Linear low-density polyethylene (LLDPE) is produced by copolymerising ethylene with 1-butene, 1-hexene, or 1-octene in gas-phase, solution, or slurry-loop polymerisation. Commercial film resins are specified primarily by density and melt index, but these two values do not encode molecular architecture. Two resins with 0.918 g/cm³ density (ASTM D1505) and 1.0 g/10 min melt index (ASTM D1238, 190°C, 2.16 kg) may differ in comonomer type, short-chain branch distribution, molecular weight distribution (ASTM D6474), catalyst residues, and additive package. LLDPE Grade Equivalence Across Producers: Building a Drop-In Map therefore treats a resin as a vector of measurable parameters and maps producer-specific grades onto a common framework. The objective is to determine whether a candidate resin can be substituted without altering extrusion temperature settings, screw speed, die geometry, cooling air, or downstream converting behaviour.

In blown-film conversion, the first indication of an incomplete equivalence map is rarely a failed tensile test. On a low-stalk line using a 90 mm grooved-feed extruder with 30:1 L/D and a 250 mm die, bubble geometry is controlled by extensional viscosity, not by single-point melt index. A candidate with a lower I21/I2 flow-rate ratio at equal MI will generate higher melt pressure at the same screw speed because its apparent shear viscosity at die-lip shear rates of 100 s⁻¹ to 500 s⁻¹ is higher. The bubble may become wider and the frost line height may rise. Operators then adjust internal bubble pressure, air-ring airflow, and line speed. The same resin may produce acceptable film on a cast line because the extensional draw is different. A drop-in map must therefore be specific to the conversion route and not portable between blown and cast film without validation.

Flow-rate ratio I21/I2, sometimes called melt index ratio, is a single-number proxy for shear sensitivity. In LLDPE, I21/I2 values typically range from 15 to 35, but producer-specific grades at the same MI can differ by 2–5 units. A higher I21/I2 generally indicates broader molecular weight distribution or different short-chain branching; this lowers melt pressure at high screw speed but can reduce dart impact at equal density. The drop-in map therefore records I21/I2 alongside MI and density, and treats it as a process-critical variable rather than a derived datasheet footnote.

Does a Matching Melt Index and Density Guarantee a Drop-In LLDPE Grade?

No. Single-point rheology and density are screening values only. A butene-based LLDPE and a hexene-based LLDPE at 0.918 g/cm³ and 1.0 g/10 min MI differ in tie-molecule population. The hexene branch, being longer, is more effective in connecting adjacent lamellae, which raises dart impact and Elmendorf tear. This difference appears at 25 µm film thickness and is measurable by ASTM D1709A and ASTM D1922. The map therefore adds comonomer type, molecular weight distribution, melting peak, and flow-rate ratio. The following table lists the minimum analytical set for a producer-neutral comparison.

Differential scanning calorimetry per ASTM D3418 should be run at 10°C/min. A difference of 2–4°C in peak melting temperature changes heat-seal initiation and may require modification of the seal bar set point on form-fill-seal equipment. Seal initiation temperature is not a single value; it is defined as the temperature at which a seal strength of 0.5 N/15 mm is achieved in a tensile peel test per ASTM F88. Equivalence mapping also requires dynamic oscillatory shear at 190°C using parallel-plate geometry of 25 mm diameter, 1% strain, and a frequency sweep from 0.01 rad/s to 100 rad/s. The crossover frequency and low-frequency storage modulus separate grades with a high-molecular-weight tail. A candidate with lower crossover modulus may show lower bubble stability even when MI and density are within ±5%.

After film is produced, the equivalence map is tested against product specifications: tensile properties per ASTM D882 at a crosshead speed of 500 mm/min, Elmendorf tear per ASTM D1922, dart impact per ASTM D1709A, puncture resistance per ASTM D5748, haze per ASTM D1003, and gloss per ASTM D2457 at 60° angle. A candidate may pass all tensile tests and still fail dart impact because film gauge variation is higher. Gauge variation is analysed by high-speed capacitance scanning across the web; control limits are typically set at ±5% of nominal thickness. A drop-in map therefore includes not only average properties but also process capability indices for thickness uniformity.

