FIELD OF THE INVENTION
The present invention is concerned with a method for producing
a reinforced polymer by introducing carbon nanotubes into the polymer. The invention
also relates to reinforced polymers produced by the present methods and the use
of carbon nanotubes for improving the mechanical properties of polymers.
BACKGROUND TO THE INVENTION
It has been known for many years that blending fibres,
such as carbon fibres, with polymers can significantly improve the mechanical properties
of the blends (see
Polymer Composites, April 1987, Vol. 8, No. 2, 74-81
J. Composite Materials, Vol. 3, October 1969, 732-734
Polymer Engineering and Science, January 1971, Vol. 11, No. 1, 51-56
discloses a method of reinforcing polymers by the incorporation of long
fibres of material such as metal, glass or asbestos. The advantage of carbon fibres
is that they are very light, but despite this exhibit relatively great mechanical
strength. In particular they exhibit very high stiffness.
More recently, since the discovery of Buckminsterfullerene
(C60), it has been found that carbon tubes (often termed carbon nanotubes
because of their diminutive dimensions) having a structure related to the structure
of C60 exist, which have the potential to be used in similar ways to
carbon fibres. In particular, the structure of carbon nanotubes makes their aspect
ratio (length/diameter, L/D) comparable to that of long fibres. Typically the aspect
ratio of carbon nanotubes is >10000. Thus, the aspect ratio of carbon nanotubes
is generally much greater than that of conventional short fibres, such as short
glass fibres and short carbon fibres. In addition, the tubes can potentially be
lighter than conventional carbon fibres, whilst being stronger and stiffer than
the best conventional carbon fibres (see
P. Calvert "Potential application of nanotubes" in Carbon Nanotubes, Editor
T. W. Ebbeson, 297, CRC, Boca Raton, Florida 1997
Depending on their diameter, helicity, and number of layers
(single-wall v. multiple-wall) carbon nanotubes have electronic properties between
those of conductors and semi-conductors. They may thus be added to an electrically
insulating polymer to increase its conductivity.
discloses an electrically conductive polymer composition containing carbon
nanotubes. In addition, carbon nanotubes have great mechanical strength, being cited
as having bending modulus values of from 1000-5000 GPa. Moreover they have been
mentioned in connection with new, highly efficient, fracture micromechanisms which
would prevent pure brittle failure with a concomitant low strain. Thus, carbon nanotubes
have been envisaged for use in many applications in recent years (see
P. Calvert "Potential application of nanotubes" in Carbon Nanotubes, Editor
T. W. Ebbeson, 297, CRC, Boca Raton, Florida 1997
T. W. Ebbeson, "Carbon Nanotubes", Annu. Rev. Mater. Sci., 24, 235, 1994
Robert F. Service, "Super strong nanotubes show they are smart too", Science,
281, 940, 1998
B. I. Yakobson and R. E. Smalley, "Une technologie pour le troisième
millénaire: les nanotubes", La Recherche, 307, 50, 1998
However, in the past when producing polyolefin composites
by incorporating carbon nanotubes, tangling of the nanotubes and consequent randomising
of the orientations of the nanotubes has caused problems (see
M. S. P. Shaffer, X. Fan, A. H. Windle, "Dispersion of carbon nanotubes: polymeric
analogies", poster 39, p. 317 in Proceedings of Polymer '98", September 1998, Brighton
P. M. Ajayan, "Aligned carbon nanotubes in thin polymer films", Adv. Mater.,
7, 489, 1995
H. D. Wagner, O. Lourie, Y. Feldman and R. Tenne, "Stress-induced fragmentation
of multi-wall carbon nanotubes in a polymer matrix", Appl. Phys. Lett., 72 (2),
K. Yase, N. Tanigaki, M. Kyotani, M. Yomura, K. Uchida, S. Oshima, Y. Kuriki
and F. Ikazaki, Mat. Res. Soc. Symp. Proc., Vol. 359, 81, 1995
). In particular, tangling can give rise to a reduction in the homogeneity
of fibre/polymer blends since it is difficult for the fibres to distribute themselves
evenly within the surrounding polymer matrix. This reduces the mechanical strength
of the blends, since lack of homogeneity introduces weak points in a blend at positions
where, for instance, there is a relatively low concentration of fibre and a high
concentration of polymer. Moreover the randomising of the orientation of the fibres
also reduces the mechanical strength of the blends. This is because (for example)
the maximum resistance to strain in a given direction will be achieved when all
of the fibres in the blend are oriented with their longitudinal axes aligned in
that direction. The further that a blend deviates from such an ideal orientation,
the less the resistance to strain of the blend in that direction. However, up to
present it has not been possible to control the orientation of the fibres to a degree
sufficient to improve mechanical properties.
