Patent Application
20160186012
1. Field of the Invention
The present invention relates generally to a novel composition, which is thermally conductive, electrically insulating, high temperature resistant and flame retardant, and an adhesive sheet containing this composition. In particular, the adhesive composition and the adhesive sheet are tested under a standard test for flammability, i.e. UL 94 test, and are able to be classified into the highest flame retardant group, i.e. UL 94V-0.
2. Description of the Prior Art
The general purpose of an adhesive composition is to make two objects firmly adhere to each other. The special adhesive composition may be used on computer motherboards. For example, the adhesive composition is used to make a heat-generating device firmly adhere to a heat-dissipating device. Also, the adhesive composition can be used to transmit heat from the heat-generating device to the heat-dissipating device.
There are already double-sided adhesives, which are commercially available and known as thermally conductive adhesives, to conduct the heat coming from the heat source to the heat-dissipating device. These thermally conductive and double-sided adhesives are generally substrates coated with acrylic-based adhesive. However, since the main ingredients of the adhesives are acrylic-based organic compounds, these thermally conductive double-sided adhesives have drawbacks. For example, it is neither high temperature resistant (not over 100° C. in general) nor flame retardant. Also, the thermally conductive property is not good enough.
In addition, although there are also commercially available double-sided and thermally conductive adhesives with silicone-coated substrates, and they are temperature resistant (not over 180° C. in general) , but these silicone-based adhesives are not flame retardant. Also, although there are many commercially available thermally conductive adhesives, the mechanical property of the adhesives is still not high enough due to its thin thickness.
Accordingly, there is still a need to provide a composition and an adhesive sheet which are thermally conductive, electrically insulating, high temperature resistant and flame retardant. Moreover, they should pass the standard test for flammability, i.e. UL 94 test, and be classified into the highest flame retardant group, i.e. UL 94V-0.
In view of the above, the present invention discloses an adhesive composition, which is thermally conductive, electrically insulating, high temperature resistant and flame retardant, and an adhesive sheet containing this composition. The features of the composition and the adhesive sheet of the present invention are that they are not only highly thermally conductive and electrically insulating, but also rated to the specification of UL 94V-0 standard, which is fully able to meet the strict and stringent industrial demands.
According to one embodiment of the present invention, a thermally conductive, electrically insulating, high temperature resistant and flame retardant adhesive sheet is disclosed and includes a substrate, an adhesive layer disposed on the substrate, metal hydroxide and metal oxide. The adhesive layer is formed from an adhesive composition, which is made of from 10 to 90 wt. % of a solvent-free and pressure sensitive siloxane resin. The siloxane resin includes hydrophilic functional groups, and the weight percentage of the hydrophilic functional groups in the siloxane resin is in the range of 1 to 10 wt. %.
According to another embodiment of the present invention, a method of fabricating a thermally conductive, electrically insulating, high temperature resistant and flame retardant adhesive sheet is disclosed. First, at least a layer of vinyl-functional coupling agent is coated on the surface of a substrate. A mixture made by mixing a solvent-free siloxane, a metal hydroxide and a metal oxide is then coated on the surface of the substrate. The composition of the solvent-free siloxane includes at least a hydrophilic functional group, and the weight percentage of the hydrophilic functional group in the solvent-free siloxane is in the range of 1 to 10 wt. %. Finally, the substrate is heated so as to cure the mixture coated on the substrate.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
The present invention provides a thermally conductive, electrically insulating, high temperature resistant and flame retardant composition and an adhesive containing the composition. The adhesive of the present invention contains at least a silicon oxide resin, a metal hydroxide and a metal oxide, and optional nitride or aluminum powder. In addition, a coupling agent based on silane is optionally employed when a silicon-free substrate is used.
