AGGLOMERATED SOLID MATERIAL MADE FROM LOOSE CARBON NANOTUBES

Abstract

An agglomerated solid material comprising loose carbon nanotubes and that is free from organic compounds is described, as well as the method of preparation thereof, and uses thereof, where the agglomerated solid material consists of a continuous network of carbon nanotubes comprising aggregates of carbon nanotubes with an average size d50 under 5 m, in a proportion below 60% by area, determined by image analysis by electron microscopy and has an apparent density between 0.01 g/cm.sup.3 and 2 g/cm.sup.3.

Claims

1. Agglomerated solid material in any rough shape whose smallest dimension is greater than 1 millimetre, comprising loose carbon nanotubes (CNTs) that are free from organic compounds, consisting of a continuous network of carbon nanotubes comprising aggregates of carbon nanotubes with an average size d50 under 5 m, in a proportion below 60% by area, determined by image analysis by electron microscopy characterized in that it has an apparent density of between 0.01 g/cm.sup.3 and 2 g/cm.sup.3.

2. Material according to claim 1, characterized in that it comprises at least one chemical compound of an inorganic nature intimately incorporated in the continuous network of carbon nanotubes.

3. Material according to claim 1, characterized in that it has an apparent density between 0.1 and 1.0 g/cm.sup.3.

4. Method for preparing an agglomerated solid material as defined according to claim 1, characterized in that it comprises at least one step of compression of a carbon nanotube powder in the presence of at least one sacrificial substance, and optionally of at least one inorganic compound, followed by high-shear mixing of the powder in the compressed state, then forming to obtain an agglomerated solid material and final removal of the sacrificial substance.

5. Method according to claim 4, characterized in that it comprises at least the following steps: a) charging a compounding device with carbon nanotubes in the powdered state and at least one sacrificial substance in a weight ratio from 10/90 to 40/60, and optionally at least one inorganic compound; b) mixing the carbon nanotubes and the sacrificial substance in said device to form a mixture in an agglomerated physical form; c) recovering the mixture in the form of agglomerated solid material; d) removing the sacrificial matrix.

6. Method according to claim 4, characterized in that the sacrificial substance is a solvent that does not leave any residue after it is removed by drying the agglomerated solid material, an organic substance that does not leave any residues after pyrolysis of the agglomerated solid material, or a substance in the supercritical state.

7. Method according to claim 4, characterized in that the carbon nanotubes in the powdered state are crude, purified and/or oxidized.

8. Method according to claim 4, characterized in that the inorganic compound comprises entities of a metallic nature, carbon, silicon, sulphur, phosphorus, boron, and other solid elements; metal oxides, sulphides, or nitrides; hydroxides and salts; ceramics of complex structure or mixtures of all these inorganic materials.

9. Agglomerated solid material obtainable by the method as defined according to claim 4, characterized in that its percentage porosity corresponds to the volume fraction of the sacrificial substance implemented in the method.

10. Use of the agglomerated solid material according to claim 1 for incorporating carbon nanotubes in water-based or organic liquid formulations.

11. Use of the agglomerated solid material according to claim 1 for manufacturing composite materials, of the thermoplastic or thermosetting type.

12. Use of the agglomerated solid material according to claim 1 for preparing elastomer compositions.

13. Use of the agglomerated solid material according to claim 1 for making components of batteries and supercapacitors.

14. Use of the agglomerated solid material according to claim 1 for preparing electrode formulations for lithium-ion batteries, lithium-sulphur batteries, sodium-sulphur batteries, or lead-acid batteries or other types of energy storage systems.

15. Use of the agglomerated solid material according to claim 1 for preparing catalyst supports making up electrodes.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0062] FIG. 1 is an SEM image of the morphology of the agglomerated solid material according to the invention.

[0063] FIG. 2 is an SEM image of the morphology of a CNT powder (comparative).

DETAILED DESCRIPTION OF THE INVENTION

[0064] The invention is now described in more detail, and non-limiting, in the description given hereunder.

[0065] The loose carbon nanotubes making up the agglomerated solid material according to the invention may be of the single-walled (SWNT), double-walled (DWNT) or multiwalled (MWNT) type.

[0066] Carbon nanotubes usually have an average diameter from 0.1 to 200 nm, preferably from 0.1 to 100 nm, more preferably from 0.4 to 50 nm, and better still from 1 to 30 nm, or even from 10 to 15 nm, and advantageously a length of more than 0.1 m and advantageously from 0.1 to 20 m, preferably from 0.1 to 10 m, for example of about 6 m. Their length/diameter ratio is advantageously greater than 10 and most often greater than 100.

