WO2015051354A1

WO2015051354A1 - Thermally conductive electrically insulating particles and compositions

Info

Publication number: WO2015051354A1
Application number: PCT/US2014/059239
Authority: WO
Inventors: Yuji Saga; Takashi Hirahara
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2013-10-04
Filing date: 2014-10-06
Publication date: 2015-04-09
Also published as: CN105764969B; JP2017504177A; JP6654562B2; KR20160068762A; CN105764969A

Abstract

A thermally conductive particle comprising (a) a composite core prepared by compression shear mixing a plurality of thermally conductive core particles and an organic binder, and (b) an insulating material that coats at least partially the composite core, wherein the volume resistivity of the thermally conductive particle ranges from at least 1x10⁴ Ω•cm to 1x10¹⁰ Ω•cm.

Description

THERMALLY CONDUCTIVE ELECTRICALLY INSULATING PARTICLES AN D COMPOSITIONS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Japanese Pat. App. No. 2013- 209304, filed 4 October 2013, and currently pending.

OVERVIEW

Described herein are thermally conductive particles that are also electrically

insulating, resin compositions comprising these particles, and methods for making them.

In recent years, electronic devices, like LED modules and hand helds, have become not only more miniaturized and integrated but also of greater power, which has demanded greater heat dissipation and electrical insulation of the components in these devices.

Consequently, a thermally conductive particle with excellent electrical insulating properties has been sought as a building block for materials used in electronics devices.

Int. Pat. App. Pub. No. WO2011/027757 discloses ceramic coated carbon particles, which are thermally conductive, and formed by adding a slurry of carbon particles to a slurry of ceramic particles, whereby the ceram ic particles adhere to the carbon particles. U.S. Pat. No. 5,246,897 discloses coated graphite particles formed by a mechanical impact method in which graphite particles and coating particles are collided using high speed gas flow. U.S. Pat. No. 7,588,826 discloses coated graphite particles formed by mechanical melting in the presence of an interactive functional agent.

Described herein are thermally conductive particles that comprise a composite core and insulating material that at least partially coats the composite core, wherein the

composite core contains thermally conductive core particles bound together by an organic binder via mechanofusion processing, and the volume resistivity of the thermally

conductive particle ranges from at least ixio⁴ Ω · cm to ixio¹⁰ Ω - cm. Also described herein are resin compositions comprising thermoplastic resin, thermosetting resin, aramid resin, rubber, or mixtures of these, and 10 to 70% by volume of these particles. Also described herein are methods of making these particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional drawing of thermally conductive particles recited in the claims.

FIG. 2 is a schematic of an apparatus that measure volume resistivity of thermally conductive particles.

FIG. 3 is a top-view photograph of a molded product comprising thermally

conductive particles recited in the claims.

FIG. 4 is a cross-sectional photograph of a molded product comprising thermally conductive particles recited in the claims.

DETAILED DESCRIPTION

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.

Definitions

As used herein, the term "volume resistivity" refers to electrical resistivity of a material and determines 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. In the examples herein, a voltage of 500 V was applied through the terminals to measure the resistivity of the particles described herein.

As used herein, the term "aspect ratio" of a particle refers to the ratio of the particle's greatest length divided by its width, that is, its greatest thickness.

As used herein, the term "composite core" refers to thermally conductive core particles bound to an organic binder by the process of mechanofusion.

As used herein, the term "thermally conductive" or "thermal conductivity" (often denoted k, λ, or κ) refer to the property of a material to conduct or transfer heat. Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity is typically measured as thermal conductance, which refers to the quantity of heat that passes in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one kelvin. For a plate of thermal conductivity k, area A and thickness L, the conductance calculated is kA/L, measured in W/m-K and equivalent to: W/°C.

As used herein, the term "thermal diffusivity" refers to the thermal conductivity of an article divided by the density and specific heat capacity of the article at constant pressure, and measures the ability of the article to conduct thermal energy relative to its ability to store thermal energy. It has the SI unit of m²/s. Thermal diffusivity is usually denoted a. The formula is: ot = k , where

p c_P,

k is thermal conductivity (W/(m K));

p is density (kg/m³); and

c_p is specific heat capacity (J/(kg K)).

Abbreviations

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

As used herein, "vol %" refers to volume percent.

As used herein, "hrs" refers to hours; "m" refers to minute; "s" refers to seconds

As used herein, "g" refers to grams.

As used herein, "μιη" refers to microns.

As used herein, "nm" refers to nanometers.

As used herein, "rpm" refers to revolutions per minute.

As used herein, "mm" refers to millimeters.

As used herein, "cm" refers to centimeters

As used herein, "ml" refers to milliliters

As used herein, "V" refers to volts

As used herein, "Ω - cm" refers to ohms - centimeters.

As used herein, "W" refers to watts.

As used herein, "m" refers to meter

As used herein, "K" refers to Kelvin

As used herein, "mPa - s" refers to millipascal second

Ranges

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 such pairs are separately disclosed herein. The processes and articles described herein are not limited to the specific values disclosed in defining a range in the description.

