Seal initiation temperature testing for metallocene LLDPE: what to measure is not a single dial setting but a complete seal-strength curve generated at controlled jaw pressure, dwell time, specimen width, and film thickness. In linear low-density polyethylene (LLDPE) converting lines, the value most often cited is the jaw temperature at which a cooled seal reaches 0.5 lb/in (1.31 N/15 mm) or 1.0 lb/in (2.62 N/15 mm); the threshold force must be stated because no universal pass/fail value exists across packaging structures. Metallocene LLDPE (mLLDPE) shifts the curve lower than Ziegler-Natta LLDPE because single-site catalyst insertion creates a narrower crystallite thickness distribution and fewer high-melting, slow-sealing domains. The reported temperature is therefore not interchangeable with a generic polyethylene softening point. A complete data set requires heat seal preparation per ASTM F2029, cooled seal strength per ASTM F88/F88M, hot tack per ASTM F1921, conditioning per ASTM D618, and thickness verification per ISO 4593 or ASTM D6988.

Which Measurements Separate Seal Initiation Temperature from Hot Tack Initiation Temperature?

Seal initiation temperature records the thermal point at which sufficient polymer has interdiffused across the interface to create a measurable cold-seal strength. Hot tack initiation temperature records the thermal point at which a just-formed, still partially molten seal resists peel force during package filling or gusset opening. The two tests are mechanically different: ASTM F88/F88M measures a cold seal after crystallization, while ASTM F1921 measures a hot seal after a defined delay from jaw opening, typically 0.1 s to 0.5 s. A metallocene LLDPE film may show SIT at 85 °C but hot tack initiation at 95 °C under the same sealing parameters. Conversely, a film with sufficient cold-seal strength can still fail on a high-speed packager if hot tack is below the force required to hold the package contents during filling. For this reason, the measurement set must include both the SIT curve and the hot tack curve, not a single point from a heated jaw test.

The cooled seal strength is more sensitive to molecular interdiffusion, crystallite bridging, and contact area. The hot tack force is more sensitive to melt cohesion, squeeze-out, and the viscosity of the molten layer. For mLLDPE, the narrow molecular weight distribution can reduce melt strength, but the uniform comonomer distribution broadens the hot tack window relative to conventional LLDPE. A hot tack tester, rather than a standard tensile machine, is required because the force must be recorded before the seal has cooled below the deformation temperature. Published data for mLLDPE hot tack commonly show a broader temperature plateau than Ziegler-Natta LLDPE, but only when the test delay and jaw-release speed are held constant.

Specimen preparation and reporting conventions create more interlaboratory variation than the resin itself. Film thickness, sealant-layer thickness, backing-layer thermal conductivity, and slip-agent bloom must be controlled. Specimens conditioned at 23 ± 2 °C and 50 ± 5 % RH per ASTM D618 give more repeatable heat-transfer response than films equilibrated in a warehouse. A 25 mm wide specimen is standard for many ASTM F88 and ASTM F2029 evaluations; narrower specimens reduce force resolution, while wider specimens amplify edge effects and misalignment. The number of seals per temperature step should be at least three; at 2 °C steps near the SIT threshold, five replicates reduce the effect of film gauge variation. Thickness should be measured with a digital micrometer per ISO 4593, not assumed from the supplier data sheet. A 60 µm film can show SIT 4 °C to 8 °C lower than a 30 µm film of the same resin at the same jaw pressure because the thicker film retains heat longer during sealing.

Jaw-face temperature calibration is the primary hidden variable in seal initiation measurement. Setpoint controllers monitor heater block temperature, not the actual sealing surface in contact with film. A surface-contact thermocouple should be placed at the center and at positions 25 mm from each jaw edge after thermal equilibration. Industrial jaw faces often show a spread of 2 °C to 5 °C; for an mLLDPE seal curve with a steep onset, that spread shifts reported SIT by nearly one temperature step. Ceramic-coated or profiled jaws lag behind the controller by 5 °C to 10 °C at high cycling rates because of thermal resistance. Calibration should be performed at the same cycling frequency used for the seal test, not under static heating. A calibrated contact thermocouple with ±0.5 °C accuracy is recommended, and the jaw temperature should be recorded in the test report.