[0067] They may have closed and/or open ends. These nanotubes are generally obtained by chemical vapour deposition. Their specific surface area is for example between 100 and 300 m.sup.2/g, advantageously between 200 and 300 m.sup.2/g, and their apparent density may notably be between 0.01 and 0.5 g/cm.sup.3 and more preferably between 0.07 and 0.2 g/cm.sup.3. Multiwalled carbon nanotubes may for example comprise from 5 to 15 sheets and more preferably from 7 to 10 sheets.

[0068] An example of crude CNTs in the powdered state usable for preparing the loose CNTs according to the invention is notably the trade name Graphistrength C100 from the Arkema company.

[0069] According to one embodiment of the invention, the loose CNTs comprise metallic or mineral impurities, in particular the metallic and mineral impurities derived from the synthesis of crude CNTs in the powdered state. The amount of non-carbon impurities may be between 2 and 20 wt %.

[0070] According to one embodiment of the invention, the loose CNTs are free from metallic impurities, and result from crude CNTs in the powdered state that have been purified in order to remove the impurities inherent in their synthesis.

[0071] The crude or ground nanotubes may be purified by washing with a solution of sulphuric acid, so as to remove any residual mineral and metallic impurities from them, for example such as iron derived from the method of preparation. The weight ratio of nanotubes to sulphuric acid may notably be between 1:2 and 1:3. The purification operation may, moreover, be carried out at a temperature from 90 to 120 C., for example for a time from 5 to 10 hours. This operation may advantageously be followed by steps of rinsing with water and drying the purified nanotubes. The nanotubes may as a variant be purified by thermal treatment at high temperature, typically above 1000 C.

[0072] According to one embodiment of the invention, the loose CNTs are oxidized CNTs.

[0073] Oxidation of the nanotubes is advantageously carried out by bringing them into contact with a solution of sodium hypochlorite containing from 0.5 to 15 wt % of NaOCl and preferably from 1 to 10 wt % of NaOCl, for example in a weight ratio of nanotubes to sodium hypochlorite from 1:0.1 to 1:1. Oxidation is advantageously carried out at a temperature below 60 C. and preferably at room temperature, for a time from a few minutes to 24 hours. This operation of oxidation may advantageously be followed by steps of filtration and/or centrifugation, washing and drying of the oxidized nanotubes.

[0074] The loose CNTs form a continuous network comprising CNT aggregates with an average size d50 under 5 m, in a proportion below 60% by area determined by image analysis by electron microscopy.

[0075] The proportion of aggregates with an average size d50 under 5 m is preferably below 40% by area, more preferably below 20% by area, or even below 10% by area.

[0076] The continuous network of CNTs preferably represents more than 60% by area, more preferably more than 80% by area, or even more than 90% by area, according to image analysis by electron microscopy.

[0077] The loose CNTs are free from organic compounds on their surface.

[0078] A method for preparing the loose CNTs making up the agglomerated solid material of the invention uses a compounding device to compress a CNT powder and shear the CNT aggregates so as to reduce their size and the entanglement of the CNTs.

[0079] Examples of co-kneaders usable according to the invention are the BUSS MDK 46 co-kneaders and those of the series BUSS MKS or MX, marketed by the company BUSS AG, which all consist of a screw shaft provided with flights, arranged in a heating barrel optionally consisting of several parts and whose inside wall is provided with kneading teeth suitable for interacting with the flights to produce shearing of the material being kneaded. The shaft is rotated, and is provided with oscillating motion in the axial direction, by a motor. These co-kneaders may be equipped with a granule-producing system, fitted for example at their discharge orifice, and which may consist of an extrusion screw or a pump.

[0080] The co-kneaders usable according to the invention preferably have a screw ratio L/D in the range from 7 to 22, for example from 10 to 20, whereas the co-rotating extruders advantageously have a ratio L/D from 15 to 56, for example from 20 to 50.

[0081] To achieve optimum shearing of the CNT aggregates as well as minimum entanglement of the CNTs in the aggregates, it is generally necessary to apply considerable mechanical energy, which is preferably above 0.05 kWh/kg of material, in the compounding device.

[0082] According to the method of the invention, compounding of the powder is carried out in the presence of a sacrificial substance in a weight ratio from 10:90 to 40:60, preferably from 10:90 to 32:68, or even from 20:80 to 30:70, so as to obtain agglomerated particles comprising loose CNTs and the sacrificial substance, the sacrificial substance then being removed to form the loose CNTs free from organic compounds. It has in fact been shown that in this ratio, optimum compounding is possible for a wide range of sacrificial substances.

