KR20160124354A - Polymer films having dispersed vermiculite fine particles and Methods for preparing the same - Google Patents

Abstract

The present invention provides a fire retardant polymer film in which fine vermiculite particles having a fire retardant feature, a flexible feature, and a porous feature are dispersed, an isolation film for a lithium ion electrode including the same, and a manufacturing method of the retardant polymer film. According to the present invention, the polymer film has excellent fire prevention performance to endure at even a high temperature of 400C or higher for several minutes so that the polymer film can be a battery isolation film to prevent explosion or fire of a lithium ion battery. Furthermore, according to the present invention, the fire retardant polymer film has flexibility to be applied to flexible lithium ion battery development.

Description

TECHNICAL FIELD [0001] The present invention relates to a flame-retardant polymer film in which vermiculite fine particles are dispersed and a method for preparing the same.

The present invention relates to a flame retardant polymer film in which vermiculite fine particles are dispersed, a separator for a lithium ion battery comprising the same, and a process for producing the flame retardant polymer film.

Batteries are used in a variety of electrical devices, from cell phones and laptops to electric cars. To obtain higher power, a battery may be stacked to form a battery pack that typically houses a plurality of cells connected in series or in parallel from several batteries.

The temperature of the battery may rise during charging and discharging. If the heat can not be sufficiently dissipated from the battery, the battery may cause a fire or explode. The risk of ignition or explosion is higher when the battery is damaged or deformed. This problem is particularly relevant to the field of electric vehicles in which high power lithium ion batteries are currently used.

In a lithium ion battery, an electrolyte exists between the electrodes, and the electrodes are separated from each other by a separator through which the ions can pass. It is typically desirable to increase the active surface area of the electrode by reducing the particle size of the material, i. However, it has been noted that when the cell is being used, the particles begin to agglomerate, i. E., The particle size begins to grow. In the worst case, this can lead to breakage of the separator and a short circuit in the cell, and such a situation may occur even when the impurity metal particles occasionally present in the cell break the separator.

After an in-cell short circuit, internal overpressure and explosion in the cell may occur due to ignition of the organic carbonate used as the electrolyte and rapid rise of the temperature.

Currently commercially available battery membranes are known to cause heat shrinkage upon exposure to 150 ° C for 1 hour [ ^1] . DSC thermograms analysis also showed that the shutdown temperatures of the polypropylene and polyethylene separators were 165 ° C and 135 ° C, respectively ² . Therefore, the two separators do not have thermal stability at 200 ° C or higher. Accordingly, if the battery separator can withstand a temperature of 400 DEG C or more for several minutes, it is very promising as a lithium ion battery separator material.

Numerous papers and patent documents are referenced and cited throughout this specification. The disclosures of the cited papers and patent documents are incorporated herein by reference in their entirety to better understand the state of the art to which the present invention pertains and the content of the present invention.

 Cho, Tae-Hyung, et al. Journal of Power Sources 181.1 (2008): 155-160.  Venugopal, Ganesh, et al. Journal of Power Sources 77.1 (1999): 34-41.

The present inventors have made efforts to develop a novel material which can be used not only as a separator for a lithium ion battery but also has a flame retardant performance capable of withstanding at a high temperature (200 ° C or higher). As a result, the present invention has been accomplished by preparing a polymer film having flame retardance, flexibility and porosity characteristics through a simple method of mixing photo-curing after mixing vermiculite fine particles, photopolymerizable polymer and photoinitiator into a solvent.

Accordingly, an object of the present invention is to provide a flame-retardant polymer film.

Another object of the present invention is to provide a separator for a lithium ion battery.

Another object of the present invention is to provide a method for producing a flame-retardant polymer film.

Other objects and advantages of the present invention will become more apparent from the following detailed description of the invention, claims and drawings.

According to one aspect of the present invention, the present invention provides a polymer film formed by a photopolymerizable polymer; And a flame-retardant polymer film comprising vermiculite fine particles dispersed in the polymer film.

The present inventors have made efforts to develop a novel material which can be used not only as a separator for a lithium ion battery but also has a flame retardant performance capable of withstanding at a high temperature (200 ° C or higher). As a result, a polymer film having flame retardance, flexibility and porosity characteristics was prepared by mixing a vermiculite fine particle, a photopolymerizable polymer and a photoinitiator in a solvent and then inducing photo - curing.

