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Recyclable plastic for the printer: A step toward a circular economy for synthetic polymers

The possibilities for material design and production of plastic components are being expanded through 3D printing technology. However, there is a shortage of recyclable polymers that meet the performance requirements.

In the journal , a research team has introduced a new class of polymers called polythioenones, which are mechanically and chemically recyclable and suitable for 3D printing. They also demonstrate better mechanical properties than conventional polyolefins—thanks to a special, ring-shaped building block.

Using digital modeling, complex structures can be precisely constructed layer by layer with 3D printers. Extensive customization and rapid prototyping open new possibilities in fields such as biomedical engineering, automotive manufacturing, and consumer product design.

In 3D printing with the fused filament fabrication (FFF) process, a threadlike thermoplastic material is pressed through a hot nozzle, where it melts. It is then applied in layers until the desired three-dimensional component is produced—with a minimal waste of material. The downside to this method is the lack of suitable polymers that can be recycled.

Chemically recyclable polymers, which can be split apart into their building blocks (monomers) and repolymerized, would help reduce environmental problems resulting from long-lasting plastic waste and help conserve fossil-derived feedstocks.

The synthesis of novel thermoplastic polymers with improved recyclability begins and ends with the design of suitable monomers. A team led by Will R. Gutekunst and H. Jerry Qi at the Georgia Institute of Technology (Atlanta, U.S.) has now developed a novel family of monomers: cyclic thioenones (CTE), rings made of seven carbon atoms and one sulfur atom.

The rings contain one C=C double bond and one carbonyl group (C=O) and can readily be modified by the addition of different side groups. The monomers can be polymerized through a ring-opening reaction in which monomers are added one by one to the end of the growing chain. Known as a thia-Michael addition, this reaction is reversible, meaning that the resulting polythioenones (PCTE) can be depolymerized back to the starting monomer.

One of the synthesized polymers, PCTE-Ph, proved to be particularly interesting. It is made from a CTE-monomer with an aromatic six-membered carbon ring (phenyl ring) as its side group.

PCTE-Ph is a thermally stable thermoplastic with outstanding mechanical properties. Colorants and fillers can be incorporated, and it can be processed by customary methods. This new material is especially well suited for 3D printing.

Components printed with this material can be mechanically recycled by simply melting the material and processing it again while maintaining its advantageous properties, such as tensile strength and thermal stability. In addition, it can be catalytically depolymerized back to the initial monomer at 90% yield. This recovered monomer is then available for additional rounds of polymerization.