Low-density polyethylene (LDPE) is assigned resin identification code #4 under ASTM D7611, but the code is a material identifier rather than a collection instruction. LDPE is a branched thermoplastic polyolefin with density between 0.910 g/cm³ and 0.940 g/cm³ when measured under ASTM D792. Melt flow index for typical #4 film extrusion grades, determined by ASTM D1238 at 190 °C with 2.16 kg weight, commonly falls between 0.25 g/10 min and 2.0 g/10 min; coating and injection grades can be designed with higher flow. Differential scanning calorimetry places the crystalline melting peak between 105 °C and 115 °C, below the melting range of high-density polyethylene, which allows lower-energy extrusion pelletization. The practical #4 category also includes linear low-density polyethylene grades with density at or below 0.940 g/cm³. Crosslinked polyethylene, however, is not melt-reprocessable within the same stream because it has a thermoset network.

LDPE differs from HDPE in chain architecture, crystallinity, and melt strength. The branched structure reduces density, lowers tensile yield, and increases melt elasticity compared with linear polyethylene of similar melt index. These properties carry into reclaimed material. Post-consumer #4 film frequently contains a mixture of conventional LDPE and LLDPE stretch film; LLDPE lowers average melt strength and can create edge instability in blown film at high recycle content. The resin code does not distinguish LDPE from LLDPE in mixed film. Published data for the exact LDPE/LLDPE ratio in retail store drop-off film is limited, but reclaimers observe batch-to-batch shifts in melt strength that require formulation adjustment. Color also affects sorting. Carbon-black pigmented films can absorb near-infrared radiation and evade NIR identification at some material recovery facilities, so black #4 film may be rejected or manually separated despite being chemically LDPE.

Is LDPE Recyclable? What the #4 Code Actually Accepts

Technical recyclability is separate from municipal acceptance. Under ISO 18604, LDPE can be considered material-recyclable when a controlled collection, sorting, and reprocessing route returns the polymer to productive use without fundamental loss of function. Post-industrial LDPE trim, edge scrap, and rejected film rolls are routinely reclaimed because the source is homogeneous and contamination is low. Post-consumer #4 film is technically recoverable but requires dedicated collection, usually through retail drop-off, agricultural film depots, or commercial backhauling programs. Most municipal curbside systems do not accept LDPE film because film wraps around rotating material recovery facility equipment such as disc screens, star screens, and conveyor bearings, causing unplanned downtime and maintenance overload.

What the #4 code actually accepts is narrow. Under ASTM D7611, the mark identifies that the article is manufactured from LDPE resin or a linear low-density ethylene polymer within the LDPE density specification. The code does not accept a package format, cleanliness state, barrier layer composition, ink coverage, adhesive burden, or reclamation route. A soiled multi-layer film carrying #4 may be technically valid as a resin identification while remaining unsuitable for any available PE film reclaimer. The code therefore accepts a resin class, not a product. This is the central operational boundary for recyclers and packaging converters.

Store drop-off programs accept clean, dry polyethylene film forms: grocery bags, produce bags, bread bags, case wrap, newspaper sleeves, dry-cleaning film, air pillows, stretch wrap, and bubble wrap. Films must be free of food residue, organic matter, abrasive label adhesives, and non-PE components. Polyvinylidene chloride cling film, metalized snack wrappers, and multi-layer films containing nylon or EVOH are excluded from the LDPE film stream even where an LDPE layer carries the #4 mark. The exclusion is process-based: metalized and barrier layers can survive sink-float separation in the PE float fraction and generate gels, black specks, or melt-pressure anomalies during extrusion. Published data for specific MRF bale composition is limited, but reclaimers impose these restrictions to protect extruder screen packs and downstream pellet quality.

