At the specification stage, Polyethylene Packaging: Matching Resin and Form to Shelf, Shipping and Cost requires selecting among low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, and high-density polyethylene grades whose density spans 0.912 g/cm³ to 0.965 g/cm³ and whose melt index spans 0.2 g/10 min to 150 g/10 min when measured under ISO 1133-1:2022 or ASTM D1238-20. The packaging forms—blown film, cast film, extrusion coated board, blow moulded bottles, injection moulded tubs, thermoformed sheet, foam, and bulk liners—are not interchangeable. Film extrusion commonly uses melt indices above 0.5 g/10 min, blow moulding uses 0.3 g/10 min to 0.8 g/10 min to maintain parison hang strength, and injection moulding uses 20 g/10 min to 60 g/10 min to fill thin walls. Crystallinity increases with density: raising density from 0.918 g/cm³ to 0.960 g/cm³ increases flexural modulus from roughly 200 MPa to 1,200 MPa under ASTM D790-17 and reduces moisture vapour transmission rate by approximately 60% to 70% at equal gauge. Cost is governed by density, gauge, and processing speed, but downgauging without addressing toughness and seal integrity produces field failures in distribution.

What Limits Shelf Life in PE Barrier Structures?

Moisture vapour transmission rate for LDPE and LLDPE film measured at 37.8 °C and 90% RH under ASTM F1249-20 commonly ranges from 0.8 g·mil/(100 in²·day) to 1.5 g·mil/(100 in²·day), while HDPE at 0.950 g/cm³ typically ranges from 0.35 g·mil/(100 in²·day) to 0.50 g·mil/(100 in²·day). Oxygen permeation remains the major shelf-life boundary: unfilled polyethylene exhibits oxygen transmission rate above 2,000 cm³·mm/(m²·day·atm) at 23 °C and 0% RH under ASTM D3985-17. Polyethylene alone is therefore used for moisture protection rather than oxygen scavenging, and oxygen-sensitive food packages are configured as coextruded or laminated structures with EVOH, PVDC, metallized PET, or aluminium foil, with PE serving as sealant and moisture barrier. Sealant thickness is typically 18 µm to 50 µm; the exact distribution of tie layers and barrier cores is proprietary in many commercial specifications, so published data for this specific configuration is limited. Environmental stress crack resistance measured by ASTM D1693-21 Condition B in 10% Igepal CO-630 remains the controlling property for HDPE bottles containing surfactants or aggressive liquids. Blow moulded household chemical bottles commonly require F50 values above 100 h, while thin-wall food tubs may be acceptable with F50 below 10 h because of lower in-use stress.

During palletized freight handling, distribution packaging shifts selection toward tear propagation resistance, dart impact, puncture energy, and low-temperature ductility. A 50 µm blown film produced from hexene LLDPE with density of 0.920 g/cm³ and melt index of 1.0 g/10 min typically exhibits dart impact between 400 g and 800 g under ASTM D1709-16a, machine-direction Elmendorf tear between 15 N and 30 N under ASTM D1922-15, and slow puncture energy between 0.5 J and 1.2 J. Low-temperature performance is tested by conditioning at −20 °C before impact; LLDPE retains ductile failure more effectively than LDPE, but certain Ziegler-Natta hexene grades show brittle failure below −30 °C. Surface coefficient of friction influences pallet load stability; a machine-direction and transverse-direction kinetic coefficient of friction between 0.20 and 0.40 under ASTM D1894-14 reduces shingling and sliding on packaging lines.

Resin Density, Melt Index and Conversion Cost in Rigid Packaging

Rigid HDPE packaging exploits the inverse relationship between density and environmental stress crack resistance. Blow moulding grades with melt index 0.3 g/10 min to 0.7 g/10 min and density 0.953 g/cm³ to 0.958 g/cm³ are extruded at melt temperatures from 180 °C to 230 °C on shuttle or wheel machines. Increasing density to 0.962 g/cm³ raises top load and allows wall thickness reduction of 10% to 15%, but ESCR measured under ASTM D1693-21 may fall by 50% or more, creating a cliff-edge when the bottle contains a stress-cracking liquid. Top load measured under ASTM D2659-16 for a 1 L HDPE bottle at 28 g wall weight commonly exceeds 250 N when density is 0.958 g/cm³. Thin-wall injection moulding of round tubs and lids uses high-flow HDPE with melt index from 30 g/10 min to 60 g/10 min, requiring clamp force of 2,000 kN to 5,000 kN per multi-cavity tool and fill times below 0.4 s. At melt index above 60 g/10 min, impact strength measured under ISO 179-1:2023 or ASTM D256-23 may fall sufficiently to cause lid hinge failures in field use. Cost reduction is therefore controlled by lightweighting within the top-load and ESCR boundaries, not by arbitrarily raising melt index.

