Explosive connecting cord
US 3125024 A
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March 17, 1964 M. A. HICKS ETAL, 3,125,024 EXPLOSIVE CONNECTING CORD Filed May 3. 1962 Inventors: Mervyn A. HICKS William MANN flan/ML F Agnt United States Patent 3,125,024 EXPLOSIVE CONNECTING CORD Mervyn Anthony Hicks and William Mann, Brownsburg,
Quebec, Canada, assignors to Canadian Safety Fuse Company Limited, Montreal, Quebec, Canada, a corporation of tianada Filed May 3, 1962, Ser. No. 192,111 Claims priority, application Canada Mar. 23, 1962 6 Claims. (Cl. 102-27) This invention relates to an improved explosive connecting cord. More particularly it relates to an improved explosive connecting cord of the recently developed type which has a uniform velocity of detonation like detonating fuse but which is considerably less brisant and produces less noise than detonating fuse.
As is well known, detonating fuse consists of a core of high explosive such as pentaerythritol tetranitrate (PETN) or cyclotrimethylene trinitramine (RDX) confined within a textile sheath surrounded by a protective and reinforcing covering. A typical detonating fuse has a uniform velocity of detonation of the order of 6000 meters per second and comprises a core of 30-50 grains per foot of PETN, a textile sheath surrounding said core to contain the same, an asphalt layer surrounding said sheath to waterproof it and prevent said core from sifting, a tape covering for said asphalt layer, a second textile sheath surrounding said tape covering to protect it, and a waterproofing layer of wax surrounding said second textile sheath. Other typical detonating fuses have only a thermoplastic covering immediately over the initial textile sheath.
Because of the high brisance (shattering power) and considerable detonation noise which are caused by the high concentration of explosive in its core, it has recently been proposed to replace detonating fuse by a new type of explosive connecting cord which is commonly referred to in the trade as low energy detonating cord (LEDC). This newly proposed type of cord comprises a core having from 0.1 to 10 grains of high explosive (e.g. PETN, RDX, nitromannite, lead azide, TNT, HMX, lead styph nate or tetryl) per foot of length encased in a flexible metal sheath (e.g. a lead sheath). This metal sheath, the presence of which is essential and critical to ensure the progagation of the detonation, may be reinforced with a fabric countering for strengthening purposes, with or without a wire reinforcing to provide resistance to shear abrasion and general abrasive handling.
Due to the confinement of the high explosive core by the metallic sheath, LEDC has a velocity of detonation which is uniform like that of detonating fuse despite the lower concentration of high explosive in the said core. Furthermore, because of this lower concentration of explosive in its core, LEDC produces very little noise upon detonation and has little brisance, which latter property makes it eminently suitable for bottom-hole priming, i.e. for use in blasting operations where initiation of the bottom charge in a borehole prior to initiation of the remaining charges is desired. The high concentration of explosive in the core of ordinary detonating fuse results in such a high brisance of the fuse that it cannot be used for bottom-hole priming: the upper charges of explosive in the hole are initiated by the passage of the detonating impulse if dynamite is used as the blasting explosive, whereas, if the very insensitive ammonium nitrate explosives are used, the ammonium nitrate, if loaded loosely in the borehole in a dry location, is disturbed thus reducing the continuity of the charge or, if loaded in waterproof containers, as is done in wet locations, the containers are destroyed and their contents scattered, thus subjecting the soluble salts to the moisture Within the borehole and again reducing the continuity of the charge.
However, despite its superiority over detonating fuse from the standpoints of brisance and noise, LEDC is still inferior to detonating fuse from the following standpoints.
Firstly, LEDC is manufactured by loading a tube of metal with crystals of a high explosive and thereafter reducing the diameter of the tube to the desired size by drawing the tube in several steps. This limits the length of the final detonating cord to that which can be obtained from an original tube necessarily limited in length because of the awkwardness and difficulties of manipulating long tubes. Detonating fuse, on the other hand, can be produced continuously in unlimited length.
Secondly, it is a known deficiency of LEDC that it cannot be initiated by direct attachment to enother length of detonating fuse, which is the most common way of assembling a net work of detonating fuse lines in preparation for a blasting operation. Because of this insensitivity to initiation by normal means, LEDC requires several different types of connecting devices for the preparation of the said net wor with resultant increase in costs, loss of time and higher probability of the detonation failing to propagate.
Thirdly, the metal sheathed detonating cord is electrically conductive along its length due to the conductivity of its sheath, so that an electrical charge can be carried along the cord to be discharged into any explosive into which the cord is inserted or with which it is in contact. This discharge can 'cause the initiation of the said explosive (e.g. a commercial blasting cap) resulting in a potentially dangerous premature explosion. Such electrical charges can be induced by electrical storms, static build-up through handling, static build-up through powdered or granular explosives being pneumatically loaded into bore-roles, or current leakage from adjacent electrical equipment or cables. Detonating fuse, on the other hand, does not generally contain any metal and is completely non-conductive.
An object of the present invention is to provide an improved explosive connecting cord which combines all the advantages of low energy detonating cord over detonating fuse with all the advantages of detonating fuse over low energy detonating cord. Additional objects will become apparent as this invention is more fully described hereinafter.
It has indeed been surprisingly found that if the specific surface of the high explosive in the core of low energy detonating cord is increased from its normal 500 square centimeters per gram to at least about 900 square centimeters per gram and if the explosive is present in a concentration of at least 1.5 grains per foot for specific surfaces of up to 3400 square centimeters per gram, a uniform velocity of detonation can still be obtained but without the presence of the metallic sheath. The high explosive core can instead be confined within the ordinary textile sheath and protective and reinforcing covering of detonating fuse with the resultant aforesaid advantages of detonating fuse over low energy detonating cord.
