Development of UV stable LED encapsulants

In recent years, high power white LED devices (LEDs) have been developed and widely used in display, display backlight module and general lightings. UV LED pumped RGB phosphors is one of the methods to generate white light. It shows some advantages, such as: excellent color rendering index (CRI), tolerant to LED variation, etc. However, the high energy of UV or near UV based LED lighting source leads to encapsulated transparent epoxy resin chain scission and forms some chromatic sites. The discolored epoxy materials decreases the light output and accumulates heat inside to speed up material degradation. Moreover, LED display and LED lighting devices also suffer UV irradiation from sunlight in the outdoor applications. In order to avoid the UV-degraded problem of LED encapsulating materials, we developed UV resistant material technology. Our developing strategies included evaluating UV stabilizer, synthesis of silicone modified epoxy or curing agent to enhance the bonding energy of encapsulating material. Transparent epoxy resin and anhydride curing agent systems are commonly used in LED encapsulation applications. In this work, UV resistant properties of different epoxy resin structures were evaluated, firstly. It was found that aromatic type epoxy system discolored more quickly than the other epoxy systems due to the UV absorption of the conjugated structure. Secondly, we evaluated the effect of UV absorbers (UVAs) for the UV resistant properties. Modification with silicone materials has been utilized as the most effective method to improve the toughness, thermal and UV stability of transparent epoxy encapsulant. Therefore, we also developed silicone-modified reactive compounds to improve UV stability of current epoxy based encapsulant. The results showed excellent UV resistant properties of cycloaliphatic epoxy systems by the addition of silicone-modified epoxy. The technologies of UV absorbers and silicone-modified epoxy systems demonstrate excellent UV resistant properties for high performance LED applications.