LLDPE 218WJ and 218NJ: The SABIC Film-Grade Family Compared requires separating base-resin equivalence from surface-additive divergence. Both grades are butene-copolymer linear low-density polyethylene resins supplied for blown-film extrusion with a nominal density of 0.918 g/cm³ under ISO 1183-1 and a melt flow rate of 2.0 g/10 min under ISO 1133-1:2022. They are frequently treated as interchangeable base resins in polyolefin film converting, but substitution without adjusting slip, antiblock, and downstream packaging parameters creates measurable differences in blocking force, coefficient of friction, haze, and gloss.

Because the two grades are produced on the same butene-based LLDPE platform, lot-to-lot density variation is typically controlled within ±0.001 g/cm³ and MFR variation within ±0.2 g/10 min. This narrow specification window allows converters to use the same screw design, die geometry, and downstream haul-off without modifying the melt-processing recipe. What cannot be assumed is surface behavior after winding: slip and antiblock additives have different residence-time requirements and air-exposure sensitivities.

The 218 series is based on a Ziegler-Natta-catalyzed ethylene-butene copolymer. Butene incorporation creates short-chain branching that disrupts crystallinity more effectively than traditional high-pressure LDPE long-chain branching at the same density. The crystalline fraction is therefore lower than an LDPE of equivalent density, which reduces stiffness and increases dart impact, tear, and puncture. The melt rheology of the base resin is shear-thinning; under film extrusion shear rates of 100–500 s⁻¹, apparent viscosity falls sufficiently to allow high output on grooved-feed extruders. However, the absence of long-chain branching also lowers melt tension, so bubble stability is more sensitive to frost-line geometry and internal bubble cooling than LDPE.

What Limits the Interchangeability of 218WJ and 218NJ in Blown Film?

The primary converter-visible difference is additive package. 218WJ is supplied with a low-slip, low-antiblock surface system, whereas 218NJ is specified with a migratory slip and antiblock package that reduces film-to-film friction after bloom. In high-density packaging operations, a film with kinetic coefficient of friction above 0.5 can wander on polished metal rollers and generate enough static charge to disrupt photoelectric sensors. The slip additive in 218NJ migrates to the surface over 24–48 h at ambient temperature and more rapidly at sustained winding temperatures of 40–50°C. During this migration period, COF changes from near-neat resin values to the equilibrium datasheet value, and corona treatment must be delayed or compensated if lamination or printing is planned.

Slip additives in polyethylene film are typically long-chain fatty amides such as erucamide or oleamide. They bloom to the surface because of limited compatibility with the polyethylene matrix; migration kinetics follow a diffusion-controlled process accelerated by temperature. At 40°C, equilibrium COF may be reached in 8–12 h, while at 23°C the same film may require 48–72 h. Antiblock agents, often synthetic silica or zeolite, act by creating surface micro-roughness that reduces film-to-film contact area. The tradeoff is optical: surface scattering increases haze and reduces 45° gloss. 218NJ is therefore not a direct substitute for 218WJ in clear display overwrap, even though both films share the same base resin.

On a production line using a 65 mm grooved-feed single-screw extruder with 30:1 L/D and a 250 mm diameter spiral-mandrel die, both grades process stably at melt temperatures of 180–220°C and die gaps from 1.8 mm to 2.5 mm. Lower melt temperatures preserve slip additive activity but raise die lip shear stress, increasing the risk of sharkskin on high-output lines. Higher die gaps reduce shear stress but coarsen the melt memory at the die exit and require a larger blow-up ratio to rebalance orientation. A common operating set point is a BUR of 2.5:1 with a frost-line height of 6–10 die diameters. At these settings, the 2.0 MI base resin yields a stable bubble with sufficient transverse orientation for balanced tear properties. When 218WJ is run at high stalk heights without internal bubble cooling, bubble instability is observed above 3.0:1 BUR because the low melt tension cannot resist the air pressure differential. The same instability occurs with 218NJ, but slight changes in slip migration at the outer bubble surface can reduce the perceived stickiness of the collapsing frame, improving bubble tracking over metal guide rolls.

