Textile Mercerisation Nonuniform Swelling and Lustre at Caustic Concentrations Below 24°Bé

Textile mercerisation conducted with sodium hydroxide solutions below 24°Bé at 20°C—equivalent to approximately 18% w/w NaOH—represents a concentration boundary at which cellulose swelling is initiated but often fails to propagate uniformly through the yarn. The conversion from Baumé to specific gravity for this solution follows the relation °Bé = 145 − 145/sp gr, yielding a specific gravity near 1.198 at 20°C and a nominal NaOH content of 18.0% w/w. Continuous mercerising lines that operate below this concentration typically do so to reduce tensile strength loss, limit alkali attack on size residuals, or accommodate cellulosic blends with heat-sensitive components. The process consequence is that radial alkali penetration becomes the rate-limiting step, and the resulting swelling is frequently nonuniform across the cotton fibre cross-section. This nonuniformity produces a mixed population of rounded, lustrous fibres and residual ribbon-like, convoluted fibres that scatter light differently. On production-scale slasher and chainless mercerising machines, the incomplete transition is observed as centre-to-selvedge differences in lustre and as differential dye uptake after subsequent reactive dyeing. The following technical scenarios examine the causes, measurement, and process boundaries of this low-Baumé condition without extrapolating beyond published physical data and standard test methods.

What Limits Radial Alkali Penetration When the Caustic Concentration Falls Below 24°Bé?

Radial alkali penetration into a twisted cotton yarn is governed by the concentration gradient between the external caustic bath and the fibre interior, the pore structure of the secondary cell wall, and the hydrodynamic resistance of the yarn capillary system. At concentrations below 24°Bé, the driving force for diffusion is lower than in a conventional 28–30°Bé mercerising bath, and the alkali front advances more slowly through the cuticle and primary wall. The initial surface swelling can partially close the narrow capillary channels between fibres, further restricting convective transfer into the yarn core. In scoured and bleached cotton, residual wax removal and fibre wettability improve the rate of penetration, but heavily twisted yarns such as Ne 40/2 sewing thread show a persistent core-sheath effect when dwell time is below 30 s. Field data from chainless mercerising ranges indicate that dark selvedges and lighter centre sections after reactive dyeing are often traced to a caustic concentration drift of only ±0.5°Bé combined with uneven squeeze pressure across the pad. AATCC TM20 cross-sectional microscopy of these samples typically reveals circular fibres at the yarn periphery and flattened, unlustred fibres at the core. The practical implication is that concentration below 24°Bé cannot be compensated solely by increasing line speed; it requires longer immersion, mechanical working of the fabric, and wetting agents stable in high-salt alkaline systems.

On a chainless mercerising range, the fabric enters a caustic saturator through guided rollers and is passed under submerged rolls to achieve an immersion length between 8 m and 12 m. The saturator bath is maintained at 15–20°C by a plate-and-frame heat exchanger, because swelling enthalpy and viscous heating from pump recirculation can raise the bath temperature into the low-swelling regime. After impregnation, the fabric passes through a pair of rubber-covered squeeze rollers with a nip pressure configured to produce a wet pick-up between 80% and 100% on cotton fabric. The sheet then travels through an air passage and onto a tenter frame where width is restrained during the final stage of swelling and initial stabilisation washing. Tension in the tenter section is maintained at a level sufficient to prevent width shrinkage, typically by controlling the rail spacing and speed differential between saturator and stabiliser. When the caustic concentration is below 24°Bé, this equipment configuration is more sensitive to minor variations in nip pressure, roll camber, and bath circulation because the reduced alkali reservoir in the fabric cannot mask uneven distribution. The result is a characteristic stripe or moiré-like lustre pattern that may not be visible until the dried fabric is examined under collimated light.

When Low-Baumé Caustic Is Applied Without Zincate Additives in Continuous Mercerising

Low-Baumé mercerising baths are often operated without zincate or other heavy-metal swelling promoters, which places the entire wetting and penetration burden on the surfactant system. The surfactant must remain soluble and surface-active in 15–20% w/w sodium hydroxide, resist salting out at low temperatures, and lower the interfacial tension between the strongly alkaline phase and the cotton surface. Sulfated fatty alcohol and cresol-derived mercerising assistants are used at typical addition levels between 2 g/L and 5 g/L, but their effectiveness falls as caustic concentration decreases because the surfactant partitions differently between the bulk solution and the fibre boundary. If the wetting agent is overused, foaming in the saturator can create alkaline mist and uneven wetting; if underused, the alkali front becomes irregular and the resulting swelling is localized. Temperature control within ±2°C of the set point is required at 18% w/w NaOH because the degree of cellulose swelling is strongly exothermic and because the equilibrium between alkali-cellulose formation and free alkali changes with temperature. Production experience shows that low-Baumé baths are more prone to temperature stratification than conventional 28°Bé baths when circulation pumps are undersized or when heat exchanger flow is throttled. This stratification can produce a measurable density difference between top and bottom of the saturator, leading to fabric entering a lower-density zone at the top and exiting through a higher-density zone at the bottom.

