Linear Low-Density Polyethylene (LLDPE) film grades are typically specified with density from 0.918 g/cm³ to 0.940 g/cm³ and melt-flow index from 0.5 g/10 min to 2.0 g/10 min as determined by ISO 1133-1:2022 or ASTM D1238. In blown-film and cast-film lines running post-consumer recyclate LLDPE (rLLDPE) at addition levels from 10 wt% to 50 wt%, the phrase “gel and odour problems from rLLDPE: root causes in film lines” describes a recurring pattern of bubble instability, pressure rise across the screen pack, visible gel specks, and odour carryover into wound film. The defects are not inherent to the ethylene-α-olefin copolymer fraction; they arise from contaminant inclusions, prior oxidative ageing, and the shear-temperature path within the extruder. A typical single-screw grooved-barrel blown-film line with 30:1 L/D barrier geometry, a 20/40/60 mesh screen set, and a die gap of 1.8 mm to 2.4 mm may show a clean-pressure baseline near 45 bar and a blinding threshold above 120 bar when gel accumulation is established. The interfacial area of the bubble amplifies the visual and sensory consequences of even low concentrations of volatile or particulate defects.

A three-layer A/B/A blown-film configuration can hide some rLLDPE gel population in the core layer, but it does not suppress volatile migration. Odour compounds partition from the core into the virgin LLDPE skins during bubble cooling and winding, often becoming detectable only after the reel has rested for 24 h to 72 h. Film gauge also changes the perception of gel defects: a gel above 100 µm in a 50 µm film is visible as a lump or fisheye, whereas the same particle in a 150 µm film may be less obvious but can still initiate dart-impact failure under ISO 7765-1.

What Converts Residual Contamination in Post-Consumer LLDPE into Fused Gel Domains During Extrusion?

Gel bodies recovered from rLLDPE film can be classified by hot-stage microscopy and differential scanning calorimetry under ISO 11357-3:2018. Oxidised polyethylene gels are usually translucent or amber, deform readily at 140 °C, and exhibit reduced melt index relative to the matrix. Crosslinked polyethylene gels are non-deformable and do not flow at 200 °C; they remain as elastic inclusions after heating. Crystalline contaminants such as polypropylene, high-density polyethylene, or nucleated LLDPE appear as discrete domains with separate melting endotherms. Barrier polymer residues from polyamide or ethylene-vinyl alcohol show brown or black discoloration and produce non-melting char-like particles.

The root causes on film lines are multiple. Low-level oxidative damage in the recyclate consumes the original antioxidant package. When oxidation induction time measured under ISO 11357-6:2018 at 210 °C falls below 5 min on rLLDPE powder or pellet, the melt-phase stabiliser is effectively exhausted, and additional chain scission and macroradical recombination convert high-molecular-weight fractions into microgels during processing. Virgin LLDPE film grades typically show isothermal OIT values above 20 min under the same conditions. Immiscible higher-melting polypropylene or polyamide domains are not fully homogenised at LLDPE melt temperatures below 250 °C. These domains may pass through a 100 µm screen pack in a softened state and then bridge across die lands. Paper fibre, label adhesive, and ink binder particles can carbonise in stagnant zones near the screw tip or die lip, creating black gels that dislodge periodically. The melt-filtration circuit is only partially effective because deformable oxidised gels can extrude through the interstitial spaces of a 20/40/60 mesh screen, while the pressure drop across the filter increases as rigid gels form a packed bed.

Melt pressure is not a reliable surrogate for gel absence. A screen-changer differential of 35 bar can coexist with small deformable gel counts because the gel particles align and pass through the filter medium. Only optical film-inspection data below 200 µm equivalent circular diameter can distinguish aesthetic gel failures from upstream contamination. Many converting specifications combine optical inspection with ISO 7765-1 dart-impact loss after gel-induced film puncture. Published data for exact gel-count reject limits in post-consumer rLLDPE film is limited; thresholds are usually site-specific.

Oxidation during die-lip stagnation is a film-line source of gel formation that is distinct from incoming recyclate quality. When gel specks appear only after 45 min of continuous operation and correlate with visible die deposit, the appropriate diagnostic is a purge study with virgin LLDPE under identical melt temperatures. If the gel count returns to below a site-defined limit, the recyclate is supplying oxidisable precursors rather than preformed gels. If the gel count remains elevated, die-lip or screw-tip dead zones are converting stagnant polymer into oxidised material. Line-speed changes from 10 m/min to 40 m/min alter residence time distributions and can therefore change the visual gel size distribution without changing the recyclate concentration.

