WO2009053470A1

WO2009053470A1 - Conductive polymer composite

Info

Publication number: WO2009053470A1
Application number: PCT/EP2008/064460
Authority: WO
Inventors: Hua Deng; Antonius Andreas Johannes Maria Peijs
Original assignee: Queen Mary And Westfield College, University Of London
Priority date: 2007-10-24
Filing date: 2008-10-24
Publication date: 2009-04-30

Abstract

A method of preparing a conductive multi-component material, comprises the steps of : preparing an initial multi-component material, the initial multi-component material including a continuous first polymeric component comprising conductive filler and a first polymer, and a second polymeric component comprising a second polymer, the first polymeric component having a lower melting temperature than the second polymeric component; subjecting the initial multi-component material to an orienting process; and annealing the oriented multi-component material to form the conductive multi-component material.

Description

CONDUCTIVE POLYMER COMPOSITE

The present invention relates to a method of preparing a conductive multi-component composite, a conductive multi-component composite and use of a conductive multi-component composite.

Conductive polymer composites (CPCs) can be made by adding conductive filler into an insulating polymer matrix, where it forms a conducting network.

Such composites are useful, for example, for antistatic, electrostatic discharge (ESD) , electrostatic painting and electromagnetic-radio frequency interference (EMI) protection purposes [1], particularly in the automotive and textile industries. Applications include anti -static uniforms to avoid sparks caused by static electricity (for example, in chemical plants or petrol stations); anti-static carpets; anti-static conveyor belts in airports; and car parts (where the use of conducting material can save paint) . Such composites are also useful as components in electrical circuits, for example in electric fences for animals .

It has been demonstrated that a sudden jump in CPC conductivity occurs when a critical loading (the "percolation threshold") of conductive filler is added into the matrix [2] . The percolation threshold of fibre reinforced CPC has been shown both experimentally and theoretically to decrease with increasing filler aspect ratio [3, 4] . CPCs based on carbon nanotubes have attracted much attention in the last few years (WO 91/03057, WO2006/072741 , EP 1349179A1) . It has been shown that CPCs based on multi- wall nanotubes (MWNTs) can have a percolation threshold as low as 1.44 wt% in melt compounded polymers [5] and 0.0025 wt% in epoxy matrix [6] . This is low compared with conventional conductive filler such as metal powders and low structure carbon black, where the percolation threshold is typically between 5 wt% to 30 wt% [7] . in order to process a CPC into a desired shape, the CPC must undergo process steps such as injection moulding, spinning, extrusion or stretching. in these processes the conducting network is deformed to varying extents. Work has been carried out to investigate the influence of a deformed conducting network on conductivity [4, 8-11] . An inhomogeneous conductive network was observed for an injection moulded sample because of shear flow during processing. To simulate deformation of the CPC melt during extrusion or injection moulding, Alig I., et al. [8, 9] have performed a study to investigate the influence of shearing on the conductivity of molten CPC. The study shows that conductivity is lost when the network is deformed, but it slowly recovers after the shearing is stopped and the molten CPC is annealed. This is explained by re-aggregation of MWNTs in the melt. However, the time needed to reform the network in these experiments is nearly

3 hours, which is too long for commercial extrusion or injection moulding processes. Similar observations have been reported for carbon black filled polymer composites [12-14] .

Bin et al . [15, 16] showed that thermal annealing conducted by scanning temperatures ranging from room temperature to near melting temperature could increase the conductivity of highly oriented CPC fibre based on ultrahigh molecular weight polyethylene (UHMWPE) and MWNTs. In addition, recently, Miaudet P. et al . [17] carried out an investigation on the effect of thermal treatment on the conductivity of CPC fibres containing polyvinyl alcohol (PVA) (a semi-crystalline polymer} and MWNTs. It was found that thermal annealing between the glass transition temperature (T_g) and melting temperature (T_m) of the polymer dramatically increased the conductivity of the fibre. An x- ray diffraction (XRD) study showed that the increased conductivity might be due to increased mobility of the polymer chain, leading to mobility of carbon nanotube which could increase the quality and the density of nanotube contacts. However, the effect of thermal annealing on the mechanical properties of these fibres [15-17] was not reported, and it is most likely that the mechanical properties and integrity of these highly oriented polymer fibres or tapes will be lost due to polymer relaxation during thermal annealing.

There remains a need for CPC materials which have good conductivity and good mechanical properties.

In a first aspect, the present invention relates to a method of preparing a conductive multi-component material, comprising the steps of: preparing an initial multi -component material, the initial multi-component material including a continuous first polymeric component comprising conductive filler and a first polymer, and a second polymeric component comprising a second polymer, the first polymeric component having a lower melting temperature than the second polymeric component; subjecting the initial multi-component material to an orienting process; and annealing the oriented multi-component material to form the conductive multi-component material . The term "multi-component material" as used herein includes or refers to a material wherein separate components do not form a continuous homogeneous phase. The materials are at least to some extent spatially separated within the material. They may be present as identifiable separate phases under SEM/TEM or as clearly demarcated layers.

The term "continuous" as used herein includes or refers to a component in which a path can be traced from one side of the material to another without moving without moving to another component.

The term "melting temperature" as used herein includes or refers to the temperature at start of the melting peak (or onset of the melting peak) in the case of semi-crystalline polymer as determined by differential scanning calorimetry (DSC) . In the case of amorphous polymers, which do not have a melting temperature, the term "melting temperature" includes or refers to the glass transition temperature as determined by DSC.

The term "annealing" as used herein refers to or includes treating material at elevated temperature for a certain period of time. Annealing is typically carried out above the melting temperature of the material. Annealing is used to mobilise the polymer chains and thus the conductive network. Preferably, the second polymeric layer is also continuous .

The multi-component may take various forms, for example film, tape, fibre or yarn (wherein the term "yarn" includes a collection of fibres) . Preferably, the multi -component material is a multilayer material, wherein the first polymeric component is a first polymeric layer and the second polymeric component is a second polymeric layer. Preferred dimensions for such materials and arrangements of layers are discussed further below.

Alternatively, however, the multi-component material may be a non-layered material. For example, the first polymeric component and second polymeric component may form a co-continuous blend.

Suitable dimensions for the multi -component material are as follows :

Film

Width: 1 mm to 10 m. Thickness: 1 μm to 10 mm

Preferably, the width of the film is from 20 mm to 1 m, thickness is from 10 μm to 5 mm. Highly preferably, the width of the film is from 20 mm to 100 mm (it may be limited by the width of the die) . The highly preferred thickness of the film is from 10 Um to 3 mm (it may be limited by the thickness of the die and picking up speed of the roller) .

Tape

Width: 1 mm to 1 m. Thickness: 1 μm to 10 mm

Preferably, the width of the tape is from 1 mm to 500 mm, thickness is from 5μm to 5 mm. Highly preferably, the width of the tape is from 1 mm to 100 mm (it may be limited by the width of the die and the draw ratio) . The highly preferred thickness of the film is from 10 μm to 3 mm (it may be limited by the thickness of the die, picking up speed of the roller and draw ratio) .

Yarn/Fibre Diameter: 1 U m to 5 mm

Preferably, the diameter of the yarn/ fibre is from 10 μ m to 3 mm. Highly preferably, the diameter of the yarn/fibre is from 20 μ m to 2 mm (it may be limited by the diameter of the die and draw ratio) .

These materials can be continuously produced in long lengths .

The multi -component material may comprise two, three or more components. A single first component and a single second component may be present (A: B structure) .

In a multi-layer material, a single central second layer with a first layer on each side may be present (sandwich structure) , in which case the first layers may be the same (A:B:A structure) or different (A:B:C structure) . Other examples of possible structures include A:B:A:B, A:B:C:D, A:A:B:D, A:C:C:D, B:C:D:D, A:B:C:D:E:F, etc., with one or more conductive components being present.

In the case of a yarn/ fibre, one or more core layers/ fibres and one or more sheath layers may be present, which may or may not be co-axial. The core layer (s) /fibre (s) may have various cross-sectional shapes e.g. circular, elliptical, Y-shape, H-shape, X-shape. Alternatively or additionally, one or more segments may be present.

Various possible structures for the multi-component material are sketched in Figs. 3 and 4. Other more complex structures are of course possible.

The first component and second component are suitably present in amounts of A: B = 1:99 to 99:1 (preferably 1:48 to 1:1) by weight in the case of A: B structure multi -component material. The conductive filler may have an aspect ratio of 1 or more. Aspect ratio is preferably determined by transmission electron microscopy (TEM) , scanning electron microscopy

(SEM) or atomic force microscopy (AFM) on the conductive filler alone. Preferably, the aspect ratio of the conductive filler is 10 or more. More preferably, the aspect ratio of the conductive filler is 30 or more. The carbon black clusters used in this study have an average aspect ratio of 30 according to TEM study. Highly preferably, the aspect ratio of the conductive filler is 100 or more. The MWNTs used in this study have an average aspect ratio of about 150.

