Plastic-bonded Explosives Research Articles

Review of the Effects of Polymer Binder Properties on Microstructure and Irreversible Volume Growth of Plastic Bonded Explosives Formulations

AbstractThe rational design of effective polymeric binders for the formulation of plastic‐bonded explosives (PBX) is challenging due to their inherent compositional complexity. The composites comprise irregularly shaped energetic material (EM) powders coated with low weight fractions of polymer via non‐equilibrium processes such as slurry coating. Defects can deleteriously affect PBX stability and performance: nano‐ to micrometer‐scale voids can act as loci for hot spots, lowering deflagration and detonation temperatures in unpredictable ways. Furthermore, some nominally desirable polymer properties are at odds with each other: e. g. good flow characteristics are desirable for coating and adhesion, but mechanical stiffness is needed to prevent deformation and cracking of PBX under mechanical stress. Good binder adhesion is critical, but the best means to predict and measure adhesion in PBX is not obvious. Experimental methods of determining binder adhesion on model surfaces may not capture polymer structural configurations relevant to deposition during coating. Molecular dynamics‐based computational models have predicted key observables in PBX formulation, suggesting that they may be powerful tools for binder selection. In this review, primarily recent (∼2006 and later) literature on polymeric binders for insensitive HE (IHE) is surveyed. We focus on how binder properties influence observable PBX properties as resistance to irreversible volume growth and void formation in PBX formulations mainly (but not exclusively) featuring 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) as the EM (e. g. PBX 9502, LX‐17 and others). Conclusions from these studies yield useful guidelines for choosing HE binder candidates, as well as for the general design of highly‐filled polymer composite materials. Finally, studies describing challenges in PBX formulation with the newer and more energetically dense high explosive, LLM‐105, will be discussed.

Evaluation of thermodynamic closure models for partially reacted two-phase mixture of condensed phase explosives

One of the key fundamental issues that is crucial in the continuum modeling of reactive flow phenomena is the thermodynamically consistent description of reaction mixture properties. To define the mixture properties, thermodynamic closure rules that relate the properties of the individual reaction components to the mixture properties are required. In the context of reactive two-phase modeling approaches, various strategies to define the thermodynamic closures have been adopted such as pressure temperature (PT) equilibrium between the individual reaction components, pressure (specific) volume (PV) equilibrium, etc. The choice of closure rules determines the relative distribution of specific volume and energy across the reaction components that comprise the mixture. Therefore, depending on the choice of the closure, the mixture thermodynamic behavior can vary. The present work examines the effect of different closure approaches on the thermodynamic properties of the reaction mixture. The analysis is performed for a condensed phase HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) based plastic bonded explosive (PBX) 9501 explosive using four different thermodynamic closures, viz., PT equilibrium, PV equilibrium, volume temperature (VT) equilibrium, and pressure (P) equilibrium with reactants on an isentrope. The relative variations in the thermodynamic properties of the mixture are analyzed and compared under both compression and expansion loading regimes. It is shown that out of the four closure models, only PT equilibrium and P equilibrium closures lead to a thermodynamically accurate description of the mixture under both compression and expansion.

Quasistatic anisotropic induced behaviour of a tatb-based plastic-bonded explosive

The constitutive law of a plastic-bonded explosive (PBX) based on TATB crystals is studied in the quasistatic loading range. The experimental data highlight a viscoelastic behaviour with damage, and an anisotropy induced by plastic flow. The existing models for PBXs being not appropriate, a constitutive law accounting for the observed mechanisms is proposed. The linear viscoelasticity is integrated in a microplane model describing an effective anisotropic damage. A multilayer viscoplasticity is used to yield a non-linear kinematic hardening. A von Mises yield criterion is defined on each surface and a dilatancy function describes the volumetric plastic yield. This law is implemented in the Abaqus/Standard finite element code in the form of a UMAT subroutine. The model is calibrated on ten tests. The simulations performed to compare the model to each experimental test correctly reproduce the behaviour of the studied explosive composition.

Anisotropic Reaction Properties for Different HMX/HTPB Composites: A Theoretical Study of Shock Decomposition.

