Silicone-grafted core-shell particles, polymer matrix, and LED containing same

Abstract

A silicone-grafted core-shell particle is described wherein the silicone-grafted core-shell particle comprises a core of an inorganic particle and a shell of a grafted poly(dimethylsiloxane) polymer formed from a bi-terminated poly(dimethylsiloxane) having reactive groups at each terminal end. The silicone-grafted core-shell particles may be dispersed in a polysiloxane polymer matrix and employed as an LED encapsulant.

Claims

1. An LED comprising an LED die encapsulated in a polymeric matrix comprising a plurality of silicone-grafted core-shell particles dispersed in a polysiloxane polymer.

2. The LED of claim 1 wherein the polymeric matrix further contains a luminophor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a synthetic procedure for the preparation of silicone-grafted core-shell particles.

(2) FIG. 2 is a graph of the calculated thermal conductivity of the composites having three different fillers.

(3) FIG. 3 is a graph of the experimental thermal conductivity values for both pure silicone and silica-silicone hybrid particles with 50% SiO.sub.2 and 62% SiO.sub.2 by weight.

(4) FIGS. 4 and 6 show thermogravimetric analyses in air of SiO.sub.2-PDMS hybrids (dashed line), along with pure PDMS (solid line), and PDMS+50% SiO.sub.2 (dotted line).

(5) FIGS. 5 and 7 are graphical comparisons of the transmission of light through 1 mm cast films of SiO.sub.2-PDMS hybrids and PDMS+50% SiO.sub.2.

(6) FIG. 8 shows heat release rates, as measured by combustion microcalorimetry, for SiO.sub.2-PDMS hybrids compared to PDMS alone.

(7) FIG. 9 is a transmission electron micrograph of a SiO.sub.2-PDMS.

(8) FIG. 10 is a schematic illustration of an LED with a silica-containing polymer encapsulant according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

(10) FIG. 1 illustrates a synthetic procedure for the preparation of the silicone-grafted core-shell particles and, in particular, poly(dimethyl siloxane) grafted core-shell particles. The circle represents an inorganic particle; R represents a functional end group reactive toward the surface of the particle (see Table 1); R.sup.1 represents a product of reaction of the functional group R with the surface moieties of the particle; n is the degree of polymerization of the polymer; and x is the number of grafted chains per particle. Preferred materials for the core-shell particles include silicon dioxide (silica), titanium dioxide (titania), aluminum oxide (alumina), calcium carbonate, aluminum nitride, as well as other materials with a thermal conductivity of 1 W/mK and higher. Possible reactive end-functionalized groups are presented in Table 1. The groups react with the surface of particles resulting in a covalent or non-covalent bond. Depending on the ligand, the process may require a catalyst. The synthesis may or may not involve a solvent.

(11) TABLE-US-00001 TABLE 1 Examples of the functional end groups Name of the group R Structure of the group R Epoxyalkyl: n = 1 . . . 10. Covalent addition; catalyst: salts of tin or titanium. Hydroxy. Covalent addition; catalyst: HO salts of tin or titanium. Trialcoxyalkyl: R, R = methyl, ethyl; a = 1 . . . 3; b = (3 a); n = 1 . . . 10. Covalent addition. Alkylphosphonic acid: n = 1-10. Covalent and non-covalent addition. Alkylphosphonic acid, dialkyl ester: R = methyl, ethyl. Covalent and non-covalent addition. Carboxylic acid. Covalent and HOOC(CH.sub.2).sub.n non-covalent addition. Sulfide. Covalent and HS non-covalent addition. Catechol: n = 1 . . . 10. Non-covalent addition.

(12) The thermal conductivity of a mixture of particles in polymer matrix follows the effective medium approximation. According to the Bruggeman model, the effective thermal conductivity of the composite is derived from the Equation 1, where X.sub.f is the volume fraction of the filler, and k.sub.f, k.sub.m, k.sub.c are thermal conductivity values for filler, matrix, and composite respectively.