Lustre evaluation on mercerised fabric is based on the angular distribution of reflected light rather than on a single reflectance value. A goniophotometer with collimated illumination at 45° incidence and a detector scanned from 0° to 75° from the specular angle is used to record the specular and near-specular reflectance. Fabrics that have undergone uniform swelling and tension show a narrow reflectance peak at the specular angle and low diffuse background; fabrics with partial swelling below 24°Bé show broader peaks, reduced maximum reflectance, and higher diffuse scattering due to the remaining twisted, flattened fibre segments. Colour yield after reactive dyeing is quantified by Kubelka-Munk K/S values using a spectrophotometer with D65 illuminant and 10° observer according to AATCC EP6; uneven swelling produces K/S differences across the fabric width that can exceed the repeatability limit of the spectrophotometer, normally below ±0.05 K/S units. AATCC TM110 is used for whiteness index where optical brighteners or residual alkali may influence the assessment. Published goniophotometric thresholds for acceptable lustre at concentrations below 24°Bé are limited; therefore, mills commonly establish internal reference swatches from a controlled mercerising trial and compare collimated-light photographs rather than relying on absolute specular reflectance values.

Fibre Cross-Sectional Transformation and Lustre Grading Criteria

Microscopic examination of mercerised cotton cross-sections remains the most direct method for distinguishing uniform swelling from partial core mercerisation. Unmercerized cotton fibres exhibit flat, twisted ribbon-like profiles with visible lumen and convolutions. Uniform mercerisation under tension produces near-circular cross-sections, reduced convolution count, and collapse of the lumen over most of the fibre population. Fibres treated below 24°Bé often show a mixed morphology: outer regions of the yarn contain round fibres, while inner regions retain the original bean-shaped or collapsed geometry. Quantitative image analysis of cross-sections uses shape descriptors such as circularity, defined as 4πA/P², where A is the cross-sectional area and P is the perimeter; a perfect circle approaches 1.0, while unmercerized cotton may fall below 0.5 depending on fibre position. The proportion of fibres with circularity above 0.8 is a useful process control metric, although published data for this specific configuration is limited. Lustre grading itself is conducted under collimated light in a dark room, with a series of reference samples arranged at a fixed viewing distance and angle; production operators compare the tested fabric to a set of five internal grades covering fully matte to high-lustre. AATCC TM20 provides the fibre identification and cross-sectional procedures used to support this evaluation, but it does not define an absolute lustre index for mercerised cotton.

Nonuniform Swelling Produces Detectable Differential Dye Absorption and Tensile Modulus Shifts

Differential dye absorption is the most sensitive production indicator of nonuniform swelling at caustic concentrations below 24°Bé. The selective conversion of cellulose I to cellulose II in the swollen outer regions increases accessible hydroxyl groups and changes dye substantivity, while the unmercerized core retains its original dyeing behaviour. When the treated fabric is dyed with a medium- to high-substantivity reactive dye such as C.I. Reactive Blue 19 or C.I. Reactive Black 5, the contrast between the mercerised shell and the less accessible core becomes visually evident as dark streaks, selvedge-to-centre shade variation, or moiré effects. The K/S difference across the width is measured spectrophotometrically according to AATCC EP6 and may exceed 1.0 K/S unit in severe cases, although accepted production tolerances are typically narrower. Tensile testing according to ISO 13934-1 or ASTM D5035 often shows higher breaking force in the fully mercerised zones and lower elongation at break in the tension-restrained zones. The nonuniformity also influences dimensional stability measured according to AATCC TM135 or ISO 5077, because residual alkali and differential swelling leave localized relaxation shrinkage. The stress-strain curve of partially mercerised cotton under low-Baumé conditions is therefore not representative of the fabric as a whole; it reflects a serial combination of high-modulus mercerised fibre segments and extensible unmercerized fibre segments.

Polyester/cotton blended fabrics mercerised below 24°Bé present a different failure mode. The cotton component swells and shrinks while the polyester component is largely unaffected by sodium hydroxide under these conditions, leading to shear stresses at the fibre boundary and possible fabric curl or skew. The lower caustic concentration reduces the risk of polyester surface hydrolysis and weight loss, but it also reduces the plasticising effect on the cotton, so the tension applied during stabilisation must be more precisely controlled to avoid creasing and width variation. Wet pick-up on polyester/cotton blends is lower than on 100% cotton because the hydrophobic polyester fibres do not imbibe the alkaline solution, and the cotton component may be unevenly wetted unless a low-foaming caustic-stable wetting agent is present. Production-scale observations indicate that selvedge pinning on the tenter frame can induce local tension differences of 2–5 N/cm across the sheet, and these differences become visible when the cotton component is only partially mercerised. The process boundary is therefore defined not only by caustic concentration but also by fabric construction, blend ratio, and the uniformity of tension across the tenter width.

Continuous low-Baumé mercerising is most prone to nonuniform swelling when the caustic concentration, temperature, pad expression, and dwell time drift independently. Inline sensors for Baumé density and refractive index are installed in the saturator recirculation loop, with automatic dosing of fresh 50% w/w caustic to maintain the target concentration within ±0.2°Bé. The bath is checked against a calibrated glass hydrometer at 20°C, and temperature correction is applied because density-based Baumé readings deviate substantially with temperature changes of even 5°C. Squeeze rollers are inspected for camber loss and re-covered when the rubber hardness changes by more than 5 Shore A points across the face. Wash water in the stabiliser is maintained at low residual alkali to avoid redeposited caustic streaks. In cases where these controls are not maintained, the fabric may pass tensile and dimensional standards but still exhibit a visible nonuniform lustre band that is unacceptable for solid-shade apparel. The operational boundary is therefore defined by the combination of equipment alignment, bath homogeneity, and testing according to the standards listed in the matrix above.