GB1582903A - Explosive connecting cord and cord-manufacturing method and apparatus - Google Patents

Abstract

The improved low-energy detonating fuse is lightweight, flexible, strong and non-conducting. It detonates at a high rate and is adapted to modern rapid production methods. It has a continuous core (2) made of deformable, bound explosive containing a crystalline high-explosive compound, preferably fine-particulate pentaerythritol tetranitrate mixed with a binder. The crystalline, high-explosive compound is present in an amount of approximately 0.1 to 2 g/m. The core is surrounded by a plastic protective sleeve (4) which is approximately 0.13 to 3.18 mm thick. Preferably the binder-containing explosive core is reinforced at its circumference between the core and the plastic sheath by at least one yarn strand (3); in particular the fuse contains a core reinforcement consisting of at least one endless yarn strand at the circumference of the core and extending virtually parallel to the longitudinal axis of the core, the reinforcement having a tensile strength such that the core is prevented from contracting to the yield point under the effect of the forces normally occurring during charging of a borehole. <IMAGE>

Description

PATENT SPECIFICATION ( 11) 1 582 903

^ ( 21) Application No 2614/78 ( 22) Filed 23 Jan 1978 ( 19) ( 31) Convention Application No's 762824 ( 32) Filed 26 Jan 1977 842096 17 Oct 1977 in ' " > ( 33) United States of America (US) tn ( 44) Complete Specification Published 14 Jan 1981 S k ( 51) INT CL 3 CO 6 C 5/04 ( 52) Index at Acceptance F 3 A C 5 A C 1 D 6 A 2 A 6 A 2 D 6 A 2 G 6 A 2 H 6 A 2 J 6 A 2 L 6 B 2 ( 54) EXPLOSIVE CONNECTING CORD AND CORD-MANUFACTURING METHOD AND APPARATUS ( 71) We, E I DU PONT DE NEMOURS AND COMPANY, a corporation organised under the laws of the State of Delaware, USA, of Wilmington, State of Delaware 19898, United States of America, do 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: 5

This invention relates to an explosive connecting cord for use in transmitting a detonation wave to an explosive charge, and more particularly to an explosive connecting cord of the type known as "low-energy detonating cord".

Divided from this application is our copending Application No 34414/78 (Serial No.

1582904) which describes and claims an apparatus and method for making detonating cords 10 such as those of this invention The apparatus of the divisional application comprises:(a) a first extrusion means, for forming a mass of a deformable bonded detonating explosive composition into a continuous solid core; (b) means for orienting strands of yarn substantially parallel to one another in annular array; 15 (c) means for drawing the substantially parallel strands under tension sufficient to form them into a moving cage, the strand-orienting means being so positioned with respect to the first extrusion means that, in operation of the apparatus, the cage entrains internally thereof and conveys the core emanating from the first extrusion means; (d) a second extrusion means, for applying a soft plastics material in the form of a 20 sheath to a substrate moving relative to said second extrusion means, the second extrusion means being so positioned with respect to the first extrusion means and the strand-orienting means that, in operation of the apparatus, the caged core moves through the second extrusion means so as to constitute the substrate for the sheath appklication with substantially no previous, coincident, or subsequent reduction in core diameter; and 25 (e) Means for receiving the sheathed, caged core for hardening the plastics material.

The method of the divisional application comprises:

(a) forming a mixture of cap-sensitive crystalline high explosive compound and a binding agent therefor into a continuous solid core(b) drawing strands of yarn under tension sufficient to form a moving cage of 30 substantially parallel longitudinal strands; (c) allowing the moving cage to entrain the core within it, whereby the cage becomes a conveyor for the core; (d) applying a layer of soft plastics material around the moving cage while effecting sustantially no change in the diameter of the core after its entrainment within the cage; and 35 (e) hardening the plastics material.

The hazards associated with the use of electrical initiation systems for detonating explosive charges in mining operations i e, the hazards of premature initiation by stray or extraneous electricity from such sources as lightning, static, galvanic action, stray currents, radio transmitters, and transmission lines, are well-recognized For this reason, non-electric 40 initiation through the use of a suitable detonating fuse or cord has been looked upon as a widely respected alternative A typical high-energy detonating cord has a uniform detonation velocity of about 6000 meters per second and comprises a core of 30-50 grains per foot ( 6 to 10 grams per meter) of pertaerythritol tetranitrate (PETN) covered with various combinations of materials, such as textiles, waterproofing materials, plastics, etc 45 1 582 903 However, the magniture of the noise produced when a cord having such PETN core loadings is detonated on the surface of the earth, as in trunklines, often is unacceptable in blasting operations in developed areas Also, the brisance (shattering power) of such a cord may be sufficiently high that the detonation impulse can be transmitted laterally to an adjacent section of the cord or to a mass of explosive which, for example, the cord contacts 5 along its length In the latter situation, the cord cannot be used to initiate an explosive charge in a borehole at the bottom (the "bottomhole priming" technique), as is sometimes desired.

Low-energy detonating cord (LEDC) was developed to overcome the problems of noise and high brisance associated with the above-described 30-50 grains per foot cord LEDC 10 has an explosive core loading of only about 0 1 to 10 grains per linear foot ( 0 02 to 2 grams per meter) of cord length, and often only about 2 grains per foot ( 0 4 gram per meter) This cord is characterized by low brisance and the production of little noise, and therefore can be used as a trunkline in cases where noise has to be kept to a minimum, and as a downline for the bottomhole priming of an explosive charge It has been generally considered essential to 15 the reliable detonation of LEDC that it should be provided with a metal or heavy textile sheathing but this has the disadvantage that manufacture in continuous lengths is difficult and costly.

The present invention provides an improved low-energy detonating cord comprising (a) a continuous solid core of a deformable bonded detonating explosive composition 20 comprising at least 55 percent by weight of a cap-sensitive crystalline high explosive compound selected from organic polynitrates and polynitramines admixed with a binding agent, the particles of crystalline high explosive compound in the composition having their maximum dimension in the range of from 0 1 to 50 microns, the average maximum dimension generally being no greater than 20 microns, and the core containing from 0 5 to 25 grains of crystalline high explosive compound per foot ( 0 1 to 2 grams per meter) of length; and (b) enclosing the core, a protective sheathing formed from one or more layers of plastics material, preferably having a total thickness of about from 0 005 to 0 075 inch ( 0 127 to 1 905 mm), the plastics material being one which is capable of flowing at a 30 temperature not exceeding the melting point of the crystalline high explosive compound by more than 750 C For example, the plastics material should desirably flow at a temperature not exceeding 200 C when the high explosive compound is PETN.

