Preparation and properties of vinylphenyl-silicone resins and their application in LED packaging

Abstract

Three types of vinylphenyl silicone resins, vinyl-terminated, vinyl-capped, vinyl-terminated and vinyl-capped silicone resins, were synthesized through co-condensation and poly-condensation methods, and were characterized by nuclear magnetic resonance spectroscopy (¹H NMR, ²⁹Si NMR) as well as Fourier transform infrared spectroscopy (FTIR). The cured phenyl-silicone resins were characterized by thermogravimetric analysis (TGA), a electron universal testing machine at different temperatures, spectrophotometry, and transmission electron microscopy (TEM). The TGA results indicated that the thermal stability of cured phenyl-silicone resins prepared from the vinyl-terminated silicone reins was better than that of vinyl-capped silicone resin. In addition, all of these cured phenyl-silicone resins exhibited an onset decomposition temperature higher than 500 °C. The values of elongation at break were above 20% at −40 °C, indicating that these materials possessed a certain toughness at a relatively low temperature. The results of TEM revealed that the materials with a higher homogeneity degree of morphological structure could obtain 98.7% transmittance at a wavelength of 680 nm. Reliability tests, including a dye test, thermal shock test, and wet and high temperature operation life (WHTOL) test, demonstrated a wide range of applications of these materials in LED packaging.

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Introduction

Owing to its striking features, such as low power consumption, long lifetime, versatility in having a variety of colors, environmental friendliness and short response time, the light emitting diode (LED) has attracted great attention. LED lighting is considered to be the next-generation light source after the compact fluorescent lamp (CFL). With the continuous development of LED technology, the application of LEDs has penetrated into many aspects of human life, including lighting, displays, communication, signage, medical services and general illuminations in recent years, which brings a variety of convenience, brightness and color to people.^1–10

Among the components of LED devices, LED encapsulation materials play a vital role in protecting the chip and providing light channels. Since the development and commercialization of LEDs, the evolution of LED encapsulation materials has experienced a change from epoxy resin to silicone.^11–15 In the early time, thermosetting epoxy resins such as diglycidyl ether of bisphenol-A or cycloaliphatic epoxy resins were widely used as encapsulation materials of LEDs due to their overall properties and cost.^13,16 Unfortunately, on one hand, diglycidyl ether of bisphenol-A epoxy resins cannot maintain their transparency at the junction temperature as a result of thermal decomposition, which would result in a serious luminous attenuation performance.^17,18 On the other hand, although the cycloaliphatic epoxy resin could overcome the yellowing phenomenon, thermal residual stress is another unfavorable factor for epoxy resins, which would result in packaging materials cracked during the LED application process, especially in freezing outdoor environment. At present, to address the issue of yellowing and thermal stress, silicone resins are regarded to be superior to epoxy ones in terms of thermal and mechanical characteristics for high-power LED.^19,20 Furthermore, their outstanding performances, including stable thermo-optical property, a wide range of service temperatures, good transparency and minimal moisture absorption, make silicone resins suitable for LED packaging.^21,22

The refractive indexes (RI) of silicone encapsulant generally range from 1.41 to 1.55 depending on the number of phenyl contents, which are relatively low compared with LED chip.²³ The RI gap between the chip and the encapsulant brings in significant internal reflection, resulting in reduced light extraction efficiency of conventional LED packages.^23–27 It seems that higher RI silicone encapsulant is a better material for LED package. However, it is usually found that low RI silicone resin are preferred to high RI silicone resin for high power LED device packaged. This is because higher RI silicone encapsulant with more rigid phenyl would bring about great internal stress during polymerization process of encapsulant. Because of the existence of the internal stress, several phenomena would take place, such as the decrease of adhesive strength between encapsulant and LED die (the mismatch of thermal expansion coefficient is another factor), fatigue of wire ball bond and solder joint, crack of packaging materials, etc. What's more, the cold-heating thermal alternation as well as internal stress would change when the device exposed to harsh environment, and cause encapsulant cracked and wire fractured. Therefore, in order to avoid the existence of great internal stress, methyl silicone resins were usually selected as packaging materials for high-power LED devices with the light efficiency sacrificed. Various efforts have also been devoted to improving the light output and gas barrier, such as the loading of nanoparticles with higher refractive index, but their effects on the reliability need further study.^28–30

