Notes on Plastics

The following is by no means a comprehensive guide to industrial chemistry. This is just an amateur exploration, driven by curiosity about which modern plastics could be produced without fossil fuels¹ (Nearly all of them, it turns out!)

¶ What is plastic?

Plastic is a word which simply means that something is easy to shape. When we use the word as a noun, we’re usually talking about a specific group of plastic materials made of man-made polymers.

Polymers

The name means “many-parts”. Polymers are long chains of simple molecules called monomers. Polymer materials are strong and flexible for much the same reason that cloth is strong and flexible. The long chains are tough to snap but can bend and slide past each other.

(Except that’s a bit of a cheaty metaphor because the natural fibers used to make fabric are themselves polymers. In fact, there are lots of polymers in nature, and some early modern plastics were developed as replacements to their natural alternatives.)

Monomers

The name means “one part”. These are the pieces that link together to make a polymer. Simple sugar is one of the most common monomers.

¶ Naturally occuring polymers

Polysaccharides

The name means “many sugars”. These are polymers made by chaining together simple sugars called glucose. Although simple sugar monomers taste sweet and are easy to break apart, they become very strong and rigid when hooked together. Starch, cellulose, and chitin are all examples of polysaccharides.

¶ Plant polymers

Cellulose

It’s what makes plants rigid, and it’s the part of plants we turn into paper. Ever held a cotton ball? That’s basically pure cellulose. Some animals, like cows, have bacteria in their guts to break cellulose back down into sugar to use as food. The human gut can’t do that, but we can still need to eat cellulose as dietary fiber (which adds bulk to stool).

Lignin

Lignin is a wonky tangly polymer made by linking together several kinds of monomers. Wood is mostly cellulose, mixed with lignin to hold the cellulose together. The lignin increases the hardness and rot resistance of the material.

Latex

Latex is a sticky substance made of polymers mixed with water. Plants make the stuff to protect themselves from predators. Certain kinds of latex can be processed into materials like natural rubber and chicle.

¶ Bug polymers

Chitin

A polymer used in the bodies of many animals and fungi. It’s made by altering the sugar with nitrogen before linking it together. Chitin is used by insects and crustaceans to make their exoskeletons, and by fish to make their scales. Many scientists are trying to figure out a way of manufacturing chitin on large scales so that we can mold things out of it. The best we’ve figured out so far is to grind up shrimp shells and mix them with a bit of lye.

Lac

A substance secreted by certain kinds of insects as protection while they feed on plants. People harvest lac and dissolve it in alchol to form shellac. It can be brushed onto a surface to form a glossy water-resistant coating (and similar synthetic materials are still called lacquers). It can also be molded into complex shapes. Before the invention of vinyl, records were typically made of shellac.

Silk

This is the stuff spiders use to spin their webs. The spider keeps the liquid containing the silk monomers in its silk glad, and then squeezes the liquid through a twisted narrow passageway called the spinneret while pulling on the other end. This process causes the components of the silk to connect together into a tough fiber. Many insects, most famously the silk worm, use a similar process to build a shelter out of silk. Natural silk cloth is made by combining the silk from thousands of silk worm coccoons.

¶ The Polymers in our Body.

Protein

Long chains of monomers called amino acids. There are 20 different amino acids use by the human body and the order in which they are connected together changes their shape and function. Proteins are used for all sorts of purposes, and only some of them are used as building material. (DNA, which stores the information on how to assemble these proteins, is also technically a polymer.)

Scleroprotein

Very long proteins that connect together into sheets or fibers. Due to their structure, these proteins behave the most like what we would typically call “plastic”. Important scleroproteins include collagen keratin, Fibrin, and Elastin.

Collagen

Collagen is the most common protein in the human body. It’s tough and flexible, and used to make ligaments, tendons, cartilege and blood vessels. When mixed with a mineral called hydroxyapatite, it can be used to make bones and teeth. The name means “glue-maker”, because we can boil it down to make some kinds of glue.

