NO313324B1 - Ammonium nitrate propellant composition - Google Patents

Description

The present invention relates to ammonium nitrate propellant core compositions. More precisely, it is aimed at age-stabilized and/or reinforced ammonium nitrate propellant compositions.

Propellant charge compositions are suitable for a majority of applications. One such application is in vehicle airbag devices. In such devices, it is important to reduce the toxicity of the gases produced by the combustion of the propellant charge. It is also desirable that the propellant composition burn in a smokeless or nearly smokeless manner because the presence of smoke can cause various problems. For example, after an accident in which an airbag has been deployed, smoke not only obstructs visibility, it also interferes with any ongoing rescue attempt. Thus, it is desirable that the propellant mixture combustion products are smokeless or almost smokeless.

Another application of propellant compositions is their use in rockets and in other munitions as propellant propellant compositions. Combustion of propellant compositions in rockets and the like provides the energy required to transport them over long distances towards a given target. During combat, maintaining the advantage of surprise and stealth is critical. Therefore, it is desirable that rockets powered by propellant propellant combustion are as undetectable as possible during launch and during approach.

To maintain these advantages of secrecy and surprise, it is important that the propellant fuel composition is smokeless or near smokeless during combustion. In an effort to meet the desire for a smokeless combustion propellant composition, several compositions have been developed that U.S. Pat. Military (the US military). Among the compositions that have been developed are the "dual base" propellant core compositions. As is known in the art, "dual base" refers to a propellant composition containing both nitroglycerin (NG) and nitrocellulose (NC). Dual-base propellants tend to prematurely detonate or prematurely explode in response to various unplanned stimuli (eg, fire, heat, shell casing, projectiles, other fragments, etc.) that may occur in combat. In addition, for propulsion applications, the energy yield from the combustion of dual-base propellants is sometimes insufficient. Thus, the addition of energy-rich additives such as cyclotetramethylenetetranitramine (HMX) and/or cyclotrimethylenetrinitramine (RDX) is often required to provide the energy yield sought during combustion. However, the addition of such high-energy additives exacerbates the already hazardous tendency of dual-base propellants to premature explosion or premature explosive fire.

Nevertheless, to meet the smokeless and high energy requirements of propellant propellants, as defined herein, propellant compositions including dual base propellants are sought at the expense of safety, particularly with respect to naval operations. Consistently, the U.S. The Navy has taken the lead in formulating a series of standards regarding non-sensitive ammunition requirements, formalized as MIL-STD-2105B, incorporated herein in its entirety by reference. Equivalent insensitive ammunition standards have been adopted by most major military powers (eg England, France, Germany, etc). These standards require that propellant compositions meet and exceed insensitivity as ammunition safety standards for the weapon platforms for which they are designed.

Furthermore, with respect to military propellant applications, several different smoke characteristics required of propellant compositions have been strictly defined. Based on the empirical work conducted by the U.S. Missile Command at Redstone Arsenal and some of their counterparts in other countries, industry accepted definitions for "minimum smoke" and "reduced smoke" promulgated in STANAG 6016 (Nato Standardized Agreement Solid Propellant Smoke Classification). STANAG 6016 is incorporated herein in its entirety by reference. The smoke emission calculated using the number of thermo-chemical codes that are well known within the industry. For example, the STANAG 6016 classifications "AA" and "AC" correspond respectively to the minimum smoke and reduced smoke definitions. The terms "smoke-free", "almost smoke-free" and/or "substantially smoke-free" as used herein are synonymous with the definition of minimum smoke (ie code AA).

To meet these requirements (i.e. smokeless-minimum smoke according to STANAG 6016, high energy yield and safety according to insensitive ammunition requirements formalized as MIL-STD-2105B) attempts have been made to develop non-dual base propellant compositions which are smokeless and yet ensure to handle. For example, ammonium nitrate, metal nitrate, alkaline earth metal nitrate, ammonium perchlorate and metal perchlorate propellant compositions and the like have been used. However, these propellant compositions present several problems. Metal nitrates typically produce solid particles upon combustion. These solid particles form a visible smoke referred to as "primary smoke" which is undesirable. Ammonium or metal perchlorates produce hydrogen chloride during combustion. Hydrogen chloride reacts with moisture in the surrounding air to become a liquid/gas aerosol. The aerosol forms another visible smoke referred to as "secondary smoke". Either "primary smoke" or "secondary smoke" formed as an emission from the combustion of a propellant propellant composition negates the advantage of surprise. The smoke trail helps opposing forces in the destruction or other counterplay against the incoming missile. In addition, such emissions of smoke point towards the firing position. During combat, such smoking areas expose the staff to a greater risk of potential successful retaliation, e.g. in case of retaliatory firing of battery.

Ammonium nitrate as a propellant ingredient can produce a propellant that produces no primary or secondary smoke upon combustion. However, ammonium nitrate presents other setbacks as a propellant component. In principle, ammonium nitrate is known to undergo several crystal phase changes at various well-known temperatures. Pure ammonium nitrate undergoes a series of structural and volumetric crystal phase transformations over typical operating temperature ranges. In pure ammonium nitrate, crystal structure phase transitions are observed at -18°C, 32.3°C, 84.2°C and 125.2°C, respectively. The phase transition at around 32.3°C is particularly problematic. A large volumetric change (about 3.7%) in the crystal phase of ammonium nitrate is observed when the temperature varies between above and below about

32.3°C (ie the transition between phase IV (below 32.3°C) and phase m (above 32.3°C)). When the ammonium nitrate moves between phase IV and phase Ul it expands and contracts. Repeated variations through phase IV to phase m are the transition temperature (ie about 32.3°C) associated with ammonium nitrate grain growth and destruction of grain purity. The result is that there is porosity and loss in mechanical strength of ammonium nitrate-based drivladrungsairmen sets.

