Linear low-density polyethylene (LLDPE) is a copolymer of ethylene and an α-olefin, commonly but-1-ene, hex-1-ene, or oct-1-ene, produced by low-pressure gas-phase, solution, or slurry polymerisation. Its short-chain branching distribution imparts a density range of 0.915 g/cm³ to 0.940 g/cm³ measured in accordance with ISO 1183-1:2019. Chinese converter specifications repeatedly list LLDPE 7042 and LLDPE 7050, making the phrase LLDPE 7042 and 7050: The Two Chinese Codes Converters Meet Most Often a practical sorting rule in flexible packaging enquiries. The 7042 designation is widely associated with a butene-based film resin having a melt mass-flow rate near 2.0 g/10 min under 190 °C/2.16 kg per ISO 1133-1:2022 and a density near 0.920 g/cm³. The 7050 designation appears in domestic film-grade datasheets with similar melt-flow and density targets, but additive composition can differ among suppliers, so equivalence must be verified from the certificate of analysis before line introduction.

Why Must a Converter Separate the 7042/7050 Code from Comonomer Type?

The 7042 and 7050 codes are frequently butene-based film-grade LLDPE; however, a grade code is not a comonomer declaration. Butene-based LLDPE generally displays lower hot-tack strength, lower dart impact, and lower Elmendorf tear when compared with hexene-based LLDPE of matching density and melt-flow rate. Tensile properties on 25 μm films should be measured per ISO 527-3:2018, dart impact per ISO 7765-1:2004 or ASTM D1709-22, Elmendorf tear per ISO 6383-2:1983 or ASTM D1922-15, and seal initiation per ASTM F1921/F1921M-18. Published data for 7050-specific comonomer disclosure are limited; therefore the converter should perform Fourier-transform infrared spectroscopy per ASTM D6645-18 for ethylene comonomer identification and differential scanning calorimetry per ISO 11357-3:2018 for melting peak confirmation when the certificate is ambiguous. A 7042-class resin with density of 0.920 g/cm³ is commonly a butene copolymer, but 7050 may be sourced from a different line with a different short-chain branching distribution. The practical risk is not a density disagreement but a film toughness shift that appears only after conversion.

On a conventional monolayer blown-film line using a 30:1 L/D barrier screw and a spiral mandrel die, LLDPE of the 7042 class is usually started at barrel zone settings of 150 °C, 170 °C, and 185 °C, with adapter and die temperatures from 190 °C to 210 °C. Measured melt temperature at the die inlet should be held between 190 °C and 230 °C; operation below 190 °C may raise melt pressure and increase sharkskin severity, while sustained operation above 230 °C increases the risk of oxidation gel formation. LLDPE has lower extrudate swell than high-pressure LDPE at equivalent die shear rate, so die pin geometry and die gap may require adjustment when a converter switches from a LDPE-heavy blend to a 7042 or 7050 resin.

Air Ring, Die Gap, and Frost Line Geometry in Monolayer Blown Film

Die gap for butene-based 7042-class LLDPE typically is set at 1.6 mm to 2.5 mm, and blow-up ratio is held between 2.0:1 and 3.0:1. Because LLDPE has a narrow molecular weight distribution and less strain hardening than LDPE, the bubble is less self-supporting at low frost line heights. A frost line height of 4 to 8 die diameters, combined with dual-lip air ring cooling, stabilises the bubble and controls MD/TD orientation balance. Elmendorf tear in the machine direction generally decreases as blow-up ratio increases; converters that require balanced tear values should verify by ISO 6383-2:1983 rather than adjusting frost line height by sight. A die gap below 1.4 mm can reduce bubble stability and increase melt fracture, while a gap above 2.6 mm may lower transverse direction stretch and reduce film flatness.

In coextruded barrier structures, LLDPE 7042 is often used as a sealant skin coextruded with EVOH or polyamide tie-layer cores. Sealant layer thickness of 10 μm to 25 μm in a total film thickness of 70 μm is common. Hot-tack testing per ASTM F1921/F1921M-18 should be run at sealing temperatures from 105 °C to 125 °C; butene-based LLDPE usually develops lower hot-tack force than hexene-based LLDPE at equivalent seal temperature. A converter should not assume that 7050 provides higher seal strength without a certified hexene incorporation value or a differential scanning calorimetry trace. For multi-layer lines with external deckles, the lower shear sensitivity of LLDPE relative to LDPE can increase back pressure at the same screw speed; actual head pressure should be recorded and compared against the die manufacturer’s maximum allowable pressure rating before increasing output.

When a Converter Must Re-Certify COF and Haze at 2.5 wt% Antiblock Loading

Slip and antiblock additive packages are compounding additions that are not always visible from the grade code. Coefficient of friction, both kinetic and static, is measured per ASTM D1894-14; haze is measured per ASTM D1003-13; gloss at 60° is measured per ASTM D2457-21. A conventional silica antiblock at 2.5 wt% alters haze and gloss, with the magnitude dependent on median particle size, particle size distribution, and refractive index match with the LLDPE matrix. Slip agents such as erucamide migrate to the film surface over time; COF should be evaluated after 48 h aging at 23 °C ± 2 °C and 50 % ± 10 % RH per ISO 291:2008. A film tested within 1 h of blowing may show COF above 0.50, while the same film after slip bloom may reach 0.15 to 0.25; the exact values depend on additive masterbatch concentration and screw dispersion. Converters who switch from 7042 to 7050 should re-run COF and haze on a 25 μm film sample because a different additive package can shift haze by a measurable amount even when density and MFR remain within the same specification window.

Where converters reduce raw material cost by blending 7042-class LLDPE with reclaimed LDPE or post-industrial film, addition levels of 10 wt% to 40 wt% recycled material are common. The recycled fraction shifts melt flow rate, gel count, and transparency. A high-shear single-screw mixer with a Maddock section operating at 200 °C to 220 °C is recommended because low-shear distributive mixing may leave high-viscosity LLDPE domains incompletely dispersed in the melt. Tensile properties and dart impact of the blend should be measured per ISO 527-3:2018 and ISO 7765-1:2004 because reclaimed content can reduce dart impact at a rate that is not linear with recycle percentage. Published data for 7050 in recycled-content blends are limited.

Feed Throat Temperature Limits and Pellet Handling Before On-Line Blending

LLDPE pellets are not inherently hygroscopic, but condensation on cold pellet surfaces can create feed-throat bridging below 10 °C pellet temperature. Pre-drying is generally unnecessary; if pellets have been stored outdoors at relative humidity above 60 %, a hopper dryer at 60 °C for 2 h removes surface moisture. Feed throat temperature should be kept between 20 °C and 50 °C; lower temperatures reduce screw intake and can generate flow instability in the metering zone. Purging between 7042 and 7050 deliveries on the same line is performed with a high-MFR LDPE purge resin at 190 °C to 210 °C. Because both grades are close in MFR and density, a 20 min purge at 2 kg/h to 5 kg/h is often sufficient, but a clear film sample after purging is required to confirm the absence of die-lip residue and unmelted pellets.

On-line gel counts using an optical gel counter should be recorded after each grade change and after any excursion above 230 °C. LLDPE 7042 and 7050 should not be blended with ionomer-based reclaim or certain amine-based processing aids without verification because acidic or amine residues can destabilise slip additives and shift surface COF. Residues from previous high-temperature polyamide purges can remain in the die adapter if purge temperature exceeded 250 °C; these residues appear as black specks in subsequent LLDPE film. Published data for 7050-specific interaction with amine processing aids are limited, so a laboratory film trial with full optical inspection is required before large-scale blending or sustained coextrusion operation.