Looking at the challenges and benefits of various recycling technologies, this article uses polymethyl methacrylate (PMMA) as an example to analyze the economics and design considerations for depolymerization processes
Plastics are a valuable commodity in society and provide versatile materials for a variety of industries. Millions of tons of plastic are produced worldwide each year for applications in many industries, including building and construction, packaging, automotive, consumer electronics
and healthcare.
However, as plastics have become more versatile, there are growing challenges related to several aspects of plastic production and use, including the impact of greenhouse gas (GHG) emissions from production and disposal, and the effect of landfilling, incineration and littering on the environment. Waste and disposal have become such a leading concern worldwide that regulators and industry organizations are now focused on developing a more circular economy. This can be seen in the European Green Deal and forthcoming end-of-life vehicle (ELV) directive, as well as others in North America and Asia. Many of these regulations focus on how waste-plastic materials can be reintegrated back into the value stream to reduce (and perhaps one day eliminate) dependency on virgin, fossil-based raw materials. The industry’s first look at the ELV directive last year is a prime example, as it could mandate automakers to use 25% recycled plastics, a quarter of which must come from ELVs.
Circularity has significant buy-in from industry organizations that are delivering strategies to align chemical manufacturing with these forthcoming regulations, such as Plastics Europe’s roadmap released in 2023. The first strategic pillar of this roadmap is to make plastics circular, which it considers one of the “fastest, most affordable, effective and reliable methods for reducing GHG emissions from the plastics system [ 1].”
However, to develop a circular economy, the chemical process industries (CPI) must first reduce their dependence on virgin, fossil-based raw feedstocks through recycling technologies that recover high-quality and high-purity materials that can be integrated into the manufacturing process. It is instrumental that these economies incorporate recyclers that can collect, sort and process waste streams to generate recycled feedstocks for use in demanding applications.
While mechanical recycling has been the hallmark recycling method for decades, the time has come for investment in new technologies that enable more product design flexibility and meet performance standards. Chemical and physical recycling technologies are catalysts for change and must be incorporated as part of the CPI’s investment in a circular-economy infrastructure. Recycling technologies and processes must complement one another to facilitate a vibrant plastics economy that is not dependent on virgin raw materials.
Types of recycling processes
All recycling technologies fall into three main categories: mechanical, physical or chemical. These technologies can be used on many plastic materials to yield different results or to complement each other, depending on the final products.
Mechanical recycling has been the preferred recycling method to date [2]. It requires waste materials to be processed, sorted and treated or cleaned before they undergo grinding and compounding, so that the material maintains its chemical structure and original color.
Physical recycling, also known as selective dissolution, is currently gaining interest with several polymers. Like mechanical recycling, it preserves the molecular weight of the polymer. In comparison to mechanical recycling, this process enables a broader acceptance of lower waste quality. Similarly, though, in some cases, the process can yield nearly 100% material recovery.
However, there are some drawbacks to mechanical and physical recycling technologies because they do not return the material to its original monomer. First, the characteristics — such as the polymer chain length — can potentially become degraded over time, as it continues to be recycled. This can affect the mechanical and optical properties over repeated recycling cycles so that its performance is no longer adequate, and the recyclate needs to be downcycled [2].
Second, mechanically recycled materials are typically more likely to contain undesired additives and inhibitors, as well as polymeric impurities, which cannot be removed during the process [ 2]. Vehicle parts are a prime example of this — the materials are colored and produced with an anticipated lifecycle of at least 10 years. Those older vehicles could contain chemicals that have since been banned from use or include contaminants that cannot be removed during recycling. With this in mind, materials utilized for automotive applications or other long-life applications can be difficult to recycle via mechanical methods, since color and additives cannot be removed during this process. For colored materials, these wastes are processed for use in all-black and dark applications. Alternatively, those materials that include additives and banned chemicals should not be mechanically recycled and typically end up being incinerated or landfilled.
Third and final, some materials simply cannot be mechanically recycled. An example of this is polymethyl methacrylate (PMMA) cast sheets, since they begin to degrade at the temperatures generated by mechanical recycling due to their unique properties and high molecular weight. By limiting the PMMA waste streams that can effectively be mechanically recycled, a significant PMMA waste portion is landfilled or incinerated.
