“Several years ago, an explosion took place in a customer facility during a grease-filling procedure,” Schmidt-Amelunxen told the conference. This highly viscous silicone oil based grease was applied with a standard high-pressure grease pump. No high temperatures or other unusual operating conditions were present as the grease was being pumped, yet a sudden explosion rocked the room. One worker was injured, and others described an extremely loud bang, followed by a black cloud of smoke.

“Due to the high pressure of the explosion, objects — even quite heavy ones — were blown through the room,” reported Schmidt-Amelunxen. “A piece of hose immediately after the outlet of a high pressure grease dispenser was determined to be the source of the explosion. Surprisingly, the hose was not torn off completely, and only a relatively small hole in the hose was found.” The white, food-grade grease inside the hose was clean, without black particles, but the wall nearby was blackened by carbon residue.

Silicone greases like the one involved in the explosion are niche products, used in specialty applications such as sanitary, food and beverage filling applications, or on beer keg valves, for example. Such greases often are thickened with PTFE, to add extreme pressure properties. However, both silicone oil and PTFE are inert raw materials, unlikely to explode spontaneously.

Unable to pinpoint a cause, the accident investigators finally assumed an old or wrong type of hose had been used, and closed the case. Best they could guess, a “diesel” effect may have been caused by small air bubbles in the grease, or perhaps improper cleaning of the equipment had led to the blast.

A couple years later though, two similar accidents happened at other production sites. Again, the explosions were located directly after the grease pump outlet and happened during a high-pressure grease filling process. “The power of one of the explosions was so heavy that a metal fitting of the grease pump was torn off, and another piece of metal was stuck in the concrete wall several meters away,” said Schmidt-Amelunxen. “Luckily, no one was hurt by these accidents.”

Kluber began a deeper investigation, with the aim of averting such problems in the future. It wanted to know the source of the black residue from the explosion, and why the explosions were so very focused.

Digging deeper, Schmidt-Amelunxen and his Kluber colleague J. Muhlemeier saw some links: The site of the blasts were all just behind the grease pump. Only highly viscous PTFE thickened greases were affected. No flames were observed, but there was strong pressure and a heat wave, combined with a loud “bang.” The flung-out grease was charred like carbon black, but the grease inside the equipment was still white and unchanged. Pressures in the pumps shot up by 400 to 500 bar on start-up. And in most cases, the explosions occurred after a new grease container was hooked up, or after the weekend.

A close look at the grease itself was puzzling. “No exothermic chemical reaction is known between dimethyl polysiloxanes [silicone] and PTFE,” Schmidt-Amelunxen stressed. No evidence of impurities or irregularities in the grease was found. The black residue was identified as carbon black — yet the grease was all white.

The company questioned if some kind of chemical decomposition reaction were taking place, involving the depolymerization of the PTFE into tetrafluorethylene (TFE), which in turn breaks down into tetrafluoromethane (CF4) and carbon black (C). TFE is very difficult to handle and its decomposition can lead to violent explosions, but normally such decomposition requires several hundred degrees Celsius to take place, Schmidt-Amelunxen explained.

In fact, that decomposition may be occurring, Kluber’s subsequent research suggests. “The presence of air bubbles is a key factor in the decomposition of the PTFE, and there have been cases where adiabatic compression of air bubbles can lead to very high temperatures of several hundred degrees C,” Schmidt-Amelunxen said. (A simpler example can be seen in hydraulic and compressor applications, where the air and pressure can mean higher oxidative degradation of lubricants.)

To test this theory, Kluber sponsored “falling hammer tests” on grease samples, conducted under the eye of Martin Gosewinkel at the Wilhelm-Jost-Institute in Hamm, Germany, a safety testing and explosion prevention center. Here, silicone/PTFE greases containing varying amounts of air bubbles were subjected to heavy blows with weights; after each blow of the hammer, the grease was inspected and the presence of black spots was interpreted as a reaction caused by the energy impact. Sure enough, greases with air bubbles exploded and threw off carbon black, and did so at much lower-pressure blows from the hammer than greases which were free of air.

“It was clearly seen that the presence of air bubbles caused the safe energy level to drop dramatically,” the testers concluded. The grease was degrading due to air — but whether that came from oxidation or thermal decomposition is still unknown.

In the end, the falling hammer tests may explain why some PTFE greases explode, while others seem to stay intact. Silicone greases without PTFE don’t appear to be a danger, nor are silicone greases with only small amounts of PTFE. The triggering factor is the presence of air bubbles in the grease, which may be introduced when the drums are filled — or especially by lifting and lowering of follower plates that are used with high-pressure grease pumps when the grease is dispensed. Another source of air bubbles might be certain homogenizer equipment used in grease manufacturing.

In any case, Schmidt-Amelunxen advised, the source of the adiabatic compression is probably a high pressure pump, where shock-like pressure peaks can trigger the oxidation reaction and lead to the bubble implosions.

To combat this risk, he advised pumping equipment be thoroughly degassed before each use, and pump pressures and speeds be reduced as well. Anywhere grease is handled, there should be protection against drawing in of air, and avoidance of shock-like pressures. Kluber has already added these and other safety recommendations on its MSDS, product information leaflets and labels.

“This is not a quality problem or a problem of a specific grease supplier,” Schmidt-Amelunxen emphasized, “but concerns all PTFE thickened silicone greases of all manufacturers.” One silicone grease supplier in the NLGI audience was quick to demur, pointing out that the key factor seemed to be PTFE, not silicone. Maybe so, said Schmidt-Amelunxen, but that was the only grease type in each of the cases Kluber investigated.

To a follow-up question, Schmidt-Amelunxen replied that he believes other grease types with high PTFE levels could see the same violent reactions, if subjected to the same air bubbles and high-pressure shocks. In fact, he suggested such explosions may have already occurred, but the link to PTFE decomposition has been overlooked by investigators. More research and openness may bring other cases to light, he hopes, so the risk can be understood and avoided in the future.