EP2328736A1 - Method for producing composite materials having reduced resistance and comprising carbon nanotubes - Google Patents

Abstract

The invention relates to a method for producing a composite material having a reduced surface resistance and comprising carbon nanotubes.

Description

 Process for the preparation of carbon nanotube-containing composite materials with reduced resistance

The invention relates to a method for producing a composite material with reduced surface resistance comprising carbon nanotubes.

Carbon nanotubes are referred to below as "CNT" (carbon nanotubes) CNT are microscopic tubular structures (molecular nanotubes) made of carbon The diameter of the tubes is usually in the range of 1-200 nm. Depending on the detail of the structure is the electric Conductivity inside the tube is metallic or semiconducting: In addition to the electrical properties, the mechanical properties of carbon nanotubes are outstanding: CNTs have a density of 1.3 - 2 g / cm ³ and a tensile strength of 45 GPa the current carrying capacity and the thermal conductivity are interesting: the former is estimated to be 1000 times higher than copper wires, the latter at room temperature with 6000 W / (m * K) is almost twice as high as that of Diamant (3320 W / (m * K)).

CNT may be added to materials to enhance the electrical and / or mechanical and / or thermal properties of the materials. Such composites comprising CNT are known in the art.

WO-A 2003/079375 claims polymeric material which exhibits mechanically and electrically improved properties by the addition of CNT.

WO-A 2005/015574 discloses compositions containing organic polymer and CNT which form rope-like agglomerates and contain at least 0.1% impurities. The compositions are characterized by a reduced electrical resistance and a minimum of notched impact strength.

It is known that nanoparticles form agglomerates which must be comminuted in order to obtain the most homogeneous possible distribution of the nanoparticles in the composite material (A. Kwade, C.

Schilde, Dispersing Nanosized Particles, CHEManager Europe 4 (2007), page 7; WO-A

94/23433). By introducing shear forces into the dispersion, CNT agglomerates can be broken up (WO-A 94/23433).

Of glass fibers which are added to plastics to improve the mechanical and thermal properties, it is known that they are due to a stress, such as For example, occurs at entry of shear forces, a shortening experience (F. Johannaber, W. Michaeli, Manual injection molding, 2nd ed., Carl Hanser Verlag 2004, Chapter 5.8.6). Preferred are CNTs with a high ratio of length / diameter d (aspect ratio) because of their better electrical properties (Zhu et al., Growth and electrical characterization of high-aspect-ratio carbon nanotube arrays, Carbon, Volume 44, Issue 2, February 2006, pages 253-258). It is feared that a shortening can occur due to excessive stress on the CNT as with glass fibers. In the publication WO-A 05/23937, therefore, the energy input in the extruder is explicitly limited in order not to shorten the CNT (see eg page 6 lines 8-34 or page 11 lines 7-13).

As required for the dispersion of the CNT agglomerates, in addition to sufficient shear penetration, the medium is considered to penetrate into the interior of the CNT agglomerates (infiltration) (G. Kasaliwal, A. Göldel, P. Pötschke, Influence of Processing Conditions in Small scale melt mixing and compressing molding on the resistivity of polycarbonate-MWNT composites, Proceedings of the Polymer Processing Society, 24th Annual Meeting, PPS24, June 15-19, 2008 Salerno, Italy; WO-A 94/23433). For a reduction of the CNT agglomerate size, Kasaliwal et al., Citing the infiltration process considered necessary, is described in the aforementioned publication. expressly called a high viscosity as a hindrance. In the publication WO-A 94/23433 it is recommended to increase the temperature in the extruder at the beginning of the dispersion in order to improve the wetting behavior and the penetration of the medium into the interior of the CNT agglomerates. For the same reasons, preference is given to lower viscosity or processing viscosity masterbatch formulations with CNT (see, for example, WO-A 94/23433, page 13 lines 11 to 24).

Starting from the prior art, the object is to provide a method for producing composite materials comprising reduced carbon nanoparticles (CNT) which disperses CNT agglomerates in a fluid material and distributes them homogeneously in the material such that the CNT forms a three-dimensional network in the CNT Form material. In particular, the number of CNT agglomerates having a sphere equivalent diameter greater than 20 μm in the composite per square millimeter should be less than 20 multiplied by the percent CNT concentration (ie, less than 100 for 5% CNT). More preferably, the number of CNT agglomerates having a sphere-equivalent diameter greater than 20 μm in the composite per square millimeter should be less than 2 multiplied by the concentration in percent. Furthermore, the process should be readily modifiable (applicable) for throughputs on an industrial scale, ie scalable to large throughputs on a ton scale. Furthermore, the process should not cause any appreciable reduction in CNT.

Surprisingly, it has been found that the object can be achieved by subjecting the CNT agglomerates for dispersion in a fluid material to a minimum stress, which leads to comminution of the CNT agglomerates, without the CNTs being appreciably shortened, the minimum stress being dependent of the required size distribution of the CNT in the composite but independent of the selected fluid material.

The present invention therefore provides a process for producing a composite material with reduced electrical resistance comprising carbon nanotubes (CNT) with a predeterminable size distribution, characterized in that a mixture comprising at least CNT and a fluid material in a dispersing machine an empirically determined depending on the predetermined size distribution Under stress is subjected to stress preferably occurring in the dispersing machine highest shear stress is to be understood.

The term "carbon nanotube" is understood to mean substantially cylindrical compounds consisting mainly of carbon The substantially cylindrical compounds may be single-walled carbon nanotubes (SWNTs) or multi-walled (multiwalled carbon nanotubes, MWNTs) a diameter d between 1 and 200 nm and a length / which is a multiple of the diameter, Preferably the ratio Ud (aspect ratio) is at least 10, more preferably at least 30. The term "carbon nanotubes" is understood here to mean compounds which consist entirely or mainly of carbon. Accordingly, carbon nanotubes containing "foreign atoms" (e.g., H, O, N) are also to be understood as carbon nanotubes, and such carbon nanotubes of the present invention are herein abbreviated to CNT.

