High impact PP is polypropylene toughened by a rubber phase: particles of ethylene-propylene rubber dispersed through a polypropylene matrix, either formed in the polymerisation reactor or blended in afterwards. The rubber particles make it much tougher than homopolymer, in the cold as well, at some cost in stiffness.
Datasheets call it impact copolymer, heterophasic copolymer (HECO) or, under an older name some still print, block copolymer. The ISO 1043-1 abbreviation is PP-HI, preferred over the common HIPP. The similar abbreviation HIPS belongs to a different polymer, high-impact polystyrene.
How High Impact PP Is Made
High impact PP is made in one of two ways: the rubber is polymerised together with the polypropylene in the reactor, or it is compounded into finished polypropylene resin in an extruder.
There is no single formula, because high impact PP is a mixture of phases. A published fractionation of one commercial impact grade found it about three-quarters isotactic polypropylene, with the rest ethylene-propylene rubber and semicrystalline ethylene-propylene copolymers.
Impact Copolymer Made in the Reactor
A reactor impact copolymer is made in two stages, in reactors in series, on a Ziegler–Natta catalyst. The first stage makes the polypropylene matrix, usually homopolymer; the powder then passes to a second stage, typically a gas-phase reactor, where ethylene and propylene are copolymerised into rubber within the same powder.
A random copolymer also contains ethylene, but built into the polypropylene chains in a single phase.
A general-purpose reactor grade such as K8003 goes into household articles, industrial containers and battery cases, and high-flow grades such as K9928H into automotive compounds and appliance housings; low-flow grades are also extruded into pipe.
High Impact PP Made by Compounding
Compounded high impact PP is made by melt-blending polypropylene with an elastomer in a twin-screw extruder. The traditional elastomers are ethylene-propylene rubber and EPDM. Polyolefin elastomers (POE) such as ethylene-octene copolymers have increasingly replaced them.
The compounder sets the rubber level by how much elastomer goes in, and can add mineral filler and colour in the same pass. Talc- and calcium-carbonate-filled impact grades are made this way. Compounded impact PP is widely used for car bumpers and interior trim.
A datasheet does not always say which route a grade took: ISO 19069-1 puts both routes in one impact class, so a PP-B designation under it, or a description that says only “high impact”, leaves it open. A description such as “reactor impact copolymer”, or a name that joins polymers with a plus sign such as PP+EPDM, does say.
Properties of High Impact PP
Against a homopolymer of similar melt flow, high impact PP has several times the notched impact strength at 23 °C and keeps useful impact strength at −20 °C, a temperature at which homopolymer datasheets typically report none. The trade-off is lower stiffness and yield strength; the melting point is the same.
The table sets one producer’s homopolymer and impact copolymer side by side, TotalEnergies PPH 3060 and PPC 3660, at melt flows of 1.8 and 1.3 g/10 min.
| Property | Test method | PPH 3060 (homopolymer) | PPC 3660 (impact copolymer) |
|---|---|---|---|
| Notched Izod impact, 23 °C (kJ/m²) | ISO 180 | 6 | >50 |
| Notched Izod impact, −20 °C (kJ/m²) | ISO 180 | not reported | 7 |
| Flexural modulus (MPa) | ISO 178 | 1300 | 1200 |
| Tensile strength at yield (MPa) | ISO 527-2 | 35 | 24 |
| Melting point (°C) | ISO 3146 | 165 | 165 |
| Heat deflection temperature, 0.45 MPa (°C) | ISO 75 | 100 | 90 |
Heat deflection temperature has no fixed direction: it is lower than the homopolymer’s in the table’s pair and higher in another matched pair from the same producer.
Standard impact copolymer is also much hazier than homopolymer.
TotalEnergies’ impact grades run from 0.3 to 130 g/10 min, and their notched Izod (ISO 180, 23 °C) falls from above 80 kJ/m² at the low-flow end to 4.5 at the high-flow end.
An impact figure compares only with one measured by the same method at the same temperature. ISO 180 reports notched Izod in kJ/m² and ASTM D256 in J/m or ft-lb/in, and ASTM’s own text says D256 resembles ISO 180 in title only, so converting units does not make the two comparable. Charpy (ISO 179) is a third test with its own numbers.
An entry such as “NB” means the specimens did not break, and it cannot be ranked against a value from specimens that broke. Check the method, its specimen type and the temperature before setting one supplier’s figure against another’s, or a certificate of analysis against the datasheet.