US3017300A - Pelleted igniter composition and method of manufacturing same - Google Patents

Description

3,9113 Patented Jan. 16,1962

3,017,300 PELLETED IGNTTER KIOMPQSITHUN AND METHQD OF MANUFACTURING SAME Liles G. Herring, McGregor, Tex, assignor to Phillips Petroleum Company, a eorporation of Delaware No Drawing. Filed June 21, 1956, Ser. No. 592,995 17 Claims. (Cl. 149-41) This invention relates to an igniter composition. In

one aspect, it relates to a method for manufacturing a pelleted, pyrotechnic igniter composition. Ina further aspect it relates to a pelleted, pyrotechnic igniter composition particularly adapted as an igniter charge for initiating combustion of solid rocket propellants, such as the type employed in those rocket motors used for assisting the take-off of aircraft. i

Although my novel pelleted igniter composition has a variety of uses, it is particularly adapted for initiating the combustion of solid rocket propellants and the following discussion will illustrate it as applied thereto. However, it is to be understood that my invention is not to be unduly limited to such application since it is generally applicable with pyrotechnic devices in general where an igniter charge having relatively low burning rate is required.

Solid propellants utilized in rockets, such as IATO units, comprise a fuel and an oxidant for oxidizing the fuel. Ammonium nitrate and ammonium perchlorate are often utilized as the oxidant while the fuel component is usually a hydrocarbon material which acts as a binder for bonding the particles into a solid grain as well as acting as a fuel. Binder-fuel materials include asphalt, rubber and other tacky hydrocarbon-containing materials.

Recently, it has been discovered that superior solid propellant mixtures are obtained comprising a major proportion of a solid oxidant such as ammonium nitrate or ammonium perchlorate and a minor proportion of a rubbery binder material such as a ccpolymer of a conjugated diene and a vinylpyridine or other substituted heterocyclic nitrogen base compound, which after incorporation is cured by a quaternization reaction or a vulcanization reaction. Solid propellant mixtures of this nature and a process for their production are disclosed and claimed in copending application Serial No. 284,447, filed April 25, 1952, by W. B. Reynolds and J. E. Pritchard.

These solid propellant mixtures, especially where the solid oxidant employed is ammonium nitrate, are inherently difficu-lt to ignite. Ammonium nitrate-binder propellants have a relatively high auto ignition temperature (about 600 F.). They are highly susceptible to changes in pressure and a pressure of about 200 psi. is necessary to sustain combustion. Their specific heat is high but their heat transfer coefficient is low and heat must be maintained at the burning surface for a considerable time in order to initiate combustion and make it selfsustaining. The igniter charges heretofore employed have been found wanting because they did not provide reproducible ignition of ammonium nitrate-type propellants at 75 F. and below. Therefore, there has arisen a need for an igniter composition which will satisfactorily initiate combustion of these solid propellant materials, especially those employing ammonium nitrate as the oxidant.

An igniter composition suitable for initiating the combustion of these propellants must satisfy certain criteria. The components of the igniter composition must remain in intimate mixture and not segregate. Segregation of the components is usually attributed to the different densities of the components and when it occurs, the burning rate .of the igniter composition is not constant. Combustion of propellants ignited by these unsuitable igniter compositions is not uniform and hazards arise in the operation of rocket motors so charged.

The igniter composition employed must not exhibit any substantial brisance, that is, upon ignition the igniter composition must not result in a deleterious shock or explosion. This is very important in that such shocks or explosions tend to disintegrate portions of the propellant desired to be ignited with the consequent uncontrolled burning of the propellant. In some cases, igniter compositions characterized by high brisance have resulted in expulsion of propellant material from the rocket motor.

The burning rate of the igniter composition must be uniform and controllable so as to produce the ignition products over a period of time sufiicient to ignite all exposed burning surfaces of the propellant. This is especially true when igniting propellants containing ammonium nitrate oxidizers since heat must he applied for a period of time up to 500 milliseconds to adequately ignite the burning surfaces.

