I've recently come across the following (european) patent:
It seems to have expired already due to non-payment of patent fees, but I'm not a patent lawyer, so I can't really be sure. But afaik it only matters for commercial uses anyways, so using it for a personal, non-commercial project shouldn't be a problem.
The patent discusses the use of different types of replacement fluids, namely:
- perfluorinated liquids:
- Fluorinert FC-40
- Salt+Water solutions:
- sodium chloride (table salt)
- magnesium sulfate
- cesium sulfate
- cesium chloride
Let's take a look at the perfluorinated liquids first:
Fluorinert FC-40: can be bought on ebay for about 250$/litre, has density of 1.9g/cm^3 and is non-toxic. But the material safety sheet says it has a high global warming potential, since its emissions have a long atmospheric lifetime.
perfluorodimethylcyclohexane: has a density of 1.788g/cm^3, is slightly toxic as well but it costs around 800+$ for 1 litre, though I couldn't find a supplier for private use.
trichlorotrifluoroethane: is out of the question, since it depletes the ozone layer (like other CFC's) and afaik it's only permitted for medical uses.
So I'd disregard the perfluorinated liquids for price, health and environment reasons.
Moving on to different salt solutions (densities are for saturated solutions at ~20°C):
Table Salt/sodium chloride: Several forum members have used table salt water before with good results, so I won't go into too much details. It has a density of around 1.2g/cm^3, so only slightly above the 1.1g/cm^3 of most resins and is otherwise unproblematic.
magnesium sulfate: non-toxic and non-hazardous for waste disposal. It's used in gardening as a magnesium source for soil. Density is 1.3g/cm^3. You can buy 4.5kg for 25$ on amazon, as a bath salt or mild laxative
cesium sulfate: I've found one source selling 1kg for 201.95, though most places sell it for around 50g/50$, and it's toxic when ingested at around 2g/kg bodyweight (in mice and rats). Density is 1.65g/cm^3
cesium chloride: Low toxicity in humans, 2.3g/kg bodyweight orally in mice (in powder form), but doesn't seem to be an irritant otherwise (i.e. as long as you don't drink it, it should be safe to handle). Concerning disposal, it's kinda hard to come by information (most MSDS don't mention any specifics), but http://arctic.cbl.umces.edu/sbi/web-content/Cruise%20Link%20Information/Hazardous%20waste.htm lists it as being disposable down the drain. Density is 1.88g/cm^3. 500g cost 850$ on Amazon, but you can get it for 50-100$/kg from wholesalers (I found several on alibaba).
Now, given all that, I think Magnesium Sulfate could be an obvious contender for table salt, since it has a higher density, is easy to aquire and otherwise uncomplicated.
Cesium chloride would be very interesting, given it's high density, though it's price/difficulty aquirying larger quantities, while not being too bad, have to be taken into consideration.
The benefit of higher density liquids is that the resin and liquid layers separate quicker and are less easily disturbed. And I think higher density leads to less waves when the print platform moves, so less wait time for the resin to settle (though this might be wrong, I couldn't find any specific information on this). It should also make it easier to stir the resin (if the pigment has separated) without having to wait as long for everything to settle (though this might be negligible anyways).
One more substance that I can think of that could be interesting as resin replacement is corn syrup or other sugar solutions. Corn Syrup has a density of 1.38g/cm^3, a viscosity some 1000 times higher than water, is cheap and harmless. But I couldn't find any information on chemical or physical interactions of sugar and photopolymers.
Has anyone experimented with any of these or other substances as resin replacement? And could any chemist chime in to verify the information I've gathered?
I'm currently waiting for parts for my new top-down dlp printer (atm I only have a RepRap Prusa) so I can't test this myself, but I'll do some experiments once the resins arrive.