China is the world's largest LED application market, but the development of high-end LED and other fields is relatively lagging behind, and the production capacity is insufficient, which is far from meeting the needs of the market. Corti, Philips, Osram, Kyocera, Sumitomo Electric, Nichia and other foreign brands have long-term Has occupied the domestic high-end market. In addition, the core technology of high-end LED materials is basically in the hands of European and American enterprises. Most domestic enterprises are at the low end of the industry, and it is difficult to gain the upper hand in competition with foreign companies. Moreover, the trend of homogenization competition in some areas began to appear, and the phenomenon of disrupting market order by sub-filling and malicious price reduction further weakened the market competitiveness of domestic enterprises, thus limiting the rapid development of the domestic LED material industry.
The continuous development of LED packaging materials has led to the continuous improvement of the corresponding packaging technology. The packaging material has an important influence on the function of the LED chip . The poor heat dissipation or low light extraction rate will cause the function of the chip to fail. Therefore, the packaging material must have high thermal conductivity, high transmittance, good heat resistance and UV resistance. (UV light) shields good features. With the rapid development of white LEDs, the outer packaging materials must maintain high transparency in the visible range and have good absorption of ultraviolet and visible light (to prevent ultraviolet and visible radiation).
The traditional epoxy resin (EP) has the disadvantages of easy yellowing, large internal stress and poor thermal stability, so it cannot meet the packaging requirements of white LEDs, and is replaced by a silicone material with excellent performance. Silicone encapsulants have excellent thermal stability, water resistance and light transmissibility due to their organic and inorganic groups. They have become the focus of research on LED packaging materials at home and abroad; however, organic Silicon still has some disadvantages (such as poor UV aging resistance, low thermal conductivity, etc.). In recent years, researchers at home and abroad have used nanotechnology to modify silicones and have received extensive attention.
POSS modified EP packaging material
EP refers to a polymer compound containing two or more reactive epoxy groups in the molecule, which can be crosslinked with an amine, an acid anhydride, and a PF (phenolic resin) to form an insoluble polymer having a three-dimensional network crosslinked structure. Object. Therefore, EP has advantages such as excellent adhesion, good sealing properties, and low cost, and is a main material for packaging such as LEDs and integrated circuits. However, with the rapid development of technology, the performance requirements of packaging materials are getting higher and higher. Traditional EP packaging materials have the disadvantages of fast aging rate, easy discoloration and brittle materials, so the modified EP packaging materials are bound to be Row.
POSS is a compound composed of an inorganic core composed of silicon and oxygen and an organic peripheral group, and has a three-dimensional structure and a machine-inorganic hybrid nanocage structure. Compared with traditional inorganic nanoparticles, POSS has good heat resistance, structural stability and thermodynamic properties due to its special molecular structure, and the POSS molecular structure can be "cut" and "assembled" as needed. Therefore, the use of POSS modified EP can overcome the shortcomings of traditional EP packaging materials.
Xiao et al first synthesized (3-oxidized glycidyl propyl) dimethylsiloxy POSS and vinyl epoxy cyclohexane dimethylsiloxy POSS, followed by 4,4'-diphenylmethane and tetra After mixing the methyl phthalic acid, a corresponding hybrid material is prepared. The results show that with the increasing assimilation temperature, the hardness of POSS modified EP decreases gradually. Although the thermal expansion coefficient of composites is higher than that of bisphenol A type EP (DGEBA) at low temperature, the dependence of expansion coefficient on temperature is higher. It is small and its coefficient of thermal expansion is still low, so that it can effectively improve light stability and heat resistance.
Fu et al. use POSS modified EP containing propylidene. Studies have shown that although the Tg (glass transition temperature) of the modified EP is significantly reduced, the high temperature light stability, heat resistance and UV aging resistance of EP are significantly improved.
Based on g-(2,3-epoxypropoxy)propyltrimethoxysilane, Zhou Lijun and other preparations of epoxy silsesquioxane (SSQ-EP) with good compatibility with DGEBA were prepared by hydrolysis polycondensation. Then, the hybridization of SSQ-EP and DGEBA improves the disadvantages of poor heat resistance and yellowing of DGEBA, and enables DGEBA/SSQ-EP hybrid materials to maintain high light transmittance. The research shows that when m(DGEBA):m(SSQ-EP)=1:1, the comprehensive performance of hybrid materials is relatively optimal (the refractive index is 1.51, the transmittance is 92.07% and the heat aging resistance is excellent). .
