In addition, the weight average molecular weight of the rubber-based resin is preferably adjusted within an appropriate range. The rubber-based resin with a low molecular weight is advantageous in designing the heat dissipation filler with a high content, but may be disadvantageous in terms of thermal conductivity. The rubber-based resin with a high molecular weight is advantageous for thermal conductivity, but may make it difficult to design a high content of the heat dissipation filler in the heat dissipation sheet.

In addition, the content of the heat dissipation filler may vary depending on the type of the main resin. In terms of the content of the heat dissipation filler, it may be better to use the main resin having a low density. For example, it may have a density of 1 g/m³or less. If the main resin having the density exceeding the range is used, it may be difficult to design the content of the heat dissipation filler in the heat dissipation sheet to be high, and accordingly, it may be difficult to achieve sufficient heat radiation characteristics.

On the other hand, in addition to the above-mentioned rubber-based resin, other types of resin may be additionally contained. Even in this case, it may be better to use a resin having a low density in order to increase the content of the heat dissipation filler in the matrix, and for example, the density may be less than 1 g/m³. However, even when other types of resin are included as an auxiliary, the resin is preferably used in an amount of 10% by weight or less based on the total weight of the matrix. The other types of resins may be, for example, one or more selected from the group consisting of high-density polyethylene, polycarbonate, polyamide, polyimide, polyvinyl chloride, polypropylene, polystyrene, polyisobutylene, modified polypropylene ether (PPE), polyethyleneimide (PEI), polyetheretherketone (PEEK), acrylonitrile-butadiene-styrene (ABS), epoxy-based, acrylic-based, and polyurethane.

On the other hand, although the rubber-based resin has a different degree of elastic restoring force depending on the specific type, the resin has an elastic restoring force of a certain level or more, so it is advantageous to provide a high content of heat dissipation filler in the sheet, but it is not easy to implement the sheet with a thin thickness. That is, it is not easy to increase the density of the heat dissipation sheet, and the heat dissipation sheet undergoes a process of pressing the heat dissipation sheet to increase the density and to remove the pores contained in the matrix. However, it is not easy to increase the density of the implemented sheet including the rubber-based resin as it is restored to the thickness before compression by the elastic restoring force after a predetermined time has elapsed even after the sheet is compressed to a desired thickness. Accordingly, the matrix of the present invention includes a crosslinked rubber-based resin, through which the density after compression can be maintained even over time, and mechanical strength can be improved by increasing the bonding force between the components constituting the matrix. In addition, since the distance between the heat dissipation fillers is close in the thickness direction of the heat dissipation sheet or the contact between the heat dissipation fillers can be significantly increased, it may be more advantageous to improve the thermal conductivity in the vertical direction.

The crosslinking can be achieved through a crosslinking agent. In consideration of the type of rubber-based resin selected, a crosslinking agent that is known to be suitable for crosslinking may be used without limitation. For example, the crosslinking agent may be one or more selected from the group consisting of polyolefin-based, isocyanate-based, and peroxide-based. From the viewpoint of being advantageous in minimizing the increase in thickness that may occur under various conditions of use after crosslinking the rubber-based resin, particularly styrene-butadiene rubber, and in maintaining an initially set density, the crosslinking agent may preferably be one or more of an isocyanate-based and a peroxide-based type. On the other hand, in terms of mass production, the isocyanate-based crosslinking agent may be more advantageous in terms of storage stability of the sheet forming composition and the surface quality during sheet formation. In the case of the isocyanate-based crosslinking agent, a known crosslinking agent may be used, and for example, a block isocyanate-based crosslinking agent may be used.

In addition, the crosslinking agent may be contained in an amount of 1 to 10 parts by weight, more preferably 3 to 7 parts by weight, based on 100 parts by weight of the rubber-based resin. If the crosslinking agent exceeds 10 parts by weight, flexibility is reduced, and matrix hardness and brittle characteristics are increased, so that damage such as matrix breakage may easily occur. In addition, when the amount of the crosslinking agent is less than 1 part by weight, the sheet formability, shape stability, and heat resistance of the heat dissipation sheet may be deteriorated, and it may be difficult to implement the density of the heat dissipation sheet to a desired level.

The above-mentioned heat dissipation filler including graphite may be provided in 80% by weight or more, 90% by weight as another embodiment, or 92% by weight as another embodiment, of the total weight of the heat dissipation sheet. Thus, even in a state in which the heat dissipation filler is provided with a high content, breakages or cracks may not occur, and excellent flexibility can be expressed. If the heat dissipation filler exceeds 92% by weight, it may be difficult to form a sheet. In addition, there is a possibility that the heat radiation characteristics may be deteriorated as the heat insulating effect is expressed due to the inclusion of pores in the matrix. In addition, the heat dissipation sheet may have the density of 1.7 g/m³or more, more preferably 1.8 g/m³or more, through which the heat dissipation filler may be dispersed at a high filling rate, and the sheet can be implemented in a very thin thickness, thereby having advantages in achieving excellent heat radiation characteristics.

In addition, the heat dissipation sheet may have a thickness of 5 to 200 μm, and may be 20 to 100 μm, but is not limited thereto, and may be appropriately changed in consideration of an application place, heat radiation performance, and the like.

