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Novel polyamide nanocomposites based on silicate nanotubes of the mineral halloysite
Composites Science and Technology 69 (2009) 330–334
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Novel polyamide nanocomposites based on silicate nanotubes of the mineral halloysite Katrin Hedicke-Höchstötter a,*, Goy Teck Lim b, Volker Altstädt c a
Polymer Engineering, Technical University of Hamburg-Harburg, Harburger Schloßstr. 36, D-21079 Hamburg, Germany Polymer Science, University of Akron, Goodyear Polymer Center, Akron, OH 44325, USA c Polymer Engineering, University of Bayreuth, Universitätsstr. 30, D-95447 Bayreuth, Germany b
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 6 October 2008 Accepted 13 October 2008 Available online 1 November 2008 Keywords: A. Nanocomposites A. Nanoparticles Silicate nanotubes B. Mechanical properties B. Thermomechanical properties
a b s t r a c t In this study, the silicate nanotubes of the mineral halloysite will be used as reinforcement in polyamide6 (PA 6). The nanocomposites based on PA-6 and as-received halloysite were prepared by melt extrusion and an adjacent injection moulding process. Mechanical and thermomechanical properties have been investigated by tensile testing and dynamic mechanical analysis. The results show an increased strength and stiffness as well as an enhanced elongation at break at low halloysite content. To evaluate the potential of halloysite as a new candidate in the class of nanofillers, the properties of the halloysite nanocomposites has been compared to those of conventional nanocomposites based on organically modified montmorillonite. From this comparison it can be seen, that both types of nanocomposites show enhanced tensile properties as well as an increased storage modulus, but the increase in tensile strength is more pronounced in the organoclay nanocomposites, whereas the raise of the storage modulus is more prominent in the halloysite nanocomposites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Since the first fundamental research activities of the Toyota Research Group on the field of nanocomposites based on layered silicates [1], extensive research has been carried out at universities as well as in the industry on this subject [2–6]. The potential of this new class of materials is great, already small amounts (65 wt%) of organically modified layered silicates can increase tensile strength and stiffness as well as flexural strength and modulus [2,3], and in some cases also with comparable impact strength as neat polyamide-6 [2,7–9]. Simultaneously, physical properties of polymer matrices like heat resistance, barrier properties and flame retardance can be enhanced [9,10]. Focusing on siliceous mineral nanofillers commonly adopted for research, there are two typical shapes: spherical (e.g. quartz) or layered structures (e.g. montmorillonite). The application of a tubular, silica based, natural occurring nanoparticle as reinforcing material is still new. The mineral used in this study is halloysite, a clay mineral with the chemical composition Al2(OH)4Si2O5(2H2O). Halloysite is a weathering product of volcanic rocks of rhyolitic up to granitic composition and occurs in great deposits. Common halloysites can be found in form of fine, tubular particles [11,12], as can be seen from Fig. 1, which shows the halloysite used in this study. * Corresponding author. Tel.: +49 40 42878 3928; fax: +49 40 42878 3929. E-mail address: [email protected] (K. Hedicke-Höchstötter). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.10.011
As a kaolonite mineral, halloysite contains interlayered water between its tetrahedral and octahedral sheets to disrupt the hydrogen bonds across the interlayers. The stress between the slightly disproportionate dimensioned tetrahedral and octahedral sheets result in a rolling of the layers to acquire the tubular shape [13]. Deposits of high-quality halloysite nanoparticles can be found in Australia and New Zealand. The particles have a thickness of around 30–100 nm with a varying length of 500 nm up to 10 lm [14]. Given this high length–diameter aspect ratio of the longer tubes, halloysite can be an excellent nano-reinforcement for polymer nanocomposites. Furthermore, compared to the thin exfoliated montmorillonite nanoplatelets that are often used for polymer nanocomposites, these halloysite nanotubes have a higher intrinsic stiffness. Although, there exist no detailed informations about the mechanical properties of halloysite, it can be easily visualized, that the thicker halloysite nanotubes, which consists of several silicate layers, have a higher stiffness than the only 1 nm thin silicate platelets of montmorillonite. The field of halloysite nanocomposites is still new in polymer science; there exist only a few scientific publications about polymeric nanocomposites based on halloysite up to now. These nanocomposite systems are based on epoxy resin [15,16], respectively on polypropylene [17–20]. In this study, the first results due to the mechanical and thermomechanical behaviour of halloysite nanocomposites, based on a commercial grade of polyamide-6 and an unmodified halloysite should be presented in a scientific journal. To estimate the potential of this new kind
