From structure to energy: Resorcinol-based benzoxazine polymers for modern ammunition

Abstract

Today’s ammunition comprises sparingly about 30 wt% of payloads like RDX-based explosive meanwhile, the bulk of the mass consists of structural materials such as metals (e.g., steel and aluminum), which do not contribute energetically. This highlights a significant potential for the improvement of reactive structural materials (RSMs). This class of advanced materials is designed to serve dual purposes: providing structural integrity and releasing chemical energy upon activation. Rigid polymers with glass transition temperature (T_g) higher than the operating temperatures are thought as a new emerging candidate to replace the heavy and metal-based materials (e. g., alloys, thermites). According to this vision, we report the synthesis and characterization of resorcinol-based benzoxazine (Re-Bz) energetic polymer utilizing resorcinol as a renewable material. The assessment of the energetic properties was reported as well. Overall, the newly developed polymer showed promising energetic performances with deflagration temperatures of about 270-275°C, where the combustion heat released during the deflagration was found to be 20.15 kJ·g⁻¹. The thermal analysis results highlighted that these polymers can be effectively used as reactive structure materials (RSMs) owing to their combination of rigidity above operational temperatures and remarkable energetic performances.

Keywords

reactive structural materials RSM benzoxazine resorcinol glass transition temperature energetic polymer

Introduction

In the realm of modern ammunition, a significant portion over 50 wt% is comprised of structural materials primarily made up of metals. While these metals provide essential structural integrity, they contribute little to the energetic performance of the ammunition.^1,2 This inherent limitation presents a substantial opportunity for enhancement through the use of reactive structural materials. RSMs are a class of advanced materials engineered to fulfill dual roles: maintaining structural integrity and releasing chemical energy upon activation. The exploration and development of these materials promise to revolutionize the field of ammunition by integrating energetic functionality with structural support, thereby optimizing performance and efficiency.³ RSMs are commonly expected to be used in systems like kinetic penetrators, reactive fragments, reactive bullets, munition casings, and reactive armor.^4,5

The conventional approach to constructing ammunition relies heavily on metal-based materials such as alloys and thermites. Although effective in providing mechanical strength, these materials add significant weight and do not contribute to the energetic output.^3,6 In contrast, RSMs have the potential to reduce weight and improve energetic efficiency, making them an attractive alternative for modern ammunition design.^5,7 The first attempt to decrease the weight of the structural materials is to replace the metallic structure with a polymeric structure that can contribute to decreasing the ammunition weight. Subsequently, the purpose was to add energetic groups to the polymer backbone in a way that they can contribute energetically upon initiation.^8,9

Among the various candidates for RSMs, rigid polymers with a glass transition temperature above operational temperatures have emerged as a promising solution. These polymers not only meet the structural requirements but also offer significant energetic benefits.^10–12 The development of RSMs has evolved significantly since the initial composites of polytetrafluoroethylene and aluminum, which can be seen as precursors for modern polymer-based RSMs. Historically, research has focused predominantly on metal-based materials such as thermites and alloys. These materials release additional energy but are often limited by the slow nature of these reactions and the need for gaseous oxidizers like atmospheric oxygen, which restricts their application range.^13–15

One of the notable advancements in this area is the development of energetic polymers with high glass transition temperatures that can endure the operating conditions with no alteration of the RSMs performances. 3-(2,4,6-Trinitrophenoxy)oxetane (TNPO), was the first monomer reported by Born’s group.¹⁶ They synthesized it following one-pot reaction process using picryl chloride and lithium oxetan-3-olate. After characterization they find out that this polymer presents very low sensitivity, sufficient thermostability, and performances comparable to TNT. In 2022, Klapötke et al., reported the synthesis of its homopolymer and investigated its polymerization behavior by copolymerization with prior art energetic oxetanes.² They found that DSC revealed the high effect of the TNPO repeating unit on the glass transition temperature. One more rational approach is to develop thermosetting energetic polymers encompassing high T_g and intriguing energetic contribution.^17,18 For this purpose, resorcinol-based benzoxazine (Re-Bz) energetic polymer emerges as the best practical candidate.¹⁹

Resorcinol, a renewable material, serves as a key component in the synthesis of these polymers, aligning with the growing emphasis on sustainable materials in advanced technological applications.^20,21 Resorcinol, also known as 1,3-dihydroxybenzene, is a naturally occurring phenol that can be derived from natural sources such as lignin, a complex polymer found in the cell walls of plants, via fermentation or chemical extraction processes. Its molecular structure allows for creating bis-functional monomers, which are essential for producing thermosetting resins and high-performance polymers.^22,23 Besides, the presence of two hydroxyl groups allows resorcinol to form strong bonds with other molecules, enhancing the mechanical properties and durability of the resulting polymers. The Re-Bz polymer exemplifies the potential of using renewable resources to create high-performance materials that meet the stringent demand of modern ammunition.^24,25 Herein, we report the synthesis and characterization of (Re-Bz) energetic polymer using the para-nitroaniline as a source of explosophores groups through a one-step Mannich reaction process. The synthesized monomer was confirmed by ¹H, FTIR, and elemental analysis. The thermal properties were assessed by DSC-TGA analyses. The energetic performances as well as the physico-chemical properties were studied by bomb calorimeter, deflagration tester, and electronic densimeter.

