Light-emitting diodes are currently encapsulated by thermally curable epoxy resins. Thermal curing systems require long curing cycles at high temperatures. Further, because of viscoelastic behavior of the resin, the resin tends to "creep" along the connecting wires (Weisenberg effect), which causes solderability problem. The cured resin should be removed manually, which is time consuming and labor intensive. These problems are solved by the ultraviolet radiation curable systems. UV curing is an ultrafast reaction and takes place at room temperature. No creep behavior occurs due to the rapidity of the curing. The UV curing technique can result in higher productivity and energy saving than the thermal process. This article presents results on the development of UV curable formulations based on cycloaliphatic diepoxide, diglycidyl ether of bisphenol A, and epoxidized novolac induced by cationic photoinitiators. "Mixture experimental design" was employed to arrive at the optimum composition, which meets the stringent demands of performance characteristics and durability.

No abstract

This work aims to synthesize new series of polysiloxane copolymer through hydrosilylation reaction based on different level of crosslink densities and phenyl content. The results revealed that polysiloxanes resins with phenyl rings increased viscosity up to 3800 cps while displaying a relatively high refractive index of 1.531. The asymmetric phenyl substitution onto chain backbone led to formation of densely packed structure resulting in reduced water absorption and gas permeability of PDMS resins. High level of crosslinking contributed to a high shore A hardness and adhesion strength. It displayed transparency of 97% whose percentage reduction was 1.6% under thermal and 0.6% UV aging. No obvious discoloration was observed during accelerated thermal aging up to 210 min at 80 C. These results are key attributes that make the synthesized polysiloxane series as ideal candidate for high brightness LED encapsulant.

A series of high refractive index of α,ω-vinyl silyl terminated polydimethylsiloxane-co-polydiphenylsiloxane was synthesized by the equilibration ring opening – anionic polymerization of cyclic organo-siloxane oligomers in the present of 1,3-divinyltetramethyldisiloxane as terminating agent. Structural modifications were characterized by use of FTIR, H-NMR, and solution viscosity. UV-visible spectroscopy was analyzed which reveal a good transmittance in the region 400 – 750 nm radiation for all polysiloxanes.The presence of diphenylsiloxane unit results in a linear increase of the refractive index of the resin. These features favor good application as encapsulant for high brightness LED (HBLED) packaging.