Effect of multiwall nanotube on the properties of polypropylenes A. M. Ádámné1, K. Belina1 1
Kecskemét College, Institute of Metal and Polymer Technology – Kecskemét, Izsáki u. 10., H-6000, Hungary URL: www.gamf.hu e-mail:
[email protected];
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ABSTRACT: The variety of plastics is enormous; however in some cases there is no pure material to fulfill all the requirements. In these cases it is important to prepare the desired material by mixing pure polymeric materials. Different composition of carbon nanotubes and polymers were produced by a special mixing unit called Infinitely Variable Dynamic Shear Mixer (IDMX). In the experiments polypropylene and polycarbonate polymers were used as matrix materials. Nanotube masterbatches were used to prepare different compositions. Concentration series were manufactured by the dynamic mixer. The nanotube composites were granulated and test pieces were injection moulded. The prepared materials were characterised by scanning electron microscopy. Mechanical, electrical and burning properties of the materials were also determined. The order of magnitude increase of the conductivity of the composites was 8 compared to the pure polymer. The ultimate elongation decreases with increasing nanotube concentration. The composites show more rigid behaviour than the pure materials. Key words: Polymer composites, Carbon nanotube, Mechanical properties
1 INTRODUCTION In the last ten years carbon nanotube composites are in the focus of the researchers of polymer blends. The carbon nanotube is one type of fullerenes. The usage of carbon nanotube-polymer composites is very important because of the increase of the conductivity of the polymer [1]. Carbon nanotube decreases the resistance of the polymer, so electrostatic discharge can be avoided. It was also found that the mechanical properties (modulus, strength) can be enhanced by adding carbon nanotube to virgin polymer [2,3]. In addition, other properties like thermal stability, fire behaviour and others can be influenced favourably by using carbon nanotube [4-6]. Mixing of polymers is carried out in melt to achieve homogeneous properties. A special mixing unit called Infinitely Variable Dynamic Shear Mixer (IDMX) is used to produce blends in our department. This is a new type of dynamic mixer containing stationary and rotating elements to
improve the shear and strain fields in the polymeric melts [7]. Now melt mixing of nanotubes into polymeric materials plays an increasing role [8,9]. This method seems to be the preferred method for industrial manufacture of carbon nanotube polymer composites. In our experiments different nanotube composites were prepared. Polypropylene and polycarbonate polymers were used as matrix materials. Nanotube master batches (Hyperion Catalysis) were used to prepare different compositions. The nanotube composites were granulated and test pieces were injection moulded. The composites were investigated by scanning electron microscopy. Mechanical, electrical and burning properties were also studied. 2 EXPERIMENTAL Polypropylene homopolymer (TIPPLEN H649FH, TVK Rt) and polycarbonate with glass fiber (LEXAN 2110, GE) were used for the experiments.
Carbon nanotube master blends (Hyperion Catalyst, USA) were used to prepare the different composites by the dynamic shear mixer. The temperature profiles of the extruder were 190 °C, 210 °C, 220 °C, 220 °C for the polypropylene composites, 270 °C, 280 °C, 290 °C, 290 ºC for polycarbonate composites from the inlet to the dynamic mixer. The mixer temperature was set at 220 °C, 220 °C and 260 °C, 245 ºC for the PP and PC composites, respectively. Test pieces were injection moulded by ARBURG Allrounder 270 U 350-70. The melt temperature was 220 ºC and 285 ºC for the polypropylene and for the polycarbonate, respectively. The mould temperature was 50º C in both cases. Scanning Electron microscopic pictures were made by field-emission scanning electron microscopy (FESEM, Hitachi-S4700). The materials were broken under liquid nitrogen.
The carbon nanotube can be seen on the fractured surfaces. It is important to emphasise that the distribution of the nanotube in the matrix material is more or less uniform. We did not find any sign of agglomerates in the materials. Having smaller magnification, it is clear from the SEM investigation that the structure of the polypropylene material has changed. It also supports that the carbon nanotube has nucleating effect on polypropylene [11]. As the diameter of the nanotube is in the range of a few ten of nanometers, we assume that some kind of prearranged polypropylene chains are inside the tube. This can enhance the crystallisation rate of the material.
