FR2915109A1 - PROCESS FOR PREPARING AN ALCOHOLIC SUSPENSION OF CARBON NANOTUBES AND SUSPENSION THUS OBTAINED - Google Patents

Abstract

The subject of the present invention is thus a process for the preparation of an alcoholic suspension of carbon nanotubes, comprising: placing in contact with said nanotubes an alcoholic medium with at least one dispersant consisting of a homo- or copolymer of vinylpyrrolidone, in a weight ratio of the carbon nanotubes to said dispersant ranging from 0.1 to less than 2, and the mechanical treatment of the mixture thus obtained. It also relates to the suspension thus obtained and to its uses.

Description

Process for the preparation of an alcoholic suspension of carbon nanotubes

and suspension thus obtained

  The present invention relates to an alcoholic suspension of carbon nanotubes, its method of preparation and its uses.

  Carbon nanotubes (or CNTs) are known and possess particular crystalline structures, tubular, hollow and closed, composed of atoms arranged regularly in pentagons, hexagons and / or heptagons, obtained from carbon. CNTs generally consist of one or more coiled graphite sheets. One can distinguish single wall nanotubes (SWNTs) and multiwall nanotubes (Multi Wall Nanotubes or MWNTs).

  CNTs are commercially available or can be prepared by known methods. There are several methods of synthesis of CNTs, including electrical discharge, laser ablation and chemical vapor deposition or CVD (Chemical Vapor Deposition) which ensures the production of large quantities of carbon nanotubes and therefore obtaining them at a cost price compatible with their massive use. This process consists precisely in injecting a source of carbon at relatively high temperature over a catalyst which may itself consist of a metal such as iron, cobalt, nickel or molybdenum, supported on an inorganic solid such as alumina, silica or magnesia. Carbon sources may include methane, ethane, ethylene, acetylene, ethanol, methanol or even a mixture of carbon monoxide and hydrogen (HIPCO process).

  Thus, the application WO 86 / 03455A1 of Hyperion Catalysis International Inc. describes in particular the synthesis of CNTs. More particularly, the process comprises contacting a metal-based particle, such as in particular iron, cobalt or nickel, with a carbon-based gas compound at a temperature of between approximately 850 ° C. and 1200 ° C. the dry weight proportion of the carbon-based compound relative to the metal-based particle being at least about 100: 1.

  From a mechanical point of view, the CNTs have both excellent stiffness (measured by the Young's modulus), comparable to that of steel, while being extremely light. In addition, they have excellent electrical and thermal conductivity properties that make it possible to consider using them as additives to impart these properties to various materials, in particular macromolecular materials, such as polyamides, polycarbonate, polyesters, polystyrene, polyethylether ketones and polyethylene imine.

  However, CNTs are difficult to handle and disperse, because of their small size, their powderiness and possibly, when they are obtained by the CVD technique, their entangled structure, all the more important that the we are trying to increase their mass productivity in order to improve production and reduce the residual ash content. The existence of strong Van der Waals interactions between single-wall nanotubes also affects their dispersibility and the stability of the suspensions obtained.

  To remedy the poor dispersibility of CNTs, which significantly affects the characteristics of the composites they form with the polymer matrices into which they are introduced, various solutions have already been proposed in the state of the art. These include sonication, which has only a temporary effect, or ultrasonication which has the effect of partially cutting the nanotubes and creating oxygen functions that may affect some of their properties.

  It has furthermore been suggested to carry out mixtures in a CNT solvent with dispersing agents such as surfactants including sodium dodecyl sulphate (EP-1 495 171, VIGOLO B. et al., Science, 290 (2000), 1331). WANG J. et al., J. of Chem., Society, 125, (2003), 2408, MOORE, VC et al, Nanoletters, 3, (2003), 2408). These, however, do not allow to disperse large quantities of CNT, satisfactory dispersions can be obtained only for CNT concentrations of less than 2 or 3 g / l. In addition, the surfactants are capable of desorbing entirely from the surface of the CNTs during the dialysis step generally used to remove the excess of surfactant in the solution, which has the effect of destabilizing the suspension obtained.

