WO2009150690A1

WO2009150690A1 - Polymer and carbon nanotubes composite materials as low-cost temperature sensors

Info

Publication number: WO2009150690A1
Application number: PCT/IT2009/000261
Authority: WO
Inventors: Heinrich Christoph Neitzert; Andrea Sorrentino; Luigi Vertuccio; Liberata Guadagno; Vittoria Vittoria
Original assignee: Universita' Degli Studi Dl Salerno
Priority date: 2008-06-13
Filing date: 2009-06-12
Publication date: 2009-12-17
Also published as: EP2285888A1; ITSA20080014A1

Abstract

The invention concerns a system based on polymeric composites with carbon nanotubes to obtain low-cost and high accuracy temperature sensors. The composite material is obtained by homogeneously dispersing a suitable amount of carbon nanotubes having conductive characteristics within a polymer matrix. The sensing device is obtained by connecting at least two electrodes to the polymer-carbon nanotubes composite and measuring the electrical resistance of the system obtained. The composite material and the articles consisting of the said material can be applied in all those situations requiring a low-cost sensor having even complex geometrical shape.

Description

POLYMER AND CARBON NANOTUBES COMPOSITE MATERIALS AS LOW-COST TEMPERATURE SENSORS

DESCRIPTION

Field of the invention

The present invention concerns composite materials made of polymer and carbon nanotubes, to be applied as low-cost temperature sensors. More particularly, the invention relates to a system to obtain temperature sensors having reduced cost but high sensitivity, to the preparation of polymer-carbon nanotubes composites and to their use as temperature sensors. Background of the invention

Several thermosensitive elements are presently available on the market, such as, for example, the common resistance thermometers (or resis- tance temperature detectors) and the thermocouples. All these devices are able to provide an electrical signal proportional to the measured temperature. Even if many of said thermosensitive elements may be obtained at a low-cost due to the modern manufacturing techniques employed, when the same elements are to be used for a specific application or the electrical signal provided by them is to be used in a specific control circuit, the assembly costs become very high, while a poor sensitivity to the particular temperature variability ranges that have to be detected may frequently occur.

Therefore, there was a felt need to have available a simple system to obtain low-cost temperature sensors with however complex geometries, which could be adapted to any specific application, and also a need to have a system for realizing tailored temperature sensors having each time the desired shape and sensitivity features.

To that aim, according to the present invention, it was taken in consideration the possibility of using the particular electric properties of carbon nanotubes to realise versatile and cheap temperature sensors, by incorporating such elements into a polymeric matrix of the desired geometry.

It is well known that carbon nanotubes are solid inorganic structures that may be considered allotropic forms of carbon, essentially formed by one or more rolled graphite layers, with a diameter comprised between 1 and 50 nm and a length/diameter ratio equal or above 1000. When the structure is formed by a single rolled layer of graphite it is called single-walled nanotube (SWNT), and when the graphite layers are multiple, coaxially rolled on one another, the structure is called multi-walled nanotube (MWNT). The structural organization of the carbon atoms provides to the carbon nanotubes several particular physical properties, such as a greater mechanical resistance compared to any other material, a thermal conductivity in the direction of the main axis that is better than any other material, an electrical conductivity that may be different according to the dimensional and structural characteristics of the nanotube, and which allows to use such structures as optimal conductors, with conductivities comparable to copper or gold, or also as semiconductors.

The particular properties of carbon nanotubes have already been used, in particular, for the production of composite materials by combining them with polymeric-type matrixes to obtain improved and specific mechanical, thermal and electrical characteristics. Examples of composite materials consisting of single-walled nanotubes dispersed or encapsulated in polymeric matrixes are described in the international patent application publ. No. WO02/16257 (William Marsh Rice University). This document hypothesizes a huge variety of possible composition variants and preparation methods, but the only potential applications proposed in the document for the composite polymer-carbon nanotubes are the use in antennae and electro-optical and electromagnetic devices. Summary of the invention

An object of the present invention is to realize a new temperature sensor characterised by a low cost and, at the same time, extremely sensible to temperature variations, and, preferably, a sensor which, in addition to being very sensible, is also extremely versatile and cheap when used in different applications, without any particular loss of performances.

