Composite

D e s c r i p t i o n

Technical field

The invention relates to a composite, an electrostatic shielding device, an electromagnetic shielding device, a conductor, an electrostatic painting, a construction material and a method of forming a composite.

Background and related art

Research on single-walled carbon nanotubes (SWCNT) has rapidly grown since their scientific discovery, because of their extraordinary physical properties. SWCNT are cylindrical nanostructures with a high aspect ratio of usually above 10,000. They can conceptually be constructed from rolling up a large 2D graphene sheet. The extended pi-conjugated architecture provides the tubular carbon nano clusters with extraordinary

electronic properties, like ballistic conductance in metallic nanotubes and possible current densities of 10 ¹³ A/m ² .

Probably, the first major commercial application of carbon nanotubes will become the usage as electrically conducting fillers in polymer composites, aiming to replace today's state of the art carbon black (CB). The market of carbon black is vast. Reports reveal the world carbon black demand will reach 9.6 million metric tons in 2008. Of course, carbon nanotubes, especially SWCNT, are expensive nowadays, but mainly owing to the lack of mass production.

Carbon nanotubes are a promising filler material for polymeric matrices in order to realize a strong lightweight material for practical macroscopic applications. Carbon nanotubes as filler material in a polymeric matrix have outstanding advantages concerning the electrical matter. Already loadings in intrinsically insulating host systems initiate electric percolation and conduction much earlier as for example carbon black which is state of the art for many practical applications nowadays. The lower loadings and the high aspect ratios of nanotubes allow electronic conductivity to be achieved while avoiding or minimizing the degradation of other performance aspects, such as the mechanical properties. It is even possible to enhance simultaneously the electrical and mechanical properties due to the outstanding combined electrical and mechanical properties of nanotubes.

Unfortunately, the firmness of the carbon nanotubes can often not be transferred to the composite or the matrix system, respectively. One reason is many times the weak adherence of the filler to the host. This is amplified by impurities (like amorphous carbon or graphite particles) and by too short tubes. The first obstacle, the impurities can theoretically be removed by a purification of the nanotube material. Another approach to get rid of both obstructions simultaneously is the adaption and tuning of the nanotubes synthesis process. However, both approaches are not trivial because firstly purification also comes along with some defect introduction into the nanotubes themselves, and, secondly, the synthesis mechanisms are not yet completely understood, so that the mentioned kind of fine-tuning of the synthesis process is not possible at present.

Another potential approach to take advantage of the tube's firmness in the whole material system is to functionalize the nanotubes chemically. This functionalization should then fulfill more than one function. The first function is that added molecules to the shell of the tube can (covalently) bind to the matrix molecules. The second function

is that these molecules also support the untangling/unbundling of the nanotube agglomerates/bundles to achieve a good dispersion in the matrix system, especially while for example melt extrusion processes. This is important because larger agglomerates of nano fillers in the matrix are starting points for material failure.

Adding carbon nanotubes to the polymer matrix does not only improve the electrical and mechanical properties of such a material: in general, adding of carbon nanotubes to a polymer matrix generally is also able to increase the fire resistance of the polymer. However, the glass transition temperature of thermo-plastics is decreasing to lower values, which negatively influences the firmness and stability of the material at higher temperatures.

Another way to enhance the electrical and mechanical properties of a polymeric matrix is to incorporate carbon fibers into the polymeric matrix. However, compared to carbon nanotubes, the electrical conductivities of carbon fibers is rather moderate and the thermal conductivity is far below the thermal conductivity of carbon nanotubes. Nevertheless, mechanically, carbon fibers provide a substantial increase in stability and firmness of the total system. Due to the structure of carbon fibers, carbon fibers can also be functionalized considerably more easy compared to carbon nanotubes.

US 2004/0071949 A1 discloses a conformal coating comprising an insulating layer and a conducting layer containing electrically conductive material. The conducting layer comprises materials that provide electromagnetic interference shielding such as carbon black, carbon buckyballs, carbon nanotubes, chemically modified carbon nanotubes and combinations thereof.

