Both polyethylene and polypropylene land in your warehouse as off-white pellets with a density near 0.91 and a single MFI on the COA. The pellets tell you almost nothing about which one belongs in your part.
A converter choosing between the two isn’t really picking a material family. The choice is decided by the part’s single most demanding requirement — service temperature, low-temperature impact, stiffness, clarity, or chemical contact. Each requirement points to a specific sub-grade and MFI band, not just “PE” or “PP.”
How Polyethylene and Polypropylene Differ on the Spec Sheet
Polypropylene runs stiffer and hotter; polyethylene runs tougher and colder. PP holds a melt point of 165-175 °C against PE’s 105-130 °C, carries a higher flexural modulus, and resists solvents and stress cracking better. PE answers back with impact toughness that survives to roughly -80 °C, where PP homopolymer turns brittle near 0 °C.
That family-level verdict is accurate, and it resolves nothing for a real part. The spread inside each family is wide enough to swallow the difference between them.
| Property (ASTM) | Polypropylene | Polyethylene |
|---|---|---|
| Density, D792 | 0.89-0.91 | 0.91-0.97 |
| Melt point | 165-175 °C | 105-130 °C |
| Flexural modulus, D790 | 13,000-25,000 psi (90-170 MPa) | 29,000-174,000 psi (200-1,200 MPa), LDPE→HDPE |
| Notched Izod, D256 | 21-747 J/m (homo→impact) | LDPE no-break, HDPE 1.3 ft-lb/in |
| Brittleness onset | ~0 °C (homopolymer) | to ~-80 °C (HDPE) |
| ESCR | excellent | poor (LDPE) to excellent (HDPE) |
Read the impact row before anything else. PP’s notched Izod runs from a brittle 21 J/m homopolymer to a 747 J/m impact copolymer — a 35-fold internal range that overlaps deep into PE territory.
The family name is a starting point. The sub-grade is the answer.
How the PP and PE Sub-Grades Differ
Each family fans out across a property range that the family label hides, and the sub-grade choice often decides the part more than the PE-versus-PP line does. Three PP grades and three PE grades carry the real decision variables.
On the PP side, the divider is ethylene comonomer content:
- Homopolymer (0% ethylene) — maximum stiffness and the highest melt point, but a glass transition near 0 °C makes it brittle in the cold. This is monofilament, woven-bag tape, and rigid injection work. A homopolymer monofilament grade like PetroChina T30S sits here.
- Random copolymer (1-7% ethylene) — trades some stiffness for clarity, heat-sealability, and a lower melt point. This is the PP-R hot-and-cold water pipe grade; PetroChina Dushanzi T4401 is the typical reference, with Sinopec equivalents in a similar MFI band.
- Impact / block copolymer (5-15% ethylene) — the rubber phase keeps it tough at low temperature. Freezer parts, automotive, and cold-service injection live here.
On the PE side, the divider is density and chain branching:
- LDPE (0.910-0.940) — heavily branched, soft, flexible, poor stress-crack resistance. Film, bags, liners.
- LLDPE (0.915-0.940) — a linear backbone with short branches delivers better tear and puncture resistance plus good ESCR at the same density. Stretch and agricultural film. A grade like LL0209AA outperforms LDPE on puncture despite the overlapping density.
- HDPE (0.941-0.970) — tightly packed, stiffest PE, excellent stress-crack resistance. Its flexural modulus of 174,000 psi is roughly six times LDPE’s 29,000 psi and rivals PP. Pressure pipe (PE100, such as TUB121N3000) and blow molding.
The within-family numbers prove the point. HDPE is six times stiffer than LDPE, and impact copolymer PP carries up to ten times the notched impact of homopolymer. Treating LLDPE and LDPE as interchangeable because both are low-density is the single most common selection error here, and it costs film converters real puncture failures.
Choosing Polyethylene or Polypropylene by the Part’s Governing Requirement
Start with the part’s single most demanding requirement and let it pick the sub-grade. One requirement governs; the rest are constraints you check afterward.
- Service temperature — for hot-fill, steam, or hot-water duty, PP wins outright: homopolymer for the highest heat-deflection temperature, or random copolymer for PP-R pipe. PE’s maximum operating temperature tops out near 71 °C (LDPE) to 82 °C (HDPE), so it’s off the table for anything hot.
- Low-temperature impact — this points away from PP homopolymer every time. A freezer or cold-service part needs PE (HDPE or copolymer, tough to roughly -80 °C) or PP impact copolymer, where 5-15% ethylene drops the brittleness onset to -20 °C or below. Specifying homopolymer for a sub-zero drop-test part is the classic failure; raising its MFI won’t save it.
- Stiffness and rigidity — HDPE or PP homopolymer, both far above LDPE’s 29,000 psi flexural modulus. If the part also has to take cold impact, that conflict is exactly why impact copolymer exists.
- Clarity — random copolymer PP is the clearest option in this set. HDPE reads translucent-to-milky, so it loses on any see-through part.
- Chemical contact — PP carries broad solvent resistance and very high stress-crack resistance; industry estimates put its ESCR roughly 20-100× that of HDPE. That suits PP to aggressive solvent and acid contact, while HDPE earns pipe duty only because PE100 grades are bimodal-engineered specifically to raise ESCR.
The processing method then sets the MFI band — high MFI to fill thin-wall injection and blow molds, low MFI for pipe and extrusion. One trap on the COA: a PP MFI is measured at 230 °C/2.16 kg and a PE MFI at 190 °C/2.16 kg, so the two numbers are not directly comparable across families.
Read the test condition, not just the number. MFI alone also won’t tell you whether a grade draws clean — you need the molecular-weight distribution behind it. Once the family and sub-grade are set, the PP grade-pick narrows further across homopolymer, random, and impact options.
Telling PE from PP at Incoming Inspection
A 60-second bench check confirms which resin a received lot actually is, and it does not start with the float test. PP sits at 0.895-0.92 g/cm³ and LDPE/LLDPE at 0.910-0.940, so both float in water. The common “PP floats, PE sinks” shortcut only separates the heavier HDPE, which sinks toward 0.96, and leaves a low-density-PE-versus-PP band unresolved.
Melt point is the clean separator. PP melts at 165-175 °C against PE’s 105-130 °C, a gap wide enough that a hot-plate or DSC reading settles the call on its own.
Burn behavior confirms it without instruments. PP burns with a candle-wax smell carrying a dirty-engine-oil undertone, drips, and produces water vapor and CO₂; PE smells like clean paraffin wax.
Run at least two of these — density screen, melt point, burn — before you trust the lot. Confirming the resin and sub-grade on arrival keeps a homopolymer-for-copolymer or PE-for-PP substitution from reaching the line.
Making the Polyethylene-or-Polypropylene Pick
Identify the one requirement the part cannot compromise on, and let that requirement choose the sub-grade and MFI band before you compare material families. The family question answers itself once you know whether you need PP’s heat and stiffness or PE’s cold toughness.
The within-family choice — homopolymer versus copolymer, HDPE versus LLDPE — usually decides the part more than the PE-versus-PP line does. Neither resin is the better material; the part’s governing requirement is.
The COA gives you four numbers, the spec sheet gives you the window, and the application tells you which corner you need — a melt-point and burn check on arrival confirms it.