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The Polymerist

Solving The Plastic Waste Problem

A Working Proposal

This is a collection of Tuesday posts I wrote for my newsletter The Polymerist. A free newsletter that publishes twice a week with one in-depth article on Tuesdays and a roundup and honest analysis of what happened in the world of chemicals on Fridays. Join the subscriber list to get free updates with honest analysis from someone with a PhD in chemistry from the industry.

I decided to put this together into one place where people interested in plastic waste could read it, make comments, highlight important sections, and all around tear it up. You should not need a degree in chemistry to understand it either. This will appear long, but I feel it is actually quite short for how nuanced this problem is on a global scale.

About 50% of total plastic we produce is polyethylene and polypropylene. China has severely restricted how much post-consumer plastic they will take from the United States so we find ourselves in a bit of a conundrum of what to do with it all. Today I will attempt to give some background on how these materials were invented so that we might better understand how to recycle them.

My training as a chemist focused primarily on being able to make synthetic polymers that have really useful properties. A polymer is a generic term that encompasses a broad class of molecules such as cellulose, proteins, plastics, and specialty polymers. There are six types of modern plastics that make up what most people consider to be plastics and then there are a myriad of different types of polymers that work behind the scenes that are lesser known and are known as “specialties.”

In terms of total volume of polymers produced the “big six” plastics represent ~75% of all synthetic polymers. The “big six” are high density polyethylene, low density polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyethylene terephthalate.

This is an edited excerpt from a book I’ve been writing about polymers, society, and how we might be able to solve today’s problems with the ingenuity and science that got us into the problems we now face. If you were stuck on Mars would you want to be stuck there with the rocket scientist that got you there in the first place?

The Big Six Plastics

Polyethylene and Polypropylene:

It is difficult to write about polyethylene and not write about polypropylene. These two types of plastics represent half of the big six with low density polyethylene (LDPE), high density polyethylene (HDPE), and polypropylene (PP). In recycling symbols or numbers for plastic they would be numbers 4, 2, and 5 for LDPE, HDPE, and PP respectively. Limitations with making LDPE have yielded pathways to making both HDPE and PP through similar methods.

Low density polyethylene was invented in 1933 at Imperial Chemical Industries (ICI). The idea that polymers were these big long molecules was still a very controversial topic at that time. Herman Staudinger would not win his Nobel Prize in chemistry for polymers until 1953, which means that in 1920 he published his first paper on the concept of “polymers.” While academics were debating if polymers were real many chemists in the industry were racing ahead and doing fundamental research that would lead to the discovery of plastics.

In the 1920s ICI decided to embark on basic research and to pursue reactions under high pressure. At the same time DuPont had hired Wallace Carothers (inventor of Nylon) to work on how to make new commercial products. This investment into R&D represented a start in the pursuit of finding better ways to utilize oil at many industrial chemical companies.

The original goal of ICI chemists before they made polyethylene was to add ethylene into benzaldehyde, but instead they got a waxy solid that was the first low molecular weight polyethylene and the unreacted benzaldehyde. In further trying to refine their methods the ICI chemists improved the conditions necessary to get ethylene to polymerize with itself and made the discovery that some oxygen was important due to a small leak in their initial reactor. That oxygen was key in being able to make a high molecular weight polymer of ethylene.

Sometimes scientists make discoveries by accident, but the fundamentals of having a hypothesis, being observant, taking good notes, and trying to understand why initial hypotheses didn’t work often yields what is needed to succeed. The Rolling Stones line of:

You can’t always get what you want. But if you try sometime you find. You get what you need.

Often, in my own experience as well as some stories I’ll share through here I find that the best discoveries are often those that occur right in front of us and are unexpected. We have to be able to get out of our own way.

The word polymer deserves unpacking as it is a mash-up of the Greek word polus, which translates to many or multiple and meros, which means part. So a polymer is multiple iterations of a single part. In the case of polyethylene the starting molecule, ethylene, is polymerized in a set of chain reactions that forms polyethylene.

Polyethylene would become critical in WWII for being able to insulate telecommunication cables that stretched across the bottom of the Atlantic ocean from the US to England for faster telecommunication and insulation of other wires such as radar cable insulation. Polyethylene and polyolefins in general are used as coatings for wires because they are great at insulating conducting wires and they are able to deflect water as well.

