Plastics are omnipresent and are among the most common materials of all. However, the efficient recycling of these important materials has so far been limited. In order to offer new solutions to this problem, chemists at the University of Konstanz led by Prof. Dr. Stefan Mecking have developed a more sustainable process for the chemical recycling of polyethylene-like plastics. The researchers use "predetermined breaking points" at the molecular level to break down the plastic into its molecular components. The new process does not require extreme temperatures, is therefore more energy-efficient and has a significantly higher recovery rate (of around 96% of the original material) than established processes. The research results were published in the scientific journal Nature on February 17, 2021.
Mechanical recycling vs. chemical recycling
"What often stands in the way of the direct reuse of plastics is the fact that this so-called mechanical recycling only works to a limited extent in practice - because the plastics are contaminated and mixed with additives, which impairs the properties of the recycled material," explains Stefan Mecking. One alternative is "chemical recycling", in which the used plastic is broken down into its molecular building blocks using a chemical process. These can then be used to synthesize a new plastic.
Hurdles in the chemical recycling of polyethylene
Especially in the case of polyethylene - the most commonly used plastic - chemical recycling is, however, complex. Plastics consist of long molecular chains on a molecular level. "The polymer chains of polyethylenes are very stable and not so easy to break down into small molecules again," explains Stefan Mecking. Temperatures of over 600 degrees Celsius are required, which makes the process energy-intensive. At the same time, the recovery rate is limited (sometimes less than ten percent of the original material).
How chemical recycling of polyethylene can become more sustainable
Stefan Mecking and his team are now reporting on a process that can be used to chemically recycle polyethylene-type plastics in a much more energy-efficient way and with a high recovery rate of around 96% of the starting material. The chemists use "predetermined breaking points" at molecular level, which enable the polymer chain to be broken down into smaller molecular building blocks. "The key to our process is plastics with a low density of predetermined breaking points in the polyethylene chain, so that the crystalline structure and material properties are not impaired," explains Stefan Mecking and adds: "This class of plastics is also well suited for 3D printing."
Stefan Mecking's research team demonstrated the chemical recycling process on polyethylene-like plastics based on vegetable oil. The process only requires temperatures of around 120 degrees. The chemists also demonstrated chemical recycling from mixtures with other plastics, such as those found in waste streams. The properties of the recycled materials are on a par with the original material, explains Stefan Mecking. "Recyclability is an important aspect of future technologies based on plastics. It makes a lot of sense to reuse such valuable materials as efficiently as possible. With our research, we want to make a contribution to making chemical recycling processes for plastics more sustainable and more productive," concludes Stefan Mecking.
