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The conundrum

I have a security printing ink that I make by adding an IR-absorbing dye to a 50/50 mix of Isopropyl Alcohol and distilled water. The ink solution is pale green but dries virtually invisible to the human eye.

The problem is that when the ink is applied to the stamp the ink evaporates within seconds but I need it to remain viable for ~10 minutes. Even if I shortcut the 10 minutes to just a few seconds, the ink evaporates enough to cause an uneven distribution (i.e., it is faded in some areas.)

Requirements

  1. Provides an even distribution of ink (to me, this means viscous)
  2. As little bleeding as possible (to me, thin coat on stamp, also viscous)
  3. Stays viable on a stamp for ~10 minutes
  4. Allows the final inked product to be virtually invisible after ~24 hours

A solution that works (almost)

I have solved this problem before with a different ink. It was a UV-reactive ink which uses an identical IPA/water solvent. I was able to meet all of my requirements by adding 14% Glycerin to the solution. The increased viscosity really helped with consistent and clean (no bleed) marks. The slower evaporation gave me plenty of time to stamp. There was much rejoicing.

However, the Glycerin has an unwanted side effect with the IR ink in that it remains visible on the substrate. I'm guessing this is because the Glycerin prevents the ink from fully drying. I tried heating final product to see if that dried out the Glycerin but it didn't help.

You may ask why the UV ink ended up being invisible (requirement #4). Simple - that ink is clear to begin with.

My questions

  1. I think what I need is a viscous fluid (maybe not as viscous as Glycerin) that evaporates slowly but fully in ~24 hours. Does such a wonder exist?
  2. Glycerine solves all of my problems and creates one (visible marks.) Is there a way to use Glycerin, while still getting an invisible mark? (I've tried varying the amount of glycerin, but it's a trade-off of usability and visibility.)
  3. Is there another chemical (hopefully easily obtainable by an individual) that could work as well as Glycerine while also allowing invisible marks?
  4. I would love to understand this problem better so I can troubleshoot it myself (I'm a software engineer, not a chemist.) Any random insights would be welcome.
  5. It seems to me that viscosity is inversely proportional to evaporation rate. Is that true?

Background info

I am trying to apply this ink to long thin strips (~7mm each) of a porous paper substrate (wrapped around plastic.) This needs to be done hundreds of times. To simplify the alignment process, I have a large rubber stamp with a series of ~100 duplicate strips embossed into it. This stamp is placed face up into a rig with alignment grooves on either side. From here, I apply the ink to the stamp and then slide each strip, one by one, into a pair of adjacent grooves and press down to stamp. This is much faster than re-inking a single thin stamp for every strip. This process takes about 10-15 minutes.

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  • $\begingroup$ (1) Am I correct to assume that you do not want to change your inking stamp setup which you already have designed? (2) To process a batch of strips takes 10-15 minutes, so am I correct to assume that most of that time is fiddling with loading the paper strips? (3) How long between inking the stamp and stamping the strips? (ie can you load strips, then ink stamp, then stamp to reduce cycle time between inking and stamping?) $\endgroup$ – MaxW Apr 22 at 22:15
  • $\begingroup$ (1) Before arriving at this solution, I tried many things, including hacked up inkjet printer. I'd prefer to keep using it if possible. (2) Time is relative to the number of items. Just a few seconds each: pick one up, line it up in the proper adjacent grooves, stamp evenly, set it aside, grab another. (3) Once I ink the stamp, I start to stamp immediately. First one lands on the stamp ~4 seconds after the stamp is fully inked. The problem is, by the time I've finished inking the entire stamp, the first part I inked is nearly dry with pure IPA because it's a thin coat of ink to avoid bleeding. $\endgroup$ – pauln Apr 22 at 22:26
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This is more of comment than a solution.

It has often been noted here that chemistry is an experimental science so often you just need to try things.

There are a number of considerations here. It seems like it takes on the order of 10 minutes to stamp a batch of "strips" (whatever that is...) and that:

  • You need a stamp with multiple "stamp faces" to stay wet that long.
  • You also want the ink solvent to "dry" so that the ink solvent doesn't leave a noticeable "watermark" on the paper.
  • You want to use a minimal amount of solvent so that the dye stamp doesn't bleed into the paper.

(1) The dye must be fairly polar to dissolve in a solution of 50/50 mix of isopropyl Alcohol (2-propanol) and distilled water. You can look at a table of solvents with their polarities listed at this webpage. You can try other solvents that you have, or mixtures that you can make, to get a rough idea of how polar the solvent (mixture?) must be.

  • Reasonable to assume about 1 gram/ml for any of the solvents listed. Estimate polarity of a mixture by assuming ideal solution, ie that total solvent polarity is equal to molarity of a solvent times its mole fraction. (ie 50 wt % is not the same as 50 mole %.)

  • You can check to see if the ink dissolved by seeing if the solution is "clear" - in other words shine a bright light through it. If you see scattering then the dye didn't dissolve.

  • You can probably estimate from your work with glycerine how concentrated the dye must be to be invisible when dried, but visible under fluorescence. (I'm guess that the dry dye itself is green. So you don't want to "paint" the surface with particles of the dye. Rather you want the dry to be carried into the fibers of the paper as the solvent carrying the dye is adsorbed.)

(2) Look at other solvents that have been used for inks, for example on this webpage. Choose some solvents to test which have a polarity that works from step (1). Now look at solvents vs vapor pressure. The lower the vapor pressure the slower the solvent will be to evaporate.

  • Note that a mixture won't evaporate homogeneously. So with an acetone/glycerin mixture the acetone evaporates preferentially to the glycerin.

  • There is also an effect here in that the ink solvent is adsorbed into the paper not just sitting on the surface of the paper like a drop of such solvent on glass. So the correlation between vapor pressures and drying time would be "good" but certainly not perfect.

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  • $\begingroup$ I mix it in the IPA first (91%) then add water. It takes ~1.5 hours until the particles are no longer visible in the IPA. It does not dissolve in water. (I discovered the use of a flashlight almost right away.) I have tried many things (my kitchen table is full of chemicals) including Hypromellose, xanthan gum, and Acrylates C10-30 alkyl acrylate cross polymer. . $\endgroup$ – pauln Apr 23 at 0:18
  • $\begingroup$ This is good info for things to look into and learn from. Thank you. Side question: the flashlight works for solvency, but how do you know if two miscible liquids are fully mixed? $\endgroup$ – pauln Apr 23 at 0:36
  • $\begingroup$ If there are two phases (two liquids that don't mix) you should be able to see a line between the too. Think of mixing a salad dressing with oil and water. The oil layer floats on top. $\endgroup$ – MaxW Apr 23 at 3:26
  • $\begingroup$ OK, just wanted to make sure. I have been waiting for the dye to dissolve, but just shaking the bottle for a few seconds for liquids until the solution looked homogenous. $\endgroup$ – pauln Apr 23 at 5:00
  • $\begingroup$ You said, "The higher the vapor pressure the slower the solvent will be to evaporate." The murov table seems to indicate the opposite. Am I missing some basic understanding? My understanding: A substance with a low vapor pressure will have less volatility, reaching equilibrium at lower pressures (due to less volatility pushing against the pressure.) This adds up to lower evaporation rates. $\endgroup$ – pauln Apr 23 at 15:59

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