Prior to polyethylene insulation of wires people used what are known as drying oils and were naturally derived such as linseed or soybean oil. These same oils are used in oil based paints. The word “drying” in this context refers to the ability of the oil to harden over time, which is actually a chemical reaction of the oils reacting with themselves and making an interconnected network or a polymer. This process without the addition of heat or sunlight can take a long time to occur and the method is prone to defects in the coating, which is not something you want when running a telecommunications cable across the bottom of the Atlantic. This same reaction of reacting oils takes place when seasoning a cast iron pan in the kitchen. The preferred oils for seasoning are typically olive oil or soybean oil.

A fun home experiment would be to try and season a cast iron pan with coconut oil (an oil without the polymerizable part) and olive oil. Try different temperatures for seasoning and different methods, but be prepared for generating smoke. What oil more readily yields a nice non-stick coating after a light washing of the cool cast iron pan? Does one oil need more heat than the other?

The invention of polyethylene (patent granted in 1939 to ICI) also helped displace the need to use natural rubber from the rubber tree. Think about waterproof or water resistant clothing at the time. The only way to make waterproof boots or ponchos was to use rubber that people extracted from the rubber tree. Growing rubber trees and extracting the rubber is both labor and time intensive. Further, rubber trees do not grow readily in much of the climate of North America or Europe so that natural rubber would have to be shipped from Mexico, Central America, Asia, or other climates that facilitate rubber tree growth.

Once ICI was able to demonstrate to the world that they could make polyethylene there was a lot of interest in being able to make polyethylene that did not infringe on ICI’s patented process, which was very specific. Karl Ziegler was considered one of the first to come up with an alternate route to polymerize polyethylene via a catalyst that he discovered on accident from one of his high pressure reactors. An emulsified metal in Ziegler’s reactor after it was cleaned helped yield a low molecular weight polyethylene that did not utilize the conditions of ICI’s process.

While Ziegler was at the Max Plank institute for Carbon Research he and his team would eventually discover they were able to make a polyethylene of exceptional purity and was able to organize itself into a higher density. Ziegler’s discovery was that certain transition metals were able to facilitate the polymerization of ethylene into polyethylene that was free of the branches typical of the radical process. This branch free polyethylene allowed for the polyethylene chains to associate with themselves more readily and were better able to pack together, which ultimately led to a higher density. High molecular weight polyethylene without branching is known as high density polyethylene (HDPE).

Ziegler would share his knowledge of this catalyst with an Italian chemist Guilio Natta who with the financial backing from the Italian chemical company Montecatini would be able to polymerize propylene into polypropylene. Being able to find a use for propylene at the time was significant as it was not seen as a very valuable gas, much in the way that ethylene was prior to the invention of polyethylene.

In 1951 two Phillips Petroleum chemists, Robert L. Banks and J. Paul Hogan, discovered a route to polymerize propylene and ethylene via an organochromium catalyst on silica. Banks and Hogan would go on to win the Perkin Medal for their discovery, but would ultimately not share the Nobel prize with Ziegler and Natta. About ⅓ of the world’s HDPE is produced via the Philips catalyst as well as a significant amount of the world’s polypropylene.

Ziegler, Natta, Banks, and Hogan would set the stage for a whole new research topic for academic and industrial chemists around the world, which was transition metal mediated polymerizations and a whole branch of chemistry.

Polyethylene and polypropylene are the first and second most produced polymers in the world with a combined global market value in 2019 of over $200 billion dollars. Their starting materials were of not much use in the 1930s, but in the 21st century have become essentially for many of the goods we do not think about such as wire coatings, toys, face masks, food packaging, automobiles, bikes, trains, buses, airplanes, footwear, agriculture products, and more in less than 100 years.

Being able to produce ethylene and propylene at scale is one of the key abilities of companies able to make polyethylene and polypropylene. The route to making the starting monomers is most economically feasible at the time of this writing via steam cracking. Some integrated oil companies such as ExxonMobil can make their own polyolefins and some chemical companies can pipe in ethane and propane feedstocks in order to crack their own monomers.

Low oil prices generally favor making polyolefins because it boosts the profit margins. An oil company might view making chemicals as a high margin business that can offset their fuels and lubricants business. A chemical company might look at making commodity polymers a way to keep the lights on while they pursue higher margin specialty products.

