Cotton mercerising in the caustic soda concentration window of 18–30 wt% is a swelling-controlled, tension-dependent transformation of cellulose I into cellulose II rather than a superficial surface treatment. The industrial effect depends on the activity of water in the sodium hydroxide solution, the lateral order distribution of the cotton fibre, the residence time in the caustic zone, and the mechanical restraint applied during swelling. Below 16 wt% NaOH, the hydrated alkali clusters are generally too small to disrupt the interchain hydrogen-bond network of native cellulose I, and the lattice remains largely unconverted. Above 30 wt% NaOH, the dynamic viscosity of the liquor rises sharply, the diffusion coefficient of sodium hydroxide into the fibre core decreases, and the steep concentration gradient between fibre surface and interior can produce a shell-core structure with a swollen, plasticised outer layer and an unswollen, stiff core. In a chain mercerising range processing plain-weave cotton fabric at 80–120 m/min, the dwell time in the caustic trough is commonly 35–70 s, and the cotton is saturated to a wet pickup of 80–110 wt% before the clip chain applies width control. Caustic concentration is expressed both as wt% NaOH and as g/L NaOH; at 20°C, a 20 wt% NaOH solution has a density of approximately 1.219 g/mL, giving 244 g/L NaOH and an Na₂O equivalent of 15.5 wt%. A 30 wt% NaOH solution has a density near 1.328 g/mL, a caustic content of approximately 398 g/L, and an Na₂O equivalent of 23.3 wt%. Hydrometer scales are still used in some mill specifications; the 18–30 wt% NaOH window corresponds to approximately 24–36°Bé at 20°C. Temperature is not an independent variable: a reduction from 30°C to 15°C shifts the lower boundary downward because alkali cellulose formation is exothermic, and cooling favours both swelling and lattice penetration. Consequently, the same degree of mercerisation can be obtained with 18 wt% NaOH at 15°C and with 21–22 wt% NaOH at 35°C, provided tension and dwell time remain constant. The practical concentration window is therefore an envelope that moves with the thermal balance of the range, the water carryover from the entering fabric, and the liquor circulation rate in the saturator.
The lower bound of the caustic window is set by the degree of swelling required to disrupt the hydrogen-bonded sheets of cellulose I and to permit the sodium ion to enter the lattice as a hydrated ion pair. In cotton fibre cross-sections, a NaOH concentration below 10–12 wt% produces only inter-fibrillar swelling without lattice conversion; the fibre retains its bean-shaped cross-section, and the X-ray diffraction pattern continues to show cellulose I reflections near 14.8°, 16.5°, and 22.7° 2θ with Cu Kα radiation. Between 12 wt% and 16 wt%, partial conversion occurs, but the degree of mercerisation becomes highly sensitive to residual sizing agents, natural waxes, and spinning lubricants, all of which reduce the effective local concentration at the cotton surface. At 15°C, the lower limit for consistent tension mercerising is approximately 16–18 wt% NaOH; however, mill-scale ranges commonly set the target at 20–22 wt% because the liquor in the impregnation trough is diluted by residual water carried by the dry fabric. A fabric entering the caustic bath with 6–8 wt% moisture and a residence time of 50 s can reduce the boundary-layer concentration by 1.0–1.5 wt% NaOH, which is sufficient to drop a 18 wt% bath below the effective lower boundary. Caustic flow in the trough must therefore be maintained with a circulation rate of at least 3–5 times the liquor volume per minute, and the tank volume is typically sized at 1.5–2.0 m³ per metre of fabric width to prevent local depletion. In a saturator-type chainless merceriser, the fabric passes through a pair of heavy squeeze rolls with a nip pressure of 0.4–0.8 MPa, then through a dwell zone of 25–40 m before the first tension-controlled wash. The degree of mercerisation can be assessed by barium activity number, which compares the barium hydroxide adsorption of the treated cotton with that of untreated cotton; values above 115 % are generally considered adequate, while values above 125 % indicate pronounced conversion to cellulose II. Below 16 wt% NaOH at 15°C, the barium activity number seldom exceeds 105 %, and the lustre gain is insufficient to justify the capital cost of the range. The lower concentration bound is therefore determined not by the fibre alone but by the water content of the incoming fabric, the liquor circulation rate, the wetting rate, and the analytical method used to verify concentration.
