Linear Low-Density Polyethylene (LLDPE)

Linear low-density polyethylene (LLDPE) is a copolymer of ethylene and an alpha-olefin, typically 1-butene, 1-hexene, or 1-octene, with a density range of 0.915 g/cm³ to 0.940 g/cm³ measured according to ASTM D792-20 or ISO 1183-1:2019. Commercial grade codes are not fixed molecular definitions; a single grade number can refer to a gas-phase butene LLDPE with a broad molecular weight distribution, a solution-process hexene LLDPE with intermediate branching, or a metallocene-catalysed octene LLDPE with a narrow comonomer distribution. When a grade code is discontinued, the replacement search must move beyond the resin data sheet and into the converting line’s rheological, thermal, optical, sealing, and regulatory requirements. Discontinued LLDPE grade codes frequently remain in converter specifications for years, creating a live replacement problem that cannot be resolved by matching only melt index and density.

Why Do Discontinued LLDPE Grade Codes Produce Non-Linear Replacement Failures?

The melt index reported by ASTM D1238-20 Procedure A or ISO 1133-1:2022 Method A at 190°C and 2.16 kg is a single-point viscosity measurement that does not capture shear-thinning differences between a broad molecular weight distribution butene LLDPE and a narrow molecular weight distribution hexene LLDPE. Two resins with an identical melt index of 1.0 g/10 min and identical density of 0.918 g/cm³ can differ in zero-shear viscosity, extensional viscosity, melt strength, comonomer distribution, and seal initiation temperature by amounts that exceed production process windows. A live replacement search therefore requires orthogonal polymer characterisation: high-temperature gel permeation chromatography for molecular weight distribution, crystallization elution fractionation or temperature rising elution fractionation for short-chain branching distribution, and parallel-plate rheometry from 0.1 rad/s to 100 rad/s for zero-shear viscosity and shear-thinning index.

On a production-scale blown film line, replacement failures are often non-linear rather than gradual. A candidate resin with equivalent melt index and density may exhibit a decrease in dynamic melt strength measured by a Rheotens device from 18 cN to 11 cN at 190°C, causing bubble instability at blow-up ratios above 2.2:1 even though the standard melt index specification remains unchanged. Manufacturer technical bulletins routinely report that narrow-MWD metallocene LLDPE requires a lower melt temperature for stable bubble cooling but produces a higher die pressure than a Ziegler-Natta butene LLDPE of the same melt index. A live replacement search must therefore begin with the converting line’s measured response variables—output rate, die pressure, melt temperature, frost-line height, and web tension—not with the resin data sheet alone.

In coextruded cast film where a discontinued 0.920 g/cm³ butene LLDPE was run on a 90 mm barrier screw with L/D 30:1, die width 2,400 mm, and chill-roll temperature 21°C, a live replacement candidate with the same melt index can shift neck-in from 35 mm to 52 mm per edge if the molecular weight distribution narrows. The edge trim scrap rate at 2,500 m/min line speed rises proportionally with increased neck-in, and the draw resonance threshold may drop below the required draw ratio because the extensional viscosity at high strain rates is insufficient. Measurements of extruder head pressure and specific output at constant screw speed provide a more sensitive screening criterion than density or melt index alone: a replacement grade that reduces head pressure by more than 15% at 100 rpm while increasing melt temperature by more than 6°C indicates a rheological mismatch that cannot be corrected by barrel profile adjustment alone.

When a Discontinued Hexene-LLDPE Grade Is Replaced in Blown Film

Blown film processing of a discontinued hexene-LLDPE with a density of 0.918 g/cm³ and a melt index of 0.9 g/10 min requires a candidate that matches not only the quench rate response but also the tear balance in machine and transverse directions. The replacement should be evaluated on a laboratory blown film line with a 45 mm grooved-feed extruder, L/D 25:1, die diameter 100 mm, die gap 1.2 mm, and blow-up ratio adjustable from 2.0:1 to 3.0:1. The frost-line height must be held constant to within ±5 mm; otherwise the dart impact values shift because of changes in orientation balance and crystallinity. A comparative test on a 40 µm film showed that a replacement with equivalent melt index but lower hexene content failed the dart impact specification of ≥ 150 g at -20°C despite passing all room-temperature tensile requirements under ASTM D882-22.

