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I'm working with a 3D printer that uses PVA filament as a support material. After a 3D printed part is made, it is put in water for some time to dissolve away the PVA supports.

I'm trying to speed up the process by using a heated magnetic stirring plate with a pump connected to a filter. The filter is for removing the already dissolved PVA from the water solution so that the solution doesn't get saturated.

I plan on using a 2L container of water and heating it to about 60-80 degrees C. There would be about 10-50 grams of PVA being dissolved at a time.

I have two main questions:

  • What kind of filter(s) could I use for PVA in a water solution?
  • Do I really need a filter? I.e. would 50 g of PVA have issues dissolving in 2L of 80C water?
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  • $\begingroup$ PVA? $\endgroup$
    – Martin - マーチン ♦
    May 31, 2017 at 6:52
  • $\begingroup$ I don't have a ready answer for you, but I don't think that a 2.5% w/w would pose much of a problem and the whole filtering process is probably completely unnecessary. This publication DOI 10.1007/0-306-46915-4_3 might give you more information, I don't have access to it. $\endgroup$
    – Martin - マーチン ♦
    May 31, 2017 at 7:20
  • $\begingroup$ I tend to agree with @Martin-マーチン, a 2.5% w/w solution isn't going to be a problem, even with a moderate amount of repeat uses. $\endgroup$
    – J. Ari
    May 31, 2017 at 17:04
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    $\begingroup$ Since you mentioned 3D printing, have you considered there's a separate stackexchange site for it? $\endgroup$
    – Pritt says Reinstate Monica
    Jun 3, 2017 at 4:27
  • $\begingroup$ You might be able to add a brine solution and crash out the PVA. $\endgroup$
    – A.K.
    Jun 24, 2018 at 0:35

3 Answers 3

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Rig up a column that you can fill with granular activated carbon (GAC). Throw batting on either end to prevent carbon fines from entering your dissolution chamber. Replace the carbon when you reckon it is no longer working. If you can afford the heat, throw a cooling coil in front of the filter.

Another method is indicated in this patent: US4078129. They add 0.25g sodium borate and 0.6g sodium sulfate per liter to precipitate between 0.03 and 3% PVA. This may not be a good solution if you are prone to overdosing, as it would say, goop your soup, when you go to do the next batch.

A third method seems to be already popular at the large scale: ultrafiltration. There are many papers discussing its usage for waste water discharge applications. Use the smallest pore UF you can find (50kDa is what I've seen in the literature). The pressures required to operate it are pretty low. Be sure the temperature of the solution is <50 degrees C prior to entering the UF (almost all membrane applications fail >50 degrees C). Run it outside-in. Don't dead-head it. You will generate a small concentrate stream. Perhaps try method 2 on that to reduce waste generation.

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The first thing that comes to my mind is Reverse Osmosis, since it can exclude large molecules dissolved in water. I don't think that a typical filter (eg: paper filters for buchner apparatus) would work if the polymer is actually dissolved in water and it's not a precipitate.

An alternative (maybe cheaper) idea is the following: since your mixture is heated at 60-80°C, you could attach a side apparatus, separed by a valve from the main one, and gradually distill at 100°C the water from it, with supplemental heating (or even at 60-80°C, if you use a vacuum system). The solid residue will be your polymer. The clean, distilled water could be driven back to your mixture. The valve will then be opened again, and the procedure repeated.

This would allow you to perform the process without using any filter.

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  • $\begingroup$ distillation could work but would not be practical as it would be as slow as the PVA dissolving and thus wouldn't provide a significant speed up. $\endgroup$
    – Conor Patrick
    May 28, 2017 at 3:06
  • $\begingroup$ Why all the way to RO? A UF membrane should reject a polymer of significant size (>50kDa). A carbon filter might even do the trick. $\endgroup$
    – Pete
    Jun 4, 2017 at 10:06
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Back in the day we used carbon filters on process water that was used to wash circuit boards. We also used PVA solder masking dots. The PVA would accumulate on top of the carbon filters and increase the pressure drop until we had to change them. These were 12 Cu ft filters and you could see the carbon media compress over time as a white film (PVA) would build up on top of the filter bed. On a small scale I think that the carbon filters would be readily available & semi inexpensive to try.

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