The siloxane resin is used as the carrier for the high temperature resistant property, and the metal hydroxide as well as the metal oxide may be used to enhance the flame retardant property. At least one of the nitride and aluminum powder may be used to enhance the thermally conductive property. The coupling agent such as silane may be used alone or mixed with a solvent to form the chemical bonding with the silicon-free substrates to enhance the bonding strength. When the metal oxide is present in a higher ratio, it is possibly more thermally conductive and flame retardant. Preferably, the composition in the adhesive of the present invention may be not fully cured to be sufficiently sticky.
The siloxane resin may be 10% to 90% by weight of the overall adhesive composition, preferably from 20% to 50% by weight. These siloxane resins are materials which are able to withstand a temperature above 150° C. without substantially cracking at a temperature around 150° C., and its hardness does not substantially change with a change of the temperature. The term “substantially” used in the present invention is considered as the overall hardness change of the siloxane resin less than 10 degrees (shore A) as the temperature changes, which meets the international standard of UL157 of 0-ring and sealing gasket. In other words, the functions of the adhesive composition are not deteriorated accordingly. Supposing the change of the hardness of the thermoplastic material is within ±10 degrees, the function of the material is not compromised. The siloxane resin may be the products from USA MOMENTIVE™ (GE), USA Dow Corning® or Japan Shin-Etsu. For example, the MOMENTIVE™ series, which is used in pressure sensitive adhesives (PSA series), may be the siloxane resin for use in the present invention. Preferably, the siloxane resins are solvent-free and pressure sensitive siloxane resins and includes hydrophilic functional group. The weight percentage of the hydrophilic functional group in the solvent-free and pressure sensitive siloxane resin is in the range of 1 to 10 wt. %. The hydrophilic functional group is selected from the group consisting of a hydroxyl group, a silanol group, a carboxyl group, and an amino group, but not limited thereto. As an example, the solvent-free and pressure sensitive siloxane resins may be resins of MOMENTIVE™ PSA series, such as Silopren Gel 8-5900-PSA. The term “solvent-free siloxane resin” means that the siloxane resin contains less than 3 wt. % of solvent, and preferably contains no solvent. By using the solvent-free siloxane resin as the main component of the adhesive layer or sheet, the adhesive layer or sheet can show better flame-retardant property compared with convention adhesive layers or sheets.
The metal hydroxide may be 0.01% to 75% (wt.) of the overall adhesive composition, preferably from 20% to 50% by weight. The metal hydroxide generates water and a corresponding metal oxide when heated. The metal hydroxide may be, for example, at least one of zinc hydroxide, aluminum hydroxide and magnesium hydroxide. The metal hydroxide helps to improve the flame retardant property of the overall adhesive composition.
The metal oxide may be 0.01% to 80% (wt.) of the overall adhesive composition, preferably from 20% to 50% by weight. The metal oxide may be a main group metal oxide or a transition metal oxide, such as at least one of alumina (aluminum oxide), zinc oxide and magnesium oxide. The metal oxide helps to improve the thermally conductive, flame retardant and electrically insulating properties of the overall adhesive composition.
The optional nitride may be 0.01% to 30% (wt.) of the total adhesive composition, preferably from 10% to 20% by weight. The nitride may be a metal nitride, or a non-metal nitride, which helps to improve the thermally conductive property of the overall adhesive composition. For example, aluminum nitride or boron nitride may be added to the siloxane resin to make it more thermally conductive. The aluminum nitride has a thermal conductivity of 70˜210 W/mK in the form of poly crystalline, and the single crystal boron nitride has a thermal conductivity about 100 W/mK.
The aluminum powder may be 0.01% to 3% by wright in the total adhesion composition, preferably from 0.5% to 1% by weight. The aluminum powder of high purity, high sphericity and less oxidized status may has an average particle size between 1 micrometer to 50 micrometers. The aluminum powder may have a heat transfer coefficient about 230 W/mK and helps to improve the thermally conductive property of the overall adhesion composition.
The optional coupling agent may be 0.01% to 5% by wright in the total adhesion composition, preferably from 1% to 3% by weight.
