Production control of crimp frequency and crimp stability in nylon 6 carpet staple fibre requires simultaneous management of fibre rheology, moisture, finish-on-fibre level, and the steam-setting energy input delivered after stuffer-box deformation. Caprolactam spin finish chemistry is distinctive because the fibre-forming polymer is not fully inert with respect to the finish phase; residual ε-caprolactam and cyclic oligomers migrate from the fibre interior to the surface during drawing at temperatures between 70°C and 190°C, altering the chemical composition and film-forming behaviour of the applied spin finish. A finish formulation that performs within specification at start-up can move outside the processing window as caprolactam accumulates in the recirculated finish bath. The result is often visible on the production floor as crimp non-uniformity, high crimper torque variability, steam-tube streak, and intermittent fibre breakage at the breaker bar. The processing window should therefore be defined not as a single set point but as a bounded matrix in which finish-phase composition, fibre surface energy, friction, and thermomechanical load interact. The polymer side is typically a nylon 6 homopolymer with relative viscosity measured in 96% sulfuric acid according to ISO 307:2019, with a viscosity number between 180 mL/g and 260 mL/g for carpet staple; the melt viscosity is also checked by ISO 1133-1:2022 at 275°C under 5.0 kg, where values outside 20 cm³/10 min to 60 cm³/10 min trigger a chip evaluation before extrusion. The fibre-forming polymer is melted in a single-screw or twin-screw extruder with an L/D ratio of 24:1 to 30:1 and filtered through a pack with 20 µm to 40 µm media before spinning. Spin finish applied to the tow typically contains a base lubricant, an emulsifier system, an antistatic component, and a pH-buffered water phase. In caprolactam-containing finishes, the residual monomer acts as a polar cosolvent and can increase the cloud point of non-ionic emulsifiers while also increasing the conductivity of the aqueous phase. Because caprolactam is hygroscopic and water-soluble, it modifies the drying behaviour of the finish film after water removal. The interaction is not trivial: the finish film must remain coherent and uniform across the tow width, must not deposit hard varnish on the draw rolls, and must not plasticize the fibre surface so heavily that crimp retention collapses after steam setting. Field experience from production lines with stuffer-box crimpers indicates that a finish-on-fibre level measured by solvent extraction at 0.80 wt% to 1.40 wt% is a common starting window for nylon 6 carpet staple, but the allowable range narrows when caprolactam concentration in the recycled finish exceeds 15 wt% to 20 wt% of the non-aqueous phase.

The finish formulation itself must remain stable under recirculation, mechanical shear, and biological load, particularly when the plant operates closed-loop finish recovery. Caprolactam can be consumed by microbial growth in the finish bath if biocides are not maintained; bacterial degradation of caprolactam releases ammonia and amine compounds that shift pH and destabilize emulsifiers. This pH shift, from a typical finish emulsion pH of 5.5 to 7.5, can cause dispersed droplets to coalesce, resulting in non-uniform oil pick-up across the tow width. The emulsion particle size distribution is monitored by dynamic light scattering, with a target median droplet diameter below 1.0 µm to prevent creaming; once caprolactam concentration rises above the solubility limit of the emulsifier system, the droplet size can increase to 5.0 µm to 20.0 µm, and the coefficient of variation of finish-on-fibre across the tow may exceed 8% to 12%. This is one reason why finished fibre is extracted with solvent according to enterprise methods aligned to ISO 1833-1:2020 or ASTM D2257 for extractable matter; the acceptance band is often tighter than the nominal 0.80 wt% to 1.40 wt% because downstream crimp uniformity is affected by even small deviations in surface lubricant film thickness.

Why Does Caprolactam Partition into Aqueous Spin Finish Emulsions During Two-Stage Drawing?

