Something optically clear, temperature range of -40C to +60C (70C would better) that can tolerate daily changes of ~30C for ~20 years. Ideally, adherent to a variety of surfaces but I can compensate, if necessary.
And, not exhorbitantly priced (I need gallons of it)
Rigid or flexible?
I take it you've ruled out things like
bob prohaska
Neither.
As a student (1980's) I worked on pulsed magenetic field coils which had to be reinforced with fiberglass or kevlar and encapusulated to keep them from blowing up. The usual encapsulant was an epoxy resin sold under the name Epocast 202. It was chosen for low viscosity, minimal shrinkage on cure and reasonable strength. It happened to be clear when carefully degassed before casting, though in this case optics mattered less than dielectric strength.
Like most cured epoxies, it set glass-hard when the hardener/resin ratio was stoichiometric, so it fails your requirement, but hardness can be tuned somewhat by varying the ratio.
PDMS resins (really, elastomers) are generally fairly soft (think RTV silcone) but a skilled formulator might be able to adjust that if desired. Any opacity comes from fillers.
Epocast 202 was sold by a company called E. V. Roberts, now apparently GracoRoberts. It's surely not now the outfit it was then, but you might inquire anyway. I think you'll find a chemist more help than anybody else.
hth,
bob prohaska
All the epoxies I remember were at least lightly tinted amber. If "water clarity" is needed a PDMS formulation comes closer.
Good luck,
bob prohaska
IIRC the 3 to 5 years was for most of day exposure in sunny southern climates, and due to the fact that the UV "inhibitors" are actually absorbers, thus the very top of the coating slowly degrades while the bulk is protected. High quality 2 part polyurethane is extremely abrasion resistant, this is what they use for automotive clearcoat, for example
Some clear epoxies claim to be UV resistant
Both 2 part polyurethanes and silicones (PDMS) have toxicity issues, usually specced 'for professional use only', ignore MSDS and mfgr's application info at your own peril.
Next best thing to glass as a front surface might be UV resistant acrylic sheet, will eventually dull from UV like polyurethane but last a long time in low UV situation, might be bondable to clear epoxy which bonds poorly to glass, removal of factory release agent could be required. Closer CTE to epoxy also, should reduce stress on bond.
Many vendors offer application assistance which could save you a lot of time.
So use water clear epoxy topped off with a used cell phone UV resistant "Gorilla Glass" top.
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