Depolymerization study of PP and PET to solve “plastic waste problem”

04 May 2023 --- A recent study has revealed a catalyst-free, far-from-equilibrium thermochemical depolymerization method that can generate monomers from commodity plastics PP and PET through pyrolysis. The study describes depolymerization as a promising strategy for recycling waste plastic into constituent monomers for subsequent repolymerization.

The research successfully depolymerized PP and PET to their monomers with yields of about 36% and about 43%, respectively. 

However, many commodity plastics cannot be selectively depolymerized using conventional thermochemical approaches, which are difficult to control. Usually, catalysts can improve selectivity but are susceptible to performance degradation. However, this study takes out the need for a catalyst. 

“The approach reported in this paper offers a potential solution to the plastic waste problem. Specifically, the approach can be applied to recycle a range of polyolefin-based plastics to their constituent monomers that can be used for subsequent repolymerization, potentially closing the loop in plastic usage of the packaging industry,” Qi Dong, an assistant research scientist at the University of Maryland, US, and one of the study authors tells PackagingInsights. 

The study is published in Nature. 

Researchers peg depolymerization as successful way to reduce plastic waste.Non-catalyst depolymerization
The depolymerization method was realized by two features, a spatial temperature heating (STH) gradient and a temporal heating profile. 

“The approach is based on electrified heating, which offers better tunability and thus improved performance compared with conventional thermochemical methods for recycling plastic wastes,” says Dong.

“It could also be integrated with commercially available tools for continuous operation using renewable energy sources for higher energy efficiency and reduced CO2 emissions. In addition, the approach holds good potential for converting a range of polymeric wastes into value-added chemicals.”

The scientists use a temperature gradient that promotes continuous melting, wicking, vaporization and reaction of the plastic as it encounters the increasing temperature traversing the bilayer, enabling a high degree of depolymerization. 

Meanwhile, a pulsing electrical current is put through the top heater layer generating a temporal heating profile that features periodic high peak temperatures to enable depolymerization. The heating duration can suppress unwanted side reactions.

Reduced CO2 emissions
The electrified STH approach could be further integrated with commercially available tools for continuous operation using renewable energy sources for higher energy efficiency and reduced CO2 emissions, reveals the research. 

Dong explains what makes depolymerization a better solution compared to other plasticing recycling options. “Selective depolymerization to monomers holds great economic incentive, as the generated monomers can be reused in subsequent polymerization reactions for another product life cycle or as chemical feedstocks for other processes in the chemical industry.”

The study continues that the electrified STH approach holds great potential for converting a range of plastics, biomass and other supramolecules toward the sustainable, energy-efficient and scalable manufacturing of value-added chemicals. Study uses depolymerization without catalyst to recycle PP and PET.

“Overall, this electrified STH approach potentially offers a solution to the global plastic waste problem,” writes the study. 

The researchers say that common but highly unsustainable methods of removing plastic waste include burning or landfilling, with the former being extraordinarily carbon-heavy and air-polluting. In contrast, the latter can cause long-term damage to the environment.

Alternatively, mechanical processing can recycle many thermoplastics into low-end but functional materials. However, mechanical plastic-recycling approaches often suffer from limited product value. With this, the scientists aim to create a more sustainable way to recycle plastics. 

Ramping up depolymerization
Companies have been investigating and investing in depolymerization as a means to recycle plastic as it has proven promising as opposed to chemical and mechanical recycling. 

Similarly, plant biology researchers at the University of York, UK, developed a biologically-bound nickel catalyst (ni-phytocat) to accelerate the depolymerization of PE.

Last month, Citeo signed a nine-year contract with Paprec to chemically recycle packaging by depolymerization technology to allow reuse for contact with food. 

Loop Industries owns patented and proprietary technology that depolymerizes no- and low-value waste PET plastic and polyester fiber, including plastic bottles and packaging, carpets and textiles of any color, transparency or condition. It can also recycle ocean plastics degraded by the sun and salt from its base building blocks. 

By Sabine Waldeck