Post-Consumer #4 Film Contamination Thresholds at the Reclaimer

Commercial reclaimers audit baled post-consumer film against inbound specification matrices. Moisture is limited because water above 0.5 wt% can flash into steam during extrusion, producing pinholes and melt pump surging. Non-PE polymer film is restricted because polypropylene and other polyolefins shift melt rheology and narrow the processing window. LDPE and polypropylene both float in water near 1.00 g/cm³, so sink-float separation alone does not remove polypropylene from the LDPE fraction; near-infrared sorting or manual separation is required. PVC and PVDC are more critical contaminants because thermal degradation can release hydrogen chloride and accelerate screw and barrel corrosion. Practical inbound limits for PVC or PVDC in PE film are often held below 0.5 wt% to 1 wt%. Melt filtration is then used at the extruder to remove remaining non-melt particles and high-molecular-weight gels.

In a typical post-consumer #4 film line, baled film is fed into a shredder with water spray to reduce agglomeration and then passed through air classification to remove paper fiber, dust, and label fragments. Friction washers and hot wash tanks remove polar surface contamination; rinsing is followed by sink-float separation to reject PET and PVC. The LDPE-rich fraction is dried and metered into a single-screw reclaim extruder with length-to-diameter ratio near 30:1. Barrel set points are maintained between 160 °C and 200 °C, while die melt temperature is held below 220 °C to reduce oxidative chain scission. Vacuum venting after the compression zone strips volatile residuals from printing inks and residual moisture. A melt pump stabilizes polymer flow to the screen changer, and the filtered melt is pelletized and cooled. Batch-to-batch variability in post-consumer LDPE pellet melt flow index is higher than in post-industrial reclaim, and compounding may be required to move the final MFI into specification.

When Curbside Programs Reject #4 Film but Accept Rigid LDPE Containers

Rigid #4 containers such as squeeze bottles, dropper bottles, and some dispensing closures present lower handling risk than film. Their three-dimensional geometry does not wrap around disc-screen shafts or star-screen fingers in the same manner as thin film, so they can be sorted with rigid plastic bottles. Once separated, rigid LDPE is processed through mixed plastics or polyethylene bottle lines. Sink-float separation floats LDPE along with HDPE and polypropylene; isolating an LDPE-rich fraction requires additional density or NIR sorting. Where that separation is not cost-effective, mixed polyolefin fractions are produced for less demanding applications. The #4 code identifies the resin but does not force a receiving facility to isolate LDPE. Curbside acceptance of rigid #4 depends on sorting equipment, end-market contract, and densification economics, not solely on the resin mark.

Melt Filtration and Vacuum Venting Do Not Remove All Odor Compounds

Melt filtration removes gels, sand, and non-melting fragments, but low-molecular-weight odor bodies from degraded inks, fatty residues, and detergent films can survive vacuum venting when their vapor pressure is too low at the vent port. Recycled LDPE pellets can therefore meet density, melt flow, and filtration specifications while retaining odor unsuitable for high-value packaging. Reclaimers may use hot wash additives, activated carbon filtration of vent gases, or melt-phase stripping in twin-screw equipment to reduce odor, but these steps are not universal. Consequently, post-consumer #4 film is often recycled into refuse sacks, construction film, agricultural film, and non-appearance injection-molded parts rather than direct food-contact packaging. Direct food-contact use of post-consumer LDPE requires compliance with FDA 21 CFR 177.1520 for olefin polymers and an appropriate recycling process review, which many film reclaimers do not hold.

Recycled LDPE pellet quality is verified by ASTM D1238 for melt flow index, ASTM D792 for density, ASTM D638 for tensile yield and elongation, and ASTM D3418 for melting peak. Post-consumer recycled LDPE film pellets typically exhibit density between 0.915 g/cm³ and 0.935 g/cm³ and melt flow index between 0.5 g/10 min and 2.0 g/10 min, but these values shift with source mix and extrusion history. In blown film conversion, high-letdown recycled LDPE above 30 wt% can reduce bubble stability because volatile residues and crosslinked gel particles act as stress concentrators. In injection molding, reclaimed #4 is used for closures, base cups, and industrial parts where mechanical demands are moderate. Published data for specific high-letdown multi-layer extrusion trials using post-consumer LDPE is limited, and formulation adjustments are validated on a trial-line basis before production commitments are made.