Keratin

Long coiled protein polymers used to make claws, fingernails, hair, feathers, reptile scales, tortoise shells, and rhino horns. It’s very hard and tough. If a part of your skin is repeatedly subjected to stress, your body will put some keratin in the skin there to form a callus.

Fibrin

This scleroprotein helps you stop bleeding when you are injured. First, it blocks the damaged blood vessel to slow the flow of blood, and then it’s used to build a protective scab to cover the wound while its healing.

Elastin

Kind of like our body’s version of rubber. It’s what makes our lungs and bladder so stretchy, and its mixed with collagen to help absorb shocks. Our skin contains elastin fibers, and the loss of these causes our skin to get saggy as we age.

¶ Early Man-Made Polymers: Repurposing Cellulose and Formaldehyde

¶ Cellulose-derived Plastics

Rayon (1894)

Rayon is chemically the same as cellulose, but arranged into much smoother, more regular fibers, which makes it great for use in textiles. Made by treating wood pulp with lye and carbon disulfide (CS2, originally called “liquid sulfur”). The treated cellulose is then dissolved, and extruded through tiny holes into an acid bath. Because of the similarity between the extruder and a spider’s spinnerets, rayon is sometimes called “artificial silk”.

Cellophane (1912)

Cellophane is made the same way as rayon, but a narrow slit is used instead of a spinneret. The name is a combination of “diaphanous” (thin enough to be transparent) and “cellulose”. It’s used in food packaging, and other situations where clear, slightly breathable wrapping is desired.

Nitrocellulose (1846)

AKA cellulose nitrate, gun cotton, flash paper, and many other names.

Nitrocellulose is one of the earliest truly synthetic polymers. It’s made by using a mixture nitric acid and sulfuric acid to add nitrogen atoms into cellulose. The ingredients required have been known to chemists for hundreds of years, but the actual recipe wasn’t discovered until the 1840s. (One apocryphal story goes that a chemist wiped up an acid spill with a cotton apron, and the apron later spontaneously exploded after being hung up to dry.)

Nitrocellulose has several valuable properties. Firstly, it’s explosive. Secondly, unlike normal cellulose, it can be easily dissolved and then allowed to dry into a shiny hard plastic coating. This is used for all sorts of stuff, from playing cards to nail polish. (Yes, the stuff that makes your nails shiny happens to be an explosive.)

Celluloid (1862/1868)

Celluloid is a mixture of nitrocellulose with camphor, which is a stinky wax harvested from the camphor laurel. The camphor makes the nitrocellulose stronger and easier to work with. Celluloid is a thermoplastic, which means that it can be heated to become soft, dissolved or molded into shape (injection molded even), and then cooled back down to become hard.

Celluloid’s first application was as a replacement for the ivory (elephant tooth, itself a polymer composite) previously used to make billiard balls, and soon became widely popular as a replacement for horn and ivory in all sorts of applications. Celluloid film was also the first flexible material which could hold photographs. This flexibility allowed many small photographs to be stored on a long coiled strip of film, and led to the rise of moving pictures as an art form.

Oh, but do remember that this stuff is basically gunpowder. The camphor makes the nitrocellulose significantly less explosive, but it’s still a tiny wee bit explosive. Early celluloid billiard balls would sometimes explode on impact, and the tendency for decaying film to spontaneously burst into flame is a substantial problem for the presevation of early video footage. If you have any regulation ping-pong balls from before 2014, make sure to keep them away from open flame.

Cellulose Acetate (1865)

Similar to nitrocellulose, cellulose acetate is made by using an acide bath to modify the composition of cellulose fibers. The process for making this stuff starts by soaking cellulose in a mixture of sulfuric acid, acetic acid (vinegar), and acetic anhydride.

