Notes on flash powder bulking agents



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Hi all, in my experience with perchlorate based aluminium flash powders, the fumed silicas are best. I prefer grade EH-5 Cab-O-Sil. However, because the bulking agent has an important physical role, it seems incorrect to call these bulking agents extenders, fillers or diluents, etc… The number of ideas on the actual role of a bulking agent in flash mixtures gives me an opportunity to comment on a little known rational in their use; especially as in some of the high-performance military/aerospace flash mixes… The physical chemistry function is to reduce bulk density by increasing the separation distance between clusters of fuel and oxidiser. Fumed silica does this by promoting an equalisation of electrical charge. Ideally, the particles of fuel are uniformly mixed among oxidiser particles, but to get this to happen evenly, and safely, there must be a specific particle size ratio between them, with a minimum electrostatic charge to promote agglomeration; using what is in essence a charged humidifier. Following this, flake aluminium (fine bright waxed) is used as a cluster coater leading to groups of aluminium coated clusters. To act as a charge disperser (as thus a bulking agent) fumed silica is finally added with light tumbling to separate clusters and decrease the bulk density; it also neutralises charge to minimise the electrostatic hazard: that’s how it works as a bulking agent… Specifically, approximately uniform clusters of potassium perchlorate with an atomised spheroidal aluminium receive a monolayer of a suitable flake aluminium. Flakes work as the coating where atomised spheroidal particles work best as interstitial fuel; filling the spaces between oxidiser particles. The particle sizes lead to near stoichiometric ratio between the oxidiser and interstitial fuel. To make the ‘charged’ clusters separate (remember that the waxing on the flakes acts as a dielectric) fumed silica is added to act as an intermediary charge disperser. The separated clusters then leave the fumed silica between them causing the mixture to bulk, which thus increases fire passage. A greatly magnified ideal flash mixture may look similar to the following illustration; of course, a mixture can only approach the ideal with the correct selection of particle sizes, shapes and the proper mixing techniques; like when clusters are coated with flake they are ‘warm’ tumbled to promote the waxed aluminium’s cohesion, which leads to an improvement in the robustness of the clusters – it’s also part of a critical post-humidification safe drying regime before adding the charge neutralising fumed silica… *. O* .* OA *O. * A.A*.O * AOA * AA.* *A* *AOA. .AAAAAAA . * AAAAAAA * A AAAAAAAP * A..AAAAAAAO * A.O.O.O.A A AO.O.O.OA .O A.O.O.O.A O * AO.O.O.OA AO.O.O.OAO * A.O.O.O.A * OAO.O.O.OA.A A.O.O.O.AO * A.O.O.O.A .AO.O.O.OA A A.O.O.O.A A* AO.O.O.OA AO.O.O.OA * A.O.O.O.A . . OAAAAAAAO O A.O.O.O.A * A.O.O.O.A A AAAAAAA *.O.* * AO*. OAAAAAAA. .AAAAAAAO.O * .O. * AO.O.A O . * .A. * O . * . * * * AAA .O. * A AAAAAAA. * * *.*AAAAAO A* AAA O.AO.OA AAAAAAA * AO.O.O.OA O.*A AO.O.OA AO.OA A.O.A AO.O.O.OA * AAAAAAAA *.O * * A.O.O.A * A.O.A AO.OA A.O.O.O.A O.* AO.O.O.A *A . AO.O.OA * AO.OA AAA AO.O.O.OA OA.AA.O.O.OA AO. * AAAAA .O. A.O.A.O * .O A.O.O.O.A * AO.O.O.A. * A.O* A * * AAA * AAAO AAAAAAA * A * AAAAAA O*.O *AO. * .OO. * * AOAO O* . A* O .* .*. A Where the coating aluminium flakes (A) surround the small clusters: the oxidiser particles (O) and the much smaller particles of interstitial spheroidal aluminium (.) with a size chosen to fit neatly between the oxidiser particle’s spaces to produce a dense humidified structure. The fumed silica (*), is added last to the ‘structured’ mix – being dispersed between the clusters of bi-modal aluminium with oxidiser – which causes the clusters to separate. This is the way such a well-designed flash mixture appears with a stereo microscope. This illustrates the roles of particle sizes and shapes and the sequence and method of mixing to obtain optimum structures. Indeed, the use of humidifiers with some special blending techniques allow very specific compositional structures to be engineered, not just flash powders… Given that flake aluminium coats an almost stoichiometric mixture of spheroidal aluminium and oxidiser, most excess aluminium of the flash is the flake aluminium. A lot more can be written on this subject, although the above should be sufficient to allow others to comment…

Author: unknown

Source: http://www.passfire.com/

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