Silicone compounds for LED and optical components

Silicone rubber is an elastomer (rubber-like material) that is widely used in various industries. This material is usually non-reactive, stable, and resistant to extreme temperatures from -55 °C to +300 °C while still maintaining its properties. Because of its properties and ease of processing, silicone elastomers are found in a growing number of products, including: automotive applications, household items and packaging as well as in apparel, sportswear and footwear.

The rapidly growing demand for silicone elastomer with special properties prompted Munich-based chemical company WACKER to develop encapsulants for light emitting diodes (LED) and optical components. The compounds are made of highly transparent silicone elastomers with a high refractive index. The products withstand high operating temperatures and strong light radiation without yellowing. That makes them ideal for manufacturing LEDs with high light efficiency. They also protect the LED chip against corrosive gases, which greatly extends the component’s service life. The silicone encapsulation compounds can be processed in industrial metering processes such as dispensing.

WACKER offers two new HRI silicones for encapsulating high-performance LEDs. The encapsulants stand out due to their high refractive index, excellent transparency and low gas permeability. LUMISIL® 590 and LUMISIL® 591 are highly transparent, addition-curing silicone elastomers with a refractive index of 1.53, which means they rank among the high-refractive-index (HRI) encapsulants. Such grades are particularly suitable for manufacturing highly efficient LEDs. The HRI silicone protects the sensitive LED chip against mechanical influences and corrosive gases. At the same time, the silicone’s high refractive index enables optimum light efficiency.

High refractive property
Semiconductor chips used to generate light in LEDs have a high refractive index. To maximize the amount of light emitted by the LED chip that can pass through the encapsulation, the refractive index of the chip and encapsulant must be roughly the same value. With their refractive index of 1.53, LUMISIL® 590 and LUMISIL® 591 make LEDs highly efficient. The highly transparent silicones are almost completely transparent for light in the visible spectral range (approx. 400 to 700 nm) and do not yellow even when radiation is extremely intense. Transmission tests with LUMISIL® 590 show that a one-millimeter-thick layer lets over 91 percent of visible light through.

LUMISIL® 590 and LUMISIL® 591 protect the LED chip reliably against environmental influences. Corrosive gases such as hydrogen sulfide can damage the LED chip and reduce its performance. Tests show that LED chips encapsulated with LUMISIL® 590 and LUMISIL® 591 are protected against such damage longer and have a prolonged service life.

The  HRI silicones are easy to process, heat-resistant and absolutely tack-free after curing. They also exhibit optimized flow and crosslinking characteristics. With a viscosity of the mix of 2,000 and 2,500 mPa*s respectively, they enable efficient, cost-effective processing. Both products are suitable for encapsulating the LED chip via contact-free dispensing processes. LUMISIL® 590 and LUMISIL® 591 form cured rubber grades of varying hardness. With a hardness of Shore A 65, LUMISIL® 590 is relatively soft, whereas LUMISIL® 591 is formulated to be significantly harder at Shore D 40.

The LUMISIL® product line comprises optical high-performance silicones for LED manufacturers with high requirements. All LUMISIL® products enable very efficient, cost-effective production and exhibit optimized flow and crosslinking characteristics, as well as high photostability. As an encapsulation material, they protect the LED chip reliably against environmental influences and thereby extend its service life.

Rising capacity  
The WACKER Group has expanded its production capacity for functional silicone fluids at its Burghausen site. With the new expansion stage, which came on stream in early January, WACKER is meeting the rising global demand for specialty fluids and silicone emulsions in the coatings, paper, textile, cosmetics and personal care industries. Functional silicone fluids serve as important precursors here. Investments in the expansion amount to around €26 million.

The expansion measure is of particular importance for WACKER SILICONES. At the performance silicones plant in Burghausen, the precursor siloxane and other raw materials are used to make highly specialized functional silicone fluids. They are important starting materials for antifoam, impregnating and release agents and for additives for the construction, coatings, paper, cosmetics and personal care industries. The expansion is increasing the plant’s total production capacity by around 70 percent.

WACKER SILICONES is one of the largest silicone manufacturers worldwide with over 3,000 highly specialized and innovative products. The division’s portfolio ranges from silicone fluids, emulsions, resins, elastomers and sealants to silanes, silane-terminated polymers and pyrogenic silica. These stand out due to their significant value-adding potential, enhancing both the benefits and performance of customers’ end products. WACKER SILICONES’ products find application in such sectors as construction, chemicals, cosmetics, textiles, automotive, paper, electronics and medical engineering. In 2014, WACKER SILICONES generated around 36 percent of Group sales.