COMBUSTION

This invention relates to compositions capable of combustion at very high temperatures, and incendiary devices employing those compositions. The aluminothermic or thermite reaction between aluminium powder and iron oxide as first described by Goldschmidt in German Patent 96,317 (1895) produces large quantities of heat at high temperatures and has been widely used in industry, for example in welding and in the production of pure carbon-free metals such as manganese and chromium.

The thermite reaction may be represented as

8A1 + 3Fe ₃ 0 ₄ ⁴ 2 ⁰ 3 ^{+ 9Fe + 3} ' ^3KJ/ 9

(solid) (solid) (solid) (liquid) Thermites can be difficult to ignite, particularly those based on aluminium as the sole fuel, but this problem can be overcome by including magnesium as part of the fuel and also by including more reactive oxidants, for example barium nitrate, in the formulation. However, known thermite formulations still have disadvantages for particular purposes.

It is an object of the present invention to provide an improved thermite-type composition. Thermite compositions typically contain

a fuel and an oxidant.

The criteria to be applied in selecting components for a thermite-type composition may be summarized as follows. The selection of the most suitable fuels for use in thermites is based on maximum energy output and can best be achieved with the use of materials of high calorific values. Thus fuels used in thermites should possess the following properties.

1. High heat of combustion energy and high combustion temperature.

2. Be readily oxidised, while requiring minimal oxygen for their combustion. 3. Form combustion products that provide for the best special effect of the composition, in this case the ormation of a metal slag.

4. Be chemically and physically stable at normal operating temperatures, and non-hygroscopic.

5. Be non-toxic, readily available and easy to process.

Oxidants used in thermites ideally should have the following characteristics.

1. Minimal heat of formation.

2. Highest possible specific gravity.

3. Be reduced to a metal having a low melting point and high boiling point.

4. Contain at least 25% oxygen (or equivalent oxidant) .

The most suitable combustible for thermites, having regard to the energy output, combustion temperature and the realtively low fusion temperature of the oxide, is aluminium, but thermites based on aluminium as the sole fuel are difficult to ignite. Normally a magnesium-based thermite is used to ignite the aluminothermic reaction. It has been found experimentally that the magnesium content of the fuel should be at least 30% to consistently ignite the mixture. As oxidants, oxides of metal having an intermediate atomic weight (approximately 40 to 80) appear to be the most suitable for use in thermites.

The maximum amount of heat is evolved when the chemical bonds being severed are weak and the new compound bonds are strong. Thus potentially oxygen and fluorine are the best oxidants but in a solid state reaction

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(as with a thermite combustion) there are ^• relatively few solid compounds of oxygen and fluorine containing weak bonds.

When combining with fluorine many active metals like magnesium and aluminium evolve even more energy than when combining with oxygen.

We have found that polytetrafluoroethylene (PTFE) , also known by the trade name of TEFLON, may be employed with advantage as a component of thermite—type compositions.

PTFE is a non-hygroscopic solid with a fluorine (oxidant) content of 76%. In effect it "sensitizes ¹¹ the thermite by its ability to react vigorously (even violently) with both magnesium and aluminium and this also improves the ease with which the thermite can be ignited.

Accordingly, in one aspect the present, invention provides a thermite-type composition comprising polytetrafluoroethylene. Further, by choice of suitable binders, we have found that a pliable thermite may be produced, i.e. a composition able to be shaped into any form.

Prior to the present invention, only thermites in the form of powder, pressed pellets or petroleum

gels were available.

The present invention in a further aspect provides a pliable thermite containing a polymeric resin as a binder. The following Table 1 gives the critical surface tension values of resins we investigated as binders in developing the present invention.

TABLE 1 CRITICAL SURFACE TENSION VALUES OF RESINS MATERIAL γ (refs. 5, 6)

(n/m)

PTFE (teflon) 1.8

Silicone rubber 2.4 - 2.6

Open-chain methyl silicones* 1.9 - 2.0

Polyethylene 3.1

Polystyrene 3.3

Polyvinylchloride (PVC) 3.9

Polymethylmethacryl te 3.9 (Perspex)

Open-chain aliphatic 2.7 - 2.9 monoesters

Open-chain aliphatic 2.8 - 3.4 diesters (alkyd)

Starch 3.9

Cellulose 4.5

Nylon 4.6

This group includes dimethylpolysiloxane.

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Of the resins studied the best results were obtained with silicone resins.

The selection of a silicone as a dispersing medium has several advantages viz 1. Low surface tension, a useful characteristic which minimises tackiness when the mixture is handled (Table 1) , and

2. High water resistancy.

Of the silicone resins investigated the most consistent products were otained when a dimethyl- polysiloxane resin having a viscosity of about

300 poise was used. This silicone resin does not contain reactive groups and consequently has a low surface tension. A preferred embodiment of the invention accordingly provides a pliable thermite composition containing PTFE and employing a silicone resin as binder, more preferably a dimethylpolysiloxane resin. In one aspect of the investigations carried out in the development of the present invention we aimed to produce a composition that would be capable of igniting oil and a wide range of petroleum products even when submerged in oil; capable of ignition by an electrically-actuated device, and

30/6

able to burn without being affected by high winds.

In this connection the use of a thermite-type composition offered the following advantages:- 1. The high temperature produced in the combustion reaction. For most thermites the combustion temperature lies in the range (2000-3000°C) which is more than twice the temperature (700-900°C) generated by liquid incendiary devices. 2. The almost complete absence of gaseous reaction products, resulting in flameless combustion, and

3. The formation of molten slags which increased the possibility of ignition. Nearly all pyrotechnic devices or formulations

(including thermites) are composed of fuels, oxidants and additives, the ratio depending on the specific purpose of the device. The reactivity of the mixture depends largely on the chemical nature of the individual components and on their particle size.