Preferably, the conductive filler is carbon nanotubes, carbon black, graphite, graphene, conductive nano-wires, carbon fibre, carbon nanofibre, metal powders or wires (such as copper, steel or silver) , conductive polymer, or a combination of any of these materials. Carbon materials (carbon black, carbon nanotubes, carbon fibre, carbon nanofibre, graphite, graphene, or a combination of any of these materials) are particularly preferred. Carbon nanotubes, for example multi-wall nanotubes, double-wall nanotubes or single wall nanotubes are used in a preferred embodiment of the invention. The nanotubes may be functionalised or coated with polymer or oxidised. Carbon black is used in another preferred embodiment of the invention.

Preferably, the first polymeric component contains conductive filler in an amount of 0.0001 wt . % to 50 wt . % based on weight of the first polymeric component. More preferably, the first polymeric component contains conductive filler in an amount of 1 to 20 wt% based on weight of the first polymeric component. Highly preferably, the conductive filler is present in an amount of 1 wt . % to 6 wt . % for conductive filler with an aspect ratio of 100 or more (e.g. MWNTs), or in an amount of

5 wt. % to 20 wt. % for conductive filler with an aspect ratio smaller than 100 (e.g. carbon black) .

The first polymer and second polymer may independently be crystalline or amorphous in nature. Typically the first polymer will have lower crystallinity or less complicated polymer chains than the second polymer. Preferably, the first polymer comprises polyolefin, polyester, polyamide, polycarbonate, poly(methyl methacrylate) (PMMA) and/or elastomer. Preferred polymers include polypropylene co-polymer (co-PP) , polypropylene, polystyrene, polycarbonate, polybutylene terephthalate (PBT), polyethylene (PE, e.g. LDPE, HDPE or LLDPE), polyethylene terephthalate co-polymer (co-PET) and polyamide-6 co-polymer (co-PA6) . Most preferably, the first polymer comprises polypropylene co-polymer (co-PP) , PA6 copolymer elastomer (co-PA6) , PET copolymer (co-PET) , LDPE or PMMA as demonstrated in example A and B.

Preferably, the second polymer comprises polyolefin, polyester, polyamide, polycarbonate and/or elastomer.

Preferred polymers include polypropylene (PP) , polyethylene

(PE) , polyethylene terephthalate (PET) , polyamide-6 (PA6) and polyamide-6 copolymer (co-PA6) . The second polymer may optionally include conductive filler.

Particularly preferred combinations of first and second polymers respectively include co-PP/ PP, LDPE/HDPE, CO-PA6/PA6, co-PET/PET, LDPE/PP, and CO-PA6/PET. For an A:B:C structure, preferred combinations are LDPE/PA6/co-PET, co-PET/PET/co-PAβ, co-PP/PP/co-PP, co-PET/PA6/co-PA6 , co- PA6/PA6/CO-PA6, LDPE/PET/CO-PET, and LDPE/PET/CO-PA6. Preferably, the difference in melting temperature between the first polymeric component and the second polymer polymeric component is at least 5 ^aC. Highly preferably, the difference in melting temperature between the first polymeric component and the second polymer polymeric component is at least 10 ²C. Most preferably, the difference in melting temperature between the first polymeric component and the second polymer polymeric component is at least 30 ²C. Preferably, the first step of preparing a multi- component material is carried out by co-extrusion, hot pressing or spinning the first polymeric component and second polymeric component together, or filament winding the first polymeric component and second polymeric component and subsequently welding them together, or film stacking the first polymeric component and second polymeric component and then welding them together, or phase separation of different polymers during spinning.

Preferably, the orienting process comprises one or more of drawing, extrusion and spinning.

The terms "drawing" and "stretching" as used herein refer to or include applying tensile force to a material, optionally through a die. The term "solid drawing" or "solid state drawing" as used herein refers to or includes drawing below the DSC melting temperature peak of the drawn material . The term "spinning" as used herein refers to or includes drawing and/or simultaneously drawing and twisting a material, for example from a polymer in the melted state. The term "extruding" as used herein refers to or includes forcing material through a die under pressure.

The orienting process may comprise one step or more than one step. The draw ratio used in the orienting process may be 1 or more than 1, and may be up to the maximum achievable for the polymer (e.g. 500) . Preferably, the draw ratio is from 1 to 10 for film or tape; from 3 to 100 for solid state drawn yarn/fibre or tape; and from 100 to 500 for melt spun yarn/ fibre or tape (which are spun from polymers in their melt state and optionally subsequently further drawn in their solid state) . Highly preferably, the draw ratio is from 1 to 5 for as-extruded film or tape; from 3 to up to 50 for the yarn/ fibre or tape. Most preferably, the draw ratio for film is 1, and the draw ratio for yarn/ fibre or tape is from 3 to 24.

A draw ratio of 1 may be used in the case of extrusion. The term "draw ratio" as used herein is a measure of the degree of stretch during the orientation of a sample, expressed as the ratio of the length of drawn material to that of undrawn material.

Where more than one orienting step is used, the overall draw ratio is the product of the draw ratios for each step. Optionally, the steps of preparing the multi- component material and subjecting the multi -component material to an orienting process are carried out simultaneously, for example by co-extrusion (which may be followed by one or more drawing steps) . These steps may alternatively be carried out separately.

Suitable annealing temperatures will depend on the melting temperature of the first polymer and the second polymer, since annealing is conducted near to or above the melting temperature (for semi-crystalline polymer) or glass transition temperature (for amorphous polymer) . The best results are obtained when the annealing temperature is as high as possible (close to the maximum temperature at which the whole multi -component material maintains its integrity during annealing) for a fixed amount of annealing time. Preferably, the annealing is conducted a temperature at least 30 ⁰C above the melting temperature of the first polymeric component.

For co-PP and PP as first polymer and second polymer respectively, an annealing temperature in the range of 120 ⁰C to 165 ⁰C has been found to be appropriate. Good results are obtained at long annealing times {such as 1 hour) . To achieve similar results, annealing time reduces with increasing temperature. Thus, suitable annealing temperatures could be as high as 400 ⁰C with a very short annealing time, for example 0.1 s, to maintain the integrity of the whole film, yarn/ fibre or tape. The cooling after annealing has been found to have negligible effect on the conductivity.

Suitably, annealing can be conducted using one or more heated rollers, ovens or baths (for example water or oil baths) . Preferably, annealing takes place as part of a continuous process. One or more annealing steps may be used.

Preferably, the method is conducted in a continuous fashion. However, it may also be conducted in a batch fashion. In a second aspect, the invention relates to a conductive multi -component material prepared by the method described above.

Preferably, the multi -component material has a conductivity (l/resistivity) of 1-E9 S/m to 10000 S/m. More preferably, the multi-component film, tape or yarn/fibre has a conductivity of 1-E9 S/m to 1000 S/m. Preferably, the multi -component material has a tensile strength of 20 MPa to 10 GPa and a Young's modulus of 100 MPa to 300 GPa. More preferably, a multi-component tape or yarn/fibre has a tensile strength of 100 MPa to 3 GPa and a Young's modulus of 1 GPa to 200 Gpa, and a multi - component film {typically with a low draw ratio) has a tensile strength of 20 MPa to 200 MPa and a Young's modulus of 100 MPa to 5 GPa. Most preferably, a multi -component tape or yarn/ fibre has a tensile strength of 100 MPa to 3 GPa and a Young's modulus of 2 GPa to 150 GPa, and a multi -component film has a tensile strength of 20 MPa to 100 MPa and a Young's modulus of 100 MPa to 2 GPa.

In a preferred embodiment, the Young's Modulus and/or tensile strength of the conductive multi-component material are at least twice as high as the Young's Modulus and/or tensile strength of the initial multi-component material, and/or the conductivity of the conductive multi-component material is at least 100 times as high as the conductivity of the oriented multi-component material before annealing. In a third aspect, the invention relates to a multi- component material comprising: a first polymeric component comprising conductive filler in an amount of 1 to 20 wt% based on the weight of the first polymeric component and a first polymer,- and a second polymeric component comprising a second polymer; wherein the first polymeric component has a lower melting temperature than the second polymeric component; and the composite has a conductivity of at least 1-E9 S/m. in a fourth aspect, the invention relates to an article made from the conductive multi-component material described above. The article may for example be formed from a woven or non woven textile or a rigid material. The multi -component material may be pressed or woven together in the article. The article may for example be an article of clothing or a car component. Suitably, the article has anti-static properties .

