US3157127A - Solid propellant grain with cellulose acetate coating - Google Patents

Description

Nov. 17, 1964 w, PROELL 3,157,127

SOLID PROPELLANT GRAIN WITH CELLULOSE ACETATE COATING Filed Marfch 15, 1962 INVENTOR. Wayne A. Proe/l United States Patent 3,157,127 OlLlD hRGPELLANT GRAZN WETH CELLULGSE ACETATE CUATKNG Wayne A. Email, eymour, had, assignor to Standard @il Company, Chicago, llh, a corporation of lndiana Filed Mar. 15, 1962, fies. No. 13%4923 6 Claims. (Cl. 192-98) This invention relates to ammonium nitrate articles, and particularly ammonium nitrate propellant grains suitable for use in gas generators and rockets.

The explosive industry utilizes ammonium nitrate as a principal component of explosives. The ammonium nitrate may be used in conjunction with oxidizable organic materials, or ammonium nitrate particles may be combined with other explosives. It is advantageous frequently to isolate the ammonium nitrate from direct contact with other components of the final explosive, or simply to protect the ammonium nitrate from contact with at mospheric moisture. At present, it is common to apply a sealer coat of hydrocarbon wax to the ammonium nitrate particles. This is not a satisfactory answer because the particles tend to cohere in storage.

Ammonium nitrate propellant compositions are now widely used for the generation of gas at high pressure, which gas is used to drive turbines and the like, or to propel rockets. These propellant compositions comprise ammonium nitrate, an oxidizable organic binder which permits forming the propellant to configurations of definite shape, and an ammonium nitrate combustion catalyst. it is necessary to control the direction of burning of the propellant body; this is done by applying to the surface(s) of the grain which are not desired to burn, a coat of a combustion restrictor. The restrictor coat must be non-porous and free of holes of any size because hot gases in the motor chamber will pass through these holes and ignite the propellanP-the burning of the propellant at the holes causes uneven pressure maintenance in the motor. Also, the restrictor coat must be stable at prolonged storage and not develop cracks. Some restrictor materials fail on storage owing to reaction with exposed ammonium nitrate in the propellant body or even with a binder component. In such a case it is customary to apply a sealer coat of some material-usually a restrictor in its own right-to the surface of the grain. Then the restrictor coat is applied to the sealer coat.

Cellulose acetate may be adhered to ammonium nitrate and functions, in coats of suitable thickness, as a restrictor. It is difiicult to build up a coating of suiiicient thickness by the use of solutions of cellulose acetate which are applied to the surface to be restricted (coated). Commonly sheets of cellulose acetate are used as restrictors and are cemented to the grain surface by the use of a cellulose acetate solution cement. Such cementing techniques are barely satisfactory and frequently result in restrictor failure during the burning of the propellant.

When cellulose acetate is applied from solution, it is frequently difiicult to obtain a non-porous barrier. The coat is frequently brittle and cracks during handling of the coated grain before the application of the restrictor coat. In any case, considerable skill, care and expense goes into the production of a satisfactory adherent coating of cellulose acetate when using a solution, such as cellulose acetate and acetone, as the source of the cellulose acetate coat.

An object of the invention is an ammonium nitrate particle having a cellulose acetate sealer coat adhered to a portion of, or even the entire surface thereof. A particular object is an ammonium nitrate propellant having a sealer coat on at least a portion of the surface thereof, which sealer coat is formed of cellulose acetate. Still anilhilli? Patented Nov. 1?, 196 4:

other particular object is a shaped ammonium nitrate propellant grain provided with a cellulose acetate sealer coat and a restrictor coat of material tending to react with ammonium nitrate, said sealer coat barring contact between exposed ammonium nitrate and the restrictor coat. Other objects of the invention will become apparent from the detailed description thereof.

It has been discovered that ammonium nitrate itself or the surface of ammonium nitrate type propellant can be provided with a satisfactory sealer coat of cellulose acetate by the following procedure. First, a layer of a liquid capable of plasticizing cellulose acetate is applied to the surface to be covered. Second, powdered cellulose acetate is applied to the liquid covered surface; sufiicient of the acetate is applied to form a seal coat on the surface after the baking treatment. Lastly, the cellulose acetate covered surface is baked at a temperature of about C. for a time sutiicient to form a coat of cellulose acetate on said surface.

The sealer coat present on the article or propellant of the invention is formed from cellulose acetate. Particularly suitable for this purpose are the cellulos acetates which have a combined acetic acid content of about 50-53 weight percent. A preferred cellulose acetate with a combined acetic acid content of about 55% is made by The Celanese Company as grade HLFS 93. Another preferred cellulose acetate is that sold to the trade as lacquer grade containing 54-56% combined acetic acid.

