Patent Application
20080207824
The present invention relates to the field of composite materials, and more particularly to nanocomposites. The invention relates to an improved method for dispersing carbon nanotubes in a polymer matrix.
Among polymer matrix composites, distinction may be made, according to the size of the particles, between “microcomposites” where the three dimensions of the charge are greater than or equal to a micrometre, and “nanocomposites” in which at least one of the three dimensions of the charge is less than 100 nm, or even of the order of one to a few tens of nanometres.
Nanocomposites are of particular interest in the industrial sphere since they have remarkable properties for relatively low charge rates, i.e. less than 10% by weight. In fact, they significantly improve the mechanical or electrical properties of the polymer matrix. In addition, unlike reinforcement of a fibrillar type, they reinforce the polymer matrix in all spatial directions.
Nanocomposites comprising carbon nanotubes as particle charges have already been proposed for various applications.
However, in practice, the use of carbon nanotubes as charges in polymer matrices in order to create nanocomposites runs into problems. In fact, it turns out that carbon nanotubes tend to aggregate together to form very stable “bundles”, also known as aggregates or agglomerates. The presence of such aggregates adversely affects the physical and mechanical properties of the composites in which they are present.
In order to avoid this aggregation phenomenon and to preserve the physical, mechanical and electrical properties of the polymer matrices forming such composites, coating the carbon nanotubes was thought of. Thus, document WO 02/16257 discloses a composition comprising single-wall carbon nanotubes coated in a polymer. Mention may also be made of document WO 2005/040265 that discloses a composition comprising a polymer matrix and 0.1 to 10% by weight of nanotubes coated in polyaniline.
However, these solutions allow to limit the size of aggregates without entirely preventing their formation, in order to obtain homogeneous dispersion on a nanoscopic scale.
In addition, document WO 2005/012170 discloses a particular coating method that allows to increase the compatibility of carbon nanotubes with the polymer matrix in which they are to be dispersed. This allows to obtain a homogeneous and stable dispersion of the carbon nanotubes in a polymer matrix. This method is characterised by the fact that the carbon nanotubes are used as catalytic supports due to the settling at their surfaces of a co-catalyst/catalyst couple, in order to form a catalytic system. The catalytic system is activated before polymerisation occurs on the surface of the carbon nanotubes in order to create a coating around these carbon nanotubes.
However, the coating polymer used in the above-described method uses a coating polymer that is miscible with the polymer matrix of the composite. Now, according to the present invention, it is not necessary, and may even prove counterproductive, to use miscible polymers for the matrix and the coating.
The present invention aims to provide a solution that does not have the drawbacks of the state of the art.
In particular, the present invention aims to provide an improved method for the dispersion of carbon nanotubes in a polymer matrix that is either non-miscible compatible, or non-miscible incompatible with the polymer for coating the carbon nanotubes.
The present invention also aims to provide the use of the improved method of dispersion in order to obtain a nanocomposite in which the carbon nanotubes are homogeneously dispersed in a polymer matrix on a nanoscopic scale.
The present invention relates to a method for dispersing carbon nanotubes within a host polymer matrix comprising the following steps:
By “host polymer matrix” is meant a polymer which forms the matrix of a composite in which particles, also called charges, are dispersed.
Two polymers are said to be non-miscible compatible or incompatible when, on various measurement scales, a phase-separation effect is observed. This phase separation may be observed on the micrometre scale by viewing the mixture with a scanning electron microscope, which often shows nodules of the minority polymer in the majority polymer. On a quasi-molecular scale, the non-miscibility of two polymers can be observed by the presence of two vitreous transition temperatures that are characteristic of the two polymers making up the mixture. These vitreous transitions may be measured by various techniques such as differential scanning calorimetry or dynamic mechanical analysis.
Two polymers are said to be non-miscible when the free energy of the mixture (ΔGmix) is greater than or equal to zero.
Two polymers are said to be non-miscible incompatible when the free energy of the mixture is greater than or equal to zero, when no modification of the respective vitreous transition temperatures (Tg) of the partners can be observed, when the mixture has a Flory-Huggins parameter χ (chi) greater than zero, and when the interface tension is high. The interface tension, which is proportional to the square of the Flory-Huggins parameter χ (chi), is considered “high” when it is greater than 2 mN/m.
Two polymers are said to be non-miscible compatible when the free energy of the mixture is greater than or equal to zero, when modifications of the respective vitreous transition temperatures (Tg) of the partners can be observed, when the mixture has a Flory-Huggings parameter χ (chi) that is low but greater than zero, and when the interface tension is low, i.e. between 0 and 2 mN/m. The interface tension, which is proportional to the square of the Flory-Huggings parameter χ (chi) is considered “low” when it is between 0 and 2 mN/m.
