Blown-film conversion of SABIC LLDPE 218WJ for heavy-duty industrial sack applications is carried out on monolayer or two-layer blown-film lines equipped with a grooved-feed single-screw extruder with a 30:1 L/D ratio and a barrier screw with a mixing element. The grade, a butene-copolymer linear low-density polyethylene with a melt flow rate of 2.0 g/10 min at 190 °C/2.16 kg per ISO 1133-1 and a density of 0.918 g/cm³ at 23 °C per ISO 1183-1, is processed through a barrel profile from 180 °C at the feed throat to 200 °C at the die adapter. On a production line with a 75 mm grooved-feed extruder and a 250 mm internal bubble cooling die, the die gap is maintained between 1.2 mm and 1.8 mm, while the blow-up ratio is held between 2.2:1 and 2.8:1. Head pressure during steady operation falls in the 280–340 bar range, and the line output reaches 180–220 kg/h depending on screw speed, frost line height, and internal bubble cooling airflow. The film gauge is controlled at 50–80 µm, with the frost line height set between 650 mm and 800 mm above the die face to avoid low stalk wobble and random gauge variation.

Failure modes observed on high-output sack lines include frost-line oscillation at gauge tolerances below ±5% and irregular transverse tear propagation when the internal bubble cooling differential exceeds 15 °C across the bubble circumference. Dart impact, measured on a 50 µm monolayer film according to ASTM D1709 Method A, is highly sensitive to die geometry; widening the die gap from 1.2 mm to 1.8 mm at constant output increases dart impact but degrades haze from approximately 8% to 12% as measured by ASTM D1003. Elmendorf tear tested per ASTM D1922 is normally lower in the machine direction than in the transverse direction under high-stalk processing, and the imbalance can be reduced by lowering the frost line height by 100 mm or increasing annular air velocity. Converting operations on this film use thermal impulse sealers at 115–130 °C, with seal strength verified by ASTM F88. The film must remain free of low-molecular-weight processing aids that migrate to the seal area during storage and reduce seal strength after 28 days of ambient aging.

What Limits Bubble Stability When LLDPE 218WJ Is Processed on High-Stalk Agricultural Film Lines?

Agricultural greenhouse covers and crop-protection films produced from LLDPE 218WJ are run with a high-stalk bubble because increased molecular orientation in the machine direction improves tensile strength per ISO 527-3 and permits downgauging of 150 µm sheet. The resin is commonly blended with 5–8% UV stabilizer masterbatch in a low-density polyethylene carrier, plus 2–4% white or infrared additive masterbatch depending on crop requirements. The processing window narrows because a high-stalk configuration operates with a free stalk length of 7–9 die diameters before expansion; LLDPE 218WJ exhibits lower melt tension than hexene-copolymer LLDPE, so the bubble is prone to snap at blow-up ratios above 3.0:1. When the stalk length exceeds nine die diameters, bubbles show periodic diameter pulses of ±8 mm and internal bubble pressure fluctuates by more than 12 Pa, corresponding to gauge variation exceeding ±10% across the layflat width. The die gap for agricultural film lines is set between 1.6 mm and 2.0 mm to prevent melt fracture in the low-shear skin layer, while melt temperatures are held at 190–205 °C. Above 210 °C the low-molecular-weight tail of the molecular weight distribution causes film blocking on the collapsing frame and downstream embossing rollers.

At the winder, the film is treated by corona discharge to a surface tension of 38–40 mN/m measured with test inks per ISO 8296. Producers who coextrude the grade as the core layer and use metallocene-catalyzed LLDPE skins at 10–15% of total thickness obtain the necessary dart impact for 150 µm greenhouse sheet. All films intended for multi-season crop covers must be evaluated for tensile elongation retention after xenon arc weathering according to ISO 4892-2 and for tear propagation before and after UV exposure. Published data for this exact grade configuration under xenon arc exposure is limited, and converters are required to verify weathering performance on their own film structures rather than relying on resin-only ultraviolet resistance data. Processors must also avoid blending the grade with high levels of low-molecular-weight amine stabilizers because such additives can migrate to the film surface, raise the coefficient of friction above 0.40 measured by ISO 8295, and create blocking during multi-layer storage at 40 °C.

For thermal lamination sealant webs produced on tandem blown-film lines, LLDPE 218WJ is converted into sealant films for lamination to aluminium foil and oriented polypropylene in flexible packaging. On tandem extrusion equipment, the sealant layer is run at 15–30 µm and is combined in a three-layer structure with an LDPE core and a metallocene LLDPE skin. The 218WJ layer is processed at 200–210 °C because butene short-chain branching depresses the melting point to approximately 122 °C, as determined by differential scanning calorimetry using ISO 11357-3. The heat seal initiation temperature of the finished laminate is therefore lower than that of LDPE-only sealants, and converters test hot tack strength on a laboratory hot tack tester according to ASTM F1921 at 110–120 °C. The film must survive downstream extrusion lamination at line speeds of 150–200 m/min without wrinkling; this requires a coefficient of friction of 0.25–0.40 measured by ISO 8295 and a gauge profile within ±6%.

Surface treatment is required before lamination, and the converter typically targets a wetting tension above 40 mN/m according to ISO 8296. If the sealant web is stored longer than 14 days, corona treatment decays and can fall below 38 mN/m, producing delamination at the adhesive interface. The sealant layer must also be free of migratory slip additives above 800 ppm because migration into the adhesive tie layer reduces laminate bond strength measured by ASTM F904. Plant-scale runs show that an 800 ppm erucamide slip package causes a 15–20% drop in bond strength after 28 days at 40 °C when compared with a low-slip formulation, although published data for this exact grade configuration is limited. Lamination converters therefore specify a non-migratory slip system or use an anti-block additive based on synthetic silica at 2000–4000 ppm to maintain stable reel unwind and adhesive wet-out without compromising bond performance.

