Propellant with very high burning rate acrylate polymer binder and NF plasticizer

Abstract

Composite propellant compositions having NF binders selected from a mixture comprised of the plasticizer 1,2,3-tris[1,2-bis(difluoroamino) ethoxy] propane in combination with a terpolymer constituted of 2,3-bis(difluoramino) propyl acrylate, carboranyl methyl acrylate, and acrylic acid, or a copolymer constituted of ethyl acrylate and acrylic acid with added plasticizer, 1-isopropenyl-carboranyl-acetone, are disclosed which have high impulses, high densities, high burning rates, and which employ conventional ammonium perchlorate oxidizer grinds.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 866,067, filed Sept. 30, 1969, now abandoned.

Description

BACKGROUND OF THE INVENTION

Current high burning rate composite propellants (with burning rates of about 15 inches/second at 10,000 psi) employ fine ammonium perchlorate (AP) (e.g., 2 to 4 microns, diameter) along with derivatives of dicarbaclovododecaborane (carborane) as plasticizers. Carborane plasticizers generally have a high specific volume resulting in a propellant with low density.

Difluoramino compounds belong to a class of compounds which are known to be useful as propellant ingredients for obtaining high density and high impulse. Derivatives of carborane belong to a class of compounds which are known to be effective ballistic modifiers for obtaining high burning rates. However, most derivatives of carborane are not miscible with the standard NF binder constituted of 1,2,3-tris-(1,2-bis-difluoroamino) ethoxy propane (TVOPA) and 2,3-bis(difluoramino) propyl acrylate (NFPA).

Thus, a need exists for a high impulse, high density, high burning rate propellant employing conventional ammonium perchlorate oxidizer grinds and carborane derivatives which are miscible with NF compounds.

The object of this invention is to provide a combination binder having carborane derivatives which are miscible with NF compounds which are also compatible with other propellant ingredients.

Another object is to provide composite propellants having the characteristic features of high impulse, high density, and high burning rates while employing conventional ammonium perchlorate oxidizer grinds.

SUMMARY OF THE INVENTION

One representative propellant of this invention is constituted of a binder comprised of a blend of TVOPA and NFPA-carboranyl methyl acrylate (CMA)-acrylic acid (AA) terpolymer, an inorganic oxidizer of ammonium perchlorate, a curing agent and optionally, aluminum metal fuel. Another representative propellant of this invention is constituted of a binder comprised of ethyl acrylate (EA)-acrylic acid (AA) copolymer (designated EA-AA), and a mixed plasticizer comprised of a blend of TVOPA and 1-isopropenyl-carboranyl-acetone (IPCA), an inorganic oxidizer of ammonium perchlorate, a curing agent, and optionally, aluminum metal fuel.

A number of new compounds used in the propellant formulations of this invention have resulted from research in the fields of NF and carborane chemistry. NF refers to Nitrogen-Fluorine and NF propellants are those propellants containing the difluoramino group NF.sub.2. The new compounds: TVOPA, IPCA, and NFPA, resulting from research efforts in the NF and Carborane Chemistry fields, may be prepared by procedures set forth below.

PREPARATION OF TVOPA

TVOPA, 1,2,3-tris[1,2-bis(difluoramino)ethoxy] propane, is prepared by reacting tris(vinoxy)propane (prepared in accordance with U.S. Pat. No. 2,969,400) with tetrafluorohydrazine. TVOPA contains two high energy difluoramino groups, NF.sub.2, in each of the three vinoxy groups of the starting compound, tris(vinoxy)propane. The reaction of tetrafluorohydrazine with tris(vinoxy)propane to form TVOPA is conducted under pressure in the range of 500 mm of mercury up to about 600 psig and temperature range of about 0.degree. to 120.degree.C. The reaction is conducted in the presence of an inert volatile organic solvent, preferably one that is a suitable solvent for both the TVOPA as well as the reactant. Aromatic and aliphatic hydrocarbons, chlorinated hydrocarbons, ethers and ketones may be employed as the solvent. Typical solvents include diethyl ether, dipropyl ether, pentane, hexane, chloroform, carbon tetrachloride, methylene chloride, benzene, toluene, xylene, and acetone.