As used herein, the term "aging stabilized" refers to a state of ammonium nitrate where the crystal phases ffl-IV and the associated volumetric changes upon thermal cycling are substantially reduced. Thus, the durability of ammonium nitrate propellant core composition is significantly increased from about 1-2 years to about 5-20 years or more.

Furthermore, the term "strengthened" as used herein refers to a state of ammonium nitrate propellant where the tensile strength of the propellant is increased without undue sacrifice of elongation or, alternatively, is associated with an increase in elongation. The reinforced ammonium nitrate propellant shell composition is essentially resistant to physical degradation of the propellant.

Also as used herein, the term "safe" refers to an ammonium nitrate propellant composition that meets or exceeds the sensitivity munitions requirements promulgated in MDL-STD-2105B in which the tendency for violent explosive combustion or explosion is substantially reduced and the shelf life is substantially increased from about 1-2 years to about 5-20 years or more. Furthermore, the term "safe" as used herein refers to an ammonium nitrate propellant composition in which the tendency to form grain cracks due to crystal phase changes is substantially reduced or completely eliminated.

It is feared that non-reinforced/non-age-stabilized ammunition charge compositions that have been stored (eg, either in ammunition or in vehicle airbag retention devices) for more than about 1 to 2 years may have undergone several crystal phase changes to the extent that the physical integrity of the propellant may have been compromised and the propellant will no longer function in the desired manner. Accordingly, the useful shelf life of prior art ammonium nitrate propellant compositions is disadvantageously shortened. Thus, it is desirable to formulate a smokeless (or substantially smokeless) and nevertheless safe ammonium nitrate propellant composition with an extended shelf life.

Typically, a propellant or gas generating device containing a propellant composition requires a storage time of from about 5 to about 20 years or more. The storage time for the device is essentially dependent on the storage time for the propellant composition contained therein. Typically, a desirable storage time for an ammunition (propellant) propellant composition or a vehicle airbag (gas-producing) propellant composition is about 5 or more years, preferably from about 7 to 20 years. In order to achieve a longer storage time for ammonium nitrate propellant compositions, measures have been directed towards solving the crystal phase stabilization problem (ie that for ammonium nitrate). For example, several patents and publications suggest the use of KNO 3 , KF, metal dinitramide, or metal oxides such as MgO, NiO, CuO and/or ZnO as additives providing phase-stabilized ammonium nitrate. See e.g. U.S. Patent No. 4,158,583 to Anderson, U.S. U.S. Patent No. 5,076,868 to Doll et al. U.S. Patent 5,271,778 to Bradford et al. Patent No. 5,292,387 to Highsmith et al. And the U.S. Mishra, U.S. Patent No. 4,552,736 Patent No. 5,545,272 to Poole et al. See also Choi, C.S. and Prask, H.J., Phase Transitions in Ammonium Nitrate, J. Appl. Cryst., vol. 13, p. 403-409 (1980). However, several problems are associated with the use of the aforementioned phase stabilizing additives. For example, the use of potassium nitrate leads to the formation of large quantities of unwanted residues as combustion products. See U.S. Patent No. 4-552,736 to Mishra. When KF is used, it must be added to the molten phase (I) of ammonium nitrate. The KF-modified ammonium nitrate is then cooled. The requirement for melting ammonium nitrate before the addition of KF is cumbersome, expensive and takes a lot of time. In addition, the emission from such a device using such a propellant charge is corrosive, smoky (with an increased radar cross-section) and toxic.

The use of the metal oxides also has several disadvantages. E.g. solid particles are formed during combustion when MgO, NiO, CuO and/or ZnO are used. Solid particles, as noted earlier, contribute to the formation of primary smoke which is undesirable. Furthermore, NiO is carcinogenic. Furthermore, NiO and CuO pose environmental hazards. In addition, both NiO and ZnO are only marginally effective. That is once exposed to moisture, these oxides are no longer effective ammonium nitrate phase stabilizers. Furthermore, NiO and ZnO increase the detonation ability of the ammonium nitrate, which is undesirable. Furthermore, the production of propellant compositions comprising NiO and/or ZnO is more expensive. Similarly, the use of metal dinitramides (see '387 by Highsmith et al.) also leads to the formation of primary smoke on combustion. Thus, none of the known ammonium nitrate phase stabilizers is completely satisfactory for the formation of a safe, age-stable and smokeless ammonium nitrate propellant composition with a long shelf life.

The occurrence of phase Ul in ammonium nitrate depends on the presence of water, e.g. down to as little as about 0.1% by weight relative to the ammonium nitrate. See Choi et al., J. Appl. Cryst., vol. 13, page 403 (1980). In particular, according to the '736 patent by Mishra above (in column 2, lines 66-68), a high moisture content is said to favor Ni-IV phase transitions. Furthermore, according to the U.S. patent no. 4,86,396 to Kjohl et al., (in column 1, lines 30-32), these phase transitions make the ammonium nitrate less stable against thermal fluctuations.

U.S. Patent No. 5,061,511 to Baczuk ('511) suggests the use of aluminosilicate molecular sieves (having a pore size of less than about 10 angstroms) as a stabilizer in propellant compositions such as single base or dual base propellant. In particular, the '511 patent is directed to propellant compositions which give off gases during the aging process. These propellant compositions include nitrocellulose and nitroglycerin, high energy flour containing propellants, single or double base nitrate ester propellants and composite propellants such as ammonium perchlorate/AL with rubber binders. The unwanted gases released from these propellant charges during firing include N2, CO2, CO, Nox and F2. Likewise, the U.S. suggests patent no. 4,045,261 by Baczuk ('261) the use of a molecular sieve

(with a pore size of 10 angstroms or more) as part of a stabilization system for a urethane cross-linked double-base propellant composition to capture nitric acid.

