Evaluating Viscoelastic Relaxation Kinetics and Heat Setting Efficiency in Air Jet Spun Polyurethane Core Woven Fabrics

Nozzle

Air-jet vortex spinning relies on swirling high-pressure compressed air to wrap outer staple fibers around a central elastomeric core. A continuous elastomeric filament enters the hollow spindle through a guide needle placed directly in the vortex zone. When this core filament strays off-center relative to the surrounding fiber stream, the outer staple fibers cannot build a complete helical sheath.

That asymmetry exposes patches of bare elastane, altering how the core and sheath slide against each other during weaving.

Polyurethane core draft ratios in air-jet spinning stay within tight margins, typically 2.8 to 4.2 times original filament length. Pre-drafting the core before assembly establishes the baseline elastic tension retained in the finished yarn. Nozzle air pressure holds the core centered: below 0.35 MPa, injection pressure generates insufficient centripetal force to bind cotton or synthetic staple fibers firmly over the stretched core.

Above 0.52 MPa, turbulence inside the vortex chamber ejects staple fiber through the waste extractor, producing an uneven wrapper layer.

Air-jet core yarn construction differs fundamentally from ring-spun yarn. Ring spinning puts true twist through the full yarn cross-section, locking staple fibers around the elastomeric center in a tight matrix. In vortex spinning, the central core filament remains straight along the axis while twist is confined to the outer wrapper fibers.

Because the sheath secures the core through surface friction instead of full twist integration, the core can suffer localized micro-slippage during weaving stretch. Tensile testing at a constant rate of extension measures the yarn’s initial modulus.

Controlling warp tension on the loom is critical for air-jet core-spun yarns. Since wrapper fibers encase a straight core under high internal strain, slight variations in reed beat-up or warp let-off trigger rapid viscoelastic distortion. Maintaining greige loom tension above 0.25 cN/tex per warp end keeps the polyurethane core continuously stressed inside the shed, driving early stress decay before the fabric reaches wet processing.

If construction specs ignore this initial tension loss, off-loom greige width expands beyond tolerance, distorting finished mass per unit area.

Outer staple fiber selection heavily dictates friction between the sheath and the polyurethane core. A sheath of 100 percent carded cotton relies on natural convolutions to grip the smooth elastomeric core. Smooth regenerated cellulosics such as viscose or modal provide far less friction, allowing the polyurethane core to retract inside the yarn matrix during off-loom relaxation.

This internal retraction drives strip-back at cut ends as bare elastane snaps inward, curling selvages and distorting roll width.

Nozzle air pressure governs wrapper fiber alignment density. Higher pressure increases wrapper turns per unit length, boosting clamping force on the core and checking micro-slippage, though it restricts free stretch in the polyurethane filament. Lower pressure yields a softer hand but leaves the core vulnerable to migration during high-tension wet processing like desizing and scouring.

Technical dossiers for air-jet stretch fabrics must specify fixed nozzle pressures, core draft ratios, and delivery speeds.

Vortex speed and core feed tension combine to create uneven stress distributions across wound packages. Taking up yarn onto paper tubes at delivery speeds near 450 meters per minute imparts residual micro-torsion to the polyurethane core via centrifugal forces in the spinning chamber. Low-tension unwinding during warping then causes snarling.

Managing package density and warehouse climate prevents thermal degradation while greige yarn awaits weaving. How specific nozzle aperture geometries affect long-term micro-slippage in synthetic staple blends wrapping high-dtex cores remains an active question.

Rheology

Polyurethane core filaments store elastic energy under weaving tension while gradually dissipating stress over time. This viscoelastic behavior stems from the contrasting thermodynamics of the polymer’s soft and hard segment domains. The soft segments ~ usually long-chain polyether or polyester diols with molecular weights between 1,000 and 3,000 g/mol ~ are amorphous and flexible at room temperature, giving the yarn its stretch and quick recovery.

