Fluoropolymer Binders Research Articles

Fluorinated semi-interpenetrating polymer networks for enhancing the mechanical performance and storage stability of polymer-bonded explosives by controlling curing and phase separation rates

Herein, the effect of fluoropolymer binders on the properties of polymer-bonded explosives (PBXs) was comprehensively investigated. To this end, fluorinated semi-interpenetrating polymer networks (semi-IPNs) were prepared using different catalyst amounts (denoted as F23-CLF-30-D). The involved curing and phase separation processes were monitored using Fourier-transform infrared spectroscopy, differential scanning calorimetry, a haze meter and a rheometer. Curing rate constant and activation energy were calculated using a theoretical model and numerical method, respectively. Results revealed that owing to its co-continuous micro-phase separation structure, the F23-CLF-30-D3 semi-IPN exhibited considerably higher tensile strength and elongation at break than pure fluororubber F2314 and the F23-CLF-30-D0 semi-IPN because the phase separation and curing rates matched in the initial stage of curing. An arc Brazilian test revealed that F23-CLF-30-D-based composites used as mock materials for PBXs exhibited excellent mechanical performance and storage stability. Thus, the matched curing and phase separation rates play a crucial role during the fabrication of high-performance semi-IPNs; these factors can be feasibly controlled using an appropriate catalyst amount.

Effects of the crystallinity of fluoropolymer binders components in polymer-bonded explosives on shock Hugoniots: A computational study

Fluoropolymers play a crucial role as binders in polymer-bonded explosive (PBX) formulations. However, there is a lack of clear understanding of the effects of increased fluoropolymer crystallinity on the shock response of PBXs in the service environment. This study investigated the shock Hugoniots of two widely applied fluoropolymer binders: (1) F2314 from China—a copolymer with a molar ratio of vinylidene fluoride (VDF) to chlorotrifluoroethylene (CTFE) of 1:4 and (2) F2313 from the United States, also known as Kel F-800, with a VDF to CTFE molar ratio of 1:3. The Hugoniot curves of both fluoropolymers were calculated based on equilibrium molecular dynamics (MD) and a mixing rule. Furthermore, the corresponding P–V curves were obtained through fitting using the Tait equation of state (EOS). Their calculated parameters, including zero-pressure bulk modulus (κo) and sound velocity (co), agreed well with experimental data. The results reveal that the Hugoniots of amorphous F2314 and F2313 exhibited negligible differences. However, increasing crystallinity significantly impacted the Hugoniot curves of both fluoropolymers, especially for F2314 with high crystallinity. The obtained macroscopic characteristic parameters, namely κo and co, exhibited an exponential dependence on crystallinity. Physically, this phenomenon can be attributed to a reduction in the compressible free volume of the fluoropolymers due to a more orderly chain arrangement. Additionally, under the same compression ratio, the shock temperature of the fluoropolymers increased with the crystallinity, posing potential safety risks to explosives. These findings establish a correlation between the crystallinity of fluoropolymers and the shock properties of PBXs, providing a theoretical reference for the formulation design of fluoropolymer-based PBXs.

(Invited) Advanced Fluoropolymer Binder for Solvent-Free Manufacturing of Lithium-Ion Battery Electrodes

With rising demand of lithium-ion batteries for consumer electronics and electric vehicles, battery manufacturers are eager to increase energy density, reduce cost of manufacturing and minimize environmental footprints. Solvent-free electrode manufacturing technology has advantages over conventional wet electrode casting methods. It can save the cost of solvent, reduce energy consumption associated with drying off the solvent and eliminate capital investment for solvent recovery facilities. It also can reduce the environmental impact and mitigate health concerns of organic solvents, such as N-methyl-2-pyrrolidone, used in wet slurry process. In addition, the solvent-free process enables manufacturing of high-quality thick electrodes; This is challenging for wet slurry process due to the sedimentation of particles and redistribution of binders during drying. Teflon™ PTFE fluoropolymer is the top binder candidate for a solvent-free electrode process due to its strong fibrillation property. The fibrils provide the binding force necessary to maintain the structural integrity of electrodes and make it feasible to scale up a roll-to-roll production process.Chemours is a global market leader in advanced fluoropolymers with strong capabilities in molecular design, synthesis, scale-up and manufacture; This ensures the delivery of the high-quality products to meet customer needs. In this presentation, we will give an overview of Chemours advanced fluoropolymer products for solvent-free manufacturing of anode and cathode electrodes. By tailoring the physical and chemical properties of fluoropolymers, electrode processibility and battery performance can be significantly improved.