For further information about the preparation of TVOPA refer to U.S. Pat. application Ser. No. 363,680, filed Apr. 27, 1964.

PREPARATION OF IPCA

The derivative of carborane, 1-isopropenyl-carborane, is further modified to form 1-isopropenyl-carboranyl-acetone which is referred to as IPCA. Preparative procedure for IPCA is as follows: after flushing the reaction flask and accessory equipment, with dry nitrogen, 50 grams of isopropenylcarborane and 300 milliliters of anhydrous ethyl ether are combined in a three necked flask equipped with two addition funnels, cooling and heating accessory equipment, and accessory equipment for reflux procedure. Butyl lithium (171 milliliters of 1.60 molar) is placed into one addition funnel, and 40 milliliters of propylene oxide is placed in the other one. The flask is cooled to 0.degree.C, and the butyl lithium is added dropwise. After the butyl lithium is added, the flask is heated to reflux and the propylene oxide is added dropwise. After the addition is complete, the flask is refluxed for an additional 1 1/2 hours. Contents of the flask are then poured into a separatory funnel and washed with an equal portion of water. The water layer is drained off and discarded and the ether layer is washed with two 200-ml portions of 10% HCl in water. The acid layers are discarded and the ether layer dried over magnesium sulfate. The ether is removed on a rotary evaporator. The remaining oil is washed into another three-neck flask with 100-ml of ether and treated dropwise with a chromic acid solution of 95.6 grams K.sub.2 Cr.sub.2 O.sub.7, 42.6 ml conc. H.sub.2 SO.sub.4, and 120 ml water after the flask is cooled to 0.degree.C. Throughout the addition the ice bath should remain around the flask as heat is generated in this reaction. After the addition of the chromic acid, the ice bath is removed and the reaction stirred at room temperature over night.

The reaction mixture is poured into a separatory funnel and washed with water. The water layer is discarded. The ether layer is then washed with saturated sodium bicarbonate-water solutions. The water layer is discarded and the ether layer dried over magnesium sulfate. The ether is removed on an evaporator and the remaining oil distilled to give IPCA (1), bp 140.degree.C at 0.025 mm, 35 grams yield. ##SPC1##

Additional characteristics of IPCA are revealed by the NMR spectrum which consists of four absorptions centered at 1.95, 2.18, 3.07, and 5.57 ppm. the 5.57 ppm absorption, due to the vinylic protons, has the gross appearance of a quartet (J = 1 cps) and a singlet superimposed on one another. The 3.07 and 2.18 ppm absorptions, due to the CH.sub.2 and CH.sub.3 of the acetone group are singlets while the 1.95 ppm peak assigned to isopropenyl CH.sub.3 group has the appearance of a doublet (J = 1 cps).

PREPARATION OF 2,3-BIS(DIFLUORAMINO)PROPYL ACRYLATE (NFPA)

To a 200 ml. three-necked flask fitted with mechanical stirrer, condenser, thermometer and dropping funnel (all outlets covered with Drierite drying tubes) is introduced 7.72 parts (0.029 mole) of triphenylphosphine and 100 ml. anhydrous diethyl ether (commercial grade). To this solution is added 11.0 parts (0.029 mole) of a reactant 2,3-bis(difluoramino)propyl-.alpha.,.beta.-dibromopropionate. (The difluoramino groups in the specified reactant are positioned within the alcohol moiety of the compound and paired on vicinal carbon atoms. The reactant is prepared by reacting propenyl-.alpha.,.beta.-dibromopropionate with tetrafluorohydrazine at a temperature of about 50.degree.C to 120.degree.C.) A slightly exothermic heat of reaction (23.degree.C to 27.degree.C) and an immediate precipitate of triphenylphosphine dibromide are noted. The addition is completed within a 10-minute period. The reaction is continued for a period of 24 hours. The solids are removed by filtration, and the ether is removed at reduced pressure on a rotary evaporator. The residue, containing a small quantity of solids, is treated with 100 ml. of pentane and filtered again. The pentane is removed and the residue first examined by gas chromatography and then distilled. The gas chromatogram showed the product to be approximately 95% pure. The product (b.p. 52.degree.C. 1.1 mm. of mercury) is obtained in a yield of 4.22 parts (67%). The infrared spectrum of the product is identical in every respect to that of known 2,3-bis(difluoramino)propyl acrylate. The product gives the following analysis:

