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Novel UV-transparent 2-component polyurethane resin for chip-on-board LED micro lenses

Joachim Bauer, Marko Gutke, Friedhelm Heinrich, Matthias Edling, Vesela Stoycheva, Alexander Kaltenbach, Martin Burkhardt, Martin Gruenefeld, Matthias Gamp, Christoph Gerhard, Patrick Steglich, Sebastian Steffen, Michael Herzog, Christian Dreyer, and Sigurd Schrader

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Abstract

In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (11)
Fig. 1.
Fig. 1. Schematic representation of the simulation of reflection Rsim and transmission Tsim considering the multiple incoherent reflections within the substrate and coherent interference effects within the thin film [41]. R0 is the air side and R1 the substrate side reflection from the thin film, T0 is the air to substrate side and T1 the substrate to air side light transmission through the thin film, R2 is the substrate/air reflection and T2 the transmission, respectively, and Tsub is the substrate transmission (for quartz Tsub = 1).

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Fig. 2.
Fig. 2. TGA curve of the Polyurethane, Temperature Program: Heat from 20 °C to 600 °C with a heating rate of 10 °C/min, in Nitrogen atmosphere with a purge rate of 10 mL/minute. The 5% weight loss temperatures Td5% are marked by circles (o).

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Fig. 3.
Fig. 3. DSC measurements at temperatures from -150 to 500 °C. The marked points indicate the glass transition temperatures Tg and the degradation temperatures Td of the polyurethanes. The polymer shows an exothermic reaction at 150 °C for resPUR-OT-3000 and 220 °C for resPUR-OT-T24000 and resPUR-OT polyurethane, probably caused by a crystallization process or postreaction (secondary reaction of isocyanates).

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Fig. 4.
Fig. 4. Contact angles of polyurethane resPUR-OT on different modified substrates: (left) glass with the surface tension of σ d = 35 dyn/cm and σ p = 31 dyn/cm and (right) polytetrafluorethylene (PTFE) modified glass substrate with the surface tension of σ d = 14 dyn/cm and σ p = 2 dyn/cm, measured by contact angle measurements with water and methylene iodide. Both surfaces are used for the determination of the surface tension of the liquid polyurethane according to [38–40].

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Fig. 5.
Fig. 5. Experimental and simulated (red) reflection spectra of polyurethane thin films. High-resolution spectra in a wavelength range of 400 - 600 nm were normalized to the low-resolution spectra (200-890 nm) and show typical thin-film interferences, from which the film thicknesses d was determined in the fitting procedure resulting in d = 13.86 µm (resPUR OT-T24000), d = 4.88 µm (resPUR-OT-3000) and d = 6.87 µm (resPUR-OT).

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Fig. 6.
Fig. 6. Experimental and simulated (red) transmission spectra of the PU thin films as shown in Fig. 5 measured with the spectrometer in the spectral range of 200–890 nm. resPUR-OT has the maximum band gap energy of 5,02 eV corresponding to ∼ 247 nm.

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Fig. 7.
Fig. 7. Optical constants of Polyurethane, (a) refractive index, and (b) extinction coefficient by R&T measurement of thin films and thick PU rods. The relatively high k in the long wavelength region of resPUR-OT-T24000 is caused by the exciton absorption effect.

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Fig. 8.
Fig. 8. Frequency doubled 532 nm Nd:YAG laser beam transmitted through the 8 mm thick PU rods of resPUR-OT-3000 (a), resPUR-OT-T24000 (b) and resPUR-OT (c). Photographs taken at right angle to beam direction demonstrate fluorescence (a), strong (b) and weak (c) scattering.

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Fig. 9.
Fig. 9. Experimental transmission spectra of 8 mm thick PU rods. The deviation from the maximum transmission in the long wavelength range can mainly be explained by scattering, which is quite high with resPUR-OT-T24000 and resPUR-OT-3000. In contrast, the optimized material resPUR-OT shows very low scattering.

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Fig. 10.
Fig. 10. Transmission of optical polymers [9] compared to polyurethane resPUR-OT at a film thickness of 3.174 mm. The transmission was calculated using the optical constants n and k from Fig. 7(a) and (b).

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Fig. 11.
Fig. 11. Dome-Type package of InGaN-CoB-LED with Polyurethane resPUR-OT lens (a), (b) together with measured and simulated (red) light distributions at λ = 525 nm (c).

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Tables (4)

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Table 1. Polyurethane material and component specification.

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Table 2. Thermal characteristics of polyurethane resPUR.

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Table 3. Surface tension of liquid polyurethanes resPUR.

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Table 4. Summary of some key characteristics of optical properties of polyurethane resPUR.

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Equations (3)

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S ( n , k ) = ∑ λ 1 λ m ⁡ { [ R s i m ( n ( λ ) , k ( λ ) , d ) − R e x p ( λ ) ] 2 + [ T s i m ( n ( λ ) , k ( λ ) , d ) − T e x p ( λ ) ] 2 } .
T s i m = T 0 T 2 T s u b 1 − R 1 R 2 T s u b 2
R s i m = R 0 − R 2 T s u b 2 ( R 1 R 0 − T 1 T 0 ) 1 − R 1 R 2 T s u b 2 .
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