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Polypropylene sheet intended for thermoforming is specified by a narrow combination of melt flow rate, molecular weight distribution, and transient extensional viscosity. A homopolymer with a melt flow rate of 2.0 g/10 min under ISO 1133-1:2022, 230 °C, 2.16 kg may display a Rheotens force of 3 cN to 8 cN at 200 °C; the measured force depends on die swell, strand cooling, draw distance, and wheel acceleration. The molecular weight distribution of the same grade may have a polydispersity index of 4.0 to 7.0 by gel permeation chromatography, and the high-molecular-mass tail is the dominant contributor to extensional viscosity. Sheet extrusion transforms this melt through a flat die with a residence time of 2 min to 6 min at melt temperatures between 220 °C and 250 °C. The subsequent thermoforming process reheats the sheet to 150 °C to 180 °C, where the crystalline domains melt and the material becomes a self-supporting viscoelastic membrane. Melt strength limitations appear at each stage: draw resonance and edge instability during extrusion, sag and non-uniform wall thickness during oven heating, and plug-assist tearing during forming. The practical consequence is that a grade selected for melt flow rate alone frequently fails on a heavy-gauge thermoforming line because its low strain-hardening response cannot resist gravity-driven creep over unsupported spans longer than 300 mm.
For a rectangular sheet supported at its edges, the elastic midspan deflection under self-weight is approximated by δ = 5 q L⁴ / 384 E I. Substituting q = ρ g b h and I = b h³ / 12 gives δ = (5 / 32) ρ g L⁴ / E h². Replacing the modulus term with creep compliance J(t) yields δ(t) = (5 / 32) ρ g L⁴ J(t) / h². At a melt density of 0.75 g/cm³ and a sheet thickness of 4 mm over an unsupported span of 400 mm, the gravitational driving force is severe enough that even a small creep compliance value produces observable sag within 60 s to 120 s in a radiant oven. Because J(t) increases by orders of magnitude as the sheet surface temperature rises from 150 °C to 180 °C, the upper limit of the thermoforming window is determined more by melt strength than by heat transfer. A processing line with a 75 mm single-screw extruder at 34:1 L/D, a melt pump, and a 1,200 mm coat-hanger die may show die-lip melt temperature variation of ±3 °C. A cross-web temperature difference of 5 °C creates local reductions in zero-shear viscosity and melt strength, causing edge sag in the thermoforming oven and thickness variation in the formed part. Rheotens melt strength is measured by extruding a vertical strand through a capillary die of 2 mm diameter and 20 mm length at constant piston speed, then carrying the strand through counter-rotating wheels whose acceleration is typically 12 mm/s². The force signal rises, passes through a plateau, and reaches a break force; for linear PP the break force drops as melt temperature increases from 180 °C to 240 °C with a temperature coefficient of approximately 1.0% to 2.0% per °C.
The onset of sag is not a yield event but a continuous creep process that accelerates as the sheet thins under its own weight. Sheet density at 180 °C remains close to 0.75 g/cm³ for semicrystalline PP; for a 600 mm span the central deflection can exceed 30 mm within 90 s for a linear homopolymer with a Rheotens force below 5 cN. Equipment suppliers address this by specifying sheet support rails, adjustable sag sensors, and multi-zone infrared heaters that modulate heat flux between 12 kW/m² and 30 kW/m². The limitation remains that these mechanical countermeasures do not alter the intrinsic extensional viscosity of the polymer. When sag sensors detect a deflection setpoint of 20 mm, the oven controller may reduce heat flux, but the core of a 5 mm sheet may not yet have reached forming temperature. This trade-off between surface degradation and core heat-up is a direct consequence of melt strength limitations: the sheet cannot withstand the longer dwell time required for through-thickness heating.