[0083] The following may be used as sacrificial substances, although this is not an exhaustive list: a solvent that does not leave any residue after it is removed by drying the agglomerated solid material, or an organic substance that does not leave any residues after pyrolysis of the agglomerated solid material, or a substance in the supercritical state that does not leave any residue after degassing, for example supercritical CO.sub.2.

[0084] Preferably water, an alcohol, or other hydrophilic solvents, as well as mixtures thereof, preferably water, are used as the solvent.

[0085] Preferably, a polymer such as a polypropylene PP, a polyethylene terephthalate PET, a polycarbonate PC, a polyamide PA, preferably a polypropylene PP, is used as the organic substance.

[0086] According to one embodiment, it is possible to add inorganic compounds such as metal oxides or salts in the compounding device, in order to obtain an agglomerated solid material of loose CNTs comprising mineral compounds that are beneficial for the intended application. We may mention for example soda, zinc oxide or titanium oxide, a carbonate, a hydroxide, a metal oxide or sulphide for example of lithium, manganese, nickel, or cobalt.

[0087] It is also possible to add other carbon-containing nanofillers such as graphene, graphite, or carbon black at a content suitable for the intended application.

[0088] The invention will now be illustrated by the following examples, which do not aim to limit the scope of the invention, which is defined by the accompanying claims.

EXAMPLES

Example 1: Preparation of an Agglomerated Solid Material of Loose CNTs With a Polypropylene (PP) as the Sacrificial Substance

[0089] A PP homopolymer, grade PPH 155 (produced by BRASKEM) was used as the sacrificial substance. The carbon nanotubes (Graphistrength C100 from ARKEMA) and the PPH 155 were introduced in a weight ratio of 25/75 by means of two gravimetric feeders into the hopper of a BUSS MDK 45 co-kneader equipped with a recovery extrusion screw and a granulator.

[0090] The temperature of the two heating zones of the co-kneader is 290 C. and 240 C. The profile of each kneader zone includes the restriction ring ensuring compression of the material undergoing mechanical shearing applied by the screw of the co-kneader. The recovery extruder was set at 250 C. The final composition was then formed into granules of cylindrical shape with a diameter of 3.5 mm and a length of 3-4 mm.

[0091] 500 g of granules were fed into a 3-litre vertical cylindrical kiln, heated gradually at 10 C./min to 400 C. under a nitrogen stream, held at 400 C. for 1 hour, and then cooled to room temperature. Granules of the same size as the starting formulation were discharged.

[0092] TGA measurement performed on this agglomerated solid material of loose CNTs demonstrates absence of weight loss between 150 and 250 C., indicating absence of organic substance that could be present after thermal decomposition.

[0093] The density of the agglomerated solid material obtained is found to be 0.24 g/cm.sup.3.

[0094] FIG. 1, based on electron microscopy (SEM), shows the morphology of this agglomerated solid material. According to this image, the proportion of CNT aggregates with an average size d50 under 5 m represents 3% by area.

[0095] For comparison, FIG. 2 illustrates the morphology of a crude CNT powder, characterized by the presence of more than 90% by area of CNT aggregates with an average size d50 under 5 m.

Example 2: Preparation of an Agglomerated Solid Material of Loose CNTs With Water

[0096] In this example, the sacrificial matrix used is demineralized water.

[0097] The equipment used is identical to that in example 1.

[0098] The CNTs (Graphistrengthx C100 from ARKEMA) were introduced into the hopper of the co-kneader by the gravimetric feeder, and water, preheated to 60 C., was injected by the piston pump into the 1st zone of the co-kneader. The proportion of CNTs was set at 25 wt % relative to the water. The temperature of the mixture was kept below 100 C.

[0099] The mixture was formed into granules with a diameter of 4 mm and a length of 4-5 mm. Then the granules were fed into a ventilated stove heated to 130 C. After drying for 3 h, the agglomerated solid material of CNTs in the form of granules has the same appearance as the material obtained in example 1.

[0100] The density is found to be 0.22 g/cm.sup.3.

Example 3: Production of Polymer-Based Formulations With an Agglomerated Solid Material of Loose CNTs According to the Invention

[0101] EPDM rubber, grade VISTALON 2504N, was used as the polymer base.

