Ldpe 4 Vs Hdpe 2 Vs Pet 1 Which Code Goes In Which Bin
Low-Density Polyethylene (LDPE): Code 4 Sorting Boundaries in Rigid and Flexible Plastic Streams
The comparison embedded in the phrase “LDPE 4 vs HDPE 2 vs PET 1: Which Code Goes in Which Bin” is only partly answered by resin identification codes. ASTM D7611/D7611M-21 assigns numeral 1 to polyethylene terephthalate, numeral 2 to high-density polyethylene, and numeral 4 to low-density polyethylene. The actual bin assignment is determined by article form—film, bottle, tray, closure—and by the unit operations installed at the receiving material recovery facility, not by the code alone.
At a single-stream material recovery facility equipped with near-infrared optical sorters operating in the 1000–1700 nm wavelength range, PET 1 containers are separated by the ester carbonyl absorption signature. HDPE 2 and LDPE 4 require subsequent density separation because their C-H overtone signatures overlap in some classifiers. A sink-float tank using water at 1.0 g/cm³ separates PET 1 as the sink fraction from HDPE 2 and LDPE 4 as float fractions; however, that same water-density gate cannot distinguish HDPE 2 from LDPE 4 because both have densities below 1.0 g/cm³.
What Do RIC 1, 2, and 4 Actually Denote in ASTM D7611/D7611M-21?
ASTM D7611/D7611M-21 provides resin identification code assignment for plastic manufactured articles. The code is a triangle-enclosed numeral, not a recycling certification. Under ISO 11469:2016, the article is marked with the material abbreviation >LDPE< and the resin identification numeral 4; this is a material marking system, not a municipal sorting certificate. PET 1 is a condensation polyester with repeat unit ethylene terephthalate and typical bottle-grade intrinsic viscosity of 0.80–0.84 dL/g. HDPE 2 is a linear polyethylene produced by Ziegler-Natta, chromium-oxide, or metallocene catalysis with a density range of 0.940–0.970 g/cm³ and a typical blow-molding melt flow rate of 0.2–0.7 g/10 min at 190°C/2.16 kg under ASTM D1238-20. LDPE 4 is a branched polyethylene produced by high-pressure radical polymerisation in tubular or autoclave reactors; it has a density range of 0.910–0.940 g/cm³ and typical film-grade melt flow rate of 0.5–2.0 g/10 min at 190°C/2.16 kg.
| Property / method | PET 1 | HDPE 2 | LDPE 4 |
|---|---|---|---|
| Density (ASTM D1505) | 1.38–1.40 g/cm³ | 0.940–0.970 g/cm³ | 0.910–0.940 g/cm³ |
| Peak melting temperature (DSC, ASTM D3418) | 245–265°C | 120–140°C | 105–115°C |
| Melt flow / solution viscosity | Intrinsic viscosity 0.80–0.84 dL/g | MFR 0.2–0.7 g/10 min | MFR 0.5–2.0 g/10 min |
| Tensile yield strength (ASTM D638-14) | 55–75 MPa | 20–32 MPa | 8–15 MPa |
| Tensile modulus (ASTM D638-14) | 2800–3100 MPa | 800–1200 MPa | 100–500 MPa |
| Water absorption (ASTM D570, 24 h) | 0.1–0.2% | <0.01% | <0.01% |
| Water sink-float behaviour | Sinks in water | Floats in water | Floats in water |
Polymer architecture controls the failure boundaries more than the resin identification numeral. LDPE 4 is produced at reactor pressures of 1000–3500 bar and temperatures of 150–300°C; the long-chain branches reduce crystallinity to 40–55% and impart high melt strength in blown-film extrusion. HDPE 2 has a linear or lightly branched backbone and crystallinity of 60–80%. PET 1, after stretch blow moulding, develops strain-induced crystallinity in the sidewall of 25–35%; its amorphous phase density is 1.33 g/cm³, which pushes bottle density above water.