K. Hedicke-Höchstötter et al. / Composites Science and Technology 69 (2009) 330–334
of nano-reinforcing in polymers, the halloysite nanocomposites will be compared to more conventional nanocomposites which are based on organically modified montmorillonite. All nanocomposites used in this study has been melt compounded and injection molded into tensile testing bars. The results show, that the halloysite nanocomposites have improved tensile properties, compared to the neat polyamide-6 and show also a better thermomechanical behaviour. 2. Experimental details 2.1. Materials The tubular halloysite shown in Fig. 1 (Halloysite A) has been provided by Imerys Tableware, New Zealand and had been used without any chemical modification. The halloysite nanotubes have a cation exchange capacity of around 10 meq/g. The matrix material was a polyamide-6 (Ultramid B4Ò) from BASF AG, Germany. Melt compounding of nanocomposites with 2 and 5 wt% Halloysite A has been carried out with a twin-screw extruder (Berstorff ZE 25) under the following conditions: a rotation speed of 220 rpm, a flow rate of 40 kg/h and a processing temperature of 260 °C. Pure PA was also prepared using the extrusion process to ensure the same thermomechanical history like the nanocomposites. Finally, injection moulding of standard dumbbell specimens (S2 bars type 5A DIN EN ISO 527-2) with 2 mm thickness was performed with the following conditions: a cylinder temperature of 270 °C, a rotation speed of 200 rpm and a flow velocity of 70 mm/s (Arburg Allrounder 420C 800-250 Jubilee). For the comparison of halloysite nanocomposites, nanocomposites based on a montmorillonite modified with octadecylbenzyldimethylammonium-chloride (Nanofil 919, Südchemie AG, Germany) with clay contents of 2 and 5 wt% (normalized to the silicate content) have also been produced using the similar processing conditions. 2.2. Characterisation Dynamic mechanical analysis (Rheometrics Scientific, RDA II) has been done under torsion with a frequency of 1 Hz, a strain of 0.1%, and a heating rate of 3 k/min with rectangular specimens (ligaments of the tensile specimens) in a temperature interval from room temperature (RT) up to 220 °C. Prior for testing, specimens was dried for 72 h at 80 °C in vacuum.
Fig. 1. TEM picture of Halloysite A used in this study.
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S2 tensile testing bars were used for tensile tests according to DIN EN ISO 527. The measurements have been carried out at room temperature and with a crosshead speed of 5 mm/min (Universal testing machine Zwick 1455 with a 10 kN force cell). For each material a minimum of 5 samples were tested to determine the average values. The specimens has been dried for 72 h at 80 °C in vacuum before testing. TEM images have been taken with a Phillips EM 400 transmission electron microscope to show the small-sized nanoparticles. For TEM investigations of the pure halloysite, the samples were prepared as a methanol/water suspension and sprayed onto a carbon TEM grid. For the TEM image of the nanocomposite, the samples were ultramicrotomed into thin sections with a thickness of 50–100 nm. 3. Results and discussion The storage modulus (G0 ) of the halloysite nanocomposites has been investigated by DMA as a function of halloysite content and temperature. In Fig. 2, the positive influence of halloysite on G0 depending on the temperature can be seen. The curve progression shows clearly, that the stiffness of the halloysite nanocomposites increases significantly compared to the pure polyamide matrix due to the influence of the silicate nanotubes. This increase can be attributed to the high intrinsic stiffness of the halloysite nanotubes, resulting from their tube-like geometry. Considering the progression of G0 with increasing temperature it becomes clear, that the decrease of G0 with increasing temperature is less pronounced in the halloysite nanocomposites, i.e., the storage modulus can be maintained at a relatively higher level above an increased temperature range. Thus, the silicate nanotubes have the same influence on the polyamide matrix like glass fibres, as they enhance the heat dimensional stability. In Fig. 3, representative stress–strain curves of polyamide-6 and the halloysite nanocomposites are shown. As can be seen, a halloysite content of only 2 wt% leads to a significant increase of the yield stress from 71.5 MPa in the pure polyamide-6 up to 80.2 in the nanocomposite (Table 1). Interestingly, an enhancement of the halloysite content up to 5 wt% leads just to a marginal further increase of the yield stress. The same effect could also be observed from the work of Lopez Manchado et al. [21] on single-wall carbon nanotube (SWCNT) composites, which attributes this effect on a stronger agglomeration of the tubes at higher SWCNT content. The formation of agglomerates results in a reduction of the effective surface area, which can undergo matrix-filler interactions. In the case of
Fig. 2. Storage modulus of Ultramid B4 and the halloysite nanocomposites.
