Solving the Plastic Waste Problem: Part 1
Understanding how our plastics industry works is the first step to understanding how to fix it.
One thing I am seeking to do here in addition to summarizing chemistry news is to try and educate. At this point in time the majority of the world understands there is a plastic waste problem. An easy feel good solution might be to swear “I’ll never buy plastic bags again,” and while good intentioned does not actually contribute to solving the problem of plastic waste because its bigger than changing how a consumer consumes. Another solution is to increase our recycling, but mixed recycling streams are difficult to separate and then partition into useful raw materials, but we as a society can work on that.
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In the News
The American Chemistry Council has announced a partnership with Titus Material Recovery Services to do secondary separation of recycled materials at material recovery facilities (MRFs). Secondary separation means isolating paper, glass, and specific plastic types together. Having pure recycled plastic means that it is easier to reprocess that plastic into other forms of material such as water bottles, films, wire coatings, and textiles. Having the paper on its own means its easier to use that material for generating chemicals like Levulinic Acid or biofuels like ethanol and butanol.
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?
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.
So How Does This Help Solve Plastic Waste?
This sets the stage for us to know how polyethylene and polypropylene are made and that we only discovered how to do it less than 100 years ago. The American Chemistry Council and Titus are taking the first step towards solving the problem and that is doing the job of separating the recyclables into single sources. Polyethylene goes with polyethylene and polypropylene goes with polypropylene. We can think of this separation as the first step to solving the plastic waste problem. In the Tuesday newsletters I will discuss strategies on the different recycling strategies for these plastics.
To fix a problem you need to understand it.
Tony
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The views here do not represent those of my employer nor should they be considered investment advice.