Calculated for C.sub.6 H.sub.8 F.sub.4 N.sub.2 O.sub.2 (NFPA): %C, 33.33; %H, 3.70; %F, 35.19; %N, 12.96; Found: %C, 33.25; %HH, 4.42; %F, 35.1; %N, 14.35.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fast burning NF propellants result when a binder comprised of about two parts TVOPA and one part NFPA -- 4% acrylic acid (AA) copolymer modified by adding carboranyl methyl acrylate (CMA) to the copolymer, is utilized in the formulation. The modified binder contains 2 parts TVOPA and one part NFPA/CMA/AA = 71/25/4 terpolymer. The composition, burning rate, density, and calculated specific impulse for the modified and control propellants are listed in Table I below.

Fast burning NF propellants result when IPCA is formulated with TVOPA along with moderate or high amounts of ammonium perchlorate to provide control of the final density and impulse of the propellant composition. Propellant samples based on an ethyl acrylate-acrylic acid (EA-AA) copolymer and a mixed plasticizer comprised of a blend of TVOPA and 1-isopropenyl-carboranyl-acetone (IPCA) and containing only ammonium perchlorate in the solids phase are illustrated in Table II below. The formulations illustrate the trade-off of density and impulse for higher burning rates while utilizing a moderate amount of ammonium perchlorate.

Propellants with high ammonium perchlorate oxidizer concentrations are illustrated in Table III below. Increasing the oxidizer loading usually increases the density of propellant. Addition of aluminum will increase both density and impulse. Table III also illustrates that the density and impulse of a propellant can be increased with only a small loss in burning rate when NF compounds and aluminum are added to a carborane propellant.

