WO2018093987A1

WO2018093987A1 - Method for the manufacture of thermally conductive composite materials and articles comprising the same

Info

Publication number: WO2018093987A1
Application number: PCT/US2017/061926
Authority: WO
Inventors: Li Zhang; Murali Sethumadhavan; Lei Liu; Jing Jiang; Yang Zhong
Original assignee: Rogers Corporation
Priority date: 2016-11-16
Filing date: 2017-11-16
Publication date: 2018-05-24

Abstract

A process for making a thermal interface composite material includes combining a plurality of sol-gel coated boron nitride particles comprising boron nitride particles comprising an outer layer of inorganic silica, with a precursor polymer matrix composition, to form a mixture; forming the mixture into a shape; and hardening the shaped mixture to obtain the thermal interface composite material comprising the plurality of sol-gel coated boron nitride particles distributed in a polymer matrix.

Description

METHOD FOR THE MANUFACTURE OF THERMALLY CONDUCTIVE COMPOSITE

MATERIALS AND ARTICLES COMPRISING THE SAME

BACKGROUND

[0001] Disclosed herein are thermally conductive composite materials, which are useful for providing heat management, in association with electronic devices, or in providing a substrate for circuit materials.

[0002] Circuit designs for electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly smaller and thinner. Smaller electronic components are being more densely packed into smaller spaces and, moreover, the increasing power of such electronic devices has resulted in more intense heat generation. Consequently, manufacturers are continuously facing the challenge of dissipating heat in electronic devices.

[0003] In particular, temperature-sensitive elements in an electronic device need to be maintained within a prescribed operating temperature in order to avoid significant performance degradation or even system failure. A thermal interface layer or "pad" can be used in electronic equipment, where the pad is positioned between a first heat transfer surface and a second heat transfer surface to provide a thermal pathway therebetween. The first heat transfer surface can be a component designed to absorb heat, such as a heat sink or an electronic circuit board, and the second (opposed) heat transfer surface can be a heat generating source, for example, a heat generating electronic such as an LED (light emitting diode) or power semiconductor.

[0004] New designs have been developed for thermal management in electronic devices, including thermal interface pads that are both compressible and contain thermally conductive particles. For example, US6591897 discloses a heat sink for electronic devices comprising a foam block surrounding heat-conducting columnar pins mounted on a spreader plate. US 2012/ 0048528 discloses a compressible, thermally conductive foam pad filled with ceramic filler, for example alumina (AI2O3) or boron nitride (BN) particles in an amount from 20 to 80% of the total weight of the foam pad. The foam pad can further include various elastomeric materials, including silicone or polyurethane. As load on a compressible thermal interface pad increases, the void volume decreases, resulting in increasing thermal conductivity with increasing load. The foam pad can have a thermal conductivity of 0.5 W/m-K or more.

[0005] Another approach to improving thermal conductivity in thermal interfaces has been with respect to the thermally conductive fillers used to increase heat transfer through such materials. A problem with the use of thermally conductive particles has been the fact that they tend to be highly inert and one-dimensional in properties, which can lead to poor cohesive strength of the boron-nitride-filled composite material. US7524560 discloses BN particles that are reacted with a silane, siloxane, or a carboxylic derivative, followed by calcining of the particles. In some embodiments, BN particles are coated with smaller nanoparticles of colloidal silica as a first coating in order to increase reactive sites, followed by surface functionalization with a silane compound and then optional sintering. The particles showed improved or lower viscosity and improved thermal performance.

[0006] Other prior art related to silicon-containing coatings on BN particles include US7527859, which discloses BN coated with an organosilicon compound following by calcining. JP2013136658 discloses spherical agglomerates of BN particles made with an inorganic binder such as alumina or silica (silicon dioxide, or Si0₂.) US2006/0121068 discloses spherical agglomerates of BN to improve viscosity and thermal conductivity, wherein the binder includes polymers or colloidal silica. Spray drying and treatment in a fluidized bed are also mentioned, wherein BN is coated with a layer of aluminum oxide by contact with AlCb vapors. The coated particles can be heat treated at very high temperatures to facilitate crystal growth. US2001/0021740 also discloses BN particles that are agglomerated together with an organic binder. The agglomerated particles are made by spray drying a slurry.

[0007] US2010/0110608 to Wei et al. discloses core-shell particles having a conductive core, including silicon nitride. The particles are made by coating a core particle with a dielectric material using sol-gel technology, followed by annealing or sintering at high temperature. This reference, however, does not relate to improved thermal conductivity but rather to multilayer capacitors. US2010/0213131, although relating to chromatography, discloses a core particle such as BN coated with smaller shell particles such as silicon carbide or alumina using a polymer coating to adhere the shell particles.

[0008] Nonetheless, there remains a need in the art for methods for the manufacture of thermal management management materials for electronic devices, or in circuit materials, wherein improved heat transfer efficiency increases heat dissipation from heat generating elements.

SUMMARY

[0009] A process for making a thermal interface composite material includes combining a plurality of sol-gel coated boron nitride particles comprising boron nitride particles comprising an outer layer of inorganic silica, with a precursor polymer matrix composition, to form a mixture; forming the mixture into a shape; and hardening the shaped mixture to obtain the thermal interface composite material comprising the plurality of sol-gel coated boron nitride particles distributed in a polymer matrix. [0010] In another aspect, a thermal interface comprises a first and a second heat transfer surface, and further comprising the above-described thermal interface composite material, wherein a bulk thermal conductivity of the thermal interface is at least 0.5 W/m K.

[0011] A thermal management assembly comprises the above described thermal interface disposed between a first adjacent external surface of a heat-generating member and second adjacent external surface of a heat-dissipative member to provide a thermally conductive pathway therebetween.

[0012] A dielectric substrate comprises the above-described thermally conductive composite material, wherein the dielectric substrate has a bulk thermal conductivity of at least 0.5 W/m K, a UL-94 rating of at least V-1, and a dissipation factor (Df) of less than 0.006 at 10 GHz.

[0013] The above-described and other features are exemplified by the following figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Referring now to the exemplary drawings wherein like elements are numbered alike in the figure:

[0015] FIG. 1 is a schematic of a single clad laminate;

[0016] FIG. 2 is a schematic of a double clad laminate;

[0017] FIG. 3 is a schematic of a double clad laminate with patterned conductive layer;

[0018] FIG. 4 is a scanning electron microscopy image of an h-BN powder of Example 3 before the silica coating;

[0019] FIG. 5 is a scanning electron microscopy image of an h-BN powder of Example 3 before the silica coating;

[0020] FIG. 6 is a scanning electron microscopy image of a silica-coated h-BN powder of Example 3;

[0021] FIG. 7 is a scanning electron microscopy image of a silica-coated h-BN powder of Example 3;

[0022] FIG. 8 is a Fourier transform infrared spectra of h-BN powder of Example 3before coating with silica; and

[0023] FIG. 9 is a Fourier transform infrared spectra of the silica-coated h-BN of Example 3. DETAILED DESCRIPTION

[0024] The inventors have discovered that use of a sol-gel method for fully or partially coating BN particles with silica improves the properties of the particles in a thermal composite material. Benefits include improved mechanical strength and improved peel strength of the composite material when used in a circuit material. Another benefit is improved thermal efficiency of the composite material. In particular, sol-gel deposition of a silica coating has been found to modify the surface of the particles to make them more compatible with the polymers in the polymeric matrix, thereby providing higher cohesive strength and improved peel strength to copper or other metal layers in a thermal management assembly or a circuit subassembly.

[0025] A plurality of the sol-gel coated BN particles are present as a filler dispersed in the polymer matrix, and provide a thermally conductive composite that is useful as a thermal interface. The thermal interface can have first and second heat transfer surfaces and can obtain a bulk thermal conductivity of at least 0.5 Watts per meter-Kelvin (W/m K), preferably up to 200 W/m K, more preferably 0.5 to 10 W/m K. The thermal conductivity can be measured using various test standards, for example, ASTM C518-10, ASTM E1225-13, ASTM E1530-11(2016), and ASTM D5470-12. The sol-gel coated BN particles can increase the thermal conductivity of the composite material without adversely affecting its peel strength unduly, which may otherwise occur due to incompatibility between BN and polymers in the composite material.

[0026] The sol-gel coating of the BN particles, either partially or fully coating each particle, can be achieved through a sol-gel reaction to form a silica outer layer on each core particle of BN. A micro-fluidizing process can be efficiently utilized to dry the coated BN particles during manufacture.

[0027] Boron nitride particles for making the coated BN can vary as to the kind of crystalline type and size, and the distribution of the foregoing. Regarding crystalline type, BN particles can comprise a structure that is hexagonal, cubic, wurtzite, rhombohedral, or any other synthetic structure. Among the various structures, BN particles of hexagonal structure (hBN) can obtain superior heat conductivity of 10 to 300 W/m K or more and particles of cubic structure can obtain an extremely high heat conductivity of 1300 W/m K maximum, although hBN particles may be more readily obtained from a variety of commercial sources. Hexagonal BN (hBN) has a layered structure, analogous to graphite, in which the layers are stacked in registration such that the hexagonal rings in layers coincide, according to Edgar, Properties of Group III Nitrides, Chapter 1, p. 8 (Feb. 1994). The positions of N and B atoms alternate from layer to layer.

[0028] Boron nitride particles, crystalline or partially crystalline, can be made by processes known in the art. These include, for example, BN particles produced from the pressing process disclosed in US5898009 and US6048511; the BN agglomerated particles disclosed in US2005/0041373; and the highly delaminated BN particles disclosed in US6951583. A variety of BN particles are commercially available, for example from Momentive under the tradename PolarTherma™ BN.

[0029] The BN particles can comprise either single particles (primary particles) or aggregates (secondary particles) containing a plurality of particles. In some embodiments, the BN particles (either primary or aggregates of particles) have an average particle size of 0.1 to 1000 micrometers (μιη), preferably a particle size of 5 to 500 μιτι, more preferably an average particle size of 10 to 250 μιτι, most preferably an average particle size of 25 to 150 μιη. In some embodiments, the BN particles comprise irregularly shaped hBN platelets, having an average size above 10 μιη. In some embodiments, BN particles are a blend of different BN types, e.g., 10 to 40 volume percent (vol%) of BN particles having an average particle size of 5 to 50 μιη and 60 to 90 vol% of BN particles having an average particle size of 75 to 100 μιη. "Particle size" as used herein refers to the mean diameter or equivalent diameter as best determined by standard laser particle measurement. Particle size D50 is known as the median diameter or the median value of the particle size distribution; it is the value of the particle diameter at 50% in the cumulative distribution.

[0030] In some embodiments, the BN particles are in the form of platelets having an average aspect ratio (the ratio of width to length of a particle) of 4:5 to 1 :300, preferably 1 :2 to 1 :300, more preferably 1 :2 to 1 :200, and in some embodiments 3 :5 to 1 : 100. The BN platelets can have a hexagonal structure with a crystallization index of at least 0.12, preferably 0.20 to 0.55, and more preferably 0.30 to 0.55. In an embodiment, the platelets are substantially single particles, rather than aggregates.

[0031] The exact shape of the BN platelets is not critical. In this regard, the BN particles can have irregular shapes, although the term "platelets" as used herein is generally descriptive of any thin, flattened particles, inclusive of flakes.

[0032] In some embodiments, the coated BN particles are prepared by immersing the particles in a sol-gel precursor solution, catalyzing a sol-gel reaction to coat silica onto the surface of the particles. The sol-gel reaction can involve the reaction of a silica precursor compound such as alkyl orthosilicates and combinations thereof. The alkyl can be a Ci-C₈ alkyl group, preferably C1-C4 alkyl group, for example methyl or ethyl. In some embodiments, the coated BN particles have been made by immersing BN platelets in a solution comprising ethyl orthosilicate dissolved in aqueous alcohol, in the presence of a catalyst. The catalyst can be an acid or a base. In some embodiments a strong acid such as H2SO4, HQ, or H3PO4 is used. [0033] After coating, the coated BN particles can be subjected to an elevated temperature to dry and harden the silica coating. Hardening can be by introducing the surface-coated particles into a spray tower or fluidized bed to harden the silica coating on each particle.

Optionally, the coated BN particles can be sintered. In some embodiments the sol-gel coated boron nitride particles are subjected to a maximum temperature of 1600°C during manufacture.

[0034] It is not necessary for all of the boron particles to be coated, or for the coating to completely cover each particle. Particles that are at least substantially coated can therefore be used. For example, in a given batch of particles, at least 60% of the total surface area of the particles, preferably at least 70%, more preferably at least 80%, and even more preferably at least 90% of the total surface area of the particles can be coated. Coating thickness can vary widely. In some embodiments, the thickness of the coating is 0.004-0.2 mils (0.1-5

micrometers), preferably 0.02-0.1 mils (0.53-3 micrometers). In some embodiments, sufficient coating material is used to increase the equivalent diameter of the uncoated BN core filler particles, on average, by at least 1% percent, preferably 3-40%.

[0035] In some cases the sol-gel coated boron particles can be in the form of

agglomerates. For example, in some embodiments substantially separate platelets are coated, and the coated particles are produce as agglomerates. In other embodiments, aggregated platelets are coated as such to provide agglomerates. In some embodiments it is desirable to limit or avoid agglomerating individual particles of coated BN particles. Hence, in some embodiments, the median size of the coated BN particles, dispersed in the composite material, is less than five times, preferably less than four times, more preferably less than three times, most preferably less than two times, the median size of the individual (primary) BN particles. Agglomerated particles can be minimized during coating by use of primary (unaggregated) particles, or by vigorous mixing during coating. Alternatively, agglomerates produced after coating can be reduced by processes such as ball milling or hammer milling.

