US3177101A - Carboxyl-terminated linear polyester gas-generating composition and method of preparaion - Google Patents

Description

United States Patent ()filice 3,1711% Patented Apr. 6, 1965 3,177,101 CARBUXYL-TEATED LINEAR PGLYESTER GAS-GENERATING CQMPGSITKQN AND METH- D Oi PREPARATIQN Calvin W. Vn'esen, Erookside, Del., assignor to Thiokol Chemical Corporation, Bristol, Pa., a corporation of Delaware No Drawing. Filed July 2, 1962, Ser. No. 207,963 9 Claims. (Cl. 149-19) This invention relates to gas-generating compositions of the type used in auxiliary power sources to generate gases 7 under pressure for operating a variety of pressure fluid actuated devices.

In the aeronautical and astronautical fields lightweight, gas-generating auxiliary power sources are extensively used for a wide variety of applications, particularly those applications wherein a rather substantial amount of power of relatively shortv duration is required to actuate some mechanical device. For example, such gas generators are used as starters for jet engines, hydraulic pumps and alternators in electrical power systems, as well as starters for the large turbo-pump systems of liquid propellant rockets. They are also used as a power source for small reciprocating piston pumps for hydraulic systems. In some cases the generated gas under pressure is applied directly to a liquid to be moved as for example in the pressurization of the fuel tanks of a liquid propellant rocket system.

While the compositions used in such gas generators are functionally akin to the materials used as ballistic and rocket propellants, they differ in important respects from the compositions commonly used as propellants. Thus gas-generating compositions have a substantially lower burning rate than conventional propellants, just as propellants have a substantially lower burning rate than explosives. .Also gas-generating compositions should have relatively low flame temperatures and should be essentially nonerosive and noncorrosive to prevent damage to valves, turbine blades and the like with which they may come in contact. For such applications as jet engine starters, it is important that the composition burn substantially completely to produce an essentially smoke-free gaseous product.

Like conventional rocket propellants, gas-generating compositions must remain ballistically stable over a relatively Wide temperature range and must be shock-resistant at both high and low temperatures. Also they must be ignitable at temperatures as low as 70 to 75 F. In general, both types of compositions should have substantial elasticity to provide the desired stability during storage and use.

Like conventional rocket propellants, gas-generating compositions usually are essentially composed of a major portion of inorganic oxidizer and minor portion of organic fuel binder. However, in gas-generating compositions it is customary to use ammonium nitrate as the oxidizer rather than a more energetic oxidizer such as ammonium perchlorate in order to achieve the desired greater stability in storage and low flame temperature during combustion. Also Whereas in a conventional rocket propellant the oxygen content of the fuel binder is kept to a minimum in order to maximize the energy developed during combustion of the propellant, binders employed in gas-generating compositions usually contain substantial amounts of oxygen to reduce the flame temperature and to insure substantially smoke-free combustion.

It has previously been suggested that polyester resins be used as fuel binders in gas-generating compositions. Such polyester resins have long been used for general purpose molding applications. They are formed by condensing pclyhydric alcohols with unsaturated polybasic acids to form linear polyesters having unsaturated group-s therein. The unsaturated groups of the polyesters are then reacted with a compound having vinyllic unsaturation, e.g., styrene, in the presence of a peroxide catalyst to form a solid res'm. In making gas-generating compositions, the liquid unsaturated polyester has been mixed with the oxidizer, a quantity of styrene, a peroxide catalyst, and a plasticizer. The vinyl group of the styrene reacts with the unsaturated groups of the polyester to produce a solid binder. Compositions of this type are disclosed for example in Marti Patent No. 3,013,871. As shown in the Marti patent, substantial quantities of plasticizer must be employed in such compositions, since the polyester resin is a relatively rigid material and in the absence of plasticizer does not yield the desired physical properties.

The use of a plasticized polyester resin a the fuel binder of a gas-generating composition is rather unsatisfactory because of the tendency of the plasticizer to bleed out of the resin binder. Hence there is a need in the gas generator art for a fuel-binder which contains the necessary amount of oxygen to insure complete combustion and which will provide the desired physical properties without the addition of a separate plasticizer. It is an object of the. present invention to provide a gasgenerating composition incorporating such a fuel-binder and a method of making such a gas generator composition. The present invention is predicated on the finding that an improved gas generator composition can be made by employing as the fuel binder thereof a carboxyl-terminated linear polyester that is cured through the carboxyl terminal thereof. In accordance with the method of the present invention, a liquid polyester having a molecular weight of 500 to 5,000 is prepared by condensation of a polycarboxylic acid and a polyhydric alcohol to produce a liquid carboxyl-termina-ted copolymer. The liquid copolymer is mixed with ammonium nitrate powder and a curing agent for the copoly-mer selected from the group consisting of polyimines and polyepoxides. Upon heating to a moderately elevated temperature, say to F., the polyester cures to a solid binder. The resulting compositions upon ignition burn relatively slowly and with a low flame temperature to give essentially smoke-free combustion products. The cured polyesters have substantial elasticity and thus confer good shockresistance and low temperature properties on thegas generator composition, even though no separate plasticizer is incorporated therein. I 1

