Re: Burn rate of ammonium perchlorate based mixtures.

I think that quick burning and ease of burning with saltpetre as
oxidizer has much to do with its low m.p. AND the m.p. of the
ingredients usually combined with it. As noted, saltpetre-based
antimonial white flame compositions are not that easy to ignite,
because of the relatively high m.p. of antimony sulphide or regulus.
The m.p. of potassium perchlorate is significantly higher than that of
the chlorate, but the more exothermic heat of decomposition of the
chlorate is also a reason for quick burning of chlorate compositions.

Ammonium perchlorate's pattern of decomposition may have something to
do with the typical ignition and burning speed properties of
compositions based on it; however, bear in mind that barium chlorate
when used alone as an oxidizer can yield rather slow burning, even
though it melts at only about 50 deg. C higher temperature, and has an
even more negative (exothermic) heat of decomposition than does
potassium chlorate. The liquid phase is important but does not explain
everything.



On Mar 7, 10:14 am, matthew <matthewpr...@xxxxxxxxxxx> wrote:
On Mar 5, 5:03 pm, mikes2653 <mikes2...@xxxxxxxxxx> wrote:



On Mar 5, 3:39 pm, matthew <matthewpr...@xxxxxxxxxxx> wrote:

Yep. Would it be accurate to say that this equation is primarily
useful in evaluating the ability of a fuel/oxidizer mixture to engage
in a stable pyrotechnic reaction?

Yes. There are several more elaborate calculations that are supposed
to reflect 'rate laws' but as they relate to the speed of pyrotechnic
reactions even these are often thrown into a cocked hat by simple
variations in particle size and intimacy of mixture.

It seems to me that one thermodynamic factor, that being the heat of
decomposition of oxidizers, is fairly useful in predicting the
sensitivity of a pyrotechnic mixture to mechanical action. Is this a
correct observation?

This is true, but again it does not tell the whole story. Some
oxidizer/fuel combinations in which the oxidizer has a high positive
heat of decomposition can still be unexpectedly sensitive to
mechanical action. The red lead/magnalium crackle composition is an
example. The barium nitrate/aluminum/sulphur luce forte compositions
are another.

As for making the simple complicated - do you want to make fireworks
or are you just interested in theory? If it's the former, let me
suggest on the basis of long experience that a lot of the science of
pyrotechnics is rather remote from everyday application, and will not
repay in practical results the effort required to master it. I have
run into quite a few people over the years who have been distracted by
playing with the chemistry of formulae to the point that they have
never learnt how to make reliably functioning shells, rockets, etc. It
is not just a question of 'whatever turns your crank,' unless for you
this is purely an armchair pursuit and you do not intend to make any
actual fireworks. Mastering the craftsmanship required is necessary to
safety; mastering the science is not.  It is easily possible to adjust
compositions to give the results you want without extensive
thermodynamic calculations.

My pursuit of pyrotechnic knowledge is at this point an "armchair
pursuit." I used to make fireworks, (I was quite sucessful in making
small skyrockets that carried "payloads" of cut stars, both color and
charcoal. I was most sucessful with small rockets, but I also have
made decent fountains, wheels, and small starmines). At this point in
time, I am just interested in theory.- Hide quoted text -

- Show quoted text -

Thanks for the good information.

Now, regarding my original post regarding the burn rate of ammonium
perchlorate based mixtures, thermodynamic considerations aside, what
accounts for the lower burn rate in comparison to similar mixtures
that use potassium chlorate or perchlorate? In the first post, I
speculated that it has to do with the fact that AP doesn't melt in the
same way as the potassium salts. It seems to me that this could
account for both the sluggish ignition and slow burn rate. In
contrast, I believe that black powder and potassium chlorate mixtures
ignite easily and burn quickly largely because of the low melting
point of potassium nitrate and chlorate. It seems to me that the use
of oxidizers with low melting points can result in extensive reactions
in the liquid phase (this is cited as one reason why black powder
burns quickly in low pressure situations), which tends to result in
fast and stable pyrotechnic reactions. Ammonium perchlorate, from what
I understand, does not melt, but decomposes directly into gases. It
seems to me that there would be little or no liquid phase reaction
zone in AP based mixtures, and the fuel would need to decompose into
gases and mix with AP decomposition products (gases) before any
combustion could occur, rather than the combustion reactions begining
in a liquid phase. It seems to me that this would increase the size of
the reaction zone somewhat, and slow the reaction by requiring the
fuel to reach higher temperatures before begining combustion.  Do I
have my facts straight? Does my thinking make sense? Thanks much.

.