US3018204A - Propellants with improved burning rate - Google Patents

Description

3,918,264 Patented Jan. 23, 1962 3,018,204 PROPELLANTS WITH IMPROVED BURNING RATE Charles C. Bice, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed June 23, 1958, Ser. No. 743,957 8 Claims. (Cl. 149-19) This invention relates to burning rate.

In the preparation of propellants, particularly propellants for use in missiles, maximum performance per unit weight of propellant is desired. One drawback in the acceptance of solid propellants has been a lower burning rate than that obtainable with liquid propellants. The present invention relates to a propellant composition which has an improved burning rate and wherein the performance per unit weight is materially increased as a result of a synergistic effect of ingredients in the propellant.

The following are objects of my invention.

An object of this invention is to provide a propellant having a fast burning rate. A further object of my invention is to improve the performance of propellants. A further object of my invention is to provide a propellant comprising a binder, an oxidant, and which contains boron carbide and activated charcoal.

Other objects and advantages of my invention will be apparent to one skilled in the art upon reading this disclosure.

Broadly, the invention is directed to solid propellants using a binder such as asphalt, various resins, plastics, orrubber. A solid inorganic oxidizing salt is used as the oxidant. The improvement is based upon the discovery that the performance of the propellant can be improved by incorporating, on a weight basis, 0.1 to 30, preferably 0.5 to 10, parts of finely divided boron carbide per 100 parts of binder plus oxidizer and 0.1 to 10 parts of activated charcoal, preferably 0.2 to parts, per part of said boron carbide. The total weight of carbide and charcoal should be less than 110 parts per 100 parts of binder plus oxidizer. The addition of the boron carbide improves the burning rate results in an increase in performance per unit weight when compared to the same propellant without the carbide.

Synthetic rubbers and particularly the polymers prepropellants with improved pared by polymerizing a major amount of 1,3-butadiene with copolymerizable monomers, such as styrene and various vinylpyridines, are frequently preferred polymers for use in preparation of the binder. The polymers of 1,3 butadiene and 2-methyl-S-vinylpyridine, such as the 90/ copolymer have been widely used.

As those skilled in the art will understand, the conjugated dienes which can be employed are, in addition to butadiene, those which contain from 4 to 6, inclusive, carbon atoms per molecule and include 1,3-butadiene, isoprene (2-methyl-1,3-butadiene), piperylene, 2-methyl-1,3- pentadiene, 2,3-dimethyl-l,3-butadiene, and others. Mixtures of conjugated dienes can be used.

The polymerizable heterocyclic nitrogen bases which are applicable in our invention are those of the pyridine and quinoline series which are copolymerizable with a conjugated diene and contain one and only one substituent wherein R is either hydrogen or a methyl group. That is, the substi-tuent is either a vinyl or an alpha-methylvinyl (isopropenyl) group. Of these compounds, the pyridine derivatives are of the greatest interest commercially at present. Various substituted derivatives are also applicable but the total number of carbon atoms in the nuclear substituted groups, such as alkyl groups, in addition to the vinyl or alpha-methylvinyl group, should not be greater than 12 because the polymerization rate decreases somewhat with increasing size of the alkyl group. Compounds where the alkyl substituents are methyl and/or ethyl are available commercially.

