Patrick Brousseau

Polymer-Grafted Metal Nanoparticles for Fuel Applications

Ultrafine metal powders have been identified as very promising fuels for future energetic material formulations. However, the large specific surface area that gives these powders a high reactivity also makes them particularly difficult to remain in a nonoxidized state. They also agglomerate easily during compounding processes due to strong particle-to-particle interactions. The coating of the particles with a polymer may offer a solution to these problems. We investigated two in situ polymerization processes using thermoplastic and thermoset coatings. Polyolefins such as polyethylene and polypropylene were obtained using a modified Ziegler-Natta reaction scheme. This process was found to be flexible enough to control the amount of polyethylene grafted onto the powders. The second type of coating was based on polyurethane chemistry. Nanometric-sized aluminum and boron powders were treated and characterized by means of thermogravimetric analysis, electronic microscopy, and x-ray photoelectron microscopy. The barrier properties of the polymer layer grafted onto the particles were evaluated using a chemical digestion method and thermoanalytical techniques. Polyethylene-coated particles showed a better resistance to early aging under stringent conditions of humidity and temperature and therefore would be expected to demonstrate a longer shelf life in a propellant formulation.

The dispersion and polymer coating of ultrafine aluminum powders by the Ziegler Natta reaction

Ultrafine aluminum powders have been identified as very promising fuels as part of energetic materials formulations. However, the large specific surface area that gives these powders a high reactivity also makes them particularly difficult to maintain in an unoxidized state. They also agglomerate easily during compounding processes. The coating of the particles by a polymer could offer a solution to these problems. We carried out the in-situ synthesis of high density polyethylene on the surface of aluminum nanoparticles by the Ziegler-Natta process. The process was found to be flexible enough to control the amount of polyethylene grafted on the powders. Ultrasounds were applied during the polymerization process to enhance the dispersion of the particles in the solvent. The resulting characteristics of the coated powders are discussed by means of thermogravimetric characterizations and electronic microscopy.

Measurements of the Temperature Inside an Explosive Fireball

This paper discusses the development of a fiber optic probe that can obtain temperature measurements from the interior of explosive fireballs, which are generated when unreacted detonation products react with oxygen in the surrounding air. Signatures of the thermochemical environment and chemical species involved can often be deduced from their light emissions, but the limited optical depth of fireballs means that remote sensing techniques can only sample emissions from the outer shell. By developing a protected fiber optic probe that can be placed adjacent to an exploding charge, giving it the ability to become enveloped by the fireball, the thermal radiation from the interior of the fireball can be sampled. Measurement from five shots using Detasheet-C explosives were carried out and could be obtained over the course of about 20 ms. Blackbody-type radiation with temperatures in the 1600 K to 1900 K range were observed, peaking at about 1850 K after 12 ms. The magnitude and time behavior of the temperature was not significantly different when taken at different locations within the fireball, indicating that temperature is fairly uniform throughout. The lack of specific spectral emission lines implies that in the interior of the fireball any combustion that occurred was probably primarily with carbonaceous soot, though differences in optical depth at different locations in the fireball indicate that it was much more fuel-rich closer to the center.

Formation of Additive-Containing Nanothermites and Modifications to their Friction Sensitivity

ABSTRACT Nanothermites can provide high energy densities and reaction rates but can also display extreme friction sensitivities. Additives that provide friction modification offer the potential to reduce the friction sensitivity of nanothermites. In the present work, MoS2, graphene, and hexadecane additives were dispersed in MoO3 prior to nanothermite formation with the aim of reducing friction sensitivity. Nanothermites were subsequently prepared using a palmitic acid–passivated nano-aluminum (L-Al) and additive-containing nano-MoO3 by the resonant acoustic mixing of dry powders. In general, the incorporation of additives results in a reduction in friction sensitivity with the baseline minimum ignition friction rising from 10 to 120 N using 0.5% wt/wt micrometer-sized MoS2 or 5% wt/wt hexadecane. However, the relationships between loading and performance are complex and vary by additive; for example, the friction sensitivity dependence using micrometer-diameter MoS2 displays a maximum at 0.5% wt/wt and declines to 7 N using 5% MoS2.

Conversion of oscillatory shear data from highly filled polymers by direct numerical integration

AbstractA procedure was developed to enable the direct numerical integration of the Fourier integral transform equation relating G(t) to G′(ω) by considering integration limits that vary as a function of time and which define a range of discrete sub-intervals within the complete frequency domain data set. The method provides results that are in very close agreement to results determined from a relaxation spectrum. However, at low values of time the solution to the variable limit integral transform is sensitive to the absence of a contribution beyond the upper experimental limit of the frequency domain data. G(t) results determined from the conversion of shifted master G′(ω) experimental data using the variable limit integral transformation and the relaxation spectrum compared favourably with actual shifted master G(t) experimental data. The former curves were characterised by the same form and trend as the experimental results, confirming that the underlying viscoelastic behaviour is well represented. While the variable limit Fourier integral transform procedure provides a good approximation to relaxation spectrum results, the latter is clearly the more robust method of converting data from the frequency to the time domain.It was observed that the time-temperature superposition procedure used in the construction of shifted master curves can magnify potential differences between the shifted G(t) values determined from the conversion of G′(ω) data and the actual experimental G(t) results, when compared to data that has not been shifted to a master curve.

Polymer-Coated Ultra-Fine Particles

Ultra-fine metallic particles have demonstrated recently their potential in tailoring the performance of energetic materials. DRDC Valcartier has explored methods to create controllable nanometric coatings on metallic particles and has opted to use polymers to treat the particles. Those coatings can have multiple positive effects. For example, in the case of aluminium, small particles are very reactive and tend to cause interations with the surrounding media. One example is the ageing of aluminium nanoparticles in the presence of air and humidity. Ultra-fine particles age much faster than micron-size particles. The long-term stability of energetic material mixes containing ultra-fine particles will be affected by this reactivity, and coatings would help to solve this problem. Another example is the interaction of aluminium nanoparticles with nitramines that causes gassing. Three coating methods will be presented: by thermoplastics using a Ziegler-Natta reaction, by thermosets through a polycondensation reaction initiated at the surface of the particles, and in-situ coating of particles by on-line polymerization during the plasma production of powders. The results of coating experiments using those methods will be presented. It will be shown that, for aluminium particles, adequate dispersion is a challenge and affects the results of the coating experiments. To assess the performance of the coating methods, ageing tests were carried out on coated and uncoated nanoparticles. The results of ageing tests with those methods will be presented and compared. It will be shown that the polymer coatings reduce significantly the loss of active metal content during accelerated ageing tests. Since the purpose of the powders is to be used in energetic materials, a study on the rheological effects of the coated particles in polymeric solutions will be presented as well. Coated particles increase the relative viscosity of HTPB-Al solutions by a factor of 100 at low shear rates, but much less with PPG.