Patent Application
20100307648
The subjects of the present invention are:
The present invention relates more precisely to the coating of crystals of energetic explosive materials with an inorganic (metal) and/or organic (polymer) film so as to reduce the shock sensitivity, friction sensitivity and/or static electricity sensitivity of said crystals and to lower their deflagration-to-detonation transition threshold.
The field of application of the invention covers the whole field of application of energetic materials, especially for the defense industry, the space industry and automotive safety.
To desensitize these crystals and make them easier to use under satisfactory safety conditions, a known technique is to modify the surface of said crystals by coating them so as:
The coating material is in general a polymer which may either be inert (U.S. Pat. No. 4,043,850 and DE 37 11 995) from the pyrotechnic standpoint or energetic (WO 2000/73245 and GB 2 374 867). There are cases in which the coating material consists of a polymeric binder filled with a metal in pulverulent form, said metal being involved with regard to electrostatic charges (EP 1 500 639).
The coating methods according to U.S. Pat. No. 4,043,850, DE 37 11 995, WO 2000/73245, GB 2 374 867 and EP 1 500 639 involve wet processing. The same applies to the method described in Journal of Polymer Materials, 21, 377-382, 2004 for the coating of CL-20. The technical problem of removing all traces of solvent then inevitably arises.
Moreover, when implementing all these coating methods, the quality of the coating (thickness and continuity of the film, morphology, etc.) cannot be satisfactorily controlled.
In general, the prior art offers no solution for controlling the deposition of a small predetermined amount of coating material on an explosive crystalline substance. However, controlling the quality and the thickness of the coating layer is of paramount importance for optimizing the compromise between the level of desensitization and the energetic capability of the coated explosive substance.
Those skilled in the art are therefore always seeking a method for minimizing the amount of coating, so as to obtain a continuous layer which is as thin as possible. The objective is in fact not to overly relativize the amount of active product to the detriment of an energetically less active or inactive material (the coating).
Of course, the method must also meet, on the one hand, the handling criteria for explosive materials (i.e. be able to be worked at temperatures low enough not to modify the structure of the molecules or crystals) and, on the other hand, environmental criteria relating to the use of volatile solvents (for example VOC emissions).
The supercritical fluid deposition method for coating with a thin metal layer consists in depositing a nanostructured thin metal layer (a layer ranging from an organization of nanoparticles to a uniform nanostructured film) on organic or inorganic compounds. This deposition is carried out by dissolving a metal precursor in a solvent. Said precursor, when decomposed, results in the precipitation of the metal on the compound to be coated. The method is described in patent application WO 2000/59622. The article “Design at the nanometer scale of multifunctional materials using supercritical fluid chemical deposition” by Samuel Marre et al., Nanotechnology, Volume 17, Number 18, Sep. 28, 2006, pp 4594-4599 describes the implementation of this method for depositing a copper film (consisting of copper nanoparticles) on submicronic silica beads. The method is carried out at temperatures ranging from 100° C. to 150° C. and under a pressure of 24 MPa. It consists in:
The method of coating with a thin continuous polymer layer in a supercritical medium is also well known, especially in the pharmaceutical and cosmetic fields. The deposition is carried out by dissolving the coating agent in a solvent and then by precipitating said coating agent on the compound to be coated by an antisolvent effect. Through this approach, it is possible for the deposited layer to be very finely controlled. Application WO 2004/91571 describes a method for depositing a polymer coating on particles using a supercritical fluid, for example supercritical carbon dioxide, as antisolvent, to which a polymer solution and an organic solvent are added, the particles being dispersed in said organic solvent. The coating is deposited when the supercritical fluid and the suspended particles are combined to cause the polymer to precipitate on the particles to be coated.
Within such a context, the inventors have made the valuable contribution of selecting a coating technique under pressure and temperature setpoints, transposing it to the field of energetic explosive molecules, showing that said technique is suitable for depositing metal layers and polymer layers on the surface of the crystals of such molecules and showing that this technique can be implemented for generating such coatings, which are thin, continuous and uniform, over the entire surface of said crystals, in such a way that said crystals are desensitized without their energetic performance being significantly impaired.
According to the first subject of the present invention, this therefore relates to a method of desensitizing crystals of an energetic explosive substance by coating them. Characteristically, said method comprises:
Under such conditions—outside normal temperature and pressure conditions, advantageously outside said normal conditions, in the liquid state of the fluid in question, very advantageously (i.e. preferably) under supercritical conditions—it is possible to obtain the expected result without having to reach elevated temperatures that the temperature-sensitive energetic explosive substance in question could not withstand.