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Fig. 5. Storage modulus of Ultramid B4, the halloysite nanocomposites and the organoclay nanocomposites containing 2 wt%. Fig. 3. Stress–strain curves of Ultramid B4 and the halloysite nanocomposites.
the material containing only 2 wt% halloysite, well-dispersed halloysite particles of different length are observable (Fig. 4), what results in a higher effective surface area as in the 5% nanocomposites. From Fig. 3 it is also observable, that the incorporation of 2 wt% halloysite leads to an increase in elongation at break compared to the pure polyamide-6 matrix. At higher halloysite contents (5 wt%), the elongation at break decreases again, what could also be ascribed to the formation of agglomerates. This effect is also known from the more conventional organoclay nanocomposites and has been often described in the literature (e.g. [22–26]). To estimate the potential of halloysite as a new nano-reinforcement in polymers, the mechanical and thermomechanical proper-
ties of the halloysite nanocomposites will be compared to those of organoclay nanocomposites. As can be seen from Fig. 5, the storage modulus of the nanocomposites with 2 wt% of halloysite is much higher than the storage modulus of the nanocomposite based on organically modified montmorillonite. This effect is attributed to the higher intrinsic stiffness of the siliceous nanotubes compared to the thinner and more flexible nanoplatelets of the organoclay. Especially it should be emphasized, that in contrast to the organoclay nanocomposites, these results could be achieved without any organic modification of the halloysite. At the higher filler content of 5 wt%, the further improvement in storage modulus of the halloysite nanocomposites is not so pronounced as in the organoclay nanocomposites; now the differences in stiffness between the both nanocomposites at room temperature becomes smaller (Fig. 6). This could be attributed to the absence of an organic modification in the used halloysite, because distinctive interactions between the polymer matrix and the surface of the silicate nanotubes are missing. In the case of the nanocomposites based on organically modified montmorillonite, the interactions between the silicate surface of the well exfoliated and dispersed nanoplatelets (Fig. 7) and the polyamide matrix leads to a further increase in storage modulus with increasing organoclay content. Obviously, two factors are responsible for the increase of stiffness. The intrinsic stiffness of the particles contrib-
Fig. 4. TEM picture of the halloysite nanocomposite containing 2 wt%.
Fig. 6. Storage modulus of Ultramid B4, the halloysite nanocomposites and the organoclay nanocomposites containing 5 wt% clay.
Table 1 Mechanical variables from tensile testing of PA-6 and the nanocomposites. Material
Yield stress (MPa)
E-modulus (MPa)
Strain at failure (%)
PA-6 2% Halloysite 5% Halloysite 2% Organoclay 5% Organoclay
71.5 ± 0.6 80.2 ± 0,4 81.8 ± 1.5 84.8 ± 0.5 87.2 ± 0.5
2882 ± 130 3640 ± 58 3788 ± 127 4298 ± 315 4362 ± 286
98.8 ± 18.2 186.0 ± 15.0 96.0 ± 54.3 65.9 ± 16.1 63.8 ± 13.4
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Fig. 9. Stress–strain curves of Ultramid B4 and the nanocomposites containing 5 wt% clay. Fig. 7. TEM picture of the organoclay nanocomposites containing 5 wt% clay.
Fig. 8. Stress–strain curves of Ultramid B4 and the nanocomposite containing 2 wt% clay.
