| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 19.09.2000 |
| PATENT TITLE |
Method of making an activated fabric of carbon fibers |
| PATENT ABSTRACT |
A fabric made of fibers of a carbon-precursor cellulose material is impregnated with a composition containing at least one inorganic ingredient having a function of promoting dehydration of cellulose, and the fabric is subjected to heat treatment. This treatment consists in raising temperature at a speed lying in the range 1.degree. C./min to 15.degree. C./min followed by keeping the temperature constant in the range 350.degree. C. to 500.degree. C., and it is followed by a step of washing the fabric. This produces directly an activated fabric of carbon fibers having a specific surface area of not less than 600 m.sup.2 /g, without subsequent activation treatment at a higher temperature. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | September 13, 1999 |
| PATENT CT FILE DATE | March 12, 1998 |
| PATENT CT NUMBER | This data is not available for free |
| PATENT CT PUB NUMBER | This data is not available for free |
| PATENT CT PUB DATE | September 24, 1998 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED |
Lai et al, ACS Symp. Ser. (1977), 48 (Cellul. Chem. Technol., Symp.), pp. 256-272. Zakrzewska, Koks, Smola, Gaz (1981), 26(4), pp. 105-108. Gavrilov, Khim. Drev. (1983), (2), pp. 23-26. Derwent Abstract, AN 85-287343, JP 60 198 166A, Oct. 1985. Derwent Abstract, AN 83-49756k, JP 57 167 716, Oct. 1982. Derwent Abstract, AN 77-24525y, JP 52 025 120A, Feb. 1977. Derwent Abstract, AN 89-169593, JP 01 111 022A, Apr. 1989. Derwent Abstract, AN 96-299267, TW 274 567, Apr. 1996. Derwent Abstract, AN 77-52947y, JP 52 070 121A, Jun. 1977. Derwent Abstract, AN 78-22845a, JP 53 014 831 A, Feb. 1978. Patent Abstract of Japan JP 55 010472A, Jan. 1980. Derwent Abstrat, An 97-436057, CN 1 115 796A, Jan. 1996. |
| PATENT CLAIMS |
What is claimed is: 1. A method of making an activated fabric of carbon fibers, the method comprising the steps that consist in providing a fabric of fibers of a carbon-precursor cellulose material, impregnating the fabric with a composition containing at least one inorganic ingredient having a function of promoting dehydrating of cellulose, and performing heat treatment on the impregnated fabric at a temperature which is sufficient to cause the precursor cellulose to be transformed essentially into carbon, and obtaining a fabric of carbon fibers; the method being characterized in that the heat treatment consists n raising temperature at a speed lying in the range 1.degree. C./min to 15.degree. C./min followed by keeping the temperature constant in the range 350.degree. C. to 500.degree. C., and followed by a step of washing the fabric, thereby directly obtaining an activated fabric of carbon fibers having a specific surface area of not less than 600 m.sup.2 /g, without subsequent activation treatment at a higher temperature. 2. A method according to claim 1, characterized in that the duration of the constant temperature heat treatment as not more than 1 h. 3. A method according to claim 1, characterized in that the heat treatment is performed under an inert atmosphere. 4. A method according to claim 1, characterized in that the heat treatment is performed under a partially-oxidizing atmosphere. 5. A method according to claim 1, characterized in that the carbon precursor cellulose material is selected from the group consisting of rayons, spun viscose, solvent spun celluloses, cotton, and bast fibers. 6. A method according to claim 1, characterized in that the carbon precursor cellulose material is selected from textile rayons and spun viscose. 7. A method according to claim 1, characterized in that the composition for impregnating the fabric of cellulose material fibers contains at least one inorganic ingredient and solid fillers. 8. A method according to claim 7, characterized in that the inorganic fillers are selected from the group consisting of antimony, iron, titanium, and silicon. 9. A method according to claim 1, characterized in that the steps of heat treatment and of washing are performed continuously on the fiber fabric. 10. A method according to claim 1, characterized in that the washing is performed in water and comprises a first stage of solubilizing any excess ingredient of the impregnation composition, and a second stage of rinsing. 11. A method according to claim 1, characterized in that the fabric of cellulose material fibers is impregnated with a composition containing at least phosphoric acid, in such a manner that the mass of pure phosphoric acid fixed on the fabric lies in the range 10% to 22% of the mass of the fabric in the dry state. 12. A method according to claim 2, characterized in that: the heat treatment is performed under an atmosphere selected from the group consisting of an inert atmosphere and a partially oxidizing atmosphere; the carbon precursor cellulose material is selected from the group consisting of rayons, textile rayons, spun viscose, solvent spun celluloses, cotton, and bast fibers; the composition of the liquid for impregnating the fabric of cellulose material fibers contains at least one inorganic ingredient and solid fillers; the inorganic fillers are selected from the group consisting of antimony, iron, titanium, and silicon; the steps of heat treatment and of washing are performed continuously on the fiber fabric; the washing is performed in water and comprises a first stage of solubilizing any excess ingredient of the impregnation composition, and a second stage of rinsing; and the fabric of cellulose material fibers is impregnated with a composition containing at least phosphoric acid, in such a manner that the mass of pure phosphoric acid fixed on the fabric lies in the range 10% to 22% of the mass of the fabric in the dry state. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