In an embodiment of the invention, a low energy detonating cord comprises a bonded detonating explosive composition formed into an elongate deformable core which has a 35 loading cap-sensitive crystalline high explosive compound of from 0 1 to 2 grams per metre of core length and comprising at least 55 per cent by weight of said high explosive compound as superfine organic polynitrate or superfine organic polynitramine in admixture with a binding agent, a protective sheath enclosing said core along its length and comprising at least one layer of a plastics material capable of flowing at a temperature not exceeding by 40 more than 750 C the melting temperature the crystalline high explosive compound, and reinforcement means comprised of one or more mono or multi-filaments, extending axially and non-helically outside of the core and devoid of substantial engagement by weaving or other means with any other such filament whereby the tensile strength of the cord is increased 45 In a preferred cord of the invention, the bonded explosive core is reinforced peripherally by at least one strand of yarn between the core and the plastic sheath, or in or around the sheath, and most preferably the cord contains core-reinforcement means consisting essentially of at least one continuous strand of yarn on the periphery of the core and running substantially parallel to the core's longitudinal axis, the strand(s) having sufficient 50 tensile strength as to prevent the core from necking down to a failure point under forces normally encountered in borehole loading, e g, to provide the reinforced core with a tensile strength of at least about 10 pounds ( 4 5 kilograms), and preferably at least about 20 pounds ( 9 kilograms), to enable it to withstand more unusual forces.

A particularly preferred cord of the invention is one in which the crystalline high 55 explosive compound in the bonded composition is pentaerythritol tetranitrate (PETN), the PETN loading in the core is about from 2 to 10 grains per foot ( 0 4 to 2 grams per meter) of length, the plastic material is a polyolefin extrudable at a temperature of about 1750 C, and at least about four reinforcing strands of a polyamide or polyester yarn are substantially uniformly distributed on the periphery of the core 60 The invention will now be described by way of example only, with reference to the accompanying drawings in which:Figures 1 and 5 are perspective view in partial longitudinal crosssection of sections of different embodiments of the connecting cord of the invention:

Figure 2 is a schematic representation of apparatus suitable for use in making cords 65 3 1 582 903 3 according to the invention; Figures 3 and 4 are cross-sectional views of different embodkments of portions of the apparatus shown in Figure 2.

Referring to the section of low-energy detonating cord 1 shown in Figure 5, the cross-sectioned portion shows a continuous solid core 2 of a low energy deformable bonded 5 detonating explosive composition, as described earlier, e g superfine PETN admixed with a binding agent such as plasticized nitrocellulose, the diameter and explosive content of the core being such that about from 0 5 to 10 grains of explosive are present therein per foot ( 0.1 to 2 grams per meter) of length; and a protective plastic sheath 4, e g, about from 0 005 to 0 075 inch ( 0 127 to 1 905 mm) thick, which encloses core 2 In the section of cord 10 shown in Figure 1, core-reinforcement means 3 consisting of a mass of filaments derived from multi-filament yarns around and in contact with the periphery of core 2 runs parallel to the longitudinal axis of core 2, and sheath 4 encloses core 2 and corereinforcing filaments 3 In another portion of Figure 5 sheath 4 has been removed to reveal the peripheral appearance of core 2, and in other portions of Figure 1, sheath 4 has been removed to 15 reveal the peripheral appearance of filaments 3 on core 2, and filaments 3 removed to reveal the peripheral appearance of core 2.

The low-energy detonating cord of this invention combines the features of a continuous solid (i e, non-hollow) core of a deformable bonded detonating explosive composition having a low-loading, i e, 0 5 to 10 grains per foot ( 0 1 to 2 grams per meter) of length, of 20 crystalline high explosive in a binder therefor, and only a light protective plastic sheath enclosing the core An additional feature, which is preferred, is longitudinal fiber reinforcement of the core external thereto It has been found that, contrary to the teachings of the prior art on low-energy detonating cords, a deformable bonded detonating explosive in the form of a cord can be made to propagate a detonation reliably even in loadings below 25 5-10 grains per foot, at a rate that is useful in blasting operations, e g, above about 4000 meters per second, without confinement in a metal or woven textile sheath, spirally wound textile, plastic, or metal strands or filaments, or a thick plastic sleeve It has been found that the just-mentioned confinement is unnecessary if the core is a continuous solid rod of bonded explosive, e g, a plastic-bonded explosive, containing at least about 55 percent of 30 explosive by weight, and a "superfine" crystalline high explosive component (as will be described later), and any reinforcement means for the core is external thereto In the cord of this invention the explosive particles in the core are held together with a binding agent, e.g, an organic polymeric composition, and this has been found to have a beneficial effect in assuring a uniform, high core density and consequently reliability of detonation, high 35 density being an important consideration particularly in small-diameter, low loading cords of low brisance Relative to the bonded core it has been found that, despite the fact that central, internal reinforcement has been reported to be preferred in highloading cords made of self-supporting explosives (U S Patent 3,338,764), external reinforcement filaments are important for the proper functioning of low-loading cords made with this type 40 of core Furthermore, when the bonded-explosive core of the cord of this invention is reinforced, the external reinforcing means, e g, textile yarns, preferably are longitudinal, running substantially parallel to the cord axis Such a cord is readily adapted to be made by high-speed continuous manufacturing techniques in contrast to those lowenergy detonating cords of the prior art which employ woven or wound textile reinforcement 45

The bonded explosive composition which constitutes the explosive core in the cord contains at least one finely divided cap-sensitive crystalline high explosive compound, which can be an organic polynitrate such as PETN or mannitol hexanitrate, or polynitramine such as cyclotrimethylenetrinitramine (RDX) or cyclotetramethylenetetranitramine (HMX) PETN is the most readily available of these compounds and is 50 satisfactory for use under conditions most commonly encountered in blasting, and for these reasons is the preferred crystalline explosive in the bonded explosive core The crystalline high explosive compound is admixed with a binding agent, which can be a natural or synthetic organic polymer, e g, the soluble nitrocellulose described in U S Patent 2,992,087, or the mixture of an organic rubber and a thermoplastic terpene hydrocarbon 55 resin described in U S Patent 2,999,743 The compositions described in these patents can be used for the core of the present, cord, and the disclosures of these patents are incorporated herein by reference Other ingredients may be present in the composition, such as additives used for plasticizing the binder or densifying the composition Other compositions which can be used are those described in U S Patents 3,338, 764 and 60 3,428,502, the disclosures of which also are incorporated herein by reference.

