Polypropylene. Little did Karl Zeigler or Giulio Natta realise, 60 years ago, when they were developing a catalyst system to produce a useful thermoplastic from the inexpensive monomer, propylene, that their work would have such far reaching consequences.
The early propylene homopolymer had similar properties to polyethylene but with higher stiffness and a higher temperature working range. Soon it was discovered that particulate mineral fillers (eg talc) gave even higher stiffness, better elevated temperature performance, reduced mould shrinkage and better dimensional stability. Coupling agents ensured that glass fibre filled grades not only had high stiffness but also good strength properties.
The unique ability to form a ‘living hinge’ fascinated designers….and frustrated moulders. Nucleating agents were added to provide higher transparency, while biaxial orientation during processing gave thin film with high strength and high clarity, quickly displacing packaging film based on cellulosic thermoplastics. Uniaxial orientation led to polypropylene tape and fibre, replacing jute and other fibres in sacking, carpets and sports surfaces.
Incorporating ethylene as a comonomer yielded copolymers, both block and random, which greatly improved low temperature impact resistance, the Achilles heel of the polypropylene homopolymer. Higher ethylene content in the copolymers led to a range of thermoplastic elastomers. These had better chemical resistance than the styrene based TPEs.
Homopolymer grades are favoured by the film and fibre sectors while copolymer grades are used more in injection moulding. Today, thermoplastics based on propylene can be considered the workhorses of the plastics industry. However, one wonders why propylene copolymers with higher olefins and more polar monomers have not been more developed.