Preparation And Properties Of Optical Resin With High Refractive Index For LED

Optical resins with high refractive index used for encapsulant in light emitting diode（LED） is essential in avoiding stress shock and humidity corrosion, improving light extractionefficiency, reducing heat elimination and prolonging the service life of LED chip. Manyresearches showed that introducing sulfur element, by thioalcohol and thioether, into thechemical structure of polymer is regarded as one of the most effective methods to improverefractive index of optical resins.However, there are some problems, such as high cost, poor stability or difficultprocessing, in optical resin with high refractive index up to now reported. The opticalmaterials with high refractive index, lower cost and better comprehensive performance areextremely needed to develop. Therefore, the main contents and results of this paper are asfollowing:First, diglycidyl ether of thiodibenzenethiol （DGETDBT） and diglycidyl ether ofthiobis-phenol （DGETP） were synthesized using4,4’-thiodibenzenethiol,4,4’-thiobis-phenoland epichlorohydrin by two-step method. Their chemical structures were characterized withFTIR and1H NMR spectrometer. Using m-xylylenediamine （MXDA） andmethylhexahydrophthalic anhydride （MHHPA） as curing agent, curing behavior and thermalstability of DGETDBT and DGETP were studied by DSC and TGA and their lighttransmittance and refractive index were analyzed by UV–Vis scanning spectrophotometer andAbbe refractometer, compared with those performances of diglycidyl ether of bisphenol A（DGEBA）.The refractive indices of DGETDBT and DGETP were1.665and1.612.The refractiveindices of cured products with MXDA were1.698and1.645,1.628and1.595with MHHPA.It was all significantly higher than the corresponding values of DGEBA （1.604and1.568）.The refractive index test showed that sulfur atoms in DGETDBT and DGETP contributed tothe obvious increasing of their refractive indices. The refractive indexes of cured productswere relevant to the refractive indices and proportion of epoxy prepolymer and curing agent.The visible light transmittance of cured products of DGETDBT, DGETP and DGEBAwith MXDA and MHHPA were all exceeded85%, increasing in order. The transmittanceszero region of DGETDBT/MXDA and DGETDBT/MHHPA were200³70nm and200³90nm. It might protect people from UV radiation when these resins were used inencapsulant of light emitting diode （LED）.DGETDBT、DGETPå’ŒDGEBA three kinds of prepolymer had the same curing behavior and the cured products of these prepolymers and curing agent MXDA and MHHPA had agood thermal stability. The temperature at1%weight loss of these cured products allexceeded250℃，which is much higher than the normal usage temperature of LED.The refractive index of DGETP （1.612） is higher than that of DGEBA （1.570） at thesame time it has the preferable thermal stability and acceptable price. Therefore DGETP canbe expected to be a kind of high rate of quantity and price encapsulant material for LED.Second, thiol curing agent can possess high refractive index and high curing speed andimprove the toughness of cured product. Although thiophenol brings a high refractive index,the complex synthesis of thiophenol leads to the extremely high price. While aliphatic thiolcan be synthesized by thiourea of which the costing is acceptable.O-benzenedimethanethiol（OBDMT） was synthesized using o-benzenedimethanechlorinevia thiourea-method in this paper. Its chemical structure was characterized with FTIR and1HNMR spectrometer. OBDMT was derived as a pale yellow or colorless crystal with arefractive index of about1.628. The refractive index of cured DGETP/OBDMT was1.654,the transmittance is near to85%and the glass transition temperature is close to28.7℃.Corresponding values for cured DGEBA/MHHPA were1.595,88%and64.9℃, respectively.The product of OBDMT has higer refractive index than the product of MHHPA.Third, PETTA/PETTG and PETTA/OBDMT cured resin were prepared via an clickchemistry method in which pentaerythritol tetraacrylate （PETTA） respectively reated withpentaerythritol tetrathioglycolic （PETTG） and OBDMT. The thiol-ene addition reaction in thecuring course was mainly carried out by heat-curing or UV-curing.PETTA/PETTG and PETTA/OBDMT cured resin both have low glass transitiontemperature. The refractive index of PETTA/OBDMT cured resin is1.597, but thermalstability and transmission of PETTA/OBDMT are lower than those of PETTA/PETTG curedresin. PETTA/PETTG cured resin have outstanding thermal stability （the temperature at1%mass loss thermal is324.2℃） and the higher transmission （over90%）, which is on a par withthe silicone resin. The refractive index of PETTA/PETTG cured resin reached1.556，higherthan mostof silicone resin （the refractive index only1.410-1.530）.Fouth, the relationship between curing degree, curing time and refractive index ofDGETP/triethylene tetramine（TETA） mixture was studied by in-situ reaction method. Thereare the clearly exponential relation between the refractive index and curing degree or curingtime. The change of the refractive index and curing degree is bigger during its initial curingperiod, but those became smaller in the middle and later curing periods because molecular number and molecular volume had no evident changes. The result proved that the refractiveindex was mainly influenced by its molecular volume and the inherent molar refractivityaccording to the Lorentz–Lorenz equation.The curing kinetics of DGETP/TETA cured product was studied by FTIR andnon-isothermal DSC．The reaction activation energy was52.6kJ·mol^-1,56.3kJ·mol^-1and59.3kJ·mol^-1, calculated by Kissinger equation, Ozawa equation and FTIR method,respectively. Three datas were conincident and could be validated with each other. the resultsfrom the non-isothermal curing kinetics in DSC showed that the reaction activation energy ofDGETP/MHHPA mixture was highest, followed by DGETP/TETA, and least byDGETP/OBDMT. The reaction order of DGETP/OBDMT was equal to that ofDGETP/TETA, different from that of DGETP/MHHPA. These results have been reflected bythe DSC curves.The reaction activation energy, onset temperature and peak exothermic temperature ofDGEBA and DGETBBA have the high value, while corresponding values of DGEBATP andDGETDBT retained the lower value.The end, the form-stable phase change materials （FSPCM） based on thermoplasticelastomers, such as POE, EVA and EPDM, were respectively preparated with paraffin,aimming at the heat problem of LED. The stabilities of FSPCM were measured via heatresistance test test, hot water bath test, temperature cycle test and differential scanningcalorimetry （DSC） test. The results showed that the heat storage and stable performances ofthree thermoplastic elastomer/wax FSPCMs were obvious better than those of HDPE/waxFSPCM.