The CNTs to be used preferably have an average diameter of 3 to 100 nm, preferably 5 to 80 nm, particularly preferably 6 to 60 nm.

Common processes for the production of CNT include arc discharge, laser ablation, chemical vapor deposition (CVD) and vapor deposition (CCVD). Preference is given to using CNTs obtainable from catalytic processes, since they generally have a smaller proportion of, for example, graphite- or carbon black-like impurities. A particularly preferred method for the production of CNT is known from WO-A 2006/050903.

The CNTs usually accumulate in the form of agglomerates, the agglomerates having a sphere-equivalent diameter in the range of 0.05 to 2 mm.

The inventively incorporated in the composite CNT the electrical resistance of the material is reduced, ie, the conductivity is increased. By "reduced electrical resistance" is meant a surface resistance of less than 10 ⁷ ohms / sq (Ω / sq) (for measurement of surface resistance, see Figure XX).

By "fluid" material is meant a viscous material or a viscoelastic material or a viscoplastic material or a plastic material or yield point material. "Fluid" material, in particular, means suspensions, pastes, liquids and melts. Accordingly, for the production of CNT composite materials according to the invention, materials are used which are in a "fluid" state, can be converted into a "fluid" state or have a "fluid" precursor Materials which can be used are, for example, suspensions, pastes , Glass, ceramic masses, metals in the form of a melt, plastics, plastic melts, polymer solutions and rubber compounds, plastics and polymer solutions are preferably used, particularly preferably thermoplastic polymers, preferably at least one of the series polycarbonate, polyamide, polyester, in particular polybutylene terephthalate and polyethylene terephthalate, polyethers, thermoplastic polyurethane, polyacetal, fluoropolymer, in particular polyvinylidene fluoride, polyethersulfones, polyolefin, in particular polyethylene and polypropylene, polyimide, polyacrylate, in particular poly (methyl) methacrylate, polyphenylene oxide, polyphe nylon sulfide, polyether ketone, polyaryl ether ketone, styrene polymers, in particular polystyrene, styrene copolymers, in particular styrene-acrylonitrile copolymer, acrylonitrile-butadiene-styrene block copolymers and polyvinyl chloride. Also preferably used are so-called blends of the listed plastics, by which the expert understands a combination of two or more plastics.

Other preferred feedstocks are rubbers. As the rubber, at least one of styrene-butadiene rubber, natural rubber, butadiene rubber, isoprene rubber, ethylene-propylene-diene rubber, ethylene-propylene rubber, butadiene-acrylonitrile rubber, hydrogenated nitrile rubber, etc. is preferably used. Butyl rubber, halobutyl rubber, chloroprene rubber, ethylene-vinyl acetate rubber, polyurethane rubber, thermoplastic polyurethane, gutta-percha, arylate rubber, fluororubber, silicone rubber, sulfoed rubber, chlorosulfonyl polyethylene rubber used. A combination of two or more of the listed rubbers, or a combination of one or more rubber with one or more plastics is of course also possible.

For the production according to the invention of a composite material with reduced resistance, CNT is mixed in the form of agglomerates with at least one further material. The material is optionally heated before, during or after the addition of CNT to bring the material into a "fluid" state, and it is also conceivable to achieve the "fluid" state by introducing mechanical energy.

According to the invention, the CNT agglomerates are comminuted by a minimum stress of the mixture comprising at least CNT and a fluid material. The minimum stress is made by introducing energy into the mixture. For this purpose, a dispersing machine is used whose task is the dispersion of CNT in a material.

As a dispersing machine, e.g. The following machines are used: single-screw extruder, twin-screw or twin-screw extruders, in particular co-rotating twin-screw extruders such as the ZSK 26 from Coperion Werner & Pfleiderer, planetary roller extruder, internal mixer, ring extruder, kneader, calender, co-kneader or a combination of at least two the mentioned machines.

Dispersing machines introduce energy into the mixture comprising at least CNT and a fluid material which results in comminution of the CNT agglomerates and dispersion of the CNT in the fluid material. In a variety of dispersing machines, it is preferable to have shear stresses that lead to this desired effect. The skilled person is, however, clear that the stress of the mixture instead of shear stress (shear stress) can also be done as pressure or Dehnbeanspruchung or as any stress combination. Accordingly, shear stress is generally understood to mean a stress that has an analogous effect, such as a shear stress, i. which leads to a comminution of the CNT agglomerates and a distribution of CNT in the material (see also Equations 1 and 2). In a preferred embodiment, the minimum stress is expressed by the maximum shear stress occurring in the dispersing machine used.

The minimum stress is preferably determined empirically. In this case, microscopically or macroscopically measurable characteristic target values can be defined. For example, it is possible to define a minimum conductivity at a given CNT concentration. As the expert As is known, the conductivity of a CNT composite material increases as the CNT agglomerates shrink, thereby increasing the amount of deagglomerated CNT dispersed in the material. Accordingly, it makes sense to require a minimum conductivity, which occurs at a minimum stress. Empirically, it can be determined which minimum stress is required to achieve the required minimum conductivity. The conductivity or its reciprocal of the resistance (preferably the surface resistance) is understood as macroscopically measurable size.

Likewise, it is possible to directly monitor the comminution of the CNT agglomerates and to define a characteristic size distribution of the CNT agglomerates as the target size. The measurement of the size distribution of the CNT agglomerates may be e.g. with a microscope, which is why the characteristic size is understood as a microscopically measurable size.

