Thermally conductive silicone composition and cured product thereof

Abstract

The present invention relates to a heat-conducting silicone composition which is liquid and has curability, and which contains (A) a liquid silicone, (B) a hydrosilylation catalyst, and (C) a filler, and which is liquid and has curability, the filler contains at least (C-1) a thermally conductive filler having a true density greater than the density of the liquid silicone and (C-2) a low-density filler having a true density equal to or less than the density of the liquid silicone, and the particle size distribution of the filler is controlled so that the filler is dispersed in a state close to the closest filling in the matrix containing the liquid silicone.

Description

Technical Field

[0001] The present invention relates to a thermally conductive silicone composition and a cured product thereof. Background Art

[0002] In recent years, the miniaturization and high integration of electrical and electronic components have been continuously promoted, and the heat generation has gradually increased. Similarly, in secondary batteries, their capacity and output power are gradually increasing, and the heat generation is also increasing. For heating elements such as electrical components, electronic components, and secondary batteries, it is necessary to prevent their temperature from rising excessively, so a structure is adopted to dissipate the heat generated in the heating element to a heat sink such as a radiator or an outer packaging shell. The heat conductive composition or its cured product is sandwiched between the heating element and the heat sink to improve the thermal coupling between the heating element and the heat sink, and is used to efficiently transfer the heat from the heating element to the heat sink. As a heat conductive composition, a heat conductive silicone composition containing silicone as a matrix and various fillers added to the silicone matrix is ​​widely used. The heat conductive silicone composition or its cured product is sold in the market in various forms such as heat dissipation grease and heat sink. Among them, thermally conductive silicone compositions and their cured products, which are called gap fillers, are liquid when applied to heating elements and heat sinks, and have excellent followability to uneven surfaces (i.e., gap filling properties), enabling precise coating. They also cure over time, thereby preventing pumping out and dripping, and are therefore widely used.

[0003] In order to improve the heat dissipation performance by sandwiching a thermally conductive silicone composition or its cured product between the heating element and the heat sink, it is first important that the thermally conductive silicone composition or its cured product has high thermal conductivity. In order to improve the thermal conductivity, a thermally conductive filler with high thermal conductivity is mixed into the thermally conductive silicone composition. In addition, in order to improve the heat dissipation characteristics, it is also important to reduce the thermal resistance at the interface between the heating element and the thermally conductive silicone composition or its cured product, and at the interface between the heat sink and the thermally conductive silicone composition or its cured product. However, when the mixing ratio of the thermally conductive filler is increased in order to obtain high thermal conductivity, the viscosity of the thermally conductive silicone composition increases, and as a result, the adhesion of the thermally conductive silicone composition or its cured product when in contact with the heating element or the heat sink decreases, and the thermal resistance at the interface increases. In order to reduce the viscosity, a method of diluting the uncured thermally conductive silicone composition with an organic solvent is sometimes used. However, if the thermally conductive silicone composition and its cured product contain volatile components due to the use of an organic solvent, bubbles are generated inside the thermally conductive silicone composition or its cured product, or voids are generated at the interface with the heat sink or the heating element over time, making it difficult to transfer heat. For the same reason, it is also not preferable that the thermally conductive silicone composition or its cured product generate volatile components due to chemical reactions.

[0004] In addition, if the mixing ratio of the thermally conductive filler in the thermally conductive silicone composition is increased, the hardness of the cured product also increases. Therefore, not only the impact absorption capacity is reduced, but also the effect of alleviating the stress caused by the expansion and contraction of electrical components, electronic components, secondary batteries, etc. due to temperature changes is reduced. However, as a thermally conductive filler, a filler composed of an inorganic material with a large thermal conductivity, i.e., an inorganic filler, is generally used. However, the density of the inorganic filler is quite high compared to silicone. Therefore, when the mixing ratio of the thermally conductive filler in the thermally conductive silicone composition is increased, the density of the thermally conductive silicone composition and its cured product also increases, resulting in an increase in the mass of electronic equipment and automobiles. If the mass increases, for example, the portability will decrease in the case of electronic equipment, and the cruising range will be shortened in the case of automobiles. Therefore, it is desired to reduce the weight of the thermally conductive silicone composition combined with the thermally conductive filler.

[0005] As a document on a heat-conductive composition containing a heat-conductive filler, Patent Document 1 discloses that in an insulating heat-dissipating film using a cross-linked rubber, in order to reduce the amount of inorganic filler blended into the rubber and achieve high thermal conductivity, particles composed of organic substances, i.e., organic fillers, are added to the rubber in addition to the inorganic filler. In the technology described in Patent Document 1, boron nitride, which is a plate-like particle, is used as the inorganic filler, and polyamide particles or cross-linked polyacrylate particles having a larger average particle size than the inorganic filler are used as the organic filler, thereby positioning the inorganic filler in the gaps between the organic fillers in the cross-linked rubber. Since the inorganic filler is positioned in the gaps between the organic fillers, it is easy to form a heat transfer path based on the inorganic filler, and even if the amount of the inorganic filler blended is small, an insulating heat-dissipating film with improved thermal conductivity and excellent mechanical properties can be obtained.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2012-224711

[0009] Non-patent literature

[0010] Non-patent document 1: Jong-Woo, et al., "The properties of AlN-filled epoxy molding compounds by the effects of filler size distribution", J. Mater. Sci., 35, 5907-5913 (2000) Summary of the invention

[0011] Problems to be solved by the invention

[0012] In the technology described in Patent Document 1, boron nitride, which is a plate-like particle, is used as an inorganic filler having a high thermal conductivity. However, the plate-like filler increases the viscosity of the uncured thermal conductive composition, resulting in a high hardness of the cured product and a loss of flexibility. In addition, in order to properly arrange the inorganic filler in the gap between the organic fillers, the volume percentage of all fillers in the thermal conductive composition, i.e., the total volume percentage of the fillers, which is the sum of the inorganic filler and the organic filler, needs to be greater than a certain value. Due to these factors, in the technology described in Patent Document 1, the viscosity of the uncured thermal conductive composition tends to increase. If the viscosity of the thermal conductive composition is high, the hardness of the cured product may also increase.

[0013] Therefore, an object of the present invention is to provide a thermally conductive silicone composition which is lightweight and has high thermal conductivity, has a low viscosity of an uncured composition, and has a low hardness of a cured product and has excellent flexibility, and a cured product thereof.

[0014] Means for solving problems

[0015] The inventors have found that when a first filler having high thermal conductivity and a second filler having low density are mixed with silicone to obtain a thermally conductive silicone composition, the flexibility of the thermally conductive silicone composition and its cured product can be improved by controlling the particle size distribution of the filler, thereby completing the present invention. That is, according to one embodiment of the present invention, the thermally conductive silicone composition is a liquid and curable thermally conductive silicone composition containing (A) liquid silicone, (B) a hydrosilylation catalyst, and (C) a filler.

[0016] The filler includes at least (C-1) a thermally conductive filler having a true density greater than that of the liquid silicone, and (C-2) a low-density filler having a true density less than that of the liquid silicone.

[0017] The number of particle sizes at the calculation points of the cumulative frequency in the cumulative particle size distribution based on the volume of the filler is denoted as R, the minimum and maximum particle sizes of the thermally conductive filler among the particle sizes at the calculation points of the cumulative frequency are denoted as P (μm) and Q (μm), respectively, and the particle size of the filler at the nth calculation point determined so as to satisfy the following equations (a1) and (a2) is denoted as x. n (μm) (where n is an integer satisfying 1≤n≤R), the above cumulative particle size distribution and the above particle size x n The cumulative frequency of the corresponding particle size of the thermally conductive filler on a volume basis is defined as c n (%), regarding the particle size distribution of the filler, the minimum value E of the mean square error E represented by the following formula (a3) ​​is min and the minimum value E min The value a0 of the coefficient a in the corresponding following equation (a4) satisfies:

[0018] 0≤E min ≤40, and

[0019] 6.0≤a0≤13.0.

[0020] [Mathematical formula 1]

[0021]

[0022] According to another aspect of the present invention, the cured product is a cured product obtained by curing the thermally conductive silicone composition described above by a hydrosilylation reaction.

[0023] Effects of the Invention

[0024] According to the present invention, a thermally conductive silicone composition and a cured product thereof can be obtained which are lightweight, have high thermal conductivity, have low viscosity of an uncured composition, and have low hardness of a cured product and excellent flexibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram showing the difference in the distribution of the thermally conductive filler depending on whether or not a low-density filler is added and the total volume percentage of the filler.

[0026] Figure 2 This is a graph showing the relationship between the particle size of the thermally conductive filler and the cumulative frequency.

[0027] Figure 3 This is a graph showing the relationship between the particle size of the thermally conductive filler and the cumulative frequency.