Film extrusion of LDPE 4 on a single-screw extruder with an L/D ratio of 24:1 to 30:1 and a barrier screw typically operates at melt temperatures of 160–220°C. Bubble stability in a blown-film line is governed by melt tension, blow-up ratio, and frost-line height; a collapse-frame angle below 20° and a die gap of 0.8–2.0 mm are used to control gauge uniformity. Moisture absorption data under ASTM D570 separate the three resins: LDPE 4 and HDPE 2 absorb less than 0.01% after 24 h, whereas PET 1 absorbs 0.1–0.2%. This difference dictates pre-drying requirements: PET 1 preforms must be dried below 50 ppm moisture before injection moulding; LDPE 4 and HDPE 2 require surface moisture removal only at relative humidity above 60%.
Curbside Bin Logic for RIC 1, 2, and 4
In a curbside collection programme accepting plastic containers, PET 1 and HDPE 2 bottles are placed in the containers bin because they are rigid, blow-moulded articles compatible with material recovery facility disc screens and optical sorters. LDPE 4 film is generally excluded from this container bin. The exclusion is not caused by chemical incompatibility but by unit-operation failure: thin film wraps around disc screens, trommel shafts, and conveyor bearings, reducing on-line run time in single-stream plants. Some municipalities operate a separate curbside film collection stream; film must be clean, dry, and bagged in a clear LDPE bag with no polypropylene or paper labels above the acceptance threshold specified by the hauler.
| Resin code and article form | Common curbside container bin | Separate film drop-off | Key restriction |
|---|---|---|---|
| PET 1 bottle/jar | Accepted | Not applicable | Remove full-sleeve PVC labels if surface coverage exceeds 50% |
| HDPE 2 bottle/jug | Accepted | Not applicable | Drain residual liquids; closure may be PP 5 |
| LDPE 4 film/bag | Rejected in most single-stream programmes | Accepted at retail store front-of-house | No food residue; thickness varies by programme |
| LDPE 4 rigid container | Programme-dependent | Not typical | Verify with local hauler; near-infrared cannot always separate from HDPE 2 |
Regulatory food-contact status does not dictate bin placement. FDA 21 CFR 177.1520 permits olefin polymers under specified extractables limitations; FDA 21 CFR 177.1630 applies to PET 1. European food contact requirements under Regulation (EU) No 10/2011 address overall migration limits; they do not specify which curbside bin receives code 4 film. Recycled resin must independently comply with the applicable food-contact standard before re-entering packaging applications.
When LDPE 4 Enters a PET 1 Reclamation Stream
When LDPE 4 film is mistakenly deposited in a PET 1 bottle bale, the most severe failure occurs in the hot caustic wash step. PET reclamation lines use a wash solution of 0.5–2.0 wt% NaOH at 85–95°C before granulation. LDPE 4 begins to soften near its Vicat softening point of 80–100°C, and film fragments can shrink around PET flake or adhere to auger flights. Because LDPE 4 does not hydrolyse under these conditions, it remains as a persistent low-density contaminant that can survive downstream elutriation. At a 2 wt% LDPE 4 contamination level in PET flake, extruded sheet exhibits pinholes and gel-like specks at draw ratios above 3:1; published data for commercial lines at specific contamination levels is limited, but equipment manufacturers report that removal of LDPE 4 before hot wash is required to maintain film-grade PET clarity.
In HDPE 2 bottle reclamation, LDPE 4 film contamination is often removed by a cold-water rinse before regrind; if not, the lower-melting phase creates differential melting at the extruder because LDPE 4 melts at 105–115°C, which is below HDPE 2’s 120–140°C melting peak. The lower-melting phase can encapsulate higher-melting HDPE 2 granules and create melt-pressure fluctuations in the blow-molding accumulator head. The Association of Plastic Recyclers design guidance for HDPE bottles classifies LDPE 4 pressure-sensitive labels as a non-preferred material if they do not separate in the wash.