The igniter composition must be in a physical form which will retain the physical arrangement of the pyrotechnic components and not pack due to vibration and the like which will result in voids throughout the material. The existence of voids in the igniter-composition results in uncontrolled burning which will eventually have some effect on the thrust developed by the rocket motor. Consequently, powdery igniter material is unsuitable since it does not only result in packing but leads to hazards in processing and handling.

A further criterion for a suitable igniter composition is that it must be unaffected by the presence of moisture.

Accordingly, an object of my invention is to provide a novel igniter composition. Another object is to provide a method of manufacturing pelleted igniter material. A further object is to provide an igniter composition characterized by a permanent intimate mixture of its components, low brisance, safety in processing and handling, resistance to the contamination of moisture, and a controlled and uniform burning rate. A further object is to provide an efficient method of manufacturing a pelleted igniter composition which is particularly suited for initiating the combustion of solid rocket propellants such as disclosed in the aforementioned copending application, and thereby insure eificient operation of a rocket motor charged with such a propellant. Other objects and advantages of my invention will become apparent, to those skilled in the art, from the following discussion and appended claims.

Broadly contemplated, the above-mentioned objects are achieved by providing a pelleted igniter composition com prising a plurality of discrete pellets comprising in turn powdered metal, powdered inorganic oxidizing material and a binding agent, ethylcellulose, which retains the powdered materials in intimate contact and which retards the burning rate of the same. The pelleted igniter composition can be placed in a suitable container which is positioned in proximity to the material desired to be ignited, for example, a solid rocket propellant loaded in a rocket motor. A container suitable for this purpose is a retiform cup or basket coated with plastic or rubber, such as that disclosed and claimed in the pending US. application Serial No. 605,904, filed August 23, 1956, by O. D. Ratliff.

According to my invention, these pelleted igniter compositions are manufactured as follows. The binding agent, ethylcellulose, is normally dissolved in a solvent, such as acetone, to produce, for example, a solution of 1 to 10 parts of ethylcellulose per parts of solvent. The powdered ingredients are dry blended and the solution of binding agent is added to the blended powdered pyrotechnic ingredients (metal and oxidizing material) and mixed therewith to form a slurry. The resulting slurried mass is screened. During the mixing and screening part of the solvent is evaporated.

The resulting blend of binding agent and powdered ingredients is then extruded or pressed to form pellets which are then dried to remove excess solvent. Dry air 15 preferably used for drying the pellets and reduced pressure or vacuum treatment can be employed to hasten the removalof the excess solvent.

Though, I prefer to employ a solvent, it is not necessary since the binding agent, ethylcellulose, can be dry blended with the powdered ingredients in a suitable mixer. However, if a solvent is not employed, usually greater pelleting' pressures will be required.

Alternately, the solvent can be added to a blend of the powdered ingredients and ethylcellulose and the resulting mixture worked up into a slurry suitable for extrusion or pressing.

Organic solvents which can be employed include acetone, methylene chloride, chloroform, ethyl alcohol, methylethyl ketone and the like. The particular solvent employed should dissolve the ethylcellulose, spread the binding agent over the metal and oxidizer particles in a satisfactory manner, and should be non-toxic, inex pensive, and easy to evaporate. I igniter compositions generally encompassed by my invention comprise at least one powdered metal and at least one powdered inorganic oxidizing agent, the combination of which is referred herein and in the appended claims as pyrotechnic component or pyrotechnic ingredients, together with ethylcellulose which serves as a binding agent for the powdered pyrotechnic ingredients. Examples of such compositions, useful in the practice of my invention are set forth in the following table. This table illustrates the diversity of igniter compositions in which ethylcellulose can serve as a binding agent. The latitude in the ranges of amounts given for the listed components permits variation in the burning rates and the amount of heat developed.

