WO2013155571A1

WO2013155571A1 - Polymeric composites containing highly aligned carbon nanotubes and method for making them

Info

Publication number: WO2013155571A1
Application number: PCT/AU2013/000413
Authority: WO
Inventors: Menghe Miao; Qun Gu; Jing Wang
Original assignee: Commonwealth Scientific And Industrial Research Organisation; Ningbo Institute Of Materials Technology And Engineering, Chinese Academy Of Sciences
Priority date: 2012-04-19
Filing date: 2013-04-19
Publication date: 2013-10-24

Abstract

A method of producing a carbon nanotube reinforced polymer product includes merging a web of generally aligned carbon nanotubes into a liquid or gel extruded from a solution containing a polymer or polymer blend and compatible with the carbon nanotubes so as to form an intermediate product in which the nanotubes of the web are in contact with the liquid or gel and are generally aligned in a longitudinal direction of the intermediate product. The intermediate product is processed to obtain a carbon nanotube reinforced polymer product. Also disclosed is a carbon nanotube reinforced polymer product in which the carbon nanotubes are highly aligned in the longitudinal direction of the product, the carbon nanotube loading is greater than 5%, and the average length of carbon nanotubes is greater than 100 micron.

Description

Polymeric composites containing highly aligned carbon nanotubes and method for making them

Field of the invention

This invention relates generally to the production of polymeric composites containing carbon nanotubes and in particular to carbon nanotube (CNT) reinforced polymer products such as, for example, fibres, films and membranes. In this context, 'fibre" is interchangeable with "filament" and both terms may appear herein.

Background of the invention

Virtually since they were first observed and fabrication techniques developed, carbon nanotubes have been of interest as a reinforcement for polymer composites. This interest has arisen from the exceptional mechanical, electrical and thermal properties of carbon nanotubes, coupled with their very high aspect ratio. For example, the tensile strength of multi-walled carbon nanotubes (MWNT) is between 11 and 150GPa and their stiffness or Young's modulus is between 0.27 and 0.95TPa (1TPa=1000GPa). In comparison, Kevlar™ fibre has a tensile strength of 3.6 to 3.8GPa and a modulus of 0.06 to 0.18TPa. Gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibres or filaments (these terms are used interchangeably herein) are known for their high strength and stiffness: their strength and Young's modulus are similar to Kevlar™, but specific strength is higher because of their lower density. Carbon nanotubes are thus several times stronger and stiffer than the highest performing polymer fibres on these measures, and so are considered to be effective reinforcement material for polymers. Lightweight strong materials are ideal for many applications, such as armours for personal protection, aerospace, sports and military equipment.

Published methods of manufacturing CNT reinforced polymeric composites generally include dispersing carbon nanotubes in a polymer matrix and then processing the composite material in substantially the same manner as for the pure polymer. This approach has several inherent disadvantages. The carbon nanotubes, due to their size and to the strong van der Waals force between them, tend to agglomerate and are difficult to disperse. This limits the fraction ratio of carbon nanotubes that can be mixed into a polymer matrix. To achieve uniform dispersion, the carbon nanotubes are commpnly modified to make them compatible with the polymer. These modifications commonly result in damage to the perfect atomic arrangement in the carbon nanotubes and result in shortened CNT length and reduced CNT mechanical properties (i.e. strength, modulus and toughness). CNT fraction ratios with acceptably uniform dispersion have been claimed up to 5% but have often been much less, while CNT lengths in the composites are typically in the order of a few micrometres.

Another difficulty in manufacturing strong CNT reinforced polymeric composites is that the dispersed carbon nanotubes are random in orientation in the polymer matrix, preventing the carbon nanotubes from making full contribution to the composite in the direction of applied load. Although composites have been drawn to improve CNT alignment in the composite, the maximum draw ratio is usually limited to a very low level that will not lead to sufficiently high alignment of the carbon nanotubes in the final composite. Carbon fibres possess high mechanical properties and have been extensively used as reinforcement material for polymers in applications such as aerospace and sport equipment. Further improvement of carbon fibre strength is limited by micro- and nano- scale flaws perpendicular to the axis of the fibre (SC Bennett, et al. Strength-structure relationships in PAN-based carbon fibres. Journal of Materials Science 1983, vol.18, 3337-3347). Carbon nanotubes may be introduced into carbon fibre precursor to "bridge" these structural flaws in the final carbon fibre. Likewise, the lack of carbon nanotube alignment in the axial direction of the polymeric precursor reduces the effectiveness of the bridging effect of the carbon nanotubes in the final carbon fibres.

