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Latest developments in carbon nanotubes based nanocomposites.

A carbon nanotube (CNT) is a tube-shaped material, made of carbon, that has a diameter measuring on the nanometer scale (which corresponds to a one-billionth of a meter, or about one ten-thousandth of the thickness of a human hair). The graphite layer appears somewhat like a rolled-up sheet with a continuous unbroken hexagonal mesh and carbon molecules at the apexes of the hexagons (figures l a and lb). Carbon nanotubes have many structures, differing in length, thickness, type of helicity and number of layers. Although they are formed from essentially the same graphite sheet, their electrical characteristics differ, depending on these variations, acting either as metals or semiconductors. Carbon nanotubes typically have a diameter ranging from below 1 nm up to 50 nm. Their length is typically several microns, but recent progress has made them much longer, into the centimeter range. A longer form of carbon nanotubes is usually an assembly of straight segments of carbon atoms, which results in a zigzag structure at the microscale (figure lb). This structure is typical of commercially available multi-wall carbon nanotubes.

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Early forms of carbon nanotubes were synthesized for the first time during the beginning of the eighties by industrial research chemist Howard Tennent, who developed a process for catalytically growing nanotubes using hydrocarbon feedstocks. The lack of high resolution for analytical and observation instruments did not allow at that time to really estimate the full potential of this novel form of carbon. Their exceptional properties started to be really investigated in the early '90s when Sumio Iijima from Nippon Electric published a study of high-resolution electron microscopy of carbon soot containing multi-wall carbon nanotubes (ref. 1). This was the first real evidence of this nanostructured tubular form of carbon. Their intrinsic properties were measured further and revealed unexpected results. Indeed, the measurement of their intrinsic mechanical and transport properties position them as ultimate carbon fibers. In tables 1 and 2, these properties are compared to other advanced and engineering materials. Carbon nanotubes show a unique combination of stiffness, strength and tenacity compared to other fiber materials, which usually lack one of these properties. Thermal and electrical conductivity is also very high and comparable to other conductive materials. Carbon nanotubes can thus provide many useful properties that could be of interest for enhancing polymer properties as an additive.

Its expertise in tuning the carbon nanotubes properties and morphologies to obtain the best out of their intrinsic potential has made Nanocyl one of the most recognized producers of specialty carbon nanotubes and of materials and technologies using carbon nanotubes. Moreover, this knowledge and expertise has lead Nanocyl to exploit further the CNT advantages by preparing concentrates, dispersions and semi-formulated products out of various material families (thermoplastics, thermosets, elastomers, silicones, liquids, etc.). Table 3 shows the characteristics and morphologies of the MWCNT produced by Nanocyl (Nanocyl 7000), and table 4 lists some of the company's key product lines. Nanocyl 7000 is now a well accepted product and known as one of the most conductive MWCNT present on the market (figure 2).

The application of CNT is nowadays increasing very fast. A lot of applications are using the key properties of CNT, including:

* Electrical conductivity--ESD applications such as fuel lines in automotive, electronic packaging, e-painting, etc.;

* Mechanical reinforcement--structural composites, C/C parts, sporting goods, ceramics, etc.;

* Thermal conductivity--thermal management; and

* Flame retardancy--paints, FR compounds. However, the main ones are exploiting the electrical conductivity at low loadings of the CNTs.

