US study “revolutionizes” chemical plastic recycling with table salt as researchers seek industry partners

12 Sep 2023 --- US researchers have chemically recycled mixed and metalized plastics using table salt (NaCI). The study authors say the findings have the potential to “revolutionize” plastic chemical recycling by offering a reusable, low-cost and efficient catalyst – NaCl – as an alternative to conventional catalysts, which are often toxic.

In the investigation, a polyolefin blend of high-density PE, low-density PE , linear low-density and PP, as well as metallized films, were used as testbed polymers. 

NaCl, two different pyrolysis (Pt) catalysts and a control were also investigated. For the metalized films, table salt and the Pt/alumina catalyst showed “excellent” conversion efficiencies. 

For the polyolefin blend, this ratio is chosen because it has a similar composition to plastics in municipal solid waste in the US.

Image showing the aluminum-wrapped autoclave employed for the pyrolysis and the products obtained from the reaction (Image credit: Michigan State University).Muhammad Rabnawaz, associate professor and College of Agriculture & Natural Resources Faculty Laureate in the School of Packaging at Michigan State University, US, tells Packaging Insights that the advantages of using table salt for recycling plastic waste lie in performance and cost. 

“Salt offered a high yield of desired oil, minimizing wax production as compared to other pyrolysis. Furthermore, table salt prices at US$230 per ton versus Pt (US$1070 per 373.3 g). Salt is also recyclable after use by washing it with water,” highlights Rabnawaz, who conceived the idea and supervised the project team. 

A low-cost transformation 
The research demonstrated that table salt offers effective and efficient pyrolysis of polyolefin blends and metalized films. The polyolefin blend was 100% converted to oil (86%) and gas (14%) products without any undesirable wax. Table salt outperformed the expensive catalysts under the given conditions, noting that catalysts were used at lower loadings than table salt.

In addition, table salt enhanced oil and gas fractions when used along with Pt catalyst. For the metalized films, table salt yielded a pyrolysis product with a total oil and gas content of 84%, and this content was further increased to 91% when combined with the Pt/carbon catalyst. 

While the Pt/Al catalyst yielded a pyrolysis product with a 97% oil/gas content, the addition of table salt to the Pt/Al catalyst for metalized films reduced the gas and oil fractions. 

The current findings suggest that table salt can offer a transformative approach to chemical recycling by offering a low-cost, efficient pyrolysis methodology for mixed plastics and metalized film waste to produce useful hydrocarbon products. Further studies are in progress to expand the scope of table salt to different waste streams, says the team. Table salt offers effective and efficient pyrolysis of polyolefin blends and metalized films, finds the study. 

“Salt can potentially generate revenue that’s threefold compared to control pyrolysis. Using Pt is prohibitively expensive for waste plastics, making it less viable. This advantage could serve as a pivotal factor for the widespread adoption of our work,” asserts Rabnawaz.

Aiming for industry collaboration 
To find the optimized pyrolysis conditions, several experiments were carried out at different temperatures, using different reactor types (glass versus metal), pyrolysis with and without NaCl, as well as pyrolysis with selected Pt catalyst, as well as pyrolysis in the presence of NaCl/Pt catalyst. 

“Once the system was optimized, we selected two testbed samples (mixed polyolefins and metalized films) for the pyrolysis reaction. Pyrolysis was carried out in the presence of NaCl, Pt/carbon and Pt/alumina. As a benchmark, we used a control pyrolysis process for the blends and metalized films without any catalysts,” explain the scientists. 

“We aim to analyze the minor gas fractions to determine their composition and potential utility. Collaborating with the industry to integrate this approach into commercial pyrolysis reactors is something we’re eagerly anticipating,” he concludes.

By Natalie Schwertheim