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Author: Subject: Hello and help for lightsensitive paint
fra77x
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[*] posted on 10-11-2021 at 17:28
Hello and help for lightsensitive paint


Hello, this is my first post here.

I am interested to make some photosensitive paint (UV). I have read some wikipedia articles about silver and dichromates and I wonder if someone can tell me if it is at all possible at my level and (non existent) equipment and perhaps if it is, to point me to some directions.

Regards,
John

[Edited on 11-11-2021 by fra77x]
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walruslover69
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[*] posted on 10-11-2021 at 22:01


Hi John, It's definitely possible but it depends on what your actual end goal is. Silver chloride/bromide/iodide are commonly used are is how film and polaroids work. These reactions are also not reversible, and require a little bit of developing. Usually a suspension of silver chloride or silver bromide suspended in gelatin is used. But photosensitivity is dependent on the wavelength of light. It really depends on what your end goal is.
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fra77x
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[*] posted on 11-11-2021 at 01:00


Thanks. I have read about gelatin and I have some here. My goal is to prepare some photosensitive paint so I can use it to cover copper boards and expose them with a mask to UV light (like sun or UV LEDS) perhaps I will also use some lens to make the mask appear smaller So I can get greater detail by printing bigger masks and using lens to focus).

Now I read that I can make silver chloride by mixing silver nitrate with salt. And to make silver nitrate I have to dissolve some silver metal with nitric acid. And to make nitric acid I can use HCl, nitrate salt and copper.

So now my question is how to make a nitrate salt. I read that any source of nitrate can work like sodium nitrate,potassium nitrate, ammonium nitrate but I have no idea how/where to get such chemicals.

Is it possible to make ammonium nitrate by using ammonia and sodium bicarbonate? How should I proceed?

[Edited on 11-11-2021 by fra77x]
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[*] posted on 11-11-2021 at 01:25


What you are looking for is a photocured polymer - quite different from a silver photographic emulsion. Totally different chemistry.
If all you want is to make PCBs on the cheap (its quite hard to beat the prices out of China these days), I'd suggest you look at toner transfer. Laser printer + some transfer paper (I prefer "Press'n'Peel Blue" - gives excellent results) + a hot plate. It takes a bit of work to get the process right (true of every method of PCB production), but once you do you can turn out a board in 30 minutes. And you can get high levels of detail with a little care - I've achieved 5mil/5mil track/space on 1/2oz double-sided Kapton flex. Apologies for the imperial units - electronic cad seems stuck in the past sometimes.




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fra77x
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[*] posted on 11-11-2021 at 01:32


I never had success with laser printer transfer method. I have also tried to use a photosensitive coating but I could never apply it correctly. I am also interested to make a photosensitive paint as a way to spend some time with chemistry.

So it is a different process. I believe I have read I need to make some type of polymer, I have read about dichromate.
Is this achievable with my means?
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[*] posted on 11-11-2021 at 12:32


Start here: https://en.wikipedia.org/wiki/Photopolymer
That will give you a broad overview of the chemistry, and offer a starting point for your research.
I looked at that ( for making 3d printing resin), and threw it in the "too hard" basket.




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fra77x
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[*] posted on 11-11-2021 at 14:44


I have read this article and hopped there was still a chance to be able to do something, that's the reason I asked here. So creating photopolymers is too hard, that's what I wanted to hear.

Thanks
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[*] posted on 12-11-2021 at 03:31


Making a photoresist is unfortunately hard, and if you are just beginning with chemistry I would suggest to stay away from carcinogenic substances such as dichromates.

If you want to make printed circuit boards you can buy presensitized boards that already have a photoresist coar atop the copper.
I have a video on my channel explaining the process https://youtu.be/5d9JvZiYhEg but there are many similar ones online.

If for some reason you need to apply a photoresist to an existing surface, I had most luck with gelled photoresist rolls you can buy from ebay. They are like double sided tape, you peel one of the two protective films and apply it to the surface, expose it to UV light through a mask, let the resin cure, peel the other protective film and develop it.
I show how to use this kind of photoresist in my video where I explain how to make double sided printed circuits at home (including plated-through holes) https://youtu.be/Kn92cLf69iw using a process I developed that can be done in an amateur lab.

