Cu is used to prevent the oxidation of silver in glass mirrors. Would it be possible to add a layer of Cu to the silver via PVD after the silver has been deposited on a polyester film in order to provide a similar protection mechanism? Could an acrylic film like Korad 05005 be used as the substrate instead of polyester to provide a tough yet clear surface as well as excellent UV protection? I have zero experience with PVD coatings so I am sorry if this is a silly suggestion.
Your suggestion is far from silly. In some cases it could be used but in most cases there are some reasons why it is not appropriate.
The silver onto PET for mirrors is generally used with the silver being used as the front surface whereas the silver on glass usually uses the glass as the front surface and so the silver is on the back and can be protected using copper and usually a lacquer too.
The use of reflectors on flexible substrates sound simple but there are more factors than are often talked about. The cost of manufacture is one. Silver on glass is a good high performance system but it is perceived that it is much cheaper to coat a roll of polymer film than the heavier sheets of glass and the losses in transport due to breakages is much less with the polymer substrates.
The down side of this change in substrate is that the polymer is not a barrier for oxygen or moisture that glass is and so the silver is prone to degradation. For some applications the high reflectivity of glass is not required and so aluminium can be used and the aluminium forms its own native oxide that is a hard protective layer. Where the superior silver reflectivity is required other strategies have to be considered. In some cases an anti-reflective over coating is used on the silver which also protects the silver from oxygen and moisture but this is a more difficult material to deposit and also is an additional material and so the manufacturing costs increase. Alloys of silver have also been used to slow down the corrosion but this too increases the deposition cost as this needs to be a sputtered coating and the deposition rates are anywhere up to 1000x slower than evaporation.
Using different substrates has been done but generally PET is preferred for the all round performance including the film stability during the deposition process. PET also have UV stabilised grades and optical grades that have high clarity. Again where the optical transmittance has to be maximised an anti-reflection coating on the polymer surface has been used.
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