JP7527742B2 - Silicone rubber composition - Google Patents

Description

The present invention relates to a silicone rubber composition that has excellent fire resistance, shape retention and ignition resistance, particularly at high temperatures such as during a fire.

Traditionally, fire-resistant construction materials have been made from rubber materials such as chloroprene rubber and ethylene-propylene-diene rubber. However, although these rubber materials have excellent flame retardancy, when exposed to high temperatures during a fire, they produce large amounts of smoke, impairing visibility, and sometimes contain toxic components. In addition, their shape retention at high temperatures is not sufficient. For these reasons, silicone rubber, which has excellent heat and weather resistance and creep resistance, has recently come to be used as a fire-resistant construction material. For the same reason, the use of silicone rubber as an electric wire coating material is also increasing.

Normal silicone rubber burns and turns to ash when exposed to high temperatures such as flames for long periods of time, so it does not function adequately as a gasket.

In order to improve the above-mentioned drawbacks of fire-resistant materials made of silicone rubber, the following have been considered: a combination of zinc oxide and/or aluminum hydroxide with a platinum compound (Patent Document 1); and a combination of at least one selected from manganese carbonate, mica, and black iron oxide with zinc oxide and/or quartz powder and a platinum compound (Patent Document 2). Furthermore, it has been proposed to add magnesium hydroxide (Patent Document 3) or calcined mica (Patent Document 4) to organopolysiloxane to sinter (ceramize) it at high temperatures. However, these silicone rubbers have insufficient dimensional stability at high temperatures, and the rubber may partially foam, causing the siloxane gas contained therein to burn.

Japanese Patent Publication No. 141778/1983 Special Publication No. 5-73158 JP 2000-169706 A International Publication No. 2015/011971

Therefore, the object of the present invention is to provide a silicone rubber composition that exhibits excellent shape retention even when exposed to high temperatures for a long period of time.

As a result of intensive research to achieve the above-mentioned objective, the inventors have discovered that a silicone rubber composition that is particularly excellent in shape retention at high temperatures can be provided by using a composition containing an organopolysiloxane having a degree of polymerization of 100 or more and having two or more alkenyl groups bonded to silicon atoms in each molecule, reinforcing silica having a specific surface area measured by the BET method of 50 ^m2 /g or more, and a curing agent, and further using in combination calcined mica of a specific particle size and a metal hydroxide, and have thus completed the present invention.

Accordingly, the present invention provides the following silicone rubber composition.
[1]
(A) an organopolysiloxane having a degree of polymerization of 100 to 10,000 and having two or more alkenyl groups bonded to silicon atoms in each molecule: 100 parts by mass,
(B) reinforcing silica having a specific surface area of 50 to 500 m ² /g as measured by the BET method: 10 to 100 parts by mass,
(C) Calcined mica having an average particle size of 0.5 to 20 μm: 1 to 100 parts by mass,
A silicone rubber composition comprising: (D) 0.3 to 15 parts by mass of a metal hydroxide having an average particle size of 0.5 to 20 μm; and (E) an effective amount of a curing agent.
[2]
The silicone rubber composition according to claim 1, wherein the calcined mica of component (C) is obtained by calcining pulverized muscovite.
[3]
The silicone rubber composition according to claim 1 or 2, wherein the metal hydroxide of component (D) is aluminum hydroxide or magnesium hydroxide.
[4]
The silicone rubber composition according to any one of [1] to [3], which is for use in a fire-resistant gasket or a fire-resistant electric wire.

The present invention provides a silicone rubber composition that has excellent fire resistance and shape retention, particularly at high temperatures.

FIG. 2 is a schematic diagram of a testing machine used to measure breaking strength in the examples of the present invention.