The coupling agent mainly functions to enhance the interaction between the siloxane resin and a non-silicone substrate. The coupling agent may be a silane with single component such as from USA MOMENTIVE™ (GE), USA Dow Corning®, Japan Shin-Etsu or Germany Degussa AG, or a blended silane coupling agent with a solvent. The solvent may be toluene, xylene, isopropanol, butanone or ISOPAR®. The silane-based coupling agent may combine the silane layer with the non-silicone substrate through a hydrolysis reaction. The cross-linking reaction helps to enhance the interaction between the siloxane resin and a non-silicone substrate. Preferably, the coupling agent is a silane-based coupling agent mainly composed of vinyl-tris(2-methoxyethoxy)silane, vinyl-trimethoxy or gamma-glycidoxypropyl trimethoxy silane, but is not limited thereto.
After appropriately formulated, the adhesive composition of the present invention is thermally conductive, electrically insulating, high temperature resistant and flame retardant at the same time. The formulation may be done by a closed mixer or an open mixer. The thermal resistance of the adhesive composition may be less than 5° C./w, and the thermal conductivity may be well adjusted in accordance with the addition of different ingredients. The volume resistivity of the adhesive composition is at least greater than 108 ohm-cm, usually greater than 1011 ohm-cm, preferably greater than 1014 ohm-cm. The adhesive composition of the present invention is rated to an international standard, such as the specification of UL 94V-0.
UL 94V is the international standard test for flammability of plastic materials for parts in devices and appliances. It rates the plastic and rubber flammability by judging the afterflame time, afterglowtime, burning rate and the falling flaming particles or drops. The term “afterflame” means persistence of flaming of a material, after the ignition source has been removed, and the term “afterflame time” is used to indicate the length of time for which a material continues to flame, under specified conditions, after the ignition source has been removed. In addition, the term “afterglow” means persistence of glowing of a material, after cessation of flaming or, if no flaming occurs, after removal of the ignition source. The term “afterglow time” indicates the length of time for which a material continues to glow under specified test conditions, after the ignition source has been removed and/or cessation of flaming.
Furthermore, the tested materials can be classified to three classes, V-0, V-1, and V-2, and the V-0 class is the highest class in the test. If a material is classified in the UL 94V-0 class, its afterflame time for each individual specimen is less than 10 seconds, its total afterflame time for any condition set consisting of 5 specimens is less than 50 seconds, its afterflame plus afterglow time for each individual specimen after the second flame application is less than 30 seconds, afterflame or afterglow of any specimen up to the holding clamp would not be happened, and a cotton indicator under the specimen would not be ignited by flaming particles or drops of the specimen. Because the details of UL 94V-0 are available to the public, these details are omitted for the sake of simplicity.
Optionally, the layer of adhesive composition as well as the adhesive sheet of the present invention may have an extremely small thickness. For example, the thickness of the layer of the adhesion composition may be between 0.05 mm to 20 mm, typically between 0.5 mm to 0.05 mm, preferably between 0.2 mm to 0.05 mm. Therefore, the adhesive composition of the present invention may forma single-sided or a double-sided adhesive sheet with extremely small thickness.
In one embodiment of the present invention, the adhesive of the present invention may form a single-sided product of extremely small thickness.
The adhesive composition 12 covers one side of the substrate 11, and the release film 13 covers the other side of the adhesion composition 12, so that the adhesion composition 12 is sandwiched between the substrate 11 and the release film 13. Tear the release film 13 off to adhere the adhesive composition 12 to a target object (not shown).
In another embodiment of the present invention, the adhesive sheet of the present invention may form another single-sided adhesive product.
The adhesive composition 22 covers one side of the substrate 20, while the release film 23 coves the other side of the adhesive composition 22, so that the adhesive composition 22 is sandwiched between the substrate 20 and the release film 23. Tear the release film 23 off to adhere the adhesive composition 22 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may form a single-sided adhesive product of a minimum thickness.
