WO2003002644A1

WO2003002644A1 - Thermally conductive sheet

Info

Publication number: WO2003002644A1
Application number: PCT/US2002/016743
Authority: WO
Inventors: Maya Igarashi; Mitsuhiko Okada; Tomoaki Uchiya
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2000-06-29
Filing date: 2002-05-24
Publication date: 2003-01-09
Anticipated expiration: 2003-12-29
Also published as: JP2002030212A

Abstract

A thermally conductive sheet comprising a binder resin and thermally conductive fillers dispersed in said binder resin, characterized in that said binder resin is an acrylic polyurethane resin. The thermally conductive sheet has flexibility and conformability and also has excellent thermal conductivity and well-balanced adhesion performance.

Description

THERMALLYCONDUCTIVE SHEET

The present invention relates to a thermally conductive sheet and, more particularly, to a thermally conductive sheet which has an excellent thermal conductivity and a well-balanced adhesion performance and is suited for use as a heat transfer medium for electronic parts.

It has recently become a matter of concern to remove heat from a heating element in various fields. It is particularly a serious problem to remove heat from built-in heat generating electronic parts (e.g. integrated circuit (IC) chip) and other parts (hereinafter referred generically to as "heat generating parts) in various devices such as electronic devices and personal computers. This is true because the probability of a malfunction tends to enhance exponentially with the increase of the temperature of parts in various heat generating parts. The size reduction of electronic heating generating parts and the demand for increased processing speeds have highlighted the need for improved heat dissipation properties.

To release heat in heat generating parts heat dissipating materials such as a heat sink, a heat dissipating fin, and a metallic heat dissipating plate have conventionally been mounted to heat generating parts. Furthermore, thermally conductive sheets have been used as heating spacers to operate as a heat transfer medium interposed between the heat generating parts and heat dissipating material.

A thermally conductive sheet should have a high thermal conductivity and be sufficiently conformable to the surfaces of heat generating parts having various shapes, such as an IC chip. It is also necessary that the thermally conductive sheet has electrical insulation properties. To satisfy these requirements, almost all of conventional thermally conductive sheets are prepared by mixing silicone rubber with a filler for enhancing the thermal conductivity. As the filler, for example, alumina, silica (quartz), boron nitride and magnesium oxide are used. Specifically, Japanese Unexamined Patent Publication (Kokai) No. 56-837 discloses a heat dissipating sheet comprising, as a principal component, an inorganic filler and a synthetic rubber, the inorganic filler comprising two components (A) boron nitride and (B) alumina, silica, magnesia, zinc white and mica.

Japanese Unexamined Patent Publication (Kokai) No. 7-111300 discloses an insulating heat dissipating sheet having a thickness of not less than 1 μm and comprising a silicone rubber and a boron nitride powder dispersed in the silicone rubber. These thermally conductive silicone rubber sheets can exhibit high thermal conductivity, but have some problems to be solved. For example, silicone rubber itself has a high price so that the cost of the heat dissipating sheet depends on its high price. Additionally, a long time is required to form a sheet because silicone rubber exhibits a low curing rate. Also, a thin sheet can not be made with high accuracy because a large amount of the filler is added to improve the thermal conductivity. Complicated production processes and complicated large-scale production apparatuses are required including a hot-air oven and a press. Moreover, because a conventional silicone rubber sheet is hard and not readily conformable to a special shape, any unevenness of a heat generating part or a heating material causes the thermal resistance to increase by the resulting air gaps. In the case where the rubber sheet is forcibly pressed to remove the air gaps, functional troubles are caused by excess pressure on fine electronic parts.

