Partially oriented yarn (POY) destined for air-jet texturing is characterized by an elongation at break that is measured under constant rate of extension according to ISO 2062:2009 using a 500 mm gauge length and 500 mm/min test speed after conditioning at 20 °C ± 2 °C and 65 % ± 4 % relative humidity per ISO 139:2005. For polyester POY spun at take-up speeds between 2800 m/min and 3400 m/min, published mill data and supplier technical bulletins typically report breaking elongation values between 110 % and 140 %, while polyamide 6 POY from high-speed spinning at 3800 m/min to 4500 m/min frequently records values from 60 % to 90 %. These elongation windows are not specification targets in isolation; they reflect the thermomechanical history imposed by spin-line stress, quench air velocity, spin pack filtration, and winding tension. In air-jet texturing, the POY is not drawn through a heated draw zone; instead, filament loops are generated by overfeeding the yarn into a high-velocity air stream inside a fixed-gap jet such as the Heberlein HemaJet series, where a turbulent shock front produces entanglements and loop formation. Consequently, POY elongation exerts a direct influence on the energy required to separate filaments, the bending strain during loop collapse, and the probability of filament rupture when the yarn is subjected to instantaneous velocity gradients. If the breaking elongation is too low, edge filaments fracture and form broken filaments that wrap on the jet baffle; if the elongation is too high, the loop structure lacks dimensional stability and the resultant air-jet textured yarn exhibits excessive boiler shrinkage or low package hardness. The relationship between POY elongation and air-jet texturing efficiency therefore cannot be reduced to a single scalar value; it must be interpreted alongside spin finish type, filament denier per filament, jet air pressure, and overfeed ratio.

What Process Window Governs POY Elongation Before Air-Jet Texturing?

The process window is bounded by spin-line stress and the resulting amorphous orientation. In high-speed spinning of polyester, an increase in take-up speed from 3000 m/min to 3400 m/min increases stress-induced orientation and lowers the measured elongation at break by approximately 10 to 20 percentage points, depending on polymer intrinsic viscosity and quench configuration. The corresponding orientation changes are expressed as birefringence values that may rise from 0.02 to 0.06 while the density remains below 1.37 g/cm³ for POY. For air-jet texturing, a PET POY with an elongation below 100 % is generally associated with higher filament break rates due to the bending and torsional strains in the jet; a POY above 140 % is associated with loop instability because the oriented amorphous network is insufficient to retain loop geometry after the yarn exits the jet. These boundaries are equipment-dependent. Production-scale texturing frames fitted with fixed-gap jets and air pressures of 0.7 MPa to 1.0 MPa may tolerate a narrower elongation band than laboratory systems operating at 0.5 MPa. Thus the operative process window is defined by the intersection of POY elongation, jet air pressure, and overfeed ratio. For a typical 167 dtex / 48 filament polyester POY, industrial settings often use overfeed ratios from 12 % to 22 %; for polyamide 6 POY of 78 dtex / 24 filament, overfeed ratios may be set between 8 % and 15 % because the lower elongation and higher moisture sensitivity of polyamide change the loop formation dynamics. Published data for specific air-jet texturing configurations is limited; equipment suppliers commonly furnish jet-specific starting parameters, but the interaction between elongation and loop geometry is not captured by a single standard.