The next three plastics in the Big Six are polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS). There are three easy examples to illustrate the ubiquity of these plastics. If you have ever drank something out of a plastic bottle there is a 95% chance it was polyethylene terephthalate. If you have ever looked under a sink and saw some white piping that was polyvinyl chloride. If you have ever received something with some sort of molded foam packaging or drank something out of a red Solo cup then you have touched polystyrene.

The routes to these plastics are a bit more complex than polyethylene and polypropylene and there isn’t a nice little story about how invention of one led to another and then someone won a Nobel prize. The similarity that these plastics do have with one another is that they are dependent on the concept and production of platform chemicals.

Platform Chemicals

As the name might suggest these are chemicals from which other chemicals can be derived. One of the most important platform chemicals out there is benzene. When oil is distilled and fractionated there is a process called “catalytic reforming” where an oil fraction called “naptha” further processed with a catalyst to reform the chemical structure. Primarily, the goal is to get benzene, toluene, and xylenes otherwise known as BTX.

The toluene can be used as a solvent, but a major use is to transform it into benzene. Xylenes can also be used as a solvent, but para-xylene is often the most sought after. The other platform chemical relevant to this chapter would be ethylene. From ethylene, benzene, para-xylene, and chlorine we can make PET, PS, and PVC.

Polystyrene

React benzene with ethylene correctly and ethylbenzene is produced. Remove two hydrogens from the ethyl portion of that ethylbenzene and styrene is achieved. That is the way it has been done for just under 100 years, but in 2015 a new route to polystyrene was achieved in a single step. Vaughn and coworkers reported this significant discovery in Science — one of the most prestigious scientific journals on the planet. The differences in processes is shown in the figure below.

Rhodium is the most expensive metal on the planet and thus hinders the immediate commercial viability of the process. If the catalytic process can be done with a different metal then the synthesis of polystyrene becomes more efficient than the world’s best chemical engineers could achieve with the old process. If the new process were to become commercial it would then see decades of process efficiency gains as chemical engineers work to make it better.

Is making styrene and eventually polystyrene in this way better for our world? Well, it would make the process more efficient, lower prices, and if adopted by the current styrene producers it could provide significant shareholder value. This new process could also make it more difficult for any new plastic or polymer to compete and take its place.

Polystyrene is typically made via radical polymerization and is a common experiment performed by undergraduate chemistry majors. The polymerization methods for making polystyrene are not too dissimilar as polyethylene. Special catalysts (similar to Zieglar-Natta catalysts) can also produce different orientations of how the styrene monomers add together and can be used to help fine tune their properties for engineering applications.

The largest consumer of polystyrene is expanded or extruded polystyrene foam. Extruded polystyrene foam is made through an extrusion process and is more applicable to construction insulation products while expanded polystyrene foam is made from expanding small polystyrene beads in a mold. Expanded polystyrene foam is the form of polystyrene that most people are familiar with and it is also one of the most difficult polymers to recycle due to the low density as a result of the foam.

In terms of an insulating material extruded polystyrene foam has a difficult time breaking down, has a low cost, is light, is an excellent insulator and if building a home that should last for 30+ years is a good option for insulation. For sending me a computer monitor in the mail expanded polystyrene seems to only be used due to being able to absorb shock and because its low cost.

Unlike the other plastics in the Big Six expanded and extruded polystyrene are not typically accepted by recyclers. One issue with expanded polystyrene foam is that to transport it significant distance is uneconomical because it takes up significantly more volume than it weight compared to other plastics. Even if you were to get the foam all in one place it requires significantly more processing compared to the other plastics in order to be recycled.

Polyvinyl Chloride

The only plastic out of the big six that contains an atom other than carbon, oxygen, and hydrogen. Polyvinyl chloride or PVC is made from vinyl ethylene in a polymerization process similar to making polyethylene or polystyrene. The synthesis of vinyl chloride is non-trivial not because it is difficult, but because chlorine gas and hydrochloric acid are dangerous. The main routes to vinyl chloride is thermal cracking of ethylene dichloride with a byproduct of hydrochloric acid (HCl). The platform chemical of note here once again is ethylene and is needed by all of the plastics in this post.