US 2003/0213939 A1 discloses an electrically conductive composition comprising a polymeric foam and carbon nanotubes.

US 2002/0035170 A1 discloses an electromagnetic shielding composite comprising nanotubes.

WO 2007/044889 A2 discloses a textile-reinforced composite friction material that includes a nonwoven needle punched fiber mat, a resin matrix within and onto the fiber mat and a carbon nanomaterial dispersed within the resin matrix. Thereby, the carbon nanomaterial is preferably carbon nanotubes and/or carbon nanofibers.

Summary of the invention

The present invention provides a composite comprising a polymer matrix, nanotubes and fibers, wherein the fibers are microfibers or nanofibers. Combining these three materials in one composite material has the advantage, that a synergetic effect can be achieved which allows to combine the outstanding properties of all three kinds of materials. The synergetic effect also allows to use the fibers as a kind of secondary matrix to the nanotubes providing attachment sites for the nanotubes which reduces underling effects, enhanced fine distribution of the nanotubes on the fiber surface and therewith low percolation thresholds while enhancing the mechanical and electrical properties of the system.

Microfibers or nanofibers has to be understood as fibers with a mean diameter less than 400 μm.

Since due to the presence of the fibers, the fibers support the untangling or unbundling of the nanotubes agglomerates or bundles, an extra chemical functionalization of nanotubes to support such kind of untangling or unbundling is not necessary anymore, which reduces the production costs and allows to concentrate chemical modification of nanotubes with respect to a perfect incorporation of said nanotubes into the polymer matrix.

In accordance with an embodiment of the invention, the nanotubes are aligned relative to the fibers. Thereby, this alignment relative to the fibers is preferably a self-alignment of the nanotubes. However, such an alignment relative to the fibers can also be achieved by external magnetic or electric fields. This also allows to simultaneously align nanotubes and fibers with respect to the externally applied alignment forces.

Alignment has the further advantage that it is possible to achieve directional effects regarding the electrical, thermal or mechanical properties of the composite according to the invention. This allows to concentrate the enhancement of physical properties of the composite well defined in the direction, which later on in application will be used as the preferred direction where for example external forces are applied on. This allows to yield special (an-)isotropic properties of the total system. Therewith, a well-targeted adaptation of the composite with respect to later applications can be provided.

In accordance with an embodiment of the invention, at least a fraction of the nanotubes are attached to the surface of the fibers. Thereby, it is possible that the fibers are only

covered by the nanotubes or that even the fibers are coated by the nanotubes. Such a coating or covering of the fibers by the nanotubes is thereby a self-controlled process. This is a surprising scientific discovery of nanotubes self-attachment on nanofibers or microfibers. It is due to adhesive forces between the fibers and the nanotubes: the nanotubes self-arrange themselves on the fibers. Such a coating or covering of the fibers by the nanotubes has the same advantage as already mentioned regarding the self-alignment of the nanotubes relative to the fibers: an untangling or unbundling of nanotube agglomerates or bundles occurs which drastically reduces the nanotube percolation threshold. Therefore, the contact among the nanotubes is enhanced, which means that for example for applications relating to the electric conductivity of the nanotubes the conductivity of the total system is drastically enhanced while the amount of added nanotubes can be kept to a low value.

In accordance with an embodiment of the invention, the fibers and/or the nanotubes are interwoven. Such an interweaving of the nanotubes enhances the overall structural integrity of the system, since regarding mechanical properties, forces on the total system are uniformly distributed among all nanotubes staying in contact to each other. Regarding electrical or thermal conductivity properties, interwoven nanotubes lead to a homogenously distribution of respective thermal or electrical currents over all nanotubes, which enhances the thermal or electrical conductivity of the total system. Also, burning of individual nanotubes due to too high electrical currents can be reduced by interweaving the nanotubes, since interweaving leads to a parallel arrangement of the nanotube conductors.

