Amorphous energetics

Abstract

Stabilized, amorphous high energetic compositions having crystallization inhibiting polymers dispersed throughout the solid composition. The compositions disclosed herein are an improvement over crystalline high energetic compositions in that such disclosed compositions are stable and possess physical properties desirable in propellant and high explosive applications.

Claims

1. An energetic solid solution comprising: secondary high explosive wherein the amorphous secondary high explosive is cyclotetramethylene-tetranitramine (HMX), hexanitrohexaazaisowurtzitane (CL-20), and trimethylenetrinitramine (RDX) or a combination of at least two thereof, and at least one polymer selected from the group consisting essentially of poly(benzimidazolone sulfone), polyvinyl acetate, cellulose acetate butyrate, cellulose acetate, polyimide, and nitrocellulose and, wherein the secondary high explosive and polymer are present as a single-phase glassy state.

2. The energetic solid solution of claim 1, wherein the polymer is polyvinyl acetate.

3. The energetic solid solution of claim 1, wherein the polymer is cellulose acetate butyrate.

4. The energetic solid solution of claim 1, wherein the polymer is cellulose acetate.

5. The energetic solid solution of claim 1, wherein the polymer is polyimide.

6. The energetic solid solution of claim 1, wherein the polymer is nitrocellulose.

7. The energetic solid solution of claim 1, wherein the polymer is poly(benzimidazolone sulfone) (PBIS).

8. The energetic solid solution of claim 1, wherein the polymer has a T.sub.g of about 100 C. to about 400 C.

9. The energetic solid solution of claim 1, wherein the amorphous, secondary high explosive is 60-99% by weight and the polymer is 1-40% by weight percent.

10. The energetic solid solution of claim 1, further comprising blast enhancing additives selected from the group consisting of aluminum, graphite and carbon nanotubes.

11. The energetic solid solution of claim 1, wherein the solid solution is compacted into a dense form.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) New Materials, Properties, Applications

(2) The amorphous secondary high explosive materials prepared by the present inventive method are readily used in current military munitions as replacements for the munitions main charges, boosters, and detonator output charges. As stated above, the subject amorphous explosive materials offer significant improvements in safety (sensitivity to inadvertent initiation) as well as explosive performance versus the commonly used crystalline explosives from which these materials are derived.

(3) As also stated above, the amorphous materials which are the subject of this invention are prepared from at least one commonly used crystalline secondary high explosive materials, such as HMX, RDX, or CL-20. More specifically, the commonly used crystalline secondary high explosive material from which the inventive amorphous material are manufacturedcan be a pure secondary high explosive material (such as HMX, RDX, or CL-20), or blends of two or more such crystalline secondary high explosive materials.

(4) The pure or blended crystalline secondary high explosive material must itself be blended with a polymeric additive toas stated above, inhibit crystallization of, secondary high explosive, or blends thereof, during the spray drying and to improve long term stability of the resulting amorphous phase. The chemical structure, molecular weight, density or other relevant characteristic of the polymeric additive is important to obtain the desired stability and reactivity of the end product.

(5) In selecting stabilizing polymers, it is desirable to select polymers having high glass transition temperatures (T.sub.g) (of at least about 100 C. to about 400 C.) that also have common solvents with the desired secondary high explosives to be used in the final amorphous composition. It has been determined that such polymers provide the amorphous explosive the ability to survive intensive thermal aging conditions. When such amorphous explosive compositions are stored below their glass transition temperature, crystallization can be avoided. By selecting a polymer having a relatively high T.sub.g, the amorphous composition itself will have a very high T.sub.g, thereby enhancing the stability of the amorphous phase with respect to crystallization

(6) An exemplary high Tg polymer may include poly(benzimidazolone sulfone) (PBIS) which are random copolymers of poly(benzimidazole sulfone) with biphenolsulfone. The PBIS compounds are formed by the condensation of three distinct monomeric units. Aabid A. Mir et al, Synthesis and Properties of Polymers Containing 2H-Benzimidazol-2-one Moieties: Polymerization via NC Coupling Reactions, ACS Macro Lett. 2012, 1, 194-197 details the T.sub.g relationship to the ratio of the copolymer constituents, such Tg polymers are incorporated herein by reference in its entirety.

(7) Additional exemplary polymers include, polyvinyl acetate, cellulose acetate butyrate, cellulose acetate, polyimide, and nitrocellulose.

(8) Additional additives such as burn rate modifiers, stabilizers, plasticizers may be added to the stabilized high energetic compositions.