High adhesion of silicone rubber to parts having a complicated shape has been attempted. For example, Japanese Unexamined Patent Publication (Kokai) No. 10-

189838 discloses a thermally conductive gel suited for use as a heat dissipating sheet, which is cured in the form of a gel at normal temperature, the thermally conductive gel being prepared by adding a silicone oil and thermally conductive fillers such as boron nitride, silicon nitride, aluminum nitride and magnesium oxide to a condensation type gel such as condensation curing type liquid silicone gel. In this heat dissipating sheet, however, a two-part curing type silicone gel contains molecules which have no functional group and are not crosslinked. It has been pointed out that those having a low molecular weight can diffuse into electronic apparatuses to exert an adverse influence on the operation of other electronic parts because they are liable to be volatilized. A hard disc whose capacity has recently been increased is broken down when a trace amount of silicone is oxidized and fixed to a head in the hard disc so that it has been known that such a device is a part damaged easily by silicone among electronic parts. Therefore, it has been required to develop a thermally conductive sheet which does not contain any silicone. A heat dissipating sheet using a silicone gel as a binder has tackiness on the surface thereof, but can not be firmly fixed to heat generating parts and a heat dissipating material because it hardly exhibits enough adhesive strength when incorporated into an adhesive sheet to be used for bonding. A thermally conductive sheet using, a urethane resin binder in place of the silicone rubber and gel described above also has been used (see, Japanese Unexamined Patent Publication (Kokai) Nos. 2-74545, 5-162296 and 11-111899). However, since all thermally conductive sheets disclosed in these patent publications have an object of improving the thermal conductivity and are not intended to afford a function for a bonding sheet, they have not enough performances for the adhesive sheet similar to the thermally conductive sheet using a silicone rubber and a silicone gel.

There is also known a thermally conductive sheet having thermal conductivity and adhesion properties wherein an acrylic adhesive is used as a binder. For example, the present applicant has suggested a thermally conductive electrical insulating pressure-sensitive adhesive comprising (a) a polymer prepared from a monomer mixture containing (i) an alkyl acrylate or methacrylate and (ii) a polar monomer which is copolymerizable with the alkyl acrylate or methacrylate, and (b) thermally conductive electrical insulating particles dispersed in the polymer in Unexamined Patent Publication (Kokai) No. 6-88061. When using this pressure-sensitive adhesive, there can be obtained effects such as comparatively simple production process, low thermal resistance, excellent adhesion properties and the like. However, it is necessary for the thermally conductive sheet using such a pressure-sensitive adhesive to cure the monomer mixture by irradiating with ultraviolet light after forming into a sheet. Therefore, a polymer (resin) is not cured when the thermally conductive electrical insulating particles to be dispersed in the polymer are black particles through which ultraviolet light easily penetrates. The amount of the thermally conductive electrical insulating particles to be added is from about 20 to 30 volume % at most, and it is difficult to further improve the thermal conductivity by mixing a larger amount of the thermally conductive electrical insulating particles. When using a white thermally conductive particles made of aluminum oxide, the center portion of the sheet is not likely to be sufficiently cured by increasing the thickness of the sheet.

An object of the present invention is to solve the above-described problems of the prior art and to provide a thermally conductive sheet which has flexibility and is conformable to a special shape such as unevenness and curved surface, thereby making it possible to secure high adhesion and heat dissipation, and to improve the thermal conductivity and obtain well-balanced adhesive performance by mixing a large amount of thermally conductive electric insulating particles with a binder. The present inventors have intensively studied to solve the object described above and obtained such a finding that, when using an acrylic polyurethane resin as a binder resin of a thermally conductive sheet, a thermally conductive filler can be mixed in a high content and a thermally conductive sheet having high thermal conductivity can be provided.

That is, the present invention resides in thermally conductive sheet comprising a binder resin and thermally conductive fillers dispersed in said binder resin. As described above, the thermally conductive sheet comprises a binder resin and a thermally conductive filler dispersed in the binder resin, characterized in that an acrylic polyurethane resin is used as the binder resin. The acrylic polyurethane resin used herein is prepared by the polyaddition reaction of (a) an acrylic oligomer having at least two hydroxyl groups in a molecule and (b) a polyfunctional isocyanate having at least two isocyanate groups in a molecule, optionally in the presence of a catalyst. For the acrylic polyurethane resin of the present invention, an acrylic oligomer having at least two hydroxyl groups in a molecule is used as a polyol component. Since the acrylic oligomer has a function of imparting high adhesive performance to a polyurethane resin prepared therefrom and is also acrylic-based, the cured resin can exhibit sufficiently high heat resistance and weathering resistance without adding an antioxidant.

As the acrylic oligomer, having at least two hydroxyl groups in a molecule, useful in the present invention (hereinafter referred to as "acrylic oligomer"), for example, there can be advantageously used an acrylic oligomer that is generally used in an acrylic adhesive. Such an acrylic oligomer typically has two hydroxyl groups in a molecule, but may be a mixture with those having one or three or more hydroxyl groups. As used herein, the term "acrylic oligomer" includes both of acrylic and methacrylic oligomers. The acrylic oligomer useful in the present invention usually has an adhesive component and a cohesive component in the molecule. Useful materials for the adhesive component of the acrylic oligomer include, but are not limited to, acrylate monomers such as isooctyl acrylate (IOA), 2-ethylhexyl acrylate (2EHA), n-butyl acrylate (n-BA), methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, methoxypolyethylene glycol mono(meth)acrylate, ethoxypolyethylene glycol mono(meth)acrylate, ethoxypolypropylene glycol mono(meth)acrylate, ethoxypolybutylene glycol mono(meth)acrylate and nonylphenoxypolyethylene glycol mono(meth)acrylate. Useful materials for the cohesive component of the acrylic oligomer include, but are not limited to, acrylate monomers such as methyl methacrylate (MMA), ethyl methacrylate (EMA), methyl acrylate (MA), ethyl acrylate (EA), vinyl acetate (VAc) and acrylamide (AAm). The method of introducing a hydroxyl group relating to the polyaddition reaction with polyisocyanate into the molecule of the acrylic oligomer includes, for example, method using a polymerization chain transfer agent having a hydroxyl group, method using a polymerization initiator having a hydroxyl group and method of copolymerizing a monomer having a hydroxyl group.

The acrylate monomer may be used alone, or two or more of them may be used in combination. A polyether or polyester polyol, conventionally useful as a raw material for a polyurethane resin, may be added to the acrylic oligomer or combination of acrylic oligomers.

The viscosity of the acrylic oligomer can vary depending on the characteristics required of the acrylic polyurethane resin to be derived therefrom, but is preferably within a range from about 500 to 10,000 cps. When the viscosity of the acrylic oligomer is less than 500 cps, the adhesion performance of the acrylic polyurethane resin after curing may become poor because of too small molecular weight. On the other hand, when the viscosity of the acrylic oligomer exceeds 10,000 cps, the fluidity of a compound obtained by mixing the oligomer with a thermally conductive filler may be lowered, thereby making it difficult to prepare the acrylic polyurethane resin. Because the viscosity of the acrylic oligomer varies with the composition and molecular weight of its constituent monomers, an optimum viscosity can be obtained by controlling these factors.

The polyfunctional isocyanate, which is subjected to the polymerization addition reaction with the acrylic oligomer to form an acrylic polyurethane resin, is not specifically limited as far as it has a predetermined number of hydroxyl groups in a molecule, in addition to at least two isocyanate (-NCO) groups, and there can be used an aromatic or aliphatic polyisocyanate compound used generally in the preparation of the polyurethane resin. Preferable polyfunctional isocyanate includes, but is not limited to, hexamethylene diisocyanate (HDI), tolylene diisocyanate (TDI), diphenylenemethane diisocyanate (MDI), isophorone diisocyanate (IPDI), tolidine diisocyanate (TODI), naphthalene diisocyanate (NDI) and xylylene diisocyanate (XDI). These polyfunctional isocyanates may be used alone, or two or more of them can be used in combination. On selection of the polyfunctional isocyanate in the preparation of each thermally conductive sheet, it is recommended to select a most preferable one in consideration of the pot life and curing rate of a thermally conductive compound before curing.

The amount of the polyfunctional isocyanate to be added to the acrylic oligomer varies depending on the desired effect and conditions of the polyaddition reaction to be applied, but is preferably an amount that the equivalent amount of the hydroxyl group of the acrylic oligomer is almost the same as that of the isocyanate group of the polyfunctional isocyanate. When the polyfunctional isocyanate is added so that the equivalent amount of the isocyanate group is slightly larger than that of the hydroxyl group, reaction efficiency can be enhanced. On the other hand, when the equivalent amount of the isocyanate group is smaller than that of the hydroxyl group, a harmful influence is likely to be exerted on the cured state.

The polyaddition reaction between the acrylic oligomer and polyfunctional isocyanate is carried out in the presence of a suitable catalyst. The catalyst used herein may be a catalyst used generally in the preparation of the acrylic polyurethane resin and is not specifically limited. Catalysts includes, for example, organometallic catalysts such as stannous octoate, dibutyltin diacetate and lead octeate; and amine catalyst such as triethylenediamine. These catalysts may be used alone, or two or more kinds of them may be used in combination. On selection of the catalyst in the preparation of each thermally conductive sheet, it is recommended to select a most preferable one in consideration of the pot life and curing rate of a thermally conductive compound before curing, similar to the selection of the polyfunctional isocyanate. The amount of the catalyst can vary depending on the kind of the catalyst and polymerization conditions, but is preferably from about 0.01 to 0.5% wt% based on the amount of the binder component in the thermally conductive compound (polymerizable composition) before curing.

Using the acrylic oligomer and polyfunctional isocyanate as starting materials, the polyaddition reaction can be carried out under various reaction conditions in the presence of the above catalyst. This polymerization process is carried out with heating in consideration of the above starting materials according to the manner used generally in the preparation of the polyurethane resin. The heating can be conducted by putting the starting materials in an oven, or can be conducted by heating using an infrared heater in some cases. The heating temperature varies depending on the kind of the starting material, but is usually from about 25°C to 150°C.

According to the present invention, the polymerization process described above, in order to easily obtain a sheet comprising an acrylic polyurethane resin and a thermally conductive filler dispersed uniformly in the acrylic polyurethane, may be preferably carried out in such a manner that the thermally conductive filler is previously contained in a polymerizable composition including starting materials and the mixture is formed into a sheet before polymerization, and then the sheet is cured with heating.

In the thermally conductive sheet of the present invention, various fillers used generally for a thermally conductive sheet can be used as the thermally conductive filler. An inorganic filler in the form of particles is preferred. Particles of the inorganic filler are not specifically limited as far as they are superior in thermal conductivity and resist settling due to gravity during the storage of the resulting thermally conductive sheet, and also they can be dispersed uniformly in the polymerizable composition on polymerization of the above acrylic oligomer and polyfunctional isocyanate. Preferably the particles of the inorganic filler include, but are not limited to, particles of oxides such as aluminum oxide, silicon dioxide and titanium dioxide; particles of carbonates such as silicon carbide; and particles of metals such as copper and aluminum. These particles of inorganic fillers may be used alone, or two or more kinds of them may be used alone.

The thermally conductive sheet of the present invention is worthy of note in the respect that a white filler can be used similar to a conventional thermally conductive sheet when the thermally conductive filler is dispersed in a binder resin, and that there can also be used a filler colored in black or other colors, that has never been used by using the technique disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-88061 (cited above). In case of a conventional thermally conductive sheet, since curing of the binder resin depended on photopolymerization, the white filler that does not interfere with light irradiation was exclusively used. In the present invention, however, the filler is not limited to the white filler because thermal polymerization is conducted. Therefore, not only the degree of freedom in selection of fillers and the like markedly increases, but also a filler capable of contributing drastically to an improvement in the thermal conductivity can be used easily without any limitation.

The shape of particles of the thermally conductive filler to be used is not specifically limited and particles include, for example, spherical and flaky particles. The size (average particle diameter) of these particles is usually within a range from about 1 to 200 μm, and can range from about 10 to 100 μm. When the particle diameter of the filler particles is less than 1 μm, it may become complicated to prepare the sheet or becomes difficult to uniformly disperse the filler particles in the binder resin. On the other hand, when the particle diameter of the filler particles exceeds 200 μm, it may become necessary to increase the film thickness of the thermally conductive sheet contrary to the requirement to reduce the thickness as much as possible. When using particles of these thermally conductive fillers, the same kinds of the filler particles having different particle diameters may be used or different kinds of the filler particles having the same or different particle diameters may be used, if they are preferred.