Improving the mechanical performances of polymer bonded explosives using monomer tuned polythioureas

To improve the mechanical properties of 2,4,6-trinitrobenzene-1,3,5-triamine (TATB)-based polymer bonded explosives (PBXs), four kinds of polythiourea binders, namely the polyetherthiourea (P1), aliphatic polythiourea (P2), aromatic polythiourea (P3) and silane polythiourea (P4), were prepared and used in the PBXs. These four polythioureas were synthesized via the copolymerization of carbon disulfide (CS2) and diamines with various structures under mild conditions. They were then characterized using nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR). The interfacial binding energy between polythioureas and TATB was calculated via molecular dynamics simulations. The tensile and compression mechanical properties of various PBXs (PBX-Pn, n ​= ​1–4) were studied through Brazilian and compression tests and were then compared with those of the PBX with a conventional fluoro polymer binder (PBX-FP). The results show that the PBX-P1 possessed the strongest interfacial interaction energy and the best mechanical properties among all the prepared PBXs. Its Brazilian strength and strain was 11.0 ​MPa and 0.56%, which was 89% and 256% higher than those of the PBX-FP with the same binder proportion, respectively. Furthermore, PBX-P1 showed excellent compression mechanical properties, with a compression strength of 40.7 ​MPa and a compression strain of 3.53%. Moreover, its Brazilian and compression fracture energy was 600% and 101% higher than those of PBX-FP, respectively, which was beneficial for improving the PBX stability. The results of this study provide a new idea for designing TATB-based PBX with improved mechanical properties using polyetherthiourea binders.

In situ coating of energetic crystals: A compact core-shell structure with highly reduced sensitivity

The weak interfacial interaction and high sensitivity is a longstanding problem for nitramine explosives. Herein, polyhydroxyethyl acrylate nitrate (PANE) was adopted to passivate three nitramine explosives through a facile and efficient in situ polymerization method. The core-shell structure of the PANE-coated composites was confirmed after detailed morphological and structural characterization. As a result, we find that, the hydrophobic PANE shell improved the adhesion work between the explosives and fluoropolymer (F2314) binder because of the enriched interfacial interactions. Besides, 2% PANE can retard the crystal transformation of CL-20 and HMX explosives by 10.7 °C and 4.3 °C, but does not affect the thermal decomposition behavior. The mechanical sensitivity of PANE-coated composites decreased more significantly than raw explosives and the mixed samples. As a typical example, the impact and friction sensitivity of CL-20@PANE composite with 2.0% shell content is practically halved when compared to raw CL-20 with the characteristicheight (H50) of 32.2 cm and the explosionprobability (P) of 50%. This study offers a simple, effective and universal method for design and preparation of energetic composites with high safety performance.

Interfacial reinforcement of core-shell HMX@energetic polymer composites featuring enhanced thermal and safety performance

