In automotive interior thermoforming of needle-punched and carded nonwoven fabrics, the copolyester (CoPET) sheath of a sheath-core bicomponent fiber functions as a controlled-flow bonding phase while the polyethylene terephthalate (PET) core preserves dimensional stability. The bicomponent fiber is typically spun through a sheath-core spinneret after compounding the CoPET on a co-rotating twin-screw extruder with an 25:1–40:1 L/D ratio and a melt filtration pack rated at 25–40 µm. The fiber denier ranges from 2.2 dtex to 6.7 dtex, with sheath-to-core mass ratios of 30:70 to 50:50, and the PET core generally has an intrinsic viscosity of 0.60–0.80 dL/g. The melt plateau is the temperature interval bounded at its lower edge by the completion of crystalline melting in the CoPET sheath and at its upper edge by the onset of PET core softening, thermal-oxidative chain scission, or unacceptable surface degradation. For a typical low-melting CoPET sheath containing isophthalic acid or diethylene glycol comonomer, the differential scanning calorimetry endotherm under a heating rate of 10 K/min in accordance with ISO 11357-3 exhibits a peak melting temperature of 110–150 °C, an onset of 95–125 °C, and a melt completion of 150–170 °C. The thermoforming plateau, however, is not equivalent to the DSC endotherm; it requires the polymer to reach a sufficiently low melt viscosity for flow into mold contours, typically a complex viscosity below 1 × 10⁴ Pa·s at 1 rad/s, while the PET core remains below its softening region to prevent structural collapse. Published data for specific commercial CoPET sheath grades is frequently limited because suppliers disclose only melting range and melt flow index rather than full rheological master curves, but production-scale trials on carded nonwoven lines indicate that a usable plateau of 15–35 °C exists when the sheath melt flow index is between 15 g/10 min and 60 g/10 min at 190 °C/2.16 kg. The width of this plateau governs the practical operating window for multi-zone IR heating, contact plate tooling, and hot-air impingement systems used to form floor carpets, wheel arch liners, trunk side trims, and parcel shelf supports. If the plateau is narrower than ± 5 °C, batch-to-batch variation in fiber denier, sheath-to-core ratio, and regrind content produces localized underfusion or core shrinkage. If the plateau is excessively broad, the sheath may undergo excessive flow and migrate to the part surface, increasing gloss, surface tack, and fogging propensity. Therefore, the plateau limits are not merely a thermodynamic property but a manufacturing boundary influenced by fiber geometry, heating rate, mold temperature, and dwell time.

Generic dry blending of uncolored CoPET sheath fibers with a masterbatch of identical sheath chemistry does not alter the plateau unless the carrier resin melt index differs by more than 10 g/10 min.

What Defines the Lower Melt Plateau Boundary in CoPET Sheath Systems?

The lower boundary is conventionally assigned to the temperature at which the CoPET sheath attains sufficient viscous flow to wet and bond adjacent fibers under the applied thermoforming pressure, typically 0.2–0.8 MPa for decorative nonwoven laminates. This is not the onset of melting because the initial melt fraction at the DSC onset is insufficient to create continuous adhesive necks; rather, flow initiates when approximately 80–90 % of the sheath crystallinity has been converted to an amorphous melt. In practice, the lower limit is identified by hot-stage microscopy and by measuring the peel adhesion of a two-layer nonwoven assembly after a 60 s dwell at controlled platen temperatures. A sheath with 12–18 mol% isophthalic acid content typically displays a lower plateau limit of 115–135 °C, while a sheath modified with 30–45 mol% diethylene glycol can depress the lower limit to 85–105 °C, but at the cost of increased hydrolytic sensitivity and reduced recrystallization resistance. The lower boundary also depends on heating rate: at 40–60 K/min in an IR tunnel, the effective melting shift can increase the lower boundary by 3–8 °C relative to the 10 K/min DSC measurement because of thermal lag through the nonwoven thickness. Therefore, a lower plateau limit derived solely from DSC must be corrected by thickness-dependent heat transfer modeling or instrumented surface temperature measurement. Table 1 summarizes representative DSC-derived plateau boundaries for three CoPET sheath configurations; the values are aggregated from supplier technical data and published DSC profiles and should be revalidated for each lot because catalyst residues, comonomer sequence distribution, and solid-state post-polymerization alter melting behavior.