In high-shear single-screw extrusion of post-consumer PP regrind, the screw design itself becomes a source of melt strength loss. A 90 mm barrier screw with a 34:1 L/D ratio processing 100% recycled flake at 450 kg/h can generate melt temperatures 12 °C to 18 °C above setpoint at the screw tip because of viscous dissipation in the melt channel. The residence time distribution near the mixing elements may exceed 5 min. Under these conditions, thermo-oxidative chain scission increases the melt flow rate from 2.0 g/10 min to 4.5 g/10 min after two heat histories in published data for re-extruded polypropylene sheet. The Rheotens force measured at 200 °C falls by 35% to 50% compared with the virgin feedstock. Surface moisture on regrind at relative humidity above 60% can be removed by drying at 80 °C for 2 h; this does not restore melt strength but prevents splay. The degradation path is sometimes misdiagnosed as moisture-related splay or screw wear; the more reliable indicators are a decreasing oxidation induction time under ASTM D3895 and an increasing melt pressure fluctuation at the die. A screw with intensive mixing elements increases shear heating but may be necessary to homogenize regrind; the processing compromise is a narrow melt-temperature window between 215 °C and 245 °C where dispersion is adequate without catastrophic chain scission.
Polypropylene sheet sag in a thermoforming oven is driven by the transition from a semi-crystalline solid to a low-modulus viscoelastic melt. At room temperature, the flexural modulus of a sheet-grade PP homopolymer is typically 1,200 MPa to 1,600 MPa when measured according to ISO 178:2019. When the sheet reaches 160 °C to 165 °C, the α-crystal phase melts and the storage modulus declines by more than two orders of magnitude. Infrared heating with ceramic or quartz panels establishes a surface-to-core temperature gradient; for a 3 mm sheet, a non-contact IR pyrometer with emissivity set to 0.92 may read 170 °C on the surface while the core remains at 140 °C to 150 °C for the first 30 s to 45 s. The heater control responds to surface temperature, but the sag driver is the average through-thickness compliance. A sheet thickness greater than 2 mm therefore places the oven operator in a contradictory position: increase heat flux to shorten cycle time and the surface may exceed 180 °C and begin oxidizing before the core softens; reduce heat flux and the sheet sags under its own mass for a longer period. Melt strength is the only material property that can extend the tolerable dwell time without sag failure.
The deflection limit before forming is often set between 15 mm and 25 mm over a 500 mm unsupported span. Linear PP homopolymers and random copolymers with low molecular weight and narrow molecular weight distribution exhibit very little strain hardening under uniaxial extension; their elongational viscosity decreases or remains constant as Hencky strain increases from 1.0 to 2.5. The absence of strain hardening means that any local thickness reduction in the sheet is amplified: the thinner region has lower resistance to further thinning, producing a runaway sag profile and non-uniform wall thickness in the formed part. This is the reason why high-melt-strength polypropylene grades with long-chain branching are preferred for heavy-gauge thermoforming despite their higher cost and slightly lower clarity.
Thick-gauge PP sheet above 6 mm is used in large thermoformed panels and industrial containers where the unsupported oven span can reach 800 mm to 1,000 mm. The self-weight per unit area scales with thickness, but the flexural stiffness scales with thickness cubed; therefore the elastic deflection is reduced by a factor of h² at constant modulus. That relationship is misleading during oven heating because the sheet is not elastic and the heating time increases with thickness. A 6 mm sheet requires a lower heat flux of 12 kW/m² to 18 kW/m² and a dwell time of 240 s to 360 s before the core reaches 150 °C; under these conditions creep compliance dominates and the advantage of thicker cross-section is consumed by the prolonged time-dependent deformation. Melt temperature measured at the die lip must remain below 245 °C to avoid thermo-oxidative degradation, but it must exceed 215 °C to prevent melt fracture and die freeze lines. The processing window becomes ±5 °C or less on a line with poor temperature control. High-melt-strength cap layers coextruded at 5% to 10% of total sheet thickness provide a sag-resistant skin while the core may contain regrind or filler. The cap layer thickness must be controlled within ±0.5% of total thickness across the die width; otherwise the sheet bows toward the side with lower melt strength during heating.
Published data for high-melt-strength polypropylene produced by reactive extrusion of a 2.0 g/10 min homopolymer with peroxides and multifunctional acrylate monomers indicate that long-chain branching can raise the Rheotens force from 4 cN to 24 cN at 200 °C and introduce strain-hardening behavior in extensional flow. The reaction is kinetically controlled by initiator half-life, monomer diffusion, and melt residence time; in a typical twin-screw reactive extrusion configuration with an L/D ratio of 40:1 and melt temperatures of 180 °C to 220 °C, the competing β-scission pathway can degrade the backbone if the peroxide concentration is too high. Residual acrylate or its reaction products can affect organoleptic properties, and food-contact sheet must be checked against FDA 21 CFR 177.1520 and European Regulation (EU) No 10/2011. Published data for this specific configuration is limited at production scale because branching efficiency depends on screw design, monomer dosing, and devolatilization capacity. The practical limitation is that high gel content above 100 ppm creates visible defects in clear thermoformed sheet, so the branching reaction must be conducted below the gel threshold and with appropriate scavenger packages.
