Suitable (cheap) mid IR polymer window

[dsiap]

Hi. I'm looking to make a window that is IR transparent in the 3-5 micron range. I found a paper online that said teflon/Polytetrafluoroethylene had near 100% transmissivity here, but another paper that said exactly the opposite for a thinner sample.

It doesn't look like there's a ton of research in this area or at least I haven't found it and I'm pretty new to this. Can anyone in the field help me out?

Thanks!

 

[IRstuff]

Teflon-like polymers tend to be transmissive in the longwave. The only thing that works well in the longwave and is cheap is some form of Saran wrap. Most others tend to be translucent, rather than transparent; q.v. typical motion detectors. You've not specified what you want this window to do; nonetheless, if there was a polymer that worked, they would already be on the market.

TTFN
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[dsiap]

For clarity, what I need is IR transparency in the 3-5 micron range (mid range), and some small level of rigidity. I know that I can use a crystal like say CaF2 for this but according to the FTIR it seems like both Polytetrafluoroethylene and polystyrene should work. However according to some case studies I've found that doesn't seem to be the case. I'm not sure whether this is due to impurities or surface issues or non chemical material properties.

Oh and the window is going to be mounted on a wind tunnel.

For the record, I did find some manufacturers that claim to have some proprietary polymer like iriss but they don't seem like a good alternative for my application - more industrial large order and pricey.

D

 

[IRstuff]

The big question is whether you are trying to image through it, and if so, what, and how hot is it? Even window glass is somewhat transmissive in the MWIR; I've seen lit cigarettes through storefront-type windows. It's possible that if you get a sufficiently thin, say 1/16"-1/8", window glass, it might be adequate for your, as yet unknown, application.

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[dsiap]

So, to reiterate, the window is going to be mounted on a wind tunnel.

We are still in the planning phase so actively heating whatever's inside the tunnel is not out of the question, but obviously not to combustion temperatures... It's unclear exactly what kind of temperature gradients we will see and obviously this depends on our heating method. All that being said, I believe it's fairly obvious with an IR window you want to maximize your transmissivity while minimizing things like cost, chance of failure, etc. Given that CaF2 is a good solution with high cost, I'm hoping someone in materials would know of alternative solutions or at least a solid analytical reason why the flourocarbons I mentioned don't work in practice.

From what I've seen of the viable solid polymers in general mechanical failure doesn't seem to be an issue as the absolute max pressure we see is around .2 psi so we are free to go relatively thin on the window.

Hope that helps and thanks...

 

[IRstuff]

But, are you trying to image with your sensor? 3 posts in, and you still have not specifically and unequivocally stated what you are trying to do. If you already have a camera, why not just grab a bunch material and try them?

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[dsiap]

Yes we are trying to image.

I'm currently gathering stuff and trying em... just figured I'd get a good handle on exactly what to gather.

 

[Demon3]

Each type of chemical bond has its own IR frequency where it adsorbs. That's why you can use IR to identify polymers.

What that means to you is that you want a polymer with as few types of bonds as possible.

So, for example polyethylene is great as it just has C-C and C-H bonds.

PTFE is great as it only has C-C and C-F bonds.

PVC less good, three types of bonds, PMMA bad, PS not so good.

You can find the IR spectra of some polymers free at

http://www.sigma-aldrich.com

You can find books e.g. from RAPRA with the FTIR spectra of all polymers.

Additives in the polymer can adsorb large amounts, e.g. UV stabilizers etc. so work with your supplier to get a grade with low amounts of additives.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/

 

[dsiap]

Hi Dr DeArmitt,

So I've worked with IR spectrometry before, and I am aware that both ptfe and polystyrene have almost 0 absorption in the range I'm interested in. Polyethylene in fact is something I want to stay away from as there is a strong C-H mode there.

I see now I wasn't clear in my initial post.

I see that the IR transmissivity for PTFE and polystyrene between 3-5 microns should be near 100%. However experimentally based case studies/journal papers I've found report transmissivity a fraction of that. My question is: is there a theoretical basis for this lack of transmissivity? By theoretical I mean assuming a pure sample excluding things such as impurities and surface finishes, etc. For example are there characteristics in the internal microstructure that cause diffusion similar to a grain boundary in metals? That example probably doesn't make sense, but I hope you get the gist.

Best

 

[Demon3]

Aha. Yes, there are factors just like that. In particular, semi-crystalline plastics (PE, PP, nylons, PET, PBT, PLA to name a few) will have crystal boundaries that could induce scattering so you would get a loss albeit not due to absorption.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/

 

[IRstuff]

The way that most people measure transmissivity, scatter ought not substantially affect that. That said, PTFE and polystyrene are, by their very nature, will scatter and therefore degrade image quality, possibly to the point of unusability. I think that PTFE would be the worst of the lot. Polystrene could possibly be made transparent and minimally scattering in the visible.

Another possibility is polycarbonate, which can be made to optical quality in the visible, so there's some hope there

TTFN
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[Pud]

Would this be any use?

Topas (COC)

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[dsiap]

Thanks for all the replies guys.

"The way that most people measure transmissivity, scatter ought not substantially affect that" What method would that be? As I understand it, measurement of diffuse transmission in the IR spectrum is fairly problematic and the error is significant.

I started with polycarbonates, specifically the Lexans which seem to be the best of the bunch, but unfortunately their transmittance drops off drastically past the near IR so 1200 nm+ They are good for the near IR though, and cheap. I'm not really concerned with the visible, only 3-5 micron.

I emailed the vendor about the COC. I'm hoping, but looking at the chemistry I'm suspicious about a dip in the transmittance around 4 microns due to the C-H. Their transmittance graph only goes to about 800nm unfortunately.

Actually... I've reread the experimental study and I'm not sure if I believe it anymore, first of all it's 53 years old, and second they attribute their peak to a vibrational mode of C-F at 4.2 microns = 2381 cm^-1 whereas I believe it should be around half of that. So I'm just going to try and see what kind of results I get with whatever samples I can drum up. If the scattering isn't too catastrophic maybe I can tune the camera to compensate as it should be constant across the spectrum - or at least the 2 micron range I'm interested in. Overall it's looking more like there isn't an obvious suitable candidate other than of course the crystals.

D

 

[IRstuff]

"it's looking more like there isn't an obvious suitable candidate other than of course the crystals."

Which is why there aren't any for sale.


Go as thin as you can, possibly even to the level of Saran wrap; that will give you as good a transmission as possible, and as good an image quality as possible.


TTFN
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[TomDOT]

How big a window do you need? You can get a stock 50mm diameter CaF2 window from Pike for less than $200. If that meets your needs - better than screwing around with polymer sheeting for the next week. Or month.

If that doesn't meet your needs - give them a call. These guys are spectroscopy experts, but much more reasonably priced than the "big name" instrument manufacturers.

https://www.piketech.com/Windows-Disks-KBr.html