PetroChina Dushanzi KunLun mLLDPE HPR3518CB is a metallocene-catalysed linear low-density polyethylene formulated as a 1-hexene copolymer for blown film conversion. The manufacturer’s public datasheet lists a nominal density of 0.918 g/cm³ determined in accordance with ASTM D1505 and a nominal melt index of 3.5 g/10 min measured at 190°C under 2.16 kg load according to ASTM D1238. The resin is intended for film structures in which downgauging, impact resistance, seal integrity, and optical consistency must be balanced against the processing constraints of metallocene polyethylene. In comparison with conventional Ziegler-Natta butene-based LLDPE of the same nominal density, the hexene comonomer and single-site catalyst architecture of HPR3518CB narrow the molecular weight distribution and modify the distribution of short-chain branching across the molar mass envelope.

Molecular Architecture and Comonomer Sequence Distribution in the Hexene Copolymer

The metallocene active site produces a statistically more uniform incorporation of 1-hexene than a conventional Ziegler-Natta catalyst. The resulting molecular weight distribution for gas-phase metallocene LLDPE of this class is typically in the range 2.0–2.5 as a polydispersity index, whereas conventional Ziegler-Natta butene-based LLDPE commonly ranges from 3.5–4.5. The narrower distribution reduces the concentration of low-molecular-weight polymer chains that can migrate to film surfaces and alter coefficient of friction and haze during storage. The density of 0.918 g/cm³ per ASTM D1505 reflects a controlled level of short-chain branching; in a 1-hexene copolymer the branch is a butyl pendant group, which is more effective at forming tie chains between crystalline lamellae than the ethyl branch of a 1-butene copolymer at equal density. The melt index of 3.5 g/10 min at 190°C and 2.16 kg per ASTM D1238 places HPR3518CB in a moderate-flow regime suitable for coextruded film lines, but the melt flow value alone does not describe die-lip viscosity because narrow molecular weight polymers exhibit different shear thinning behaviour.

How Does the Melt Rheology of HPR3518CB Set Extrusion Boundaries?

At extrusion shear rates typical of blown film dies, commonly 100–500 s⁻¹, metallocene LLDPE with a narrow molecular weight distribution exhibits less shear thinning than Ziegler-Natta LLDPE. The consequence is that a melt index of 3.5 g/10 min at low shear does not translate into equivalent flow at die-lip shear rates. On conventional single-screw extruders with 30:1 to 40:1 L/D and Maddock or barrier mixing sections, high-shear viscosity can remain elevated enough to increase melt pressure and screw torque when switching from a Ziegler-Natta C4 resin of equivalent melt index. Field operation records from blown film lines processing gas-phase metallocene hexene copolymers indicate that melt temperatures of 190–230°C are typical. Below 190°C, high-molecular-weight species may persist as visible gel defects; above 230°C, oxidative gel formation and stabiliser consumption become measurable over extended run times. The neat resin is not hygroscopic and does not require pre-drying under normal dry storage, but reclaimed film with surface moisture should be dried to <0.05 wt% moisture before re-extrusion.

Bubble stability is the principal processing constraint. Single-site catalysed polyethylene lacks the long-chain branching of low-density polyethylene, so extensional strain hardening is lower. At blow-up ratios above 3.0 or at film gauges below 20 μm, bubble flutter and draw resonance can increase in the absence of an appropriate process compromise. A common production modification is the addition of 20–30 wt% LDPE. LDPE contributes melt strength and stabilises the frost line, but the addition raises seal initiation temperature and reduces dart impact relative to the neat metallocene resin. Die gaps of 1.5–2.5 mm are often used for metallocene LLDPE film to manage extruder head pressure while controlling morphology; narrower gaps can increase shear heating and reduce bubble stability, whereas wider gaps may require higher draw-down and can alter optical properties.