Under such conditions—outside normal temperature and pressure conditions, advantageously outside said normal conditions, in the liquid state of the fluid in question, very advantageously (i.e. preferably) under supercritical conditions—it has proved possible to generate a metal coating and/or (generally or) a polymer coating on the surface of the crystals. It has also proved possible to generate such a thin, continuous and uniform coating over the entire surface of the crystals.
In the context of implementing the method of the invention, characteristically:
The method in question, characteristically implemented (for depositing the coating material) under pressure and temperature setpoints, is of the type described for application in other fields (see above): method based on the reduction of a metal precursor in a medium under pressure and temperature setpoints, preferably a supercritical medium, for depositing a metal film; antisolvent method for depositing a polymer film. These two implementation variants will be discussed later.
The fluid, under pressure and temperature setpoints during implementation of the method of the invention, is advantageously carbon dioxide (CO2). In general, this is the fluid most often used when it is intended to work under supercritical conditions since it has easily achievable critical co-ordinates (Tc=31° C. and Pc=7.38 MPa). Moreover, it is inexpensive, nontoxic and chemically stable. However, it is not excluded from the scope of the invention to carry out the method with the use, under supercritical conditions, of a fluid other than CO2.
As indicated above, the method of the invention makes it possible to obtain thin, continuous uniform layers over the entire surface of the crystals. In particular, it is suitable for depositing:
Thus, the method of the invention is suitable in particular for depositing a layer of metal (Cu) particles with a thickness of around 50 nm, which corresponds to a measured mass content of 2.6%. The metal layer in question is a cover (continuous layer) consisting of nanoparticles.
Incidentally, it should be noted here that the method of the invention is not limited to obtaining coating layers as thin as this, but the fact that it does enable such layers to be obtained—which are thin but also continuous and uniform—is of particular interest.
The method of the invention is advantageously implemented for depositing a metal film of at least one metal chosen from nickel, copper, aluminum, titanium and zirconium and/or at least one oxide of such a metal. The metal film in question contains the corresponding metal(s) or the corresponding oxide(s), or else a mixture thereof.
The composition of the coating film is controlled by controlling the parameters of the method and more particularly the pressure and the temperature for implementing the method and the composition of the reaction medium.
The method of the invention, implemented for depositing a metal film, is advantageously of the type described in WO 2000/59622 and is based on the reduction of a metal precursor. This method comprises:
The method therefore comprises the contacting, under temperature and pressure setpoints, of crystals with a medium containing the dissolved precursor. By heating the medium, the precursor is decomposed on the surface of the crystals, causing a (metal) film to form.
Under the conditions indicated, i.e. outside normal temperature and pressure conditions (advantageously under such conditions, in the liquid state of the fluid in question; very advantageously under supercritical conditions), the fluid used is therefore a solvent for the solution containing said at least one precursor.
Said at least one precursor is advantageously chosen from metal acetates and acetylacetonates, advantageously from metal hexafluoroacetylacetonates. Such acetylacetonates have a high solubility in supercritical CO2.
Said at least one precursor consists very advantageously of copper hexafluoroacetylacetonate.
Advantageously, the precursor is reduced in the presence of hydrogen (a reducing agent). Advantageously, a catalyst (such as Pd) may also be involved.
Purely by way of illustration, one way of implementing this variant of the method of the invention is explained below:
In general, it may be pointed out that the thickness of the deposited metal film is controlled by, inter alia, the temperature, the contacting time and the concentration. The temperatures involved in the reduction vary depending on the precise nature of the precursors in question. They generally vary between 70° C. and 270° C., thereby enabling the energetic explosive substances to be below their decomposition temperatures.
The method of the invention is advantageously implemented for the deposition of a polymer film of polybutadiene, especially of a hydroxytelechelic polybutadiene (HTPB), of polyurethane (PU), especially of a poly(diethylene glycol adipate) (PDEGA), of a polyoxyethylene/polyoxypropylene (POE/POP) copolymer, of polyglycidyl azide (PGA) or of a mixture of such polymers.
The method of the invention, implemented for depositing a polymer film, is advantageously of the type described in WO 2004/91571. As indicated above, this is an antisolvent method, which comprises:
The crystals are dispersed in a solution of at least one polymer. This solution is placed in a reactor, which is then pressurized with an antisolvent (missible with the first solvent), thereby causing said at least one polymer to precipitate on the surface of the crystals.