Plastic-bonded explosives (PBXs) consisting of explosive grains and a polymer binder are commonly synthesized to improve mechanical properties and reduce sensitivity, but their intrinsic chemical behaviors while subjected to stress are not sufficiently understood yet. Here, we construct three composites of β-HMX bonded with the HTPB binder to investigate the reaction characteristics under shock loading using the quantum-based molecular dynamics method. Six typical interactions between HMX and HTPB molecules are detected when the system is subjected to pressure. Although the initial electron structure is modified by the impurity states from HTPB, the metallization process for HMX does not significantly change. The shock decompositions of HMX/HTPB along the (100) and (010) surface are initiated by molecular ring dissociation and hydrogen transfer. The initial oxidations of C and H within HTPB possess advantages. As for the (001) surface, the dissociation is started with alkyl dehydrogenation oxidation, and a stronger hydrogen transfer from HTPB to HMX is detected during the following process. Furthermore, considerable fragment aggregation is observed, which mainly derives from the formation of new C−C and C−N bonds under high pressure. The effect of cluster evolution on the progression of the following reaction is further studied by analyzing the bonded structure and displacement rate.

The behavior of antioxidant irganox 1010 during the thermal degradation of a plastic bonded explosive

The effect of water concentration on the aging behavior of blend components in plastic bonded explosive (PBX) 9501 is investigated when samples were aged up to 24 months under various conditions. The blend components studied here are: poly(urethane ester) (Estane®5703) (Estane), nitroplasticizer (NP), and antioxidant Irganox 1010 (Irg1010). The experimental results reveal that NP is prone to thermally degrading and producing H2O, NOx, and HNOx species, which are the predominant species to consume Irg1010 during PBX 9501 aging under inert environment. As Irg1010 is completely consumed, Estane degrades through oxidation and NP addition, in addition to well anticipated hydrolysis. The competition among hydrolysis, oxidation, and NP addition results in non-monotonical changes in the molecular weight of Estane over the aging process.

Impact and contact problems of explosives by ‘Mixed’ meshless local Petrov-Galerkin finite volume method

The ‘Mixed’ Meshless Local Petrov-Galerkin Finite Volume method (MLPG5) is developed for solving low-velocity impact problems of plastic bonded explosives (PBX). Due to the ductility and plasticity of PBX, there is large deformation instead of pulverization or fragmentation. In this paper, the impact dynamics equations are solved by the meshless approach (MLPG5), both stress as well as displacements are interpolated, through the moving least squares (MLS) interpolation schemes, which eliminates the expensive process of domain integrals and differentiating the shape function. The collocation method is applied to enforce strain-displacement relationships only at each nodal point, instead of the subdomain. In the present mixed method, the independent interpolation of stress improves accuracies of both the plastic stress and contact stress. The contact force is obtained by penalty function method via iteration scheme. Due to the non-linear constitutive relation, the incremental stress is obtained via radial return schemes through Newton Raphson iteration. Finally, several numerical examples are given to demonstrate the feasibility and the accuracy of the present numerical approach compared with the finite element method. Numerical examples also demonstrate the advantages of the present methods: (i) smaller support sizes can be used; (ii) higher accuracies of stress are obtained.

Coefficient of Thermal Expansion Evolution during Ratchet Growth of PBX 9502 and Neat TATB

AbstractThe Plastic‐Bonded Explosive (PBX) 9502 is comprised of the insensitive high explosive TATB (triamino trinitrobenzine) crystals coated in FK‐800 polymer binder and isostatically pressed. The TATB crystals have a graphitic, plate‐like morphology and possess significant thermal and mechanical anisotropy. Compactions of neat TATB or TATB‐based composites (like PBX 9502) have been shown to have TATB crystals that are oriented during the pressing process giving rise to TATB texture and resulting in micro‐ and macro‐scale anisotropy of the compaction. Thermal cycling of the compactions results in irreversible volume expansion (or ratchet growth) likely by creating stress points on the microscale. However, the actual mechanism behind this expansion is not fully understood. Previous characterization of the ratchet growth phenomenon has focused on the magnitude of the irreversible strain as measured before and after a thermal cycle. In the work presented here, we analyze the evolving thermal expansion behavior during the ascending and descending temperature ramps and compare differences observed due to the presence/absence of binder in the compaction.

High throughput tabletop shock techniques and measurements

Although shock experiments are traditionally performed in large facilities, tabletop experiments that provide convenient high-throughput shock testing have been growing in importance. Here, we describe tabletop experiments using a shock compression microscope that features a pulsed 0–6 km/s laser flyer plate launcher and a photon Doppler velocimeter. We also describe methods to mass-produce flyer plates and targets to achieve high throughput. We explain how to condition a laser beam to launch flyers that provide reproducible short-rise time impacts with minimal tilt, and we present a number of applications including measuring shock propagation in nanoporous media, a simple way to describe shock wave energy absorption, the use of photoemissive probes such as organic dyes or quantum dots to study shocked inhomogeneous media, the development of an apparatus to measure optical absorption in shocked media, methods to study and measure the temperature of shocked energetic materials in the form of plastic-bonded explosives and in a form that allows us to observe hot spots in real time, and studies of the shocked interface between a metal fuel and a ceramic oxidizer. Finally, a brief perspective is presented describing new possibilities for future research of a diverse set of applications including the chemistry of shocked water and biological systems, dense plasmas, and the use of laser-launched flyer plates as surrogates for hypersonic vehicles.

