Rayon Grade Caustic Soda

Alkali Cellulose Steeping and Pressing Constraints in Viscose Staple Lines

Rayon-grade caustic soda entering the viscose staple line is specified against narrow impurity ceilings because sodium chloride, sodium carbonate, iron, and silica carry through alkali cellulose into the xanthation step and alter ripening rate, filtration pack life, and fibre chroma. Typical merchant certificates for rayon-grade membrane-cell sodium hydroxide require NaOH ≥ 50.0 wt%, Na₂CO₃ ≤ 0.20 wt%, NaCl ≤ 0.005 wt%, Fe₂O₃ ≤ 0.0005 wt%, and SiO₂ ≤ 0.002 wt% on a 100% NaOH basis; trace nickel, copper, and chromium are commonly limited to ≤5 mg/kg total. Compliance documentation for European and North American textile lines references REACH Regulation (EC) No 1907/2006, the ZDHC MRSL v3.1, and OEKO-TEX Standard 100; total alkalinity is determined by ASTM E291-21, trace metals by ISO 11885 ICP-OES, and chloride by potentiometric titration against an AgNO₃ standard.

In the steeping aisle, dissolving pulp sheets or shredded pulp are contacted with NaOH at 17.5 wt% to 19.5 wt% and 45 °C to 55 °C for 30 min to 60 min. The liquor-to-pulp ratio is maintained at 12:1 to 20:1 by weight to ensure uniform alkali penetration and hemicellulose extraction. After steeping, the alkali cellulose is pressed on double-roll or screw presses to a press ratio of 2.8 to 3.2 times the bone-dry pulp weight, resulting in a NaOH-to-cellulose ratio of approximately 0.55 kg to 0.70 kg NaOH per kilogram cellulose and residual hemicellulose below 4 wt%. The pressed alkali cellulose is shredded and aged at 25 °C to 30 °C until the degree of polymerization falls to 400–600; sulfidation with carbon disulfide at 28 wt% to 36 wt% based on alpha-cellulose converts the alkali cellulose to cellulose xanthate, which is dissolved in dilute NaOH at 5 °C to 7 °C to yield a dope containing 8.0 wt% to 9.5 wt% cellulose and 5.0 wt% to 6.5 wt% NaOH.

After ripening, filtration, and deaeration, the viscose is extruded through spinnerets into a coagulation bath containing 95 g/L to 110 g/L H₂SO₄, 220 g/L to 320 g/L Na₂SO₄, and 10 g/L to 15 g/L ZnSO₄, held at 48 °C to 55 °C. On production-scale lines the bath is recirculated through flash evaporators and acid recovery systems; sodium sulfate concentration excursions above 320 g/L produce brittle filaments and spin-line breaks, whereas zinc below 10 g/L reduces skin-core differentiation and lowers wet modulus. Finished staple classifications include regular viscose staple, high wet modulus staple, modal staple, and flame-retardant viscose staple, each varying in spinning speed, drawing ratio, and aftertreatment. Operational boundaries are strict: iron above 3 mg/kg in the caustic feed increases dark pigment formation in the viscose dope, and chloride above 0.005 wt% accelerates ripening and destabilizes salt index control, requiring compensatory reductions in carbon disulfide input.

In continuous viscose filament production, rayon-grade caustic soda functions less as a bulk alkali and more as a dissolution and coagulation modifier, because the finished yarn must retain wet modulus and fatigue resistance under dynamic load. The viscose dope is formulated at 6.0 wt% to 7.0 wt% cellulose, 6.0 wt% to 7.5 wt% NaOH, and carbon disulfide at 30 wt% to 34 wt% of alpha-cellulose; for high-tenacity filament for tire cord, an amine-based modifier is additionally dosed at 0.5 wt% to 1.5 wt% on cellulose, though compatibility must be verified because residual iron and chloride alter modifier adsorption and ripening kinetics. Spinning is carried out into a coagulation bath containing 130 g/L to 160 g/L H₂SO₄, 260 g/L to 320 g/L Na₂SO₄, and 12 g/L to 18 g/L ZnSO₄ at 45 °C to 50 °C; spinneret hole diameters range from 40 μm to 100 μm, with draw ratios of 1.5 to 2.5 in the coagulation step and subsequent hot stretching under controlled tension.

