JPWO2001005703A1 - Heat-resistant expandable graphite sheet and its manufacturing method - Google Patents

Abstract

(57) [Abstract] The objective of this invention is to provide an expanded graphite sheet that has a low rate of oxidation attrition in air and excellent heat resistance, even when exposed to high-temperature conditions of 700°C or higher for long periods of time, and that also satisfies the various other performance requirements of this type of expanded graphite sheet. This objective is achieved by incorporating phosphorus pentoxide and phosphate into the expanded graphite sheet.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates to an expanded graphite sheet and a method for producing the same, and more particularly to an expanded graphite sheet having extremely excellent heat resistance and oxidation wear resistance, and a method for producing the same. BACKGROUND ART Expanded graphite sheets are produced by treating graphite such as natural graphite, kish graphite, or pyrolytic graphite with concentrated sulfuric acid,
An intercalation compound is formed by treating graphite particles with a strong oxidizing agent such as concentrated nitric acid, concentrated sulfuric acid and potassium chlorate, concentrated sulfuric acid and potassium nitrate, or hydrogen peroxide, or a halide such as bromine or aluminum chloride. The graphite particles (acid-treated graphite raw material) with this intercalation compound formed are then rapidly heated, for example, at a high temperature of 950°C or higher for 1 to 10 seconds to generate decomposition gas. The gas pressure expands the spaces between the graphite layers to form expanded graphite particles, which are then compression molded or roll-formed with or without a binder. Expanded graphite sheets produced in this manner have various excellent properties and are effectively used in a wide range of fields, such as gaskets, sealing materials, heat insulating materials, and cushioning materials. Conventionally, expanded graphite particles used in this type of expanded graphite sheet have a low expansion ratio, for example, about 20 to 70 times, which makes them difficult to mold into a sheet without a binder and requires the use of an adhesive, which has the drawback of reducing purity and various physical properties. In contrast, expanded graphite particles with a high expansion ratio, usually about 200 to 300 times, can be used to produce sheets using only graphite, resulting in high purity and various excellent physical properties. Therefore, currently, expanded graphite particles with a high expansion ratio are usually used to produce expanded graphite sheets. However, conventional expanded graphite sheets have a problem with heat resistance in air, particularly in high-temperature air above 700°C, resulting in a fatal drawback of a high rate of oxidation attrition, which causes oxidation attrition of the graphite. To solve this problem, expanded graphite sheets have been developed that use low-expansion graphite particles and are treated with phosphoric acid or phosphates to inhibit oxidation (Japanese Patent Publication No. 54-30).
No. 678). This publication discloses that low-expansion graphite particles can be formed into sheets without adhesives by using phosphoric acid or phosphates. However, even if sheets can be formed, since no adhesive is basically used, various physical properties required for expanded graphite sheets, particularly mechanical properties and sheet uniformity, are not satisfactory. Furthermore, although it is stated that oxidation resistance is improved, this is also insufficient, and particularly when exposed to long periods of time, oxidation wear is severe, making it by no means satisfactory. The present invention has been made in view of the above points, and its object is to develop a sheet of low-expansion graphite particles that can be formed into sheets without adhesives at temperatures up to 700°C.
The present inventors have conducted extensive research to achieve the above object, and as a result, have found that an expanded graphite sheet containing phosphorus pentoxide and phosphate in a predetermined ratio has excellent heat resistance and can withstand temperatures of 70°C or more.