Materials, preparation and characterization

Materials

Resorcinol (96%) and para-nitroaniline (97%) were purchased from Shanghai Aladdin Reagents (China). Paraformaldehyde (96%), sodium hydroxide (97 %), and anhydrous magnesium sulfate (MgSO₄, 98 %) were purchased from Chengdu Kelong Chemical Co. The acetone, chloroform (99 %), and 1,4-dioxane (99.8 %) solvents were supplied by Honeywell Riedel-de-Haen (Germany). All chemicals were used as received without additional purification.

Synthesis of Re-Bz benzoxazine monomer

Re-Bz energetic monomer was synthesized via the typical solvent method utilizing para-nitroaniline, paraformaldehyde, and resorcinol, as highlighted in Scheme 1. To a 500 ml two-necked round-bottom flask equipped with magnetic stirrer, thermometer, and 1,4 dioxane solvent, a mixture of paraformaldehyde (3 g, 19.72 mmol) and para-nitroaniline (2.72 g, 19.84 mmol) were placed under a reflux condenser at a heating temperature of 100°C for 1 h until the complete dissolution of the starting materials. A solution of dissolved resorcinol (1.18 g, 39.44 mmol) was then carefully poured. Next, the reaction mixture was refluxed at 130°C for an additional 12 h until the reaction mixture was turned yellow. After that, the resulting product was cooled at room temperature and then diluted with 80 ml of a chloroform/acetone mixture (1:2) and washed with 2N NaOH solution and distilled water to remove impurities and unreacted chemicals. The formed organic layer was subsequently dried over anhydrous magnesium sulfate (MgSO₄) and the solvent was removed under reduced pressure. The resulting product shows a yellowish solid with a yield of 87%.

Scheme 1.

Synthesized symmetric and asymmetric Re-Bz monomers.

On the other hand, the synthesis of resorcinol-based benzoxazine monomers leads to the formation of two constitutional isomers namely RE-dnph-aS (asymmetric) and RE-dnph-S (symmetric), arising from the asymmetric substitution pattern of the resorcinol framework. This behavior is well-documented for resorcinol-derived benzoxazines and is generally unavoidable under conventional synthesis conditions. The isomeric composition was analyzed via 1H NMR spectroscopy, where the presence of both isomers is confirmed by the duplication of the diagnostic benzoxazine methylene signals and aromatic resonances. Specifically, the methylene bridge protons (Ar-CH2-N and O-CH2-N) appear as two sets of distinct singlets. Integration of these diagnostic signals indicates that the two isomers are present in comparable proportions, approximately equimolar within experimental error. This distribution suggests that the substitution at the 2,4-positions and 4,6-positions of the resorcinol ring occurs with similar statistical probability under the current reaction parameters.

Polymerization process

The polymerization process of Re-Bz monomer was meticulously conducted by placing the monomer into a steel mold, and the structure of the resulting polymer backbone is highlighted in Scheme 2. The mold was subjected to a controlled heating program to ensure proper polymerization. Initially, the temperature was maintained at 140°C for 60 min to facilitate the onset of the polymerization process. This was followed by a gradual increase to 160°C for an additional 60 min, allowing the reaction to progress further. Subsequently, the temperature was elevated to 180°C for 30 min to ensure thorough polymerization. Finally, the system was heated to 200°C and maintained for 60 min to complete the polymerization, ensuring the formation of a robust polymer network. This precise temperature regimen is critical for achieving the desired mechanical properties and stability in the resulting Re-Bz polymer.

Scheme 2.

Cationic polymerization of asymmetric Re-Bz(aS) and symmetric Re-Bz(S) resorcinol-based benzoxazine monomer.

Physicochemical characterization

Proton ¹H NMR spectra was recorded using a Bruker NMR400 spectrometer, functioning at 400.13 MHz. The solvent utilized was dimethyl sulfoxide (DMSO-d₆), with tetramethylsilane (TMS) serving as the internal standard. Elemental analysis for carbon, hydrogen, nitrogen, and oxygen (C, H, N, O) was performed with a Thermo Flash EA 1112-NCHSO analyzer.

Fourier transform infrared (FTIR) spectroscopy was conducted with a Bruker Vector 22 FTIR instrument, which included a deuterated triglycine sulfate (DTGS) detector and KBr optics. Transmission spectra were obtained within the range of 4000 to 500 cm^-1, with a resolution of 5 cm^-1. The sample was first finely ground and then pressed into thin disks using KBr powder.

Thermal and curing properties of the samples were analyzed through differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). TGA measurements were carried out using a NETZSCH STA 409 PC/PG analyzer under a nitrogen flow of 100 ml/min, with a heating rate set at 10°C/min. DSC tests were conducted on a Perkin-Elmer DSC8000, also at a 10°C/min heating rate, with a nitrogen flow rate of 70 ml/min.