Partitioning of caprolactam from nylon 6 fibre into the spin finish phase is driven by temperature, relative humidity, fibre residence time on heated godets, and the aqueous solubility of the monomer. The fibre after spinning and washing is not free of monomer; depending on extraction efficiency, residual caprolactam and cyclic dimer/trimer content may lie between 0.1 wt% and 0.6 wt% before drawing. During two-stage draw at draw ratios of 2.8 to 3.8, the fibre structure becomes oriented and the amorphous fractional free volume declines, reducing the equilibrium solubility of caprolactam in the polymer. At the same time, the tow surface temperature rises to 140°C to 180°C on the second draw stage, providing thermal energy for diffusion of monomer to the surface. Water present in the finish emulsion dissolves the migrating monomer, and as water evaporates from the finish film, the caprolactam remains behind. Cyclic oligomers, especially cyclic dimer and trimer, are less water-soluble than ε-caprolactam, but they can be suspended in the finish film and can accumulate on heated rolls as a white deposit. The partition coefficient of caprolactam between fibre surface and finish film is not a single constant; it depends on the polarity of the lubricant base, the ethoxylation degree of the emulsifier, and the presence of amine-terminated polyamide oligomers that can form hydrogen bonds with caprolactam. Published equilibrium data for caprolactam partitioning in commercial spin finish systems is limited; however, production monitoring shows that the rate of caprolactam build-up in recycled finish increases when the finish bath temperature is maintained above 45°C because the monomer diffuses faster from the fibre and the aqueous phase has higher saturation solubility.

The effect of caprolactam on the finish film is principally rheological and interfacial. At concentrations below 5 wt% of the non-aqueous finish phase, caprolactam may act as a hydrotrope and reduce emulsion droplet size; at concentrations above 10 wt% to 15 wt%, it can raise the effective glass transition temperature of dried finish film after water removal, causing the film to become hard and brittle. This transition is measurable in the crimper as an increase in the force required to pre-compress tow and as a change in the stuffer-box pressure signature. The finish film must retain enough cohesion to control fibre-to-metal friction but must not become so tacky that it builds up on the crimper roll surface. For carpet staple fibre, a dynamic fibre-to-metal friction coefficient between 0.28 and 0.42 measured at 50 m/min according to ASTM D3108-13 is commonly targeted, but caprolactam migration can increase the measured coefficient by 0.05 to 0.15 if the film dries into a sticky oligomer layer. The crimper torque controller often records a rise in baseline torque of 3% to 8% before visible deposits appear on the roll. Plant trial records show that the torque signal starts to oscillate at frequencies of 0.1 Hz to 0.5 Hz when finish film composition shifts, corresponding to stick-slip at the metal-tow interface.

On a typical staple fibre line, finish is applied after the draw zone or on the draw zone feed section using a kiss roll, a metering pump, and a recirculating finish tank. The finish application system for carpet staple fibre typically consists of a stainless steel kiss roll with a diameter of 200 mm to 300 mm, a doctoring blade to control film thickness, and a variable-speed metering pump with a flow range of 3 L/min to 20 L/min. The finish concentration in the bath is often set at 8 wt% to 20 wt% active matter, while the target finish-on-fibre level is achieved by controlling the kiss roll speed and the gap between the blade and roll. The finish emulsion is held in a jacketed tank at 25°C to 45°C; if the tank temperature exceeds 50°C, evaporation of water increases the active concentration and accelerates caprolactam extraction from the fibre surface. Recirculation flow through the tank should provide a constant bath turnover with a residence time below 10 min to prevent local concentration gradients. The finish application uniformity is verified by extracting fibre samples across the tow width with a solvent such as methanol or a methanol/water mixture at 80°C according to enterprise methods aligned to ISO 1833-1:2020; the coefficient of variation for finish-on-fibre across the tow should be below 5.0% for consistent crimping. If the finish bath becomes enriched in caprolactam above 15 wt% of the non-aqueous phase, the pick-up can become non-linear with kiss roll speed because the emulsion viscosity and droplet size change; in that condition, increasing kiss roll speed may not produce the expected increase in finish-on-fibre because the film on the roll breaks and no longer transfers uniformly.