(9) Further, as also stated above, due to the disordered arrangement of molecules in an amorphous state of the present inventive amorphous secondary high explosive materialsexcess stored energy from configurational strain is present. This excess stored energy renders the material more reactive in relation to crystalline analogs. The enhanced reactivity and higher specific free energy of the amorphous material both are manifested in improved detonation and initiation behavior. This includes smaller critical detonation failure dimensions, improved corner turning, shorter shock to detonation transition (SDT) and shorter deflagration to detonation transition (DDT). These properties are especially important in explosive components with small charge dimensions. Another benefit of amorphous energetics of the present invention is enhanced burn rate which can be exploited in propellant applications ranging from rocket motors to microthrusters.

(10) While the amorphous explosives of the present invention are more reactive and more energetic than the crystalline secondary explosive or explosives from which they are formedimportantly their initiation sensitivity can be surprisingly very low. The shock sensitivity, for example, can be effectively eliminated all together when the material is loaded at full density, with effectively no porosity. In such an instance, a highly homogeneous explosive charge is achieved, devoid of heterogeneities such as cracks, voids, and even grain boundaries and dislocations which are all characteristically present in crystalline charges and to which initiation sensitivity is attributed.

(11) An important embodiment of the present invention is the capability of the highly insensitive amorphous phase to become sensitized on demand by heating, such that the material undergoes crystallization resulting in a heterogeneous, porous structure which can be readily initiated by existing initiation technology. Selective heating or application of light to portions of the amorphous composition can revert those treated sections back to a crystalline state.

(12) Also, improved loading of amorphous explosive is another advantage over traditional, crystalline forms thereof. This is a direct result of the glass transition phenomenon common to amorphous materials in general. Above the glass transition temperature (Tg), softening occurs such that the material exhibits liquid-like viscous flowin which state, molding of amorphous explosives into casings can be accomplished at much lower pressures than when molding the conventional crystalline analogs. Further, and surprisingly, densities as high as ca. 100% of TMD are readily achievable.

(13) Inventive Method

(14) The inventive method for creating the amorphous energetic/secondary explosive materials of the present invention is based on the rapid precipitation of a crystalline secondary explosive material from solution. At very high precipitation rates, the conditions become favorable for the formation of the desired amorphous phase of the subject secondary explosive materials. The capacity to form an amorphous phase depends on the molecular structure including size and conformational flexibility of the particular crystalline explosive material being converted. In general, smaller molecules tend to be less likely to form an amorphous phase than larger molecules. Another factor is the melting point. Materials with lower melting points can be rendered amorphous by rapid melt quenching. Common secondary high explosive materials tend to have relatively small molecular sizes and therefore are more challenging to convert to the desired amorphous state. To overcome this difficulty, blending of explosives and/or addition of polymeric additives is employed. This is achieved by spray drying a solution containing the desired materials including the secondary high explosive and the polymeric additive, to rapidly precipitate the desired amorphous form.

(15) Rapid precipitation from solution is achieved using conventional spray drying technology. During the spray drying, the feed solution is atomized into fine droplets within a flowing drying gas (usually hot nitrogen or air). Due to the high surface area of the liquid droplets, rapid evaporation can be attained. This consequently leads to rapid precipitation of the solutes within the droplets. It has been shown that the highest precipitation rate occurs at the outer surface. As precipitation progresses, a shell-like structure forms, containing the remainder of the solution within. As the shell thickens, droplet evaporation slows due to impeded mass transfer caused by the shell. As the evaporation rate decreases so does the precipitation rate. Since at slower precipitation rates, formation of crystals becomes increasingly likely, it is important to set the spray drying conditions such as the solution droplet size and heating gas temperature so that no crystalline product is formed.

(16) Selection of the atomizer setting and thereby the droplet size of the atomized solution, will determine the final size of the amorphous particles. This is an important consideration when control over microstructure is necessary, for example the void size distribution in a pressed charge made from the amorphous powder. Particle size will also influence the specific surface area which has a strong effect on the propagation of combustion and detonation of the final amorphous secondary explosive being created.

(17) Further, a critical result of this inventive process is the stabilization of the amorphous explosiveas, as stated above, the subject amorphous materials typically tend to readily convert to the more thermodynamically favorable crystalline state. The desired stabilization is achieved by the addition of polymers. The degree of stability is tied to the amount of the additive, as well as its chemical structure and molecular weight. Modification of these variables enables tuning the product stability to desired levels. Stabilization of the amorphous material is also enhanced by compaction of amorphous powder to form pellets or any other compressed configuration. Maximum stability is achieved with such pellets or other form at densities near the TMD value of the given material.

(18) The drying gas temperature and flow rate during spray drying should be selected such that the solution droplets are completely dried within the drying chamber. The temperature should not exceed temperatures at which decomposition of the product may take place. Typically, a temperature at or above the boiling point temperature of the solvent is used.

(19) The dry powder can be effectively separated from the drying gas stream using a cyclone separator, however, alternatively a bag filtration may be employed.