[0028] Figure 4 This is a graph showing the relationship between the particle size of the thermally conductive filler and the cumulative frequency. DETAILED DESCRIPTION

[0029] The method for implementing the present invention is described. The thermally conductive silicone composition based on the present invention is liquid, and is sandwiched between the heating element and the heat sink in its original state (uncured) or in a cured state (i.e., in the form of a cured product) to improve the thermal coupling between the heating element and the heat sink. The thermally conductive silicone composition is usually a composition formed by combining a liquid silicone containing a silicone polymer with a hydrosilylation catalyst for causing the silicone polymer to produce a cross-linking reaction and a thermally conductive filler that improves thermal conductivity. Since the thermally conductive filler is usually formed of an inorganic material with a high density, the mass of the thermally conductive silicone composition and its cured product will also increase if the amount of the thermally conductive filler is increased. Therefore, in the thermally conductive silicone composition based on the present invention, in addition to the thermally conductive filler, a low-density filler having a true density equal to or less than that of the liquid silicone is also mixed into the liquid silicone. It should be noted that the volume of the filler in this specification is the true volume of the filler as a particle, and the true density and volume percentage are determined based on the true volume. Although there are cases where a hollow body is used as a low-density filler as described later, the true volume of the low-density filler as a hollow body is the volume including the hollow part thereof, and the true density of the low-density filler is the value obtained by dividing the mass of the low-density filler by the true volume. Even if the volume percentage of the thermally conductive filler in the thermally conductive silicone composition is the same, the proportion of the volume of the thermally conductive filler relative to the volume of the liquid silicone as the matrix becomes higher due to the presence of the low-density filler. Therefore, the thermally conductive fillers are close to each other in the matrix, and the thermal conductivity of the thermally conductive silicone composition and its cured product is improved. By adding the low-density filler in this way, a high thermal conductivity can be obtained by adding a small amount of the thermally conductive filler, so that the thermally conductive silicone composition and its cured product can be lightweight.

[0030] However, when dispersing the particles in the matrix, the space surrounded by the large particles is filled with small particles, and the space surrounded by the small particles is further filled with tiny particles, thereby controlling the particle size and the addition ratio of each particle size in a manner close to the closest packed state. It is known that by setting it in this way, the viscosity and hardness of the matrix can be reduced while filling the particles at a high density. In the thermally conductive silicone composition based on the present invention, the viscosity of the thermally conductive silicone composition is reduced while obtaining sufficient thermal conductivity, and the hardness of the cured product is reduced by optimizing the particle size distribution of all fillers including the thermally conductive filler and the low-density filler, thereby obtaining sufficient flexibility. The optimization of the particle size distribution of the filler of the present invention will be described later.

[0031] Figure 1This is a schematic diagram showing the difference in the distribution of thermally conductive fillers in a thermally conductive silicone composition due to the presence or absence of low-density fillers added to a matrix containing silicone and the size of the total volume percentage of the fillers. As shown in the figure, when the total volume percentage of the fillers is small, even if low-density fillers are added, the distance between the thermally conductive fillers does not change much, and the improvement in thermal conductivity is also small. In contrast, when the total volume percentage of the fillers reaches a certain level or more, the distance between the thermally conductive fillers is drastically shortened by the addition of low-density fillers, and the thermal conductivity is greatly improved.

[0032] Hereinafter, main components constituting the thermally conductive silicone composition will be described.

[0033] <(A) Component: Liquid silicone (silicone oil)>

[0034] The liquid silicone that is the component (A) and constitutes the matrix of the liquid silicone composition may be any liquid silicone as long as it is formed of a silicone polymer that undergoes a crosslinking reaction using a hydrosilylation catalyst. However, in general silicone compositions, silicone obtained by mixing an organopolysiloxane having an alkenyl group with an organohydrogenpolysiloxane having a silicon-bonded hydrogen atom in a molecular chain side chain is often used as the liquid silicone. The present inventors have also studied the application of such a liquid silicone used in a general silicone composition to a thermally conductive silicone composition.

[0035] The liquid silicone used in a general silicone composition is cross-linked by a hydrosilylation reaction in the presence of a catalyst to form a cured product. In order to reduce the viscosity of the thermally conductive silicone composition, it is necessary to reduce the viscosity of the liquid silicone that serves as the matrix. As one method for reducing the viscosity of the liquid silicone, there is a method of using a liquid silicone with a small molecular weight. However, if the liquid silicone with a small molecular weight is cross-linked, the cross-linking density becomes high, and the hardness of the cured product becomes high. On the other hand, in order to reduce the hardness of the cured product while using a low molecular weight liquid silicone, it is necessary to add a large amount of organic hydrogen polysiloxane having silicon-bonded hydrogen atoms in the molecule to the liquid silicone to extend the molecular chain. However, in this case, the amount of silicon-bonded hydrogen atoms relative to alkenyl groups will increase. In addition, the silicon-bonded hydrogen atoms present in the side chains of the molecular chain of the organic hydrogen polysiloxane cannot react completely due to steric hindrance, and are also likely to remain after curing. Due to these factors, when the ratio of the organohydrogenpolysiloxane in the liquid silicone is increased, a large amount of silicon-bonded hydrogen atoms remain even after curing by cross-linking.

[0036] The silicon-bonded hydrogen atoms that remain after curing react with water in the presence of a catalyst to become silanol groups. Thereafter, a crosslinking reaction is promoted between the generated silanol groups and the remaining silicon-bonded hydrogen atoms. Therefore, even if the cured product of the thermally conductive silicone obtained by crosslinking an organopolysiloxane having an alkenyl group with an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms in the side chain of the molecular chain shows low hardness at the initial stage, the hardness gradually increases after long-term use. Therefore, in the liquid silicone used in the present invention, it is preferred to use an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the end of the molecular chain as a crosslinking extender, and to use an organopolysiloxane having at least 3 alkenyl groups in the molecule as a crosslinking point. By using an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the end of the molecular chain instead of at the side chain of the molecular chain, it is possible to prevent the reduction of reactivity due to steric hindrance and reduce the remaining silicon-bonded hydrogen atoms. By extending the molecular chain and reducing the crosslink density in this way, the cured product has low hardness and a small number of remaining silicon-bonded hydrogen atoms, and thus can maintain its low hardness for a long time.

[0037] As described above, the liquid silicone used in the thermally conductive silicone composition according to the present invention preferably comprises:

[0038] (A-1) an organopolysiloxane having alkenyl groups at both ends of the molecular chain;

[0039] (A-2) an organopolysiloxane having an alkenyl group at one end of the molecular chain and having no reactive functional group at the other end;

[0040] (A-3) an organopolysiloxane having at least three alkenyl groups in the molecule; and

[0041] (A-4) Four types of polymers, namely, organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the molecular chain terminals and no silicon-bonded hydrogen atoms at the molecular chain side chains. Among the polymers of the components (A-1) to (A-4), the polymer of the component (A-1) which is an organopolysiloxane having alkenyl groups at both molecular chain terminals serves as a base polymer of the liquid silicone.

[0042] In addition, the polymer of the component (A-2) of the organopolysiloxane having an alkenyl group at one end of the molecular chain and no reactive functional group at the other end has an alkenyl group that is the object of the hydrosilylation reaction at one end of the molecular chain and no reactive functional group at the other end, so only one end is cross-linked, and the other end can move freely. By including the polymer of the component (A-2) in the liquid silicone in this way, the degree of cross-linking can be reduced while suppressing bleeding, and a soft thermally conductive silicone composition can be obtained. As an example of the polymer of the component (A-2), an organopolysiloxane having one end of the molecular chain capped by a vinyldimethylsilyl group and the other end capped by a trimethylsilyl group or a tert-butyldimethylsilyl group can be exemplified. In addition, the addition amount of the component (A-2) is preferably 10 to 90 parts by mass in 100 parts by mass of the liquid silicone. If the amount of component (A-2) added is less than 10 parts by mass, the effect of reducing the hardness of the cured product is insufficient. If the amount of component (A-2) added is more than 90 parts by mass, the crosslinking degree when the liquid silicone is cured by hydrosilylation reaction is too low to be cured. It should be noted that examples of the reactive functional group mentioned here include alkenyl, silicon-bonded hydrogen, silanol, amino, isocyanate, epoxy, acrylic, mercapto, and alkoxy groups.