Finish filtration is critical when caprolactam oligomers desorb from the fibre. The recirculating loop typically includes a stainless steel mesh filter with a pore size of 25 µm to 100 µm, and a filter bypass alarm is set at differential pressure of 1.5 bar to 2.5 bar. If the filter loads with oligomer gel, the finish pressure drops and the kiss roll application becomes uneven. The filter pressure trend is therefore a leading indicator that caprolactam is migrating into the bath from under-extracted fibre. Some lines use a two-stage filter: a coarse 100 µm bag filter for agglomerates and a fine 25 µm disc filter for gel-like particles. The service life of the fine filter drops from more than 48 h to less than 8 h when caprolactam oligomer deposition is high, and this interval is recorded in the process log as an early warning.

Interfacial Friction, Film Thickness, and Crimper Load Stability

Under controlled crimping conditions, the stuffer-box crimper relies on a balance between fibre-to-metal friction and fibre-to-fibre cohesion. The tow is delivered to the crimper by a pair of nip rolls with a surface temperature typically between 60°C and 100°C, which heats the finish film and softens the lubricant phase. If the finish film is too thick as a result of excessive finish-on-fibre or high caprolactam content, hydrodynamic lubrication develops at the nip roll and the tow slips, reducing the stuffing-box pressure. If the finish film is too thin or too glassy as a result of caprolactam oligomer enrichment, boundary lubrication dominates, the fibre-to-metal friction increases, and the crimper motor load rises. The crimper speed for carpet staple is commonly set at 120 m/min to 300 m/min, while the stuffing-box pressure is maintained by a flapper or wedge that can be set to 0.5 bar to 3.0 bar depending on fibre decitex. The measured fibre-to-metal friction coefficient by ASTM D3108-13 is only one component of the system; the fibre-to-fibre static friction and the hydrodynamic film thickness under shear are also important. When the finish contains caprolactam as a polar cosolvent, the film thickness at the crimper nip under load may be from 0.05 µm to 0.50 µm depending on finish concentration and temperature. Below 0.05 µm, asperity contact dominates and stick-slip occurs; above 0.50 µm, the tow hydroplanes and crimp frequency declines. These boundaries are critical for crimp frequency control and are not easily predicted from bulk viscosity alone.

The relationship between finish viscosity and crimper load is nonlinear. A kinematic viscosity of the diluted finish at 40°C between 10 mm²/s and 30 mm²/s is often used for nylon 6 staple fibre, but caprolactam can increase the aqueous phase viscosity and simultaneously reduce the dried film flexibility. On a controlled torsion rheometer, the dried finish film may show a complex viscosity at 1 Hz and 25°C from 100 Pa·s to 1000 Pa·s for a normal film, while oligomer-enriched films can exceed 5000 Pa·s and exhibit solid-like behaviour. The production impact is observed as a shift in the crimper flapper pressure set point required to maintain the same apparent crimp frequency. When flapper pressure is adjusted upward repeatedly during a run without changing finish concentration, the cause is often a change in film rheology rather than a mechanical drift. Maintenance records from lines with heated nip rolls show that roll cleaning intervals shorten from 7 days to 2 days if caprolactam oligomers are allowed to accumulate; the hardened film on the roll can be removed only with hot water at 80°C to 90°C or with an aqueous caprolactam solution, which acts as a solvent for the oligomer phase.

Electrostatic charge accumulation in the stuffer box becomes more severe when the relative humidity in the crimping room falls below 45% at 20°C to 25°C. Nylon 6 staple fibre has a low bulk electrical conductivity unless a humectant or ionic antistat is retained on the surface. The spin finish formulation for caprolactam-containing systems typically includes an ethoxylated phosphate ester or a quaternary ammonium compound as an antistatic agent. The static half-life, measured according to an enterprise method aligned to ISO 18080-1:2015, should be below 1.0 s for the finished tow before entering the crimper. If caprolactam in the finish phase binds water, the electrical conductivity of the finish film may initially improve at high relative humidity. However, when caprolactam films dry in slow-moving air, a hygroscopic surface layer can become sticky and increase fibre-to-fibre cohesion, leading to uneven stuffer-box fill and localised plugging. The cohesion force between fibres is affected by the surface energy of the finish film; caprolactam-rich films can develop ionic clusters that act as weak physical crosslinks. In the crimper, excessive fibre-to-fibre cohesion produces dense, twisted clumps and non-uniform crimp frequency from edge to centre. Field observations indicate that when the finish aqueous phase conductivity exceeds 2000 µS/cm, the static half-life is acceptable, but if the conductivity is obtained by using high concentrations of caprolactam and alkali metal salts, drier films become sticky; a better balance is achieved with a phosphate ester antistat and a caprolactam concentration in the aqueous finish below 10 wt%.

