US2936225A - Ammonium nitrate propellant - Google Patents

Description

2,936,225 AMMONIUM NITRATE PROPELLANT I Jack Linsk and Robert W. Todd, Highland, Ind., a'ssignors of Indiana s Claimsn ((31.52-5-1'5) to Standard Oil Company, Chicago, 111., a corporation This invention relates to new and improved compositions for the generation of a gas and more particularly to compositions wherein ammonium nitrate is the oxidizer component. Still more particularly the invention relates to compositions comprising a predominant amount of ammonium nitrate, an oxidizable binder, at least one combustion catalyst and an amine N-oxide to promote the combustion of said compositions in adaptation of the compositions to rocket propulsion.

Ammonium nitrate is widely used as a component of high explosives. However, it is insensitive and cannot be detonated by the local application of heat. When ignited ammonium nitrate does not sustain flame propagation consistently. Normally it is mixed With oxidizable materials such as sulfur, carbon, hydrocarbons, cellulosie materials, etc. to utilize the excess oxygen available in the ammonium nitrate. These mixtures are either insensitive, or even when provided with a catalyst may not burn at a rate sufficient for the service desired.

' When ammonium nitrate is mixed with oxidizable mate rialjsuchasplasticized synthetic' resins to function. as

binder material, and a catalyst, the mixtures can be molded into grains which will burn uniformly. Such grainsmay be used in assist" take-off service for aircraft, for rocket propulsion or missile application'sprovided the burning rate is adjusted to the desired rate States Patent 0 for the particular service. The most commonly used method for increasing the. burning rate of ammonium nitrate compositions is toadd a combustion catalyst. Among. the catalysts which have been used are chromates, iron oxides, dichromates and; the iron cyanide types; While certain of these catalysts find use for combustion control of compositions used in propulsion of missiles and rockets andin assist take-off application, promoters are required to obtain high burning rates. In service where high temperature combustion gases con tact nretalj parts erosion problems may arise and organic tzoinbustiohcatalysts are preferred; It is frequently de- .isi'rabile increase the burning rate produced by such anicca'tal'ystsj. The turbine and compressor in large h ines require auxiliary starting. The gas produced lurid'erfpressureby the combustion of the grains may be used to loperate the turbine offan auxiliary starter. However, lthe'jgas so produced must be substantially nonfeliosive aud tion-corrosive to the blades and bearingsof the tauxiliary turbine.

An object .ofthis invention is an improved ammonium nitrate gas-forming composition comprising ammonium nitrate,.,oxidizable material, a combustion catalyst. and a promoter toincrease the burning rate of said conipositions. Another object is a gas-producing compositioncomprisinga major amount of ammonium nitrate, 1

a combustible binder, a combustion catalyst and a pro- :niot'erffor. increasing the burning. rate.- Still another'objectlof theinventionis an improved catalyst-promoter combination for increasing the combustion rate of a gas- V forming.composition capable of being shapedintograins Patented M 10,.1960

and comprising a predominant amount of ammonium nitrate, minor amounts of oxidizable organic components and said catalyst-promoter combination, the combustion of which will not cause erosion to metal surfaces at high temperatures and pressures. A still further object of the invention is to provide an organic combustion promoter which when present in a composition comprising ammonium nitrate, oxidizable binder and a combustion catalyst increases the combustion rate substantially. Other objects will appear from the description of the invention below.

The gas-producing composition of this invention comprises, on a Weight basis, about 10% to about 25%."dfoxidizable binder material, about 0.5% to about 10% of at least one combustion catalyst selected from the class consisting of Prussian blue, ammonium dichromate, finely divided carbon, magnesium oxide and a sulfurized substituted diphenylamine blue dye having a color index of 956, about 0.5% to about 5% of an amine 'N-oxide promoter selected from the class consisting of pyridine, N-oxide, 4-methyl pyridine N-oxide, N methylmorpholine N-oxide, triisopropanolamine' N-oxide,1triethanolamine N-oxide, Z-diethylaminoethanol N-oxide, triethylamine N-oxide and tetrahydroxyethyl ethylene diamine N-oxide, the remainder of said composition consistingv essentially of ammonium nitrate. Minor amounts-, of other oxidizable organic materials may be added to improve the properties of grains molded from the composition.