The weak interface interaction and solid-solid phase transition have long been a conundrum for 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX)-based polymer-bonded explosives (PBX). A two-step strategy that involves the pretreatment of HMX to endow –OH groups on the surface via polyalcohol bonding agent modification and in situ coating with nitrate ester-containing polymer, was proposed to address the problem. Two types of energetic polyether—glycidyl azide polymer (GAP) and nitrate modified GAP (GNP) were grafted onto HMX crystal based on isocyanate addition reaction bridged through neutral polymeric bonding agent (NPBA) layer. The morphology and structure of the HMX-based composites were characterized in detail and the core-shell structure was validated. The grafted polymers obviously enhanced the adhesion force between HMX crystals and fluoropolymer (F2314) binder. Due to the interfacial reinforcement among the components, the two HMX-based composites exhibited a remarkable increment of phase transition peak temperature by 10.2°C and 19.6°C with no more than 1.5% shell content, respectively. Furthermore, the impact and friction sensitivity of the composites decreased significantly as a result of the barrier produced by the grafted polymers. These findings will enhance the future prospects for the interface design of energetic composites aiming to solve the weak interface and safety concerns.

Simultaneously Altering the Energy Release and Promoting the Adhesive Force of an Electrophoretic Energetic Film with a Fluoropolymer.

Energetic coatings have attracted a great deal of interest with respect to their compatibility and high energy and power density. However, their preparation by effective and inexpensive methods remains a challenge. In this work, electrophoretic deposition was investigated for the deposition of an Al/CuO thermite coating as a typical facile effective and controllable method. Given the poor adhesion of the deposited film and the native inert Al2O3 shell on Al limiting energy output, further treatment was conducted by soaking in a Nafion solution, which not only acted as a fluoropolymer binder but also introduced a strong F oxidizer. It is interesting to note that the adhesion level of Al/CuO films was improved greatly from 1B to 4B, which was attributed to Nafion organic network film formation, like a fishing net covering the loose particles in the film. Combustion and energy release were analyzed using a high-speed camera and a differential scanning calorimeter. A combustion rate of ≤3.3 m/s and a heat release of 2429 J/g for Al/NFs/CuO are far superior to those of pristine Al/CuO (1.3 m/s and 841 J/g, respectively). The results show that the excellent combustion and heat release properties of the energetic film system are facilitated by the good combustion-supporting properties of organic molecules and the increase in the film density after organic treatment. The prepared Al/NFs/CuO film was also employed as ignition material to fire B-KNO3 explosive successfully. This study provides a new way to prepare organic-inorganic hybrid energetic films, simultaneously altering the energy release and enhancing the adhesive force. In addition, the Al/NFs/CuO coating also showed considerable potential as an ignition material in microignitors.

Flexoelectricity in fluoropolymer/aluminium reactives

AbstractThe development of smart reactive materials is of significant interest to the reactives community in order to tailor energy delivery and enable ON/OFF capability. Of the polymer bound reactives, those with fluoropolymer binder have shown potential as smart materials due to the polymer's strong electroactive characteristics. This could be due to piezoelectricity or flexoelectricity but has not been well characterized. This effort measures the flexoelectric coefficient for two fluoropolymer/aluminium (Al) reactive systems and details the role processing, configuration and fluoropolymer type have on their electroactive behavior. Specifically, poly(vinylidene fluoride) (PVDF) and tetrafluoroethylene‐hexafluoropropylene‐vinylidene fluoride (THV) were characterized with Al (fuel) loadings of 0, 20, 33 and 50 wt%. Two additive manufacturing routes, direct ink writing (DIW) and fused filament fabrication, were used to determine how processing and print direction may influence the electroactive behavior. A linear increase in flexoelectric coefficient as a function of Al loading was observed for both fluoropolymer systems ranging from 5.03 ± 0.7 nC m–1 to 7.83 ± 1.1 nC m–1. Processing via DIW resulted in samples with the highest flexoelectric coefficients, 8.48 ± 1.0 and 11.62 ± 2.5 nC m–1 for THV/Al and PVDF/Al, respectively, attributed to increased porosity. These results indicate that non‐piezoelectric polymer bound reactives have promise as smart reactives with calculated apparent piezoelectric coefficients up to 189 ± 12 pC N–1. © 2021 Society of Industrial Chemistry.

