Plastic recycling efforts have stagnated with rates at around 9% in the US, according to the US EPA1, and other nations’ rates are not much higher. To significantly increase these rates, companies are scaling up advanced recycling methods that could expand the amounts and types of plastics recycled beyond what conventional recycling methods can handle. But many experts studying the plastic waste problem believe that technological advances alone aren’t sufficient. “At the end of the day, it’s all an economic discussion as to why recycled materials aren’t used more broadly,” says Scott Trenor, technical director at the Association of Plastics Recyclers, a non-profit organization that works to improve recycling. Government policies must help incentivize investment in recycling infrastructure and level the economic playing field so that manufacturers pick recycled plastics over virgin materials.
Many factors contribute to today’s low rates of plastics recycling, including hurdles to efficient and effective collection of plastic waste. But the most-used recycling process, called mechanical recycling, has significant limitations that hamper its ability to turn a wide range of plastic waste into useful materials.
During mechanical recycling, plastic waste gets washed, shredded into flakes, and then melted and extruded to eventually form pellets that can be used to make new plastic products. The polymer chains of the plastics mostly stay intact in this process, but almost all of what goes into the melting and extruding process is what comes out in the pellets. That includes contaminants.
As a result, the quality of the plastic pellets produced by mechanical recycling depends on the purity of the incoming plastic waste. The process works best with non-coloured bottles made from polyethylene terephthalate (PET) or high-density polyethylene (HDPE) because these products are typically made from similar grades of PET and HDPE, respectively. Other types of packaging like potato chip bags contain mixtures of plastics and even other materials, like metals. Mechanical recycling can’t separate out those various materials into pure pellets.
The contamination problem isn’t just about mixtures of different plastic types. There are various grades of polypropylene (PP), for example, and each type has been engineered for desired properties for specific applications. So even if a mechanical recycler received bales of all PP, the pellets produced would contain a mix of types of PP. “You kind of get a mix of all properties, which isn’t always useful,” when compared to a single grade of PP developed for a specific application, says Trenor.
The other contaminants that affect mechanical recycling are plastic additives, such as dyes, plasticizers, and ultraviolet stabilizers. “Even things that are added in very small weight percents can kind of frustrate the ability to use that resin like a virgin resin,” says Margaret Sobkowicz-Kline, a professor of plastics engineering at the University of Massachusetts Lowell.
These different forms of contamination mean that the properties of mechanically recycled plastic pellets don’t match those of virgin plastic, which affects recycling economics. But companies have been developing and scaling up recycling technologies that get around some of these shortcomings. None of these so-called advanced recycling technologies would replace mechanical recycling, experts say, but they could expand the scope of what gets recycled and yield higher-quality plastics.
The technology that is operating at the largest scale so far is pyrolysis. During pyrolysis, plastic waste that has been shredded gets heated to temperatures around 300 to 700 °C without oxygen, sometimes in the presence of a catalyst, such as a zeolite or metal oxide, to tune the reaction products. This process breaks down the polymer chains into smaller molecules that resemble the hydrocarbons found in petroleum products like naphtha. This resulting pyrolysis oil can then go through refining steps like petroleum components do to produce chemical feedstocks, which can be used to produce the monomers that make up plastic polymers. These refining steps can also generate fuels.
Pyrolysis facilities can handle mixtures of mostly polyolefins, like PP and polyethylene. For example, ExxonMobil’s facility in Baytown, Texas had processed more than 45,000 metric tons of plastics by May this year. Still, the scale of pyrolysis for plastics is relatively small: The global pyrolysis oil market was about US$300 million in 2020, while the global oil market was US$6.6 trillion in 2022, according to a report from the US National Institute of Standards and Technology (NIST).
Despite pyrolysis’s ability to handle mixtures of plastics that mechanical methods can’t, these facilities still have to worry about what they feed into their kilns. At first, pyrolysis “was really exciting, because the idea was you can throw any mixed mess of stuff into the pyrolysis chamber,” Sobkowicz-Kline says. “The problem is that a lot of the catalysts and the processes are sensitive as well to contamination.” For example, pyrolysis of PET can create acids and other chemicals that clog machinery, and pyrolysis of polyvinyl chloride (PVC) can form corrosive compounds that damage equipment.
These contaminants also affect the quality and therefore the value of the pyrolysis oil produced, so refiners can’t treat it exactly as they would conventional petroleum products.
To maintain the quality of pyrolysis oils, some companies rely on products like BASF’s PuriCycle, which consists of catalysts and absorbents that remove contaminants like oxygen and halogen compounds. Some companies also add catalysts during the pyrolysis process to better tune the chemical composition of the oil they produce to make it more attractive for customers.
While pyrolysis takes plastic waste back to chemical feedstocks that could make plastic monomers, depolymerization methods unzip polymers directly into these building blocks. This process often involves a solvent that reacts with the polymer chains with the help of a catalyst, such as metal acetates, metal oxides, and even engineered enzymes.
Eastman operates a plant in Kingsport, Tennessee that has the capacity to depolymerize about 110,000 metric tons of PET per year. The company focuses on PET from sources that are unlikely to end up in mechanical recycling streams, such as coloured shampoo and pharmaceutical bottles, as well as polyester-based carpet fibres and textiles.
In the depolymerization process, methanol solvent breaks apart PET into ethylene glycol and dimethyl terephthalate. This methanolysis process was adapted from earlier technology the company used to recycle polyester X-ray films, says Christopher Layton, Eastman’s director of circular policy strategy. Eastman uses ethylene glycol and dimethyl terephthalate to make new PET.
The methanolysis process allows them “to create essentially a recycled finished product that has the same performance, quality and safety as virgin material,” he says. “So it allows us to get recycled content into applications that probably wouldn’t be easily achieved with mechanical.”
Dissolution is another advanced recycling method that is being scaled up to commercial operation. In these processes, plastics go through many of the same steps as in mechanical recycling, but the plastic waste flakes get dissolved in a solvent to allow for filtration and separations to purify the material. These steps allow dissolution to produce purer plastic pellets than mechanical recycling can.
PureCycle operates a plant in Ironton, Ohio that has the capacity to recycle about 48,500 metric tons of PP per year via dissolution. The PureCycle process uses a non-toxic solvent to produce PP that is similar to virgin PP when analysed by standard techniques, says Pete Dias, senior director of sales at the company.