During our initial .experimental work it was observed that it was necessary to sustain an average combustion temperature of about 2500°C for about 20 seconds to ensure that crude oil

« _m * &

and other petroleum products of high viscosity and low vapour pressure would consistently ignite. Lengthening the period of combustion can be achieved by a number of factors, one of the more significant being to increase the average particle size of the primary fuel, aluminium. However the coarser the particle size, the lower the specific surface area and the more difficult aluminium is to ignite.

We found that if a small proportion of a third fuel, titanium, was added and the average particle size of the aluminium kept relatively low the combustion period was lengthened and the combustion temperature maintained.

Taking into account the factors governing the choice of the various constituents as described above, the following formulation was chosen in a preferred embodiment of the invention.

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tø_ OM

> . ^lP

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wt%

FUELS Aluminium 22 to 30 Magnesium 10 to 15 Titanium 0 to 4

2. OXIDANTS

Iron Oxide 12 to 16, preferably about 14

Teflon 16 to 22

3. BINDER

Silicone resin 22 to 30

It is preferred that the total fuels constitute about 40% of the composition.

A specific preferred formulation is as follows:-

MRL-X410 Thermite Formulation

Mean Particle

Diameter (y ) Wt%

1. FUELS

Aluminium 55 ¹ 26.2

Magnesium 55 ¹ 11.9

Titanium 115 ¹ 2.4

2. OXIDANTS Iron Oxide Red ³ < 2" 9.5

Black ^* < 2- 4.8

Teflon 6-8' 19.0

3. BINDER

Siliconeresin 26.2

100.0

NB. 1. As measured on a Malvern ST 3300 Particle Size Analyser.

2. As measured on a Fisher Sub-sieve Sizer.

3. The use of iron oxides in this ratio gives a brown product. Any colour between red and black could be produced by adjusting the ratio appropriately. Other colours could be produced by using the appropriate dye.

OM

In this formulation the teflon powder was initially dispersed in the viscous silicone resin and then the other oxidant, iron oxide, previously dried and sieved, was added and uniformly dispersed. Finally, the fuels which had been previously sieved-mixed, were incorporated and the resultant mixture stirred until a homogeneous product resulted.

This formulation is fuel-rich because of the difficulty of incorporating the stoichiometric amount of iron oxide. The stoichiometric fuel- oxidant ratio is approximately 40% fuel and 60% oxidant, i.e. the mixture should contain a significantly higher content of iron oxide and/or teflon; but thermitic formulations containing high contents of iron oxide dispersed in a silicone resin tend to have poor cohesive properties.

The properties of this formulation are summarize as follows.

Pliability and Environment Effects The material developed is plastic and can be shaped into any form by hand. When exposed to the hot sun or to severe environmental testing (ISAT B) conditions some tackiness

was observed due to 'sweating' of the resin.

This 'sweating* effect is a common occurrence in putties, mastics, luting compounds and even plastic explosive, but this material can be reworked manually and the thermite is apparently restored to its original condition with no noticeable effect on its incendiary behaviour.

Ignitability Under Water Like virtually all silicone resins, the dispersing medium in this.material is water- repellant. Immersion of the thermite in water for up to 72 hours does not affect its ignition or combustion performance. It can be ignited under water and will sustain combustion under water.

Once ignited, the plastic thermite is very difficult to extinguish with conventional fire-fighting materials (foam, water) . Resistance to Hydrocarbons

When immersed in petrol or oil for thirty minutes some resin from the surface is dissolved and the solvent is discoloured, but subsequent ignitability and performance of the thermite do not seem to be affected.

O PI

Ignitability Under Oil

The thermite was successfully ignited when submerged under thirty centimetres of oil using either an electrically triggered igniter or safety fuse known as plastic igniter cord

(PIC, (slow) , supplied by ICI, Australia) .

When submerged under thirty centimetres of water the thermite will ignite using either of the above initiating devices. Penetration of Aluminium

Another important characteristic of the plastic thermite itself is that it generates enough heat to penetrate aluminium or aluminium alloy plate. For example, a 50 gm cube of plastic thermite will penetrate aluminium

1 mm thick. However, it was not possible to penetrate comparable thicknesses of steel, presumably because of the significant differences between the melting point of aluminium (659°C) and that of steel (about 1550°C) .

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During testing of the material for the purpose of igniting oil it became apparent that the material was most effective when ignited on or close to the surface of the oil. It would 5 therefore be advantageous if it could be modified to float on oil which would have the inherent advantage that it could be useful in the event of an oil spill. However, the specific gravity of the formulation has been calculated to be

3 10 2.78 Mg/ and the bulk density was measured at 2 Mg/m . and it will not float on oil, water, or any of the hydrocarbon fuels.

We decided that it was not possible with commonly available materials to reformulate

15 the thermite to reduce the specific gravity to about .8 Mg/m which approximates to the specific gravity of hydrocarbon fuels. Nor is it possible to reduce the density sufficiently by including such additives as phenolic or organic microballoons

29 (which have a bulk density of about .2 Mg/m ) without affecting the cohesion of the product.

We then decided to contain the incendiary in a canister designed in such a way that it just floated on oil.

The invention accordingly includes a device containing an appropriate quantity of a thermite-type composition as described hereinabove, a quantity of bouyancy material sufficient to cause the canister to float on oil, an igniter, and aperture adapted in use to release molten liquid products of the thermite reaction into the said oil.

It will be clearly understood that the invention in its general aspects is not limited to the specific details described hereinabove.