US 3498856 A
Description (OCR text may contain errors)
United States Patent 3,498,856 COMBUSTION CATALYST FOR SOLID AMMONIUM NITRATE PROPELLANT GRAIN Samuel W. Harris, Chicago, 11]., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana 7 No Drawing. Filed July 24, 1967, Ser. No; 655,328
Int. Cl. C06d 1/00 US. Cl. 149-19 Claims ABSTRACT OF THE DISCLOSURE Propellant grain containing a major amount of am monium nitrate as the primary oxidizer, polymeric olefin as the primary binder and an' effective amount of combustion catalyst comprising a hydrocarbon having a boiling point of between 150 C. and 500 C. and which is capable of forming a peroxide at a temperature in the range of the combustion of the propellant.
BACKGROUND OF THE INVENTION This invention relates to a new class of propellant compositions more particularly to ammonium nitrate compositions having a polymeric olefin binder and a novel combustion catalyst.
Ammonium nitrate is' classified as a high explosive even though it is extremely insensitiveand cannot be readily ignited by the local application of heat or by a blasting cap. Even when ignited, ammonium nitrate alone does not conistently sustain the propagation of combustion. In order to utilize the excess oxygen available from the decomposition" of ammonium" nitrate, 'oxidizable' materials" are mixed with the ammonium nitrate. Generally, these mixtures of ammoniumnitrate and oxidizable material are more sensitive thanthe ammonium nitrate alone, but the mixtures are also insensitive and slow burning. The utilization of certain binder materials, which provide excellent stability and good ballistic properties, generally create even more insensitive compositions due to thestability. Such a class of binder systems, as for example,
polymericolefins, e.g. polyethylene,-and polyethylene de-- rivatives, are highly'desirable, but incertain. applications, it is necessary to incorporate into the composition a combustion .catalyst'which offers control of the burn rate.
One requirement forasolid propellant suitablefor military use is that it be ballistically stable after long periods of storage at temperatures as high as 160 F. and as low as -'65 F. Anotherrequirement is that the'burning of the composition following storage at either of these extremes, be 'unireiar'ana not erran aiiiarnany"near the burning characteristics of such composition which has not been subjected to these conditions. The proper choice of a combustion catalyst, therefore, requires a consideration of the ability of the catalyst to .aid in comr bustion only 'when called upon to do so. For this reason, it is necessary for the catalyst to be stable at all temperatures common to these storage conditions, but readily active at the initiation of combustion upon ignition.
SUMMARY OF THE INVENTION tion and in the claims is intended to mean either ordinary commercial grade ammonium nitrates such as conventionally grained ammonium nitrate containing a small amount of impurities which may be coated with a small amount of moisture-resisting material such as petrolatum or paratfm, or to mean military grade ammonium nitrate or mixture of minor amounts (usually less than 10%) of other organic or inorganic nitrates such as guanidine nitrate or sodium nitrate or potassium nitrate with the ammonium nitrate in a mixture of finely ground and unground or coarsely ground ammonium nitrate is preferred. The major proportion of the ammonium nitrate should be finely ground, but it is well known in the propellent art that particle size and size distribution within the composition are related to the ballistic properties. The amount of grinding then, will depend upon the desired properties of the propellant.
The polyolefin binder material may be any polymeric olefin known to the art as a binder material for use in ammonium nitrate based solid propellants, for example, polyethylene; Surlyn A type polymers (which is the subject of copending application, S.N. 655,300 filed July 24, 1967); etc. The Surlyn A compositions may be obtained commercially from E. I. du Pont de Nemours and Company.
The combustion catalyst is a hydrocarbon-containing compound having at least one active site capable of forming a peroxide radical at temperatures in the range of the combustion of the composition into which the catalyst is incorporated. Substantially all active sites are reacted at said temperature. However, the catalyst should be stable, i.e. exhibit substantially no peroxide formation, at temperatures to which the grain is subjected during storage. It is preferred that the active site be capable of oxidation to form the peroxide radical. The catalyst, therefore, should undergo substantially complete oxidation to form peroxide at a temperature in the range of approximately 125 C. to 250 C., and substantially no oxidation below about 125 C. The catalyst must have a boiling point in the range of 150 C. to 500 C., in order to provide the desired volatility. It is preferred that the catalyst contain .also contain atoms other than carbon and hydrogen, for
tertiary hydrogen, since these compounds are known to readily form peroxides. Exemplary of the compounds suitable for use in this invention are partially hydrogenated naphthalenes, e.g. tetrahydronaphthalene; paritially hydrogenated anthracenes, e.g. octahydroanthracene;
cumene; substituted cumenes, e.g. phenylcumene; polybutene containing tertiary hydrogen and having a molecular weight above 300, preferably in the range of 300 to 2,000; etc. The compounds suitable in this invention may example, nitrogen and nitrogen-containing groups.
