The Plastic Pollution Conundrum and The Closed-Loop Economy
Of all the challenges staring down humanity in the 21st century, plastic pollution portends severely dire consequences. For example, the vast majority of people know that plastic waste collects in the oceans, subsequently killing swaths of marine life. Moreover, micro levels of plastic also accumulate inside all sea-life-eating lifeforms. But, because plastic has benefited human wellbeing by keeping food fresh and serving as the foundation for other complex materials vital to the modern world, a complete decoupling from plastic use is impossible. Thus, the proliferation of the closed-loop or circular economy has become commonplace in recent years.
From Collection to Reprocessing
What is the circular or closed-loop economy? Both are synonyms for the same system. The following scenario best describes this system. First, consumers purchase goods packaged in plastics. Then, after the consumers use the goods, a public or private utility company collects the plastic containers that once held the products. The collecting company brings them to a material-recovery facility (MRF—pronounced "murf"). At the MRF, plastic is sorted, baled, then sold to a processor for reprocessing.
Depending upon the type of bales sold at the MRF, the processor might sort the material one more time to isolate certain plastic types. Then, actual reprocessing of the material begins: shredding the plastic into flakes (also called regrind) followed up by washing and separating the flakes from any other type of plastic in the batch (which is usually cap and label content). Next, using high-heat pelletizing machines, the processor transforms the flakes into pellets (also called granules or resin). Finally, the processor sells resin to a manufacturer who uses the resin to produce end products. The manufacturer sells the end products to consumers or businesses.
Businesses, consumers, collectors, processors, and manufacturers repeat this process over and over again, circulating the material in different forms from player to player. Ideally, manufacturers do not use any new feedstock products: they use only what has gone into the original circulation. We industry professionals call this a "circular economy" because the same inputs, in raw material or end-product form, circulate from consumer to processor to businesses and back. Additionally, we also call this system a "closed-loop system" because manufacturers use the raw materials (inputs) to produce the same end products; the manufacturers do no introduce any new raw materials into the loop. They only use what has already been introduced.
The circular economy heavily relies on the interplay between the groups mentioned above. First, consumers need to sort plastics and other recyclables from non-recyclable waste. To do this, they must possess a certain level of knowledge, which they usually obtain in primary or elementary school. Next, the same consumers usually rely on companies to collect waste and deliver it to a MRF. Consumers usually pay for this service on a subscription basis.
Crucially, businesses must have the know-how and machinery required to produce high-quality recycled pellets on par with prime-virgin plastic material. While not impossible, producing high-quality, comparable-to-virgin output is neither easy nor cheap. For example, some companies manufacture incredibly sophisticated pelletizing machines capable of making highly coveted resins. Equipment of this stature sells at over $1 million per unit.
Professionals refer to the sort of recycling described above as "mechanical recycling" because processors use physical machinery to separate, shred, wash, and melt down plastic feedstock. Mechanical recycling forms the backbone of the current circular economy. Unfortunately, while the workings of the circular economy appear ideal on paper, several impediments occlude perfection. The following fact accentuates a startling reality: less than 10% of all plastic waste reaches recycling plants, making waste management and recycling as a whole almost insignificant.
Mechanical Recycling's Drawbacks
Unfortunately, mechanical recycling is not without its drawbacks. In the end, these drawbacks contribute to the disquieting fact that less than 10% of all plastic waste flows circularly and sustainably.
Drawback 1: Rarities of Plastic Type
In addition to the popular types of commonly used plastics like high-density polyethylene (HDPE), low-density polyethylene (LDPE), poly-propylene (PP), poly-vinyl chlorate (PVC), and poly-ethyl terephthalate (PET), there are several other types that are unknown to most consumers. A few examples of these include poly-vinyl butyrate (PVB: used to make car windshields shatter-proof), poly-chromate (PC: used for various coatings and water-cooler bottles), and linear-low-density polyethylene (LLDPE: another polymer used for bag applications).
Many MRFs don't have the capabilities to sort these "more unknown" plastic varieties. Moreover, only a minuscule number of recyclers that specialize in processing these materials exist throughout the world. As a result, a large percentage of these plastics end ups in landfills.
Drawback 2: Lifespan
Recent research found that commonly plastics like PET, HDPE, PP, and LDPE have a "biological clock." This means that there is a natural limit for the number of times producers can reprocess each material before it becomes unrecyclable. However, not all plastic types have an equal degradation time. While HDPE, PP, and LDPE can be processed many times before polymer quality degrades to the point of non-recyclability, PET has severe limitations.
