November 2019 SPRAY 25
my experience, a failure at 130°F indicates keeping a
close watch on the results of the room temperature
and 100°F checks in the long term. My personal
(undocumented and untested) opinion is that
there is frequently (but not always) a correlation
between a stability failure at 130°F and later
failures at 100°F and room temperature. However, I
(and some of my colleagues) will be quick to point out
that this is not always a direct correlation. The exception
here is when “pitting” corrosion is observed in the 130°F
samples. My process is to label any pitting type corrosion as unacceptable
and the stability of that particulate aerosol variable (formula/
propellant/can combination) is immediately terminated.
Pitting is the most severe form of can corrosion wherein the “pit”
can rapidly corrode completely through the can wall leading to a
release of the product/propellant.
Back to my story…after an exhaustive investigation the following
points were noted:
• The oven was set to 130°F and the heating system was
functioning properly.
• The five cans in the oven were insecticide; the propellant
was a flammable hydrocarbon blend.
• The event occurred approximately three weeks into the
test. One of the cans developed significant levels of pitting-
type corrosion.
• The flammable contents leaked from this can and the
hydrocarbon propellant fraction, which is heavier than air,
accumulated on the bottom of the oven.
• At this point, the four required elements for an explosion
to occur were present:
1. Fuel: The hydrocarbon propellant.
2. Oxygen: Naturally present in the chamber of the oven.
3. LEL achieved: Lower Explosion Limit (the mixture of
fuel/air must be: A not too rich and B not too lean
for an explosion to occur).
4. Ignition source: a faulty (exposed) heating element in the
oven.
• Next, the oven’s temperature controller sensed a
need to turn on the heating element (located at the
bottom of the oven) to maintain the temperature
set-point. The heating element was not fully
shielded/gasketed from the oven’s internal
environment and, when energized, acted as an
ignition source, causing the explosion in the
oven chamber. This was the root cause of the explosion.
• While only one can leaked, three cans exploded due
to the concussive force of the propellant explosion. Two of
the five cans remained intact. We speculated that the force
(shock wave) of the gas explosion instantly raised the
internal pressure of the three cans beyond the structural
integrity limits of the cans and the buckle/burst transition
happened instantly.
Let me define “buckle/burst” for those who may not be familiar
with the term. Aerosol containers are regulated by the U.S.
Dept. of Transportation (DOT). Specifically, the DOT regulates
pressure requirements of consumer product aerosols (refer to the
Code of Federal Regulations–CFR Title 49).
The buckle pressure specification represents the maximum
pressure that the aerosol can is expected to withstand before
any structural deformation occurs (with no release of product).
Examples are top or bottom bulging (but not separating from the
cylindrical container sidewall). The burst pressure is the maximum
pressure that an aerosol can is expected to withstand before
any release of contents occurs from a structural failure of the
container. This release is typically exhibited as a forceful release of
contents through a tear in the sidewall of the can, a top (or bottom)
seam separation or forceful evacuation of contents through a
pressure release mechanism.
TABLE 1
U.S. DOT Aerosol Can Specifications
Specification Buckle (psi) Burst (psi) Labeling
DOT “Standard” 140 210 None
DOT “2P” 160 240 “DOT 2P”
DOT “2Q” 180 270 “DOT 2Q”
Lessons learned
The resulting lessons and corrective actions from the investigation
are recommendations I routinely present to clients.
Benchtop stability oven and oven controls.