UV curable resins

Although lately I have mostly been busy mixing heat triggered curing resins (Furfuryl Alcohol and such) I have made some very important progress on the UV curing resins as well.

I was toying around mixing different kind of resins and seeing how those compounds improved the polymer properties. The two big problems I had remaining on the researched mixes I discovered were 1) surface wrinkling due to shrinkage of the top layer, 2) poor depth curing due to UV screening of the top layer.

Both these issues seem to get solved by adding synthetic varnish of the alkydic kind (very common, like the one from Titanlux) to Polyester resin.
You actually need to add very little of it. I tried with 1 gram of varnish to 10 grams of PEst but you could surely add even less. The mix gels in some way and gives the resulting resin a more viscous texture.
The curing catalyzer is still Benzophenone which needs 253nm UV light, aka germicidal UV.

Once subjected to UV rays, the resin hardens quite nicely with no wrinkling and the layer can be deeper and still cure through.

I'll be posting further on these issues.

Edited 1 time(s). Last edit at 01/12/2008 11:51AM by Fernando.

fantastic. UV resins sound like they would be very useful. i'm sure we could push out resin much faster than plastic which would speed up printing quite a bit.

That sounds cool.

What is the curing time, is it long enough to ensure that the next layer to be printed would bond to the first ??

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Actually, th etime is more than enough. Stopping UV irradiation stops the curing process.
Curing time is currently below 10 minutes and can be adjusted to greater values if needed. I have spotted a lamp and am designing a hyperbolic reflector that will reduce this time to below 3 minutes and improve curing depth.

I'm currently also working on accelerating the curing process to below 1 minute irradiation.
This would allow us to print a layer, irradiate it just enough to harden it, then print the next layer on top of that. The resins I'm working with have excellent adhesion, so that interlayer bonding is not a problem. You will be able to use Polyester, Epoxy and Acrylic resins with this system. Last week I have spotted 5kg of Polyester resin for around 7

So, eventually when this gets to working, I'll have to build a cover for my printer to prevent myself from getting a mid-winter tan right?

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Jay

Fernando

Sorry for this another quick question.

For depositing the resin it should be possible to use a motor driven syringe and standard deposition needles.

What is the compatability like between the resin and standard syringe type plastics, ie will the uncured resin react with/dissolve or ugger up a standard plastic type syringe ??

Cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Hi Andy,

... the plastic body of the syringe shouldn't be a problem, i used them with aggressive acids and highly corrosive paste-materials.

For moving the material i used the standard plastic piston-heads too.

It's the needle, what sometimes made problems - some of my ingredients battled with the steel, others even with titanium-needles, so i use standard-plastic-tips when i dispense highly corrosive materials ...

Viktor

@ Jay: Yes, even for the UV-C type radiation which doesn't penetrate deep into the skin, and generally speaking, UV radiation is not good for you. The hyperbolic reflector is thought to direct the rays towards the printed resin parts only, but there still will be stray radiation escaping the apparatus. The best way to shield the thing, short from having the RepRap in a closed room, is to build a box around it, with a glass lid, to see what's happening inside.

@ Andy & Viktor: plastic syringes are made of HDPE, wich doesn't react stick to anything. The needles are usually stainless steel, and I will see to stabilise my resins towards metal reaction. Some mixes harden as soon as they get in contact with metals, and some resins I research are acidic, allthough specifically the ones I use should be OK with stainless steel and titanium. The biggest syringes I got from the pharmacy are 20ml. That's rather little and I have seen other types of cartridge systems with pushers that could hold bigger amounts of resin (a la silicone cartridges). Those would come in handy for bigger operations.

Most excellent.

What are you using for your UV source, are LED's viable ??

(Sorry the questions keep on coming)

I was asking re compatibility, because I expected the resin to have some form of thinner or solvent as part of the mix. I spent some time once doing some GRP work during the summer break, styrene monomer is the main thinner in the resin. We also used acetone to clean the tools at the end of each run/session.

All of it bad stuff, f**ked with your mind and wasn't great in contact with the wrong materials. I remember having to use metal buckets for this reason.

Cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

The cheapest types of photoinitiators work well with what is called germicidal UV fluorescent lamps. This is the cheapest combination I have found by now.
There are some Leds I hav found lately that would fit into some photoinitiator spectra, but they have quite a low intensity (watt/cm

... maybe it's a good idea to salvage the diodelasers from PS3-Bluray-HD-drives?

They seem to be in the interesting range between blue and near-UV ...

Viktor

Actually near UV photoinitiators are the most expensive ones.
I have just received a couple of chemicals that are less expensive and that I will test and hope to see some results in those wavelengths. But I'm not sure yet they will work well enough.

