Anyone familiar with photochemistry, or curing resin with UVB/UVC (not UVA)?

[pipe2null]

I'm in the process of building a resin UV curing box with not-so-cheap budget.

I have a Phrozen Mighty 8k on it's way to my doorstep, and the cheapest curing machines for that build volume start upwards of $250 - $700.  For a box...  With blacklights lights in it....  Yes, there is more to it than that, but not really much at all...  So, I'm building my own, and my budget is what I WOULD have spent on the cheapest commercial unit.

I found a local supplier of UV transmissive quartz glass to use as the curing plate to hold work pieces above the UV source (cure the bottom without shuffling it around).  Heh, that stuff is not cheap for a hunk of glass, but still well within budget compared to the cheapest big-enough commercial curing machine.

Now on to the UV source.  Yes, I have seen a million videos on how to cure resin with a bucket and $15 spools of black light LEDs (UVA), and curing with sunlight, and....  There are a ton of non-commercial DIY solutions out there that use UVA spectrum sources.

I came across the following article talking about UV photo initiation wavelengths of some polymers being in the UVC and UVB range:
https://polymerinnovationblog.com/uv-curing-part-2-tour-uv-spectrum/

So apparently many (most?) photo activated polymers require UVC or UVB wavelengths, NOT UVA. So some type of special sauce gets added to 3D printer resin to allow it to cure using the lower energy UVA spectrum, but that doesn't mean that the same chemical goop would not cure with UVC or UVB.  Obviously it comes down to the specific chemistry of the specific color of the specific type of the specific maker of the resin, but perhaps it's interesting enough to try curing with other wavelengths.  Back at the beginning of the whole pandemic thing, for some crazy reason I was really interested in UVGI.  A good book on the subject suggested by a fellow eevblog-er is by Wladyslaw Kowalski "Ultraviolet Germicidal Irradiation Handbook".  UVGI uses UVC wavelength sources to kill all kinds of viruses, bacteria, and other nasty little buggers.

PSA WARNING: FYI for those not familiar, UVC is very nasty.  It destroys DNA molecules and will cause cancer and various nastiness.  Stay away from UVC unless you know what you are doing.

LOOONG story short, I have a small pile of UVC sources, both bulbs and LED lights, as well as UVC sensitive photodiodes to both verify the output of UVC sources as well as verify that shielding is not leaking, etc.  And I used all of these previously, and used the sensors to verify the sources and the shielding I was using.

I think that I might get some (properly shielded) use out of my old UVC germicidal LEDs for curing my 3D resin prints.

To be clear, I'm not jumping into using UVC for this resin curing project to simply save $15 on blacklights.  My thought is that UVC could possibly cure better, faster, and maybe have deeper penetration.  Or not at all, thus I'm asking if there's someone knowledgeable might either encourage this course of action or comment on why it won't work (in more or less layman's terms).


Can anyone familiar with photo chemistry or photo activated polymers comment on the subject?

Thanks!

[jpanhalt]

First off, I have not done any 3D printing.  I studied photochemistry while in university about 50 years ago, and while out of date, I still have a passing interest in it.

So apparently many (most?) photo activated polymers require UVC or UVB wavelengths, NOT UVA.

Can you provide a link to support that assertion?  The references I have seen all use 405 nm (UVA).  There is a difference between what you may think the native resin needs and what is actually used, as photosensitizers may be added.  Basically, they absorb at longer wavelength, cross to a triplet, and transfer that to the resin to initiate polymerization.  You can consider that similar to the polymerization of polyester resins initiated by a radical from MEK peroxide.  Another example is novolak resin used for making photosensitive masks for etching PCB's.  The reason those work at 405 nm is addition of a photosensitizer.

Second, any photochemical reaction can be inhibited by self absorption.  Let's assume you have a resin that absorbs strongly at 254 nm (like most aromatics).  If you use 254 nm light, it will not penetrate very deeply. (See: EDIT) Longer wavelength light will penetrate much more deeply.  That is another reason photosensitizers are used and at relatively low concentration.

Finally, very short wavelength light can increase the proportion of degradation and unwanted reactions.

I recommend sticking with what the resin manufacturer recommends.

EDIT:  Take styrene as an example.  Its molar extinction coefficient at 254 nm is 10^4, molecular weight = 104 (about 100) and specific gravity 0.9 (i.e., 900 g/l = 9 molar).

Rearranging the Beer-Lambert equation, OD = εlc to give length (in cm) ---> OD/εc = l
For an OD =1 (i.e., 10-fold reduction in intensity) , l = 10^-4 mm
That is thinner than most layers used for resin 3D printing, which is usually 0.01 mm or more.

