WO2015031570A1

WO2015031570A1 - Making thermally conductive particles

Info

Publication number: WO2015031570A1
Application number: PCT/US2014/053076
Authority: WO
Inventors: Yuri NOMA; Yuji Saga
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2013-08-29
Filing date: 2014-08-28
Publication date: 2015-03-05
Also published as: CN105705239A; JP2015044718A; EP3038754A1

Abstract

Methods of making thermally conductive particles include mixing in a solvent carbon particles, a cationic surfactant or an amphoteric surfactant, a hydrolysis catalyst, and a silica precursor to result in mixed solution containing silica-coated carbon particles having volume resistivity of 1 x 10⁶ Ω-cm or more. Compositions comprising these particles and a polymeric medium, which, when molded, exhibit a combined property of thermal conductivity of at least 1 W/mK, and volume resistivity of at least 1 x 10⁸ Ω-cm.

Description

MAKING THERMALLY CONDUCTIVE PARTICLES

OVERVIEW

[0001] Described herein are methods of making thermally conductive particles having volume resistivityof at least 1 at least 1 x io⁶ Ω-cm as well as insulating compositions containing these thermally conductive particles.

[0002] Electronic devices, such as light emitting diodes (LEDs) have been developed to be more powerful and generate greater electrical output at the same time that they have become miniaturized and more integrated into ever smaller housings. In particular, for LEDs, the higher the light output of the LED, the greater the electrical energy requirement and the greater the thermal output. Thermal management of high power electronic devices, such as LEDs, is crucial to maintain long-term functioning and safe performance of the device.

Thus, thermal management of such high power devices has resulted in the need to dissipate heat from within the housing while preserving electrical insulation to avoid shock. In some LEDs in particular, the housing acts as a heat sink and aluminum is commonly used as the heat sink material. However, such metal housing is relatively heavy and electrically conductive.

[0003] To help solve the need for thermal management in housings of high power electronic devices while reducing the electrical conductivity of the housing , thermally conductive yet electrically insulating particles have become a current research interest . The aim is to blend these into polymeric compositions to thereby provide polymeric compositions suitable for housings and other elements of high power electronic devices.

[0004] JP Pat. App. Pub.2010-024406 discloses a method of forming a film of silicon dioxide hydrate on the surface of natural graphite in which natural graphite and tetraethoxy silicate, a coupling agent, are added to isopropanol. JP Pat. App. Pub.2011/089216 discloses a graphitized short fiber having a silicon carbide layer on its surface and used as a thermally conductive material that has insulating properties. The silicon carbide layer is coated by firing at over 1000 °C in silicon monoxide gas. JP Pat. App. Pub.2009-235650 discloses forming an insulating coating on a fibrous carbon system material. JP Pat. App. Pub.09-309710 and JP 08-259838 disclose the preparation of nonconductive carbonaceous powders. U.S. Pat. App. Pub.

2011/0129672 discloses a silane coating process for non-spherical hollow particles for cosmetic applications. U.S. Pat. No.8,110,284 discloses microcapsules which are encapsulated with a silane compound. U.S. Pat. No.6,919,106 discloses the preparation of porous SOG films using silane. compounds.

[0005] Described herein are are thermally conductive particles that exhibit thermal conductivity with electrical insulation made by a method of mixing either a cationic surfactant or an amphoteric surfactant, a hydrolysis catalyst, and a silica precursor. Also described herein are electrically insulating polymeric compositions containing these thermally conductive particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 shows results of analysis by Auger electron spectroscopy (AES) in the depth direction of thermally conductive particles of Example 1.

Figure 2 depicts the device used to measure the volume resistivity of thermally conductive particles made in the Examples.

Figure 3 depicts a thermally conductive particle made by methods described herein.

DETAILED DESCRIPTION

Definitions

[0007] The following definitions and abbreviations are to be used to interpret the meaning of the terms discussed in the description and recited in the claims.

As used herein, the terms "light-emitting diode" or "LED" refer to a device comprising at least one light-emitting semiconductor diode, an electrical connection capable of connecting the diode to an electrical circuit, and a housing partially surrounding the diode. The LED may optionally have a lens that fully or partially covers the LED.

As used herein, the terms "LED housing" or "housing" refer to a structural element of an LED of which at least part, preferably all, of the structural element comprises a polymer composition and coated carbon-based particles disclosed herein and wherein the housing partially or completely surrounds the diode so as to form a cavity around the diode with the housing having an opening for the light emitted by the diode to exit.

As used herein, the term "carbon-based particle" refers to carbon based particles that are not in the form of fibers. Carbon-based particles also include carbon powders and carbon flakes. The carbon-based particle can be naturally occurring carbon or synthetic carbon. Non-fibrous carbon-based particles have an aspect ratio (length to width ratio) of less than 2. Such particles are typically round, oval, flat, or irregular in shape.

As used herein, the term "graphite flake" refers to graphite particles that are not in the form of fibers. Graphite flakes also includes graphite powder and graphite particles. The graphite can be naturally occurring graphite or synthetic graphite. Non-fibrous graphite or graphite flake has an aspect ratio (length to width ratio) of less than 2. Such flakes are typically round, oval, flat, or irregular in shape.

As used herein, the term "amorphous silica precursor" refers to compounds or materials which when exposed to a catalyst, results in the formation or generation of a silica based material which is useful for coating particles to make the particles electrically insulating and thermally conductive. As used herein, the term "collected" refers to a process by which coated carbon-based particles are separated and isolated from the solution in which the particles are coated.

As used herein, the term "coated" refers to a carbon-based particle which has on its entire surface a layer of silica based material such as a Si0₂ coating. The layer of silica material completely encapsulates or encloses the particle.

As used herein, the term "coated carbon-based particles" refers to particles in which the exterior surface of the particle may be completely or partially coated with a material that renders the particle electrically insulating and thermally conductive.