Conversely, LDPE 4 film cannot be processed in a PET 1 injection stretch blow moulding barrel at 260–285°C because LDPE 4 undergoes oxidative chain scission rapidly above 250°C in air, producing off-gassing and black specks. This thermal incompatibility reinforces the need for resin separation before melt conversion.
Density separation cannot separate LDPE 4 from HDPE 2 because both float in water; a hydrocyclone using a medium density of 0.930–0.950 g/cm³ can float LDPE 4 and sink HDPE 2. This approach is used in some plastics recycling plants but is not a standard curbside bin operation. Mixing LDPE 4 with HDPE 2 in blow-moulded bottles alters environmental stress crack resistance measured by ASTM D1693; the effect depends on long-chain branching distribution and melt index, so no universal threshold can be assigned without pilot testing on the target blow-moulding line.
Rheological conflicts in mixed polyethylene recycle are measurable by capillary rheometry under ASTM D3835. At a shear rate of 100 s⁻¹ and 190°C, HDPE 2 blow-moulding grades and LDPE 4 film grades exhibit different shear-thinning slopes because of differences in molecular weight distribution and long-chain branching. The resulting melt-pressure instability at the screen changer is a known limitation in mixed-polyethylene extrusion; the exact fluctuation amplitude depends on regrind ratio and screen-pack configuration.
Why Do Some Municipalities Accept LDPE 4 in Curbside Bins While Others Reject It?
Acceptance of LDPE 4 in curbside bins is determined by the material recovery facility’s primary sorter type. Single-stream plants using disc screens and ballistic separators reject film because it binds the disc shafts. Dual-stream or source-separated programmes may accept LDPE 4 bags if a manual pre-sort station is staffed. Equipment manufacturer technical bulletins for film recovery systems report LDPE 4 recovery in the 60–80% range when film is presented in a separate bag or container and residual contamination is below 5% by mass. If residual contamination exceeds that threshold, film-to-film extrusion becomes economically unfavourable because recycled melt flow rate may increase beyond 2.0 g/10 min and extruder pressures become unstable at melt filtration levels below 125 µm.
LDPE 4 film recycling at a post-consumer film line uses a preconditioning guillotine, a wet granulator, a friction washer, and a vented twin-screw compounding extruder with melt filtration at 100–150 µm. The recycled pellet has a melt flow rate typically 0.5–1.5 g/10 min and is blended with virgin LDPE or LLDPE at addition levels of 10–30 wt% to control gel content and output. Because of oxidation during reprocessing, the melt flow rate may increase beyond 2.0 g/10 min and stress at break may decline; blown-film lines processing high recycled content often require a finer melt screen and reduced take-off speed to avoid bubble tears. The exact tolerance limits depend on the film product’s target thickness and the extruder’s backpressure profile.
Household Bin Assignment Is Not Identical Across Programs
At the household bin, the operative rule is: PET 1 bottles and HDPE 2 bottles go to the containers bin; LDPE 4 film goes to the store drop-off or to a separate curbside film bin if the municipal contract includes film-specific sorting. This rule is reversed if the LDPE 4 article is rigid, in which case local acceptance must be verified with the hauler because near-infrared and density systems cannot consistently distinguish rigid LDPE 4 from HDPE 2. A closure or label made of a different resin must be evaluated against the Association of Plastic Recyclers design guidance for the target stream; a PET 1 bottle with a full-surface LDPE 4 shrink sleeve can be mis-sorted as LDPE 4 and ejected into the wrong bale.
At a retail store front-of-house film collection point, LDPE 4 bags, pallet wrap, and air pillows are bulked into a clear recycling bag and transported to a film recycler. The typical output is a recycled pellet sold for refuse sacks, agricultural film, and composite lumber. The store bin is not intended for PET 1 bottles or HDPE 2 jugs; placing those rigid articles in the film bin contaminates the film reprocessing line because the granulator and agglomerator are not designed for rigid, high-melting articles.