The release films 33 cover both sides of the adhesive composition 32 so that the adhesive composition 32 is sandwiched between the release films 33. Tear the release film 33 off to adhere the adhesive composition 32 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may also form a double-sided product with a substrate.
The first adhesive composition 42 and the second adhesive composition 44 respectively cover the first side 40a and the second side 40b of the substrate 40, so that the substrate 40 is sandwiched between the first adhesive composition 42 and the second adhesive composition 44. The release films 43 cover both the first adhesive composition 42 and the second adhesive composition 44 at the same time so that the substrate 40, the first adhesive composition 42 and the second adhesive composition 44 are also sandwiched between the release films 43. Tear the release film 43 off to respectively adhere the first adhesive composition 42 and the second adhesive composition 44 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may also forma double-sided adhesive product with a substrate.
The first adhesive composition 52 and the second adhesive composition 54 respectively cover the first side 50a and the second side 50b of the substrate 50, so that the substrate 50 is sandwiched between the first adhesive composition 52 and the second adhesive composition 54. The release films 53 cover both the first adhesive composition 52 and the second adhesive composition 54 at the same time so that the substrate 50, the first adhesive composition 52 and the second adhesive composition 54 are also sandwiched between the release films 53. Tear the release film 53 off to respectively adhere the first adhesive composition 52 and the second adhesive composition 54 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may also forma double-sided adhesive product with a substrate.
The first adhesive composition 62 and the second adhesive composition 64 respectively cover the first side 60a and the second side 60b of the substrate 60, so that the substrate 60 is sandwiched between the first adhesive composition 62 and the second adhesive composition 64. The release films 63 cover both the first adhesive composition 62 and the second adhesive composition 64 at the same time so that the substrate 60, the first adhesive composition 62 and the second adhesive composition 64 are also sandwiched between the release films 63. Tear the release film 63 off to respectively adhere the first adhesive composition 62 and the second adhesive composition 64 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may forma compressible single-sided adhesive.
The adhesive composition 72 covers one side of the substrate 71, and the release film 73 covers the other side of the adhesive composition 72, so that the adhesive composition 72 is sandwiched between the substrate 71 and the release film 73. Tear the release film 73 off to adhere the adhesive composition 72 to a target object (not shown).
In another embodiment of the present invention, the adhesive of the present invention may form a double-sided adhesive with a substrate.
The compressible, flexible, temperature resistant and electrically insulating adhesive sheet 74 of the present invention is particularly for use in assembling electronic products or in a heating source whose surface may not be even. Please refer to the above for the details of the adhesive compositions 72. Tear the release film 73 off to adhere the adhesive composition 72 to a target object (not shown). Due to its superior adhesion, the target object would not come off or dislocate during transportation or being fetched after the adhesive composition adheres to a target object.
Table 1 summarizes the properties of the single-sided or double-sided adhesive sheets of the present invention formed by the adhesion composition of the present invention with various substrates, in particular low thermal resistance, high temperature resistance, high flame resistance and high voltage resistance. The adhesion composition of the present invention is thermally conductive, electrically insulating, high temperature resistant and flame retardant.
Beside, several experiments are conducted to compare the flame retardant property of the commercially available adhesive sheets with those of one of the adhesive sheets disclosed in the present embodiments. The result is shown in Table 2 below.
Specifically, the composition of the exemplary embodiment −1 shown in Table 2 is:
In addition, the composition of the exemplary embodiment −2 shown in Table 2 is:
In brief, the adhesive sheet of the present invention has the following advantages:
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
This application is a continuation-in-part of U. S. application Ser. No. 13/744,427, filed on Jan. 18, 2013, which claims the benefit of U.S. provisional application Ser. No. 61/588,661, filed on Jan. 20, 2012. The entire contents of these related applications are incorporated herein by reference and made a part of this specification.
| Number | Date | Country | |
|---|---|---|---|
| 61588661 | Jan 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13744427 | Jan 2013 | US |
| Child | 15065880 | US |