The present invention will now be described in further detail.
[(A) Alkenyl-containing organopolysiloxane]
The component (A) used in the silicone rubber composition of the present invention is an organopolysiloxane having a degree of polymerization (or the number of silicon atoms in the molecule) of 100 to 10,000 and having two or more alkenyl groups bonded to silicon atoms in one molecule. This component (A) acts as the main component (base polymer) of the composition. The alkenyl-containing organopolysiloxane of component (A) is preferably a raw rubber-like component (i.e., a non-liquid component with high viscosity and no self-flowability) at room temperature (25°C). The silicone rubber composition of the present invention, which contains such a raw rubber-like alkenyl-containing organopolysiloxane as the main component, usually gives a millable type composition (i.e., a raw rubber-like composition that can be uniformly kneaded under shear stress by a kneader such as a roll mill). The alkenyl-containing organopolysiloxane of component (A) is preferably, for example, one represented by the following average composition formula (I):
R _a SiO _(4-a)/2 (I)
(In formula (I), R represents the same or different monovalent hydrocarbon group having 1 to 12 carbon atoms, and a is a number from 1.95 to 2.05.)

In the above average composition formula (I), R represents the same or different monovalent hydrocarbon group having 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, and specifically includes alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, and octyl groups, cycloalkyl groups such as cyclopentyl and cyclohexyl groups, fluoroalkyl groups such as 3,3,3-trifluoropropyl groups, alkenyl groups such as vinyl, allyl, and propenyl groups, aryl groups such as cycloalkenyl groups, phenyl groups, and tolyl groups, and aralkyl groups such as benzyl and 2-phenylethyl groups. Methyl, vinyl, phenyl, and trifluoropropyl groups are preferred, and methyl and vinyl groups are particularly preferred. It is preferable that 80% or more, and particularly 90% or more of all R groups are methyl groups, and it is further preferable that all R groups except for alkenyl groups are methyl groups.

Specific examples of the alkenyl-containing organopolysiloxane of component (A) include organopolysiloxanes whose main chain is made of dimethylsiloxane units, and dimethylpolysiloxanes whose main chain has diphenylsiloxane units having phenyl groups, vinyl groups, 3,3,3-trifluoropropyl groups, etc., methylvinylsiloxane units, methyl-3,3,3-trifluoropropylsiloxane units, etc., introduced into part of the main chain.

The organopolysiloxane of component (A) has two or more alkenyl groups, preferably vinyl groups, per molecule (usually 2 to 50, particularly about 2 to 20). For example, it is preferable that 0.01 to 10%, particularly 0.02 to 5%, of all R groups in the average composition formula (I) above are alkenyl groups. The alkenyl groups may be bonded to silicon atoms at the ends of the molecular chain, or to silicon atoms in the middle of the molecular chain (non-terminal), or both. However, it is preferable that the organopolysiloxane of component (A) has alkenyl groups bonded to silicon atoms at at least both ends of the molecular chain. Specifically, it is preferable that both ends of the molecular chain are blocked with dimethylvinylsilyl groups, methyldivinylsilyl groups, trivinylsilyl groups, or the like.

In the above average composition formula (I), a is a number from 1.95 to 2.05, preferably from 1.98 to 2.02, and more preferably from 1.99 to 2.01. The molecular structure of the organopolysiloxane of component (A) is generally a straight-chain structure in which the main chain is basically composed of repeating diorganosiloxane units (R ₂ SiO _2/2 ) and both ends of the molecular chain are blocked with triorganosiloxy groups (R ₃ SiO _1/2 ), but it may also have a branched structure containing a small amount of branch units (RSiO _3/2 ) in the main chain within a range that does not impair rubber elasticity (R is as defined above).

The degree of polymerization of the organopolysiloxane of component (A) is 100 to 10,000, preferably 1,000 to 10,000, more preferably 2,000 to 10,000, even more preferably 3,000 to 8,000, and particularly preferably 5,000 to 8,000. If the degree of polymerization is less than 100, sufficient rubber strength cannot be obtained. In the present invention, the degree of polymerization is the number of silicon atoms in the organopolysiloxane, and can be determined as the weight-average degree of polymerization (or weight-average molecular weight) in terms of polystyrene in GPC (gel permeation chromatography) analysis using toluene or the like as a developing solvent. In the present specification, the degree of polymerization is a value determined as the weight-average degree of polymerization from the number-average molecular weight in terms of polystyrene in gel permeation chromatography (GPC) analysis measured under the following conditions.
[Measurement condition]
Developing solvent: toluene Flow rate: 0.6 mL/min
Detector: Refractive index detector (RI)
Column: TSK Guard column Super H-H
TSKgel SuperH5000 (6.0mm I.D. x 15cm x 1)
TSKgel SuperH4000 (6.0mm I.D. x 15cm x 1)
TSKgel SuperH3000 (6.0mm I.D. x 15cm x 1)
(Both manufactured by Tosoh Corporation)
Column temperature: 40°C
Sample injection volume: 50 μL (toluene solution with a concentration of 0.5% by mass)

The organopolysiloxane of component (A) may be used alone or in combination with two or more different types with different molecular structures or degrees of polymerization.