I have found some leds that emit in th enear UV bands too. They come with colimating lenses and I guess it's probably cheaper and easier to build that getting them from a DVD burner:
[www.nichia.com]
[www.nichia.com]

I haven't tested them with my range of photoinitiators and don't know if they are powerful enough.

Double whammy bingo success!!!!!

You know those chemicals I have recieved? Well from the four, two have excellent curing behavour! They react with UVB light and in combination with Benzophenone and combines UVC and UVB irradiation I get fantastic curing at depths of +/- 1mm below 10 minutes of irradiation!!

Here is the cheapest of both, called Benzil:
[www.acros.com]
This one needs combined UVC/UVB irradiation and to be mixed with Benzophenione to work right. But the price is a real winner.

The more expensive on is Benzoin isobutyl ether:
[www.acros.com]
This one could work alone and with UVB irradiation only (the fluorescent lamps are 4 times cheaper cheaper although clumsier). But the price of the chemical is 3 times more than the one above.

This is the breakthrough I was looking for, a real winner! It works with cheap Polyester resins and will also work on Acrylic and Epoxy resins. It also opens the door to possible led-based curing devices.

Yayyyy!! Where is my Cognac bottle, got to celebrate....

Hi Fernando,

... glad to hear this

Then i should search some UV-sources with micro-spots ...

Viktor

Thanks Viktor!
I really think this will open the door for really accurate printing applications.
As a reference, when I speak of UVC, it's 254nm. UVB is 365nm.

maybe I should mail you or post the corresponding spectra of these photoinitiators, so that you see what UV sources would be valid.

A very usefull link on UV leds
[www.biouvled.com]

Edited 1 time(s). Last edit at 02/08/2008 07:24PM by Fernando.

Continuing tests on the recently discovered photoinitiator mixes:

I made a series of tests I figured to see which of both mixes fares better.
Don't forget, Benzil+Benzophenone is the cheap mix, Isobutybenzoinesther+Benzophenone is the expensive one. The layer thickness is 1mm in every test. No mentionable shrinkage will occur in any test.

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 5 minutes of UVB followed by 2 minutes of UVC
results: both deal very similarly. Result more solid than th eexample before

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 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 afetr th efirst 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.

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.

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 wich will cut the times by at least 3 and distribute the rays more evenly.

I will post these findings in the Builder's blog and hope to add some pictures.

Cheers!!

This is very exciting work and looking very close to a working solution excellent.

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Ian
[www.bitsfrombytes.com]

Actually I'm seriously considering building a cartesian robot right now.
I have almost all I need and the issues that I have to solve now are more of a construction of the robot with efficient UV lamps type.
I will open a thread with my inputs on UV lamps. Maybe there are some ideas out there that will help me out.

Clever stuff

I had originally suggested LED's because I was envisaging a ring of leds around the deposition type nozzel/needle. The tool head can then Deposit, Deposit & Cure or Cure only and can be selective about which areas get cured and for how long.

Whilst a syringe is an option, providing the tubing needles etc were screened to prevent UV penetration a feed line could be used instead giving much more volume without pausing to refill.

Reading your thoughts etc though, I think a closed box (polished aluminum maybe so it reflects) with lamps over head but angled might give best results.

The deposition tool head will cast shadows but if it is pretty much always on the move or is withdrawn periodically this might work.

Other thoughts ideas are perhaps making a hollow object with the usual FDM technology then filling and UV curing with the resin mix on a dual head system.

Thoughts for what they are worth...

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Good ideas!

If you don't mind, I will copy this to the other thread about the UV RepRap

By all means, the thoughts are freeware and made available strictly to GPL hehehehe.

For a deliver system I was thinking along the lines of a pressure pot (Like they use for bulk spray painting) with pickup filter in the pot and with the metering orifice and fluid valve on depositing head just prior to needle/nozzle.

I don't know if a standard LuLock to tube adapter is available or makable but this would allow interchangeable tips and diameters for nozzle.

Variable pressure on the pressure pot could give varying delivery rates when taken in conjunction with the metering orifice.

A pressure pot itself if not available cheaply (surplus or spray painting supplier) is something that is easily welded up using standard pipe and fittings of appropriate diameter.

Hope this is useful

Cheers

aka47

Going off to play with bits of pipe and clamps.

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Necessity hopefully becomes the absentee parent of successfully invented children.

Hi Andy,

aka47 Wrote:
-------------------------------------------------------
> ...
> I don't know if a standard LuLock to tube adapter
> is available or makable but this would allow
> interchangeable tips and diameters for nozzle.
> ...

You can buy LuerLock-adapters with metric- or inch-threading - the cheaper ones are from plastic, so a bulk order should work fine ...

Viktor

Zach asked me on the builder's blog what the shelf-life of these resins was.
That's quite an important issue as nobody wants resins to harden in your gear overnight or even worse, while you are printing.

