EP0966558B1 - Method for producing an activated carbon fibre texture - Google Patents

Description

Field of the invention

The present invention relates to the production of activated textures in carbon fibers.

Such textures are especially usable for the filtration of fluids, for example the treatment of gaseous or liquid effluents.

Invention background

We know various methods of making fiber textures. carbon from a texture of cellulosic fibers which is impregnated with a liquid composition containing a constituent having a function of promoter of dehydration of the cellulose, before being heat treated at a sufficient temperature to transform cellulosic fibers essentially carbon fiber.

These constituents which promote the dehydration of cellulose are also known as cellulose flame retardants. They allow carbonization of the cellulosic precursor with better yield and faster kinetics.

Achieving an activated carbon fiber texture includes then a treatment to activate the carbon fiber texture by the action of an oxidizing gas, for example carbon dioxide, water vapor or air, to a temperature above 500 ° C, typically 600 ° C to 1000 ° C, i.e. a temperature higher than that of charring. An activation technique of carbon fabric in an oven is described in the document FR-A-2 741 363.

Reference may also be made to "Database WPI documents, Derwent Publications Ltd. ", London, 4B, n ° AN96-299 267 (TW-A-274 567), n ° AN77-52947Y (JP-A-52-070121), n ° AN85-287 343 (JP-A-60-198 166), n ° AN83-49756K (JP-A-57-167716) and "Patents Abstracts of Japan", vol. 4, n ° 38 (C-004) (JP-A-55-010472) which describe the activation of fiber textures carbon previously obtained by carbonization of a cellulosic precursor to which a cellulose dehydration promoter (chloride ammonium, phosphoric acid and zinc chloride, for example).

These activation techniques require heat treatment specific. They have a relatively low overall mass yield, by compared to the texture of cellulosic fibers, the activation treatment having for effect of creating a microporous network by elimination of carbon. The price of returns is very high, the carbon fiber textures activation supports being themselves expensive. In addition, activation significantly affects the mechanical qualities of carbon fibers, and we observe that the documents cited above do not, in general, state the properties mechanics of activated carbon fibers.

Objects and summary of the invention

The present invention aims to provide a method for obtaining activated textures in carbon fibers, from fibers in carbon precursor of the cellulosic type more economically and with a much higher yield than in the prior art.

The invention also aims to provide a method for to obtain activated textures in carbon fibers having good hold mechanical and retaining high flexibility allowing them to be shaped, for example by draping.

According to the invention, a process for producing an activated texture of carbon fibers, comprising the steps which consist in providing a texture in fibers of cellulosic material, carbon precursor, impregnating the texture with a composition containing at least one constituent mineral having a function of promoter of the dehydration of the cellulose, and carrying out a heat treatment of the impregnated texture at a temperature sufficient to cause the transformation of the cellulose precursor essentially into carbon and to obtain a texture in carbon fibers, is characterized in that the carbon precursor cellulosic material is chosen from rayon, fibrane, solvent celluloses, cotton and bast fibers, and the heat treatment is carried out under an inert or partially oxidizing atmosphere, consists of a rise in temperature at an average speed between 1 and 15 ° C / min followed by a plateau at a temperature between 350 ° C and 500 ° C, and is followed by a step of removing residual phases of the impregnation composition and of degradation products of the cellulosic material by washing the texture, so that the an activated carbon fiber texture is obtained directly having a specific surface at least equal to 600 m ² / g, without subsequent activation treatment at a higher temperature.

Thus, the invention is remarkable in that the phases of carbonization and activation are carried out in a single heat treatment, at a temperature moderate and result in an activated texture with a very specific surface high. In addition, the yield measured by the ratio between the texture mass activated and the starting texture mass of cellulosic fibers is greater than 30%, typically between 35% and 45%, therefore high. Furthermore, like this is apparent from the examples given below, it is possible to obtain textures activated with carbon fibers retaining excellent mechanical strength.

