How are soft plastics recycled

Recycling: Infinitely reusable plastic

A new class of plastics is set to solve one of the biggest problems in plastic recycling. The material known as PBTL is suitable as an engineering plastic and can be recycled indefinitely, reports a working group led by Eugene Chen from Colorado State University. As the team writes in “Science Advances”, the technology is based on tailor-made sulfur-containing molecules, the bicyclic thiolactones. Not only can they be combined to form the long molecular chains of plastic, but they can also be easily removed from the chains as individual molecules. This means that, unlike today's plastics, the material can be reprocessed without any loss of quality.

Plastics currently in use, such as polyethylene, cannot be recycled without a loss of quality because the original building blocks can no longer be separated from one another. The molecular chains inevitably get shorter and shorter during the recycling process. As a result, the properties of the plastic deteriorate, for example it becomes softer and more unstable, so that it can only be used for less demanding purposes. This is known as "downcycling"; the material is not used in the cycle, but in a kind of downward spiral that inevitably ends in incineration or in the landfill.

This article is contained in Spectrum Compact, Recycling - How circular economy protects the environment

PBTL escapes this spiral because in recycling you do not reuse the finished plastic, i.e. the chains, but the individual components. As with other plastics, the bicyclic thiolactones line up in rings or chains and form a material with new, desirable properties. In contrast to conventional plastic, however, the reaction can simply be reversed by heating the substance with a suitable auxiliary molecule, a catalyst. During this process, PBTL breaks down again into the original bicyclic thiolactones. These can then be processed into new plastic without any loss of quality.

The principle of recyclable plastics is not new. However, no such material has so far met the conflicting requirements for recycled plastic. On the one hand, it must be able to be decomposed again without undue effort, but on the other hand it must also be stable and elastic enough for technical applications. In addition, it must be possible to precisely control the exact structure of the chains and thus the properties of the substance. This is in principle possible with the new material, writes Chen's team. But there is one problem that PBTL does not overcome either: it can only be reused if it is available as a pure substance. A large part of the plastic, however, is processed into composites of various substances.