utes as well as the interfacial bonding between the matrix and the particles to the enhancement of the mechanical properties [25]. In the halloysite nanocomposites only the stiffness of the silicate tubes is responsible for the increase in modulus, because only limited interactions between the polymer chains and the surface of the halloysite can occur without an appropriate organic modification [27]. Comparing the tensile properties of halloysite nanocomposites with those of organoclay nanocomposites, it can be seen from Figs. 8 and 9, that the incorporation of both reinforcements into the polyamide-6 matrix results in a significant enhancement of tensile strength (increase from 71.2 MPa up to 80.2 MPa in the 2% nanocomposites, respectively, 81.8 MPa in the 5% nanocomposites), but this tendency is more pronounced in the nanocomposites based on organically modified montmorillonite, where the tensile strength increase up to 84.8 MPa, respectively, 87.2 MPa. This can be ascribed to the missing of an organic modification on the halloy-
site nanotubes and therefore weaker interactions between the silicates and the polymeric matrix, as it has been already explained in the prior section. Furthermore, the influence of a possible nucleating effect of the halloysite as well as of the organoclay on the PA-6 matrix can be excluded, because all nanocomposites have a similar degree of crystallinity like the neat matrix material, calculated from the melting enthalpy, which was determined by differential scanning calorimetry (not shown). On the other hand, at clay contents of 2 wt%, the incorporation of the organoclay lead to a decrease in elongation at break, while from the influence of the halloysite nanotubes, the elongation at break is enhanced (Fig. 8). This effect can be attributed to crack-bridging effects resulting from interactions between the silicate nanotubes and the PA-6 matrix, as can be seen from the SEM pictures in Fig. 10. In Fig. 10a two single silicate nanotubes, completely covered with matrix, can be seen. This indicates, that the surface of the nanotubes and the matrix interact with each other and an interface between filler and matrix is formed. From this interactions, it is possible, that the crack-bridging halloysite-matrix-fibres which are shown in Fig. 10b are formed. These special failure mechanisms are responsible for the improved ductility of the halloysite nanocomposites. On the other hand, in the higher filled nanocomposites beside the crack-bridging effects large agglomerates can be found, as it is shown in Fig. 10c. These agglomerates are responsible for the decrease in ductility of the material containing 5 wt% halloysite. An overview about the mechanical properties of the nanocomposites is given in Table 1. 4. Conclusions In this study, investigations on halloysite nanocomposites based on polyamide-6 are presented. Injection molded specimens of nanocomposites containing 2 and 5 wt% halloysite has been investigated by dynamic mechanical analysis and tensile testing, and the mechanical and thermomechanical properties of the halloysite nanocomposites has been compared to those of nanocomposites based on organically modified clay. Already these first, fundamental results show clearly the potential of the halloysite nanotubes as reinforcing material in polymers. Beside the enhancement of tensile strength and elastic modulus of the polyamide-6 matrix from the incorporation of halloysite, the toughening effect of the silicate nanotubes especially should be
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Fig. 10. SEM pictures of the fracture surfaces from tensile testing of the halloysite nanocomposites. (a) Halloysite nanotubes covered with matrix (2 wt% halloysite), (b) crack bridging effects (2 wt% halloysite) and (c) agglomerate (5 wt% halloysite).
emphasized. This predestines the halloysite nanocomposites for applications, which demand a simultaneous improvement of strength and toughness. Remarkable is, that these results could be achieved with unmodified halloysite, which suggests that there lies great potential for further improvement of the properties of halloysite nanocomposites with a suitable chemical surface treatment. Acknowledgements We want to thank the DFG for the financial grant of this project (Reference No. AL 474/13-1). Special thanks go to Mr. David Gittins and Mr. Chris Paynter from Imerys Ltd. UK for their support. References [1] Okada M, Kawasumi T, Karauchi O, Kamigaito O. Synthesis and characterization of a nylon 6-clay hybrid. Polym Preprints 1987;28(2):447–8. [2] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993;8(5):1185–9. [3] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [4] Zilg C, Reichert P, Dietsche F, Engelhard T, Mühlhaupt R. Nanocomposites based on a synthetic layer silicate and polyamide-12. Acta Polym 1998;49:116–23. [5] Hedicke K, Wittich H, Mehler C, Gruber F, Altstädt V. Crystallisation behaviour of polyamide-6 and polyamide-66 nanocomposites. Compos Sci Technol 2006;66:571–5. [6] Uribe-Arocha P, Mehler C, Puskas JE, Altstädt V. Effect of sample thickness on the mechanical properties of injection molded polyamide-6 and polyamide-6 clay nanocomposites. Polymer 2003;44:2441–6. [7] Okada A, Usuki A. The chemistry of polymer-clay hybrids. Mater Sci Eng 1995;3:109–15. [8] Yano K, Usuki A, Okada A. Synthesis and properties of polyimide-clay hybrid films. J Polym Sci A: Polym Chem 1997;35:2289–94. [9] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng 2001;28:1–63. [10] Gilman JW. Flammability and thermal stability studies of polymer layered silicate (clay) nanocomposites. Appl Clay Sci 1999;15:31–41.
