Corona discharge polyester subbing

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by hrst, Nov 28, 2009.

  1. hrst

    hrst Member

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    I have now built a working corona discharge unit in order to try subbing polyester (PET) sheets, to prevent the emulsion from loosening during processing.

    It consists of 24V DC supply, a HV source made from flyback transformer (from color CRT television), one power transistor and two resistors. Transistor's base current is taken from feedback winding and it starts to oscillate, generating 5 kV voltage (measured at 0.6 mA current). The complete unit also contains a meter for current (0 - 1 mA).

    High voltage + is then fed to a corona wire taken from a copier, and ground is connected to a copper plate under the corona wire.

    A PET sheet is placed on the ground plate. Push-button is pressed to enable HV, and then you drag the sheet.

    This seems to work: the sheet is heavily attracted by the ground plate and it needs much force to drag the sheet, when the button is pressed. During this, unit produces quite loud hiss noise from corona discharge and draws HV current of 0.2 mA, making the total corona power of 1 Watt at 5 kV. And a familiar ozone smell like with copiers & laser printers. :smile:

    I made some coatings on corona-charged sheets. I coated them in a few minutes after coronizing. I will post the results in a few days after they have dried&hardened so I can try to process them.

    I attached some images of this cool unit :D.
     

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  2. Emulsion

    Emulsion Member

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    Well done! Unit looks great. Please keep us informed of how well it works.

    Did you do a test of coating using the corona discharge preparation versus coating without the preparation?

    You have set me thinking of alternative High Voltage sources that are readily available.
    -Neon sign transformer with a rectifier on the output. (5-30kV).
    -A car ignition coil with a driver circuit
    -A corona module from an older scrapped photocopier. (The one I have is 5kV)

    Emulsion.

     
  3. Photo Engineer

    Photo Engineer Subscriber

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    What you have to do for "proof of concept" is to coat on a sheet of the film before discharge and on a sheet after discharge. These both have to be identical coatings. Then you have to demonstrate the adhesion properties of one vs the other. This proves that the sheet "before" did not have some property that enabled the coating process.

    Very good work.

    PE
     
  4. hrst

    hrst Member

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    I coated one sheet which I coronized only for the lower half. This should allow me to compare with the same coating.

    In addition, I have coated about 3-4 or more these very same A4 PET sheets before, with same emulsion and same coating blade, and every time the emulsion has started to loosen at least at the edges, after 2-5 minutes of processing. If this seems to stick, I'll process longer and longer. If it survives a longer-than-normal process with vigorous agitation then it's probably okay. That will be my test.

    If it doesn't work, then I'll try to rise the voltage. The current design can give at least 8 kV. It's possible to do but the risk of arcing will become higher.

    My coatings still need to improve, though. Our last emulsion (the one that fogged, good for practicing though) is also way too dilute and thus hard to coat. (Of course I could add gelatin.)

    Emulsion, thanks for list of HV sources; In fact, if this works, I think that the whole unit can be found in a photocopier or laser printer; just remove the melting unit and toner unit and you have a complete product! But that would have been too easy, I had to construct something :wink:.
     
  5. Kirk Keyes

    Kirk Keyes Member

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    How about a photo of it running with the room lights turned down low so we can see the glow?!
     
  6. hrst

    hrst Member

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    Failure!!

    The two identical coatings, one with corona discharge and one without, both started loosening after 5 minutes of processing. Loosening looks very identical in both samples.

    Coating thickness doesn't seem to affect much. Thin coating took 1 minute to diffuse thoroughly (darkened from the base side in developer), and thicker took 5 minutes to diffuse, but both started loosening at the same time, after 5 mins.

    But what's even more interesting, I made another coating so that I dragged the PET base four times through the coronizer. When I did this, the first time I needed much force to drag, the second time was much easier and the two last times it went through without any force. After this, I could feel the static electricity. BUT! This coating had loosened completely already during drying, which is quite interesting!

    Currently I'm giving positive corona discharge (meaning that the corona wire has the positive voltage), side to be coated facing the corona wire. Maybe I should try changing the polarity? Or then again, I could rise the voltage to give stronger corona discharge. I can't even see any glow even in dark now.
     
  7. Photo Engineer

    Photo Engineer Subscriber

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    Look at some of the Kodak patents. They may have more specific information. I'm sorry I can't help, but this was way outside of my field of work.

    PE
     
  8. wildbillbugman

    wildbillbugman Member

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    Goodby R-O-O-O-SIE, Queen of Corona !
    Sorry! But when the Spirit moves me, I godda let go.
    Bill
     
  9. richard ide

    richard ide Subscriber

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    Just an idea transposed from a process I did many years ago. What would happen if you abraded the sheet with something like diatomaceous earth. I used something similar on plastic sheet but the material I was coating was solvent based with a water base photo resist on top. It worked very well.
     
  10. Photo Engineer

    Photo Engineer Subscriber

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    Richard;

    This should not be needed. The corona discharge, if done properly, (right polarity and voltage) will do the trick all on its own.

    PE
     
  11. richard ide

    richard ide Subscriber

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    PE
    Thanks. Very interesting procedure to make a surface hydrophylic.
     
  12. hrst

    hrst Member

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    :D

    I understand well, getting sudden urges to sing loudly in the darkroom every now and then, too....



    I'm happy that electricity has only two possible polarities to try out :smile:.
     
  13. Photo Engineer

    Photo Engineer Subscriber

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    I get tendancies to scream in the darkroom.

