NZ243509A

NZ243509A - Pyrotechnic composition comprising a fuel and coated particles of barium peroxide or lead dioxide, the coating being a surface active agent; low energy shock tubes and delay elements

Info

Publication number: NZ243509A
Application number: NZ243509A
Authority: NZ
Inventors: Geoffrey Frederick Brent; Malcolm David Harding; John Cooper
Original assignee: Ici Plc
Priority date: 1991-07-11
Filing date: 1992-07-10
Publication date: 1994-12-22
Also published as: AU1866792A; ZA924895B; AU643895B2; GB9114985D0; MX9204050A; PH30801A; ZM3492A1; ZW10392A1; JPH05238866A; CA2073543A1

Description

New Zealand Paient Spedficaiion for Paient Number £43509 24 35 0 9 ! F. /©• 7)' ^ ■ Cass: , Ces»<<=> |if;. .< Ofe»'3.3.3i /oc^j PubliCG-icn p-;:: 2. fl^.Q P.O. Jo'jrn:', No.: Date: L dS NEW ZEALAND PATENTS ACT, 1953 NO DRAWINGS COMPLETE SPECIFICATION PYROTECHNIC COMPOSITION We, IMPERIAL CHEMICAL INDUSTRIES PLC, a British company of Imperial Chemical House, Millbank, London SW1P 3JF, England, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (followed by page 1A) /V<£ c_ 3b^Hqf//V2 24 35 09 Pyrotechnic Composition This invention relates to the field of blasting and is particularly concerned with means for transmitting an initiating signal (non-electrically) to an explosive device 5 to remotely detonate same in accordance with a predetermined delay period.

There have been many proposals for achieving remote detonation of explosives by means of non-electric methods of initiating signal transmission. These include low energy 10 fuses such as the so-called "shock wave conductors" or "shock tubes", which consist of plastics tubing containing a fine dusting of particulate chemicals capable of reacting to propagate a percussion wave throughout the length of the tubing, as currently available commercially under the Trade 15 Marks "Nonel" and "EXEL" (hereinafter simply referred to as "shock tube"). Reactive combinations of chemicals that have to date achieved sufficiently reliable and reproducible performance for practical systems mostly have signal propagation velocities of around 2000 m.s"1, too fast for 20 the shock tube itself to be conveniently used as inter-hole delay elements. Achievement of desirable slower propagation velocities is dependent upon availability of suitable, reliable, precise, reactive compositions. For an inter-hole delay of, say, 10 milliseconds at, for example, 5 metres 25 inter-hole separation a propagation velocity of from around 500 m sec"1 to, at most, say 1000 m sec"1 would be desired for a shock tube fuse to allow for short or at least manageable lengths of tubing to be used. At 20 milliseconds inter-hole delay the desired maximum propagation velocity 30 would drop correspondingly to about 400 to 500 metres per second. These velocities are intermediate between classical "Nonel" and safety fuse.

Whereas in the past it has been proposed to reduce the overall signal transmission rate of shock tube systems by 35 using discrete pyrotechnic delays or mechanical artifacts interposed in the tubing, and the literature describes chemical compositions purporting to lower signal transmission rates, our co-pending patent applications 24 3 5 0 9 including NZ 232429, GB-A-2 237 101, EP-A-0 384 630 propose improvements in low velocity shock tubes by using barium peroxide (BaC>2) as oxidant in a typical reactive chemical composition. The advantage of barium peroxide as 5 oxidant is that it has a thermal decomposition temperature (circa 800°C) that is exceptionally well suited for the supply of oxygen to sustain a stable low speed propagation. In that context stable means providing predictable and reliable firing. However it has been found that if a 10 pyrotechnic composition containing Ba02 is stored over extended periods of time in unsealed systems e.g. in a shock tube assembly having a tube wall permeable to water vapour and carbon dioxide there is an observed decrease in peroxide concentration. It is considered that ambient carbon dioxide 15 and water vapour penetrate the mixture and together exert a deleterious effect on its stability by producing basic carbonates at the expense of peroxide in the composition.