As used herein, the term "flame retardant" means the property of a material that is difficult to burn to fire, and the flame retardancy can be expressed in terms of "flame retardancy "," fire resistance ", and "heat resistance ".

The polymer film can be produced through photo-curing. For example, a polymer film can be prepared by photo-curing a mixture containing a photopolymerizable polymer (photo-crosslinkable polymer), a vermiculite fine particle, a solvent and a photoinitiator by irradiation with light (for example, UV).

For photocuring, cationic photoinitiators or radical photoinitiators may be used. For example, Darocure 1173, Darocure 4265, Darocure 1664; Cyracure UVI-6990 from Union Carbide, Cyracure UVI-6974; Degusa's Degacure; Asahi Denka's SP-55, SP-150, SP-170; Photoinitiators such as Irgacure 261, Irgacure 184, Irgacure 819, Irgacure 907 and Irgacure 2959 of Ciba-Geigy can be used.

According to an embodiment of the present invention, the weight ratio of photopolymerizable polymer: vermiculite fine particles in the mixture is 1: 1-3.5.

In one specific example, the weight ratio of photopolymerizable polymer: vermiculite microparticles in the mixture is 1: 2.5-3.5.

According to another embodiment of the present invention, the weight ratio of the photopolymerizable polymer: vermiculite fine particle: solvent: photoinitiator in the mixture is 1: 1-3.5: 1-2: 0.1-0.5.

In one specific example, the weight ratio of photopolymerizable polymer: vermiculite fine particle: solvent: photoinitiator in the mixture is 1: 2.5-3.5: 1.1-1.5: 0.1-0.3.

In one specific example, the weight ratio of photopolymerizable polymer: vermiculite microparticles in the mixture is 1: 3.0-3.5, 1: 3.1-3.5, or 1: 3.2-3.5.

According to an embodiment of the present invention, the photopolymerizable polymer is poly (ethylene glycol) dimethacrylate (PEGDMA).

According to an embodiment of the present invention, the solvent is an alcohol solvent. For example, ethanol, isopropyl alcohol or methanol may be used in the present invention.

According to one embodiment of the present invention, the solvent is a volatile solvent such as acetone.

According to one embodiment of the present invention, the vermiculite fine particles are obtained by grinding the vermiculite and sieving the vermiculite. For example, the diameter of the vermiculite fine particles obtained by sieving is 45 占 퐉 or less.

According to the present invention, a flame-retardant polymer film having various thicknesses can be obtained through the above-described photo-curing. For example, the thickness of the flame-retardant polymer film is 20-200 m, 20-190 m, 20-180 m, 20-170 m, 20-160 m, 20-150 m, or 20-140 m.

According to one embodiment of the present invention, the flame-retardant polymer film has a porous structure (see FIG. 10). The porous structure provided in the flame-retardant polymer film can pass through the lithium ion, and the flame-retardant polymer film of the present invention can be developed as a separator for a lithium ion battery.

In addition, the flame-retardant polymer film of the present invention can be applied to the development of fireproof bags, fireproof tapes, wrist flame retardant films, mobile phone fire retardant films, carpet protective films, film fire retardants, and flame retardant adhesive tapes.

According to another aspect of the present invention, there is provided a separation membrane for a lithium ion battery to which the flame retardant polymer film is applied.

According to another aspect of the present invention, there is provided a lithium ion battery including the separator.

The polymer film of the present invention has excellent fire resistance capable of withstanding more than a few minutes at a high temperature of 400 DEG C or more, so that the explosion of the lithium ion battery And as a battery separator for the purpose of preventing fire. Furthermore, since the flame-retardant polymer film of the present invention has flexibility, it can be applied to the development of a flexible lithium ion battery.

The lithium ion battery of the present invention includes a separation membrane to which a flame-retardant polymer film is applied, two electrodes (+, - electrode) and an electrolytic solution. The electrolytic solution is a lithium salt _{(e.g., LiPF 6, LiBF 4, LiSbF} 6, LiAsF 6, LiN (SO 3 C 2 F 5) 2, LiC 4 F 9 SO 3, LiClO 4, LiAlO 2, LiAlCl 4, LiCl , etc.) And organic solvents.