The spiral-mandrel die distributes the 2.0 MI melt into a circular film bubble. Melt pressure at the die entry is commonly 25–38 MPa depending on die gap, screw speed, and temperature profile. Specific energy input is near 0.22–0.27 kWh/kg for a 65 mm grooved-feed extruder at 120 kg/h. These conditions are below the shear stress threshold for melt fracture in the base resin, but antiblock particulates in 218NJ can create micro-scratches on the die lip over prolonged campaigns. Die lip cleaning intervals are therefore shorter when running 218NJ continuously; accumulated antiblock at the die exit may produce die lines in the film. When die lines exceed 2–3 µm in depth, haze and weak spots become measurable. In practice, purging with a higher-MI LDPE or a commercial die-cleaning compound at shift changes is used to remove these deposits.

Mechanical and Optical Property Divergence Appears After Surface Additive Migration

Bulk mechanical properties of films made from the two grades are close because the base resin parameters are identical. Divergence appears in optical and surface measurements after additive bloom. Antiblock particulates in 218NJ scatter light and reduce gloss, while the absence or low loading of such particulates in 218WJ preserves gloss and clarity. However, 218WJ’s higher surface friction increases blocking force, particularly in warm warehouse conditions or high winding tension. The table below lists typical comparison values obtained from 25 µm blown film produced at 2.5:1 BUR and conditioned for 48 h at 23°C and 50% RH. Values are typical datasheet figures, not specification limits.

Seal behavior of the base resin is dominated by melting range and not by surface additives until the seal jaws contact the film surfaces. The butene LLDPE melting endset near 121°C allows seal initiation below 110°C in bench testing according to ASTM F88. In hot-tack tests per ASTM F1921, peel forces after sealing remain measurable over a window from approximately 95°C to 115°C. 218NJ’s slip additive can slightly reduce hot-tack if a thick additive bloom layer is present, but the effect is generally smaller than the reduction caused by over-treatment of the film surface with corona discharge. Converters must control backside treatment to avoid surface oxidation that interferes with heat-seal fusion.

When 218NJ Replaces 218WJ in High-Speed Form-Fill-Seal Lines

In vertical form-fill-seal and horizontal flow-wrapping equipment, the film is pulled over forming collars under constant tension. If the film-to-metal COF is too high, intermittent stick-slip causes product misalignment and seal-jaw misplacement. Replacing 218WJ with 218NJ typically stabilizes film transport after the slip additive has bloomed, allowing line speed to be held at the equipment’s rated throughput. However, the lower blocking force of 218NJ also changes roll formation; a softer roll profile may require lower winding tension to prevent telescoping. Seal parameters should not be transferred automatically from 218WJ to 218NJ. A starting seal-jaw temperature of 115–125°C, a dwell time of 0.5–0.8 s, and 2.5 bar pneumatic jaw pressure are common reference values, but grade-specific trials on the actual packaging machine are necessary because jaw size, serration geometry, and film gauge alter heat transfer. Published data for this specific configuration is limited.

Compliance statements for both grades are anchored to olefin polymer regulations. SABIC technical literature refers to FDA 21 CFR 177.1520(c) 3.2a for food-contact use and EU Regulation 10/2011 with specific migration limits. Neither grade is formulated with heavy-metal-based pigments, and both are typically considered compliant with REACH and the RoHS Directive 2011/65/EU as unpigmented polyolefin materials. The table below summarizes standards relevant to grade comparison and film qualification.

Thin-gauge greenhouse and silage films often require high cling and low slip to remain in contact with metal frames, reduce air ingress, and allow film layers to be tied or sealed without slipping. 218WJ is used in these applications because its surface retains a higher coefficient of friction and does not carry the antiblock-induced haze penalty. Roll blocking is managed by winding tension below 0.5 N/mm of web width and by warehouse storage below 35°C. Heavy-duty sack and construction film require puncture resistance and tear-propagation resistance at low temperatures. The shared 0.918 g/cm³ butene LLDPE base retains ductility at freezer temperatures, but the low additive loading in 218WJ can produce blocking in gusseted sacks stacked under load. 218NJ, with antiblock, reduces such blocking but may lower surface friction below the level required for stable pallet stacking of sacks in high-humidity warehouses. Selection is therefore application-specific: the film surface requirement, not the base-resin density or MFR, determines which grade is suitable.