H20 2Ba02 + 2 CO2 2 BaC03 + O2 Barium hydroxide (Ba(0H)2) would also be formed after exposure to a humid atmosphere. Storage of a similar composition in an air-tight container results in no significant decomposition of the peroxide. ^ Deterioration during prolonged storage is also evident in compositions containing lead dioxide (Pb(>2 - which is also referred to as lead peroxide or superoxide), when similarly exposed to carbon dioxide and water vapour with formation of PbC03 and Pb(0H)2-) Such undesirable reaction of these higher oxides with atmospheric carbon dioxide and water vapour can lead to signal transmission failure in systems based on such oxidants, whether shock tubes or other delay devices.

Thus it is an object of the present invention to provide further improvements in delay compositions 35 containing BaC>2 or Pb02 as oxidant. It is a further object of this invention to provide a storage stable intermediate velocity shock tube delay element for use in a blasting system. • 3 243509 Accordingly this invention provides a pyrotechnic composition of enhanced storage stability comprising mixed particles of a fuel and barium peroxide or lead dioxide as oxidant wherein the said oxidant particles have been treated 5 with a surface active material that attaches to surface regions of the barium peroxide or lead dioxide particles to provide a surface array of hydrocarbyl chains which acts as a barrier to impede ingress of water vapour and or carbon dioxide. The surface active material may exhibit properties 10 characteristic of an anionic surfactant or perform in use as though it were an anionic surfactant. Chelating agents such as modified polyalkylene glycols may also form the basis of suitable materials capable of performing a stabilising role.

A preferred stabilising material is obtainable from a 15 long-chain fatty acid or a derivative thereof. Thus the surface active material may comprise any fatty acid including monocarboxylic, dicarboxylic and polycarboxylic acids or a derivative thereof which is freely convertible to the free acid or a salt thereof under conditions of 20 application, e.g. an ester such as a lower alkyl (C^-Cg) e.g. methyl ester, or an acid anhydride or acid ghloride may be effective for the purpose of this invention.

The resulting pyrotechnic composition exhibits a surprising improvement in storage stability enabling its 25 application in intermediate velocity shock tubing without any need to take other precautions to protect such tubing from ambient air either during the production process or in storage of the tubing. The composition can also be stored in bulk for extended periods and remain functional. 3 0 The surface active material may be applied by mixing into a dry particulate form of the oxidant or into the dry powder pyrotechnic mixture itself. Such dry mixing could be enhanced by grinding e.g. as in ball-milling. Alternatively the material may be applied by a vapour deposition method. 35 Preferably it is applied to the oxidant as a solution in a non-aqueous solvent. The solvent may be any convenient organic solvent e.g. an aliphatic hydrocarbon such as hexane, an aromatic such as xylene or an alcohol or mixtures 24 35 09 thereof. If dry mixing with the oxidant itself the surfactant treatment may be augmented by warming of the dry components before or during mixing.

The preferred surface active materials tested to date 5 may be classified as generally anionic surfactants, but use of a surfactant which would be classed as non-ionic but has surface adhering polar groupings with pendant hydrocarbyl chains (as tails or loops) to protect the oxidant particles and possibly additionally provide preferential polar sites 10 for scavenging of water molecules by for example hydrogen bonding is also considered within the scope of this invention. Particularly suitable surfactants identified so far include those having a carboxylic functional group such as an aromatic carboxylic acid or aliphatic carboxylic acid 15 having branched or unbranched long alkyl chains, (so-called fatty acids) and derivatives thereof, especially stearic acid which is particularly effective in both dry mix and solvent processes. Small amounts of carboxylic acid or carboxylate surfactant may suffice, say from 0.25 to 3% m/m 20 to obtain substantial reduction in peroxide loss. Barium and lead carboxylates would appear to be formed quite readily but this cannot be conclusively stated to be the only mechanism of protection. Other anionic surfactants including organic phosphate or sulphate surfactants also 25 exhibit similar but less startling protective properties.

Non-ionic surfactants such as sorbitan monooleate show some improvement but are not as effective as stearic acid. Poly(isobutylene) succinic anhydride (PIBSA) which would not as such be classified as anionic performs remarkably well 30 and in this case it is considered that the PIBSA may form protective layers over the peroxide particles or in the course of application or use react with hydroxyl species present at the surface of the particles so as to perform as a dicarboxylic acid.

Classical cationic surfactants tested so far are without any appreciable beneficial effect in meeting the problem of stability over a period of storage. However this 24 3509 does not exclude the possibility that selected mixed anionic and cationic surfactants or surfactants having both cationic and anionic functional groups in the same molecule will be found to be effective.

Beneficial effects are anticipated by selecting or preparing polymeric surface active materials capable of adopting conformations geometrically suited to providing required attachment points with outwardly pendant hydrocarbyl chains in close array.

The pyrotechnic composition is preferably in the form of a substantially continuous fine powder dusting on an inner surface of the tubing. The core loading in a tubing having an internal diameter of around 1-2 mm suitably lies in the range of from about 10 to 100 mg. m"1, preferably 15 from about 15 to about 50 mg.m-1, depending on the fuel component (s) chosen and the amount of any adjuvants also present. The ratio of fuel component (s) to oxidant when, as is preferred, Ba02, t*16 sole solid oxidant present may be from about 2:98 to about 80:20, preferably from about 10:90 20 to 55:45. The fuel may be one or a mixture of metals and pseudo-metals combustible in oxygen e.g. B, Al, Si, Se, Ti and W. As is already recognised in the art, important variables of these systems are atomic weight of the fuel, and its particle size and proportions of ingredients in the 25 reactive compositions relative to notional stoichiometric amounts.

Stable reproducible (within 5%) propagation speeds at selected values lying in the range of around 400 m sec"1 to around 800 m sec"1 have been achieved using different 30 metal/pseudo metal fuels and/or different relative proportions of fuel and BaC^. The controlling signal transmitting reaction is combustion of dispersed fuel "dust" with this liberated oxygen, although any oxygen already present in the tube, e.g., as air, will also become 35 involved.

This invention is especially directed at shock tube having a signal propagation speed intermediate between conventional "Nonel" tubing (circa 2000 ms"1) and safety 6 24 3 5 09 fuse cord (less than 1 m sec-1) and in that context while mixed fuels may be readily considered, mixtures of Ba02 and other solid oxidants need to be selected with caution. However, in the broader context of shock tubing for which 5 inherent delay timing is not an important issue BaC>2 may usefully be used in admixture with other solid oxidants. It will be evident that this invention also provides a delay unit which comprises tubing as aforesaid connected at one end (at least) to an instantaneous cap or a delay detonator 10 in a manner known per se in the art.

The invention will now be further described by way of the following examples.

Reference Example Production of a low energy fuse of the type with which 15 this invention is concerned involves adding a mixture of a fuel e.g. fine aluminium and an oxidant e.g. barium peroxide, in a weight ratio of 10:90, in a manner known per se in the art to a 1.5 mm I.D. tubing made of "Surlyn" (a trade mark of Du Pont). The core load per linear metre was 20 about 50 mg. A velocity of about 760 m.s"1 was recorded. This result was repeatable to within 5%.

The following examples illustrate the improvements obtainable by virtue of the invention and particularly describe treatment of barium and lead per/dioxides which may 25 be used as oxidant in a shock tube as described in the reference example above. In these examples the per/dioxides were placed in a bell jar and subjected to an accelerated ageing test over a 24 hour period under the following conditions: relative humidity 100%; partial pressure of 30 carbon dioxide 0.5 atm., temperature 20°C, and a titrametric method was used to determine per/dioxide concentrations before and after exposure to such conditions. Results are reported as losses in per/dioxide concentration after several days exposure to the test. Generally it was 35 found that where losses were observed they would be significant initially but thereafter reach a plateau. Applying this test to untreated barium peroxide resulted in dramatic reductions in peroxide concentration (80-100% 24 3 5 0 9 losses) within 24 hours. Infra-red spectroscopic analysis clearly showed the formation of BaCC>3 after such exposure. Under the same test conditions untreated lead dioxide showed up to 10% loss of dioxide.

Example 1 Dry mixing and grinding of barium peroxide with stearic acid (octadecanoic acid Ci8H36^2) at a level of 2% m/m resulted in a 20% peroxide loss over the period of test.

Example 2 Example 1 was repeated using only 1% stearic acid but mixing was accompanied by heating above the melting point of stearic acid (80°C). A significant improvement was apparent since the peroxide loss was only 5% Example 3 Solutions of stearic acid in hexane were prepared containing 0.5%, 1% and 3% m/m stearic acid respectively and applied to particulate barium peroxide to obtain treated oxidant which when exposed to the accelerated ageing test showed only 5% reduction in peroxide in each case.

Example 4 Dry mixing and grinding of lead dioxide with stearic acid (octadecanoic acid C18H35O2) at a level of 3% m/m resulted in only 1% dioxide loss over the period of test.

Example 5 Solutions of stearic acid in hexane were prepared containing 0.25%, 1% and 3% m/m stearic acid respectively and applied to particulate lead dioxide to obtain treated oxidant which when exposed to the accelerated ageing test showed no reduction in dioxide at all.

Example 6 An ethanol solution containing 3% (m/m) of the organic phosphate surfactant EMPHOS was applied to particulate barium peroxide to obtain treated oxidant which showed only a 10% reduction in peroxide after accelerated ageing.

Example 7 A solution containing 3% polyisobutylene succinic anhydride in hexane/xylene mixed solvent was made up and applied to particulate barium peroxide to obtain treated

Claims

24 35 09 oxidant which when exposed to the accelerated ageing test showed only a 6% reduction in peroxide. Example 8 The aromatic carboxylic acid, fluorescein acid (3,4-5 dehydroxyfluoran resorcinolphthalein C20Hi2Os) was added by dry mixing to ground barium peroxide at a level of 2% m/m and exposure of the thus treated oxidant to the accelerated ageing test showed a 45% peroxide loss over the period of test. 10 Comparative Example 1 A solution of 3% m/m sorbitan monooleate in hexane was prepared and applied to particulate barium peroxide whereupon the treated oxidant when exposed to the accelerated ageing test showed a 55% peroxide loss. 15 Comparative Example 2 A 3% m/m solution of the cationic surfactant ARMAC-T, an amine acetate, in hexane was prepared and applied to particulate barium peroxide which when exposed to the accelerated ageing test showed an 80% peroxide loss. 20 Preferred treated compositions according to the invention are those which in the said accelerated ageing test show a per/dioxide reduction of less than 35%, more preferably less than 20%, and most preferably less than 10% (in the case of Ba02 - containing compositions) and less than 25 2% (in the case of PbC>2 - containing compositions) relative to the untreated equivalent compositions. It will be appreciated that the treatments herein described should be regarded as hindering reaction of the per/dioxide rather than rendering the per/dioxide non-30 reactive to moisture and carbon dioxide. The invention has been highlighted in the context of shock tube applications but is broadly applicable to Ba02 and Pb02~ containing pyrotechnic compositions where an initiating signal is transmitted by a deflagration front or 35 a shock front. » ' 243509 ■Claims WHAT ^/WO CLAIM JC:

1. A pyrotechnic composition of enhanced storage stability comprising mixed particles of a fuel and barium peroxide or lead dioxide as oxidant wherein the said oxidant particles 5 have been treated with a surface active material that attaches to surface regions of the barium peroxide or lead dioxide particles to provide a surface array of hydrocarbyl chains which acts as a barrier to impede ingress of water vapour and or carbon dioxide. 10

2. A pyrotechnic composition according to claim l wherein the said oxidant has been treated with a surface active material formed from(a high^olecularf weight carboxylic acid or a derivative thereof. 15

