WO2017012118A1

WO2017012118A1 - Thermally conductive composite comprising boron nitride-thermoset particles

Info

Publication number: WO2017012118A1
Application number: PCT/CN2015/084936
Authority: WO
Inventors: Yunfeng Yang; Hongyu Chen; Yan Huang; Libo DU
Original assignee: Dow Global Technologies Llc
Priority date: 2015-07-23
Filing date: 2015-07-23
Publication date: 2017-01-26

Abstract

Polymer composite material containing a matrix resin and particles comprising a thermoset and hexagonal boron nitride is disclosed. The polymer composite material is useful for a thermal management component because of its isotropic thermal conductivity.

Description

THERMALLY CONDUCTIVE COMPOSITE COMPRISING BORON NITRIDE-THERMOSET PARTICLES

Field of the invention

The present invention relates to thermally conductive composite materials comprising particles of boron nitride and a thermoset， which is useful for a thermal management component of electronic devices.

Background of the invention

Thermal management is critical in every aspect of the microelectronics space， such as integrated circuits (IC) ， light-emitting diode (LED) ， power electronics， displays and photovoltaics. The performance of these devices can be directly affected by operating temperature. Lowering the operating temperature of these devices often increases lifetime and improves performance， as compared to operations at higher temperatures.

In solid state lighting technologies， there is a strong need to improve thermal management. Proper dissipation of heat in LED devices is critical to their reliable long-term operation. Failure to adequately manage the heat can have an undesirable impact on the performance of LEDs. Prolonged exposure to excessive operating temperatures can lead to irreversible damage to the semiconductor components within the LED die， resulting in lowered light outputs， changes to the color rendering index， and significantly reduced LED lifetimes. Therefore， a material with higher thermally conductive property is desired for thermal management of LED devices.

Although polymer materials are light and easy to processing， their low thermal conductive property is a barrier of using polymer materials to a themal management component of electronic devices which is required thermal conductivity. To increase thermal conductivity of polymer materials， thermally conductive fillers are added in polymer materials. Hexagonal boron nitride is thought to be an excellent thermally conductive filler because of its high thermal conductivity， low electrical conductivity and no hydrolysis properties. However， polymer materials comprising hexagonal boron nitride fillers often show anisotropic thermal conductivity (i.e. different thermal conductivity along different axes) because (1) the intrinsic thermal conductivity of hexagonal boron nitride is very different in its in-plane direction and thickness direction， and (2) due to its plate-like morphology (platelet boron nitride) ， hexagonal boron nitride tends to orient in a flow field during forming polymer materials. Especially when polymer materials have sheet or plate like shapes， thermal conductivity in through-plane direction and in-plane direction differs a lot. Such strong anisotropy of thermal conductivity will lead to different heat dissipation performance in different directions and thus certainly limit the application of boron nitride based thermally conductive polymer materials.

To resolve such problem， agglomerated hexagonal boron nitride or a mixture of hexagonal boron nitride and other thermal conductive filler has been used for thermal conductive fillers in a polymer materials， see e.g. US6,794,435B， US6,764,975B， US6,645,612B， US5,898,009A， US8,394,489A， US5,854,155A， JP04906243B， US2012-0046387A， CN101318636B， JP2008-001536A and US2006-0127422A. However， many additional steps are needed to prepare agglomerated hexagonal boron nitride. On the other hand， when adding another filler in addition to hexagonal boron nitride to get improved thermal conductivity， the contents of total thermal conductive fillers are increased and cause difficulty of processability because of increased viscosity. Therefore， isotropic thermal conductive filler both for thorough-plane direction and in-plane direction and a polymer material with easy processability and high thermal conductivity suitable for a thermal management component of electronic devices is desired.

Summary of the invention

The present inventors have now found a technical approach to get isotropic thermal conductivity using fillers comprising hexagonal boron nitride， which can be obtained by a simple method. This approach uses particles comprising hexagonal boron nitride and a thermoset as thermally conductive fillers. Surprisingly， a polymer composite material comprising the particles has much higher through-plane thermal conductivity than the one comprising an agglomerated hexagonal boron nitride without decreasing in-plane thermal conductivity. A polymer composite material comprising the particles has isotropic thermal conductivity which is useful for thermal management components of electronic devices.

