Within the metallocene linear low-density polyethylene portfolio of PetroChina Daqing Petrochemical Company, the KunLun grade HPR18H10AX is classified as a hexene-based mLLDPE for blown-film extrusion; the certificate of analysis should confirm comonomer type for each lot. Published specification data identify a melt mass-flow rate of 1.0 g/10 min at 190 °C under 2.16 kg load and a nominal density of 0.918 g/cm³; these values position the material in high-toughness packaging film rather than rotational moulding or injection moulding. The resin is supplied as pellets and is intended for monolayer or multilayer film structures where optical clarity, dart impact resistance, and low seal initiation temperature are required simultaneously. Because the material is metallocene-catalysed, its molecular architecture differs from conventional Ziegler-Natta LLDPE of the same density and melt index in extrusion pressure, bubble stability, heat seal response, and extractable content. The table below reproduces the two specification values that are available from the public datasheet; film properties are deliberately separated because they depend on gauge, blow-up ratio, frost line height, and die geometry.

Molecular Architecture and Catalytic Pathway

The defining feature of HPR18H10AX is single-site catalysis rather than a multisite Ziegler-Natta system. In multisite LLDPE, the distribution of short-chain branches is heterogeneous; high-molecular-mass chains often contain fewer branches, while lower-molecular-mass chains contain more, producing a broad crystallinity distribution. In the metallocene variant, the comonomer insertion is more uniform across molecular weight fractions, giving a narrower composition distribution and a reduced fraction of high-crystallinity, high-seal-temperature species. This architectural shift is relevant to film converters because it lowers the heat seal initiation temperature relative to a Ziegler-Natta LLDPE of equal density and melt index, typically by 5–15 °C depending on film gauge and seal dwell time. The same uniformity reduces extractable oligomer content and improves optical haze, but it also reduces melt elasticity. Consequently, screw torque and head pressure on a 45 mm single-screw extruder may be 10–30 % higher than for a Ziegler-Natta butene-LLDPE of equivalent melt index, particularly when the die gap is below 1.2 mm. No additivation-specific molecular weight data for HPR18H10AX is reproduced here; the converter should obtain the certificate of analysis for lot-specific values.

On a 55 mm grooved-feed single-screw blown-film line with L/D of 28:1 and a 200 mm spiral-mandrel die, the lower melt strength of HPR18H10AX becomes a practical constraint in frost line height management. When the frost line is set above 4D from the die face, bubble oscillation and gauge bands may appear because the narrow molecular weight distribution cannot sustain the same extensional strain hardening as a low-density polyethylene or a broad-MWD autoclave LDPE. A die gap of 1.5–2.5 mm is usually specified for metallocene LLDPE film; reducing the die gap below 1.0 mm raises wall shear stress and can initiate sharkskin melt fracture at the die lip. The corrective measure on production lines is not to lower the melt temperature below 170 °C, because unmelts may persist in the inner screw channels of a barrier screw, but to add 0.02–0.05 wt% of a fluoropolymer processing aid and to increase the die gap or reduce take-off speed. The barrel profile for this grade is typically 180 °C in the feed zone, 210 °C in the metering zone, and 220 °C at the adapter and die, with melt temperature kept below 240 °C to avoid oxidative gel formation and surface defects.

Apparent shear viscosity at 190 °C and an apparent wall shear rate of 100 s⁻¹ is not reproduced in the public datasheet; however, metallocene LLDPE of equivalent density and melt index typically exhibits a shear-thinning exponent of 0.4–0.6 over the shear rate window 100–1000 s⁻¹, which means that head pressure on a spiral die can be 15–35 % higher than Ziegler-Natta LLDPE of the same melt index. A capillary rheometer with a 1.0 mm diameter die and 20:1 length-to-diameter ratio is recommended for lot-to-lot flow verification when changing from pellets to regrind. The critical wall shear stress for sharkskin onset in narrow-MWD mLLDPE is often in the range 0.10–0.30 MPa at 190 °C; once die lip deposits build on the outer lip, the local shear stress exceeds the critical value and the defect appears as chevron or sharkskin roughness. On a 90 mm single-screw extruder running at 120 kg/h, a head pressure increase of 10–15 bar after 8 h of continuous operation usually indicates die lip fouling rather than a change in melt index.

What Limits Bubble Stability in High-Stalk Extrusion?

High-stalk extrusion of HPR18H10AX is constrained less by the melting point than by the strain-hardening deficit of a narrow molecular weight distribution. In low-density polyethylene, long-chain branching generates elongational viscosity that rises rapidly at high Hencky strain; HPR18H10AX lacks this mechanism, so the maximum stable blow-up ratio is generally lower than that of an LDPE-rich blend. On a high-stalk line with a die diameter of 160 mm and a bubble diameter of 400 mm, the frost line may fluctuate by ±15 mm if the ambient air temperature varies by 5 °C, producing gauge variation of ±8 % at the collapse frame. To compensate, converters often pre-blend 10–20 wt% of a high-pressure LDPE with melt index 0.3–0.5 g/10 min; this restores some strain hardening but reduces dart impact and raises haze. If a pure HPR18H10AX structure is required, a dual-lip air ring and internal bubble cooling are recommended, and the blow-up ratio should be limited to 2.0–2.8 unless line trials demonstrate otherwise. Published data for HPR18H10AX-specific bubble stability are limited; the values above are based on industrial practice with metallocene LLDPE of the same nominal density and melt index.