In accordance with an embodiment of the invention, the fibers are bundled nanotubes. By using fibers adapted as bundled nanotubes, the outstanding mechanical and conductive properties of bundled nanotubes can be used in combination with the prominent mechanical and electrical properties of individual nanotubes which can be attached to the fiber surface as already mentioned above. This might for example be necessary, if composites with extremely high demands regarding the mechanical stability and electrical or thermal conductivity of the composite have to be designed.

Using bundled nanotubes as the fiber has further the advantage, that in case any kind of chemistry has to be performed on both the nanotubes covering the fibers, as well as on the fibers themselves, no extra kind of chemistry due to the different surface morphology of fibers and nanotubes has to be performed.

In accordance with an embodiment of the invention, the fibers are carbon fibers and/or biological fibers and/or mineral fibers and/or glass fibers and/or plastic fibers and/or metal fibers. Thereby, a fiber can for example be a single individual filament of a bundle of multiple filaments. A carbon fiber may for example comprise predominantly graphite- like structures (>93 %). In another embodiment of the invention, the nanotubes are carbon nanotubes and/or boron-nitride nanotubes and/or metal oxide nanotubes and/or carbon-boron nanotubes. Also, the carbon nanotubes can be adapted as single-walled carbon nanotubes and/or multi-walled carbon nanotubes. With such kind of richness in materials that can be combined to the composite according to the invention, it is possible to design individually composites which satisfy requirements regarding material properties for many kinds of industrial applications.

Therewith, a universal applicable material can be designed, which can fulfill special electrical, mechanical, thermal and chemical criteria. Thereby, electrical criteria may for example be the electrical conductivity, electromagnetic shielding and attenuation and sensor applications. Mechanical criteria may be the Young's modulus, tensile strength, firmness, mechanical attenuation and also sensor applications. Thermal criteria may be the thermal conductivity and thermal stability and chemical criteria may also include for example criteria regarding the chemical stability of such a composite system and sensor applications.

Fields of applications are aerospace, automotive engineering, railway engineering, housing engineering, general mechanical engineering, medical and microsystems engineering as well as building construction and civil engineering.

In accordance with an embodiment of the invention, the fibers are at least partially chemically functionalized. Also, the nanotubes are at least partially chemically functionalized and/or the nanotubes are at least partially doped. Thereby, the functionalization of the fibers can be adapted to provide docking sites to the nanotubes and/or the functionalization of the nanotubes can adapted for providing docking sites to the fibers. Such a functionalization may be chemically or physically. A functionalization allows an improved docking ability of the nanotubes and the fibers among each other, as well as an improved docking-ability of the nanotubes and the nanofibers to the polymer matrix via ionic, atomic or metallic bindings. Also, functionalization allows to improve wetting and adhesion properties of nanotubes and fibers due to physical effects and pre-processing (for example plasma treatment).

In accordance with an embodiment of the invention, the polymer matrix comprises synthetic carbon based polymers and/or copolymers and/or blockpolymers and/or biopolymers and/or synthetic polymers and/or polycarbonates and/or polyetherketone and its derivates and/or acrylnitril-butadien-styrol. Other polymer matrices comprise Polystyrene (PS), Polyaniline (PANI), Polypyrrole (PPY), epoxy, Poly(methyl methacrylate) (PMMA), and Polyurethane (PU). Moreover, Polycarbonate (PC) is an industrially important amorphous thermo-plastic which is widely used for housings of electronic devices, which also has to be especially considered for electromagnetic shielding purposes for usage as a polymer matrix in combination with nanotubes and fibers.

In accordance with an embodiment of the invention, the portion of nanotubes is in between 0.1 and 15 weight percent of the composite, typically 4 weight percent. In accordance with another embodiment of the invention, the portion of fibers is in between 0.2 and 50 weight percent of the composite, typically 5 weight percent.

