A DETONATION INITIATING DEVICE

Abstract: This invention provides a detonation initiating device for a detonator, the device including two spaced electrical conductors; a resistive substance extending between the electrical conductors, for releasing energy upon application of an electrical current to the resistive substance; and an explosive composition including diazodinitrophenol (DDNP), covering the resistive substance. The explosive composition includes DDNP in fine particulate form as well as in a relatively larger particulate form. The detonation initiating device according to the invention, is not only very stable and resistant to accidental explosion, but also very reliable due to the presence of the fine particulate DDNP. Furthermore, the explosive composition of the detonation initiating device can be initiated even at the low energy levels at which electronic initiating devices usually operate.

Description: A DETONATION INITIATING DEVICE INTRODUCTION AND BACKGROUND TO THE INVENTION This invention relates to a detonator and to a detonation initiating device for a detonator. Conventional electric or electronic detonators are usually provided with a detonation initiating device, which often comprises a metal resistive wire embedded in primary explosives compositions based on lead styphnate or lead picrate. Alternatively, the metal resistive wire may be embedded in pyrotechnic compositions based on intimate mixtures of heavy metais such as cerium, tantalum or boron in combination with an oxidiser. Such detonation initiating device is commonly known as a fuse head. Alternatively to the fuse head, various other detonation initiating devices, for use with electrical or electronic detonators, are known. Examples of these are thick film, printed, resistive bridge initiators; vacuum deposited thin film bridge initiators; and so-called SCB initiating devices. A thick film bridge initiator usually comprises a printed resistor extending between two conductors which are insulated from each other, the resistor being either polymeric or ceramic in nature, as is common in hybrid electronic technology. The bridge is trimmed or printed to value. A thin film bridge usually comprises a central conductor; and an annular conductor surrounding the central conductor, the conductors being spaced by a glass substrate. A metal layer forming a resistor is vacuum deposited on the glass substrate to connect the conductors, with the metal layer often being trimmed to value. A SCB initiating device uses silicon doping compositions to generate plasma between two spaced conductors, when an electrical current is passed through. To amplify the electric energy flowing through the resistors or plasma of the above initiating devices, into a flame or detonation able to initiate explosives in proximity to the resistor, the resistor or SCB is disposed in close contact with either a primary explosive or pyrotechnic compositions of the type commonly used in fuse head manufacture. A disadvantage of the explosive or pyrotechnic compositions of the type commonly used with fuse heads or other initiating devices is that they are usually mechanically and electrically sensitive to uncontrolled initiation. Another disadvantage of such compositions is that toxic heavy metal particles are released into atmosphere upon explosion of the initiating explosives. OBJECT OF THE INVENTION It is accordingly an object of the present invention to provide a detonator and a detonation initiating device for a detonator with which the aforesaid disadvantages may be overcome or at least minimised. SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a detonation initiating device for a detonator, the device including: two spaced electrical conductors; a resistive substance extending between the electrical conductors, for releasing energy upon application of an electrical current to the resistive substance; and an explosive composition including diazodinitrophenol (DDNP), covering the resistive substance. Diazodinitrophenol (DDNP) has been used in the past as a main explosive charge in detonators. However, because it is as such relatively insensitive to mechanical and electrical initiation, it has thusfar not been considered as an initiating explosive in detonation initiating devices, for electric and electronic detonators. The resistive substance may constitute a thick film bridge for an electronic detonator. Alternatively the resistive substance may constitute a thin film bridge. Further alternatively the resistive substance may comprise a dope substance for creating plasma upon the application of the electrical current. The explosive composition may further include a binder. The binder may be an energetic material. Preferably the binder is selected from the group comprising nitro-cellulose and polyvinyl nitrate. Alternatively the binder may be non- energetic. Preferably the binder is selected from the group comprising acrylate and natural gum. Yet further according to the invention, the explosive composition includes DDNP in fine particulate form as well as in a relatively larger particulate form. The DDNP in fine particulate form may have a particle size which is magnitudes smaller than the DDNP in