Draft MSc project proposal below. Comments (constructive ones!) welcome...
*Investigation of Shock Sensitivity and Critical Diameter*
*in Fire-Damaged Ammonium Perchlorate*
*Background*
Ammonium perchlorate has been involved in a number of accidental
explosions, some of them fatal, most notably the PEPCON disaster in Nevada
in 1988. AP is normally considered an oxidiser at larger particle sizes
(e.g. 200 um), and is not considered an explosive until it is reduced to a
very fine particle size (< 15 um). The cause of AP explosions is often
attributed to mixing of AP with a fuel (e.g. asphalt, plastic storage
drums) during a fire, thus creating an intimate fuel-oxidiser mixture which
is much more sensitive and with a smaller critical diameter than AP alone.
However, when heated up it is well known that AP partially decomposes and
sublimes, losing approximately 30% of its mass, leaving a very porous
crystal structure. We speculate that this porous AP could be significantly
more sensitive and have a reduced critical diameter when compared to the
original material, and that this could in fact be the cause of explosions
of AP of a nominally large particle size that is involved in accidental
fires.
This project will look to investigate whether this is a plausible mechanism
for explosions of AP in a fire.
*Project Plan*
1. Heat treat a quantity of AP to obtain samples of “porous” AP samples.
2. Characterise this “porous” AP in terms of particle size, surface area,
bulk density, …
3. Subject the “porous” AP to critical diameter tests and subsequently,
assuming that we can get the “porous” AP to detonate at all,
shock-sensitivity (gap) tests.
4. Possibly Fragment Attack tests (maybe the EMTAP small-scale version,
dependent on critical diameter)…
*Risks & Potential Problems*
1. Any tests that do not fully detonate the AP samples will scatter
unreacted material all over the range, upsetting the tree-huggers. Potentially
quite a bit of AP…
2. We can’t get the “porous” AP to detonate at all. It won’t be much of a
project for the student…
On 23 July 2015 at 16:50, James Padfield <james.padfield@xxxxxxxxx> wrote:
Peter, if you don't mind I'll email you off list - I feel, as interesting
as this might be to some people, that we may be drifting a bit OT. Unless
anyone thinks it is relevant and asks us to keep it on-list...
On 23 July 2015 at 16:26, Peter Fairbrother <zenadsl6186@xxxxxxxxx> wrote:
On 23/07/15 13:58, James Padfield wrote:
Peter, that sounds like it could be an interesting student project...
Investigate the shock sensitivity and critical diameter of "pristine" AP
crystals, and then heat treat them to produce the strange porous
crystals we are talking about. What do you think? I still have a good
relationship with my previous employer, perhaps we could come up with a
plan, I can contact them and we could do some research into the matter.
Sounds good to me, please let me know if I can help. Can do math :).
I can't do this myself, for legal (and practical) reasons, here in the UK
- if I could, I would.
btw, if there's a published paper, I wouldn't mind 4th author - I need to
up my publication status by mid-2017 if possible. Not something I usually
care a jot about, but it has become a little more relevant recently.
-- Peter Fairbrother
On 23 July 2015 at 13:55, Peter Fairbrother <zenadsl6186@xxxxxxxxx
<mailto:zenadsl6186@xxxxxxxxx>> wrote:
On 23/07/15 11:39, James Padfield wrote:
Peter: I helped with a student project a couple of years ago at
the
university I was working at. She was examining baking AP (I
don't
recall the initial particle size) at fairly high temperatures
(200,
maybe 220 degC) to alter the particle porosity. As you say,
some mass
is lost, 30% sounds about right, and the resulting particles are
no
longer regular crystals, but porous. She took some nice Scanning
Electron Microscope photos of the particles. She then
formulated them
(IIRC) with PVC/DOA binder system (easier than mucking about
with a
curable HTPB/IPDI system). Don't recall how far she progressed
with the
project, I don't believe it got as far as measuring burnrates...
James: What I'd really like to know is how the detonatability
changes. The product has submicron particles, which would be easy to
detonate if they were normal crystals - but they aren't, and I don't
know what happens.
IMO this could be very significant, for AP in fires ..
John: Thinking again about AP flowing. while it does not melt in
bulk, it may flow as a bulk powder in some circumstances. If it is
for instance on top of and reacting with tarmac or polythene, the
reaction will produce lots of gas, at a prodigious rate.
This effusion of gas may be fast enough to fluidise any overlying AP
powder, which would then flow almost like a liquid.
-- Peter Fairbrother
