Cold Mercerization of Cotton Fabric in 18 to 25 Percent NaOH Liquor

Cold mercerization of cotton fabric in sodium hydroxide liquor at 18–25% w/w is an open-width caustic swelling process in which the fabric is saturated, held in a temperature-controlled plastic state, restrained to width and length, washed hot, neutralized, and dried. The primary structural result is the conversion of native cellulose I to cellulose II, a change that reduces fibre convolution, lowers crystallinity from roughly 70–80% to 50–60%, and increases the number of accessible hydroxyl sites available for water sorption and dye fixation. The working concentration range is not arbitrary; it corresponds to the stability field of soda cellulose I at temperatures from 15 to 20°C. The fabric is processed on a mercerizing range equipped with a caustic saturator, a multi-bowl dwell chamber, a clip tenter or chain mercerizer, and a countercurrent washer. The reaction is exothermic because sodium hydroxide dilution in the bath and penetration into the fibre both release heat. Without external cooling, the liquor and fabric temperature can rise by 8–12°C, and the resulting loss of swelling efficiency is non-linear, producing streakiness, moiré, or reduced barium activity number. The process is therefore run with recirculation cooling through a plate heat exchanger served by 8–12°C chilled water. The degree of mercerization is measured by barium activity number using AATCC 89-2008; values below 115–120 indicate incomplete conversion, while values from 130 to 150 are typical of uniform tension-mercerized fabric. The process is incompatible with starch-based sizes that have not been fully removed, with siloxane defoamers that deposit on fibre surfaces, and with amine-based softeners applied before caustic treatment.

Which Variables Determine the Lower and Upper NaOH Concentration Boundaries?

Sodium hydroxide solutions in the 18–25% range have elevated density and viscosity that affect both wetting and diffusion. At 20°C, the density of 18 percent NaOH is approximately 1.197 g/cm³, 20 percent approximately 1.219 g/cm³, and 25 percent approximately 1.274 g/cm³. The corresponding dynamic viscosities are approximately 3.5, 4.5, and 7.0 mPa·s. The higher viscosity at 25 percent reduces wetting speed, so the practical upper boundary is often determined not by cellulose chemistry alone but by the need to achieve uniform penetration within 25–60 seconds of dwell. The lower boundary near 18 percent is set by the sodium hydroxide concentration necessary to form soda cellulose I quickly; below approximately 14 percent the lattice expansion is insufficient, and mercerization is incomplete. The upper boundary near 25 percent is set by the economic cost of caustic recovery, the risk of forming higher alkali complexes, and the loss in tear strength that occurs when the swollen structure is over-restrained. For fabrics with high yarn twist or dense construction, the concentration is commonly maintained at 20–22 percent w/w rather than at the upper end, because the limiting factor is penetration rather than fibre swelling. An inline Coriolis density meter or automatic titration according to ISO 979:2023 is used to maintain the concentration within ±0.5 percentage points. Concentration should not be inferred from Baumé alone because density varies with temperature; a temperature-compensated density meter or direct acid titration is the control method of record.

Fabric entering the caustic saturator at residual moisture above 8% dilutes the first bath and creates a low-concentration boundary layer at the fibre surface that retards swelling. In production practice, the last drying step before mercerization is controlled to 6–8% moisture by weight, measured by ASTM D2495-07(2019), and the moisture profile across the width is checked by a dielectric scanner. A moisture spike of ±2% across 10 cm of width can reduce the effective caustic concentration at that band by 1.5–2.5 percentage points and produce a visible stripe after dyeing. The absorbency of the prepared fabric is also checked by a water drop penetration test; if a 0.1 mL water drop does not penetrate within 3 seconds, the fabric is re-dried or re-scoured. The mercerizing liquor is recirculated through 50–100 µm lint filters to remove fibre fragments, and the nip rolls are cleaned at each shift to prevent the accumulation of alkali-starch gel. Foaming in the saturator is controlled by selecting low-foam caustic-stable wetting agents at 2–5 g/L; the absence of foam is confirmed visually at the overflow weir because foam trapped at the nip produces unmercerized spots. Common wetting-agent chemistries for this bath are phosphate esters and short-chain alkoxylates with cloud points above 100°C in the alkaline environment.

