WO2010081821A1

WO2010081821A1 - Production of composite material

Info

Publication number: WO2010081821A1
Application number: PCT/EP2010/050345
Authority: WO
Inventors: Alexander Bismarck; Milo Sebastian Peter Shaffer; Mike Q Tran; Steven Lamoriniere; Angelika Menner; Emile Greenhalgh
Original assignee: Imperial Innovations Ltd
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2009-01-14
Filing date: 2010-01-13
Publication date: 2010-07-22
Anticipated expiration: 2011-07-14
Also published as: GB0900560D0

Abstract

The present invention relates to a process for the production of a composite, by the impregnation of a fibre tow with a suspension comprising nanocomposite particles and a non-solvent liquid phase or an emulsion comprising droplets of nanocomposite in a non-solvent liquid phase. The invention further relates to the composite produced by the process of the invention.

Description

PRODUCTION OF COMPOSITE MATERIAL

The invention relates to a process for the production of hierarchically-reinfoTcod composite materials, based on the impregnation of a continuous fibre tow, composite materials produced by said process and an apparatus for carrying out said process.

Composites, traditionally, have consisted of a micron-scale reinforcement imbedded in a continuous matrix. The resulting material possesses improved properties, such as mechanical strength, thermal conductivity, and electrical conductivity, over the individual constituents. With the relatively recent discovery of nanoscalc reinforcements, such as carbon nanotubes and nanocJays, a hierarchical reinforcement scheme based the mesoscale and nanoscale reinforcement has been realised. In this reinforcement scheme, conventional mesoscale reinforcements, such as unidirectional fibres, provide axial strength and stiffness whilst the nanoscale reinforcement provides additional property benefits, particularly transverse to the mesoscale reinforcement. In particular, the nanc-reinforcemcnt addresses critical problems associated with conventional composites, especially those related to matrix-dominated properties, such as in-plane shear, compression and off-axis strength. Functional advantages can also accrue through improvements in electrical conductivity (relevant to lightening strike protection, electromagnetic shielding, structural health monitoring and static discharge), thermal conductivity, processing, helping to ensure uniform cure, inhibiting manufacturing induced defects, glass transition temperature (maximum operating temperature), solvent resistance, fire retardance, and so on.

To achieve these properties, high aspect ratio nanofillers are desirable or necessary. Unfortunately, the inclusion of nanoscale reinforcements into the matrix further complicates processing due to greatly increased viscosity and what has been termed "self-filtration" when using conventional matrix infusion techniques. High viscosity occurs due to the large hydrodynamic volume of high aspect ratio nanoparticles; above typically around lvol% (depending on the aspect ratio), the individual nanoparticles begin to interact ('entangle' or 'percolate'), giving rise to a rapid .increase in viscosity. The self-filtration phenomenon can be best described as a log jam of nanoparticles, which occurs as a nanoscale reinforced resin is infused into continuous fibre tows as illustrated in figure 1; the high aspect ratio particles accumulate outside the fibre tows, and fail to infiltrate the structure along with the resin. These factors have limited existing efforts to create multi-scale composites, combining conventional fibres with nanofiUed matrices, to low aspect ratios and/or very low loading fractions. A further problem with the infusion technique is that the high aspect ratio particles tend to become aligned with the resin flow direction, which is often parallel to the primary fibres; a more desirable orientation for the nanofϊller is usually normal to the primary fibres.

The first aspect of the invention provides a process for the production of a composite, said process comprising impregnation of a fibre tow with a suspension comprising nanocomposite particles and a non-solvent liquid phase or an emulsion comprising droplets of nanocomposite in a non-solvent liquid phase. The process of the first aspect further comprises removal of the non-solvent Non-solvent removal is usually carried out by evaporation with or without heating. The nanocomposite particles arc then consolidated to form a continuous nanocomposite matrix around the fibre. Consolidation occurs by merging of the nanocomposite particles (or droplets) after the removal of the non-solvent. Consolidation may occur with or subsequent to the removal of the non-solvent. Alternatively or additionally, it may be encouraged by heating and/or the use of shear forces (applied though pins or rollers). The resulting composite material can then be further processed using standard techniques for composite materials, to produce components, as required.

