The comparative assessment of virgin vs recycled LDPE granules: quality, odour and certification begins with feedstock identity. Low-density polyethylene is specified by density 0.910–0.940 g/cm³ per ASTM D792-20 and by melt flow rate determined at 190 °C under 2.16 kg load in accordance with ISO 1133-1:2022. Virgin LDPE granule quality is controlled at reactor level through comonomer incorporation, branching density and additive masterbatch consistency. Recycled LDPE granule quality reflects post-consumer film composition, thermal-oxidative history and decontamination efficiency. Production-scale recycling using a single-screw extruder with screw length-to-diameter ratio 33:1 and dual vacuum vents reduces volatile odourants, but it cannot eliminate non-melting contaminants below the melt filtration threshold.

Virgin LDPE is produced in high-pressure autoclave or tubular reactors at 1500–3000 bar and 180–300 °C, generating short- and long-chain branching that controls melt strength and drawdown. Recycled LDPE does not modify this branching architecture, but multiple heat cycles at 180–240 °C during reprocessing generate crosslinked gels and carbonyl intermediates. Density alone is insufficient to separate virgin from recycled LDPE because contamination with linear low-density polyethylene or ethylene-vinyl acetate shifts density in either direction.

Quality comparison distinguishes post-industrial regrind from post-consumer recyclate. Post-industrial LDPE edge trim and film scrap generally retains a narrower melt flow ratio and a lower ash burden because it has not undergone consumer contamination. Post-consumer LDPE from mixed flexible packaging contains polar films, tie layers, pigments and food residues; these components persist as ash and cause screen pack pressure excursions above 30 bar on melt filtration units. Batch-to-batch variance is therefore higher for recycled LDPE, with melt flow index variability reported as ±0.3 g/10 min in controlled post-industrial streams and up to ±1.5 g/10 min in mixed post-consumer streams.

How Does Recycled LDPE Granule Quality Compare Under Melt Rheology Testing?

Under laboratory capillary rheometry, virgin LDPE film grades exhibit a shear-thinning viscosity curve governed by weight-average molecular weight and long-chain branching. In recycled LDPE, residual high-molecular-weight gel structures increase apparent viscosity at low shear rates, while chain scission from repeated extrusion lowers viscosity at high shear rates. A recycled lot may therefore pass the 2.16 kg melt flow rate specification but still produce melt pressure instability in a blown-film die due to gel-induced flow disturbances. Extensional viscosity measured by a capillary breakup extensional rheometer is useful because it correlates with blown-film bubble stability. Recycled LDPE lots with longer extruder residence time can exhibit lower extensional viscosity at 1 s⁻¹ due to chain scission, while gel-rich lots show spurious extensional hardening from suspended gel domains.

The practical limit for gel content in film below 30 µm is often set at fewer than 10 gels of 200–400 µm per 100 cm² of film, although quantitative correlation with fisheye density remains equipment-specific. Published data for this specific configuration is limited because optical inspection systems use different grey-level thresholds, and gel counts are not fully harmonised across ISO 1853 and ASTM D3596 procedures.

Ash content is a decisive quality indicator. Inorganic residues from calcium carbonate fillers, titanium dioxide pigments and aluminium metallisation contribute to elevated ash in recycled LDPE. At ash loading above 0.5 wt%, blown-film bubble stability deteriorates because solid particles act as bubble-break nucleation sites. This is observed on production lines as increased stalk oscillation and film thickness variation exceeding ±5 % when measured online with a capacitance gauge. Pre-drying at 70–80 °C for 2–3 h is required when residual moisture exceeds 0.05 wt% to prevent surface splay and melt line defects. Formulators avoid combining recycled LDPE containing acidic degradation residues with amine-based slip masterbatches, because the acid-base reaction can reduce melt stability and release odourous species at the die.

In practice, converters dilute post-consumer LDPE with virgin film grade at 20–30 wt% recycled content to maintain bubble stability and mechanical property consistency. Below 20 wt% recycled content, gel-induced fisheyes are less visible, but odour may still be detectable in the final film.

Because odour in recycled LDPE granules is a primary constraint for flexible packaging converters, organoleptic testing is performed on dried pellet samples after extrusion pelletising. Post-consumer LDPE may contain volatile degradation products including acetic acid, 2,4-decadienal, limonene and fatty acid oxidation by-products; these compounds are released from the melt at the die face and can be partially removed by degassing. Under VDA 270 B3, a specimen stored in an odour-neutral vessel at 80 °C for 2 h is assessed on a 1–6 intensity scale. Recycled LDPE of odour grade ≤3 is frequently required for household packaging applications, while virgin LDPE typically exhibits grade 1–2. A vacuum degassing extruder operated at residual pressure below 100 mbar lowers volatile load, but polar odourants such as short-chain carboxylic acids can remain adsorbed on inorganic pigments and are not fully removed by melt extraction.