[11] Baile SW. Hallyosite – a critical assessment. In: 9th International clay conference. Straßbourg: Sciences Geologiques Memoirs; 1989. [12] Beutelsbacher H, van der Marel HW. Atlas of electron microscopy of clay minerals and their admixtures: a picture atlas. Elsevier Publishing Co.; 1968. [13] Singh B. Why does halloysite roll? A new model. Clays Clay Miner 1999;44(2):191–6. [14] Norrish K. An unusual fibrous halloysite clay: controlling the environment. In: 10th International clay conference, Adelaide, 1993. [15] Ye Y, Chen H, Wu J, Ye L. High impact strength epoxy nanocomposites with natural nanotubes. Polymer 2007;48:6426–33. [16] Liu M, Guo B, Du M, Cai X, Jia D. Properties of halloysite nanotubes-epoxy resin hybrids and the interfacial reactions in the systems. Nanotechnology 2007;18:455703. [17] Du M, Guo B, Jia D. Thermal stability and flame retardant effects of halloysite nanotubes on poly(propylene). Europ Polym J 2006;42:1362–9. [18] Du M, Guo B, Liu M, Jia D. Preparation and characterization of polypropylene grafted halloysite and their compatibility effect to polypropylene/halloysite composite. Polym J 2006;38(11):1198–204. [19] Liu M, Guo B, Zou Q, Du M, Jia D. Interactions between halloysite nanotubes and 2,5-bis(2-benzoxazolyl)thiophene and their effects on reinforcement of polypropylene/halloysite nanocomposites. Nanotechnology 2008;19:205709. [20] Ning N, Yin Q, Luo F, Zhang Q, Du R, Fu Q. Crystallization behavior and mechanical properties of polypropylene/halloysite composites. Polymer 2007;48:7374–84. [21] Lopez Manchado MA, Valentini L, Biagiotti J, Kenny JM. Thermal and mechanical properties of single-walled carbon nanotubes-polypropylene composites prepared by melt processing. Carbon 2005;43:1499–505. [22] Cho JW, Paul DR. Nylon 6 nanocomposites by melt compounding. Polymer 2001;42(3):1083–94. [23] Huang JC, Zhu ZK, Ma XD, Qian XF, Yin J. Preparation and properties of montmorillonite/organo-soluble polyimide hybrid materials prepared by a one-step approach. J Mater Sci 2001;36:871–7. [24] Shah RK, Paul DR. Nylon 6 nanocomposites prepared by a melt mixing masterbatch process. Polymer 2004;45:2991–3000. [25] Shelley JS, Mather PT, DeVries KL. Reinforcement and environmental degradation of nylon-6/clay nanocomposites. Polymer 2001;42:5849–58. [26] Hedicke-Höchstötter K, Langenfelder D, Demchuk V, Altstädt V. Fatigue crack propagation behaviour of polyamide-6 nanocomposites based on layered silicates. J Plast Technol 2007;2007(2):1–22. [27] Bureau MN, Denault J, Cole KC, Enright GD. The role of crystallinity and reinforcement in the mechanical behavior of polyamide-6/clay nanocomposites. Polym Eng Sci 2002;42(9):1897–906.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Novel polyamide nanocomposites based on silicate nanotubes of the mineral halloysite Katrin Hedicke-Höchstötter a,*, Goy Teck Lim b, Volker Altstädt c a
Polymer Engineering, Technical University of Hamburg-Harburg, Harburger Schloßstr. 36, D-21079 Hamburg, Germany Polymer Science, University of Akron, Goodyear Polymer Center, Akron, OH 44325, USA c Polymer Engineering, University of Bayreuth, Universitätsstr. 30, D-95447 Bayreuth, Germany b
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 6 October 2008 Accepted 13 October 2008 Available online 1 November 2008 Keywords: A. Nanocomposites A. Nanoparticles Silicate nanotubes B. Mechanical properties B. Thermomechanical properties
a b s t r a c t In this study, the silicate nanotubes of the mineral halloysite will be used as reinforcement in polyamide6 (PA 6). The nanocomposites based on PA-6 and as-received halloysite were prepared by melt extrusion and an adjacent injection moulding process. Mechanical and thermomechanical properties have been investigated by tensile testing and dynamic mechanical analysis. The results show an increased strength and stiffness as well as an enhanced elongation at break at low halloysite content. To evaluate the potential of halloysite as a new candidate in the class of nanofillers, the properties of the halloysite nanocomposites has been compared to those of conventional nanocomposites based on organically modified montmorillonite. From this comparison it can be seen, that both types of nanocomposites show enhanced tensile properties as well as an increased storage modulus, but the increase in tensile strength is more pronounced in the organoclay nanocomposites, whereas the raise of the storage modulus is more prominent in the halloysite nanocomposites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Since the first fundamental research activities of the Toyota Research Group on the field of nanocomposites based on layered silicates [1], extensive research has been carried out at universities as well as in the industry on this subject [2–6]. The potential of this new class of materials is great, already small amounts (65 wt%) of organically modified layered silicates can increase tensile strength and stiffness as well as flexural strength and modulus [2,3], and in some