HEAT DISSIPATION FILM, DISPERSION LIQUID FOR HEAT EMISSION LAYER, METHOD FOR PRODUCING HEAT DISSIPATION FILM AND SOLAR CELL

Abstract

The present invention provides a heat dissipation film having high mechanical strength and flexibility, which is obtained by laminating a heat emission layer excellent in heat dissipation by infrared radiation, electrical insulation, and heat resistance on a metal film having excellent heat transfer efficiency. The present invention also provides a dispersion for heat emission layers for use in the production of the heat dissipation film, a method for producing a heat dissipation film using the dispersion for heat emission layers, and a solar cell including the heat dissipation film. The present invention provides a heat dissipation film including a heat transfer layer; and a flexible heat emission layer laminated on the heat transfer layer, the heat transfer layer being a metal film, the heat emission layer containing a water-insoluble inorganic compound and a heat-resistant synthetic resin, the amount of the water-insoluble inorganic compound in the heat emission layer being 30 to 90% by weight relative to the total weight of the heat emission layer, the heat emission layer having a thermal emissivity of at least 0.8 and a dielectric breakdown strength of at least 10 kV/mm.

Claims

1. A heat dissipation film comprising: a heat transfer layer; and a flexible heat emission layer laminated on the heat transfer layer, the heat transfer layer being a metal film, the heat emission layer containing a water-insoluble inorganic compound and a heat-resistant synthetic resin, the amount of the water-insoluble inorganic compound in the heat emission layer being 30 to 90% by weight relative to the total weight of the heat emission layer, the heat emission layer having a thermal emissivity of at least 0.8 and a dielectric breakdown strength of at least 10 kV/mm.

2. The heat dissipation film according to claim 1, wherein the heat emission layer is laminated on one side of the heat transfer layer, and an insulating layer is laminated on the other side of the heat transfer layer, the insulating layer contains a water-insoluble inorganic compound and a heat-resistant synthetic resin, and the amount of the water-insoluble inorganic compound in the insulating layer is 30 to 90% by weight relative to the total weight of the insulating layer.

3. The heat dissipation film according to claim 1, wherein the water-insoluble inorganic compound comprises at least one selected from the group consisting of a silica compound, a silica alumina compound, an aluminium compound, a calcium compound, a nitride, and coal ash.

4. The heat dissipation film according to claim 1, wherein the heat dissipation film contains a phyllosilicate mineral as the water-insoluble inorganic compound.

5. The heat dissipation film according to claim 4, wherein the phyllosilicate mineral is a non-swelling clay mineral.

6. The heat dissipation film according to claim 5, wherein the non-swelling clay mineral is at least one selected from the group consisting of talc, kaolin, pyrophyllite, and non-swelling mica.

7. The heat dissipation film according to claim 1, wherein the heat-resistant synthetic resin is a polyimide resin or a polyamideimide resin.

8. The heat dissipation film according to claim 1, wherein the heat emission layer is classified as 0 to 2 for adhesion with the metal film used as the heat transfer layer, as determined by a cross-cut test according to JIS K 5600.

9. The heat dissipation film according to claim 1, wherein the heat emission layer is classified as VTM-0 for flammability, as determined by the UL-94 VTM test, and the thickness of the heat emission layer is 100 m or less when the heat emission layer is classified as VTM-0 for flammability.

10. The heat dissipation film according to claim 1, wherein the thickness of the heat emission layer is 20 to 100 m.

11. The heat dissipation film according to claim 1, wherein the metal film as the heat transfer layer is an aluminium film or a copper film.

12. The heat dissipation film according to claim 1, wherein the thickness of the heat transfer layer is 10 to 1000 m.

13. The heat dissipation film according to claim 1, wherein the heat dissipation film exhibits a cooling temperature of at least 15 C. when the heat dissipation film having the same area as a 2.4-cm square, 0.5- to 1.5-mm-thick ceramic heater is placed on the top of the ceramic heater generating heat with a 3 W power supply.

14. The heat dissipation film according to claim 1, wherein the mandrel diameter at which cracking occurs in the heat emission layer of the heat dissipation film is 10 mm or less, as determined by a bend test by a cylindrical mandrel method according to JIS K 5600-5-1 (1999).

15. The heat dissipation film according to claim 1, wherein the water vapor permeability of the heat dissipation film at 40 C. and 90% RH is lower than 0.01 g/m.sup.2.Math.day.