[0036] A plurality of the coated BN particles are incorporated into a polymer matrix to provide the thermal composite material. In an embodiment the coated BN particles are uniformly dispersed in a polymeric matrix to provide homogeneous mechanical and thermal properties.

The composite materials can comprises the plurality of coated BN particles in an amount or proportion by weight sufficient to provide the degree of thermal conductivity desired for an intended application. Generally the loading can be in an amount of from 1-85 weight percent

(wt%) of the coated BN particles, preferably 25-80 wt%, more preferably 30-70 wt%, most preferably 40-60 wt% of coated BN particles (inclusive of the weight of the sol-gel coating on the particles when mixed with the polymeric composition forming the matrix of the dispersed particles), each based on the total weight of the thermal interface composite material. [0037] In any of the embodiments herein, the sol-gel coated BN particles provide the primary means of thermal conductivity in the composite material, but optionally other thermally conductive particles can be present. Other thermally conductive particulate fillers can include metal and non-metal oxides, other nitrides, carbides, borides, and graphite particles, and mixtures thereof, and more particularly titanium diboride, aluminum nitride, silicon carbide, graphite, metal oxides such as aluminum oxide, magnesium oxide, zinc oxide, beryllium oxide, antimony oxide, and mixtures thereof. Such optional additional fillers characteristically exhibit a thermal conductivity of at least 20 W/m-K. For example, some proportion of aluminum oxide, i.e., alumina, can be used for reasons of economy, whereas for reasons of improved thermal conductivity alone, the BN particles provide superior thermal conductivity.

[0038] In addition to the optional additional thermally conductive fillers, still other optional fillers can be added to the formulation for the composite material. For example, non- thermally conductive fillers (alumina trihydrate, silica, talc, calcium carbonate, clay, and so forth), pigments (for example titanium dioxide and iron oxide), and so forth, as well as combinations comprising at least one of the foregoing additives, can also be used. Reinforcing fillers such as woven or non-woven webs, silica, glass particles, and the like can be used, particular when the composite material is intended for use as a circuit dielectric substrate.

[0039] In addition to the above-mentioned fillers, various additives can be included in the formula for the composite material. Such additives can include flame retardants. Exemplary flame retardant materials are magnesium hydroxides, nanoclays, and brominated compounds. In some embodiments, flame retardance of the composite material meets certain Underwriter's Laboratories (UL) standards for flame retardance. For example, the composite material can have a rating of V-0 under UL Standard 94. Still other additives that can be present in the composite material include dyes, antioxidants, ultraviolet (UV) stabilizers, catalysts for cure of the polymer, crosslinking agents, and the like, as well as combinations comprising at least one of the foregoing additives.

[0040] In general, the process for making a thermal interface composite material comprises combining a plurality of the sol-gel coated boron nitride particles with a precursor polymer matrix composition to form a mixture; forming the mixture into a shape; and hardening the shaped mixture to obtain the thermal interface. The details of the process depend on whether the precursor polymer matrix composition is a thermosetting composition or a thermoplastic composition. When the precursor polymer matrix composition comprises a prepolymer or thermosetting resin, the hardening can be by curing or crosslinking. When the precursor polymer matrix composition comprises a thermoplastic polymer, the hardening can be by cooling or removing solvent from the precursor polymer matrix composition. The details of the process further depend on whether the thermal interface composite material is a foam or a solid, as described in further detail below.

[0041] In some embodiments of the process for producing a foam thermal interface composite material, a precursor polymer matrix composition used to form the foam is combined with the sol-gel coated BN particles (and any other optional additives) and then formed into a shape, e.g., a layer, having a first heat transfer side and an opposite second heat transfer side. Depending on the polymer, foaming can be performed prior to casting, during casting, or after casting. The polymer foam layer can be formed by casting the foam or foamable composition onto a carrier. Thus, a first (bottom) carrier can be provided, and a layer having a first heat transfer surface and an opposite second heat transfer surface can be formed on the carrier, wherein the first heat transfer surface of the foam layer can be disposed on the first carrier. A second (top) carrier can be disposed onto the second heat transfer surface of the layer. The first carrier, the second carrier, or both can be a removable layer, or can be provided with a removable layer, such that the removable layer is in contact with the first heat transfer surface of the cast layer, the second heat transfer surface of the cast layer, or both. The first or second layer can be a release layer. In some embodiments, after hardening (cure or solidification of the polymer foam layer), removal of the removable layer exposes the BN particles at the surface of the foam layer in contact with the removable layer. In another embodiment, removing the removable layer is configured to also remove a portion (e.g., thin layer) of foam from the foam layer, thereby exposing more of the conductive particles on the surface of the foam layer than would be exposed when the removable layer is removed without also removing a portion of the foam layer.

[0042] A foam material can have the benefits of retaining compressibility and compression-set properties of the foam substrate while significantly improving thermal conductivity by means of compression. Being compressible, a foam thermal interface pad can also readily conform to first and second heat transfer surfaces, whether these surfaces are regular or irregular in shape. Thus, the surface of the compressible thermally conductive sheet can be generally planar, multi-planar, curved, or complex curved, indented, etc. As the foam is compressed, the thermal conductivity increases, thereby enhancing the heat transfer from an electronic component or other heat-generating element to a heat sink or the like.

[0043] As used herein, "foams" refers to materials having a cellular structure. Suitable foams can have densities lower than 65 pounds per cubic foot (pcf), preferably less than 55 pcf

(881 kg/m³), more preferably less than 25 pcf (400 kg/m³). The density can be determined in accordance with ASTM D 3574-95, Test A. Alternatively, the foam can have a void volume content of at least 20 to 99%, preferably 30 to 85%, based upon the total volume of the foam. In some embodiments, the foam has a density of 5 to 30 pounds per cubic foot (lb/ft³) (80 to 481 kg/m³), a 25% compression force deflection (CFD) 0.5 to 20 lb/in² (0.3 to 1.41 kg/m²), and a compression set at 70°F (21°C) of less than 10%, preferably less than 5%. The compression force deflection can be determined in accordance with ASTM D1056-14.

[0044] Polymers for use in the foams can be selected from a wide variety of

thermoplastic or thermoset polymers. Blends comprising different polymers can be used.

Examples of thermoplastic polymers include ethylene propylene rubbers (EPR), polyacetals, polyacrylics, polyamides (such as Nylon 6, Nylon 6,6, Nylon 6, 10, Nylon 6, 12, Nylon 1 1 or Nylon 12), polyamideimides, polyarylates, polyarylsulfones, polycarbonates, polyesters (such as polyethylene terephthalates, polybutylene terephthalates), polyether etherketones,

polyetherketones, polyether ketone ketones, polyetherimides, polyethersulfones, polyimides, poly(meth)acrylates, polyolefins (which includes polyethylenes, polyethylene-propylene, polytetrafluoroethylenes, fluorinated polyethylene-propylenes, polychlorotrifluoroethylenes), polyphenylene sulfides, polystyrenes (which includes styrene-acrylonitrile, acrylonitrile- butadiene-styrene), polysulfones, polyurethanes, polyvinyl chlorides, polyvinylidene fluorides, polyvinyl fluorides, and the like, or a combination comprising at least one of the foregoing polymers.

[0045] Examples of blends of polymers that can be used in polymer foams include acrylonitrile-butadiene-styrene/ nylon, polycarbonate/ acrylonitrile-butadiene-styrene,

acrylonitrile butadiene styrene/polyvinyl chloride, polyphenylene ether/polystyrene,

polyphenylene ether/nylon, polysulfone/acrylonitrile-butadiene-styrene,

polycarbonate/thermoplastic urethane, polycarbonate/polyethylene terephthalate,

polycarbonate/polybutylene terephthalate, thermoplastic elastomer alloys, polyethylene terephthalate/polybutylene terephthalate, styrene-maleic anhydride/acrylonitrile-butadiene- styrene, polyether etherketone/polyethersulfone, styrene-butadiene rubber, polyethylene/nylon, polyethylene/polyacetal, ethylene propylene rubber (EPR), and the like, or a combination comprising at least one of the foregoing blends.

[0046] Examples of thermoset polymers that can be used in polymers foam include polyurethanes, epoxies, phenolics, reactive polyesters, polyamides, silicones, and the like, or a combination comprising at least one of the foregoing.

[0047] In some embodiments, the composite material is a polyurethane foam, such as an open cell, low modulus polyurethane foam, which can have an average cell size of 50 to 250 μπι, as may be measured, for example, in accordance with ASTM D 3574-95; a density of 5 to 30 lbs/ft³, preferably 6 to 25 lbs/ft³, a compression set of less than 10%, and a force-deflection of between 1 to 9 psi (7 to 63 kPa). Such materials are marketed under the name PORON4700 by Rogers Corporation. PORON foams have been formulated to provide an excellent range of properties, including compression set resistance. Foams with good compression set resistance provide cushioning, and maintain their original shape or thickness under loads for extended periods.

[0048] A polyurethane foam can be manufactured from a reactive composition. The reactive compositions can comprise an organic isocyanate component reactive with an active hydrogen-containing component, optionally a surfactant, and a catalyst. The organic isocyanate components generally comprise polyisocyanates having the general formula Q(NCO)i, wherein "i" is an integer having an average value of greater than two, and Q is an organic radical having a valence of "i". Q can be a substituted or unsubstituted hydrocarbon group (e.g., an alkane or an aromatic group of the appropriate valency). Q can be a group having the formula Q^Z-Q¹ wherein Q¹ is an alkylene or arylene group and Z is -0-, -O-Q^S, -CO-, -S-, -S-Q^-S-, -SO- or - SO2-. Exemplary isocyanates include hexamethylene diisocyanate, 1,8-diisocyanato-p-methane, xylyl diisocyanate, diisocyanatocyclohexane, phenylene diisocyanates, tolylene diisocyanates, including 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, and crude tolylene diisocyanate, bis(4-isocyanatophenyl)methane, chlorophenylene diisocyanates, diphenylmethane-4,4'- diisocyanate (also known as 4,4'-diphenyl methane diisocyanate, or MDI) and adducts thereof, naphthalene- 1,5 -diisocyanate, triphenylmethane-4,4',4"-triisocyanate, isopropylbenzene-alpha-4- diisocyanate, polymeric isocyanates such as polymethylene polyp henylisocyanate, and combinations comprising at least one of the foregoing isocyanates.

[0049] Q can also represent a polyurethane radical having a valence of "i", in which case Q(NCO)i is a composition known as a prepolymer. Such prepolymers are formed by reacting a stoichiometric excess of a polyisocyanate as set forth hereinbefore and hereinafter with an active hydrogen-containing component as set forth hereinafter, especially the polyhydroxyl-containing materials or polyols described below. Usually, for example, the polyisocyanate is employed in proportions of 30 to 200% stoichiometric excess, the stoichiometry being based upon

equivalents of isocyanate group per equivalent of hydroxyl in the polyol. The amount of polyisocyanate employed will vary slightly depending upon the nature of the polyurethane being prepared.

[0050] The active hydrogen-containing component can comprise polyether polyols and polyester polyols. The polyether polyols are obtained by the chemical addition of alkylene oxides (such as ethylene oxide, propylene oxide, and so forth, as well as combinations comprising at least one of the foregoing), to water or polyhydric organic components (such as ethylene glycol, propylene glycol, trimethylene glycol, 1,2-butylene glycol, 1,3-butanediol, 1,4- butanediol, 1,5-pentanediol, 1,2-hexylene glycol, 1, 10-decanediol, 1,2-cyclohexanediol, 2- butene-l,4-diol, 3-cyclohexene-l, l-dimethanol, 4-methyl-3-cyclohexene-l,l-dimethanol, 3- methylene- 1,5 -pentanediol, diethylene glycol, (2-hydroxy ethoxy)- 1 -propanol, 4-(2- hydroxyethoxy)- 1 -butanol, 5-(2-hydroxypropoxy)- 1 -pentanol, 1 -(2-hydroxymethoxy)-2- hexanol, l-(2-hydroxypropoxy)-2-octanol, 3 -allyloxy- 1,5 -pentanediol, 2-allyloxymethyl-2- methyl- 1,3 -propanediol, [4,4 - pentyloxy)-methyl]-l,3-propanediol, 3-(o-propenylphenoxy)-l,2- propanediol, 2,2'-diisopropylidenebis(p-phenyleneoxy)diethanol, glycerol, 1,2,6-hexanetriol, 1, 1, 1-trimethylolethane, 1, 1,1-trimethylolpropane, 3 -(2-hydroxy ethoxy)- 1,2-propanediol, 3-(2- hydroxypropoxy)-l,2-propanediol, 2,4-dimethyl-2-(2-hydroxyethoxy)-methylpentanediol-l,5; l, l, l-tris[2-hydroxy ethoxy) methyl]-ethane, l,l, l-tris[2-hydroxypropoxy)-methyl] propane, diethylene glycol, dipropylene glycol, pentaerythritol, sorbitol, sucrose, lactose, alpha- methylglucoside, alpha-hydroxyalkylglucoside, novolac resins, phosphoric acid,

benzenephosphoric acid, polyphosphoric acids such as tripolyphosphoric acid and

tetrapolyphosphoric acid, ternary condensation products, and so forth, as well as combinations comprising at least one of the foregoing). The alkylene oxides employed in producing polyoxyalkylene polyols normally have 2 to 4 carbon atoms. Propylene oxide and mixtures of propylene oxide with ethylene oxide are preferred. The polyols listed above can be used per se as the active hydrogen component. A useful class of polyether polyols is of the formula:

