PP homopolymer or copolymer: which type fits your application?

Three types, one family

Polypropylene comes in three basic types, which producers describe as homopolymer, random copolymer and impact (heterophasic) copolymer; ISO 19069-1 is the designation system and basis for specifications of PP moulding and extrusion materials. The type names describe the structure of the material — and the structure sets tendencies, not guaranteed values. The actual properties depend on the grade and its data sheet.

Homopolymer (PP-H) is made from propylene alone. Among comparable grades it is usually the stiffest; its impact strength at low temperature is usually lower than that of impact copolymers.

Random copolymer (PP-R) contains a small amount of a comonomer (most often ethylene), which disrupts the crystallinity of the polymer. In return it is usually the clearest and the most flexible of the three types, with good gloss — hence its popularity in packaging, where appearance counts.

Impact copolymer, heterophasic (PP-B) — in the trade also called “block copolymer”, “PP-BC” or simply “impact” — is a two-phase system: ethylene-propylene rubber (EPR) in the form of microscopic nodules dispersed in a PP matrix. Thanks to that it withstands impacts much better, at room temperature and in the cold, at the cost of some stiffness and clarity; as a rule it is cloudy or opaque. The name “block copolymer” is a common misnomer: the material is a heterophasic system, not a classic block copolymer in the chemical sense. A supplier who writes “PP copolymer” without a qualifier has not yet told you whether random or impact is meant.

What it means for the part

• Stiffness and temperature — more often the domain of the homopolymer. Parts that have to be stiff and work in the warm are usually started from PP-H or from a compound based on it.
• Impact strength, especially in the cold — more often the domain of the impact copolymer: housings, containers used outdoors, parts exposed to impacts.
• Clarity and gloss — more often the domain of the random copolymer: packaging, containers, products in which the contents are meant to be visible.
• Integral hinges — polypropylene is known for them, but the life of a hinge depends on the type, the grade, orientation and design; that is a subject for a trial, not an assumption.

What the abbreviation “PP” hides

• The PP type is often left out of enquiries — yet it is the type that sets the balance between stiffness and impact strength. The MFR alone will not replace that information; see MFR / MFI: what it means — and what it does not tell you.
• Fillers and reinforcements (talc, calcium carbonate, glass fibre) are added on top of the base type. “PP with talc” can be a homopolymer or a copolymer — the difference in impact strength remains.
• Stabilisation: black does not automatically mean UV-stabilised. Resistance to outdoor conditions has to be confirmed for the specific grade and its documentation.
• PP regranulates from mixed streams are often a mixture of types; their behaviour is assessed by a trial, not by analogy with a virgin grade — see PP and PE recycled pellets: what to check before buying.

Typical mistakes

1. Choosing by price instead of by type. A homopolymer can be cheaper, but in a part exposed to impacts in the cold it may crack. The cost of a complaint outweighs the price difference.
2. Expecting clarity from an impact copolymer. The rubber phase scatters light — for transparent products the random copolymer is usually the choice.
3. Switching type without a trial. Even at a similar flow rate, PP-B and PP-H are different grades: they can differ in stiffness and impact strength, among other things. According to LyondellBasell, non-nucleated PP types shrink roughly the same, but a different grade may be nucleated differently or carry a different pigment, and that can shift a dimension. A switch is checked in a trial — as part of the qualification.
4. Judging impact strength “from the data sheet” without the conditions. Impact strength depends on the temperature, the test method and the notch (e.g. Charpy to ISO 179-1); compare only values measured in the same way.

Example: a housing of a device used outdoors

Requirements: stiff, but it must not crack on impact in frost; black; matt surface; outdoor use. The starting point is rather an impact copolymer (impact strength in the cold), possibly with a mineral filler for stiffness and dimensional stability, in a grade with confirmed UV stabilisation — the black colour alone does not provide it. Black also opens the way to a regranulate from a well-described post-industrial stream — after a trial. An unmodified homopolymer would be risky here despite its higher stiffness.

What the data do not settle: qualification in your application

The type narrows the choice; it does not finish it. Values in a data sheet are typical values measured on standard specimens, not acceptance limits for your part. Whether a grade is suitable is confirmed by a sample or a trial in your production — with your mould, your cycle and your requirements — and, where your customer requires it, by their approval. Availability and the terms of a specific lot are confirmed with the supplier for each enquiry.

How this translates into an enquiry

Instead of “PP MFI 20”, write what you make, whether the part has to be stiff or impact-resistant, at what temperature it works, whether it has to be transparent and what colour it is. Those answers let us narrow the PP type and the form of material right away. What to send us for PP is also listed on the page Polypropylene — PP / rPP.

Limits of this guide

• The descriptions of the three types are tendencies within comparable grades, taken from the designation standard and the producer’s descriptions of its resin types; they are not values for a specific grade. Check the data sheet of the grade you are offered.
• The guide gives no typical values of stiffness, impact strength, melting point or shrinkage and no MFR ranges for particular products — those follow from the grade, the process and a trial.
• SEMANAT does not carry out tests and does not produce compounds; a material or blend proposal is a starting point for trials.