The film industry eventually switched from nitrocellulose to cellulose acetate for making film stock, calling the new stuff “safety film”. Safety film isn’t immune to decay, but instead of becoming explosive in the process - as celluloid film can - decaying safety film just turns into vinegar.

¶ Formaldehyde Resins

This next group of plastics are made using some stuff called formaldehyde. Formaldehyde is made from wood alcohol (methanol), and is used as a medical preservative. Wood alcohol is usually made from fossil fuels, but it can also, as the name suggests, be made by distilling wood, though the process is inefficient.

Galalith (1893)

An early plastic made from milk and formaldehyde.

Casein is the stuff in milk that’s good for your teeth. Turns out, formaldehyde can react with casein, linking it together into long chains. The resulting material was called Galalith, meaning “milk stone”, and was machined into jewelry and buttons.

Milk stone was hot stuff for much of the first half of the twentieth century, but was eventually displaced by the development of newer, cheaper materials with better physical properties. Galalith is still sometimes used to make buttons for clothes, however.

Bakelite (1907)

Bakelite was the earliest fully synthetic resin, developed as an alternative to lac

Bakelite is made by mixing formaldehyde with phenol, which is one of the earliest chemical antiseptics. Phenol originally comes from coal tar, so we are finally moving into fossil-fuel derived plastics here.

UF Resins

Urea-formaldehyde resins are the most common plastics made using formaldehyde.

As the name suggests, UF resins are made by combining formaldehyde and urea. Although most urea nowadays is synthesized from natural gas, the process of isolating urea crystals from urine was discovered all the way back in the 1200s.

UF resin can be molded into solid shapes, but is most commonly used in combination with cellulose. It’s mixed in with bits of wood to form particle board. And it’s used as a strengthening additive to paper, cotton, and rayon. It used to be used on its own as foam insulation, but there were concerns about formaldehyde gas being released into homes.

MF Resins

Melamine formaldehyde resins are a bit higher quality than UF resins. When you write on a white-board, it’s likely you’re writing on MF resin. Melamine is manufactured from urea.

¶ Ethylene-derived Plastics

The bulk of the plastics in the world today are derived from a monomer called ethylene (or ethene). In fact, by volume, humanity manufactures more ethylene than any other organic molecule (unless we’re counting agriculture as manufacturing, in which case obviously glucose/cellulose wins by a landslide).

¶ Naturally Occuring Ethylene

Ethylene on its own is a simple gas which naturally occurs in small amounts.

It’s an important hormone that triggers aging in plants. It’s what causes flowers to develop and fruit to ripen. It’s also what causes leaves to fall from trees. My spouse is very particular in how they store fruit, claiming for example that putting apples next to the kiwis will make the kiwis taste sweeter. Turns out, that’s not just a weird tradition from grandma; apples fart out ethylene! Small scale ethylene production is often used for this purpose, generating ethylene gas in an enclosed room to rapidly ripen fruits.

Old-timey kerosene lamps produce small amounts of ethylene in their smoke, leading to a bizarre phenomenon in the late 1800s and early 1900s where street lights were killing the trees along city streets.

Oh yeah, and it’s also psychoactive. It smells sweet and induces euphoria when inhaled, so was tried for a time as an anesthetic alternative to chloroform. Doctors stopped using it that way as soon as better alternatives were found. There’s also speculation that the prophetic visions of the Oracle of Delphi were triggered by ethylene inhalation.

Finally, it shouldn’t surprise you to learn that the stuff can explode at high concentrations.

¶ Manufacturing Ethylene in Bulk

The main source of ethylene production is from fossil fuels, in a process called steam-cracking.