Spin finish is applied to POY before winding at take-up, and the after-winding oil-on-yarn value for polyester POY typically falls between 0.30 wt% and 0.50 wt%, while polyamide 6 POY is frequently finished to 0.60 wt% to 0.90 wt% to offset higher static charge generation and moisture uptake. The finish is an emulsion or neat liquid metered onto the yarn through a kiss roll or metering pump; the active concentration and emulsion particle size determine whether the finish forms a uniform film or discontinuous droplets on the filament surface. For air-jet texturing, the finish film must provide sufficient filament-to-metal lubrication to reduce guide and jet-body friction, while preserving enough filament-to-filament cohesion to prevent the filament bundle from opening prematurely before the jet. A typical finish formulation contains a synthetic ester or polyoxyethylene fatty acid ester as the lubricant component, an ethoxylated C12–C14 alcohol as the emulsifier, an alkyl phosphate potassium salt as the antistat, and a low-molecular-weight polymeric cohesion agent to control yarn bundle integrity. The viscosity of the neat finish at 25 °C is often specified between 20 mPa·s and 50 mPa·s using a rotational viscometer per ISO 2555:2018; diluted emulsions may have viscosities below 5 mPa·s. When the oil-on-yarn exceeds 0.60 wt% on PET POY, finish transfer to guides and jet surfaces increases, and the resulting residue can reduce entangling efficiency. When the oil-on-yarn is below 0.25 wt%, static charging and filament breakage become process bottlenecks. These values are not universal specifications but are derived from production-scale finishing trials on POY spinning machines and air-jet texturing frames.

When Spin Finish Deposits Shift Entangling Performance in Air-Jet Texturing

Deposit formation in an air-jet texturing nozzle is governed by the shear stress at the filament finish interface and the thermal stability of the finish film. The finish is exposed to transient air velocities that can exceed 300 m/s in a narrow gap, which strips weakly adsorbed emulsifier and low-viscosity fractions from the yarn surface. The stripped material accumulates on the jet body, needle, and baffle surfaces, where it is subjected to repeated compression and frictional heating. As the deposit layer grows, the effective jet gap narrows and the differential air pressure across the jet increases for a given supply pressure. Texturing frames instrumented with upstream and downstream pressure transducers frequently record a pressure rise of 0.1 bar to 0.3 bar before yarn quality deteriorates; the cleaning interval is commonly set by this pressure differential rather than by a fixed time. Finishes containing unsaturated fatty acid esters with iodine values above 10 g I₂/100 g are especially prone to oxidative varnish formation on jet surfaces. The addition of antioxidant packages based on hindered phenolic compounds can increase the oxidative induction time, but the antioxidants themselves may leave residues if their molecular weight is too low. Mineral-oil-based finishes generally offer good lubrication but can be difficult to emulsify and may generate smoke at elevated jet temperatures. Synthetic ester finishes provide better thermal stability but poorer static dissipation unless combined with an effective antistat. The coefficient of friction between yarn and a polished chromium-oxide-coated guide, measured per ASTM D3108-13, is frequently maintained between 0.20 and 0.35 for POY finishes intended for air-jet texturing; values below 0.15 indicate insufficient filament-to-filament cohesion, while values above 0.45 can cause filament damage at high-speed guides.

The measurement of POY elongation itself is sensitive to conditioning history and instrument settings. A polyester POY specimen tested at 500 mm gauge length under 0.5 cN/tex pre-tension will generally report a slightly higher elongation than a specimen tested at 100 mm gauge length because the longer gauge integrates more weak points along the filament. Polyamide 6 POY conditioned at 65 % RH absorbs approximately 4.0 wt% to 4.5 wt% moisture, increasing elongation at break by 2 to 5 percentage points compared with the dry state. The practical consequence is that a PA6 POY doffed at 20 °C and 45 % RH may pass an incoming inspection criterion based on 70 % minimum elongation, but the same package reconditioned at 65 % RH may exceed 75 % and produce a noticeably softer air-jet textured yarn loop. Therefore, air-jet texturing plants that convert both PET and PA6 POY should not apply the same elongation specification to both polymers; each material requires a separate receiving specification that accounts for moisture regain, spin finish type, and package storage time. The conditioning protocol per ISO 139:2005 should be enforced before both tensile and finish extraction testing. For finish extraction, n-hexane or petroleum ether is commonly used in a Soxhlet apparatus, and the oil-on-yarn is calculated as the mass of extracted residue divided by the conditioned yarn mass multiplied by 100.

