Linear low-density polyethylene cast film grade selection is usually driven by die pressure limits, draw resonance suppression, and the need to maintain puncture toughness after down-gauging. SABIC LLDPE C2820T is an ethylene/α-olefin copolymer supplied for chill-roll cast film and lamination. The manufacturer specifies a melt flow rate of 2.8 g/10 min (ISO 1133-1, 190°C/2.16 kg) and a density of 0.918 g/cm³ (ISO 1183-1). The pellet is not formulated with slip or antiblock additives, which shifts the additive selection burden to the converter and permits independent control of coefficient of friction in multilayer structures.

Why Is Melt Flow Rate Positioned at 2.8 g/10 min for Cast Film?

The melt flow rate of 2.8 g/10 min places the grade between conventional blown-film LLDPE resins, often below 1.0 g/10 min, and high-speed extrusion coating or cast film coating resins that may exceed 5.0 g/10 min. This intermediate viscosity reduces motor load and head pressure on single-screw extruders with 30:1 L/D used in cast film; it also retains sufficient melt tension to limit draw resonance when the air gap and chill roll speed are properly balanced.

Because the melt flow test applies a low shear rate, typically below 100 s⁻¹, it does not capture the high-shear viscosity that governs die pressure. Capillary rheometry at 230°C over 100–1000 s⁻¹ is recommended before die design changes. Published data for this specific grade across the full shear-rate range is limited; the melt flow rate should be used for lot acceptance and approximate viscosity ranking, not for die pressure calculations.

On a 90 mm single-screw extruder fitted with a barrier screw, melt temperatures at the die are typically set between 240°C and 270°C. Melt pressures vary with die gap, throughput, and melt temperature, so process limits should be taken from the extruder manufacturer’s maximum continuous head pressure rating rather than from a fixed value. Prolonged melt residence above 280°C increases oxidative gel formation; the resulting gel count in the finished cast film may exceed 0.5 mm in diameter when thermally degraded material detaches from screw or die surfaces.

A gear pump between the screw and die can lower melt temperature by 5–10°C because it reduces the need for excessive screw speed and shear heating. The pump also stabilises die pressure and narrows gauge variation; however, it does not eliminate the need for a clean die lip and proper internal deckle adjustment.

Extrusion Temperature, Residence Time, and Moisture Boundaries

At relative humidity above 60%, surface moisture on pellets can be a source of film defects because the polymer itself is non-hygroscopic but the pellet surface can transport free water into the feed throat. Hopper drying at 60–70°C for 2 h is sufficient in high-humidity conditions; drying above 80°C is unnecessary and may cause pellet agglomeration in the hopper. Bypassing the dryer when the ambient dew point is below 10°C is generally acceptable, but the feed throat must be kept free of condensation from plant air.

In multilayer cast film, the melt stream is split after the extruder or in a feedblock. The core layer viscosity should be within approximately ±20% of the skin layer melt viscosity at the die shear rate to avoid layer thickness instability. C2820T, with its 2.8 g/10 min MFR, is normally paired with skin-layer resins in the 1.0–4.2 g/10 min MFR range; larger mismatches may require adjustment of skin layer melt temperature rather than reformulation.

In cast film lines configured with a 75 mm or 90 mm extruder, a polish-roll stack, and an air-knife/pinning system, C2820T is run as the bulk or core layer in two-layer and three-layer structures. The resin’s lack of slip and antiblock in the pellet means that the skin layer can be modified with amide or silica masterbatch while the core remains free of migratory additives; this split-layer design reduces the total masterbatch required to achieve a target coefficient of friction because the additive is concentrated at the surface rather than dispersed through the entire film cross-section.

Chill roll temperatures between 18°C and 30°C are commonly used for the first cooling roll. Lower temperatures accelerate quench and can improve haze, but the roll surface must remain above the dew point of the production hall. Under high-humidity ambient conditions, condensation on the chill roll causes localised quench interruption and produces pit-like surface defects that cannot be removed without increasing melt temperature.

Draw ratio and air gap must be adjusted together. A long air gap above 25 cm raises melt draw-down and may induce neck-in or draw resonance; a very short air gap below 8 cm can force the melt into contact with the chill roll before sufficient tension is developed. The optimum is machine-specific and is normally determined by monitoring film width loss and thickness variation across the web.

Optical, Mechanical, and Surface Properties After Chill-Roll Quenching

Cast film quenched on a polished chill roll generates a different crystalline morphology from blown film. The rapid quench suppresses large spherulite growth and typically lowers haze relative to air-cooled blown film of equivalent density. However, surface finish of the chill roll, die deposit accumulation, and melt temperature strongly affect final optical values. Published data for this specific configuration is limited; film producers should generate statistically valid measurements on their own line using ISO 14782 for haze and ISO 2813 for gloss.

Mechanical performance is direction-dependent. Tensile yield and elongation at break should be measured on cast film according to ISO 527-3, with specimen width and initial gauge recorded because cast film properties vary with draw ratio. Elmendorf tear is tested according to ISO 6383-2, and dart drop impact according to ISO 7765-1; both are thickness-dependent, so comparison between grades requires normalization to film thickness and film density.

