Cotton Mercerization Swelling, Chain Relaxation, and Residual Alkali Recovery

During continuous cotton mercerization, the fiber is immersed in sodium hydroxide at a concentration between 18 wt% and 25 wt% and at a wet-bulb temperature maintained between 15 °C and 25 °C on a chainless roller-bed mercerizer. The swelling response is driven by the penetration of hydrated sodium ions and hydroxide ions into the cellulose network, disrupting inter-sheet hydrogen bonds in amorphous regions and then attacking crystallite surfaces. The result is a radial expansion of the cotton fiber cross-section, a reduction in the lumen void volume, and a shift from the collapsed bean-shaped cross-section toward a more circular geometry. In a production-scale pad-nip unit with elastomer roll hardness in the 75–85 Shore A range and a nip pressure between 4.5 bar and 6.0 bar, fabric wet pickup after the alkali bath typically ranges from 80% to 130% relative to dry fiber mass, depending on fabric mass per unit area, pre-wetting, and residual size. Typical production speed for a chainless mercerizer on woven sheeting is 40 m/min to 80 m/min, which imposes an alkali dwell between 40 s and 60 s in a roller bed of 30 m to 50 m fabric path length. If the fabric path is shortened or line speed raised above the dwell limit, the swelling front does not reach the fiber core of ring-spun yarns, and the Barium Activity Number can remain below 115 even though the bath concentration is correct. This high swelling state is stable only while the alkali concentration remains above the threshold for sodium-cellulose I complex formation. Water washing below approximately 10 wt% sodium hydroxide converts the swollen alkali-cellulose structure into cellulose II, which is the crystalline modification associated with increased dye sorption and improved luster. The degree of conversion is monitored by AATCC TM 89 barium activity number, where a value between 130 and 150 is considered indicative of full mercerization for high-value woven shirting, while values below 115 usually indicate under-mercerization. Fiber cross-section rounding and luminosity development are further influenced by the initial cotton maturity; immature fibers with thin secondary walls swell more rapidly but can collapse after wash-off, producing nonuniform dye strike when immature and mature cotton are blended.

What Governs Chain Relaxation During Sodium Hydroxide Deswelling?

Chain relaxation in mercerization is the molecular-scale process by which tensile stress stored in cellulose crystallites is released after alkali penetration breaks the native hydrogen-bond network. In slack mercerization, the absence of machine-direction tension permits the individual fibrils and whole fiber assemblies to shrink longitudinally by 10% to 20%, while the width increases by 20% to 40%; the exact balance depends on yarn twist multiple, fabric construction, and alkali dwell time. In tensioned mercerization on a chain mercerizer, the fabric is held at a defined machine-direction strain, often 2% to 5% extension relative to greige length, during the dwell period. This mechanical constraint prevents macroscopic fiber shortening and forces the relaxation to occur primarily as radial swelling and crystallite reorientation. The resulting tensile behavior differs: slack mercerized fabric typically gains 10% to 15% in breaking force but loses 20% to 30% in elongation at break when tested according to ISO 13934-1:2013; tensioned fabric may gain 15% to 25% in breaking force with a smaller elongation loss of 5% to 15%. The chain relaxation rate is temperature dependent. Below 15 °C, sodium hydroxide penetration is slowed by increased solution viscosity, and above 30 °C uncontrolled swelling can produce uneven fabric width shrinkage and selvedge curl. Relaxation also depends on the degree of tension mobilization at the molecular level. Native cellulose I has an X-ray diffraction main reflection near 22.6° 2θ; after slack mercerization and wash-off, the cellulose II main reflection shifts to approximately 20.5° 2θ, allowing quantitative conversion estimates by Segal crystallinity calculation. Water retention value after centrifugation at 3000×g for 15 min can increase from 35% to 45% for untreated cotton to 55% to 70% after slack mercerization, but this method is sensitive to centrifuge radius and pad pressure.

