Many polyester plastics have not yet been recycled because they are too complex and cannot be sorted. These include sports clothing or plastic parts of household appliances. But with a new recycling method, these waste could also be recycled in the future. The complex polymers do not have to be sorted in advance, but can simply be melted and cooled as a mixture. This reduces the crystallization in the plastics and makes it accessible to enzymes, which the polymers can then disassemble in an environmentally friendly manner.
Plastics are polymers from different building blocks, so -called monomers. Plastic bottles and CO consist of, for example, long chains of polyethylenterephthalate (PET). These pure waste can be collected easily, separated from other plastic waste and broken down into their monomers via enzymes. These can then be recycled for new PET objects. This type of recycling is efficient and environmentally friendly, since there are no sharp chemicals or extreme reaction conditions.
However, this recycling does not work for other polyester such as polybutylenterephthalate (PBT) and polytrimethylenterephthalate (PTT), which are often used for plastic components and housings of household appliances or carpets. Fibers and textiles from PET, such as non-woven fabrics, bathing or sports clothing, which make up most of the PET waste, cannot be recycled. Because these polyester consist of different monomers or contain additives such as dyes or plasticizers, which means that the waste is difficult to sort and separate. In addition, these polymers are difficult to access enzymes due to their crystalline structure – in contrast to the variety of varieties and semicry stalls pets of the plastic bottles.
Mixed heating lowers crystallization rate
Researchers around Hernan Garate from the University of Sorbonne in Paris have now developed a new method to also reduce such polyester waste. To do this, they specifically mixed various polyester waste and melted them at 270 degrees Celsius. Copolymers were created, which were put together in random combinations. This chemical reaction made this chemical reaction to residues of antimontrioxide and other catalysts, which were still present in the polyesters of their manufacturing process. The resulting copolymer networks then crystallized significantly less when cooling in ice pools and thus became accessible to enzymes. As a result, they could be grinded and broken down with the same enzymes that are also used for recycling semi-stalline PET bottles. This resulted in building blocks that are suitable for the production of new polyester products, as the team reports.
Her technology tested Garate and his colleagues under different conditions. They found that the addition of an epoxy added increased the degree of networking of the copolymers, further lowered the crystallization rate and improved the resulting yield on monomers. The same was true for longer mixed times. The method was successfully used with PBT and PTT waste as well as with polyester textile waste, the most effective was a combination of several such complexes with varieties with varieties of PET particles made of plastic bottles. The different polymers were thus dismantled with a yield of 90 percent or more and broken down into their individual parts, as the team determined.
Mix the garbage instead of separate
“The reactive mix of a highly stubborn, heterogeneous PET waste with a completely resistant PBT polyester enables almost complete enzymatic depolymerization,” write Garate and his colleagues. “This statement is contrainting and suggests that the mixture instead of sorting, which is often expensive or impossible, could be the way to the future.” Accordingly, the new mixed enzyme process is suitable in order to again add the plastic cycle economy again. “Synergistic mixing can replace sorting and expand the scope of the enzymatic recycling to unruly, heterogeneous and unsortable waste,” writes the team.
Source: Hernan Garate (Sorbonne Université) et al.; Proceedings of the National Academy of Sciences, DOI: 10.1073/PNAS.2505611122