Preferred Variants

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

Described herein are thermally conductive particles, one of which is shown as element 10 in FIG. l, which comprise a composite core made up of core particles n bonded together with an organic binder 12; and insulating material 13, which at least partially coats the composite core.

Also described herein are resin compositions that comprise the thermally conductive particles described herein and a thermoplastic resin, thermosetting resin, aramid resin, rubber, or mixtures of these.

Also described herein are methods of coating at least partially a composite core with insulating material, wherein:

the composite core comprises a plurality of core particles and an organic binder binding the core particles together,

the core particles are thermally conductive and selected from the group consisting of metal particles, ceramic particles, carbon-based particles, and mixtures of these,

the insulating material is selected from the group consisting of sericite, boehmite, talc, and mixtures of these, and

the thermally conductive particle exhibits a volume resistivity, when measured on a cylinder of said thermally conductive particles having a 10 mm diameter and a height of 3.0 mm at 500V applied voltage, which ranges from at least ixio⁴ Ω - cm to 1 x 10¹⁰ Ω - cm.

Thermally Conductive Particles

Binding a plurality of core particles with an organic binder using a compression shear mixing method results in a composite core, which can be prepared to have a desired particle size or shape. The volume of organic binder per ioo parts by volume core particles ranges from l to 30 parts by volume, preferably from 2 to 26 parts by volume, and more preferably from 4 to 22 parts by volume.

The composite cores are coated, either partially or entirely, with an insulating layer, the thickness of which ranges from 0.1 to 10 μιη, and preferably from 0.5 to 6 μιη. The volume of insulating material per 100 parts by volume of the composite core ranges from 3 to 48 parts, preferably from 5 to 35 parts, and more preferably from 10 to 32 parts. Such volume concentrations of the insulating material and thicknesses of the insulating coating impart to the thermally conductive particle sufficient thermal conductivity and the desired volume resistivity as recited in the claims.

The average particle size of the thermally conductive particles described herein ranges from 0.5 to 300 μιη, preferably from 20 to 250 μιη, and more preferably from go to 190 μιη. Average particle size may be determined by measuring its longest axis 14 via scanning electron microscopy (SEM).

Aspect ratio of the thermally conductive particles described herein ranges from 1 to 100, preferably from 2 to 50, more preferably from 3 to 35, and still more preferably from 4 to 15. FIG. 1 shows aspect ratio as the particle length (along long axis 14) divided by particle thickness 15. The aspect ratio of the thermally conductive particles described herein is preferably greater than 2 and preferably ranges from 3 to about 7 When the thermally conductive particles have a flat shape, molding them with a resin into an article can give the molded article anisotropic thermal conductivity.

The volume resistivity of the thermally conductive particles described herein ranges from at least 1x10 Ω - cm Ω - cm and ranges preferably from i.oxio⁴ to i.oxio¹⁸ Ω - cm, more preferably from i.oxio⁴ to i.oxio¹⁰ Ω - cm.

In any of the thermally conductive particles, compositions or methods described herein, any or all of the following elements may be combined to result in a wide ranging number of variants, each of which is contemplated as a recited invention:

the core particles may be carbon-based particles; and/or

- the carbon-based particles are graphite; and/or

the core particles are naturally occurring graphite and flaky carbon-based particles in a ratio of 3:2 to 99:1; and/or

when a mixture of graphite and flaky carbon-based particles are used, the flaky carbon-based particles have an average thickness less than that of the naturally occurring graphite; and/or

the core particles are 100 parts by volume of the thermally conductive particles described herein; and/or

the organic binder ranges from 3 to 25 parts by volume of the volume of the core particles; and/or

- the insulating material ranges from 4 to 48 parts by volume of the core particles; and/or the organic binder is a thermosetting resin; and/or

average particle size of the thermally conductive particles described herein ranges from 0.5 μιη to 300 μιη; and/or

the insulating material is selected from the group consisting of sericite, boehmite, talc, mica, and mixtures of these; and/or

in the thermally conductive resin compositions described herein, the thermally conductive particles range from 10 to 70% by volume of the total volume of the

composition; and/or

the thermally conductive resin compositions may comprise thermoplastic resin, thermosetting resin, aramid resin, rubber, or mixtures of these. a) Composite Cores

The composite cores exhibit sufficient thermal conductivity of at least loo W- m^" K^"1 and preferably less than or equal to 800 W - m^"1 - K^"1. The maximum value occurs with composite cores having anisotropic thermal conductivity.

Core Particles

Composite cores may comprise core particles of metal, ceramic, or carbon or mixtures of these. Metal particles include, copper, silver, nickel, aluminum, of alloys of these. Ceramic particles include alum inum nitride, silicon carbide, and alumina. Carbon- based particles include graphite, carbon nanotubes, fullerene, graphene, carbon black, glass carbon, carbon fibers, amorphous carbon, boron carbide, or mixtures of these.

Preferably, the composite cores include carbon-based particles. Carbon-based particles preferably contain graphite, which may be natural or synthetic, with natural being preferred because of its thermal conductivity and cost.