For a polyethylene/EVA mixture, the interface tension is considered low when it is of the order of 2 mN/m and, as regards a polyethylene/polyamide or polyethylene/polycarbonate mixture, it is considered low when it is greater than 2 mN/m.
According to particular embodiments, the invention has one or several of the following features:
The present invention also discloses the use of a polymer for coating carbon nanotubes, that is non-miscible compatible or incompatible with a host polymer matrix, in order to obtain homogeneous dispersion of said carbon nanotubes within a host polymer matrix on a nanoscopic scale.
By homogeneous dispersion of the carbon nanotubes “on a nanoscopic scale” is meant the homogeneous distribution of the carbon nanotubes on a scale of billionths of a metre. Carbon nanotubes are, on that scale, essentially separated from each other and practically form no agglomerates or aggregates.
By “nanocomposites” is meant composite materials having a polymer matrix and incorporating nanoparticles as a charge, that is particles of which at least one of the dimensions is less than or equal to 100 nm. It may also be the case that at least one of the dimensions of the particles is of the order of one to a few tens of billionths of a metre.
The use of a coating in order to obtain carbon nanotubes in a dispersed state within a polymer matrix is well known.
The originality of the present invention is based on the use of a polymer for coating carbon nanotubes, that is non-miscible compatible or incompatible with the polymer matrix. Surprisingly, this allows to obtain a homogeneous dispersion of carbon nanotubes within said polymer matrix on a nanoscopic scale. Moreover, this allows to improve the electrical characteristics of nanocomposites comprising carbon nanotubes dispersed as in the invention whilst at the same time preserving the mechanical properties of the polymer matrix forming these nanocomposites.
The method for coating carbon nanotubes using the dispersion method as in the invention may be that known by the name of the “Polymerisation Filling Technique” or “PFT” (
The coating used in the present invention may be that claimed in claim 1 of document WO 2005/012170.
The carbon nanotubes are preferably pre-treated in the way described in claim 2 as well as in paragraphs 97 and 98 and paragraphs 116 to 125 of document WO 2005/012170. The pre-treatment consists in settling a catalyst, known to catalyse the polymerisation of the monomer used for the coating, to the surface of the carbon nanotubes, the polymerisation is subsequently started directly on the surface of the nanotubes.
The catalyst and the catalyst/co-catalyst couple are preferably selected according to claims 6 to 9 of document WO 2005/012170 and advantageously according to the examples given in paragraphs 104 to 106, and the polymerisation of the coating polymer may be achieved according to the method described in paragraphs 126 to 130 of document WO 2005/012170.
The polymerisation, achieved at the surface of the nanotubes in order to obtain a coating polymer, allows the dissolution of bundles, agglomerates or aggregates of nanotubes that usually form during the production of nanocomposites comprising carbon nanotubes. This coating has the effect of forcing the carbon nanotubes to separate from each other and thereby causing the dissolution of nanotube bundles.
Once they are coated, even with a small quantity of polymer, the carbon nanotubes can then be dispersed in a host polymer that is commercially available by traditional methods (internal blender, extruder, etc.). The dispersion obtained is homogeneous on a nanoscopic scale.
Surprisingly, and as shown in
Moreover, nanocomposites comprising carbon nanotubes dispersed by means of the method as in the invention have electrical conductivity equivalent to the composites described in the state of the art, but this electrical conductivity is however obtained with a quantity of carbon nanotubes well below that required in the case of the nanocomposites of the state of the art. In fact, whereas 1% by weight of carbon nanotubes (MWNT N700,
This surprising improvement in the properties of electrical conductivity, observed for a nanocomposite whose carbon nanotubes have been dispersed by means of the method as in the invention, lies in the use for coating of a polymer that is non-miscible compatible or incompatible with the polymer matrix of the composite. This non-miscibility or incompatibility between the coating polymer and the polymer matrix will allow the coating to play the part of a “transporter of carbon nanotubes” and thereby bring about homogeneous dispersion. The coating, and more particularly the polymerisation of the coating polymer achieved at the surface of the nanotubes, will allow each carbon nanotube to be kept separate. Then, during the incorporation of these carbon nanotubes into the polymer matrix, the coating, due to its non-miscibility (compatible or incompatible) and due to the fact that it does not have a covalent bond with the carbon nanotubes, will be literally “chased” off the surface of the carbon nanotubes. Therefore, as shown in
In a first embodiment example of the invention, the multi-wall carbon nanotubes (MWNTs) are coated in high-density polyethylene (HDPE) (
The non-miscibility and compatibility of the HDPE and the EVA at room temperature hav been the subject of numerous studies and is documented in particular in the “Polymer Handbook” 4th edition, Ed. J. Wiley and Sons, New York, 1999, by J. Bandrup, E. H. Immergut and E. A. Grulke.