This is the national stage of International Application No. PCT/FR98/00504, filed Mar. 12, 1998. 1. Field of the Invention The present invention relates to making activated fabrics out of carbon fibers. Such fabrics are usable in particular for filtering fluids, e.g. for processing gaseous or liquid waste. 2. Background of the Invention Various methods are known for making carbon fiber fabrics starting from a cellulose fiber fabric which is impregnated with a liquid composition containing an ingredient whose function is to promote dehydration of the cellulose, prior to being subjected to heat treatment at a temperature which is high enough to transform the cellulose fibers essentially into carbon fibers. Such ingredients that promote the dehydration of cellulose are also known as fire-proofing agents for cellulose. They enable the cellulose precursor to be carbonized with better efficiency and at a faster rate. The making of an activated fabric out of carbon fibers then includes activation treatment of the carbon fiber fabric by the action of an oxidizing gas, e.g. carbon dioxide, water vapor, or air, at a temperature greater than 500.degree. C., typically in the range 600.degree. C. to 1000.degree. C., i.e. at a temperature higher than the carbonizing temperature. A technique for activating a carbon fabric in an oven is described in document FR-A-2 741 363. Reference can also be made to the following documents: "Database WPI, Derwent Publications Ltd.", London, 4B, No. AN96-299 267 (TW-A-274 567), No. AN77-52947Y (JP-A-52-070121), No. AN85-287 343 (JP-A-60-198 166), No. AN83-49756K (JP-1-56-167716), and "Patents Abstracts of Japan", Vol. 4, No. 38 (C-004) (JP-A-55-010472) which describe the activation of carbon fiber fabrics previously obtained by carbonizing a cellulose precursor to which there has been added a cellulose dehydration promoter (ammonium chloride, phosphoric acid, zinc chloride, . . . ). Those activation techniques require special heat treatment. Their overall mass efficiency is relatively low compared with the cellulose fiber fabric, since the activation treatment has the effect of creating an array of micropores by eliminating carbon. Cost price is relatively high since the carbon fiber fabrics to which activation is applied are themselves expensive. In addition, activation has a major effect on the mechanical qualities of the carbon fibers, and it can be seen that the above-mentioned documents do not, in general, mention the mechanical properties of activated carbon fibers. OBJECTS AND BRIEF SUMMARY OF THE INVENTION An object of the present invention is to provide a method enabling activated fabrics of carbon fibers to be obtained starting from cellulose type carbon-precursor fibers, and to do so in a manner that is less expensive and with much greater efficiency than in the prior art. Another object of the invention is to provide a method enabling activated fabrics of carbon fibers to be obtained having good strength and conserving a high degree of flexibility, enabling them to be shaped, e.g. by being draped. The invention provides a method of making an activated fabric of carbon fibers, the method comprising the steps that consist in providing a fabric of fibers of a carbon-precursor cellulose material, impregnating the fabric with a composition containing at least one inorganic ingredient having a function of promoting dehydrating of cellulose, and performing heat treatment on the impregnated fabric at a temperature which is sufficient to cause the precursor cellulose to be transformed essentially into carbon, and obtaining a fabric of carbon fibers, which method is characterized in that the heat treatment consists in raising temperature at a speed lying in the range 1.degree. C./min to 15.degree. C./min followed by keeping the temperature constant in the range 350.degree. C. to 500.degree. C., and followed by a step of washing the fabric, thereby directly obtaining an activated fabric of carbon fibers having a specific surface area of not less than 600 m.sup.2 /g, without subsequent activation treatment at a higher temperature. Thus, the invention is remarkable in that the carbonization and activation stages are performed in a single heat treatment stage, at a moderate temperature, giving rise to an activated fabric having very high specific surface area. In addition, its efficiency, measured as the ratio between the mass of the activated fabric and the mass of the initial cellulose fiber fabric, is greater than 30%, and typically lies in the range 35% to 45%, and is therefore high. Furthermore, as can be seen from the examples given below, it is possible to obtain activated fabrics of carbon fibers that conserve excellent strength. The duration of the constant temperature heat treatment stage is preferably no greater than 1 h. The heat treatment is performed under an atmosphere that is inert or partially oxidizing. The carbon-precursor cellulose material constituting the fibers of the starting fabric is selected from: rayons, spun viscose, solvent spun celluloses, cotton, and bast fibers; and preferably from textile rayons and spun viscose. Also advantageously, the composition of the liquid for impregnating the fabric of cellulose material fibers contained at least one inorganic ingredient and solid fillers, e.g. selected from antimony, iron, titanium, and silicon. Also advantageously, the steps of heat treatment