It is also desired that the composition contain a carbon component. The finely divided carbon may be added in amounts up to 10% by weight of the composition, preferably in amounts of 1 to 5% by weightforthe purpose of improving the burning rate of the composition. Highly adsorptive activated carbons such as Norite and Nuchar well known inthe art as activated carbon of vegetable origin, make up one class of efiective burning rate components. A second general class of carbon useful for increasing the burning rate of compositions are the carbon blacks, roughly classified as the channel blacks and contain adsorbed hydrogen and oxygen. Bead type carbon blacks, such as Micronex Beads and Statex Beads, v
are also suitable. A third type of carbon which may be used is finely ground petroleum coke, particularly petroleum coke obtained as residue in the pipe-stilling of midcontinent heavy residuums.
The oxidizer and binder should be present in the composition in essentially stoichiometric amounts, in order to best utilize the excess oxygen obtained from the oxidizer, and an effective amount of catalyst must be present. A major amount of oxidizer should be present. Suitable propellent compositions comprise:
Percent by weight (a) Binder 5 to 40 (b) Catalyst 0.5 to 5 (0) Carbon 0 to (d) Remainder of composition essentially ammonium nitrate.
Preferred compositions are as follow:
Percent by weight (a) Binder to (b) Catalyst 1.5 to 3.5 (0) Carbon 1 to 5 (d) Remainder of composition essentially ammonium nitrate.
Other additives which may be added are present in an amount sufficient to provide effective improvement of the properties for which they are added.
In preparing the compositions of this invention, any procedure known to the art for the preparation of ammonium nitrate grains containing a thermoplastic binder may be used.
With respect to the properties of a solid propellant, the velocity at which a solid propellant is consumed during operation is called the burning rate. It is measured in a direction normal to the propellent surface and is usually expressed in inches per second. The burning rate may be expressed by the following relation, in which the influence of all performance parameters is small compared to the chamber pressure and the initial grain temperature.
The burning rate or velocity of propellent consumption r is usually given in inches per second; the chamber pressure p in pounds per square inch; a and n" are constants. The constant a varies with the initial propellent temperature, and thus the burning rate is a function of the temperature of the grain prior to combustion. The lower the value of n, the less is the detonating character of the decomposition of a gas producing composition and the more even and smooth is the burning rate of the propellent grain. Thus, a sustained thrust rather than a detonation is obtained by smooth burning of the grain.
The temperature sensitivity for different solid propellants is usually expressed as the percentage change of thrust per unit of temperature change. Temperature changes effect the equilibrium pressure and the burning rate. The definitions of the temperature coefficients are given by Sutton, Rocket Propulsion Elements (2nd ed. 1958).
Here 'n' is the temperature sensitivity coefficient of equilibrium pressure at a particular value of K (K is the ratio of the burning surface to the throat area), expressed in percent pressure change per degree temperature change. Mathematically it is defined as the partial derivative of the natural logarithm of the equilibrium chamber pressure p with respect to temperature T. The other temperature sensitivity coeflicient a refers to the change in burning rate r of a solid propellant with respect to temperature T at a particular value of chamber pressure p It is also known as the burning rate temperature coefficient, while 'lr is known as the temperature sensitivity of pressure.
For most propellent applications, as low a temperature coefficient as possible is desirable and even required for engineering design consideration. Lower pressure levels over a given temperature level allows a sizable weight savings for most missile applications.
DESCRIPTION OF PREFERRED EMBODIMENTS Weight Example Formulation Percent Ammonium nitrate Carbon black III Carbon black Surly nit--. Cumene The Surlyn A of the above formulations have a softening point of about C., and contain sodium as the primary metallic ion. All of the above formulations were molded into grains and cut into strands for testing in a Crawford bomb. The results of the tests are shown in the table below.
TABLE Ballistic Properties Percent, F.
Product of Example n ap 1rx r1000 (hr/sec.)
What is claimed is:
1. A propellent composition comprising:
(a) a major amount of ammonium nitrate;
(b) a binder selected from the class consisting of polyethylene, and a polymeric material having a softening point in the range of from 80 C. to C., said polymeric material comprising a crossed-linked thermoplastic resin contaniing:
(i) a major amount of ethylene units; and
(ii) interchain forces developed between carboxyl groups pendant from an alpha olefin chain and metallic ions selected from the group consisting of sodium, potassium, calcium, magnesium, and Zinc; and
(c) from 0.5 to 5.0 weight percent of a combustion catalyst selected from the class consisting of partially hydrogenated naphthalenes, partially hydrogenated anthracenes, cumene, substituted cumenes, polybutenes containing tertiary hydrogen and having a molecular weight greater than 300.
5 6 2. The composition of claim 1 wherein said binder is References Cited present in the range of 5 to 40 weight percent; said catalyst is present in the range of 0.5 to 5.0 weight percent; 7 UNITED STATES PATENTS carbon is present in the range of 0 to 10 weight percent; $147,161 9/ 1964 et a1 14919 and the remainder of said composition is essentially amr 3,305,523 2/1967 Burnslde 14919 X 0 3,338,764 8/1967 Evans 14919 monium nitrate.
3. The com sition of claim 1 wherein said catal st is tetrahydmapggmene y BENJAMIN R. PADGETT, Primary Examiner 4. The composition of claim 1 wherein said catalyst is octahydroanthracene. 10 149 20 5. The composition of claim 1 wherein said catalyst is phenylcumene.