WO2018013624A1 - Isotropic boron nitride, method of manufacture thereof and articles made therefrom - Google Patents

Abstract

In an embodiment, a method of making isotropic boron nitride includes combining anisotropic boron nitride comprising an interstice with a supercritical fluid; penetrating the interstice with the supercritical fluid, expanding the interstice to form the isotropic boron nitride, and optionally repeating the combining, penetrating or expanding.

Description

ISOTROPIC BORON NITRIDE, METHOD OF MANUFACTURE THEREOF AND

ARTICLES MADE THEREFROM

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of Chinese Application No.

201610546816.3 filed July 12, 2016. The related application is incorporated herein in its entirety by reference.

BACKGROUND

[0002] This disclosure relates to isotropic boron nitride, methods of manufacture thereof and articles made therefrom.

[0003] Thermally conductive composites are used in a broad variety of applications, for example printed circuit boards. Printed circuit boards (PCBs) are used to mechanically support and electrically connect electronic components using conductive pathways laminated onto a non-conductive substrate. Some thermally conductive composites have included a boron nitride filler. Boron nitride is generally known to impart thermal conductivity to a variety of materials including polymers.

[0004] Boron nitride (BN) is a layered material that can exist in different lattice structures. Common lattice structures include a hexagonal, cubic, wurtzite, and turbostratic structures. The hexagonal lattice structure includes stacked layers of interconnected B-N hexagons. Key properties of hexagonal boron nitride include high thermal conductivity, low thermal expansion, good thermal shock resistance, high electrical resistance, low dielectric constant and loss tangent, microwave transparency, low toxicity, easy machinability, good lubricity, and chemical inertness.

[0005] Strong covalent bonding within the hexagonal boron nitride layers leads to high mechanical strength and very good thermal conductivity in-plane. On the other hand, adhesion between the boron nitride layers is weak. This weak bonding results in lower mechanical strength and thermal conductivity perpendicular to the plane, i.e., along the Z- axis. Consequently, these and other properties of hexagonal boron nitride are directionally dependent, i.e., anisotropic. In general, incorporation of anisotropic boron nitride into a substrate will result in composites with anisotropic thermal conductivity, for example very good thermal conductivity in-plane but lower conductivity along the Z- axis perpendicular to the plane. [0006] 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. The increasing power of such electronic components has resulted in increasing heat generation. Moreover, smaller electronic components are being densely packed into ever smaller spaces, resulting in more intense heat generation. At the same time, temperature- sensitive elements in an electronic device may need to be maintained within a prescribed operating temperature in order to avoid significant performance degradation or even system failure. Consequently, manufacturers are continuing to face the challenge of dissipating heat generated in electronic devices.

[0007] It is well known in the art that isotropic boron nitride possesses uniform thermal conductivity, electrical resistance, and other characteristics, which properties and characteristics are uniform when measurements are taken in any direction, whether longitudinally, horizontally, or through the thickness dimension of a sample of the product. These uniform characteristics e.g. thermal conductivity can be useful in addressing the demand for improving efficiency and heat dissipation in electronic devices.

[0008] Accordingly, there remains a need for isotropic boron nitride, methods of making isotropic boron nitride and articles containing isotropic boron nitride, and thermally conductive composites containing isotropic boron nitride.

BRIEF SUMMARY

[0009] A method of making isotropic boron nitride is achieved by combining anisotropic boron nitride including an interstice with a supercritical fluid; penetrating the interstice with the supercritical fluid; expanding the interstice to form the isotropic boron nitride; and optionally repeating the combining, penetrating, or expanding.

[0010] An isotropic boron nitride is achieved by the above-described method.

[0011] Further described is an article including the above-described isotropic boron nitride.

[0012] Described herein is also a thermally conductive composite including the above-described isotropic boron nitride, and articles comprising the composites.

[0013] The above described and other features are exemplified by the following Detailed Description. DETAILED DESCRIPTION

[0014] Generally, boron nitride includes boron and nitrogen atoms forming interconnected hexagons. Each hexagon includes three boron atoms and three nitrogen atoms. Boron and nitrogen alternate in the ring. Each of these atoms is trivalent and is covalently bonded to its neighbor. This arrangement results in a highly oriented material including interconnected hexagons. The most common grade of boron nitride is

manufactured by hot pressing. Hot-pressing is a uniaxial densification method. This uniaxial densification results in a platelet structure, making the boron nitride anisotropic. Usually, the crystal structure of hot-pressed boron nitride is hexagonal. The crystals are separated by interstices.

[0015] The inventors hereof have found that isotropic boron nitride is obtained by combining anisotropic boron nitride with a supercritical fluid. The combining allows the supercritical fluid to enter the interstices present between adjacent boron nitride crystals. These interstices are expanded after the supercritical fluid enters and also fills them. Without wishing to be bound by theory, it is believed that expanding the interstices in accordance with the methods herein disrupts the anisotropic platelet structure, thereby resulting in isotropic boron nitride. Expanding the interstices also expands the boron nitride itself, and the resultant expanded boron nitride is isotropic.

[0016] This isotropic boron nitride has the advantage of having uniform properties, i.e., in all three dimensions, for example one or more of uniform thermal conductivity, thermal expansion, thermal shock resistance, electrical resistance, dielectric constant, and loss tangent.

[0017] As used herein, the term "anisotropic" refers to an anisotropic characteristic or property having a different value when measured in a different direction, for example an x-, y-, or z-axis. An anisotropic property or characteristic is directionally dependent. As used herein, the term "isotropic" refers to for instance an anisotropic characteristic or property that has the same value when measured along an x-, y-, or z-axis. An isotropic property or characteristic is directionally independent. As used herein, the term "supercritical fluid" refers to any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, and where the critical point is the end point of the thermodynamic phase-equilibrium curve.

[0018] In the methods herein, the supercritical fluid is combined with anisotropic boron nitride, preferably hexagonal boron nitride. The anisotropic boron nitride can be in the shape of a flake. Preferably, the hexagonal anisotropic boron nitride is in the shape of a flake. In some applications, a fluid that is not a supercritical fluid is combined with anisotropic boron nitride, preferably hexagonal boron nitride. Combining this fluid with the anisotropic boron nitride allows the fluid to enter the interstices surrounding crystals of boron nitride. Once these interstices are filled with the fluid, the anisotropic boron nitride is exposed at conditions effective to convert the fluid into a supercritical fluid, i.e., a first pressure and a first temperature. The converting can be in any suitable vessel, for example an autoclave. As the supercritical fluid forms in the interstices, its volume expands and thereby also the interstices expand, resulting generally in expanding the boron nitride itself.

Expanded boron nitride has the advantage of being isotropic.

[0019] The first pressure can be 10 to 80 megapascals (MPa), or 15 to 60 MPa, or 25 to 40 MPa. The first temperature can be 100 to 600 degrees Celsius (°C), or 150 to 500°C, or 200 to 400°C. Upon decreasing the first pressure to a lower, second pressure, for example to atmospheric (for example 1 atm), or less than or equal to 0.1 MPa, or lowering the first temperature to a second temperature, for example 0 to 100°C, or 0 to 25°C, or 0 to 10°C, the supercritical fluid returns to the normal state, thereby re-forming the fluid. The steps of combining anisotropic boron nitride with the fluid, converting the fluid to the supercritical fluid, and re-forming the fluid from the supercritical fluid can be repeated to provide an isotropic boron nitride having a predetermined degree of expansion.

[0020] Generally, the supercritical fluid comprises an organic solvent. Exemplary organic solvents are benzyl benzoate; a C_1-8 alcohol, for example ethanol, isopropanol, or n- butanol; an N-(C_1-12 alkyl)-2-pyrrolidone, for example N-methyl-2-pyrrolidone, N-vinyl-2- pyrrolidone, N-octyl-2-pyrrolidone, or l-lauryl-2-pyrrolidone; γ-butyrolactone;

dimethylacetamide; l,3-dimethyl-2-imidazolidinone; dimethylformamide; dimethylsulfoxide; acetone; or a combination comprising at least one of the foregoing.

[0021] In some embodiments, the supercritical fluid includes water. For example, the amount of the water present in the supercritical fluid can be 0.1 to 15 weight percent, or 1 to 10 weight percent, or 1 to 5 weight percent, based on weight of the supercritical fluid.

[0022] Optionally, the combining of the anisotropic boron nitride with the

supercritical fluid, the penetrating the interstice with the supercritical fluid, or the expanding the interstice is repeated 1 to 15 times, or 2 to 10 times, or 5 to 10 times.

[0023] In a further step, the supercritical fluid can be removed from the isotropic boron nitride. Usually, supercritical fluid is removed from the isotropic boron nitride at conditions effective for removal of supercritical fluid, for example at a temperature of 60 to 250°C, or 80 to 150°C, or 80 to 120°C; optionally under vacuum or using sonication. [0024] The isotropic boron nitride provided by the methods herein is a single layer isotropic boron nitride, for example a single layer isotropic hexagonal boron nitride or an expanded single layer isotropic hexagonal boron nitride. Generally, the isotropic boron nitride is thermally conductive. The isotropic boron nitride can have a thermal conductivity of 1 to 2,000 watts per meter-Kelvin (W/m K), or 10 to 1,800 W/m K, or 100 to 1,600 W/m K, or 1,500 to 2,000 W/m K, or 50 to 1,500 W/m K, or 100 to 1,000 W/m K according to ASTM E1225-13. The isotropic boron nitride can also have an electrical resistivity of 5 to 15 ohm-centimeters (Ω-cm) at room temperature (for example 25°C), or 8 to 12 Ω-cm, a dielectric constant of 3 to 4, for example 3.01 to 3.36 at room temperature at 5.75 x 10⁹ hertz (Hz), and a loss tangent of 0.0001 to 0.001, or 0.0003 to 0.0008 at room temperature at 5.75 x 10⁹ Hz.

[0025] The isotropic boron nitride can be crystalline, polycrystalline, amorphous, or a combination thereof, and is in form of a flake, a fiber, a crystal, a powder, a nanofiber, a nanotube, a nanoplate, or a combination thereof. Isotropic boron nitride in the form of a flake or a nanotube is specifically mentioned. The nanofiber can be solid. The nanotube can have one wall or can be multiwalled, and can have a hollow core.

[0026] The isotropic boron nitride can have any suitable dimensions. The isotropic boron nitride can have an average largest dimension of 1 nanometer (nm) to 1,000 micrometers (μπι), or 20 nm to 80 μπι, or 50 nm to 1 μπι. In some embodiments, the isotropic boron nitride is a nanoparticulate having an average largest dimension of 1 to 100 nm, or 2 to 50 nm. The isotropic boron nitride can have an aspect ratio, calculated as a largest dimension/cross-sectional dimension of 2 to 1,000,000, or 50 to 50,000, or 100 to 1,000. The cross-sectional dimension can be a diameter of a fiber or tube, or a thickness of a plate. The average largest dimension and the cross-sectional dimension can be determined using image analysis of a plurality of particles, for example by determining the respective length scales of 10 to 100 particles and calculating the average. An average particle size of the isotropic boron nitride, as measured by laser light scattering for example, can be 10 nm to 1000 micrometers (μπι), or 20 nm to 500 μπι, or 40 nm to 250 μπι.

