Polypropylene is possibly one of the most recognizable plastics due to the trend, begun in the 1990s, for using it in all kinds of coloured, matt translucent products. It’s also one the easiest plastics to identify and remember: just think of the waxy feel of PP on the butterfly hinge on a toothpaste tube – PP is the best commodity plastic for withstanding repeated bending.
This, and its ability to be coloured are two of its key characteristics. It also performs well at high temperatures, but less well at low temperatures. It has good toughness, chemical resistance and the ability to create a built in ‘live hinge’, as on the toothpaste cap, which opens hundreds of times without shearing. It is, like other commodity plastics such as polyethylene and polystyrene, highly cost effective, which means it is used prolifically in all levels of consumer products, particularly in the billion plastic baskets that are sold every day at shops across the globe.
Polypropylene has similar properties to polyethylene, but a lower density and higher softening point of 160ºC (320ºF), compared with 100ºC (212ºF) for PE. It differs from PE in that it has only one main form; however, it is available in different grades, such as one that changes it from its naturally milky appearance into a version that is closer to crystal clear PET. Polypropylene is particularly good at receiving glass-fibre reinforcement to give it superior strength and rigidity, which makes it ideal for large-scale applications such as furniture.
Image: Chop2Pot by Joseph and Joseph
Key features
•Versatile processing
•Good high-temperature resistance
•Tough
•Inexpensive
•Blends well with other materials
•Excellent flex resistance
•Food grade
•Good chemical resistance
•Accepts filler and reinforcements
•Recyclable
Sources
Widely available from multiple global suppliers.
Cost
£1.55 ($2.45) per kg.
Sustainability issues
As one of the main commodity plastics, it occupies number 5 in the recycling triangle symbol and as such can be recycled where an effective recycling programme is in place.
Production
As a commodity plastic it can be widely processed through a variety of techniques including injection moulding, thermoforming and foaming extrusion. In extruded sheet form it can be die-cut, folded and creased. Moulded PP also readily accepts fillers and reinforcement such as minerals and glass for increased stiffness.
Typical applications
It is used for anything that displays a durable, live hinge, such as toothpaste tube lids, through to packaging for fast food that can be placed in a microwave.
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–Versatile, easy to work –Can withstand repeated bending –Tough and resistant –Colourfast –Recyclable |
–Poor UV resistance: additives required for external applications |