Polypropylene is a resin, and which comparison you are actually making depends on which sense of the word is in front of you. In the plastics trade, “resin” is the polymer as it is sold — PP, PE and PET are all resins, and the only real comparison is between grades. In the craft, casting and 3D-printing market, “resin” means an epoxy, polyester or UV system: a liquid that cures into a thermoset, a different class of material from PP.
Is Polypropylene a Resin?
Yes. In the plastics trade “resin” is the polymer as it is sold, in powder, granules or pellets, and polypropylene, polyethylene and PET all sit in that category.
Whether polypropylene counts as a plastic gets the same answer from the other side: it does.
The 5 inside the triangle on a moulded part is a resin identification code, and it identifies the polymer as polypropylene. Two parts marked 5 can be different polypropylene resins to buy: the code separates one polymer from another and says nothing about melt flow, family or additive package.
Polypropylene vs Thermoset Resin
PP can be melted and re-melted, and a casting resin — epoxy, polyester, polyurethane or a UV system — cures once into a thermoset that cannot be melted at all.
Processing: Melt and Mold vs Cast and Cure
PP is shaped with heat and pressure and finished by cooling, while a casting resin is poured without pressure and finished by chemistry. A PP part comes out of injection moulding, extrusion, blow moulding or thermoforming, and the cycle ends when the part is cool enough to eject.
One aluminium-filled epoxy tooling system gives a pot life of two to three hours and a de-mould time of 24 hours, with a post-cure schedule on top of that.
Moving a cast part to PP therefore means funding a tool before the first shot, while a casting route needs little tooling and costs more per part in labour and mould occupancy.
Properties: Where Each One Wins
Which one wins is decided by the grade and by the test condition. On the same method and temperature — notched Izod, ASTM D256, 23 °C — a PP homopolymer datasheet reads 0.8 ft·lbf/in and a cured epoxy 0.7, which is the same band.
An impact copolymer reads No Break on that test at 23 °C, and on ductility the gap is wide: these PP grades draw to 145–321 % elongation at break, while the cured thermosets break under 10 %.
The thermoset side leads on stiffness and on heat under load, provided the load is quoted. At 0.45 MPa a PP homopolymer reads 93–100 °C deflection temperature; at 1.82 MPa the order is PP impact copolymer 52 °C, vinyl ester 82 °C, epoxy 132 °C.
PP’s own weak points are known and mostly manageable: it embrittles under UV without a stabiliser package, and it crystallises as it cools, giving 0.8–2.5 % linear mould shrinkage that the tool and the part design have to absorb.
Cost and Recycling
Resin price and part cost move for different reasons, so comparing the two materials per kilogram answers the wrong question. Commodity PP is priced per tonne and tracked on published monthly indices; a casting system is priced per kilogram together with its hardener.
PP remelts and moves as a recovered rigid stream, while a cured thermoset cannot be remelted; a peer-reviewed review reports that grinding recovers only low-performance filler and that the vast majority of cured thermoset waste is landfilled.
What “Resin” Means on a Quotation
On a quotation, a datasheet or a COA, “resin” means one specific grade, and the grade is what is being compared. The designation carries the producer, which polypropylene family it belongs to, and — under ISO 19069-1, which replaced the withdrawn ISO 1873-1 in 2015 — three designatory properties: tensile modulus, impact strength and melt mass-flow rate.
For PP, melt flow rate is measured at 230 °C under a 2.16 kg load, and both the temperature and the load are selected per material and form part of the result, so two figures taken at different loads never belong in the same column.
ASTM D1238 states that it and ISO 1133 “differ in technical content”, so a COA figure measured to one and a datasheet figure measured to the other are not automatically comparable. The figure indicates only lot-to-lot uniformity of flow, so check the COA against the datasheet line by line.
When a quotation says only “PP resin”, three questions close the gap:
- Producer and grade code: which plant made it and under what designation, since PP grades from different producers rarely substitute one-for-one on the machine.
- MFR with its full condition: 230 °C / 2.16 kg, and whether the figure is reported to ISO 1133 or to ASTM D1238.
- Additive package: which stabiliser, nucleator or slip system is in the pellet, because the datasheet properties assume it.