PetroChina Daqing KunLun mLLDPE HPR18H27DX enters the blown film process with a density of 0.918 g/cm³ when measured per ISO 1183-1 and a melt flow rate of 2.7 g/10 min under 2.16 kg at 190°C per ISO 1133-1:2022. Those two values alone do not explain its processing behavior. The metallocene catalyst system produces a narrow molecular weight distribution, typically with a polydispersity index below 3.0, and the hexene comonomer is incorporated more uniformly along the chain than in a Ziegler-Natta butene LLDPE of the same density. On a 65 mm grooved-feed single-screw extruder with a 30:1 L/D ratio and a barrier mixing section, the resin generates melt pressure at the die that is 20–35% higher than a conventional 0.918 g/cm³ butene LLDPE at the same screw speed. The practical consequence is that a standard annular die gap of 1.0 mm—acceptable for a Ziegler-Natta film resin—produces melt fracture, elevated head pressure, and bubble instability. The corrective sequence begins with widening the annular die gap to 2.0–2.5 mm, not with raising barrel temperature. A barrel profile of 180/190/200/205/205°C from feed throat to die, with adapter and die held at 205°C, produces a melt temperature of 190–205°C. When melt temperature exceeds 215°C, melt tension drops and bubble wander increases; when melt temperature falls below 185°C, screw torque rises and gel particles may appear as arrow-shaped fisheyes in a 25 µm film. Bubble stability is the controlling process conflict at a blow-up ratio of 2.0–2.5. Unlike a high-melt-strength LDPE, the metallocene narrow molecular weight distribution does not sustain a high stalk. Frost line height is therefore held at 1–2 die diameters, and internal bubble cooling is used to counteract the low melt tension. A 20–30 wt% addition of high-pressure LDPE with a density of 0.924 g/cm³ and a melt flow rate below 0.5 g/10 min stabilizes the bubble and reduces neck tension but lowers dart impact. This is the central trade-off: each 10 wt% increment of LDPE above 20 wt% typically reduces blown film dart impact measured per ASTM D1709 Method A by 8–15%, while Elmendorf tear measured per ASTM D1922 shifts from TD-dominant to more balanced MD/TD values. The film is also evaluated for haze per ASTM D1003 and for coefficient of friction per ISO 8295; films containing slip and antiblock additives require aging for 48–72 h before stable surface values are reached. Pellets do not require pre-drying unless outdoor storage above 60% relative humidity has created surface condensation; if pellet surface moisture is present, a hopper dryer at 65–70°C for 2 h is sufficient because the resin is not hygroscopic. Incompatibility appears when the mLLDPE is processed on screws designed for high-melt-strength LDPE: localized polymer residence times above 230°C create oxidation gels, which show as fisheyes in film below 50 µm. A fluoropolymer processing aid at 300–800 ppm is used to remove melt fracture, but its feed must be continuous from start-up; intermittent addition produces cyclic head pressure.

What Lowers Seal Initiation Temperature in Hexene-Based mLLDPE Compared with Ziegler-Natta Grades?

The uniform comonomer incorporation in the hexene-based metallocene resin narrows the melting endotherm and shifts the seal initiation temperature downward relative to a Ziegler-Natta butene LLDPE of the same 0.918 g/cm³ density. Seal initiation temperature, defined as the seal bar temperature at which seal strength reaches 4.4 N/15 mm, is measured per ASTM F2029; published data for the 0.918 g/cm³ hexene metallocene class at a sealant thickness of 15 µm typically place the value between 100°C and 105°C, which is 10–15°C lower than a 0.918 g/cm³ Ziegler-Natta butene LLDPE. In three-layer polyethylene structures for vertical form-fill-seal machines, the mLLDPE is coextruded as a 10–20 µm sealant web against a Ziegler-Natta LLDPE core, with the outer skin chosen from HDPE or LDPE for heat resistance and printability. The processing advantage is not limited to lower energy input; the hot-tack plateau measured per ASTM F1921 remains above 2 N/15 mm over a wider seal-temperature window, which permits sealing through milk powder or detergent dust contamination on a filler running 60 packs/min. The sealant layer must be cooled to below 40°C before the film enters the forming collar; otherwise the film puckers at the gusset fold and produces leakers. Seal bar pressure is set to 0.5–0.8 MPa depending on jaw design, and dwell time is not reduced below 0.4 s when running laminated polypropylene/polyethylene structures because the mLLDPE surface requires thermal penetration through a reverse-printed film. Addition of 20 wt% LDPE to the sealant layer raises seal initiation by 3–7°C but broadens the hot-tack plateau and helps in high-speed horizontal machines where the film path is unsupported. At the opposite extreme, sealant layers below 10 µm show position-sensitive hot tack; misalignment of the seal jaw by as little as ±0.5 mm can reduce the measured hot tack by more than 30%. The operational boundary is therefore maintained by continuous jaw alignment verification rather than by increasing seal dwell time.

