Acetal copolymer (POM-C) and polypropylene (PP) both list a maximum service temperature near 100 °C. Yet under a 1.8 MPa load, POM-C holds its shape to about 110 °C while PP softens at roughly 55 °C — half the number.
That gap is the whole comparison in miniature: the two materials look interchangeable on a one-line spec sheet and behave nothing alike once a part is doing work.
Most selection rules of thumb collapse this to “acetal handles load, PP handles cost,” which steers engineers toward POM-C by reflex on anything that moves. The real decision turns on a single governing constraint.
POM-C earns its premium when stiffness-under-load, surface wear, or tight dimensional tolerance rules the part. The moment the controlling factor becomes chemical resistance, weight, cost, or a flexing hinge, PP is the correct call — not a downgrade.
Where POM-C Outperforms Polypropylene
POM-C wins on stiffness, friction, and creep — the properties that decide whether a moving or load-bearing part holds up, and the axes where PP cannot follow.
Stiffness Under Load
POM-C’s flexural modulus runs about 2.5–2.6 GPa (ASTM D790) against PP’s 1.2–1.5 GPa, so an unfilled acetal part is roughly twice as stiff for the same wall section.
That stiffness is why a POM-C gear tooth or a snap-fit boss deflects less under the same force.
Friction and Wear
POM-C is self-lubricating, with a dynamic coefficient of friction around 0.21, where PP runs higher and carries no self-lubrication.
Pair two acetal gears, a cam, or a bushing and they slide cleanly dry; the same geometry in PP wears and squeals.
Creep Under Sustained Load
POM-C is among the most creep-resistant of the crystalline thermoplastics — under a sustained clamp load or a press-fit, it holds dimension.
PP creeps and relaxes under the same load over time, so a tight fit goes loose. For any part governed by load, precision, or sliding wear — gears, bearings, cams, fasteners, conveyor components — POM-C is the right material and the premium buys real performance.
Where Polypropylene Is the Right Material
PP beats POM-C on chemical resistance, weight, cost, and living-hinge fatigue — and on chemical contact it is the tougher material, not the budget substitute. These constraints govern most fluid-handling and high-volume parts.
Chemical Resistance
POM-C resists solvents, fuels, oils, and weak acids, but it has poor resistance to strong acids and strong bases, and sustained hot-water service risks hydrolysis.
PP shrugs off the strong acids and bases that attack acetal, which is why it dominates lab benches, chemical tanks, and semiconductor fluid handling. On a chemical-contact part, specifying POM-C over PP is the actual downgrade.
Weight and Cost
PP runs about 0.905 g/cm³ against POM-C’s 1.41 — roughly 36% lighter.
On a high-volume molded part, that density gap cuts both shipped mass and per-part material cost — PP is a commodity polyolefin priced well below engineering-grade acetal.
The Living Hinge
The living hinge belongs to PP alone. Acetal is not ductile enough for a 90-degree integral hinge beyond a few tens of flex cycles — it fatigues and cracks.
PP and PE own the integral hinge for any closure that opens repeatedly. If your part flexes at a built-in pivot, the material question is already answered.
On the PP side, homopolymer polypropylene is the stiffest PP family. Copolymer polypropylene trades some rigidity for impact, and is the chain architecture behind PP-R pipe and living-hinge closures.
Two POM-C vs PP Specs That Mislead
Two datasheet numbers cause more bad POM-vs-PP decisions than any others: the shared ~100 °C service temperature, and the claim that POM-C is “more dimensionally stable.” Both invert under real duty.
Equal Max Service Temperature, Unequal Under Load
Both materials cite a maximum service temperature near 100 °C, so they read as thermally equal. Load them and they separate.
Heat deflection temperature at 1.8 MPa is about 110 °C for POM-C versus roughly 55 °C for PP (ASTM D648) — nearly double. Equal max-service-temp does not mean equal load capability at temperature, so any part carrying stress in a warm environment lands on POM-C.
Moisture Stability Runs the Other Way
The dimensional-stability claim quietly inverts a real number. POM-C does hold tighter tolerance under sustained load and thermal cycling — but not because it absorbs less water.
PP absorbs less than 0.03% moisture (ASTM D570); POM-C takes up 0.22% at 24 hours and roughly 0.8% at saturation. On raw moisture uptake, PP wins.
POM-C’s dimensional edge is creep resistance and stiffness retention under load, not moisture. State it the other way around and you have specified the wrong material for a humid low-load part.
When to Choose Acetal Copolymer and When to Choose Polypropylene
Match your part’s single dominant requirement to the material; do not average the properties. Each row maps a governing constraint to the correct choice and the numeric gap behind it.
| Governing constraint | POM-C | PP | Goes to |
|---|---|---|---|
| Stiffness under load (flexural modulus, D790) | ~2.6 GPa | ~1.3 GPa | POM-C |
| Surface wear / low friction (COF) | ~0.21, self-lubricating | higher, none | POM-C |
| Creep under sustained load | among the best | creeps, relaxes | POM-C |
| Load capability at temperature (HDT @1.8 MPa, D648) | ~110 °C | ~55 °C | POM-C |
| Strong acid / base resistance | poor | resistant | PP |
| Weight / density (D792) | 1.41 g/cm³ | 0.905 g/cm³ | PP |
| Living hinge / repeated flex | excluded | owns it | PP |
| Material cost per part | engineering-priced | commodity | PP |
| Moisture uptake (D570) | 0.22–0.8% | <0.03% | PP |
The split is clean: precision mechanical hardware on the left, fluid contact and weight, cost, and flex on the right. A part rarely needs both, which is why the dominant-requirement rule resolves nearly every borderline case.
When the constraint lands on PP and the part is a PP-R pipe fitting, PetroChina Dushanzi T4401 random copolymer is the typical reference grade. The equivalents from Sinopec sit in a similar MFI band.
Picking the right PP family is its own decision — the three main PP grades split along stiffness, clarity, and low-temperature impact.
The Bottom Line
Decide by the single requirement that governs the part, not by which material has the better all-around datasheet. If the part lives or dies on stiffness, sliding wear, creep, or load at temperature, POM-C earns its premium.
If the controlling factor is chemical contact, weight, cost, or a flexing hinge, PP is the materially correct answer — and on the chemical axis, the tougher material, not the cheaper compromise.
Fluid-handling components, snap-and-flex closures, and high-volume molded parts where every gram and every cent counts all belong to PP — the large share of real parts. For those, the next question is simply which PP family fits: the stiffness of a homopolymer, the clarity of a random copolymer, or the sub-zero impact of an impact grade.