Breaking down polymers

Researchers from the Adolphe Merkle Institute’s Polymer Chemistry and Materials group are investigating the recycling and reuse of polymers with the goal of reducing their impact on the environment and increasing circularity.

Since the beginning of the 20th century, synthetic plastics and other polymeric materials have become an integral part of our daily lives, but their production from petrochemical feedstock and current end-of-life scenarios are not sustainable. Moreover, conventional polymers, such as those used in plastic bottles and other packaging applications, are not easily degradable and contribute to environmental pollution. 

“Plastic pollution is a really important issue, so devising energy- and cost-efficient strategies for their depolymerization and the recovery of their structural components in high yield and purity is key to a circular plastics economy,” explains Prof. Christoph Weder.

Recycling is particularly difficult for polymer networks, which are used in applications where high durability is important, for example, automotive tires, and wind turbine blades. These products create large amounts of waste that is difficult or impossible to recycle. One approach to solve this problem is to form the networks with dynamic bonds, which under certain conditions can break and reform easily. This concept gained significant traction in the last decade when researchers started producing a class of plastics known as vitrimers. Despite their network structure, these materials flow when heated and can, therefore, be reshaped– i.e., recycled – multiple times without a loss in mechanical properties, which is one of the key problems in the mechanical recycling of conventional plastics.

To create such vitrimers, researchers have exploited different kinds of reversible bonds, including so-called vinylogous urethanes (VU). This bond type has been used to make many different vitrimers that were demonstrated to be mechanically recyclable. Interestingly, the chemical recycling of such materials has been largely ignored. In chemical recycling processes, the polymer is chemically degraded to its building blocks, i.e., the monomers, which can subsequently be used to make new polymers. Chemical recycling affords products of higher quality than mechanical recycling, but it is generally less cost- and energy-effective than mechanical recycling and has a larger environmental footprint.

The AMI researchers recently demonstrated that polymers based on VU bonds can be chemically recycled at ambient temperatures upon reaction with water. This approach allows for the recovery of the original materials, even when the polymers are originally part of a waste mix that emulates typical household waste. “The depolymerization rate can be easily adjusted using factors such as the reaction temperature, the amount of water, and the nature and ratio of the two monomers,” explains Weder. “We can also fine-tune the mechanical properties of the resulting materials.”

The fact that the conditions under which the VU based polymers are degraded by water can be widely controlled is important. The AMI researchers demonstrated that it is, on the one hand, possible to design polymers that degrade in water within hours, and this can help to reduce the environmental impact when such materials are released into the environment. On the other hand, they also produced materials that are highly stable in pure water. In this case, an additional solvent that transports the water into the polymer is required to degrade the material.  This approach affords water-proof materials, which, however, can be recycled on demand. “We hope our results will encourage researchers to pursue the further development of chemical recycling of VU-based polymers,” says Weder.

The reseachers have also been investigating the recycling of polyurea thermosets, which are widely used in the automotive, marine, aerospace, and construction industries because of their stability and strength. But these same qualities make them next to impossible to recycle. One approach to overcoming this is to add dynamic covalent bonds, which can break and reform. The AMI researchers have achieved this by creating special types of ureas that can heal and be reshaped at room temperature or higher.

They devised a simple “drop-in” method to add dynamic bonds to polyureas. Testing showed that this method can be implemented successfully on molecules of varying sizes. These innovative polymers can also be reshaped and healed easily, making them candidates for recycling, while also proving to be stronger.

References:

Ma, Y.; Jiang, X.; Shi, Z.; Berrocal, J. A.; Weder, C. Closed-Loop Recycling of Vinylogous Urethane Vitrimers. Angewandte Chemie International Edition 2023, 62 (36), e202306188.

Ma, Y.; Jiang, X.; Yin, J.; Weder, C.; Berrocal, J.A.; Shi, Z.; Chemical Upcycling of Conventional Polyureas into Dynamic Covalent Poly(aminoketoenamide)s. Angewandte Chemie International Edition 2023, 62, e202212870.