16. A dispersion for heat emission layers for use in the production of the heat dissipation film according to claim 1, the dispersion for heat emission layers comprising: a dispersion medium; and nonvolatile components including a water-insoluble inorganic compound and at least one of a heat-resistant synthetic resin or a heat-resistant synthetic resin precursor; the amount of the water-insoluble inorganic compound being 30 to 90% by weight relative to the total weight of the nonvolatile components, the amount of the nonvolatile components being more than 18% by weight and not more than 65% by weight relative to the total weight of the dispersion for heat emission layers.

17. The dispersion for heat emission layers according to claim 16, wherein the dispersion medium is at least one selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, and sulfolane.

18. A method for producing a heat dissipation film, the method comprising: step (1-1) of mixing the dispersion medium and the nonvolatile components including a water-insoluble inorganic compound and at least one of a heat-resistant synthetic resin or a heat-resistant synthetic resin precursor to prepare the dispersion for heat emission layers according to claim 16; step (1-2) of spreading the prepared dispersion for heat emission layers on a metal film that serves as a heat transfer layer, followed by standing still; and step (1-3) of removing the dispersion medium from the dispersion for heat emission layers spread on the metal film, forming a film, and obtaining a laminate film.

19. The method for producing a heat dissipation film according to claim 18, wherein in step (1-2), the thickness of the dispersion spread on the metal film is at least 30 m.

20. The method for producing a heat dissipation film according to claim 18, wherein in step (1-3), the temperature at which the dispersion medium is removed from the dispersion for heat emission layers is 20 C. to 150 C.

21. A method for producing a heat dissipation film, the method comprising: step (2-1) of mixing the dispersion medium and the nonvolatile components including a water-insoluble inorganic compound and at least one of a heat-resistant synthetic resin or a heat-resistant synthetic resin precursor to prepare the dispersion for heat emission layers according to claim 16; step (2-2) of spreading the prepared dispersion for heat emission layers on a substrate, followed by standing still; step (2-3) of removing the dispersion medium from the dispersion for heat emission layers spread on the substrate, forming a film, and separating the film from the substrate to obtain a film as a heat emission layer; and step (2-4) of adhesively laminating the film as a heat emission layer on the metal film as a heat transfer layer by hot pressing to obtain a laminate film.

22. The method for producing a heat dissipation film according to claim 21, wherein in step (2-2), the thickness of the dispersion spread on the substrate is at least 30 m.

23. The method for producing a heat dissipation film according to claim 21, wherein in step (2-2), the substrate is made of glass, polyethylene terephthalate, polyimide, polyethylene, or polypropylene.

24. The method for producing a heat dissipation film according to claim 21, wherein in step (2-3), the temperature at which the dispersion medium is removed from the dispersion for heat emission layers is 20 C. to 150 C.

25. The method for producing a heat dissipation film according to claim 21, wherein in step (2-4), the film as a heat emission layer is adhesively laminated on the metal film at a heat-pressing temperature of 50 C. to 200 C. and a pressure of 10 to 100 kgf/cm.sup.2.

26. The heat dissipation film according to claim 1, wherein the heat dissipation film is used to dissipate heat generated from built-in IC chips or LEDs in electronic devices.

27. A solar cell comprising: the heat dissipation film according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0121] FIG. 1 is a schematic cross-sectional view illustrating an example of the solar cell of the present invention.

[0122] FIG. 2(a) is a photograph of the front side of a solar cell module produced using a heat dissipation film produced in an example; and FIG. 2(b) is a photograph of the back side of the solar cell module.

DESCRIPTION OF EMBODIMENTS

[0123] The present invention is described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1

Synthesis of Polyamide Acid Varnish

[0124] A 2-L reaction vessel equipped with a stirrer and a thermometer was charged with 140.1 g (0.70 mol) of 4,4-diaminodiphenylether and 1433.3 g of N-methyl-2-pyrrolidone, and these components were dissolved at 30 C. to 40 C. Subsequently, 72.5 g (0.33 mol) of pyromellitic anhydride and 97.8 g (0.33 mol) of 3,3,4,4-biphenyltetracarboxylic dianhydride were added to the reaction vessel over 40 minutes while the temperature was maintained at 45 C. to 50 C. After stirring at the same temperature for 60 minutes, 183.3 g (0.02 mol) of a 2.4% by weight solution of pyromellitic anhydride in N-methyl-2-pyrrolidone was added to the reaction vessel to adjust the viscosity, and 7.5 g (0.002 mol) of a 4.2% by weight solution of phthalic anhydride in N-methyl-2-pyrrolidone was added thereto to terminate the reaction. Thus, 1933.9 g of polyamide acid varnish having a concentration of 16.3% and a viscosity of 6.2 Pa.Math.s was obtained.

