That Methyl Methacrylate Tank

I wasn’t posting over the long weekend, so I missed the saga of the Garden Grove methyl methacrylate tank. The immediate danger seems to abated, fortunately, but let’s talk about it to illustrate some widely applicable chemistry. People wrote to me to ask if methyl methacrylate itself is on my “Things I Won’t Work With” list, but it’s a long way from being in that category, to be honest. I’ve worked with it, as well as related compounds like methyl acrylate, acrylic acid, acrolein, acrylonitrile, etc., but I’m not using them for the purpose that they’re used for most of the time in the chemical industry. That, folks, is the one word of advice that Dustin Hoffman’s character got in “The Graduate”: plastics. More precisely, polymers. All of the compounds listed above share a common feature: they have a carbon-carbon double bond, unsubstituted at one end, and with the other end bearing some sort of electron-withdrawing group (ester, acid, aldehyde, nitrile and so on). This makes that double bond more chemically active, pulling electron density out of it and making it more favorable for (say) a single-electron free radical species to attack it, and there comes the interesting part. When such a free radical attacks that unsubstituted end of the double bond, a new single-electron free radical is produced at the other end (up next to the carbonyl). That one can now turn right around and attack another molecule’s double bond, which makes another free radical, which can attack another. . .and you’re off. That is free-radical chain polymerization, and in the 1800s and early 1900s it was rather a mystery. Some of the early syntheses of acrylate esters and such compounds were accompanied by observations that if they were left sitting exposed (particularly in sunlight) they turned irreversibly into clear, hard materials, but no one was quite sure how. Poly(methyl methacrylate) or PMMA itself was first commercialized in the early 1930s, and it’s familiar under a lot of early brand names like Plexiglas, Perspex, and Lucite. Acrylic paint is PMMA in water with pigments, along with some additives to keep everything homogeneous.

We now understand the details of polymerization, and boy are there a lot of them. Polymer chemistry is almost infinitely complicated, because you can polymerize mixture (of two, three, or more components in all sorts of ratios), and you can polymerize them under all sorts of conditions (still castings, extrusion, and more) and at different temperatures and stirring conditions. (Not to mention that there are other ways beside free radical chains to get this to happen, like ionic polymerization and the methods that don't depend on chain reactions at all). These will affect the geometry and length of the polymer chains you’re making, and these possibilities give rise to a crazy diversity of products with properties all up and down the scale.  Hardness, transparency, flexibility, chemical and thermal stability, resistance to cracking or impact or abrasion - you name it. We are utterly surrounded by these things in our daily lives by now, to the point where we don’t even notice them. But people from earlier eras would have been puzzled indeed by these strange materials which aren’t glass, pottery, stone or wood or anything else they could recognize. There’s a key energetic aspect to these polymerizations: they’re thermodynamically favorable, and these bond formations give off a bit of heat as they occur. This heats up the solution as a whole, and that in turn speeds up the reactions all by itself! Which means even more heat as even more molecules polymerize. . .and now you see why storage of large quantities of these monomer compounds is not for the unwary. You have to keep them away from anything that can start a free-radical chain reaction, and that means light and heat for starters, but also not letting them sit in contact with many metals and alloys, etc.  Now the instincts of many bench-level organic chemists (like me!) would be to keep something like methyl methacrylate away from oxygen (a notorious source of free radical chemistry), but the commercial material has polymerization inhibitors added to it that are activated by the presence of oxygen. So you don’t store the stuff on an industrial scale under inert atmosphere unless you’re in the mood for trouble - I believe that you need at least 5% oxygen around for the inhibitors to work.

Their amount and type (mixtures of hydroquinones and substituted phenols like BHT) is generally tailored to how long you’re planning on keeping the stuff around and at what temperature, but they’re usually good for a few months. If the storage temperatures go too high, these inhibitors will get used up, same as if you leave things in storage too long. Corrosion in a tank can provide a lot of free-radical-initiating species that will use up your safety margin, too. You also want to make sure that you haven’t gradually developed a separate aqueous phase at the bottom of your tanks, because some of these inhibitors will tend to sequester there and make the rest of the tank less stable (and it can lead to corrosion as well). Increased heat is obviously a concern; MMA tanks and such are often painted white to try to minimize heating by sunlight. In general, any temperature rise in a methyl methacrylate tank is a bad sign that needs immediate attention, because it strongly suggests that polymerization is taking place - and if so, it’ll only get worse. I have read that a simple test for the presence of polymer is to dissolve a sample of the tank contents in methanol - any cloudiness indicate the presence of polymeric species. But a steadily warming tank doesn’t need any such confirmation - a rise of a degree or two C per hour needs immediate attention, and a rise of five degrees per hour is a red light and siren. There are several options, depending on how bad things have become. A “short stopping” polymerization inhibitor like phenothiazine is definitely an option, and such storage facilities are supposed to have equipment to add that in case of emergency. Phenothiazine can kill off the chain reaction (without any need for oxygen) at the cost of maybe making the tank contents unusable, but you’re probably at that point anyway. However, if there’s too much overpressure on the tank already, you may not have the option of pumping more stuff into it. At that point, the main thing to do is to try to cool things down with sprayed water - the contents of the tank are going to polymerize anyway, so your job is to keep them from doing it so quickly that the tank ruptures and sprays a toxic and flammable mixture of monomer and polymeric goo all over the landscape.

This is exactly what played out in Garden Grove over the weekend, and fortunately it looks like things have been contained. That MMA tank and its contents are surely a complete loss, but that is infinitely preferable to having everything suddenly distributed across the suburban Los Angeles area. The chemical industry has been handling this stuff (and other reactive monomers) for a long time now, and many such incidents have occurred over the decades, but hard-won experience has taught us how to make them ever more infrequent. I look forward to the eventual incident report for the root causes of this one - assuming that we do such things any more here in the #$%@! Golden Age in which we find ourselves - because that, too, will make things safer going forwards. One hopes.