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PETTA/PETTG optical resin, a kind of LED encapsulating resin with high refractive index, was prepared via click chemistry method in this study. Optical and thermal properties of this resin were investigated with UV–Vis scanning spectrophotometer, Abbe refractometer and thermogravimetric analyses (TGA), respectively. The results show that the light transmitance of this resin can arrive up to 93% and its refractive index is 1.556, which is higher than those of silicone resins. Meanwhile, the cured PETTA/PETTG resin demonstrates the equal thermal stability to silicone resins, and its 5% weight loss temperature was about 350 °C. Therefore, the cured PETTA/PETTG resin could be used as an alternate of expensive silicone resins in LED encapsulation.
© 2015 Optical Society of America
1. Introduction
Recently, LEDs have garnered increasing attention as a new solid light source. However, there may be a loss of light because the GaN chip in LEDs has a high refractive index (~2.2). To overcome this constraint and to improve the light extraction efficiency, the refractive index of encapsulating materials needs to be further increased [1].
Currently, epoxy and silicone resin are widely used as the main encapsulating materials in LEDs. Epoxy resin exhibits a high refractive index, but its heat stability is poor, and it easily undergoes decomposition and discoloration. Compared to epoxy, silicone resin is increasingly used for encapsulating LEDs due to its excellent thermal stability, but the refractive index of silicone is too low and very difficult to be improved up to the level of epoxy resin. The refractive index of silicone resin can be regulated by the amount of phenyl groups that are connected to the silicon atom, and its index value is in the 1.400~1.570 range [2]. However, its resistance to UV decomposition and thermal oxidation are reduced because of the high phenyl group content. Therefore, the preparation of the encapsulating material with a high refractive index and a good thermal stability must be the main direction and object.
Click chemistry is a term applied to chemical synthesis tailored to generate substances quickly and reliably by joining small units together. Click chemistry is not a single specific reaction, but describes a way of generating products that follows examples in nature, which also generates substances by joining small modular units. Based on the C-X bonding reaction, click chemistry is one of the most effective ways of synthesizing a target compound [3]. The click chemical reaction between a thiol and an ethylene is simple, and the chemical structure of the target compounds could be easily regulated. In addition, the obtained thioether structure is beneficial for improving the refractive index [4–6]. Moreover, the reaction between thiol group and ethylene group can be triggered by radicals produced from heating and UV illumination, so it is a simple and effective method [7~11].
Rie Okutsu et al. synthesized a thermoplastic poly(sulfide sulfone) film with a high refractive index by a click chemistry method using 2,5-dihydroxy-1,4-dithiane and divinyl sulfone as sources [12]. 5% weight loss temperature of the film was 275 °C under N2 atmosphere, and its refractive index was 1.686. Using the same method, Yasuo Suzuki synthesized the flexible and transparent polymer film with a maximum refractive index of 1.651 [13~15]. In addition, Hironori Matsushima and his associates investigated the click chemistry reaction among isocyanate, acrylate, and thiol [16]. They synthesized a series of polysulfide ammonia ester films, and the refractive index could achieve 1.550.
These resins prepared by click chemistry method showed high refractive indexes. However, most of the reported work had focused on thin films with very small thickness (normally less than 50 μm) which can’t be used in LED encapsulation. In addition, the complex preparation process and high price in the reported work have limited the application of these resins in LED encapsulation. In contrast, pentaerythritol tetraacrylate (PETTA) and pentaerythritol tetrathioglycolic (PETTG) are suitable to preparing encapsulation materials because they have integrated features such as perfect transparency, stability, extensive raw material, active group, and rapid click chemistry reaction. It can be seen from Fig. 1 that cross-linked network structures can be formed by the click chemistry reaction between PETTA and PETTG because of the four active groups existed in each molecule structure. This kind of optical resin is expected to have the higher stability, refractive index and transparency. In our experiment, the cured PETTA/PETTG resin was successfully prepared and the optical and thermal properties were investigated in detail.
2 Experimental
2.1 Materials
PETTA (CR Grade) was purchased from the Guangzhou Deco Composite Technology Co., Ltd. PETTG (CR Grade) was purchased from the Qingdao Jiahua Chemicals Industry Co., Ltd. Azobisisobutyronitrile(AIBN, AR Grade) was purchased from the Tianjin Damao Chemical Reagent Factory.