Density measurements were obtained with an Accupyc 1340 II Pycnometer. The procedure involved using helium as the displacement gas to measure the free volume within the samples, and this value was input into the software along with the sample mass. The Pycnometer completed 10 full evacuation cycles before calculating the final density. Each sample was measured three times.

The energetic properties were measured using a Parr 6200 bomb calorimeter to determine the heat of combustion (ΔU_comb) of the synthesized polymer in an oxygen-rich environment, ensuring that the conditions were suitable for complete combustion. The experiments showed that Re-Bz samples ignited successfully without needing to be pressed with benzoic acid. For each polymer, three separate measurements were taken under identical conditions, and the average value was used.

The deflagration temperature of the synthesized polymer was also determined using a standardized deflagration tester (OZM Type: V.69179.64000), operating between 25 and 425°C. Each sample, weighing 500 mg, was crushed smoothly using mortar and pestle and placed in the combustion chamber, with the heating rate set to 5°C/min.

Results and discussion

Structural characterization

The structural characterization of the Re-Bz monomer was conducted with a specific focus on validating the successful ring-closure of the oxazine moiety. It is important to note that the energetic nitro (NO₂) functionalities were introduced into the system via the p-nitroaniline precursor. Since these explosophoric groups remain chemically intact throughout the synthesis, the following spectroscopic interpretation primarily targets the transition from the starting materials to the cyclic benzoxazine structure. Specifically, the analysis of FTIR and ¹H NMR spectra emphasize the emergence of the characteristic methylene bridges and ether linkages, which serve as definitive evidence of the Mannich-like condensation between the phenolic hydroxyl groups, the primary amine, and formaldehyde.

The synthesized monomer was subjected to Fourier Transform Infrared (FTIR) spectroscopy analysis to confirm its molecular structure. The resulting spectrum is shown in Figure 1. The FTIR spectrum was carefully analyzed to match characteristic absorption bands to corresponding chemical bonds, thereby ensuring the successful synthesis of the targeted monomer. The following section presents and discusses the FTIR results obtained from the analysis. The benzoxazine ring was observed for Re-Bz with the characteristic absorptions at 1259 cm⁻¹ (asymmetric stretching vibration of C–O–C), 1082 cm⁻¹ (symmetric stretching of C–O–C), 1158 cm⁻¹ (asymmetric stretching of C–N–C).^18,26 The absorption band at 1470–1500 cm⁻¹ is due to the different modes of vibration within the aromatic ring, because in tetrasubstituted benzene, the substitution pattern can affect the absorption bands of the C = C stretching vibrations due to changes in the symmetry and electronic environment of the benzene ring.^27,28 The peaks 1480 cm⁻¹, 1498 cm⁻¹, and 1585 cm⁻¹ (s) (υ_{(C = C)} benzene ring vibration). The bands at 830 cm⁻¹ (s), 755 cm⁻¹ (s), and 695 cm⁻¹ (m) are assignable to the C–H bonding of the tetrasubstituted benzene.²³

Figure 1.

FTIR spectrum of resorcinol-based benzoxazine monomer.

On the other hand, Crucial evidence for the successful formation of the benzoxazine ring is found in the fingerprint region of the FTIR spectrum. Specifically, a distinct absorption band is observed at 948 cm⁻¹, which is assignable to the N–C–O asymmetric stretching and the out-of-plane C–H bending of the benzene ring fused to the oxazine moiety. This peak, frequently cited as the characteristic ‘oxazine ring vibration,’ serves as a primary indicator of successful ring closure. The presence of this band, coupled with the C–O–C stretching at 1259 cm⁻¹ and 1082 cm⁻¹, provides unambiguous spectroscopic confirmation of the Re-Bz monomer structure. Furthermore, the absence of this specific vibration in the spectrum of the cured thermoset (Figure 2) confirms the complete ring-opening polymerization of the monomer into the polybenzoxazine network.

Figure 2.

FTIR spectrum of resorcinol-based benzoxazine polymer.

The structure of the new benzoxazine Re-Bz monomer, was examined using ¹H NMR spectroscopy, as depicted in Figure 3. Resonance signal assignments were made based on the ¹H NMR spectra of the starting materials and prior studies involving resorcinol-based benzoxazine monomers. Peaks observed in the ranges [4.69-4.71 ppm] and [5.69-4.73 ppm] are likely associated with the –CH₂–N– and –O–CH₂–N–groups of the oxazine ring of both isomeric forms, respectively, confirming the successful formation of the benzoxazine monomer.^29–31 Additional signals, labeled 3, 4, 5, and 6 provide further evidence of the formation of the targeted benzoxazine structure containing the paranitroaniline moiety, as detailed in Table 1.

Figure 3.

¹H NMR spectrum of the resorcinol-based benzoxazine monomer (Re-Bz).

Table 1.

¹H signals of the symmetric benzoxazine Re-Bz and asymmetric Re-Bz respectively.

