between 24°Bé and 28°Bé (18.5% w/w to 22.4% w/w NaOH at 20°C). The immersion path length in the first trough rarely exceeds 4.0 s, creating a radial concentration gradient within individual yarns: outermost fibres contact bulk solution at 22–23% NaOH while core fibres may not reach equilibrium beyond 16–17% NaOH before the first squeeze-nip. That gradient propagates downstream as the fabric moves through the timing cylinder and subsequent baths, but differential swelling already imprinted during the initial shock swelling phase (<10 ms after contact) is irreversible. Fibres that swell under 19% NaOH exhibit only 40–50% of the crystallite conversion to Na‑cellulose I compared with fibres immersed at 24%, as tracked via wide-angle X‑ray diffraction in a 2021 spinning-beamline study (Diamond Light Source session mx₂‑214). The consequence is yarn‑to‑yarn tensile coefficient of variation (CV%) that escalates from 3.2% on a well‑balanced range (Benninger Dimensa with multi‑pass liquor circulation) to 8.7% when the primary lye feed pressure fluctuates by ±0.3 bar. Tensile testing according to ISO 13934‑1:2013 on warp‑strips extracted from the left, centre, and right selvedge often reveals a centre‑to‑selvedge breaking force delta of up to 68 N when the trough concentration drifts asymmetrically by 1.2°Bé across the width.

Control strategies applied on high‑production ranges incorporate continuous refractive‑index monitors (Maselli MIS‑03 series) sampling at 2‑second intervals and feeding a proportional‑integral loop that drives a metering pump for 50% stock lye. In a 23‑month production run at a Pakistani denim mill (L/C ratio 1:1.2), reducing the standard deviation of the first‑trough concentration from 0.8°Bé to 0.2°Bé lowered the warp tensile CV% from 7.9% to 3.5% and eliminated an annual reject rate of 2.3% related to seam‑slippage failures under ASTM D434‑95. When the lye temperature rises above 22°C, however, the refractive‑index signal becomes confounded because the temperature coefficient of the NaOH solution refractive index (−0.00018/°C in the 20–25% range) is often inaccurately compensated in the factory‑calibrated instrument, producing a systematic offset of 0.4°Bé that masks real concentration dips. This measurement artifact was confirmed by laboratory titration (0.1 N HCl, phenolphthalein endpoint) on 50 dips sampled over 8 hours; the resulting Bland‑Altman analysis showed a bias of +0.38°Bé with limits of agreement spanning −0.12°Bé to +0.88°Bé. Correcting the temperature compensation algorithm with a third‑order polynomial derived from the CRC Handbook lye density tables brought the bias to within ±0.05°Bé and restored the correlation between trough concentration CV and fabric tensile CV.

Mercerisation Tension, Lye Reluctance, and the 18.5% Threshold

When cotton cellulose is swollen without restraint, fibre diameter increases by 25–30% while length contracts 5–8%. Under controlled longitudinal tension, length contraction is arrested and crystalline reorganisation occurs under strain, which raises the molecular orientation factor (f_c) from around 0.72 in raw cotton to 0.88–0.92 in tension‑mercerised yarn. The tensile gain manifests as an increase in single‑yarn tenacity from 15–18 cN/tex to 21–26 cN/tex (Uster Tensorapid 4 at 500 mm gauge, 5,000 mm/min extension) when mercerisation is performed at 22% NaOH and a stretch ratio of 1.03–1.04. Below 18.5% NaOH, intracrystalline swelling is incomplete; the Na‑cellulose I lattice forms only in a minority of crystallites, and the applied tension merely elastically deforms the fibre without producing the permanent fines‑structural change that translates into elevated post‑wash tenacity. The threshold is sharp: differential scanning calorimetry on microsamples (0.5 mg cotton fibres, sealed Al pans, 10°C/min under N₂) shows the mercerisation endotherm peak shifting from 118°C at 18.0% NaOH to 124°C at 18.8% NaOH, indicating the onset of a cooperative lattice transition. A mill that attempted to economise by running one bath at 17.5% NaOH (reasoning that overall caustic recovery would improve) documented a 14% drop in warp‑way tensile strength retention (ASTM D5034) after only 7 production batches, with the reduction concentrated in selvedge regions where the bath temperature ran 1.5°C cooler due to heat loss through uninsulated trough ends, further lowering effective swelling power.

The interaction between lye concentration and applied tension creates a narrow processing window: at 20% NaOH and 1.06 stretch ratio, fibre breakage during mercerisation (measured as fly generation in the timing‑cylinder section) reaches 0.8% of total fibre input, compared with 0.12% at 23% NaOH and 1.03 stretch. The mechanism is shear‑induced fibrillation at incompletely swollen fibre surfaces; scanning electron micrographs (reported by Hashemizad et al., Cellulose 2020) show axial splits 50–200 µm long that nucleate at pits in the primary wall when the mercerising tension exceeds 0.5 cN/dtex and the lye concentration falls below 20%. Process engineers therefore program chain‑driven clip expanders to maintain a dynamic tension envelope of 0.35–0.45 cN/dtex when the first‑trough concentration drops transiently below 20% during start‑up, ramping to 0.50–0.60 cN/dtex only after the bulk reaches 22%.

Direct measurement of in‑process tension on a contemporary mercerising range (Küsters 2R-2000, 220 cm working width) employs three-axis load cells integrated into the last compensator roller before the stabilising wash zone. Data logged at 100 Hz reveal that tension in the centre of the web is consistently 12–18% lower than at the clip‑engagement edges, a disparity arising from differential heat accumulation in the centre region where alcohol‑soluble sizes leach and raise the local bath temperature by 1.2–1.8°C. Since lye viscosity falls from ~18 mPa·s at 18°C to ~12 mPa·s at 22°C (24% NaOH solution, Brookfield DV‑II+), the centre‑to‑edge tension gradient is compounded by a viscosity gradient that accelerates liquor drain and reduces wet‑pickup in the centre by 6–8% relative to the edges. Wet‑pickup data derived from microwave moisture analysis (TAPPI T‑550 om‑09) correlate with breaking force retention: when centre wet‑pickup falls below 90% of the edge value, post‑mercerisation warp tensile CV climbs above 6%, triggering a stop‑mark inspection protocol under the mill’s ISO 9001:2015 quality plan.

Applications where the mercerisation process is applied to ring‑dyed indigo denim consignments with heavy metal salt pre‑treatment demand even tighter control of the lye concentration profile. In a production campaign monitored over 14 consecutive lots (Each lot 5,000 m), lot‑average tensile strength retention following both mercerisation and subsequent bleaching ranged from 87% to 96% of the unmercerised grey strength. Process CUSUM analysis identified that the 87% lots correlated with an average first‑trough concentration of 21.3% NaOH (target 23.5%), a condition traced to a faulty densitometer that had drifted by −2.1°Bé over 45 operating hours. Recalibration and replacement of the sensor head restored the CUSUM to within target range within 3 lots.