These heterocyclic nitrogen bases have the formula where R is selected from the group consistingof hydrogen, alkyl, vinyl, alpha-methylvinyl, alkoxy, halo, hydroxy, cyano, aryloxy, aryl, and combinations of these groups such as haloalkyl, alkylaryl, hydroxyaryl, and the like; one and only one of said groups being selected from the group consisting of vinyl and alpha-methylvinyl; and the total number of carbon atoms in the nuclear substituted groups in addition to the vinyl or alpha-methylvinyl group, being not greater than 12. Examples of such compounds are 2vinylpyridine; 2-vinyl-5rethylpyridine; Z-methyl-S-vinylpyridine; 4-vinylpyridine; 2,3,4-trimethyl- 5-vinylpyridine; 3,4,5,6 tetramethyl-Z-vinylpynidine; 3- ethyl-S-VinyIpyridine; 2,6 diethyl-4-vinylpyridine; 2-isopropyl-4-nonyl-5-vinylpyridine; Z-methyl 5 undecyl-3- vinylpyridine; 2,4 dimethyl-5,6-dipentyl-3-vinylpyridine; Z-decyl-S- (alpha-methylvinyl pyridine; 2-vinyl-3-methyl 5 ethylpyridine; 2 methoxy 4 chloro 6 vinyl pyridine; 3-vinyl-5-ethoxypyridine; 2-vinyl-4,5-dichloropyridine; 2- alpha-methylvinyl -4-hydroxyl6-cyanopyridine; 2-vinyl-4-phenoxy 5 methylpyridine; 2-cyano-5- (alpha-methylvinyl)pyridine; 3-vinyl-5-phenylpyridine; 2- (para-methylphenyl)-3-vinyl-4-methylpyridine; 3-vinyl-5- (hydroxyphenyl)-pyridine; 2-vinylquinoline; 2-vinyl-4- ethylquinoline; 3-vinyl-6,7-di-n-propylquinoline; 2-methyl- 4 nonyl 6 vinylquinoline; 4-(alpha-methylvinyl)-8-' dodecylquinoline; 3-vinylisoquinoline; 1,6-dimethyl-3- vinylisoquinoline; 2 vinyl 4-benzylquinoline; 3-vinyl-5- chloroethylquinoline; 3-vinyL5,6-dichloroisoquinoline; 2- vinyl-6-ethoxy-7-methylquin0line; 3-vinyl 6 hydroxymethylisoquinoline; and the like. Mixtures can be used. The copolymers are prepared by polymerizing a major amount of the diene with a minor amount of the heterocyclic nitrogen base by any suitable method. Preferably, the ultimate product has a Mooney viscosity (ML-4) of 10 to 40 although higher Mooney polymers are frequently made and then mixed with extender oils. t

In the preparation of propellants in accordance with this invention a suitable binder can be prepared using these rubbery polymers in conjunction with well-known rubber compounding ingredients. Carbon black is commonly used as a rubber reinforcing agent and as a fuel in amounts to 100 parts and more for each 100 parts of rubber. Stabilizers, softeners, sulfur or other curative systems are mixed with the polymer to give binder compositions which can be vulcanized by heating. Vulcanization is effected after admixing the binder with the oxidizer and other propellant ingredients by heating, commonly to temperatures in the range of 150 to 300 F. for l to 48 hours. Sulfur cure is not required and other methods can be used. Cure by quaternization is one such sulfur-free cure. This method is disclosed in Reynolds and P-nitchard application Serial No. 284,447, filed April 25, 1952.

In one method of preparing the propellant, a solvent mix technique is used. In this process the binder is dispersed in a solvent such as, n-hexane, cyclohexane, methylcyclohexane, and the like. The oxidizers and other propellant components are mixed with the solvent-binder mixture. When a uniform blend has been obtained the solvent is removed and the propellant formed by molding. Conventional milling procedures can also be used.

By boron carbide I mean compositions represented by the formula BxCy where the ratio x/ y varies between 1.0 and 6.0. The preparation and properties of these boron carbides have been reviewed by Glaser and Moskowitz, Ioumal of Applied Physics, 24, No. 6, pages 731-3 (June 1953). These compounds are described as solid solutions of varying amounts of carbon in a slightly distorted boron lattice. The values for the ratio of x/y need not therefore be a whole number within the range of 1.0 to 6.0. Examples of compounds which are useful in the practice of this invention can be represented as BC, B C, B C, B C, B C. Mixtures of these compounds can also be utilized. Carbides having a particle size less than about 200 microns, preferably less than 100 microns, are preferred.

