US3113057A - Stabilized ammonium nitrate propellant - Google Patents

Description

with a matrix former or binder. sitions must produce gas at a uniform rate; therefore, to

the burning of the ammonium nitrate.

d S P O Umte tates atent, Ice meme, Dec,

other elements in addition to carbon and hydrogen, for 3,113,957 example, as in Thiokol rubber and neoprene. The stoi- STABIUZED AMMDNIUM NITRATE chiometry of the composition is improved, with respect PROPELLANT Walter W. Butcher, South Bend, Ind, assignor to Standand Oil Company, Chicago, Ill., a corporation of Indiana No Drawing. Filed June 30, 196i), Ser. No. 40,161

6 Claims. (Cl. 14919) This invention relates to gas-generating compositions or propellants which are based upon ammonium nitrate as the oxidizer.

the grain there is added a catalyst adapted for promoting When the binder contains a cellulose ester such as cellulose acetate as a W s-major component, the ammonium nitrate composition suffers the disability of instability at elevated temperatures.

The Armed Forces of necessity must store gas-generators and propellants over the entire surface of the earth. It is common for storage buildings in the tropics to reach temperatures of 150 F. In order'to meet these conditions, the Armed Forces require ammonium nitrate propellants to be storage stable at temperatures on the order of 170 F. for a period of one year. After prolonged storage at these temperatures, cellulose ester-containing ammonium nitrate propellants tend to evolve gas; in extreme cases the gas evolution is sufiicient to produce fissures in the grain and even to break the grain into several pieces.

Gas-generating or propellant compositions based upon ammonium nitrate as the oxidizerv basically comprise ammonium nitrate, an oxidizable organic binder material, a catalyst adapted for promoting the burning rate of the composition, and a gas evolution stabilization additive adapted for increasing the storage stability of the composition at elevated atmospheric temperatures. It has now been discovered that salts of nitrilotriacetic acid derived from a nitrogen base are effective to improve the storage stability of such compositions.

The improved stabilizer composition of the invention contains ammonium nitrate as the major component. The ammonium nitrate may be ordinary commercial ammonium nitrate such as is used for fertilizers. This commercial grade material contains a small amount of impurities and the particles are usually coated with moisture resisting material such as paraflin wax. Military grade ammonium nitrate which'is almost chemically'pure is particularly suitable. The ammonium nitrate is preferably in a finely divided particulate form which may be either produced by prilling or by grinding. The ammonium nitrate is the major component of the gas-generator composition and usually the composition will contain between about 55 to 80 percent of ammonium nitrate. (It is to be understood that all percentages set out herein are percent by weight of the total composition.)

In order to permit the shaping of the ammonium nitrate composition into definite configurations, a matrix former or hinder material is present. When ammonium nitrate decomposes, free-oxygen is formed. Advantage of the existance of this free-oxygen istaken, and oxidizable organic materials are used as the binders. These oxidizable organic materials may contain only carbon and hydrogen,

for example, high molecular weight hydrocarbons such as asphalts or residuums, and rubbers, either natural or synthetic. Or, the oxidizable organic material may contain Gas-generator compoto smoke production, by the use of oxygenated organic materials as the binders. The binder or matrix former may be a single compound such as a rubber or asphalt, or it may be a mixture of compounds. The mixtures are particularly suitable when special characteristics are to be imparted to the grain which cannot be obtained by the use of a single compound.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of alkanoic acids containing from 2 to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrate and cellulose propionate; the polyvinyl resins such as polyvinylchloride and polyvinyl acetate are also good bases; styrene acrylonitrile is an example of a copolymer which forms a good base material. In general, the binder contains between about 15% and 45% of the particular polymeric base material.

The plasticizer component of the binder is broadly defined as an oxygenated hydrocarbon. The hydrocarbon base may be aliphatic or aromatic or may contain both forms. The oxygen may be present in the plasticizer in either linkage and/or hydroxyl group and/or carboxyl groups; also the oxygen may be present in inorganic substituents, particularly nitro groups. In general, any plasticizer which is adapted to plasticize the particular polymer may be used in the invention. Exemplary classes of plasticizers which are suitable are set out below.

It is to be understood that these classes are illustrative only and do not limit the types of oxygenated hydrocarbons which may be used to plasticize the polymer.

Di-lower alkyl-phthalates, e.g., dimethyl phthalate, dibutyl phthalate, dioctyl phthalate and dimethyl nitrophthalate.

Nitrobenzenes, e.g., nitrobenzene, dinitrobenzene, nitrotoluene, dinitrotoluene, nitroxylene, and nitrodiphenyl.

Nitrodiphenyl ethers, e.g., nitrodiphenyl ether and 2,4-

dinitrodiphenyl ether.