In addition, the heat dissipation sheet may have a surface resistance of 0.1 to 100Ω/□, a relative permittivity of 100 or less at a frequency of 28 GHz, and a thermal conductivity of 60 to 150 W/m·k, more preferably, 80 to 150 W/m·k. As described above with reference to FIG. 1, the heat dissipation sheet exhibiting high thermal conductivity may interfere with the wireless signal transmission and reception of the low frequency antenna, and the heat dissipation sheet with the surface resistance of 0.1 to 100Ω/□ may be more suitable for minimizing or preventing the effects of signal disturbance, signal attenuation, and/or signal blocking on the wireless signal transmission/reception of the low frequency antenna. In particular, if the surface resistance exceeds the above range and the electrical conductivity increases, there is a risk that the wireless signal transmission efficiency may be significantly reduced or the function may be lost. In addition, when the heat dissipation sheet has the thermal conductivity is 60 to 150 W/m·k while having a level of surface resistance that does not affect the transmission and reception of wireless signals, it can dissipate the heat generated from the low frequency antenna to an outside more quickly, and thus it is advantageous to prevent the deterioration of the characteristics of the low frequency antenna due to the generated heat or the deterioration or function degradation of the surrounding parts.

In addition, the heat dissipation sheet may have a relative permittivity of 100 or less at a frequency of 28 GHz, thereby further minimizing the influence of transmission and reception of wireless signals. More specifically, the relative permittivity may be 100 or less at a predetermined frequency, for example, 1 GHz, 5 GHz, 10 GHz, 15 GHz, 20 GHz, 25 GHz, 28 GHz, 30 GHz, or 35 GHz.

The above-described heat dissipation sheet may be manufactured by a manufacturing method described later, but is not limited thereto.

The heat dissipation sheet according to an embodiment of the present invention may be manufactured by the method including the steps of (1) preparing a heat dissipation filler including graphite, and (2) preparing a preliminary sheet by mixing the heat dissipation filler with a matrix forming component.

First, as the step (1), the step of preparing the heat dissipation filler having graphite is performed. In this case, the graphite may have been subjected to a process for modifying a surface. The surface modification may be performed by appropriately employing a known method in consideration of the type of the modification. For example, after wetting the heat dissipation filler using an organic solvent such as ethanol, it is mixed with a silane compound and stirred at 40 to 80° C. for 3 hours or more, and then washed and dried to obtain the heat dissipation filler with the modified surface.

Next, as the step (2) of the present invention, the step of preparing a preliminary sheet by mixing the prepared heat dissipation filler with a matrix forming component is performed.

The preliminary sheet may be manufactured by a conventional method for manufacturing a heat dissipation sheet using a conventional heat dissipation sheet forming composition including a matrix forming component and a heat dissipation filler. Particularly, the heat dissipation sheet forming composition may further include a known solvent suitable for dissolving the matrix forming component. For example, toluene, xylene, methyl ethyl ketone, and the like may be used as the solvent. For example, the content of the solvent may be 100 to 1,000 parts by weight based on 100 parts by weight of the matrix forming component, and the content may be adjusted in consideration of an appropriate viscosity or the type of the matrix forming component according to a sheet forming method.

In addition, the matrix forming component may be, for example, a rubber-based resin, and the heat dissipation sheet forming composition may further include a crosslinking agent capable of crosslinking the rubber-based resin. The crosslinking agent may be provided in an amount of 1 to 10 parts by weight, more preferably 3 to 7 parts by weight, based on 100 parts by weight of the rubber-based resin. When the content of the crosslinking agent is less than 1 part by weight, the sheet formability, shape stability, and heat resistance of the heat dissipation sheet may be reduced, and it may be difficult to achieve a desired level of density of the heat dissipation sheet. In addition, when the content of the crosslinking agent exceeds 10 parts by weight, there is a risk that the hardness increases and the flexibility decreases. On the other hand, the rubber-based resin in the preliminary sheet may be in a state in which the crosslinking reaction by the crosslinking agent has not occurred at all, but may be in a B-stage state in which a partial crosslinking reaction has occurred.

The heat radiation forming composition may further include known additives such as a pH adjuster, a flame retardant, a leveling agent, a dispersant and an antifoaming agent, and the present invention is not particularly limited with respect to the specific type and content of the additive.

The heat dissipation sheet forming composition may be subject to a stirring process using a 3-Roll-Mill and/or PL mixer to uniformly disperse the heat dissipation filler and obtain an appropriate viscosity. The stirring process may use a high-power disperser such as 3-Roll-Mill to improve the dispersibility of the heat dissipation filler, and to improve the thermal conductivity, density and flexibility of the heat dissipation sheet.

In addition, a defoaming process for removing bubbles generated in the stirring process may be performed together with the stirring process or after the stirring process.

Thereafter, the homogeneously prepared heat dissipation sheet forming composition may be prepared as a sheet phase by a conventional method, for example, may be processed on a substrate to form a sheet shape. A method of treating the heat dissipation sheet forming composition on the substrate may employ a known coating method. For example, knife coating using a comma coater may be used, but is not limited thereto.