Briefly, the new temperature sensor of the present invention comprises a conventional polymeric matrix where a sufficient quantity of carbon nanotubes having a high electric conductivity are homogeneously dispersed. At least two electrodes are then applied to the composite, in a possibly symmetric position with respect to the zone to be monitored. Finally, wires of conductive material, with a suitable cross-section, are electrically connected to the electrodes and used to collect the electric signal provided by the sensor.

Therefore, there is proposed, according to the invention, a composite material consisting of carbon nanotubes with characteristics of high electrical conductivity incorporated in a polymeric matrix in such a way as to provide a material the electrical resistance of which varies in a regular manner with the temperature variation. The corresponding temperature sensors may be obtained by connecting at least two electrodes to the composite material, in a proper position with respect to the zone where the temperature has to be detected. Given its constructive features, the proposed sensor may be easily applied in all those situations where a low-cost sensor is required, having even complex geometries.

The polymeric matrix may be obtained either from a thermoplastic polymer or from a thermosetting polymer. In the first case, the introduction of the carbon nanotubes in the polymeric matrix may be obtained by heating the thermoplastic matrix at a temperature higher than the melting temperature or the glass transition temperature of the polymer, and by vigorously stirring it with the carbon nanotubes, manually or by means of any mechanical tool, until obtaining a homogeneous mixture. During the subsequent cooling the composite obtained can be shaped and formed so as to reach the desired shape. In the case of thermosetting matrixes, the nanotubes dispersion is obtained at room temperature by directly mixing the nanotubes with the matrix precursors. Once a good dispersion is obtained, the composite is heated to the cross-linking temperature and left to polymerise until a material with the desired mechanical characteristics in obtained. Also in this case, the curing reaction may be carried out in such a manner to obtain an article with the desired geometrical features.

As a result, a temperature sensor suitable for many applications may be obtained; in addition, although using inexpensive components it is possible - A -

to model the sensor in the shape most suitable to each single purpose, and the performance desired may be obtained, in terms of sensitivity and accuracy of the measures.

Another object of the present invention is a system to improve the per- formance of a polymer matrix, wherein the carbon nanotubes, either single or agglomerated, are dispersed to form a three-dimensional network. Such network limits the sliding of the polymer chains and enhances the mechanical features of the material obtained.

According to a further aspect thereof, the present invention also com- prises a process for the preparation of the temperature sensor proposed, which process comprises dispersing the conductive carbon nanotubes in the polymer matrix and shaping the same according to the particular application sought.

The articles made by the system according to the invention, which are endowed with the property of presenting an electrical conductivity variable as a function of the temperature, may be in the form of film, pipes, panels, seals, tubs, bottles, bags, multilayer articles or any other articles having any suitable shape. Such articles may be not only in a solid form, but also in semi-solid or liquid form, etc., and may also be characterised by mechanical properties optimised as a function of the final intended use.

Still further objects of the invention are the articles made or coated with a single or multiple layer of a material according to the invention. Detailed description of the invention

Therefore, the present invention specifically provides a composite temperature sensing material comprising: (i) a polymer matrix; (ii) electrically conductive carbon nanotubes homogeneously dispersed within the said matrix.

The electrical conductivity of the composite material provided can vary as a function of the temperature due to the presence of the carbon nanotubes. Thus, through a proper choice of the nanotubes characteristics and of the material composition, it is possible to realize with such materials suitable temperature sensors. Typically, the system proposed according to the invention comprises from about 90% to 99.99% by weight of polymer matrix (i) and from 0.01% to 10% by weight of carbon nanotubes (ii). The preferred compositions are from 95% to 99.99% by weight of polymer matrix (i) and from 0.01% to 5% by weight of carbon nanotubes, and more specifically, from 97% to 99.90% by weight of polymer matrix (i) and from 0.1 % to 3% by weight of carbon nanotubes.