One approach to addressing the alignment problem is disclosed, for example, in R Haggenmueller et al. Journal of Nanoscience and Nanotechnology 2003, vol. 3 no. 1/2, 105-110. Here, single walled carbon nanotubes were mixed into molten polyethylene or polystyrene by means of a twin screw compounder, and the resultant composite meltspun to produce a filament in which the carbon nanotubes were found to be highly aligned. In recent years, a number of techniques have been disclosed for producing webs of generally aligned carbon nanotubes by drawing the webs from a forest of the nanotubes on a substrate. Such disclosures include International patent publication WO 2007/015710, in which both twisted and untwisted webs are described, and US patent application 2004/0053780, which discloses an untwisted web. A carbon nanotube polymer composite derived from the latter structure is disclosed in US patent application 2009/0092813. A carbon nanotube film is drawn from a forest of carbon nanotubes; multiple layers of the films are overlayed on a substrate, which is then dipped into a polymer solution (for example an epoxy polymer or a polypropylene) to obtain the composite.

It is an object of this invention to provide an improved carbon nanotube reinforced polymer composite.

It is a further object of the invention to provide a method of making carbon nanotube reinforced polymer composites that at least in part alleviates one or more of the aforementioned difficulties.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

The present invention entails a realisation that webs of generally aligned carbon nanotubes such as those drawn from a forest of carbon nanotubes may be combined in a novel way with a polymer that is an efficient mode of fabrication and moreover is able to obtain a satisfactory composite product. More specifically, the concept is to merge the web of generally aligned nanofibres into a liquid or gel extruded from a solution containing a polymer or polymer blend, and preferably to thereafter longitudinally draw the merged composite. More particularly, in one aspect, the invention provides a method of producing a carbon nanotube reinforced polymer product, comprising: merging a web of generally aligned carbon nanotubes into a liquid or gel extruded from a solution containing a polymer or polymer blend and compatible with the carbon nanotubes so as to form an intermediate product in which the nanotubes of the web are in contact with the liquid or gel and are generally aligned in a longitudinal direction of the intermediate product; and processing the intermediate product to obtain a carbon nanotube reinforced polymer product. The intermediate product may be, for example, a filament, a film or a membrane. The carbon nanotube reinforced polymer product may be, for example, a filament, a film or a membrane.

The liquid or gel extruded from a solution containing a polymer or polymer blend is preferably in a gel state. The processing to obtain a carbon nanotube reinforced polymer product may typically include solidification, e.g. gel-crystallisation, and extraction of solvent to obtain a carbon nanotube polymer composite.

The extrusion of the liquid or gel may usually have immediately preceded the merging step, but in one or more particular embodiments the gel, e.g. a gel filament, may have been pre-prepared and stored in an appropriate package from which it is withdrawn for the merging step.

Advantageously, the processing includes longitudinal drawing. For example, the intermediate product, and/or the carbon nanotube polymer composite post gel- crystallisation, may be longitudinally drawn. In the former case, the intermediate product with the merged carbon nanotubes may be longitudinally drawn while still in a liquid or gel state. This is found to be useful in improving carbon nanotube alignment in the longitudinal direction of the final carbon nanotube reinforced polymer product. This longitudinal drawing may include inserting twist into the intermediate product.

Longitudinal drawing of the carbon nanotube polymer composite post gel-crystallisation to obtain the carbon nanotube reinforced polymer product is found to be useful in improving carbon nanotube alignment, and in bringing out the mechanical properties of the polymer, through molecular orientation. When the CNT content is very high, the mechanical properties of the polymer become less important, so the gel-state draw plays a more important role than the post gel-crystallisation draw in enhancement of the properties of the final composites. In one or more embodiments, the longitudinal drawing is effected on the intermediate product, but twisting is employed rather than further drawing of the composite post gel- crystallisation.

In one or more embodiments, the invention further includes preparing the web of generally aligned carbon nanotubes by drawing from a forest of the nanotubes on a substrate. The direction of drawing is preferably between 60° and 90° to the direction of alignment of the nanotubes on the substrate.

In one or more alternative embodiments, the web merged into the liquid or gel is a web that has been drawn from a forest of the nanotubes on a substrate, and may be a twisted array or yarn. The or each step of longitudinal drawing may be a superdraw, i.e. substantially higher than draw ratios used in melt spinning, for example, entailing a draw ratio preferably greater than 15 and more preferably greater than 30, most preferably greater than 50.