Therefore， one aspect of the invention relates to a polymer composite material comprising (a) a matrix resin and (b) particles comprising 50 to 95 volume％boron nitride and a 5 to 50 volume％thermoset based on the volume of the particles.

Other aspects of the invention relate to methods of forming and using the polymer composite material， including an electronic device comprising a thermal management component formed form the polymer composite material.

Brief description of the drawings

Fig. 1 is a SEM photograph for cross-section of injection molded plaques obtained in Comparative example 1.

Fig. 2 is a SEM photograph for cross-section of injection molded plaques obtained in Inventive example 2.

Detailed description of the invention

As used throughout this specification， the abbreviations given below have the following meanings， unless the context clearly indicates otherwise： g＝gram (s) ； mg＝milligram (s) ； m＝meter (s) ； mm＝millimeter (s) ； cm＝centimeter (s) ； min. ＝minute (s) ； s＝second (s) ； hr. ＝hour (s) ； ℃＝degree (s) C＝degree (s) Celsius； K＝kelvin； W＝watt； Ω＝ohm； wt％＝weight percent (s) ； vol％＝volume percent (s) ； . Throughout this specification， the words ‘resin’ a nd ‘polymer’ a re used interchangeably. Throughout this specification， the words ‘hexagonal boron nitride’ ， boron nitride platelet (s) ’ a nd ‘hexagonal boron nitride platelet (s) ’ a re used interchangeably. Throughout this specification， the word “thermosetting resin” covers both a resin prior to curing and a cured resin， while the word “thermoset” means a material that irreversibly cured.

The matrix resin is a polymeric material. Before final formation of the article in which the polymeric composite is used the resin is processable-e.g. by melt processing or the like. The matrix resin can be thermoplastic， thermosetting， or mixture thereof. Examples of the matrix resin include polyamide (PA) ， polyester， polyoxymethylene (POM) ， polycarbonate (PC) ， polyurethane， polyolefin， polyphenylene sulfide (PPS) ， polyphenylene oxide (PPO) ， liquid crystal polymer (LCP) ， polysulfone， epoxy， polyurethane， melamine， cyanate esters， vinyl ester polymer resins， silicone， phenolics， ureas， cyanate ester， bismaleimide， polyimide， acrylate copolymers， nylons， polyacetal， polystyrene (PS) ， polyethylene telephthalate (PET) ， acrylonitrile-butadiene-stylene (ABS) ， polytetrafluoroethylene (PTFE) ， polyvinyl fluoride (PVF) ， polybutadiene terephthalate (PBT) ， polyetheretherketone (PEEK) ， polyetherimide (PEI) and polymethyl methacrylate (PMMA) . The content of the matrix resin in the polymer composite material is preferably 15 volume％or more， more preferably 20 volume％or more based on the entire volume of the polymer composite material. At the same time， the content of the matrix resin in the polymer composite material is preferably 80 volume％or less， more preferably 60 volume％or less.

The particles used in the invention comprises from 5 to 50 volume％of thermoset and from 50 to 95 volume％of hexagonal boron nitride based on the volume of the particles. As disclosed before， hexagonal boron nitride has plate-like morphology. The plate-like morphology of hexagonal boron nitride comes from its hexagonal crystal structure， i.e. a layer of sheets formed by boron atoms and nitrogen atoms. Normally， the vertical direction against the plane of a plate-like morphology is short while the horizontal direction is long. In this application， the size of hexagonal boron nitride is disclosed as an average particle size measured by laser diffraction method although hexagonal boron nitride has plate-like morphology.

Preferably， the size of hexagonal boron nitride is 2 micrometers or more， more preferably 5 micrometers or more. At the same time， the size of hexagonal boron nitride is 50 micrometers or less， more preferably 30 micrometers or less.

The content of the hexagonal boron nitride in the particles is 50 volume％or more， preferably 60 volume％or more， more preferably 65 volume％or more based on the entire volume of the particles. At the same time， the content of the hexagonal boron nitride in the particle is 95 volume％or less， preferably 85 volume％or less， more preferably 80 volume％or less based on the entire volume of the particles.

The thermoset in the particles is a cured resin thus it is crosslinked. Prior to curing， a thermosetting resin works as a glue or an adhesive to fix many hBN together with random directions. After curing process， the hexagonal boron nitrides in the particles maintain its random directions. Therefore， the polymer composite material comprising the particles comprising hexagonal boron nitride and a thermoset has isotropic thermal conductivity.

The content of the thermoset in the particles is 5 volume％or more， preferably 15 volume％or more， more preferably 20 volume％or more based on the entire volume of the particles. At the same time， the content of the thermoset in the particles is 50 volume％or less， preferably 40 volume％or less， more preferably 35 volume％or less based on the entire volume of the particles.

Examples of the thermoset include epoxy resins， polyurethane resins， melamine resins， cyanate ester resins， cross-linked polyester， vinyl ester polymer resins， silicone， phenolic resins， ureas， rubbers， cyanate ester， bismaleimide， polyimide， and acrylate copolymers. Preferably， the thermoset is an epoxy resin.

The particles can comprise other additives such as flame retardents， coupling agents and toughening agents.

The particles are basically formed by the following steps： (i) preparing a mixture of hexagonal boron nitride and a thermosetting resin prior to curing， (ii) heating the mixture to the temperature of curing temperature of the thermosetting resin， and optionally (iii) milling the cured mixture to sufficient sizes as thermal conductive fillers. The mixture can comprise a curing agent and/or a catalyst to help the cure of thermosetting resin. When the mixture comprises a curing agent and/or a catalyst， the contents of those are 1 to 15 wt％and 0.01 to 0.2 wt％respectively based on the mixture. The mixing of the mixture can be conducted by any known methods， such as speed mixer， high-shear mixer or static mixer. The mixing speed is 500 rpm or more. At the curing step， the temperature can be increased step by step. After the resin B in the mixture is fully cured， the obtained cured mixture is cooled then optionally milled by any known method such as ball miller. The size of the particles is 10 micrometers or more， preferably 20 micrometers or more. The size of the particles is 500 micrometers or less， preferably 200 micrometers or less. The size of the particles is average size and it is measured by laser diffraction method.

As disclosed before， the particles of hexagonal boron nitride and a thermoset are used like thermally conductive fillers in the polymer composite material of the invention. The content of the particles in the polymer composite material is preferably 20 volume％or more， more preferably 25 volume％or more， and further more preferably 35 volume％or more based on the polymer composite material. The content of the particles in the polymer composite material is preferably 80 volume％or less， more preferably 60 volume％or less， further more preferably 50 volume％or less based on the polymer composite material.

The polymer composite material of the invention can comprise further thermal conductive fillers. Examples of the thermal conductive fillers include hexagonal boron nitride， aluminum oxide (Al₂O₃) ， aluminum nitride (AlN) ， magnesium oxide (MgO) ， zinc oxide (ZnO) ， silicon nitride (Si₃N₄) ， aluminum powder and graphite. When the polymer composite material comprises hexagonal boron nitride as other thermal conductive fillers in addition to hexagonal boron nitride in the particles， a part of the hexagonal boron nitride is located in the particles and the rest is located in a matrix resin. Such structure increases through-plane thermal conductivity of the polymer composite material. Therefore， it provides more isotropic thermal conductivity than the one all of hexagonal boron nitride is located in a matrix resin. When the polymer composite material comprises further thermal conductive fillers， the content of the thermal conductive fillers is 5 volume％or more， preferably 10 volume％or more， more preferably 15 volume％or more based on the polymer composite material. The content of the further thermal conductive fillers is 45 volume％or less， preferably 35 volume％or less， more preferably 25 volume％or less based on the polymer composite material.

The polymer composite material of the invention can comprise other additives such as flame retardant， antioxidant， UV stabilizer， plasticizer， coupling agent， mold release agent， pigment and dye. Those additives can be added in a matrix resin.

Examples of flame retardant include antimony oxides， halocarbon， halogenated ester， halogenated ether， brominated flame retardant agent， and halogen free compounds such as organophosphorus compounds， organonitrogen compounds， intumescent flame retardants.

Examples of antioxidant include sodium sulfite， sodium pyrosulfite， sodium hydrogen sulfite， sodium thiosulfate and dibutyl phenol.

Examples of UV stabilizer include benzophenones， benzotriazoles， substituted acrylates， aryl esters and compounds containing nickel or cobalt salts.

Examples of plasticizer include phthalates benzoates， dibenzoates， thermoplastic polyurethane plasticizers， phthalate esters， naphthalene sulfonate， trimellitates， adipates， sebacates， maleates， sulfonamides， organophosphates， polybutene.

Examples of coupling agent include chrome compound， silane coupling agent， titanate coupling agent， zirconium coupling agent， magnesium coupling agent and tin coupling agent.

Examples of mold release agent include inorganic mold release agent such as talcum powder， mica powder， argil and clay； organic mold release agent such as aliphatic acid soap， fatty acid， paraffin， glycerol and vaseline； polymer mold release agent such as silicone oil， polyethylene glycol and polyethylene.

Examples of pigment or dye include chromate， sulfate， silicate， borate， molybdate， phosphate， vanadate， cyanate， sulfde， azo pigment， phthalocyanine pigent， anthraquinone， indigo， quinacridone and dioxazinedyes.

The polymer composite material is obtained by blending， extruding， pouring， or other common mixing methods the matrx resin with the particles. To form an article the material is shaped， e.g. by molding， extruding， coating， etc. and then cooled. Ifthe matrx resin includes a thermosetting resin， a curng step is further added to the above process. The polymer composite material of the invention has isotropic thermal conductivity for diferent directions. The in-plane thermal conductivity of the polymer composite material is 6.0 W/m·K or more， and preferably 8 W/m·K or more. The through-plane thermal conductivity of the polymer composite material is 2.5 W/m·K or more， preferably 3.0 W/m·K or more， and more preferably 4.0W/m·K or more.

The polymer composite materal of the invention can be used for theral management components of electronic devices. Examples of such electronic devices comprise integrated circuit (IC) chip， light-emitting diode (LED) ， power electronics， displays and photovoltaics.

The polymer composite materal ca be used for a heat sink or connecting materal with heat source and heat sin. Heat sink is used to cool electronics components or semiconductor components such as high-power semiconductor devices， and optoelectronic devices such as higher-power lasers and light emitting diodes (LEDs) . Since the polymer composite materal of this invention has isotropic thermal conductivity， the heat generated by the heat source is effectively transferred and removed from the heat source.

Other examples of the theral management components are， electronic packaging materal， sealing material， adhesive material， electric switch， printed circuit board and wire coating.

The polymer composite material of the invention can be used for a substrate with electronics element or semiconductor element such as IC chips or power electronics (heat source) and plastic substrate or plastic flm contacted to such heat source (athermal management component) . IC chips or other electronics elements are normally mounted on a laminated plastic substrate such as epoxy or polyimide resin. Ceramic substrate such as aluminum or aluminum nitrate is also used as a substrate for power electronics because of the need for heat management generated by the power electronics. Since ceramic substrate is dififcult to laminate or process， plastic substrate with high thermal conductivity is desired. The polymer composite material of our invention can be used for the purpose.

The polymer composite material of the invention can be used for a system comprising electronics device (heat source) and a covering thermosetting resin of the device (athermal management component) . To protect electronics devices from mechanical damages， electronics devices are covered by a material such as thermosetting resin. Since electronics devices generate heat， thermal management of the material is required. The polymer composite material of our invention with isotrophic thermal conductivity can be used for the purpose. Example of such article is LED lightning system with LED light encapsulated by the cured thermosetting resin.

The polymer composite material of the invention can be used for a solid state lightening system comprising LED light (heat source) and a base which is mounted the LED light (athermal management component) . In solid state lightning system， LED light is mounted on a base and surrounded by a side-wall. Since the LED light generates heat， thermal management of the solid state lightning system is required. The polymer composite material of our invention with isotrophic thermal conductivity can be used for the purpose.

Examples

Preparation of a particles of hexagonal boron nitride and thermoset

Preparation of Sample A

Hexagonal Boron nitride (220.6g， PCTP30， D50＝30 micrometer， supplied from Saint-Gobain) ， epoxy (15.4g， DER331， supplied from Dow Chemical) ， a curing agent (13.9g， methyl tetrahydrophothalic anhydride (MTHPA) ， supplied from Dow Chemical) and a catalyst (0.15g， benzyldimethylamine (BDMA) ， provided by Dow Chemical) were mixed together using a speed mixer for 5 min at 2000 rpm， and then for 5 min at the increased speed 5600 rpm. The weight ratio of DER331/MTHPA/BDMA was 10/9/0.1. The epoxy in the mixture was fully cured via heating at 90 ℃ for 2h， 100 ℃ for 2h； 120 ℃ for 2h； 140 ℃ for 4h， 160 ℃ for 4h. The volume ratio of epoxy /hexagonal boron nitride was 20/80. The original hexagonal boron nitride has been agglomerated by the cured epoxy.

Preparation of Samples B and C

The same process as disclosed above was conducted excepting for hexagonal boron nitride was changed to another source (RH-S， D50＝15 micrometer， supplied from Dandong Rijin Science and tech nology Co. Ltd) or the volume ratio of epoxy /hexagonal boron nitride was changed as shown in Table 1. For the mixture of epoxy/hexagonal boron nitride volume ratio was 35/65， the cured powder was milled at 500 rpm for 20-30 minutes in a ball miller until all the milled powder can pass through a 60 mesh sieve.

Table 1

Inventive examples 1 to 5 and Comparative examples 1 to 3 Particles A to C were used as a part of thermally conductive filler. The content of each component is shown in Tables 3 and 4. All fillers including hexagonal boron nitride， epoxy /hexagonal boron nitride particles and Al₂O₃ for Inventive example 5 and Comparative example 3 were mixed together by dramatic shaking. A laboratory scale HAAKE mixer was initially set at 280 ℃ and a rotor speed of 50 revolutions per minute (rpm) . Polyamide 66 (PA 66， Zytel 101 F， supplied from Dupont) was first loaded into the mixer until it melted completely， and then the filler or already mixed fillers were added slowly and totally mixed for 15 min at 50 rpm. 0.3 g of antioxidant Irganox 1010 was added for each batch (～50 ml composite) .

The samples (thin plaque) for TC measurement and morphology observation were prepared by injection molding (Machine： FanucФ28 mm， Clamp Tonnage： 200. ) ： Conditions are listed in the Table 2. Size of the thin plate mold was 1mm*60mm*60mm. The thickness of injection plaques was 1 mm.

Table 2

Property evaluation methods

1. Thermal conductivity (TC)

The thermal diffusivity α (mm²/s) was determined with Netzsch Nanoflash LFA 447 instrument according to ASTM D 1461-01. The measurement temperature was 25 ℃. The samples for TC measurement were cut from the injection molded plaques. Diameter 11.8-12.6 mm， thickness ～1.0 mm for through-plane TC measurement and diameter 24.8-25.4 mm， thickness ～0.5 mm for in-plane TC measurement. A laser light absorbing spray was applied to surfaces of disk-shaped samples， and then the samples were dried in air. Four laser flash shots were conducted and then the average α and standard deviation was obtained. The density ρ (g/cm³) of the samples at room temperature was measured by hydrostatic weighing， in which the displacement of water due to a submerged object was used to determine the density of the object. The heat capacity Cp (J/g C) at 25 ℃ of the samples was determined by DSC (DSC-Q2000) according to ASTM E 1269-11. The thermal conductivity (W/m·K) was calculated according to the following equation： TC＝α*ρ*Cp

2. Morphology observation

Injection molded plaques were cut into pieces for cross-section imaging. Pieces were embedded in epoxy and metallurgically polished prior to imaging. Polished pieces were observed by back scattering electron detector using Nova Nano630 SEM.

Property evaluation results were shown in Tables 3 and 4. As presented in Table 3， epoxy /hexagonal boron nitride (35/65 (vol％) ) particles (Particles C) was used to replace part of original hexagonal boron nitride (keep total amount of hexagonal boron nitride the same， 60 vol％in total composition) in Inventive examples 1 and 2 compared with Comparative example 1. It can be seen that the through-plane thermal conductivity was increased from 2.4 W/mK (Comparative example 1) to 3.8 W/mK (Inventive example 1) and even 4.6 W/mK (Inventive example 2) . Although the in-plane TC was decreased from 21 W/mK to about 16 W/mK， the in-plane TC was still maintained in a pretty high level.

