PetroChina Guangdong KunLun mLLDPE HPR3518CB is a metallocene-catalysed ethylene–α-olefin copolymer supplied in pellet form for high-clarity cast film, lamination film, and coextruded sealant web applications. Supplier technical literature identifies the nominal density as 0.918 g/cm³ when measured under ISO 1183-1:2019 or ASTM D1505-20, and the melt mass-flow rate as 3.5 g/10 min at 190 °C under a 2.16 kg load when tested to ISO 1133-1:2022 or ASTM D1238-20. The metallocene catalyst platform yields a narrow molecular weight distribution and homogeneous short-chain branching distribution, which lowers crystalline haze and reduces the seal initiation temperature relative to conventional Ziegler–Natta C4-LLDPE of equal density. For food-contact applications, the resin is classified as an olefin polymer under 21 CFR 177.1520(c) and EU Regulation (EU) No 10/2011; migration testing under the intended simulant and time–temperature condition remains the responsibility of the converter. The resin follows polyolefin recycling streams under SPI code 4. Published data for this specific configuration is limited to standard resin characterisation and converter film trials, so lot-specific certificates of analysis and supplier technical bulletins should be consulted before qualification.

The narrow molecular weight distribution of HPR3518CB also changes the crystallisation rate during cast film quenching. Uniform short-chain branching distribution suppresses the formation of mixed lamellae populations; as a result, the quenched film is less prone to surface roughness from lamellar isotropy and exhibits lower internal haze than a C4-LLDPE with the same density. In thermal analysis, the crystallinity of a metallocene LLDPE at 0.918 g/cm³ is typically between 40% and 48% when measured by differential scanning calorimetry at a heating rate of 10 °C/min under ISO 11357-3:2018, but the exact value depends on cooling history and film annealing. The lower melting peak temperature and narrower endotherm are the primary drivers of low-temperature sealability; however, published data for this specific configuration is limited to the supplier certificate of analysis, which may not report full thermal curves.

What Distinguishes the Metallocene Architecture from a Ziegler–Natta LLDPE?

The metallocene architecture of HPR3518CB reduces the high-molecular-weight tail that is responsible for gel-like defects and melt memory in heterogeneous Ziegler–Natta resins. In 30 µm cast film, haze values are typically reported below 8% under ASTM D1003-21, and 45° gloss values fall near 80–85 units under ASTM D2457-21. The uniform comonomer insertion narrows the melting endotherm and shifts seal initiation to approximately 98 °C at a seal strength threshold of 4.4 N/15 mm when tested under ASTM F88/F88M-21. By comparison, a conventional C4-LLDPE of density 0.918 g/cm³ typically requires a sealing jaw temperature 7–10 °C higher to reach the same strength. The narrow distribution also limits shear thinning; at the same melt temperature and throughput, older single-stage extruders with shallow metering sections may record higher drive current and higher die pressure than with a broad-MWD C4-LLDPE of equal melt index.

In extrusion, the practical consequence of the narrow MWD is a different shear-thinning exponent. A broad-MWD ZN-LLDPE may show a Carreau shear-thinning exponent that reduces viscosity by several decades between 10 s⁻¹ and 1000 s⁻¹, whereas a metallocene LLDPE retains a higher high-shear viscosity. This is why processors observe higher die pressure and torque when replacing a 1.0 g/10 min C4-LLDPE with a 3.5 g/10 min mLLDPE on the same high-shear cast line, despite the higher MFR. Screw cooling, barrel zone profiling, and die land temperature should be re-optimised rather than relying on melt index alone. Melt fracture onset in a narrow-MWD resin is often delayed relative to a broad-MWD resin at equal melt temperature, but once melt fracture occurs it can appear as fine-scale sharkskin on the chill roll surface rather than gross melt rupture; this defect is best addressed by raising die lip temperature or reducing draw resonance through a vacuum box rather than by increasing melt temperature excessively.

Table 1 values are typical converter-reported film data and do not constitute a product specification; the certificate of analysis controls lot-specific values.

Cast Film Processing Window and Equipment Requirements

On cast film lines using barrier screws with L/D 30:1 or greater, the melt temperature at the die is typically set between 220 °C and 240 °C. Because HPR3518CB exhibits a narrow molecular weight distribution, the viscosity at high shear is higher than that of a broad-MWD Ziegler–Natta LLDPE of equal melt index; die pressure is therefore maintained below 350 bar by using screw geometries with moderate compression and a metering section that does not over-shear the melt. Die gap settings of 0.6–1.2 mm are commonly used for monolayer cast film, and chill roll temperatures in the range 15–25 °C increase quench rate to improve optics. If the production area exceeds 60% RH, hopper pre-drying at 70–80 °C for 2–4 h is recommended to remove surface condensation, even though the resin is not hygroscopic; residual moisture can produce micro-bubble defects at the die exit.