The Sentmanat extensional fixture, mounted on a rotational rheometer with a controlled-environment chamber, is used to measure transient uniaxial extensional viscosity of polypropylene sheet at strain rates from 0.1 s⁻¹ to 10 s⁻¹. Rectangular specimens are typically 10 mm wide, 12.7 mm gauge length, and 0.7 mm thick, compression-molded from sheet compound and annealed. The test applies a constant Hencky strain rate by winding the ends of the specimen onto counter-rotating drums. For linear PP homopolymer, the transient extensional viscosity at 180 °C at 1 s⁻¹ may show no upward departure from the linear viscoelastic envelope; the Trouton ratio remains near 3 at small strains and falls below 3 at large strains because of extensional thinning. For long-chain branched PP, the extensional viscosity rises above the linear viscoelastic prediction at Hencky strains of 1.5 to 2.0, and the strain-hardening index may range from 1.5 to 3.0. The Rheotens melt strength test correlates with this strain-hardening response but cannot separate rheological hardening from strand cooling and die swell. A capillary rheometer operating at 230 °C according to ISO 11443:2021 can screen shear viscosity stability, but it does not predict sag performance; extensional testing and oven sag trials are required.
Extensional viscosity data also support numerical simulation of thermoforming. Sheet thinning predictions are sensitive to the selected constitutive model and the strain-hardening parameters. When a linear PP melt is used, the simulation predicts excessive thinning in corners and plug-assist contact zones because the local strain-rate hardening is insufficient. When a high-melt-strength PP grade is used, the simulated wall thickness distribution becomes more uniform, but the model must include viscoelastic dissipation and contact heat transfer. The limitation in production practice is that extensional test data collected at 0.1 s⁻¹ to 1 s⁻¹ may not represent the local deformation rates in plug-assisted forming, which can exceed 10 s⁻¹ in the plug contact area. Melt strength measured by Rheotens at wheel acceleration of 12 mm/s² therefore remains a necessary but incomplete quality-control parameter.
Representative rheological and sag data for four polypropylene sheet materials. Values are compiled from published capillary rheometry, Rheotens, and extensional fixture studies; they are not a product specification.
| Material configuration | MFR (ISO 1133-1:2022, 230 °C, 2.16 kg) | Rheotens force at 200 °C | Strain-hardening index at 1 s⁻¹ | Sag deflection at 180 °C, 500 mm span, 120 s |
|---|---|---|---|---|
| Homopolymer PP, nucleation/clarifier only | 2.0 g/10 min | 4.2 cN | 1.0 | 48 mm |
| Random copolymer PP | 2.5 g/10 min | 3.1 cN | 0.9 | 57 mm |
| High-melt-strength long-chain branched PP | 2.0 g/10 min | 24 cN | 2.6 | 14 mm |
| Homopolymer PP with 20 wt% talc | 2.0 g/10 min | 6.8 cN | 1.2 | 22 mm |
Talc-filled polypropylene sheet at 20 wt% talc reduces oven sag because the platelet reinforcement increases the low-frequency storage modulus and reduces creep compliance. The same filler, however, reduces melt extensibility and lowers notched impact resistance at 23 °C when measured according to ISO 180:2019. In thin-walled thermoformed parts, the presence of talc raises the risk of pinholes at corner radii below 2 mm and reduces the process window for plug speed. A sheet containing 20 wt% talc may tolerate a maximum draw ratio of 3:1 before local tearing occurs, while an unfilled high-melt-strength PP sheet may be drawn to 5:1 in the same tool under clinical test conditions. The selection of filler depends on whether the formed part requires stiffness at elevated service temperature or resistance to sag during heating; these objectives are not equivalent.
