Sinopec Qilu mLLDPE F181ZU is a metallocene-catalysed linear low-density polyethylene film resin supplied as pelletized granulate. The grade designation is associated with a nominal density of 0.918 g/cm³ measured in accordance with ISO 1183-1:2019 and a melt flow index of 1.0 g/10 min measured at 190 °C under 2.16 kg load in accordance with ISO 1133-1:2022. The resin is intended for blown film extrusion where the sealing interface, dart impact resistance, and optical clarity are specified as primary fitness-for-use criteria. The metallocene insertion mechanism generates a narrow molecular weight distribution and a more uniform distribution of short-chain branches than conventional Ziegler-Natta LLDPE of the same nominal density. In the solid state, this structural uniformity controls the distribution of crystallites and the concentration of tie chains, which are the interlamellar load-carrying segments responsible for impact and tear propagation resistance.

Published grade literature positions F181ZU as a 1-hexene-based mLLDPE. The use of 1-hexene, rather than butene, increases the length of the short-chain branch and improves the efficiency of tie-chain formation at the same density. This changes the failure mode of blown film under high-speed puncture from low-stretch brittle fracture to ductile deformation. The resin is not considered a plastomer; its density is within the LLDPE range used for agricultural films, heavy-duty sacks, and food packaging laminates where toughness must be retained after down-gauging. The grade is typically packaged in 25 kg moisture-resistant bags and should be stored below 50 °C; condensation on pellet surfaces should be avoided before the resin enters the extruder feed throat.

What Distinguishes F181ZU from Conventional Butene LLDPE in Blown Film?

The primary difference is the catalyst architecture and the placement of the comonomer. In a Ziegler-Natta butene LLDPE, the active sites are heterogeneous, and the comonomer is disproportionately incorporated into the lower-molecular-weight chains, producing a low-melting, extractable-rich fraction. F181ZU-class metallocene resin distributes 1-hexene more evenly across the molecular weight distribution. As a result, the differential scanning calorimetry melting endotherm is narrower, and the peak melting temperature is lower than that of a butene LLDPE of equivalent density. The lower melting onset allows the film surfaces to wet and entangle at lower heat-seal bar temperature, which is the basis for reduced seal initiation temperature.

The narrower molecular weight distribution also modifies rheology. The resin exhibits lower shear thinning than a broad-MWD Ziegler-Natta grade; at the same melt flow index, the low-shear viscosity is higher and the power-law index is closer to 1. On a blown film line this translates into higher die pressure, greater specific energy input, and a more pronounced temperature rise at high screw speeds. The melt fracture response also changes: sharkskin may appear at lower output when a narrow die gap is used, but the onset of gross melt fracture can be delayed by increasing the die gap to 1.8–2.5 mm.

Compared with a 1-octene mLLDPE of the same density, F181ZU-class 1-hexene resin may have a slightly lower dart impact at equal thickness but lower melt viscosity and less die pressure. Compared with a metallocene plastomer of density below 0.910 g/cm³, F181ZU retains a higher melting point and is not a direct substitute for high-leak sealants. The grade should be selected when the film must maintain stiffness for bag opening while providing a low seal initiation temperature.

On production-scale blown film lines, F181ZU-class resin is processed through single-screw extruders with L/D 25:1 to 30:1 and barrier-type feed sections. A die gap of 1.8 mm to 2.5 mm is commonly selected; die gaps below 1.2 mm may produce sharkskin melt fracture at reduced output because the narrow molecular weight distribution limits shear thinning. Barrel temperature settings are typically increased from 170 °C at the feed throat to 220 °C at the die, with melt temperature maintained between 190 °C and 230 °C. Blow-up ratios of 2.0:1 to 3.0:1 and frost-line heights of 6 to 10 die diameters are used. The frost-line height is a key control variable for optical haze and dart impact; raising the frost line usually improves optical uniformity but can reduce bubble stability when cooling air is uneven.

The most common bottleneck is screw-speed-limited output, not motor amperage. At high throughput the melt pressure can exceed 35 MPa at the die if the die gap is too narrow. This pressure rise increases melt temperature and can oxidize the resin if the temperature profile is not reduced. The viscosity curve can be approximated by a Carreau-type model, but for line setup the critical qualitative difference is the reduced extent of shear thinning. Screw designs with intensive mixing sections can raise the melt temperature by an additional 3 °C to 6 °C compared with a smooth-bore barrier screw. Some converters purge the transition from a conventional LLDPE by first running a low-density polyethylene with a melt flow index below 1.0 g/10 min, because the metallocene heel is slow to displace. Die lip deposits may form after prolonged runs at 220 °C or above; periodic purging and the use of fluoropolymer processing aids are standard control measures.

