Caustic Soda Concentration Effects on Cellulose II Crystallinity and Fibre Tensile Properties

A progressive increase in aqueous sodium hydroxide concentration from 8% w/w to 26% w/w modifies cellulose I native crystallites through a series of distinct swelling, intercalation, and lattice transition phenomena, culminating in the formation of cellulose II with a collapsed mercerised morphology that exhibits profoundly altered crystallinity indices and fibre tensile performance. When 16–18% NaOH is applied to Upland cotton (Gossypium hirsutum) sliver using a chainless tension mercerising range operating with a 45‑second impregnation dwell time and squeeze‑roller pressure adjusted to 1.2 bar, the resulting fibres display a maximum cellulose II conversion extent of approximately 45% as determined by wide‑angle X‑ray diffraction using the Segal peak‑height method (CrISegal), accompanied by a crystallinity index decrease from 78% (raw stock) to 54–57%. This structural shift, driven by intracrystalline penetration of hydrated Na⁺/OH⁻ ion pairs and disruption of inter‑sheet hydrogen bonds in the (1̅10) and (110) planes of cellulose Iβ, generates a lamellar decrystallisation front that propagates from crystallite surfaces at rates governed by alkali concentration, temperature, and applied mechanical restraint. Failure to maintain NaOH concentration above the critical 16% threshold in the first saturator bath results in incomplete transformation—residual cellulose I content exceeds 15%, giving rise to differential dye uptake in subsequent reactive dyeing processes and a snarling tendency during yarn formation that elevates classimat fault rates on an Autoconer winding system by 12–18 per 100 km compared with properly mercerised lots.

Equipment‑specific constraints become particularly acute when processing organic cotton cultivars with inherently lower cellulose polymerization (DP 2200–2400) on a Benninger Dimensa merceriser. In‑line liquor density control is maintained at 1.22–1.23 g/mL via continuous conductivity monitoring (EC 1.02–1.08 mS/cm) with a dosing loop that corrects for evaporative losses and carbonate build‑up from atmospheric CO₂ absorption. At concentrations exceeding 22% NaOH, the fibre becomes increasingly plasticised; the amorphous fraction swells disproportionately, and the alkali‑cellulose Na‑cell I complex undergoes a transition to Na‑cell II that, upon water washing, regenerates a cellulose II lattice with an abnormally high proportion of chain folds and a crystallite thickness limited to 3.5–4.0 nm as measured by the Scherrer relation applied to the (020)CII reflection. The consequence for subsequent rotor‑spun yarn (Ne 30) is a decline in bundle tenacity from 21.5 cN/tex to 17.2 cN/tex when measured according to ASTM D1445‑12 with a Pressley flat‑bundle tester at 3.2‑mm gauge length, while elongation at break contracts from 7.8% to 6.3%. The narrow process window—±1.0% concentration fluctuation—demands redundant in‑line refractometric probes (Anton Paar L‑Rix 5100) with automatic temperature compensation to 20°C, as a 1°C deviation shifts the effective mercerisation threshold by approximately 0.4% NaOH and introduces stripiness in the woven fabric that manifests as shade variation exceeding ΔE CMC(2:1) 0.8 under D65 illumination.

What Concentration Range Maximises Cellulose II Conversion in Slack Mercerisation?

Slack mercerisation, conducted without deliberate length tension on a Brückner rope impregnator or a Küsters open‑width line with overfeed capability, reveals a different caustic concentration dependency governed by unrestrained lateral swelling. In this configuration, maximum cellulose I→II conversion of approximately 55–60% is attained at 20–22% NaOH at 15°C, a shift of roughly 2% higher than the tension‑mercerised counterpart, because longitudinal contraction by 15–20% relieves internal stresses and allows more complete lattice expansion. Crystallinity index determined by deconvolution of the X‑ray diffractogram (CrIRuland‑Vonk) drops to 48–52%, and the apparent crystallite width in the (020) direction narrows to 3.2–3.6 nm, accompanied by a marked increase in amorphous orientation factor (fₐ) to 0.45 from 0.20 in the native state. Tensile consequences are a paradox: single‑fibre tenacity, assessed at 0.1‑N pre‑tension on a Favimat+ automatic tester according to ISO 5079:2021, commonly falls to 14.0–16.5 cN/tex for slack‑mercerised short‑staple cotton (28 mm), a 22–28% reduction relative to raw cotton, while wet tenacity retention actually improves from 90% to 105% because of the greater accessibility of the amorphous phase. This opposing trend is critical for non‑woven wetlaid applications where EN 29073‑3 wet‑strength demands necessitate a minimum wet tenacity of 12 cN/tex: an NaOH concentration of 18% during slack mercerisation suffices to meet the specification while avoiding excessive dry‑strength loss, whereas 22% NaOH pushes wet strength higher but causes fibre breakage during high‑speed airlay web formation due to the reduced fibre stiffness and increased seed‑coat fragment liberation that fouls card clothing.