In a fifth aspect, the invention relates to use of an article described above to provide anti -static properties; electrostatic properties (for example for use in electrostatic discharge (ESD) , electrostatic painting or electromagnetic-radio frequency interference (EMI) protection) or as a component in an electrical circuit.

The invention will be further described with reference to non- limiting examples, and to the Figures, in which:

Fig. 1 shows a circuit used for measurement of resistivity

(Example A) ;

Fig. 2 shows schematically the arrangement of MWNTs within the first polymer layer (a) before drawing and annealing; (b) after drawing but before annealing; and (c) after drawing and annealing;

Fig. 3 is a sketch of the structures of various possible multi-component tapes of the invention; and

Fig. 4 is a sketch of various possible multi -component yarns/fibres of the invention.

Examples

A: SMALL SCALE (ABOUT 10 g) EXAMPLES

MATERIALS AND METHODS Materials

Nanσcyl^® 7000 MWNTs were provided by Nanocyl S. A.

(Belgium) . The average aspect ratio of these MWNTs is 150 according to the data sheet. The average diameter is 10 run. The polypropylene used for the core layer of the tape was Dow H507 (MFI=3.2 g/10mins) from DOW Chemical Ltd. Its melting temperature peak occurs at 168 ⁰C according to

Differential Scanning Calorimetry (DSC) measurement.

The polymer used in the skin layer (s) of the tape was PP copolymer (co-PP) Adsyl^® 5C39F from Basell. This is a random copolymer contain ethyl and polypropylene. Its melting temperature peak occurs at 114.6 ⁰C according to

Differential Scanning Calorimetry (DSC) measurement.

The conductive carbon black (CB) used was Printex^® XE-2 which was supplied by Grolman Ltd.

General Method

MWNTs or CB were melt blended with co-PP in a mini- extruder (DSM Micro 15) at 200⁰C, 200 rpm for 10 minutes. The extruded strand was then hot pressed into film with a thickness of 150 μm at 200⁰C for 5 minutes. The filler content of the CPC was measured by thermogravimetric analysis (TGA) . Two outer layers of CPC film with co- PP/MWNTs (or carbon black) and one central layer of PP film were then pressed into a sandwich structure at 155⁰C. The sandwich tape was then hot drawn into tapes at different draw ratios at 12O⁰C on a tensile test machine Instron 5584 equipped with an environmental chamber. Thermal annealing was conducted on mechanically constrained tape at 155⁰C for 15 minutes in the same environmental chamber. The thickness of the solid state drawn tapes, or solid state drawn and annealed tapes ranges from 30 μ m to 100 μm as they have different draw ratios. The width of the tape can range from 1 mm to 50 mm for different draw ratios or different starting width for the initial film.

DSC and TGA measurement

Differential scanning calorimetry (DSC) was performed under nitrogen gas flow in a temperature range of 20-200⁰C for co-PP, MWNT/co-PP and PP using a METTLER Toledo DSC 822^e apparatus. Samples were heated to 200 ⁰C and held at this temperature for 5 minutes to remove the previous thermal history, then cooled to 20 ⁰C. The DSC scanning rate was 10°C/min. Thermo-gravimetric Analysis (TGA) was carried out under either air or nitrogen atmosphere in a temperature range of 20-1000 ⁰C using a TA TGA Q500 apparatus. The heating rate used was 20 °C/min.

Electrical Measurement

DC electrical resistivity was measured for the composites at different stage of processing using the circuit shown in Fig. 1. A two point resistivity measurement method with voltage scan from 1 volt to 10 volt was conducted using an Agilent 6614C programmable voltage source in combination with a Keithley 6485 picoammeter . The equipment was interfaced with a computer to record the I-V curve and thus to calculate the resistivity. In this type of measurement (not online monitoring electrical properties during thermal annealing) , the oven was kept at room temperature. Silver paint was applied on both ends of the sample to ensure contact. Thus, contact resistance was much smaller than sample resistance. The dimensions of the sample were not uniform in this study because the thickness and width vary with draw ratio. For the specimens having resistivity higher than 5^χlO⁷ Ohm.m, the electrical resistivity is not measurable using this set-up and the specimens are considered non-conductive or not conductive.

Online Monitoring of Electrical Properties during Annealing

The automated online electrical measurement set up is schematically shown in Fig. 1. The voltage scan was chosen to be 1 V to avoid a strong electric current through the sample. In order to avoid a constant electric field on the sample, 300 ms internal voltage (0 Volt) was applied for every 700 ms at 1 Volt. Silver paint was applied on both ends of the sample to ensure good contact. High temperature resistant polymer coated copper wire was used in the oven to connect the sample with electrodes and wires outside the oven. The oven was heated to a targeted temperature and kept there until a relative stable current value was obtained. Then the heating was stopped and the oven cooled down to room temperature by air cooling (effectively by opening the door of the oven) . Therefore, the total annealing time was different for different type of specimens. The temperature was recorded during heating and cooling and at the same time the current data was recorded in order to calculate the resistivity.

Morphology Studies

Morphology studies were carried out by scanning electron microscopy (SEM) using a JEOL JSM-6300F SEM apparatus in order to investigate the form of the conductive network during tape fabrication and after annealing. A high accelerating voltage {25 kv) was applied for the SEM study, which is known to be contrast mode [18] .

Transmission electron microscopy (TEM) imaging of cross -sectional cut samples was performed. The as-prepared nanocomposite samples were sectioned at room temperature using an ultra-microtome (Reichert-Jung Ultracut E) . The TEM was operated in bright-field mode at 80 kv to increase the contrast between the MWNTs and the surrounding polymer matrix.

Tensile tests

Tensile tests were performed on an Instron 5566 apparatus equipped with a video extensometer . The gage length and cross-head speed used for drawn or un-drawn film, tapes or yarn/ fibres were 30 mm and 30 mm/min respectively.

At least five specimens were tested to obtain the average value .

EXAMPLE Al - INITIAL STUDY

The effect of thermal annealing on the conductivity and mechanical properties of CPC tape containing carbon nanotube or carbon black was studied. Multilayer sandwich tapes with CPC outer layers and neat polymer central layer were used.

DSC study showed that the melting temperature for co- PP starts (onset) at 70 ⁰C (the same value for different content of MWNTs in co-PP} , and the melting temperature peak occurs at 134.6 ⁰C (146 ⁰C for different content of MWNTs in co-PP) . The melting temperature peak for PP starts (onset) at 140 ⁰C and its peak occur at 168 ⁰C. The I-V curves were noticed to be linear. The drawn and annealed sample had resistivity at least 3 orders of magnitude below that of the drawn sample.

Table 1 shows the resistivity results for MWNTs/co- PP.

TABLE 1

The isotropic CPC specimens lost their conductivity after solid drawing, but annealing after solid drawing made them even more conductive than the original isotropic film. The annealing process appears to have restored the conductive network after a large deformation. Resistivity was compared with carbon black samples. The results are shown in Table 2 :

TABLE 2

Morphology studies were carried out as described above. The results are shown in Table 3:

TABLE 3

Sample Appearance of sample containing Appearance of sample preparation MWWTs containing CB process

Extrusion some orientation of MWNTs on Isotropic CB network the surface of the sample (due to shear during extrusion)

Hot pressed film network, of randomly isotropic Isotropic CB network dispersed MWNTs bundles solid state highly orientated MWNTs Orientated CB structure drawing bundles, and highly oriented

MWNTs inside the bundles

Solid state hairy orientated bundles: Isotropic CB network drawing and highly oriented bundles, but annealing locally random oriented MWNTs inside the bundles

The structures of the MWNT/co-PP samples are sketched in Fig. 2. Results from TEM study confirmed that the hairy orientated bundles consisted of highly oriented bundles and randomly oriented MWNTs inside these bundles.

Whilst the inventors do not wish to be bound by this theory, they believe that the mechanism of increased conductivity following drawing and annealing is as follows. A theoretical study into percolating anisotropic networks was performed by Munson-McGee in [19] where cylinder shaped fillers are considered in 3D, 2D planar and ID oriented arrays, respectively. In the fully aligned ID system, the radius of the cylinder is shown to increase linearly (when the length of the filler is fixed) with increasing probability of two cylinders intersecting in a unit volume

(thus, decreasing resistivity for fixed content of conductive filler in the matrix) ., This probability is considered as the ability of fillers to form a conductive network in a polymer matrix. The oriented MWNT bundles in a solid state drawn tape can be considered as aligned cylinders in such a system, where the radius is the half of the diameter of the MWNT bundles. After annealing, the diameter of the MWNT bundles increases dramatically. Approximately, their diameter increase from 70-190 nm to 500-1000 nm. With the MWNTs bundles becoming "hairy" {as shown in Figure 2) . This is believed to be due to the largely random orientation of MWNTs within the bundles, which remain largely oriented at a mesoscopic scale. These oriented "hairy" bundles not only allow for more MWNTs being effectively aligned in a particular direction, but also provides the MWNT bundles with lateral inter-bundle connections through individual nanotubes to reduce tunnelling resistance.