It was discovered that even when exposed to high-temperature conditions of 0°C or higher for long periods of time, the rate of oxidative loss in air is low, and that various other performance requirements for this type of expandable graphite sheet can be satisfied. The present invention was completed based on this discovery, and the gist of each invention is as follows. A heat-resistant expandable graphite sheet according to a first aspect of the present invention contains phosphorus pentoxide and a phosphate. The heat-resistant expandable graphite sheet according to the first aspect contains phosphorus pentoxide and a phosphate, so that the sheet has excellent heat resistance and, even when exposed to high-temperature conditions of 700°C or higher for long periods of time, the rate of oxidative loss in air is low, making it suitable for various applications under high-temperature conditions. Furthermore, the sheet has various properties required for this type of expandable graphite sheet and can satisfy various other performance requirements. A heat-resistant expandable graphite sheet according to a second aspect of the present invention is the heat-resistant expandable graphite sheet according to the first aspect, containing 0.05 to 5.0 wt % of phosphorus pentoxide and 1 to 16 wt % of phosphate. According to the heat-resistant expandable graphite sheet of the second aspect of the present invention, if the content of phosphorus pentoxide in the sheet is less than 0.05 wt%, the oxidation loss rate of the sheet cannot be significantly reduced, and even if the content exceeds 5.0 wt%, the oxidation loss rate is not reduced. Furthermore, if the content of phosphate is less than 1 wt%, the oxidation loss rate is not sufficiently reduced, and if the content exceeds 16 wt%, the sheet tends to become hard, impairing the flexibility of the expanded graphite sheet. According to the heat-resistant expandable graphite sheet of the third aspect of the present invention, the phosphate contained in the heat-resistant expandable graphite sheet of the first or second aspect is selected from lithium monophosphate, lithium diphosphate, calcium monophosphate, calcium diphosphate, aluminum monophosphate, and aluminum diphosphate. According to the heat-resistant expandable graphite sheet of the fourth aspect of the present invention, the heat-resistant expandable graphite sheet of any of the first to third aspects has an oxidation loss rate of less than 10% when exposed to air at 700°C for 3 hours. According to the fourth aspect of the present invention, the heat-resistant expandable graphite sheet contains phosphorus pentoxide and phosphate in a predetermined ratio, resulting in excellent heat resistance and an extremely low oxidation loss rate even when exposed to high temperatures for a long period of time. According to the fifth aspect of the present invention, a method for producing a heat-resistant expandable graphite sheet involves treating a graphite raw material with a strong acid and phosphoric acid, adding phosphate to the acid-treated graphite raw material, drying the material, and subsequently expanding it to obtain expanded graphite powder, which is then compression-molded or roll-molded into a sheet. According to the sixth aspect of the present invention, a method for producing a heat-resistant expandable graphite sheet involves treating a graphite raw material with a strong acid, adding phosphoric acid and phosphate to the acid-treated graphite raw material, drying the material, and subsequently expanding it to obtain expanded graphite powder, which is then compression-molded or roll-molded into a sheet. Examples of the strong acid used in this method include sulfuric acid. The expansion treatment is carried out at an expansion temperature of 900°C or higher, preferably about 950 to 1200°C, for 200 to 30 minutes.
It is preferable to expand the graphite sheet by about 0 times. In a seventh aspect of the present invention, in the methods for producing a heat-resistant expandable graphite sheet of the fifth and sixth aspects, the phosphoric acid is selected from orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and polymetaphosphoric acid. In the method for producing a heat-resistant expandable graphite sheet of the seventh aspect of the present invention, phosphoric acid uniformly blended into the acid-treated graphite raw material generates phosphorus pentoxide (P ₂ O ₅ ) by a dehydration reaction in the expansion treatment step by heating, and this is contained in a predetermined amount in the sheet by compression molding or roll molding. In an eighth aspect of the present invention, in the method for producing a heat-resistant expandable graphite sheet of the fifth or sixth aspect, the phosphate is selected from lithium monophosphate, lithium diphosphate, calcium monophosphate, calcium diphosphate, aluminum monophosphate, and aluminum diphosphate. According to the ninth aspect of the method for producing a heat-resistant expandable graphite sheet of