LDPE #4 Recycling: What Actually Happens to Bags and Wraps

Post-consumer LDPE #4 film enters reclamation as baled grocery sacks, pallet wrap, mattress bags, furniture wrap, and back-of-house stretch film. The resin identification code #4 under ASTM D7611-21 classifies low-density polyethylene, but the actual post-consumer film stream is a mixed ethylene-polymer feed because high-pressure LDPE and linear low-density polyethylene are both present in bags and wraps and are not separated by store drop-off infrastructure. The density of this feedstock typically falls between 0.917 g/cm³ and 0.930 g/cm³, the film thickness ranges from roughly 15 µm to 100 µm, and the melt flow rate at 190 °C/2.16 kg commonly falls between 0.5 g/10 min and 2.0 g/10 min. The melting peak of the LDPE fraction is normally between 105 °C and 115 °C by differential scanning calorimetry per ASTM D3418-21. The reclaimed value of the material is governed less by the base LDPE chemistry than by the load of paper labels, pressure-sensitive tape, food residue, barrier films, and rigid fragments that enter before baling. Compared with rigid bottle reclaimers, film reclaimers face a higher surface-area-to-mass ratio: the film surface carries more aqueous food residue and label adhesive per kilogram, so moisture and contamination cannot be treated as minor trimming losses.

What Material Configuration Is Present in Store Drop-Off and Back-of-House Bales?

Bale composition is heterogeneous. A sorted load of post-consumer polyethylene film commonly contains 80% to 95% PE film by mass, with the balance split among shrink labels, paper receipts, tape, PET bottles, mixed-plastic strapping, and non-PE food wrappers. The non-PE fraction is not measured solely by mass; it is measured by the melt-filtration burden and visual-defect count it imposes downstream. Near-infrared sorting arrays can separate polypropylene from polyethylene and reject some PVC on the whole-bale or flake line, but thin multilayer structures and adhesives are partially opaque to NIR and require wet density separation. Baled film is not handled well by material recovery facility disc screens and star screens, because the film wraps around rotating shafts and increases downtime; this is the operational reason why many curbside programs exclude bags and wraps and why store drop-off and back-of-house recovery dominate the LDPE #4 stream. The receiving sequence follows the general recovery path described in ISO 15270:2008, with visual inspection and bale audit preceding mechanical processing. A bale that passes the audit is loaded by a hoist or conveyor into a guillotine or shear, and the cut sections are metered to the first granulator at a rate set by the density of the baled film, which is typically between 300 kg/m³ and 500 kg/m³.

Baled film is charged through a guillotine or single-shaft shredder before entering a granulator fitted with a screen in the 10 mm to 20 mm range. The reduction sequence releases paper labels from film surfaces but also generates fines that carry adhesive and clay coatings into downstream baths. A wet granulator may be used to reduce folding damage and initiate partial surface cleaning. The shredded flake then enters a prewash or friction washing line where high-pressure water and rotating paddles remove loose contaminants. Flake density after shredding remains below 1.0 g/cm³ for unfilled LDPE and LLDPE, allowing a sink-float tank to separate floating PE from sinking PET, PVC, and filled films. Filled LDPE film containing calcium carbonate at sufficient loading exceeds 1.0 g/cm³ and reports to the sink fraction; this limits filler content in recyclable bags. The sink-float system is operated with controlled water temperature and surface surfactants to reduce density interference from trapped air bubbles and hydrophobic surfaces. Polyethylene film with a density at or above 1.0 g/cm³ is lost to the sink fraction, which is a known yield-loss mechanism for film that incorporates mineral filler, paper coatings, or metal layers. The float fraction is dewatered and conveyed to the hot-wash circuit; the sink fraction is dewatered separately and sent to residual disposal or a mixed-plastics recycle path.

Hot-Wash Chemistry, Friction Action, and Sink-Float Separation Limits

After the initial density cut, the floating PE stream is transferred to a hot-wash tank. Caustic soda or potassium hydroxide is metered to maintain a pH above 10, and water temperature is held between 60 °C and 80 °C. The combination of alkalinity, heat, and residence time removes protein-based food residues, carbohydrates, and some paper labels. Residence times are set according to the input bale quality; plants processing heavily contaminated residential bag bales may need longer residence, whereas back-of-house stretch wrap requires shorter exposure. The wash tank is typically agitated and followed by a friction washer that scrubs the film surfaces. Dissolved organic load in the wash water is treated by dissolved-air flotation or membrane filtration before reuse. The separation limit is not sharp: multilayer film with an EVOH or nylon tie layer remains in the PE float fraction because the combined density is usually below 1.0 g/cm³, and it is only removed later as melt filtration gels or black specks. After hot washing, the flake passes through a friction washer and a cold rinse. Mechanical dewatering and thermal drying then reduce residual moisture to below 0.1 wt%. Drying is required because moisture above 0.2 wt% can generate steam pressure at the extruder feed and produce porosity in the filtered melt.