It can also easily be made from ethanol. Ethanol (the stuff in wine that gets you drunk, or “aqua vitae” as an alchemist might call it) is the hydrate of ethylene. To vastly oversimplify, ethanol = ethylene + water, and converting between the two is fairly straightforward with the right catalysts. In fact, the supply chain can go either way:

Until about 1950, ethylene was expensive and was obtained from fermented ethanol by dehydration - the reverse of the above processes. With the advent of cheap ethylene from steam cracking, the petrochemical route to ethanol became more economical than fermentation. By the early 1970s, scarcely any industrial ethanol was made by fermentation in the United States, although there was a legal requirement in most countries that potable ethanol be made in the traditional way. In the United States in the 1980s, this trend was reversed when government subsidies were introduced to facilitate the production of ethanol by fermentation of corn starch. The product went into automotive fuel known as gasohol. … With the advent of huge production facilities for fermentation ethanol, new plants have been built in Brazil for its dehydration to ethylene.
-From Industrial Organic Chemicals, 3rd edition (Wittcoff Reuben Plotkin 2013)

¶ Turning Ethylene into Plastics

Polyethylene

Polyethylene is the name for any polymer of ethylene, essentially just a bunch of ethylene hooked together into a chain, and is the most common kind of plastic. You’re probably touching some of it right now.

There are several different kinds of polyethylene, and all of them require modern technology to manufacture. After all, there’s a lot of ethylene floating around in nature. If it were easy to polymerize the stuff, you’d think polyethylene would show up in living creatures.

Low Density Polyethylene (LDPE)

LDPE is flexible plastic made by simply compressing and heating ethylene. But this requires pressures of thousands of atmospheres.

High Density Polyethylene (HDPE)

HDPE (which is harder and more rigid than LDPE) can be made at near-atmospheric pressures and relatively low temperatures, but the process requires exotic catalysts

Linear Low Density Polyethylene (LLDPE)

LLPDE is a successful attempt to make flexible Low Density PE using the more reasonable temperatures and pressures of the HDPE process. LLDPE requires a intermediate step where some of the ethylene is processed into oligomers (very short chains) with names like “ethylethylene”.

Polypropylene (PP)

PP is the second most common polymer being manufactured today. The monomer propylene isn’t typically made from ethylene, but it is possible to do via a convoluted multi-stage conversion, which seems to be the only way of making the stuff without using fossil fuels. The more typical process creates the propylene monomers from propane gas.

PolyVinyl Chloride (PVC)

PVC is usually just called “vinyl”. It’s the third most common modern plastic. The vinyl monomer can be made by from reacting chlorine with ethylene over hot pumice, then the vinyl monomer is pressurized to 13 atmospheres and heated to make the vinyl polymer. In the textbook I was reading, it seems like most of the effort in synthesizing vinyl is finding a way to deal with the horrible awful deadly toxic byproducts.

Polyester

Polyesters are a large group of polymers, but the term is often used to refer specifically to PET, short for PolyEthylene Terephthalate.

PET is the fourth most common synthetic polymer, and is made from ethylene in several steps. First, ethylene is combined with oxygen over a silver catalyst at 15 times atmospheric pressure to form ethylene oxide. The ethylene oxide is then mixed with water to form ethylene glycol and a bunch of other stuff which has to be filtered out. (The ethylene glycol is useful on its own as antifreeze.) The antifreeze is then combined with terephthalic acid under heat and pressure to form PET.

That’s certainly a lot more complicated than the production of nitrocellulose, which could probably be made by a medieval alchemist. Modern supply chains are terrifyingly complex beasts.

¶ Sources

Here are some resources I found very useful, and might be useful if you want to read more about plastic or modern industrial chemistry:

• Guide to the Business of Chemistry by the American Chemistry Council. There are some great appendices at the back.
• Industrial Organic Chemicals, 3rd Edition, by Wittcoff, Reuben, and Plotkin. A textbook that I found enjoyable as a lay reader. Includes lots of details about chemical reactions, of course, but also about the history, politics, and economics of chemistry as well. Expensive, but maybe you can find a copy in the library like I did.
• The Essential Chemical Industry Online. A lovely little reference website, and a good starting point if you want to learn about a particular chemical.
  • Here’s their index on basic chemicals.
  • And here’s their index on polymers.