In this article, we keep trying to use phenyl silicone resins as encapsulant for high-power LED devices. In order to reduce internal stress of encapsulant, three different structures of phenyl methyl silicone resins were obtained through chemical structure regulation. The phenyl trimethoxysilane [PhSi(OCH₃)₃] (T unit), methylphenyl dimethoxysilane [MePhSi(OCH₃)₂] (D unit), 1,3-divinyltetramethyl-siloxane (Me₂ViSiOSiViMe₂) (M unit) and vinyltrimethoxysilane [ViSi(OCH₃)₃] (t unit) were used for the preparation for silicone resins through co-condensation and poly-condensation methods. The structures of prepared liquid silicone resins were characterized by ¹H NMR, ²⁹Si NMR, and FTIR. Thermal properties of the cured silicone resins were studied by TGA and DSC. Electron universal testing machine was employed to characterize the mechanical properties of materials at different temperatures. The transmittances were investigated with spectrophotometer, and morphological structures of materials were observed by transmission electron microscopy (TEM). As we all know, the properties of materials could be modified by changing the chemical structure of materials, such as changing the proportion of different structure units.³¹ It was found that the mechanical strength of phenyl-silicone resins was significantly influenced by the ratio of T/D in the structure. Moreover, the suitable phenyl silicone encapsulants for high-power LED device (1 W) were successfully obtained through regulating T/D ratio in the silicone structure.

Experimental

Materials

1,1,3,3-Tetramethyldivinyldisiloxane (TMDVS), vinyltrimethoxysilane (VTMS), phenylmethyldimethoxysilane (PMDMS), phenyltrimethoxysilane (PTMS), and γ-(2,3-epoxypropoxy)propyltrimethoxysilane (EPTS) were supplied by Jingzhou Jianghan Fine Chemical Co. Ltd (China). Methylphenyl hydrogen-containing silicone resin SH305 (H = 0.46%, η = 120 mPa s, n²⁵_D = 1.536) was obtained from Zhejiang Runhe Organic Silicon New Materials Co., Ltd., Ningbo, China. Ethanol, toluene, magnesium sulfate (MgSO₄) and 36.5 wt% HCl solution were purchased from Beijing Chemical Works, China. 1 wt% of platinum(0)–1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution (200 ppm of Pt) was supplied by Sigma-Aldrich Co. LLC. 5050 SMD lead frame (5 mm × 5 mm) and C₂ COB LED lead frame (11 × 11 mm) were supplied by Foshan Nationstar Optoelectronics Co., LTD, Foshan, China. Adhesion promoter (PVEOS) was prepared in our laboratory (ESI†).

Synthesis of vinyl-terminated phenyl-silicone resins TDM

To a 1000 mL three-necked round-bottom flask equipped with a reflux condenser, a thermometer and a mechanical stirrer, TMDVS (0.20 mol), PMDMS (0.80 mol), PTMS (1.00 mol), toluene (80 mL), ethanol (50 mL) were added. The temperature of the reaction mixture was raised to 58 °C, and 100.00 g of 10% HCl was added dropwise for about 2 h. The temperature was raised to 80 °C and the reaction was continued for another 1 h, and then the reaction mixture was poured into a 1000 mL separating funnel. The upper layer of aqueous solution was removed. The organic phase was washed with deionized water to neutral pH. The solution was dried over anhydrous magnesium sulfate, and the solvent was removed under 150 °C/0.09 MPa to get colorless TDM in 72.5% yield.

Synthesis of vinyl-capped phenyl-silicone resins TDt

To a 500 mL three-necked round-bottom flask equipped with a reflux condenser, a thermometer and a mechanical stirrer, VTMS (0.20 mol), PMDMS (0.60 mol), PTMS (1.00 mol), toluene (50 mL), and ethanol (50 mL) were added. The temperature of the reaction mixture was raised to 51 °C, and 86.00 g of 10% HCl was added dropwise for about 3 h. The temperature was raised to 80 °C and the reaction was continued for another 3 h, and then the reaction mixture was poured into a 500 mL separating funnel. The upper layer of aqueous solution was removed. The organic phase was washed with deionized water to neutral pH. The solution was dried over anhydrous magnesium sulfate, and the solvent was removed under 150 °C/0.09 MPa to get colorless TDt in 74.7% yield.

Synthesis of vinyl-terminated and capped phenyl-silicone resin TDMt

To a 1000 mL three-necked round-bottom flask equipped with a reflux condenser, a thermometer and a mechanical stirrer, TMDVS (0.10 mol), VTMS (0.20 mol), PMDMS (0.80 mol), PTMS (1.00 mol), toluene (80 mL), and ethanol (50 mL) were added. The temperature of the reaction mixture was raised to 54 °C, and 108.00 g of 10% HCl was added dropwise for about 2 h. The temperature was raised to 80 °C and the reaction was continued for another 1 h, and then the reaction mixture was poured into a 1000 mL separating funnel. The upper layer of aqueous solution was removed. The organic phase was washed with deionized water to neutral pH. The solution was dried over anhydrous magnesium sulfate, and the solvent was removed under 150 °C/0.09 MPa to get colorless TDMt in 79.1% yield.