The hard segments, created by reacting aromatic diisocyanates like 4,4′-methylene diphenyl diisocyanate with short-chain diol extenders, organize into rigid, hydrogen-bonded crystalline micro-domains that serve as physical crosslinks, preventing permanent plastic flow under load.

Unstretched polyurethane core segments inside relaxed greige fabric lock yarn intersection points before thermal setting can take place.

When an air-jet core-spun fabric is stretched, the immediate elastic response comes from amorphous soft segments uncoiling in the core. Holding that extension triggers stress relaxation, which is commonly modeled using multi-element viscoelastic systems like the four-element Burgers model (a Maxwell element in series with a Kelvin-Voigt element). The initial spring modulus governs immediate elastic behavior, whereas delayed elastic strain and viscous flow reflect hydrogen bonds slowly rearranging inside the hard-segment crystallites under stress.

Stress relaxation kinetics are measured over 86,400 seconds to map long-term decay under standard conditions of 20 degrees Celsius and 65 percent relative humidity. The stress decay function fits a dual-exponential kinetic equation:

sigma(t) = sigma_1 exp(-t / tau_1) + sigma_2 exp(-t / tau_2) + sigma_infinity

where sigma(t) is instantaneous stress at time t, sigma_1 and sigma_2 are relaxation stress magnitudes for short-term soft-segment orientation and long-term hard-segment restructuring, tau_1 and tau_2 are their time constants, and sigma_infinity is residual equilibrium stress. In raw, unfixed core yarns, tau_1 usually falls between 1.2 and 4.8 seconds, while tau_2 ranges from 3,600 to 14,400 seconds depending on the backbone’s soft-to-hard segment ratio.

Higher temperatures accelerate stress relaxation by introducing thermal energy into the chain network. As conditions approach the glass transition temperature of the hard segment phase, hydrogen bonds in those domains rapidly break and reform under strain. This rearrangement drops the instantaneous modulus and hastens the shift from delayed elastic recovery to permanent viscous drift.

If greige fabric is stored under tension on batching rollers in unconditioned warehouses, ambient heat spikes can cause early strain decay in the core, draining stretch power before wet processing even starts.

The outer staple wrap in an air-jet yarn provides mechanical damping that alters the core’s apparent relaxation behavior. Unlike a bare polyurethane filament, which relaxes freely, the core in a core-spun yarn experiences continuous pressure from surrounding staple fibers. When the core tries to contract after extension, wrapper fibers press inward, creating inter-fiber friction that resists recovery.

That friction adds a mechanical hysteresis loop on top of the polymer’s intrinsic molecular hysteresis, so stress relaxes more slowly in the finished fabric than in an isolated bare core.

Moisture actively plasticizes polyether- and polyester-based polyurethanes. Water molecules diffuse into the amorphous phase and hydrogen-bond with carboxyl or ether oxygens, increasing chain mobility and shortening relaxation time constants. In wet operations like scouring or desizing, hot water immersion accelerates stress relaxation by orders of magnitude compared to dry states.

Held under warp tension in an 80 degrees Celsius wet bath, the core can lose up to 45 percent of its initial tension in 300 seconds through moisture-assisted relaxation. Controlling wet bath tension is essential to preserve fabric recovery forces.

Core degradation occurs when physical rearrangement gives way to irreversible chemical bond cleavage. Thermal breakdown accelerates above 180 degrees Celsius as urethane linkages dissociate into original isocyanates and alcohols or form secondary amines. Chemical degradation follows exposure to oxidizers, chlorine, or extreme pH levels in the dye bath: polyester-based cores suffer hydrolytic cleavage of ester linkages in hot acidic or alkaline liquors, while polyether-based cores resist hydrolysis but remain vulnerable to UV photo-oxidation and dry heat breakdown.

A practical rule governs this stage: fabric allowed to relax fully in hot water before heat setting retains higher recovery force than fabric pulled tight while dry.

