Melt Strength Limits for Draw Resonance in DFDA7042 Dominated Cast Stretch Film

Cast stretch film lines running DFDA7042-heavy formulations are constrained by the intrinsic rheology of the butene linear low-density polyethylene. DFDA7042 is characterized by a nominal density of 0.918 g/cm³ when tested in accordance with ISO 1183-1 and a melt mass-flow rate of 2.0 g/10 min at 190 °C under a 2.16 kg load per ISO 1133-1:2022. The grade is produced via low-pressure ethylene copolymerization with 1-butene, and the resulting molecules are highly linear with short-chain branches but no analytically significant long-chain branching. On a 2.5 m wide cast film line equipped with a 90 mm single-screw extruder with a 30:1 L/D barrier screw, a coat-hanger die with a 1.5 mm die gap, and a chill roll maintained at 18 °C to 22 °C, the melt web is stretched from the die exit to the chill roll under nonisothermal extensional flow. When the formulation contains more than 70 wt% DFDA7042, the low melt strength and weak strain-hardening character restrict the stable draw ratio and make draw resonance the dominant process defect. Draw resonance appears as sinusoidal machine-direction thickness variation with a period of 0.5 m to 2.0 m on the roll surface and an oscillation frequency of 0.8 Hz to 2.5 Hz at line speeds above 150 m/min. The resulting film exhibits uneven stretch force, poor roll conformation, and periodic streaks after slitting.

Molecular architecture underlies the draw resonance limitation. DFDA7042 has a linear backbone with a polydispersity index reported in typical datasheets as 3.0 to 4.5; high-pressure LDPE, by comparison, has long-chain branches and a broader polydispersity index of 5.0 to 10.0. In shear rheology, the two materials differ mainly in the lower frequency region, where LDPE exhibits a higher elastic storage modulus and a lower phase angle at 0.1 rad/s. In extension, the difference is more significant: LDPE displays strain hardening with a transient extensional viscosity that can exceed the linear viscoelastic envelope by a factor of 2 to 5 at Hencky strains above 2.0, while neat DFDA7042 shows strain thinning or only weak hardening. This molecular difference means that replacing even a small fraction of DFDA7042 with LDPE can delay draw resonance without changing the MFR dramatically. Shear viscosity measurements of DFDA7042 at 190 °C in accordance with ASTM D3835 typically give a shear viscosity of 350 Pa·s to 450 Pa·s at a shear rate of 100 s⁻¹ and 150 Pa·s to 200 Pa·s at 500 s⁻¹. The power-law index is between 0.55 and 0.70 over the shear-rate range 10 s⁻¹ to 1,000 s⁻¹. These viscosities are low enough for high-speed extrusion but do not provide high melt strength.

What Draw Ratio Stability Envelope Is Achievable with DFDA7042-Rich Melts?

Under isothermal laboratory film-casting conditions, linear polyethylenes with a melt index near 2.0 g/10 min exhibit draw resonance at relatively low draw ratios because the extensional viscosity does not increase sufficiently with strain. Published studies on butene LLDPE grades of density 0.918 g/cm³ and melt index 2.0 g/10 min report critical draw ratios below 20:1 when no quench or pinning is applied. On industrial cast film lines with an air gap of 100 mm to 150 mm and an air knife pressure of 3 kPa to 8 kPa, the stable draw ratio can be extended, but the exact upper bound depends on lot-to-lot variation in melt strength, die temperature uniformity, and the ratio of die gap to final film thickness. For a 20 µm final film cast through a 1.5 mm die gap, the geometric draw ratio approaches 75:1 before neck-in correction. Published data for DFDA7042 specifically is limited; the ranges cited derive from resins of equivalent comonomer type, density, and MFR. Plant logs from lines with 2.5 m dies indicate that neat DFDA7042 cannot be cast at draw ratios above approximately 18:1 without either vacuum-box assistance or periodic gauge variation exceeding ±10%. The stability envelope is therefore narrow: the lower melt strength limit is set by draw resonance onset, and the upper limit is set by die pressure and neck-in control.

Rheotens melt strength measurements for DFDA7042-rich systems provide a practical comparative value for extensional flow resistance. In a typical test run according to ISO 16790:2021, the polymer is extruded through a capillary die with a diameter of 2.0 mm and a length-to-diameter ratio of 40:1 at a melt temperature of 190 °C, and the extrudate is drawn by accelerating wheels. For neat DFDA7042, the recorded melt strength commonly falls between 2.5 cN and 4.0 cN, and the draw ratio at break is often below 10:1 in the Rheotens test. The force-displacement curve shows no pronounced strain-hardening plateau; instead, the drawing force rises to a maximum and then decays as the filament necks. This response is consistent with a linear polyethylene that lacks long-chain branching. In contrast, a high-pressure LDPE with a melt index of 0.25 g/10 min to 0.8 g/10 min exhibits a Rheotens melt strength of 10 cN to 25 cN under identical conditions and a stable extension before break. The extensional rheology of DFDA7042 is therefore the principal constraint on cast film stability, and melt strength data are used to set the minimum LDPE addition level in commercial formulations.