The non-miscibility between EVA and HDPE is documented in detail in the following three publications:
In addition, the use of the HDPE and EVA (28% VA) couple allowed to perform tests on the dispersion of nanotubes coated in HDPE in the EVA matrix at different temperatures, allowing the two polymers either to stand both in the molten state, or for the HDPE to remain in a solid state and for the EVA matrix to be in a molten state. The difference in the melting temperatures between the coating HDPE and the EVA matrix (28% VA) being about 40° C.
The dispersion tests at two different temperatures nevertheless gave the same result in terms of the carbon nanotube dispersion on a nanoscopic scale (
For composites based on EVA (28% by weight of vinyl acetate), the mechanical properties are compared with those of a nanocomposite based on clay (Cl 30B): montmorillonite organomodified by methyl bis (2-hydroxyethyl) ammonium tallow) of an exfoliated type (nanosheets of clay homogeneously dispersed in a nanoscopic state)—see Table I.
These results show the transfer to the EVA matrix of the properties of the carbon nanotubes, that have been coated in HDPE. Compared with nanocomposites based on clay, carbon nanotubes that have been coated show an increase in the Young modulus, which indicates an excellent transfer to the EVA matrix of the rigidity properties of the carbon nanotubes.
In a second embodiment example of the invention, the multi-wall carbon nanotubes (MWNTs) are coated in a polyethylene polymer and incorporated into a polycarbonate matrix (Iupilon E 2000, Mitsubishi Plastics, Japan).
The polycarbonate and the multi-wall carbon nanotubes are premixed in the form of powder, the polycarbonate being dried at 120° C. for at least 4 hours, before being mixed under heat (280° C.) with a DACA Micro Compounder blender for 15 minutes at 50 revolutions per minute. The plates obtained after pressing at 280° C. have a thickness of 0.35 mm and a diameter greater than 65 mm.
Surprisingly, and as shown in
Thus, the quantity of polymer for coating the nanotubes affects the dispersion quality of the carbon nanotubes within the matrix and as a result, affects the electrical conductivity characteristics of the nanocomposite; in fact, for a composite with a polycarbonate matrix comprising 0.25% by weight of MWNTs that are coated then dispersed by means of the method as in the invention, the use of a polyethylene coating, being 78% by weight of the total weight of the coated carbon nanotubes, allows to obtain a nanocomposite with better electrical conductivity than a nanocomposite in which the coating is only 56% by weight of the total weight of the coated carbon nanotubes.
As the electron microscope images (
In a third embodiment example of the invention, multi-wall carbon nanotubes (MWNTs) are coated with high-density polyethylene and dispersed by means of the method as in the invention in a polyamide matrix (Capron 8202).
The polyamide and the multi-wall carbon nanotubes are mixed under heat (240° C.) with a DACA Micro Compounder blender for 15 minutes at 50 revolutions per minute. The plates obtained after pressing at 240° C. have a thickness of 0.6 mm and a diameter greater than 65 mm.
As shown in
As shown in
In a fourth embodiment example of the invention, the multi-wall carbon nanotubes (MWNTs) are coated in high-density polyethylene and dispersed by means of the method as in the invention in a PEEK matrix.
Table II shows the influence of the carbon nanotubes, dispersed by means of the method as in the invention or dispersed in the usual way, on the behaviour of the nanocomposite subjected to the tensile modulus test and to the bending modulus test.
As shown in this table, after the successive stages of extrusion, injection and casting, the nanocomposite with a PEEK matrix thus obtained and which comprises 1.5% by weight of coated multi-wall carbon nanotubes (Table II) has performances in the tensile modulus test that are comparable to those obtained by a nanocomposite with a PEEK matrix comprising 5% by weight of uncoated multi-wall nanotubes (Table II). This observation is equally valid with regard to the results obtained in the bending modulus test.
In the four above-described embodiment examples of the invention, the coating of the carbon nanotubes may be achieved by the method described in document WO 2005/012170.
The dispersion of carbon nanotubes as in the invention achieves particular properties in polymer matrices into which they are incorporated. As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 05447125.5 | May 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BE06/00061 | 5/24/2006 | WO | 00 | 11/26/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60783784 | Mar 2006 | US |