In particular, the carbon-based particles comprise a mixture of graphite that is more of less spherical as well as platy shaped, aka as flaky, in a volume ratio of 3:2 to 99:1 and preferably in a volume ratio of 1:1 to 6:1 of spherical to flaky. Flaky carbon-based particles that are thinner than natural graphite include expanded graphite, graphene, and mixtures of these. Expanded graphite is a flaky natural graphite in which the interlaminar space is expanded by chemical and heat treatment to expand and separate graphite in the direction in which the lamina are stacked. Graphene is a flaky particle in which the carbon atoms are aligned into a hexagonal lattice. T, natural graphite is several μιη thick, while expanded graphite and graphene are less than 1 μιη thick.

Mixing natural graphite that is relatively thick, with flaky carbon-based particles forms a more or less flat thermally conductive composite core. Molded articles formed with resin compositions comprising more or less flat thermally conductive particles impart anisotropic paths of thermal conduction, which increases the thermal conductivity of the molded article.

The average particle size of the composite core ranges from 1 to 150 μιη, preferably from 15 to 100 μιη, and more preferably from 30 to 90 μιη. The average particle size of the composite core is determined by measuring the longest axis of the composite core by scanning electron microscopy (SEM).

The aspect ratio of the composite core ranges from preferably 1 or more, more preferably 2 or more, and even more preferably 5 or more. The composite core may include two or more types of particles with different aspect ratios. Organic binder

The composite core may be formed into a desired shape by binding one or more kinds of core particles together with an organic binder. The organic binder is a polymer with a weight average molecular weight of at least 300. The weight average molecular weight is preferably less than 1,000,000.

The type of organic binder is not specifically limited, but preferably a thermoplastic resin, thermosetting resin, or mixtures of these and more preferably a thermosetting resin. Thermosetting resins include epoxy, novolac, isothiocyanate, melamine, urea, imide, aromatic polycarbodiimide, phenoxy resin, phenols, methacrylates, unsaturated polyesters, vinyl esters, urea urethane, resol, and silicone, and mixtures of these. The thermosetting resin is preferably melamine.

The organic binder may be in solution or dispersed in a solvent. The preferred solvent is water; aqueous organic binder solutions are preferred.

Preparing Composite Cores

The composite core may be formed by mixing core particles and an organic binder to bind and mix the core particles to the organic binder. The organic binder fills in the gaps between core particles and increases the mechanical strength of the core particles.

Compression shear mixing can be used when blending the core particles and organic binder and conjugating the core particles. Compression shear mixing is a method that the compression force and shear force are exerted on a plurality of particles of different materials (the mixture of core particles and organic binder), causing the particles to bind to one another.

A Mechanofusion System (Hosokawa Micron Ltd.) or Theta Composer System

(Tokuju Corp.) may be used to provide compression shear mixing. When the

Mechanofusion System is used, rotating blades inside the compression vessel press core particles and organic binder against the vessel inner wall and imparting intense

compression and shear force to bind and mix the core particles with the organic binder.

When a dry particle composing machine, such as NOB-130^® (available from

Hosokawa Micron Ltd.), is used for conjugation, the gap between the blades and the vessel wall ranges from 1 to 3 mm and the rotational speed of the blades ranges preferably from 1,000 rpm to 6,000 rpm, and more preferably from 1,800 rpm to 4,500 rpm.

When the Theta Composer System (THC model Theta Composer, available from Tokuju Corp) is used, the vessel of the system rotates in one direction while elliptical rotors inside the vessel rotate in the opposite direction, so that compression and shear are exerted in the gap between the vessel wall and the rotors to bind and the core particles with the organic binder. When the gap between the rotors and the vessel wall ranges from 1 to 3 mm, the rotational speed of the blades ranges preferably from 1,000 rpm to 6,000 rpm, and more preferably from 1,800 rpm to 4,500 rpm.

The organic binder may also be thinned in advance with a solvent before mixing with core particles. By thinning the organic binder with a solvent, it is possible to bind the carbon particles with less binder. The solvent may be any type as long as the solvent dissolves the organic binder. Solvents include water, isopropyl alcohol (IPA), methanol, ethanol, methylethyl ketone (MEK), methyl isobutyl ketone (MIBK), propylene glycol monomethyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), monoethanolamine (MEA), dipropylene glycol diacrylate (DPGDA), or mixtures of these. It is preferable that the solvent is a solution in which the organic binder will be uniformly dispersed. If the organic binder is water soluble, the solvent is preferably water.

The temperature of the mixing of the core particles and organic binder is not specifically limited, but depends on the type and viscosity of the organic binder. In particular, if the organic binder is an aqueous solution, the mixture temperature ranges preferably from io°C to less than 8o°C, and more preferably from 25°C to less than 50°C to avoid evaporation of the water.

The viscosity of the organic binder or mixture of organic binder and solvent ranges preferably from 0.5 to 1,000 mPa · s at 2o°C. The composite core may achieve a desired shape by adjusting the viscosity of the organic binder and solvent during mixing. The composite core is more or less flat at 0.5 to 100 mPa · s at 2o°C, and becomes more spherical in shape at 100 to 1,000 mPa · s.