3. A pyrotechnic composition according to claim 2 wherein the acid is an aromatic carboxylic acid or aliphatic carboxylic acid having branched or unbranched long alkyl chains, or a derivative thereof. 20

4. A pyrotechnic composition according to claim 2 or claim 3 wherein the acid is a polycarboxylic acid.

5. A pyrotechnic composition according to claim 2 or 25 claim 3 wherein the acid is a dicarboxylic acid.

6. A pyrotechnic composition according to claim 2 or claim 3 wherein the acid is a monocarboxylic acid. 30

7. A pyrotechnic composition according to claim 6 wherein the acid is stearic acid.

8. A pyrotechnic composition according to claim 6 wherein the acid is fluorescein acid. 35

9. A pyrotechnic composition according to claim 2 wherein the acid derivative is an anhydride. 10 10 24 3 50 9

10. A pyrotechnic composition according to claim 9 wherein the anhydride is a poly talk (en) yl] succinic anhydride.

11. A pyrotechnic composition according to claim 10 wherein the anhydride is poly(isobutylene) succinic anhydride.

12. A pyrotechnic composition according to claim 1 wherein the said oxidant has been treated with a surface active material formed from an anionic surfactant.

13. A pyrotechnic composition according to claim 12 wherein the surfactant is a phosphate glycerol monoester.

14. A pyrotechnic composition according to any one of the 15 preceding claims wherein the surface active material is applied by mixing into a dry particulate form of the oxidant or into the dry powder pyrotechnic mixture itself.

15. A pyrotechnic composition according to claim 14 wherein 20 the application of surface active material is enhanced by crushing during mixing.

16. A pyrotechnic composition according to any one of claims 1 to 15 wherein the surface active material is 25 applied to the oxidant as a solution in a non-aqueous solvent.

17. A pyrotechnic composition according to claim 16 wherein the solvent is an organic solvent selected from aliphatic 30 hydrocarbons or an aromatic or an alcohol or mixtures thereof.

18. A pyrotechnic composition according to. claim 16 wherein the solvent is hexane. 35

19. A pyrotechnic composition according to claim 16 wherein the solvent is xylene.

20. A pyrotechnic composition according to any one of claims 14 to 19 wherein the dry oxidant or powder is heated before or during applica' of the surface active material. * V •243509

21. A pyrotechnic composition according to any one of the preceding claims wherein the amount of surface active material is from 0.25 to 3% m/m (based on oxidant). 5

22. A pyrotechnic composition according to any one of the Examples 1 to 8 hereinbefore.

23. A low energy shock tube of the type which comprises tubing having throughout its length an inner surface 10 carrying a sufficient amount of pyrotechnic composition to enable transmission of an initiation signal from one end of said tubing to the other, wherein the pyrotechnic composition is as claimed in any one of claims 1 to 20. 15

24. A low energy shock tube according to claim 23 wherein the pyrotechnic composition contains BaC>2 as the oxidant and the weight ratio of fuel component(s) to Ba02 is from 2:98 to 80:20. 20 25. A low energy shock tube according to claim 23 or 24 wherein the composition of fuel component (s) and oxidant provides a signal propagation speed of from 1 m.s"1 to 2000 m.s-1.

25

26. A low energy shock tube according to claim 23 or 24 wherein the composition of fuel component(s) and oxidant provides a signal propagation speed of from 80 m.s-1 to 400 m.s-1. 30

27. A low energy shock tube according to any one of claims 23 to 26 wherein the fuel component (s) comprise (s) B, Al, Si, Se, Ti or W.

28. A delay element which comprises a tube according to any 35 one of claims 23 to 27 connected at one end thereof, least, to an instantaneous cap or delay detonator. 12 24 3 50

29. A low energy shock tube as defined in claim 23 substantially as herein described with reference to any example thereof.

30. A delay element as defined in claim 28 substantially as herein described with reference to any example thereof. -T-J ;,c;c;-.L3 ,, f. v- -'iv Si SON