����ý

Although traditional mechanical recycling is still the dominant form of reprocessing polymers, a diversity of technologies is used on a smaller scale and could make significant changes to the quality and quantity of recycled polymers available in the future. Mechanical recycling is well established and very efficient — both in terms of yield and resource consumption — but still faces challenges in many applications. Mixed and contaminated streams, such as those from municipalities, are particularly challenging because shear stress and heat degrade the mechanical performance of the polymer; sorting and washing processes are required; and any additives compounded with the material will end up in the recycled product (most obviously, color).

Chemical recycling technology for recycling plastic includes pyrolysis and a variety of depolymerization methods. Pyrolysis creates an oil that can be used to make fuel or processed into industrial chemicals. Depolymerization methods use solvents, heat, pressure, enzymes or a combination of methods to produce monomers. The advantage of these methods is a truly fresh start — the opportunity to build a new polymer with no past. That is, free of the additives, contaminants or degradation which a polymer may pick up in its previous life.

Featured Content

A Purge Solution for the Blow Molding and Compounding Industries READ MORE

Automatic Conveying with AI Optimization READ MORE

A Simpler Way to Calculate Shot Size vs. Barrel Capacity READ MORE

Depending on what technology is used and how, these processes can be energy intensive and/or inefficient from a yield/input standpoint. Comparisons of recycling technologies, such as the one conducted by the NREL in 2022, suggest that the plastic recycling challenge has no single technological solution, because each approach has its own trade-offs, and functional results depend heavily on the polymer type, quality and mixture of feed material.

Still other recycling methods, including dissolution technologies, lie in between mechanical recycling methods with products as good or as bad as their inputs, and chemical recycling methods that produce an oil or other feedstock materials. These methods do not break the polymer back down to create a new monomer, but they do process out contaminants and additives, providing a clean material. Operators of dissolution projects describe it as a type of mechanical recycling (rather than chemical) because the intent is to leave the polymer molecule intact. For some feedstocks, it could provide the shortest path to a quality recycled product. 

Meeting the Recycling Challenge of Polypropylene

The very versatility of polypropylene (PP) that makes it so ubiquitous creates a challenge when it comes to recycling. PP packaging appears in every imaginable color in service to consumer brands. This makes a recycling method that can make a refreshed plastic without going all the way back to propylene especially attractive. PP is compounded with a multitude of additives, including every color, and removing these additives could be the key that unlocks greater value for recycled PP.  

Clear PP pellets can be produced from postindustrial or postconsumer feedstreams by dissolution mechanical recycling. Source: Purecycle

Three distinct pathways are compared for recycling PP in a paper that appeared in the journal last year, by Benjamin Caudle, Thu TH. H. Nguyen and Sho Kataoka. Solvent-antisolvent methods use a solvent to dissolve the plastic at high temperature, then reduce the temperature and add in an antisolvent to precipitate the clean polymer. The temperature swing method follows a similar path but omits the antisolvent. A third method uses a solvent at supercritical conditions, then lowers the temperature and pressure to induce precipitation. In this study, the supercritical solvent method was concluded to have the lowest carbon emissions per kg of rPP produced. It is also a method that has reached the stage of commercial sale.  

, previously covered in the June 2021 and April 2023 issues of Plastics Technology, uses a supercritical solvent recycling technology that CEO Dustin Olson calls a “molecular wash.” At supercritical conditions, solvents have no defined phase, exhibiting both gaseous and liquid properties. These conditions facilitate separation of any additives and contaminants from the polymer. Last year, Purecycle received an expanded , which indicates its process produces a material of sufficient purity to be used in food-contact applications.

Purecycle announced its first major sale in January, when Drake Polymers purchased nearly 500,000 pounds of resin that was compounded with Purecycle’s PureFive recycled PP. Using recycled material for fiber is particularly challenging due to the fine diameter threads. “The fiber threads are almost invisible,” Olson says. “So you can imagine if you’re bringing in a recycled stream with a lot of molecular contaminants — it’ll break the fiber; the die will plug; you’ll have loads of problems. But we’ve been able to just run, and we’re excited about that because it’s an indication of the product quality.”

The company’s flagship production facility in Ironton, Ohio, has run at up to 12,500 lbs/hr. Purecycle also has a sorting facility in Denver, Pennsylvania, that sorts and prepares material and has acquired key equipment for its next production site in Augusta, Georgia. The company also plans to expand internationally with a facility in Antwerp, Belgium. 

Dow Program Targets Dissolution Recycling of Polyethylene

In 2024, Dow Chemical announced a collaboration with to use dissolution technology on reclaimed polyethylene (PE) materials. The technology also takes advantage of the properties of supercritical solvents. The goal of the program is to develop a technology capable of recycling collected packaging from households into near-virgin quality PE. 

At this year’s , Dow described the process as being at a low level of technological readiness and stated that a target date for commercialization had not been established. The companies’ plans for the process even include PE films, which can be challenging to recycle due to their low density and frequently complex design. Both companies bring expertise to the arrangement, with Dow providing process engineering and P&G providing dissolution experience, having developed the dissolution technology being used by Purecycle.

High Yield Dissolution for Styrenics

began development of dissolution recycling methods for styrenics such as general-purpose polystyrene (GPPS) and acrylonitrile butadiene styrene (ABS) in 2017, with the aim of developing technology that consumes relatively low levels of energy and produces high polymer-to-polymer yield. In 2019, the company began working on dissolution recycling for polylcarbonate (PC) and PC/ABS blends.

“With all waste streams there are key selection criteria to find the right solvent or combination of solvents — and that depends of course on the polymer that needs to be recovered, but also on the typical architecture of how a part or device is made,” says Pascal Lakeman, director of SBS R&D at Trinseo. “All of these affect the selection of the solvents system and process conditions — this makes dissolution-based recycling a very technologically-intensive process.”

Processes for recycling styrenics need to enable deep volatile removal or volatile consumption to curb the formation of free styrene. This is necessary for preparing a material appropriate for food contact, which Trinseo has achieved and verified through challenge testing. Challenge testing involves adding contaminants to the feedstock and demonstrating they are effectively removed by the recycling process. Results demonstrated that the rPS resins meet European food-contact standards.

Trinseo hopes to expand its PC recycling facilities in the future, and develop dissolution-based recycling of ABS, HIPS and possibly EPS. “On rubber-containing materials such as ABS and HIPS, recycling is a bit more complicated compared to materials that are not impact-modified,” Lakeman says.

Closing the Solvent Loop

After the dissolution process removes unwanted impurities, the recycler has a polymer/solvent mixture which needs to be separated. is offering a technology called DEVO, adapted from its methods for devolatilizing virgin resin. It uses a heat exchanger to bring the mixture up to temperature, then passes it into a degassing chamber where volatiles are removed. Depending on the polymer type, multiple stages may be needed. According to Sulzer, the process reduces cost and polymer degradation compared to degassing extrusion. Sulzer’s first implementation of the technology in a recycling context is currently in the engineering phase.

The majority of recycling today consists of mechanical recycling of postindustrial or postconsumer materials: anything from press-side regrind to a sophisticated operation that can include washing, pelletization and recompounding. Dissolution/precipitation recycling and other technologies offer the possibility of expanding what is possible with postconsumer materials, especially polyolefins and styrenics, in situations where it is desirable to remove additives and contaminants to achieve properties like clarity while minimizing energy demand.