Carbon black is used as a conductive filler for comparison because it has much a smaller aspect ratio than MWNTs. The aspect ratio of the filler plays an important role during the drawing process, as shown in the literature [4, 10] . For carbon black, the resistivity of drawn and annealed tape is similar to that of hot pressed film. This can be understood as the orientated CPC having returned to isotropic state, with no difference between a hot pressed film and a drawn and annealed sample because of the smaller aspect ratio of carbon black which results in less physical interaction between the filler clusters, giving an isotropic conductive network after annealing. In the entangled network (see Table 3 and Fig. 2) formed by MWNTs due to its large aspect ratio (near 150 in this study) , the MWNTs bundles are believed to be under strain (possibly different strain for different parts of the bundle) after solid state drawing as they were deformed during stretching. When the polymer is annealed above its melting temperature, the constrained MWNT bundles tend to return to their original state as the polymer around them allows them more mobility during annealing. Therefore, they become locally random oriented, but in large scale they are still oriented to give exceptional electrical properties in the drawing direction.

EXAMPLE A2 - EFFECT OF FILLER CONTENT

Resistivity was measured for MWNT/co-PP composites with a draw ratio of 8 over a variety of filler contents . The annealing conditions were 155 ⁰C for 15 mins . The results are shown in Table 4:

TABLE 4

Resistivity in [Ohm.in]

It can be seen from these results that annealing has reduced significantly the resistivity of highly oriented tapes containing different amounts of MMNTs. Consequently, the percolation threshold of the oriented tape is reduced significantly.

EXAMPLE A3 - EFFECT OF DRAW RATIO

Resistivity was measured for 5.3 MWNT/co-PP tape and 10 wt.%, 15 wt. % and 20 wt . % CB/co-PP tapes over a variety of draw ratios. Samples were thermally annealed at 155 ⁰C for 15 mins. Results are shown in Tables 5 and 6:

TABLE 5

TABLE 6

EXAMPLE A3a - EFFECT OF DRAW RATIO WITH ONLINE MONITORING OF ELECTRICAL PROPERTIES Resistivity was measured for 5.3 MWNT/co-PP tape over a variety of draw ratios. Samples were thermally annealed at 165 ⁰C. Electrical properties were monitored online during annealing. Results are shown in Table 6a:

TABLE 6a

It can be seen from the results of Examples A3 and A3a that the effects of the annealing process on resistivity apply for the range of draw ratios.

EXAMPLE A4 - EFFECT OF AlSINEiUiING TEMPERATURE

Resistivity was measured for 2.3 wt . % MWNT/co-PP tape with a draw ratio of 24 and an annealing time of 15 mins at different annealing temperatures. Results are shown in Table 7: TABLE 7

EXAHPLE A4a - EFFECT OF ANNEALING TEMPERATURE WITH ONLINE MONITORING OF ELECTRICAL PROPERTIES

Resistivity was measured for a 5.3 wt . % MWNT/co-PP tape with a draw ratio of 21. Samples were thermally annealed. Electrical properties were monitored online during annealing. Results are shown in Table 7a:

TABLE 7a

The results of thermal annealing on several 5.3 wt.% MWNT/co-PP tapes with a draw ratio of 20 during annealing were measured. Annealing was conducted at different temperatures for each of these tapes. The results are shown in Table 7b.

TABLE 7b

2 Q O

The results of Examples A4 and A4a show that the effect of annealing is more pronounced at higher temperatures . Annealing can be conducted at a relative low- temperature (above than the melting temperature of the first polymeric material) for a relatively long period of time; or, alternatively, annealing can be conducted at a higher temperature for a relatively short period of time.

10 EXAMPLE A5 - ANISOTROPY STUDY

An anisotropy study was conducted on the conductive network formed by MWNTs {5.3 wt% MWNT/co-PP) or carbon black (10 wt% CB/co-PP) . Anisotropy index (RT/RL) is defined as:

I b R-Transverse / R-Longitudinal ' Where R-Transverse and

are the resistivity of the CPC at transverse and longitudinal direction to the drawing direction, respectively. The higher the RT/RL, the more anisotropy the network has. The results are shown in Tables 8 and 9 :

TABLE 8

MWNT/co-PP

TABLE 9

CB/CO-PP

As expected, the anisotropy of tape from CPC loaded with carbon black was decreased by the annealing process. RT/RL after drawing and annealing is approaching 1, indicating that the composite was nearly isotropic.

By contrast, RT/RL after drawing and annealing for CPC loaded with MWNTs only dropped from 100 to 10 at draw ratio 6, showing that some anisotropy remains in the network after annealing.

Thus, the electrical anisotropy study confirmed the morphological study (Example Al) : the network constructed by carbon black showed some anisotropy after solid drawing, but decreased to nearly isotropic after annealing; the network formed by MWNTs showed significant anisotropy in solid drawn tape, and some anisotropy was kept after annealing due to orientated MWNT bundles in large scale being maintained.

EXAMPLE A6 - INVESTIGATION OF PERCOLATION THRESHOLD

The effect of solid state drawing and annealing (draw ratio = 8) on the percolation threshold of CPC was investigated.

The results are shown in Tables 10 and 11:

TABLE 10 - MWNT/co-PP composites NC = Not Conductive

TABLE 11 - CB/co-PP composites NC = Not Conductive

In the MWNTs/co-PP system, the percolation threshold is increased from between 1.44 wt . % and 1.82 wt . % to between 3.1 wt% and 5 wt% after solid drawing, but is decreased to between 1.15 wt% to 1.44 wt . % after the drawn tape is annealed, as a sudden drop of resistivity is observed between these filler contents. Thus, the resistivity of drawn and annealed tape is even lower than the resistivity of isotropic CPC. By contrast, the percolation threshold of the carbon black/co-PP system increases from near 3 wt% to 10 wt% by solid drawing, and thermal annealing brings the percolation threshold back to 3 wt%. The resistivity of the drawn and annealed tape is the same as isotropic film. However, in both the cases shown in Table 10 and 11, a significant drop in resistivity is observed for solid state drawn tapes after annealing.

EXAMPLE A7 - MECHANICAL PROPERTIES

The mechanical properties of the co-extruded tape are mainly contributed by the middle neat polymer layer. The outer CPC layers are expected to have very low mechanical properties comparing with the highly orientated PP tape, because they have been annealed above their melting temperature, so that the orientation of the polymer chain is reduced significantly.

Therefore, the mechanical properties of highly drawn (draw ratio = 20) neat PP drawn tape were investigated before and after annealing. The results are shown in Table 12:

TABLE 12

Solid drawn Annealing at Annealing at 165 ^aC for 165 ^aC for 1000 seconds 4000 seconds

Young ' s 10. 53 + 0. 38 9.7678 ±0.41 9.36±1.0 modulus [GPa]

Tensile 471 .6± 23 .85 425.2± 6.89 434.6±28.38 strength [MPa]

Strain at 8.7 ±0. 49 9.6 + 1. 59 12.33±1.38 break [%]

This shows a minor decrease in Young's modulus and ultimate tensile strength (UTS) after annealing. A slight increase in strain at break is also observed.

EXAMPLE B: SEMI-INDUSTRY SCALE EXAMPLE

Materials

A 10 wt% MWNT/co-PP master batch was provided by Nanocyl S. A. The master batch was prepared by mixing co-PP

(as above) with NC 7000 MWNTs (as above) in a twin-screw extruder at 70 rpm and 220 ⁰C. 2 wt% MWNT/co-PP and 5 wt%

MWNT/co-PP were diluted at 100 rpm and 220 ⁰C from the master batch. NC 7000 MWNTs (as above) were mixed with LDPE, co-PET, CO-PA6 and PMMA in a twin-screw extruder at an initial content of 10 wt . % to make respective master batches. The conditions used were 70 rpm and 200 ⁰C for LDPE, 70 rpm and 160 ⁰C for co-PET, 70 rpm and 230 ⁰C for co-PA6, 70 rpm and 240 ⁰C for PMMA. These materials were diluted to lowered concentrations at 100 rpm at the same temperatures (as described above), respectively. The polymers used here are oven dried for 24 h before processing or compounding at 50 ⁰C for 5 wt. % MWNT/LDPE, 5 wt . % MWNT/co-PET, 5 wt . % MWNT/co- PA6; 80 ⁰C for PET, PA6 , PC and PMMA.

