PRIMING COMPOSITION

Abstract: A priming composition suitable for making percussion caps which meet military standards, including stability and sensitivity, and which may comprise no toxic heavy metals, comprises 21 % to 52 % by weight of an explosive composition, the explosive composition comprising at least 85 % potassium dinitrobenzofuroxan (KDNBF) as a main energetic ingredient; and no more than 15 % tetrazene as a sensitizer, 48 % to 70 % by weight of anhydrous strontium nitrate and/or potassium nitrate as an oxidant; and optionally up to 15 % by weight of a fuel such as calcium silicide. The composition may be made by dry mixing of the components.

Description: PRIMING COMPOSITION The present invention relates to a novel priming composition, and particularly but not exclusively to a priming composition for percussion caps for small .arms ammunition (SAA) capable of meeting military standards and a method for the manufacture thereof. As is well known, the main propellant charge contained within the cartridge case of a round of ammunition is relatively insensitive and requires an energetic input of considerable magnitude for successful rapid and gener ly complete ignition. This input is provided by a primer located at the rear of the cartridge case, such primer comprising an energetic material which is relatively highly sensitive to an energy input such as percussion, or heat generated by friction, and provides in response thereto a flame (hot gas) and/or hot particulates for activating the propellant charge. Suitable design of the primer enables appropriate direction of the flame and/or particulates, and suitable composition of the energetic material determines its sensitivity .and the nature of its fl.ame/particulate output. A percussion primer typically comprises a cap formed of a cup and a priming composition within the cup, commonly covered with varnished paper or the like to exclude moisture, the cap being placed within a pocket in the casing of the round. In a Boxer primer there is also an anvil pressed into the open end of cup, whereas in a Berdan primer the anvil is integral with the pocket of the casing. In either case, ignition of the priming composition is initiated by impacting the firing pin of a weapon against the central portion of the cup, thereby compressing the priming composition between anvil and cup. Heat generated by compression and/or friction causes the composition to ignite almost instantaneously, and to burn very rapidly. Hot gases and /or particles are directed to the propellant in the casing by flash holes in the casing pocket Commonly a priming composition for use in a percussion primer comprises a main energetic ingredient, an oxidant and a sensitiser which increases susceptibility of the main ingredient to the effect of the impact from the firing pin of the weapon. Other components, such as secondary energetic ingredient(s) and fuels which modify the output of the primer, for example by providing more heat or by producing hot particulates, may also be present. With certain rounds, depending on design and propellant, the production only of a flame or hot gas can be found to be insufficient to promote efficient ignition of the propellant, insofar as the gas tends to impact and activate only the surface region thereof. Activation of the remaining volume of the propellant is therefore delayed, and depends on propagation of the reaction from the surface region. In such cases, the production of hot particulates can be beneficial, since these can be directed so as to penetrate the propellant and to activate it throughout a larger volume substantially simultaneously. The components of many priming compositions contain heavy metal elements. For example, a common main energetic component is lead styphnate, a common oxidant is barium nitrate, and a common fuel is antimony sulphide. It will be understood that the presence of heavy metals is now considered undesirable on health and environmental grounds. Typically, known priming compositions are mixed by a wet mixing process in which the resulting wet mixture is pressed into a perforated plate to form pellets for loading into primer cups, and left to dry in the cups. In a typical wet mixing process, a binder is added to the active ingredients of the mix and the resulting dough is forced into a charge plate, extracted by the use of a dowel, and forced into the cap. The filled cap is subsequently dried in a drying house to remove the volatile components of the binder . It is necessary for personnel to add wet caps to the stock in the drying house and to remove dried caps therefrom, and at any time it is common for a large number of caps, with a large explosive potential, to be present. If an incident occurs, it is likely to be on a scale sufficient to cause fatalities. Dry mixing processes are known in which dry ingredients are blended in a jelly mould mixer and, in a filling cubicle, the resulting composition placed on a volumetric dispensing charge plate. A spatula sweeps back and forth over the charge plate, the volume of composition retained in a hole in the plate being of the correct weight for subsequent transfer to the cap. In the dry process, the most significant risk of an explosive incident is associated with the mixing and filling cubicles, where bulk compositions are present. To minimise risk, these areas are enclosed behind thick reinforced concrete walls and entry by personnel is prohibited when filling is in progress. Thereafter, the amount of powder composition being transferred around other stations can be kept to a minimum, with pressing operations taking place behind guards. Whilst an incident occuring in a dry mixing process is also likely to cause fatalities, by taking the basic safety precautions described the likelihood of such an incidence is significantly reduced over that in a wet mixing pricess. Thus, a dry mixing process is inherently a much safer production method for producing a priming composition. It is necessary to maintain homogeneity of a priming composition to ensure