WO2011156015A2

WO2011156015A2 - Thermally conductive and electrically insulative laminate

Info

Publication number: WO2011156015A2
Application number: PCT/US2011/022204
Authority: WO
Inventors: Wei De Liu; Chi Kwan Wu; Zeming Li; Cecil V. Francis
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2010-01-25
Filing date: 2011-01-24
Publication date: 2011-12-15
Anticipated expiration: 2012-07-25
Also published as: WO2011156015A3; CN102136327B; CN102136327A

Abstract

The present invention provides a thermally conductive and electrically insulative laminate, comprising a first layer of silicone rubber; a second layer of silicone rubber, and a center layer between the first layer and the second layer of silicone rubber, and the center layer is made of a fiberglass cloth coated with a thermally conductive coating or a polymer film comprising thermally conductive filler.

Description

THERMALLY CONDUCTIVE AND ELECTRICALLY INSULATIVE LAMINATE

Technical Field

The present invention relates to a thermally conductive and electrically insulative laminate, and more specifically relates to an electrically insulative laminate having improved thermal conductivity.

Background

A thermally conductive and electrically insulative laminate is a thin dielectric laminate usually made of silicone rubber with a thickness generally in the range of 0.1 mm-0.3 mm. For example, U.S. Patent Nos. 4,574,879, 4,602,678 and 4,685,987 described such a thermally conductive and electrically insulative laminate having a basic structure comprising two layers of thermally conductive silicone rubber and an electrically insulative layer between the two layers of thermally conductive silicone rubber. In order to enhance its mechanical properties and voltage breakdown strength, polyimide film and fiberglass cloth are commonly used as a mechanically reinforcing center layer. For example, fiberglass cloth is used as an electrically insulative center layer in U.S. Patent Nos. 4,602,678 and 4,685,987, and polyimide film is used as an electrically insulative center layer in U.S. Patent Nos. 4,574,879 and 4,685,987. However, since both polyimide film and fiberglass cloth have poor thermal conductivity (generally lower than 0.3W/m.K), the thermally conductive and electrically insulative laminate made thereof also has poor thermal conductivity and may not satisfy applications where good thermal conductivity is required.

Thus, there is still a need to develop an electrically insulative laminate having good thermal conductivity.

Summary

The inventors of the present invention have discovered that using a fiberglass cloth coated with a thermally conductive coating or a polyimide film comprising thermally conductive filler as the center layer of an electrically insulative laminate can significantly improve thermal conductivity of the electrically insulative laminate, while maintaining mechanical properties and voltage breakdown strength of the electrically insulative laminate. In accordance with one aspect of the present invention, there is provided a thermally conductive and electrically insulative laminate, comprising: a first silicone rubber layer; a second silicone rubber layer; and a center layer between the first layer and the second layer of silicone rubber, the center layer being made of a fiberglass cloth coated with a thermally conductive coating or a polymer film comprising thermally conductive filler.

The thermally conductive and electrically insulative laminate of the present invention possesses good thermal conductivity, electrical insulating properties and contact wettability with other substrate surface, and can be used for various types of

electronic/communication products that require high heat dissipation properties.

Brief Description of the Drawings

Figure 1 is a top, perspective view of a high performance thermally conductive and electrically insulative laminate in accordance with an embodiment of the present invention.

Figure 2 is a top, perspective view of a high performance thermally conductive and electrically insulative laminate in accordance with another embodiment of the present invention.

Figure 3 is a top-sectional view of the center layer of a high performance thermally conductive and electrically insulative laminate in accordance with an embodiment of the present invention as described above.

Figure 4 is a top-sectional view of the center layer of a high performance thermally conductive and electrically insulative laminate in accordance with another embodiment of the present invention as described above.

Detailed Description

Unless specifically indicated otherwise, the term "fiberglass cloth" used in the present invention refers to woven fabrics or non- woven fabrics made of glass fibers.

Unless specifically indicated otherwise, the term "rolling" used in the present invention refers to pressurized processing of a specimen using twin-roll technology while maintaining a certain gap between the rolls in order to provide compactness of the filler material in the specimen. Unless specifically indicated otherwise, the term "D50" used in the present invention refers to median particle size, that is, the corresponding particle size when the percentile of the particles in particle size distribution of a sample reaches 50%. Its physical significance is that the quantity of particles having a particle size larger than the median accounts for 50%, and the quantity of particles having a particle size smaller than the median also accounts for 50%.

