Hardening gelatin with eosyn and TEA

[koraks]

I'm experimenting with this and getting somewhere, but not quite where I'd like to be, yet. Specifically, I feel I need too much TEA to get the job done.

Take for instance an emulsion composed of:

• Gelatin 20% w/v
• TEA 20% w/v
• Eosyn 1% w/v solution, 4% thereof (so effectively 0.04% w/v eosyn)
• Some pigment; I use 1% Pb15:3 and then something like 20% thereof

Swell, melt and brush onto paper. Dry. Expose to intense light in visible spectrum (peak sensitivity at ca. 550nm) for a few minutes; I use a bright LED desk lamp for 10 minutes or so. Direct sunlight is also great. Then melt unexposed gelatin away in warm water. During the exposure, most of the exposed eosyn will turn colorless (or just break down, IDK).

So in principle this works. Now, what I don't like about this is how much TEA it takes. Currently I'm using a 1:1 ratio of gelatin:TEA, and if I reduce the TEA by even 50%, the whole thing breaks down. I can't imagine it should take this much TEA to get the job done.

Can anyone explain in layman's terms what the hardening process is in this case, and what factors may likely improve overall efficiency?
Thanks to @Cor for providing me with some eosyn to play with!

PS: I can find one mention on Photrio of this here: https://www.photrio.com/forum/threads/getting-dichromates-in-scandinavia-norway.142488/post-1894532
A more recent mention was on the Carbon groups.io, here: https://groups.io/g/carbon/topic/das_sensitized_gelatin/110999961
I suspect it's the same 'jim' in both cases.

 

[koraks]

Can anyone explain in layman's terms what the hardening process is in this case, and what factors may likely improve overall efficiency?

Answering part of my own question using this source: https://pmc.ncbi.nlm.nih.gov/articles/PMC5614854/

Visible light excites Eosin Y from the ground state into a triplet state. This then extracts hydrogen atoms from amine-functionalized co-initiators, such as TEA. The deprotonated TEA radical then initiates the formation of a radical center on the methacryloyl groups of GelMA.

The problem here of course is that they're not discussing gelatin as such, but methacryloylated gelatin. In this case, it's not really the gelatin that does the hardening - it's really the MA functional group added to it. The idea of GelMA is to use gelatin for its biocompatibility (i.e. it works nicely in the human body), but to add a group to it that modifies its crosslinking behavior so non-toxic agents can be used. If gelatin would crosslink particularly well without those groups, I'm sure they would use it that way.

So I'm starting to wonder if this route will work in the first place for printing. Then again, an initial experiment does show some promising results. Here's a snap of one of the tests I did yesterday - this was by far the most convincing one:

Exposure was in direct sunlight for something like half an hour. This was sufficient to also destroy the dye in the exposed areas, while the dye in the unexposed areas washed out quite well.

I'm still puzzled by what I read elsewhere about very short exposure times; this is certainly something I've not been able to replicate so far: https://groups.io/g/carbon/topic/carbrontransfer_re_non/9640857

Concerning this:

Currently I'm using a 1:1 ratio of gelatin:TEA, and if I reduce the TEA by even 50%, the whole thing breaks down. I can't imagine it should take this much TEA to get the job done.

I've now found the original message by 'Jim' (James?) that gives a ballpark for the amounts he uses:

It looks like a gram of dye will make 100 milliliters of sensitizer solution, and about 5 milliliters are needed for sensitising 100 milliliters of gelatin solution along with 20 milliliters of triethanolamine 1% solution, so a little goes a long way.

(the groups.io thing is an absolute nightmare to navigate, especially the archived threads...)

Working this into a ratio like I did above, it seems it would go something like this, assuming his gelatin load is around 10% w/v:

• Gelatin 10% w/v
• Eosin 1% w/v thereof 5% v/v, so effectively around 0.05% w/v
• TEA 1% thereof 20%, so effectively around 0.2% w/v

Let's calculate this back to ratios per gelatin weight:

(w/w or v/w to dry gelatin)	My random guess	Jim's 2016 ratios
Eosin 1%	0.2%	0.5%
TEA	100%	2%

So he uses more eosin, but in particular much, much less TEA.
A similar exercise was done by a discussant ('Rudy') here: https://groups.io/g/carbon/message/7574
He concluded Jim's mix is 31mg eosin to 0.1% TEA in a 100-125ml volume, which makes 0.1g TEA and 32mg eosin. But Rudy also notes that in an academic article, the optimal ratio for hardening PEG was 0.01mmol eosin and 0.1%TEA.

Looking at another reference provided by Jim, I find this:

20% gelatin with 0.0289 mM Eosin Y and 90 mM TEOA

OK, that actually talks about gelatin and mentions specific amounts - yay!
eosin = ca. 692g/mol, so 0.0289mmol = 20mg/liter, or 0.2mg/g gelatin (20% w/v gelatin = 200g/liter)
TEOA = 149 g/mol, so 90mmol = 13.41g/liter, or 0.067g/g gelatin

 

[koraks]

After having run some more experiments, I've not made much progress. Some observations:

It's hard to figure out exactly what "Jim" did (i.e. which exact ratios he used) and what results he got. There are two sets of TEA:eosin ratios that he may have used, and he mentions a 1-minute exposure time (which he believed was overexposed). None of the ratios I've tested give any hardening whatsoever with a 1-minute exposure. The best hardening I obtained was with the dramatically high 1:1 ratio of gelatin:TEA with also a fairly large amount of eosin. Even then, a 1-minute exposure time seems exceedingly optimistic - and it's certainly not convenient or economic.

Moreover, reading the literature some more, much of it focuses on hardening not gelatin, but GelMA, i.e. modified gelatin with a methacryloyl group attached to it. In those cases, it's actually that group that is involved in the crosslinking. The gelatin is just used as a backbone for biocompatibility. Had it been possible to effectively and swiftly harden unmodified gelatin using dyes like eosin, surely, the biomedics would have opted for that route?

To be sure, I've tested also a porcine gel just to make sure that for some freak reason, it doesn't work with a bovine gel. No luck and no difference noted.

I also note that the other guy on groups.io didn't seem to have been able to replicate Jim's success. I did find one mention of @keesbran who noted that a student of his had been experimenting with riboflavin in a similar way, which did allow for hardening, but apparently also very, very slowly. That aligns with my experience so far.

Mind you, I can sort of see how the eosin-approach might work for e.g. hardening final support papers for a single transfer dichromate-based process. You could indeed sensitize the gelatin with eosin and TEA, then hang them up for a day or so during which they will effectively harden. If there's an excess of TEA, this will allow all the eosin to decolorize (the TEA is essential for this!) and you end up with no dye stain. What the TEA may or may not do in the long term to the stability of the gelatin, I really don't know. But eosin + TEA might be a feasible (non-toxic, eco-friendly) alternative to e.g. formalin, chrome alum, glutaraldehyde etc. for this kind of hardening.

For actual printing/image formation...I'm far from convinced at this point.

To be continued...maybe?