Issue Date: September 11, 2017 | Web Date: September 7, 2017
What made Arkema’s peroxides unstable in Harvey’s aftermath?
Organic peroxides, the chemicals that led to evacuations and explosions at the flooded Arkema chemical plant in Crosby, Texas, are well-known for their instability—the characteristic that provides both their chemical utility and their hazardous properties.
Organic peroxides contain the peroxide functional group ROORʹ.The O–O bond is inherently weak; the bond dissociation energy of CH3O–OCH3 is 157.3 kJ/mol compared with 335 kJ/mol for CH3–OCH3.
Exactly why O–O bonds are so weak is an “excellent question,” says Weston T. Borden, a computational chemist at the University of North Texas, “because I do not think that the correct answer is known with any degree of certainty.”
Likely, the weak peroxide bond arises from repulsion between the lone pairs of electrons on the oxygen atoms, combined with the atoms’ high electronegativity, or tendency to attract electrons. “Covalent bond formation involves transfer of electron density to the region between the two atoms that are covalently bonded, and this transfer should be more energetically costly as the electronegativity of the atoms increases,” Borden says.
The instability of the peroxide bond means that it takes little energy, such as in the form of heat, to get peroxides to break apart. That easy breakup is where organic peroxides’ chemical utility originates. Organic peroxides readily decompose into free organic radicals, RO•, that may then serve as initiators for polymerization.
But peroxides’ effortless split into highly reactive radicals also means that they must be stored well below their “self-accelerating decomposition temperature” (SADT). Above that temperature, a dangerous cycle will develop that leads to a thermal runaway: Peroxide decomposition releases heat that causes more peroxide decomposition that releases more heat. The temperature at which emergency procedures should be implemented is 10 °C below the SADT, according to Arkema’s storage guidelines.
Media reports suggest that the exploding peroxide containers at the Arkema plant were a “textbook example” of a thermal runaway, says explosives expert Jimmie Oxley of the University of Rhode Island. Arkema’s storage of the materials in thousands of containers from 3.8 to 18.9 L, which prevented easy neutralization, may have been part of a safe storage strategy, Oxley adds. “If you have a larger mass, it’s easier for the material to self-heat” because the heat won’t dissipate unless the material is stirred, Oxley says.
Oxley thinks it’s unlikely that the Arkema organic peroxides detonated. Instead, the explosions were more likely pressure ruptures from buildup of gaseous peroxide decomposition products. If the decomposition products were flammable, they could also have autoignited from the heat.
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