US3551225A - Solid propellant composition with a polymeric binder containing ammonium perchlorate and aluminum - Google Patents

Description

United States Patent 3,551,225 SOLID PROPELLANT COMPOSITION WITH A POLYMERIC BINDER CONTAINING AM- MONIUM PERCHLORATE AND ALUMINUM David C. Sayles, Huntsville, Ala., assignor to the United States of America as represented by the Secretary of the Army No Drawing. Original application Dec. 2, 1965, Ser. No. 513,146, now Patent No. 3,506,713, dated Apr. 14, 1970. Divided and this application Dec. 10, 1968, Ser.

Int. Cl. C06b 11/00 US. Cl. 149-19 2 Claims ABSTRACT OF THE DISCLOSURE Solid propellant compositions that contain aluminum, ammonium perchlorate, diisocyanate cross linking agent and a polymeric binder that contains oxidizer and fuel material.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This application is a division of applicants copending application Ser. No. 513,146, filed Dec. 2, 1965 now US. Pat. 3,506,713.

This invention relates to solid propellants and more particularly to a novel polymeric binder material.

The solid propellants concerned in the present case are generally referred to as composite-type propellants which are comprised of. several distinct classes of materials including binders, oxidizers, metal, fuel and additives. The main bulk of the propellant essentially must be the fuel and oxidizer. Fuel is supplied both by an organic resin binder and by a metallic fuel component, while the oxidizer is a compound of high oxygen content such as a perchlorate. Several perchlorate salts have been used as oxidizers; however, ammonium perchlorate is the preferred oxidizer. A fuel binder is an essential part which holds the propellant ingredients together and usually burns as fuel. The metallic fuel, normally aluminum in the form of finely divided and dispersed particles throughout the composition releases a large amount of energy upon burning and accelerates the burning rates. Additives are materials provided in relatively small proportions to impart or improve desirable properties or suppress undesirable properties, for example, inorganic salts to impart desired ballistic properties or trace amounts of materials such as lecithin to improve casting or mixing operations.

The binder should possess good burning characteristics if it is to contribute properly to the total specific impulse of the propellantv The binder should be one that is easily cured by its ability to form linkage with other organic compounds, and it should be compatible with other propellant ingredients. The binder must be able to impart the required physical characteristics such as rubber-like character and high strength. It should have proper viscosity during mixing operations and during the curing period in order to maintain homogeneity of ingedients. The material selected for binder should be reasonably plentiful, preferably commercially available or one easily prepared from a commercially available starting material.

Another desirable feature for the binder is a chemical composition which includes oxygen in the polymer struc- Patented Dec. 29, 1970 ice ture. The oxygen present in the binder would contribute to the total amount of oxygen in the propellant compositron and would thus allow a reduction in the amount ofoxidizer required for complete combustion. Since the oxidizer is a finely divided crystalline material without adhesive properties, a decrease in this component and a corresponding increase in binder content would provide improved mechanical properties.

It is an object of this invention to provide an oxygencontaining binder so as to substantially reduce the amount of oxidizer salts required in the propellant formulation.

Another object is to provide a binder which can be conveniently and easily produced from a commercially available starting composition.

Still another object will be to provide a binder possessmg good chemical and physical properties for solid propellant requirements.

In accordance with this invention, a new composition of matter has been produced which is a prepolymer useful as a binder-oxidizer-tuel ingredient for solid propellants. The new composition of matter has a basic structure as illustrated herewith:

The value of x is a whole number, at least one.

When x is a whole number greater than one, x represents the number of basic units of the new composition polymerized to yield an average molecular weight from about 2,000 to about 4,000. For simplicity, the above composition of matter, poly{l,4-bis[(difluoroamino)- methyl] 3,5 dioxa 2 hydroxy-methylpentane}, will be hereinafter referred to as DDHP.

DDHP has been found to allow a substantial reduction in the amount of ammonium perchlorate oxidizer while maintaining and even increasing the specific impulse in the resulting propellant composition. This function is believed to result from reaction between the fluorine in the subject binder with aluminum metal, and in addition, the oxygen present in the DDHP burns the carbon to add additional energy which also increases the specific impulse of the propellant.