The duration of the stage of the heat treatment is preferably at most equal at 1 a.m.

Advantageously, the liquid composition for impregnating the texture in fibers of cellulosic material contains at least one mineral constituent and solid fillers, these being for example chosen from antimony, iron, titanium and silicon.

Advantageously also, the heat treatment and washing steps are carried out continuously, thanks to the good mechanical strength of the texture

Brief description of the drawings

• figures 1A, 1B, 1C : une représentation très schématique d'une installation industrielle permettant la mise en oeuvre du procédé ; et
• figure 2 : une courbe illustrant le profil de température du four de traitement thermique de la figure 1B.

Embodiments of the method will be described below for information, but not limitation. Reference will be made to the appended drawings which illustrate:

• FIGS. 1A, 1B, 1C: a very schematic representation of an industrial installation allowing the implementation of the process; and
• Figure 2: a curve illustrating the temperature profile of the heat treatment oven of Figure 1B.

Detailed description of preferred embodiments

The process can be implemented on different fibrous textures, in particular wires, cables, fabrics, unidirectional layers of wires or multidirectional, felts, mats, knits, sheets and films.

The initial fibrous texture is made of carbon precursor fibers of cellulosic type, for example in rayon multifilaments, in yarn of viscose (fibranne), of cellulose fibers or filaments containing solvent, of cotton or even bast fibers.

As will emerge from examples given below, it is preferable, for obtain an activated carbon fiber texture with good mechanical strength, using precursor fibers made of cellulosic material with a low degree of orientation and a low level of crystallinity. We will rather choose fibers of textile rayon or fibranne.

The texture of cellulosic fibers is impregnated with a composition containing at least one constituent having a function of promoting the dehydration of cellulose. Such constituents are well known in themselves and are at least, for some, also used as flame retardants for cellulose. One or more mineral constituents may be used chosen from phosphoric acid (H ₃ PO ₄ ), sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCI), diammonic phosphate ((NH ₄ ) ₂ HPO ₄ ), sodium phosphate (Na ₃ PO ₄ ), potassium sulfate (K ₂ SO ₄ ), ammonium chloride (NH ₄ CI), zinc chloride (ZnCl ₂ ), any phosphorus salt or boron, and generally Lewis or Brönsted acids.

A mixture of several constituents may have a beneficial effect on the mechanical strength of the final texture obtained, if they are chosen for promote the dehydration of cellulose at different times of the treatment thermal and therefore make the reaction less violent.

Different solid fillers can be added to the composition impregnation in order to constitute impurities which favor the development of the microporous network during heat treatment. We could for example use particles of antimony, iron, titanium, silicon. These heteroatoms are placed between and / or within the structural units of carbon during the formation of the carbon network, thus increasing the microporosity.

The content of catalyst component (s) in the dehydration of the cellulose depends on the nature of the constituents. She is chosen, in a way general, high enough to generate a large specific surface in the activated texture, but without being excessive, which would lead to a texture fragile (brittle) and rigid.

The heat treatment includes a first phase of rise progressive in temperature, followed by a plateau.

The temperature rise must be fast enough to obtain a large specific surface, but without excessive speed for achieve controlled degradation of the cellulose and obtain an activated texture final with good mechanical strength. The average rate of climb in temperature is between 1 and 15 ° C / min, the temperature increase not necessarily being linear over time.

The plateau at the final heat treatment temperature allows complete the degradation of cellulose. It is important, however, not to exceed a maximum value beyond which a risk of closure of microporosities has been observed. Final processing temperature is understood between 350 ° C and 500 ° C.

The heat treatment (temperature rise and level) is carried out under an inert atmosphere, for example under nitrogen, or partially inert. In the latter case may be present oxygen from the air, dioxide of carbon, water vapor or other oxidizing agents, especially generated by the decomposition of the constituent (s) of the impregnation composition. In the temperature range of the heat treatment, air, dioxide carbon or any water vapor involved in the decomposes cellulose but does not behave as oxidants direct carbon and have no function of activating agents, like this would be the case at much higher temperatures.