R[(OCH_nH2n)zOH]a wherein R is hydrogen or a polyvalent hydrocarbon radical; "a" is an integer equal to the valence of R, "n" in each occurrence is an integer of 2 to 4 inclusive (preferably 3), and "z" in each occurrence is an integer having a value of 2 to 200, or, more preferably, 15 to 100. Desirably, the polyether polyol comprises a mixture of one or more of dipropylene glycol, 1,4-butanediol, and 2-methyl- 1,3 -propanediol, and so forth. The polyether polyols are obtained by the chemical addition of alkylene oxides (such as ethylene oxide, propylene oxide, and so forth, as well as combinations comprising at least one of the foregoing), to water or polyhydric organic components (such as ethylene glycol, propylene glycol, trimethylene glycol, 1,2- butylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5 -pentanediol, 1,2-hexylene glycol, 1, 10- decanediol, 1,2-cyclohexanediol, 2-butene-l,4-diol, 3-cyclohexene-l,l-dimethanol, 4-methyl-3- cyclohexene-l,l-dimethanol, 3 -methylene- 1,5 -pentanediol, diethylene glycol, (2- hydroxy ethoxy)- 1 -propanol, 4-(2-hydroxy ethoxy)- 1 -butanol, 5-(2-hydroxypropoxy)- 1 -pentanol, 1 -(2-hydroxymethoxy)-2-hexanol, 1 -(2-hydroxypropoxy)-2-octanol, 3 -allyloxy- 1 , 5 -pentanediol, 2-allyloxymethyl-2-methyl- 1,3 -propanediol, [4,4 - pentyloxy)-methyl]- 1,3 -propanediol, 3-(o- propenylphenoxy)- 1,2-propanediol, 2,2'-diisopropylidenebis(p-phenyleneoxy)diethanol, glycerol, 1,2,6-hexanetriol, 1, 1,1-trimethylolethane, 1, 1, 1-trimethylolpropane, 3-(2- hydroxy ethoxy)- 1 ,2-propanediol, 3 -(2-hydroxypropoxy)- 1 ,2-propanediol, 2,4-dimethyl-2-(2- hydroxyethoxy)-methylpentanediol-l,5; 1, 1, 1 -tris [2-hydroxy ethoxy) methyl] -ethane, 1,1, 1- tris[2-hydroxypropoxy)-methyl] propane, diethylene glycol, dipropylene glycol, pentaerythritol, sorbitol, sucrose, lactose, alpha-methylglucoside, alpha-hydroxyalkylglucoside, novolac resins, phosphoric acid, benzenephosphoric acid, polyphosphoric acids such as tripolyphosphoric acid and tetrapolyphosphoric acid, ternary condensation products, and so forth, as well as

combinations comprising at least one of the foregoing). The alkylene oxides employed in producing polyoxyalkylene polyols normally have 2 to 4 carbon atoms. Propylene oxide and mixtures of propylene oxide with ethylene oxide are preferred. The polyols listed above can be used per se as the active hydrogen component. A useful class of polyether polyols is represented generally by the formula: R[(OCH_nH2n)zOH]_a wherein R is hydrogen or a polyvalent

hydrocarbon radical; "a" is an integer equal to the valence of R, "n" in each occurrence is an integer of 2 to 4 inclusive (preferably 3), and "z" in each occurrence is an integer having a value of 2 to 200, or, more preferably, 15 to 100. Desirably, the polyether polyol comprises a mixture of one or more of dipropylene glycol, 1,4-butanediol, and 2-methyl-l,3-propanediol, and so forth.

[0051] Exemplary polyester polyols are inclusive of polycondensation products of polyols with dicarboxylic acids or ester-forming derivatives thereof (such as anhydrides, esters and halides), polylactone polyols obtainable by ring-opening polymerization of lactones in the presence of polyols, polycarbonate polyols obtainable by reaction of carbonate diesters with polyols, and castor oil polyols. Exemplary dicarboxylic acids and derivatives of dicarboxylic acids which are useful for producing polycondensation polyester polyols are aliphatic or cycloaliphatic dicarboxylic acids such as glutaric, adipic, sebacic, fumaric and maleic acids; dimeric acids; aromatic dicarboxylic acids such as phthalic, isophthalic and terephthalic acids; tribasic or higher functional polycarboxylic acids such as pyromellitic acid; as well as anhydrides and second alkyl esters, such as maleic anhydride, phthalic anhydride and dimethyl terephthalate.

[0052] Additional active hydrogen-containing components are the polymers of cyclic esters. The preparation of cyclic ester polymers from at least one cyclic ester monomer is well documented in the patent literature as exemplified by US3021309, US3021317; US3169945; and US2962524. Exemplary cyclic ester monomers include δ-valerolactone; ε-caprolactone; zeta-enantholactone; and the monoalkyl-valerolactones (e.g., the monomethyl-, monoethyl-, and monohexyl-valerolactones). In general the polyester polyol can comprise caprolactone based polyester polyols, aromatic polyester polyols, ethylene glycol adipate based polyols, and combinations comprising at least one of the foregoing polyester polyols, and especially polyester polyols made from ε-caprolactones, adipic acid, phthalic anhydride, terephthalic acid or dimethyl esters of terephthalic acid. [0053] Another type of active hydrogen-containing material can be obtained by polymerizing ethylenically unsaturated monomers in a polyol as described in US3383351.

Exemplary monomers for producing such compositions include acrylonitrile, vinyl chloride, styrene, butadiene, vinylidene chloride, and other ethylenically unsaturated monomers. The polymer polyol compositions can contain 1 to 70 wt%, or, more preferably, 5 to 50 wt%, and even more preferably, 10 to 40 wt% monomer polymerized in the polyol, each based on the total weight of the polyol. Such compositions are conveniently prepared by polymerizing the monomers in the selected polyol at a temperature of 40 to 150°C in the presence of a free radical polymerization catalyst such as peroxides, persulfates, percarbonate, perborates, azo compounds, and combinations comprising at least one of the foregoing.

[0054] The active hydrogen-containing component can also contain polyhydroxyl- containing compounds, such as hydroxyl-terminated polyhydrocarbons (US2877212); hydroxyl- terminated polyformals (US2870097); fatty acid triglycerides (US2833730); hydroxyl- terminated polyesters (US2698838, US2921915, US2591884, US2866762, US2850476, US2602783, US2729618, US2779689, US2811493, US2621166 and US3169945);

hydroxymethyl-terminated perfluoromethylenes (US2911390 and US2902473); hydroxyl- terminated polyalkylene ether glycols (US2808391; GB733624); hydroxyl-terminated polyalkylenearylene ether glycols (US2808391); and hydroxyl-terminated polyalkylene ether triols (US2866774). Other polyols are disclosed in JP Sho 53-8735.

[0055] The polyols can have hydroxyl numbers that vary over a wide range. In general, the hydroxyl numbers of the polyols, including other cross-linking additives, if used, can be 28 to 1,000, and higher, or, more preferably, 100 to 800. The hydroxyl number is defined as the number of milligrams of potassium hydroxide required for the complete neutralization of the hydrolysis product of the fully acetylated derivative prepared from 1 gram of polyol or mixtures of polyols with or without other cross-linking additives. The hydroxyl number can also be defined by the equation:

OH = 56.1 x 1000 x f

Mw

wherein: OH is the hydroxyl number of the polyol,

f is the average functionality, that is the average number of hydroxyl groups per molecule of polyol, and

Mw is the weight average molecular weight of the polyol based on polystyrene standards.

[0056] A number of the catalysts capable of catalyzing the reaction of the isocyanate component with the active hydrogen-containing component can be used in the foam preparation. Exemplary catalysts include phosphines; tertiary organic amines; organic and inorganic acid salts of, and organometallic derivatives of bismuth, lead, tin, iron, antimony, uranium, cadmium, cobalt, thorium, aluminum, mercury, zinc, nickel, cerium, molybdenum, vanadium, copper, manganese, and zirconium. Specific examples of such catalysts include dibutyltin dilaurate, dibutyltin diacetate, stannous octoate, lead octoate, cobalt naphthenate, triethyl amine, triethylenediamine, Ν,Ν,Ν',Ν'-tetramethylethylenediamine, 1, 1,3,3-tetramethylguanidine, Ν,Ν,Ν'Ν'-tetramethyl- 1 , 3 -butanediamine, Ν,Ν-dimethylethanolamine, N,N-di ethyl ethanolamine, 1,3,5-tris (N,N-dimethylaminopropyl)-s-hexahydrotriazine, o- and p-(dimethylaminomethyl) phenols, 2,4,6-tris(dimethylaminomethyl) phenol, N,N-dimethylcyclohexylamine,

pentamethyldiethylenetriamine, 1,4-diazobicyclo [2.2.2] octane, N-hydroxyl-alkyl quaternary ammonium carboxylates and tetramethylammonium formate, tetramethylammonium acetate, tetramethylammonium 2-ethylhexanoate, and so forth, as well as combinations comprising at least one of the foregoing catalysts.

[0057] Metal acetyl acetonates based on metals such as aluminum, barium, cadmium, calcium, cerium(III), chromium(III), cobalt(II), cobalt(III), copper(II), indium, iron(II), lanthanum, lead(II), manganese(II), manganese(III), neodymium, nickel(II), palladium(II), potassium, samarium, sodium, terbium, titanium, vanadium, yttrium, zinc and zirconium. A common catalyst is bis(2,4-pentanedionate) nickel (II) (also known as nickel acetylacetonate or diacetylacetonate nickel) and derivatives. Ferric acetylacetonate (FeAA) is particularly preferred, due to its relative stability, good catalytic activity, and lack of toxicity. Added to the metal acetyl acetonate can be acetyl acetone (2,4-pentanedione), as disclosed in commonly assigned US5733945. The amount of catalyst present in the reactive composition can be 0.03 to 3.0 wt%, based on the weight of the active hydrogen-containing component.

[0058] A wide variety of surfactants can be used for purposes of stabilizing a

polyurethane foam before it is cured, including mixtures of surfactants. Organosilicone surfactants are especially useful, such as a copolymer consisting essentially of S1O2 (silicate) units and

(trimethylsiloxy) units in a molar ratio of silicate to trimethylsiloxy units of 0.8: 1 to 2.2: 1, or, more preferably, 1 : 1 to 2.0: 1. Another organosilicone surfactant stabilizer is a partially cross-linked siloxane-polyoxyalkylene block copolymer and mixtures thereof wherein the siloxane blocks and polyoxyalkylene blocks are linked by silicon to carbon, or by silicon to oxygen to carbon, linkages. The siloxane blocks comprise hydrocarbon-siloxane groups and have an average of at least two valences of silicon per block combined in the linkages. At least some portion of a polyoxyalkylene block comprises oxyalkylene groups and is polyvalent, i.e., have at least two valences of carbon or carbon-bonded oxygen per block combined in said linkages. Any remaining polyoxyalkylene blocks comprise oxyalkylene groups and are monovalent, i.e., have only one valence of carbon or carbon-bonded oxygen per block combined in said linkages. Additional organopolysiloxane-polyoxyalkylene block copolymers include those described in US2834748, US2846458, US2868824, US2917480, and US3057901.

Combinations comprising at least one of the foregoing surfactants can be use. The amount of the surfactant can vary over wide limits, e.g., 0.5 to 10 wt%, preferably 1.0 to 6.0 wt%, based on the weight of the active hydrogen component.

[0059] The polyurethane foams can be manufactured from the reactive composition, which can be mixed prior to or concomitant with foaming. Foaming can be by mechanical frothing or blowing (using chemical or physical blowing agents, or both), or a combination of mechanical frothing and blowing (using chemical or physical blowing agents, or both). Chemical blowing agents include, for example, water, and chemical compounds that decompose with a high gas yield under specified conditions, for example within a narrow temperature range.

Desirably, the decomposition products do not effloresce or have a discoloring effect on the foam product. Exemplary chemical blowing agents include water, azoisobutyronitrile,

azodicarbonamide (i.e. azo-bis-formamide) and barium azodicarboxylate; substituted hydrazines (e.g., diphenylsulfone-3,3'-disulfohydrazide, 4,4'-hydroxy-bis-(benzenesulfohydrazide), trihydrazinotriazine, and aryl-bis-(sulfohydrazide)); semicarbazides (e.g., p-tolylene sulfonyl semicarbazide and 4,4'-hydroxy-bis-(benzenesulfonyl semicarbazide)); triazoles (e.g., 5- morpholyl-1,2,3,4- thiatriazole); N-nitroso compounds (e.g., Ν,Ν'- dinitrosopentamethylene tetramine and N,N-dimethyl-N,N'- dinitrosophthalmide); benzoxazines (e.g., isatoic anhydride); as well as combinations comprising at least one of the foregoing, such as, sodium

carbonate/citric acid mixtures. The amount of blowing agent can be 0.1 to 10 wt%, based upon a total weight of the reactive composition.

[0060] Exemplary physical blowing agents include the CFC's (chlorofluorocarbons) such as 1, 1-dichloro-l-fluoroethane, l, l-dichloro-2,2,2-trifluoro-ethane,

monochlorodifluoromethane, and l-chloro-l, l-difluoroethane; the FC's (fluorocarbons) such as

1, 1, 1,3,3,3-hexafluoropropane, 2,2,4,4-tetrafluorobutane, 1, 1, l,3,3,3-hexafluoro-2- methylpropane, 1, 1,1,3,3-pentafluoropropane, 1, 1,1,2,2-pentafluoropropane, 1, 1,1,2,3- pentafluoropropane, 1, 1,2,3, 3-pentafluoropropane, 1,1,2,2,3-pentafluoropropane, 1, 1,1,3,3,4- hexafluorobutane, 1, 1, 1,3,3-pentafluorobutane, 1, 1, 1,4,4,4-hexafluorobutane, 1, 1, 1,4,4- pentafluorobutane, 1,1, 2,2,3, 3-hexafluoropropane, 1,1, 1,2,3, 3-hexafluoropropane, 1, 1- difluoroethane, 1,1, 1,2-tetrafluoroethane, and pentafluoroethane; the FE's (fluoroethers) such as methyl- 1, 1,1-trifluoroethyl ether and difluoromethyl-l, l, l-trifluoroethyl ether; hydrocarbons such as n-pentane, isopentane, and cyclopentane; and well as combinations comprising at least one of the foregoing. As with the chemical blowing agents, the physical blowing agents are used in an amount sufficient to give the resultant foam the desired bulk density. The physical blowing agents can be used in an amount of 5 to 50 wt% of the reactive composition, or, more preferably,

10 to 30 wt%.

[0061] In a method of producing the polyurethane foams, the components for producing the foams, i.e., the isocyanate component, the active hydrogen-containing component, surfactant, catalyst, optional blowing agents, thermally conductive, flame retardant filler, and other additives can be first mixed together then subjected to mechanical frothing with air.

Alternatively, the ingredients can be added sequentially to the liquid phase during the

mechanical frothing process. The gas phase of the froths is most preferably air because of its cheapness and ready availability. However, if desired, other gases can be used which are gaseous at ambient conditions and which are substantially inert or non-reactive with any component of the liquid phase. Such other gases include, for example, nitrogen, carbon dioxide, and

fluorocarbons that are normally gaseous at ambient temperatures. The inert gas is incorporated into the liquid phase by mechanical beating of the liquid phase in high shear equipment such as in a Hobart mixer or an Oakes mixer. The gas can be introduced under pressure as in the usual operation of an Oakes mixer or it can be drawn in from the overlying atmosphere by the beating or whipping action as in a Hobart mixer. The mechanical beating operation preferably is conducted at pressures not greater than 7 to 14 kg/cm² (i.e., 100 to 200 pounds per square inch or "psi"). Readily available mixing equipment can be used and no special equipment is generally necessary. The amount of inert gas beaten into the liquid phase is controlled by gas flow metering equipment to produce a froth of the desired density. The mechanical beating is conducted over a period of a few seconds in an Oakes mixer, or 3 to 30 minutes in a Hobart mixer, or however long it takes to obtain the desired froth density in the mixing equipment employed. The froth as it emerges from the mechanical beating operation is substantially chemically stable and is structurally stable but easily workable at ambient temperatures, e.g., 10 to 40°C.

[0062] After frothing, the reactive mixture is transferred at a controlled rate through a hose or other conduit to be deposited onto a first carrier. For convenience, this first carrier can be referred to as "bottom carrier," and is generally a moving support that can or cannot readily release the cured foam. A second carrier, also referred to herein as a "surface protective layer" or

"top carrier," can be placed on top of the froth. The top carrier is also a moving support that also can or cannot readily release from the cured foam. The top carrier can be applied almost simultaneously with the froth. Before applying the top carrier, the foam can be spread to a layer of desired thickness by a doctoring blade or other suitable spreading device. Alternatively, placement of the top carrier can be used to spread the foam and adjust the frothed layer to the desired thickness. In still another embodiment, a coater can be used after placement of the top carrier to adjust the height of the foam. After application of the top carrier, the frothed foam is blown under the influence of a physical or chemical blowing agent. In some embodiments, the carriers impart a substantially flat surface to the surface of the foam. The assembly of the carrier and foam layer (after optional blowing) is delivered to a heating zone for curing the foam. The temperatures are maintained in a range suitable for curing the foam, for example at 90 to 220°C, depending on the composition of the foam material. Differential temperatures can be established for purposes of forming an integral skin on an outside surface of the foam or for adding a relatively heavy layer to the foam.

[0063] After the foam is heated and cured, it can then be passed to a cooling zone where it is cooled by any suitable cooling device such as fans. Where appropriate, the carriers are removed and the foam can be taken up on a roll. Alternatively, the foam can be subjected to further processing, for example lamination (bonding using heat and pressure) to the carrier layer.

[0064] In a specific embodiment, a method of manufacturing a polymer foam comprises frothing a liquid composition comprising a polyisocyanate component, an active hydrogen- containing component reactive with the polyisocyanate component, a surfactant, a catalyst, and a filler composition comprising a plurality of coated BN particles; casting the froth on a removable layer to form a polymer layer having a first surface adjoining the removable layer and an opposite second surface; and curing the layer to produce a polyurethane foam having a density of 1 to 125 pounds per cubic foot, an elongation of greater than or equal to 20%, and a compression set of less than or equal to 30%. Even further, the removable layer can be removed so as to as to expose the thermally conductive particles at the first surface. Even further, this method comprises disposing the removable layer on a carrier.

[0065] In another embodiment, the polymeric matrix can be a silicone foam. Silicone foams can be produced as a result of the reaction between water and hydride groups in a polysiloxane precursor polymer matrix composition with the consequent liberation of hydrogen gas. This reaction is generally catalyzed by a noble metal, preferably a platinum catalyst. In some embodiments, the polysiloxane polymer has a viscosity of 100 to 1,000,000 poise at 25°C and has chain substituents comprising hydride, methyl, ethyl, propyl, vinyl, phenyl,

trifluoropropyl, or a combination comprising at least one of the foregoing. The end groups on the polysiloxane polymer can be hydride, hydroxyl, vinyl, vinyl diorganosiloxy, alkoxy, acyloxy, allyl, oxime, aminoxy, isopropenoxy, epoxy, mercapto groups, or other known, reactive end groups. Suitable silicone foams can also be produced by using several polysiloxane polymers, each having different molecular weights (e.g., bimodal or trimodal molecular weight

distributions) as long as the viscosity of the combination lies within the above specified values. It is also possible to have several polysiloxane base polymers with different functional or reactive groups in order to produce the desired foam. In some embodiments, the polysiloxane polymer comprises 0.2 moles of hydride (Si-H) groups per mole of water.

[0066] Depending upon the chemistry of the polysiloxane polymers used, a catalyst, generally platinum or a platinum-containing catalyst, can be used to catalyze the blowing and the curing reaction. The catalyst can be deposited onto an inert carrier, such as silica gel, alumina, or carbon black. In some embodiments, an unsupported catalyst selected from among chloroplatinic acid, its hexahydrate form, its alkali metal salts, and its complexes with organic derivatives is used. Exemplary catalysts are the reaction products of chloroplatinic acid with

vinylpolysiloxanes such as 1,3-divinyltetramethyldisiloxane, which are treated or otherwise with an alkaline agent to partly or completely remove the chlorine atoms; the reaction products of chloroplatinic acid with alcohols, ethers, and aldehydes; and platinum chelates and platinous chloride complexes with phosphines, phosphine oxides, and with olefins such as ethylene, propylene, and styrene. It can also be desirable, depending upon the chemistry of the

polysiloxane polymers to use other catalysts such as dibutyl tin dilaurate in lieu of platinum based catalysts.

[0067] Various platinum catalyst inhibitors can also be used to control the kinetics of the blowing and curing reactions in order to control the porosity and density of the silicone foams. Examples of such inhibitors include polymethylvinylsiloxane cyclic compounds and acetylenic alcohols. Physical or chemical blowing agents can also be used to produce the silicone foam, including the physical and chemical blowing agents listed above for polyurethanes. Other examples of chemical blowing agents include benzyl alcohol, methanol, ethanol, isopropyl alcohol, butanediol, and silanols. In some embodiments, a combination of methods of blowing is used to obtain foams having desirable characteristics. For example, a physical blowing agent such as a chlorofluorocarbon can be added as a secondary blowing agent to a reactive mixture wherein the primary mode of blowing is the hydrogen released as the result of the reaction between water and hydride substituents on the polysiloxane.

[0068] In the production of silicone foams, the reactive components of the precursor polymer matrix composition are typically stored in two packages, one containing the platinum catalyst and the other the polysiloxane polymer containing hydride groups, which prevents premature reaction. In another method of production, the polysiloxane polymer is introduced into an extruder along with the thermally conductive particles, water, physical blowing agents if necessary, and other desirable additives. The platinum catalyst is then metered into the extruder to start the foaming and curing reaction. The use of physical blowing agents such as liquid carbon dioxide or supercritical carbon dioxide in conjunction with chemical blowing agents such as water can give rise to foam having much lower densities. In yet another method, the liquid silicone components are metered, mixed and dispensed into a device such a mold or a continuous coating line. The foaming then occurs either in the mold or on the continuous coating line.

[0069] In some embodiments, a method of manufacturing a composite material comprising silicone foam comprises spreading or extruding a reactive polymer mixture onto a first removable layer, the mixture comprising a polysiloxane polymer having hydride

substituents, a blowing agent, a platinum based catalyst, and a filler composition comprising a plurality of sol-gel coated BN particles, preferably sol-gel coated BN platelets, and other optional additives or particles; and (b) blowing and curing the mixture.

[0070] Alternatively to the above-described silicone foam formulations, a soft, thermally conductive silicone composition can be formed by the reaction of a precursor polymer matrix composition comprising a liquid silicone composition comprising a polysiloxane having at least two alkenyl groups per molecule; a polysiloxane having at least two silicon-bonded hydrogen atoms in a quantity effective to cure the composition; a catalyst; and optionally a reactive or non- reactive polysiloxane fluid having a viscosity of 100 to 1000 centipoise. Suitable reactive silicone compositions are low durometer, 1 : 1 liquid silicone rubber (LSR) or liquid injection molded (LIM) compositions. Because of their low inherent viscosity, the use of the low durometer LSR or LIM facilitates the addition of higher total filler quantities, and results in formation of soft foam.

[0071] In some embodiments, the non-reactive polysiloxane fluid remains within the cured silicone and is not extracted or removed. The reactive silicone fluid thus becomes part of the polymer matrix, leading to low outgassing and little or no migration to the surface during use. In some embodiments, the boiling point of the non-reactive silicone fluid is high enough such that when it is dispersed in the polymer matrix, it does not evaporate during or after cure, and does not migrate to the surface or outgas.

[0072] In some embodiments, LSR or LIM systems are provided as two-part

formulations suitable for mixing in ratios of 1 : 1 by volume. The "A" part of the formulation comprises one or more polysiloxanes having two or more alkenyl groups and has an extrusion rate of less than 500 g/minute. Suitable alkenyl groups are exemplified by vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl, with vinyl being particularly suitable. The alkenyl group can be bonded at the molecular chain terminals, in pendant positions on the molecular chain, or both.

Other silicon-bonded organic groups in the polysiloxane having two or more alkenyl groups are exemplified by substituted and unsubstituted monovalent hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; aryl groups such as phenyl, tolyl, and xylyl; aralkyl groups such as benzyl and phenethyl; and halogenated alkyl groups such as 3-chloropropyl and 3,3,3-trifluoropropyl. Exemplary substituents are methyl and phenyl groups.

[0073] The alkenyl-containing polysiloxane can have straight chain, partially branched straight chain, branched-chain, or network molecule structure, or can be a mixture of two or more selections from polysiloxanes with the exemplified molecular structures. The alkenyl- containing polysiloxane is exemplified by trimethylsiloxy-end-blocked dimethylsiloxane- methylvinylsiloxane copolymers, trimethylsiloxy-end-blocked methylvinylsiloxane- methylphenylsiloxane copolymers, trimethylsiloxy-end blocked dimethylsiloxane- methylvinylsiloxane-methylphenylsiloxane copolymers, dimethylvinylsiloxy-end-blocked dimethylpolysiloxanes, dimethylvinylsiloxy-end-blocked methylvinylpolysiloxanes, dimethylvinylsiloxy-endblocked methylvinylphenylsiloxanes, dimethylvinylsiloxy-endblocked dimethylvinylsiloxane-methylvinylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-methylphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane-diphenylsiloxane copolymers, polysiloxane comprising R3 S1O1/2 and S1O4/2 units, polysiloxane comprising RS1O3/2 units, polysiloxane comprising the R2S1O2/2 and RS1O3/2 units, polysiloxane comprising the R2S1O2/2, RS1O3/2 and S1O4/2 units, and a mixture of two or more of the preceding polysiloxanes. R represents substituted and unsubstituted monovalent hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; aryl groups such as phenyl, tolyl, and xylyl; aralkyl groups such as benzyl and phenethyl; and halogenated alkyl groups such as 3-chloropropyl and 3,3,3-trifluoropropyl, with the proviso that at least 2 of the R groups per molecule are alkenyl.

[0074] The "B" component of the LSR or LIM system comprises one or more polysiloxanes that contain at least two silicon-bonded hydrogen atoms per molecule and has an extrusion rate of less than 500 g/minute. The hydrogen can be bonded at the molecular chain terminals, in pendant positions on the molecular chain, or both. Other silicon-bonded groups are organic groups exemplified by non-alkenyl, substituted and unsubstituted monovalent hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; aryl groups such as phenyl, tolyl, and xylyl; aralkyl groups such as benzyl and phenethyl; and halogenated alkyl groups such as 3-chloropropyl and 3,3,3-trifluoropropyl. Exemplary substituents are methyl and phenyl groups.

