Popcorn Polymer: Introduction
A conjugated diolefin is a certain type of hydrocarbon whose molecular structure makes it more reactive than other hydrocarbons. The simplest conjugated diolefin is 1,3-butadiene (BD), and its physical and electronic structure make it particularly active towards free radical attack. Physically, the small hydrogen atoms bound to the carbon backbone offer little steric hindrance (physical blocking) towards an attack from a molecular free radical - being a planar molecule, the carbon backbone can be accessed from above or below the plane since the small hydrogen atoms do not get in the way.
Electronically, the alternating double bonds allow an unpaired electron (aka radical) to “delocalize,” or occupy multiple locations simultaneously. This can be visualized using resonance structures, with each possible structure contributing to the overall quantum mechanical “molecular orbital,” which is related to the probability of finding an electron at a particular spot.
This makes BD particularly susceptible to unwanted free radical polymerization reactions. Such reactions proceed via a chain reaction mechanism. Once initiated, the polymerization reaction repeats, and will continue until it either reacts with a terminating radical or all monomer is consumed.
BD polymer can grow in different ways, resulting in several morphologies. The popcorn morphology is by far the most problematic and insidious. Popcorn polymer grows via the proliferous polymerization mechanism: an abnormal free radical mechanism with a constantly accelerating rate. It is highly cross-linked, and significantly less dense than other types of polymer, which means less monomer is required to occupy the same volume. Not only does this rapid growth adversely affect unit performance by blocking fluid flow, but it poses a major safety risk: popcorn polymer has the power to bend or break metal pipes, bolts and even vessels as it grows.
While 1,3-butadiene is the most common identified in reports of popcorn polymer coming from the chemical industry, other conjugated molecules can undergo proliferous polymerization to form popcorn, including isoprene, piperylene and styrene. Some monomers, such as styrene, require a cross-linker to be present to form popcorn since cross-linking is a key requirement for popcorn formation. Cross-linking occurs when two or more growing polymer backbones react with each other via dangling vinyl groups.
Popcorn polymer is problematic in part because of its forceful expansion during growth - this is what caused the 2019 explosion at a TPC butadiene plant n Port Neches, TX, for example. The reason why popcorn polymer is so insidious is that it grows from the inside out, with free radical active sites created every time it “pops,” i.e. breaks bonds somewhere in the polymer network; that newly created pair of radicals is buried inside the polymer matrix. Not only does this cause the polymer to expand very forcefully, but the polymer itself acts like a “living seed,” since the radical site is attached to the polymer itself and terminating agents have difficulty reaching the radical to quench it.
This “living seed” concept has extremely important ramifications for process safety and reliability. A piece of popcorn polymer can be removed from the monomer solution it was grown from, transferred to another flask with fresh monomer and continue growing right where it left off. No separate initiation step is required - the free radical is stabilized inside the chunk of polymer, and in the absence of oxygen or other reactive agents the lifetime of the radical site can be measured in years. That means if polymer is not thoroughly cleaned from the tower, active seeds will remain and resume growth once hydrocarbon is introduced.
Popcorn is a notorious troublemaker within the ethylene industry, particularly for those who operate butadiene extraction units. As is the case with many notorious characters, there are a few myths clouding the subject of popcorn polymer, and to counter those myths trade groups such as the Ethylene Producer’s Committee (EPC) in the US and its European counterpart the EEPC share incident reports and best practices for managing popcorn polymer to help improve the safety of all.
