Thursday, February 23, 2006


Grinding Frozen Polymorph

I put about 20g of Polymorph granules (~3 mm in size) in my freezer (-20 C) to see if that would make it easier to grind. I discovered two things:

1. Polymorph is incredibly tough - the coffee grinder blades make the granules go round and round, but they don't break up, abrade, or indeed change in any way at all, and

2. If you put a whole coffee grinder in a freezer its shaft locks up when the lubricant freezes...

Yes that CAPA is REALLY tough. I found that feeding a few granules at a time through a hand cranked meat grinder does the trick...

except that you have to run a sample back through 20-25 times to get it fine enough to go through my 6.35 mm pump. With 13.5 that shouldn't require that degree of fineness.

Adrian! Why don't you lean on your Solvay guy for some CAPA 6806 that's already powdered? Mind if you want shoulders somewhere between those sported by Russel Crowe and Arnie Schwarzenegger the meat grinder route is a good one! :-o
Regarding methods that cut rather than shatter, have you tried warming the stuff up a little so that it cuts easier?
I put a beaker of water (to absorb the radiation if nothing else did and prevent the magnetron xpldng) and a beaker of polymorph into the microwave. Yes - you can melt polymorph/CAPA 6800 in a microwave. This may well be generally useful, as it gives a way to melt it without wetting it and without flames.
PS After the freezer bit, now this, my family are giving me the sorts of looks to which Vik must be innured after his concrete-in-the-oven experiments...
Perhaps a goofy idea, but since CAPA melts at below the boiling point of water, it ought to be feasible to finely divide it by running it through a blender with hot water, and then quenching the suspension with some colder water. Then just filter it out with a coffee filter. This would pretty well saturate it with water to the extent that's possible, but we don't yet know if that's really a problem.
It's denser than water.
Have you tried a rock tumbler or similar device? Any sort of drum with some sort of grit in it might do. Would probably take a while to grind up the plastic and then there's the task of separating the powder from the grit. I guess a ball mill probably wouldn't work.
//it ought to be feasible to finely divide it by running it through a blender with hot water//

I've actually done this with HDPE. Made a hell of a racket and the plastic was barely affected. Might work differently with polymorph.
I remember from a long time ago that one of the people that regularly commented on the blog mentioned that you could dissolve polyolefins in acetone and then recover them as an extremely fine powder by filtering(?).

Do you suppose that dodge would work with polymorph or CAPA?
If actually disolved, I wouldn't expect that to work, but if you were to spray the solution, and let the droplets evaporate in midair, you'd probably get a very fine powder.
If it's acetone, I'd recommend spraying into water rather than air, to reduce the fire hazard. Water will also pull the acetone out of the polymer faster than air does (a trick I learned from an ex-girlfriend who was impatient in drying her nail polish), and make it easier to contain the dust.
On second thought, forget the spray gun: blend dissolved polymorph quickly (violently?) with an excess of water, and the smaller droplets in the suspension will lose their acetone the fastest, giving a relatively monodisperse particle size.
Assuming there was some chemical reason for not using the water method, you could spray a highly dilute solution of CAPA into a stream of warm CO2 or argon (available at any welding store) to prevent combustion. Pipe the CO2 through a hair drier or heat gun first. Would require a drop column or other method of collecting the dried powder.
Alternative to spraying you could place the solvented polymorph in a commercial ultrasonic humidifier available at walmart for 35$ but you'd have to coat the plastic surfaces to prevent them from dissolving. This produces particle sizes in the lower microns believe it or not. This method is currently at use in the production of micron scale particles of zinc oxide.
***Here is another idea.***

Yup, I thought of that one some time ago. Initially, I was going to alter a standard cooking pressure cooker for a prototype. That didn't work out so well because you need a submersible heater rather than applying heat to the bottom of the pot. I didn't have the money to build a purpose built device at that time.

There is no reason that the concept won't work and work rather well. While it is not a continuous process like the screw extruder it can hold a relatively large charge of polymer. It also has the advantage of being a very low tech device that could handle scrap plastic in large chunks.

I see only two problems with the concept.

First, You need about 40 atm to extrude the polymer I'm not sure you could get that at the temperatures that you need to melt it.

Second, it won't work the same way for polymers that are less dense than water. Sadly, those polymers are tending to be much less expensive than what we are using now.
Steve, the point of using the hot water in the blender is that the CAPA would be a fluid at the time the blender was turned on; Melting point below the boiling point of water, remember? Running the blender should produce fine droplets of CAPA suspended in the water, which could then be quenched by mixing with colder water.

However... Look, the point of using a screw to extrude is that it lets you generate the really high pressures we use to push decent flows of viscous materials through complex dies. Often thousands of psi.

But, still, it's just a *pump*.

Since the CAPA can be melted at perfectly reasonable temperatures, and we're only talking about a few tens of psi, why not just use an off the shelf pump out of a pot of molten resin on the stove?
I thought someone would bring this up by now, but how about a shot tower? melt polymer however you like, dump chunks into a cylinder with a perforated bottom and apply pressure from top. You might or might not have to spin a blade set underneath to break up the threads (depends on surface tension and desired ball size) allow the results to fall 10-20 feet into cold h2o. uniform granueles(sp) with medival tech.

