Polypropylene plastic is a semi-crystalline thermoplastic polyolefin, polymerised from propylene gas and sold as pellets. It is the second-largest commodity thermoplastic by volume once the polyethylene grades are counted as one family.
The base polymer is cleared for food contact in the US under 21 CFR 177.1520. BPA is a building block of polycarbonate and of epoxy can linings, while polypropylene is polymerised from propylene alone. What differs between two PP articles is the additive package and the converting, both of which the buyer specifies.
Polypropylene is also assigned resin identification code 5.
What Polypropylene Plastic Is Made Of and How It Is Made
Polypropylene is made from a single monomer, propylene, a gas recovered from the steam cracking of refinery gas and the catalytic cracking of heavy oil, with propane dehydrogenation as the third route.
Nearly all commercial PP is polymerised on heterogeneous Ziegler-Natta catalysts. Metallocenes are the single-site alternative, with tighter control of molecular weight, end groups and stereoregularity.
The methyl groups can take several spatial arrangements along the chain, and only the isotactic form, with all of them on the same side, is marketed in significant quantity.
The resin leaves the reactor as powder. It is homogenised in a twin-screw extruder, cut into pellets under water, and blended by lot before it ships.
Properties of Polypropylene Plastic
Polypropylene is semi-rigid: stiffer and harder than polyethylene, with a higher melting point, and more flexible than ABS or nylon — where it sits among the commodity plastics.
| Property | Typical homopolymer value | Test method |
|---|---|---|
| Density | 0.905 g/cm³ | ISO 1183 |
| Melting point | 160–165 °C | ISO 3146 |
| Flexural modulus | 1,200–1,450 MPa | ISO 178 |
| Tensile strength at yield | 30–33 MPa | ISO 527-2 |
| Heat deflection temperature | 100 °C at 0.45 MPa; 55 °C at 1.80 MPa | ISO 75-2 |
| Notched Charpy impact, 23 °C | 6 kJ/m² | ISO 179 |
| Melt flow rate | 3 g/10 min (general-purpose grade) | ISO 1133, 230 °C / 2.16 kg |
At 0.905 g/cm³ polypropylene is the lightest of the commodity plastics.
Service temperature is far below the melting range. A loaded PP part runs continuously between 0 and 80 °C, and reaches about 100 °C only where it carries no significant mechanical stress.
Melt flow rate is most of what a PP grade code tells you.
Two MFR figures taken at different loads or temperatures never belong in the same column, and the two standards are not interchangeable either — ASTM D1238 notes that it and ISO 1133 address the same subject but differ in technical content. Check which standard a certificate of analysis was written against before you set it beside a datasheet.
What Are the Downsides of Polypropylene
Polypropylene’s downsides are low-temperature brittleness, UV sensitivity, oxidation, mould shrinkage and flammability. Each takes a standard correction, either a grade change or an additive package, and none of them calls for a different polymer.
- Low-temperature brittleness: the homopolymer’s notched Charpy is 6 kJ/m² at 23 °C. An extrusion-grade heterophasic copolymer still returns 8 kJ/m² at −20 °C, but read the impact row at your own temperature.
- UV sensitivity: unpigmented PP discolours and loses mechanical properties outdoors. UV absorbers, light stabilisers or carbon black extend it.
- Oxidation: the tertiary carbon on every second backbone atom is where thermal degradation begins. Every commercial grade carries an antioxidant package.
- Mould shrinkage and warping: shrinkage is high for a commodity thermoplastic and direction-dependent. Nucleation and mineral filler, usually talc or glass fibre, are the standard corrections.
- Flammability: with a limiting oxygen index around 17, PP burns readily in air. Flame-retardant grades exist for the applications that require a classification.
The Three Types of Polypropylene
The three commercial types are homopolymer, random copolymer and impact (heterophasic) copolymer. They are separated by comonomer — what is fed to which reactor.
| Type | What is polymerised | What it changes | Where it goes |
|---|---|---|---|
| Homopolymer | Propylene alone | Highest stiffness and melting point, weakest impact below room temperature | Raffia tape, BOPP film, fibre, general injection moulding |
| Random copolymer | Propylene with 0.1–8 mol% ethylene in the same reactor | Stays single-phase, so clarity rises and the melting point falls; softer | Clear containers, hot- and cold-water pipe |
| Impact copolymer | Homopolymer first, then an ethylene-propylene rubber phase in a gas-phase reactor | Low-temperature impact strength, paid for in stiffness and clarity | Crates, automotive parts, cold-store packaging |
PP compounded after the reactor with a bought-in elastomer is a separate product on a separate route.
What Is Polypropylene Plastic Used For
Polypropylene is converted on every commodity route — injection moulding, film, fibre, pipe and sheet — and each route takes a different polypropylene grade, set first by melt flow and then by type.
| Converting process | Typical products | Type usually specified |
|---|---|---|
| Injection moulding | Closures with living hinges, thin-wall containers, crates, automotive interior and under-bonnet parts | Homopolymer for thin-wall, impact copolymer for crates and automotive |
| Film | BOPP and CPP film for packaging and lamination | Homopolymer |
| Fibre and raffia | Woven sacks and FIBC, spunbond nonwovens, carpet yarn, rope | Homopolymer |
| Pipe | Hot- and cold-water pipe systems, specified by ISO 15874 | Random copolymer (PP-R) |
| Sheet and thermoforming | Corrugated sheet, fabricated tanks and ductwork, thermoformed pots and cups | Homopolymer or copolymer sheet grades |