In slack mercerising, the fabric is allowed to shrink freely in both directions during saturated exposure to 20–26 wt% NaOH, and the concentration window shifts toward the higher end because the absence of tension increases the rate of caustic penetration but also increases the risk of non-uniform shrinkage. Cotton knitted goods, processed in open-width or tubular form, are commonly treated in a slack state at 22–26 wt% NaOH for 10–20 min in batch machines because longitudinal shrinkage may reach 12–18 % and width shrinkage may reach 8–14 %. At caustic concentrations below 20 wt%, the swelling force generated by alkali cellulose formation is insufficient to remove the flattening and convolution of the cotton fibre, and the residual ribbon-like morphology produces a dull, uneven surface. At concentrations above 28 wt%, the high viscosity of the liquor impedes rapid drainage from the fabric, and the subsequent washing step must be prolonged because the alkali absorbed in the fibre interior is not easily removed. The slack mercerising window is therefore narrower than the tension-mercerising window for routine mill operation: 22–26 wt% NaOH at 20–25°C with a washing sequence of 4–6 counterflow boxes at 70–95°C is a typical production formulation. When the liquor temperature falls below 18°C, the rate of caustic diffusion through the primary wall is reduced, and the alkali may remain on the fibre surface even though the fabric is fully saturated. When the temperature exceeds 30°C, the swelling equilibrium shifts to lower alkali uptake, and the fabric develops variable shrinkage because the edges cool faster than the centre. Mechanical agitation and garment rotation in batch slack mercerising also alter the effective liquor-to-fabric ratio; a reduction from 10:1 to 5:1 may deplete the effective caustic concentration by 1.5–2.0 wt% because the cellulose consumes sodium hydroxide in the formation of alkali cellulose. Published data for oval winch machines operating at 5:1 liquor ratio with 24 wt% NaOH indicate that the bath concentration can fall to 22 wt% within 30 min if no replenishment stream is provided. The process cannot be controlled by initial concentration alone; the concentration at the end of the batch is the more meaningful parameter, and it should be verified by titration against 1.0 N hydrochloric acid using phenolphthalein and corrected for sodium carbonate interference.
The temperature-concentration relationship in cotton mercerising is dominated by the hydration of the sodium ion and the water activity of the caustic solution. At 20°C, the dynamic viscosity of the liquor increases from approximately 3.5 mPa·s at 20 wt% NaOH to approximately 6.5 mPa·s at 30 wt% NaOH, and the self-diffusion coefficient of water decreases accordingly. The sodium ion in dilute solution is surrounded by a primary hydration shell of 4–6 water molecules; in concentrated sodium hydroxide, the number of free water molecules is insufficient to complete the hydration shells of both the sodium cation and the hydroxide anion. This hydration-shell depletion is one of the central rate-limiting factors in the industrial mercerising window. Native cotton fibre has a moisture regain of approximately 8–9 % at 65 % relative humidity and 20°C; mercerised cotton, after neutralisation and rinsing, can reach a moisture regain of 10.5–12.0 % under the same conditions because cellulose II has a more accessible fibre surface and lower lateral order. The uptake of NaOH by cotton increases with caustic concentration up to a plateau near 26–28 wt%; beyond this concentration, the quantity of alkali absorbed continues to rise, but the degree of lattice conversion does not increase proportionally because the highly viscous solution cannot penetrate the smaller pores of the fibre. At 28–30 wt% NaOH, the surface of the cotton swells extremely rapidly, forming a swollen outer layer that blocks the inward diffusion of additional caustic; this shell-core morphology is observable in cross-section microscopy as a sharply defined boundary between the swollen outer zone and the unswollen core. The shell-core structure is undesirable because it produces a high gloss on the fibre surface but low dye uptake in the interior, and it can lead to differential scorching or yellowing during subsequent drying. For this reason, ranges processing dense woven cotton twills add a pre-wetting stage that activates the primary wall and allows the caustic to reach the secondary wall before the outer layer skin forms. Pre-wetting with water at 70–85°C and subsequent vacuum extraction to a moisture content of 60–70 wt% are used to open the fibre structure; the wet fabric is then passed into 20–24 wt% NaOH at 18–24°C. The pre-wet caustic concentration is held higher than the target because the water film on the fibre dilutes the boundary layer; a 24 wt% bath is often specified when the desired effective concentration at the fibre surface is 21–22 wt%. The heat of dilution of the caustic liquor also affects the process. Diluting 50 wt% NaOH to 24 wt% releases a substantial quantity of heat; unless the dilution tank is cooled, the resulting liquor can reach 35–40°C and reduce the swelling efficiency. In a continuous mercerising range, a caustic cooler with cooling water at 8–12°C is therefore installed after the dilution skid to maintain the working concentration at 18–22°C. Temperature fluctuations of more than ±2°C at fixed caustic concentration have been shown in production trials to produce visible selvedge-to-centre differences in lustre; the edge of the fabric cools on the clip chain while the centre retains heat from the exothermic reaction, resulting in a widthwise gradient. The concentration window must therefore be specified with a simultaneous thermal tolerance; in a compact mercerising range with a 30 m dwell zone, a temperature variation of ±1.5°C is the maximum allowable for a uniform product at 23 wt% NaOH.