The suspension (or emulsion) is prepared by pre-mixing nanoparticles into a matrix and dividing the mixture into particles (or droplets) of similar diameters to the primary fibres (the selection of the diameters of the particles or droplets will therefore depend on the fibre type), thereby forming the nanocomposite particles (or droplets of nanocomposite). It will be appreciated that the nanocomposite particles or droplets will be provided as a distribution of diameters, although a narrow distribution is preferred. The references to diameters of the particles or droplets therefore relate to the average diameter of the particles or droplets in the distribution. The particles and/or droplets have a diameter of around 1 to 100 microns, preferably 5 to 50 microns, more preferably 5, 10, 20, 30, 40 or 50 microns. Where the nanocompositc particles or droplets are provided for use with carbon fibres, they will typically have a diameter of around 5 to 10 microns, such as 5, 6, 7, 8, 9 or 10 microns. Where the nanocomposite particles and/or droplets are provided for use with glass/oxide fibre, they will typically have a diameter of around 10 to 50 microns, such as 10, 20, 30, 40 or 50 microns adjusted approximately to match the primary fibre diameter. The nanocomposite particles can be prepared by various means, for example cryogenic grinding of bulk, extruded (or other shear mixed) nanocomposite thermally-induced phase segregation or coprecipitation from solution, or other methods known in the art. The nanocomposite particles (or droplets) are then suspended in a non-solvent liquid phase, in which they have low solubility (or miscibility). Water is a preferred non- solvent for convenience, although other organic solvents can be used (such as hcxane, ethanol, propanol, butanol, DMF, NMP, acetone, etc). Surfactant may be used to stabilise the suspension (or emulsion). The nanocomposite particles can contain high loading fractions of high aspect ratio nanoparticles, and may have high viscosities, either at room temperature or in any subsequent melt phase. The nanocomposite particles (or droplets) are infiltrated, whole, throughout the fibre tows and arc thus fully and uniformly distributed through the fibre composite matrix. The continuous matrix is formed by subsequent consolidation of the particles or droplets which involves flow of the nanocompositc matrix material only over very short distances (on the order of the primary fibre diameters). In this context, the high viscosity does not present a problem, there is no opportunity for self filtration, and an approximately random orientation of the nanoparticles can be retained (preferable to an alignment normal to the primary fibres).

The particle suspension is preferably an aqueous or other suspension of fine nanocomposite particles, or a nanocomposite emulsion. Where the invention relates to a suspension of fine nanocompositc particles, the nanocomposite particles arc solid materials which arc solid at the impregnation temperature and/or in the impregnation solvent. The suspension therefore provides a high aspect ratio nanoparticle dispersed into the desired primary matrix. The first aspect of the invention alternatively provides a process comprising impregnation of a fibre tow with an emulsion. The emulsions consist of liquid droplets suspended in an immiscible non-solvent liquid phase, wherein the droplets contain suspended/dissolved nanoscale reinforcements in a liquid material that will subsequently form the primary matrix by solidification or reaction (either with components already present or with additional molecular curing agents introduced subsequently). The droplets therefore contain nanoparticles in a liquid dispersed phase or in a semi-solid phase. For the purposes of the present invention, the term nanoparticles relates to particles having at least one dimension less than lOOnm.

For the purpose of this invention, the matrix may be a thermoplastic, a thermoset (or a component of a thermoset) resin, an elastomer, an inorganic ceramic or glass, or a metal. The nanoparticle phase may include nanotubes (e.g. carbon, boron nitride, vanadium oxide, (W,Mr>χS,Se)₂, etc), nanoplatelets (clays, mica, talc, graphite), nanorods (ZnO, TiO₂, Si, Ge, etc). The fibre can be any continuous fibre, such as carbon, ceramic, glass, oτ aramid fibre, compatible with the matrix and associated processing.