Preparation of silicone resin-type encapsulation materials

TDM, TDt, and TDMt resin were mixed with methylphenyl hydrogen containing silicone resin SH305 at a Si–H/Si–Vi mole ratio of 1.05, respectively. 1 wt% of adhesion promoter and 3 ppm Karstedt's catalyst were added into mixture under stirring. Then the mixtures were degassed in vacuum and poured into a cup-like LED bracket or dumbbell mould. The polymerization of silicone resins were conducted at 80 °C for 0.5 h and then at 150 °C for 2 h. The corresponding products were labeled as PTDM, PTDt, and PTDMt.

Characterizations and measurements

Proton (¹H) and silicon (²⁹Si) nuclear magnetic resonance (NMR) spectra were recorded using a Bruker AM-400S NMR spectrometer at a proton frequency of 400 MHz and the corresponding silicon frequency. Deuterated chloroform was used as a solvent and tetramethylsilane as an internal standard.

Fourier transform infrared spectra (FTIR) were obtained with a Nicolet Avatar 320 FT-IR Spectrophotometer. Samples were finely ground with KBr powder and pressed into disk.

The dynamic curing reactions of encapsulation were monitored with a Diamond/Pyris DSC differential scanning calorimeter operating in a dry nitrogen atmosphere. All the samples (about 10 mg in weight) were heated from 50 to 300 °C and DSC curves were recorded at the heating rate of 10 °C min⁻¹.

Glass translation temperature (T_g) was investigated by Diamond/Pyris DSC operating in a dry nitrogen atmosphere. All the samples (about 10 mg in weight), firstly, were quickly heated up to 150 °C and lasted for 5 min, and then cooled down to −70 °C in 20 min, afterwards heated up to 150 °C with the heating rate of 10 °C min⁻¹.

Thermal stability of the cured samples were investigated by a Perkin-Elmer TGA-6 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ from 50 to 800 °C under nitrogen atmosphere at a flow rate of 90 mL min⁻¹ in all cases.

Light transmission properties were determined on 2 mm thick specimens. Transmittance spectra were obtained with a Shimadzu UV-2550 (Shimadzu Corporation, Kyoto, Japan) spectrophotometer in the range of 300–800 nm, using air as the reference.

Internal morphological structure of cured polymers was observed by transmission electron microscopy (TEM) performing on a JEOL JEM-2010 transmission electron microscope at an acceleration voltage of 50 kV. The samples were trimmed using an ultrathin microtome machine, and the sectioned samples (ca. 50 nm in thickness) were placed in 200 mesh copper grids for observations.

The viscosity was measured with a model DV-II+ cone and plate viscometer (Brookfield Co., USA) at 25 °C. Refractive index (n²⁵_D) was measured with an Abbe refractometer at 25 °C. Tensile properties of cured encapsulant were carried out with a microcomputer controlled electron universal testing machine CM74304 (Shen Zhen Suns Technology Stock Co., LTD. Shen Zhen, China) according to ISO37:2005. The hardness was measured with an LX-A ShoreA durometer (Shanghai Liuling Instrument Factory. Shanghai, China) according to ASTM D 2240.

The dye test with red ink experiment was carried out in boiled red ink for 6 h, and then checked up whether the red ink penetrated into LED bracket packaged with produced silicone materials. No red ink penetration indicated packaging materials passed the dye test; otherwise, it was a failure. The red ink was made up of a volume of commercial red ink, a volume of water, and a volume of ethyl alcohol.

Wet and high-temperature operation life (WHTOL) of LED devices packaged with silicone materials was carried out under the condition of 85 °C/85% RH and 350 mA of forward current. Lighting time of LEDs was recorded.

Thermal shock cycling test for LED devices packaged with produced silicone materials was carried out by the following procedure: a cycle consist of −40 °C × 15 min and 125 °C × 15 min. The failure percentage and luminance of LEDs were inspected after different test conditions. The cycle times were recorded until the LEDs went out.

Results and discussion

Synthesis and characterization of the vinylphenyl silicone resins TDM, TDt, and TDMt

Silicone resin could be synthesized through co-condensation and poly-condensation of chlorosilanes or alkoxysilane.^23,32 The synthetic method using alkoxysilane has a much more environmentally friendly process than chlorosilane-method. Because the corrosive nature of chlorosilanes would bring about significant risks in handling and disposal. Some acid or alkaline compounds are used as catalyst for the hydrolysis, such as HCl, H₂SO₄, NaOH and (CH₃)₄NOH. However, the gel particles easily form in the products using these catalysts, which would make the light transmittance of cured products depressible. The gel particles were usually produced from self-condensation of different reactant, such as phenylsilsesquioxane, an insoluble substance formed through self-condensation of phenyltrimethoxysilane.³¹ The different reactivity of each reactant should be responsible for the phenomenon of self-condensation. Therefore, we believe that minimizing the reactivity difference of each reactant may be an effective way to solve the products heterogeneity problem.

