The material commonly called number 2 plastic is high-density polyethylene (HDPE), a predominantly linear ethylene homopolymer or ethylene/α-olefin copolymer with a density range of 0.941–0.967 g/cm³ as determined by ASTM D1505-18. The polymer is distinguished from low-density polyethylene by a short-chain branch frequency below 10 branches per 1,000 total carbon atoms, which permits a crystalline fraction of 60–80 % and a differential scanning calorimetry melting peak near 125–135 °C. Molecular weight distribution and comonomer selection—commonly 1-butene, 1-hexene, or 1-octene—control stiffness, slow crack growth resistance, and melt processing behavior. Resin identification code 2 therefore does not define a single molecular architecture; HDPE grades span blow molding, injection molding, film, pipe, sheet, and geomembrane converting operations.

What Is Number 2 Plastic? HDPE, the Recycling Code and Where It Shows Up

Under ASTM D7611/D7611M-21, the code 2 is assigned to high-density polyethylene packaging and is marked as the numeral 2 inside the triangular chasing-arrows symbol. The code identifies base resin type for sorting and is not a certification of recycled content, recyclability, or acceptance by a specific material recovery facility. Where the material shows up is a direct function of melt rheology, density, and environmental stress crack resistance: blow-molded dairy and detergent containers, injection-molded caps and crates, pails, corrugated drainage pipe, pressure pipe, geomembranes, blown film, and thermoformed sheet. Grades intended for rigid packaging are specified by melt flow rate under ASTM D1238-20 and density under ASTM D1505-18. High-load melt index measured at 190 °C and 21.6 kg is used for blow molding resins because the 2.16 kg melt index may fall below 0.1 g/10 min and becomes too low for reliable process control.

Blow Molding-Grade HDPE in Dairy and Household Chemical Containers

Extrusion blow molding of HDPE converts parisons into milk jugs, juice bottles, shampoo and conditioner containers, household chemical bottles, and windshield washer reservoirs. Continuous shuttle and reciprocating screw machines with L/D ratios of 24:1–32:1 are operated at melt temperatures of 180–220 °C. Parison programming, die ovalization, and pre-blow timing are adjusted to maintain wall thickness tolerance because HDPE exhibits limited melt strength relative to fractional-melt, high-molecular-weight polyethylene grades. Bimodal molecular weight distribution resins with high-load melt index of 8–12 g/10 min are selected for large-part containers where sag resistance is required. Detergent bottle grades are evaluated by ASTM D1693-22, Condition B, because the liquid stress-cracking agents in formulations can induce brittle failure at seams and pinch-off areas. F50 values above 600 h are typical for qualified household chemical packaging grades. The pinch-off weld must be cooled under compression; failure at the tail weld is a production-scale defect when regrind content exceeds 20 % without adequate melt temperature adjustment. Oxygen barrier is low in monolayer HDPE, so milk jugs are pigmented and designed for short shelf life. For oxygen-sensitive contents, coextrusion with ethylene-vinyl alcohol and an HDPE regrind layer is used.

When HDPE is processed in reciprocating screw injection machines, clamp force per projected area is commonly 3–5 tons/in² for multicavity closures, crates, pails, and storage totes. Melt temperatures are held at 180–250 °C and mold temperatures at 10–30 °C, with cold-runner or hot-runner tooling. High-flow HDPE grades with melt flow rates of 8–30 g/10 min are chosen for thin-wall caps and lids to reduce injection pressure and cycle time; tensile yield strength remains near 23–30 MPa when measured under ASTM D638-14. Differential shrinkage of 1.5–2.0 % in the flow and transverse directions causes warpage in flat pails when mold cooling is nonuniform, a production bottleneck addressed by conformal cooling and gate repositioning. Batch-to-batch variation in melt flow can widen fill variation in high-cavitation hot-runner molds; off-line melt flow testing at 190 °C/2.16 kg and process monitoring of cushion and recovery time are used to segregate lots. Low-temperature toughness for freezer-grade containers is evaluated by notched Izod impact at 23 °C and −30 °C under ASTM D256-23e1. Room-temperature values of 20–50 J/m are typical, but the drop at −30 °C narrows the operating window unless higher molecular weight grades are selected.

Pipe, Extrusion, and Geomembrane Service Conditions

In pressure piping applications, HDPE grades classified under ASTM D3350-21 and ISO 17855-1 are characterized by long-term hydrostatic strength rather than tensile yield alone. PE4710 or PE100 materials are specified by minimum required strength; ISO 12162 assigns PE100 a minimum required strength of 10.0 MPa at 20 °C for 50 years. Standard dimension ratios SDR 11 and SDR 17 are common in water and gas distribution. Extrusion of solid-wall HDPE pipe uses grooved-feed extruders with L/D ratios of 30:1–36:1 and melt temperatures of 190–225 °C; vacuum calibration and multi-stage cooling tanks control outside diameter and wall thickness to ISO 11922-1 tolerances. Textured HDPE geomembranes with nominal thicknesses of 1.5 mm and 2.0 mm are installed in landfill liners, heap leach pads, and tailings impoundments. Seam strength is tested by ASTM D6693-20, oxidative induction time by ASTM D3895-19, and stress crack resistance by the notched constant tensile load method ASTM D5397-20. Long-term performance is limited by exposure to strongly oxidizing acids and by uncontrolled hydrocarbon contact, which can plasticize the amorphous phase and reduce yield stress.

For blown film and cast sheet converting, HDPE grades with melt flow rates below 1.5 g/10 min and density above 0.949 g/cm³ are used to produce crinkly packaging film, produce bags, butcher wrap, cereal liners, and thermoformed sheet. In blown film lines, blow-up ratios of 3:1–5:1 and frost line heights of 6–10 die diameters balance dart drop impact and tear anisotropy. Dart drop impact is evaluated under ASTM D1709-22; Elmendorf tear in the machine and transverse directions under ASTM D1922-23. HDPE film has low water vapour permeability but high oxygen permeability, so modified atmosphere packaging requires coextrusion or lamination with barrier polymers. Gauge uniformity at thicknesses below 10 µm requires tight die gap control and dual-lip air rings; production-scale variation often rises above ±5 % when melt temperature drifts or when screen pack backpressure changes.

When Post-Consumer Number 2 Material Is Melt Reprocessed

Post-consumer HDPE from collection streams is granulated, washed in hot caustic or detergent systems, and separated from heavier PET by sink-float tanks because HDPE density below 0.965 g/cm³ permits flotation in water. Polypropylene contamination is not removed by water flotation; near-infrared sorting or density separation in water-alcohol or water-glycol media is required where PP concentrations above 5 wt% degrade weld-line integrity in injection-molded items. Melt filtration of recycled HDPE is performed with continuous screen changers using screens from 80–250 µm to remove paper fiber, aluminum foil, and gel particles. Recycled HDPE pellets are characterized by melt flow rate under ASTM D1238-20, density under ASTM D1505-18, and oxidation induction time under ASTM D3895-19 because oxidative history from prior melt cycles shortens further processing stability. Post-consumer HDPE intended for direct food contact is not automatically compliant with FDA 21 CFR 177.1520(c); secondary use in food packaging generally requires a food-contact substance notification, a letter of no objection, or a functional barrier layer that prevents contaminant migration. Multiple melt reprocessing cycles reduce environmental stress crack resistance and increase gel formation. Published data indicate that after three to five closed-loop extrusion cycles, F50 values may fall below the virgin-material specification unless stabilizer packages are replenished. The exact retention depends on prior consumer exposure, washing efficiency, and extruder oxygen ingress.