The mixing time may be adjusted so that the core particles and organic binder suitably bind and mix with one another. The mixing time ranges preferably from 1 minute to 30 minutes, and more preferably from 3 minutes to 10 minutes. b) Insulating material

The insulating material of the thermally conductive particles described herein has a volume resistivity of at least ixio⁹ Ω - cm at 20°C and preferably ixio²² Ω - cm or less.

Suitable insulating material is not specifically limited, may be naturally occurring or synthetic and includes, without limitation, metal oxides, metal carbonates, carbonate minerals, metal nitrides, metal sulfides, phosphate minerals, clay minerals, silicate minerals, glass materials, or mixtures of these.

Metal oxides include aluminum oxide (Al₂0₃), zinc oxide (ZnO), titanium oxide (Ti0₂), iron oxide (FeO), magnesium oxide (MgO), silicon oxide (Si0₂), boehmite (ΑΙ₂0₃ · H₂0), or mixtures of these. Metal carbonates include calcium carbonate (CaC0₃), magnesium carbonate (MgC0₃), or mixtures of these. Carbonate minerals include calcite (polymorphic CaC0₃), aragonite (crystalline CaC0₃), dolomite (CaMg(C0₃)₂), hydrotalcite (Mg₅AI₂C0₃(OH)_l5 - (H₂0)), pyroaurite (Mg₅Fe₂(C0₃)(OH)_l5 - 4(H₂0)), manasseite

(Mg₅AI₂(C0₃)(OH)_l5 - H₂0), or mixtures of these. Metal nitrides include boron nitride (BN), aluminum nitride (AIN), silicon nitride (Si₃N₄), or mixtures of these. Metal sulfides include molybdenum sulfide (MoS₂), tungsten sulfide (WS₂), zinc sulfide (ZnS), or mixtures of these. Phosphate minerals include apatite (Ca₅(P0₄)₃(F, CI, OH)), hydroxyapatite (Ca₅(P0₄)₃(OH)), or mixtures of these. Silicate minerals include monoclinic crystalline clay-like minerals (such as serpentine ((Mg, Fe)₃Si₂0₅(OH)₄), pyrophyllite (AI₂Si₄O_ao(OH)₂), kaolin clay, sericite (KAI₂AISi₃0₁₀(OH)₂), montmorillonite ((Na, Ca)₀.₃₃(AI, Mg)₂Si₄0₁₀(OH)₂ - nH₂0), chlorite group minerals, talc, verm iculite, and smectite group minerals), mica, diatomite (Si0₂ - nH₂0), or mixtures of these.

Since chemical formulas of the insulating materials listed above may apply to classes of insulating material, any species of insulating material identified by a chemical formula is contemplated in the thermally conductive particles described herein.

Suitable insulating material include aluminum oxide (alumina), zinc oxide, talc, magnesium oxide, silicon dioxide, boehmite, boron nitride, mica, aluminum nitride, silicon nitride, zinc sulfide, or mixtures of these. More preferable insulating material include talc, boehmite, serisite, mica, or mixtures of these. Talc is particularly preferable.

The average particle size of the insulating material is preferably from 10 nm to 50 μιη, more preferably from 100 nm to 30 μιη, and more preferably from 300 nm to 15 μιη.

The volume concentration of the insulating material to thermally conductive core particle ranges from about 4 to about 40 volume percent, preferably from about 5 to about 30 volume percent, more preferably from about 8 to about 30 volume percent, and most preferably from about 10 to about 25 volume percent.

Coating the Composite Core with Insulating Material

The composite core is at least partially coated by insulating material, which preferably has a volume resistivity of ixio⁹ to ixio²⁰ Ω - cm.

Similar to forming the composite core, a Mechanofusion System or Theta

Composer System may be used to coat the composite core with insulating material, which is in essence fused to the surface of the composite core by compression and shear force. Alternatively, different methods could be used to coat the composite core. However, to maximize manufacturing efficiency, it is preferable to use the same system to form the composite core and coat it with insulating material. Preferably, a mechanofusion process is used to form and coat the composite core.

It is preferable to engage in a two-step process whereby the core particle and organic binder first undergo compression shear mixing to form the composite core, to which is added the insulating material, which again undergoes compression shear mixing to result in the thermally conductive particles described herein. The temperature at which the composite core is coated with insulating material ranges preferably from less than or equal to the curing temperature of the organic binder, and if solvent is used, less than or equal to the boiling point of the solvent. The coating temperature preferably ranges from io°C to less than 8o°C, and more preferably from 25°C to less than 50°C. The insulating material will at least partially coat the surface of the composite core, thereby resulting in thermally conducting particles described herein.

Additionally, the duration of compressive shear mixing of the composite core with the insulating material may be adjusted so that the insulating material fuses to surface of the composite and ranges preferably from 5 seconds to 5 minutes, and more preferably, from 10 seconds to 120 seconds.

