WO2010068957A2

WO2010068957A2 - Slow burning pyrotechnic delay composition

Info

Publication number: WO2010068957A2
Application number: PCT/ZA2009/000109
Authority: WO
Inventors: Walter Wilhelm Focke; Olinto Guiseppe Del Fabbro; Darren Swanepoel
Original assignee: African Explosives Limited
Priority date: 2008-12-09
Filing date: 2009-12-09
Publication date: 2010-06-17
Also published as: WO2010068957A3

Abstract

A pyrotechnic time delay composition which comprises a particulate mixture formed at least of manganese and an oxidant which is an oxide of manganese.

Description

SLOW BURNING PYROTECHNIC DELAY COMPOSITION

BACKGROUND OF THE INVENTION

[0001] This invention relates to a time delay element for a detonator.

[0002] A delay element usually includes a small diameter tube which contains a compacted pyrotechnic composition located between an ignition source and an ignition transfer system. The composition is typically a mixture of an oxidising agent and a fuel which is capable of an exothermic oxidation-reduction reaction. After the composition has been ignited by the ignition source a combustion wave is propagated along the tube, ideally at a constant velocity, to transfer an initiation impulse to a detonator, connected to the ignition transfer system, after a precisely controlled time interval.

[0003] The actual time delay is determined by a number of factors such as the nature and stoichiometry of the pyrotechnic composition, the length and diameter of the pyrotechnic composition inside the tube and the material and dimensions of the tube.

[0004] For time delays of a short duration a fast burning composition is used, e.g. silicon as a fuel in combination with red lead oxide. For a long time delay a slow burning composition is used e.g. barium sulphate. In each instance the reaction should be exothermic, self-sustained and self-contained. The composition should, when burnt, evolve less than 1001 of gas per kilogram of mixture and should burn at a constant predetermined and repeatable rate. [0005] For a long time delay it is commonplace to make use of a BaSO₄-based mixture which is loaded into a bore of a lead tube and then compacted, The tube is then rolled to a desired diameter. During the rolling process the pyrotechnic composition is compacted yet further. This arrangement works satisfactorily.

[0006] The use of lead does however carry certain disadvantages. For example lead is expensive and has a toxic characteristic. For this reason there has been a move away from lead tubes to rigid aluminium tubes. It has been found however that the slow burning BaSO₄-based mixtures which are used in lead tubes show variable ignition performance when used in aluminium tubes.

[0007] An object of the present invention is to provide a slow burning pyrotechnic composition which displays reliable ignition behaviour in a rigid aluminium tube.

SUMMARY OF INVENTION

[0008] The invention provides a pyrotechnic time delay composition which comprises a particulate mixture formed at least of manganese and an oxidant which is an oxide of manganese.

[0009] Preferably the oxide is MnO₂.

[0010] The manganese may have a mean particle size (d.50) of from 4μm to 8μm, preferably of the order of 6μm; a surface weighted mean particle size of from 10μm to 15μm, preferably of the order of 13,2μm; and a BET surface area of from 0,4m²/g to 0,8m²/g, preferably of the order of 0,6 m²/g. [0011] A diluent of any appropriate type may be included in the mixture. For example, the diluent may be selected from hollow glass spheres and fumed silica. The diluent may be added, to the mixture, in differing amounts, depending on a predetermined burn rate required for the delay composition.

[0012] The spheres may have a nominal diameter of 75μm and may be of the kind sold under the trade name Ballotini Q-CeI 2106. The fumed silica may be of the kind supplied by Degussa under the trade name Aerosil 200.

[0013] The manganese dioxide may have a mean particle size of from 8μm to 12μm, preferably of the order of 10μm.

[0014] Preferably the composition includes the following constituents, in the following ranges, specified on a percentage mass basis: Mn : 40% to 80%;

MnO₂ : 25% to 55%; diluent : 2% to 10%.

[0015] The precise constituent composition of the pyrotechnic composition depends on the required burn rate for a particular application of the pyrotechnic composition.

[0016J The invention also extends to a time delay element which includes a tubular enclosure and a pyrotechnic time delay composition of the aforementioned kind located in a bore of the enclosure.

[0017] The delay element may include an ignition source exposed to the pyrotechnic composition. The ignition source may be a G-type pyrotechnic. [0018] The tubular enclosure is metallic. The enclosure may be lead but preferably is a rigid aluminium tube.