A known suitable polymer for the method of the invention is ultra-high molecular weight polyethylene (UHMWPE). Other ultra-high molecular weight polymers, such as Polyvinylalcohol (PVA), Polypropylene (PP) and Polyacrylonitrile (PAN), are also suitable for this method. Advantageously, the method includes forming the solution of the polymer in a suitable solvent compatible with both the polymer and the carbon nanotubes, and gel extrusion of said liquid or gel from this solution. More generally, the polymer is selected for its very high drawability so that the carbon nanotube polymer intermediate or composite can be drawn to a high ratio in order to achieve an enhanced level of carbon nanotube alignment along the processing direction in the product. In one or more embodiments, the merging of a web of generally aligned carbon nanotubes may entail merging of a plurality of separate such webs simultaneously or sequentially.

In one or more applications of the carbon nanotube reinforced polymer product, the polymer may be removed by melting, dissolving or other methods or combinations of methods to produce materials consisting of highly aligned carbon nanotubes.

In a further aspect, the invention is directed to a carbon nanotube reinforced polymer filament, film or membrane produced by the method of the invention. Because the inventive method, at least in one or more preferred embodiments, is such as to preserve the length of the carbon nanotubes, the average CNT length where the CNTs are derived from a drawable nanotube forest will be of an order similar to their length in the forest. Relative to known composites, the product is characterised by a carbon nanotube loading ratio greater than 5%, more preferably greater than 10% and most preferably greater than 20%, and further characterised by an average length of carbon nanotubes in the product greater than 100 micron, preferably greater than 500 micron, more preferably greater than 1mm.

In a still further aspect, moveover, the invention provides a carbon nanotube reinforced polymer product in which the carbon nanotubes are highly aligned in the longitudinal direction of the product, the carbon nanotube loading is greater than 5%, preferably greater than 10% and most preferably greater than 20%, and the average length of carbon nanotubes is greater than 100 micron, preferably greater than 500 micron, more preferably greater than 1 mm. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Brief description of the drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagram of the principal steps in a method according to an embodiment of the present invention;

Figure 2 (a) and (b) are SEM images showing a carbon nanotube reinforced UHMWPE composite fibre prepared according to an embodiment of the invention;

Figure 3 is an SEM image showing a ruptured end of the same composite fibre;

Figure 4 is a diagram of the principal steps in a method according to another embodiment of the present invention;

Figure 5 (a) and (b) are SEM images showing a carbon nanotube reinforced UHMWPE composite fibre prepared according to another embodiment of the present invention; and

Figure 6 is a diagram of tensile curves of neat UHMWPE fibre (lower curve) and CNT / UHMWPE composite fibre (upper curve).

Detailed description of the embodiments

The process illustrated in Figure 1 depicts a known process for gel spinning filaments or fibres of ultra-high molecular weight polyethylene (UHMWPE), modified according to an embodiment of the present invention. Film or membrane can be produced in substantially the same manner using an extrusion pack of different design. UHMWPE polymer is particularly suitable for the purposes of the present invention because it can be superdrawn and because of the small ratio required in preparing the polymer-solvent gel for extrusion. Other known suitable polymers for gel spinning and superdrawing include polyvinylalcohol (PVA), polypropylene (PP) and polyacrylonitrile (PAN).

The polymer is first dissolved (12) in a suitable solvent, and a gel filament, film or membrane 14 extruded from this solution via a suitable extrusion pack or die 16. Various solvents can be used to dissolve UHMWPE, including liquid paraffin (a mixture of alcanes) and decalin (decahydronaphthalene), but the solvent must be compatible with, and preferably have good compatibility with, not only the polymer but also the carbon nanotubes to be introduced into the gel. It has been found that liquid paraffin and decalin meet this requirement. In the normal gel spinning process, the next step is to allow the gel filament, film or membrane 14 to solidify (30). In accordance with a preferred embodiment of the present invention, however, a step 20 is introduced at which a web 22 of generally aligned carbon nanotubes is merged into the gel filament, film or membrane. This web may have been separately produced but is conveniently just derived by being drawn from a forest 18 of the carbon nanotubes on a substrate 19. The drawing is typically lateral (e.g. between 60° and 90°) to the direction of alignment of the carbon nanotubes on the substrate. Suitable webs for the present invention, their manner of production from a nanotube forest, and modes of preparing the forest, are disclosed, for example, in the aforementioned international patent publication WO 2007/015710 and US patent application 2004/0053780. The web may be a "yarn", twisted or untwisted, or a broader sheet or ribbon. In the web, integrity of the CNT array is believed to be primarily due to van der Waals forces between the carbon nanotubes.

