KR20230073728A - A composite material composition of high dielectric constant and low dielectric loss rate - Google Patents

Abstract

The present invention relates to a composite material composition exhibiting high permittivity and low dielectric loss characteristics, which includes liquid crystal polyester as a matrix and expanded graphite as a filler, and optionally includes TiO_2 additionally. According to the present invention, when expanded graphite is added alone or a composite material of a ceramic filler is added to the liquid crystal polyester resin composition, high permittivity of 10.0 or more and low dielectric loss characteristics of 0.1 or less in the 2.5 GHz region are simultaneously achieved.

Description

Composite material composition having high dielectric constant and low dielectric loss rate {A COMPOSITE MATERIAL COMPOSITION OF HIGH DIELECTRIC CONSTANT AND LOW DIELECTRIC LOSS RATE}

The present invention relates to a composite material composition exhibiting high dielectric constant and low dielectric loss characteristics, and further relates to a composite material composition comprising liquid crystal polyester as a matrix, expanded graphite as a filler, and optionally additionally containing TiO ₂ . .

[0002] In the transition period to the advanced information age in which the Internet of Things (IoT) era is approaching, a technology for transmitting very high-capacity data at high speed is required.

Smartphones and tablet PCs, which play a key role in information and communication devices, are evolving further in the direction of miniaturization, light weight, and multifunctionality. In addition, the application of high-speed transmission parts to quickly acquire and control sensor and camera information obtained during driving due to the development of EVs and the expansion of automatic driving technology is expected to increase in the automotive field.

In addition, the high-frequency wideband shift of the transmission signal progresses due to the increase in the amount of transmission information, and according to this change, the demand for high-performance and high-reliability electronic components that can adapt to high-frequency regions such as microwave and millimeter wave is increasing.

On the other hand, due to the multiplexing of electronic devices, interference and noise between radio communication devices and devices are emerging. In order to solve this problem, it is necessary to develop a material that has a permittivity in a certain range in a frequency domain of 1 GHz or more (2.5 GHz) and minimizes communication interference by removing noise of electromagnetic waves.

BACKGROUND OF THE INVENTION LCP (Liquid Crystal Polymer, or liquid crystal polyester) is a material having excellent heat resistance and low dielectric properties, and has been attracting attention as a component material for information and communication devices according to the development of such innovative information and communication technologies.

Specifically, the LCP material is a material with great potential as a polymer material for realizing a high dielectric constant and a low dielectric loss factor. Since LCP material has low dielectric properties, it can be developed as a composite material that can be applied in the high frequency region because it can increase dielectric constant and maintain low dielectric loss rate by adding inorganic filler such as ceramic filler.

On the other hand, conventionally, inorganic fillers or graphite (graphite) having high permittivity such as BaTiO ₃ , TiO ₂ , TiO ₃ , ZnO were mainly used as ceramic fillers. There was a disadvantage that a material satisfying all three functions could not be manufactured, and the development of a new composite material to overcome these disadvantages is required.

An object of the present invention is to provide a composite material that satisfies high dielectric constant and low dielectric loss characteristics in a high frequency signal region of 2.5 GHz or higher.

In one embodiment of the present invention to achieve the above object, in the composite material composition composed of a mixture of a matrix and a filler, the matrix includes a liquid crystal polyester, the filler is expanded graphite or expanded graphite and TiO ₂ Of Including the mixture, it provides a composite material composition having a high dielectric constant of 10.0 or more and a low dielectric loss factor of 0.1 or less at 2.5 GHz.

As another embodiment of the present invention, when expanded graphite is used alone as the filler, the present invention provides a composite material composition characterized in that the expanded graphite is included in 7 to 10wt% based on the weight of the total composite material.

As another embodiment of the present invention, when a mixture of expanded graphite and TiO ₂ is used as the filler, the weight ratio of the expanded graphite and TiO ₂ is 1: 4 to 1: 80, characterized in that the composite material composition to provide.