Residual alkali recovery on a continuous textile mercerizing range begins at the first vacuum extraction zone after the alkali dwell loop. The expressed liquor, typically containing 10 wt% to 18 wt% sodium hydroxide, hemicellulose fragments, cotton wax residues, and short fiber fines, is collected in a recovery tank and passed through a rotary drum filter with a 25 µm to 50 µm wedgewire screen before entering the evaporator feed. Failure to remove fiber fines above the filter rating accelerates scale deposition on the calandria tubes of a multiple-effect evaporator and reduces steam economy from a clean-system value of 3.2 kg to 4.2 kg of water evaporated per kilogram of steam to below 2.5 kg water per kilogram of steam after 400 h to 600 h of continuous operation. Recovery of sodium hydroxide from wash water is normally carried out through countercurrent washing, in which the weakest wash liquor is used for initial desizing and the most concentrated wash liquor is sent to the caustic recovery evaporator. A three-stage countercurrent wash train can reduce fresh water consumption to 3 L to 6 L per kilogram of fabric, depending on fabric mass per unit area and final residual alkali specification. The recovered caustic solution is typically concentrated to 40 wt% to 50 wt% sodium hydroxide before metered reintroduction into the mercerizing bath, where it is blended with fresh 50 wt% caustic to the target working concentration. Residual hemicellulose in recovered alkali modifies the viscosity of the mercerizing solution; solution viscosity at 20 °C may rise from approximately 40 mPa·s for fresh 20 wt% sodium hydroxide to 55 mPa·s or higher after multiple recovery cycles, requiring either higher pump energy or a purge stream of 5% to 10% of the circulating inventory per shift to maintain stable wet pickup. Accumulation of sodium carbonate from atmospheric carbon dioxide absorption is an additional recovery limit; recovered caustic exposed to air can reach 1.5 wt% to 3.0 wt% sodium carbonate and must be periodically withdrawn to prevent precipitation in heat exchangers.

When the sodium hydroxide concentration drops below 18 wt% during continuous treatment

When the sodium hydroxide concentration in the impregnation bath falls below 18 wt% because of dilution from wet fabric carry-over or insufficient replenishment, the swelling pressure generated inside the cotton fiber drops disproportionately. At 15 wt% to 17 wt% sodium hydroxide, the fiber cross-section becomes only partially rounded, and under transmitted light, mercerized fibers no longer show the uniform circular cross-section expected for full conversion. The first visible consequence on a continuous mercerizing range is a reduction in fabric width contraction control after the hot wash; the fabric often exits the wash section with width variation exceeding ±2.5 cm on a 180 cm wide sheeting line. Dye uptake after washing also shifts. Reactive dye exhaustion on under-mercerized fabric can decline by 10% to 20% at 1.0% owf dye depth, requiring either a higher dye concentration or an additional 30 min of dyeing time to reach the same visual depth. Production control is typically performed with an inline density meter or automatic refractometer, with a tolerance band of ±0.5 wt% sodium hydroxide. The density of 20 wt% sodium hydroxide at 20 °C is approximately 1.219 g/mL; a fall to 1.180 g/mL typically corresponds to a concentration in the 15 wt% to 17 wt% range, which is insufficient for uniform cellulose II conversion. When the bath concentration is restored to the target, the swelling response returns only if the fabric receives sufficient dwell time; transient production segments from the correction period frequently show a Barium Activity Number below 115 and are cut away as seam-to-seam shade mismatches after dyeing. Published data for the exact dye-uptake loss in a specific fabric construction is limited because the effect depends on cotton maturity, dye class, and residual alkali neutralization uniformity.

Thermal degradation pathways in caustic recovery evaporators limit recovered caustic brightness

Thermal degradation pathways in caustic recovery evaporators limit recovered caustic brightness and impose a practical ceiling on the amount of recycled alkali that can be returned to the mercerizing bath. In multi-effect evaporators concentrating recovered mercerization wash liquor from 8 wt% to 50 wt% sodium hydroxide, first-effect steam temperature is usually maintained between 120 °C and 140 °C, while the last effect operates under vacuum at 50 °C to 60 °C. These conditions promote alkaline hydrolysis of cotton-derived organics and thermal degradation of reducing sugars into colored organic acids, which raise the total organic carbon load of the recovered alkali. The resulting concentrated liquor may acquire a brown-to-black color, and when reintroduced into the mercerizing bath above 5% of the total circulating volume, it can shift the fabric base shade enough to require re-bleaching. The operational boundary for recovered caustic return is therefore set by a maximum total organic carbon concentration, typically measured by high-temperature combustion according to ISO 8245:1999 or equivalent; values above 2,000 mg/L in the recovered 50 wt% concentrated alkali frequently correlate with visible white-goods yellowing after continuous drying at 120 °C. Carbon steel piping and storage tanks in hot caustic service must be stress-relieved and operated below 80 °C to minimize stress corrosion cracking at weld zones; many plants use 316L stainless steel with molybdenum content above 2.5 wt% for the hot alkali loop. Published data for the exact ΔE shift on optically bleached goods in a single recovery cycle is limited, and most production sites establish their own purge ratio based on hourly titration of recovered caustic strength and weekly total organic carbon measurement.