Bead milled spray dried nano-explosive

Abstract

A method for manufacturing nano-sized insensitive high explosive molding powder usable as a booster HE is provided herein. The method preferably involving the steps of dissolving a binder in a liquid and suspending crystalline high explosive to said liquid, grinding that suspension in a bead mill until the crystalline high explosive is nano-sized, and precipitating the binder and crystalline high explosive using a spray dryer to produce granules containing nano-sized crystalline high explosive. The liquid may be water or an organic solvent so long as the binder is highly soluble in the liquid and the crystalline high explosive is generally insoluble in the liquid. A fatty alcohol, water defoaming/dispersant/surfactant agent can be added to the dissolved binder/suspended crystalline high explosive, to aid in the manufacturability.

Claims

1. A method of manufacture of an insensitive high explosive molding powder comprising: adding a binder, and a crystalline high explosive into a liquid to form a mixture prior to milling the mixture; and agitating the mixture such that the binder is generally dissolved in the liquid and the crystalline high explosive is generally, uniformly suspended in the liquid prior to milling the mixture; milling the mixture in a bead mill until the crystalline high explosive has an average particle size of less than 1 μm; and spray drying the mixture containing the nano-sized crystalline high explosive material to produce powder granules wherein the granules comprise nano-sized crystalline high explosive material uniformly coated with the binder.

2. The method of claim 1, wherein the liquid is an organic liquid selected from the group consisting of ethyl acetate, acetone, ethanol, nitromethane, acetonitrile, hexane, benzene, diethyl ether, toluene, pentane, cyclopentane, chloroform, methanol, acetic acide, n-proponal, n-butanol, cyclohexane, dioxane, dichloromethane, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, propylene carbonate, isopropanol, and n-proponaol.

3. The method of claim 1, wherein the crystalline high explosive is at least one crystalline high explosive selected from the group consisting of RDX, HMX and CL-20.

4. The method of claim 1, wherein the binder is selected from the group consisting of polyvinyl alcohol, polyethylene glycol, polyvinyl acetate, viton, and cellulose acetate butyrate.

5. The method of claim 1, wherein the mixture further comprises a defoaming, dispersant, plasticizer or surfactant agent.

6. The method of claim 1, wherein the mixture further comprises a fatty alcohol.

7. The method of claim 1, wherein the liquid is ethyl acetate, the crystalline high explosive is HMX, and the binder is cellulose acetate butyrate.

Description

DETAILED DESCRIPTION

(1) The present inventive method provides an effective, efficient, and inexpensive means of manufacturing insensitive high explosive molding powders formed of granules, containing from about 50 to 99 weight percent of a crystalline high explosive material. The balance of the weight percentage being a non-energetic binder; wherein the crystals within the high energy explosive material are nano-sized and uniformly coated with a non-energetic binder or non-energetic binder system, and wherein the final granules range from about 0.5 to about 20 microns in size.

(2) The subject inventive method of manufacture involves first creating a solution of a non-energetic binder, or a binder system, i.e. including any desired plasticizer or surfactant with the binder dissolved in a liquid, to form an aqueous solution and then adding a crystalline high explosive material (which crystalline HE material will be held in-suspension within the aqueous binder solution). Then bead milling the mixture until the crystalline explosive material is nano-sized, i.e. having a mean crystal size below 1000 nm in diameter. If desired, in addition to the binder, HE crystalline material and liquid, an effective quantity of a defoamer/dispersant/surfactant can be added to the solution (prior to adding the crystalline high explosive thereto and prior to milling of the mixture thereof), preferably an alcohol dispersant, most preferably isobutanol or similar.

(3) The desired final binder/explosive molding powder is then recovered from the aqueous solution/suspension mixture by spray drying using commercially available spray drying technology. The relative amounts of the crystalline explosive and binder/binder system ingredients which are dissolved in the liquid to form the aqueous solution/suspension should be chosen to reflect the desired composition of the resulting molding powder, as the composition of the resulting molding powder granules will be nearly identical to the relative composition of such ingredients initially placed in solution. Preferably, the inventive formulation consists of 50 to 99 weight percent crystalline HE and the balance being the binder, or binder system, containing desired additive(s), such as a plasticizer and/or surfactant.

