Construction of self-reinforced polymer based energetic composites with nano-energetic crystals to enhance mechanical properties

Abstract

Nano-fillers have attracted a large amount of attention in enhancing the mechanical properties of polymer bonded explosive (PBX), due to the strong interfacial interactions. However, the addition of inert components reduces the energy output and makes the processing, recycling and re-utilization of energetic composites quite unfavorable. To improve the mechanical properties while maintaining detonation performance, a novel self-reinforced PBX was constructed with very low content of nano-1,3,5-triamino-2,4,6-trinitrobenzene (TATB) served as modifiers. The incorporation of nano-TATB particles into PBX evidently increased the storage modulus, strength, and toughness. The enhanced mechanism could be ascribed to the nano effect of nanoparticles and “interlocking block” formed on the rough surface of nano-TATB, which provided stronger interfacial interaction. This work demonstrates the successful application of nano-energetic crystals to self-reinforce PBXs, thereby opening up new perspectives in structure design and modification of mechanical properties for energetic composites without changing the intrinsic components of energetic composites.

Introduction

As typical polymer-based functional composites, polymer bonded explosives (PBXs) have been extensively applied for decades in both civil and military applications. Generally, PBXs are composed of high loading of solid explosive crystals (90–95% by weight) dispersed in a minimal amount of polymer binder matrix [1], [2], [3]. PBXs are subject to various loads and prone to deformation and damage during the storage and usage, which severely threaten the safety, reliability and long-term performances of the weapon systems. Consequently, considerable efforts have been devoted to design and prepare novel PBXs with high mechanical performance.

To fine-tune the mechanical performance, several approaches have been developed, primarily concerning preparation technology, crystal quality and morphology, interfacial structure, as well as polymer binder. One approach is developing a novel granulation method, such as emulsion solvent evaporation (ESV) [4], which gives high quality coverage and superior toughness properties. However, the high ratio of water and explosive crystals limits the efficiency of large-scale production. As far as crystal quality and morphology are concerned, Yang et al. [5] have found that spherical morphology was profitable for both the mechanical strength and creep resistance, especially for the samples with rough surfaces. Unfortunately, it is challenging to develop a universal method to control the morphology of different explosive crystals. Another approach is to tune the interfacial structures with coupling agents [6], neutral polymeric bonding agents (NPBA) [7], [8], or bio-inspired PDA coating [9], [10], [11], [12]. Furthermore, the addition of nano-fillers such as carbon nanotubes [13], graphene [14], graphene nanoplatelets [15] in the polymer matrix, is efficient in the modification of mechanical properties. Whereas the addition of inert constituents reduces the energy output, cause complexity for the preparation process, and is unfavorable for the recycling and re-utilization of PBXs. Thus, the path to research of enhancing the mechanical properties of energetic materials is still fraught with challenges.

As environmentally benign materials, self-reinforced polymeric materials (SRPMs) have attracted more and more attention due to their unique properties, such as perfect interfacial bonding between fiber and matrix, pure chemical functionality, and ease of recycling [16], [17], [18]. Excellent compatibility and bonding strength could be achieved at the interphase that comprises the same chemical structure but different crystalline and/or supermolecular properties. The effective stress transfer from the “weak” matrix to the “strong” reinforcing structure gave rise to higher stiffness and strength of SRPMs than that of the raw matrix.

As a two-phase system, the self-reinforced PBXs could be aquired either from the energetic crystals or the polymer matrix. However, in the conventional preparation process involving a water suspension granulation method [5], the polymer binder is dissolved in a solvent, bringing difficulty in producing different crystalline and/or supermolecular properties between the reinforced phase and matrix phase. Consequently, energetic crystals with the same chemical structure but different morphology are potential candidates to be used as the reinforced phase in PBXs.

Generally, nano-energetic materials exhibit size-dependent properties different from conventional micro-sized ones. These include thermal decomposition, sensitivity, combustion, detonation, and compaction behaviors [19], [20], [21]. Therefore, extensive studies have been carried out on the preparation and characterization of nano-structured energetic materials, such as HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)/estane nanocomposites, nano-Keto RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), nano-HNS (2,2′,4,4′,6,6′-hexanitrostilbene), nano-CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), nano-FOX-7 (1,1-diamino-2,2-dinitroethene), nano-NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one), and nano-LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) [22], [23], [24], [25], [26]. The reduction in the particle size of energetic materials from micron to nano-sized is suitable for obtaining reduced sensitivity. Wang et al. [22] have found that nanocrystalline HNS is less sensitive than synthesized HNS (50 μm) to impact and shock stimuli. It has also been revealed that the impact sensitivity of nano-CL-20 decreases compared to micrometer-sized CL-20 [23]. The thermal behaviors of nano-energetic materials have been extensively studied. For nano-FOX-7 prepared via an ultrasonic spray-assisted electrostatic adsorption method, the first exothermic peak is increased by 45 °C and has a faster energy releasing efficiency and releases more energy than the original and sub-micro particle crystals, indicating a potential alternative application for micro-electromechanical systems, micro thrusters and so on [24]. The researches on the relationship between decreasing particle size of explosives and their thermal stability confirm that a size reduction may reduce the thermal stability of the explosives [25], [26].

As one of the favorite high explosives, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) has attracted intensive interest in science and engineering during the past decade due to moderate energy output and excellent thermal and shock stability [27], [28], [29]. With excellent comprehensive properties, TATB is widely used in high energy and insensitive formula. For example, it has been widely used as an insensitive explosive or a desensitizer in mixed explosives [30]. Having a very broad application prospects, it becomes the focus of researches in the explosive community.

TATB is a high explosive with extremely low sensitivity to heat, impact, shock, and electric sparks. Literally, TATB has a great advantage in safety performance over other conventional explosives [30]. Taking TATB, HMX and pentaerythritol tetranitrate (PETN) for instance, these explosives represent three different types of explosives (i.e., nitro aromatic, nitroamino and nitric ether), respectively. They display increasing sensitivity to heat (i.e., their largest heat release peak temperature decreases from 374 to 275 and 202 °C), impact (i.e., their H₅₀, which is indicative of a height corresponding to a 50% explosion probability with a drop hammer of 2.5 kg, decreases from 4.9 to 0.32 and 0.16 cm), shock (i.e., their shock ignition inputs decrease from 18 to 11 and 4 GPa) and electric sparks (their spark energy, denoting static electricity spark sensitivity, decreases from17.75 to 2.89 and 1.74 J) [31].

Though the researches of nano-TATB in military field have proliferated in recent years [32], [33], [34], the high cost and the easy aggregation significantly limit the potential application. To address this long-standing problem, it is essential to develop a facile and effective method to achieve uniform dispersion of TATB nanoparticles and strong interfacial bonding with matrix. In this work, the nano-energetic materials were used as the reinforced phase to prepare self-reinforced PBXs. Excellent dispersion of nano-energetic materials in the polymer matrix could be achieved with ultrasonic irradiation. Slight content of nano-energetic materials required for the purpose also reduces the cost. This unique self-reinforced structure can effectively enhance the interfacial interaction and achieve high mechanical properties. The present work potentially provides a novel and universal method for enhancing mechanical properties of energetic composites without changing the constituents.