that correct takeover to the propellant takes place on intiation. Despite the inherent safety advantages associated with a dry mixing process, the wet mixing process has previously been preferred as it has provided more easily, a uniform, homogenous mixture of the priming composition components. This homogeneity is important in ensuring fast propogation of energy throughout the primer and in providing the optimum sensitivity and flame particulate output characteristics. Both potassium dinitrobenzofuroxan (KDNBF) and tetrazene are well known energetic ingredients which do not contain heavy metal elements. The use of KDNBF as the sole energetic ingredient ("primary explosive") in a priming composition has been found to lead to compositions which are either too inconsistent or too insensitive US 4 693 201 teaches the addition of a proportion of tetrazene to priming compositions comprising primary explosives such as KDNBF and diazodinitrophenol (DDNP), the tetrazene being employed as a sensitiser increasing the overall energetic potential of the composition. US 4 693 201 advocates quantities of 4-8% tetrazene in a composition comprising 20 to 40% by weight of primary explosives, that is the explosive part of the composition comprises approximately 17% terazene and 83% primary explosive. For military applications, it is necessary to have a priming composition which, inter alia, is stable over prolonged storage periods under a wide range of environmental conditions as laid down in STANAG 4170. The presence of relatively large amounts of tetrazene in priming compositions such as that described in US 4 693201 has been found to lead to failure of the vacuum stability test dictated by STANAG 4170. For these purposes, it is preferred that the proportion of tetrazene to primary explosive in the explosive portion of the composition is less than 15%, and preferably less than 10% . However, previous knowledge suggested that such low levels of tetrazene would provide insufficient increases in the sensitivity of the primary explosives. US 4 693 201 promotes the use of strontium nitrate as an oxidant for its proposed priming compositions due to the increased flame temperature it provides over the insoluble and amphoteric dioxides and peroxides previously investigated as alternatives to the toxic lead styphnate. Strontium is considered to be a relatively acceptable metal in relation to health and environment. In particular, US 4 693 201 advocates a multi-step wet mixing process involving the hydration of commercially available anhydrous strontium nitrate to a total moisture content of 10 - 13% as a preliminary step prior to mixing with a wet mix of the primary explosive and tetrazene, pressing and then drying the caps. Thus while a relatively non-toxic and good performance priming composition results, the manufacturer is faced with an expensive multi-step process with the associated safety risks previously mentioned for wet mixing processes. The present invention provides a priming composition comprising up to 52 % by weight of an explosive composition, the explosive composition comprising at least 85% potassium dinitrobenzofuroxan (KDNBF) as a main energetic ingredient and no more than 15% tetrazene, 48 to 70 weight percent of anhydrous strontium nitrate and/or potassium nitrate as an oxidant, and optionally up to 15 weight percent of a fuel. Where there is no added fuel, the minimum amount of KDNBF in the overall composition will be 27 weight percent. Preferably the explosive composition comprises less than 10% tetrazene and more than 90% KDNBF. Most preferred is that the explosive composition comprises less th.an about 6% tetrazene and at least 94% KDNBF. It has now been found that, by a suitable choice of ingredients, and, in particular, the physical form of those ingredients that it is possible to provide a priming composition of minimal toxicity in which KDNBF is combined with a suitably low amount of tetrazene, which retains desirable performance characteristics, and in particular an adequate sensitivity and stability, including the ability to conform to the vacuum stability test. The KDNBF is preferably provided to give a crystal habit comprising aggregates of amorphous granules (orange/red in colour in their natural state) which permit a homogeneous composition to be formed on dry mixing with the other ingredients. One particular feature which enables better homogeneity is the bulk density of the KDNBF crystals. Preferably the KDNBF will have a bulk density of up to 0.4 gem '3 , most preferably the bulk density will be about 0.2gcm "3 . KDNBF with these characteristics has been found to sit conveniently well with the preferred oxidants and provides for good homogeneity in a dry mix of the oxidant and explosive. The improved homogeneity of the composition helps improve the overall performance characteristics of the primer allowing the primary explosive to release its energy more efficiently when the primer is ignited and thus reducing the need for such large quantities of tetrazene. Thus a primer is provided which has comparable perfomance characteristics to the known compositions whilst having the stability to enable it to conform with the vacuum stability test. The skilled reader will appreciate that this novel composition could be manufactured by the conventionally used wet mixing processes, however, the composition has the significant advantage that the desired properties can be achieved by the safer dry mixing process. It is preferred that the KDNBF provides at least 28 weight percent and/or at most 36 weight percent of the priming composition, and more preferably 32 to 36 weight percent of the combined components excluding any fuel which may be present. The relatively low amount of tetrazene facilitates compliance with the vacuum stability test. It is preferably present in an amount of between about 3% and 10 percent by weight of the explosive part of the priming composition, more preferably 3 to 7 percent by weight. A preferred oxidant is anhydrous strontium nitrate, which retains