Chemical recycling, on the other hand, utilizes methods like pyrolysis (heat treatment in the absence of oxygen) and solvolysis (heat treatment in presence of a chemically active agent) to return the polymer to its monomer form. Gasification (heat treatment in the presence of oxygen) generates synthesis gas (syngas), a mix of carbon monoxide and hydrogen, from which multiple products can be made, the most common being methanol, ethanol and ammonia. However, it does not often yield the monomer or one of its building blocks. Research shows, within this family of processes, that depolymerization is the most sustainable option because, by returning the material to monomer form, there is, in theory, no limitation to the recyclability of the solution in a closed-loop system [2]. In this context, closed-loop refers to when waste plastic can be used in exactly the same application, whereas in an open-loop system, the waste is recycled into another, less demanding application. Additionally, this enables the recycling of contaminated or difficult waste. To date though, chemical recycling contributes marginally to the total recycled volumes, with less than 3 wt.% of European plastics being recycled undergoing chemical processing [2].
When recycling processes are operated with a high mass yield, they keep the carbon in the economy for a longer time, for multiple cycles. For example, a process operating at 90 wt.% mass yield in a closed-loop system will retain 50% of the mass after six recycling operations (0.96 ≈0.5). PMMA and polystyrene pyrolysis processes operate at much higher mass yields than mixed polyolefins/plastics pyrolysis and are therefore more appropriate for an extended number of recycling cycles.
A model for depolymerization
End-of-life PMMA is an example of a waste stream that can benefit from the expansion of chemical recycling via depolymerization (Figure 1). PMMA has a forecasted market value of 4 million tons/yr by 2025 and will be worth $6 billion by 2027 [3]. Sold under the brand names of Perspex, Plexiglas and Altuglas, PMMA (also known as acrylic) is a transparent thermoplastic that can be used as an alternative to glass because it is resistant to ultraviolet (UV) light, is transparent and has high light transmission. Research shows that each year, 10% of produced acrylic materials end up as post-production collected waste [ 3], but that as much as 90% could be collected as post-consumer waste, so there is significant room for growth in the recycling of this material.
FIGURE 1. A European chemical manufacturing facility that includes a PMMA depolymerization plant is shown here
Depolymerization has long been known as a desirable method for recycling PMMA materials. Just 17 years after acrylic was first synthesized, an archaic form of the depolymerization process was patented [3]. However, since PMMA and its monomer form — methyl methacrylate (MMA) — represent just a small percentage of the market share for all plastics [4], investment into advancing this recycling technology has been limited. That said, PMMA provides opportunities to remove valuable plastics from the waste stream for recycling. The geographic distribution of acrylic recyclers around the world, and their capacity to process the material, shows that worldwide, 100,000 to 150,000 tons of PMMA can be recycled each year across fewer than 75 facilities [5]. Additionally, historically in Europe, waste streams have been exported to China for processing that generate low-quality PMMA, but this method cannot persist. Upcoming regulations would limit the export of many plastics waste streams outside of European borders, so the industry must adapt and find methods for generating usable recyclate that meets global directives. Depolymerization enables manufacturers to achieve realistic recycling goals that can feed a closed-loop system.
The latest advancements in PMMA circularity have come from a chemical industry consortium, MMAtwo (www.mmatwo.eu), which was funded by the European Union’s Horizon 2020 research and innovation program. This research has been monumental in making depolymerization a more effective recycling process for acrylics that keeps them from being downcycled.
Driving depolymerization
Depolymerization falls under the pyrolysis family of chemical recycling, and from this, there are a variety of methods to produce regenerated MMA (rMMA). The most common method uses high temperatures to generate a liquid monomer. This method generates a vapor of the monomer, which is later condensed. When operating a depolymerization facility, there can be some inherent safety concerns depending on the methodology in place, as follows:
- The recycling technology may operate at a higher temperature than the MMA self-ignition temperature
- The potential formation of solid residues that could be pyrophoric
- The required cleaning operations
- The amount of material that is immobilized within the reactor
Technology evolution
The technology has come a long way since its initial patent, which was for a “heat-transfer bath process [3].” While molten-metal baths are still in practice at some depolymerization facilities, there are now many different technologies based on the significant variations that recyclers and chemical manufacturers use to depolymerize acrylic polymers. There are currently 11 technologies that have been developed for the depolymerization of PMMA, which MMAtwo explored at 24 sites around the world to better understand how companies were generating rMMA. The advantages and disadvantages of each technology are illustrated in Table 1. MMAtwo explored these technologies to understand how high-quality rMMA could be generated and remain profitable.