Conductive composite applications

The use of carbon nanotubes for antistatic and conductive applications in polymers is already a commercial reality and is growing in sectors such as electronics and the automotive industry. Figure 3 shows a typical resistivity plot for an engineering thermoplastic. The loading with carbon nanotubes can be 5-15% lower than with carbon black grades for conductive plastics applications (< [10.sup.6] ohm-cm). This can be explained by the theory of percolation (ref. 3). A path for electron flow is created when the particles are very close to each other or have reached a percolation threshold. Fibrous structures with high aspect ratio (length/diameter) increase the number of electrical contacts and ensure a smoother pathway. The geometric aspect ratio of carbon nanotubes is typically greater than 100 compared to short carbon fiber (<30) and carbon black (~1) in the final product (e.g., injection molded part). This explains the lower content needed for a given resistivity. This behavior has been reproduced with carbon nanotubes in several thermoplastic and thermoset materials (refs. 4-8). Besides improving electrical conductivity, thermal conductivity can also be increased at low content of carbon nanotubes (refs. 9 and 10). This is of interest for thermal management applications with improved performance. A lower loading of additive can offer several advantages, such as better processability, aspect of surface, reduced sloughing and relative maintaining of the mechanical properties of the original polymer. Table 5 shows the mechanical properties of PC/ABS conductive compounds designed with a volume resistivity below 103 ohm-cm. The mechanical properties of the original polymer are better maintained with multi-wall carbon nanotubes than with carbon black or PAN carbon fibers. This can be critical in applications where these properties are important, such as impact resistance for exterior automotive parts. Combinations with conductive fillers (e.g., carbon black and graphite) can also reduce the total amount of conductive carbon content, especially in applications where the loading is very high (e.g., bipolar fuel cell plates), and thus enhance the processability, which can favor the economics of the process.

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A lower percolation threshold also gives new possibilities, such as antistatic transparent thin films or coatings with permanent conductivity (refs. 11 and 12). This is possible using a concentration below 1 wt. % and thickness in the order of 10 microns. Higher conductivity can be reached, but the thickness needs to be reduced much more in order to maintain transparency at a wavelength of 500 nm. Competitive materials for this application are usually inherently conductive polymers (e.g., polyaniline or PANI) or antistatic agents (e.g., alkyl amine salts). The latter can be problematic where contamination has to be avoided. Inherently conductive polymers can easily provide transparency, but can deteriorate the mechanical properties of the film for some systems due to the larger amount to be incorporated in the composite matrix.

Structural composite applications

Carbon nanotubes used for conductive applications are usually the multi-wall type because their price/performance ratio is more attractive than a single tube structure. They do not need special surface treatments, and conventional blending techniques used for melt-blending and solutions mixing are satisfactory in most of the cases. Some mechanical improvements can already be measured, but are far below what is expected. However, rubbery materials (e.g., elastomers) can be reinforced significantly without any particular surface treatments (figure 4).

The real use of the exceptional strength and stiffness of carbon nanotubes is still at an early stage of development. Both single-wall (SW) and multi-wall (MW) nanotubes have high stiffness and strength. Young's modulus of 1,000 GPa and tensile strength of 60 GPa were measured. However, SW nanotubes are usually in a rope or bundle form that limits their interaction with a surrounding matrix, and MW nanotubes are only efficient in their outermost layer during the stress transfer.

The main issues for gaining mechanical enhancement in polymers are their proper dispersion and distribution within the matrix, and increasing their interactions with the polymer chains. A loading of a few wt. % can already provide significant enhancement by optimizing processing conditions and surface chemistry of the carbon nanotubes. The largest efficiency is expected in oriented structures, like films and fibers, that maximize their axial properties. Continuous fibers with loading over 60% of SW have been achieved, and outstanding toughness was measured (ref. 13). Also, engineering fibers with only a few % of MW or SW have shown a major strength increase. Common fibers have only a few micrometers in diameter, and thus only nanoscale additives could be used for their reinforcement. Epoxy matrix-carbon fiber composites were made by reinforcing epoxy with CNTs (ref. 14). Figure 5 shows a significant improvement (over 100%) in fracture toughness for the composite laminate produced with epoxy matrix modified by the addition of CNTs.

The influence of CNTs on the fracture toughness of the composite laminates shows that even at relatively low CNT concentrations (less than 0.5 wt. %) in the epoxy, there is a significant improvement in the fracture toughness. Such improvement is due to the efficiency of CNTs in underlying mechanisms, such as bridging (ref. 15).

In general the influence of carbon nanotubes on the mechanical properties of composites is strongly dependent upon their weight fraction, dispersion and interracial interaction between carbon nanotubes and the matrix. The additional factors, such as the alignment of CNTs in composites, fiber alignment in the laminates and non-uniformity in surface modification of CNTs with functional groups, can also contribute towards the final mechanical performance of composite laminates.

Another example of mechanical reinforcement has been observed in poly(vinyl alcohol) (PVA) and polypropylene composites (ref. 16). It is fairly easy to disperse carbon nanotubes in PVA, making the composite a good candidate for applying and verifying different composites models. Table 6 summarizes the results (ref. 17). For the PVA composites, no functionalization was carried out, but microscopic observations showed that a thin crystalline coating is coupling the nanotube with the matrix. This interlayer enhances the stress transfer during the mechanical loading. A model predicts the increase of the composite strength with the thickness of this coating. For polymers that do not crystallize around the nanotubes, the authors suggest that they have to be functionalized with adequate chemistry. In this case, the multi-wall nanotubes were first functionalized with butyllithium groups and then further reacted with chlorinated polypropylene to give nanotubes that were covalently bonded. The toughness of the composite increased by a factor four, thanks to the greatest interfacial interactions. A much better dispersion for the nanotubes was also achieved. The value for the chlorinated polypropylene is between the value for Kevlar (33 J/g) and dragline silk (165 J/g). This is one of the best results so far achieved at this level of loading. PVA composite fibers loaded with 60 wt. % of single-wall nanotubes have reached 570 J/g (ref. 17). This organometallic approach was also applied with success to other polymers, such as polystyrene, PVC and polyamides.

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A recent development uses homogeneous surface coating of long carbon nanotubes by in situ polymerization of ethylene, as catalyzed directly from the nanotube surface-treated by a highly active metallocene-based complex (figure 6). This allows the break-up of the native nanotube bundles, leading upon further melt blending with HDPE, to high-performance polyolefinic nanocomposites (ref. 18). The resulting polymer-coated multi-wall nanotubes have been used as masterbatches that have been dispersed in various polymers. The improvement in multi-wall nanotubes' dispersion arising from the coating has allowed increasing of the mechanical properties and the electrical properties with comparable success in commodity, engineering and high performance plastics.

This system, currently developed at Nanocyl, marks an evolution in the field of the carbon nanotube nanocomposites, since it drastically improves the dispersion and the properties (same or better properties at lower loading) and generates masterbatches of never-before-obtained high loading (up to 60% of carbon nanotubes) totally compatible with the usual processing methods and much easier to handle.

Dimensional properties

Studies were first dedicated to checking the impact on injection molding of some properties carbon nanotubes could provide. This led to the observation that they stabilize polymers during the injection molding process. That means adding carbon nanotubes greatly reduces injection issues such as shrinkage and war-page (figures 7a and 7b).

Thermal properties

Beyer, et al, was the first to show the unexpected use of multi-wall carbon nanotubes as a very effective flame retardant at low loadings in a polymer (ref. 19). A synergistic effect was observed with organo-modified Montmorillonite (nanoclays) in ethylene-vinyl acetate copolymer (EVA), and a cable prototype of 2 kilometers long was made in collaboration with Nanocyl in 2003. The cable was tested according to fire standards used in the European cable industry. The nanocomposite formulation containing carbon nanotubes and nanoclays showed outstanding fire performance. The flame retardant effect of carbon nanotubes was also demonstrated by other groups (refs. 20 and 21), but the mechanism of the nanotubes is still not yet fully understood. Several polymers have been validated with this effect (EVA, PP, LDPE). They observed a strong char formation during the combustion of the material, resulting in a closed-crack surface. Current work in this area focuses on the development of fire-proof textiles and coatings.

Concerning thermal properties related to thermal conductivity, some experiments have revealed CNTs can impart 1 to 5 W/m.K to PP, for instance. It opens the way to replace highly filled polymer or metallic structures.

Aging

Carbon nanotubes have also revealed an influence on ageing, as it has been demonstrated that they could provide some stabilization to the thermoplastic matrices to which they were added. Some examples can be provided for PE (figures 8 and 9). Figure 8 shows that the more carbon nanotubes are added, the less carbonyl groups are released in a given time. It seems that carbon nanotubes promote a delay in releasing degradation products that result from aging (samples tested in forced mode--UV, moisture, temperature).

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Photo-oxidation tests show that carbon nanotubes provide some stability to polyethylene. We can assume this stability is afforded physically by the nanotubes, as they may prevent easy movements into the matrix; moreover, they might act as a physical barrier.

At last, carbon nanotubes' impact on thermal properties have also been studied in order to see if they afford some stabilization to the parts they are included in. As for weathering and photo-oxidation, they have demonstrated they could stabilize PE, PP or PB (figures 10 and 11).

Conclusions

Carbon nanotubes seem to be complete products, as they afford many advantages to the matrices they are incorporated in. If we briefly summarize what has been done, wherever nanotubes are added we can observe positive impacts (thermoplastic, rubbers, thermosets), on different kinds of properties (electrical, mechanical, dimensional, thermal ones or flame retardancy). Even in aging they afford some stabilization.

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Nanocyl is really keen on investigating further all these fields and improving the nanotubes' impact on matrices they are added in. The way to proceed can be either on carbon nanotubes' specificity (tailoring them) or on carbon nanotubes' introduction and mixing into matrices.

This article is based on a paper presented at a SmithersRapra Technology conference (www.rapra.net/conferences).

References

(1.) S. Iijima, Nature 1991, 354, 56.

(2.) P. Potschke and S. Pegel, IPF Dresden, Olivier Decroly and Fanny Liegeois, Nanocyl, "Comparison of different commercial MWNT in composites with polycarbonate produced by melt mixing," presentation CNT-net meeting, Szczecin, Poland, 6.-8. October 2004.

(3.) P. Potschke, Sergej M. Dudkin and lngo Alig, "Dielectric spectroscopy on melt processed polyearbonate multiwalled carbon nanotube composites, "Polymer 44 (2003) pp. 5,023-5, 030.

(4.) O. Meincke, D. Kaempfer, H. Weickmann, C. Friedrich, M. Vathauer and H. Warth, "Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blend with acrylomitrile/butadiene/styrene," Polymer 45 (2004) pp. 739-748.

(5.) P. Potschke and Arup R. Bhattacharyya, "Carbon nanotubes filled polycarbonate composites produced by melt mixing and their use in blends," Polymer Preprints 2003, 44(1), p. 761.

(6.) M. Wu and Leon L. Shaw, "On the improved properties of injection-molded, carbon nanotubes-filled PET/PVDF blends," Journal of Power Sources 136 (2004) pp. 37-44.

(7.) M. Rutkofsky, H. Ait-Haddou, M. Banash and R. Rajagopal, "Using carbon nanotube additive to make a thermally and electrically conductive polyurethane," Zyvex Application Note, www.zyvex.com.

(8.) M. Rutkofsky, M. Banash, R. Rajagopal and J. Chen, "Using carbon nanotube additive to make an electrically conductive commercial polymer composites," Zyvex Application Note, www.zyvex.com.

(9.) C.H. Liu, H. Huang, Y. Wu and S.S. Fan, "Thermal conductivity improvement of silicone elastomer with carbon nanotube loading."

(10.) Olivier Decroly, CRIF, Nanocomposites Project Final Report, 2000-2002.

(11.) S. H. Foulger and R. Gregory, National Textile Center Annual Report: November 2001, M00-C07.

(12.) Muataz Ali Atieh, Nazlia Girun, Fakhru 'l-Razi Ahmadun, Chuah Teong Guan, El-Sadig Mahdi and Dayang Radia Baik, "Multi-wall carbon nanotubes/natural rubber nanocomposite, "posted on http://www.azonano.com.

(13.) W.A. Lee and R.A Rutherford, in Polymer Handbook 2nd ed, J. Brandup and E.H. Immergut, Wiley, New York 1975.