Such organopolysiloxanes can be obtained by known methods, for example, by (co)hydrolytic condensation of one or more organohalogenosilanes, or by ring-opening polymerization of cyclic polysiloxanes using an alkaline or acidic catalyst.

[(B) Reinforcing Silica]
The reinforcing silica of component (B) is exemplified by reinforcing silica fine powders that are usually used in silicone rubber compositions. For example, fumed silica, dry silica such as calcined silica, and wet silica such as precipitated silica are exemplified, and among these, fumed silica is preferred from the viewpoint of heat resistance. The specific surface area measured by the BET method is 50 to 500 m ² /g, preferably 100 to 400 m ² /g, and particularly preferably 100 to 300 m ² /g. If the specific surface area is less than 50 m ² /g, the mechanical strength is insufficient. If the specific surface area is greater than 500 m ² /g, industrial production becomes difficult.

If necessary, the surface of this reinforcing silica may be hydrophobized with a known treating agent such as chlorosilane or hexamethyldisilazane.

The amount of reinforcing silica in component (B) added is 10 to 100 parts by mass, preferably 20 to 70 parts by mass, and particularly preferably 30 to 60 parts by mass, per 100 parts by mass of the organopolysiloxane in component (A). If the amount of component (B) added per 100 parts by mass of component (A) is less than 10 parts by mass, the amount is too small to achieve a sufficient reinforcing effect, and if it exceeds 100 parts by mass, processability may deteriorate and mechanical strength may decrease.

[(C) Calcined mica]
The calcined mica of component (C) is a component that imparts fire resistance (i.e., sinterability (shape retention) after combustion) to the silicone rubber obtained by curing the silicone rubber composition. The calcined mica used in the present invention is obtained by heat treating wet mica or dry mica at a temperature of 800°C or higher. An example of such calcined mica is calcined mica manufactured by Repco Co., Ltd., which is manufactured by calcining pulverized muscovite. Non-calcined mica has the disadvantage that it easily adheres to the metal roll surface during kneading with two rolls. Furthermore, non-calcined mica may cause significant foaming during molding of silicone rubber or when the molded product is exposed to high temperatures.

The calcined mica has an average particle size of 0.5 to 200 μm, preferably 1 to 100 μm, and particularly preferably 1 to 50 μm. If the average particle size of the calcined mica is less than 0.5 μm, it is prone to agglomeration and is difficult to handle, while if it is more than 200 μm, when the silicone rubber molded article is heated at high temperatures (combustion or sintering), the ceramicization after sintering is insufficient, and the shape retention of the incinerated (sintered) silicone rubber molded article is reduced.
In the present invention, the average particle size is determined as the cumulative weight average diameter (or median diameter, d50) in particle size distribution measurement by laser diffraction method.

The amount of component (C) added is 1 to 100 parts by mass, and preferably 10 to 80 parts by mass, per 100 parts by mass of the organopolysiloxane of component (A). If the amount of component (C) added per 100 parts by mass of component (A) is less than 1 part by mass, sufficient shape retention will not be obtained, and if it exceeds 100 parts by mass, the plasticity of the material will be high, making kneading and molding difficult.

[(D) Metal hydroxide]
As the metal hydroxide of component (D), aluminum hydroxide or magnesium hydroxide is preferred. By adding an appropriate amount of metal hydroxide to the composition, fine bubbles can be intentionally generated uniformly and large bubbles can be suppressed. If a metal hydroxide is not added to the composition, large swelling (foaming) may occur when exposed to high temperatures of 500°C or higher. This foaming is thought to be caused by moisture and low molecular weight siloxane derived from silica and calcined mica, and the swelling part becomes thin and the strength is reduced. In particular, in the case of a gasket, there is a risk of a gap being created between the glass and the gasket and hot air passing through. In addition, the swelling part is filled with flammable siloxane gas, and if it bursts at high temperatures, the gas may burn.

