WO2006008706A2

WO2006008706A2 - An alternate oxidant for a delay composition

Info

Publication number: WO2006008706A2
Application number: PCT/IB2005/052329
Authority: WO
Inventors: Lonji Kalombo; Olinto Giuseppe Del Fabbro; Cornelius Du Plooy Conradie; Walter Wilhelm Focke
Original assignee: University Of Pretoria
Priority date: 2004-07-14
Filing date: 2005-07-13
Publication date: 2006-01-26
Also published as: WO2006008706A3; ZA200700236B

Abstract

This invention relates to an alternate oxidant for a delay composition, and more particularly to a delay composition including a fuel and an oxidant in the form of antimony hexitatridecoxide having the formula Sb6O13. The invention also relates to the use of antimony hexitatridecoxides as an oxidant for a delay composition.

Description

AN ALTERNATE OXIDANT FOR A DELAY COMPOSITION

FIELD OF THE INVENTION

This invention relates to a delay composition and to an oxidant for use in such a composition.

BACKGROUND TO THE INVENTION

Pyrotechnics applications include military and civilian devices such as flares, tracers, signals, smokes, incendiaries and fireworks. Delay compositions are generally used, inter alia, in delay elements for which there are a great deal of possible compositions known to those skilled in the art.

The purpose of the delay composition is to provide for a time delay between ignition (e.g. from a shock tube) to the initiation of an explosive charge. The time delay is governed by a number of factors, most especially the burn time of the delay composition. Typical delay times may vary between 20ms to 10s.

Whilst it will be appreciated that there are many variations in the art, delay compositions generally comprise a fuel and an oxidant. Without wishing to be bound by theory, the delay composition must be capable of a highly exothermic, propagating oxidation-reduction (redox) reaction. Although many oxides exist, few are suitable for delay compositions, especially when the reaction must not result in the production of large amounts of gaseous by-products that may interfere with the burn rate of the delay composition.

It is further known to those skilled in the art that attempts have and are continually being made in the explosives industry to reduce the use of toxic substances so as to avoid problems during the preparation of a delay composition as well as to reduce harmful emissions and to minimise exposure to the toxic substances.

Common oxidants in delay compositions are red lead (Pb₃O₄) and lead oxide (PbO₂) which are generally considered to be toxic. Barium based oxidant compounds, such as Barium Sulphate (BaSO₄), are also common oxidants, however they are slow burning and have variable performance.

OBJECT OF THE INVENTION

It is an object of the invention to provide an alternate oxidant for delay compositions.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided a delay composition including a fuel and an oxidant in the form of antimony hexitatridecoxide (Sb₆Oi₃).

The delay composition may comprise from 10 to 90 wt% (based on the total weight of the delay composition) of Sb₆O₁₃, preferably from 50 to 80 wt% of Sb₆Oi₃, more preferably from 55 to 75 wt% of Sb₆O₁₃ and most preferably 60 wt% of Sb₆O₁₃.

The fuel may be chosen from the group consisting of metals and metalloids. Preferably the fuel is silicon, and more specifically milled silicon.

The silicon is preferably in particulate form, the silicon particles having a specific surface area of from 1 to 15 m².g^"1' preferably from 2.50 to 10.5 m².g^~\ more preferably a specific surface area of 6.3m².g^"1.

The delay composition may comprise from 10 to 90 wt% (based on the total weight of the delay composition) of fuel, preferably silicon, preferably from 20 to 50 wt% of fuel, more preferably from 25 to 40 wt % of fuel and most preferably 40 wt% of fuel.

The delay composition may also include at least one additive, preferably selected from the group consisting of a retardant, an accelerator, a catalyst, a fluxing agent, a phlegmatising agent, a corrosion inhibitor and filler.

The retardant may be fumed silica or Fuller's Earth. The retardant, and in a preferred embodiment, the fumed silica, may be present in the delay composition in the weight percentage, based on the total weight of the delay composition of from, 0 to 15 wt%.