Filtration and deaeration are critical upstream unit operations: rayon-grade NaOH with NaCl ≤ 0.005 wt% and SiO₂ ≤ 0.002 wt% extends candle-filter service and reduces spinneret orifice fouling, whereas carbonate above 0.20 wt% introduces gassing and uneven ripening. Compliance for filament destined to apparel and tyre applications is assessed against BISFA viscose filament yarn testing rules, ISO 2062 for tensile properties, REACH (EC) No 1907/2006, and ZDHC MRSL v3.1 for chemical management. Depending on aftertreatment and denier, the terminal product slate includes continuous viscose textile filament for linings, high-tenacity rayon filament for tire cord and conveyor belting, and embroidery-grade filament with controlled lustre and evenness. In process audits, the main batch-to-batch variance occurs in ripening index when caustic soda chloride or carbonate drift is not compensated; this shifts coagulation demand and can reduce yarn elongation by 2–4 percentage points under standard BISFA-conditioned tensile testing.

What Limits Regenerated Cellulose Film Clarity in Slit-Die Casting?

The limiting variables in cast regenerated cellulose film are not cellulose concentration alone but the interaction of NaOH purity with dope ripening, salt index, and particulate haze. For plain and coated cellophane, the viscose dope is prepared at 6.5 wt% to 8.0 wt% cellulose, 6.0 wt% to 7.0 wt% NaOH, and carbon disulfide in the range of 28 wt% to 34 wt% on alpha-cellulose. The dope is ripened to a salt index of 6 to 8 before slit-die extrusion into a coagulation-regeneration bath containing 120 g/L to 180 g/L H₂SO₄ and 200 g/L to 300 g/L Na₂SO₄ at 35 °C to 50 °C. Rayon-grade caustic soda is preferred because chloride and iron directly degrade optical transmission; iron above 3 mg/kg forms visible specks in film thinner than 25 μm, and silica particles larger than 2 μm create fisheye defects during casting.

After coagulation, the film web passes through regeneration, washing, desulfurization, bleaching, and plasticizing sections; glycerol or polyethylene glycol is applied at 8 wt% to 15 wt% on cellulose to prevent brittleness, and the film is dried on multi-bank steam-heated cylinders. Regulatory compliance for food-contact uses references 21 CFR 177.1200 for cellophane, 21 CFR 184.1763 for sodium hydroxide as a food substance, and EU Regulation (EC) No 1935/2004 Article 3; additional hygiene requirements are verified by EN 1186 migration testing when the film contacts fatty or aqueous food simulants. Finished product types include plain regenerated cellulose film, nitrocellulose-coated or PVDC-coated barrier cellophane, twist wrap for confectionery, and semipermeable membrane for battery separator or medical packaging. The most common production bottleneck is bath sulfate accumulation above 300 g/L, which reduces regeneration rate and leaves residual alkali in the film, shifting colour toward yellow and increasing extractable sodium.

When sodium carboxymethylcellulose is produced by slurry etherification, rayon-grade caustic soda is used for alkalization because the chloride ceiling of technical-grade caustic can raise ash content and interfere with monochloroacetate conversion. In a conventional ethanol or isopropanol slurry process, alkalization is carried out at a NaOH-to-anhydroglucose unit molar ratio of 0.9 to 1.5, with NaOH supplied as a 20 wt% to 30 wt% solution in the organic medium; the solvent-to-cellulose ratio is typically 8:1 to 15:1, and sodium monochloroacetate is charged at 0.7 mol to 1.0 mol per anhydroglucose unit. Etherification proceeds at 50 °C to 70 °C for 60 min to 120 min in a sigma-blade kneader or paddle dryer, followed by neutralization with acetic acid to pH 6.5 to 7.5, washing with 70–80% ethanol, and vacuum drying.

