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.
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.