The amine N-oxides which are etfective promoters are members of the class of N-oxides of pyridine and homologs of pyridine, N-oxides of aliphatic amines including aliphatic polyamines and N-oxides of heterocyclic amines which may have oxygen as Well as nitrogen in the cyclic nucleus. When used alone as the sole means of promoting the combustion of ammonium nitrate .based' grains the amine l -oxides are ineffective. They are nottrue catalysts but are classed as promoters. Moreover,

not all amine N-oxides are eifeetive promoters. combustion rate of a composition containing Prussian catalysts, broadly defined as Prussian blue, are efiective for the combustion of ammonium nitrate grains containing an oxidizable binder.

either or both of these With magnesium oxide maybe used. Magnesium oxide not only functions as a catalyst but is also an excellent stabilizer against chemical decomposition at elevated temperatures. When used alone the amount of these inorganic catalysts is inthe range of 0.5 to 4.0% by Weight, and these catalysts may be used with carbon. 'Finely divided carbon when .usedis-fin-A cluded in the propellant composition; in amounts within the range of about 0.5 to 4% by Weight, preferably in amounts of 0.5 to about 3.0% by Weight of the finished grain. Highly adsorptive activated carbon such: as Notit and Nuchar make up one class of carbon catalysts. A second general type of carbon forum ingasgeneration compositions are the carbon'- blacks roughly classified as channel blacks and furnace combustion blacks. These are characterized by low ash content and small particle size, 50-5000 A. Finely ground. petroleum coke, obtained as residue in thexpipestilling.of'Mid con i I i Mixtures of ammonium (ii-- chromate and insoluble Prussian blue, or mixtures of nucleus.

tinent heavy residium, which contains less than about 1% ash, may be used. The magnesium oxide in the compositions would be in the form of a very fine powder which would pass through a #200 US. Standard sieve.

The amine N-oxide promoter may also be used in conjunction with sulfurized substituted diphenylamine blue dyes which are catalysts for the combustion of the compositions of this invention. The blue dye has a color index of 956 and usually conttains from to 70% by weight of an alkali metal carbonate preferably from about 10 to about 35% by weight of sodium carbonate. The preferredblue dye which may be used as a catalyst component in the composition is known commercially as Pyrogene Direct Blue RL-OF. It may be prepared by heating 2,4-dinitro-4-hydroxy-diphenylamine or by heat- .ing 2-amino-4-nitro-4-hydroxy diphenylamine with sodiutntetrasulfide in alcohol at a temperature of 135 145 C. under a pressure of about 8 to 10 atmospheres for a period of 3 to 4 hours. Methods of preparing this dye are described in US. 665,726 and in Color Index, Society of Dyers and Colorists, 1st edition, January 1924, under. the specific color index. Examples of commercially available sulfurized substituted diphenylamine blue dyes which are effective catalysts are listed in the Tech. Manual and Yearbook of American Textiles and Colorists, pages 240-1 under heading color index 956, specifically Pyrogene Direct Blue RL-CF and Sulfogene Direct Blue EN. The amount of these organic catalysts in the compositions is in the range of 0.5 to 4.0% by weight of the composition. Total amount of the catalyst components in the compositions of this invention is not more than 10% by weight and usually will not be more than 7% of the weight of the grain.

The binder of the gas-producing propellant composition is a thermoplastic material comprising at least one organic synthetic resin plasticized with at least one oxidizable organic plasticizer. Preferred synthetic resins are the cellulose acetates and cellulose acetate-butyrates and polyvinyl acetate. Cellulose acetate having an acetic acid content of 51 to 57% by weightt of acetic acid may be used. Cellulose acetate-butyrate having a butyric acid content of 16 to 61% by weight and acetic acid content of 7 to 55% by weight is also a suitable synthetic resin for the binder of the composition. Polyvinyl acetate resins suitable for binders are solid homopolymers of relatively high molecular weight, the lowest molecular weight resin preferably having a softening point above about 190 C. Although these synthetic resins which contain relatively high oxygen content are preferred, other synthetic resins maybe used as binder components. Mixtures of the resins may be used. Compatibility with the plasticizer and oxygen content are of primary importance in the choice of specific resins or mixtures thereof as base materials.