Comparing the shock sensitivity of insensitive energetic materials

We present a tabletop method to study the shock sensitivity of plastic-bonded explosives that are considered shock insensitive using high dynamic range optical emission spectroscopy with laser-launched km/s flyer plate impacts (2, 3, and 4 km/s), which measures the spectral radiance (the emission spectrum vs a calibrated intensity standard) every 0.8 ns in the nanosecond and microsecond regimes. The explosives were TATB (1,3,5 trinitro, 2,4,6 triamino benzene), FOX-7 (1,1-diamino-2,2-dinitroethylene), LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide), and NTO (nitrotriazolone), all with 20% Sylgard 182 binder. Time resolving emission from shocked explosives produces a unique fingerprint at each shock pressure, and the emission temporal profile can be used to understand each material's shock response and the underlying mechanisms within the framework of the well-known shock ignition and growth models. Ignition was characterized by the emission intensity during nanosecond hot spot ignition. The growth process, which occurs after the shock has unloaded, was characterized by the time derivative of microsecond emission intensity. Singular-value decomposition was used to determine a characteristic spectral radiance over the entire time range, and this radiance was fit to a graybody model to determine a characteristic temperature Tc. Even though these insensitive explosives have similar shock sensitivities, our method can clearly distinguish their shock response. FOX-7 and NTO were the most sensitive, with FOX-7 having the greatest shock ignition sensitivity, growth rate, and Tc. LLM-105 was much less sensitive than FOX-7 and NTO, and TATB consistently showed the lowest sensitivity by any measure.

Shock Pressure Dependence of Hot Spots in a Model Plastic-Bonded Explosive.

Shock initiation of plastic-bonded explosives (PBX) begins with the formation of so-called "hot spots", which are energetic reactions localized in regions where the PBX microstructure concentrates the input shock wave energy. We developed a model PBX system to study hot spots which consists of a single crystal of the high explosive HMX (cyclotetramethylene-tetranitramine) embedded in a transparent polyurethane binder (J. Phys Chem. A, 2020, 124, 4646-4653). In the current work we use this model system to study the influence of input shock pressure (12-26 GPa) on hot spot generation using micrometer-resolved high-speed imaging and nanosecond-resolved optical pyrometry. By shocking ∼100 HMX single crystals (HMX-SC), two distinct shock pressure thresholds were observed. The threshold for producing single hot spots in some crystals was 15 GPa. At 23 GPa, hot spot density was sufficiently high to lead to rapid deflagration of the entire HMX-SC. It takes about 25 ns after the shock passes for the hot spots to appear to our visible-light detection apparatus which has a noise floor at about 2000 K. That indicates the shock produces nascent hot spots that undergo a thermal explosion that reaches temperatures >2000 K in 25 ns. The initial hot spot temperature is roughly 3800 K which settles down to 3400 K, the adiabatic flame temperature of HMX. The higher initial temperature is attributed to release of stored interfacial strain energy produced by the shock. An initial estimate for the velocity of the flame front originating at an HMX hot spot is 550 m/s.

A Micromechanics Pressurization Model for Cookoff

AbstractA micromechanics pressurization (MMP) model has been derived for explosive decomposition models that are pressure‐dependent. The model includes volumetric thermal strain and internal pressurization using well‐known solutions of elastic equations that include displacement of the condensed phase. The model is based on observations of a heated, high‐density, plastic bonded explosive (PBX) containing 95 wt% triaminotrinitrobenzene (TATB) with 5 wt% chlorotrifluoroethylene/vinylidene fluoride binder (Kel‐F). The model was developed for explosives that are either permeable or impermeable to decomposition gases. The MMP model is based on pore mechanics which describe reaction nucleation, decomposition chemistry, and elastic volumetric expansion. The model accounts for the expansion or swelling of the explosive into the surrounding gas‐filled ullage space. The pressurization model was used in conjunction with a simple decomposition model to determine ignition time and internal temperatures for the TATB‐based explosive at 1881 kg/m3. The MMP model was used to predict pressure, specific surface area, and gas volume fraction. A Latin hypercube sensitivity analysis showed that prediction of ignition time was most sensitive to the maximum pore pressure which defines the threshold between permeable and impermeable explosive layers. The MMP model coupled to a pressure‐dependent chemistry model can predict accurate ignition times for high‐density PBX's exposed to high temperatures and may be useful for more general application scenarios.

