Mechanical method uses collisions to break down plastic for sustainable recycling

While plastic helps provide modern living standards, its accumulation in landfills and the environment continues to become a global problem.

Polyethylene terephthalate (PET) is one of the world's most widely used plastics, with tens of millions of tons produced annually in bottles, food packaging and clothing fibers. The durability that makes PET so useful also means that it is more difficult to recycle effectively.

Now researchers have developed a method to break down PET using mechanical forces instead of heat or harsh chemicals. Published in the magazine Chemistry, their conclusions demonstrate how the “mechanochemical” method – chemical reactions caused by mechanical forces such as collisions—can quickly transform PET back into its basic building blocks, paving the way for faster, cleaner recycling.

The research team, led by postdoctoral fellow Kinga Golombek and professor Carsten Sievers of Georgia Tech's School of Chemical and Biomolecular Engineering, hit solid pieces of PET with metal balls with the same force they would experience in a machine called a ball mill.

This can cause PET to react with other solid chemicals such as sodium hydroxide (NaOH), generating enough energy to break the plastic's chemical bonds at room temperature, without the need for dangerous solvents.

“We show that mechanical stress can help break down plastic into its original molecules in a controlled and efficient way,” Sievers said. “This could make plastic recycling a more sustainable process.”

Impact Mapping

To demonstrate this process, the researchers used controlled single-impact experiments, as well as advanced computer simulation to map how collision energy is distributed throughout the plastic and triggers chemical and structural transformations.

These experiments showed changes in the structure and chemical composition of PET in tiny zones that experience different pressures and temperatures. By mapping these transformations, the team gained new insight into how mechanical energy can cause rapid and effective chemical reactions.

“This understanding can help engineers design industrial-scale waste treatment systems that are faster, cleaner and more energy efficient,” Golombek said.

Plastic destruction

Each collision created a tiny crater, the center of which absorbed most of the energy. In this zone, the plastic stretched, cracked and even softened slightly, creating ideal conditions for chemical reactions with sodium hydroxide.

High-resolution images and spectroscopy showed that the normally ordered polymer chains became disordered in the center of the crater, with some chains breaking into smaller fragments, increasing the surface area exposed to the reagent. Even without sodium hydroxide, mechanical force alone caused little chain scission, showing that mechanical force alone can cause chemical changes.

The study also showed the importance of the amount of energy transferred with each strike. Low energy collisions only slightly damage PET, but higher impact impacts cause cracks and plastic deformationexposing new surfaces that can react with sodium hydroxide for rapid chemical decomposition.

“Understanding this energy threshold allows engineers to optimize mechanochemical processing, maximizing efficiency and minimizing unnecessary energy consumption,” Sievers explained.

Closing the loop of plastic waste

These results point to a future in which plastics can be completely recycled back into their original building blocks, rather than recycled or thrown away. By using mechanical energy instead of heat or harsh chemicals, waste recycling can become faster, cleaner and more energy efficient.

“This approach could help close the cycle of plastic waste,” Sievers said. “We could imagine recycling systems in which everyday plastics are processed mechanochemically, repeatedly giving waste new life and reducing environmental impact.”

The team now plans to test real waste streams and see if similar methods can work with other hard-to-recycle plastics, bringing mechanochemical recycling closer to industrial use.

“With millions of tons of PET produced annually, improving recycling efficiency could significantly reduce plastic pollution and help protect ecosystems around the world,” Golombek said.