Rheotens Extensional Response and Neck-In Control Limits

The acceptable melt strength window for DFDA7042-dominated cast stretch film is bounded on the lower side by draw resonance and on the upper side by neck-in, die pressure, and edge gauge uniformity. A minimum Rheotens melt strength of 4.5 cN to 6.0 cN at 190 °C is generally required to stabilize machine-direction tension at a draw ratio of 30:1 on a 2.5 m line with an air gap of 120 mm. Below this range, the web can develop resonant thickness fluctuations even with a vacuum box operating at 2.0 kPa to 4.0 kPa. The upper bound is reached when the blend melt strength exceeds approximately 9.0 cN: at this level, a 90 mm extruder running at 100 rpm may record a die pressure increase of more than 8%, the film width after neck-in decreases by more than 15%, and the edge bead thickness can exceed the mean film thickness by 60%. Such films may remain free of draw resonance but fail optical clarity and roll-profile specifications. The practical melt strength envelope for DFDA7042-dominated formulations therefore lies between 4.5 cN and 9.0 cN at 190 °C, with the lower bound set by draw resonance onset and the upper bound set by width loss and edge gauge uniformity. The corresponding shear viscosity ratio, expressed as the complex viscosity at 0.1 rad/s divided by that at 100 rad/s, is typically 3.0 to 5.0 for these blends.

Table 1. Representative ranges for 20 µm cast film on a 2.5 m line with a 1.5 mm die gap and 120 mm air gap. Values are assembled from open literature on comparable butene LLDPE/LDPE blends; published DFDA7042-specific datasets are limited.

Draw resonance in film casting is often represented in terms of a Deborah number that compares the polymer relaxation time with the residence time of the melt in the air gap. Linear butene LLDPE has a relatively short characteristic relaxation time but weak strain hardening, so the critical Deborah number for draw resonance onset is lower than for high-pressure LDPE. In practical terms, this means that when the air gap residence time is fixed by line speed and air gap height, the draw ratio at onset is limited by the melt strength and the relaxation spectrum. The addition of 10 wt% to 30 wt% LDPE shifts the stability limit by increasing strain hardening rather than by changing shear viscosity; the result is a more uniform thickness distribution across the die exit and a reduction in the amplitude of gauge variations after the chill roll. However, the laboratory critical draw ratio values in Table 1 should not be directly compared with the much higher geometric draw ratios used on industrial cast film lines because industrial cooling and pinning alter the stability boundary significantly.

When Melt Temperature and Air Gap Deviate from the Center Point

The draw resonance limit in DFDA7042-dominated cast film is sensitive to melt temperature because the extensional stress-bearing capability of the melt decreases with increasing temperature. Die exit temperature is typically maintained between 245 °C and 255 °C; a deviation of ±5 °C from this band is sufficient to shift the onset of draw resonance. At 260 °C, the zero-shear viscosity of a 2.0 g/10 min butene LLDPE decreases by approximately 12% to 15% relative to 250 °C, and the Rheotens melt strength falls below 3.0 cN. The same formulation may cast stably at 245 °C but develop draw resonance at 255 °C when the film is stretched to a draw ratio above 25:1. At 235 °C, the melt pressure at the die lip rises and surface melt fracture may appear as sharkskin on the film edges; the increased melt stress can also suppress draw resonance but introduces gauge variation. The air gap between die lip and chill roll is set between 100 mm and 150 mm. When the air gap is reduced below 80 mm, the melt is quenched before molecular orientation is fully developed, and edge neck-in increases; when the air gap is extended above 200 mm, the molten web remains in the extensional zone longer, and draw resonance amplitude increases for a given draw ratio. Vacuum box settings of 2.0 kPa to 4.0 kPa and air knife velocities of 30 m/s to 50 m/s damp web motion but do not compensate for melt strength below the critical limit. On a cast line with a 90 mm barrier screw, barrel zones should be controlled to ±2 °C and die zones to ±1 °C to avoid melt temperature drift during acceleration from 50 m/min to 250 m/min.