Since ammonium nitrate is not a propellant of the class described by Baczuk (ie, See '511) and does not indicate the gases N2, C02, CO, Nox, or F2 (ie, see '511) during aging, there is no general expectation that molecular sieves, even less those with a pore size of 10 angstroms or less will stabilize ammonium nitrate. Similarly, since ammonium nitrate is not a urethane cross-linked double base propellant (ie see '261) there is correspondingly no expectation that molecular sieves, e.g. with a pore size of 10 angstroms or more will stabilize ammonium nitrate against volumetric crystal phase changes.

Nevertheless, since water is associated with the undesirable crystal phase changes of ammonium nitrate, Kjohl et al., supra, used porous additives capable of absorbing water to stabilize ammonium nitrate. They further discovered that the presence of water absorbing porous particles resulted in no movement of water in the ammonium nitrate particles and that during thermal oscillation only a small amount of swelling of ammonium nitrate was observed. Kjohl et al., however, state that the porous particles should be added to the ammonium nitrate after the ammonium nitrate has been dried. Finally, they state that not just any type of porous particles is suitable for stabilizing ammonium nitrate. For example, according to Kjohl et al., silicates of the molecular sieve type can bind water, but it has been found difficult to assign such particles the desired particle size and binding to the ammonium nitrate particles. As a result, Kjohl et al. conclude that molecular sieves work poorly for stabilizing ammonium nitrates (see column 3, lines 18-23).

Thus, there is still an existing need to provide a safe, age-stabilized and/or reinforced ammonium nitrate discharge composition with a long shelf life that is sufficiently smoke-free when burned, as well as a method for its production.

It is therefore an object of the present invention to provide a safe, age stabilized, sufficiently smokeless ammonium nitrate propellant composition which has a long shelf life (eg up to about 20 years or more).

It is another object of the present invention to provide a reinforced, substantially smokeless ammonium nitrate propellant composition having a long shelf life.

It is yet another object of the present invention to provide a reinforced and age-stabilized, substantially smokeless ammonium nitrate propellant composition having a long shelf life.

Surprisingly, these and other purposes can be achieved by the addition of silicates of the molecular sieve type to ammonium nitrate prior to grinding the ammonium nitrate present in the propellant composition. The invention thus includes an age-stabilized ammonium nitrate propellant composition, consisting essentially of ammonium nitrate, which has a crystalline phase present in an amount from approx. 40% to approx. 85%, a synthetic zeolite that has a pore size of approx. 3 angstroms to approx. 13 angstroms present in an amount from approx. 0.2% to approx. 0.4%, and a binder present in an amount from approx. 3% to approx. 40% by weight of the composition, and wherein said ammonium nitrate, synthetic zeolite and binder are each present in an amount effective to stabilize the crystalline phase of said ammonium nitrate, to give the composition a shelf life of at least approx. 5 years, and to produce a composition that is essentially smoke-free when burned.

One possible method of forming a safe, age-stabilized ammonium nitrate propellant composition includes the steps of providing a quantity of ammonium nitrate, adding a sufficient amount of a silicate molecular sieve to absorb water from the ammonium nitrate, grinding the ammonium nitrate with the molecular sieve, maintaining contact between the ammonium nitrate and the sieve. , then adding at least one binder (apart from any curing agent, e.g. isocyanate curing agent), maintaining contact between the milled molecular sieve and the other ingredients and finally adding curing agent, if any, to achieve the safe, age stabilized ammonium nitrate propellant composition with a long shelf life.

Alternatively, these and other purposes are achieved by the addition of a strengthening agent to a composition according to the invention in order to obtain a reinforced and age-stabilized ammunition nitrate propellant composition.

In addition, an age-stabilized ammonium nitrate composition can be formed by adding a molecular sieve to ammonium nitrate (eg at least about 1 gram of molecular sieve per 0.454 kg of ammonium nitrate) and then grinding the mixture. Then the mixture can be stored safely without harmful changes for an extended period of time in a sealed container.

According to the present invention, there are at least three embodiments of the ammonium nitrate propellant composition. The first embodiment of the present invention relates to an age stabilized ammonium nitrate propellant composition. The second embodiment relates to an age-stabilized and reinforced ammonium nitrate propellant composition. Furthermore, each embodiment can be used with certain modifications as a gas-producing ammonium nitrate propellant composition or as a propellant ammonium nitrate propellant composition. The gas-producing ammonium nitrate propellant compositions are designed to be used in vehicle airbag systems and the like where gas production is of greatest importance. The propellant ammomummtrate d^rivladmng compositions are designed to be used in rockets and other ammunition where the energy effect is of greatest importance.

In the first (ie age stabilized) embodiment, the substantially smokeless ammonium nitrate propellant composition comprises ammonium nitrate, a synthetic zeolite which is a molecular sieve and a binder. Optionally, the first embodiment may contain one or more of a plurality of additives. These additives include, but are not limited to, a nitroplasticizer (e.g. nitramines and/or nitrate esters which are in a liquid phase when added, typically at room temperature such as about 25°C), an energy-rich additive (e.g. nitramines which are in solid form when added, typically at room temperature), a nitrate ester stabilizer, a curing agent, a curing accelerator, a feed agent, and a polymer protector (eg, an antioxidant). In the first embodiment, the ammonium nitrate can be present as small particles, pearls, granules and the like. The size of the ammonium nitrate can vary between about 5 µM and about 5,000 µM (or any value in between) in thickness. However, a particle thickness is preferably from about 5 µM to about 400 µM and most preferably from about 30 to about 50 jjM.

