Linear low-density polyethylene (LLDPE) used as a sealant layer is produced primarily as a copolymer of ethylene with 1-butene, 1-hexene, or 1-octene. Comonomer content and distribution control short-chain branch frequency, which in turn influences melting enthalpy, crystallization rate, seal-initiation temperature, and hot-tack plateau width. Typical blown-film sealant grades are characterized by density of 0.916–0.922 g/cm³ according to ISO 1183-1:2019 and melt mass-flow rate of 0.5–2.0 g/10 min at 190 °C under 2.16 kg according to ISO 1133-1:2022. On a production heat-sealing jaw, the interface reaches temperature through time-dependent conductive transfer, and seal strength is measured as force per 25.4 mm width after peeling in a tensile tester following ASTM F88/F88M-21. The result is not a resin property alone; it depends on dwell time, jaw pressure, film gauge, backing layer stiffness, and failure mode. Because LLDPE has a lower peak melting endotherm than low-density polyethylene—commonly 105–128 °C when measured by ISO 11357-3—the seal-initiation temperature can be set 10–25 °C lower than that required for a comparable LDPE sealant, but the actual setpoint must be verified on the specific film structure.

Hot tack is the seal’s ability to resist opening force before the melt has recrystallized. ASTM F1921/F1921M-20 measures hot tack by sealing a specimen and subjecting it to a defined peel force after a cooling delay often set at 100–400 ms. In vertical form-fill-seal machines, the delay between jaw opening and product drop can be as short as 150 ms, so the hot-tack peak temperature and plateau width control machine throughput. LLDPE sealant layers generate a broad hot-tack plateau when the molecular weight distribution and short-chain branch distribution are sufficiently wide, but the plateau narrows if the resin is blended with highly crystalline HDPE. The seal bar temperature is therefore selected not for seal strength alone; it is selected for the midpoint of the hot-tack plateau to allow for thermal drift across the jaw width.

What Controls Hot Tack in High-Speed Polyolefin Sealant Layers?

Hot tack is governed by the rate of interfacial diffusion, the melt’s extensional viscosity, and the time available before crystallization locks the seal. At a seal-bar temperature above the melting peak, hot tack initially rises with temperature because interdiffusion accelerates. Above the optimum, the melt viscosity is too low to resist peel forces and the seal fails cohesively. The peak is therefore a balance between molecular diffusion and load-bearing viscosity. In Ziegler-Natta LLDPE, broad melting and relatively slow crystallization can extend the hot-tack window, but the seal-initiation temperature may be less precise. Metallocene LLDPE with narrow composition distribution gives a sharper melting peak and lower seal initiation, yet it may exhibit lower melt strength unless its molecular architecture is modified. Longer α-olefin comonomers—1-hexene and 1-octene—introduce ethyl and butyl branches that reduce crystallinity more efficiently than 1-butene at equivalent mole fraction. This shifts seal initiation lower and can sustain the amorphous interfacial layer for a longer time. However, hot-tack force is not simply inversely related to density. Load-bearing tie chains and melt elasticity contribute more at high deformation rates. Increasing weight-average molecular weight and multimodal molecular weight distribution improves melt strength and hot tack, but raises extruder head pressure and torque. The conflict is managed on the machine by selecting barrel profiles, screw speed, die gap, and output to keep shear heating below the oxidation threshold. In practice, melt temperature at the die exit is maintained between 190 °C and 230 °C for LLDPE film lines; below this range, die pressure can exceed the limit and produce sharkskin at the die lip, while above it, oxidation and gel formation increase, especially in metallocene grades with low antioxidant retention.

Seal, Hot Tack and Coextrusion with LLDPE: Machine-Level Decisions

In blown-film coextrusion, the LLDPE sealant skin is commonly paired with an HDPE or polypropylene core for stiffness, temperature resistance, and downgauging. The first machine-level decision is layer ratio. A sealant skin of 5–15 µm in a total film of 40–80 µm is typical for high-speed packaging; below 5 µm, seal strength becomes gauge-dependent and jaw-pressure-sensitive. The coextrusion feedblock or multimanifold die must maintain layer stability. If the LLDPE skin has a melt mass-flow rate of 1.0 g/10 min and the HDPE core has 0.5 g/10 min, pressure-drop mismatch across the die can produce nonuniform layers. Processors compensate by raising the sealant layer melt temperature to 220–240 °C while holding the core at 200–220 °C. This practice improves flow matching but can reduce hot tack and increase extractables if residence time is excessive. Alternative corrections include selecting a higher melt index LLDPE for thin skins, using a multimanifold die rather than a feedblock, or increasing die gap.