As another embodiment of the present invention, the present invention provides a composite material composition, characterized in that the expanded graphite is included in 0.5 to 8wt% based on the weight of the total composite material.

As another embodiment of the present invention, the total weight of the filler including the expanded graphite and TiO ₂ is less than 50wt% relative to the weight of the total composite material, characterized in that it provides a composite material composition.

As another embodiment of the invention, the filler provides a composite material composition characterized in that it further comprises at least one material selected from the group consisting of glass bubbles, silica, titanium oxide, talc and calcium carbonate.

As another embodiment of the present invention, a composite material composition is provided in which the expanded graphite has a diameter of 50 μm or less.

The composite material composition according to the present invention may exhibit high dielectric constant and low dielectric loss characteristics by applying expanded graphite as a filler alone or in combination with a ceramic filler containing TiO ₂ based on a liquid crystal polyester resin.

When expanded graphite is applied as a filler, it was confirmed that the dielectric constant is greatly increased compared to general graphite, and it is possible to realize a high dielectric constant with a smaller filling amount than when TiO ₂ is used alone, which is effective in improving mechanical properties and reducing weight.

1 is a view showing the surface of a composite material composition according to an embodiment of the present invention

Hereinafter, the present invention will be described in detail using drawings so that those skilled in the art can practice it. However, the content described below is not limited to this as an embodiment of the present invention, and changes, combinations, etc. are possible according to circumstances.

The composite material composition of the present invention may use a liquid crystal polyester resin as a matrix, and may be formed in a form containing a filler based on the liquid crystal polyester resin.

In the case of the liquid crystal polyester, due to its excellent physical properties such as high strength and high elongation, it is suitable for use as a base composition for a resin exhibiting high dielectric constant and low dielectric loss characteristics.

The liquid crystal polyester resin may be prepared by stirring the raw material monomers, injecting a reactive solvent thereto to proceed with an acetylation reaction, and then raising the temperature to proceed with an esterification polycondensation reaction, followed by solid phase polycondensation.

At this time, it is preferable that the raw material monomers include hydroxybenzoic acid, biphenol, terephthalic acid and isophthalic acid, and in preparing the liquid crystal polyester resin, 1.08 to 1.12 equivalents of acetic anhydride are added relative to the total raw material monomers. It is most preferable to proceed with the manufacturing process.

When the amount of acetic anhydride is less than 1.08 equivalent, the degree of polymerization cannot be sufficiently increased in the solid-state polymerization process, and thus the physical properties of the prepared resin cannot be sufficiently increased. On the other hand, if the amount of acetic anhydride exceeds 1.12 equivalents, the polymerization reaction occurs too quickly, making it difficult to control the polymerization rate, and acetic anhydride may remain on the surface of the resin to deteriorate overall physical properties, which is not preferable.

When the liquid crystal polyester resin is manufactured through the above method, it is possible to manufacture a resin having very excellent physical properties such as high strength and elongation. Therefore, when the resin is used as a matrix, it is suitable for use as an electronic component because it can maintain relatively excellent physical properties even when materials such as inorganic fillers for adding high dielectric properties are mixed.

Meanwhile, the dielectric constant may be improved by mixing a filler with the above matrix.

Specifically, in order to be used as an electronic component, the dielectric constant (Dk) is preferably 10.0 or more at 2.5 GHz.

In addition, the dielectric loss factor (Df) is preferably adjusted to 0.1 or less at 2.5 GHz.

If the dielectric constant is lower than the above range or the dielectric loss factor is high, communication interference may occur, and thus there is a limit to its use as an electronic component.

In order to improve the dielectric constant as described above, inorganic fillers having a high dielectric constant, such as BaTiO ₃ , TiO ₂ , TiO ₃ , ZnO, or the like, have been used as ceramic fillers, or natural graphite has been mainly used.

