Anti Static Pyro Compositions

Hello All,

I have been doing extensive research on pyrotechnic compositions and their sensitivity to static electricity... and have not been satisfied with any potential solutions.

Obviously mixtures with metal particles are more static sensitive, such as 70/30 flash. And the smaller the metal particles the more sensitive to static (I presume this is because a smaller particle will melt and vaporizes with a smaller energy input such as static). In regards to flash, I have also heard it is static sensitive because the oxide layer is non conductive, and the inside Al is... essentially forming a capacitor.

So... lets take 70/30 (2-5 micron flake Al as an example... if I don't want to use a larger Al size because performance will drop... what are my options?

1) Mixing in a non conductive clay, although I hear this reduces performance. Has anyone tried this?

2) Perhaps coating the Al powder with a thin (micron to nanometer) coat of wax or animal fat, sterin? Again, has anyone tried this?

3) I can't find any information on this in regards to pyrotechnics but I know copper nano particles (5-7nm) are used in making anti static plastics. If this were mixed in with the flash powder wouldn't it create a Gaussian cage?? surly at 5-7nm all of the particles would be evenly coated and surrounded. Perhaps mill the Al with copper nano particles?

That is all of my ideas..

Bert your response is mandatory because you have experience in all of these things, or you can always point me in the right direction...

I don't care if the solution is expensive... lets partake in some thought experiments, or actual experiments!!

Quote: Originally posted by MineMan

Philou, Thank you for your reply... I understand what you are saying but have a few questions. If the best thing is for the metallic particles to touch each other than why do compositions become more static sensitive when nano particles are induced... as they completely coat a 120 micron oxidizer particle? How would nano copper or silver not help with this, especially if they were 5-7nm, they would coat everything, and silver, being a noble metal will not oxidize easily? So for this flash powder, would it be better to use spherical Al 500nm instead of the 2-5 micron flake?... but then it will become more impact sensitive... ahhh!

Usullay they don't coat efficiently...have you made a simultation or a calculation to say this? Can you show me the calculation to see if it is correct?

Apparently...it is hard to understand or visualize (again linked to the "pin point power"): if you have two different particle size of the same material brought at the same electric potential...the radius reduction of the particle reduces the overal electric charge of the particle propotionnally...and at the same distance (bigger than R) the electric field is also lower
So if R = 2*r
$$q R = V × 4 π ε 0 R {\displaystyle q_{R}=V\times 4\pi \varepsilon _{0}R} {\displaystyle q_{R}=V\times 4\pi \varepsilon _{0}R}$$

$${\displaystyle q_{r}=V\times 4\pi \varepsilon _{0}r={\frac {V\times 4\pi \varepsilon _{0}R}{2}}={\frac {q_{R}}{2}}}$$

$${\displaystyle E_{R}={\frac {V\times R}{d^{2}}}}$$

$${\displaystyle E_{r}={\frac {V\times r}{d^{2}}}={\frac {V\times R/2}{d^{2}}}={\frac {1}{2}}{\frac {V\times R}{d^{2}}}={\frac {E_{R}}{2}}}$$

so at the distance d (what is further than R itself twice as big as r)...the electric field of the particle r is reduced by a factor 2 vs particle R.

But if you go in close contact with the external layer for the tinier particle r you can go much closer than into the R case since R is unpenetrable solid and charges are onto the outer shell/surface.
So
$${\displaystyle E_{R}={\frac {V\times R}{R^{2}}}}$$
$${\displaystyle E_{r}={\frac {V\times r}{r^{2}}}={\frac {V\times R/2}{(R/2)^{2}}}={\frac {V\times R/2}{R^{2}/4}}=2{\frac {V\times R}{R^{2}}}=2E_{R}}$$

And in fine, we observe that the electric field in close viccinity to the surface of the tinier particle will be twice bigger than the one in close viccinity of the larger one.
By extension...if the radius is n times smaller...the electric field at the surface will be n times bigger.
It comes from the fact that the sum of the charges q set on a spherical surface submitted to an electric potential V is increasing linearly with the increasing radius while the electric field decreases at the same time as a function of 1/d².

If you consider a pin point as a very small diameter half sphere (what is the case if you look at it under strong magnification)...the electric field obtained at that point has a value that tends to the infinity...and this very high field will contribute to air ionisation, and electric arc triggering.

Last thing to consider for R vs r (=1/2*R)...if at the surface of r you have an electric field twice bigger...for the same weight of metal (at the same density) you have also the same volume...
So
V_R = 4/3*π*R³
and
V_r = 4/3*π*r³
thus
V_r = 4/3*π*(R/2)³ = 1/2³*4/3*π*R³ = 1/8*V_R
You thus have potentially 8 times more electric field per equivalent weight without counting the already double value...
In the case of a reduction of factor n (R=n*r) you thus have for each particule r an electric field n*E_R bigger but also n³ times more particles than into the R case...and this makes a serious increase of n³*n*E_R = n⁴*E_R
So reducing the radius by a factor 2 increases the risk of static by 16...I let you imagine what happens when passing from micro to nano sized metallic powder....

Silver and copper are near "noble metals" they don't oxidise easily from air or oxygen...but they are oxidised by trace poluants from air (ammonia, CO2, H2SO4, H2S)...silver has the bad habit to turn dark grey/black upon time...but this will normally not lead to connectivity troubles only a % reduction (how much I don't know).

In theory from all what is explained here above...mixes with larger spherical particles will be less prone to static...and flakes will increase static because of their sharp edges at the end of the plannar surface acting like a pin point all arround the flake.



[Edited on 28-2-2017 by PHILOU Zrealone]

Quote: Originally posted by phlogiston

Speculating here, but for large sparks, it is also a requirement that the charge can move quickly towards the point of discharge. That is why you can get long bright sparks from the metal sphere from a van der graaf accelerator, but not from the bottom plastic roller even though the amount of charge stored in each should be roughly the same. Translating that to your question, at higher concentrations of metal powder, bigger networks of interconnected particles can form that can transport a larger amount of the charge stored in the powder quickly to the location where it is dissipated in the form of a spark. Maybe adding a poorly conducting powder (graphite?) will help, by dissipating the energy gradually along the way as ohmic heating rather than as a localised, high-energy spark?

Quote: Originally posted by MineMan

Yes, my experiments are showing that graphite may be valuable.... I just wish there was an easy non expensive way to test static sensitivity... The only way I know of is using a BBQ Piezo spark..