D. Mark Hoffman

Irreversible volume growth in polymer-bonded powder systems: effects of crystalline anisotropy, particle size distribution, and binder strength

Pressed-powdered crystallites of intrinsically anisotropic materials have been shown to undergo irreversible volume expansion when subjected to repeated cycles of heating and cooling. In a previous letter publication [R. H. Gee et al., Appl. Phys. Lett. 70, 254105 (2007)], we developed a coarse-grained (micron-scale) interaction Hamiltonian for such a system and quantitatively reproduced experimentally observed irreversible growth through explicit molecular dynamics simulations. In this paper, we report (1) recent experiments with a high-density fluoropolymer binder that significantly lowers irreversible growth, (2) identification of a critical interaction parameter of our model that has a strong correlation with binder properties, (3) sensitivity of irreversible growth to the details of particle size and alignment distribution, and (4) a physical picture of irreversible growth in terms of particle displacements.

Comparison of New and Legacy TATBs

Two newly synthesized versions of the insensitive high explosive (IHE) 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were compared to two legacy explosives currently used by the Department of Energy. Except for thermal analysis, small-scale safety tests could not distinguish between the different synthetic routes. Morphologies of new TATBs were less faceted and more spherical. The particle size distribution of one new material was similar to legacy TATBs, but the other was very fine. Densities and submicron structure of the new TATBs were also significantly different from the legacy explosives and the densities of pressed pellets were lower. Recrystallization of both new TATBs from sulfolane produced nearly hexagonal platelets with improved density and thermal stability.

Dynamic mechanical signatures of aged LX-17-1 plastic bonded explosive

Abstract The complex shear modulus of the plastic bonded explosive (PBX) LX-17-1 from stockpile returns, core tests and historical billets was measured over the temperature range from −150 to 120°C at five frequencies from 0.1 to 10 Hz. LX-17-1 is composed of 92.5% insensitive high explosive triaminotrinitro-benzene (TATB) and 7.5% plastic binder, KF-800. Three relaxations were observed as peaks in the loss modulus and tangent delta in various samples of LX-17-1. The low temperature β-relaxation, a very broad relaxation, occurred between −50 and −65°C depending on sample history. The glass transition of the binder was observed as a peak in the loss modulus at 30 ± 2°C at 1 Hz. No evidence of plasticization or anti-plasticization caused by the explosive filler or binder crystallization was observed. When LX-17-1 was stored above its glass transition, the binder crystallized and occasionally the α-relaxation, associated with melting of the crystal, could be resolved as a peak in the loss tangent at approximately 80°C. More often only a change in slope or broadening in the loss peak was observed. The shear storage modulus increased depending on binder crystallinity by a factor of up to 3.2 at temperatures between the glass transition and melting point of the binder. Crystallization is slow so the dynamic moduli of LX-17-1 with amorphous binder could be measured before recrystallization occurred. The weak relaxation of the pure TATB at about 34°C was hidden by the glass transition of the polymer binder. Secondary crystallization of the binder may be responsible for the PBX stiffening over time as indicated by a very slight increase in storage modulus of aged LX-17-1 from stockpile and accelerated aging tests. Some increase in modulus was also found below Tg in aged samples that may indicate other mechanisms were responsible for the observed increase in stiffness.

Partial Phase Behavior of HMX/DMSO Solutions

The phase diagram of HMX/DMSO and the solvated complex HMX·(DMSO)2 has been investigated by visual observation, high performance liquid chromatography (HPLC), differential scanning calorimetry (DSC), and optical microscopy. Visual observations showed the solved complex exhibited an incongruent melting point, i.e., the melting point of the solvated crystal occurs within the region of the phase diagram where HMX itself precipitates. If the solvated crystal is removed from solution and heated, it reverts to solid HMX and DMSO at the peritectic temperature (∼70°C). Using HPLC the change in concentration of the liquid phase was monitored as a function of temperature to confirm the phase line of the DMSO/solvated complex based on visual observation. Polarized light microscopy of the dissolution of seeded solutions of different concentrations was also used to verify these results. It is possible that a 1:1 complex also exists, but no evidence for such a complex was found in these experiments.