Purely by way of illustration, one way of implementing this variant of the method of the invention is presented below:
This variant of the method of the invention, implemented under supercritical conditions (the preferred method of implementation) may be termed the SAS (Supercritical AntiSolvent) variant.
Characterization techniques have demonstrated the uniform character of the layer (in the context of HTPB-coated or PGA-coated CL20). The quantity of layer deposited is expressed in percentages by weight, which are perfectly measurable and are familiar to a person skilled in the art (see above). HTPB layers deposited on silica beads according to the method of the invention have thicknesses of 7±2 nm for a mass content of 3% (the density of silica is obviously not that of CL20).
The methods of the invention, as presented above, are advantageously implemented:
The combined deposition of at least one metal and at least one polymer is not completely excluded within the context of the invention. Of course, controlling such a hybrid deposition is more difficult. This has to involve, upstream, at least one metal precursor and at least one polymer in solution and the temperature and pressure conditions have to be determined, in particular when the at least two intended reactions (reduction of said at least one precursor into at least one metal, and precipitation of said at least one polymer) take place.
Finally, with reference to the method of the invention, it should be pointed out that it is advantageously implemented for coating, under pressure and temperature setpoints, an energetic explosive substance of the organic secondary explosive type, especially chosen from:
By implementing the perfectly controllable and reproducible method of the invention, the coating on the explosive substances may be characterized by unmatched uniformity and thinness. The sensitivity of the coated explosive substances may thus be reduced, while still maintaining energetic levels close to those of the uncoated substance.
The second subject of the present invention relates to coated crystals of an energetic explosive substance, which crystals are obtainable by the method described above, namely a method of coating them with a metal film and/or (advantageously or) a polymer film, implemented, characteristically, outside the normal temperature and pressure conditions. Said coated crystals are new because of the nature of the film in question and/or because of the characteristics thereof (quality [it is uniform and continuous over the entire surface of the crystals] and/or amount deposited).
The crystals coated with a metal film are new per se.
The crystals coated with a polymer film (or even a hybrid metal/polymer film) are new because of the characteristics of the coating. Said characteristics, which are new and particularly advantageous, result from the new implementation of the coating, under pressure and temperature setpoints with a suspension, containing, in solution, the coating material or at least one precursor thereof (said material or said at least one precursor thereof having been dissolved upstream in a solvent (the nature of said solvent and the concentration of said material or of said at least one precursor within said solvent possibly having been optimized) (see above).
The coated crystals of the invention advantageously have:
In view of the above description of the coating method, it will be understood that the coated crystals of the invention are advantageously:
Finally, the third subject of the present invention relates to energetic materials incorporating in their composition crystals of the invention, i.e. crystals coated per se and/or desensitized crystals obtained by the method of the invention. Said energetic materials contain an effective amount of said coated or desensitized crystals. In fact, they generally consist of said crystals or contain them, in an effective amount, in a binder.
The invention will now be illustrated in an entirely nonlimiting manner, by the appended figures and the following examples:
In the case of the examples, the reaction was carried out, in batch mode, in a high-temperature high-pressure reactor of 255 cm3 internal volume.
Example 1 relates to the application of the method of the invention to the coating of the explosive substance 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW or CL20) with a copper film.
The trial was carried out by preparing a solution comprising 15 ml of isopropanol+1.2 g of Cu(hfac)2.H2O+60 mg of Pd(hfac)2.H2O. Added to the solution were 3.5 g of CL20, the suspension obtained then being placed at the bottom of the reactor. Said reactor was then injected with H2 at 2.5 MPa, the pressurization up to 9 MPa then being completed with CO2. The chamber was then brought to the desired conditions (100° C., 22 MPa) and held there for a relatively short exposure time (45 minutes). Next, the reactor was cooled and then depressurized. The powder, corresponding to copper-coated CL20, was recovered from the bottom of the reactor in isopropanol.
The copper-coated CL20 had a gray/black color (coppery red under an optical microscope), whereas the CL20 before coating was white.
The copper coating on the CL20 crystals was characterized by 5 techniques:
The morphology of the copper-coated CL20 crystals was characterized by scanning electron microscopy (SEM). In addition, EDX (X-ray analytical technique associated with SEM) was used for surface elemental analysis so as to determine the elements present (
The SEM image obtained shows that the surface of the CL20 crystals is covered with nanoparticles having a size between 50 and 300 nm (
The EDX analysis provided surface maps of the specimen for each element (
To determine the degree of oxidation of the copper on the surface of the CL20 crystals, XPS characterizations have been carried out (
In this figure, peaks corresponding to the binding energies of carbon, oxygen, nitrogen, copper and palladium atoms may be clearly distinguished. The small amount of palladium comes from the palladium precursor (Pd(hfac)2) that catalyzes the reduction of the copper precursor. The enlargement of the copper peak reveals the proportions of metallic copper (right-hand peak) and of copper in oxidized form (precursor, copper oxide—left-hand peak). The metallic copper is predominantly present on the surface of the CL20.