[0027] The isotropic boron nitride can be undoped or doped to provide a desired property. For example, the isotropic boron nitride can be doped to increase the dielectric properties of the polymer, for example with silver, carbon, or fluorine. Alternatively, the isotropic boron nitride can be doped to provide an n-doped or a p-doped boron nitride. The doped boron nitride can comprise an element effective to provide an isotropic boron nitride having semiconducting properties. Representative dopants include carbon, oxygen, sulfur, a halogen (for example F), a transition metal (for example Ag, Zr, or Ti), or a metalloid (for example Si, Ge, As, Sb, or Te). A combination comprising at least one of the foregoing dopants can be used. A content of the dopant can be 0.001 to 20 wt%, or 0.01 to 10 wt%, or 0.1 to 1 wt%, based on a total weight of the isotropic boron nitride. Representative doped boron nitrides include Si doped BN comprising 0.1 to 10 wt% Si based on the total weight of the boron nitride.

[0028] The isotropic boron nitride can be contained in the composite in an amount sufficient to provide the composite suitable thermal conductivity, dielectric constant, and mechanical properties. Usually, the isotropic boron nitride is contained in the composite in an amount of 0.1 to 90 weight percent (wt%), or 1 to 85 wt%, or 5 to 80 wt%, based on a total weight of the composite. The composite can have a thermal conductivity of 1 W/m K or more, or of 2 W/m K or more, or 4 W/m K or more, or 1 to 50 W/m K measured according to ASTM E1225-13. The composite can have a dielectric constant of 1.5 to 15, or 3 to 12, or 4 to 10, measured for example at room temperature at 5.75 x 10⁹ Hz. The composite can have a coefficient of thermal expansion of 1 to 50 parts per million per degree Celsius (ppm/°C), or 2 to 40 ppm/°C, or 4 to 30 ppm/°C.

[0029] The composite can comprise any polymer suitable for the intended end use. Examples of thermoplastic polymers that can be used include polyacetals (for example polyoxy ethylene and polyoxymethylene), poly(Ci-6 alkyl)acrylates, polyacrylamides

(including unsubstituted and mono-N- and di-N-(Ci-8 alkyl)acrylamides), polyacrylonitriles, polyamides (for example aliphatic polyamides, polyphthalamides, and polyaramides), polyamideimides, polyanhydrides, polyarylene ethers (for example polyphenylene ethers), polyarylene ether ketones (for example polyether ether ketones (PEEK) and polyether ketone ketones (PEKK), polyarylene ketones, polyarylene sulfides such as polyphenylene sulfides (PPS), and polyarylene sulfones such as polyethersulfones (PES) and polyphenylene sulfones (PPS)), polybenzothiazoles, polybenzoxazoles, polybenzimidazoles, polycarbonates

(including homopoly carbonates and polycarbonate copolymers such as polycarbonate- siloxanes, polycarbonate-esters, and polycarbonate-ester-siloxanes), polyesters (for example polyethylene terephthalates, polybutylene terephthalates, polyarylates, and polyester copolymers such as polyester-ethers), polyetherimides (including copolymers such as polyetherimide-siloxane copolymers), polyimides (including copolymers such as polyimide- siloxane copolymers), poly(Ci-6 alkyl) methacrylates, polymethacrylamides (including unsubstituted and mono-N- and di-N-(Ci-8 alkyl)acrylamides), cyclic olefin polymers (including polynorbornenes and copolymers containing norbornenyl units, for example copolymers of a cyclic polymer such as norbornene and an acyclic olefin such as ethylene or propylene), polyolefins (for example polyethylenes, polypropylenes, and their halogenated derivatives such as polytetrafluoroethylenes), and their copolymers, for example ethylene- alpha-olefin copolymers, polyoxadiazoles, polyoxymethylenes, polyphthalides, polysilazanes, polysiloxanes (silicones), polystyrenes (including polystyrene copolymers such as

acrylonitrile-butadiene-styrene (ABS) and methyl methacrylate-butadiene-styrene (MBS)), polysulfides, polysulfonamides, polysulfonates, polysulfones, polythioesters, polytriazines, polyureas, polyurethanes, vinyl polymers (including polyvinyl alcohols, polyvinyl esters, polyvinyl ethers, and polyvinyl halides (for example polyvinyl fluoride), polyvinyl ketones, polyvinyl nitriles, polyvinyl thioethers, and polyvinylidene fluorides). A combination comprising at least one of the foregoing thermoplastic polymers can be used.

[0030] Thermoset polymers can be used. Thermoset polymers are derived from thermosetting prepolymers (resins) that can irreversibly harden and become insoluble with polymerization or cure, which can be induced by heat or exposure to radiation (for example ultraviolet light, visible light, infrared light, or electron beam (e-beam) radiation). Thermoset polymers include alkyds, bismaleimide polymers, bismaleimide triazine polymers, cyanate ester polymers, benzocyclobutene polymers, diallyl phthalate polymers, epoxies,

hydroxymethylfuran polymers, melamine-formaldehyde polymers, phenolics (for example phenol-formaldehyde polymers such as novolacs and resoles), benzoxazines, polydienes such as polybutadienes (including homopolymers and copolymers thereof, such as poly(butadiene- isoprene)), polyisocyanates, polyureas, polyurethanes, silicones, triallyl cyanurate polymers, triallyl isocyanurate polymers, polyimides, certain silicones, and copolymerizable

prepolymers (for example prepolymers having ethylenic unsaturation, such as unsaturated polyesters or unsaturated polyimides). The prepolymers can be copolymerized or crosslinked with a reactive monomer, for example styrene, alpha-methylstyrene, vinyltoluene, chlorostyrene, acrylic acid, (meth)acrylic acid, a (Ci-6 alkyl)acrylate, a (Ci-6 alkyl)

methacrylate, acrylonitrile, vinyl acetate, allyl acetate, triallyl cyanurate, triallyl isocyanurate, acrylamide, or a combination comprising at least one of the foregoing. The weight average molecular weight of the prepolymers can be 400 to 10,000 Daltons based on polystyrene standards.

[0031] In an embodiment, the polymer is a dielectric polymer suitable for use in electronic materials, for example a polyimide, a polyphenylsulfone, a polyethersulfone, a polytetrafluoroethylene, a poly(arylene ether), or an epoxy. The polymer can be a

homopolymer or a copolymer, and can comprise a graft or a block copolymer. The polymer can be crosslinked. A useful poly(arylene ether) comprises 2,6-dimethyl-l,4-phenylene ether units, optionally in combination with 2,3,6-trimethyl-l,4-phenylene ether units. The polymer can be functionalized. PPE-MA from Asahi (a maleinized poly(arylene ether)), and Blendex HPP820 from Chemtura Corporation (Philadelphia, PA) (an unmodified poly(arylene ether)) are representative.

[0032] In an embodiment, the polymer can be a polybutadiene or a polyisoprene suitable for use in circuit materials. A "polybutadiene or polyisoprene polymer" as used herein includes homopolymers derived from butadiene, homopolymers derived from isoprene, and copolymers derived from butadiene or isoprene or less than 50 wt% of a monomer co-curable with the butadiene or isoprene. Suitable monomers co-curable with butadiene or isoprene include monoethylenically unsaturated compounds, for example acrylonitrile, ethacrylonitrile, methacrylonitrile, alpha-chloroacrylonitrile, Ci-6 alkyl

(meth)acrylates, acrylamide, methacryl amide, maleimide, N-methyl maleimide, N-ethyl maleimide, itaconic acid, (meth)acrylic acid, and combinations comprising at least one of the foregoing monoethylenically unsaturated monomers. The polybutadiene or polyisoprene can further comprise an elastomeric block copolymer and optionally other components. Amounts and components of polybutadiene or polyisoprene compositions that can be used are described, for example, in US 5571609; US 585887; US 5982811; US6048807; and

US6071836.

[0033] A suitable dielectric polymer can conform to one or more industry

specifications for PCBs. For example, the dielectric polymer can conform to IPC-4104B/21 specifications, or the relevant industry specifications for the particular PCB being

manufactured.

[0034] The polymer can be solid or in the form of a foam. Polymers that can be formulated to provide foams include polyolefins, fluoropolymers, polyimides,

polyarylketones, polyarylether ketones, silicones, and polyurethanes.

[0035] The composite can further comprise an additional filler, for example a filler to adjust the dielectric properties of the composite. A low coefficient of expansion filler, for example glass beads, silica, or ground micro-glass fibers, can be used. A thermally stable fiber, for example an aromatic polyamide or a polyacrylonitrile, can be used. Representative fillers include titanium dioxide (rutile and anatase), barium titanate (BaTi0 ), strontium titanate, fused amorphous silica, corundum, wollastonite, aramide fibers (for example KEVLAR™ from DuPont), aiT Ow, fiberglass, quartz, aluminum nitride, silicon carbide, beryllia, alumina, magnesia, mica, talcs, nanoclays, aluminosilicates (natural and synthetic), and fumed silicon dioxide (for example Cab-O-Sil, available from Cabot Corporation), each of which can be used alone or in combination.

[0036] The fillers can be in the form of solid, porous, or hollow particles. The particle size of the filler affects a number of important properties including coefficient of thermal expansion, modulus, elongation, and flame resistance. In an embodiment, the filler has an average particle size of 0.1 to 15 micrometers, or 0.2 to 10 micrometers. A

combination of fillers having a bimodal, trimodal, or higher average particle size distribution can be used. The filler can be included in an amount of 0.1 to 80 wt%, or 1 to 65 wt%, or 5 to 50 wt%, based on a total weight of the composite.

[0037] To improve adhesion between the isotropic boron nitride, the filler if present, and the polymer, a coupling agent, for example a silane, a zirconate, or a titanate can be used. The isotropic boron nitride and the filler, if present, can be pretreated, or the coupling agent can be included in the polymer. The coupling agent, when present, can be present in amounts of 0.01 to 2.0 wt%, based on the total weight of the composite.

[0038] The composition used to form the composite or the composite can further optionally comprise additives, for example cure initiators, crosslinking agents, viscosity modifiers, wetting agents, flame retardants, and antioxidants. The particular choice of additive depends upon the particular application of the composite, and the desired properties for that application, and are selected so as to enhance or not substantially adversely affect the electrical properties of the circuit subassemblies, for example thermal conductivity, dielectric constant, dissipation factor, dielectric loss, or other desired properties.

[0039] Exemplary cure initiators include those useful in initiating cure (cross-linking) of the polymers, in the composite. Examples include, but are not limited to, azides, peroxides, sulfur, and sulfur derivatives. Free radical initiators are especially desirable as cure initiators. Examples of free radical initiators include peroxides, hydroperoxides, and non-peroxide initiators, for example 2,3-dimethyl-2, 3-diphenyl butane. Examples of peroxide curing agents include dicumyl peroxide, alpha,alpha-di(t-butylperoxy)-m,p- diisopropylbenzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane-3, and 2,5-dimethyl-2,5-di(t- butylperoxy)hexyne-3. The cure initiator, when used, can be present in an amount of 0.01 to 5 wt%, based on the total weight of the composite.

[0040] Crosslinking agents are reactive monomers or polymers that increase the cross-link density upon cure of the dielectric material. In some embodiments, such reactive monomers or polymers are capable of co-reacting with the polymer in the composite.

Examples of suitable reactive monomers include styrene, divinyl benzene, vinyl toluene, divinyl benzene, triallylcyanurate, diallylphthalate, and multifunctional acrylate monomers (for example Sartomer compounds available from Sartomer Co.), among others, all of which are commercially available. Useful amounts of crosslinking agents are 0.1 to 50 wt%, based on the total weight of the composite.

[0041] Exemplary antioxidants include radical scavengers and metal deactivators. A non-limiting example of a free radical scavenger is poly[[6-(l, l,3,3-tetramethylbutyl)amino- s-triazine-2,4-dyil][(2,2,6,6,-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6- tetramethyl-4-piperidyl)imino]], commercially available from BASF AG under the tradename Chimassorb 944. A non-limiting example of a metal deactivator is 2,2-oxalyldiamido bis[ethyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] commercially available from

Chemtura Corp. (Middlebury, CT) under the tradename Naugard XL-1. A single antioxidant or a combination of two or more antioxidants can be used. Antioxidants are typically present in amounts of up to 3 wt%, or 0.5 to 2 wt%, based on the total weight of the composite.

[0042] Coupling agents can be present to promote the formation of or participate in covalent bonds connecting a metal surface or filler surface with a polymer. Exemplary coupling agents include 3-mercaptopropylmethyldimethoxy silane, 3- mercaptopropyltrimethoxy silane, and hexamethylenedisilazane.

[0043] Representative flame retardant additives include bromine, phosphorus, and metal oxide containing flame retardants. Suitable bromine containing flame retardants are commercially available from, for example Albemarle Corporation under trade names Saytex BT-93W (ethylenebistetrabromonaphthalamide), Saytex 120

(tetradecabromodiphenoxybenzene), and Chemtura Corp. under trade name BC-52, BC-58, Esschem Inc under the trade name FR1025. Suitable phosphorus containing flame retardants include various organic phosphorous-based compounds, for example an aromatic phosphate of the formula (GO) P=0, wherein each G is independently an Ci- 6 alkyl, cycloalkyl, aryl, alkylarylene, or arylalkylene group, provided that at least one G is an aromatic group. Two of the G groups can be joined together to provide a cyclic group, as for example in diphenyl pentaerythritol diphosphate. Other suitable aromatic phosphates can be, for example phenyl bis(dodecyl) phosphate, phenyl bis(neopentyl) phosphate, phenyl bis(3,5,5'-trimethylhexyl) phosphate, ethyl diphenyl phosphate, 2-ethylhexyl di(p-tolyl) phosphate, bis(2-ethylhexyl) p- tolyl phosphate, tritolyl phosphate, bis(2-ethylhexyl) phenyl phosphate, tri(nonylphenyl) phosphate, bis(dodecyl) p-tolyl phosphate, dibutyl phenyl phosphate, 2-chloroethyl diphenyl phosphate, p-tolyl bis(2,5,5'-trimethylhexyl) phosphate, and 2-ethylhexyl diphenyl phosphate. A specific aromatic phosphate is one in which each G is aromatic, for example triphenyl phosphate, tricresyl phosphate, isopropylated triphenyl phosphate, and the like. Examples of suitable di- or polyfunctional aromatic phosphorous-containing compounds include resorcinol tetraphenyl diphosphate (RDP), the bis(diphenyl) phosphate of

hydroquinone and the bis(diphenyl) phosphate of bisphenol-A, respectively, and their oligomeric and polymeric counterparts. Metal phosphinate salts can also be used. Examples of phosphinates are phosphinate salts, for example alicyclic phosphinate salts and

phosphinate esters. Further examples of phosphinates are diphosphinic acids,

dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, and the salts of these acids, for example the aluminum salts and the zinc salts. Examples of phosphine oxides are isobutylbis(hydroxyalkyl) phosphine oxide and l,4-diisobutylene-2,3,5,6-tetrahydroxy- 1,4-diphosphine oxide or l,4-diisobutylene-l,4-diphosphoryl-2,3,5,6- tetrahydroxycyclohexane. Further examples of phosphorous-containing compounds are

H1197™ (Chemtura), H1511™ (Chemtura), NcendX™ P-30 (Albemarle), Hostaflam™ OP5500 (Clariant), Hostaflam™ OP910 (Clariant), EXOLIT™ 935 (Clariant), and

Cyagard™ RF 1204, Cyagard™ RF 1241 and Cyagard™ RF 1243R (Cyagard™ are products of Cytec Solvay Group). In a particularly advantageous embodiment, a halogen-free composite has excellent flame retardance when used with EXOLIT™ 935 (an aluminum phosphinate). Still other flame retardants include melamine polyphosphate, melamine cyanurate, Melam, Melon, Melem, guanidines, phosphazanes, silazanes, DOPO (9, 10- dihydro-9-oxa-10 phosphenathrene-10-oxide), and DOPO (10-5 dihydroxyphenyl, 10-H-9 oxaphosphaphenanthrenelo-oxide). Suitable metal oxide flame retardants are magnesium hydroxide, aluminum hydroxide, zinc stannate, and boron oxide.

[0044] The composite can be manufactured by combining the polymer or prepolymer composition, the isotropic boron nitride, and any additives to manufacture the thermally conductive composite. The combining can be by any suitable method, for example blending, mixing, or stirring. In an embodiment, the components used to form the composite, including the polymer or prepolymer composition and the multiphase boron nitride composition, can be combined by being dissolved or suspended in a solvent to provide a coating mixture or solution. The solvent is selected so as to dissolve the polymer or pre-polymers, disperse the isotropic boron nitride and any other optional additives that can be present, and to have a convenient evaporation rate for forming and drying. A non-exclusive list of possible solvents is xylene, toluene, methyl ethyl ketone, methyl isobutyl ketone, hexane, and higher liquid linear alkanes (for example heptane, octane, and nonane), cyclohexane, isophorone, and various terpene-based solvents. Specific exemplary solvents include xylene, toluene, methyl ethyl ketone, methyl isobutyl ketone, and hexane, specifically xylene and toluene. The concentration of the components of the composition in the solution or dispersion is not critical and will depend on the solubility of the components, the filler level used, the method of application, and other factors. In general, the solution comprises 10 to 50 wt% solids (all components other than the solvent), or 15 to 40 wt% solids, based on the total weight of the solution.

