LDPE is widely used in packaging applications but only around 5% gets recycled. This polymer is produced at extremely high pressures and temperatures (2,500 to 4,000 bar and 250 C), requiring high amounts of energy. Moreover, its chemical structure consists of robust carbon bonds that are difficult to break down during depolymerisation.
Now, scientists at Universität Bayreuth in Germany have developed a process to mimic the chemical structure of low-density polyethylene (LDPE) while making it easier to chemically recycle.
The team used a synthesis protocol called coordinative chain transfer polymerisation (CCTP) that allows for the controlled and efficient polymerisation of ethylene and related monomers. Using a low-pressure and low-temperature environment (2 bar and 70 C), they produced a material consisting of two macromonomers.
“The new material, called LDPE-mimic, is close in chemical structure to commercial LDPE,” said Prof. Rhett Kemp, the leader of the study. “The key to success is the use of our new catalysts, which produce defined building blocks of a certain size under appropriate mild reaction conditions. The new material consists of two different macromonomers, a backbone and potential long-chain branches. The branches can be reversibly attached to the backbone and cleaved under acidic and basic conditions,” Kemp explained.
The scientists used zirconium as a catalyst for the CCTP synthesis of the two macromonomers. They then introduced what they called ‘recycled points’, or ester linkages, to facilitate the grafting (polymerisation) and cleavage (depolymerisation) of the macromonomers via acidic esterification and basic saponification. Both the backbone and branch are ‘well soluble in a variety of organic solvents for separation from other polymers such as HDPE or iPP’, the scientists wrote.
The team used commercial Lupolen LDPE 1800P for comparison. Results show that the mimic LDPE material has similar melting point, density, crystallisation temperature, and tensile behaviour. The scientist further noted that the introduction of recycled points did not ‘lead to much earlier thermal decomposition’.
The team shared their findings in ‘A Closed-Loop Recyclable Low-Density Polyethylene,’ recently published in Advanced Science.