The present invention provides a thermally conductive and electrically insulative laminate, comprising: a first layer of silicone rubber; a second layer of silicone rubber; and at least one center layer between the first layer and the second layer of silicone rubber, the center layer being made of a fiberglass cloth coated with a thermally conductive coating and/or a polymer film comprising thermally conductive filler.

Silicone rubber is a compliant insulating material well known in the art and possesses outstanding dielectric properties. Silicone rubbers suitable for the present invention include, but are not limited to: methyl silicone rubber, dimethyl silicone rubber, methylvinyl silicone rubber, cyano-silicone rubber, fluorinated silicone rubber and the like. For example, standard brand products that can be purchased from Momentive Performance Materials Group, Nanjing Dongjue Silicone Group Co. Ltd. and other manufacturers are methyl silicone rubber 101, methylvinyl silicone rubber 110-1, methylvinyl silicone rubber 110-2, methylvinyl silicone rubber 110-3, methylphenylvinyl silicone rubber 120-1, methylphenylvinyl silicone rubber 120-2, cyano-silicone rubber 130-2, fluorinated silicone rubber SF-1, fluorinated silicone rubber SF-2, fluorinated silicone rubber SF-3, dimethyl silicone rubber MQ1010, methylvinyl silicone rubber MVQ1101, methylvinyl silicone rubber MVQ1102, methylvinyl silicone rubber

MVQ1103, and the like. Particularly, the thickness of the first layer and the second layer of silicone rubber may be the same or different, and is no greater than 10 mm, more particularly no greater than 1 mm, even more particularly no greater than 0.5 mm, and most particularly no greater than 0.2 mm.

In the present invention, the at least one center layer between the first and second silicone rubber layers is made of fiberglass cloth coated with a thermally conductive coating and/or a polyimide film comprising thermally conductive filler. The thickness of the center layer is particularly no greater than 5 mm, more particularly no greater than 1 mm, and most particularly no greater than 0.1 mm. Figure 1 presents a top, perspective view of an embodiment of the present invention. In this embodiment, the fiberglass cloth 2 coated with a thermally conductive coating is between the two silicone rubber layers 1. Figure 2 presents a top, perspective view of another embodiment of the present invention. In this embodiment, the polyimide film 2' comprising thermally conductive filler is between the two silicone rubber layers 1. Although it is not shown with in the figures, it should be understood that the thermally conductive and electrically insulative laminate of the present invention may comprise more than one center layer, and these center layers may be the same or different. For example, all of them can use fiberglass cloth coated with a thermally conductive coating or all of them can use a polyimide film comprising thermally conductive filler. Alternatively, some of the layers can use the fiberglass cloth coated with a thermally conductive coating, while some other layers can use the polyimide film comprising thermally conductive filler. The thermally conductive and electrically insulative laminate of the present invention can optionally contain a conventional fiberglass cloth or polyimide film.

The fiberglass cloth coated with thermally conductive coating may be obtained by directly coating a thermally conductive coating material on a fiberglass cloth.

Alternatively, the thermally conductive coating material may be coated on glass fibers first, and then the fiberglass cloth coated with thermally conductive coating can be made from the glass fibers coated with thermally conductive coating. The coating may be carried out via various processes, including, but not limited to: dip coating, spray coating, scrape coating, brush coating, flow coating, vacuum coating, chemical deposition, and the like. By means of coating, a uniform layer of thermally conductive coating with a certain thickness having dielectric properties can be formed on the fiberglass cloth surface. There are no particular restrictions as to the thermally conductive coating material suitable for the present invention, which may contain a polymer base material, a thermally conductive material and an optional solvent and various types of additives. The polymer base material, for example, may be polyester resin, silicone rubber, flexible modified polyethylene, elastic thermoplastic rubber, and the like. There are no particular restrictions as to the thermally conductive material, but preferably, ceramic powder materials having high thermal conductivity are used, such as aluminum oxide (A1203), boron nitride (BN), silicon nitride (SiN), and the like. If the center layer is made of fiberglass cloth, a solution containing a certain proportion of thermally conductive particles may be formulated first according to specification. When it is determined that dispersion of the particles becomes uniform, the fiberglass cloth are coated by dip-coating and then dried by baking in a desired

temperature range so that a coating comprising thermally conductive powders can be formed on the fiberglass cloth surface. Its structure is as shown in Figure 3. Figure 3 presents a top-sectional view of the center layer (fiberglass cloth) in an embodiment of the present invention, wherein the outer surface of the fiberglass cloth 4 is covered with a layer of thermally conductive coating 3. In accordance with a particular embodiment using a continuous process, the fiberglass cloth is first passed through a bath of thermally conductive coating material for dip-coating, and then the fiberglass cloth impregnated with the thermally conductive coating material is passed through an oven and dried to provide the fiberglass cloth covered with the thermally conductive coat.

There are no particular restrictions as to the thickness of the thermally conductive coat, but the thickness of the thermally conductive coating is particularly in the range of 50 micron, and more pparticularly in the range of 1-10 micron. The content of the thermally conductive material in the thermally conductive coating is particularly 5%-90% by weight.

There are no particular restrictions as to the fiberglass cloth suitable for the present invention. Any fiberglass cloth commonly used in the art can be used for the present invention. For example, they can be fiberglass cloth 1080 and 1060, which are commercially available from Shanghai Porcher Industries Co., Ltd.

As to the center layer made from a polymer film comprising thermally conductive filler, the film may be made by stretching a polymer comprising thermally conductive filler. For example, thermally conductive particles in the order of nanometers

(particularly, their size is mostly smaller than 100 nm) may be used and added

proportionally to a mother solution of the polymer film. When the particles have been effectively dispersed, an appropriate film forming process is used to carry out film formation, thus to form a film product having excellent flexibility, insulating properties and thermal conductivity. Polymers suitable for the present invention are preferably heat resistant polymers, including, but not limited to: polyimide, polytetrafluoroethylene, ultrahigh molecular weight polyethylene and the like, which can resist temperature over 100 degrees Celsius. Among them, polyimide is mostly preferred. Thermally conductive fillers suitable for the present invention include but are not limited to aluminum oxide (A1203), boron nitride (BN), silicon nitride (SiN), silicon carbide (SiC), and the like. There are no particular restrictions as to the size and shape of the thermally conductive filler, but its preferred crystalline structure is hexagonal crystal, whose crystal lattices are so arranged to form a anisotropic molecule-based material. For example, Figure 4 presents a top-sectional view of the center layer (polyimide film) in an embodiment of the present invention, wherein the polyimide film 2' contains an evenly distributed thermally conductive filler 5. SiC in the order of nanometers (for example, 40 nm) is used as the thermally conductive filler, which is weighed, added and uniformly mixed before formation of the polyimide film, and stretched together with the polyimide film to provide the thermally conductive polyimide film comprising thermally conductive filler as shown in Figure 4. There are no particular restrictions as to the content of the thermally conductive filler in the center layer, but preferably it accounts for 5-60% of the total weight of the center layer.

The thermally conductive and electrically insulative laminate of the present invention can be used as a solution of heat dissipation for various types of

electronic/communication products, and possesses excellent thermally conducting capacity, electrical insulating properties and contact wettability with other substrate surface.

This invention is illustrated in more details by the following examples. However, it should be indicated that the present invention is not limited by these examples. In the following examples and comparative examples, unless specified otherwise, all parts, proportions, percentages are based on weight, and temperature is in Celsius degrees.

Examples

The raw materials used in the present invention and their sources are summarized in Table 1 below. Table 1 : List of the Raw Materials

Comparative Example: Preparation of Reference Samples 1-3

The raw materials were weighted respectively according to the proportions as shown in Table 4 and Table 5 and added into a vessel after being mixed. After being stirred at high speed (800-2000 rpm) for 3 min, the mixture was charged onto a three-roll grinder and was grinded at least twice to ensure uniform dispersion. Finally, additives were added and the mixture was stirred again in an agitator at (800 rpm) to provide a thermally conductive silicone rubber material that can be used for dip-coating. Then, the fiberglass cloth 1080 without any pre-treatment is coated in a dip-coating process. The coated fiberglass cloth with a thickness of 2 mm is used as reference samples 1-3. In the reference samples 1-3, the thermally conductive silicone rubber was only coated on both sides of an ordinary non-thermally conductive fiberglass cloth (1080/1060), whose glass fiber in fact does not have the external thermally conductive coating structure as shown by the present invention. Example 1 : Preparation of Thermally Conductive Coating Materials Suitable for Use With Fiberglass Cloth

The thermally conductive coating materials were formulated respectively according to the three different proportions as indicated in Table 2 below. First of all, polyester resin R-961 was melted. Then, the nano silicon carbide powder (40 nm) was added. After the mixture was stirred for a while, sodium p-dodecylbenzenesulfonate was added according to the proportions as indicated in Table 2. The mixture was stirred again at 800-1200 rpm to provide the thermally conductive coating material 1-3 that can be used for improving thermal conductivity of the fiberglass cloth (1080/1060).

