Free Alkali Level Ranges Maintaining Stable Soap Noodle Extrudability

Free caustic alkali in sodium soap noodles is most frequently reported as percentage sodium hydroxide on an as-received basis using the hot ethanol dissolution and acid titration procedure of ISO 456:1973 or the free acid/free alkali method of AOCS Da 4a-48. Commercial specifications for extrudable toilet soap base typically place the acceptable free NaOH band between 0.02 % and 0.15 %, with the tighter control range of 0.04–0.08 % reserved for high-speed stamping lines operating at 150–250 bars/min and plodder cone exit temperatures below 48 °C. The free alkali value interacts with moisture, sodium chloride, and glycerine content in the lamellar gel phase of the noodle; at a fixed moisture of 11.5 % and salt content of 0.5 % NaCl, an increase in free NaOH from 0.03 % to 0.10 % reduces the gel viscosity and shifts the onset of wall slip to lower screw speeds but simultaneously lowers the critical stress for surface tearing. A global review of supplier data shows that the most frequently specified band is 0.05–0.10 % NaOH for standard vacuum plodding, while high-shear twin-screw refining lines with roll gaps of 0.12–0.18 mm and cone tip pressures above 60 bar are associated with maximum free NaOH values of 0.07 % because higher residual alkalinity produces sticky mill chips and increases refiner roll coating after 20–30 min of continuous operation. The purpose of controlling free alkali is therefore not merely chemical stability but the preservation of a stable rheological window between brittle fracture at the low-alkali boundary and adhesive wall slip at the high-alkali boundary.

In continuous saponification plants using distilled palm oil fatty acid, the free alkali is controlled by the excess sodium hydroxide added after the final neutralization stage; subsequent vacuum drying at 80–90 °C under 35–50 mbar absolute pressure can reduce free caustic values by 0.01–0.02 % NaOH through carbonation and neutralization by traces of free fatty acid. Noodle samples taken from the top, middle, and bottom of a 1000 kg supersack therefore exhibit spatial free alkali variation of up to 0.03 % NaOH unless the drying and cooling air is conditioned to a dew point below 10 °C. Because the titration method of ISO 456:1973 excludes carbonate interference, the reported free NaOH value is not inflated by sodium carbonate formed during storage, but it is equally unable to warn the operator of a carbonate-rich headspace in the sealed package unless total alkalinity is separately measured by ISO 685:1975. The distinction between free caustic and total alkali becomes operationally important when extruding high-moisture soap noodles above 12.0 % moisture, because carbonate can act as a stronger dehydrating salt than hydroxide and produce a falsely hard noodle that still fails at the stamping die.

Free Caustic Alkalinity Above 0.10 % NaOH Drives Adhesive Migration of Sodium Soap Lamellae at the Cone Wall

At free NaOH concentrations greater than 0.10 %, the sodium ion activity in the continuous aqueous phase compresses the electrical double layer around soap crystallites, weakening electrostatic repulsion and promoting the formation of a continuous, sticky lamellar film at the metal surfaces of the plodder cone. This film lowers the apparent viscosity at the metal interface, causing the extrudate to slip rather than undergo cohesive plug flow; the cone pressure therefore falls from 35–42 bar to 22–28 bar on a 150 mm single-stage vacuum plodder running at 800–1,200 kg/h with a screw L/D ratio of 10:1 and barrel jacket temperatures of 38 °C in the feed zone, 42 °C in the compression zone, and 46 °C at the cone. The loss of pressure is not accompanied by improved throughput; instead, throughput variability increases because the slip layer intermittently ruptures, producing a sawtooth torque trace with amplitude ±8 A on a 90 kW drive. Surface defects visible on the extrudate correspond to the stick-slip transition: transverse ridges spaced 5–15 mm apart appear when the free alkali exceeds 0.12 % NaOH and the moisture exceeds 12.0 %. In severe cases at 0.15 % NaOH, the extrudate surface roughness measured by profile stylus increases from an Ra of 0.9–1.1 μm to 1.8–2.4 μm, and the billet requires 2–4 °C lower cooling water temperature to regain hardness for cutting. The elevated free alkali also increases the equilibrium moisture uptake of the noodles at 50 % RH by 1.0–1.5 percentage points, which can reduce the total fatty matter below label claims if the specification is not rebalanced. Mechanistically, this behaviour is consistent with published phase maps for sodium palmitate/sodium oleate/water systems in which the hexagonal-to-lamellar transition temperature is lowered by the presence of excess hydroxide, expanding the temperature range over which the soap remains in a soft, low-yield-stress mesophase.

The following comparative data from a single six-point gradient trial on a 90/10 palm stearin/palm kernel olein noodle at 11.5 % moisture illustrates the pressure and surface response to free alkali variation. Each value is the mean of three samples taken after 30 min of steady-state plodding, with free NaOH determined by ISO 456:1973.

The shift in screw current and pressure at 0.02 % NaOH is accompanied by a characteristic increase in the refiner roll temperature from 32 °C to 38 °C after 45 min of recirculation, while the same run at 0.05 % NaOH remains below 34 °C. This thermal drift is an early-warning indicator that appears before visible edge cracking and is used on production lines to trigger alkali adjustment before the stamping defect rate exceeds 2 %.

What Free NaOH Range Keeps Cone Pressure Fluctuation Within ±3 bar During a 12-Hour Continuous Plodding Shift?