The heat dissipation sheet forming composition processed into a sheet phase on the substrate may be dried at 70 to 130° C. In another example, the heat dissipation sheet forming composition may be dried at an initial temperature of 70 to 85° C., and then the drying temperature may be increased up to a final temperature of 110 to 130° C. for completing the drying. In addition, since the drying time may vary depending on the drying temperature, the present invention is not particularly limited thereto. On the other hand, the thickness of one dried preliminary sheet may be 80 to 150 μm, but is not limited thereto.

Next, as step (3), when the heat dissipation sheet forming composition includes a crosslinking agent, a crosslinking process may be further performed for the rubber-based resin included in the prepared preliminary sheet.

The crosslinking may be performed by an appropriate method depending on the type of the rubber-based resin and the type of the crosslinking agent. For example, it may be a thermal crosslinking reaction by heat treatment or a photo crosslinking reaction by light irradiation. For example, when the crosslinking reaction is induced by heat treatment, it may be carried out by applying heat of 120 to 170° C.

The step (3) according to an embodiment of the present invention may be performed by including the step of stacking several preliminary sheets and then applying heat and/or light, and pressure to the preliminary sheets in a stacked state.

The case where heat and/or light and pressure are applied to the several stacked preliminary sheets may be advantageous in implementing the desired level of thickness and density of the heat dissipation sheet, compare to the case where heat and/or and pressure are applied to one preliminary sheet. In addition, the orientation of the heat dissipation fillers within the heat dissipation sheet is improved through the applied pressure, and the separation distance between the heat dissipation fillers in the thickness direction, that is, the vertical direction can be significantly reduced, which can be advantageous for improving the thermal conductivity in the vertical and horizontal directions.

Preferably, 2 to 5 preliminary sheets may be stacked. If more than 5 preliminary sheets are stacked, the preliminary sheets may be pushed during the pressurization process of the stacked preliminary sheets, so uniform pressure may not be performed, and there is a risk of thickness variation depending on the location of the heat dissipation sheet.

On the other hand, if the thickness of one preliminary sheet is very thin (for example, 40 μm or less), it may not be easy to stack the preliminary sheets in the process. Therefore, it is preferable to perform the step (3) for one sheet, without stacking multiple sheets.

On the other hand, the preliminary sheet manufactured through the step (2) and the heat dissipation sheet manufactured through the step (3) may have the thickness reduction rate of 20% or more, preferably 25%, more preferably 40% or more, calculated according to Equation 1 below. Accordingly, the high content and high density design of the heat dissipation filler in the heat dissipation sheet is possible, which may be more advantageous in improving heat radiation characteristics.

thickness
⁢

reduction
⁢

rate
⁢

(
%
)

=

[

Equation
⁢

1

]

thickness
⁢

of
⁢

preliminary
⁢

sheet
⁢

(
μm
)

-

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

(
μm
)

thickness

of

preliminary

sheet

(
μm
)

In addition, although in the heat dissipation sheet manufactured through the step (3), a matrix is implemented using a rubber-based resin, a thickness change rate calculated by Equation 2 below after left at 40° C. for 50 hours may be 10% or less, preferably 5% or less, more preferably 2% or less, even more preferably 1% or less, even more preferably 0.5% or less. Through this, there is an advantage in minimizing the deterioration of the heat radiation characteristics that occur as the thickness increase or the quality deterioration due to a shape deformation such as thickness fluctuates or non-uniform thickness after the implementation of the heat dissipation sheet.

thickness
⁢

change
⁢

rate
⁢

(
%
)

=

[

Equation
⁢

2

]

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

after
⁢

left
⁢

(
μm
)

-

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

before
⁢

left
⁢

(
μm
)

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

after
⁢

left
⁢

(
μm
)

The step (3) according to an embodiment of the present invention may include the steps of crosslinking while applying heat and pressure with respect to at least one preliminary sheet, and cooling the crosslinked preliminary sheet.

The crosslinking step can induce a thermal crosslinking reaction while applying pressure. Through this, in addition to realizing the desired thickness, the density of the heat dissipation sheet can be increased, and the content of the heat dissipation filler per unit volume can be further increased, and at the same time, the distance between the heat dissipation fillers can be shortened depending on the pressure, so there is an advantage that heat radiation characteristics can be further improved. In addition, when the heat dissipation filler has a plate shape, the orientation in the horizontal direction within the heat dissipation sheet is improved, and the vertical distance between the heat dissipation fillers is shortened, so that both the horizontal and vertical heat radiation characteristics can be improved. At this time, the applied pressure may be 2.5 to 5 kgf/mm², so that it may be advantageous in achieving the desired effect of the present invention.

In addition, the heat applied in the crosslinking step may be 100 to 180° C., preferably 110 to 170° C., more preferably 150 to 180° C., the execution time may be 10 to 60 minutes, more preferably 15 to 55 minutes.

In addition, the cooling step is to avoid the problems of density reduction and non-uniform thickness due to matrix expansion occurring when left at room temperature after crosslinking through heat, and has the advantage of realizing the heat dissipation sheet having a higher density and uniform thickness. In addition, it is possible to realize the heat dissipation sheet having a better surface quality through the cooling step. As shown in FIGS. 9 and 10, it can be confirmed that the surface quality of the heat dissipation sheet of FIG. 10 in which the cooling step is performed is superior to the surface of the heat dissipation sheet of FIG. 9 in which the cooling step is not performed after crosslinking through heat.