One possible characteristic target would be e.g. a number of CNT agglomerates with a sphere equivalent diameter greater than 20 microns in the composite per square millimeter less than 20 multiplied by the CNT concentration in percent (for 5% CNT content thus less than 100). A particularly preferred target size is a number of CNT agglomerates having a sphere equivalent diameter greater than 20 μm in the composite per square millimeter less than 2 multiplied by the concentration in percent. It has been empirically found that such a size distribution of CNT in the composite material leads to a reduced electrical resistance. To determine the number of CNT agglomerates above or below certain sizes, CLSM (Confocal Laser Scanning Microscopy) wells are very well suited.

By Kasaliwal et al. (G. Kasaliwal, A. Göldel, P. Pötschke, Influence of Processing Conditions in Small Scale Melting and Compressive Molding on the Resistivity of Polycarbonate-MWNT Composites, Proceedings of the Polymer Processing Society, 24th Annual Meeting, PPS24, June 15- 19, 2008 Salerno, Italy) defines a dispersion grade DG ("macro dispersion index"). The dispersing quality DG is determined on the basis of microscope images of the CNT composite material. It is calculated from the ratio of the area A occupied by agglomerates with an area greater than a certain limit (Kasaliwal et al., As the limit value 1 μm ² ) to the total area A _{0 of} the evaluated micrograph of the CNT composite material according to the following formula : DG = \ \ -fά! J ±. | .100%. (Equation 12)

I ^y J

Here, f is a factor that is correlated with the actual volume of the filler, for CNT is described by Kasaliwal et al. f = 0.25 indicated. The value v indicates the volume fraction of the CNTs in percent. This can be easily calculated from the mass fraction of CNTs, according to Kasaliwal et al. For example, the density of CNTs is about 1.75 g / cm ³ . A value of the dispersing quality of 100% means that there are no agglomerates in the compound that exceed the selected limit. This indicates the state of very good dispersion. Kasaliwal et al. limit DG to positive values and set the value of dispersion quality to zero if the area fraction of large CNT agglomerates becomes so large that DG becomes negative according to the calculation formula. Small values of DG thus describe a poor degree of dispersion. The dispersing quality DG can also be used as a characteristic, microscopically measurable variable and a corresponding target size can be defined.

Surprisingly, it has been found that a minimum stress e.g. in the form of a minimum shear stress is necessary to achieve maximum conductivity at a given CNT content. Increasing the stress (shear stress) to a value above the minimum stress (minimum shear stress) does not lead to increased conductivity. Surprisingly, it has been found that the stress within the mixture comprising CNT and fluid material is the decisive factor for achieving maximum conductivity. Moreover, it was surprising that the found relationship between minimum stress and maximum conductivity is independent of the material used.

The CNT agglomerates are comminuted by energy input in the dispersing machine. According to the invention, the mixture of CNT and at least one further material is subjected to a minimum stress. As known to those skilled in the art, and textbooks of fluid and continuum mechanics may be taken, the stress state in a fluid may be described by a stress tensor that determines the shape

Has. This tensor is symmetric, ie τ = τ and the same applies to all other components apart from the main diagonal. The invention for comminuting the CNT Agglomerates to be used stress can by the representative voltage τ according to Eq. 2 expressing any stress state:

Where sp is the trace operator, that is, the sum of the elements of the tensor diagonal. The

Square of the tensor T is performed according to the well-known rules of matrix multiplication. A person skilled in the art is from G. Böhme, Fluid Mechanics of Non-Newtonian Fluids, Stuttgart Teubner, 1981, 1st ed., ISBN 3-519-02354-7 discloses that the stress tensor T of Newtonian fluids is linear from the strain rate tensor

D = - (grad v + (grad v) ^T J (equation 3)

depends:

T = 2ηD (equation 4)

For non-Newtonian media, the law of material associating the stress tensor with the deformations is more complex and may involve both a dependence of the viscosity on the strain rate tensor and a dependence on past deformations of the fluid (G. Böhme, Fluid Mechanics of Non-Newtonian Fluids , Stuttgart Teubner, 1981, 1st ed., ISBN 3-519-02354-7).

The theological properties of various materials and the various methods for measuring the viscosity of the skilled worker, for example, in Gleißle (M.Pahl, W. Gleißle, H. -M. Laun, Practical rheology of plastics and elastomers, 1st ed., VDI -Verlag 1991). The viscosity can be determined, for example, via capillary rheometer.

As known to those skilled in the art, the Cox-Merz rule, which combines viscosity measured in oscillatory rheometers with the shear viscosities measured in capillary or cone-plate rheometers, strictly speaking applies only to unfilled polymers. Nevertheless, oscillatory measured viscosities can serve as reference values for shear viscosities of the mixtures comprising at least CNT and a fluid material. The estimation of a maximum shear stress for some dispersing machines is easily possible for a person skilled in the art on the basis of mechanical variables. For a trained pipe flow in a pipe of length L with the radius R and the pressure loss Ap, the maximum wall shear stress is the same

Ap R

T = (equation 5) L l

For a flow-through gap of height dog of length L is the maximum shear stress

ApH

T = (equation 6) L l

For an aperture of length L in the area of the laminar inlet, the wall shear stress is

τ ⁼ Ε ^p «- ^(G17) where Re is the Reynolds number and / _{dyn is} the dynamic pressure. For dynamic printing:

with the density of the fluid p and the speed u. For the Reynolds number: Re = - ^U ± P- (equation 9)

For a wall of a gap moving with the wall velocity u (the other wall is stationary) of the height h, the maximum occurring shear stress is obtained from τ = -η (equation 10) h

For the viscosity η, in the preceding equations, the actual viscosity of the mixture occurring during dispersion is at least CNT and a fluid Use material at processing temperature and actual shear rate in the dispersing machine.