Polyhydric alcohols that can be'use d in preparing the linear polyesters include ethylene glycol, diethylene glycol, propylene glycol, polypropylene lycol, butanediol,

polybutylene glycols, glycerol, trimethylolpropane, 1,2,6- hexane triol, castor oil and mixtures of these polyols. Suitable polybasic acids for reacting with the foregoing alcohols to form the linear polyesters include oxalic, adipic, sebac-ic, phthalic, isophth-alic, terephthalic, malonic, succinic, maleic, furnariqdimer and itaconic acids and mixtures of these acids. The alcohols and acids having more than two functional groups can be used in varying amounts in the reaction mixture to produce varying amounts of cross-linking. Polyesters prepared by reacting a molar excess of adipic acid with diethylene glycol to form a carboxyl-terminated linear polyester have been found especially satisfactory. Since the linear polyesters are well known in the art, it-is deemed unnecessary to describe in detail methods for making them.

In general, the present compositions are prepared by mixing ammonium nitrate in powder form with the carboxyl-terminated polyester and a suitable curing agent therefor to form a fluid composition that can be cast at atmospheric temperature in a mold of the desired configuration, and then heating to a temperature of 100 to 150 F. to cure the polyester polymer to form a solid grain of the gas generator composition. While any of the curing agents previously employed for the curing of carboxyl-terminated polyesters may be used, good results have been obtained with compounds having two or more heterocyclic groups that are reactive with carboxyl groups, e.g., polyepoxides and polyimines. The diglycidyl ether of bis-phenol A sold under the name Epirez 510 is typical of the commercial polyepoxides that may be used, and the alkyleneimine tris[1-(2-methyl) aziridinyl]phosphine oxide sold under the trade name MAPO is typical of the commercial imine curing agents that can be employed in making the present compounds. Other curing agents include the diglycidyl ether of hisphenol F [diglycidyl ether of bis-(4-hydroxy phenyl) methane]; tris [I-(Z-methyl) aziridinyl] phosphine sulfide; N,N' hexamethylene-bis-l-aziridinecarboxamide; 1,3,5-[fi-(2-methyl).aziridinyl]propionoxy pentane; 1,2- [6 (2- methyl)aziridinyflpropionoxy ethane; N,N'- (methylphenylene) bis-l-aziridinecarboxamine; 1,1'-(sulfonyldiethylene) bis-2-methylaziridine; and 1,1'-terephthaloyl-bis-aziridine. Curing is eifected by reaction of the heterocyclic groups of these curing agents with the carboxyl terminals of the polyester.

As indicated by the specific examples given below, the present compositions may contain various minor constituents in addition to the ammonium nitrate, polyester and curing agent. Thus the reaction mixture prior to curing may contain any of various known curing catalysts such as for example 4,4'- methylene bis(2-chloroaniline); 4,4 methylene dianiline; m-phenylene diamine; diamino toluenes; triethylene .tetramine; tetraethylene pentamine; diethylene triamine; tri(dimethylaminomethyl) phenol and mixtures of these amines. Also it has been found desirable to use a minor proportion of a combustion catalyst to improve the reliability of ignition of the composition and to insure continued combustion after ignition. It has been found that a small amount of b-arbituric acid in the composition is especially useful for this purpose.

In preparing the present compositions, a relatively large proportion of the inorganic oxidizer, in this case ammonium nitrate, should be employed. More particularly, the cured composition desirably contains from 50% to 80% by weight of ammonium nitrate powder and 20% to 50% by Weight of the cured binder and minor constituents. In the fluid mixture prior to curing, the liquid polyester component may comprise from 15% to 45% by weight and the curing agent from, say 3% to 10% by weight. The combustion catalyst, e.g., barbituric acid, may comprise from 0.5% to 5% by weight of the mixture.

In order to point out more fully the nature of the pres ent invention, the following specific examples are given of procedures for making the present compositions, and certain of the properties of the cured compositions are set forth.

Example 1 The polyester used in this example was prepared by condensing 9.4 mols of adipic acid with 8.94 mols of diethylene glycol. The resulting polyester polymer had an acid number of 60 and a viscosity of 90 poises at 80 F.

A vertical planetary mixer was charged with 17.35 parts of this polyester, 8.65 parts of the diglycidyl ether of bisphenol A (Epi-rez 510), 1 part of 4,4 methylene dianiline, and 1 part of barbituric acid. These ingredients were mixed for minutes. Thereafter 43.2 parts of ammonium nitrate (3600 r.p.m. grind) and 28.8 parts of ammonium nitrate (16,800 r.p.m. grind) were added, and mixing continued for an additional period of one hour.

One portion of this mixture was cured at F. for 96 hours and then sawed and stamped into dumbbellshaped specimens. Thereafter the specimens were subjected to a tensile test and exhibited a tensile strength of 33 p.s.i. and an elongation of 42%. The modulus of the material was 112 p.s.i.

Another portion of the mixture was cured in a gas generator motor, and the burning rate after cure was determined to be 0.054 in./sec. at 1000 p.s.i.a. with a burning rate exponent of 0.45.

