April 1942 ET-190
United States Department of Agriculture
Bureau of Entomology and Plant Quarantine
THE PREPARATION OF INSECTICIDAL AEROSOLS BY THE USE
OF LIQUEFIED GASES
By L. D. Goodhue, Division of Insecticide Investigations,
and W. N. Sullivan, Division of Control Investigations
Insecticidal aerosols (smokes or fogs) can be prepared by
several methods, one of which requires the use of liquefied gas.
The insecticide to be dispersed is dissolved in a low-boiling
solvent, such as dichlorodifluoromethane or methyl chloride, and
discharged into the atmosphere of the infested space. The vapor
pressure of the solvent produces the necessary spraying pressure,
which does not decrease as long as a drop of liquid is present.
As the liquid containing the insecticide is sprayed, a fine mist
forms, and the solvent evaporates almost instantly, leaving the
insecticide suspended in the air as an aerosol. Such an aerosol
is nontoxic to man and animals, noninflammable, and easily applied
without the use of heat or power, and yet it is highly toxic to
mosquitoes, flies, and many other insects.
For an aerosol having these desirable properties dichloro-
difluoromethane is used as the solvent. This liquid boils at
-21.7 F. (-29.8 C.) and has a vapor pressure of 82 pounds per
square inch at 68 F. (20 C.). Pyrethrum oleoresin (20 percent
total pyrethrins) in amounts sufficient to make about 4 mg. of
total pyrethrins per gram of solution is used as the insecticide.
It must be filtered to remc-. e sEall particles that might clog the
nozzle. Refined sesame oil is added at the rate of 10 mg. per gram
of solution to increase the toxicity.
Cylindrical tanks commonly used to hold 5 pounds of di-
chlorodifluoromethane can be purchased at almost any local re-
frigeration supply store. Each tank is equipped with a valve having
a 1/8-inch female pipe connection. If the tank is to be used with
the valve at the top, a siphon tube must be added, since the
solution, not the gas, must be sprayed. For most purposes it is
almost as convenient to spray with the tank inverted. The apparatus
is shown in figure 1.
The nozzles employed most successfully have been those
designed for small oil burners. A Monarch nozzle with an 80 cone,
having a capacity of 2 gallons per hour, is suitable for enclosures
up to 10,000 cubic feet. With a nozzle of this size 1 or 2 minutes'
spraying is sufficient to kill adulJ mosquitoes in 10,000 cubic feet,
with the proper distribution and exposure. Coir.mercially available
adapters and screens are used with these nozzles. Other nozzles
have been used, one that is designed for spraying vegetables being
satisfactory. The space from the valve to the orifice of the
nozzle should be small so that not much solution is left in this
compartment when the valve is closed after spraying. Any nozzle
must have its orifice smaller than the other constrictions to
prevent boiling in the nozzle. It can be of iron, brass, or
An oil-burner nozzle, adapter, and screen attached to a
small laboratory test apparatus are shown in figure 2.
Preparation of Solution
A clean tank is evacuated with a water pump, a funnel is
attached, and the required amounts of pyrethrum oleoresin and
sesame oil are introduced. The valve is closed before any air is
drawn in. The tank is then connected to a supply tank containing
dichlorodifluoromethane which has been turned upside down, and
both valves are opened to allow the liquid to flow into the evac-
uated tank. The supply tank must be warmed or the other tank must
be cooled to cause enough liquid to flow. A small amount of a
waxy substance in the resin is not soluble in dichlorodifluoro-
methane, but since it adheres to the inside of the tank it does
The airount of dichlorodifluororr.elhane introduced into' the
tank containing the insecticide is obtained by difference in
weight. The required amount can be determined most easily if a
flexible hose connection is used bet.',een the two tanks. No tank
should be completely filled, lest it burst from the pressure of
the expanding liquid should the temperature rise. Shaking causes
the insecticide to dissolve, and after the nozzle is attached the
package is ready for use.
For experimental work the amount delivered by the nozzle can
be determined by loss in weight duriii spraying for a certain time
interval. After this calibration the amount used can be determined
Other Liquefied Gases
Next to dichlorodifluoro.cethane methyl chloride is the most
convenient solvent. It is, in fact a much better solvent for many
solid insecticides. Methyl chloride is less expensive, its vapor
pressure is satisfactory (69.9 pounds per square inch at 68 F.),
and it has the advantage of a lower density. Our tests on beans,
snapdragons, and carnations indicate that it, as well as dichloro-
difluoromethane, can be used where plants are present, since only
a small amount is necessary to produce an aerosol. Methyl chloride
does have some toxicity to man, about one-fourth that of chloroform,
but when insecticides that are toxic to man are to be used, it is
not necessary to employ a nontoxic solvent. A large number of
insecticides are now being tested in this solvent.
Propane, which is inexpensive, may have a use especially for
the dilution of dichlorodifluoromethane, and can be added to a
point just below inflammability. It has a vapor pressure of 130
pounds per square inch at 68 F. Dimethyl ether might be used in
mixtures with other less inflammable solvents. Carbon dioxide
diluted with acetone or alcohol to reduce the high vapor pressure
(1,000 pounds per square inch at room temperature) might be em-
ployed, but no attempts have been made to use these mixtures.
Other less volatile or nonvolatile solvents have been added
to dichlorodifluoromethane. Among those tried are refined kerosene,
orthodichlorobenzene, acrylonitrile, acetone, and petroleum ether.
At equal parts by volume most of these mixtures show a vapor
pressure of more than 50 pounds per square inch, which is still a
suitable spraying pressure.
Since a considerable pressure is still retained after
dichlorodifluoromethane or methyl chloride has been diluted with
slightly volatile or nonvolatile solvents, this method is applica-
ble to the dispersion of some of the less volatile fumigants, such
as orthodichlorobenzene, which require a rather high concentration
to be effective.
Figure 1.-A diagram showing the apparatus used in the dispersion of
insecticidal aerosols: 1, Tank; 2, siphon tube; 3, valve; 4,
connection; 5, adapter; 6, nozzle (oil-burner); 7, screen.
Figure 2.-Laboratory apparatus for testing solubilities of insecticides
in dichlorodifluoromethane and methyl chloride and for testing mall
amomts of insecticides in aerosol form: 1, Heavy-wailed glass test
tube; 2, celluloid guard; 3, modified 1/8-inch pipe union; 4, leather
washer; 5, rubber washer; 6, needle valve; 7, nozzle; 8, screen; 9,
adapter for nozzle, screen, and 1/8-inch pipe connection.
UNIVERSITY OF FLORIDA
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