PP Foam Extrusion: Why Foaming PP Is Harder Than PE or PS

Where the difficulty lies

Foaming requires a melt that holds together while bubbles expand. PP is a semi-crystalline linear polymer with a sharp melting point, low melt strength, and elongational viscosity that does not rise with strain — it lacks strain hardening — so bubbles rupture or coalesce easily.

LDPE has long-chain branching and PS has higher melt strength, making both far easier to foam.

Three routes

One, raise melt strength using high-melt-strength PP (HMS-PP) modified with long-chain branching — currently the most effective route.

Two, blending: PP with LDPE or EVA improves foaming somewhat, at the cost of losing part of PP's heat and stiffness advantage.

Three, process control: supercritical CO₂ or butane as blowing agent, with die temperature and pressure held within about ±3 °C.

Advantages and applications

PP foam resists heat well — above 120 °C — along with chemicals, and it is recyclable, outperforming PE and PS foams. It is used for automotive cushioning layers, air ducts, building insulation and packaging.

Note that foam properties depend heavily on cell density and closed-cell ratio, which is process know-how; products differ widely between makers.