cases also with comparable impact strength as neat polyamide-6 [2,7–9]. Simultaneously, physical properties of polymer matrices like heat resistance, barrier properties and flame retardance can be enhanced [9,10]. Focusing on siliceous mineral nanofillers commonly adopted for research, there are two typical shapes: spherical (e.g. quartz) or layered structures (e.g. montmorillonite). The application of a tubular, silica based, natural occurring nanoparticle as reinforcing material is still new. The mineral used in this study is halloysite, a clay mineral with the chemical composition Al2(OH)4Si2O5(2H2O). Halloysite is a weathering product of volcanic rocks of rhyolitic up to granitic composition and occurs in great deposits. Common halloysites can be found in form of fine, tubular particles [11,12], as can be seen from Fig. 1, which shows the halloysite used in this study. * Corresponding author. Tel.: +49 40 42878 3928; fax: +49 40 42878 3929. E-mail address: [email protected] (K. Hedicke-Höchstötter). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.10.011
As a kaolonite mineral, halloysite contains interlayered water between its tetrahedral and octahedral sheets to disrupt the hydrogen bonds across the interlayers. The stress between the slightly disproportionate dimensioned tetrahedral and octahedral sheets result in a rolling of the layers to acquire the tubular shape [13]. Deposits of high-quality halloysite nanoparticles can be found in Australia and New Zealand. The particles have a thickness of around 30–100 nm with a varying length of 500 nm up to 10 lm [14]. Given this high length–diameter aspect ratio of the longer tubes, halloysite can be an excellent nano-reinforcement for polymer nanocomposites. Furthermore, compared to the thin exfoliated montmorillonite nanoplatelets that are often used for polymer nanocomposites, these halloysite nanotubes have a higher intrinsic stiffness. Although, there exist no detailed informations about the mechanical properties of halloysite, it can be easily visualized, that the thicker halloysite nanotubes, which consists of several silicate layers, have a higher stiffness than the only 1 nm thin silicate platelets of montmorillonite. The field of halloysite nanocomposites is still new in polymer science; there exist only a few scientific publications about polymeric nanocomposites based on halloysite up to now. These nanocomposite systems are based on epoxy resin [15,16], respectively on polypropylene [17–20]. In this study, the first results due to the mechanical and thermomechanical behaviour of halloysite nanocomposites, based on a commercial grade of polyamide-6 and an unmodified halloysite should be presented in a scientific journal. To estimate the potential of this new kind
K. Hedicke-Höchstötter et al. / Composites Science and Technology 69 (2009) 330–334
of nano-reinforcing in polymers, the halloysite nanocomposites will be compared to more conventional nanocomposites which are based on organically modified montmorillonite. All nanocomposites used in this study has been melt compounded and injection molded into tensile testing bars. The results show, that the halloysite nanocomposites have improved tensile properties, compared to the neat polyamide-6 and show also a better thermomechanical behaviour. 2. Experimental details 2.1. Materials The tubular halloysite shown in Fig. 1 (Halloysite A) has been provided by Imerys Tableware, New Zealand and had been used without any chemical modification. The halloysite nanotubes have a cation exchange capacity of around 10 meq/g. The matrix material was a polyamide-6 (Ultramid B4Ò) from BASF AG, Germany. Melt compounding of nanocomposites with 2 and 5 wt% Halloysite A has been carried out with a twin-screw extruder (Berstorff ZE 25) under the following conditions: a rotation speed of 220 rpm, a flow rate of 40 kg/h and a processing temperature of 260 °C. Pure PA was also prepared using the extrusion process to ensure the same thermomechanical history like the nanocomposites. Finally, injection moulding of standard dumbbell specimens (S2 bars type 5A DIN EN ISO 527-2) with 2 mm thickness was performed with the following conditions: a cylinder temperature of 270 °C, a rotation speed of 200 rpm and a flow velocity of 70 mm/s (Arburg Allrounder 420C 800-250 Jubilee). For the comparison of halloysite nanocomposites, nanocomposites based on a montmorillonite modified with octadecylbenzyldimethylammonium-chloride (Nanofil 919, Südchemie AG, Germany) with clay contents of 2 and 5 wt% (normalized to the silicate content) have also been produced using the similar processing conditions. 2.2. Characterisation Dynamic mechanical analysis (Rheometrics Scientific, RDA II) has been done under torsion with a frequency of 1 Hz, a strain of 0.1%, and a heating rate of 3 k/min with rectangular specimens (ligaments of the tensile specimens) in a temperature interval from room temperature (RT) up to 220 °C. Prior for testing, specimens was dried for 72 h at 80 °C in vacuum.