It is known to the person skilled in the art that the maximum occurring shear stress can not be subjected to all material elements. The stress which material elements see in a dispersing machine is subject to a distribution function. For a Newtonian fluid, 50% of all material particles in a shear gap experience at least half of the maximum stress. For a co-rotating twin-screw extruder (eg ZSK from Coperion Werner & Pfleiderer) Kirchhoff (K. Kohlgrüber, The co-rotating twin-screw extruder, Carl Hanser Verlag, 1st edition, Munich 2007, Chapter 9.3) shows for realistic parameters that even for a L / D ratio of 10 (L = length of the extruder in the axial direction, Z> = housing diameter) in the middle of each fluid element flows 3.5 times over the shear-intensive comb gap. For real extruders with an L / D ratio well above 10, statistically significantly more than 50% of the fluid particles, i. the majority of the material particles experience at least half of the maximum stress.

By repeating the load two or more times (for example, by repeatedly loading the CNT composite on the same machine), each pass increases the proportion of CNT composite material that has experienced more than a certain shear stress. Experimentally, this could be demonstrated for CNT agglomerates (see Example 2).

Preferably, the minimum stress in the dispersing machine is expressed by the maximum shear stress, as these are easily calculated as shown above and can be easily varied in a dispersing machine. It is clear to the person skilled in the art that comminution does not necessarily require the maximum shear stress occurring in a dispersing machine. The shear stress actually required for comminuting a CNT agglomerate will be somewhat smaller than the maximum shear stress occurring in the dispersing machine; but it is not so easy to determine / specify. For this reason, the minimum stress is preferably expressed by the maximum shear stress occurring in the dispersing machine.

In a preferred embodiment of the method according to the invention are CNT-containing

Composite materials having a number of CNT agglomerates per square millimeter surface with a sphere-equivalent diameter greater than 20 microns of less than 20 multiplied by the CNT concentration, ie for 5% CNT content, the CNT Agglomerate number, which has a sphere-equivalent diameter greater than 20 microns, that is less than 100. More preferably, the number of CNT agglomerates having a sphere-equivalent diameter greater than 20 μm in the composite material per square millimeter surface should be less than 2 multiplied by the concentration in percent.

The process according to the invention is characterized in that a mixture comprising at least CNT and a fluid material is subjected to a minimum stress of 75,000 Pa, stress being understood to mean preferably the maximum shear stress occurring in the dispersing machine. Preferably, the minimum stress is greater than 90,000 Pa, more preferably greater than 100,000 Pa. At the top, the load is limited, otherwise irreversible damage to the CNT polymer composite material is to be expected. The upper limit of 2,000,000 Pa seems reasonable.

For apparatus in which the maximum shear stress that occurs can not readily be calculated (for example, when dispersed in a nozzle in which the flow is turbulent), the approach of Equation 11 is used; It will be held in place of the maximum

Shear stress indicates the mean shear stress required to achieve the desired sizes.

In general, if a power P is dissipated in an apparatus with a volume V, then the mean shear stress is:

τ =, / - (Eq. 11)

V F

In a preferred method, the specific mechanical energy input in the dispersing machine to a value in the range of 0.1 kWh / kg to 1 kWh / kg, preferably from 0.2 kWh / kg to 0.6 kWh / kg and the minimum residence time on set a value in the range of 6 seconds to 90 seconds, preferably 8 seconds to 30 seconds.

The person skilled in the art is aware that, for example, when incorporating carbon blacks into materials, a high shear stress with a short residence time has the same effect as a low shear stress with a long residence time. For CNT agglomerates, the shear stress required for dispersion is markedly higher than that of conventional fillers (for example, carbon blacks), which is why dispersing CNTs in low-viscosity polymer melts does not work well. Therefore, without sufficiently high shear stress, CNT dispersion can not be economical. In a preferred embodiment of the method according to the invention, the minimum residence time of the mixture comprising at least CNT and a fluid material in the dispersing machine is Range from 6 s to 90 s, preferably 8 s to 30 s. Higher residence times are generally no longer economical. Accordingly, a high stress is required to ensure effective crushing of CNT agglomerates. In a preferred embodiment, the inventive method is characterized in that the minimum stress is achieved by a correspondingly high shear rate and / or a correspondingly high viscosity.

The minimum stress in terms of the highest shear stress occurring in the dispersing machine may be a product of shear rate (highest shear rate occurring in the dispersing machine) of the mixture (at least comprising CNT and a fluid material) and viscosity (actual viscosity of the mixture during dispersion at processing temperature and actual shear rate in the dispersing machine). In a preferred embodiment of the method according to the invention, in which the occurring highest shear rate is specified by the apparatus by the apparatus, the viscosity of the mixture is selected so that the product of viscosity and shear rate is greater than or equal to the minimum stress, the minimum stress preferably greater than or equal to 75,000 Pa, more preferably greater than 90,000 Pa, most preferably greater than 100,000 Pa. In a further preferred embodiment of the method according to the invention, in which the viscosity of the mixture is predetermined, the shear rate of the dispersing machine is selected such that the product of the highest shear rate occurring in the dispersing machine and the viscosity is greater than or equal to the minimum stress, the minimum stress being preferred greater than or equal to 75,000 Pa, more preferably greater than 90,000 Pa, most preferably greater than 100,000 Pa.

It is well known in the art to introduce high shear forces for the fragmentation of CNT agglomerates. However, a low viscosity is required in the prior art in order to ensure good wetting of the agglomerates and penetration of fluid into the agglomerates. Surprisingly, it has been found that a high viscosity is advantageous in the comminution of the agglomerates.