Rheological Response Across the Melt Plateau Dictates Cavity Fill and Wall Thickness Uniformity

Once the lower plateau boundary is exceeded, the CoPET sheath transitions from a semicrystalline solid to a viscoelastic melt whose shear-thinning behavior controls how the nonwoven preform conforms to the tool cavity. Oscillatory shear rheometry following ISO 6721-10 with a 25 mm parallel-plate geometry and a 1 mm gap shows that the complex viscosity of a typical CoPET sheath falls from 1.5 × 10⁴ Pa·s at 115 °C to 2 × 10³ Pa·s at 145 °C at 1 rad/s. Above the plateau midpoint, the flow activation energy derived from Arrhenius fitting is typically 80–120 kJ/mol, but this value is not constant because the melt contains residual crystallites that act as physical crosslinks until the upper end of the DSC endotherm. Elongational deformation during cavity filling is more severe than simple shear, and CoPET sheaths with narrow molecular weight distributions exhibit strain hardening at Hencky strains above 1.5, which stabilizes the cell walls of low-density acoustic nonwovens but can also produce thickness variation when the strain rate exceeds 10 s^-1 in deep-draw areas. Melt flow index alone, typically measured at 190 °C/2.16 kg according to ISO 1133-1:2022, is insufficient for plateau definition because it reports a single-point viscosity at a shear rate far below thermoforming deformation rates; a sheath with an MFI of 25 g/10 min can still exhibit a tenfold increase in elongational viscosity if its molecular weight distribution contains a high-molecular-weight tail. Production-scale thermoforming trials on hydraulic presses with clamping forces of 300–600 t indicate that cavity pressures between 0.4 MPa and 1.2 MPa are required to consolidate a 600–1,200 g/m² nonwoven preform, but the pressure window is narrower when the sheath plateau is eutectic-like and the viscosity drops by more than two decades over 10 °C. In such systems, a temperature overshoot of 5 °C at the preform surface can cause the sheath to migrate from the core network and accumulate at the part surface, producing visible surface striations and a reduction in peel strength below 15 N/50 mm when tested in accordance with ISO 11339. Consequently, the rheological plateau must be mapped across the entire thermoforming shear-rate range and not inferred from melt flow index alone.

Typically, the most severe process conflicts arise when a multi-zone IR heating tunnel is coupled to a high-cycle automotive interior line running floor carpets at net output speeds of 4–8 m/min. The preform, often a crosslaid needle-punched web of 1,000–1,800 g/m², passes beneath ceramic or quartz IR emitters with peak radiation wavelengths of 2.5–3.5 µm. The surface temperature is measured by pyrometers at the exit of each zone, and the closed-loop control toggles emitter power to hold a setpoint of 160–190 °C at the embossed face. Differential heating is necessary because the backside of a floor carpet is frequently a heavier fiber matrix that must remain below 120 °C to avoid premature bonding and loss of acoustic loft. The CoPET sheath melt plateau imposes a specific constraint: if the lower boundary is 130 °C and the upper boundary is 175 °C, the usable surface setpoint corridor is only 45 °C wide, but thermal gradients across a 12 mm thick preform can consume 30–40 °C of that corridor. On production lines, this results in a characteristic failure pattern where the top surface is fully fused and glossy while the core remains unbonded, detectable as a drop in burst strength below 250 N when tested according to ASTM D3786. Another failure mode is edge tearing during clamping when the preform outside the mold cavity loses its green strength because the sheath has not melted, causing an increase in edge trim scrap above 12 %. To mitigate this, tooling engineers install heated edge clamps held at 100–120 °C and use draft angles of 3–6 ° to reduce the tensile stress on the molten web. The dwell time under pressure is typically 30–60 s for decorative trunk side trims and 45–90 s for thick acoustic wheel arch liners; longer dwells permit the PET core to anneal and increase crystallinity, which raises the part modulus but also increases the risk of core shrinkage when the part is ejected above 85 °C. Batch-to-batch variation in CoPET sheath melt flow index of ± 10 % shifts the lower plateau boundary by 2–4 °C, and regrind concentrations above 25 wt% broaden the melting endotherm by blending sheaths with different thermal histories, which can erase the plateau sharpness and require real-time adjustment of the IR power profile. Published production line case studies for this specific configuration are limited, but equipment manufacturers recommend that the melting peak of the sheath be at least 40 °C below the PET core melting onset to avoid the upper plateau boundary encroaching on the core during radiant heat overshoot.