[0019] The aluminium tube may have an outer diameter in the range 6.0 to 6.6mm, an inner diameter in the range 3.0 to 3.4mm and a length in the range 30 to 60mm.

[0020] The pyrotechnic composition within the aluminium tube may be compacted by application of a pressure to the composition which is greater than 7 MPa and preferably is from 10 MPa to 12 MPa.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] A number of candidate metal fuel/metal oxide oxidant compositions, for use in a pyrotechnic time delay composition, were identified, theoretically, based on thermodynamic feasibility considerations and adiabatic reaction temperature (ART) calculations.

[0022] An example of such a pyrotechnic composition candidate, so identified, is Mn/MnO₂ which will react to produce a product of MnO as follows: Mn + MnO₂ → 2MnO. This composition was initially identified, as a possible candidate, as MnO theoretically is thermodynamically more stable than MnO₂. Thus the above equation, once initiated, should proceed spontaneously towards the right i.e. towards formation of MnO.

[0023] However for a thermodynamically spontaneous reaction to be sustainable, the combustion of the composition requires that energy which is released from the reactive system should exceed or equal the heat loss experienced by the system to the environment. The ART is a theoretical indicator of this energy release.

[0024] Therefore, for a composition to qualify as a pyrotechnic composition candidate, the anticipated reaction of the composition must be thermodynamically feasible and must have a relatively high ART.

[0025] The following compositions, tabulated in Table 1 , qualified as candidate compositions based on these criteria. Cobalt and copper, with relatively low ARTs, were also included as controls in an attempt to determine if the ART of a composition is a suitable indicator of the feasibility of the composition as a pyrotechnic time delay composition.

Table 1

[0026] An open burn test, which is a relatively quick test used to determine if a candidate composition will ignite on application of a heat source substantially to completion, was conducted on each of the candidate compositions.

[0027] In conducting the tests, a homogenous mixture of each of the candidate compositions was made by brushing a powdered mixture of the composition through a 125μm sieve, three or four times, to break up any agglomerates and to effect good mixing.

[0028] Thereafter, each homogenous mixture was placed on an asbestos slab and a heat source, from a blow torch, was applied. If the candidate composition sample did not ignite, a small amount of starter composition (G-type pyrotechnic) was used as an ignition aid.

[0029] Each candidate composition was tested over a metal fuel range of 20%, i.e. 10% on each side of a mass determined by the stoichiometric ratio. The results of the open burn tests are shown in Table 2.

Table 2

[0030] The relatively high ARTs of antimony, manganese, molybdenum and vanadium produce an expectation that the candidate compositions containing these metals should combust during the open burn tests. However, surprisingly only the compositions of Mn/MnO₂ and Nb/Nb₂O₅ would combust under the test conditions. The other candidate compositions did not combust, irrespective of the respective metal fuel constituent mass. [0031] These tests indicate that only compositions of MnZMnO₂ and Nb/Nb₂O₅ remain as viable pyrotechnic time delay compositions.

[0032] Further analysis and experimentation shows that only manganese reacts in the required manner, i.e. Mn + MnO₂ → 2MnO. The niobium produced a combustion reaction with oxygen in the atmosphere, and did not give rise to a true pyrotechnic reaction.

Claims

1. A pyrotechnic time delay composition which comprises a particulate mixture formed at least of manganese and an oxidant which is an oxide of manganese.

2. A time delay composition according to claim 1 wherein the manganese has: a) a mean particle size (d.50) of from 4μm to 8μm; b) a surface weighted mean particle size of from 10μm to 15μm; and c) a BET surface area of from 0,4m²/g to 0,8m²/g

3. A time delay composition according to claim 1 or 2 which includes a diluent selected from hollow glass spheres and fumed silica.

4. A time delay composition according to any one of claims 1 to 3 wherein the oxide is Mnθ₂ with a mean particle size of from 8μm to 12μm.

5. A time delay composition according to any one of claims 1 to 4 which includes the following constituents, in the following ranges, specified on a percentage mass basis:

Mn : 40% to 80%;

MnO₂ : 25% to 55%; diluent : 2% to 10%.

6. A time delay element which includes a tubular enclosure and a pyrotechnic time delay composition located in a bore of the enclosure wherein the pyrotechnic time delay composition is according to any one of claims 1 to 5. A time delay element according to claim 6 wherein the tubular enclosure is a rigid aluminium tube with an outer diameter in the range 6.0 to 6.6mm, and inner diameter in the range 3.0 to 3.4mm and a length in the range 30 to 60mm.