JP2000044219A - Production of thermally expandable graphite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the total sulfur content of a thermally expandable graphite after thermal expansion to such a level as to produce no harmful effect on corrosiveness. SOLUTION: When a graphite is acid-treated with a mixture of concd. sulfuric acid and a strongly oxidizing agent and it is washed and dried to produce a thermally expandable graphite, >=1,000 ppm phosphorus is left in the thermally expandable graphite by subjecting the thermally expandable graphite to phosphorus treatment. The total sulfur content of the resultant thermally expandable graphite can be reduced to such a level of <=1,700 ppm as to produce no harmful effect on corrosiveness and the graphite can be used for a graphite sheet used as a hermetically sealing member such as packing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing thermally expandable graphite which expands 100 to 300 times when heated to 1000 DEG C. and has a small residual sulfur content after thermal expansion.

[0002]

2. Description of the Related Art When heat-expandable graphite is heated to a high temperature, it expands instantaneously to tens to hundreds of times its original volume, and when it is pressed, it is a highly flexible graphite sheet without using a binder. Is obtained. In addition, if it is added to a polyurethane foam or the like, it can impart flame retardancy, and is therefore known as an industrially important material.

[0003] Thermally expandable graphite is usually treated with a mixture of concentrated sulfuric acid and a strong oxidizing agent (hereinafter, this treatment is called an acid treatment) of graphite such as natural graphite, pyrolytic graphite, and quiche graphite. After the compound is produced, it can be obtained by washing with water, filtering and drying. As the acid treatment method,
It is generally based on concentrated sulfuric acid such as concentrated sulfuric acid and nitric acid, concentrated sulfuric acid and potassium permanganate, concentrated sulfuric acid and perchloric acid, and concentrated sulfuric acid and hydrogen peroxide. A method using only fuming nitric acid is also known.

Recently, a method for producing thermally expandable graphite by electrolytic oxidation has also been proposed. In this method, graphite is placed in an acid bath such as sulfuric acid, formic acid, sulfuric acid and acetic acid, and anodizing or anodizing cathode reduction of the graphite is performed by an electrolytic reaction to form an intercalation compound between the graphite layers. It is obtained by filtering off and drying.

[0005] The heat-expandable graphite produced by the above-described chemical oxidation and electrolytic oxidation, when concentrated sulfuric acid is used in the production process,
Even after expansion, the sulfur content remains on the order of several thousand ppm.
After thermally expanding the heat-expandable graphite, for example, when formed into a sheet and used for sealing members such as packing, sliding members such as bearings, corrosion problems due to residual sulfur,
The SOx problem has recently gained much attention and is a major obstacle for this application.

[0006] Graphite sheets are used as gasket packing for automobiles, and the automotive industry in the United States defines residual sulfur in packings as 1200 ppm or less. In order to stably supply the heat-expandable graphite to the user for the graphite sheet, it is urgently necessary to reduce the sulfur remaining after heating the heat-expandable graphite.

[0007] In order to eliminate the residual sulfur of the heat-expandable graphite after heating, fuming nitric acid may be used for the acid treatment. However, not only harmful NOx is generated at the time of thermal expansion, but also because of fuming nitric acid, recycling is not possible. Impossible, waste acid treatment is essential. Further, if formic acid is used as an acid bath at the time of electrolytic oxidation, residual sulfur can be eliminated, but there is a problem in economical efficiency as an industrial production method.

As a method for reducing the residual sulfur of the order of several thousand ppm after the expansion of the thermally expandable graphite, expanded graphite containing residual sulfur (thermally expanded graphite) is added to 1 gram of the expanded graphite. A method in which air is oxidized for a holding time of 0.2 to 15 hours while being exposed to an atmosphere consisting of an air flow of 0.1 to 10 liters / minute and a temperature of 400 to 650 ° C. (Japanese Patent Laid-Open No. 60-118618). Gazette) has been proposed.

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the method disclosed in Japanese Patent No. 8618, expanded graphite that has been thermally expanded is air-oxidized at a temperature of 400 to 650 ° C., so that the graphite is partially oxidized and burned, and the yield is reduced. In addition, if an air oxidizing device at a temperature of 400 to 650 ° C. is incorporated in the graphite sheet manufacturing process, the air oxidizing retention time is long, which hinders the graphite sheet manufacturing.

On the other hand, a graphite sheet used for a sealing member such as a packing has a total sulfur of 1 after thermal expansion at 1000 ° C. for 10 seconds.
If it is 700 ppm or less, it is reduced to 1200 ppm or less at the stage of forming into a graphite sheet, and it has been confirmed that the corrosiveness due to residual sulfur does not pose a problem.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide a level of 1200 ppm or less when formed into a graphite sheet, which does not adversely affect corrosivity, that is,
An object of the present invention is to provide a method for producing thermally expandable graphite that can reduce the total sulfur content after thermal expansion to 1700 ppm or less.