Fig. 1. TEM picture of Halloysite A used in this study.
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S2 tensile testing bars were used for tensile tests according to DIN EN ISO 527. The measurements have been carried out at room temperature and with a crosshead speed of 5 mm/min (Universal testing machine Zwick 1455 with a 10 kN force cell). For each material a minimum of 5 samples were tested to determine the average values. The specimens has been dried for 72 h at 80 °C in vacuum before testing. TEM images have been taken with a Phillips EM 400 transmission electron microscope to show the small-sized nanoparticles. For TEM investigations of the pure halloysite, the samples were prepared as a methanol/water suspension and sprayed onto a carbon TEM grid. For the TEM image of the nanocomposite, the samples were ultramicrotomed into thin sections with a thickness of 50–100 nm. 3. Results and discussion The storage modulus (G0 ) of the halloysite nanocomposites has been investigated by DMA as a function of halloysite content and temperature. In Fig. 2, the positive influence of halloysite on G0 depending on the temperature can be seen. The curve progression shows clearly, that the stiffness of the halloysite nanocomposites increases significantly compared to the pure polyamide matrix due to the influence of the silicate nanotubes. This increase can be attributed to the high intrinsic stiffness of the halloysite nanotubes, resulting from their tube-like geometry. Considering the progression of G0 with increasing temperature it becomes clear, that the decrease of G0 with increasing temperature is less pronounced in the halloysite nanocomposites, i.e., the storage modulus can be maintained at a relatively higher level above an increased temperature range. Thus, the silicate nanotubes have the same influence on the polyamide matrix like glass fibres, as they enhance the heat dimensional stability. In Fig. 3, representative stress–strain curves of polyamide-6 and the halloysite nanocomposites are shown. As can be seen, a halloysite content of only 2 wt% leads to a significant increase of the yield stress from 71.5 MPa in the pure polyamide-6 up to 80.2 in the nanocomposite (Table 1). Interestingly, an enhancement of the halloysite content up to 5 wt% leads just to a marginal further increase of the yield stress. The same effect could also be observed from the work of Lopez Manchado et al. [21] on single-wall carbon nanotube (SWCNT) composites, which attributes this effect on a stronger agglomeration of the tubes at higher SWCNT content. The formation of agglomerates results in a reduction of the effective surface area, which can undergo matrix-filler interactions. In the case of
Fig. 2. Storage modulus of Ultramid B4 and the halloysite nanocomposites.
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Fig. 5. Storage modulus of Ultramid B4, the halloysite nanocomposites and the organoclay nanocomposites containing 2 wt%. Fig. 3. Stress–strain curves of Ultramid B4 and the halloysite nanocomposites.
the material containing only 2 wt% halloysite, well-dispersed halloysite particles of different length are observable (Fig. 4), what results in a higher effective surface area as in the 5% nanocomposites. From Fig. 3 it is also observable, that the incorporation of 2 wt% halloysite leads to an increase in elongation at break compared to the pure polyamide-6 matrix. At higher halloysite contents (5 wt%), the elongation at break decreases again, what could also be ascribed to the formation of agglomerates. This effect is also known from the more conventional organoclay nanocomposites and has been often described in the literature (e.g. [22–26]). To estimate the potential of halloysite as a new nano-reinforcement in polymers, the mechanical and thermomechanical proper-
ties of the halloysite nanocomposites will be compared to those of organoclay nanocomposites. As can be seen from Fig. 5, the storage modulus of the nanocomposites with 2 wt% of halloysite is much higher than the storage modulus of the nanocomposite based on organically modified montmorillonite. This effect is attributed to the higher intrinsic stiffness of the siliceous nanotubes compared to the thinner and more flexible nanoplatelets of the organoclay. Especially it should be emphasized, that in contrast to the organoclay nanocomposites, these results could be achieved without any organic modification of the halloysite. At the higher filler content of 5 wt%, the further improvement in storage modulus of the halloysite nanocomposites is not so pronounced as in the organoclay nanocomposites; now the differences in stiffness between the both nanocomposites at room temperature becomes smaller (Fig. 6). This could be attributed to the absence of an organic modification in the used halloysite, because distinctive interactions between the polymer matrix and the surface of the silicate nanotubes are missing. In the case of the nanocomposites based on organically modified montmorillonite, the interactions between the silicate surface of the well exfoliated and dispersed nanoplatelets (Fig. 7) and the polyamide matrix leads to a further increase in storage modulus with increasing organoclay content. Obviously, two factors are responsible for the increase of stiffness. The intrinsic stiffness of the particles contrib-
Fig. 4. TEM picture of the halloysite nanocomposite containing 2 wt%.
