Sheet comprising a composite material of a polymer, thermally conductive particles and electromagnetically absorbing particles and processes for producing the same

Abstract

The present disclosure relates to a sheet comprising a thermally conductive electromagnetically absorbing composite material comprising a polymer, electromagnetically absorbing particles and thermally conductive particles, wherein the thermally conductive particles comprise platelet-shaped thermally conductive particles, and wherein the platelet-shaped thermally conductive particles are oriented in a direction perpendicular to the direction of the plane of the sheet, and wherein the thermally conductive particles are electrically insulating, and wherein the composite material comprises at least 30 percent by weight of the thermally conductive particles, based on the total weight of the composite material, and wherein the sheet has a through-plane thermal conductivity of more than 12 W / m*K. The present disclosure further relates to processes for producing said sheet.

Description

[0001] SHEET COMPRISING A COMPOSITE MATERIAL OF A POLYMER, THERMALLY CONDUCTIVE PARTICLES AND ELECTROMAGNETICALLY ABSORBING PARTICLES AND PROCESSES FOR PRODUCING THE SAME

[0002] Technical Field

[0003] The present disclosure relates to a sheet comprising a composite material comprising a polymer, electromagnetically absorbing particles and thermally conductive particles, wherein the thermally conductive particles comprise platelet-shaped thermally conductive particles being oriented in a direction perpendicular to the direction of the plane of the sheet.

[0004] Background

[0005] Thermally conductive polymer compounds are used for thermal management solutions. For electronic devices, like in mobile devices, for LED technology, for electric vehicles, and for 5G technology, there is a growing demand for thermally conductive and electrically insulating polymer materials. To improve performance of these materials, thermal conductivity needs to be increased. To this end, thermally conductive fillers are used such as boron nitride, alumina, aluminum nitride, silicon carbide, silicon nitride, magnesium oxide or minerals. With increasing load of thermally conductive fillers, higher values for thermal conductivity can be obtained. The maximum loads of fillers in compounds are typically limiting thermal conductivities that can be achieved.

[0006] For thermal management applications for electronic devices, materials are needed that have high thermal conductivity and electrical insulation. For many electromagnetic applications, these thermal interface materials are needed in the form of a thin film or sheet. Oftentimes it is desired that thermal conductivity of these films or sheets is as high as possible in a direction perpendicular to the plane of the film, i.e., a high through-plane thermal conductivity is required.

[0007] For many future thermal management applications for electronic devices, thermally conductive materials are needed which have electromagnetically absorbing properties, as densely packed devices are causing electromagnetic interference (EMI) issues and there is a need for improved signal to noise ratio.

[0008] US 2010 / 0200801 Al discloses a thermal interface material comprising a base matrix comprising a polymer and 5 to 90 wt.%, preferably 20 to 60 wt.%, of boron nitride filler having a platelet structure, wherein the platelet structure of the boron nitride particles is substantially aligned for the thermal interface material to have a bulk thermal conductivity of at least 1 W / m*K. The thermal interface material is extruded into sheets. As a second step, the sheets may be stacked, pressed, cured and sliced in a direction perpendicular to the stacking direction, or the sheet may be compression rolled into a roll, cured and sliced into a plurality of circular pads in a direction perpendicular to the rolling direction. WO 2019 / 097445 Al discloses a polymer matrix composite comprising a porous polymeric network, and a plurality of thermally conductive particles distributed within the polymeric network structure.

[0009] WO 2021 / 198849 Al discloses a thermally conductive electromagnetically absorbing material comprising a plurality of particles dispersed in a binder, the plurality of particles having a particles size distribution comprising at least three peaks, wherein at least a majority of particles within a half width at half maximum (HWHM) of one, but not the other ones, of the at least three peaks are at least partially coated with an electromagnetically absorbing coating.

[0010] US 5,389,434 discloses a non-electrically-conductive electromagnetic radiation absorbing material, comprising a plurality of dissipative particles and a dielectric binder through which the dissipative particles are dispersed. Any of the dissipative particles comprises a core particle, a dissipative layer located on the surface of the core particle, and an insulating layer overlaying the dissipative layer.

[0011] There is a need for thermally conductive and electromagnetically absorbing thermal interface materials having a high through-plane thermal conductivity, and good dielectric properties, i.e., low to moderate dielectric constant for achieving low reflection and moderate to high dielectric dissipation factor for achieving high electromagnetic absorption.

[0012] As used herein, "a", "an", "the", "at least one" and "one or more" are used interchangeably. The term “comprise” shall include also the terms “consist essentially of’ and “consists of’.

[0013] Summary

[0014] In a first aspect, the present disclosure relates to a sheet comprising a thermally conductive electromagnetically absorbing composite material comprising a polymer, electromagnetically absorbing particles and thermally conductive particles, wherein the thermally conductive particles comprise platelet-shaped thermally conductive particles, and wherein the platelet-shaped thermally conductive particles are oriented in a direction perpendicular to the direction of the plane of the sheet, and wherein the thermally conductive particles are electrically insulating, and wherein the composite material comprises at least 30 percent by weight of the thermally conductive particles, based on the total weight of the composite material, and wherein the sheet has a through-plane thermal conductivity of more than 12 W / m*K.

[0015] In another aspect, the present disclosure also relates to a process for producing a sheet as disclosed herein, the process comprising providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a suspension of electromagnetically absorbing particles and thermally conductive particles in a polymer-solvent solution, wherein the polymer in the polymer- solvent solution has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted at a temperature above the melting point of the polymer in the polymer-solvent solution, and below the boiling point of the solvent, forming the suspension into a fdm, wherein the platelet-shaped thermally conductive particles are oriented in a direction parallel to the direction of the plane of the fdm, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the fdm to obtain a porous fdm, optionally compressing the porous fdm to obtain a densified fdm, stacking multiple layers either of the porous fdm or of the densified fdm one upon another to obtain a fdm stack, pressing the fdm stack to obtain a bonded fdm stack, and slicing a sheet from the bonded fdm stack in a direction perpendicular to the planes of the stacked fdm layers.

[0016] In another aspect, the present disclosure also relates to a process for producing a sheet as disclosed herein, the process comprising providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a slurry, wherein the slurry is a suspension of the polymer, the electromagnetically absorbing particles, and the thermally conductive particles in the solvent, and wherein the polymer has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted below the melting point of the polymer, and below the boiling point of the solvent, forming the slurry into a fdm, wherein the platelet-shaped thermally conductive are oriented in a direction parallel to the direction of the plane of the fdm, heating the fdm in an environment to retain at least 90 percent by weight of the solvent in the fdm, based on the weight of the solvent in the fdm, and solubilizing at least 50 percent by weight of the polymer in the solvent, based on the total weight of the polymer, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the fdm to obtain a porous fdm, optionally compressing the porous fdm to obtain a densified fdm, stacking multiple layers either of the porous film or of the densified film one upon another to obtain a film stack, pressing the film stack to obtain a bonded film stack, and slicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

[0017] The sheet disclosed herein comprises highly oriented thermally conductive platelet-shaped particles and has consequently highly anisotropic properties, particularly highly anisotropic thermal conductivity properties.

[0018] The sheet disclosed herein comprises thermally conductive platelet-shaped particles oriented perpendicularly to the plane of the sheet and has a high through-plane thermal conductivity.

[0019] The sheet as disclosed herein allows to remove heat faster and more efficiently, due to the high through-plane thermal conductivity. Compared to other polymer sheets filled with thermally conductive particles, the sheets disclosed herein have a higher through-plane thermal conductivity than in-plane thermal conductivity.

[0020] In addition, the sheet disclosed herein has electromagnetically absorbing properties.

[0021] Furthermore, the sheet disclosed herein has good dielectric properties, specifically a low or moderate dielectric constant and moderate or high dissipation factor.

[0022] Typically, the sheet disclosed herein is free of silicone.

[0023] “Phase separation”, as used herein, refers to a process in which particles are uniformly dispersed in a homogeneous polymer-solvent solution that is transformed (e.g., by a change in temperature or solvent concentration) into a continuous three-dimensional composite material, i.e., a polymer matrix composite. In the first process disclosed herein, phase separation is achieved via solvent induced phase separation (SIPS) using a wet or dry process, or thermally induced phase separation (TIPS) processes. In the second process disclosed herein, the desired article (i.e., a film) is formed before the polymer becomes miscible with the solvent and the phase separation is a thermally induced phase separation process.

[0024] “Miscible” as used herein refers to the ability of substances to mix in all proportions (i.e., to fully dissolve in each other at any concentration), forming a solution, wherein for some solvent- polymer systems heat may be needed for the polymer to be miscible with the solvent. By contrast, substances are immiscible if a significant proportion does not form a solution. For example, butanone is significantly soluble in water, but these two solvents are not miscible because they are not soluble in all proportions.

[0025] Typically, the maximum particle loading that can be achieved in traditional particle-filled composites (dense polymeric films, adhesives, etc.), is not more than about 40 to 60 vol.%, based on the volume of the particles and the binder. Incorporating more than 60 vol.% particles into traditional particle-filled composites typically is not achievable because such high particle loaded materials cannot be processed via coating or extrusion methods and / or the resulting composite becomes very brittle. Traditional composites also typically fully encapsulate the particles with binder, preventing access to the particle surfaces and minimizing potential particle-to-particle contact. Typically, the thermal conductivity of a thermally conductive particle -fdled composite increases with particle loading, making higher particle loadings desirable. Surprisingly, the high levels of solvent and the phase -separated morphologies, obtained with the processes described herein, enable relatively high particle loadings with relatively low amounts of high molecular weight binder. Although not wanting to be bound by theory, it is believed that another advantage of embodiments of the composite material described herein, is that the particles are not fully coated with binder, enabling a high degree of particle surface contact, without masking due to the porous nature of the binder. Compression of the fdm significantly enhances the particle-to-particle contact.

[0026] Brief Description of the Drawings

[0027] The present disclosure is explained in more detail on the basis of the drawings, in which Figure 1 schematically shows the stacking and slicing steps of the processes for producing a sheet as disclosed herein.

[0028] Detailed Description

[0029] The sheet as disclosed herein comprises a composite material which is thermally conductive and electromagnetically absorbing. The composite material comprises a polymer, electromagnetically absorbing particles and thermally conductive particles. The thermally conductive particles comprise platelet-shaped thermally conductive particles. Platelet-shaped thermally conductive particles may also be referred to as flake-shaped or scale-like thermally conductive particles.

[0030] The thermally conductive particles are electrically insulating.

[0031] The platelet-shaped thermally conductive particles have a basal plane. The basal plane of the platelet-shaped thermally conductive particles is oriented perpendicularly to the direction of the plane of the sheet. In other words, in the sheet as disclosed herein, the platelet-shaped thermally conductive particles are oriented in a direction perpendicular to the direction of the plane of the sheet.

[0032] The orientation in a direction perpendicular to the direction of the plane of the sheet is evidenced by scanning electron microscopy and by X-ray diffraction measurements, showing that thermally conductive particles in the composite material are oriented in a direction perpendicular to the direction of the plane of the sheet.

[0033] Typically, at least 50 percent by weight of the thermally conductive particles are plateletshaped thermally conductive particles, based on the total weight of the thermally conductive particles. Preferably, at least 80 percent by weight of the thermally conductive particles are platelet- shaped thermally conductive particles, based on the total weight of the thermally conductive particles. It is also possible that all of the thermally conductive particles are platelet shaped. A portion of, or even all of, the platelet-shaped thermally conductive particles may be agglomerated to thermally conductive agglomerates. The platelet-shaped thermally conductive particles may also be non-agglomerated.

[0034] The sheet as disclosed herein comprises a composite material comprising a polymer. The polymer may be selected from the group consisting of polyurethane, polyester, polyamide, polyether, polycarbonate, polyimide, polysulfone, polyethersulfone, polyphenylene oxide, polyacrylate, polymethacrylate, polyacrylonitrile, polyolefin, styrene, styrene-based copolymer, styrene-base copolymer, chlorinated polymer, fluorinated polymer, copolymers of ethylene and chlorotrifluoroethylene, and combinations thereof. The styrene-based copolymer may be a styrene- based random copolymer, or a styrene-based block copolymer. The polyolefin may be an ultra-high molecular weight polyethylene (UHMWPE) or a polypropylene.