relatively larger particulate form. The DDNP in particulate form may be a precipitate of a mixture of DDNP and said binder. The DDNP in particulate form may be in a crystalline or modified crystalline form. The DDNP in particulate form may be prepared substantially as described in the patent specification of DE 4117717. The DDNP in particulate form may include compounds selected from the group comprising explosives, oxidisers, fuels and/or reducing agents. The oxidisers may be selected from the group comprising chlorates, nitrates, perchlorates, and oxides and the fuels may be selected from the group comprising metals and metal sulphides. Said resistive substance may constitute a resistive bridge and the fine particulate DDNP may have a particle size which is at least five times smaller than the smallest dimension of the resistive bridge. Preferably the fine particulate DDNP has a particle size which is at least ten times smaller than the smallest dimension of the resistive bridge. The ratio between the fine particulate DDNP and the DDNP in relatively larger particulate form may be between 1 :99 and 20:80 on a mass per mass basis. Preferably the ratio between the fine particulate DDNP and the DDNP in relatively larger particulate form is 5:95 on a mass per mass basis. The arrangement may be such that when an external current is applied to the contacts, the fine particulate DDNP ignites, to initiate ignition of the DDNP in relatively larger particulate form, which in turn ignites a detonator charge. Further according to the invention, the DDNP in fine particulate form may be applied separately to the resistive bridge, in ink of the type used to print polymer thick film bridges substantially as disclosed in South African patent number 95/1680. This increases the reliability of ignition of the explosives in contact with the resistive bridge. Further according to the invention, the DDNP is enclosed. Preferably the DDNP is enclosed by a layer comprising a polymeric material or a resin. According to a second aspect of the invention there is provided a detonator including a detonation initiating device according to the first aspect of the invention. BRIEF DESCRIPTION OF THE DRAWING The invention will now be described further by way of a non-limiting example with reference to the accompanying drawing, which is a diagrammatical cross-sectional view of a detonation initiating device 10 according to a preferred embodiment of the invention for a detonator (not shown). DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The detonation initiating device 10 includes two spaced electrical conductors 12 located on an insulating substrate 14; a thick film resistive bridge 16 extending between the electrical conductors 12, for releasing energy upon application of an electrical current to the resistive bridge 16; and an explosive composition, generally designated by reference numeral 18, covering the resistive bridge 16. The explosive composition 18 is prepared from an energetic nitro-cellulose binder 17 and diazodinitrophenol (DDNP), substantially as described in the patent specification of DE 4117717, to include DDNP in fine particulate form 18.1 as well as in a relatively larger particulate form 18.2. The DDNP in fine particulate form 18.1 has a particle size which is magnitudes smaller than the DDNP in relatively larger particulate form 18.2 and at least ten times smaller than the smallest dimension X of the resistive bridge 16. The ratio between the fine particulate DDNP 18.1 and the DDNP in relatively larger particulate form 18.2 is 5:95 on a mass per mass basis. The DDNP in particulate form is preferably in a crystalline state and the explosive composition 18 includes additional compounds such as other explosives, oxidisers, fuels and/or reducing agents. The oxidisers are selected from the group comprising chlorate, nitrate, perchlorate, and oxides and the fuels are selected from the group comprising metals and sulphides. Preferably the oxidiser is lead monoxide (PbO) and the fuel metal is zirconium. The DDNP in fine particulate form 18.1 may alternatively be applied separately to the resistive bridge 16, in ink of the type used to print polymer thick film bridges substantially as disclosed in South African patent number 95/1680. The explosives composition 16 is enclosed by a polymeric layer 20. In use, an external current is applied to the contacts 12 to energise the resistive bridge 16, which ignites the fine particulate DDNP 18.1. The fine particulate DDNP 18.1 initiates ignition of the DDNP in relatively larger particulate form 18.2, which in turn ignites a detonator charge (not shown). The detonation initiating device according to the invention, is not only very stable and resistant to accidental explosion, but also very reliable due to the presence of the fine particulate DDNP. Furthermore, the explosive composition of the detonation initiating device can be initiated even at the low energy levels at which electronic initiating devices usually operate. It will be appreciated that no heavy metal particles are released by the detonation initiating device according to the invention, during explosion. It will be appreciated further that variations in detail are possible with a detonator and a detonation initiating device according to the invention without departing from the scope and/or spirit of the appended claims