In this work, we chose HCl acid as a catalyst to synthesize vinylphenyl silicone through co-condensation and poly-condensation from PTMS, PMDMS, TMDVS, VTMS, and water in a toluene/ethanol mixture. Fig. 1 showed the synthesis routine of vinylphenylsilicon resins. In order to avoid gel particles formation in product, the condensation reactivity difference of each reactant was minimized by tuning the co-condensation temperature. The effect of co-condensation temperature on appearance of products (TDM) was investigated, and results were listed in Table 1. It was found that the reaction product would become turbid when the co-condensation temperature was higher than 61 °C or lower than 52 °C. The appearance of product prepared at 61 °C or 52 °C was transparent, but transmittance of their cured material was not very high (discussed in optical performance section). Finally, it was found the optimum co-condensation temperature for synthesizing TDM resin was about 58 °C. It indicated condensation reactivity of PhSi(OCH₃)₃ and MePhSi(OCH₃)₂ was the closest at this temperature. Likewise, we found the optimum co-condensation temperatures for TDt and TDMt were 51 °C and 54 °C, respectively.

Fig. 1 Synthesis routine of vinylphenylsilicone resins.

Table 1 The parameters of TDM silicone resins synthesized in different reaction temperature^a

Samples	*Co-condensation temperature (°C)	Yield (%)	Appearance	Viscosity (mPa s)
a Conditions: M : D : T = 10 : 8 : 2. *The co-condensation reaction time was lasted for 2 hours for each experiment, standstilled and separated out the hydrophilic phase, and carried out poly-condensation reaction for 1 hour, washed with deionized water until the Cl^− content lower than 10 ppm, took off low-boiling residues under 150 °C/0.09 MPa for 2 hours.
TDM40	40	46.7	Mist viscous liquid	2872
TDM50	50	51.3	Translucent viscous liquid	3068
TDM52	52	56.8	Transparent viscous liquid	4217
TDM58	58	72.5	Transparent viscous liquid	4876
TDM61	61	74.6	Transparent viscous liquid	5123
TDM63	63	77.4	Translucent viscous liquid	5368
TDM70	70	82.1	Translucent viscous liquid with gel particles	5178

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The vinylphenyl silicone resins with different ratio of T/D were prepared as shown in Table 2. With the decrease of T/D value, the parameters including phenyl content, vinyl content, refractive index, and viscosity all increased. Considering the viscosity of packaging material and vinyl content requirements, TDM58-2, TDt51-2, and TDMt54-2 were chosen to study in our work.

Table 2 The parameters of vinylphenyl silicon resin^a

Sample		T : D : M : t	Ph/R (%)	Yield (%)	Vi (mol g^−1)	n^25D	Viscosity (mPa s)
a Conditions: co-condensations of TDM, TDt, and TDMt were carried out at 58 °C, 51 °C, and 54 °C, respectively, for 2 hours. The poly-condensations were carried out at 80 °C for 1 hour, then reaction systems were washed with deionized water until the Cl^− content of hydrophobic phase lower than 10 ppm, then took off low-boiling residues under 150 °C/0.09 MPa for 2 hours and obtained colorless viscous liquid.
TDM	TDM58-1	100 : 100 : 20 : 0	58.8	64.8	0.00124	1.5413	3928
	TDM58-2	100 : 80 : 20 : 0	60.0	72.5	0.00136	1.5428	4876
	TDM58-3	100 : 60 : 20 : 0	61.5	74.6	0.00152	1.5447	7238
	TDM58-4	100 : 40 : 20 : 0	63.6	73.8	0.00171	1.5478	22 658
TDt	TDt51-1	100 : 100 : 0 : 40	62.5	73.6	0.00126	1.5456	5835
	TDt51-2	100 : 80 : 0 : 40	64.3	74.7	0.00139	1.5479	14 480
	TDt51-3	100 : 60 : 0 : 40	67.7	75.2	0.00155	1.5501	Solid
TDMt	TDMt54-1	100 : 100 : 10 : 20	60.6	78.3	0.00125	1.5421	4238
	TDMt54-2	100 : 80 : 10 : 20	62.1	79.1	0.00138	1.5448	6879
	TDMt54-3	100 : 60 : 10 : 20	64.0	76.8	0.00153	1.5465	47 895