[0102] The reference formulation without carbon-containing additives is as follows:

TABLE-US-00001 Stearic acid 2 phr ZnO 5 phr ZDTP (Mixland + 50GA F500) 3.1 phr TBBS (Mixland + 75GA F500) 2.67 phr S80 (Mixland S80 GAF500) 1.5 phr

[0103] CNTs were added at 3 phr and at 7 phr in 4 different forms: [0104] Formulation 1: agglomerated solid material according to the invention from example 1 [0105] Formulation 2: agglomerated solid material according to the invention from example 2 [0106] Formulation 3: CNTs in the form of powder, commercial grade from ARKEMA Graphistrength C.100 [0107] Formulation 4: Commercial masterbatch from ARKEMA: Graphistrength C EPDM 20, containing 20 phr of CNT Graphistrength C100

[0108] Preparation of the Formulations

[0109] 1st Mixing Step

[0110] The mixer used has a mixing capacity of about 260 cm3 (FIG. 1). The chamber of the kneader has two rotors of the Banbury tangential type. The rotors are controlled by a motor equipped with a speed variator.

[0111] Mixing Protocol:

[0112] T tank=90 C.

TABLE-US-00002 TABLE 1 Time (min) n (rpm) Action 0 90 Introduction of the rubber 1 50 Introduction of the various forms of CNTs 2 min30 70 Introduction of stearic acid and ZnO 5 min30 90 Discharge

[0113] 2nd Step: Formulation With the Vulcanization Additives in the External Mixer

[0114] The open mill consists of two cylinders rotating in opposite senses of rotation at identical or different speeds. The ratio of the 2 speeds is called the coefficient of friction.

[0115] The external mixer is used here to achieve a dispersive state in the mixture and introduce the vulcanization system (sulphur and accelerators).

TABLE-US-00003 TABLE 2 T cylinders = 50 C. Gap (mm) AV (rpm) AR (rpm) Recovery 1.2 20 22.4 Introduction Extrusion 18 0.9 offcuts Outlet 1.9-2

[0116] The densities were measured on the crude materials after introduction of the vulcanization system, on a helium pycnometer. The mixtures with more CNT filler are logically denser than the mixtures with a lower level of filler.

TABLE-US-00004 TABLE 3 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 Density, g/cm.sup.3 0.91 0.92 0.88 0.91 0.86 0.89 0.89 0.91 at 20 C.

[0117] Formulations 1 and 2 prepared with the agglomerated solid material comprising loose CNTs have comparable density values.

[0118] Formulation 3 prepared with CNTs in the form of primary aggregates (Graphistrength C100) is characterized by a lower density due to the defects of the possible dispersion.

[0119] A Mooney MV One instrument (TA instruments) is then used for characterizing the viscosity. This test consists of measuring the torque required to turn a flat rotor at constant speed (2 rev.Math.min.sup.1) in a hermetic cylindrical chamber filled with rubber, with a volume equal to 25 cm.sup.3, and heated at constant temperature.

[0120] The resistance exerted by the rubber to this rotation corresponds to the Mooney consistency of the elastomer. It is expressed as an arbitrary unit proportional to the measured torque, called Mooney unit (MU).

[0121] It is established that 1 Mooney unit is equal to 0.083 N.m.

[0122] The introduction of a higher level of CNTs leads to an increase in the Mooney ML(1+4) 100 C. for each formulation. The more the viscosity increases, the better is the distribution of the CNTs in the volume.

TABLE-US-00005 TABLE 4 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 ML(1 + 4) 37.3 45.3 36.5 42.4 29.8 33.4 35.6 38.7 100 C.

[0123] Formulations 1 and 2 are superior to formulation 3 comprising crude CNTs introduced in the form of powder.

[0124] Formulations 1 and 2 are superior to formulation 4, which comprises CNTs already pre-dispersed in a masterbatch.

[0125] These results confirm that the loose CNTs present in the agglomerated solid material according to the invention display superior dispersibility relative to crude CNT powder, and also superior relative to crude CNTs already pre-dispersed in the same polymer matrix.

Example 4: Vulcanized Materials Containing the Agglomerated Solid Material According to the Invention

[0126] Forming of the elastomer-based formulations obtained in example 3 was done by thermocompression on a 30T platen press. The crude mixture is positioned in a frame with a thickness of 2 mm between two Teflon papers, in their turn sandwiched between two steel plates. The forming temperature is fixed at 165 C., and the vulcanizing time is determined by measurement of kinetics performed on the RPA measuring instrument.

[0127] Kinetic monitoring of the vulcanization of the mixtures was performed within a moving-chamber rheometer. An RPA Elite rheometer made by TA Instruments was used.

[0128] The sample, with a volume equal to 4 cm.sup.3, is placed in a thermally regulated chamber. The variation of the resisting torque opposed by the rubber at low-amplitude oscillation (0.2; 0.5; 1; 3 of arc) of a twin-cone rotor is measured. The frequency of oscillation is fixed at 1.67 Hz.