Volatile compounds in recycled LDPE can also originate from label adhesives, printing inks and surfactant residues from washing. Adhesive components such as rosin esters and terpene oligomers are not fully removed by cold washing; they degrade at extrusion temperatures to form odour-active monoterpenes. A two-step washing process with a centrifuge dryer and a hot air dryer reduces residual water to below 0.10 wt%, but odour performance still depends on the efficiency of melt degassing. Hot washing at 70–80 °C with surfactant dosing reduces limonene carry-over from detergent bottles, but washing alone is insufficient for fatty food residues. Published data for specific volatile organic compound concentrations in recycled LDPE granules is limited because contaminant profiles vary by input bale composition, weather during curbside storage and regional collection scheme.

Volatile Degradation Products and Organoleptic Test Limits

Regulatory and industrial odour specifications for recycled LDPE are not harmonised globally. Automotive applications rely on VDA 270, whereas flexible packaging converters may specify sensory evaluation according to ISO 8586 assessors or chamber emission testing under ISO 16000-6. The odour threshold concentration for a contaminant such as nonanal is in the low parts-per-billion range in air, but measured headspace concentrations in recycled LDPE vary by five orders of magnitude depending on washing media temperature and venting efficiency. A post-consumer LDPE lot with odour grade 4 may fail direct food-contact packaging due to taint transfer even when total volatile organic compound content measured by ISO 16000-6 remains below 500 µg/m³, because sensory thresholds are lower than common emission screening values.

Certification of recycled LDPE granules creates a chain-of-custody link between post-consumer waste collection and final resin conversion. Under EN 15343:2007, recycled content is verified through mass-balance control of input waste streams, product output and accumulated rejects; the associated audit includes sorting line yield, wash water retention and extruder throughput. Third-party schemes such as RecyClass and EuCertPlast evaluate process conformity, while ISO 14021:2016 defines self-declared environmental claims and requires that recycled content be stated as a percentage of total mass. For food-contact applications, FDA 21 CFR 177.1520(c) applies to olefin polymers, but recycled LDPE feedstock must be shown to be of suitable purity and is typically evaluated via a no-objection letter or equivalent regional framework. In the European Union, Regulation (EC) No 1935/2004 requires that the final material not transfer constituents to food in quantities that endanger human health or alter organoleptic properties.

For European converter declarations, a recycled LDPE supplier may provide a certificate of conformity combining EN 15343:2007 data with batch test results for density, melt flow rate and ash. Some packaging buyers also require evidence of good manufacturing practice under Regulation (EC) No 2023/2006, particularly for food-contact materials. The absence of a fully harmonised EU regulation for recycled plastic food-contact materials means that many LDPE recyclates are used in non-food applications such as refuse sacks, construction film and transport packaging.

Absence of REACH Annex XVII restricted substances is generally confirmed by supplier declaration, but post-consumer LDPE may carry legacy substances such as certain brominated flame retardants if the input bale contains non-LDPE products. Screening by gas chromatography–mass spectrometry is therefore performed before lot acceptance. The certification position of recycled LDPE is thus process-dependent: a granulate may conform to EN 15343:2007 traceability and still fail an odour specification or an application-specific impurity limit.

When Recycled LDPE Granules Are Processed on Blown Film and Cast Film Lines

On a blown-film line equipped with a die gap of 0.8–1.2 mm and a blow-up ratio of 2.5:1, recycled LDPE with an elevated gel count produces visible fisheyes. These gel particles are typically crosslinked high-molecular-weight LDPE or degraded polypropylene contaminants that do not melt at the 170–210 °C barrel profile. A converter can mitigate fisheyes by using a continuous screen changer with filtration size 70–100 µm, but this increases melt temperature and pressure, and may reduce output by up to 10 % compared with virgin LDPE under the same screw speed.

In cast film extrusion, recycled LDPE with ash above 0.3 wt% creates die lip buildup at the 240–260 °C adapter zone, requiring more frequent lip cleaning. The use of a gear pump after the screen changer stabilises melt pressure to within ±2 bar and is standard practice for recycled LDPE to minimise thickness variation. For cast film lines running high-speed winding at 150–300 m/min, recycled LDPE with inconsistent melt viscosity creates gauge bands and blocking tendencies. An online gauge profiler using 2-sigma limits of ±3 % is difficult to hold with post-consumer LDPE at 100 % content. Converters often reduce line speed to 80–90 % of the virgin LDPE benchmark and increase chill roll temperature by 5–10 °C to improve film clarity and flatness.

Extrusion operations processing 100 % post-consumer LDPE frequently set a pre-drying boundary at relative humidity above 60 % because granulate moisture uptake increases rapidly under ambient storage. Without pre-drying, moisture above 0.05 wt% creates hydrolysis of residual ester-containing contaminants and generates additional volatile species at the die. Published data on exact output loss percentages for each filtration mesh is limited and equipment-specific.