Enhancing the Safety of Nitramine Explosives via Interfacial Polymerization of Melamine‑Urea‑Formaldehyde Resin

Background

Modern weapon systems demand munitions that combine high performance with robust safety. Conventional high‑energy nitramines—RDX, HMX, and CL‑20—excel in power but remain highly sensitive, posing handling risks. The development of insensitive high explosives (IHEs) has therefore become a priority for defense research [1–3]. Researchers have explored refinement, coating, and eutectic approaches to mitigate sensitivity [4–9]. Coating technologies—physical or chemical—envelop energetic particles with a protective layer, aiming to reduce impact and friction sensitivities while preserving performance [15]. The effectiveness of a core‑shell design hinges on coverage uniformity, mechanical strength, and inhibition of self‑nucleation [15]. The choice of binder critically influences particle morphology, size distribution, and energetic performance. Melamine‑formaldehyde (MF) resins, though effective, suffer from brittleness, high cost, and limited shelf life [16]. In contrast, melamine‑urea‑formaldehyde (MUF) resins combine the mechanical advantages of urea‑formaldehyde with the nitrogen‑rich nature of melamine, offering superior bonding and thermal properties. Recent studies have demonstrated that MUF can enhance the safety and stability of nitramines when used as a coating binder [16–18]. Ultrasonic assistance has proven beneficial in polymer‑based coating processes, promoting uniform dispersion and reducing agglomeration [19–20]. Building on this, we synthesized a green MUF binder via a two‑step process and applied it to HMX, RDX, and CL‑20 cores. We compared three fabrication routes—physical mixing, an improved drying‑bath method, and an optimized interfacial polymerization technique—each producing nine distinct composites (five‑percent MUF content). The interfacial polymerization approach yielded the most uniform, densely coated particles with superior thermal stability and markedly reduced sensitivities.

Methods

Materials

High‑purity HMX, RDX, and CL‑20 were sourced from Gansu Yinguang Chemical Industry Group Co. Raw materials were recrystallized following protocol [21]. Dimethyl sulfoxide, Tween 80, Span 80, triethanolamine (TEOA), urea, formaldehyde, hydrochloric acid, resorcinol, ammonium chloride, and polyvinyl alcohol (PVA) were acquired from reputable suppliers. Deionized water was used throughout.

Two‑Step Synthesis of MUF Resin

Step 1: Urea‑formaldehyde prepolymer was formed by reacting 0.62 g urea with 1.87 g 37 % formaldehyde under stirring at 65 °C. The pH was adjusted to 8.5–9.5 using TEOA, then the mixture was cooled, and HCl was added dropwise to reach pH ≈ 3.5. Step 2: 1.87 g of the prepolymer was emulsified in 35 mL deionized water, followed by the addition of 8 % PVA, 0.01 g melamine, 0.125 g resorcinol, and 0.06 g ammonium chloride. The pH was again adjusted to ≈ 3.5, then the reaction proceeded at 65 °C for 3–4 h. After cooling, the mixture was filtered, washed with deionized water, and vacuum‑dried to yield high‑quality MUF resin (~0.3 g per batch).

Preparation of Explosive/MUF Composites by Interfacial Polymerization and Drying‑Bath Methods

Both routes used the same initial MUF prepolymer protocol, differing only in the second step.

• Interfacial polymerization: 6 g of explosive was dispersed in 35 mL deionized water with 0.01 g Span 80. The emulsion was sheared at 7 000 rad min⁻¹ for 30 min to form a stable explosive emulsion, which replaced the water in the MUF synthesis sequence.
• Drying‑bath method: 6 g of explosive was dissolved in 35 mL DMSO at 65 °C to form an explosive solution, which replaced the water in the MUF synthesis. After 3–4 h, the emulsion was dried at 70 °C for 48 h to obtain core‑shell particles.

Preparation of Explosive/MUF Composites by Physical Mixing

For comparison, 6 g of explosive was mixed with the pre‑synthesized MUF binder in 35 mL deionized water and stirred at 65 °C for 2 h. The mixture was then allowed to stand, filtered, and dried to yield physically mixed composites.