The final washing of the activated texture is preferably carried out immediately after heat treatment to avoid blockage of microporosities created, an obstruction that can result from the crystallization of a excess of constituents of the impregnation composition in the micropores, the kinetics of dissolution of these crystals being very slow.

Washing with water can include a first phase of solubilization of the constituent (s) of the impregnating composition present in excess on the final texture, then a second rinsing phase. Washing eliminates not only residual phases of the composition impregnation, but also degradation products of the cellulosic material carbon precursor.

Particular examples of implementation of the method will be now described.

Example 1

Rayon fabric samples consisting of a multifilament viscose, containing less than 0.03% size. The fabric is obtained from 190 tex threads woven in 15 x 15 texture (15 threads per cm in chain and weft).

The fabric is steamed at 120 ° C for 1 hour in a ventilated oven and then cooled 1/2 hour in a desiccator. The surface mass of the fabric is then equal to 530 g / m ² .

A tissue sample is then soaked in an aqueous solution phosphoric acid at 200 g / l, for 2 h, then drained flat on a grid for at least 24 hours. The acid content on the fabric is 17%, measured by the ratio between the mass of pure phosphoric acid on the tissue and the mass of dry fabric before impregnation.

The impregnated fabric is wound on itself and arranged in a ceramic nacelle which is introduced into a quartz tube of an oven heat treatment.

A heat treatment is carried out under a nitrogen flow of 10 l / h at atmospheric pressure. The treatment includes a rise in temperature to a speed of approximately 10 ° C / min up to 400 ° C followed by a 30 min plateau at this temperature.

After cooling, the fabric is washed in order to remove products of degradation of the celluloses phases of the initial precursor and / or excess of additive acid. Washing is carried out by circulating distilled water for 5 h and the fabric washed is dried in air at 160 ° C for 2 h.

• une surface spécifique élevée : environ 1 000 m²/g,
• une masse surfacique d'environ 350 g/cm² après séchage,
• une résistance mécanique à la rupture en traction de bon niveau, environ 1 daN/cm, tant en sens trame qu'en sens chaíne,
• un diamètre moyen de pores égal à environ 0,6 nm,
• un volume total de pores d'environ 0,6 cm³/g,
• un taux de carbone de 80 % environ, et
• un rendement moyen de 40%, obtenu par mesure du rapport entre le poids de tissu activé en fibres de carbone obtenu et le poids du tissu de rayonne étuvé et séché (un tel rendement est plus du double de celui obtenu avec des procédés de l'art antérieur évoqué en tête de la description dans lesquels une activation à température élevée est réalisée après carbonisation).

The activated carbon fiber fabric finally obtained has the following remarkable characteristics:

• a high specific surface: around 1000 m ² / g,
• a surface mass of approximately 350 g / cm ² after drying,
• good mechanical tensile strength, around 1 daN / cm, both in the weft direction and in the warp direction,
• an average pore diameter equal to about 0.6 nm,
• a total pore volume of approximately 0.6 cm ³ / g,
• a carbon level of around 80%, and
• an average yield of 40%, obtained by measuring the ratio between the weight of activated carbon fiber fabric obtained and the weight of the steamed and dried rayon fabric (such a yield is more than double that obtained with methods of prior art mentioned at the head of the description in which activation at high temperature is carried out after carbonization).

The above example is shown in the first row A of Table 1 below.

Examples 2 to 12

The procedure is as in Example 1, but by varying the concentration of the phosphoric acid solution, or the conditions of the heat treatment (temperature rise speed, bearing temperature, duration of the stage, possible addition of water vapor to the atmosphere under which heat treatment is carried out).

Examples 2 to 12 are given in rows B to L of Table 1.