The cellulose acetate is applied to the wet surface in the form of a powder and the degree of fineness of the powder is determined by the type of acetate used, the type of plasticizer, the thickness of the sealer coat desired, and the baking temperature and time. Thicker coats may be made up of two or more layers, formed by repeated applications of plasticizer and cellulose acetate.

The ultimate thickness of the sealer coat will be that needed to meet the requirements of the particular task. A paper-thin coat may b suificien't to act as a barrier between exposed ammonium nitrate and a restrictor coat which is capable of reacting with ammonium nitrate. U.S. Patent No. 3,012,507 of Robert A. Masher and William G. Stanley describes in detail the use of a sealer coat (undercoat) as a barrier between ammonium nitrate and restrictor coat material reactive therewith. When it is desired to produce an ammonium nitrate article covered with a sealer coat to act as a barrier against absorption of atmospheric humidity, the coat will be tailored to suit the humidity conditions faced by the ammonium nitrate article in storage. In other instances, the sealer coat will function as a restrictor coat also. ln such a case, the thickness is great enough to withstand the hot gases from ammonium nitrate decomposition for the time needed to protect the surface beneath the sealer coat segment. When used as a chemical barrier material, the cellulose acetate sealer coat may be in general or less. When used as a restrictor coat, in general, the thickness will be on the order of although in severe cases, thicker coats may be necessary.

A liquid capable of plasticizing cellulose acetate is ape plied to the surface to be covered by the sealer coat as the first step in the formation of the sealer coat. It may be necessary to heat the plasticizer in order to apply it to the surface to be covered. In general, the temperature at which the plasticizer is applied, preferably, is lower than the lower baking temperatures. The amount of plasticizer applied is dependent upon the particular plasticizer and also upon the thickness of the final sealer coat desired. in general, a layer of liquid exhibiting complete wetting of the surface, to visual observation, is sufficient for the formation of a sealer coat suitable for use as a-chemical barrier. The liquid pla ticizer may be applied to the surface by brushing, spraying or dipping.

Any compound capable of plasticizing cellulose acetate may be used. Plasticizers may be classified generally as polymeric esters, esters of polyhydric alcohols, ethers of nitrophenols, nitromonocyclic aromatics, esters of polycarboxylic acids, alkyl ethers of polyglycol, and polyglycols. Specific examples of such plasticizers are ethylene glycol diglycolate, acetin (mono, di and tri), nitromethylpropanediol diacetate, triethylene glycol di-Z-ethylbutyrate, triethylene glycol di-Z-ethylhexoate and polyethylene glycol di-Z-ethylhexoate. Specific examples of esters of polycarboxylic acids are alkyl citrates such as triethyl citrate, acetyl triethyl citrate, trimethyl citrate,

' acetyi trimethyl citrate, and the dialkyl phthalates such as dibutyl phthalate and dioctyl phthalate. Specific examples of nitrophenyl ethers which may be incorporated in the binder of the body portion are dinitrophenyl allyl ethers, dinitrophenoxyethanols and the dinitrodiphenyl ethers such as bis(dinitrophenoxy)ethane and 2,4 dinitrodiphenyl ethers. Such nitromonocyclic aromatics as dinitrotoluenes and dinitrobenzenes may be used. Combinations of two or more of the plasticizers may be used.

In those cases when the viscosity of the plasticizing liquid is too low to provide a sufficiently thick layer of liquid on the surface to be covered, the viscosity of the liquid may be increased by dissolving some cellulose acetate therein. It is to be understood that even considerable bodying of the plasticizer with cellulose acetate will not permit formation of satisfactory sealer coats for most applications. Coats prepared from plasticizer solution only tend to be porous.

The acetate-plasticizer surface covering is converted to a hard, tough, pore-free, adherent coating by heat treatment (baking) at a temperature of about 90-120 C; The heat treatment may be directed only at the covered surface, or the entire article may be heated to the necessary temperature. The heat treatment is continued for a time sufiicient to convert the cellulose acetate particles to a continuous coating. The presence of the plasticizer liquid appears not only to provide adherency, but also assists in the melting of the solid particles and their fusion into a tough, continuous, nonporous sealer coat. The exact time needed will be dependent upon the temperature, the particle size and the thickness of the acetate-liquid covering prior to heat treatment.