  Similarly, it has been suggested to use water-soluble polymers such as gum arabic or a copolymer of styrene and t-butyl acrylate to disperse single-walled carbon nanotubes in water or ethanol, respectively ( WO 02/76888 and WO 2005/073305). The first of these processes requires: however, the use of large amounts of polymer and does not stabilize more than 0.5 g / l of CNT.

  Another solution, proposed in particular in Applications EP-1,359,121 and EP-1,359,169, was to make a dispersion of CNTs in a solvent and a monomer and to carry out an in situ polymerization leading to the obtaining of NTC functionalized. This solution is however complex and can be expensive depending on the products used. Moreover, the grafting operations may damage the structure of the nanotubes and, consequently, their electrical and / or mechanical properties (GARG A. et al., Chem Phys Lett, 295 (1998), 273).

  Examples of such grafting methods have been described in particular by HADDON et al in Science, (1998), 282, p. 95-98 and J. Phys. Chem., B2001, 105, p. 2525-2528; SUN et al in Chem. Mater., 2001, 13, p. 2864-2869; CHEN et al. in Carbone, (2005), 43, 1778-1814; QUIN et al in Macromolecules, 2004, 37, p. 752-757; ZHANG et al. in Chem. Mater., 16 (11) (2004), 2055-2061.

  Among these methods, some allow more particularly to disperse CNTs in ethanol optionally mixed with water. This is the case of those described by FERNANDO et al. in Nanoscience and Nanotechnology, 5 (7), (2005), 1050-1054, leading to functionalization of CNTs by means of polyvinyl alcohol and by LIU et al in Carbon, 43 (7), (2005), 1470 1478, which allow a grafting of trifluoroacetic groups on CNTs.

  It will be noted that the various techniques above are carried out in the presence of a large amount of solvent which is not always compatible with the envisaged applications and must be subsequently eliminated.

  Yet another solution is to oxidize single-walled nanotubes with nitric acid and then disperse them in ethanol. This method, described by LEE et al. in J. Phys. Chem. B, 109, (36), (2005), 17128-17133, has the disadvantage of rejecting acid effluents, which is undesirable from an environmental point of view or requires an additional processing step.

  Another method of chemical treatment of CNTs with a view to improving their dispersibility, which is the case in ethylene glycol, has been disclosed in the application US 2002/0100578, which consists in treating CNTs with a solution of hypochlorite of sodium before acidifying them and treating the surface hydroxyl groups with 2-chloroethanol. This process is relatively complex, especially because of its number of steps.

  There remains therefore the need to provide a simple, inexpensive and environmentally friendly process for preparing suspensions of carbon nanotubes, particularly multiwall NTC, at concentrations of the order of 0.1 to 10 g / 1, without substantially affecting their mechanical and electrical properties. It would be furthermore advantageous for these suspensions to be homogeneous, sufficiently fluid so that their viscosity, measured using a capillary-type or Couette-type viscometer, does not exceed 10 cP at 25.degree. C. and that they do not exhibit grain of more than 30 pm or more than 20 pm.

  The Applicant has discovered that this need could be satisfied by the implementation of a process comprising contacting carbon nanotubes with a homo- or copolymer of vinylpyrrolidone, followed by mechanical treatment.

  The subject of the present invention is thus a process for the preparation of an alcoholic suspension of carbon nanotubes, comprising: placing in an alcoholic medium said nanotubes with at least one dispersant consisting of a homo- or copolymer of vinylpyrrolidone, in a weight ratio of the carbon nanotubes to said dispersant ranging from 0.1 to less than 2, and the mechanical treatment of the mixture thus obtained.

  For the purpose of the present invention, the term "alcoholic medium" means any medium containing at least one monoalcohol such as ethanol or isopropanol and / or a glycol such as ethylene glycol or propylene glycol and optionally a solvent. miscible with alcohol such as water. It is preferred that this medium consists only of the aforementioned constituents and advantageously that it contains only mono-alcohol such as those mentioned above and possibly water in an amount of less than 10% by weight.

  The carbon nanotubes (hereinafter NTC) that can be used according to the invention can be of the single-wall, double-wall or multi-wall type. Double-wall CNTs may especially be prepared as described by FLAHAUT et al in Chem. Com. (2003), 1442. The multi-walled CNTs may in turn be prepared as described in WO 03/02456.