Figure 1 shows a SEM image of cross-section of injection molded plaques of Comparative example 1. Figure 2 show a SEM image of cross-section of injection molded plaques of Inventive example 2. Clearly， in Figure 1 the hexagonal boron nitride oriented in the flow direction of injection molding. However， in Figure 2 less hexagonal boron nitride oriented in the flow direction， which was caused by the fact that much hexagonal boron nitride was in the state of agglomerate. Some hexagonal boron nitride tended to orient in the through-plane direction， which was responsible for the significant increase of through-plane thermal conductivity.

In Inventive example 3， another epoxy /hexagonal boron nitride (20/80) particles (Particles A) was used. Still， an obvious increase of through-plane thermal conductivity was achieved compared with Comparative example 1.

Table 3

Another epoxy/hexagonal boron nitride particles (Particles B) was used in Inventive example 4. The particle size of RH-S hexagonal boron nitride was smaller than the one of PCTP30 hexagonal boron nitride. As can be seen from Table 4， the through-plane thermal conductivity was improved from 2.4 W/mK to 4.1 W/mK， a 71％increase. The inventive composite had more balanced thermal conductivity in both directions (through-plane direction and in-plane direction) .

For Inventive example 5 and Comparative example 3， hexagonal boron nitride and Al₂O₃ were used as other fillers. Twenty (20) vol％of hexagonal boron nitride with epoxy /hexagonal boron nitride particles was used. However， the total hexagonal boron nitride content in the two formulations is the same. Both formulations contain 25 vol％of spherical alumina. The through-plane thermal conductivity was again increased and also similar in-plane thermal conductivity was achieved. Therefore， more balanced thermal conductivity was achieved.

Table 4

Comparative examples 4 and 5

To compare the particles of epoxy /hexagonal boron nitride with commercially available agglomerated boron nitride， Comparative examples 4 and 5 were conducted. CTS 2M agglomerated boron nitride (D50＝35 micrometer) and CTS 7M agglomerated boron nitride (D50＝120 micrometer) both supplied from Saint-Gobain were used for Comparative examples 4 and 5 respectively. The content of each component is shown in Table 5. The total contents of boron nitride and resin in Comparative examples 4 and 5 are same as the one in Inventive example 2. Inventive example 2 has much higher through-plane thermal conductivity than Comparative examples 4 and 5， although in-plane thermal conductivity of those are silimar. It means that using the particles of this invention has obvious advantage over using commercial agglomerated boron nitride.

Table 5

Claims

A polymer composite material comprising (a) a matrix resin and (b) particles comprising 59 to 95％ by volume of hexagonal boron nitride and 5 to 50％ by volume of a thermoset， based on the volume of the particles.
The polymer composite material of claim 1， wherein the content of the particles is from 20 to 80 volume ％.
The polymer composite material of claims 1 or 2， wherein the thermoset is selected from epoxy， polyurethane， melamine， cyanate esters， cross-linked polyester， vinyl ester polymer resins， silicone， phenolics， ureas， rubbers， cyanate ester， bismaleimide， polyimide， acrylate copolymers.
The polymer composite material any of claims 1 to 3， wherein the resin A is selected from polyamide， polyester， polyoxymethylene， polycarbonate， polyurethane， polyolefin， polyphenylene sulfide， polyphenylene oxide， liquid crystalline polymer， polysulfone， epoxy， polyurethane， melamine， cyanate esters， vinyl ester polymer resins， silicone， phenolics， ureas， cyanate ester， bismaleimide， polyimide， acrylate copolymers.
The polymer composite material any of claims 1 to 4， further comprising (c) thermal conductive fillers.
The polymer composite material any of claims 1 to 5， wherein through-plane thermal conductivity of the polymer composite material is 2.5 W/mK or higher.
The polymer composite material any of claims 1 to 6， wherein the average size of the particles is from 10 to 500 micrometers.
The polymer composite material any of claims 1 to 7， wherein the hexagonal boron nitride is randomly oriented in the particles.
A method of forming the composite material any of claims 1 to 8 comprising

(a) forming the particles by mixing the hexagonal boron nitride with a thermosetting resin and curing the resin

(b) mixing the matrix resin with the particles.
The method of claim 9 further comprising forming the material into a desired shape.
The method of claim 10 wherein the matrix resin is a thermosetting material which is cured after forming the material into the desired shape.
An article made by the method of claims 10 or 11.
An electronic device comprising a thermal management component formed form the polymer composite material any of claims 1 to 8.