Propellant Binder

Abstract

This invention relates to solid propellant formulations. More particularly, it relates to a novel binder for utilization in such solid propellants.

Description

Solid propellants found in application today are divided into two distinct categories; double-base propellants and composite propellants. The distinction is based on the physical characteristics of the propellants and their composition. Most propellants contain a sufficient amount of oxygen such that upon ignition they can be readily burned and converted to gaseous product. Thus, without any additional source of oxygen a solid propellant can sustain continuous burning producing a great amount of pressure to be utilized in producing thrust.

In general, a double-base propellant contains as its principle ingredient, nitrocellulose, and an explosive plasticizer normally nitroglycerin. Double-base propellants can contain inert plasticizers, stabilizers, ballistic modifiers and other high energy materials which may, in fact, be highly explosive themself. The composite propellants embrace a solid particulate oxidizer material dispersed throughout a matrix of plastic or resinous-type material. The matrix may provide the fuel for the combustion, however, normally today a solid particulate fuel is dispersed within the matrix. This fuel is normally a finely ground metal or metal wire. The composite propellants are also known as hydrocarbon systems because of the normally hydrocarbon binder material used. The three types of binders previously used in these hydrocarbon systems include polyurethane, polybutadiene-acrylic acid or PBAA and more recent, carboxy-terminated linear polybutadiene. Use of these binders, as well as the double-base system binders, often introduces several problems.

Particularly concerned with the processing of the propellant, data has indicated that double-base systems based on nitrocellulose and high energy plasticizers such as trimethylolethanetrinitrate or triethyleneglycoldinitrate with oxidizers such as ammonium perchlorate, hydrazine nitroform or nitronium perchlorate and fuels such as aluminum hydride can produce very high specific impulses. However, in processing it is difficult to obtain the necessary level of solids loading in the double-base system. That is, it is often impossible to incorporate successfully the high percentages of the solid particulate oxidizer and fuel necessary to produce the theoretical impulse that could be derived from such a system. Additionally, double-base systems are characterized by the extreme sensitivity to impact. This results in a serious drawback in the processing of such propellants due to the extreme danger in handling.

With regard to the composite propellants, the presence of polyurethane, polybutadiene acrylic acid or carboxy-terminated linear polybutadiene binders require very high total solids loading to achieve desired high impulse goals. However, the impact sensitivity of the final propellant is much lower than that of the double-base systems and thus processing from the sensitivity standpoint is improved. Generally, though, the impulse to be derived from the composite systems is lower than that obtained from the double-base ones.

On the basis of performance, it is apparent that the highest impulse is obtainable only with a double-base binder. There is generally little difference among the hydrocarbon systems except that higher solids loading is possible with the carboxy-terminated linear polybutadiene binder. However, it is much more difficult to achieve desired high impulse with the hydrocarbon binders. From a safety point, the hydrocarbon binders appear to be considerably superior to the double-base ones. Processing problems are normal with the double-base binder but are more difficult with the hydrocarbon ones due to the fact that the high total solids loading is required to obtain high impulse.

Thus, it is an object of this invention to provide a novel propellant binder system having high performance with regard to specific impulse.

Another object of this invention is to provide a solid propellant binder that has good safety for handling and processing.

A further object of the invention is to provide a new solid propellant binder having ease of processing.

The above and other objects are accomplished by a binder system based upon a linear polymethylenenitramine. The nitramine backbone has the following formula which will be represented as Z:

it may appear from two to six times in the monomers utilized to prepare the final polymeric product. It is preferred that it appear three times in the monomer. The monomers that will form the polymeric binder of this invention have the following general formula:

Wherein x may vary from two to six and R is a functional group capable of entering into cross-linking or polymerization. Thus, R can be selected from the class consisting of: ##SPC1##

wherein R' is selected from the class consisting of H and CH.sub.3 and R" is an alkylene radical of from one to five carbon atoms.

