Ensuring DIY electric igniter coating consistency

[deer]

Hi!

 

While I did my first batch of E-Matches they turned out quite consistent and reliable, but were a terrible waste of material. I did like 10 or so matches and then discarded most of the coating mix. If I do 40..50 ones (amount required to use up all my mix) it starts to dry out and adding solvent causes ingredient ratios to change.

 

Now, basically I make dark chlorate flash, e.g. 500mg, drop 100mg of NC lacquer (low nitrated, commercial one used for wood) and 100mg of acetone. Then dip the heads (PCB chip with ~1.5Ohm 40AWG nichrome bridge) in this slurry. When it starts to dry out I add acetone just enough to keep the consistency visually similar, but obviously the lacquer content gets lower and the Al - chlorate ratio changes too.

 

At 1Amp they just smoke for minute without igniting. At 2A some pop so fast that only tiny amount of FP around the very bridge wire burns off while others smoke for good 5..6 seconds before lighting up. I think this is both due to change in ratios as well as inability to achieve consistent coat thickness because of the changing slurry properties.

 

Maybe there is a NC lacquer solvent that doesn't evaporate so fast, so it would dry out e.g. over night instead of few minutes?

Edited June 30, 2015 by deer

[MrB]

Now, basically I make dark chlorate flash, e.g. 500mg, drop 100mg of NC lacquer (low nitrated, commercial one used for wood) and 100mg of acetone. Then dip the heads (PCB chip with ~1.5Ohm 40AWG nichrome bridge) in this slurry. When it starts to dry out I add acetone just enough to keep the consistency visually similar, but obviously the lacquer content gets lower and the Al - chlorate ratio changes too.

It shouldn't change in any significant way. Your adding solvent, which evaporates. The end result will be identical, regardless of if you add a teaspoon or a bucket of acetone. Your main problem is staying within a range that lets your composition stay suspended in the slurry.

B!

[deer]

It does. First dips take 50:50 solvent/lacquer out of the mix, when solvent evaporates, I start taking more lacquer, reducing FP/lacquer ratio. This lack of lacquer needs to be compensated by more solver, to achieve the same consistency, thus making binder thinner. Too thin and fine aluminium flakes start to float and stick to match more, while chlorate grains sink and stay in cup skewing the ratios even more. With right amount of lacquer it all stays in one sticky, homogeneous mass.

 

Anyway, did some reading and it seems that too fast drying NC lacquer is a common issue and there even is a whole article about it: http://news.thefinishingstore.com/index.php/understanding-solvents-part-iii-laquer-thinner/

 

Now I just need to figure out what are the matching local brands and see which ones doesn't interfere with ignition. I mean they all seem to be flammable, but some might mess with aluminium or chlorate.

 

Also, now that I explain the process to you, it arose to me, that making a larger cup of lacquer/solvent mix that I use to re-wet FP mix might help.

Edited June 30, 2015 by deer

[stix]

...Now, basically I make dark chlorate flash, e.g. 500mg, drop 100mg of NC lacquer (low nitrated, commercial one used for wood) and 100mg of acetone...

 

Are you making these for igniting core burner rockets? If that's the case, I can't see why you would need an exotic pyrogen such as dark chlorate flash. What's wrong with basic bp+ncl?

 

My pyrogen consists of the basic bp 75/15/10 mix + 2% 100mesh Al + ping pong NC. I make a slurry similar to the consistency of paint. When I dip the wire in, a few drops may run off back into the dish, I'll then re-orient the wire to ensure the remaining wet slurry can move around and coat it as evenly as possible - I gently blow on it with my mouth to help it set.

 

I usually do 3-4 coats with 1/2hr of warm air between. As with painting, more coats will give a more even result than just one. The only time I've ever needed to make a "hotter" pyrogen was when I was experimenting with fuels, NH4ClO4 and using various epoxy resins as a binder - it was very hard to light.

 

To alleviate any problems with the pyrogen "popping" and breaking off half-lit or not at all, I use the "spiral wrap method". I'ts a method I've developed myself but also read about some time ago. It's rarely spoken about. Most people just make the bridge wire connections and then dip it in a few times thinking all should be fine.

 

In essence the so called "spiral wrap method" is this. The ignitor wire is wrapped around a piece of pyrogen and a final thin layer is applied for stability and protection. The benefits of this method is that the amount of fire can be controlled by the initial thickness, and instead of bits popping off, it burns from the outside in - creating a wonderful localised ball of heat, if you like that sort of thing - perfect IMO . This also helps bring the motor up to pressure quickly.

 

Is it possible that you are over-complicating things - or is there some other reason that I'm unaware of? If so, please ignore the above.

 

[EDIT] btw. There should be no need to discard any unused pyrogen. If it's dry, just break it up a bit and add acetone. I have some old dry stuff from years ago that I use - it still works perfectly.

 

Cheers.

Edited July 1, 2015 by stix

[deer]

Many reasons. To name few in order of importance - moisture resistance, sensitivity / min. firing current, ping-pong ball price (plain wood lacquer would just kill BP), ability to light sub-par fuel (moist rcandy) and the reason that I’m still just working on my BP mill and currently do mostly rcandy model rocketry.

 

What resistance range does this spiral wrap yield? I imagine that would require quite some power to get it up to burning temperatures. 1.5Ohm of AWG40 nichrome glows orange at 1A from 3.7V Li-Ion. And even then I sometimes need to increase power to burn the wire completely for the FP to ignite. It's not that sensitive really.

 

As for the discarding - main issue is I cant be sure of the exact composition of remaining mixture. Is it mostly FP, or lacquer? Is the FP part even close to 70/30 etc.

[stix]

Ok, fair enough. I didn't mean to come across as though the 'spiral wrap' method was the only method - It works well for me though.

 

The wire I'm using is tungsten. http://www.amateurpyro.com/forums/topic/10719-this-wire-im-using-for-my-electronic-igniters-what-is-it/

I don't know what the gauge is but it's around hair thickness. The best I could measure with my crappy multimeter was 1ohm over about an inch. It ignites easily over 10mtrs of standard speaker cable with 2 x 9v batteries in series. The tungsten was a pretty lucky find.

 

Years ago I did use nichrome similar to yours but it was ignited using a car battery. The car was also used as the bunker.

 

 

As for the discarding - main issue is I cant be sure of the exact composition of remaining mixture. Is it mostly FP, or lacquer? Is the FP part even close to 70/30 etc.

 

I still don't get this. As B! already pointed out, the same ratio of ingredients should still be there after the acetone has evaporated. Are there other methods and processes that you have that we don't know about?

 

Hope you get it sorted.

 

Cheers.

 

[EDIT] It's important to point out that with the "spiral wrap" method, I'm winding about 4 turns over 1cm - it's not tightly wound as that might cause issues with resistance.

Edited July 3, 2015 by stix