Symmetric isomer (1)			Asymmetric isomer (1)
Numbering	Structure	¹H, δ (ppm)	Numbering	Structure	¹H, δ (ppm)
1	N-CH₂	4.71	1	4.69	N-CH₂
2	O-CH₂-N	5.69	2	5.73	O-CH₂-N
3	= C_aryl	6.55	3	6.52	= C_aryl
4	= CH-C	7.17	4	7.14	= CH-C
5	= C_arylH	7.28	5	7.30	= C_arylH
6	= CH-C	8.22	6	8.26	= CH-C

The chemical structure and successful formation of the resorcinol-based benzoxazine (Re-Bz) were further confirmed by ¹³C NMR spectroscopy (Figure 4). The characteristic carbon resonances corresponding to the oxazine ring methylene groups, specifically Ar–CH₂–N and O–CH₂–N, were clearly observed at 50.1 ppm and 82.4 ppm, respectively. These signals are the primary indicators of a closed-ring benzoxazine structure. The aromatic carbon signals originating from the resorcinol core and the amine precursor were identified in the range of 105.8–156.4 ppm. Specifically, the quaternary carbons attached to the oxygen atoms of the resorcinol backbone appeared at the lower field (approximately 154 ppm), consistent with the deshielding effect of the phenolic oxygen. The high resolution and absence of extraneous peaks in the ¹³C NMR spectrum further substantiate the high purity of the synthesized Re-Bz monomer, supporting the assignments made in the ¹H NMR analysis.

Figure 4.

¹H NMR spectrum of the resorcinol-based benzoxazine monomer (Re-Bz).

The stoichiometry of the targeted energetic benzoxazine was validated through elemental analysis; the observed percentages of Carbon, Hydrogen, and Nitrogen were found to be in close proximity to the theoretical values, as detailed below:

Elemental analysis: (%) calculated for C₂₂H₁₈N₄O₆ (434) C, 60.83; H, 4.15; N, 12.90; found: C, 60.69; H, 4.22; N, 13.12.

Thermal analyses

DSC-TGA analyses were performed to study the thermal phenomena associated with the curing process of the Re-Bz monomer. The DSC-TGA thermograms are shown in Figures 5 and 6. The DSC thermogram of the monomer reveals distinct thermal transitions that characterize its behavior upon heating. Initially, the monomer exhibits an endothermic melting peak at 140°C Table 2. As the temperature increases, an exothermic peak was found in the temperature range [175 – 185°C] which was attributed to the ring-opening polymerization of Re-Bz. During this process, the oxazine ring is cleaved under heating.^18,32 The cleavage process happened preferentially on the O–C bound due to the heavy molecular weight of the dangling groups of the nitroaniline attached to the carbon of the oxazine ring as highlighted in Figure 5. At a maximum peak temperature of 230°C, the thermogram shows an exothermic degradation peak, marking the temperature at which the polymer begins to decompose.

Figure 5.

DSC curve of resorcinol-based benzoxazine monomer.

Figure 6.

TGA-DTG thermogram of resorcinol-based benzoxazine monomer.

Table 2.

Thermal parameters from the DSC analysis.

Monomer	1st exotherm		2nd exotherm	Total exotherm (J/g)
Monomer	Onset (°C)	Max (°C)	Max (°C)	Total exotherm (J/g)
Re-Bz	167.8	180.3	230	1080

On the other hand, the TGA thermogram of the synthesized Re-Bz monomer is presented in Figure 6. This thermogram exhibits a unique well-resolved weight loss at the T_5% of 188°C which may be attributed to the evaporation of any volatile components and residual solvent present in the sample. The maximum mass loss was observed at the DTG thermogram with a temperature of 235°C.³³ This step corresponds to the thermal decomposition of the sample, where larger molecular fragments or components begin to break down. Notably, at 600°C, the thermogram indicates a char yield of 36%, suggesting that a substantial portion of the sample has converted into char rather than volatilizing completely. This high char yield indicates the presence of stable carbonaceous residues from the thermal degradation process.

Thermal degradation behavior

The thermal stability and decomposition profile of the cured Re-Bz polymer were evaluated using TGA-DTG under a nitrogen atmosphere. As illustrated in Figure 5 the thermogram Re-Bz polymer reveals two distinct weight loss events, indicating different stages of thermal degradation. The first weight loss occurs at 273.75°C, which can be attributed to the initial decomposition of the polymer backbone. This temperature suggests that the polymer begins to break down at this stage, likely through the cleavage of weaker bonds, such as the energetic groups attached to the backbone, which are less thermally stable under heating. This process represents the primary degradation phase, where the polymer structure starts to collapse.

The second weight loss, occurring at 379.86°C, corresponds to the decomposition of the remaining degradation products. At this stage, the byproducts of the initial breakdown, which are likely more resistant fragments or more complex molecular structures, undergo further decomposition. This could involve the breaking of stronger covalent bonds within the polymer’s core structure, or the volatilization of any residual organic compounds left behind from the first phase of degradation. The higher temperature of this second event indicates that these residuals require more energy to decompose fully, reflecting the strength of the remaining molecular fragments after the initial breakdown.³⁴

In addition, the TGA-DTG analysis shows relatively high thermal stability for Re-Bz, particularly with its second degradation phase occurring at nearly 380°C. This is appealing property for reactive structural materials (RSMs), as it endorses that the polymer can withstand high temperatures before complete decomposition by providing both energetic output and structural integrity in a high-heat environment. Moreover, the well-defined two-step decomposition pattern also implies that the material degrades in a controlled way, which is beneficial for applications where gradual energy release is needed (Figure 7).