(4) The liquid utilized in the present invention can be any liquid that can dissolve the binder while also not dissolve or cause ripening of the crystalline HE material. Proposed liquids include water or organic solvents such as ethyl acetate, acetone, ethanol, nitromethane, acetonitrile, hexane, benzene, diethyl ether, toluene, pentane, cyclopentane, chloroform, methanol, acetic acide, n-proponal, n-butanol, cyclohexane, dioxane, dichloromethane, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, propylene carbonate, isopropanol, and n-propanol. Exemplary liquids and crystalline HE combinations that avoids the dissolution of the crystalline HE or ripening of the crystalline HE material include: 1) chloroform with RDX, 2) chloroform with CL-20, or 3) ethyl acetate and HMX.

(5) Exemplary binders include non-energetic, inert polymer binders—such as polyethylene glycol (PEG), polyvinyl alcohol (PVOH), polyvinyl acetate, viton, and cellulose acetate butyrate.

(6) The required bead milling to form the nano-sized crystalline HE is done in a commercially available bead mill which accepts the aqueous solution of crystalline explosive material, with or without the binder/binder system in the aqueous solution, and provides the desired nano-sized explosive HE crystals. Particularly useful bead mills include the DMQX™ Horizontal Bead Milling System, available from Union Process Inc, of Akron; the MicroMedia™ Nano bead mill, from Bühler Inc., Plymouth, Minn.; the UltraApex Mill type UAM-015 manufactured by Kotobuki Ind. Co. Ltd., Joto-ku, Osaka, Japan; and preferably the Netzsch Bead Mill (Microseries) available from NETZSCH Premier Technologies, Inc., Exton, Pa.—among others.

(7) In the present method, as is common in spray drying, the precipitation of the dissolved ingredients occurs and the formation of granules is achieved by atomizing the aqueous binder solution/HE explosive material suspension into droplets and drying such droplets in a flowing stream of heated gas—preferably air or nitrogen. Most commercially available spray dryers may readily be used in this invention. Depending on the desired grain size of the molding powder, several spraying approaches can be selected. The atomization of the feed solution may be achieved using a variety of standard atomizers including compressed gas, ultrasonic, and rotary disk. The droplet size distribution may be varied by manipulation of the solution feed rate, and by nozzle settings. For example, the commonly used gas atomized nozzle, the nozzle diameter and the atomizing gas flow rate may be adjusted to get the desired droplet size—to result in a particular granule size. In the case of the ultrasonic nozzle, the nozzle frequency and amplitude may be used as the control parameter.

(8) In the subject inventive spray drying process, the precursor solution/suspension may be fed to the atomizer using a variety of available liquid pumps, however, for product uniformity, it is desired that the pumping be relatively steady, rather than pulsating. Preferred pump types include, but are not limited to: centrifugal, peristaltic, piston, and diaphragm type pumps.

(9) Furthermore, in the subject spray drying process, the temperature of the drying chamber should be selected such that the solution droplets are completely or nearly completely dried within the drying chamber. The temperature should not exceed that at which decomposition of the product may occur—preferably less than 150 degrees Centigrade.

(10) Finally, the molding powder granules obtained from the subject inventive spray drying process are separated and recovered from the gas stream using a cyclone separator, filtration, or other known means.

(11) To aid in the understanding of the subject inventive method, the following examples are provided as illustrations —however, they are merely examples and should not be construed as limitations on the claims:

Example 1

(12) An explosive molding powder containing 95 wt. % HMX and 5 wt. % PVOH binder was prepared. The preparation of this molding powder began by mixing 6.7 wt. % FEM HMX (the smallest particle size HMX that is commercially available), 0.35 wt. % PVOH, and 2.3 wt. % isobutonal with 90.65 wt. % water—where the PVOH and isobutonal dissolved easily and the HMX remained in suspension. The mixture was milled using a Netzsch Agitator Bead Mill with 300 micron yttria stabilized zirconia beads, available from Netzsch Inc., Exton, Pa. The mill was set to a speed of 6,800 rpm and the mixture was milled for approximately 1 hour. The mean crystal size of the milled HMX as determined by dynamic light scattering was 300 nm. The suspension was then spray dried using a Buchi B290 spray dryer (Buchi Labortechnik AG, Switzerland), equipped with a two fluid nozzle gas atomization configuration. The inert drying gas (N.sub.2) inlet temperature was set at 140 degrees Centigrade. The final, desired, insensitive molding powder product was collected using a cyclone separator.