I have found out that on my standard, high reactivity resins (composed of high amounts of any of the new catalyzers, between 4-5 weight% of new catalyzers along with 4-5w% Benzophenone (BP) and 2w% N-Methyldiethanolamine) they have a pot life of a mere 5 days.
This means you have to clean out your gear with solvents say, every 2 days.

I know a couple of additives that will lengthen pot-life. One of them is a stabiliser called Hydroquinone. This is a very effective chemical that will stabilize the final resin-mix. I have done a test batch with around 0.01w% of it and these are the results:

The mix containing Benzoinisobutylbether (aka, the expensive mix) has not improved noticeably in pot-life.
The mix containing Benzil (aka, the cheap mix), after 10 days, the resin is still very fluid, but the reactivity is quite lower.

Conclusions: the expensive mix still has the same problem, and has lowered reactivity. The cheap mix has at least double pot-life, but the reactivity is reduced as well.

Where to go from here: I have to make tests with even less concentrations of Hydroquinone.
I have to get myself some EDTA, another chemical that will probably longer pot-life without hopefully reducing reactivity.

Hi all,

Myself Nishant, I'm research student of Polymer technology. I'm working on UV curable pressure sensitive adhesive. My objective is to improve the pot life of my UV curable emuslion resin. When I apply my UV formulated emuslion on substrate it get cure without UV curing. I want to know how can i improve pot life. My emulsion contains monomers like 2-EHA, MMA and AA. its two component system. For making it UV curable I mix this emulsion with TMPTA and Photoinitiator Irgacure 184.

Your reply is awaited.

Nishant Bhore

Nishant,

Do you have florescent lighting in your lab? The UV from them may be initiating the polymerization. You may have to put up UV blocking filters/curtains on your windows and lights. You might also try turning off the lights and using another, non-uv light source when working with the uncured resin. Examples of non-uv light sources include incandescent yellow "bug lights", most colors of LED lights, etc. (I don't remember how much uv normal incandescent light bulbs put out.)

Thank you Mr. Sabastien,

I have conveyor belt tipe UV curable machine which has the facility of both IR and UV radiations. UV bulb is high vapor pressure mercury. I will try method as you suggested in darken condition also. But is there any additive which can stabilize the formulation before UV curing.

Nishant

Hello Nishant,

You can try Hydroquinone which should act as a radical blocker. Very low doses around 0.01% could be effective. Work upwards from that dose to increase potlife.

Do you know if your resin mix is sensitive to metal ions? If yes, adding EDTA would also reduce the sensitivity of the resin mix.

Good hunting!

Hi Fernando,

... i didn't get the chance to experiment with UV-curing until now - lack of time, of a scale exact enough and the new job started in May ...

But i hope to get round the next weeks, so there is hope

The unmixed high-temp silicone has an excellent potlife - the remainings on top of the pot are fluid as if new after 4 weeks in open air!

But i think that it's completely dark for UV-light, so the curing would only affekt the skin of the tray, not the complete slice.

So i will buy some clear epoxy afor testing ...

Viktor

Thank you Mr. Fernando,

I tried it with hydroquinone. It gave good results. Is there any literature available on this method. So that I can put reference in my literature survey. Can we use slow crosslinker also to increase the pot life. I'm trying with aliphatic isocynate. Is that feasible to use. My emulsion contains ammonia which i added for storing stability. After adding hydorquinone it changes colour of emulsion from milky white to somewhat lavender.

Thank you
Nishant

@Viktor
No worries, mate. i myself am extremely busy now, changing jobs as well, wich leaves me very little free time for much else. So, whenever you have time, have a go at the initiators I sent you and post it on the forum, as I will always be checking.
To bad about the silicone being UV opaque! That would really have been a great application. Did you see what happens if lightly heated? Maybe using infra-red instead of UV would be a usefull trick that wouldn't need photoinitiators in that special case... just a hunch.

@Nishant
Great to hear that Hydroquinone worked for you!! For me it was very effective in Polyester resins as well, although just leaving the mixed resins in the dark until used also proved to be as good an option. Yes, Hydroquinone will often generate colored derivates when reacting. This shouldn't be a problem unless you need to attain a specific target color.
Here are some references that you may look up for the use of Hydroquinone:
[www.freepatentsonline.com]
[www.freepatentsonline.com]
[www.freepatentsonline.com]

Using slow crosslinkers will also slow down the curing process and increase potlife, but it will not detain it. So you will get a larger range of viscosities during the potlife time and maybe that's not desirable for your application. Also slowing down the curing with different crosslinkers will result in an end-polymer with different properties. You would need to find out if the new properties are useful or counterproductive. The common knowledge is that a resin that polymerizes slower has a greater toughness, but anything could happen, really.
I hope this will be useful for you.