For the purposes of the first aspect of the invention, the matrix is preferably a polymer thermoplastic or thermoset matrix. The polymer thermoplastic matrix is preferably selected from one or more of PA, PVdF, PEEK, PEK, PES, PI or PP. The polymer thermoset matrix is preferably Epoxy: Phenolic: polyester. The nanoparticles arc preferably carbon nanotubes, particularly multi-walled carbon nanotubes. In a particularly preferred feature of the first aspect of the invention, the material comprises a polymer thermoplastic or thermostat matrix in combination with carbon nanotubes, particularly multiwalled carbon nanotubes, reinforced by conventional carbon fibres.

The material can be provided with a loading of 10 wt% or above nanoparticles in the matrix, preferably 5wt% or above nanoparticles in the matrix. The loading is preferably 1 to 10 wt%, preferably 2, 3, 4, 5, 6, 7, 8 9 or 10 wt%. The material can particularly be provided with 10wt% or above, preferably 5wf% or above in the matrix, where the nanoparticles arc carbon nanotubes.

It is a feature of the present aspect to provide composites which are reinforced, more preferably nanoreinforced to reduce cost and to improve formability and/or damage tolerance. The fibres can therefore be provided as two-dimensional woven materials for example, the fibre tows can be woven into fabrics using texture technologies or non-crimp fabric (NCFs), in which unidirectional layers of tows are held together with thermoplastic fibre stitches to produce dry fibre preforms. The resulting fabric layers can then be stacked and the suspension or emulsion can be infused into the composite. Alternatively, films of the suspension or emulsion can be stacked between the laminate and infused into the laminate, foτ example by heating. Alternatively, nanoreinforced thermoplastic fibres can be prepared and then can be woven into the fibre preform and then infused into the laminate for example by heating. It will be appreciated that these processes can be used to produce nanoreinforced 3 dimensional braided composites.

The second aspect of the invention provides a composite prepared according to the process of the first aspect of the invention.

The third aspect of the invention provides an apparatus for the production of a composite comprising an impregnation batb housing in use a suspension or an emulsion comprising nanocompositc particles and a matrix, wherein said bath has an inlet for the introduction of a fibre tow and an outlet for the removal of a fibre tow.

The apparatus further comprises a fibre pretensioner in communication with the inlet of the impregnation bath. The apparatus further comprises a drying section in communication with the outlet of the impregnation bath. The apparatus further comprises a collection device in communication with the drying section, said collection device being provided in use for the collection or collection and storage of the composite. In a preferred feature of the third aspect of the invention, the apparatus comprises a fibre pretensioner, impregnation bath, a drying section, a belt puller, and a take-up device. After passing the fibre tow through the matrix and nanoreinforcement containing media, excess carrier such as water or organic solvent is then removed in the drying section (for example a drying oven). The configuration of the production line can be adjusted to suit any thermoplastic, thermoset, glass, ceramic, or metal matrix system that can be converted to, or is available as_> a fine powder, or emulsion. The nature of the consolidation step varies with the nature of the matrix. For instance, the inclusion of a second oven, shear impregnation pins, and consolidation rollers would accommodate the requirements for processing thermoplastic composites. Another advantage of the disclosed impregnation procedure is the ability to control rtre fibre volume content of the product by simply controlling the impregnation bath concentration and/or production speed The impregnation procedure is capable of producing continuous lengths of continuous, unidirectional fibre reinforced composite tape. The hierarchically-reinforced composite tape, which can be considered as a prepreg in polymer matrix composition terminology, is highly versatile and can be post-processed by a number of well-established procedures, such as compression moulding, filament winding, tape placement/laying, and roll-trυsion, to form a number of final product geometries, structures, such as wound pipe or compression moulded laminates.