Plasticizer components for the 'binder of the ammonium nitrate gas-forming compositions consisting of highly oxygenated organic materialsare preferred, other factors being equal. One or more of a group of nitro substituted compounds are preferred as a part of the plasticizer. This group inclpdes dinitrobenzene, dinitrotoluene, and nitro-substituted aromatic ethers, these preferably containing on the average not more than 2.5 nitro groups per molecule and not more than 2 nitro groups per phenyl The 'phenyl nucleus of the nitro-substituted aromatics ma also contain in addition to nitro groups as substituent groups, methyl, alkoxy, phenoxy, nitrophenoxy and propeneoxy groups. Examples are 2,4-dinitrodiphenyl ether, 2,4-dinitrophenylpropyl ether, 2,4-dinitrophenyl allyl ether, 2,4-dinitroanisole and di-(2,4-dinitro.- phenyl) -polyglycol ether.

It is desirable to use along with the nitro-aromatic com.- pounds at least one non-nitro group containing plasticizer. These co-plasticizers are preferably high in oxygen content. Esters of polyhydric aclohols, polyoarboxylic acids, aliphatic hydroxycarboxylic acids and polyglycols, are

suitable. Examples are diallylphthalate, trialkyl citrates (methyl, ethyl, propyl and butyl), diethyl and dibutyl tartrate, triethylene glycol di-Z-ethylbutyrate, triglycol dioctanoate and polyglycol dioctanoate. These commercial products are described by Buttrey in Plasticizers" (1950), pages 49-50. Asphalt may be added to the binder although such additives have a poor oxygen balance. Asphalt obtained by air blowing of a petroleum residuum, e.g. a Mid-Continent petroleum residuum, to obtain a product of flash point of at least 150 F. (Cleveland Open Cup) and a softening point of 2l5-235 F. is preferred. Generally the binder material contains from about 10 to about 60 parts by weight of the synthetic resin plasticized with to 40 parts by weight of plasticizer. The relative amounts of different plasticizers in a given composition may be varied over a wide range. Thus, the plasticizer may consist entirely of 2,4-dinitrodiphenyl ether or mixtures of this ether with 2,4-dinitrotoluene, or the plasticizer may comprise di-(dinitrophenyl)-polyglycol ether as the sole nitro aromatic plasticizer or the plasticizer may be mixed with other nitro-substituted plasticizers. In general. the non-nitro group-containing plasticizers constitute not more than 70% by weight of the plasticizer components of the binder.

As indicated above the propellant composition comprises a major proportion of ammonium nitrate. The term ammonium nitrate as used in the specification and claims is indicated to mean either commercial grade ammonium nitrate such as conventionally grained ammonium nitrate containing a small amount of impurities which may be coated with a small amount of moistureresisting material such as petrolatum or parafiin, or to mean military grade ammonium nitrate or a mixture of minor amounts (usually less than 10%) of other inorganic nitrates such as sodium nitrate or potassium nitrate with ammonium nitrate. A mixture of finely ground and coarsely ground ammonium nitrate is preferred, the major proportion of the nitrate being finely ground.

The burning characteristics of ammonium nitrate-oxidizable binder are dependent upon the temperature and pressure in the combustion chamber. The burning rate as affected by pressure is defined by the pressure exponent, the numerical value equal to the slope of the curve of the burning rate in inches per second obtained by plotting the burning rate at various pressures, the burning rates being plotted vertically and pressures plotted horizontally. The lower the value of the pressure exponent the less is the tendency to develop excessive pressures and safety disc rupture in firing the grains shaped from the compositions, and the more even and smooth is the burning of the propellant grain. Pressure exponents below about 0.8 are desirable and preferred exponents are below about 0.7. Thus a sustained thrust rather than a detonation is obtained by smooth burning of the grain and a sustained flow of gas from the gas generator is obtained if the pressure exponent of the composition is low.

One method of preparing the compositions is as follows. The binder is prepared by heating the plasticizer material to a temperature of -120 C. with stirring, and adding the resin, and milling until plasticization is complete. Not more than one hour is required for this operation. The ammonium nitrate, promoter and catalyst are then milled with the softened binder at a temperature not in excess of about C. until a homogeneous product is obtained. The material is then molded into desired shapes.

In preparing burning rate strands the composition is molded at 2000 pounds pressure into slabs of about one inch by three-quarters inch cross-sectional dimension. These strips are cut into test strands about 5 inches in length, and the test strands are coated with lacquer grade cellulose acetate to inhibit surface burning. Strand burning rates are determined by burning in a Crawford bomb under varying pressures of nitrogen.