Compatibility with jet materials adds another constraint. Many air-jet texturing jets use hardened stainless steel bodies, ceramic needles, and polyurethane baffles. Finish components that hydrolyze to acidic species can corrode chromium-containing surfaces, while polymeric cohesion agents may swell polyurethane baffles and alter the shock geometry. The surface resistivity of the finish film, measured as a dried film on a metal coupon according to ASTM D257-14, should be maintained between 1 × 10^8 Ω/sq and 1 × 10^10 Ω/sq for reliable static dissipation without excessive moisture sensitivity. Formulations that depend on high-humidity activation of antistatic components may fail in winter conditions when relative humidity falls below 35 %. Conversely, polyamide POY with finish levels above 0.90 wt% can develop visible oil streaking on package edges and promote sloughing during unwinding. A compatible finish system must therefore be evaluated not only by bench-scale tensile and friction tests but also by a controlled production run on the specific air-jet texturing frame, because jet gap, air pressure, and baffle geometry amplify small differences in finish rheology.

Thermal Degradation Pathways in POY Finish Films at Air-Jet Nozzle Surfaces

Thermogravimetric analysis of POY spin finishes according to ISO 11358-1:2022 under nitrogen at a heating rate of 10 °C/min reveals that many aliphatic synthetic ester lubricants undergo 5 % mass loss between 180 °C and 220 °C. Although bulk air temperatures in unheated air-jet texturing are typically below 50 °C, the local temperature at the yarn-to-metal contact line inside a venturi can be substantially higher due to frictional dissipation. The degradation products include low-molecular-weight aldehydes, carboxylic acids, and unsaturated species that can recombine into high-viscosity varnish. Finishes formulated with unsaturated fatty acid derivatives exhibit earlier mass-loss onsets and are generally not recommended for high-speed air-jet texturing. Polyethylene glycol monostearate components may volatilize at elevated temperatures and leave a sticky film on the jet baffle. In contrast, trimethylolpropane trioleate and similar hindered esters have higher decomposition onsets, but they may not provide adequate antistatic performance unless the formulation includes an alkyl phosphate or quaternary ammonium antistat. The selection of finish for air-jet texturing therefore requires a simultaneous evaluation of lubricity, static dissipation, emulsifiability, and deposit tendency. Thermogravimetric data alone cannot predict deposit formation because tribochemical reactions and metal-catalyzed oxidation at the chromium or ceramic surface alter the degradation pathway. A finish with acceptable thermal stability may still deposit if it contains non-volatile emulsifier fractions that are not removed by the air stream. Conversely, a finish with moderate thermal stability may perform acceptably if the oil-on-yarn is kept below 0.40 wt%.

On a production air-jet texturing frame, the practical operating limit is detected through differential pressure rise rather than through laboratory finish values alone. A jet fitted with a maintenance pressure gauge upstream of the air supply and another in the texturing chamber will show a pressure drop that increases as deposit films accumulate. When the differential pressure at a constant supply pressure of 0.8 MPa rises by 0.2 bar, the cleaning interval is reached. Attempting to compensate for deposit-induced pressure loss by increasing supply pressure may temporarily restore loop formation but accelerates finish stripping and can increase filament breakage. The acceptable oil-on-yarn range for a specific POY product is therefore not a single number; it is a function of the finish formulation, the jet geometry, the air pressure, and the pressure-rise criterion. A finish that performs acceptably at 0.45 wt% on a 167 dtex / 48 filament polyester POY may be unsuitable at 0.40 wt% on a 78 dtex / 24 filament polyamide POY running through the same jet because the surface area and static propensity differ. Field experience on texturing frames with automatic jet cleaning systems indicates that deposit-related yarn breaks can be minimized by selecting a finish with a narrow molecular-weight distribution and by storing POY packages at 20 °C and 55 % RH or below. The operational boundary is further narrowed by the incompatibility of amine-based additives with polyester hydrolysis products; finishes containing polyalkylene glycol amines can react under humid storage conditions to form colored, insoluble residues that are difficult to remove from jet surfaces. Therefore, air-jet texturing trials on new POY lots should include a short-duration deposit test using a dedicated jet body followed by gravimetric residue measurement after a fixed number of kilograms.