An example of the apparatus of the third aspect of the invention is disclosed in figure 2, which illustrates a fibre prctensioner, impregnation bath, oven, belt puller, and take-up device. The impregnation of the continuous fibre tow, as set out in the first aspect of the invention, is carried out by immersing the fibre tow in the impregnation bath, which contains the suspension or emulsion comprising nanocomposite particles within a matrix. The impregnation bath preferably has a number of fibre guides which spread the fibre tow, increase residence time within the bath, and promote homogeneous impregnation of fibres by the nanocomposite matrix or nanoreinforcment carrying media. Next, the carrier is removed from the impregnated fibre tow in a drying section, such as an oven, which can consist of any type of heating d ements such as resistance or infrared emission. The resulting product can be either collected and stored as a "prcpreg" as in the case of thermosetting matrices or additional processing segments can be added to the production line for other types of matrices. For instance, if a second oven, shear impregnation pins, and consolidation rollers are added between the drying oven and belt puller (see Fig 3), the line can be used to produce an unidirectional thermoplastic composite tape. Furthermore, the addition of an inline fibre modification system, such as a plasma spray device, permits the continuous modification of the fibres to enhance fibre matrix interaction. The modular design of the production line enables segments to be added or removed to accommodate the desired composite system.

All preferred features of each of the aspects of the invention apply to all other aspects mutatis mutandis.

The invention may be put into practice m various ways and a number of specific embodiments will be described by way of example to illustrate the invention with reference to the accompanying drawings, in which:

Figure 1 shows a schematic diagram of self-filtration of carbon nanotubes during typical resin transfer/infusion techniques, Figure 2 shows a schematic diagram of the continuous, unidirectional, hierarchically reinforced composite production line configured for thermosetting matrices;

Figure 3 shows a schematic diagram of the continuous, unidirectional, hierarchically reinforced composite production line configured for thermoplastic matrices;

Figure 4 shows a schematic diagram of loss-in-weight metering system used to control impregnation bath concentration;

Figure 5 shows a SEM micrograph of the fracture surface of AS4-GP/CNT/PVDF under (A) low and (B) high magnification;

Figure 6 shows an optical micrograph of the HexFlow RTM6/DMF/G-CNT in hcxanc emulsion; Figure 7 shows a schematic diagram of an apparatus for the production of laminates by filament winding; Figure 8 shows a SEM micrograph of the fracture surface of hierarchically reinforced epoxy resin composite laminate;

Figure 9 shows the particle size distribution of pure PEEK powder and carbon nanotubes (CNT) incorporated PEEK powder processed using the "powder" route; and

Figure 10 shows a representative SEM micrograph 5% CNT loading in PEEK powder at low (figure 10a, left hand picture) and high (figure 10b, right hand picture) magnification.

The present invention will now be illustrated by reference to one or more of the following non-limiting examples.

EXAMPLES

1. Hierarchically-reinforced thermoplastic composites : PVdF/CNT/CF

Materials:

The carbon fibres used for the production of the continuous, unidirectional fibre reinforced PVDF composite was AS4-GP (Hexcel, UK) which was sized with 1.0 wt-% uncured epoxy. Commercially- available multi- walled carbon nanotubes (Arkcma, Lacq, France) were used as the nanoscale reinforcement in the hierarchical reinforcement scheme and were used without further purification. The carbon nanotubes had a diameter of approximately 10-20 nm and a length of at least 5 μm (manufacturer's claim). Kynar 711 (Arkema, Serquigny, France), a PVDF homopolymer, was used as the matrix.

Nanocoraposite powder preparation:

A PVDF-carbon nanotube powder was produced by the precipitation of a mixture of carbon nanotubes and PVDF from dimethyl forrnamide (DMF) through the use of a watcr/DMF non-solvent. This procedure produced an aggregated powder of homogeneously dispersed carbon nanotubes in PVDF which is suitable for the disclosed impregnation procedure (sec , M. Tran, M. Shaffer, A. Bismarck, Macromol. Mat. & Eng., 293 (3), 188-193, 2008). The nanocornposite powder can be readily re-suspended in water by adding 2.0 wt-% Crcmaphor A25 (with respect to the nanocomposite/polymer weight, BASF AG, Ludwigshafen, Germany) and homogenising for 1 h at 4000 RPM (L2H, Silverson, Chesham Bucks, UK).

Hierarchically-reinforced PVDF composite tape production:

Directly prior to production, a 2.0 L polymcr/nanocomposite suspension with a 5 wt- % solid content was prepared for the impregnation bath. The suspension was placed in the bath and agitated using two magnetic stir bars (500 RPM, RCT, IKA, Staufen, Germany). In addition, a 2 L concentrate polymer/nanocomposite suspension with a 10 wt.-% solid content was prepared. A Ioss-in-weight metering system containing the concentrated polymcr/nanocomposite suspension was placed above the impregnation bath. The loss-in-weight system consisted of a 500 mL addition runnel which was held on a ring stand. The addition funnel and ring stand were placed on a balance and the outlet of the addition funnel was placed over the impregnation bath (see Fig 4). The concentrated polymer/nanocomposite suspension was added to the addition ftmnel and agitated with an overhead stirrer (300 RPM).

The carbon fibre tow was threaded from the fibre pretensioner, through the fibre guides of the impregnation bath, through the drying and melting ovens, over the shear pins, and finally in between the belts of the puller. The fibre preteosioner was then set to 100 g force, the drying oven was set to 120°C_> melting oven was set to 180°C and the shear impregnation pins were set to 200°C. The ovens and shear pins were allowed to equilibrate for 30 min prior to production.

The target fibre volume content for the PVDF composite and hierarchical fibre reinforced PVDF tape was 60 ± 5%. To monitor the fibre volume content prior to and during sample collection to ensure production remained in control, the fibre-volume content was determined gravimetrically. The fibre volume content was measured by precisely cutting 1.00 m sections of tape and measuring the mass of the product. Given 1 m of the 12k carbon fibre tow weighed 0.858 g (manufacturer's claim which was confirmed experimentally) and the density of both the carbon fibre and PVDF are 1.79 g cm^{" l} (manufacturers' claims), the following equation was used to determine the fibre volume content:

where Vf is the volume of the fibres within the composite, V_«-_av is the volume of the composite, πif is 0.858 g, Df is the density of the carbon fibres, m_Pvi>- is the mass of PVDF within the composite tape, ppvw is the solid density of bulk PVDF, and m^^ is the mass of 1.0 m composite tape. A i m section of composite tape weighing between 1.317 g and 1.556 g corresponded to fibre volume contents between 65% and 55%, respectively. Composite tape outside of this weight range, typically the result of twists within the fibre tow, was discarded.

After thermal equilibrium was reached, the puller was set to run at I m min^"1 and tape production commenced. Since the powder particle size of the polymer and nanocomposite suspensions are known to directly affect impregnation of the fibre tow and therefore the fibre volume content, slight adjustments to the concentration of the impregnation bath had to be made to account for the variation in particle size prior to sample collection. The initial approximate fibre volume content of the composite tape was determined using the above mentioned procedure 5 min after starting the impregnation line. If the fibre volume content was too low, water was added. This led to an increase in the fibre volume content. If the fibre volume content was too high, a 100 g aliquot of the concentrated suspension was added and the composite tape was re-sampled after allowing the line to stabilise for 5 min. The composite tape was not collected until three consecutive sections had the required fibre volume content The composite tape was measured for the fibre volume content every 30 min to ensure that the process remained in control and tape with the required fibre volume content was produced.

If the (hierarchical) composite tape had the required fibre volume content, the concentration of the impregnation bath had to be maintained. The rate of depletion of the impregnation bath was dependant on the fibre volume content as well as the production speed. In order to replenish the powder within the impregnation bath, aliquots of the concentrated polymer/nanocompositc suspension was added to the impregnation bath. For instance, to produce a composite tape with a 60% fibre volume content at 1 m min^"1, the impregnation bath depleted at a rate of 0.569 g min^"1 as determined by a mass balance (for VV 60%, one metre of composite tape weighs 1.427 g of which 0.858 g is carbon fibre). To accommodate for the depletion, a 40 g aliquot of the concentrated suspension, which was equivalent to 4.00 g of polymer/nanocomposite, was added every 7 min to the bath to maintain the bath concentration.

