TPPO: A Replacement Candidate for Red Phosphorous

Joseph A. DomanicoChief, Pyrotechnics Team

Building E3580 Beach Point RoadAMSSB-REN-SP

Edgewood Chemical Biological CenterAberdeen Proving Ground, MD 21010-5424Voice: 410-436-2180 FAX: 410-436-4941joseph.domanico@sbccom.apgea.army.mil

ABSTRACT

The Edgewood Chemical Biological Center has been actively pursuing a replacement for redphosphorous for use in military smoke producing ammunition. Both burning and burstinggrenade designs using Triphenylphospine Oxide (TPPO) as the smoke material have shown thepotential for nearly matching the amount of smoke produced from a similar sized device filledwith red phosphorous. End burning hand grenade sized munitions provide a sizable smoke cloudfor a substantial amount of time. Core burning devices provide a significant increase in smokedensity and a reduced amount of time. TPPO is currently under evaluation for both its smokeproducing capabilities, and also for its potential to reduce the environmental pollution caused bythe use of red phosphorous in military ammunition. This paper will highlight the technical data,photographs, and videotape of the research and development efforts.

Introduction

The use of red phosphorous in military smoke ammunition gave an alternative to thehexachloroethane (HC) smoke system. HC smoke has been used for many years in generalwarfare to provide a visual screen which hides troop movements during battle. The zinc chloridebased smoke cloud is quite toxic due to the formation of hydrochloric acid in the lungs when asufficient quantity of smoke is inhaled. Additionally, the hexachloroethane used in theproduction of the grenade is now considered a carcinogen.

Background

Original formulations using red phosphorous were evaluated by the armed forces many yearsago. Red phosphorous proved to be difficult to use in burning type grenades due to its lowTamman temperature. This characteristic of red phosphorous resulted in the original burningformulations to react during storage severely reducing their shelf life. Eventually, a formulationwas devised using magnesium as the fuel and manganese dioxide as the oxidizer. The additionof zinc oxide as a curing catalyst and linseed oil as the binder allowed this formulation to besuccessfully used in a day/night marine marker. The large flame and copious smoke productionallowed its use a a rescue signal. A starter composition consisting of lead dioxide, copper oxide,and silicon successfully passed the long term storage tests for compatibility.

Reformulation efforts using a boron and calcium sulfate mixture to replace the magnesium andmanganese dioxide (fuel / oxidizer system) were successful at the Edgewood ChemicalBiological Center and at other research and development facilities. The additional cost of theformulation was considered too high a price to pay for the hydrogen gas which was evolved fromthe magnesium during storage. This effort was subsequently dropped for financialconsiderations.

An additional effort was successfully completed which used a binder which could prevent thepre-oxidation of the red phosphorous with the oxidizer during storage. The new compositionused aluminum as the fuel and sodium nitrate as the oxidizer. This formulation was successfullyused in burning wedges which were bare on all sides. There was no compatibility issues with theignition composition as it was sealed inside the round and had no direct contact with the redphosphorous mix.

Several of these types of formulations were fabricated in the traditional smoke grenade sized cansfor use as a substitute for the HC smoke grenade by ground troops. The large flame produced bythe red phosphorous grenades was a significant fire hazard and was considered the main reasonpreventing adoption of the system. Additionally, the red phosphorous based grenade could beaccidentally ignited if the canister was impacted with sufficient pressure. It was possible that thelevel of impact required could be obtained by a soldier “hitting the ground” from a standingposition. The subsequent flame and smoke would make him a dissatisfied customer as well asfurther highlighting his position to the enemy.

If a substitute for HC smoke grenades was to be found, and phosphorous was to be used, simplydesigning burning grenades in a manner similar to air burst munitions would not be successful.The search for a substitute for HC, and subsequently RP, lead to the use of triphenylphospineoxide. Other materials are currently under evaluation, but sufficient data is not available at thistime from publication. The use of triphenylphospine oxide as a successful substitute for redphosphorous in grenades and pellets may also allow its use as a substitute for HC smokegrenades.

Materials and Equipment

One of the goals of this effort was to only use techniques which could easily be scaled up intoproduction level. Typical pyrotechnic handling procedures would be evaluated if they could beutilized with existing equipment or by slight modification of existing equipment. Specialhandling procedures such a the use of protective masks and respirators, and protective gloveswould be used as simple precaution measures for the material handlers. Only a little is currentlyknown on the toxicity of handling triphenylphospine oxide and its subsequent combustion andvaporization products.

Dry blending of the components for manufacturing the smoke compositions was quite successful.The resulting mixtures burned well but the mixture contained excess amounts of trapped air.This resulted in a high degree of difficulty during the consolidation of the smoke blend. Theexcess trapped air makes the mixture “fluffy”. The direct result of pressing the loose powder into

a cake was extremely difficult as the mixture had a tendency to “flow” around the pressing ramand out of the can. The result was that a significant quantity of mixture could be found on thefloor and not inside the canister.