Wetting additives for mercerising liquors must remain soluble and surface-active in the presence of 20–30 wt% NaOH, which is a more demanding requirement than in ordinary scouring. Anionic sulfated alcohols and phosphate esters are frequently used at 0.2–0.5 g/L, and their effectiveness is verified by wetting time measurements according to AATCC TM17-2005; a wetting time below 5 s is needed for high-speed ranges operating above 60 m/min. Non-ionic surfactants based on alkylphenol ethoxylates are generally avoided because they can phase-separate at high caustic concentration, producing oily spots and uneven dyeing. The wetting agent reduces the contact angle between the caustic solution and the hydrophobic cotton cuticle; without a wetting agent, the fabric may float on the surface of the caustic trough and the effective dwell time is reduced. The wetting agent must also be stable to hydrolysis: at 28 wt% NaOH and 25°C, some sulfated alcohols undergo hydrolysis within 4–6 h, and the resulting alcohol is insoluble in the alkaline liquor. The mill standard procedure is to test the wetting agent in the actual working liquor at the target concentration and temperature for 8 h before a production run; this avoids the discovery of phase separation after the liquor has been charged. Sulphated castor oil derivatives may improve fibre swelling but can reduce the interfacial tension too sharply and produce excessive foaming in the wash section. Alcohol-based antifoams are typically limited because they may be salted out by the caustic and deposit on the fabric; silicone emulsions with a pH stability above 13 are used only at very low dosage if foam control is essential. The concentration of wetting agent is not a direct substitute for caustic concentration: it modifies the kinetics of wetting but does not alter the equilibrium swelling of cellulose. When an ester-based wetting agent is added at 0.3 g/L to a 20 wt% NaOH bath, the contact time required for full fibre wetting can be reduced from 8 s to 2 s, but the barium activity number remains unchanged if the caustic concentration is below the lower threshold. Thus, the wetting agent is particularly important in boundary-layer depletion zones near the squeeze nip and at high processing speed, but it cannot rescue an under-concentrated bath.
The mechanical restraint applied during caustic treatment determines the distribution of swelling and the residual tension in the fabric. In a clip chain merceriser, the width is normally adjusted to a value slightly below the original grey width to control dimensional stability; the applied tension is, however, not uniform across the fabric width. Selvedge-to-centre tension differentials as low as 8–12 % can be measured with embedded strain sensors in a production range, and the effect on caustic uptake is magnified because tension reduces the accessible free volume of the fibre. At the selvedge, the fabric is held firmly by the clips and cannot shrink, so the fibre is stretched longitudinally during swelling; this increases the orientation of the cellulose chains and produces a high lustre. At the centre, the fabric is relatively slack, and the cotton is free to shrink and convolute, producing a duller surface. The caustic concentration required to achieve uniform lustre therefore depends on the tension profile: a 20 wt% NaOH bath may produce a uniform effect only when the clip chain is precisely levelled and the bow and skew are controlled to ±0.5 % of fabric width. If the guiding system allows a deviation of 1.0° in the clip angle, the local tension at one edge can rise to 18–20 daN per clip, while the centre remains below 10 daN. The resulting variation in caustic penetration is not corrected by increasing the dwell time, because the tension difference persists through the duration of the reaction zone. In commercial ranges, the width of the fabric is monitored by a web guiding scanner with a resolution of ±1 mm, and the speed of the clip chain at each side is adjusted to maintain the required tension. A tension sensor output of 0–100 N across the fabric is calibrated against a load cell in the control system; when the span exceeds 12 % of the mean value, an alarm is generated, and the differential is reduced by adjusting the weft straightener. The caustic concentration window cannot be widened without limit to compensate for mechanical non-uniformity; at 26 wt% NaOH, the swelling force is so high that the fabric becomes stiff and difficult to control, and at 30 wt% NaOH the differential shrinkage may cause permanent bow. Process capability studies on a chainless merceriser with a 3.2 m working width have shown that the standard deviation of fabric width after the alkali zone can be held below 0.8 % when the NaOH concentration is controlled to ±0.3 wt% and the clip tension to ±5 %. When the NaOH concentration is allowed to drift to ±0.6 wt%, the width standard deviation rises to 1.4 %, and the fabric exhibits visible selvedge-to-centre lustre banding. The process is therefore controlled by a closed-loop caustic analyser tied to a mass-flow meter on the dilution skid; the analyser samples the trough liquor every 10–15 s and adjusts the water addition to maintain the setpoint. Online density measurement with a Coriolis meter is preferred because it is insensitive to air bubbles and fibre fines, but the density signal must be corrected for temperature because a thermal shift of 5°C changes the measured density by approximately 0.005 g/mL in the 20–30 wt% NaOH region.