When Jaw Pressure and Dwell Time Drift Outside the Qualification Window

Seal initiation temperature is process-dependent, not an intrinsic resin property. On a laboratory heated-jaw sealer, dwell time and pressure are often fixed at 0.5 s and 0.275 MPa (40 psi), but on a vertical form-fill-seal machine the actual dwell can be 0.25 s to 0.40 s at 80 bags/min and jaw pressure may vary by ±0.03 MPa as pneumatic cylinders cycle. A film qualified at 0.5 s dwell may require a 5 °C to 8 °C higher jaw setpoint to reach the same seal strength on a high-speed line. Conversely, excessive pressure can squeeze molten mLLDPE out of the seal area and produce edge thinning, reducing seal strength even while the seal area reaches the setpoint.

Because mLLDPE has a narrow crystallite distribution, the temperature gap between initial seal formation and film distortion or burn-through is smaller than for conventional LLDPE. A processing window of 8 °C to 12 °C between the 1.0 lb/in threshold and visible jaw sticking is common for thin films. Setpoint drift of ±5 °C therefore consumes a substantial portion of the window. This is the central reason why SIT must be measured as a curve with controlled pressure and dwell rather than as a single pass/fail sealing temperature. The test report should record jaw pressure to ±0.014 MPa (±2 psi), dwell time to ±0.05 s, and actual jaw face temperature at three positions.

Seal Curve Reproducibility, Comonomer Distribution, and Failure-Mode Classification

Stepwise seal curves should begin below the expected SIT and advance in 5 °C intervals until the seal strength rises above the threshold. Near the onset, 2 °C intervals are necessary because mLLDPE seal strength can increase from below the threshold to film tear within 6 °C. At each temperature, seals should be prepared with at least three replicates and tested after cooling to 23 °C. The failure mode must be recorded: interfacial peel, cohesive film deformation, or film tear. A transition from peel to film tear indicates the lower bound of the usable sealing plateau, not the SIT itself. For mLLDPE, the peel-to-tear transition may occur at 3 °C to 6 °C above the 0.5 lb/in threshold, while a Ziegler-Natta resin may require 8 °C to 14 °C above threshold. This narrower transition is both an advantage and a limitation; it allows lower setpoints but leaves less room for temperature surge.

Differential scanning calorimetry per ISO 11357-3 or ASTM D3418 supports SIT interpretation but does not replace seal testing. The main melting endotherm shows the temperature range of crystallite melting, but the seal initiation process also involves welding time, pressure, and chain interdiffusion. A resin with a broad melting endotherm may have a broad seal curve, but only if the sealing parameters do not overheat the film surface. The melt mass-flow rate per ASTM D1238 at 190 °C and 2.16 kg is useful for incoming resin verification but is a low-shear viscosity index and cannot predict high-speed hot tack. Density per ASTM D1505 or ISO 1183 indicates the overall comonomer content, but not the uniformity of that comonomer distribution. For mLLDPE, the catalyst-driven uniformity is the primary difference, and it must be confirmed by the shape of the seal curve rather than by a density value alone.

On a production vertical form-fill-seal machine, the seal initiation temperature must be verified against the jaw setpoint with film from each incoming roll. Slip and antiblock additives bloom to the sealant surface over time and can raise SIT by 2 °C to 5 °C relative to freshly extruded film. Corona treatment, used for print adhesion, oxidizes the surface and can change the seal response if the treated side is placed against the jaw. A production trial therefore includes seal strength sampling at start-up, after 30 min, and after each roll change. The lower control limit is the cold-seal threshold, typically 1.0 lb/in (2.62 N/15 mm), not the laboratory SIT. If the seal strength falls below the threshold at the qualified jaw setpoint, the setpoint is raised in 5 °C increments. If edge thinning or stringing appears before the threshold is reached, dwell time and jaw pressure are checked before changing the resin. Flat-jaw SIT data should not be extrapolated to crimp-seal packages without a comparative trial on the actual jaw profile. Published data for specific crimp-jaw configurations is limited, so interlaboratory comparisons must be limited to identical jaw geometry and film construction.