Sinopec Qilu Petrochemical Co., Ltd. supplies the film-grade resin mLLDPE F2703S as part of its metallocene-catalysed linear low-density polyethylene series. The grade is supplied as white pellets and is intended for blown film extrusion where the converter requires a combination of low seal initiation temperature, high dart impact resistance, and reduced extractable content. The product code uses the F-series film designation; the numeral block identifies the nominal melt flow range. The polymer is produced under single-site metallocene catalysis, which yields a narrower molecular weight and comonomer distribution than conventional multi-site Ziegler-Natta LLDPE. The resin contains a stabiliser and neutraliser package; the exact formulation is lot-dependent and should be obtained from the supplier when organoleptic or migration testing is planned.

The nominal melt index under ASTM D1238/ISO 1133-1:2022 at 190 °C and 2.16 kg is 2.7 g/10 min. The density measured under ASTM D1505/ISO 1183-1 is approximately 0.927 g/cm³. These values place the grade in the medium-melt-index blown film window and the upper density portion of the linear low-density polyethylene range. The density indicates sufficient crystallinity for stiffness and blocking resistance while retaining toughness from single-site comonomer placement. Exact melt flow and density tolerances should be taken from the vendor specification, because commercial lot data may vary around the nominal values.

What Distinguishes Metallocene LLDPE F2703S from Ziegler-Natta Film Resins?

The decisive difference is the distribution of short-chain branches. In a conventional Ziegler-Natta LLDPE, comonomer tends to be concentrated in the lower molecular weight fraction, leaving higher molecular weight chains with fewer branches and therefore higher crystallinity. In F2703S, the single-site catalyst produces a more uniform comonomer distribution across the molecular weight distribution. The consequence is that at equal density and melt index, the metallocene resin develops a lower peak melting temperature and a lower seal initiation temperature than a Ziegler-Natta LLDPE of similar molecular weight. The shift in seal initiation under ASTM F1921 is typically 5–10 °C for film of 25 µm thickness.

The second difference is molecular weight distribution breadth. F2703S has a narrow molecular weight distribution characteristic of metallocene LLDPE; this reduces the high molecular weight tail. The practical consequence is lower melt elasticity, less shear thinning, and a higher shear viscosity at processing shear rates than a Ziegler-Natta LLDPE of the same melt index. Extrusion pressure drop across a spiral mandrel die is therefore higher, and the grade is more sensitive to die-gap selection and melt temperature. Film made from F2703S shows improved dart impact and tear relative to a Ziegler-Natta LLDPE of equal density and melt index when measured under ASTM D1709 and ASTM D1922. Optical properties also improve: haze values are lower and 45° gloss values are higher due to the absence of large crystalline aggregates.

Extractables are reduced because the narrow comonomer distribution minimises low molecular weight, highly branched chains that can migrate. Compliance with 21 CFR 177.1520 and EU Regulation 10/2011 is therefore more readily demonstrated than with older Ziegler-Natta grades, provided the antioxidant package is correctly selected. The lower soluble fraction also reduces seal contamination and taste/odour transfer in food packaging.

Table 1 lists representative physical property values compiled from published grade literature for F2703S. The values are not contractual specification limits and should be confirmed against the lot certificate.

When F2703S Is Extruded on High-Stalk Blown Film Lines

Processing behaviour differs from both LDPE and conventional LLDPE. F2703S requires a wider die gap than an LDPE grade of comparable melt index. On high-stalk blown film towers fitted with spiral mandrel dies, a die gap of 1.5–2.5 mm is typically used, compared with 0.8–1.2 mm for LDPE. The wider gap reduces shear stress at the die lip and delays melt fracture. If a narrow gap die is retained, the onset of sharkskin melt fracture moves to lower output rates because the higher shear viscosity of the metallocene resin increases die-lip shear stress. The critical shear stress for melt fracture of similar metallocene LLDPE resins has been reported in the range 0.1–0.3 MPa; published data for F2703S under specific die geometries is limited.

Barrel temperature settings on a grooved-feed extruder with L/D 30:1 and a barrier screw should be set to give a melt temperature of 190–220 °C at the die entry. Temperatures above 240 °C are not recommended due to the risk of oxidative degradation and gel formation, unless the line is equipped with a nitrogen purge and a melt degassing system. The bubble is less stable than an LDPE bubble because the resin has limited long-chain branching and a narrow molecular weight distribution; the high-stalk configuration provides a longer stretching distance and stabilises the frost line. Blow-up ratios of 2.0–3.0 and frost line heights of 2–8 die diameters are typical. Internal bubble cooling is recommended at high output to remove heat and stabilise the frost line. On lines without internal bubble cooling, maximum output is limited by bubble instability rather than extruder capacity.