HDPE was HDPE VL4580 produced by Borealis A/S. LDPE was LDPE 352E produced by The DOW Chemical Company.

Co-PET was Griltex D1616E, a co-polyester hotmelt adhesive.

C0-PA6 was Pebax 5533 SA 01, a thermoplastic elastomer made of flexible polyether and rigid polyamide. PET was Futura PET resin- 1.1 lv.

PA6 was Ultramid B33L from BASF.

The polycarbonate (PC) was Calibre 201-10 from The DOW Chemical Company.

PMMA was Altuglas V825T 101 produced by Altuglas International of Arkema Inc.

Method

A set of extruders and co-extrusion tape die (with a width of 100 mm and thickness of 3 mm) produced by Dr. Collin GmbH were used to make A:B, A:B:A and A:B:C type tapes. Three single-screw extruders (2 TEACH-LINE E20T SCD15 and 1 SCDl5 equipped with a melt pump) were connected to the co-extrusion die directly through three heated melt pipes. Temperature control was realised by adding thermal couples to monitor the temperature for different heating zones through the extruders. A melt pump was used between one of the single-screw extruder and the co-extrusion die to control the melt pressure in the die as well as to have more thermal couples available. A temperature controlled pick up roller (TEACH-LINE CR72T) was used to collect the as-spun film or tapes . Co-extrusion was conducted under different temperatures for different polymers. As these three single extruders were the input for the co-extrusion die, they were set at suitable temperatures for different polymers or compound, but the co-extrusion die was set to a temperature which allows the polymer with the highest melting temperature to flow smoothly (for example in the A:B:A tape of MWNT/co- PA6 :PA6 :MWNT/CO-PA6, the temperature of the die was set to 240 ⁰C to allow good flow of PA6, so the temperature of the die was much higher than the melting temperature peak of the MWNT/CO-PA6 which is 165 ⁰C according to DSC measurement) . As a result, the as-extruded tapes had relatively low resistivity because they were already annealed to some extent by the high temperature die during extrusion. The thickness ratio of the co-extruded tape was controlled by the adjusting the extrusion speed of the single screw extruders or the melt pump output. The component of the tape was changed by simply changing input materials in the extruders . The as extruded film or tape was produced by collecting film or tapes from the co-extrusion die to a pick up roller, which typically consisted of several temperature controlled rollers. The film or tape collected here was considered as extruded film or tape with a draw ratio of 1. In the case of polymer blends between 5 wt . % MWNT/LDPE and PA6 , the blends were compounded in the twin screw extruder under conditions of 30 rpm and 240 ⁰C, then added into the single screw extruder equipped with melt pump (above) and die (diameter of 2 mm), then spun at 240 ⁰C. Multi-component as-spun fibre or yarn/fibre (with circular cross-section) was collected by the pick up roller (above) . Drawing was conducted on a drawing line (TEACH-LINE MDO-BT and MDO-A) produced by Dr. Collin GmbH. The rollers were water cooled and heated with high pressure water.

Annealing was conducted in an environmental chamber equipment with the Instron machine 5584 as described before.

The specimens were typically mechanically constrained and annealed for 20-30 mins in the oven at different suitable temperatures for different polymers as stated below.

Electrical measurement, mechanical test, SEM, DSC and TGA were conducted as described above in example A. TGA was used to determine the actual content of conductive filler in the material. However, the temperature scan range of DSC measurement for various polymers were different: 20 ⁰C to

200 ⁰C for LDPE (with or without MWNTs) , HDPE, co-PA6 (with or without MWNTs), co-PET (with or without MWNTs), PMMA

(with or without MWNTs) and PC; 20 ⁰C to 280 ⁰C for PA6 and

PET.

EXAMPLE Bl - INITIAL STUDY

DSC study showed similar melting temperature for the MWNT/co-PP as in example A.

Extrusion followed by 2 stage drawing (120 ^aC/draw ratio = 6 then 120-140 ^eC/draw ratio = 1.33) was used to prepare 1:4:1 tapes of MWNT/co-PP: PP:MWNT/co-PP, thickness 0.08-0.09 mm. Annealing was carried out at 165⁰C for 30 mins. MWNT contents as determined by TGA were 5.58 wt% and 2.4 wt% .

The results are shown in Table 13 :

TABLE 13

Little difference was found after drawing and annealing between extruded samples (Example B) and hot pressed samples (Example A) for MWNT content from 2 wt% . Thus, the thermal history of the tape appears to have a very small effect on the final resistivity. However, extruded samples were found to have a lower resistivity before annealing.

The 5 wt. % MWNT/co-PP as extruded tape or film (draw ratio of 1) was annealed at different temperatures with online monitoring of electrical properties during annealing. Results are shown in Table 14:

TABLE 14

Annealing Annealing target Annealing target time temperature 1 65 ⁰C temperature 180 ⁰C

[Seconds]

Mechanical properties of the 5 wt% MWNT/co-PP co- extruded tape with a draw ratio of 8 were investigated. Results are shown in Table 15 :

TABLE 15

Again, this confirmed that mechanical properties were little affected by annealing.

EXAMPLE B2 - DIFFERENT POLYMER MATERIALS

The melting temperature was studied by DSC for different polymers, with results as follows:

Melting temperature (onset/start of melting peak) :

50 ⁰C for LDPE (70 ⁰C for MWNT/LDPE) , 60 ⁰C for co-PET and MWNT/co-PET, 80 ⁰C for co-PA6 and MWNT/co-PA6, 100 ⁰C for HDPE, 220 ⁰C for PET, 200 ⁰C for PA6.

Melting peak:

110 ⁰C for LDPE (117 ⁰C for MWNT/LDPE) , 95 ⁰C for co-PET and MWNT/co-PET, 165 ⁰C for co-PA6 and MWNT/co-PAβ, 145 ⁰C for HDPE, 253 ⁰C for PET, 223 ⁰C for PA6.

The glass transition temperature (T_g) is 104 ⁰C and 170 0C for the PMMA and PC, respectively, according to DSC study.

Various multi-component tapes or yarns/ fibres were prepared. The results are shown in Tables 16 onwards. Annealing was found to decrease the resistivity of the oriented tapes significantly. However, the mechanical properties of the tapes were tested and found to be little affected by annealing.

TABLE 16

5 wt. % MWNT/CO-PA6 : PA6 : 5 Wt . % MWNT/CO-PA6. Tape structure: 1:4:1, Dr=4.19, Drawing temperature 140 ⁰C Annealing at 170 ⁰C for 20 mins Annealing at 200 ⁰C for 30 mins

TABLE 17

5 wt. % MWNT/CO-PA6 : PA6 : 5 Wt . % MWNT/CO-PA6. Tape structure: 1:48:1, Dr=4.26, Drawing temperature 140 ⁰C

TABLE 17a

TABLE 17b

TABLE 18

5 wt. % MWNT/ CO-PET : PET Tape structure 1:10, Dr=S.27, Drawing at 80 ⁰C

TABLE 19

5 wt. % MWNT/CO-PA6 : PET : 5 wt . % MWNT/CO-PA6 Tape structure 1:10:1, Dr=4.85, 70°C

TABLE 20

5 Wt. % MWNT/LDPE : PP Tape structure 1:4, Dr=8, Drawing at 120 ⁰C

TABLE 21

3.5 wt. % MWNT/LDPE : PA6

Tape structure 1:2, Dr=5.8, 120 ⁰C

TABLE 22

5 wt. % MWNT/co-PΞT : PET : 5 wt . % MWNT/LDPE Tape structure 1:10:1, Dr=5.19, Drawing at 80 ⁰C

TABLE 23

5 wt. % MWNT/co-PET : PET : 2 Wt . % MWNT/CO-PA6 Tape structure 1:10:1, Dr=5.76, Drawing at 80⁰C

TABLE 24 5 wt. % MWNT/PMMA : PC

Tape structure 1:2, Dr=4, 120 ⁰C

The results from different polymers combinations (example B2 ) show that annealing reduces the resistivity of oriented tapes consisting of a wide range of polymers, including polyester, polyamide, polyolefin and amorphous polymers. Tapes including two or three polymers coming from different polymer types have been tested. The mechanical properties of the whole tape are largely retained after annealing, indicating that the polymer orientation in the middle layer is not significantly reduced. Therefore, it is expected that the results would be applicable to any polymer type.

