Congmei Lin

High-temperature creep properties of TATB-based polymer bonded explosives filled with multi-walled carbon nanotubes

In order to investigate the effects of multi-walled carbon nanotubes (MWCNTs) content and loading stress on the high-temperature creep properties of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based polymer bonded explosives (PBXs), three-point bending creep behavior was studied by dynamic mechanical analyzer at 60 °C. The experimental results showed that under low stress (4 MPa), the constant creep strain rates of the MWCNTs modified formulation reduced with MWCNT concentration. However, due to the agglomeration of MWCNTs, the existence of intertube sliding or stick-slip resulted in an increase in creep strain. Under higher stress (6 MPa and 7 MPa), with only 0.25 mass% of MWCNTs, the creep resistance of TATB-based PBX could be significantly improved with reduced constant creep strain rate. Additionally, compared with TATB-based PBX without MWCNTs, the creep lifetime of the nanocomposites with a content of 0.25 mass% MWCNTs had been considerably extended by over 33.3% and 750% under 6 MPa and 7 MPa, respectively. The nanocomposites with a content of 0.5 mass% MWCNTs displayed a lower creep resistance compared to that of the TATB-based PBX without MWCNTs. The creep resistance of the TATB-based PBXs depends significantly on loading stress. A six-element model could be used to simulate the high-temperature creep behaviors of the TATB-based PBXs with and without MWCNTs. The constitutive equations of the creep curves under different stresses at 60 °C were obtained. Among the fitting parameters, the elastic modulus of high elastic deformation E3 and the viscosity of Voigt units η3 were sensitive and promising for the evaluation of the creep strain of the TATB-based PBXs.

The dependence of the non-linear creep properties for TATB-based polymer bonded explosives on the molecular structure of polymer binder

The influences of molecular structure of polymer binders on the mechanical properties and non-linear time dependent creep of the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based polymer bonded explosives (PBXs) at different temperatures and stresses were investigated. A copolymer of chlorotrifluoroethylene and vinylidene fluoride (PF1) and a copolymer of chlorotrifluoroethylene, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene (PF2) were used as polymer binders. An increase of the storage modulus and glass transition temperature was observed for PF2, compared to that of PF1. The compressive and tensile properties of TATB-based PBX with PF2 were higher than the one with PF1 at both ambient temperature and elevated temperature. The creep resistance also showed clear dependence on the molecular structure of polymer binders. It was found that the incorporation of tetrafluoroethylene and hexafluoropropylene comonomers in PF2 resulted in a decrease of the constant creep strain rate and the maximal creep strain values and an increase of creep rupture time for TATB-based PBX. Non-linearity in the creep response was modeled using the six-element mechanical model. The predicted theoretical results coincided quite well with the experimental data. Compared with the formulation containing PF1 as binder, an increase in the elastic modulus E2, E3 and bulk viscosity η4 was observed for TATB-based PBX with PF2 under the same conditions. Three-point bending master curves of creep strain were constructed using a time–temperature superposition (TTS) concept. The formulation with PF2 showed consistently lower creep strain than the formulation with PF1 in the entire time scale.

Enhanced non-linear viscoelastic properties of TATB-based polymer bonded explosives filled with hybrid graphene/multiwalled carbon nanotubes

In this work, hybrid graphene/multiwalled carbon nanotubes (MWCNTs) nanofillers were selected to improve to non-linear viscoelastic properties of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based polymer bonded explosive (PBX). The morphology, mechanical properties, and creep behaviors of TATB-based formulations were studied. The results were compared with the corresponding composites with individual graphene nano-additives. Scanning electron microscopy observation results indicated graphene particles were fairly well dispersed in the nanocomposite filled with hybrid graphene/MWCNTs, while graphene sheets were prone to aggregation in the PBX filled with individual graphene. Hybrid graphene/MWCNTs modified PBX exhibited higher storage modulus in the whole temperature range. The compressive fracture energy (Wc) and tensile fracture energy (Wt) at 20 °C were up to 31.6% and 89.6% higher than that of PBX without nanofillers. The creep responses of the composites were determined by short-time creep tests at various temperatures and stresses. The creep compliance curve for TATB-based PBXs showed a remarkable synergetic effect between graphene and MWCNTs in improving the creep resistance. The better dispersion of graphene nanoparticles and higher interfacial zones, which produced strong interfacial interaction between the graphene and polymer matrix to restrict the mobility of polymer chains, were considered as crucial factors for the improvement in the creep resistance for TATB-based PBXs.