In accordance with an embodiment of the invention, the composite is substantially optically transparent to visible light and/or infrared light and/or UV light. Having an optically transparent composite has the advantage, that the composite can be utilized for example for window panes, which is necessary in case an electromagnetic shielding of rooms exhibiting window panes has to be realized. This is for example necessary for usage in magnetic resonance imaging facilities, where typically the magnetic resonance magnet is located in a special electromagnetically shielded room, wherein said room is separated from a control room by such a transparent window pane. Another application of transparent composites may for example be security glass, which can be applied due to improved mechanical properties due to the incorporated nanotubes for high security protection of endangered objects like for example in banks, exhibitions etc.

In another aspect, the invention relates to an electrostatic shielding device comprising the composite according to the invention. Such kind of electrostatic shielding is necessary in order to avoid destruction of for example electronic components like integrated circuits, chips, etc.

In another aspect, the invention relates to an electromagnetic shielding device comprising the composite according to the invention. Electromagnetic shielding devices are highly demanded devices nowadays, since electromagnetic compatibility has to be ensured for many devices. Attenuation or shielding, respectively, of electromagnetic

waves to protect electronic devices from failure and malfunction, or to protect humans from harm, is an important task for today's engineering. An important example are mobile phones, where waves should not propagate towards the person's head. Other examples are cables and measuring devices, where the influences of electromagnetic waves can cause misinformation or even severe fault. This addresses in particular the automotive and aviation industries.

In another aspect, the invention relates to a conductor comprising the composite according to the invention, wherein the conductor is a thermal conductor or an electrical conductor. Such kind of for example electrical conductor could combine the properties of high mechanical stability and high conductivity. This allows to design new kinds of transmission lines, which are less vulnerable to mechanical strains like for example occurring during a heavy storm. At the same time, due to the combination of a suitable polymer matrix, the nanotubes and the fibers it is possible to design a high conductive, extremely lightweight material which overtops current mechanical and electrical properties of state of the art transmission lines.

In another aspect, the invention relates to an electrostatic painting comprising the composite according to the invention. Such kind of electrostatic painting could be for example applied to the interior of automobiles, to furniture, to window panes etc, which reduces the risk of experiencing an electric shock due to electrostatic charging of surfaces. Also, since an electrostatic charging of such kind of surfaces is suppressed, attraction of dust is reduced, which enhances the cleanliness of such surfaces.

In another aspect, the invention relates to a construction material comprising the composite according to the invention. Such kind of construction material can thereby possess extraordinary mechanical properties, as well as specially adapted processing properties, which allows to reduce processing costs. For example, concrete and metals can be substituted by such kind of new composites, which possess similar mechanical properties, but however are moldable as polymers to special shapes. Machine costs are reduced, and wear marks are significantly reduced.

In another aspect, the invention relates to a method of forming a composite, the method comprising providing nanotubes, providing fibers, wherein the fibers are microfibers or nanofibers and incorporating the nanotubes and the fibers in a polymer matrix.

In accordance with an embodiment of the invention, the method further comprises premixing of the nanotubes and the fibers. Premixing of the nanotubes and the fibers has the advantage, that in advance a self-arrangement of the nanotubes with respect to the fibers can be achieved. Also, a homogeneous mixing of the nanotubes and the fibers, which is necessary due to the small size of nanotubes and fibers, is guaranteed.

In accordance with an embodiment of the invention, the method further comprises at least partially chemically functionalizing the nanotubes and/or the fibers.

In accordance with an embodiment of the invention, the method further comprises coating the fibers with the nanotubes.

In accordance with an embodiment of the invention, the coating is performed during incorporation of the nanotubes and the fibers in the polymer matrix. Thereby, preferably, a self-coating occurs during the incorporation of the nanotubes and the fibers in the polymer matrix, which means that due to adhesive forces between the nanotubes and the fibers, which can be additionally enhanced by chemical functionalization, the nanotubes attach themselves to the surface of the fibers.

In accordance with an embodiment of the invention, the method further comprises aligning of the nanotubes relative to the fibers. Such kind of alignment could for example be performed by flow techniques, applying external magnetic forces or applying external electrical forces.