Table I Ignitcr ingredients Parts by Weight Weight percent Supplemental pressure component Black powder Tetranitrocarbozole Binding agent: ethylcellulose It is evident from the above table that the various powdered metals and oxidizing materials can be employed; their use will be dictated by the particular propellant desired to be ignited as well as economic considerations. These powdered metals generate heat and carry heat to the propellant surfaces as Well as determine to some extent the burning rate of the igniter composition.

The black powder and tetranitrocarbozole act as pressure generators and cause more rapid rupturing of the igniter container. Their incorporation into the igniter composition is not critical but will depend upon the particular container employed, etc. A particularly useful black powder formulation is that sold under the name FFFg, comprising potassium nitrate, charcoal, and sulfur. A useful ethylcellulose compound is that sold under the name Ethocel.

The pyrotechnic ingredients are powdered so as to pass through a fine mesh, e.g., 250 to 325 mesh. The zirconium/nickel component can be a powder alloy in which the respective metals are present in the ratios of 25/75 to 75/25 weight percent. Although the presence of this component is not critical, I have found it to be particularly useful and efficient, and, accordingly, prefer to employ it in my igniter compositions in amounts in the range between 25 and parts by Weight of the pyrotechnic component. Similarly, as oxidizing material, I prefer to employ a mixture of potassium perchlorate and barium nitrate in which 0 to 40 parts by Weight of the former and 0 to 70 parts by Weight of the latter are mixed to provide a mixture which is about 30 to 70 parts by weight of the pyrotechnic component.

As previously mentioned, my invention finds particular applicability in initiating ammonium nitrate-binder solid rocket propellants such as disclosed in the aforementioned copending application, especially when these propellants are used for charging rocket motors employed for assisting takeoff of aircraft. A preferred igniter composition particularly suited for this purpose is set forth in the following table.

Acetone used as solvent in preparation.

As an example of the above igniter composition, the pellets made according to my invention will be 0.24 to 0.26 inch in diameter, 0.08 to 0.095 inch in thickness, and have a density of 0.121 to 0.138 pound cubic inch.

For initiating the combustion of an ammonium nitratebinder solid rocket propellant having an internal-external burning surface and weighing about 60 pounds, about pellets of the above igniter composition can be employed. This amount of igniter material will be suitable to provide a necessary working pressure, for example, 1000 p.s.i.g., in the combustion chamber of the rocket motor during the first milliseconds. Any type of primer, such as an electrical squib, can be employed for firing the igniter material.

The following table is an example of an igniter composition wherein one of the other metals, aluminum, is incorporated to supplement the carrying of heat by the zirconium/ nickel to the propellant burning surfaces.

B No solvent employed in preparation.

In reducing my invention to practice by actually firing rocket motors, equipped with the igniter charges of my invention, such as that described above, the objects of my invention have been achieved. Rocket motors employing my novel igniter material as an igniter charged are characterized by improved and desirable operational performance.

Various modifications of my invention will become apparent, to those skilled in the art, without departing from the spirit and scope of my invention and it is to be understood that the examples set forth herein are merely illustrative of my invention and do not unduly limit my invention.

I claim:

1. As an article of manufacture, a pelleted igniter composition consisting of a mixture of 98 to 85 weight percent of a pyrotechnic component and 2 to 15 weight percent of ethylcellulose which serves as a binder for said pyrotechnic component, said pyrotechnic component consisting of a major amount of powdered potassium perchlorate and a minor amount of a powdered mixture of a 25/ 75 to 75/25 zirconium/ nickel alloy and aluminum.

2. As an article of manufacture, a pelleted igniter composition consisting of a major amount of pyrotechnic component which consists of a mixture of powdered metal comprising zirconium and nickel and powdered inorganic oxidizing material, and a minor amount of ethylcellulose.

3. An article of manufacture according to claim 2 wherein said metal further comprises aluminum, and said zirconium and nickel are present as an alloy.