The detonating cord of this invention is a "low-energy" cord, i e, one which, when detonated, produces relatively little noise and exhibits relatively low brisance Therefore, for a given core composition, the core diameter is such that about from 0 5 to 10, preferably at least about 2, grains of the crystalline high explosive compound are present per foot of 65 1 582 903 core length ( 0 1 to 2, preferably at least about 0 4, grams per meter) With trunklines containing cores of higher loadings, the noise level is likely to be a problem in certain areas.

Below bout 0 5 grain per foot ( 0 1 gram per meter), the reliability of complete propagation of detonation is low unless a high-energy binding agent and/or plasticizer is included in the core composition With such a composition, e g, with a composition having a high-viscosity 5 nitrocellulose binder plasticized with trimethylolethane trinitrate, as is described in U S.

Patent 3,943,017, loadings of particulate high explosive in the core as low as about 0 1 grain per foot ( 0 02 gram per meter) may be feasible Loadings of about from 2 to 10 grains per foot ( 0 4 to 2 grams per meter) have been found to be particularly advantageous for downline and trunkline cords With explosive cores of low loading, as in the present cord, it 10 is important that the crystalline high explosive component be in the "superfine" particle size range, i e, the maximum dimension of the particles should be in the range of about from 0 1 to 50 microns, and generally the average maximum dimension should be no greater than about 20 microns Larger explosive particles, extreme variations in particle size, and particulate foreign matter are undesirable inasmuch as they interfere with the 15 uniform propagation of detonation in the core A preferred explosive for use in the core is one having microholes, as made by the process described in U S Patent 3, 754,061, the disclosure of which is incorporated herein by reference.

The explosive loading of the core is a function of the crystalline high explosive content of the bonded composition and the core diameter The crystalline high explosive content can 20 vary, e g, from about 55 percent up to about 90 percent by weight of the core composition.

Although a low explosive content can to some extent be compensated for by a large core diameter, it is more efficient and the propagation of detonation more reliable if, for any given loading, the explosive content is as high as possible, preferably at least about 70 percent by weight of the core composition For explosive contents in the range of about 25 from 55 to 90 percent, core diameters of about from 0 010 inch ( 0 025 cm) to 0 060 inch ( 0.152 mm) will be used to achieve core loadings of 0 5 to 10 grains per foot ( 0 1 to 2 grams per meter) of core length A diameter of about 0 027 inch ( 0 069 cm) is used to achieve the preferred core loading of 2 grains per foot ( 0 4 gram per meter) The explosive composition also contains about 1 to 10 percent, preferably 2 to 5 percent, by weight of a binding agent, 30 and in addition a plasticizer if needed, to make the composition extrudable, and to provide cohesiveness in the core.

The density of the core varies with the specific particulate explosive and binding agent used and their content, and the nature and amount of other additives, if present Generally, cores based on the compositions described in the aforementioned U S Patents 2,992,087 35 and 2,999,743 will have a density of about 1 5 grams per cubic centimeter A core density of this magnitude, in contrast to densities of only about 1 2 grams per cubic centimeter attained with particulate cores, has the advantage of affording a better transmission of the detonation wave and hence a higher detonation velocity for a given diameter Although the shape of the core's cross-section is not critical to the proper functioning of the cord, it is 40 usually preferred to use a core of substantially circular cross-section to facilitate the production of cords having the circular configuration in common use.

The bonded explosive core is enclosed in sheathing as a means of protecting it against abrasion or other damage that can occur during handling and preparations for blasting.

Inasmuch as the sheath is primarily protective, it is relatively thin, i e, in the range of about 45 from 0 005 to 0 075 inch ( 0 013 to 0 191 cm), except that a sheath up to about 0 125 inch ( 0.318 cm) thick may be used if the cord is to be subjected to extremely stressful conditions, as are encountered in surface quarry operations Uniform protection is difficult to provide with sheaths which are thinner than about 0 005 inch ( 0 013 cm) A sheath which is thicker than about 0 125 inch ( 0 318 cm) is not required in the present cord, and, in any event, adds 50 unnecessarily to the thickness and cose of the cord, limits its flexibility, and may be difficult to load into small-diameter boreholes A sheath thickness of about from 0 020 to 0 050 inch ( 0.051 to 0 127 cm) is preferred from the point of view of ease of applicability to the core and degree of protection afforded Thus, with preferred core loadings of 2 to 10 grains per foot and preferred sheath thicknesses of 0 020 to 0 050 inch, the ratio of the core loading 55 (gr/ft) to sheath thickness (in) is 40/1 to 500/1 ( 3/1 to 39/1 for core loadings of 0 4 to 2 grams per meter and sheath thicknesses of 0 051 to 0 127 cm).

Within the useful sheath thickness range, it often is advisable to use a thicker sheath when the explosive loading in the core is near the low end of the core loading range, inasmuch as in such cases this may assure reliable initiation and propagation of the 60 detonation Also, as the core explosive loading increases, increasing the sheath thickness may assure a continuity of detonation through knots and half-hitches.

The sheath consists solely of one or more plastic layers This means that any layer of which the sheath is constructed consists essentially of plastic and that no confining metal or woven textile layer is present in the sheath, either adjacent to or separated from the core 65 1 582 903 5 The sheath is made of a plastic, i e, deformable, substance that is capable of flowing, e.g, is extrudable, at a temperature not greatly in excess of the melting point of the explosive in the core, i e, not more than about 750 C above the explosive's melting point.

This allows the plastic sheath to be applied to the core, e g, by extrusion or other conventional coating procedure, without causing a deleterious transformation of the 5 explosive The plastic should be flexible and tough when hardened Although the temperature of the plastic which can be used during the application of the sheath to the core will vary depending on the time of contact between the core and the overlying soft plastic, on the rate of heat exchange between the core and plastic, and on the stability of the binding agent in the core, with a PETN-containing core the plastic should be fluent at a 10 temperature not exceeding about 200 'C The plastic substance can be a thermosetting material such as a rubber or other elastomer, or a thermoplastic material such as wax, asphalt, or one or more polyolefins, e g, polyethylene or polypropylene; polyesters, e g, polyethylene terephthalate; polyamides, e g, nylon; polyvinyl chloride; ionomeric resins, e g, ethylene/methacrylic acid copolymer metallic salts; etc Thermoplastic sheaths are 15 preferred, and most preferably polyethylene, on the basis of availability, ease of application, etc.