The delay composition may have a burn rate of from 2 mm.s'¹ to 25 mm. s^"1. According to a second aspect of the invention there is provided a method for preparing a delay composition, the method comprising the steps of:

a. providing silicon as a fuel; b. providing an oxidant in the form of antimony hexitatridecoxide having the formula Sb₆Oi₃; and c. dry mixing the fuel and oxidant together to form a mixture of fuel and oxidant.

The method may further comprise the step of wet mixing the mixture with a binding and/or phlegmatising agent, which thereafter may be dried at a temperature range of from 40⁰C to 70⁰C.

The mixture of step c may be subjected to wet granulation which may take place through a sieve or an orifice plate and may subsequently be dried at a temperature range of from 40⁰C to 70⁰C .

Further steps may involve dry granulation of the mixture through a sieve and ageing of the resultant granules for five days.

According to a third aspect of the invention, there is provided the use of antimony hexitatridecoxide as an oxidant in a delay composition.

According to a fourth aspect of the invention, there is provided a delay element including a delay composition comprising a fuel and an oxidant in the form of antimony hexitatridecoxide. According to yet a further aspect of the invention, there is provided a detonator including a delay composition, the delay composition comprising a fuel and an oxidant in the form of antimony hexitatridecoxide.

These and other features of the invention are described in more detail below.

DETAILED DESCRIPTION OF THE INVENTION

Without thereby limiting the scope of the invention and by way of example only, embodiments of the invention will now be described with reference to the following non limiting examples and figures.

Figure 1 is a schematic representation of a detonator device and the equipment used to determine the burn rate of the delay compositions according to the invention;

Figures 2 and 3 are graphs indicating the burn rate of the delay compositions set out in Tables 1 and 2 below, wherein a lead tube was used in the delay element;

Figure 4 is a graph indicating the burn rate of the delay compositions set out in Table 3 below, wherein an aluminium tube was used in the delay element.

Figure 5 is a graph indicating the burn rate of the delay compositions set out in Table 4 below, wherein fumed silica was used as an additive.

Preparation of SbsOi?.

Antimony hexitatridecoxide (Sb₆O₁₃) was prepared by heating white colloidal Sb₂O₅ powder in a crucible which was covered with a steel lid with a small hole to allow gases to escape there from. The Sb₂O₅ was subjected to an 8 hour thermal treatment at ca. 315⁰C in a convection oven, where-after the resultant product was allowed to cool in the furnace back to room temperature and was confirmed to be cubic Sb₆Oi₃ by X-ray Diffraction.

Preparation of the delay compositions

Various delay compositions, set out in the tables 1 to 4 were prepared by blending the different ingredients in a tumble mixer for 4 hours. There after the mixture was passed gently through a 125μm sieve using a soft brush so as to ensure break-up of any agglomerates formed during blending. It will be appreciated from the tables below that the delay compositions comprise silicon as the fuel, Sb₆O₁₃ as the oxidant, which oxidant was prepared in terms of the method set out above, and in the case of table 4, an additional additive in the form of fumed silica, which acts as a retardant.

Table 1: Silicon fuel ratios and masses used for silicon having a surface area of 6.30 rn².g^"1 and resultant burn rate in a delay element including a lead tube.

Table 2: Silicon fuel ratios and masses used for silicon having a surface area of 10. 1 m².g^'1and resultant burn rate in a delay element including a lead tube.

Table 3: Silicon fuel ratios and masses used for silicon having a surface area of 10. 1 m^z.g^'1 and resultant burn rates in a delay element including an aluminium tube.

Percentage Mass of Si (9) Mass of Sb -,O₁₃ Burn rate in a weight ratio of (9) lead tube as

Silicon to total delay element

The effect on the burn rate of a delay composition as a result of the amount of fuel used, the surface area of the fuel and the type of delay element used is represented graphically in Figures 2 to 4. From the above results it is clear that the longest burn rate was achieved for a delay composition having 40 wt% of silicon, based on the total weight of the delay composition, 60 wt% Sb₆Oi₃ and a silicon surface area of 6.30 m².g-¹.