4. An article of manufacture according to claim 2 wherein said oxidizing material is potassium perchlorate.

5. An article of manufacture according to claim 3 wherein said oxidizing material is potassium perchlorate and barium nitrate.

6. As an article of manufacture, a pelleted igniter composition consisting of a mixture of 98 to 85 weight percent of a pyrotechnic component and 2 to 15 weight percent of ethylcellulose, said pyrotechnic component comprising the following powdered material, in parts by weight:

Zirconium/nickel alloy 25 to 80 Potassium perchlorate Up to 40 Barium nitr Up to 70 Aluminum Up 120 33 Magnesium Up to 47 Titanium Up to 65 8. A method for the manufacture of a pelleted igniter composition, comprising the steps of (1) blending a major amount of a mixture of powdered metal comprising zirconium and nickel with powdered inorganic oxidizing material, (2) mixing the resulting blend with a minor amount of ethylcellulose, and (3) pelleting the resulting mixture.

9. The method according to claim 8 wherein said powdered metal further comprises aluminum, and said zirconium and nickel are present as an alloy.

10. The method according to claim 8 wherein said oxidizing material is potassium perchlorate.

11. The method according to claim 9 wherein said ox idizing material is a mixture of potassium perchlorate and barium nitrate.

12. The method according to claim 8 wherein an organic solvent for said ethylcellulose is added to said resulting blend.

13. The method according to claim 8 wherein said ethylcellulose is dissolved in an organic solvent.

14. The method according to claim 13 where said organic solvent is acetone.

15. A method for the manufacture of a pelleted igniter composition, comprising the steps of (1) dry blending 98 to 85 weight percent of a pyrotechnic component comprising the following powdered ingredients in parts by weight:

Zirconium/nickel alloy 25 to 80 Potassium perchlorate up to 40 Barium nitrate up to 70 the sum parts of said potassium perchlorate and barium nitrate being 30 to 70 parts by Weight of said pytrotechnic component, said zirconium and nickel being present in an amount of at least 25 weight percent of said alloy, (2)

mixing the resulting blend with 2 to 15 weight percent of ethylcellulose, and (3) pelleting the resulting mixture.

16. A method for the manufacture of a pelleted igniter composition comprising the steps of (1) dry blending 98 to 85 weight percent of a pyrotechnic component comprising the following powdered ingredients in parts by weight:

the sum parts of said potassium perchlorate and barium nitrate'being 30 to parts by weight of said pyrotechnic component, said zirconium and nickel each being present in an amount of at least 25 weight percent of said alloy, (2) mixing the resulting blend with 2 to 15 weight percent of a solution of ethylcellulose in an organic solvent, to form a slurry, (3) pelleting the resulting slurry, and (4) drying the resulting pellets to remove the excess of said solvent.

17. The method according to claim 16 wherein said solution is a 1 to 10 percent solution of ethylcellulose in acetone.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,860 Bennett et al. Jan. 4, 1949 2,477,549 Van Loenen July 26, 1949 2,643,611 Ball June 30, 1953 FOREIGN PATENTS 655,585 Great Britain July 25, 1951

Claims (1)

1. AS AN ARTICLE OF MANUFACTURE, A PELLETED IGNITER COMPOSITION CONSISTING OF A MIXTURE OF 9, TO 85 WEIGHT PERCENT OF A PYROTECHNIC COMPONENT AND 2 TO 15 WEIGHT PERCENT OF ETHYLCELLULOSE WHICH SERVES AS A BINDER FOR SAID PYROTECHINIC COMPONENT, SAID PYROTECHNIC COMPONENET CONSISTING OF A MAJOR AMOUNT OF POWDERED POTASSIUM PERCHLORATE AND A MINOR AMOUNT OF A POWDERED MIXTURE OF A 25/75 TO 75/25 ZIRCONIUM/NICKEL ALLOY AND ALUMIUM.