WO2003093336A1

WO2003093336A1 - Acrylic-based thermally conductive composition, sheet, and process

Info

Publication number: WO2003093336A1
Application number: PCT/US2003/013391
Authority: WO
Inventors: Yoshinao Yamazaki; Yuji Hiroshige; Mitsuhiko Okada; Tomoya Tanzawa
Original assignee: 3M Innovative Properties Company
Priority date: 2002-05-02
Filing date: 2003-04-30
Publication date: 2003-11-13
Also published as: JP4995403B2; AU2003225236A1; TW200400226A; JP2006111644A

Abstract

A composition for forming an acrylic-based thermally conductive composition, containing a thermal polymerizing binder component comprising (i) at least one (meth)acrylic monomer or its partially polymerized polymer and (ii) a thermal polymerization initiator, and a thermally conductive filler, the composition including the thermally conductive filler in an amount of 30-90 vol% of said acrylic-based thermally conductive composition. The invention also provides a process for producing an acrylic-based thermally conductive composition comprising adding a thermally conductive filler to a thermal polymerizing binder component comprising at least one (meth)acrylic monomer or its partially polymerized polymer and a thermal polymerization initiator to form a thermal polymerizing mixture, and heating said thermal polymerizing mixture for polymerization to form an acrylic-based thermally conductive composition, wherein said thermally conductive filler is added in an amount of 30-90 vol% in said acrylic-based thermally conductive composition.

Description

ACRYLIC-BASED THERMALLY CONDUCTIVE COMPOSITION,

SHEET, AND PROCESS

Background The present invention relates to an acrylic-based thermally conductive composition-forming composition, to a thermally conductive sheet obtained therefrom and to a process for their production.

Radiating release of heat generated by electronic and electrical devices such as personal computers is a matter of major importance. Thermally conductive sheets are used as heat radiating means to allow heat from heat generating parts of electronic and electric devices to escape to heat radiating parts such as heat sinks and metal covers. They are also used for anchoring between electronic parts and heat radiating parts.

Silicone resins have commonly been used as binder components in thermally conductive sheets. Ηowever, silicone resins generate low-molecular-weight siloxanes which adhere to equipment and can result in poor contacts. Acrylic resins are possible alternatives to silicone resins as binders. Patent Publications WO 98/24860 and Japanese Unexamined Patent Publication (Kokai) No. 11-269438 describe a thermally conductive tape obtained by coating a film with a composition comprising an acrylic-based polymer, a solvent and a filler, and removing the solvent by drying. Because a solvent is used, however, a high degree of loading of the filler results in precipitation and separation of the filler such that a higher loading cannot be achieved, while the presence of the solvent makes it impossible to form thick sheets.

Patent publications WO 98/24860, Japanese Unexamined Patent Publication (Kokai) No. 6-88061, Japanese Unexamined Patent Publication (Kokai) No. 9-316388, Japanese Unexamined Patent Publication (Kokai) No. 10-324853, and Japanese

Unexamined Patent Publication (Kokai) No. 2001-279196 describe that a thermally conductive sheet is obtained by sheeting a solvent-free composition comprising an acrylic- based monomer or its partially polymerized polymer and a filler, and conducting photopolymerization reaction with ultraviolet rays or the like. However, because this composition requires light transmittance in order to promote the polymerization reaction, the filler content and sheet thickness are restricted. It has therefore only been possible to fabricate sheets with low thermal conductivity or thin sheets. In particular, it has not been possible to achieve a high degree of loading with black, green or other colored fillers that have low light transmittance.

On the other hand, patent publications Japanese National Patent Publication (Kohyo) No. 9-512054 and Japanese Unexamined Patent Publication (Kokai) No. 2000-34453 describe a solventless acrylic resin obtained by a thermal polymerization reaction, but addition of a thermally conductive filler in large amount is not pursued.

Summary

It is therefore an object of the present invention to provide a composition which allows formation of thick acrylic-based thermally conductive sheets with high thermal conductivity, as well as thermally conductive sheets obtained from it and a process for their production.

According to the present invention there is provided a composition for formation of an acrylic-based thermally conductive composition, containing (1) a thermal polymerizing binder component comprising (a) at least one

(meth)acrylic monomer or its partially polymerized polymer and (b) a thermal polymerization initiator, and

(2) a thermally conductive filler, the composition including the thermally conductive filler in an amount of 30-90 percent by volume (vol %) of the acrylic-based thermally conductive composition.

Thermal polymerization reaction of the composition allows formation of a composition with high thermal conductivity as a thick sheet product.

Detailed Description The composition for formation of an acrylic-based thermally conductive composition according to the invention (hereunder referred to as "thermally conductive composition precursor") is a composition for formation of a thermally conductive composition by thermal polymerization using substantially no solvent. It is thereby possible to obtain a thermally conductive composition with high thermal conductivity that has not been achievable by the prior art. The composition of the present invention can be used as a thermally conductive sheet article, as well as a thermally conductive adhesive wherein it is filled in the location or area between the portions to be adhered as a liquid and then thermally polymerized. The thermally conductive composition of the invention may or may not have a pressure-sensitive adhesiveness.

The thermally conductive composition precursor contains a thermal polymerizing binder component comprising (a) at least one (meth)acrylic monomer or its partially polymerized polymer and (b) a thermal polymerization initiator. The (meth)acrylic monomer or the (meth) acrylic monomer for its partially polymerized polymer is not particularly restricted, and any monomer commonly used for formation of acrylic polymers may be used. Specifically, the (meth)acrylic monomer used is a (meth)acrylic monomer with an alkyl group of 20 carbons or less, and more specifically there may be mentioned ethyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl

(meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, decyl (meth)acrylate and dodecyl (meth)acrylate. For increased cohesive force of the resulting thermally conductive composition, it is preferred to additionally use a (meth)acrylic monomer with a homopolymer glass transition temperature of 20°C or higher. As such monomers there may be mentioned carboxyl acids and their corresponding anhydrides, such as acrylic acid and its anhydride, methacrylic acid and its anhydride, itaconic acid and its anhydride, and maleic acid and its anhydride. Other examples of (meth)acrylic monomers with homopolymer glass transition temperatures of 20°C or higher include cyanoalkyl (meth)acrylates, acrylamide, substituted acrylamides such as N,N'-dimethylacrylamide, and polar nitrogen-containing materials such as N-vinylpyrrolidone, N-vinylcaprolactam,

N-vinylpiperidine and acrylonitrile. Other monomers include tricyclodecyl (meth)acrylate, isobornyl (meth)acrylate, hydroxy (meth)acrylate and vinyl chloride. The (meth)acrylic monomer with a glass transition temperature of 20°C or higher is preferably included in an amount of no more than 100 parts by weight to 100 parts by weight of the (meth)acrylic monomer with an alkyl group of 20 carbons or less.

The acrylic-based thermally conductive composition precursor of the invention contains a thermally conductive filler. With conventional compositions obtained by photopolymerization with ultraviolet rays or the like, light transmittance for polymerization could be ensured only if the thermally conductive filler comprised less than 45 vol % of a white filler or less than 10 vol % of a colored filler, but since the acrylic-based thermally conductive composition precursor of the invention is polymerized by thermal polymerization, it is possible to add a thermally conductive filler in an amount of 10 vol % or greater of the obtained thermally conductive composition, irrespective of the color of the filler. The amount of the thermally conductive filler will normally be 30- 90 vol %. If the amount of the thermally conductive filler is less than 30 vol %, the thermal conductivity is reduced, while if it is greater than 90 vol %, the strength of the sheet is weakened.

As thermally conductive fillers there may be used ceramics, metal oxides, metal hydroxides, metal and the like. There may be mentioned thermally conductive fillers such as aluminum oxide, silicon oxide, magnesium oxide, zinc oxide, titanium oxide, zirconium oxide, iron oxides, silicon carbide, boron nitride, aluminum nitride, titanium nitride, silicon nitride, titanium boride, carbon black, carbon fiber, carbon nanotube, diamond, nickel, copper, aluminum, titanium, gold, silver. Crystalline form thereof can be any crystalline form that can be formed by the respective chemical species, such as hexagonal or cubic system. The filler particle size will usually be 500 μm or smaller. If the filler particle size is too large, the sheet strength is reduced. It is preferred to combine a portion of large-sized powder with a portion of small-sized powder. This is because the small particle size portion will fill the space between the large particle size portion, thus increasing the amount of filler that can be loaded. The particle size of the large particle size portion is preferably 10-150 μm, and the particle size of the small particle size portion is below that of the large particle size portion, and preferably less than 10 μm. For improved sheet strength, a filler which has been surface-treated with silane, titanate or the like may be used. In addition, a filler coated thereon with a coating such as water resistant or insulative coat made of ceramics or polymers. The term "particle size" used here means the dimension of the longest length as measured in a straight line passing through the center of gravity of the filler particle. The filler particle shape may be regular or irregular, for example, polygonal, cubic, elliptical, spherical, needle-like, plate-like or flaky, or combination of these shapes. Further, a filler can be a particle formed by aggregating crystalline particles. The filler shape may be selected based on the viscosity of the thermal polymerizing binder component and ease of working the final thermally conductive composition after polymerization. Further, an electromagnetic wave absorbing filler can be added in order to impart an electromagnetic wave absorbing property. An electromagnetic wave absorbing filler includes soft ferrite compounds such as Ni-Zn ferrite, Mg-Zn ferrite, Mn-Zn ferrite, magnetically soft metals such as carbonyl iron and Fe-Si-Al alloy (sendust), and carbon black. Since an electromagnetic wave absorbing filler itself is also thermally conductive, an electromagnetic wave absorbing filler can be used alone, or as a mixture with a thermally conductive filler. The thermal polymerizing binder component of the invention comprises (a) at least one (meth)acrylic monomer or its partially polymerized polymer and (b) a thermal polymerization initiator. Because the viscosity of the (meth)acrylic monomer will generally be low, the filler will often settle when a thermally conductive filler is mixed with the (meth)acrylic monomer-containing binder component. In such cases, it is preferred to partially polymerize the (meth)acrylic monomer beforehand to increase the viscosity. Such partial polymerization is preferably carried out to a viscosity of about 100-10,000 centipoise (cPs) in terms of the thermal polymerizing binder component. The partial polymerization may be carried out by any of various methods such as, for example, thermal polymerization, ultraviolet polymerization, electron beam polymerization, γ-ray polymerization and ionizing irradiation.

A thermal polymerization initiator or photopolymerization initiator is commonly used for such partial polymerization. As thermal polymerization initiators there may be used organic peroxide free radical initiators such as diacyl peroxides, peroxyketals, ketone peroxides, hydroperoxides, dialkyl peroxides, peroxy esters, peroxy dicarbonates and the like. Specifically there may be mentioned lauroyl peroxide, benzoyl peroxide, cyclohexanone peroxide, l,l-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane and t-butylhydroperoxide. Alternatively, persulfate/bisulfite combinations may also be used. As photopolymerization initiators there may be mentioned benzoin ethers such as benzoin ethyl ether or benzoin isopropyl ether, anisoin ethyl ether and anisoin isopropyl ether, Michler's ketone (4,4'-tetramethyldiaminobenzophenone), or substituted acetophenones such as 2,2-dimethoxy-2-phenylacetophenone (for example, KB-1 by Sartomer; Irgacure™ 651 by Ciba-Specialty Chemical) and 2,2-diethoxyacetophenone. In addition there may be mentioned substituted -ketols such as 2-methyl-2- hydroxypropiophenone, aromatic sulfonyl chlorides such as 2-naphthalenesulfonyl chloride, and photoactive oxime-based compounds such as l-phenone-l,l-propanedione-

2-(o-ethoxycarbonyl)oxime. Any combination of the foregoing thermal polymerization initiators or photopolymerization initiators may also be used. The amount of the initiator used for partial polymerization is not particularly restricted, but will normally be 0.001-5 parts by weight to 100 parts by weight of the (meth)acrylic monomer.

For the partial polymerization, the thermal polymerizing binder component may include a chain transfer agent to control the molecular weight and the content of the polymer included in the obtained partially polymerized polymer. Examples of the chain transfer agent include mercaptans, disulfides, carbon tetrabromide, carbon tetrachloride or the like, and combination thereof. If used, the transfer agent is generally used in an amount of 0.01-1.0 parts by weight based on 100 parts by weight of the (meth)acrylic monomer.

A crosslinking agent may be used to increase the strength when the obtained thermally conductive composition is processed into a sheet form or the like. As crosslinking agents, heat-activated crosslinking agents may also be used. Also included are lower alkoxylated aminoformaldehyde condensates with 1-4 carbon atoms in the alkyl group, hexamethoxymethylmelamine (for example, Cymell™ 303 by American

Cyanamide), tetramethoxymethylurea (for example, Beetle™ 65 by American Cyanamide) or tetrabutoxymethylurea (Beetle™ 85). Other useful crosslinking agents include polyfunctional acrylates such as 1,6-hexanediol diacrylate and tripropyleneglycol diacrylate. The crosslinking agent will usually be used in an amount of 0.001-5 parts by weight to 100 parts by weight of the monomer. Combinations of the foregoing crosslinking agents may also be used.

The above binder component comprises a thermal polymerization initiator. After partial polymerization, a thermal polymerization initiator is added to a partially polymerized polymer, or a mixture of a monomer with its partially polymerized polymer. As thermal polymerization initiators to be added may be organic peroxide free radical initiators such as diacyl peroxides, peroxyketals, ketone peroxides, hydroperoxides, dialkyl peroxides, peroxy esters, peroxy dicarbonates and the like. Specifically there may be mentioned lauroyl peroxide, benzoyl peroxide, cyclohexanone peroxide, l,l-bis(t- butylperoxy)-3,3,5-trimethylcyclohexane and t-butylhydroperoxide. Alternatively, persulfate/bisulfite combinations may also be used. An amount of the thermal polymerization initiator included in the thermal polymerizable binder component is 0.001 to 5 parts by weight per 100 parts by weight of (meth)acrylic monomer or its partially polymerized polymer, or a mixture of the monomer with its partially polymerized polymer.

For polymerization of the thermal polymerizing binder component, the thermal polymerizing binder component may include a chain transfer agent to control the molecular weight of the obtained acrylic polymer. As such chain transfer agents there may be mentioned mercaptans, disulfides, carbon tetrabromide, carbon tetrachloride, and the like. If used, the chain transfer agent will generally be used in an amount of 0.01-1.0 part by weight (phr) to 100 parts by weight of the (meth)acrylic monomer or its partially polymerized polymer. The thermally conductive composition of the invention may also contain additives such as tackifiers, antioxidants, plasticizers, flame retardants, anti-settling agents, thickeners such as acryl rubber and epichlorhydrin rubber, thixotropic agents such as micronized silica powder, surfactants, antifoaming agents, coloring agents, electric conductive particles, antistatic agents and the like, so long as the thermal conductivity endures. Combinations of the foregoing additives also may be used.

A thermal polymerizing mixture (thermally conductive composition precursor) is formed by combining a thermal polymerizing binder component comprising the aforementioned (meth)acrylic monomer or its partially polymerized polymer obtained by partial polymerization of the (meth)acrylic monomer, or a mixture of the aforementioned monomer and partially polymerized polymer, and a thermal polymerization initiator, with a thermally conductive filler, and an optional crosslinking agent, chain transfer agent or additives. The same thermal polymerization initiators mentioned above for the partially polymerized polymer may be used for this polymerization. Two or more thermal polymerization initiators with different half-lives may also be used to form the thermal polymerization mixture.

Further, the thermally conductive composition precursor is deaerated and mixed by a planetary mixer or the like. The resulting thermal polymerizing mixture can be used as a thermally conductive adhesive, wherein it is filled in the location or area between the portions to be adhered and then thermally polymerized at 50-200°C. Alternatively, the thermal polymerizing mixture is heated at about 50-200°C for thermal polymerization reaction to obtain a thermally conductive sheet according to the invention. The same thermal polymerization initiators mentioned above for the partially polymerized polymer may be used for this polymerization. Two or more thermal polymerization initiators with different half-lives or different activation temperatures may also be used to form the thermal polymerization mixture.

As a (meth)acrylic monomer, (meth)acrylic monomer having any polarity of acidic, neutral or basic polarity in a molecule can be used. Further, a thermally conductive filler used may be acidic, neutral or basic. A (meth)acrylic monomer and a thermally conductive filler, when used together, can have both the same polarity, or different polarity. When a thermally conductive composition is prepared using an acidic thermally polymerizable binder component including an acidic (meth)acrylate such as acrylic acid and a peroxide thermal polymerization initiator and a mixer including iron such as stainless steel is used, iron ion is dissolved into the acidic solution and it can be a catalyst for the peroxide thermal polymerization initiator. Consequently, polymerization reaction will proceed while mixing and the resulting composition cannot be molded. This problem can be avoided by lowering a viscosity of the thermally polymerizable binder component by reducing a proportion of the partially polymerized polymer. Alternatively, the problem can be avoided by using a mixer made from non-iron material or a mixer coated with a resin on its metal surface.

When a thermally conductive sheet is prepared, the polymerization is preferably carried out after application or coating of the composition onto a support surface such as a liner and forming a sheet by calendaring or press molding, in order to obtain a thermally conductive sheet according to the invention. The sheet may be formed in an inert atmosphere of nitrogen or the like in order to prevent inhibition of polymerization by oxygen. Since the invention allows loading of the thermally conductive filler to a much higher degree of loading than by the prior art, it is possible to produce a composition with a high thermal conductivity of 2 W/mK or greater. Such a thermally conductive sheet may be used for adhesion of heat sinks or heat radiators to electronic parts, and particularly semiconductor/electronic parts such as power transistors, graphic IC, chip sets, memory chips, central processing units (CPUs) and the like.

The thickness of the sheets is mainly determined by considering a thermal resistance of the portions to be applied. Typically, the sheets preferably have a thickness of 5 mm or less. However, when filled into a gap between a larger heat generating part and heat dissipating part, or applied to conform to irregularity of a part surface, sheets having a thickness greater than 5 mm may be suitable. When sheets having a thickness greater than 5 mm are suitable, thickness of the sheets is preferably less than 10 mm.

The thermally conductive sheet is provided by forming a thermally conductive composition layer on a release treated support or base which is releasable with respect to the thermally conductive composition. In this case, release of the support or base from the sheet during use will allow the latter to serve as a free-standing film. The thermally conductive sheet may also be used while anchored to the support or base for improved sheet strength. Polymer films are typical as supports or bases, and for example there may be used films of polyethylene, polypropylene, polyimide, polyethylene terephthalate, polyethylene naphthalate, polytetrafluoroethylene, polyether ketone, polyethersulfone, polymethylterpene, polyetherimide, polysulfone, polyphenylene sulfide, polyamidoimide, polyesterimide and aromatic amides. When heat resistance is a particularly required, a polyimide film or polyamidoimide film is preferred. The thermal conductivity may also be increased by adding a thermally conductive filler to the support or base. As supports or bases there may be mentioned metal foils of aluminum, copper or the like, or woven, nonwoven fabrics or scrims formed from glass fibers, carbon fibers, nylon fibers or polyester fibers, or such fibers that have been coated with a metal. The support or base may be present on one or both surfaces of the sheet, or it may be embedded in the sheet. The acrylic-based thermally conductive composition of the invention has a high thermally conductive filler content, and satisfactory thermal conductivity. The thermal conductivity and the dynamic properties such as tensile strength and compression properties are especially important when the thermally conductive composition is to be worked into a thermally conductive sheet. For example, the sheet must have sufficiently high tensile strength so as not to tear during attachment or reattachment of the thermally conductive sheet, and it must also have sufficiently low compression stress so as not to exert an excessive load on electronic parts when it is incorporated into an electronic device. In order to obtain a thermally conductive sheet exhibiting suitable dynamic properties, it is important to control the chemical structure of the acrylic-based polymer composing the binder. Here, an acrylic-based polymer suitable as a binder was discovered by crosslinking an acrylic polymer chain with low entanglement using a crosslinking agent such as a polyfunctional acrylate. In a thermally conductive sheet having a composition with a thermally conductive filler at 30-90 vol %, the viscoelastic properties of the acrylic-based polymer serving as the binder may be a shear storage elastic modulus (G') of 1.0 x 10³ - 1.0 x 10⁵ Pa and a loss tangent (tanδ) in the range of 0.2-0.8 measured at room temperature (about 20°C) and frequency of 1 Hertz. These viscoelastic properties represent a range suitable for crosslinking. On the other hand, the degree of entanglement of the polymer chain is largely dependent on its molecular weight, with low molecular weight polymer chains having low entanglement. When considering a polymer chain with no crosslinking, a number average molecular weight of less than 200,000 will result in a suitable degree of entanglement.

If the shear storage elastic modulus G' is lower than the range specified above, the tensile strength is too low, whereas if it is higher than the range specified above, the compression distortion at a given compression stress is low, i.e., the compression stress at a given distortion tends to be too high. If the loss tangent tanδ is lower than the range specified above, the compression distortion is low, whereas if it is higher than the range specified above, the tensile strength tends to be too low.

In other words, the acrylic-based polymer binder is the acrylic-based polymer obtained from the aforementioned (meth)acrylic-based monomer, and it has a polymer chain number average molecular weight of less than 200,000 and is crosslinked so as to have a shear storage elastic modulus (G¹) of 1.0 x 10³ - 1.0 x 10⁵ Pa and a loss tangent

(tanδ) in the range of 0.2-0.8 at 20°C and frequency of 1 Hertz.

As general methods for obtaining low molecular weight polymers by thermal radical polymerization there may be mentioned a method of increasing the amount of the thermal polymerization initiator, a method of carrying out the polymerization at a higher temperature than the decomposition temperature of the thermal polymerization initiator used, or a method of using a chain transfer agent. Under such conditions, a greater amount of radicals are generated at the start of the reaction, allowing effective consumption of the generated radicals for polymerization to obtain a low molecular weight polymer. Specifically, in cases where (1) the thermal polymerization initiator is added at 0.1-10 parts by weight (phr) to 100 parts by weight of the (meth)acrylic-based monomer for polymerization, (2) lauroyl peroxide (10-hour half-life decomposition temperature: 61.6°C) is used as the thermal polymerization initiator and polymerization is conducted at 80-200°C, (3) a chain transfer agent is added at 0.01-0.1 phr for the polymerization or (4) the polymerization is conducted with a combination of the above methods, it is possible to obtain an acrylic-based polymer with the molecular weight satisfactorily controlled to less than 200,000. Under such conditions, using a crosslinking agent in an amount of 0.01-5 parts by weight to 100 parts by weight of the (meth)acrylic-based monomer will give an acrylic-based polymer with the viscoelastic properties described above.

It has been found that the flexibility, pliability and adhesion during service are enhanced by using a specific low molecular weight acrylic polymer in place of an optional conventional plasticizer in the acrylic-based thermally conductive composition, which contains a large amount of thermally conductive fillers and has high thermal conductivity, of the present invention, resulting in reduced thermal resistance at the contact interface and thus an acrylic-based thermally conductive composition having high thermal conductivity can be obtained. If the larger amount of a thermally conductive filler is included in the acrylic thermally conductive adhesive composition of the present invention, the more prominent is the above mentioned effect of inclusion of the low molecular weight acrylic polymer compared to a composition which does not include the low molecular weight acrylic polymer. Further, the low molecular weight acrylic polymer has such an advantage that the polymer does not ooze out because of high compatibility with the composition as compared with a conventional plasticizer, and that the polymer is not substantially volatilized because of high molecular weight as compared with a conventional plasticizer and thus it does not cause contamination during service.

The low molecular weight acrylic polymer, which can be used in the present invention, is in the form of liquid at normal temperature and has Tg of 20°C or lower. Such an acrylic polymer comprises an acrylate ester monomer as a main component and has an ester moiety with 1-20 carbons. Examples of acrylate esters having an ester moiety with 1-20 carbons include alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, s-butyl acrylate, t-butyl acrylate, neopentyl acrylate, 2-ethylhexyl acrylate, isodecyl acrylate, lauryl acrylate, tridecyl acrylate, stearyl acrylate and the like. These acrylate esters may be used alone, or two or more may be used in combination. The low molecular weight acrylic polymer can be copolymerized with a copolymerizable monomer, in addition to the acrylate ester. Examples of copolymerizable monomers include vinyl-based monomers such as methacrylate ester, -olefins, vinyl esters and vinyl ethers. The low molecular weight acrylic polymer can be produced by a conventional process such as suspension polymerization or emulsion polymerization in an aqueous medium, or solution polymerization in an organic solvent or bulk polymerization. The glass transition temperature of the acrylic polymer is 20°C or lower, and preferably 0°C or lower. The weight-average molecular weight is preferably 500 or more and 100,000 or less, and more preferably 700 or more and 20,000 or less. If the glass transition temperature is higher than 20°C, a thermally conductive sheet having high flexibility and high adhesion may not be obtained. If the weight-average molecular weight exceeds 100,000, sufficient plasticity is not achieved, resulting in poor processability into a thermally conductive sheet. On the other hand, when the weight-average molecular weight is less than 500, cohesive force of the sheet is lowered, resulting in poor handling properties. The low molecular weight acrylic polymer can be produced by using the chain transfer agent as described above in an amount of 0.01-1.0 parts by weight based on 100 parts by weight of the (meth)acrylic monomer. Upon polymerization, particularly partial polymerization of the thermally conductive composition precursor for production of the acrylic-based thermally conductive composition of the present invention, a low molecular weight acrylic polymer suited for use as a plasticizer can also be formed in situ in the composition by adding the chain transfer agent. The amount of the low molecular weight acrylic polymer is preferably 1-100 part by weight, and more preferably 5-70 parts by weight, based on 100 parts by weight of the monomer or the partially polymerized polymer. If the amount is less than 1 part by weight, the effect of the plasticizer disappears. On the other hand, if the amount is more than 100 parts by weight, handling properties are deteriorated because of excess stickiness and physical strength such as tensile strength is also lowered.

The term "acrylic polymer which has substantially no functional group" with respect to the low molecular weight acrylic polymer means that the polymer has substantially no functional group which can react with functional groups on the (meth)acrylate monomer or its partially polymerized polymer, or the thermal polymerization initiator or the crosslinking agent.

The acrylic-based thermally conductive composition precursor of the present invention provides, by thermal polymerization, thermally conductive compositions with high thermal conductivity that has been unachievable by the prior art. Further, an acrylic thermally conductive sheet having superior mechanical properties such as compression property and tensile strength are obtained using an acrylic polymer as a binder. Furthermore, by using the low molecular weight acrylic polymer as the plasticizer, the flexibility of the acrylic-based thermally conductive composition is enhanced and the adhesion with the body to be contacted is improved, and thus the thermal conductivity is further improved.

The features of the embodiments of the invention are further illustrated in the following non-limiting examples.

Examples

These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight unless indicated otherwise. Parts by weight is abbreviated "pbW", and parts by volume is abbreviated "pbV".

Test Methods

Thermal Conductivity Using a thermal conductivity detector QTM-D3 (product of Kyoto Electronics

Manufacturing Co., Ltd.), the thermal conductivity of the sheet itself was measured.

Thermal Resistance

A sample was cut to a size of 10 mm x 11 mm and sandwiched between a heating element and a cooling plate, the difference in temperature between the heating element and the cooling plate was measured when a constant load of 7.6 N/cm" and an electric power of 4.8 W were applied and the thermal resistance was determined by the following equation:

Thermal resistance (°C*cm²/W) = difference in temperature (°C) x area (cm²)/electric power (W). Tensile Strength

The tensile strength was measured according to JIS K6251. The thickness of each sample was 1 mm, and a #2 dumbbell was used as the punch die.

Compression Strain

The thermally conductive sheets used for the Tensile Strength test were used for the compression strain test. Each 1 ram-thick sample was cut to 10 mm x 10 mm and compressed at a compression rate of 0.5 mm/min, and the compression strain ( = compression/ original thickness x 100) was measured at the point where the compression stress reached 50 N/cm².

Molecular Weight

The obtained thermally conductive sheets were subjected to gel permeation chromatography (GPC) and the molecular weight distributions were determined in terms of polystyrene.

Viscoelastic Properties

The viscoelasticity measurement was accomplished using an RDA II Dynamic Analyzer by Rheometrics Co., and the shear elastic modulus (G) (G^τ: storage elastic modulus, G": loss elastic modulus; units: Pa) and the loss tangent (tanδ) at 20°C with a frequency of 1 Hz were determined.

Weight Loss

The thermally conductive sheet was heated at 150°C for 120 minutes and the weight loss was measured and the percent weight loss was calculated.

Materials Used In The Examples

2-EHA - 2-ethylhexyl acrylate BA - n-butyl acrylate DMA - N,N'-dimethylacrylarnide

TCDA - tricyclodecyl acrylate IBOA - isobornyl acrylate HDD A - 1,6-hexanediol diacrylate 2-EHTG - 2-ethylhexyl thioglycolate LPO - lauroyl peroxide

Perloyl™ TPC - (bis(4-t-butylcyclohexyl)peroxydicarbonate) thermal polymerization initiator available from Nihon Yushi Co., Ltd.

Irgacure™ 651 - (2,2-dimethoxy-2-phenylacetophenone) available from Ciba-Specialty Chemicals.

Irganox™ 1076 - (octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) available from Ciba-Specialty Chemicals.

2,2'-azobis(2,4-dimethylvaleronitrile) - polymerization initiator l,l-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane - polymerization initiator n-dodecylmercaptan - chain transfer agent

UP-1000 - liquid polyacrylate having a molecular weight of 3000 and Tg of -55°C available from Toagosei Co., Ltd..

DOP - di(2-ethylhexyl) phthalate

BPTC - l,l-bis(t-butylperoxy)3,3,5-trimethylcyclohexane S 151 - titanate-based coupling agent (available from Nippon Soda Co., Ltd.)

Silicon carbide - mean particle size of 80 μm.

Aluminum hydroxide - mean particle size of 2 μm and titanate-treated.

Alumina 1 - alumina with a mean particle size of 50 μm.

Alumina 2 - silane-treated alumina with a mean particle size of 2 μm. Aluminum nitride -mean particle size of 70 μm, and surface-coated with boron nitride.

Boron nitride - hexagonal crystalline system having an average particle diameter of

8 μm.

Ferrite - Ni-Zn ferrite having an average particle diameter of 5 μm.

Examples 1-9 and Comparative Example 1 Preparation of partially polymerized polymer:

An ultraviolet polymerization initiator (Irgacure™ 651) was added in an amount of 0.04 wt % to a (meth)acrylic monomer of either 2-EHA or BA, and the mixture was exposed to ultraviolet (UN) rays at an intensity of 3 mW/cm² using UV light source having maximum intensity at wave length of 300 nm to 400 nm to obtain a partially polymerized polymer with a viscosity of approximately 1000 centipoises (cPs). The partially polymerized polymer was a thickened liquid as a result of polymerization of 10- 20% of the total monomer.

Fabrication of thermally conductive composition: Thermally conductive composition precursors obtained by deaerating and kneading each of the components in the compositions listed in Table 1 below by a mixer were sandwiched between two polyethylene terephthalate (PET) liners coated with a silicone releasing agent and subjected to calendering.

In Examples 1 to 9, the molding was followed by heating in an oven at 150°C for 15 minutes for thermal polymerization, to obtain a thermally conductive sheet, hi

Comparative Example 1, the molding was followed by irradiation with UN rays at an intensity of 3 mW/cm² using UV light source having maximum intensity at wavelength of 300 nm to 400 nm for 5 minutes for UN polymerization. Sheets with thicknesses of 0.5 mm to 4 mm were obtained in Examples 1 to 9. The sheets were released from the liners and the thermal conductivity was measured using the test method described above.

The results are shown in Table 1 below.

In Comparative Example 1 (CE-1), polymerization of the (meth)acrylic monomer was insufficient, and therefore the sheet lacked cohesive force and could not be released from the liner. In the following table, "ppp" means partially polymerized polymer, and "Ν/M" means not measurable.

Table 1

The results shown above demonstrate that thick acrylic-based thermally conductive sheets with high thermal conductivity can be obtained utilizing a thermal polymerization reaction.

Example 10

Production of partially polymerized polymer:

TM _,

Upon combining 100 parts by weight of 2-EHA and 0.04 part of Irgacure^llvl 651, the mixture was exposed to ultraviolet rays at an intensity of 3 mW/cm² using ultraviolet light source having maximum intensity at wavelength of 300 nm to 400 nm to obtain a partially polymerized polymer with a viscosity of approximately 1000 centipoise (cPs).

Fabrication of thermally conductive sheets:

A mixer was used for deaerating and kneading of 40 pbV of a polymerizing binder component consisting of 45 pbW of 2-EHA partially polymerized polymer prepared as described above, 40 pbW of 2-EHA monomer, 15 pbW of TCDA monomer, 0.3 part by wt. of Irganox™ 1076, various amounts of HDD A (added in the weights listed in Table 2 below) and 0.5 part by wt. of LPO, with 40 pbV of silicon carbide and 20 pbV of aluminum hydroxide, and the resulting compositions were sandwiched between liners and subjected to calendering. The molding was followed by thermal polymerization reaction under the four conditions of 180°C for 15 minutes, 150°C for 15 minutes, 100°C for 30 minutes and 80°C for 2 hours to fabricate thermally conductive sheets.

The tensile strengths of the obtained thermally conductive sheets were measured according to the test method described above and the results are shown in Table 2.

Table 2. Tensile Strength (N/cm²)

A suitable tensile strength is 10 N/cm or greater. A low strength indicates the minimum degree of crosslinking.

The thermally conductive sheets used for the tensile test were subjected to a compression strain test according to the test method described herein. The results are shown in Table 3.

Table 3. Compression Strain (%)

A suitable compression strain is 10% or greater. The composition with 1.0 part by wt. of HDDA exhibited low compression strain, and this therefore represented the maximum degree of crosslinking.

Example 11

The process of Example 10 was used to fabricate thermally conductive sheets comprising 40 parts by volume of a polymerizing binder component consisting of 45 pbW of 2-EHA partially polymerized polymer prepared as described in Example 10, 40 pbW of 2-EHA monomer, 15 pbW of TCDA monomer, 0.3 part by wt. of Irganox™ 1076 and 0.5 part by wt. of LPO, with 40 pbV of silicon carbide and 20 pbV of aluminum hydroxide. The thermal polymerization conditions are shown in Table 4. The obtained thermally conductive sheets were tested for molecular weight distributions. The results are shown in Table 4. Table 4. Molecular Weight

(a) Mn = number average molecular weight

(b) Mw = weight average molecular weight

Based on the correlation between the compression property test results and the molecular weight, it was concluded that an increase in molecular weight (lower polymerization temperature conditions) results in lower compression strain. It was demonstrated that undesirably low compression strain is exhibited when the number average molecular weight is 200,000 or greater.

Example 12

A composition containing no thermally conductive filler was polymerized under various polymerization conditions to prepare samples for measurement of the viscoelasticity.

The acrylic composition contained 45 pbW of 2-EHA partially polymerized polymer of Example 10, 40 pbW of 2-EHA monomer, 15 pbW of TCDA, 0.3 part by wt. of Irganox™ 1076 and 0.5 part by wt. of LPO, and the crosslinking agent HDDA as listed in Table 5.

Thermal polymerization was conducted under the four conditions of 180°C for 15 minutes, 150°C for 15 minutes, 100°C for 30 minutes and 80°C for 2 hours to obtain 100 μm-thick samples. The polymerization temperature profiles under each of the conditions were confirmed to be the same as for fabrication of heat-release sheets. The viscoelastic properties were determined according to the test method described above. The results are shown in Table 5. Table 5. Viscoelastic Properties

For the viscoelastic properties of the acrylic-based polymers, a shear storage elastic modulus (G¹) of 1.0 x 10³ - 1.0 x 10⁵ Pa and a loss tangent (tanδ) in the range of 0.2-0.8 at room temperature (20°C) and frequency of 1 Hertz were found to be satisfactory.

Example 13

A composition with an acrylic-based polymer glass transition temperature (Tg) of -7.8°C was examined. The composition of the polymerizing binder component was 15 pbW of 2-EHA partially polymerized polymer of Example 10, 45 pbW of 2-EHA monomer, 40 pbW of TCDA monomer, 0.3 part by wt. of Irganox™ 1076, 0.05 part by wt. of HDDA and 0.5 part by wt. of LPO. The Tg of the acrylic-based polymer, calculated by the Fox Equation (T.G. Fox, Bull. Am. Phys. Soc, 1, 123(1956)) was -7.8°C. Thermally conductive sheets were obtained in the same manner as Example 10 from a composition comprising 40 pbV of the aforementioned polymerizing binder component, 40 pbV of silicon carbide and 20 pbN of aluminum hydroxide.

The viscoelastic properties, tensile strength and compression strain were carried out by the test methods described above. The molecular weight measurement was conducted in the same manner as Example 11, using a sample separately fabricated from the aforementioned composition containing no HDDA. The results are shown in Table 6.

All of the obtained thermally conductive sheets exhibited favorable dynamic properties.

Example 14 A composition with an acrylic-based polymer Tg of -50°C was examined.

The composition of the polymerizing binder component was 60 pbW of 2-EHA partially polymerized polymer of Example 10, 35 pbW of 2-EHA monomer, 5 pbW of TCDA, 0.3 part by wt. of Irganox™ 1076 and 0.12 part by wt. of HDDA, with 0.25 part by wt. of LPO and 0.75 part by wt. of l,l-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane as the initiator. The Tg of the acrylic-based polymer, calculated by the Fox Equation, was

-50°C.

Thermally conductive sheets were obtained in the same manner as Example 10 from a composition comprising 40 pbV of the aforementioned polymerizing binder component, 40 pbV of silicon carbide and 20 pbV of aluminum hydroxide. The viscoelastic properties, tensile strength and compression strain were carried out according to the test methods above. The molecular weight measurement was conducted in the same manner as Example 10, using a sample separately fabricated from the aforementioned composition containing no HDDA. The results are shown in Table 7.

Table 7. Properties

All of the obtained thermally conductive sheets exhibited favorable dynamic properties. The thermally conductive sheets prepared in Examples 10-14 all have thermal conductivity of 2.1 W/mK.

Examples 15-17 and Comparative Examples 2-4

Examples utilizing a low molecular weight acrylic polymer as a plasticizer were prepared as follows: Production of partially polymerized polymer:

After mixing 100 pbW of 2-EHA with 0.04 pbW of Irgacure™ 651, the mixture was exposed to ultraviolet rays at an intensity of 3 mW/cm² using an ultraviolet light source having maximum intensity at wavelength of 300 nm to 400 nm to obtain a partially polymerized polymer with a viscosity of approximately 1000 centipoise (cPs). The partially polymerized polymer was a thickened liquid as a result of polymerization of 3- 20% of the total 2-EHA monomer.

Fabrication of thermally conductive composition:

The thermally conductive composition precursor obtained by deaerating and kneading each of the components in the compositions listed in Table 8 below by a mixer were sandwiched between two polyethylene terephthalate (PET) liners coated with a silicone release agent and subjected to calendering. The molding was followed by heating in an oven at 150°C for 15 minutes for thermal polymerization to fabricate a thermally conductive sheet. In Comparative Examples 2 and 3, thermally conductive sheets containing no low molecular weight acrylic polymer were fabricated. In Comparative Example 4, a thermally conductive sheet containing DOP as a conventional plasticizer was fabricated in the same manner. All sheets thus fabricated had a thickness of 1 mm.

The sheets were tested for thermal conductivity, and weight loss according to the test methods described herein. The results are shown in Table 8.

Table 8

As compared with the sheets of Comparative Examples 2 and 3 wherein the low molecular weight acrylic polymer as the plasticizer was not added, all sheets of Examples 15-17 exhibited low thermal resistance. These results show that the sheets having high flexibility and high adhesion during service could be obtained by the addition of the low molecular weight acrylic polymer as the plasticizer in Examples 15-17. As described above, in Examples 15-17, the thermal conductivity is remarkably improved even in case of the same content of the thermally conductive filler as compared with Comparative Examples 2 and 3. The sheet containing a conventional plasticizer of Comparative Example 4 has low thermal resistance as well as high flexibility and high adhesion, but exhibits large heating loss which is considered to be caused by volatilization of the plasticizer, resulting in a fear of contamination of the portion of the body to which the sheet is applied due to the plasticizer.

As is apparent from the above results, the flexibility and adhesion during service of the sheet are enhanced by the addition of the liquid low molecular weight acrylic polymer and thus an acrylic thermally conductive sheet having high thermal conductivity can be obtained.

Example 18

Production of low molecular weight acrylic polymer by solution polymerization:

A low molecular weight acrylic polymer was produced by the solution polymerization. 40 pbW of 2-EHA monomer, 0.5 pbW of 2,2'-azobis(2,4- dimethylvaleronitrile), 1.2 pbW of n-dodecylmercaptan and 50 pbW of ethyl acetate were mixed with stirring. After replacing dissolved oxygen by a nitrogen gas while bubbling the nitrogen gas into the obtained solution for 10 minutes, the solution was polymerized at 50°C. The solvent was removed from the solution and the molecular weight was measured according to the test method described herein. As a result, the weight-average molecular weight was 14,000.

Fabrication of thermally conductive sheet

A solution prepared by mixing 20 pbW of the low molecular weight acrylic polymer obtained by the solution polymerization as described above, 30 pbW of the partially polymerized polymer produced in Examples 15-17, 50 pbW of 2-EHA monomer,

0.3 pbW of HDDA, 0.25 pbW of LPO, 0.25 pbW of BPTC and 3.0 pbW S 151 was used as a binder component. Using a precursor composition of 25 pbV of the binder component,

50 pbV of Alumina- 1, and 25 pbV of Alumina-2, a thermally conductive sheet was fabricated in the same manner as in Examples 15-17. In the same manner as in Examples 15-17, the thermal resistance, the thermal conductivity and the weight loss were measured. As a result, the thermal resistance of the thermally conductive sheet was 3.39 °C cm²/W, the thermal conductivity was 6.0 W/mK, and the weight loss was 0.20 % by wt.