On a mono-layer cast film line with a 120 mm extruder, 0.5 mm die gap, and 305 mm chill roll, the map is governed by melt curtain stability. The critical processing parameters are draw ratio, neck-in, and melt temperature at the die lip, typically 220–260°C for LLDPE. Neck-in is measured as the difference between die width and final film width, divided by die width. Published data from cast-film studies show that butene LLDPE necks in more than octene LLDPE at equal MI and density because extensional viscosity is lower. A drop-in candidate with higher neck-in requires either a wider die or additional edge trimming, changing net yield. Thickness uniformity is checked by ISO 4593, and deviation is typically controlled within ±5% of target web thickness.

When a Converter Switches From Butene to Hexene LLDPE on Low-Stalk Blown Film

Low-stalk and high-stalk configurations respond in opposite directions to melt strength changes. In low-stalk film, a butene-to-hexene switch increases dart impact and tear resistance but may require a larger air-ring opening because the higher extensional viscosity reduces bubble diameter at constant internal pressure. In high-stalk film, the longer stalk residence time amplifies differences in melt elasticity; a candidate with lower Mz/Mw can sag and lose stalk stability before the frost line. The following table summarises typical film property shifts for 25 µm monolayer film at 1.0 g/10 min MI and 0.918 g/cm³ density.

The values in the table are screening ranges assembled from producer datasheets and technical literature and should not replace a grade-specific certificate of analysis. When a specific pairing lacks published comparative data, a pilot line trial using a 60 mm extruder and 100 mm die is the minimum validation before commercial conversion. The trial should record bubble diameter, frost line height, melt pressure, output, and draw-down speed at three screw speeds, and the film should be tested after 48 h conditioning at 23°C and 50% relative humidity per ASTM D618. An octene grade may require a die gap increase of 0.2 mm to remain within the line’s maximum backpressure; that change alters drawdown ratio and machine-direction orientation, so it cannot be ignored in the map.

Where LLDPE is specified for injection moulding of thin-wall lids or caps, the equivalence map uses injection moulding grades with MI from 20 g/10 min to 50 g/10 min. Mould shrinkage is compared on plaques per ISO 294-4 after 24 h and 48 h at 23°C. A candidate with the same MI and density can exhibit mould shrinkage differing by 0.3–0.8% due to comonomer type and nucleating additives; tooling may not be drop-in if critical dimensions are tighter than ±0.5%. Rotational moulding uses LLDPE with MI in the 3–8 g/10 min range and density near 0.935–0.940 g/cm³. Drop-in mapping for rotomoulding compares environmental stress-cracking resistance per ASTM D1693 condition B using 100% Igepal CO-630, low-temperature brittleness per ASTM D746, and warpage on a standard cube mould. Published data for butene-to-hexene substitution in rotomoulding is limited; validation therefore requires a full moulding cycle and cooling-rate comparison on the target machine.

Additive Package, Compliance, and Organoleptic Boundaries

Base-resin equivalence does not transfer additive performance. Slip agents such as erucamide migrate to the film surface over time; coefficient of friction is compared per ASTM D1894 or ISO 8295 after 72 h ageing at 40°C. Antiblock type—synthetic silica, talc, or zeolite—affects haze per ASTM D1003 and film-to-film separation. Food-contact compliance must be revalidated for the candidate grade under FDA 21 CFR 177.1520 and EU Regulation 10/2011, including specific migration limits for the actual additive package. A producer-specific food-contact approval for the incumbent resin does not automatically cover the drop-in candidate, even when base density and MI match. Organoleptic testing may be required for sensitive packaging; published data for this specific configuration is limited, so internal sensory panel protocols vary by converter.

Extruder screw design also limits grade interchangeability. LLDPE requires a barrier screw with a Maddock-style mixer and an L/D ratio of 24:1 to 30:1. A producer swap from a narrow-molecular-weight Ziegler-Natta grade to a bimodal grade may lower torque at 80 rpm but raise melt pressure fluctuations; the map must include power draw per kilogram per hour. Lines with grooved feed sections are less sensitive to melt index variations than smooth-bore extruders because positive conveying dominates. Rheological fingerprinting in a quality control laboratory provides the most sensitive discriminator between grades that appear identical on a datasheet. Capillary rheometry at 190°C with a 20:1 die and apparent shear rates from 10 s⁻¹ to 1000 s⁻¹ generates the shear viscosity curve. Extensional viscosity measured with a Sentmanat extensional rheometer fixture at 150°C and strain rates of 0.1 s⁻¹ to 10 s⁻¹ discriminates melt strength. A drop-in map remains valid only inside the production line’s established maximum melt pressure, maximum motor load, and maximum screw speed. When a candidate grade exceeds one of those boundaries, the map classifies it as a reformulation project, not a drop-in substitution.