TABLE I ______________________________________ BURNING RATES, DENSITIES, AND SPECIFIC IMPULSE FOR NF PROPELLANT CONTAINING CMA MODIFIER Control Modified Propellant Propellant Component Composition A Composition B ______________________________________ NFPA 12.5 9.2 CMA None 3.3 AA 0.5 0.5 TVOPA 26.0 26.0 AP 46.0 46.0 Al 15.0 15.0 Additive: UNOX 221 (dicyclo diepoxy carboxylate curing agent) 1.5 1.5 Density (lb./in.sup.3) 0.0662 0.0652 I.sub.sps (lb./-sec/lbm).sup.a 268.8 266.0 Burning Rate (in/sec) 55 micron oxidizer 20,000 psia 12.8 21.0 15,000 psia 10.0 16.2 10,000 psia 7.0 11.1 5,000 psia 3.8 5.9 Burning Rate (in/sec) 8 micron oxidizer 20,000 psia 14.2 28.5 15,000 psia 11.5 19.9 10,000 psia 8.5 14.2 5,000 psia 5.0 9.2 ______________________________________ .sup.a Calculated impulse, 1000 psia chamber pressure, standard conditions. __________________________________________________________________________ BURNING RATES, DENSITIES, AND SPECIFIC IMPULSES FOR ETHYL ACRYLATE PROPELLANTS CONTAINING IPCA MODIFIER Control Propellant Modified Propellants Component Composition C Composition D Composition E __________________________________________________________________________ EA 6.6 6.6 6.6 AA 0.5 0.5 0.5 TVOPA 42.9 35.75 21.45 IPCA None 7.15 21.45 AP (8 micron) 50.0 50.0 50.0 Additive: UNOX 221 (dicyclo diepoxy carboxylate curing agent) 1.7 1.7 1.7 Density (lb/in.sup.3) 0.0608 0.0590 0.0556 I.degree.sps (lbf-sec/lbm).sup.a 258.7 253.1 249.9 Burning Rate (in/sec) 20,000 psia 11.9 20.3 24.9 15,000 psia 9.7 16.2 18.3 10,000 psia 7.3 11.9 11.3 8,000 psia 6.2 10.0 11.3 __________________________________________________________________________ .sup.a Calculated impulse, 1000 psia chamber pressure, standard conditions. __________________________________________________________________________ BURNING RATES, DENSITIES, AND SPECIFIC IMPULSES FOR ETHYL ACRYLATE PROPELLANTS CONTAINING TVOPA MODIFIER Control Propellant Modified Propellants Component Composition F Composition G Composition H __________________________________________________________________________ EA 2.67 2.67 3.4 AA 0.20 0.20 0.2 TVOPA None 4.89 18.3 IPCA 21.48 16.59 3.1 AP (8 micron) 75.65 75.65 70.0 Al -- -- 5.0 Additive: UNOX 221 (dicyclo diepoxy carboxylate curing agent) 0.5 0.5 0.5 Density (lbm/in.sup.3) 0.0594 0.0607 0.0653 I.degree.sps (lbf-sec/lbm).sup.a 256.9 254.0 263.7 Burning Rate (in/sec) 20,000 psia 24.2 23.2 15,000 psia 20.0 18.9 10,000 psia 14.9 13.8 13.0 8,000 psia 12.8 11.5 11.3 __________________________________________________________________________ .sup.a Calculated impulse, 1000 psia chamber pressure, standard conditions.

The procedure for propellant mixing as conventionally used in the propellant manufacturing art is employed in making the propellant grains of this invention. For example, the propellant formulation ingredients are blended together under vacuum until a homogeneous mixture is obtained and then the propellant grain is formed and cured. The metal powder is usually mixed with the binder or prepolymer [e.g., copolymers (EA-AA) or terpolymer (NFPA/CMA/AA)] and the mass mixed until the metal powder is wetted. The inorganic oxidizing salt and additives are next added, and the mixture is mixed under vacuum until a homogeneous mixture is obtained. The resulting mixture is cast into a suitable mold or rocket motor to form the propellant grain and to cure. The grain is cured for several days at about 60.degree.C until a cured grain is obtained. The curing may vary somewhat, but generally in about 3 days the grain is properly cured.

The inorganic oxidizing salts which are useful in the solid propellant compositions of this invention are those oxygen containing salts which readily give up oxygen. These oxidizing salts include ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids, and mixtures thereof. Representative of the oxidizing salts referred to are ammonium perchlorate, ammonium nitrate, sodium nitrate, potassium perchlorate, potassium chlorate, lithium perchlorate, lithium chlorate, calcium nitrate, calcium chlorate, barium perchlorate, strontium chlorate, and strontium perchlorate. Ammonium perchlorate is preferred since less smoke results from lower solids present in exhaust gases from a rocket motor wherein used. Lower smoke solids is required for military purposes of concealment. Ammonium perchlorate of particle size from the range of about 8 microns to the range of about 55 microns is suitable for use in the propellant compositions of this invention to achieve the burning rates and special properties as disclosed in the Tables listed hereinabove.

Powdered metals, as additive, useful in the propellant compositions of this invention include in addition to aluminum, magnesium, titanium, zirconium, boron, and beryllium. Alloys and mixtures of the aforesaid metals may also be employed. Aluminum metal powder is the preferred metal additive.