Non-linear viscoelastic properties of TATB-based polymer bonded explosives modified by a neutral polymeric bonding agent

The neutral polymeric bonding agent (NPBA) was selected to enhance the interface adherence between 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) crystals and fluoropolymer. The interfacial performance of the composites was investigated by the measurement of contact angle and the interfacial bonding mechanism was studied by XPS analysis. The results indicate that a hydrogen bond between TATB and NPBA is formed. The mechanical analysis of the TATB-based polymer bonded explosives (PBXs) revealed that the storage modulus, the mechanical strength and the elongation at break of the formulation modified by NPBA were improved. The creep behaviors of the TATB-based PBXs with and without NPBA were also investigated at different temperatures and stresses. Reduced creep strain and steady-state creep strain rate and prolonged creep failure time were observed for the modified formulation, suggesting enhanced creep resistance performance. The creep experimental data were evaluated using a six-element mechanical model and the long-term creep performance of the materials was predicted using the time–temperature superposition principle. The creep behavior up to 6.0 years at 30 °C/4 MPa could be predicted by the short-term experimental data (5400 s) acquired at 30–80 °C under 4 MPa. The application of NPBA provides an efficient route to reinforce, toughen, and improve the creep resistance of explosive composites, such as TATB-based PBXs in this study.

Evaluation of the fracture behaviors of fluoropolymer binders with the essential work of fracture (EWF)

The fracture behaviors of fluoropolymer binders with different molecular structures used in polymer bonded explosives (PBX) were first investigated by the essential work of fracture (EWF). The molecular motions of four selected fluoropolymers were measured by dynamic mechanical analysis (DMA). The incorporation of chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) comonomers, would retard the molecular mobility of the fluoropolymer chain, resulting in the glass transition temperature (Tg) shifting to a higher value. Through EWF testing, the effect of the molecular structure on the fracture behaviors for fluoropolymer binders was investigated. The specific essential work of fracture, we, decreased with Tg increasing, while the specific plastic deformation work, βwp, had an opposite change trend. By fracture energy partitioning, the energy distribution during the fracture process of fluoropolymers was found. The dominant factors which affected we and βwp for the studied systems were the necking terms, as the majority of the fracture energy was dissipated in the necking and fracture process. In addition, the Brazilian test, or diametrical compression, was used to study the tensile properties of PBX. The influences of the obtained fracture parameters of polymer binders by EWF on the mechanical properties of PBX were analyzed.

Construction and thermal properties of nano-structured polymer bonded explosives with graphene

1,3,5-Triamino-2,4,6-trinitrobenzene (TATB)-based polymer bonded explosives (PBXs) modified with graphene were prepared by the water suspension methods. The effects of coating methods and graphene content on the thermal conduction properties were investigated. Reference raw TATB-based PBX samples were also tested and compared with the results obtained from the nanocomposites filled with graphene. The experimental results indicated that the dispersion and concentration of graphene in the nanomaterials could play an important role in the thermal behaviors. Compared with the nanocomposites with outer-coating graphene, the corresponding PBXs with inner-coating graphene presented a higher thermal conductivity due to better dispersion of graphene in the composites with the aid of ultrasound vibration. The nonlinear dependence of the thermal conductivity of TATB-based PBXs on the graphene content was observed. When the content of inner-coating graphene came to 2 wt%, the thermal conductivity of TATB-based PBX increased by 105.5%, with values of 1.122 W m−1 K−1. The Agari model was adopted to analyze the thermal behaviors of TATB-based PBXs filled with graphene. With the incorporation of graphene, the thermal conduction mechanism of TATB-based PBXs changed from a series model to partial parallel model. Additionally, the testing temperatures had a dual effect on the thermal conductivity of TATB-based PBXs. With increasing temperature, the thermal conductivity initially exhibited a sharp drop, and then a little fluctuation was observed in the temperature range of 50–70 °C. Finally, the thermal conductivity of PBXs reduced with temperature again.

The dependence of the non-linear creep properties of TATB-based polymer bonded explosives on the molecular structure of the polymer binder: (II) effects of the comonomer ratio in fluoropolymers

1,3,5-triamino-2,4,6-trinitrobenzene (TATB) based polymer bonded explosives (PBXs), with three polymer binders containing different molecular structures, were studied by non-linear time dependent creep tests at different temperatures and stresses. Three fluoropolymers, i.e. F2311, F2313, and F2314 with molar ratios of comonomer vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE) of 1 : 1, 1 : 3, and 1 : 4 were chosen as polymer binders. The experimental results suggested that all of the materials showed temperature, stress and molecular structure sensitivity. With the decrease of temperature and stress, the creep resistance of the three TATB-based PBXs was improved with reduced creep strain, decreased steady-state creep strain rate, and prolonged creep failure time. Replacement of F2311 with F2313 in the binder system lead to a creep strain decrease and creep failure time rise. With further increasing of the CTFE content in fluoropolymers from 75% to 80%, the creep resistance performances were enhanced for TATB/F2314 composites under pressures from 1 to 9 MPa, compared with TATB/F2313 composites. The creep strain–time plots for TATB-based PBXs could be accurately fit using the six-element mechanical model. The long-term creep behaviors of TATB-based PBXs were predicted based on the time-temperature superposition (TTS) concept. In addition, the dynamic behaviors and mechanical properties of fluoropolymers and TATB-based PBXs were also studied and analyzed in detail.