[0129] The measurements were performed at a temperature of 180 C. for 20 minutes with an angle of 0.5 of arc.

[0130] The values of t95 measured by the RPA are given in the following Table 5:

TABLE-US-00006 TABLE 5 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 t95 9 min 34 s 8 min 16 s 10 min 20 s 9 min 17 s 8 min 30 s 8 min 7 s 8 min 52 s 7 min 46 s

[0131] Plates were moulded at 180 C. at t95 on the 30T platen press. Mechanical testing was carried out according to standard ISO37 on the INSTRON Universal Tensile tester at room temperature. The standardized test specimens were cut out beforehand:

[0132] As shown by the results in Table 6, formulations 1, 2 and 4 are all superior in tensile properties relative to formulation 3 made with CNT powder.

[0133] The loose CNTs present in the agglomerated solid material prepared in example 2 in the hydrophilic medium have slightly lower performance than those obtained with the loose CNTs present in the agglomerated solid material prepared in example 1 in the hydrophobic medium.

TABLE-US-00007 TABLE 6 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 M10% (MPa) 0.32 0.44 0.30 0.40 0.26 0.32 0.34 0.42 M50% (MPa) 0.97 1.24 0.90 1.17 0.74 0.86 1.00 1.26 M100% (MPa) 1.38 1.80 1.26 1.67 1.01 1.20 1.43 1.89 M200% (MPa) 2.01 2.71 1.77 2.38 1.42 1.97 2.09 2.88 M300% (MPa) 2.90 3.88 2.41 3.23 2.11 2.75 2.89 4.06 R rupture 2.85 3.47 2.68 2.94 1.64 2.07 2.72 4.57 (MPa) Elongation 292 320 326 331 344 321 333 333 at rupture, %

[0134] The mechanical behaviour at 60 C. was evaluated for the 4 formulations.

[0135] The scanning tests in deformation (Table 7) were carried out at 10 Hz and 60 C. on samples crosslinked for 10 minutes at 180 C., vulcanization carried out in the RPA.

TABLE-US-00008 TABLE 7 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 G*max(MPa) 0.91 1.20 0.86 1.09 0.74 0.93 0.93 1.01 G*min(MPa) 0.65 0.78 0.56 0.65 0.51 0.66 0.63 0.67 G*(MPa) 026 0.42 0.30 0.44 0.23 0.27 0.29 0.34 Tan max 0.13 0.17 0.13 0.17 1.42 1.97 0.12 0.15

[0136] As expected, the PAYNE effect or non-linearity, represented by delta G*, is greater for the filled mixtures. This parameter is connected with the state of dispersion. According to this criterion, the loose CNTs according to example 2 give a very good result for dispersibility, better than the masterbatch of the prior art (formulation 4). The tensile test results for formulation 2, which are lower, can be explained more by the more favourable CNT/EPDM interfaces in the hydrophobic systems.

Example 5: Electrical Performance of the Formulations

[0137] Measurements of electrical resistance R are carried out on plates with a thickness of 2 mm, with a size of 100100 mm. In this case either surface conductivity or volume conductivity may be measured. The resistivity (.Math.Cm) or the electrical conductivity =1/ (S.Math.cm.sup.1) is calculated from the measurement of the resistance and according to the geometry of the test specimen and of the probe. Or a volume measurement is obtained using strips of crosslinked mixtures on which an electrode is painted with silver paint.

[0138] The results obtained for the 4 formulations are presented in Table 8 below.

TABLE-US-00009 TABLE 8 Formulation 1 Formulation 2 Formulation 3 Formulation 4 CNTs, phr 3 7 3 7 3 7 3 7 Volume conductivity, 5.5E12 1.1E4 3.5E12 2.1E3 8E13 4E7 2E13 3.1E3 S .Math. cm.sup.1
The agglomerated solid material of the invention makes it possible to approach the antistatic domain, even at the low level of 3 phr, by marking the start of percolation.
At 7 phr, it is formulation 2 that displays performance at the same level as formulation 4 of the prior art, prepared from a masterbatch comprising a pre-dispersion of the crude CNTs, which is to date the best technological approach, transferable to the industrial scale.

[0139] The agglomerated solid material of the invention makes it possible to obtain similar or superior results relative to this reference from the prior art, in terms of mechanical or electrical properties.

[0140] The agglomerated solid material of the invention is usable for a large choice of polymer matrices, and thus becomes a universal solution for efficiently introducing CNTs, in contrast to the masterbatch approach, which requires a similar nature of the matrix of the CNT concentrate and of the polymer matrix of the application.