The resin does not contain slip or antiblock additives, so the intrinsic coefficient of friction of a monolayer film is higher than that of a formulated grade. For measurement, ISO 8295 is applied after the film has been conditioned at 23°C and 50% relative humidity for at least 40 h. Values before additive migration are not representative of final reel performance.

In comparative evaluations, C2820T should be treated as a base resin platform rather than as a finished-film recipe. The addition of slip and antiblock masterbatch changes both surface optics and mechanical energy absorption; therefore, any tabulated mechanical property without additive loading and film gauge is not sufficient for specification purposes.

When Slip and Antiblock Additives Are Absent from the Pellet

Because the base resin is uncompounded, the coefficient of friction of the final film is governed by the converter’s additive masterbatch. Erucamide or oleamide slip masterbatches are typically dosed at 0.5–2.0 wt% in the skin layer, depending on film thickness, storage temperature, and the target coefficient of friction. The amide molecule migrates to the film surface over time; migration kinetics are slower at low storage temperatures and faster in thin films. Final coefficient of friction should be measured after 24–48 h of ageing because values measured immediately after winding understate the developed surface lubricity.

Blocking resistance is controlled separately with antiblock masterbatch based on silica or synthetic aluminosilicate. Dosing levels between 0.5 wt% and 1.5 wt% are common in monolayer film. Above 2.0 wt%, haze can increase and die lip deposit formation may become noticeable. The exact threshold depends on masterbatch carrier resin and particle size distribution, not only on the loading level.

If corona treatment is used to raise surface energy for lamination or printing, the absence of migratory slip from the pellet is advantageous because there is no pre-existing surface layer to interfere with oxidation. Target dyne levels are usually set between 38 dyn/cm and 42 dyn/cm; the specific value depends on ink and adhesive chemistry and should be verified by contact angle or dyne pens immediately before coating.

Machine roll cast stretch film lines use C2820T as the core layer in formulations where a metallocene-catalysed or higher-alpha-olefin skin supplies the stretch force and puncture resistance. The intermediate melt flow rate stabilises the melt curtain during start-up and gauge changes; the absence of slip additive in the core prevents uncontrolled migration into the skin that would otherwise reduce cling performance in the stretch film.

In lamination film, the grade is selected when the converter wishes to control surface energy independently of base resin additives. The corona-treated surface retains predictable wetting characteristics because the pellet does not release erucamide or oleamide from the internal layers. The film is further characterised by its ability to accept solvent-based or solventless adhesives after standard surface preparation, although the specific bond strength depends on the adhesive, coating weight, and lamination nip conditions.

Hygienic film backsheets and textile packaging applications require a combination of gauge uniformity, softness, and adequate tear resistance. C2820T can be run in these structures when the final film is thicker than approximately 15 µm and when tearing forces are not the dominant design criterion. At very low gauge, the film must be evaluated for pinholing and dart drop impact because the mechanical reserve decreases with thickness.

Die lip deposit formation is influenced by melt temperature, antioxidant package, and additive masterbatch. In cast film, visible die lip deposits can transfer to the web as drag lines. When running C2820T without slip masterbatch, die lip deposits are generally lower than with precompounded slip-containing grades, but the difference is process-dependent. Periodic die cleaning is required at intervals determined by visual inspection of the film surface under polarised light or dark-field imaging.

Distinguishing C2820T from Blown Film LLDPE and LDPE Extrusion Coating Grades

Relative to a general-purpose blown-film butene LLDPE with a melt flow rate below 1.0 g/10 min, C2820T has lower melt viscosity and lower bubble stability. Blown film lines operating at high stalk or large blow-up ratios should not assume direct interchangeability; the grade can be blended with a low-MFR LLDPE to increase melt strength, but the blend will also change dart drop impact and film haze.

Compared with LDPE extrusion-coating grades, C2820T has a linear molecular structure and lacks the long-chain branching that provides strain-hardening under extensional flow. This means neck-in and draw resonance are more pronounced in single-layer extrusion coating. Coextrusion with an LDPE skin may be required when the same grade is used at high line speed in a coating configuration.

When compared to hexene-1 or octene-1 based LLDPE grades of equivalent density and melt flow rate, the butene-1-based chain architecture generally produces lower transverse-direction Elmendorf tear. The difference arises from the shorter comonomer branch length, which influences tie-molecule concentration and stress distribution in the amorphous region. Film converters comparing grades should measure tear on the same orientation and at the same thickness; differences in chill roll temperature and draw ratio can mask the resin effect.

If the application requires lower seal initiation temperature or higher dart impact at equivalent density, the converter should evaluate metallocene-catalysed grades or higher-alpha-olefin copolymers. Published data for the specific comparison between C2820T and metallocene alternatives is limited; therefore a three-lot trial on the target line is required to capture batch-to-batch variance in melt flow rate and film toughness.

Regulatory verification is performed on the finished article rather than on the resin alone. The following references apply to food-contact and chemical management documentation:

The operational boundary for C2820T is defined by the combination of melt temperature, residence time, and additive loading. At melt temperatures above 280°C, oxidative degradation can raise gel counts and shift melt flow rate; at additive loadings above 2.0 wt% of slip or antiblock masterbatch, surface bloom and optical losses may become significant. Films intended for food contact must be tested as finished articles because the migration behaviour of the final structure cannot be predicted solely from the base resin specification.