Catechol-modified polymers for surface engineering of energetic crystals with reduced sensitivity and enhanced mechanical performance

Surface engineering has been widely implemented in materials science for multipurpose functionalization. However, the rational design of surface structure with both adhesiveness and toughness remains a huge challenge. Herein, a facile and general strategy has been demonstrated for coating of energetic crystal 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) via in situ copolymerization of catechol and amino functionalized monomers. The design principle is on the basis of bioinspired supramolecular chemistry, providing the synergistic effect of covalent and noncovalent interactions. Compared with the pristine HMX crystal, the coated HMX crystals exhibited significantly improved phase-transition stability and reduction of impact/friction sensitivity owing to the reinforced interfacial strength and coating roughness. Moreover, thanks to incorporation of the catechol–containing synthetic polymer (CSP) interlayer, the interfacial interaction between HMX crystal and fluoropolymer binder was noticeably enhanced, leading to markedly improved mechanical strength (both tensile and compression strength were improved by over 40%), as well as creep resistance (the creep strain was decreased by over 55%) for the resulted polymer bonded explosive (PBX) composite. Therefore, the present work provides an effective and universal method for the construction of environmental adaptive energetic materials on account of their reduced sensitivity, enhanced mechanical strength and thermal stability, which were favorable for their practical applications.

Incorporation of high explosives into nano-aluminum based microspheres to improve reactivity

Nano-aluminum, as a potential replacement of micro-aluminum, has attracted significant research interests in energetic fields due to its high reactivity. However, the intrinsic high surface area of nanoparticles drives nano-aluminum encounter challenges in handling process and prone to large aggregation, significantly impeding the further application of nano-aluminum. In this work, nano-aluminum based microspheres incorporating hexanitrohexaazaisowurtzitane (CL-20) are directly fabricated by electrospray deposition. These as-prepared microspheres were in high monodispersity and their size ranges from 8 to 16 μm. The surface morphology of such microspheres was adjustable by using different polymers as binder. Thermal behavior results show that CL-20 has no appreciable effect on the reactivity of nano-aluminum while the fluoropolymer binder (F2314) facilitates the oxidation of nano-aluminum. Nevertheless, the presence of CL-20 can significantly enhance the reactivity of nano-aluminum in the combustion behavior tests, where electrosprayed microspheres exhibit outstanding performance in comparison to physical mixed sample, presenting a high pressurization rate, peak pressure and short burning time. These excellent properties are possibly attributed to their uniform microstructure. Therefore, the use of electrospray method can offer a promising strategy to fabricate the microstructure controllable microspheres.