[0075] The hydrogen-containing polysiloxane component can have straight-chain, partially branched straight-chain, branched-chain, cyclic, network molecular structure, or can be a mixture of two or more selections from polysiloxanes with the exemplified molecular structures. The hydrogen-containing polysiloxane is exemplified by trimethylsiloxy-end-blocked methylhydrogenpolysiloxanes, trimethylsiloxy-end-blocked dimethylsiloxane- methylhydrogensiloxane copolymers, trimethylsiloxy-end-blocked methylhydrogensiloxane- methylphenylsiloxane copolymers, trimethylsiloxy-end-blocked dimethyl siloxane- methylhydrogensiloxane-methylphenylsiloxane copolymers, dimethylhydrogensiloxy-end- blocked dimethylpolysiloxanes, dimethylhydrogensiloxy-end-blocked

methylhydrogenpolysiloxanes, dimethylhydrogensiloxy-end-blocked dimethylsiloxanes- methylhydrogensiloxane copolymers, dimethylhydrogensiloxy-end-blocked dimethylsiloxane- methylphenylsiloxane copolymers, and dimethylhydrogensiloxy-end-blocked

methylphenylpolysiloxanes.

[0076] The hydrogen-containing polysiloxane component is added in an amount sufficient to cure the composition, preferably in a quantity of 0.5 to 10 silicon-bonded hydrogen atoms per alkenyl group in the alkenyl-containing polysiloxane.

[0077] The silicone composition further comprises, generally as part of Component "A," a catalyst such as platinum to accelerate the cure. Platinum and platinum compounds known as hydrosilylation-reaction catalysts can be used, for example platinum black, platinum-on-alumina powder, platinum-on-silica powder, platinum-on-carbon powder, chloroplatinic acid, alcohol solutions of chloroplatinic acid platinum-olefin complexes, platinum-alkenylsiloxane complexes and the catalysts afforded by the microparticulation of the dispersion of a platinum addition- reaction catalyst, as described above, in a thermoplastic resin such as methyl methacrylate, polycarbonate, polystyrene, silicone, and the like. Mixtures of catalysts can also be used. A quantity of catalyst effective to cure the present composition is generally from 0.1 to 1,000 parts per million (by weight) of platinum metal based on the combined amounts of alkenyl and hydrogen components.

[0078] The composition optionally further comprises one or more polysiloxane fluids having a viscosity of less than or equal to 1000 centipoise, preferably less than or equal to 750 centipoise, more preferably less than or equal to 600 centipoise, and most preferably less than or equal to 500 centipoise. The polysiloxane fluids can also have a viscosity of greater than or equal to 100 centipoises. The polysiloxane fluid component can be added for the purpose of decreasing the viscosity of the composition, thereby allowing at least one of increased filler loading, enhanced filler wetting, and enhanced filler distribution. Use of the polysiloxane fluid component obviates the need for an extra step during processing to remove the fluid, as well as possible outgassing and migration of diluent during use. The polysiloxane fluid should not inhibit the curing reaction, that is, the addition reaction, of the composition, but it may or may not participate in the curing reaction.

[0079] The non-reactive polysiloxane fluid has a boiling point of greater than 500°F

(260°C), and can be branched or straight-chained. The non-reactive polysiloxane fluid comprises silicon-bonded non-alkenyl organic groups exemplified by substituted and unsubstituted monovalent hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; aryl groups such as phenyl, tolyl, and xylyl; aralkyl groups such as benzyl and phenethyl; and halogenated alkyl groups such as 3-chloropropyl and 3,3,3-trifluoropropyl. Exemplary substituents are methyl and phenyl groups. Thus, the non-reactive polysiloxane fluid can comprise R3S1O1/2 and S1O4/2 units, RS1O3/2 units, R2S1O2/2 and RS1O3/2 units, or R2S1O2/2, RS1O3/2 and S1O4/2 units, wherein R represents substituted and unsubstituted monovalent hydrocarbon groups such as alkyl, methyl, ethyl, propyl, butyl, pentyl, hexyl, aryl, phenyl, tolyl, xylyl, aralkyl, benzyl, phenethyl, halogenated alkyl, 3-chloropropyl, or 3,3,3-trifluoropropyl. Because the non-reactive polysiloxane is a fluid and has a significantly higher boiling point (greater than 230°C (500°F)), it allows the incorporation of higher quantities of filler, but does not migrate or outgas. Examples of non-reactive polysiloxane fluids include DC 200 from Dow Corning Corporation.

[0080] Reactive polysiloxane fluids co-cure with the alkenyl-containing polysiloxane and the polysiloxane having at least two silicon-bonded hydrogen atoms, and therefore can themselves contain alkenyl groups or silicon-bonded hydrogen groups. Such compounds can have the same structures as described above in connection with the alkenyl-containing polysiloxane and the polysiloxane having at least two silicon-bonded hydrogen atoms, but in addition have a viscosity of less than or equal to 1000 centipoise (cps), preferably less than or equal to 750 cps, more preferably less than or equal to 600 cps, and most preferably less than or equal to 500 cps. In some embodiments, the reactive polysiloxane fluids have a boiling point greater than the curing temperature of the addition cure reaction.

[0081] The polysiloxane fluid component can be present in the formulation for the composite material amount effective to allow the addition, incorporation, and wetting of higher quantities of total filler including the coated BN particles or to facilitate incorporation of the thermally conductive particles, for example to facilitate detangling or dispersion. In some embodiments, the polysiloxane fluid component is added to the composition in an amount of 5 to 50 weight parts per 100 weight parts of the combined amount of the polysiloxane having at least two alkenyl groups per molecule, the polysiloxane having at least two silicon-bonded hydrogen atoms in a quantity effective to cure the composition, and the catalyst. The amount of the polysiloxane fluid component is preferably greater than or equal to 5 weight parts, more preferably greater than or equal to 7.5 weight parts, and even more preferably greater than or equal to 10 weight parts of the combined amount of the polysiloxane having at least two alkenyl groups per molecule, the polysiloxane having at least two silicon-bonded hydrogen atoms in a quantity effective to cure the composition, and the catalyst. Also desired is a polysiloxane fluid component of less than or equal to 50 weight parts, more preferably less than or equal to 25 weight parts, and more preferably less than or equal to 20 weight parts of the combined amount of the polysiloxane having at least two alkenyl groups per molecule, the polysiloxane having at least two silicon-bonded hydrogen atoms in a quantity effective to cure the composition, and the catalyst.

[0082] Silicone composite materials can further optionally comprise a curable silicone gel formulation. Silicone gels are lightly cross-linked fluids or under-cured elastomers. They are unique in that they range from very soft and tacky to moderately soft and only slightly sticky to the touch. Use of a gel formulation decreases the viscosity of the composition, thereby allowing at least one of increased filler loading, enhanced filler wetting, or enhanced filler distribution, and increased softness. Suitable gel formulations can be either two-part curable formulations or one-part formulations. The components of the two-part curable gel formulations is similar to that described above for LSR systems (i.e., an organopolysiloxane having at least two alkenyl groups per molecule and an organopolysiloxane having at least two silicon-bonded hydrogen atoms per molecule). The main difference lies in the fact that no filler is present, and that the molar ratio of the silicon-bonded hydrogen groups (Si-H) groups to the alkenyl groups is usually less than one, and can be varied to create an "under-cross linked" polymer with the looseness and softness of a cured gel. Preferably, the ratio of silicone-bonded hydrogen atoms to alkenyl groups is less than or equal to 1.0, preferably less than or equal to 0.75, more preferably less than or equal to 0.6, and most preferably less than or equal to 0.1. An example of a suitable two-part silicone gel formulation is SYLGARD™ 527 gel commercially available from the Dow Corning

Corporation.

[0083] For many applications, the foam thermally conductive composite material, for example, a thermal interface foam pad, can have an average thickness of 0.1 mm to 25 mm, preferably 0.25 to 15 mm or 10 to 1000 mils (0.254 to 25.4 mm), and typically, but not necessarily, will be small relative to the extents of the lengthwise or widthwise dimensions of foam pad as defined along the x- and y-axes. Non-foamed composite materials that serve as circuit materials such as dielectric substrates can have even lesser thicknesses, for example, 50 to 1000 micrometers.

[0084] Regarding the above-described foamed composite materials, the properties (e.g., density, modulus, compression load deflection, tensile strength, tear strength, and so forth) can be adjusted by varying the components of the reactive compositions. In general, the foam can have a density of 50 to 500 kg/m³, preferably at least 70 kg/m³, more preferably 90 to 400 kg/m³.

The physical properties of such foam materials can be designed for use in a particular

application of a thermally conducive foam pad. Such foam materials can typically have a compression set resistance of less than or equal to 10%, or, more preferably, less than or equal to 5%. For example, a specific compressible thermally conductive composite sheet can have one or more of the following properties: a thermal conductivity of at least 1 W/m K, compression set of less than or equal to 10% after 50% compression for 22 hours at room temperature; and a compression force deflection at 25% of 1 to 20 psi, preferably 2 to 15 psi.

[0085] In some embodiments, the average cellular diameter of the foam can be 10 micrometers (μπι) to 1 millimeter (mm), or, more preferably, 50 to 500 micrometers. In open- celled foams where at least a portion of the cells extend through the sheet, through holes can be distinguished from such open cells on the basis of size. For example, in mechanically frothed foams, the smallest diameter of a through hole is at least ten times larger than the largest diameter of a cell. In a blown foam, or non-microcellular foam, the smallest diameter of a through hole is at least twice as large as the largest diameter of the cell.

[0086] In still other embodiments, a solid material comprising the sol-gel coated BN particles dispersed in a polymer matrix can be formed by casting, extrusion, molding, or other conventional process. Polymers suitable for the solid thermal interface composite material include those described above.

[0087] In an embodiment, the process of making a thermal interface composite material for various applications can generally comprise mixing the sol-gel coated BN particles with a precursor polymer matrix composition; forming the mixture into a material having a flattened profile (a layer) comprising first and second opposing heat transfer surfaces in the x-y plane; and subsequently hardening the precursor polymer matrix composition to form a solid polymer matrix. The sol-gel coated BN particles can be more randomly dispersed in the composite material, in terms of orientation, compared to the use of the same core uncoated BN particles.

[0088] In this general process, the BN particles can be mixed with a polymer solution, a polymer melt, or a reactive thermosetting composition or solution that forms the polymeric matrix of the composite material fully upon curing. Other additives can be included in the mixture as described above. The process of forming the filled mixture into a material having a flattened profile can comprise casting or extruding the filled mixture (for example, a circuit substrate) into a continuous or semi-continuous sheet, which can have a patterned or non- patterned surface. Other methods of shaping the precursor polymer matrix composition include injection molding and the like.

[0089] In a specific embodiment, the sol-gel coated BN platelets are made by immersing the BN particles in a sol-gel precursor solution, catalyzing a sol-gel reaction to coat or deposit silica onto the surface of the BN particles, and introducing the surface-coated particles into a spray tower or fluidized bed to harden or the silica coating. The coated BN particles are mixed with a precursor polymer matrix composition that is uncured or only partially cured. The mixture is formed into a layer comprising first and second heat transfer surfaces (opposed surfaces each having a surface area that is on average at least 10 times the average cross-sectional area between surfaces). The shaped material can then be hardened to obtain a thermal interface composite material or circuit substrate layer as described below, which material can optionally be divided into individual units. The bulk thermal conductivity of the interface material is at least 0.5 W/m-K.

[0090] In an aspect, the thermal interface composite material can be used as a circuit material, including dielectric substrate layers in circuits and circuit laminates, especially in association with a heat-generating component. Such thermally composite materials are typically not foamed and comprise a polymeric matrix that comprises a low polarity polymer. As used herein, a circuit material is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit subassemblies, bond plies, resin-coated conductive layers, unclad dielectric layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric layer. Double clad circuit laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuits together using bond plies, by building up additional layers with resin coated conductive layers that are subsequently etched, or by building up additional layers by adding unclad dielectric layers followed by additive metallization. After forming the multilayer circuit, known hole-forming and plating technologies can be used to produce useful electrical pathways between conductive layers.

[0091] In particular, a thermally conductive composite material can form a dielectric layer in which, in addition to the thermal properties controlled by means of the BN particles, the dielectric and other relevant electronic and physical properties (for example, the mechanical properties of a dielectric substrate for a conductive layer) are controlled by the use of additional mineral or ceramic particulate fillers. Additional fillers can be selected to provide a low dielectric constant (Dk) (also known as the relative permittivity) and other desired electrical properties, while maintaining the total filler volume necessary for preservation of mechanical properties. In some embodiments, a specific desire can be to obtain filler having electrical properties necessary for high frequency applications that require a low dissipation factor in circuit subassemblies.

[0092] In some embodiments, a thermally conductive composite material as disclosed herein, when designed for use as a dielectric substrate layer, capable of supporting a conductive layer and any electronic components, can have a dielectric constant of less than 3.5 and a dissipation factor of less than 0.006 at 10 GHz and 23°C.

[0093] The thermally conductive composite materials for use as a dielectric substrate, optionally part of a multilayer circuit material, can optionally include one or more additional particulate fillers other than BN for the purpose of providing desired electronic properties. For example, additional types of fillers can be used to determine or control the dielectric constant, dissipation factor, coefficient of thermal expansion, and other relevant properties of the dielectric composite material, which can be fine-tuned to meet strict requirements. Examples of such additional particulate fillers can include, without limitation, titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (including fused amorphous silica), corundum, wollastonite, aiT Ow, solid glass spheres, synthetic glass or ceramic hollow spheres, quartz, beryllia, nanoclays, mica and metal oxides such as alumina, alumina trihydrate, magnesia, talcs, and magnesium hydroxide. A combination of particulate fillers can be used to provide a desired balance of properties. Optionally, the fillers can be surface treated with a silicon-containing coating, for example, an organofunctional alkoxy silane coupling agent. Alternatively, a zirconate or titanate coupling agent can be used. Such coupling agents can improve the dispersion of the filler in the polymeric matrix and reduce water absorption of the finished composite circuit substrate.

[0094] The thermally conductive composite material, when used as a dielectric substrate, especially layers containing non-polar or low polarity polymeric compositions, can also contain constituents useful for making the material resistant to flame. Such constituents can be present in overall composite volumes ranging from 0 to 30 volume percent (vol%). These flame retarding agents can be halogenated or not. The choice of flame retardant, however, can influence the loading required to achieve the desired level of flame resistance.

[0095] The total filler component in a thermally conductive composite material when used to make a dielectric substrate can comprise 5 to 70 vol% of the coated BN particles and 1 to 90 vol% of one or more other fillers, preferably 25 to 75 vol% of other filler, based on the total composition of 100 percent. In some embodiments, the filler component comprises 5 to 50 vol% of the BN and 70 to 30 vol% of silica, aluminum oxide, magnesium oxide, or combinations thereof as filler based on the total volume of the filler.

[0096] [In the case of the composite material used to make circuit material, exemplary polymer matrix materials can include low polarity, low dielectric constant and low loss polymer resins, including those based on thermosetting and thermoplastic resins such as 1,2- polybutadiene (PBD), polyisoprene, polybutadiene-polyisoprene copolymers, polyetherimide, fluoropolymers such as polytetrafluoroethylene, polyimide, polyetheretherketone, polyamidimide, polyethylene terephthalate, polyethylene naphthalate, polycyclohexylene terephthalate, polyphenylene ethers, and those based on allylated polyphenylene ether resins. Such polymeric materials exhibit the desirable features of low dielectric constant that can be further improved by addition of filler. Combinations of low polarity resins with higher polarity resins can also be used, non-limiting examples including epoxy and poly(phenylene ether), epoxy and poly(ether imide), cyanate ester and poly(phenylene ether), and 1,2-polybutadiene and polyethylene.

[0097] Suitable fluoropolymer matrix materials for use in a dielectric layer can include fluorinated homopolymers, e.g., polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE), and fluorinated copolymers, e.g. copolymers of tetrafluoroethylene with

hexafluoropropylene or perfluoroalkylvinylethers, copolymers of tetrafluoroethylene with vinylidene fluoride, vinyl fluoride, or ethylene, and copolymers of chlorotrifluoroethylene with hexafluoropropylene, perfluoroalkylvinylethers, vinylidene fluoride, vinyl fluoride, or ethylene. Blends of these fluoropolymers and terpolymers formed from the above listed monomers can also be used as the polymer matrix material.

[0098] Other specific polymer matrix materials include thermosetting polybutadiene or polyisoprene resin. As used herein, the term "thermosetting polybutadiene or polyisoprene resin" includes homopolymers and copolymers comprising units derived from butadiene, isoprene, or mixtures thereof. Units derived from other copolymerizable monomers can also be present in the resin, for example, optionally in the form of grafts. Exemplary copolymerizable monomers include, but are not limited to, vinylaromatic monomers, for example substituted and

unsubstituted monovinylaromatic monomers such as styrene, 3 -methyl styrene, 3,5- diethylstyrene, 4-n-propylstyrene, alpha-methyl styrene, alpha-methyl vinyltoluene, para- hydroxystyrene, para-methoxystyrene, alpha-chlorostyrene, alpha-bromostyrene,

dichloro styrene, dibromo styrene, tetra-chlorostyrene, and the like; and substituted and unsubstituted divinylaromatic monomers such as divinylbenzene, divinyltoluene, and the like. Combinations comprising at least one of the foregoing copolymerizable monomers can also be used. Exemplary thermosetting polybutadiene or polyisoprene resins include, but are not limited to, butadiene homopolymers, isoprene homopolymers, butadiene-vinylaromatic copolymers such as butadiene- styrene, isoprene-vinylaromatic copolymers such as isoprene-styrene copolymers, and the like.

[0099] The thermosetting polybutadiene or polyisoprene resins can also be modified. For example, the resins can be hydroxyl-terminated, methacrylate-terminated, carboxylate- terminated resins or the like. Post-reacted resins can be used, such as epoxy-, maleic anhydride-, or urethane-modified butadiene or isoprene resins. The resins can also be crosslinked, for example by divinylaromatic compounds such as divinyl benzene, e.g., a polybutadiene-styrene crosslinked with divinyl benzene. Mixtures of resins can also be used, for example, a mixture of a polybutadiene homopolymer and a poly(butadiene-isoprene) copolymer. Combinations comprising a syndiotactic polybutadiene can also be useful.

[0100] The thermosetting polybutadiene or polyisoprene resin can be liquid or solid at room temperature. Suitable liquid resins can have a number average molecular weight greater than 5,000 gram/mole (g/mol) but generally have a number average molecular weight of less than 5,000 g/mol (most preferably 1,000 to 3,000 g/mol) based on polystyrene standards.

Thermosetting polybutadiene or polyisoprene resins include resins having at least 90 wt% 1,2- addition, which can exhibit greater crosslink density upon cure due to the large number of pendent vinyl groups available for crosslinking.

[0101] The polybutadiene or polyisoprene resin can be present in the polymer matrix composition of the thermally conductive composite circuit material in an amount of up to 100 wt%, preferably up to 75 wt%, more preferably 10 to 70 wt%, even more preferably 20 to 60 or 20 to 70 wt%, based on the total weight of the composite circuit material.

[0102] Other polymers that can co-cure with the thermosetting polybutadiene or polyisoprene resins can be added for specific property or processing modifications. For example, in order to improve the stability of the dielectric strength and mechanical properties of the electrical substrate material over time, a lower molecular weight ethylene propylene elastomer can be used in the resin systems. An ethylene propylene elastomer as used herein is a copolymer, terpolymer, or other polymer comprising primarily ethylene and propylene. Ethylene propylene elastomers can be further classified as EPM copolymers (i.e., copolymers of ethylene and propylene monomers) or EPDM terpolymers (i.e., terpolymers of ethylene, propylene, and diene monomers). Ethylene propylene diene terpolymer rubbers, in particular, have saturated main chains, with unsaturation available off the main chain for facile cross-linking. Liquid ethylene propylene diene terpolymer rubbers, in which the diene is dicyclopentadiene, can be used.

[0103] The viscosity average molecular weights of the ethylene propylene rubbers can be less than 10,000. Suitable ethylene propylene rubbers include an ethylene propylene rubber having a viscosity average molecular weight (MV) of 7,200, which is available from Lion

Copolymer, Baton Rouge, LA, under the trade name TRILENE CP80; a liquid ethylene propylene dicyclopentadiene terpolymer rubbers having a viscosity average molecular weight of

7,000, which is available from Lion Copolymer under the trade name of TRILENE 65; and a liquid ethylene propylene ethylidene norbornene terpolymer, having a viscosity average molecular weight of 7,500, which is available from Lion Copolymer under the name TRILENE

67. The ethylene propylene rubber can be present in an amount effective to maintain the stability of the properties of the substrate material over time, in particular the dielectric strength and mechanical properties. Typically, such amounts are up to 20 wt% with respect to the total weight of the polymer matrix composition, more preferably 4 to 20 wt%, even more preferably 6 to 12 wt%.

[0104] Another type of co-curable polymer is an unsaturated polybutadiene- or polyisoprene-containing elastomer. This component can be a random or block copolymer of primarily 1,3 -addition butadiene or isoprene with an ethylenically unsaturated monomer, for example a vinylaromatic compound such as styrene or alpha-methyl styrene, an acrylate or methacrylate such a methyl methacrylate, or acrylonitrile. The elastomer can be a solid, thermoplastic elastomer comprising a linear or graft-type block copolymer having a

polybutadiene or polyisoprene block and a thermoplastic block that can be derived from a monovinylaromatic monomer such as styrene or alpha-methyl styrene. Block copolymers of this type include styrene-butadiene-styrene triblock copolymers, for example, those available from Dexco Polymers under the trade name VECTOR 8508M, from Enichem Elastomers America, under the trade name SOL-T-6302, and those from Dynasol Elastomers under the trade name CALPRE E 401; and styrene-butadiene diblock copolymers and mixed triblock and diblock copolymers containing styrene and butadiene, for example, those available from Kraton

Polymers under the trade name KRATON Dl 118 a mixed diblock / triblock styrene and butadiene containing copolymer that contains 33% by weight styrene.

[0105] The optional polybutadiene- or polyisoprene-containing elastomer can further comprise a second block copolymer similar to that described above, except that the

polybutadiene or polyisoprene block is hydrogenated, thereby forming a polyethylene block (in the case of polybutadiene) or an ethylene-propylene copolymer block (in the case of

polyisoprene). When used in conjunction with the above-described copolymer, materials with greater toughness can be produced. An exemplary second block copolymer of this type is KRATON GX1855 (commercially available from Kraton Polymers, which is believed to be a mixture of a styrene-high 1,2-butadiene-styrene block copolymer and a styrene-(ethylene- propylene)-styrene block copolymer.

[0106] The unsaturated polybutadiene- or polyisoprene-containing elastomer component can be present in the polymeric matrix in an amount of 2 to 60 wt% with respect to the total polymer matrix composition, more preferably 5 to 50 wt%, or even more preferably 10 to 40 or 20 to 50 wt%.

[0107] Still other co-curable polymers that can be added for specific property or processing modifications include, but are not limited to, homopolymers or copolymers of ethylene such as polyethylene and ethylene oxide copolymers; natural rubber; norbornene polymers such as polydicyclopentadiene; hydrogenated styrene-isoprene-styrene copolymers and butadiene-acrylonitrile copolymers; unsaturated polyesters; and the like. Levels of these copolymers are generally less than 50 wt% of the total polymer in the matrix composition.

[0108] Free radical-curable monomers can also be added for specific property or processing modifications, for example to increase the crosslink density of the resin system after cure. Exemplary monomers that can be suitable crosslinking agents include, for example, di, tri-, or higher ethylenically unsaturated monomers such as divinyl benzene, triallyl cyanurate, diallyl phthalate, and multifunctional acrylate monomers (e.g., SARTOMER resins available from Sartomer USA, Newtown Square, PA), or combinations thereof, all of which are commercially available. The crosslinking agent, when used, can be present in the resin system in an amount of up to 20 wt%, preferably 1 to 15 wt%, based on the total polymer matrix composition.

[0109] A curing agent can be added to the resin system for the polymeric matrix to accelerate the curing reaction of polyenes having olefinic reactive sites. Preferably useful curing agents are organic peroxides such as, for example, dicumyl peroxide, t-butyl perbenzoate, 2,5- dimethyl-2,5-di(t-butyl peroxy)hexane, a,a-di-bis(t-butyl peroxy)diisopropylbenzene, and 2,5- dimethyl-2,5-di(t-butyl peroxy) hexyne-3, all of which are commercially available. Carbon- Carbon initiators can be used in the resin system, for example, 2,3-dimethyl-2,3 diphenylbutane. Curing agents or initiators can be used alone or in combination. Typical amounts of curing agent are 1.5 to 10 wt% of the total polymer matrix composition.

[0110] In some embodiments, the precursor polymer matrix composition comprises a polybutadiene or polyisoprene polymer that is carboxy-functionalized. Functionalization can be accomplished using a polyfunctional compound having in the molecule both (i) a carbon-carbon double bond or a carbon-carbon triple bond, and (ii) one or more of a carboxy group, including a carboxylic acid, anhydride, amide, ester, or acid halide. A specific carboxy group is a carboxylic acid or ester. Examples of polyfunctional compounds that can provide a carboxylic acid functional group include maleic acid, maleic anhydride, fumaric acid, and citric acid. In particular, polybutadienes adducted with maleic anhydride can be used in the thermosetting composition. Suitable maleinized polybutadiene polymers are commercially available, for example from Cray Valley under the trade names RICON 130MA8, RICON 130MA13, RICON 130MA20, RICON 131MA5, RICON 131MA10, RICON 131MA17, RICON 131MA20, and RICON 156MA17. Suitable maleinized polybutadiene-styrene copolymers are commercially available, for example, from Sartomer under the trade names RICON 184MA6. RICON

184MA6 is a butadiene-styrene copolymer adducted with maleic anhydride having styrene content of 17 to 27 wt% and number average molecular weight (M_n) of 9,900 g/mole. [0111] In a dielectric substrate layer, the relative amounts of the various polymers in the precursor polymer matrix composition, for example, the polybutadiene or polyisoprene polymer and other polymers, can depend on the particular conductive metal layer used, the desired properties of the circuit materials and circuit laminates, and like considerations. For example, it has been found that use of a poly(arylene ether) can provide increased bond strength to a conductive metal layer, particularly copper. Use of a polybutadiene or polyisoprene polymer can increase high temperature resistance of the laminates, particularly when these polymers are carboxy-functionalized. Use of an elastomeric block copolymer can function to compatibilize the components of the polymer matrix material. Determination of the appropriate quantities of each component can be done without undue experimentation, depending on the desired properties for a particular application.

[0112] In addition to the polymer matrix and sol-gel coated BN, the composite material for use in forming a dielectric substrate can optionally also include an unwoven or woven, thermally stable web of a suitable fiber, preferably glass (E, S, and D glass) or high temperature polyester fibers. Such thermally stable fiber reinforcement provides a circuit laminate with a means of controlling shrinkage upon cure within the plane of the laminate. In addition, the use of the woven web reinforcement renders a circuit substrate with a relatively high mechanical strength.

