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 ;). 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.
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.
What about a bath in some water. Perhaps with a little salt added to increase the conductivity? Grounded metal tank, of course.
Originally Posted by hrst
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.
Originally Posted by Kirk Keyes
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.
AH!! This explains the mottle in rev.RA4! I've been wondering it so much. Thanks for the explanation.
Don't know if we are talking about the same technology.
I used to print on PET roll material, and before the first ink roller is applied, the substrate undergoes corona treatment.
If we had any problems with the ink not sticking on the substrate, we would have to measure the effectiveness of surface treatment. We use a test solution (most accurate), or test pens (good enough) to test surface tension, measured in dynes/cm. As a rule of thumb, Printing on PET needs a minimum of 42dyne/cm.
My understanding of the corona treatment is that the surface is deformed to increase surface area. But I think there is another school that thinks corona treatment creates microscopic pinhole in the substrate.
There is a lot of information about Corona Treatment from the world of Narrow Web Printing, perhaps some would be useful to you.
We screen print conductive ink onto polyester sheets. All of our print lines have an 'anti-static bar' to remove the static charge from the sheet prior to printing. This is probably the same as the system you are describing although for us, it is more as a handling aid than for ink adhesion.
Originally Posted by Hamster
Manual handling of polyester sheets can result in a lot of static shocks. Not so much of a problem with a web based process.
The corona treatment is not permanent. If it created defects or whatever, this treatment would perhaps be permanent, but the corona discharge treatment dissipates and is gone in about 24 hours. The film or RC must be re-bombarded.
Good news; I was able to reduce contact angle to about 30...50 degrees throughout the whole sheet and many times in a row. Water behaved completely differently now on the surface, forming large (~1 cm) puddles instead of small drops. This contact angle should be really enough for coating, but after so many failures I'm not sure yet :). I made a test coating again and will report soon.
The key was a new anode electrode made of metal mesh to increase its surface area. And, the surprising part, which I accidentally found, was that no matter what, the wrong side of the sheet seems to get the treatment. So, the side facing the anode (mesh) does not work, but the side facing the dielectric-coated plate (cathode) shows a great hydrophilicity. Changing the polarity didn't change this; the side facing the plate was treated. Oh well, this doesn't matter really, if it just works, but I was surprised. The voltage has still to be very high, 18 kVrms, despite the patents speaking about much lower voltages.