the requisite sensitivity and reproducibility of results for military use. Potassium nitrate is a suitable alternative oxidant for milit.ary purposes. Preferably the particles of oxidant will have dimensions of from 40 microns to 70 microns this enables better distribution of the oxidant througout the mixture. Option ly, a small quantity of carbon black can be added to the oxidant to prevent the formation of larger aggregates which may affect the homogeneity of the mix. Preferably the quantity of carbon black added will be added in a weight to weight proportion of less than one part carbon black to 9 parts oxidant, more preferably less than 1 part carbon black to 19 parts oxidant. The inclusion of a fuel is dependent on the application of the priming composition. Where present, the fuel is preferably an inorganic compound. A preferred fuel is calcium suicide, other commonly used fuels .are boron, aluminium and titanium. Examples of the invention will be described hereafter. Example 1 A first example of a composition according to the present invention comprises 31.0 weight percent KDNBF, 56.1 weight percent strontium nitrate, 1.8 weight percent tetrazene and 11.1 weight percent calcium suicide. The calcium suicide is a fuel component which is intended to provide hot p.articles for embedment in the propellant charge. This composition was found to be suitable for use in 5.56mm Boxer caps. By the use of appropriate cap and anvil, and filled with a nominal charge weight of 20mg, the cap was found to comply with the NATO sensitivity limits of: (Mean drop height + 5 SD) < 450mm (Mean drop height - 2 SD) > 75mm where SD = standard deviation. When combined with a commercially available propellant for the NATO 5.56mm round, acceptable ballistics were obtained: Mean chamber pressure @ 21 ° C 3800 Bar (max) Mean chamber pressure + 3 SD @ 21 °C 4200 Bar (max) Mean port pressure - 3 SD @ 21°C 880 Bar (min) Mean action time - 5SD @ -52° C 3ms (max) Furthermore, this composition, or a cap containing it, has been shown to be compatible with other parts of the system, including the brass/nickel plated cap and the varnished paper; stable and falling within the vacuum stability requirement; capable of lighting surface moderated cut tubular and ball powder propellants, both single and double base, across the temperature range and beyond; and capable of lighting heavily deterred and slow burning propellants suitable for 0.50" rounds. This composition essentially also gives rise to non-toxic combustion by-products. Analysis of the latter by Fourier Transform Infrared (FTIR) spectroscopy identified carbon dioxide and water vapour as the principal gaseous products from a series of test firings, and all other gases, including carbon monoxide were below the detection limit for this method of analysis. Compared with the known priming composition VH2, it was found that there was a more rapid rise in breech pressure, and a reduction in the concentration of toxic organic combustion products, and both these factors are believed to be the result of a higher flame temperature with the composition of this Example. In a further comparison with VH2, the storage characteristics were investigated, over 12 and 24 weeks at 60C. Factors such as the figure of insensitiveness, the figure of friction, temperature of ignition, ease of ignition, responses to an electric spark, ball and disc test, emery paper friction, and various mallet tests, were investigated in addition to the vacuum stability. For the whole of the 24 week period there was no significant change in the results for the composition of this example, but VH2 became slightly more sensitive in at least some of the tests. The results from the vacuum stability test show the composition of this Ex.ample to be more stable than VH2 when tested after 40 hours at 100°C. Example 2 There are different requirements for a 9mm Boxer cap for use in pistol ammunition. Sparks (muzzle flash) should not extend beyond the length of the short (75mm) barrel; the strike energy of the weapon is less, thus requiring a more sensitive primer; and there is a different set of ballistic criteria. These requirements can be met by adapting the composition of Example 1, by altering the proportions of the components, including removing the calcium suicide fuel, by reducing the thickness of the cap base, and by using a pistol propellant. Thus this example of a composition according to the present invention and suitable for use in short barrelled pistol type weapons comprises 35.1 weight percent KDNBF, 63.1 weight percent strontium nitrate, and 1.8 weight percent tetrazene. Testing of the cap/round indicated that: Weapon 9mm SMG .and 9mm pistol In all cases (2160 rounds) the weapon functioned correctly. Climatic storage tests on rounds with primers comprising the composition of this example, performed satisfactorily after conditions of exposed desert, continuous heating and continuous arctic, as detailed in "Manual of Proof and Inspection Procedures for NATO Ammunition". Example 3 A third example of composition according to the invention uses potassium nitrate as the oxidant. It comprises 53.0 weight percent potassium nitrate, 45 weight percent KDNBF and 2 weight percent tetrazene. During testing of this composition, it was found necessary to reduce the cap charge weight to 10 mg, and this change reduced the Crown of Composition. Since this would result in the formation of an undesirable air gap if a conventional anvil was used, an appropriately lengthened anvil was employed. A Bruceton assessment of sensitivity indicated that this build standard has the required mean firing level. Caps containing the composition of this Example were loaded into two sets each of 30 cartridge cases, respectively containing the same two propellants as in Example 1. Each set was split into three equal subsets which were conditioned at -54°C, +21°C and +52°C, prior to firing in an EPVAT set-up. Ballistics of the test ammunition showed no difference relative to reference ammunition across the temperature range.

