High-Temperature Chemistry of Polypropylene Pyrolysis: Millisecond Reaction Kinetics and Visualization

Abstract

The ubiquity of plastics in modern life is evident by the rapid and continual growth of global plastic production. Polypropylene is one of the most widely produced and used plastic materials, accounting for approximately 20% of global polymer production. Billions of tons of plastic waste have been produced as a byproduct of the widespread use of plastics and are insufficiently managed under the current linear plastic economy, with the majority of plastic waste accumulating in landfills or the environment. To allow for the continued use of plastics in a sustainable fashion, a transition must be made towards a circular plastic economy, wherein end-of-life plastics are recycled in a closed loop, fully regenerating the original polymers. To realize a circular plastic economy, new recycling techniques must be developed. Pyrolysis, the thermal conversion of a material in an inert atmosphere, is a high-potential technology to help enable a circular plastic economy. Currently, the fundamental understanding of plastic pyrolysis is limited but will be essential for the development of industrially relevant waste management solutions. The quantification of the intrinsic reaction kinetics of plastic pyrolysis is an ongoing challenge, owing to the complexity of pyrolysis chemistry and the limitations of existing analytical techniques. In this work, a new Pulse-Heated Analysis of Solid Reactions (PHASR) technique was developed that is uniquely capable of operation under reaction-controlled conditions absent transport limitations to measure the millisecond intrinsic kinetics of polyolefin pyrolysis. The capabilities of this reactor to pyrolyze polyolefins under kinetically limited, isothermal conditions with millisecond scale control were extensively validated. A second, Visual PHASR reactor system was developed that enables in situ observation of reaction polyolefins via high-speed photography. Observations of reacting polyolefins revealed the presence of reaction phenomena, including a potential Leidenfrost effect. The intrinsic millisecond reaction kinetics of polypropylene pyrolysis were successfully quantified. The overall reaction kinetics were described by a lumped first-order consumption model with an activation energy of 242.0 ± 2.9 kJ mol-1 and a pre-exponential factor of 35.5 ± 0.6 ln(s-1). Additionally, the production of the solid residues formed during polypropylene pyrolysis was investigated, revealing a secondary kinetic regime.