Hygroscopicity, problems, solutions and thoughts

[Arkangel]

Working recently as I have with Sodium Chlorate and sugar the problem of hygroscopicity is in my mind. A rocket ramming tool, left overnight with a small amount of fuel on the base, by next morning had a wet, transleucent blob of goo instead. Some materials are "thirstier" than others, but keeping things dry can be a problem for virtually every powder you work with (although some you WANT to keep wet <img border="0" title="" alt="[Wink]" src="wink.gif" /> ).

When working with such materials, I always try to keep the workplace as warm as possible, but is this a good idea? Most materials are far more soluble at higher temperatures, so does this compound the problem? Lots of people describe heating wet comps to dry them, yet it barely working. Is this because they are able to hold more moisture at higher temperatures? Heating things further in an oven might evaporate all the water, but then you're at grave risk of a cook off ignition. So maybe a cool dry environment would be a better idea. Also, I have a gas powered heater (not used when gases or powder could be present in the air). These kick off quite a bit of water vapour, so really I need to look at an electric heater of some kind.

My solution to drying out comps is to get a dry bag of silica gel, and put it into a sealed plastic tub with the comp.

This

<a href="http://www.geejaychemicals.co.uk/usingsachets.htm#USING%20SILICA%20GEL%20SACHETS." target="_blank">http://www.geejaychemicals.co.uk/usingsachets.htm#USING%20SILICA%20GEL%20SACHETS.</a>

links to a UK supplier of Silica Gel, and describes the ratio of the stuff needed to keep things dry. Since I'm using 1lb bags in a container of 1lb of comp, I'm fairly certain that even if I have to change gel bags for new dry ones every now and again, I can dry out even the most soaking wet comp. (I have some testing right now) As long as the container is sealed, and the silica gel is more hygroscopic than whatever you're trying to dry, it should work. If so, then you might even be able to safely cast a BP or other fuel grain, then put all your motors in a sealed container like this, and leave them for a couple of weeks to dry?

Once you have dry comps (or if you started with them dry in the first place), you might want to store them for a while. One solution is, again, a sealed container, along with some dry bags of gel. Another, especially if you want to leave the ordinance in some tactical place for an extended period, would be to construct the device, and then seal it with varnish, a laquer or even paint. Has anyone an idea of how effective the latter might be? (Or any other thoughts)

[Anthony]

I realise what a pain NaClO3/sugar can be. I used to dry it under a radiator and it worked ok. A desk lamp might work well, and you can vary the temperature by how cose you put it to the comp.

Sealing finished devices is a good idea, a drop of wax over the nozzle of a finished rocket motor would make it storeable and the motor will have no trouble clearing it on ignition.

I briefly experimented with casting BP grains, they tended to shrink away from the tube walls on drying and subsequently CATO on ignition <img border="0" title="" alt="[Frown]" src="frown.gif" />

[vulture]

Why would you buy silicagel when you can use cheap CaCl₂ or MgSO₄ which are available in bulk quantities?

[VX]

Yeah,as vulture has said CaCl2 is far more easily avalibe, and cheaper in the UK at least. (Focus and B&Q sell bags of the stuff as refils for those damp trap things).

Magnesium Sulphate is also avalable en mass from garden shops fairly cheaply, but it is sold in it's hydrated form, so to be any use as a desicant it must first be dried out in an oven, and kept in an airtight container untill it is needed.

[Sparky]

In some whistle compositions such as those containing sodium salicylate have vaseline in them. This keeps it dry and lubricates it when it is being pressed.

On that page that talks about Bates in the "Rockets" thread, the author has done some experiments with coating his grains with NC laquer. The conclusion is that it works well but needs to not have any cracks in the coating. I also have seen a silicone grease/blend on firefox used as a sealant. Maybe you could use vaseline. I wonder if you could get the fuel wet, while it's still in powder form, with NC laquer then let it dry in a thin layer. Then granulate and use that to make your rocket.

If you work in an enclosed space, maybe a dehumidifier would help (increases static though).

Potassium chloride is incredibly cheap as a water softener. I saw a 20 kilo bag of it in a grocery store right by my house: $5. It might have been a typo or something. Convert your sodium chlorate to potassium chlorate.

For drying stars I am thinking of using a vacuum oven. My dad brought me one from his office, they were throwing it out. I need a vacuum pump though.

Mostly I try to avoid hygroscopic compositions. For example instead of using sodium chlorate/sugar I would use potassium nitrate/sugar, ball mill it and use a longer core. If I were to have hygroscopic chemicals that I want to stay dry I would them in mason jars. I guess you could seal with wax it for long term storage.