SABIC LLDPE 518NT is a butene-copolymer linear low-density polyethylene pellet with a nominal density of 0.918 g/cm³ when measured to ISO 1183-1:2019 and a melt flow rate of 1.0 g/10 min at 190 °C/2.16 kg when measured to ISO 1133-1:2022. The grade is supplied as a natural resin without slip or antiblock additives. The absence of surface modifier packages permits the converter to set slip, antiblock, and process aid doses independently. In heavy-duty industrial liner production, the resin is run on monolayer or three-layer blown film lines with die diameters from 100 mm to 400 mm and die gaps between 1.8 mm and 2.4 mm. A grooved-feed extruder with a 30:1 L/D barrier screw and a Maddock mixing section is common. Melt temperatures are held between 190 °C and 230 °C. Higher temperatures do not improve optical clarity significantly and increase the risk of antioxidant depletion. The bubble is inflated to a blow-up ratio of 2.5–3.5. Internal bubble cooling is used when layflat width exceeds 900 mm because the low melt tension of butene-copolymer LLDPE reduces bubble stability under high throughput. The frost line height is set between 500 mm and 900 mm to balance MD/TD orientation and dart impact. Thickness gauges are profiled with a rotating die or oscillating haul-off. Gauge variation below ±5% is necessary for liners that carry UN dangerous goods. In this segment, film thickness ranges from 60 µm to 150 µm. The lower limit is set by puncture resistance during filled-bag handling. Dart impact is measured to ASTM D1709-16a. Puncture resistance is measured to ASTM D5748-19. A 5 wt% carbon black masterbatch is typically added when the liner carries light-sensitive intermediates. The masterbatch carrier is an LLDPE or LDPE with compatibility matching the base resin. Calcium carbonate filler is added at 5–20 wt% only in low-value liners. Filler above 20 wt% causes a measurable loss in dart drop and tear propagation. Film blocking in warm warehouses must be addressed by adding a slip/antiblock masterbatch. A starting point of 2–4 wt% of a masterbatch containing 5% erucamide and 10% synthetic silica is used on several conversion lines. That dose is not universal and must be reduced when the liner is printed with solvent-based inks because high slip migration can degrade ink adhesion.

The main processing conflict in this application is melt fracture. In spiral die gaps between 1.2 mm and 2.0 mm, sharkskin defects can appear when the wall shear stress exceeds the critical value for the resin. On production-scale dies, the onset is observed as a progressive loss of gloss and a rough matte surface on the film. The defect is more severe on the inner bubble surface where air-side cooling reduces surface temperature. Converters suppress sharkskin by opening the die gap to 2.2–2.4 mm, lowering extruder speed, or adding a fluoropolymer-based polymer processing aid at 200–500 ppm. The processing aid is normally added as a 2–3% masterbatch in LDPE. The exact dosage depends on the die shear rate. Published data for this specific configuration is limited. On a 65 mm extruder running 120 kg/h, melt pressure at the breaker plate is typically observed between 25 MPa and 35 MPa. A filter pack with 80/120/80 mesh screens reduces gel transmission but increases pressure drop. If pressure drop exceeds 10 MPa, the pack must be changed early because screen blinding causes shear heating and gel formation. The resin should not be run at melt temperatures above 240 °C for prolonged residence times. Thermal degradation produces crosslinked gels that appear as hard spots in the film and cause downgauging scrap. Processing stability is assessed by monitoring melt pressure and motor load. Batch-to-batch variation is normally low for this grade, but variations in supplier lot stabilizer content can slightly shift melt pressure.

Mechanical design for heavy-duty liners starts with tensile and tear data. Tensile properties of the finished film are measured to ISO 527-3:2018. Elmendorf tear resistance is measured to ASTM D1922-15. In practice, MD tear values are lower than TD tear values in high blow-up-ratio film. A BUR of 3.0 produces a more balanced shrink and impact profile. If the application requires flat drop resistance from 1.5 m, a light calcium carbonate load or a thicker film is used. Heavy-duty liners carrying sharp-edged regrind, glass, or metal turnings require puncture values above 30 N. Such values are compared after conditioning at 23 °C and 50% RH to minimize moisture effects. The resin is not suited to long-term UV exposure. Outdoor storage of filled liners beyond 6 months requires a UV masterbatch.