The insulating material preferably covers or coats the entire surface of the composite core. Alternatively, the insulating material may cover or coat a sufficient part of the surface of the composite core to maintain a volume resistivity of the thermally conductive particles of at least ixio⁴ Ω - cm. Resin compositions

Compositions described herein include the thermally conductive particles described herein dispersed in a thermoplastic resin, thermosetting resin, aramid resin, rubber, or mixtures of these. Compositions described herein exhibit both sufficient thermal conductivity and volume resistivity for use in molded products, films, sheets, adhesives and the like. For instance, as the compositions described herein may be prepared as an insulating film and applied to the surface of an electronic part, or may be injection molded and used as the housing for heat-generating electronic parts, such as LED bulb components, and the like.

As for thermoplastic resins, any suitable resin may be used in the compositions described herein and may include polyolefin resins such as polyethylene and polypropylene, polyamide resins such as nylon 6, nylon 66, nylon 610, nylon 12, and aromatic polyamides, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polycyclohexylmethylene terephthalate, cyclic polyester oligomers, ABS resin,

polycarbonate resin, denatured polyphenylene ether resin, polyacetal resin, polyphenylene sulfide resin, wholly aromatic polyester resin, polyether etherketone resin, polyether sulfone resin, polysulfone resin, and polyamide-imide resin. Copolymers obtained by selectively combining constituents that constitute these resins may also be used.

Thermoplastic resins may be combined. Preferably, the thermoplastic resin is selected from the group consisting of polyamide resin, polyester resin, polyphenylene sulfide resin and wholly aromatic polyester resin.

As for thermosetting resins, any suitable resin may be used in the compositions described herein and may include epoxy resin, novolac resin, isothiocyanate resin, melamine resin, urea resin, imide resin, aromatic polycarbodiimide resin, phenoxy resin, phenol resin, methacrylate resin, unsaturated polyester resin, vinylester resin, urea urethane resin, and resol resin. Preferably, the thermosetting resin contains melamine resin, epoxy resin, or unsaturated polyester resin.

Any suitable solvent may be added to the compositions described herein so long as it dissolves the resin or rubber or adjusts its viscosity. It is expected that most of the solvent will evaporate in a drying step. Suitable solvents include water, isopropyl alcohol (IPA), methanol, ethanol, methylethyl ketone (MEK), methyl isobutyl ketone (MIBK), propylene glycol monomethyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), monoethanolamine (MEA), dipropylene glycol diacrylate (DPGDA), or mixtures of these. The volume of the thermally conductive particles described herein relative to the total volume of the compositions described herein ranges preferably from lo to 70% by volume, more preferably from 15 to 50% by volume, and even more preferably from 20 to 38% by volume. Sufficient thermal conductivity and volume resistivity will be obtained when the volume percent of the thermally conductive particles relative to the total volume of the compositions falls within these ranges

Compositions described herein may contain additives, such as antioxidants, glass fibers, and lubricants.

The volume resistivity of articles made of the compositions described herein, when measured at 500 V applied voltage, ranges preferably from ι.οχιο¹⁰ to 1.0x1ο¹⁸ Ω - cm, and more preferably from 1.0x1ο¹³ to 1.0x1ο¹⁵ Ω · cm. The thermal conductivity of the surface of a 1 mm-thick molded product made of the compositions described herein, as estimated by the laser flash method, preferably ranges from 0.5 W/mK to 10.0 W/mK and more preferably from 1.0 W/mK to 5.0 W/mK or less.

Methods of Making the Thermally Conductive Particles Herein

The thermally conductive particles described herein exhibit the recited volume resistivity because of the way they are made, that is, because of a two-step process in which: first, a composite core is made by binding together thermally conductive core particles and organic binder, as shown in Fig. 1, and then subsequently the composite core is coated at least partially by insulating material. Both steps-of preparing composite cores and of coating the composite cores— are done by compression shear mixing.

In particular, the thermally conductive particles described herein are made by:

first, compression shear mixing in a dry particle compounding apparatus, such as NOB-130^® (available from Hosokawa Micron Ltd.) thermally conductive core particles and an organic binder, both as described herein, for 5 minutes at a rotational speed of 3,000 rpm to form a composite core; and

then, compression shear mixing in the dry particle compounding apparatus the composite core and insulating material so that the surface of the composite core becomes at least partially coated by the insulating material, thereby to form the thermally conductive particles.

The first compression shear mixing duration may range from about 30 seconds to about 90 minutes; and the organic binder may be dissolved or suspended in a solvent, preferably water, to make an organic binder solution or suspension. The second shear mixing time may range from less than 30 seconds to about 10 minutes.

Preferably, these particles may then be heat treated for three hours at i2o°C to eliminate moisture. If the organic binder was a thermosetting resin, such as methylol melamine, the heating treatment may cause that resin to further crosslink. The thermally conductive particles may then be added to any suitable resin or polymer as described herein to be prepared into thermally conductive resin compositions.

EXAMPLES

The present invention is illustrated by, but not limited to, the following examples (E) and comparative examples (C).

Materials

Natural Graphite Particles - flake graphite having an average particle size of 60 μιη available as graphite X-100 fromJTO Graphite Co. Ltd.

Methyol Melamine (MM) - organic binder available as Nikaresin S-i76 from Nippon Carbide Industries Co. Inc.

