Polypropylene (PP)

Polypropylene was first polymerised in the 1950s by Giulio Natta at the Politecnico di Milano, using a catalyst system developed with Karl Ziegler. The pair shared the 1963 Nobel Prize in chemistry for the work, and polypropylene went on to become the second most produced plastic in the world. Today it is everywhere, from medical syringes and yoghurt tubs to chemical tanks, lab benches and the Tic Tac box.

Chemically, polypropylene is similar to polyethylene but with a methyl group hanging off every second carbon along the backbone. That small change makes a big difference. Polypropylene is stiffer than HDPE, has a higher melting point, and crucially, can be flexed back and forth at a thin section thousands of times without breaking. This last property is what makes the famous living hinge possible. A properly designed polypropylene hinge can survive a million open and close cycles without failure, which is why almost every flip top container and folding clamshell you have ever used is made from PP.

The density of polypropylene is around 0.90 g/cm3, which is the lowest of any common plastic. It floats high in water, which makes it useful for marine applications and adds a useful weight advantage in any application where every gram counts. The maximum continuous service temperature is around 100 C, which is meaningfully higher than HDPE and high enough for hot water plumbing, steam sterilisation, dishwasher service and hot food contact. This is why PP is the standard material for autoclavable medical containers and hospital equipment housings.

Polypropylene comes in two main forms. Homopolymer PP (PP-H) is the stiffer, more rigid version, used for chemical tanks, fume hoods and structural fabrications. Copolymer PP (PP-C) has small amounts of ethylene added to the polymer chain, which improves impact strength and low temperature performance at a small cost in stiffness. Copolymer is the better choice for parts that need to flex without cracking, particularly in cold weather. Both grades are available as standard sheet in Australia, typically supplied in natural (off white), white, grey and black.

Chemical resistance is one of polypropylene's headline properties. It handles a similar range of acids, bases and solvents to HDPE, and in some cases (notably concentrated sulfuric and nitric acids at moderate temperatures) it outperforms HDPE. This is why polypropylene is the standard material for laboratory benches, fume hoods, acid resist tanks and pickling line equipment. Food grade PP is also widely used for reusable food containers, beverage caps and surfaces that need to withstand hot wash cycles.

Like HDPE, polypropylene is heat weldable. Hot air welding and extrusion welding are the standard joining methods for tank and fume hood fabrication. Mechanical fasteners work well, and threaded inserts can be installed for repeated assembly. What polypropylene will not do is bond reliably with conventional adhesives and paints. The surface energy is too low. Specialist polyolefin primers can be applied to enable adhesive bonding, but for most jobs the design is better adjusted to use welding or mechanical fasteners.

The cost of polypropylene sits in a similar range to HDPE, generally cheaper than ABS or polycarbonate. PP-H and PP-C are priced similarly to each other, with PP-C usually carrying a small premium for the improved impact performance. Specialty grades such as natural pharmaceutical PP and electrostatic dissipative PP cost more.

Outdoor performance is a weakness in standard grades. Polypropylene is more UV sensitive than HDPE and will degrade in direct sunlight within months unless UV stabilisers are added during manufacture. UV stabilised PP is available and will last 5 to 10 years outdoors, but for long outdoor service in Australian conditions HDPE is generally the better and cheaper choice. PP also becomes brittle in cold weather, particularly below 0 C, which limits its use in cold climates compared to HDPE or polyethylene grades.

A special note on Coreflute. Coreflute is the trade name for twin wall corrugated polypropylene sheet, widely used in Australia for real estate signs, election signs, packaging boxes and temporary displays. It is the same base polymer as solid PP sheet but extruded as a hollow profile, giving a very light, stiff panel at a fraction of the cost of solid sheet. We supply Coreflute for signage applications and CNC route it to shape.

Environmentally, polypropylene is the resin code 5 in the kerbside recycling triangle and is one of the more widely recycled plastics. Clean offcuts and post consumer waste can be reprocessed into new sheet and pellet stock with little property loss. Polypropylene's combination of food safety, chemical resistance and heat tolerance makes it a strong candidate for long service applications.

When not to use polypropylene: outdoors in cold climates without UV stabiliser. In structural applications where impact toughness matters and HDPE or ABS would do a better job. Where solvent welding or paint adhesion is required, and acrylic, ABS or PVC would be more practical. And for any clear or transparent application, where the standard milky translucent appearance of PP rules it out.