WO2009122354A1

WO2009122354A1 - A method to produce functional carbon fibres fabrics

Info

Publication number: WO2009122354A1
Application number: PCT/IB2009/051339
Authority: WO
Inventors: Giovanni Paladini; Roberto Cingolani; Dario Pisignano
Original assignee: Fondazione Istituto Italiano Di Tecnologia
Priority date: 2008-04-01
Filing date: 2009-03-31
Publication date: 2009-10-08
Also published as: ITTO20080252A1

Abstract

A process for the production of a non- woven fabric formed by a plurality of layers or tapes of carbon fibres or of composite carbon fibres; said layers or tapes and the fibres of said layers are bonded together by deposition between said layers or tapes of a polymeric, or¬ ganic, inorganic or hybrid-organic/inorganic material, applied by electro-spinning or elec¬ tro-spraying.

Description

A method to produce functional carbon fibres fabrics

Field of the invention

The present invention relates to the production of fabrics based on carbon fibres, obtained by superposition of two or more layers or webs of fibres which are bonded together by deposition in sequential or concurrent steps of an organic, polymeric or hybrid material between said layers.

The invention specifically relates to the process for producing said fabrics and has been developed by giving specific attention to the use of said process for the production of unidirectional fabrics based on carbon fibres.

Description of the related art

Carbon fibres are one-dimensionally elongated structures, with diameters in the range of a few micrometers (smaller diameters are also possible in the case of carbon nanofibres). In particular, a carbon fibre is a fibrous carbon material having a micro-graphite crystalline structure, and it is typically made by means of the fibrillation of acrylic resins, which are well known textile materials, or from oil/coal pitch and subsequent specific heat treatments.

Carbon fibres are generally classified as polyacrylonitrile based fibres (in the following PAN), pitch-based and rayon-based. Among them, PAN-based carbon fibres are currently the largest in terms of production volumes and the most often used. In the beginning of 1970's, the commercial production of PAN-based and isotropic pitch-based carbon fibres started on a large scale.

In the latter half of 1980's, anisotropic pitch-based carbon fibre manufacturers also broke into the market.

PAN-based carbon fibres are a class of fibres which are produced by the carbonization process of PAN precursors. They exhibit high tensile strength and high elastic modulus, and they are extensively applied for structural material composites and reinforcement in aerospace and industrial fields and for the realization of sporting/recreational goods and the like.

Another type of fibre is instead produced by the carbonization of oil/coal pitch precursor. This class of fibres exhibit extensive properties from low elastic modulus to ultra high elastic modulus. In particular, fibres with ultra-high elastic modulus are extensively adopted in high stiffness components and various uses that need high thermal conductivity and/or electric conductivity.

In general, carbon fibres exhibit a traction resistance and a stiffness that are twice or three times larger than those of steel, and a specific weight which is about 25% of that of steel (Tensile elastic modulus: 600-200 GPa; Tensile strength: 2,500-3,500 MPa). The typical density of carbon fibres is 1750 kg/m³.

Carbon fibres are generally supplied on the market in the form of composite fibres made by a carbon core embedded within a thermoplastic or thermocurable organic material. Such an architecture is functional for providing the fibres with more processability and handling possibility.

Such a composite material is employed in all the industrial and technological fields for which the ratio stiffness/weight is important or crucial, where high resistance to traction and high elastic modulus are required, and where building materials have to absorb large amounts of mechanical energy without structural partitioning or breaking.

Traditional materials, such as ceramics and metals, do not typically fulfil these requirements.

These applications have been first related to aeronautics and defence industries, utilising this material within structural components. More recently, many other industrial fields have been utilizing carbon fibres, including naval and automotive sectors, the sport articles field, the industry of building construction and industrial plants.

Carbon fibres are utilised as conventional textile fibres for producing cloths and fabrics with different weave geometries and thicknesses. A typical carbon fabric is composed by weft and warp. Components based on carbon fibres are generally realized by the impregnation of the fabrics by means of a polymeric matrix, that is then cured by thermal/high- pressure treatments. Pressure is used to make the component more compact and to eliminate defects due to air or excess polymer.

The features of high tensile strength and stiffness of carbon fibres rely on the intrinsically anisotropic nature of carbon fibres, in which the mechanical performances are peculiarly enhanced along the direction of the fibre length. Hence, fibres have to be preferentially oriented along the direction corresponding to the maximum structural and mechanical strain.

Misaligned fibres do not participate in the functionality of structural strain/support, but instead can determine defects, beside an increase of weight, in the components.