Brief description of the drawings

In the following preferred embodiments of the invention are described in greater detail by way of example only making reference to the drawings in which:

Figure 1 is a block diagram of an embodiment of a composite according to the invention,

Figure 2 is a dynamical analysis diagram of composites according to the invention,

Figure 3 is a diagram displaying an E-Modulus analysis of a composite according to the invention,

Figure 4 is a diagram displaying the shielding of electromagnetic waves in the farfield of various composites according to the invention,

Figure 5 is a flowchart of a method of forming a composite according to the invention.

Detailed description

Fig. 1 is a block diagram of an embodiment of a composite according to the invention. The composite comprises a polymer matrix 100, fibers 102 and nanotubes 104. Thereby, the nanotubes 104 are coating the fibers 102. The nanotubes 104 are attached to the fibers 102 and arranged in a network like structure. The attachment can be for example due to a self-arrangement of the nanotubes on the fibers. Such kind of network is necessary, in order to ensure that the nanotubes are contacted with each other in order to provide a high overall thermal or electrical conductivity. Also, such kind of network is necessary in order to provide a high mechanical stability in terms of for example a high Young's modulus. Also, at least some of the fibers 102 are in contact to other fibers. Such kind of contact may be for example due to chemical groups present on the fibers which lead to an attachment of ends of fibers to each other, or it might be some kind of entanglement of the fibers 102 to form a fiber network. Even a connection between the fibers due to chemical bondings is possible, which also further enhances the overall mechanical stability of the system.

Also a stable and resistive interconnection between the polymer matrix 100 and the fibers 102 can be achieved. This has the advantage to prevent an unmixing of the components incorporated in terms of the fibers 102 and the nanotubes 104 in the polymer matrix 100.

It has to be noted, that the coated arrangement of the nanotubes 104 on the surface of the fibers 102 is only one possible embodiment of the invention. In another embodiment, the nanotubes are arranged in a network-like structure detached from the arranged structure of fibers 102 in the polymer matrix 100. However, even if the nanotubes 104 are detached from the surface of the fibers 102 it has to be ensured, that the nanotubes 102 are still arranged in a network-like structure in the polymer matrix 100. Again, this is necessary in order to obtain for example a high thermal and/or electrical conductivity and high mechanical stability of the total system.

Fig. 2 is a diagram of a dynamical analysis of composites according to the invention. The abscissa of fig. 2 indicates the temperature in between 2O ⁰ C and 18O ⁰ C, wherein the ordinate in fig. 2 displays the dynamical E-Modulus at 1 Hz in units of MPa. Curve 200 in fig. 2 displays the result of the dynamical E-Modulus of pure polycarbonate (PC) obtained by dynamical mechanical analysis (DMA). Curve 202 is recorded for composites comprising PC, nanotubes and fibers in various ratios at a constant loading of 12.5 weight percent of nanotubes and fibers. Comparing curve 200 to the curve 202, a pronounced increase in the dynamical E-Modulus can be observed for the composites. This clearly shows, that by incorporating nanotubes together with fibers in a polymer matrix, the mechanical properties of such a polymer matrix can be significantly enhanced.

Fig. 3 shows a diagram of an E-Modulus analysis of a composite comprising a polymer matrix, nanotubes and fibers according to the invention. Thereby, the bottom abscissa depicts the loading of single-walled nanotube material while the top abscissa shows the loading of carbon fibers. The total loading of nanotubes and fibers is always kept to 12.5 weight percent. From fig. 3 it becomes clear, that the Young's modulus, which is a measure of the stiffness of the composite, is almost constant over a large range of nanotube-fiber ratios.

From fig. 3 it becomes also clear, that by varying the ratios of carbon nanotubes and carbon fibers the physical properties of the composite can be tuned: for example, if no nano material and no carbon fibers are present in the polymer matrix (in the present example PC), the composite mechanical properties in terms of the Young's modulus is rather unsatisfying. By incorporating carbon fibers into the PC matrix without additionally incorporating any single-walled carbon nanotubes, the Young's modulus of the obtained composite is slightly improved. However, it has to be expected that the electrical conductivity of the system is still rather bad. This is due to the bad conductivity of PC and the carbon fibers.

