Toxic gas control for RF absorber fires

A method of controlling toxic outgassing of carbon impregnated polyurethane nd other radio-frequency-absorbing materials when burning or smouldering. Ammonia gas and water or steam are sprayed onto smouldering material to combine with hydrogen cyanide (HCN) and hydrogen chloride (HCl) formed duringburning to yield ammonium cyanide plus ammonium chloride which yields relatively harmless precipitates when cooled.

BACKGROUND OF THE INVENTION 
This invention relates to a method of controlling fires and more 
particularly to a method of controlling toxic outgassing of certain 
burning or smouldering materials. 
In recent years polymeric materials have been used more and more for 
structural components and for interior furnishings of buildings. Many of 
these materials contain halogen, cyanide, and/or other nitrogenous 
mixtures, which can react during combustion or pyrolysis to form toxic 
products such as hydrogen chloride (HCl) and hydrogen cyanide (HCN). 
Because of the danger to life and property, the response of these 
materials to fire needs to be readily controlled. 
Experience has shown that one must be careful in interpreting the 
terminology used to describe varying degrees of fire resistance. 
Self-extinguishing materials burn as long as a source of external heat is 
present but are incapable of sustaining combustion after removal of the 
heat source. The fire characteristics of a self-extinguishing grade of 
polyurethane foam can be drastically changed by the processing it 
undergoes to produce a good microwave absorber. Treatment with fire 
retardants does not produce a nonflammable material; it only reduces 
flammability. A nonflammable material will not burn when exposed to a 
flame; however, foams that have been painted with a fire-retardant paint 
can lose their fire-retardant characteristics when the paint is destroyed, 
either mechanically or by fire. 
The interior walls and ceiling of many modern anechoic chambers are covered 
with a nitrogen-containing polymer-polyurethane foam. To serve as a good 
microwave absorber, the foam is impregnated with a carbon and a latex 
binder to impart good electrical characteristics. The resulting material 
has low electrical resistance and is thus a good conductor. The foam is 
further treated with enough of a fire retardant, such as a 
chlorine-containing compound, to impart fire resistance with degrading 
electrical properties. When involved in fires, these treated foams may 
release toxic combustion products containing nitrogen and halogen, whose 
presence could significantly increase the toxicity of fire gases. 
Further, it has been determined that these polymers release the irritant 
gas, hydrogen chloride (HCl). When air containing moderate concentrations 
of HCl gas is inhaled, the HCl is normally removed by the upper 
respiratory tract and does not penetrate into the lungs. However, it has 
been shown that the gas may be sorbed in hazardous quantities on airborne, 
fire-generated particles. If the particles are properly sized when 
inhaled, they carry the irritant gas past the defenses of the upper 
respiratory tract and deposit in the lungs. Desorbing from the particles, 
the gas then insults the delicate tissue of the lungs and causes pulmonary 
edema. This mechanism is believed to have caused deaths that occured 24 to 
48 hours after exposure in certain fire situations. 
Airborne soot and water particles are the suspected carriers of the 
irritant gas in fires. In burning experiments of polyvinyl chloride with 
polyethylene, soot particles have been shown to carry the irritant. When 
filtered from the fire gases, 2 percent by weight of the particles have 
been found to be loosely bound HCl. Electron micrographs show that the 
particles are sperically shaped with diameters of from 0.03 to 0.11 
microns. According to theory, such particles cluster into assemblies that, 
for periods of 30 to 60 minutes, are of sizes that penetrate the 
respiratory defenses; 20 to 40 percent of the inhaled particles succeed. 
Atmospheric analysis of different types of foams indicate that the 
combustion products contain toluene diisocyanate, HCN, HCl, CO.sub.2 and 
CO. These gases will cause discomfort in a short period of time and even 
death after an hour in some cases. Experimental tests have been reported 
in an NRL Report 7793 entitled "Flammability and Toxic-Gas Production from 
Urethane Foams used in Anechoic Chambers", by Patricia A. Tatem and 
Frederick W. Williams, Naval Research Laboratory, Washington, D.C. 20375. 
The almost opaque airborne soot compounds the fire fighter's problem. 
Visibility is so poor that personnel must be continually accounted for and 
the buddy system used. 
The problem with burning foam is it will burn as long as heat is applied, 
even if it has a fire retardant in the foam. There is little firemen can 
do to control smouldering RF absorber material other than contain products 
of combustion and wait for smouldering to gradually stop. Normal use of 
water, foam, CO.sub.2 and soda acid extinguishers do nothing to control 
outgassing. In most RF absorbers, even fire retardant polyurethane, 
smouldering continues unless the temperature is lowered because the 
material contains its own oxidizer and can burn slowly for hours. 
Therefore, some method must be used to extinguish the burning and 
smoulding foam materials while preventing toxic outgassing. 
SUMMARY OF THE INVENTION 
This invention provides a method by which burning or smouldering RF 
absorber materials may be controlled to avoid toxic outgassing.