What Limits Seal Strength Retention Below 25 µm in High-Speed FFS Packaging?

Form-fill-seal operations at film thickness below 25 µm demand consistent resin homogeneity. The seal layers must flow uniformly through the die. A gel or poorly dispersed additive cluster in the seal lip creates a seal leak. In high-speed FFS lines running 60–80 bags/min, the seal bar contact time is short. Dwell times between 0.3 s and 0.8 s are typical for vertical FFS packaging of dry granular goods. The heat seal strength of the film is evaluated to ASTM F88/F88M-21. Hot tack is evaluated to ASTM F1921-18. These two methods are not interchangeable. A film can have high ultimate seal strength but fail hot tack because the molten interface opens under fill weight before crystallization completes.

The seal initiation window of butene-copolymer LLDPE occurs within the melting range. Differential scanning calorimetry to ISO 11357-3:2018 shows a broad melting endotherm for LLDPE. On FFS lines, seal bar settings are normally 10–15 °C above the film melting peak. For thin film, heat transfer through the structure is rapid. The seal bar temperature is reduced by 5–10 °C compared with thick film. If the seal bar temperature is too high, thinning at the seal interface reduces seal strength. If it is too low, the interface does not fully wet. The optimum window is validated by a seal curve generated at 0.5 s dwell and 0.5 MPa seal pressure. The operating boundary is narrow. A temperature change of ±5 °C can shift seal strength from acceptable to peel failure. The presence of migrating slip additives can also reduce seal strength. If a high erucamide level is used, the additive blooms to the surface and weakens the weld. In down-gauged FFS film, total erucamide content should not exceed 800 ppm in the seal layer unless sealing performance is revalidated.

Production-scale equipment for FFS film often uses a 75 mm extruder with a 30:1 L/D screw. The die gap is 1.8 mm. BUR is kept between 2.0 and 2.8 to limit thickness variation. A low BUR improves MD strength. The bubble is positioned to stabilize frost line height. If the frost line oscillates by more than ±50 mm, the film thickness profile changes and seal failure occurs at the bag corners. The grade has low gel tendency when the resin is fresh. However, dusty plant conditions or recycled edge trim introduce contaminants. Melt filtration with 120 mesh screens is required for film below 25 µm. The use of post-industrial regrind in the seal layer is limited to 10 wt% because higher levels increase gel count. The finished FFS film is typically corona-treated to 38–42 mN/m on the reverse-print surface. The seal surface is not treated.

Frozen Food Film: Thermal Stabilizer, Haze, and Puncture

At freezer temperatures, film must remain puncture-resistant. Frozen food packaging is produced at 25–40 µm and is tested at -20 °C or below. Dart impact is measured according to ASTM D1709-16a Method A. At low temperatures, polyethylene becomes more rigid, but LLDPE retains higher toughness than LDPE. The use of SABIC LLDPE 518NT as the main film resin is based on its combination of density and melt flow rate. The lower density reduces crystallinity. A lower crystallinity improves low-temperature crack resistance. The film is often run as a monolayer on a 160 mm die with a 2.0 mm die gap. Blow-up ratio is set to 2.2–2.6. High blow-up ratios increase TD strength but reduce MD tear and can produce an unbalanced film for heavy frozen goods. The grade does not include slip or antiblock. A separate masterbatch is necessary. For frozen food, the antiblock loading is kept low to avoid haze. Haze is measured to ASTM D1003-21. Gloss at 45° is measured to ASTM D2457-21. The target haze depends on the customer specification but is typically below 15% for a 25 µm film. The converter can reduce haze by lowering the frost line height and increasing quench rate. The trade-off is lower mechanical strength in the MD direction.