(13) $\begin{array}{cc}\frac{k_c}{k_m}=\left(\frac{k_f-k_c}{k_f-k_m}\right)*\frac{1}{{\left(1-X_f\right)}^3}& \left(1\right)\end{array}$

(14) FIG. 2 is a graph of the calculated thermal conductivity of the composites having three different fillers, silica (SiO.sub.2), titania (TiO.sub.2), and alumina (Al.sub.2O.sub.3), that are dispersed in a silicone matrix with a thermal conductivity of 0.15 W/mK. The values for thermal conductivity are given at the temperature of 300K.

(15) A core-shell particle (also referred to as a hybrid particle) interacts with light as a particle with the effective refractive index that is a combination of the refractive indices of core and shell materials (Equation 2):

(16) $\begin{array}{cc}{n}_{c-s}=\left(n_c-n_s\right)\frac{r^3}{{\left(r+h\right)}^3}+n_s& \left(2\right)\end{array}$

(17) Here, n.sub.c-s, n.sub.c, and n.sub.s are refractive indices of the core-shell particle, the material of the core and the shell respectively, r is the radius of the core and h is the thickness of the shell. When the overall particle size is much smaller than the wavelength of light and n.sub.c-s is equal to the refractive index of the polymer matrix, the incorporation of such particles into the matrix will result in a non-scattering transparent material.

(18) Table 2 presents the calculated required core size for four different materials coated with a shell comprised of poly(dimethyl siloxane) with a refractive index of 1.395 and thickness of 5 nm and a matrix comprised of poly(diphenyl siloxane)-co-(dimethyl siloxane) copolymer with a refractive index of 1.460 or 1.537. The calculations are based on Equations (1) and (2). The size of core particles refers to the required particle diameter in order to have the effective refractive index of core-shell particles equal to that of the matrix. The highest volume fraction of filler refers to the volume fraction with no matrix polymer added.

(19) TABLE-US-00002 TABLE 2 Calculation for core size and volume fraction of transparent composites Highest Thermal Size of volume Thermal conductivity thickness core fraction of conductivity at the highest of shell, Core particles, filler, of the filler, volume fraction, n.sub.matrix n.sub.shell nm material n.sub.core nm % W/mK W/mK 1.460 1.397 5.0 SiO.sub.2 1.532 34.5 46.6% 1.5 0.63 1.537 1.397 5.0 TiO.sub.2 2.432 10.5 13.5% 7.0 0.23 1.537 1.397 5.0 AlN 2.170 13.0 18.1% 30.0 0.27 1.537 1.397 5.0 Al.sub.2O.sub.3 1.755 27.1 39.0% 30.0 0.65

(20) In a preferred embodiment, the polysiloxane surface modifying agent has a formula:

(21) ##STR00006##
wherein A, R, and B represent the following functional groups:

(22) A: OH, H

(23) B: OH, H, CHCH.sub.2

(24) R: CH.sub.3, C.sub.2H.sub.5, C.sub.6H.sub.5, CH.sub.2CH.sub.2CF.sub.3

(25) The index n is the degree of polymerization of the polymer and x is the number of the grafted chains per particle. A and B represent reactive terminal groups at the terminal ends of the siloxane polymer chain. The grafting occurs via reaction of the surface hydroxyl groups of the particles with the reactive terminal groups of the polymer chains, e.g., hydride or silanol groups. The reaction may be catalyzed by salts of tin and the solvent may be any good solvent for polysiloxanes, such as toluene, xylene, ethylbenzene, methyl isobutyl ketone (MIBK), tetrahydrofurane (THF), chloroform, or methyl ethyl ketone. The bi-terminated poly(dimethyl siloxane) with reactive groups at both terminal ends is used to form the grafted particles in order to have the capability of cross-linking the grafted particles into elastomer networks.