Low-Temperature Ductility in Frozen Food Packaging Structures

Frozen vegetable and potato product lines that fill at -20°C impose flexural fatigue on film during auger dosing and vibratory settling. A 45 µm reverse-printed laminate with a 15 µm sealant web of HPR18H27DX is evaluated for dynamic puncture after conditioning for 48 h at -20°C using ASTM D3420 with a 4.3 mm probe, and for Elmendorf tear per ASTM D1922 on the same conditioned specimens. The hexene comonomer fraction in the 0.918 g/cm³ metallocene resin reduces low-temperature brittleness compared with a butene LLDPE of equal density; film dart impact values measured per ASTM D1709 Method A on a 25 µm monolayer typically exceed 800 g at ambient and remain above 500 g after conditioning, based on published data for the density/MFR class. On a horizontal form-fill-seal machine with a film draw length of 90 mm, the film path must avoid corona treatment on the sealant side; if discharge treatment is applied to the sealant web, slip agent migration changes the coefficient of friction against stainless steel from below 0.25 to above 0.35 within 72 h, and the film stalls at the forming collar. The specified seal temperature is 115°C for a 0.4 s dwell to achieve 30 N/25 mm seal strength per ASTM F88. This subambient application is not limited by the mLLDPE layer itself but by the ink binder used on the reverse-printed outer layer; solvent-based polyurethane ink binders can embrittle below -30°C, so nitrocellulose-based flexo systems are substituted where storage is below -25°C.

Heavy-duty sack conversion lines that run 50–60 sacks/min through gusset folding and bottom-sealing stations require film toughness at 100–180 µm thickness without splitting along the gusset crease. The sack structure is coextruded at a blow-up ratio of 2.2–2.5, with the HPR18H27DX layer comprising 60–70 wt% of the total structure and the remainder a high-melt-strength LDPE or Ziegler-Natta LLDPE. The dart impact of a 125 µm film measured per ASTM D1709 Method A is commonly above 800 g, and Elmendorf tear per ASTM D1922 is above 20 N/mm in both directions, which are threshold values required by polymer granule and fertilizer filling operations. The bottom seal is made by thermal sealing through the outer layers; to obtain a seal strength of 40 N/25 mm per ASTM F88, the seal jaw temperature is reduced by 15–20% relative to an LDPE-only sack because the mLLDPE sealant reaches plateau strength at lower temperature. Dwell below 0.8 s creates stringy seal failure at the bottom fold, a defect amplified when the sack contains dusty fill or when the film has been stored above 30°C and slip has bloomed to the surface. For outdoor intermediate bulk containers, a 2 wt% carbon black masterbatch is added to the core layer, but the addition reduces dart impact by 10–15% and forces a thickness increase to 150–180 µm to meet the same drop-test requirement.

When Agricultural Film Requires Anti-Fog Agent Compatibility at Low Addition Levels

Greenhouse cover films in the 150–200 µm thickness range are normally produced as three-layer coextrusions with EVA skins carrying anti-fog and UV-stabilizer packages. Replacement of the core layer with a 0.918 g/cm³ hexene-based mLLDPE changes anti-fog migration kinetics because the core layer crystallinity is lower than that of a conventional LLDPE core, and the uniform comonomer distribution allows faster diffusion of glycerol ester anti-fog additives toward the surface. In practice, the anti-fog loading in the mLLDPE core must stay below 1.5 wt% to avoid plate-out on haul-off rolls, while the EVA skins carry 2–4 wt% of the surface-active concentrate. The three-layer line is run with a die gap above 2.0 mm and the melt temperature between 190°C and 200°C to prevent pre-migration of anti-fog additives in the die. Light transmittance is measured per ASTM D1003 after 1000 h of accelerated weathering in a QUV chamber per ASTM G154, and tensile retention is measured per ISO 527-3 after 5000 h of xenon-arc exposure. Published data for this specific grade and anti-fog system are limited; however, field observations from three-layer greenhouse film lines indicate that recycled polyethylene content above 10 wt% creates gel streaks that scatter light and produce visible haze bands across the web. The operational boundary is therefore a regrind limit of 10 wt% and a filter pack of 60/80/100 mesh in the core extruder.

Cast film lines running above 300 m/min with polished chill rolls maintained at 22°C subject the molten web to high longitudinal extension rates that favor anisotropic tensile properties. The HPR18H27DX resin is cast at a melt temperature of 230–250°C through a 0.5 mm slot die onto a chill roll with an air knife; the resulting 12–15 µm film is used for machine-wrap and hand-wrap stretch film with a pre-stretch ratio of 200–250%. The low extractable fraction and narrow melting range reduce plate-out on the chill roll, but the same narrow molecular weight distribution lowers the draw-resonance margin when the draw-down ratio exceeds 25:1. To maintain web stability, processors blend up to 15 wt% LDPE or reduce line speed to 250 m/min. Tensile elongation at break is measured per ASTM D882 and puncture resistance per ASTM D5748; published data for 0.918 g/cm³ hexene metallocene cast film place elongation at break above 450% at 12 µm.

Why Extrusion Lamination on BOPP Substrates Requires an Air Gap Above 120 mm

For extrusion lamination of biaxially oriented polypropylene or polyester substrates with a thin mLLDPE sealant web, the melt is drawn through an air gap of 120–180 mm at 260–280°C; published data for this specific configuration is limited.