Production bottlenecks in frozen food film arise at the winder. Without antiblock, the warm film layers block tightly and tear during unwinding. But excessive silica increases haze and reduces cold-seal strength. The optimum silica particle size in the masterbatch is between 3 µm and 6 µm. Larger particles create visible lumps. The masterbatch is dry-blended before the hopper by a gravimetric dosing unit. If dosing accuracy is below ±0.5%, the local antiblock concentration varies and blocking occurs in patches. On high-speed flexographic printing lines, the film surface is corona-treated to 38–42 mN/m. Wetting tension is checked to ISO 8296.

The compliance position of the grade must be confirmed by the converter for the finished package. LLDPE manufactured from ethylene and butene falls under FDA 21 CFR 177.1520(c) for olefin polymers. For EU food contact, the finished film is evaluated against EU No 10/2011. Overall migration is assessed against the limit of 10 mg/dm² using the assigned food simulant. In the United States, the food type and use condition define specific migration limits. The grade itself does not provide a halal or kosher certification unless documented in the supplier lot certificate. For direct food contact, the converter must ensure that slip and antiblock masterbatches are also food-contact approved. The thermal stabilizer package in 518NT is designed for processing protection. Long-term exposure to hot food or retort conditions is outside the normal application envelope.

Greenhouse film production uses SABIC LLDPE 518NT as a middle or inner layer in a three-layer structure. The outer layer is usually LDPE or EVA loaded with UV stabilizers. The inner layer may contain anti-drip and anti-fog additives. The function of the LLDPE layer is to provide puncture resistance and recoverable stretch under wind pressure. A typical three-layer line uses die gaps of 2.0 mm to 2.6 mm. The blow-up ratio is set between 2.5 and 3.5 to balance the mechanical load. The die diameter on a production line ranges from 200 mm to 600 mm. A rotating die or oscillating haul-off distributes gauge variation. Film thickness for greenhouse use ranges from 100 µm to 200 µm. Below 100 µm, the structure loses puncture resistance to hail and mounting hardware. Above 200 µm, the light transmission falls unless additives are reformulated.

Light transmission of the finished greenhouse film is measured to ASTM D1003-21 or ISO 13468-1:2019. The base resin contributes little haze in a well-quenched film. A high frost line height increases crystallinity and reduces light transmission. The converter must therefore set a frost line height from 400 mm to 700 mm depending on the die diameter. The anti-drip additive is polar. It migrates to the surface over the first days of service. In a three-layer structure, the LLDPE layer is used as a barrier to control and slow the migration of anti-drip from the inner layer into the outer layer. This is not a formal permeability specification. It is an observed production behavior on lines running EVA-based inner layers.

The base resin does not contain a UV stabilizer package. Outdoor exposure of unstabilized film leads to chain scission and a rapid drop in elongation. Accelerated weathering is conducted to ISO 4892-3:2016 using UVA340 lamps. The failure criterion is often 50% retained elongation at break. Greenhouse film must contain an appropriate UV masterbatch based on HALS and benzophenone or triazine chemistry. The loading depends on the target service life. A typical service life of 3 years requires a higher additive dose than a one-season mulch film. The converter must not use 518NT as the only outer layer without UV protection. Contact with sulfur-based pesticides can also degrade the surface if chlorine from PVC spray lines contaminates the regrind. Polyethylene film is incompatible with PVC thermal stabilizer residues that generate acidic species at extrusion temperatures. These interactions are a known boundary condition for greenhouse film converters.