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In addition, due to the polarity difference of reactants, the reaction system was separated into two phases containing hydrophobic phase and hydrophilic phase. The hydrophobic phase included toluene, silicon and a part of reactant at earlier stage, and hydrophilic phase consisted of alcohol, water, and catalyst. During the reaction process, the products of silicone could transfer to hydrophobic phase, and by-products (methanol) went into hydrophilic phase. Owing to the strong polarity, the majority of chloridions (Cl⁻) were hidden in the hydrophilic phase. Therefore, it was fortunately that the Cl⁻ content in the products was lower than 10 ppm after repeatedly washing the hydrophobic phase 4–6 times.

FTIR, ¹H NMR, and ²⁹Si NMR spectra of vinylphenyl silicone resin were shown in Fig. 2–4. In the FTIR spectra, the characteristic symmetric stretching absorption peaks of Si–O–Si were located at 1080 cm⁻¹. The characteristic absorption peaks of vinyl were not obvious because of their relatively low content. The peaks at 3075 cm⁻¹ were assigned to stretch vibration absorption of vinyl group (C=C–H) and peaks at 1657 cm⁻¹ were belonged to stretch vibration absorption of vinyl group (C=C) appeared in the partially enlarged FTIR spectra. A little difference was found in fingerprint area for three vinylphenyl silicones, out-plane vibration absorption peak of C=C–Si(CH₃)₂ [originating from TMDVS] at 836 cm⁻¹ appeared in FTIR spectra of TDM and TDMt. Out-plane vibration absorption peak of C=C–Si(O)₃ [originating from VTMS] at 802 cm⁻¹ appeared in FTIR spectra of TDt and TDMt. Meanwhile, some small peaks located at 3650 cm⁻¹ were ascribed to stretch vibration absorption of Si–OH, which indicated that a small amount of silicon hydroxyl in their structure.

Fig. 2 FTIR spectra of vinylphenyl silicone resins.

Fig. 3 ¹H NMR spectra of vinylphenyl silicone resins.

Fig. 4 ²⁹Si NMR spectra of vinylphenyl silicone resins.

Three vinylphenyl silicones almost displayed similar absorption peak in their ¹H NMR spectra. The signals at 7.05–8.00 ppm were the characteristic absorption peaks for phenyl groups. The characteristic signals located at 5.54–6.23 ppm were attributed to the protons of vinyl group, –CH=CH₂ and the chemical shifts of 0–0.82 ppm were assigned to the methyl groups, –CH₃. The proton peaks of silicon-hydroxyl and silicon-methoxy located at 2.62 ppm and 3.41 ppm, respectively.

The ²⁹Si NMR spectra were able to clearly reflect skeleton structure of silicone resins. TDM silicone resins have the cross-linked structure unit (RSiO_3/2) containing Si atoms whose chemical shifts are located at −80.3 ppm, and chemical shifts of some Si atoms bonded with unreacted hydroxyl were located at 72.1 ppm, liner structure unit (R₁R₂SiO) embodying Si atoms of which chemical shifts are located at about −33.2 ppm, the signal located at −3.2 ppm belonged to the chemical shifts of Si atoms in terminal structure unit (R₁R₂R₃SiO). Based on the above analysis, it was speculated that TDM silicone resins presented vinyl-terminated ladder network structure. Only liner structure units (chemical shifts of Si atoms located at −33.2 ppm) and cross-linked structure units (chemical shifts of Si atoms located at −80.3 ppm) were found in the ²⁹Si NMR spectra of TDt, which indicated TDt silicone resin possessed vinyl-capped network structure. The similar ²⁹Si NMR spectra of TDMt and TDM implied that they probably had similar chemical structure. In consideration of the number of terminal units of TDMt less than TDM, it was inferred TDMt silicone resins possibly exhibited semi-cage network structure.