The structure of the DDHP enables it to react easily with certain compounds having propellant formulation and curing stages. The compound contains a hydroxyl group in the side chain in a favored position for crosslinkage, thus allowing curing by cross-linking agents such as isocyanates. In formulations with isocyanates, it readily forms linkages which enable the prepolymer to bind the propellant ingredients into a stable and homogeneous mass capable of being shaped and retained in the desired shape. When cured, the resulting propellant is capable of withstanding the stresses which are normally encountered during rapid burning of the propellant.

The precursor for the prepolymer of the poly (difluoroamino) binder-oxidizer-fuel is available commercially fiom Miles Chemical Company as Sumstar or poly 3 ,5 -dioxa-2-hydroxymethyl- 1,4-pentanedial dial referring to di-aldehyde structure. Poly(3,5-dioxa 2 hydroxymethyl-1,4-pentanedial) is produced commercially by the oxidation of starch in accordance with the processes disclosed in Pats. Nos. 2,648,629 and 2,713,553. This oxidation technique was originally applied to starch by Jackson and Hudson as disclosed in Journal of American Chemical Society 5922049 (1939) and 602989 (1938). The oxidation reaction of the starch proceeds as illustrated below:

(IIILOH ll 1! x DDHP is formed by reacting the above pentanedial with difluoroamine in the presence of concentrated sulfuric acid. The reaction is believed to take place by an attack at the aldehyde group which results in an addition of NF at the 1,4 positions of the starting structure and is illustrated by following stoichiometric reaction scheme, where x is at least one and n is equal to four times x:

(DDHP) The molar ratio of difluoroamine to the reacting dial must be at least 4:1 since four moles of NF are required for each mole of the dial. An excess of the NF compound, for example, a proportion of 5:1 is preferred to ensure complete reaction. The reaction proceeds at room temperature, and the temperature is not critical, except that at temperatures above 30 C. the sulfuric acid tends to produce charring of carbon by too severely attacking at the aldehyde groups.

Since the reaction takes place in sulfuric acid, it is necessary to have an adequate volume of sulfuric acid to permit the addition of diifuoroamine to the sulfuric acid. The dial is added slowly at the same time as the difluoroamine, and the temperature is controlled, preferably in range of about l020 C. in a suitable reactor pot. A volume of about 1.5 mls. of fuming sulfuric acid (30%) to each gram of dial provides suitable contact of reactants. The volume of sulfuric acid used is not critical; however, the separation and washing of product following reaction completion is more easily accomplished when excessive acid is not used.

The DDHP can be separated by extracting with a suitable solvent such as diethyl ether which retains the DDHP in the ether layer. The material is then washed with Water, followed by washing with aqueous sodium bicarbonate and further rinsing with water. The extraction solvent easily evaporates and the DDHP, a viscous material of medium viscosity, is dried over anhydrous sodium sulfate to remove any remaining moisture.

Since DDHP has the capability of forming linear linkage because of resulting end functionalities, the average molecular weight can vary over a range of about 2,000 to 4,000. Linear linkage and cross-linkage are necessary to complete the functions as a binder and yield a rubber-like texture in the finished product.

Cross-linkage between DDHP structures is easily accomplished by reactions with isocyanates. Any isocyanate cross-linking agent can be used, and toluene diisocyanate is preferred. For example, when toluene diisocyanate is used at a weight ratio of about 1 to 5 of the DDHP in the propellant formulation, a rubber-like texture results when the propellant is cured at room temperature. The crosslinkage is believed to be formed between an active hydroxyl group in the DDHP side chain and a cyanate group. Thus, toluene diisocyanate having two cyanate radicals easily bridges the gap between adjacent DDHP units in the mix. Substitution of other isocyanates can be made for diisocyanates if different physical characteristics are desired.

The DDHP is a prepolymer when separated from the reactants, and as such, it is ready for immediate use in a propellant formulation consisting of cross-linking com: pound, oxidizers and additives. When used in propellant mix, the preploymer, DDHP, enables one to begin with a medium high viscosity mix, and after very little mixing action is completed, the propellant can be shaped and cured at room temperature to a uniform mass. Medium high viscosity during mixing and curing is important for two major reasons. First, power requirements for mixing are excessive if material is too viscous, and sedimentation is a problem encountered in propellant formulation and curing if the viscosity is too low. The medium viscosity resulting from using DDHP means both preferred conditions for proper lrnixing conditions and retardation of sedimentation of ingredients during curing conditions.