Because of the preferred method of synthesis of the monomers, R is normally a substituted alkoxy group. Thus, an ether linkage is normally present connecting the terminal group to the backbone. Additionally, an ester linkage can be formed instead of the ether using a carboxy acid instead of an alcohol in the synthesis of the monomers, however, this type of ester linkage is less desirable than the ether one from the standpoint of hydrolytic stability. The preferred terminal functional groups capable of chain extension and polymerization which are present on the R's are the epoxy, carboxy and hydroxy groups because of their case of cross-linking to form good propellant binders. The number of carbon atoms in the R chain should be kept generally to a minimum, preferably not more than four though up to six may be usable. A resin having the minimum number of carbon atoms is desired to produce a polymer having the highest percentage of the nitramine backbone to give high energy. The presence of the carbon atoms will tend to detract from this desired property.

It is important in the formation of solid propellant binders that in the polymerization reaction there be no condensation material which must be removed from the propellant. As a result, the particular terminal functional groups present must be ones which are capable of entering into an addition-type polymerization so that no byproducts such as water or the like are formed which would deleteriously effect the propellant.

It is found that particularly good results are obtained when one of the monomers that is reacted to form the polymeric binder of the invention is a compound having epoxy terminal groups. The most preferred monomer having the epoxide end groupings is 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane having the following formula:

This monomer may in turn be reacted with another monomer having the nitramine backbone and having one of the functional groupings above described such as carboxy or hydroxy and the like. It is preferred to react the particular preferred diepoxide monomer with either an acid anhydride such as maleic anhydride and 3,11-dioxa-5,7,9-trinitrazatridecane-1,13-diol or 3,11dioxa-5,7,9-trinitrazatridecane-1,13-dioc acid and a suitable cross-linker such as citric acid.

The nitramine binders of this invention are particularly adapted to propellant processing in that they are liquid at processing temperatures before curing or polymerization. This provides for their ease of casting. When combined with a nitroplasticizer, particularly, the binders yield propellant that is castable, stable (Class II) and has a high delivered impulse. For example, an unmetallized propellant would have a delivered impulse of above 240 sec., with aluminum--above 245 sec., with beryllium--above 255 sec., with aluminum hydride--above 255 sec., and with beryllium hydride--above 265 sec.

The starting material for the synthesis of the monomers is hexamethylenetetramine or hexamine as it is commonly called. The compound is commercially available in quantity. The first step in the synthesis is to prepare the compound 1,7-diacetoxy-2,4,6-trinitrazaheptane called BSX. This is carried out by nitrolysis according to the following reaction. ##SPC2##

The resultant compound, BSX, and the above method of preparation are well known in the literature. BSX is then reacted with HCl to form GSX.

like BSX, GSX and its method of preparation are well known in the literature.

The chloro groups of GSX are very reactive because of activation by the nitramine group. GSX reacts readily with hydroxy compounds, splitting out hydrogen chloride and forming an ether. The hydroxy-terminated monomer used in the invertion is synthesized by reacting GSX with ethyleneglycol according to the following reaction.

This above reaction and the compound called TNDO are known. The carboxy-terminated monomers can be prepared by reacting GSX ethylacetate as known in the literature.

The preparation of the desired epoxy-terminated monomers called TNDE are formed by reacting GSX with glycidol according to the following reaction. ##SPC3##

The detail synthesis of the diepoxide is as follows in Example I.

EXAMPLE I

The starting material GSX must be free from organic contaminants which it may pick up during its synthesis and metal autoclaves. One recrystallization from ethylene dichloride is adequate if the hot solution of GSX in ethylene dichloride is filtered through diatomaceous earth before allowing the GSX to crystallize. An 18 -liter glass reactor was equipped with electric mantle, magnetic stirrer, thermometer and gas outlet. The reactor was then charged with the following.

2,387 grams GSX (recrystallized)

9,320 ml. glycidol

2,400 grams CaCO.sub.3 (low alkali)

1,200 grams MgSO.sub.4 (anhydrous)

4,000 ml. dioxane

The mixture was stirred and heated until the temperature reaches 60.degree. C. The temperature was then maintained between 60.degree. and 65.degree. C. by the use of heat or water bath as necessary. An initial exotherm was experienced somewhere in the vicinity of 60.degree. C. After 5 hours above 60.degree. C., the reaction mixture was cooled to room temperature with an ice bath and filtered immediately through diatomaceous earth. The clear filtrate was then placed in a rotary evaporator, and the dioxane removed at 65.degree. C. with a water aspirator. The majority of the glycidol was then removed using the same apparatus but evacuating to below 2 mm. with a vacuum pump. A total of 4,223 grams of glycidol was removed leaving a 7,067-gram residue. The residue was diluted with 8 liters of ethylene dichloride, cooled in a refrigerator overnight and then filtered. The filtrate was washed, 2 liters at a time, with three 500 ml. portions of cold water. The wet solution was placed in a rotary evaporator and the ethylene dichloride removed with a water aspirator and a bath temperature of 65.degree. C. The residue was then brought to a weight loss of only 1.4 grams in two hours at 65.degree. C. and less than 2 mm. vacuum. The product was completely colorless and slightly milky at ambient temperature. The yield was 1,906 grams with an epoxy equivalent of 304. The diepoxide is a mixture of monomer and various polymers formed during the diepoxide synthesis. The epoxy equivalent may vary from 141 to 400 by variation in processing techniques.

The reaction of two nitramine monomers of this invention capable of polymerizing produces a linear polymer. The same result can occur by using a single nitramine monomer and reacting it with an agent that is capable of chain extension. The agent would react with the terminal groups of the nitramine monomer forming a linear polymer. For utilization as solid propellant binders, it is generally necessary that the linear polymers are cross-linked. Cross-linking can be accomplished by utilization of additional compounds containing three or more functional groups capable of entering into the polymerization. The cross-linking produces a solid propellant matrix having a higher tensile strength and better modulus of elasticity and other improved physical properties. For example, the diepoxide monomer can be reacted with citric acid. The diepoxide monomer can be both extended linearly and cross-linked by the addition of trifunctional compound citric acid. The percent of the carboxy equivalents from the citric acid can be from 0 to 100 percent. Any of the monomers of the invention can be extended and cross-linked by the addition of various known cross-linking additives which are capable of reacting with the terminal groups present on the nitramine monomers. Additionally, linear polymers can be formed by the reaction of two of the nitramine monomers having terminal groups capable of polymerization with each other. The resultant linear polymers will generally only have one reactive site on each end and cannot cross-link. As a result, to accomplish such cross-linking, it is generally necessary to add an agent capable of causing such a transpire.

When the preferred diepoxide monomer is reacted with a dicarboxy-terminated monomer, citric acid may be used as a cross-linking agent. Other cross-linking agents include: 1,2,3,4-butane tetracarboxylic acid, trimesic acid, trimellitic acid and 1,2,3-propane tricarboxylic acid. It should be apparent that not all of the above cross-linking agents are compatible with all the nitramine monomers of the invention. The method of selecting the proper cross-linker is well within the skill of the art. As the selection of appropriate nitramine monomers, the choice of a cross-linking agent is based on ones that can react with the terminal functional group of the nitramine monomers forming an addition-type polymer.

The proportion of the three ingredients is dependent on the end functional groups of each. It is desirable that substantially all of the end functional groups are reacted. For example, the ratio of epoxy to carboxy equivalents would for most practical formulations be from 1:1 to 4:1. The amount of cross-linker present can be from 0 to 100 percent of total carboxy equivalents present depending on properties desired.