Tri-lower alkyl-citrates, e.g., triethyl citrate, tributyl citrate and triamyl citrate.

Acyl tri-lower alkyl-citrates where the acyl group contains 2-4 carbon atoms, e.g., acetyl triethyl citrate and acetyl tributyl citrate.

Lower alkylene-glycol oxolates, e.g., diethylene glycol oxoiate and polyethylene glycol (200) oxolate.

Lower alkylene-glycol maleates, e.g., ethylene glycol maleate and bis-(diethylene glycol monoethyl ether) maleate.

Lower alkylene-glycol diglycollates, e.g., ethylene glycol diglycollate and diethylene glycol diglycollate.

Miscellaneous diglycollates, e.g., dibutyl diglycollate, dilmethylalkyl diglycollate and methyl Carbitol diglycolate.

Lower alkyl-phthalyl-lower alkyl-glycollate, e.g., methyl phthalyl ethyl glycollate, ethyl phthalyl ethyl glycollate and butyl phthalyl butyl glycollate.

Di-lower alkyloxy-tetraglycol, e.g., dimethoxy tetra glycol and dibutoxy tetra glycol.

Nitrophenylether of lower alkylene glycols, e.g., dinitrophenyl ether of triethylene glycol and nitrophenyl ether of polypropylene glycol.

Nitrophenoxy alkanols wherein the alkanol portion is derived from a glycol having a molecular weight of not more than about 200. These may be pure compounds or admixed with major component bis(nitrophenoxy) alkane.

A single plasticizer may be used or more usually two or more plasticizers may be used in conjunction. The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used.

The mixture of ammonium nitrate, polymeric base and oxygenated hydrocarbon is essentially as insensitive to shock as is ammonium nitrate itself. It is extremely difficult to get this particular mixture to burn. Smooth burning is attained by the addition of a catalyst to the mixture. This catalyst is distinguished from the well-known sensitizers. For example, nitro starch or nitroglycerine may be added to ammonium nitrate in order to increase its sensitivity to shock and enable it to be more easily detonated for explosive use. Catalysts as a class do not promote sensitivity and are used to cause the ammonium nitrate composition to burn for example like a cigarette. The effectiveness of the catalyst is in general measured by its ability to impart a finite burning rate to a strand of ammonium nitrate composition. The burning rate is specified as inches per second at a given pressure and temperature; usually these burning rates are obtained by a bomb procedure operating at 1000 p.s.i. and about 75 F. temperature.

Many catalysts which promote the burning of ammonium nitrate compositions are known. The inorganic chromium salts form the best known classes of catalysts. The better known members of this class are ammonium chromate, ammonium polychromate, the alkali metal chromates and polychromates, chromic oxide, chromic nitrate, and copper chromite. Ammonium dichromate is the most commonly used chromium salt. Various hydrocarbon amine chromates such as ethylene diamine chromate and piperidine chromate are also excellent chromium catalysts. Certain heavy metal cyanides, particularly those of cobalt, copper, lead, nickel, silver and zinc are effective catalysts. The cyanamides of barium, copper, lead mercury and silver are effective catalysts. The various Prussian blues are excellent catalysts.

In addition to the above primarily inorganic catalysts, various organic catalysts are known. The organic catalysts are particularly useful when it is desired to have combustion products which are gases or vapors and thereby do not erode gas exit orifices. Two catalysts which do not contain any metal components are pyrogene blue (Color Index 95696l) and methylene blue. Particularly suitable catalysts are the alkali metal barbiturates.

The chromium salts and Prussian blue promote the rate of gas evolution of ammonium nitrate compositions containing cellulose esters and therefore the gas evolution prevention additive of the invention is particularly applicable to compositions containing these catalysts.

The catalysts are present in the composition in an amount determined by its use and also by the particular catalyst. In general between about 1 and 12 percent of catalyst is present and more usually between about 2 and 4 percent.

In addition to the basic components, i.e., ammonium nitrate, binder and catalyst, the composition may contain other materials. For example, materials may be present to improve low temperature ignitability, for instance oximes may be present or asphalt may be present. Surfactants may be present in order to improve the coating of the nitrate with the binder and to improve the shape characteristics of the composition. Various burning rate promoters, such as finely divided carbon, which are not catalyst per se, may also be present.