The merging step produces an intermediate product, in this case a filament, film or membrane, 24 of gel consistency in which the carbon nanotubes of the web are in contact with the gel and are generally aligned in the longitudinal direction of the filament, or in the processing direction of the film or membrane. It will be appreciated that in the merging step the carbon nanotube web may be introduced into the polymer gel from an approximately parallel direction, as illustrated, or from an approximately perpendicular direction, ^or from any angle relative to the processing direction. In a possibly preferred arrangement, the point of merge may be at or close to the point of emergence of the gel filament from the extruder spinneret, or the gel film or membrane from the die, respectively. It is however also possible that a gel filament prepared and stored in an appropriate package in advance is used to merge with the carbon nanotube web or webs. Typically, the gel intermediate 24 containing the carbon nanotubes is subjected in succession to conventional steps including gel-crystallisation (30), a preliminary drawing or stretching (32), extraction (33) and evaporation (36) of the solvent to obtain a solidified carbon nanotube polymer composite 34. In an embodiment of the invention, this composite is longitudinally drawn under heat (38) and subjected to heat setting (40) to obtain a carbon nanotube reinforced polymer filament, film or membrane 54, which is finally wound onto a suitable package (42). These steps can be carried out in substantially the same way as in pure polymer gel spinning.

Alternatively or additionally, the gel intermediate is longitudinally drawn while still in a liquid or gel state, and the drawn gel intermediate is subjected to the aforementioned conventional steps including gel-crystallisation (30), a preliminary drawing or stretching (32), extraction (33) and evaporation (36) of the solvent.

To fully realise the potential mechanical properties of the polymer, the post gel- crystallisation draw is preferably a superdraw. This is especially important for composites containing low ratios of carbon nanotubes.. The superdrawing ratio is preferably greater than 20 and more preferably greater than 30, most preferably greater than 50. The superdrawing may conveniently be carried out in two or more successive draws, preferably at successively higher temperatures.

Following the longitudinal drawing step 38, the carbon nanotube reinforced polymer filament 54 obtained is characterised by a very high degree of alignment of the carbon nanotubes with the longitudinal direction of the filament, film or membrane.

In a typical drawable nanotube forest used for preparation of carbon nanotube webs, the mean length of the carbon nanotubes is of the order of 100 microns to more than 1 milimetre. Because the described and illustrated process preserves the length and properties of the carbon nanotubes, it is expected that the average CNT length in the final product 54 is of a similar order. It is certainly greater than 100 micron and can be greater than 1mm. This contrasts sharply with the lengths of the bulk carbon nanotubes typically employed in existing processes involving dispersion of carbon nanotubes into a polymer, i.e. of the order of only a few microns.

Example 1

In one of our experiments, carbon nanotube reinforced polymer fibre was made from ultra-high molecular weight polyethylene polymer in accordance with the following procedure. 1.9g of the polymer, of molecular weight 3.6x10⁶ and antioxidant of 0.019g, were dispersed in 50ml of paraffin oil and heated to 110°C with mechanical stirring for 48 hours. The temperature was then raised to 180°C with mechanical agitation and maintained for 6 hours. The result was a gel-like solution.

Carbon nanotube forest was grown on a silicon wafer substrate using chemical vapour deposition as described in Huynh CP, Hawkins SC, "Understanding the synthesis of directly spinnable carbon nanotube forests", Carbon 2010;48(4): 1105-15. The CNTs had 7+2 walls, with an outer diameter of 10+_3nm and an inner diameter of 4+1 nm. The length of the CNTs was approximately 350pm.

The solution (gel) was now fed to a mini scale extrusion machine for filament extrusion at 160°C. Carbon nanotube web drawn laterally from the forest of carbon nanotubes was directly introduced to the gel filament that continuously emerged from the extruding machine. It was observed that surface tension pulled the carbon nanotubes towards the surface of the newly extruded filament into contact with the gel, to form an intermediate filament of mixed polymer gel and carbon nanotubes.

The UHMWPE-carbon nanotube gel fibre then crystallised and was stored for 24 hours. Dichloromethane (CHCI₂) was used to extract the paraffin oil solvent from the UHMWPE-carbon nanotube gel filament. The product was then placed in an oven set to 60°C to evaporate any residual CHCfe. The resultant solidified composite fibre was longitudinally drawn in three steps, with a progressively increasing temperature but reducing draw ratio. In a first experiment, the draw ratio for the first drawing was 5.5 at 125°C, for the second drawing 4.5 at 137°C, and finally 1.5 at 142°C. These drawing conditions were merely exemplary and can be further optimised.

