A NEW GENERATION OF CONSTRUCTION MATERIALS: CARBON NANOTUBES INCORPORATED TO CONCRETE AND POLYMERIC MATRIX

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1 A NEW GENERATION OF CONSTRUCTION MATERIALS: CARBON NANOTUBES INCORPORATED TO CONCRETE AND POLYMERIC MATRIX Javier Grávalos, Juan Manuel Mieres and Santiago González R&D Department, NECSO Entrecanales Cubiertas S.A., Spain Abstract Due to their exceptional mechanical, physical, thermal and electrical properties, carbon nanotubes have a great potential in materials sciences are being part of many technical papers and are turning into reality in some commercial products. NECSO Entrecanales Cubiertas, at the head of innovation in construction, is developing this nanotechnological project to study new possibilities in construction. The difficulty to incorporate carbon nanotubes to create new materials with improved properties is to achieve a good dispersion in the matrix. This project studies the effects of the variables which can affect the dispersion, to decide the best way to fabricate a new generation of materials. A Tests Program is drawn up, to measure mechanical and physical properties of carbon nanotubes composites At the end of the project will be available a complete study of the factors that have to be used to achieve the better properties. This is needed to create high-performance materials to be part of materials in construction. Keywords: Nanotubes, Dispersion, Mechanical, Electrical, Properties, Characterization. 1. INTRODUCTION Carbon Nanotubes (CNT) are probably the most interesting material discovered in the last decades. Their outstanding properties have caught the attention both of the scientific community and of several types of industries. It can be said that this material is, currently, the best known material of this revolution that is just starting and called: nanotechnology. So far, many experimental studies have been performed on the properties of the nanotubes, measured with transmission electronic microscopic devices (TEM) and atomic force microscopic devices (AFM), which have resulted in theoretical estimates of the Young s

2 Modulus of the order of 1 TPa. 1 By way of comparison, this value for a conventional carbon fibre amounts to 400 GPa 2. Another interesting property of these materials is their elasticity (they can withstand large strains before they actually break and they extraordinarily recover their properties when the load is taken away). In addition to excellent mechanical properties, their electrical and thermal conductivity is very high. All this gives these materials a great potential, both for microelectronics and power applications, as well as for the creation of new materials. It is in this field where the construction industry, in general, and NECSO, in particular, shows great interest. The construction industry was one of the first to recognise nanotechnology as a future line of investigation. It is currently lagging behind other industries, such as the electronic, automotive and chemical industries, where very promising results have been obtained and where marketing of these products has already started. 2. INTEREST IN THE CONSTRUCTION INDUSTRY The application of these materials in the construction industry is still very low and fragmented and has only been developed in scientific circles. Even so, new materials are being developed through the incorporation of nanomaterials, such as carbon nanotubes. The potential for application to the construction industry is enormous: paints, pigments and coatings, as well as for the improvement of the properties of polymers, steel and concrete. More specifically, the research efforts on nanotechnology for application to the construction industry have focused on the study of new materials, based on the incorporation of nanostructures into traditional materials, with the aim of improving their electrical, thermal and mechanical properties, that would allow the develop of final products, such as sensors, structural materials, new fibres, self-cleaning materials, storage structures, power transmission, etc. Furthermore, the mechanical properties of the carbon nanotubes make them ideal reinforcements for the fabrication of composites. The matrices into which they can be incorporated could be: cements, polymers and glass, materials in which the construction industry has special interest. The problem that presents itself is that it will be necessary to combine the macroscopic properties of the end products with the difficulties that arise from the nanoscopic nature of these new materials (some effects at the molecular and quantum level of the matter), with the dispersion in the matrices and the adhesion to them being the primary examples of the obstacles we shall have to overcome if we want to boost the properties of the carbon nanotubes in the fabrication of new materials. 3. GOAL OF THE RESEARCH The goal of the nanotechnology project, developed by NECSO s Research & Development Department, is to expand current knowledge in the nanotechnology field, using a comprehensive study on the effects caused by the incorporation of Carbon Nanotubes to traditional materials, widely used in the construction industry, such as cement and epoxy resins and, from this basis, to develop the ability to fabricate multifunctional materials, with their properties enhanced thanks to nanotechnology. Within the term of this project two types of matrices have been used: cementitious and epoxide matrices, as well as carbon nanotubes, of the Multi-walled NanoTube (MWNT) type, from which three different typologies have been chosen for the study. The process of incorporating the nanotubes to the matrices was regarded as critical for the project initial

