PP is a resin used across plastics processing — mainly injection molding and extrusion. This page covers what it actually does and the grade properties that matter when buying it.
What is it?
PP is classified under ASTM D4101 into three categories: homopolymer, copolymer, and elastomer-modified grades. Its density — 0.900–0.910 g/cm³ — is the lowest among commonly used commodity plastics. Melting/crystallization behavior for semicrystalline polymers like PP is measured using differential scanning calorimetry, per ASTM D3418.
Why should I use it?
- The lightest common commodity plastic, which matters directly for material cost and part weight in molded products.
- A defined battery of grade-qualifying tests, including one most buyers don’t think to ask about — see Limitations.
How it’s used
In Plastics & Polymers, PP is the base resin for injection-molded and extruded products, tested under ASTM D4101’s property battery — flow rate, tensile stress at yield, flexural modulus, Izod impact resistance, and heat deflection temperature.
Limitations — when it isn’t the right choice
- Low-temperature impact performance is a real consideration, not a given. ASTM D4101 specifically tests a multiaxial impact ductile-brittle transition temperature — the point at which a grade shifts from ductile to brittle failure under impact — precisely because this varies by grade and matters for outdoor or cold-chain applications. It’s a property to check per grade, not something to assume.