In the first embodiment, the amount of ammonium nitrate included depends on the application for which the propellant composition is designed. For example, in propulsion applications, additives that increase the energy effect (e.g. nitro softeners and/or energy-rich additives) in the first embodiment are preferably inhibited herein. On the other hand, if designed for gas-producing applications, such nitrosofteners and/or high-energy additives are often omitted from the initial design. However, nitroplasticizers and/or energy-rich additives may optionally be included herein. The amount of ammonium nitrate included in the first embodiment varies depending on the presence or absence of nitro plasticizers and/or high energy additives therein.

Unless otherwise specifically indicated, the weight percent values of the various propellant compounds given below refer to a percent of the total weight of the propellant composition. In the first embodiment when used for gas producing applications, the ammonium nitrate (when a nitro plasticizer and/or high energy additive is added) is present in an amount of at least about 60%. When nitroplasticizers and high-energy additives are absent from the initial design designed for gas-producing applications, the amount of ammonium nitrate present can vary from about 65% to about 85%.

However, for driving applications of the first embodiment when combined with the optional nitro plasticizers and/or high energy additives, the amount of ammonium nitrate added may vary from about 40% to about 80% (or any value in between). In the absence of such nitroplasticizers and energy-rich additives, the amount of ammonium nitrate added to the first version designed for driving applications varies from about 65% to about 85%.

Furthermore, as noted, the first embodiment of the invention includes a molecular sieve. One type of molecular sieve is the aluminosilicate type molecular sieve, commonly referred to as a zeolite molecular sieve. See, for example, Breck, D.W., Crystalline Molecular Sieves, Journal of Chemical Education, Vol. 41, page 678 (December 1964). See UOP, Product Information Sheet, Union Carbide Molecular Sieves, Molecular Sieve Type 4A. See also "Linde<®> Molecular sieves Data", "Linde<®>" Molecular Sieve Type 4A. See also Cotton, F.A. and Wilkinson, G., Advanced Inorganic Chemistry-A Comprehensive Text, pages 390,392,4. Ed., John Wiley and Sons (New York 1980). Typical zeolites have the formula Mex/n ((AlO 2 ) x (SiO 2 )y). Z H2O where Me = metal cation and x, y and n are integers. Z is zero or a positive real number. Z indicates the number of hydrate waters associated with a given zeolite. Typically y/x ranges from about 1 to about 5. An example of a type A synthetic zeolite has the formula Nai2 ((AIO2) (SiO^n) • 27H2O. As noted by Cotton & Wilkinson, above, a molecular sieve is obtained by heating of a zeolite to about 350°C under a vacuum to remove the water of hydrate. Thus, once the water is removed from a typical molecular sieve such as Nai2((Ali2Sii2048) - 27H20), a type A zeolite with anhydrous cubic microcrystals is formed.

For a particular zeolite to be suitable for use in the first embodiment it must have two properties. It must be a more active absorber of water than any other compound (eg, the ammonium nitrate, the binder, etc.) of the ammonium nitrate propellant core composition with the exception of the curing agent (ie, in the presence of water, eg, isocyanate curing agents react typically fast with this). Furthermore, the zeolite must retain the absorbed water molecules so that the water is not available to any of the other compounds in the ammonium nitrate propellant core composition, especially the ammonium nitrate. Thus, the retention of the water in the zeolite/water adduction must be extremely robust.

In particular, without being bound by theory, it is assumed that the water molecules must not be simply absorbed to the surface of the zeolite. It is believed that zeolites keep the water molecules inside the pores present in the sieve. Furthermore, without being bound by theory, it is assumed that the water molecules can first be absorbed to the surface of the zeolite. However, after a short period of time (eg up to about 48 hours) the water molecules are transported to the interior of the zeolite via its pores. Again without being bound by theory, it is believed that if the sieve has a suitable pore size (eg, typically about 13 angstroms or less such as from about 3 to about 13 angstroms or any value in between), then the water can be absorbed in the interior of the zeolite. Thus, the other compounds in the ammonium nitrate propellant composition (eg, binders, nitroplasticizers, high-energy additives, nitrate ester stabilizers, curing agents, curing catalysts, matting agents, and/or antioxidants) are sterically isolated from the absorbed water.

As noted, for the zeolite/molecular sieve to effectively age stabilize ammonium nitrate, it is required that the zeolite has a pore size sufficient to absorb and ultimately retain water. Therefore, the absorbed water becomes unavailable to the ammonium nitrate and the other compounds in the propellant composition. Typically, a pore size suitable for this application is around 13 angstroms or less. Preferably, the pore size is from 3 to about 13 angstroms or any value in between. More preferably, the pore size is from about 3 to about 5 angstroms. More preferably, the pore size is about 4 angstroms. Examples of zeolites/molecular sieves compatible with the first embodiment of the invention include, but are not limited to molecular sieves respectively of the type 3 A, 4A, 5 A and 13X. These sieves are manufactured by various companies, including Union Carbide (N.Y., N.Y.) which sells its molecular sieves under the trade name "LINDE<®>". Molecular sieve 4A is a sodium form of the type A crystal structure. It is an alkali metal alumino-silicate. The Type 4A sieve will absorb molecules with critical diameters up to about 4 angstroms.

In the first embodiment of the propellant composition, the zeolite is present in an amount from about 0.2% to about 0.4% (or any value in between). Preferably, the zeolite/molecular sieve is present from about 0.20% to about 0.22%. The preferred zeolite/molecular sieve is the type 4A sieve.

Binders compatible with the first embodiment of the present invention include, but are not limited to, thermoplastic elastomers (eg, "Finaprene™", "Kraton™" or mixtures thereof) and a set-consolidating material. Examples of cure-consolidating materials include, but are not limited to, a hydroxy-terminated polybutadiene (HTPB), hydroxy-terminated polyether (HTPE), polyglycol adipate (PGA), glycidyl azide polymer (GAP), poly bis-3,3'-azidomethyloxetane (BAMO), poly-3 -nitratomethyl-3-methyloxetane (PNMMO), polyethylene glycol (PEG), polypropylene glycol (PPG), cellulose acetate (CA) or mixtures thereof. An example of a binder is a mixture of 7 parts by weight of BAMO and 3 parts by weight of PNMMO. However, the preferred binder is PGA. It is noted that other binders well known in the art can be used.