It has been discovered that a composition containing ammonium nitrate, a polymeric base, particularly a cellulose ester, an oxygenated hydrocarbon adapted to plasticize the polymeric base, and a burning rate catalyst may be effectively stabilized against gas evolution during storage at elevated atmospheric pressures by the addition of a salt of nitrilotriacetic acid, the salt being derived by reacting a nitrogen base with the acid. Illustrative examples of suitable nitrogen bases from which a salt may be made are ammonia; ammonium hydroxides; hydrazine; quaternary ammonium hydroxides, such as tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, tetrapropyl ammonium hydroxide, ethyl trimethyl ammonium hydroxide, and phenyl trimethyl ammonium hydroxide; the aliphatic amines, such as the primary, secondary and tertiary amines have methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl and hexyl radicals, e.g., methyl amine, methyl ethyl amine, triisopropyl amine; naphthenic amines, such as cyclohexyl amine (hexahydroaniline); the aromatic amines, such as phenyl amine (aniline) diphenyl amine, toluidine, xylidine, and mesidine; mixed aliphatic-aromatic amines, such as phenyl methyl amine and tolyl ethyl amine, wherein both the aliphatic and aromatic radicals are adapted directly to the nitrogen atom; and also mixed aliphatic-aromatic amines, such as benzyl amine, where the aromatic nucleus is separated from the nitrogen atom by an alkyl group; heterocyclic amines, such as pyridine and quinoline; and aliphatic, naphthenic and aromatic diamines, such as ethylene diamine, hexamethylene diamine and benzidine (4,4'-biphenyl diamine). Ammonium hydroxide and hydrazine are particularly suitable nitrogen bases from which to form a salt of ni-trilotriacetic acid. It is desirable that the salt used give a neutral or somewhat alkaline pH when dissolved in water.

The salts of nitrilotriacetic acid may be prepared by mixing in an aqueous solution the acid with the desired nitrogen base, evaporating the liquid therefrom, and drying the resulting salt. Evaporating and drying temperatures of not higher than about 300 F. are preferred, in order that the salt not decompose. The salts also may be prepared by mixing a nitrogen base with nitrilotriacetic acid in an organic solvent, such as an alcohol, and filtering and drying the resultant precipitant.

The amount of the above-described salt to be used in an ammonium nitrate composition is determined by the instability of the particular composition or by the specific requirements for a particular composition. Increasing the specified storage time or the specified temperature at which the composition must remain stable increases the amount of the salt to be used in the composition. In general, increasing the amount of burning catalyst in the composition requires an increase in the amount of salt necessary to maintain constant gas evolution characteristics. Normally, the amount of salt used will be between about 0.1 and about 10 weight percent, more normally between about 0.5 and 3 percent. Various catalysts affect the gas evolution rate to differing degrees.

The above-described salt may also be used in conjunction with other known gas evolution stabilization additives, such as aromatic amines, e.g., toluene diamine, diamine phenyl amine, naphthalene, and toluene triamine, and/or a morpholine, such as N-phenylmorpholine, or a morpholine having an alkyl-substituted phenyl group, such as N-tolylmorpholine. In compositions which are particularly difiicult to stabilize, use of both an aromatic amine and a phenylmorpholine together with an abovedescribed salt is advantageous. The amounts of these additional gas evolution stabilization compounds to be used range between about 0.1 and percent, advantageously 0.5 to 2 percent.

It is to be noted that the aromatic amine used as a supplemental gas evolution stabilization additive may be the same compound, or of the same structural class of compounds, as the nitrogen base used to prepare the above-described salt, e.g., toluene diamine.

Broadly the composition will contain between about 20 and 35 weight percent of binder when the polymeric base material is a cellulose ester of an alkanoic acid containing 2 to 4 carbon atoms and an oxygenated hydrocarbon plasticizer therefor. A particularly useful composition consists of cellulose acetate, about 6l2%; acetyltriethylcitrate, about 6-l2%; a mixture having two to four parts of dinitrophenoxyethanol to one part of bis(dinitrophenoxy)ethane, about 6l2%; carbon, about 24%; toluene diamine, about /2 N-phenylmorpholine, about /2 an ammonium salt of nitrilotriacetic acid, about 2 4%; and catalyst, about 24%.

TESTS Two ammonium nitrate-type compositions were tested for burning rate, storage stability, and other characteristics required by military specifications.

Each composition was prepared by mixing together for one hour in a laboratory mixer a 300-gram batch having ingredients in the proportions indicated in the table set forth below. The mixing temperature was about 100 C. Lacquer grade commercial cellulose acetate analyzing about 55 percent of acetic acid equivalent was the polymer base. Two plasticizers were used. One plasticizer contained about three parts of dinitrophenoxy ethanol and one part of bis(dinitrophenoxy)ethane, obtained by the reaction of dinitrochlorobenzene and ethylene glycol in the presence of aqueous sodium hydroxide solution. The other plasticizer was acetyl triethyl citrate. Both compositions contained about 3% of sodium barbiturate as a burning rate catalyst, and about /2% each of toluene diamine and N-phenylmorpholine. Composition B contained in addition about three weight percent of an ammonium salt of nitrilotriacetic acid as a gas evolution stabilization additive.

