Novel Pyrotechnics for Incendiary Applications

T T Griffiths, QinetiQ, Fort Halstead, Sevenoaks, UKE L Charsley, J J Rooney and H M Markham, Huddersfield University, Huddersfield, UKP D Howe, CEH Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, UK

AbstractMany of the incendiary compositions used in ammunition contain potassium perchlorate which is recognized as a potential hazard to human health. In response to SON PPSON-05-02, new formulations containing environmentally acceptable oxidizers are being investigated.

BackgroundPerchlorates are high energy oxidizers used in a wide range of military applications including rocket propellants and pyrotechnics. Many ammunition incendiary compositions containing potassium perchlorate are formulated to produce either an incandescent flash (and smoke) to mark an impact point or act as ignition sources for flammable liquids.

Potassium perchlorate has a high solubility in water which results in very low retardation in aquifers. As a result, any groundwater plumes can be extensive and pose severe remediation problems. It can be released into the environment as a result of spillages during manufacture, demilitarization, or when ammunition fails to function correctly.

The presence of perchlorates in drinking water is recognized as a potential hazard to human health; their ingestion is known to inhibit iodide uptake by the thyroid gland. A number of US states control the perchlorate content of drinking water; in California a final Public Health Goal of 6 ppb has been introduced, and in Massachusetts the acceptable perchlorate level is only 2 ppb.

Work is in progress to formulate new high energy pyrotechnic compositions for incendiary ammunition applications. The composition that they will replace have been found to have burning rates of between 4 mm s-1 and 7 mm s-1.

Materials and MethodsFormulation studiesA range of binary and ternary pyrotechnic compositions were prepared using aluminum, magnesium or magnesium-aluminum alloy. The oxidants examined were the nitrates of barium, potassium, sodium and strontium, and the sulfates of barium, calcium, potassium, sodium and strontium.

The compositions were prepared by blending the ingredients together in a conducting pot using a Turbula mixer.

Pyrotechnic Burning rateFlares were prepared by pressing four 3 g increments of each composition into 12 mm diameter cardboard tubes at 6.7 kN. The column length of the pressed composition was determined and the flare burning time measured.

Exothermicity MeasurementsExothermicity measurements were performed using a Parr Model 1425 semi-micro combustion calorimeter. Duplicate experiments were carried out on 100 mg powder samples in an argon atmosphere under a pressure of 1 atmosphere. The samples were ignited in quartz dishes using a hot wire.

DSC studies under ignition conditionsDifferential scanning calorimetry (DSC) experiments were performed on the compositions using a high temperature DSC developed for pyrotechnic studies. The measurements were made on 20 mg samples which were heated at 50 °C min-1 in 2 cm tall quartz crucibles in an upward flowing argon atmosphere.

DiscussionCompositions containing aluminumThe combustion studies show that initially as barium nitrate is replaced by potassium or sodium nitrate a large decrease in burning rate occurs (1). In contrast, the addition of strontium nitrate had little effect on the burning rate of the compositions.

When sulfates are used as the second oxidant the burning rate are found to increase for calcium and strontium sulfate (2) but decrease for barium, potassium and sodium sulfate.

The DSC curves for binary compositions containing 50% aluminum and 50% nitrate show that the fusion of the aluminum at around 660 ˚C influences the exothermic activity following the melting of both strontium and barium nitrate (3).

Compositions containing magnesiumA complex series of burning rate curves are obtained for the compositions where barium nitrate is replaced by a sulfate (4).

The maximum burning rate is observed for the binary composition containing magnesium and calcium sulfate. A similar burning rate is observed for the formulations where half of the barium nitrate had been replaced by barium or strontium sulfate but at higher sulfate levels the burning rates are lower.

Only small changes in burning rate are observed for the compositions containing barium nitrate and sodium sulfate. The compositions containing potassium sulfate generally show a decrease in burning rate with increasing sulfate content.

The exothermicity of the compositions decrease when barium nitrate is replaced by a sulfate (5). The greatest reduction is observed for barium sulfate.

Except for potassium sulfate, the DSC curves for the binary magnesium-sulfate compositions (6) show significant exothermic activity.

Compositions containing magnesium-aluminum alloyTernary compositions based on magnesium-aluminum alloy, barium nitrate and either potassium, sodium or strontium nitrate show a decrease in burning rate as the barium nitrate content is reduced (7).

In general, the burning rates of ternary compositions containing barium, calcium, potassium, sodium or strontium sulfate increase with increasing sulfate content (data not shown).

A small increase in exothermicity is observed as barium nitrate is replaced by one of the other nitrates (8). In contrast, adding the sulfates decreases the exothermicity of the compositions (9).

ConclusionsCompositions with a wide range of burning rates and exothermicities have been formulated; several could be suitable as replacement incendiary compositions. The effect on burning rate of adding a second nitrate or sulfate varied widely, depending of the fuel used in the composition.

The fusion of aluminum influences the exothermic activity of binary aluminum-nitrate compositions.

AcknowledgementsThese studies were funded by the Strategic Environmental Research and Development Program (SERDP).

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Results

Aluminum-barium nitrate-oxidant compositions

1. Burning rate

7. Burning rate

4. Burning rate

Magnesium-aluminum alloy-barium nitrate-oxidant compositions

Magnesium-barium nitrate-oxidant compositions

3. Ignition DSC2. Burning rate

6. Ignition DSC5. Exothermicity

9. Exothermicity8. Exothermicity