The'preferred activated charcoal for use in the practice of this invention is material designated as decolorizing or deodorizing charcoal or carbon. Suitable activated charcoals are prepared from various carbonaceous products including wood, sawdust, peat, fruit pits, lignite, pulpmill waste, coconut shells, blood, bone and coal. These products are carbonized in the absence of air at temperatures below about 600 C. Carbonization is also effected in the presence of steam or carbon dioxide at temperatures of 800 to 900 C. The carbonized product is preferably ground to a size less than about 400 microns.

The oxidizers which are used in the preparation of the propellant include ammonium nitrate, potassium nitrate, ammonium perchlorate, potassium perchlorate and other salts of nitric and perchloric acid, These oxidizers can be used alone or. in admixture. Mixtures of these o-xidizerswith other modifiers can be desirable. Thus, I can use in admixture with the oxidizer; metallic oxides, such as PbO metallic nitrates, such as Ba(NO metallic chromates and bichromates, such as (NI-10 G KgCr o and K CrO metal powders, such as aluminum, magnesium, iron and boron.

One particularly suitable oxidizer is phase stabilized ammonium nitrate. Phase stabilized ammonium nitrate can be prepared by admixing the ammonium nitrate with potassium nitrate, and 1) cocrystallizing the nitrates from water solution; (2) heating a moist mixture at 50 C.; and (3) by fusing a physical mixture of the nitrates. Commonly, the phase stabilized ammonium nitrate is prepared with about 5 to about 15 percent by weight potassium nitrate.

For the'practiceof my invention I prefer to mix the boron carbide and the activated charcoal carbon with the oxidizer. This mixture is then blended with the binder to achieve 'a uniform mixture. Mixing can be effected conveniently in an internal mixer, such as the Baker-Perkins and the Banbury mixer and on a roll mill.

In the process of making the propellant the binder is commonly used in amounts from about 3 to about 25 parts, to correspondingly 97 to 75 parts of oxidizer or mixtures of oxidizers, exclusive of other modifiers. For each 100 parts by weight of binder pilus oxidizer, I prefer to use 0.5 to 10 parts by Weight of boron carbide although smaller and greater amounts can give beneficial effects. For example, noticeable improvement is observed with as little as 0.1 part of the carbide. Amounts greater than 10 parts, for example 20 to 30 parts, are sometimes less effective per unit weight. For each part of boron carbide, I prefer to use at least 0.1 part to 10 parts, more preferably 0.2 to 5 parts, of activated charcoal. The total weight of carbide plus charcoal is kept below parts per 100 parts of binder plus oxidizer. The activated charcoal with the boron carbide permits formulation of propellants having enhanced burning rates. The significance of this invention is illustrated by the following example.

Example Propellant compositions were mixed and burning rate strands were prepared according to the procedure given below. For the binder, a rubbery 90/10 copolymer of 1,3-butadiene/2-methyl-5-vinylpyridine was prepared, blended with carbon black and stabilizer. This binder was mixed with the oxidizer (phase stabilized ammonium nitrate), cathalyst (ammonium dichromate), and other additives (boron carbide and activated charcoal) in the process of preparing the propellant strands. Details of the process are given below. p v

The rubbery polymer was prepared by emulsion poly merization at 41 F. of 90 parts 1,3-butadiene and 10 parts of 2 methyl-5- vinylpyridine. The rubbery polymer was latex masterbatched with furnace carbon black and treated with antioxidant.

composition: I

Parts by weight. 90/10 copolymer 100 Carbon black (Philblack A) 22 Antioxidant (Flexamine) 3 The oxidizer was prepared by blending 90 parts by weight of ammonium nitratewith 10 parts by weight of potassium nitrate. A small amount (about 0.2 part per 100 parts of the oxidizers) of water was added and the mixture was blended. The product was then dried in'air at a temperature of about l60 F. and ground to a particle size less than about 150 microns. The weight average particle size was about 50 to 60 microns. oxidizer was mixed with other dry ingredients.

The boron carbide, B C, was a commercial product substantially less than about 44 microns (325 mesh) in size. having a weight average particle size of about 18 microns. The activated charcoal, Norit- A, was a' dry, powdery product.