The amount of the thermally conductive filler to be mixed with the binder resin can vary widely depending on the effect of the addition required to the filler, but is generally within a range from about 10 to 70% by volume. When the amount of the filler is less than 10% by volume, the thermal conductivity may become poor. On the other hand, when the amount exceeds 70% by volume, the fluidity of the thermally conductive compound of the present invention may be lowered, thus making it difficult to form into a sheet.

In the production of the thermally conductive sheet of the present invention, additives used generally in the polymer chemistry may be added to the polymerizable composition, if necessary. For the purpose of controlling the adhesion performance of the sheet, for example, tackifiers and plasticizers may be added. For the purpose of improving the heat resistance, antioxidants may also be added. Since the polyol component is acrylic-based in the present invention, excellent heat resistance and weathering resistance can be imparted to the resin after curing without adding antioxidants. Other additives include, for example, modifiers, thermal stabilizers, and colorants such as pigments and dyes.

Describing more specifically, the thermally conductive sheet of the present invention can be prepared preferably by mixing an acrylic oligomer, a polyfunctional isocyanate, a catalyst and arbitrary additives in each predetermined amount to obtain a liquid mixture (binder component), and adding a predetermined amount of a thermally conductive filler in the form of particles to the binder component. The thermally conductive filler may also be added on preparation of the liquid mixture, if necessary. After the completion of the addition, the mixture is uniformly kneaded under stirring using a kneader such as planetary mixer. Then, the resulting compound is interposed between two liners (e.g. polyester film treated with silicone) treated with a release agent and formed into a sheet by pressing using a press whose gap was controlled to a value to secure a predetermined thickness. The sheet-liked uncured compound may be placed in an oven and cured by heating at a predetermined temperature. As a result, a thermally conductive sheet is obtained.

Alternatively, the compound may also be continuously formed into a sheet by passing it through two gap-controlled rolls. Furthermore, a multi-layer structure thermally conductive sheet may also be prepared by passing a core material interposed between layers of the compound through rolls.

In the thermally conductive sheet thus prepared, particles of the thermally conductive filler are dispersed uniformly in the binder component and the particles do not settle during storage to cause non-uniform dispersion. This thermally conductive sheet is a two-part curing type acrylic polyurethane resin wherein the binder is free from a solvent, and a thermally conductive filler can be mixed in a high mixing ratio. Therefore, the thermally conductive sheet can exhibit excellent thermal conductivity of 0.5 W/m ^• K or more.

In the thermally conductive sheet of the present invention, since the acrylic polyurethane resin is not obtained by curing due to irradiation with ultraviolet light, a tackifier capable of absorbing visible light and ultraviolet light can be used, thereby making it possible to obtain enhanced adhesion performance. By selecting the kind and amount of the tackifier, not only the adhesion performance can be freely controlled, but also well-balanced adhesion performance can be realized by selection of the polyfunctional isocyanate and addition of the plasticizer.

The thermally conductive sheet of the present invention is also worthy of note in the respect that it can be formed in a different thickness according to the portion applied because of no limitation of the thickness due to the curing reaction. The thermally conductive sheet of the present invention can be usually formed in the form of a thin film having a thickness with a range from 0.01 to 4.0 mm. When the thickness of the sheet is less than 0.01 mm, it may become difficult to obtain enough adhesive strength, resulting in poor heat dissipation properties. On the other hand, when the thickness exceeds 4.0 mm, the heat resistance of the thermally conductive sheet may be enhanced, resulting in deterioration of the heat dissipation properties.

The thermally conductive sheet of the present invention is a self-supporting sheet and, therefore, it can be used advantageously as a heating means as it is. If necessary, this sheet may be used in combination with a substrate. Preferable substrate includes, for example, plastic film, woven fabric, nonwoven fabric and metallic foil.

Plastic films suitable for use as the substrate are a polyolefin film, and a plastic film having good thermal conductivity, good weathering resistance and comparatively high substrate strength can be used. Preferable polyolefin film includes, but is not limited to, polyethylene film, polypropylene film, ethyl ene -vinyl acetate copolymer (EVA) film, ethylene-acrylic acid copolymer (EAA) film and ionomer film. Among these polyolefin films, high crystalline and high density polyethylene and ultra high molecular weight polyethylene can be used most preferably because of high strength even as a thin film and comparatively high thermal conductivity. The thickness of the polyolefin film can vary widely depending on various factors, but is preferably within a range from about 1 to 25 μm.