According to the present invention, by "polymeric matrix" a solid polymer that may derive from a single polymer or from a mixture of homo- or co- polymers, possibly partially or totally crosslinked, and wherein another

Polymers that may advantageously be employed according to the invention are synthetic or natural biodegradable or biocompatible polymers. Some non-exhaustive examples of the classes of polymers that may be used are as follows: polyolefins, polyethylene glycols, polycaprolactones and poly- esters, polylactides, polyanhydrides, polyvinylpyrrolidones, polyurethanes, pblysiloxanes, polyaminoacids, polyacrylates and polymethacrylates, polyam- ides, polyimides, polyanilines, polyacrylonitriles, silicones, polyether-ketones, polyether-ether-ketones, high density and low density polyethylenes, polypro- pylenes, polystyrenes; natural polymers such as polysaccharides in general, starches, celluloses, chitins, chitosans, pectins, gelatins, proteins, polypeptides; phenolic resins, ureic resins, melamine resins, epoxy resins, unsaturated polyester resins and thermosetting polymers in general, individually taken or in mixture with one another.

The mentioned polymers may be possibly functionalized and option- ally partially or totally crosslinked. In addition, the matrix may further comprise one or more additives selected from the group consisting of antioxidants, stabilizers and plasticizers.

The carbon nanotubes of the composite are chosen from known types, namely single-walled nanotubes (SWNT) a nd multiple-walled nano- tubes (MWNT) according to their characteristics relevant to the invention, in particular, thermal conductivity and dependency of electrical resistivity on temperature. In particular, some specific types of single-walled carbon nano- tubes may be chosen referred to as chiral nanotubes, zigzag nanotubes, armchair nanotubes, as well as oxidized, purified or functionalized nanotubes. Said constitutive elements of the composite may be dispersed at a micromet- ric or nanometric level in the polymer matrix. It has been observed that the incorporation of the intercalation compounds according to the invention into the polymer matrix enhances its mechanical properties (e.g. the elastic modulus and the tensile strength), its thermal properties (e.g., the glass transition temperature and the thermal degradation temperature of the polymer) and its properties of gas- and va- pour-permeability, this allowing the manufacture and processing of articles having a high mechanical modulus and a good tenacity.

The carbon nanotubes may be superficially treated, oxidized, purified or functionalized with organic molecule in order to improve their dispersion in the polymeric matrix chosen. The said carbon nanotubes are then incorporated in a selected polymer matrix. The latter may be biodegradable or non-biodegradable, according to the intended use. The said matrix can also be a copolymer or a mixture of polymers and/or copolymers, as pointed out in the foregoing.

Some parameters to be taken into account in the preparation of the system according to the invention are as follows: a) type of polymeric system; b) concentration of carbon nanotubes in the polymeric matrix - as noted, said concentration may advantageously range from 0.01 % and 10% by weight referred to the matrix; c) type of incorporation process, the conditions of which are chosen by the person skilled in the art according to his personal knowledge. The incorporation process may be one of the following ones:

^■ from solvent, by casting;

^■ from the melt, by die-casting or extrusion; ■ manual mechanical mixing or mixing by means of a mill;

^■ by sonication.

Once the system according to the invention is obtained, it can un- dergo further working steps in order to obtain the articles intended for the final use. Some of such working procedures and some final articles are shown by way of example below, and are accessible to any person skilled in the art.

Films and membranes, either compact or porous, may be obtained by die-casting or by solvent casting.

Specific articles, having a prefixed shape, can be obtained by injection- moulding in the specific form, or they can be worked on a lathe, extruded or shaped.

Multilayer articles, wherein at least one layer is made of the composite material according to the invention.

The electrical features of the system, on which the sensitivity of the proposed sensor depends, can be varied and controlled in a wide range. By using the indications in the instant description and his/her own knowledge in the field, a person skilled in the art is able to find the most suitable conditions to carry out the invention. Generally, the aspects to be evaluated are the following ones:

• type of carbon nanotube;

• type of functionalization of the carbon nanotube;

• type of polymer matrix, which may comprise homopolymers, copolymers or their mixtures;

• concentration of carbon nanotubes in the polymer matrix;

• dispersion grade of the carbon nanotubes in the polymer matrix;

• type of working of the article (compact or porous).

Therefore, it is evident that by suitably choosing and controlling the above parameters it is possible to produce articles suitable for a great number of applications.

In addition, according to the percentage of carbon nanotubes present therein, the system of the invention acquires unexpected mechanical properties (increase of the compression resistance, of the resistance to high tem- peratures, of tensile strength, increase of the moldability into even complex forms).