Additionally, a few companies sell liquid photoresist in spray cans, such as positiv 20
https://www.tme.eu/Document/b7a6a16ed14d2aa6149ed9a14fb1b03d...
but it is hard to find and hard to apply without a spin coater.
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[*] posted on 12-11-2021 at 05:02



Quote:

Making a photoresist is unfortunately hard


I hopped it was different.
I have used several methods for PCB, most of what one can find in tutorials. I know about graphic designing and I wanted to develop my own way to design PCB (I like to use illustrator) I also like to experiment with focused lensing. Also I thought if sink printing uses a photosensitive paint (is it right?) then perhaps it wasn't so hard to get/ or even to make. It seems I guessed wrong. In general I just wanted to have the freedom of making my own photosensitive paint so to use it in "experiments" with printing, etching and things like that. I will investigate further and thanks for the tip that dichromates are carcinogenic and I should avoid them.

Is there any other way to make a photoresistive paint? I mean without dichromates?

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[*] posted on 12-11-2021 at 14:32


Prior to bichromate bitumen was once as a photoresist. There are probably lots of other compounds compounds that could be used. Try searching for historic photoengraving methods.

The gelatin bichromate process is simple and effective I have made prints using it but yes dichromate is carcinogenic. It was once common in chemistry sets. Depending on where you live you may not be able to buy it these days.

Presumably you have to buy the copper clad board so I suggest you buy a can of spray on photoresist or even better dry film photoresist, £4.34 for a sheet 0.3m x 2m
see: https://www.ebay.co.uk/itm/224482571845_trkparms=ispr%3D1&am...




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[*] posted on 12-11-2021 at 15:33


Bitumen of Judea or Syrian asphalt... the first photograph... photolithography.. cool stuff. Thanks wg48. I found some ammonium bichromate for 50 euro per 500 gr. Too expensive I think. I will investigate a bit the bitumen process.

I will also try gelatin with carbon.
I have tried with dry film photoresist and I couldn't get reliable results mainly due to the ripples that were unavoidable during the coating.
Photoresist spray seems interesting.
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[*] posted on 28-12-2021 at 18:24


Have you tried pre sensitized boards? That’s what I’ve always used with decent results, even with smd, down to 0402 passives and chips.

I’m trying a different exposure method right now, using a SLA printer (resin 3d printer), trying to eliminate the whole laser jet/transparency thing.

I’ve watched a few videos on the dry film method, and to prevent wrinkles they spray the bare copper with a fine spray of distilled water and squeegee it out before laminating.
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[*] posted on 28-12-2021 at 20:04


Photographic silver emulsion leaves suspended silver oxide particles in the gelatin, and washes the unbruised silver salts into solution, and away where they can't change the image later. Think all the trouble of making your own film for a big camera. Not exposing it accidentally, getting it even coatings, etc. Not sure the clear parts wash all gelatin away so you'd have a clean etch?

Silk screen resist isn't that expensive, and in my experience ( screen printing) it's easy. Use a rudimentary stick or 1" putty knife to get some on the screen, start with a small blob. Skreet it side to side with a rubber blade print wiper, untill it's even. Let dry in a dark cabinet with forced air. Set your negative on it and expose to sunlight by watch count.

A proper framed print screen, shirt contacting side up with your velum on it image down and a board with foam under the print screen to supply back pressure against the image. Place a glass sheet over the velum for intimate contact between image and dried emulsion. This ensures no bubbles that take away sharpness, and also that your tightest marks are going to be face down on the PCB ... Not up where you load your paint for the masking yet to come. You can expose it. Wash it out while the black covered parts are soft and the exposed parts are hard. Too early and you have pinholes everywhere, soft edges and too much washing ruins the screen. Too late and your supposedly soft parts under the image still won't wash out easy. Once it's cleaned up, you can use a dab or two of the emulsion to fix pinholes.

Start printing in acrylic paint on the PCB with a rubber printing squeegee, or something easy to wash out. Don't let it set on your screen! Washing early is easier than scrubbing later. You can get a lot of use out of a wood frame, couple yards of screen, small jar of resist and some acrylic paint. If the design is good, it's kinda hard to wear out the screen. It gets harder the more things you sub in or change, results vary, with practice/skill

I've used hand drawn fingernail polish, numerous runs of a black sharpie, pitch and l tried super glue. If you want detail, do it right or buy them. It is infuriating to try and do surface mount parts by hand I tell you. Also don't drill the holes too big if it's through hole parts. Closing the gap is a pain. I've deff down it wrong more times and ways than right. But I have also printed t-shirt for a few years when in college, by the thousands. So if reproducibility and precision is key, buy at least purpose made items. Now the frames and screens can be made by any idiot, I should know. The sun is free and works on a cloudy day too. The squeegee, resist material and fabric for the screen are not trivial accessories.
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