[0043] In addition, the polymer of the component (A-3) which is an organopolysiloxane having an average of at least 3 alkenyl groups in the molecule reacts with hydrogen atoms bonded to silicon atoms and acts as a crosslinking point. The components (A-1) and (A-2) are chain polymers, and the component (A-3) may be a chain polymer or a cyclic polymer, or may have a branched structure. Examples of the component (A-3) include cyclic polymers such as 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, and straight-chain dimethyl silicone oil having 4 vinyl groups in the side chain or at the end. The component (A-3) may exist in the liquid silicone without forming a complex, or may exist in the form of a platinum complex such as a platinum (0)-2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane complex. In addition, a non-complex type component (A-3) and a complex type component (A-3) may coexist in the liquid silicone. The amount of component (A-3) added is preferably 0.01 to 2 parts by mass based on 100 parts by mass of the liquid silicone. If the amount of component (A-3) added is less than 0.01 parts by mass, the number of crosslinking points is too small and curing does not proceed. On the contrary, if the amount of component (A-3) added is more than 2 parts by mass, the number of crosslinking points is too large and the hardness of the cured product becomes high.

[0044] In addition, the polymer of the component (A-4) which is an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the molecular chain ends and not having silicon-bonded hydrogen atoms at the molecular chain side chains reacts with each polymer component (A-1) to (A-3). The component (A-4) is a chain polymer with or without a branched structure. As the component (A-4), there can be exemplified a straight-chain organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain and not having silicon-bonded hydrogen atoms at the molecular chain side chains; an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the molecular chain ends but not having silicon-bonded hydrogen atoms at the molecular chain side chains and having a branched structure, etc. By using the component (A-4), the polymer of the component (A-1) can be extended by a hydrosilylation reaction, and even when a low-viscosity alkenyl-containing organopolysiloxane is used, the crosslinking density of the cured product does not increase, and a cured product with low hardness can be obtained as a cured product of the thermally conductive silicone composition. In addition, since the silicon-bonded hydrogen atoms in the component (A-4) are not present in the side chains of the molecular chain but in the ends of the molecular chain, it is not easy to reduce the reactivity due to steric hindrance, and it is not easy for unreacted silicon-bonded hydrogen atoms to remain in the cured product. However, it is preferred to add the component (A-4) so ​​that the silicon-bonded hydrogen atoms contained in the component are 0.6 to 1.6 moles relative to 1 mole of alkenyl groups in the liquid silicone. If the amount of silicon-bonded hydrogen atoms relative to 1 mole of alkenyl groups is less than 0.6 moles, it will not cure. On the other hand, if it is greater than 1.6 moles, the remaining silicon-bonded hydrogen atoms increase, and the silicon-bonded hydrogen atoms react with water in the presence of a catalyst to generate silanol groups. Since the silanol groups generated in this way are cross-linked with the silicon-bonded hydrogen atoms that still remain, it becomes the cause of the increase in the hardness of the cured product when used for a long time.

[0045] It should be noted that the liquid silicone as component (A) may contain an organopolysiloxane represented by the following formula (1) as a polymer of component (A-5) on the basis of the polymers of the components (A-1) to (A-4) described above. Component (A-5) does not have reactive functional groups such as alkenyl groups and silicon-bonded hydrogen atoms, and thus can reduce the crosslinking density when the liquid silicone is cured, thereby reducing the hardness of the cured product. The amount of component (A-5) added is preferably 0 parts by mass or more and 80 parts by mass or less in 100 parts by mass of the liquid silicone. If the amount of component (A-5) added is greater than 80 parts by mass, the crosslinking density is too low and the shape cannot be maintained.

[0046] [Chemistry 1]

[0047]

[0048] The viscosity of the liquid silicone containing each component (A-1) to (A-4) or the liquid silicone containing each component (A-1) to (A-5) is preferably 10 to 10000 mPa·s. If the viscosity is less than 10 mPa·s, the thermally conductive filler as the component (C-1) is likely to settle during storage of the thermally conductive silicone composition. If the viscosity is greater than 10000 mPa·s, the viscosity of the thermally conductive silicone composition increases, and the thermally conductive silicone composition and its cured product have poor adhesion to the heating element and the heat dissipating element, thereby increasing thermal resistance.

[0049] <Component (B): Hydrosilylation catalyst>

[0050] In order to promote the crosslinking reaction in the liquid silicone described above, a hydrosilylation catalyst as component (B) is added to the thermally conductive silicone composition. Examples of the hydrosilylation catalyst include metal-containing catalysts such as platinum-based catalysts, rhodium-based catalysts, and palladium-based catalysts. More specifically, examples include platinum (0)-2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane complexes and platinum (0)-1,3-divinyltetramethyldisiloxane complexes. One of these catalysts may be used alone, or two or more catalysts may be used in combination. The content of the hydrosilylation catalyst in the thermally conductive silicone composition is preferably 0.1 to 1500 ppm relative to the liquid silicone in terms of the mass of the metal element contained in the catalyst. If the amount of the hydrosilylation catalyst added is less than 0.1 ppm, curing will not proceed, and if it is greater than 1500 ppm, the curing speed is too fast, making the operation complicated. It should be noted that the amount of the hydrosilylation catalyst added can be appropriately adjusted to obtain a desired curing speed.

[0051] <(C) Ingredient: Filler>

[0052] The filler added as the component (C) in the thermally conductive silicone composition according to the present invention includes at least two filler components, namely, a thermally conductive filler as the component (C-1) and a low-density filler as the component (C-2). In addition to the components (C-1) and (C-2), a filler component that is neither the component (C-1) nor the component (C-2) may be added to the thermally conductive silicone composition.

[0053] The thermally conductive filler as the component (C-1) is used to impart thermal conductivity to the thermally conductive silicone composition and its cured product. For example, it is a filler obtained by forming a material having a thermal conductivity higher than that of liquid silicone into a filler particle shape. The thermally conductive filler has a true density greater than the density of liquid silicone. As the thermally conductive filler, for example, a single metal, an alloy, a metal oxide, a metal hydroxide, a metal nitride, a metal carbide, a metal carbonate, silicon dioxide, boron nitride, silicon carbide, etc. are used. These substances can be used alone or in combination of two or more. As the single metal used in the thermally conductive filler, gold, silver, copper, aluminum, magnesium, etc. can be mentioned, and as the alloy, brass, various aluminum alloys, etc. can be mentioned. In addition, as metal oxides, aluminum oxide, zinc oxide, magnesium oxide, calcium oxide, etc. can be mentioned, as metal hydroxides, aluminum hydroxide, magnesium hydroxide, etc. can be mentioned, as metal nitrides, etc. can be mentioned, and as metal carbonates, anhydrous magnesium carbonate, etc. can be mentioned. The thermally conductive filler added to the liquid silicone may be a single thermally conductive filler or may be used in combination of two or more thermally conductive fillers.

[0054] In addition, from the viewpoint of reducing the viscosity of the thermally conductive silicone composition and reducing the hardness of the cured product, the shape of the thermally conductive filler is preferably not any of plate-like, needle-like and fibrous, but preferably spherical, polyhedral and amorphous. The aspect ratio of the shape of the thermally conductive filler is preferably close to 1. In the case of using a plate-like, needle-like or fibrous filler, since the contact probability and contact area between the filler particles increase, the mechanical interaction between the fillers also increases, the viscosity of the thermally conductive silicone composition increases, and the hardness of the cured product also increases. Moreover, the thermally conductive filler is preferably formed of a soft material, and the Mohs hardness of the material constituting the thermally conductive filler is preferably low. If the Mohs hardness increases, the thermally conductive filler will scrape the inner surface of the sizing machine used in the coating of the thermally conductive silicone composition and the mixing device used in the mixing, and it is easy to cause foreign matter to mix into the thermally conductive silicone composition or the maintenance frequency of the device to increase. In addition, in order to obtain a lightweight and high thermally conductive silicone composition and a cured product thereof, it is preferred that the thermally conductive filler itself is also lightweight and high in thermal conductivity.

[0055] Examples of the thermally conductive filler used in the thermally conductive silicone composition of the present invention include aluminum hydroxide (Mohs hardness 3, density 2.42 g / cm 3 , thermal conductivity 5~10W / (m·K)), anhydrous magnesium carbonate (Mohs hardness 3.5, density 3.04g / cm 3, thermal conductivity of about 15W / (m·K)), etc. Regarding the amount of thermally conductive filler added, it is preferably an amount that makes the volume percentage of the thermally conductive filler in the thermally conductive silicone composition and its cured product range from 50.0 volume % to 72.0 volume %. If the amount added is less than 50.0 volume %, although the density of the thermally conductive silicone composition and its cured product becomes lower, the thermal conductivity becomes lower. On the other hand, if the amount added is more than 72.0 volume %, although the thermal conductivity of the thermally conductive silicone composition and its cured product becomes higher, the density increases. Thermally conductive silicone compositions and their cured products are often used in electrical and electronic components. In this case, electrical insulation is sometimes required to prevent electrical short circuits. It is also preferable to use a material with high electrical insulation in the thermally conductive filler as the (C-1) component.

[0056] The low-density filler as component (C-2) is added to liquid silicone in order to achieve high thermal conductivity by promoting the formation of multiple thermal conduction paths based on the thermal conduction filler in the matrix of liquid silicone while achieving lightweight in the thermal conductive silicone composition and its cured product. The thermal conductivity of the low-density filler is generally lower than that of the thermal conduction filler. By adding the low-density filler, even if the volume percentage of the thermal conduction filler in the thermal conductive silicone composition and its cured product is the same, the volume percentage of the thermal conduction filler relative to the liquid silicone increases, so the distance between the thermal conduction fillers becomes shorter and heat can be efficiently transferred. As a result, a thermal conductive silicone composition and its cured product that are lightweight and have high thermal conductivity can be obtained. From the perspective of lightweight, the low-density filler as component (C-2) has a true density that is equal to or lower than the density of the liquid silicone. As an example, since the typical density of liquid silicone is 0.97 g / cm 3 Therefore, the true density of the low-density filler is preferably 0.97 g / cm 3 In addition, the true density of the low-density filler is preferably 0.01 g / cm 3 If the true density is less than 0.01g / cm 3 , low-density fillers are easy to fly in the air during weighing and mixing, making the operation complicated.

[0057] As for the low-density filler, since the purpose of adding the thermally conductive fillers is to bring them close to each other in the thermally conductive silicone composition and its cured product, it can be any of a solid body, a hollow body, and a porous body. As long as the true density has the desired value, there is no restriction on the material of the low-density filler. As a specific example, polyethylene solid particles, polypropylene solid particles, resin hollow particles also called resin hollow balls, glass hollow particles also called glass hollow balls, etc. can be used as low-density fillers. The low-density filler formed by polyethylene solid particles is excellent in load resistance and in the following aspects, that is, the possibility of deformation of the filler particles due to shearing during kneading is low, and polyethylene has a higher thermal conductivity than the liquid silicone as a matrix. The thermal conductivity of liquid silicone is about 0.2W / (m·K), and the thermal conductivity of polyethylene is about 0.3~0.5W / (m·K). Although the low-density filler formed by hollow resin particles or hollow glass particles is not easy to obtain the effect of improving the thermal conductivity of the heat-conducting silicone composition when it is added, since the true density is particularly low compared with liquid silicone, it is excellent in terms of lightweight. The shape of the low-density filler is preferably spherical. In the case of a spherical low-density filler, since the contact area between the filler particles becomes smaller, the mechanical interaction between the filler particles becomes smaller. As a result, the viscosity of the heat-conducting silicone composition becomes lower, and the hardness of its cured product also becomes lower. As a low-density filler added to the liquid silicone, one or more low-density fillers can be used.

[0058] In the thermally conductive silicone composition based on the present invention, a thermally conductive filler as a component (C-1) and a low-density filler as a component (C-2) are added to liquid silicone. At this time, the median diameter ratio, which is the ratio of the volume-based median diameter (D50) of the low-density filler to the volume-based median diameter (D50) of the thermally conductive filler, that is, [D50 of low-density filler / D50 of thermally conductive filler], is preferably greater than 0.3 and less than 20. If the median diameter ratio is less than 0.3, the low-density filler fills the gaps between the particles of the thermally conductive filler, so that the effect of the thermally conductive fillers approaching each other due to the low-density filler pushing the thermally conductive filler is reduced. On the other hand, if the median diameter ratio is greater than 20, the thickness of the thermally conductive silicone composition cannot be reduced, so the thermal resistance becomes large, and it is difficult to dissipate heat from the self-heating body.

[0059] The volume percentage of the filler as the component (C) in the thermally conductive silicone composition and its cured product is preferably 70.0% by volume or more and 80.0% by volume or less. If the volume percentage of the filler relative to the thermally conductive silicone composition and its cured product is less than 70.0% by volume, even if a low-density filler is added, the volume percentage of the thermally conductive filler relative to the liquid silicone is not large enough, and the thermally conductive fillers cannot be sufficiently close to each other. On the other hand, if the volume percentage of the filler is greater than 80.0% by volume, the viscosity of the thermally conductive silicone composition becomes high and the cured product becomes hard. Regarding the amount of thermally conductive filler and low-density filler added, the volume ratio, which is the ratio of the volume of the thermally conductive filler to the volume of the low-density filler in the filler added to the liquid silicone, is preferably 1.67 or more and 22.0 or less. If the volume ratio is less than 1.67, the possibility of forming a network structure based on the low-density filler in the matrix and cutting off the heat transfer path based on the thermally conductive filler becomes high, resulting in difficulty in transferring heat. On the other hand, if the volume ratio is larger than 22.0, the effect of the low-density filler pushing the thermally conductive filler and bringing the thermally conductive fillers closer to each other becomes smaller, and the effect of improving the thermal conductivity becomes smaller.

[0060] In the thermally conductive silicone composition described herein, fillers are blended into liquid silicone at a high filling rate in such a manner that the volume percentage of the fillers as a whole relative to the volume thereof is, for example, 70% by volume or more. In order to obtain a thermally conductive silicone composition with low viscosity and a cured product with low hardness while blending fillers at a high filling rate, it is important to properly control the particle size and addition ratio of the fillers to form a structure close to the closest packing in which small particles enter the gaps between large particles, thereby optimizing the particle size distribution of the fillers. It should be noted that the so-called particle size distribution of the fillers refers to the particle size distribution when all the fillers added to the liquid silicone are regarded as one group. Although the particle size distribution is not optimized for each type of filler, from the perspective of establishing a heat transfer path, after using a filler with a relatively uniform particle size as a low-density filler and using fillers with different particle sizes as a thermally conductive filler, the particle size distribution of the filler as a whole is optimized, thereby significantly improving the thermal conductivity while ensuring the softness of the composition.

[0061] However, in order to make a structure close to the closest packing, particles of different particle sizes are mixed, and various studies have been conducted on the preferred mixing ratio of these particles. However, in most cases, the particles are assumed to have a single particle size, or only the average particle size is used to discuss the optimization of the particle size and the addition ratio. However, whether it is a thermal conductive filler or a low-density filler, for commercially available fillers, the particle size is not single, but there is a range in the particle size distribution. Moreover, depending on the type of particles and the manufacturing method, even with the same average particle size, the particle size distribution is different. Therefore, if only the addition ratio of each average particle size is studied, sometimes it is impossible to reduce the viscosity of the thermal conductive silicone composition or the hardness of the cured product. In particular, when the amount of filler added is increased, it is difficult to take into account both low viscosity and low hardness. Therefore, the inventors focused on the cumulative particle size distribution on a volume basis after mixing multiple fillers with different particle sizes and studied the optimal particle size distribution. The following is a description of the inventor's research content and results.

[0062] <Research Content>

[0063] Non-patent document 1 discloses that, regarding aluminum nitride (AlN) filler mixed into epoxy resin, a high filling rate close to closest packing can be achieved by setting the addition ratio of large-diameter particles to small-diameter particles to 0.735:0.265 on a volume basis. From the perspective of achieving a state close to closest packing, since the aluminum nitride filler mixed into epoxy resin is in the same state as the filler mixed into liquid silicone, it is considered that large-diameter particles and small-diameter particles are mixed at a volume ratio of 0.735:0.265.

[0064] Figure 2The middle broken line A represents the cumulative frequency of the particle size when particles of a single particle size of 10 μm and particles of a single particle size of 1 μm are mixed in a manner of a volume ratio of 0.735:0.265, that is, the cumulative particle size distribution. In addition, the broken line B represents the cumulative particle size distribution when particles of a single particle size of 50 μm, 5 μm, and 0.5 μm are mixed in a manner of setting the volume ratio of particles of 50 μm to particles of 5 μm to 0.735:0.265 and setting the volume ratio of particles of 5 μm to particles of 0.5 μm to 0.735:0.265, respectively. Similarly, line C represents the cumulative particle size distribution when particles of single particle sizes of 80 μm, 4 μm, and 0.2 μm are mixed in a manner such that the volume ratio of particles of 80 μm to particles of 4 μm is set to 0.735:0.265, and the volume ratio of particles of 4 μm to particles of 0.2 μm is set to 0.735:0.265. However, even if the average particle size is used to specify the type of particles, the actual particles also show a particle size distribution with a certain degree of fluctuation. Even if multiple fillers with different average particle sizes are simply mixed in the above ratios, it is difficult to obtain a particle size distribution with a certain degree of fluctuation. Figure 2 The thermally conductive silicone composition has a cumulative particle size distribution represented by broken lines A to C.

[0065] Therefore, the present inventors focused on the fact that, when the cumulative frequency on a volume basis is y (%) and the particle size is x (μm), a can be set as a positive parameter, i.e., a coefficient, and the following equation (2) can be used to roughly approximate: Figure 2 The cumulative particle size distributions represented by the broken lines A to C. For example, Figure 2 The broken line A can be approximated by a curve with a=19.3 in equation (2). Similarly, the broken line B can be approximated by a curve with a=9.6, and the broken line C can be approximated by a curve with a=7.6. Figure 2 The curves indicated by the dotted lines in are approximate curves expressed by equation (2) when the coefficient a is set to various values. Equation (2) can be considered to be an equation expressing the optimal cumulative particle size distribution corresponding to the closest packing.

[0066] [Mathematical formula 2]

[0067]

[0068] In formula (2), ax 1 / 2 When ≥100, y = 100% because the maximum value of the cumulative particle size distribution is 100% by definition. Figure 2As shown, if the coefficient a is increased, the approximate curve of formula (2) moves toward the smaller particle size side, and if the coefficient a is reduced, it moves toward the larger particle size side. By mixing a variety of fillers with different average particle sizes in a manner that makes the particle size distribution as close as possible to formula (2), a thermally conductive silicone composition having high thermal conductivity and low viscosity can be obtained, and by curing such a thermally conductive silicone composition, a cured product having high thermal conductivity and low hardness can be obtained. Regarding the cumulative particle size distribution on a volume basis after mixing a variety of fillers, it can be calculated for each type of filler based on the particle size distribution of the filler and its mixing ratio. The measurement results of the particle size distribution performed by the manufacturer of the filler can be used as the particle size distribution of the filler.

[0069] When a plurality of fillers are mixed in practice, it is preferred to introduce an index showing the extent to which the actual cumulative particle size distribution of the mixed fillers deviates from the cumulative particle size distribution represented by formula (2), and it is preferred to set the filler mixing ratio in such a way that the deviation represented by the index is as small as possible. As an index representing the degree of deviation in the cumulative particle size distribution, the mean square error E is an effective index. When considering the cumulative particle size distribution, the particle size as the starting point (minimum value) of the cumulative frequency calculation is set to P (μm), and the particle size as the end point (maximum value) is set to Q (μm) (where Q>P), and k is set to a constant satisfying k>1, and is set to 1≤n≤R (n is an integer), so as to satisfy the following formulas (3) and (4), and R particle sizes {x n}.

[0070] x n =k n-1 P, (3)

[0071] x R =Q (4)

[0072] Obviously, the particle size {x n} is a value obtained by determining R particle diameters in a relationship of forming a geometric progression with a common ratio of k between the particle diameter P as the starting point and the particle diameter Q as the end point, including the starting point and the end point, and is a calculation point for calculating the cumulative frequency.

[0073] Furthermore, the particle size x n The actual cumulative frequency of the filler at n (%), substitute x in equation (2) into x n The cumulative frequency is set as y n , the mean square error E is determined as shown in the following formula (5).

[0074] [Mathematical formula 3]

[0075]

[0076] It can be said that the larger the mean square error E, the greater the deviation between the actual particle size distribution in the filler and the particle size distribution expressed by equation (2), and the smaller E, the smaller the deviation. In practice, since the mean square error E is a quadratic function of the coefficient a, the minimum value of the mean square error E when the coefficient a is changed can be obtained, and this minimum value is set as E. min , find the minimum value E min The corresponding value of coefficient a is taken as a0.

[0077] <Research Results>

[0078] The result of the above study is that in order to make E min When a plurality of fillers are mixed so that a and a0 simultaneously satisfy the following formulae (7) and (8), a thermally conductive silicone composition having a low viscosity and a low hardness of a cured product can be obtained.

[0079] 0≤E min ≤40 (7)

[0080] 6.0≤a0≤13.0 (8)

[0081] It should be noted that if the minimum value of the mean square error E min If a0 is greater than 40, the deviation from the ideal particle size distribution represented by formula (2) becomes larger, so the viscosity of the thermally conductive silicone composition increases and the hardness of its cured product also increases. When a0 is less than 6.0, the particle size distribution is excessively biased toward the larger particle size side. Therefore, when the thermally conductive silicone composition is sandwiched between the heating element and the heat sink, the distance between the heating element and the heat sink increases, and the heat transfer efficiency deteriorates. On the other hand, if a0 is greater than 13.0, the particle size distribution is excessively biased toward the smaller particle size side. Therefore, the surface area of ​​the filler increases, the viscosity of the thermally conductive silicone composition increases, and the hardness of its cured product also increases. Formulas (2) to (5) are derived only based on the particle size distribution of the filler. E described here min The optimal range of a0 can be applied to both low-density fillers and high-density fillers, that is, regardless of the type of filler, and can also be applied when all fillers mixed into liquid silicone, including low-density fillers and high-density fillers, are combined and treated as one filler group. The thermally conductive filler can be one type, or two or more thermally conductive fillers can be used in combination. In addition, with respect to two or more thermally conductive fillers, any combination can be used in terms of average particle size and material. Similarly, as a low-density filler, one type can be used, or two or more types can be used in combination. In addition, with respect to two or more low-density fillers, any combination can be used in terms of average particle size and material.

[0082] As an example, when the starting particle size P in the particle size distribution of the filler is set to 0.33 μm, the end particle size Q is set to 204 μm, and R is set to 47, k is obtained according to equations (3) and (4) as k≈1.15. The starting point P (μm) and the end point Q (μm) of the particle size for calculating the mean square error E can be selected as any value as long as it includes the range where the cumulative particle size frequency of the filler is greater than 0% and less than 100%. R can be appropriately determined based on the actual cumulative particle size distribution and the required calculation accuracy in the calculation of the mean square error E of the cumulative particle size distribution based on equation (2). If R is increased, the accuracy becomes better, but the amount of calculation increases. If R is reduced, the amount of calculation can also be reduced, but the accuracy slightly deteriorates.

[0083] Figure 3 This is a diagram for explaining the difference between the actual particle size distribution of the filler and the particle size distribution represented by formula (2) when the cumulative particle size distribution is considered, with the starting particle size P being 0.33 μm, the ending particle size Q being 204 μm, and R being 47. Figure 3 In the figure, the solid curve represents the actual particle size distribution, and the dashed curve represents the particle size distribution shown in equation (2). The lengths of the line segments between the two curves and extending in the vertical direction are |y n -c n |. Figure 4 This shows how the relationship between the particle size distribution calculated based on equation (2) and the actual particle size distribution changes when the value of coefficient a is changed. Figure 4 As shown in Figure 2, the actual particle size distribution when a=9.2 is consistent with the particle size distribution based on formula (2). Figure 4 It can be seen that when a=5 and a=15, the actual particle size distribution deviates greatly from the particle size distribution based on the formula (2).

[0084] <Surface treatment agent for filler>

[0085] For the filler as component (C) including the thermally conductive filler as component (C-1) and the low-density filler as component (C-2), in order to improve the wettability with the liquid silicone as the matrix, the surface can be hydrophobized by using a surface treatment agent. By improving the wettability with the liquid silicone, the filler can be dispersed in the liquid silicone with a high degree of dispersion, the viscosity of the resulting thermally conductive silicone composition becomes lower, and the hardness of its cured product also becomes lower. It is particularly preferred to treat the thermally conductive filler with a surface treatment agent. As the surface treatment agent used in the filler, alkoxysilanes having an alkyl group with more than 6 carbon atoms in the molecule such as octyltriethoxysilane and decyltrimethoxysilane, polyorganosiloxane having a hydroxyl group in the molecule, polyether-modified silicone oil, polyglycerol-modified silicone oil, fatty acid glycerol esters such as glycerol monooleate, fatty acid sorbitan esters such as sorbitan monooleate and sorbitan trioleate, fatty acids with more than 6 carbon atoms such as stearic acid and oleic acid, etc. can be exemplified. The amount of the surface treatment agent added is preferably 0.005% by mass or more and 2.5% by mass or less relative to the mass of the filler. If the amount of the surface treatment agent added is less than 0.005% by mass, the treatment effect of the surface treatment agent becomes small and the viscosity of the thermally conductive silicone composition becomes high. On the other hand, if the amount added is greater than 2.5% by mass, the hydrosilylation reaction is hindered and the thermally conductive silicone composition may be difficult to cure. Among various surface treatment agents, surface treatment agents such as alkoxysilanes that can chemically bond to hydroxyl groups (-OH) present on the surface of the filler improve the storage stability of the thermally conductive silicone composition compared to surface treatment agents that bond using hydrogen bonds or intermolecular forces, and are therefore preferably used.

[0086] <Other additives>

[0087] The liquid silicone constituting the thermally conductive silicone composition may contain other additives such as a curing retardant and a pigment as necessary.

[0088] <Method for producing thermally conductive silicone composition and cured product thereof>

[0089] The thermally conductive silicone composition according to the present invention can be produced by adding a hydrosilylation catalyst as a component (B) and a filler as a component (C) to a liquid silicone as a component (A) and kneading the mixture. However, when a liquid silicone containing components (A-1) to (A-4) or components (A-1) to (A-5) is used, in order to suppress the progress of the crosslinking reaction before use of the thermally conductive silicone composition, for example, the polymer components constituting the liquid silicone except for the component (A-4) are preferably mixed, and a thermally conductive filler as a component (C-1) is added little by little thereto and kneaded, and then the kneaded mixture is divided into two, and a hydrosilylation catalyst as a component (B) and a low-density filler as a component (C-2) are added to one side as an agent A, and a component (A-4) and a low-density filler as a component (C-2) are added to the other side as an agent B, and the agents A and B are mixed at the time of use or immediately before use. The cured product of the thermally conductive silicone composition can be obtained by curing the thermally conductive silicone composition at room temperature or by curing it by heating.

[0090] Example

[0091] The present invention is described in detail below based on examples and comparative examples in which a thermally conductive silicone composition and a cured product thereof are actually manufactured and evaluated. As evaluation items, the degree of plasticization is measured for the physical properties (uncured physical properties) of the thermally conductive silicone composition, and the hardness, density, thermal conductivity, and change in hardness over time are measured for the physical properties (cured physical properties) of the cured product. In addition, for the examples and comparative examples, the minimum value E of the mean square error E specified by the above formula (5) is calculated. min The coefficient a and the minimum value E min The corresponding value is a0.

[0092] <Method for producing thermally conductive silicone composition and cured product>

[0093] In the examples and comparative examples, a thermally conductive silicone composition and a cured product thereof were prepared as follows. First, the polymer components except for the component (A-4) (in the comparative example, a methylhydrogen polysiloxane having a silicon-bonded hydrogen atom in the side chain of the molecular chain) in the liquid silicone were mixed, and the thermally conductive filler and the surface treatment agent as the component (C-1) were added little by little thereto, and the mixture was heated and kneaded at 150°C in a 600cc Banbury mixer. After the obtained composition was cooled, it was divided into two, and a hydrosilylation catalyst as the component (B) and a half amount of a low-density filler as the component (C-2) were added to one side to form the A component, and the component (A-4) and a half amount of a low-density filler as the component (C-2) were added to the other side to form the B component. Thereafter, the A component and the B component were mixed in a vacuum mixer while being vacuum degassed, and then filled into a mold, and heated and cured at 90°C for 30 minutes to obtain a cured product. By this production method, a cured product having a size of 100 mm×100 mm×10 mm in thickness was obtained and used as a test piece.

[0094] <Evaluation method of thermally conductive silicone composition and cured product thereof>

[0095] (1) Plasticity determination method

[0096] The plasticization degree of the thermally conductive silicone composition was measured as follows: 2 cm 2 of each of the A and B agents were collected. 3 , measured using a Williams plasticometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) in accordance with JIS K6249. Under an environment of 23±2°C, the values ​​of Agent A and Agent B were measured 5 minutes after the load was applied. Thereafter, the average value of the plasticization degree of Agent A and the plasticization degree of Agent B was set as the plasticization degree of the thermally conductive silicone composition. The smaller the value of the plasticization degree, the lower the viscosity of the composition.

[0097] (2) Hardness measurement method

[0098] The 10 mm thick test piece obtained by the above-mentioned production method was conditioned at 23±2° C. and then measured using an Asker C durometer (manufactured by Kobunsu Keiki Co., Ltd.) in accordance with JIS K7312. The maximum value after pressing the durometer was used as the hardness value.

[0099] (3) Density measurement method

[0100] The test piece having a thickness of 10 mm obtained by the above-mentioned production method was conditioned in an environment of 23±2° C., and then the density was measured using a densitometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) in accordance with JIS K6249.

[0101] (4) Thermal conductivity measurement method

[0102] The surface of the 10 mm thick test piece obtained by the above-mentioned manufacturing method was covered with plastic wrap (manufactured by Ube Film Co., Ltd.) while being careful not to trap bubbles, and the test piece was stabilized at 23±2°C. The test piece wrapped with plastic wrap was placed on a reference plate (manufactured by Kyoto Electronics Industry Co., Ltd., material: zirconium oxide, thermal conductivity: 3.36 W / m·K), and the thermal conductivity was measured by connecting the probe PD-13 (manufactured by Kyoto Electronics Industry Co., Ltd.) to the thermal conductivity measuring device QTM-500 (manufactured by Kyoto Electronics Industry Co., Ltd.). The current value is set to I 2 =9.000A 2 .

[0103] (5) Method for measuring the change of hardness over time

[0104] The hardness H0 of the test piece was measured by the method described in the "Hardness Measurement Method" of (3) above. After that, the test piece was left to stand for 500 hours in a thermostatic chamber set at a temperature of 85°C and a relative humidity of 85%, and then conditioned in an environment of 23±2°C. The "Hardness Measurement Method" of (3) above was again implemented to measure the hardness H1 of the test piece. Thereafter, the hardness change over time ΔH was calculated using the following formula (9).

[0105] ΔH=H1-H0 (9)

[0106] (6) Minimum value of mean square error E min And the calculation method of the value a0 of the corresponding coefficient a

[0107] Using 1.15 as k in the above formula (3), a geometric progression (…, 0.0076, 0.0087, 0.0100, 0.0115, 0.0132, …) including a particle size of 0.0100 μm as an element (term) of the sequence and a common ratio of 1.15 is considered, and each term of the geometric progression is used to represent a candidate for the calculation point of the cumulative particle size. Thereafter, from the candidates for the calculation point, the starting point P (μm) and the end point Q (μm) of the particle size for calculating the mean square error E are selected. The starting point P (μm) is set to the maximum particle size at which the cumulative frequency of the filler as the (C) component is substantially considered to be 0%, and the end point Q (μm) is set to the minimum particle size at which the cumulative frequency of the filler as the (C) component reaches 100%. After that, after P and Q are determined in this way, in the geometric progression of the common ratio k mentioned above, each element between the starting point P and the end point Q is set as the calculation point of the cumulative particle size. The number of calculation points set in this way is R. Then, the mean square error E is calculated based on formula (5), and the minimum value E of the mean square error E is calculated. min and the minimum value E min The corresponding value of coefficient a is a0.

[0108] [Example 1]

[0109] In the preparation of the thermally conductive silicone composition, a dimethylpolysiloxane having vinyl groups at both ends of the molecular chain (DMS-V21, viscosity 100 cSt, manufactured by Gelest, Inc.) was blended as the component (A-1), a dimethylpolysiloxane having vinyl groups only at one end of the molecular chain (RH-Vi305B, viscosity 1000 cSt, manufactured by Zhejiang Runhe Silicone New Material Co., Ltd.) was blended as the component (A-2), a cyclic siloxane having four vinyl groups in the side chain of the molecular chain, namely, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (manufactured by Sigma-Aldrich Co. LLC) was blended as the component (A-3), and a dimethylpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain (FLD620V20, viscosity 20 mmHg) was blended as the component (A-4). 2 / s, manufactured by Elkem Japan Co., Ltd.) as the (A-4) component, dimethyl polysiloxane having no reactive functional group (KF-96-100CS, viscosity 100 cSt, manufactured by Shin-Etsu Chemical Co., Ltd.) as the (A-5) component, and platinum (0)-2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane complex (SIP6832.2, manufactured by Gelest, Inc.) as a platinum catalyst as the (B) component. In Table 1, the dimethylpolysiloxane having vinyl groups at both ends of the molecular chain as the component (A-1) is described as “chain polysiloxane having vinyl groups at both ends”, the dimethylpolysiloxane having vinyl groups only at one end of the molecular chain as the component (A-2) is described as “chain polysiloxane having vinyl groups at one end”, the cyclic siloxane having four vinyl groups in the side chain of the molecular chain as the component (A-3) is described as “cyclic siloxane having four vinyl groups”, the dimethylpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain as the component (A-4) is described as “chain polysiloxane having silicon-bonded hydrogen atoms at both ends”, and the dimethylpolysiloxane having no reactive functional group as the component (A-5) is described as “chain polysiloxane having no reactive functional group”. In addition, although dimethylpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain is used as the (A-4) component here, since it is dimethylpolysiloxane and has silicon-bonded hydrogen atoms at both ends of the molecular chain, it can be said that this component is an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the ends of the molecular chain and not having silicon-bonded hydrogen atoms in the side chains of the molecular chain.

[0110] In addition, aluminum hydroxide (BW53, true density 2.42 g / cm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), aluminum hydroxide with an average particle size of 10 μm (BW103, true density 2.42 gcm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), aluminum hydroxide with an average particle size of 1 μm (BF013, true density 2.42 g / cm 3 , amorphous, and Nippon Light Metal Co., Ltd.), and polyethylene particles with an average particle size of 155 μm (HI-ZEX MILLION 630M, true density 0.93 g / cm 3 , spherical, manufactured by Mitsui Chemicals, Inc.), and these fillers were added in the blending ratios shown in Table 1. In addition, octyltriethoxysilane (Silquest A-137, manufactured by Momentive Performance Materials Japan Co., Ltd.) and glycerol monooleate (RHEODOL MO-60, manufactured by Kao Corporation) were used as surface treatment agents for the thermally conductive fillers, and these components were added in the blending amounts shown in Table 1.

[0111] The results obtained for the thermally conductive silicone composition and its cured product of Example 1 are shown in Table 2. The mean square error E calculated based on the particle size distribution and addition ratio of each filler added to the thermally conductive silicone composition according to Formula (5) is minimized when a = 9.2 (i.e., a0 = 9.2). The value of E at this time (i.e., E min ) is 6.6. The plasticization degree of the obtained thermal conductive silicone composition is as low as 39 and the viscosity is low. The initial hardness of the cured product is 15 degrees. The hardness after 500 hours under the conditions of 85°C and 85% relative humidity has only changed by +2 points compared with the initial value. That is, the change in hardness under high temperature and high humidity conditions is small. The density of the cured product is as low as 1.93g / cm 3 It is lightweight and has a thermal conductivity of 2.7 W / (m·K). These values ​​are satisfactory values ​​for thermally conductive silicone compositions and cured products thereof. It should be noted that in Table 2, as the volume percentage of fillers, the volume of thermally conductive fillers, the volume of low-density fillers and the total volume of fillers relative to the volume of the thermally conductive silicone composition, calculated based on the filler mixing ratio, and the ratio of the volume of thermally conductive fillers relative to the total volume of fillers are shown. In addition, in Table 2, as a quantity that characterizes the particle size distribution of the filler, the median diameter (D50) of the thermally conductive filler, the median diameter (D50) of the low-density filler, and the median diameter (D50) of the filler as a whole (i.e., the thermally conductive filler and the low-density filler) are also shown.

[0112] [Examples 2 to 4]

[0113] A thermally conductive silicone composition was produced and evaluated in the same manner as in Example 1 except that the blending amounts of each thermally conductive filler, low-density filler, and surface treatment agent were changed as shown in Table 1. The results are shown in Table 2.

[0114] [Example 5]

[0115] In the preparation of the thermally conductive silicone composition, a dimethylpolysiloxane having vinyl groups at both ends of the molecular chain (DMS-V31, viscosity 1000 cSt, manufactured by Gelest, Inc.) was blended as the (A-1) component, a dimethylpolysiloxane having vinyl groups at one end of the molecular chain (MCR-V25, viscosity 500 cSt, manufactured by Gelest, Inc.) was blended as the (A-2) component, a cyclic siloxane having four vinyl groups in the side chain of the molecular chain, namely, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (manufactured by Sigma-Aldrich Co. LLC) was blended as the (A-3) component, and a dimethylpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain (FLD620V20, viscosity 20 mm 2 / s, Elkem Japan Co., Ltd.) as the (A-4) component, and a platinum catalyst platinum (0)-1,3-divinyltetramethyldisiloxane complex (P2075, Tokyo Chemical Industry Co., Ltd.) as the (B) component. Dimethylpolysiloxane having no reactive functional group as the (A-5) component was not used. The thermal conductive filler and low-density filler used the same substances as in the example in Example 1 in the blending amounts shown in Table 1. In addition, octyltriethoxysilane (SilquestA-137, Momentive Performance Materials Japan Contract Co., Ltd.) and stearic acid (Stearic acid 50S, Shin Nippon Rika Co., Ltd.) were used as surface treatment agents for the thermal conductive filler in the blending amounts shown in Table 1. Thereafter, the evaluation was performed in the same manner as in Example 1. The results are shown in Table 2.

[0116] [Example 6]

[0117] A thermally conductive silicone composition was produced and evaluated in the same manner as in Example 5 except that the blending amounts of each thermally conductive filler, low-density filler, and surface treatment agent were changed as shown in Table 1. The results are shown in Table 2.

[0118] [Example 7]

[0119] In the preparation of the thermally conductive silicone composition, a dimethylpolysiloxane having vinyl groups at both ends of the molecular chain (DMS-V31, viscosity 1000 cSt, manufactured by Gelest, Inc.) was blended as the (A-1) component, a dimethylpolysiloxane having vinyl groups at one end of the molecular chain (MCR-V25, viscosity 500 cSt, manufactured by Gelest, Inc.) was blended as the (A-2) component, a cyclic siloxane having four vinyl groups in the side chain of the molecular chain (2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, manufactured by Sigma-Aldrich Co. LLC) was blended as the (A-3) component, and a dimethylpolysiloxane having silicon-bonded hydrogen atoms at both ends of the molecular chain (FLD620V20, viscosity 20 mmHg) was blended as the (A-4) component. 2 / s, manufactured by Elkem Japan Co., Ltd.) as the (A-4) component, dimethyl polysiloxane having no reactive functional group (KF-96-100CS, viscosity 100 cSt, manufactured by Shin-Etsu Chemical Co., Ltd.) as the (A-5) component, and platinum (0)-2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane complex (SIP6832.2, manufactured by Gelest, Inc.) as the (B) component as the platinum catalyst. The thermally conductive filler and the low-density filler used the same materials as those in Example 1, and the surface treatment agent for the thermally conductive filler used the same materials as those in Example 5, and these fillers were mixed in the amounts shown in Table 1. Thereafter, the evaluation was performed in the same manner as in Example 1. The results are shown in Table 2.

[0120] [Example 8]

[0121] A thermally conductive silicone composition was produced and evaluated in the same manner as in Example 7 except that the blending amounts of each thermally conductive filler, low-density filler, and surface treatment agent were changed as shown in Table 1. The results are shown in Table 2.

[0122] [Examples 9 to 11]

[0123] A thermally conductive silicone composition was produced and evaluated in the same manner as in Example 5 except that the blending amounts of each thermally conductive filler, low-density filler, and surface treatment agent were changed as shown in Table 1. The results are shown in Table 2.

[0124] [Example 12]

[0125] As the low-density filler as the component (C-2), polyethylene particles (HI-ZEXMILLION 030S, true density 0.95 g / cm 3, spherical, manufactured by Mitsui Chemicals, Inc.), except that the thermally conductive filler, low-density filler and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0126] [Examples 13-14]

[0127] As the low-density filler as component (C-2), polyethylene particles (MIPELON XM-330, true density 0.94 g / cm 3 , spherical, manufactured by Mitsui Chemicals, Inc.), except that the thermally conductive filler, low-density filler and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0128] [Example 15]

[0129] As the low-density filler as component (C-2), polyethylene particles (MIPELON XM-220, true density 0.94 g / cm 3 , spherical, manufactured by Mitsui Chemicals, Inc.), except that the thermally conductive filler, low-density filler and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 5. The results are shown in Table 2.

[0130] [Example 16]

[0131] As the low-density filler as component (C-2), hollow resin particles made of acrylonitrile copolymer with an average particle size of 60 μm and coated with calcium carbonate (Matsumoto Microsphere MFL-UPR60, true density 0.12 g / cm 3 , spherical, Matsumoto Oil & Fats Pharmaceutical Co., Ltd.), except that the thermally conductive filler, low-density filler and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0132] [Example 17]

[0133] As the thermally conductive filler as component (C-1), aluminum hydroxide (BW53, true density 2.42 g / cm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), aluminum hydroxide with an average particle size of 10 μm (BW103, true density 2.42 g / cm 3, amorphous, Nippon Light Metal Co., Ltd.) and anhydrous magnesium carbonate with an average particle size of 1 μm (Magsarm (Japanese: マグサーモ) MS-S, true density 3.04 g / cm 3 , polyhedral, and manufactured by Kamishima Chemical Industry Co., Ltd.), and except that the thermally conductive filler, low-density filler, and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 16. The results are shown in Table 2.

[0134] [Example 18]

[0135] As the low-density filler as component (C-2), hollow glass particles (Glass Bubbles S38, true density 0.38 g / cm 3 , spherical, 3M JAPAN Co., Ltd.), except that the silicone polymer, thermally conductive filler, low-density filler and surface treatment agent were blended in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0136] [Example 19]

[0137] As the low-density filler as component (C-2), hollow glass particles (Glass Bubbles iM30K, true density 0.60 g / cm 3 , spherical, 3M JAPAN Co., Ltd.), except that the thermally conductive filler, low-density filler and surface treatment agent were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Example 18. The results are shown in Table 2.