2.2 Synthesis of PETTA/PETTG cured resin
A solution was prepared by dissolving 0.01 g of AIBN into 0.01 mol of PETTG. The resulting solution was mixed with 0.01 mol PETTA by stirring and then injected into the silicone rubber mold to solidify. The molecular structure of the reactants and the reaction equation are shown in Fig. 1.
The mixture was heated at 60 °C for 2 h and subsequently exposed to 140 °C for 0.5 h. Or the mixture was transferred into the UV-curing machine to solidify for 20 seconds and then annealed at 140 °C for 0.5 h to further solidify.
2.3 Characterization
Fourier transform infrared spectra (FTIR) were obtained by a Nicolet 6700 FTIR spectrometer in order to identify the chemical structures of specimens, which were prepared by attaching the samples to a potassium bromide (KBr) disc. Non-isothermal curing kinetics was studied by DSC with a TA DSC Q100 calorimeter under the protection of nitrogen flow (flow velocity of 50 mL/min). After the sample cooled to the room temperature, the glass transition temperature was characterized under the same heating rate at 10 °C /min from −40 to 250 °C. Calibration of the calorimeter with regards to temperature and energy was achieved by using the temperature and enthalpy measurements of the melting of indium and lead as the standard material. Thermogravimetric analysis (TGA) was carried out using TA Instruments TGA Q50 analyzer from the United States. The samples were heated from 25 to 550 °C at a rate of 10 °C/min under the protection of nitrogen flow (balance purification velocity is 40mL/min,flow rate in the sample room is 60 mL/min). Transmittance spectra of the samples with the thickness of 1 mm were recorded with an UV–Vis scanning spectrophotometer (UV2450) coming from SHIMADZU Co., Ltd. of Japan. The refractive indices were determined using NAR-1T solid Abbe refractometer according to reflection method, purchased from ATAGO Co., Ltd. of Japan. The samples of refractive index were cut from the bulk sample and burnished to mirror, size about 10*15*20mm. The UV-curing machine was obtained by Dongguan Concept Photoelectric Technology Co., Ltd and the curing time was 20 seconds.
3. Results and discussion
3.1 FTIR Analysis
FTIR spectra of PETTA, PETTG, and the uncured PETTA/PETTG mixture were tested in the transmission model. FTIR spectrum of the PETTA/PETTG cured resin was directly tested by the intelligent reflection accessory of the FTIR spectrometer.
In Fig. 2, the peaks at 3017 cm−1, 3038 cm−1, 1635 cm−1, 1618cm−1, 1061 cm−1, 984 cm−1, and 808cm−1 are attributed to the stretching vibration mode of -CH = CH2, and the peak at 2566 cm−1 is attributed to the stretching vibration mode of -SH in the PETTG molecule. The peaks at 2566 cm−1,1635 cm−1, 1061 cm−1 and 808cm−1 all weaken or disappear after the PETTA/PETTG mixture were cured by UV or heating, which indicates that the -CH = CH2 groups did react with -SH groups. FTIR spectra of the PETTA/PETTG mixture before and after reaction indicate that the curing reaction has happened and the crosslinked chemical structure has been formed. Moreover it can be seen from the curves of the mixtures cured by UV and heating that the ultraviolet curing and thermal curing treatment can achieve the same chemical structures.
Fig. 2 FTIR spectra of PETTA, PETTG, and the mixture of PETTA/PETTG before and after solidification.
3.2 Optical property
The refractive indexes of PETTA, PETTG, the uncured PETTA/PETTG mixture and the cured PETTA/PETTG mixture were 1.487, 1.541, 1.521 and 1.556, respectively. These results indicated that the sulfate atoms were introduced into the resin by the click chemistry reaction between the sulfhydryl group and ethylene.