When Core PET Softening Overlaps the CoPET Plateau Upper Boundary, Part Distortion Ensues

Although the CoPET sheath is designed to melt well below the PET core melting temperature, the upper plateau boundary is not solely defined by the core melting onset. In nonwoven preforms with a sheath-to-core ratio of 50:50 or higher, the optical absorption of the PET core in the near-infrared region is significant, and the core can reach its glass transition temperature of 69–75 °C early in the heating cycle. The upper boundary for dimensional stability is therefore the temperature at which the core begins to shrink and crystallize under the applied tension, typically 160–180 °C for PET homopolymer with an intrinsic viscosity of 0.65–0.80 dL/g measured in 60:40 phenol/tetrachloroethane at 25 °C according to ISO 1628-5. If the CoPET sheath plateau extends to 185 °C because of a high-melting comonomer or a post-polymerized sheath, the upper boundary may overlap the PET core shrinkage onset, causing the formed part to curl, delaminate, or exhibit severe gloss variation. Differential scanning calorimetry of the sheath and core is therefore inadequate as a standalone tool; the upper plateau limit must be experimentally determined by measuring shrinkage of a restrained preform at 150 °C and 175 °C for 15 min in accordance with ISO 11501. A core shrinkage force above 0.02 N/tex at 175 °C is generally incompatible with deep-draw geometries exceeding a draw ratio of 1.5:1. In production, the upper boundary is also influenced by residence time in the heating tunnel. A preform that dwells for 120 s at 170 °C may experience the same core shrinkage as one exposed for 20 s at 190 °C, so the plateau must be expressed in time-temperature space rather than as a single temperature. This is particularly critical for multi-layer laminates with a PE or EVA backing film, because the backing layer can thermally decompose or exude plasticizer above 150 °C, generating odor and fogging defects that are detected by the tropical climate test DIN 75201 or the chamber emission method VDA 278. When the upper plateau boundary is approached, mold release agents based on fatty acid esters can degrade and form deposits on the tool surface, increasing the cleaning frequency from 5,000 cycles to 2,000 cycles or fewer. Thus, the upper melt plateau limit is not merely a material property; it is an operational boundary imposed by the core polymer, the laminate backing, and the tooling surface chemistry.

At ambient relative humidity above 60 %, the CoPET sheath absorbs moisture rapidly, and pre-drying of the needle-punched preform is mandatory to prevent hydrolytic molecular weight loss during the brief period above the lower plateau boundary. Moisture regain for CoPET sheaths with high diethylene glycol content can reach 0.4–0.8 wt% at 23 °C/65 % RH, and even 0.2 wt% is sufficient to reduce the intrinsic viscosity by 0.03–0.06 dL/g during a 30 s dwell at 180 °C. Production-scale thermoforming operations therefore use desiccant dryers or recirculating air ovens to bring the preform moisture content below 0.005 wt% before entering the IR tunnel. The lower plateau boundary is depressed by hydrolysis because chain scission reduces the melt viscosity, causing the sheath to flow at temperatures 5–10 °C below the normal onset, which can produce surface sticking and contamination of the mold face. This effect is particularly severe with DEG-modified CoPET sheaths; the ester linkages adjacent to the ether oxygen are more susceptible to nucleophilic attack, and the resulting oligomer content increases the fogging value measured by DIN 75201. The use of amine-based processing additives or color concentrates is incompatible with CoPET sheaths because aminolysis accelerates chain cleavage at the melt plateau and shifts the apparent upper boundary downward by broadening the melt endotherm. Regrind streams that have absorbed moisture during warehouse storage must be re-dried at 110–120 °C for 4–6 h to achieve a dew point below -30 °C in the hopper. These operational boundaries are not optional at high relative humidity; they prevent lot-to-lot shifts in the plateau that would otherwise require constant tool temperature adjustments.