[0138] [Example 20]

[0139] As the thermally conductive filler as component (C-1), aluminum hydroxide (SB93, true density 2.42 g / cm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), aluminum hydroxide with an average particle size of 10 μm (BW103, true density 2.42 g / cm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), aluminum hydroxide with an average particle size of 1 μm (BF013, true density 2.42 g / cm 3 The thermally conductive silicone compositions were prepared and evaluated in the same manner as in Example 5 except that the thermally conductive filler, the low-density filler and the surface treatment agent were added in the amounts shown in Table 1. The results are shown in Table 2.

[0140] As shown in Table 2, similarly to Example 1, in any one of Examples 2 to 20, the minimum value E of the mean square error E based on Formula (5) is min and E min The corresponding coefficient a value a0 satisfies the above-mentioned formulas (7) and (8), and the obtained thermally conductive silicone composition has a low plasticity and low viscosity. In addition, the hardness and density of the cured product are low, the change in hardness is small, and the thermal conductivity is high.

[0141] [Comparative Example 1]

[0142] In the preparation of the thermally conductive silicone composition, dimethylpolysiloxane (SF3000EDK001, viscosity 1000 cSt, manufactured by KCC Corporation) having vinyl groups at both ends of the molecular chain (corresponding to the component (A-1) in each example) and methylhydrogenpolysiloxane (HMS-501, viscosity 10 to 15 cSt, manufactured by Gelest, Inc.) having silicon-bonded hydrogen atoms in the side chains of the molecular chain (referred to as "chain polysiloxane having silicon-bonded hydrogen atoms in the side chains" in Table 1) were used as liquid silicones in the amounts shown in Table 1, and platinum (0)-1,3-divinyltetramethyldisiloxane complex (P2075, manufactured by Tokyo Chemical Industry Co., Ltd.) was added as the platinum catalyst as the component (B). In addition, aluminum hydroxide (BW53, true density 2.42 g / cm) having an average particle size of 54 μm was added as the thermally conductive filler as the component (C-1) in the amounts shown in Table 1. 3 , amorphous, Nippon Light Metal Co., Ltd.) and aluminum hydroxide with an average particle size of 10 μm (BW103, true density 2.42 g / cm 3 , amorphous, manufactured by Nippon Light Metal Co., Ltd.), glycerol monooleate (RHEODOL MO-60, manufactured by Kao Corporation) was added as a surface treatment agent for the thermally conductive filler. The obtained thermally conductive silicone composition and its cured product were evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0143] The mean square error E calculated based on the formula (5) based on the particle size distribution and addition ratio of each thermally conductive filler added to the thermally conductive silicone composition reaches the minimum when a=8.1 (i.e., a0=8.1). The value of E at this time (i.e., E min ) is 96.4. Therefore, the thermally conductive silicone composition of Comparative Example 1 does not satisfy the above formula (7). In addition, the plasticization degree of the thermally conductive silicone composition is 119 and the viscosity is high, and the initial hardness of the cured product is 65 degrees and is relatively hard. Although the density of the cured product is as low as 1.93 g / cm 3, however, the thermal conductivity is as low as 2.2 W / (m·K), which is not a sufficient value. The hardness of the cured product changes as much as +26 points. This is considered to be because if methyl hydrogen polysiloxane having silicon-bonded hydrogen atoms in the side chains of the molecular chain is used to achieve low viscosity and low hardness, the amount of remaining silicon-bonded hydrogen atoms increases, and therefore, the cross-linking reaction between the silanol groups generated by the reaction of silicon-bonded hydrogen atoms and the remaining silicon-bonded hydrogen atoms proceeds slowly.

[0144] [Comparative Example 2]

[0145] A thermally conductive silicone composition was prepared and evaluated in the same manner as in Comparative Example 1 except that the blending amounts of each thermally conductive filler and surface treatment agent were changed as shown in Table 1. The results are shown in Table 2. The thermally conductive silicone composition of Comparative Example 2 does not satisfy the above formula (7). When the blending amount of the thermally conductive filler is increased in order to improve the thermal conductivity, the thermal conductivity is as high as 3.0 W / (m·K), but the density is also as high as 2.05 g / cm 3 The thermally conductive silicone composition of Comparative Example 2 also has a high plasticity of 135 and a high viscosity, and the hardness of the cured product is also high at 81 degrees. For the same reasons as described in Comparative Example 1, the change in the hardness of the cured product is also as large as +11 points.

[0146] [Comparative Example 3]

[0147] As the thermal conductive filler of component (C-1), boron nitride (UHP-1K, plate-shaped, manufactured by Showa Denko K.K.) with an average particle size of 8 μm was used, and as the low-density filler of component (C-2), polyethylene particles (MIPELON XM-220, true density 0.94 g / cm 3 , spherical, Mitsui Chemicals, Inc.), except that the thermally conductive filler and the low-density filler were added in the amounts shown in Table 1, a thermally conductive silicone composition was produced and evaluated in the same manner as in Comparative Example 1. The results are shown in Table 2. The thermally conductive silicone composition of Comparative Example 2 does not satisfy any of the above-mentioned formulas (7) and (8). When boron nitride is used as a plate-like filler, the plasticization degree of the thermally conductive silicone composition is as high as 306 and the viscosity is high. In addition, the hardness of the cured product is also as high as 83 degrees.

[0148] By comparing the examples and comparative examples, the thermally conductive silicone composition according to the present invention, which satisfies the conditions of the above formulas (7) and (8), has a plasticity of less than 110 and a low viscosity, and also exhibits a good thermal conductivity of more than 2.3 W / (m / K) in its cured product, and a density of less than 2 g / cm 3The hardness based on Asker C is less than 70 degrees, and the change in hardness is less than ±5 points. That is, it can be seen that the present invention can obtain satisfactory products as thermally conductive silicone compositions and cured products thereof.

[0149]

[0150]

[0151]

[0152]

[0153]

Claims

1. A thermally conductive silicone composition comprising: A: Liquid silicone, B: Hydrosilylation catalyst, and C: Filler, and A liquid and curable thermally conductive silicone composition, wherein: The filler at least comprises: C-1: a thermally conductive filler having a true density greater than that of the liquid silicone, and C-2: a low-density filler having a true density lower than the density of the liquid silicone, The number of particle sizes that are points for calculating the cumulative frequency in the cumulative particle size distribution based on the volume of the filler is defined as R, The particle sizes of the thermally conductive filler that are the smallest and largest among the particle sizes that are the calculation points of the cumulative frequency are defined as P and Q, respectively. The particle size of the filler at the nth calculation point determined so as to satisfy the following formula (a1) and formula (a2) is defined as x n , The cumulative particle size distribution and the particle size x n The cumulative frequency of the corresponding particle size of the thermal conductive filler on a volume basis is set as c n When P, Q and x n The unit is μm, n is an integer satisfying 1≤n≤R, and c n In %, Regarding the particle size distribution of the filler, the minimum value E of the mean square error E represented by the following formula (a3) ​​is min and the minimum value E min The value a0 of the coefficient a in the corresponding following equation (a4) satisfies: 0≤E min ≤40, and 6.0≤a0≤13.0， 。 2. The thermally conductive silicone composition according to claim 1, wherein The volume percentage of the filler C in the thermally conductive silicone composition is 70.0 volume % or more and 80.0 volume % or less, A ratio of the volume of the thermally conductive filler in the filler C to the volume of the low-density filler is 1.67 or more and 22.0 or less.

3. The thermally conductive silicone composition according to claim 1 or 2, wherein: The low-density filler includes at least one of polyethylene solid particles, resin hollow particles and glass hollow particles.

4. The thermally conductive silicone composition according to claim 1 or 2, wherein: The thermally conductive filler includes a filler formed of aluminum hydroxide.

5. The thermally conductive silicone composition according to claim 1, wherein The liquid silicone comprises: A-1: an organopolysiloxane having alkenyl groups at both ends of the molecular chain; A-2: an organopolysiloxane having an alkenyl group at one end of the molecular chain and having no reactive functional group at the other end; A-3: an organopolysiloxane having at least 3 alkenyl groups in the molecule; and A-4: an organohydrogenpolysiloxane having silicon-bonded hydrogen atoms at the molecular chain terminals and having no silicon-bonded hydrogen atoms at the molecular chain side chains.

6. The thermally conductive silicone composition according to claim 5, wherein: The liquid silicone comprises an organopolysiloxane represented by the following general formula (1): 。 7. A cured product obtained by curing the thermally conductive silicone composition according to claim 1 or 2 by a hydrosilylation reaction.

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