Thickening and/or slowing evaporation of IPA-based ink

Question

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.

UPDATE

When I originally wrote this post, I was unable to openly discuss the underlying application, which hindered my ability to describe the problem clearly. I have since open-sourced the application, which is a technology for card magic that uses a small computer to scan bar codes marked on the edges of cards.

If this stimulates your interest in answering this question, you will find the GitHub project here. In the documentation, I explain the process of marking the edges of cards using a stamp and a variety of inks.

The section on IR Absorbing Inks covers the challenges with working with this ink. Specifically, this section outlines these challenges and details the extensive process that I use to mitigate them.

As describe in the original question, I was able to mitigate these same issues for UV reactive inks by using Glycerine, which is outlined here.

The intent of this question was to find a way to simplify the mitigation process of working with the IR absorbing inks, similar to working with the UV reactive ink.

Answer 1

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.

Answer 2

The vapor pressure of IPA at 2.4 degrees C is 1/4 that of its room temperature value (ref).

The viscosity of IPA also increases dramatically as temperature cools to ~0 (ref).

Could you simply conduct the evaporation with the same solution but in a colder environment? A normal household fridge should be in the right range for what you're looking for.

The viscosity will also likely increase as evaporation happens since the concentration of the dye will increase, so you may be able to start the evaporation process in the fridge and then let warm to room temperature (or higher if needed) to fully drive off the solvent within your time constraints.

A possible process could be to pre-cool your solution to below 0 degrees Celcius, conduct the stamping at room temperature (within your 10 minute window), place the stamped substrates in the fridge to allow for slow evaporation over 8-18 hours, then warm to room temperature to drive off remaining solvent.