The ammonium salt of nitriloacetic acid used as a gas evolution stabilizer in Composition B was prepared by adding ammonium hydroxide to nitrilotriacetic acid until a pH of 7 was obtained, evaporating the solution to dryness, and drying the resulting salt in an oven at about 250 F. An aqueous solution of the salt had a pH of essentially 7.

After mixing the resulting pasty mass was compression molded into a slab approximately one-half inch in thickness. The slab was subsequently sawed into strips for the burning rate test and broken into smaller pieces for the storage stability test.

The burning rate tests were conducted in a Crawford bomb pressured at 1000 p.s.i.g. and 25 C.

The high temperature storage stability or the compositions was determined in a laboratory test as follows. A small sample, about three grams, of the composition was placed in a vessel connected by tubing to a mercury manometer system which was so arranged that differential readings of the manometer were translatable into volume changes in the system. Since volume change of the composition sample itself can be disregarded, the volume change in the system corresponds to the amount of gaseous decomposition products from the sample. The vessel was inserted into a metal block provided with electrical heating elements and controls which permit the b oc t b a nta ned a a empe ature of 150 C. A

period of 15 minutes was allowed for the sample to come to the temperature of C., at which time the manometer was zeroed. While the sample was maintained at 150 C., the time (induction period) was measured between the zeroing of the manometer and the time when the gas evolution rate reached one cc. per gram per hour. Also the time was measured between the zeroing of the manometer and the time at which a gas evolution rate of five cc. per gram per hour was reached.

The foilowing ta'ole summarizes the ingredients of each composition tested and the results of the burning rate and high temperature stability tests conducted thereon:

Table I Composition N-phenyl morphol Ammonium salt of nitriloacetic ac1:l

Totals Test Results:

Burning Rate, inches/see Pressure Exponent... Induction Period, hours Tine to gas evolution rate of 5 ce./g./hr., hours Thus having described the invention, what is claimed l. A composition consisting essentially of (a) ammonium nitrate as the predominant component, (b) an oxidizable organic binder material wherein said binder material consists essentially of a polymeric base selected from the class consisting of cellulose esters of alkanoic acids containing from two to four carbon atoms, polyvinyl chloride, polyvinyl acetate and styrene-acrylonitrile and an oxygenated hydrocarbon adapted to plasticize said polymer, (0) a catalyst adapted for promoting the burning of said ammonium nitrate, and (d) a salt of nitrilotriacetic acid, said salt being derived from a nitrogen base and being used in an amount at least suflicient to improve the stability of said composition with respect to gas evolution.

2. The composition of claim 1 wherein said salt is an ammonium salt of nitrilotriacetic acid.

3. The composition of claim 1 wherein said salt is derived from hydrocarbon amines having between one and about ten carbon atoms per molecule.

4. The composition of claim 1 wherein said salt is derived from hydrazine.

5. The composition of claim 1 wherein there is included between about one-half to about five weight percent of a compound selected from the class consisting of aromatic hydrocarbon amines, N-phenylmorpholine and mixtures thereof.

6. A composition consisting essentially of (a) ammonium nitrate, (b) cellulose acetate, about 6l2%, (c) acetyl triethyl citrate, about 642%, (d) about 6-12% of an about 2:1 mixture of dinitrophenoxyethanol and bis- (dinitrophenoxy)ethane, (e) carbon, about 2-4%, (1) sodium barbiturate catalyst, about 24%, (g) toluene diamine, about /2 (h) N-phenylmorpholine, about /2%, and (i) an ammonium salt of nitrilotriacetic acid, about 24%,

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Claims (1)

1. A COMPOSITION CONSISTING ESSENTIALLY OF (A) AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT, (B) AN OXIDIZABLE ORGANIC BINDER MATERIAL WHEREIN SAID BINDER MATERIAL CONSISTS ESSENTIALLY OF A POLYMERIC BASE SELECTED FROM THE CLASS CONSISTING OF CELLULOSE ESTERS OF ALKANOIC ACIDS CONTAINING FROM TWO TO FOUR CARBON ATOMS, POLYVINYL CHLORIDE, POLYVINYL ACETATE AND STYRENE-ACRYLONITRILE AND AN OXYGENATED HYDROCARBON ADAPTED TO PLASTICIZE SAID POLYMER, (C) A CATALYST ADAPTED FOR PROMOTING THE BURNING OF SAID AMMONIUM NITRATE, AND (D) A SALT OF NITRILOTRIACETIC ACID, SAID SALT BEING DERIVED FROM A NITROGEN BASE AND BEING USED IN AN AMOUNT AT LEAST SUFFICIENT TO IMPROVE THE STABILITY OF SAID COMPOSITION WITH RESPECT TO GAS EVOLUTION.