The average particle size of the metal hydroxide is 0.5 to 20 μm, preferably 1 to 15 μm, and particularly preferably 2 to 10 μm. If it is smaller than 0.5 μm, it will not be easily filled into the composition, and if it is larger than 20 μm, it may foam non-uniformly at high temperatures or the strength of the silicone rubber molded product may decrease.
The average particle size is determined, for example, as the cumulative weight average diameter (or median diameter, d50) in particle size distribution measurement by laser diffraction method.

The amount of component (D) added is 0.3 to 15 parts by mass, and preferably 0.5 to 10 parts by mass, per 100 parts by mass of the organopolysiloxane of component (A). If the amount of component (D) added is less than 0.3 parts by mass per 100 parts by mass of component (A), the effect is insufficient, and if it exceeds 15 parts by mass, the amount of foaming at high temperatures increases significantly and the roll processability of the silicone rubber composition may also deteriorate.

[(E) Curing agent]
The curing agent of component (E) is not particularly limited as long as it can cure the silicone rubber composition of the present invention. Therefore, known vulcanizing agents (curing agents) for silicone rubber, such as organic peroxide vulcanizing agents (curing agents) and addition vulcanizing agents (curing agents), can be used.

Examples of organic peroxides used as organic peroxide vulcanizing agents (curing agents) include benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,4-dicumyl peroxide, 2,5-dimethyl-bis(2,5-t-butylperoxy)hexane, di-t-butyl peroxide, t-butyl perbenzoate, 1,6-hexanediol-bis-t-butylperoxycarbonate, etc. These may be used alone or in combination of two or more.
The amount of organic peroxide added may be any amount sufficient to effectively cure the silicone rubber composition, but is preferably 0.1 to 10 parts by weight, and more preferably 0.2 to 5 parts by weight, per 100 parts by weight of component (A).
When an organic peroxide vulcanizing agent (curing agent) is used as the curing agent, a platinum-based catalyst, which will be described later, may be used in combination in order to improve heat resistance. When used in combination, the amount of platinum-based catalyst is preferably 1 to 2,000 ppm, and more preferably 10 to 1,000 ppm, calculated as the mass of platinum group metal per 100 parts by mass of component (A).

When curing by addition reaction (hydrosilylation addition reaction), a combination of organohydrogenpolysiloxane (curing agent or crosslinking agent) and platinum catalyst (curing catalyst) is used as the addition vulcanizing agent (curing agent).

Examples of platinum-based catalysts include platinum element alone, platinum compounds, platinum complexes, chloroplatinic acid, alcohol compounds of chloroplatinic acid, aldehyde compounds, ether compounds, complexes with various olefins, and platinum group metal catalysts such as compounds of rhodium, palladium, and ruthenium similar to the platinum compounds.
The amount of platinum catalyst added is preferably 1 to 2,000 ppm, and particularly preferably 10 to 1,000 ppm, calculated as the mass of platinum group metal relative to the organopolysiloxane of component (A).