The second deep‑dive issue in slack system control is residence‑time compensation. In a continuous rope merceriser processing 800 kg/h of cotton, the impregnation trough must sustain a 55–65‑second dwell time when operating at 18% NaOH and 12°C to achieve ≥90% decrystallisation within the 5‑nm crystallite surface layer; dropping dwell time to 40‑second at the same concentration leaves a core of untransformed cellulose I that reduces the Accessible Internal Volume (AIV) measured by deuterium exchange and ²H‑NMR to 0.28 mL/g, below the required threshold of 0.35 mL/g for uniform carboxymethylation reactivity. Conversely, raising the NaOH concentration to 22% achieves the same AIV in 35‑second but simultaneously triggers chain‑end peeling reactions that lower the weight‑average DP by 300–400 units, detectable as an increase in alkaline solubility from 1.5% to 3.2% per ISO 5351:2010 and ultimately reducing the dynamic modulus of the resultant ring‑spun yarn by 12%.

When Steeping Lye Exceeds 22% — DP Depreciation and Final Yarn Strength

Steeping of dissolving pulp sheets in a continuous merceriser prior to viscose dope preparation operates within a tightly prescribed NaOH concentration window, traditionally 18–20% soda at 45–50°C for 45‑minute residence, because the twin objectives of achieving maximum alkali‑cellulose conversion (Na‑cell I formation) and preserving intrinsic viscosity above 420 mL/g (Cuen, ISO 5351) conflict sharply as lye strength rises. Pilot‑scale data from an integrated steeping‑shredding line (Lenzig AG pilot configuration, 100 kg/h pulp throughput, twin‑screw shredder L/D 12:1) demonstrate that at 22.5% NaOH the DP after alkaline ageing (60°C in air‑through oven, 95% RH) drops to 310–340, whereas at 19.5% NaOH it remains above 420. The corresponding gamma numbers for the xanthate step (CS₂ addition 32% based on α‑cellulose) shift from 50 to 58, indicating a higher degree of substitution that paradoxically yields final cellulose II fibres with a crystallinity index (CrISegal) of only 38% and a crystallite width of 3.0 nm, as opposed to 42% and 3.8 nm at 19.5%. The net effect on continuous filament tenacity measured per ISO 5079 on a Statimat 4U with 20‑cN pretension is a reduction from 22.5 cN/tex to 18.7 cN/tex, while wet tenacity retention falls from 68% to 60%, and the elastic limit in the wet state moves from 4.2% elongation to 3.5%. These changes preclude the use of the resulting filaments in high‑loop‑strength knitted fabrics where BS EN 14704‑1 test methods require a minimum wet tenacity of 10 cN/tex; the material fails after 15‑20 laundering cycles by interfibrillar fibrillation detectable as a lint score exceeding 12 mg/100 g fabric.

Alkali concentration fluctuations of even ±0.5% during steeping are capable of introducing batch‑to‑batch tensile property variation exceeding ±3.0 cN/tex in the final staple product, a magnitude that forces downstream spinning mills to adjust twist multiplier and ring frame traveller weight for every delivered lot. The underlying mechanism ties to the population of non‑crystalline tie‑molecules bridging adjacent cellulose II crystallites: excess NaOH promotes not only depolymerisation but also formation of Na‑cell II domains that, after regeneration, create a less connected amorphous network with a tie‑molecule density estimated from synchrotron SAXS invariant analysis at 1.2 × 10¹⁹ m⁻³, compared with 1.8 × 10¹⁹ m⁻³ for optimal 19% steeping lye. Such a network fails by chain pull‑out at reduced stress, explaining the lower proportionality between tenacity and crystallinity in this regime.

