IndependentBoffin
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The initiation and de-sensitisation of explosives
Dear all,
I am going to make a few statements regarding explosive sensitivities and welcome debate/peer review on them.
The objective of my post is to provide clear statements to facilitate understanding of how explosives are physically sensitised and how to mitigate
this, so that we may store/handle/manipulate these energetics in a consistently safe manner.
I will leave the chemical mechanisms of sensitisation to others (e.g. amines + nitromethane) more qualified to discuss in this area.
The key sources of my understanding of the initiation mechanism for explosives comes from:
Bowden, F. P. and O. A. Gurton (1949). "Birth and Growth of Explosion in Liquids and Solids Initiated by Impact and Friction." Proceedings of the
Royal Society of London. Series A. Mathematical and Physical Sciences 198(1054): 350-372.
http://www.noveldefence.com/pdfs/birth_and_growth_of_explosi...
Bowden, F. P. and O. A. Gurton (1949). "Initiation of Solid Explosives by Impact and Friction: The Influence of Grit." Proceedings of the Royal
Society of London. Series A. Mathematical and Physical Sciences 198(1054): 337-349.
http://www.noveldefence.com/pdfs/initiation_of_solid_explosi...
Yoffe, A. (1949). "Influence of entrapped gas on initiation of explosion in liquids and solids." Proceedings of the Royal Society A: Mathematical,
Physical and Engineering Science 198(A): 373 - 388.
http://www.noveldefence.com/pdfs/influence_of_entrapped_gas_...
Statement 1: All explosive compounds have at least one exothermic decomposition process
Statement 2: Decomposition of all explosive compounds is ultimately heat-driven
Discussion: Mechanisms of shock, friction or electrical initiation of explosives can be unified into the generation of localised hot
spots. For example shock initiation is due to adiabatic heating of gas pockets, friction initiation is due to hot spots where explosive crystals rub
against each other and electrical initiation is by the generation of a hot spots at electrical arc points.
Statement 3: If the rate of heat generation from a hot spot exceeds the rate of heat dissipation, a self-sustaining reaction is
produced (deflagration or detonation). If the rate of heat generation is less than the rate of heat dissipation, then the reaction is quenched.
Discussion: From statements 2 + 3 we can draw the following conclusions:
1) Sensitive explosives have a low decomposition temperature
and/or
2) Sensitive explosives produce intense hot spots upon decomposition of a small quantity of them in the bulk body of the explosive (either by a high
heat of reaction or a poor heat dissipation mechanisms)
Statement 4: All explosives have some endothermic processes which antagonise the exothermic decomposition process.
Discussion:
A specific chemistry-based example from Richard Nakka's website:
| Quote: |
When pure AN is heated to a temperature range of 169o to approximately 200<sup>o</sup>C, essentially no decomposition occurs. When heated
further, to the range of 200-250C, the following exothermic reaction (heat is released) primarily occurs:
NH<sub>4</sub>NO<sub>3</sub> => N<sub>2</sub>O + 2 H<sub>2</sub>O, ΔH= -8.8 kcal/mol
From the above equation, for 100 grams of AN, 45 grams of H<sub>2</sub>O is produced, with the remaining 55 grams being gaseous nitrous
oxide. This reaction is exothermic, with the release of 110 calories/gram.
Simultaneous to this reaction, a dissociating reaction occurs endothermically (heat is absorbed) whereby the AN breaks down into ammonia and nitric
acid.:
NH<sub>4</sub>NO<sub>3</sub> => NH<sub>3</sub> + HNO<sub>3</sub>, ΔH= +44.6 kcal/mol
The combination of these two effects results in a steady-state, or self-limiting temperature, provided the decomposition process is carried out with
the gaseous reaction products allowed to freely escape (in particular the HNO<sub>3</sub> . As such, if pure AN is heated at a moderate rate in the open air with no confinement, the temperature cannot rise
appreciably beyond its melting point.
Under steady-state conditions, the endothermic dissociation of AN into gaseous NH<sub>3</sub> and HNO<sub>3</sub> absorbs all
the heat available from decomposition. Thus, when heat is added to AN at atmospheric pressure and even from a very hot source, the temperature of the
AN is limited by its own dissociation to values at which decomposition rate is comparatively moderate. At elevated pressures, however, the
dissociation reaction is repressed and the rate of decomposition accelerates.