    It might not be the polarity of the electricity, but rather the Higg's Bosons you are trapping that cause things to stick. Maybe gluons are doing something too. :D

    PE
     
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  15. richard ide

    richard ide Subscriber

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    If the gluons were doing their job properly; the emulsion should have no tendency to lift. :D:D
     
  16. hrst

    hrst Member

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    The setup I described above was indeed a bit ineffective with corona power of 1 W.

    Now I made a new one with two neon tube transformers (3 kV 50 mA!) in series. Now I see the corona discharge in dark and AHHH, the smell of ozone!! It takes 50 mA at 6 kV, that is 300 watts of power! I'll test subbing with this one soon - as soon as I add a rectifier and capacitors to make DC from this.

    Disclaimer: The previous was quite innocent one compared to this -- this is really a lethal weapon. Don't do this at home like me if you are not sure what you are doing! :smile:.
     
  17. BetterSense

    BetterSense Member

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    I have a lot of experience gluing coroplast, which is a low-surface energy plastic that is really hard to glue, especially to itself. In order for most glues to work, the surface has to be prepared, or it will stick not at all. Scrubbing with scotchbrite pads has worked somewhat in the past, but a quick pass with a propane torch seems to make it work very well with CA glues, like magic. I wonder if you could prepare the surface with a quick pass of a flame or a low-energy plasma discharge.
     
  18. Photo Engineer

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    The things I remember about the equipment we used was the noise, the ozone and the blue glow it emitted. So, you are going in the right direction. :D

    PE
     
  19. hrst

    hrst Member

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    Yes, now this unit gives a bright blue glow you can see even room lights on. And a huge SSSSSHHHHHHH and ozone that gives you a headache in a few seconds! It takes about 4 mA @ 18 kV = 72 W.

    I just coated a quick test with gelatin & color & glyoxal & surfactant. Let's see results tomorrow.

    Do you remember the film base giving you very uncomfortable static discharges / jolts after this treatment? This can be a problem since it probably will expose film. And I have to either use protective gloves or invent new swear words. If the static electricity goes off by just waiting, then it's fine if I handle freshly coated film carefully to avoid discharges.
     
  20. Photo Engineer

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    See my other post about corona discharge causing fog. There is a patent by Yost et al about this problem. I posted this elsewhere, but had forgotten it.

    The film was highly charged and would raise the hair on your head and arms.

    PE
     
  21. Kirk Keyes

    Kirk Keyes Member

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    Have you tried laying/touching the freshly treated sheets onto a grounding plate, i.e. a metal plate that is connected to ground to discharge the static charge?
     
  22. hrst

    hrst Member

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    This work is getting a bit complicated. I have tried both positive and negative corona, with different power levels and distances. I have read many old patents now and they describe a very easy way to measure the level of hydrophobicity; place a drop of water on the surface and look it carefully from the side and measure the angle of water drop at the point it separates from the film. Well then, I've looked these water drops with microscope and the angle is around 70...80 degrees, not quite enough according to patents. With this method, I don't have to make test coatings every time.

    Now comes the interesting part; as I was making tests, one sample, to which I had given negative corona discharge for a minute or so, did show a significant difference compared to unprocessed sheet; the angle of many of the drops was something like 60 degrees, which would be enough for coating. But, I wasn't able to repeat this results, no matter what I tried -- longer or shorter treatments, different distances, both polarities.

    Well then, I've read patents and they describe quite well the apparatus used. I'm going to try that next. They describe an AC voltage with a large DC offset level and positive corona; well, that's exactly what I've been doing. But they describe a totally different type of electrodes than just a wire and plate.

    Kirk, grounding the plate gets rid of most of the static charge but there still remains so much. At least it's fun. You can stick the sheets to walls and give them to your friends so they can get funny static jolts etc :wink:. And when you take the sheets near to any galvanometer, the needle dances around. The sheets have such a high electric field.

    PE; I searched for patents by Yost and found many, and they were good resources, but didn't find the one about corona discharge causing fog. I found patents that describe corona discharge causing mottling in color coupler materials if the corona treatment is fresh. Maybe I'm just blind, or the search engine I use at my university lacks some patents? Do you have the patent number?

    I also found patents about purely chemical subbing layer for PET, but it included components not readily available.
     
  23. Photo Engineer

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    Try Yost and Heidke or Heidke and Yost. It is there. I remember their work. I'll see if I can find the number.

    Our rolls of bombarded support had quite a charge. When I lifted small rolls, I could feel my hair stand up. We also grounded them and as I mentioned, the discharge was quite visible.

    And yes, the mottle problem is severe on RC paper if the discharge is too fresh or if you don't add certain chemicals to the coating or subbing.

    PE
     
  24. Kirk Keyes

    Kirk Keyes Member

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    What about a bath in some water. Perhaps with a little salt added to increase the conductivity? Grounded metal tank, of course.
     
  25. hrst

    hrst Member

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    Yep. That's exactly what I'm going to do. Almost any ions may be fine. It have been worked for me. I just hope it won't affect the surface treatment. But as soon as I find the right patent PE was referring to, I believe it will give me an answer.
     
  26. Photo Engineer

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    We did not use any post discharge treatment. It went from the bombardment to the coating stage directly through a light trap to prevent UV fogging the film or paper.

    Any preventative measures were in the emulsion.

    BTW, if you cross process Endura you will see a tiny amount of mottle that is caused by the "incorrect" process of Endura to a reversal image. The Yost-Heidke method was engineered in Endura for neg-neg processes. It is different for pos-pos processes.

    PE