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Chemically grinding ABS scrapMonday, November 24. 2008It appears that acetone can be used to reduce ABS scrap to a
fine powder for reprocessing into new filament...
I
did a little exploratory work into grinding HDPE scrap with rotary files and
rasps some months ago. While I was able to demonstrate that it was indeed
feasible to grind HDPE down into a fluffy powder, it involved a considerable
expenditure in electricity and created quite a lot of noise.
This last weekend I recalled a conversation that went on at the very
beginning of the Reprap project back in July 0f 2005, well before we actually
had anything that worked. As a timeline, Adrian created the Mk II extruder, the
first practical one, in November of that year. In any case, the conversation got
into the notion of dissolving plastics with solvents and the potential for their
use in the printing of plastics. Steve DeGroof was participating and in the time
that followed I'd managed to attribute the solvents idea to him. A note to Steve
this morning set me straight on that matter and enabled me to recover the blog posting
that had the conversation.
Most of the solvents notions were put forth by a guy named Mike, who I've
not heard from since. Somewhere along the way he'd got the idea that you could
dissolve HDPE in acetone. I have an affinity for printing with HDPE, probably a
vain hope, but one I nurture all the same. This morning I had a few moments so I
purchased a pint of acetone and made an attempt to dissolve some HDPE scrap.
Sadly, after being submerged for three hours in pure acetone, the scrap
came out in pretty much the same shape it went in.
To my delight, the acetone turned milky with dissolved ABS within a few seconds. After stirring and leaving the mix for about half an hour I saw that about half of the ABS filament had dissolved. Another half of an hour found the mix in about the same state, so I presumed that some sort of equilibrium between solid ABS and ABS in solution had been reached. Mike had suggested that... Another way would be to add an excess of acetone, make a slurry and then drip the slurry into water to form the powder. the finer the drops the finer the powder. While he was dead wrong about acetone dissolving HDPE the method seemed to me worth trying with the dissolved ABS. I poured the dissolved ABS/acetone mix into a pyrex tray containing about four cups of water.
A froth of finely divided ABS formed on top of the water. I was a bit nonplused about this since ABS is supposed to be more dense than water. I got a bit of the undissolved ABS out of the measuring cup and tossed it in to the water bath and it floated, too. Mind, not all of the powder floated. Some of it appeared to be neutrally bouyant. Most, however, collected at the top of the water bath. This did not appear to be a surface tension issue. Just to be certain, I cut a small piece of ABS off of my roll and tossed it into the water bath. After poking it with an old spoon to break the surface tension, it promptly sank. I'm not sure what to make of all of that. The undisolved residue of ABS quickly dried into a few lumps and a thin, transparent film.  I carefully collected the floating ABS in the waterbath with a ceramic spoon and ran it through a coffee filter. We appeared to have caked, powdered ABS. 
A few moments with a hot air paint stripper on its lowest setting yielded
an ABS powder with the consistency of fine cake flour.
It appears that dissolving scrap ABS in acetone and then separating it with
water yields a very high quality ABS powder that could easily be converted back
into filament or, for the more optimistic, used directly for printing.
I did, however, a few back-of-the-envelope calculations. It appears that I
could dissolve something like 25-40 cm^3 of ABS scrap in a gallon of acetone.
Given that I can buy a gallon of acetone at a retail price of about $15.00, it
seems obvious to me that I must have some method of recovering the acetone in
order for the method to be economic. Fortunately, acetone boils at 52 C, so
simply boiling it out of the water bath and collecting it in a condenser should
be no problem. It's latent heat of vapourization is about a quarter of water's
so the energy cost of recovery should not be excessive.
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I use acetone for pcb making. My experience is acetone evaporates quickly at room temperature.
Womans use them as nail polish remover, and they do not need to wash their hands afterwards because acetone just evaporates.
So I just does not get you boiling idea.
Yes, it will evaporate off by itself out of water at room temperature if I wait long enough. If I raise it to 52C, however, it comes out of the water immediately and I can recover the acetone in a condenser kept at room temperature. I want to get that acetone back, because it is expensive.
Think of a distillation system to make whiskey and you will get the general idea. :-)
And the HDPE not dissolving might bode well for a reprappable recycler!
The recycler would even be stable above the acetone's boiling point, making it conceivable that the recycler could recover powder AND the acetone...
Neat!
Acetone is extremely flamable. Please use extra caution. I would not try to boil it.
Yes, I've read the safety sheets on acetone. It is worrying. I'm certainly not going to try anything like that indoors.
Instead of trying to boil off the acetone, you might try sparging it off. Sparging is where you bubble dry air through through the solution, the solution component with the higher vapor pressure gets removed from the solution(in this case acetone). Then you pass the air that has passed through the solution through a condenser to collect the acetone.
Sparging takes a while, but it is perfectly safe. Also please note you must use DRY AIR! To dry out air pass it through some salt.
The bubbling might also help with the mixing process....
While there is no question that you could do that, I wonder how one would be able to produce sufficient amounts of dry air. If you use dessicants as you propose you are of course going to have to recharge the dessicants periodically.
The fact that some of the particles sank, some floated and some were neutrally buoyant suggests that this process is separating the three components of ABS (Acrylonitrile Butadiene Styrene). It would be interesting to investigate the mechanical properties of extruded plastic made from this powder.
After a day, there were very few neutrally buoyant particles of ABS left in the tray. Virtually all of the ABS that had not been skimmed off the top had sunk into a loose structure at the bottom of the tray.
I seem to remember apparatus for extracting essential oils from plants that used easily evaporated solvents like acetone. And operated in a recirculating fashion.
You don't actually need that much acetone if you create a recirculating system.
Acetone from a reservoir drops onto the the stock to be recovered which is in a mesh container allowing the solvent/plastic solution to drain through into the warm water bath.
The warm water bath would be indirectly warmed (double boiler or heater jacket)
The water being warm drives of the acetone leaving plastic and water.
The acetone vapour is condensed using a cold plate/finger (maybe a peltier unit, with the hot side providing the heat for the water) over the top of the reservoir container.
hmmmmm.
That's pretty much the system I have in mind. In that acetone boils at about 52 C, however, I'd planned to use a simple air-cooled condenser instead of a Peltier unit.
Acetone is sold in bottles made of HDPE.
I think the "floating" particles may well have been finer. I don't believe any of the three components of ABS would float in their pure form.
You can test whether there has been chemical separation by sintering (or re-melting) the powder and seeing how tough it is. This separation could not have been complete, because there is significant grafting (covalent bonding) holding together some fraction of the various types of polymer.
The variability of density in the separated ABS may be due to filler powders being used in your source ABS.
Likely present:
Fillers to add volume (I think talc is common)
Fillers to add strength: cotton strands or glassfibre strands
Fillers to add colour: dyes or pigments.
I checked with the supplier. No additives of any sort. He's always been very up front in the past and I have no sense that he is being anything but totally hones.
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In which your narrator continues his efforts to get something for Adrian to put in his granule extruder... This time I prepared 5 cm^3 of ABS from 3 mm filament by cutting it into 4-5 mm bits To this I added 20 cm^3 of pure aceto
Tracked: Dec 12, 07:35