Scientists can already make new products from 100% recycled material, that have even better properties than the initial products and can be used for food-grade applications. In addition, plastic waste that we cannot recycle at the moment can be recycled this way. And even toxic additives that are banned nowadays and passed on to the recycled product with classical (mechanical) recycling methods, could be removed and recovered. Here, we explain what chemical recycling is.
What is Chemical Recycling
Plastics are made up of polymers, long molecules that are produced by connecting a lot of monomers into a chain.
Have you ever wondered why flower pots are black?
Classically, plastic is recycled by melting and re-extrusion, but this way plastic can only be remade into something of less value, like colored shampoo bottles and black flower pots.
Colorants, antioxidants and other additives cause the plastic to look pretty grimy after this recycling step. They also cause the plastic to appear brittle and it has to be mixed with new plastic granulates to retain its properties.
In addition, only certain plastic types can be recycled this way. And they have to be separated well, because otherwise a brittle plastic product would result.
That is why we need a different way to recycle plastic…
Instead of melting the plastic and shaping it into something else, in chemical recycling the plastic is ‘cut’ into its monomer units, which can be used to make new polymers.
There are different ways to chemically recycle plastic. Solvolysis only works for certain kinds of polymers, while pyrolysis and dissolution/precipitation work for a wider range of polymers. Recycling polymers like PVC might require pre-treatment steps prior to chemical recycling
Thermochemical Recycling Routes
The most researched thermal recycling route is pyrolysis. In this process, the polymer chains are broken purely by thermal energy. Normally, when plastics are heated just in the open air, they start burning, which means that they react with the oxygen to produce CO2 and CO. This can be avoided, if instead of air, pure nitrogen is used. In that case the polymer chains are broken into liquids and gasses that resemble diesel and gasoline, but other chemicals like aromatics can be produced as well. Which products are obtained, depends on the temperature, whether a catalyst is used and other parameters.
Through solvolysis monomer can be recovered by exposing it to liquid cleavage agent. After heating, a liquid is obtained, which contains the monomers and the rest of the liquid cleavage agent. This technique only works for certain polymers, PET, PU and PA, also called nylon.
Polymers can be dissolved in certain liquids and are unsolvable in others. This can be exploited to remove colorants and other additives from plastic.
Other technologies for waste handling include gasification to produce CO and H2, a mixture that is called syngas and can be used to make chemicals in a process called Fischer-Tropsch. And there might be even better ways to chemically recycle plastic, which is why research into this topic is so important.