Claims: CLAIMS 1. A priming composition comprising 21 to 52 weight percent of an explosive composition, the explosive composition comprising at least 85% potassium dinitrobenzofuroxan (KDNBF) as a main energetic ingredient; and no more than 15 weight percent tetrazene as a sensitizer, 48 to 70 weight percent of an anhydrous oxidant; and optionally up to 15 weight percent of a fuel. 2. A priming composition as claimed in claim 1 wherein the oxidant comprises anhydrous strontium nitrate and/or potassium nitrate. 3. A priming composition as claimed in claim 1 or claim 2 wherein the KDNBF is provided in a form having a crystal habit comprising aggregates of amorphous granules which permit a homogeneous composition when mixed wet or dry with the other ingredients 4. A priming composition as claimed in any preceeding claim wherein the bulk density of the KDNBF is 0.4 gem '3 or less. 5. A priming composition as claimed in claim 4 wherein the bulk density of the KDNBF is 0.2gcm '3 . 6. A priming composition according to any preceding claim wherein said fuel is present and is wholly inorganic. 7. A priming composition according to any preceding claim wherein said fuel is present and is calcium suicide. 8. A priming composition according to any one of claims 1 to 5, wherein no fuel is present. 9. A priming composition according to any preceding claim wherein the KDNBF component provides at least 28% of the total weight of the priming composition. 10. A priming composition according to any preceding claim wherein the KDNBF component provides no more than 36 % of the total weight of the priming composition. 11. A priming composition as claimed in any preceding claim wherein the explosive composition comprises less than 10% tetrazene and more than 90% KDNBF. 12. A priming composition as claimed in claim 11 wherein the explosive composition comprises less than 6% tetrazene and at least 94% KDNBF. 13. A priming composition substantially as hereinbefore described with reference to Example 1, Example 2 or Example 3. 14. A priming composition according to any preceding claim which conforms to the military standard STANAG 4170. 15. A priming composition according to any preceding claim wherein carbon black is added to the oxidant. 16. A priming composition as claimed in claim 15 wherein the carbon black is added in a proportion by weight of 1 part carbon black to 9 parts oxidant or less. 17. A priming composition as claimed in claim 16 wherein the carbon black is added in a proportion by weight of 1 part carbon black to 19 parts oxidant or less. 18. A method of making a priming composition comprising mixing the ingredients according to any preceding claim using a dry mixing process. 19. A method of making a primer cap, comprising mixing the dry components of the composition according to any one of claims 1 to 17, placing a predetermined .amount of the dry mixture in a primer cap, and pressing the dry composition therein. 20. A primer cap comprising a priming composition according to any one of claims 1 to 17. 21. A primer cap according to claim 20 in the form of a Boxer or Berdan cap.

Inventors: SCOTT HAROLD ARTHUR, SCOTT, HAROLD, ARTHUR
Applicants: ROYAL ORDNANCE PUBLIC LIMITED, SCOTT HAROLD ARTHUR, ROYAL ORDNANCE PUBLIC LIMITED COMPANY, SCOTT, HAROLD, ARTHUR
Application year: 1998
Main classification: C06C7
IPC codes: C06C7