Testing rLLDPE: MFI, Ash Content and Contamination Screening

Linear low-density polyethylene is an ethylene/α-olefin copolymer with a density from 0.915 g/cm³ to 0.940 g/cm³; butene, hexene, and octene are the conventional comonomers, and the material is processed predominantly into blown film, cast film, and rotomoulded articles. Recycled linear low-density polyethylene, referred to here as rLLDPE, retains the same density envelope but introduces heterogeneity in molecular weight distribution, melt viscosity, inorganic residues, and dispersed contaminants that is not captured by a single bulk property. Mill evaluation of rLLDPE therefore requires three linked data classes: melt flow index under polyethylene standard conditions, ash content by furnace ignition, and contamination screening across multiple analytical scales. These measurements determine whether a recycled lot can be extruded through fine-mesh screen packs, whether pellet stability meets a defined lot-acceptance criterion, and whether residual catalyst fragments, pigments, or crosslinked gels will generate film pinholes or pressure-rise failures. Testing rLLDPE: MFI, ash content and contamination screening cannot be separated from the downstream conversion step, because each parameter influences melt pump stability, die lip fouling, and bubble stability in different ways.

What Does Melt Flow Indexing Reveal in Post-Consumer rLLDPE Lots?

Melt flow index is determined according to ISO 1133-1:2022 or ASTM D1238-23 using an extrusion plastometer equilibrated to 190 °C with a 2.16 kg piston load for standard polyethylene grades. The die has a hardened steel bore of 2.095 mm diameter and 8.000 mm length; the piston foot diameter is 9.474 mm. Barrel temperature control is required to maintain ±0.1 °C. rLLDPE is typically reported as melt mass-flow rate in g/10 min using Procedure A timed cutoffs. Because recycled lots often contain fractions with differing comonomer contents, a single MFI value may conceal bimodality. To expose this, high-load melt flow testing at 5.0 kg or 10.0 kg is paired with the standard 2.16 kg result to compute a flow rate ratio. Film-grade LLDPE commonly carries a nominal MFI of 0.5 g/10 min to 2.0 g/10 min, but rLLDPE can shift to 0.3 g/10 min or above 4.0 g/10 min depending on the proportion of low-density polyethylene, metallocene LLDPE, and degraded polyolefin in the feed. A lot with a high-load flow rate ratio above 30 often contains crosslinked microgel or high molecular weight components that are not visible in pellet inspection.

On a 60 mm grooved-feed single-screw extruder with a 38 D screw, a 1.0 g/10 min rLLDPE lot may require melt temperatures of 220 °C to 240 °C to maintain specific output. If MFI exceeds 3.0 g/10 min, bubble stability in blown film deteriorates because melt strength is too low; the frost line height must often be reduced by 15–25 % to maintain gauge uniformity. For cast film, high-MFI rLLDPE permits lower melt temperature but raises neck-in; this trade-off is managed through die lip adjustments and air knife position rather than by accepting broad MFI variability. Moisture and volatile residues can generate gas bubbles during MFI testing, producing artificially low apparent flow; therefore, recycled pellets with excessive surface moisture should be dried in dehumidified air at 60 °C until the moisture content is below 0.05 % by mass before measurement.

Ash Content as a Fingerprint for Filler, Catalyst, and Metal Residues

Ash content in rLLDPE is measured by ASTM D5630-22 or ISO 3451-1:2019 using a muffle furnace capable of maintaining 600 °C ± 25 °C, with a porcelain or quartz crucible conditioned in a desiccator to constant mass. A 2 g to 10 g sample is heated in air to complete combustion of the polyolefin matrix; the residue is weighed after cooling in a controlled low-humidity environment. Virgin film-grade LLDPE normally exhibits ash below 0.1 % by mass, whereas rLLDPE may range from 0.2 % to more than 8 % depending on the presence of calcium carbonate anti-block masterbatch, titanium dioxide concentrate, residual catalyst residues, and tramp metal fragments. Ash values above 1.5 % generally indicate inorganic filler carried over from printed film or mixed-polyolefin contamination, and the condition cannot be compensated solely by screw design.

Elevated ash is not only a quality parameter but a wear parameter. In twin-screw compounding, a shift from 0.1 % to 1.0 % ash raises barrel and screw wear rates, particularly in the melting section. On a 50 mm co-rotating twin-screw extruder, hard metal residues from post-consumer waste impose additional torque spikes; equipment operators therefore specify a maximum ash of 0.5 % for high-clarity film applications. For less demanding applications such as bin liners or construction film, ash contents up to 3.0 % are sometimes tolerated, but screen-pack service intervals shorten proportionally. When calcium carbonate is present as a deliberate anti-block agent, its loading should be controlled through masterbatch records and verified by thermogravimetric analysis; residual calcium oxide after ignition may overstate the true filler level if the test temperature is not tightly controlled.