To enable the cord to retain its structure and dimensions under field use, it is preferred that reinforcement means be used to enhance the tensile strength of the cord and prevent the core from necking down to a failure point under forces normally encountered in 20 borehole loading While such reinforcement can be provided by a material suspended in the plastic layer(s) of the protective sheath, e g, by fragments or strands of yarn held therein, for example, in the manner shown in U S Patent 2,687,553, or on the outer periphery of the sheath, it is preferred that the core be reinforced by at least one, and usually preferably four or more, continuous strands of yarn which are substantially in contact with the 25 periphery of the core and run substantially parallel to the core's longitudinal axis.

The presence of the yarn strands between the core and the sheath is preferred to yarn strands within the plastic layer of the sheath because heat is less readily transferred from the plastic to the core when hot plastic is extruded onto the core The term "yarn" is used herein in the sense given in Standard Definitions of Terms Relating to Textile Materials, 30 ASTM Designation D 123-74 a, where "yarn" is defined as a generic term for a continuous strand of textile fibers, filaments, or material occurring as a number of fibers twisted together, a number of filaments laid together without twist, a number of filaments laid together with more or less twist, a single filament (monofilament) with or without twist, or one or more strips made by the lengthwise division of a sheet of material such as a natural or 35 synthetic polymer with or without twist Varieties of yarn included in this definition are single yarn, plied yarn, cabled yarn, cord, thread, fancy yarn, etc The strand(s) of yarn are held in place around the core by the plastic sheath, which encloses the core and peripheral strand(s) Any yarn can be used which has a high enough tensile strength as to prevent the core from necking down under forces normally encountered in borehole loading to such a 40 degree that it fails to propagate a detonation This usually requires that the core be provided with a tensile strength of at least about 10 pounds For added assurance that the cord will withstand more extreme forces, a tensile strength of at least about 20 pounds in the reinforced core is preferred The yarn material, filament count, and denier, and the number of yarns, will be selected so as to provide the required tensile strength 45 Multifilament yarns may be preferred inasmuch as these, in contrast to monofilaments, tend to spread out around the core, providing an insulating effect in the coating operation and a more widespread caging effect Also, fewer strands and lower deniers can be employed with stronger fibers Yarns larger than 2000 denier are not preferred, as these add unduly to the thickness of the cord While any natural fiber can be used in the yarn, 50 synthetic fibers of the polyester, polyamide, and polyacrylic types are preferred on the basis of their superior strength Especially preferred are nylon, polyethylene terephthalate, and the all-aromatic polyamide made by the condensation of terephthalic acid and paraphenylenediamine These fibers in deniers of 800 or higher have tensile strengths of at least about 10 pounds and thus a single strand or yarn thereof in the present cord is adequate 55 Multiple yarns give added strength, however, and therefore are preferred Also, they can be used in lower deniers, e g, down to about 400 denier In the preferred cord, at least four multifilament yarns are spaced substantially uniformly around the core's periphery, resulting in a uniform distribution of reinforcement about the core There is no significant advantage to be gained by placing multifilament yarns adjacent to one another in the cage 60 prior to the application thereto of the plastic sheath, the cage-drawing and plastic coating operations in any case causing a spreading-out or diffusion of the filaments in multifilament yarns which may cause the yarns to blend around the core For this reason, and in consideration of the circumference of the core and the denier of the yarns, the use of more than about twelve yarns is superfluous Usually the layer of filaments will be no thicker than 65 6 1 582 903 6 about 0 010 inch ( 0 025 cm).

Textured and multiplex yarns (as described in U S Patent 3,338,764) are especially effective core reinforcing means inasmuch as they can become firmly bonded to the plastic sheath which surrounds them Application of an adhesive coating, e g, a soft wax, to the strands also improves the bonding between the strands and plastic sheath, decreasing yarn 5 mobility and possible resulting interference with the core, as well as increasing the peel strength of the sheath.

The apparatus shown in Figures 2-4 of the drawings and its use will now be described In Figures 2 and 3, 5 is a ram or piston-type extruder having a ram or piston 6, and a cylindrical chamber or barrel 29, which is surrounded by heating coils 7 Extruder chamber 29 is 10 provided with vacuum port 25, and screen 26, which is mounted on one side of a multi-apertured support plate 27 A mass 28 of a deformable bonded detonating explosive composition is shown in extruder chamber 29 and in the apertures of plate 27 Plate 27 has its other side adjacent to the reduced-diameter die portion of chamber 29 into which explosive mass 28 is forced by the action of ram 6 and is shaped into a solid rod or core 2 15 Adjacent to the die portion of extruder 5 is strand-orienting plate 8, which is a means for orienting strands of yarn including 9 and 10 into a substantially parallel annular array Plate 8 has an axial channel, and strand-receiving radial grooves in one surface communicating with the axial channel, the grooved surface of the plate curving as it meets the axial channel.

Plate 8 is supported in such a position that its grooved surface meets the surface of extruder 20 so that the plate's axial channel is coaxial with the core 2 emanating from the die portion of extruder 5 by action of ram 6 Strands 9 and 10 are drawn off respective spools 11 and 12 by capstan 13, which constitutes a means for drawing or pulling strands under tensionsufficient to form them into a moving cage 14 Core 2 emanating from extruder 5 is entrained within cage 14 and becomes conveyed thereby Capstan 13 draws cage 14 25 (containing core 2) through extrusion die 15 of a second extruder, whereby a plastics matrial is applied around the cage in the form of a sheath 4 Extrusion die 15 has an annular outer portion 17 and an inner tubular member 16, so positioned that a soft plastics material delivered to die 15 through the wall of 17 by known means (not shown) is formed into a tube between facing surfaces of outer portion 17 and inner tubular member 16, and cage 14 30 moves through the axial channel in tubular member 16 Vacuum port 18 passes through the wqall of tubular member 16 and opens into the latter's axial chamber Tubular member 16 and strand-orienting plate 8 are maintained in spaced-apart, coaxial relationship, and are joined together by connecting tube 19, which surrounds cage 14 in the space between plate 8 and tubular member 16 35 The sheathed core-containing cage (cord 1) formed at the exist of die 15 moves through vessel 20, e g, a water tank, which is a means for hardening the plastics sheath material.

The cord, after passing over capstan 13, subsequently is collected on windup 22, the winding of the cord being facilitated by its passage over tension-control means 21, e g, an air dancer Extruder piston 6 is connected to sensing means 23, which senses the speed of 40 the piston and emits a signal in accordance therewith to signal processor 24, which is connected to the drive means for capstan 13 and to the drive means for windup 22 and adjusts their speeds in accordance with the signal received from sensing means 23.