Odour in rLLDPE film is best understood as a devolatilisation failure rather than a single-compound problem. When rLLDPE is conditioned and analysed by headspace gas chromatography according to ASTM D4526-20, the volatile fraction commonly contains terpenes from citrus and fragrance packaging, short-chain carboxylic acids from spoiled dairy or bio-waste, printing-ink solvents such as ethyl acetate and toluene, and aldehydes from lipid oxidation or polyethylene thermo-oxidation. These species are retained in the amorphous phase of the recyclate and are released when the melt temperature reaches 230 °C to 250 °C. The mechanical work in a 30:1 L/D barrier screw raises the local melt temperature at the die by 5 °C to 15 °C above set point, which accelerates the formation of additional aldehydes and ketones from fatty acid decomposition.

A single-stage blowing line without a vented degassing zone discharges these volatiles at the bubble exterior and into the wound film. Vacuum degassing at 80 kPa to 90 kPa below atmospheric, placed before the metering zone, strips low-boiling solvents and some aldehydes, but its effectiveness falls as molecular mass of the volatile compound rises above 150 g/mol. Residual butyric acid and long-chain aldehydes persist because they are partially associated with polar melt contaminants and are not fully stripped at typical melt viscosities. Sensory evaluation under ISO 13302:2003 or an equivalent panel method therefore remains necessary, because no single headspace concentration reliably predicts perceived intensity in converted food packaging. Compliance with 21 CFR 177.1520 for olefin polymers or REACH restrictions does not define acceptable odour; sensory failures can occur even when migration and composition limits are met. The bubble geometry also matters: a high-stalk film line with a 2.4 m frost line may trap volatiles in condensate and recycle them through the collapsing frame, while a shorter low-blow-up line may discharge them to the factory air.

Published data for specific volatile threshold concentrations in post-consumer LLDPE film is limited; pass/fail levels are commonly set by customer sensory panels and gas-chromatography intensity rather than a single standard. The operational boundary is narrow: when rLLDPE content exceeds 30 wt% in a non-vented monolayer line, odour complaints increase unless the recyclate is first washed at elevated temperature and the extruder is fitted with at least one atmospheric vent or vacuum port. Alkaline washing reduces free fatty acids but may leave terpenes and print solvents unchanged, while steam stripping in the recycling plant reduces low-boiling solvent residues.

When rLLDPE Shifts Melt Viscosity and Head Pressure in Grooved-Feed Film Extruders

The introduction of rLLDPE changes the shear-thinning behaviour of the melt because the recycled fraction often contains branched and thermally degraded molecules plus small amounts of lower-viscosity polypropylene. Melt-flow index of a blended feed may not predict the apparent viscosity in the high-shear region of a screw. A blend with a melt index of 1.0 g/10 min can still exhibit melt-pressure swings of 10 bar to 25 bar if the feed is not homogenised before the grooved feed zone. In film lines, head pressure fluctuations above 15% of the mean value create thickness variation and bubble instability. The root cause is often upstream: inconsistent rLLDPE bulk density, pellet geometry, and contaminant moisture all alter the feed-conveying mechanism. A grooved feed section with 0.5 mm groove depth is especially sensitive to inconsistent compaction, whereas a smooth-bore feed section may lose specific throughput before the pressure fluctuation appears.

When the screen pack blinds with gels, the operator typically sees a progressive pressure rise rather than a hard spike. At 120 bar differential, melt residence time in the screw increases and the risk of oxidised gel formation rises because the boundary layer remains at high temperature. The die land acts as a secondary stagnation volume: gel particles that deform through the filter can adhere to the die lip and oxidise over 20 min to 60 min, producing periodic release of yellow or brown specks into the film. The correct process response is to reduce melt temperature at the die to 220 °C or lower, if bubble stability permits, and to replace the screen pack before pressure differential exceeds 80 bar. In multi-layer lines, the rLLDPE should be placed in a core layer behind virgin LLDPE skins; this does not remove the gel source but reduces die-lip oxidation and visual defect density. Barrier-screw designs with a Maddock mixing section improve gel dispersion up to a point, but they also increase melt temperature by 3 °C to 8 °C and may reduce film tear resistance if the gels are broken into fine domains that still act as stress concentrators.

On cast-film lines using conventional chilled-roll quenching, gel-related defects appear as surface protrusions or die streaks rather than bubble fracture. The same root-cause hierarchy applies, but cast lines are less tolerant of high-melt-temperature oxidation because the melt reaches the die at lower draw and longer residence time. Gel defects larger than 100 µm are visible in films below 50 µm, while odour is often reduced because the thin melt curtain exposes the polymer to a larger surface-to-volume ratio before quenching. However, the chilled roll does not strip non-volatile fatty acids; oxidation products can condense on the roll and transfer back to the film. A film-line audit should therefore collect pressure, melt-temperature, and volatile data before changing the recyclate source or increasing filtration depth.