By incorporating additional nano material up to 5 weight percent and reducing therewith the ratio of carbon fibers down to 7.5 weight percent (and keeping up the constant ratio between carbon material and PC to 12.5 weight percent), the improved mechanical properties of the system can be maintained, while due to the high conductivity of the single-walled carbon nanotubes the electrical properties of the system can be drastically enhanced.

Fig. 4 shows a diagram displaying the shielding of electromagnetic waves in the far field of various composites according to the invention. Thereby, the abscissa shows various composites comprising polymers, single-walled nanotubes and carbon fibers at different mixing ratios. The ordinate displays the electromagnetic wave attenuation in dB. For the measurement, the range between 100 and 1100 MHz was scanned. As can be seen in fig. 4, the presence of the single-walled carbon nanotubes strongly effects the attenuation of electromagnetic waves.

The samples measured in fig. 4 are melt extruded composites. Since the carbon fibers used for the experiment in fig. 4 are extremely bad conductors, the increase in attenuation with increasing amount of carbon fibers while keeping the amount of carbon nanotubes almost constant cannot be attributed to for example an enhanced interconnection of carbon fibers, since high amounts of carbon fibers in PC do not provide any significant attenuation of electromagnetic waves. Therewith, from fig. 4 it becomes clear, that the combination of carbon fibers with the carbon nanotubes is the key for obtaining the high attenuation values displayed in fig. 4. The carbon nanotubes coat the carbon fibers, which enhances the contact among the better conducting single- walled nanotubes, which allows for an effective shielding of electromagnetic waves. This cannot be obtained in state of the art composites comprising only the polymer matrix and single-walled nanotubes or carbon fibers: the combination of fibers and nanotubes is absolutely necessary in order to obtain the strong attenuation of the electromagnetic waves.

Fig. 5 is a flowchart of a method of forming a composite according to the invention. In step 500 nanotubes are provided, which are functionalized in step 502. In parallel, in step 504 fibers are provided which are also functionalized in step 506. However, the functionalization steps 502 and 506 are optional. The nanotubes and fibers provided in the steps 500-506 are then premixed in step 508 in order to obtain an homogeneous mixture of nanotubes and fibers. In step 510, the premixed nanotubes and fibers are incorporated in a polymer matrix. Thereby, such kind of incorporation in the polymer matrix can be performed by any state of the art technique like melt extrusion, coagulation etc.

Not shown here in the steps 500-510 is the possibility to align the nanotubes relative to the fibers or to align the nanotubes independently to the fibers. This could for example

be performed as already mentioned above by external forces due to electric or magnetic fields or by flow techniques.

Also not shown in the steps 500-510 is a coating of the fibers with the nanotubes. Such kind of coating could for example occur before or after the premixing step 508, and it can be self-driven and enhanced by special kinds of functionalization steps 502 and 506 adapted to make the nanotubes and fibers self-adhering to each other.

Using the method of forming a composite depicted in fig. 5, it is possible to design a functional material with outstanding structural properties. The material can thereby comprise relatively lightweight and/or heavyweight polymers and the nanotubes and fiber filling material. The designed composite can thereby exhibit special functional properties like electrical conductivity, electromagnetic attenuation or shielding, sensor properties, thermal conductivity, mechanical attenuation etc. At the same time the outstanding structural improvements can be realized in terms of mechanical elasticity, tensile strength, hardness, thermal and chemical stability etc.

Such kind of material according to the invention is designable with any kind of polymer as matrix, like synthetic polymers, biopolymers even comprising cellulose, wood, paper, starch, chitin, as well as synthetic polymers for example based on silicon as matrix.

By tuning the combination of a special polymer matrix, nanotubes and fibers it is also possible to reduce or improve the permeability and the absorption properties for various fluids. The mechanical attenuation (mechanical dynamical loss modulus) can be improved, the adhesion properties of colors on the composite surface can be improved and the color pigment adhesion properties in the composite can be improved.

List of Reference Numerals