[0012]

According to the method for producing thermally expandable graphite of the present invention, graphite is subjected to an acid treatment with a mixture of concentrated sulfuric acid and a strong oxidizing agent, followed by washing with water and drying to produce thermally expandable graphite. The method is characterized in that the obtained thermally expandable graphite is subjected to a phosphorus treatment. By subjecting the obtained thermally expandable graphite to a phosphorous treatment, the total sulfur content after thermal expansion can be reduced to a level that does not adversely affect the corrosiveness of 1700 p.
pm or less can be obtained, and it can be used as a graphite sheet for a sealing member such as packing.

The concentration of phosphorus remaining in the heat-expandable graphite is 10
When the content is not less than 00 ppm, it is possible to obtain a heat-expandable graphite in which the total sulfur content after thermal expansion is reduced to 1700 ppm or less, which is a level that does not adversely affect corrosiveness, and is used for a graphite sheet used for a sealing member such as packing. It can be used.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The graphite used as a raw material in the present invention is not particularly limited.
Although it is possible to use those used in the production of ordinary thermal expansion graphite such as pyrolytic graphite and quiche graphite, natural scaly graphite which is easily available is preferred. The particle size of graphite is 2
What is pulverized to 0 to 100 mesh, especially particle size 30 to
80 mesh is preferred.

The concentrated sulfuric acid used in the present invention has a concentration of 90% or more, preferably 95 to 98%. As the strong oxidizing agent, hydrogen peroxide or nitric acid is used. The concentration of hydrogen peroxide is 30% to 60%.
%, Preferably 60%. As nitric acid,
Commercially available 64% nitric acid can be used.

The amount of concentrated sulfuric acid, hydrogen peroxide or nitric acid, which is a strong oxidizing agent, is based on 100 parts by weight of graphite and 300 parts by weight of concentrated sulfuric acid.
-500 parts by weight, hydrogen peroxide or nitric acid is concentrated sulfuric acid 100
It is preferably 1 to 10 parts by weight based on parts by weight. In acid-treating graphite with concentrated sulfuric acid to which hydrogen peroxide or nitric acid as a strong oxidizing agent has been added, hydrogen peroxide or nitric acid is first mixed with concentrated sulfuric acid while cooling to 60 ° C. or lower, preferably 20 ° C. or lower.

In the acid treatment of graphite, the raw material graphite is charged while stirring the mixture adjusted as described above to form a slurry, and the slurry is further stirred at 50 ° C. or lower for 10 minutes to 3 minutes.
When the reaction is performed for 0 minutes, preferably for about 15 minutes, an intercalation compound is generated between the graphite layers. The liquid phase portion is separated and removed from the slurry of the acid-treated graphite, poured into water, filtered to collect the acid-treated graphite, and dried at 100 ° C. for 90 to 150 minutes, preferably about 120 minutes to obtain a thermal expansion. Graphite can be obtained. After washing with water and filtering the slurry,
Neutralization can also be performed by adding an alkali such as ammonia.

In the water washing, a liquid phase portion such as sulfuric acid is separated and removed from the slurry after the acid treatment, and the acid-treated graphite separated as a solid content is added to a large amount of water, and the mixture is thoroughly stirred and washed with water. If necessary, washing with water can be repeatedly performed. After washing with water, the acid-treated graphite from which the liquid phase has been separated and removed is recovered as a solid content. When the recovered acid-treated graphite is neutralized with ammonia water, the acid-treated graphite is usually added to a tank containing ammonia water, and the mixture is thoroughly stirred and then separated and collected.The filtration process of separating and removing the liquid phase after washing with water In this method, ammonia water is sprinkled directly on the recovered material as a residue to make contact therewith. The concentration of aqueous ammonia is usually 3 to 4%. Washing process,
If necessary, the acid-treated graphite obtained by neutralization treatment is usually dried until the volatile matter becomes 1% by weight or less by a method such as tray drying, flash drying, fluidized drying, and the like, to form a thermally expandable graphite. Become.

In the phosphorus treatment applied to the heat-expandable graphite, neutralization and phosphorus treatment can be performed simultaneously with an aqueous solution of ammonium phosphate in the neutralization step of the acid-treated graphite after washing with water, or the acid treatment filtered off after the neutralization. A phosphoric acid aqueous solution may be sprayed and mixed on graphite. Further, a phosphoric acid solution or an aqueous solution of phosphoric acid can be sprayed and mixed on the thermally expandable graphite obtained by drying. In the phosphorus treatment performed before drying, if a methanolic phosphate solution is used, methanol is volatilized in the drying process after the phosphorus treatment, and there is a risk of explosion. Therefore, an aqueous phosphoric acid solution is used. Further, in the phosphorus treatment performed after drying, if an aqueous phosphoric acid solution is used, it is necessary to dry again after the phosphorus treatment. Therefore, a methanolic phosphate solution is used. When a methanolic phosphate solution is used, methanol is volatilized during the mixing, and there is no need to re-dry.