The metallic foil suited for use as the substrate includes, for example, foils of various metallic materials such as aluminum, copper, gold, silver, lead and stainless steel. As used herein, the term "foil" refers to those substrates having a small thickness, thus including those referred to as metallic sheet and metallic foil. The thickness of the metallic foil can vary widely depending on various factors, but is preferably small as possible similar to the above plastic film and is usually within a range from 1 to 20 μm.

EXAMPLES The present invention will be further described with reference to the following non-limiting examples. In the following examples, "parts" are by weight unless otherwise stated.

EXAMPLE 1 Preparation of thermally conductive sheet:

As described in the following Table 1, 100 parts acrylic oligomer (manufactured by Soken Chemistry Co., product No.: UT-100), 13 parts polyfunctional isocyanate (dimerized hexamethylene diisocyanate (HDI)(manufactured by Soken Chemistry Co., product No.: HV-01), 12 parts polyfunctional isocyanate (HDI having three functional groups, manufactured by Soken Chemistry Co., product No.: HN-01) and 0.1 parts catalyst (manufactured by Soken Chemistry Co., product No.: UT-100) were mixed to prepare a binder. This binder was a viscous liquid.

Then, 150 parts of particles of a thermally conductive filler were added to 100 parts of the binder. The particles of the thermally conductive filler used herein are prepared by mixing silicon carbide (SiC) (average particle diameter of 75 μm, manufactured by Nanko Ceramic Co.) with SiC (average particle diameter of 10 μm, manufactured by Nanko Ceramic Co.) in a weight ratio of 1 :3 as described in Table 1 below. The binder and particles of the filler were uniformly mixed with defoaming under reduced pressure.

The resulting mixed solution was interposed between two polyester films (thickness of 50 μm), the surface of which was treated with silicone, followed by formation into a sheet using a press under the conditions that the thickness of the sheet after forming becomes 1 mm. The resulting sheet was put in an oven at 120° C and heated for three minutes. As a result, a thermally conductive sheet having a thickness of 1 mm was obtained.

Evaluation of characteristics of thermally conductive sheet:

The thermally conductive sheet thus obtained was evaluated by the following procedures with respect to (1) cured state, (2) thermal resistance, (3) shear force and (4) adhesive force to aluminum (Al). The resulting evaluation results are shown in Table 2 below. (1) Cured state

After putting the thermally conductive sheet on the hand, it was judged by stretching or judged visually whether or not it has enough sheet strength to secure handling.

Sr: state where operation can be conducted smoothly because of enough sheet strength

X : uncured state where curing has not been completed, or state where sheet is cured but its shape can not be retained In the case of this example (Example 1), as shown in Table 2 below, it was judged that the cured state is good (&). (2) Thermal resistance

After the thermally conductive sheet was interposed between a central processing unit (CPU) and an aluminum plate, the sheet was pressed against the CPU by applying a fixed pressure and a voltage of 7.0 V was applied to the CPU. After a lapse of five minutes, the difference in temperature between the CPU and aluminum plate and the thermal resistance was calculated by the resulting value. The thermal resistance of the thermally conductive sheet of Example 1 was 1.07 K- in²/W (3) Shear force

After the thermally conductive sheet was cut to form a rectangular sample (10 mm X 10 mm), the rectangular sample was interposed between two stainless steel plates and its shear force was measured according to the procedure defined in JIS-Z-0237. The shear force of the thermally conductive sheet of this example (Example 1) was 11.7 N/cm².

(4) Adhesive force to Al

After the thermally conductive sheet was backed with a single-coated tape (product No. #851 A, manufactured by Sumitomo 3M Co.), the sheet was applied to an aluminum plate and the 90° peel adhesive force was measured according to the procedure defined in JIS-Z-0237. The sheet was peeled off at a testing rate of 300 mm/min. The adhesive force to Al of the thermally conductive sheet of Example 1 was 1.6 N/cm.