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The following non-exhaustive list describes other physical endothermic processes that compete against the exothermic generation of heat:
1) Raising the temperature of the body of explosive - specific heat capacity
2) Phase changes - melting (latent heat of fusion) and boiling (latent heat of vaporisation)
Statement 5: Desensitisation of explosives can therefore be achieved by the following methods
1) Inert bulking materials to absorb the heat from hot spots, e.g. plasticisers
2) Additives that absorb the heat from hot spots. Subject to chemical compatibility with the explosive we are planning on desensitising, low boiling
point additives (well below the decomposition temperature of the explosive) such as alcohols or ethers should serve to desensitise them by quenching
any hot spots that occur.
3) Methods that prevent the hot spots forming in the first place. Examples are embedding the explosive crystals in an inert plastic matrix which
minimise entrapped gases, inter-crystalline friction or intra-crystalline fracture (compared to free flowing crystals)
I can sell the following:
1) Various high purity non-ferrous metals - Ni, Co, Ta, Zr, Mo, Ti, Nb.
2) Alkex para-aramid Korean Kevlar analogue fabric (about 50% Du Pont's prices)
3) NdFeB magnets
4) High purity technical ceramics
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watson.fawkes
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I disagree
with this statement categorically, in particular with the word "all". I also think its dangerous and absurd to constrict examination of a wide range
of physical behavior to a single point of analysis. Human ignorance is a varied and wide-ranging fact, and if you're basing a safety regime on only a
single possibility, you are then also willfully blind to potential effects that you are only accidentally mitigating.
As to the physical facts, I have to imagine that detonation can also be induced both by pressure and by high (local) electric fields. Consider
pressure. An octahedral press is a big machine with hydraulic pistons pointed inward to a point and mating anvil presses. These things can be used to
generate very high pressures at ambient temperatures, or below. You can produce artificial diamonds in them. I can't believe that there's no high
explosive that won't detonate under such treatment. I'm not claiming that all of them will. But you are claiming that none of them will, a claim I
find completely implausible.
As for high electrical field, there are some ordinary sources: external static discharge and piezoelectric stress. High electric fields move
electrons. Moving electrons can rearrange bond structures. I've got to believe that some explosives can detonate purely from electrical stimulus with
no temperature elevation.
Furthermore, you're making an implicit claim that a detonation wave propagates because of high temperature, as opposed to merely in concert with high
temperature. I find it just as plausible that propagation for certain materials stems principally from pressure, not temperature. Designing mitigation
measures based solely on temperature will lead you away from whole categories of techniques.
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hissingnoise
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And high-energy radiation will also detonate an explosive . . .
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hissingnoise
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I must apologise IB, my comment was offensive and I should have known better than to post it.
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IndependentBoffin
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Haha don't worry no offense taken. I tend to assume humour unless proven otherwise 
I will reply to the other posts in time, and after due thought.
Yes I am aware of some very rare explosives that can be initiated by stimuli other than heat, e.g. light initiated silver acetylide - silver nitrate
explosives, or nitrogen triiodide by alpha radiation. But I think these are exceptions to the rule of heat based initiation and self-sustaining
propagation.
I am testing some inventions of mine tomorrow at a gun range and am busy making final touches to them, but will endeavour to reply after tomorrow 
I should have a second unique product for sale soon
I can sell the following:
1) Various high purity non-ferrous metals - Ni, Co, Ta, Zr, Mo, Ti, Nb.
2) Alkex para-aramid Korean Kevlar analogue fabric (about 50% Du Pont's prices)
3) NdFeB magnets
4) High purity technical ceramics
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hissingnoise
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Way to go IB!
I'm just a sarcastic old git!
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watson.fawkes
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That's true; I should have mentioned it. The
mechanism here is to create an excited electron or even an ion, changing the occupancy of molecular orbitals and thus reactivity.
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hissingnoise
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And I could have been more accurate by saying that; high-energy radiation can lead to the detonation of certain explosives.
And we've all heard of peroxides going off in sunlight . . .