| NaOH concentration | Bath temperature | Barium activity number | Slack shrinkage, length | Tensile strength change | Moisture regain at 65% RH |
|---|---|---|---|---|---|
| 12 wt% | 20°C | 95–105 % | 2–4 % | +5–10 % | 8.5–9.0 % |
| 16 wt% | 20°C | 105–115 % | 6–10 % | +10–15 % | 9.0–9.5 % |
| 20 wt% | 20°C | 115–125 % | 10–14 % | +15–25 % | 10.5–11.0 % |
| 24 wt% | 20°C | 125–140 % | 12–18 % | +20–30 % | 10.5–12.0 % |
| 28 wt% | 20°C | 130–145 % | 14–20 % | +15–20 % | 11.0–12.5 % |
Residual alkali control after mercerising determines final dye uptake, because sodium hydroxide left in the fibre raises the pH above the isoelectric point of cotton and produces alkaline hydrolysis of reactive dyes during subsequent dyeing. The washing sequence after the caustic zone is designed to reduce the alkali content of the fabric to below 0.05 wt% NaOH on the weight of the fibre, which corresponds to an aqueous extract pH below 8.5 as measured by ISO 3071:2005. The washboxes are operated in counterflow with a temperature gradient from 70°C in the first box to 25°C in the final box; a neutralising sour bath of acetic acid at 0.5–1.0 g/L, pH 4.5–5.0, is inserted before the final hot and cold rinses. Dye uptake of mercerised cotton compared with untreated cotton can be determined by the Kubelka-Munk K/S value; production trials report an increase of 15–25 % for reactive dyes applied at 2.0 % on weight of fibre under identical dyeing conditions and measured according to ISO 105-J01:1997. The combined effects of caustic concentration and residual alkali are not additive: a fabric mercerised at 28 wt% NaOH and poorly washed to pH 9.5 will dye unevenly even though the fibre is highly swollen, because the high pH accelerates dye hydrolysis and causes the dye to react with alkali in the liquor before it can diffuse into the cotton. The dye yield loss can reach 10–20 % in continuous pad-dry-steam dyeing, and the fastness of the resulting dyeing is reduced because the hydrolysed dye is poorly fixed. For this reason, the mercerising range operator monitors the pH of the final wash water and titrates the alkali content of the fabric every 30–60 min; if the residual alkali exceeds 0.05 wt%, the fabric is rewashed before drying.