Gas Diffusion Media with Optimized Water Transport

The water management in polymer electrolyte fuel cells (PEFC's) needs to be properly balanced to maximize the power output of the fuel cell. The polymer electrolyte membrane requires to be hydrated in order to be proton conductive. However, since the water transport and gas reactant transport occurs through the same media, the so called gas diffusion media (GDM), accumulation of water in the GDM can block the access of reactant gases to the catalyst limiting the performance of the cell. The GDM is usually composed of two layers: the gas diffusion layer (GDL) and the microporous layer (MPL). In the majority of cases the GDL consist of a highly porous structure made of carbon fibers coated with a fluoropolymer to enhance its hydrophobicity. The GDL has a porosity of around 80% and a mean pore size diameter of 20 µm. The MPL, on the other hand, is formed by carbon particles and fluoropolymer. The porosity is around 50% and the mean pore size is for most MPL types around 50 nm [1]. Most of the optimization strategies for water management comprise modification on the GDM. Several groups have developed techniques aiming at increasing the water permeability of the GDM locally in order to improve water transport while keeping free pathways for the reactant gases [2]. Our group in particular has developed such a material based on a chemical modification of the GDL coating. We use radiation grafting to selectively alter regions on the GDL, generating a GDL with a hydrophilic and hydrophobic pattern [3]. Our previous studies show that the incorporation of the hydrophilic regions to the GDL improves the cell performance in comparison with the base material. We have also studied the incorporation of a self-standing MPL to the patterned GDL, see Figure 1 a). In this case the overall performance of both the base and the patterned GDL increase and the patterned GDL still shows better performance than the base GDL. Nevertheless, when using an MPL the effectiveness of the pattern in the GDL is reduced due to the difference in water injection caused by the presence of the MPL [4]. The MPL requires a much higher capillary pressure for the water to permeate through, therefore any special feature such as cracks, holes or regions with a lower thickness define the injection points into the GDL. Once the water reaches the GDL it will preferentially be transported through the hydrophilic areas, but since it had previously gone through the MPL which imposes a higher capillary pressure barrier the effectiveness of the pattern is limited.. To resolve this issue, we created GDM's with MPL's with chemically modified MPL fluoropolymer binder in the hydrophilic regions of the GDL (see Figure 1 b) and locally increased porosity in regions of the hydrophilic pattern in the GDL (see Figure 1 c). Here, we will present our synthetic methods and the material characterization, as well as the influence of the modifications on water distribution and cell performance. A. El-Kharouf, T. J. Mason, D. J. L. Brett, B. G. Pollet, J. Power Sources, 218, (2012), 393–404.R. Omrani, B. Shabani, Int. J. Hydrogen Energy, 42, (2017), 47, 28515–28536.A. Forner-Cuenca, J. Biesdorf, L. Gubler, P. M. Kristiansen, T. J. Schmidt, P. Boillat, Adv. Mater., 27, (2015), 41, 6317–6322.A. Forner-Cuenca, J. Biesdorf, V. Manzi-Orezzoli, L. Gubler, T. J. Schmidt, P. Boillat, J. Electrochem. Soc., 163, (2016), 13, F1389–F1398. Figure 1

Effects of Fluoropolymer Binders on the Mechanical Properties of TATB‐Based PBX

AbstractThe relationship between the molecular structure of polymeric binders and mechanical properties of a TATB‐based polymer bonded explosive (PBX) were studied by using five different fluoropolymers as binders to compare the quati‐stastic and dynamic mechanical property, as well as the three‐point bending creep property of the PBX. F‐ binder A, which was composed of vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE), endowed the PBX−A with the best mechanical strength and creep resistance ability. The fracture surfaces of the PBXs after the Brazilian tests were investigated by a field emission‐scanning electron microscope (FE‐SEM). The rupture mode in PBX−A and PBX−B was transcrystalline fracture together with interfacial debonding, but only interfacial debonding in PBX−C and PBX−D. It was explained that the C−Cl bond in CTFE played the key role to keep the polymeric binder rigid and increase the strength of the PBX.

Evolution of Carbon Clusters in the Detonation Products of the Triaminotrinitrobenzene (TATB)-Based Explosive PBX 9502

The detonation of carbon-rich high explosives yields solid carbon as a major constituent of the product mixture, and depending on the thermodynamic conditions behind the shock front, a variety of carbon allotropes and morphologies may form and evolve. We applied time-resolved small-angle X-ray scattering (TR-SAXS) to investigate the dynamics of carbon clustering during detonation of PBX 9502, an explosive composed of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and 5 wt % fluoropolymer binder. Solid carbon formation was probed from 0.1 to 2.0 μs behind the detonation front and revealed rapid carbon cluster growth which reached a maximum after ∼200 ns. The late-time carbon clusters had a radius of gyration of 3.3 nm which is consistent with 8.4 nm diameter spherical particles and matched particle sizes of recovered products. Simulations using a clustering kinetics model were found to be in good agreement with the experimental measurements of cluster growth when invoking a freeze-out temperature, and tempora...