Because the nitroplasticizers are incompatible with HTPB (ie, they are not soluble in each other) they are preferably not combined in any of the embodiments of the propellant composition. When HTPB is the binder of choice, energetic additives (e.g., solid phase nitramines such as RDX, HMX) and/or other plasticizers such as dioctyl adipate (e.g., in an amount of about 3 to about 10% or any value in between) is used. Other plasticizers compatible with HTPB are well known to those skilled in the art and may be used herein.

The thermoplastic elastomer binders compatible with the first, i.e. all stabilized, embodiments of the present invention are those having melting points or plasticizing melting points above the expected use and storage temperatures of the propellant compositions. Typically, the application and storage temperatures range from about -54°C to about 93°C. Furthermore, the thermoplastic elastomers must melt in their plastic state below the decomposition temperature of ammonium nitrate and/or in any nitroplasticizer present therein. In the first embodiment, the binder is present in from about 3% to about 40% (or any value in between), preferably from about 5% to about 30%.

Furthermore, as noted earlier, the first embodiment may further contain an energy-rich additive (i.e., a solid phase component that increases the energy effect, e.g., such as nitramines) and/or a nitroplasticizer (i.e., a liquid phase component that increases the energy effect, e.g., as nitrate esters and as nitramines). Typical nitroplasticizers compatible with the age-stabilized ammonium nitrate propellant composition (ie, the first embodiment) of the present invention include, but are not limited to, trimethylolethane trinitrate (TMETN), triethylene glycol dinitrate (TEGDN), triethylene glycol trinitrate (TEGTN), butanetriol trinitrate (BTTN) , diethylene glycol dinitrate (DEGDN), ethylene glycol dinitrate (EGDN), nitroglycerin (NG), diethyleneglycerin trinitrate (DEGTN), dinitroglycerin (DNG), nitrobenzene (NB), N-butyl-2-nitratoethylnitramine (BNEN), methyl-2-nitratoethylnitramine (MNEN) , ethyl-2-nitratoethylnitramine (ENEN) or mixtures thereof. The preferred nitroplasticizer is a 50-50% by weight mixture of TMETN and TEGDN. In the first embodiment, the nitroplasticizer is optionally present in up to about 40% by weight. Examples of high-energy additives compatible with the first embodiment include, but are not limited to, dinitroxydiethylnitramine (DNDEN), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), or mixtures thereof. The preferred energy-rich additives are RDX, HMX and mixtures thereof. In the first embodiment, they are preferably present in up to 40%. As similar nitroplasticizers and high-energy additives become commercially available, they may be included in this list as those of ordinary skill in the art will recognize.

Because nitroplasticizers and high-energy additives tend to increase the energy output, flame temperature, and explosive nature of an armononium nitrate-(Mvladmngs) composition, these materials are not always included in the first embodiment of the invention when used in gas-producing applications, but soon they may be included herein. In the gas-producing applications, the nitroplasticizer and/or the energy-rich additive may be present (ie as an optional additive) in an amount of up to about 35%. Used, it is desirable to use nitroplasticizers and/or energy-rich additives in the driving applications of the first embodiment where increased energy output is of greatest importance. In the propellant munitions applications the nitro plasticizer and/or high energy additive is typically present in an amount from about 5% to about 25% or any value in between. The amounts of binder plus any preferably nitroplasticizer and energy-rich additive must make up at least around 20% in total to form a physically acceptable first embodiment and preferably from about 20% to about 35%. As used herein, "physically acceptable" means a composition that can be formed into a plurality of desired shapes, (eg, grains, etc.) and that can be maintained in those shapes.

When a nitroplasticizer which is a nitrate ester is included in the first embodiment, it is preferred that a nitrate ester stabilizer is also added. When a nitroplasticizer is not used or when an energy-rich additive (e.g. a nitramine such as RDX, HMX or mixtures thereof) is included without a nitroplasticizer in the propellant core composition, the nitrate ester stabilizer can be omitted from the propellant composition. Thus, in the first embodiment according to the present invention, the nitrate ester stabilizer may be present in an amount of up to about 3%, more preferably from about 0.1 to about 2% and most preferably from about 0.35% to about 0.5%. Nitrate ester stabilizers compatible with the first embodiment of the present invention include, but are not limited to N-methyl-4-nitroaniline (MNA), 2-nitrodiphenylamine (NDA), ethyl centralite (EC) or mixtures thereof. The preferred nitrate ester stabilizer is a mixture of MNA and NDA, preferably in a weight ratio of about 1:1.

Curing agents compatible with the first embodiment of the present invention include, but are not limited to, hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), dimeryl diisocyanate (DDI), diphenylmethane diisocyanate (MDI), naphthalene diisocyanate (NDI), dianisidine diisocyanate (DADI ), phenylene diisocyanate (PDI), xylylene diisocyanate (MXDI), other diisocyanates, triisocyanates, higher isocyanates than the triisocyanates, polyfunctional isocyanates (e.g. Desmodur N 100), other polyfunctional isocyanates or mixtures thereof. It is preferred that the isocyanate has at least two reactive isocyanate groups. If there is no binder component with a functionality greater than 2, then the curing functionality (eg, the number of reactive isocyanate groups per molecule of isocyanate curing agent) must be greater than 2.0. The quantity of the curing agent is determined by the desired stoichiometry (ie stoichiometry between curable binder and curing agent). The curing agent is present in an amount of up to about 5%. However, if a curable binder (eg, binders with reactive hydroxy groups such as HTPB) is used, the curing agent is present from about 0.5% to about 5%.

