Mapping Polyethylene and Polypropylene to Real Applications: A Buyer's Grid

Polyethylene (PE) is specified not as a single material but as a family of semicrystalline olefin resins whose density, comonomer type, and molecular weight distribution control long-term crack resistance, permeation, and melt processability. The selection framework in Mapping Polyethylene and Polypropylene to Real Applications: A Buyer's Grid separates PE from PP primarily by slow crack growth resistance, low-temperature ductility, and continuous service temperature limits. The scenarios below provide selection thresholds, test standards, and production equipment constraints observed on manufacturing lines.

For buried pressure pipe, a bimodal high-density polyethylene with density 0.950–0.960 g/cm³ and melt flow rate 0.2–0.7 g/10 min at 190°C/2.16 kg is selected because the high-molecular-mass fraction contributes tie molecules across lamellae, raising slow crack growth resistance. Pipe-grade PE100 resin is extruded on a single-screw grooved-feed extruder with L/D 30:1–40:1 and a spiral-mandrel die, melt temperature 200–230°C, and die head temperature 190–210°C. Hydrostatic design basis follows ISO 9080 and ISO 4427-2, with PE100 assigned an MRS of 10 MPa; a 110 mm SDR 11 pipe is pressure-rated at 16 bar at 20°C. The same pipe derates to 10 bar at 60°C and is not recommended for continuous service above 60°C in chlorinated water without oxidative stabiliser verification. ESCR per ASTM D1693-15, condition B, 10% Igepal CO-630, F50 exceeds 1000 h for bimodal PE100 resins. Melt pressure before the screen changer is monitored to detect gel build-up; a rise of 10–15% from initial line pressure triggers screen replacement. Without 2.0–2.5 wt% carbon black or an adequate stabiliser package, outdoor storage of pressure pipe leads to UV embrittlement within 6–12 months.

Crosslinked HDPE for hydronic heating is produced by peroxide-initiated crosslinking of an HDPE base resin with a gel content of 65–80% per ASTM D2765-16. PEX-a pipes extruded on a single-screw extruder with L/D 30:1 and a crosslinking zone at 180–220°C are rated for 0.6 MPa at 110°C for 50 years under ISO 15875-2. Oxygen diffusion through a 2 mm PEX wall is typically 0.1–0.5 g/m³/day at 40°C, requiring an EVOH barrier layer when the pipe is embedded in ferrous concrete systems. PP-R pipe, covered by ISO 15874-2, is joined by socket fusion at 260°C, while PEX requires crimp or expansion fittings. PP-R offers 0.6 MPa service at 70°C but lower low-temperature impact resistance than PEX.

What Determines Parison Sag and Die Swell in Extrusion Blow Moulding of HDPE?

Extrusion blow moulding of HDPE containers requires a resin with a low melt flow rate of 0.3–1.5 g/10 min at 190°C/2.16 kg and a density of 0.945–0.955 g/cm³ to maintain parison strength. Die swell measured on a bottle line with a diverging die typically ranges 30–50%, and parison sag is controlled by extruder head tooling temperature and melt strength; excessive sag above 15% of parison length causes wall-thickness variation. Drop impact is tested according to ASTM D2463-15 at 23°C and -20°C. ESCR for jerrycan resins must exceed 300 h under ASTM D1693-15, condition B, 10% Igepal. Accumulator-head machines with shot capacity 2–10 L use clamp force 150–600 kN; continuous shuttle machines are typically limited to containers below 2 L. When PP is considered for hot-fill bottles, PP-H with MFR 0.8–2.0 g/10 min at 230°C/2.16 kg provides higher heat distortion, but PP homopolymer exhibits brittle failure under ASTM D2463-15 at -20°C. HDPE therefore remains preferred for industrial containers carrying aggressive liquids at sub-zero ambient; PP is limited to hot-fill food packaging up to 90°C where ESCR is not limiting.

On a three-layer coextrusion blown film line configured with a 40:1 L/D barrier screw and a 250 mm spiral-mandrel die, LLDPE with density 0.918–0.925 g/cm³ and melt index 0.8–1.2 g/10 min is blended with 10–30 wt% LDPE to reduce melt pressure and postpone sharkskin. Die gap is set at 1.5–2.5 mm, blow-up ratio 2.0–3.0, frost-line height 4–8 die diameters, and melt temperature 190–210°C. Dart drop impact of a 50 µm film per ASTM D1709-16a increases from 200 g to 600 g as LLDPE content is raised, while tensile elongation at break measured per ASTM D882-18 remains above 500% in the machine direction and 700% in transverse direction. Seal initiation temperature of 85–110°C is determined by DSC and hot-tack testing, allowing PE to serve as the sealant layer in polyethylene/polypropylene laminates. PP cast film, by contrast, gives lower water vapour transmission rate of 5–10 g/m²/day at 38°C/90% RH per ISO 15106-3 and higher gloss, but its seal initiation is higher and its low-temperature impact is inferior. Operational boundary: film line operators must hold die lip temperatures above 215°C during LLDPE runs because melt fracture appears as sharkskin at low die temperatures; at screw speeds above 120 rpm, melt temperature can rise beyond 240°C and initiate gel formation.