  They usually have a diameter ranging from 0.1 to 100 nm, preferably from 0.4 to 50 nm, and more preferably from 1 to 30 nm and advantageously from 0.1 to 10 μm. Their length / diameter ratio is advantageously greater than 10 and most often greater than 100. Their specific surface area is, for example, between 100 and 300 m 2 / g and their apparent density may especially be between 0.05 and 0.5 g / g. cm3 and more preferably between 0.1 and 0.2 g / cm3. The multiwall carbon nanotubes may for example comprise from 5 to 15 sheets and more preferably from 7 to 10 sheets.

  An example of crude carbon nanotubes is in particular commercially available from ARKEMA under the trademark Graphistrength "C100.

  These nanotubes may be purified and / or oxidized and / or milled before being used in the process according to the invention.

  The grinding of the CNTs can in particular be carried out cold or hot and be carried out according to the known techniques used in devices such as ball mills, hammers, grinders, knives, gas jet or any other grinding system likely to reduce the size of the entangled CNT network. It is preferred that this grinding step is performed according to a gas jet grinding technique and in particular in an air jet mill.

  The purification of the crude or milled CNTs can be carried out by washing with a sulfuric acid solution, so as to rid them of any residual mineral and metal impurities originating from their preparation process. The weight ratio of CNTs to sulfuric acid may especially be between 1: 2 and 1: 3. The purification operation may also be carried out at a temperature ranging from 90 to 120 ° C., for example for a period of 5 to 10 hours.

  This operation can advantageously be followed by steps of rinsing with water and drying the purified CNTs.

  The oxidation of the CNTs is advantageously carried out by putting them in contact with a solution of sodium hypochlorite containing from 0.5 to 15% by weight of NaOCl and preferably from 1 to 10 by weight of NaOCl, for example in a weight ratio of CNTs to sodium hypochlorite ranging from 1: 0.1 to 1: 1. The oxidation is advantageously carried out at a temperature of less than 60 ° C. and preferably at ambient temperature, for a duration ranging from a few minutes to 24 hours. This oxidation operation may advantageously be followed by filtration and / or centrifugation, washing and drying steps of the oxidized CNTs.

  In the first step of the process according to the invention, the CNTs (crude or crushed and / or purified and / or oxidized) are brought into contact with a dispersant comprising a homopolymer or copolymer of vinylpyrrolidone, preferably containing at least 50% by weight. moles of vinylpyrrolidone monomer. According to the invention, it is preferred that the dispersant be composed of polyvinylpyrrolidone. This may especially have a number average molecular weight ranging from 10,000 to 100,000. Such a polymer is especially available from the company CARBIOCHEM. The weight ratio of the dispersant to the CNTs used in this step of the process ranges from 0.1: 1 to less than 2: 1 and for example from 0.15: 1 to 1.5: 1, preferably from 0.2. : 1 to 1: 1 and more preferably 0.25: 1 to 0.5: 1. In addition, it is preferred that the total mass of dispersant and CNT is 0.1 to 5% and more preferably 0.5 to 2% of the weight of the alcoholic medium.

  In the second step of the process according to the invention, the mixture of the CNTs and the dispersant in the alcoholic medium is subjected to a mechanical treatment. This treatment can be of any kind, provided that it allows to obtain a homogeneous dispersion. It is preferred according to the invention that this treatment comprises ultrasonic treatment or shearing of the mixture, advantageously using a rotor-stator system.

  Such a rotor-stator system generally comprises a rotor driven by a motor and provided with fluid guiding systems perpendicular to the axis of the rotor, such as blades or blades arranged substantially radially or a flat disk provided with peripheral teeth, said rotor optionally being provided with a ring gear, and a stator arranged concentrically with respect to the rotor, and at a short distance outside thereof, said stator being provided on at least a portion of its circumference of openings , formed for example in a grid or defining between them one or more rows of teeth, which are adapted to the passage of fluid sucked into the rotor and ejected by the guide systems to said openings. One or more of the aforementioned teeth may be provided with sharp edges. The fluid is thus subjected to high shear, both in the gap between the rotor and the stator and through the openings in the stator.

  Such a rotor-stator system is in particular marketed by SILVERSON under the trademark Silverson "L4RT Another type of rotor-stator system is marketed by IKA-WERKE under the trade name Ultra-Turrax".