When a curing agent is used, a curing catalyst is preferably added to the propellant composition. The curing catalyst is used to accelerate the curing reaction between the curable binder and the curing agent. Curing catalysts compatible with the first embodiment of the present invention include, but are not limited to, a tin dilaurate (eg an alkyl tin dilaurate, butyl tin dilaurate, isopropyl tin dilaurate etc), metal acetylacetonate, triphenyl bismuth, maleic anhydride, magnesium oxide and mixtures thereof. A preferred curing catalyst is an equal weight % mixture (ie 33 1/3%) of each of triphenylbismuth, maleic anhydride and magnesium oxide. The curing catalyst is present in up to about 0.3% by weight. Furthermore, one matting agent compatible with the first embodiment is carbon black. The matting agent is present in up to about 2%. Those skilled in the art are aware of other matting agents that can be used.

Antioxidants can also be added to the first embodiment of the present invention. Antioxidants compatible with the first embodiment of the present invention include, but are not limited to 2,2'-bis (4-methyl-6-tert-butylphenol), 4,4'-bis (4-methyl-6-tert- butylphenol) or mixtures thereof. Other antioxidants well known in the art are within the scope of the present invention. The antioxidant is present in an amount of up to about 1%.

As shown in table JJ, as the percentage of reinforcing agent is increased to 15%, both the maximum stress and the percentage elongation increase from the control values. Increased maximum stress and percentage elongation indicate a tougher propellant composition more resistant to premature explosive fire or explosion from unplanned stimuli such as dust or multiple fragments e.g. projectiles and the like. The maximum stress and elongation measurements were performed according to the test procedure described in Chemical Propulsion Information agency (CPIA) publication 21, section 4.3.2 (Supplement) using a Class C specimen. CPIA Publication 21 is incorporated in its entirety herein by reference.

Now referring to the third embodiment of the invention (ie, an age-stability/enhanced propellant composition), this embodiment comprises ammonium nitrate, a molecular sieve, a reinforcing agent and a binder. The same ammonium nitrates, molecular sieves, reinforcing agents, binders, nitroplasticizers, high-energy additives, nitrate ester stabilizers, curing agents, curing catalysts, matting agents and/or antioxidants compatible with the first embodiment are similarly compatible with the second embodiment.

The addition of a booster to a propellant composition reduces its impulse. To compensate for the loss of impulse, nitro plasticizers and/or energy-rich additives are added. Typically, ammonium nitrate propellant compositions lose their detonable characteristic when the impulse is less than or equal to about 2246 N s/kg. However, if a sufficient amount of one or more nitroplasticizers and/or high energy additives is added to an amiomummtrate oMloading composition containing a reinforcing agent, then the benefits of the reinforcing agent are obtained without loss of impulse. The effect of adding a fortifying agent to the following ammonium nitrate recharge compositions (Table I) produced the results indicated below (Table II). These ciriloading core compositions are not in themselves part of the invention, but are included only to describe the effect of enhancers.

As previously noted unless otherwise indicated, all compounds are indicated as a weight % of the total weight of the propellant composition. Furthermore, all components in the propellant composition add up to a total of 100% by weight. Preferred enhancing agents compatible with the second embodiment of the present invention include, but are not limited to, azodicarbonamide, dicyandiamide, oxamide, or mixtures thereof. The most preferred reinforcing agent is azodicarbonamide. In the second embodiment of the present propellant composition, the enhancing agent is present in an amount of from about 2 to about 20%. Preferably, the enhancing agent is present from about 3% to about 12% and most preferably from about 8 to about 12%.

Further, unless otherwise indicated below, the amounts of ammonium nitrate, nitroplasticizer, energy-rich additive, nitrate ester stabilizer, curing agent, curing catalyst, matting agent and/or antioxidant described with respect to the first embodiment are similarly applicable to the second embodiment. It should be noted that the second embodiment contains both a molecular sieve and a reinforcing agent. The amounts of the molecular sieve/zeolite added to the first embodiment are similarly applicable to the second embodiment. Finally, in the second embodiment, the binder is present in an amount from about 5% to about 30%. The binder plus any nitroplasticizer and energy-rich additive must total at least about 20% to form a physically acceptable second embodiment and preferably from about 20% to about 35%.

In the following, methods for forming the various versions of the composition according to the present invention are described in detail. Initial drying of all components is accomplished according to typical industrial practice, which is well known in the art. For example, the ammonium nitrate is dried (e.g. in an oven) at approx. 110°C for about 16 hours to remove surface water. Appropriate initial drying procedures for all propellant components are well known to those skilled in the art. As previously indicated herein and as indicated immediately below for embodiments containing a molecular sieve (the first and second embodiments), significant additional drying is achieved with the molecular sieve.

Before the other components of the driving fluid composition are added, at least a portion of the molecular sieve which is ultimately to be added is mixed with the ammonium nitrate. For example, after initial drying, the molecular sieve is added to the extent that at least about 1 gram per 0.4536 kg of ammonium nitrate. The mixture of the ammonium nitrate and the molecular sieve can then be allowed to stand for a first aging period. The first aging period is up to about 48 hours or longer, preferably from about 0.25 hours to about 16 hours and more preferably as close to zero as possible. If the mixture of the molecular sieve and the ammonium nitrate is exposed to the ambient air (ie including the moisture therein) it is preferably ground immediately after mixing (ie the first aging period is zero minutes or so). If, however, the mixture is kept in a closed container (ie with limited exposure to ambient air and the moisture therein) then the mixture can be maintained indefinitely without painting. For example, the first aging period can be up to about 48 hours (or more), such as from about 4 to about 16 hours. However, it is preferred to grind the mixture immediately (or, for example, as soon as is practical to do so in a production or assembly line) after mixing to obtain a first mixture. Although not bound by theory, it is believed that the paint (mixture) allows the molecular sieve to be in greater physical proximity to the ammonium nitrate and the water associated therewith. Thereby, it is further assumed that the paint allows the molecular sieve to more efficiently and effectively absorb (and retain) water away from the ground ammonium nitrate.