Figure 7.

TGA-DTG thermogram of resorcinol-based benzoxazine polymer.

The char yield observed at the end of the process also highlights the formation of a carbonaceous residue, indicating that not all of the material sublimates, further contributing to the material’s stability and performance under thermal stress. This dual-phase degradation behavior makes Re-Bz a strong candidate for applications where both mechanical and energetic properties are critical.

Energetic performances

In order to assess the energetic performances of the synthesized resorcinol and paranitroaniline-based polymer, the heat of combustion was measured using bomb calorimetry. Following this, the enthalpy of formation was calculated by applying Hess’s law. During the combustion reactions, it was assumed that the water formed was in a liquid state, while the carbon dioxide remained in a gaseous state. The standard heats of formation were taken as −286 kJ·mol⁻¹ for the formed water H₂O (l) and −394 kJ·mol⁻¹ for the gaseous carbon dioxide CO₂ (g) as depicted in Figure 8.^35,36 These evaluations provide a comprehensive understanding of the thermal properties and energy efficiency of the renewable resorcinol-based polymers.

Figure 8.

Combustion reaction of Re-Bz benzoxazine polymer.

Oxygen balance

The oxygen balance is an important factor in evaluating the efficiency of energetic polymers used in Reactive Structural Materials (RSMs), as it indicates how much oxygen is available to fully oxidize fuel components like carbon and hydrogen. For resorcinol-based benzoxazine (Re-Bz), which has an oxygen balance of −173.27, the negative value insinuates that there is a lack of oxygen supply for this polymer.³⁷

A negative oxygen balance, as seen with Re-Bz, is common in energetic materials designed for controlled environments, such as ammunition or explosives, where external oxidizers are integrated into the overall system.³⁸ While the lack of sufficient oxygen within the polymer may seem like a limitation, this property can actually contribute to highly energetic reactions when paired with the appropriate oxidizer. In practical applications, Re-Bz can still perform efficiently by using oxidizers in the RSM’s composition, allowing it to release significant energy upon activation. The relatively low oxygen balance may also make Re-Bz suitable for applications where a delayed or controlled release of energy is desired, avoiding overly explosive reactions. Overall, the balance between its fuel-rich composition and external oxidizers enables Re-Bz to remain a promising material for enhancing both structural integrity and energetic output in modern ammunition systems.³⁹

It is well established that the heat of formation and heat of combustion are critical factors in determining the energetic potential and stability of polymers used in RSMs. For resorcinol-based benzoxazine (Re-Bz), the heat of formation is measured at −3738 kJ/mol, while its heat of combustion is 7504.1 kJ/mol. These values indicate significant energy release upon decomposition, with the heat of combustion representing the total energy liberated when the polymer is fully oxidized.

Moreover, the heat of combustion of 7504.1 kJ/mol (Table 3) is notably high compared to other RSM materials, which indicates that Re-Bz can deliver a substantial energy output. This makes it highly suitable for applications where energy release is critical. However, this high energy release must be carefully balanced with the material’s structural role. It should be pointed out that in RSMs that are designed to combine both mechanical support and energetic contribution, an overly energetic material could risk causing structural damage or premature decomposition, which would be undesirable in controlled environments.³⁹

Table 3.

Comparative physical and energetic properties of Re-Bz and 4-NBZ benzoxazine polymers.

Sample	Re-Bz	4-NBZ¹⁸
Formula (repeating unit)	C₂₂H₁₈N₄O₆	C₃₁H₂₈N₄O₆
Molecular mass (g·mol⁻¹)	434	552
Appearance	Yellowish powder	Yellowish powder
Heat of combustion	20.15	29.21
−ΔUcomb (kJ·g⁻¹)	20.15	29.21
Enthalpy of combustion	7504.1	16123.92
−Δ_Hcomb (kJ.mol⁻¹)	7504.1	16123.92
Heat of formation	3738	1709.7
−Δ_fH (kJ.mol⁻¹)	3738	1709.7
Oxygen balance OB (%)	−173.27	−202.90
Deflagration temperature T_def (°C)	274	267
Density ρ (g.cm⁻³)	1.64	1.36
Glass transition Temperature Tg (°C)	186.2	185.77

The heat of formation provides insight into the material’s stability and energy storage capabilities. Heat of formation of −3738 kJ/mol (Table 3), is relatively low value and suggests that Re-Bz is thermodynamically stable in its unreacted form, making it suitable for safe handling and long-term storage, a critical factor in practical applications like ammunition.

Together, the high heat of combustion and controlled heat of formation make Re-Bz an ideal candidate for RSMs, where the goal is to strike a balance between structural integrity and energetic performance. The material can provide significant energy output while maintaining stability and strength, ensuring safe and controlled energy release without damaging its structure.

