Silver and silver alloy have been in use for a long time as noble metals by utilizing their excellent optical and electrochemical properties for ornaments, coins, tableware, electronic materials, lighting devices, and dental materials. There have recently been rapidly increasing demands for them as reflecting materials for light-emitting diodes (LED). The light-emitting diodes have been in use as light sources replacing fluorescent lamps or incandescent light bulbs for lighting devices, automobile lights, and the like, while these light-emitting devices provide a substrate with a light-reflecting layer such as a silver-plated layer, so as to improve light extraction efficiency.
However, silver and silver alloy are chemically so unstable that they tend to react easily with oxygen in the air, moisture, hydrogen sulfide, sulfurous acid gas, and the like, thereby producing silver oxide and silver sulfide, which blacken the silver surface by discoloring (corroding).
As a method for preventing silver from discoloring (corroding) as such, organic corrosion inhibitors, for example, have been proposed (see, for example, Patent Literatures 1 and 2). However, these organic corrosion inhibitors have a drawback that they are less resistant to ultraviolet rays and likely to discolor when exposed to the ultraviolet rays for a long period of time. The light-emitting diodes used for lighting devices and automobiles employ near-ultraviolet rays, which make it difficult to apply the organic corrosion inhibitors.
As a material replacing the organic corrosion inhibitors, a modified silicone material having high gas sealability and ultraviolet resistance has been proposed to be applied as a sealing material for light-emitting diodes (see, for example, Patent Literature 3).