Figure 4 show an alternative extrusion die 15 which can be used in the present apparatus in conjunction with an extruder for forming the explosive core This particular die includes 45 a means for orienting yarn strands into a substantially parallel annular array and thus can be used in the apparatus shown in Figure 2 without strand-orienting plate 8 In this embodiment an axial channel in extrusion die 15 has a cylindrical portion 31 and a conical portion 32 A hollow conical insert 33 is positioned so that its apex portion is nested within conical die portion 32 with a small spacing between facing surfaces Capstan 13 draws yarn 50 strands 9 and 10 through apertures in the yarn-guide ring 34, and thence along the inner surface of adjacent conical insert 33 The strands converge in the conical portion of inset 33 and thereafter become oriented substantially parallel to one another and formed into a cage by passage through a cylindrical portion at the apex of insert 33.

Explosive core 2 moves into the cylindrical portion of insert 33, where it is entrained by 55 the yarn cage formed therein Plastics material 30 is introduced into an annulus formed between the walls of conical insert 33 and extrusion die 15 This annulus communicates with cylindrical die portion 31 via the space between conical die portion 32 and the apex portion of insert 33 The cylindrical portion of insert 33 communicates coaxially with cylindrical die portion 31 The core-containing cage 14 formed in the cylindrical portion of insert 33 is 60 drawn through a stream of plastics material 30 which flows through cylindrical die portion 31 having entered there from the space between conical die portion 32 and the apex portion of insert 33 Plastics material 30 is formed into a sheath around corecontaining cage 14 to form cord 1.

The preparation of a preferred cord of the invention is illustrated by the following 65 1 582 903 1 582 903 Example.

Example 1

A Referring to Figure 3, mass 28 in extruder chamber 29 is a 1-lb ( 455-g) slug of a deformable bonded explosive composition consisting of a mixture of 76 5 % superfine 5 PETN, 20 2 % acetyl tributyl citrate, and 3 3 % nitrocellulose prepared by the procedure described in U S Patent 2,992,087 The superfine PETN is of the type which contains dispersed microholes prepared by the method described in U S Patent 3,754, 061, and has an average particle size of less than 15 microns, with all particles smaller than 44 microns.

The temperature of chamber 29 is maintained at 630 C by heating coils 7 to assist in 10 maintaining the extrudability of the explosive composition therein After the slug of explosive has been placed in chamber 29, ram 6 is advanced to seal off chamber 29, and a cavuum is drawn through port 25 A vacuum level of -29 2 inches of mercury is maintained for 1 minute This is done to prevent the entrapment of air in the explosive composition, a condition which can cause discontinuities in the extruded core, deleteriously affecting its 15 ability to propagate a detonation Ram 6 is then advanced further until explosive mass 28 is compressed but not yet to such a degree as to cause extrusion to occur.

Strands 9 and 10, and four additional strands (not shown), are threaded into the radial grooves of plate 8, and are drawn through the axial channels of plate 8 and tubular member 16 by actuating the drive on capstan 13 Each of the six strands is a 1000denier strand of 20 polyethylene terephthalate yarn, and their tension is controlled at four ounces (each) by tension-control means 21 At the same time, the drive on windup 22 and the means for moving plastics material 30 are actuated Plastics material 30 is lowdensity polyethylene at a temperature of 1500 C Vessel 20 is a two-compartment trough containing water at 81 WC in the first compartment through which the cord passes, and water at 21 MC in the second 25 compartment This two-zone cooling assists in providing a more uniform cooling of the plastics sheath and in promoting a tighter fit of the sheath on the cage The diameter of the portion of extruder 5 wherein core 2 is formed is 0 030 in ( 0 076 cm) The spacing between the facing surfaces of outer portion 17 and inner tubular member 16 of die 15 is such as to produce a polyethylene sheath 4 having a thickness of 0 035 in ( 0 089 cm) 30 After capstan 13, tension-control means 21, windup 22, and vessel 20 are operating, ram 6 is advanced at a rate of 0 500 in ( 1 270 cm) per minute Explosive mass 28 is forced through screen 26, which screens out particles larger than 0 010 inch, and through the apertures in plate 27, and is formed into solid core 2 having a 0 030 in ( 0 076 cm) diameter.

The core moves out of extruder 5 at a rate of 248 feet per minute, and the speed of the cage 35 being advanced by capstan 13 and wound on windup 22 is matched to the core extrusion rate by signals received from signal processor 24 A vacuum is drawn through port 18 to assist in collapsing the plastics sheath onto the core-containing cage 14 passing through tubular member 16 A vacuum level of -5 9 inches of mercury is maintained in tubular member 16 40 Cord 1 accumulated on windup 22 has an outer diameter of 0 100-in ( 0 254 cm), a 0.030-in ( 0 076-cm) diameter core, and a 0 035-in ( 0 089-cm)-thick polyethylene coating.

The PETN loading in the core is 2 5 gains per foot ( 0 533 g/m) (gt/ft PETN per in coating= 71/1; g/m PETN per cm coating= 6/1), and the core density is 1 5 grams per cubic centimeter The strands of yarn surround the core substantially completely as shown in 45 Figure 1 The cord is flexible and light, and has a tensile strength of 100 pounds ( 45 kilograms).

The cord, initated by a No 6 blasting cap, the end of which is in coaxial abutment with the exposed end of the cord, detonates at a celocity of 6900 meters per second The cord does not initiate itself from one section spliced together side-by side with another 50 Detonation of a continuous length of the cord is propagated through knots of various types.

Also, the cord is difficult to initiate if the cap-to-cord abutment is not coaxial.

B The same cord is made by the procedure described in Section A above, except that extrusion die 15 shown in Figure 4 replaces die 15 and strand-orienting plate 8 shown in Figure 3 In this procedure, capstan 13 draws four strands of yarn through the cylindrical 55 portion of insert 33 under tension sufficient to form them into a moving cage of longitudinal substantially parallel strands; the cage entrains the core; and the corecontaining cage is drawn through the stream of polyethylene flowing through the cylindrical portion of the die's axial channel whereby a sheath of soft polyethylene is applied around the cage As in the procedure described in Section A, substantially no reduction in the diameter of the core 60 occurs during the operation.

Cords having different core diameters, sheath thicknesses, and numbers of reinforcing yarn strands can be produced by the procedures described above by suitable modification of die sizes and extrusion rates.

The use of the low-energy detonating cord of the invention, and the effects of various 65 1 582 903 parameters such as core loading and diameter, sheath thickness and composition, and number and type of reinforcing yarns, are shown by the following examples.

