Encapsulants for light‐emitting diodes from visible light‐cured epoxy monomers

Abstract

During the last decade, light‐emitting diodes (LEDs) have replaced incandescent, fluorescent, and neon lamps due to their ability to produce high luminosity at low currents and voltages. LEDs are currently encapsulated by thermally curable epoxy resins. However, long periods of curing at high temperature result in high consumption of energy and require stringent process control to avoid failure of the devices. In addition, the thermal cure results in yellowing of the encapsulant, which decreases the efficiency of the LED. In recent years, photoinitiated polymerization has received much interest as it congregates a wide range of economic and ecological benefits. Cationic photoinitiators, such as diaryliodonium salts, generate Brønsted acid in situ, which initiates polymerization. The process can be triggered on demand by irradiating the mixture with light. Results from the present research reveal that cycloaliphatic epoxy monomers, photoactivated with an iodonium salt and Camphorquinone, polymerize readily under visible light irradiation (470 nm) in the absence of external heating. The partial replacement of cycloaliphatic epoxy with aromatic diglycidyl ether of bisphenol‐A (DGEBA) is an effective means of improving the refractive index of the material and consequently the efficiency of the photoemission. Visible light polymerization of DGEBA pure proceeds at a slow rate; however, it is enhanced by the increase in temperature during the polymerization of the highly reactive cycloaliphatic monomer. From results obtained in the present research, it may be concluded that visible light polymerization of epoxy monomers is a promising route for the processing of LED encapsulants. Copyright © 2013 John Wiley & Sons, Ltd.

Monitoring of visible light photopolymerization of an epoxy/dimethacrylate hybrid system by Raman and near-infrared spectroscopies

“…The conversion profiles of methacrylate monomers in thick specimens were calculated from the decay of the absorption band located at 6165 cm À1 [12]. The extent of reaction of epoxy monomer at the surface of 1.5 mm thick specimens was evaluated from the decay of the band at 745 cm À1 by attenuated total reflectance [13] using a diamond crystal (4 cm À1 , 64 scans).…”

Quantitative Analysis of Curing Mechanisms of Epoxy Resin by Mid- and Near- Fourier Transform Infra Red Spectroscopy

“…Same principle can be used in nIR region taking 4530 cm -1 peak as proof of epoxy group remaining 5, 13,20,29,36 . This peak speaks for combination of the second overtone of the epoxy ring with fundamental C-h stretching Fig.…”

Synthesis of phenyl silicone resin with epoxy and acrylate group and its adhesion enhancement for addition-cure silicone encapsulant with high refractive index

“…In recent years, light emitting diode (LED) has gradually taken the place of incandescent, fluorescent, and neon lamp owing to its many attractive properties such as high luminous efficiency, driving at low voltage, high resistance to vibration, low power consumption, superior lifetime, light weight, [1][2][3][4] and is widely used in lighting and display fields. The encapsulant is an important component of LED devices.…”