GB2329380A - Priming composition - Google Patents

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 20 to 49 weight percent of potassium dinitrobenzofuroxan (KDNBF); 50 to 70 weight percent of anhydrous strontium nitrate and/or potassium nitrate as oxidant; 1 to 3 weight percent tetrazene; and optionally up to 15 weight percent 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 priming composition, and particularly but not exclusively to a priming composition for percussion caps for small arms ammunition (SAA) capable of meeting military standards.

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 generally 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 flame/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.

Both potassium dinitrobenzofuroxan (KDNBF) and tetrazene are well known energetic ingredients which do not contain heavy metal elements, and they are listed as alternatives in EP 0 699 646. A specified composition therein employs both tetrazene and pentaerythritol tetranitrate (PETN) in a 3:1 ratio by weight, but there is no specific examples of tetrazene combined with KDNBF. The oxidant is specified as stannic oxide.

EP 0 704 415 indicates that the use of KDNBF as the sole energetic ingredient ("primary explosive") had previously been found to lead to compositions which are either too inconsistent or too insensitive, with tetrazene needing to be employed as a sensitiser. The claims of this specification embrace compositions comprising a single energetic ingredient such as KDNBF (among others), an oxidising agent such as barium or potassium nitrate, and a "friction agent" exemplified as glass powder or crystalline boron.

The use of a main energetic ingredient of diazodinitrophenol (DDNP) or KDNBF, and preferably the former, together with tetrazene as a sensitiser is described for example in US 4 693 201. The amount of tetrazene is 4 to 8%, and the sole example comprises 6% tetrazene.

This document also specifies strontium nitrate as an oxidant, preferably comprising 10 to 13% total moisture, and an organic nitrate ester fuel, such as a mixture of nitrocellulose and nitro-glycerine. Strontium is considered to be a relatively acceptable metal in relation to health and environment.

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. However, the presence of relatively large amounts of tetrazene in priming compositions, as in US 4 693 201, tends to lead to failure of the vacuum stability test.

In a first aspect, the present invention provides a priming composition compnsing 20 to 49 weight percent of potassium dinitrobenzofuroxan (KDNBF) as a main energetic ingredient; 50 to 70 weight percent of anhydrous strontium nitrate and/or potassium nitrate as an oxidant; and 1 to 3 weight percent tetrazene, and optionally up to 13 weight percent of a fuel. Where there is no added fuel, the minimum amount of KDNBF will be 27 weight percent.

It has now been found that, by a suitable choice of other ingredients, in particular the oxidant, that it is possible to provide a priming composition in which KDNBF is combined with a suitably low amount of tetrazene, whilst retaining desirable charactenstics, and in particular an adequate sensitivity and stability, including the ability to conform to the vacuum stability test.

Furthermore, the other ingredients are selected such that the priming composition comprises substantially no toxic heavy metals.

The KDNBF is produced to give a crystal habit comprising aggregates of amorphous granules, orange red in colour in their natural state, which permit a homogeneous composition when mixed with the other ingredients. Preferably the KDNBF will have a bulk density of upto 0.4 gum~3 for the desirable level of homogeneity, most preferably the bulk density will be about 0.2gcm3..It is also preferred that it 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 1.4 and 2.2 weight percent, more preferably 1.6 to 2.0 weight percent. The embodiments comprise 1.8 weight percent ofthis component.

A preferred oxidant is anhydrous strontium nitrate, which retains the requisite sensitivity and reproducibility of results for military use. Potassium nitrate is an alternative 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 silicide, other commonly used fuels are boron, aluminium and titanium.

US 4 963 201 relates to a wet mixing process of the type in which the resulting mixture is pressed into a perforated plate to form pellets for loading into primer cups, followed by drying.

In a 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 to remove the volatile components of the binder in a drying house. 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.

In a dry mixing process, the 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 dangerous areas are the mixing and filling cubicles, where bulk compositions are present. These are prohibited areas when filling is in progress, and are enclosed behind thick reinforced concrete walls, so that personnel are safe should an incident occur at this stage. Thereafter, the amount of powder composition being transferred around other stations can be kept to a minimum, with pressing operations taking place behind guards.

Thus, on the grounds of safety, a dry mixing process is advantageous for producing a composition according to the first aspect of the invention. Compared with the preferred composition of US 4 963 201 which incorporates nitroglycerin and a partially hydrated form of strontium nitrate, the composition of the present invention does not involve liquid or hydrated components and thus provides for safer storage of the volatile tetrazene.

Accordingly, in a second aspect, the invention provides a method of making a priming composition according to the first aspect, in which the components of the composition are mixed dry.

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 silicide.

The calcium silicide is a fuel component which is intended to provide hot particles 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 &commat; 21 C 3800 Bar (max) Mean chamber pressure + 3 SD gas 21"C 4200 Bar (max) Mean port pressure - 3SD &commat; 21"C 880 Bar (min) Mean action time - 5SD ( -32 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 (STIR) 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 Example 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 estend 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 silicide 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: a) although a small flash emanated from the barrel, no sparks were seen, and this was judged acceptable.

b) the sensitivity complied with the all-fire and no-fire levels.

c) acceptable ballistics were obtained when the round was fired in the EPVAT equipment at ambient.

d) a series of weapon function trials were completed satisfactorily, with the variables being: Temperature -340C, +21"C, +52"C, +70"C Rate of Fire Automatic, Single Shot Elevation Horizontal, elevated, depressed 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 -540C, +2 10C 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.

It is necessary to maintain homogeneity of the composition to ensure that correct takeover to the propellant takes place.

Claims (16)

1. A priming composition comprising 20 to 49 weight percent of potassium dinitrobenzofuroxan (KDNBF); 50 to 70 weight percent of anhydrous strontium nitrate and/or potassium nitrate as oxidant; I to 3 weight percent tetrazene; and optionally up to 15 weight percent of a fuel.

2. A priming composition as claimed in claim 1 wherein the KDNBF is provided in a form having a crystal habit comprising aggregates of amorphous granules which permit a homogeneous composition when mixed with the other ingredients

3. A priming composition as claimed in claim 1 or claim 2 wherein the bulk density of the KDNBF is 0.4 gcm3 or less.

4. A priming composition as claimed in claim 3 wherein the bulk density of the KDNBF is 0.2gem9.

5. A priming composition according to any preceding claim wherein said fuel is present and is wholly inorganic.

6. A priming composition according to any preceding claim wherein said fuel is present and is calcium silicide.

7. A priming composition according to any one of claims 1 to 4, wherein no fuel is present.

8. A priming composition according to any preceding claim wherein the KDNBF component provides at least 28% of the composition.

9. A priming composition according to any preceding claim wherein the KDNBF component provides no more than 36 weight percent of the priming composition.

10. A priming composition according to any preceding claim wherein the tetrazene is present in an amount of between 1.4 and 2.2 weight percent.

11. A priming composition substantially as hereinbefore described with reference to Example 1, Example 2 or Example 3.

12. A priming composition according to any preceding claim which conforms to the military standard STANAG 4170.

13. A method of making a priming composition according to any preceding claim, in which the components of the composition are mixed dry.

14. A method of making a primer cap, comprising mixing the dry components of the composition according to any one of claims 1 to 12, placing a predetermined amount of the dry mixture in a primer cap, and pressing the dry composition therein.

15. A primer cap comprising a priming composition according to any one of claims 1 to 12.

16. A primer cap according to claim 15 in the form of a Boxer or Berdan cap.