Sciencemadness Discussion Board - Density and initiation

Density and initiation

agent_entropy - 18-7-2006 at 07:57

Hello all. I’ve been lurking about the forum for a week or two now and I finally decided to register.

I’m a bit confused by the various discussions of the density of an explosive. What is being referred to? The density of the compound as a function of the crystal structure (ie. the density of each crystal)? Or the density of the charge as a whole? Which one has more effect on the quality of detonation?

And how does this relate to compounds that can be melted and cast such as TNT? Does the compound usually crystallize in its new shape or does it assume some amorphous state? How beneficial is the absence of spaces in between crystals of the compound in a melted and cast form? Does the above perform better somehow than a collection of crystals packed together? (and is the large, now probably more brittle, shape more sensitive to fracture?)

Also, either in melted and cast or packed crystal form, is it best for the primary to be next to, on top of, engulfed by the main charge … etc…?

simply RED - 19-7-2006 at 13:08

Molten TNT is amorphous.

Modern explosives formulations consist of small chrystals embeded in polymer matrix. No big chrystals are used due to unstability.
When calculating properties the density of the final product should be considered.

The chrystal density and the density of powdered and pressed material usually does not differ much (10-15% most).
VoD depends from the density of the explosive charge (not chrystal density).
RDX at 1,6 fires at higher VoD than RDX at d=1,4, for example.

nitro-genes - 19-7-2006 at 14:30

Quote:

Originally posted by agent_entropy
How beneficial is the absence of spaces in between crystals of the compound in a melted and cast form? Does the above perform better somehow than a collection of crystals packed together? (and is the large, now probably more brittle, shape more sensitive to fracture?)

A meltcast explosive containing no air spaces will allow the detonation wave to "run through" the charge at full speed so to say. The airspaces cannot transmit the detonation wave very well and quench the speed of the detonation wave. A value that is used by many on this forum is ~1500 m/s which is an approximation since it depends on the diameter of the airgap. (If the airgaps would become very large, the maximal transfer speed would eventually slow down to the soundspeed in air at ambient pressure and temperature.)

So if there is, say, 10% of air mixed in a cast explosive with a VoD of 8000 m/s at crystal density (which is the maximum density), then the approximate VoD of the charge would be: 0.9 * 8000 + 0.1 * 1500 = 7350 m/s. And the density of the charge would be about 90% of the explosives crystal density in this case. Even a meltcast will contain considerable amounts of air mixed in, therfore meltcasting is usually done under vacuum...

The performance of meltcasts is not better in any case, many of the most brisant explosives are not castable, that is why they use molten TNT in octol and cyclotol to fill up the empty spaces. But TNT is not very brisant compared to RDX or HMX, therefore they switched to pressed polymer bonded explosives (PBX) Red was talking about for applications demanding the highest brisance possible, like shaped charges for example. The polymer matrix also provides better mechanical strength and handling properties than meltcast explosives...

[Edited on 19-7-2006 by nitro-genes]

Rosco Bodine - 19-7-2006 at 16:21

Quote:

Originally posted by nitro-genes

The performance of meltcasts is not better in any case,

Bull . It takes more to get'em going , but they damn sure
go faster and more brisant when they go at their max potential . When the cystallization grain has been modified
optimized , this is even more true and similar pressed
compositions won't touch a good cast charge in terms of
bulk power and velocity and brisance .

nitro-genes - 19-7-2006 at 16:35

I meant that most very high brisance explosives, like HMX or RDX are not castable. Only in combination with lower brisance explosives, like TNT they are. That is why for example a pressed composition of 95% HMX and 5% estane (LX-14) has a considerably higher brisance than a melt cast of HMX with TNT (octol)...
But you're right that if you compare pressed TNT with cast TNT, the cast TNT will have the higher brisance.

[Edited on 20-7-2006 by nitro-genes]

Axt - 19-7-2006 at 18:40

You sure, the cast density is most often (if not always) lower then a pressed density. Its liquid state is going to less dense then solid so one must assume some voidspace opening up somewhere.

I've also assumed that a greater degree of disorganisation in the crystal lattice from crystalising from a melt is also responsible for a lowered density. Is this true?

From LLNL, nominal density of TNT; 1.5-1.6g/cm3 when cast, 1.63-1.64g/cm3 when pressed. 1.654g/cm3 TMD.

The worst I've seen for this was the dichloro-EDNA which when cast at ~45°C expanded considerably on solidification. (and the crystalised form was more dense then the melt)

This is in theory, of course pressing a large charge to near TMD is terribly impractical.

[Edited on 20-7-2006 by Axt]

Rosco Bodine - 19-7-2006 at 19:12

IIRC even cast TNT using sulfur and TNX as crystallization
modifers has an output about 10% greater than plain pressed TNT .

agent_entropy - 19-7-2006 at 19:29

So, how hard are crystals generally pressed for a given charge? Say, for PETN, would it be possible (or a concern) to press the charge too hard and risk detonating it?

