Last updated: 2026-09-01 · 5 min read

PP Homopolymer vs PP Copolymer: Which Grade Fits Your Molded Part?

Polypropylene homopolymer (PP-H) is polymerized from propylene monomer alone — it produces a stiff, high-crystallinity resin with good chemical resistance and a higher heat-deflection temperature. Polypropylene copolymer (PP-C) incorporates ethylene comonomer into the polymer chain, which disrupts the crystalline structure and produces a tougher, more impact-resistant resin with lower stiffness and a lower heat-deflection temperature. The choice between the two depends on whether the part needs stiffness and heat resistance (PP-H) or impact resistance and clarity (PP-C).

Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

Key Facts

  • PP-H has a crystallinity of 50–60 % and a melting point of 160–168 °C. PP-C (random copolymer) has a crystallinity of 30–45 % and a melting point of 135–155 °C, depending on the ethylene content (typically 1–7 % by weight). The lower crystallinity of PP-C gives it better optical clarity, lower shrinkage, and higher impact strength at low temperatures, but lower stiffness and heat-deflection temperature.
  • The notched Izod impact strength of PP-C (5–15 J/m at 23 °C) is 2–5 times higher than PP-H (2–4 J/m at 23 °C). At sub-zero temperatures (-20 °C), the difference widens — PP-H becomes brittle (impact strength drops below 1 J/m), while PP-C retains usable toughness (3–8 J/m). For parts used in cold environments (automotive interiors in winter, refrigerator components, outdoor enclosures in cold climates), PP-C is the default choice.
  • PP-H has a higher shrinkage rate (1.0–2.5 %) than PP-C (0.8–1.8 %). The difference matters for dimensional control in multi-cavity molds — a PP-H part may require a larger mold cavity to compensate for the higher shrinkage, and the shrinkage variation between resin lots is wider for PP-H than for PP-C.

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    Property Comparison

    Property PP Homopolymer PP Random Copolymer
    Melting point 160–168 °C 135–155 °C
    Crystallinity 50–60 % 30–45 %
    Flexural modulus 1,300–1,800 MPa 800–1,200 MPa
    Notched Izod (23 °C) 2–4 J/m 5–15 J/m
    HDT @ 0.45 MPa 100–110 °C 75–90 °C
    Mold shrinkage 1.0–2.5 % 0.8–1.8 %
    Optical clarity translucent to opaque transparent to translucent
    Chemical resistance excellent (both) excellent (both)

    When PP Homopolymer Is the Right Choice

    1

    Parts requiring stiffness and rigidity

    PP-H has a flexural modulus 40–60 % higher than PP-C. For structural components that must resist bending under load — appliance housings, automotive battery trays, storage containers that stack — PP-H provides the stiffness without adding wall thickness or ribs.

    2

    Parts exposed to elevated temperatures

    The heat-deflection temperature (HDT) of PP-H is 15–25 °C higher than PP-C. For parts near heat sources — dishwasher components, microwave-safe containers, under-hood automotive parts (non-structural), hot-fill containers — PP-H retains its shape at temperatures where PP-C would deflect.

    3

    Living hinges and snap-fits

    PP-H is the default resin for living hinges (the thin, flexible section between two rigid sections of a molded part, such as a flip-top cap). The high crystallinity of PP-H gives the hinge section excellent fatigue resistance — it survives millions of flex cycles without cracking. PP-C is softer and more ductile, which makes it less suitable for hinges that need a crisp snap-back.

    When PP Copolymer Is the Right Choice

  • Parts used in cold environments — PP-C retains impact strength at sub-zero temperatures where PP-H becomes brittle.
    Refrigerator drawers, automotive interior trim in cold climates, outdoor enclosures in northern latitudes, and shipping containers that experience winter transit temperatures all specify PP-C for its low-temperature toughness.
  • Transparent or translucent parts — PP-C’s lower crystallinity produces a resin that transmits more light than PP-H.
    For containers, packaging, and consumer-product housings where visual clarity or translucency is a design requirement (without adding a clarifying agent), PP-C is the starting point.
  • Parts requiring drop-impact resistance — consumer electronics housings, protective cases, tote bins that are dropped during handling, and medical-device enclosures that must survive a drop from table height.
    PP-C’s higher notched Izod impact strength absorbs the impact energy without cracking.
  • Dimensional stability in multi-cavity molds — PP-C’s lower and more consistent shrinkage (0.8–1.8 % vs 1.0–2.5 % for PP-H) reduces cavity-to-cavity dimensional variation in high-cavitation molds.
    For programs with tight dimensional tolerances across 32 or 64 cavities, PP-C is easier to hold within tolerance.

    • Frequently Asked Questions

    What about PP impact copolymer — is it different from random copolymer?

    Yes.
    PP impact copolymer (also called heterophasic copolymer or block copolymer) is a different material from PP random copolymer. Impact copolymer has discrete rubber domains (ethylene-propylene rubber, EPR) dispersed in a PP-H matrix. The rubber domains absorb impact energy, producing impact strengths of 15–80 J/m (far higher than random copolymer). Impact copolymer is used for automotive bumpers, pails, and crates. It is opaque and has lower stiffness than PP-H.
    Random copolymer has ethylene units randomly distributed in the chain, producing lower but more balanced property changes.

    Can PP-H and PP-C be blended at the molding machine?

    In principle, PP-H and PP-C pellets can be blended at the hopper to produce an intermediate property set. In practice, blending at the machine introduces inconsistency — the blend ratio varies from shot to shot, and the resulting properties are not documented on the resin supplier’s data sheet. The correct approach is to select a grade from the resin supplier that already has the target balance of stiffness and impact (suppliers offer dozens of PP grades with intermediate properties) rather than blending at the machine.

    Does the mold need to change when switching from PP-H to PP-C?

    The mold cavity dimensions may need to change because the shrinkage rates differ. PP-H shrinks 1.0–2.5 %; PP-C shrinks 0.8–1.8 %. A mold cut for PP-H (with 1.5 % shrinkage compensation) will produce a slightly larger part in PP-C (which shrinks less). For non-critical dimensions, the difference is within tolerance. For critical dimensions, the mold may need to be recut or the part may need to be re-qualified. The processing parameters (melt temperature, mold temperature, hold pressure) also change between the two grades.

    Key Takeaways

    1. PP-H is stiffer, has a higher heat-deflection temperature, and is the default for living hinges and structural parts. PP-C is tougher, clearer, and retains impact strength at low temperatures.
    2. The shrinkage difference (PP-H 1.0–2.5 %, PP-C 0.8–1.8 %) affects mold design. A mold optimized for one grade may produce out-of-tolerance parts in the other. Switching grades after the mold is cut requires re-qualification of critical dimensions.
    3. The choice between PP-H and PP-C should be based on the most demanding property requirement — if stiffness dominates, choose PP-H; if impact resistance dominates, choose PP-C. Impact copolymer (heterophasic) is a third option for extreme toughness requirements.

    A tooling engineer who knows which PP grade the part will run before the mold is designed avoids the most common source of dimensional surprises at T1: a mold cut for one shrinkage rate running a resin with a different one. The resin grade should be specified on the drawing, confirmed in the DFM, and locked before the cavity is cut — not selected after T1 based on what was in the hopper.

    Sources
    LyondellBasell — polypropylene grade selection
    Borealis — PP material data
    Plastics Technology — resin processing guides

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet