WO2010106287A2 - Method for dispersing carbon nanotubes, system for implementing same, and uniform dispersion thus obtained - Google Patents

Abstract

The invention relates to a method for dispersing CNTs in a liquid medium, consisting of mixing the CNTs in the liquid medium using two types of physical mixing, one preferably using a ball mill (10) and the other an ultrasound device (20).

Description

 PROCESS FOR DISPERSION OF CARBON NANOTUBES; SYSTEM FOR IMPLEMENTATION AND DISPERSION UNIFORM SO OBTAINED.

The invention relates to a method for dispersing carbon nanotubes. It also relates to a system for implementing the method and a uniform dispersion thus obtained.

Classically referred to as carbon nanotubes NTC, any nanoparticle with a carbon-based tubular structure, in the form of rolled graphene sheets, which has a diameter of between 2 and 200 nm. When the tubular structure is full, it is called nano carbon fiber NFC. The length / diameter ratio of the CNTs or NFCs is much greater than 1, and typically greater than 10.

Carbon nanotubes are used in the manufacture of nanocomposite materials. Nanocomposites are of particular interest in the industrial field because they have remarkable properties for relatively low charge rates, that is to say less than 10% by weight. Indeed, CNTs provide a significant improvement in the mechanical and electrical properties of the polymer matrix. Moreover, unlike fibril-type reinforcements, CNTs reinforce the polymeric matrix in all directions of space.

Therefore, a good state of dispersion is a key issue to hope to take full advantage of the exceptional intrinsic properties of CNTs.

However, in practice, the use of carbon nanotubes as fillers in polymer matrices for making nanocomposites encounters difficulties because carbon nanotubes tend to aggregate together to form very stable agglomerates, also called aggregates. The presence of such aggregates alters the physical and mechanical properties of the composites in which they are present.

In order to allow a transmission of the physical and chemical properties of the carbon nanotubes to the matrix, it is necessary to be able to distribute the nanotubes homogeneously in this matrix. For this, it is first necessary to prepare dispersions of carbon nanotubes in a liquid medium and make the dispersions stable, so that it there is no aggregation or precipitation of the nanotubes in the liquid medium, the adhesive forces between the particles to be minimized.

To this end we can add a solvent. Unfortunately, it is found that the solubility of carbon nanotubes NTC (or NFC carbon nanofibers), is very low regardless of the solvent used.

In some cases, it is also possible to add a surfactant or polymer-type dispersing agent. But this addition is generally not sufficient to disperse the carbon nanotubes. Indeed, it is also generally necessary to provide energy to the dispersion to fight against Van der Waals interactions that bind the nanotubes or nanofibers to each other.

For dispersions operated with a solvent, it is also necessary to provide agitation by a source of energy. Ultrasound is usually the source of energy used. Reference can be made to the state of the art constituted by the patent application US 2005/0025694, in which a dispersion of carbon nanotubes in a liquid medium is described with the combined use of a dispersing agent and a agitation which is preferably obtained by ultrasound.

Indeed, ultrasound can reduce the size of aggregates of carbon nanotubes and therefore disperse them.

Unfortunately ultrasounds have several drawbacks: the necessary powers involved in obtaining a good state of dispersion of the nanotubes cause their partial destruction. For some applications, obtaining a sufficient dispersion state leads to CNT sizes of about 200 nm,

- In addition, to be effective, this technique is limited to relatively low viscosity levels of solutions, which implies also low concentrations of NTC.

The state of the art constituted by the US Pat. No. 5,744,235 issued on April 28, 1998 in the name of the Hyperion Company is also known. This patent describes a process for preparing a composite material containing fillers such as carbon or glass fibers, whiskers (whiskers) of carbon or silicon carbide, particles of silica or carbon black, or fibrils of carbon in a metal matrix, ceramic, or organic resin such as elastomers, thermoplastic resins or thermosetting resins. This method comprises a step a) of mixing the fillers in the matrix and then subjecting this mixture to a shearing and force applying step b) so as to reduce the size of the charge aggregates by a value of not more than 1000 times the size of the aggregates of departure. Step b) is carried out with a conventional ball mixer. Unfortunately, this method is not at all suitable for reducing the size of the carbon nanotubes obtained by a catalytic vapor phase process (Catalyst Chemical Vapor Deposition in English terminology), to a size allowing their introduction in a homogeneous manner in a organic matrix.