The ingredients of the propellants of this invention may be varied to provide propellants having a wide range of high densities, high impulses, and high burning rates. Satisfactory ranges for the ingredients are inorganic oxidizing salts from about 45 to about 76 weight percent, metal additive up to about 20 weight percent, ethyl acrylate from about 2 to about 7 weight percent, acrylic acid from about 0.20 to about 7 weight percent, TVOPA from about 4 to about 36 weight percent, carborane derivatives (e.g., IPCA and CMA) from about 3 to about 22 weight percent, NFPA up to about 10 weight percent, and curing agent in the range of about 1 to 2 parts per 100 parts of the total propellant ingredients. Although, the ingredients are specified in weight percent of the total propellant mix, the binder is conveniently formulated of a mixture of about 2 parts of TVOPA to one part of terpolymer constituted of NFPA/CMA/AA = 71/25/4 in one representative propellant formulation. Other representative propellant formulations based on ethyl acrylate - acrylic acid (EA-AA) copolymer employ a mixed plasticizer comprised of a blend of TVOPA and 1-isopropenyl-carboranyl-acetone (IPCA). The mixed plasticizer varies in TVOPA ratio to IPCA from about 1/1 to about 6/1. IPCA is used at a ratio of about 1 part to about 6 parts for each part of copolymer used.

A number of dicyclo diepoxy carboxylates are suitable for use as the curing agent for the propellants of this invention. One procedure for preparation is given hereinbelow. This type curing agent is also sold under brand names of UNOX 221 and ERL 4221.

The dicyclo diepoxy carboxylate curing agent used in compositions A-H is more extensively identified under UNOX 221 in "The Condensed Chemical Dictionary," Seventh Edition, Copyright 1966 by Reinhold Publishing Corporation. The dicyclo diepoxy carboxylate which is sold under Union Carbide Corporation's trademark UNOX 221 has the following properties: Sp. gr. 1.1750 (20/20.degree.C); b.p. 354.degree.C (760mm); vapor pressure 0.01 mm hg. at 20.degree.C; f.p. -20.degree.C (sets to glass below this temperature); at 20.degree.C, 0.03% by wt. soluble in water; water in, 2.8% by wt; wt/gal 9.76 at 20.degree.C; flash point 400.degree.F.

The disproportionation of two molecules of an aldehyde (usually aromatic) brought about by the action of sodium or potassium hydroxide to yield the corresponding alcohol and acid is known as the Cannizzaro reaction. A review of the Cannizzaro reaction is given by T. A. Geissman, in "Organic Reactions" (R Adams, ed.), Vol. II, p. 94, Wiley, New York, 1944.

One method of synthesis to produce a dicyclo diepoxy carboxylate curing agent of the type sold under trademark UNOX 221 employs a Cannizzaro-type redox reaction which is a reaction between an aldehyde and an alcoholate. The reaction is summarized by the chemical equation which follows the "Procedure for the Synthesis of 4,5-Epoxycyclohexylmethyl 4',5'-epoxycyclohexylcarboxylate" set forth hereinbelow.

PROCEDURE FOR THE SYNTHESIS OF 4,5-EPOXYCYCLOHEXYLMETHYL

4'5'-epoxycyclohexylcarboxylate ##SPC2##

Metallic sodium (3g., 0.13 g-atom) is dissolved by warming for one-half hour in 4,5-epoxycyclohexylmethyl alcohol (82g. 0.65 mole). After the mixture has cooled to room temperature, the solution is added gradually, with thorough mixing, to 4,5-epoxycyclohexaldehyde (529g, 4.3 moles) (which contains less than 1% of 4,5-epoxycyclohexycarboxylic acid). The reaction mixture has a tendency to become warm, but the temperature should be maintained somewhat below 50.degree.-60.degree., by cooling, if necessary. A pasty, gelatinous mass results. After about one-half hour, the temperature of the mixture no longer rises; it is then warmed on a water bath for 1-2 hours with occasional shaking.

The cooled reaction product is treated with water (200 ml.); the layer of oil is separated; washed once with a second portion of water, and subjected to distillation under reduced pressure. The first fraction of the distillate contains recovered epoxycyclohexylmethyl alcohol together with unchanged epoxycyclohexaldehyde, as well as a small quantity of water. The temperature then rises rapidly to the boiling point of the epoxycyclohexylmethyl epoxycyclohexylcarboxylate. The product boils at 184.degree.-185.degree./5mm or in the range of about 350.degree.-360.degree.C/760mm. The yield is 410-420g (75-80% of the theoretical amount).

The resulting product has a viscosity of 350-450 centipoise at 25.degree.C; a specific gravity of 1.16-1.18; its color on the Gardner Color Comparator is less than 1.

The method of synthesis is by a Cannizzaro-type redox reaction, as summarized by the following chemical equation: ##SPC3##

Claims

We claim:

1. A solid propellant composition comprising: a mixture of an inorganic oxidizing salt selected from ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids and mixtures thereof; an NF binder comprised of the plasticizer 1,2,3-tris(1,2-bis-difluoramino) ethoxy propane in combination with a material selected from a terpolymer of 2,3-bis(difluoramino) propyl acrylate, carboranyl methyl acrylate, and acrylic acid, or a copolymer of ethyl acrylate and acrylic acid with the added plasticizer, 1-isopropenylcarboranyl-acetone; and a dicyclo diepoxy carboxylate curing agent.

2. The solid propellant composition of claim 1 and additionally comprising a metal additive selected from the powdered metals consisting of aluminum, magnesium, titanium, zirconium, boron, and beryllium.

3. The solid propellant composition of claim 1 and wherein said inorganic oxidizing salt is ammonium perchlorate present in amount from about 45 to about 76 weight percent; said NF binder is comprised of the plasticizer 1,2,3-tris(1,2-bis-difluoramino) ethoxy propane from about 4 to about 36 weight percent of said propellant composition in combination with a terpolymer of 2,3-bis(difluoramino)-propyl acrylate up to about 10 weight percent of said solid propellant composition, carboranyl methyl acrylate from about 3 to about 22 weight percent of said solid propellant composition, and acrylic acid from about 0.20 to about 7 weight percent of said solid propellant composition; and said curing agent is present in an amount from about 1 to about 2 parts per 100 parts of the total propellant ingredients; and wherein said composition further comprises a metal additive of aluminum in the amount up to about 20 weight percent.

4. The propellant composition of claim 3 wherein said ammonium perchlorate has particle size in the range from about 8 micron to about 55 micron; and wherein said terpolymer is comprised of about 71 parts 2,3-bis(difluoramino) propyl acrylate, of about 25 parts carboranyl methyl acrylate, and of about 4 parts acrylic acid, said terpolymer being present in an amount of about 1 part to about 2 parts of said 1,2,3-tris(1,2,-bis-difluoramino) ethoxy propane.

5. The propellant composition of claim 1 and wherein said inorganic oxidizing salt is ammonium perchlorate present in an amount from about 45 to about 76 weight percent; said NF binder is comprised of 1,2,3-tris(1,2-bis-difluoramino) ethoxy propane from about 4 to about 36 weight percent of said propellant composition, in combination with a copolymer comprised of ethyl acrylate from about 2 to about 7 weight percent of said propellant composition and of acrylic acid from about 0.20 to about 7 weight percent of said propellant composition with added compound 1-isopropenyl-carboranyl-acetone from about 3 to about 22 weight percent of said propellant composition; and said curing agent is present in an amount from about 1 to about 2 parts per 100 parts of the total propellant ingredients.

6. The propellant composition of claim 5 and wherein said ammonium perchlorate has particle size in the range from about 8 micron to about 55 micron, and said 1-isopropenyl-carboranyl-acetone being used in a ratio of from about 1 part to about 6 parts to each part of copolymer used.

7. The propellant composition of claim 6 and wherein said 1-isopropenyl-carboranyl-acetone is used in a ratio with said 1,2,3-tris(1,2-bis-difluoramino) ethoxy propane from about 1/1 to about 1/6; and wherein said composition further comprises a metal additive of aluminum in the amount of about 5 weight percent.