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I have recently acquired a 1 L bottle of liquid flux. I only realised upon receipt that it is quite flammable and that I may not be able to provide the proper storage conditions.

For now I have just put in on the bookshelf in the study nook. I live in an apartment so I have limited storage opportunities apart from shelving.

I am quite paranoid about whether I should be storing this at all or whether I should just look to dispose it at a chemical disposal site.

I have temperature sensor near where the bottle is stored which read a max of 28°C over the last week.

Should I be worried?

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    \$\begingroup\$ It's not worse than anything else flammable like cleaning chemicals. It's not going to magically combust on its own. You should probably worry more about keeping it away from children's reach etc. \$\endgroup\$
    – Lundin
    Commented Mar 2, 2022 at 9:34
  • \$\begingroup\$ I have put it on a book shelf for now. High enough so its out of reach of our kids. \$\endgroup\$
    – shimon
    Commented Mar 2, 2022 at 9:39
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    \$\begingroup\$ It it more flammable than a bottle of vodka or whiskey? Not likely ... So take similar precautions. \$\endgroup\$
    – brhans
    Commented Mar 2, 2022 at 12:58
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    \$\begingroup\$ @brhans: vodka and whiskey are not flammable, you've seen too many movies. you can heat them up (like flambé) and ignite the pure alcohol vapor, and you can burn high-proof spirits like everclear/151, but 40% alcohol doesn't burn, even with wicks... \$\endgroup\$
    – dandavis
    Commented Mar 2, 2022 at 21:00
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    \$\begingroup\$ Flux is stored in a flux capacitor, surely? \$\endgroup\$ Commented Mar 3, 2022 at 3:14

2 Answers 2

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Liquid flux is typically flammable (when it is) because there is some alcohol-like carrier (eg. butanol). You can look up the MSDS for the particular flux you have and find out what that material is. Here is one example, and you can and should refer to the specific one for your product. That shows an NFPA fire hazard rating of 3.

Sure, it's flammable, like a bottle of isopropanol. It won't burst into flames in contact with a hot soldering iron (below about 400°C typically, so your 28°C seems on the safe side) so it's not very easy to get going. I suggest storing it with the same care that you'd store a 1-liter bottle of 99% isopropanol (rubbing alcohol)- a liquid which has similar scary markings when sold for industrial use (there are a lot of exceptions for products sold for consumer use- you will see products piled on shelves that would require an expensive special solvent cabinet in a factory situation). Here is an MSDS for 99% isopropanol (also NFPA fire hazard rating of 3).

Incidentally, it's a good idea to be careful with solvents such as lacquer thinner, acetone, isopropanol etc (including, apparently, your flux). If they are spilled and find an ignition source they are definitely flammable and some burn with a flame that's not very visible (like lighter fluid).

Here is a photo of a dispenser bottle I use with a small brush for spot cleaning. The bottle holds only 100ml. The top has a hinged cap that snaps down, and each depression of the cap mechanically pumps a small amount from the main reservoir into the small 'dish' in the cap for immediate use. It's spill-proof, even if you knock the bottle over it won't spill hardly any liquid.

enter image description here

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    \$\begingroup\$ oh that's really handy! Now I'll have to look for something like that :) \$\endgroup\$ Commented Mar 2, 2022 at 10:52
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    \$\begingroup\$ @MarcusMüller They're the bees knees. I prefer the glass ones from Menda., I have like 4 but wish I had more for reasons. They are pricey though. \$\endgroup\$
    – DKNguyen
    Commented Mar 2, 2022 at 14:27
  • \$\begingroup\$ @MarcusMüller, They are called "ESD dispenser bottles", the most common brand in my experience is "Mechanic". \$\endgroup\$
    – Phil
    Commented Mar 3, 2022 at 16:30
  • \$\begingroup\$ 'Mechanic' is the Asian variety. You can find them at places like McMaster, however the price is 4x-5x higher (maybe a bit better quality though). \$\endgroup\$ Commented Mar 3, 2022 at 16:33
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The main ingredient is alcohol, and possibly water. It will not spontaneously combust. A bookshelf is safe enough, for reasonable amounts (and 1l sounds reasonable). I'd look that it doesn't stand in the brightest sunlight, but mostly because that heat might both lead to pressure and make the plastic brittle - over decades. 28 °C is not a high temperature. 55 °C would be. So, not an urgent problem.

Word of advice, though: find a second, smaller bottle to store some (~20ml?) amount for daily work in - simply because tipping over a bottle while working happens, and while this stuff can usually nicely be cleaned up with a cloth and a second wipe of a paper towel with alcohol, it still is a sticky mess. Don't ask me how I know.

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