Example 2

Four grains ( 0 26 g) of the superfine PETN described in Example 1 is placed in a 0 003-in 5 ( 0.08-mm)-thick coined bottom aluminium shell, the end of which is butted against the side of a 10-foot ( 3-meter) length of the cord described in Example 1 A, with the exception that the cord in this case has a 0 050-in-( 0 127-cm) diameter core having a PETN loading of 7 gr/ft ( 1 49 g/m) This cord serves as a trunkline One end of a 5-foot ( 1 5-meter) length of the cord described in Example 1 A (downline) is inserted into the aluminium shell (the 10 booster) so as to contact the PETN The other end of the downline is butted with its side against the percussion-sensitive element of a percussion-type delay cap The trunkline is detonated by means of a No 6 blasting cap having its end in coaxial abutment with the exposed end of the cord The detonation is transmitted from the trunkline to the booster, from the booster to the downline, and from the downline to the percussiontype delay cap 15 The same results are obtained with trunkline cords having 10 gr/ft ( 2 13 g/m) and 4 4 gr/ft ( 0.938 g/m), i e, 0 060-in ( 0 152-cm) and 0 040-in ( 0 102-cm) diameter, cores; and with downline cords having 3 0 gr/ft ( 0 638 g/m) and 2 2 gr/ft ( 0 469 g/m), i e, 0 033-in-( 0 084cm) and 0 028-in ( 0 07-cm) diameter, cores.

20 Example 3

The following tests show the kinds of abusive treatment with respect to knotting, tension, and abrasion the cord of this invention is capable of withstanding.

A One end of a 60-foot ( 18-meters)-long downline of the cord described in Example 1 A is butted with its side against the percussion-sensitive element of a percussion-type delay 25 cap The cap is embedded in a 2-pound ( 0 9 kg) chub cartridge (flexible film tube having constricted sealed ends), 2 inches ( 5 cm) in diameter and 16 inches ( 41 cm) in length, containing a nonexplosive composition simulating a water gel explosive The cap and cord are secured in position in the film cartridge by two half hitches The cartridge is lowered into a simulated 50-foot ( 15-meter)-deep borehole under various loading conditions which 30 could be encountered in field use, the simulated hole being the inside of a vertical 5-inch ( 13-cm)-diameter steel pipe The other end of the 2 5 gr/ft downline is connected to the booster and 7 gr/ft trunkline as described in Example 2 After the pipe has been loaded under the described conditions, the trunkline is detonated as described in Example 2 The downline detonates completely, and the percussion-sensitive delay cap detonates within its 35 designed timing, after the downline-cap-cartridge assembly has been subjected to the following loading conditions.

I The cartridge is allowed to fall freely for the entire length of the downline.

II The free fall of the cartridge is stopped suddenly every 15 feet ( 4 6 m).

III The cord moves against the rough edge of the steel pipe as the assembly is lowered 40 into the pipe.

IV Conditions II and III are combined.

V A 7-pound ( 3 2 kg) sand bag is dropped into the pipe, removed and dropped again for a total of five times, onto the assembly positioned in the pipe in each of cases I, II, III, and IV, the sand bag scraping against the cord in its fall 45 B A knot is tied in the cord described in Example 1 A, and a 7-pound ( 3 2 kg) weight is suspended from the end of the cord The weight is dropped into the 50-foot pipe described in Part A above, while the free fall of the weight is stopped 5 times, thereby exerting increased tension on the knot Five cords handled in this manner subsequently detonate completely, with no cut-offs at the knots 50 Example 4

The use of the cords described in Examples 1 and 2 to transmit detonation waves to the bottom charge of a column of blasting explosive charges in boreholes is as follows:

Six 25-foot ( 7 6-meter)-deep, 3-inch ( 7 6-cm)-diameter boreholes spaced 8 feet ( 2 4 55 meters apart are each loaded with three aligned 2 x 16 inch ( 5 x 41 cm) chub cartridges of a water gel explosive described in U S Patent 3,431,155, wrapped in polyethylene terephthalate film Embedded in the bottom cartridge in each hole is a percussion-type delay cap, connected to the cord (downline) described in Example 1 B in the manner described in Example 2 The other end of each downline is connected to the trunkline cord 60 described in Example 2 (except having four yarn strands) in the manner described in Example 2 No stemming is used Detonation of the trunkline results in sequential detonation of the charges in the holes starting with the bottom charge, as concluded from the delay timing of the caps used There is no evidence of column disruption.

051 051 9 1 582 903 9 Examples 5-10

Cords are made as described in Example 1 The core explosive composition is 76 1 % superfine PETN, 20 3 % acetyl tributyl citrate, and 3 6 % nitrocellulose (by weight) Four strands of the same yarn as that described in Example 1 are used The same plastic coating composition as that described in Example 1 is used The core is extruded to different 5 diameters, and coatings of different thicknesses are applied The detonation properties of the cords (initiated as described in Example 1) are summarized in the following table:

C Core Diam.

Ex in (cm) O 013 ( O 033)(") 6 O 020 ( O 051) 7 O 030 ( O ( 076) 8 O 040 ( O 102) 9 O 050 ( O 127) O 060 ( O 152) PETN gr./ft.

(g./m) 0.5 ( O 107) 1.( O ( O 213) 2.5 ( O 533) 4.4 ( O 938) 7.0 ( 1 49) 10.0 ( 2 13) Detonation Velocity (m /sec) at Cord O.D Specified 0.070 O 080 0 090 O 100 ( 0.178) ( O 203) ( O 229) ( 0 254) 6700 6800 6800 6800 6800 6600 6600 6600 6600 6700 6800 7000 7000 0.125 ( 0.318) 6700 7200 xo 00 t'Q (a) This cord is initiated and propagates a detonation in 50 % of the trials made; all other cords detonate reliably.

Outer diameter in inches (cm).

CD 1 582 903 These examples show that the detonation velocity of the cords tested is within the 6900 meters per second 5 % range regardless of the PETN loading and the thickness of the plastic coating With this particular core composition and a coating thickness of 0 044 inch ( 0.112 cm), however, reliability of detonation becomes compromised somewhat at the minimum PETN loading and core diameter 5 Examples 11-14 The explosive cord described in Examples 5-10 is tested in three different core loadings and diameters for reliability of initiation and continued propagation with minimum coating thicknesses 10 No of Detonations Out of 10 Trials at Coating Thickness PETN Specified 0 010 O 015 0 025 gr /ft Core Diam ( 0 025) ( O 038) ( 0 064) 15 Ex (g /m) in (cm) 0 11 0 5 ( 0 107) 0 013 ( 0 033) 0 10 12 1 0 ( 0 213) 0 020 ( 0 051) 4 10 13 2 5 ( 0 533) 0 030 ( O 076) 8 10 20 14 7 0 ( 1 49) O 050 ( 0 127) 10 in (cm) These examples show that as the core diameter and PETN loading increase, the plastic 25 coating has a diminishing effect on the cord's ability to be initiated and propagate a detonation.

