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Polypropylene (PP)

Polypropylene is a low-cost, lightweight semi-crystalline plastic with outstanding chemical resistance and a top corrosion rating. At just 0.91 g/cm3 it is among the lightest plastics, with 35 MPa tensile and 100 percent elongation that give it fatigue resistance ideal for living hinges. It is soft and inexpensive, favoring chemical and forming uses over precision machining.

How Polypropylene (PP) machines

PP machines at 3.5/5 but is soft, semi-crystalline, and gummy, so it benefits from very sharp tools, positive rake, and high speeds to shear cleanly without smearing. It deflects under cutting force and stress-relaxes, making tight tolerances difficult. Most PP parts are molded rather than machined; machining suits prototypes and one-offs.

Manufacturing & processing

PP is primarily injection molded, exploiting its fatigue resistance for integral living hinges that flex repeatedly without breaking. It is readily heat- and hot-gas weldable for tanks and ducts, but resists adhesives without surface treatment. It cannot be solvent-bonded, so welding or mechanical fastening is the rule.

Typical applications

PP is used for chemical tanks, fittings, ductwork, fluid containers, and labware thanks to its broad chemical resistance, plus living-hinge enclosures, caps, and snap-closures. Its low cost and light weight suit high-volume consumer and packaging parts, and weldability makes it a staple for fabricated chemical-process equipment.

When to choose it

Choose PP for low-cost chemical resistance, living hinges, and weldable chemical-handling fabrications where strength demands are modest. If you need more stiffness or wear resistance, step up to acetal or nylon. Avoid PP where tight machined tolerances, adhesive bonding, or higher strength are required. It wins on chemistry, weight, and price.

Suitable surface finishes

Common finishes for Polypropylene (PP): bead blasting, powder coating. Use the finish selector →

FAQ

Why is PP used for living hinges?
Polypropylene's semi-crystalline structure and 100 percent elongation let a thin molded web flex hundreds of thousands of cycles without cracking. The hinge orients its molecules on first bend, building fatigue resistance. No other commodity plastic matches PP for integral, one-piece flexing closures molded directly into a part.
Can polypropylene be glued?
Not easily. PP's low surface energy rejects most adhesives without flame, plasma, or chemical surface treatment. The reliable joining methods are heat welding and hot-gas welding, widely used for chemical tanks and ductwork, or mechanical fastening. Design PP assemblies around welding or fasteners rather than bonding.
Is PP or HDPE better for a chemical tank?
Both resist most chemicals and weld well. PP handles higher temperatures, around 100 C versus HDPE's 80 C, and is stiffer. HDPE is tougher, more impact-resistant at low temperature, and slightly cheaper. Choose PP for hotter fluids and rigidity, HDPE for impact toughness and cold service.

Property values are typical/nominal for early guidance and vary by temper, grade, supplier and heat treatment. Confirm critical specs against a certified datasheet or with an mfgiq engineer.