SUMMARY OF THE INVENTION
It is an aim of the present invention to overcome the problems
associated with the above blends and methods. Accordingly, the present invention
provides a method for the production of a reinforced polymer, which method comprises:
- (a) introducing carbon nanotubes into a polymer to provide a mixture of the
polymer and the nanotubes;
- (b) stretching the mixture at or above the melting temperature (Tm)
of the polymer; and
- (c) stretching the mixture in the solid state so as to orient the carbon nanotubes.
The present invention further provides use of oriented
carbon nanotubes in a polymer to reinforce the polymer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the present invention, the polymer is not particularly
limited as long as the polymer can be oriented in the molten and in the solid state.
In a preferred embodiment, the polymer is preferably a polyolefin, such as a polyethylene
or a polypropylene or blends thereof. When the polyolefin is a polymer of an olefin
monomer having 3 or more carbon atoms, such as polypropylene, the polyolefin may
be isotactic or syndiotactic. A particularly preferred polymer is isotactic polypropylene,
iPP. Other polymers which can be used in the present invention include polyesters,
such as PETs and PEEKs, polyamides, PVCs, and polystyrenes.
The present invention is advantageous in that it succeeds
in orienting the carbon nanotubes within the polymer such that their longitudinal
axes are more aligned with each other than would otherwise be the case. In this
invention 'orienting' is intended to mean a degree of disentangling of the carbon
nanotubes and/or a degree of aligning of the carbon nanotubes. Not only are the
nanotubes oriented, but also the individual polymer molecules undergo a degree of
orientation in the present method. Orientation of the nanotubes leads to a greater
homogeneity and less tangling in the resulting blends, and a consequent significant
improvement in the mechanical properties of the blends. In particular, superior
tensile modulus (as measured at 10 % strain, hereafter termed modulus (10 %)) and
tenacity can be achieved by the present blends as compared with known blends, whilst
still retaining a relatively high toughness (the product of tenacity and strain).
For these reasons, the reinforced polymers of the present
invention are useful in a wide variety of applications involving the reinforcement
of polymers, including use in fishing gear, tyres, safety belts, sewing thread,
protective clothing, durable man-made fibre, and in cement paste, mortar or concrete.
The reinforced polymers of the present invention are particularly useful in high
tenacity polyolefin fibres and filaments as a replacement for conventional reinforcing
agents (see, for example,
M. Ahmed, "Polypropylene Fibres - Science and Technology", Textile Science
and Technology 5, High tenacity industrial yarns 389-403 and 665-681, Elsevier Amsterdam
The stretching procedure of the present method comprises
two sequential steps: stretching the polymer/nanotube mixture in the molten state
(step b) and subsequently stretching the solidified material (step c).
Stretching step (b) may comprise one of many traditional
ways of processing polyolefins, including fibre melt spinning, extrusion, thermoforming
and injection moulding. In a preferred embodiment of the present method, stretching
step (b) comprises extruding the polymer/nanotube mixture. The extrusion step is
carried out at a temperature at or above the melt temperature (Tm) of
the polymer, so that the polymer is molten. Extrusion can be adapted to form a polymer
fibre (e.g. by melt spinning) or to form a polymer sheet (e.g. by extrusion through
a slit die or by compression moulding). In a preferred embodiment, the extrusion
step is carried out by extruding the mixture through a spinneret to produce a fibre.