On three-roll chill stacks running at 600 m/min to 900 m/min, cast stretch film operations select metallocene LLDPE with narrow molecular weight distribution and melt index from 2.0 g/10 min to 4.0 g/10 min. The narrow distribution raises the critical shear rate for melt fracture; however, shear viscosity under ISO 11443:2021 is lower than that of a Ziegler-Natta hexene copolymer of equal melt index, so extruder backpressure can decrease by 10% to 20% on a 90 mm single-screw extruder with L/D of 30:1. A 20 µm machine film is frequently downgauged to 12 µm to 15 µm while maintaining F2 puncture values above 0.8 J in stretch wrapping. On air-cooled blown film lines, bubble stability with mLLDPE is lower; die gap is narrowed to 1.4 mm to 2.0 mm, frost-line height is raised by 15% to 25%, and blow-up ratio is kept between 2.0:1 and 2.5:1 to avoid oscillation. Published data for this specific configuration is limited where screw design and die gap are proprietary, but the operational boundary is well established on production lines.

For extrusion coating and lamination, autoclave LDPE with melt index 4 g/10 min to 12 g/10 min is selected because tubular LDPE gives higher neck-in at equivalent melt index. Neck-in on a 300 m/min coating line is typically maintained below 25 mm per side by using melt temperature 315 °C to 330 °C and a die-to-nip distance of 100 mm to 150 mm. Adhesion to paperboard or aluminium foil depends on oxidation in the air gap; over-oxidation increases odour, while under-oxidation reduces peel strength measured under ASTM D1876-08. Coating weights of 12 g/m² to 25 g/m² are standard for liquid packaging board, where PE provides moisture resistance and sealability but not oxygen barrier.

Heat-sealing windows for PE packaging on vertical form-fill-seal machines are defined by hot-tack force and seal initiation temperature. Ethylene-alpha-olefin copolymers with density 0.918 g/cm³ to 0.925 g/cm³ initiate hot-tack above 90 °C and maintain seal strength above 1.5 N/15 mm over a 20 °C to 30 °C window when tested at 0.2 s dwell and 0.4 MPa seal pressure under ASTM F1921-12. Resins with a lower melting peak measured by differential scanning calorimetry under ISO 11357-3:2018 permit higher packaging line speeds but reduce the hot-tack plateau, producing seal failures when dusty products contaminate the seal area. The cost trade-off is between faster line speed and higher downgauging versus added rework from leakers.

Thermoforming Windows Narrow When Melt Strength Is Insufficient

As sheet gauge drops below 0.4 mm, the processing window narrows because sag under radiant heating increases and plug-assist forming produces uneven wall thickness. HDPE sheet for thermoformed trays and tubs is extruded at 200 °C to 230 °C from sheet grades with melt index 0.8 g/10 min to 2.0 g/10 min and density 0.945 g/cm³ to 0.960 g/cm³. Sheet surface temperature at forming is maintained between 125 °C and 145 °C; below 120 °C the sheet may stress-whiten, and above 150 °C melt strength is insufficient for deep-draw ratios above 1.5:1. Tooling temperature is held at 40 °C to 70 °C to avoid sink marks while keeping cycle time below 6 s per stroke. Melt strength is measured indirectly by sag tests or extensional viscosity; conventional low-sag HDPE grades are formulated with broad molecular weight distribution.

In shipping and corner protection, expanded polyethylene foam is produced at densities from 1.2 lb/ft³ to 6.0 lb/ft³, corresponding to 19 kg/m³ to 96 kg/m³. Thermal conductivity at 24 °C typically ranges from 0.036 W/(m·K) to 0.042 W/(m·K) under ASTM C518-21. Compressive creep is monitored at 23 °C and 50% RH with static stress of 0.03 MPa; published data for specific densities is limited, but lower-density foam below 2.0 lb/ft³ shows greater thickness loss after prolonged pallet stacking. For reusable bulk containers, HDPE sheet of 3 mm to 6 mm is welded or thermoformed; corner impacts at −10 °C may initiate cracking if the resin lacks sufficient high molecular weight fraction to resist crack growth.

Post-consumer recycled PE blended into packaging reduces cost but increases gel density and creates film tear variability. Recyclate addition above 20% in blown film often raises gel counts and lowers dart impact by 30% to 50%; filters of 100 mesh to 200 mesh are installed on extruders to control gels. The melt-flow ratio and density of recycled HDPE vary batch-to-batch, requiring pellet blending and increased frequency of melt index testing under ASTM D1238-20. Incoming recycled pellet limits are defined at moisture below 0.05% and ash below 2% to avoid screen pack pressure spikes.

For food-contact compliance, PE packaging is governed by FDA 21 CFR 177.1520 and EU 10/2011 with overall migration limit 10 mg/dm², while specific migration testing follows EN 1186-1:2002 and EN 13130-1:2004. Halogen-free and heavy-metal limits for packaging used in electronics shipping are evaluated under IEC 62321-8:2017 and RoHS Directive 2011/65/EU where recycled content is incorporated. HDPE bottles are not suitable for retort sterilization above 121 °C because heat deflection temperature under ASTM D648-18 at 0.455 MPa is typically below 100 °C; hot-fill applications are limited to 60 °C to 80 °C unless sidewall ribs or vacuum panels compensate for paneling. Ethylene-vinyl alcohol coextruded structures require a continuous PE moisture layer at least 10 µm thick on each side to prevent EVOH plasticization and oxygen barrier loss under high humidity.