Fogging, Odor, and Interior Air Quality Compliance Matrix

Automotive interior thermoforming of CoPET sheath nonwovens introduces volatile and semi-volatile degradation products when the melt plateau is exceeded or when dwell time at the upper boundary is prolonged. The main chemical pathways are ester bond hydrolysis, thermo-oxidative chain scission, and thermal decomposition of finish oils and spin finishes. Compliance is verified through a matrix of test methods that connect the processing plateau to cabin air quality. The emanation of condensable volatiles is measured gravimetrically under DIN 75201, with typical acceptance values below 2 mg for a 100 cm² test specimen after 16 h at 100 °C. The headspace emission method VDA 278 quantifies volatile organic compounds and fogging compounds in two thermal desorption steps; automotive OEM specifications often require VOC below 100 µg/g and FOG below 250 µg/g for interior nonwovens. Residual monomers such as ethylene glycol and diethylene glycol are monitored by high-performance liquid chromatography following extraction, with limit values typically below 500 mg/kg for total glycol content. These thresholds impose a direct constraint on the melt plateau: prolonged exposure of the CoPET sheath above its endset temperature increases the concentration of acetaldehyde and formaldehyde, the latter being regulated in cabin air by ISO 16000-6 or regional vehicle cabin air quality standards. The compliance matrix in Table 2 summarizes the test methods most commonly applied to thermoformed CoPET sheath parts; each method must be performed on specimens cut after thermoforming because the thermal history during molding alters the emission profile relative to the raw fiber.

Because the CoPET sheath remains the primary load-transfer phase in a thermoformed nonwoven composite, the lower melt plateau boundary also determines the final part’s acoustic impedance and compressive recovery. Under-heated parts with incomplete sheath melting exhibit higher air permeability, typically above 80 L/m²/s at 100 Pa, which reduces sound absorption in the 500–2,000 Hz frequency band and creates a harsh cabin acoustic signature. Over-heated parts with excessive sheath flow become dense and reflective, with airflow resistance above 1,200 N·s/m³, shifting absorption toward low frequencies but increasing mass and reducing resilience. The service temperature boundary for a CoPET sheath part is governed by the onset of recrystallization and thermomechanical creep of the bonded network. Continuous exposure at 85–95 °C, typical of a parked vehicle in a hot climate, can anneal the amorphous CoPET phase and increase crystallinity, causing a loss of elongation at break below 20 % and a rise in flexural modulus that makes the part more brittle. Cyclic testing between -40 °C and 95 °C, as specified in OEM thermal cycle standards, requires the sheath to maintain adhesion to the PET core without cracking; this is usually achieved when the sheath plateau upper boundary is at least 15 °C above the maximum in-service temperature and when the part is ejection-cooled below 60 °C to prevent post-mold crystallization. Moisture exposure at 40 °C/95 % RH can hydrolyze CoPET sheaths with high diethylene glycol content, reducing the molecular weight and lowering the plateau viscosity over repeated heat cycles. Therefore, melt plateau limits, once defined, must be embedded into the plant control architecture with temperature alarms at the lower and upper boundaries, and the acceptable lot-to-lot variation in sheath melting peak should not exceed ± 5 °C unless a full revalidation of the thermoforming window is performed.