As used herein, the term "volume resistivity" refers to electrical resistivity of a material and is a method for determining the electrical insulating capacity of a material. Volume resistivity is measured by placing the sample carbon particles in a transparent cylinder between two electrodes with terminals. The surface area of the electrode is 0.785 cm². A voltage of 1000 V was applied through the terminals and the resistivity of the particles measured. The packing ratio is calculated from the weight and volume of the particles.

As used herein, the term "aspect ratio" of a particle refers to the ratio of the particle's length over its width.

As used herein, the term "colloidal silica" refers to suspensions of fine amorphous, nonporous, and typically spherical silica particles suspended in a liquid phase, and is a silica precursor used in the methods described herein. The liquid is typically H₂0.

As used herein, the term "water glass" is any number of related sodium silicate substances dissolved in water.

Abbreviations

[0008] As used herein, "PDMS" refers to polydimethylsiloxane.

As used herein, "SiO_x» refers to silica.

As used herein, Aq Amphitol^® and Aq Capstone^® refer to aqueous solutions of Amphitol^® and of Capstone^® respectively, which are described in detail in the Materials section.

As used herein, "I PA" refers to an aqueous solution of water and isopropyl alcohol as described in Solvents section.

As used herein, "Aq NH₃" and "ammonia water" refer to Aqueous Ammonia Solution, used in the methods described herein as a Hydrolysis Catalyst.

As used herein, "Aq Snowtex^®" refers to an aqueous solution of Snowtex^® as described in the materials section.

As used herein, "HCI" refers to hydrochloric acid.

As used herein, "Water Glass" refers to a common name for any sodium silicate compounds having the formula Na₂₍Si0₂)_nO, available in aqueous solution.

As used herein, "wt%" refers to weight percent. As used herein, "μιη" refers to micrometers.

As used herein, "nm" refers to nanometers.

Ranges

[0009] Any range set forth herein expressly includes its endpoints unless explicitly stated otherwise. Setting forth an amount, concentration, or other value or parameter as a range specifically discloses all ranges formed from any pair of any upper range limit and any lower range lim it, regardless of whether any specific range of each such possible pairs of upper and lower limits are expressly disclosed herein. To be clear, the processes, compositions, methods and articles described herein are not limited to only those specific ranges expressly stated herein.

Preferred Variants

[0010] The disclosure herein of any variants in terms of materials, methods, steps, values, and/or ranges, etc.— whether identified as preferred variants or not— of the processes, compositions and articles described herein is specifically intended to disclose any process and article that includes ANY combination of such materials, methods, steps, values, ranges, etc. For the purposes of providing photographic and sufficient support for the claims, any such disclosed combination is specifically intended to be a preferred variant of the processes, compositions, and articles described herein.

Generally

[0011] Described herein are methods of making thermally conductive, silica-coated carbon particles having a volume resistivity of at least 1 x 10⁶ Ω-cm, which include mixing in a solvent carbon particles, a cationic surfactant or an amphoteric surfactant, a hydrolysis catalyst, and a silica precursor to result in mixed solution containing silica-coated carbon particles. Such mixing results in chemically reacting the silica precursor to form a silica layer on the surface of carbon particles. The silica precursor should be in colloidal form, and the carbon particles would preferably be coated by the solid silica precursor that has lost fluidity through promotion of the reaction. In addition, the thermally conductive particles may be removed by filtration from the mixture solution.

[0012] Also described herein are compositions that comprise the silica-coated carbon particles made by the methods described herein and at least one polymer.

[0013] In any of the methods or compositions described herein, any or all of the following variations may be included:

- only a cationic surfactant is used; and/or - only an amphoteric surfactant is used; and/or

- when a cationic surfactant is used, it is selected from the group consisting of quaternary ammonium salts, alkylamine salts, pyridinium salts, and mixtures of these; and/or

- the carbon particles are selected from the group consisting of graphite particles, carbon nanotubes, fullerene particles, carbon black, glass carbon particles, carbon fibers, silicon carbide particles, amorphous carbon, expanded graphite particles, boron carbide particles , and mixtures of these; and/or

- the silica precursor is silicon alkoxide; and/or

- the mixing of the mixed solution occurs when the temperature of the mixed solution ranges from 35°C to less than ioo°C; and/or

- the silica-coated carbon particles have a thickness of the silica layer ranging from 30 nm to 500 nm; and/or

-the composition, when molded, exhibits a combined property of a thermal conductivity of at least l W/mK, and a volume resistivity of at least ι χ 10⁸ Ω-cm; and/or

- the polymer is selected from the group consisting of organic polymers, inorganic polymers, organic-inorganic hybrid polymers, and mixtures of these; and/or

-the polymer is selected from the group consisting of polybutylene terephthalate, polyethylene terephthalate, polytrimethylene terephthalate, and at least one polyamide; and/or

- the polymer is selected from polybutylene terephalate; and/or

-the polymer is selected from at least one polyamide.

Mixing

[ooi/v] The methods described herein result in thermally conductive yet electrically insulating carbon particles coated with silica via s surface treatment with a silica precursor in a solvent.

[ooi5] To a solvent are added a catalyst, carbon particles, a surfactant— either cationic or amphoteric— and a silica precursor. The surfactant and the carbon particles may be added first to the solvent and stirred, followed by the addition of the silica precursor. Since the silica precursor reacts with water, hydrolysis can be initiated when the carbon particles and the surfactant are uniformly present in the solvent and then the silica precursor can be effectively added, particularly when the solvent is aqueous.

[0016] The mixing results in a mixed solution. The hydrolysis reaction could be promoted by regulating the temperature of the mixed solution during mixing.

[0017] The temperature at which mixing occurs may be adjusted and is a function of the boiling point of the solvent used. For example, the temperature of the mixed solution during mixing may range from 35°C to less than ioo°C. Alternatively, the temperature of the mixed solution may range from 45°C to less than 8g°C. Adjusting the temperature of the mixed solution to a range from 40°C to under 8o°C is desirable as this promotes the hydrolysis reaction of the silica precursor.

[ooi8]There is no specific limitation on the mixture duration since the hydrolysis reaction rate varies with the type of hydrolysis catalyst and mixting temperature. For example, mixing may range from 30 minutes to 10 hours. Alternatively, mixing may range from lto 8 hours or range from 1.5 to 5 hours. The operational efficiency of mixing improves when using a stirrer.

[0019] During mixing, a condensation polymerization reaction of hydrolyzed silica precursors results in the surface coating of the carbon particles with silica. The cationic surfactant or amphoteric surfactant acts as a binder of the silica to the carbon particles and silica.

[0020] The silica coating may be modified in various ways. For example, mixing may occur once or be repeated to facilitate a thicker silica coating. In addition, a silicon rubber may be combined with the silica precursorto impart elasticity and more strength to the silica coating. The amount of silicon rubber added should be in the range of 0.5 to 20 weight parts per 100 weight parts of silica precursor.

[0021] Additionally, when it is contemplated to make a thermoplastic polymer composition from the silica-coated graphite particles described herein and a polymer, a silane coupling agent may beneficially be added to the mixed solution in orderto improve the compatibility of the particles with the polymer. When adding a silane coupling agent to the mixed solution, mixing should occur by stirring for 30 minutes to 2 hours at a temperature of the solvent ranging from 30 to ioo°C .The amount of silane coupling agent may range from 1 to 10 weight parts per 100 weight parts of carbon.

[oo22]There is no specific limitation on the type of silane coupling agent used, but particulary suitable are: vinyl trimethoxy silane, vinyl triethoxy silane,

2- (3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane,

3- glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane,

3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane,

3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane,

3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane,

N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane,

3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(i,3-dimethyl butyl idene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane,

N-(vinylbenzyl)-2-aminoethyl-3-aminopropyltrimethoxysilane hydrochloride,

3-ureidopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane,

3-mercaptopropyltrimethoxysilane, bis (triethoxysilylpropyl) tetrasulfide, and

3-isocyanatepropyltriethoxysilane. It is within the knowledge of those of skill in the art to select si lane coupling agents for compatibility with the polymeric medium of an insulating composition.

Filtering and Silica Layer

[0023] After mixing, the coated carbon particles may be filtered by pouring the mixed solution through a filter with a mesh smaller than the particle diameter of the coated carbon particles. Silica-coated carbon particles would collect on the filter while the the solvent and the hydrolysis catalyst dissolved in the solvent pass through. However, during filtration, it is expected that some hydrolysis catalyst may remain in the filtered carbon particles and may be removed by washing the filtrate with alcohol or water and then drying. Drying preferably occurs at a temperature under 2oo°C. For example, the particles may set out to dry at ambient

temperature for 24 hours.

[0024] Now with reference to Fig. 3, the resultant particle 30 include a carbon particle 31 and a silica surface coating or silica layer 32. At this point, particle 30 is thermally conductive yet electrically insulating and has a volume resistivity of at least 1 x 10⁶ Ω-cm. It is expected that these methods result in a silica coating that covers the entire surface of each carbon particle. Nonetheless, even if some of the resultant particles are only partially silica-coated, the volume resistivity of each resultant particle is expected to be at least 1 x 10⁶ Ω-cm. And, the volume resistivity of the resultant particles may range from 5.0 x 10⁶ Ω-cm to 1 x 10¹³ Ω-cm.

[0025] Silica layer 32 covering carbon particle 31 in thermally conductive particle 30 contains a surfactant in silica layer 32, which is residual from the silica coating step.

[oo26] There is no specific limitation on the thickness of the silica layer, but preferably ranges from 3onm to 500 nm because this thickness provides adequate electrical insulation.

Carbon particles used in these methods

[0027] Carbon particles contain carbon, which includes carbon isotopes or carbon compounds. Carbon particles form the core of thermally conductive particles made by the methods described herein. Carbon material with thermal conductivity above 100 W-m^"1 -K^"1 would be formed into particle shape.

[0028] Carbon particles used in the methods described herein may be selected from graphite, carbon nanotubes, fullerene, carbon black, glass carbon, carbon fibers, silicon carbide, amorphous carbon, expanding graphite, boron carbide, and mixtures of these.

[002g] The diameter of the carbon may range from ι μιη to 300 μιη, or from 5 μιη to 50 μιη, or from 15 μιη to 100 μιη. The particle size distribution is determined via laser diffraction and the particle diameter is reported as the median of the distribution, known as Dso. The microtrack (X-100) can be used as a commercial particle size distribution measurement apparatus.

[0030] Desirable carbon particles in the methods described herein are graphite or carbon fibers. Graphite has a non-fibrous shape and may have an aspect ratio of less than two, meaning the particle's length is less than twice as long as its width. Graphite typically has a flat or plate shape, and would have length and width at least 2.5 times the thickness. The length or width of graphite may be 1 μιη to 300 μιη, or 5 μιη to 150 μιη, or 15 μιη to 100 μιη. The aspect ratio may be less than 1.5 or less than under 1.0. The minimum thickness of graphite may be 0.5 μιη and maximum thickness may be determined by the length and width of flake-shaped particles.

[0031] Carbon fibers may have a diameter ranging from 0.5 to 50 μιη and an aspect ratio ranging from 3 to 15 or 4 to 10. Desirable carbon fibers may be pitch-based carbon fibers. The thickness, length, and width of graphite and the diameter of carbon fibers may be measured with an electron microscope.

Silica precursors used in these methods

[0032] The silica precursor in the methods described herein is the source of the silica that coats graphite partices. Silica or SiO_x is a silicon oxide and may be crystalline or amorphous.

Amorphous silica may be used because the silica coating may be formed at low temperature. The silica used may contain in some part crystalline silica. Differentiation of crystalline or non-crystalline silica is done via X-ray analysis; peaks revealing crystalline structure do not appear in X-ray analysis of amorphous silica.

