[pepsi_max2k]

Hi all, first time here (it's the first Chemistry related forum I found), and have a question regarding Aluminium Oxide and it's dissipation in water.

I'm trying to reproduce a simple polishing liquid for polycarbonate; and a good excuse to learn some basic chemistry at the same time . As far as I can tell, it contains Aluminium Oxide (~20%), Aluminium Nitrate Nonahydrate (~2%) and water.

The nonahydrate's use has somewhat baffled me, but I've come to the conclusion it's probably not entirely required for the primary function.

The water & oxide's use is obvious, however I'm having trouble mixing them together - namely the Al2O3 I have is the size of fine sugar, and just sinks to the bottom of the water without really mixing in.

So my question is: What particle size would be suitable for Al2O3 (or any substance) to become suitably suspended within water than it'll take a few hours+ (or longer) to seperate to the bottom again? Eg. 0.1 microns / 100 nanometers? 1 microns? 10 microns? Don't really have a clue, or the experience to make any kind of guess.

And if anyone would like a guess, what's the relevance of the Aluminium Nitrate Nonahydrate in the solution? A few people have suggested it's used as chemical polish, but again opinion seems to vary widely and I've no idea myself

Thanks.

[SirRoderick]

Ehm, well Al2O3 is insoluble in water mate. It won't happen.

And the Aluminium Nitrate is an oxidizing agent, a rather strong one.
So perhaps THAT is the actual active ingredient. The Al2O3 is probably just an abrasive.

[Enthalpy]

Alumina is a standard choice as a polishing paste, because it's hard, cheap, chemically inert...

It won't dissolve into water, and shall not in order to polish. If it's fine enough, it will stay suspended in water for a long time, true.

I had powders of tungsten carbide and of graphite, both around 30nm size of individual particles (graphite makes bigger clumps). Very dilute in water, tungsten carbide takes days to settle partially and can be mixed again. Lighter graphite stays longer. More viscous liquids would keep them suspended longer.

Indian ink is a graphite suspension but finer, and stays for years with little settling.

Now, polishing requires a paste rather than a dilute suspension, doesn't it? I'd say settling won't happen then. But to get an optical finish, you need grains much smaller than a quarter of a 500nm wavelength, so <<100nm is the right direction, for instance 30nm.

Not fun at all: nanoparticles or TiO₂ have been recently suspected of being unhealthy. And they're everywhere, including in toothpaste and sunscreen creams.