Blending rLLDPE into Film: How Much Before Spec Breaks

Linear low-density polyethylene (LLDPE) is specified for blown and cast film through a narrow set of physical, optical, and sealing properties. Density is typically controlled within 0.915–0.930 g/cm³ according to ASTM D792 or ISO 1183-1, while melt index is measured at 190°C/2.16 kg under ASTM D1238-20 or ISO 1133-1:2022. Incorporation of recycled LLDPE (rLLDPE) obtained from post-industrial trim or post-consumer film modifies the molecular weight distribution, gel content, and comonomer composition. The addition level at which a converter specification fails is not fixed; it is a function of the base resin grade, the recycled stream source, the film gauge, and the converting process. In most blown-film lines, the first specification limits to fail are dart impact, haze, seal initiation temperature, or gel-related visual defects, not tensile strength. Published data for a single universal allowable addition level is limited; however, extrusion trials and incoming lot data indicate that a 20–30 wt% rLLDPE replacement can be tolerated in heavy-gauge overwrap film, while thin-gauge stretch and hygiene films often break specification at 10–15 wt%.

Before let-down into a virgin butene-copolymer LLDPE grade with a melt index of 1.0 g/10 min and density of 0.918 g/cm³, post-consumer rLLDPE should be characterised for density, melt index ratio, ash, and polymer contamination. Batch-to-batch variation in rLLDPE from flexible packaging often ranges from 0.5 g/10 min to 2.0 g/10 min melt index and 0.915–0.925 g/cm³ density, with the lower melt index typically reflecting HDPE or thick-gauge film contamination. The melt index ratio, measured as 21.6 kg/2.16 kg under ASTM D1238-20, is a better predictor of processability than single-point melt index because it captures the breadth of the molecular weight distribution. When the rLLDPE melt index falls below 0.7 g/10 min, the blend viscosity increases sufficiently to raise melt pressure and reduce throughput on a 90 mm, 30:1 L/D barrier screw equipped with a 100/120/60 mesh screen pack. The result is not a uniform property decline but a change in the residence time distribution that accelerates gel formation. Pre-drying is not normally required for polyethylene; however, rLLDPE containing washed post-consumer film should be processed with a vented extruder or moisture-removal vacuum if surface moisture exceeds 0.1 wt%. Incoming lots with ash content above 0.5 wt% after 600°C ignition should be rejected for high-clarity film because the inorganic fraction will not melt, can block screen packs, and produces visible specks.

Is Dart Impact the First Specification to Fail at 25 µm?

In a 25 µm monolayer blown-film structure, dart impact measured according to ASTM D1709 method A is commonly the first mechanical property to cross the lower specification boundary because rLLDPE contains a broader distribution of chain lengths and occasional crosslinked gels. A neat butene LLDPE may exhibit 120–160 g dart impact at 25 µm. Replacement with 15 wt% rLLDPE can reduce the average value by 10–20% depending on gel count and the base resin’s tie-chain population. At 30 wt%, dart impact may drop below 80 g in recycled streams containing high levels of low-molecular-weight film scrap, although published data for this specific configuration is limited. Tensile elongation at break under ASTM D882 is generally more tolerant than dart impact because LLDPE film draws through the amorphous phases rather than requiring the same localised crack initiation resistance. The shift from ductile to brittle fracture becomes visually apparent at a critical gel count above 500 gels/m² when the film is inspected using a 50 µm resolution camera system. In practice, specification breaks on dart impact are not caused solely by the recycle fraction; they are driven by the concentration of high-gel post-consumer material and the extrusion temperature profile. Raising the melt temperature from 210°C to 240°C can disperse gels but also increases oxidative degradation, shifting the film’s yellowness index and reducing the time before the film fails sensory specification.

Gel Formation, Melt Pressure Instability, and Screen Pack Filtration Limits

Across a 90 mm blown-film line running a 1.8 mm die gap and 2.5:1 blow-up ratio, melt pressure at the screen changer provides an earlier specification limit than mechanical testing. When the screen pack is 100/120/60 mesh, a rLLDPE addition of 20 wt% can increase the screen pressure from 180–220 bar to 260–310 bar within 6–8 hours of continuous operation if the recycled fraction contains paper fibre, foil, or nylon. The pressure rise is accompanied by a reduction in bubble stability and an increase in visible gels. Operators may compensate by raising the melt temperature from 210°C to 230°C, but this shifts the oxidative induction time and can increase odour. Filtration downstream of the breaker plate using a continuous screen changer is the only robust mitigation at addition rates above 15 wt%; otherwise the bubble becomes unstable and the film gauge profile exceeds ±8% variation. Gel counts above 300 gels/m² for 200–400 µm gel diameters are often unacceptable in printed film because the gels leave un-inked voids. In coextruded structures, the rLLDPE should not be placed in the outer skin layer if gel appearance is critical; placing the recycled fraction in the core layer can delay visible defects to addition rates above 30 wt%.