Chain, Clip, and Washwater Temperature Management

After the dwell chamber, the fabric enters a clip tenter or rubber-belt chain arrangement that restrains the selvedges and controls width. The warp direction is simultaneously controlled by speed differential between the entry and exit rollers. In the plastic state, cotton fabric allowed to shrink freely in 20 percent NaOH loses 8–15 percent of its length; by applying warp extension of 2–4 percent and width relaxation of 0–2 percent from the incoming width, the range converts that shrinkage tendency into improved fibre orientation and luster. Tensile strength measured by ASTM D5035-11(2019) increases by 15–30 percent in the warp for a 120–150 g/m² plain-weave fabric, while the weft direction may show only 5–15 percent increase because the weft yarns are not subjected to the same axial stress. The tension must not be increased to the point of squeezing the yarn structure; tear strength measured by ASTM D1424-21 can fall by 20–35 percent under excessive cross-direction restraint. Bow rollers and a weft straightener are used immediately after the clip zone because any bow or skew present during the plastic state becomes set into the cellulose II structure. The first wash bay is operated at 65–75°C to dilute sodium hydroxide and decompose the sodium cellulose complex while the fabric remains under tension. If the fabric is released from tension before the caustic concentration has fallen below approximately 6–8 percent, uncontrolled shrinkage occurs and gloss is reduced. Countercurrent washing in 5 to 8 bays reduces caustic carryover to below 0.1 percent w/w before the neutralization slot. The neutralization slot is dosed with acetic acid or citric acid to bring the fabric pH to 6.5–7.5, measured by AATCC 81 or ISO 3071:2020. A final fabric pH above 8.0 is a reject condition for reactive-dyed goods because it causes dye hydrolysis and storage yellowing.

When Caustic Recovery Falls Below 70 Percent

The economic and environmental boundary of cold mercerizing in 18–25% NaOH is the recovery of the dilute wash liquor. A countercurrent washing train produces a weak caustic stream at 5–12 percent NaOH that must be filtered, evaporated, and remixed with concentrated caustic to restore working concentration. When recovery efficiency drops below 70 percent, the specific caustic consumption rises above approximately 300 kg NaOH per tonne of fabric, wastewater pH exceeds 12, and the neutralization load of acetic acid, sulfuric acid, or carbon dioxide becomes disproportionate. The recovery loop in a modern range consists of a self-cleaning rotary screen filter for lint removal, a storage balance tank, a steam-jacketed evaporator under vacuum of 60–80 kPa, and an inline density sensor. The evaporator is run in double-effect or thermal recompression mode to minimize steam use. The recirculated caustic must be monitored for sodium carbonate, formed by reaction with atmospheric carbon dioxide; sodium carbonate levels above 1.5–2.0 percent w/w in the working bath reduce swelling efficiency and can deposit as scale on heat exchanger plates. The caustic bath is therefore blanketed or ventilated to limit CO₂ ingress, and the recovered liquor is periodically purged. Carbonate concentration is measured by dual-endpoint titration with phenolphthalein and methyl orange. The recirculation loop is also protected from ferric iron and hardness because these form insoluble precipitates in strong alkali and can deposit on the clip chains. Chelating agents used in the caustic bath must be compatible with 25 percent NaOH at 20°C; phosphate-based chelants are preferred over EDTA because EDTA loses effectiveness at high alkalinity.

Following neutralization and drying, the cold-mercerized cotton has a higher equilibrium moisture regain of 10–14 percent compared with 7–9 percent for untreated cotton. The reduction in crystallinity and the transformation to cellulose II increase the internal pore volume accessible to direct and reactive dyes. In exhaust dyeing, the first-cycle exhaustion of CI Reactive Blue 19 onto mercerized cotton can be 15–25 percent higher than on untreated cotton. This higher substantivity increases the risk of unlevel dyeing. The dyeing procedure is therefore adjusted by injecting salt in three or four progressive additions rather than a single dump, starting dyeing at 30–40°C instead of 50–60°C, and raising the temperature at 1°C/min. Color strength is evaluated by K/S using a reflectance spectrophotometer according to AATCC Evaluation Procedure 6, and wash fastness is tested by ISO 105-C06:2010 with a minimum shade-change rating of 4. Because the mercerized cotton has a higher water retention capacity, drying after dyeing requires 15–25 percent more evaporation energy per kilogram of fabric. This increase must be included in finishing cost calculations; published data for specific equipment configuration is limited, but the heat-load increase is directly measurable from dryer steam flow. The fabric is not compatible with amine-based softeners in the same bath as reactive dye because the softener can exhaust unevenly and reduce fastness. Softening is applied only after dye fixation and soaping.

On production lines where cellulosic blends containing elastane are processed, exposure to 18–25 percent NaOH at 15–20°C is usually tolerated, but hot washing stages above 70°C can reduce elastane recovery force by 10–20 percent after repeated exposure. Such fabrics require a modified first-wash temperature of 55–60°C and a reduced dwell time. Polyester-containing blends are less critical at the saturator stage, but caustic carried into steam-heated drying cylinders can hydrolyze polyester surfaces and create oligomer deposits; thorough washing before drying is therefore mandatory. The mercerizing bath is also incompatible with residual polyvinyl alcohol size at high concentration because polyvinyl alcohol precipitates at alkaline pH and forms a film on the fabric. In all cases, the operational limit is set by the combination of caustic concentration, temperature, dwell time, and tension. A change of ±0.5 percentage point in NaOH concentration, ±2°C in liquor temperature, or ±1 percent in warp extension is sufficient to produce a measurable difference in barium activity number and final dye depth.