[0113] A dielectric substrate can be produced by means known in the art, wherein sol-gel coated BN particles are added to the formulation with other fillers. The particular choice of processing conditions can depend on the polymer matrix composition selected. For example, where the polymer matrix composition is based on a fluoropolymer such as PTFE, the polymer matrix composition can be mixed with a first carrier liquid. The mixture can comprise a dispersion of polymeric particles in the first carrier liquid, i.e. an emulsion, of liquid droplets of the polymer or of a monomeric or oligomeric precursor of the polymer in the first carrier liquid, or a solution of the polymer in the first carrier liquid. If the polymer component is liquid, then no first carrier liquid may be necessary.

[0114] The choice of the first carrier liquid, if present, is based on the particular polymeric matrix material and the form in which the polymeric matrix material is to be introduced to the dielectric composite material. If it is desired to introduce the polymeric material as a solution, a solvent for the particular polymeric matrix material is chosen as the carrier liquid, e.g., N-methyl pyrrolidone (NMP) would be a suitable carrier liquid for a solution of a polyimide. If it is desired to introduce the polymeric matrix material as a dispersion, then a suitable carrier liquid is a liquid in which the matrix material is not soluble, e.g., water would be a suitable carrier liquid for a dispersion of PTFE particles and would be a suitable carrier liquid for an emulsion of polyamic acid or an emulsion of butadiene monomer.

[0115] Optionally, the total filler component of the composite material, including the coated BN, can be dispersed in a suitable second carrier liquid, or mixed with the first carrier liquid (or liquid polymer where no first carrier is used). The second carrier liquid can be the same liquid or can be a liquid other than the first carrier liquid that is miscible with the first carrier liquid. For example, if the first carrier liquid is water, the second carrier liquid can comprise water or an alcohol. In an exemplary embodiment, the second carrier liquid is water.

[0116] The filler dispersion can include a surfactant in an amount effective to modify the surface tension of the second carrier liquid to enable the second carrier liquid to wet the BN particles and other filler particles. Exemplary surfactant compounds include ionic surfactants and nonionic surfactants. Triton X-100™, commercially available from Rohm & Haas, has been found to be an exemplary surfactant for use in aqueous filler dispersions. Filler dispersions can comprise 10 to 70 vol% of filler and 0.1 to 10 vol% of surfactant, with the remainder comprising the second carrier liquid.

[0117] The mixture of the polymeric matrix material and first carrier liquid and the filler dispersion in the second carrier liquid can be combined to form a casting mixture. In an exemplary embodiment, the casting mixture comprises 10 to 60 vol% of the combined polymeric matrix material and BN particles and optional or other fillers and 40 to 90 vol% combined first and second carrier liquids. The relative amounts of the polymeric matrix material and the filler component in the casting mixture are selected to provide the desired amounts in the final composition as described below.

[0118] The viscosity of the casting mixture can be adjusted by the addition of a viscosity modifier and to provide a dielectric composite material having a viscosity compatible with conventional laminating equipment. Exemplary viscosity modifiers suitable for use in aqueous casting mixtures include, e.g., polyacrylic acid compounds, vegetable gums, and cellulose based compounds. Specific examples of suitable viscosity modifiers include polyacrylic acid, methyl cellulose, polyethyleneoxide, guar gum, locust bean gum, sodium carboxymethylcellulose, sodium alginate, and gum tragacanth. The viscosity of the viscosity-adjusted casting mixture can be further increased, i.e., beyond the minimum viscosity, on an application by application basis to adapt the dielectric composite material to the selected laminating technique. In an exemplary embodiment, the viscosity-adjusted casting mixture exhibits a viscosity of 10 to 100,000 cp, preferably 100 to 10,000 cp. It will be appreciated by those skilled in the art that the foregoing viscosity values are room temperature values (for example, taken at 23°C). Alternatively, the viscosity modifier can be omitted if the viscosity of the carrier liquid is sufficient to provide a casting mixture that does not separate during the time period of interest.

[0119] A layer of the casting mixture can be cast on a substrate by conventional methods, e.g., dip coating, reverse roll coating, knife-over-roll, knife-over-plate, and metering rod coating. Examples of carrier materials can include metallic films, polymeric films, ceramic films, and the like. Specific examples of carriers include stainless steel foil, polyimide films, polyester films, and fluoropolymer films. Alternatively, the casting mixture can be cast onto a glass web, or a glass web can be dip-coated.

[0120] The carrier liquid and processing aids, i.e., the surfactant and any viscosity modifier, are removed from the cast layer, for example, by evaporation or by thermal decomposition in order to consolidate the composite material.

[0121] Circuit subassemblies comprising the composite material, e.g., laminates, can be formed by means known in the art. In some embodiments, the lamination process entails placing one or more layers of the composite material between one or two sheets of coated or uncoated conductive layers (an adhesive layer can be disposed between at least one conductive layer and at least one dielectric substrate layer) to form a circuit substrate. The conductive layer can be in direct contact with the dielectric substrate layer or optional adhesive layer, preferably without an intervening layer, wherein an optional adhesive layer is less than 10% of the thickness of the dielectric substrate layer. The layered material can then be placed in a press, e.g., a vacuum press, under a pressure and temperature and for duration of time suitable to bond the layers and form a laminate. Lamination and curing can be by a one-step process, for example using a vacuum press, or can be by a multi-step process. In an exemplary one-step process, for a PTFE precursor polymer matrix composition, the layered material is placed in a press, brought up to laminating pressure (e.g., 150 to 400 psi) and heated to laminating temperature (e.g., 260 to 390°C). The laminating temperature and pressure are maintained for the desired soak time, i.e., greater than or equal to 20 minutes, and thereafter cooled (while still under pressure) to below 150°C.

[0122] In an exemplary multiple-step process suitable for thermosetting materials such as comprising polybutadiene or polyisoprene, a conventional peroxide cure step at temperatures of 150°C to 200°C can be conducted, and then a partially cured stack can be subjected to a high temperature cure step. Use of a two-stage cure can impart an unusually high degree of cross- linking to the resulting laminate. The temperature used in the second stage is typically 250°C to 300°C, or the decomposition temperature of the resin. This high temperature cure can be carried out in an oven but can also be performed in a press, namely as a continuation of the initial lamination and cure step. Particular lamination temperatures and pressures will depend upon the particular adhesive composition and the substrate composition, and are readily ascertainable by one of ordinary skill in the art without undue experimentation.

[0123] In accordance with an exemplary embodiment, FIG. 1 shows an exemplary circuit subassembly, in particular a single clad laminate 110 comprising a conductive metal layer 112 disposed on and in contact with a thermally conductive composite material, comprising coated BN, functioning as a dielectric layer 114. The dielectric substrate layer 114 can comprise a polymer matrix having a total particulate filler content of 10 to 70 vol%, including coated BN particles, preferably coated BN platelets or agglomerates of platelets. An optional glass web (not shown) can be present in dielectric substrate layer 114. It is to be understood that in all of the embodiments described herein, the various layers can fully or partially cover each other, and additional conductive layers, patterned circuit layers, and dielectric layers can also be present. Optional adhesive (bond ply) layers (not shown) can also be present, and can be uncured or partially cured. Many different multi-layer circuit configurations can be formed using the above substrates.

[0124] FIG. 2 shows another embodiment of a multilayer circuit assembly 210, preferably a double clad circuit layer 210 that comprises conductive layers 212, 216 disposed on opposite sides of a thermally conductive composite material, comprising coated BN filler, functioning as a dielectric substrate layer 214 comprising coated BN particles, preferably coated BN platelets. Dielectric substrate layer 214 can comprise a woven web (not shown).

[0125] FIG. 3 shows a circuit subassembly 310 comprising a circuit layer 318 and a conductive layer 316 disposed on opposite sides of a thermally conductive composite material, comprising coated BN particles, functioning as a dielectric substrate layer 314. Dielectric substrate layer 314 can also comprise a woven web (not shown).

[0126] In addition to circuit materials and circuit subsassemblies, the thermal interface composite materials can be used as a thermal interface in a heat management assembly. The thermal interface can be in a wide variety of two- or three-dimensional shapes depending on the use. Further shaping of the thermal interface composite material can comprise thermoforming, or compression rolling (pressing) an extruded sheet and then dividing the sheet into a plurality of individual thermally conductive units for a given application.

[0127] In an embodiment, the heat management assembly comprises a thermal interface material as described above, wherein the thermal interface is in contact with at least one external heat transfer surface to conduct heat away from the external heat transfer surface. In preferred embodiments the heat management assembly comprises a thermal interface having two heat transfer surfaces, a first heat transfer surface that is in association with a heat generating surface, preferably part of an electronic component, and a second heat transfer surface that is in association with a thermal dissipation element, for example, a heatsink for the heat generated by the electronic component. In some embodiments, a primer or pressure-sensitive (PSA) or other adhesive can be used to secure the thermal interface (in the form of a sheet or other applicable shape or surface configuration) in place between the first and second heat transfer surfaces.

[0128] The at least one transfer surface, or the first and the second heat transfer surfaces can be one or more components in a wide variety of electronic equipment or devices, inclusive of an LED, insulated-gate bipolar transistor (IBGT), integrated circuit, in consumer electronics such as cell phones, computer monitors, plasma TVs, automotive electronic components and systems, with circuit boards, card cages, vents, covers, PCMCIA cards, back or face planes, shielding caps or cans, to I/O connector panels of an electronic device, or of an enclosure or cabinet therefore. It will be appreciated that aspects can also find advantageous use in various other applications requiring a thermally conductive sheet material.

[0129] The following examples, which are meant to be exemplary, not limiting, illustrate compositions and methods of manufacturing of some of the various embodiments of the thermally conductive composite materials described herein.

EXAMPLES

[0130] Two testing devices are used to measure the thermal conductivity of samples, an Anter Unitherm™ 2022 (ASTM El 530) test apparatus, and a T.I.M. test apparatus by Analysis Tech (ASTM D5470). Both devices present thermal conductivity in W/m K. (It is noted that thermal conductivities of compressible materials vary with the pressure and gap during testing.) Both methods use three measurements, applying fixed pressure or a fixed gap to the material. Thermal conductivities and physical properties are obtained. Values in thermal conductivity here are based off the 20 psi measurement.

[0131] CFD is measured by calculating the force in pounds per square inch (psi) required to compress the sample to 25% of the original thickness in accordance with ASTM D1056-14. Compression set can be determined by measuring amount in percent by which a standard test piece of the foam fails to return to its original thickness after being subjected to a 50% compression for 22 hours at the specified room temperature.

[0132] Copper peel strength is tested in accordance with the "Peel strength of metallic clad laminates" test method (IPC-TM-650 2.4.8). The laminate is tested for solder float by floating them on a pot of molten solder at a temperature 288°C for 10 seconds. This procedure is repeated five times on each sample. A failure in the solder float test is noted if there is blistering or delamination of the copper foil from the laminate surface. EXAMPLE 1

[0133] Thermally conductive composite materials comprising at least one silicone composition (preferably, a gel or LSR) and coated BN particles are prepared.

[0134] Due to the high surface area of the BN particles, the viscosity can increase viscosity quickly. Viscosity of loaded mixes can peak over 200,000 cps. An organic solvent such as toluene can be added to decrease the viscosity; for example, a workable viscosity range of around 80,000 cps can be obtained. Viscosity is measured with a Brooks meter with #6 spindle and 2 rpm, 10 rpm and 25 rpm settings. Formulations are optimized for workable viscosity, maximum thermal conductivity, and compression force deflection. In general, the bulkier the BN particles, the less viscous the precursor polymer matrix composition.

[0135] The silicone compositions used in the experimental formulations are Dow Corning Sylgard™ 527, a two-part heat-cured soft gel with low viscosity; Momentive LIM 6010™, a two-part heat-cured LSR to provide toughness; and Nusil 213™, a two-part heat- cured firm gel with low viscosity. Two formulation examples are shown in the Tables 1 and 2 below. Formulation 1 used toluene to reduce formulation viscosity, and toluene was driven off during the curing process. Formulation 1 contained no additional solvent.

Table 1 (Formulation 1)

Table 2 (Formulation 2)

[0136] Part A and Part B of each formulation are first mixed separately using a high speed mixer (Flacktec™) until both were homogeneous. Parts A and B are then mixed together using the mixer.

[0137] Using an LC-100™ counter top lab coater, the BN particle-mixed thermally conductive silicone composition is cast at a controlled thickness on a polycarbonate carrier. The sample layers are then cured in an XP oven at 1 10°C for 10 minutes.

[0138] The thermally conductive materials exhibit effective thermal conductivity in the z-direction. EXAMPLE 2

[0139] A dielectric substrate formed from a composite material is prepared using the component materials in Table 3.

Table 3

[0140] A dielectric substrate is prepared using the formulation in Table 4 below.

Table 4

[0141] Conventional dielectric substrate manufacturing methods are employed.

Preferably, the composition of Table 4 is coated onto glass fabric and dried to make a 2-10 mil prepreg sheet. After the prepreg is plied up to a specific thickness, a stack is mace with adhesive- coated copper placed on both sides of the prepreg and laminated using heat and pressure to make a double clad laminate. The laminate is densified and cured via flat-bed lamination; typical cure temperature ranges are from 325°F (163°C) and 525°F (246°C) employing a pressure of 300 to 1200 psi.

[0142] The dielectric substrates exhibit improved thermal conductivity in the z-direction and comparatively superior cohesive strength and peel strength.

