Two impact copolymer quotes sit on the desk. One reports notched Izod at 115.1 J/m, the other a Charpy floor of 40 kJ/m². Dividing an energy by a length and dividing it by an area do not produce the same quantity, so no arithmetic turns one figure into the other.
The grade still has to fill a long flow path, hold shape under its own weight after ejection, and survive whatever temperature the assembled part sees in service. Melt flow rate, the stiffness-impact balance, and the test method behind the impact number are the datasheet properties that decide the grade, read in that order.
What Melt Flow Rate Does an Impact Copolymer Need for Thick Sections?
An impact copolymer part’s flow-length-to-wall-thickness ratio sets the melt flow band the resin must provide. Unfilled polypropylene runs to about 250:1 as a planning guideline. A large fascia at a 3 mm wall section fed from one gate can demand more flow than a smaller part at half that thickness.
MFR is measured under ISO 1133 or ASTM D1238 at a stated condition (230 C and 2.16 kg for polypropylene). A different test load on the same resin gives a different number.
PetroChina Dushanzi PP K9928H sits at MFR 28.8 g/10 min and is positioned for wall sections above 3 mm (barrel 160-240 C, melt 190-220 C). PetroChina Lanzhou PP EP531N runs 20-25 g/10 min and targets bumper fascias, door-trim substrates, and instrument-panel carriers. Thin-wall packaging grades push MFR above 50 g/10 min, a different design envelope.
Buying more flow generally costs impact performance. In ExxonMobil’s 2017 Asia Pacific ICP slate, notched Izod at 23 C dropped from no-break at MFR 4 g/10 min to 72 J/m at MFR 30. One nucleated grade at MFR 50 broke the slope at 94 J/m, which is why grade design, not MFR alone, sets impact.
The mechanism is molecular weight: visbreaking shortens the chains that carry impact energy. An impact copolymer that holds toughness at high flow is a formulation achievement, not a dial setting.
How Impact Copolymer Stiffness and Toughness Trade Off
Flexural modulus (ISO 178) and notched impact sit on opposite ends of a design lever inside the grade, not on a fixed quality ladder. Two ExxonMobil grades at the same MFR of 30 g/10 min in the 2017 slate illustrate this: one reads 993 MPa flexural modulus with no-break Izod, the other 1 310 MPa with 72 J/m.
That lever is what makes the gap between K9928H at 1.32 GPa flexural modulus and 23.6 MPa tensile yield, against PP EP531N at a floor of 800 MPa, meaningful rather than a quality ranking. K9928H’s figures are measured typical values. EP531N’s are guaranteed minimums under its internal producer standard Q/SY LS0305-2017.
ExxonMobil states the convention on the face of its own grade slate: “Values given are typical and should not be interpreted as specifications.” Ranking a typical against a floor compares a measurement with a promise.
Why Two Polypropylene Impact Figures May Not Be Comparable
ASTM D256 and ISO 179 divide absorbed impact energy by different specimen dimensions and report different physical quantities. An Izod figure in J/m and a Charpy figure in kJ/m² cannot be ranked against each other.
The Two Scales
ASTM D256 divides absorbed energy by the specimen thickness and reports J/m on a 64 x 12.7 x 3.2 mm bar. ISO 179 refers absorbed energy to the cross-sectional area at the notch and reports kJ/m². The two numbers are not merely in different units; they measure different things.
ISO 179-1:2023 states in its principle clause that results from specimens of different dimensions or notches are not comparable and that the method is not intended as a source of data for design calculations. ISO 10350-1:2017 exists because datasheet data from different sources are not necessarily comparable even when the same standard test was used.
K9928H publishes notched Izod at 115.1 J/m (23 C) and 63.5 J/m (-20 C), measured under PetroChina’s internal standard Q/SY DS 0517. EP531N publishes a Charpy impact floor of 40 kJ/m² at 23 C under Q/SY LS0305-2017. The two figures sit on different scales, against different specimen geometries, and are different kinds of promise.
The Sub-Zero Gate
Only K9928H publishes a temperature below ambient in its impact column. Automotive specifications typically call a notched Charpy figure at -30 C per ISO 179/1eA, a designation that encodes specimen type, blow direction, and notch shape. Producers do publish sub-zero impact, but often on a different scale, such as Gardner falling-dart impact in joules.
If a datasheet is silent below room temperature, the buyer must request the test at the temperature and geometry the part actually sees. A 23 C figure alone does not predict where the brittle-ductile transition sits.
What an Automotive Part Adds Beyond Mechanics
Cabin odor and VOC emissions are specified requirements on automotive impact copolymer polypropylene grades, not optional extras. EP531N is marketed as a low-odor grade for exactly this reason.
Color and cleanliness figures decide whether a part can be molded in a light color without post-processing. K9928H publishes a yellow index of -3.6 and ash of 0.0278%, both tightening the allowable pigment window.
EP531N specifies pellet size at 2-5 mm with a defect count of 10 pcs/kg or fewer per SH/T 1541-2006. On a large shot, a single degraded pellet marks the surface.
What an Impact Copolymer TDS Must Show Before You Order
The TDS must name the producer, the plant, the test method and condition behind every figure, and whether each number is a typical value or a specification limit. A grade code alone does not pin the material.
K9928H is produced by PetroChina at the Dushanzi plant on an Innovene gas-phase horizontal stirred-bed line. EP531N is produced at the Lanzhou plant on a Spheripol-II dual-loop plus single gas-phase line. Same parent company, different processes, different internal standards.
Before releasing a container-load order, demand:
- The producer AND the plant, not just the brand
- Whether each published figure is a typical value or a specification limit
- The test method, specimen geometry, notch type, and test temperature behind every impact number
- The internal producer standard the supplier is contractually bound to (Q/SY DS112-2017 for K9928H, Q/SY LS0305-2017 for EP531N)
The COA gives you four numbers; the spec sheet gives you the window; the application tells you which corner of the window you actually need.
Where Datasheet Comparisons Go Wrong
The common mistake is reading two impact figures as though they sit on the same scale. A buyer comparing 115.1 J/m against 40 kJ/m² is comparing a measurement to a guarantee, across two specimen geometries, under two producer standards, at one temperature only.
The grade that prints the larger number can be the weaker material at the temperature the part actually has to survive.
Start with melt flow rate and wall section. Confirm the stiffness-impact balance against the part’s structural duty. Then read the test method and the temperature behind every impact number before ranking anything.
A number without its test condition and method is not a specification.