“Homopolymer” is one word on a PP quote that locks in a property window before you read any other number. It tells you the chain is built from a single monomer — propylene only, no comonomer disrupting the regularity.
That regularity is the whole story. The all-propylene chain packs into high crystallinity, and that same crystallinity is the source of both the stiffness and heat resistance you want and the cold-impact brittleness you have to design around.
What “Homopolymer” Means on a PP Grade Spec
Homopolymer polypropylene is polypropylene polymerized from a single monomer — propylene, with no second comonomer added to the chain. That single-monomer structure lets the methyl groups sit on the same side of the backbone regularly, making the chain highly isotactic.
High isotacticity drives high crystallinity. Commercial homopolymer grades run roughly 60-70% crystalline, with the most ordered grades pushing past 75%.
The chains fold into tight crystalline lamellae because nothing interrupts the propylene sequence. A random copolymer scatters ethylene units that each break up that folding; homopolymer has none of them.
Tight crystalline packing also sets the density near 0.905 g/cm³, the lowest of the commodity plastics.
Homopolymer is one of three branches of the polypropylene resin family, alongside random copolymer and the heterophasic impact grades.
What Homopolymer PP’s Property Numbers Predict
The 60-70% crystallinity produces a predictable spec window: high stiffness, high tensile strength, strong heat resistance, near-zero moisture pickup, and the lowest density in the commodity range.
Tensile strength runs 31-41 MPa (ASTM D638 / ISO 527). Flexural modulus — the number that tells you how rigid the part feels — sits at 117-172 MPa under ISO 178, above any copolymer grade.
Water absorption is 0.01-0.03% over 24 hours (ASTM D570), effectively zero — homopolymer holds dimensional stability in wet service where a hygroscopic polymer like nylon would swell.
Heat resistance is a two-number story, and reading only one of them gets parts wrong. Under light load (0.45 MPa) the heat deflection temperature reaches 107-121 °C; under structural load (1.80 MPa) it drops to 49-60 °C (ASTM D648 / ISO 75).
A homopolymer crate holds its shape near boiling water when nothing is pushing on it, but starts to deflect well below 70 °C once you load it. The peak melting point sits around 160-165 °C — the wider 210-290 °C “range” on some sheets includes thermal decomposition, not true Tm, so don’t read it as a service ceiling.
These ranges are wide on purpose. MFI moves the molecular weight across that window for fiber versus injection, but it does not move the architecture — a 2 MFI and a 35 MFI homopolymer are the same chain at different lengths.
Why Homopolymer PP Turns Brittle Below 0 °C
The same crystallinity that delivers the stiffness and heat resistance is the direct cause of homopolymer’s defining weakness — it turns brittle in cold impact. The glass transition temperature (Tg) hovers right around 0 °C, and below it the amorphous chain segments lose mobility and the polymer goes glassy.
Dense crystalline lamellae give a crack a clean, streamlined path to propagate. With no rubber phase and little amorphous region to absorb the energy, a sharp impact below freezing finds a fracture path instead of yielding.
The numbers make the gap concrete. Notched Izod impact for homopolymer runs 21-75 J/m at room temperature (ASTM D256 / ISO 180) — and it falls further as temperature drops.
Cold-impact brittleness is the reason homopolymer is the wrong call for a freezer-compartment part or any cold-service component that takes a knock. “Strong” is true for static tensile load; it is not true for impact, and the two failure modes diverge sharply below 0 °C.
High crystallinity also makes homopolymer haze more than random copolymer. The same large, well-formed crystalline spherulites that stiffen the part scatter light, so an unmodified homopolymer film reads cloudier than an RCP one. Parts chasing optical clarity move to clear polypropylene grades built on random copolymer plus a clarifier, not on homopolymer.
Where Homopolymer Polypropylene Fits
Homopolymer’s stiffness, draw-ability, and low cost make it the workhorse for monofilament, woven bag and FIBC tape, fiber and non-woven, and general rigid injection — applications where impact below freezing is not the duty. The high crystallinity that hurts cold-impact performance is exactly what lets a tape or filament draw to high tenacity without necking unevenly.
Homopolymer grades like PetroChina T30S are the monofilament and raffia workhorse, drawn into woven bags, carpet backing, geotextiles, baler twine, and ropes — places where stiffness and clean draw matter more than impact. For the woven-bag and FIBC tape slot, an S1003-class homopolymer carries the same job with good stretchability for tape extrusion.
Fiber and non-woven spunbond extrusion runs on a homopolymer like S2025, where consistent draw at high line speed sets the slot. Higher-MFI homopolymer grades cover general injection and caps.
One caveat that catches buyers: MFI alone does not tell you whether a grade will draw clean — you also need molecular-weight distribution data. Two homopolymer grades at the same MFI can behave differently in monofilament because the distribution governs draw behavior, so the application slot, not MFI alone, picks the grade.
How Homopolymer Differs from Copolymer at a Glance
Homopolymer leads on stiffness, tensile strength, and heat resistance because of its high crystallinity, but copolymer beats it on impact toughness and clarity because a comonomer deliberately disrupts that crystallinity. The single cleanest number proving the gap is notched Izod: homopolymer sits at 21-75 J/m, while copolymer grades run 59-747 J/m.
That spread is why “homopolymer is stronger” holds only for static load. A random copolymer scatters ethylene along the chain to enlarge the amorphous, shock-absorbing zones; an impact copolymer disperses a rubber phase inside a homopolymer matrix to do the same job without losing matrix stiffness.
When your part takes a cold knock, needs to weld cleanly, or has to be transparent, the brittleness and haze trade-off points you to the copolymer side of the binary.
Picking between homopolymer, random copolymer, and impact copolymer is a separate sourcing decision with its own grade grid, which the three-grade selection guide works through family by family.
What This Means in Practice
The next time “homopolymer” shows up on a spec or quote, you can read most of the resin off that one word. It implies high crystallinity, density near 0.905, and tensile in the low-40s MPa. Heat resistance splits between 49-60 °C loaded and 107-121 °C unloaded, and the notched Izod warns you off any cold-impact part.
The trap is the word “stronger.” Homopolymer wins on static stiffness and heat, but its 21-75 J/m impact floor means a sub-zero or drop-tested part needs a copolymer, not a higher-MFI homopolymer.
Match the application slot to the grade first. The COA gives you four numbers, the spec sheet gives you the window, and the application tells you which corner you actually need.