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.)
- Provides an even distribution of ink (to me, this means viscous)
- As little bleeding as possible (to me, thin coat on stamp, also viscous)
- Stays viable on a stamp for ~10 minutes
- 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.
- 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?
- 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.)
- Is there another chemical (hopefully easily obtainable by an individual) that could work as well as Glycerine while also allowing invisible marks?
- 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.
- It seems to me that viscosity is inversely proportional to evaporation rate. Is that true?
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.
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.
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.