Rotational Moulding and the 35-Mesh Powder Window

Rotational moulding grades are cryogenically ground to a powder with a target of 80–95 wt% below 425 µm 35 mesh, bulk density 0.92–0.94 g/cm³, and dry flow 25–35 s/100 g. The mould is heated in an oven at 280–320°C until the internal air temperature reaches 190–210°C; peak internal temperature above 210°C for LLDPE increases oxidation and interior surface roughness. Cooling rates of 10–15°C/min in the slow-cool stage reduce warpage; rapid water quench can reduce cycle time but raises shrinkage variation across thick sections. LLDPE rotational moulding grades with density 0.925–0.940 g/cm³ and melt index 3–6 g/10 min at 190°C/2.16 kg are common because they balance impact and processability. Low-temperature impact per ASTM D1998-21 on a 6 mm wall section shows no break at -40°C for properly sintered LLDPE. PP rotational moulding is limited due to narrower sintering window and oxidative degradation at long oven residence times; published industrial data for PP-specific rotational moulding remains limited. For outdoor tanks, PE requires UV stabilisation with 2 wt% hindered amine stabiliser and 2.0–2.5 wt% carbon black; without a UV package, surface chalking occurs within 12–18 months in subtropical exposure.

Because extrusion coating exposes the melt to air at 315–325°C for 0.5–1.5 s before the nip, LDPE with melt index 4–8 g/10 min at 190°C/2.16 kg is selected for its low melt elasticity and stable neck-in. A 4.5 m-wide coating line with an air-assisted die and chill roll at 15–20°C runs 300–500 m/min; neck-in is held to 25–50 mm per side. Adhesion to paperboard is measured by a 180° peel test at 300 mm/min and must exceed 1.5 N/15 mm for liquid packaging; low-density polyethylene achieves this through high melt temperature oxidation and, where required, an aqueous primer or corona treatment of the paperboard. Coating weight is controlled by beta gauge at 15–30 g/m². PP extrusion coating requires higher melt temperatures of 320–340°C and yields lower neck-in but poorer adhesion and heat seal unless copolymerised or coextruded with a PE skin. For lidding film, PE is used as the sealant at 85–110°C seal initiation, while PP provides steam sterilisation resistance up to 121°C. Operational boundary: silicone rubber nip rolls degrade above 0.25 MPa nip pressure and 30°C surface temperature, causing release failure.

When Crosslinked UHMWPE Replaces PP in Load-Bearing Orthopaedic Components

For load-bearing orthopaedic components, ultra-high-molecular-weight polyethylene with a molecular weight above 3.1 million g/mol, tensile yield strength 20–25 MPa, tensile strength 38–48 MPa, and elongation 200–350% is selected over PP due to its combination of low wear rate and impact toughness. Oxidation index measured by Fourier transform infrared spectroscopy per ASTM F2102-17 remains below 1.0 after sequential irradiation and annealing; unirradiated UHMWPE bearing surfaces have a volumetric wear rate of 1000–2000 mm³/million cycles in multidirectional pin-on-disc testing, while highly crosslinked formulations reduce this to below 500 mm³/million cycles. Injection-moulded PP for orthopaedic housings is limited to disposable, non-load-bearing components because its continuous service temperature of 90–100°C is adequate for autoclaving at 121°C for 20 min, but its flexural modulus of 1200–1500 MPa and low crack resistance under cyclic load are unsuitable for bearing applications. Tibial inserts machined from UHMWPE require sterilisation by ethylene oxide or gas plasma; gamma sterilisation above 50 kGy without post-irradiation annealing increases oxidation potential. Dimensional stability is critical: UHMWPE creep under 10 MPa load at 37°C is 4–8% after 24 h, which influences locking mechanism design tolerances. PP syringe barrels, by contrast, are produced by injection moulding with clamp force 1000–3500 kN and a melt temperature of 200–230°C, using a 0.8–1.2 mm wall; they pass ISO 7886-1:2017 for sterile hypodermic syringes.