The cooling step may be terminated when the manufactured heat dissipation sheet is cooled to a temperature of 60° C. or less, preferably 18 to 60° C., and more preferably 18 to 50° C. In addition, the cooling step may be performed for 10 to 60 minutes, preferably 15 to 55 minutes. In addition, the cooling rate may be, for example, 5 to 30° C./min. If the cooling temperature exceeds 60° C., there is a risk of thickness fluctuation, and in the cooling process, the heat dissipation sheet may be attached to a cooling device, for example, a surface of a press and not easily detach from it, and this significantly increases the deterioration of the surface quality of the heat dissipation sheet and there is a risk of lowering productivity.

In addition, the cooling step may also be performed while applying a pressure, so that there is an advantage that can minimize the thickness variation of the heat dissipation sheet. At this time, the applied pressure may be, for example, 2.5 to 5 kgf/mm².

On the other hand, the steps of crosslinking and cooling may be performed while applying pressure through a first press and a second press having different temperatures. In this case, productivity can be further improved compared to when the temperature condition is changed using one press, and the time between the crosslinking step and the cooling step can be minimized or easily adjusted to a desired level, so there is advantage of improving the quality of the heat dissipation sheet. The temperature and pressing time of the first press may be the temperature and execution time of the above-described crosslinking step, and the temperature and pressing time of the second press may be the temperature and execution time of the above-described cooling step.

The heat dissipation sheet manufactured through the above-described method may implement a low frequency antenna module, together with a low frequency antenna having an operating frequency of 50 kHz to 350 kHz and a magnetic field shielding sheet.

The low frequency antenna has a frequency band of 50 kHz to 350 kHz as an operating frequency, and may be, for example, a wireless power transmission (WPT) antenna or a magnetic secure transmission (MST) antenna. The wireless power transmission antenna may be a Qi standard or PMA standard antenna operating in a magnetic induction method in the frequency band. Alternatively, the magnetic security transmission antenna may be an antenna of a well-known standard capable of generating a non-directional electromagnetic wave that can pass through a reader head in a POS terminal although it is spaced apart a certain distance from the POS terminal, so that card information is transmitted to the POS terminal so that payment can be performed using a portable device such as a smartphone, in the same way as a magnetic card by the magnetic security transmission technique.

Hereinafter, a case in which the low frequency antenna 200 is a wireless power transmission antenna will be described in detail. When described with reference to FIG. 2, the wireless power transmission antenna may be configured to include a circular, oval, or rectangular flat-plate coil 210 in which a conductive member having a predetermined length is wound a plurality of times in a clockwise or counterclockwise direction, and a support member 220 supporting the coil. The support member 220 may be a circuit board including a circuit for controlling a wireless signal transmitted or received by the coil with the purpose of supporting the coil 210. Alternatively, the support member 220 may be a heat radiation support member designed to have heat radiation characteristics for transferring the heat generated from the coil downward, together with the purpose of supporting it. The heat radiation support member may be, for example, a heat radiation plastic that contains a known heat conductive filler and is injection molded.

Meanwhile, as shown in FIGS. 3 and 4, the wireless power transmission antenna may be configured as a flat-plate coil 210 without the support member 220. In this case, the flat-pate coil 210 may be directly disposed on a magnetic field shielding sheet 300 or the heat dissipation sheet 100.

Alternatively, the wireless power transmission antenna may be implemented by patterning a conductor such as copper foil in a loop shape on one surface of the circuit board or forming a loop-shaped metal pattern using conductive ink. Here, the circuit board may be a flexible circuit board made of a material such as PI or PET, or a rigid circuit board made of a material such as FR4. When the wireless power transmission antenna is a metal pattern patterned on the circuit board, the heat dissipation sheet according to an embodiment of the present invention may be attached to the opposite surface of the one surface of the circuit board on which the metal pattern is formed, or may be attached to directly contact the metal pattern.

Next, the magnetic field shielding sheet 300 shields the magnetic field generated by the wireless signal induced in the low frequency antenna and increases the collection speed of the magnetic field in a required direction, thereby improving the transmission efficiency of the wireless signal. In addition, the magnetic field shielding sheet may be a plate-shaped member having a predetermined area.

In addition, regarding the magnetic field shielding sheet 300, any known magnetic field shielding sheet used together with an antenna may be used without limitation, and for example, a magnetic material may be used. The magnetic material may be a soft magnetic material, and non-limiting examples thereof include pure iron or alloy steel such as silicon steel sheet and permalloy, or Fe-based alloy of amorphous or nanocrystalline grains such as Fe—Co-based alloy, Fe—Si-based alloy, etc. Alternatively, it may be a known soft ferrite such as Mn—Zn, Ni—Zn, Mg—Zn.

In addition, the magnetic field shielding sheet 300 may have a form in which a plurality of magnetic sheets is stacked in multiple layers via an adhesive layer, and the plurality of magnetic sheets may be flake-treated and separated into a plurality of fine pieces, Neighboring micro-pieces may be entirely insulated or partially insulated.

In addition, the magnetic field shielding sheet 300 may be provided with an appropriate size to cover an area corresponding to the low frequency antenna 200 so as to increase the performance of the low frequency antenna 200.

