Question from June's Chemical Processing
A modified-banbury mixer is used to blend a new solid rocket propellant. It is a DOT class 1.1, double-base, nitroglycerine propellant. During the first step, where only polymer and plasticizer are present, everything went well. HMX and ammonium perchlorate are used in the formulation. Following industry practice, the coarse oxides are added first, then the finer oxides and aluminum. The bonding agent and cross-linker are added normally. The mixer is maintained under a slight vacuum and chilled water cools the side of the vessel. The bottom and top are not chilled. The interior surface of the vessel and mixer are polished to a pharmaceutical finish. Later, as solids are added, approaching 90%, the mixer explodes. Fifteen minutes before the explosion, the viscosity unexpectedly spiked. There was a sharp rise in viscosity after that (Figure 1). Fortunately, nobody was hurt. What caused the incident and what can we do to improve the operation?
Consider a number of possibilities
I hope I can offer some insights to your problem. I used to work in the propellant industry, but with J.H. Day mixers, not Banburys. First of all, I’m glad no one was hurt in the incident. Protecting people should be your number one concern. Look at this experience as a learning opportunity to make sure this never happens again. An Ishikawa diagram should help explain my thoughts on the cause of the accident (Figure 2).
Figure 2. This chart highlights various factors that could have led to solid propellant explosion. (Click to enlarge.)
What initiates explosive mixtures? Heat, friction and electrostatic discharge are a likely cause. The mix temperatures may be a clue here. I would review the records to confirm that they were within their normal ranges. Also, the maintenance history of the temperature elements and data acquisition systems should be reviewed to make sure temperature readings were accurate. The grounding procedures of the solid feeding vessels (e.g., sifter) should be confirmed. Make sure the solids feeding vessels were grounded to the mixer, and they were both attached to a building ground, so there would be no build up of electrostatic potential. Interviews with operators should be conducted to make sure they were doing this important step correctly. Feeding fine HMX, AP and aluminum powder can result in large voltages if not grounded correctly.
Next, I would look at the mechanical equipment. How was the mix viscosity monitored? Was it from power demand on the motor? Normally, the reactions of the urethane and the crosslinker cause an exponential increase in the batch viscosity (with time). Handling of the propellant isn’t difficult; casting is usually done well before the material sets up. A viscosity spike is unusual. I assume that quality control is good, i.e., that the isocyanate and crosslinker were checked before adding them to the batch. This leaves only mechanical problems. What could cause the spike? I see two possibilities: a foreign object in the mix or slugging of the solid feed.
Since the blade-to-blade and blade-to-bowl clearances are usually very small in these high-shear mixers, a foreign object could have wedged itself between the blades and accounted for a viscosity spike (if measured by motor power demand). Of course, this could have resulted in high friction which could have initiated the explosion. The investigator should look for paths of foreign material entry to the mixer. For instance, are all solids screened prior to addition? Are all tools accounted for? Do operators empty their pockets prior to entering the mix building?
Solids slugging could result in a viscosity spike. The solids may have rushed into the mixing vessel and caused a temporary increase in power demand. Then, as the solids wetted, power demand would have dropped. Does the mix system have a history of solids slugging? Slugging could result in friction or electrostatic discharge.
Again, getting back to the curative (the isocyanate), there may be a few unexplored problems. Was the proper amount and strength of curative used? Too much curative has caused high mix viscosity and temperature and (if memory serves me correctly) has resulted in mixer fires and explosions. Review the batch records for procedures. Inspect the ingredient lots for curative strength, composition and general condition of the containers and storage area.
Is there any evidence from the debris which could be used to locate the exact spot in the mixer where the explosion initiated? For instance, are their any burn marks near a bearing, on a blade, etc.? Collection and reassembly of the remaining mixer components could be helpful.
Edward Giugliano, team leader
AES Warrior Run, LLC, Cumberland, Md.
An impurity caused a chain reaction
I believe that a polymerization chain reaction occurred. Possibly the heat of mixing or poor heat transfer in the jacket may have set it off. Typically, a chain reaction is blamed on free radicals. A polymerization reaction doesn’t have to involve formation of free radicals to occur. It can be caused by an ionic mechanism, either cationic or anionic. Since aluminum metal is an ingredient in your batch that seems the likely culprit. If some oxidizing potential exists, or existed, whereby the aluminum could have been converted to a cationic agent this could cause the chain reaction. Humidity, combined with the presence of sulfur compounds, like SO3, can oxidize aluminum. There’s an organic reaction that was identified just about 15 years ago called the “manich” which involves ammonia/amines, formaldehyde and carbonyls. If your organics are di-carbonyl functionalized I can see a lot of possibilities for cationic polymerization to occur if there’s any water in the mixture to allow ionic, acidic, species to exist. I think there must have been an impurity in the batch ingredients, possibly from the ammonium perchlorate, aluminum, or HMX, or something else, that may have caused the aluminum to initiate a chain reaction.