Extrusion pressure is higher than for a Ziegler-Natta LLDPE of identical melt index. The pressure drop across a spiral mandrel die can increase by 20–40 % under the same throughput and die gap. The extruder barrel, screen changer, and die should therefore be rated for continuous operation above 350 bar. Melt filtration with a screen pack of 60–100 mesh is used to remove degraded particles and incidental contamination. Frequent screen changes may be required if gels are formed by thermal degradation at high melt temperature.

Blown film of 25–80 µm thickness from F2703S is used in food pouches, freezer film, heavy-duty sacks, agricultural greenhouse film, and lamination webs. In high-speed form-fill-seal packaging, the low seal initiation temperature permits sealing at 95–110 °C with 0.5 s dwell, which reduces heat input and allows line speeds above 40 cycles/min on rotary sealers. The dart impact values above 200 g for 25 µm film support downgauging from thicker conventional LLDPE films. Elmendorf tear measured under ASTM D1922 and low-temperature puncture resistance under ASTM D5748 should be verified for frozen food packaging applications.

For heavy-duty sacks, blending with LDPE or HDPE is common. LDPE improves bubble stability and melt strength but reduces dart impact when tested under ASTM D1709; HDPE raises modulus and reduces elongation when tested under ASTM D882. Blends with 10–20 wt% LDPE are used on conventional low-stalk lines to improve bubble stability without losing the seal-initiation advantage. In agricultural film, UV stabilisers should be added via masterbatch; the carrier resin must be compatible with metallocene LLDPE to avoid gel formation detectable under ASTM D7310.

Seal Initiation Temperature and Hot-Tack Window

Seal initiation temperature is measured by heat sealing film samples at 0.5 N/mm² jaw pressure and 0.5 s dwell, then testing seal strength under ASTM F88. For 25 µm film, the temperature at which seal strength reaches 10 N/15 mm is typically 95–105 °C. The hot-tack plateau, measured under ASTM F1921 with a 0.5 s seal time and 0.2 s delay, is reached at 110–120 °C. These values are lower than conventional Ziegler-Natta LLDPE of similar density, which typically requires an additional 5–10 °C to achieve equivalent seal strength. The lower seal initiation temperature reduces dwell time on high-speed vertical form-fill-seal machines and decreases the thermal load on heat-sensitive contents.

The hot-tack window is narrower than that of LDPE because the metallocene resin begins to melt over a shorter temperature interval. Converters should verify seal jaw temperature uniformity across the full width of the sealing bar; temperature variation greater than ±5 °C can produce weak regions at the edge of the seal. For packages with gussets or thick seams, the seal bar pressure should be increased to 0.8–1.0 N/mm² to compensate for the higher melt viscosity and ensure complete seal interface wetting.

Polyolefins are not hygroscopic, but condensation on pellets stored in unheated warehouses can introduce surface moisture. If pellet surface relative humidity exceeds 60%, the resin should be dried in a hopper dryer at 70–80 °C for 2–4 h before processing to prevent bubble defects and hydrolytic degradation of ester-containing additives. The grade should not be blended with high loadings of polar tackifiers or amine-based antistatic concentrates without evaluating film quality, because incompatible additives can increase die lip build-up and reduce seal strength.

Storage temperature should remain below 40 °C and the resin should be protected from direct sunlight to prevent photo-oxidation of the stabiliser package. Lot-to-lot variation in melt index and density should be monitored by the converter; excursions greater than ±0.3 g/10 min in melt index or ±0.005 g/cm³ in density may require adjustment of barrel temperatures and frost line height to maintain bubble stability and film gauge uniformity.

Regulatory Status Under Food Contact and Recycling Frameworks

Compliance statements should be obtained from the supplier for the specific lot. In general, F2703S can be formulated to meet olefin polymer food contact requirements under 21 CFR 177.1520 and EU Regulation 10/2011. Table 2 lists the main regulatory references and the applicable test or limit.

In mechanical recycling streams, the narrow comonomer distribution of F2703S permits incorporation into recycled LLDPE blends at typical addition levels up to 20 wt% without large shifts in melt index or density; however, the blend should be tested for gel count and film appearance using ASTM D7310 and ASTM D3351 before commercial use. Published data for the recyclability of F2703S under specific collection schemes is limited.