The above-described low frequency antenna modules 1000 and 1000′ may be a transmission module for transmitting a wireless signal or a reception module for receiving a wireless signal.

The arrangement of the heat dissipation sheet 100, the low frequency antenna 200 (or the flat-plate coil 210) and the magnetic field shielding sheet 300 will be described with reference to FIGS. 2 to 4.

The low frequency antenna modules 1000 and 1000′ of FIGS. 2 and 3 are disposed such that the heat dissipation sheet 100 directly covers the upper portion of the flat-plate coil 210 of the low frequency antenna 200. In this case, the upper direction of the flat-plate coil 210 may be the receiving direction of the wireless signal when the low frequency antenna modules 1000 and 1000′ are the reception antenna modules. In addition, the magnetic field shielding sheet 300 may be disposed on the lower portion of the flat-plate coil 210 to increase the collection speed of the wireless signal received from the upper portion of the flat-plate coil 210. As described above, since the heat dissipation sheet 100 according to the present invention has little effect on the reception of the wireless signal even if it is positioned on the path through which the wireless signal is received as shown in FIGS. 2 and 3, it enables the expression of heat radiation performance without being restricted by the arrangement position, and through this, the module can be freely designed, compared to the conventional heat dissipation sheet used together with the antenna.

Alternatively, as shown in FIG. 4, the low frequency antenna module 1000″ is disposed such that the heat dissipation sheet 100 supports the lower surface of the flat-plate coil 210 of the low frequency antenna 200, and the magnetic field shielding sheet 300 can be provided on the lower portion of the heat dissipation sheet 100. Meanwhile, a low frequency antenna module may be implemented by changing the positions of the heat dissipation sheet 100 and the magnetic field shielding sheet 300 of FIG. 4.

On the other hand, although it has been described that the low frequency antenna modules 1000 and 1000′ include the magnetic field shielding sheet 300, the magnetic field shielding sheet may be omitted in the module configuration.

In addition, the above-described heat dissipation sheet 100 or the low frequency antenna module (1000, 1000′, 1000″) provided with the heat dissipation sheet 100 can be provided to implement an electronic device. The electronic device may be a known electronic device having an antenna capable of transmitting and receiving wireless signals. For example, the electronic device may be various electronic devices such as electronic devices having communication means such as smart phones, tablet PCs, notebook PCs, etc., and home appliances to which communication technology can be broadly applied according to IoT technology. For example, the heat dissipation sheet 100 may be very useful for electronic devices, such as 5G smartphones or tablet PCs, which generate high heat and of which performance may be degraded due to the high heat.

EXAMPLES

The present invention will be described in more detail through the following examples, but the following examples are not intended to limit the scope of the present invention, which should be construed to aid understanding of the present invention.

Example 1

Based on 100 parts by weight of SBR (200° C. MFR 5 g/min, weight average molecular weight of 900,000), 3 parts by weight of bis(tert-butylphenoxy-2-isopropyl)benzene, which is a peroxide-based crosslinking agent, 1200 parts by weight of graphite flake having an average particle diameter of 80 μm and having a surface modified with 3-aminopropyltriethoxysilane that is amino silane compound, and toluene as a solvent were mixed and stirred to prepare a heat dissipation sheet forming composition having a viscosity of about 2500 cps. In this case, the graphite flake whose surface was modified with an amino silane compound was prepared by soaking the graphite flake in ethanol, mixing with 3-aminopropyltriethoxysilane, stirring at 60° C. for 4 hours, washing and drying. In the finally obtained graphite flake of which surface is modified with the amino silane compound, 2.5 parts by weight of the amino silane compound was provided with respect to 100 parts by weight of the graphite flake.

Thereafter, the sheet was prepared using a comma coater, and then, dried at 120° C. to prepare a preliminary sheet having a thickness of 110 μm. After stacking two preliminary sheets, a release film was attached on the uppermost preliminary sheet, and a pressure of 3.1 kgf/mm²was applied using a first press of a temperature of 160° C. to induce a thermal crosslinking reaction for 40 minutes. Thereafter, a cooling process was performed for 40 minutes in a state of applying the pressure of 3.1 kgf/mm²using a second press of a temperature of 50° C. to prepare the heat dissipation sheet having the final thickness of 50 μm and the content of the heat dissipation filler of 90% by weight.

Example 2

It was prepared in the same manner as in Example 1, except that the modified graphite flake was changed to the artificial graphite modified in the same manner to prepare a heat dissipation sheet of the same thickness.

Comparative Example 1

It was prepared in the same manner as in Example 1, except that the modified graphite flake was changed to the boron nitride modified in the same manner to prepare a heat dissipation sheet of the same thickness.

Comparative Example 2

A copper foil having a thickness of 0.05 mm was prepared as a heat dissipation sheet.

Experimental Example 1

The following physical properties of the heat dissipation sheets according to Examples 1, 2 and Comparative Examples 1, 2 were measured and shown in Table 1 below.

1. Relative Permittivity

The dielectric constant at a frequency of 28 GHz was measured using a network analyzer equipment.

2. Thermal Conductivity

It was measured in LFA by using ASTM E1461.

In addition, after placing the LED at a predetermined interval on the circumference of a circle with a diameter of 25 mm, a thermometer was placed in the center of the circle, and a measuring equipment was manufactured so that a predetermined voltage could be applied to the LED. The measuring equipment was placed in an acrylic chamber of 32 cm×30 cm×30 cm in width, length, and height, respectively, and the temperature inside the acrylic chamber was adjusted to be 25±0.2° C. After placing a heat dissipation sheet on the LED of the measuring equipment, a predetermined input power was applied to the LED, and after a predetermined time had elapsed, a thermal image was taken from the upper portion of the heat dissipation sheet and the temperature of the thermometer in the measuring equipment was measured. Afterwards, the average temperature of the portion of the heat dissipation sheet corresponding to the LED was calculated to show the average temperature through the results of thermal imaging, and the temperature calculated through the thermometer in the measuring equipment was shown as the T.C value, respectively.

In addition, as a standard for evaluating heat radiation performance, the same input power was applied to the LED of the measuring equipment in the absence of the heat dissipation sheet, and after the same time had elapsed, thermal imaging was taken from the upper portion of the LED and the temperature of the thermometer in the measuring equipment was measured, and the result was defined as a default value, and the average temperature taken in the thermal image without the heat dissipation sheet was 53.8° C., and the T.C. was 66.3° C.

Experimental Example 2

The wireless power transmission antenna was used as a low frequency antenna, the wireless power transmission efficiency and heat radiation characteristics of the heat dissipation sheets according to Examples 1, 2, Comparative Examples 1, 2 were evaluated, and the results were shown in Table 1 below. In addition, a photograph of the thermal image taken during the evaluation of the heat radiation characteristic was shown in FIG. 7. On the other hand, when there was no heat dissipation sheet, wireless power transmission efficiency and heat radiation characteristics were tested together, and the result value was used as a default value.

Specifically, after preparing a transmission antenna and a reception antenna for wireless power transmission having a frequency of 105 to 205 kHz as an operating frequency of a low frequency antenna, the heat dissipation sheet was placed on the opposite side of the side where the flat-plate coil was arranged in the transmission antenna as shown in FIG. 5. The transmission antenna and the reception antenna were placed so that each of the flat-plate coils faced each other, and then, the separation distance between the flat-plate coils in each antenna was 1 mm. After applying a voltage of 12V to the transmission antenna for wireless power transmission, the efficiency was measured by adding a load for each power (W) of the reception antenna for wireless power transmission.

In addition, as for the heat radiation characteristics, the temperature of the hottest point in the thermal image was measured using a thermal image taken from the upper direction of the reception antenna for wireless power transmission after applying a voltage of 12V to the transmission antenna for wireless power transmission.

TABLE 1

Wireless power transmission

Heat radiation

Relative
Thermal
Wireless power
characteristic

permittivity
Conductivity
transmission
(Maximum

(@28 GHz)
(W/m · K)
efficiency (%)
temperature (° C.))

Default
—
—
80.5
54.8

Example 1
90.2
89.2
80.8
50.3

Example 2
85.0
1500
79.1
51.2

Comparative
3.2
42.1
81.0
53.2

Example 1

Comparative
Not measurable
400
77.2
56.1

Example 2

As seen in Table 1, in Examples 1 and 2 using graphite as the heat dissipation filler, the efficiency during wireless power transmission was increased compared to the default case in which the heat dissipation sheet was not provided, and the heat generated from the wireless power reception antenna was rapidly transferred to an outside. Accordingly, the temperature of the highest point was measured at a low level of 3.6 to 4.5° C., which confirmed that the heat radiation characteristics were very excellent.

However, it was confirmed that the wireless power transmission efficiency in Comparative Example 1 was at the same level or somewhat superior to the wireless power transmission efficiencies in Examples 1 and 2, but the heat radiation characteristic was remarkably poor.

In addition, it was confirmed that although the thermal conductivity in Comparative Example 2 was very excellent compared to Example 1, when employed in wireless power transmission, the wireless power transmission efficiency was also significantly reduced, and the heat radiation characteristics were rather increased compared to the default in which the heat dissipation sheet was not employed.

Experimental Example 3

In the same manner as in Experimental Example 2, the wireless power transmission efficiency and heat radiation characteristics according to Examples and Comparative Examples were evaluated, but the arrangement position of the heat dissipation sheet was placed between the flat-plate coil of the transmission antenna for wireless power transmission and the flat-plate coil of the reception antenna for wireless power transmission, and the distance between the flat-plate coils on the heat dissipation sheet and the transmission antenna for wireless power transmission was changed to 1 mm, as shown in FIG. 6. The results of the evaluated wireless power transmission efficiency and heat radiation characteristics were shown in Table 2 and FIG. 8.

TABLE 2

Wireless power transmission

Heat radiation

Relative
Thermal
Wireless power
characteristic

permittivity
conductivity
transmission
(Maximum

(@28 GHz)
(W/m · K)
efficiency (%)
temperature (° C.))

default
—
—
80.5
54.8

Example 1
90.2
89.2
79.9
49.0

Example 2
85.0
1500
0
Not measurable

Comparative
3.2
42.1
80.5
59.9

Example 1

Comparative
Not measurable
400
0
Not measurable

Example 2

As seen in Table 2, when the heat dissipation sheet was positioned on the movement path of the wireless power transmission signal, the copper foil heat dissipation sheet of Comparative Example 2 did not operate the wireless power transmission function, which suggested that the heat dissipation sheet blocked the wireless power transmission signal.

In addition, the wireless power transmission efficiency of the heat dissipation sheet of Comparative Example 1 was somewhat superior to that of Example 1, but its heat radiation characteristic was significantly increased compared to the default in which the heat dissipation sheet was not employed, which confirmed that the heat radiation characteristic itself was not expressed.

On the other hand, when the case of Example 2 using artificial graphite as the heat dissipation filler was positioned on the movement path of the wireless power transmission signal, it was confirmed that the wireless power transmission signal was blocked. Accordingly, it was confirmed that the case of Example 2 could be used as the heat dissipation sheet adopted for a lower frequency antenna, only when it was positioned in a place other than the movement path of the wireless power transmission signal. However, it was confirmed that the heat dissipation sheet according to Example 1 exhibited excellent heat radiation characteristics while minimizing the decrease in wireless power transmission efficiency even when positioned on the movement path of the wireless power transmission signal. These results showed that the limitation of the conventional heat dissipation sheet in which the position must be arranged in consideration of the movement path of the wireless power transmission signal was resolved, and it was confirmed that the heat dissipation sheet using graphite flakes was very suitable for realizing the low frequency antenna module.

Example 3

It was prepared in the same manner as in Example 1, except that the thickness of the final heat dissipation sheet was changed to 60 μm to prepare a heat dissipation sheet as shown in Table 3 below.

Comparative Example 3

Manufactured in the same manner as in Example 3, but 3 parts by weight of DICY as a curing agent and 200 parts by weight of methyl ethyl ketone as a solvent were mixed based on 100 parts by weight of the bisphenol A epoxy component (Kukdo, YG-011) as a matrix forming component, to prepare a sheet forming composition, and the prepared sheet forming composition was treated on a substrate to a predetermined thickness using a comma coater, cured at 150° C. for 30 minutes, and then cooled in the same manner as in Example 1 to prepare the heat dissipation sheet having the final thickness of 60 μm, and the content of the heat dissipation filler of about 90% by weight.

Comparative 4

The same heat dissipation filler as in Example 1 was used, but it was carried out in the same manner as in Example 1, except that the matrix forming component was changed to thermoplastic polyurethane (TPU). The prepared heat dissipation sheet had the final thickness of 60 μm, and the content of the heat dissipation filler of about 90% by weight.

Experimental Example 4

100 each of heat dissipation sheets according to Example 3 and Comparative Examples 3 and 4 were prepared in the same size. Then, among 100 specimens, the number of specimens with cracks or breakage and the number of specimens with pores on the surface or shrinkage were counted, and the results were shown as a percentage in Table 3 below.

TABLE 3

Comparative
Comparative

Example 3
Example 3
Example 4

Matrix forming component
Rubber-based
Epoxy component
Polyurethane

component (SBR)

component

Percentage (%) of cracked/broken sheets
0
80
0

Percentage (%) of shrunk/porous sheets
1
0
53

As seen in Table 3, it was confirmed that the heat dissipation sheet of Example 3 using the rubber-based component as the matrix forming component did not generate cracks or breakages, did not change shape such as shrinkage, and had excellent surface quality.

Example 4

It was prepared in the same manner as in Example 3, except that the content of the surface-modified graphite was changed to 1200 parts by weight to prepare a heat dissipation sheet as shown in Table 4 below.

Example 5

It was prepared in the same manner as in Example 3, except that the type of crosslinking agent was changed to hexamethylene diisocyanate to prepare a preliminary sheet, and a heat dissipation sheet as shown in Table 4 was prepared through crosslinking and cooling processes.

Comparative Example 5

It was prepared in the same manner as in Example 3, but without adding a crosslinking agent, a heat dissipation sheet as shown in Table 4 was prepared.

Experimental Example 5

The following physical properties were evaluated for the heat dissipation sheets according to Examples 3 to 5 and Comparative Example 5, and the results are shown in Table 4.

Specifically, after measuring dimensions such as thickness and weight immediately after manufacturing the heat dissipation sheet, a thickness reduction rate was calculated according to Equation 1 below. In addition, a thickness change rate was calculated according to Equation 2 below after the manufactured heat dissipation sheet was left at 40° C. for 50 hours.

thickness
⁢

reduction
⁢

rate
⁢

(
%
)

=

[

Equation
⁢

1

]

thickness
⁢

of
⁢

preliminary
⁢

sheet
⁢

(
μm
)

-

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

(
μm
)

thickness

of

preliminary

sheet

(
μm
)

thickness
⁢

change
⁢

rate
⁢

(
%
)

=

[

Equation
⁢

2

]

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

after
⁢

left
⁢

(
μm
)

-

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

before
⁢

left
⁢

(
μm
)

thickness
⁢

of
⁢

heat
⁢

dissipation
⁢

sheet
⁢

after
⁢

left
⁢

(
μm
)

TABLE 4

Comparative

Example 4
Example 5
Example 5

Matrix forming component
SBR
SBR
SBR

Type of crosslinking agent
Peroxide-based
Isocyanate-based
Not used

Thickness of preliminary sheet(μm)
140
140
140

Thickness of heat dissipation sheet
82.0
82.0
101.0

immediately after manufactured (μm)

Thickness reduction rate (%)
41.4
41.4
27.9

Density (g/m³)
1.81
1.81
1.71

Thickness of heat dissipation sheet
82.2
90.2
138

after left for 50 hours (pm)

Thickness change rate (%)
0.3
10.0
36.6

As seen in Table 4, in Example 4 in which a peroxide type crosslinking agent was used and Example 5 in which an isocyanate type crosslinking agent was used, the thickness change rate after preparation was 10% or less, which was superior to that in Comparative Example 5.