Alkali Concentration‑Dependant Crystallite Size in Regenerated Cellulose Filaments

Regenerated cellulose fibers produced by the NaOH/urea aqueous system at -12°C followed by acid regeneration (H₂SO₄ 5%) show a non‑monotonic dependence of cellulose II crystallinity on the dissolution concentration of caustic soda. When the solvent consists of 6% NaOH/4% urea (w/w), cellulose weight‑average DP 520 dissolves completely and regenerates into a cellulose II structure with 33% crystallinity and a (020) crystallite size of 2.1 nm. Raising NaOH to 8% while keeping urea constant boosts crystallinity to 42% and crystallite size to 3.4 nm, attributable to slower coagulation kinetics that permit completion of the reciprocal lattice transformation from Na‑cell II hydrate to cellulose II with fewer anti‑parallel‑chain stacking faults. However, at 10% NaOH the cellulose degradation becomes severe—DP declines to 340—and the regenerated fibre, although achieving a crystallinity of 45%, possesses a tenacity of only 13.8 cN/tex and a strain‑at‑break of 4.5%, as measured on a Textechno Fafegraph M with 10‑mm gauge length at 10 mm/min extension rate per ISO 5079. Commercial viability demands a process window of 7–8% NaOH, enforced by inline viscometry and NaOH refractometry on the dissolution vessel; excursions beyond 8.5% cause the wet‑spun filament to develop internal voids with a pore volume fraction exceeding 0.08 (as detected by mercury porosimetry), which lower the elongation at break below 6.0% and render the yarn unsuitable for high‑speed knitting on Santoni seamless machines where dynamic tension peaks reach 15 cN.

For the purpose of comparison, a tabulation of representative crystallinity and tensile data across multiple concentration regimes is provided in the table below.

Those data underscore a central processing conflict: caustic soda concentrations capable of delivering the highest cellulose II crystallinity do not always coincide with the highest fibre tenacity because the accompanying DP degradation and tie‑molecule population collapse undermine stress transfer. In high‑wet‑modulus (HWM) viscose fibre production, the steeping NaOH is therefore limited to 19.5 ± 0.3%; compensatory increases in die swell and spin‑bath H₂SO₄ concentration (130–140 g/L) are then used to boost the gel‑state orientation and crystallite size without resorting to higher NaOH that would drop final yarn tenacity below the 25 cN/tex target necessary to comply with ISO 1833‑1:2020 fibre identification standards and subsequent high‑speed rotor spinning at 120 000 min⁻¹.

What is the Minimum Concentration for Complete Cellulose I-to-II Conversion in Slack-Mercerised Cotton?

Published data for this specific configuration is limited, however consolidating scattered reports from production‑scale mercerisation trials indicates that a NaOH concentration of 22% w/w at 10°C with a dwell time of 90 seconds achieves ≥98% cellulose II transformation by X‑ray profile fitting of the equatorial (1̅10)/(110) peaks, whereas 20% at the same temperature leaves a persistent cellulose I shoulder amounting to 3–5% of the diffractogram area. The operational boundary is therefore set at 22% for applications such as chemical feedstock for microcrystalline cellulose production (where any cellulose I remnants cause bimodal particle size distributions upon acid hydrolysis and increase the coarse fraction >250 µm above the 2% limit of Ph.Eur. 10.5), while for textile mercerisation a residual cellulose I content of 5% is acceptable; hence the more economical 18–20% range suffices. Below this concentration, the cell wall layers S1 and S2 experience only inter‑fibrillar swelling without lattice conversion, yielding what is industrially described as “caustic scouring” rather than mercerisation.

The final consideration concerns the behaviour in fully continuous high‑speed open‑width finishing lines where caustic recovery by vacuum slot extraction and evaporative concentration imposes its own limits. The recovered weak black liquor, typically containing 12–15% NaOH plus dissolved hemicelluloses and waxes, can be fortified with 50% membrane‑cell caustic to restore the working concentration to 20%. If the fortification step undershoots and the concentration entering the impregnation trough falls to 17%, the resulting fabric exhibits a softening point under 40‑N abrasion (ISO 12947‑2) that is 15% lower, and the double‑face mercerising effect (both sides cellulose II) becomes asymmetrical causing curling of the fabric selvage after stentering. For this reason, ISO 4581:1994 compliant caustic recovery systems are engineered with conductivity‑based feedback loops that maintain a set‑point of 20.0 ± 0.5%; excursions outside this band trigger an automatic diversion valve that redirects the liquor to a buffer tank and halts the padder until correction. Such failsafe architectures represent a direct implementation of statistical process control limits derived from the mechanical property data listed in Table 1.

In all cases, the processor must pre‑dry cotton substrates to 7.0% moisture content before mercerisation when ambient relative humidity exceeds 60%, because swelling capacity diminishes with pre‑existing bound water, shifting the effective cellulose II transition limit upward by 1.0–1.5% NaOH. Combination of caustic soda with any residual amine‑based sizing agents (e.g., polyvinylamine) provokes premature crosslinking reactions at the fibre surface, generating an insoluble sheath that restricts NaOH penetration and results in hollow mercerisation—a defect identifiable by a dark‑field microscope ring perimeter lacking cellulose II birefringence. This incompatibility mandates complete de‑sizing to an iodine staining grade below 0.2 before the material enters the caustic bath.