Curing of TDM, TDt, and TDMt

The hydrosilation reactivity of vinylphenyl silicone resins could be reflected from the DSC thermograms. DSC traced the polymerization behavior with the heat rate of 10 °C min⁻¹, as given in Fig. 5. With the increase of temperature, exothermic reaction process covered a wide temperature range which corresponded to the polymerization of silicone resins through hydrosilation of hydrogen-containing silicone resins and vinyl-containing silicone resins. The outset reaction temperatures appeared at 85 °C for TDM58-2, 92 °C for TDt51-2, and 88 °C for TDMt54-2, respectively. The exothermic peaks of three samples appeared at 122.7 °C for TDM58-2, 126.3 °C for TDt51-2, and 129.8 °C for TDMt54-2, respectively. The values of heat enthalpy for polymerization were 347 J g⁻¹ (247.85 kJ mol⁻¹) for TDM58-2, 129 J g⁻¹ (161.25 kJ mol⁻¹) for TDt51-2, and 268 J g⁻¹ (223. 33 kJ mol⁻¹) for TDMt54-2, respectively. From the outset reaction temperatures, the hydrosilation reactivity of vinylphenyl silicone resins was in the order of TDM58-2 > TDMt54-2 > TDt51-2. The exothermal peak temperature of TDMt54-2 was higher than that of TDt51-2. It was probably because polymerization of TDt51-2 at low temperature, which could restrict the mobility of most reactive functional groups of CH₂=CH– and Si–H. The order of heat enthalpy of polymerization (TDM58-2 > TDMt54-2 > TDt51-2) indicated that some reactive functional groups in PTDt and PTDMt were not reacted.

Fig. 5 DSC thermograms of silicone resins with heat rate 10 °C min⁻¹ in N₂ atmosphere.

Although the samples had experienced heat polymerization process at 80 °C for 0.5 h and then at 150 °C for 2 h, the characteristic absorption peak of silicone–hydrogen bond at 2136 cm⁻¹ were still observed in the FTIR spectra of PTDt51-2 and PTDMt54-2 as shown in Fig. 6. It implied that the vinyl groups of silicone resins were not completely polymerized. As we discussed in the section of ²⁹Si NMR, cage structure of the precursor for PTDt51-2 and PTDMt54-2 could wrap some vinyl groups. Consequently, those wrapped vinyl groups were retained during the polymerization process because they did not touch Si–H groups.

Fig. 6 FTIR spectra of the cured vinylphenyl silicone resins.

Structure and properties of cured silicone resins

Thermal properties. Owing to the high bond energy of Si–O bond, organic silicon polymers usually have good thermal stability and weatherability. Meanwhile, they also have excellent low temperature performance because of their flexible chain segment. The glass transition temperatures (T_gs) of PTDM58-2, PTDt51-2, and PTDMt54-2 were 11.5 °C, 21.5 °C and 19.5 °C, respectively, which were acquired from the DSC thermograms shown in Fig. 7. Because more crosslink site from VTMS than that from TMDVS, the values of T_g was in the order of PTDM58-2 < PTDMt54-2 < PTDt51-2.

Fig. 7 DSC thermograms of cured silicone resins with heat rate 10 °C min⁻¹ in N₂ atmosphere.

The thermal stability of cured silicone resins was evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere. The weight-loss curves are shown in Fig. 8. The 5% and 10% weight loss temperatures (T₅ and T₁₀) for PTDM58-2 are 461.2 °C and 541.5 °C, respectively. For PTDt51-2, T₅ and T₁₀ were decreased to 387.1 °C and 503.3 °C, respectively. T₅ and T₁₀ of PTDMt54-2 were slightly lower than those of PTDM58-2, which were 458.4 °C and 541.3 °C, respectively. The char yield at 800 °C of three silicone resins in nitrogen atmosphere was 79.8% (PTDM58-2), 78% (PTDt51-2) and 79.2% (PTDMt54-2), respectively. The high decomposition temperatures indicated the excellent thermal stability of three silicone resins, and the high char yield suggested the outstanding flame retardancy of these silicone resins.

Fig. 8 TGA thermograms of cured silicone resins in N₂ atmosphere.

From the TGA thermograms of three silicone resins, it can be clearly seen that either the values of weight loss temperature or the char yield were in the order of PTDM58-2 > PTDMt54-2 > PTDt51-2. To the best of our knowledge, the first stage of decomposition for three cured silicone resins was ascribed to the aliphatic chain section originating from C=C bonded to three-dimensional structure (SiO_3/2) in PTDt, and linear structure (SiO) in PTDM broken off from the silicone center. The second stage decomposition was ascribed to methyl broken off at about 570 °C.³³ Obviously, the stress of aliphatic chain of PTDt was stronger than that of PTDM during the heating process. Therefore, the cleavage of bond between aliphatic chain and silicone in PTDt was easier than that of PTDM, and the starting decomposition temperature of PTDt was lower than that of PTDM. Although aliphatic chain sections bonded to three-dimensional structures also existed in the structure of PTDM, the starting decomposition temperature was higher than that of PTDMt. This was probably because the stress of aliphatic chain was reduced during the heating process by introducing the flexible aliphatic chains bonded to linear structure.