Poly(Dimethyl Siloxane) Grafted Silica Particles

Example 1

(26) Silica nanoparticles (MIBK-ST, 30 wt % dispersion in methyl isobutyl ketone, average size 20 nm, obtained from Nissan Chemicals) in the amount of 6 mL of dispersion (2 g of SiO.sub.2) were diluted with 14 mL of methyl isobutyl ketone (MIBK), and 30 mg of SnCl.sub.2.2H.sub.2O was dissolved in the mixture. The mixture was transferred into a flask filled with nitrogen and 0.4 mL of a 0.5 g/mL MIBK solution of hydride-terminated poly(dimethyl siloxane)

(27) ##STR00007##
(DMS-H21, 6,000 g/mol, obtained from Gelest, Inc.) was added dropwise. The flask was connected to a nitrogen gas line, immersed into an oil bath at 100 C., and the contents stirred for 2 hours under nitrogen protection, followed by slow addition of a further amount of the 0.5 g/mL MIBK solution of the hydride-terminated poly(dimethyl siloxane) intended to yield a desired fraction of silica in the final product. To obtain a product with a lower fraction of silica, 3.6 mL of the PDMS solution was used, resulting in a total of 2 g of the polymer added to the flask. To obtain a product with a higher fraction of the silica, 1.6 mL of the PDMS solution was used, resulting in a total of 1 g of the added polymer.

(28) After stirring the contents for 40 hours, the flask was cooled to room temperature and the contents poured into 70 mL of methanol. A white precipitate formed which was separated by centrifugation at 5000 g for 40 minutes and then re-dissolved in 30 mL of toluene. The solution was sonicated in a bath for 5 min, filtered through a 0.45 um polypropylene syringe filter, and concentrated on a rotary evaporator to 5 mL. To the resulting solution, 70 mg of a cross-linking agent (1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) and 40 L of a 1 g/L THF solution of a platinum catalyst (platinum carbonyl cyclovinylmethylsiloxane complex) was added. The residual solvent was evaporated in vacuum at room temperature and the solid was allowed to cure for 24 h, yielding a transparent elastomer.

(29) The composition of the samples was determined by analyzing the amount of carbon in the material, considering that poly(dimethyl siloxane) contains 32.4% carbon (by weight). The sample with the lower silica content contained 50% SiO.sub.2 and 50% PDMS polymer by weight, while the sample with the higher silica content had 62% SiO.sub.2 and 38% PDMS polymer by weight.

(30) FIG. 3 represents the experimental thermal conductivity values for both pure silicone and the silica-silicone hybrid particles with 50% SiO.sub.2 and 62% SiO.sub.2 by weight. The solid line represents the calculated values based on the Bruggeman effective media model. The best fit to the experimental data was enabled considering that thermal conductivity of the individual colloidal silica nanoparticles is 1.2 W/mK. The mass fraction is used instead of volume fraction.

Example 2

(31) Silica nanoparticles (MIBK-ST, 30 wt % dispersion in methyl isobutyl ketone, average size 20 nm, obtained from Nissan Chemicals) in the amount of 6 mL of dispersion (2 g of SiO.sub.2) were diluted with 14 mL of methyl isobutyl ketone (MIBK), and 30 mg of SnCl.sub.2.2H.sub.2O was dissolved in the mixture. The mixture was transferred into a flask filled with nitrogen and 0.4 mL of a 0.5 g/mL MIBK solution of silanol-terminated poly(dimethyl siloxane)

(32) ##STR00008##
(DMS-S31, 26,000 g/mol, obtained from Gelest) was added dropwise. The flask was connected to a nitrogen gas line, immersed into an oil bath at 100 C., and the contents stirred for 2 h under nitrogen protection, followed by slow addition of 3.6 mL of the 0.5 g/mL MIBK solution of the silanol-terminated poly(dimethyl siloxane), resulting in a total of 2 g of the polymer added to the flask. After stirring the contents for 40 h the flask was cooled to room temperature and the contents poured into 70 mL of methanol. The white precipitate was separated by centrifugation at 5000 g for 40 min and re-dissolved in 30 mL of toluene. The solution was sonicated in a bath for 5 min, filtered through a 0.45 um polypropylene syringe filter, and concentrated on a rotary evaporator to 5 mL. The residual solvent was evaporated in vacuum at room temperature and the solid was allowed to cure for 24 h, yielding a transparent elastomer.

