Method for producing thermally conductive filler and method for producing thermally conductive composition

Abstract

This method for producing a thermally conductive filler from a thermally conductive composition raw material containing a thermosetting resin and a thermally conductive filler involves: a thermal decomposition step for heating the thermally conductive composition raw material in a low oxygen atmosphere having an oxygen concentration of 10% by volume or less to thermally decompose the thermally conductive composition raw material; and a pulverizing step for pulverizing a residue after the thermal decomposition, wherein the pulverizing step is executed so that the median particle diameter becomes 0.1-300 μm, and a thermally conductive filler is thus produced. The obtained thermally conductive filler is mixed with a thermosetting resin to provide a thermally conductive resin composition. Consequently, the thermally conductive filler is recovered from waste of the thermally conductive composition, which has conventionally been discarded, and reused.

Description

Method for producing thermally conductive filler and method for producing thermally conductive composition

[0001] The present invention relates to a method for producing a thermally conductive filler useful for thermally conductive compositions suitable for being interposed between a heat-generating portion of an electric or electronic component or the like and a heat sink, and a method for producing a thermally conductive composition.

[0002] Thermally conductive silicone materials using silicone are used as heat dissipation materials for electronic components. Conventionally, scraps generated during manufacturing and used materials are disposed of in landfills as industrial waste without being reused due to quality assurance and other factors. In particular, when using thermosetting polymers, it is necessary to separate the polymer and the thermally conductive filler for reuse, but an effective separation process for heat dissipation materials has not yet been put into practical use. A process has been proposed in which the polymer is thermally decomposed into monomers in a high-temperature alkaline aqueous solution, recovered, and then repolymerized, but the residues, such as inorganic reinforcing materials, that remain as precipitates are discarded. When heat dissipation materials are treated using this process, the residual thermally conductive filler can be reused by washing it, but this is not practical from the perspective of cost and environmental impact. In recent years, recycling technologies suitable for heat dissipation materials are desired in response to the increasing demand for Sustainable Development Goals (SDGs). Patent Document 1 proposes pyrolyzing silicone rubber, separating it into volatile siloxanes and silicic acid, and recovering them. Patent Document 2 proposes catalytic pyrolysis of silicone rubber with alcohol to recycle the polymer.

[0003] Japanese Patent Laid-Open No. 5-271416 Japanese Patent Laid-Open No. 2002-187976

[0004] However, there have been no effective proposals for recycling thermally conductive fillers, and most of them have been disposed of by landfill, resulting in high disposal costs.

[0005] In order to solve the above-mentioned conventional problems, the present invention provides a method for producing a thermally conductive filler and a method for producing a thermally conductive composition, which can recover and reuse a thermally conductive filler from waste thermally conductive compositions that have previously been discarded.

[0006] One embodiment of the present invention relates to a method for producing a thermally conductive filler from a thermally conductive composition raw material containing a thermosetting resin and a thermally conductive filler, the method including: a pyrolysis step of heating and pyrolyzing the thermally conductive composition raw material in a low-oxygen atmosphere having an oxygen concentration of 10 vol.% or less; and a pulverization step of pulverizing a residue after the pyrolysis, wherein the pulverization step pulverizes the material to a median particle size of 0.1 to 300 μm.

[0007] Another embodiment of the present invention relates to a method for producing a thermally conductive resin composition, in which the thermally conductive filler obtained by the above-mentioned method for producing a thermally conductive filler is mixed with a thermosetting resin to form a thermally conductive resin composition.

[0008] The present invention includes a pyrolysis step in which a thermally conductive composition raw material is heated in a low-oxygen atmosphere with an oxygen concentration of 10 vol.% or less to pyrolyze the raw material, and a pulverization step in which the residue after the pyrolysis is pulverized. In the pulverization step, the raw material is pulverized to a median particle size of 0.1 to 300 μm. This allows the production of a thermally conductive filler using waste material from thermally conductive compositions that would previously have been discarded as raw materials, thereby making it possible to provide a method for producing a thermally conductive filler and a method for producing a thermally conductive composition that enable the reuse of waste material.

[0009] Figure 1 is a schematic cross-sectional view showing a method for using a thermally conductive sheet according to one embodiment of the present invention. Figures 2A-B are schematic explanatory diagrams showing a method for measuring the thermal conductivity of a sample according to one embodiment of the present invention. Figure 3 is an SEM photograph (magnification: 500x) of the filler of Example 1 after pyrolysis and before pulverization. Figure 4 is an SEM photograph (magnification: 500x) of the filler of Comparative Example 1 after pyrolysis and before pulverization.

[0010] The present invention produces a thermally conductive filler from a thermally conductive composition raw material containing a thermosetting resin and a thermally conductive filler. Examples of the thermosetting resin raw material include silicone resin, epoxy resin, and phenolic resin. Among these, silicone resin is preferred. Examples of the thermally conductive filler are preferably at least one inorganic particle selected from the group consisting of aluminum oxide (alumina), zinc oxide, magnesium oxide, aluminum nitride, boron nitride, aluminum hydroxide, and silicon carbide. These inorganic particles have high thermal conductivity and are suitable as TIMs (thermal interface materials). Among these, alumina and / or aluminum nitride are preferred.

[0011] The method of the present invention includes the following steps. 1. Pyrolysis Step (1) Pyrolysis Atmosphere The thermally conductive composition raw material is heated and pyrolyzed in a low-oxygen atmosphere with an oxygen concentration of 10 vol.% or less. While the normal oxygen concentration at sea level is approximately 21 vol.% and nitrogen approximately 78 vol.%, in the present invention, pyrolysis is carried out in a dilute or oxygen-free environment. This is because the presence of oxygen causes organic matter to burn, leaving behind residue. If residue remains, it becomes difficult to reuse the recycled thermally conductive filler. The oxygen concentration is preferably 7 vol.% or less, more preferably 5 vol.% or less, and even more preferably 3 vol.% or less. The low-oxygen atmosphere is preferably an inert gas flow, a mixed gas flow of 51 to 100 vol.% inert gas and 0 to 49 vol.% air, under reduced pressure, or in superheated steam. The thermally conductive composition raw material is heated and pyrolyzed in a low-oxygen atmosphere with an oxygen concentration of 10 vol.% or less. Heating and pyrolysis in a low-oxygen atmosphere of 0.1% or less can reduce the amount of residual polymer components. The residual polymer components are preferably 15 wt. % or less of the polymer mass before treatment, more preferably 10 wt. % or less, and even more preferably 8 wt. % or less. Note that the amount of residual polymer components is relative to the polymer mass before treatment, which is 100%. (2) Treatment Temperature: In the pyrolysis step, heating to a temperature of 400°C to 1600°C is preferred. Temperatures below 400°C are inefficient. Temperatures above 1600°C are undesirable because silica and alumina react to produce mullite. From the perspective of energy conservation, 450 to 1000°C is preferred, more preferably 500 to 800°C. (3) Treatment Time: The heating retention time depends on the heating temperature, but is preferably 10 minutes to 10 hours, more preferably 20 minutes to 5 hours, and even more preferably 30 minutes to 3 hours. In the pyrolysis step, volatile components generated by pyrolysis may be separated. Volatile components may also be reusable. 2. Pulverization process The residue after pyrolysis is pulverized. In the pulverization process, the particles are pulverized to a median particle size of 0.1 to 300 μm. The pulverization process is carried out using a ball mill, pin mill, cutter mill, jet mill, bead mill, hammer mill, or automatic mortar.

[0012] After the pulverization step, it is preferable to carry out classification to make the particle size uniform. Classification can be carried out by sieving using a mesh, air classification, water classification, vortex classification, etc. Among these, sieving using a metal mesh is preferable because it can accurately make the particle size uniform.

[0013] The Si atom concentration of the thermally conductive filler obtained by the thermally conductive filler manufacturing method of the present invention (hereinafter also referred to as "recycled filler"), as determined by SEM-EDX analysis (scanning electron microscope-energy dispersive X-ray spectroscopy), is preferably 1 at% or less, more preferably 0.5 at% or less, and even more preferably 0.1 at% or less. This allows the purity of the target thermally conductive filler to be increased.

[0014] The thermally conductive composition raw material preferably contains 100 to 4,000 parts by mass of thermally conductive filler per 100 parts by mass of thermosetting resin, more preferably 200 to 3,000 parts by mass, and even more preferably 400 to 2,000 parts by mass. If the thermally conductive filler is in this range, it can be efficiently recovered and is effective for reuse.

[0015] On the surface of the recycled filler, RaSi(OR') 4-a(wherein R is an unsubstituted or substituted hydrocarbon group having 8 to 12 carbon atoms, R' is an alkyl group having 1 to 4 carbon atoms, and a is 0 or 1) may be added. Examples of alkoxysilane compounds (hereinafter simply referred to as "silane") of the above chemical formula include methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, pentyltrimethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, decyltrimethoxysilane, decyltriethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, octadecyltrimethoxysilane, and octadecyltriethoxysilane. The above silane compounds can be used alone or in combination. In particular, if fine thermally conductive particles with a volume-based median diameter D = 50 of 1 μm or less are surface pretreated with a coupling agent in advance, they will have better mixability when compounded, improving workability, and will not adsorb platinum catalysts during the compounding process, preventing interference with the silicone curing reaction of the addition reaction cure.

[0016] The method for producing a thermally conductive resin composition of the present invention involves mixing the thermally conductive filler obtained by the method for producing a thermally conductive filler described above with a thermosetting resin to produce a thermally conductive resin composition. The thermosetting resin is preferably a silicone polymer. Silicone polymers have high heat resistance and are suitable as thermally conductive materials: TIMs (Thermal Interface Materials). Examples of preferred silicone polymers include addition-curable silicone polymers, peroxide-curable silicone polymers, and condensation-curable silicone polymers. These may be used alone or in combination. Commercially available silicone polymers can be used. They can be in the form of rubber, gel, grease, putty, liquid, and the like.

[0017] The thermally conductive composition preferably contains 100 to 4,000 parts by mass of the thermally conductive filler per 100 parts by mass of the thermosetting resin, more preferably 200 to 3,000 parts by mass, and even more preferably 400 to 2,000 parts by mass, which allows for a thermally conductive composition with high thermal conductivity to be obtained.

[0018] The thermally conductive composition is produced, for example, by the following steps: (1) Thermally conductive composition: A thermosetting resin, a thermally conductive filler, a curing catalyst such as platinum if necessary, other pigments, RaSi(OR') 4-a (where R is an unsubstituted or substituted hydrocarbon group having 8 to 12 carbon atoms, R' is an alkyl group having 1 to 4 carbon atoms, and a is 0 or 1) and other alkoxysilane compounds are mixed and stirred using a mixing device such as a planetary mixer to form a compound (composition). Liquid compositions such as grease are made into products in this state. (2) Sheet molding When molding into a sheet, the compound is degassed for 1 to 10 minutes under reduced pressure, for example, at about -0.1 Pa. Next, the compound is sandwiched between polyethylene terephthalate (PET) films that have been treated with a release agent, rolled to a specified thickness using constant speed rolls, and cured by heating at 100°C for about 10 minutes to form a thermally conductive sheet.

[0019] The recycled filler obtained by the present invention can be reused in a thermally conductive composition and / or a thermally conductive sheet, and is suitable as a thermally conductive filler for a thermal interface material (TIM), a heat dissipation material interposed between a heat-generating portion of an electrical or electronic component or the like and a heat sink. A thermally conductive sheet will be used as an example of the heat dissipation material. FIG. 1 is a schematic cross-sectional view of a heat dissipation structure 10 incorporating a thermally conductive sheet according to one embodiment of the present invention. The thermally conductive sheet 11b dissipates heat generated by an electronic component 13, such as a semiconductor device. The thermally conductive sheet 11b is fixed to the main surface 12a of the heat spreader 12 facing the electronic component 13, and is sandwiched between the electronic component 13 and the heat spreader 12. The thermally conductive sheet 11a is sandwiched between the heat spreader 12 and a heat sink 15. The thermally conductive sheets 11a and 11b, together with the heat spreader 12, constitute a heat dissipation member that dissipates heat from the electronic component 13. The heat spreader 12 is formed, for example, in the shape of a rectangular plate, and has a main surface 12a facing the electronic component 13 and side walls 12b extending along the outer periphery of the main surface 12a. A thermally conductive sheet 11b is provided on the main surface 12a surrounded by the side walls 12b, and a heat sink 15 is provided on the other surface 12c opposite the main surface 12a via the thermally conductive sheet 11a. The electronic component 13 is, for example, a semiconductor element such as a BGA, and is mounted on a wiring board 14.

[0020] The thermally conductive composition raw material containing the thermosetting resin and the thermally conductive filler used in the present invention may be of any type, such as process waste, waste scrap, recovered products, inventory after the expiration of the usage period, prototypes, etc. The properties of the raw material may also be of any type, such as rubber, gel, grease, putty, or liquid.

[0021] The present invention will be described below using examples, but is not limited to these examples.

[0022] Various parameters were measured using the methods described below. <Thermal Conductivity> The thermal conductivity of the thermally conductive silicone rubber sheet was measured using a hot disk (compliant with ISO 22007-2:2008). As shown in Figure 2A, this thermal conductivity measuring device 1 sandwiches a polyimide film sensor 2 between two samples 3a and 3b, applies a constant power to the sensor 2, and generates a constant amount of heat, analyzing the thermal characteristics from the temperature rise of the sensor 2. The sensor 2 has a 7 mm diameter tip 4 and, as shown in Figure 2B, has a double spiral electrode structure with an applied current electrode 5 and a resistance electrode (temperature measurement electrode) 6 located at the bottom. The thermal conductivity is calculated using the following equation (Equation 1): <SEM-EDX analysis> SEM-EDX analysis (scanning electron microscope-energy dispersive X-ray spectroscopy analysis) was performed using a Hitachi High-Tech TM4000 Plus II tabletop electron microscope. The atomic concentration obtained as the analysis result represents the relative atomic ratio of elements in the sample. The analysis range depends on the material, but generally, at a magnification of 500x, the analysis depth is 1-2 μm, and the analysis range is several μm to several tens of μm.