Claims: CLAIMS 1. A detonation initiating device for a detonator, the device including two spaced electrical conductors; a resistive substance extending between the electrical conductors, for releasing energy upon application of an electrical current to the resistive substance; and an explosive composition including diazodinitrophenol (DDNP), covering the resistive substance. 2. A detonation initiating device according to claim 1 wherein the resistive substance constitutes a thick film bridge for an electronic detonator. 3. A detonation initiating device according to claim 1 wherein the resistive substance constitutes a thin film bridge. 4. A detonation initiating device according to claim 1 wherein the resistive substance comprises a dope substance for creating plasma upon the application of the electrical current. A detonation initiating device according to any one of the preceding claims wherein the explosive composition further includes a binder. 6. A detonation initiating device according to claim 5 wherein the binder is an energetic material. 7. A detonation initiating device according to claim 6 wherein the binder is selected from the group comprising nitro-cellulose and poly vinyl nitrate. 8. A detonation initiating device according to claim 5 wherein the binder is non-energetic. 9. A detonation initiating device according to claim 8 wherein the binder is selected from the group comprising acrylate and natural gum. 10. A detonation initiating device according to any one of the preceding claims wherein the explosive composition includes DDNP in fine particulate form as well as in a relatively larger particulate form. 11. A detonation initiating device according to claim 10 wherein the DDNP in fine particulate form has a particle size which is magnitudes smaller than the DDNP in relatively larger particulate form. 12. A detonation initiating device according to claim 10 or claim 11 wherein the DDNP in particulate form is a precipitate of a mixture of DDNP and a binder. 13. A detonation initiating device according to any one of claims 10 to 12 wherein the explosives composition includes compounds selected from the group comprising other types of explosives, oxidisers, fuels and/or reducing agents. 14. A detonation initiating device according to claim 13 wherein the oxidisers are selected from the group comprising chlorates, nitrates, perchlorates, and oxides and wherein the fuels are selected from the group comprising metals and metal sulphides. 15. A detonation initiating device according to any one of claims 10 to 14 wherein the DDNP in particulate form is in a crystalline or modified crystalline form. 16. A detonation initiating device according to any one of claims 10 to 15 wherein the DDNP in particulate form is prepared substantially as described in the patent specification of DE 4117717. 17. A detonation initiating device according to any one of claims 10 to 16 wherein said resistive substance constitutes a resistive bridge and wherein the fine particulate DDNP have a particle size which is at least five times smaller than the smallest dimension of the resistive bridge. 18. A detonation initiating device according to claim 17 wherein the fine particulate DDNP have a particle size which is at least ten times smaller than the smallest dimension of the resistive bridge. 19. A detonation initiating device according to any one of claims 10 to 18 wherein the ratio between the fine particulate DDNP and the DDNP in relatively larger particulate form is between 1 :99 and 20:80 on a mass per mass basis. 20. A detonation initiating device according to claim 19 wherein the ratio between the fine particulate DDNP and the DDNP in relatively larger particulate form is 5:95 on a mass per mass basis. 21. A detonation initiating device according to any one of claims 10 to 20 wherein the DDNP in fine particulate form is optionally applied separately in ink of the type used to print polymer thick film bridges substantially as disclosed in South African patent number 95/1680. 22. A detonation initiating device according to any one of claims 10 to 21 wherein the arrangement is such that when an external current is applied to the contacts, the fine particulate DDNP ignites, to initiate ignition of the DDNP in relatively larger particulate form, which in turn ignites a detonator charge. 23. A detonation initiating device according to anyone of the preceding claims wherein the DDNP is enclosed. 24. A detonation initiating device according to claim 23 wherein the DDNP is enclosed by a layer comprising a polymeric material or a resin. 25. A detonation initiating device substantially as herein described with reference to the accompanying drawing. 26. A detonator including a detonation initiating device as claimed in any one of claims 1 to 25.

Inventors: GOOSEN ADRIAAN JOHANNES, GOOSEN, ADRIAAN, JOHANNES
Applicants: GOOSEN ADRIAAN JOHANNES, SMI TECHNOLOGY PROPRIETARY LIM, GOOSEN, ADRIAAN, JOHANNES, SMI TECHNOLOGY (PROPRIETARY) LIMITED
Application year: 1999
Main classification: C06C7
IPC codes: C06B45, C06C7