US2908559A - Ignition mixtures and electric initiators - Google Patents

Description

Oct. 13, 1959 D. T. ZEBREE mumou MIXTURES AND ELECTRIC mmmons Filed March 30, 1956 DAVID T. ZE BREE INVENTOR.

United States Patent Ofilice IGNITION MIXTURES AND ELECTRIC INITIATORS David T. Zebree, Kingston, N.Y., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware Application March 30, 1956, Serial No. 575,026 19 Claims. 01. 52-21 tion is a continuation-in-part of my copendingapplication, Serial Number 78,383, filed February 25, 1949, now abandoned.

In the maufacture of delay electric initiating devices one of the most commonly used and most satisfactory igniton compositions is a mixture of finely-divided lead and selenium. These mixtures are substantially gasless during combustion, and, for the most part, have proved most satisfactory in igniting fuse elements containing delay trains composed of such difficultly ignitable mixtures as barium peroxide-selenium.

A very substantial problem has been encountered, however, due to the deterioration is quality of these mixtures which results from the oxidation of the lead. This deterioration is most pronounced during warm weather storage and its rate is accentuated by the presence of moisture. Such deterioration has two very undesirable effects on the performance of a delay electric blasting initiator which flow directly from the dimunition in calorific content of the mixture due to the progressive oxidation.

First, irregularity in firing times results with a consequent overlapping among delay periods in series firing. Second, actual fuse ignition failures result in numbers directly proportional to the degree of oxidation of the lead which has taken place. As a result of such deterioration, it has been necessary to purchase special and more expensive grades of lead powder which have a very low initial oxide content. It is then necessary to exercise meticulous care in the handling and storage of both the ignition mixture and the completed initiating devices. Despite all such precautions, however, a progressive deterioration in quality may still take place in time, with the undesirable conseqences above set forth.

A further problem has been encountered when delay electric blasting initiating devices containing lead-selenium mixtures are fired in an inverted position. Even when the ignition mixtures contain no appreciable oxide content, occasional failures are still encountered during inverted firing due to the tendency of the 'leadfselenium mixture to shrink during combustion. The failures are due to an actual shrinking away from the delay fuse to the extent that the ignited ignition powder does not make sufiicient contact 'for ignition of the delay train.

Therefore, an object of this invention is a delay electric initiating device of greatly improved performance which is not subject to failure due to deterioration of its ignition mixture. Another object of this invention is a delay electric initiating device comprising an ignition mixture containing lead and selenium which is not subject to irregularity in firing time due to improper ignition. Another object of this invention is a delay electric initiating device Patented Oct. 13, 1959 2 comprising an ignition mixture containing lead and selenium which is not subject to ignition failures during inverted firing. Still another object is to provide leadselenium ignition mixtures exhibiting improved ignition characteristics. Yet another object is to provide a method for efiectingignition in the detonation of an explosive employing a primer. Other aspects and objects will 'be apparent from the accompanying disclosure and the appended claims.

In accordance with this invention an ignition mixture is provided which comprises lead and selenium in ignitable proportions, as the major ingredients, and an amount of silicon capable of imparting increased heat of combustion when said lead and selenium are burned. Still in accordance with this invention is provided an electric initiating device having in combination a delay fuse element and a loose ignition .mixture in contact with the delay fuse element, said mixture comprising lead 'and sele-' nium as the major ingredients and an amount of silicon capable of imparting increased heat of combustion when said lead and selenium are burned. Further, in accordance with this invention is provided a method for effecting detonation of a primer composition in a blasting cap, comprising placing an ignition mixture in communication with said primer, in said cap, so as to effect detonation of said primer responsive to heat formed when said mixture is burned, said ignition mixture comprising lead and selenium in ignitable proportions, as the major ingredients, and an amount ofsilicon capable of imparting increased heat of combustion when said lead and selenium are burned, and igniting said mixture.

Although it is within the scope of my invention to utilize any ignitableproportions of lead and selenium, i.e., capable of undergoing combustion when cont-acted with a heated bridge wire as discussed hereinafter, it is now preferred for optimum results that the lead and selenium be present in the mixture in substantially stoichiornetric proportions. Such a chemically equivalent mixture contains, by weight, 72.4% lead and 27.6% selenium. However, mixtures containing any suitable proportions generally between about 64% and about lead and between 20% and 36% selenium can be employed.