When the preferred diepoxide is used with a nitramine diol and an anhydride, maleic anhydride being preferred, a cross-linked polymer is formed. The anhydride reacts with the diol to form a dicarboxy acid which then reacts with the diepoxide. This polymerization would give only a linear polymer except that when the ester linkage is made, a hydroxy group is produced which can react with excess anhydride to form a carboxy group along the chain for a cross-linking site. Other usable anhydrides include: phthalic anhydride, hexahydrophthalic anhydride, succinic anhydride and glutaric anhydride.

It should be apparent that not all of the above anhydrides are compatible with all the nitramine monomers of the invention. The method of selecting the proper anhydride is well within the skill of the art. This anhydride must primarily be an anhydride of a dicarboxy acid. The proportion of the three ingredients is dependent on the end functional groups of each. It is desirable that substantially all of the end functional groups are reacted. For example, the ratio of equivalents of diol: maleic anhydride: diepoxide are 1:0.75-2:1-3 for the most practical formulations.

The metal fuel may constitute 0.2 to 32 weight percent propellant grain of this invention, preferably is one or more of the metals of Groups I-A, II-A, III-A and Groups I-B through VII-B, and Group VIII of the Periodic Table. Thus, the metal may be Group I-A elements such as lithium, and Group II-A metals such as beryllium or magnesium. Illustrative of the Group III-A metals is aluminum. The metals of Group I-B through VII-B include copper, silver, zinc, cadmium, titanium, zirconium, vanadium, niobium, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, ruthenium, rhodium, osmium, palladium, and platinum. Additionally, hydrides of the metals such as beryllium hydride and aluminum hydride are contemplated.

To aid in the processing of the propellant, it is often advisable to employ plasticizers in the preparation and utilization of the polymeric and plastimeric materials employed in the invention. The plasticizers can be up to 75 percent by weight of the binder composition. It is preferred that energetic plasticizers be used with the polymers of this invention to give higher impulse. Such preferred ones include the nitroplasticizers such as bis-dinitropropylformal, bis-dinitroproplyacetal, and 2,2-nitro propane. Other plasticizers may be of the general type of inert and explosive plasticizers. Examples of inert plasticizers include triacetin, the various phthalates such as diethyl phthalate, dibutyl phthalate, dioctyl phthalate, di-(methoxyethyl) phthalate, methyl phthalyl ethyl glycolate, ethyl phthalyl ethyl glycolate and butyl phthalyl butyl glycolate, sebacates such as dibutyl and dioctyl sebacates, adipates such as dioctyl adipate and di(3,5,5-trimethylhexyl)adipate, glycol esters of higher fatty acids, organic phosphate esters such as tributoxyethyl phosphate, and the like. The explosive plasticizers include nitroglycerin, butane triol trinitrate, diglycol dinitrate, ethylene glycol dinitrate, and the like.

The solids oxidizing agents utilized can be compounds such as metal perchlorates and metal nitrates. The metal perchlorates employed as oxidizing agents or oxygen carriers in the compositions are anhydrous and have the general formula M(ClO.sub.4) , wherein M is NH.sub.4 or a metal and x is the valence of M. Since the propellant composition is required to withstand high-temperature storage, it is preferable that the melting point and the decomposition temperatures of the oxidizer be as high as possible. The perchlorates of the Group I-A, Group I-B, and Group II-A metals are found to have the required high-temperature stability and are employed in the preparation of propellant compositions by the process of this invention. Hence, the metal perchlorates used in the preparation of the propellant compositions include lithium perchlorate, sodium perchlorate, potassium perchlorate, rubidium perchlorate, and cesium perchlorate which are the perchlorates of the metals of Group I-A of the Periodic Table of Elements; silver perchlorate which is a perchlorate of the Group I-B metal; and magnesium perchlorate, calcium perchlorate, strontium perchlorate, and barium perchlorate which are the perchlorates of the Group II-A metals. In addition to the metal perchlorates, the compounds ammonium perchlorate and nitronium perchlorate find extensive use in propellant compositions. Examples of the nitrates of the Group I-A, and I-B and II-B which are employed in preparing propellant compositions by the process of this invention are compounds such as lithium nitrate, sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate, barium nitrate, strontium nitrate, etc. Ammonium nitrate is also used. The amount of oxidizer present falls within the range of 25 to 80 weight percent of the propellant composition.