In the phosphorus treatment performed before drying,
For example, after a phosphoric acid aqueous solution is sprayed and mixed on 1/10 to 1/20 of the acid-treated graphite, the mixture is added to the remaining acid-treated graphite, and a scraping-up type mixer is used. Further, in the phosphorus treatment performed after drying, 1/10 to 1
/ 20 is sprayed with methanolic phosphate solution and mixed.
By adding this to the remaining heat-expandable graphite and mixing using a scraping-type mixer, methanol is volatilized during mixing, and there is no need to dry the heat-expandable graphite again.
When the phosphoric acid aqueous solution is sprayed and mixed after drying, it is necessary to dry again.

In the phosphorus treatment, 0.5 parts by weight or more, preferably 1 part by weight or more, of phosphoric acid is dissolved or diluted in methanol or water with respect to 100 parts by weight of the heat-expandable graphite. The addition and mixing are performed so that phosphorus is left in the heat-expandable graphite at 1000 ppm or more. The residual phosphorus concentration in the thermally expandable graphite is 100
If it is less than 0 ppm, the total sulfur content after expansion after heating at 1000 ° C. for 10 seconds is said to be a level that does not adversely affect corrosiveness when formed on a graphite sheet, 1700 pp
m cannot be reduced.

As a phosphorus source used for the phosphorus treatment, ammonium salts such as phosphoric anhydride, orthophosphoric acid, polyphosphoric acid or ammonium phosphate, ammonium pyrophosphate and ammonium metaphosphate can be used. Phosphoric acid, polyphosphoric acid, or ammonium salts thereof are used after being dissolved or diluted in methanol or water.

The heat-expandable graphite obtained by the above method has the following properties:
Total sulfur content after expansion after heating at 000 ° C. for 10 seconds is
When formed into a graphite sheet, the content is reduced to 1700 ppm or less, which is said to be a level that does not adversely affect corrosivity. The reason is not clear, but by phosphorus treatment,
Phosphorus adheres not only to the graphite surface but also to the crystal ends of the graphite crystal in the direction perpendicular to the layer surface (C axis), and the phosphorus forms a glassy substance by heating to coat the crystal ends. It is considered that as a result of suppressing the formation of -CS bonds, -COS bonds, etc., the total sulfur content after expansion is reduced.

[0024]

Example 1 300 parts by weight of 98% sulfuric acid and 8 parts by weight of 60% hydrogen peroxide are mixed while cooling to 60 ° C. or lower, preferably 20 ° C. or lower. While stirring this mixture, 20-
100 parts by weight of 100 mesh natural scaly graphite,
After the slurry was formed, the mixture was further reacted for 30 minutes with stirring at 50 ° C. or lower to generate an intercalation compound between the graphite layers. A liquid phase portion such as sulfuric acid was separated and removed from the slurry after the acid treatment, and the acid-treated graphite separated as a solid was added to a large amount of water, followed by thorough stirring and washing with water. After the water washing, the acid-treated graphite from which the liquid phase was separated and removed was added to a tank containing 3% ammonia water, stirred sufficiently, and then separated and collected.

With respect to 100 parts by weight of the obtained acid-treated graphite,
A phosphoric acid aqueous solution in which 0.4 to 5 parts by weight of phosphoric acid having a concentration of 98% is dissolved in 2 parts by weight of water is sprayed, mixed using a V-type blender, and dried at 105 ° C. for 2 hours with a hot air drier. A heat-expandable graphite treated with phosphorus was obtained. Each of the obtained heat-expandable graphites was measured for the residual phosphorus concentration and at 1000 ° C.
And expanded for 10 seconds, and the degree of expansion (cc / g) at 1000 ° C. and the residual sulfur content after expansion were measured. Table 1 shows the results. For comparison, the degree of expansion of the thermally expandable graphite not subjected to the phosphorus treatment at 1000 ° C. (c
c / g) and the residual sulfur content after expansion was measured. Table 1 shows the results.

The residual phosphorus concentration was determined by extracting phosphorus from thermally expanded graphite subjected to phosphorus treatment, and then measuring the phosphorus concentration of the extract by ion chromatography. The degree of expansion is 10
Volume 15 held for 5 minutes or more in an electric furnace held at 00 ° C
A 0 cc quartz beaker is taken out of the furnace, and a 0.5 g sample of the heat-expandable graphite is immediately charged, and immediately placed in an electric furnace maintained at 1000 ° C. After holding for 10 seconds as it is, it is taken out of the furnace, allowed to cool naturally, and the volume after expansion is measured by a scale attached to a beaker. As the degree of expansion, the volume after expansion relative to the weight of the sample (cc / g) was indicated. Also,
The residual sulfur content was measured using an SC-432 sulfur analyzer (combustion-absorption method) manufactured by LECO, using an expanded graphite whose expansion degree was measured as a sample.