Example 19 Production of partially polymerized polymer consisting of low molecular weight acrylic polymer and acrylic monomer

100 pbW of 2-EHA monomer, 0.04 pbW of Irgacure™ 651 and 0.30 pbW of 2-EHTG chain transfer agent were mixed together. After replacing dissolved oxygen by a nitrogen gas while bubbling the nitrogen gas into the obtained solution, the solution was exposed to UV rays to obtain a partially polymerized polymer with a viscosity of 1000 centipoise (cPs). This partially polymerized polymer was a mixture of a low molecular weight acrylic polymer and an acrylic monomer, which contains 70 % by wt. of an acrylic polymer having a weight-average molecular weight of 85,000.

Fabrication of thermally conductive sheet

A solution prepared by mixing 60 pbW of the partially polymerized polymer obtained above, 40 pbW of a 2-EHA monomer, 0.2 pbW of HDDA, 0.25 pbW of LPO,

0.25 pbW of BPTC and 3.0 pbW of S 151 was used as a binder component. The binder component contains 42 pbW of the low molecular weight acrylic polymer (70 % x 60). A thermally conductive sheet consisting of 25 pbV of this binder component, 50 pbV of

Alumina- 1 and 25 pbV of Alumina-2 was fabricated in the same manner as in

Examples 15-17.

In the same manner as in Examples 15-17, the thermal resistance, the thermal conductivity and the weight loss were measured. As a result, the thermal resistance of the thermally conductive sheet was 2.97°C cm²/W, the thermal conductivity was 6.4 W/mK, and the weight loss was 0.21 % by wt.

Example 20

Production of partially polymerized polymer consisting of low molecular weight acrylic polymer and acrylic monomer

100 pbW of 2-EHA monomer, 0.04 pbW of Irgacure™ 651 and 0.40 pbW of 2-EHTG were mixed. After replacing dissolved oxygen by a nitrogen gas while bubbling the nitrogen gas into the obtained solution, the solution was exposed to UN rays to obtain a partially polymerized polymer with a viscosity of 1000 centipoise (cPs). This partially polymerized polymer was a mixture of a low molecular weight acrylic polymer and an acrylic monomer, which contains 50 % by weight of an acrylic polymer having a weight- average molecular weight of 65,000.

Fabrication of thermally conductive sheet

A solution prepared by mixing 60 pbW of the partially polymerized polymer obtained above, 40 pbW of a 2-EHA monomer, 0.2 pbW of HDDA, 0.25 pbW of LPO, 0.25 pbW of BPTC and 3.0 pbW of S 151 was used as a binder component. This binder component contains 30 pbW of the low molecular weight acrylic polymer (50% x 60).

A thermally conductive sheet consisting of 25 pbN of this binder component, 50 pbN of

Alumina- 1 and 25 pbN of Alumina-2 was fabricated in the same manner as in

Examples 15-17. In the same manner as in Examples 15-17, the thermal resistance, the thermal conductivity and the weight loss were measured. As a result, the thermal resistance of the thermally conductive sheet was 3.13 degCcm²/W, the thermal conductivity was

6.0 W/mK, and the weight loss was 0.21 % by wt.

While the various features of the preferred embodiment of the invention have been described in detail, changes to these features and to the described embodiment may be apparent to those skilled in the art. Such changes or modifications to are believed to be within the scope and spirit of the invention, as set forth in the following claims.

Claims

1. A composition for forming an acrylic-based thermally conductive composition, containing (a) a thermal polymerizing binder component comprising (i) at least one (meth)acrylic monomer or its partially polymerized polymer and (ii) a thermal polymerization initiator, and

(b) a thermally conductive filler, the composition including the thermally conductive filler in an amount of 30-90 vol% of said acrylic-based thermally conductive composition.

2. A composition according to claim 1, wherein said (meth)acrylic monomer includes a (meth)acrylic monomer with an alkyl group of 20 carbons or less.

3. A composition according to claim 1, wherein said (meth)acrylic monomer includes a (meth)acrylic monomer with an alkyl group of 20 carbons or less and a (meth)acrylic monomer with a homopolymer glass transition temperature of 20°C or higher.

4. A composition according to any one of claims 1 to 3, wherein said thermal polymerization initiator is an organic peroxide.

5. A composition according to any one of claims 1 to 4, wherein it further comprises an acrylic polymer which includes as a main component an acrylate ester having an ester moiety with 1-20 carbons and has a glass transition temperature of 20°C or lower and a weight-average molecular weight of 500 or more and 100,000 or less, and also which has substantially no functional group.

6. An acrylic-based thermally conductive composition obtained by polymerizing a thermal polymerizing binder component of a composition according to any one of claims 1 to 5.

7. An acrylic-based thermally conductive composition according to claim 6, wherein said polymerizing binder component further comprises (c) a crosslinking agent, and wherein the acrylic polymer of the binder obtained by polymerizing and crosslinking said polymerizing binder component has a polymer chain with number average molecular weight of less than 200,000 and is crosslinked so as to have a shear storage elastic modulus (G¹) of 1.0 x 10³ - 1.0 x 10^s Pa and a loss tangent (tanδ) in the range of 0.2-0.8 measured at 20°C and frequency of 1 Hz.

8. A composition according to any one of claims 1-7, wherein the thermal conductivity of the composition is 2 W/mK or greater.

9. A thermally conductive sheet obtained by forming a composition according to any one of claims 6-8 into a sheet.

10. A thermally conductive sheet according to claim 9, which has a base material on the surface and/or in the interior of said thermally conductive sheet.

11. A process for producing an acrylic -based thermally conductive composition comprising: adding a thermally conductive filler to a thermal polymerizing binder component comprising (a) at least one (meth)acrylic monomer or its partially polymerized polymer, and (b) a thermal polymerization initiator to form a thermal polymerizing mixture; and heating said thermal polymerizing mixture for polymerization to form an acrylic-based thermally conductive composition, wherein said thermally conductive filler is added in an amount of 30-90 vol% in said acrylic-based thermally conductive composition.