On high-output three-layer blown-film lines running SABIC LLDPE 121WJ for chemical and fertiliser liners, the melt temperature is held between 195 °C and 215 °C, with the lower boundary set by sharkskin onset in the inner seal layer and the upper boundary by oxidative gel formation in stagnant regions of the spiral mandrel die. The die gap is set at 2.0 mm to 2.5 mm, and a blow-up ratio of 2.2:1 to 2.7:1 is maintained to balance machine-direction and transverse-direction tensile behaviour. At a total film thickness of 120 µm to 180 µm, the core layer is typically 100% SABIC LLDPE 121WJ, while the outer layers may carry 5–10 wt% clean post-industrial reclaim, provided the reclaim has an MFR within ±0.15 g/10 min of the virgin material and is screened through a 125 µm mesh screen pack. Frost line height is stabilised at 7–9 die diameters by a closed-loop internal bubble cooling system with stack air temperature controlled to 12–18 °C. Tensile properties are monitored against ASTM D882-18, and Elmendorf tear is monitored against ISO 6383-2:1983. The characteristic failure mode in liners produced from this grade is not dart puncture but low-frequency fatigue tearing at folded gusset creases; this is controlled by maintaining transverse-direction elongation at break above 750% at a test speed of 50 mm/min. Pre-drying of the pellets is not required because bulk density and surface condensation are controlled when silo temperature is kept at least 3 °C above ambient dew point.

The grade is supplied as a butene-based linear low-density polyethylene with published data sheet values of melt flow rate at 1.0 g/10 min under 190 °C/2.16 kg in accordance with ISO 1133-1:2022, and density at 0.918 g/cm³ in accordance with ISO 1183-1:2019. The melting peak measured by ISO 11357-3:2018 lies in the range of 121–124 °C. These values constrain the processing window in thick-gauge blown film because the low density reduces melt stiffness relative to HDPE, while the 1.0 g/10 min flow rate requires a grooved-feed extruder or an aggressively cooled feed throat on a smooth-bore machine to prevent partially molten pellets from blocking the intake zone. On a 55 mm three-layer line running at 180–220 kg/h, the resulting die pressure is normally 18–24 MPa, and the extruder discharge temperature is kept below 220 °C to limit gel accumulation on the die lip during runs longer than 48 h.

Does HDPE blending above 20 wt% degrade 121WJ dart impact faster than tear propagation?

Published data for this specific configuration is limited; however, comparative data for butene LLDPE/HDPE systems with similar MFR and density indicate that the relationship between HDPE addition and dart impact retention in heavy-duty sack films is non-linear and depends on melt viscosity mismatch between the two polyethylenes. In a 70/20/10 structure of SABIC LLDPE 121WJ, HDPE with a density of 0.950 g/cm³ and an MFR of 0.5 g/10 min at 190 °C/2.16 kg, and HDPE reclaim, dart impact at 100 µm under ASTM D1709A retains 80–88% of the pure 121WJ baseline when the resins are melt-blended in a grooved-feed single-screw extruder with a barrier-flighted screw of L/D 30:1 and a shearing Maddock section operating at a specific energy input of 0.25–0.30 kWh/kg. Above 20 wt% total HDPE, dart impact retention drops below 75%, and Elmendorf tear in the transverse direction falls to less than 65% of the virgin 121WJ value, although secant modulus at 1% strain rises from approximately 190 MPa to 250 MPa when measured by ISO 527-3:2018.

The practical processing boundary is the heat-seal jaw temperature. The seal initiation temperature shifts from 112 °C for the pure 121WJ layer to 121 °C at 20 wt% HDPE, and the sealing window narrows from approximately 20 °C to 14 °C. This occurs because the HDPE-rich phase raises the average melting point of the seal layer and reduces interdiffusion at the seal interface. End products are 25 kg and 50 kg shipping sacks for granular fertiliser and polymer resin, where the outer layer is HDPE-rich for stacking stiffness, the core is 121WJ for puncture resistance, and the inner layer remains a 30–40 µm 121WJ-rich layer for seal integrity. On production-scale lines, the dominant failure mode in these blends is not seal failure but interlayer instability in the spiral die when the viscosity ratio between the HDPE-rich skin and 121WJ-rich core exceeds 1.8, producing wavy interfaces that lower tear resistance.

Oscillating haul-off and die gap selection in stretch hood films containing 121WJ

Stretch hood and silage cover films require high retention force and puncture resistance, so SABIC LLDPE 121WJ is placed in the core of a three-layer cast or blown structure while the skin layers are formulated with metallocene LLDPE or EVA to provide cling and low-temperature seal. On a 65 mm five-layer cast line with an L/D 33:1 barrier screw and a 0.8 mm primary die gap, the 121WJ-rich core is processed at a melt temperature of 220–235 °C; the higher temperature is required because the elongational viscosity of butene LLDPE at a draw ratio of 4.5:1 suppresses neck-in only when the melt reaches a shear viscosity below 1,200 Pa·s at 100 s⁻¹. Edge-trim is normally recycled at up to 15 wt% into the core layer after granulation and passing through a 100 µm non-woven screen pack. The main operational boundary is cling additive migration: glycerol monooleate or polyisobutylene tackifiers in the skin layers migrate into 121WJ within 72 h, reducing the coefficient of friction of the core from 0.55 to 0.38 without affecting tensile properties.

Films produced in this configuration are used as stretch hoods for palletised ceramic tile and as silage stretch films of 120 µm thickness. The 121WJ core contributes to tear resistance measured by ISO 6383-2:1983, while the metallocene skin layers control cling force in the range of 15–25 N/25 mm when tested by ASTM D4649-20. A process conflict arises on oscillating haul-off units because the low melt stiffness of 121WJ can produce edge flutter above 45 m/min line speed if the die gap is wider than 2.2 mm; reducing the die gap to 1.6–1.8 mm raises shear heating but restores bubble or web stability. Published data for this specific configuration is limited, so line speed and die gap must be verified on the actual haul-off geometry, particularly when the oscillating angle exceeds 30°.

