Vacuum metallizing Plastics With Silver

A. Silver is not a suitable coating for good reflectivity as it tarnishes over time. Aluminum coating is generally used for telescope mirrors; but it needs a glossy surface. Plastics are first lacquered to get a glossy surface and then aluminum is deposited by evaporation in a vacuum chamber. You may contact those who aluminise automobile headlight reflectors.

A. Hi, Bob.

Now I'm confused. What "road reflectors" are you speaking of? Retroreflective surfaces are neither aluminum nor silver, they are just transparent or translucent plastic of a geometrical shape that causes the light to be retro-reflected. The beads they put into highway paint stripes to brighten them are just transparent glass balls without any coating.

Regards,

A. Traditionally silver is indeed used to get good reflectivity from mirrors. But the silver is on one side protected by the transparent glass and on the other side by some kind of cement or paint. Though silver is expensive, silvering technology - a chemical process - is not. For scientific and technical applications a refractory glass in front of the coating is not desirable as it distorts the image. Coating has to be on the front surface. Aluminum is used in such cases. Though aluminum is inexpensive, the coating process requires vacuum pumps, chambers, power supplies, etc., which are pretty expensive. But it is an environmentally friendly process.
If you want to make an opaque object reflective either you have to go for aluminising or electroplating techniques and compromise on reflectivity. You may also have silver coating with a protective layer of, say, lacquer if the resulting quality is acceptable.

A. Hi again. Just for completeness (since electroplating was mentioned), rather than as a practical answer to Bob's question, rhodium is a very highly reflective electroplateable precious metal which will not tarnish.

Regards,

A. Hi again. The process used to make the silver coating on mirrors and thermos bottles -- I think it's called the Brashear process -- is centuries old and, as H.R. noted, not expensive.

In principle you simply mix silver nitrate and a reducing agent to deposit a thin layer of silver, then you coat the silver so it can't oxidize (the silver in a thermos bottle is under glass on both sides sealed in a vacuum). I don't know if it adheres well to PMMA, but presume it does. I also don't know if PMMA has any outgassing properties that are deleterious towards causing the silver deposit to oxidize. Good luck.

Regards,

A. Hi David. Sorry, I know next to nothing about vacuum metallization and can't help you.

But I suspect that before anyone else can help you they'll need a lot of input from you including exactly what you want to metallize and why. That will determine what metal you'll be using ("vacuum metallizing" is sometimes presumed to mean "vacuum metallizing with aluminum"), whether you'll need a basecoat and a topcoat, and what general types of machines might work (presumably vacuum metallizing rolls of mylar foil involves rather different mechanics than metallizing discrete parts). Good luck, but the first investment I'd probably make if I were you would be in a vacuum metallizing consultant :-)

Regards,