Examples 6 to 8

It was prepared in the same manner as in Example 4, but the cooling temperature was changed as shown in Table 5 below to prepare a heat dissipation sheet as shown in Table 5 below.

Experimental Example 6

For the heat dissipation sheets of Example 4 and Examples 6 to 8, the thickness change rate was calculated in the same manner as in Experimental Example 5. In addition, among a total of 1000 manufactured heat dissipation sheets for each Example, the number of heat dissipation sheets attached to the second press was counted and shown in Table 5 below.

TABLE 5

Example 4
Example 6
Example 7
Example 8

Matrix forming components
SBR
SBR
SBR
SBR

Type of crosslinking agent
Peroxide-based
Peroxide-based
Peroxide-based
Peroxide-based

Cooling temperature (° C.)
50
60
65
70

Thickness of preliminary sheet (μm)
140
140
140
140

Thickness of heat dissipation sheet
82.0
82.0
82.0
82.0

immediately after manufactured (μm)

Thickness reduction rate (%)
41.4
41.4
41.4
41.4

Thickness of heat dissipation sheet
82.2
82.74
83.80
86.76

after left for 50 hours (μm)

Thickness change rate (%)
0.3
0.9
2.2
5.8

Number of heat dissipation sheets
0
3
26
100

attached to a second press

As seen in Table 5, it was confirmed that in Examples 7, 8 in which the cooling temperature exceeded 60° C., the number of heat dissipation sheets adhered to the second press increased, and the thickness change rate also increased.

Example 9

A heat dissipation sheet was manufactured in the same manner as in Example 3, except for performing a cooling process.

Experimental Example 7

For the heat dissipation sheets according to Examples 4 and 9, surface SEM photographs were taken, and the results were shown in FIGS. 9 (Example 9) and 10 (Example 4).

As seen in FIGS. 9 and 10, it was confirmed that FIG. 9 which was a photograph of the heat dissipation sheet of Example 9 that has not undergone the cooling process showed a rough surface, whereas the surface quality of the heat dissipation sheet of Example 4 that has been subjected to the cooling process showed excellent.

Examples 10 to 21

The heat dissipation sheet as shown in Tables 6 and 7 was manufactured in the same manner as in Example 4, except that the particle size of the heat dissipation filler and the content of the heat dissipation filler were changed as shown in Tables 6 and 7 below.

Experimental Example 8

The following physical properties were evaluated for the heat dissipation sheets according to Example 4 and Examples 10 to 21, and were shown in Tables 6 and 7 below.

1. Evaluation of Heat Radiation Characteristics

Thermal conductivity was calculated using the thermal diffusivity measured using LFA, specific heat measured using DSC, and the density of the heat dissipation sheet.

In addition, after placing the LED at a predetermined interval on the circumference of a circle with a diameter of 25 mm, a thermometer was placed in the center of the circle, and a measuring equipment was manufactured so that a predetermined voltage could be applied to the LED. The measuring equipment was placed in an acrylic chamber of 32 cm×30 cm×30 cm in width, length, and height, respectively, and the temperature inside the acrylic chamber was adjusted to be 25±0.2° C. After placing a heat dissipation sheet on the LED of the measuring equipment, a predetermined input power was applied to the LED, and after a predetermined time had elapsed, a thermal image was taken from the upper portion of the heat dissipation sheet and the temperature of the thermometer in the measuring equipment was measured. Afterwards, the average temperature of the portion of the heat dissipation sheet corresponding to the LED was calculated to show the average temperature through the results of thermal imaging, and the temperature calculated through the thermometer in the measuring equipment was shown as the T.C value, respectively.

In addition, as a standard for evaluating heat radiation performance, the same input power was applied to the LED of the measuring equipment in the absence of the heat dissipation sheet, and after the same time had elapsed, thermal imaging was taken from the upper portion of the LED and the temperature of the thermometer in the measuring equipment was measured, and the result was defined as a default value, and the average temperature taken in the thermal image without the heat dissipation sheet was 53.8° C., and the T.C. was 66.3° C.

2. Surface Quality

In order to evaluate the amount of heat dissipation fillers come off from the surface of the heat dissipation sheet, an adhesive sheet of the same size as the manufactured heat dissipation sheet was attached to one surface of the heat dissipation sheet and then removed. The peeled adhesive sheet was divided into 10 horizontally and vertically to partition them into a total of 100 cells, and the number of cells on which the heat dissipation filler was adhered was counted.

3. Flexibility Assessment

For a total of 1000 heat dissipation sheets for each example, it was observed whether cracks or breakage occurred when bent with a curvature of 20 mm, and the number of sheets in which cracks or breakage occurred was counted. Then, based on the number of Example 10 as 100%, the number of broken sheets in the remaining examples was expressed as a relative percentage.