[wizard69]

While I can't discuss the resin, one of the engineers at work said that it transformed at a very specific wave length and that 99% of the UV / visible light spread (from our source) did very little to nothing to transform the resin.   Chemistry is not my thing so I left it at that.   This is not a 3D printed product but I'd have to believe most UV cured materials are similar.

What ever  you  do make sure you have the proper safeties designed in.   UV burns can be hideous and some sources have no visible component.   Plus you can easily blind yourself.   I mention this because I see a lot of dodgy UV and laser based systems, on line, from the DIY community.   Make sure  any enclosure you make use of can block UV totally.

It probably wouldn't hurt to  get the tech specs for the resin and figure out what wavelength the resin likes.   Your supplier should have that info.

[mclute0]

I think you may be over thinking the process. Over the past few years, most of the fledgling industry has standardized on 405nm. At 405nm most simple full spectrum lightbulbs may be used for curing as well as sunlight. The resin color is by far the most influential variable in the process, but it depends on your application.

Dental applications seem to be driving the market and research at the moment.

There is a good amount of information on resins here ... including exposure times and steps. Always follow the resin manufacturers recommendations unless failure is acceptable.

https://photocentricgroup.us

[NiHaoMike]

High power 405nm LEDs are relatively cheap. No reason to use anything else for this application.

[pipe2null]

Thanks for some much-needed schooling, and helping me avoid a big chunk of wasted time and effort.  And great info to dig into more!

For my project, I'll stick with the UV wavelengths that the curing machine manufacturers advertise supporting: 365nm, 385nm, and 405nm, most models only advertise to use 405nm.  Or far more likely, just sticking with off-the-shelf LED black lights or full spectrum sources and call it a day. 


@jpanhalt:
I freely admit many words in the article I linked in my original post are well over my head , but an interesting learning experience none-the-less.  I have very little if any real understanding of photo initiated polymerization, if that is even the correct term.  I think I understand most of your reply, and thanks for the detailed lesson.

The variance in supported wavelengths of different pro/consumer-grade curing machines is what sent me down the rabbit hole: I incorrectly assumed that the wavelengths chosen by machine manufacturers had more to do with newb-grade consumer safety than with ideal photochemistry, thus started my hunt for the ideal-but-potentially-dangerous UV wavelengths to use.  Then with a very cursory internet search I found the link I posted earlier talking about 2 photoinitiators (benzophenone and triarylsulfonium hexafluorophosphate salt) that have peak absorption in the UVC range.  And since I already had a pile of UVC sources on hand...  But yup, I was wrong on my assumptions.

[NiHaoMike]

Then with a very cursory internet search I found the link I posted earlier talking about 2 photoinitiators (benzophenone and triarylsulfonium hexafluorophosphate salt) that have peak absorption in the UVC range.  And since I already had a pile of UVC sources on hand...  But yup, I was wrong on my assumptions.

Give it a try on a test piece and see how it works out.

[jpanhalt]

UVA goes to 315 nm according to Wikipedia.  365 nm is pushing a tungsten lamp, but the old Cary 12 UV/VIS spectrometer could use its tungsten source down to 350 nm as I recall.

All of the common photosensitizers with which I am familiar can be excited with UVA (basically an n -->pi* transition (n = non-bonding electron, e.g., the non-bonding electrons in double bonded oxygen  >C=O )).  Those transitions are "not allowed" and have relatively low extinction coefficients.  They are great as radical polymerization initiators, e.g., benzophenone and related.

In brief, there is no need to get esoteric and use lamps capable of UVB or UVC.  Ordinary old-fashioned fluorescent lamps have plenty of UV due to the mercury vapor.  Phosphor coatings inside fluorescent lamps give visible light, but there is still a lot of UV, which is why they are not used in art museums, etc.  As also mentioned, modern LED's do too.   

[jpanhalt]

Sorry, I was writing when NiHaoMike responded and didn't see his post.

I searched for photoinitiated polymerization, and this is one of the more recent articles from the ACS -- a reputable group -- that popped up:
https://pubs.acs.org/doi/10.1021/acsapm.8b00165

Go to section 3.1.1.  NB: The table showing wavelengths gives the lambda max (wavelength of maximum absorption) not the wavelength needed for an n-->pi* transition.  For reasons I described before (self-quenching/absorption) you would be best to pick a wavelength where the photoinitiator absorbs to do its thing and the resin doesn't, if you still want to go down that route.