On the other hand, in the case of such an inorganic filler, there is a problem in that the increase in dielectric constant is low, so in order to increase the dielectric constant to 10.0 or more at 2.5 GHz, the amount of the inorganic filler relative to the weight of the total resin must be filled with a high amount of more than 50 wt%. As the amount of the inorganic filler filled as above increased, the content of the base resin decreased, resulting in a problem of lowering the mechanical properties of the composite material.

Specifically, when the strength of the material is less than 8 kgfcm / cm, damage occurs even with a small impact, making it difficult to practically use it as an electronic component.

In addition, when TiO ₂ is used, the specific gravity increases as the filling amount of the high specific gravity filler compared to the resin increases, so there is a disadvantage that the purpose of using a plastic material that replaces metal for the purpose of weight reduction is also far away.

On the other hand, when natural graphite was added, the dielectric constant increased significantly compared to inorganic fillers such as TiO ₂ . However, dielectric loss also increases as natural graphite is introduced, so there is a problem in that data loss increases when applied to a part that accepts the same frequency as an antenna.

Specifically, in order to increase the dielectric constant to 10.0 or more at 2.5 GHz by using natural graphite as a filler, the amount of natural graphite should be filled in an amount of more than 10 wt% relative to the weight of the total resin. At 2.5GHz, the dielectric loss factor is higher than 0.1. On the other hand, in order to adjust the dielectric loss factor to 0.1 or less at 2.5 GHz, the amount of natural graphite relative to the weight of the total resin should be adjusted to less than 10 wt%. When the content is adjusted as above, the dielectric constant is less than 10.0 at 2.5 GHz .

That is, it is difficult to manufacture a material that satisfies both characteristics of high permittivity and low dielectric loss through natural graphite.

Accordingly, in the present invention, expanded graphite was used as a filler mixed with the matrix material.

In the case of the expanded graphite, the dielectric constant compared to the same content is higher than that of natural graphite, and the dielectric loss is similar. That is, when using the expanded graphite, a desired high permittivity can be achieved even with a small amount compared to natural graphite, and the dielectric constant can be adjusted to 10.0 or more while adjusting the dielectric loss factor to 0.1 or less at 2.5 GHz. In addition, when using the expanded graphite, it was possible to achieve an excellent impact strength of 8 kgfcm / cm or more, more preferably 12 kgfcm / cm or more, which could not be achieved through TiO ₂ , and the purpose of developing a lightweight material by reducing specific gravity was able to achieve

A method for producing the expanded graphite is as follows.

First, chemical agents such as sulfuric acid and nitric acid are electrolytically treated between layers of graphite, or chemicals such as sulfuric acid and nitric acid are inserted in the presence of an oxidizing agent to prepare an interlayer compound.

When the interlayer compound is reduced, the physical properties of the compound are different from those of conventional graphite, and when the compound is heat-treated again, the graphite molecules expand tens to hundreds of times in the c-axis direction. Expanded graphite can be manufactured through the above process.

The expanded graphite manufacturing method may include oxidizing graphite using SO ₃ gas generated from fuming sulfuric acid or sulfuric anhydride, and heat treatment in the interlayer compound manufacturing process of the above process is performed within a range of 400 to 800 ° C. It is desirable to

When heat treatment is performed at less than 400 ° C, it is undesirable because sufficient degree of expansion cannot be obtained. It is most preferable to

In particular, in the case of the expanded graphite used as the filler of the present invention, it is preferable to use a material having a diameter of 50 μm or less, more preferably 45 μm or less, and a surface area of 20 m ² /g, more preferably 25 m ² /g It is preferable to control more than that.

In the case of dielectric constant (Dk)/dielectric loss factor (Df), it is affected by the size and surface area of the added filler, so to achieve the excellent Dk/Df ratio to be achieved in the present invention, the size and surface area of the particles as described above It is desirable to adjust

In addition, the density is preferably adjusted to 0.1 to 0.4 g/cm ² so that mixing with the LCP resin can proceed properly. If the density is too low, mixing may be difficult, and on the contrary, if the density is too high, the amount of expanded graphite added per unit weight is reduced, resulting in poor economic feasibility.

Additionally, for the stability of the overall material, the tensile strength of the expanded graphite is 90 to 110 MPa, the tensile elongation is 7500 to 8500 MPa, and the impact strength is most preferably adjusted to 30 to 35 KJ/m ² .

In the case of the expanded graphite adjusted as described above, the permittivity is 20 to 25 under the same content conditions, and may have a permittivity about 1.5 to 2 times higher than that of general natural graphite showing a permittivity of 12 to 16.

As the filler of the present invention, when the expanded graphite is used alone, it is preferable to add the expanded graphite in an amount of 5 to 10 wt% based on the total weight of the resin. When the expanded graphite is added less than the above range, a sufficient high permittivity cannot be obtained, and when the expanded graphite is added above the above range, the dielectric loss factor is excessively high like conventional natural graphite, which is not preferable.

Meanwhile, as a filler, the expanded graphite may be used alone or in combination with expanded graphite and TiO ₂ .

Compared to adding expanded graphite alone, when TiO ₂ , which is cheaper than expanded graphite, is mixed, the cost of the filler can be reduced, and the resin can be manufactured more economically.

On the other hand, when using a mixture of expanded graphite and TiO ₂ , it is preferable to add expanded graphite in an amount of 0.5 wt% to 8 wt% relative to the total resin, and the ratio of expanded graphite and TiO ₂ is 1: 4 to 1: 80, more preferably 1:10 to 1:60. When TiO ₂ is contained higher than the above ratio, the overall physical properties of the material may be deteriorated, and on the contrary, when TiO ₂ is contained below the above ratio, a problem that it is difficult to expect an additional dielectric constant improvement effect due to the addition of TiO ₂ actually occurs. can

In addition, when the total filler content was added in combination of 50 wt% or more, the composite material had a high specific gravity, resulting in poor economic feasibility, and low impact strength, resulting in weakened physical properties. There is a problem of lack of space for adding a reinforcing filler to reinforce the bar, the total filler content including the expanded graphite and TiO ₂ is preferably adjusted to less than 50wt%.

Meanwhile, the filler may further include at least one material selected from the group consisting of glass bubbles, silica, titanium oxide, talc, and calcium carbonate.

Hereinafter, specific examples are presented to aid understanding of the present invention. However, the following examples are only provided to more easily understand the present invention, and the content of the present invention is not limited by the examples.

Preparation Example: Preparation of liquid crystal polyester resin

13,000 g (127.3 mol) of acetic anhydride was added to a batch reactor with a capacity of 200 L, and 20,000 g (144.8 mol) of monomeric parahydroxy benzoic acid (HBA), 9,000 g (48.3 mol) of biphenol (BP), terephthalic acid ( After adding 5,400 g (32.6 mol) of TPA) and 2,600 g (15.7 mol) of isophthalic acid (IPA), 14,100 g (138.1 mol) of acetic anhydride was further added to ensure good mixing in the batch reactor. Thereafter, 2.8 g of potassium acetate catalyst and 11.2 g of magnesium acetate catalyst were added, and nitrogen was injected to make the interior space of the reactor inactive. Thereafter, the temperature of the reactor was raised to a temperature at which acetic anhydride inside the batch reactor was refluxed over 1 hour, and the hydroxy groups of the monomers were acetylated at this temperature for 2 hours. Acetic acid generated in the acetylation reaction and excessively added unreacted acetic anhydride were removed while raising the temperature to 320 ° C at a rate of 0.5 ° C to produce liquid crystal polyester, discharged through the lower valve, cooled and solidified, and first pulverized to 32,000 got g. Thereafter, the mixture was subjected to secondary pulverization using a fine mill, put into a rotary heating device, and heated to 200° C. for 2 hours while flowing nitrogen at a flow rate of 25 L/min. After maintaining at this temperature for 2 hours, the temperature was raised to 285 °C at a rate of 0.2 °C and maintained for 3 hours to conduct a solid-state polymerization reaction, through which a liquid crystal polyester resin was obtained. The melting point of the prepared resin was 330°C.

<Example 1: Manufacture of high dielectric composite material containing 10wt% of expanded graphite as a filler>

10wt% of expanded graphite having an average particle size of 45㎛ was added as a filler to the liquid crystal polyester resin prepared in the previous preparation example, mixed for 30 minutes with an automatic mixer (Jil Industrial Equipment, JITD-50KW), and hot air dryer (Jeil Industrial Equipment , JIB-100KW) after drying at 150 ° C for 2 hours, melt kneading was performed at a rotational speed of 350 rpm at a barrel temperature of 340 ° C through a twin screw extruder. In addition, during the melt-kneading, a vacuum was applied to the twin-screw extruder to remove gas and by-products generated during the melt-kneading at a high temperature.

The composite material in the form of a pellet obtained through the melt kneading was injection molded into a rectangular specimen with a thickness of 2 mm, and then the dielectric constant and dielectric loss factor were measured through a dielectric constant evaluation device (SPDR) of 2.5 GHz, and the impact strength was measured by the ASTM D256 method. , The specific gravity was measured according to the ASTM D792 method.

<Example 2: Manufacture of high-permittivity composite material containing 5 wt% of expanded graphite as a filler>

A composite material in the form of a pellet was prepared in the same configuration as in Example 1, except for adding 5 wt% of expanded graphite as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Example 3: Manufacture of high dielectric composite material using expanded graphite and TiO ₂ as a filler>

A composite material in the form of a pellet was prepared in the same configuration as in Example 1, except that 0.5 wt% of expanded graphite and 30 wt% of TiO ₂ were added together as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 1: Manufacture of high dielectric composite material using 13wt% of expanded graphite>

A composite material in pellet form was prepared in the same configuration as in Example 1, except for adding 13 wt% of expanded graphite as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 2: Manufacture of a composite material using 2 wt% of expanded graphite

A composite material in the form of a pellet was prepared in the same configuration as in Example 1, except for adding 2 wt% of expanded graphite as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 3: Manufacture of high dielectric composite material using only TiO ₂ as a filler>

A composite material in pellet form was prepared in the same configuration as in Example 1, except for adding 60 wt% of TiO ₂ as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 4: Manufacture of high dielectric composite material using only TiO ₂ as a filler>

A composite material in pellet form was prepared in the same configuration as in Example 1, except for adding 40 wt% of TiO ₂ as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 5: Preparation of a high dielectric composite material using only 10 wt% of natural graphite as a filler

A composite material in the form of a pellet was prepared in the same configuration as in Example 1, except for adding 10 wt% of natural graphite as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

<Comparative Example 6: Preparation of a high dielectric composite material using only 15 wt% of natural graphite as a filler

A composite material in the form of a pellet was prepared in the same configuration as in Example 1, except for adding 15 wt% of natural graphite as a filler.

After injection molding the pellet-type composite material in the form of a rectangular specimen with a thickness of 2 mm, the dielectric constant and dielectric loss factor were measured through a 2.5 GHz dielectric constant evaluation device (SPDR), and the impact strength was measured by the ASTM D256 method and the ASTM D792 method. Specific gravity was measured.

Table 1 summarizes the composition of each experimental example.

LCP resin (wt%) Expanded graphite (wt%) TiO ₂ (wt%) Natural graphite (wt%) Example 1 90 10 - - Example 2 95 5 - - Example 3 69.5 0.5 30 - Comparative Example 1 87 13 - - Comparative Example 2 98 2 - - Comparative Example 3 40 - 60 - Comparative Example 4 60 - 40 - Comparative Example 5 90 - - 10 Comparative Example 6 85 - - 15

Table 2 summarizes the results of measuring dielectric constant (Dk), dielectric loss factor (Df), impact strength and specific gravity of the composite resin composition prepared based on each experimental example and comparative example.

