Water-based process turns plastic waste into organic acids

Researchers have developed a catalyst-free process using water, heating, and stirring to convert plastic waste, including PE and PP, into organic acids that could be used as building blocks in the chemical industry. 

A catalyst-free method is noteworthy as many conventional chemical recycling processes rely on catalysts that can be poisoned or deactivated by additives and contaminants found in plastic waste. 

Researchers from Zhejiang University, China, Cardiff University in the UK, and the University of Tokyo in Japan mixed PE waste from low-density PE bags, high-density PE caps, and mixed PE with water in a stirred pressure reactor at 125 degrees Celsius under 2 MPa oxygen. 

Heating and stirring melted and dispersed the plastic into microdroplets, creating a large interface between the molten plastic and water. At this interface, reactive hydroxyl radicals acted as “chemical scissors,” breaking strong polymer chains and converting the plastic into short-chain organic acids.

Packaging Insights speaks to Yong Wang, corresponding author from Zhejiang University, and Graham Hutchings, corresponding author from Cardiff University, to learn more about the method and its implications for the plastic packaging industry. 

“Flexible and multilayer packaging is difficult to mechanically recycle because it often contains mixed polymers, additives, pigments, fillers, or metalized layers,” says professor Wang. “Our method shows tolerance to common additives and inorganic impurities, and can process commercial PE bags, mixed polyolefins, and multilayer packaging films.”

He adds that in the method, organic polymer fractions can be converted into acid products, while inorganic or metal-rich residues can be separated into solids. 

“This suggests a potential route for packaging waste streams that are currently low-value or difficult to recycle mechanically.”

The researchers stress that plastic waste poses a “global threat” to the environment and public health. They highlight that while chemical recycling “holds promise,” they suggest that its industrial adoption is limited by additive-induced catalyst deactivation, feedstock heterogeneity, process inflexibility, and limited economic viability. 

Catalyst-free chemical recycling 

Many chemical recycling processes use metal or acid catalysts to help break polymer chains. However, catalysts can often be poisoned or deactivated by additives or contaminants found in mixed plastics, disrupting the process and reducing successful conversion. 

Professor Hutchings adds: “Because the process does not use external catalysts or organic solvents, it can avoid catalyst poisoning, catalyst regeneration, and some downstream complexity.”

The study suggests a possible chemical-recycling route for dirty, mixed, or additive-containing plastics without a catalyst such as metal or acid, reducing sensitivity to additives and contaminants and broadening the type of plastic waste that can be treated. 

The experiment also converted PP, polystyrene, multilayer packaging, and rubber tires using variations of this method. 

Additionally, “compared with conventional pyrolysis, which often requires temperatures above 400 degrees Celsius and produces complex hydrocarbon mixtures, our PE model reaction operates at 125 degrees Celsius under 2 MPa oxygen and converts plastic mainly into short-chain dicarboxylic acids,” says Hutchings. 

Scaling outside the lab 

The research, published in Nature, found that a five-liter reactor containing 300 g of PE achieved 89% conversion and more than 52% saturated-diacid yield after 48 hours. The researchers suggest that this indicates that the reaction can be scaled beyond gram-scale experiments. 

“The main challenge is not simply increasing reactor volume, but maintaining efficient and stable water–molten plastic interfacial contact at a larger scale,” adds Wang. 

In the researchers’ current system, molten plastics are dispersed in water to form microdroplet interfaces, where reactive species are generated in situ. 

“For scale-up, reactor engineering will be crucial, including mixing, oxygen transfer, heat management, continuous feeding, product separation, and long-term operational stability,” he adds. 

Commercial adoption

There are several steps needed before the method can be commercially viable, according to Hutchings. 

“First, the process must be validated using real packaging waste streams with variable composition and contamination levels. Second, continuous or semi-continuous reactor operation needs to be demonstrated beyond laboratory scale. Third, product separation, water recycling, oxygen management, and safety protocols must be optimized.” 

Finally, Wang tells us that the recovered chemicals need to meet downstream quality requirements and fit into existing chemical supply chains. 

He concludes: “We see this technology as complementary to mechanical recycling: clean, single-polymer waste should still be mechanically recycled where possible, while complex or contaminated polyolefin packaging could be directed to chemical upcycling.”