the present invention, the phosphate uniformly blended into the acid-treated graphite raw material remains substantially unchanged in the sheet as the phosphate even during the expansion treatment by heating. Therefore, the heat-resistant expandable graphite sheet produced by this method contains phosphorus pentoxide and phosphate in predetermined proportions. Hereinafter, embodiments of the present invention will be described in detail. A method for producing a heat-resistant expandable graphite sheet will be described. The acid-treated graphite raw material of the present invention refers to a raw material prepared by treating graphite with sulfuric acid and an oxidizing agent according to a conventional method, followed by drying according to a conventional method, and is a conventionally used raw material. More specifically, the acid-treated graphite is prepared by acid-treating graphite with a strong acid, such as sulfuric acid, using a conventionally known oxidizing agent such as hydrogen peroxide, and then drying the resulting graphite according to a conventional method, typically at about 100 to 120°C. The present invention also includes graphite treated with phosphoric acid together with sulfuric acid. The acid-treated graphite raw material is described in more detail below. (A) Acid-treated graphite raw material obtained by treating graphite raw material with sulfuric acid (B) Acid-treated graphite raw material obtained by treating graphite raw material with sulfuric acid and phosphoric acid In the present invention, any of the above acid-treated graphite raw materials is used. Using any of these acid-treated graphite raw materials, in the case of (A), phosphoric acid and a phosphate are added simultaneously, or phosphoric acid is added first and then the phosphate is added, and in the case of (B), the phosphate is added, and then, in accordance with a conventional method, the graphite is expanded by about 200 to 300 times at an expansion temperature of preferably about 950 to 1200°C, and then formed into a sheet. As the graphite in this case, various graphite raw materials that have been conventionally used, such as natural graphite, kish graphite, and pyrolytic graphite _, are widely used. The phosphoric acid to be uniformly blended into the acid-treated graphite powder is orthophosphoric acid ( _H3PO4 )
The phosphoric acid salt _{is selected} from linear condensed phosphoric acids such as metaphosphoric acid ( _HPO3 ), polyphosphoric acid, specifically _{pyrophosphoric acid (H4P2O7 )} and _{tripolyphosphoric} acid ( _H5P8O10 ), and cyclic condensed phosphoric acids such as polymetaphosphoric acid, specifically trimetaphosphoric acid and tetrametaphosphoric acid, and is usually used in the form of an aqueous solution. Furthermore, primary phosphates and secondary phosphates are used as phosphates to be uniformly blended with the phosphoric acid, and among these, alkali metal salts and alkaline earth metal salts are preferred, with lithium and calcium being particularly preferred. Furthermore, aluminum salts can also be used as the metal salt. Specifically, lithium primary phosphate ( _LiH2P
O ₄ ), dibasic lithium phosphate (Li ₂ HPO ₄ ), monobasic calcium phosphate [Ca(H
Examples of _suitable phosphates include _dibasic calcium phosphate ( _CaHPO4 ), monobasic aluminum phosphate (Al( _H2PO4 ) _{3 )} , and dibasic aluminum phosphate ( _Al2 ( _HPO4 ) ₃ ). These phosphates are used in the form of an aqueous solution or a suspension. Next, the expanded graphite raw material is treated at a high temperature of preferably 950 to 1200°C for 1 _to 10 seconds to generate decomposition gas, and the gas pressure expands the spaces between the graphite layers to 20°C.
After forming expanded graphite particles expanded by about 0 to 300 times, the expanded graphite particles are compression molded or roll molded to produce an expanded graphite sheet. The expanded graphite sheet thus obtained contains phosphorus pentoxide ( _P2O5 ) and phosphates produced by the dehydration reaction of phosphoric acid. The amount of phosphorus pentoxide and phosphates contained in the expanded graphite sheet determines the heat resistance of _the expanded graphite sheet, and therefore the oxidation loss rate of the expanded graphite sheet. In the present invention, the phosphorus pentoxide content is 0.05 to 5.0% by weight, preferably 0.2 to 100% by weight.