When Dew Point in the Steam Setting Tunnel Exceeds 90°C

After stuffer-box deformation, crimp fixation in nylon 6 staple fibre is performed in a steam setting tunnel or autoclave. The crimp is set by heating the fibre with saturated steam at 90°C to 130°C for a residence time of 10 s to 60 s, depending on tow mass and packing density. The steam not only heats the fibre but also interacts with the spin finish film. When the dew point in the tunnel exceeds the saturation temperature of the incoming steam, condensation forms on the fibre surface; the condensate extracts caprolactam and finish components from the fibre, producing a draining liquid that can accumulate in the tunnel bottom and on the conveyor belt. If the finish contains non-ionic emulsifiers with a cloud point below the steam temperature, the emulsion may phase-separate on the fibre, leaving an oily deposit on the rolls after the tunnel and reducing crimp retention. The caprolactam component is particularly mobile under wet steam because it is water-soluble; it can be washed out of the finish film and deposited as a white crystalline crust on the tunnel walls, where it absorbs moisture and becomes sticky. This fouling requires weekly cleaning with hot water and a detergent; the cleaning interval is shorter when the dew point in the tunnel is not continuously monitored. The seam welds and support rails show corrosion if the condensed water contains caprolactam derivatives that lower pH. The steam setting tunnel is therefore operated with a dew point sensor at the exit, and the dew point is controlled to 85°C to 95°C for an atmospheric tunnel; when the value exceeds 95°C, the finish film becomes a wetting liquid rather than a lubricating film, and the fibre surface can be damaged by dragging against the conveyor surface.

The post-steaming finish-on-fibre level may drop by 0.05 wt% to 0.20 wt% due to steam washing; the amount depends on the finish formulation and the degree of caprolactam extraction. If the finish film is too water-soluble, the final finish-on-fibre after setting can fall below the minimum required for downstream carding lubrication. For this reason, caprolactam-based spin finish formulations are often designed with a hydrophobic ester component and an emulsifier package that resists steam extraction. The finish film is evaluated by extraction of finished staple after steam setting, and the retained finish-on-fibre should be within 0.80 wt% to 1.40 wt%. If retained finish is below 0.60 wt%, the fibre will exhibit high carding friction and may accumulate static charge; if retained finish is above 1.60 wt%, the fibre may generate excessive deposits on card clothing and draw rolls.

Analytical monitoring of caprolactam in spin finish can be performed using high-performance liquid chromatography with ultraviolet detection at 210 nm or gas chromatography after derivatization. The finish emulsion is extracted from fibre with methanol/water at 80°C; the extract is filtered through a 0.45 µm membrane and injected into the HPLC system. The caprolactam peak area is calibrated with ε-caprolactam reference material of purity greater than 99.0%. The limit of quantitation is typically 0.05 wt% in the finish non-aqueous phase. In addition to caprolactam, the cyclic dimer and trimer can be detected by gel permeation chromatography or HPLC with a reversed-phase column and acetonitrile/water gradient. The amount of cyclic oligomer in the finish is an indicator of fibre surface migration rather than simple wash-off; cyclic dimer is less water-soluble and more likely to remain as a film-forming deposit on heated metal surfaces. Process control then includes a frequency of every 4 h for finish bath samples when a new finish batch is introduced, and every 8 h during steady-state operation.