Influence of fluoropolymer binders on the electrochemical performance of C-LiFePO4 based cathodes

The polymer binder plays a relevant role in the performance of battery electrodes. This work proposes poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, as a polymer binder for C-LiFePO4 based cathodes and investigates its effect in the electrochemical properties of the cathodes.Morphological, electrical, swelling and electrochemical properties were investigated for this polymer binder and the results were compared with the ones from two other fluoropolymers, poly(vinylidene fluoride), PVDF, and poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, already reported in the literature as binders for C-LiFePO4 cathodes.It is demonstrated that cathode morphology and electrical conductivity are similar, independently of the polymer binders, but that the swelling values depend on the selected binder. The voltage difference between the two redox peaks of the C-LiFePO4 based cathodes with PVDF, PVDF-HFP or PVDF-TrFE as binders are 0.46, 0.79 and 0.83V, respectively. Also, the impedance values show the following order: PVDF<PVDF-HFP≤PVDF-TrFE. Finally, the discharge capacities values are 129mAh·g−1, 75.5mAh·g−1 and 94.3mAh·g−1 for PVDF, PVDF-HFP and PVDF-TrFE binders after 50cycles at C-rate. Thus, it is concluded that PVDF-TrFE binder shows good rate capability and cycle performance, being therefore adequate for the development of C-LiFePO4 cathodes in lithium-ion batteries.

Layered double oxide (LDO) particle containing photoreactive hybrid layers with tunable superhydrophobic and photocatalytic properties

Inorganic/organic hybrid layers have been prepared having superhydrophobic as well as photoreactive properties. The hybrid thin films with micro- and nanosized dual-scale surface roughness consist of ∼25μm layered double oxide (LDO) photocatalyst particles and low surface energy poly(perfluorodecyl acrylate) [p(PFDAc)] fluoropolymer binder material. The application of [p(PFDAc)] resulted in the decrease in the surface free energy of the hydrophilic LDO. The structured surface LDO with ∼12% ZnO phase content were synthesized from layer double hydroxide (LDH) spheres. The determined excitation wavelength and the calculated band gap energy values were 386nm and 3.23eV, respectively. The hybrid thin films were prepared by a simple spray-coating method, which is a low-cost, fast and scalable film-forming technique. The surface roughness and also the wetting properties of the two-component hybrid layers proved to be finely adjustable by the LDO:fluoropolymer ratio. It was found that at 80–90wt% LDO content, the thin films with a surface free energy value of ∼12mJ/m2 displayed superhydrophobic behaviour (Θ>150°) with satisfactory photocatalytic properties. This means special photoreactive surfaces with superhydrophobic properties instead of the conventional superhydropilic photocatalyst layers. According to the benzoic acid photodegradation test experiments of benzoic acid, the hybrid layers with 80–90wt% LDO content photooxidized 22–24% of the initial test molecule concentration (0.17g/L) under UV-A (λmax=365nm) illumination.

Probing the Reaction Mechanism of Aluminum/Poly(vinylidene fluoride) Composites.

Energetic thin films with high mass loadings of nanosized components have been recently fabricated using electrospray deposition. These films are composed of aluminum nanoparticles (nAl) homogeneously dispersed in an energetic fluoropolymer binder, poly(vinylidene fluoride) (PVDF). The nascent oxide shell of the nAl has been previously shown to undergo a preignition reaction (PIR) with fluoropolymers such as polytetrafluoroethylene (PTFE). This work examines the PIR between alumina and PVDF to further explain the reaction mechanism of the Al/PVDF system. Temperature jump (T-jump) ignition experiments in air, argon, and vacuum environments showed that the nAl is fluorinated by gas phase species due to a decrease in reactivity in a vacuum. Thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) was used to confirm the occurrence of a PIR, and gas phase products during the PIR and fluorination of nAl were investigated with temperature jump time-of-flight mass spectrometry (T-jump TOFMS). Results show a direct correlation between the amount of alumina in the PVDF film and the relative signal intensity of hydrogen fluoride release (HF). Although the PIR between alumina and PVDF plays an important role in the Al/PVDF reaction mechanism, burn speeds of Al/PVDF films containing additional pure alumina particles showed no burn speed enhancement.