PET, the polymer used overwhelmingly for single-use bottle applications, has bleak drawbacks because each reprocessing session substantially decreases its quality. Therefore, pellet makers can only reprocess the same PET bottles no more than three times before molecular degradation disqualifies PET’s material integrity.
Drawback 3: Difficult to Recover Applications
Plastic bottles are sometimes processed into non-bottle applications, which pose extreme difficulties for mechanical recycling. For example, many PET beverage bottles get made into fiber (polyester). Fiber has many textile applications, including clothing, luggage bags, upholstery, and more. Unfortunately, PET-textile applications made from recovered PET bottles are particularly arduous, if not impossible, to reprocess. As a result, most polyester and other PET textiles often end up in landfills.
Like polyester made from PET bottles, many non-bottle applications for repurposed HDPE bottles exist. A few examples of these applications include plastic lumber and HDPE piping. But, like polyester made from PET bottles, reprocessing plastic lumber and HDPE pipes is daunting when compared to reprocessing HDPE plastic bottles.
Mechanical Recycling's Lingering Advantage
Despite the above drawbacks, mechanical recycling still prevents both the landfilling of millions of tons of waste as well as more plastic pollution. Mechanical recycling also provides resources to keep the closed-loop economy "closed," and mechanical recycling companies employ many people worldwide. Unfortunately, the fact remains that less than 10% of plastics make it into the closed-loop system. However, a glimmer of hope has emerged over the past few years. This glimmer originates from chemical laboratories, which have made pivotal strides toward innovating an almost futuristic kind of reprocessing: chemical recycling.
An Overview of Chemical Recycling
Unlike mechanical recycling, chemical recycling utilizes chemical reactions to break plastic down into either base hydrocarbons or long-chain molecules. The general method processing proceeds as follows.
Chemical recyclers insert post- consumer waste into their machinery. After a series of reactions, different precipitates come into fruition. The form of these precipitates depends on the type of chemical recycling applied. Some precipitates may be the liquid polymer form, which is very similar to what virgin processors create after they crack naphtha during the virgin plastic making process. Other times, the chemical recycling processors further deconstruct plastic into a gaseous, more-simple-than-polymer molecular form.
Thus, chemical recycling is an umbrella term for several different methods that deconstruct plastic waste to either its original polymer or molecular form. So just how exactly do these types of chemical recycling work? And what are the types of chemical recycling?
Feedstock Recycling
Feedstock Recycling is the first type of chemical recycling. In feedstock recycling, the methods employed break plastic material down to hydrocarbons, a group of molecules made up of carbon and hydrogen atoms. Because hydrocarbons are only single molecules, they're also known as "monomers." Importantly, hydrocarbons are the building blocks of all fossil fuels and therefore plastics.
All hydrocarbons form due to the natural long-term decomposition (millions of years) of organic matter. Replicating this natural process on an industrial scale, Feedstock Recyclers create hydrocarbon monomers in a much more advantageous-to-humanity time duration. Feedstock Recyclers employ three modern industrial engineering methods to make circular-economy-fueling monomers. We explain each of these methods below.
Pyrolysis
Pyrolysis, a process commonly used to produce fuel from bio-waste, has recently become an effective method for chemical recycling. The word “pyrolysis” comes from two Greek roots meaning “fire” (pyro) and “to break down” (lysis). Therefore, producers using pyrolysis break plastic waste down into single hydrocarbon monomers using extreme heat (between 350°C to 550°C) in the complete absence of oxygen (meaning this process is anaerobic). Thus, pyrolysis is similar to "cracking," an essential step in producing virgin plastic from crude oil. Because pyrolysis produces monomers like those made from crude oil, industry professionals have given this liquid end-product a special name: pyrolytic oil. Industrial-level production of pyrolytic oil has mammoth potential.
Pyrolytic oil is indistinguishable from crude oil, so a wealth of applications abounds. First, refineries can potentially transform pyrolytic oil monomers into all sorts of fuel for automobiles, engines, and other gas-requiring apparatuses. More telling for the plastics industry, however, is the fact that polymer-engineers can also transform pyrolytic oil into some plastic types.
While PET is not compatible with pyrolysis, many other mainstream plastic types, as well as a few rare ones, can go into pyrolytic oil production: polyethylene (HDPE, LDPE, LLDPE, etc.), polypropylene (PP), polybutylene (PB), and polymethylmethacrylate (PMMA). Most importantly, polystyrene (PS-the infamous plastic also known as Styrofoam), one of the most problematic plastics to mechanically recycle, is a prime candidate for pyrolysis.
