US3150020A - Gasless igniter composition - Google Patents

Description

United States Patent (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to an igniter mixture for pyrotechnic delay compositions and, more specifically, to a gasless igniter composition comprising zirconium and lead monoxide.

Explosive compositions are customarily initiated by combination electro-explosive devices containing gasless delays which, almost exclusively, use primary explosives heated by a bridge wire to effect the initial reaction. The primary explosive may or may not be formulated to produce a delay in the timing of the initiation. When the primary explosives react, however, liberal quantities of gaseous combustion products are formed. If these gases are allowed to materially increase the internal pressure of the electro-explosive device, the burning rate of the gasless delay powder composition, which is pressure sensitive, is considerably increased over its rate of burning at ambient pressure and inefficient use of the delay with respect to time/length results. Additionally, if the internal gas pressure is not controlled, time uniformity may not be obtained. To overcome these difficulties, practical designs have allowed for venting the primary explosive gases through vent holes to the exterior or have incorporated a fully contained internal vent chamber. The former expedient is objectionable since it reduces the shelf life of the electro-explosive device in moist environments and also prevents potting or encapsulating the device. The latter expedient overcomes the moisture and potting difficulties but is objectionable in that it requires increased volume or length (for the vent chamber) and this space in a weapon system is often at a premium.

Another method of overcoming the foregoing problems would be to eliminate the primary explosive and substitute an igniter material sensitive to ignition by a hot wire and which would have essentially gasless combustion products. Such igniter materials have not, however, been generally available. igniter mixes such as A-lA (65% Zr, 25% Fe O Superfioss) and FA-878 (40% Zr, 20% Ba(NO 20% PbO 20% PETN) were known to be marginally close to the hot wire sensitivity desired but could not be relied upon for reproducibly reliable performance.

The igniters presently in use in the explosive arts are not of a gasless nature and are relatively insensitive to ignition by a hot wire. Moreover, the present igniters are composed of a large number of chemical components and, being gas-producing mixtures the igniters have caused erratic functioning times of pyrotechnic delay trains when used in no-free-volume, nonvented systems. The igniters now in use in the art must therefore be used in a vented or large-free-volume system and must be 3,15,020 Patented Sept. 22, 1964 ignited by some sensitive or primary explosive or by another igniter mix rather than by a hot wire. Accordingly, in view of the present state of the art, a pyrotechnic igniter having the following characteristics is needed:

(1) Little or no gas evolution while burning, thus being operable in a no-free-volume, nonvented system.

(2) Capability of igniting (a) another igniter, or (b) a pyrotechnic delay composition.

(3) Capability of ignition by another igniter or by a combination of primary explosive and igniter.

(4) Capability of hot wire ignition.

(5) Simplicity and reliability.

It is known in the prior art to provide ignition compositions comprising a metal powder and a metal oxide with and without other additives. For example, a known igniter composition comprises PbO CaSi Zr and S. Another is comprised of P00 Zr and Ba(NO Still another igniter composition comprises Zr with a metallic oxide such as M00 Pb0 or red lead (Pb O The main disadvantage of the prior art compositions is that they are not of a gasless nature and electro-explosive devices utilizing such gas-producing compositions must either be vented or must incorporate a fully contained internal vent chamber. The disadvantages of these expedients have been pointed out previously.

It has now been unexpectedly found that an igniter mix capable of hot wire ignition and consisting of zirconium powder and lead monoxide appears to act gasless when utilized to ignite pyrotechnic delays based on metals lighter than tungsten, such as molybdenum and manganese based delay compositions. Whether the combustion roducts of the Zr/PbO igniter mix of the present invention are absolutely of a nongaseous nature is not precisely known. The igniter mix, however, behaves as if it were gasless since the mix may be used in nonvented, no-freevolume systems with reliability and with none of the attendant problems of prior art mixes.

The igniter composition of this invention may be ignited not only by a hot wire but may also be ignited by another igniter, such as A-lA or FA878, and also may be ignited by a combination of a primary explosive such as lead azide and/or lead styphnate with an igniter such as A-lA or FA-878. Conversely, the novel igniter mix of this invention may be used to ignite not only pyrotechnic delay compositions but Will also ignite other igniters such as A-lA or FA878.

The following example is illustrative of a specific embodiment of the invention and should not be construed as a limitation thereon.

EXAMPLE Sample igniter mixes of zirconium powder and lead monoxide were prepared having Zr/PbO ratios as shown in Table I below. The mixes were prepared by screening reagent grade PbO through a 325 mesh screen (44 microns or less) and admixing the PhD with Zr powder having an average diameter of about 3 microns by fluid blending in a Waring Blender, using ethyl acetate as suspending medium. The components were blended for 15 minutes, filtered through a vacuum filter and dried overnight in an oven at a temperature of about C. Neither the drying temperatures nor the fluid blending medium is critical, convenience being the only consideration.

3 SENSITIVITY TESTING Sensitivity tests were conducted on the prepared igniter mixes by pressing the mixtures about a Tophet-C wire, having a length of 0.050 inch and a diameter of 0.0004 inch, and then determining the potential needed, on a 1 microfarad capacitor, to fire the sample. The results of this testing are summarized in Table I.