(Preparation of Dispersion for Heat Emission Layers)

[0125] A plastic airtight container was charged with 6.0 g of talc (Talc RA available from Nippon Talc Co., Ltd.) and 24.5 g of the synthesized polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g). These components were then stirred with a planetary centrifugal mixer (ARE-310 available from Thinky Corporation) in a mixing mode (2000 rpm) for 10 minutes and in a deaeration mode (2200 rpm) for 10 minutes. Thus, a uniform dispersion for heat emission layers was obtained in which the proportion of the talc was 60.0% by weight relative to the total weight of the nonvolatile components and the proportion of the nonvolatile components was 32.8% by weight relative to the total weight of the dispersion.

(Production of Heat Dissipation Film)

[0126] The obtained dispersion for heat emission layers was applied with a bar coater having a groove depth of 200 m to a 200-m-thick aluminium film having a smooth and rectangular bottom. The applied dispersion was dried in a forced air oven at 90 C. for two hours while the aluminium film was held horizontally, whereby a film as a heat emission layer was formed on the aluminium film. The laminate film was heated at 120 C. for 30 minutes, at 150 C. for 5 minutes, at 200 C. for 5 minutes, at 250 C. for 5 minutes, and at 350 C. for 60 minutes in the stated order, thus obtaining a heat dissipation film having a 49.2-m-thick heat emission layer containing talc and a polyimide resin in which the amount of the water-insoluble inorganic compound (talc) was 60.0% by weight relative to the total weight of the heat emission layer.

Example 2

[0127] A uniform dispersion for heat emission layers was obtained as in Example 1, except that 36.0 g of talc was added and 22.8 g of N-methyl-2-pyrrolidone was further added in (Preparation of dispersion for heat emission layers). In the obtained dispersion for heat emission layers, the proportion of the talc was 90.0% by weight relative to the total weight of the nonvolatile components, and the proportion of the nonvolatile components was 48.0% by weight relative to the total weight of the dispersion.

[0128] A heat dissipation film having a 57.6-m-thick heat emission layer containing talc and a polyimide resin was obtained using the obtained dispersion for heat emission layers as in Example 1, except that a bar coater with a groove depth of 100 m was used. The amount of the water-insoluble inorganic compound (talc) was 90.0% by weight relative to the total weight of the heat emission layer.

Example 3

[0129] A uniform dispersion for heat emission layers was obtained as in Example 1, except that 1.7 g of talc was added in (Preparation of dispersion for heat emission layers). The proportion of the talc was 30.0% by weight relative to the total weight of the nonvolatile components, and the proportion of the nonvolatile components was 21.8% by weight relative to the total weight of the dispersion.

[0130] A heat dissipation film having a 43.6-m-thick heat emission layer containing talc and a polyimide resin was obtained using the obtained dispersion for heat emission layers as in Example 1, except that the dispersion was applied with a bar coater having a groove depth of 200 m, the dispersion medium was removed by drying in a forced air oven at 90 C. for 30 minutes, and the dispersion was further applied to the same surface with a bar coater having a groove depth of 150 m. The amount of the water-insoluble inorganic compound (talc) was 30.0% by weight relative to the total weight of the heat emission layer.

Example 4

[0131] A plastic airtight container was charged with 3.0 g of talc (Talc RA available from Nippon Talc Co., Ltd.), 3.0 g of coal ash (Clean Ash available from Soma Environment Service Corporation), 0.2 g of carbon black (MA-100 available from Mitsubishi Chemical Corp.), and 24.5 g of the polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g) synthesized in Example 1. These components were mixed under stirring as in (Preparation of dispersion for heat emission layers) of Example 1. Thus, a uniform dispersion for heat emission layers was obtained in which the total proportion of the talc, coal ash, and coloring agent (carbon black) was 60.8% by weight relative to the total weight of the nonvolatile components and the proportion of the nonvolatile components was 33.2% by weight relative to the total weight of the dispersion.

[0132] A heat dissipation film having a 49.2-m-thick heat emission layer containing talc, coal ash, carbon black, and a polyimide resin was obtained using the obtained dispersion for heat emission layers as in Example 1. The total amount of the talc, coal ash, and coloring agent (carbon black) was 60.8% by weight relative to the total weight of the heat emission layer.