[0033] The silica precursor is silicon alkoxide represented by formula (I):

(R_a)_n Si(OR₂ )₄._n , where

R_a represents hydrocarbons with 1 to 8 identical or different, substituted or unsubstituted carbon atoms, n represents 0, 1, 2, or 3, and R₂ represents hydrocarbons with l to 8 carbon atoms. The silicon alkoxide is reacted with water and the hydrolysis catalyst to create silica, which is the entity that coats the carbon particles. [0034] The silicon alkoxide may be tetraalkoxysilane. More specifically, the tetraalkoxysilane may be tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, tetrabutoxysilane, tetraamyloxysilane, tetraoctyloxysilane,tetranonyloxysilane,dimethoxy diethoxy silane, dimethoxy diisopropoxy silane, diethoxy diisopropoxy silane, diethoxy dibutoxy silane, diethoxy ditrityloxy silane, or mixtures of these.

[0035] When the silicon alkoxide is tetraethoxysilane (TEOS, Si(OC₂H₅)₄)) , the hydrolysis reaction is:

nSi(OC₂H₅)₄+nH₂0 -» nSi(OH)(OC₂H₅)₃+nC₂H₅OH

TEOS ultimately becomes Si(OH) as the hydrolysis reaction proceeds. A condensation polymerization reaction proceeds between two hydroxide molecules created here, and silica is created as shown below.

Si(OH)₄+Si(OH)₄ -» (OH)₃Si-0-Si(OH)₃+H₂0

The silica precursor may range from 50 to 200 weight parts per 100 weight parts of carbon.

Surfactants used in these methods

[0036] The methods described herein may use cationic surfactants with hydrophilic groups that dissociate in aqueous solution into cations or amphoteric surfactants that dissociate in aqueous solution into both anions and cations. These surfactants are used in these methods as binders of carbon particles and silica.

Amphoteric

[0037] Exam ples of amphoteric surfactants used in these methods include lauryl dimethyl amino acetic acid betaine, stearyl dimethyl amino acetic acid betaine, lauryl dimethyl amine oxide, lauric acid amido propyl betaine, lauryl hydroxy sulfobetaine,

2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine,

N-lauroyl-N'-carboxymethyl-N'-hydroxyethyl ethylene diamine sodium, N-coconut oil fatty acid acyl-N'-carboxyethyl-N'-hydroxyethyl ethylene diamine sodium,

oleyl-N-carboxyethyl-N-hydroxyethyl ethylene diamine sodium, cocamidopropyl betaine, lauramido propyl betaine, myristamidopropyl betaine, palm kernelamidopropyl betaine, lauramidopropyl hydroxysultaine, lauramidopropyl amine oxide, and hydroxyalkyl (C12-14) hydroxyethyl sarcosine.

[0038] Amphoteric surfactants may be amphoteric fluorinated surfactants with intramolecular perfluoroalkyls. An example is perfluoroalkyl betaine. Commerical examples of amphoteric fluorinated surfactants include Ftergent 400SW, available from Neos Co., Japan, Saffron S-231, available from AGC Chemicals Co., Japa n, and Capstone^® TMFS-50, available from E. I. du Pont de Nemours and Company, Wilmington, DE.

Cationic

[0039] Cationic surfactants may be selected from quaternary ammonium sa Its, alkylamine salts, and pyridinium salts. Quaternary ammonium salts and alkylamine salts are represented by formula (II) .

r

S^: , where

R represents identical or different alkyls, and X represents the halogens fluorine (F), chlorine (CI), and bromine (Br).

[0040] Examples of quaternary ammonium salts used in these methods include hexadecyl trimethyl am monium chloride, hexadecyl trimethyl ammonium bromide, octyl trimethyl ammonium chloride, octyl trimethyl ammonium bromide, decyl trimethyl a mmonium chloride, decyl trimethyl ammonium bromide, dodecyl trimethyl ammonium chloride, dodecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium chloride, octadecyl trimethyl ammonium bromide, stearyl trimethyl ammonium chloride, stearyl trimethyl ammonium bromide, cetyl trimethyl am monium chloride, cetyl trimethyl ammonium bromide, distearyl dimethyl ammonium chloride, distearyl dimethyl ammonium bromide, benzalkonium chloride, benzethonium chloride, cetyl pyridinium chloride, decalinium chloride, and iodofluoroalkyl trimethyl am monium. Among these, long-chain monoalkyl (or alkenyl) quaternary ammonium salts with 10 to 20 carbon atoms, and tri-short chain alkyl quaternary ammonium salts with 1 to 3 carbon atoms would be preferable.

[0041] Examples of alkylamines used in these methods include trioctylamin e hydrochloride, trioctylamine hydrobromide, tridecylamine hydrochloride, tridecylamine hydrobromide, tridodecylamine hydrochloride, tridodecylamine hydrobromide, trihexadecylamine

hydrochloride, trihexadecylamine hydrobromide, trioctadecylamine hydrochloride, and trioctadecylamine hydrobromide.

[0042] Pyridinium salts have a pyridine ring and are represented by general formula [III].

^{, ! ! ί ?} , where

R represents an alkyl, and X represents the halogens fluorine (F), chlorine (C I), and bromine (Br).

[0043] Exam ples of pyridinium salts used in these methods include pyridinium chloride, cetylpyridinium chloride, cetylpyridinium bromide, myristyl pyridinium chlcride, myristyl pyridinium bromide, dodecylpyridinium chloride, dodecylpyridinium bromide, ethylpyridinium chloride, ethylpyridinium bromide, hexadecylpyridinium chloride, hexadecylpyridinium bromide, butyl pyridinium chloride, butyl pyridinium bromide, methyl hexyl pyridinium chloride, methyl hexyl pyridinium bromide, methyl octyl pyridinium chloride, methyl octyl pyridinium bromide, dimethyl butyl pyridinium chloride, and dimethyl butyl pyridiniumi bromide.