The deflagration temperature of the Re-Bz polymer is recorded at 274°C as indicated in Table 3. This temperature dictates the polymer’s response to heat and external initiation sources. The deflagration process in energetic polymers like Re-Bz occurs when the material is heated or subjected to an external stimulus, resulting in rapid combustion and energy release. The deflagration temperature of 274°C places Re-Bz within a temperature range that ensures it remains stable under typical storage conditions, significantly higher than the maximum temperatures encountered in storage facilities. This high deflagration temperature minimizes the risk of accidental autoignition, making it safe for long-term storage.⁴⁰

Furthermore, it is also worthwhile to know that a deflagration temperature of 274°C is ideal for energetic materials used in ammunitions, as it ensures a rapid and reliable response upon intentional initiation. A lower deflagration temperature could lead to a preemptive ignition. While an excessively high one might result in slower or incomplete energy release. Re-Bz’s deflagration temperature compromises between safety during storage and effective performance when needed.

To evaluate the impact of the phenolic precursor on the final properties of the energetic thermoset, the physical and energetic parameters of Re-Bz were compared with the p-nitroaniline-based bisphenol-A benzoxazine (4-NBZ) reported in the literature. The comparative data is summarized in Table 3. The results indicate that the resorcinol-based architecture in Re-Bz offers significant advantages for energetic applications. Re-Bz exhibits a substantially higher density (ρ = 1.64 g·cm⁻³) compared to 4-NBZ (ρ = 1.36 g·cm⁻³), which is a critical factor for increasing the detonation pressure and velocity in energetic materials.

Additionally, the oxygen balance (OB) of Re-Bz (−173.27%) is markedly improved over that of 4-NBZ (−202.90%), stemming from the higher oxygen-to-carbon ratio in the resorcinol backbone. Thermal stability is also enhanced in the resorcinol system; Re-Bz demonstrates a deflagration temperature (T_def) of 274°C, whereas 4-NBZ degrades at 267°C. This improvement is likely due to the higher cross-linking density achievable with the smaller, more compact resorcinol-based repeating unit (434 g·mol⁻¹) compared to the bulkier bisphenol-A unit (552 g·mol⁻¹). These findings confirm that transitioning from a bisphenol-A to a resorcinol-based framework successfully optimizes the energetic performance while maintaining superior thermal integrity.

To further evaluate the performance of the target Re-Bz polymer within the context of Reactive Structural Materials (RSMs), a comparative study was conducted against several benchmark energetic polyoxetane-based polymers (P1–P6). As summarized in Table 4, the molar heat of combustion (ΔH_comb) of Re-Bz was found to be 7504.1 kJ/mol. This value is notably higher than those of TNPO (4669.3 kJ/mol) and 3AO (6599.9 kJ/mol), and it remains highly competitive with established energetic binders such as NIMMO (7452.1 kJ/mol).²

Table 4.

Physicochemical, Thermodynamic, and Thermal properties of Re-Bz compared to benchmark polymers P1–P6.²

Polymer	Formula (repeating unit)	Monomer name	Heat of combustion -ΔH_C (kJ/mol)	Glass transition temperature (°C)
(P1) TNPO	C₉H₇N₃O₈	3-(2,4,6-trinitrophenoxy) oxetane	4669.3	63
(P2) 3AO	C₁₂H₁₂N₆O₉	3-azidooxetane	6599.9	25.2
(P3) AMMO	C₁₂H₁₆N₆O₉	3-azidomethyl-3-methyloxetane	7685.1	0.7
(P4) BAMO	C₁₄H₁₅N₉O₉	,3-bis(azidomethyl)oxetane	8229.4	23.4
(P5) NIMMO	C₁₄H₁₆N₄O₁₂	-nitratomethyl-3-methyloxetane	7452.1	36
(P6) AMNMO	C₁₄H₁₅N₇O₁₂	3-azidomethyl-3-nitratomethyloxetane	8026.5	42.4
(P7) Re-Bz	C₂₂H₁₈N₄O₆	Resorcinol-based benzoxazine monomer	7504.1	186.2

While the energy density of Re-Bz is comparable to high-performance azido-substituted polymers like AMMO (7685.1 kJ/mol), a critical distinction arises when considering thermal-mechanical stability. The benchmark materials P1–P6 are characterized by low glass transition temperatures (T_g) ranging from 0.7 to 63°C, which typically classifies them as soft binders or semi-solids. In stark contrast, the Re-Bz polymer developed in this study exhibits a T_g of 186.2°C.

This significant increase in T_g represents a successful departure from the traditional trade-off between energy content and structural rigidity. By maintaining a high-strength glassy state at temperatures where traditional energetic polyoxetanes would soften, Re-Bz demonstrates superior potential as a multifunctional RSM capable of maintaining structural integrity under extreme thermal loads without sacrificing energetic performance.

Conclusion

The study presented here highlights the potential of resorcinol-based benzoxazine as an innovative material for use in reactive structural materials. In modern ammunition, where structural materials often account for a significant portion of the mass without contributing to the energetic performance, there is a strong drive to develop materials that can provide both structural integrity and energetic output. Re-Bz represents a promising advancement in this area due to its dual functionality and renewable raw material basis.