Figure 2 (a) and (b) are SEM images showing the nanotube reinforced UHMWPE fibre after the multistep drawing. The majority of carbon nanotubes now appear to be straightened and aligned along the fibre axis/longitudinal direction. Figure 3 is a SEM image of a rupture end of a nanotube reinforced UHMWPE fibre by tensile loading. The carbon nanotubes appear to embed in the polymer matrix with a higher concentration in the outer layer.

UHMWPE films and membrane containing highly aligned carbon nanotubes can be produced in substantially the same way.

In a second experiment, CNT / UHMWPE composite fibre containing 9wt% CNTs was produced with a net hot draw ratio of 34.5. Figure 6 compares the tensile behaviours of this composite fibre (upper curve) with that of a neat UWHMWPE fibre (hot draw ratio 37) (lower curve). The composite fibre showed a 22% higher modulus and 30% higher failure strength than the neat UHMWPE fibre. The neat UHMWPE fibre strength achieved in this study was similar to that of previously reported fibres produced at similar hot draw ratios and closely matched the strength of some commercial UHMWPE fibres, such as Dyneema SK25 and Spectra 900.

Example 2

Polymer fibres containing much higher ratio of carbon nanotubes were made according to the following procedures, shown schematically in Figure 4. The preparation of the gel-like solution of ultra-high molecular weight polyethylene polymer in paraffin oil was essentially the same as in Example .

A number of carbon nanotube webs drawn laterally from carbon nanotube forests were introduced, simultaneously or sequentially, to a ready-made gel filament or a gel filament that continuously emerged from the extruding machine. Up to eight carbon nanotube webs were successfully combined with the gel filament. In this way, the ratio of carbon nanotubes in the intermediate gel filament 24 was increased by several folds in comparison with that achieved in Example 1 , producing final composites containing up to 70 % carbon nanotubes by weight.

The carbon nanotube-UHMWPE gel filament, i.e. still in a gel state, was drawn longitudinally in one or more steps, at draw ratios up to 10 for each step, at room temperature to improve alignment of carbon nanotubes in the gel filament. The upper limit of these draw ratios is only limited by the thickness of the starting gel filament and the desired thickness of the final filament. Insertion of twist to the carbon nanotube- UHMWPE gel filament during drawing was found to be beneficial to the production of an even and void-free composite filament (60). Additional twist may be inserted after the drawing process to enhance the structural integrity and mechanical properties of the final composites, as in the production of yarns from conventional textile fibres. n-Alkanes (typically 5 to 7 carbons) was then used to remove the paraffin oil solvent from the initially drawn UHMWPE-carbon nanotube gel filament (33). The product was placed in an oven set to 60 - 10°C to evaporate any residual n-Alkanes and to obtain a solidified composite fibre.

Figure 5 shows scanning electron microscope (SEM) images of the carbon nanotube- UHMWPE composite obtained by laying six layers of carbon nanotube webs taken after the paraffin oil had been removed and before the heated drawing step 38. The carbon nanotubes appear to be straightened and aligned very well along the fibre axis.

The solidified composite fibre was then drawn in one or more steps at elevated temperatures (38) in a similar manner as in Example 1. Additionally, the polymer in the resulting composites may be removed by melting, dissolving or other methods or combinations of methods to produce materials consisting of close to 100% highly aligned carbon nanotubes. Example 3

It is well known that high molecular weight polyacrylonitrile (PAN) can be dissolved in solvents (for example, thiocyanates) and extruded into a gel filament according to a principle similar to that with ultra-high molecular weight polyethylene polymer. Web or webs of carbon nanotubes can be combined with the PAN gel filament in the same way as in Examples 1 and 2. The combination of carbon nanotubes and PAN gel is then drawn to achieve high levels of carbon nanotube alignment as stated in Examples 1 and 2. After removal of the solvent, the gel spun carbon nanotube/PAN fibre is carbonised in substantially the same manner as in conventional carbon fibre manufacture to obtain a carbon nanotube reinforced carbon fibre. Iri this way, the carbon nanotubes are aligned to the axis of the carbon fibre to provide maximum bridging effect to the structural cracks that appear perpendicular to the axis of conventional carbon fibres.