[0027]

[Table 1]

As shown in Table 1, the phosphorus-treated test N
o. 2-5, the residual phosphorus concentration in the heat-expandable graphite is 1
Test No. of 000 ppm or more. The thermally expandable graphites 3 to 5 have a total sulfur after expansion of 1700 ppm or less. On the other hand, the residual phosphorus concentration in the heat-expandable graphite is 10%.
Test No. less than 00 ppm Test No. 2 and the test No. In the thermally expandable graphite of No. 1, the total sulfur after expansion is 1800 ppm or more, which adversely affects the corrosiveness.

Example 2 300% by weight of 98% strength concentrated sulfuric acid and 64% strength nitric acid 8
After mixing the parts by weight while cooling the mixture to 20 ° C or lower, 100 parts by weight of natural scaly graphite pulverized to 20 to 100 mesh is added while stirring the mixture, and the mixture is slurried, and further stirred at 50 ° C or lower. While reacting for 30 minutes, an interlayer compound was generated between the graphite layers. In the slurry after the acid treatment, a liquid phase portion such as sulfuric acid was separated and removed, and the acid-treated graphite separated as a solid was added to a large amount of water, and the mixture was thoroughly stirred and washed with water. After the water washing, the acid-treated graphite from which the liquid phase was separated and removed was added to a tank containing 3% ammonia water, stirred sufficiently, and then separated and collected. The obtained acid-treated graphite was dried with a hot air drier at 105 ° C. for 2 hours to obtain thermally expandable graphite.

To 100 parts by weight of the obtained heat-expandable graphite, a methanol solution of polyphosphoric acid in which 0.5 to 5 parts by weight of polyphosphoric acid having a concentration of 105% was dissolved in 2 parts by weight of methanol was sprayed. After mixing, the residual phosphorus concentration was measured. In the same manner as in Example 1, the obtained phosphorus-treated thermally expandable graphite was measured for the degree of expansion (cc / g) at 1000 ° C. and the residual sulfur content after expansion. Table 2 shows the results. Also, for comparison,
Regarding the heat-expandable graphite not subjected to the phosphorus treatment, the residual phosphorus concentration and the degree of expansion (cc / g) at 1000 ° C. were measured, and the residual sulfur content after expansion was measured. Table 2 shows the results. In addition, methanol volatilized at the time of mixing in the V-type blender, and there was no need to dry the thermally expandable graphite.

[0031]

[Table 2]

As shown in Table 2, a test N in which phosphorus was treated to leave phosphorus in the heat-expandable graphite in an amount of 1000 ppm or more was carried out.
o. The thermally expandable graphite of 7 to 10 has a total sulfur of 17 after expansion.
It is less than 00 ppm. On the other hand, in test No. In Comparative Example No. 6, the total sulfur after expansion is 2300 ppm which adversely affects the corrosiveness.

[0033]

The method for producing thermally expansible graphite of the present invention is a method for producing thermally expansible graphite by subjecting graphite to acid treatment with a mixture of concentrated sulfuric acid and a strong oxidizing agent, followed by washing and drying. By subjecting the obtained heat-expandable graphite to a phosphorous treatment and leaving phosphorus in the heat-expandable graphite at 1000 ppm or more,
It is possible to obtain thermally expandable graphite having a total sulfur content after thermal expansion of 1700 ppm or less, which is a level that does not adversely affect corrosiveness, and can be used as a graphite sheet used for a sealing member such as packing. .

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Shigeo Kawamura 1850 Minato, Wakayama-shi, Wakayama Sumikin Chemical Wakayama Office (72) Inventor Mikio Oka 1850 Minato, Wakayama-shi, Wakayama Sumikin Chemical Wakayama Business In-house F-term (reference) 4G046 EA05 EB06 EC02 EC05 EC08 4H017 AA29 AC13 AC16 AD01 AE05 4J037 AA01 CA08 CA18 CA20 CA22 CB04 DD20 EE02 EE33 EE35 EE43 FF13 FF17

Claims (2)

[Claims] 1. A method for producing heat-expandable graphite by subjecting graphite to acid treatment with a mixture of concentrated sulfuric acid and a strong oxidizing agent, followed by washing with water and drying, wherein the obtained heat-expandable graphite is subjected to a phosphorus treatment. A method for producing thermally expandable graphite, comprising: 2. The method according to claim 1, wherein the concentration of phosphorus remaining in the heat-expandable graphite is 1%.
2. The method for producing thermally expandable graphite according to claim 1, wherein the concentration is not less than 000 ppm.