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IndependentBoffin
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Really sorry for the ultra-late response guys  . I've kind of started seeing a girl
(there goes a few hours/day, although I consider it well spent ) and my
company has just launched two new products which I was busy testing and am now promoting.
Anyway, first off I'm attaching four pages from the book "Blast & Ballistic Loading of Structures" by Smith & Hetherington. Its early chapters
discuss explosives in general and it has been a good introduction to explosives for me.
Here are the salient quotes:
Pg 18:
| Quote: |
All explosive materials are sensitive to heat and various methods used to initiate explosives make use of heat energy. |
Pg 19:
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If the temperature of the charge rises to between about 150 and 350<sup>o</sup>C, common explosive materials are likely to ignite. The
basis of all systems which are used to initiate explosives deliberately is the supply of heat energy to the material. |
So, watson.fawkes...
| Quote: |
I disagree with this statement categorically, in particular with the word "all". I also think its dangerous and absurd to constrict examination of a
wide range of physical behavior to a single point of analysis. Human ignorance is a varied and wide-ranging fact, and if you're basing a safety regime
on only a single possibility, you are then also willfully blind to potential effects that you are only accidentally mitigating.
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I think with few exceptions, the statement still applies and is backed up by literature as I have shown. It is just incumbent on explosives users to
be familiar with what can cause hotspots in your explosive material.
Friction, adiabatic heating of cavities, electric arcing, resistance heating and light pulses (PETN by lasers or silver acetylide-silver nitrate by
light) are cited examples. Nitrogen triiodide is the only known explosive detonable by ionising radiation and doesn't quite hold to this "initiation
by heat" rule.
Edit: Actually, come to think of it, I think there is a critical volume of hotspots for a given temperature that is characteristic to an explosive
composition. In the case of both NI<sub>3</sub> (prepared via BN + 3IF → NI<sub>3</sub> + BF<sub>3</sub> and the NH<sub>3</sub>.NI<sub>3</sub> complex, prepared using
ammonia + iodine, the critical volume is below that which is caused by energetic alpha particles. Indeed it has been noted that the initiation of
NI<sub>3</sub> by alpha particles is a statistical process, see Bowden, F. P. (1958). "The Initiation of Explosion by Neutrons,
alpha-Particles and Fission Products." Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 246(1245): 216-219.
| Quote: | | As to the physical facts, I have to imagine that detonation can also be induced both by pressure and by high (local) electric fields. Consider
pressure. An octahedral press is a big machine with hydraulic pistons pointed inward to a point and mating anvil presses. These things can be used to
generate very high pressures at ambient temperatures, or below. You can produce artificial diamonds in them. I can't believe that there's no high
explosive that won't detonate under such treatment. I'm not claiming that all of them will. But you are claiming that none of them will, a claim I
find completely implausible. |
Yes but 3-4 hotspot generation mechanisms occur when you put a high explosive in such an anvil. One is compression of any cavities/air pockets. The
second is inter-crystalline friction. The third is inter-crystalline shear and fracture. A fourth possible one is the presence of deviatoric stress
resulting in plastic deformation of the sample. Your press must apply perfect hydrostatic pressure so that there is no plastic flow -> plastic work
heating.
If you remove all these mechanisms - I would wager a drink for you that you can subject the sample to enormous hydrostatic pressures and not get it to
blow up.
| Quote: |
As for high electrical field, there are some ordinary sources: external static discharge and piezoelectric stress. High electric fields move
electrons. Moving electrons can rearrange bond structures. I've got to believe that some explosives can detonate purely from electrical stimulus with
no temperature elevation. |
Well, the book I am quoting state:
| Quote: |
There are several ways of converting electrical energy to heat energy based on resistance heating. The most common is the bridgewire in which a
filament is heated to ignite an attached small quantity of explosive. In the exploding bridgewire method a high tension pulse causes a filament to
vaporise producing both high temperature and a shock wave which is often sufficient to initiate a secondary explosive. Finally, in a so-called
'conducting cap', a primary explosive is mixed with carbon to make it conducting but of high resistance: a low tension source produces an essentially
instantaneous temperature rise to ignition. |
| Quote: | | Furthermore, you're making an implicit claim that a detonation wave propagates because of high temperature, as opposed to merely in concert with high
temperature. I find it just as plausible that propagation for certain materials stems principally from pressure, not temperature. Designing mitigation
measures based solely on temperature will lead you away from whole categories of techniques. |
The distinction is quite chicken and egg in an adiabatic context. High temperature implies high pressure in a condensed explosive, and vice versa.