(14.) A. Godara, L. Mezzo, F. Luizi, A. Warrier, S. V. Lomov, I. Verpoest, A.W. VanVurre and P. Moldenaers, "Influence of carbon nanotube reinforcement on the processing and the mechanical behavior of carbon fiber/epoxy composites," (Manuscript in communication).

(15.) F.H. Gojny, M.H.G. Wichmann, U. Kopke, B. Fiedler and K. Schulte, "Carbon nanotube-reinforced epoxy-composites--enhanced stiffness and fracture toughness at low nanotube contents," Compos. Sci. Technol. 2004; 64 pp. 2,363-71.

(16.) J.N. Coleman, M. Cadek, et al., "High-performance nanotube-reinforced plastics: Understanding the mechanism of strength increase," Advanced Functional Materials, vol. 14, issue 4, pp. 791-798, 2004.

(17.) Alan B. Dalton, et al., "Super-tough carbon-nanotube fibers," Nature, brief communication, vol. 423, 12 June 2003, p. 703.

(18.) D. Bonduel, M. Mainil, M. Alexandre, F. Monteverde and P. Dubois, "Supported coordination polymerization." A unique way to potent polyolefin carbon nanotube nanocomposites," 2005, Chem. Commun., pp. 781-783.

(19.) G. Beyer, "Improvements of the fire performance of nanocomposites," presented at the Thirteenth Annual BCC Conference on Flame Retardancy, Stamford CT, June 2002.

(20.) T. Kashiwagi, E. Gulke, J. Hilding, R. Harris, W. Awad and J. Douglas, "Thermal degradation and flammability properties of poly(propylene)/carbon nanotubes composites," Macromolecular, Rapid communication, 2002, 23, No13.

(21.) S. Bocchini, et al. (Sergio Bocchini, Alberto Frache, Giovanni Camino and Michael Claes), European Polymer Journal 43 (2007) pp. 3,222-3,235.

Michael Claes, Geraldine Dupin and Frederic Luizi, Nanocyl SA (http://www.nanocyl.com)
Table 1-mechanical properties of high aspect ratio materials

Material Specific E (TPa) Strength Strain at
 density (GPa) break (%)

Carbon nanotubes 1.3-2 1-1.7 10-60 10-30
 Nano-sized
 materials
Single silicate layer 2.8-3 0.17 1 <10
 (Montmorillonite)
Vapor grown carbon 2 0.4-6 3-7 <10
 fiber (VGCF)
Graphite nanoplatelet 2 1 10-20 n.a.
 Micron-sized
 materials
Alumina whiskers 3.9 0.4-0.55 14-28 <10
Wollastonite 2.96 0.3-0.53 2.7-4.1 <10
 Continuous
 fiber
 materials
Carbon fiber-PAN 1.7-2 0.2-0.6 1.7-5 0.3-2.4
Carbon fiber-pitch 2-2.2 0.4-0.96 2.2-3.3 0.27-0.6
E/S-glass 2.5 0.07/0.08 2.4/4.5 4.8
HS steel 7.8 0.2 4.1 <10
Kevlar 49 1.4 0.13 3.6-4.1 2.8

Table 2-transport properties of, conductive materials

Material Thermal Electrical
 conductivity conductivity
 (W/m. k) (S/m)

Carbon nanotubes >3,000 [10.sup.5]-[10.sup.7]
Copper 400 6 x [10.sup.7]
VGCF 1,950 2 x [10.sup.6]
Carbon fiber-pitch 1,000 1-5 x [10.sup.5]
Carbon fiber-PAN 8-105 5-10 x [10.sup.4]

Table 3-Nanocyl 7000 data sheet

NC7000
Outer diameter (nm) 9.5
Mean lengths ([micro]m) 1.5
C purity (%) 90
BET surface ([m.sup.2]/g) >300