Hierarchically-reinforced PVDF composite characterisation:

The produced unidirectional composite tape was compression moulded to form test samples. Sections measuring 198 mm in length, 11 mm in width, and 0.80 mm thick were cut from the produced tape. A mould, which measured 200 mm x 12 mm, was made in order to minimise expansion during compression moulding, thereby preventing excessive fibre waviness and misalignment. Samples for mode I fracture toughness (Double Cantilever Beam) samples consisted of 40 plies separated in the mid-plane by a 60 mm x 12 mm x 13 μm thick Upilex release film at one end. The insert was used to make a partial dclamination from which the mode 1 crack was grown. The plies were stacked into the mould and preheated in the hot press for 5 min at 190°C. The test bars were then compression moulded at 190°C and 10 MPa of pressure for 10 min. The mould was then transferred to a press set at 80°C and held at 5 MPa to control the rate of cooling. After moulding, the bars were machined to have final dimensions of 190 mm long by 10 mm wide by 2 mm thick. To facilitate the introduction of the mode I loading, end blocks with an 8 mm hole which measured 20 mm long by 20 mm high by 10 mm wide were attached to the samples with cyanoacrylatc glue. Mode I fracture toughness testing was performed in accordance to ASTM D5528-01 at a test speed of 2 mm/min (1 kN load cell, 4502, Instron, Norwood, US).

Scanning electron microscopy (SEM, 7-10 kV, Gemini FEG-SEM, LEO, Germany) was used to observe the fracture surface of the hierarchical ly-reinforced PVDF samples. The fracture surfaces were sputter-coated with Au (15 ran thickness, 1,5 min, 20 mA, Edwards, Crawley, UK) and clearly show randomly oriented, homogeneously dispersed carbon nanotubes throughout the fracture surface (see Fig 5A and 5B). This proved that the disclosed impregnation procedure was successful in producing hierarchically-reinforced carbon fibre/carbon nanotube/PVDF composites and eliminated self-filtration of the nanoscale reinforcement. Moreover, the random carbon nanotube orientation suggested that although the shear stress induced by the shear impregnation pins was parallel to the fibre axis, it had not promoted carbon nanotube alignment.

2. Hierarchically reinforced epoxy resin composites

Materials:

The carbon fibres used for the production of the continuous, unidirectional fibre reinforced composite were T700 (Toray, Japan) which were sized with 1.0 wt.-% uncured epoxy. Commercially-available multi-walled carbon nanotubes (Arkema, Lacq, France) were surface grafted with 2.5 wt-% glycidyl methacrylate (G-CNT) as described in PCT/GB2009/001655. HexFlow RTM6 was used as the matrix. Dimethyl formamide (DMF) and hexane were used without further purification.

Preparation of the emulsion impregnation bath: The HexFlow RTM67DMF/G-CNT in hexane emulsion, which was subsequently used as fibre impregnation bath, was produced as follows. The internal phase of the emulsion consisted of 67 wt-% HexFlow RTM6, 31 wt-% DMF and 2 wt.-% G- CNTs. The viscosity of HexFlow RTM6 was reduced via the addition of DMF. The G-CNTs were dispersed within the mixture using firstly an ultrasonic probe (amplitude 0.8, 0.8 cycles) for 2 min and secondly a homogeniser at 5000 rpm for 5 min. Afterwards, the described internal phase was gradually added into the continuous phase, which consisted of 0.64 wt-% Hypcrmer B246sf with respect to HexFlow RTM6 dissolved in hexane, whilst the mixture was stirred at 15000 rpm. The microscope image of the resulting emulsion is shown in Figure 6. The droplet size ranges from 10 micrometre 100 to micrometre, the G-CNTs are dispersed within the droplets.

Hierarchically-reinforced epoxy resin composite laminate production:

The laminates were produced by filament winding (Fig. 7). Therefore, the carbon fibre tow was passed through a fibre prctensioner, which was set to 700 g force, and then through the fibre guides of the impregnation bath, which contained 500 mL of the above described emulsion. Afterwards, the filament was wound around a plate mandrill, which was rotated by a motor. The impregnation bath and feed eye passed back and forth across the plate at a relative speed of 0.63 to the speed of the spinning plate, thereby, forming a single layer. The single layer was dried at 80°C under reduced pressure for 20 min. Afterwards, it was cut off the plate whilst the plate was held in a hot compressor at 20 N/crn² to form 2 individual "preprcg" layers. 7 prepreg layers were laid up into a laminate, which was compressed to 2 mm in height and cured at 160°C for 75 min and 180°C for 2 h in a hot compressor at 20 N/cm². In order to complete the crosslinking reaction the laminate was cured for further 40 h at 180°C under reduced pressure.

