Optical Cement/Adhesive

I need to join a plano-plano interface between two filters, and I'm wondering what the best optical adhesive would be. Wavelengths of interest are all within visible light. One filter
has high UV-A transmittance, so UV-cure would be acceptable. Service conditions are home/office environment, so no special environmental considerations apply.
My principal concern is longevity of the bond. I would guess that epoxy would outperform polyurethane, because of the good adhesion of epoxy to glass.
All other things being equal, I would guess that UV-cure is better than thermal cure, both because of the better pot life of UV-cure and because the stress-free temperature would be room temperature (as opposed to thermal cure, which crosslinks at elevated temperature).
As far as the polymer family, epoxy has good adhesion and good mechanical strength but possibly susceptible to moisture infiltration. Polyurethane is cheaper and mechanically weaker. Silicone optical adhesives only seem to be used by NASA, and they use Dow Corning 93-500 (very expensive). Silicones are mechanically weak and only have good adhesion with the addition of silane coupling agents. If I recall correctly, the bonds formed by silane coupling agents are subject to slow hydrolytic attack by moisture. (I may be wrong about any of the "factoids" in this posting -- feel free to correct me.)
Have I missed any important decision factors? I won't be applying the adhesive myself, so ease-of-use and easy clean-up are not significant factors, except that they may affect how much the optical shop charges me.
What does the military use?
I prefer materials with a long track record, which right now makes me favor thermally cured epoxy over UV-cure polyurethane.
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Mark,
I think you are stressing over this decision a little too much. Almost any decent optical cement would meet the criteria you state. (Although you do not mention the size of the filters. You would need a flexible cement if they are large.)
UV cured is almost always the most convenient, and believe me, there is no problem with adhesion to most glasses. UV cements do have a long track record. By the way, I do not think most UV cured optical cements are, in fact, polyurethanes. Look at the info on the Norland products or Summers Optical websites.
If you are having this done by an optical shop, they will usually have a favorite cement that they use by default and have the most experience with. I would use that.
Depending on your application, it might be more important to specify things such as flatness, parallelism, and bubbles rather than the exact type of optical cement.
--
Adam Norton

Norton Engineered Optics
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Adam Norton wrote:

Assuming that the filters transmit the UV!
Cheers,
Phil Hobbs
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Of course! But the O.P. said that this was the case.
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Adam Norton wrote:

Quite right, replied too fast. Flexible epoxy is a good choice, provided that it's really properly cured. That's sometimes easier to arrange with two-part. In any case, a nice 100C bake for an hour will cover a multitude of sins.
Cheers,
Phil Hobbs
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Adam Norton wrote:

Quite right, replied too fast. Flexible epoxy is a good choice, provided that it's really properly cured. That's sometimes easier to arrange with two-part. In any case, a nice 100C bake for an hour will cover a multitude of sins.
Cheers,
Phil Hobbs
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Mark Thorson wrote:

Best in what regard. If the optics are costly and service acceptable Canadian Balsam is almost infinitely repairable.
If the lenses are large and are subject to sudden cooling, shock or both a cement that stays flexible all it is life is the thing.
In the long term a cheap cement at the topical shop could cost you your business.
Gordon Couger Stillwater, OK www.couger.com/gcouger
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In my experience, this decision often comes down to quantity - does the assembler have enough volume to justify the purchase of a UV curing lamp (or does he already have one)? If yes, then UV adhesives (which are primarily acrylated urethanes) are perfect, and quite robust. See www.dymax.com. If not, 2 part optical epoxies are avialble at RT cure. See www.intertronics.co.uk/products/optintro.htm. Proviso is overall size - you will need increasing resiliency/flexibility with greater bond area/expected thermal excursions. One assumes both filters are glass.
Peter
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