In a particular embodiment of the present invention, DDHP is used as a binder in a composite propellant containing aluminum fuel, ammonium perchlorate oxidizer and diisocyanate cross-linking agent. The DDHP may comprise 15 to 35 weight percent of the composition; aluminum, 5 to 25 weight percent; ammonium perchlorate, 55 to weight percent and the diisocyanate, 3 to 7 weight percent. The preferred values are 15 weight percent DDHP with 3.5 weight percent toluene diisocyanate, 20 weight percent aluminum and 61.5 weight percent amrnonium perchlorate. A minor proportion of other additives such as burning rate catalysts and wetting agents may be provided in the composition as required to meet specific needs.

EXAMPLE The starting materials for production of DDHP consisted of poly (3,S-dioxa-Z-hydroxymethyl-1,4-pentanedial), difiuoroamine (generated when used), and concentrated sulfuric acid.

The preparation of DDHP was accomplished by following procedures and amounts.

Difluoroamine (0.25 mole) was generated from the gradual addition of aqueous diffuorourea ml., 2 moles) to boiling aqueous sulfuric acid (120 ml., 2 moles) in a diffuoroamine generator. The difiuoroamine, thus generated, was slowly condensed into fuming sulfuric acid (30%, 11 ml.). The rate of addition was controlled so that the reaction temperature was maintained at 10 to 20 C. Concurrent with the generation of the diffuoroamine, the pentanedial (0.05 mole, 8.0 gms.) was charge slowly while the pot temperature was controlled (1020 C.), as well as the rate of generation of difluoroamine, so that it was under constant reflux. After complete addition of reactants, the reaction mixture was stirred an additional 30 minutes. The pot temperature was then increased from 20 to 30 C. and the excess difluoroamine vented, and finally purged with nitrogen. The reaction product was diluted with diethyl ether, (200 ml.) Washed with water, aqueous sodium bicarbonate, water and then dried over anhydrous sodium sulfate to remove any trace of moisture remaining.

The new compound, DDHP, was found to blend easily with propellant ingredients in varying amounts from about 15 to 35%. A 15% binder proportion was selected to permit a substantial variation in quantity of aluminum content and oxidizer required to yield desired properties in finished propellant. Therefore, the 15% DDHP was selected for formulations which were cast for propellant testing evaluation purposes.

Propellant samples using the prepolymer, DDHP, were compounded using conventional techniques. Five formulations comprising 15% DDHP and 3.48% toluene diisocyanate along with variable aluminum content of from to 25%, the remainder of the composition being ammonium. perchlorate ranging from 57% to 77% yielded calculated specific impulse and flame temperature values as shown in Table I.

The propellants noted in Table I were all considered to be satisfactory as far as physical requirements were concerned. All samples were cast in a strand casting block and cured at room temperature to yield test samples measuring A" by A" by 8". Samples of formulations No. 3 and No. 4 were selected from the group as the preferred samples with comparable specific impulse values.

Laboratory-scale burning tests showed that samples of strands from formulations No. 3 and No. 4 burned at a rate if 1.2 inches per second at 1,000 p.s.i. pressure. The measured specific impulse varied in the range of 243 to 247 pounds-seconds of force per pound of propellant on separately tested strands burning under standard pressure oi 1,000 p.s.i.

The above example is merely illustrative and is not to be understood as limiting of the invention which is limited only as indicated by the appended claims.

6 What is claimed is: 1. A solid propellant comprising a cured intimate mixture of 15 to 35 weight percent of a polymer having the formula where the value of x is such that the average molecular weight of said polymer is from 2,000 to 4,000, 5 to 25 weight percent finely divided aluminum, to 80 weight percent ammonium perchlorate and 3 to 7 weight percent diisocyanate cross-linking agent.

2. The propellant of claim 1 wherein the respective proportions in weight percent of said polymer, aluminum, ammonium perchlorate and diisocyanate cross-linking agent are 15, 20, 61.5 and 3.5.

References Cited UNITED STATES PATENTS 3,147,160 9/1964 McCrone 14976X 3,332,811 7/1967 Guthrie et al 149-76X 3,332,812 7/1967 Guthrie 14976X 3,346,621 10/1967 Petry 149--76UX 3,441,549 4/1969' Gardiner et al. 14976X CARL D. QUARFORTH, Primary Examiner S. J. LECHERT, ]R., Assistant Examiner US. 01. X.R 149-20, 42, 44,