Plastic How Plastic Waste Is Turned Into Higher-Quality Products

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ETH Zürich

FAULHABER Drive Systems

A team of researchers at ETH Zurich (Switzerland) has developed a process that transforms waste made of polyethylene and polypropylene—two of the world’s most commonly used plastics—into a strong adhesive. 

Approximately 450 million metric tons of plastics are produced worldwide each year. The majority of this is used only once and then ends up in an incinerator or a landfill. This also applies to so-called polyolefins, which include polyethylene (PE) and polypropylene (PP). As packaging materials and films, they account for about half of our plastic consumption.

Although polyolefins can be melted down and remolded, this process results in a loss of quality. The dyes in the waste darken the recycled material, and heating damages its quality: As a result, the material is only suitable for low-grade applications such as planters or floor tiles. To date, it has not been possible to selectively break down polyolefins into their basic building blocks—the monomers—from which new, high-quality polymers could be synthesized. Olefins, which consist exclusively of carbon and hydrogen, lack the oxygen, nitrogen, or chlorine atoms necessary for targeted cleavage reactions. To date, only random breakdown through an energy-intensive heating process—known as pyrolysis—is possible. This process produces crude oil feedstocks from which the monomers must be rebuilt.

Upcycling into a High-Performance Adhesive

Now, researchers led by polymer chemist and ETH professor Tae-Lim Choi have developed a process that allows polyolefins to be upcycled into higher-value plastics, rather than simply being recycled in a way that reduces their value. To do this, they graft additional polymer chains with new functionalities onto the plastics.  Hongsik Kim, the senior scientist in Choi’s group who was primarily responsible for developing the method, first produced adhesives from commercially available plastic bags and bottles. Their adhesive strength with various materials—such as metal, glass, or wood—meets the requirements for an industrial adhesive used in load-bearing and high-stress joints in automotive and equipment manufacturing. However, the range of potential applications is much broader: Through the additional polymer chains, the entire spectrum of chemical functionalities can be introduced into polyolefins—ranging from polar groups that interact with water to varying degrees, to acids and bases, all the way to chemical groups capable of highly specific reactions. 

Easy and Precise to Control 

The key chemical component of the process is chlorinated solvents. When irradiated with short-wavelength light, these solvents can generate highly reactive individual chlorine atoms, known as radicals. These radicals transfer their reactivity to carbon atoms in the polyolefin chains. This creates carbon radicals that can be used as starting points for the synthesis of additional polymer chains. The idea of using chlorinated solvents to create reactive sites in the polyolefin chains stems from a collaboration between Choi and the group led by Athina Anastasaki, professor of sustainable polymers at ETH Zurich. She applied this principle to the recycling of acrylic glass, also known under the brand name Plexiglas.
The patented method requires no expensive specialty reagents or catalysts, nor does it require high processing temperatures. The entire process takes place in a single step: The plastic waste is dissolved in a chlorinated solvent, and then the monomers needed to synthesize the desired graft polymer are added. Furthermore, the process can be precisely controlled: “We can control how many radicals are generated—and thus how many sites in the polyolefin chains are activated for grafting—by adjusting the degree of chlorination of the solvent and the wavelength of the light as needed,” explains Kim. 

Plastics with New Properties Are Becoming Possible 

In addition to the adhesive that has now been produced, many other applications of the technology are conceivable, Choi explains: “The adhesive is a first-generation product. In the future, plastics with entirely new properties will be possible. In principle, additional polymers with every conceivable chemical functionality can be grafted onto polyolefins.”  Since the patent was published six months ago, the researchers have already established many contacts with industry. One specific collaboration involves food packaging made from polyolefin plastics that are not only waterproof but also prevent oxygen from penetrating. As a result, food oxidizes less in the freezer (“doesn’t discolor as much”) than in today’s polyethylene and polypropylene packaging.