Mechanical properties. Suitable mechanical properties for encapsulant are necessary. On one hand, enough mechanical strength is needed to protect the LED devices. From Table 3, it is found that the hardness of these silicone resins were about at 55A, the tensile strength were about 3 MPa, and the elongation at break were about 85% at normal temperature (25 °C), which indicated these silicone polymers possess excellent mechanical strength. On the other hand, the toughness is also important for LED encapsulants, which could decrease the generation of internal stress during the process of cold and hot alternative. Generally speaking, the toughness of materials could be affected by environment temperature and structure. The toughness of polymers would be improved with the increase of environmental temperature. And the toughness would be sacrificed at a relatively low environmental temperature because the chain segment movements were frozen at glassy state. The tensile strength and elongation at break change trend in Table 3 also verified above principle. Although the value of tensile strength and elongation at break decrease with the decrease of the test temperature, the values of elongation at break were 28% for PTDM58-2, 20% for PTDt51-2, and 24% for PTDMt54-2 at −40 °C, respectively, indicated that these materials still have certain toughness at relatively low environmental temperature. Above phenomena were positive signals for these materials used as encapsulant.

Table 3 Mechanical properties of the cured silicone resins

Properties	T/D	Samples
		PTDM58-2	PTDt51-2	PTDMt54-2
a Experiments were carried out at 125 °C.b Experiments were carried out at 25 °C.c Experiments were carried out at −40 °C.
Hardness (shore A)	100 : 100	44^b	49^b	42^b
	100 : 80	52^b	58^b	55^b
	100 : 60	56^b	61^b	58^b
Tensile strength (MPa)	100 : 100	1.27^a	0.86^a	0.74^a
	100 : 80	1.48^a	1.35^a	1.52^a
	100 : 60	1.61^a	1.43^a	1.72^a
	100 : 100	2.87^b	2.23^b	2.45^b
	100 : 80	3.45^b	2.87^b	3.12^b
	100 : 60	3.51^b	3.07^b	3.42^b
	100 : 100	6.63^c	6.14^c	7.26^c
	100 : 80	8.38^c	7.27^c	8.83^c
	100 : 60	9.12^c	8.56^c	10.18^c
Elongation at break (%)	100 : 100	242^a	218^a	228^a
	100 : 80	188^a	164^a	174^a
	100 : 60	121^a	107^a	123^a
	100 : 100	92^b	88^b	95^b
	100 : 80	84^b	74^b	81^b
	100 : 60	65^b	54^b	71^b
	100 : 100	37^c	29^c	34^c
	100 : 80	28^c	20^c	24^c
	100 : 60	17^c	14^c	15^c

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In addition, T/D unit of the polymer structure also affected the mechanical properties of cured silicone resins. As shown in Table 3, the hardness and tensile strength increase with the increase of the T/D value and the elongation decreases with increasing of T/D value. This is because the phenyl content and the crosslink density (vinyl content) would decrease with the decrease of T/D value (Table 2). That is to say, the toughness of these materials could be improved though increasing the D unit content in their structure.

Optical properties. Generally, in order to reduce RI gap and improve the luminous efficiency, high RI silicone encapsulants were prepared for LED packaging under condition of other properties suited. Table 4 listed the RIs of liquid silicone resins and their corresponding cured materials. The RIs of these materials were above 1.538; and the values of RI were in the order of TDM58-2 < TDMt54-2 < TDt51-2. For silicone materials, the value of RI is determined by their phenyl content, which could range from 1.41 to 1.55 as the phenyl groups increased in their structure.²³ Another phenomenon was that the RIs of liquid silicone resins were slightly higher than those of their corresponding cured materials. On one hand, except for vinyl-silicone resins, the raw materials of cured materials contained hydrophenyl-silicone resins of which RI was 1.536. This could lower the RI of the cured system. On another hand, due to the hydrosilylation, the decrease of vinyl also would make the RI of cured silicone depressible.

Table 4 The refractive index of liquid vinylphenyl-silicone resins and their cured materials

Samples	Liquid vinylphenyl-silicone resin			Cured material
	TDM58-2	TDt51-2	TDMt54-2	PTDM58-2	PTDt51-2	PTDMt54-2
RI	1.5428	1.5479	1.5448	1.5385	1.5410	1.5402

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Light transmittance is another optical performance requirement for LED packaging materials. Fig. 9 shows the transmittance spectra in the wavelength range from 300 nm to 800 nm. The light transmittances were 90.3% for PTDt51-2, 92.8% for PTDMt54-2, 94.2% for PTDM58-2 at wavelength of 450 nm, and 96.5% for PTDt51-2, 97.7% for PTDMt54-2, 98.7% for PTDM58-2 at wavelength of 680 nm, respectively. The transmittances of PTDM-52 and PTDM-61 were 75.6% and 79.4% at wavelength of 450 nm, and 79.2% and 82.4% at wavelength of 680 nm, respectively.