Processing Window Boundaries at 20 wt% Caprolactam in the Non-Aqueous Finish Phase

At concentrations near 20 wt% caprolactam in the non-aqueous finish phase, the finish film transitions from a plasticized lubricant to a sticky, hygroscopic solid at ambient temperatures. In a recirculating finish bath, the non-aqueous phase is the portion remaining after water is evaporated at 105°C for 2 h; this phase includes the lubricant base, emulsifiers, antistats, and extracted fibre species. At caprolactam concentrations below 10 wt%, the caprolactam is usually dissolved homogeneously in the polar components of the finish and may improve the conductivity and wetting characteristics. Between 10 wt% and 20 wt%, the dried film softens and tack increases; the coefficient of friction of the finished tow may remain within the target range, but fibre-to-fibre cohesion increases enough to change the stuffer-box pressure signature. Above 20 wt%, the film can become supersaturated with caprolactam at room temperature, leading to solid crystal growth on the finish roll, the crimper nip, and the steam tunnel entrance. The observed result is a shift in the finish application set point needed to maintain the same finish-on-fibre level; the metering pump speed may need to be raised by 5% to 15% to compensate for the greater viscosity and reduced transfer efficiency. The steam-setting tunnel fouling rate also increases, and the pressure drop across the finish filter rises. The process window narrows to less than ±5°C in the finish bath temperature because the solubility of caprolactam in the polar phase is strongly temperature dependent; a bath temperature excursion from 35°C to 45°C can change the amount of crystallized caprolactam in the tank and cause a feed line blockage. A temperature-controlled jacket with a control accuracy of ±1°C is therefore specified for finish tanks operating near this threshold. The upper processing boundary for caprolactam in the recycled finish is usually set at 15 wt% to 20 wt%; above this value, the finish bath is purged and replaced with fresh emulsion. Published data for the exact rheological and tribological transitions in proprietary finish systems is limited; therefore, the boundary is validated on each production line by solvent extraction, friction measurement, and visual inspection of the crimper rolls.

The kinetic path to high caprolactam concentration is accelerated by several operating conditions: low residual moisture in the fibre before finishing, high draw-zone temperature, and long residence time in the finish bath. If the undrawn tow is stored in unheated creels at ambient temperature, caprolactam diffusion is slow; if the tow is preheated to 60°C to 80°C before finishing, migration increases. The finish bath residence time and the ratio of finish bath volume to tow throughput determine the steady-state concentration. For a line producing 5000 kg/h of tow with a finish pick-up of 1.0 wt%, the finish bath may cyclically accumulate caprolactam at a rate of 0.05 kg/h to 0.3 kg/h depending on the residual monomer content of the polymer; the resulting steady-state concentration can be estimated from the purge rate. In practice, a purge-and-replenish programme of 5% to 10% of the bath volume per hour is used. If the line operates closed-loop, the bath may need an adsorbent or membrane-based caprolactam removal step; published data for industrial-scale removal of caprolactam from finish baths is limited, and the most common intervention remains partial dilution with fresh finish followed by bath replacement at the threshold. The process control system should alarm when the finish bath refractive index or density deviates from the calibration range; refractive index at 25°C is often used as a rapid estimator of caprolactam concentration because the refractive index of caprolactam solutions increases linearly by approximately 0.0001 to 0.0002 per wt% depending on the base finish composition.

The daily process log for a carpet staple fibre crimping line should record the following paired data to separate finish chemistry effects from mechanical drift: crimper nip roll temperature, crimper motor load, stuffing-box flapper pressure, finish bath temperature, finish bath active concentration, caprolactam concentration in the non-aqueous phase, and the filter pressure differential. When the flapper pressure set point must be adjusted by more than 0.2 bar during a shift to maintain crimp frequency within 4.0 cm⁻¹ to 6.0 cm⁻¹, the finish batch and finish-on-fibre level are extracted immediately. The fibre is sampled at the crimper inlet and after steam setting; the after-setting sample is additionally checked for retained finish-on-fibre and for the presence of white deposits on the fibre surface. The crimp frequency is measured according to ASTM D3937-18, and the crimp recovery is measured after conditioning at 20°C and 65% RH for 24 h. A loss of crimp recovery below 70% after steam setting indicates that the finish film or the steam condition has plasticized the fibre insufficiently or excessively. In such cases, the dew point in the steam tunnel is lowered and the finish active concentration is reduced by 0.1 wt% to 0.3 wt%. If the problem persists, the finish formulation is analysed for caprolactam content by high-performance liquid chromatography; concentrations above 20 wt% in the non-aqueous phase warrant immediate bath replacement.