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Plastics designed with recycling in mind

21 May 2019

An acid bath recovers a plastic’s original monomers, which can be reassembled into different polymers.

It’s easy to toss a plastic soda bottle into the recycling bin rather than the trash can. But it’s difficult to recycle material from the bottle later. That’s in part because the properties that make plastics so suitable for packaging drinks and food—strength, low density, inertness, impermeability, and so on—arise from a chemical process, polymerization, that is energetically costly or impossible to undo. Now Brett Helms of Lawrence Berkeley National Laboratory and his colleagues have designed a plastic that can be easily depolymerized back to its original state.

A series of images showing the progress of the plastic dissolving

In their plastic, called poly(diketoenamines) or PDK, individual triketone monomers click together like Legos when mechanically ground at room temperature with compounds called amines. PVC, polystyrene, and other conventional plastics are held together by strong covalent bonds. Triketones and amines in physical proximity spontaneously form dynamic covalent bonds, which have the strength of a conventional covalent bond but are reversible: A triketone can covalently bond to one amine then switch to bond with another. Triketones build up a network with amines as the bonding agents.

When exposed to a strong aqueous acid, such as sulfuric acid, the bond between triketones and amines breaks, leaving behind triketones identical to the original ones. Manufacturers can then extract the triketones to use in new plastics with properties that are different from the previous manifestations.

The PDK plastic takes on different colors or other properties through additives such as dyes or flame retardants. But the additives’ presence doesn’t affect triketone extraction. For example, when a flame-retardant fiberglass cloth suffused with PDK was submerged in acid, it separated into an intact cloth, retardant, and triketones. The presence of other plastics also doesn’t impede the recycling process. Helms and his team treated PDK and a range of other common plastics (left two panels of the figure); the other plastics remained intact, whereas the PDK depolymerized as it did when treated alone (two right panels). Thus a recycling plant processing a whole range of plastics wouldn’t need to presort the PDK to successfully recycle it. (P. R. Christensen et al., Nat. Chem. 11, 442, 2019.)

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