  Other rotor-stator systems still consist of colloid mills, deflocculating turbines and high-shear rotor-stator type mixers, such as the apparatus marketed by IKA-WERKE or the company ADMIX. It is preferred according to the invention that the rotor speed is set to at least 1000 rpm and preferably at least 3000 rpm or even at least 5,000 rpm. In addition, it is preferred that the width of the gap between the rotor and the stator is less than 1 mm and preferably less than 200 μm, more preferably less than 100 μm and more preferably less than 50 μm. less than 40 pm. Moreover, the rotor-stator system used according to the invention advantageously confers a shear of 1,000 to 109 seconds.

  The present invention also relates to the suspension that can be obtained according to the method as described above.

  The suspension according to the invention can in particular be used for the reinforcement of polymeric matrices; for the manufacture of packaging materials of 1: 5 electronic components (for example for electromagnetic shielding and / or antistatic dissipation), such as housings for mobile phones, computers, electronic devices on motor vehicles, rail or air; for the manufacture of inks for the electrical connection between two electronic components; or for the manufacture of medical instruments, fuel lines (gasoline or diesel), adhesive materials, antistatic coatings, thermistors, or electroluminescent diode electrodes, photovoltaic cells or supercapacitors.

  The present invention therefore also relates to the use of the suspension as defined above for the aforementioned purposes.

  The invention will now be illustrated by the following nonlimiting examples.

  EXAMPLES Example 1: Preparation of a sample of crude CNTs

  A sample of NTC is prepared by vapor phase chemical deposition (CVD) from ethylene at 650 ° C., which is passed over an iron catalyst supported on alumina. The product resulting from the reaction contains a ash content, measured by loss on ignition at 650 ° C in air, of 7%. This sample, which will be designated subsequently by NTC1, contains 3% of Fe2O3 and 4% of Al2O3, determined by chemical analysis.

  Example 2 Preparation of a Sample of Purified CNTs

  18.5 g of CNTC, obtained as described in Example 1, are subjected to a purification operation in 300 ml of 14% by weight sulfuric acid for 8 hours at 103 ° C. After washing with water and dried, a product, identified by NTC2, containing an ash content of 2.6% (2.5% Fe2O3 and 0.1% Al2O3, determined by chemical analysis) is obtained.

  Example 3 Preparation of Alcoholic Suspensions of Crude NTCs in the Presence of PVP

  Different suspensions of CNTs were prepared according to the following method: polyvinylpyrrolidone (PVP) of molecular weight 40,000 sold by the company CARBIOCHEM was added to a beaker before the volume was added to 100 ml with ethanol. 1 g of carbon nanotubes of NTC1 type as prepared in Example 1 were then added and the mixture was sonicated for 15 minutes at a frequency of 20 KHz using a Vibracell apparatus of the company. BIOBLOCK, having a displayed electric power of 300 W.

  After standing for 5 days at room temperature, the concentration of nanotubes in the supernatant was measured and the state of the dispersion, in particular its fluidity and the possible presence of grains, was observed.

  The different suspensions tested, as well as the results obtained, are summarized in Table 1 below.

  Table 1 Evaluation of CNT1 / PVP suspensions in ethanol Ex. Mass Time Aspect PVP stirring (g) (min) Presence Fluidity NTC of the in the grains supernatant suspension (g / 1) 1 0.1 15 Few Pasty but many re-homogenized well 2 0.1 30 Very very pasty 8.6 many 3 0.3 15 Absent Easy fluid 9.7 homogenize 4 1 15 Little Reasonably 9 Many fluid but difficult to homogenize 5 2 15 Numerous Numerous - As is apparent from this table, the suspensions according to the invention, having a weight ratio of CNTs to PVP of from 0.1 to less than 2, lead to suspensions more or less fluid, with few grains and a good concentration of NTC in the supernatant, which reflects the good dispersion of CNTs in ethanol.

  EXAMPLE 4 Ultrasonic Preparation of an Alcoholic Suspension of Purified NTCs in the Presence of PVP

  A suspension of NTC (1 g) of NTC2 type, obtained as described in Example 2, in ethanol was prepared, in the same manner as in Example 3, in the presence of PVP (0.3 g), which was subjected to 15 minutes of ultrasonic agitation.

