SABIC® LLDPE 218B is a butene-comonomer linear low-density polyethylene pellet grade supplied for blown film extrusion. The nominal melt flow rate is 2.0 g/10 min at 190 °C under 2.16 kg load per ISO 1133-1:2022, and the nominal density is 0.918 g/cm³ per ISO 1183-1:2019. The resin is converted into monolayer film with thicknesses typically between 15 µm and 80 µm. The grade is stabilized with a phenolic/phosphite antioxidant package and does not contain deliberately added slip or antiblock; surface friction and blocking behaviour are controlled by introducing a converter-selected masterbatch at the feed throat or through a side feeder. On a 45 mm grooved-feed extruder with 25 L/D, 1.2 mm die gap, and 2.5:1 blow-up ratio, stable bubble operation is reported at melt temperature set points between 185 °C and 210 °C. Published data for configurations outside this range is limited; converters are advised to establish a process window through a trial with continuous gauge monitoring.

What Distinguishes Butene-Based 218B from Hexene and Octene LLDPE Grades in Film Toughness?

The short-chain branch type is the primary molecular variable that differentiates 218B from C6 and C8 LLDPE resins. Butene introduces ethyl side chains onto the polyethylene backbone, whereas hexene and octene introduce butyl and hexyl side chains, respectively. At the same nominal density and melt flow rate, the longer comonomer branch in C6 and C8 grades increases the concentration of load-bearing tie chains and reduces the frequency of sharp lamellar boundaries because the longer side chains are excluded from the crystal lattice more effectively. In 218B, the resulting solid-state structure produces lower puncture and tear resistance, but also lower shear viscosity at moderate screw speeds, which can reduce motor load and die pressure on undersized extruders. The trade-off is most visible in the comparative 25 µm monolayer data shown in Table 1.

The data in Table 1 are representative film values, not specification limits, and are influenced by die geometry, frost line height, and ambient air temperature. In applications where dart impact at 25 µm below 150 g is unacceptable, 218B is typically displaced by a C6 or C8 grade of the same density. For thin-gauge liners and produce bags where tear resistance is less critical than package cost per thousand bags, the C4 architecture remains technically viable.

The difference in melt strength also matters. Butene-based LLDPE tends to exhibit lower extensional viscosity at typical haul-off strain rates, which narrows the stable operating envelope under high blow-up ratios. Converters who attempt to operate 218B at a blow-up ratio above 3.0:1 may observe bubble sway and film gauge bands larger than ±8% unless internal bubble cooling is used. A C8 grade with the same density can often be run at 3.5:1 or higher because its longer comonomer branches introduce additional chain entanglements. This is a direct consequence of branch length, not molecular weight distribution alone.

Differential scanning calorimetry at 10 °C/min according to ISO 11357-3:2018 typically shows a peak melting temperature of 122 °C to 126 °C for 218B, which is similar to other 0.918 g/cm³ LLDPE resins. The cooling run gives a crystallization onset near 106 °C to 109 °C. These values affect heat sealing and blocking; lower peak melting temperature permits lower heat seal onset but narrows the high-temperature dimensional stability window.

Grooved-Feed Extrusion Pressure, Melt Temperature, and Bubble Cooling Limits

On grooved-feed extruders between 24 L/D and 30 L/D, 218B exhibits a melt viscosity sufficiently low to reduce die pressure relative to a 1.0 g/10 min LLDPE of equal density. Typical die pressure for a 250 mm die with a 1.2 mm gap is between 180 bar and 240 bar at an output of 80 kg/h; the exact value depends on adapter geometry, screen pack condition, and melt temperature. The melt temperature should be maintained between 185 °C and 210 °C. Below 175 °C, melt fracture and incomplete homogenization can occur. Above 220 °C, oxidative degradation in stagnation zones can generate gel particles that appear as specks in thin film. In practice, the temperature profile is set flat or slightly reverse from feed to die to avoid overheating the already viscous melt.

Bubble cooling is the limiting unit operation for 218B because low melt strength increases sensitivity to air turbulence. For a 250 mm die running at 60 kg/h to 90 kg/h, a frost line height of 3 to 5 die diameters is reported to produce gauge profiles within ±6% when combined with a dual-lip air ring. If the frost line is raised beyond 6 die diameters, the bubble enters a metastable region where diameter oscillation and film blocking on the collapsing frame become more frequent. If the frost line is forced below 2 die diameters, the film may exhibit excessive optical haze because the polymer is quenched before sufficient strain-induced orientation is imposed.