4 TESTING IN DELAY DEVICES Tests in delay devices, both vented and nonvented, were conducted with delay mixes of various burning rates. Comparisons were made between burning times in the vented and nonvented devices. Test devices contained 100 milligrams of 50/ 50 Zr/PbO igniter mix and 5 grams of molybdenum or manganese base delay mix. The ig niter mix was loaded at 10,000 p.s.i. while the delay compositions were loaded at 30,000 p.s.i. The units were tested over a temperature range of 65 F. to +160 F. The energy from a l-microfarad capacitor charged to 60 volts was passed through the T ophet-C bridge wire to produce ignition and the ability of the mixture to ignite the delay composition and the delay times were determined. Table II summarizes the results.

As with any explosive, sensitivity and rate of burning of the igniter mix decreases with inc .asing pressure and it has thus been found that the optlnum pressing pressure *The lower voltage was the lowest value at which any sample was range is from about 10,000 to 20,000 psi,

observed to fire. The upper voltage was the lowest value at which all samples fired.

Table II follows.

Table II OBSERVED DELAY TIMES FOR DELAY COMPOSITIONS IGNITED FROM 50/50 ZIROONIUM/ LEAD MONOXIDE IGNITER MIXTURE Non-vented Vented Inverse Delay Galen burning column lated de- Temp, Type of delay powder rate length lay time "F. Number Mean Number Mean (sec/in.) (inches) (seconds) of delay of delay samples time samples time seconds) (seconds) Molybdenum base 0.22 0. 270 0. 059 165 5 0. 0654 5 0. 0661 0. 22 0. 270 0. 059 Room 5 0. 0716 5 0. 0710 0. 22 0. 270 0. 059 65 5 0. 0825 5 0. 0805 Manganese base 2. 53 0. 233 0. 589 160 10 0.519 10 0.539 2. 53 0. 233 0. 580 Room 10 0. 565 10 0. 578 2. 53 0. 233 0. 589 65 10 0. 585 10 0. 674

From this table it is seen that the 20/80 and 50/50 Zr/PbO mixes could be ignited without suflicient difference in sensitivity to choose between them. The 95/5 mix could not be fired even at 6 microfarads and 250 volts.

HOT WIRE IGNITION TESTING The 20/80, /60, and /50 Zr/PbO mixes were subjected to hot wire ignition tests in which the light output was viewed as a function of time using a photoelectric detector tube and an oscilloscope. The 20/80 Zr/PbO mix showed no output for the full sweep of the oscilloscope, which was 5 milliseconds in these tests, although the mix, of course, was ignited. The 40/60 mixture and the 50/50 mixture were united within the sweep time.

The sensitivity of the 50/ 50 Zr/PbO mix to ignition by a hot wire was determined by a capacitor discharge Bruceton type test using a fixed l-microfarad capacitor and a variable potential. In this type of test, ignition of a number of samples is attempted at preselected voltages and the number of fires and failures at each voltage is determined. By this method it may be determined, for each voltage, what percentage of the samples will fire. The mean firing potential for the 50/ 50 Zr/PbO mixture was determined to be 57.4 volts, corresponding to an energy of 16,500 ergs. Thus at an energy of 16,500 ergs 50% of the samples tested were ignited. The calculated 99.9% firing point was at 63.1 volts, corresponding to an energy of 19,900 ergs.

Inspection of Table II discloses that the maganese delay compositions were reliably initiated over a temperature range of F. to 65 F. in both the vented and nonvented systems. In the case of the molybdenum based delays, there was no significant difference in the observed delay times between the vented and nonvented systems. However, for the manganese delay compositions there was a small reduction of the burning time in the nonvented system. In addition to the Mo and Mn base delays, the igniter mix of this invention may be used to ignite other metals lighter than tungsten.

Thus it may be seen that the igniter mix of this invention has the following advantageous characteristics:

(1) Operability in a nonvented, no-free-volume system, having no detectable gas evolution.

(2) Capability of igniting another igniter or a pyrotechnic delay composition.

(3) Capability of ignition by another igniter or combination of primary explosive and igniter.

(4) Capability of hot wire ignition.

(5) Simplicity and reliability.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Having thus described the invention, What i claimed and desired to be secured by Letters Patent of the United States is:

1. A gasless pyrotechnic delay igniter composition, capable of being ignited by a hot wire, consisting essentially of zirconium powder and lead monoxide.

2. The composition of claim 1 wherein the Zr/PbO ratio is in the range of from about 1/1 to about 1/4 by Weight.

3. The composition of claim 1 wherein the zirconium powder has an average diameter of about 3 microns.

4. A pyrotechnic device comprising a delay powder based on a metal lighter than tungsten and an igniter composition consisting essentially of zirconium powder and lead monoxide.

5. The device of claim 4 wherein the Zr/PbO ratio is in the range of from about 1/1 to 1/4 by Weight.

6. The device of claim 4 wherein the average diameter of the zirconium particles is about 3 microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,309,978 Pratt Feb. 2, 1943 2,375,742 Kalil et a1. May 8, 1945 2,478,501 Patterson Aug. 9, 1949 2,740,703 Mulqueeny Apr. 3, 1956 FOREIGN PATENTS 596,725 Great Britain Jan. 9, 1948

Claims (1)

1. A GASLESS PYROTECHNIC DELAY IGNITER COMPOSITION, CAPABLE OF BEING IGNITED BY A HOT WIRE, CONSISTING ESSENTIALLY OF ZIRCONIUM POWDER AND LEAD MONOXIDE.