The method of producing the precursor composite (the mixture
produced prior to solid state stretching) may be important, since it can affect
the efficiency and the ease of orientation of the composites. For example, fast
cooling isotactic polypropylene melt will produce the smectic crystalline form,
which can deform much more easily in the solid state than the monoclinic crystalline
form. Thus, in a preferred embodiment, after stretching in the molten state a desired
cooling protocol is selected in order that the most desirable precursor composite
In step (c), the polymer is preferably stretched at a temperature
at or above its glass transition temperature, Tg, and below its melting
temperature, Tm. Preferably this stretching step comprises solid state
drawing. This typically involves drawing the polymer in the form of a fibre or sheet
across a series of rollers. If desired the stretching step can be repeated one or
more times. Optionally in the case of a polymer film, stretching can be performed
biaxially, with longitudinal and transverse stretching taking place either sequentially
or simultaneously. The stretching conditions can be chosen depending upon the nature
of the polymer used, including the thermal characteristics, the molecular weight
and/or the molecular weight distribution of the polymer. The conditions to be chosen
for a specific polymer are readily determined according to standard solid state
As mentioned above, carbon nanotubes are used for reinforcement
in the present invention. By carbon nanotubes it is meant carbon tubes having a
structure related to the structure of Buckminsterfullerene (C60). Although
often termed carbon nanotubes because of their diminutive dimensions, the carbon
nanotubes used in the present invention need not necessarily have dimensions of
the order of nanometers in size. The dimensions of the nanotubes can be much greater
than this. However, it is preferred that the nanotubes are of a diameter from 1-50
nm, more preferably about 10 nm. Preferably the nanotubes are 1 µm or more
in length, more preferably about 10 µm in length. Thus, it is preferred that
the nanotubes are endowed with a high aspect ratio, having a length/diameter (L/D)
of 100 or more, preferably 103 or more and most preferably 104
or more. Therefore, composites containing these nanotubes should, when the nanotubes
are properly aligned, have mechanical properties which behave similarly to those
of composites containing continuous carbon fibres.
Increasing the aspect ratio of the nanotubes (at constant
nanotube volume fraction and orientation) leads to enhanced strength and stiffness
in the composite. A long aspect ratio makes it less important to functionalise the
polymeric matrix as well as the nanotubes to give good covalent binding and ensure
good interfacial shear strength.
It should be noted that the use in the present invention
of effectively non-continuous nanotubes (short in comparison to regular carbon fibres)
rather than continuous fibres, allows access to typical processing techniques useful
for thermoplastics. These techniques permit high throughput production and fabrication
of high quality, complex shaped composites. Furthermore, due to their high strength
and small diameter, the high aspect ratio of the nanotubes will not be decreased
by typical polymer processing operations such as extrusion and injection. Thus polymer
composites comprising nanotubes can provide the best of both worlds: high mechanical
strength and ease of processing.
The quantity of carbon nanotubes added to a given quantity
of polymer is not particularly limited. Typically less than 50 % wt. of carbon nanotubes
or less is added to the polymer. Preferably 30 % wt. or less and more preferably
20 % wt. or less of nanotubes is added. It is most preferred that 5 % wt. or less
of nanotubes is added. A very small quantity of nanotubes is capable of beneficially
affecting the properties of a polymer, such that very small quantities can be used,
depending on the intended use of the polymer. However, for most applications it
is preferred that 0.1 % wt. of nanotubes or greater is added.
The method of manufacture of the polymers used in the present
invention is not especially limited. In one preferred embodiment, the carbon nanotubes
are mixed with the monomer, prior to carrying out the polymerisation procedure.
Thus, in this embodiment the polymer is formed in situ in the presence of
the carbon nanotubes.
The present invention extends to a reinforced polymer obtainable
according to the methods of the present invention. The reinforced polymers of the
present invention have superior mechanical properties, as has been discussed above.
These properties are particularly marked in the case where the polymers are spun
into fibres. Typically, fibres formed from the present reinforced polymers have
a high modulus (10%), a high tenacity whilst still retaining a relatively high toughness.
It is preferred that the modulus (10%), the tenacity and/or the toughness of fibres
formed from the present reinforced polyolefin are greater by at least 15 %, as compared
with the equivalent properties of the same polymer not comprising carbon nanotubes
after undergoing the same stretching procedure.
Any additives typically introduced into polymers can be
included in the present reinforced polymers, provided that the additives do not
prevent the enhanced mechanical properties of the present polymers being obtained.
Thus, additives such as pigments, anti-oxidants, UV-protective HALS, lubricants,
anti-acid compounds, peroxides, grafting agents and nucleating agents can be included.
Embodiments of the present invention will now be described
in the following by way of example only.