In this table, the "acid content" is the ratio between the mass of pure acid fixed on the fabric after impregnation and the mass of dry fabric before impregnation the "yield" is the weight of the activated carbon fabric washed and dried compared to by weight of the steamed and dried rayon fabric, and the tensile strength is that measured in the weft or warp direction on the activated carbon fabric obtained. Activated fabric Acid content
% Heat treatment yield
% Specific surface
m ² / g Tensile strength
daN / cm AT 17 10 ° C / min
400 ° C, 1/2 h 40.7 1040 0.9 B 17 10 ° C / min
500 ° C, 1/2 h 39.5 690 0.8 VS 17 10 ° C / min
600 ° C, 1/2 h 42.3 260 1 D 17 10 ° C / min
400 ° C, 1/2 h
+ water vapor at 31% vol. 39.4 985 0.45 E 17 10 ° C / min
500 ° C, 1/2 h
+ water vapor at 31% vol. 36.5 930 0.85 F 17 1 ° C / min
400 ° C, 1/2 h 42.5 870 2.6 G 17 6 ° C / min
400 ° C, 1/2 h 41.6 910 0.55 H 17 15 ° C / min
400 ° C, 1/2 h 33.7 1070 0.3 I 17 20 ° C / min
400 ° C, 1/2 h 40.4 1065 (brittle) J 25.5 10 ° C / min
400 ° C, 1 h 40.3 1280 (brittle) K 25.5 1 ° C / min
400 ° C, 1 h 41.6 1095 0.76 The 34 10 ° C / min
400 ° C, 1 h 39.7 1660 (very brittle)

The results observed show that the level of acid fixed on the rayon fabric must preferably remain within a certain limit, otherwise the resistance of the activated carbon fabric becomes low, or even zero. Likewise, the heat treatment must be relatively moderate, in terms of rate of temperature rise, as well as temperature and bearing time. We also note that the bearing temperature must not exceed 500 ° C if we want to guarantee a relatively high specific surface, in any case greater than 600 m ² / g.

In addition, the presence of water vapor in the atmosphere under which heat treatment is carried out increases the surface specific.

Example 13

A semi-continuous treatment is carried out on a rayon fabric at means of the installation shown very diagrammatically in FIGS. 1A, 1B and 1 C.

We start from a strip of textile rayon fabric 10 (FIG. 1A) based on viscose unwound from a spool 12. The fabric contains less than 0.03% of size, has a width of 1000 mm and a surface mass dry of about 530 g / m ² .

After drying by passing over heating rollers 14 to one temperature of about 120 ° C, the fabric is impregnated, using the technique of padding, with a composition containing a mixture of phosphoric acid pure (18% by weight), sodium phosphate (2% by weight) and borate sodium (1.5% by weight), the remainder being water. The fabric is conveyed in a tray 16 containing this composition, then is expressed between two rolls 18 applied against each other with a pressure set at approximately 2 bars. The running speed of the fabric strip is approximately 0.5 m / min. The fabric impregnated is dried at a temperature of 30 ° C to 85 ° C, for example by passing over heating rollers 20, in order to remove the water from the composition impregnation then passes through a traction system 21 of omega type before to be wound on a reel 22 to be stored for about 24 hours.

The impregnated fabric is taken from the reel 22 by means of a system omega type pulling device 24 (FIG. 1B) and passes through a puppet 26 allowing to guarantee constant tension during the whole process.

The fabric passes through a sealing box 32 and a drainage box effluents 34 located in front of the entrance to a heat treatment oven 30. In out of the oven, the fabric passes through an effluent discharge box 36 and a box seal 38.

The sealing boxes 32, 38 are crossed by a transverse flow of nitrogen under pressure. The effluent discharge box 34 is fixed to the upstream wall of the oven and to its wall traversed by a rod 35 allowing in particular the supply of the internal volume of the oven with nitrogen, the heat treatment being carried out in a neutral atmosphere . The effluent discharge box 36 is fixed to the downstream wall of the furnace 30. The boxes 34 and 36 have outlets 34 a , 36 a for the evacuation of gaseous effluents. Screens 40, allowing passage to the strip of fabric, are provided at the entrance and at the exit of the oven 30 to limit the thermal radiation towards the outside.

