Small Optically clear fluoropolymer beads w RI ~1.36

[JCSI]

Hello,
Am new to this board. Looking for a source for small (~0.200mm), optically clear plastic beads with a refractive index ca. 1.36, which I assume would probably be a fluoropolymer. For example, Norton FEP would be OK, but seems to be supplied only as powder or film?
Jonathan

 

[JCSI]

P.S.
Nafion beads are a bit too expensive for this application, but would like the beads to be wettable by an ethanol-water solution. Teflon AF seems not to be wettable...CYTOP might work?

 

[patprimmer]

Many years ago, ICI used to make some acrylic (Methyl Methacrylate (PMMA)) polymer in small bead form. From memory, maybe 0.3 or 0.4 mm dia. The trade name was Diakon (I think LH and MH).

Maybe it is still available somewhere.

PMMA is very clear and a lot easier to wet than are Fluorocarbon based polymers. I think it's refractive index is about 1.36. I know it is somewhat higher than glass as lenses made from it can be thinner for the same power.

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[Demon3]

I often need to know refractive indices of fillers and polymers so I keep a list for common materials. None of the common ones come close to 1.36, as you realise that's very low and you need probably fluorpolymers to get there.

This looks promising as it says fluoroacrylates will work (I checked and PMMA is too high at 1.49).

http://weekoflearning.pstc.org/papers.php

"Electro-Optical Light Management Material: Low Refractive Index Adhesive
Eng-Pi Chang
Principal Scientist
Avery Dennison

A prototype pressure sensitive adhesive, which has a low refractive index (< 1.36) and is optically clear, has been developed. The refractive index is a key feature in the application of optical polymers and this pressure sensitive adhesive is particularly useful for light transmitting devices. This pressure sensitive adhesive is a fluorosubstituted monoacrylate adhesive. Fluoropolymers are known to have low refractive index, but typically the fluoropolymers are not sticky and are opaque due to crystallinity. The design of this novel fluorosubstituted monoacrylate adhesive polymer is based on the rheological marriage of fluoropolymers and pressure sensitive adhesives."

You would still have the wetting problem though so why not add a tiny amount of surfactant? The best one I know of for wetting low energy surfaces is Dow Corning Additive 67. It's silicone based.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[nbucska]

Glass beads are made for sintered chemical filters
in about this size by the lb-s. Wouldn't it do?




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[JCSI]

Several glasses are used to make beads, but the most common are soda lime and borosilicate, whose RI's are ca. 1.523 and 1.474.
Am trying to find a combo of spheres and liquid for which the spheres in the liquid would be invisible, i.e. matched RI's. This for imaging sub mm delicate museum specimens normally stored in 75% EtOH (~1.36).
Specimens tend to float in liquids near 1.474 (e.g. glycerol), and, anyway, would be best to leave them in the EtOH.
Hence the search for beads of low refractive index, which probably will have to be a fluoropolymer.
My problem is unfamiliarity with FP's and their appearance and the forms in which they are supplied, e.g. clear and ~0.2mm, would be best.

 

[Demon3]

Can't you add a thickening agent to the glycerol to stop your samples from floating? Might be a much simpler, cheaper solution.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[JCSI]

You'd think, but no. This is about photography and posing the subject. If you can't stop the subject from drifting, cold, it does drift, however slowly, and that is endlessly frustrating.

 

[Demon3]

The thickeners I know of for water will completely stop settling / movement for smaller particles anyway. Perhaps you can describe why you need the subject to be that still. What exposure are you using for the photographs?


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Demon3]

In case it helps, I should mention that I have recently been doing some work on matching refractive indices and it might interest you to know that changing the temperature can help match the RI perfectly. RI depends on density which means upon heating or cooling the RI of the liquid will change a lot but that of the polymer won't. So even if you don't find a polymer with perfect RI match to the liquid medium you can still match them by temperature change.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[JCSI]