Grinding is achieved on a ball mill, fluid energy mill or micropulverization. Other painting methods well known in the art can also be used. Furthermore, as long as it is compatible as a feed material to the particular grinding process used, the particle characteristics (eg particle thickness, particle size, particle shape-grain, pearls, crystal size, etc.) of the ammonium nitrate are not critical. The size of the ammonium nitrate should be as previously noted from about 5 µm to about 400 (J.M preferably from about 30 µm to about 50 µM in thickness.

After both painting and then aging (or just painting if first aging period = zero hours), the remaining components of the propellant composition (except for any curing agents) are added to the first mixture to obtain a second mixture. The second mixture is then allowed to stand for another aging period.

The second aging period allows the molecular sieves to absorb (and retain) a sufficient amount of the water present to age stabilize the second mixture. The second aging period is up to about 48 hours or longer, preferably from about 0.25 hours to about 24 hours and most preferably from about 16 to about 24 hours. Finally, the curing agent, if any, (eg, isocyanate curing agent) is optionally added to the second mixture to complete and form the final age-stabilized ammonium nitrate propellant composition or the final age-stabilized/reinforced propellant composition. The propellant composition in example 8 below was prepared where the first aging period was set to zero, 2 hours and 48 hours, respectively. The effect of variation in the second aging period on hardness (Shore A), ultimate tensile strength (psi) and elongation at break (%) of the propellant composition from Example 8 below is given in Table HI below.

Elongation is an indication of elasticity. It indicates the length to which the drive charge assembly can be stretched before it breaks. An increase in tensile strength with a concomitant increase in elongation indicates an increase in "toughness". The increase in "toughness" indicates that less damage will occur in projectile or fragment impact scenarios. Less damage means less surface area that is burned and therefore the reaction to unplanned stimuli (e.g. projectiles or fragment collisions) will be less violent. The aging period significantly increases the shelf life (eg, to 20 years or more) of the ammonium nitrate propellant composition.

Having described the invention, the following examples are set forth to illustrate specific applications thereof, including the best method now known to carry out the invention. These specific examples are not intended to limit the scope of the invention described herein.

EXAMPLES

The following propellant core compositions were prepared using the ingredients in the amounts indicated below. However, where indicated, the examples are prophetic. It should be noted that in all of the prophetic examples, a nitrate softener and a curing catalyst are included where appropriate as previously explained. Furthermore, all components in each formulation add up to a total of 100% by weight. Prophecy Examples 9,10,11 and 12 all indicate stabilized and mildly boosted propellant compositions with a minor loss in propellant composition impulse. Prophecy Examples 13,14,15 and 16 indicate age-stabilized and moderately enhanced propellant core compositions with a moderate loss in propellant composition impulse. Prophecy Examples 17, 18, 19 and 20 indicate age-stabilized and highly enhanced propellant compositions with a significant loss in propellant composition impulse. Other minor ingredients that may be included in all prophetic examples include matting agents, nitrate ester stabilizers, antioxidants and curing catalysts. These minor components increase age stabilization, ballistic uniformity, accelerated cure, and the like well known in the prior art.

Example 1

(Aging stabilized)

Example 2

(Aging stabilized)

Prophetic example 3

(Aging stabilized)

Prophetic example 4

(Aging stabilized)

Prophetic example 5

(Aging stabilized)

Prophetic example 6

(Aging stabilized)

Example 7

(Aging stabilized and reinforced)

A first mixture was prepared according to Example 21 below. All ingredients (except the curing agent) in the amounts indicated immediately below were combined in a propellant mixer and mixed for 15 minutes without vacuum. The mixing was then stopped and the mixture was kept at 60°C for 18 hours with the mixing pan sealed. The curing agent was then added and then mixed therein under vacuum for 15 minutes. The mixture was then scraped down and the propellant mixed under vacuum for another 15 minutes. This mixture was then safely cast into a block mold tool or into engines. The propellant composition was then cured within 48 hours at 60°C.

The addition of 4% dicyanodiamide to the above propellant composition more than doubled the tensile strength and also significantly increased the strain capacity (ie elongation) thereof.

Example 8

(Aging stabilized)

Prophetic example 9

(Aging stabilized and slightly reinforced)

Prophetic example 10

(Aging stabilized and slightly reinforced)

Prophetic example 11

(Aging stabilized and slightly reinforced)

Prophetic example 12

(Aging stabilized and slightly reinforced)

Prophetic example 13

(Aging stabilized and moderately reinforced)

Prophetic example 14

(Aging stabilized and slightly reinforced)

Prophetic example 15

(Aging stabilized and moderately reinforced)

Prophetic example 16

(Aging stabilized and moderately reinforced)

Prophetic example 17

(Aging stabilized and moderately reinforced)

Prophetic example 18

(Aging stabilized and moderately reinforced)

Prophetic example 19

(Aging stabilized and moderately reinforced)

Prophetic example 20

(Aging stabilized and moderately reinforced)

Example 21

(First mix)

0.454 kg of ammonium nitrate (AN) was dried at 110°C for 16 hours. One gram of 3A molecular sieve was added and the mixture ground in a ball mill for 10 minutes. This produced a first mixture of AN with an average particle size of about 10 µM. The first mixture is best used immediately, but can be stored in a sealed container for at least about a year.