3 RESULTS AND DISCUSSION Figure 1 shows a characteristic SEM micrograph of the fractured surface of the pure polypropylene. The sample preparation was carried out in the same way as in [10]. Fig. 2.a. SEM micrograph of the fractured surface of PP containing 0,5%carbon nanotube.
Fig. 1. SEM micrograph of the fractured surface of PP.
The surface of the fractured sample is relatively smooth, and can not be seen any characteristic structure in it. SEM micrograph of the fractured sample of 0,5% and 8% carbon nanotube containing polypropylene composite is shown in Fig. 2.a. and 2.b. Comparing the fractured surfaces of the virgin and the nanotube containing polypropylenes it is evident that the latter one has a different structure.
Fig. 2.b. SEM micrograph of the fractured surface of PP containing 8% carbon nanotube.
The relative polycarbonate shown in Fig. polycarbonate
Charpy impact strength of the and polypropylene composites are 3. It can be seen that in the case of the impact strength does not change
up to 1% carbon nanotube content, while definite decrease can be noticed in polypropylene composites. This also support that the crystal structure of the polypropylene has been changed. The impact strength of the crystalline polymers are connected to the number of tie molecules. These molecules are in the amorphous phase, therefore the reduction of it increases the rigidity of the material.
The order of magnitude increase of the conductivity of the composites was 8. It is very important that only 0,5% carbon nanotube increases dramatically the electric conductivity of the materials. It was found that the nanotube concentration has negligible effect on the conductivity of the composites in the range of 0,5% - 8%, however some increase can be seen in the case of polycarbonate (Figure 5.). It suggests that there are some inhomogenity in these materials.
Fig. 3. Relative impact strength of PP and PC composites having different carbon nanotube content.
The tensile mechanical properties show similar changes. The change of the yield strength by the nanotube content is shown in Figure 4. When the polymer matrix is polypropylene the increasing content of the carbon nanotube does not influence the yield strength significantly. It also enforces the nucleating effect of the nanotube in polypropylene. In the case of polycarbonate, the yield strength decreases dramatically above 2% nanotube content. We assume that homogeneity of these samples were not satisfactory.
Fig. 4. Change of the yield strength by the concentration.
Fig. 5. Change of the specific resistance of PP and PC composites having different carbon nanotube content.
We also found that the burning properties decrease with the increasing concentration of carbon nanotube Fig. 6. Polypropylene nanotube composites above 4% nanotube content were not dripping. It is most probable due to some kind of “cross-linked” structure of the material. We assume that the carbon nanotube forms a physical network. This assumption is supported by the MFI measurements of the composites. Increasing the nanotube content of the materials, the MFI value drops dramatically from the original 4.7 g/10 min (virgin polypropylene) to 0,7 g/10 min (8% nanotube content). This change means that the viscosity of the polypropylene composites increases with increasing carbon nanotube content. As there is no chemical change in the material, a physical network should be formed. It supports the non-dripping properties of the material.
also thank to Desi Csongor for the support of the IDMX® mixer. This project is sponsored by Baross-3-2005-0008, Gábor Baross programme of the National Office for Research and Technology, and Kecskemét College Faculty of GAMF.
REFERENCES 1. Fig. 6. Burning speed of polypropylene nanotube composites.
4 CONCLUSIONS Polypropylene and polycarbonate carbon nanocomposites were prepared and investigated. The structures of the composites were characterised by SEM technique. Test pieces were injection moulded. Mechanical, electrical and burning properties were studied. It was found that the mechanical properties do not show significant change up to 2% nanotube concentration. Exceeding this nanotube content the materials became brittle and the yield strength decreases as well, however there is some difference between the different polymers. The electric resistance decreases with increasing carbon nanotube content. The main change happens up to 1% of nanotube. Further increase of the nanotube content does not increase further the conductivity. The flammability of the composites is significantly smaller than the original polymers. We assume that the carbon nanotube creates a physical network in the polymers. ACKNOWLEDGEMENTS We thank to Béla Hopp and Tamás Csákó (Szeged University) for the SEM sample preparation and SEM measurements. We
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