Paint The World
From cave drawings to anti-corrosion coatings on bridges. It's all plastic (sorta).
Paint as you know it today is really just a synthetic polymer (a plastic) formulated with a bunch of other stuff. Most people can pick out a plastic in a line-up, but I see all synthetic polymers as coming mostly from the same place: crude oil and natural gas. Modern paints and coatings are not biodegradable, but because of their fillers they can often sink to the bottom of a waterway. Paint dust doesn’t get the same attention as other microplastics, but I digress.
Perhaps the earliest paint ever used was developed by early homo sapiens about 40,000 years ago. Humans had figured out that putting a pigment into a binder such as egg, oil, or milk could be used to coat things in color. That must have been an exciting day.
A paint is a coating that contains a pigment. A liquid that can be applied to a surface that forms a film can be called a coating. If you take the pigments out of a paint you still have a coating, but a pigment isn’t going to stay stuck to a wall on it’s own. Artists might call a coating a “binder” or a medium that holds (binds) a pigment. They might call a clear coating that gives a shiny or matte appearance a varnish. Coatings and varnishes are the same thing.
My very first internship during my undergraduate degree was at the Smithsonian Museum Conservation Institute. I worked for an art conservator and we were trying to understand how to conserve historic plastics such as cellulose acetate. Conservation is not restoration. I spent a lot of time reading about early synthetic plastics such as cellulose acetate and cellulose nitrate, but I would always overhear conversations about paintings, varnishes, and different types of oils.
Plant oils with high degrees of unsaturation, such as linseed oil, eventually became ideal oils to use as binders for paints because they dried films. This process of drying an oil is a slow crosslinking reaction that happens between the unsaturation within the triglyceride structure and are known colloquially as “drying oils.” If you ever get a chance to refinish a piece of furniture try and varnish it with some pure tung oil. You just wipe the wood down with the oil with a rag and you can actually feel the heat of that polymerization as it takes place. Be careful not to bunch up the rags after coating because it can be difficult to dissipate that much heat.
Drying oils were good, but could be difficult to work with and eventually we got alkyd resins from General Electric in the early 1900s. This was before Wallace Carothers figured out how to make nylon and around the same time that Leo Bakeland would figure out Bakelite. Alkyds tend to use a polyol (glycerol, ethylene glycol, etc) and an anhydride (phthalic anhydride, maelic anhydride, etc) to make a polymer, but some of those polyols are often made up from fatty acids derived from saponification of their unsaturated oils. The important thing about having these unsaturated oils around is that they then become part of the alkyd polymer and can perform that drying reaction. Alkyds had the benefit of some of the binder being partially polymerized and then the drying reaction from the oil takes place after the film or coating has been formed. Alkyds are still used today due to their low cost, versatility, and ability to adhere to many different surfaces. We have even seen shortages over the last year.
Alkyds, in the early days, were often suspended in a solvent to ease their application to a surface. You can buy an alkyd at the hardware store and when working with it you might notice that it smells a bit more than the normal waterborne paints (I’ll get there in a few paragraphs). The solvent was/is important to dissolve the synthetic polymer and the pigments and leveling agents and fillers within the paint. In the early/mid 1900s, solventborne paints were everywhere and when a house got painted you really couldn’t be in there until the paint finished drying. When people think of paint “drying” it is in reference to the solvents evaporating and not the “drying reaction.”
Solvents mainly served a purpose of thinning out the synthetic polymer, but solvent can only take us so far. The length of the synthetic polymer is ultimately restricted by many things, but when cooking in a solvent it’s going to be an issue of solubility and viscosity. Further, the longer the polymer the more difficult it became to get the solvent out due to vapor pressures changing. You can even do a molecular weight determination via this phenomena, but I digress. We needed another way to polymerize paint binders.
In the 1920s/1940s some chemists figured out how to do an emulsion polymerization. I say the 1920s because Goodyear was doing this for rubber latex and in the 1940s by Smith and Ewart proposed a good mechanistic model with styrene. An emulsion, like a good salad dressing or mayonnaise, is the mixing of two different immiscible phases. A stable emulsion is when those two phases can coexist without seperation. The problem of polymer molecular weight not being able to get high enough with alkyds due to solvent limitations essentially gets solved (no pun intended) with an emulsion polymerization.