[0035] The polymer may comprise, consist essentially of, or consist of at least one thermoplastic polymer. Exemplary thermoplastic polymers include polyurethane, polyester (e.g., polyethylene terephthalate, polybutylene terephthalate, and polylactic acid), polyamide (e.g., nylon 6, nylon 6,6, nylon 12 and polypeptide), polyether (e.g., polyethylene oxide and polypropylene oxide), polycarbonate (e.g., bisphenol-A-polycarbonate), polyimide, polysulphone, polyethersulfone, polyphenylene oxide, polyacrylate (e.g., thermoplastic polymers formed from the addition polymerization of monomer(s) containing an acrylate functional group), polymethacrylate (e.g., thermoplastic polymers formed from the addition polymerization of monomer(s) containing a methacrylate functional group), polyolefin (e.g., polyethylene and polypropylene), styrene and styrene-based, random and block copolymer, chlorinated polymer (e.g., polyvinyl chloride), fluorinated polymer (e.g., polyvinylidene fluoride; copolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride; copolymers of ethylene, tetrafluoroethylene; hexafluoropropylene; and polytetrafluoroethylene), and copolymers of ethylene and chlorotrifluoroethylene. In some embodiments, thermoplastic polymers include homopolymers or copolymers (e.g., block copolymers or random copolymers). In some embodiments, thermoplastic polymers include a mixture of at least two thermoplastic polymer types (e.g., a mixture of polyethylene and polypropylene or a mixture of polyethylene and polyacrylate). In some embodiments, the thermoplastic polymer may be at least one of polyethylene (e.g., ultra-high molecular weight polyethylene), polypropylene (e.g., ultra-high molecular weight polypropylene), polylactic acid, poly(ethylene-co-chlorotrifluoroethylene) and polyvinylidene fluoride. In some embodiments, the thermoplastic polymer is a single thermoplastic polymer (i.e., it is not a mixture of at least two thermoplastic polymer types). In some embodiments, the thermoplastic polymers consist essentially of, or consist of, polyethylene (e.g., ultra-high molecular weight polyethylene). In some embodiments, the thermoplastic polymer used to make the composite material of the sheet disclosed herein are particles having a particle size less than 1000 (in some embodiments, in the range from 1 to 10, 10 to 30, 10 to 50, 30 to 100, 10 to 200, 10 to 500, 100 to 200, 200 to 500, 500 to 1000) micrometers.

[0036] In some embodiments, the polymer used to make the composite material of the sheet disclosed herein has a number average molecular weight in a range from 5 x 104to 1 x 107(in some embodiments, in a range from 1 x 106to 8 x 106, 2 x 106to 6 x 106, or even 2 x 106to 5 x 106) g / mol. For purposes of the present disclosure, the number average molecular weight can be measured by known techniques in the art (e.g., gel permeation chromatography (GPC). GPC may be conducted in a suitable solvent for the thermoplastic polymer, along with the use of narrow molecular weight distribution polymer standards (e.g., narrow molecular weight distribution polystyrene standards). Thermoplastic polymers are generally characterized as being partially crystalline, exhibiting a melting point. In some embodiments, the thermoplastic polymer may have a melting point in a range from 120 to 350 (in some embodiments, in a range from 120 to 300, 120 to 250, or even 120 to 200) °C. The melting point of the thermoplastic polymer can be measured by known techniques in the art (e.g., the on-set temperature measured in a differential scanning calorimetry (DSC) test, conducted with a 5 to 10 mg sample, at a heating scan rate of 10 °C / min., while the sample is under a nitrogen atmosphere).

[0037] In some embodiments, the polymer used to make the composite material of the sheet disclosed herein is an ultra-high molecular weight polyethylene (UHMWPE) having a number average molecular weight in a range from 5 x 104to 1 x 107(in some embodiments, in a range from 1 x 106to 8 x 106, 2 x 106to 6 x 106, or even 2 x 106to 5 x 106) g / mol.

[0038] The thermally conductive particles and the electromagnetically absorbing particles are dispersed in the polymer, i.e., the polymer is a matrix material for the thermally conductive particles and the electromagnetically absorbing particles. The thermally conductive particles have a direct particle-to-particle contact in the composite material, i.e., continuous paths of thermally conductive particles are formed in the composite material. The direct particle-to-particle contact and the continuous paths of thermally conductive particles are evidenced by the high through-plane thermal conductivity of the composite material of the sheet.

[0039] The composite material comprises at least 30 percent by weight of the thermally conductive particles, based on the total weight of the composite material.

[0040] The composite material may comprise at least 32, or at least 35, or at least 40, or at least 42, or at least 45, or at least 50, or at least 52, or at least 55 percent by weight of the thermally conductive particles, based on the total weight of the composite material.

[0041] The composite material may comprise from 30 to 88, or from 40 to 88, or from 50 to 88, or from 55 to 88, or from 30 to 85, or from 40 to 85, or from 50 to 85, or from 55 to 85, or from 30 to 80, or from 40 to 80, or from 50 to 80, or from 55 to 80, or from 30 to 75, or from 40 to 75, or from 50 to 75, or from 55 to 75, or from 50 to 70, or from 52 to 68, or from 55 to 70, or from 55 to 65 percent by weight of the thermally conductive particles, based on the total weight of the composite material.

[0042] The composite material may comprise at least 10, or at least 15, or at least 20, or at least 25, or at least 30, or at least 35, or at least 40, or at least 45 percent by weight of the electromagnetically absorbing particles, based on the total weight of the composite material.

[0043] The composite material may comprise from 10 to 68, or from 10 to 60, or from 10 to 55, or from 10 to 50, or from 15 to 68, or from 15 to 60, or from 15 to 55, or from 15 to 50, or from 15 to 45, or from 20 to 68, or from 20 to 60, or from 20 to 55, or from 20 to 50, or from 20 to 45, or from 20 to 40, or from 25 to 68, or from 25 to 60, or from 25 to 55, or from 25 to 50, or from 25 to 45, or from 25 to 40, or from 25 to 35 percent by weight of the electromagnetically absorbing particles, based on the total weight of the composite material.

[0044] The composite material may comprise at least 2 percent by weight of the polymer, based on the total weight of the composite material. The composite material may comprise at most 30, or at most 20 percent by weight of the polymer, based on the total weight of the composite material. The composite material may comprise from 2 to 30, or from 2 to 20, or from 2 to 15, or from 2 to 10, or from 2 to 9, or from 2 to 8, or from 2 to 7, or from 2 to 6, or from 2 to 5, or from 2 to 4 percent by weight of the polymer, based on the total weight of the composite material.

[0045] The sheet disclosed herein has a through-plane thermal conductivity of more than 12 W / m*K. The through-plane thermal conductivity of the sheet may be at least 15 W / m*K, or at least 18 W / m*K, or at least 20 W / m*K, or at least 25 W / m*K, or at least 30 W / m*K, or at least 35 W / m*K, or at least 40 W / m*K, or at least 45 W / m*K, or at least 50 W / m*K. The through-plane thermal conductivity may be from more than 12 W / m*K to 25 W / m*K, or from more than 12 W / m*K to 30 W / m*K, or from more than 12 W / m*K to 35 W / m*K, or from more than 12 W / m*K to 40 W / m*K, or from more than 12 to 50 W / m*K, or from 15 W / m*K to 25 W / m*K, or from 15 W / m*K to 30 W / m*K, or from 15 W / m*K to 35 W / m*K, or from 15 W / m*K to 40 W / m*K, or from 15 W / m*Kto 50 W / m*K, or from 20 W / m*Kto 30 W / m*K, or from 20 W / m*Kto 35 W / m*K, or from 20 W / m*K to 40 W / m*K, or from 20 W / m*K to 50 W / m*K, or from 30 to 50 W / m*K, or from 40 W / m*K to 50 W / m*K.

[0046] The through-plane thermal conductivity of the sheet is higher than the in-plane thermal conductivity of the sheet. Typically, the ratio of the through-plane thermal conductivity to the inplane thermal conductivity of the sheet is at least 2, and may be at least 3, or at least 4, or at least 5. The ratio of the through-plane thermal conductivity to the in-plane thermal conductivity of the sheet may be from 2 to 10.

[0047] The through-plane thermal conductivity of the sheet can be measured using the laser flash analysis method according to ASTM E1461 (2013). The through-plane thermal conductivity can also be measured according to ASTM D-5470-17 (standard test method for thermal interface materials). The through-plane thermal conductivity can be measured on sheet samples.

[0048] The in-plane thermal conductivity of the sheet may be at least 2 W / m*K. The in-plane thermal conductivity of the sheet can be measured using the laser flash analysis method according to ASTM E1461 (2013).

[0049] The composite material comprised in the sheet disclosed herein may be electrically insulating. The electrical resistivity of the composite material comprised in the sheet may be at least 1 x 1010 *m.

[0050] The thermally conductive particles for use herein, i.e., the thermally conductive particles that are comprised in the composite material of the sheet disclosed herein, may be selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide, graphene oxide, and combinations thereof.

[0051] Graphene oxide (GO) is a 2D atomic sheet of graphite that contains oxygenated functional groups on its basal plane. It is typically made by chemical oxidation of graphite. GO is an electrically insulating material with a band gap of 3.5 eV and high thermal conductivity reported (-800 W / m K). It is hydrophilic in nature. Electrically insulating graphene oxide (GO) is different from reduced graphene oxide (rGO) which is an electrically conducting material with a band gap of 1.2 eV, made by processing graphene oxide under thermal or chemical reducing conditions.

[0052] Preferably, the thermally conductive particles comprise hexagonal boron nitride particles. The hexagonal boron nitride particles comprise platelet-shaped hexagonal boron nitride particles.

[0053] The degree of orientation of the platelet-shaped hexagonal boron nitride particles in the sheet can be characterized by the orientation index, measured on a sheet sample. The orientation index of hexagonal boron nitride with isotropic orientation of the platelet-shaped hexagonal boron nitride particles, thus without preferred orientation, has a value of 1. For platelet-shaped hexagonal boron nitride particles being oriented parallelly to the plane of the sheet, the orientation index decreases with the degree of parallel orientation in the sheet sample and has values less than 1. For plateletshaped hexagonal boron nitride particles being oriented perpendicularly to the plane of the sheet, the orientation index increases with the degree of perpendicular orientation in the sheet sample and has values greater than 1. As used herein, by “the platelet-shaped hexagonal boron nitride particles are oriented perpendicularly to the plane of the sheet” it is to be understood that the orientation index is greater than 4.0. As used herein, by “the platelet-shaped hexagonal boron nitride particles are oriented parallelly to the plane of the sheet” it is to be understood that the orientation index is at most 0.5.

[0054] The orientation index of the sheet as disclosed herein is greater than 4.0. The orientation index of the sheet may be at least 4.5, or at least 5, or at least 6, or at least 7, or at least 8, or at least 10. Preferably, the orientation index of the sheet is at least 6. The orientation index is determined by X-ray diffractometry. For this, the ratio of the intensities of the (100) and of the (002) reflection of hexagonal boron nitride (hBN) measured on X- ray diffraction diagrams of a sheet sample is determined and is divided by the corresponding ratio for an ideal, unoriented, i.e., isotropic, hBN sample. This ideal ratio can be determined from Powder Diffraction Pattern (PDF) #01-073-2095 of the International Centre for Diffraction Data (ICDD) data (2020) and is 0.147. The theoretical peak positions of (002) and (100) reflections are at 26.7 and 41.6 degrees in two theta for Cu Kai radiation source, respectively. Peak intensities of (002) and (100) reflections are measured from peak area at these positions. The orientation index (OI) can be determined from the formula:

[0055] 1(100), sample / 1(002), sample 1(100), sample / 1(002), sample

[0056] 01 = -

[0057] 1(100), theoretical / 1(002), theoretical 0.147

[0058] The electromagnetically absorbing particles for use herein, i.e., the electromagnetically absorbing particles that are comprised in the composite material that is comprised in the sheet disclosed herein, may be selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof.

[0059] Suitable metal oxide particles may be selected from the group consisting of doped particles of tin oxide, undoped particles of tin oxide, ferrous oxide particles, ferric oxide particles, zinc oxide particles, manganese oxide particles, lead oxide particles, nickel oxide particles, cobalt oxide particles, silver oxide particles, antimony oxide particles, copper oxide (CuO) particles, titanium monoxide (TiO) particles, and combinations thereof.

[0060] Examples of metal nitride particles that are suitable include doped and undoped particles of tantalum nitride, titanium nitride, vanadium nitride, and zirconium nitride. Mixtures of metal nitride particles are also suitable.

[0061] Examples of metal carbide particles that are suitable include doped and undoped particles of tungsten carbide, niobium carbide, titanium carbide, vanadium carbide, molybdenum carbide, silicon carbide, zirconium carbide, boron carbide, and titanium silicon carbide. Mixtures of metal carbide particles are also suitable.

[0062] Examples of metal sulfide particles that are suitable include doped and undoped particles of copper sulfide, silver sulfide, iron sulfide, nickel sulfide, cobalt sulfide, lead sulfide, and zinc sulfide. Mixtures of metal sulfide particles are also suitable. Examples of metal silicide particles that are suitable include doped and undoped particles of chromium silicide, molybdenum silicide, cobalt silicide, vanadium silicide, tungsten silicide, and titanium silicide. Mixtures of metal silicide particles are also suitable.