Cookoff of Powdered and Pressed Explosives Using a Micromechanics Pressurization Model

AbstractCookoff experiments of powdered and pressed TATB‐based plastic bonded explosives (PBXs) have been modeled using a pressure‐dependent universal cookoff model (UCM) in combination with a micromechanics pressurization (MMP) model described in a companion paper. The MMP model is based on the accumulation of decomposition gases at nucleation sites that load the surrounding TATB crystals and binder. This is the first cookoff model to use an analytical mechanics solution for compressibility and thermal expansion to describe internal pressurization caused by both temperature and decomposition occurring within closed‐pore explosives. This approach produces more accurate predictions of ignition time and pressurization within high‐density explosives than simple equation‐of‐state models. The current paper gives details of the reaction chemistry, model parameters, predicted uncertainty, and validation using experiments from multiple laboratories with errors less than 6 %. The UCM/MMP model framework gives more accurate thermal ignition predictions for high density explosives that are initially impermeable to decomposition gases.

Thermal Decomposition and Kinetics of Cast Plastic Bonded Explosives Based on NTO and HMX

In this work, we studied the influence of the ratio of NTO and HMX on the thermal decomposition performance of the series of cast PBXs. DSC was used to study the nonisothermal thermal decomposition performance of different PBXs, and ARC was used to study the adiabatic thermal decomposition. Both DSC and ARC results show that among the five different PBXs, N30 is the cast PBX with the best thermal stability.

A finite-deformation constitutive model of particle-binder composites incorporating yield-surface-free plasticity

Abstract We present a constitutive model for particle-binder composites that accounts for finite-deformation kinematics, nonlinear elasto-plasticity without apparent yield, cyclic hysteresis, and progressive stress-softening before the attainment of stable cyclic response. The model is based on deformation mechanisms experimentally observed during quasi-static monotonic and cyclic compression of mock Plastic-Bonded Explosives (PBX) at large strain. An additive decomposition of strain energy into elastic and inelastic parts is assumed, where the elastic response is modeled using Ogden hyperelasticity while the inelastic response is described using yield-surface-free endochronic plasticity based on the concepts of internal variables and of evolution or rate equations. Stress-softening is modeled using two approaches; a discontinuous isotropic damage model to appropriately describe the softening in the overall loading-unloading response, and a material scale function to describe the progressive cyclic softening until cyclic stabilization. A nonlinear multivariate optimization procedure is developed to estimate the elasto-plastic model parameters from nominal stress-strain experimental compression data. Finally, a correlation between model parameters and the unique stress-strain response of mock PBX specimens with differing concentrations of aluminum is identified, thus establishing a relationship between model parameters and material composition.

Brazilian disk compression testing of plastic-bonded idoxuridine mock explosive

ABSTRACT Recently, there has been increased interest in developing new inert surrogate materials, or “mocks,” for plastic-bonded explosives (PBX). Idoxuridine (IDOX) has similar density and mechanical behavior to the explosive HMX, and formulated plastic-bonded IDOX showed promise for emulating the HMX-based PBX 9501 in preliminary mechanical experiments. Several improvements to the manufacture of plastic-bonded IDOX have been made and are characterized here by Brazilian Disk testing. Compression tests were performed on these disk specimens under quasi-static loading conditions and at three different temperatures, , , and . The full-field deformation over the sample surface during the compression was obtained using the optical digital image correlation (DIC) technique. In this article, we summarize the Brazilian disk compression test results of the 2020 batch of IDOX mock specimens. We present the overall response, as well as the local deformation measurement. We discuss in detail, the sequence of deformation events occurring in the disk specimen during the compression process. Elastic constants, including shear modulus, Young’s modulus, and Poisson’s ratio, of the IDOX mock material, are extracted from the Brazilian disk compression test data. Finally, we compare the Brazilian disk compression test results with the data obtained from the baseline PBX 9501, tested in 2007, as well as the data from a previous batch of IDOX mock material tested in 2017. We hope that such comparisons would point out the path of improvement for the formulation and pressing of the new IDOX mock.

