Low-density polyethylene (LDPE) is a branched, semicrystalline polyolefin with a density of 0.910 g/cm³ to 0.925 g/cm³, a peak melting range of 103 °C to 115 °C, and crystallinity commonly between 40% and 55%. Recycled LDPE (rLDPE) is recovered from post-industrial edge trim, converter scrap, shrink film, agricultural cover film, and post-consumer flexible packaging. Unlike virgin LDPE, its specification must include contamination limits because incoming bales contain polypropylene caps, paper labels, inks, adhesives, metallised film, and food residues. A typical post-consumer mixed flexible bale entering a mechanical recycling plant contains 60 wt% to 70 wt% LDPE/LLDPE, 10 wt% to 15 wt% polypropylene, 5 wt% to 8 wt% paper fibre, 3 wt% to 5 wt% PET, and 2 wt% to 4 wt% aluminium or metallised fragments. Grades are separated by melt mass-flow rate (MFR), density, filtration level, and residual contamination; a single MFR point can nonetheless overlook bimodal flow behaviour created when low-viscosity post-consumer film is blended with high-viscosity edge trim. Characterisation of polyethylene recyclates follows EN 15344:2021, with MFR measured at 190 °C under 2.16 kg load by ISO 1133-1:2022 and density by ISO 1183-1:2019. A capillary rheometry sweep from 10 s⁻¹ to 1000 s⁻¹ at 190 °C on a 25:1 L/D single-screw extruder is required to expose the shear-viscosity differences hidden by an equivalent melt index. This article defines rLDPE grades through the interaction of melt flow, density, contamination limits, and downstream process thresholds, then maps those grades to end uses where specific failure modes are observed.

What Separates Natural Film, Black Film, and Moulding rLDPE Grades When Melt Flow Index and Density Overlap?

At mechanical recycling plants, rLDPE is commonly divided into natural film grade, black film grade, and moulding grade. Natural film rLDPE is produced from sorted transparent LDPE and LDPE/LLDPE films that have passed through hot caustic washing at 70 °C to 90 °C, friction washing, hydrocyclone separation, and melt filtration through a 120 µm or 150 µm stainless steel screen pack. The target MFR is 0.5 g/10 min to 2.0 g/10 min at 190 °C/2.16 kg, and density is held between 0.915 g/cm³ and 0.925 g/cm³. Black film rLDPE tolerates a wider MFR band of 0.5 g/10 min to 3.0 g/10 min and filtration at 150 µm to 200 µm because carbon black masterbatch masks residual ink and colour variation. Moulding-grade rLDPE is filtered at 250 µm to 400 µm and has an MFR of 4 g/10 min to 20 g/10 min; this high-flow grade is filled into thin-walled injection moulds on machines with clamp forces from 800 kN to 3,000 kN. The grade boundaries are not absolute because recyclers blend high-MFR post-consumer material with low-MFR post-industrial scrap to reach the contractual MFR. That blending produces a broader molecular weight distribution. Gel permeation chromatography of natural film rLDPE often shows a dispersity index of 6 to 12, compared with 4 to 7 for virgin LDPE, because repeated heat histories generate simultaneous chain scission, long-chain branching, and crosslinked gel domains. A batch extruded on a co-rotating twin-screw line with a 44:1 L/D and a vent at barrel 7 can show MFR drift of ±0.5 g/10 min as bale composition shifts from carrier bag film to agricultural film. Vacuum degassing at −80 kPa removes residual moisture before pelletising, but degassing alone cannot correct polymeric contamination. The extruder is typically coupled to a melt pump and a backflush screen changer to maintain pellet uniformity.

Contamination limits in rLDPE are governed by three classes: particulate, polymeric, and volatile. Particulate contamination includes paper fibre, sand, aluminium fragments from metallised film, and carbonised polymer generated during reprocessing. Polymeric contamination is primarily polypropylene from caps and labels, with occasional ethylene-vinyl acetate, polyamide, or polyethylene terephthalate from multilayer laminates. Volatile contamination includes printing ink solvents, adhesive residues, fatty food soils, and degradation products from processing stabilizers. A post-consumer LDPE film bale containing 8 wt% moisture and 12 wt% residual food soil can deliver 2 wt% to 5 wt% ash after washing and extrusion if the wash line lacks a friction washer and a hot caustic bath at 70 °C to 90 °C. For natural film rLDPE, the moisture ceiling is 0.10 wt%; higher residual moisture causes steam pitting at the die lip and bubbles in cast film above 170 °C melt temperature. Ash is limited to 1.5 wt% for natural film, 2.5 wt% for black film, and 3.5 wt% for moulding grade. These values are determined by ISO 3451-1:2019 on a 5 g sample combusted at 600 °C. Moisture is measured by ISO 15512:2019 using a Karl Fischer coulometer; a 0.2 g flake sample is heated to 160 °C in a nitrogen carrier. Polymeric contamination is assessed by differential scanning calorimetry: a melting endotherm near 160 °C identifies polypropylene, and a second endotherm near 250 °C identifies polyethylene terephthalate. Sink-float separation in water cannot remove polypropylene because both LDPE and polypropylene float below water density; LDPE density extends to 0.925 g/cm³ and polypropylene density to 0.905 g/cm³. Upstream separation therefore depends on near-infrared sorting, ballistic separators, and hydrocyclone density sorting. On film lines, a polypropylene limit of 3 wt% is common because higher levels reduce tear resistance below 25 N/mm in ISO 6383-2 and make dart impact in ISO 7765-2 erratic.