Gasification
Like pyrolysis, chemical recyclers apply heat to plastic waste for gasification. Gasification, however, requires much higher heat levels (about 1,000°C to 1,500°C) than pyrolysis, as well as a minor presence of oxygen. Another difference from pyrolysis is the precipitate of the gasification process. Unlike pyrolysis, gasification does not yield a liquid monomer but a gaseous product called "syngas," which is a mixture of hydrogen, carbon dioxide, and carbon monoxide gasses.
Processors already use gasification technologies to produce base molecules and convert them into ammonia and fertilizers for the chemical and agriculture industries. And, like pyrolytic oil, manufacturers utilize syngas to make fuels like diesel, petroleum, and natural gas. Chemical recyclers can also isolate hydrogen gas from syngas and then use it to power hydrogen fuel cells.
There is some promise that utilizing gasification can produce plastics in the same way that pyrolysis can. In the mid-2010s, a Japanese company began work on a syngas production plant, which, upon completion, will make plastic resin. An aspect that makes gasification particularly advantageous is that all types of plastics– HDPE, LDPE, PET, compounded plastics, and even polyester– can be used to produce syngas.
Hydrothermal (Hydrolysis)
Hydrothermal recycling, the last type of Feedstock Recycling, functions in a manner more resembling gasification than pyrolysis. Like gasification, hydrothermal recycling uses heat (between 160°C to 240°C) and the combination of a very common molecule to turn a plastic-waste pile into a monomer pool. What is that monomer-creating molecule? It’s water; hence the “hydro” in hydrothermal. Because hydrothermal recycling uses water to break down the material, some also call it hydrolysis recycling.
Hydrothermal recycling is very new to the chemical recycling universe. Scientists are still aiming to discover ideal conditions and applications for industrial-scale hydrothermal recycling. An example of this research comes from a study, which concluded last year, claiming that hydrothermal recycling can transform PET to synthetic petroleum. Since hydrothermal recycling is currently in a nascent state, scientists are still determining to what extent it will work in chemically recycling other plastics.
Depolymerization
Depolymerization is the second and most complex type of chemical recycling. The complexities arise from the number of chemicals added to plastic waste to complete the Depolymerization process. While the three types of Feedstock Recycling rely on heat and (usually) one other molecule or compound’s presence, Depolymerization utilizes various chemical additives. Like Feedstock Recycling, Depolymerization also applies high temperatures, which deconstruct plastic waste polymer chains into either monomers or shorter polymer chains called “oligomers.” Industry professionals call Depolymerization “chemolysis,” as it relies on many chemicals to function. Essentially, the compounds added to the plastic waste work in a way precisely opposite from which the original polymer was formed or “polymerized.” Thus, the title of “Depolymerization” suits this particular chemical recycling process.
Unfortunately, Depolymerization only functions with a handful of polymers. These include PET, polyamides (PA, PLA), and polyurethanes. However, an advantage of Depolymerization is that manufacturers can successfully depolymerize polyester products, provided that processors carefully remove all other polymer types from each batch. As a result, Depolymerization of PET and polyester is picking up steam, with a few industrial depolymerization plants scattered across the globe.
Purification
Purification, the third and final type of chemical recycling, and has many similarities to Depolymerization. Like the Depolymerization, manufacturers must first sort plastics by type before Purification can begin. Then, the facility places the pure plastic-type into a solvent bath. However, unlike Depolymerization, which breaks the plastic down into monomers or oligomers, Purification breaks the plastic back down into its long-chain, original polymer form--the form that came from fossil fuels. Purification also does not require extreme levels of heat.
Purification can potentially work with HDPE, LDPE, LLDPE, PVC, and PP. Like Depolymerization, however, Purification is still in its infancy, as industrialists have yet to implement large-scale Purification processes.
Conclusion: Chemical Recycling’s Long-Term Outlook: March’s Topic
As explained above, chemical recycling is an amalgam of five different processes which go beyond the classic shred-wash-pelletize steps of mechanical recycling. Chemical recycling is also a broad-spectrum term that can include up to five methods. These methods differ from mechanical recycling because they break plastics into hydrocarbons, monomers, oligomers, or polymers. In addition, chemical recycling can rely upon high levels of heat and external additives, or combinations of heat and additives, to function thoroughly.
Now that we’ve hashed out the differences between each type of chemical recycling, how exactly are companies planning on implementing it to solve the waste management problem? How far have waste-management or chemical companies progressed in their efforts to enact chemical recycling on a massive scale? The answers to these questions and the effect Island Leaf believes that chemical recycling will have on creating a genuinely closed-loop-waste-management economy will be the topics of next month’s blog.
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