[0044] Cationic surfactants may include fluorinated surfactants that have fluoroalkyls, for example, perfluoro alkyl trimethyl ammonium salts. Commercially available surfactants include Ftergent 300 or Ftergent 310, available from Neos Co., and Saffron S-221, available from AGC Semichem ical Co. Desirable cation ic surfactants include hexadecyl trimethyl ammonium bromide, stearyl trimethyl ammonium bromide, dodecyl trimethyl ammonium bromide, trimethyl stearyl ammonium bromide, cetyl trimethyl ammonium chloride, distearyl dimethyl ammonium chloride, and mixtures of these. In addition, hexadecyl trimethyl ammonium bromide having formula (IV) may also be desirable.

[0045] Cationic surfactants may contain one or more or combinations of quaternary ammonium salts, alkyl amine salts, and quaternary ammonium hydroxide. The amount of cationic surfactant may range from 0.5 to 10 weight parts per 100 weight parts of cairbon. The molecular weight of surfactant range from 50 to 5000, or from 100 to 1000, or from 300 to 500.

Hydrolysis catalysts used in these methods

[0046] Hydrolysis catalysts promote the hydrolysis reaction of silica precursors as acidic hydrolysis catalysts or basic hydrolysis catalysts. The methods described he rein may use acidic hydrolysis catalysts or basic hydrolysis catalysts. Acidic hydrolysis catalysts are proton (H⁺) donors that promote the hydrolysis reaction through protonation of oxygen atoms, whereas basic hydrolysis catalysts are proton (H⁺) acceptors that promote the reaction by enabling nucleophilic addition through proton transfer from carbon atoms in hydrolysis. [0047] Acidic hydrolysis catalysts may be used as the sole catalyst. When repeating the silica coating process as described above, basic hydrolysis catalysts and acidic hydrolysis catalysts may be alternated, which is expected to increase the strength of the silica coating.

[0048] Hydrochloric acid may be preferable as an acidic hydrolysis and ammonia may be preferable as a basic hydrolysis catalyst. The amount of hydrolysis catalyst ranges from 0.5 to 10 weight parts per 100 weight parts of carbon.

Solvents used in these methods

[0049] Effective solvents in these methods facilitate solubility of the surfactants. A particularly suitable solvent may be an aqueous solution that uniformly disperses the solute. Carbon particles, surfactants, and silica precursors can uniformly react by being uniformly dispersed.

[0050] Besides water, suitable solvents in these methods include isopropyl alcohol (IPA), methanol, ethanol, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), propylene glycol monomethyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA),

monoethanolamine (MEA), dipropylene glyol diacrylate (DPGDA), and mixtures of these.

[0051] Another particularly suitable solvent is an aqueous solution of water and one or more of the following: isopropyl alcohol (IPA), methanol, ethanol, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), propylene glycol monomethyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), monoethanolamine (MEA), dipropylene glyol diacrylate (DPGDA). In particular, the solvent may be a mixture of water and one or more of the following: isopropyl alcohol (IPA), methanol, and ethanol. When the solvent is an aqueous solution of water and IPA, methanol, or ethanol, the amount of solvent ranges from 300 to 2000 weight parts per 100 weight parts of carbon and the amount of water ranges from 4 to 70 weight parts per 100 weight parts of carbon.

Compositions Described Herein

[oo52]Compositions having electrically insulating properties may be prepared by dispersing in a polymeric medium the thermally conductive particles made by methods described herein. These compositions have both suitable electrical resistivity and suitable thermal conductivity. Media include polymers and other suitable media as well as combinations of media.

[0053] Suitable polymeric media include organic polymers, inorganic polymers,

organic-inorganic hybrid polymers, and any combination of these. Suitable organic polymers include thermoplastic resins, thermosetting resins, aramid resins, and rubber, and more specifically: polyolefin resins such as polyethylene and polypropylene; polyamide resins such as nylon 6, nylon 66, nylon n, nylon 12, and aromatic polyamide; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polycyclohexylmethylene

terephthalate; cyclic polyester oligomers, ABS resin, polycarbonate resin, modified

polyphenylene ether resin, polyacetal resin, polyphenylene sulfide resin, wholly aromatic polyester resin, polyether ether ketone resin, polyethersu lfone resin, polysulfone resin, polyamide imide resin, polyimide resin, polytrimethylene terephthalate resin, fluorine resin, epoxy resin, novolak resin, isothiocyanate resin, melamine resin, urea resin, imide resin, aromatic polycarbodiimide resin, phenoxy resin, phenol resin, methacrylate resin, unsaturated polyester resin, vinyl ester resin, urea urethane resin, and resol resin. Copolymers in which the constituents including these resins are arbitrarily combined may also be used. These organic polymers may be used alone or in combinations. Particularly suitable organic polymers include polyamide resin, polyester resin, polyphenylene sulfide resin, and wholly aromatic polyester resin.

[0054] Suitable inorganic polymers include, but are not limited, to silicon resin.

Organic-inorganic hybrid polymers are polymers with silica partially compounded in the carbon framework of organic polymers. While not restricted to specific polymers, a suitable example is epoxy resin - silica hybrid polymer.

[0055] Other suitable media may include organic media as necessary to dissolve these polymers or to regulate viscosity of the composition. The organic media may be evaporated by drying the insulating composition.

[0056] The amount of thermally conductive particles in these compositions may range from 10 to 80 wt°/o, or from 15 to 7owt°/o, or from 20 to 60 wt°/o, of the total weight of the composition. These compositions may also contain additives, including antioxidants, glass fiber, and lubricants. Because of their combined property of thermal conductivity and electrical conductivity, these compositions are particularly suitable in housings or components for LED lamps as well as for insulating film applied to a substrate for the installation of electronic components.

EXAMPLES

[0057] The methods described herein are further illustrated by, but not limited to, the examples, denoted by "E" in the following data tables. "CE" denotes comparative examples.