The thermal and energetic characterization of Re-Bz reveals several key properties that make it an ideal candidate for RSMs. The polymer’s high glass transition temperature (above operational conditions) ensures that it maintains rigidity and structural integrity under stress, while its high deflagration temperature of 274°C ensures safe storage and controlled energy release when initiated. Moreover, the high heat of combustion (7504.1 kJ/mol) indicates a significant energy output upon activation, making Re-Bz highly suitable for applications where both mechanical support and energetic performance are critical.

The negative oxygen balance (−173.27 %) suggests that, while Re-Bz is oxygen-deficient, this property can be leveraged in applications where additional oxidizers are present, such as in ammunition or reactive armor systems. This allows Re-Bz to release energy effectively when paired with external oxidizers, making it a flexible and efficient material in controlled environments.

Overall, the combination of high energy release and thermal stability makes Re-Bz a promising candidate for replacing traditional metal-based structural materials in munitions and other defense-related applications. By utilizing a renewable raw material like resorcinol, Re-Bz also aligns with the growing emphasis on sustainability in advanced materials development. Future research could further optimize its energetic performance and explore its integration into various RSM applications, paving the way for more efficient and lightweight ammunition systems.

Footnotes

ORCID iDs

Oussama Mehelli

Slimane Abdous

Mehdi Derradji

Abdelwahad Berrouane

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

References

Hastings

Dreizin

. Reactive structural materials: preparation and characterization. Adv Eng Mater 2018; 20(3): 1700631. https://doi.org/10.1002/adem.201700631

Born

Plank

Klapötke

. Energetic polymers: a chance for lightweight reactive structure materials? Propellants, Explos Pyrotech 2022; 47(4): e202100368. https://doi.org/10.1002/prep.202100368

Wang

L-y.

Jiang

J-w.

, et al. Improving the damage potential of W-Zr reactive structure material under extreme loading condition. Def Technol 2021; 17(2): 467–477. https://doi.org/10.1016/j.dt.2020.03.001

Bacciochini

Radulescu

Yandouzi

, et al. Reactive structural materials consolidated by cold spray: Al–CuO thermite. Surf Coating Technol 2013; 226: 60–67. https://doi.org/10.1016/j.surfcoat.2013.03.036

Lee

Mock

Jr Carney

, et al. Reactive materials studies. In: AIP Conference Proceedings, American Institute of Physics 2006, pp. 169–174.

Zheng

, et al. Damage effects of double-spaced aluminum plates by reactive material projectile impact. Int J Impact Eng 2017; 104: 13–20. https://doi.org/10.1016/j.ijimpeng.2017.01.023

Zygmunt

. Explosive properties of the Mg-Al/PTFE composition. Chin J Energetic Mater 2011; 15(6): 592–596.

Ang

Pisharath

. Energetic polymers: binders and plasticizers for enhancing performance. John Wiley & Sons, 2012.

Klapötke

. Chemistry of high-energy materials. Walter de Gruyter GmbH & Co KG, 2022.

10.

Wang

R-M

Zheng

S-R

Zheng

. Polymer matrix composites and technology. Elsevier, 2011.

11.

Fink

. Reactive polymers: fundamentals and applications: a concise guide to industrial polymers. William Andrew, 2017.

12.

Pantoya

Levitas

Granier

, et al. Effect of bulk density on reaction propagation in nanothermites and micron thermites. J Propul Power 2009; 25(2): 465–470. https://doi.org/10.2514/1.36436

13.

Guo

Liang

Islam

, et al. Highly reactive thermite energetic materials: preparation, characterization, and applications: a review. Molecules 2023; 28(6): 2520. https://doi.org/10.3390/molecules28062520

14.

Badgujar

Talawar

Asthana

, et al. Advances in science and technology of modern energetic materials: an overview. J Hazard Mater 2008; 151(2-3): 289–305. https://doi.org/10.1016/j.jhazmat.2007.10.039

15.

Klapoetke

. Reactive structure materials. Journal of Engineering Science and Military Technologies 2020; 4(2): 171–177.

16.

Born

Voggenreiter

. New energetic oxetane monomers based on nitroaromatic scaffolds. New Trends in Research of Energetic Materials (NTREM) 2019.

17.

Abdous

Derradji

Khiari

, et al. Design of advanced and highly effective partially bio-energetic polybenzoxazines as the next generation of reactive structure materials. J Energetic Mater 2023; 43: 1–21. https://doi.org/10.1080/07370652.2023.2236608

18.

Abdous

Derradji

Mehelli

, et al. Mannich reaction as an interesting synthetic route for the development of energetic benzoxazine polymers. Mater Today Commun 2021; 29: 102878. https://doi.org/10.1016/j.mtcomm.2021.102878

19.

Derradji

. Unravelling the role of energetic polybenzoxazines in modern reactive structure materials. Int J Energ Mater Chem Propuls 2024; 23(4): 75–85. https://doi.org/10.1615/intjenergeticmaterialschemprop.2024052744

20.

Pilato

. Phenolic resins: a century of progress. Springer, 2010.

21.

