picture example, an aerosol, sprayed upon a
large flat surface, produced yellow-tan clouds of
isobutane, between 8" and 12" (250 to 300mm)
high, and having roiling, rounded tops around
6" to 10" in diameter—with little or no evidence
of gradation into gas-air mixtures. There appeared
to be some degree of cohesiveness. The
surface characteristics could be likened to those
of unstable, drained and severely coalesced
foams with macro-bubbles—except these
rounded gas structures are vastly bigger.
The aerosol industry also uses pentanes, hexanes
and heptanes, often as isomeric mixtures
(Shell, et. al.) or as distillation cuts (ExxonMobil,
et. al.). For example, n-heptane vapor is
3.45 times as heavy as air and has a flash point
of 25°F (-4°C). The vapor is relatively slow
to evaporatively generate, but once formed,
can progress along floors and down ramps or
stairways in a relatively thin layer of supra-LEL
content.
In one warehouse episode, a gaseous mixture
of hexanes and heptanes drifted across a
concrete floor for a distance of almost 80'
(24 meters) before contacting an electrical
switchboard that ultimately caused it to ignite.
Figure 6
If a floor fire does occur, the flames will travel fanwise and back to the source at the typical rate of about 12.6' per
second (3.85 m/s), making it virtually impossible for people to get out of harm’s way.
Serious plant explosions have occurred as a result of traveling supra-LEL gas-air mixtures—not only of the volatile
hydrocarbons, but also of ethanol, diethyl ether, dimethyl ether and acetone. In fact, diethyl ether vapors have an
insidious degree of cohesiveness that will allow the vapors emanating from a small, open container to travel over
26' feet (8 meters) along a laboratory work-bench to an ignition site. This unusual characteristic has caused most
toll packers to eschew the filling of diethyl ether (DEE) into cans of engine starter and similar products.
The volatile liquid hydrocarbons, now used rather widely in mold releases, certain adhesives, certain de-dusting
type cleaners and other specialty aerosols, are often treated the same as the normally gaseous types; for instance,
stored in pressure resistant bulk tanks, particularly in the case of n. and isopentanes. Drums of mixed hexanes
(boiling range beginning at 121.5°F or 49.7°C) are known to deform—and in at least two cases, to leak—if stored in
the hot sun during summer months. The volatile liquid hydrocarbons are markedly different than the normally
gaseous ones in that they can remain as liquids during aerosol productions. Many years ago, during the production
of a hair spray that included 15.0% isopentane and 5.5% carbon dioxide, two major safety hazards were encountered.
Upon their release from the T-t-V gasser about 1.0 gram of isopentane sprayed out and onto the gas house
floor. It formed a pool of liquid, cooled by the evaporation of a portion. The ventilation system was obviously
incapable of removing the liquid, which was being continually replenished as production continued. Inevitably it
got onto the boots worn by the gas house operator, making him even more nervous.
The cans then traveled into the main building and into the hot water bath. They still contained some liquid
isopentane in the well of the valve cup, which sizzled into gas as soon as the can submerged in the bath. It was
estimated that about 1.0 lb. (454g) of isopentane vapors were generated in the hot tank every three minutes or
so. When a “cup blower” was installed between the gas house and the main establishment, liquid isopentane was
blasted out of the cup—solving the hot-tank problem—but the mist landed on the ground below the blower. This
could be tolerated fairly well during the hot summer months. However, as winter weather developed, the isopentane
evaporation rate decreased precipitously, and some actually seeped through holes or porous areas of snow to
remain for long periods as trapped (flammable) gas. Much to the disappointment of the marketers, these formulas
had to be changed to eliminate the isopentane ingredient.
Summary
The LEL of flammable gases and liquids is one of the most important safety concepts of the aerosol industry.
Around the world, many industrial and consumer accidents could have been avoided if LEL aspects were better
understood and implemented. Since over 10 billion aerosols per year (worldwide) (Editor’s note: More than 15 billion
in 2018) are now made using hydrocarbon gases or volatile liquids, LEL technology affects virtually every aerosol
plant on the planet, and it is an important factor to be considered in our efforts to provide continuing consumer
protection. Spray
42 Spray January 2019