Permittivity (Dk) Dielectric loss factor (Df) Impact strength (kgfcm/cm) importance Example 1 17 0.09 12.7 1.48 Example 2 11 0.009 13.4 1.53 Example 3 12 0.08 9.2 1.91 Comparative Example 1 23 0.15 12.5 1.45 Comparative Example 2 6 0.005 13.8 1.55 Comparative Example 3 11 0.06 5.7 2.25 Comparative Example 4 8 0.05 8.6 1.95 Comparative Example 5 8 0.09 12.7 1.57 Comparative Example 6 11 0.14 10.6 1.53

The experimental results of Table 2 are summarized as follows.

In the case of general graphite, the increase in dielectric constant is higher than that of TiO ₂ , but the dielectric loss also increases, so it is not suitable as a material for antennas. On the other hand, it was confirmed that when expanded graphite was applied, the permittivity was greatly increased compared to natural graphite. In this way, it was confirmed that the expanded graphite can realize a high permittivity with a smaller filling amount than TiO ₂ and is effective in improving mechanical properties and reducing weight.

Specifically, expanded graphite exhibits the same dielectric loss compared to natural graphite for the same content, but its dielectric constant is 1.5 times higher. Therefore, the dielectric constant can be adjusted to have a dielectric loss factor of less than 0.1 while increasing. Specifically, when expanded graphite is added in the range of 5 to 10 wt% relative to the weight of the total resin, the dielectric constant is 10 or more in the 2.5GHz region and the dielectric constant is less than 0.1. A composite material having a loss rate can be manufactured.

On the other hand, when the expanded graphite is applied as a hybrid with TiO ₂ , it compensates for the low permittivity of TiO ₂ and shows a permittivity of 10 or more in the 2.5GHz region and also shows a low dielectric loss, so it remains to be seen that it is possible to develop a material suitable for use for antennas. Could know. Specifically, when applied as a hybrid with expanded graphite and TiO ₂ in this way, by adding expanded graphite at 0.5 to 8 wt% and adjusting the weight ratio of the expanded graphite and TiO 2 to 1: 4 to 1: 80, A composite material satisfying the same permittivity and dielectric loss ratio can be manufactured.

On the other hand, when the total filler content was added in combination of 50 wt% or more, the composite material had a high specific gravity, resulting in poor economic feasibility, and low impact strength, resulting in weakening physical properties. There was a problem that there was not enough space to add a reinforcing filler to reinforce the.

Therefore, in the present invention, the total filler content is adjusted to less than 50 wt% using expanded graphite to prepare a composite material having excellent physical properties with an impact strength of 8 kgfcm / cm or more and satisfying both high dielectric constant and low dielectric loss rate.

Claims (7)

In the composite material composition composed of a mixture of a matrix and a filler,
The matrix includes liquid crystal polyester,
The filler includes expanded graphite or a mixture of expanded graphite and TiO ₂ , and has a high dielectric constant of 10.0 or more and a low dielectric loss factor of 0.1 or less at 2.5 GHz, a composite material composition.
According to claim 1,
When expanded graphite is used alone as the filler, the expanded graphite is a composite material composition characterized in that it is included in 5 to 10 wt% based on the weight of the total composite material
According to claim 1,
As the filler, when a mixture of expanded graphite and TiO ₂ is used, the weight ratio of the expanded graphite and TiO ₂ is 1: 4 to 1: 80 Composite material composition
According to claim 3,
The expanded graphite is a composite material composition, characterized in that contained in 0.5 to 8wt% relative to the weight of the total composite material
According to claim 3,
Composite material composition, characterized in that the total weight of the filler containing the expanded graphite and TiO ₂ is less than 50wt% relative to the weight of the total composite material
According to claim 1,
The filler is a composite material composition characterized in that it further comprises at least one material selected from the group consisting of glass bubbles, silica, titanium oxide, talc and calcium carbonate
According to claim 1,
Composite material composition, characterized in that the diameter of the expanded graphite is 50 μm or less

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