JP3746587B2 - Speaker - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a loudspeaker diaphragm used in audio equipment and the like, and more particularly to a technique for effectively insulating heat generated by eyelets provided on the diaphragm.
[0002]
[Prior art]
The speaker is a device that converts an electric signal from an amplifier into a sound wave, and as shown in FIG. 7, a magnetic circuit 20 including a magnet 21, a center pole 22 and a yoke 23, a voice coil 31, and a voice coil bobbin 32. And the damper 40, the speaker frame 50, the diaphragm 12, and the like.
[0003]
In the speaker shown in FIG. 7, a magnetic field is generated between the center pole 22 and the yoke 23 by the magnet 21. When a signal current is passed through the voice coil 31 provided so that a current flows perpendicularly to the direction of the magnetic field generated uniformly, a force is applied to the voice coil 31 and the voice coil bobbin 32. The diaphragm 12 bonded to the voice coil bobbin 32 moves together with them to oscillate air and emit sound waves.
[0004]
In the speaker configured and functioning as described above, it is necessary to connect a wire (not shown) for guiding a signal current from the outside to the voice coil 31. This wire is guided and connected to the voice coil 31 from a terminal 51 installed in the speaker frame 50. As the connection method, the following two methods are used. The first method is a method in which a copper wire extending from the voice coil 31 is soldered to the eyelet 11 provided on the diaphragm 12, and the eyelet 11 to the terminal 51 are connected by a gold thread wire. The second method is a coil method. This is a method in which a wire and solder-bonded gold thread wire is directly bonded to the voice coil bobbin 32 to connect the voice coil 31 to the terminal 51 with the gold thread wire.
[0005]
Wiring via eyelets (hereinafter referred to as “wiring via eyelets”), which is the first method, prevents abnormal amplitude caused by eccentricity of the vibration system due to the weight of the gold thread wire, and also prevents distortion due to the resonance of the gold thread wire. Therefore, it is used for a loudspeaker that requires a large input with a large input or a large amplitude. The second method is mainly used for a small-diameter, low-amplitude speaker with a small input.
[0006]
In a loudspeaker configured using this “wiring via grommet”, when an excessive current exceeding the rated input flows through the wire, a grommet with a large resistance value compared to copper or aluminum used for the wire Heat generation may occur in the solder part of the eyelet. When heat is generated in the eyelet part and a rapid temperature rise occurs, the heat is transmitted from the eyelet to the diaphragm, and the temperature of the diaphragm rises, which may cause a failure of the speaker.
[0007]
In this case, if the speaker performs a normal operation, the heated eyelet and its solder portion are cooled by the flow of air generated by the amplitude of the diaphragm, so that there is no particular problem.
[0008]
In addition, in order to block heat transfer between the eyelet and the diaphragm, attempts have been made to crimp the eyelet by sandwiching a heat resistant rubber, a heat resistant fiber, a heat resistant resin film, or the like between the eyelet and the diaphragm.
[0009]
[Problems to be solved by the invention]
However, in the loudspeaker according to the above-described prior art, the voice coil bobbin sometimes rides on the top plate or deforms and is fixed within the magnetic gap when a large input is made in a loudspeaker having a large diameter and a large amplitude. In such a state, the diaphragm cannot perform a normal operation and no air flows. If it does so, the heat | fever which generate | occur | produced in the eyelet part will be transmitted from an eyelet to a diaphragm, the temperature of a diaphragm will rise, and there exists a possibility that failure may occur in a speaker.
[0010]
In addition, in the configuration in which heat-resistant rubber or the like is sandwiched between the eyelet and the diaphragm, the ignition temperature of paper pulp or the like constituting the diaphragm and the decomposition of the polymer material constituting the heat-resistant rubber, heat-resistant fiber and heat-resistant resin film, etc. The breakdown temperature is approximately equal to about 400 ° C., respectively, while the temperature that is instantaneously generated when an excessive current flows through the coil reaches the 1000 ° C. level, which is not practical.
[0011]
Furthermore, although it is the “eyelet via wiring” having such problems, it is difficult to substitute another wiring method in view of the acoustic characteristics of a speaker having a large diameter and a large amplitude.