Any amount of silicon can be employed in the practice of this invention which is capable of increasing heat of combustion when thelead and selenium are burned. Generally such amount is within the range of about 0.3 to 20 Wt. percent of the ignition mixture, from about 1 to 5 percent being now preferred. The upper limit of silicon concentration will be set by its etfect on the sensitivity of the ignition mixture, i.e., by its tendency todesensitize the mixture, which may take place at concentrations some- ,what above 20 percent, dependent upon the specific leadselemum proportions utilized.

Having generally described the present invention, a more specific description is presented with reference to the accompanying drawing which represents a sectional view of a delay electric blasting cap in accordance with one embodiment of the invention. t

The drawing represents a ventless delay electric blasting cap comprising a-bronze shell 1 into the bottom of nium as the iiuse powder 4 compressed to the proper density in a lead tube 5. A loose charge of ignition mixture '6 is superimposed on the'fii se element; The'ignition mixtureis composed'of finely-divided lead and selenium .(72. 4-27.6)/ silicon in a ratio of 98/2. A bridge wire-Tisaflixed to'the terminal ends of a pair of lead illustration.

' Having now described ,and illustrated the invention,

the followin'giexamplesare given for purposes of further Example Ventlessdelay electric blasting: initiators were constructed similar to that shown in the drawing. More specifically, the shell was-of bronze with a 0.259 inch inside diameter. .A base charge of 0.40 g. of pentaerythritol tetram'trate was pressed to a height of inch. A

priming chargeof 0.30 g. 'diazodinit'rophenol/potassium chlorate'(75725) was pressed on top of the base charge to a height of inch.' The fuse, 4 inch in length, consisted of a lead tube filled with a mixture of barium peroxide-selenium 'in the ratio of 84/ 16.. The superimposed ignition compound comprised lead-selenium in stoichiometric proportions (72.427.6%, respectively)/ silicon, 98/ 2.- This ignition compound occupied a space of .inch. sulfur ignition plug inch long was placed over the ignition powder in such a manner that .the leg'wire ends, which were connected by a bridge wire, extended into the. loose powder to a distance of inch. A charge of waterproofing was placed on top of the-plug and occupied a space of inch. A -inch column of sulfur completed the cap.

One hundred eighty such caps were made and 180 others were made which were similar in all respects except that the ignition charge was lead-selenium in stoichiometric proportions. In all 360 caps a lead powder was used which had a lead oxide content of 41%. When these caps were fired, 8 of those with lead-selenium ignition mixtures failed to fire. 7 All 180 caps having leadselenium-silicon ignition mixtures fired normally after the predetermined period of delay.

Example 2 Ventless delay electric blasting caps were made similar to those of Example 1. Twohundred caps had leadselenium ignition charges while another 200 caps had the lead-selenium-silicon ignition charges. In all 400, the lead powder used was of top quality with a very low oxide content. When these 400 caps were fired in an inverted position, 2 vof those having lead-selenium ignition charges, failed, while all those having lead-seleniumsilicon charges fired normally. In'the two failures, the fused ignition mixture was found to have shrunk away from the delay fuse. I Y

' Example 3 v In the tabulation below are recorded the results of incorporation of 1 percent-and 5 percent of silicon in a lead-selenium mixture; A lead powder was employed which had an oxidecontent-of 41%. This oxidized powder was mixed with selenium in stoichiometric proportions (72.4% lead to 27.6% selenium) as if no oxidation had taken place. The delay electric blasting caps employed were those described in Example 1. Corresponding data for lead-seleniumaloneare also set forth. In each test, 25 such. caps were fired in series with a firing current of one ampere.