Materials

[0058] Carbon Particles: Flake shaped graphite particles having:

- a diameter (D50) of 35 μιη OR - a diameter (D50) of 150 μιη

Solvents:

-Deionized H₂0 ;

-Deionized H₂0 in Isopropyl alcohol ["I PA"];

-Ethanol

Anionic Polymer Coating: In comparative examples, an aqueous solution of 0.35 g having 30 wt % poly (4-sodium styrene sulfonate) of mean molecular weight of 200,000 and available from Sigma-Aldrich, St. Louis, MO, 2.5 g of graphite flakes, and 50 g of water was mixed for five minutes at room temperature in order to form an anionic polymer coating on the surface of graphite particles. The coated graphite particles were collected by filtering this mixed solution and then further treated.

Cationic Polymer Coating: In comparative examples, an aqueous solution of 0.25 g having 20 wt % poly-diallyldimethylammonium chloride aqueous solution ofmean molecular weight of 200,000 to 350,000 and available from Sigma-Aldrich, St. Louis, MO, 50 g of deionized water, and 1.46 g of sodium chloride was mixed and the graphite particles coated with anionic polymer were added thereto and stirred for five minutes at room temperature to achieve a cationic polymer coating over the anionic polymer coating.

Surfactants, Cationic: Indicated in the Tables by (C)

(1) Hexadecyltrimethylammonium bromide [CTAB], CAS No. 57-09-0

(2) Stearyltrimethylammonium bromide [STAB], CAS. No. 1120-02-1

(3) Dodecyltrimethylammonium bromide [DTAB], CAS No. 1119-94-9

(1), (2), and (3) are all available from from Sigma-Aldrich, St. Louis, MO

(4) Fluorinated cationic surfactant: Ftergent 300, available from Neos Co., Tokyo, Japan

(5) Polymer from reaction of 2-methyl-2-oxazoline and 2 g of acrylic acid

Surfactants, Amphoteric: Indicated in the Tables by (AP)

(6) Lauryl dimethyl amino acetic acid betaine: 31 weight percent aqueous solution of lauryl dimethyl amino acetic betaine (a.k.a Lauryl betaine), available as AMPHITOL 20BS, from Kao Corp., Tokyo, Japan. For the examples, 0.071 grams of lauryl dimethyl amino acetic betaine were added to water to create an aqueous solution that had the same amount of

surfactant-0.022 grams-as used in Example 1 with CTAB

(7) Fluorinated amphoteric surfactant: 27 weight percent aqueous solution of Capstone^® FS-50, available from E.I. du Pont de Nemours and Company, Wilmington, DE. For the examples, 0.081 grams of Capstone^® FS-50 were added to water to create an aqueous solution that had the same amount of surfactant-0.022 grams-as was used in Example 1 with CTAB

Surfactants, Anionic: Indicated in the Tables by (AN)

(8) Sodium Palmate, available from Tokyo Kasei Kogyo Co., Japan.

Surfactants, Nonionic Indicated in the Tables by (N)

(9) Polyoxyethylene (10) cetyl ether, available as Brij^® C10 from Sigma-Aldrich, St. Louis, MO Silica precursor:

-Tetraethoxysilane [TEOS] (a.k.a. tetraethyl orthosilicate), available from Sigma-Aldrich, St. Louis, MO.

Other Silica sources:

- Aqeous solution of 2.5 g colloidal silica, available as Snowtex^® from Nissan Chemical, Japan, in 50 g deionized H2O

- Water Glass, also known as liquid glass, is a common name for sodium silicate compounds having the formula Na₂Si0₂)_nO, and available in aqueous solution.

Silane Coupling Agent:

- 3-glycidoxypropyltriethoxysilane, available as KBE-403 from Shin-Etsu Chemical Co., Japan Polydimethylsiloxane [PDMS], CAS. No. 70131-67-8, available from Wako Pure Chemical

Industries, Ltd., Osako, Japan

- y-(2-aminoethyl) aminopropyltrimethoxysilane, available from Sigma Aldrich, St. Louis, MO. Catalyst:

-Ammonia Aqueous Solution

-Sodium Sulfate Solution

-Aqueous hydrochloric acid solution

Methods

General Preparation of Silica Coated Carbon Particles

[0059] Carbon particles in the form of flake graphite particles were subjected to surface coating by the following method: To a solvent was added a catalyst and a surfactant, followed by the addition of flake shaped graphite with a diameter (D50) of 35 μιη or a diameter (D50) of 150. μιη. A silica precursor was added, followed by stirring for two hours at a certain temperature, either 6o°C or 8o°C, using a magnetic stirrer to result in a mixed solution in which the graphite particles become at least partially coated. The mixed solution was then filtered, the graphite particles removed and dried for one day at room temperature. The resulting graphite particles were investigated by Auger electron spectroscopy (AES), which revealed the thickness of the silica layer to be about 100 nm (see FIG. 1).

Measurement of Volume Resistivity of Silica Coated Carbon Particles

[0060] The volume resistivity of the coated carbon particles in the examples and the comparative examples was measured by the two terminal method using the device 100 shown in FIG. 2. Carbon particles 12 were packed to a height of 30 mm in a clear transparent cylinder 11 bonded to two terminal electrodes 10 on both sides. The amount packed was 0.4 g. The area of the contact surface of one terminal electrode 10 with the transparent cylinder 11 was 0.785 cm². Voltage of 1000 V was applied to the cylinder between the two terminals and volume resistivity was determined.

Methods of Making Insulating Compositions Comprising Silica Coated Carbon Particles [0061] Silica-coated carbon particles of Example 1 were dispersed in organic solvent to prepare an insulating composition. Such compositions may be applied to at least part of a surface of an article to result in an insulated surface.

Method of Making Molded Articles from Silica Coated Carbon Particles and Polymeric Medium

[0062] Silica-coated carbon particles were mixed with polybutylene terephthalate and then subjected to molten kneading and injection molding using the micro compounder from DSM Research Xplore Co. and a desk-top injection molder to prepare a molded article 16 mm wide x 16 mm high x 16 mm thick.