and of washing are performed continuously on the fiber fabric, which is made possible by the good strength of the fabric. BRIEF DESCRIPTION OF THE DRAWINGS Implementations of the method are described below by way of non-limiting indication. Reference is made to the accommodating drawings, in which: FIGS. 1A, 1B, and 1C are a highly diagrammatic representation of an industrial installation enabling the method to be implemented; and FIG. 2 is a graph showing the temperature profile of the heat treatment oven shown in FIG. 1B. DETAILED DESCRIPTION OF THE PREFERRED IMPLEMENTATIONS The method can be implemented using various fiber fabrics, in particular fabrics made of threads, tows, woven cloth, sheets of unidirectional or multidirectional threads, felts, mats, knits, sheets, films, . . . . The starting fiber fabric is made of carbon-precursor fibers of the cellulose type, e.g. rayon multifilaments, spun viscous fibers (fibranne), fibers or filaments of solvent spun cellulose, cotton fibers, or indeed bast fibers. As appears from the examples given below, in order to obtain an activated fabric of carbon fibers that presents good strength, it is preferable to use precursor fibers made of a cellulose material having a small degree of orientation and a small amount of crystallinity. It is then preferable to select textile rayon or spun viscose. The cellulose fiber fabric is impregnated by a composition containing at least one ingredient whose function is to promote dehydration of the cellulose. Such ingredients are well known per se and at least some of them are also used as agents for fire-proofing cellulose. One or more inorganic compounds can be used selected from phosphoric acid (H.sub.3 PO.sub.4), sulfuric acid (H.sub.2 SO.sub.4), hydrochloric acid (HCl), dibasic ammonium phosphate ((NH.sub.4).sub.2 HPO.sub.2), sodium phosphate (Na.sub.3 PO.sub.4), potassium sulfate (K.sub.2 SO.sub.4), ammonium chloride (Na.sub.4 Cl), zinc chloride (ZnCl.sub.2), any salt of phosphorus or of boron, . . . , and in general Lewis acids or Br.o slashed.nsted acids. A mixture of several ingredients can have a beneficial effect on the strength of the resulting final fabric by selecting the ingredients so as to promote cellulose dehydration at different moments in the heat treatment, and consequently causing the reaction to be less violent. Various solid fillers can be added to the impregnation composition so as to provide impurities that enhance the development of arrays of micropores during the heat treatment. By way of example, it is possible to use particles of antimony, iron, titanium, or silicon. These heteroatoms occupy places between and/or within structural units of carbon during the formation of the carbon lattice, thereby increasing its microporosity. The concentration of ingredient(s) for catalyzing dehydration of cellulose depends on the nature of the ingredients. As a general rule, concentration is selected to be high enough to generate a large specific surface area in the activated fabric, but without being excessive since that would lead to a fabric that is fragile (brittle) and rigid. The heat treatment has a first stage during which temperature is caused to rise progressively, followed by a stage at which temperature is kept constant. Temperature should rise fast enough to obtain a large specific surface area, but not too fast so as to ensure that the cellulose is degraded under controlled conditions, thereby obtaining a final activated fabric that has good strength. The average rate of temperature rise lies in the range 1.degree. C./min to 15.degree. C./min, and it is not necessary for temperature to increase in linear manner over time. The final constant temperature portion of the heat treatment serves to finish off degrading the cellulose. Nevertheless, it is important not to exceed a maximum value beyond which it has been observed that there is a risk of the micropores closing. The final treatment temperature lies in the range 350.degree. C. to 500.degree. C. The heat treatment (temperature rise followed by constant temperature) is performed under an inert atmosphere, e.g. nitrogen, or an atmosphere that is partially inert. With a partially inert atmosphere, the following may be present: oxygen from the air, carbon dioxide, water vapor, and other oxidizing agents, in particular agents generated by decomposition of the ingredients in the impregnation composition. In the constant temperature portion of the heat treatment, any air, carbon dioxide, or water vapor, that may be present, participates in decomposing the cellulose, but they do not behave as direct oxidants of carbon and they do not act as activation agents, as would be the case at temperatures that are much higher. Final washing of the activated fabric is preferably performed immediately after the heat treatment so as to prevent the newly-created micropores becoming obstructed, which could otherwise arise because of excess ingredients of the impregnation composition crystallizing in the micropores. Immediate washing is important because the rate at which such crystals dissolve is very slow. Washing performed in water can Include a first stage of solubilizing the ingredient(s) of the impregnation composition present in excess on the final fabric, followed by a second stage of rinsing. Washing makes it possible to eliminate not only the residual impregnation composition, but also to eliminate the products of degrading the carbon-precursor cellulose material. Particular implementations of the method are described below. |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
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