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  1. #1

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    Gelatin coating modern pigmented inkjet prints

    Hi,

    I'm another one of those hybrid people trying to use traditional processes to solve modern problems. I am researching and testing the process of using photographic grade gelatin to coat modern pigmented inkjet prints. The problem we are trying to solve is that although pigmented inkjet prints offer great light fastness, the various inkjet coatings are porous even after printing and are susceptible to pollutants and the pigmented ink sits on the surface of the paper/coating and is susceptable to scuffing and scratching. Commercially available poly coatings change the aesthetic of the surface in a way that many of our clients do not like.

    The test samples include three different paper types: 1. Fine Art Rag paper that is matte. 2. Fine Art paper with a semigloss coating and baryta layer. 3. Commercial RC base, with semigloss coating.

    I have tested various dilutions (1%, 3%, and 7%) of photographic grade gelatin using two different hardeners (Glutaraldhyde and Chrome Alum). I have begun to achieve interesting results, but with complications that leads me to several questions.

    The initial results are encouraging because the gelatin coating does increase the scratch and scuff resistance of the pigmented inkjet print. The maximum benefit seems to be achieved at 3% dilution and the 7% just increases the possibilities for defects. The FA matte paper retains it's matte look while getting the benefit of the scuff resistant gelatin coat. The FA baryta paper, results in a more even semi-gloss and any gloss differential or bronzing characteristic of the uncoated print disappears. The Commercial RC paper also results in a more even semi-gloss and any gloss differential or bronzing characteristic of the uncoated print disappears.

    The dissolved gelatin while still at 30˚C or so, is very liquid or water like in viscosity. I found that trying to use a coating rod (No. 90 Coating Rod. Stainless Steel, and 5/8 inch diameter) is not working because of the low viscosity leaves too much solution on the print so I either roll on the coating with a HD foam roller, or just dip the print in the solution. In any case I let the excess solution sun off of the print and I hang the print for drying. But, the gelatin solution does not drip off perfectly evenly and I get some streaks, and some dried bubbles. I've tried adding a wetting agent (hyper wet) but at various dilutions I'm not seeing an improvement, and there is a residual film visible on the print surface from the hyper wet solution. So, I'll be experimenting with using isopropyl alcohol instead. Also, it seems that there are more streaks in the Dmax areas. I'm not sure if this is because of adhesion differences or other problems. Any opinions on wetting agents?

    I also have questions about hardeners. I understand that one benefit of hardening is protection form fungus growth... I assume there is a retained effect in the dried print.
    When I coat prints the gelatin is still warm, so it is very liquid. When the gelatin cools it firms up; some solutions more than others. I have varied firmness depending on gelatin dilution and hardener... actually, I don't know if the hardener is affecting that. Since the coating seems to dry fine on the print surface even if the remainder of the gelatin does not dry to a firm state (it will run if tipped to 45˚ angle) does this matter? Should I be trying to achieve a gelatin that will dry to firm state when left in an open container at room temperature? When carbron transfer people are talking about achieving the proper gelatin hardness, what are they looking for and does this matter with this application?

    I'm leaning towards using Glutaraldhyde for hardening an drop experimenting with the chrom alum, especially do to the caustic nature. Any opinions on the best hardener or if it matters?

    Lastly, I'd also like to add UV blocking to the gelatin. Does anyone have experience or knowledge about adding UV Blockers to gelatin coatings. I've considered using nano fine ZnO, but it's still a particle and does not dissolve. So I don't know if the small amount I'm able to suspend in solution adds any UV blocking. Any input?

    Best,
    Ken Allen
    kenallenstudios.com

  2. #2
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    Ken;

    To coat gelatin, the best concentration is about 5 - 10% and the best temperature is about 40 deg C. You will need a spreading agent, as the gelatin will spread unevenly without one. Glutaraldehyde will harden too fast in most cases. There are UV absorbers that are transparent and can be suspended in gelatin for a clear overcoat. Most analog films and papers contain these UV blockers. I don't have a chemical name at the moment.