On the other hand, the organohydrogenpolysiloxane used as the curing agent (crosslinking agent) may be an organohydrogenpolysiloxane containing two or more (usually 2 to 200), preferably three or more (for example, 3 to 150, preferably 4 to 100) hydrogen atoms bonded to silicon atoms (hydrosilyl groups) in one molecule. The organohydrogenpolysiloxane may have a linear, branched, cyclic, or three-dimensional network structure. The degree of polymerization of this organohydrogenpolysiloxane is preferably 300 or less, more preferably 2 to 300, and particularly preferably 3 to 150. Specific examples of this organohydrogenpolysiloxane include 1,1,3,3-tetramethyldisiloxane, 1,3,5,7-tetramethylcyclotetrasiloxane, 1-propyl-3,5,7-trihydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1,5-dihydrogen-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane, tris(hydrogendimethylsiloxy)methylsilane, tris(hydrogendimethylsiloxy)phenylsilane, methylhydrogencyclopolysiloxane, methylhydrogensiloxane-dimethylsiloxane cyclic copolymer, methylhydrogenpolysiloxane blocked at both ends with trimethylsiloxy groups, dimethylsiloxane-methylhydrogensiloxane copolymer blocked at both ends with trimethylsiloxy groups, and copolymers of dimethylsiloxane and methylhydrogensiloxane blocked at both ends with dimethylsiloxy groups. Methylhydrogensiloxy group-blocked dimethylpolysiloxane, dimethylsiloxane/methylhydrogensiloxane copolymer blocked at both ends with dimethylhydrogensiloxy groups, methylhydrogensiloxane/diphenylsiloxane copolymer blocked at both ends with trimethylsiloxy groups, methylhydrogensiloxane/diphenylsiloxane/dimethylsiloxane copolymer blocked at both ends with trimethylsiloxy groups, methylhydrogensiloxane/methylphenylsiloxane/dimethylsiloxane copolymer blocked at both ends with trimethylsiloxy groups, methylhydrogensiloxane/dimethylsiloxane/diphenylsiloxane copolymer blocked at both ends with dimethylhydrogensiloxy groups, methylhydrogensiloxane/dimethylsiloxane/methylphenylsiloxane copolymer blocked at both ends with dimethylhydrogensiloxy groups, (CH Examples of the copolymer include a copolymer consisting of ( _CH3 ) _{2HSiO1 /2} units, ( _CH3 ) _{3SiO1 /2} units and SiO4 _/2 units, a copolymer consisting of ( _CH3 ) _2HSiO1 / ₂ units and SiO4/ ₂ units, and a copolymer consisting of ( _CH3 )2HSiO1 _{/2 units} , SiO4 _/2 units and ( _C6H5 ) _{3SiO1 /2 units} .

The amount of organohydrogenpolysiloxane added as a curing agent is preferably such that the molar ratio of hydrogen atoms (hydrosilyl groups) directly bonded to silicon atoms in the organohydrogenpolysiloxane is 0.5 to 5 mol/mol relative to the alkenyl groups in the organopolysiloxane of component (A). When curing the composition of the present invention, a co-vulcanization type can be used in which the above-mentioned organic peroxide vulcanizing agent (curing agent) and an addition vulcanizing agent (curing agent) are used together as the curing agent of component (D).

In addition to the above components, the silicone rubber composition of the present invention may contain optional components, as necessary, within the range that does not impair the effects of the present invention, such as dispersants for the reinforcing silica (B) (e.g., α,ω-silanol-blocked low-polymerization diorganopolysiloxanes, etc.), platinum compounds (e.g., platinum compounds similar to the platinum-based catalysts exemplified as components in the addition vulcanizing agent of the above-mentioned component (D)), flame retardants such as iron oxide, titanium oxide, carbon, and halogen compounds, heat resistance improvers, anti-aging agents, ultraviolet absorbers, colorants, and mold release agents, and other additives known in silicone rubber compositions.

The method for producing the silicone rubber composition of the present invention is not particularly limited, but it can be obtained by kneading the predetermined amounts of the above-mentioned components with a known kneading machine such as a two-roll (roll mill), kneader, or Banbury mixer. If necessary, heat treatment (kneading under heat) may also be performed. Specifically, a method is preferred in which components (A) and (B) are kneaded, and if necessary, heat treatment is performed, and then component (E) is added at room temperature. In this case, components (C) and (D) may be mixed either before or after the heat treatment. When heat treatment is performed, the heat treatment temperature and time are not particularly limited, but it is preferable to perform the heat treatment at 100 to 250°C, especially 140 to 180°C, for about 30 minutes to 5 hours.

The silicone rubber composition of the present invention may be molded in a manner that is appropriate for the intended use (molded product). Specific examples include compression molding, injection molding, transfer molding, normal pressure hot air vulcanization, and steam vulcanization. There are no particular limitations on the curing conditions, which may be appropriately selected depending on the curing method and molded product. Curing conditions are generally 80 to 600°C, particularly 100 to 450°C, for a few seconds to several days, particularly about 5 seconds to 1 hour. Secondary vulcanization may also be performed if necessary. Secondary vulcanization is usually performed at 180 to 250°C for about 1 to 10 hours.

The silicone rubber composition of the present invention is useful as a construction gasket material (fire-resistant gasket material), and also as a wire coating material (fire-resistant wire material) that requires fire resistance, a rubber material for vehicles and/or on-board use, and a battery protection material.