Example 5

[0133] A plastic airtight container was charged with 6.0 g of non-swelling mica (SJ-010 available from Yamaguchi Mica Co., Ltd.) and 24.5 g of the polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g) synthesized in Example 1. Further, 2.8 g of N-methyl-2-pyrrolidone was added thereto. These components were mixed under stirring as in (Preparation of dispersion for heat emission layers) of Example 1. Thus, a uniform dispersion for heat emission layers was obtained in which the proportion of the water-insoluble inorganic compound was 60.0% by weight relative to the total weight of the nonvolatile components and the proportion of the nonvolatile components was 30.0% by weight relative to the total weight of the dispersion.

[0134] A heat dissipation film having a 45.0-m-thick heat emission layer containing non-swelling mica and a polyimide resin was obtained using the obtained dispersion for heat emission layers as in Example 1. The amount of the water-insoluble inorganic compound (non-swelling mica) was 60.0% by weight relative to the total weight of the heat emission layer.

Comparative Example 1

[0135] A plastic airtight container was charged with 6.0 g of talc (Talc RA available from Nippon Talc Co., Ltd.), 24.5 g of the polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g) synthesized in Example 1, and 25.1 g of N-methyl-2-pyrrolidone, and these components were mixed under stirring as in (Preparation of dispersion for heat emission layers) of Example 1 in such a manner that the proportion of the talc was 60.0% by weight relative to the total weight of the nonvolatile components and that the proportion of the nonvolatile components was 18.0% by weight relative to the total weight of the dispersion. The talc deposited several minutes after standing still, and thus a uniform dispersion could not be obtained.

[0136] Further, an attempt was made to produce a film as in Example 1; however, the talc deposited, and thus a uniform film could not be obtained.

Comparative Example 2

[0137] A plastic airtight container was charged with 24.5 g of the polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g) synthesized in Example 1, and put in an oven at 90 C. to evaporate the solvent until the total amount was 10.8 g. Thus, a 37.0% by weight solution of a polyamide acid in N-methyl-2-pyrrolidone (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 6.8 g) was obtained. To the solution was added 9.3 g of talc (Talc MS-K available from Nippon Talc Co., Ltd.), followed by mixing under stirring as in (Preparation of dispersion for heat emission layers) of Example 1. Thus, a uniform dispersion for heat emission layers was obtained in which the amount of the water-insoluble inorganic compound (talc) was 70.0% by weight relative to the total weight of the nonvolatile components and the amount of the nonvolatile components was 66.2% by weight relative to the total weight of the dispersion.

[0138] The obtained dispersion for heat emission layers had little fluidity, and thus could not be applied to a substrate. Thus, a heat dissipation film could not be produced.

Comparative Example 3

Synthesis of Polyamide Acid Varnish

[0139] A 500-mL reaction vessel equipped with a stirrer and a thermometer was charged with 29.4 g (0.10 mol) of 3,3,4,4-biphenyltetracarboxylic dianhydride and 80.8 g of N,N-dimethylacetamide, and these components were dissolved at room temperature. Then, the solution was cooled to 0 C., and a mixture of 21.0 g (0.10 mol) of 4,4-diaminodicyclohexylmethane and 37.0 g of N,N-dimethylacetamide was added to the solution at 0 C. to 25 C. over 2 hours. Subsequently, the mixture was stirred at room temperature for one week, and 0.7 g (0.0002 mol) of a 4.2% by weight solution of phthalic anhydride in N,N-dimethylacetamide was added to the mixture to terminate the reaction. Thus, 168.9 g of polyamide acid varnish having a concentration of 29.8% and a viscosity of 10 Pa.Math.s was obtained.

(Production of Heat Dissipation Film)

[0140] A plastic airtight container was charged with 30.7 g of the synthesized polyamide acid varnish (polyamide acid: 9.1 g; N-methyl-2-pyrrolidone: 21.6 g), followed by deaeration in a deaeration mode (2200 rpm) for 10 minutes using a planetary centrifugal mixer. Then, a heat dissipation film having a 65.2-m-thick heat emission layer free of a water-insoluble inorganic compound was obtained as in (Preparation of dispersion for heat emission layers) of Example 1, except that the polyamide acid varnish was applied to a substrate to a thickness of 250 m using a doctor blade.