Yes, I do realize that temperature is very important to the RI of liquids. The working temp will be very close to 20C, but I could consider a heating stage, I suppose. If a polymer of ri ca. 1.34-1.48 could be identified, however, I could get the same precision by mixing liquids of diff RIs.
The full context is that I work for the Smithsonian, and we have embarked on a very large project to image our most valuable biological specimens. Many of these are stored in 75% EtOH and are small (~500um). It is difficult to take decent pictures of small things submerged in liquids. For the photos to be useful scientifically, the specimens must be posed in individually idiosyncratic ways. The informal solution has been to pose them in plain sand (i.e. partially submerged or nestled), but sand as a background is aesthetically inferior.
Thus the quest for clear, "sand-like" particles immersed in a liquid of matching refractive index. Spheres are the optimal shape.
The lowest glass RI's are ~1.45-1.474, but liquids that match are not particularly compatible with 75% EtOH, are oils, or toxic, etc. It's do-able, but not ideal.
So rather than high-tech liquids, how about high-tech spheres? Hence the question about fluoropolymer beads w RI ~1.36?
The component in the adhesive mentioned above in the abstract sounds good, but it's from an abstract at a meeting given by a corporate chemist reporting R&D--likely proprietary and probably unavailable.
Nafion reportedly would work, but it is extremely expensive. Other optical fluoropolymers seem to come in bars or films, and there are thousands of them.
I thought someone reading this forum might happen to know of some promising leads.

 

[JCSI]

P.S.
It's not so much that we can't stop motion w strobes, it's that a moving specimen drifts out of the plane of focus, and we are using 3D image 'stacking' technology to produce composite photos with great depth of field.

 

[Demon3]

Thanks for the clarification. I think there is a solution to your problem if someone is willing to put in some effort. For example there are published tables of RI for all polymers in the Polymer Handbook and other places. The data for the liquids is even more readily available. I will see if I get time to find a pair that might work.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[JCSI]

Thanks! A lot of plastics, I know, are, in fact, supplied as drums of powders that are often various sizes of beads--problem may to be get them before they are turned into films. But is next to impossible to find out their appearance (clear or no) and size, except by calling manufacturers one by one.

 

[Demon3]

So I am having a look now.

First have a look at this, it's Teflon AF from DuPont. It's amorphous and therefore transparent.

http://www2.dupont.com/Teflon_Industrial/en_US/products/product_by_name/teflon_af/properties.html

The curve of refractive index versus temperature is here:

http://www2.dupont.com/Teflon_Industrial/en_US/assets/images/teflon_af_fig3.jpg

It seems to be in the right range for your application.

Of lowest RI of any hydrocarbon polymer (non fluoro) is TPX® Polymethylpentene with 1.46

http://www.boedeker.com/tpx_p.htm

Here's a nice document of refractive index matching for LEDs. They use silicones (RI 1.40)and fluorosilicones (RI 1.38) and silicone thermosets (RI 1.38 – 1.57).

http://www.polytec-pt.com/ger/_file...cone_for_High_Brightness_LED_Packaging(1).pdf

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Demon3]

I found Google hits for "FEP pellets" as you said that would work.

[PDF] Teflon(R) FEP 160, technical informationFile Format: PDF/Adobe Acrobat
FEP 160 is supplied as pellets and is. available in 55-lb (24.9-kg) multilayer kraftbags. with an integral polyethylene liner. U.S. Freight Classification ...

http://www2.dupont.com/Teflon_Industrial/

en_US/assets/downloads/h26581.pdf - Similar pages
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There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[JCSI]

Dupont is sending me some FEP--they say it is "milky." Will let you know how it goes. Teflon AF is priced like nafion--ca. 1K/gram.
Some time ago I spoke to the NuSil folks, who were kind enough to send me a sample, but the stuff cured to a rather rubbery, sticky solid, which I milled to small particles, but they stay sticky, even when wet. Can't have sticky. It may be that the hardener didn't properly mix. But you're right, NuSil is another promising technology.
Have been measuring the temp--it's gone up from 26 to 29C in about two hours, due to fiber optic lighting of EtOH bath.

 

[Demon3]

Did you try mixing some solvent with high refractive index into your ethanol? Benzoic acid or phenol would probably mix and do a lot to increase the refractive index to the level where you can match the RI of the solvent mixture to common polymers.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[JCSI]

No, but I will keep that in mind. Was also thinking of using Benzyl Alcohol, which is miscible with ethanol.

 

[Demon3]

Benzyl alcohol is a good choice. As you probably know, anything with a phenyl/benzene ring in it will give a high refractive index.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell