Declaring Recycled Content in Polyethylene: Traceability and Audit Evidence

In the mechanical recycling of polyolefins, the declaration of recycled content in polyethylene is not a single analytical result. It is a chain-of-custody claim that must be supported by traceability records, mass balance data, supplier declarations and selective physical testing. High-density polyethylene, low-density polyethylene and linear low-density polyethylene are chemically identical to their virgin counterparts after recompounding, because the base polymer chain is not altered sufficiently to create an unambiguous recycled marker. Consequently, the audit file rather than laboratory spectroscopy carries the primary evidence burden for a declared recycled mass fraction. Under ISO 14021:2016, 7.7.4, a self-declared recycled content claim is expressed as a mass percentage and requires the calculation boundary, source category and verification method to be documented. The term “post-consumer resin” is acceptable in commercial communication only if it is mapped in the audit record to a defined source category, such as post-consumer bottle bales or post-consumer film waste, and distinguished from pre-consumer converter scrap.

The recycled mass fraction is conventionally calculated on the dry polymer mass after moisture and non-polymer residues have been excluded. Certification records under EN 15343:2007 require the recycler to state the input material source, the recycling process, the output grade and the modelled chain-of-custody mechanism. Without this documentation chain, a laboratory-determined melt flow rate or density value cannot prove that a pellet contains 30% post-consumer HDPE. The ambiguity is operational: scrap generated inside the same converting plant may have identical rheology and density to external post-consumer waste, but the two categories carry different sustainability and regulatory value.

Which Chain-of-Custody Model Fits Post-Consumer PE Granulate?

Chain-of-custody modelling under ISO 22095:2020 provides the legal and accounting structure for transferring a recycled content claim from sorted waste to finished polyethylene products. Identity preserved, segregated and controlled blending models differ in the degree of physical separation required and in the method by which recycled attributes are allocated. Identity preserved is rarely used for post-consumer PE because it demands that a specific bale or flake batch remain continuously separated from other recyclate and virgin polymer through every silo, hopper and package. Segregated systems permit mixing of certified recycled batches but not mixing with virgin material. Controlled blending, often called mass balance in industry practice, permits physical mixing of recycled and virgin polyethylene while the recycled attribute is allocated by input-output mass ratios over a defined booking period.

An audit against controlled blending must demonstrate that the declared recycled tonnage transferred to finished polyethylene does not exceed the certified recycled input minus documented process losses. Process losses arise in hot washing fines, label removal, melt filter bleed, start-up purges and off-spec pellet generation. Conversion factors are site-specific and must be recalculated at least monthly where the input bale composition varies. Published data for single-site conversion factors across all PE film recyclers is limited; therefore audit sampling must capture the actual shredder throughput, wash line yield and extruder output for the declaration period.

Mechanical recycling lines processing post-consumer polyethylene film typically deploy shredding, hot washing, hydrocyclone density separation, granulation and extrusion. Polyethylene and polypropylene both float in water, so float-sink separation is not sufficient to remove PP contamination from PE flake; near-infrared sorters are required for HDPE, LDPE and PP discrimination. Recyclers must record the sorting unit settings, ejection valve response and manual quality-control draws for each shift. In single-screw recycling extruders with 30:1 to 40:1 L/D ratios, melt filtration through screen changers removes rigid particles, paper fibre and aluminium residues. The screen pack pressure upstream of the filter, measured in MPa or bar, is a process parameter that should be assigned to the production batch and retained as audit evidence, because progressive pressure rise indicates contaminant loading in the incoming flake.

When Supplier Declarations Are Not Sufficient for High-Purity PE Film

The audit evidence for recycled polyethylene is weakest when a supplier certificate is accepted without batch-level mass balance and without incoming material testing. A supplier declaration under EN 15343:2007 may state the recycled content, source category and chain-of-custody model, but it does not by itself prove that a specific delivered lot carries that recycled fraction. High-purity blown film applications using post-consumer LDPE or LLDPE demand additional records because gel defects, odour and melt pressure excursions in the film extruder are directly linked to the quality of the recyclate. The buyer must compare the certificate scope against the delivery note lot number, the silo or big-bag identifier, and the internal melt flow rate result.

For controlled blending systems, the mass balance ledger is the highest-risk document. An auditor must trace the opening recycled stock, the certified input deliveries, the production outputs labelled as recycled, and the closing stock. If the recycler operates a continuous extrusion line with multiple silos, the ledger must specify the allocation method used when a single output silo receives material from both recycled and virgin feeders. The absence of a documented allocation rule means that the recycled content claim cannot be assigned to any particular pellet lot. Batch-level testing of melt flow rate and density does not correct this defect, because the physical test values are independent of the book-and-claim allocation.