Comparative Example 4

[0141] A plastic airtight container was charged with 3.0 g of talc (Talc RA available from Nippon Talc Co., Ltd.), 3.0 g of coal ash (Clean Ash available from Soma Environment Service Corporation), 2.0 g of carbon black, 24.5 g of polyamide acid varnish (polyamide acid: 4.0 g; N-methyl-2-pyrrolidone: 20.5 g) synthesized in Example 1, and 4.5 g of NMP. These components were mixed under stirring as in Example 1. Thus, a uniform dispersion for heat emission layers was obtained in which the amount of the water-insoluble inorganic compound (talc+coal ash) and the coloring agent (carbon black) was 66.7% by weight relative to the total weight of the nonvolatile components, and the amount of the nonvolatile components was 32.4% by weight relative to the total weight of the dispersion.

[0142] A heat dissipation film having a 48.6-m-thick heat emission layer containing talc and polyimide resin was obtained using the obtained dispersion for heat emission layers as in Example 1. The amount of the water-insoluble inorganic compound (talc+coal ash) and the coloring agent (carbon black) was 66.7% by weight relative to the total weight.

Comparative Example 5

[0143] Cerac (available from Ceramission Co., Ltd.) in a liquid form containing a mixture of materials such as silicon oxide and aluminium oxide and a silicone resin as a binder was applied with a bar coater to a 200-m-thick aluminium film having a smooth and rectangular bottom (i.e., a heat transfer layer) to a thickness of 50 m. The coating was dried in a forced air oven at 90 C. for one hour while the aluminium film was held horizontally, whereby a heat emission layer was formed on the aluminium film. This laminate film was heated at 120 C. for 20 minutes to obtain a heat dissipation film having a 50.0-m-thick heat emission layer containing a mixture of materials such as silicon oxide and aluminium oxide and a silicone resin. The amount of the water-insoluble inorganic compound (mixture of silicon oxide and aluminium oxide) was 56.0% by weight relative to the total weight of the heat emission layer.

Comparative Example 6

[0144] A heat dissipation film having a 49.2-m-thick heat emission layer containing talc and a polyimide resin was obtained as in Example 1, except that a 135-m-thick graphite sheet (available from Japan Matex Co. Ltd.) was used as the heat transfer layer instead of the 200-m-thick aluminium film in (Production of heat dissipation film). The amount of the water-insoluble inorganic compound (talc) was 60.0% by weight relative to the total weight of the heat emission layer.

[0145] Table 5 shows the composition of each of the dispersions for heat emission layers prepared in Examples 1 to 5 and Comparative Examples 1 to 4, the portion of the nonvolatile components relative to the total weight of the dispersion, and the portion of the water-insoluble inorganic compound relative to the total weight of the nonvolatile components.

TABLE-US-00005 TABLE 5 Com- Com- Com- Com- parative parative parative parative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 1 ple 2 ple 3 ple 4 Disper- Composition Non- Water- Talc 6.0 36.0 1.7 3.0 6.0 9.3 3.0 sion for (parts by volatile insoluble Coal ash 3.0 3.0 heat weight) compo- inorganic Non- 6.0 emis- nents compound swelling sion mica layers Coloring Carbon 0.2 2.0 agent black Heat-resistant Polyamide 4.0 4.0 4.0 4.0 4.0 4.0 4.0 9.1 4.0 synthetic acid resin precursor Dispersion medium N-methyl-2- 20.5 43.3 20.5 20.5 23.3 45.6 6.8 21.6 25.0 pyrrolidone Amount of nonvolatile components relative 32.8 48.0 21.8 33.2 30.0 18.0 66.2 29.8 32.4 to the total weight of the dispersion (wt %) Amount of water-insoluble inorganic compound 60.0 90.0 30.0 60.8 60.0 60.0 70.0 0 66.7 and coloring agent relative to the total weight of nonvolatile components (wt %)

<Evaluation>

[0146] The heat dissipation films obtained in the examples and the comparative examples were evaluated as follows. For the evaluation of the thermal emissivity, dielectric breakdown strength, adhesion, and flammability, only a heat emission layer of a heat dissipation film was produced and evaluated.

[0147] The following evaluation was not carried out for Comparative Example 1 and Comparative Example 2 in which a heat dissipation film could not be produced.

[0148] Table 6 shows the results.

(Thermal Emissivity)

[0149] The thermal emissivity was measured using a thermal emissivity meter TSS-5X available from Japan Sensor Corporation.

(Dielectric Breakdown Strength)

[0150] The dielectric breakdown voltage (kV) was measured by a method according to ASTM D 149 using a dielectric breakdown tester HAT-300 available from Hitachi Chemical Co., Ltd., and the dielectric breakdown strength (kV/mm) was calculated.