By specific surface is meant the surface area per unit weight of the high explosive in powder form. It is measured by the standard nitrogen adsorption test using the BET equation and apparatus. (See Encyclopedia of Chemistry, Reinhold 1957 or Small Particle Statistics by G. Herdan, Buttleworth, 1960.)
The improved explosive connecting cord of this invention thus comprises essentially a core of high explosive having a specific surface of at least about 900 sq. cm./ g. in a concentration of at least 1.5 grains per foot of length for specific surfaces of up to 3400 sq. cm./g., confined within a textile sheath surrounded by a protective and reinforcing covering. This covering may be a thermoplastic layer or may consist of a series of Waterproofing and reinforcing materials, e.g. an asphalt layer, a tape covering, a second textile sheath and a wax layer in successive order.
In order to illustrate this invention more fully, reference is now made to the accompanying drawing wherein FIG. 1 is a sectional view of a finished connecting cord of this invention, FIG. 2 is an end view of the same cord, FIG. 3 is a sectional view of another type of finished connecting cord of this invention, and FIG. 4 is an end View of the cord shown in FIG. 3.
Referring to FIGS. 1 and 2 in greater detail, 1 represents the core of high explosive having the increased critical specific surface, 2 represents the textile sheath confining core 1, 3 is an asphalt layer surrounding sheath 2, 4 is a tape covering over layer 3, 5 is a textile countering surrounding tape 4, and 6 represents a wax layer around countering 5.
In FIGS. 3 and 4, 7 represents the core of high explosive, 8 represents the textile sheath confining core 7, and 9 is a thermoplastic covering over sheath 8.
To describe this invention further, reference is now made to the following specific example, which of course, is only illustrative and is not intended to limit the invention to the embodiments described therein.
Example Lengths of explosive connecting cords of the type described above by reference to FIGS. 1 and 2 (samples 10, 15, 16, 17, 19, and 21) and FIGS. 3 and 4 (samples 1 to 9, 11 to 14, 18 and 22 to 29) of the drawing were made wherein the high explosive of the core was PETN of different specific surfaces and was present in the core in different concentrations. These cords were initiated by conventional means and the results of the initiations are given in the following table in terms of uniformly propagated detonation velocity, the expression failures meaning that initiated cords failed to propagate detonation.
PETN, Detonation Samples PETN, Specific velocity,
grains/toot surface, meters/ sq. cm./g second 15 500 failures 23. 9 500 failures 7 670 failures 11 670 failures 8. 2 890 failures 10 890 4, 975 9. 1 1, 102 3, 875 9. 3 1, 320 4, 545 7. 8 1, 320 3, 525 10 1, 460 5, 565 9. 5 2, 830 5. 256 G. 1 2, 830 4, 994 3. 2 2, 830 4, 470 2. 0 2, 830 3, 100 8. 5 3, 070 5, 380 6. 5 3, 070 5, 450 4. 1 3,070 5, 375 3. 6 3, 070 5, 145 8. 2 3, 150 5, 380 3. 9 3, 150 5, 040 3. 1 3, 150 5, 534 4. 2 3, 300 5, 460 4. 0 3, 300 5, 300 4. 5 3, 380 5. 430 3. 5 3, 380 5, 560 2. 5 3, 380 5, 420 2. 0 3, 380 4, 770 1. 6 3, 380 4, 700 1. 4 3, 380 failures To those skilled in the art, it will be apparent that by varying the concentration of high explosive in the core of the explosive connecting cord and/or by modifying the confinement of the said core through changes in the thickness and/or density of its sheath, one can vary the detonation velocity of the cord. It will also be apparent that increasing the specific surface of the high explosive in the core allows one to decrease the concentration of explosive therein with the result that with explosives having a specific surface greater than 3400 sq. cm./g., one can decrease their concentration in the core below 1.5 grains/foot and still obtain cords which will propagate detonation with a uniform velocity.
What we claim is:
1. An improved explosive connecting cord comprising essentially a core of pentaerythritol tetranitrate having a specific surface of from about 900 to 3400 square centimeters per gram in a concentration of from 1.5 to 10 grains per foot of length, confined within a textile sheath surrounded by a protective and reinforcing covering.
2. An improved explosive connecting cord as claimed in claim 1 wherein the protective and reinforcing covering is a thermoplastic layer surrounding the textile sheath.
3. An improved explosive connecting cord as claimed in claim 1 wherein the protective and reinforcing covering consists of the following materials in successive order: an asphalt layer, a tape covering, a second textile sheath and a wax layer.
4. An improved explosive connecting cord comprising essentially a core of pentaerythritol tetranitrate having a specific surface of about 3380 square centimeters per gram in a concentration of from 3.5 to 4.5 grains per foot of length confined within a textile sheath surrounded by a protective and reinforcing covering.
5. An improved explosive connecting cord as claimed in claim 4 wherein the protective and reinforcing covering is a thermoplastic layer surrounding the textile sheath.
6. An improved explosive connecting cord as claimed in claim 4 wherein the protective and reinforcing cover ing consists of the following materials in successive order: an asphalt layer, a tape covering, a second textile sheath and a wax layer.
References Cited in the file of this patent UNITED STATES PATENTS 2,239,051 Pearsall et a1 Apr. 22, 1941 2,380,312 Johnson July 10, 1945 2,891,475 Dolan et al June 23, 1959 2,982,210 Andrew et a1 May 2, 1961 FOREIGN PATENTS 592,050 Canada Feb. 2, 1960 839,832 Great Britain June 29, 1960