Furthermore, it was expected that with increasing energy input, the CNT will be better isolated, but the length of the CNT decreases steadily. Since the electrical conductivity according to the general theory with decreasing length / diameter ratio decreases - with constant CNT content and degree of dispersion - it should increase with increasing energy input initially due to the better separation of the CNT, but then due to the decreasing Ud ratio of CNT decline again. Surprisingly, it was found that even at high energy input on industrial, continuous dispersing machines the electrical conductivity does not drop again. This was found for typical residence times in dispersing machines (for example extruders) of 6 to 9O s. Kasaliwal et al. (G. Kasaliwal, A. Göldel, P. Pötschke, Influence of Processing Conditions in Small Scale Melting and Compressive Molding on the Resistivity of Polycarbonate-MWNT Composites, Proceedings of the Polymer Processing Society, 24th Annual Meeting, PPS24, June 15- 19, 2008 Salerno, Italy) report a partial decrease in conductivity at high speeds in the microcompounder, although the CNT agglomerates are better dispersed at high speeds and consequently the conductivity should be better. Here, a reduction of CNTs could occur because Kaliwal et al. have chosen a long residence time in the microcompounder of five minutes. The residence time in continuously operated industrial dispersing machines (for example twin-screw extruders) is considerably shorter. For example, the average residence time on a co-rotating ZSK 26 Mc twin-screw extruder from Coperion Werner & Pfleiderer with an L / D ratio of 36 at a throughput of 20 kg / hr is approximately 30 seconds. At a constant filling level, the extruder would have to be made longer by a factor of 5 (LJD = 180) in order to achieve at least half of the residence time of 5 minutes. Common industrial compounding extruders have an L / D ratio of 20 to 40.

A high viscosity of the dispersion can be achieved, for example, by selecting the material. For example, if the material is a polymer, a higher viscosity can be achieved by selecting a type with a higher content of longer chain molecules.

It is likewise conceivable to increase the viscosity of the dispersion by adding further materials, for example by adding fillers such as (nanoscale) fumed silica, carbon black, graphite, lime, talc, (glass) fibers, mica, kaolin, CaCC> ₃ , glass flakes, Dyes and pigments (such as titanium dioxide or iron oxide) or other substances increase. High viscosity can also be affected by the amount of fillers (CNT and / or others), with viscosity generally increasing with increasing filler content.

Since the viscosity generally decreases sharply with increasing temperature (for example the viscosity of polymer melts), in a preferred embodiment of the process according to the invention the viscosity is increased by a low processing temperature. It is clear to the person skilled in the art that the highest viscosities occur in the melting zone of the dispersing machine for thermoplastic polymers. A preferred embodiment of the method according to the invention consists in setting the temperature of the dispersing machine (for example twin-screw extruder) low, in particular in the region of the melting zone. In general, the Temperature of the thermoplastic polymers in dispersing machines at the beginning lowest, so that there the viscosities are higher due to the low temperature.

A preferred embodiment of the process according to the invention consists in dispersing the CNT agglomerates in one pass on a dispersing machine, since this is particularly economical. The smaller the desired size of the CNT agglomerates remaining in the compound, the higher the stresses are necessary. If the desired stress (shear stress) and, associated therewith, a desired size of CNT agglomerates in a first pass on a dispersing machine can not be achieved (for example for polymers with low viscosity), in a preferred embodiment the CNT compound material is used in the first pass on the Dispersing was obtained, again (two or more times) processed on the dispersing machine. According to the invention, the viscosity of the CNT compound material is increased with each pass through the higher proportion of dispersed CNTs, which in turn increases the stress (shear stress) in the next pass and thus improves the dispersing quality.

In a further preferred embodiment of the method according to the invention, in a first step, a higher concentration of CNT is incorporated into the material than is provided for the later composite material and in a second step added a further amount of material for "dilution" of the CNT concentration of the dispersion The second step can be carried out downstream on the same dispersing machine, but it can also be carried out on the same or another dispersing machine as an extra process step The addition of the higher concentration of CNT in the first step has the same effect as the addition of fillers: The viscosity of the dispersion increases, and if shearing forces are introduced into the dispersion to break down the CNT agglomerates, the shear stress is higher than if a smaller amount of CNT had been added to the dispersion Wind speed reached, or is the shear stress in the case of higher concentrated CNT dispersion higher. According to the CNT agglomerates are effectively crushed without causing a significant reduction of CNT. In a second step, then the amount of a same and / or further material is added, which is necessary in order to obtain the composite material with the desired CNT concentration. In addition, the material added in the second step may have a different viscosity. In a preferred embodiment of the method according to the invention, a material having the same or lower viscosity is added in the second step, since low viscosities are advantageous for the further processing of the CNT compound. In addition to the viscosity, the shear rate can be increased to the necessary

Minimum stress. One possibility for increasing the shear rate in a dispersing machine (for example single-screw extruder, identical or counterrotating twin or multi-screw extruder - in particular co-rotating twin-screw extruder such as, for example, the ZSK 26 Mc from Coperion Werner & Pfleiderer, planetary roller extruder, internal mixer, ring extruder, kneader,

Calender, co-kneader) consists e.g. through the use of higher speeds. The gap width in the

Machines can be made small as another way to increase the shear rate. Particularly narrow gaps in which very high shear rates occur, e.g. Calender on.

In a further preferred embodiment of the method according to the invention CNT are together with a thermoplastic polymer in solid phase the main entry of a single-screw extruder or an equal or opposite twin- or multi-screw extruder (as an example here is a co-rotating twin-screw extruder ZSK 26 Mc Coperion Werner & Pfleiderer called) or a planetary roller extruder or an internal mixer or a ring extruder or a kneader or a calender or a co-kneader. The CNTs are predispersed in the feed zone by solids friction to form a solids mixture. In a melting zone following the intake zone, the polymer is melted, and the CNT in this melting zone is further dispersed predominantly by hydrodynamic forces and distributed homogeneously in further zones in the polymer melt.