Measurement of Thermal Conductivity of Molded Articles

[0063] The in-plane thermal conductivity of the molded articles was measured using a xenon flash analyzer from NETZSCH Co.

Measurement of Volume Resistivity of Molded Articles

[0064] The volume resistivity of the molded articles was measured at 500 V applied voltage using a Hiresta UP (MCP-HT 50) resistivity meter from Mitsubishi Analytic Co.

Tables

[0065] Table 1 shows examples of coated carbon particles made by the methods described herein. In Example 1, graphite particles were subjected to surface coating by the following method. The solvent was a mixture of deionized water in isopropyl alcohol. Ammonia water was added, followed by the addition of CTAB as the surfactant and of flake-shaped graphite having a diameter (D50) of 35 μιη. Finally, the silica precursor , tetraethoxysilane (TEOS), was added to this mixture, followed by stirring for two hours at 6o°C using a magnetic stirrer. The mixed solution was then filtered, followed by the removal of graphite particles and drying for one day at room temperature. The resulting silica-coated carbon particles were investigated by Auger electron spectroscopy (AES), which revealed the thickness of the silica layer to be approximately 100 nm (see FIG. 1).

[0066] Example 2 was prepared as in Example 1, except that STAB was substituted as a cationic surfactant. Example 3 was prepared as in Example 1, except that dodecyl trimethyl ammonium bromide was substituted as a cationic surfactant. Example 4 was prepared as in Example 1, except a fluorinated surfactant-Ftergent 300 was substituted as a cationic surfactant.

[0067] Example 5 was prepared as in Example 1, except an aqueous solution of 31% lauryl dimethyl amino acetic acid betaine— in particular, AMPHITOL^® 20BS— was substituted as an amphoteric surfactant. The amount of lauryl dimethyl amino acetic acid betaine added was set at 0.071 g of a lauryl dimethyl amino acetic acid betaine aqueous solution so as to reach the same 0.022 g quantity as CTAB.

[0068] Example 6 was prepared as in Example 1, except that a fluorinated surfactant (27 wt% aqueous solution Capstone^® FS-50) was substituted as an amphoteric surfactant. The amount of fluorinated surfactant added was set at 0.081 g of a fluorinated surfactant aqueous solution so as to reach the same 0.022 g quantity as CTAB in Example 1.

[0069] Example 7 was prepared as in Example 1, except ethanol was the solvent. Example 8 was prepared as in Example 1, except the mixing occurred at 8o°C.

[0070] Example 10 was prepared as in Example 7, except for the further addition of 0.05 g (4 weight parts) of silane coupling agent and the change of solvent amount from 18 g (1565 weight parts) to 4.5 g (391 weight parts). TEOS was added to the mixed solution and reacted for two hours, followed by the addition of the silane coupling agent 3-glycidoxypropyltriethoxysilane and further heating for one hour at 6o°C .

[0071] Example 11 was prepared as in Example 7, except for the further addition of 0.05 g (1.6 weight parts) of PDMS, change of the solvent amount from 18 g (1565 weight parts) to 4.5 g (391 weight parts), and the use of graphite having a diameter (D50) of 150 μιη. The PDMS was mixed with TEOS beforehand and then added to the mixed solution.

Table i: Examples of Coated Graphite Particles Made by Methods Described Herein

[oo72]Table 2 shows Comparative Examples of coated carbon particles made by methods NOT described or recited herein. CEi was prepared as in Example 1, except without the CTAB. CE2 was prepared as in Example 1, except sodium palmitate was substituted as an anionic surfactant. CE3 was prepared as in Example 1, except polyoxyethylene (10) cetyl ether— Brij® Cio was substituted as a nonionic surfactant.

[0073] In CE4, anionic coated graphite particles were prepared from 0.35 g of an aqueous solution of 30 wt.% poly (4-sodium styrene sulfonate), 2.5 g of graphite flakes, and 50 g of water, that had been mixed forfive minutes at room temperature in orderto form the anionic polymer coating on the surface of graphite particles. Anionic coated particles were collected by filtering the mixed solution. A total of 0.25 g of an aqueous solution of 20 wt.% poly-dia I lyldi methyl- ammonium chloride, 50 g of deionized water, and 1.46 g of sodium chloride (to cause colloidal silica to polymerize and form the coating on the graphite particle) were mixed; the anionic coated particles were added thereto and stirred forfive minutes at room temperature to achieve a cationic polymer overcoat on top of the anionic polymer coating. Subsequently, the mixed solution was filtered to collect the coated graphite particles. Then, the overcoated graphite particles were mixed with 50 g of deionized water and 2.5 g of colloidal silica, i.e., Snowtex®, forfive minutes at room temperature to achieve an outer silica coating. The coated graphite particles were removed by filtering, then dried for one day at room temperature.

[0074] In CE5, a total of 120 g of graphite particles were suspended in 4.8 liters of deionized water (2.4 wt.%). The pH of the suspension was adjusted to PH9.3 with sodium sulfate. The suspension was heated to 95°C, and both 1 liter of 5.25% sodium silicate aqueous solution and 1 liter of 1.57 wt% sulfuric acid solution were added concurrently at a constant rate over two hours to result in the suspension having pH 9.5. The graphite particles were then removed by filtering, washed, and then dried for one day at room temperature.

[0075] In CE6, 5 g of graphite particles were dispersed in 95 g of ethanol. Then, 3 g of

2-methyl-2-oxazoline and 2 g of acrylic acid were added and stirred for eight hours at 25°C.87 g of mixed solution were removed, after which 10 g of tetraethoxy silane and 3 g of 0.001 N dilute hydrochloric acid were added and mixed for 10 hours at 2 ° . The mixed solution was filtered to remove the graphite particles, which were washed and then dried for one day at room temperature.