Thakur

Kessler

. Handbook of composites from renewable materials, biodegradable materials. John Wiley & Sons, 2017.

22.

Sauget

Zhou

Pizzi

. Tannin-resorcinol-formaldehyde resin and flax fiber biocomposites. J Renew Mater 2014; 2(3): 173–181. https://doi.org/10.7569/jrm.2013.634128

23.

Arnebold

Schorsch

Stelten

, et al. Resorcinol‐based benzoxazine with low polymerization temperature. J Polym Sci Polym Chem 2014; 52(12): 1693–1699. https://doi.org/10.1002/pola.27169

24.

Sam

Durairaj

, et al. Synthesis of biomass-based carbon aerogels in energy and sustainability. Carbohydr Res 2020; 491: 107986. https://doi.org/10.1016/j.carres.2020.107986

25.

Godino-Ojer

Blazquez-García

Matos

, et al. Porous carbons-derived from vegetal biomass in the synthesis of quinoxalines. Mechanistic insights. Catal Today 2020; 354: 90–99. https://doi.org/10.1016/j.cattod.2019.06.043

26.

Abdous

Derradji

Mehelli

, et al. Synthesis and characterization of nitrogen-rich polybenzoxazines for energetic applications. High Perform Polym 2022; 34(4): 455–464. https://doi.org/10.1177/09540083211073654

27.

Berthon-Fabry

Hildenbrand

Ilbizian

. Lightweight superinsulating Resorcinol-Formaldehyde-APTES benzoxazine aerogel blankets for space applications. Eur Polym J 2016; 78: 25–37. https://doi.org/10.1016/j.eurpolymj.2016.02.019

28.

Derradji

Mehelli

Khiari

, et al. High performance green composite from vanillin-based benzoxazine containing phthalonitrile and silane surface modified basalt fibers. High Perform Polym 2022; 34(9): 989–997. https://doi.org/10.1177/09540083221103762

29.

Zhao

Hao

, et al. Thermal latent and Low-Temperature polymerization of a Bio-Benzoxazine resin from natural renewable chrysin and furfurylamine. Eur Polym J 2022; 166: 111041. https://doi.org/10.1016/j.eurpolymj.2022.111041

30.

Zhang

. Synthesis and properties of biobased mono-benzoxazine resins from natural renewable pterostilbene. Eur Polym J 2021; 156: 110607. https://doi.org/10.1016/j.eurpolymj.2021.110607

31.

Hao

Liu

, et al. Synthesis, polymerization kinetics and thermal properties of benzoxazine resin containing ortho-maleimide functionality. Macromol Res 2021; 29: 24–32. https://doi.org/10.1007/s13233-021-9006-7

32.

Lochab

Varma

Bijwe

. Thermal behaviour of cardanol-based benzoxazines: monomers and polymers. Journal of Thermal analysis and Calorimetry 2010; 102: 769–774. https://doi.org/10.1007/s10973-010-0736-6

33.

García-Martínez

Gude

Ureña

. Understanding the curing kinetics and rheological behaviour of a new benzoxazine resin for carbon fibre composites. React Funct Polym 2018; 129: 103–110. https://doi.org/10.1016/j.reactfunctpolym.2017.02.005

34.

Sini

Bijwe

Varma

. Thermal behaviour of bis-benzoxazines derived from renewable feed stock'vanillin. Polym Degrad Stabil 2014; 109: 270–277. https://doi.org/10.1016/j.polymdegradstab.2014.07.015

35.

Silva

Almeida

Ribeiro da Silva

, et al. On the enthalpy of formation and enthalpy of sublimation of dihydroxylammonium 5, 5′‐bitetrazole‐1, 1′‐dioxide (TKX‐50). Propellants, Explos Pyrotech 2023; 48(7): e202200361. https://doi.org/10.1002/prep.202200361

36.

Klapötke

Stierstorfer

. Triaminoguanidinium dinitramide—calculations, synthesis and characterization of a promising energetic compound. Phys Chem Chem Phys 2008; 10(29): 4340–4346. https://doi.org/10.1039/b804504f

37.

Zhang

Shreeve

. 3, 3′-Dinitroamino-4, 4′-azoxyfurazan and its derivatives: an assembly of diverse N–O building blocks for high-performance energetic materials. J Am Chem Soc 2014; 136(11): 4437–4445. https://doi.org/10.1021/ja501176q

38.

Ribeiro da Silva

Monte

Rocha

, et al. Energetic study applied to the knowledge of the structural and electronic properties of monofluorobenzonitriles. J Org Chem 2012; 77(9): 4312–4322. https://doi.org/10.1021/jo3002968

39.

Zhao

Wang

, et al. Design and synthesis of energetic materials towards high density and positive oxygen balance by N-dinitromethyl functionalization of nitroazoles. J Mater Chem A 2016; 4(15): 5495–5504. https://doi.org/10.1039/c6ta01501h

40.

Dreizin

Schoenitz

. Mechanochemically prepared reactive and energetic materials: a review. J Mater Sci 2017; 52: 11789–11809. https://doi.org/10.1007/s10853-017-0912-1