Topcoat For Plastic Reflectors

Question:

I work for a major manufacturer (made in the USA) of emergency warning lights for fire trucks, ambulances, and police cars. We mold our own reflectors out of Polycarbonate, Amodel, Lexan, Apec, and Ultem and have them metallized and coated by a subcontractor. Some of these lights are used in “lightbars” which have separate lenses and some are used with lenses directly attached to the reflector (with gaskets). The operating temperatures run as high as 300F. We occasionally have problems with topcoat failure which leads to a quick failure of the metallizing. Needless to say our customers aren’t too happy when they see black (the base material color) instead of metallizing. Humidity seems to be the root cause. Any suggestions on picking the best coating for this application would be appreciated. B. F.

Answer:

When I had a real job, I worked on a metallized plastic light reflector project for my company’s lighting division. I looked in my archives for references to this work, but couldn’t find any. However, since the important information developed is intellectual property belonging to my former employer, I couldn’t share it with you anyway.

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I am not sure of the adhesion of the metallizing to the various plastic substrates. Perhaps you should look into this as a possible cause for adhesion failures. It may be necessary to prime coat a particular plastic to solve this problem. Many of the high temperature clear coatings are yellowish, not water white, and are unsuitable for your application, however, a silica filled silicon topcoat used on plastic lenses may also be suitable as a reflector topcoat. Unfortunately, I don’t have a specific material recommendation.

Although I can’t name them, there are suppliers of coatings developed specifically for your products. Suppliers who can help you are listed in the Products Finishing 2003 Directory and Technology Guide, under Chemical Coatings, on pages 320 to 321. Another source is under Hardcoatings, clear, scratch-resistant on page 376 in the Directory.