EXAMPLE B3 - CONDUCTIVE YARN/FIBRE PRODUCED FROM POLYMER BLENDS

Yarn/ fibre was produced from 5 wt.% MWNT/LDPE: PA6 = 50:50 by weight, draw ratio of 4.5, drawing temperature of 80 ⁰C, annealing conducted at 150 ⁰C for 20 mins. The results are shown in Table 25: TABLE 25

According to SEM study on the cold fractured surface of the cross-section of the yarn/ fibre, a continuous phase was observed for 5 wt . % MWNT/LDPE, as the non conductive phase (PA6) appeared brighter under SEM at contrast mode than the conductive phase. Annealing was found to reduce the resistivity of oriented multi-component yarn/fibre significantly.

The tapes or yarns /fibres of the Examples have the following advantages: good mechanical properties; - good electrical properties,- ease of production; good recyclability.

Good mechanical properties are provided by the neat polymer component. Solid state drawing orients this component to give good mechanical properties. Annealing takes place below the melting temperature peak of this layer, so that the orientation is not disturbed.

Good electrical properties are provided by the CPC component (s) . Drawing worsens the electrical properties by disturbing the conductive network of MWNTs, but this is overcome by subsequent annealing. It was a surprising finding of the present inventors that after annealing the electrical properties are better than before drawing and annealing. The low percolation thresholds mean that only a small amount of MWNTs is needed.

By comparison of the MWNT and carbon black examples, it can be seen that electrical properties after annealing are particularly good for MWNTs. For carbon black, electrical properties are improved after annealing but are typically about the same as before drawing and annealing. Without wishing to be bound by this theory, the inventors believe that this is because of the higher aspect ratio of MWNTs .

The material can potentially be produced on a large scale, as demonstrated by Example B. A continuous process using co-extrusion and inline annealing could potentially be used.

Good mechanical properties are often provided by using fibres of a different material to the host polymer e.g. glass fibres in polypropylene. These make it very difficult to recycle the material in which they are incorporated. In the examples, the core layer is of a single polymer material. In addition, in many of the examples the polymers of the core and CPC layer are sufficiently similar that the tape qualifies as a mono-component material (such as co-PP/PP, LDPE/HDPE, co-PET/PET, CO-PA6/PA6) , which is an advantage in recycling because the material can be simply re-melted and reused again. The small amount of conductive filler remaining in the co-polymer layer, leading to even smaller amounts of conductive filler in the overall tape, has no significant effect on recyclability . The CPC components are capable of welding multiple tapes together to make composites.

Whilst the invention has been described with reference to the Examples, it will be appreciated that modifications are possible within the scope of the invention.

References

I. Ramasubramaniam R, Chen J, and Liu HY. Applied Physics Letters 2003 ; 83 : 2928-2930. 2. Stauffer D and Aharony A. introduction to percolation theory: Taylor & Francis, 1985.

3. Celzard A, McRae E, Deleuze C, Dufort M, Furdin G, and Mareche JF. Physical Review B 1996 ; 53 : 6209-6214.

4. Bin Y, Mine M, Ai K, Jiang X, and Masaru M. Polymer 2006;47:1308-1317.

5. Potschke P₇ Dudkin S, and Alig I. Polymer 2003 ; 44 : 5023- 5030.

6. Sandler JKW, Kirk JE, Kinloch IA, Shaffer MSP, and Windle AH. Polymer 2003 ; 44 : 5893-5899. 7. Probst N. Conducting carbon black. In: Donnet J. B. BRC, Wang M., editor. Carbon black Science and Technology, 2nd ed. New York: Marcel Dekker, 1993. pp. 271-288. 8. Alig I, Lellinger D, Dudkin SM, and Potschke P. Polymer 2007 ,-48:1020-1029. 9. Alig I, Skipa T, Engel M, Lellinger D, Pegel S, and

Potschke P. Phys . stat. sol. 2007; 244 {11) : 4223-4226. 10. Deng H, Zhang R, Bilotti E, Peijs T, and Loos J. Journal of applied polymer science 2008; Accepted.

II. Villmow T, Pegel S, Potschke P, and Wagenknecht U. Polymer 2008 ; 68 : 777-789.

12. Sumita M, Abe H, Kayaki H, and Miyasaka K. Journal of Macroraolecular Science and Physics 1986 ,-25:171-184.

13. Asai S and Sumita M. Journal of Macromolecular Science and Physics 1995 ; 34 : 283-294. 14. Wu GZ, Asai S, Zhang C, Miura T, and Sumita M. Journal of applied physics 2000; 88 : 1480-1487.

15. Bin Y, Kitanaka M, Zhu D, and Matsuo M. Macromolecules 2003;36:6213-6219.

16. Bin Y, Chen QY, Tashiro K, and Matsuo M. Physical Review B 2008; 77 : 035419.

17. Miaudet P, Bartholome C, Derre A, Maugey M, Sigaud G, Zakri C, and Poulin P. Polymer 2007 ; 48 : 4068-4074.

18. Loos J, Alexeev A, Grossiord N, Koning CE, and Regev O. Ultramicroscopy 2005 ,-104:160-167. 19. Munson-McGee SH. Physical Review B 1991 ; 43 : 3331-3336. ANNEX TO DESCRIPTION

CONDUCTIVE POLYMER COMPOSITE WITH REDUCED

ELECTRICAL RESISTIVITY BY THERMAL OR CHEMICAL TREATMENT

Abstract

This study describes a method fabricating mechanically strong and conductive polymer/MWNTs tape, which consists of melt compounding, drawing and annealing process. Highly oriented carbon nanotube structure is observed in highly drawn composites tape, the annealing process after drawing has relaxed the structure into orientated hairy MWNTs bundle. This structure is even more conductive than the random dispersed isotropic film from the concentration. It has shown for the first time that the orientated MWNTs conductive network in polymer fibre/tape has contribute more network to drawing direction, which makes it more conductive at that direction than the isotropic film before drawing. The percolation threshold in conductive polymer tape is decreased from near 5.03 wt% to 1.51 wt% without sacrifice the mechanical properties by annealing. It is the lowest in literature for highly orientated polymer tape according to our knowledge. This method has solved the problem of losing conductive network when polymer/MWNTs undergo deformation during fibre/ tape processing. It is expected to be useful for conductive nanotube composites tape application.

Introduction

Conductive polymer composites (CPC) based on carbon nanotubes have attracted much attention in last few years.

It is well known that CPC can be made by adding conductive filler into insulating polymer matrix. It has been demonstrated that a sudden jump of conductivity can be reached when a critical loading of conductive filler is added into the matrix [1] . This phenomenon is described as percolation threshold. The percolation threshold of fibre reinforced CPC have been shown both experimentally and theoretically that decrease with the filler aspect ratio [2, 3] . Carbon nanotubes have been extensively investigated since 1991 [4-8]. It has been shown that CPC based on MWNTs can reach percolation threshold as low as 0.0025 wt% in epoxy matrix [9] . Though, melt compounding polymer with MWNTs have only reached percolation threshold of 1.44 wt% [10] . It is still relatively low comparing with conventional conductive filler where the percolation threshold is between 20 wt% to 30 wt%.

In order to process CPC containing carbon nanotubes into desired shape, CPC has to undergo processing, such as: injection moulding, spinning or stretching. The conducting network is deformed to different extent during these processing methods. Some work have been carried out to investigate the influence of deformed conducting network on the conductivity [11-13] . Inhomogeneous conductive network was observed for injection molded sample, it is due to shear flow during processing [13] . To simulate the shear during the extrusion or injection molding, Alig I., et al . [11, 12] has performed study to investigate the influence of shear on the conductivity of CPC in the melt. It is shown that the conductivity was lost when the network is deformed, but it slowly comes back after the shear is stopped and CPC is annealed in the melt. It is explained by re-aggregation of MWNTs in the melt. But the time needed to reform the network is nearly 3 hours, which is too long for extrusion or injection moulding process. It is also shown that certain amount of filler content is vital to overcome the deformation during extruding or injection molding [11] .

Highly orientated polymer tape/ fibre have been investigated extensively since 1970s [14] . Stretching CNTs polymer composites was firstly considered by Ajayan et al . [15] as a method to align CNTs in the same direction. Orientated CNTs is believed to be more efficient at mechanically reinforcing composites than they do in bulk composites, successful examples have been review in [4-6, 8] . Orientated CPC tape or fibre have also been studied [16-19] . During the drawing or spinning process of CNTs polymer composites, the lowest resistivity occurs for slightly aligned, rather than isotropic CNTs network as studied by Du F.M. et al . [18] . It has been shown that certain amount of filler is vital in order to keep the conducting network during stretching [17, 19] . The resistivity of the tape/fibre has been shown increasing with increasing draw ratio, and the anisotropy of the conductive network is pronounced during stretching. More interestingly, high aspect ratio filler such as: MWNTs, is found to be more efficient to keep the conductive network during stretching than low aspect ratio filler such as: carbon black [17]. Morphological study has shown highly orientated carbon nanotube network can be obtained by stretching the CPC to high drawn ratio [17] . Lately, Miaudet P. et al . [20] carried out an investigation on the effect thermo treatment on conductivity of CPC fibre containing PVA and MWNTs. It is found that thermo annealing near the glass transition temperature of the polymer have dramatically increased the conductivity of the fibre. XRD study has shown that the increased conductivity might due to the increased mobility of polymer chain, it can leads to mobility of carbon nanotube which could increase the quality and the density of nanotube contacts.