The ratio of total solids-to-polymeric binder material in a propellant falls in the range of from about 1:1 to about 9:1 with an optimum ratio of about 7:3.

Other substances which are employed in the preparation of propellants by the process of this invention include minor amounts of burning catalysts, well known in propellant compositions. These are composed of one or a mixture of two or more metal oxide powders in amounts sufficient to improve the burning rate of the composition. The amounts usually range from about 0.01 to about 3 weight percent, based on the weight of the oxidizer employed. The particle size of the powders can range from about 10 to about 250 microns in diameter. Nonlimiting examples of metals that serve as burning catalysts are copper, vanadium, chromium, silver, molybdenum, zirconium, antimony, manganese, iron, cobalt, and nickel. Examples of metal oxide burning catalysts are ferric oxide, aluminum, copper oxide, chromic oxide, as well as the oxides of other metals mentioned above.

Burning rate depressants and modifiers are also sometimes advantageously added to the solid propellant grain of this invention. These are generally compounds added in the amounts of 0.05 to 5 weight percent of the propellant composition. These compounds tend to inhibit burning reaction rates or absorb heat and include specifically carbonyl chloride, oximide, nitroguanidine, guanidine nitrate, and oxalic acid.

Curing catalysts are often added in minor amounts to cure the polymer in the performance of the process of this invention. Nonlimiting examples of catalysts used for this purpose are aluminum chloride, tristrimethylsilyl borate, benzoyl peroxide, and other catalysts well known in the curing of plastics, resin, polymers, and rubbers. Examples of various catalysts may be found in text books such as "Synthetic Rubber," by G. S. Whitley, pp. 892-933, 1954 Ed., published by John Wiley and Sons, Inc., New York. The curing catalysts are added in amounts of from 0.1 to about 10 weight percent based on the weight of the polymer, resin or elastomer. The particular catalyst and amount employed depend on the state of cure desired and the nature of the polymeric material employed in the composition.

The following example indicates the preparation of a propellant formulation utilizing the polymeric binder of this invention.

EXAMPLE II

The following constituents were utilized to form the propellant grain. ---------------------------------------------------------------------------

Binder Grams __________________________________________________________________________ TNDE 38.90 Maleic anhydride 4.78 TNDO 13.48 BDNPF-BDNPA 40.30 __________________________________________________________________________

(mixture 50 percent: 50 percent of bis-dinitropropylformal: bis-dinitropropyl acetal)

Oxidizer Grams Ammonium perchlorate 149.3 Aluminum 78.0 __________________________________________________________________________

All of the binder ingredients are placed in a vertical or horizontal mixer such as the Baker-Perkins mixer which has been previously heated to 150.degree.-175.degree. F. The binder ingredients were then mixed for a period of time sufficient to wet all the ingredients. Vacuum is then placed on the mixer and mixing is continued for 20 minutes. At this point, the mix was very fluid and transparent. The aluminum powder is then added and mixed until wet. Vacuum was then placed on the mixer and mixing was continued for about 2 minutes until the aluminum was thoroughly dispersed. The mixer was then stopped and the oxidizer was added in three equal increments. After each of the first two increments, the ingredients were mixed for 10 minutes under vacuum. After the last increment of oxidizer was added, the final ingredients were mixed for 30 minutes under vacuum. The mixer is then finally stopped and the ingredients are ready for pouring into a mold to be cast. The propellant mixed is poured into a mold and cured at atmospheric pressure at 24 to 120 hours at 120.degree. F.