For monolayer and coextruded LLDPE blown film, die gap is set wider than for LDPE—commonly 1.5–2.5 mm versus 0.8–1.2 mm for LDPE—to reduce die-lip shear stress and mitigate melt fracture. Blow-up ratio is typically 2.0:1–3.0:1. Single-screw extruders with grooved feed sections and L/D ratios of 25:1–30:1 provide stable feed and melting, but screw speed is often limited by motor load and melt temperature rather than output. Internal bubble cooling and dual-lip air rings increase cooling and output; the frost line height then becomes a separate control variable for orientation and sealing performance. Separate extruders allow different barrel temperatures, but the die body is common, so a thin LLDPE skin can reach the die body temperature faster than a thick core. Low-residence-time die designs and streamlined flow paths reduce degradation. In cast film coextrusion, die gap is 0.4–0.9 mm, and chill roll temperatures of 15–30 °C produce rapid quenching. Faster quench can preserve low crystallinity and improve hot tack, but it may increase blocking if slip and antiblock additives are not adjusted.

When a Three-Layer Coextrusion Line Pairs LLDPE with HDPE for Stiffness

In a three-layer A/B/A or A/B/C structure, the measured seal curve is not determined by the LLDPE skin alone. The HDPE core typically melts between 125–135 °C; when sealing through the structure, the jaw must raise the sealant interface above its seal-initiation temperature without collapsing the oriented core. The seal bar temperature is often set 5–10 °C above the LLDPE seal initiation but below the point at which the core begins to shrink and wrinkle. If the skin is too thin, the peel force can be governed by delamination from the tie layer or by tensile elongation of the HDPE backing; the seal may appear intact but fail destructively under product load. On high-output lines, coextruded film with an LLDPE skin and HDPE core also exhibits asymmetric layer thermal expansion, so the sealant can wrinkle at the seal if the jaw release is too early. The operational fix is to increase dwell time or reduce the temperature of the heat-seal jaw while maintaining hot tack above the product-drop force.

Die Gap and Frost Line Height Are Not Independent Variables

On a high-stalk LLDPE bubble, widening the die gap reduces shear stress at the die lip and permits higher output, but the bubble must then be rebalanced through the air ring and frost line height. Die-lip shear rates for LLDPE can range from 200 s⁻¹ to 1,200 s⁻¹ depending on output, die diameter, and die gap. When die-lip shear stress exceeds 0.1–0.2 MPa, sharkskin can appear, with metallocene grades often showing the defect at the lower end of that range. A wider die gap reduces this shear stress, but the thicker melt web carries more heat and shifts the cooling load downstream. If frost line height is increased, the film solidifies under higher draw, increasing orientation. High orientation can shift seal initiation upward and reduce the hot-tack plateau because the oriented structure relaxes unevenly in the seal jaw. Conversely, a very low frost line height reduces orientation but can produce blocking and gauge bands. These variables interact with the sealant’s molecular structure; published data for specific LLDPE structures on high-stalk lines is limited, so start-up must include seal tests on actual line samples rather than resin data alone.

Slip and antiblock masterbatches in the LLDPE sealant skin alter seal performance. Erucamide slip at 500–1,200 ppm migrates to the surface over days and can interfere with seal initiation, producing variable seal strength in aged film. Synthetic silica antiblock at 1,000–5,000 ppm reduces blocking after fast quenching, but surface asperities can weaken seal initiation at low jaw temperatures. These additive interactions are measured by storing film at 40 °C for 7–14 days and retesting with ASTM F88/F88M-21. The processing window for LLDPE sealant films is therefore bounded on one side by melt fracture and die-pressure limits, and on the other by additive migration, thermal oxidation, and seal contamination. Published data for specific coextruded configurations with exact additive packages is limited; plant trials remain the final verification step.