Table 2: Nano Silicon Carbide Test Solution for Fiberglass Cloth (unit: parts)

Example 2: Preparation of Thermally Conductive Coating Materials Suitable for Use With Fiberglass Cloth

The thermally conductive coating materials were formulated respectively according to the three different proportions as indicated in Table 3 below. First of all, polyester resin R-961 was melted. Then, the hexagonal boron nitride powder (3 micron) was added, and the mixture was stirred evenly at 800-1500 rpm to provide the thermally conductive coating materials 4-6 suitable for use with fiberglass cloth. Table 3: Hexagonal Boron Nitride Solution for Fiberglass Cloth (unit: parts)

Example 3: Preparation of Thermally Conductive Coating Materials Suitable For Use With Fiberglass Cloth

The thermally conductive coating materials were formulated respectively according to the three different proportions as indicated in Table 4 below. First of all, the raw materials were weighted respectively according to the proportions and added into a vessel after being mixed. After being stirred at high speed (800-2000 rpm) for 3 min, the mixture was charged onto a three-roll grinder and was grinded at least twice to ensure uniform dispersion. Finally, additives were added and the mixture was stirred again in an agitator (at 800 rpm) to provide thermally conductive silicone rubber materials 7-9 that can be used for dip-coating.

Table 4: Ingredients and Their Proportions in the Nano Silicon Nitride Rubber Solution

Example 4: Preparation of Thermally Conductive Coating Materials Suitable For Use With Fiberglass Cloth

The thermally conductive coating materials were formulated respectively according to the formulations with three different proportions as indicated in Table 5 below. First of all, the raw materials were weighted respectively according to the proportions, and added into a vessel after being mixed. After being stirred at high speed (800-2000 rpm) for 3 min, the mixture was charged onto a three-roll grinder and was grinded at least twice to ensure uniform dispersion. Finally, additives were added and the mixture was stirred again in an agitator (at 800 rpm) to provide thermally conductive silicone rubber coating materials 10-12 that can be used for dip-coating of glass fibers.

Table 5 : Ingredients and Their Proportions in the Hexagonal Boron Nitride Silicone

Rubber Solution

Coating Examples 1-4: Fiberglass Cloth

According to the method as described below, the above-described thermally conductive coating materials 1-12 were coated respectively on fiberglass cloths to form fiberglass cloths 1-12 of 0.055 mm in thickness with a thermally conductive coating (using the fiberglass cloth 1080 of Shanghai Porcher Industries Co., Ltd.). A solution of thermally conductive coating material was prepared according to the requirement, and the fiberglass cloth was inserted into a dip-coating devise. Parallelism and tension of the fiberglass cloth were adjusted and coating speed was controlled at less than 2 meter/min. The thickness of the coat is ensured by adjusting pinch rolls. The baking temperature was 60-180°C, and baking time is 1 min-5 min.

Thermal resistance and temperature difference of the fiberglass cloth 1-6 were measured in accordance with ASTM D 5470, and the results are summarized in Table 6-8 below.

Table 6: Solution Corresponding to the Formulation Proportion of 50% by Weight

Table 7: Solution Corresponding to the Formulation Proportion of 60% by Weight

The voltage breakdown strength of the fiberglass cloths 1-6 was tested with 750-

2/D 149-30B Breakdown Voltage Tester in accordance with ASTM D 2240. The testing results are shown in Table 9.

Table 9: Test Results of the Voltage Breakdown Strength

Comparative Example: Preparation of Reference Samples 4-6

The raw materials were weighted respectively according to the proportions of silicone rubber/thermally conductive powders and so on as shown in Table 4 and Table 5, and added into a vessel after being mixed. After being stirred at high speed (800-2000 rpm) for 3 min, the mixture was charged onto a three-roll grinder and was grinded at least twice to ensure uniform dispersion. Finally, additives were added and the mixture was stirred again in an agitator (at 800 rpm) to provide a thermally conductive silicone rubber material that can be used for dip-coating. Then, the thermally conductive silicone rubber was coated by dip-coating process on an ordinary polyimide film made by biaxial orientation stretching (such a polyimide film with a thickness 25 micron may be purchased directly from Jiangyin Tianhua Technology Co., Ltd.), and the thickness of coatings on both sides is 1 mm each with a total thickness of 2 mm. The coated films were used as reference samples 4-6. In reference samples 4-6, thermally conductive silicone rubber was only coated on both sides of ordinary thermally non-conductive polyimide film (without thermally conductive particles being added).

Preparation Example: Thermally Conductive Polymer Films 1-3

A quantity of silicon nitride powder (particle size 40 nm) accounting for 20%, 30%, 50% of the total weight of the thermally conductive polymer film was added respectively into molten polyimide and mixed uniformly at ambient temperature, and then was processed by biaxial orientation stretching to form thermally conductive polymer films 1-3 with a thickness of 0.24 μιη.

The thermal resistance and temperature difference of the thermally conductive polymer film 1-3 was tested in accordance with ASTM D 5470. The test results are summarized in Table 10 below.

Table 10: Thermal Resistance Test Results of the Thermally Conductive Polymer Film 1-3

Compared with the reference sample 2, the thermal resistance of the thermally conductive polymer film 1-3 is decreased by about 20% in average.

The voltage breakdown strength of the thermally conductive polymer film 1-3 was tested with 750-2/D149-30B Voltage Breakdown strength Tester in accordance with ASTM D 2240. The testing results are shown in Table 11.

Table 11 : Voltage Breakdown Strength Test

Claims

1. A thermally conductive and electrically insulative laminate, comprising:

a first layer of silicone rubber;

a second layer of silicone rubber; and

at least one center layer between the first layer and the second layer of silicone rubber, the center layer being made of a fiberglass cloth coated with a thermally conductive coating and/or a polymer film comprising thermally conductive filler.

2. The thermally conductive and electrically insulative laminate according to claim 1, wherein the at least one center layer is made of a fiberglass cloth coated with a thermally conductive coating.

3. The thermally conductive and electrically insulative laminate according to claim 1, wherein the at least one layer center layer is made of a polymer film comprising thermally conductive filler.

4. The thermally conductive and electrically insulative laminate according to claim 1, wherein the thermally conductive coating comprises a polymer base material and a thermally conductive material.

5. The thermally conductive and electrically insulative laminate according to claim 4, wherein the polymer base material is a polyester resin.

6. The thermally conductive and electrically insulative laminate according to claim 5, wherein the thermally conductive material is at least one material selected from the group consisting of aluminum oxide, boron nitride, silicon carbide and silicon nitride.

7. The thermally conductive and electrically insulative laminate according to claim 6, wherein the thermally conductive material is silicon carbide.

8. The thermally conductive and electrically insulative laminate according to any one of claims 5-7, wherein the D50 size of the thermally conductive material is no greater than 1 μιη.

9. The thermally conductive and electrically insulative laminate according to claim 8, wherein the D50 size of the thermally conductive material is no greater than 0.1 μιη.

10. The thermally conductive and electrically insulative laminate according to claim 8, wherein the D50 size of the thermally conductive material is no greater than 0.01 μιη.

11. The thermally conductive and electrically insulative laminate according to claim 1 , wherein the thermally conductive filler is at least one material selected from the group consisting of boron nitride, aluminum oxide, silicon carbide and silicon nitride.

12. The thermally conductive and electrically insulative laminate according to claim 1, wherein the weight percentage of the thermally conductive filler is 5%-90% based on the total weight of the center layer.

13. The thermally conductive and electrically insulative laminate according to claim 1, wherein the polymer is selected from the group consisting of polyimide,

polytetrafluoroethylene, ultrahigh molecular weight polyethylene, or a blend thereof.

14. The thermally conductive and electrically insulative laminate according to claim 1, wherein the polymer is polyimide.

15. The thermally conductive and electrically insulative laminate according to claim 1, wherein the fiberglass cloth is in a form selected from woven fabrics, non- woven fabrics or film.

16. The thermally conductive and electrically insulative laminate according to claim 1, wherein the thickness of the first layer and the second layer of silicone rubber is no greater than 10 mm, respectively.

17. The thermally conductive and electrically insulative laminate according to claim 1, wherein the thickness of the center layer is no greater than 5 mm.

18. The thermally conductive and electrically insulative laminate according to claim 4, wherein the content of the thermally conductive material in the thermally conductive coating is about 5%-90% by weight.

19. The thermally conductive and electrically insulative laminate according to claim 1, wherein the content of the thermally conductive filler in the polymer film is about 5%-60% by weight.