In the furnace 30, the fabric passes inside a quartz tube 30 a while resting on a scale 30 b also made of quartz. The useful length of the quartz tube is approximately 1.3 m. The oven 30 has several heating zones, for example four successive zones I, II, III, IV and the heating is controlled so that the fabric reaches a temperature of approximately 400 ° C., approximately 40 min after entering the the oven, after gradual rise in temperature, and remains at this temperature for about 30 min before leaving the oven. Figure 2 shows the temperature profile in the oven as a function of the residence time. The rate of temperature rise up to the 400 ° C plateau is approximately 10 ° C / min.

At the outlet of the sealing box 38, the fabric passes over a roller 42 (figure 1C) associated with a strain gauge, allowing to measure the tension on the fabric.

The fabric then reaches a washing station comprising a tank 50 divided into two upstream and downstream compartments 50 a , 50 b . Before entering the compartment 50 a , the fabric is watered by permuted water by means of nozzles 52 with flat jet, which feed the compartment 50 a , in which the excess of constituents of the impregnation composition still present on the tissue can be dissolved. The fabric then passes into compartment 50 b where it is rinsed with demineralized water sprayed onto the fabric by means of nozzles 54 located at the outlet of compartment 50 b , above the latter.

The washed fabric passes through a traction system 56 of the omega type, in which it is also expressed, before being dried at a temperature about 120 ° C by passage between two radiant plates 58. The speed drive by the traction system 56 is chosen slightly greater than that imposed by the traction system 24, to take account of the shrinkage of the fabric during charring.

This example is shown in line M of Table 2 below. An activated carbon fiber fabric is obtained having a specific surface of approximately 1000 m ² / g and a tensile strength, both in the warp and in the weft, of approximately 1 daN / cm.

Examples 14 to 16

We proceed as in Example 13, but by varying different parameters: phosphoric acid content, rate of rise temperature and duration of the stage of heat treatment.

Examples 14 to 16 are shown in rows N to P of Table 2. In this table, the phosphoric acid content is the ratio between the mass of pure acid fixed on the fabric after impregnation and the mass of dry fabric before impregnation and the tensile strength expresses the tensile breaking strength in the chain direction. Activated fabric H ₃ PO _{4 content}
% Heat treatment Specific surface
m ² / g Tensile strength
daN / cm M 17 10 ° C / min
400 ° C, 1/2 h 1,000 1 NOT 1 to 7 10 ° C / min
400 ° C, 1/2 h 150 to 250 5.2 O 17 0.01 to 0.1 ° C / min
400 ° C, 1/2 h 200 1.1 P 17 10 ° C / min
400 ° C, 2 to 12 h 170 to 130 0.9

The results obtained with N tissues show that a low not enough phosphoric acid to create microporosity substantial. Referring also to the fabrics J, K, L of Table 1, we can consider that the phosphoric acid content should preferably be understood between 10 and 22%. Note that a small amount of phosphoric acid is reflected by an increase in mechanical resistance: the carbon structure is closed, which goes against the concern of creating a porous network.

The results obtained with O tissues show that a very low speed of temperature rise also does not allow porosity to be obtained satisfactory. Observation of the results obtained with the tissues F, G, H, I of table 1 indicates that the average rate of temperature rise must be between 1 and 15 ° C / min, preferably between 1 and 10 ° C / min, if you do not want not penalize mechanical strength (fabric I).

The results obtained with P tissues show that one step too many long duration leads to a quasi-closure of the developed microporosity previously. This is why it is preferable to limit the duration of the stage to at plus 1 hour.