Gas-producing grains are fabricated by molding under and flexible.

Pr s ure of. about zoooto sooo sr. and, at a temper};

a'tute of about 105 C. The size and shape 'of the depend, of course upon-their intended use. The

taining pyridine N-oxide promoter andcontrols contain -s de;

ingonly catalysts components and not amine promoter, are shown in Table I.

Control compositions.

grains provided a hole or opening ex- I tending. lengthwise of the grain to provide an aperture j which maybecircular, starform', cruciform, etc., to afford increased burning "surface. Such "grains may be mounted "onventional case and may be ignited or fired by' elec r cal or. other known means. The combustion of the I grainsmay be restricted with respect toburning' area by coating with a material such as cellulose acetate or asphalt, thus causingfithe grain to burnonly from the unrestricted surfaces." The temperature of the combustion gases produced by burning ofthe grain may be of the order of 1500 to 2500 F. butpreferably not above about 2100 F.

The following examples are illustrative of propellant compositions which maybe used in'the propelling of rockets and in assist take-01f operations as described above.

TEST SERIES A phenyl propyl ether and parts by weight of 2,4-dinitrodiphenyl ether and stirring the-mixture to homogeneity.

To this stirred mixture was then added 40 parts by weight of polyvinyl acetate and stirring was continued until the plastic was homogeneous. The binder was tough To the prepared binder was then added at a temperature of 105 C."finely ground ammonium nitrate containing intimatelymixed therewith the combustion catalyst, and alsointermixed therewith, amine oxide as indicated in Table I. The mixtures were stirred to homogeneity and during the mixing 1.0% based on the total weight of the composition, of a chemical stabilizer,

' m-phenylene diamine, was added to those mixtures which did not contain magnesium oxide catalyst. The compositions were molded into slabs and strands were burned in the bomb: to determine burning rates and pressure rate was obtained using 3% 'Prussian blue, 1% magi-i Referring to Table I the p without promot about 0.18 per second at 100 0 pounds pressure, This tained by the use of catalyst alone nesium oxide and 2% by weight of carbon ascatalyst. (see Test 2). When 2% of the ammonium nitrate was substituted by 2% of added pyridine N-oxide the burning rate was raised to 0.24" per second at 1000 pounds pressure and by increasing the amount of carbon from, I

2% to 3%, still holding the pyridine N-oxide at 2%;by weight concentration in the compositionfthe burning rate was increased to 0.26" per second "at 1000 psi. (see Tests 3 and 4). In a test not shown in the table, increasing the carbon from 2% to 5% in a composition in vother respects like the composition of Test 2 of Table I, the burning rate was increased only from 0.179 to 0.182. Compositions which do not contain conventional inorganic combustion catalysts such as Prussian blue or ammonium dichromate do not burn as rapidly as compositions which contain these catalysts. Thus compositions such as those used in Tests 5, 6 and 9 wherein the catalysts'used were magnesium oxide, carbon or a mixture of these, the burning rates are relatively low. However, inclusion of pyridine N-oxide in such compositions produces marked improvement in the burning rates (Test 7 vs. Test 6) Without changing the pressure exponent.

TEST SERIES B oxide addition and by external cooling when necessary.

After stirring over night at room temperature vunreacted exponents. The results obtained from compositions conamlne and water were removed by vacuum topping. The

Table II Catalyst Components Burnin Binder rate at Pressure Test (Percent) Amine N-oxide of (wt. percent) 1,000 x- Prussian MgO Carbon p.s.i. ponent Blue I 13 3 1 3 Pyridine (3) 0.263. 0.68 13 3 1 2 4-111ethyl pyridine (2) 0. 225 0. 59 13 3 1 3 Triisopropanol (3) 0.251 0.60 13 3 1 8 2- hylaminoethanol 0.265 0. 61 13 3 1 3 0.271 0. 66 13 3 1 3 0.237 0. 65 13 3 1 2 I 0.243 0.61 13 3 1 3 Tetrahydroxyethyl ethylenedlamine (3). 0. 240 0. 55

amine oxide product was not distilled. The pyridine N- oxi'deof Test Series A and B and 4-methylpyridine N oxide were commercial products. The results of burning rate tests of compositions containinga wide range of amine N-oxide are given in Table II.