[0076] In CE7, 5 g of graphite particles were dispersed in 95 g of ethanol.4 g of y-(2-aminoethyl) aminopropyltrimethoxysilane were added to this dispersion and stirred.0.03 g of 0.005N dilute hydrochloric acid were then added and mixed for four hours at 6o°C.The mixed solution was filtered; the graphite particles removed, washed, and dried for one day at room temperature. Table 2 Comparative Examples of Coated Graphite Particles NOT Prepared by Methods Described Herein

*Three step process using 50 g water in each step; superscripts i,2,and 3 indicate the step in which material was added

Discussion ofTables 1 and 2

[oo77] The volume resistivity of the graphite particles used in the methods described herein is typiclly 1 Ω-cm. The higher volume resistivity rates exhibited in both Tables 1 and 2 result from the silica layer coating on both the exemplary and the comparative particles.

[0078] Table 1 shows that the volume resistivity of the particles of Ei to E8 and Eio and E11 was at least 10⁴ and up to 10⁸ times higher than that of any comparative example in Table 2. In addition, CEi to CE3 differed from the examples only in that they used neither a cationic nor amphoteric surfactant. Thus, the absolute increase in volume resistivity of Ei to E8 and Eio and E11 over that of CEi to CE3 clearly shows the greater effectiveness of cationic or amphoteric surfactants in forming silica coatings in the methods described herein.

[0079] CE4 shows that colloidal silica as the source of silica in combination with a cationic surfactant produces silica coated graphite particles having very poor volume resistivity. CE5 shows that sodium silicate as the source of silica in combination with a hydrolysis catalyst in the absence of a surfactant produces silica coated graphite particles below the recited volume resistivity. CE4 and CE5 together show that, when the source of silica for coating is not a silica precursor, or, when a surfactant is not used, the silica coated graphite does not attain the recited volume resistivity.

[0080] CE6 shows that, when using a silica precursor with a cationic surfactant but insufficient hydrolysis catalyst, the resulting silica coated graphite particle does not attain the recited volume resistivity. CE7 shows that, when a silane coupling agent is used in the absence of a surfactant and a silica precursor, and with insufficient hydrolysis catalyst, the resulting graphite particle does not attain the recited volume resistivity.

Table 3: Compositions with or without Coated Graphite Particles Made by These Methods

[0081] Eg was prepared from a composition comprising polybutylene terephthalate as the polymer medium and silica-coated graphite particles as prepared in Example 1. The polybutylene terephthalate and the coated graphite particles were mixed and melt blended. The melt blend was injection molded using the micro compounder from DSM Research Xplore Co. and a desk-top injection molder to derive a molded test article 16 mm wide x 16 mm high x 16 mm thick. A molded test article CE8 was prepared as for Eg, except the composition included graphite particles that had not been coated with silica. A molded test article CEg was prepared as for Eg, except the composition lacked graphite particles.

Discussion ofTable

[oo82] Table 3 presents the thermal conductivity and the volume resistivity of molded test articles Eg, CE8, and CEg. Although Eg and CE8 exhibited the same thermal conductivity, Eg had more than 10¹⁰ times the volume resistivity of CE8, which shows the insulating property of silica coated graphite particles made by the methods described herein. Although CEg exhibited a substantially similar volume resistivity as Eg, its thermal conductivity was reduced five-fold. Thus, Eg shows that molded compositions containing graphite particles coated by the methods described herein exhibit a combined property of thermal conductivity and electrical insulation sufficient to whisk away or transfer heat from inside an LED housing or other high temperature electronic device while preventing electric shock.

Claims

What is claimed is:

1) A method of making thermally conductive particles, comprising:

mixing in a solvent carbon particles, a cationic surfactant or an amphoteric surfactant, a hydrolysis catalyst, and a silica precursor to result in a mixed solution containing silica-coated carbon particles,

wherein the silica-coated carbon particles have a volume resistivity of at least 1 x io⁶ Ω-cm.

2) The method of claim 1, wherein a cationic surfactant is used.

3) The method of claim 1, wherein an amphoteric surfactant is used.

4) The method of claim 1, 2,or 3, wherein the cationic surfactant is selected from the group consisting of quaternary ammonium salts, alkylamine salts, pyridinium salts, and mixtures of these.

5) The method of claim 1, 2, 3, or 4, wherein the carbon particles are selected from the group consisting of graphite particles, carbon nanotubes, fullerene particles, carbon black, glass carbon particles, carbon fibers, silicon carbide particles, amorphous carbon, expanded graphite particles, boron carbide particles , and mixtures of these.

6) The method of claim 1, 2, 3, 4, or 5, wherein the silica precursor is silicon alkoxide.

7) The method of claim 1, 2, 3, 4, 5, or 6, further comprising removing silica-coated carbon particles from the mixed solution by filtering.

8) The method of claim 1, 2, 3, 4, 5, 6, or 7, wherein the mixing occurs when the temperature of the mixed solution ranges from 35°C to less than ioo°C.

9) The method of claim 1, 2, 3, 4, 5, 6, 7, or 8, wherein the silica-coated carbon particles have a thickness of the silica layer ranging from 30 nm to 500 nm.

10) A composition comprising:

thermally conductive particles made by the method of claim 1, 2, 3, 4, 5, 6, 7, 8, or g; and a polymer. n)The composition of claim 10, wherein the polymer is selected from the group consisting of organic polymers, inorganic polymers, organic-inorganic hybrid polymers, and mixtures of these.

12) The composition of claim 10 or 11, wherein, when molded, exhibits a combined property of a thermal conductivity of at least i W/mK, and a volume resistivity of at least 1 x io⁸ Q-cm.

13) The composition of claim 9, 10, n,or 12, wherein the polymer is selected from the group consisting of polybutylene terephthalate, polyethylene terephthalate, polytrimethylene terephthalate, and one or more polyamide.

14) The composition of claim 13, wherein the polymer is selected from polybutylene terephalate.

15) The composition of claim 13, wherein the polymer is selected from at least one polyamide.