MMAtwo looked at a variety of factors when deciding the advantages and disadvantages of each method, including safety, economic feasibility, scalability, process and more. With some, while there were benefits, environmental, health and safety (EHS) concerns far outweighed these, and for others, the potential of additional steps to remove residue or lingering elements would be cost-prohibitive. The research by the consortium showed that the twin-screw extruder process was the most advantageous. This process, which is a variation of the dry distillation method, utilizes a plug-flow stirred reactor so that the heat transfer can be maximized [5].
In this method, liquid rMMA is produced utilizing high electrical heat and a shearing from the mixing screw. During the recycling process, rMMA is captured by the twin-screw extruder, cooled in a heat exchanger, and then purified. MMAtwo found that this method was inherently safer than other methodologies since it was self-cleaning and the reactor had less PMMA holdup at any given time, so the polymer could more readily be removed in the case of an emergency shutdown.
MMAtwo utilized this depolymerization model to develop a new value chain for handling and recovering PMMA waste. While previous models could generate purity levels from 91 to 99 wt.%, the MMAtwo models have consistently generated 99.5 to 99.8 wt.% (and even more, in some cases) [3]. By building on an already successful model, the consortium was able to develop rMMA product with high purity that can be used in the same applications as virgin MMA, such as in vehicle taillights or windows that require high optical quality.
Results of the MMAtwo study not only showed that the products were of high quality, but also that they had a reduced environmental impact compared to their virgin counterparts. Most of the facilities that utilized depolymerization technology, and which were evaluated by the consortium, had at least a 70% reduced carbon footprint compared to those facilities producing virgin MMA [ 5]. Additionally, this form of PMMA recycling for rMMA consumed less energy and water than the virgin process that uses acetone as feedstock [5]. This could have a significant impact on those companies looking to reduce their environmental impact.
As regulations continue to evolve, and the chemical industry pursues more circular solutions, depolymerization provides a unique opportunity to reuse wastes that would otherwise be incinerated or landfilled. Next-generation depolymerization technologies could completely close the loop by yielding continuous high-quality monomer that has no discernible difference from its virgin counterpart.
References
1. Plastics Europe, The Plastics Transition, 2023, www.plasticseurope.org/wp-content/uploads/2023/10/PlasticsEurope_Summary_24.10.pdf.
2. De Tommaso, J., Galli, F., Weber, R., Dubois, J.-L., Patience, G.S., Total Capital Investment of plastic recycling plants correlates with energy losses and capacity, ChemSusChem, January 2024.
3. De Tommaso, J and Dubois, J.-L., Risk analysis on PMMA Recycling Economics, Polymers, August 2021.
4. Dubois, J.-L., PMMA Depolymerization: Scale-up and Industrial Implementation, Proceedings of the MMAtwo Virtual Workshop on Polymer Recycling, September 2020.
5. D’hooge, D., Marien, Y., Dubois, J.-L. (edited by), “Polymer Circularity Roadmap: Recycling of Poly(methyl methacrylate) as a Case Study,” 2022, pp. 65–86.
Author
Jean-Luc Dubois currently serves as principal scientist at Trinseo PLC (Email: [email protected]). Prior to joining Trinseo, he worked for Arkema for more than 18 years, and started chairing the Executive board and the Advisory Board of the MMAtwo project, a completed European Union-funded project. He led the workstream on depolymerization technology of the project which developed a new value chain to process different types of PMMA waste, including the most challenging waste, into high quality regenerated monomer. Dubois joined Trinseo in 2023 after the company’s acquisition of Arkema’s PMMA business and helped implement the PMMA depolymerization technology. He is the author and co-author of more than 150 scientific publications and 190 patent families. Dubois holds a Ph.D. from Pierre & Marie Curie University.