In practice, the encapsulation materials are simultaneously exposed to heat and light – particularly blue light in the case of white LEDs. The aging process that this causes can be simulated in the laboratory by the blue-flux test, in which disk-shaped test specimens of the cured encapsulants are irradiated at a temperature of 85 °C with blue LEDs operated at a power of 5 watt. At certain intervals, the light transmittance and yellowing of the test disks are measured. Tests show that LUMISIL® 770 retains its high light transmittance even after 1,000 hours. As with every polymer, slight yellowing is detectable in silicone, too. However, it is not apparent to the naked eye and can be considered negligible.
Corrosive gases, too, cause LEDs to age. This is particularly noticeable with the effect of hydrogen sulfide, which is mainly emitted in exhaust fumes in large towns and can pose a problem for exterior lighting applications. In the interior of the LED packages, many surfaces are silvered so that they reflect the incident light toward the encapsulant. However, silver tarnishes immediately in the presence of hydrogen sulfide – even trace amounts – due to the formation of black silver sulfide. This sulfur corrosion may be severe enough even to show through the encapsulation. The encapsulation should therefore also act as a barrier to hydrogen sulfide and other gases that can form hydrogen sulfide.
The sulfur corrosion test shows that the two high-refractive silicones act as a particularly effective gas barrier – due to the presence of phenyl groups in the molecular structure. “The best performance is exhibited by LUMISIL® 591,” says Dr. Yang, who subjected the silicone to extensive applications testing at COEE in Seoul. “The light output of LEDs encapsulated with this particularly hard material decreased least in the test as a function of the exposure time to sulfurous gases.” He added that, with this product, the extremely high crosslink density had an additional inhibiting effect on gas diffusion.