[0063] Examples of metal boride particles that are suitable include doped and undoped particles of chromium boride, molybdenum boride, titanium boride, zirconium boride, niobium boride, and tantalum boride. Mixtures of metal boride particles are also suitable.

[0064] Examples of particles of multiferroic compounds that are suitable include doped and undoped particles of bismuth ferrite (BiFcCE). bismuth manganate (BiMnCE), and rare earth-iron oxides (MFe2O4 where M is a rare earth element, such as, for example, L11FC2O4). Mixtures of particles of multiferroic compounds are also suitable.

[0065] Examples of mixed ceramic particles include particles with a mixture of metal or metalloid elements. Suitable examples include doped and undoped particles of silicon carbide and beryllium oxide, silicon carbide and aluminum nitride, copper oxide (CuO) and aluminum oxide, aluminum nitride and glassy carbon, and Si — Ti — C — N ceramics.

[0066] Examples of chalcogenide glass particles include glassy materials based on As — Ge — Te and Se — Ge — Te.

[0067] Particularly suitable electromagnetically absorbing particles for use in the composite material that is comprised in the sheet of this disclosure are metal oxide particles, especially copper oxide (CuO) and titanium monoxide (TiO).

[0068] The electromagnetically absorbing particles for use herein may be particles with substantially isotropic properties, such as spherical particles or otherwise isotropic particles.

[0069] The electromagnetically absorbing particles for use herein, i.e., the electromagnetically absorbing particles that are comprised in the composite material that is comprised in the sheet disclosed herein, may comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle.

[0070] The electromagnetically absorbing particles for use herein, i.e., the electromagnetically absorbing particles that are comprised in the composite material that is comprised in the sheet disclosed herein, may comprise a core particle, an electromagnetically absorbing coating located on the surface of the core particle, and an electrically insulating coating located on the electromagnetically absorbing coating.

[0071] The electromagnetically absorbing particles may further comprise a further inorganic coating which is located on the surface of the core particle, with the electromagnetically absorbing coating being located on the further inorganic coating. In other words, the further inorganic coating if present is a first coating on the core particle, and the electromagnetically absorbing coating is a second coating which is coated on the first coating. If an electrically insulating coating is present, the further inorganic coating if present is a first coating on the core particle, and the electromagnetically absorbing coating is a second coating which is coated on the first coating, and the electrically insulating coating is a third coating which is coated on the second coating.

[0072] The core particles can have any suitable shape (e.g., at least one of flakes, platelets, spheres, spheroids, ellipsoids, irregularly shaped particles). In some embodiments, at least a majority of the core particles are substantially spherical. A particle can be considered substantially spherical if its outline fits within the intervening space between two, concentric, truly spherical outlines differing in diameter from one another by up to about 30% of the diameter of the larger of these outlines. In some embodiments, each particle in at least a majority of the particles fits within the intervening space between two, concentric, truly spherical outlines differing in diameter from one another by up to about 20% or 10% of the diameter of the larger of these outlines.

[0073] The core particles may be selected from the group consisting of alumina, hollow glass microspheres, solid glass microbeads, and combinations thereof. By “solid glass microbeads” it is meant that the glass microbeads are dense particles, i.e., non-hollow particles.

[0074] The electromagnetically absorbing coating may comprise a metal or a semiconductor. The electromagnetically absorbing coating may comprise a material selected from the group consisting of tungsten, aluminum, titanium, steel, chromium, nickel, and combinations thereof. In some embodiments, the electromagnetically absorbing coating comprises or consists of tungsten. It is typically desired that when the electromagnetically absorbing coating is a metal that it is sufficiently thin that it results in significant electromagnetic absorption in a desired frequency range (e.g., via dielectric relaxation as described in Bowler, “Designing Dielectric Loss at Microwave Frequencies using Multi-Layered Filler Particles in a Composite”, IEEE Transactions on Dielectrics and Electrical Insulation Vol 13, No. 4, pp. 703-711, August 2006). When the metal layer is sputtered onto the core particle, metals such as tungsten that tend to form a monolayer are typically preferred.

[0075] The core particles may be at least partially coated with the electromagnetically absorbing coating. A partial coating can include a plurality of islands of the coating material and can, in some embodiments, include varying densities of the coating material in the islands. The core particles may also be fully coated with the electromagnetically absorbing coating.

[0076] The electromagnetically absorbing coating can be said to be electromagnetically absorptive if the electromagnetically absorbing coating contributes substantially to the electromagnetic absorptivity of the composite material which includes the electromagnetically absorbing particles (e.g., in some embodiments, replacing the electromagnetically absorbing particles with otherwise equivalent particles not having the electromagnetically absorbing coating would reduce the electromagnetic absorptivity of the composite material by at least 20%, or at least 40%, or at least 60%). Particles included in the composite material which do not contribute substantially to the electromagnetic absorptivity of the composite material can be referred to as substantially non- absorptive particles. In some embodiments, most of the electromagnetic absorptivity of the composite material arises due to the presence of the electromagnetically absorbing particles. The core particles may be further at least partially coated with the electrically insulating coating, i.e., the electrically insulating coating is coated on the electromagnetically absorbing coating. In some embodiments, the electromagnetically absorbing coating may be discontinuous, and the electrically insulating coating is continuous or substantially continuous.

[0077] The electrically insulating coating may be a non-conductive metal oxide coating. The electrically insulating coating may be selected from the group consisting of an aluminum oxide (AlOx) coating, a silicon oxide ( Si O2) coating, a zirconium oxide (ZrCf) coating, and combinations thereof. The electrically insulating coating provides good electrical insulation properties for the electromagnetically absorbing particles.

[0078] The further inorganic coating may be a metal coating such as an aluminum coating. The further inorganic coating, if present, is coated on the core particles, and the electromagnetically absorbing coating is coated on the further inorganic coating.

[0079] The thickness of the electromagnetically absorbing coating may be from 0.1 to 20 nm, or from 0.5 nm to 15 nm, or from 1 to 10 nm, for example.

[0080] The thickness of the electrically insulating coating may be from 1 nm to 100 nm, or from 1 nm to 20 nm, for example.

[0081] The electromagnetically absorbing coating and the electrically insulating coating may be applied by physical vapor deposition (e.g., magnetron sputtering), for example, as generally described in U.S. Pat. Nos. 4,612,242 (Vesley et al.); 5,389,434 (Chamberlain et al.); 7,727,931 (Brey et al.); 8,698,394 (McCutcheon et al.), for example.

[0082] Typically, the thermally conductive particles do not comprise an electromagnetically absorbing coating located on the surface of the thermally conductive particles.

[0083] The mean particle size (dso) of the thermally conductive particles used for the sheet disclosed herein may be from 0.5 to 500 pm, or from 3 to 500 pm.

[0084] The mean particle size (d o) of the platelet-shaped thermally conductive particles used for the sheet disclosed herein may be from 0.5 to 100 pm, or from 3 to 100 pm.

[0085] Preferably, the mean particle size (dso) of the thermally conductive particles is at least 5 pm, more preferably at least 8 pm. In some embodiments, the mean particle size (dso) is from 5 to 50 pm, or from 5 to 30 pm, or from 8 to 30 pm, or from 30 to 60 pm, or from 40 to 50 pm, or from 8 to 15 pm, or from 10 to 12 pm. The mean particle size (dso) can be measured by laser diffraction.

[0086] The mean aspect ratio of the platelet-shaped thermally conductive particles typically is at least 5. The aspect ratio is the ratio of the diameter to the thickness of the platelet-shaped thermally conductive particles. As used herein, the platelet-shaped thermally conductive particles are also referred to as thermally conductive platelets. The aspect ratio of the thermally conductive platelets may be at least 10, or at least 15, or at least 20. The mean aspect ratio of the thermally conductive platelets may also be up to 40, or up to 100. The mean aspect ratio of the thermally conductive platelets may be from 7 to 20, or from 20 to 40, or from 7 to 40, or from 10 to 40, or from 50 to 100, or from 5 to 500. Typically, the mean aspect ratio of the thermally conductive platelets is at most 500. The mean aspect ratio can be measured by scanning electron microscopy (SEM), by determining the aspect ratio of 20 particles, and calculating the mean value of the 20 individual values determined for the aspect ratio. The aspect ratio of an individual thermally conductive platelet is determined by measuring the diameter and the thickness of the thermally conductive platelet and calculating the ratio of the diameter to the thickness. Required magnification of the SEM images used to measure diameter and thickness of thermally conductive platelets depends on the size of the platelets. Magnification should be at least lOOOx, preferably at least 2000x. Where appropriate, i.e., for smaller platelets with a mean particle size (dso) of 5 to 10 pm, a magnification of 5000x should be used.

[0087] The thermally conductive particles may comprise agglomerates of thermally conductive primary particles, the primary particles comprising platelet-shaped thermally conductive particles. The thermally conductive particles may also consist of agglomerates of thermally conductive primary particles, the primary particles comprising platelet-shaped thermally conductive particles. The agglomerates of thermally conductive primary particles may also be referred to as “thermally conductive agglomerates”. The mean particle size (dso) of the thermally conductive agglomerates may be at most 500 pm, or at most 250 pm, or at most 150 pm, or at most 100 pm. The mean particle size (d n) of the thermally conductive agglomerates may be at least 20 pm, at least 30 pm, or at least 50 pm. The mean particle size (dso) of the agglomerates may be from 20 to 500 pm, or from 20 to 400 pm, or from 30 to 500 pm, or from 30 to 300 pm, or from 50 to 400 pm, or from 30 to 50 pm, or from 50 to 100 pm, or from 100 to 150 pm, or from 100 to 200 pm, or from 200 to 400 pm. The mean particle size (dso) of the thermally conductive primary particles may be from 3 to 50 pm, or from 3 to 30 pm, or from 5 to 30 pm, or from 8 to 30 pm, or from 8 to 15 pm, or from 10 to 15 pm, or from 10 to 12 pm. The mean particle size (dso) of the thermally conductive agglomerates and of the primary particles can be measured by laser diffraction. The thermally conductive agglomerates may have any shape, e.g., spherical, irregularly shaped or flake-shaped. The flake-shaped agglomerates may have an aspect ratio of from 1 to 20.

[0088] In some embodiments, the thermally conductive particles may comprise, or consist of, agglomerates of thermally conductive primary particles, the primary particles comprising plateletshaped thermally conductive particles, wherein the mean particle size (dso) of the agglomerates is from 30 to 300 pm, and wherein the mean particle size (dso) of the primary particles is from 8 to 15 pm.

[0089] In some embodiments, if thermally conductive agglomerates are used as thermally conductive particles, the agglomerates may be partially or fully disintegrated into the primary particles in the final product, i.e., the sheet. In some embodiments, if thermally conductive agglomerates are used as thermally conductive particles, such as flake-shaped thermally conductive agglomerates, a portion or all of the agglomerates may still be present in the form of agglomerates in the final product, i.e., the agglomerates may not have disintegrated into the primary particles in the sheet.

[0090] The thermally conductive particles may also comprise non-agglomerated platelet-shaped thermally conductive particles. The thermally conductive particles may also consist of nonagglomerated platelet-shaped thermally conductive particles. The mean particle size (dso) of the non-agglomerated platelet-shaped thermally conductive particles may be from 3 to 100 pm, preferably from 5 to 30 pm. The mean particle size (dso) of the non-agglomerated platelet-shaped thermally conductive particles may also be from 10 to 100 pm, or from 10 to 50 pm, or from 30 to 70 pm, or from 30 to 60 pm, or from 40 to 50 pm.

[0091] In some embodiments, mixtures of agglomerates and non-agglomerated platelet-shaped thermally conductive particles may be used.

[0092] The mean particle size (dso) of the electromagnetically absorbing particles used for the sheet disclosed herein may be from 50 nm to 100 pm, or from 50 nm to 50 pm, or from 1 to 10 pm.

[0093] The composite material and the sheet may have a porosity of up to 20%. The composite material and the sheet may have a porosity of 0%, or of at least 1%, or of at least 2%, or of at least 3%, or of at least 4%, or of at least 5%, or of at least 8%, or of at least 10%. The composite material and the sheet may have a porosity of from 0.5 to 20%, or from 1 to 20%, or from 2 to 20%, or from 5 to 20%, or from 0 to 10%, or from 0 to 8%, or from 0 to 5%.

[0094] The composite material and the sheet may have a density of at least 80%, or of at least 85%, or even of at least 90% of theoretical density. The composite material and the sheet may have a density of up to 100%, or of up to 99.5%, or of up to 99%, or of up to 98%, or of up to 97%, or of up to 96%, or of up to 95%, or of up to 92%, or of up to 90% of theoretical density. The composite material and the sheet may have a density of from 80 to 95%, or from 80 to 98%, or from 80 to 99%, or from 80 to 99.5%, or from 80 to 100% of theoretical density. The theoretical density can be calculated from the known densities of the components of the composite material and the sheet, and from the weight fractions of the components.