It will also be possible to refer to the closest state of the art constituted by the document Dl: WANG ET AL titled "A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio" of CARBON, ELSERVIER, OXFORD, GB, vol.41, No. 15, January 1, 2003 (2003-01-01), pages 2939-2948, XP004470635 ISSN: 0008-6223 in which are listed several methods for separating nanotubes (NTC ) which tend to aggregate together to form stable agglomerates.

These different methods are stated Column 2 page 2940: In paragraph 3, a sample of ground NTC carbon nanotubes was obtained by treating dry crude CNTs in a ball mill.

In Section 5, a sample of crude CTs treated with ultrasound was obtained using an ultrasound apparatus to disperse the crude CNTs into a water bath containing a dispersing agent (SDS).

In paragraph 6, a sample of ultrasonically treated CNTs and ball mills is obtained by using the method of ultrasonication described in paragraph 5 on dry-milled NTCs by the method described in paragraph 3. Dry-milled NTCs are placed in the water bath with the dispersing agent and this bath is subjected to ultrasound.

Thus, the treatment carried out by the ball mill is carried out on dry crude CNTs. The method described in this document makes it possible to separate the agglomerates of CNTs with a ball mill but does not carry out a mixture of CNTs in a liquid medium as described in the present invention. In addition, on page 2943, column 1, it is clearly described that even with the combination of the two methods given in paragraph 3 and 6, grinding and then ultrasound, a stable suspension could not be obtained even after several hours of treatment with ultrasound. The present invention solves this problem.

Reference can also be made to document D2, EP 1 862 432. This document describes a method for obtaining single-stranded NTCs after filtration of a solution in which single-stranded NTCs have been ultrasonically dispersed, the solution comprising a dispersing agent such as CMC (carboxymethylcellulose). The problem described in this document is to obtain single-walled NTC by filtration of a solution of CNTs and to obtain films containing single-wall NTCs and CMC after drying the solution.

This document does not describe the process according to the invention. In addition, the problem we are trying to solve in this document is different.

Description of the invention

The problem that the applicant has sought to solve by the present invention is to provide a method for dispersing carbon nanoparticles, NTC or NFC, without the disadvantages of the processes just described.

In the following description, between X and Y means terminals included.

In addition, it is called liquid medium, it being understood that the liquid medium may have a greater or lesser fluidity depending on its composition. The present invention applies to a liquid medium such as a fluid matrix whose viscosity ranges from 0.1 to 10000 mPa.s.

The present invention relates to a method of dispersing carbon nanotubes in a liquid medium which makes it possible to obtain an improved dispersion at a given CNT load, which has a stability that does not degrade over time. The dispersion process can also be carried out in a liquid medium having a viscosity of between 0.1 and 10,000 mPa.s.

The subject of the invention is more particularly a method for dispersing CNTs, in a liquid medium consisting in mixing the CNTs in the liquid medium mainly characterized in that the medium comprises a fluid matrix and a dispersing agent with a mass ratio rI / r2 between the CNT load in the medium and the dispersing agent between 0.25 and 7, the mixture consisting of at least a first physical agitation performed with a ball mill and then a second physical agitation performed by an ultrasonic apparatus.

Thus, this method comprises an operation consisting in mixing the CNTs in the liquid medium by at least a first physical agitation of a first type and at least a second physical agitation of a type different from the first;

The first physical agitation is performed by a ball mill;

The second physical agitation is performed by an ultrasound apparatus;

The ball mill agitation is carried out on the liquid medium containing the CNTs, and the stirring is carried out by the ultrasonic apparatus. It is expected that the CNTs are previously introduced into the liquid medium. The mixture will then be agitated by one or more successive passages in the ball mill, and the stirring is carried out ultrasonically.