The present invention will be specifically described below with examples and comparative examples, but the present invention is not limited to the following examples. In the following examples, parts indicate parts by mass. The degree of polymerization indicates the weight average degree of polymerization in terms of polystyrene in GPC (gel permeation chromatography) analysis using toluene as the developing solvent. The average particle size indicates the cumulative weight average diameter; d50 in particle size distribution measurement by laser diffraction method.

[Examples 1 to 6, Comparative Examples 1 to 3]
100 parts of an organopolysiloxane consisting of 99.825 mol % dimethylsiloxane units, 0.15 mol % methylvinylsiloxane units, and 0.025 mol % dimethylvinylsiloxy units and having a degree of polymerization of 7,000, 45 parts of fumed silica (Aerosil-200, manufactured by Nippon Aerosil Co., Ltd.) having a specific surface area of 200 ^m2 /g as measured by the BET method, and 5 parts of dimethylpolysiloxane having silanol groups at both ends and a degree of polymerization of 5 were mixed in a kneader and heat-treated at 150°C for 2 hours to produce a silicone rubber compound.
To 100 parts of the obtained rubber compound, calcined mica (manufactured by Repco Corporation, average particle size 18 μm) produced by calcining pulverized muscovite, aluminum hydroxide (manufactured by Huber Engineered Materials, average particle size 1 μm or 8 μm), magnesium hydroxide (manufactured by Kyowa Chemical Industry Co., Ltd., average particle size 0.8 μm or 1.7 μm), and titanium oxide (manufactured by Nippon Aerosil Co., Ltd., specific surface area by BET method ⁵⁰ mm2/g) were added in the amounts shown in Table 1, and further 0.1 parts of an alcohol solution of chloroplatinic acid (Pt concentration 2% by mass) was added using a two-roll mill, followed by the addition of 1.3 parts of a 50% by mass paste of p-methylbenzoyl peroxide to prepare a silicone rubber composition.

This composition was press molded at 120°C for 10 minutes to obtain a 2 mm thick rubber sheet, which was then post-cured at 200°C for 4 hours. Four pieces of 3.5 cm x 3.5 cm were cut out of this rubber sheet and heated in a high-temperature furnace at 800°C for 5 minutes to obtain four sintered bodies as test pieces for each example. The obtained sintered body 2 was placed on the base 3 of a testing machine (with a cylindrical tip and a diameter of 4 mm) equipped with a load cell 1 as shown in Figure 1, and was broken at a breaking speed of 120 mm/min (the arrow in Figure 1 indicates the load direction). The test force measurement results of the four sintered bodies (sintered body 1 to sintered body 4) for each example are shown in Table 1.

It was confirmed that the shape of the sintered body was stable in each of the Examples. Furthermore, in each of the Examples, the test force values of the test pieces of Sintered Body 1 to Sintered Body 4 were stable and high.
On the other hand, Comparative Example 1, which did not contain (D) metal hydroxide, showed excellent sinterability, but large swelling was observed in some places in the sintered body. Comparative Examples 2 and 3, which contained excessive amounts of aluminum hydroxide and magnesium hydroxide, respectively, showed large swelling overall after sintering, and the strength of all test pieces was significantly reduced. Also, stickiness was observed during rolling.

1 Load cell 2 Sintered body 3 Base

Claims (4)

(A) an organopolysiloxane having a degree of polymerization of 100 to 10,000 and having two or more alkenyl groups bonded to silicon atoms in each molecule: 100 parts by mass,
(B) reinforcing silica having a specific surface area of 50 to 500 m ² /g as measured by the BET method: 10 to 100 parts by mass,
(C) Calcined mica having an average particle size of 0.5 to 20 μm: 10 to 80 parts by mass,
A silicone rubber composition comprising: (D) 0.5 to 10 parts by mass of a metal hydroxide having an average particle size of 0.5 to 20 μm; and (E) an effective amount of a curing agent. The silicone rubber composition according to claim 1, wherein the calcined mica of component (C) is obtained by calcining pulverized muscovite. The silicone rubber composition according to claim 1 or 2, wherein the metal hydroxide of component (D) is aluminum hydroxide or magnesium hydroxide. The silicone rubber composition according to any one of claims 1 to 3, which is used for a fire-resistant gasket or a fire-resistant electric wire.

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