Fig. 9 Transmittance of cured silicone resins originated from vinylphenyl silicone resins.

From Fig. 10, it could be found that the morphologies of PTDM58-2, PTDt, and PTDMt exhibited homogeneous structure but that of PTDM-52 and PTDM-61 presented cloud structure of heterogeneous features. As well known, light scattering would take place when light passes through the material with heterogeneous structure, leading to depressed transmittance.³⁴ So it was found transmittances of PTDM-52 and PTDM-61 are significantly lower than that of PTDM58-2.

Fig. 10 TEM micrographs of cured silicone resins and picture of test samples with thickness of 2 mm.

As described in Table 1, the appearances of TDM52, TDM58 and TDM61 were transparent, whose transmittance were all more than 99.8%. But the transmittance of their cured materials existed huge differences. It was considered that TDM52 or TDM61 was made up of multi-structural mixtures which probably include some amounts of similar self-condensation precursor. These multi-structural mixtures had good compatibility with each other, and exhibited good transparent appearances. However, due to existence of these different structural differences, polymerization induced phase separation would take place during their thermal curing reaction, which could make the cured material possess heterogeneous structure. This further strengthened the importance of structural homogeneity of the precursor for optical transparency of cured silicone resins.

Reliability test

Reliability tests including dye test, thermal shock test, and WHTOL test for three types of silicone resins applied in LED packaging were carried out. Table 5 lists the test results for each experiment. No permeation took place either in COB leadframe packaged or SMD leadframe packaged during the course of the dye test. It suggested the encapsulants mainly consisting of silicone resins in our work possessed excellent adhesive strength with LED leadframe.

Table 5 Reliability test results for silicone resins applied in LED packaging

Test project	Lamp type	Test results
		PTDM58-2	PTDt51-2	PTDMt54-2
Dye test (pass or failure)	SMD	Pass	Pass	Pass
	COB	Pass	Pass	Pass
Thermal shock test (times)	SMD	284	197	256
	COB	232	164	206
WHTOL test (h)	SMD	>1008	>1008	>1008
	COB	>1008	>1008	>1008

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The LED package structure, in particular the bonding wire, suffered from significant thermo-mechanical stresses and strains during the thermal shock testing.³⁵ The results of thermal shock test indicated the prepared silicone resin-type encapsulants showed good thermal resistance. Because of smaller size of package for SMD, the thermal shock time was obvious more than that of COB. The thermal shock times were all about 200 for PTDM58-2 and PTDMt54-2 used in 1 W COB lamps. It was demonstrated that these materials were suitable for high-power LED packaging. The thermal shock time of PTDt51-2 (164) was obvious less than that of PTDM58-2 and PTDMt54-2. This indicated that larger internal stress was generated in PTDt during thermal shock testing compared to PTDM and PTDMt. The results also further suggested the toughness of PDTM and PTDMt was superior to that of PTDt, which was consistent with mechanical properties as discussed above.

WHTOL test is a steady-state temperature humidity life test for the purpose of evaluating the reliability of packaged solid-state device in humid environment.³⁶ The penetration of moisture through the encapsulant materials or along the interface between encapsulants and lead frames was accelerated in the condition of temperature and humidity. From the results of WHTOL test, the encapsulant prepared in our work exhibited good bonding durability and low moisture permeability under wet and high-temperature environment.

Conclusions

A series of vinylphenyl silicone resins, such as vinyl-terminated silicone (TDM), vinyl-capped silicone (TDt), vinyl-terminated and vinyl-capped silicone (TDMt), were synthesized through co-condensation and poly-condensation. The homogeneity of their structure was regulated by tuning the co-condensation temperature which could narrow the reactivity difference of each reactant. The optimal co-condensation temperatures for TDM, TDt, and TDMt were 58 °C, 51 °C and 54 °C, respectively. The curing temperatures of these vinylphenyl silicone resins with hydrophenyl silicone ranged from 80 °C to 150 °C and the exotherm peak centered at about 125 °C. And the hydrosilation reactivity of vinylphenyl silicone resins was in the order of TDM > TDMt > TDt. The onset decomposition temperatures of cured materials from TGA were as high as 450 °C and the char yields were close to 80% at 800 °C. These materials showed excellent mechanical properties and good toughness. The values of elongation at break were above 20% at −40 °C. Those materials possessed higher homogeneity degree of morphological structure and consequently higher transmittance which reached up to 98.7% at wavelength of 680 nm. The results of reliability tests including dye test, thermal shock test, and WHTOL test demonstrated that these materials were suitable for using in the field of high-power LED packaging.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08295e

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