(Adhesion)

[0151] The heat emission layer of each obtained heat dissipation film was subjected to a cross-cut test according to JIS K 5600. The heat emission layer was cut to form a square grid with 25 cells using a single-blade cutting tool, and a transparent adhesive tape was applied to the grid. Then, the tape was unfailingly peeled off at an angle close to 600 within 0.5 to 1.0 seconds. The degree of peeling of the heat emission layer on the grid area was visually categorized in six levels from 0 to 5, with 0 being the least degree of peeling, according to Table 1.

(Flammability Classification Based on VTM Test)

[0152] The heat emission layer of the resulting heat dissipation film was subjected to the UL-94 vertical flammability test for thin materials (VTM test).

[0153] Five specimens (length: about 200 mm; width: 50 mm) of each heat emission layer were used for each criterion shown in Table 2. The size of flame was 20 mm.

[0154] The flame application time was three seconds, and the afterflame time after flame application was measured for each specimen. As soon as the fire was out, the second flame application was carried out for three seconds, and the afterflame time after flame application was measured for each specimen as in the first measurement. Further, whether or not the cotton placed below the specimen was ignited by flaming drips from the specimen was observed at the same time. The benchmark was at a position 125 mm from the bottom of the specimen, and the marking cotton was placed 300 mm below the bottom of the specimen.

[0155] As for the flammability classification based on the VTM test, the degree of flame retardancy decreases as the number increases from VTM-1 to VTM-2, with VTM-0 being the most flame-retardant. Specimens not corresponding to any of the ranks of VTM-0 to VTM-2 were evaluated as failed.

(Flammability Classification Based on V Test)

[0156] The heat emission layer of the resulting heat dissipation film was subjected to the UL-94 vertical flammability test (V test).

[0157] Five specimens (length: 127 mm; width: 13 mm) of each heat emission layer were used for each criterion shown in Table 3. The size of flame was 20 mm.

[0158] The flame application time was 10 seconds, and the afterflame time after flame application was measured for each specimen.

[0159] As soon as the fire was out, the second flame application was carried out for 10 seconds, and the afterflame time after flame application was measured for each specimen as in the first measurement. Further, whether or not the cotton placed below the specimen was ignited by flaming drips from the specimen was observed at the same time.

[0160] The flammability classification was determined based on the burning time in the first and second tests, ignition of the cotton, and the like according to the UL-94-V standard. As for the flammability classification based on the V test, the degree of flame retardancy decreases as the number decreases from V-1 to V-2, with V-0 being the most flame-retardant. Specimens not corresponding to any of the ranks of V-0 to V-2 were evaluated as failed.

(Flammability Classification Based on 5V Test)

[0161] The heat emission layer of the resulting heat dissipation film was subjected to the UL-94 125-mm vertical flammability test (5V test).

[0162] Specimens (length: 127 mm; width: 13 mm) were used for each criteria shown in Table 4. The size of flame was 125 mm.

[0163] The flame application time was five seconds, and the afterflame time after flame application was measured for each specimen.

[0164] As soon as the fire was out, the second flame application was carried out for five seconds, and the afterflame time after flame application was measured for each specimen as in the first measurement. This procedure was repeated five times. Further, whether or not the cotton placed below the specimen was ignited by flaming drips from the specimen was observed at the same time.

[0165] The flammability classification was determined based on the burning time in the first to fifth tests, ignition of the cotton, and the like according to the UL-94-5V standard. Those that passed criteria were further subjected to a plate flammability test.

[0166] Plate specimens (length: 150 mm; width: 150 mm) were evaluated in a plate flammability test. The size of flame was 125 mm.

[0167] The flame application time was five seconds. As soon as the fire was out, the second flame application was carried out for five seconds. This procedure was repeated five times. The presence or absence of a hole was determined in each plate specimen after flame application. Those without a hole were evaluated as 5V-A, and those with a hole was evaluated as 5V-B.

(Cooling Temperature)

[0168] Each obtained heat dissipation film was evaluated for its cooling temperature by the following method.

[0169] A 2.4-cm square, 0.5- to 1.5-mm-thick ceramic heater (BPC10 available from BI Technologies Japan Ltd.) (hereinafter also simply referred to as a heater) placed on a substrate (MODEL ICB-88G available from Sunhayato Corp.) was placed in a plastic airtight container (body: polypropylene; lid: polyethylene; container size: 188 mm225 mm; distance from the ceramic heater to the lid: 18 mm). A coated electric wire was soldered to an end portion of the heater so as to connect the heater to a DC stabilized power supply (AD-8724D available from A&D Company, Limited). On a heat spot (12 mm19 mm) of the heater, an aluminium film (thickness: 1 mm) having the same area as the heat spot was placed in order to prevent contact between the soldered portion and the heat dissipation film. Also, polystyrene foam as a thermal insulating material was placed at the lower portion of the airtight container.

