M42 Primer Processing with Potassium and Copper Salt Formulations

In this application, a primer is an igniter that is used to initiate smokeless powder/delay compositions. Its composition is usually consisted of a lead based explosive, specifically lead styphnate, fuel, and oxidizer(s). Lead is known to be toxic to the environment and to human health. The use of lead in ammunition, pollutes training ranges and exposes manufacturers and users to serious health hazards. At times, the operator’s exposure to lead is so high that many ranges or shoot houses have been shut down [1-4].

There have been many attempts to replace lead styphnate in percussion primers. Several different potential lead  replacement mixtures include: i) metastable interstitial composites (MIC) [5], ii) red phosphorous [6], iii) DDNP based mixtures [7], iv) DBX-1 based mixtures [8-10], and v) KDNP based mixtures [11]. MIC and DBX-1 based mixtures are not only suitable for replacing lead but also provide opportunities for automation of the manufacturing process. Current primer manufacturing processes are manual, inherent with safety and health risks as well as increased manufacturing defects. Automation of the primer loading process eliminates occupational safety hazards related to worker’s exposure to toxic and dangerous materials. MIC water-based processing and automated slurry loading have been developed and demonstrated [12-13]. The DBX-1 based mixtures have also shown great promise for processing and loading in slurry form, using a binary solvent of water and alcohol. The tested copper and potassium salt formulations, described in this text, were studied as a precursor to the DBX-1 based mixtures currently under development.

The potassium and copper salts were synthesized at Ludwig- Maximilians University of Munich (LMU), the formulations were developed at US Army Armament Research, Development and Engineering Center (ARDEC), and the primers were manufactured and tested at Innovative Materials and Processes, LLC (IMP). The work was performed collaboratively by ARDEC/ IMP/LMU.

Materials and Methods

The methods used for characterization analysis of the Cu-salt and K-salt compounds, including Differential Thermal Analysis (DTA), Infrared (IR), Elemental Analysis (EA), Impact Sensitivity (IS), Friction Sensitivity (FS), Electro Static Discharge Sensitivity (ESD), Differential Scanning Calorimetry (DCS), Mass to Charge Ratio (m/z), and Raman, was performed using accepted procedures described in the AOP-7 document [14].

The M42 primers were manufactured at IMP using the following methods. The primer cups were manually loaded with the appropriate energetic formulation. The dry mix was loaded into the primer cups using a primer cup holder and a slide on cover attachment with a built in funnel. The dry loading setup is shown in Figure 1.

Figure 1. Dry loading setup for M42 primers.

Between 20.1 mg and 23.3 mg of the dry mixture was weighed out on a balance and loaded in dry powder form into M42 primer cups. The individually loaded cups were then flat pressed using a flat punch attachment in a pneumatic press (air-hydraulics, model: AP-1200) set to 241 kPa (1356 N). The pressed material heights, measured from the underside of the primer cup to the top of the pressed material, ranged from 1.079 mm to 1.219 mm.

The pressed material within the primer cup was then wetted with 3 μL of isopropyl alcohol. The wetting of the surface allowed for the paper foil to be inserted using a flat hand press and slightly adhere to the surface of the wetted material. After the alcohol has evaporated from the surface of the pressed material, the anvils were inserted into the primer cups. M42 type anvils were inserted into each cup to a total primer height (cup + anvil) of 2.972 ± 0.025 mm.

The full primer production process is shown in step by step format in Figure 2.

Figure 2. M42 primer processing, in step by step format.

The consolidated M42 primers were inserted into the test application (5.56 case stubs) using a pneumatic press set to 241 kPa (1356 N). The primer was inserted so that it bottomed out in the application primer pocket. The press stop was set to the maximum allowable insertion depth below flush, 0.152 mm in this case. All primers were inserted into the 5.56 test cases to 0.102 mm – 0.152 mm below case flush insertion depth.

Primer sensitivity testing was conducted using a ball drop device and the Neyer D-optimal testing method [15]. The Neyer method sensitivity testing provides output statistics of the average drop height (H_avg) and standard deviation (σ) from that height. A sample set of 30 to 50 primers is normally sufficient for Neyer sensitivity testing. The testing parameters included a 55 g drop ball and a 1.016 mm diameter firing pin. A photograph of the ball drop device is shown in Figure 3 (left).