(33) The synthesized material (SiO.sub.2-PDMS hybrid) was compared to the pure poly(dimethyl siloxane) (pure PDMS) and poly(dimethyl siloxane) blended with amorphous silica in the ratio of 1:1 by weight (further: PDMS+50% SiO.sub.2). FIG. 4 presents a thermogravimetric analysis in air of the SiO.sub.2-PDMS hybrid (dashed line), along with pure PDMS (solid line), and PDMS+50% SiO.sub.2 (dotted line). FIG. 5 compares the transmission of light through 1 mm cast films of SiO.sub.2-PDMS hybrid and PDMS+50% SiO.sub.2. The transmittance of the SiO.sub.2-PDMS hybrid is significantly higher compared to PDMS+50% SiO.sub.2 prepared by mechanical blending, due to the minimized aggregation of the silicon dioxide particles in SiO.sub.2-PDMS provided by chemical modification of the particles surface.

Example 3

(34) Silica nanoparticles (MIBK-ST, 30 wt % dispersion in methyl isobutyl ketone, average size 20 nm, obtained from Nissan Chemicals) in the amount of 6 mL of dispersion (2 g of SiO.sub.2) were diluted with 14 mL of methyl isobutyl ketone (MIBK), and 30 mg of SnCl.sub.2.2H.sub.2O was dissolved in the mixture. The mixture was transferred into a flask filled with nitrogen and 0.4 mL of a 0.5 g/mL MIBK solution of hydride-terminated poly(dimethyl siloxane) (DMS-H21, 6,000 g/mol, obtained from Gelest) was added dropwise. The flask was connected to a nitrogen gas line, immersed into an oil bath at 100 C., and the contents stirred for 2 h under nitrogen protection, followed by slow addition of 3.6 mL of the 0.5 g/mL MIBK solution of the hydride-terminated poly(dimethyl siloxane), resulting in a total of 2 g of the polymer added to the flask. After stirring the contents for 40 h the flask was cooled to room temperature and the contents poured into 70 mL of methanol. The white precipitate was separated by centrifugation at 5000 g for 40 min and re-dissolved in 30 mL of toluene. The solution was sonicated in a bath for 5 min, filtered through a 0.45 um polypropylene syringe filter, and concentrated on a rotary evaporator to 5 mL. To the resulting solution, 70 mg of a cross-linking agent (1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) and 40 L of 1 g/L THF solution of a platinum catalyst (platinum carbonyl cyclovinylmethylsiloxane complex) was added. The residual solvent was evaporated in vacuum at room temperature and the solid was allowed to cure for 24 h, yielding a transparent elastomer.

(35) FIG. 6 presents a thermogravimetric analysis in air of the resulting SiO.sub.2-PDMS hybrid (dashed line), along with pure PDMS (solid line), and PDMS+50% SiO.sub.2 (dotted line). FIG. 7 shows the transmission of light through 1 mm cast films of SiO.sub.2-PDMS hybrid and PDMS+50% SiO.sub.2. A significant improvement in optical transmittance of the hybrid material over the mechanical blend can be observed, which is attributed to the minimized aggregation of particles.

(36) FIG. 8 shows the heat release rates, as measured by combustion microcalorimetry, for the SiO.sub.2-PDMS hybrids prepared in Examples 2 and 3 compared to a sample of PDMS (no silica). Each sample was heated in air from 75 C. to 750 C. at the rate of 1 C. per second. The peak on the plot indicates the burning process. A significant delay in the ignition and burning of SiO.sub.2-PDMS hybrid materials compared to the unfilled PDMS indicates a greater flame resistance for the SiO.sub.2-PDMS hybrid materials. The much lower heat release rates further indicate a lower flammability for the SiO.sub.2-PDMS hybrid materials. FIG. 9 is a transmission electron micrograph of the SiO.sub.2-PDMS hybrid material of Example 3.

(37) FIG. 10 is an illustration of an LED package 10 wherein the LED die 20 is mounted in a carrier 14 having reflective sides 18. The LED die 20 is encapsulated in a polymeric matrix 12 which contains a dispersion of silicone-grafted core-shell particles. The matrix 12 further contains particles of a luminophor 16, e.g., a phosphor or quantum dot material.

(38) While there have been shown and described what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.