Thermal decomposition mechanisms of LLM-105/HTPB plastic-bonded explosive: ReaxFF-lg molecular dynamics simulations

ABSTRACT Thermal decomposition is not only related to the explosive’s ignition and detonation performances but also to their sensitivity to the surrounding environment. Based on the molecular dynamics method, the interaction and the thermal decomposition mechanism of plastic-bonded explosives (PBXs) composed by 2,6-diamino-3,5-dinitropyrazine-1-oxidated (LLM-105) and hydroxyl-terminated polybutadiene (HTPB) were investigated using the reactive force field molecular dynamics (ReaxFF-lg MD) simulations at 2000 K-4000 K. The interaction between the explosive and the binder effectively stabilizes the explosive crystal and reduces its sensitivity. The predicted activation energy of LLM-105/HTPB is 39.86 kcal/mol, more than that of the pure LLM-105 (36.57 kcal/mol), which reduces the initial decomposition reaction rate of LLM-105 after adding HTPB binder. Products distribution in thermal decomposition shows that the main initial decomposition mechanisms of LLM-105 include the C-NO2, C-NH2 bonds breaking, nitroso rearranging, NH2 elimination and H atom transferring. The H ions from the HTPB dehydrogenation delay the chemical bond breaking to inhibit the decomposition of LLM-105, and the reversible H transfer reactions provide an energy buffering area to reduce sensitivity.

The effect of active aluminum content on the detonation performance of aluminized explosives

ABSTRACT For aluminized explosives, it is important to investigate the effect of the active aluminum content on their detonation performance. Ten formulations of plastic bonded explosives (PBXs) containing aluminum/ammonium perchlorate/1,3,5-trinitro-1,3,5-triazine (Al/AP/RDX) are prepared where their active aluminum contents are in the range 59–99%. Measurements of detonation velocity for these formulations are done by a 430 g cylindrical charge. It is shown that there is a strong linear relationship between the measured detonation velocities and active Al contents in different samples. The measured detonation velocities of the formulations with a higher content of active Al are greater than the other compositions. Thus, it can be expected that the contribution of Al in the reaction zone is higher for the formulations with a higher content of active Al. The results of this work show that the coating of the surface of submicron-sized Al reduces the oxidation rate and increases their service life.

Composite binder, processing, and particle size effects on mechanical properties of non-hazardous high explosive surrogates

Plastic-bonded explosives (PBX) are polymer matrix composites that are highly loaded with micron-scale explosive crystals. The thermal and mechanical properties of these materials must be well-understood so that appropriate engineering models can be developed, but testing such properties under many scenarios can be hazardous and expensive. A non-hazardous PBX surrogate (“mock”) may be used in place of a PBX in many experiments if the thermomechanical properties are well-matched. Our previous research has shown that 5-iodo-2′-deoxyuridine (IDOX) is a promising crystalline filler material for the explosive cyclotetramethylene-tetranitramine (HMX). Here, we investigate how the binder composition, variations in the IDOX particle size distribution, and pressing temperature affect the compressive strength and elastic modulus. We observe that while the binder system used in the composite dominates the mechanical response, the particle size distribution and pressing conditions do have a measurable effect on the material compliance and strength.

The investigation of NTO/HMX-based plastic-bonded explosives and its safety performance

3-nitro-1,2,4-triazol-5-one (NTO) is the main component of insensitive munitions (IM) formulation because of its outstanding insensitive properties. In this paper, a series of NTO/HMX-based composite explosives were prepared and characterized. The study focuses on the effect of NTO on the performance of the formulations, especially the safety performance. The results revealed that the mechanical sensitivity of formulations was associated with NTO content, as well as the thermal conductivity, specific heat capacity and Arrhenius parameters. Then, the high amount of NTO using in formulation was proved to be helpful for NTO/HMX-based formulation to exhibit good thermal safety. Besides, by accelerating rate calorimeter (ARC) and a modified cook-off equipment, the pressure and pressure rise rate were proved as the important indicator for judging the thermal safety performance in confined spaces. Finally, the numerical simulation was used as a credible method for predicting the respond temperature of cook-off experiment.

Modeling Microstructural Effects on Heterogeneous Temperature Fields within Polycrystalline Explosives

AbstractThe paper addresses the role of crystal anisotropy on the evolution of heterogeneous temperature fields in plastic‐bonded explosives (PBXs) under conditions of weak shock. The modeling approach is based on simplified idealizations of PBX microstructure including RDX grains and estane binder regions subjected to velocity boundary conditions representative of impact conditions in situ. The constitutive description of the microstructure constituents includes a dislocation‐based, anisotropic, single crystal plasticity model for the explosive grains and a linear viscoelastic model to represent the estane binder. Large suites of simulations were used to systematically study the correlation in heating of the grains with local wave dynamics, crystal orientation, and the microstructural neighborhood. These correlations were identified through the selection of characteristics of crystal anisotropy including oriented wave speeds and Taylor factor. A key observation is that the wave dispersion within a certain distance from the impact interface plays a dominant role in the temperature field. Beyond this distance, individual crystal orientations play a more dominant role, but cannot entirely account for the observed heterogeneity without consideration of the local microstructure neighborhood.