For products made from commodity plastics, a transition toward a more circular economy could mean less packaging, a transition from nonrecyclable materials such as composites or multi-materials to more recyclable alternatives, and an increased use of recycled materials.
Recycling is not a perfect process. Each time many plastics are recycled, they degrade in quality. However, according to a February 2021 report published in the journal Nature, researchers used polyethylene-like polymers derived from plant oils to demonstrate a chemical recycling process that offers a closed-loop recycling alternative that retains the materials’ high-performance properties.
According to the report, this process requires less energy to recycle polyethylene compared with existing methods, which involve heating the material to 800°C in order to break carbon bonds. The report noted that “of central importance are low densities of break points in the polyethylene chain such that the crystalline structure is not affected, and consequently properties like mechanical strength are not compromised … the alcoholysis reactions used here are essentially thermoneutral equilibrium reactions shifted to depolymerization by an excess of solvent.”
Closed-Loop Recycling
Referring to the chart above, here’s how the closed-loop system works.
- Monomers obtained from the biorefining of plant or microalgae oils (“bio-based,” top inset) are polymerized to give polycarbonates and polyesters polyethylene-like properties.
- These materials are then used in manufacturing (such as 3D printing).
- After the useful service life of the polyethylene-like polymers (the diagram shows a test part used for tensile testing), they are recycled.
- Chemical recycling via a solvolysis process to the underlying monomers is enabled by the functional groups present in the polymer backbone. The recovered monomers can be repolymerized to materials with properties on a par with the initial polymers for an overall recycling rate that is greater than 96 percent.
The researchers used polyester-18 and polycarbonate-18 in their testing. Commercially available, renewable 1,18-octadecanedioic acid, produced from large-scale biorefining of plant oils by olefin metathesis technology, was employed as a starting material.
In addition to the tensile testing bars, the researchers manufactured several more functional and varied items from the plastic materials, such as the smartphone case shown above. A smartphone case is an excellent example of a product for which this recycling process could be desirable, due to the life cycle of the average smartphone.
Real-World Recycling
Of course, the researchers understood that in the real world, recycling is hardly as ideal as the example shown in the leftmost image above. (However, waste material from a manufacturing process is more likely to consist of a single material than post-consumer recycling. For example, waste from an injection molding process, or chips from a milling process, could reasonably be a source of just one polymer.) Material to be recycled could come from a waste stream with several different materials, and it could also include colorants, fibers and other impurities. The researchers demonstrated that chemical recycling could occur using a realistic polyolefin waste stream that includes a variety of plastic materials. As noted in the report:
“In real-life sorting of mixed plastic waste streams, separation occurs physically by sink-flow density separation or mechanical sorting after near-infrared analysis. Both methods could include our polyethylene-like polymers in the polyolefin stream together with polypropylene (PP) and polyethylene. Thus, the ability to separate our polyethylene-like polymers from these polyolefins in the chemical recycling process is desirable. To this end, pieces of commercial PP (re-usable hot beverage cup), HDPE (solvent bottle cap) and blue-coloured PC-18 were subjected to the above depolymerization process. Whereas the polyethylene-like PC-18 was completely depolymerized and dissolved, the PP and HDPE pieces were recovered in their original unaltered state ... colourants and carbon fibres, which even in small amounts are detrimental to recycling in general, could be removed completely.”
The Additive Manufacturing Industry’s Relationship with Recycling
Closed-loop recycling that does not degrade material performance is particularly desirable in the additive manufacturing industry. Better recycling would build on the sustainability advantages that additive manufacturing already has compared with traditional manufacturing. For example:
- A distributed economy enabled by additive manufacturing can produce lower carbon emissions compared to a traditional global shipping strategy.
- On-demand production reduces waste from unused inventory.
- Additive manufacturing can facilitate repairs, reducing waste.
Aside from what’s described in this paper, the additive manufacturing industry has produced other promising recycling technologies. For example, machines have been developed that grind and re-extrude 3D printer waste and plastic parts into usable filament. One such example is the ReDeTec ProtoCycler. According to the company website, this desktop machine integrates a grinder, extrusion, spooler and software control that can control melt and extrusion parameters in real time. As a desktop machine, the ProtoCycler seems best suited for small-scale 3D printing activities, such as for the hobbyist, or for use in education rather than in additive production.
Many vendors in the industrial additive space have published marketing materials describing a commitment to sustainability. For example, HP claims that it’s shifting to a system where the company can:
- Keep products and materials in use—Designing products for long life, offering service-based solutions that improve customer value and decrease environmental impacts, and recapturing products and materials at end of service for repair, reuse and recycling.
- Create a low carbon future—Improving product energy efficiency to reduce customers’ energy consumption and decrease product use carbon and water footprints.
- Design out waste and use materials responsibly—Increasing material efficiency, using more recycled content, and replacing materials of concern.
- Regenerate natural systems—Partnering to actively strengthen the natural systems that sustain life, with a focus on tackling ocean plastic pollution and protecting and restoring global forests.
New Opportunities for Use of Recycled Materials
Green-conscious consumers may wonder why manufacturers still use virgin plastics at all, given the relatively preferable environmental impacts of recycled material.
One factor is that certain recycled materials may contain harmful impurities, making them unsuitable for some applications such as food and beverage packaging. A 2017 paper published in the International Journal of Health Sciences and Research showed that samples of recycled plastic contained heavy metals such as cadmium and arsenic, and concluded that “recycled plastics products are not recommended for use in products intended to come in contact with foodstuffs.” Because chemical recycling can be used to isolate the pure monomer, it may present an opportunity to produce “cleaner” recycled materials that don’t have such impurities.
Another factor that contributes to the use of virgin plastics compared to recycled plastics is cost. Virgin plastics are produced from petrochemical sources extremely cheaply. According to a 2019 paper on recycling, “Historically, companies have used post-consumer resin (PCR) because it was a lower cost feedstock than virgin. In recent years, however, pricing for virgin plastic (mostly ‘wide spec’ resin) has fallen below that of PCR (mostly high quality PCR that is suitable for food contact).”
A third reason for using virgin plastics rather than recycled plastics is the downgrading of properties and performance that occurs. This is one factor that is mitigated by chemical recycling.
Despite the disadvantages of recycled plastic that often result in the selection of virgin plastic for manufactured products, chemical recycling and the use of plant-derived materials could present viable alternatives for manufacturers looking to transition to circular economies.
From the Nature report: “The approach demonstrated here provides a way to use fully recyclable polyethylene-like materials suitable for high quality applications. This is an important potential building block for a resource-saving low emission economy beyond the currently used technology.”
For more on circular economies and manufacturing, check out Prioritizing Circularity in the Design and Manufacturing World.