Kim et al. [17] prepared some silicone material for LED encapsulation,which transmittance at 450 nm is 90% with a thickness of 2 mm. In particular, it has good thermal stability against discoloration to yellow by aging even at 200 °C, which is a key factor for the long lifetime of a LED encapsulant. As shown in Fig. 3, the light transmittance of the PETTA/PETTG cured resin with a thickness of 1 mm is excellent. The light transmittance is more than 90% in the visible light region, and the maximum value is 93%. The solidified sample does not contain any allochroic groups, and the sample shows excellent heat aging resistance. Therefore, the aging and allochroic phenomena are not observed in the solidified progress, and the targeted products are colorless and transparent, which means it is suitable for application to LED encapsulation.
3.3 DSC analysis
The curing progress of PETTA/PETTG and the glass transition temperature of the cured PETTA/PETTG were tested by differential scanning calorimetry (DSC), and the obtained results are shown in Fig. 4 and Table 1.
Table 1. DSC results of PETTA/PETTG and PETTA/OBDMT
As shown in Fig. 4 and Table 1, the onset temperature of the curing progress is 60.2 °C, and the fastest curing rate is observed at 96.1 °C. The terminal temperature is 137.2 °C, and 353.2 J/g heat is released in the progress. As can be seen in the figure, a small melting peak observed in the region between 30 °C and 50 °C is assigned to the fusion of PETTA, and a large amount of heat is produced in the curing progress. Moreover, necessary measures should be taken to exclude the opacity samples with bubbles. The resin was cured by low reaction rate at 60 °C, and the produced heat could be released. Then, the cured reaction could be carried out at 140 °C, and the residual groups could be consumed at last.
The DSC curve of the cured PETTA/PETTG at second heating indicates that the glass transition temperature of the cured sample is only at 25.6 °C, and the low Tg implies excellent flexibility. The excellent flexibility would be highly beneficial to avoiding the circuit malfunction caused by encapsulating resin and substrate cracking.
The glass transition temperature of the silicone material synthesized by Kim et al [17] was about 27 °C~39 °C.Their work pointed out that the softness of the material was desirable for LED encapsulation applications because it protected the delicate wire connections between the PCB and LED, which could be easily broken due to mechanical stress induced by fluctuations at high temperature.
3.4 TG analysis
The high thermal stability of the silicone materials is necessary for application as a kind of LED encapsulant. The temperature at 5% weight loss of the silicone material synthesized by Kim et al [17] was about 350 °C, under N2 condition with a heating rate of 5 °C/min. The thermostability of the cured PETTA/PETTG resin was tested by the TGA according to the aforementioned method, and the obtained results are shown in Fig. 5.
As shown in Fig. 5, the cured PETTA/PETTG resin shows high thermostability. The sample does not decompose below 300 °C, and its T1 (The temperature of 1% weight loss) is and T5 is 318.3 °C and 352.7 °C, respectively. The temperature at 5% weight loss of the cured PETTA/PETTG resin was improved about 20 °C comparing with that of the material prepared by Gao N [18] and it is equal with the thermal stability of silicon resin using for LED encapsulation. Once thoroughly solidified, all the chemical bonds reached a saturated state, and a crosslinked structure was formed, therefore exhibiting excellent thermostability. From the viewpoint of thermostability, the cured PETTA/PETTG resin would be a suitable substitute for traditional silicon resin as LED encapsulating material. Also, the sample exhibited no color change and no significant brightness reduction after 8 hours thermal ageing test at 100 °C.
4. Conclusions
The cured PETTA/PETTG resin has prepared via the click chemistry method by UV or heating. The results from FTIR indicate that two kinds of curing methods have similar effect on the reaction between the sulfydryl and double bond, and both could achieve the designed chemical structures.
Transmittance of the cured PETTA/PETTG resin is more than 90% for visible light, with a maximum value of 93% and a refractive index of 1.556. The lower glass transition temperature of the cured sample suggests that the sample has good flexibility. In addition, the cured PETTA/PETTG resin shows the excellent thermostability and the temperature of 5% weight loss is about 350 °C. Therefore, the cured PETTA/PETTG optical resin can be a potential substitute for the traditional silicon resin as LED encapsulating material.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. U1201243, 51273221, and 51173047), the Shenzhen Polytechnic Science Foundation (601422K27003), the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (2012LYM_0121) and the Fundamental Research Funds for the Central Universities SCUT (NO. 2014ZZ0004, 2014ZM0008).
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