Ammunition and explosives – Igniting devices and systems – Fuse cord
Reissue Patent
1998-04-22
2002-05-07
Johnson, Stephen M. (Department: 3641)
Ammunition and explosives
Igniting devices and systems
Fuse cord
C102S275100, C102S275110
Reissue Patent
active
RE037689
ABSTRACT:
The present invention relates to an improved, low energy fuse for use in commercial blasting, improved materials useful in its manufacture and to a method for producing such a fuse.
The use of non-electric explosives initiation systems is now well known in the blasting art. Generally, these systems comprise the use of one or more lengths of detonating fuse cord each having attached at one end thereof an instantaneous or delay blasting cap. When the opposite end of the cord is initiated by means of an explosive initiator, such as a cap or priming trunk line fuse cord, the detonating fuse is detonated and an explosive wave is transmitted along its length at high velocity to set off the attached blasting cap. The use of such a system is generally chosen where there may be hazards involved in using an electric initiation system and electric blasting caps.
In the past, many improvements have been made in the quality and reliability of non-electric initiation systems and in detonating fuse cord. An early but significant development was disclosed in our British patent No 808 087 (equals U.S. Pat. No. 2,993,236). This provided a solution to the problem of how to safely incorporate an explosive core in a thermoplastic tubular sheath during extrusion. The technique disclosed therein can be widely applied to production of tubular products for use in initiation systems. One such product is shown in British Patent No. 1 238 503 (equals U.S. Pat. No. 3,590,739; CA 878 056) which discloses a detonating fuse which comprises a tube having only a thin layer of a reactive substance coated on the inner area thereof rather than a core. Such a fuse is marketed under the registered trade mark “NONEL”. Commonly, this type of fuse has come to be known as a shock wave conductor and will be referred to as such hereinafter.
The production of shock wave conductors of small diameter has been restricted to use of a limited number of polymers due to the principal properties sought for the product. The product development trend in the art to meet such problems has been to provide laminated plastics tubes comprising an inner and outer layer of differing plastics to satisfy requirements of reactive substance adhesion and mechanical strength respectively. A shock wave conductor in the form of a two-ply laminated tube, the outer ply of which provides reinforcement and resists mechanical damage, is disclosed in GB 2 027 176 (U.S. Pat. No. 4,328,753; CA 1 149 229). Likewise in U.S. Pat. No. 4,607,573, a method is described for the manufacture of a two-ply or multiply shock tube wherein the outer covering is applied only after the inner tube has been stretched to provide the desired core load per unit length. Further examples of such over coated tubes are disclosed in U.S. Pat. No. 4,757,764 which proposes use of the tubes of the type disclosed in the above-mentioned U.S. Pat. No. 4,607,573 with non-self-explosive reactive material within the tube. Other disclosures of the use of non-self-explosive reactive material are to be found in Brazilian Patent No. PI 8104552, CA 878 056, GB 2 152 643 and U.S. Pat. Nos. 4 660 474 and 4 756 250.
While the invention of the shock wave conductor has been an important contribution to the art of blasting, the known shock wave conductors are not without disadvantages. Since the reactive substance within the tube only comprises a thin surface coating which adheres to, but is not bound to the tube, then only certain special plastics have in practice been found suitable to provide the necessary adhesion. Such special plastics tend to be both expensive and to lack mechanical strength. When protected by an outer layer of material, as disclosed in U.S. Pat. Nos. 4,328,753 and 4,607,573, the mechanical properties are improved.
SUMMARY OF THE INVENTION
A need has arisen, therefore, for a shock wave conductor which retains all the explosive properties of the tubes currently in use and which is also possessed of great mechanical and tensile strength but at low production cost.
According to the present invention, a low energy shock wave conductor is provided which comprises an extruded single-wall, dimensionally stable plastic tube having an inner surface coated with a particulate reactive energetic material, the plastic of the said tube comprising a substantially homogeneous blend of a major amount of a draw orientable polymer resin lacking adequate reactive material-retaining properties, and a minor amount of a modifier which is a miscible or compatible material which imparts an enhanced reactive material-retaining capability to the said extruded plastic tube.
Most favourable results are achieved in most instances when the polymer is substantially orientated linearly and this is best achieved by cold drawing the tube after melt consolidation. As used herein the term “cold drawing” means irreversible extension with a localised draw point of the extruded tube at any stage after the polymer has left the extruder and cooled sufficiently to consolidate a permanent tubular structure but remains plastic or sufficiently so to permit stretching under applied stress to thereby orientate the crystallites in the direction of tube length. Thus cold drawing may be carried out at any stage after the tube has taken shape after extrusion and has begun to cool from its extrusion temperature. Therefore it should be noted that the temperature of “cold drawing” lies suitably in the range of from about ambient room temperature to about 180° C. or higher depending on the polymer(s) chosen and it will be recognised that the temperature profile of the cold drawing stage(s) need not be uniform so that the post-extrusion temperature treatment of the tube may be variable. Additionally, intermediate or terminal relaxation stages may be employed, as are well known in the synthetic fibre art, to “stress relieve” the cold drawn tube and thereby impart improved dimensional stability to the tube. It is envisaged that normally artificial cooling of the extruded tube will be applied such as forced air and/or water cooling to control the temperature during post extrusion treatment. The resulting tube is safe to handle and is easily reeled for storage or transport. Of course the finished tube may be treated externally with agents to improve resistance to water and oil, especially diesel, permeability. Ordinarily a thin film or coating will suffice. Alternatively, the polymer blend may include a further resin to improve oil resistance. The tube can be overcoated with another layer of polymer as in the prior art tubes but there is no perceived advantage in doing so.
Tests, including microscopic examination, carried out on the improved tubes made so far in accordance with the invention indicate that the draw-orientable polymer resin is in the form of a continuous matrix whilst said compatible material is mostly present within the matrix as discrete noncontiguous particles, sized about 0.5 &mgr;, or fibrils a few microns in length, with aspect ratios typically of from about 6 up to about 10 oriented along the tube axis. The structural state of said miscible material is less certain because inherently there are no clear phase boundaries to be highlighted by electron microscopy. However we have noted that those miscible polymeric materials that impart good particle adhesion properties at the inner tube surface appear to be present to a substantial extent as indistinctly segregated zones of more concentrated material. Thus electron microscopy (viewing regions up to 20&mgr; across) reveals arbitrary random microstructure in the plastic matrix consistent with such zoning. It has further been observed that in many instances the miscible or compatible material is, following melt extrusion, distributed such that it has a greater concentration at the inner surface of the tube than in the body of the matrix which provides optimum exposure to interaction with the reactive material and favorable performance in the resulting shock wave conductor. The distribution of the miscible or compatible material will vary depending on the physical and che
Greenhorn Robert C.
Stewart Ronald F.
Welburn David J.
Welsh David M.
Johnson Stephen M.
Klee Maurice M.
Orica Explosives Technology Pty. Ltd.
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