Melt Rheology and Screw Design Limits for High rLLDPE Fractions

Melt index is a single-point indication of viscosity, but the specification limit for rLLDPE blending is controlled by high-shear viscosity and extensional rheology. A blend containing 20 wt% post-consumer rLLDPE with a melt index of 0.6 g/10 min can shift the high-load melt index ratio, 21.6 kg/2.16 kg under ASTM D1238-20, from 5.5 to 6.2. This broadens the molecular weight distribution, raising the zero-shear viscosity and increasing die land pressure. On a 50 mm single-screw extruder with a 25:1 L/D general-purpose screw, the same blend has shown an output loss of 7–12% compared with neat LLDPE at 230°C. When the rLLDPE fraction reaches 40 wt%, melt fracture at the die lip may appear as sharkskin if the shear stress exceeds approximately 0.14 MPa. Converters therefore do not only blend by weight; they blend by melt index and by melt index ratio. A useful incoming-lot rule is to hold the blend’s melt index within ±10% of the neat resin value. If the base LLDPE has a melt index of 1.0 g/10 min, the maximum rLLDPE fraction for a stream with 0.6 g/10 min melt index is approximately 11–14 wt% under this rule. In cast-film extrusion, shear heating is lower than in blown film, but the die residence time is shorter; therefore, the same rLLDPE lot may pass cast-film specifications at 5–10 wt% higher addition than in blown film.

Seal initiation temperature and hot tack window shifts are observed before visible clarity loss in many coextruded films. The seal initiation temperature measured according to ASTM F1921 increases by 2–4°C at 20 wt% rLLDPE and by 5–8°C at 40 wt% when the recycle stream contains branched polyethylene or LDPE contamination. This is critical for form-fill-seal lines running at 45–60 packages/min because the dwell time is insufficient to re-establish a strong seal when the sealant layer no longer melts within the specified temperature window. Hot tack strength under ASTM F1921 may fall below 1.0 N/25 mm if the rLLDPE fraction exceeds 25 wt% and the sealant layer contains 5–8% LDPE. The operational boundary is therefore not the bulk mechanical specification; it is the heat-seal window and the available cooling time on the packaging line. Machine-direction sealability is also affected by contamination from paper labels and adhesive residues, which create low-molecular-weight species that plasticise the seal surface but weaken the hot tack.

When the Recycled Fraction Approaches 40 wt% in 12 µm Stretch Film

In 12 µm machine-film and stretch-film constructions, the first specification to break is often not dart impact but puncture resistance and elongation at break. A 40 wt% addition of post-consumer rLLDPE can reduce elongation at break under ASTM D882 from 450–550% to 300–380% because the recycled fraction contains short chains and oxidative degradation products. The same film may retain a tensile strength above 25 MPa in the machine direction, but the elongation loss prevents the film from meeting converter specifications. Tear resistance measured by ASTM D1922 may fall by 20–35% at 40 wt%, with the cross-direction tear often becoming the limiting parameter. The cast-film line used for this grade typically employs a 75 mm extruder and a 2.0 mm die gap, with chill roll temperature held at 25–30°C. At 40 wt% rLLDPE, die lip build-up increases and surface haze under ASTM D1003 can exceed 5–8% in structures that require <3%. Published data for this specific configuration is limited, so the limit is generally established by a design of experiments within the converter’s own line. The addition of 5 wt% LDPE to improve stretch can reduce the melt strength, but it worsens the already reduced tear resistance and should be avoided at rLLDPE levels above 30 wt%.

Compliance Verification Matrix for rLLDPE-Containing Film Structures

When rLLDPE is used in food-contact, hygiene, or pharmaceutical packaging, the verification burden extends beyond mechanical properties. The following matrix summarises the standard designations and typical acceptance windows relevant to rLLDPE let-down. The values are not universal specifications; they are representative of a monolayer LLDPE film at 25 µm.

For cast-film converters, the gel count and optical clarity boundary is often more restrictive than the mechanical boundary. A high-clarity polypropylene- or LLDPE-based lamination film may permit a gel count of only 100 gels/m² larger than 200 µm, while rLLDPE-containing film can reach 300–500 gels/m² after 20 wt% addition. The increase in haze under ASTM D1003 is not linear; at 10 wt% rLLDPE the haze may remain below 5%, but at 30 wt% it can exceed 12% because of dispersed PP, nylon, and crosslinked gel domains. These domains do not melt at conventional LLDPE processing temperatures of 200–230°C, producing localised optical defects. A laser gel counter with 25 µm resolution is used to monitor the film surface; the specification is often a pass-fail gel rating rather than an absolute count. Converters may compensate by adding a higher level of processing stabiliser, but the additive itself can migrate and change the coefficient of friction.

Recycled LLDPE inevitably dilutes the slip and antiblock masterbatch contained in the original film, but the effect is not predictable from the nominal rLLDPE fraction alone. A base film containing 500 ppm erucamide slip additive and 1000 ppm silica antiblock may lose 20–30% of its kinetic coefficient of friction control when the rLLDPE fraction reaches 30 wt% because the recycled fraction has already consumed part of the slip additive. The kinetic coefficient of friction under ASTM D1894 can increase from 0.18 to 0.30, causing film-to-film blocking on high-speed wrapping lines. The addition of fresh slip masterbatch can restore the coefficient of friction, but it also creates a migration pathway that may alter seal initiation. This is an operational boundary that is frequently omitted from laboratory tensile data but dominates field performance on high-speed packaging lines.

In a high-speed food-packaging line running 55 packages/min with a 25 µm LLDPE overwrap, a 25 wt% rLLDPE lot containing 0.9 g/10 min melt index and 0.920 g/cm³ density may pass the tensile specification but fail sensory and seal-start tests under FDA 21 CFR 177.1520 if the recycled stream includes printed film. Extractables testing under Commission Regulation (EU) No 10/2011 may exceed 10 mg/dm² when migrants from inks and adhesives are present, despite acceptable gel and mechanical data. The operational boundary in such a package is therefore determined by the seal initiation window and the extractables limit, not by a universal rLLDPE addition rate.