The process conflict at fine filtration is shear heating. A 120 µm screen pack can raise melt pressure from 3 MPa to 7 MPa within 4 h when gel content is high; the resulting viscous dissipation increases melt temperature by 5 °C to 10 °C and accelerates radical oxidation. Continuous backflush screen changers are required to maintain throughput. Stabilizer depletion is measured by oxidation induction time according to ISO 11357-6:2018. A residual oxidation induction time below 10 min at 200 °C indicates insufficient stabiliser for further processing; film-grade rLDPE should remain above this threshold to prevent gel formation during extrusion. Melt pressure transducers positioned before and after the screen pack record the pressure differential, and the data are used to schedule screen change cycles. When the differential exceeds 3.5 MPa on a 150 µm filter, the operator either reduces throughput or switches to a larger screen. This operational boundary is more restrictive than the pellet specification alone, because a pellet can pass visual inspection and yet blind a screen pack rapidly due to deformable gel particles smaller than the filtration aperture that coalesce at the screen surface.

When rLDPE Re-enters Blown Film Lines, Gel Formation and Die-Lip Deposits Set the Practical Ceiling

Gel particles in rLDPE are crosslinked or oxidised domains that do not fully melt at standard film extrusion temperatures. They arise from degraded coatings, metal-induced oxidation, and repeated extrusion of unstabilised material. At a melt temperature of 190 °C and a die gap of 1.2 mm, gel particles above 250 µm disturb the melt web and initiate bubble tear. Although a 120 µm screen removes some gel body, a fraction is soft and deformable and can pass through or blind the screen. Film extruders operating a 30:1 L/D grooved-barrier single-screw extruder at 80 kg/h report die-lip deposit formation when rLDPE contains more than 0.05 wt% low-molecular-weight oxidate. Selection of a barrier screw with a Maddock mixer and melt temperature control at 180 °C to 200 °C reduces residence-time degradation but cannot eliminate pre-existing gels. Blends of 30 wt% rLDPE in virgin LDPE are common for refuse sacks; inclusion above 50 wt% requires a stabiliser masterbatch and a bubble stabiliser configured for low gel tolerance. The practical ceiling is therefore a coupled function of gel particle size distribution, residual volatile load, and residual oxidation induction time. A blown film line running 80 µm film with a blow-up ratio of 2.5:1 and a frost line height of 500 mm will show loss of bubble symmetry when gel count exceeds roughly 50 particles per 10 m² in the 300 µm to 500 µm size class. Published data for this specific configuration is limited.

Mechanical property retention is directly governed by the melt filtration step. On a cast film line producing 40 µm film from 30 wt% natural film rLDPE, tensile strength and elongation are tested in machine and transverse directions by ISO 527-3. Published recyclability studies indicate that retention of elongation at break typically falls to 70%–85% of virgin film when gel content is low; if the film contains polypropylene above 3 wt%, transverse elongation drops more sharply due to immiscible domains oriented in the machine direction. Dart impact retention measured by ISO 7765-2 follows a similar trend, with gel particles acting as stress concentrators. In practice, the converter must adjust the die gap and melt temperature to compensate for the broader relaxation spectrum of rLDPE; a die gap increase from 1.2 mm to 1.8 mm reduces melt fracture but lowers drawdown capability. These adjustments are process-specific and cannot be captured in a single pellet certificate. Therefore, end-user specifications for film-grade rLDPE increasingly include a gel count, a filtration pressure-rise test, and an oxidation induction time measurement in addition to the basic MFR and density values.

End uses for rLDPE follow a contamination tolerance hierarchy. Thick refuse sacks of 60 µm to 120 µm accept black film rLDPE filtered at 150 µm to 200 µm because the product is not load-bearing for food contact and colour variation is masked. Construction damp-proof membrane and temporary cover film accept 2.5 wt% ash and 0.15 wt% moisture when the melt is filtered at 200 µm. Agricultural greenhouse film and mulch film demand natural film rLDPE filtered at 120 µm, with 1.5 wt% ash and retained UV stabilizer packages; aluminium pigment contamination creates localised heat absorption and premature failure under ISO 4892-3 QUV exposure. Injection-moulded closure bodies and caps operate at MFR of 4 g/10 min to 20 g/10 min and accept 3.5 wt% ash, but polypropylene contamination above 5 wt% reduces weld-line strength and causes surface delamination. Extrusion blow moulding of non-food rLDPE bottles requires MFR of 0.7 g/10 min to 1.5 g/10 min and sufficient melt strength; parison sag is controlled by gel-free material and a minimum oxidation induction time of 10 min at 200 °C. Each end use is therefore specified not solely by MFR but by a combination of filtration aperture, ash limit, moisture limit, polymeric contamination, and residual stabiliser content. For these applications, published data for specific inclusion rates are limited because the mechanical response depends on feedstock history, washing line configuration, and melt filtration rather than grade name alone.