Over the long weekend, a storage tank of methyl methacrylate in Garden Grove, California, started to warm up — and as the chemist Derek Lowe lays out in a post on the chemistry of the saga, a warming MMA tank is exactly the wrong thing to see. The stuff wants to turn into plastic, and the reaction that does it gives off heat, which makes it run faster, which gives off more heat. "Now you see," Lowe writes, "why storage of large quantities of these monomer compounds is not for the unwary."
Methyl methacrylate is the monomer used to make PMMA, the clear plastic sold as Plexiglas, Perspex, and Lucite; acrylic paint is the same material in water. Left alone, the molecules link into chains in a self-feeding reaction, and "we are utterly surrounded by these things in our daily lives by now," Lowe writes, "to the point where we don't even notice them."
So a tank that's heating up means that reaction has started, and it only speeds up from there. "A rise of a degree or two C per hour needs immediate attention," Lowe writes, "and a rise of five degrees per hour is a red light and siren." There's a counterintuitive catch with oxygen: the commercial monomer's stabilizers only work in air, so it's stored with oxygen in the tank rather than under an inert blanket. Do otherwise, Lowe warns, and you're "in the mood for trouble."
Crews can dump in a chain-stopping chemical like phenothiazine or hose the tank down to cool it. The contents polymerize either way; the job, Lowe writes, is to keep the tank from rupturing and spraying "a toxic and flammable mixture of monomer and polymeric goo all over the landscape."
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