Aqueous Polyester (PE) - a water soluble polyester available as Z730 from Goo Chemical. Polyamide (PA) - a polyamide suspension in water available as PA200 from

Sumitomo Seika Chemicals.

Sodium Polystrene Sulfonate (SPS) - a 20% aqueous solution of sodium

polystyrene sulfonates, available as PS-50 from Tosoh Organic Chemical.

Talc 1 - having average D50 particle size of 0.65 μιη, available as HTP Ultra 5 from IMI FABI Co., Ltd.

- Talc 2 - having average D50 particle size of 2.4 μιη, available as HTP 2C from IMI

FABI Co., Ltd.

Talc 3 - having average D50 particle size of 5 μιη, available as LMS200 from Fuji Talc Industrial.

Sericite - fine grained mica, having average D50 particle size of 1.6 μιη, available as Sericite J from Kinsei Matic, Co. Ltd.

Mica - having average D50 particle size of 5 μιη, available from Yamaguchi Mica.

Boehmite - having average D50 particle size of 3-5 μιη, available as BMF from

Kawai Line Industry.

Titanium Dioxide (Ti0₂) - having average D50 particle size of greater than 0.5 μιη, available as Tipure^® R108 from E.I. du Pont de Nemours and Company [DuPont],

Wilmington, DE.

Boron Nitride (BN) - having average D50 particle size of 12 μιη available as SGP from Denki Kagaku.

Calcium difluoride (CaF₂) - having average D50 particle size of 6 μιη available as

HO#ioo from Sankyo Seifun.

Expanded graphite - obtained from Ito Kokuen Co., Ltd. as EC300.

Polybutylene terephthalate (PBT) - available as Crastin^® from DuPont. Methods

Measurement of Volume Resistivity

The volume resistivity of thermally conductive particles in the Examples and

Comparative Examples was measured by the two-terminal method using the device 20 in FIG. 2. A transparent cylinder 22 having a 10 mm inside diameter connected to terminal electrodes 21 in two locations on both sides was filled with graphite particles 23 to a height of 3.0 mm. The filled amount was 0.2 g. The surface area of the surface of one of the terminal electrodes 21 in contact with the transparent cylinder 22 was 0.785 cm². The volume resistivity was obtained by applying a 500 V voltage on the cylinder between the two terminals.

Methods of Preparing Thermally Conductive Particles Described Herein

All examples of thermally conductive particles set forth in the Tables below were made using Method A. Comparative example, Ci, was made using Method B; Comparative example, C2, was made using Method B.

Method A: 2 compression shear mixing steps to produce thermally conductive particle

The first step was compression shear mixing graphite and organic binder to produce the composite core; followed by a second step of compression shear mixing the composite core and insulating material.

Natural graphite particles and organic binder were added into a dry particle compounding apparatus (NOB-130®, available from Hosokawa Micron Ltd.) and

compression shear mixed for 5 minutes at a rotational speed of 3,000 rpm to form a composite core. The organic binders in the Tables were obtained by dissolving or suspending the specific binder in a quantity of water used as solvent to result in an organic binder solution or suspension.

Insulating material, which included talc, boehmite, sericite, and mica, was then added to a dry particle compounding apparatus containing the composite core, and subjected to a second compression shear mixing for 30 seconds at a rotational speed of 3,000 rpm to cause an insulating layer to partially form on the surface of the composite core, thereby to result in thermally conductive particles. The thermally conductive particles were heat treated for 3 hours at i20°C to remove remaining solvent. When methylol melamine was used as the organic binder, heating step caused the methylol melamine to further crosslink.

Method B: Single compression shear mixing step: Pre-mixing organic binder and insulating material, followed by adding thermally conductive core particles and then compression shear mixing all elements

Talc and aqueous solution of methylolmelamine were pre-m ixed together to form a talc/methylol amine mixture. This pre-mixing was not a compression shearing step. The pre-mixture was then added to a dry particle compounding apparatus as described in Method A along with natural graphite particles and compression shear mixed for 5 minutes to form thermally conductive particles. The thermally conductive particles were heat treated for 3 hours at i2o°C to eliminate solvent, resulting in thermally conductive particles having a volume resistivity of 8xio² Ω - cm at 500V applied voltage.

This method relies on ONLY one compression shear mixing step. Therefore, in Method B, NO composite cores were first produced. Method C: Single compression shear mixing step: Compression shear mixing organic binder, thermally conductive core particles and insulating material together This method has no pre-mixture step and all elements were added together and compression shear mixed. Natural graphite particles, organic binder (melamine aqueous solution) and insulating material (talc) were added together to a dry particle compounding apparatus as described in Method A and compression shear mixed for 5 minutes to form thermally conductive particles. The thermally conductive particles were then heat treated for 3 hours at i2o°C to eliminate solvent, resulting in thermally conductive particles having a volume resistivity of 8xio³ Ω - cm at 500V applied voltage. Like Method B, Method C also relies on ONLY one compression shear step and does NOT produce com posite cores.