In particular, for many application fields (i.e. bicycle frames, masts etc.), one often needs a material ideally characterised by excellent, very resistant mechanical properties just along one direction, thus allowing one to not waste high-performance materials along directions which intense, possibly disruptive forces do not pass through.

For many applications, a fabric based on carbon fibres, without presenting or minimising the material used for weft, would be very desirable, since this would result in a remarkable improvement in terms of reduction of overall weight of the fabric and of the final components, and of costs, without degrading any structural performance.

Moreover, a presence of weft fibres in fabric structure can create local misalignments in the warp fibres due to the thickness of weft fibres.

To date, unidirectional structures based on carbon fibres have been realized by cables and wires, to be employed as structural reinforcement for biomedical devices and prostheses and as lifting components for high-tension electric wires.

On the contrary, conformable fabrics realized by uniformly aligned carbon fibres are hardly produced, since the lack of weft could cause the exfoliation of the fabric.

Summary of the invention

The object of the present invention is to provide a process for the production of a non- woven fabric, based on carbon fibres or composite carbon fibres, wherein the bonding functionality between the weft fibres and the overall structural support of the fabrics itself which, according to the convention art is provided by the warp, is instead provided by the deposition of an organic material, polymeric, or hybrid, in the form of fibres, films or discrete sprays or spots, having a size generally below a few micrometers, which has a binding function between layers or webs of said fibres.

In view of the above-mentioned object, the subject of the invention is a process having the features defined by the following claims.

Particularly, for the deposition of said binding material, the present invention makes use of the electro-spinning process (spinning in an electrical field) or electro-spraying (spraying in an electrical field).

The electro-spinning technique has been implemented and developed for the first time at the beginning of 1900, and has been the subject of an increasing interest during the 1990's. Today, electro-spinning is the main technique for the production of nanofibres. In comparison with other industrial processes, such as melt-blowing, electro-spinning is a low cost technique which is simple and economical.

Organic micro- and nanofibres realised by electro-spinning found various applications in tissue engineering, especially for producing scaffolds for cell cultures, mechanics and electronics, and molecular filtering devices.

With respect to bulk polymers, micro- and nanofibres exhibit better mechanical features along their length, and a very high surface/volume ratio (with exposed areas up to 10³ m²/g). A typical electro-spinning setup is a simple apparatus and it is build up by a reservoir containing the solution to be processed, connected to a needle extruding the polymeric jet. The needle and a collector are biased by a voltage (generally larger than 5 kV), determining the formation of a jet by the electrified solution. More than a needle may be connected to a single high tension power source; as a consequence, electro-spinning may be engineered to constitute a particularly economical technique (a commercial high tension power source normally requires a few hundreds Watts), particularly in comparison with techniques, such as melt-blowing, which require high temperatures and therefore high electric power consumptions.

The collector is typically grounded, and the extruding needle is instead positively biased. For achieving an optimal control of the electro-spun material, and for keeping constant the rate of jetting solution over time, one often needs to maintain a constant overpressure in the reservoir (typically by means of syringe pumps).

The formation of polymeric micro-nanofibres by Electro-spinning relies on the stretching undergone by the viscoelastic solution when the external electric field is applied. The repulsive force due to the electrical charging of the external surface of the extruding drop of polymer solution becomes larger than the liquid surface tension, hence one or more jet polymeric filaments start forming.

Upon stretching during its flying towards the collector, which is concomitant to the solvent evaporation, the continuous jet produces thin unidimensional structures with diameters in the range between tens of micrometers and tens of nanometres. The repulsive force due to the electrical charge on the external surface of the extruding drop of the polymeric solution is responsible for the simultaneous formation of a plurality of fibres directly from a single needle.

The fibre producing process becomes even faster with the use of a device having a plurality of deposition needles.

The main process parameters to be controlled in a standard Electro-spinning procedure are the solution viscosity and conductivity, the bias applied between the needle and the collector, the needle-collector distance, the feeding rate for the polymer solution and some ambient parameters such as humidity and temperature.

The resulting fibre diameter decreases upon increasing the applied bias, the needle- collector distance and the solution conductivity, whereas it increases upon increasing the solution viscosity. The process atmosphere humidity affects the resulting electric field and hence the morphology of the resulting fibres. Temperature affects the solvent evaporation rate.

Specific changes of the standard setup include the possibility of collecting treated polymer in different ways in order to obtain various fibrous assemblies and of not employing a spinneret.

In the process according to the invention, polymeric fibres, drops ("spots"), layers or films obtained by electro-spinning or by electro-spraying are deposited on carbon fibres during spinning/spraying or thereafter. In particular, carbon fibres are electrically conductive, therefore they can be directly utilised as collectors for electro-spun polymeric/organic fibres or electro-sprayed polymeric/organic spots.

Hence, one can directly deposit, on a weft-free fabric of carbon fibres, thermoplastic organic elements, polymeric fibres or nets, etc., by electro-spinning or electro-spray. The typical size of the organic features produced in this way has to be comparable with those of the carbon fibres or lower.

The high flexibility of the electro-spinning technique allows using the same deposition apparatus for the production of polymeric fibres with different diameters, depending on the size of the unidirectional fibres of the fabric.

After deposition, the binding material by electro-spinning or electro-spraying, two or more carbon fibres or carbon fibres layers or tapes are placed in contact, and this sandwiched structure is then heated up to a temperature higher than the glass transition temperature of the plastic deposited material. Before, concomitantly, or after heating, the system can be pressed to favour a better mutual adhesion between different carbon fibres surfaces. Upon cooling, the very fine and thin morphology of the deposited binding material allows one to achieve a standing and conformable fabric (preferably unidirectional); the plastics component is preferably below 10% by weight because of the very tiny size of the organic/polymeric elements produced by electro-spinning or electro-spraying.

In a preferred embodiment of the present invention, a polymeric network of organic fibers is realized at the interface between two layers or tapes of carbon fibers, mutually aligned along the same direction.

In particular, in a preferred embodiment of the present invention a unidirectional fabric is produced by the following steps (see Figure 2): preparation of a solution by a thermoplastic polymer, preferably selected from polymethylmethacrylates, polyethylenes, polystyrenes, polyamide (nylon®), cycloolefin polymers (Topas®, Zeonex® etc.), or by a composite solution or suspension including a polymer, such as cited above, together with particles, micro-nanopowders, micro-nanocrystals, and/or functional molecules etc.; organic solvents such as chloroform, dichloromethane, toluene, formic acid, with polymer concentrations in the range of 1-1000 mg of polymers per ml of solvent are typically employed; deposition of a network of thermoplastic fibres by electro-spinning or electro- spraying on a carbon fibres layer or tape; a syringe pump with suitable pumping rate (1-10⁷ microliters per minute, depending on the solution and polymer characteristics and on the desired final throughput) can be used to generate controlled overpressure in the solution reservoir; the voltage applied between the needle/deposition head and the collecting surface is provided by a high-tension supplier, optionally tuneable, able to supply bias of the order of tens of kVolts; one electrode from the power supplier is dipped into the solution or connected with the extruding needle/head; the latter is made by a metal, with one or more jetting holes with diameters in the range 1 millimeter- 10 micrometers; the amount of deposited material depends on the spinning/spraying time, and on the volumetric rate of solution exiting the needle/deposition head; it is possible to optimise the process parameters in order to avoid the formation of polymeric/organic residues at the needle during or after spinning; mutual adhesion of two layers or tapes of carbon fibres (one of which, or all of which previously placed in contact or covered with the polymeric/organic/composite features) with simple contact or assisted by an applied pressure (for instance in the range 10^~3-l kN/cm ), at room temperature or upon heating for melting the bonding polymer; the applied pressure and the pressure/heating time intervals depend on the areas and on the thickness of the employed materials; heating the system determines the softness of the thermoplastic polymer, strongly adhering to the carbon fibres, embedding some of them, and acting as bonding point between different layers o tapes of carbon fibres; heating also helps to avoid the incorporation of bubbles or of other defects in the final resulting composite; typical glass transition temperatures of thermoplastic materials are below 300°C; in case of heating process, cooling the system up to room temperature.

In a particular embodiment of the present invention, a single needle/deposition head is employed for electro-spinning or electro-spraying, but also two or more needles/deposition heads can work in parallel to deposit bonding fibres or spots on larger areas.

In a particularly preferred embodiment of the present invention, two layers/tapes of carbon fibres are aligned along the same direction before bonding, and bonding results in the production of unidirectional carbon fibres based fabric.

In a particularly preferred embodiment of the present invention, the bonding process is implemented by means of different classes of polymers or organic molecules, such as plastic materials, epoxy resins, etc., to be employed instead of thermoplastic polymers as bonding elements.