Synthesis of nanotubes
The nanotubes used in the present invention were obtained
by catalytic decomposition of acetylene on a metallic catalyst (cobalt salt deposited
onto zeolites or silica gels) according to
K. Hernadi et al, Catalysis Letters, 48, 229, 1997
. Silica-supported catalyst samples (prepared from Co-acetate solution
of different pH) were tested in the decomposition of acetylene. Catalysts with various
pre-treatments (different calcination temperature, time and atmosphere) were compared
for nanotube yield and quality (quantity of turbostratic straight and helical tubes).
In the following examples, melt compounding of the polyethylene
(PE) and polypropylene (PP) with the nanotubes was carried out using a MiniMax mixer
B. Maxwell, "SPE J., 28, 24, 1972
Films were produced by compression moulding between Mylar®
(Du Pont) PET sheets above the melting points of the polyolefins (typically below
200°C). Injection moulded tensile bars were produced with the MiniMax moulder.
Fibres were produced by blending and melting the polyolefin
in the MiniMax, cutting pieces from an extruded strand, feeding the pieces into
the barrel of a capillary rheometer (a CEAST 1000) and melting. The extruded strand
was wound onto a rotating wheel, whose rotation speed determined the final diameter
of the monofilament.
Solid state stretching was carried out on a MiniMat from
Rheometrics Scientific, formerly produced by Polymer Laboratories, which is a miniature
tensile tester equipped with an oven (see
F. Biddlestone et al "The application of the MiniMat to plastic fracture:
a review", Polymer Testing, 6, 163, 1986
The titer of the fibre is the mass of the fibre per unit
length, measured in g/10 km. The titer was determined using a Lenzing vibroscope.
Tensile Properties Measurements
The tensile properties of the melt-spun filament, the films,
and the injection-moulded bars, were tested at room temperature in a tensile machine
(a MiniMat for the films and the bars and a Lenzing for the fibres). The methods
employed were carried out according to ISO 5079.
The conditions of testing were thus as follows:
Gauge length 10 mm
Cross head speed 10 mm/min.
Tensile Parameter Definitions
Rigidity was estimated at 10 % strain (10xforce at 10 %
strain divided by the titer of the unstretched fibre). Force is expressed in cN.
The tenacity is the force at break divided by the titer of the fibre. For iPP (used
in the following Examples) 1cN/Tex is equivalent to 9 MPa.
The strain at break was expressed in %: (L/L0)*100, with L0
being the gauge length (10mm). The maximum strain was 600 % and was limited by the
machine maximum traverse length.
The birefringence (&Dgr;n) is defined as the difference
between the refractive index (n) of a fibre measured parallel to its axis, and that
measured perpendicularly to its axis. It was measured in the following Examples
using a Berek compensator (Zeiss - Germany) and utilised according to the method
described in "
Polarised light microscopy: principles, instruments, applications", W. J.
Patzelt, 3rd edition, E. Leitz Wetzlar (1985) p65-67
Birefringence is a measure of degree of orientation in
the polymer. The greater the birefringence, the greater the degree of orientation
in the polymer.
Stretch Ratio (SR)
The solid state drawing carried out in the following Examples
was conducted by selecting a specific SR for stretching. The SR is the ratio of
the lengths of the fibre after and before the drawing step. It is equivalent to
the ratio of the cross section (or titre) after and before the drawing step. Thus
SR 2.5 refers to a 2.5 fold increase in the fibre length as a result of stretching.
SR max refers to the maximum stretching possible without breaking the fibre. In
each case in the following Examples the solid state drawing step was conducted at
To 10 g of isotactic polypropylene (iPP) reactor powder
(having a melt flow index (MFI) of 10, determined at 230°C with a 2.16 kg load
according to ISO 1133) 0.3 g (3 % wt. based on the weight of the polymer) of carbon
nanotubes were added. Also added were 3000 ppm of anti-oxidant (B215 from Ciba)
and 500 ppm of calcium stearate. The iPP powder, the additives and the nanotubes
were well dispersed using a high shear mixer. 1.6 g of the blended material was
introduced into the mixing chamber of a MiniMax moulder, which was heated at 220°C.
The mixture was maintained at 220°C for 1 minute under an argon atmosphere
and the axial rotor was rotated at 80 rpm for 2 minutes, allowing further intimate
blending of the nanotubes and the iPP.