The organic solvent includes, for example, phosphates such as lithium hexafluorophosphate; Cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; Chain carbonates such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate; Esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and? -Butyrolactone; Ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane and 2-methyltetrahydrofuran; Nitriles such as acetonitrile; Dimethylformamide, and the like.

According to another aspect of the present invention, the present invention provides a method for producing a flame-retardant polymer film, comprising the steps of:

(a) mixing a photopolymerizable polymer, a vermiculite fine particle and a photoinitiator in a solvent; And

(b) irradiating the mixture of step (a) with light to induce polymerization reaction to form a polymer film in which vermiculite fine particles are dispersed.

Since the production method of the present invention relates to a method for producing the above-mentioned flame retardant polymer film, the common content between the two inventions is omitted in order to avoid the excessive complexity of the present specification.

In step (a), a photopolymerizable polymer, a particulate microparticle and a photoinitiator are added to the solvent to obtain a mixture.

According to an embodiment of the present invention, in the step (a), the weight ratio of photopolymerizable polymer: vermiculite fine particles is 1: 1-3.5.

In one particular example, the weight ratio of photopolymerizable polymer: vermiculite microparticles in step (a) is 1: 2.5-3.5.

According to another embodiment of the present invention, in the step (a), the weight ratio of the photopolymerizable polymer: vermiculite fine particle: solvent: photoinitiator is 1: 1-3.5: 1-2: 0.1-0.5.

In one specific example, the weight ratio of photopolymerizable polymer: vermiculite fine particle: solvent: photoinitiator in step (a) is 1: 2.5-3.5: 1.1-1.5: 0.1-0.3.

In one specific example, the weight ratio of photopolymerizable polymer: vermiculite microparticles in the mixture is 1: 3.0-3.5, 1: 3.1-3.5, or 1: 3.2-3.5.

Thereafter, in step (b), the mixture is irradiated with light (for example, UV) to induce photo-curing, thereby forming a polymer film in which vermiculite fine particles are dispersed (see FIG.

The features and advantages of the present invention are summarized as follows:

(I) The present invention provides a flame retardant polymer film in which vermiculite fine particles having flame retardance, flexibility and porosity are dispersed, a separator for a lithium ion battery comprising the same, and a process for producing the flame retardant polymer film.

(Ii) Since the polymer film of the present invention has an excellent fire resistance ability to withstand more than several minutes even at a high temperature of 400 ° C or higher, it can be utilized as a battery separator for the purpose of explosion and fire prevention of a lithium ion battery. Further, Since the polymer film has flexibility, it can be applied to the development of a flexible lithium ion battery.

(Iii) Using the method of the present invention, a flame-retardant polymer film can be produced within a short time (about 10 minutes).

Figure 1 shows the grinding (a) of bulk vermiculite, the resulting product (c), and the bulk vermiculite sample (b).
Fig. 2 shows a process (a) for sieving vermiculite particles and the resulting product (b).
Figure 3 shows a vermiculite / PEGDMA film.
Fig. 4 shows a vermiculite / PEGDMA film sample having different thicknesses used for fire resistance evaluation.
Fig. 5 shows the result of measuring the thickness of the produced vermiculite / PEGDMA film.
Figure 6 shows front and back SEM images of three vermiculite / PEGDMA film samples.
Figure 7 shows a SEM image of a cross section of a vermiculite / PEGDMA film sample.
Fig. 8 shows optical microscope images before and after immersing a specimen of vermiculite / PEGDMA film having different thicknesses (20 mu m, 40 mu m, 60 mu m) into the electrolyte solution.
Fig. 9 shows optical microscope images before and after immersing a specimen of vermiculite / PEGDMA film having different thicknesses (80 mu m, 100 mu m, 120 mu m) in an electrolytic solution.
Figure 10 shows the porous structure of a vermiculite / PEGDMA film sample.

Hereinafter, the present invention will be described in more detail with reference to Examples. It is to be understood by those skilled in the art that these embodiments are only for describing the present invention in more detail and that the scope of the present invention is not limited by these embodiments in accordance with the gist of the present invention .

Example

1. Materials and Experimental Methods

Preparation of vermiculite microparticles

The bulk vermiculite (Shinsung Mineral Co., LTD) was grinded in a concave vessel for several hours and then sieved for 2 hours using Daedok Science to prepare vermiculite fine particles (< 45 탆). FIG. 1 shows a grinding process, and FIG. 2 shows a screening process.