It has been confirmed that an expanded graphite sheet containing 2.0 wt% phosphorus pentoxide and 1 to 16 wt%, preferably 2 to 10 wt%, of phosphate has excellent heat resistance and, consequently, excellent resistance to oxidation loss. If the phosphorus pentoxide content in the expanded graphite sheet is less than 0.05 wt%, the oxidation loss rate of the expanded graphite sheet cannot be significantly reduced. Furthermore, if the content exceeds 5.0 wt%, the oxidation loss rate is not reduced and white smoke is generated during the dehydration reaction of phosphoric acid to produce phosphorus pentoxide, which is undesirable from an environmental hygiene perspective. Furthermore, if the phosphate content is less than 1 wt%, the oxidation loss rate is not sufficiently reduced. Furthermore, if the phosphate content exceeds 16 wt%, the expanded graphite sheet tends to become hard when formed into a graphite sheet, impairing its flexibility. The properties of the expanded graphite sheet of the present invention containing 0.05 to 5.0 wt% of phosphorus pentoxide and 1 to 16 wt% of phosphate are as follows: [0023] Thickness (mm): 0.2-1.5 Bulk density (g/ ^cm3 ): 0.8-1.1 Tensile strength (kgf/ ^cm2 ): 40-60 Compression ratio (70 kgf/ ^cm2 ,%): 10-25 Recovery ratio (70 kgf/ ^cm2 ,%): 25-45 [0024] Figure 1 shows the results of tests on the oxidation loss rate of the expanded graphite sheets of the present invention, which contain 4 wt% phosphate (aluminum monophosphate) and have various amounts of phosphorus pentoxide. The graph showing the test results shows that the expanded graphite sheets containing phosphorus pentoxide and phosphate have an extremely low oxidation loss, i.e., weight loss rate of less than 10%, even under harsh conditions such as 700°C for 3 hours. [0025] Industrial Applications [0026] The heat-resistant expandable graphite sheet of the present invention has excellent heat resistance and oxidation loss resistance, and is therefore suitable for use in gaskets, sealing materials, heat insulating materials, cushioning materials, etc. BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[Examples 1 to 20] While stirring 300 parts by weight of concentrated sulfuric acid with a concentration of 98%, 5 parts by weight of a 60% aqueous solution of hydrogen peroxide was added as an oxidizing agent to prepare a reaction liquid.
The temperature was maintained at ° C., and 100 parts by weight of natural flake graphite powder having a particle size of 30 to 80 mesh was added, followed by a reaction for 30 minutes. After the reaction, the acid-treated graphite was separated by suction filtration.
The acid-treated graphite was stirred with 300 parts by weight of water for 10 minutes and then suction filtered. This washing procedure was repeated twice to thoroughly remove sulfuric acid from the acid-treated graphite. Next, the acid-treated graphite from which the sulfuric acid had been thoroughly removed was dried for 3 hours in a drying oven maintained at a temperature of 110°C, and this was used as an acid-treated graphite raw material. While stirring 100 parts by weight of the acid-treated graphite raw material, 0.16 to 3.5 parts by weight of an aqueous solution of orthophosphoric acid with a concentration of 84% as phosphoric acid and 5 parts by weight of phosphate were added to the acid-treated graphite raw material.
A solution of 2 to 38 parts by weight of a 0% aqueous aluminum monophosphate solution diluted with 10 parts by weight of methanol was sprayed and uniformly stirred to obtain a wettable mixture. This wettable mixture was dried for 2 hours in a drying oven maintained at 120°C. The mixture was then treated at 1000°C for 5 seconds to generate decomposition gas, and the gas pressure expanded the spaces between the graphite layers to produce expanded graphite particles (expansion ratio 240 times). During this expansion treatment, orthophosphoric acid in the components underwent a dehydration reaction to produce phosphorus pentoxide, and it was confirmed that aluminum monophosphate remained almost unchanged and coexisted with phosphorus pentoxide. The expanded graphite particles were then mixed at a roll nip of 0.33 m.
The expanded graphite sheets were rolled at a rolling mill at 1000°C to produce expanded graphite sheets with a thickness of 0.36 mm. The composition of the expanded graphite sheets thus obtained and the results of tests on the oxidation loss rates of the expanded graphite sheets are shown in Tables 1 to 5. The numerical values of the composition in the tables are expressed in weight %. The oxidation loss rate of the expanded graphite sheets was evaluated by leaving the expanded graphite sheets in air maintained at a temperature of 700°C for 3 hours, and then expressing the weight loss rate (%) of the expanded graphite sheets.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

