WO2008145910A2

WO2008145910A2 - Method for preparing an alcoholic suspension of carbon nanotubes and suspension thus obtained

Info

Publication number: WO2008145910A2
Application number: PCT/FR2008/050696
Authority: WO
Inventors: Dominique Plee
Original assignee: Arkema France
Priority date: 2007-04-20
Filing date: 2008-04-18
Publication date: 2008-12-04
Also published as: WO2008145910A3; FR2915109A1; FR2915109B1

Abstract

The invention relates to a method for preparing an alcoholic suspension of carbon nanotubes, that comprises: contacting in an alcoholic medium said nanotubes with at least one dispersant including a vinylpyrrolidone homo- or copolymer according to a weight ratio between said carbon nanotubes and said dispersant from 0.1 to less than 2; and mechanically processing the mixture thus obtained. The invention also relates to the suspension thus obtained and to the uses thereof.

Description

Process for preparing 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 can 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 Hypeπon 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 gaseous compound based on carbon at a temperature of between approximately 850 ^° C. and 1200 ^° C. ⁰ C, the proportion by dry weight of the carbon-based compound with respect 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.

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 impairs 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 in 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 oxygenated functions that can 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-I 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 / 1. 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 for dispersing single-wall carbon nanotubes in water or ethanol, respectively (WO 02/76888 and WO 2005/073305). The first of these processes, however, requires the use of large amounts of polymer and does not allow to stabilize more than 0.5 g / l of CNT.

Another solution, proposed in particular in applications EP-I 359 121 and EP-I 359 169, consisted in producing a dispersion of CNTs in a solvent and a monomer and in situ polymerization leading to obtaining CNTs. 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 Carbon, (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 processes, 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 J ^" of Nanoscience and

Nanotechnology, 5 (7), (2005), 1050-1054, leading to -Punctionalize CNTs by means of polyvinyl alcohol and LIU et al in Carbon, 43 (7), (2005), 1470-1478, which allow a grafting of tπfluoroacetic 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-wall 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, that is to say 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 is therefore still a need to provide a simple, inexpensive and environmentally friendly process for preparing suspensions of carbon nanotubes, in particular multiwall NTCs, for 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. no grain larger than 30 μm or more than 20 μm.

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 preparing an alcoholic suspension of carbon nanotubes, comprising:

Contacting said nanotubes in an alcoholic medium with at least one dispersant consisting of a vinylpyrrolidone homo- or copolymer, in a weight ratio of carbon nanotubes to said dispersant ranging from 0.1 to less than 2, and

the mechanical treatment of the mixture thus obtained.

For the purposes 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, if appropriate, an alcohol-miscible solvent 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 optionally 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-walled, double-walled or multi-walled type. Double-walled CNTs can in particular be prepared as described by

FLAHAUT et al in Chem. Corn. (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 better still from 1 to 30 nm and advantageously a length of 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. / cm ³ and more preferably between 0.1 and 0.2 g / cm ³ . 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 especially commercially available from Arkema under the trade name 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 may especially be carried out cold or hot and be carried out according to the known techniques used in apparatus 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 can in particular 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 may 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 below 60 ^° C. and preferably at room temperature, for a duration ranging from a few minutes to 24 hours. This oxidation operation may advantageously be followed by filtration steps and / or centrifugation, washing and drying 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: 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, or a passage of the mixture in a ball mill or ball. A rotor-stator system that can be used according to the invention generally comprises a rotor driven by a motor and provided with fluid guide systems perpendicular to the axis of the rotor, such as blades or blades arranged substantially radially or a flat disc provided with teeth. peripheral, 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 equipped on at least a portion of its circumference 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 sold especially by the company Silverson under the trade name Silverson ^® L4RT.

Another type of rotor-stator system is sold by IKA-WERKE under the Ultra-Turrax ^® trade name.

Other rotor-stator systems still consist of colloid mills, deflocculating turbines and high-shear mixers of the rotor-stator type, such as the apparatus marketed by the company IKA-WERKE or the company ADMIX. According to the invention, the rotor speed is preferably set to at least 1,000 rpm and preferably at least 3000 rpm or even at least 5,000 rpm.

In addition, it is preferred that the width of the air gap between the rotor and the stator be less than 1 mm and preferably less than 200 μm, more preferably less than 100 μm, and better still less than 50 μm. less than 40 μm. Moreover, the rotor-stator system used according to the invention advantageously confers a shear of 1,000 to ⁹ s ¹ .

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 for electronic components (for example for electromagnetic shielding and / or antistatic dissipation), such as housings for mobile telephones, 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 Chemical Vapor Deposition (CVD) from ethylene at 650 ^° C., which is passed over a catalyst consisting of iron 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 NTCl, contains 3% of Fe ₂ O ₃ and 4% of Al ₂ O ₃ , determined by chemical analysis.

Example 2 Preparation of a sample of purified CNTs

18.5 g of NTCl, obtained as described in Example 1, are subjected to a purification operation in 300 ml of sulfuric acid at 14% by weight for 8 hours at 103 ^° C. Once washed with water and dried, there is obtained a product, identified by NTC2, containing an ash content of 2.6% (of which 2.5% of Fe ₂ O ₃ and 0.1% of Al ₂ O ₃ , determined by chemical analysis) .

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

Different suspensions of CNTs were prepared according to the method following: was added in a beaker of polyvinylpyrrolidone (PVP) molecular weight of 40,000 marketed by the company CARBIOCHEM before completing the volume to 100 ml with ethanol. 1 g of NTC1-type carbon nanotubes 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 from 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.

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 more or less fluid suspensions, with few grains and a good concentration of NTC in the supernatant, which reflects the good dispersion of CNTs in the ethanol.

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

A suspension of NTC2 (1 g) NTC, obtained as described in Example 2, was prepared in the ethanol in the presence of PVP (0.3 g), analogously to Example 3. 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 NTC (5 g) of 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-stator system. Silverson ^{11 *} 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, ie 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 to obtain 10 g / l of carbon nanotubes. After 5 days of rest at ambient 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

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. Method according to any one of claims 1 to 3, characterized in that the carbon nanotubes have a length of 0.1 to 10 microns.

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 of sodium hypochlorite and / or crushed using a grinder and air.

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 ultrasonic treatment or shearing of the mixture using a rotor-stator system or a passage of the mixture in a mill ball or ball.

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 microns, more preferably less than 100 microns and, better, less than 50 microns or even less than 40 microns.

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 10 ⁹ s ^{~ x} .

13. 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 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.