The practical lower and upper free alkali limits for stable extrudability are best defined by the amplitude of pressure fluctuation at the plodder cone rather than by any single viscosity measurement. On a 200 mm twin-screw vacuum plodder with independent screw temperature control at 40 °C and cone jacket temperature at 48 °C, a free NaOH value of 0.05–0.07 % maintains the cone pressure within ±3 bar of the 38 bar set point for a 90/10 palm stearin/palm kernel olein blend at 13.0 % moisture and 0.4 % sodium chloride. When the free NaOH drifts to 0.03 %, the pressure increases to 44–48 bar and the screw motor current rises by 9–12 %; when the free NaOH drifts to 0.11 %, the pressure drops to 26–30 bar and the pressure signal fluctuates by ±7 bar because of repeated slip-stick at the cone wall. The stable bracket reported in mill trial records is therefore often expressed as 0.04–0.08 % NaOH for toilet soap lines using refined and bleached palm stearin with iodine value 32–35 and lauric content below 5 %. For high-lauric formulations containing 20–30 % palm kernel oil, the upper boundary is lowered to 0.06 % NaOH because laurate soaps swell more strongly in the presence of free hydroxide, producing soft extrudates that collapse under their own weight at the die exit. This formulation dependence demonstrates that a universal free alkali specification is not appropriate; the tolerable range must be re-established whenever the carbon chain distribution shifts from C16/C18 to C12/C14 or when the moisture is changed by more than 0.5 percentage points.

Low-Free-Alkali Brittleness, Refiner Roll Fouling, and Bar Edge Cracking at 0.02 % NaOH

At free NaOH values below 0.03 %, the soap noodle mass tends toward a more elastic, less plastic rheology, with a higher yield stress that must be overcome in the refiner nip before homogeneous shear is achieved. Measurements on a three-roll refiner with 300 mm roll diameter and nip gaps of 0.15 mm, 0.12 mm, and 0.08 mm show that a reduction in free NaOH from 0.07 % to 0.02 % raises the refiner motor load by 12–18 % and increases the frequency of recirculating chip carry-over, which appears as unmixed white specks in the extruded billet. The low-alkali condition also leaves the soap with insufficient buffering capacity to neutralise trace free fatty acids released by hydrolysis during storage at 35 °C and 70 % RH; the resulting development of 0.05–0.15 % free fatty acid as oleic shifts the rheology further toward brittle fracture and increases edge cracking at the stamping die after 6–8 weeks of carton storage. In high-moisture systems above 12.5 % moisture, the lower boundary can be relaxed to 0.02 % NaOH because the plasticising effect of water suppresses brittle failure; in low-moisture systems below 10.5 % moisture, the lower boundary should be raised to 0.04 % NaOH to avoid audible refiner chatter and visible transverse cracking on the extruded bar. These operational boundaries assume a free alkalinity method that excludes sodium carbonate interference, as the carbonate fraction exerts a stronger dehydrating effect on the soap gel than hydroxide and produces false low free NaOH readings when not excluded.

Before a free alkali range is fixed for a new soap noodle grade, the triglyceride and fatty acid distribution of the feed must be linked to the phase behaviour of the finished soap. Palm stearin with a C16/C18 ratio above 1.8 and iodine value below 35 produces sodium palmitate-rich soaps that develop adequate beta-phase crystallinity and tolerate free NaOH values up to 0.10 % without severe die adhesion, provided the moisture is held at 10.5–11.5 %. The same free alkali upper limit cannot be transferred to a 70/30 palm stearin/palm kernel oil blend with iodine value 25–28 and C12–C14 content of 18–22 %; the lauric soaps hydrate more rapidly and the extrudate surface becomes tacky at free NaOH values above 0.06 %. Distilled coconut oil fatty acid saponification systems, in which C12 content can reach 45–50 %, are controlled at free NaOH values of 0.02–0.05 % to maintain a cone penetrometer hardness of 4–6 mm at 25 °C and to limit wet-bar smear after 2 h immersion in water at 30 °C. Tallow-based toilet soap bases with high stearate content and saturated fat of 45–50 % are commonly produced at 0.05–0.09 % NaOH; the higher upper limit reflects the lower solubility of sodium stearate at plodding temperatures of 38–45 °C. These feedstock-specific ranges are not derived solely from laboratory rheometry but from the cumulative effect of free alkali on the refiner, plodder, cutter, and stamping stages. Published data for continuous plodder trials across all of these formulations is limited; therefore the ranges above should be validated on the target line using the same analytical method.

When Free Alkali Falls Below 0.03 % NaOH and Citric Acid Is Present in the Noodle Formula, Extrudate Cracking Appears Before Bar Stamping

Citric acid is frequently added as a chelating agent at 0.02–0.10 % by weight to sequester trace iron and copper that would otherwise discolour the finished bar. Because citric acid reacts with free sodium hydroxide to form sodium citrate and water, its addition consumes a stoichiometric fraction of the free alkali and can shift a nominally acceptable 0.04 % NaOH noodle into the brittle region below 0.02 % NaOH. In a typical ultra-mild toilet bar formulation containing 0.05 % citric acid monohydrate, the equilibrium free NaOH falls by approximately 0.015–0.025 % after 24 h of mixing and storage, depending on the moisture available for ionisation. The resulting extrudate often exhibits longitudinal cracks at the die exit when the free alkali drops below 0.02 % NaOH; these cracks widen into bar-edge defects after stamping because the low-alkali soap cannot maintain cohesive plasticity under the 8–12 t stamping force of a rotary press. The corrective action is not necessarily the removal of citric acid but the adjustment of the free alkali specification to 0.06–0.09 % NaOH before chelant addition so that the post-reaction value remains above 0.04 % NaOH. This pre-reaction offset must be verified by titration of the finished noodle because the reaction rate depends on moisture, temperature, and the crystalline form of the citric acid.

The following compliance matrix consolidates the principal analytical and process parameters that must be monitored to maintain stable extrudability when free alkali is used as the primary control variable.