Wacker, a chemical company based in Munich, Germany, has introduced two new high refractive index silicones for encapsulating LEDs. In addition to their high refractive index, the silicones boast excellent transparency and low gas permeability. Lumisil® 590 and Lumisil® 591 are highly transparent, addition-curing silicone elastomers. Both Lumisil 590 and 591 feature a very high refractive index of 1.53. Grades of silicon with high refractive indexes are especially suitable for manufacturing highly efficient LEDs. The silicone protects a sensitive LED chip against mechanical influences and corrosive gases. The silicone’s high refractive index enables optimum output.
An encapsulate with a high refractive index helps maximize the amount of light emitted by the LED chip that can pass through the encapsulation. Wacker points out that the refractive index of the chip and the encapsulant must be roughly the same value. Highly transparent silicones allow almost all light in the visible spectral range of about 400 to 700 nm to pass. The company says that the robust encapsulants do not yellow even when radiation is extremely intense. Wacker reports that a one-millimeter-thick layer of Lumisil 590 show lets over 91 percent of visible light through, according to transmission tests.
Lumisil 590 and Lumisil 591 protect the LED chip reliably against environmental factors such as corrosive materials and gases. Wacker asserts that its tests found that LED chips encapsulated with Lumisil 590 and Lumisil 591 are longer protected against such damage and have a prolonged service life.
According to Wacker, the new silicones are easy to process, absolutely tack-free after curing, and heat-resistant. They also have optimized flow and crosslinking characteristics. The a viscosity of the encapsulants is of 2,000 and 2,500 mPa*s respectively, enabling efficient, cost-effective processing. Both silicones allow contract free dispensing processes. Once cured Lumisil 590 and Lumisil 591 form rubber grades of varying hardness. Cured Lumisil® 590 is relatively soft with a hardness of Shore A 65, and cured Lumisil 591 is is significantly harder at Shore D 40.