[0045] The composition can be formed into an article by known methods, for example extruding, molding, or casting. For example, the composition can be formed into a layer by casing onto a carrier from which it is later released, or alternatively onto a substrate, for example a conductive metal layer that will later be formed into a layer of a circuit structure. Where a foam is formed, the composition can be foamed by methods known in the art, for example by mechanical frothing, and molding to form an article or casting to form a layer. Alternatively, the composition can be foamed by chemical or physical blowing agents and formed into an article before or after foaming.

[0046] After the article or layer is formed, any solvent is allowed to evaporate under ambient conditions, or by forced or heated air, to form the composite. The layer can be uncured or partially cured (B-staged) in the drying process, or the layer can be partially or fully cured, if desired, after drying. The layer can be heated, for example at 20 to 200°C, or 30 to 150°C, or 40 to 100°C. The resulting composite can be stored prior to use in a process, for example the process can comprise laminating (for example to a foam), partially curing, and storing; or laminating, fully curing, and then storing.

[0047] The composite can have a dissipation factor of less than or equal to 0.02 measured at 10 gigahertz; or less than or equal to 0.01 measured at 10 gigahertz; or less than or equal to 0.005 measured at 10 gigahertz.

[0048] The composite can have a relatively low modulus and high elongation. These properties are particularly helpful in the reliability of copper interconnects to prevent excessive stress from being imparted on the walls of copper plated vias as the parts are thermally cycled. In some embodiments, the tensile modulus of the composite is less than 3,000 megapascals (MPa), or 100 to 3,000 MPa, or 200 to 2,500 MPa. In another embodiment, the elongation at break of the composite is greater than or equal to 5%.

Depending on the thickness, the tensile properties can be determined in accordance with ASTM D882-12 or ASTM D638-14.

[0049] The composite can further have low moisture absorption, which results in a substrate that is less sensitive to environmental conditions both in use and during storage. In an embodiment, the moisture absorption is 0.05 to 0.3 wt% at 24 hours immersion in water at 23°C.

[0050] The thickness of the composite layer will depend on its intended use. In an embodiment, the thickness of the composite is 5 to 1,000 micrometers, or 5 to 500 micrometers, or 5 to 400 micrometers. In another embodiment, when used as a dielectric substrate layer, the thickness of the composite is 25 to 400 micrometers, or 50 micrometers to 200 micrometers, and or 75 micrometers to 150 micrometers.

[0051] The composite can be used in a variety of applications, for example in circuit materials. 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 substrate layers, free films, and cover films. Circuit subassemblies include circuit laminates having a conductive layer, for example copper, fixedly attached to a dielectric substrate 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.

[0052] In particular, the composite can be used a buildup layer, a bonding layer, a dielectric substrate layer, or a combination comprising at least one of the foregoing. Useful conductive layers for the formation of the circuit materials circuit laminates can include, without limitation, stainless steel, copper, gold, silver, aluminum, zinc, tin, lead, transition metals, and alloys comprising at least one of the foregoing, with copper being exemplary. Suitable conductive layers include a thin layer of a conductive metal such as any copper foil presently used in the formation of circuits, for example an electrodeposited copper foil.

[0053] In an embodiment, the laminate is formed by placing one or more layers of the dielectric 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 with or without an intervening layer.

Alternatively, an adhesive or bond ply layer can be located between the conductive layer and the dielectric layer. The bond ply layer can be less than 10 percent of the thickness of the dielectric layer.

[0054] The layered material can then be placed in a press, for example 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 if applicable) can be by a one-step process, for example using a vacuum press, or by a multiple-step process. In an exemplary one-step process, for a PTFE polymer matrix, for example the layered material can be placed in a press, brought up to laminating pressure (for example 150 to 400 pounds per square inch (psi)) and heated to laminating temperature (for example 260 to 390°C). The laminating temperature and pressure are maintained for the desired soak time, for example 10 to 30 minutes, and thereafter cooled (while still under pressure) to below 150°C.

[0055] In an exemplary multiple-step process involving cure, for example a partial peroxide cure step at 150°C to 200°C can be conducted, and the partially cured stack can then be subjected to a high-energy electron beam irradiation cure (E-beam cure) or a high temperature cure step under an inert atmosphere. 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 can be, for example 250 to 300°C, or less than or equal to the decomposition temperature of the polymer. 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.

[0056] The methods herein provide isotropic boron nitride that has uniform properties, including thermal conductivity and improved heat dissipation. Advantageously, this combination of uniform thermal conductivity and high heat dissipation is useful in electronic devices that generate higher heat, for example chips that run at high clock speeds, high power, larger sized chips, or optoelectronic components with heat generating laser diodes. The composite can be used in a variety of circuit subassemblies, and can be used as a buildup layer, as a dielectric substrate layer in a multilayer core, or a combination thereof.

[0057] The present disclosure is further illustrated by the following non-limiting examples. PROPHETIC EXAMPLES

Example 1.

[0058] h-Boron nitride (2 grams (g)) is combined with dimethylformamide (DMF, 10 g) in an autoclave. The mixture is exposed at a first pressure and a first temperature effective for converting DMF to supercritical DMF. After 1 hour the temperature is lowered to room temperature (25°C) and the pressure is relieved to atmospheric, whereby natural-state DMF re-forms from supercritical DMF. The steps of converting natural-state DMF to supercritical DMF and re-forming natural-state DMF from supercritical DMF are each repeated five times before the solid is filtered off and dried in vacuo at 80°C. The dry solid has the desirable properties associated with isotropic boron nitride.

Example 2.

[0059] Poly(phenylene ether) (Noryl 640-111, 1 g), butadiene-styrene copolymer (Ricon 184 MA-6, 1 g), a curing agent (Perkadox, 0.5 g), and the boron nitride of Example 1 (1 g), are combined in xylene and then cast to form a layer, and the layer dried to form a composite. The composite has desirable thermal conductivity, dielectric properties, and mechanical properties.