Results

Table 1: Comparison of Mechanofusion Process and Conditions

Table 1 compares the volume resistivity of thermally conductive particles prepared by mechanofusion processes A, B, and C, having the same rotating speed, curing temperature and duration . Ei was prepared by mechanofusion process A and

demonstrates that mechanofusion process A achieved thermally conductive particles of the recited volume resistivity, while mechanofusion process B or C achieved C2 and C3, which did exhibit the recited volume resistivity.

Table 2: Effect of Varying Organic Binder Amount; Using Two Kinds of Graphite

All examples and the comparative example in Table 2 were prepared by method A and had an average length along the long axis of 131.7 μιη and average thickness of 22.5 μιη, giving an aspect ratio of 5.85. Each of E2 through E5 had a different quantity of organic binder in the composite core. C3 lacked an organic binder and exhibited a volume resistivity 5 Ω - cm. This contrasts with the minimum volume resistivity of E2 through E5, which was 1.5x1ο⁷ Ω· cm. Table 2 shows that, when the volume percent of the organic binder relative to the amount of thermally conductive core particles, ranges from 3 to 20, the recited volume resistivity of at least ixio⁴ Ω - cm is obtained.

Table 2 also shows a thermally conductive particle with two types of graphite in the composite core, E6, formed in the same manner as in E2, except that 20 parts by volume of the 100 parts by volume natural were substituted with expanded graphite. The volume resistivity of E6 was 5.5x1ο⁵ Ω· cm, an improvement of almost five times over that of E2.

Table 3: Effect of Varying Amount of Insulating Material

IM: Talc (parts by volume) 0 4.8 9.6 1 29

Preparation Method A A A A A

Volume Resistivity (n* cm) 20 2.2X10⁵ 6.9x1ο⁷ >i.7xio⁹ >i.7xio⁹

The examples in Table 3 were formed in the same manner as the examples in Table 2, except for varying the amount of insulating material. Each of E7 through E10 had differing amounts of insulating material, that is, talc. C4 lacked talc. The volume resistivity of C4 and of E7 through E10 was measured as for Examples 2 through 6.

Table 3 shows that: the volume resistivity of E7 through E10 increases as the amount of talc increase. The volume resistivity of E10, containing double the amount of talc as in Eg, was the same as that of Eg. These results show that the relatively more effective amount of insulating material ranges from about 10 to about 20 parts by volume of the total volume of the thermally conductive particles.

Table : Effect of Varying Particle Size of Talc Used as Insulating Material

The thermally conductive particles in Table 4 were formed in the same manner as for Examples 2 through 10, except that the average particle size of the talc insulating material was varied. E11 had talc particles of about the same average size as in Eg. E11, E12, and E13 had talc of average particle size of 650 nm, 2.4 μιτη, and 5.05.0 μηη, respectively. Thus, the average talc particle size for E12 was about 4 times greaterthan that for E11 (Eg). The average talc particular size for E13 was about twice that for E12 and about 8 times larger than that for E11 (E8). The volume resistivity of E11 (Eg) and of E12, regardless of the 4 fold difference in average insulating particle size, was substantially similar. However, the decrease in volume resistivity of E13 relative to that of E12 implies that, when talc is the insulating material, optimum results are achieved when the average talc particle size for ranges between 600 nm and about 3.0 μηη. Nonetheless, in all three examples, E11 through E13, the volume resistivity of the thermally conductive particles was greater than ι.ο^χιο⁸ Ω - cm, a 10,000 fold increase over the minimum recited value. Table 5: Effect of Varying Insulating Material

In Table 5, C5, C6, and C7, having as their insulating material titanium dioxide, boron nitride, and calcium fluoride, respectively, did not achieve thermally conductive particles with the recited volume resistivity. Calcium fluoride has a Mohs hardness of 4.0; boron nitride has a particle size of about 12 μιη; and the titanium oxide in C5 had a spherical shape with an aspect ratio of about 1.

By contrast, Table 5 shows that E14, E15, and E16, having as their insulating material sericite, mica, and boehmite, respectively, achieved thermally conductive particles, each having a volume resistivity of at least the recited value of ι.οχιο⁴. Boehmite, sericite, and mica all have Mohs hardness values below 3.5 and particle sizes of about 5 μιη or less.

Table 6: Effect of Varying Organic Binder

Solvent: Water (ml) 10 10 10 10 10

IM:Talc g(vol%) 25 (21) 25(21) 25 (21) 25 (21) 25 (19)

Preparation Method A A A A A

Rotating speed (rpm) 3000 3000 3000 3000 3000

1^st Compression Shear Mixing

5imin 5 min 5imin 5 min Duration 5 min

2^nd Compression Shear Mixing

1 min 30 sec 30 sec 30 sec Duration 30 sec

Curing temperature (°C) 120 120 120 120 120

Heat treatment duration (hr) 3 3 3 3 3

Volume Resistivity (Ω^■ cm) 8.OX10⁰ 2.5x1ο⁹ 5.0x1ο⁶ 2.OX10⁹ 3.5x1ο⁴

Table 6 shows the volume resistivity of thermally conductive particles having different organic binders. Each organic binder used in Table 6 produced thermally conductive particles that exhibited a volume resistivity of at least the recited value, ixio⁴ Ω-cm at 500V applied voltage. Using methylol melamine or aqueous polyesters as the organic binder gave thermally conductive particles with the best volume resistivities.

