Catalysis for Compatibilizers: Upcycling Plastic Waste into Value-Added Materials

The environmental impact of plastics is a Jekyll-and-Hyde conundrum. While plastic pollution poses a very real environmental danger, plastics have also reduced CO2 emissions across Europe by a factor of 5-9, partly through their use as lightweight components in transport vehicles and as building insulation materials. Plastics also address the global challenge of food security by reducing food waste by 20%.

However, current production methods are unsustainable. Of the 8300 Mt of plastic produced globally by 2015, 4900 Mt has been discarded. Moving from a linear plastic production model to a circular economy is of urgent importance and requires innovative scientific technologies.

Over 99% of plastics are currently produced from crude oil. By 2050, annual production is predicted to use 20% of global oil reserves and generate twice as much CO2 as the aviation industry. To conserve oil stocks and avoid energy intensive cracking processes, it is essential to find alternative feedstocks.

Discarded plastic waste offers an attractive alternative feedstock, by upcycling this waste material into useful value-added products. Yet only 10% of plastics are currently recycled. This is partly because "recycled" materials are generally downcycled, with a reduction in material properties such as strength and flexibility leading to progressively lower value applications until recycling is rapidly no longer cost effective.

Further complicating this complex process, plastic products often contain multiple types of plastic. For example, milk cartons are made of polyethylene (PE) but the lids and labels consist of polypropylene (PP); these plastics require separation and subsequent recycling as individual components. Creative scientific solutions are crucial to address these problems.

A game-changing technology has emerged in the past two years. The creation of "chemical zips" has enabled efficient recycling of a combination of PE and PP, avoiding the need for separation of these two materials. The "zip" is a molecule designed with different and alternating "teeth"; one set of teeth interacts with PE while the other set interacts with PP to stitch these two plastics together.

Remarkably, the new hybrid plastics produced by combining PE, PP and the "zip" give upcycled materials that are stronger than any of the individual components. The industrial commercialisation of these "zips" is set to revolutionise the end-of-life treatment of PE and PP. However, the technology developed to date is limited.

While chemical zips have been developed for PE and PP, analogous systems for polyesters remain unexplored. Translating this concept to polyesters is an exciting target, as the use of degradable polyesters such as polylactic acid (PLA) has doubled in the past four years. Commercial PLA products include disposable drinking cups, films and food packaging, and some PLA products involve mixed plastic systems such as PLA food trays with a PE film.

Recycling companies have reported increasing contamination of recycling streams with PLA, which is problematic as the current separation technologies are less well developed than for conventional plastics, and this challenge is set to grow.

The production of new chemical zips that could amalgamate PLA with PE, PP or polystyrene (PS) would revolutionise current recycling processes and create a range of new upcycled plastic materials from waste, yet the development of these systems has been hindered by the significant scientific challenge of producing such zips. This project will enable the production of bespoke chemical zips for PLA by designing a catalyst that can switch between two different chemical reactions to build up these zips one tooth at a time.

These transformative zips will then be applied to the combination of PLA with PE, PP and PS to create a new class of desirable, diverse and valuable materials from plastic waste, with important economic, environmental and energy benefits for society.