The density of a sample of the cured material was determined to be 0.056 lb./in.

Example 2 The procedure of Example 1 was followed except that the proportions were changed as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 45.00 Ammonium nitrate (16,800 r.p.m. grind) 30.00 Polyester prepolymer 15.86 Polyepoxide 7.14 4,4 methylene dianiline 1.00 Barbituric acid 1.00

Samples of this formulation were cured as in Example 1 and exhibited the following tensile properties: tensile strength-65 p.s.i., e1ong-ation-16%, modulus 580 p.s.i. The burning rate was determined by burning strands of the cured material in a Crawford Strand Burner and was found to be 0.04 in./sec. at 1000 p.s.i.a. with a burning rate exponent of 0.59.

Example 3 The procedure of Example 1 was followed except that the sample was cured 40 hours at 150 F. and the proportions of ingredients were changed as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 42.60 Ammonium nitrate (16800 r.p.m. grind) 28.40 Polyester prepolymer 20.80 MAPO 4.02 N,N'-hexamethylene-bis-1-aziridine-carboxamide 2.18 Barbituric acid 2.00

Tensile results obtained with cured samples were: tensile strength-46 p.s.i., elongation23%, modulus129 p.s.i. The burning rate, determined on material cast and cured in a generator, was found to be 0.04 in./sec., at 1000 p.s.i.a., and the burning rate exponent was found to be 0.55.

A flame temperature of 1975 F. was measured during the burning test by means of a thermocouple.

Example 4 The procedure of Example 1 was followed except that samples were cured for 40 hours at 150 F. and the proportions of ingredients were changed as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 31.95 Ammonium nitrate (16800 r.p.m. grind) 39.05 Polyester prepolymer 22.94 MAPO 1.63 1,1'-(su1fonyldiethylene)bis-2-methyl-azi1idine 2.43 Barbituric acid 2.00

Tensile properties of cured specimens were found to be: tensile strength-61 p.s.i., elongation4l%, modulus-220 p.s.i. The burning rate, determined on material cast and cured in a generator, was found to be 0.035 in./sec. at 1000 p.s.i.a. with an exponent of 0.55.

It is of course to be understood that the foregoing examples are intended to be illustrative only and that numerous changes can be made in the ingredients, proportions and conditions set forth therein without departing from the spirit of the present invention as defined in the annexed claims.

I claim:

1. A gas-generating composition essentially composed of ammonium nitrate powder and a cured carboxylterminated essentially linear polyester binder, said linear polyester prior to curing having a molecular weight of 500 to 5000 and being the condensation product of a polycarboxylic acid and a polyhydric alcohol.

2. A gas-generating composition essentially composed of ammonium nitrate powder and a cured carboxyl-terminated essentially linear polyester binder, said polyester prior to curing having a molecular weight of 500 to 5000 and being the condensation product of a polycarboxylic acid and a polyhydric alcohol, said polyester being cured by the reaction of the carboxyl groups thereof with a curing agent selected from the group consisting of of 50% to 80% by weight of ammonium nitrate powder and 20% to 50% by Weight of a cured carboxyl-terminated essentially linear polyester binder, said linear polyester prior to curing having a molecular weight of 500 to 5000 and being the condensation product of a polycarboxylic acid and a polyhydric alcohol.

7. A gas-generating composition essentially composed of from 50% to 80% by Weight of ammonium nitrate powder, from 0.5% to 5% by weight of barbituric acid as a combustion catalyst, .and a binder which is a cured carboxyl-terrninated essentially linear polyester, said linear polyester prior to curing having a molecular weight of 500 to 5000 and being the condensation product of a polycarboxylic acid and a polyhydric alcohol. 8. The method of making a gas-generating composition which comprises mixing from to by weight of ammonium nitrate powder with 15% to 45% by weight of a carboxyl-terminated essentially linear polyester having a molecular weight of 500 to 5000 and 3% to 10% by weight of a curing agent for curing said polyester through the carboxyl groups thereof, casting the resulting mixture into a desired configuration, and curing said mixture to cause the polyester and curing agent to react to form an elastomer.

9. A method according to claim 8 and wherein said curing agent is selected from the group consisting of polyepoxides and polyimines.

References Cited in the file of this patent UNITED STATES PATENTS 3,000,714 Batchelder et al Sept. 19, 1961 3,000,715 Lawrence Sept. 19, 1961 3,031,288 Roberts Apr. 24, 1962 OTHER REFERENCES Chem. and Eng. News, August 1, 1960, p. 35. Chem. and Eng. News, August 8, 1960, p. 53.

Claims (1)

1. A GAS-GENERATING COMPOSITION ESSENTIALLY COMPOSED OF AMMONIUM NITRATE POWDER AND A CURED CARBOXYLTERMINTATED ESSENTIALLY LINEAR POLYESTER BINDER, SAID LINEAR POLYESTER PRIOR TO CURING HAVING A MOLECULAR WEIGHT OF 500 TO 5000 AND BEING THE CONDENSATION PRODUCT OF A POLYCARBOXYLIC ACID AND A POLYHYDIRC ALCOHOL.