1 The ignition powder burned but failed to generate enough heat to and selenium, results in greatly enhanced performance. These results are believed due to the great afii nity that silicon has for lead oxide and to the fact its addition causes the burning ignition'mixture to expand rather than contract. While the reaction between lead oxide and silicon in the presence of lead-selenium is not violent, it is exothermic. When silicon as a corrective material is used in accordance with this invention, there is no adverse eifect on the performance of the ignition mixture despite 'the' 1 degree to which deterioration in quality of the lead-selenium mixture has taken place. To the contrary, as has been shown, the presence of as little as 2% of silicon has been found to completely compensate for the presence of as much as 41% of lead oxide.

For example, a marked improvement in lead-selenium mixtures is noted when 0.3% silicon is incorporated. When 1% silicon is incorporated, fuse ignition failures in both upright and inverted firing are eliminated. When 2% of silicon is incorporated, all aberration in firing timesis eliminated. Consequently, while silicon may be incorporated up to 20% and higher without adversely affecting ignition of the delay fuse or firing time of the cap, 2%.and up to 5% of silicon is generally sufiicient to compensate for the formation of up to 41% of lead oxide and any contraction of the lead-selenium mixture during combustion.

While any of the usual types of delay fuse may be employed in the delay initiator embodiment of this invention, the barium peroxide-selenium type of fuse employed in the examples is preferred. Any of the normal base-charges and priming explosives are also operable although pentaerythritol tetranitrate-base charges and diazodinitrophenol/potassium chlorate priming charges are preferred. The usual types of shell, plug, leg'wires, and sealing compound may be used although those employed and illustrated herein are preferred.

Deterioration of lead-selenium ignition mixtures due to the oxidation of the lead, therefore, no longer constitutes a problem in the improved electric initiators of this invention. 'When the prescribed amounts of silicon as the corrective material are incorporated in the ignition mixture as described and illustrated, it is no longer necessary to select highest grade load powders or to exercise meticulous care in handling or storing the ignition mixtures themselves or the completed initiators. Furthermore, the failures of delay electric initiators containing leadselenium ignition mixtures during inverted firing are also elminated. Delay electric initiators made in accordance with the present invention are therefore easier and less expensive to manufacture and store and exhibit greatly improved performance.

A feature of this invention is in the broad rangeof proportions of silicon in my ignition compositions. Thus, when employing a metal such as aluminum or magnesiumv as a corrective material in a lead-selenium ignition mixture to counteract presence of lead oxide therein, there is a critical relatively low maximum limit of concentration, i.e., 3%, above which a violent burnmg reaction takes place with development of flame which generally results in bursting of the shell and concomitant failure. Therefore, when employing aluminum or magnesium or other such flame-producing metals, there is always the inherent danger that the initiator will inadvertently contain more than the maximum allowa'ble metal'andreact violently, when fired, with concomitant failure. Furthermore, manufacture of such ignition mixtures (containing aluminum or magnesium or the like) is hazardous and requires highly precise procedures and equipment for handling such materials particularly to assure that the said critical limit is not exceeded. On the other hand, although silicon present in a lead-selenium ignition mixture in a concentration of say 2-3 percent often provides an optimum amount of heat liberation, no violent burning reaction takes place when such concentration is exceededeven to as high as 20 weight percent or higherprecluding thereby any possibility that bursting of theshell willv take place.

The initiator of this invention is therefore, in all events, free from any violent burning of the ignition mixture and concomitant bursting of the shell to assure against failures. Indeed, if desired, relatively approximate amounts of silicon can be utilized in the said ignition mixture without the need for precise measuring and handling.

Thus, as seen in the following tabulation, aluminum employed in a concentration of 3% in a lead-selenium ignition mixture functions without setting up a violent reaction. However, when 3.5%, and 4% aluminum are employed, failure can take place as result of the violent reaction with bursting of the cap shell.

Ignition powder burned but failed to generate suflicient heat to ignite the fuse.

I Ignition powder burned so violently that the shell was forcibly ruptured and the delay fuse was not ignited.

With reference to Example 3 hereinabove, it is seen that concentrations of silicon as high as 5% gave no violent reaction. Similarly, silicon concentrations as high as 20% and higher can be employed as described hereinabove, in no event there occurring failure resulting from violent reaction involving silicon.

The foregoing clearly illustrates the nonequivalency of silicon with flame-producing metals such as aluminum and magnesium in ignition mixtures of this invention.