Example 1. Preparation of Amorphous CL-20/HMX/PVAc Compositions

(20) Amorphous embodiments of the present invention were prepared using a combination of crystalline secondary explosive materials CL-20 and HMX. Initially the respective secondary crystalline explosive materials were placed into solution with the solvent acetone. The solutions were prepared at room temperature with the following alternative crystalline secondary high explosives and polymer ratios: CL-20/HMX/polymer ratios: 50/45/5 and 60/35/5 wt %, with the solvent to HMX weight ratio fixed at 50/1. The preferred polymer added to the mixture/solution was polyvinyl acetate (PVAc) with a 100,000 M.W. The solutions, with the added PVAc, were spray dried using a Buchi model B-290 laboratory spray dryer equipped with a two-fluid gas nozzle (0.7 mm diameter). N.sub.2 was used for atomization as well as the drying gas. The drying gas inlet temperature was set to 90 C. The drying gas flow rate was set to 35 m.sup.3/hour. The liquid feed rate was set to 5 ml/min. The product was collected from the gas stream using a cyclone separator.

(21) The products from Example 1 were analyzed using Powder X-ray Diffraction (P-XRD), and the X-ray diffraction patterns showed a broad diffraction halo without sharp peaks. Therefore, it can be concluded that the materials are highly amorphous. Further, SEM images were taken of the CL-20/HMX/PVAc (60/35/5) productafter the spray drying. The image shows spherical particles with a very small mean size and having smooth surfacesconsistent with amorphous structure.

(22) Stability Analysis

(23) As detailed above, a critical element of the stabilization of the inventive amorphous powder is use of a stabilizing polymeric additive. For example, when the amorphous composition of the present invention was prepared with polyvinyl acetate (100,000 M.W.), CL-20/HMX/PVAc (60/35/5), the as prepared powder when kept in an oven at 100 C. for 16 hours completely converted to a crystalline material as was confirmed by P-XRD analysis. The use of NC as the polymeric additive appears to greatly stabilize the amorphous phase. For example, the amorphous composition prepared with CL-20/NC (70/30) when heated at 100 C. for 16 hours did not show signs of crystallization when inspected by P-XRD.

(24) A second mode of stabilization of the amorphous phase, as disclosed above, is via compaction. The CL-20/HMX/PVAC (60/35/5) composition when pressed into a cylindrical pellet or other configuration, at a density of 1.65 g/cc exhibited greatly improved thermal stability in comparison to the lose powder of the same material. No conversion of the amorphous material to crystalline was observed following exposure to 100 C. for 16 h.

Example 2. Preparation of Amorphous CL-20 with Nitrocellulose

(25) A solution was prepared containing the secondary explosive material CL-20 and the polymeric additive nitrocellulose (NC) (70/30 wt %), with acetone as the solvent. The ratio of CL-20 to acetone was 1/10. The spray drying conditions were the same as described in Example 1. After spray drying to remove the acetone and rapidly precipitate the explosive materialsPXRD analysis on the produced material was done and it was concluded, from the lack of sharp Bragg peaks, that the produced material has an amorphous structure.

Example 3. Preparation of Amorphous HMX/CL-20 with PBIS

(26) A mixture consisting of 15% Poly(benzimidazolone sulfone)s and 4,4-Biphenol Copolymer with 21.25% HMX, and 63.75% CL:20 (% mass) was dissolved in 80 C. DMF and spray dried. This yielded a composition having a T.sub.g above 85 C. and This amorphous powder was pressed into a pellet and stored at 70 C. over an extended period of time and was found to not crystallize.

Example 4. Preparation of Amorphous CL-20/HMX with Cellulose Acetate

(27) An amorphous propellant consisting of 60 wt % CL-20, 20 wt % HMX, and 20 wt % cellulose acetate (CA) 30,000 MW, was prepared by spray drying an acetone solution containing 6 wt % CL-20, 2 wt % HMX, and 2 wt % CA. The solution was spray dried with a Buchi, model B-290 laboratory spray dryer. The solution was atomized using a two-fluid pneumatic nozzle (Model 044698, Buchi) with nitrogen as the atomizing gas. The inlet drying gas temperature was set to 95 C., with a flow rate of 35 m m.sup.3/hr. The solution feed rate was set to 5 ml/min. The product was separated from the gas stream with a cyclone separator. The collected material was analyzed by powder X-ray diffraction. The powder pattern showed a broad halo-like pattern with no sharp peaks, suggesting the material is completely amorphous.

(28) While the present invention was described using certain exemplary, specific embodiments, those skilled in the art will recognize that the teachings presented herein are not limited to these specific embodiments. The preferred embodiments of the invention are provided for the purpose of explaining the principles of the present invention and its practical applications, thereby enabling others skilled in the art to understand the invention. Various embodiments and modifications are contemplated within the scope of the present invention.