Samples were designated as follows: interfacial polymerization (sample 1), drying‑bath (sample 2), and physical mixing (sample 3).

Characterization

SEM (MIRA3 LMH, 10 kV) visualized particle morphology. XRD (DX‑2700, Cu‑Kα, 1.5418 Å) scanned 5°–50° (0.03° steps, 6 s). FT‑IR (Nicolet FT‑IR 8700) collected 32 scans at 4 cm⁻¹ resolution. DSC (DSC‑131) measured at 10 °C min⁻¹. Impact sensitivity was assessed via a drop hammer (H₅₀, 30 mg sample, 25 trials). Friction sensitivity employed a WM‑1 instrument (20 mg sample, 25 trials). Particle size was measured by a QICPIC dynamic analyzer under controlled temperature and humidity.

Results and Discussion

Morphology of the Samples

SEM images show raw nitramines as polygonal crystals with uneven size distribution, whereas the MUF binder alone appears spherical but partially hollow. The composite particles differ markedly depending on the fabrication route:

• Physical mixing: Exposed cores and uneven coating indicate poor binder distribution.
• Drying‑bath: Dense, irregular particles arise from solvent evaporation and limited dispersant solubility in DMSO, leading to re‑aggregation and non‑spherical shapes.
• Interfacial polymerization: Uniform, smooth, near‑spherical particles with dense shells, reflecting effective wetting and steric hindrance provided by PVA and the reduced Hamaker constant.

Crystal Structure of Samples

XRD patterns confirm that the crystal phases of HMX, RDX, and CL‑20 remain unchanged after MUF coating. Peaks characteristic of MUF (≈ 27° 2θ) appear in all composites, indicating successful shell formation. Minor peak broadening and attenuation are attributed to the amorphous nature of MUF, which disrupts long‑range order in the composite lattice. FT‑IR spectra show all key functional groups of both binder and core, further verifying that the energetic phase is preserved.

Thermal Properties

DSC analysis reveals that coating reduces the decomposition temperature of the nitramines by 2.6–10 °C. The interfacial polymerization route produces the smallest temperature drop (≈ 2.6 °C for HMX), reflecting its more compact and uniform shell. This suggests that a well‑coated core mitigates heat transfer and preserves thermal stability. In contrast, the drying‑bath composites show larger reductions, likely due to uneven coverage.

Sensitivities

Impact sensitivity (H₅₀) increases dramatically for interfacial polymerization composites: HMX from 21.6 cm to 73.4 cm, RDX from 31.8 cm to 85.6 cm, and CL‑20 from 15.3 cm to 64.0 cm. Friction sensitivity follows a similar trend, with the interfacial composites exhibiting the lowest values. The enhanced safety is attributed to the uniform shell, which buffers mechanical stimuli and delays hotspot formation. These results surpass previous reports employing MF or other binders [7, 18, 26].

Conclusions

We fabricated nine core‑shell composites (HMX/MUF, RDX/MUF, CL‑20/MUF) using three fabrication routes with a 5 % MUF binder. XRD and FT‑IR confirmed phase integrity; DSC showed modest decomposition‑temperature reductions, minimal for interfacial polymerization. Most notably, the interfacial composites achieved a 3–4× increase in H₅₀, evidencing superior safety. Their dense, uniform shells also improved thermal stability compared to the other routes. For future energetic formulations, selecting MUF as a binder and employing interfacial polymerization is recommended to produce high‑energy, low‑sensitivity munitions suitable for advanced weaponry.

Abbreviations

CL-20:

Hexanitrohexaazaisowurtzitane

DSC:

Differential scanning calorimetry

ESV:

Emulsion solvent evaporation

FI-IR:

Fourier‑transform infrared spectra

GPBX:

Green polymer‑bonded explosives

HMX:

Cyclotetramethylenetetranitramine

IHEs:

Insensitive high explosives

MF:

Melamine formaldehyde

MUF:

Melamine‑modified urea‑formaldehyde

NC:

Nitrocellulose

PF:

Phenolic resin

RDX:

Cyclotrimethylenetrinitramine

SEM:

Scanning electron microscopy

UF:

Urea formaldehyde

XRD:

X‑ray diffraction