Where HPR18H10AX replaces a conventional Ziegler-Natta butene LLDPE of 0.918 g/cm³ density and 1.0 g/10 min MFR in a lamination film, the principal differences are observed in heat seal strength and optical haze. With a jaw sealer set at 110 °C, 0.5 s dwell, and 0.3 MPa pressure, the metallocene grade can produce seal strengths above 5 N/15 mm on a 50 µm blown film, whereas the Ziegler-Natta control may require 120–130 °C to reach equivalent strength; this is a published class difference, not a grade-specific HPR18H10AX guarantee. Haze measured by ASTM D1003 on the same 50 µm film is generally below 8 % for the metallocene grade and above 12 % for the Ziegler-Natta control, although surface roughness and die lip deposits influence the result. In heavy-duty sack applications, dart impact measured by ASTM D1709 method A at 25 µm may improve by 20–50 % relative to a butene Ziegler-Natta LLDPE of equal density, but the improvement is reduced when the structure contains more than 20 wt% recycled post-industrial film because contamination particles dominate puncture initiation. No claim is made for hot tack without specifying seal temperature, dwell time, and peel speed; the converter must generate a hot tack curve on the actual sealing equipment.

Hot tack performance is more sensitive than seal initiation to molecular architecture. Because HPR18H10AX has a narrow composition distribution, the disappearance of the crystalline lattice over a narrow 5–10 °C window produces a sharper hot tack peak. That peak can exceed 2.5 N/25 mm on a 40 µm film at 110 °C with 0.5 s dwell and 0.2 MPa pressure, but the seal strength may fall rapidly above 130 °C because the film enters the melting region and loses elastic recovery. Converters running vertical form-fill-seal machines should set the seal bar temperature to the rising edge of the hot tack curve rather than to the maximum hot tack value. Grade-specific published hot tack data for HPR18H10AX are limited; the values cited are class-typical for metallocene LLDPE of 0.918 g/cm³ density and 1.0 g/10 min MFR.

In three-layer coextrusion, HPR18H10AX is ordinarily placed in the core or skin layer, not in a tie layer. When paired with an ethylene-vinyl alcohol barrier layer, the melt temperature of the mLLDPE should not exceed 230 °C at the adapter to avoid thermally degrading the EVOH; a melt temperature split of 10–15 °C between the mLLDPE and the barrier layer is common on a five-layer line. The grade is not suitable as a skin over nylon if the die temperature must exceed 250 °C, because local gel formation may occur in dead spots behind the spiral mandrel. In cast coextrusion of polypropylene and HPR18H10AX, the rheology mismatch between PP and mLLDPE is lower than between PP and high-pressure LDPE, but edge encapsulation and layer thickness uniformity should be monitored by cross-web gauge profiling at intervals of 30 min.

When Hexene Distribution Replaces Ziegler-Natta Heterogeneity

If a processor substitutes HPR18H10AX for a Ziegler-Natta hexene LLDPE of identical density and melt index, the immediate change is a reduction in seal initiation temperature and an increase in film clarity, but not a uniform improvement in all mechanical properties. Machine-direction tear strength may be lower in the metallocene grade because the narrower composition distribution suppresses the high-molecular-weight species that resist tear propagation; Elmendorf tear measured by ASTM D1922 on a 25 µm film can shift downward by 10–25 % relative to the Ziegler-Natta hexene control, while transverse-direction tear may remain within 5 % or improve. This asymmetry is a known trade-off in mLLDPE film design and is not a defect. If a converter requires high MD tear for perforated packaging films, HPR18H10AX may need to be blended with 20–30 wt% of a broad-MWD LLDPE or LDPE, but this blend raises haze and lowers dart impact. The reduction in low-molecular-weight extractables is the primary regulatory advantage; hexane extractables tested under FDA 21 CFR 177.1520(c) conditions are expected to be lower than those of a Ziegler-Natta LLDPE of the same density, but the final article must be tested because extrusion temperatures above 240 °C can generate oxidative by-products that alter the extractable profile.

For cast film and extrusion coating operations, HPR18H10AX requires different die edge stabilisation than a high-pressure LDPE. Neck-in is typically greater because the narrow molecular weight distribution lacks long-chain branching; a vacuum box and edge bead reduction are therefore used. Pellets should be kept below 50 °C in closed silos and protected from ultraviolet exposure, and regrind addition above 15 wt% may cause gel defects if the regrind contains oxidised film from high-scrap side trim. No pre-drying is required when bags are intact and ambient relative humidity is below 80 %. Incompatibility with strongly oxidising acids or aromatic hydrocarbon solvents at elevated temperature is a general polyethylene limitation; HPR18H10AX is not an engineering polymer for continuous chemical exposure.