Fatigue of lx-14 and lx-19 plastic bonded explosives

Abstract The DOD uses the plastic bonded explosive (PBX) LX-14 in a wide variety of applications including shaped charges and explosively formed projectiles.1 LX-19 is a higher energy explosive, which could be easily substituted for LX-14 because it contains the identical Estane 5703p binder and more energetic CL-20 explosive. Delivery systems for large shaped charges, such as TOE 2, include the Apache helicopter. Loads associated with vibrations and expansion from thermal excursions in field operations may, even at low levels over long time periods, cause flaws, already present in the PBX to grow. Flaws near the explosive/liner interface of a shaped charge result in reduced performance. Small flaws in explosives are one mechanism (the hot spot mechanism) proposed for initiation and growth to detonation of PBXs like LX-14, PBXN 5, LX-04 and LX-17 among others. Unlike cast-cured explosives and propellants, PBXs cannot usually be compression molded to full density. Generally, the amount of explosive ignited by a shock wave is approximately equal to the original void volume.2 Whether or not these flaws or cracks grow during field operations to an extent sufficient to adversely affect the shaped charge performance or increase the vulnerability of the PBX is the ultimate question this effort could address. Currently the fatigue life of LX-14 under controlled conditions is being studied in order to generate its failure stress as a function of the number of fatigue cycles (S-N curve). Proposed future work, depending on the availability of funding, will address flaw and crack growth and their relationship to hot-spot concentration and explosive vulnerability to shock and/or fragment initiation.

Dynamic mechanical and thermal analysis of crystallinity development in Kel-F 800 and TATB/Kel-F 800 plastic bonded explosives: Part I. Kel-F 800

Abstract Dynamic mechanical and differential scanning calorimeter measurements were made on Kel-F 800, a copolymer of chlorotrifluoroethylene and vinylidene fluoride, as a function of time. Samples annealed over a 30 day period at 50 ° C showed reduced relaxation strength in the glass transition which directly correlated with the development of between 10 and 15% crystallinity in this polymer. Kel-F 800 samples thermally cycled from −54 to 74 ° C without annealing for 12, 24, and 36 cycles developed only about half the crystallinity of the annealed samples. Kel-F 800 which was annealed 30 days and subsequently subjected to the above thermal cycling retained almost all of the crystallinity which developed during the annealing process. These results indicate that Kel-F 800 copolymer crystallizes at an extremely slow rate, but as long as the thermal excursion remains below the major melting peak most of the crystallinity remains.

Mechanical Mocks for Insensitive High Explosives

Three mechanical mocks were formulated and tested as replacements for the current mock for insensitive explosives LX-17-1 and PBX 9502 because its binder was no longer available. The three polymers evaluated were a butyl/isobutyl acrylate copolymer, ethyl cellulose and a new fluoropolymer, PFR 91. The glass transitions of these polymers were 35, 130, and −10°C, respectively. Two inert fillers, talc and cyanuric acid, were used in the new formulations. Pressing densities and mechanical and thermal properties were used to characterize these mocks. The mock based on the acrylic copolymer most closely emulated these insensitive high explosives.

Inhibiting the Transport of Hazardous Spores Using Polymer-Based Solutions

A series of polymer solutions were developed for the purpose of immobilizing aerosolized 1–10 μ m sized hazardous biological particles. The polymer solutions were designed as tools for emergency response and remediation personnel. The inhibition of secondary aerosolization and migration of biothreat particles has important implications for public health protection and contamination cleanup. Limiting further dispersion of particles such as Bacillus anthracis spores may reduce inhalation hazards and enhance remediation efficiencies. This study evaluated film-forming polymers that have multiple functional groups capable of attracting and binding particles; these included acrylates, cellulosics, vinyl polymers, and polyurethanes. The selected polymers were combined with appropriate solvents to design solutions that met specific performance objectives. The polymer solutions were then evaluated for key characteristics, such as high adhesion, high elasticity, low density, short drying time, low viscosity, and low surface tension. These solutions were also evaluated for their adhesion to biothreat agent in a series of wind tunnel experiments using highly refined aerosolized Bacillus atrophaeus spores (a simulant for anthrax, 1–3 μ m). Results demonstrated that a polymer solution, an amphoteric acrylate identified as NS-2, was the best candidate for attaching to spores and inhibiting reaerosolization. This polymer solution was anionic, thus providing the electrostatic (coulombic) attraction to cationic spores, had low surface tension, and performed well in wind tunnel tests.

Infrared properties of three plastic bonded explosive binders

ABSTRACT Three polymers are routinely used as binders for plastic bonded explosives by Lawrence Livermore National Laboratory, FK-800, Viton A 100, and Oxy 461. Attenuated total reflectance Fourier transform infrared measurements were performed on 10 different lots of FK-800, 5 different lots of Oxy 461, and 3 different lots of Viton A-100, one sample of Viton VTR 5883 and 2 Fluorel polymers of hexafluoropropene and vinylidene fluoride. The characteristic IR bands were measured. If possible, their vibrational modes were assigned based on literature data. Simple Mopac calculations were used to validate these vibrational mode assignments. Somewhat more sophisticated calculations were run using Gaussian on the same structures.