Pressure output characterization was conducted using an IMP designed pressure cell device. The device utilizes a closed bomb pressure cell, a PCB piezoelectric pressure transducer (model #102A) and an oscilloscope to measure the pressure output. A primer was inserted into a 5.56 case stub and screwed into the pressure cell. The ball drop device was used to initiate the percussion primer using a 1.016 mm diameter firing pin at an all fire height, and the data was recorded using an oscilloscope. A photograph of the closed bomb pressure cell is shown in Figure 3 (right).

Figure 3. Photograph of the ball drop device (left) and the pressurecell device (right).

Results and Discussion

Two different compounds were considered for the replacement of lead styphnate in the formulation for the M42 percussion primers, namely tetraazido(1,2-di(1H-tetrazol-1-yl) ethane)dicopper(II) (Cu₂(N₃)₄(dte)), where dte = 1,2-di(1Htetrazol- 1-yl)ethane, and dipotassium dinitraminobistetrazole (C₂K₂N₁₂O₄ = K₂DNABT). The next section describes the synthesis procedures for both compounds, elemental analyses of final and intermediate compounds, as well as some physical properties of these compounds.

Synthesis of tetraazido(1,2-di(1H-tetrazol-1-yl)ethane)dicopper( II) (Cu-Salt)

The synthesis of this Cu-salt (see Figure 4) was done according to a previously described procedure in literature [16]. An aqueous solution (10 mL) containing sodium azide (0.4 mmol, 26 mg) and dte (0.1 mmol, 17 mg), in a tube, was layered by aqueous ethanol (50%, 5 mL), and an ethanol solution (10 mL) of CuCl₂⋅ 2 H₂O (0.20 mmol, 34 mg) was carefully added. Slow diffusion at room temperature yielded crystals of [Cu₂(N₃)₄(dte)] in 10 days.

Figure 4. Schematics of reaction steps for the synthesis of Cu₂(N₃)₄(dte) (Cu-salt).

An aqueous solution of sodium azide (260 mg, 4 mmol) was added to a 70 °C warm solution of dte (166 mg, 1 mmol) and  copper (II) sulfate pentahydrate (499 mg, 2.00 mmol) in water (15 mL). The complex started to precipitate immediately. The obtained brownish colored product was filtered off, washed with a small amount of water, and then air dried [17].

Yield: 0.39 g (0.85 mmol, 85 %). DTA (5 °C min^–1) onset: 197 °C (decomp.); IR (ATR, cm−1): Ṽ = 3365 (w), 3319 (w), 3122 (m), 3018 (w), 2976 (w), 2953 (w), 2074 (m), 2044 (vs), 1809 (w), 1513 (m), 1499 (m), 1454 (m), 1449 (m), 1387 (w), 1339 (w), 1290 (s), 1274 (m), 1183 (s), 1135 (m), 1092 (s), 907 (s), 845 (w), 806 (w), 696 (s), 664 (s); EA calc. (%) for C₄H₆Cu2N₂0 (461.32 g mol−1): C 10.41, H 1.31, N 60.72; found: C 11.01, H 1.32, N 59.45; IS: 1 J; FS: 5 N; ESD: 14 mJ (at grain size 100–500 μm).

Synthesis of dipotassium dinitraminobistetrazole (K-salt)

Compound 2 (see Figure 5) was obtained by a similar procedure, using btzb (1,2-ditetrazolylbutane) instead of btze.

Figure 5. Schematics of reaction steps for the synthesis of K2DNABT (K-salt).

Dichlorodimethoxycarbonylglyoxal bishydrazone (Compound 3)

10.1 g (50 mmol) of Compound 2 were suspended in 300 mL DMF and 20 g (150 mmol) NCS were added. The mixture was stirred overnight at room temperature and filtered, washed with water, ethanol, and ether. The dry product weighed 9.79 g.

Diazidodimethoxycarbonylglyoxal bishydrazone (Compound 4)

960 mg (3.452 mmol) of Compound 3 were suspended in 10 mL DMF and cooled to 0–5 °C. 650 mg (10 mmol) of sodium azide were added and the mixture was stirred overnight at room temperature. The mixture was diluted with 30 mL of ice-water and filtered, washed with water, ethanol, and ether yielding 387 mg (38 %) of Compound 4.