The liquid medium constitutes a matrix whose viscosity is between 0.1 and 10000 mPa.s. The level of charge in CNT in the liquid medium may vary from 0.1 to 5%.

The charge rate of CNT in the liquid medium is preferably between 0.1% and 3% by mass.

The liquid medium comprises a fluid matrix and preferably a dispersing agent. For example, the fluid matrix may be water and the dispersing agent an anionic polymer. The liquid matrix may comprise 1% by weight of anionic polymer, for example Sodium Carboxy Methyl Cellulose (CMC).

The weight ratio r1 / r2 between the charge in NTC and dispersing agent is between 0.25 and 7; expression in which rl = mass of CNT / mass M of the liquid medium (for example water), and r2 = mass of the dispersing agent / mass M of the liquid medium (for example water).

The dispersion may therefore for example relate to water containing 1% of NTC by mass and 2% of polyacrylic acid.

The invention also relates to a system for implementing the method mainly characterized in that it comprises a mechanical stirring apparatus and a cavitation stirring apparatus.

The mechanical stirring apparatus is ball mill.

The cavitating apparatus is an ultrasonic apparatus. The invention also relates to a homogeneous dispersion of CNTs in a liquid medium having a viscosity of between 0.1 and 10,000 mPa.s, obtained by the method described.

Other features and advantages of the invention will become clear from reading the description which is given below and which is given by way of illustrative and non-limiting example and with reference to the figures in which: FIG. an optical microscope photograph of NTC dispersions obtained by a ball mill, FIG. 2 represents an optical microscope photograph of ultrasonic NTC dispersions;

FIG. 3 represents an optical microscope image of CNT dispersions obtained according to the invention; FIG. 4 represents the evolution curves of the total intensity transmitted by aqueous CNT dispersions obtained with different processes, curve C4 corresponding to the result obtained with the present invention; FIG. 5 shows an optical microscope slide of a dispersion of CNT obtained by physical agitation carried out by an apparatus imposing high shear rates;

FIG. 6 shows an optical microscope slide of an ultrasound-derived NTC dispersion;

FIG. 7 shows an optical microscope slide of a dispersion of CNT obtained by an apparatus imposing high rates of shearing and then ultrasound; FIG. 8 represents evolution curves of the total intensity transmitted by the aqueous NTC dispersions obtained for the C5 curve, by an apparatus imposing high shear rates and then an ultrasonic apparatus and, for the curve C6, by a grinder ball and then an ultrasound machine; FIG. 9 represents diagrams of the evolution of the conductivity of the CNT dispersions as a function of the dispersion methods used;

- Figure 10, a schematic diagram of the dispersion system.

It is recalled that a ball mill is an apparatus for making mixtures, consisting of a cylindrical chamber in which balls whose nature may vary. are rotated and in which are introduced the various constituents to mix or grind.

It is recalled that an ultrasonic stirring apparatus makes it possible to subject a mixture, in this case the liquid medium and the CNTs, to ultrasonic waves for a determined duration and according to different pulse modes: continuous or discontinuous pulses. Conventionally, to avoid excessive heating of the medium subjected to ultrasonic agitation, a mode of discontinuous pulses is preferred with for example pulses of 40s every minute for several tens of minutes. Two types of devices can be used to generate ultrasound: ultrasonic baths and probes.

The method for dispersing CNTs in a liquid medium according to the invention consists in mixing the CNTs in the liquid medium by means of two types of physical agitation, one being operated preferentially by a ball mill and the other by an ultrasound machine. In a preferred embodiment, the method consists in using in combination the ball mill, and then the ultrasonic apparatus for dispersing carbon nanotubes in any liquid matrix whose viscosity is between 0.1 and 10000 mPa.s.