According to some other of its aspects, the present invention thus concerns a device for temperature sensing comprising a composite material as defined above, electrically connected to two or more electrodes, as well as the use of a composite material as defined above, electrically connected to two or more electrodes, as well as the use of a composite material as defined above as a temperature sensor.

As noted before, the method of preparing a composite temperature sensing material as defined above essentially comprises the following steps: treating the carbon nanotubes so as to make them compatible with the polymer matrix and to thus obtain a nanometric filler, and then mix said filler with the polymer matrix. The mixing can be carried out with the proper polymer, at the suitable temperature, in the event that the matrix is thermoplastic, or with some polymer precursors in the event that a thermosetting matrix is involved. In the latter case polymerization is obtained by adding the proper cross-linking agent to the mixture of polymer precursor and nanotubes already prepared. According to some further aspects thereof, the present invention also concerns articles realized by means of the process described above and shaped in such a prefixed geometrical shape, articles and products in the form of compact or porous membranes, film, gel, sponges and multilayer articles. In particular, multilayer coatings may be foreseen, comprising at least one layer manufactured according to the procedure defined above and containing composite temperature sensing materials according to the invention.

The specific features of the present invention, as well as its advantages and the corresponding ways of operating it, will be more evident with reference to the examples presented for non limitative purposes further on, together with the results of some experimentation carried out on the invention.

Brief description of the drawings

The operating examples and the corresponding results are also shown in the enclosed drawings, wherein:

Figure 1 shows the schematic picture of a temperature sensor ac- cording to the invention, equipped with copper electrodes previously inserted in a liquid composite material and then left to cross-link together with the composite; and Figure 2 shows the dependency of the current from the temperature with a linear fit, measured with a "Keithley 2400" type Source Measurement Unit, during the cooling down of the sample from 16O⁰C to 45°C and applying a voltage of 200 V. EXAMPLE 1

The non-functionalised "multi-walled" carbon nanotubes (Nanocyl®- 3100) were purchased from NANOCYL S.A. (Rue de I'Essor, 4 B-5060 Sam- breville, Belgium). They had been produced by "catalytic carbon vapour deposition" (ccvd). Such nanotubes were purified until obtaining a carbon percent- age above 95%. The polymeric matrix consisted of an epoxy matrix of di- glycidyl ether of bisphenol A (DGEBA) supplied by Sigma-Aldrich. Such resin is formed through a condensation reaction between epichlorohydrine and bisphenol A, catalyzed by a base (NaOH). The reaction is carried out with an excess epichlorohydrine, so as to limit the production of high molecular weight products.

The resin used is mainly formed, i.e. 87-88%, by di-glycidyl ether. This mixture gives rise to a resin with average molecular weight of about 370 g/mol, epoxy equivalent weight (EEW) of 180-200, viscosity at 25°C of 1000- 1800 Pa*s and density of 969 kg/m³, the overall n being about 0.2. As the curing agent an aromatic primary diamine has been chosen, i.e. DDS.

DGEBA has been first heated to 50-80⁰C and then degassed for 45 min at 7O⁰C in oven under vacuum. Then, an amount of 0.5% of nanotubes has been incorporated with the resin by sonication over 20 minutes; this tech- nique involves applying mechanical energy through the application of ultrasounds to the material to be mixed.

The mixture obtained, placed in an oil bath at 125°C, was added with DDS, in different amounts equal to 100% of the stoichiometric amount, calculated on the equivalent weight of the epoxide. Then the mixture was stirred with a magnet (at 400 rpm) for about one hour until a homogeneous solution was obtained. The compound obtained was cured by keeping it in oven 180°C for 3 hours.

As shown in Figure 1 of the enclosed drawings, the samples obtained, in the form of a parallelepiped with sides of 35x10x3 mm, were provided at the farthest ends thereof with two copper electrodes, previously in- serted in the liquid composite and left to cross-link together with the composite (Sample A).