[0095] The composite material comprised in the sheet disclosed herein, and the sheet, has good dielectric properties. Typically, the composite material and the sheet has a dielectric dissipation factor (Df) ofat least 0.01 at 10 GHz. The dielectric dissipation factor (Df) of the composite material and the sheet may be from 0.01 to 0.5 at 10 GHz, or from 0.05 to 0.4 at 10 GHz, or from 0.05 to 0.2 at 10 GHz. Typically, the composite material and the sheet has a dielectric dissipation factor (Df) of at least 0.01 at 40 GHz. The dielectric dissipation factor (Df) of the composite material and the sheet may be from 0.01 to 0.5 at 40 GHz, or from 0.05 to 0.4 at 40 GHz, or from 0.05 to 0.2 at 40 GHz. The dielectric dissipation factor of these ranges may also be referred to as “moderate to high dielectric dissipation factor”. The dielectric constant or relative permittivity (Dk) of the composite material and the sheet typically is at most 6.0 at 10 GHz and may be from 2.0 to 6.0 at 10 GHz. The dielectric constant or relative permittivity (Dk) of the composite material and the sheet typically is at most 6.0 at 40 GHz and may be from 2.0 to 6.0 at 40 GHz. The dielectric constant or relative permittivity of these ranges may also be referred to as “low to moderate dielectric constant”.

[0096] The presence of pores in the composite material and the sheet may improve the dielectric properties of the composite material and the sheet. Furthermore, the presence of pores in the composite material and the sheet allows the composite material and the sheet to be compressible which may be desirable for some applications. The pores in the composite material and the sheet should be of small size and homogeneously distributed in the composite material and the sheet.

[0097] In some embodiments of the sheet disclosed herein, the composite material comprised in the sheet does not comprise fibers, such as carbon fibers, glass fibers or fibers made from other materials.

[0098] The sheet disclosed herein may have a Shore D hardness of at most 90. The Shore D hardness of the sheet may be from 20 to 90, or from 30 to 90. The sheet may also have a lower or higher hardness.

[0099] The thickness of the sheet disclosed herein may be from 0.01 mm to 6 mm, or from 0.05 mm to 6 mm, or from 0.1 to 3 mm, or from 0.5 to 2 mm. The size of the hexagonal boron nitride particles may be selected depending on the film thickness.

[0100] The width and the length of the sheet may be up to several inches (e.g., 1 inch, 5 inches, 10 inches, 50 inches) or larger. Typically, the sheet has a constant thickness over the width and the length of the sheet. The sheet also may have a variable thickness, i.e., the thickness of the sheet is not constant over the width and length of the sheet.

[0101] In some embodiments of the sheet disclosed herein, the composite material may further comprise an organic liquid. The amount of organic liquid comprised in the composite material may be from 0.1 to 10 percent by weight, based on the total weight of the composite material. Useful organic liquids that may be comprised in the composite material are non-volatile organic liquids that are of sufficient surface energy to wet the pores or stay entrapped in the pores. The organic liquid that may be comprised in the composite material may be, for example, at least one of mineral oil, paraffin oil / wax, orange oil, vegetable oil, castor oil, or palm kernel oil. The organic liquid comprised in the composite material may reduce the hardness of the composite material and the sheet and make it softer.

[0102] Small quantities of other additives may be added to the composite material to impart additional functionality or act as processing aids. These include viscosity modifiers (e.g., fumed silica, block copolymers, and wax), plasticizers, thermal stabilizers (e.g., such as available, for example, under the trade designation “Irganox 1010” from BASF, Ludwigshafen, Germany), antimicrobials (e.g., silver and quaternary ammonium), flame retardants, antioxidants, dyes, pigments, and ultraviolet (UV) stabilizers. Also thermally conductive particles such as carbon or hexagonal boron nitride with a low particle size (e.g., an average particle size (dso) of less than 1 pm) may be used as viscosity modifiers. Optionally, elastomers may be added to the composite material to improve elasticity, i.e., to reduce brittleness, of the composite material. Any elastomer that can be mixed or blended in with the polymer and the hexagonal boron nitride particles may be used. Suitable elastomers are, e.g., Santoprene 8211-35, available from Celanese (Irving, TX, US), Kraton 1645, available from Kraton (Houston, TX, US), Vector Thermoplastic Elastomers 2518, available from TSRC Corporation (Taiwan), Kraton DI 119 PT, available from Kraton (Houston, TX, US), Pebax 4033, available from Arkema (Colombes, France), and PIB additives Oppanol and Glissopal, available from BASF, Florham Park, NJ, US.

[0103] Typically, the composite material of the sheet and the sheet disclosed herein do not comprise a silicone.

[0104] The composite material of the sheet disclosed herein is obtained by densifying a material comprising a porous network of the polymer.

[0105] Typically, the electromagnetically absorbing particles, i.e., the electromagnetically absorbing particles that are comprised in the composite material that is comprised in the sheet disclosed herein, are different from the thermally conductive particles, i.e., from the thermally conductive particles that are comprised in the composite material that is comprised in the sheet disclosed herein.

[0106] In some embodiments of the sheet disclosed herein, the thermally conductive particles are selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide particles, graphene oxide particles, and combinations thereof, wherein the thermally conductive particles do not comprise an electromagnetically absorbing coating located on the surface of the thermally conductive particles, and either

[0107] (i) the electromagnetically absorbing particles are selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof, wherein the metal oxide particles are selected from the group consisting of doped particles of tin oxide, undoped particles of tin oxide, ferrous oxide particles, ferric oxide particles, zinc oxide particles, manganese oxide particles, lead oxide particles, nickel oxide particles, cobalt oxide particles, silver oxide particles, antimony oxide particles, copper oxide (CuO) particles, titanium monoxide (TiO) particles, and combinations thereof; or

[0108] (ii) the electromagnetically absorbing particles comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle. In some embodiments of the sheet disclosed herein, the thermally conductive particles are selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide particles, graphene oxide particles, and combinations thereof, wherein the thermally conductive particles do not comprise an electromagnetically absorbing coating located on the surface of the thermally conductive particles, and either

[0109] (i) the electromagnetically absorbing particles are selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof, wherein the metal oxide particles are selected from the group consisting of doped particles of tin oxide, undoped particles of tin oxide, ferrous oxide particles, ferric oxide particles, zinc oxide particles, manganese oxide particles, lead oxide particles, nickel oxide particles, cobalt oxide particles, silver oxide particles, antimony oxide particles, copper oxide (CuO) particles, titanium monoxide (TiO) particles, and combinations thereof; or

[0110] (ii) the electromagnetically absorbing particles comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle, wherein the core particles are selected from the group consisting of alumina, hollow glass microspheres, solid glass microbeads, and combinations thereof, and wherein the electromagnetically absorbing coating comprises a material selected from the group consisting of tungsten, aluminum, titanium, steel, chromium, nickel, and combinations thereof.

[0111] First process

[0112] A first process for producing the sheet as disclosed herein comprises providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a suspension of electromagnetically absorbing particles and thermally conductive particles in a polymer-solvent solution, wherein the polymer in the polymer- solvent solution has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted at a temperature above the melting point of the polymer in the polymer-solvent solution, and below the boiling point of the solvent, forming the suspension into a film, wherein the platelet-shaped thermally conductive particles are oriented in a direction parallel to the direction of the plane of the film, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the film to obtain a porous film, optionally compressing the porous film to obtain a densified film, stacking multiple layers either of the porous film or of the densified film one upon another to obtain a film stack, pressing the film stack to obtain a bonded film stack, and slicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

[0113] For producing the sheet as disclosed herein by the first process, polymers, thermally conductive particles and electromagnetically absorbing particles as described above more in detail may be used.

[0114] The solvent is typically selected such that it is capable of dissolving the polymer and forming a polymer-solvent solution, i.e., the solvent needs to be miscible with the polymer. Heating the solution to an elevated temperature may facilitate the dissolution of the polymer. In some embodiments, combining the polymer and solvent is conducted at a temperature in a range from 20 °C to 350 °C. The thermally conductive particles and the electromagnetically absorbing particles may be added at any or all of the combining, before the polymer is dissolved, after the polymer is dissolved, or at any time there between.

[0115] The solvent is selected such that it forms a polymer-solvent solution. The solvent may be a blend of at least two individual solvents. In some embodiments, when the polymer is a polyolefin (e.g., at least one of polyethylene and polypropylene), the solvent may be, for example, at least one of mineral oil, tetralin, decalin, orthodichlorobenzene, cyclohexane-toluene mixture, dodecane, paraffin oil / wax, kerosene, isoparaffinic fluids, p-xylene / cyclohexane mixture (1 / 1 wt. / wt.), camphene, 1,2,4 trichlorobenzene, octane, orange oil, vegetable oil, castor oil, or palm kernel oil. In some embodiments, when the polymer is polyvinylidene fluoride, the solvent may be, for example, at least one of ethylene carbonate, propylene carbonate, or 1,2,3 tricetoxypropane.

[0116] The polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles may be combined using conventional mixing aggregates such as a twin screw extruder, planetary extruder, conical twin screw extruder, kneader, or industrial mixer such as but not limited to a double planetary mixer.

[0117] In some embodiments of the first process, the polymer in the polymer-solvent solution has a melting point, and the solvent has a boiling point, and combining the polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles may be conducted at a temperature above the melting point of the polymer in the polymer-solvent solution, and below the boiling point of the solvent. By combining the polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles, a suspension of thermally conductive particles and the electromagnetically absorbing particles in a polymer-solvent solution is formed.

[0118] After forming the suspension of thermally conductive particles and electromagnetically absorbing particles in a polymer-solvent solution, the suspension is formed into a film. In the film, the platelet-shaped thermally conductive particles are oriented in a direction parallel to the direction of the plane of the film. The film may be a continuously formed film, or a film or layer comprising microreplicated or macroreplicated structures and made by microreplication or microreplication techniques. The forming of a film may be conducted using techniques known in the art, including, knife coating, roll coating (e.g., roll coating through a defined nip), and extrusion (e.g., extrusion through a die (e.g., extrusion through a die having the appropriate layer dimensions (i.e., width and thickness of the die gap))).

[0119] In one exemplary embodiment, the polymer-solvent solution has a paste-like consistency and is formed into the film by extrusion (e.g., extrusion through a die having the appropriate layer dimensions (i.e., width and thickness of the die gap)).

[0120] After forming the suspension into a film, where the polymer is miscible in its solvent, the polymer is then induced to phase separate. Several techniques may be used to induce phase separation, including at least one of thermally induced phase separation or solvent induced phase separation. Thermally induced phase separation may occur when the temperature at which induced phase separation is conducted is lower than the combining temperature of the polymer, solvent, the thermally conductive particles and the electromagnetically absorbing particles. This may be achieved by cooling the polymer-solvent solution, if combining is conducted near room temperature, or by first heating the polymer-solvent solution to an elevated temperature (either during combining or after combining), followed by decreasing the temperature of the polymer-solvent solution, thereby inducing phase separation of the polymer. In both cases, the cooling may cause phase separation of the polymer from the solvent. Solvent induced phase separation can be conducted by adding a second solvent, a poor solvent for the polymer, to the polymer-solvent solution, or may be achieved by removing at least a portion of the solvent of the polymer-solvent solution (e.g., evaporating at least a portion of the solvent of the polymer-solvent solution), thereby inducing phase separation of the polymer. Combining of phase separation techniques (e.g., thermally induced phase separation and solvent induced phase separation), may be employed. Thermally induced phase separation may be advantageous, as it also facilitates the dissolution of the polymer when combining is conducted at an elevated temperature. In some embodiments, thermally inducing phase separation is conducted at a temperature in a range from 5 to 300 (in some embodiments, in a range from 5 to 250, 5 to 200, 5 to 150, 15 to 300, 15 to 250, 15 to 200, 15 to 130, or even 25 to 110) °C below the combining temperature. In some embodiments of the first process, the polymer in the polymer-solvent solution has a melting point, and the inducing phase separation is conducted at a temperature less than the melting point of the polymer in the polymer-solvent solution.

[0121] During the induced phase separation, a polymeric network structure may be formed. In some embodiments, the phase separation is induced thermally (e.g., via thermally induced phase separation (TIPS) by quenching to lower temperature), chemically (e.g., via solvent induced phase separation (SIPS) by substituting a poor solvent for a good solvent), or change in the solvent ratio (e.g., by evaporation of one of the solvents). Other phase separation or pore formation techniques known in the art, such as discontinuous polymer blends (also sometimes referred to as polymer assisted phase inversion (PAPI), moisture induced phase separation, or vapor induced phase separation, can also be used. The polymeric network structure may be inherently porous (i.e., have pores). The pore structure may be open, enabling fluid communication from an interior region of the polymeric network structure to an exterior surface of the polymeric network structure and / or between a first surface of the polymeric network structure and an opposing second surface of the polymeric network structure.

