Polyethylene (PE) is the largest-volume thermoplastic polymerised from ethylene, with global annual nameplate capacity above 120 million tonnes. The polymer family includes HDPE at 0.941–0.970 g/cm³, LLDPE at 0.915–0.940 g/cm³ and LDPE at 0.910–0.925 g/cm³. Conversion processes include blown-film extrusion, cast-film extrusion, blow moulding, injection moulding, rotomoulding and extrusion coating. Bio-Attributed and Recycled Polyethylene: Separating Real Tonnes From Paper Claims requires an audit trail that distinguishes a physical bio-based or post-consumer tonne from a fossil tonne carrying a certificate. Polyethylene is nonpolar and chemically simple, so Fourier-transform infrared spectroscopy cannot distinguish a sugarcane-derived PE molecule from a fossil PE molecule. The functional distinction is therefore made by chain-of-custody documentation, carbon-14 measurement or mass-balance accounting.

When a Tonne of Bio-Attributed PE Is Not a Tonne of Bio-PE

Under ISCC PLUS or REDcert² mass-balance certification, bio-attributed PE is generally a fossil PE molecule with biogenic feedstock credits allocated from an earlier production stage. ISO 22095:2020 defines chain-of-custody models, including identity preserved, segregated, controlled blending and mass balance. In the mass-balance model, the certified output tonnage can be sold as bio-attributed even when the physical stream contains little or no bio-based carbon, provided the renewable feedstock was consumed within the defined mass-balance period. The product may therefore show a fossil-like percent modern carbon by ASTM D6866-22 Method B, while carrying a certificate for 30 wt%, 70 wt% or 100 wt% bio-attribution. Some mass-balance mechanisms permit free attribution within defined limits, further separating the certified tonnage from physical content. Sugarcane-derived ethanol is dehydrated to bio-ethylene at 300–500°C over alumina catalysts, and the ethylene is polymerised in gas-phase, slurry-loop or high-pressure LDPE processes without changing molecular architecture. A physically segregated bio-PE grade from this route typically exhibits a biogenic carbon fraction above 95 pMC by ASTM D6866-22; an ISCC PLUS bio-attributed grade with the same nominal percentage may exhibit a biogenic carbon fraction below 5 pMC. Buyers should request the chain-of-custody model, the mass-balance period statement, and a carbon-14 certificate if the specification requires physical bio-carbon. Claims under ISO 14021:2016 must be clear and substantiated; a label stating “bio-attributed” without declaring mass balance is open to challenge because the physical product may be fossil-based.

Recycled Feedstock Verification and Melt-Filtration Limits

Mechanical recycled PE is governed by EN 15343:2007 for recycled content and traceability, supported by ISO 14021:2016 for self-declared environmental claims. Distinguishing real post-consumer recycled tonnes from post-industrial or pre-consumer material requires physical origin records and sorting-line mass balances. A certified rHDPE lot from post-consumer bottle caps should carry the collection source, input weight, yield after hot-wash and dry separation, extrusion output, and final sieve/filter records. Real PCR tonnes commonly retain non-PE components at low percentages: polypropylene caps, PET fines, EVOH barrier platelets and aluminium foil residues. A well-optimised mechanical recycling line can reduce non-PE contamination to below 3 wt%, but film grades may exceed 7 wt% without additional melt filtration. The filtration step uses slide-plate or continuous backflush screen changers with screens from 80 µm to 250 µm; finer filtration controls gels but increases melt temperature and pressure, particularly with HDPE of low melt index. Post-industrial off-spec material is not post-consumer recycled; real post-consumer tonnes require proof of collection from end-use waste rather than factory scrap.

Mass-balance recycled PE from pyrolysis oil is generated by feeding mixed plastic waste to a pyrolysis unit and then processing the resulting naphtha in a steam cracker. Because the cracker pools fossil and recycled naphtha, the delivered PE tonne may carry a recycled credit but contain negligible plastic-waste-derived carbon. This is another paper-claim pathway unless the chain of custody is clearly stated. The following property spread is representative of supplier technical data sheets aggregated for virgin HDPE blow-moulding grade and recycled HDPE from post-consumer bottles.

