Summary for vacuum metallizing plastics with silver Discussion
Vacuum metallizing plastics with silver was discussed in the context of PMMA light-pipe development and a claimed 98% reflectivity target. Bob Harshaw asked whether silver or a silver alloy could be vacuum metallized onto PMMA and whether any supplier could do it. H.R. Prabhakara said silver is not a good long-term reflective coating because it tarnishes, and noted that aluminum is generally used for telescope mirrors after the plastic is lacquered to create a glossy surface; he suggested contacting companies that aluminize automobile headlight reflectors. Bob then reported conflicting information that road reflectors are “silver” metallized. Ted Mooney clarified that road reflectors are neither silver nor aluminum, but retroreflective plastic structures, and that glass beads in road paint are uncoated. For the light-pipe application, Ted said silver coatings are old technology, can be made by reducing silver nitrate, and must be protected from oxidation; he was unsure about adhesion to PMMA and possible outgassing effects.
November 3, 2011
Q. I am seeking some information on the vacuum metallization of PMMA using Silver or Silver alloy. I need a reflective index of 98% and this is the only material that indicates that possibility. First, if anyone has any other option for achieving the 98% RI, please provide that option. Second, I need to find a company that can provide vacuum metallization using Silver, not Aluminum.
Bob Harshaw
Inventor/Product Development – Ottawa, Kansas, USA
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.
H.R. Prabhakara – Consultant
Bangalore Plasmatek – Bangalore Karnataka India
November 9, 2011
Q. Thank you so much for your response. Actually I am told that road reflectors are “silver” metallized because aluminum has a lower reflective index. I have received indications from both sides of the fence. Numerous responses similar to yours and then others that indicate instances where silver is indeed used. I assume the silver surface is coated to prevent any oxidation. Could you please elaborate on your experience with this topic as I remain quite confused.
Thank you,
BFH
Bob F. Harshaw [returning]
– Ottawa, Kansas
December 14, 2011
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,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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December 15, 2011
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.
H.R. Prabhakara – Consultant
Bangalore Plasmatek – Bangalore Karnataka India
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,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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December 16, 2011
Q. The Part that is under development is a “Light Pipe” or light guide. The exposed surface is contained within a light fixture and a a top coat is totally acceptable. The actual reflective surface is protected by PMMA The back side of the finish can be coated with whatever is most effective. I have found several companies that specialize in “silver” metallization, however I am careful not to totally rely on sales based tech data. If you have any additional information on using a reflective surface finish within a light pipe configuration, I would greatly appreciate your thoughts.
Thank you,
Bob
Bob F. Harshaw [returning]
– Ottawa, Kansas, USA
December 31, 2011
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,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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Need to know complete process of vacuum plastic metallization
November 2, 2014
Dear Ted, I hope you are good.
We are initiating a business related to Vacuum Plastic metallization. Being new (as investor) in this business we require following assistance:
1- Chemical Names (utilized in Vacuum plastic metallization)
2- Complete Process of Vacuum Plastic metallization (Step By Step)
3- Names of the machines (required in the process)
Your earliest response will be highly appreciated.
Regards,
David Anderson
– Palo Alto California
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,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
November 2014