What polymers are usually used in polymer bonded explosives? Does anyone know of any high explosive polymers (other than nitrocellulose)?

The other thing that bothers me is ANFO, I've seen it being merely poured into bore holes (haven't seen it being packed at all), how can the shockwave/detonation propagate across the gaps between the prills efficiently enough to make it a practical explosive?

Chris The Great - 19-7-2006 at 23:15

You have a lot of ANFO and use a really big shockwave

There are quite a few energetic polymers beyond nitrocellulose, offhand I can think of polyvinyl nitrate and polyGLYN, however there are a few azide ones as well. Somewhat harder to prepare, I have done a fair amount of research on these polymers but the polymerization process is not easy to do at home (completely qanhydrous, BF3 polymerization initiator, etc).

I think polyvinyl nitrate would be easy though, PVC + NaOH -> polyvinyl alcohol, then nitrate.

simply RED - 20-7-2006 at 03:01

I doubt PVC will yield the desired product with NaOH. As PVC is insoluble in NaOH...

Do a search for "energetic polymers" there are quite a lot of them, none with any use in practics. Usually polymers are added to desensitize explosives. Energetic polymers are simply not good desensitizers.
(NC is also very bad to mix with RDX, PETN...)

Boomer - 20-7-2006 at 03:14

Something to remember is that the closer to TMD, the harder a HE is to initiate. That's why you need a booster of pressed TNT (or pentolite etc) to kick-start cast TNT.

An even more striking example are EBW (exploding bridge wire) detonators. Here, a thin gold wire is *exploded* (not only melted) by passing say 1000 Amps through it in a few micro seconds. This creates a shock that directly initiates PETN. These detos are vastly safer than the old style with a primary explosive on top of the PETN.

Now, the PETN pellet used has a low density, something like 0.88 g/ccm. The discharge energy needed for initiation grows with something like the fourth (seventh ?) power of the relative density (or rather reciprocal to remaining void space). So for d=1.4, you need ten times the energy, for 1,6 a hundred times and at 1.78 it gets nearly infinite. Don't shoot me for the numbers, they are from memory, I can look it up if you really want me to. It is just that there is an incredible increase if you get close to crystal density.

agent_entropy - 20-7-2006 at 08:42

Boomer:
If you don't mind looking it up I'd like to see how to calculate the discharge energy needed for initiation of a given HE.

Further questions:
How can one be sure how much of the energy from the primary is actually being transferred into the main charge? If the primary is merely next to the main charge, and the shockwave propagates in a more or less spherical fasion, won't almost half of the energy from the shockwave be lost since that part of the wave is moving away from the main charge?

If the primary is in the middle of the main charge (and there is no containment) how does one know that the primary will not simply fracture the main charge into pieces without detonating it? Maybe some degree of containment could be useful so that the shockwave originating in the middle of the charge can compress the explosive against the sides of the container. Does this make sense?

I'm currently working on making some PETN and I would like to try detonating some of it. Can anyone provide thoroughly detailed, non-k3wl instructions on how one might go about this? (the only primary I have right now is silver acetylide, might this work? or will I need something else?)

Rosco Bodine - 20-7-2006 at 10:12

There is actually a method of loading sequential layers of initiator and base charge in a detonator which is reported to reduce the total amount of initiator required for producing the same strength of detonation of the base charge , by utilizing the shockwave from both sides
of the initiator which is sandwiched between layers of the
base charge . During detonation , sympathetic multipoint
initiations occur from the compression wave which pinches
all the layers together ....so the whole column of layers
detonates at once and at maximum density and velocity .

When very small masses of materials are being detonated against other small masses , a good external containment is necessary to keep the charges in contact for the energy transfer to occur , but when larger masses of material are involved , typically at a point of a few grams , there is sufficient " inertial mass " to the charge
that containment becomes a non-issue , as the mass of the charge is sufficient to keep it in place for sufficient time that the impulse from an adjacent blow will couple
and transfer energy to the larger mass with no disruption by any gap formed from any pushing away of the target mass , as could occur with a smaller target . Working with
anything except very small charges , you have no problems getting good transfer of the detonation wave .
But it is a very real factor to be reckoned with in the early elements in a firing train .

Chris The Great - 20-7-2006 at 14:13

Quote:

Originally posted by simply RED
I doubt PVC will yield the desired product with NaOH. As PVC is insoluble in NaOH...

I was thinking boiling it or something, sorta like that reaction with polyethylene bottles with NaOH that gives an acid product. Sorry that I don't remember it, but basically a bit of time and heat caused the plastic to hydrolyse and dissolve. Though it did takea fair bit of heat IIRC....