For low viscosity to medium viscosity media having a zero viscosity at room temperature in the range of 0.1 mPas to 500 Pas or materials with a yield value up to 500 Pa, the CNTs are e.g. processed according to the invention by means of one or a combination of several of the following apparatuses to a composite material: jet disperser, high pressure homogenizers, rotor-stator systems (sprocket dispersing machines, colloid mills, ...), stirrers, nozzle systems, ultrasound.

In the case of low-viscosity media (containing CNT), high stress can be achieved, for example, by ultrasound. The resulting cavitation generates pressure surges of over 1000 bar, which causes an effective crushing of CNT agglomerates. Low-viscosity media (containing CNT) can, for example, even under high pressure (eg 10 bar - 1000 bar) through narrow gaps (eg 0.05 - 2 mm) or correspondingly small holes or correspondingly small slots (fixed components or with moving components) be promoted, creating high Stresses occur. It is clear to the person skilled in the art that for such flows, even if they are turbulent, for example, a shear stress according to Eq. 7 or Eq. 10 can be calculated.

The process according to the invention offers the advantage that CNT composite materials with homogeneously distributed CNTs and reduced electrical resistance, high thermal conductivity and very good mechanical properties can be produced on an industrial scale in an economically efficient manner. The process of the invention can be operated both continuously and discontinuously; it is preferably operated continuously.

The invention also provides a CNT composite material obtained from the process according to the invention.

The invention furthermore relates to the use of the CNT composite material obtained by the process according to the invention as an electrically conductive, electrically shielding or electrostatically dissipative material.

In the following the invention will be explained in more detail with reference to exemplary embodiments and drawings without restricting them to them.

Show it

Fig. 1 is a process diagram of a system for carrying out the method

Fig. 2 is a schematic longitudinal sectional view of the system used in the Fig. 1

Twin-screw extruder Fig. 3 shows a measuring arrangement for determining the surface electrical resistance of the CNT composite materials

FIG. 4. Microscopic image of CNT from Example 1 (untreated, Experiment No. 1)

FIG. 5 micrograph of CNT from example 1 (acid-treated (HCl), experiment no.

2)

Fig. 6 Light micrographs of CNT agglomerates Fig. 7 Viscosities of the PE types used in Example 3 Fig. 8 Microscopic image of a mLLDPE-CNT compound from Example 3, experiment

No. 4 Fig. 9 Micrograph of an LLDPE-CNT compound from example 3, experiment no.

5

10 Microscopic image of a HDPE-CNT compound from Example 3, Experiment No. 6 FIG. 1 Micrograph of an LDPE-CNT compound from Example 3, Experiment No. 7

Examples

The plant gem. Fig. 1 consists in the core of a twin-screw extruder 1 with a

Filling funnel 2, a product discharge 3 and a degassing 4. The two co-rotating screw shafts (not shown) of the extruder 1 are driven by the motor 5. The constituents of the CNT composite (e.g., polymer 1, additives (e.g.

Antioxidants, UV stabilizers, mold release agents), CNT, optionally polymer 2) are required via metering screws 8-11 in the feed hopper 2 of the extruder 1. The from the

Nozzle plate 3 emerging melt strands are cooled in a water bath 6 and solidified and then crushed with a granulator 7.

The twin-screw extruder 1 (see FIG. 2) has, inter alia, a housing consisting of ten parts, in which two worm shafts (not shown) rotating in the same direction and meshing with one another are arranged. The components to be compounded including the CNT agglomerates are fed to the extruder 1 via the feed hopper 2 arranged on the housing part 12.

In the area of the housing parts 12 to 13 is a feed zone, which preferably consists of threaded elements with a pitch of 2 times the screw shaft diameter (short: 2 DM) to 0.9 DM. Through the threaded elements, the CNT agglomerates are promoted together with the other components of the CNT composite material to the melting zone 14, 15 and the CNT agglomerates by frictional forces between the solid-state polymer granules and also in the solid phase CNT powder intense mixed and predispersed.

In the area of the housing parts 14 to 15 is the melting zone, which preferably consists of Knetblöcken; Alternatively, depending on the polymer, it is also possible to use a combination of kneading blocks and tooth mixing elements. In the melting zone 14, 15, the polymeric constituents are melted and the predispersed CNT and additives are further dispersed and mixed intensively with the remaining composite components. The heating temperature of the extruder housing in the region of the melting zone 14, 15 is set to a value which is greater than the melting temperature of the polymer (in the case of semicrystalline thermoplastics) or the glass transition temperature (in the case of amorphous thermoplastics).

In the area of the housing parts 16 to 19, a post-dispersion zone is provided downstream of the melting zone 14, 15 between the conveying elements of the screw shafts. She shows kneading and mixing elements, which cause a frequent rearrangement of the melt streams and a broad residence time distribution. In this way, a particularly homogeneous distribution of CNT is achieved in the polymer melt. Very good results were achieved with tooth mixing elements. Furthermore, for mixing the CNT and screw mixing elements, eccentric discs, return elements, etc. can be used. Alternatively, several Nachdispergierzonen can be connected in series to intensify the fine dispersion. In any case, the combination of predispersion in the solid phase, the main dispersion during the melting of the polymer (s) and the downstream fine dispersion, which takes place in the liquid phase, is important for achieving the most uniform possible CNT distribution in the polymer.

The removal of volatile substances takes place in a degassing zone in the housing part 20 via a degassing opening 4, which is connected to a suction device (not shown). The degassing zone consists of threaded elements with a pitch of at least 1 DM.