Flake discharged from the dryer is metered into a vented single-screw extruder with an L/D ratio in the 30:1 to 36:1 range. Barrel zone settings typically progress from 180 °C near the feed throat to 230 °C at the die, with melt temperature measured at 210 °C to 240 °C. A screen changer with meshes from 60 to 200 or a continuous scrap-saving filter removes non-melting contaminants. Pressure upstream of the screen pack increases as gels, paper fibers, and adhesive particles build up; production-scale reclaimers use the screen-change frequency and pressure rise curve as an indirect contaminant index. Vacuum degassing removes moisture and low-molecular-weight odor species. The filtered melt is then pelletized via strand or underwater pelletizer. Melt flow rate is checked after compounding because repeated extrusion of LDPE can produce both chain scission and long-chain branching/gel formation; the melt flow rate may shift upward or downward depending on oxygen, temperature, and residence time. The recycled pellet may be restabilized with an antioxidant package at 0.1 wt% to 0.3 wt% if the target application demands it. Melt flow rate is determined according to ASTM D1238-20 or ISO 1133-1:2022. Film-grade post-consumer LDPE often contains volatile compounds from printing inks and food residues; vacuum degassing is therefore sized for higher volatile load than virgin pellet lines. The pelletizer operator monitors die pressure and pellet water temperature because high water temperature can cause pellet agglomeration and low melt strength can cause strand breaks. This unit operation determines the upper limit on contamination: a screen pack that blocks in minutes rather than hours forces a shutdown or feed reject.

When Multilayer Barrier Films, PVC, and Adhesive Matter More Than the Base Polymer

Multilayer packaging film is the principal process conflict in LDPE #4 reclamation. EVOH and polyamide barrier layers, aluminum foil laminates, and pressure-sensitive adhesives do not melt into a homogeneous blend at LDPE processing temperatures. EVOH degrades above its processing window and creates brown gels and odor; polyamide may remain as hard, unmelted inclusions at LDPE barrel temperatures. PVC labels and cling films generate hydrochloric acid at normal PE extrusion temperatures and must be excluded before melt processing. The Association of Plastic Recyclers PE Film Design Guide identifies EVOH barrier layers, PVDC coatings, metal foil laminates, and degradable additives as stream-contaminating features that require removal or redesign. A production-scale consequence is screen-pack blinding and frequent screen changes; plants report elevated gel counts and black specks when the barrier film fraction in feed exceeds a few percent by mass, although exact published thresholds vary by extruder design and filtration capacity. This is the operational boundary where reclaimers reject a bale: the incremental value of additional flake is eliminated by the cost of a filter cleanout and subsequent down-gauging loss. In addition, adhesives are not chemically inert; oxidized pressure-sensitive adhesives can produce sticky residues that accumulate on the die lip and cause die lines. Silicone-coated release liners from label backing can generate melt defects at low concentration and are often difficult to detect in bale audits because they are thin and transparent. The operational objective is not absolute elimination of contamination but control of incoming film categories through supplier specifications, visual inspection, and flake sorting.

Recycled LDPE #4 pellets are not direct substitutes for virgin film resin in all applications. They are typically dry-blended or compounded with virgin LLDPE to bring melt flow ratio and dart impact back into film extrusion range. Blown film evaluations use ASTM D1238-20 for melt flow rate, ASTM D638-14 for tensile properties, ASTM D1003-21 for haze, and dart impact methods such as ASTM D1709-22. Recycled LDPE is allocated to opaque trash bags, can liners, construction film, agricultural film, and thick plastic lumber profiles, where minor gel particles and odor are within end-use tolerances. The pellet is also reformulated with carbon black masterbatch or calcium carbonate compounds to standardize density and opacity. Blown-film conversion of recycled LDPE typically uses a die gap in the 0.8 mm to 1.5 mm range and blow-up ratios between 2:1 and 4:1, with low melt strength narrowing the stable bubble window. It is not allocated to food-contact packaging unless the reclaimer holds a specific Food and Drug Administration letter of no objection and the feedstock is controlled under the requirements for olefin polymers in 21 CFR 177.1520. A reclaimer may also certify recycled content under a chain-of-custody standard such as ISO 22095:2020, although this certification is separate from fitness-for-use testing. The control matrix below links incoming material limits to standard test methods.

Operational boundaries for LDPE #4 reclaimers are narrower than the resin label suggests. Feedstock moisture above 0.2 wt% can produce vent foaming; filler loadings above 10% to 15% by mass can shift flake density above 1.0 g/cm³ and remove reusable film to the sink fraction; silicone-coated release liners and PVC labels create melt defects at parts-per-hundred to parts-per-thousand levels. Reclaimers therefore apply incoming bale audits, flake density checks, and post-extrusion gel counts rather than relying on the #4 code alone. Because published data for this specific configuration is limited, the process window is typically developed by each plant using its own pressure-rise and gel-count history. The output is sold into durable and non-food film applications where the process-induced shift in melt rheology, odor, and gel level is acceptable.