Candidate B passes the listed film mechanics, but the replacement was rejected on a 3-layer coextrusion line because the slip additive package contained erucamide at 1,100 ppm and required 72 h of controlled storage at 23°C to reach the specified coefficient of friction. The converter required a coefficient of friction below 0.25 within 24 h of winding, measured according to ASTM D1894-24, because the film bags are filled on vertical form-fill-seal equipment with a jaw cycle of 55 packs/min. Published data for additivation time-dependent migration in this specific sealed-bag configuration is limited, so the replacement qualification required an industrial trial on the actual packaging line.

For rotational molding of a discontinued octene-LLDPE grade with a density of 0.936 g/cm³ and melt index of 5.5 g/10 min, live replacement screening must account for high-temperature powder sintering and bubble removal. A candidate with equivalent melt index but a broader molecular weight distribution may cure acceptably in a 3.2 mm wall section at 232°C peak internal air temperature, but the spiral-flow length on a 50 mm cast aluminum mold may decrease by 18% due to reduced zero-shear viscosity. In an industrial trial, the replacement produced voids at the mold parting line when the peak internal air temperature was held below 238°C; only by raising the oven set point by 8°C and extending the cook cycle by 4 min did the porosity fall below the acceptance limit of 0.5% cross-sectional void area. This is a critical threshold risk because the original grade code had been validated for a 10.2 kg part with a maximum wall thickness variation of ±0.4 mm.

High-Speed Cast Film Neck-In and Seal Initiation Temperature

Cast film produced from LLDPE depends on the melt flow response in a shallow die and a short air gap. A discontinued grade code can be replaced only if the candidate’s melt strength and stress relaxation time are sufficiently close to maintain edge stability at line speeds above 300 m/min. The neck-in observed on a 2,000 mm die at 600 mm/s line speed is controlled by the extensional viscosity at strain rates near 10 s⁻¹ to 100 s⁻¹; melt index from ASTM D1238-20 is measured at wall shear rates below 10 s⁻¹ and therefore cannot predict edge bead formation or draw resonance. A replacement with a lower zero-shear viscosity but a higher shear-thinning slope may process acceptably at high speed but lose seal strength because the molecular architecture contains fewer high-molecular-weight chains that participate in interdiffusion during heat sealing. Seal initiation temperature should be measured by ASTM F88/F88M-21 on films sealed at 0.5 s dwell time and 0.275 MPa jaw pressure; a difference of more than 5°C relative to the discontinued grade will generally require line modifications or package integrity retesting.

A Live Replacement Search Must Include Regulatory and Food-Contact Additive Equivalence

Even when melt processing and film mechanical properties match, a discontinued LLDPE grade code cannot be replaced without verification of the antioxidant, neutralizer, slip, and antiblock additives. LLDPE intended for food contact in the United States must satisfy 21 CFR 177.1520, and the replacement must not introduce a substance outside the original clearance. In the European Union, the relevant overall migration limit is 10 mg/dm² for food-contact plastics under EU Regulation 10/2011, and specific migration limits apply to erucamide, oleamide, and silica. A converter changing from a discontinued grade to a live candidate should request a complete regulatory statement from the resin supplier and compare the declared additive package with the extraction test reports from the previous grade.

The replacement candidate should be qualified only after the converter has compared batch-to-batch melt index variation, density variation, and additive loading against the historical control range of the discontinued grade. For a 24-month production record, a discontinued butene LLDPE exhibited a melt index standard deviation of 0.03 g/10 min and a density standard deviation of 0.0002 g/cm³; a live replacement with a wider standard deviation may pass a single lot qualification and then produce periodic film splits during seasonal ambient humidity changes. Qualification must therefore include at least three non-consecutive reactor campaigns and a summer/winter comparison if the converting hall is not humidity-controlled.