US3384516A - Augmenting ignitability of solid propellant grains by coating with a metalloorganic complex - Google Patents

Description

United States Patent 3,384,516 AUGMENTING IGNITABILITY OF SOLID PROIEL- LAN T GRAINS BY COATING WITH A METALLU- ORGANIC COMPLEX David C. Sayles, Huntsville, Ala., assiguor to the United States of America as represented by the Secretary of the Army No Drawing. Filed Aug. 11, 1965, Ser. No. 480,250 4 Claims. (Cl. 149-4) ABSTRACT OF THE DISCLOSURE The ignitability of solid propellants having a polybutadiene-acrylic acid-acrylonitrile polymer binder is augmented by reacting the burning surface of the propellant grain with a metal carbonyl compound. Ignition of the resulting propellant can be achieved with a smaller igniter charge, and heat and pressure requirements are decreased. This treatment also serves to render the burning surface resistant to loss of ignitability upon aging.

The invention described herein may be used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to solid propellants and more particularly to a method of augmenting ignitability of solid propellants.

Solid propellant rocket motors are normally fired by means of a small pyrotechnic igniter charge containing explosive bridge wires in the igniter. Upon application of current, the wires explode, causing the igniter to detonate, producing hot gases and burning solid particles under pressure. The burning surface of the propellant is in turn ignited by contact with the pressurized hot gases and solid particles. In a typical design, the igniter charge is placed at the forward end of a propellant grain having burning surfaces along a internal perforation extending throughout the length of the grain.

One of the problems presented in ignition of solid propellants is selection of an igniter charge which produces sufiicient heat and pressure to ensure reliable ignition without creating an excessive initial pressure surge in the rocket motor. :Many solid propellants require substantial amounts of heat and pressure to initiate sustained burning, and if the igniter charge is too small, such propellants would not be ignited. On the other hand, too large an igniter charge would produce an excessive pressure surge upon detonation, resulting in fracture of the propellant grain or motor case. Improved ignitability of the propellant burning surface would allow use of a smaller igniter charge, and the requirements for withstanding pressure in the design of the propellant grain and motor case would be substantially decreased.

Another problem in ignition of solid propellants is loss of ignitability due to aging. Some types of propellants, for

example, composite propellants having a polybutadiene acrylic acid-acrylonitrile polymer binder, undergo chemical changes at the burning surface after extended storage, resulting in significantly decreased ignitability. The prob ability of failure to ignite becomes high for such propellants after a period of aboue five years, or less under extreme temperature conditions. It is desired to provide a means of retaining a high degree of ignitability in these propellants so that the shelf life of the rocket motors utilizing them can be extended.

It is therefore an object of this invention to provide a method of augmenting the ignitability of solid propellants.

Another object is to provide a method of treating the burning surface of propellants having a polybutadiene- 3,384,516 Patented May 21, 1968 acrylic acid-acrylonitrile polymer binder to enhance the ignitability thereof.

Still another object is to provide a method of rendering the burning surface of a propellant having a polybutadiene-acrylic acid acrylonitrile polymer binder resistant to loss of ignitability upon aging.

Other objects and advantages will be apparent from the following description.

In the present invention the ignitability of solid propellants having a polybutadiene acrylic acid-acrylonitri-le polymer binder is augmented by reacting the burning surface of the propellant with a metal carbonyl compound. Ignition of the resulting propellant can be achieved with a smaller igniter charge, and heat and pressure requirements are decreased. This treatment also serves to render the burning surface resistant to loss of ignitability upon aging.

Although this invention is not to be understood as limited to a particular theory, it is postulated that the metal carbonyl reacts with nitrile groups in the polybutadiene-acrylic acid-acrylonitrile polymer to produce a new and highly reactive metalcontaining complex, which provides a large number of ignition sites on the burning surface.

The present invention is applicable to any propellant composition having nitrile groups available in the binder. Nitrile-containing binders currently employed in solid propellants are primarily polybutadiene-acrylic acidacrylonitrile c-opolymers composed of about 30 to 70 weight percent polybutadiene and the balance acrylic acid and acrylonitrile, the amount of either of the latter two ingredients being 20 to weight percent of the total of these two ingredients.

The remaining components of the propellant are not critical, and any of the previously used propellants having a polybutadiene-acrylic acid-acrylonitrile binder may be treated by this method, One type of propellant employing this binder comprises 12 to 20 weight percent polybutadiene-acrylic acid-acrylonitrile binder, 15 to 20 weight percent finely divided aluminum fuel, 70 to weight percent ammonium perchlorate oxidizer and a minor proportion, 'for example, one to five percent, of additives such as a burning rate catalyst and casting and curing agents.