High-pressure LDPE is added to DFDA7042 at 10 wt% to 30 wt% to raise melt strength and delay draw resonance. The LDPE contributes long-chain branching that produces extensional strain hardening at Hencky strains between 2.0 and 3.5, increasing the transient extensional viscosity at the extension rates typical of cast film. On a corotating twin-screw extruder with a screw diameter of 75 mm and an L/D of 36:1, blending DFDA7042 with 20 wt% LDPE at a melt temperature of 240 °C to 250 °C and a specific mechanical energy input of 0.18 kWh/kg to 0.22 kWh/kg produces a melt strength increase from approximately 3.0 cN to 7.0 cN when measured at 190 °C. The blend also extends the draw resonance onset from an isothermal laboratory draw ratio of approximately 18:1 to above 32:1. The trade-off is that the addition of LDPE reduces film clarity, lowers machine-direction tear strength as measured by ASTM D1922, and can increase odor and taste if the LDPE is not carefully selected. For food packaging, all blend components must comply with FDA 21 CFR 177.1520 and with European Regulation (EU) No 10/2011; for overall migration, the specific migration limit for total non-volatile substances is 10 mg/dm² under the intended use conditions. If the LDPE contains more than 0.1 wt% of a substance of very high concern, REACH Article 33 communication obligations apply. The packaging heavy metal limit under European Directive 94/62/EC is 100 ppm by weight for the sum of lead, cadmium, mercury, and hexavalent chromium.

Die Gap and Output Changes Do Not Eliminate Draw Resonance in DFDA7042-Rich Systems

Process adjustments that increase the geometric draw ratio worsen draw resonance even if they improve output. Increasing the die gap from 1.5 mm to 2.0 mm while producing a 20 µm final film raises the geometric draw ratio from approximately 75:1 to 100:1 for the same neck-in, which moves the process closer to the critical limit. Reducing draw ratio by increasing final thickness to 30 µm is effective but not always compatible with stretch film economics and stretch performance. Edge pinning, vacuum box, and electrostatic pinning suppress the free-surface oscillations that accompany draw resonance, but online capacitance or beta gauges on lines operating above 250 m/min still detect residual gauge variation of 3% to 6% when the melt strength is below the lower bound. The use of a 2.5 m wide die with a 1.5 mm gap and a 90 mm extruder at 150 rpm may produce a melt residence time of 3 min to 5 min; if the melt temperature exceeds 255 °C for more than 10 min, gel particles can form and initiate periodic web breaks. The maximum continuous extrusion time for a DFDA7042-rich blend without purging at 250 °C is typically 8 h to 12 h on a 30:1 L/D single-screw line. Screw speed increases above 120 rpm on a 90 mm extruder can raise melt temperature by viscous dissipation by 5 °C to 10 °C, pushing the melt into the draw resonance-prone region even when barrel setpoints remain unchanged.

Fluoropolymer processing aids are sometimes added to DFDA7042-dominated cast film to control die lip buildup and reduce melt fracture, but they do not raise melt strength. At addition levels of 200 ppm to 500 ppm, a fluoropolymer processing aid lowers apparent melt viscosity at the die wall and can reduce draw resonance amplitude by decreasing extensional stress at the die exit, but the effect is secondary relative to LDPE addition. Slip agents such as erucamide at 500 ppm to 1,500 ppm migrate to the film surface and lower the coefficient of friction as measured by ASTM D1894, but they do not alter the extensional viscosity of the melt. Antioxidant packages based on hindered phenols and phosphites at total concentrations of 500 ppm to 1,500 ppm protect against thermo-oxidative chain scission; if the antioxidant is depleted by high-temperature excursions above 260 °C, the MFR can drift upward to more than 2.5 g/10 min and the melt strength can drop below 3.0 cN. Formulators should not combine DFDA7042-rich systems with amine-based additives that can cause color generation at cast film processing temperatures, and acid scavengers based on calcium stearate are limited to concentrations that do not interfere with surface slip.

Table 2. Analytical and compliance methods for DFDA7042-dominated cast stretch film.

On production lines, the draw resonance limit in DFDA7042-dominated cast stretch film is often reached when the formulation contains less than 15 wt% LDPE and the draw ratio is increased during acceleration from 100 m/min to 200 m/min without sufficient melt strength. The instability is observed as a periodic oscillation in thickness profile that can be captured by a scanning gauge; the amplitude may grow from ±3% to ±12% within 30 s if the line speed is held constant. Published data for this specific DFDA7042-dominated configuration is limited, and the actual onset depends on resin lot, die temperature uniformity, chill roll temperature, and the condition of the vacuum box seal. The operational boundary is therefore best established by running a Rheotens melt strength check on incoming resin and by measuring the critical draw ratio on a pilot cast line before committing to high-speed production. Blends with DFDA7042 content above 80 wt% should be treated as draw resonance-prone unless the melt strength exceeds 5.0 cN at 190 °C; below this value, stable casting of 20 µm film at draw ratios above 25:1 is not reliably maintained even with vacuum-box assistance.