In blown film operations, die gaps of 1.5–2.5 mm and blow-up ratios of 2.0–2.5 are used. Bubble stability is maintained with a dual-lip air ring supplying chilled air at 8–15 °C and a frost line height lower than that used for a high-melt-strength LDPE. Published data for this specific configuration is limited for very high-stalk processes; the converter should avoid excessively high frost lines because the resin has lower strain hardening than autoclave LDPE.

Die-build-up and chill roll plate-out are managed by minimising low-molecular-weight oxidation products. The cast film line should be cleaned to remove crosslinked residues from previous high-temperature runs, because HPR3518CB is less tolerant of acidic or oxidised contamination than a broad-MWD resin. If the die exit velocity is not uniform, edge bead thickness can increase due to the narrower draw resonance tolerance; a vacuum box with adjustable edge suction is recommended for film widths exceeding 1,500 mm. The resin is not recommended for monolayer extrusion coating onto paper or aluminium foil at line speeds above 300 m/min unless blended with LDPE, because the lower melt strength increases neck-in and edge thinning.

Slip and antiblock additives influence the seal performance of HPR3518CB. Low-molecular-weight slip additives migrate to the surface and can act as a thermal barrier at the seal interface when loadings exceed 600–800 ppm; this shifts the seal initiation curve upward by 3–5 °C relative to the base resin. Antiblock particles such as silica at 2,000–4,000 ppm reduce film blocking but increase haze and lower gloss. For clear lamination film, surface roughness from antiblock is balanced against coefficient of friction; a target kinetic coefficient of friction of 0.3–0.5 measured against a stainless steel sled under ASTM D1894-14 is typical for automated packaging. The converter should qualify the film after 72 h of ageing at 23 °C and 50% RH, because additive migration kinetics are time-dependent and can continue beyond 24 h.

When Seal Integrity Is Tested at Low Temperature

The low seal initiation threshold of HPR3518CB becomes critical in high-speed vertical form-fill-seal and horizontal form-fill-seal lines where dwell time is short. Under ASTM F88/F88M-21, hot-tack strength measured on a conventional hot-tack tester at 4.4 N/15 mm threshold can be achieved at sealing temperatures near 100–105 °C, allowing the jaw temperature setpoint to be reduced. The lower setpoint decreases thermal distortion in oriented films and reduces the risk of seal-zone thinning adjacent to the seal bead. The resin is also positioned for coextruded sealant layers in polypropylene-based multilayer structures, where a polyethylene sealant of lower melting point reduces the seal initiation gap between the sealant and the core. The narrow melting range supports faster seal-strength development, but it also narrows the operating window for temperature overshoot; seal bar maintenance and heated-jaw Teflon condition become significant at line speeds above 40 m/min because local overshoot can exceed the melting point of the thinned seal layer.

The practical seal window between minimum seal strength and seal-zone distortion is narrower than for C4-LLDPE because the resin melts over a narrower temperature range. On high-speed VFFS equipment, seal jaw temperature fluctuations of ±3 °C can shift seal strength across the 4.4 N/15 mm threshold when dwell time is below 0.4 s. Packagers typically operate HPR3518CB seal layers at setpoints of 105–115 °C when dwell is 0.3–0.8 s, but thermocouple placement in the seal bar must be calibrated with an infrared surface pyrometer to avoid reading overshoot from the heater cartridge rather than the sealing face.

Resin Selection Hinges on More than Density

Conventional C4-LLDPE, general-purpose LDPE, and metallocene LLDPE can all be supplied at density 0.918 g/cm³, but the processing and end-use behaviour differ sharply. Table 2 summarises directional differences from comparative film trials; values within each family vary by catalyst generation, additive package, and film extrusion conditions.

The lower shear thinning of HPR3518CB relative to general-purpose LDPE means that it is not a direct drop-in for extrusion coating lines that require very low neck-in and high melt strength. In cast film, however, the combination of 3.5 g/10 min melt flow and narrow MWD permits higher draw rates and thinner gauge, with dart impact retention superior to C4-LLDPE at equivalent gauge. The converter can also blend HPR3518CB with up to 20–30 wt% LDPE in some multilayer sealant layers to restore melt strength and improve neck-in resistance, but the blend ratio is constrained by the increase in seal initiation temperature and the loss of optical uniformity. Published data for this specific configuration is limited above 30 wt% LDPE addition; pilot-line trials are required before commercial implementation.

Against other mLLDPE grades based on higher α-olefin comonomers, HPR3518CB offers a balanced seal-and-toughness profile for standard food packaging and hygiene film rather than maximum puncture or dart impact. The exact comonomer type in the supplier formulation is not stated in all published summaries; comparative film evaluations should therefore be performed against the specific incumbent grade on the target line rather than inferred from density and melt index alone.

For converters running high-speed cast film with thin-gauge sealant layers, the grade is typically positioned as a low-seal-initiation, high-optics alternative to conventional C4-LLDPE and as a moderate-melt-strength complement to LDPE in coextruded structures. The hot-tack window, cast roll contamination, and additive migration behaviour under the specific film ageing conditions must be qualified on production-scale equipment before specification lock.