3 assessment. For this reason, four different methods have been used to assess the degree of validity in each case. 4. TEST PROCEDURE Carbon nanotubes, of the MWNT type, with three different typologies and purities have been used on this project: Table1: Geometrical characteristics of carbon nanotubes used. Typology Diameter Length Purity 1 10 nm 5-15 μm 95-98% 2 15 nm 5-15 μm 80% nm μm 95% To increase the nanotubes dispersion rate, two different treatments are applied: one using a diluted solution of sulphuric and nitric acids H 2 SO 4 /HNO 3, and another by means of SDS surfactant (Dodecyl sodium sulphate). 4.1 Types of materials to be tested For the tests, two types of materials have been fabricated: cementitious-matrix and polymeric-matrix nanocomposites. a) Polymeric Matrix: Polymeric-matrix materials were fabricated in two stages. The objective of the first one is to assess the impact caused by the incorporation of the nanotubes upon the chemical reaction between the epoxy resin components. On a second stage, and on the basis of the results obtained, it will be developed four mixing methods, with four different doses each. The reaction between the first resin component and the hardener lasts 6 hours at 20ºC, the time being cut in half if the temperature rises ten degrees. If the CNTs are added to the first resin component, a reduction of its mechanical properties is observed, for which reason they must have an impact on the chemical reaction, either by delaying it or by acting as an inhibitor. To assess this, epoxy resin has been prepared and 10% of CNTs has been added at the following intervals: immediately, after 15 minutes, after 30 minutes, after 1 hour and after 2 hours, leaving one sample without nanotubes, for reference purposes. The results showed an improvement after 60 minutes, for which reason the tests have been repeated with 1% of nanotubes, to check the results with smaller doses.

4 E( MP a ) Figure 1: Young Modulus vs time of aggregation of CNT. On the basis of the results, four types of treatments were performed in the most appropriate manner. Under all of these treatments, samples with doses of 0, 1, 2.5 and 5%, by weight of CNT, were prepared: 1. Direct mixing. The nanotubes are mixed directly in resin, for subsequent curing, using a hand mixing method. Two series are performed, one with the nanotubes using a chemical treatment and another one with the nanotubes, previously subjected to surfactant treatment. 2. High power mixing. The nanotubes are mixed, but the mixing is done with a powered stirrer. This method is used only with those nanotubes from the series from where the best results were obtained during process Ultrasound mixing. The two series of nanotubes are subjected to dispersion, using ultrasound techniques, and then are mixed by hand. 4. High power mixing with ultrasounds. Process two is repeated, by previously dispersing the nanotubes, using ultrasound techniques. It is done with the nanotubes with acid treatment or with surfactant treatment, depending on the previous results. b) Cementitious matrix: Under the cementitious matrix, the CNTs are incorporated into the mixing water and then cement-mortar samples are prepared. Previous treatments are repeated in the same manner, in the suspension of CNTs in the water, to be subsequently incorporated into the cement and sand. The prepared samples are cured in a wet chamber until they are subjected to standard tests.

5 Figure 2: Some mortar test samples. 4.2 Tests to be performed Mechanical compression, bending and indirect tensile tests are performed. For this purpose, samples of appropriate dimensions are prepared. Figure 3: Example of the results given by the test machine. Four samples are prepared for each type of test. The tests are realized in NECSO s R&D laboratories, specifically on a multi-test press that records all data and charts the results, for subsequent study and evaluation of conclusions.

6 Figure 4: Compression tests In turn, electrical conductivity tests have been conducted on all the prepared samples, to assess these properties on the basis of the nanotube dosing, as well as on the treatments and mixing methods being used. So far, conductivity has been achieved in samples which, without nanotubes, have none. These conductivities have a wide range, from 0.01 S/m to S/m 5. CONCLUSIONS Figure 5: Electrical conductivity in epoxy test sample. Carbon nanotubes have been the subject of numerous studies and have arisen great expectations. Now, it is high time to assess all the potential they can offer. As stated above, any possible applications to the construction industry shall consist in their incorporation to conventional materials. Their subsequent development calls for a survey to characterise these

7 materials. Once this test campaign is over, sufficient data shall have been gathered to perform a realistic assessment of the suitability of the carbon nanotubes for composite materials, and to focus the investigation on the basis of the improvements we need to achieve, as far as treatments and incorporation procedures are concerned and access shall have be gained into the different variables affecting the composites final properties. The purpose of this study is to serve as a basis for the preparation of a road map that leads us to the creation of nanotechnology-based end products, aimed at the construction industry. REFERENCES [1] Zhihang Fan, Luang-Ting Hsiao, Suresh G. Advani., Experimental investigation of dispersion during flow of multi-walled carbon nanotube/polymer suspension in fibrous porous media, Carbon 42 (2004) [2] Erik T Thostenson and Tsu-wei Chou, On the elastic properties of carbon nanotubes-based composites: modelling and characterization, J. Phys. D: Appl. Phys. 36 (2003) [3] L. Roy Xu, Vikram Bhamidipati, Wei-Hong Zhong, Jiang Li and Charles M. Lukehart, Mechanical property characterization of a polymeric Nanocomposite reinforced by graphitic nanofibers with reactive linkers. Journal of composite materials, 38 18/2004 [4] S.J.V. Frankland, V.M. Harik, G.M. Odegard, D.W. Brenner, T.S. Gates, The stress-strain behaviour of polymer-nanotube composites from molecular dynamics simulation. Composites Science and Technology 63 (2003) [5] Y.J. Liu, X.L. Chen, Evaluations of the effective material properties of carbon nanotube-based composites using a nanoscale representative volume element. Mechanics of Materials 35 (2003) [6] Youngjong Kang and T. Andrew Taton, Micelle-Encapsulated carbon nanotubes: a route to nanotube composites. J. Am. Chem. Soc , [7] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 40 (1999)