Contamination Screening Demands a Multimodal Analytical Chain

Contamination in rLLDPE includes ethylene-vinyl acetate, polypropylene, polyethylene terephthalate, nylon, polyvinyl chloride, crosslinked gel, aluminium foil, paper fibre, and mineral fillers. No single method identifies all. Screening begins with visual and near-infrared-based sorting, then moves to Fourier transform infrared spectroscopy for polymer type verification according to ASTM D5576-00(2021), X-ray fluorescence for elemental contaminants according to ASTM D6247-20, differential scanning calorimetry for melting point distribution according to ISO 11357-3:2018, and melt filtration for dispersed particulates. For rLLDPE intended for film production, contamination must be assessed both on the pellet surface and after the material is converted into a thin film, because internal gels become visible only after orientation.

Melt filtration testing of rLLDPE is conducted on a small-scale single-screw extruder or on a capillary rheometer equipped with a filter pack. A clean lot under constant throughput produces a linear pressure rise with time; a nonlinear or accelerating pressure curve indicates gel formation, paper fibre, or agglomerated pigment. The test is particularly sensitive to polyvinyl chloride contamination, which degrades at polyethylene processing temperatures and liberates hydrogen chloride. PVC-contaminated rLLDPE cannot be rendered safe by dilution alone when the downstream process involves prolonged residence time above 210 °C, because HCl attack on the barrel, screw, and die generates metallic residues that then contaminate subsequent production batches.

When rLLDPE of Unknown Provenance Enters a High-Shear Compounding Line

Pre-screening should include density separation at 0.930 g/cm³ to detect high-density polyethylene and polypropylene fractions, although density alone cannot separate LLDPE from LDPE or from crosslinked polyethylene. On a 75 mm twin-screw extruder with an L/D ratio of 44 and a gear pump, contamination that passes an 80 mesh screen may still disaggregate under high shear and cause film defects. The differential pressure across the screen pack, typically 30 bar to 80 bar for clean LLDPE at 1.0 g/10 min and 220 °C, rises above 150 bar within several hours when gel or paper fibre contamination is significant. This pressure threshold triggers automatic screen-pack backflush or shutdown in continuous filtration systems. Processing aids designed to reduce melt fracture are generally ineffective when ash content exceeds 1.0 %, because inorganic particles nucleate local stress concentration and shorten the filter service interval.

The behaviour of rLLDPE in high-shear compounding is dominated by the interaction between residual moisture, ionic contamination, and ester-containing copolymers. During blown-film extrusion, melt temperatures above 230 °C accelerate hydrolysis of residual ester-based printing inks and adhesives, producing low molecular weight species that deposit on the die lip. The resulting die-lip build-up is confirmed by subsequent FTIR analysis and is often accompanied by a rise in extrusion backpressure. If a lot exhibits MFI within specification but ash content above 1.5 % and a high-load flow rate ratio above 35, the material is generally routed to less demanding sheet or agricultural film applications rather than high-clarity packaging film, where pinhole and gel tolerance is lower.

Optical inspection of rLLDPE pellets cannot detect internal gels or thin-wall contamination between overlapping pellet surfaces. A representative lot inspection requires melting the pellets into compression-moulded films of 40 µm to 60 µm thickness. Gel counts and black specks are then assessed under polarized light or automated optical scanning. The absence of film defects in a 50 g sample does not guarantee lot cleanliness because contaminant distribution in post-consumer waste is non-normal and often clustered. For this reason, acceptance sampling plans based on ISO 2859-1 are implemented when receiving rLLDPE from variable sources. Pellet fines below 500 µm segregate during sampling and can lead to MFI values 5–10 % higher than the true pellet population. Sample splitting with a rotary divider or pellet thief reduces this bias, but the operator must record the fines fraction before MFI testing because fines can also undermine bulk density and dosing consistency on the converting line.

When a converted rLLDPE film is examined after surface printing, contamination from the printing ink system must be distinguished from contamination in the polymer. Solvent-based ink residues produce surface defects that dissolve in ethyl acetate, while polymer gels remain intact after solvent extraction. This distinction matters for failure analysis but is not part of routine lot acceptance. For routine screening, the combination of MFI, ash content, and melt filtration provides a sufficient basis for downgrading or rejecting a lot before it reaches a high-clarity film line. The limits applied to rLLDPE depend on the end-use specification, but the analytical sequence remains the same: first verify melt flow against the target processing window, then quantify inorganic residue, then examine dispersed contaminants by the most materially appropriate detection method. No single numerical index substitutes for this sequence when the material originates from mixed post-consumer collection.