The synthesis of ethylene dichloride follows either a direct chlorination of ethylene with chlorine gas to make ethylene dichloride or reacting ethylene with chlorine gas in hydrochloric acid. ICIS has a good summary of the process here. Either route you still need chlorine gas — one of the first chemical weapons used in World War I. Handling of chlorine gas is dangerous, but is often done with a high degree of safety in the chemical industry.

The main uses of PVC though are not necessarily consumer focused, but rather focused on construction products such as vinyl siding, windows, and plumbing pipes. PVC is not as good of an insulator for electrical wires as polyethylene, but it can be used for some lower voltage applications.

Of the big six plastics PVC is the least likely to end up in a recycling bin because it is not used in the everyday lives of most people (unless you are a plumber). PVC can be recycled, but the chlorine content means that it needs to be completely separate of the other plastics in the big six before it can be processed.

Polyethylene Terephthalate

The last plastic to be covered by me here and maybe one of the most ubiquitous that we encounter on a daily basis is polyethylene terephthalate (PET). The properties that really make PET stand apart from the other plastics are the gas barrier properties, clarity, durability, and versatility. We use PET for just about every bottled soda, water, juice, and tea. PET also makes great fibers and has been used in fishing nets and textiles for clothing.

The main route to PET is from oxidation of para-xylene to terephthalic acid and then esterification with methanol to produce dimethyl terephthalate, which is one of the two monomers needed for PET. The other monomer, ethylene glycol, is typically made from ethylene and oxygen to get ethylene oxide and then reacting that ethylene oxide with a hydroxide (sodium hydroxide, calcium hydroxide, etc) to make ethylene glycol.

Ethylene glycol and dimethyl terephthalate can then be reacted in the presence of a catalyst to yield polyethylene terephthalate and a methanol byproduct, which can be funneled back to the the dimethyl terephthalate process. PET is the only plastic in the big six to need two different monomers and it is the only plastic to also be made through a condensation polymerization as opposed to an addition polymerization.

Condensation polymerizations are reactions where during the polymerization another chemical is produced that must be removed from the reaction. Polyethylene, polypropylene, polyvinyl chloride, and polystyrene are all made through addition polymerization of one monomer where the monomer react with itself and no atoms are lost.

PET is built on the platform chemicals of para-xylene and ethylene. Already in covering the big six we can see that ethylene, propylene, benzene, and xylenes are completely necessary. The easiest route right now to get these platform chemicals is from distillation and fractionation of crude oil. There are other platform chemicals such as phenol, which is made from benzene and propylene, that get transformed into an even wider array of chemicals that we depend on everyday.

Recycling Route 1: Mixed Plastic Pyrolysis

What is Pyrolysis?

Pyrolysis is the act of heating up something without the presence of oxygen, typically in an inert atmosphere like pure nitrogen or argon. The concept of pyrolysis has been around for quite a long time and it is how coal is transformed to coke, how polyacrylonitrile is converted to carbon fiber, and how we can make hydrogen out of methane.

Pyrolysis of most carbons produces gas, liquid, and solid products. The temperature, duration of the process, and use of a catalyst can all play a role in the composition of the yields of liquids, gases and char produced during pyrolysis. The catalyst plays an important role in determining what types of pyrolysis products come out of the pyrolysizer as well such as the composition of gases or liquids. A simplified schematic of what a pyrolysis set-up might look like can be seen below in Figure 1. Volatile liquids get captured in the cold traps, the inert gas escapes, and the mixed gas products continue on further while the char stays behind in the main heating vessel. Essentially, plastic goes in and then liquid, gases, and char come out the other end.

The thing to take away from pyrolysis of plastic waste is that under the right conditions, chemicals that resemble oil distillates can be produced. If pyrolysis is done near or next to a cracker or catalytic reformer those pyrolysis products could be transformed or purified using existing technology developed for oil based products into the feedstocks that produced the plastics in the first place or something else that the market might need.

The advantage of pyrolysis is the flexibility to produce fuels, lubricants, plastics, or more specialty chemicals. The flexibility of pyrolysis is why I think it will be a preferred method of recycling, especially when it comes to somewhat impure, but known plastic recycling waste streams. For instance in Figure 2 we can see how pure, binary, ternary, and quaternary plastic waste mixtures behave under pyrolysis conditions with different catalysts.