Table 7: Thermal Conductivity; Volume Resistivity of Article Described Herein

Table 7 shows a resin composition in which thermally conductive particles had been dispersed in a resin and the resin composition injection molded to form an article.

A resin composition as described herein was prepared as follows:

67 parts by volume of polybutylene terephthalate resin and 7 parts by volume of polyester elastomer (of the total volume of the resin composition) were mixed for 2 minutes at 2go°C using an MC15 micromixer, manufactured by DSM Xplore. 26 parts by volume of thermally conductive particles from E10 (of the total volume of the resin composition) were added and stirred for another 30 seconds at 2go°C to produce a resin composition.

The resin composition was injection molded at a i25°C mold temperature to produce a 21 mm long, 16 mm wide, and 1 mm thick molded article (FIG. 3). Volume resistivity at the surface of the molded article of at least 1.0x1ο¹⁴ Ω - cm was measured with an ohmmeter (Hiresta-UP, manufactured by Mitsubishi Chemical Corp.). In addition, thermal diffusivity in the injection direction of the molded article was measured by the laser flash method in a 15 mmxi5 mm area using a Bruker AXS model LFA 447 NanoFlash^® and found to be 1.4 W/mK, when thermal diffusivity was calculated by the specific heat and density of the molded article.

When a cross section of this molded article was observed with an electron microscope, thermally conductive particles containing a graphite-containing composite 41 and insulative layer 42 coating the surface thereof were observed as shown in FIG. 4. In addition, the thermally conductive particles were aligned nearly parallel in resin 43.

Claims

CLAIMS What is claimed is:

1. A thermally conductive particle comprising:

a composite core ; and,

an insulating layer, wherein:

the core particles are thermally conductive and selected from the group consisting of metal particles, ceram ic particles, carbon-based particles, and mixtures of these;

the insulating material coats at least part of the composite core, and

the thermally conductive particle exhibits a volume resistivity, when measured on a cylinder of said thermally conductive particles having a 10 mm diameter and a height of

3.0 mm at 500V applied voltage, which ranges from at least ixio⁴ Ω - cm to 1 x 10¹⁰ Ω - cm.

2. The thermally conductive particle of claim 1, wherein the core particles are carbon-based particles.

3. The thermally conductive particle of claim 1, wherein the carbon-based particles are graphite.

4. The thermally conductive particle of claim 1 or 2, wherein:

the core particles are naturally occurring graphite and flaky carbon-based particles in a ratio of 3:2 to 99:1, and

the flaky carbon-based particles have an average thickness less than that of the naturally occurring graphite.

5. The thermally conductive particle of claim 1 or 2, wherein:

the core particles are 100 parts by volume,

the organic binder ranges from 3 to 25 parts by volume of the volume of the core particles, and the insulating material ranges from 4 to 48 parts by volume of the core particles.

6. The thermally conductive particle according to claim 5, wherein:

the core particles are naturally occurring graphite and flaky carbon-based particles in a ratio of 3:2 to 99:1, and the flaky carbon-based particles have an average thickness less than that of the naturally occurring graphite.

7. The thermally conductive particle according to claim 1 or 2, wherein:

the organic binder is a thermosetting resin.

8. The thermally conductive particle of claim 6, wherein:

the organic binder is a thermosetting resin.

9. The thermally conductive particle according to claim 1 or 2, wherein:

average particle size of the thermally conductive particle ranges from 0.5 μιη to 300 μιη.

10. The thermally conductive particle of claim 8, wherein:

11. The thermally conductive particle of claim 1 or 2, wherein:

the insulating material is selected from the group consisting of sericite, boehmite, talc, mica, and mixtures of these.

12. The thermally conductive particle of claim 10, wherein: the insulating material is selected from the group consisting of sericite, boehmite, talc, mica, and mixtures of these.

13. A method of making thermally conductive particles, comprising:

coating at least partially a composite core with insulating material,

wherein:

the core particles are thermally conductive and selected from the group consisting of metal particles, ceram ic particles, carbon-based particles, and mixtures of these, the insulating material is selected from the group consisting of sericite, boehmite, talc, mica, and mixtures of these, and

14. The method of claim 13, wherein:

the composite core was formed by applying compression force and shear force, causing the core particles to mix with the organic binder.

15. A resin composition comprising:

thermally conductive particles and

a resin selected from the group consisting of: wherein:

the thermally conductive particles comprise a composite core and an insulating layer, the composite core comprises a plurality of core particles and an organic binder binding the core particles together,

the insulating material coats at least part of the composite core,

the thermally conductive particles exhibit a volume resistivity, when measured on a cylinder of said thermally conductive particles having a 10 mm diameter and a height of

3.0 mm at 500V applied voltage, that ranges from at least ixio⁴ Ω - cm to 1 x 10¹⁰ Ω - cm; and the thermally conductive particles range from 10 to 70% by volume of the total volume of the resin composition.