When 518NT Is Used as a Skin Layer in Extrusion Lamination

In extrusion lamination, the resin is melted in a high-temperature extruder and applied as a molten web between two substrates. LLDPE is added to reduce neck-in and improve adhesion to polar substrates after ozone or corona treatment. SABIC LLDPE 518NT is not a dedicated extrusion-coating grade. It is used as a blend partner with LDPE when the melt flow rate is adjusted by the converter. A typical dry blend contains 20–40 wt% 518NT and 60–80 wt% LDPE extrusion-coating resin. The LDPE component provides drawdown. The LLDPE component contributes seal performance and toughness. The blend is processed at melt temperatures between 270 °C and 310 °C. The die gap is set from 0.5 mm to 0.8 mm for a 15–25 µm coating weight. The extruder is normally a 30:1 L/D single-screw unit with an adapter designed for stable pressure.

At these temperatures, the antioxidant package must protect the resin from gel formation. Smoke and odor are monitored at the die. If smoke increases, the melt temperature is reduced or the coating speed is raised. The adhesion of the coated laminate is tested to ASTM F904-16 for flexible laminates. The seal strength of the LLDPE-containing layer is tested to ASTM F88/F88M-21. The absence of slip and antiblock in the base grade is useful here. A slip additive at high concentration would reduce adhesion and ink bond. If a slip is required for downstream packaging, it is added at a low concentration in the finished laminate or the other film layer. The converter should not use recycled material in the skin layer because gels and contaminants cause pinholes in the thin coating.

Bond failure in extrusion laminates occurs when the molten web is cooled too quickly. The adhesion to aluminum foil is improved by priming with a food-grade imine or polyester primer. The primer thickness is controlled on the coater. The LLDPE blend does not adhere well to untreated PET or OPP. Corona treatment at 38–42 mN/m is required on the substrate. The ozone generator output is set to 20–45 g/h depending on line speed and web width. These are starting conditions based on extrusion laminator suppliers. Published data for this specific blend configuration is limited. The converter validates bond strength on a laboratory laminator before full production.

Down-Gauging of 518NT Blown Film Alters Tear Propagation

Carrier bags and T-shirt sacks are produced from film thicknesses between 20 µm and 40 µm. The switch from LDPE to LLDPE allows down-gauging while retaining load-carrying capacity. SABIC LLDPE 518NT is used in this segment as a high-toughness component. The converter dry-blends the resin with post-consumer recyclate. PCR content ranges from 10 wt% to 30 wt%. Above 30 wt%, dart impact falls sharply and bubble instability increases. The recycled material must be melt-filtered. A continuous screen changer with 120 mesh screens removes solid contaminants. The melt pressure is monitored. If the screen changer pressure rises above 15 MPa, the screens are changed. Unfiltered PCR causes pinholes and catastrophic web breaks at high haul-off speed.

Mechanical properties are evaluated before down-gauging. Tensile properties are measured to ISO 527-3:2018. Elmendorf tear is measured to ASTM D1922-15. Dart impact is measured to ASTM D1709-16a Method A. In carrier bag film, the MD tear value is a critical operator control. Too high MD orientation makes the bag split along the machine direction. The BUR is set between 2.5 and 3.5 to maintain transverse orientation. The frost line height is adjusted after the bag line reports split failures. A lower frost line reduces MD orientation. The haul-off speed is set to the maximum stable speed. The bubble must not oscillate. Oscillation creates alternating thin and thick bands. In bags, these bands fail under filled load.

The addition of PCR changes the viscosity of the melt. Post-consumer LLDPE often contains crosslinked particles and degraded stabilizing residues. The melt pressure may fluctuate by ±10%. The operator compensates by increasing the barrel temperature in the feed zone by 5–10 °C. This improves melting but can oxidize the resin if the temperature is too high. The extruder barrel zones are set from 160 °C at the feed throat to 210 °C at the die. The die temperature is held at 200–210 °C. If the die temperature exceeds 220 °C, the film gloss increases but the seal window narrows. The finished bags are sealed on side-seal or bottom-seal machines. Seal settings are validated by a seal curve at 0.4 s dwell and 0.3 MPa pressure. The presence of PCR increases the seal initiation temperature by 3–5 °C. This is a commonly observed shift in production. The bag maker revalidates the seal window after each PCR lot change.