Examples 21 to 28 Acid-treated graphite raw materials were prepared in the same manner as in Example 1.
While stirring 0.7 to 1.4 parts by weight of an aqueous solution of orthophosphoric acid having a concentration of 84% as phosphoric acid and 4.0 to 17.4 parts by weight of an aqueous solution of calcium phosphate having a concentration of 50% as phosphate, diluted with 20 parts by weight of methanol, were sprayed into the acid-treated graphite raw material, and the mixture was stirred uniformly to obtain a mixture having wettability.
Expanded graphite particles with an expansion ratio of 240 times were produced in the same manner as in Example 1, and then expanded graphite sheets were produced by treating them in the same manner as in Example 1. The component compositions of the expanded graphite sheets obtained in this manner and the results of tests on the oxidation loss rates of the expanded graphite sheets are shown in Tables 6 and 7. The values for the component compositions in the tables are in weight percent. The oxidation loss rates of the expanded graphite sheets were evaluated in the same manner as in the previous examples.

[Table 6]

[Table 7]

Comparative Examples 1 to 5 Acid-treated graphite raw materials were prepared in the same manner as in Example 1.
While stirring 0 parts by weight of the acid-treated graphite raw material, 0.3 to 1.7 parts by weight of an aqueous solution of orthophosphoric acid having a concentration of 84% as the phosphate salt was sprayed into the acid-treated graphite raw material, and the mixture was stirred uniformly to obtain a mixture having wettability. Subsequently, expanded graphite particles having an expansion ratio of 250 times were produced in the same manner as in Example 1, and subsequently, expanded graphite sheets were produced by treating them in the same manner as in Example 1. The component composition of the expanded graphite sheet obtained in this manner and the results of testing the oxidation loss rate of the expanded graphite sheet are shown in Table 8. The values of the component composition in the table are expressed in weight %, and the oxidation loss rate of the expanded graphite sheet was evaluated in the same manner as in the previous examples.

[Table 8]

Comparative Examples 6 to 9 Acid-treated graphite raw materials were prepared in the same manner as in Example 1.
While stirring 0 parts by weight of the acid-treated graphite raw material, 8.4 to 38 parts by weight of a solution prepared by diluting 8.4 to 38 parts by weight of an aqueous solution of 50% aluminum phosphate as a phosphate salt with 30 parts by weight of methanol was sprayed into the acid-treated graphite raw material and stirred uniformly to obtain a mixture having wettability. Subsequently, expanded graphite particles having an expansion ratio of 230 times were produced in the same manner as in Example 1, and subsequently, expanded graphite sheets were produced by treating them in the same manner as in Example 1. The component composition of the expanded graphite sheet thus obtained and the results of testing the oxidation loss rate of the expanded graphite sheet are shown in Table 9. The values of the component composition in the table are expressed in weight %, and the oxidation loss rate of the expanded graphite sheet was evaluated in the same manner as in the previous examples.

[Table 9] The expanded graphite sheets of Examples 1 to 28 contain phosphorus pentoxide and phosphates produced by the dehydration reaction of phosphoric acid, and therefore exhibit a synergistic effect of the two even under high-temperature conditions of 700°C, resulting in an extremely low oxidation loss rate (weight loss rate), demonstrating heat resistance. On the other hand, the expanded graphite sheets of Comparative Examples, which contain either phosphoric acid or phosphates alone in the expanded graphite, have a high oxidation loss rate (weight loss rate),
In particular, the expanded graphite sheets containing only phosphate (Comparative Examples 6 to 9) had an extremely high rate of oxidative loss and were poor in heat resistance. The properties of the expanded graphite sheets of Examples 8 and 15 are shown in Table 10.

[Table 10] From Table 10, it can be seen that the expanded graphite sheets of Examples 8 and 15, which have an extremely low oxidation loss rate and heat resistance, have properties almost equivalent to those of conventional products without impairing the properties inherent in expanded graphite sheets. However, "conventional product" in Table 10 refers to the expanded graphite sheet "product number PF-38D" manufactured by Toyo Tanso Co., Ltd. The flexibility evaluation in Table 10 was carried out using the test equipment shown in Figure 2, with a 10 mm wide sheet.
The bending strength was measured by bending a 100 mm long sample (expanded graphite sheet) alternately at 90-degree angles until the sample broke. In Fig. 2, reference numeral 1 denotes the sample, 2 denotes a 50 g weight, and 3 denotes the bending range. As is clear from the above examples, the expanded graphite sheet of the present invention contains phosphorus pentoxide and phosphate in a predetermined ratio, and therefore has heat resistance and can be used for 70
The expanded graphite sheet of the present invention exhibits an extremely low oxidation loss rate even under high-temperature conditions of 0°C or higher, and has properties equivalent to those of the expanded graphite sheet without impairing in any way the properties that the expanded graphite sheet originally possesses. INDUSTRIAL APPLICABILITY As described above, the expanded graphite sheet of the present invention has extremely excellent heat resistance and oxidation loss resistance, and is therefore effectively used in gaskets, sealing materials, heat insulating materials, cushioning materials, etc.