(13) The product granule size ranged from about 0.5 to about 10 microns. Optical and electronic microscopy revealed that the granules are primarily composed of nanocrystalline HMX with a homogeneous distribution of binder and HE. The composition of the product was also verified using HPLC analysis to match that of the original feed slurry.

Example 2

(14) Using the procedure outlined in Example 1, a molding powder consisting of 90% CL-20 and 10 wt. % polyvinyl alcohol was prepared and milled for 10 minutes, but otherwise subjected to the same process. The measured mean crystal size of CL-20 after milling was 400 nm. Optical and electron microscopy revealed that the granule size, the HE crystal size, and the uniformity of binder coating on the HE crystals was analogous to the sample described in Example 1—as desired.

Example 3

(15) An explosive molding powder containing 95 wt. % HMX and 5 wt. % polyvinyl acetate (PVAc) binder was prepared. The preparation of this molding powder began by mixing 6.7 wt. % FEM HMX (the smallest particle size HMX that is commercially available), 0.35 wt. % polyvinyl acetate (PVAc), and with 92.95 wt. % ethyl acetate—where the PVAc dissolved easily and the HMX remained in suspension. The mixture was milled using a Netzsch Agitator Bead Mill with 300 micron yttria stabilized zirconia beads, available from Netzsch Inc., Exton, Pa. The mill was set to a speed of 6,800 rpm and the mixture was milled for approximately 1 hour. The mean crystal size of the milled HMX as determined by dynamic light scattering was 300 nm. The suspension was then spray dried using a Buchi B290 spray dryer (Büchi Labortechnik AG, Switzerland), equipped with a two fluid nozzle gas atomization configuration. The inert drying gas (N.sub.2) inlet temperature was set at 140 degrees Centigrade. The final, desired, insensitive molding powder product was collected using a cyclone separator.

(16) Sensitivity Analysis

(17) HMX samples, as prepared in Example 1 were subjected to impact sensitivity tests performed using an Explosive Research Laboratory (ERL), Type 12 impact tester, with a 2.5 kg drop weight. This method is described in MIL STD 1751A, Method 1012, “Impact Sensitivity Test-ERL (Explosives Research Laboratory)/Bruceton Apparatus,” copies of which are available at http://assist.daps.dla.mil/ or from the Department of Defense, Standardized Document Order Desk, 700 Robbins Avenue, Bldg., 4D, Philadelphia, Pa. 19111-5094. The test is performed by dropping the drop weight from incremental heights and recording whether the HMX sample initiates, i.e. an explosion occurs. The drop height is repeated and adjusted in order to determine the height at which initiation probability is 50% (H30) and the impact sensitivity is given as the H50 value. The impact sensitivity of the HMX/PVOH formulation of Example 1 is >125.9 cm. This can be compared to a legacy booster material, LX 14, which has a similar amount of HMX, but a significantly worse impact sensitivity, i.e. only 26 cm.

(18) Shock sensitivity analysis was performed with the NOL Small-Scale Gap Test according to MIL-STD-1751A, Method 1042, copies of which are available at http://assist.daps.da.mil/ or from the Department of Defense, Standardized Document Order Desk, 700 Robbins Avenue, Bldg., 4D, Philadelphia, Pa. 19111-5094. Samples of Cl-20 and HMX prior art HE molding powders and molding powders produced according to the present method were pressed to comparable percentages of theoretical maximum density (% TMD). The shock sensitivity test results are summarized in Table 1, proving the formulations made with the inventive bead milled/spray dried composition are significantly less sensitive. In fact, both the prior art FEM CL-20/PVOH explosive and prior art FEM HMX/PVOH explosive were found to be a third more shock sensitive than the milled (i.e. 400 nm) CL-20/PVOH and the milled (i.e. 300 nm) HMX produced by the current inventive method.

(19) TABLE-US-00001 TABLE 1 Shock Sensitivity Wt. % Density Decibangs Explosive Binder Explosive (g/cc) (DBg) kBars FEM CL-20 PVOH 90 1.86 7.41 36.5 (prior art) milled PVOH 90 1.86 8.72 55.5 CL-20 FEM HMX PVOH 95 1.66 7.375 36.0 (prior art) milled HMX PVOH 95 1.60 8.5 51.7

(20) Although the invention has been described in general terms and using specific examples, it is understood by those of ordinary skill in the art that variations and modifications can be effected to these general and specific embodiments, without departing from the scope and spirit of the invention.