[0122] The polymeric network structure may be described as a porous polymeric network or a porous phase-separated polymeric network. Generally, the porous polymeric network (as-made) includes an interconnected porous polymeric network structure comprising a plurality of interconnected morphologies (e.g., at least one of fibrils, nodules, nodes, open cells, closed cells, leafy laces, strands, spheres, or honeycombs). The interconnected polymeric structures may adhere directly to the surface of the thermally conductive particles and the electromagnetically absorbing particles act as a binder for the thermally conductive particles and the electromagnetically absorbing particles. In this regard, the space between adjacent thermally conductive particles and electromagnetically absorbing particles may include porous polymeric network structures, as opposed to a solid matrix material.

[0123] In some embodiments, the polymeric network structure may include a 3 -dimensional reticular structure that includes an interconnected network of polymeric fibrils, In some embodiments, individual fibrils have an average width in a range from 10 nm to 100 nm (in some embodiments, in a range from 100 nm to 500 nm, or even 500 nm to 5 micrometers).

[0124] In some embodiments, the thermally conductive particles are dispersed within the polymeric network structure, such that an external surface of the individual units of the thermally conductive particles (e.g., individual particles or individual agglomerate particles) is mostly uncontacted, or uncoated, by the polymeric network structure. In this regard, in some embodiments, the average percent areal coverage of the polymeric network structure on the external surface of the individual particles (i.e., the percent of the external surface area that is in direct contact with the polymeric network structure) is not greater than 50 (in some embodiments, not greater than 40, 30, 25, 20, 10, 5, or even not greater than 1) percent, based on the total surface area of the external surfaces of the individual particles. Although not wanting to be bound by theory, it is believed that the large, uncontacted surface area coating on the thermally conductive particles enables increased particle- to-particle contact upon compression and therefore increases thermal conductivity.

[0125] After the polymer has been phase separated from the solvent by the inducing phase separation step, at least a portion of the solvent may be removed from the fdm, i.e., from the polymeric network structure of the fdm, thereby forming a porous fdm. The porous fdm has a polymeric network structure and the thermally conductive particles and electromagnetically absorbing particles are distributed within the polymeric network structure.

[0126] The solvent (e.g., a first solvent) may be removed by evaporation, high vapor pressure solvents being particularly suited to this method of removal. If the first solvent, however, has a low vapor pressure, it may be desirable to have a second solvent, of higher vapor pressure, to extract the first solvent, followed by evaporation of the second solvent. In some embodiments, in a range from 10 to 100 (in some embodiments, in a range from 20 to 100, 30 to 100, 40 to 100, 50 to 100, 60 to 100, 70 to 100, 80 to 100, 90 to 100, 95 to 100, or even 98 to 100) percent by weight of the solvent, and second solvent, if used, may be removed from the fdm.

[0127] For example, in some embodiments, when mineral oil is used as a first solvent, isopropanol at elevated temperature (e.g., about 60 °C) or a blend of methyl nonafluorobutyl ether (C4F9OCH3), ethyl nonaflurorobutyl ether (C4F9OC2H5), and trans- 1,2-dichloroethylene (available, for example, under the trade designation “NOVEC 72DE” from 3M Company, St. Paul, MN) may be used as a second solvent to extract the first solvent, followed by evaporation of the second solvent. In some embodiments, when at least one of vegetable oil or palm kernel oil is used as the first solvent, isopropanol at elevated temperature (e.g., about 60 °C) may be used as the second solvent. In some embodiments, when ethylene carbonate is used as the first solvent, water may be used as the second solvent.

[0128] In some embodiments of the first method, the formed and phase separated film, after the solvent removal, has a porosity of at least 5 (in some embodiments, at least 10, 20, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or even at least 80; in some embodiments, in a range from 25 to 80) percent. This porosity is caused by the phase separation of the polymer from the solvent, which initially leaves no unfilled voids, as the pores in the polymeric network structure are filled with solvent. After the solvent is completely or partly removed, pores are formed in the polymeric network structure.

[0129] Typically, the polymer used in the first process disclosed herein is a thermoplastic polymer.

[0130] The thickness of the porous film and of the densified film may be, e.g., from 5 to 150 mils (0.127 to 3.81 mm). The thickness of the porous film and of the densified film may also be lower than 5 mil or higher than 150 mil.

[0131] Second process

[0132] A second process for producing the sheet as disclosed herein comprises providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a slurry, wherein the slurry is a suspension of the polymer, the electromagnetically absorbing particles, and the thermally conductive particles in the solvent, and wherein the polymer has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted below the melting point of the polymer, and below the boiling point of the solvent, forming the slurry into a fdm, wherein the platelet-shaped thermally conductive are oriented in a direction parallel to the direction of the plane of the film, heating the film in an environment to retain at least 90 percent by weight of the solvent in the film, based on the weight of the solvent in the film, and solubilizing at least 50 percent by weight of the polymer in the solvent, based on the total weight of the polymer, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the film to obtain a porous film, optionally compressing the porous film to obtain a densified film, stacking multiple layers either of the porous film or of the densified film one upon another to obtain a film stack, pressing the film stack to obtain a bonded film stack, and slicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

[0133] Typically, the polymer used in the second process disclosed herein is a thermoplastic polymer.

[0134] For producing the sheet as disclosed herein by the second process, polymers, thermally conductive particles and electromagnetically absorbing particles as described above more in detail may be used.

[0135] The solvent is typically selected such that it is capable of dissolving the polymer and forming a polymer-solvent solution, i.e., the solvent needs to be miscible with the polymer. Heating the solution to an elevated temperature may facilitate the dissolution of the polymer. In some embodiments, combining the polymer and solvent is conducted at a temperature in a range from 20 °C to 350 °C. The thermally conductive particles and the electromagnetically absorbing particles may be added at any or all of the combining, before the polymer is dissolved, after the polymer is dissolved, or at any time there between. In some embodiments of the second process, the solvent is a blend of at least two individual solvents. In some embodiments, when the polymer is a polyolefin (e.g., at least one of polyethylene or polypropylene), the solvent may be at least one of mineral oil, tetralin, decalin, orthodichlorobenzene, cyclohexane-toluene mixture, dodecane, paraffine oil / wax, kerosene, p- xylene / cyclohexane mixture (1 / 1 wt. / wt.), camphene, 1,2,4 trichlorobenzene, octane, orange oil, vegetable oil, castor oil, or palm kernel oil. In some embodiments, when the polymer is polyvinylidene fluoride, the solvent is at least one of ethylene carbonate, propylene carbonate, or 1,2,3 triacetoxypropane.

[0136] The slurry may be continuously mixed or blended to prevent or reduce settling or separation of the polymer and / or particles from the solvent. Typically, there is no need for continuously mixing or blending to prevent or reduce settling or separation of the polymer and / or particles from the solvent. In some embodiments, the slurry is degassed using techniques known in the art to remove entrapped air.

[0137] The slurry, which is obtained by combining (e.g., mixing or blending) the polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles, is a suspension of the polymer and the thermally conductive particles and the electromagnetically absorbing particles in the solvent.

[0138] The polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles may be combined using conventional mixing aggregates such as a twin screw extruder, planetary extruder, conical twin screw extruder, kneader, or industrial mixer such as but not limited to a double planetary mixer.

[0139] In some embodiments of the second process, the weight ratio of solvent to polymer is at least 9: 1.

[0140] In some embodiments of the second process, combining is conducted at a temperature below the melting point of the polymer and below the boiling point of the solvent.

[0141] The polymer provided for the processes disclosed herein has a melting point, and the solvent has a boiling point. In some embodiments of the second process, combining the polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles is conducted at a temperature below the melting point of the polymer, and below the boiling point of the solvent. By combining the polymer, the solvent, the thermally conductive particles and the electromagnetically absorbing particles, a slurry, i.e., a suspension of the polymer and thermally conductive particles and electromagnetically absorbing particles in the solvent is formed.

[0142] The slurry is formed into a film using techniques known in the art, including knife coating, roll coating (e.g., roll coating through a defined nip), and coating through any number of different dies having the appropriate dimensions or profiles. These coating techniques may be performed between two liners. In some embodiments, and for ease of manufacturing, it may be desirable to form the fdm at room temperature.

[0143] In some embodiments of the second process, heating the fdm is conducted at a temperature above the melting point of the miscible polymer-solvent solution, and below the boiling point of the solvent.

[0144] In some embodiments of the second process, inducing phase separation is conducted at a temperature less than the melting point of the polymer in the slurry. Although not wanting to be bound, it is believed that in some embodiments, solvents used to make a miscible blend with the polymer can cause melting point depression in the polymer. The melting point described herein includes below any melting point depression of the polymer solvent system.

[0145] The inducing phase separation is conducted at less than the melting point of the polymer.

[0146] During the induced phase separation, a polymeric network structure may be formed. In some embodiments, the phase separation is induced thermally (e.g., via thermally induced phase separation (TIPS) by quenching to lower temperature than used during heating). Cooling can be provided, for example, in air, liquid, or on a solid interface, and varied to control the phase separation. The polymeric network structure may be inherently porous (i.e., have pores). The pore structure may be open, enabling fluid communication from an interior region of the polymeric network structure to an exterior surface of the polymeric network structure and / or between a first surface of the polymeric network structure and an opposing second surface of the polymeric network structure.

[0147] The polymeric network structure may be described as a porous polymeric network or a porous phase-separated polymeric network. Generally, the porous polymeric network (as-made) includes an interconnected porous polymeric network structure comprising a plurality of interconnected morphologies (e.g., at least one of fibrils, nodules, nodes, open cells, closed cells, leafy laces, strands, spheres, or honeycombs). The interconnected polymeric structures may adhere directly to the surface of the thermally conductive particles and electromagnetically absorbing particles and act as a binder for the thermally conductive particles and electromagnetically absorbing particles. In this regard, the space between adjacent thermally conductive particles and electromagnetically absorbing particles may include porous polymeric network structures, as opposed to a solid matrix material.

[0148] The polymeric network structure may comprise a plurality of nodes which are interconnected by a plurality of fibrils. Typically, the nodes have a diameter of from 1 to 50 pm. The diameter is measured as the maximum diameter on scanning electron micrographs of the composite material. The fibrils which connect the individual nodes with one another may have a diameter of from 80 to 2000 nm and a length of from 1 to 50 pm.

[0149] The nodes and the fibrils comprise the polymer. The nodes and the fibrils may consist of the polymer. The thermally conductive particles and electromagnetically absorbing particles may be located near the nodes or within the nodes. The thermally conductive particles and electromagnetically absorbing particles may also be located near the fibrils or may be connected to the fibrils. The fibrils may be mechanically anchored to the thermally conductive particles and electromagnetically absorbing particles, or, in other words, the thermally conductive particles and electromagnetically absorbing particles may act as nodes to which the fibrils are mechanically anchored. The porosity of the composite material results from pores that are located between the fibrils.

[0150] The fibrils of the composite material comprised in the sheet disclosed herein are different from fibers as known in the art. Fibers are a possible macroscopic shape of a material and typically have a length of at least 100 pm, e.g., at least 1 mm and up to 30 mm and a plurality of fibers may be comprised in a non-homogeneous woven or braided structure. In contrast, the fibrils of the composite material comprised in the sheet disclosed herein are microscopic structures with typical diameters of 80 to 2000 nm (e.g., 80 to 800 nm) and lengths of 1 to 50 pm, and with each of the fibrils being interconnected with the nodes.

[0151] In some embodiments, the polymeric network structure may include a 3 -dimensional reticular structure that includes an interconnected network of polymeric fibrils, In some embodiments, individual fibrils have an average width in a range from 10 nm to 100 nm (in some embodiments, in a range from 100 nm to 500 nm, or even 500 nm to 5 micrometers).

[0152] In some embodiments, the thermally conductive particles are dispersed within the polymeric network structure, such that an external surface of the individual units of the thermally conductive particles (e.g., individual particles or individual agglomerate particles) is mostly uncontacted, or uncoated, by the polymeric network structure. In this regard, in some embodiments, the average percent areal coverage of the polymeric network structure on the external surface of the individual particles (i.e., the percent of the external surface area that is in direct contact with the polymeric network structure) is not greater than 50 (in some embodiments, not greater than 40, 30, 25, 20, 10, 5, or even not greater than 1) percent, based on the total surface area of the external surfaces of the individual particles. Although not wanting to be bound by theory, it is believed that the large, uncontacted surface area coating on the thermally conductive enables increased particle -to-particle contact upon compression and therefore increases thermal conductivity.

[0153] After the phase separation, at least a portion of the solvent is removed from the film. By removing at least a portion of the solvent from the film, a porous film is obtained. In some embodiments of the second process, at least 90 percent by weight of the solvent, based on the weight of the solvent in the film, is removed. The film, before removing at least 90 percent by weight of the solvent, based on the weight of the solvent in the film, has a first volume, and the film, after removing at least 90 percent by weight of the solvent, based on the weight of the solvent in the film, has a second volume, and the difference between the first and second volume (i.e., (the first volume minus the second volume) divided by the first volume times 100) is less 10 (in some embodiments, less than 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.75, 0.5, or even less than 0.3) percent. Volatile solvents can be removed from the film, for example, by allowing the solvent to evaporate from at least one major surface of the film. Evaporation can be aided, for example, by the addition of at least one of heat, vacuum, or air flow. Evaporation of flammable solvents can be achieved in a solvent-rated oven. If the solvent (i.e., a first solvent), however, has a low vapor pressure, a second solvent, of higher vapor pressure, may be used to extract the first solvent, followed by evaporation of the second solvent. For example, in some embodiments, when mineral oil is used as a first solvent, isopropanol at elevated temperature (e.g., about 60 °C) or a blend of methyl nonafluorobutyl ether (C4F9OCH3), ethylnonafluorobutyl ether (C4F9OC2H5), and trans- 1,2-dichloroethylene (available, for example, under the trade designation “NOVEC 72E” from 3M Company, St. Paul, MN) may be used as a second solvent to extract the first solvent, followed by evaporation of the second solvent. In some embodiments, when at least one of vegetable oil or palm kernel oil is used as the first solvent, isopropanol at elevated temperature (e.g., about 60 °C) may be used as the second solvent. In some embodiments, when ethylene carbonate is used as the first solvent, water may be used as the second solvent.

[0154] In some embodiments of the second method, the film has a first and second major surface with ends perpendicular to the first and second major surfaces, and the ends are unrestrained during the solvent removal. This can be done, for example, by drying a portion of a layer without restraint in an oven. Continuous drying can be achieved, for example, by drying a long portion of a layer, supported on a belt as it is conveyed through an oven. Alternatively, to facilitate removal of nonvolatile solvent, for example, a long portion of the film can be continuously conveyed through a bath of compatible volatile solvent, thereby exchanging the solvents and allowing the film to be subsequently dried without restraint. Not all the non-volatile solvent, however, needs to be removed from the film during the solvent exchange. Small amounts of non-volatile solvents may remain and act as a plasticizer to the polymer.

[0155] In some embodiments of the second method, the formed and phase separated film, after the solvent removal, has a porosity of at least 5 (in some embodiments, at least 10, 20, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or even at least 80; in some embodiments, in a range from 25 to 80) percent. This porosity is caused by the phase separation of the polymer from the solvent, which initially leaves no unfilled voids, as the pores in the polymeric network structure are filled with solvent. After the solvent is completely or partly removed, pores are formed in the polymeric network structure.

[0156] In some embodiments of the first and second process, up to 10 percent by weight of nonvolatile solvents or organic liquids remain in the porous film after solvent removal, improving the elasticity of the porous film and of the sheet made from the porous film. It is also possible that up to 10 percent by weight of non-volatile organic liquids are added after solvent removal.

[0157] The first and the second process optionally include compressing the porous film, after removing at least a portion of the solvent from the film. By compressing the porous film, the porous film is densified. Compressing the porous film may be achieved, for example, by conventional calendering processes known in the art.

[0158] By compressing the porous film, the polymeric network structure of the porous film is plastically deformed. Vibratory energy may be imparted during the application of the compressive force. In some embodiments, the porous film is in the form of a strip of indefinite length, and the applying of a compressive force step is performed as the strip passes through a nip. A tensile loading may be applied during passage through a nip. For example, the nip may be formed between two rollers, at least one of which applies the vibratory energy; between a roller and a bar, at least one of which applies the vibratory energy; or between two bars, at least one of which applies the vibratory energy. The applying of the compressive force and the vibratory energy may be accomplished in a continuous roll-to-roll fashion, or in a step-and-repeat fashion. In other embodiments, the applying a compressive force step is performed on a discrete layer between, for example, a plate and a platen, at least one of which applies the vibratory energy. In some embodiments, the vibratory energy is in the ultrasonic range (e.g., 20 kHz), but other ranges are considered to be suitable.

[0159] The density of the densified film may be at least 60% (in some embodiments, at least 70%, at least 80%, at least 90%, or even 100%; in some embodiments, in the range from 60 to 98%, 70 to 98%, 80 to 98%, 60 to 100%, 70 to 100%, or even 80 to 100%) of theoretical density after densification. The theoretical density can be calculated from the known densities of the components of the densified porous film and from the weight fractions of the components.

[0160] By compressing the porous film, the density and the thermal conductivity of the film is increased by increasing the particle-to-particle contact of the thermally conductive particles. Compression of the porous film increases the density, which reduces the insulating air volume (or porosity) of the film, which will therefore increase the thermal conductivity. Likewise, the increased particle-to-particle contact of the thermally conductive particles can be measured by increased thermal conductivity.

[0161] By compression of the porous polymeric network structure of the film, a film having a higher density and a compressed polymeric network structure is obtained, with the thermally conductive particles and the electromagnetically absorbing particles distributed within the polymer network and with increased particle-to-particle contact.

[0162] If the porous film is not compressed before stacking multiple layers, the density and the thermal conductivity is increased by pressing the film stack, which compresses the porous polymeric network structure of the film. By pressing the film stack, the particle-to-particle contact of the thermally conductive particles particles and the electromagnetically absorbing particles is increased.

[0163] The through-plane thermal conductivity of the compressed, densified film may be in a range from 1 to 20 W / m*K, and the in-plane thermal conductivity of the compressed, densified film may be from 4.0 to 40 W / m*K. The processes for producing the sheet as disclosed herein, i.e., the first and the second process, comprise stacking multiple layers either of the porous film or of the densified film one upon another. If the porous film has not been densified before stacking, multiple layers of the porous film are stacked one upon another. If the porous film has been densified before stacking, multiple layers of the densified film are stacked one upon another. By stacking multiple layers, either of the porous film or of the densified film, a film stack is obtained.

[0164] For stacking multiple layers of the porous film, or of the densified film, respectively, the porous film, or the densified film, respectively, may be cut into film pieces of equal size and the film pieces may be stacked.

[0165] Pressing the film stack for the first and the second process is performed at a pressure and at a temperature and for a time sufficient to ensure bonding of the individual film layers to a bonded film stack, i.e., to a bonded block.

[0166] Pressing the film stack for the first and second process may be performed at a temperature of at least 110 °F (43 °C) and typically is performed at a temperature of at most 575 °F (302 °C). For example, if the polymer used to make the composite material of the sheet disclosed herein is an ultra-high molecular weight polyethylene, pressing the film stack may be performed at a temperature of at least 275 °F (135 °C) and typically is performed at a temperature of at most 400 °F (204 °C). By this pressing step, a bonded film stack is obtained.

[0167] It has been found that a minimum pressing pressure is required for pressing the film stack.

[0168] Advantageously, pressing the film stack may be performed under a pressing pressure of at least 10 MPa. By using a pressing pressure of at least 10 MPa, it is ensured that the individual layers of the film stack are bonded to one another and a bonded film stack, i.e., a bonded block, is obtained. In the bonded block, the individual film layers are seamlessly bonded to one another.

[0169] Furthermore, it has been found that, by further increasing the pressing pressure to values higher than the minimum pressing pressure of 10 MPa, through-plane thermal conductivity of the sheet obtained after the subsequent slicing step can be significantly further increased. Without wishing to be bound by theory, it is believed that this can be explained by an increased particle-to- particle contact of the thermally conductive particles from layer to layer, i.e., between originally individual adjacent film layers in the bonded film stack formed upon pressing.

[0170] Typically, pressing the film stack is performed under a pressing pressure of up to 40 MPa. Higher pressing pressures are possible, but are typically not required, as the through-plane thermal conductivity of the sheet typically is not further increased by increasing pressing pressure above 40 MPa.

[0171] Typically, pressing the film stack is performed for at least 2 minutes.

[0172] Advantageously, pressing the film stack may be performed in a pressing direction (i.e., in z- direction) while the film stack is constrained in a first and in a second direction perpendicular to the pressing direction, wherein the first direction is perpendicular to the second direction (i.e., in x- and y-direction). For example, a mold, e.g., a steel mold, may be used to constrain the stack in x- and y- direction. Another possibility is to perform pressing of the fdm stack in a pressing direction (i.e., in z-direction) while constraining the fdm stack in a first direction perpendicular to the pressing direction (i.e., in x-direction) and allowing the film stack to expand in a second direction perpendicular to the pressing direction, wherein the first direction is perpendicular to the second direction (i.e., in the y-direction).

[0173] The x-direction, or first direction perpendicular to the pressing direction, is a first direction parallel to the planes of the multiple layers of the porous or densified film. The y-direction, or second direction perpendicular to the pressing direction, is a second direction parallel to the planes of the multiple layers of the porous or densified film. The z-direction, or pressing direction, is a direction perpendicular to the planes of the multiple layers of the porous or densified film.

[0174] The processes as disclosed herein for producing the sheet of the present disclosure, i.e., the first and second process, may further comprise heating the film stack prior to pressing the film stack.

[0175] Advantageously, the film stack may be heated to a temperature from 110 °F to 575 °F (43 °C to 302 °C) prior to pressing the film stack. The temperature to be selected typically is near or above the melting temperature of the polymer binder. For example, if the polymer used to make the composite material of the sheet disclosed herein is an ultra-high molecular weight polyethylene, the film stack may be heated to a temperature from 275 °F to 400 °F (135 °C to 204 °C) prior to pressing the film stack.

[0176] By heating the film stack prior to pressing the film stack, a better bonding between the individual stacked film layers can be obtained.

[0177] The first and second process comprises slicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

[0178] Slicing (or cutting) may be performed by using a skiving knife or a diamond wire saw. A pneumatic cylinder may be used to force the bonded film stack across the knife or saw. The bonded film stack may be heated to make cutting easier. Force may also be applied to keep the bonded film stack pressed down against the knife or saw.

[0179] The sheet obtained by the first and second process comprises a composite material comprising a polymer, platelet-shaped thermally conductive particles and electromagnetically particles, wherein the platelet-shaped thermally conductive particles are oriented in a direction perpendicular to the direction of the plane of the sheet. By the orientation of the platelet-shaped thermally conductive particles in a direction perpendicular to the direction of the plane of the sheet, and by the high loadings of thermally conductive particles in the composite material, a sheet having a high through-plane thermal conductivity can be obtained. The processes as disclosed herein for producing the sheet of the present disclosure, i.e., the first and second process, may further comprise heating of the bonded film stack prior to slicing a sheet from the bonded film stack.

[0180] Advantageously, the bonded film stack may be heated to a temperature from 110 °F to 575 °F (43 °C to 302 °C) prior to slicing a sheet from the bonded film stack. The temperature to be selected typically is near or above the melting temperature of the polymer binder. For example, if the polymer used to make the composite material of the sheet disclosed herein is an ultra-high molecular weight polyethylene, the bonded film stack may be heated to a temperature from 275 °F to 400 °F (135 °C to 204 °C) prior to slicing a sheet from the bonded film stack.

[0181] By heating the bonded film stack prior to slicing a sheet from the bonded film stack, the bonded film stack is softened which makes slicing easier.

[0182] In Figure 1, the stacking of multiple layers of the porous or densified film one upon another to obtain a film stack, and the slicing of a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers is represented schematically. In a first step of the processes disclosed herein, a porous film 3 is formed comprising platelet-shaped thermally conductive particles, preferably platelet-shaped hexagonal boron nitride particles 1 which are oriented in a direction parallel to the direction of the plane of the film. The porous film 3 further comprises electromagnetically absorbing particles 2. The electromagnetically absorbing particles may be isotropic particles which are not oriented in a direction parallel or perpendicular to the direction of the plane of the film. In an optional process step, the porous film may be densified to obtain a densified film. The white arrows in Figure 1 represent the direction perpendicular to the basal planes of the hexagonal boron nitride particles, the black arrows in Figure 1 represent the direction parallel to the basal planes of the hexagonal boron nitride particles. In the drawing for the single film layer 3 (porous or densified) on the left side of Figure 1, the white arrow (representing the direction perpendicular to the basal planes of the hexagonal boron nitride particles, which is the direction of low thermal conductivity) is oriented perpendicularly to the direction of the plane of the film, i.e., in a direction through-plane of the film. The black arrow (representing the direction parallel to the basal planes of the hexagonal boron nitride particles, which is the direction of high thermal conductivity) is oriented parallelly to the direction of the plane of the film, i.e., in a direction inplane of the film. In a further step of the processes disclosed herein, multiple layers of the porous or densified film 3 are stacked one upon another to obtain a film stack 4. In the drawing of the film stack 4 of Figure 1, the white arrow (representing the direction perpendicular to the basal planes of the hexagonal boron nitride particles, which is the direction of low thermal conductivity) is oriented perpendicularly to the direction of the planes of the multiple layers of the porous or densified film. In the next step of the processes disclosed herein, the film stack 4 is pressed to obtain a bonded film stack 5. From the bonded film stack 5, a sheet 6 is sliced in a direction perpendicular to the planes of the stacked film layers. In the drawing of the sheet 7 on the right side of Figure 1, the sheet 6 which had been obtained by the slicing step is rotated by 90 degrees so that the direction of the plane of the obtained sheet is now parallel to the plane of the drawing. In this drawing of the sheet 7, the white arrow (representing the direction perpendicular to the basal planes of the hexagonal boron nitride particles, which is the direction of low thermal conductivity) is oriented parallelly to the direction of the plane of the sheet, i.e., in a direction in-plane of the sheet. The black arrow (representing the direction parallel to the basal planes of the hexagonal boron nitride particles, which is the direction of high thermal conductivity) is oriented perpendicularly to the direction of the plane of the sheet, i.e., in a direction through-plane of the sheet.