The metal carbonyl in the present invention may be a carbonyl of any of the metals in Groups VI, VII and VIII of the Periodic Table. These compounds are volatile liquids or solids which may be converted to gaseous form for reacting with the propellant by heating to a relatively low temperature, normally C. to 200 C. The preferred metal carbonyls are nickel tetracarbonyl, Ni(CO) iron tricarbonyl, Fe(CO) and iron pentacarbonyl, Fe(CO) The propellant burning surface is reacted with the metal carbonyl by contacting the surface with the carbonyl in the gaseous state, a gas pressure in excess of atmospheric is required to effect the desired reaction, and the use of an inert gas such as nitrogen mixed with the carbonyl is preferred to obtain a more uniform reaction along the length of the propellant surface. The reaction temperature is not critical, and any temperature high enough to maintain the carbonyl in the gaseous state may be used. In a preferred embodiment a carbonyl-inert gas mixture is heated to 100 C. to 200 C. and introduced into the internal perforation of a propellant grain, the grain being at room temperature. The carbonyl-nitrile reaction is largely confined to the surface layer of the propellant and does not significantly affect the interior of the grain. The reaction is continued until the burning surface is saturated by the carbonyl, a period of l to 48 hours normally being suitable for this purpose. The amount of carbonyl required will depend on the amount of burning surface area,

0 about 0.5 weight percent of the total propellant weight being sutficient for typical propellant grains.

The treated surface obtained in the present process is useful for purposes other than improvement of ignitability. The metal carbonyl-nitrile reaction product is electrically conductive so that the continuity of the propellant can be determined by passage of an electric current through the surface layer. The presence of cracks, which would cause erratic performance of the motor, can be detected by electrical measurements.

This invention is further illustrated by the following example:

EXAMPLE A propellant composition comprising 15 weight percent polybutadiene-acrylic acid-acrylonitrile polymer binder. 10 weight percent aluminum fuel and 75 weight percent ammonium perchlorate oxidizer was cast in a two-inch diameter hydraulic pipe motor to produce a propellant grain having a cylindrical internal perforation. The surface of the internal perforation was contacted was a gaseous nickel carbonyl-nitrogen mixture under super-atmospheric pressure, the mixture being heated to 100 to 200 C. The amount of nickel carbonyl was 0.5 weight percent of the total propellant.

The resulting treated propellant grain and a control untreated grain of the same composition were stored for two years at 100 F. After storage, ignition of the two grains by means of a conventional squibtype igniter was attempted. The treated grain ignited satisfactorily, but the untreated failed to ignite.

The above example is merely illustrative and is not to be understood as limiting the scope of the invention, which is limited only as indicated by the appended claims.

What is claimed is:

It. The method of augmenting the ignitability of a solid propellant grain having a nitrile-containing binder compobent and an oxidizer which comprises reacting said nitrilecontaining binder component at the burning surface of said propellant grain with a gaseous metal carbonyl, said oxidizer being compatible with said metal carbonyl to produce a highly reactive metal-containing complex.

2. The method of treating the burning surface of a solid propellant grain having a polybutadiene-acrylic acidacrylonitrile copolymer as binder component and an oxidizer to augment the ignitability thereof which comprises reacting said binder of said surface with a carbonyl of a metal in Group VI, Group VII or Group VIII of the Periodic Table in the gaseous state under superatmospheric pressure whereby said metal is deposited as a highly reactive metal-containing compiex at a multiplicity of sites on said surface, said oxidizer being compatible with said carbonyl.

3. The method of treating the burning surface of a solid propellant grain having a polybutadiene-acrylic acidacrylonitrile copoiymer as binder component and an oxidizer to augment ignitability of said surface which comprises contacting said surface with gaseous nickel tetracarbonyl, iron tricarbonyl or iron pentacarbonyl under superatmospheric pressure whereby nickel or iron is in corporated as a highly reactive metal-containing complex by reaction of nickel tetracarbonyl, iron tricarbonyl or iron pentacarbonyl with said binder component at a multiplicity of sites on said surface, said oxidizer being compati ble with said carbonyl.

4. The method of claim 3 wherein said carbonyl is heated to a temperature of 100 to 200 C prior to contacting said surface.

References Cited UNITED STATES PATENTS 2.959,00l l1/l960 Porter. 3,069,843 12/1962 Whitsel l492 X 3,070,469 12/1962 Jenkins 1495 BENJAMIN R. PADGETI, Primary Examiner.