Thermodynamics

Thermal energy reorganizes the crystalline hard segments of polyurethane to set fabric geometry and relieve stresses locked in during weaving. Heat setting efficiency directly governs dimensional stability, stretch capacity, growth, and finished fabric weight. In stenter treatment, cloth passes through temperatures typically between 180 and 195 degrees Celsius under controlled pin-chain tension and longitudinal overfeed.

The heat breaks weak hydrogen bonds in the hard segments, letting polymer chains realign into a relaxed configuration fitted to the tentered dimensions.

How Does Dwell Time Adjust Polyurethane Crystallite Size?

Dwell time inside the stenter heating zones dictates how completely hard segments melt and recrystallize. If exposure is too short, heat never reaches the inner core of the yarn, leaving residual strain and yielding low heat setting efficiency. If dwell time is too long, thermal degradation sets in ~ causing chain scission, yellowing the staple sheath, and sharply reducing core recovery force.

Optimal processing balances chamber temperature, air velocity, fabric mass, and line speed so heat reaches equilibrium across the full web thickness.

A heat setting temperature of 192 degrees Celsius for 35 seconds yields a setting efficiency of 88.5 percent in 40 dtex polyurethane core wovens.

Evaluating Heat Setting Efficiency (Ehs) involves measuring fabric dimensions before and after boiling water relaxation. The index is defined mathematically as:

E_hs = 100

where L0 is the marked distance on off-loom greige cloth, L1 is the extended distance on the stenter pins, and L2 is the distance measured after standardized boiling relaxation (such as 30 minutes in boiling water without strain). An Ehs of 100 percent represents total thermal stability with zero post-wash shrinkage, while values below 75 percent indicate poor heat fixing that leads to severe distortion after laundering.

Consistent temperature control across stenter chambers is essential for uniform recrystallization. Modern stenters use staggered gas burners or circulating oil loops to keep temperatures within +/- 1.0 degree Celsius across the working width. Cold air leaking at the entry slot or uneven air nozzle delivery creates widthwise gradients in setting efficiency.

If the center of the web receives less heat than the selvages, core fibers in the middle retain higher residual strain ~ producing uneven stretch across the roll so panels cut from the center fit differently than those cut near the edges.

Managing warp overfeed on the stenter is vital when core yarns run in the warp or both directions. Overfeed feeds cloth onto the pin chain faster than the track velocity ~ typically +10 percent to +30 percent faster. This speed difference lets warp threads contract through the heating zones, releasing locked-in weaving strains.

Proper overfeed settings allow the core to relax thermally without puckering the outer wrap, maximizing stretch per unit weight of elastane in the weave.

Excessive setting temperatures degrade both the polyurethane core and the outer staple wrap. Above 195 degrees Celsius, polyether polyurethanes suffer rapid thermo-oxidative breakdown, marked by falling intrinsic viscosity and mass loss in the soft segment blocks. Hard segments maintain thermal stability while soft segments provide elastic return; overheating breaks down both.

At the same time, cellulosic wrapper fibers like cotton or viscose yellow and lose degree of polymerization (DP), compromising tear strength and burst resistance.

Exit cooling zones physically lock in the recrystallized hard-segment morphology. As fabric exits the final heating chamber at 190 degrees Celsius, chilled air blasts or water-cooled steel cylinders rapidly quench it below the hard segment glass transition temperature, fixing the hydrogen-bonded network while the web is still pinned. Pulling cloth off the pins before it cools below 60 degrees Celsius triggers immediate thermal contraction, causing uncontrollable width loss and poor setting efficiency.

Thermal finishing of air-jet stretch wovens requires balancing hot air velocity against pin tension. Nozzle air pressure must stabilize the web on the pins without creating ripples that cause localized temperature swings. Likewise, variable moisture in incoming greige alters effective dwell time, as evaporating water delays heat ramp-up.

Running damp fabric directly into setting chambers without pre-drying causes inconsistent setting efficiency, leading to rejected lots, shade metamerism after dyeing, and high yield losses.