It will be appreciated by those skilled in the art that the burn rate of a delay composition may also be affected by the addition of additives. Accordingly delay compositions were prepared with an additional component, namely a retardant in the form of fumed silica. Table 4: Resultant burn rate of a delay composition comprising 10 % Silicon to total mass of the delay composition with varying ratios of retardant provided in the form of fumed silica, the silicon having a surface area of 10.1 m².g^'1

The effect on the burn rate of a delay composition comprising 10 wt% of silicon (to the total mass of the delay composition) is represented graphically in Figure 5. It can be seen from the burn rates, that the addition of fumed silica between 0.5 to 5 wt% (to the total mass of the delay composition) increased the burn rate.

Preparation of Lead tube delay elements

The delay composition was poured into a 165 mm long lead tube with an average inner diameter of 6 mm. The tube wall thickness was between 2 and 3 mm. The composition in the tube was consolidated and compressed by a rolling operation that reduced the diameter in ten successive steps to 6 mm.

During each rolling operation, the sealed tube was passed through a hole with a smaller diameter. This operation yielded a final tube approximately 420 mm long. The external diameter and wall thickness were 6 and 1 ,5 mm respectively. In this way, good compaction of the delay composition was ensured.

The rolled lead tube was then cut to a standard length of 45 mm to form the delay elements. After obtaining the 45mm lead tube, an increment of the initiating starter was added after removing 3mm of the core at one of the ends of the tube in contact with the shock tube.

Preparation of Aluminium tube delay elements

Aluminium delay elements were made by drilling holes through aluminium rods (6 mm in diameter), which rods were cut to a length of 45 mm. These tubes were filled incrementally. The mixture was compacted after adding each increment by inserting a punch and applying controlled compaction pressure that was measured by means of a load-cell device (HBM Komm).

For most of the elements prepared in this way, the pressure was set at approximately 42 MPa. A dwell time of about one second was used before relieving the stress. Thereafter the filled tube was sealed after introduction of different accessories including the anti-static cup, grommet and shock tubing.

The delay elements (lead and aluminium tubes) were incorporated into a standard non-electric delay detonator assembly for burn rate testing, which delay detonator is represented graphically in Figure 1.

Determination of Burn Rates

The device 10 set out in Figure 1 was employed to determine the burn rates of different delay compositions.

First the delay element 18 was placed inside a tube 14 that was sealed at one end thereof, the tube 14 having a charge 13 therein. A plastic anti-static cup 16 was introduced and pushed against the end of the tube of the delay element 18 followed by a rubber sealing grommet 20 placed into the tube 14, thereby acting as an anchor for the shock tubing 22. The latter was inserted until it touched the bottom of the anti-static cup 16. At this point the free end of the tube 14 was crimped to complete the assembly and to prevent any relative axial motion of individual elements in the assembly.

A hole (3mm) was drilled at the end of the tube of the delay element 18, where a type R (0,38mm in diameter) thermocouple was embedded for measurement of burning rate.

Measurement of Burn rates

Referring to Figure 1 again, the burn rate of the delay compositions were measured as follows:

The shock tube 22 was set off electrically, the shock tube 22 being connected to a trigger box 24, which emitted a noise when the ignition of the shock tube 22 took place. A sound sensor 26 placed in the trigger box 24 records the noise as the starting point for the burn reaction in respect of the delay composition.

The end of the burn reaction was detected by means of thermocouples 28, the thermocouple measurements being recorded via a computer interface. A Pt-Pt 13% Rh thermocouple (type R) with a diameter of wires equal to 0,38mm, joined by flame, was embedded in the composition through a 3 mm hole in the closed end. The reaction speed was then calculated as the ratio of the length of the element and the burn time-interval. The latter was taken as the time difference between the starting signal (sound signal) and the final thermal signal provided by the thermocouple.