[0060] The disclosure is further illustrated by the following non-limiting

embodiments.

[0061] Embodiment 1 : A method of making isotropic boron nitride, the method comprising: combining anisotropic boron nitride comprising an interstice with a supercritical fluid; penetrating the interstice with the supercritical fluid; expanding the interstice to form the isotropic boron nitride; and optionally repeating the combining, penetrating or expanding.

[0062] Embodiment 2: The method of embodiment 1 further comprising: first combining the anisotropic boron nitride with a fluid that is not a supercritical fluid;

converting the fluid to the supercritical fluid at a first pressure and a first temperature; reforming the fluid from the supercritical fluid at a second pressure and a second temperature; and optionally repeating the first combining, converting or re-forming.

[0063] Embodiment 3 : The method of embodiment 2, wherein the first pressure is 10 to 80 MPa, or preferably 15 to 60 MPa, or preferably 25 to 40 MPa.

[0064] Embodiment 4: The method of embodiment 2 or 3, wherein the first temperature is 100 to 600°C, or preferably 150 to 500°C, or preferably 200 to 400°C. [0065] Embodiment 5 : The method of any one or more of embodiments 2 to 4, wherein the second temperature is 0 to 100°C, or preferably 0 to 25°C, or preferably 0 to 10°C; and the second pressure is atmospheric pressure.

[0066] Embodiment 6: The method of any one or more of embodiments 1 to 5, wherein the supercritical fluid comprises an organic solvent. The organic solvent preferably comprises benzyl benzoate; a C_1-8 alcohol, preferably ethanol, isopropanol, or n-butanol; an N-(Ci-i2 alkyl)-2-pyrrolidone, preferably N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, N- octyl-2-pyrrolidone, or l-lauryl-2-pyrrolidone; γ-butyrolactone; dimethylacetamide; 1,3- dimethyl-2-imidazolidinone; dimethylformamide; dimethylsulfoxide; acetone; or a combination comprising at least one of the foregoing.

[0067] Embodiment 7: The method of any one or more of embodiments 1 to 6, wherein the supercritical fluid comprises water.

[0068] Embodiment 8: The method of any one or more of embodiments 1 to 7, wherein the combining, penetrating and expanding is repeated 1 to 15 times, or preferably 2 to 10 times, or preferably 5 to 10 times.

[0069] Embodiment 9: The method of any one or more of embodiments 1 to 8, wherein the anisotropic boron nitride is a hexagonal boron nitride, or preferably a natural flake hexagonal boron nitride.

[0070] Embodiment 10: The method of any one or more of embodiments 1 to 9, wherein the isotropic boron nitride is a single layer isotropic boron nitride, or preferably a single layer isotropic hexagonal boron nitride, or preferably an expanded single layer isotropic hexagonal boron nitride.

[0071] Embodiment 1 1 : The method of any one or more of embodiments 1 to 10 further comprising: removing the supercritical fluid from the isotropic boron nitride, preferably by heating at a removal temperature or by sonication.

[0072] Embodiment 12: The method of any one or more of embodiments 1 to 1 1, wherein the isotropic boron nitride has one or more of: a thermal conductivity of 1 to 2,000 W/m-K according to ASTM A1225, an electrical resistivity at room temperature of 5 to 15 Ω-cm, a dielectric constant of 3.01 to 3.36 at room temperature, and a loss tangent of 0.0001 to 0.001 at room temperature.

[0073] Embodiment 13 : An isotropic boron nitride made by the method of any one or more of embodiments 1 to 12.

[0074] Embodiment 14: The isotropic boron nitride of embodiment 13, wherein the isotropic boron nitride is a single layer isotropic boron nitride, or preferably a single layer isotropic hexagonal boron nitride, or preferably an expanded single layer isotropic hexagonal boron nitride.

[0075] Embodiment 15: An article comprising the isotropic boron nitride of any one or more of embodiments 1 to 14.

[0076] Embodiment 16: The article of embodiment 15 being a circuit material, integrated circuit package, printed circuit board, or thermal insulation component.

[0077] Embodiment 17: A composite comprising a thermoset polymer, a

thermoplastic polymer, or a combination comprising at least one of the foregoing; and the isotropic boron nitride of any one or more of embodiments 1 to 14.

[0078] Embodiment 18: The composite of embodiment 17, wherein the composite is in the shape of a fiber, a granule, or a film.

[0079] Embodiment 19: An article comprising the composite of embodiment 17.

[0080] Embodiment 20: The article of embodiment 19, wherein the article is a circuit material, integrated circuit package, printed circuit board, or thermal insulation component.

[0081] Embodiment 21 : The isotropic boron nitride of any one or more of the foregoing embodiments, wherein the isotropic boron nitride has one or more of an average largest dimension of 1 nm to 1,000 μπι, or preferably 20 nm to 80 μπι, or preferably 50 nm to 1 μπι; a nanoparticulate average largest dimension of 1 to 100 nm, or preferably 2 to 50 nm; an aspect ratio, calculated as a largest dimension/cross-sectional dimension of 2 to 1,000,000, or preferably 50 to 50,000, or preferably 100 to 1,000; and an average particle size of as measured by laser light scattering of 10 nm to 1,000 μπι, or preferably 20 nm to 500 μπι, or preferably 40 nm to 250 μπι.

[0082] In general, the articles and methods described here can alternatively comprise, consist of, or consist essentially of, any components or steps herein disclosed. The articles and methods can additionally, or alternatively, be manufactured or conducted so as to be devoid, or substantially free, of any ingredients, steps, or components not necessary to the achievement of the function or objectives of the present claims.

[0083] "Alkyl" as used herein means a straight or branched chain saturated aliphatic hydrocarbon having the specified number of carbon atoms, specifically 1 to 12 carbon atoms, more specifically 1 to 6 carbon atoms.