Setting stenter temperatures just five degrees below the hard-segment transition point can cause over 6 percent wash shrinkage in finished garments, driving up returns and re-finishing costs across an entire shipment.

Hysteresis

Energy lost during cyclic extension shows up as delayed recovery and permanent fabric growth. In air-jet core-spun wovens, hysteresis reflects this mechanical loss over repeated stretch and relaxation cycles. When fabric is stretched and released, the unloading stress-strain curve lags behind the loading curve; the area between them represents energy lost as heat within the polymer and dissipated through friction between the core and the staple wrapper.

Compliance with ISO 20932-1 requires fabric growth below 3.5 percent after 30 minutes of recovery following a 30 percent strain cycle.

Testing under ISO 20932-1 or ASTM D3107 measures cyclic hysteresis, recovery, and growth over repeated load cycles. Standard procedures pull fabric strips through 5 or 100 cycles to a target strain (such as 15 or 30 percent) at 500 mm/min, evaluating growth after 1 minute, 30 minutes, and 24 hours of release. High growth values show the core failed to recover its original length, driven by internal plastic flow or structural core slippage inside the yarn.

Air-jet core yarns fail differently under cyclic load than ring-spun core yarns. Because air-jet yarns rely on surface wrapper fibers rather than uniform twist throughout the cross-section, repeated extension causes the core to slide against the central sheath fibers. This sliding gradually unspools wrapper turns and drops local friction.

As wrapper clamping force degrades, the core retracts inside the sheath, causing localized bagging, surface distortion, and early elastane breakage under low working strains.

Structural breakdown in air-jet polyurethane core woven fabrics stems from specific manufacturing and material conditions:

• Inadequate Core Draft Ratio during air-jet spinning allows core filament migration under cyclic extension, accelerating fabric growth and bagging.
• Insufficient Stenter Temperature during heat setting leaves hydrogen bonds in the hard segment phase unstable, causing high permanent set after laundering.
• Excessive Warp Processing Tension on continuous dyeing lines strips wrapper fibers away from the core, exposing bare elastane to surface abrasion.
• Alkaline Hydrolysis Damage incurred during aggressive mercerization or scouring scions ester linkages in polyester-polyurethane cores, destroying recovery force.
• Low Pneumatic Nozzle Pressure during vortex yarn formation produces insufficient wrapper turns, leading to high yarn-level core slippage.

Elastic recovery drops by 14 percent when stenter temperatures cross 195 degrees Celsius. Over-stretching during tentering disrupts soft-segment orientation in the polyurethane core: pulling fabric past its elastic limit forces soft segments into alignment without hard-segment crosslinks to stabilize them. Under cyclic loading, these over-extended soft segments cannot produce the entropic force needed for elastic return, degrading fabric power and leaving garments prone to bagging after short wear periods.

Staple sheath material modifies hysteresis through internal friction. Coarser staple fibers (higher dtex per filament) increase yarn stiffness and resist elastomeric retraction. Cotton sheath fibers feature surface micro-roughness that locks wrapper turns against the core, resulting in lower growth than smooth synthetic sheaths like filament polyester or micro-denier nylon.

Applying silicone softeners reduces inter-fiber friction; while this softens the hand, it lowers sheath clamping force on the core, raising fabric growth and hysteresis losses during wear.

Mechanical fatigue accelerates hysteresis loss over hundreds of wear cycles. Repeated movement causes micro-voids to coalesce inside the polyurethane polymer matrix, creating localized stress points. In air-jet yarns, where wrapper fibers press unevenly against the core, these stress concentrations align with individual wrapper turns.

Under continuous flex fatigue, micro-tears initiate at these sites until the core snaps, forcing severed elastane ends out through the sheath as small white loops on the fabric surface.

Elevated growth is not inevitable with air-jet core yarns; proper nozzle pressure and stenter overfeed settings reliably keep growth below 2.5 percent.

