Polypropylene (PP), chemical formula (C3H6) n is an oil-derived polymer and is the world’s second most widely used commodity polymer (after polyethylene), accounting for 13% of total usage by value and 23% by volume. Polypropylene has many applications, both as a plastic and fibres, and is found in almost all of the plastic markets.
Polypropylene was first discovered by J. Paul Hogan and Robert L. Banks of Phillips Petroleum Company in 1951, while attempting to convert propylene into gasoline. In 1954, Guilio Natta produced PP using a transition metal catalyst and a metal alkyl co-catalyst of the type developed by Karl Ziegler for the polyethylene industry. In recognition of the work, catalysts involving transition metals and metal alkyl co-catalysts are known as Ziegler-Natta catalysts and the two men shared the Nobel Prize in chemistry in 1963 for their work. In fact, the vast majority of commercially produced PP utilises modern versions of the Ziegler-Natta catalysts. Commercial production of polypropylene commenced in 1957 and has continued to grow ever since, with an estimated 75 million tons produced by 2020. The demand of PP is still increasing, however, with the issues surrounding the availability of the diminishing feedstock (fossil fuels) there is pressure for alternative PP production routes.
Polypropylene has many advantageous properties, including: low cost, light weight, flexibility, high tensile strength, high melting point and high impact, and fatigue resistance, which makes it extremely attractive to a number of applications. PP can be processed into plastics via injection molding, blow molding and general purpose extrusion, and processed into fibres via melt spinning. PP is used in a range of applications, including: flexible packaging, rigid packaging, tubing, automotive interiors, consumer products (luggage, toys, appliances, etc.), clothing, carpets, fabrics, and medical sutures and mesh. The identification code for PP is 5.
While PP is widely utilised and often used as a single-use plastic, it is one of the least recycled post-consumer plastics in the world, with the majority ending up in landfills. As PP products take around 20-30 years to fully decompose, they pose a severe threat to the environment. The problem we face is that the properties that make PP so fit for purpose also make it extremely difficult to recycle. For example, the high melting point of PP means that to burn end-of-life PP products requires an extremely high energy input and generates CO2 in a precisely analogous manner to burning oil. Burning large-scale amounts of PP is just not energy-efficient or environmentally friendly. Also, as an oil-derived polymer, PP is a non-sustainable resource, and burning it only necessitates further consumption of fossil fuels to replace it.
There are some recycling technologies for PP which are primarily mechanical and/or thermal. However, these technologies result in a ‘down-cycled’ product that still contains contaminants and degraded material from the original product. Also, in most cases, the recycled PP has to be mixed with virgin PP up to 50% to achieve the desired properties.
It is clear that if the world wants to continue using PP, a cost-effective and eco-friendly method to recycle and sustainably produce PP is required.