The purpose of this work is to study the effect of thermal annealing on the conductivity and mechanical properties of CPC tape containing carbon nanotube or carbon black . Sandwich structure tape is used in this study; CPC is only applied on the outer layer, another layer of neat polymer is in the middle part. The neat polymer has higher melting temperature than the CPC, which allows the thermal annealing to be conducted above the melting temperature of CPC. The effect of thermal annealing on the resistivity of CPC was studied. Morphological study has shown highly orientated carbon nanotube bundles in solid drawn tape, some interesting orientated hairy bundles was found in drawn and annealed tape. It has been shown that the hairy orientated carbon nanotube bundles have decreased the resistivity of the tape dramatically. Therefore, the percolation threshold was decreased due to the orientated hairy bundles structure MWNTs have formed. Anisotropy of the highly orientated bundle and hairy bundle conductive network has been studied. CPC contains carbon black is investigated for comparison. In addition, mechanical properties of highly orientated neat PP tape are studied before and after the annealing, no significant decrease is found. It is expected to be useful for conductive nanotube composites tape application.

Materials and methods The MWNTs Nanocyl^® 7000 is kindly provided by Nanocyl S. A. (Belgium) . The surface area of MWNTs is 250-300 m²/g according the data sheet. Polypropylene used for middle layer in sandwich tape is Dow H507 (MFI=3.2 g/10mins} from DOW Chemical Ltd. The polymer used in outer layer is PP copolymer ( co-PP} Adsyl^® 5C39F from Basell . The conductive carbon black (CB) used is Printex^® XE-2 which is supplied by Grolman Ltd. Characterization study of several carbon blacks including Printex^® XE-2 was carried out by Pantea D. et al . in [21] , it has shown Printex^® XE-2 has comprehensive conductivity comparing with others. It has surface area of 910 m²/g. MWNTs or CB is melt blended with co-PP in mini- extruder {DSM Micro 15) at 200⁰C₇ 200 rpm for 10 minutes. The extruded rod is then hot pressed into film in the thickness of 150 μm at 200⁰C for 5 minutes. The filler content of the CPC is measured by TGA (Thermo gravimetric analysis) . Two layers of CPC film with co-PP/MWNTs {or carbon black) and one layer of PP film are then pressed into sandwich structure at 155°C with PP film in the middle. Similar co-extruded tape has been investigated extensively for all -polypropylene composites [22-29] . The sandwich tape is then hot drawn into tape at different draw ratio at 120⁰C. Thermal annealing is conducted on mechanically constrained tape at 155°C for 15 minutes.

Morphology study

Morphology studies are carried out on JEOL JSM-6300F SEM in order to investigate the informing of conductive network during tape fabrication and after annealing. High accelerating voltage is applied for SEM study, MWNTs or CB in the polymer matrix material is charged and give an enriched secondary-electron as demonstrated by Loos J. et al. in [30] . TEM imaging of cross-sectional cut samples was performed. The as-prepared nanocomposite samples were sectioned at room temperature using an ultra-microtome (Reichert-Jung Ultracut E) . The TEM was operated in bright-field mode at 80 kv to increase the contrast between MWNTs and the surrounding polymer matrix.

Electrical measurement

DC electrical resistivity is measured for the composites at different stage of processing. Two points method with voltage scan from 1 volt to 10 volt is conducted in resistivity measurement, silver paint is applied on both ends of the sample to ensure contact. Thus, contact resistance is much smaller comparing with sample resistance. The dimension of the sample is not uniform in this study due to the thickness and width varies with draw ratio. The resistances are measured by an Agilent 6614C programmable voltage source in combination with a Keithley 6485 picoammeter. All the equipments are interfaced with a computer to record the I -V curve in order to calculate the resistivity. The I-V curves are noticed to be linear in this study. For the specimen has resistivity higher than 10⁷ Ohm.m {isotropic 1.51 wt% MWNTs/co-PP and 3 wt% carbon black/co-PP) , the electrodes are applied on the top and bottle surface of the film instead of on the ends to obtain measurable resistance.

Results and discussion

The sandwich tape used in this study is shown in Figure 1. CPC is applied on the surface layer of the tape. Neat PP in the middle part has higher melting temperature than CPC. This allows three different processes to be conducted: anneal the isotropic CPC in the melt, draw the CPC in the melt, draw the CPC in the solid state and anneal it in the melt. Table 1-3 have shown the resistivity results of the tapes have undergone three different processes respectively. Dramatic resistivity decrease is observed after the isotropic tape is annealed in the melt as shown in Table 1. This could be explained by re-aggregation of MiAfMTs in the melt as demonstrated by Alig I. et al . [12] . Table 2 has shown the resistivity of the tape before and after melt drawing in which the CPC is drawn in the melt with PP substrate in the middle layer of the tape. The CPC specimens have become insulate after melt drawing. Morphological study have found rough surface on the tape (Figure 2a) , and randomly distributed MWNTs under SEM (Figure 3b) . The melt drawn CPC could be considered as similar system as the case investigated in [12] . The CPC have lost its conductivity after deformation is applied on it in the melt. Most interestingly, the isotropic CPC specimens have lost their conductivity after solid drawing, but the annealing after solid drawing has made them even more conductive than the original isotropic film. This could solve the problem reported in literature [13, 16, 17, 19], where resistivity is decreased dramatically when MWNTs is orientated by injection molding or drawing. The annealing process has brought back the conductive network after large deformation.

Orienting MWNTs has been considered as a method to increase the mechanical reinforcing efficiency in polymer composites [5] . It is thought that orientated MWNTs should be able to increasing the conductivity along its deformation direction, because more MWNTs should be contributing the network after the deformation. Theoretical study on percolation in anisotropic network has been done by Munson-McGee S. H. in

[3] , cylinder shape like filler is considered in 3D system, planar system and orientated system. In the orientated system, the radius of the cylinder is shown to be very important, (when the length of the filler is fixed) to the probability of two cylinders intersecting in a unit volume.

The probability could be considered as the ability of fillers forming conductive network. The probability increases linearly with increasing radius in aligned system. The aligned MWNTs bundles in solid drawn tape (as shown in

Figure 3c) can be considered as aligned cylinders in the system, where the radius is close to the diameter of MWNTs bundles. The diameter of bundles has increased dramatically

(as shown in Figure 3 d in SEM and Figure 4 in TEM) after annealing. It is due to the random orientation of MWNTs in the bundle, but the structure in large scale still remain orientated as shown in Figure 3d (in the inset) . It is the first time in literature to report similar nanotube structure has similar electrical property according to our knowledge. It is very interesting that the nanofillers can be organized into highly conductive and orientated structure by means of simple drawing and annealing process.

To study the morphology and electrical resistivity of this interesting conductive network structure; investigations on the informing of conductive network during different processing stages have been carried out. Carbon black is used as conductive filler for comparison due to it has much smaller aspect ratio than MWNTs has. Aspect ratio of the filler has played an important role during the drawing process as shown in literature [16, 17] , 5.03 wt% of MWNTs is added into co-PP to make CPC. Figure 5 and 6 have shown the morphological study and resistivity study on the CPC at different processing stages. The extruded strand has shown some orientation of MWNTs on the surface of the sample due to shear while extrusion (Figure 5a) . The hot pressed film obtained randomly isotropic dispersed MWWTs bundles network as shown in Figure 5b. Highly orientated MWNTs bundles are achieved by drawn isotropic film at solid state, and hairy orientated bundles are observed in drawn and annealed tape (Figure 5 c and d) . DC resistivity study on different specimens from stated processing stages are presented in Figure 6. Drawn and annealed sample has the lowest resistivity among all. The annealing procedure has dropped the resistivity more than 3 orders of magnitude. Figure 7 and 8 have shown the morphological study and resistivity study on the CPC loaded with carbon black to compare with the CPC with MWNTs. Orientated carbon black structure has been shown in highly drawn tape, and isotropic carbon black network are obtained in the rest of the processing stage as shown in Figure 7. Highly drawn CPC loaded with carbon black is not conductive. But the resistivity of drawn and annealed tape has returned to the same level as hot pressed film. It can be understood as the orientated CPC has returned to isotropic state, no difference between hot pressed film and drawn & annealed is obtained due to the small aspect ratio of carbon black which results in an isotropic conductive network after annealing.