[0170] In this state, the output current of the DC stabilized power supply was adjusted to supply a power of 3 W to the heater, and the temperature in equilibrium (heat generation temperature (A)) was measured with a data logger. Next, a 2.4-cm square flat plate-like heat dissipation film was placed on a heater of the same type used for measurement of the heat generation temperature (A), and the temperature in equilibrium (film set temperature (B)) was measured. When placing the heat dissipation film, an appropriate amount of silicone grease (SCH-20 available from Sunhayato Corp.) was applied to the heater so as to adhere the heat dissipation film to the heater. The difference in temperature (AB) between the heat generation temperature (A) and the film set temperature (B) was evaluated as the cooling temperature.

[0171] The cooling temperature was also evaluated by the same procedure using a 200-m-thick aluminium film instead of the heat dissipation film.

(Bending Resistance)

[0172] Each obtained heat dissipation film was subjected to the bend (cylindrical mandrel method) test according to JIS K 5600-5-1. As for the test method, each one specimen was tested using 1- to 10-mm-diameter mandrels from a larger-diameter mandrel to a smaller-diameter mandrel to determine the mandrel diameter at which cracking occurs for the first time in the heat emission layer of the heat dissipation film. A film which did not crack with a 1-mm mandrel was ranked as 1 mm.

(Tensile Strength)

[0173] A specimen type 5 was produced from each obtained heat dissipation film according to JIS K 7127-1. A tensile test was carried out using a table-top type precision universal tester (AGS-X available from Shimadzu Corporation) with a distance between chucks of 80 mm and a tensile speed of 5 mm/min to measure the maximum tensile strength so as to determine the tensile strength (N/mm.sup.2).

(Water Vapor Permeability)

[0174] The resulting heat dissipation film was subjected to measurement of water vapor permeability at 40 C. and 90% RH using a gas and water vapor permeation analysis system available from GTR Tec Corporation by gas chromatography according to JIS K 7126, Method A (differential pressure method).

TABLE-US-00006 TABLE 6 Com- parative Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 1 Heat Heat transfer Metal film Aluminium film 200 200 200 200 200 dissi- layer Other Graphite sheet pation (thickness film (m)) Heat emission Composition Water-insoluble Talc 6.0 36.0 1.7 3.0 Film layer (parts by inorganic Coal ash 3.0 could not weight) compound Non-swelling 6.0 be formed mica Mixture of silicon oxide, aluminium oxide, etc. Coloring agent Carbon black 0.2 Heat-resistant Polyimide 4.0 4.0 4.0 4.0 4.0 synthetic resin resin Other resin Silicone resin Amount of water-insoluble inorganic compound 60.0 90.0 30.0 60.8 60.0 and coloring agent relative to the total weight of heat emission layer (Wt %) Appearance Uniform Uniform Uniform Uniform Uniform Thickness (m) 49.2 57.6 43.6 49.8 45.0 Evalu- Heat emission Thermal emissivity 0.86 0.80 0.82 0.90 0.84 ation layer Dielectric breakdown strength (kV/mm) 47.3 25.8 43.8 18.5 52.3 Adhesion 1 2 1 1 1 Flammability VTM test VTM-0 VTM-0 VTM-0 VTM-0 VTM-0 passed passed passed passed passed V test V-0 V-0 V-0 V-0 V-0 passed passed passed passed passed 5V test 5VB 5VB 5VB 5VB 5VB passed passed passed passed passed Heat Cooling temperature ( C.) 16.8 16.8 17.4 18.4 17.6 dissipation (Values in the parentheses indicate cooling (1.2) (1.2) (1.2) (1.2) (1.2) film capability ( C.) with use of aluminium film only) Bending resistance (mm) 2 1 1 1 9 Tensile strength (N/mm.sup.2) 80.3 79.7 90.9 95.7 77.3 Water vapor permeability (g/mm.sup.2 .Math. day) <0.01 <0.01 <0.01 <0.01 <0.01 Com- Com- Com- Com- Com- parative parative parative parative parative Exam- Exam- Exam- Exam- Exam- ple 2 ple 3 ple 4 ple 5 ple 6 Heat Heat transfer Metal film Aluminium film 200 200 200 dissi- layer Other Graphite sheet 135 pation (thickness film (m)) Heat emission Composition Water-insoluble Talc Film 3.0 6.0 layer (parts by inorganic Coal ash could not 3.0 weight) compound Non-swelling mica be formed Mixture of 5.6 silicon oxide, aluminium oxide, etc. Coloring agent Carbon black 2.0 Heat-resistant Polyimide resin 9.1 4.0 4.0 synthetic resin Other resin Silicone resin 4.4 Amount of water-insoluble inorganic compound 0 66.7 56.0 60.0 and coloring agent relative to the total weight of heat emission layer (Wt %) Appearance Uniform Uniform Uniform Uniform Thickness (m) 65.2 48.6 50.0 49.2 Evalu- Heat emission Thermal emissivity 0.78 0.80 0.85 ation layer Dielectric breakdown strength (kV/mm) Electricity was conducted Adhesion 1 5 Flammability VTM test VTM-0 VTM-0 VTM-0 VTM-0 failed passed failed passed V test V-0 V-0 V-0 V-0 failed passed failed passed 5V test Heat Cooling temperature ( C.) 18.1 18.0 18.5 dissipation (Values in the parentheses indicate cooling (1.2) (1.2) (1.2) film capability ( C.) with use of aluminium film only) Bending resistance (mm) 1 1 1 2 Tensile strength (N/mm.sup.2) 104.3 60.6 23.2 Water vapor permeability (g/mm.sup.2 .Math. day) <0.01 <0.01 <0.01 0.54