Example 3

[0143] First, 4 g h-BN of is added to 400 mL ethyl alcohol, and ultrasound is applied to disperse the mixed solution for about 20 minutes. Next, 30 mL ammonia and 62 mL deionized water is added to the solution to adjust the pH value to about 9. The solution is stirred with a magnetic stirrer and heated to about 50°C, then 9 mL of tetraethylorthosilicate is added dropwise into the mixed solution over 2 hours to form the silica coating layer on the surface of h-BN. The silica-coated h-BN can be obtained by vacuum infiltration and drying.

[0144] FIG. 4 and FIG. 5 are the SEM images of h-BN powders before the silica coating is made, and FIG. 6 and FIG. 7 is an SEM image of the silica-coated h-BN. FIG. 8 shows the Fourier transform infrared (FTIR) spectra of h-BN powders before coating with silica, and FIG. 9 shows the silica-coated h-BN powder.

[0145] The disclosure is further illustrated by the following embodiments, which are not intended to be limiting.

[0146] Embodiment 1 : A process for making a thermal interface composite material, the process comprising: combining a plurality of sol-gel coated boron nitride particles comprising boron nitride particles comprising an outer layer of inorganic silica, with a precursor polymer matrix composition, to form a mixture; forming the mixture into a shaped mixture; and hardening the shaped mixture to obtain the thermal interface composite material comprising the plurality of sol-gel coated boron nitride particles distributed in a polymer matrix, wherein the thermal interface composite material has a bulk thermal conductivity of at least 0.5 W/m-K, preferably 1.0 to 200 W/m-K.

[0147] Embodiment 2: The process of embodiment 1, comprising forming the outer layer of inorganic silica by immersing the boron nitride particles in a sol-gel precursor solution;

catalyzing a sol-gel reaction to coat silica onto a surface of the particles; and hardening the silica coating to form the plurality of sol-gel coated boron nitride particles, preferably by introducing the surface-coated boron nitride particles into a spray tower or fluidized bed to harden the silica coating.

[0148] Embodiment 3 : The process of embodiment 2, wherein the sol-gel reaction comprises reacting an alkyl orthosilicate in the presence of the boron nitride particles. [0149] Embodiment 4: The process of any one or more of embodiments 1 to 3, wherein the coated boron nitride particles are made by immersing boron nitride particles in a solution comprising alkyl orthosilicate dissolved in aqueous alcohol in the presence of a catalyst.

[0150] Embodiment 5 : The process of any one or more of embodiments 1 to 4, wherein the boron nitride particles have an average aspect ratio of from 1 :2 to 1 :200.

[0151] Embodiment 6: The process of any one or more of embodiments 1 to 5, wherein the coated boron nitride particles are made by introducing the particles, after the sol-gel reaction, into a fluidized bed or spray tower to dry and harden the coated particles.

[0152] Embodiment 7: The process of any one or more of embodiments 1 to 6, wherein the boron nitride particles are in the form of platelets, agglomerates, or a combination

comprising at least one of the foregoing.

[0153] Embodiment 8: The process of any one or more of embodiments 1 to 7, wherein the coated boron nitride particles, either primary or agglomerated secondary particles, have a mean diameter of 1 to 1000 micrometers, as determined by standard laser measurement.

[0154] Embodiment 9: The process of any one or more of embodiments 1 to 7, comprising 1 to 85 wt%, preferably 10 to 80 wt%, of the sol-gel coated boron nitride particles, based on the total weight of the thermal interface composite material.

[0155] Embodiment 10: The process of any one or more of embodiments 1 to 9, wherein the coated boron nitride particles are substantially non-agglomerated, wherein the median size of the coated boron nitride particles is less than five times the median size of the uncoated boron nitride particles.

[0156] Embodiment 1 1 : The process of embodiments any one or more of 1 to 10, further comprising foaming the precursor polymer matrix composition before or during hardening.

[0157] Embodiment 12: A thermal interface comprising a first and a second heat transfer surface, and further comprising the thermal interface composite material of any one or more of embodiments 1 to 11, wherein a bulk thermal conductivity of the thermal interface is at least 0.5 W/m-K.

[0158] Embodiment 13 : The thermal interface of embodiment 12, wherein the thermal interface is in the form of a sheet having substantially flat heat transfer surfaces, wherein the first and second heat transfer surfaces are substantially in an x-y plane and the sheet thickness is substantially perpendicular to the x-y plane.

[0159] Embodiment 14: The thermal interface of embodiment 12 or embodiment 13, wherein the average thickness of the thermal interface is 0.1 to 25 millimeters.

[0160] Embodiment 15: The thermal interface of any one or more of embodiments 12 to

14, wherein the polymer matrix comprises a polyurethane, silicone, polyolefin, polyester, polyamide, fluorinated polymer, polyalkylene oxide, polyvinyl alcohol, ionomer, cellulose acetate, polystyrene, or a combination comprising at least one of the foregoing, preferably a polyolefin, fluorinated polymer, polyurethane, or silicone as the sole or primary polymer by weight percent, most preferably a heat-cured silicone composition.

[0161] Embodiment 16: A thermal management assembly comprising the thermal interface of any one or more of embodiment 12 to 15, wherein a first side of the material is in contact with at least one external heat transfer surface.

[0162] Embodiment 17: The thermal management assembly of embodiment 16, wherein the thermal interface is disposed between a first adjacent external surface of a heat-generating member and second adjacent external surface of a heat-dissipative member to provide a thermally conductive pathway there between.

[0163] Embodiment 18: The thermal management assembly of embodiment 17, wherein the heat generating member is an electronic component or a circuit board, and the heat dissipative member is a heat sink or circuit board.

[0164] Embodiment 19: A dielectric substrate comprising the thermally conductive composite material of any one or more of embodiments 1 to 1 1, wherein the dielectric substrate has a bulk thermal conductivity of at least 0.5 W/m K, a UL-94 rating of at least V-1, and a Df of less than 0.006 at 10 GHz.

[0165] Embodiment 20: The dielectric substrate of embodiment 19, comprising 30 to 90 volume percent of the polymer matrix and 5 to 70 volume percent of the sol-gel coated boron nitride particles dispersed in the polymer matrix based on the total volume of the dielectric substrate.

[0166] Embodiment 21 : The dielectric substrate of any one or more of embodiments 19 to 20, wherein the dielectric substrate comprises 1,2-polybutadiene, polyisoprene,

polyetherimide, a fluoropolymer, polytetrafluoroethylene, polyphenylene ether, polyimide, polyetheretherketone, polyamidimide, polyethylene terephthalate, polyethylene naphthalate, polycyclohexylene terephthalate, or a combination comprising at least one of the foregoing, preferably wherein the dielectric substrate is polytetrafluoroethylene, 1,2-polybutadiene, polyisoprene, or a combination of 1,2-polybutadiene and polyisoprene.

[0167] Embodiment 22: The dielectric substrate of any one or more of embodiments 19 to 21 , wherein a conductive layer is adhered to the dielectric substrate, and wherein the conductive layer is optionally etched to form a circuit.

[0168] Embodiment 23 : The dielectric substrate of any one or more of embodiments 19 to 22, wherein the dielectric substrate layer comprises a woven or non-woven fibrous web. [0169] Embodiment 24: The dielectric substrate of any one of embodiments 19 to 23, wherein the thickness of the dielectric substrate layer is 50 to 1000 micrometers and wherein the composition further comprises fused silica, metal oxide particles, or a combination comprising at least one of the foregoing, to obtain a Dk of less than 3.8 at 10 GHz.

[0170] Embodiment 25: A circuit material or a circuit comprising the dielectric substrate of any one or more of embodiments 19 to 24.

[0171] Ranges disclosed herein are inclusive of the recited endpoint and combinable (e.g., ranges of "up to 25 wt%, or, more preferably, 5 to 20 wt%", is inclusive of the endpoints and all intermediate values of the ranges of "5 to 25 wt%", etc.). "Combination" is inclusive of blends, mixtures, alloys, reaction products, and the like. Also, "combinations comprising at least one of the foregoing" clarifies that the list is inclusive of each element individually, as well as combinations of two or more elements of the list, and combinations of one or more elements of the list with non-list elements. The terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. "Or" means "and/or". It is to be understood that the described elements can be combined in any suitable manner in the various embodiments.

[0172] As used herein, the terms sheet, film, plate, and layer, are used interchangeably, and are not intended to denote size. The term "disposed on" or "disposed between" means that the articles are adjacent each other and may or may not be in direct contact, provided that the thermally conductive pathway being established is not significantly adversely affected. For example, one or more of the articles can be treated with a primer or a thermally conductive adhesive. Further as used herein, the term "silica" is intended to include all oxides of silicon formed by a sol-gel process, including silicon dioxide.

[0173] All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference

[0174] While the disclosure has been described with reference to several embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure . In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof.

Therefore, it is intended that the disclosure not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A process for making a thermal interface composite material, the process comprising:

combining

a plurality of sol-gel coated boron nitride particles comprising boron nitride

particles comprising an outer layer of inorganic silica, with a precursor polymer matrix composition, to form a mixture;

forming the mixture into a shape; and

hardening the shaped mixture to obtain the thermal interface composite material comprising the plurality of sol-gel coated boron nitride particles distributed in a polymer matrix, wherein the thermal interface composite material has a bulk thermal conductivity of at least 0.5 W/m-K, preferably 1.0-200 W/m K.

2. The process of claim 1, comprising forming the outer layer of inorganic silica by immersing the boron nitride particles in a sol-gel precursor solution;

catalyzing a sol-gel reaction to coat silica onto a surface of the particles; and

hardening the silica coating to form the plurality of sol-gel coated boron nitride particles, preferably by introducing the surface-coated boron nitride particles into a spray tower or fluidized bed to harden the silica coating.

3. The process of claim 2, wherein the sol-gel reaction comprises reacting an alkyl orthosilicate in the presence of the boron nitride particles.

4. The process of any one or more of claims 1 to 3, wherein the boron nitride particles have an average aspect ratio of 1 :2 to 1 :200.

5. The process of any one or more of claims 1 to 4, wherein the boron nitride particles are in the form of platelets, agglomerates, or a combination comprising at least one of the foregoing.

6. The process of any one or more of claims 1 to 5, wherein the coated boron nitride particles, either primary or agglomerated secondary particles, have a mean diameter of 1 to 1000 micrometers, as determined by standard laser measurement.

7. The process of any one or more of claims 1 to 6, comprising 1 to 85 wt%, preferably 10 to 80 wt%, of the sol-gel coated boron nitride particles, based on the total weight of the thermal interface composite material.

8. The process of any one or more of claims 1 to 7, wherein the coated boron nitride particles are substantially non-agglomerated, wherein the median size of the coated boron nitride particles is less than five times the median size of the uncoated boron nitride particles.

9. The process of claims any one or more of 1 to 8, further comprising foaming the precursor polymer matrix composition before or during hardening.

10. A thermal interface comprising a first and a second heat transfer surface, and further comprising the thermal interface composite material of any one or more of claims 1 to 9, wherein a bulk thermal conductivity of the thermal interface is at least 0.5 W/m-K.

11. The thermal interface of claim 10, wherein the thermal interface is in the form of a sheet having substantially flat heat transfer surfaces, wherein the first and second heat transfer surfaces are substantially in an x-y plane and the sheet thickness is substantially perpendicular to the x-y plane.

12. The thermal interface of claim 10 or claim 11, wherein the average thickness of the thermal interface is 0.1 to 25 millimeters.

13. The thermal interface of any one or more of claims 10 to 12, wherein the polymer matrix comprises a polyurethane, silicone, polyolefin, polyester, polyamide, fluorinated polymer, polyalkylene oxide, polyvinyl alcohol, ionomer, cellulose acetate, polystyrene, or a combination comprising at least one of the foregoing, preferably a polyolefin, fluorinated polymer, polyurethane, or silicone as the sole or primary polymer by weight percent, most preferably a heat-cured silicone composition.

14. A thermal management assembly comprising the thermal interface of any one or more of claims 10 to 13 disposed between a first adjacent external surface of a heat-generating member and second adjacent external surface of a heat-dissipative member to provide a thermally conductive pathway there between.

15. The thermal management assembly of claim 14, wherein the heat generating member is an electronic component or a circuit board, and the heat dissipative member is a heat sink or circuit board.

16. A dielectric substrate comprising the thermally conductive composite material of any one or more of claims 1 to 9, wherein the dielectric substrate has a bulk thermal conductivity of at least 0.5 W/m K, a UL-94 rating of at least V-l, and a D_f of less than 0.006 at 10 GHz.

17. The dielectric substrate of claim 16, comprising 30 to 90 volume percent of the polymer matrix and 5 to 70 volume percent of the sol-gel coated boron nitride particles dispersed in the polymer matrix based on the total volume of the dielectric substrate.

18. The dielectric substrate of any one or more of claims 16 to 17, wherein the dielectric substrate comprises 1,2-polybutadiene, polyisoprene, polyetherimide, a fluoropolymer, polytetrafluoroethylene, polyphenylene ether, polyimide, polyetheretherketone, polyamidimide, polyethylene terephthalate, polyethylene naphthalate, polycyclohexylene terephthalate, or a combination comprising at least one of the foregoing, preferably wherein the dielectric substrate is polytetrafluoroethylene, 1,2-polybutadiene, polyisoprene, or a combination of 1,2- polybutadiene and polyisoprene.

19. The dielectric substrate of any one or more of claims 16 to 18, wherein a conductive layer is adhered to the dielectric substrate, and wherein the conductive layer is optionally etched to form a circuit.

20. The dielectric substrate of any one or more of claims 16 to 19, wherein the dielectric substrate layer comprises a woven or non-woven fibrous web.

21. A circuit material or a circuit comprising the dielectric substrate of any one or more of claims 16 to 20.