Compliance for food and pharmaceutical grades is anchored to 21 CFR 182.1745 for sodium carboxymethylcellulose GRAS status, the USP-NF monograph for croscarmellose sodium when crosslinked grades are produced, the JECFA monograph for CMC, and EU Regulation (EC) No 1333/2008 for E466. Terminal product types include food-grade sodium carboxymethylcellulose used as a thickener and stabilizer, croscarmellose sodium used as a tablet disintegrant, and technical-grade CMC for drilling fluids and paper coatings. The operational boundary for rayon-grade NaOH in this route is sodium glycolate byproduct formation at molar ratios above 1.5 mol NaOH per anhydroglucose unit, which lowers viscosity and increases extractable impurities; published data for continuous reactive extrusion of very high degree of substitution CMC is limited, and batch qualification is required to establish the exact caustic-to-MCA split under vented extruder conditions.

When Sodium Hydroxide Purity Gates Mercerization Oxidative Damage

At NaOH concentrations below 14 wt%, cotton fibres swell but do not undergo the complete lattice transformation to sodium cellulose II; at bath concentrations of 20 wt% to 24 wt% and temperatures of 15 °C to 20 °C, mercerization produces maximum luster, dye uptake increase, and tensile strength gain. Rayon-grade caustic soda is specified in mercerization not because chemical reaction demand is extreme, but because heavy metals catalyse oxycellulose formation and produce yellowing that is unacceptable on optical white goods. Typical bath preparation includes 20 wt% to 24 wt% NaOH, 2 g/L to 5 g/L of anionic wetting agent with alkali stability, and immersion time of 30 s to 120 s. In chain mercerizers, the fabric is saturated under controlled clip width and length tension, then processed through a hot-water recovery section at 80 °C to 95 °C before acid neutralization with acetic acid at 1 g/L to 3 g/L.

Production-scale issues are dominated by caustic recovery and iron pickup. A mercerizing range with vacuum extraction slots and double-saturation pads can recover 90–95% of the applied NaOH, but evaporator scaling increases when carbonate in the recovered lye exceeds 0.20 wt%. Compliance for finished textiles is verified against OEKO-TEX Standard 100, ISO 105-E04 for colour fastness to perspiration, and the brand-level chemical restrictions in ZDHC MRSL v3.1. Terminal product types include mercerized cotton yarn for high-strength sewing thread, mercerized woven shirting and sheeting, and high-gloss knitted cotton fabric. The alkali concentration must not exceed 26 wt% without precise tension control because uncontrolled shrinkage exceeds 20% and fabric width loss becomes non-recoverable; below 16 wt%, the effect retreats to surface swelling and the intended strength and luster development is lost.

Cold caustic extraction operates in a narrow alkali concentration window to reject hemicellulose from Kraft pulp and upgrade it to dissolving pulp suitable for viscose or lyocell. The NaOH charge is typically 3 wt% to 8 wt% on oven-dry pulp, with extraction liquor at 5 wt% to 9 wt% NaOH, a liquor-to-pulp ratio of 6:1 to 10:1, temperature 25 °C to 40 °C, and residence time 30 min to 90 min. Alkali concentration above 9 wt% triggers excessive fibre swelling and increases pulp washing load without proportionally higher hemicellulose removal. The extraction is followed by countercurrent washing, screw pressing, and sometimes peroxide or oxygen reinforcement; the spent alkali is evaporated and re-causticized, so carbonate and silica in inbound caustic influence evaporator scaling and heat-transfer coefficient.

Compliance for dissolving pulp lines is governed by process and chain-of-custody standards rather than finished textile chemical restrictions; typical documentation includes ISO 9001, ISO 14001, FSC/PEFC chain of custody, and ISO 1762 for residue on ignition. Rayon-grade NaOH with Fe₂O₃ ≤ 0.0005 wt% and SiO₂ ≤ 0.002 wt% is selected to avoid metal carryover into the subsequent viscose dope. Terminal product types include viscose-grade dissolving pulp with alpha-cellulose above 95 wt% and lyocell-grade dissolving pulp with controlled hemicellulose below 5 wt%. Published data for hot-cold alkaline extraction sequences using rayon-grade NaOH in a single-stage configuration is limited; pilot digesters and pulp mill trials are used to establish the exact hemicellulose rejection curve because wood furnish and prior pulping severity shift the alkali uptake kinetics.