@agent_entropy: PETN is quite easy to detonate, silver acetylide will do fine. I'd say 0.1g would do to fire a base charge of 1g PETN, which then fires the main charge. IIRC silver acetylide detonates extremely easily and is a very powerful initiator.

agent_entropy - 20-7-2006 at 15:49

I like the initiator/base charge sandwich stack idea a lot! That definitely sounds like it uses the energy from initiator much more more efficiently. I'll have to look into that.

Chris: Thanks. Sounds easy enough, does the 0.1g silver acetylide have to be in a cap of some sort to keep it separate from the PETN until it is to be initiated?

nitro-genes - 20-7-2006 at 16:20

Polyvinylnitrate would make just as bad a binder as NC would I'm afraid. It is hard enough to nitrate it to it's highest N percentage and becomes as impact senstive as PETN or tetryl. The fully nitrated product is also very unstable IIRC, decomposing almost in the same manner as NC does without added stablizers...

I've tried several PBX compositions, with NC, Polystyrene, PVAc, but they all need solvents to become mixed in with the explosive. When the solvent evaporates, the density of the charge is deminished in a great way, or even "cracks" in the charge may occur...

The clear, slow setting resins that can be used to preserve insects are usefull though. They usually consist of benzoylperoxide and polystyrene resin and have they advantage that they dont shrink upon hardening.
I've no idea if the small amount of peroxide will pose a threat to the stability of the charge, but I only tried with small charges (2 gr) with nice results. 90% PETN with 10% of this resin in a 9 mm tube results in a density of 1.55 g/cc by pressing by hand which upon hardening becomes a very strong, plastic like material. (caseless detonator was the plan) Never tried to detonate or press it harder though...

For home experimentation PBX compositions are not very usefull I think, mechanical properties beeing not very important. 90% PETN with 10% plain wax would probably give the same density upon pressing. Pressing is not practical at all for larger charges anyway, better stick to meltcasts or (my favorite) plastiques all together...

[Edited on 22-7-2006 by nitro-genes]

Boomer - 21-7-2006 at 06:09

"...the primary will not simply fracture the main charge into pieces without detonating it?"

You have to find a new picture in your head of a detonation. It is NOT primarily a pressure wave that will push adjacent explosive away, but a superconic shock moving through the material that causes it to decompose. Since it is supersonic, the stuff ahead does not know what is coming, and therefore cannot 'give'. We speak 350 kilobar or 5 million PSI here, at ten times the speed of a rifle bullet.

OTOH, as soon as the shock pressure drops below a certain limit, exactly that happens: The main charge just gets scattered. Typical scenario if a kewl puts a firecracker into ANFO to 'detenate' it.

agent_entropy - 21-7-2006 at 07:49

Thanks Boomer, that mental picture for detonation makes much more sense than what I had been envisioning.

nitro-genes - 22-7-2006 at 04:03

Quote:

Originally posted by Axt
You sure, the cast density is most often (if not always) lower then a pressed density. Its liquid state is going to less dense then solid so one must assume some voidspace opening up somewhere.

I've also assumed that a greater degree of disorganisation in the crystal lattice from crystalising from a melt is also responsible for a lowered density. Is this true?

From LLNL, nominal density of TNT; 1.5-1.6g/cm3 when cast, 1.63-1.64g/cm3 when pressed. 1.654g/cm3 TMD.

Was the 1.5/1.6 g/cc from casting under vacuum? I mean during shrinking, the air filled spaces have to come from somewhere. So that probably only leaves the disorganization of the crystal lattice to account for the lower density indeed...

From PATR:

It is desirable not to use melt-pour temps above 95° to decrease viscosity for a variety of reasons. For example, TNT shrinks from 11 to 12% on solidification, so that if the melt is too viscous during pouring, the entrapped air may not escape prior to solidification. These bubbles are further enlarged by the volume shrinkage of TNT, resulting in a porous cast of low density Vacuum casting has been used in Germany by Boelkow B.m.m.H. to achieve more homogeneous casts and eliminate microporosity (Ref 14). Subsequently, Reichel showed that by programming the temp of the entire melting filling cycle and centrifuging the poured charge at 25000 ft sec2 for 10 to 15 minutes, crack-free charges of highest density could be produced...

From this it is vague whether centrifuging and vacuum only remove the air bubbles from pouring or that it also removes the air from shrinkage upon solidification...

50/50 pentolite, cast under vacuum, has a density of 1.70 g/cc though, which is close to the average of both at crystal density...

Your right though that for most substances, the amorphous density is lower than the crystalline density, pretty logical when you think about it indeed...

--> http://faculty.uscupstate.edu/llever/Polymer%20Resources/Den...

Although this is for polymers, striking is that the difference in density between amorphous and crystalline is highly dependant on the substance, like you noticed also.
Water, with a low density, hexagonal crystal formation, beeing the most striking example, having an amorphous density higher than crystalline. Most explosive compounds don't form hydrogenbonds like that, so for them it is very unlikely to have higher amorphous densities probably...

[Edited on 22-7-2006 by nitro-genes]