  After 5 days at room temperature, a fluid and grain-free suspension having a concentration of nanotubes in the supernatant of 10 g / l was obtained, thus in accordance with the expected result.

  Example 5 Preparation of the rotor-stator mixer of an alcoholic suspension of purified NTC in the presence of PVP

  A suspension of CNT (5 g) of the NTC2 type, obtained as described in Example 2, was prepared in ethanol in the presence of PVP (1.5 g), which was mixed in a rotor-rotor system. Silverson stator "L4RT.

  This apparatus consists of a vertical hollow rotor 31 mm in diameter and a concentric grid acting as a stator 32 mm in diameter, the dispersion flowing radially from the inside to the outside of the apparatus . The rotation speed is 7,000 rpm, a peripheral speed of about 12 m / s.

  The operation begins with a grid pierced with small square holes of 5 mm side to allow rapid pumping of the suspension, and continues, after thickening of the suspension, with a grid pierced with small square holes of 2 mm side during 30 minutes. An ethanol dilution is then carried out in order to obtain 1 g / l of carbon nanotubes. After standing for 5 days at room temperature, the suspension is fluid, no grains are observed and the concentration of nanotubes in the supernatant is 9.8 g / l, very close to the expected value.

Claims (14)

  1. A process for preparing an alcoholic suspension of carbon nanotubes, comprising: - bringing into contact with said nanotubes an alcoholic medium with at least one dispersant consisting of a homo- or copolymer of vinylpyrrolidone, in a weight ratio of nanotubes of carbon to said dispersant ranging from 0.1 to less than 2, and the mechanical treatment of the mixture thus obtained.   2. Method according to claim 1, characterized in that the carbon nanotubes are obtainable by a chemical vapor deposition process.   3. Method according to claim 1 or 2, characterized in that the carbon nanotubes have a diameter ranging from 0.1 to 100 nm, preferably from 0.4 to 50 nm and better still from 1 to 30 nm.   4. Process according to any one of Claims 1 to 3, characterized in that the carbon nanotubes have a length of 0.1 to 10 μm.   5. Method according to any one of claims 1 to 4, characterized in that the carbon nanotubes are crude nanotubes, purified with a sulfuric acid solution, oxidized with a solution sodium hypochlorite and / or crushed using an air jet mill. 17   6. Method according to any one of claims 1 to 5, characterized in that the dispersant is polyvinylpyrrolidone.   7. Method according to any one of claims 1 to 6, characterized in that the weight ratio of dispersant to carbon nanotubes used ranges from 0.15: 1 to 1.5: 1, preferably 0.2 : 1 to 1: 1 and more preferably 0.25: 1 to 0.5: 1.   8. Method according to any one of claims 1 to 7, characterized in that the total mass of dispersant and carbon nanotubes represents from 0.1 to 5% and more preferably from 0.5 to 2% of the weight of the medium. alcoholic.   9. Method according to any one of claims 1 to 8, characterized in that the mechanical treatment comprises an ultrasonic treatment or shearing of the mixture using a rotor-stator system.   10. The method of claim 9, characterized in that the speed of the rotor is set to at least 1,000 rpm and preferably at least 3000 rpm or even at least 5,000 rpm.   11. The method of claim 9 or 10, characterized in that the width of the gap between the rotor and the stator is less than 1 mm and preferably less than 200 pm, more preferably less than 100 pm and, better, less than 50 pm or even less than 40 gm.   12. Method according to any one of claims 9 to 11, characterized in that the rotor-stator system provides a shear of 1,000 to 109 s-1.   13. A suspension obtainable by the process according to any one of claims 1 to 12.   14. Use of the suspension according to claim 13 for reinforcing polymeric matrices; for the manufacture of packaging materials for electronic components (for example for electromagnetic shielding and / or antistatic dissipation), such as housings for mobile telephones, computers, electronic devices on board motor vehicles, railroad vehicles or aerial; for the manufacture of inks for the electrical connection between two electronic components; or for the manufacture of medical instruments, fuel lines (gasoline or diesel), adhesive materials, antistatic coatings, thermistors, or electroluminescent diode electrodes, photovoltaic cells or supercapacitors.