Heat, chemistry turn plastics back into monomers

By Karen Hanna 

Heat and chemicals are the most common catalysts for processes that can return end-of-life plastics either directly or indirectly to the manufacturing stream as monomers. Such approaches can break down plastics that would jam or contaminate mechanical recycling streams, as well as plastics that simply can’t be recycled using mechanical means.

Suboptimal plastics are the target feedstock for advanced recyclers. 

“If you actually have a technology like ours, where you can, for instance, take all the flexibles out of household waste, you actually can make a much lighter sorting. ... I'll just take out the PET bottles, and I’ll take out my shampoo bottles and my food trays, which is also PET, and all the rest goes in mine. So, I save a lot on the sorting. I make sure that whatever I sort is actually quite valuable, and the whole rest goes into my process,” said Eric Appelman, chief revenue officer for Aduro Clean Technologies, an advanced recycling technology company based in London, Ontario. 

Generally, advanced recycling approaches for PE and PP use heat, according to an article on the topic that was recently submitted to Plastics Machinery & Manufacturing by ICIS, a market pricing and analysis firm for the chemical and oil industries. Such materials are ofter heavily contaminated and difficult to sort.

Meanwhile, chemical depolymerization, which uses chemicals to produce monomers by unzipping polymers, is generally used to break down PET, especially colored or textured PET that is inappropriate for mechanical recycling.

Currently, plants using thermal processes have much lower capacity than those using chemical processes, according to the ICIS writers — Andrea Bassetti, Americas team lead, plastics recycling; Corbin Olson, senior analyst, plastics recycling; and Joshua Dill, analyst, plastics recycling. 

According to statistics available on the America's Plastic Makers website of the American Chemistry Council Inc., research has shown that advanced recycling could reduce fossil energy use by 97 percent compared to landfilling, and pyrolysis emits 50 percent less carbon dioxide than incineration. Manufacturing low-density PE (LDPE) derived from pyrolysis also results in significantly less emissions than production involving virgin LDPE. 

But because of the nature of the processes, scale-up takes time.  

“We are doing it fast, but it's a chemical process. You do want to be safe, you do not want to blow up, you do not want to poison the environment,” said Appelman, who stressed the safety practices within the chemicals industry. 

“We sometimes forget that the chemical industry is actually one of the safest industries in terms of injuries.” 

He described his company’s technology as a mix of heat and chemistry. At the high temperatures required for pyrolysis, recyclers are stuck with many components of plastics chemistry that require costly further processing.

 In contrast, Aduro’s process combines relatively modest temperature and catalysis and water as a coreactant, unlocking a different chemical pathway than pyrolysis.

“It is really waste plastic. It is really dirty. There’s a lot of contaminants in there. There are many different plastics, polyethylene, polypropylene, PET, whatever. Within those categories, you have variations, like LDPE and HDPE, but within that, you have even more variations with different melt flow grades, which make the whole mechanical recycling extremely challenging,” Appelman said. “Now, for our kind of processes and for pyrolysis, polyethylene, polypropylene, it is all sort of the same.”