[0183] The sheet as disclosed herein is useful as thermal interface material. Such thermal interface materials are useful for managing heat flow into or out of different components such as in electronic devices (e.g., batteries, motors, refrigerators, circuit boards, solar cells, and heaters). In some embodiments, an article (e.g., electronic device) comprises a heat source and the sheet described herein in contact with the heat source.

[0184] Examples

[0185] Test methods

[0186] Density and Porosity Test

[0187] If not otherwise indicated, the density of a sample was calculated using a method similar to ASTM F-1315-17 (2017), “Standard Test Method for Density of a Sheet Gasket Material” by cutting a 47 mm diameter disc, weighing the disc on an analytical balance of suitable resolution (typically 0.0001 grams), and measuring the thickness of the disc on a thickness gauge (obtained as Model 49- 70 from Testing Machines, Inc., New Castle, DE, US) with a dead weight of 7.3 psi (50.3 KPa) and a flat anvil of 0.63 inch (1.6 cm) diameter, with a dwell time of about 3 seconds and a resolution of + / -0.0001 inch. The density was then calculated by dividing the mass by the volume, which was calculated from the thickness and diameter of the sample. With the known densities and weight fractions of the components of the composite material, the theoretical density of the composite material was calculated by the rule of mixtures. Using the theoretical density and the measured density, the porosity was calculated as:

[0188] Porosity [%] =

[0189] (theoretical density - measured density) / (theoretical density - 0.001293 g / cm3) x 100

[0190] The density of 0.001293 g / cm3is the density of air.

[0191] For some of the samples, the density was measured using the Archimedes density method due to uneven surfaces from bandsaw cutting. Density for these samples was measured using an analytical scale (obtained as model MS 1003TS-C012187274 from Mettler Toledo (Schweiz) GmbH, Im Langacher 44, 8608 Griefensee, Switzerland). A glass container (similar to a beaker) was placed above the scale and was filled with distilled water until at least 3 / 4 full. A sample dish with a basket attached to the bottom was placed above the glass container so that the basket was fully submerged, and where both the dish and basket are attached to the scale so their mass is measured. The scale's mass was zeroed and a l” diameter disk was placed in the top dish so that its mass in air was measured. After storing this data, the sample was placed in the basket below the dish so that the mass of the sample when submerged in water could be measured. Using the difference in mass between the sample when dry and when submerged in water, as well as 0.99777 g / cm3for the density of distilled water at 22 °C, the density of the block was calculated.

[0192] Thermal Conductivity Test

[0193] Direct thermal diffusivity measurements are made using the flash analysis method as per ASTM E1461 (2013), using a light flash thermophysical properties analyzer (obtained as “HYPERFLASH LFA 467” from Netzsch Instruments North America LLC, Boston, MA, US). Each sample set included a reference sample (obtained under the trade designation “AXM-5Q POCO GRAPHITE” from Poco Graphite, Decatur, TX, US) which acted as a method control for diffusivity measurements. Samples are coated with a layer of sprayed-on graphite (3 spray passes at a distance of approximately 5 inches with graphite spray obtained under the trade designation “DGF 123 DRY GRAPHITE FILM SPRAY” from Miracle Power Products Corporation, Cleveland, OH, US) on the light impingement side and the detection side to normalize surface effusivity and absorptivity for the sample being tested. The sample’s thickness was measured on a thickness gauge (obtained as Model 49-70 from Testing Machines, Inc., New Castle, DE, US). The thickness was used to calculate the geometric density of the samples. In a single measurement, called a “shot”, a short time duration pulse of light (Xenon flash lamp, 230 V, 15 microsecond duration) was impinged onto one side of a sample, and a thermogram (time trace of measured temperature) was recorded on the opposite side of the sample, as measured by the voltage on an InSb IR detector. Thermal diffusivity was calculated from a fit of the thermogram to the Cowan Plus Pulse Correction model for through-plane and an anisotropic model for in-plane (the anisotropic model used for in-plane thermal diffusivity takes into account the through-plane data). Heat capacity was calculated by differential scanning calorimetry (DSC) using a DSC instrument (obtained under the trade designation “Q2000 DSC” from TA Instruments, New Castle, DE, US), following ASTM E1269 (2011) “Quasi-Isothermal Moderated DSC”. Sapphire was used as a reference for DSC. The through-plane diffusivity was calculated using the Cowan method with an additional correction for a finite pulse width, while in-plane diffusivity used the anisotropic model with the aid of the software (obtained under the trade designation “Proteus” from Netzsch, Selb, Germany). Samples with 1 inch diameter were used for the measurements. Three shots were obtained for each sample at 25 °C. The product of measured density (p) (geometric from 2.54 cm (1 inch) discs), specific heat capacity (cp) (by differential scanning calorimetry), and diffusivity (a) gave the thermal conductivity. That is, k (W / (m * K)) = p (g / cm3) x cp (J / K / g) x a (mm2 / s).

[0194] Hardness measurement (Shore D)

[0195] The Shore durometer hardness of samples was measured according to ASTM D2240-15 using the D scale. Testing was performed under ambient lab conditions of 23 °C and less than 20% relative humidity. Measurements reported were from stacked, bonded blocks and reported as averages of five measurements for the skived surfaces parallel to the lamination plane.

[0196] Dielectric properties measurement

[0197] Dielectric properties at 10 GHz are measured using a 7 mm coaxial waveguide via the ASTM D7449 standard. The APC-7 7 mm coaxial cables from Keysight Technologies are connected to a Keysight Vector Network Analyzer (model 8364C) and are calibrated using the short / open / line / thru (SOLT) method. Measurements of dielectric permittivity, including the real part epsilon’ and imaginary part epsilon”, are conducted between 1-18 GHz, and the 10 GHz values are selected to evaluate dielectric constant Dk and dielectric loss factor Df. Dielectric properties at 40 GHz are measured using a Ka-band rectangular waveguide via the ASTM D5568 standard. The waveguide fixture is from Maury Microwave, is connected to the same Keysight 8364C vector network analyzer via SMA coaxial cables, and is calibrated using a thru / reflect / line (TRL) method. Measurements of dielectric permittivity, including the real part epsilon’ and imaginary part epsilon”, are conducted between 27-40 GHz, and the 40 GHz values are selected to evaluate dielectric constant Dk and dielectric loss factor Df.

[0198] Orientation index measurement

[0199] The orientation index is determined by X-ray diffractometry. For this, the ratio of the intensities of the (100) and of the (002) reflection of hexagonal boron nitride (hBN) measured on X- ray diffraction diagrams of a sheet sample is determined and is divided by the corresponding ratio for an ideal, unoriented, i.e., isotropic, hBN sample. This ideal intensity ratio, or reference intensity ratio, can be determined from Powder Diffraction Pattern (PDF) #01-073-2095 of the International Centre for Diffraction Data (ICDD) data (2020) and is 0. 147. The theoretical peak positions of (002) and (100) reflections are at 26.7 and 41.6 degrees in two theta for Cu Kai radiation source, respectively. Peak intensities of (002) and (100) reflections are measured from peak area at these positions.

[0200] For measurement of peak intensities of (002) and (100) reflections, the samples were cut and loaded on zero background silicon sample holders. Reflection geometry data were collected in the form of a survey scan by use of a PANalytical Empyrean vertical diffractometer, copper Ka radiation, and PIXcel-3D detector (ID mode with 255 channels or 3.35° opening) registry of the scattered radiation. The X-ray source and detector are sitting at a circle with a radius of 240.00 mm. The diffractometer is fitted with 0.04 rad sellers at both incidence and diffraction beam sides, mask 20 and Ni filter on incidence side, programmable divergence slit at 140.00 mm sample distance to control irradiated length at the sample to 5.0 mm, receiving slit at height of 2.00 mm, and programmable anti-scatter slit to control observed length to 2.0 mm. The survey scan was conducted from 5 to 80 degrees (2q) using a 0.04 degree step size and 1200-second setting for dwell time. X- ray generator settings of 40 kV and 40 mA were employed.

[0201] The orientation index (OI) can be determined from the measured peak intensities of (002) and (100) reflections using the formula:

[0202] 1(100), sample / 1(002), sample 1(100), sample / 1(002), sample

[0203] OI = - = -

[0204] 1(100), theoretical / 1(002), theoretical 0. 147

[0205] Preparation of Tungsten (W) and Aluminum Oxide (AlOx) Coated Alumina Particles

[0206] Tungsten Coating of Alumina Particles

[0207] A 5” x 12” rectangular tungsten (W) sputter target was used to produce W thin film coated alumina particles. The apparatus used for the preparation ofW thin film coated particles is described in U.S. Pat. No. 8,698,394 (McCutcheon et al.). 5786.29 g of BAK-70 alumina particles (spherical alumina particles with a mean particle size (D50) of about 70 microns; available from Bestry Performance Materials, Shanghai, China; the mean particle size D50 of the BAK-70 particles was measured by laser light scattering using a laser particle sizer (LS-POP(6) available from OMEC Instruments, Guangdong, China)) was loaded in the particle agitator assembly positioned inside the vacuum chamber. The vacuum chamber was pumped down to a base pressure of IxlO-6torr. Tungsten was sputtered for 6 hours at 1.0 kW at an argon sputtering gas pressure of 5 millitorr. The chamber was backfdled with Argon, a small portion of the W coated alumina particles was removed, and a powder resistivity of 150 ohm -cm was measured. The estimated thickness of the W coating was 6-7 nm.

[0208] Aluminum Oxide (AlOx) Coating on Tungsten Coated Alumina Particles

[0209] An aluminum oxide coating was made to encapsulate the W thin fdm to prevent from oxidation as generally described in U.S. Pat. No. 5,389,434 (Chamberlain et al.). A 5”x8” aluminum target was used in the same sputter coater and aluminum was sputtered. The AlOx layer was coated on top by admitting oxygen gas at a rate of 25 seem (standard cubic centimeter per minute), in addition to argon sputter gas. The total pressure was kept at 10 millitorr. A cathodic power of 5.00 kW was applied for 5 hours with particle agitation of 15 rpm. At the end of 5 hours, the chamber was vented to ambient conditions and the particles were removed from the agitator. The powder resistivity of final aluminum oxide coating was >30E+06 ohm -cm range.

[0210] Examples 1 to 3 (EXI to EX3)

[0211] For Example 1, ultra-high molecular weight polyethylene (UHMWPE) powder (obtained under the trade designation “GUR-2126” from Celanese Corporation, Irvine, TX, US) was metered into the feed funnel of a twin-screw extruder (25 mm co-rotating twin screw extruder, Berstorff, Germany) at 275 rpm, 93 °C, from a powder feeder (KT20, Coperion K-Tron, Stuttgart, Germany) at the rate shown in Table 1. Tungsten and aluminum oxide coated alumina particles prepared as described above were metered into the feed funnel of the extruder from a disk feeder (Colortronic DP30 Disk Feeder) at the rate shown in Table 1. Hexagonal boron nitride particles (BN CFP 009, 3M Company, St. Paul, MN, US) were metered into the feed funnel of the extruder from a powder feeder (KT20, Coperion K-Tron, Stuttgart, Germany) at the rate shown in Table 1.

[0212] Paraffin (obtained under the trade designation “ISOPAR G” from Brenntag Great Lakes, Inc., Wauwatosa, WI, US) was pumped into the open barrel zone 2 of the extruder at 93 °C using a gear pump (Zenith Pumps, Monroe, NC, US) and Coriolis mass flow meter (Micromotion mass flow meter, Emerson Electric Co, St. Louis, MO, US) at the rate shown in Table 1, mixed and heated to 148 °C to melt the UHMWPE into the paraffin.