Hierarchically-reinforced epoxy resin composite characterisation: lnterlaminar shear strength test were carried out using a Lloyds machine equipped with a 50 kN load cd l. The tests were performed according to ASTM Standard D2344. Therefore, samples, which were 20 mm long, 10 mm wide and 2 mm thick, were loaded at a rate of l mm/min.

Scanning electron microscopy (tilt of 20^*, 10 IcV, JSM 5300, JEOL Ltd , Welwyn Garden City, UK) was used to observe the fracture surface of the hierarchically- reinforced cpoxy resin composite samples. Therefore, the fracture surfaces were gold coated using an Einitech K500 (Hailsham, East Sussex, UK) for two minutes at 20mA. The SEM images in Fig. 8 randomly oriented, homogeneously dispersed carbon nanotubes throughout the entire fracture surface). This proved that the disclosed impregnation procedure was successful in producing hierarchically- reinforced carbon fibre/carbon tianotube/epoxy resin composites and eliminated self- filtration of the nanoscale reinforcement.

3 Hierarchically-reinforced thermoplastic composites ; PEEK/CNT/CF

Carbon nanotube (CNl^") reinforced PEEK was used as the matrix for a carbon fibre composite. The CNTs were Nanocyl (Sambreville, Belgium) industrial grade (NC7000); the PEEK was grade 150 from Victrex (Thornton Cleveleys). The CNT/PEEK nanocomposites were produced by twin screw extrusion. The CNT/PEEK nanocomposite powder was produced using a temperature induced phase separation of a PEEK in DPS solution containing up to 5 wt% CNTs. The nanocomposite particles were suspended in deioniβcd water at approximately 5 g/L, with 1 wt% surfactant. Unidirectional carbon fibre reinforced (CNT nanocomposite) PEEK tape (12.5 mm wide and 0.1 mm thick) was manufactured using a powder impregnation technique used for thermoplastic composite manufacturing. The 12k AS4 carbon fibre roving was subjected to 500 g of tension from a tension controlled let-off unit (Izumi International, USA) and passed through guiding rollers and over a roller mounted on a load cell that measures the tension (the pretensioner shown in Figure 3). The fibre tows were guided into the impregnation bath, which consists of a series of fixed pins ^hat spread the fibres within the slurry. The wet polymer impregnated fibre tow then passed into a heating tunnel, and dried under infra-red heaters controlled by a thermocouple kept at 180 °C. The fibres were dried completely before melting to ensure no water is entrapped within the composite, which could cause blisters on the tapes when it is consolidated at later stages. Once the water was evaporated, the tow entered an infra-red heated melting oven. This oven was identical to infra-red heating oven in the drying stage but it is operated at 390 °C in order Io meh the polymer. The impregnated tape md ts coming out from the melting infra-red ovens were passed over and under a series of three heated pins operated at 390 °C and then pressed through a water-cooled rolling die to consolidate the tape and eliminate voids. The speed of the line was controlled by adjusting the speed of the belt drive motor in the haul-off unit, which was fixed to 1.0 m min^"1 throughout. The tape was wound up onto a spool.

An important feature of the powder material in the aqueous powder impregnation process during production of unidirectional carbon fibre, hierarchically-reinforced PEEK is the average and homogeneity of the particle sizes.