Film property evaluation for HPR3518CB must be conducted on the actual blown film because frost line height, blow-up ratio, die gap, and residence time alter orientation and crystallite morphology. Tensile properties are measured under ASTM D882; dart impact under ASTM D1709-22; Elmendorf tear under ASTM D1922-15a; and puncture resistance under ASTM D5748. Published film data for this specific grade are limited in the open literature, so conversion trials should generate control data on the target line rather than relying on neat resin melt properties alone. Heat-seal performance is assessed under ASTM F88/F88M-21. For metallocene hexene copolymers of this density, seal initiation temperature is generally lower than that of Ziegler-Natta butene copolymers at equal gauge because the narrower distribution and uniform branch placement lower the crystallite melting onset. The reduction can fall in the range 5–10°C, but seal bar temperature, dwell time, and film surface conditioning can shift the measured value. This behaviour supports higher jaw speeds on form-fill-seal lines only when seal temperature control and dwell time are validated on the specific packaging machine.

When HPR3518CB Replaces a Conventional C4 Ziegler-Natta LLDPE in Downgauging Trials

Switching from a nominal 0.918 g/cm³, 2.0 g/10 min butene-based Ziegler-Natta LLDPE such as Dushanzi DFDA-7042 to HPR3518CB alters both polymer architecture and melt flow. The higher melt index of 3.5 g/10 min may reduce extruder back pressure at low shear, but the lower shear thinning of metallocene resin can offset this at die-lip shear rates. Dart impact at equal film gauge is generally improved because the homogeneous hexene branch distribution yields more tie chains; however, achieving a stable bubble at 15–20 μm gauge often requires LDPE addition or an increase in die gap. If LDPE addition exceeds 30 wt%, seal initiation temperature rises and the metallocene seal advantage is partially lost. This threshold has been observed in production line trials with similar metallocene hexene film resins, although specific published HPR3518CB data remain limited.

Optical properties also change. Haze measured by ASTM D1003 can be lower for metallocene hexene films than for conventional ZN C4 films at the same gauge because of reduced internal scattering at crystalline boundaries. Gloss measured by ASTM D2457 typically rises. The improvement is conditional: high frost lines or low blow-up ratios can erase the optical advantage by increasing surface roughness. The resin is not intended for injection moulding, pipe extrusion, or rotational moulding because the narrow molecular weight distribution and high melt index reduce melt strength and creep resistance. In coextruded structures it typically occupies the sealant layer or core layer, while LDPE is used in outer layers to maintain bubble stability.

Primary converting equipment reported for HPR3518CB includes single-layer and coextruded blown film lines producing heavy-duty sacks, frozen food packaging, collation shrink film, and lamination films. For food-contact structures, qualification must be based on the final film and not the base resin alone. Olefin polymers of this class may be assessed under 21 CFR 177.1520 for direct food contact in the United States and under EU 10/2011 overall migration limits in the European Union. Compliance is dependent on the antioxidant and slip/antiblock formulation, the presence of tie layers, and the processing temperature history. REACH registration obligations apply to the substance imported into the European Economic Area, and the resin should be accompanied by a safety data sheet that identifies the stabiliser package and any intentionally added substances. RoHS does not normally apply to packaging, but absence of cadmium, lead, mercury, and hexavalent chromium is a common supply-chain requirement.

Compared with other metallocene LLDPE grades, the key differentiating specifications remain density and melt index. A lower melt index grade of 1.0 g/10 min at the same density will have higher dart impact but lower extrusion throughput and higher melt pressure under identical die conditions. A higher density hexene metallocene grade of 0.926 g/cm³ will have higher secant modulus and higher seal initiation temperature. HPR3518CB therefore occupies a narrow position below the modulus threshold of higher-density film resins while providing a melt index high enough for coextrusion lines with moderate screw torque. Selection should not be based solely on density and melt index; comonomer type and catalyst architecture are the differentiating variables that influence seal performance, organoleptics, and bubble stability.

Operational boundaries include melt temperatures that do not exceed 230°C during extended regeneration or residence time, screw speed limits that keep melt temperature below the oxidation onset of the formulated stabiliser system, and storage conditions below 50°C away from direct sunlight and strong oxidising agents. The resin is not biodegradable and should not be exposed to strong oxidising acids, halogens, or aromatic hydrocarbons at elevated temperatures. These limitations are relevant when the grade is blended with reclaim or when processing lines are shut down and restarted with excessive heat history. Qualification therefore depends on full film testing under the intended sealing and filling conditions rather than on neat resin properties alone.