(Production and Performance Evaluation of Solar Cell Module)

[0175] Two strings each consisting of two 156-mm square, c-Si modules in series were connected to each other by soldering 6-mm-width tabbing wire at 340 C. to produce a solar cell. An EVA Sheet (Sanvic FC available from Sanvic Inc., 40-cm square) as a sealing material was placed on a glass plate (available from Asahi Glass Co., Ltd., 40-cm square), and the solar cell was placed on the EVA sheet. A transparent Tedlar (registered trademark) having the same area as the solar cell was placed on the cell in such a manner that the cell was sandwiched between two EVA sheets as sealing materials. Lastly, the film produced in Example 4 was placed thereon, a mounting terminal was passed through a slit, and thermocompression bonding was carried out using a vacuum laminator at 135 C. for 21 minutes. EVA overflowing from the end portion of the thermocompression bonded-sheet was removed with a hot cutter. A sealing material (HAMATITE HOTMELT M-155 available from Yokohama Rubber Co., Ltd.) in a heat-molten state was used to fill a groove of an aluminium frame, and such an aluminium frame was fitted to each of the four sides of the laminated sheet. The aluminium frame was fastened with screwed at the four corners, followed by natural drying at room temperature. After drying, a terminal box (Onamba Co., Ltd.) was mounted on the heat dissipation film so as to cover the mounting terminal portion, using a sealing material (SH780 sealant available from Dow Corning Toray Co., Ltd.). After natural drying at room temperature, the mounting terminal was soldered to the terminal box at 340 C. Subsequently, 30 g of a potting agent (PV-7321 available from Dow Corning Toray Co., Ltd.) (i.e., a mixture of a base agent and a curing agent at a ratio of 10:1) was poured into the terminal box and dried naturally. After standing still about for one week, a cover for the terminal box was mounted. Thus, a finished product of a solar cell module was obtained. FIG. 2(a) is a photograph of the front side of the solar cell module produced; and FIG. 2 (b) is a photograph of the back side of the solar cell module.

[0176] The obtained solar cell module was evaluated for its performance. A fill factor of 0.65 to 0.75 was observed in the evaluation of I-V characteristics. Thus, the output performance of the finished product as a solar cell module was confirmed to be normal.

INDUSTRIAL APPLICABILITY

[0177] The present invention provides a heat dissipation film having high mechanical strength and flexibility, which is obtained by laminating a heat emission layer excellent in heat dissipation by infrared radiation, electrical insulation, and heat resistance on a metal film having excellent heat transfer efficiency. The present invention also provides a dispersion for heat emission layers for use in the production of the heat dissipation film, a method for producing a heat dissipation film using the dispersion for heat emission layers, and a solar cell including the heat dissipation film.

REFERENCE SIGNS LIST

[0178] 1 Solar cell [0179] 2 Solar cell element [0180] 3 Sealing material [0181] 4 Light-transmissive substrate [0182] 5 Heat transfer layer [0183] 6 Heat emission layer