EXAMPLE 2

In order to carry out the electric tests a sample of DGEBA+DDS without carbon nanotubes was prepared for comparison (Sample B). Sample A of the previous example (DGEBA+DDS with 0.5% of carbon nanotubes) and sample B (DGEBA+DDS without carbon nanotubes) were subjected to electric tests during several heating cycles in oven, between 30 and 16O⁰C. The two samples were analysed with a "Keithley 2400" type Source Measurement Unit, applying a constant voltage comprised between 10 V and 200 V. Figure 2 of the enclosed drawings shows the behaviour of the electric current of sample A, while sample B turned out to be a total insulator, and the electric resistance between the two electrodes, applying a voltage of 200 V, was such as to prevent any kind of measure.

Figure 2 shows, in particular, the dependency of the current on the temperature with a linear fit, measured with a "Keithley 2400" type Source Measurement Unit, during the cooling down of the sample from 16O⁰C to 45°C and applying a voltage of 200V. From the Figure it may be seen that on varying the temperature, the electric current shows a perfectly linear behaviour. In addition, it has been observed a remarkable phenomenon which represents a further advantage of the sensing material and device according to the invention, namely that the behaviour of the electric current is perfectly reproducible even after a great number of heating and cooling cycles.

The present invention has been disclosed with particular reference to some preferred embodiments thereof but it is to be understood that modifica- tions and changes may be brought to it without departing from its scope as recited in the appended claims.

Claims

1. A composite temperature sensing material comprising: (i) a polymer matrix; (ii) carbon nanotubes homogeneously dispersed within the matrix, said carbon nanotubes being electrically conductive.

2. A composite material according to claim 1, the electrical conductivity of which is variable as a function of the temperature.

3. A composite material according to claims 1 or 2, comprising from 90% to 99.99% by weight of polymeric matrix (i) and from 0.01% to 10% by weight of carbon nanotubes.

4. A composite material according to claim 3, comprising from 95% to 99.99% by weight of polymeric matrix (i) and from 0.01% to 5% by weight of carbon nanotubes.

5. A composite material according to claim 4, comprising from 97% to 99.90% by weight of polymeric matrix (i) and from 0.1 % to 3% by weight of carbon nanotubes.

6. A composite material according to any one of claims 1-5, wherein said polymeric matrix comprises at least one synthetic or natural biodegradable or biocompatible polymer selected from the group consisting of: polyolefins, polyethylene glycols, polycaprolactones and polyesters, polylac- tides, polyanhydrides, polyvinylpyrrolidones, polyurethanes, polysiloxanes, polyaminoacids, polyacrylates and polymethacrylates, polyamides, polyim- ides, polyanilines, polyacrylonitriles, silicones, polyether-ketones, polyether- ether-ketones, high density and low density polyethylenes, polypropylenes, polystyrenes; polysaccharides, starches, celluloses, chitins, chitosans, pectins, gelatins, proteins, polypeptides; phenolic resins, ureic resins, mela- mine resins, epoxy resins, unsaturated polyester resins, individually taken or in mixture with one another, optionally functionalized and optionally partially or totally crosslinked.

7. A composite material according to claim 6, wherein said matrix further comprises one or more additives selected from the group consisting of antioxidants, stabilizers and plasticizers.

8. A composite material according to any one of claims 1-7, wherein said carbo n nanotubes are selected from single-walled carbon nanotubes (SWNT), multiple-walled carbon nanotubes (MWNT), chiral nanotubes, zigzag nanotubes, armchair nanotubes, oxidized nanotubes, purified nanotubes and functionalized nanotubes.

9. A composite material according to any one of claims 1-8, wherein the carbon nanotubes are dispersed at a micrometric or nanometric level in the polymeric matrix.

10. A temperature sensing device comprising a composite material as defined in claims 1-9, electrically connected to two or more electrodes.

11. Use of a composite material as defined in any one of claims 1-9, as temperature sensor.

12. A process for preparing a composite temperature sensing material as defined in claims 1-9, comprising the steps of treating said carbon nano- tubes in such a way as to confer on them compatibility with the polymeric matrix, thus obtaining a nanometric filler, and mixing said filler with the polymeric matrix.

13. Articles manufactured by the process as defined in claim 12, and modelled in a prefixed geometrical shape.

14. Articles according to claim 13 in form of compact or porous membranes, film, gel, sponges or multilayer articles.

15. Multilayer coatings comprising at least one layer manufactured by the process as defined in claim 12.