On high-output blown-film lines running 80 wt% rLLDPE in the core layer of a three-layer die, melt-pressure variation across a 120 µm mesh filter pack is frequently the first indicator of gel and cross-linked fraction. A stable virgin LLDPE lot at 230°C barrel set-point may show extrusion head pressure in the range 180–220 bar; introduction of dried rLLDPE from mixed post-consumer film can shift head pressure to 240–300 bar within 20–40 min. The pressure rise is caused by insoluble gel particles, paper fibres and oxidised film fractions that blind the screen. Lines equipped with continuous backflush melt filters maintain longer run time, but backflush cycles increase from once per 8 h to once per 20–40 min when PCR content exceeds 50 wt%. Operators typically compensate by raising extruder temperatures 5–10°C after the feed throat, but this increases oxidation and can degrade the organoleptic profile. Melt pumps are recommended to stabilise output; without a melt pump, screw speed fluctuations of ±2 rpm can produce film thickness variation above ±8% at high PCR loading. Pre-drying at 70–80°C for 4 h is mandatory for reclaimed film with moisture above 0.2 wt% because bubbles and lens defects appear in the blown film. The die gap is typically 1.8–2.4 mm and the blow-up ratio is 2.0–2.5:1 for rLLDPE-rich film; changing these settings without melt-pressure monitoring can mask the build-up until bubble instability occurs.

Can Mechanical Recyclers Deliver Consistent MI and Density in High-Rate Coextrusion?

The limiting factor in coextrusion of recycled PE is not usually the average melt index but the lot-to-lot variation in melt viscosity. In a five-layer line producing collation shrink film at 350 kg/h, the melt-flow ratio of adjacent layers should stay below 2:1 to avoid layer encapsulation and interfacial instability. Post-consumer rLLDPE can vary from 0.8 g/10 min to 2.5 g/10 min at 190°C/2.16 kg, whereas a virgin metallocene LLDPE may be specified at 1.0 ±0.15 g/10 min. If the rLLDPE arrives with high MI, the layer thins at constant extruder speed and the die gap distribution shifts; if the MI is too low, melt fracture can occur at the same output because wall shear stress exceeds the critical value, typically in the range 0.1–0.4 MPa for LLDPE. Density shifts in rLLDPE from 0.918 g/cm³ to 0.934 g/cm³ alter crystallisation and haziness in chill-roll film. Converters should specify incoming PCR by both melt index and high-load melt index for the same lot. ISO 1133-1:2022 is the standard melt-mass flow rate method; ISO 1183-1:2019 covers density gradient column testing. Without per-lot incoming inspection, the paper certificate from the recycler may state “virgin-equivalent”, but the physical tonne may fail at the die.

Because post-consumer HDPE bottle scrap contains residual PP caps, EVOH barriers, PET bottles and aluminium foil, near-infrared sorting followed by wet density separation is needed to approach a real-tonne PCR supply. A sink-float bath with a specific gravity of 0.93–0.96 g/cm³ separates floating PP/PE from sinking PET/PVC, but EVOH and aluminium often remain in the HDPE fraction because their particle size or adhesion defeats the separation. At the extruder, residual PP forms dispersed domains that reduce tensile elongation and can cause die-lip deposits; a PP concentration above 3 wt% in HDPE PCR typically lowers notched Charpy impact by 20–50% compared with the same rHDPE without PP. The mechanical recycler therefore uses a hot-wash stage at 85–90°C with sodium hydroxide and detergent to remove adhesives and paper labels, followed by hydrocyclones and blow dryers. These stages are not optional for food-contact compliance; the output must meet EU No 10/2011 migration limits for the intended food type, and FDA 21 CFR 177.1520 for olefin polymers in food-contact articles. Published data for extrusion of heavily contaminated PCR into thin films remains limited because gel levels above 50 gel/m² at 600 µm thickness are routinely considered unacceptable for surface-critical applications; in such cases the real tonne can be certified but cannot be used in films requiring high clarity.