Monday, February 11, 2008

 

Cheap, reliable UV Resins: confirmed!

Continuing tests on the recently discovered photoinitiator mixes:

I made a series of tests I figured to see which of both mixes fares better. The resin in itself here is regular Polyester resin. i bought mine at the Drugstore. Found it for 7€ a kg.
The photoinitiators are as follow: Benzil+Benzophenone is the cheap mix, Isobutybenzoinesther+Benzophenone (BP) is the expensive one. I added N-Methyldiethanolamine (MDEA) as a co-initiator for Benzophenone. The mix proportions are: (roughly for now, I'll investigate into precise mixes later)
Polyester resin: 20g
BP: 1g
Benzil/Isobutybenzoinesther: 1g
MDEA: 2g (will try with 1g later, this stuff yellows the mix and maybe makes it to flexible)

The layer thickness is 1mm in every test. No mentionable shrinkage has occurred in any of the tests below.

Test A) 5 minutes of UVC irradiation, followed by 2 minutes of UVB
results: Benzil+BP harder than IsoBBesther+BP, the first a little, the last one still quite sticky

Test B) 5 minutes of UVB followed by 2 minutes of UVC
results: both deal very similarly. Result more solid than in A)

Test C) 5 minutes of combines UVB+UVC
results: both very similar, somewhat stickyish

Test D) 7 minutes combined UVB+UVC
results: both compounds, curing almost finished

Test E) 10 minutes combined UVB+UVC
results: curing finished in both compounds

The last test I did was an interlayer-adhesion test with and incorporated deep cure test.
What I tried to find out is how well 2 layers of resin glued together after the first one had been subjected to 5 minutes of combined UVC+UVB irradiation, to set it just enough to leave it sticky. After that I poured a similarly thick (1mm) layer of resin on top and irradiated it with 10 more minutes of UVC+UVB to simulate the last curing step of a printout, as well as an accumulated UV exposure to the lower layer.

results: Both mixes react similarly. The first layer after 5 minutes exposure is solid enough not to move or sag anymore, but the top surface remains a little sticky, like a sugar stained surface. I deposit another 1mm layer and start irradiating again for 10 minutes. After this, I try to pry between the 2 layers with a toothpick and cannot create any kind of separation. The lower layer is completely cured. The top layer is, of course, completely cured also. The expensive mix may be a tad harder.

This looks really good! I'm specially happy with the cheap Benzil+Benzophenone mix! The end product is a 1,5x2cm pad of transparent, glass-green, hardened resin. Th ereason why I chose 1mm thick layers is because I wanted this product to be able to work on thick dispensers. Nevertheless, when using such a thick layer, some sort of filler materials (silica powder) will have to be added to ensure that the 1mm thick thread will not sag while it is deposited, creating deposition errors. The resin has the perfect viscosity to be used in <0.6mm threads.
I wonder if this has a low viscosity enough to be used in one of them piezzo-electric ink-jet cartridges, hmmm.... maybe adding some styrene as a solvent/monomer would help?....

Additional data:
UVC source is a germicidal 36W, 256nm fluorescent
UVB source is a actinic or blacklight 40W, 365nm fluorescent
I will in the future purchase a 65W UVC and 4 compact 18W UVB lamps to build a reflector backed irradiating lamp which will cut the times by at least 3 and distribute the rays more evenly.

Comments:
That must be a fascinating drugstore you have. :-)

So are you planning to go for a photolithography scheme with this work, Fernando?
 
Bah, you should see my apartment Forrest! It's full of pots and bottles. I love it! Looks like a laboratory. I'm seriously considering going back to work in polymer chemistry....

As for my plans to put this in use, I think I will probably start building a cartesian robot, to start printing some objects. In the meantime, maybe I can ship some of my resin to Viktor Dirks and see if he can try this out on one of his gear, as he seems to have the right UV lamps allready.
 
Ha! We need to have a page on our website with a collage of the work spaces of various people building repraps. :-D

I probably haven't read what you have written closely enough, but I've missed how you are getting the goop to polymerize in the first place.

Are you polymerizing with UV or using a laser and then using UV to cure? Do you have a fume problem?
 
Ah! You're squirting it out of a syringe and then curing it with UV, right? Nice!

The question about fumes, still applies, though. Are the health and safety people going to cause trouble at some point?
 
Yup, you got it. I use UV fluorescent lamps to cure and harden the resin. No lasers needed, although they could potentially be used as well. The whole technique is quite flexible.

Well the additives that I use are pretty safe. They have been in use for sun protectors and such for a long time. On the other hand, resins always tend to have a relevant proportion of monomer in them. The most "worrysome" chemical in this whole issue is a component of all polyester resins, namely Styrene. But if we cure the resin soon after depositing, the amount of fumes evaporated should not be high, which is a good idea anyway. When mixing the resins I describe, it's enough to do it in a ventilated area and to keep the final mix in a tightly closed and dark bottle to be safe.
 
Hi Fernando,

i'll try next week to dive into, so we can then synchronize about common materials or sending samples ...

Viktor
 
how long does the final mix keep for?

if this turns out to be a viable route, it would make lots of sense to make pre-mixed supplies available.
 
No problem Viktor, take all the time you need. I don't want to hijack anybodie's time to my project :)

Zach: Good point. The actual mix I made is workable during 5 days, but I can probably stabilize it to last months, maybe a year.
The thing here is that polyester comes pre-accelerated, out of the box. This means it is more reactive and hardens faster. It contains Cobalt, Copper or Iron compounds that enhance the radicalization of catalyzers. You can block them quite easely with EDTA and/or stabilize the whole mix with Hydroquinone to make it last forever.

So there are 2 solutions for premixed supplies:
1) We offer the unmixed batch, you just need to add the catalyzers to the resin in a given proportion. Drawback is tha tthis resin has to be used within, say 5 days, then all gear needs to be cleansed with alcohol or another solvent.

2) We stabilize the resin mix, catalyzer and all included. This would be the best solution. Also we could offer an exhaustive list of specifications on what capabilities the resin has. How to use it, store it, harden times, lamps to use...
 
I have just now mixed stabilized versions of both the cheap and expensive resins by adding a tip of a teaspoon amount of Hydroquinone. Let's see if in 5 days it started to gel as the previous ones did.
 
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