Post-industrial multilayer plastic films represent a large waste source, due to inefficiencies in production. More than 100 million tons of multilayer thermoplastics are produced annually, 40% of which are estimated to go unused. This study introduces a method: solvent-targeted recovery and precipitation (STRAP), to chemically separate three or more resins that can be then reused in their originally intended capacity. 

Authors

Theodore W. Walker, Dept of Chemical and Biological engineering, University of Wisconsin-Madison & DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison; Nathan Frelka, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison; Zhizhang Shen, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison & DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison; Alex K. Chew, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison & DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison; Jesse Banick, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison & Amcor Flexibles; Steven Grey, Amcor Flexibles; Min Soo Kim, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison; James A. Dumesic, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison & DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison; Reid C. Van Lehn, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison & DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison; George W. Huber, Dept of Chemical and Biological Engineering, University of Wisconsin-Madison.

Highlights

• Due to the affordability and versatility of multilayer plastic films, more than 100 million tons of multilayer thermoplastics are produced globally each year.
• 40% of manufactured multilayer films go unused due to inefficiencies in the fabrication process and represent a large postindustrial waste source.
• Currently, no commercially viable technologies exist that could reintroduce these materials into the production process.
• This study shows that by using solvent-targeted recovery and precipitation (STRAP), three representative polymers can be separated with nearly 100% material efficiency.
• The STRAP process could recycle multilayer film into pure resins at a cost comparable to virgin materials.

“To be compatible with existing recycling infrastructures, multilayer plastic waste scraps would first need to be partially or fully deconstructed into their constituent resins before being cofed into processing equipment to produce reconstituted multilayer films. Currently, no commercially viable technologies exist to do so.”

RESEARCH BRIEF

What are multilayer plastics and why are they hard to recycle?

Multilayer plastic films are often used in the packaging industry and are constituted by several layers of heteropolymers. Each layer is selected for its properties that contribute to the usefulness of the plastic product, e.g., moisture or oxygen barriers. The complexity of the bulk material is increased by potential tie layers, adhesives, or additives that represent less the 1% of the material weight.

The multilayer films are widely used due to their versatility and affordability. More than 100 million tons are produced annually, 40% of which go to waste due to inefficiencies in the production process. These unused fractions represent a great postindustrial waste (PIW) source, that could relatively easily be reintroduced to the production process due to the lack of impurities that post-consumer plastics often have.

The difficulty of recycling multilayer films arises as they cannot be mechanically recycled. Each layer must be first separated then to be reconstructed again. No commercially viable technologies exist that can do so. Together with post-consumer multilayer films, this PIW source continues to contribute to the accumulation of plastic waste in the environment.

The Solution

One approach to separate the polymers is to individually dissolve them. At least two technologies based on this approach exist today, but due to the lack of scientific reporting on these or similar technologies, the deployment of them on a broader scale is hindered.

This study introduces a computationally guided platform strategy to separate multilayer films of three or more layers via series of solvent washes. This approach is named solvent-targeted recovery and precipitation (STRAP). The STRAP process selectively dissolves a polymer layer in a solvent system where the targeted polymer is solvent, while the others are not. The dissolved polymer is then filtered and precipitated by changing temperature and/or adding an antisolvent. The polymer is then recovered as a dry, pure solid. This process is repeated for each polymer.

In the test conducted by this study, the STRAP process managed to separate polymer components with nearly 100% material efficiency. Through a techno-economic analysis, it was also demonstrated that large volumes (minimum 5400 tons/year) could be recycled into pure resins at a cost comparable to virgin materials.

Some questions regarding toxicity, odor, and other properties of retained solvents are however left unanswered by this study. The quality of the recycled fraction needs thus to be studied further, to e.g., confirm the suitability for food packaging. When the right application for the recycled fractions is found, the resins produced via STRAP can compete with the production of virgin materials.

The way forward

The existing methods for multilayer film recycling, comparable with STRAP, vary in capacity and ability to separate different polymers. For example, an 8000 ton/year plant is built in Germany, where PE or polypropylene is dissolved from multilayer plastics. The plan is to separate PET, polystyrene, polyactic acid, and aluminum as well in the future.

Another planned facility with a capacity of 54,000 ton/year, is to recycle large volumes with the downside of only recovering a single recycled polymer component. Other, more complex chemical depolymerization processes have also been developed, but these are likely more costly than STRAP. The option of replacing multilayer polymers with ones that are more easily depolymerized or biodegraded is also investigated in research. However, these options are currently far from commercialization, and thus represent a long-term solution.

Selective dissolution represents a promising approach to recycling multilayer plastic films, are indicated by the multiplicity of examples. The STRAP differs from other, existing processes in that it can separate three or more polymers. However, the impact of impurities, such as additives, on the quality of the recycled resins using STRAP needs to be further studied.

In conclusion, the STRAP process can deconstruct commercial PIW with nearly 100% efficiency. The article suggests that STRAP could be quickly implemented on a large scale to target PIW sources. Existing equipment could be used combined with STRAP technology to recycle large volumes of plastic waste.

Future work should focus on studying the separation of resins not studied in this paper, such as PVC and polystyrene. The in-depth molecular modeling framework introduced in this study, is broadly applicable and will allow for the design of new solvent systems for processing multilayer plastics of almost any composition. This represents a key advancement toward battling the mismanaged plastics and accumulation of plastic waste in the environment. STRAP could also be applied to postconsumer plastics in the future, due to it being based on simple unit operations. These attributes make STRAP a promising strategy to increase closed-loop recycling of multilayer plastic films.

“…the viability of the STRAP process ultimately hinges on the ability to produce fit-for-use recycled resins while efficiently recovering and reusing the solvents so that production of the recycled polymer resins is cost-competitive with the production of the virgin materials.”

The full article is available here.