If the particles are too small, they drop out of the carbon fibre tow after exiting the impregnation bath. On the other hand if the powder is too large, it is difficult for the powder to be impregnated into the carbon fibre tow uniformly. This then leads to powders being filtered out of the carbon fibre low and result in either resin or fibre rich regions. The influence of different CNT loadings on the particle size distribution (PSD) d₍so₎ of CNT reinforced PEEK powder produced using the 'powder' route is presented in figure 9: at 1 % CNTs loading, the CNT reinforced PEEK powder has a PSD of 74.5 μm. As the CNTs loading increased to 2.5%, the PSD decreased to 40.6 μm. The PSD decreased further to 38.2 μm when the CNTs loading increased to 5 %. Comparing to the baseline of which the pure PEEK powder processing using the same 'powder' route, the PSD was 55.5 μm. The preparation of CNT/PEEK nanocomposite powder by cryogenic grinding of extruded CNT/PEEK nanocomposites to 10 μm is also possible but is less practical at the laboratory scale). A series of SEM micrographs were taken to examine the morphology of the PEEK powder processed using the 'powder' route. No CNT agglomeration in CNT/PEEK nanocomposites with 5 % CNTs loading in PEEK powder could be observed from the SEM images. Furthermore, at high magnification (Figure 10b) it can be seen that the CNTs are not condensed on the surface of the matrix but evenly distributed into the bulk of the matrix.

Unidirectional carbon fibre reinforced PEEK composite laminates test specimens were prepared from the produced tape in a mould release (McLube 1862, Aston, PA, USA) coated stainless steel frame mould. Composite tapes that measure 20 cm long were cut using a paper guillotine. A total of 34 layers of cut composite tapes were stacked and tightly wrapped using Upilex polyimide film (UBE Europe GmbH, Dusseldorf, Germany) before placing them into the steel mould (cavity dimensions: 200 mm x 12 mm x 5 mm). In order to reduce the consolidation time, two hot presses (EDBRO, Bolton, UK and George E. Moore & Son, Birmingham, UK) were used. The mould containing the stacked carbon fibre tapes was placed into the first hot press at 390 °C and pre-heated for 5 min. After which the pressure was increased to 2 MPa and held for 5 min before transferring the mould to another hot press operated at 120 °C and where it was held for 5 min at a pressure of 2 MPa. The mould was then allowed to cool to ambient temperature before the specimen was removed from the mould.

Moulded test specimens were cut into the required dimensions for mechanical testing using a diamond blade cutter (Diadisc 4200, Mutronic GmbH & Co, Rieden am Forggensee, Germany). The quality of the edges of both compression and injection moulded specimens was improved by grinding using P320 grit sandpaper. After all test specimens were trimmed and polished, the composite specimens were annealed at 240°C for 4 h and cooled to 140 °C at a rate of 10 °C h^-1 prior to mechanical testing.

Claims

1. A process for the production of a composite, said process comprising impregnation of a fibre tow with a suspension comprising nanocomposite particles and a non-solvent liquid phase or an emulsion comprising droplets of nanocomposite in a non-solvent liquid phase.

2. A process as claimed in claim 1 further comprising the step of removal of the non-solvent liquid phase,

3. A process as claimed in claim 2, further comprising the step of consolidating the nanocomposite particles or droplets of nanocomposite to form a continuous nanocomposite matrix around the fibre.

4. A process as claimed in any one of claims 1 to 3, wherein the suspension is prepared by pre-mixing the nanopartid es into a matrix, dividing the mixture into particles and suspending said particles into a non-solvent liquid phase.

5. A process as claimed in any one of claims 1 to 3 wherein the emulsion is prepared by pre-mixing nanopartid es into a matrix, dividing the mixture into droplets and suspending said droplets into a non-solvent liquid phase.

6. A process as claimed in any one of claims 1 to 5 wherein the nanopartid es are nanotubcs, nanoplatelcts or nanorods.

7. A process as claimed in any one of claims 1 to 6 wherein the nanocomposite particles or droplets have a diameter of 1 to 100 microns.

8. A process as claimed in any one of claims 4 to 7 wherein the matrix is one or more selected from a thermoplastic, a thermoset resin, an inorganic ceramic, glass or a metal.

9. A process as claimed in any one of claims 1 to 8 wherein the fibre tow is carbon, glass, inorganic oxide or polymer fibre.

10. A composite prepared according to the process as claimed in any one of claims 1 to 9.

11, A process as substantially described herein with reference to one or more of the examples.

12. A composite as substantially described herein with reference to one or more of the examples.