Highly Flexible Self-Assembled V2O5 Cathodes Enabled by Conducting Diblock Copolymers

Mechanically robust battery electrodes are desired for applications in wearable devices, flexible displays, and structural energy and power. In this regard, the challenge is to balance mechanical and electrochemical properties in materials that are inherently brittle. Here, we demonstrate a unique water-based self-assembly approach that incorporates a diblock copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-block-poly(ethyleneoxide) (P3HT-b-PEO), with V2O5 to form a flexible, tough, carbon-free hybrid battery cathode. V2O5 is a promising lithium intercalation material, but it remains limited by its poor conductivity and mechanical properties. Our approach leads to a unique electrode structure consisting of interlocking V2O5 layers glued together with micellar aggregates of P3HT-b-PEO, which results in robust mechanical properties, far exceeding the those obtained from conventional fluoropolymer binders. Only 5 wt % polymer is required to triple the flexibility of V2O5, and electrodes comprised of 10 wt % polymer have unusually high toughness (293 kJ/m3) and specific energy (530 Wh/kg), both higher than reduced graphene oxide paper electrodes. Furthermore, addition of P3HT-b-PEO enhances lithium-ion diffusion, eliminates cracking during cycling, and boosts cyclability relative to V2O5 alone. These results highlight the importance of tradeoffs between mechanical and electrochemical performance, where polymer content can be used to tune both aspects.

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

The beforehand evaluation of the fracture toughness and behaviors of the polymer binder through EWF can give a reliable feedback to the final fracture toughness of PBX.

In‐situ Remediation of Hydrogen Fluoride During a Detonation Event

AbstractHydrogen fluoride (HF) is a known product from the combustion or detonation of explosives formulated with fluoropolymer binder systems. This presents the user with elevated risk levels during unintended combustion events or detonations in confined situations. In an effort to remediate the production of HF, calcium disilicide (CaSi2) was added to explosive formulations and the amount of HF formed was monitored. Viton A/calcium disilicide mixtures were made and the thermal decomposition characteristics studied using thermal gravimetric/differential thermal analysis (TG/DTA). The activation energy ranged from 145–190 kJ mol−1, indicative of CF scission in the Viton A binder prior to calcium fluoride formation. An energetic formulation was prepared which consisted of approximately a 5/3 mass ratio of Viton A/CaSi2. Combustion calorimetry in oxygen and air, and subsequent analysis of the residues using X‐ray diffraction (XRD) showed evidence of calcium fluoride (CaF2) formation. The decrease in HF formation was determined by trapping off gases and subsequent analysis in anion exchange chromatography of combustion and detonation products. Upon introduction of calcium disilicide into the formulation, a small decrease in HF formation was observed along with appearance of calcium fluoride and free Si in the residue. Such products are consistent with the mechanism following a general decomposition path of 2F−+CaSi2→CaF2+2Si. Under detonation conditions, the decomposition path followed nearly the same route with a net ca. 30 % decrease in HF formation, but with a portion of the silicon oxidizing slightly further to SiO2.

Active layer materials coated with Teflon AF nano-films deposited from solutions in supercritical CO2 for fuel cell applications

In the present work we suggest a new approach to the thin-film design of active layers for electrodes of fuel cells with phosphoric acid electrolyte in a polymer matrix. A fluoropolymer binder is introduced into common Pt@C active layer materials using supercritical (SC) CO2 as a solvent. Unique wetting properties of this non-hazardous and environmentally friendly solvent allow one to deposit highly uniform thin fluoropolymer films on dispersed carbon supports. As a result, well-percolated gas-permeable fluoropolymer phases are produced in active layers already at comparatively small polymer loadings. Teflon AF 2400 was chosen as a stable high-molecular-weight fluoropolymer soluble in SC CO2 with high oxygen permeability and high Tg value. Fluoropolymer-containing active layer materials prepared via the SC CO2 deposition routes were studied by means of cyclic voltammetry and were tested in operating fuel cells using steady state voltammetry and electrochemical impedance spectroscopy. Polarization curves of operating fuel cells indicate that the optimal content of deposited from SC CO2 fluoropolymer in active layer is about 3–5%. Results of impedance spectra fitting yield information used to explain the detected values of optimal loading.