One thing that should hopefully jump out to you is that the polystyrene pyrolysis products predominately are oils both with the thermal and acid modified catalyst. Investors and the chemical industry have also taken note that pyrolysis of polystyrene can produce a high yield of liquid and may provide a new outlet for all of the consumer polystyrene foam that is difficult to recycle.

Is Anyone Doing Plastic Pyrolysis?

C&EN reported in late 2019 that one of many companies working on pyrolysis of plastic waste called Agilyx (I hope the wordplay on agility is not lost here) had successfully built a pilot plant capable of processing 10 tons/day of polystyrene via pyrolysis. Successful demonstration of chemical recycling at scale is the thing that most chemical companies need to see before they are willing to enter partnerships with start-ups or even begin doing the necessary R&D work to look at new products or license a new technology. Additionally, because Agilyx holds IP on their process and are investing into R&D, the ability to license their technology out to larger integrated companies like Sabic is another revenue stream on top of selling products from their growing chemical recycling business. Figure 3 shows the closed loop marketplace envisioned by Agilyx and the role their customers might play in the plastics supply chain.

One benefit that chemical recycling has over melt reprocessing of existing plastics is that it is a route to overcome the regulatory restrictions on food contact materials. The barrier properties for instance on recycled PET (rPET) will not be as robust as virgin PET. Chemical recycling enables consumer packaged goods companies to make their packaging circular while also not having to worry about any sorts of regulatory approval issues from government agencies. An example of this type of recycling happening I talked about in a Friday newsletter where a consortium of chemical and plastics companies are making multilayered packaging for meat food contact applications from mixed-use chemically recycled feedstocks.

Chemical Recycling Can Expand Recycling Of More Materials

A large portion of food packaging materials are multilayer laminates where each layer of different plastic serves a very specific purpose. The problem with laminates is that they cannot be physically recycled and are not accepted in traditional recycling streams now. Chemical recycling could in theory expand what we consider to be recyclable plastics and have the agility to create a diverse amount of end products from the oils and gases produced based on market demand. Mixed plastic waste could be the 21st century version of crude oil.

The Chemical Industry and Wants This To Happen

The American Chemistry Council is the main trade advocacy group for the chemical industry (lobbyists) and they have formed a special division called the Advanced Recycling Alliance for Plastics with the sole mission to educate the public and policy makers about how chemical recycling of plastic works and why it might be beneficial.

I expect installation of chemical recycling capacity will grow over the next decade and that single source streams of polystyrene, polyethylene, polypropylene, and polyethylene terephthalate will command premium value while well defined mixed-use plastic streams will be of lesser value.

Less Dependent On Oil Prices

Low oil prices make traditional recycling of plastic (melt reprocessing) really uneconomical. C&EN’s Alexander H. Tullo spoke to Nina Bellucci Butler of More Recycling about the challenges of low oil prices and utilizing traditional recycling methods on plastic waste.

In the early 2010s, Butler says, companies were investing in, and plastic goods makers were increasingly using, recycled material. That faded when oil prices tumbled. Recycling isn’t attractive when oil is below $100 per barrel, she says. “Companies aren’t ready to pay 20% premiums for a product that is not at the same level of quality as virgin unless there is a marketplace incentive to producing a product with a lower carbon footprint.”

I think chemical recycling could be a way around the low oil priced future I’ve spoken about and I believe oil companies themselves want to diversify their income streams away from traditional refinement and extraction of oil post Covid-19. I believe that chemical recycling of plastic and processing the products for an oil company will be a strategic move that many companies will make in the next few years. Shell announced about a year ago they planned on using a million tons of plastic waste in their plants by 2025 — before Covid-19 sent oil prices plummeting and we had negatively priced oil futures.

I suspect a small amount of public policy from governments around the world could help speed up this transition to this new recycling method and help lower the barrier of entry for building secondary sorting facilities. I will think about what types of public policy initiatives might be useful in the coming weeks.

Recycling Route 2: Mechanical Recycling

Low Demand = Low Pricing = Too Expensive to Recycle = Landfill

I believe that chemical recycling is the future and it is a key in saving us from drowning in plastic waste, but mechanical recycling is what we have known for decades and it is a valuable and viable method for recycling plastic, specifically HDPE, PP, and PET because there are uses for them.

If we look at the numbers from the EPA we can see that recycling volume is small compared to landfilling and the problem is primarily one of economics — not capability. Recycling plastics through melting is not as easy as it might sound.