Example 15

The cord described in Examples 5-10 having a O 03 () in ( 0 076 cm) core is made with 30 different coating materials and thicknesses All samples (at least 150 feet ( 46 ml) in length) of the cord with 0 020-in ( O 051-cm), 0 028-in ( O 071-cm), and O 033-in ( 0 084-cm)-thick coatings of low-density polyethylene, high-density polyethylene, and a metallic salt of a copolymer of ethylene and methacrylic acid (an ionomrneric resin) detonate reliably at a velocity of about 7200 meters per second with four strands as well as eight strands of the 35 yarn The extrusion die temperature is 175 C for applying the highdensity polyethylene, and 135 C for applying the ionomeric resin.

The minimum tensile strength is 70 lbs ( 32 kg) for all samples made with 4 strands of the yarn, and 140 lbs ( 64 kg) for all samples made with 8 strands of the yarn All samples, regardless of coating material thickness and type detonate after the following treatment A 40 6-lb ( 2 7 kg) weight is tied to one end of the cord The weight is allowed to drag the cord by gravity over the edge of a concrete block, and then the cord is dragged back to its starting 1 582 903 point This procedure is repeated five times.

Examples 16-19 The effect of core loading and sheath coating thickness on the behavior of the cord described in Examples 5-10 when knotted, as may occur in field use, is shown in the 5 following table:

Times Detonation Propagates PETN Coating through Knots 10 gr./ft Core Diam Thickness HalfEx (g /m) in (cm) in (CM) Hitch Knot 16 2 5 ( O 533) 0 030 ( 0 076) O 035 ( O 089) 15 () 5 () 17 3 0 ( O 638) O 033 ( O 084) O 034 ( O 086) 15 () 5 (b) 15 18 3 4 ( 0 723) 0 035 ( 0 089) O 033 ( O 084) 13 () 2 ( 19 4 4 ( 0 853) 0 040 ( 0 102) O 043 ( O 109) 14 () 4 (b) (a) Out of 15 trials (b) Knot tied under 10-lbs tension;out of 5 trials 20 These examples show that the specified cords propagate a detonation through knots rather than cut-off at the knots owing to excessive brisance They also show that as the explosive loading is increased an increase in the sheath thickness will assure the propagation of detonation through knots.

25 Examples 20-24 The cord described in Example 5-10 having a 0 030 in ( 0 076 cm)-diameter core is made with different numbers of multi-filament strands of polyethylene terephthalate (PET) yarn and an aramide yarn made from the condensation polymer of terephthalic acid and para-phenylenediamine (all 1000 denier per strand) The effect of these variables on cord 30 strength and the cord's ability to propagate a detonation through knots is shown in the following table:

Times Detonation Tensile Propagates 35 PET Aramide Strength Through Knots Ex Yarn Yarn of Cord HalfNo of Strands lb (kg) Ilitch Knot 20 43 ( 20) 4) 2 ( 40 214 82 ( 37) 10 Xo a 3 020) ' 2 2 8 150)( 68) lo M 3,3,330)0 23 2154) ' 21, 1 O (),0 " 24 4 198 ( 90)) 1 O '1 3,33 3 45 (a) Out of 10 trials (b) Knots tied under 1 '))20) 35) and 40 lbs.

( 4.5, 9 1 13 6 and 18 2 kg) tension respec ti\'clv: out ol 3 trials cach These examples show that the tensile strength of the cordc for a given numlber of yarn 50 strands of the same denier varies with the tensile strength of the yarnl In 1 this case, the aramide provides a higher tensile strength cord with fewer strands than the polyester The examples also show that a larger num-lber of stranlds of a Civen fiber or a stronger fiber, will increase the cordl S ability tos propagate a detonation through tighter knots.

55 2 L ample 25 A continuous solid core of a bonded explosive composition consisting ofo(by weight) 75 % superfine PETN al and 25,, of a bindinge agent consisting of a butadiene acrvlonitr ii.

niethiacirvlic id copolvrner (described in the aforementioned U S latent 3 33 t 8764) is attached to a sin ulle stsrand of armi amide yarn maetri from, the con(l dle sationl polp Omeri C of 60 terephthalic acpid and 1 ra-phen Clenecli-Iie Thc e core and supporting, strand are dragged together through a tuibular coati nge die which applies a 005-in ( 11)64cm (-t)il tick sheath of low densitv p olyethylenc arouind them 11 The hcsl lti ne cord Wh icb il PETN loadine(l of 7 grains per foot detonates at about 700)0 meters per second wlhen initiated by the procedure described ii Example I andll has a tensile strength of albouht 75 pou L nds ( 34 kg) 65 1 582 903 Example 26

The deformable bonded explosive composition described in Example 1 (except that the superfine PETN content is 76 percent, acetyl tributyl citrate twenty percent, and nitrocellulose 4 percent) is extruded so as to form ten 4-foot ( 1 2meter)-long cords, five having a 0 030-inch ( 0 076-cm) diameter ( 2 5 gr/ft or 0 533 g/m PETN) and five having a 5 0.050-inch ( 0 127-cm) diameter ( 7 0 gr/ft or 1 49 g/m PETN) The extruded cords are slipped into low-density polyethylene tubing having an inner diameter of O 060 in ( 0 152 cm) and an outer diameter of 0 080 in ( 0 20 cm) The ratios of explosive loading to wall thickness of these cords are 250/1 and 700/1, respectively, in gr/ft loadings and thicknesses in inches ( 18/1 and 50/1, respectively, in g/m loadings and thicknesses in cm) All of the 10 cords have tensile strengths of about 10 pounds ( 4 5 kilograms).

The cords are initiated by a No 6 blasting cap, the end of the cap cord being in coaxial abutment with the exposed end of the cord All of the cords detonate without cut-offs, consuming all of the plastic coating The average detonation velocity for all ten cords is 7300 meters per second 15 In making cards according to the invention, substantially no reduction is effected in the core diameter after the core hs been formed The process produces a highdensity core without requiring a reduction in the diameter of the core as is required, for example, in processes for making cords having a particulate explosive core Elimination of a change of core diameter during the process simplifies process control with respect to achieving a 20 required final core explosive loading and avoids the possible penetration of the core by the surrounding yarn strands.