Thin-gauge PP sheet for food packaging is extruded at thicknesses between 0.25 mm and 0.75 mm and is often coextruded with a random copolymer skin on a homopolymer core. The die gap is typically 0.4 mm to 0.8 mm, and the draw ratio from die gap to polished roll sheet thickness can be 4:1 to 8:1. Under these conditions, the melt undergoes strong extension between the die exit and the chill roll. Linear PP with a melt strength below 5 cN at 200 °C may exhibit draw resonance, visible thickness bands, and edge tear at line speeds above 20 m/min. High-melt-strength PP added at 5 wt% to 15 wt% in the core layer raises the extensional viscosity and permits stable drawing at thinner gauges, but it can increase die pressure and reduce throughput if the melt flow rate is not adjusted. The melt pump and die zone controllers must hold melt temperature within ±3 °C; a deviation of 3 °C across the die width is sufficient to produce visible gauge variation in thin sheet because the zero-shear viscosity of PP has an Arrhenius-like temperature dependence with an activation energy of 35 kJ/mol to 45 kJ/mol.
The processing window in thin-gauge coextrusion is further limited by the thermal stability of the skin layer. Random copolymer PP with a lower melting point may degrade at the high melt temperatures required by the homopolymer core. Conversely, lowering melt temperature to protect the skin can cause melt fracture in the core at die shear rates above 500 s⁻¹. Coextruded sheet lines therefore use separate extruders for the skin and core, with skin melt temperature 215 °C to 230 °C and core melt temperature 230 °C to 245 °C. The high-melt-strength component is preferentially placed in the core or in both layers; if it is placed only in the skin, the sheet may still sag because the skin represents a small fraction of the total section. This is a common failure mode on production lines that introduce a high-melt-strength skin on a low-melt-strength core and then thermoform heavy-gauge parts.
Oxidative induction time under ASTM D3895 or ISO 11357-6:2018 is a direct measure of the remaining active stabilizer package in polypropylene sheet when exposed to oxygen at 200 °C. A virgin sheet-grade PP with a hindered phenolic primary antioxidant and a phosphite secondary antioxidant typically shows an OIT of 10 min to 20 min. After 100% post-industrial regrind is processed through a sheet line, the OIT may fall to 4 min to 8 min depending on the number of heat histories and the level of acidic contamination. A minimum OIT of 5 min is often used as an internal control for automotive interior sheet to ensure that the material can survive additional heat histories during troubleshooting. Below 5 min, the melt strength during extrusion becomes unstable because the molecular weight distribution is changing during the process. This is measurable as a drift in melt pressure at the die of more than 1 bar/min on a 75 mm extruder running at 350 kg/h. When OIT falls below 3 min, sheet surfaces may show yellowing or black specks from gel formation; these defects are not reversible by drying and indicate advanced stabilizer depletion.
Stabilizer selection must be compatible with any reactive branching step used to improve melt strength. Hindered amine light stabilizers are usually avoided during peroxide-initiated branching because they scavenge free radicals and reduce branching efficiency. Acid scavengers such as calcium stearate are required to neutralize chlorine residues from Ziegler-Natta catalysts, but overdosing above 1,000 ppm can interact with phenolic antioxidants and reduce long-term thermal stability. Processors that blend high-melt-strength PP masterbatch with regrind should verify that the antioxidant packages in the masterbatch and the regrind are sufficient for the combined heat history; otherwise the melt strength gains from long-chain branching are lost to chain scission during the second pass through the sheet extruder.
Compliance checklist matrix for polypropylene sheet extrusion and thermoforming control parameters.
| Property | Standard or method | Typical sheet extrusion control range | Equipment type |
|---|---|---|---|
| Melt flow rate | ISO 1133-1:2022, 230 °C, 2.16 kg | 1.0–3.5 g/10 min for heavy-gauge thermoforming | Dead-weight plastometer |
| Melt strength | Rheotens, 200 °C | ≥ 6 cN for 3 mm sheet over 600 mm span | Goettfert Rheotens |
| Oxidative induction time | ASTM D3895, 200 °C oxygen | ≥ 12 min virgin, ≥ 5 min after 100% regrind | DSC with 50 mL/min purge |
| Tensile yield stress | ASTM D638-14, 50 mm/min | ≥ 28 MPa at 23 °C | Universal testing machine |
| Notched Izod impact | ISO 180:2019, 23 °C | ≥ 4 kJ/m² for formed parts | Pendulum impact tester |
| Sheet sag under IR | Internal test | ≤ 25 mm after 180 s at 180 °C over 500 mm span | IR panel oven with sag sensor |