The sealer coat formed as described hereinabove is of particular usefulness in the case of solid propellants comprising a shaped body portion formed of ammonium nitrate and an oxidizable organic binder Where the surface of said body is characterized by the presence of exposed ammonium nitrate, i.e., ammonium nitrate which has not been covered by the binder. And it is desired to utilize as the combustion restrictor coat, a material which reacts with ammonium nitrate to produce gaseous products or to produce areas of weakness which result in puncturing of the restrictor coat or brittleness. U.S. Patent No. 3,012,507 describes in some detail the restrictor materials which do face the problem of reaction with ammonium nitrate. And itis intended that the teachings of this patent and other patents on particular types of restrictor coatings such as US. Patents Nos. 2,999,462

and 3,012,506 are incorporated herein. The sealer coat of the .invention is also particularly suitable for use as an undercoat where the desired restrictor coating does not adhere well to the propellant surface because of the incompatability of the restrictor material and the binder material of the propellant.

'The'body portion of the solid propellant of the invention consists essentially of ammonium nitrate as the major component and oxidizable organic binder is also present to permit the ammonium nitrateto beformed into shaped configurations or grains.

monium nitrate is preferably in a finely divided particulate form which may be either produced by prilling or by grinding. Usually the composition will contain between about 65 and of ammonium nitrate.

When ammonium nitrate decomposes free-oxygen is formed. Advantage of the existence of this free-oxygen is taken and oxidizable organic materials are used as the binders. These oxidizable organic materials may contain only carbon and hydrogen, for example, high molecular weight hydrocarbons such as asphalts or residuums, and rubbers either natural or synthetic. Or, the oxidizable organic material may contain other elements in addition to carbon and hydrogen for example, Thiokol rubber and neoprene. The stoichiometry of the composition is improved, with respect to smoke production, by the use of oxygenated organic materials as the binders. The binder may be a single compound such as a rubber or asphalt or it may be a mixttire of compounds. The mixtures are particularly suitable when special characteristics are to be imparted to the grain which cannot be obtained by the use of a single compound.

The multi-component binder commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitably polymeric base materials are cellulose esters or alkanoic acids containing from 2 to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrate and cellulose propionate; the polyvinyl resins such as polyvinylchloride and polyvinyl acetate are also good bases; styrene-acrylonitrile is an example of a copolymer which forms a good base material.

A preferred binder component is broadly defined as an oxygenated hydrocarbon. The hydrocarbon base may be aliphatic or aromatic or may contain both forms. The oxygen may be present in the plasticizer in ether linkage and/or hydroxyl group and/or carboxyl groups; also the oxygen may be present in inorganic substituents particularly nitro groups. In general any plasticizer which is suitable for Work with the defined polymers may be used in the invention. Exemplary classes of plasticizers which are suitable are set out in US. Patent No. 2,999,462.

In addition to the basic components, i.e. ammonium nitrate binder and catalyst, the gas generator propellant composition may contain other materials. For example, materials may be present to improve low temperature ignitability, for instance oximes may be present or, asphalt may be present. Surfactants may be present in order to improve the coating of the nitrate with the binder and to improve the shape characteristics of the composition. Various burning rate promoters, Which are not catalyst per se, may also be present.

The mixture of ammonium nitrate and binder is expiperidine chromate are also excellent chromium cata-- lysts. Certain heavy metal cyanides particularly those of cobalt, copper, lead, nickel, silver and zinc are elfec tive catalysts. The cyanamides of barium, copper, lead mercury and silver are efifective catalysts. The various Prussian blues are excellent catalysts.

Organic catalysts are particularly useful when it is desired to have combustion products which are gases or vapors and thereby do not erode gas eXit orifices. Two catalysts which do not contain any metal components are pyrogene blue (color index 95 696l) and methylene blue. Particularly suitable catalysts are the alkali metal barbiturates.

A particular embodiment of the propellant utilizing the sealer coat described herein is set forth for purposes of illustration. In this embodiment, the body portion of the propellant which is a solid rod in shape consisted of GP. ammonium nitrate, 62%; sodium barbiturate catalyst, 3%; toluene diamine, 1%; carbon black, 4%; with the binder consisting of: lacquer grade cellulose acetate, 12%; acetyl triethyl citrate, 9%; and dinitrophenoxyethanol, 9%binder components based on propellant.

A sealer coat was applied to the cylindrical surface by dipping the grain into liquid acetyl triethyl citrate at room temperature. Commercial powdery cellulose acetate, lacquer grade, was dusted over the wetted surface. The grain was then placed in an oven and held at 110 C. for one hour. The propellant was covered with a hard, tough, extremely thin coat after the baking treatment. This coated grain was then positioned in a steel shell and a polyurethane restrictor applied between the sealer coat and the vessel wall by casting technique. The polyurethane restrictor material and the casting pro cedure were the same as described in US. Patent No. 3,012,507 at column 11, series A.