On the other hand, if the acetone and h2o trick will work you would get a finer particle size, less energy use and a closed system (heat h2o to drive off acetone, condense and reuse).
Gotta love Google, Capa seems to be soluble in acetone:
***Since the CAPA can be melted at perfectly reasonable temperatures, and we're only talking about a few tens of psi, why not just use an off the shelf pump out of a pot of molten resin on the stove?***

Two reasons. First, 40 atm is about 600 psi. That's not a few psi.

Second, you can't do that for the same reason you can't use an ordinary pump to pump hot water. The seals will perish.
Heh. Sorry, read "40 atm", and thought "40 psi" LOL.
Gad, life would be so nice if it was 40 psi. :-)

Still, 40 atm is a darned site better than the 600 atm that you find in a commercial extruder. Adrian and Vik have worked miracles in that respect.
Has anyone tried adding a small amount of acetone to decrease viscosity? It would have to dry from the shaped part which would entail shrinkage and possibly voids though. I also don't know what 40atm would do to such a blend but it wouldn't have to be heated to make it liquid.
The problem with acetone as I see it is the problem with using any solvent.

It costs money.

It's hard to recover and reuse.
Right. I got the kitchen blender and, under carefully controlled conditions (i.e. my wife wasn't home), I put in a few granules of CAPA. I poured in boiling water, waited till the CAPA had gone transparent, then turned on the blender. After about 20 secs I quenched the thing with a larger volume of cold water.

Result: One lump of CAPA with a few air bubbles in; just enought to make it float.

Now. Where does she keep her nail varnish remover?...
It coagulated!
Well, maybe the meat grinder + warm CAPA will work better.
What about a cheese grater? Say, something suitable for grating parmesan? Something like this might work. Add a feed mechanism and a motor to crank it. Something like that?
Having run this Universal #2 meat grinder for several hours now, I see no way that that cheese grater is going to work.

My Universal #2 has no gearing, an 8 inch radius turning handle and a VERY solid clamp connection to my work table. While I'm not a body builder, I'm not a weak person. I had to feed through just a few pellets of CAPA through at a time in order to keep the #2 turning. As well, I had to tighten the table clamping arrangement several times and make an exceptional effort not to eccentrically load the handle to keep it attached to the table.

Looking at your grater it looks to have a 3-4 inch radius turning handle and no clamping mechanism at all. As well, it has what looks like a fairly light plastic body. I don't think it would work at all, never mind well. :-(
Have you tried warming the CAPA until it is almost liquid before grinding it? I suppose you'd have to warm the grinder up too or else it would just immediately cool against the metal. This is turning out to be a challenging problem after all...
How about trying a crazy redkneck idea: load some into the end of a shotgun charge and blast it through some window screen like elmer fudd.
Of course, we could just do it the way that Solvay is doing it which is spraying it with liquid nitrogen and grinding it.

It's not like liquid nitrogen is expensive or particularly hard to come by.
liquid nitrogen is a might bit hard to come by in the backwoods of southern Oregon, much less the plains of Africa. Is this bead size the product of a standard manufacturing route? Material processing is going to be an issue whenever a stiff polymer is used. Extruding wax for lost investment casting probably won't have these issues but metals, ceramics and heavy duty polymers are all going to pose these sorts of challenges.
Of all the ideas here, spraying CAPA dissolved in acetone underwater followed by filtration sounds about the best to me for producing tiny particle sizes. Acetone evaporates and condenses at very low temps, so recovery from water shouldn't be too difficult, no worse then stilling some alkyhol plus it is almost universally available anywhere in the world that paint is used.
As for reducing ceramics and metals Steve is right on with the rock tumbler/ball mill idea. The folks over in pyrotechnics have a super do-it-yourself set of ball mill plans I'll try to find if anyone is interested.
If you've got a hospital, vetinarian's clinic or a welding shop anywhere nearby you've got a gas supplier. Those suppliers pretty much always keep liquid nitrogen. You need it for preserving semen for artificial insimination if for no other reason. I spent 12 years in Africa in my younger days, so I'm not speaking heresay on that.
Mind, it would be interesting to see if the acetone/water trick could be pulled of at small scales.
A gallon of acetone costs me about 5-6$ what's the smallest amount of liquid N cost? Then there's storage problems and frost burn dangers. If I'm gonna make a set of plastic chairs how much liquid N will I need to grind that much plastic? On the other hand cryogenic freezing doesn't have the selective chemistry problem that acetone/spraying has. Probably be a great way to grind just about any plastic or organic material. Ultimately what is really needed is a powered grinder so that it doesn't require shoulder pain.
I say what's needed is an extruder that'll take larger chunks. If it starts out with heated rollers to make a sheet, then cuts it across its width & extrudes the ribbons, it can take anything from granules to a recyclable object.

But that's probably a long way off.
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