The anisotropy study on the conductive network formed by MWNTs or carbon black is shown in Figure 9. RT/RL is defined aS : R-Transverse/R-Longitudinal / where R-Transverse ^^d R-Longi tudmal are the resistivity of the CPC at transverse and longitudinal direction to the drawing direction respectively. It is considered as an index to describe the anisotropy of the network. The higher the RT/RL is, the more anisotropy the network has. Thus, the anisotropy of tape from CPC loaded with carbon black has been decreased after annealing process. RT/RL is approaching 1 after anneal indicating that the composites is nearly isotropic as shown in Figure 9a. But the RT/RL from CPC loaded with MWNTs only drop from 100 to 10 at draw ratio 6, which has shown some remained anisotropy in the network after annealing. The electrical anisotropy study has confirmed the morphological study shown in Figure 3-5: the network constructed by carbon black has shown some anisotropy after solid drawing, but it was decreased to nearly isotropic after annealing; the network formed by MWNTs has show significant anisotropy in solid drawn tape, some anisotropy is kept after annealing due to orientated MWNTs bundles in large scale is maintained.

The effect of solid drawing (draw ratio 8) and annealing on the percolation threshold of CPC investigated in this study is shown in Figure 10. In MWNTs/co-PP system, the percolation threshold is increased from 1.51 wt% to near 5 wt% after solid drawing, but it is decreased to 1.51 wt% after the drawn tape is annealed. The resistivity of drawn and annealed tape is even lower than the resistivity of isotropic CPC. It shows the interesting orientated hairy MWNTs bundles have effectively contribute more MWNTs to the conductive network at drawing direction. The percolation threshold of carbon black/co-PP system has increased from near 3 wt% to 10 wt% by solid drawing, and the thermal annealing has brought the percolation threshold back to 3 wt%. The resistivity of the drawn and annealed tape has reached the same value as isotropic film. Due to their low aspect ratio, the network form by carbon black is not showing any significant orientation after solid drawing and annealing.

According to classical percolation theory, the increasing conductivity of composites materials with increasing conductive filler content can be described by a scaling law: σ = σ_o(P-P_cY Equation 1 P_c is the percolation threshold [1] . The exponent t is an exponent which depends on the dimensionality of the conductive network. It is expected to vary with different materials with calculated values of t~l3 and t ~ 2.0 in two and three dimensions respectively.

The percolation threshold and exponent value t from carbon black/co-PP are calculated according to Equation 1: Pc=2.99 wt%, t=2 for isotropic system and Pc=9.98 wt%, t=1.3 for solid drawn (draw ratio=8) system. It is fitted with the experimental data as shown in Figure 11. It is clear that the exponent t has decreased from 2 to 1.3 when the composites containing CB is drawn from isotropic to draw ratio 8. It indicates the conductive network has been stretched from random 3 D to 2 D network according to classical percolation theory. The percolation threshold has increased from 2.99 wt% to 9,98 wt% when the composites contain CB is drawn from isotropic to draw ratio 8. The deformation of the conductive network has dramatically increased the percolation threshold. The percolation threshold and exponent t is 1.51 wt% and 3 respectively for isotropic MWNTs /co-PP system as shown in Figure 12a. The t value is higher than the one calculated by [1] for 3D random network, but it is still within mean filed theory [31] . For the drawn and annealed MWNTs /co-PP system, the percolation threshold and exponent t values are 1.51 wt% and 1.3 respectively. The t value fits with the universal scaling value for 2D conductive network as shown in Figure 12b. It has confirmed the SEM, TEM observation and anisotropy study shown in Figure 3d, Figure 4 and Figure 9: 2D conductive network in large scale, but it remains random locally to ensure good contact between MWNTs bundles.

It is the purpose of this study to fabricate mechanically strong conductive polymer tape. Therefore, the mechanical properties of neat PP drawn tape is investigated before and after annealing as shown in Figure 13. It has shown minor decrease on Young's modulus and ultimate tensile strength (UTS) after annealing. Slight increase on strain at break is also observed. The mechanical property of co-extruded tape is mainly contributed by the middle neat polymer layer. As the out CPC layers are expected to have very low mechanical properties comparing with highly orientated PP tape, because they have been melted to isotropic after annealing above their melting temperature. The outer layers are not taking into account regarding to the mechanical properties because the thickness of outer layers have not been optimized in this study in order to maximize the mechanical properties and keep the electrical conductivity of the three layer structure. Large scale experiment is being carried out to study the industrial feasibility of conductive polymer tape.

Conclusion The effect of thermal annealing on the conductivity and mechanical properties of CPC tape containing carbon nanotube or carbon black is investigated in this paper. Sandwich structure tape is used in this study to conduct annealing above the melt temperature of CPC. Morphological study has shown highly orientated carbon nanotube bundles in solid drawn tape, interesting orientated hairy bundles was found in drawn and annealed tape. It has been shown that the hairy orientated carbon nanotube bundles have decreased the resistivity of the tape dramatically. The percolation threshold is decreased from near 5.03 wt% to 1.51 wt% in orientated tape due to the orientated hairy bundles structure MWNTs have formed. Anisotropy of the highly orientated bundle and hairy bundle conductive network has been studied. CPC contains carbon black is investigated for comparison, they have shown similar recovery of conductive network after drawing, but no orientated structure was found after annealing. In addition, mechanical properties of highly orientated neat PP tape are studied before and after the annealing, no significant decrease is found. Further more, large scale experiment is being carried out to study the industrial feasibility of conductive polymer tape. The effect of dispersion quality on the forming of orientated hairy MWNTs structure is also under investigation to minimize the percolation threshold in 2D conductive network.

Table 1 : Effect of annealing on resistivity of isotropic MWNTs/co-PP composites film

Table 2 : The effect of melt drawn on resistivity of MWNTs/co-PP composites film

Table 3 : The effect of solid drawn and annealing on isotropic MWNTs/co-PP film

Claims

1. A method of preparing a conductive multi-component material, comprising the steps of: preparing an initial multi -component material, the initial multi-component material including a continuous first polymeric component comprising conductive filler and a first polymer, and a second polymeric component comprising a second polymer, the first polymeric component having a lower melting temperature than the second polymeric component; subjecting the initial multi-component material to an orienting process,- and annealing the oriented multi-component material to form the conductive multi-component material.

2. A method as claimed in Claim 1, wherein the multi- component material is a multi -layer material, wherein the first polymeric component is a first polymeric layer and the second polymeric component is a second polymeric layer.

3. A method as claimed in Claim 1 or Claim 2, wherein the first polymeric component contains conductive filler in an amount of 1 to 20 wt% based on weight of the first polymeric component.

4. A method as claimed in any one of the preceding claims, wherein the aspect ratio of the conductive filler is 10 or more.

5. A method as claimed in any one of the preceding claims, wherein the conductive filler comprises carbon nanotubes and/or carbon black.

6. A method as claimed in Claim 1 or Claim 2, wherein the orienting process comprises one or more of drawing, extrusion and spinning.

7. A method as claimed in any one of the preceding claims, wherein the steps of preparing the initial multi - component material and treating the initial multi- component material with an orienting process are carried out simultaneously.

8. A method as claimed in any one of the preceding claims, wherein the method is a continuous method.

9. A conductive multi -component material prepared by the method of any one of the preceding claims.

10. A conductive multi-component material as claimed in Claim 9, wherein the Young's Modulus and/or tensile strength of the conductive multi -component material is at least twice as high as the Young's Modulus and/or tensile strength of the initial multi -component material, and/or the conductivity of the conductive multi-component material is at least 100 times as high as the conductivity of the oriented multi-component material before annealing.

11. A conductive multi -component material, optionally as claimed in Claim 9 or Claim 10, comprising: a first polymeric component comprising conductive filler in an amount of 1 to 20 wt% based on the weight of the first polymeric component and a first polymer; and a second polymeric component comprising a second polymer,- wherein the first polymeric component has a lower melting temperature than the second polymeric component; and the composite has a conductivity of at least 1-E9 S/m.

12. An article comprising a conductive multi-component material as claimed in any one of Claims 9 to 11.

13. Use of a conductive multi-component material as claimed in any one of Claims 9 to 11 or an article as claimed in Claim 12 to provide anti-static properties or electrostatic properties or as a component in an electrical circuit.

14. Method to reduce the electrical resistivity of conductive polymer composite comprising the steps of: extruding said conductive polymer composite,

- drawing said conductive polymer composite,

- applying a thermal or chemical treatment to said conductive polymer composite.

15. Method according to Claim 14, wherein said thermal treatment is the annealing of the conductive polymer composite.