    The best overcoat would probably be in the range of 100 - 200 mg of gelatin per square foot.

    PE

  3. #3

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    PE,

    Thanks for the information. I'm doing research and trying to acquire 3% Triton TX-100 (a nonionic surfactant) as a spreading agent.

    I assumed that Kodak used non-particle UV blockers. Many industries prefer ZnO due to the fact that it tends to stay where you put it versus more soluble substances that may tend to migrate away. I gues I will have to test to see if the small amount that I can keep suspended in the gelatin solution will provide useful UV blocking.

    Also, are you saying that hardeners may not be necessary at all?

    -Ken Allen

  4. #4
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    Ken;

    Hardeners are always useful to prevent problems with gelatin even if it is just as a Biocide. However they do help.

    ZnO is not used in photography. Either TiO2 as a base coat under the image, or an organic in an overcoat.

    PE

  5. #5

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    What about using potassium alum as a hardener?

    Don't carbon printers use this on the final support in the double transfer method?

  6. #6
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    Carbon printers also use chrome alum or formalin (formaldehyde).

    A sloppy wiki search revealed some UV absorbers - http://en.wikipedia.org/wiki/Ultravi..._and_absorbers

    Avobenzone - http://en.wikipedia.org/wiki/Avobenzone
    Octyl methoxycinnamate - http://en.wikipedia.org/wiki/Octyl_methoxycinnamate "...It is a clear liquid that is insoluble in water."

    It contasts these substances against "physical blockers" of UV radiation, such as titanium dioxide and zinc oxide. These seem to be generally opaque and used in sunblock.

    Photodegradation might be a problem with some ingredients (avobenzone it looks like), but here's some further reading... http://en.wikipedia.org/wiki/Sunscre...ve_ingredients
    If you are the big tree, we are the small axe

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    OK, I'm off to do more experimentation and testing. I'll report back... and probably have more questions.

    -Ken

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    Sandy King's article states:

    Drawing and watercolor papers for double transfer may be prepared as for single transfer with one important difference: in place of formalin or glyoxal use potassium alum as the hardener.

  9. #9

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    I don't think I can treat these papers as you would a water color paper. Because they have been coated for inkjet printing they have different properties and will behave differently.

    I'm not saying that the patossium alum (I have it in the form of Chrom Alum crystals from Photographers Formulary) is not the proper hardner. My question is how do I determine what would be the best hardener since I'm using it for a different purpose than Sandy and others.

    I'm thinking of an experiment where I start with 10% photographic grade gelatin solution, and create several batches where I add greater amounts of 5% Chrom Alum solution, and set each out in a flat tray to dry. Maybe from that I'll learn how the final solution of gelatin hardens and what mixture may work best to protect a pigmented inkjet print.

    .

  10. #10
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    I believe that potassium alum and chrome alum are totally different things. I dont' know how to determine the best hardener per se; each well established hardener will satisfy its purpose, but I'm not familiar with the differences to be honest.

    I've been able to get very good coatings of 6% gelatin with 1% sorbitol on clear sheets of melinex simply by pouring at about 115-119° onto a level surface, with the substrate flush (vacuum table ideally) and spreading quickly with a small hair-comb. For my purposes it works ok, but it can't coat to the edges evenly, so for your purpose you'd probably need something better. Trough coating, or float coating might be your best bets. Or, if you can print on a larger than necessary paper, pour gelatin and then crop to contain a perfectly coated section of the gelatin.

    10% gelatin might be too thick, but don't take my word for it.

    I would guess than any reasonable solution of a hardener will do the trick. I'd be curious if any hardener would be detrimental to the inks in an inkjet print however (just speculation).

    Please keep us updated, I think this is an interesting endeavor.
    If you are the big tree, we are the small axe

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