The last housing part 21 contains a pressure build-up zone, at the end of which the compounded and degassed product exits the extruder. The pressure build-up zone 21 has threaded elements with a pitch between 0.5 DM and 1, 5 DM.

The (in the form of granules) obtained CNT composite materials can then be further processed by all known thermoplastic processing methods. In particular, moldings can be produced by injection molding.

The measurement of the surface electrical resistance was as shown in Fig. 3. On the produced by means of injection molding circular specimen 22 with a

Diameter of 80 mm and a thickness of 2 mm, two Leitsilberstreifen 23, 24 are applied, whose length B coincides with their distance L, so that a square area sq (Square) is defined. Subsequently, the electrodes of a resistance measuring device 25 are pressed onto the Leitsilberstreifen 23, 24 and read the resistance value at the meter 25. The measuring voltage used was 9 volts for resistors up to 3x10 ⁷ ohms / sq and from 3x10 ⁷ ohms / sq

100 volts.

example 1

The incorporation of multi-walled carbon nanotubes (CNT produced by catalytic vapor deposition according to WO 2006/050903 A2, for example as a commercial product

Baytubes® C 150P, manufacturer Bayer Material Science AG, available) in polycarbonate (PC) (Commercial product: Makrolon® 2805, manufacturer Bayer MaterialScience AG) on a co-rotating twin-screw extruder type ZSK 26 Mc (Coperion Werner & Pfleiderer). In experiment 1, both the polymer granules and the CNT were metered into the extruder via the main intake or hopper 2. In experiment 2, the CNTs were cleaned by an acid wash (HCl).

The process parameters are shown in Table 1 below. The worm stock used was 23.6% kneading elements.

The melt temperature was measured with a commercially available temperature sensor directly in the emerging at the nozzle plate 3 melt strand.

The specific mechanical energy input was calculated using the equation: Specific. mechanical energy input = 2 * Pi * speed * torque of the shaft / throughput (Pi = circle number)

The number and diameter of incompletely dispersed CNT agglomerates contained in the carbon nanotube polymer composite are measured by a light microscope on a 5 cm long strand of the CNT polymer composite.

Table 1

There is no significant difference in CNT agglomerate size between both runs 1 and 2. Since the elements already have considerable wear, the real gap is about 1 mm. The surface resistance decreases with increasing shear stress, which can be traced back to the increased fraction of individual CNTs.

Example 2

In 9600 g of water, 200 g of carboxymethylcellulose (Walocel CRT 30G) and 200 g of MWNT (CNT prepared by catalytic vapor deposition according to WO 2006/050903 A2, eg. available as commercial product Baytubes® C 150P, manufacturer Bayer Material Science AG) at room temperature. The mixture is dispersed once with a jet disperser at 60 bar. The general geometry of the jet disperser is described in EP 0101007 Bl. The jet disperser used had a hole with a diameter of 1 mm. For the experiments, a membrane pump from Wagner (type Finisch 106 B-EX, maximum pressure 250 bar) was used. After dispersion, a maximum particle size of about 80 μm was detectable under the light microscope.

The further dispersion was carried out at 100 bar with a piston pump from Böllhoff (type 060.020.-DP, maximum pressure: 420 bar). A jet disperser with a 0.6 mm diameter bore was used. The throughput was about 72 kg / h. After passing through the jet disperser, the suspension was collected and the dispersing step was repeated. Overall, the dispersion was carried out in 10 passes at 100 bar. Thereafter, under the light microscope, a maximum particle size of about 20 microns was recognizable (Fig. 6, no. 1). The further dispersion was carried out at 200 bar again with the same piston pump from Böllhoff (type 060.020.-DP, maximum pressure: 420 bar). The dispersion was carried out in 10 passes with a jet disperser with a bore with a diameter of 0.35 mm. The throughput was about 47 kg / h. Thereafter, a maximum particle size of about 10 μm was recognizable under the light microscope (FIG. 6, no. 2). Subsequently, the dispersion was further dispersed at 200 bar with a jet disperser having a bore with a diameter of 0.35 mm. This dispersion took place in the circulation. That is, the dispersion was not collected after passing through the jet disperser, but fed directly to the pump. This dispersion was continued until the dispersion had a temperature of about 45 ° C. The duration was about 5 passes. Subsequently, another 15 passes were carried out at 200 bar. Again, these were "real" runs in which the dispersion was collected and then fed to the pump.

2 liters of the dispersion thus treated were filled into a receiver and homogenized at 1000 bar. This dispersion was carried out with a high-pressure driven piston pump from Maximator (type GSF250-3LVES-494, maximum static pressure: 4500 bar, maximum dynamic pressure: 2500 bar) and a perforated diaphragm with a bore diameter of 0.2 mm. The throughput was about 21 kg / h. The dispersion was collected after each run in a refrigerated kettle. After 5 passes, a maximum particle size of about 4 μm was detectable under the light microscope (FIG. 6, no. 3). After a further 5 passes (a total of 10 passes), a maximum particle size of about 3 μm was detectable under the light microscope (FIG. 6, no. 4). After a further 5 passes (a total of 15 runs), a maximum particle size of about 2 μm was detectable under the light microscope (FIG. 6, no. 5).

After a further 5 runs (a total of 20 runs), a maximum particle size of about 1 μm was detectable under the light microscope (FIG. 6, no. 6).