Recording of results

The thermocouple output was sent via an electronic cold junction 30 compensator to data capture software on an EAGLE PC 3OF personal computer 32. The gain of amplification was varied between 100 and 1000. The signal-to-noise ratio was improved by utilising a digital filter. The sound sensor output, after amplification, was also sent to data capture to get the starting signal.

It will be appreciated that other oxidants and fuels may be mixed with antimony hexitatridecoxide. It will further be appreciated that many variations in detail are possible without departing from the spirit and the scope of the invention. For example, different techniques and methods of measurement may be used to determine the burn rates.

Claims

1 . A delay composition including a fuel and an oxidant in the form of antimony hexitatridecoxide having the formula Sb₆O₁₃.

2. The delay composition of claim 1 , wherein the delay composition comprises from 10 to 90 wt%, based on the total weight of the delay composition, of Sb₆Oi₃.

3. The delay composition of claim 2, wherein the delay composition comprises from 55 to 75 wt%, based on the total weight of the delay composition, of Sb₆O₁₃.

4. The delay composition of claim 3, wherein the delay composition comprises 60 wt%, based on the total weight of the delay composition, of Sb₆O₁₃.

5. The delay composition according to any of the preceding claims, wherein the fuel is silicon.

6. The delay composition according to claim 5, wherein the silicon is milled silicon having a specific surface area from 1 to 15 m².g-¹.

7. The delay composition according to claim 6, wherein the silicon has a specific surface area of 6.30 m²,g^'1.

8. The delay composition according to claim 6, wherein the silicon has a specific surface area of 10.1 m².g^'1.

9. The delay composition according to claim 1 , wherein the delay composition comprises between 10 to 90 wt%, based on the total weight of the delay composition, of fuel.

10. The delay composition according to claim 9, wherein the delay composition comprises from 25 to 40 wt%, based on the total weight of the delay composition, of fuel.

1 1 . The delay composition of according to claim 10, wherein the delay composition comprises 40 wt%, based on the total weight of the delay composition, of fuel.

12. The delay composition according to any of the preceding claims, wherein the delay composition includes at least one additive selected from the group consisting of a retardant, an accelerator, a catalyst, a fluxing agent, a phlegmatising agent, a corrosion inhibitor and a filler.

13. The delay composition according to claim 12, wherein the retardant is fumed silica.

14. The delay composition according to claim 13, wherein the composition comprises between 0 to 15 wt%, based on the total weight of the delay composition, of fumed silica.

15. The delay composition according to claim 1 , having a burn rate of from 2 mm.s^"1 to 25 mm.s^"1.

16. Use of antimony hexitatridecoxide having the formula Sb₆O₁₃ as an oxidant in a delay composition including a fuel.

17. A delay element including a delay composition comprising a fuel and an oxidant in the form of antimony hexitatridecoxide having the formula Sb₆O₁₃.

18. A detenator including a delay composition comprising a fuel and an oxidant in the form of antimony hexitatridecoxide having the formula Sb₆O₁₃.

19. A method of preparing a delay composition, the method comprising the steps of:

a. providing silicon as a fuel; b. providing an oxidant in the form of antimony hexitatridecoxide having the formula Sb₆O₁₃₁

c. dry mixing the fuel and oxidant together to form a mixture of fuel and oxidant;

20. The method according to claim 19, wherein from 55 to 75 wt%, based on the total weight of the delay composition, of Sb₆O₁₃ is provided.

21 . The method according to claim 19, wherein the mixture of step c is followed by a wet mixing step with a binding agent.

22. The method according to claim 21 , wherein the mixture subjected to wet mixing with a binding agent is dried at a temperature of from 40⁰C to 70⁰C.

23. The method according to claim 19, wherein the mixture of step c is followed by a wet mixing step with a phlegmatising agent.

24. The method according to claim 23, wherein the mixture subjected to wet mixing with a phlegmatising agent is dried at a temperature of from 40⁰C to 70⁰C

25. The method according to claim 19, wherein the mixture of step c is subjected to wet granulation.

26. The method according to claim 25, wherein the mixture subjected to wet granulation is dried at a temperature of from 40⁰C to 70⁰C