Mechanical recycling is the dominant “circular plastics” technology today for polyolefins (LDPE, HDPE, PP) and polyester (PET). However, when the polymers endure several rounds of use by consumers and processing through recycling streams, it creates molecular damage that causes undesired effects. For polyolefins, this degradation is due to heat, shear and oxygen. For polyester, this degradation is due to hydrolysis.
The quality of recycled material is directly dependent on the quality and purity of input materials. Several approaches have been tried to improve the quality of recycled polyolefins and polyesters.
For polyesters, there has been success in using reactive chain extenders as an additive to regenerate higher molecular weight. Another solution has been introducing an additional solid state polymerization step to mitigate molecular weight loss.
For polyolefins, there has been success in preventing chain cleavage in reprocessing by adding fresh antioxidants as restabilization additives. However, any damage to the polymer’s main chain is irreversible and there is currently no solution available to achieve full end-to-end connectivity and restore the molecular weight of the polymer’s backbone.
BASF is seeking a solution that enables an increase in the molecular weight of cleaved polyolefins during recycling, with a focus on two potential approaches:
End-to-End Connectivity: Our preferred solution involves extending the main chain of polyolefins through end-to-end connectivity. This approach results in a mostly linear, long-chain structure, which enhances the polymer's ability to form entanglements. These entanglements serve as tie-chains between crystallites, ultimately improving the mechanical properties of the polymer. This is the most desirable outcome for us.
Statistical Crosslinking: We are also open to solutions involving statistical crosslinking. In this approach, radicals are induced during extrusion, leading to an increase in molecular weight through the addition of various branched structures. While this method does impact the polymer's architecture and reduces its entanglement capability and is less preferred, it remains of interest to us if it involves novel approaches that go beyond the state-of-the-art peroxides.
BASF is seeking a solution for polyolefins that will extend the chain of cleaved polyolefins in recycling.
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