Li Xueming et al. cross-linked and hybridized EP with POSS containing epoxy groups to prepare epoxy polyorganosilsesquioxane (EP/POSS) hybrid materials. Studies have shown that POSS and EP form a hybrid material after rapid in-situ hybridization in the UV curing process, and the obtained epoxy polyorganosil silsesquioxane hybrid material has high light transmittance, small thermal expansion coefficient and UV resistance. The advantages of good aging, overcome the shortcomings of poor flexibility of EP materials for LEDs, high temperature curing of silicone modified EP, etc., and are suitable for LED packaging.
Silicone packaging material
Although POSS can improve the heat resistance, thermal stability and light stability of EP, since EP itself contains epoxy groups, it is easily oxidized at high temperatures, and yellowing occurs after long-term use. Meet the requirements of high performance LED packaging materials. Compared with EP, silicone materials have good transparency, high temperature resistance, weather resistance and hydrophobicity. Currently widely studied silicone packaging materials are mainly two types of addition silicone resin and addition silicone rubber. This is because it does not produce by-products after vulcanization crosslinking and the size of the sulfide is relatively stable.
The addition molding silicone encapsulating material is a vinyl-containing silicone resin as a base polymer, a Si-H-containing silicone resin or a hydrogen-containing silicone oil as a crosslinking agent, and is subjected to crosslinking at room temperature or under heating in the presence of a platinum catalyst. Co-cured.
Kim et al. prepared a hydrogen-containing oligomeric resin (mixed with phenyl and vinyl-containing oligosiloxanes) by sol-gel condensation using vinyltrimethoxysilane and diphenyldihydroxysilane as raw materials. . Studies have shown that the purified phenyl silicone resin exhibits low shrinkage and high transparency in the curing reaction, and maintains good thermal stability and high refractive index at around 440 ° C, and is suitable as a silicone encapsulant for LEDs.
Zhang Wei et al. mixed vinyl phenyl silicone oil, hydrogen-containing silicone resin and vinyl silicone resin in proportion, and then solidified under the action of a platinum catalyst to form a silicone resin material for LED packaging. The research shows that the material has the characteristics of high refractive index (more than 1.54), good transparency, excellent heat resistance and thermal shock stability, and is suitable for silicone resin materials for LED packaging.
The addition molding liquid silicone rubber encapsulating material is prepared by a blending method using a vinyl-based linear polysiloxane-based polymer, a vinyl silicone resin as a reinforcing filler, and a hydrogen-containing silicone oil as a crosslinking agent.
Tabei et al. obtained a vinyl silicone resin by a chlorosilane co-hydrolysis condensation process, and then vulcanized it with a phenyl silicone-containing hydrogen-containing silicone oil under the action of a platinum catalyst to obtain an LED packaging material. The material has a high refractive index (up to 1.50) and UV has a small effect on its light transmission (from 95% to 92% after 500 h of radiation).
Miyosh adds fumed silica, thermally conductive filler and flame retardant to methylphenyl hydrogen silicone oil and vinyl silicone resin. After curing at 120-180 ° C for 30-180 min, the material has excellent properties (refractive index Up to 1.51; after 100 h of radiation from a 400 nm wavelength source, the light transmittance decreased from 95% to 92%, and was still 92% after 500 h of irradiation.
Shao Qian et al. prepared a cross-linking copolymerization of methylphenylcyclosiloxane (DnMe, Ph) and tetramethylcyclotetrasiloxane (D4H) to prepare a cross-linking hydrogen-containing silicone oil for LED packaging materials. Ring-opening copolymerization was carried out by adding trifluoropropylcyclotrisiloxane to DnMe, Ph. Studies have shown that the introduction of trifluoromethyl siloxane chains in the vinyl silicone oil molecular chain reduces the surface tension of the vinyl silicone oil, facilitates vacuum defoaming of the encapsulating material, and greatly increases the refractive index.
Xu Xiaoqiu et al. catalyzed DnMe, Ph with octamethylcyclotetrasiloxane and 1,3,5,7-tetramethyl-1,3 at a temperature of 100 ° C using a silicon alkoxide of tetramethylammonium hydroxide as a catalyst. A ring-opening polymerization reaction of a monomer such as 5,7-tetravinylcyclotetrasiloxane to obtain a transparent PDMS-PMPS-PMViS having a refractive index of more than 1.51 (polydimethyl-methylphenyl-methylvinyl copolymer) ()). Studies have shown that the compound prepared from the polymer has a high refractive index and light transmittance, and is a good packaging material for LED.
