Re-using and Recycling IPA in 3D Printing

Isopropyl alcohol (IPA) is the most commonly used washing agent for small scale and hobbyist resin modelling.  IPA has a boiling point of 80.37 °C (176.67 °F) and a composition of 87.7% by mass (91% by volume) isopropyl alcohol. Alcohol mixtures have depressed melting points, due to the fact that it forms an azeotrope with water.  What that means is that the boiling point of a mixture of IPA and water can be lower than the boiling point of either pure liquid. Practically, this means that simple distillation cannot completely separate all of the water from the alcohol.

IPA is not the same as ethyl alcohol, having a longer 3-carbon chain. It is the longest of the common forms of alcohol and is structurally different from the other 3-carbon alcohol.  It has a slightly bitter taste, and is not safe to drink.

Methanol	Ethanol	Propanol	Isopropanol

Recycling and Re-using IPA

Repeated usage of IPA in the washing process will result in a fairly contaminated mixture in 10 to 30 uses, depending on highly on how much resin remains print prior to washing, and the exact type and composition of the resin. The exact color will vary depending on additives and dyes in the resin, the type of resin, and the ratio of IPA to water.

The most volatile compound in the IPA (other than the IPA), will be the un-cured resins, still in monomer state.  The resins “cure” by the process of polymerization, and monomers are relatively small constituent “Lego-block” molecules.  Polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.  Polymerization can happen in many ways, either through exposure to air, to a co-reactant (like 2-part epoxy), or to light or heat (like UV resin).

This can be used to your advantage in reclaiming IPA. Try putting UV light on the IPA to precipitate the resin out. Then you can just filter out the now solid resin (I suggest a 150-micron filter, instead of the standard paint filter). The color will remain, but most of the resin will get removed.

I would let the UV treated IPA settle out for 3 to 5 days (or longer under some conditions), to allow precipitates to settle.  Do not pick up and look at your storage containers, even a small amount of agitation can set back the settling process considerably.  Also, although mason jars and gallon plastic jugs are commonly used, I would consider a large (1000 to 2000 ml) Erlenmeyer flasks for settling UV treated IPA.  The reason is in the next key step.

The next step is to filter the settled IPA.  Erlenmeyer flasks are specially designed for settling and removing precipitation from liquids. 

Make sure you follow these guidelines over a plastic tub or garbage bag lined box… IPA could spill and the compounds in the IPA can cause stains.  Always wear vinyl or latex gloves and a chemical rated respirator.

1. Starting with a standard 50 – 100 micron paint strainer, or a standard cone coffee filter, carefully pour the contents of the flask into a second container. Note, pour slowly and gently tilt the flask.  You will see the precipitates tend to stay in the angled bottom of the flask, thereby reducing the need to filter as much material, clog or even damage the filter.
2. Add a precipitation aide (also called a coagulants or flocculant) to the new container of IPA. You can find these aides online, they are called aluminum salts, like aluminum sulfate or aluminum chloride.
3. Let settle for 3 to 5 days
4. Using a 150 – 190 micron strainer, repeat step 1

The last step is distillation.  You can use a home or commercial distillation machine to distill the resulting IPA into 91% IPA.  This mixture level with water is sufficient for IPA washing, and is the best state you can reach with IPA exposed intermittently to air without humidity controls.

To create up to 99.97% IPA, you can follow treatment by using a molecular sieve.

Molecular sieves are crystalline metal aluminosilicates that have a three-dimensional interconnecting network of silica and alumina tetrahedra. They are used to remove water and other molecules with a critical diameter less than 4 Å such as NH3, H2S, SO2, CO2, C2H5OH, C2H6, and C2H4 123. The effective pore size of the sieve depends on the type of molecular sieve used. For example, a 4 to 8-mesh sieve is normally used in gas-phase applications, while the 8 to 12-mesh type is common in liquid-phase applications. The powder forms of the 3A, 4A, 5A and 13X sieves are suitable for specialized applications.

The following table summarizes the effective pore size and major applications of different types of molecular sieves:

Molecular sieves are like little traps, where the hole is the right size for some molecules but not for others.  Water happens to be a smaller molecule than any of the alcohol molecules, so water gets caught in the pocket, separating the two liquids.

The type used for removal of water from alcohol (ethyl, methyl, and IPA) is 3A or 4A.  This is often a altered form of Zeolite.   The process with iPA is simple:  add as small amount of the molecular sieve and then test with alcoholmeter, like a hydrometer or refractometer.  Keep adding a little of the sieve material to the mixture, until the alcoholmeter reads the percentage you want.

You can reclaim the molecular sieve later for re-use, by filtering it out then drying in an oven.

We hope this makes sense, and makes you feel better about using IPA in your 3D printing process.