I plotted the EPA’s data into Figure 2 to look at how recycling has gained adoption since the 1990s, but overall it seems to have peaked at about 9% in 2015 and then dropped to about 8–8.5% in 2017 and 2018.

So why does so much go plastic go to landfills and not into other stuff? This is a really complex question that I’m not sure I can answer here. My back of the envelope calculations in Figure 3 take into account what I could find on actual pricing of recycled HDPE, PP, and PET in 2020 versus the average cost to recycle plastics in the US over the last 5 years. Some of the data is a bit mismatched based on time and the price of recycled plastics fluctuates dramatically.

Essentially right now in the United States it appears to cost more to recycle color HDPE, PP, and PET whereas natural HDPE (no color) commands the highest price and seems to be economically viable. This is why shipping our plastic waste to China was beneficial, lower costs on recycling, and more advanced equipment. The figure from the start of the series might be more helpful now. I would not be surprised to learn that we are landfilling more plastic in 2020 than we have previously.

Another source that is helpful is McKinsey’s analysis of plastic recycling. They have an excellent report here with two figures that are useful to fully understand the challenges of traditional recycling. The figures come from McKinsey’s economic model.

From Figure 4 we can see that PET at the time of running the model was quite valuable in North America whereas polypropylene in Europe would yield little profit. Plastic waste from electronics and other material in Figure 4 actually represent a negative EBITDA, which means you would lose a lot of money if you tried to recycle those materials. That loss on recycling could be partially driven through trying to recycle thermosets in electronics or low density plastics like polystyrene foam.

McKinsey’s second figure worth discussing looks at return on invested capital.

My takeaway here is that to actually install more capacity to recycle more plastics only about 20% of all the plastics made right now would actually provide a return on investment. In the United States we are already nearing about half that capacity with our 8.5–9% recycling rate.

Essentially without some sort of massive government subsidy recycling plastics is not very economically viable. Economic viability of recycling is what puts a recycler in business.

The video below has a good example of what it takes to actually recycle PET from sorting to washing to cutting to disposing of the waste water.

How Does This Type of Recycling Actually Work?

Now that we have the economics somewhat covered how does this process actually work once we have the raw material ready to go?

The first step would be to have the chips of plastic fed into an extruder and then transformed into into either pellets (to be processed into its final form) or directly transformed into the final form such as the actual plastic part, which could be anything from films to bottles to fibers to cups. A good visual explanation of the extrusion and molding process can be found in the video below and covers many of the actual forming aspects of plastics production.

There are some finer details such as sometimes needing to dry the resins prior to extrusion and incorporating additives such as light stabilizers and antioxidants or some sort of master batched additive to impart color or unique properties to the end product. The final product may also be laminated with other plastics to provide gas barrier properties (think food packaging) or blended with other plastics to provide a unique set of physical properties such as impact strength.

One of the benefits of mechanical recycling work is that we do not need to do any chemistry to the plastics before they can be used again, but the end uses may primarily be relegated to non-food contact applications such as textiles and apparel. However, if regulations change food contact recycled plastics may become a reality in the near future and the current requirements for recycled plastic for food contact applications by the FDA can be found here.

We should consider mechanical and chemical recycling to be complimentary to each other and both are tools that we will need to use to solve the plastic waste problem. I’ll write about recycling method 3 next week.

Biomass To The Rescue? Not So Fast.

In the utopian future that many academic green chemists envision is one that is not dependent on oil. All of our polymers, lubricants, fuels, plastics, and anything a chemist can think of making at scale for either human or industrial use would come from biomass. Sure, this is somewhat of a hyperbolic view that I am purporting here, but I am doing this to try and demonstrate that we cannot put all of our hopes and dreams of figuring out a circular economy on the basis of biomass conversion alone.

Full disclosure here, my PhD was based on this sort of utopian dream and after about 5 years in the chemical industry I am more aware of how difficult and seemingly impossible that sort of dream is in the next 50 years, maybe even in the next 100 years.

I will use two of the most viable biomass polymers that have manufacturing at scale as examples on how plastics and polymers from biomass might become a solution to fixing part of the plastic waste problem.

Polylactic Acid (PLA) aka Poly(lactide)

What is PLA?

