Review on the pyrolysis products and thermal decomposition mechanisms of polyurethanes

Polyurethanes (PUs) are an important and versatile class of polymers that are applied in industry, building construction and in various consumer products. However, the rigid and complex structure of PU makes it challenging to deal with PU at end-of-life, while without appropriate waste treatment, disposed PU products may result in severe environmental pollution. Thermochemical recycling processes allow the production of valuable chemicals from PU waste streams and may enable circular recycling routes. This review focuses on pyrolysis of PUs studied by thermogravimetric methods as well as on experiments conducted in micro or bench-scale lab reactors, that allow to study the product distribution and the thermal decomposition chemistry relevant for pyrolysis and gasification processes. PUs typically decompose in multiple stages as function of the temperature, whereby generally the isocyanate linkers disassociate at lower temperatures and the high molecular weight polyol linkers decompose at higher temperatures. The product distributions were found to strongly depend on the composition of PU and the pyrolysis conditions, while the presence of fire retardants and other additives did hardly have an effect. By further development and optimization, it is expected that pyrolysis and gasification can be suitable alternatives to deal with growing amounts of PU waste.