The propellant was formed by the solvent mix technique. By this method the rubbery binder was first slurried in diameter, were cut from the slabs. The burning" rate was measured at a temperature of 70 F. by the Crawford bomb test. In this test the strands are burned in a nitrogen atmosphere and the rate of burning in inches per second is measured. The strand burning rate at a given pressure is obtained from agraph of the results obtained at several pressures.

The resulting binder had the The The ammonium dichromate wa a groundproduct The compositions which were prepared and their strand burning rates are set forth below.

Composition 1 shows that in the absence of either the activated charcoal or boron carbide, the strand burning rate is 0.217 in./sec. As shown by compositions 2 and 3, the charcoal or carbide alone increases the burning rate by 0.040 in./sec. to a value of 0.257 in./sec. With the combination of activated charcoal and boron carbide, as with composition 4, the burning rate is 0.362 in./sec. If one plots the burning rate versus parts by weight of activated charcoal in the propellant for runs 1 and 2, the burning rate with 3 parts of charcoal can be obtained by extrapolation. This value is 0.273 in./sec., as compared with the value of 0.362 actually found for composition 4. Thus, the high rate for this run is attributed to a synergistic effect of the activated charcoal and boron carbide.

In addition to the Norit A of the example, other activated charcoals are suitable, Commercially available at the present time are Nuchar and Darco.

As many possible embodiments can be made of this invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative and not as unduly limiting the invention.

I claim:

1. A propellant composition containing the following in parts by weight Binder 3-25 Solid inorganic oxidizing salt 97-75 Binder 3-25 Solid inorganic oxidizing salt 97-75 wherein said binder is selected from the group consisting of asphalt and rubber, the amount of binder and oxidizing salt being sufficient to total parts by weight, 0.5 to 10 parts by Weight of boron carbide per 100 parts by Weight of binder plus oxidizing salt, and 0.2 to 5 parts by weight of activated charcoal per part by Weight of boron carbide, the total Weight of boron carbide plus charcoal not exceeding parts by Weight per 100 parts by Weight of binder plus oxidizing salt.

3. The propellant of claim 1 wherein said binder is a rubbery copolymer of a conjugated diene containing 4 to 6 carbon atoms and styrene.

4. The propellant of claim 1 wherein said binder is a rubbery polymer of butadiene and styrene.

5. The propellant of claim 1 wherein said binder is a rubbery copolymer of a conjugated diene of 4 to 6 carbon atoms and a compound selected from the group consisting of pyridine and quinoline containing a wherein said binder has the following composition Parts by weight Butadiene/2-methyl-5-vinylpyridine copolymer 100 Furnace black 22 Antioxidant 3 References Cited in the file of this patent UNITED STATES PATENTS Moore et al May 14, 1957 Fox Mar. 17, 1959 OTHER REFERENCES Chem. and Eng. News, Oct. 7, 1957, pages 62-63,

Claims (1)

1. A PROPELLANT COMPOSITION CONTAINING THE FOLLOWING IN PARTS BY WEIGHT BINDER 3-25 SOLID INORGANIC OXIDIZING SALT 97-75 WHEREIN SAID BINDER IS SELECTED FROM THE GROUP CONSISTING OF ASPHALT AND RUBBER, THE AMOUNT OF BINDER AND OXIDIZING SALT BEING SUFFICIENT TO TOTAL 100 PARTS BY WEIGHT, 0.1 TO 30 PARTS BY WEIGHT OF BORON CARBIDE PER 100 PARTS BY WEIGHT OF BINDER PLUS OXIDIZING SALT, AND 0.1 TO 10 PARTS BY WEIGHT OF ACTIVATED CHARCOAL PER PART BY WEIGHT OF BORON CARBIDE, THE TOTAL WEIGHT OF BORON CARBIDE PLUS CHARCOAL NOT EXCEEDING 110 PARTS BY WEIGHT PER 100 PARTS BY WEIGHT OF BINDER PLUS OXIDIZING SALT.