The above procedure was repeated for iPP in the absence
of nanotubes and for iPP in the presence of 3 % wt. carbon black (blackpearl) in
the place of nanotubes.
For each of the above three mixtures, a strand was extruded
from the MiniMax and cut into small pieces. The pieces were fed into the heated
cylinder of a CEAST 1000 capillary rheometer at a temperature of 185°C. The
molten polymer was extruded through a capillary (length to diameter ratio of 5 to
1 mm) and the strand was wound onto a rotating wheel. For each of the above three
mixtures, nominal 11 dTex fibres (approximately 40 µm in diameter) were produced.
Before subjecting the extruded fibres to a solid state
drawing procedure, single fibres formed from each of the three mixtures were tested
for their tensile properties and their titer (dTe=g/10 km of fibre). Titer was measured
using a vibroscope. The results of the tests are shown in Table 1 under the heading
A first orientation procedure in the solid semi-crystalline
state was conducted on partially oriented melt-spun fibres in a MiniMat at 110°C
with a slow strain rate (10 %/min with a gauge length of 10 mm). The maximum elongation
was limited to a stretch ratio (SR) of 2.5. The fibres were easily stretched under
these conditions (no breaking occurred during stretching). Single fibres formed
from each of the three mixtures were again tested for their tensile properties.
The results of the tests are shown in Table 1 under the heading Example A.
An alternative orientation procedure in the solid semi-crystalline
state was conducted on unoriented melt-spun fibres at 110°C with a slow strain
rate (10 %/min with a gauge length of 10 mm). The maximum elongation was limited
to the maximum stretch ratio just before break (SR max). Single fibres
formed from each of the three mixtures were again tested for their tensile properties.
The results of the tests are shown in Table 1 under the heading Example B.
e Break (%)
E 5 (cN/Tex)
E 1 (cN/Tex)
3% C black
3% C nanotubes
SR 2.5, (110°C)
3 % C black
SR 2.5, (110°C)
3 % C nanotubes
SR 2.5, (110°C)
SR max, (110°C)
3 % C black
SR max, (110°C)
3 % C nanotubes
SR max, (110°C)
SR: Stretch Ratio
e Break: strain at break
E10: Secant modulus at 10 % strain
E5: Secant modulus at 5 % strain
E1: Secant modulus at 1 % strain
Te: tenacity at break (force at break/initial titer)
From Table 1, it can be seen that in the Comparative Example
in which no solid state drawing was carried out, no improvement in tensile properties
was seen and in fact a slight decrease in strain at break was observed.
However, performing moderate solid state stretching at
an SR of 2.5 (Example A in Table 1) led to an improvement in tensile properties.
A higher stiffness, tensile strength and energy-absorption capability was observed,
whilst the strain at break remained within acceptable limits. Thus the E10 and E5
are improved by approximately a factor of 3 in comparison with fibres stretched
under the same conditions which contain no additive or contain only carbon black.
When stretching the fibres to their maximum capability
(SR max) (Example B in Table 1) an extremely high tenacity and E 10 are observed
for the fibres containing nanotubes (approximately a 4-fold increase over the reference
fibres). The strain at break remains within acceptable limits.
In the latter example, the tenacity is approximately twice
the highest reported value for high tenacity iPP fibres (with a maximum tenacity
of 130 cN/Tex). The extension at break is still comparable to the reference fibres,
hence the reinforced fibres are endowed with enhanced toughness. The stiffness could
only be assessed at 1 % strain and was measured at approximately 2300 cN/Tex (21
GPa). The theoretical upper modulus (based on 1.4 % volume fraction of perfectly
aligned nanotubes in a highly oriented iPP fibre) should be between 38 GPa and 70
GPa depending on the value of the E modulus for the nanotubes (from 2000-5000 GPa).
Thus the reinforced polymers of the present invention have
tensile properties comparable with the theoretical maximum obtainable values and
are significantly improved over known reinforced polymers.
Furthermore, since excellent tensile properties are obtained
for fibre comprising nanotubes which have been subjected to solid state drawing
with an SR ratio of only 2.5, then thermal bonding in the normal processing temperature
window is possible for iPP fibres of modest orientation. Thus, non-woven fabrics
having high strength, stiffness and toughness can be produced using the present