The advantage of this joint association in this precise order of use is highlighted in the embodiment shown below and illustrated by the optical microscope picture corresponding to FIG. 3 compared with the pictures illustrated in FIGS. 2.

In the examples illustrated in FIGS. 1, 2 and 3, it is a dispersion of 1% by weight CNT in a 1% aqueous solution of carboxymethylcellulose (CMC). In the case of Figure 1, the dispersion is carried out by means of a ball mill, in the case of Figure 2, the dispersion is carried out under the same conditions by an ultrasonic device. In the case of Figure 3, the dispersion is carried out according to the method of the invention by first carrying out a dispersion with a ball mill and then a dispersion with an ultrasonic apparatus. Microscopy:

The microscopic observations clearly show an improvement in the state of dispersion using the combination of the two dispersion operations in the specified order as described. UV-visible spectrophotometry:

Measurements of the intensity of the radiation transmitted through the CNT dispersions make it possible to evaluate, at a fixed concentration, the state of dispersion of the CNTs within the solution. Thus, the lower the signal transmission, the better the dispersion state of the CNTs.

By way of example, the detection cell used is an integrating sphere that makes it possible to analyze the entirety of the signal transmitted through the sample.

FIG. 4 illustrates the evolution curves of total intensity transmitted by the different dispersions respectively obtained by dispersing by means of an ultrasonic cavitation apparatus: curve C1; by means of a ball mill: curve C2, by means of an ultrasonic apparatus then of a ball mill: curve C3 and finally by means of a ball mill and then of an ultrasonic apparatus: curve C4 .

It can be seen that the ball mill / ultrasound coupling provides an improvement over the use of a single dispersion operation. Note also that the order in which the two operations are used changes the results. The dispersion seems optimal when first passing the dispersion ball mill, then ultrasound.

Ultrasound leads to the gradual "crumbling" of aggregates (which explains the coexistence of aggregates of different sizes) whereas the ball mill produces more homogeneous dispersions with particles of intermediate sizes. It is therefore more advantageous to start by reducing the size of the large aggregates using the ball mill, then to complete the dispersion using ultrasound.

We can also consider two ball mill passes with a first pass with large diameter beads to reduce the size of aggregates to about ten microns and then a second pass with smaller diameter beads to refine the dispersion and finally a passage ultrasound.

In another embodiment, the mechanical stirring of the CNTs in aqueous media is carried out by means of a high-shear mixer, this operation being followed by physical stirring by means of ultrasound.

By way of example, the high-shear mixer is an apparatus marketed under the trademark ultraturax®. Ultraturax® is an ultra-fast mixer that induces large shear. In order to compare the various dispersion processes namely, a dispersion carried out by a high shear mixer, a dispersion carried out by an ultrasonic apparatus and a dispersion carried out by the high shear mixer followed by a mixing carried out by the apparatus. ultrasound. The results obtained are visible on the images obtained by optical microscope.

FIG. 5 corresponds to the dispersion carried out by the high-shear mixer (L'ultraturax®), FIG. 6 corresponds to the dispersion operated by the ultrasonic apparatus and FIG. 7 corresponds to the dispersion operated using the high-speed mixer. shear then the ultrasonic device. The high shear mixer / ultrasound coupling provides a state of dispersion similar to the dispersion state of the CNTs compared to an ultrasound system alone. However, unlike a dispersion operated by means of an ultrasonic apparatus, this coupling brings less limitation on the value of the viscosity of the medium and the concentration of CNT. When the required dispersion state will be important, the ball mill / ultrasound combination will be chosen.

The graph illustrated in FIG. 8 represents the curves of evolution of total intensity transmitted by aqueous dispersions of CNTs according to the two example embodiments described. The high shear mixer NTC dispersion followed by ultrasonic dispersion has a C6 conductivity curve in FIG. 8. The ball-milled NTC dispersion followed by ultrasonic dispersion has a C5 conductivity curve. in this figure 8.