[0012]
The present invention has been made to solve such problems, and it is an object of the present invention to provide a speaker that effectively blocks heat transfer between the eyelet and the diaphragm, is inexpensive, and is easy to manufacture. And
[0013]
[Means for Solving the Problems]
In order to achieve the above object, in the speaker according to the first configuration of the present invention, a contact portion is formed on a diaphragm, and a wire for guiding a signal current is connected to the voice coil via the contact portion, and the vibration A thermally foamable polymer resin layer is formed on the entire surface of the plate or the peripheral portion of the contact portion of the diaphragm. According to the speaker according to the first configuration, even if the contact portion is heated by a solder having a large electrical resistance value compared to a copper wire or an aluminum wire due to an excessive input voltage exceeding the rating, Since the thermally foamable polymer resin is foamed by heat and a foamed carbonized layer is formed around the contact portion, heat at the contact portion can be effectively prevented from being transmitted to the diaphragm.
[0014]
In the speaker according to the second configuration of the present invention, a first contact portion and a second contact portion are formed on the diaphragm, and a wire for guiding a signal current is connected to the first contact portion. A wire from a voice coil is connected to the second contact portion, the first contact portion and the second contact portion are electrically connected by a conductive resin film, and the temperature of each contact portion is Thermal foaming is applied to a predetermined region of the diaphragm so that the conductive resin film is broken when the predetermined value or more is exceeded and electrical connection between the first contact portion and the second contact portion is interrupted. A conductive polymer resin layer is formed. According to the speaker according to the second configuration, similarly to the first configuration, the foamed carbonized layer is formed around each contact portion, thereby effectively insulating the heat at each contact portion. At the same time, since the electrical connection between the first contact portion and the second contact portion is interrupted, the generation of heat at the contact portion connecting the contact portions can be prevented.
[0015]
Further, in the speaker according to the second configuration of the present invention, the thermally foamable polymer resin is provided on a peripheral portion including the first contact portion and the second contact portion on one surface of the diaphragm. A layer is formed, and the thermally foamable polymer resin layer is formed on a peripheral portion including only one of the first contact portion and the second contact portion on the other surface of the diaphragm. And includes the first contact portion and the second contact portion on the other surface of the diaphragm so as to overlap the thermally foamable polymer resin layer formed on the other surface of the diaphragm. It is preferable that the conductive resin film is formed in the peripheral portion. According to this preferred example, when heat is generated in the contact portions connecting the contact portions and the contact portions, the heat-foamable polymer resin is foamed by the heat, and foam carbonization is generated around the contact portions. A layer is formed, and the conductive resin film is lifted and disconnected by the foamed carbonized layer. Therefore, it is possible to effectively insulate the heat at each contact portion and to prevent generation of heat at the contact portion connecting the contact portions.
[0016]
Further, in the speaker according to the second configuration of the present invention, the thermally foamable polymer resin layer is disposed on the diaphragm so that a spark generated when the conductive resin film is broken does not directly contact the diaphragm. Preferably it is formed. According to this preferred example, even if a spark occurs when the conductive resin film is broken, the thermally foamable polymer resin layer is formed so that the spark does not directly contact the diaphragm. The diaphragm is not ignited by a spark.
[0017]
Further, in the speaker according to the second configuration of the present invention, when the conductive resin film is broken in a region between the first contact portion and the second contact portion on the other surface of the diaphragm. It is preferable that the thermally foamable polymer resin layer is formed on an extended line of the disconnected portion. According to this preferable example, since the spark generated when the conductive resin film is broken often falls on an extension line of the disconnected portion, it is possible to effectively prevent the spark from directly contacting the diaphragm. it can.
[0018]
In the speaker according to the present invention, it is preferable that at least one of the thermally foamable polymer resin layer or the conductive resin film is formed by screen printing or spray printing. According to this preferable example, the mass productivity of the diaphragm constituting the speaker can be improved.
[0019]
In the speaker according to the present invention, the thermally foamable polymer resin layer is preferably a thermally foamable polymer resin film formed on the diaphragm, and further, the thermally foamable polymer resin. The layer is preferably a thermally foamable polymer resin-impregnated layer formed by impregnating the diaphragm with a thermally foamable polymer resin. According to this preferred example, the thermally foamable polymer resin layer can be formed relatively easily.
[0020]
Moreover, in the speaker according to the present invention, it is preferable that the contact portion is configured using eyelets. According to this preferred example, the contact portion can be configured relatively easily and inexpensively.
[0021]
In the speaker according to the present invention, it is preferable that the diaphragm is configured using a base material impregnated with a halogen-based or phosphorus-based flame retardant. According to this preferable example, the nonflammability of the diaphragm is improved.
[0022]
Moreover, in the speaker according to the present invention, it is preferable that the resin constituting the thermally foamable polymer resin layer is an acrylic resin. The acrylic resin is a general term for polymers obtained by polymerizing acrylic acid and derivatives thereof, and includes polymers and copolymers such as acrylic acid and esters thereof, acrylamide, acrylonitrile, methacrylic acid and esters thereof, and the like.
[0023]
In the speaker according to the present invention, the resin constituting the thermally foamable polymer resin layer is at least one selected from a polymer of acrylonitrile, an acrylate ester, or a methyl methacrylate and a copolymer thereof. A resin or an acrylic polymer resin containing these components is preferable.
[0024]
Further, in the speaker according to the present invention, the foaming agent constituting the thermally foamable polymer resin layer is made of ammonium polyphosphate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium carbonate, amyl acetate, butyl acetate, diazoaminobenzene. It is preferable that at least one selected.
[0025]
In the speaker according to the present invention, it is preferable that the thermally foamable polymer resin layer starts foaming at a predetermined temperature and the resin portion forms a carbonized layer.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 shows a top view of a diaphragm constituting the speaker according to the first embodiment of the present invention. FIG. 2 shows a sectional view taken along line XX of FIG. Hereinafter, the manufacturing method of the diaphragm which comprises the speaker which concerns on this embodiment, and its function are demonstrated.
[0027]
The diaphragm 2 shown in FIGS. 1 and 2 is formed using, for example, paper. First, a thermally foamable polymer resin is applied to a peripheral portion of a portion (contact portion) where the eyelet 1 of the diaphragm 2 is attached. In this embodiment, by applying a thermally foamable polymer resin, the thermally foamable polymer resin film 3 having a thickness of about 100 μm and a radius of about 1.5 times the diameter of the eyelet 1 is formed on the diaphragm 2. Form on both sides. Thereafter, the thermally foamable polymer resin film 3 is dried in an atmosphere of 50 ° C. for 1 hour, and the eyelet 1 is attached to substantially the center of the thermally foamable polymer resin film 3. In addition, in this embodiment, although the state which has provided two contact parts is shown, this invention is not limited to this structure, You may provide one place or three places or more as needed. .
[0028]
The thermally foamable polymer resin used here is, for example, an acrylic resin as a base material to which a foaming agent such as ammonium polyphosphate is added. This thermally foamable polymer resin starts to foam at around 200 ° C., and at the same time, the acrylic resin as the base material forms a foam carbonized layer. The acrylic resin is preferably at least one resin selected from a polymer of acrylonitrile, an acrylate ester or methyl methacrylate, and a copolymer thereof. Examples of the foaming agent include ammonium carbonate, amyl acetate, butyl acetate, diazoaminobenzene and the like.
[0029]
The speaker according to the present embodiment is configured using the diaphragm 2 obtained as described above. The configuration of the speaker is basically the same as that shown in FIG.
Next, the function of the speaker according to the present embodiment will be described with reference to FIGS. First, a wire from a voice coil that constitutes a speaker and a gold thread wire are soldered together with an eyelet 1, and then one of the gold thread wires is joined to an external terminal (terminal). As shown in FIG. 3, a solder joint 8 is connected to the eyelet 1. Next, in order to apply a voltage to the speaker, a voltage cord is connected to the electrode of the speaker, and a slider is further connected. Then, the voltage is gradually increased while adjusting the slidac (that is, the temperature of the eyelet 1 portion is gradually increased). As a result, the foaming of the thermally foamable polymer resin film 3 around the eyelet 1 near the voltage 40V started to foam, and the foaming of the thermally foamable polymer resin film 3 continued until the voice coil was broken and broken. The ignition of the diaphragm 2 by the heat from 1 did not occur.
[0030]
This is because, even if a rapid temperature rise occurs in the eyelet 1 part, the heat foamable polymer resin film 3 is foamed by the heat generated in the eyelet 1 part to form the foamed carbonized layer 4. This is to protect the diaphragm 2 from a sudden temperature rise in the eyelet 1 part. FIG. 3 shows a sectional view of the diaphragm at this time. As shown in FIG. 3, since the foamed carbonized layer 4 is interposed between the eyelet 1 and the diaphragm 2, in the speaker according to the present embodiment (structure around the eyelet), the sharpness in the eyelet 1 portion is rapidly increased. It is possible to protect the diaphragm 2 from an excessive temperature rise.
[0031]
Hereinafter, the situation around the eyelet part in the process of forming the foamed carbonized layer will be described more specifically.
When the foamed carbonized layer is generated, water vapor and carbon dioxide are generated, so that the oxygen concentration around the eyelet portion decreases. That is, when a base material such as paper (pulp) constituting the diaphragm is burned, the oxygen concentration necessary for burning is reduced to give a suffocation effect. At the same time, it takes away heat of reaction and produces a cooling effect. Further, the formed carbonized foam layer prevents oxygen from entering from the outside and completely covers the surface of the base material.
[0032]
Further, since the foamed carbonized layer also covers the eyelet itself, the bubble cells formed inside the foamed carbonized layer have a heat insulating effect, and have the effect of delaying the time for transferring heat to the pulp of the base material. In addition, in this state where the eyelet is covered with the foamed carbonized layer, even if the temperature of the eyelet part continues to rise further than that, the base material is only steamed and carbonized inside the foamed carbonized layer. As long as the foamed carbonized layer is sufficiently covered, ignition does not occur.
[0033]
In addition, if the periphery of the contact part on the diaphragm is impregnated with a substance that imparts flame retardancy such as halogen or phosphorus, heat transfer from the contact part (around the eyelet part) to the diaphragm is further reduced. It is possible to reduce the possibility of combustion of the diaphragm.
[0034]
(Second embodiment)
FIG. 4 shows a top view of the diaphragm constituting the speaker according to the second embodiment of the present invention. FIG. 5 shows an XX cross-sectional view of FIG. Hereinafter, the manufacturing method of the diaphragm which comprises the speaker which concerns on this embodiment, and its function are demonstrated.
[0035]
The diaphragm 2 according to the present embodiment is configured by using, for example, paper, as in the first embodiment. The difference between the present embodiment and the first embodiment is that the shape of the thermally foamable polymer resin films 3a and 3b formed on the diaphragm 2 and the contact portions (first part) provided at two locations. The first eyelet 1 a and the second eyelet 1 b) are made conductive by using the conductive resin film 5. The thermally foamable polymer resin and the like used in this embodiment are basically the same as those in the first embodiment. This will be specifically described below.
[0036]
First, a thermally foamable polymer resin film 3a is formed around the first eyelet 1a on the surface 2a of the diaphragm, and an area including the first eyelet 1a and the second eyelet 2b on the back surface 2b of the diaphragm is formed. A thermally foamable polymer resin film 3b is formed around the periphery. Here, the thermally foamable polymer resin films 3a and 3b have a thickness of about 100 μm. And it is made to dry in 50 degreeC atmosphere for 1 hour.
[0037]
Next, it is an area including the first eyelet 1a and the second eyelet 2b on the surface 2a of the diaphragm, and is conductive so as to overlap the thermally foamable polymer resin film 3a formed on the surface 2a of the diaphragm. The conductive resin film 5 is formed. Here, the conductive resin film 5 has a thickness of about 100 μm. Then, it is dried at room temperature for 24 hours. On the extended line of the boundary portion 6 where the thermally foamable polymer resin film 3a and the conductive resin film 5 overlap each other, the thermally foamable polymer resin film 3a is formed slightly longer.
[0038]
The speaker according to the present embodiment is configured using the diaphragm 2 obtained as described above. The configuration of the speaker is the same as in FIG.
Next, the function of the speaker according to the present embodiment will be described with reference to FIGS. First, a wire from a voice coil that constitutes a speaker and a gold thread wire are soldered together by the first eyelet 1a and the second eyelet 1b, and then one of the gold thread wires is joined to an external terminal (terminal). As shown in FIG. 6, the solder joint portion 8 is connected to the eyelets 1a and 1b. As described above, the first eyelet 1a and the second eyelet 1b are electrically connected by the conductive resin film 5 provided on the surface 2a of the diaphragm.
[0039]
Then, as in the first embodiment, in order to apply a voltage to the speaker, a voltage cord is connected to the electrode of the speaker, and a slidac is further connected. Then, the voltage is gradually increased while adjusting the slidac (that is, the temperature of the eyelet 1 portion is gradually increased). As a result, also in the present embodiment, the foaming of the thermally foamable polymer resin film 3 around the eyelets 1a, 1b started to foam around a voltage of 40V.
[0040]
In the present embodiment, as shown in FIG. 5, the thermally foamable polymer resin films 3a and 3b and the conductive resin film 5 are applied on the diaphragm 2, and the thermally foamable polymer resin film 3a is partially applied. And the conductive resin film 5 have a two-layer structure (the thermally foamable polymer resin film 3a is the lower layer). Therefore, when the thermally foamable polymer resin film 3a is foamed, a thermally foamable polymer is formed. The conductive resin film 5 on the resin film 3a is lifted, the boundary portion 6 is broken and disconnected, and the continuity is lost, so that further temperature rise of the diaphragm 2 can be prevented. That is, in this embodiment, the two-layer structure portion of the thermally foamable polymer resin film 3a and the conductive resin film 5 functions as a so-called “fuse”. FIG. 6 shows this state. As shown in FIG. 6, in this embodiment, even if the contact portion having a high resistance value due to excessive input or the like reaches a high temperature at which a member such as a diaphragm or a voice coil is broken, the lower thermal foaming property is high. The molecular resin film 3a reacts to become the foamed carbonized layer 4 and lifts the upper conductive resin film 5, so that the boundary portion 6 is broken and a broken portion 7 is generated. In this case, the disconnection portion 7 is generated by heat generation of the conductive resin film portion rather than the contact portion. Further, as apparent from FIG. 6, the thermally foamable polymer resin film 3 foams while protecting the eyelet 1 and the diaphragm 2 to form the foamed carbonized layer 4, and the foamed carbonized layer 4 is formed. As a result, the conductive resin film 5 is lifted and a broken portion 7 is generated.
[0041]
As described above, in the present embodiment, non-combustion around the eyelet 1 using the thermally foamable polymer resin 3, and a printing type using the thermally foamable polymer resin film 3a and the conductive resin film 5 are used. By using both of them together with the “fuse”, even if the temperature near the contact point composed of eyelets etc. rises and exceeds the ignition point temperature of paper pulp, the ignition of the diaphragm will occur. Can be prevented.
[0042]
Further, in the present embodiment, when the conductive resin film 5 on the thermally foamable polymer resin film 3a is lifted to break the boundary portion 6 and the broken portion 7 is generated, a spark is generated along the fracture surface. there's a possibility that. If this spark directly contacts the diaphragm 2, the diaphragm 2 may ignite, which is not preferable. Therefore, in the present embodiment, as described above, the thermally foamable polymer resin film 3a is formed on the extended line of the boundary portion 6 slightly longer (see FIG. 4). By setting it as such a structure, the spark which generate | occur | produces along a fracture surface will fall on the thermally foamable polymer resin film 3a, and it can prevent that the diaphragm 2 and a spark contact directly. .
[0043]
In the present embodiment, the case where the boundary portion (fuse portion) is formed by forming a conductive resin film on the surface of the diaphragm has been described, but the present invention is not limited to this, and the diaphragm A boundary portion (fuse portion) may be formed by forming a conductive resin film on the back surface of the substrate.
[0044]
In each of the above embodiments, mass productivity can be improved by performing a printing process such as screen printing or spray printing when forming a resin film on the diaphragm.
[0045]
Further, in each of the embodiments described above, the case where the thermally foamable polymer resin film is formed by applying the thermally foamable polymer resin on one surface or both surfaces of the diaphragm has been described. The invention is not limited to this configuration, and for example, a configuration in which a sheet-like thermally foamable polymer resin film is disposed at a predetermined position or the diaphragm is impregnated with a thermally foamable polymer resin may be used. .
[0046]
【The invention's effect】
As described above, according to the present invention, it is possible to effectively cut off heat transfer between the eyelet and the diaphragm, and obtain a speaker that is inexpensive and easy to manufacture.