For the interpretation of the results of Table 2 with regard to the specific surface, it is useful to note that the carbons with cellulosic precursors have "naturally" (without activation) a specific surface of 50 to 150 m ² / g for treatments thermal carbonization at temperatures below 1300 ° C. Consequently, with N, O, P tissues, no very significant activation is observed beyond this "natural" microporosity.

Examples 17 to 20

We proceed as in Example 13, but using different impregnation products of rayon fabric, respectively acid phosphoric acid, a mixture of phosphoric acid and sodium borate, ammonium chloride and diammonium phosphate.

The results obtained are shown in lines Q to T of Table 3. The contents indicated in% represent the ratios between the mass of pure impregnation product fixed on the fabric and the mass of dry fabric before impregnation. Activated fabric Component of the impregnation composition Specific surface
m ² / g Tensile strength
daN / cm Q H ₃ PO ₄ (17%) 1,005 0.7 R H ₃ PO ₄ (17%) + Na ₂ B ₄ O ₇ (1.5%) 1,020 2.7 S NH ₄ CI (25%) 350 3.1 T (NH ₄ ) ₂ HPO ₄ (20%) 540 0.9

Phosphoric acid, in addition to its low cost, has the advantage of having three acid functions to promote the dehydration of cellulose and, in relation to NH ₄ Cl and (NH ₄ ) ₂ HPO ₄ , to require a lower content in view obtaining the desired porosity.

With NH ₄ Cl and (NH ₄ ) ₂ HPO ₄ , a significantly higher content is necessary to obtain a specific surface of the same order as that obtained with H ₃ PO ₄ .

Also, the use of phosphoric acid, possibly as a mixture with other constituents, will be preferred, without excluding others mineral constituents known to promote dehydration of the cellulose.

Examples 21 to 25

The procedure is as in Example 13, but using different cellulosic precursors, respectively: a textile type rayon I which naturally contains additives such as aluminum and titanium dioxide in its structure, the latter being a very disoriented crystal structure, a rayon II intermediate between textile and technical rayon, a technical rayon III, of the type used for tire reinforcements, a rayon IV of "solvent cellulose" type and a fibranne V commonly used in the 'textile industry. The results obtained are indicated in rows U to Y of Table 4. Activated fabric Precursor type Specific surface
m ² / g Tensile strength
daN / cm U Rayon I 1,310 1.5 V Ray II 1,010 3.1 W Rayon III 1,145 (rigid and brittle) X Ray IV 750 (brittle) Y Fibranne V 1,030 0.2 (flexible)

These results show that we will preferably use precursors of the textile rayon and fibranne type if one wants to obtain an outfit satisfactory mechanics (U, V and Y fabrics).

Claims (8)

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 carbon precursor cellulose material is selected from rayons, spun viscose, solvent spun celluloses, cotton, and bast fibers; and

   the heat treatment is performed under an inert or partially-oxidizing atmosphere, consists in raising temperature at an average speed lying in the range 1°C/min to 15°C/min followed by keeping the temperature constant in the range 350°C to 500°C, and is followed by a step of eliminating residual phases of the impregnation composition and products generated by the degradation of the cellulose material by washing the fabric, thereby directly obtaining an activated fabric of carbon fibers having a specific surface area of not less than 600 m²/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 is not more than 1 h.
3. A method according to claim 1 or 2, characterized in that the carbon precursor cellulose material is selected from textile rayons and spun viscose.
4. A method according to any one of claims 1 to 3, characterized in that the composition of the liquid for impregnating the fabric of cellulose material fibers contains at least one inorganic ingredient and solid fillers.
5. A method according to claim 4, characterized in that the solid fillers are selected from antimony, iron, titanium, and silicon.
6. A method according to any one of claims 1 to 5, characterized in that the steps of heat treatment and of washing are performed continuously on the fiber fabric.
7. A method according to any one of claims 1 to 6, 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.
8. A method according to any one of claims 1 to 7, 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.

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