Polymer-Coated Ultra-Fine Particles
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Polymer-Coated Ultra-Fine Particles Patrick Brousseau and Charles Dubois1 Defence Research and Development Canada – Valcartier, 2459 Pie-XI blvd. North, Val-Belair, QC, Canada G3J 1X5 1 École Polytechnique de Montréal, Department of Chemical Engineering Montreal, QC, Canada H3C 3A7 ABSTRACT 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.
INTRODUCTION The advent of nanometric metallic particles caused a great deal of excitement in the world of energetic materials, and most large conferences in this field now feature at least a session dedicated to those new ingredients. It was found in the past that aluminium nanopowders can improve burn rates in rocket and gun propellants [1-5]. They are also seen as an essential ingredient in metastable intermolecular composites, when mixed with nanometric metallic oxides [6-8]. Their application in explosives is being explored, for blast and underwater applications for example [9].
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However, the powders also suffer from serious drawbacks. One example is that they age much faster under humid conditions [8, 10, 11]. Th
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