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Hydrophobic vs. hydrophilic Aerosil


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#1 mabuse00

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Posted 16 March 2017 - 03:25 PM

When I ballmill some potassium chlorate (for example), I mostly add 0,5 - 1% of anti cake. This makes sure that when it is used maybe one year later, it's still airfloat.

 

Unfortunately I dont know what exactly I'm using.  Silicon dioxide, that's all I know, got the bag as a gift a long time ago.

 

When looking for some Silicon dioxide anti cakes, I found there are hydrophilic and hydrophibic ones. My first guess would be that hydrophobic is the right stuff for our needs, but I never stuck my nose deep enough into the chemistry of silicon to get a precise understanding of the whole thing.

Some stuff that should be treated with it is hygroscopic.

 

-> What would you use/ do you recommend?



#2 Mumbles

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Posted 16 March 2017 - 04:17 PM

I can't speak for what you have, but the suppliers in the US tend to sell Cab-o-sil M5.  It's an untreated fumed silica, which means it's hydrophillic.  Using a hydrophobic grade might seem nice for keeping things dry and free-flowing, I'd think it'd also resist wetting for making stars or other compositions too.


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#3 lloyd

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Posted 16 March 2017 - 04:31 PM

Mumb is correct.  A hydrophopic grade will cause difficulties wetting comps.  That can be remedied by adding surfactants to the water, but the 'phobic' quality of the fumed silica doesn't add one bit of protection against the other components absorbing water, so it is - at the very least - a wasted effort to use it in some sort of effort to improve the hygroscopicity of a mixture.

 

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#4 OldMarine

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Posted 16 March 2017 - 05:02 PM

Isn't one of the reasons for using Cabosil that it acts as a desiccant as well as a free flow agent? I read an article on it somewhere when studying on whether to buy perc with or without it and ran across it. I wish I'd bookmarked the article but my mind went elsewhere.

 

Derp. It was on their website. Increases adsorbency.... 

http://www.cabotcorp...des/hydrophilic


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#5 lloyd

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Posted 16 March 2017 - 06:03 PM

Patrick,

To my knowledge, glass has no dessicant properties unless it's deliberately made in fairly large globules (lumps) with micro-pores throughout.  Then, it's called "Silica Gel".   And it's usually made from sodium silicate, not by fusing glass.

 

(Oh... 'ordinary' Cab-O-Sil is made of super-thin, super-small FUSED glass bubbles, broken up some for the final product. It's glass, too, but more in the form of spherical-profile 'flakes' than globules.)

 

;)

 

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Edited by lloyd, 16 March 2017 - 06:10 PM.

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#6 OldMarine

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Posted 16 March 2017 - 06:18 PM

See Applications:

https://en.wikipedia...ki/Fumed_silica


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#7 lloyd

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Posted 16 March 2017 - 06:54 PM

I see it, but have never seen it used in practice in that role.  Also, much of the 'sil' stuff we get is not technically "fumed silica" in the amorphous form, but more like tiny bubbles that have been broken into flakes.  Maybe that's the difference.  The 'bubbly' stuff has no dessicant properties.

 

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#8 OldMarine

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Posted 16 March 2017 - 07:33 PM

That's why I ask questions.

Edit: This has led to another question (of course) that I can't find an answer for.
If the Cab-O-Sil does indeed act as a desiccant as well as a flow agent,how does the adsorbed moisture affect a finished comp's performance and it's storage?
Silly question maybe but I still want to know.

Edited by OldMarine, 16 March 2017 - 11:32 PM.

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#9 mabuse00

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Posted 18 March 2017 - 02:37 PM

Yes, anti cake wil impair the binding, but for some applications this wont hurt.



#10 OldMarine

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Posted 18 March 2017 - 04:10 PM

Looking around I found a few formulae that actually call for the addition of cabosil. Bill Ofca's titanium salute as well as Lloyd's formula for Glusatz time fuse. I also found an interesting thread on the use in salutes: 

http://www.sciencema...id=14675&page=3


Edited by OldMarine, 18 March 2017 - 04:11 PM.

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