Example 22

(Composition 7019-A with no age stabilization)

(Comparative example, not part of the invention)

Cellulose acetate (4.0 grams) was dissolved in 25 ml of acetone. AN was ground according to example 21 without the addition of molecular sieves. Ground AN (60 grams) and RDX (36 grams) were mixed dry by tumbling in a mixing pan. The mixed dry material was then added to the cellulose acetate/acetone solution and stirred by hand. Acetone was added as needed to form a thick paste. The paste was then formed into sheets, extruded into ribbons, or made into granules by molding while still moist. In the preferred form, 7019-A was formed into sheets about 0.0762 cm thick, then dried in a vacuum oven at 60°C. The sheets were then broken up into smaller pieces and then sieved through a 5 mesh sieve.

Example 23

(Composition 7019-A with age stabilization)

The procedure followed was the same as Example 22 except that the AN was formalized with molecular sieves according to Example 21.

Example 25

(Aging stabilized composition)

The same procedure as Example 7 was followed except that the reinforcing agent was omitted from the composition. The amount of binder was increased by 4.0% by weight. Otherwise, the procedure followed was identical to example 7.

Prophetic example 26

(Hi-Temp/AN composition with age stabilization)

Mix Hi-Temp<®> (100 grams) and a first mixture (100 grams - prepared according to Example 21) by tumbling in a mixing pan. Then just enough acetone is added to wet the particles and cause the ingredients to stick together. Dry the mixture at 60°C under vacuum. Sift through a 5 mesh sieve and store sealed in a sealed container.

Claims (16)

1. Aging-stabilized ammonium nitrate propellant composition, consisting essentially of ammonium nitrate, having a crystalline phase present in an amount of from about 40% to approx. 85%, a synthetic zeolite that has a pore size of approx. 3 angstroms to approx. 13 angstroms present in an amount from approx. 0.2% to approx. 0.4%, and a binder present in an amount from approx. 3% to approx. 40% by weight of the composition, and wherein said ammonium nitrate, synthetic zeolite and binder are each present in an amount effective to stabilize the crystalline phase of said ammonium nitrate, to give the composition a shelf life of at least approx. 5 years, and to produce a composition that is essentially smoke-free when burned. 2. Composition according to claim 1, further comprising at least one nitroplasticizer present in an amount of approx. 5% to approx. 25%, an energy-rich additive present in an amount of approx. 5% to approx. 25%, a nitrate ester stabilizer present in an amount of approx. 0.1% to approx. 3%, a hardener present in an amount of approx. 0.5% to approx. 5%, a curing catalyst present in an amount of up to approx. 0.3%, a matting agent present in an amount of up to approx. 2% and an antioxidant present in an amount of up to approx. 1% by weight, based on the composition. 3. Composition according to claim 2, where said synthetic zeolite has a formula Nan (AlO2)i2(SiO2)i2) • 27H20, the binder is a hydroxy-terminated polybutadiene, said nitrate ester stabilizer is N-methyl-4-nitroaniline, said curing agent is isophorone diisocyanate, said curing catalyst is selected from the group consisting of triphenyl bismuth, maleic anhydride, magnesium oxide, a tin dilaurate, metal acetyl acetonate and mixtures thereof, said matting agent is carbon black, said antioxidant is selected from the group consisting of 2,2'-bis (4-methyl-6-tert-butylphenol ), 4,4'-bis (4-methyl-6-tert-butylphenol) and mixtures thereof and said energy-rich additive is selected from the group consisting of dinitroxydiethylnitramine, cyclotrimethylenetrinitramine, cyclotetramethylenetetranitramine and mixtures thereof. 4. Composition according to claim 1, comprising a reinforcing agent. 5. Composition according to claim 4, where the strengthening agent is selected from the group consisting of azodicarbonamide, dicyandiamide, oxamide and mixtures thereof. 6. Composition according to claim 1, where the binder is selected from the group consisting of a thermoplastic elastomer, a thermoconsolidating material and mixtures thereof. 7. Composition according to claim 1, where the synthetic zeolite comprised an aluminum silicate. 8. Composition according to claim 2, wherein said nitroplasticizer is selected from the group of a nitrate ester, a nitramine and mixtures thereof. 9. Composition according to claim 8, wherein said nitroplasticizer is a nitramine selected from the group consisting of N-butyl-2-nitratoethylnitramine, methyl-2-nitratoethylnitramine, ethyl-2-nitrato-ethylnitramine and mixtures thereof. 10. Composition according to claim 8, where said nitroplasticizer is a nitrate ester and where said composition further comprises a nitrate ester stabilizer present in an amount of approx. 0.1% to approx. 2% by weight of the composition. 11. Composition according to claim 10, where said nitrate esters are selected from the group consisting of trimethylolethane trinitrate, triethylene glycol dinitrate, triethylene glycol trinitrate, butanetriol trinitrate, diethylene glycol dinitrate, ethylene glycol dinitrate, nitroglycerin, diethyleneglycerin trinitrate, dimtroglyceration, nitrobenzene and mixtures thereof. 12. Composition according to claim 10, wherein said nitrate ester stabilizer is selected from the group consisting of N-methyl-4-nitroaniline, 2-nitrodiphenylamine, ethyl centralite and mixtures thereof. 13. Composition according to claim 2, where said hardener is selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, toluene diisocyanate, trimethylxylene diisocyanate, dimeryl diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, dianisidine diisocyanate, phenylene diisocyanate, xylylene diisocyanate, other diisocyanates, triisocyanates, higher isocyanates than the triisocyanates, polyfunctional isocyanates and mixtures thereof. 14. Composition according to claim 2, wherein said curing catalyst is selected from the group consisting of triphenylbismuth, maleic anhydride, magnesium oxide, a tin dilaurate, methyl acetylacetonate and mixtures thereof. 15. Composition according to claim 2, where said matting agent is carbon black. 16. Composition according to claim 2, wherein said antioxidant is selected from the group consisting of 2,2'-bis (4-methyl-6-tert-butylphenol), 4,4'-bis (4-methyl-6-tert-butylphenol) and mixtures thereof .