The process described herein has the twin attractions of being easily adapted as a continuous process and being able to be retrofitted to existing gel spinning lines. The problematic dispersion processes of existing carbon nanotube are avoided, which itself addresses the problem of agglomeration and allows much higher nanotube loading ratios. The presentation of carbon nanotubes as an already generally aligned array further facilitates the production of a composite of enhanced strength and stiffness, and the ready ability to apply one or more drawing steps provides further benefits in terms of carbon nanotube alignment and contribution to the superior qualities of the composite product.

Claims

1. A method of producing a carbon nanotube reinforced polymer product, comprising: merging a web of generally aligned carbon nanotubes into a liquid or gel extruded from a solution containing a polymer or polymer blend and compatible with the carbon nanotubes so as to form an intermediate product in which the nanotubes of the web are in contact with the liquid or gel and are generally aligned in a longitudinal direction of the intermediate product; and processing the intermediate product to obtain a carbon nanotube reinforced polymer product.

2. A method according to claim 1 wherein said processing to obtain a carbon nanotube reinforced polymer product includes solidification, e.g. gel-crystallisation, and extraction of solvent to obtain a carbon nanotube polymer composite.

3. A method according to claim 2, wherein the processing includes longitudinal drawing of said composite.

4. A method according to claim 3 wherein the processing includes longitudinal drawing of said intermediate product while still in a gel or liquid state.

5. A method according to claim 1 or 2 wherein the processing includes longitudinal drawing of said intermediate product while still in a gel or liquid state.

6. A method according to claim 5 further including twisting said carbon nanotube reinforced polymer product.

7. A method according to claim 4, 5 or 6 wherein said longitudinal drawing includes inserting twist into the intermediate product.

8. A method according to any one of claims 3 to 7 wherein said longitudinal drawing is a superdraw.

9. A method according to claim 8 wherein said superdraw entails a draw ratio greater than 15.

10. A method according to claim 8 wherein said superdraw entails a draw ratio greater than 30.

11. A method according to claim 8 wherein said superdraw entails a draw ratio greater than 50.

12. A method according to any one of claims 2 to 11 wherein said intermediate product is a filament, and the carbon nanotube reinforced polymer product is a filament.

13. A method according to any one of claims 2 to 11 wherein said intermediate product is a film, and the carbon nanotube reinforced polymer product is a film.

14. A method according to any one of claims 2 to 11 wherein said intermediate product is a membrane, and the carbon nanotube reinforced polymer product is a membrane.

15. A method according to any one of claims 1 to 14 wherein the liquid or gel extruded from a solution containing a polymer or polymer blend is in a gel state.

16. A method according to any one of claims 1 to 15, wherein the web merged into the liquid or gel is a web that has been drawn from a forest of the nanotubes on a substrate.

17. A method according to claim 16, wherein the web merged into the liquid or gel is a twisted array or yarn.

18. A method according to any one of claims 1 to 15 further including preparing the web of generally aligned carbon nanotubes by drawing from a forest of the nanotubes on a substrate.

19. A method according to any one of claims 1 to 18 further including treating the . carbon nanotube reinforced polymer product to remove the polymer.

20. A method according to any one of claims 1 to 19, wherein said polymer or polymer blend is selected from the group consisting of ultra-high molecular weight polyethylene (UHMWPE), polyvinylalcohol (PVA), polypropylene (PP) and polyacrylonitrile (PAN).

21. A method according to any one of claims 1 to 20, wherein the extrusion of the liquid or gel immediately precedes said merging.

22. A method according to any one of claims 1 to 21 , wherein said merging of the web of generally aligned carbon nanotubes entails merging of a plurality of separate such webs simultaneously or sequentially;

23. A carbon nanotube reinforced polymer product in which the carbon nanotubes are highly aligned in the longitudinal direction of the product, the carbon nanotube loading is greater than 5%, and the average length of carbon nanotubes is greater than 100 micron .

24. A carbon nanotube reinforced polymer product according to claim 23 wherein the carbon nanotube loading is greater than 10%.

25. A carbon nanotube reinforced polymer product according to claim 23 wherein the carbon nanotube loading is greater than 20%.

26. A carbon nanotube reinforced polymer product according to claims 23, 24 and 25 wherein the average length of carbon nanotubes is greater than 500 micron.

27. A carbon nanotube reinforced polymer product according to claims 23, 24 and 25 wherein the average length of carbon nanotubes is greater than 1mm.

28. A carbon nanotube reinforced polymer product according to any one of claims 23 to 27 produced by the method of any one of claims 1 to 22.