Separator film production using various organic solvents and PEGDMA

A vermiculite film was prepared using PEGDMA as a polymer, Darocur 1173 as a photoinitiator, and various organic solvents. Water was used as the first organic solvent, but water was not suitable because it reacted with PEGDMA to form hydrogels that could cause swelling when reacted with water. Next, propylene carbonate was used as a solvent, but since propylene carbonate was very oily, there was a problem that complete evaporation of the solvent was difficult even after 2 days. To overcome these limitations, films were prepared using ethanol (Fig. 3).

Swelling of vermiculite / PEGDMA film in water

When PEGDMA is reacted with water, hydrogels are formed through UV coating with the aid of photoinitiators (San Francisco, et al., Materials Sciences & Applications 3.6 (2012)). In such a hydrogel, water can be contained inside the hydrogel, thereby causing swelling. This is problematic because when the lithium ion battery separator is placed in the electrolyte, swelling should not occur. PEGDMA + vermiculite films were prepared using water and two samples were obtained (each containing 100 mg or 200 mg of vermiculite). The film was cut in a square shape (0.4 cm X 0.4 cm) and immersed in water for one day. The final length and width were measured after one day.

Evaluation of Fire Performance of Vermiculite / PEGDMA Membrane

Fire continuity test was conducted considering that the main reason for using vermiculite as a main component of lithium ion battery separator is fire-retarding characteristic. A lighter and an alcohol lamp were prepared, and an alcohol lamp was lit. The rectangular part (0.2 cm x 0.2 cm) of each sample was fixed with a tweezers. Two types of experiments were performed. In the first experiment, we investigated the effect of film thickness. In the second experiment, we investigated the effect of the concentration of vermiculite in the film. And vermiculite particles smaller than 45 mu m were used. To start the first experiment, samples were prepared as follows. The sample was cut into a small square shape as shown in Fig.

sample # Vermiculite PEGDMA ethanol Darocur 1173 Thickness (㎛) One 200 mg 80 mg 110 mg 10 mg 20 2 200 mg 80 mg 110 mg 10 mg 40 3 200 mg 80 mg 110 mg 10 mg 60 4 200 mg 80 mg 110 mg 10 mg 80 5 200 mg 80 mg 110 mg 10 mg 100 6 200 mg 80 mg 110 mg 10 mg 120 7 200 mg 80 mg 110 mg 10 mg 140

For the second experiment, the following three kinds of samples were prepared.

sample # Vermiculite PEGDMA ethanol Darocur 1173 One 100 mg 80 mg 110 mg 10 mg 2 200 mg 80 mg 110 mg 10 mg 3 260 mg 80 mg 110 mg 10 mg

SEM image for verifying surface properties of vermiculite / PEGDMA film

SEM images of the front and back sides of three vermiculite / PEGDMA samples (all samples containing 260 mg of vermiculite, 80 mg of PEGDMA, 110 mg of ethanol and 10 mg of Darocur 1173; thickness 20-30 μm) were obtained and one vermiculite / PEGDMA sample SEM image was also obtained for the cross section of SEM analysis was carried out to confirm the surface morphology of the sample and the formation of pores. When the vermiculite / PEGDMA separator is placed on the actual battery electrolyte, the surface shape of the front and back surfaces must be similar so that proper movement of lithium ions occurs.

Optical microscope images for verifying the porous structure and surface morphology of vermiculite / PEGDMA films

A vermiculite / PEGDMA film sample (260 mg of vermiculite, 80 mg of PEGDMA, 110 mg of ethanol and 10 mg of Darocur 1173) having different thicknesses (20 탆, 40 탆, 60 탆, 80 탆, 100 탆 and 120 탆) . Microscopic images of each sample were obtained before and after immersion in an electrolytic solution (propylene carbonate), and whether or not the surface shape was deformed and whether the film (20 μm) had a porous structure was observed. Microscopic images of the samples before and after immersion in the electrolyte were observed at X5 and X10 magnifications and X20 magnification for porous structure confirmation.

2. Experimental results

The thickness of vermiculite / PEGDMA film

As shown in Table 3, a vermiculite / PEGDMA film having different concentrations of vermiculite was prepared.

Furtherance # Vermiculite PEGDMA ethanol Darocur 1173 One 100 mg 80 mg 110 mg 10 mg 2 200 mg 80 mg 110 mg 10 mg 3 260 mg 80 mg 110 mg 10 mg

For the compositions 1-3 of Table 3, the film thickness was measured using a Vernier Calliper. A film of 20-30 탆 thick was successfully prepared (Fig. 5). The thickness of 0.02 mm (20 탆) falls within the appropriate thickness range of the lithium ion battery separator.

Swelling phenomenon of vermiculite / PEGDMA film prepared by using water as solvent

Two kinds of samples were prepared as shown in Table 4 according to the above-mentioned method using water as a solvent.

sample # Vermiculite PEGDMA water Darocur 1173 One 100 mg 80 mg 110 mg 10 mg 2 200 mg 80 mg 110 mg 10 mg

Each sample was cut into square shapes (0.4 cm x 0.4 cm) and immersed in water for one day. The final length and width of the film were as shown in Table 5.

sample # Initial width First length Final width Final length One 0.4 cm 0.4 cm 0.5 cm 0.6 cm 2 0.4 cm 0.4 cm 0.5 cm 0.6 cm

Result of fire continuity test (evaluation of fire performance)

Concentration side of vermiculite

Three kinds of vermiculite / PEGDMA films were prepared as shown in Table 6.

sample # Vermiculite PEGDMA ethanol Darocur 1173 One 100 mg 80 mg 110 mg 10 mg 2 200 mg 80 mg 110 mg 10 mg 3 260 mg 80 mg 110 mg 10 mg

As a fire-insulation test, an alcohol lamp was lit and the rectangular portion (0.2 cm x 0.2 cm) of each sample was fixed with a tweezers. The time that the sample lasted before fully burned is shown in Table 7, and a commercially available Celgard separator was used as a control.

sample # Control group Sample 1 Sample 2 Sample 3 Duration (seconds) <1 One 3 > 120

As shown in Table 7, Sample 3 exhibited characteristics as a lithium ion battery membrane-like film that could withstand more than two minutes of fire. The control Celgard membrane was not able to withstand a second.

Thickness side of film

For samples 2 and 3, further evaluation of fire performance was performed in terms of film thickness. For Sample 2, each film (20 탆, 40 탆, 60 탆, 80 탆, 100 탆, 120 탆, 140 탆) was produced with a very small size (about 0.2 cm x 0.2 cm).

An alcohol lamp was lit and the time to stand until each sample film was separated into separate parts by fire was measured. The same experiment 3 was conducted on the sample 3, and the results are shown in Table 8.

sample # Thickness (㎛) Time to survive decomposition (seconds) 2 20 3 2 40 7 2 60 13 2 80 14 2 100 32 2 120 > 120 2 140 > 120 3 20 > 120 3 40 > 120 3 60 103 3 80 > 120 3 100 > 120 3 120 > 120 3 140 > 120

In case of sample 2, the fire duration time exceeded 2 minutes when the thickness was 120 μm or more. In the case of sample 3, the sample was resistant to fire for 20 minutes at a thickness of 20 μm, which is the standard cell separator.

SEM image of vermiculite / PEGDMA film

SEM images of the front (left) and back (right) sides of the three samples are shown in FIG. As shown in Fig. 6, the surface shapes of the front and back surfaces of the three samples were similar. Fig. 7 shows an SEM image of the cross section of the sample.

From the SEM image, it was found that the thickness of the film was 28-29 ㎛, which was slightly thicker than the thickness of the standard lithium ion battery membrane (25.4 ㎛). The interesting point is that there was a porous structure at the top of the film surface and there was some cracking between the vermiculite particles in the cross-section of the film, which would indicate that if the vermiculite / PEGDMA film was applied to actual battery electrolyte, Lithium ions can migrate through the vermiculite / PEGDMA film.

Optical microscope image of vermiculite / PEGDMA film

Microphotograph images of sample films before and after immersing them in an electrolytic solution (thicknesses of 20 mu m, 40 mu m, 60 mu m, 80 mu m, 100 mu m and 120 mu m) are shown in Figs. 8 and 9 (X10 magnification). As shown in Figs. 8 and 9, the surface shape of the basic film did not change before and after immersion in the electrolytic solution. Also, the sieved vermiculite particles were well distributed in the film, which shows that they can exhibit an even fire effect in each part of the film.

Figure 11 shows the porous structure of a vermiculite / PEGDMA film (20 [mu] m). The black dot in FIG. 11 shows a position where a hole exists. The pore size ranged from micrometers to nanometers. The image of Figure 11 shows the presence of a number of voids through which lithium ions can pass through the film. These results show that the vermiculite / PEGDMA film of the present invention can not only pass lyrium ions but also exhibit a fireproof effect that can withstand temperatures higher than those of conventional lithium ion battery membranes.

Evaluation of fire performance of vermiculite / PEGDMA film in two electrolytes

In previous experiments, the fire resistance of the vermiculite / PEGDMA film was measured on the film itself. Therefore, in order to investigate the fire performance in the actual situation, the battery separator was wetted with the electrolytic solution and exposed to fire, and the flame retardant performance was confirmed. Two kinds of electrolytic solutions (propylene carbonate and DEC: EC solution (diethyl carbonate: ethyl carbonate, v / v: 1: 1, LiPF ₆ ) were used. A vermiculite / PEGDMA film having the same composition as Sample 3 was used in the experiment and fire resistance performance was evaluated for various thicknesses of the film. At each time, a small piece of the film (0.2 cm x 0.2 cm) was immersed in the electrolyte solution for 1 hour, and then the film was taken out of the electrolyte solution, and then the test was conducted in an alcohol lamp to measure the time until the film was decomposed by operation. The experimental results are shown in Table 9 (propylene carbonate electrolyte) and 10 (DEC: EC solution). A cell guard membrane was used as a control.

Thickness (㎛) Time (seconds) Control group One 20 14 40 20 60 27 80 29 100 32 120 30

Thickness (㎛) Time (seconds) Control group <2 20 69 40 107 60 > 120 80 > 120 100 > 120 120 > 120

Due to the role of LiPF ₆ with adiabatic properties, the flame retardancy of the vermiculite / PEGDMA film was better in the DEC: EC electrolyte than in the propylene carbonate electrolyte. The vermiculite / PEGDMA film showed the ability to retard fire explosion for at least one minute. LiPF ₆ to without determine whether the fire performance of the vermiculite / PEGDMA loaded with oil, the same experiment was conducted without LiPF _{6, and} the results are shown in Table 11.

Thickness (㎛) Duration (seconds) Control group One 20 61 40 66 60 74 80 85 100 90 120 104

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the same is by way of illustration and example only and is not to be construed as limiting the scope of the present invention. Accordingly, the actual scope of the present invention will be defined by the appended claims and their equivalents.

Claims (12)

A polymer film formed by a photopolymerizable polymer; And
Wherein the flame retardant polymer film comprises vermiculite fine particles dispersed in the polymer film. The flame-retardant polymer film according to claim 1, wherein the polymer film is formed by irradiating light to a mixture containing a photopolymerizable polymer, a vermiculite fine particle, a solvent, and a photoinitiator. The flame-retardant polymer film according to claim 2, wherein the weight ratio of the photopolymerizable polymer: vermiculite fine particles in the mixture is 1: 1-3.5. The flame-retardant polymer film according to claim 2, wherein the solvent is an alcohol solvent. The flame-retardant polymer film according to claim 1, wherein the photopolymerizable polymer is poly (ethylene glycol) dimethacrylate. The flame-retardant polymer film according to claim 1, wherein the vermiculite fine particles are fine particles obtained by grinding and then sieving a vermiculite. The flame-retardant polymer film according to claim 1, wherein the flame-retardant polymer film has a thickness of 20 to 200 탆. The flame-retardant polymer film according to claim 1, wherein the flame-retardant polymer film has a porous structure. A separator for a lithium ion battery to which the flame-retardant polymer film of any one of claims 1 to 8 is applied. A method for producing a flame-retardant polymer film, comprising the steps of:
(a) mixing a photopolymerizable polymer, a vermiculite fine particle and a photoinitiator in a solvent; And
(b) irradiating the mixture of step (a) with light to induce polymerization reaction to form a polymer film in which vermiculite fine particles are dispersed. 11. The method of claim 10, wherein the weight ratio of photopolymerizable polymer: vermiculite fine particles in step (a) is 1: 1-3.5. The method of claim 10, wherein the weight ratio of the photopolymerizable material: vermiculite fine particle: solvent: photoinitiator is 1: 1-3.5: 1-2: 0.1-0.5 in step (a).

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