It can be seen from the graph shown in Figure 9 that the use of a ball mill significantly increases the electrical conductivity of the NTC dispersions. Despite the use of ultrasound, the levels of conductivity achieved by combining the two systems: ball mill and ultrasound, remain high, compared to levels obtained by simple use of ultrasound or the high shear mixer (L'ultraturax®) or by coupling the two.

Figure 10 illustrates the system according to the preferred exemplary embodiment of using a ball mill 10 and then an ultrasonic apparatus 20 (probe immersed in the liquid medium mixture and NTC).

The liquid matrix is introduced into the ball mill with the CNTs as indicated by the arrow in the figure. After stirring for example for one hour, the The mixture can then be pumped to be subjected to ultrasound in the apparatus 20. It is possible to proceed by successive sequences of continuous pulses of ultrasound so as to subject the mixture a dozen times for a few minutes (4 minutes for example), leaving cool the mixture between two sequences; it is also possible to carry out a single ultrasound sequence of 50 minutes, for example with pulses of 50 seconds per minute.

By way of example, it will be possible to use the ball mill 10 marketed under the reference: Dispermat® SL-25.

And the ultrasound device marketed under the reference: Branson Digital sonifier® S450-D.

Homogeneous dispersions have been obtained with other dispersants and relatively charged carbon nanotube dispersions, for example from a few tenths of% to 1 or 2% in CNT, and there are, for example, optima for the following ratios r1 / r2. :

between 0.5 and 1 for sodium dedocyl sulphate (SDS) according to the quantity of nanotubes and whether it is multiwall NTC (MW) or single-walled (SW),

* around 3 for polyacrylic acid (PAA),

* around 1, 2 for ethacryl G, * for Polyoyethylene (20) Stearyl known by its trade name Brij78

(registered trademark) currently used on direct spinning is 0, 5 for single-turn NTC (SW) and 0.75 for multi-wall NTC (MW).

Claims

1. A method for dispersing carbon nanotubes (CNTs) in a liquid medium consisting in mixing the CNTs in the liquid medium, characterized in that the medium comprises a fluid matrix and a dispersing agent with a mass ratio r1 / r2 between the charge in NTC in the medium and the dispersing agent of between 0.25 and 7, the mixture consisting of at least a first physical stirring carried out with a ball mill and then a second physical stirring carried out by an ultrasonic apparatus. 2. Process for dispersing CNTs, in a liquid medium according to claim 1, characterized in that the CNTs are introduced beforehand into the liquid medium, in that stirring is performed by one or more successive passages in the ball mill. and in that the mixing by the ultrasonic apparatus is carried out after this or these successive passages in the ball mill. 3. Process for dispersing CNTs, in a liquid medium according to claim 1 or 2, characterized in that the liquid medium constitutes a matrix whose viscosity is between 0.1 and 10000 mPa.s. 4. Process for dispersing CNTs, in a liquid medium according to any one of the preceding claims, characterized in that the CNT loading rate in the liquid medium is 0.1 to 5% by weight and preferably between 0.1% and 3% by weight. 5. Process for dispersing CNTs in a liquid medium according to claim 1, characterized in that the dispersing agent is chosen from polymers which are soluble in the fluid matrix, for example anionic polymers in water. 6. Process for dispersing CNTs in a liquid medium according to claim 1, characterized in that the liquid matrix comprises 1% by weight of anionic polymer, for example Sodium Carboxy Methyl Cellulose (CMC). 7. System for implementing the CNT dispersion method in a liquid medium, according to any one of the preceding claims, characterized in that it comprises a first physical stirring apparatus and a second physical stirring apparatus of different type, the first physical agitation apparatus (10) being a ball mill, the second physical agitation apparatus (20) being an ultrasonic apparatus. 8. Homogeneous dispersion of CNTs in an aqueous medium having a viscosity of between 0.1 and 10,000 mPa.s, obtained by the process according to any one of Claims 1 to 6.