[0023] (Example 1) (1) Manufacturing process of thermally conductive silicone rubber sheet 100 g of commercially available polyorganosiloxane, two-component room-temperature curing silicone polymer (silicone component); and a thermally conductive filler consisting of a mixture of alumina having a cumulative particle size distribution on a volume basis of D50 (median diameter) = 0.3 μm, alumina having a D50 = 2 μm, alumina having a D50 = 35 μm, and a 3.nu filler having a D50 = 75 μm, with an average particle diameter of D50 = 5.2 μm: total amount of alumina 1400 g. The above materials were mixed to form a compound, which was rolled into a sheet with a thickness of 2.0 mm and heat-cured at 100°C for 10 minutes to form a thermally conductive silicone rubber sheet. (2) Pyrolysis Step: The thermally conductive silicone rubber sheet was placed in an electric furnace and heated from room temperature (25°C) to 600°C at a rate of 100°C / hr under a nitrogen gas flow, and then maintained at 600°C for 1 hour. (3) Pulverization and Classification Step: The residue after pyrolysis was pulverized in a mortar. The pulverized material was sieved through a metal mesh with a mesh size of #150. The D50 (median diameter) of the filler obtained in this manner was 3.7 μm. SEM-EDX analysis (scanning electron microscope-energy dispersive X-ray spectroscopy) revealed that the Si atom number concentration was less than 0.1, indicating a high purity of the desired alumina. The yield of the resulting alumina was 99 wt.%. Figure 3 shows an SEM photograph of the filler of Example 1 after pyrolysis and before pulverization. The SEM-EDX analysis was performed on a point analysis of the surface of the large particles in the figure, where no small particle size fine powder was attached.

[0024] (Comparative Example 1) The same procedure as in Example 1 was carried out, except that in the pyrolysis step, heating was carried out in the air instead of under a nitrogen gas flow. As a result, the obtained filler had a D50 (median diameter) of 4.0 μm, which was close to the average particle diameter of Example 1, but the Si atom number concentration by SEM-EDX analysis was 1.1 wt.%, which was higher than that of Example 1. A high Si atom number concentration means that the purity of the target alumina is low. The yield of the obtained alumina was 99 wt.%. Figure 4 is an SEM photograph of the filler of Comparative Example 1 after pyrolysis and before pulverization. The above results are shown in Table 1.

[0025] Note*: The detection limit of SEM-EDX analysis is 0.1 to 1 at%. Although values ​​below 0.1 at% are obtained, the reliability is extremely low, so it is stated as 0.1 at% or less.

[0026] As is clear from Table 1, in Example 1, since heating was performed in a nitrogen gas flow, the concentration of Si atoms on the surface of the obtained alumina was low, and the purity was high and reusable. In contrast, in Comparative Example 1, since heating was performed in the air, the concentration of Si atoms on the surface of the alumina was high and reusability was poor.

[0027] (Example 2) 100 g of a commercially available two-component room-temperature curing silicone polymer (silicone component), which is a polyorganosiloxane; 1,400 g of the recycled filler obtained in Example 1. The above materials were mixed to form a compound, which was then rolled into a sheet having a thickness of 2.0 mm and heat-cured at 100°C for 10 minutes to form a thermally conductive silicone rubber sheet.

[0028] Comparative Example 2 The same procedure as in Example 3 was carried out except that 1400 g of the recycled filler obtained in Comparative Example 1 was used. The results are shown in Table 2.

[0029]

[0030] Example 3 The same procedure as in Example 1 was carried out, except that in the pyrolysis step, the nitrogen gas flow rate was 200 ml / min, the temperature was increased from room temperature of 25°C to 600°C at a rate of 10°C / hr, and the temperature was maintained at 600°C for 1 hour. The amount of residual polymer in the resulting combustion residue was analyzed using a thermogravimetric differential thermal analyzer (TG-DTA, manufactured by Hitachi High-Tech Science, TG / DTA7300).

[0031] Example 4 The same procedure as in Example 3 was carried out except that in the thermal decomposition step, the nitrogen gas flow rate was 180 ml / min and the air flow rate was 20 ml / min.

[0032] Comparative Example 3 The same procedure as in Example 3 was carried out except that the air flow rate in the thermal decomposition step was 200 ml / min. The results are shown in Table 3.

[0033] Note*: The residual polymer component is a relative amount when the polymer mass before treatment is taken as 100%.

[0034] The following can be seen from the table: (1) In Example 3, heating was performed under a nitrogen stream, so the amount of residual polymer components was small. (2) In Example 4, heating was performed under a mixed gas of nitrogen and air, so the amount of residual polymer components was small. (3) In Comparative Example 5, heating was performed under air, so the amount of residual polymer components was large, and the recycled filler could not be reused.

[0035] The recycled filler obtained by the present invention can be reused in a thermally conductive composition and / or a thermally conductive sheet, and is suitable as a thermal interface material (TIM) to be interposed between a heat-generating part of an electric or electronic component and a heat sink.

[0036] REFERENCE SIGNS LIST 1 Thermal conductivity measuring device 2 Polyimide film sensor 3a, 3b Thermally conductive sheet sample 4 Sensor tip 5 Electrode for applied current 6 Electrode for resistance value (electrode for temperature measurement) 10 Heat dissipation structure 11a, 11b Thermally conductive sheet 12 Heat spreader 13 Electronic component 14 Wiring board 15 Heat sink

Claims

1. A method for producing a thermally conductive filler from a thermally conductive composition raw material containing a thermosetting resin and a thermally conductive filler, comprising: a pyrolysis step of heating and thermally decomposing the thermally conductive composition raw material in a low-oxygen atmosphere with an oxygen concentration of 10 vol.% or less; and a pulverization step of pulverizing the residue after the pyrolysis, wherein the pulverization step pulverizes the material to a median particle size of 0.1 to 300 μm.

2. The method for producing a thermally conductive filler according to claim 1, wherein the thermal decomposition step involves heating to a temperature of 400°C to 1600°C.

3. The method for producing a thermally conductive filler according to claim 1 or 2, wherein the low-oxygen atmosphere is at least one atmosphere selected from the group consisting of an inert gas flow, a mixed gas flow of 51 to 100 vol. % inert gas and 0 to 49 vol. % air, a reduced pressure condition, and superheated steam.

4. The method for producing a thermally conductive filler according to any one of claims 1 to 3, wherein the pulverizing step is at least one step selected from the group consisting of a ball mill, a pin mill, a cutter mill, a jet mill, a bead mill, a hammer mill, and an automatic mortar.

5. The method for producing a thermally conductive filler according to any one of claims 1 to 4, further comprising a classification step after the pulverization step.

6. The method for producing a thermally conductive filler according to claim 5, wherein the classification step is sieving through a mesh.

7. A method for producing a thermally conductive filler according to any one of claims 1 to 6, wherein the thermosetting resin is a silicone polymer.

8. A method for producing a thermally conductive filler according to any one of claims 1 to 7, wherein the Si atom concentration of the recycled filler obtained by the method for producing a thermally conductive filler, as determined by SEM-EDX analysis (scanning electron microscope-energy dispersive X-ray spectroscopy), is 1 at% or less.

9. A method for producing a thermally conductive filler according to any one of claims 1 to 8, wherein the thermally conductive filler is at least one selected from the group consisting of alumina and aluminum nitride.

10. A method for producing a thermally conductive filler according to any one of claims 1 to 9, wherein the thermally conductive composition raw material contains 100 to 4,000 parts by mass of thermally conductive filler per 100 parts by mass of thermosetting resin.

11. A method for producing a thermally conductive filler according to any one of claims 1 to 10, wherein the thermally conductive composition raw material is process scrap, waste scrap, recovered product, inventory after the expiration of its useful life, or a prototype.

12. The recycled filler obtained by the method for producing a thermally conductive filler is further treated to form a RaSi(OR') 4-a (wherein R is an unsubstituted or substituted organic group having 8 to 12 carbon atoms, R' is an alkyl group having 1 to 4 carbon atoms, and a is 0 or 1).

13. A method for producing a thermally conductive resin composition, comprising mixing a recycled filler obtained by the method for producing a thermally conductive filler according to any one of claims 1 to 12 with a thermosetting resin to form a thermally conductive resin composition.

14. The method for producing a thermally conductive resin composition according to claim 13, wherein the thermosetting resin is a silicone resin.

15. A method for producing a thermally conductive resin composition according to claim 13 or 14, wherein the thermally conductive composition contains 100 to 4,000 parts by mass of a thermally conductive filler per 100 parts by mass of a thermosetting resin.

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