However, at a molecular scale high pressure is caused by high temperature in condensed explosives. Pressure is a bulk property of a collection of
particles; it makes no sense to say an individual atom has a pressure of 1 GPa. Temperature can be a bulk or individual property.
Attachment: blast_ballistic_loading_structs_pg_18-21.pdf (1.4MB)
This file has been downloaded 455 times
[Edited on 11-6-2011 by IndependentBoffin]
I can sell the following:
1) Various high purity non-ferrous metals - Ni, Co, Ta, Zr, Mo, Ti, Nb.
2) Alkex para-aramid Korean Kevlar analogue fabric (about 50% Du Pont's prices)
3) NdFeB magnets
4) High purity technical ceramics
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hissingnoise
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Registered: 26-12-2002
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Mood: Pulverulescent!
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| Quote: | | Really sorry . . . I've kind of started seeing a girl . . . |
And I'm really sorry to veer off-topic, but if you're "kind of seeing' a girl, then it follows that she is kind of seeing you . . .
Of course, I presume the object is, that if everything goes to plan, you will, at some point start 'seeing each others brains out!
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IndependentBoffin
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Quote: Originally posted by hissingnoise  | | Quote: | | Really sorry . . . I've kind of started seeing a girl . . . |
And I'm really sorry to veer off-topic, but if you're "kind of seeing' a girl, then it follows that she is kind of seeing you . . .
Of course, I presume the object is, that if everything goes to plan, you will, at some point start 'seeing each others brains out!
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She's amazing. We're so alike it is scary and thrilling at the same time.
When we were with some friends discussing something I just sat back and let her do the talking. It was quite amusing really, what she said is pretty
much exactly what I would have said...but she was the one being controversial and I was the one being amiable
[Edited on 11-6-2011 by IndependentBoffin]
I can sell the following:
1) Various high purity non-ferrous metals - Ni, Co, Ta, Zr, Mo, Ti, Nb.
2) Alkex para-aramid Korean Kevlar analogue fabric (about 50% Du Pont's prices)
3) NdFeB magnets
4) High purity technical ceramics
|
|
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hissingnoise
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Posts: 3940
Registered: 26-12-2002
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Mood: Pulverulescent!
|
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| Quote: | | We're so alike it is scary and thrilling at the same time. |
I'm happy for you IB, but I hope that others don't take your statement too much at face-value; it just might be a little, er, 'unsettling'?
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franklyn
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" Decomposition of all explosive compounds is ultimately heat-driven "
Heat = energy , by what action this is provided is the artificers choice. To be
broken , a bond must absorb a definite amount of heat particular to it. Only
if the heat generated by recombination of decomposition products exceeds
this threshold is detonation propagated. In the gas phase , molecules of an
explosive experience mutual collision that greatly exceeds by orders of
magnitude the impact sensitivity values of the solid explosive , yet remain
intact.
Admixture of inert matter serves to dilute and therefore decrease the ability
of the remaining energetic material to propagate any initiating energy.
A basic pH amine will induce the aci form of nitromethane the bonds of which
decompose with a lower heat input. The presence of amines in Trinitro triamino
benzene serves instead to increase the input of heat at which it decomposes.
As does Trinitrophenol neutralized with ammonia becoming Ammonium TNP.
If one seals in a vacuum explosive grains inside an envelope and then sends
this package on a fishline to the bottom of an ocean trench say 4 miles down,
it is isostatically compressed to it's absolute density.
As noted in C.O.P.A E. - page 410
" After being pressed at 25,000-30,000 pounds per square inch,
mercury fulminate becomes "dead pressed" and no longer explodes
from fire but merely burns."
Minding that there is no additional energy input , such as impact , explosives
are much more sensitive when very hot.
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PS
Thanks for the references
.
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