[jjoll]

Hello everyone,
I was just wondering if there is any type of transparent ink that could be permanently activated by heat or intense light (laser).
Preferably I am planning to use a laser light to activate the ink.
I am trying to coat a piece of metal (or paper or etc.) with this ink (which is invisible).  Then I want to write on the coating using a laser light. I am supposed to shine a laser beam to the coating in order to draw fine lines/drawings. The laser beam is supposed to activate the ink (reveal its color) as it pass through. The laser is supposed to activate the color of the ink and lock it (meaning, the color is supposed to stay on coating after laser has passes through).
I was just wondering if this is possible and if there is an ink with this kind of characteristic or if I can modify an ordinary ink to behave like this.
Thanks

[Enthalpy]

Hi jjoll,

my bet is that you'll have to wipe or wash off the untreated pigment-dye-ink after patterning, since sunlight would blacken it where the laser didn't.

A first idea is that laser printers do almost what you want. You'll have to adapt the process to your part, which isn't necessarily as easy to bend and heat as a papersheet.

Thermal printers did it too but are abandoned because the paper blackened over time. Laser printers are far better.

I could secure a printed paper sheet on metal with a cyanoacrylate glue spray over the paper, then a plastic film over the paper. The PE film doesn't adhere but lets the glue harden and gives a smooth surface. It needs some handwork, and "I didn't test" (it probably fails) over time under sunlight.

You could also cover the target with thin MMA (monomethyl methacrylate) loaded with a bit of pigment, for instance carbon black which is eternal. Operate under red light, pattern with a 405nm laser (from a DVD burner) which polymerizes the MMA to PMMA (a plastic, called Plexiglas by one manufacturer), wipe off the unexposed monomer. That should be decently durable. The MMA monomer stinks and its vapour is bad for the eyes, so you need a working area that evacuates the vapour.

Early machines for stereolithography (3D printing) use this process, except that their MMA isn't loaded with pigment, and they add a new layer on a previous one that has already the proper orientation and position as it has been obtained by the same wiper. So adapting such a machine needs limited trials.

[jjoll]

Hi jjoll,

my bet is that you'll have to wipe or wash off the untreated pigment-dye-ink after patterning, since sunlight would blacken it where the laser didn't.

A first idea is that laser printers do almost what you want. You'll have to adapt the process to your part, which isn't necessarily as easy to bend and heat as a papersheet.

Thermal printers did it too but are abandoned because the paper blackened over time. Laser printers are far better.

I could secure a printed paper sheet on metal with a cyanoacrylate glue spray over the paper, then a plastic film over the paper. The PE film doesn't adhere but lets the glue harden and gives a smooth surface. It needs some handwork, and "I didn't test" (it probably fails) over time under sunlight.

You could also cover the target with thin MMA (monomethyl methacrylate) loaded with a bit of pigment, for instance carbon black which is eternal. Operate under red light, pattern with a 405nm laser (from a DVD burner) which polymerizes the MMA to PMMA (a plastic, called Plexiglas by one manufacturer), wipe off the unexposed monomer. That should be decently durable. The MMA monomer stinks and its vapour is bad for the eyes, so you need a working area that evacuates the vapour.

Early machines for stereolithography (3D printing) use this process, except that their MMA isn't loaded with pigment, and they add a new layer on a previous one that has already the proper orientation and position as it has been obtained by the same wiper. So adapting such a machine needs limited trials.

Thanks Enthalpy for your answer,
I have a few questions. Is MMA a solution?
You said: “ cover the target with thin combination of MMA+pigment and then use 405nm laser to polymerize it. But what kind of pigment are you suggesting to add? Will the MMA+pigment be transparent under normal room condition? I am looking for something (dye, pigment,…) that is transparent and colorless in normal room condition and the color will be activated only when the laser hits it. Also is MMA toxic?
let me explain more on what I am actually trying to do. I am trying to develop a laser printing method in order to use on a sheet of multi-layered photonic crystal. what I am actually working on is: a sheet of polymer based paper that is made of 4 layers and I am trying to write/engrave on the surface of second layer so what I am thinking now is to somehow inject a transparent solution to the second layer and then emit the laser from the top (laser is supposed to go past first layer) and then write on the surface of second layer (by activating the pigments that we have injected).
thanks again

[Enthalpy]

MMA is a liquid that polymerizes (makes a solid plastic) with light
https://en.wikipedia.org/wiki/Methyl_methacrylate
it is slightly toxic but the polymer isn't. The MMA vapour irritates the eyes.

I proposed to harden MMA selectively, them keep only the cured places, because I don't know a convenient transparent material getting a colour upon exposition. You want to print a layer buried in a sandwich: would it be possible to pattern the colour layer when it is naked at the surface, and finish the sandwich later by adding the top layers?

Maybe other people here know some compound that gets coloured upon laser exposition, but I fear that sunlight exposition will colour the whole layer over time. Same difficulty if reacting to heat brought by the laser.

Sure some compounds darken to light, beginning with the silver salts that were used for photography. But after exposure, you must possibly reveal them, and for sure use some fixative to suppress the sensitivity to light. How to do that if the sensitive layer is buried in a sandwich?

In the MMA route, many pigments would work, beginning with carbon black for black colour. These pigments would not react to laser exposition: they would be removed together with the MMA where it hasn't been polymerized by light. The proportion of pigment in MMA must allow it to harden at depth, hence receive a little bit of light, while looking dark.

And how thin shall the colored layer be?

[jjoll]

thanks for the reply.

MMA is a liquid that polymerizes (makes a solid plastic) with light
https://en.wikipedia.org/wiki/Methyl_methacrylate
it is slightly toxic but the polymer isn't. The MMA vapour irritates the eyes.

I proposed to harden MMA selectively, them keep only the cured places, because I don't know a convenient transparent material getting a colour upon exposition. You want to print a layer buried in a sandwich: would it be possible to pattern the colour layer when it is naked at the surface, and finish the sandwich later by adding the top layers?

Maybe other people here know some compound that gets coloured upon laser exposition, but I fear that sunlight exposition will colour the whole layer over time. Same difficulty if reacting to heat brought by the laser.

Sure some compounds darken to light, beginning with the silver salts that were used for photography. But after exposure, you must possibly reveal them, and for sure use some fixative to suppress the sensitivity to light. How to do that if the sensitive layer is buried in a sandwich?

No,
I need to think about the MMA route. I am not sure if it is feasible due to the being toxic.
Also it needs to be approximately the same thickness as paper

[Enthalpy]

You have to remove the uncured Mma anyway because it remains sensitive to light and because it is already loaded with pigments. The remaining Pmma is not toxic: it's just the Plexiglass or other tradenames that make plastic windows.

As thin as paper: easy with carbon black for instance. Indian ink is thinner than the supporting papersheet and is black. You have to limit the concentration of carbon black so the curing light reaches the depth to polymerize the Mma there. I bet other pigments are as efficient, for instance metal oxides like cobalt blue: a few particles adhering on a papersheet suffice to color it.

Or think of colour newspapers. You don't feel any step in thickness where the colour begins.