The copper present on the surface was quantified by atomic absorption. To do this, the specimens were washed twice with isopropanol and then filtered so as to remove the copper particles not deposited on the surface of the CL20. A gray/black powder was recovered, this being dispersed in a 30% v/v nitric acid solution. The CL20 was not modified during this operation, whereas the copper dissolved to form a quantifiable Cu2+ solution. The copper(II) solution could then be quantified. The percentage of copper by weight present on the surface was 2.76% under the conditions of Example 1.5. However, this value can be varied by varying the reaction parameters (initial precursor concentration, catalyst concentration, sequences of precursor and catalyst injection into the reactor, reaction time) between 0.3 and 30%, as Table 1 shows. A person skilled in the art will know how to adjust these parameters according to his requirements.
When the amount of copper deposited is small, the SEM images show that the copper nanoparticles deposited on the surface of the crystal are non-contiguous. This is the case for the coating shown in
Sensitivity
The sensitivity of the copper-coated CL20 crystals of Example 1.5 was evaluated by carrying out the standardized shock sensitivity (SS*), friction sensitivity (FS**), electric spark sensitivity (ES***) and deflagration-to-detonation transition (DDT****) tests. Table 2 below gives the results obtained for comparison with the initial ε-CL20.
The presence of copper on the surface desensitizes the material since it is observed that there is a substantial reduction both in shock sensitivity and friction sensitivity and practically no sensitivity to static electricity. Moreover, as regards the DDT results, the critical height is increased by a factor of greater than 2.
Energetic Power
Table 3 below compares the density ρ, impulse Is and specific volume impulse Is×ρ calculated for three types of propellants comprising, in their composition, either ε-CL20 or copper-coated CL20 according to Example 1.5.
The density of the coated substance increases slightly relative to the initial substance because of the presence of about 3% copper. In contrast, the specific impulse of copper-coated CL20 according to Example 1.5 is reduced. However, the value of IS×ρ, which takes into account the specific impulse and the density of the product is practically the same as that of a composition using ε-CL20.
Example 2 relates to the application of the method for coating the explosive substance 4,10-dinitro-2,4,6,8,12-tetraoxa-4,10-diazaisowurtzitane, called TEX, with a copper film.
Table 4 below gives the two reaction parameters and the amount of copper deposited, quantified using the method described in Example 1.
Example 3 relates to the application of the method to the coating of the explosive substance 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW or CL20) with an HTPB polymer film.
Operating Method
The principle of depositing a polymer on the surface of CL20 crystals is based on an antisolvent method under supercritical conditions.
The polymer is dissolved in a dichloromethane (DCM) solution to which the crystals of explosive substance are added, these not being soluble in DCM. The crystal-laden solution is placed in the reactor, which is then pressurized with a supercritical antisolvent (scCO2, miscible with DCM), thereby precipitating HTPB on the surface of the crystals of the explosive substance. The DCM is removed by slow depressurization and flushing with an antisolvent stream for a defined time (drying time). The polymer-coated crystals are recovered from the bottom of the reactor in the form of a dry powder.
The polymer coating on the CL20 crystals was characterized by SEM and by UV-visible spectroscopy.
UV-visible spectroscopy enables the amount of polymer deposited on the surface of the crystals to be quantified. The quantification principle consists in redissolving the deposited polymer by placing a certain amount of coated crystals in DCM. The solution is then filtered and the collected polymer is quantified.
The reaction conditions of Example 3 and the percentage of HTPB are given in Table 5 below.
Characterization of the Product
The HTPB-coated CL20 crystals are of white color and have an expanded texture compared with the initial powder (
The sensitivity of the HTPB-coated CL20 crystals of Example 3 was determined by carrying out the standardized tests described in Example 1. Table 6 below gives the results obtained for comparison with the initial ε-CL20.
By coating with HTPB, it is possible to reduce the friction sensitivity FS and the static electricity sensitivity and, to a lesser extent, the shock sensitivity SS. The coating reduces the sensitivity to the DDT test very significantly.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0760036 | Dec 2007 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2008/052353 | 12/18/2008 | WO | 00 | 6/18/2010 |