Table 4-product list

Product line Grades Description

Industrial NC 7000 Carbon nanotubes powder,
 products multi-uses (tons scale capacity)
 PlastiCyl Thermoplastic masterbatches
 (tons scale capacity)
Research Nanocyl Available in small quantities
 products 11,002,100 (1-several kg)-academic uses
 3100 and and R&D
 9000
 Series
Dispersions EpoCyl Epoxy and aqueous
 BPA -MR01, dispersion -sampling stage
 EC01 (several kg or liters)
 AquaCyl
 AQ0101

Table 5-mechanical properties of PC/ABS conductive
compounds designed with a volume resistivity below
[10.sup.3] ohm-cm, comparison of CNT, CB and CF

 PC/ABS PC/ABS +
 reference multi-wall
 nanotubes
Electrical resistivity, [10.sup.16] [<10.sup.3]
 ohm-cm (ASTM D257)
Tensile modulus, 2,614 2,858
 MPa (ISO 527) ([+ or -] 205) ([+ or -] 119)
Strength at break, MPa 47.94 52.69
 ([+ or -] 3.97) ([+ or -] 1.03)
Elongation at break, % 9.55 18.98
 ([+ or -] 2.57) ([+ or -] 4.19)
Flexural modulus, 2,407 2,634
 MPa (ISO 178) ([+ or -] 11) ([+ or -] 28)
Max. flexural strength, 86.96 92.57
 MPa ([+ or -] 0.88) ([+ or -] 0.69)
Charpy impact (notched), 50.46 17.98
 KJ/[m.sup.2] (ISO 179) ([+ or -] 1.91) ([+ or -] 1.76)
Heat distortion 98.2 116.3
temperature at ([+ or -] 0.3) ([+ or -] 0.5)
 1.8 MPa, [degrees]C
 (ISO 75)

 PC/ABS + PC/ABS +
 carbon PAN carbon
 black fiber
Electrical resistivity, [<10.sup.3] [<10.sup.3]
 ohm-cm (ASTM D257)
Tensile modulus, 3,152 11,114
 MPa (ISO 527) ([+ or -] 105) ([+ or -] 439)
Strength at break, MPa 59.31 104.70
 ([+ or -] 1.86) ([+ or -] 5.69)
Elongation at break, % 3.25 1.29
 ([+ or -] 0.41) ([+ or -] 0.12)
Flexural modulus, 2,966 8,661
 MPa (ISO 178) ([+ or -] 38) ([+ or -] 129)
Max. flexural strength, 90.98 136
 MPa ([+ or -] 0) ([+ or -] 1.65)
Charpy impact (notched), 4.46 7.52
 KJ/[m.sup.2] (ISO 179) ([+ or -] 0.45) ([+ or -] 0.20)
Heat distortion 113.3 114.3
temperature at ([+ or -] 0.6) ([+ or -] 0.3)
 1.8 MPa, [degrees]C
 (ISO 75)

Table 6-mechanical properties of PVA and CI-PP composites (ref. 17)

Property PVA PVA +~1
 neat wt. % thin
 resin MWNT
 Nanocyl

E modulus 1.92 7.04
 (GPa) ([+ or -] 0.33) ([+ or -] 1.5)
Tensile strength 81.0 348
 (MPa) ([+ or -] 7.0) ([+ or -] 51)
Strain to failure 6 4
 (%)
Toughness 4.0 6.7
 (J/9) ([+ or -] 1.5) ([+ or -] 1.1)

Property PP-CI PP-CI +~1
 wt. % thin
 MWNT
 Nanocyl

E modulus 0.22 0.68
 (GPa) ([+ or -] 0.04) ([+ or -] 0.14)
Tensile strength 12.5 49
 (MPa) ([+ or -] 3.0) ([+ or -] 10)
Strain to failure 450 0
 (%)
Toughness 24.4 108
 (J/9) ([+ or -] 6.7) ([+ or -] 21)
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Author:Claes, Michael; Dupin, Geraldine; Luizi, Frederic
Publication:Rubber World
Date:Feb 1, 2009
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