DETAILED DESCRIPTION 
In carrying out this invention, ammonia gas is mixed with water or steam 
and sprayed onto any smouldering material via the ventilation system via 
existing open head sprinkler plumbing, or by a special plumbing 
arrangement including a nozzle system connected with the ammonia-water 
apparatus. When sprayed onto the smouldering material the ammonia combines 
with HCN + HCl in the formula 2NH.sub.3 + HCN + HCl .sup.cooled NH.sub.4 
CN + NH.sub.4 Cl. The process is reversible but yields relatively harmless 
precipitates when cooled by the water or steam. CO combines with O.sub.2 
to form CO.sub.2 ; 6CO + 30.sub.2 .fwdarw. 6CO.sub.2. Since NH.sub.3 is 
much lighter than air; in a closed chamber or room, the NH.sub.3 produced 
will rise to the top since it is about 60% the weight of air. The NH.sub.3 
excess can be removed by use of an exhaust fan. 
FIG. 1 illustrates a suitable, simple device for mixing ammonia gas and 
water and for spraying the mixture into a chamber. The system includes a 
pressure tank 10 within which the NH.sub.3 is stored and controlled by 
valve 11. An outlet line 12 connects at one end to the ammonia gas tank 
valve. The outlet line includes an automatic control valve 13 which may be 
made operational by heat from a flame or by hand. A pressure line 14 from 
a water tank 15 connects to the outlet line between the valve 13 and a 
venturi 16 near the end of the outlet line. A line 17 connects to the 
venturi of the outlet line and extends to near the bottom of the water 
tank so that water will be forced up and drawn up from the tank and mixed 
with the NH.sub.3 at the venturi of the outlet line due to a Bernoulli 
pressure difference as well known in the spraying art. The low pressure 
outlet end 18 of the outlet line is directed into a room or other suitable 
dispensing system and the NH.sub.3 + H.sub.2 O mixture directed onto the 
surface of interest. The mixture may be directed into the ventilation 
system or a special plumbing arrangement from which the mixture may be 
sprayed onto any smouldering or burning RF absorbing materials. Any 
NH.sub.3 gas accumulating in the upper area of the closed area should be 
exhausted by a fan. 
The ammonia gas-water mixture system may be used with an anechoic chamber. 
In the event of a fire, the valve, which may be automatic, will be opened 
and the area must be evacuated of all personnel and the chamber doors 
closed because the burning of foam produces HCl and HCN which are very 
toxic. The NH.sub.3 +H.sub.2 O cools the burning foam while reacting with 
HCl and HCN to produce NH.sub.4 CN + NH.sub.4 Cl which will be 
precipitated harmlessly. With the forming of NH.sub.4 Cl and NH.sub.4 CN 
the smouldering or burning of fire protected foams will cease and the 
personnel may return upon removal of the gases within the chamber. As set 
forth before, a fan is used to remove any NH.sub.3 in the upper area of 
the room. Thus, it is seen that the addition of a mixture of NH.sub.3 and 
H.sub.2 O will control of the burning or smouldering of RF absorber 
materials. 
In cold weather instead of using water which may freeze in storage, steam 
may be used and mixed with the NH.sub.3 prior to injection into the 
enclosed area. Expansion of the mixture in either case has an additional 
cooling effect. The strong odor of ammonia can serve as olfactory alarm 
that an automatic system has been set off and that personnel must evacuate 
the area. Prolonged containment of the products of sustained combustion 
may contribute to flash-over; therefore the exhaust fan is recommended for 
purging as well as for enabling fire fighters to reach the burning 
material. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.