Polypropylene (PP) is a rigid yet flexible thermoplastic which can be blow moulded, extruded and injection moulded.
It was developed in 1954 with increased use from 1976 when the initial patent ran out. It is now one of the most used materials.
One of the best known objects made of polypropylene is the Robin Day’s Polyprop chair. Polypropylene’s unique feature making it stand out from other materials is the ability for it to be formed into a living hinge. A living hinge is integral to the object and can be seen as a fold in the single piece of material. At the hinge the material is thinner than the rest of the object because below a certain thickness the molecules orientate themselves in the direction of flow, as opposed to a network of strands. Repeated flexing perpendicular to the orientation makes the bonds between the molecules stronger whereas other materials have a higher likelihood of breaking at this thin point.
As a thermoplastic material polypropylene can be recycled.