[Brief description of the drawings]
FIG. 1 is a top view of a diaphragm constituting a speaker according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line XX in FIG. 1 is a cross-sectional view taken along the line XX in FIG. 1 after forming the carbonized layer. FIG. 4 is a top view of the diaphragm constituting the speaker according to the second embodiment of the present invention. 6 is a cross-sectional view taken along the line XX of FIG. 4 after foaming a thermally foamable polymer resin film (after formation of a foamed carbonized layer). FIG. 7 is a schematic configuration diagram of a speaker.
DESCRIPTION OF SYMBOLS 1 Eyelet 1a First eyelet 1b Second eyelet 2 Diaphragm 2a Diaphragm surface 2b Diaphragm back surface 3, 3a, 3b Thermal foaming polymer resin film 4 Foamed carbonized layer 5 Conductive resin film 6 Boundary part 7 Disconnection 8 Solder joint

Claims (14)

A contact portion is formed on the diaphragm, and a wire for guiding a signal current is connected to the voice coil through the contact portion, and is thermally foamed on the entire surface of the diaphragm or the peripheral portion of the contact portion of the diaphragm. A speaker in which a polymer resin layer is formed. A first contact portion and a second contact portion are formed on the diaphragm, a wire for guiding a signal current is connected to the first contact portion, and a wire from a voice coil is connected to the second contact portion. Connected, and the first contact portion and the second contact portion are electrically connected by a conductive resin film, and when the temperature of each contact portion is equal to or higher than a certain value, the conductive resin film A speaker in which a thermally foamable polymer resin layer is formed in a predetermined region of the diaphragm so that the electrical connection between the first contact portion and the second contact portion is interrupted to break. On one surface of the diaphragm, the thermally foamable polymer resin layer is formed in a peripheral portion including the first contact portion and the second contact portion, and on the other surface of the diaphragm, The thermally foamable polymer resin layer is formed on a peripheral portion including only one of the first contact portion and the second contact portion, and is formed on the other surface of the diaphragm. The conductive resin film is formed on a peripheral portion including the first contact portion and the second contact portion on the other surface of the diaphragm so as to overlap the thermally foamable polymer resin layer. The speaker according to claim 2. The speaker according to claim 2 or 3, wherein the thermally foamable polymer resin layer is formed on the diaphragm so that a spark generated when the conductive resin film is broken does not directly contact the diaphragm. The thermally foamable polymer resin on an extension line of a broken portion at the time of breakage of the conductive resin film in a region between the first contact portion and the second contact portion on the other surface of the diaphragm The speaker according to claim 4, wherein a layer is formed. The speaker according to any one of claims 1 to 5, wherein at least one of the thermally foamable polymer resin layer or the conductive resin film is formed by screen printing or spray printing. The speaker according to any one of claims 1 to 6, wherein the thermally foamable polymer resin layer is a thermally foamable polymer resin film formed on the diaphragm. 7. The heat foamable polymer resin impregnated layer according to claim 1, wherein the heat foamable polymer resin layer is a heat foamable polymer resin impregnated layer formed by impregnating the diaphragm with a heat foamable polymer resin. Speaker. The speaker according to any one of claims 1 to 8, wherein the contact portion is configured using eyelets. The speaker according to any one of claims 1 to 9, wherein the diaphragm is configured using a base material impregnated with a halogen-based or phosphorus-based flame retardant. The speaker according to any one of claims 1 to 10, wherein a resin constituting the thermally foamable polymer resin layer is an acrylic resin. The resin constituting the thermally foamable polymer resin layer is at least one resin selected from acrylonitrile, an acrylate ester, a methyl methacrylate polymer and a copolymer thereof, or an acrylic resin containing these components The speaker according to any one of claims 1 to 10, which is a polymer resin. The foaming agent constituting the thermally foamable polymer resin layer is at least one selected from ammonium polyphosphate, ammonium bicarbonate, sodium bicarbonate, ammonium carbonate, amyl acetate, butyl acetate, and diazoaminobenzene. The speaker according to any one of 1 to 12. The speaker according to any one of claims 1 to 13, wherein the thermally foamable polymer resin layer starts foaming at a predetermined temperature, and the resin portion forms a carbonized layer.

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