[0084] "Aryl" means a cyclic moiety in which all ring members are carbon and at least one ring is aromatic, the moiety having the specified number of carbon atoms, specifically 6 to 24 carbon atoms, more specifically 6 to 12 carbon atoms. More than one ring can be present, and any additional rings can be independently aromatic, saturated or partially unsaturated, and can be fused, pendant, spirocyclic or a combination thereof.

[0085] "Transition metal" as used herein refers to an element of Groups 3 to 11 of the Periodic Table of the Elements.

[0086] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. "Or" means "and/or." Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this disclosure belongs. A "combination" is inclusive of blends, mixtures, alloys, reaction products, and the like. The values described herein are inclusive of an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, for example the limitations of the measurement system. The endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of "less than or equal to 25 wt%, or 5 wt% to 20 wt%," is inclusive of the endpoints and all intermediate values of the ranges of "5 wt% to 25 wt%," etc.). Disclosure of a narrower range or more specific group in addition to a broader range is not a disclaimer of the broader range or larger group. The lists of components recited herein can include a combination comprising at least one or more components from the respective lists.

[0087] Unless otherwise specified, the date of test methods is the version in effect in

2016.

[0088] 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.

[0089] While the disclosed subject matter is described herein in terms of some embodiments and representative examples, those skilled in the art will recognize that various modifications and improvements can be made to the disclosed subject matter without departing from the scope thereof. Additional features known in the art likewise can be incorporated. Moreover, although individual features of some embodiments of the disclosed subject matter can be discussed herein and not in other embodiments, it should be apparent that individual features of some embodiments can be combined with one or more features of another embodiment or features from a plurality of embodiments.

Claims

CLAIMS What is claimed is:

1. A method of making isotropic boron nitride, the method comprising combining anisotropic boron nitride comprising an interstice with a supercritical fluid; penetrating the interstice with the supercritical fluid;

expanding the interstice to form the isotropic boron nitride; and

optionally repeating the combining, penetrating, or expanding.

2. The method of claim 1 further comprising

first combining the anisotropic boron nitride with a fluid that is not a supercritical fluid;

converting the fluid to the supercritical fluid at a first pressure and a first temperature; re-forming the fluid from the supercritical fluid at a second pressure and a second temperature;

and optionally repeating the first combining, converting, or re-forming.

3. The method of claim 2, wherein the first pressure is 10 to 80 MPa, preferably 15 to 60 MPa, more preferably 25 to 40 MPa.

4. The method of claim 2 or 3, wherein the first temperature is 100 to 600°C, preferably 150 to 500° C, more preferably 200 to 400°C.

5. The method of any one or more of claims 2 to 4, wherein the second temperature is 0 to 100°C, preferably 0 to 25°C, more preferably 0 to 10°C; and the second pressure is atmospheric pressure.

6. The method of any one or more of claims 1 to 5, wherein the supercritical fluid comprises an organic solvent, preferably benzyl benzoate; a C_1-8 alcohol, preferably ethanol, isopropanol, or n-butanol; an N-Ci-i2-2-pyrrolidone, preferably N-methyl-2- pyrrolidone, N-vinyl-2-pyrrolidone, N-octyl-2-pyrrolidone, or l-lauryl-2-pyrrolidone; γ- butyrolactone; dimethylacetamide; l,3-dimethyl-2-imidazolidinone; dimethylformamide; dimethylsulfoxide; acetone; or a combination comprising at least one of the foregoing organic solvents.

7. The method of any one or more of claims 1 to 6, wherein the supercritical fluid comprises water.

8. The method of any one or more of claims 1 to 7, wherein the combining, penetrating, and expanding is repeated 1 to 15 times, preferably 2 to 10 times, more preferably 5 to 10 times.

9. The method of any one or more of claims 1 to 8, wherein the anisotropic boron nitride is a hexagonal boron nitride, preferably a natural flake hexagonal boron nitride.

10. The method of any one or more of claims 1 to 9, wherein the isotropic boron nitride is a single layer isotropic boron nitride, preferably a single layer isotropic hexagonal boron nitride, more preferably an expanded single layer isotropic hexagonal boron nitride.

11. The method of any one or more of claims 1 to 10 further comprising removing the supercritical fluid from the isotropic boron nitride, preferably by heating at a removal temperature or by sonication.

12. The method of any one or more of claims 1 to 11, wherein the isotropic boron nitride has one or more of

a thermal conductivity of 1 to 2,000 W/m K according to ASTM A1225,

an electrical resistivity at room temperature of 5 to 15 Ω-cm,

a dielectric constant of 3.01 to 3.36 at room temperature, and

a loss tangent of 0.0001 to 0.001 at room temperature.

13. The method of any one or more of claims 1 to 12, wherein the isotropic boron nitride has an average largest dimension of 1 nm to 1,000 μιτι, preferably 20 nm to 80 μιτι, more preferably 50 nm to 1 μιη.

An isotropic boron nitride made by the method of any one or more of claims 1

15. The isotropic boron nitride of claim 14, wherein the boron nitride is a single layer isotropic boron nitride, preferably a single layer isotropic hexagonal boron nitride, more preferably an expanded single layer isotropic hexagonal boron nitride.

16. An article comprising the isotropic boron nitride of any one or more of claims

1 to 15.

17. The article of claim 15 being a circuit material, integrated circuit package, printed circuit board, or thermal insulation component.

18. A composite comprising

a thermoset polymer, a thermoplastic polymer, or a combination comprising at least one of the foregoing; and

the isotropic boron nitride of any one or more of claims 1 to 15.

19. The composite of claim 18, wherein the composite is in the shape of a fiber, a granule, or a film.

20. An article comprising the composite of claim 18 or 19.

21. The article of claim 20, wherein the article is a circuit material, integrated circuit package, printed circuit board, or thermal insulation component.