According to Eq. 10, a representative (mean) shear stress for the turbulent wake zone of a jet disperser can be calculated. For this one still needs the volume of the turbulent wake zone, which can be estimated as follows: The wake zone can be described as a truncated cone with a diameter of D at the nozzle and a diameter of 3D at the end and a length of 9D. For a nozzle diameter of 0.4 mm, a throughput of 20 kg / h a pressure drop of 1000 bar (inlet and outlet pressure losses are neglected here) and a viscosity of le-3 Pas (the true viscosity is significantly increased by the CNT agglomerates ), the representative shear stress according to Eq. 10 l, 76e4 Pa. For the more realistic assumption of a real viscosity of 1 Pas, the representative (mean) shear stress is 5.57e5 Pa.

Example 3

The incorporation of multi-walled carbon nanotubes (CNT prepared by catalytic vapor deposition according to WO 2006/050903 A2, for example as a commercial product Baytubes® C 150P, manufacturer Bayer Material Science AG, available) in four different types of polyethylene (mLLDPE, LLDPE, HDPE, LDPE) ( Commercial products: LF18P FAX (mLLDPE), LXl 8 K FA-TE (LLDPE), HS GD 95555 (HDPE), LP 3020 F (LDPE), manufacturer: Basell) was carried out on a ZSK 26 Mc co-rotating twin-screw extruder (Coperion Werner & Co.). Pfleiderer). In all experiments, both the polymer granules and the CNT were metered into the extruder via the main intake or hopper 2.

The process parameters are shown in Table 2 below.

The worm stock used consisted of 28.3% kneading elements.

The melt temperature was measured with a commercially available temperature sensor directly in the emerging at the nozzle plate 3 melt strand.

The specific mechanical energy input was calculated using the equation: Specific. mechanical energy input = 2 * Pi * speed * torque of the shaft / throughput (Pi = circle number) The number and diameter of incompletely dispersed CNT agglomerates contained in the carbon nanotube polymer composite are measured by a light microscope on a 5 cm long strand of the CNT polymer composite.

Table 2

In the art, the different conductivity of different compounds of CNTs with PE types is attributed to the different degree of crystallinity (Effects of Crystallization on Dispersion of Carbon Nanofibers and Electrical Properties of Polymer Nanocomposites, SC Tjong, GD Liang, SP Bao, Polymer Engineering and Science 2008, ppl77-183, DOI 10.1002 / pen). This explanation is purely phenomenological. Surprisingly, it could be shown in the experiments carried out that there is a better explanation for the different conductivity of the various PE-type CNT compounds: Example 3 shows very different conductivities and different distributions of CNTs for the different PE types under the same compounding conditions. agglomerates. The higher the viscosity of the PE type under processing conditions (typical shear rates in an extruder are of the order of 1000 to several 1000 reciprocal seconds), the more the CNT composite is stressed and the better the CNT agglomerates are dispersed. With better dispersing quality, the conductivity also increases. Example 3 explicitly shows that a certain voltage is required for good conductivity to be achieved and for the CNT agglomerates to be below a certain size. The higher this shear stress, the smaller are the remaining CNT agglomerates. With improved CNT fragmentation, a smaller amount of CNT is needed to make the CNT-PE compounds conductive, the percolation threshold shifts to lower CNT levels. These experiments were carried out on a ZSKl 8. This machine size has a particularly high surface to volume ratio, which is why the melt is strongly cooled. The measured melt temperature at the extruder outlet says nothing about the actual melt temperatures in the machine for this machine size, so that therefore no calculation of the shear stress is dispensed with.

Since the stress on CNT dispersion is of the same order of magnitude for the first two examples, even though completely different material systems are present, the conclusion is that the highest occurring shear stress during processing is the decisive factor for the electrical conductivity of CNT composites and CNT -Dispersion is justified. This conclusion is also supported by the third example.

Claims

claims

1. A method for producing a composite material with reduced electrical resistance comprising carbon nanotubes (CNT) with a predetermined size distribution, characterized in that a mixture comprising at least CNT and a fluid material is subjected in a dispersing machine depending on the predetermined size distribution empirically determined minimum stress, wherein Stress is to be understood as meaning preferably the highest shear stress occurring in the dispersing machine.

2. The method according to claim 1, characterized in that the number of CNT Agglo- meraten with a sphere equivalent diameter greater than 20 microns in the composite per square millimeter is less than 20 multiplied by the CNT concentration in percent, particularly preferably the number on CNT agglomerates having a sphere equivalent diameter greater than 20 μm in the composite per square millimeter surface area, be less than 2 multiplied by the concentration in percent.

3. The method according to any one of claims 1 or 2, characterized in that the occurring in the dispersing machine highest shear stress is at least 75,000 Pa.

4. The method according to any one of claims 1 to 3, characterized in that the viscosity of the mixture at a occurring in the dispersing machine used highest shear rate Y is at least 75,000 Pa divided by Y.

5. The method according to any one of claims 1 to 4, characterized in that the shear rate of the dispersing machine used is at least 75,000 Pa divided by Z, where Z is the viscosity of the mixture at this shear rate.

6. The method according to any one of claims 1 to 5, characterized in that the minimum residence time of the mixture in the dispersing machine in the range of 6 s to 90 s, preferably 8 s to 30 s.

7. The method according to any one of claims 1 to 5, characterized in that the specific mechanical energy input in the dispersing machine has a value in the range of 0.1 kWh / kg to 1 kWh / kg, preferably from 0.2 kWh / kg to 0, 6 kWh / kg.

8. The method according to any one of claims 1 to 7, characterized in that the mixture is subjected to several times in the dispersing machine.

9. The method according to any one of claims 1 to 8, characterized in that the mixture is subjected in a first step of a first stress of at least 75,000 Pa, the claimed mixture is added in a second step with a material of equal or lower viscosity and another Subjected to stress, the further stress is less than the first stress.

A composite material made according to any one of claims 1 to 9.

11. Use of a composite material according to claim 10 as an electrically conductive, electrically shielding or electrostatically dissipative material.