EXAMPLE III

The process of manufacturing propellant as set forth in above example II was repeated, utilizing the following formulation. The only difference from the process of example II was between 20 and 50 p.s.i.g. pressure was used when curing the material. ---------------------------------------------------------------------------

Propellant LCH-35 Percent Grams TNDE 11.27 225.6 TNDA 4.10 82.0 Citric acid 0.33 6.4 Diethylene glycol dinitrate 14.17 283.4 Resorcinol 0.13 2.6 Ammonium perchlorate 68.00 1359.00 Aluminum 2.00 40.00 __________________________________________________________________________

In this example it is noted that nitrate ester plasticizer was used. The plasticizer was stabilized with the presence of a small amount of resorcinol.

EXAMPLE IV

A more preferred propellent formulation due to physical properties of cured propellant is shown below. It was prepared in accord with the procedure of example II. ---------------------------------------------------------------------------

propellant CLH-43-1 Percent Grams TNDE 13.42 402.6 Citric acid 2.28 68.4 Petrin (plasticizer pentaerythritol) 14.00 420.0 Ethyl centralite (stabilizer for plasticizer) 0.30 9.0 Ammonium perchlorate 46.0 1380.0 Aluminum 24.0 720.0 __________________________________________________________________________

Several 2-inch and 6-inch rocket motors were made utilizing the formulations set forth in the previous examples as well as varying the relative proportion of the constituents so as to obtain range of properties for the grains. Typical properties obtained from the formulation tested follow: ---------------------------------------------------------------------------

Mechanical Properties Elongation, e.sub.m, percent 11-37 Tensile Strength, s.sub.m, p.s.i. 95-260 Modulus E, p.s.i. 520-3400

Density, lb.-cu.-in. 0.063-0.068 Impact Sensitivity, in.-lb. 13-32 Burning Rate, in.-sec.

500 p.s.i.a. 0.178-0.266 1000 p.s.i.a. 0.229-0.351 Pressure Exponent, n 0.34-0.40 __________________________________________________________________________

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

Claims

We claim:

1. Polymers having the backbone

, said polymer formed from the addition polymerization of monomers having the formula:

wherein x is a whole integer from two to six and R is selected from the class consisting of: ##SPC4##

wherein R' is selected from the class consisting of H and CH.sub.3 and R" is an alkylene radical of from one to five carbon atoms.

2. The polymers of claim 1 further polymerized with a compound capable of entering into addition polymerization to cross-link said polymers.

3. The polymer formed from the reaction of 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane with 3,11-dioxa-5,7,9-trinitrazatridecane-1,13-diol.

4. The polymer formed from the reaction of 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane with 3,11-dioxa-5,7,9-trinitrazatridecane-1,13-dicarboxyl.

5. The polymer formed from the reaction of 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane with 3,11-dioxa-5,7,9-trinitrazatridecane-1,13-diol and maleic anhydride.

6. The polymer formed from the reaction of 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane with 3,11-dioxa-5,7,9-trinitrazatridecane-1,13-dioic acid and citric acid.

7. The polymer of claim 5 wherein the ratio of diol:anhydride:diepoxide is within the range 1:0.75-2:1-3.

8. The polymer of claim 6 wherein the ratio of dioic acid:citric acid:diepoxide is 1:0.25-2:1-3.

9. The polymer formed from the reaction of 1,2,14,15-diepoxy-4,12-dioxa-6,8,10-trinitrazapentadecane with citric acid.

10. A solid propellant binder composition comprising:

a polymer having the backbone

, said polymer formed from the addition polymerization of monomers having the formula:

wherein x is a whole integer from two to six and R is selected from the class consisting of: ##SPC5##

wherein R' is selected from the class consisting of H and CH.sub.3 and R" is an alkylene radical of from one to five carbon atoms,

a cross-linking agent for said polymer.

11. The binder of claim 10 additionally comprising:

a plasticizer in an amount up to 75 weight percent of the binder composition.

12. The binder of claim 11 wherein said plasticizer is a nitroplasticizer selected from the class consisting of: bis-dinitropropylformal, bis-dinitropropylacetal, and 2,2-nitropropane.