Blending the component chemicals with acetone and subsequently evaporating the acetoneproved to greatly reduce the trapped air and increased the free flowing density of the smokecompositions. For mixtures with either a non-organic binder or a dry binder, this was thepreferred mixing technique.

The use of binders which were soluble in acetone followed the same mixing procedure. thebinder was first dissolved in the acetone by directly adding the two components into the mixingbowl. After the binder went into solution, the dry powders were added. The oxidizer was alwaysthe final component to be added for safety reasons. (The mixture is not a pyrotechnic blend untilAFTER the powdered oxidizer is added). This reduces the exposure time of the energeticcomposition to the handlers.

Experimental

Several fuel / oxidizer combinations were blended by one of the above mentioned techniques.The dry powders were pressed directly into ‘standard” grenade cans, both in end-burningconfigurations and core-burning configurations. The standard grenade can is approximately 4.5inches in height and 2.3 inches in diameter. This is approximately the size of a 16 ounce soda (orbeer) can.

The end burning configuration is filled with smoke composition to a height of 4 inches, leavingan air space of 0.5 inches. The starter composition is pressed on the top surface. Loadingpressure varied throughout the experiment but was maintained within the 1000 pounds per squareinch and 10,000 pounds per square inch levels. The smoke exit ports for the end-burningconfiguration was 4 holes, each one 0.3 inches in diameter. The core-burning configuration isidentical in external dimensions but use a lid without any holes and a 0.5 inch diameter hollowcore in the axial center of the can.

In the end-burning configuration the smoke exits from the top of the can which is the endcontaining the igniter. In the core-burning configuration the smoke exits from the single hole atthe end opposite from the igniter. Electric igniters were used for all tests which replicatemanually functioned mechanical grenade fuzes.

Results and Discussion

End-burning configurations using a high and low consolidation pressure were tested by anindependent laboratory for particle size distribution and yield percentage. Subscale grenadesconsisting of 100 grams net weight of the TPPO based energetic smoke composition werefabricated with both small and large exit port diameters. Table 1 highlights the subscale grenadeconfigurations.

Table 1Subscale Grenade Configurations

Loading Pressure Exit Port Diameter1500 psi 1.3 cm 1.9 cm3500 psi 1.3 cm 1.9 cm

Average particle size determinations for the subscale grenades containing 40% TPPO by weightare shown in table 2. The average particle sizes usually followed the traditional bell shapedcurve. Some exceptions were noted which will not be discussed here.

Table 2Average Particle Size Range for 40% TPPO Subscale Grenades

Loading Pressure Exit Port Diameter Diameter (microns)0.5 - 1.0 1.0 - 2.0

3500 psi 1.3 cm ~50% ~50%3500 psi 1.9 cm ~30% ~70%

Mean aerosol yields of the smoke clouds were also determined to demonstrate the possibleincrease in smoke yield by the physical modification of the grenade. Table 3 shows thecomparison in the smoke yield for both end-burning and core-burning grenades. By changing theend-burning configuration to a core-burning one, the smoke yield increased from 15% to 45%for the end burner to 42% to 58% for the core burner.

Table 3Mean Smoke Aerosol Yield (Core v.s. End Burning)

Loading Pressure Physical Configuration Mean Aerosol Yield1500 psi End burning 15% to 45%1500 psi Core burning 42% to 58%

Both internal and exit port burning temperatures were measure to determine the potential for thedestruction of TPPO during its short life as a vapor inside the canister. Table 4 shows thedifferences between the exit port temperature and the internal burning temperature of thecomposition. Notice the wide difference in the temperature found inside the pressed compositionin comparison with the temperature found at the exit port for the grenade pressed at the higherpressure.

Table 4Internal Temperature v.s. Exit Port Temperature

Loading Pressure Internal Temperature Exit Port Temperature1500 psi ~380 deg C ~390 deg C3500 psi ~ 493 deg C ~345 deg C

Figure 1 shows a typical smoke cloud produced using 64% by weight of triphenylphospine oxidein a full scale smoke grenade. This configuration holds 250 grams of smoke mix with 6 grams ofstarter composition. The grenade burn time is 38 seconds. This configuration produces a thicksmoke for a moderate duration.

Figure 1Smoke Production from a 64% TPPO Grenade

Conclusions

Triphenylphospine oxide has shown its potential as a replacement for red phosphorous. It iscurrently a viable candidate based on its performance in making copious amounts of smoke.With loadings from 40 percent by weight to nearly 70 percent by weight, it has the potential todeliver copious smoke for short durations as well as high smoke production for extended smokeproduction times. The effort to increase the loading percent by weight, as well as the use ofauxiliary fuels and oxidizers may allow either more smoke production within the size constraints.