When recycling plastics, contamination and chemistry conspire to degrade the quality of each recycled generation. In a perfect world, a polymer could be converted to its monomer building blocks, then made into new plastic over and over. Researchers have been working to develop such chemistry, and Brett Helms and colleagues from Lawrence Berkeley National Laboratory now demonstrate a new approach (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0249-2).
Their polymer is based on a network of reversible diketoenamine bonds formed in a ball mill using click chemistry reactions between triketones and polyamines like tris(2-aminoethyl)amine. Helms says that the resulting plastics, which the group refers to as poly(diketoenamine)s (PDKs), have mechanical properties on par with thermoplastic polyurethanes, which are used in products such as mobile phone cases and shoe soles. PDKs are in a class of plastics called vitrimers that are characterized by a crosslinked polymer network. The materials, which behave like glasses, have been explored as recyclable plastics by other chemists.
To convert the PDKs back to their monomer building blocks, the group uses 5 M sulfuric acid. The researchers could recover more than 90% of the triketone and polyamine monomers after 12 h in the acid. They could then separate the monomers from additives and other polymers left in the acid solution. For example, they could dissolve and extract the triketones and amines from dyed plastics, mixtures of different plastic types, and plastics mixed with flame retardants.
This research shows more of what vitrimers can do, says Eric Drockenmuller, a polymer chemist at Claude Bernard University Lyon 1. “Besides showing properties typical of the best vitrimers developed so far,” he says, Helms’ group also demonstrated a new approach to so-called closed-loop recycling.
But Jinwen Zhang, who studies vitrimers at Washington State University, points out using a strong acid comes with its own environmental concerns in terms of waste disposal. He also says for PDKs to be commercially practical, they will need a cheap feedstock. Helms says that his team has made renewable feedstocks a priority from the start.