[0213] Finally, hexagonal boron nitride particles (BN CFP 009, 3M Company, St. Paul, MN, US) was fed from a powder feeder into a side staffer (Side Feeder, Century Extrusion, Travers City, MI, US) at 250 rpm connected to zone 4 of the twin screw extruder at 148 °C and was incorporated into the melt. The extruder mixed, dispersed, and increased the temperature of the melt making a stable mixture (i.e., a suspension). The mixture was then extruded through a 2” drop die with a 10 mil gap at 165 °C onto a smooth casting roll at 60 °C and the speed shown in Table 1 and quenched to form a film.

[0214] Upon quenching, the ultra-high molecular weight polyethylene phase-separated from the suspension forming a phase separated porous polymeric network structure connecting the tungsten coated alumina and hexagonal boron nitride particles and paraffin filling the voids. Results are shown in Table 1. Table 1:

[0215] For Examples 2 and 3, a cast film was prepared as described for Example 1, with the exceptions that different feed rates and casting roll speeds were used, listed in Table 2, and that additionally polyisobutene (obtained under the trade designation “Oppanol B 15 SFN” from BASF, Florham Park, NJ, US) was fed into zone 3 using a roll feeder at 300 °F at an extruder speed of 2.2 rpm and melt pump speed and feed rate listed in Table 2. Results are shown in Table 2.

[0216] Table 2:

[0217] The obtained films were placed into a solvent oven at 220 °F (104 °C) to evaporate the paraffin, thereby obtaining porous films which were subsequently cut into 1.6” x 1.6” squares. For stacking and pressing the squares, i.e., for stacking multiple layers of the porous film and for pressing the film stack, a steel mold was used to constrain the stack in the x- and y-direction. The steel mold had a cavity having a width of 1.625”, a length of 1.625”, and a height of 2.625”, and a top plunger, and was placed into an oven at 300 °F (149 °C) for 30 minutes to preheat the mold. The mold was removed from the oven and the 1.6” x 1.6” squares were stacked into the mold to a height of 2.5”. A release liner was then placed onto the top and bottom of the stack. The top plunger was inserted, and the mold and stacked film squares were placed into an oven at 300 °F (149 °C) for 15 minutes. The mold and film stack were then quickly moved from the oven to a heated hydraulic press at 350 °F (176.6 °C) and pressed by gradually increasing pressing pressure to the value shown in Table 3 over 3 minutes. The mold was then removed from the hydraulic press and opened, and additional film squares were added. The heating, pressing, and addition of additional squares was then repeated 2 more times until the film stack was approximately 1.75” high. The obtained pressed film stack was a bonded block (or bonded film stack) made from the stacked film squares and was removed from the hydraulic press and placed into a chilled hydraulic press (Carver Laboratory Press Model 2518, Menomonee Falls, WI, US) at 10 °C and of pressing pressure shown in Table 3. Sheets were cut off the bonded block by a diamond wire saw. In the sheets obtained by cutting, the hexagonal boron nitride particles are oriented in a direction perpendicular to the plane of the sheet. Results are shown in Table 3. Table 3:

[0218] Reference examples 1 to 3 Porous films prepared as described above for Examples 1 to 3 were cut into 1.6” x 1.6” squares and individual 1.6” x 1.6” squares were placed into the steel mold described previously (the same steel mold as used for stacking and pressing the squares of the film). A release liner was then placed onto the top and bottom of the individual porous film square. The top plunger was inserted, and the mold and individual porous film square were placed into a hydraulic press at 72 °F (22 °C) and pressed at 20 tons for 20 seconds. The results from these compressed films are shown in Table 4. Table 4:

Claims

Claims1. A sheet comprising a thermally conductive electromagnetically absorbing composite material comprising a polymer, electromagnetically absorbing particles and thermally conductive particles, wherein the thermally conductive particles comprise platelet-shaped thermally conductive particles, and wherein the platelet-shaped thermally conductive particles are oriented in a direction perpendicular to the direction of the plane of the sheet, and wherein the thermally conductive particles are electrically insulating, and wherein the composite material comprises at least 30 percent by weight of the thermally conductive particles, based on the total weight of the composite material, and wherein the sheet has a through-plane thermal conductivity of more than 12 W / m*K.

2. The sheet of claim 1, wherein the thermally conductive particles are selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide particles, graphene oxide particles, and combinations thereof.

3. The sheet of claim 1 or 2, wherein the thermally conductive particles are hexagonal boron nitride particles.

4. The sheet of any of claims 1 to 3, wherein the electromagnetically absorbing particles are selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof.

5. The sheet of claim 4, wherein the metal oxide particles comprise copper oxide (CuO) particles.

6. The sheet of claim 4, wherein the metal oxide particles comprise titanium monoxide (TiO) particles.

7. The sheet of any of claims 1 to 6, wherein the electromagnetically absorbing particles comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle.

8. The sheet of any of claims 1 to 7, wherein the electromagnetically absorbing particles comprise a core particle, an electromagnetically absorbing coating located on the surface of the core particle, and an electrically insulating coating located on the electromagnetically absorbing coating.

9. The sheet of claim 8, wherein the electromagnetically absorbing particles further comprise a further inorganic coating which is located on the surface of the core particle, and wherein the electromagnetically absorbing coating is located on the further inorganic coating.

10. The sheet of any of claims 7 to 9, wherein the core particles are selected from the group consisting of alumina, hollow glass microspheres, solid glass microbeads, and combinations thereof.

11. The sheet of any of claims 7 to 10, wherein the electromagnetically absorbing coating comprises a material selected from the group consisting of tungsten, aluminum, titanium, steel, chromium, nickel, and combinations thereof.

12. The sheet of any of claims 8 to 11, wherein the electrically insulating coating is selected from the group consisting of an aluminum oxide (AlOx) coating, a silicon oxide (SiO2) coating, a zirconium oxide (ZrO2) coating, and combinations thereof.

13. The sheet of any of claims 9 to 12, wherein the further inorganic coating is an aluminum coating.

14. The sheet of any of claims 1 to 13, wherein the composite material comprises at least 10 percent by weight of the electromagnetically absorbing particles, based on the total weight of the composite material.

15. The sheet of any of claims 7 to 14, wherein the thickness of the electromagnetically absorbing coating is from 0.1 to 20 nm.

16. The sheet of any of claims 1 to 15, wherein the polymer is selected from the group consisting of polyurethane, polyester, polyamide, polyether, polycarbonate, polyimide, polysulfone, polyethersulfone, polyphenylene oxide, polyacrylate, polymethacrylate, polyacrylonitrile, polyolefin, styrene, styrene-based copolymer, styrene-base copolymer, chlorinated polymer, fluorinated polymer, copolymers of ethylene and chlorotrifluoroethylene, and combinations thereof.

17. The sheet of any of claims 1 to 16, wherein the polymer is an ultra-high molecular weight polyethylene having a number average molecular weight in a range from 5 x 104to 1 x 107g / mol.

18. The sheet of any of claims 1 to 17, wherein the sheet has a dielectric dissipation factor (Df) of from 0.01 to 0.5 at 10 GHz.

19. The sheet of any of claims 1 to 18, wherein the sheet has a dielectric constant (Dk) of from 2.0 to 6.0 at 10 GHz.

20. The sheet of any of claims 1 to 19, wherein the sheet has a dielectric dissipation factor (Df) of from 0.01 to 0.5 at 40 GHz.

21. The sheet of any of claims 1 to 20, wherein the sheet has a dielectric constant (Dk) of from 2.0 to 6.0 at 40 GHz.

22. The sheet of any of claims 1 to 21, wherein the electromagnetically absorbing particles are different from the thermally conductive particles.

23. The sheet of any of claims 1 to 22, wherein the thermally conductive particles are selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide particles, graphene oxide particles, and combinations thereof, and wherein the thermally conductive particles do not comprise an electromagnetically absorbing coating located on the surface of the thermally conductive particles, and wherein either(i) the electromagnetically absorbing particles are selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof, wherein the metal oxide particles are selected from the group consisting of doped particles of tin oxide, undoped particles of tin oxide, ferrous oxide particles, ferric oxide particles, zinc oxide particles, manganese oxide particles, lead oxide particles, nickel oxide particles, cobalt oxide particles, silver oxide particles, antimony oxide particles, copper oxide (CuO) particles, titanium monoxide (TiO) particles, and combinations thereof; or(ii) the electromagnetically absorbing particles comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle.

24. The sheet of any of claims 1 to 23, wherein the thermally conductive particles are selected from the group consisting of hexagonal boron nitride particles, alumina particles, aluminum nitride particles, silicon carbide particles, magnesium oxide particles, graphene oxide particles, and combinations thereof, and wherein the thermally conductive particles do not comprise an electromagnetically absorbing coating located on the surface of the thermally conductive particles, and wherein either(i) the electromagnetically absorbing particles are selected from the group consisting of metal oxide particles, metal nitride particles, metal carbide particles, metal sulfide particles, metal silicide particles, metal boride particles, particles of multiferroic compounds, mixed ceramic particles, chalcogenide glass particles, and combinations thereof, wherein the metal oxide particles are selected from the group consisting of doped particles of tin oxide, undoped particles of tin oxide, ferrous oxide particles, ferric oxide particles, zinc oxide particles, manganese oxide particles, lead oxide particles, nickel oxide particles, cobalt oxide particles, silver oxide particles, antimony oxide particles, copper oxide (CuO) particles, titanium monoxide (TiO) particles, and combinations thereof; or(ii) the electromagnetically absorbing particles comprise a core particle and an electromagnetically absorbing coating located on the surface of the core particle, wherein the core particles are selected from the group consisting of alumina, hollow glass microspheres, solid glass microbeads, and combinations thereof, and wherein the electromagnetically absorbing coating comprises a material selected from the group consisting of tungsten, aluminum, titanium, steel, chromium, nickel, and combinations thereof.

25. A process for producing a sheet according to any of claims 1 to 24, the process comprising providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a suspension of electromagnetically absorbing particles and thermally conductive particles in a polymer-solvent solution, wherein the polymer in the polymer- solvent solution has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted at a temperature above the melting point of the polymer in the polymer-solvent solution, and below the boiling point of the solvent, forming the suspension into a fdm, wherein the platelet-shaped thermally conductive particles are oriented in a direction parallel to the direction of the plane of the fdm, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the fdm to obtain a porous fdm, optionally compressing the porous fdm to obtain a densified fdm, stacking multiple layers either of the porous fdm or of the densified fdm one upon another to obtain a fdm stack, pressing the fdm stack to obtain a bonded fdm stack, andslicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

26. A process for producing a sheet according to any of claims 1 to 24, the process comprising providing a polymer, a solvent, electromagnetically absorbing particles, and thermally conductive particles comprising platelet-shaped thermally conductive particles, wherein the thermally conductive particles are electrically insulating, combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles to form a slurry, wherein the slurry is a suspension of the polymer, the electromagnetically absorbing particles, and the thermally conductive particles in the solvent, and wherein the polymer has a melting point, and wherein the solvent has a boiling point, and wherein combining the polymer, the solvent, the electromagnetically absorbing particles, and the thermally conductive particles is conducted below the melting point of the polymer, and below the boiling point of the solvent, forming the slurry into a fdm, wherein the platelet-shaped thermally conductive are oriented in a direction parallel to the direction of the plane of the film, heating the film in an environment to retain at least 90 percent by weight of the solvent in the film, based on the weight of the solvent in the film, and solubilizing at least 50 percent by weight of the polymer in the solvent, based on the total weight of the polymer, inducing phase separation of the polymer from the solvent, removing at least a portion of the solvent from the film to obtain a porous film, optionally compressing the porous film to obtain a densified film, stacking multiple layers either of the porous film or of the densified film one upon another to obtain a film stack, pressing the film stack to obtain a bonded film stack, and slicing a sheet from the bonded film stack in a direction perpendicular to the planes of the stacked film layers.

27. The process of claim 25 or 26, further comprising heating the film stack prior to pressing the film stack.

28. The process of any of claims 25 to 27, wherein pressing the film stack is performed at a temperature of at least 110 °F.

29. The process of any of claims 25 to 28, further comprising heating of the bonded film stack prior to slicing a sheet from the bonded film stack.

30. The process of any of claims 25 to 29, wherein pressing the film stack is performed under a pressing pressure of at least 10 MPa.

31. The process of any of claims 25 to 30, wherein pressing the film stack is performed in a pressing direction while constraining the film stack in a first direction perpendicular to the pressing direction.

32. The process of any of claims 25 to 31, wherein pressing the film stack is performed in a pressing direction while constraining the film stack in a first direction perpendicular to the pressing direction and in a second direction perpendicular to the pressing direction, and wherein the first direction is perpendicular to the second direction.

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