Brief description of the annexed drawings

The invention will now be described, by way of example only, with reference to the annexed drawings, wherein: fig. 1 is a flowchart of an exemplary embodiment of the method described herein; fig. 2a is a scheme of the Electro-spinning/Electro-spraying deposition apparatus; fig. 2b is a schematic representation of a heating and pressure set-up for bonding and for fabric production; fig. 3a is a photograph of carbon fibres imaged by electronic microscope; fig. 3b is a photograph by electronic microscope of a sample of a deposit of micro- fibres and sprayed elements on carbon fibres; and figs. 4a and 4b are photographs of unidirectional carbon fibre fabrics bonded by polymeric interfacial micro-fibre webs.

Detailed description of a preferred embodiment of the invention

By referring to the exemplary schemes reported in Figures 1 and 2, in a first step 102 a needle 10 is positioned at a distance of a few (1-100) centimetres far from a collector 12 such as e.g. a collector made by a layer or by a tape of carbon fibres.

Preferably, a pumping module 14, is connected to a reservoir 16, that contains a solution of polymers, organic molecules and nano-microcomposites to be electro-spun or electro- sprayed. The pumping module 14 is also connected to the extruding needle/deposition head 10 by means of a rigid or flexible tube 18; typical pumping and injection rates are of the order of a few tens of microlitres of solution for minute.

A high- voltage supplier 20 is connected to the extruding needle/deposition head 10 by an electrical connection 22, and is set in order to provide a bias, typically in the order of a few or of tens of kVolts, between the extruding needle/deposition head 10 and the collector 12.

In a step 104, aligned carbon fibres, are preferably employed as collector 12, and they are connected to the ground potential by an electric connection 24.

In a step 106 a solution of polymers, organic molecules and nano-microcomposites is prepared and placed in the reservoir 16.

Preferably, a suitable valve 26 can be used to re-establish the pressure inside the reservoir, which is normally closed by a seal to prevent the evaporation of solvent and the contact of the solution with external atmosphere.

In a step 108 the high voltage power supplier 20 and the pumping system 14 are switched on; electro-sprayed polymer/organic/composite spots or electrospun polymeric/organic/composite fibres or elements 28 start to cover the aligned carbon fibres used as collector 12. The time during which the electro-spinning/electro-spraying process is carried out determines the quantity of polymeric/organic/composite features, spots or fibres 28 which is deposited on the aligned carbon fibres 12, on the aligned carbon fibres 30, or on both the aligned carbon fibres 12 and 30.

The deposition process may be carried out in a controlled atmosphere with horizontal or vertical deposition.

In a step 110 the high voltage power supplier 20 and the pumping system 14 are switched off, the process of electro-spinning/electro-spraying is interrupted. The processed carbon fibres are removed from the collector support.

In a step 112 a second layer or tape of carbon fibres 30 is aligned, for example in the same direction, of previously processed carbon fibres. The two layers/tapes of carbon fibres, the treated one 12 and the untreated one 30 (both the layers/tapes, 30 and 12, can be processed by electro-spinning/electro-spraying), are placed in mutual contact. At this stage, the bonding features 28 constitute an interfacial set of elements or an interfacial layer between the two layers/tapes of carbon fibres, 12 and 30. Optionally, if required by the polymeric web 28, in a step 114 the sandwich structure 32 is heated. Preferably, a thermocouple or other temperature sensor 34 positioned in proximity or in contact with the pressuring system allows one to measure the temperature of the sandwich structure 32.

Preferably, an electronic system, such as a proportional-integral-derivative controller, can be used to control the temperature and the heating/rate of the heaters, 36 and 38. Optionally only one of heaters 36 and 38, are switched on, or more than two heating elements are employed. The heaters can rely on contact, such as hot plates, optical excitation, or electromagnetic radiation, such as by infrared lamps, microwaves or laser, generation of thermal energy by batteries or chemical reaction. The temperature to be reached has to be considerably higher than the glass transition temperature of the polymeric/organic/composite features, spots or fibres 28, thus softening the features 28, and allowing them to act as bonding points between the two layers of carbon fibres, 12 and 30.

Optionally, in a step 116, pressing plates 40, tapes or rollers press the heated tape, in order to make possible the penetration between the melted polymer features 28 and the carbon fibres of the layers/tapes 12 and 30. The pressure value (for instance in the range 10^"3-l kN/cm ), pressing time intervals (which can vary from fractions of seconds, minutes, some hours) depend on the used materials. The pressing plates 40 can also incorporate the heating elements 36 and 38, or the heating elements 36 and 38 can be used after compression with the said pressing plates 40.

In a step 118 the obtained fabric is removed from the press and reeled in a spool.

Fig. 3 a shows an image of a typical carbon fibre tape. Carbon fibres are aligned along the same direction and exhibit typical diameters in the range of a few micrometers. The core fibre is made up of a carbon yarn, and this structure is covered by a polymeric shell (thermoplastic or epoxy resin for example).

Fig. 3b shows a carbon fibre tape processed by electro-spinning. A superimposed polymeric web is made by a polymethylmethacrylates solution (150 mg polymethylmethacrylates in 1 ml of chloroform). Polymer micro-fibres are randomly distributed on the collector. A lot of polymeric spots or beads are also deposited on carbon fibres as consequence of simultaneous electro-spraying process.

Figs. 4a and 4b show two photographs of a completely processed and assembled fabric. The second layer of carbon fibres is bonded with the first one, resulting in a flexible, fully conformable unidirectional carbon fibres bi-tape, as showed.

The method described herein offers a straightforward and experimentally convenient procedure to realize unidirectional or complex fabrics given by the superposition of two or more layers of carbon fibres. These results substantially improve fabrics based on carbon fibres in terms of performances for industrial application. Particularly, the application of the bonding material by means of electro-spinning provides relevant advantages with respect to other deposition techniques, particularly: energy saving, since it is not necessary to dissolve the polymer; an electro-spinning apparatus is relatively simple and industrial apparatuses are available, whereby the costs for implementation and development are rapidly decreasing; the maintenance of the extrusion heads is simple and fast, their cleaning may be carried out by sieving pure solvent; it is possible with the same feeder to work with several extrusion heads; there is no need to use devices wherein high pressures are present.

Moreover, by means of the electro-spinning or electro-spraying technique it is possible to use a material having specific functionality of a chemical, structural, mechanical-optical, electrical nature or biological properties which are provided to the thus treated carbon fibres.

Naturally, the basic principle of the invention remaining the same, the details and embodiments may widely vary with respect to what has been described by way of non-limiting example, without departing from the scope of the appended claims.

Claims

1. A process for the production of a non-woven fabric, formed by a plurality of layers or tapes of carbon fibres or composite carbon fibres, characterised in that said layers or tapes and the fibres of said layers are bonded together by deposition between said layers or tapes of a polymeric, organic, inorganic or hybrid organic/inorganic material applied by electro- spinning or electro-spraying.

2. A process according to claim 1, characterised in that said layers or tapes of carbon fibres or composite carbon fibres are formed by aligned fibres.

3. A process according to claims 1 or 2, characterised in that said polymeric material is deposited in the form of microfibers, nanofibres, spots or in the form of continuous films on said layers or tapes or in discrete areas of said layers or tapes.

4. A process according to any of the preceding claims, characterised in that it comprises the step of heating said carbon fibres or composite fibres after the deposition of said polymeric material to bond or to facilitate bonding of said fibres.

5. A process according to any of claims 1 to 4, characterised in that it comprises the step of subjecting to pressure said carbon fibres or composite carbon fibres after the deposition of said polymeric material.

6. A process according to any of claims 1 to 5, characterised in that before or after the deposition of said polymeric material, said carbon fibres or composite fibres are subjected to an optical, thermal, chemical, mechanical or electrical treatment suitable to cause or to facilitate the bonding or to confer to the material specific electrical, optical, chemical, structural and/or mechanical properties.

7. A process according to any of the preceding claims, characterised in that said polymeric material used for the electro-spinning or electro-spraying operation is previously heated above his softening temperature.

8. A process according to claim 4, characterised in that the heating of said carbon fibres or composite carbon fibres is carried out by means of an electromagnetic radiation, by means of contact with heated plates or by chemical or electrochemical means.

9. A process according to claim 5, characterised in that pressure is applied by means of pressing plates or pressing rollers.

10. A process according to any of the preceding claims, characterised in that said polymeric material is selected from the group consisting of polymethylmethacrylate, polyethylene, polystyrene, polyamides or cycloolefin polymer.

11. A process according to any of claims 1 to 8, characterised in that said polymeric material comprises a thermosetting polymeric material.

12. A process according to any of claims 1 to 8, characterised in that said polymeric material further comprises ceramic materials, microcrystals or nanocrystals.

13. A process according to any of claims 1 to 8, characterised in that said polymeric material comprises carbon nanofibres or carbon nanotubes.

14. A process according to any of the preceding claims, characterised in that said polymeric material is deposited in an amount not higher than 10% by weight referred to the weight of the fabric.