Linear low-density polyethylene (LLDPE) grades with a density of 0.918–0.940 g/cm³ and a melt flow index of 0.5–2.0 g/10 min at 190 °C/2.16 kg (ISO 1133-1:2022) are specified for blown film, cast film, and injection moulding applications. A customer request for 30% recycled content in LLDPE changes the polymer architecture, additive package, and processing limits. The term “recycled content” must be resolved before compounding: post-industrial (PIR) LLDPE from edge trim and start-up scrap is compositionally nearer to virgin resin, while post-consumer (PCR) LLDPE/LDPE film reclaim carries inks, oxygenated species, polypropylene labels, and variable melt flow. The processing route must be selected after measuring ash content (ISO 3451-1:2019), melt flow rate (ISO 1133-1:2022), and density (ISO 1183-1:2019) on the recycled lot. The request cannot be met by a simple dry blend of pellets; it requires a controlled compounding programme because the recycled fraction contains multiple polyethylene architectures and contamination species.

How does the 30% recyclate loading shift melt rheology and extruder pressure?

Adding 30 wt% recycled LLDPE or LDPE-rich reclaim to a virgin LLDPE grade with a melt index of 1.0 g/10 min typically produces a final melt index between 0.7 and 1.2 g/10 min, depending on the recyclate fraction. The reduction in melt index is observed when the recycled fraction contains gel species, crosslinked particles from oxidatively degraded film, or high-molecular-mass LDPE. In a single-screw blown film extruder, the pressure upstream of the screen pack can rise by 10–30% when the recyclate introduces gel counts above 100 counts/m² at 200 µm resolution. This pressure rise is measured at the breaker plate with a melt transducer; the operator should not compensate solely by increasing barrel setpoint, because excessive temperatures above 230 °C accelerate carbonyl index development in oxidised PCR.

Melt pressure variation across the die is amplified when 30% PCR is added without homogenisation. A 75 mm grooved-feed extruder with a 30:1 L/D screw and a barrier mixer can achieve sufficient dispersion only when the recyclate is ground to a flake size below 8 mm and pre-blended at 50–80 °C for 20–30 min. Without pre-blending, film thickness variation at the die lip is observed as a ±5–8% gauge band, measured by a capacitance thickness scanner. The recommended processing window for a 30% PCR film is barrel profile 180 °C/195 °C/210 °C/215 °C/220 °C, die 220–225 °C, and chill roll temperature 20–30 °C for cast film. Maintaining the melt temperature below 230 °C limits the formation of additional gel bodies from oxidised polymer chains.

Screen pack filtration is the first control point for 30% PCR. A continuous screen changer with a 120 mm diameter breaker plate and screen combinations of 20 mesh/60 mesh/100 mesh removes contaminants above 140 µm, but gels below that size remain. For high-clarity film, a 200 mesh (74 µm) screen is used; the resulting pressure is higher and screen change frequency can drop to 2–4 h per pack when post-consumer reclaim contains polypropylene film labels. Filtration does not remove melt-soluble contaminants. Volatile species from printed film—ketones, glycol ethers, and ink binders—are removed only with vacuum devolatilisation at -0.08 MPa to -0.09 MPa on a co-rotating twin-screw compounding extruder with an L/D of 40:1. Without a vacuum port, retained volatiles reduce bubble stability in blown film and create smoke at die temperatures above 220 °C.

When uncontrolled polypropylene contamination enters the 30% PCR blend

Polypropylene labels, caps, and rigid container fragments in the LLDPE reclaim stream are the most common cause of haze, fish eyes, and tensile brittleness at 30% loading. Polypropylene melts at 160–170 °C, but it remains immiscible in LLDPE at processing temperatures of 190–230 °C, forming dispersed PP domains that act as stress concentrators. Differential scanning calorimetry (ISO 11357-3:2018) on a 30% PCR blend should show no PP melting endotherm above 150 °C; if a peak at 163 °C is present, the lot fails the dilution standard for critical film applications. Conventional density float-sink separation in water is ineffective because both LLDPE (0.918–0.940 g/cm³) and PP (0.900–0.910 g/cm³) float. Near-infrared (NIR) flake sorting is required.

A production-scale NIR sorter with 3 mm × 3 mm pixel resolution lowers PP contamination to below 1 wt% but does not remove PP from printed film labels. In a blown film trial, PP contamination of 3 wt% in a 30% PCR blend increased gel counts from 80 counts/m² to 260 counts/m² and reduced dart impact from 110 g to 62 g using ASTM D1709-22 method A. The die lip also accumulated PP deposits after 6 h, producing machine-direction die lines. These effects are not corrected by raising the die temperature; the PP domain size increases and the mechanical property deficit remains. For every 1 wt% of uncontrolled PP in the final compound, elongation at break (ASTM D882-22) can deteriorate by 5–10%, which makes PP removal more cost-effective than additional compatibiliser dosing above 5 wt%.

Mechanical property retention and compatibiliser dosing

With a mixed-colour LDPE/LLDPE PCR fraction, tensile strength at break (ASTM D882-22) commonly falls to 80–90% of the virgin LLDPE control, while elongation at break may fall below 70%. Dart impact (ASTM D1709-22) is the most sensitive film property and can drop by 30–50% unless a compatibiliser is used. Film-grade LLDPE with 30% PCR generally requires 2–5 wt% of an ethylene-octene plastomer with a density of 0.870–0.885 g/cm³ or a maleic anhydride grafted LLDPE coupling agent. The plastomer restores some impact energy by lowering the glass transition temperature of the amorphous phase, but it also reduces modulus. For applications requiring stiffness, a high-density polyethylene fraction of 5–10 wt% can offset the modulus loss, provided the total density remains below 0.940 g/cm³ for LLDPE classification. The additive package should be dispersed in a twin-screw compounder before pelletising; single-screw dry blending of compatibiliser powder at the hopper produces localised over-dosing and film haze.

Cast film lines with 30% PCR are more stable when the die gap is kept at 0.5–0.8 mm and the melt curtain is quenched within 10–15 mm of the die lip. Blown film lines require a lower blow-up ratio of 2.0–2.5:1 instead of 3.0:1 because PCR-induced gel species initiate bubble breaks at high hoop stress. Frost line height is typically reduced to 300–500 mm from the die face. A 40 mm extruder with a 24:1 L/D screw and a Maddock mixing section may be acceptable for 15% PCR but is insufficient for 30% post-consumer film unless a static mixer or increased back-pressure is added. Published equipment trials indicate that a grooved-barrel extruder with a length of 30 L/D provides more stable conveying of low-bulk-density recycled flake. Pre-drying is required when the recycled flake moisture content exceeds 0.1 wt%; a desiccant hopper dryer at 70–80 °C for 2 h reduces surface moisture and prevents hydrolytic odour in film.

Tests, standards, and third-party attestation for recycled content claims

The 30% recycled content claim must be documented using a chain-of-custody system. For European customers, EN 15343:2007 provides the basis for recycled content calculation, and ISO 22095:2020 defines chain-of-custody models including controlled blending and mass balance. For environmental claims, ISO 14021:2016 requires that the recycled content percentage be verifiable and not overstated. In food-contact applications, the recycled fraction must meet the applicable positive-list and challenge-test requirements under FDA 21 CFR 177.1520 and Regulation (EU) No 10/2011 Annex I; a commercial PCR lot of non-food film should not be used in direct food contact without a decontamination process validated to remove migrant species below 10 µg/kg in the finished article.