Silicon nanocomposite packaging material
Pure silicone materials as packaging materials have problems such as low refractive index and poor adhesion to the substrate due to low surface energy, and cannot fully meet the requirements of high-performance LED packaging materials. Nanomaterial technology is at the heart of the development of new materials in the 21st century. Nanomaterials have the characteristics of small size, no matching atoms on the surface, and are very easy to physically and chemically interact with the polymer matrix. The use of nanotechnology can impart higher refractive index, better UV radiation resistance and comprehensive performance to silicone nanocomposite packaging materials, and thus has become the development direction of research at home and abroad. Commonly used nanocomposites combined with encapsulating materials are CeO2 (yttria), TiO2 (titanium oxide), and ZnO (zinc oxide).
Basin et al. added nano-scale TiO2 and nano-ZrO2: (zirconia) to the LED silicone packaging material for organosilicon nanocomposite. Studies have shown that the refractive index, thermal stability, UV radiation resistance, tensile strength and elastic modulus of the packaging material are significantly improved; when W (TiO2 and ZrO2) = 3% to 5% (relative to the total mass of the packaging material) In other words, the luminous efficiency of the LED can be increased by 5%. However, it is difficult to achieve high dispersion of nanoparticles in this test.
In order to meet the good UV radiation resistance of LED silicone packaging materials, nano-CeO2 is the best material for new high-efficiency UV radiation resistance. Han Ying introduced calcium-modified nano-CeO2 in the research of packaging materials, and analyzed its structure and properties. The research shows that with the increasing amount of nano-CeO2 in the silicone resin, the luminous efficiency of the LED packaging material changes significantly (the trend of rising-lowering-up); when w(CeO2)=0.12%, the luminous tendency Stable, almost unchanged under UV irradiation (indicating that CeO2 nanoparticles act to resist UV radiation). However, the compatibility of the inorganic nano-powder with the silicone resin during the test is poor, and the conventional silane coupling agent has some drawbacks in the treatment of the nano-powder.
Nano ZnO has strong absorption and scattering effect on UV. It is an excellent UV shielding agent and has the advantages of non-toxicity, high photothermal stability, etc. It can improve the UV aging resistance and thermal conductivity of silicone and prolong the LED. Service life. Sun Yuping used silane coupling agent (KH-570) to recombine nano-ZnO and silicone resin and tested its properties. The results show that the particle size of nano-ZnO has little effect on the dielectric constant of composites, but with the increase of nano-ZnO content and particle size, the thermal conductivity and UV shielding rate of nanocomposites increase. When W(ZnO)=0.15% and the average particle size is (46±0.4)nm, the thermal conductivity of the nanocomposite is 0.649 W/(m·K) which is 1.7 times that of pure silicone resin, which is suitable for high power LED. Package.
in conclusion
(1) As an LED packaging material, the defect of ordinary EP determines that it can not meet the requirements of the use of packaging materials, so the modification of the EP is imperative. The POSS molecule has both a nano-scale inorganic rigid cage structure and a reactive organic functional group, which can introduce inorganic SiO2 particles into the organic polymer chain in the form of covalent bonds, so that the performance of EP is significantly improved, thereby significantly improving The comprehensive performance of the encapsulant.
(2) Additive silicone materials have good transparency, high temperature resistance, weather resistance, insulation, hydrophobicity and UV radiation resistance, and are ideal packaging materials for white LEDs. In order to improve the refractive index and radiation resistance of the LED packaging material, an appropriate amount of a phenyl group may be added to the silicone molecule. As research continues to deepen, it will certainly be possible to develop additive-forming silicone packaging materials that meet the packaging requirements of LEDs in different environments and applications.
(3) Silicone nanocomposites not only have strong UV shielding rate, high visible light transmittance, high thermal conductivity, low dielectric constant and filling amount, but also have no effect on the mechanical properties and processing properties of the composite. The specificity of nanocomposites has attracted the attention of many experts. It is believed that in the near future, silicone nanocomposites will replace EP as the main source of packaging materials for LEDs.
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