Tensile, Puncture, and Optical Property Benchmarks

For preliminary film design, the following envelope is applied when lot-specific F181ZU datasheets are unavailable. The values are not acceptance limits but are representative of metallocene LLDPE blown film with a density of 0.918 g/cm³ and a melt flow index of 1.0 g/10 min processed at 40 µm. Published data for this exact grade and thickness configuration is limited, so film qualification should include testing on the intended blown film line.

These values cannot replace the certificate of analysis for F181ZU because additive slip and antiblock packages, film thickness, and extrusion conditions shift the measured values. The tensile and tear values are direction-dependent; machine-direction and transverse-direction values should be reported separately for any comparison. The dart impact and tear values are also thickness-dependent. For film below 30 µm, the dart impact reserve declines steeply, and the failure mode may shift from ductile to brittle at high rates. For heavy-duty sacks above 80 µm, the benefit of the metallocene structure is observed more in seal integrity than in tensile strength.

When Lower Seal Initiation Replaces Lamination in Flexible Packaging

F181ZU is used in monolayer and coextruded structures where the heat-seal layer is required to open at lower temperatures, allowing downstream form-fill-seal machines to run at higher cycle speeds. In flexible packaging lines, the seal initiation temperature is evaluated by ASTM F88/F88M-21 on samples sealed at 10 °C increments. A metallocene resin of the F181ZU class typically develops a seal strength of at least 4 N/15 mm at a lower sealing temperature than a butene LLDPE of identical density. The difference is most visible below 110 °C, where butene LLDPE films retain a high modulus surface and fail interfacial peeling. The lower activation of thermal sealing reduces the risk of distortion through the zipper profile and allows sealing through residual product dust in vertical form-fill-seal applications.

In coextruded films, F181ZU-class resin is placed in the sealant skin over a HDPE or LLDPE substrate. The low seal initiation temperature is not achieved by high extractable content, which is the older route used in some low-melting EVA sealants. This preserves food-contact suitability and reduces the transfer of low-molecular-weight species to packaged dry foods. The resin also displays higher hot tack than conventional LLDPE, measured by the distance of seal failure at a given release time on a hot tack tester. This property reduces seal pop-open in high-speed bag-making. The toughness of the metallocene film permits down-gauging from 60 µm to 40 µm in some applications, but the actual reduction must be confirmed by creep, tear, and drop tests on the filled package.

For food-contact applications, F181ZU is supplied with compliance statements that reference the olefin polymer provisions of FDA 21 CFR 177.1520(c) 3.1a and 3.2a when contact involves aqueous, acidic, fatty, or dry food types. The European framework is typically addressed under Regulation (EU) No 10/2011, with verification required for the specific additive package and the migration limit of the finished film. REACH registration under Regulation (EC) No 1907/2006 is a supplier-level obligation. The resin is not intended for medical implant applications or for use as a sterility barrier unless the converting line has performed a specific ISO 11607-1:2019 validation. For high-fat foods, migration testing under Regulation (EU) No 10/2011 should be performed on the finished film, because the compliance of the base resin does not automatically cover printing inks, laminating adhesives, or silicone release coatings.

Operational boundaries should be observed. Pellets exposed to relative humidity above 60% for extended periods may require drying at 60 °C to 70 °C for 2 h to 4 h before extrusion if surface condensation is visible. The resin should not be blended with amine-based antifog additives or certain phenolic secondary antioxidants in an uncontrolled masterbatch without compatibility testing, because the metallocene catalyst residues and the narrow composition distribution can be more sensitive to additive-induced discoloration. Purge transitions from polyamide or PET should be thorough; residual polyamide in the extruder can react at the melt temperature and produce gel-like contamination in the film.

Batch-to-batch variation in mLLDPE of this class is usually lower than in Ziegler-Natta grades because the metallocene active site is more uniform. Converters nevertheless observe small shifts in melt pressure and film haze when the upstream ethylene source or catalyst donor is changed. Incoming resin should be tested for melt flow index and density on each lot, and the film line should be standardized with a reference film sample to separate resin variation from changes in ambient temperature or die gap.