Screw design also modifies output. A barrier screw with a mixing section of 5 to 7 diameters is recommended to homogenize the melt at 190 °C to 210 °C. Pellet drying is not required; the pellet feed is dry-blended with slip or antiblock masterbatch. If the masterbatch carrier resin has a melt flow rate above 20 g/10 min, localized viscosity mismatch can produce optical gels at high let-down ratios above 5 wt%. Incompatibility with high-melt-flow carriers should be verified by a regression trial before production.

Common application fields include thin-gauge can liners, garment bags, produce bags, and general-purpose packaging film where output rate and film economics are primary constraints. In blown film lines with a 250 mm die and 2.5:1 blow-up ratio, 218B is used to run at line speeds up to 120 m/min for 15 µm to 25 µm film when the air ring and collapsing frame maintain bubble symmetry. At these line speeds, gauge variation should be monitored with a capacitance or beta gauge; deviations larger than ±8% signal bubble instability. Applications requiring heavy liquid payloads or high puncture resistance, such as biohazard liners and construction films, are more frequently produced with C6 or C8 LLDPE to avoid premature failure.

When 218B Replaces a 0.918 kg/m³ Hexene Grade in Thin-Gauge Liners

A substitution trial replacing a 0.918 g/cm³ hexene LLDPE with 218B on a 300 mm die typically shows a 10% to 15% reduction in head pressure and a 5% to 8% reduction in specific motor load at equal screw speed. The output ceiling, however, is not always higher because the lower melt strength of the butene resin shifts the onset of bubble instability to lower haul-off speeds. To maintain a stable bubble, converters commonly reduce the blow-up ratio from 3.0:1 to 2.5:1 and lower the frost line by 0.5 to 1.0 die diameters. The die gap may be tightened from 1.2 mm to 1.0 mm to increase shear stress at the die lip and reduce melt fracture.

In thin-gauge liner production at 20 µm, the C4 butene architecture leads to lower dart impact and lower machine-direction tear than the hexene reference. If the liner specification requires a dart impact above 130 g per ASTM D1709-16a Method A, the converter should either increase film thickness by 5 µm to 8 µm or blend 218B with 20% to 30% C6/C8 LLDPE. A blend at 25% C6 LLDPE often shifts dart impact upward, but published data for this specific blend ratio on production-scale lines is limited; laboratory-scale data should not be extrapolated without a full production trial.

Compared with SABIC® LLDPE 118N, a 1.0 g/10 min butene grade at the same density, 218B allows higher throughput but produces lower dart impact and tear strength. A direct comparison at 25 µm film thickness indicates that 118N can provide higher puncture resistance because the longer molecular chains and lower melt flow rate increase tie-chain concentration. 218B, however, reduces extruder head pressure and allows a smaller die size to reach a given output. Converters select between 218B and 118N based on whether the production line is torque-limited or impact-limited.

For food-contact flexible packaging in the EU, the base polymer must comply with Regulation (EU) No 10/2011. 218B is evaluated as an olefin polymer for food contact under 21 CFR 177.1520(c) and the conditions of use described in 21 CFR 176.170(c). The finished monolayer film must satisfy an overall migration limit of 10 mg/dm² when tested according to EN 1186-1:2002 or EN 1186-14:2002. For packaging intended for fatty foods, migration testing should use simulant D2 or D1 as defined in Regulation (EU) No 10/2011; for aqueous and acidic foods, simulants A and B apply. The presence of printing inks, adhesives, or recycled content introduces additional compliance duties under Regulation (EU) 2022/1616 for recycled plastic food-contact materials.

Storage before conversion should be below 40 °C and protected from direct sunlight to limit pre-oxidation and yellowing. The resin is not hygroscopic, but condensation from warehouse temperature cycling can carry surface moisture into the extruder feed throat; if surface moisture is visible, a hopper dryer at 50 °C for 2 h is sufficient to remove surface water. Prolonged purging at temperatures above 230 °C should be avoided because residence-time degradation produces black specks and off-odour. The processing window narrows further when regrind content exceeds 20%; batch-to-batch variation in regrind molecular weight and contamination increases the risk of bubble instability and gel formation. These operational boundaries are documented in production-scale blown film trials and should be reviewed against the converter’s specific die and air-ring geometry.