Emulsions are stabilized by surface tension reduction at the interface of the two phases. When you was the dishes and need to “cut through the grease” you might turn to dish soap, which reduces the surface tension between the oil (grease) phase and the water in the sink. The same is true for water and acrylics, but when we mix methyl methacrylate with a bit of butyl methacrylate in water with some surfactant (soap) and add in a water soluble initiator with some soap we can do an emulsion polymerization.
The emulsion polymerization is amazing because of it’s simplicity. It merely suspends those difficult to make polymers in a medium where they are not miscible and thus they are able to polymerize in their own little micelles. Being suspended in water enables relatively high synthetic resin loading without significantly impacting the viscosity of the emulsion. The low interaction of the two phases allows particles of polymer to slip past the water and we get low viscosity waterborne resins with high molecular weights. Emulsion polymerizations of acrylics high in methyl methacrylate tend to be more brittle and longer acrylics such as butyl or lauryl tend to give tougher films.
You should understand that this is really some high level overview stuff. People dedicate their whole careers to coatings and the chemistry that goes on in this field. A polymer chemist might spend their whole career just doing emulsion polymerizations and trying to polymerize slightly different polymers to yield slightly better end properties. I’m not trying to teach you everything about paint and coatings here. That would be impossible. I’m just trying to show that synthetic polymers make this stuff work.
There is a lot that can be done with emulsion polymerizations. The mixing and matching of different monomers and how they are feed into the reaction. The type of surfactant used to minimize the surface tension. The fact the minimal solvent is needed for these polymerizations to work has been the backbone of the waterborne coatings industry and it’s a reason why you can paint a room in your house and still live in your house at the same time.
Formulate The Paint
Emulsion polymerizations opened the door for modern coatings to flourish. This type of polymerization and the monomers used to make them have been commoditized over the last thirty years. Companies such as BASF (trade name example: Acronal) and Dow (trade name example: Rhoplex, via the old Rohm and Haas) are dominant in this space. These companies might sell their resins to a PPG or a Sherwin-Williams, which will then formulate those resins with other resins, pigments, fillers, surface leveling agents, thickeners, defoamers, and coalescing agents (e.g. Texanol via Eastman). The paint company then sells the normal consumer the paint.
Large coatings companies such as Sherwin-Williams also formulate and sell to commercial customers that want to do protective coatings. Paint is a term that everyone tends to understand, but for most people that understanding is limited to what can be bought at a hardware store. The paints and primers used on structural steel are often different than a normal consumer grade architectural paint and are usually something a bit more robust such as an epoxy resin or a polyurethane. Layering the coatings for different effects such primers with excellent metal adhesion to top coats that don’t yellow with exposure to sunlight (weatherability).
Paints and coatings are ubiquitous, but not thought of unless another coating is needed. When you paint or coat something you are essentially putting down a layer of plastic. I use the word plastic as it is commonly used to denote a synthetic polymer. Paints and coatings are not using the same resins that go into soda bottles or single use plastic forks, but the principles of how we go from crude oil to monomer to synthetic polymer are all quite similar.
Coatings and paints are a reason why our old steel bridges are still working. Coatings and paints protect their substrates and in the case of bridges that protection comes in the form of anti-corrosion. Coatings and paints protect the exterior of our houses and give us something nice to look at inside as well. Coatings keep the interior of our canned food and the exterior of our cars rust free. Paint on the roads act as a guide that we can all follow and easily see (thank you glass microbeads). For those interested in how this stuff comes from oil and gas go below the Seinfeld clip.
Hey, you made it down this far. I made a claim at the beginning that most of this stuff comes from oil and gas and down here I feel like I can talk chemical structures because this is where you, the hardcore chemistry nerd, likes to hang out.
So if we look at the synthesis of phthalic anyhdride it’s often cited from either naphalene or ortho-xylene and we get both of those from crude oil. Methyl methacylate has a few different routes. Some of them are from ethylene, which we can get from steam cracking natural gas, or potentially ethanol dehydration, but we still need acetone to make it via methyl propionate which we get from cumene, which comes from benzene and propene (both come from crude oil/nat gas). Granted, some of the alkyd building blocks can be biobased and would be challenging to make from crude oil, but once that stuff is coated it’s not biodegrading.
Making all of this stuff from nature or making a biobased equivalent would be essentially for kicking our crude oil and natural gas needs. Dean Webster’s research group at North Dakota State University is a great place to start if you are interested in making synthetic coatings from non-crude oil origins.
Let me know who is working on non-crude oil based coatings in the comments.