The gas generators were then tested for quality of the restrictor by being fired after the standard temperature cycling test. In this test, the gas generator was rapidly cooled from room temperature to -75 F. (-59.5 C.) and held at that temperature for 16 hours; the cold generator was then rapidly raised to 170 F. (79.4 C.) and held for 16 hours at this temperature; the vessel was then cooled to room temperature and fired. A time-pressure tracing was obtained during the firing and the shape of the curve illustrated the effectiveness of the restrictor coating. Six separate tests on gas generators prepared as described above showed no restrictor failures.

Tests were also made on gas generators wherein the sealer coat was derived from a solution of cellulose acetate in acetone painted on the surface of the propellant grain. In every instance, the time-pressure firing trace showed pressure surges which are conclusive proof that the sealer coat had failed at one or more sectors of the grain surface. Attempts to prepare satisfactory gas generators by the use of sealer coats laid down from cellulose acetate bodied citrate plasticizer were also failures. These tests demonstrate that the article of the invention wherein a cellulose acetate sealer coat is applied by the procedure described herein solved a very ticklish restrictor coating problem and such an article is now .in successful use on a routine production basis.

The drawing shows a side View, and a cross-sectional view taken at 22, of a cylindrical ammonium nitrate based propellant grain it to which has been applied as described herein two sealer coats 12a and 12b.

. Thus having described the invention, what is claimed 1s:

1. An article consisting essentially of an ammonium nitrate body having a portion of the surface of said body covered with a cellulose acetate coat, said coat having been obtained by (a) applying to the surface to be covered a layer of a liquid capable of plasticizing cellulose acetate, (b) applying powdered cellulose acetate to said liquid covered surface in an amount sufficient to form a seal coat thereon and (0) baking the acetate covered surface at a temperature of about 120 C. for a time suflicient to form a seal coat of cellulose acetate on said surface.

2. The article of claim 1 wherein said body consists essentially of ammonium nitrate, an oXidizable organic binder therefor and an ammonium nitrate combustion catalyst.

3. A solid propellant consisting essentially of (l) a shaped body portion formed of ammonium nitrate as the major component and an oxidizable organic binder therefor, the surface of said body being characterized by the presence of exposed ammonium nitrate which has not been covered by said binder and (II) a sealer coat adhered to and adapted to seal that part of the surface of said body where direct burning is to be prevented, where said sealer coat has been obtained by (a) applying to the surface to be covered a layer of a liquid capable of plasticizing cellulose acetate, (b) applying powdered cellulose acetate to said liquid covered surface in an amount sufiicient to form a seal coat thereon and (c) baking the acetate covered surface at a temperature of about 90- C. for a time sufficient to form a seal coat of cellulose acetate on said surface.

4. The propellant of claim 3 where a combustion restrictor coat is adhered to said sealer coat, said sealer coat barring contact between said exposed nitrate and said restrictor coat.

5. The propellant of claim 4 Where said liquid is acetyl triethyl citrate.

6. The propellant of claim 5 wherein said cellulose acetate has a combined acetic acid content of about 5058 weight percent.

No references cited.

Claims (1)

1. 3. A SOLID PROPELLANT CONSISTING ESSENTIALL OF (I) A SHAPED BODY PORTION FORMED OF AMMONIUM NITRATE AS THE MAJOR COMPONENT AND AN OXIDIZABEL ORGANIC BINDER THEREFOR, THE SURFACE OF SAID BODY BEING CHARACTERIZED BY THE PRESENCE OF EXPOSED AMMONIUM NITRATE WHICH HAS NOT BEEN COVERED BY SAID BINDER AND (II) A SEALER COAT ADHERED TO AND ADAPTED TO SEAL THAT PART OF THE SURFACE OF SAID BODY WHERE DIRECT BURNING IS TO BE PREVENTED, WHERE SAID SEALER COAT HAS BEEN OBTAINED BY (A) APPLYING TO THE SURFACE TO BE COVERED A LAYER OF LIQUID CAPABLE OF PLASTICIZING CELLULOSE ACETATE, (B) APPLYING POWDERED CELLULOSE ACETATE TO SAID LIQUID COVERED SURFACE IN AN AMOUNT SUFFICIENT TO FORM A SEAL COAT THEREON AND (C) BAKING THE ACETATE COVERED SURFACE AT A TEMPERATURE OF ABOUT 90*120*C. FOR A TIME SUFFICIENT TO FORM A SEAL COAT OF CELLULOSE ACETATE ON SAID SURFACE.

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