Where a 15–25 µm heat-seal web is required in laminated flexible packaging, SABIC LLDPE 121WJ is used as the inner blown-film substrate in the lamination structure because its low density and limited branching give a seal initiation temperature that is lower than that of HDPE and allow a wider jaw-temperature operating window. The film is produced on a three-layer blown-film line at a die gap of 2.0 mm and a blow-up ratio of 2.5:1; the film is then corona-treated to a surface tension of 38–42 mN/m according to ISO 8296:2003 and laminated by solventless adhesive to metallised PET or oriented polyamide. The seal layer is not required to provide barrier, but it must maintain a minimum seal strength of 5 N/15 mm across a sealing temperature from 115 °C to 145 °C when tested by ASTM F88/F88M-21. Migration testing under EU Regulation 10/2011, Annex V, EN 1186-1:2002, and EN 1186-3:2002 applies only after the complete laminate is evaluated, because the adhesive and printing ink contribute to overall migration.

The film used as a food-contact article in the United States falls under FDA 21 CFR 177.1520 when the olefin polymer meets the specified extractable fractions under n-hexane and xylene in the form intended for use. End products include stand-up pouches for dried soups and frozen vegetable bags, where the 121WJ layer must not be exposed directly to heat above 145 °C because of seal-through-contamination failure risk. The following matrix identifies the primary compliance boundaries applicable to qualified structures.

For laminated structures, the sealant film is not separately approved for direct food contact; the full laminate must be tested in the final form, including adhesive curing state and surface print. SABIC LLDPE 121WJ is supplied as a natural pellet without slip or antiblock, so the heat-seal layer formulation must include a separate antiblock masterbatch at 1,500–2,500 ppm silica and a slip additive at 500–800 ppm erucamide if the film is run on high-speed form-fill-seal machinery exceeding 70 cycles/min.

When the frost line is moved above 9 die diameters, gauge variation in greenhouse film reaches process capability limits

Greenhouse film lines running SABIC LLDPE 121WJ at thicknesses of 150–200 µm operate with a blow-up ratio of 2.8:1 to 3.2:1 and an internal bubble cooling system that holds frost line height at 6–8 die diameters. If the frost line is raised above 9 die diameters to increase transverse-direction orientation, the low melt strength of butene LLDPE allows the bubble to enter a metastable state in which gauge variation measured by capacitive thickness scanning across the layflat exceeds ±6% at 2-sigma, compared with ±4% when the frost line is kept below 8 die diameters. Because greenhouse films are certified under EN 13206:2017 for covering films and require at least 365 kLy UV stabilisation for a three-season service life, the base resin is dry-blended with a polyolefin masterbatch containing 1.5–2.5 wt% HALS and 0.3–0.5 wt% hindered phenol stabiliser before extrusion. The masterbatch must be dosed by weight-loss feeder at the throat, not by screw speed, because its bulk density of 0.55–0.65 g/cm³ differs from the pellet bulk density of approximately 0.54 g/cm³ for 121WJ and promotes segregation in a hopper blender.

The end product is a three-layer greenhouse cover with 121WJ in the core and EVA-rich skins for thermal infrared retention; the core provides tear resistance against hail impact, while the skins provide low-temperature flexibility. A production-scale failure mode in these structures is bubble flutter when the internal bubble cooling fan is run above 60% of rated speed, which generates asymmetric velocity fields that remain visible as transverse gauge bands after collapsing. In addition, the 121WJ core must be limited to 55–65 wt% of the total film mass when EVA skins exceed 4 wt% vinyl acetate, because higher core fractions reduce interlayer adhesion and cause delamination at fold seams during installation of the greenhouse film.

Melt filtration thresholds shift when edge-trim reuse exceeds 25 wt%

Closed-loop reuse of edge-trim from cast and blown films based on SABIC LLDPE 121WJ requires stabilisation of the melt flow rate and filtration of gel particles before the regrind stream is re-introduced into the core layer. A continuous screen changer with a 125 µm woven wire primary screen and an 80 µm sintered non-woven secondary screen is installed between the grooved-feed extruder and the melt pump because edge-trim collected from the oscillating haul-off contains starch powder, paper dust, and oxidised polymer flakes that would otherwise raise the melt pressure by more than 8 MPa in a 90 mm extruder running at 450 kg/h. The melt pump suction pressure is maintained at 6–8 MPa, and the discharge pressure is controlled to 18–22 MPa to keep residence time in the die below 120 s. If the regrind fraction exceeds 25 wt%, the MFR of the compounded material at 190 °C/2.16 kg usually rises from 1.0 g/10 min to 1.3–1.5 g/10 min, which reduces bubble stability in a downstream blown-film unit and should be compensated by lowering melt temperature by 5–8 °C or reducing the die diameter.

This approach is often applied to refuse sacks and construction sheeting where post-industrial recycle content is declared under EN 15343:2007; the final film must still meet ASTM D1709A dart impact and ISO 527-3 tensile requirements. The specific operational boundary is that the 121WJ-rich reclaim is not processed through a vacuum-vented extruder without a vent stuffer, because the low bulk density of fluff lowers throughput by 15–20% and causes vent flow instability. Additionally, regrind storage in open containers should be limited to 24 h at ambient humidity above 80% RH because surface moisture on fluff increases hydrolysis of residual ester-based tackifiers and forms microgels that pass the 80 µm secondary screen and appear as fisheyes in the final film.