[Brief explanation of the drawings]

Figure 1 is a graph showing the results of testing the oxidation loss rate of an expanded graphite sheet when the phosphate (aluminum monophosphate) content was kept constant at 4 wt % and the phosphorus pentoxide content was varied, and the oxidation loss rate of the expanded graphite sheet was expressed as the weight loss rate when the expanded graphite sheet was left to stand for 3 hours in air maintained at a temperature of 700° C. Figure 2 is a diagram showing an example of a testing device for evaluating the flexibility of an expanded graphite sheet.

───────────────────────────────────────────────────── Continued from the front page (72) Inventor: Kohei Kurose 8 Kirihara-cho, Fujisawa City, Kanagawa Prefecture, Oiles Corporation, Fujisawa Plant (72) Inventor: Hisaki Kusuyama 5-7-12 Takeshima, Nishiyodogawa-ku, Osaka City, Osaka Prefecture, Toyo Tanso Co., Ltd. (72) Inventor: Hideki Imoto 2791 Matsuzaki, Takuma-cho, Mitoyo-gun, Kagawa Prefecture, Toyo Tanso Co., Ltd., Takuma Plant (72) Inventor: Hideaki Kakimi 2791 Matsuzaki, Takuma-cho, Mitoyo-gun, Kagawa Prefecture, Toyo Tanso Co., Ltd., Takuma Plant (Note) This publication is based on the publication published internationally by the International Bureau of International Trade and Industry (WIPO). Please note that the effect of the international publication of the Japanese-language patent application (Japanese-language utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act), and is unrelated to this publication.

Claims (9)

[Claims] 1. A heat-resistant expandable graphite sheet containing phosphorus pentoxide and a phosphate. 2. The content of said phosphorus pentoxide is 0.05 to 5.0% by weight, and said phosphate is 1.
2. The heat-resistant expandable graphite sheet according to claim 1, wherein the content is in the range of 1 to 16% by weight. 3. The heat-resistant expandable graphite sheet according to claim 1, wherein the phosphate is selected from the group consisting of lithium monophosphate, lithium diphosphate, calcium monophosphate, calcium diphosphate, aluminum monophosphate, and aluminum diphosphate. Claim 4: The oxidation loss rate when exposed to air at 700°C for 3 hours is 10%
The heat-resistant expandable graphite sheet according to any one of claims 1 to 3, wherein the heat-resistant expandable graphite sheet has a viscosity of less than 1000 MPa. 5. A method for producing a heat-resistant expandable graphite sheet, comprising the steps of: treating graphite raw material with a strong acid and phosphoric acid to produce an acid-treated graphite raw material; and adding a phosphate to the acid-treated graphite raw material. 6. A method for producing a heat-resistant expandable graphite sheet, comprising the steps of: treating graphite raw material with a strong acid to form acid-treated graphite raw material; and adding phosphoric acid and a phosphate to the acid-treated graphite raw material. 7. The method for producing a heat-resistant expandable graphite sheet according to claim 5, wherein the phosphoric acid is selected from the group consisting of orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and polymetaphosphoric acid. [Claim 8] A method for producing a heat-resistant expandable graphite sheet according to any one of claims 5 to 7, wherein the phosphate is selected from lithium monophosphate, lithium diphosphate, calcium monophosphate, calcium diphosphate, aluminum monophosphate and aluminum diphosphate. 9. A heat-resistant expandable graphite sheet according to claim 5, wherein the heat-resistant expandable graphite sheet contains 0.05 to 5.0% by weight of phosphorus pentoxide and 1 to 16% by weight of phosphate.
A heat-resistant expandable graphite sheet containing the graphite in a ratio of 100% by weight.