The open access review article where I am pulling much of this information. I’ve written more about PLA previously here.

Wallace Carothers at DuPont was the first to successfully synthesize PLA in 1932 by heating lactic acid and removing water via vacuum distillation. Chemists would eventually figure out how to cyclize lactic acid into lactide, a cyclic diester comprising two lactic acid molecules, and perform ring opening polymerizations to obtain high molecular weight poly(lactide) that would have properties somewhat similar to polystyrene.

Early use of polymers made from lactic acid was actually in conjunction with glycolic acid to make bioresorbable sutures in 1974 under the trade name of Vicryl. In 2001 Dow and Cargill would come together to form the joint venture NatureWorks to make PLA under the tradename of Ingeo with the intent of pushing PLA into the world of single use plastics, packaging, and general widespread consumer adoption.

Lactic acid right now is primarily obtained from fermentation of sugar, in the US this is primarily sourced from corn, and NatureWork’s production headquarters is based in Nebraska. In 2013 NatureWorks would sell approximately a billion pounds of PLA and since then there has been widespread investment into producing more PLA globally. For instance Total Corbion has announced the planning of a second PLA facility in Europe than can produce 100,000 tons of PLA per year.

The Benefits and Uses of PLA

Poly(lactide) is essentially just a really long chain of lactic acid molecules. When it comes into contact with water and/or enzymes under the right conditions it can degrade into lactic acid, a natural acid that can be processed by our own bodies and by nature. The physical properties of PLA put it somewhere close to polystyrene, one of the big six plastics, and it is often used in single serving uses such as coffee capsules, beverage cups, utensils, and as a water barrier coating for ecofriendly paper cups.

You should understand that traditional paper based beverage cups often have a synthetic polymer coating that provides a barrier against the paper getting wet. This polymeric barrier can make it difficult for paper beverage cups to biodegrade and/or be recycled.

On a more advanced level medical grade PLA can be used to make heart stents, sutures, bone screws, tissue scaffolding, and as a vehicle for slow released drug delivery. Because of PLA’s similarity to polystyrene extensive work has been done to elevate PLA to an engineering material which entails raising its glass transition temperature, reducing creep, and improving the fracture toughness and impact strength.

The purported benefit of PLA ultimately comes into its ability to be composted, but the composting of PLA is only viable under industrial composting conditions. Trying to compost PLA in a backyard compost pile might take a a decade and even longer if it was just thrown on the ground. From a chemistry standpoint PLA is definitely compostable, but this ultimately begs the question of where could we compost it?

The Cons of PLA

Having a plastic based on food production (sugar) is problematic, especially now, when a significant portion of the world’s population is food insecure. 54 million people in the United States are facing food insecurity right now during the writing of this article in part due to Covid-19. Having a greater portion of farmland going towards making a biobased plastic with the reality of food insecurity and hunger going on seems nonsensical. But I suppose our world is often full of nonsensical ideas.

Despite PLA being compostable under very specific conditions there is no widespread network of compositing collection sites. Does PLA actually even fix anything if it ends up in a recycling stream where it cannot be mechanically recycled or if it ends up in the trash and goes to a landfill?

It is easy to be myopic about about PLA from this point of view. Having a non-food source for lactic acid and a fully developed industrial composting supply chain would remove the majority of the cons around PLA.

Cellulose Acetate

Everything Old Is New Again

James A. Moore, a professor at RPI had those words as the title of one of the last posters he would ever present before he became an Emeritus professor at Rensselaer Polytechnic Institute. I worked in his old synthetic polymer lab and his poster was about a chemical called diphenolic acid. I would later go on to use diphenolic acid as a foundational piece to my doctoral thesis and I was a teaching assistant for him in undergraduate organic chemistry labs. Diphenolic acid is obtained from levulinic acid and phenol and is what people used prior to the invention of the cumene process that produced an abundance of acetone and was used prior to bisphenol A. So despite it being very old it was new for my doctoral thesis just like cellulose acetate seems to be now.

An excellent open source review on cellulose acetate can be found here

Cellulose acetate is one of the oldest plastics ever created and the main starting material for cellulose acetate is the most abundance biopolymer on planet Earth — cellulose. Paul Schützenberger discovered that cellulose could react with acetic anhydride in 1865, but 29 years later Charles Cross and Edward Bevan developed cellulose acetate into a plastic around the same time as Leo Bakeland’s invention of Bakelite. Cellulose acetate is sort of a synthetic polymer, but not in the same ways that Bakelite or even PLA is because it actually involves disrupting and modifying cellulose, which is already a polymeric form of glucose.