In one embodiment of the method of this invention, an ignition mixture comprising lead, selenium and silicon as described herein is placed in a delay cap superposed on a suitable delay fuse such as barium peroxideselcnium. The fuse is superposed on a suitable primer such as diazodinitropheuol-potassium chlorate, the latter superposed on a high crystalline explosive such as PETN. A resistance or bridge wire is disposed in the ignition mixture and is connected by leg wires to any suitable current source. Current is then passed through the bridge wire and heat is developed responsive to which the primer is detonated, in turn detonating the PETN.

It is to be understood that the initiators of this invention are not to be limited to delay initiators, but are also inclusive of instantaneous-type initiators.

As will be evident to those skilled in the art, various modifications can be made or followed, in the light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

What I claim and desire to protect by Letters Patent 1. An ignition mixture of from 64 to 80 weight percent lead, from 20 to 36 weight percent selenium and containing an amount of silicon at least sufficient to impart increased heat of combustion when said lead and selenium are burned.

2. An ignition mixture consisting essentially, on a weight basis, of from 0.3 to 20 percent silicon, from 64 to 80 percent lead and from 20 to 36 percent selenium.

3. Anignition mixture of claim 2 containing from 1 to 5 weight percent silicon.

4. An ignition mixture of lead and selenium, in substantially a leadzselenium Weight ratio of 72.4:27.6 and containing about 2 weight percent silicon.

5. In an electric initiating device having in combination a delay fuse element and a loose ignition mixture of lead and selenium in contact with the said delay fuse element, the improvement providing as said ignition mixture, on a weight basis, from 64-80% lead, from 2036% selenium and from 0.3 to 20% silicon.

6. A method for effecting detonation of a primer composition in a blasting cap comprising placing an ignition mixture of lead, selenium and silicon in communication with said primer, in said cap, so as to effect detonation of said primer responsive to heat formed when said mixture is burned, said ignition mixture containing an amount of silicon at least sufiicient to impart increased heat of combustion when said lead and selenium are burned, and igniting said mixture.

7. A method of claim 6 wherein said ignition mixture is superposed on a delay fuse, said delay fuse being superposed on said primer.

8. A method of claim 6 wherein said ignition mixture is superposed on said primer.

9. A method of claim 6 wherein said mixture is ignited by heat from a resistance Wire in contact therewith, developed by passage of electric current through said Wire.

10. A method of claim 6 wherein said mixture contains from 0.3 to 20 weight percent silicon.

11. A method of claim 10 wherein said mixture contains from 1 to 5 Weight percent silicon.

12. A method of claim 10 wherein said ignition mixture contains lead and selenium in substantially a lead: selenium weight ratio of 72.4:27.6 and about 2 weight percent silicon.

13. A method of claim 10 wherein said ignition mixture contains from about 64 to weight percent lead, 20 to 36 weight percent selenium.

14. A delay electric initiating device having in combination a delay fuse element and a loose ignition mixture of lead, selenium and silicon in contact with the delay fuse element, the said ignition mixture consisting essentially, on a weight basis, of from 64 to 80 percent lead, from 20 to 36 percent selenium and an amount of silicon at least suflicient to impart increased heat of combustion when said lead and selenium are burned.

15. An initiating device comprising in combination a loose ignition mixture of lead, selenium and silicon in communication with a primer which detonatcs in response to being heated, so as to cause said primer to detonate in response to heat generated when said ignition mixture is burned, said ignition mixture consisting essentially, on a weight basis, of from 64 to 80 percent lead and from 20 to 36 percent selenium and containing an amount of silicon at least sufficient to impart increased heat of combustion when said lead and selenium are burned.

16. An initiating device of claim 15 wherein said ignition mixture is in direct contact with said primer.

17. A device of claim 15 wherein said ignition mixture contains from 0.3 to 20 weight percent silicon.

18. A device of claim 15 wherein said ignition mixture contains from 1 to 5 Weight percent silicon.

19. A device of claim 15 wherein said mixture contains lead and selenium in substantially a lead:selenium weight ratio of 72.4:27.6 and about 2 weight percent silicon.

References Cited in the file of this patent UNITED STATES PATENTS

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