Conversion

Sourcing elastomeric wovens requires balancing loom greige width against stenter thermal shrinkage to land target finished weights. Designing the production route for air-jet core-spun stretch fabrics determines cost, lead time, dimensional stability, and quality compliance. Converting greige cloth into finished goods involves sequential mechanical and thermal wet steps, each altering fabric dimensions, warp tension, and elastomeric strain.

Controlling finished yield per loom hour depends on accurately forecasting total area contraction across the finishing route.

Calculating proper warp overfeed ensures finished goods meet target weight while retaining elastic stretch. The relationship governing overfeed percentage (OFw) on the stenter is expressed by:

OF_w = 100

where Vin is entry roller linear speed in meters per minute and Vout is pin chain track velocity in meters per minute. For example, setting Vin to 45 meters per minute and Vout to 36 meters per minute gives +20 percent warp overfeed. This speed difference lets warp yarns contract thermally and mechanically in the heating zones, lowering residual tension and preventing unexpected lengthwise shrinkage in washing.

Qualifying a wet-processing mill for air-jet elastomeric woven conversion requires verifying key operational controls:

1. Verify that continuous desizing ranges maintain warp tension below 0.10 cN/end using multi-drive tension sensors.
2. Measure stenter temperature distribution across six heating chambers using calibrated wireless thermal dataloggers to confirm variance remains under +/- 1.0 degree Celsius.
3. Check air-jet vortex spinning production logs for consistency in polyurethane core draft ratios across all yarn lots assigned to the order.
4. Evaluate heat setting efficiency on preliminary greige rolls via boiling water relaxation tests before approving full-scale bulk processing.
5. Inspect final roll winding stations to ensure automatic tension-controlled batchers wind finished stretch rolls without inducing storage stretch.

Excessive warp overfeed on the stenter chain causes edge bowing across the central third of wide-width rolls.

Controlling tension across wet processing ranges is essential when handling air-jet core-spun fabrics. High warp tension during continuous open-width scouring or pad-steam dyeing pulls the core into extended strain while hot and wet, accelerating relaxation and causing permanent elasticity loss before heat setting. Finishing plants need low-tension wash boxes with driven guide rolls and load-cell dancer controls to keep warp tension below 0.10 cN/end across desizing, scouring, and dyeing.

Bulk procurement specifications for air-jet polyurethane core wovens should establish clear structural and performance boundaries:

• Greige and Finished Construction Parameters specifying warp/weft yarn counts, spinning type (air-jet MVS), core dtex, ends and picks per centimeter, and finished target width with +/- 1.5 cm tolerance.
• Standardized Test Protocols governing performance metrics, mandating ISO 20932-1 for elastic recovery/growth, ISO 6330 for dimensional stability, ISO 12945 for pilling resistance, and ISO 13934 for strip tensile strength.
• Thermal Heat Setting Envelope defining acceptable stenter setting temperatures (188°C – 192°C), dwell times (30 – 38 seconds), and minimum required Heat Setting Efficiency (Ehs ge 88%).
• Shade and Fastness Tolerances establishing color difference thresholds (DEcmc le 0.8 under illuminants D65, TL84, and A) and fastness ratings for washing, rubbing, and perspiration.
• Commercial Defect Allowance Limits detailing maximum allowable points per 100 square meters under ASTM D5430 Four-Point System, capped at 18 points for top-weight apparel goods.

Sourcing contracts should explicitly bind converters to heat setting efficiency targets and growth caps rather than just weight and width. Agreements lacking clear Ehs requirements leave buyers exposed when mills drop stenter temperatures to boost line speeds ~ producing cloth that hits initial width targets but shrinks badly in garment washing. Incorporating viscoelastic relaxation limits and heat setting efficiency thresholds into master purchasing dossiers ensures technical accountability, protecting yield, garment fit, and commercial performance.

All supply contracts for air-jet spun elastomeric wovens shall incorporate Clause 14.2, which mandates rejection of any bulk dye lot exhibiting fabric growth above 3.0 percent per ISO 20932-1 after 30 minutes recovery, with the converting mill bearing full landed cost replacement liabilities.