In detonating cords having small-diameter, low-loading cores, the presence of particles of foreign matter e g, sand, metal, etc, may interfere with the detonation of the cord if the particles are large enough For this reason, an important feature of the present process is 25 the provision of a core explosive composition exempt of such particles by virtue of the procedures and conditions employed in its preparation, or the presence of particlescreening means in the extruder used for making the core For cores having diameters of about 0 030 in ( 0 076 cm) and larger, particles larger than about 33 % of the core diameter should be excluded For smaller-diameter cores, prticles larger than about 0 005 in ( 0 013 30 cm) should be excluded.

In the present method when strands of yarn and the explosive core move separately into a plastic coating extrusion die, the cage formed therein usually will entrain the core and the sheath will subsequently be formed on the caged core unit However, the cage-formation, core-entrainment, and sheathing may occur substantially simultaneously Also, the two 35 extrusion means of the apparatus, i e, the core-forming die and the sheath-forming die, may be components of separate extruders, or may be positioned together in a single co-extrusion unit.

Claims (16)

WHAT WE CLAIM IS:-

1 A low-energy detonating cord comprising 40 (a) a continuous solid core of a deformable bonded detonating explosive composition comprising at least 55 percent by weight of a cap-sensitive crystalline high explosive compound selected from organic polynitrates and polynitramines admixed with a binding agent, the particles of crystalline high explosive compound in the bonded composition having their maximum dimension in the range of from 0 1 to 50 microns, and said core 45 containing from 0 1 to 2 grams of crystalline high explosive compound per meter of length; and (b) enclosing the core, a protective sheathing formed from one or more layers of plastics material, the plastics material being one which is capable of flowing at a temperature not exceeding the melting point of the crystalline high explosive compound by 50 more than 750 C.

2 A detonating cord according to Claim 1 which includes corereinforcement means located outside the core.

3 A detonating cord according to Claim 2, wherein the core-reinforcement means comprises at least one continuous strand of yarn located on the periphery of the core and 55 extending substantially parallel to the core's longitudinal axis, the reinforcement means having sufficient tensile strength as to prevent the core from necking down to a failure point under forces normally encountered in borehole loading.

4 A detonating cord according to Claim 3, wherein the core-reinforcement means comprises at least four strands of yarn substantially uniformly distributed about and in 60 contact with the periphery of the core, the strands having sufficient tensile strength as to provide the core with a tensile strength of at least 4

5 kilograms, and the sheathing enclosing the core and the strands.

A detonating cord according to Claim 3 or Claim 4, wherein the yarn is a multifilament yarn, and the filaments thereof are dispersed around the core 65 1 582 903

6 A detonating cord according to any one of the preceding claims, wherein the crystalline high explosive compound is pentaerythritol tetranitrate or cyclotrimethylenetrinitramine.

7 A detonating cord according to any one of the preceding claims, wherein the explosive composition contains at least 70 percent by weight of high explosive compound 5 and wherein the core contains at least 0 4 gram of high explosive compound per meter of length.

8 A detonating cord according to any one of the preceding claims wherein the binding agent is plasticized nitrocellulose.

9 A detonating cord according to any one of the preceding claims, wherein the

10 sheathing is made from a thermoplastics material.

A detonating cord of Claim 9, wherein the thermoplastic material is a polyolefin which is capable of flowing at a temperature below about 2000 C.

11 A detonating cord of any one of the preceding claims wherein the thickness of the sheeting is in the range of from 0 013 to O 318 cm 15

12 A low energy detonating cord comprising a bonded detonating explosive composition formed into an elongate deformable core which has a loading of capsensitive crystalline high explosive compound of from 0 1 to 2 grams per metre of core length and comprising at least 55 per cent by weight of said high explosive compound as superfine organic polynitrate or superfine organic polynitramine in admixture with a binding agent, a 20 protective sheath enclosing said core along its length and comprising at least one layer of a plastics material capable of flowing at a temperature not exceeding by more than 750 C the melting temperature of the crystalline high explosive compound, and reinforcement means comprised of one or more mono or multi filaments, extending axially and non-helically outside of the core and devoid of substantial engagement by weaving or other means with 25 any other such filament whereby the tensile strength of the cord is increased.

13 A low energy detonating cord for use as a trunkline and/or downline in a non-electric blasting assembly which cord comprises an elongate deformable core which has a loading of crystalline high explosive compound of from O 1 to 2 grams per metre of core length and comprising at least 55 per cent by weight of said high explosive compound as 30 superfine organic polynitrate or superfine organic polynitramine in admixture with a binding agent; core reinforcement means disposed outside the core to provide the core with sufficient tensile strength to prevent necking down thereof to a failure point under forces normally encountered in borehole loading; and a sheath enclosing at least the core to provide protection therefor in and prior to use, the sheath comprising one or more layers of 35 plastics material capable of flowing at an elevated temperature not exceeding by more than "C the melting temperature of the crystalline high explosive compound in the core.

14 A low energy detonating cord for use as a trunkline and/or downline and in a non-electric blasting assembly which cord comprises an elongate core formed of a deformable bonded cap-sensitive detonating explosive composition, the composition 40 comprising a binding agent and at least 55 per cent by weight of superfine organic polynitrate or superfine organic polynitramine and the explosive loading of the core being in the range from 0 4 to 2 grams per metre of core length; and a protective sheath enclosing the core along its length, the sheath comprising one or more layers of a plastics material capable of flowing at a temperature not exceeding by more than 750 C the melting 45 temperature of the superfine high explosive compound and the sheath thickness being from 0.oo 5 in to O 125 in.

A low energy detonating cord substantially as hereinbefore described with reference tok and as illustrated in, Figure 1 or Figure 5 of the accompanying drawings.

16 A low energy detonating cord as claimed in Claim 1 and substantially as 50 hereinbefore described in any one of the foregoing Examples 1, 2, 5 to 10 and 15 to 26.

BROOKES & MARTIN, Chartered Patent Agents, High Holborn House, 55 52/54 High Holborn, London WC 1 V 65 E.

Prinicd for Her Majestys Stationerm Office by Croyd-n Pintre Company Limited, Croydon, Surrey, 1980.

Published hy The Pateni Office 25 Southalmlpto-1 Butddings London, WC 2 A IAY from uhich copies mal be obtained