EXAMPLES 2 and 3

The procedure described in Example 1 was repeated. However, in these examples, the binder, the filler and the ratio of the binder to filler were changed as described in Table 1 below in the preparation of the thermally conductive sheet. The satisfactory evaluation results as shown in Table 2 were obtained.

COMPARATIVE EXAMPLES 1 to 4 The procedure described in Example 1 was repeated. However, in these examples, the binder, the filler and the ratio of the binder to filler were changed for comparison as described in Table 1 below and, in Comparative Example 3, the thickness of the sheet was increased to 3 mm from 1 mm. As shown in Table 2 below, the evaluation test could not continued because of poor uncured state. Even if the test was conducted, the evaluation results were unsatisfactory.

Table 1

(a) DOP = dioctyl phthalate

(b) IOA/AA = isooctyl acrylate/acrylic acid

(c) A1₂0₃ = aluminum oxide

Table 2

As is apparent from the results described in Table 1 and Table 2, in case of the thermally conductive sheet of the present invention, there could be obtained satisfactory results in all of the cured state, thermal resistance, shear force and adhesive force to Al.

To the contrary, in case of the thermally conductive sheets prepared in Comparative Examples 1 to 4, only unsatisfactory results were exhibited. For example, only the thermally conductive sheet of Comparative Example 1 exhibited good cured state, whereas, the thermally conductive sheets of the other Comparative Examples exhibited poor cured state so that subsequent evaluation test could not be conducted.

As described above, according to the present invention, there can be provided a thermally conductive sheet which has flexibility and is conformable to a special shape such as unevenness and curved surface, thereby making it possible to secure high adhesion and heat dissipation, and which also has excellent thermal conductivity and well- balanced adhesion performance. Since various thermally conductive materials can be mixed with a binder in this thermally conductive sheet, the thermal conductivity of the sheet can be easily controlled.

Claims

What is claimed is:

1. A thermally conductive sheet comprising a binder resin and a thermally conductive filler dispersed in said binder resin, characterized in that said binder resin is an acrylic polyurethane resin.

2. The thermally conductive sheet of claim 1 , wherein the acrylic polyurethane resin is a polymerization reaction product of an acrylic oligomer comprising at least two hydroxyl groups in a molecule and a polyfunctional isocyanate.

3. The thermally conductive sheet of claim 2, wherein the polyfunctional isocyanate is selected from the group consisting of hexamethylene diisocyanate, tolylene diisocyanate, diphenylenemethane diisocyanate, isophorone diisocyanate, tolidine diisocyanate, naphthalene diisocyanate, xylylene diisocyanate, and combinations of the foregoing.

4. The thermally conductive sheet of claim 2, wherein a viscosity of the acrylic oligomer is from 500 to 10,000 cps.

5. The thermally conductive sheet of claim 1 wherein the thermally conductive filler is an inorganic filler.

6. The thermally conductive sheet of claim 5 wherein the inorganic filler has an average particle diameter of about 1 micrometer to about 200 micrometers.

7. The thermally conductive sheet of claim 5 wherein the inorganic filler is selected from the group consisting of oxide particles, carbonate particles, metal particles, and combinations of the foregoing.

8. The thermally conductive sheet of claim 1 wherein a mixing ratio of the thermally conductive filler to the acrylic polyurethane resin is within a range from 10 to 70% by volume.

9. The thermally conductive sheet of claim 1 wherein the thickness of the sheet is from about 0.01 mm to about 4.0 mm.

10. The thermally conductive sheet of claim 1 wherein the thermal conductivity of the sheet is at least about 0.5 W/m K.

11. The thermally conductive sheet of claim 1 which is produced by a solvent- free process.

12. An article comprising:

(a) a heat generating part;

(b) a heat dissipating material; and

(c) a thermally conductive sheet comprising a binder resin and a thermally conductive filler dispersed in the binder resin, characterized in that the binder resin is an acrylic polyurethane resin, wherein the thermally conductive sheet is positioned between the heat generating part and the heat dissipating material.

13. The article of claim 11 wherein the heat generating part is an electronic part.

14. The article of claim 11 wherein the heat dissipating material is selected from the group consisting of a heat sink, a heat dissipating fin, and a metallic heat dissipating plate.