Referring to Table H the increase in burning rate of compositions containing the amine N-oxides over the burning rate of compositions containing non-promoted catalysts (see' Table I, Test 2) varied from about 25% (Test 12) to about 51% (Test 15) and this increase in burning rate was accomplished without substantial change in the pressure exponent.

Percentage compositions are expressed herein as percentages by weight unless specified otherwise.

Having thus described the invention, we claim: 1. A solid ammonium nitrate-based propellant composition which consists essentially of about 10 to about 25% by weight of an oxidizable binder material consisting essentially' of a resin selected from the class consisting of cellulose acetate, cellulose acetate butyrate, polyvinyl acetate and an oxygenated organic plasticizer therefor, about 0. -to about %f ofat-least one combustion catalyst selected from the class consisting of Prussian blue, am

moniumdichromate,"finely divided carbon, magnesium oxide and a sulfuriied diphenylamide blue dye having a color index of 956, about 0.5% to about 5% of an amine N-oxide combustion promoter selected from the class consisting of pyridine N-oxide, 4-rnethylpyridine N-oxide, N-methylmorpholine N-oxide, triisopropanolamine N- oxide, triethanolamine N-oxide, Z-diethylaminoethanol N-oxide, triethylamine N-oxide, and tetrahydroxyethyl ethylenediamine N-oxide, and ammonium remainder ofsaid composition. I

2'. The composition of claim 1 wherein the amine N- oxide combustion promoter is pyridine N-oxide.

3. The composition of claim 1 wherein the amine N- oxide combustion promoter is Z-diethylaminoethanol N-oxide.

4. The composition of claim 1 wherein the amine N- oxide combustion promoter is triethanolamine N-oxide.

5. The composition of claim 1 wherein the catalyst is finely divided carbon.

6. The composition of claim 1 wherein the catalyst is magnesium oxide.

7-. The composition of claim 1 wherein the catalyist is Prussian blue.

8. A- solid propellant composition consisting essentially .of on a weight basis (1) from about 10 to about 25% of a combustible thermoplastic binder consisting essentially of about 10% to about of polyvinyl acetate plasticized with about 40% to about of a mixture of about 5.0% of 2,4-dinitrophenylpropyl ether, about 15% of 2,4-dinitrodiphenyl ether and about 35% of triethylene glycol di-Z-ethylbutyrate, (2) from about 0.5% to about 4% of Prussian bluqifrom about 0.5 to about4%' of finely divided carbon, from about 0.5 to about 4% of magnesium oxide, (3) about 0.5% to about 5% of pyridine N-oxide and (4) ammonium nitrate as the remainder of said composition.

No references cited.

nitrate as} the

Claims (1)

1. A SOLID AMMONIUM NITRATE-BASED PROPELLANT COMPOSITION WHICH CONSISTS ESSENTIALLY OF ABOUT 10 TO ABOUT 25% BY WEIGHT OF AN OXIDIZABLE BINDER MATERIAL CONSISTING ESSENTIALLY OF A RESIN SELECTED FROM THE CLASS CONSISTING OF CELLULOSE ACETATE, CELLULOSE ACETATE BUTYRATE, POLYVINYL ACETATE AND AN OXYGENATED ORGANIC PLASTICIZER THEREFOR, ABOUT 0.5% TO ABOUT 10% OF AT LEAST ONE COMBUSTION CATALYST SELECTED FROM THE CLASS CONSISTING OF PRUSSIAN BLUE, AMMONIUM DICHROMATE, FINLEY DIVIDEC CARBON, MAGNESIUM OXIDE AND A SULFURIZED DIPHENYLAMIDED BLUE DYE HAVING A COLOR INDEX OF 956, ABOUT 0.5% TO ABOUT 5% OF AN AMINE N-OXIDE COMBUSTION PROMOTER SELECTED FROM THE CLASS CONSISTING OF PYRIMIDINE N-OXIDE, 4-METHYLPYRIDINE N-OXIDE, N-METHYLMORPHOLINE N-OXIDE, TRIISOPROPANOLAMINE NOXIDE, TRIETHANOLAMINE N-OXIDE, 2-DIETHYLAMINOETHANOL N-OXIDE, TRIETHYLAMINE N-OXIDE, AND TETRAHYDROXYETHYL ETHYLENEDIAMINE N-OXIDE, AND AMMONIUM NITRATE AS THE REMAINDER OF SAID COMPOSTION.