Food-contact polyethylene adds a separate regulatory layer. Recycled PE intended for food packaging under Regulation (EU) 2022/1616 must come from an authorised recycling process that has demonstrated decontamination performance. In that context a generic “contains recycled content” declaration is insufficient; the recycler must provide the authorisation number, the input waste stream limits and the output article scope. The audit evidence therefore shifts from generic mass balance to process-specific challenge test data, residence time records and compliance with the authorised decontamination technology. For non-food applications, such as jerrycans, drainage pipes and agricultural film, the traceability file remains the principal reference.

No standard analytical test can distinguish post-consumer fossil HDPE from virgin fossil HDPE at the molecular level after compounding. Fourier transform infrared spectroscopy in attenuated total reflectance mode can track oxidation by measuring the carbonyl absorbance near 1,715 cm⁻¹ normalised to the methylene reference near 2,918 cm⁻¹, but carbonyl index is a degradation indicator, not a recycled content meter. Similarly, differential scanning calorimetry can measure oxidation induction time, but the result reflects antioxidant depletion and processing history rather than the percentage of post-consumer input. These methods serve as incoming quality screens and should be retained alongside the mass balance file, but they must not be presented as sole evidence of recycled content.

Entrained Process Risks in Recycled HDPE Blow Moulding Grades

Extrusion blow moulding of HDPE bottles, containers and industrial packaging is sensitive to melt flow variation and contamination introduced by recycled content. Virgin blow moulding HDPE typically has a melt flow rate from 0.2 g/10 min to 1.0 g/10 min at 190 °C and 2.16 kg per ASTM D1238-20, with a density between 0.945 g/cm³ and 0.970 g/cm³ per ASTM D792-20. Post-consumer HDPE regrind derived from mixed injection and blow moulded parts can shift the melt flow rate upward or downward depending on the dominant source fraction. If the recyclate lot has a melt flow rate above the specified blow moulding target, parison sag can increase and wall thickness control deteriorates. If the melt flow rate is too low, the extruder head pressure rises and melt fracture may appear at the die lip.

Grooved-feed single-screw extruders on industrial blow moulding machines, commonly with 20:1 to 30:1 L/D, impose their own limit on recycled flake bulk density and particle size. Fines from post-consumer HDPE grinding can bridge in the feed throat and reduce output stability. Audit records should therefore include the sieve distribution of the recyclate, particularly the fraction passing 200 µm, because high fines content is a common batch-to-batch variance source on production lines. A melt filtration screen pack with a nominal aperture from 80 µm to 150 µm is often installed before the blow moulding head, but filtration alone does not eliminate dissolved volatiles, polar contaminants or residual odour compounds that originate from post-consumer soap bottles, diesel containers or food residues.

The operational boundary for recycled HDPE in blow moulding is reached when the declared recycled content raises the parison hang time variability beyond the machine’s closed-loop control range, or when the screen pack pressure increases by more than the process control limit within a single shift. These limits are equipment-specific and must be determined on the actual production line rather than transferred from a generic datasheet. Published data for the pressure-rise tolerance of specific monolayer HDPE blow moulding formulations is limited, so the audit file should retain the extruder barrel temperatures, screw speed, head pressure, parison length sensor output and screen pack differential pressure for each lot containing recycled material. If these records are missing, the producer cannot demonstrate that the declared recycled content was process-compatible under the conditions used for the final article.

Density and melt flow testing alone provide only a partial incoming inspection. A recyclate with acceptable density and melt flow can still contain polar contaminants, oxidised gel particles, residual adhesives from labels, or PP inclusions from cap material. Near-infrared sorting of HDPE from PP remains imperfect in mixed rigid bales, and PP contamination in blow moulded HDPE can reduce stress-crack resistance and pinch weld integrity. The traceability file must therefore contain the sorter rejection rate, the manual quality-control photographs of the flake, and the screen pack change interval. An audit that verifies only the mass balance but not the physical sorting evidence may approve a recycled content declaration that is mathematically valid but technically unsuitable for the intended blow moulding application.

The traceability system cannot end at the recycler’s gate. The converting plant must assign the incoming recycled pellet lot to specific production orders and retain the finished product lot records. Under a controlled blending model, the recycled mass fraction is allocated to the film or bottle lot by the mass ratio fed to the line. The declaration on the commercial invoice must be traceable to this allocation, not to a plant-wide annual average that may conceal lot-to-lot variation. Where the final customer requests evidence, the producer should be able to supply the supplier declaration, the mass balance ledger, the incoming test data and the production lot record as a single dossier. If any of those four components is missing, the declared recycled content in the polyethylene article is unverifiable regardless of the certificate displayed on the packaging.