When the Grade Is Coextruded as the Inner Layer of a Three-Layer Frozen Food Package

Because the density of 0.918 g/cm³ provides a lower brittle temperature than higher-density polyethylenes, LLDPE 218WJ is used as the inner sealing layer in symmetric A/B/C structures for frozen vegetable and seafood packaging. The film is produced at 35–60 µm total thickness with a layer distribution of 25% skin / 50% core / 25% skin, where the 218WJ layer is placed on both skins. Processing is performed at 185–200 °C to limit thermal degradation and discoloration; the extruder screw is a low-shear mixer with a compression ratio of 3.2:1 and a metering-section flight depth of 3.5 mm. The die diameter is 300 mm, the die gap is 1.8 mm, and the blow-up ratio is 2.0:1 to reduce machine-direction orientation. The film is gusseted and converted on high-speed vertical form-fill-seal equipment at 80–120 cycles/min. The contact layer must comply with food-contact requirements under FDA 21 CFR 177.1520(c) 3.2a and EU Regulation (EU) No 10/2011 as amended; converters should obtain a food-contact certificate from the resin supplier for the specific grade and batch because compliance status depends on additive package composition.

Low-temperature performance is evaluated by measuring dart impact at -20 °C after conditioning the film for 48 h according to ASTM D1709; because the standard method is written for 23 °C, the laboratory must validate the conditioning procedure using ISO 291. Seal strength of the frozen package is tested after storage at -30 °C according to ASTM F88; the typical failure mode in this structure is not seal delamination but film tear outside the seal when the seal bar temperature exceeds 125 °C. A processing boundary emerges when the coefficient of friction rises above 0.45 after the film is chilled below 10 °C, causing jamming on the forming collar. Converters therefore add a high-molecular-weight slip masterbatch at 2–3% and set the chill air temperature on the forming tube to 8–12 °C. Excessive slip addition above 4% causes plate-out on the collapsing frame and contaminates the seal jaws after 6–8 h of continuous production.

Typically, carrier bag extrusion using LLDPE 218WJ is configured around a 65 mm barrier screw with a 28:1 L/D ratio and a melt pump ahead of a 200 mm die. The base resin is dry-blended with 10–40% post-consumer recycled polyethylene in pellet form; the recyclate is pre-dried at 70 °C for 2 h when the storage environment exceeds 60% relative humidity to prevent surface moisture pinholes. The extruder profile is 170 °C at the feed, 185 °C in the compression section, and 195 °C at the adapter. Back pressure across a 200-mesh breaker plate is monitored and must remain below 240 bar; when pressure exceeds 260 bar, the recyclate batch is rejected because high gel loading causes visible spots in the film and reduced tear resistance. The film is produced at 25–40 µm thickness with a blow-up ratio of 2.5:1 and is subsequently slit and heat-welded into vest carrier bags.

The principal technological conflict in this application is the effect of recyclate contamination on Elmendorf tear strength; bags containing polypropylene caps or polyethylene terephthalate shreds cause tear deviations of more than 20% as measured by ASTM D1922. Melt filtration is therefore installed with 120-mesh screens and changed after each 8 h shift. The film surface is treated to 36–38 mN/m for printing with flexographic inks; higher treatment levels cause blocking at the bag wicket stack. The target material for 25 µm film is an elongation at break above 400% in both directions per ISO 527-3; batches that fall below this level are diverted to non-woven bag liners. The recyclate content and final film are not considered industrially compostable under EN 13432 unless separately certified in a specific formulation, and converters must not label such bags as biodegradable or compostable without independent certification.

Silage Film Seal Strength and Oxygen Transmission in Black/White Coextrusions

Silage bale and clamp covers derived from LLDPE 218WJ are produced as two-layer black/white structures with a white outer layer containing 6–10% titanium dioxide masterbatch and a black inner layer containing 4–6% carbon black masterbatch. The line configuration uses two extruders with a 300 mm coextrusion die and a die gap of 1.7 mm; the black layer is processed at 190–200 °C while the white layer is held at 185–195 °C because carbon black accelerates heat absorption and surface oxidation. The total film thickness is 80–100 µm, and the blow-up ratio is 2.3:1 to balance transverse elongation and tear resistance. The white layer must maintain a light reflectance above 70% measured by ASTM E1331; the black layer provides opacity and blocks light transmission below 1% at 550 nm using ASTM D1003. Oxygen transmission rate for a 100 µm film is controlled by the density and crystallinity of the LLDPE layers; butene-copolymer LLDPE of this density generally shows oxygen permeability in the range of 300–400 cm³·100 µm/(m²·day·atm) at 23 °C and 0% RH per ISO 15105-2, but lot-specific values must be obtained from a permeation laboratory.

At the sealing station, the film is joined by an impulse welder at 120–140 °C; seal strength must exceed 25 N/25 mm when tested according to ASTM F88. A recurrent failure is seal creep under constant load at 40 °C in the field; the specification therefore includes a 24 h sustained-load test at 15 N/25 mm according to an internal method aligned with ASTM D882. Processing staff must avoid blending more than 10% repelled silage film with oxidized surfaces because oxidized material reduces seal strength by 25% and creates pinholes at the liquid silage interface. The black inner layer must also be selected for low moisture vapor transmission rate to maintain silage density; moisture vapor transmission rate is tested at 38 °C and 90% RH per ISO 15106-3. Batches exhibiting lot-to-lot carbon black dispersion variation above 15% in optical density across the web are rejected because such variation indicates agglomerate formation that will reduce tear resistance and field service life.