The plastic that had been dominate before cellulose acetate was cellulose nitrate, which was obtained from nitration of cellulose. The main problem with cellulose nitrate was its propensity to explode and be highly flammable. Cellulose nitrate was known as celluloid and was in widespread use as guncotton in the mid 1850s and for movie films until about the 1950s. Cellulose nitrate is the basis for how Quentin Tarantino planned the assassination of Adolf Hilter in his film Inglorious Basterds through a somewhat common occurrence of a cinema fire. Eastman Kodak produced early photography film with both cellulose nitrate and then transitioned to the safety film known as cellulose acetate.

Think about that for a second. It took about 100 years for cellulose nitrate, a known explosive, to be phased out of commercial production. It should also be noted that the concept of plastic and polymers was controversial and not widely accepted at the time.

Cellulose acetate today is made primarily through reacting cellulose with acetic anhydride until enough substitution of the hydroxyl groups on the cellulose disrupts the cellulose crystallinity and renders the modified polymer soluble in different solvents. Cellulose acetate is versatile, it can be used in injection molding, it can be made into fibers, and it is often the plastic of choice for sunglasses and prescription eyewear.

Using Cellulose Acetate In New Ways

In October of 2020 Celanese, one of the oldest makers of cellulose acetate, launched a new cellulose acetate product for packaging and food contact applications under the tradename of BlueRidge. You can watch the video from Celanese below on the lifecycle of cellulose acetate.

The two major differences between cellulose acetate and PLA are that cellulose acetate can decompose in soil without any special conditions and it comes from a non-food biomass source, specifically wood pulp right now, but other cellulosic sources could also be from grasses such as switch grass or ramie.

Cellulose acetate and other cellulose derived plastics could be the sort of compostable plastic from non-food biomass that could solve a lot of our problems, especially in areas like food packaging and single use plastics. Even if we were to get widespread adoption of cellulose acetate we would still have problems with availability of composting accessibility.

The Composting Problem: We Need More Capacity

If we magically turned every plastic that is currently being produced into something compostable but still maintained all of the amazing physical properties of our current consumer grade plastics we would still probably landfill about 85–90% of this plastic. The problems that synthetic polymer chemists, polymer engineers, microbiologists, and polymer scientists can solve all revolve around producing new materials or modifying old materials to give the markets the properties that are in demand. So if the world wants a compostable plastic from biomass that can perform similar to polyethylene for the same cost I think its possible. It might take some time, but I think its possible.

If we were to solve the problem of making the materials of our dreams we still have the problem of not having a network of composting that could turn those materials back into soil or some sort of feedstock that would be useful. According to the EPA about 24% of our municipal solid waste is compostable. Right now that means it could be paper, yard trimmings, or food waste. If we were able to add in a significant amount of plastics to that percentage we could theoretically achieve >30% compostable municipal waste.

Three End Of Life Routes For Plastic

We have concluded what I consider the three routes of solving the plastic waste problem.

  1. Chemical Recycling could expand what is possible to be recycled including multi-layer plastics that are very difficult to recycle via mechanical methods and much of the current plastics that are not recycled such as polystyrene. This represents a circular economy from oil.
  2. Mechanical Recycling is our traditional route of recycling plastics and is a route that is currently under utilized and also represents a circular economy from oil.
  3. Synthesis and Composting of plastics from biomass represents a potential route for low carbon footprint plastics that could also be transformed into high value compost for agriculture and represent a cradle to grave supply chain.

From a technical standpoint I believe that this is as complete as it could be with respect to solving the plastic waste problem. I am working on what role the government might play, but I think that deserves its own post and is also complex and nuanced. I also do not know enough to write it in anytime soon. If I do write it I will add it on here. Consider this a living document.

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Tony

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Curated long form articles about chemicals, energy, oil and gas, plastics, and thoughts on how to solve some of the world’s biggest challenges.

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Anthony Maiorana

Anthony Maiorana

Writer of The Polymerist newsletter. Talk to me about chemistry, polymers, plastics, sustainability, climate change, and the future of how we live.

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