Failure Analysis of 360-Degree Explosion
In both the workplace and at home, people will often shake, tap or
hit an aerosol spray paint to loosen the mixing marble. Being only human,
people can also accidentally drop an aerosol spray paint. Shaking, tapping,
hitting and accidental dropping of aerosol spray paints are highly foreseeable
Click here to see a video
of an aerosol spray paint exploding upon impact.
Figure A presents a schematic of what is referred to as a 3-piece steel aerosol container. The three pieces are the top, body and bottom. The tops and bottoms are attached to the body by a mechanical joint known as a double seam.
Figure B schematically illustrates two types of double seams. The one on the left is the older of the two designs and is called a "straight-sided" double seam. The one on the right was introduced in about the early 1980s and is called a "necked-in" double seam. The arrow in Figure B points to the general location of the 360-degree explosive failure, which occur primarily in the necked-in design. There are no known incidents of this type of failure in the straight-sided design. Roughly half of all aerosols today use the necked-in design, the other half the straight-sided design.
There are two types of steel from which aerosol bodies are typically made. The first is referred to as "double-reduced" which is the type used most often in North America. The other is called "single-reduced" and is more common in Asia and Europe. The main (metallurgical) difference is that the double-reduced steel is further cold worked after it is annealed, while the single-reduced is not as cold worked after it is annealed. The result is that the double-reduced steel is not capable of as much deformation or bending as the single-reduced steel. While this may seem academic, it translates into very significant practical consequences with respect to aerosol safety.
Figure C illustrates how some strips of metal were cut from an aerosol spray paint container and then subjected to simple, but aggressive, bending tests. The first strip tested using double-reduced steel failed after 18 full bending cycles. This suggests that the steel of aerosol bodies should be capable of withstanding considerable tapping, hitting or dropping without exploding. However, when the metal strip was lightly scored (cross-ways) with a box cutter, the strip failed in less than one full bending cycle. This, in turn, suggests that any pre-existing defects would have a radical impact on the ability of the aerosol bodies to withstand bending and hence tapping, hitting or dropping.
When the same bending test was performed on single-reduced steel, the strip did not fail in 70 bending cycles. Likewise, when the single-reduced steel was lightly scored with the same box cutter, it did not fail. This suggests that single-reduced steel would withstand considerably more bending, tapping, hitting or dropping that double-reduced steel.
The amount of failure analysis research conducted by Chemaxx on this
failure mechanism has been extensive and only a brief summary of the
key points is presented here.
Chemaxx also believes that the necked-in design contributes to the bending that triggers the explosive failure mechanism, which is not known to occur in the straight-sided design. Drop-impact testing showed that the necked-in design could be made to explode, but that the same drop-impact tests on the straight-sided design only resulted in a leak. While a leak is undesirable and unsafe for a number of reasons, it is preferred over a catastrophic explosion.
Further failure analysis testing shows that the use of double-reduced steel contributes to the explosive failure mechanism. An aerosol made from single-reduced steel did not fail nor show any signs of incipient failure after extremely aggressive drop-impact testing. Figure E shows a metallographic cross-section of the single-reduced double seam after very abusive testing. In spite of the obvious gross deformation of the double seam region, there were no signs of cracking or incipient failure.
Additional Failure Analysis Comments
Each of the 360-degree explosive failures has occurred at near 70-90°F. The Department of Transportation (DOT) Regulations require that aerosol containers not burst at less than 1.5 times their internal pressure at 130°F. Since the aerosols are at 70-90°F when they burst, their internal pressures are clearly not even at 1.0X their internal pressures at 130°F, let alone the 1.5X the 130°F pressures. Also, there is no indication that the top domes have expanded or that the bottoms have everted outward, which further supports that these are low-pressure bursts. Hence, these aerosol containers do not meet the DOT minimum burst requirements.
The fact that these are low-temperature (and hence low-pressure) explosions increase the odds that they would be totally unexpected by the ordinary consumer and hence present an unreasonable risk to consumers.
The "bulge" shown in Figure D might suggest that the aerosols are "abused" prior to failure. However, drop-impact testing has shown that the "bulge" can be created by drop-impacts of as little as 2-3 feet. Hence, the "bulge" is not evidence of "abuse," unless a drop of 2-3 feet is considered "abuse." Another view is that accidental dropping from even 5-6 feet or tapping and hitting to loosen the mixing marble, are highly foreseeable human behaviors.
The Failure Analysis and Metallurgical Solution
Since the 360-degree catastrophic explosive failures do not occur in the straight-sided aerosol, the straight-sided aerosol is one obvious solution. The only known reason for the necked-in design is "cosmetic." In other words, the selection of the necked-in design is a choice of "looks" over "consumer safety." However, drop-impact testing has shown that the straight-sided design is susceptible to leaks, while the same drop-impact testing has shown that the use of single-reduced steel is not likely to even result in any leaks let alone an explosion. Both solutions are economically feasible and well-established existing technologies. Furthermore, the use of single-reduced steel has also been shown (Journal of Failure Analysis & Prevention, August, 2008, p.353) to significantly lessen the probability of puncture. Hence, the use of single-reduced steel would have a double benefit of reducing the probability of the 360-degree explosive failure and reducing the likelihood of puncture.
Dr. Michael Fox of Chemaxx, is accredited in Aerosol Technology by the Center for Professional Advancement as well as the British Aerosol Manufacturers Association, and certified by the DOT in the transportation of hazardous materials. (the DOT regulates aerosol containers and the products that can go in them).
Dr. Fox is a Certified Fire & Explosion Investigator with extensive experience in metallurgy, corrosion and failure analysis who is also accredited in aerosol technology.. He has made presentations at national societies on the fire and explosion hazards associated with aerosols and was the first to publish a peer-reviewed paper on aerosol failures. He now leads the field in the number of peer-reviewed papers (8) on aerosol failures, fires and explosions. His aerosol-related paper in 2007 won the Paper-of-the-Year Award from the ASM Journal of Failure Analysis & Prevention. For further details on Dr. Fox's qualifications as an aerosol expert witness click here.
©2011 CHEMAXX, INC