The caustic soda in a mercerising range is continuously extracted from the fabric by the first wash water, and the resulting weak caustic liquor is normally sent to an evaporator for recovery. The concentration of the recovered caustic, the level of sodium carbonate formed by reaction with atmospheric carbon dioxide, and the accumulation of cotton fibre fines all affect the working concentration window. Sodium carbonate is a weak base and does not mercerise cotton; its accumulation in the wash water and subsequent concentration in the evaporator can produce a recovered liquor with a carbonate content of 1.0–3.0 wt% as Na₂CO₃. When this recovered liquor is blended with fresh 50 wt% NaOH to reach the target concentration, the effective NaOH concentration is lower than the density measurement suggests, because sodium carbonate also contributes to the density. A mill producing 12,000 m of fabric per day at 2.0 m width and 220 g/m² fabric weight requires approximately 26.4 kg of fabric per minute; at 80 wt% wet pickup and 24 wt% NaOH concentration, the caustic solution pickup is approximately 21.1 kg/min, of which 5.1 kg/min is pure NaOH. The first wash stage therefore receives a large and continuous caustic load, and the wash water flow must be sized to reduce the alkali concentration in the first box from 12–15 wt% to 2–3 wt% before the drain stream. In a multi-effect evaporator with three effects under vacuum at 50–60 kPa absolute, the weak liquor is preconcentrated before final adjustment in the mixing tank. The online density sensor in the recovery loop is compensated for carbonate and temperature; a carbonate concentration of 2 wt% can change the apparent NaOH concentration by approximately 0.4–0.6 wt%, which is sufficient to move the mercerising bath from the centre of the window to the edge. Calcium and magnesium ions in the dilution water can precipitate as hydroxides and carbonates; the resulting scale on the evaporator tubes reduces heat transfer and causes the recovered liquor temperature to rise, which in turn changes the density measurement. Production-scale data indicate that the fouling resistance of the evaporator can increase by 10–15 % over 500 h of operation when the dilution water hardness exceeds 50 mg/L as CaCO₃, and the cleaning interval must be shortened. The recovered caustic is filtered through a pressure filter with a 50–100 µm screen to remove cotton fibre fines, because fines accumulate in the trough and increase the viscosity of the liquor, which shifts the effective concentration window. The control strategy is to maintain the fresh caustic addition in a ratio to recovered caustic that holds the sodium carbonate content below 1.5 wt% Na₂CO₃ and to adjust the final water addition on the basis of temperature-compensated density at 20°C. This keeps the NaOH concentration within ±0.5 wt% of the setpoint without allowing the recovery loop to shift the process into an ineffective or shell-core regime.
| Process parameter | Measurement or method | Control range | Frequency | Reference standard |
|---|---|---|---|---|
| NaOH concentration in impregnation trough | Temperature-compensated Coriolis density meter and laboratory titration | 20–24 wt% (244–293 g/L) | Continuous online; laboratory every 2 h | Internal titration against 1.0 N HCl |
| Bath temperature | PT100 probe in trough and dwell zone | 18–24°C | Continuous online | Internal calibration with certified thermometer |
| Fabric speed | Encoders on drive rollers | 40–80 m/min | Continuous online | Internal calibration |
| Residual alkali on fabric | Aqueous extraction and titration; pH of extract | ≤0.05 wt% NaOH; pH below 8.5 | Every 30–60 min | ISO 3071:2005 |
| Dimensional stability | Washing and drying cycle | Length and width shrinkage within ±2 % | Once per batch or 1,000 m | ISO 6330:2012, ISO 3759:2011 |
| Tensile strength retention | Strip tensile test on mercerised fabric | Minimum 85 % of grey fabric strength | Once per batch or 1,000 m | ISO 13934-1:2013 |
| Lustre or surface gloss | 60° specular gloss meter | Minimum 25 gloss units | Once per batch | ISO 2813:2014 |
| Barium activity number | Barium hydroxide adsorption method | 115–140 % | Once per batch | Internal method calibrated against untreated cotton |
The upper limit of the caustic window is strongly influenced by the concentration of transition metal ions in the liquor. Iron and copper leached from piping at concentrations above 2–5 mg/L catalyse the oxidative degradation of cellulose during hot washing and produce yellowing. Aluminium, zinc, and galvanised fittings are incompatible with 20 wt% NaOH; austenitic stainless steel of grade 316L or higher is used for troughs and pipework because of its resistance to stress corrosion cracking. Amine-based additives should not be used in the caustic bath because they are protonated by the alkaline system and can form coloured condensation products with reducing sugars released from the cotton. Chlorinated solvents must be excluded because they can be hydrolysed by strong alkali and generate chloride ions, which increase the risk of pitting in stainless steel. The mercerising process must also be restricted to cotton with a maturity ratio above 0.80; immature fibres swell excessively and form a gel-like surface that hinders subsequent washing. Published data for specific configurations with immature cotton and caustic concentrations above 28 wt% is limited; therefore, mills processing mixed-maturity cottons typically verify the lower end of the concentration window with a mill trial before full production.
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