Fig. 6. Storage modulus of Ultramid B4, the halloysite nanocomposites and the organoclay nanocomposites containing 5 wt% clay.
Table 1 Mechanical variables from tensile testing of PA-6 and the nanocomposites. Material
Yield stress (MPa)
E-modulus (MPa)
Strain at failure (%)
PA-6 2% Halloysite 5% Halloysite 2% Organoclay 5% Organoclay
71.5 ± 0.6 80.2 ± 0,4 81.8 ± 1.5 84.8 ± 0.5 87.2 ± 0.5
2882 ± 130 3640 ± 58 3788 ± 127 4298 ± 315 4362 ± 286
98.8 ± 18.2 186.0 ± 15.0 96.0 ± 54.3 65.9 ± 16.1 63.8 ± 13.4
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Fig. 9. Stress–strain curves of Ultramid B4 and the nanocomposites containing 5 wt% clay. Fig. 7. TEM picture of the organoclay nanocomposites containing 5 wt% clay.
Fig. 8. Stress–strain curves of Ultramid B4 and the nanocomposite containing 2 wt% clay.
utes as well as the interfacial bonding between the matrix and the particles to the enhancement of the mechanical properties [25]. In the halloysite nanocomposites only the stiffness of the silicate tubes is responsible for the increase in modulus, because only limited interactions between the polymer chains and the surface of the halloysite can occur without an appropriate organic modification [27]. Comparing the tensile properties of halloysite nanocomposites with those of organoclay nanocomposites, it can be seen from Figs. 8 and 9, that the incorporation of both reinforcements into the polyamide-6 matrix results in a significant enhancement of tensile strength (increase from 71.2 MPa up to 80.2 MPa in the 2% nanocomposites, respectively, 81.8 MPa in the 5% nanocomposites), but this tendency is more pronounced in the nanocomposites based on organically modified montmorillonite, where the tensile strength increase up to 84.8 MPa, respectively, 87.2 MPa. This can be ascribed to the missing of an organic modification on the halloy-
site nanotubes and therefore weaker interactions between the silicates and the polymeric matrix, as it has been already explained in the prior section. Furthermore, the influence of a possible nucleating effect of the halloysite as well as of the organoclay on the PA-6 matrix can be excluded, because all nanocomposites have a similar degree of crystallinity like the neat matrix material, calculated from the melting enthalpy, which was determined by differential scanning calorimetry (not shown). On the other hand, at clay contents of 2 wt%, the incorporation of the organoclay lead to a decrease in elongation at break, while from the influence of the halloysite nanotubes, the elongation at break is enhanced (Fig. 8). This effect can be attributed to crack-bridging effects resulting from interactions between the silicate nanotubes and the PA-6 matrix, as can be seen from the SEM pictures in Fig. 10. In Fig. 10a two single silicate nanotubes, completely covered with matrix, can be seen. This indicates, that the surface of the nanotubes and the matrix interact with each other and an interface between filler and matrix is formed. From this interactions, it is possible, that the crack-bridging halloysite-matrix-fibres which are shown in Fig. 10b are formed. These special failure mechanisms are responsible for the improved ductility of the halloysite nanocomposites. On the other hand, in the higher filled nanocomposites beside the crack-bridging effects large agglomerates can be found, as it is shown in Fig. 10c. These agglomerates are responsible for the decrease in ductility of the material containing 5 wt% halloysite. An overview about the mechanical properties of the nanocomposites is given in Table 1. 4. Conclusions In this study, investigations on halloysite nanocomposites based on polyamide-6 are presented. Injection molded specimens of nanocomposites containing 2 and 5 wt% halloysite has been investigated by dynamic mechanical analysis and tensile testing, and the mechanical and thermomechanical properties of the halloysite nanocomposites has been compared to those of nanocomposites based on organically modified clay. Already these first, fundamental results show clearly the potential of the halloysite nanotubes as reinforcing material in polymers. Beside the enhancement of tensile strength and elastic modulus of the polyamide-6 matrix from the incorporation of halloysite, the toughening effect of the silicate nanotubes especially should be
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Fig. 10. SEM pictures of the fracture surfaces from tensile testing of the halloysite nanocomposites. (a) Halloysite nanotubes covered with matrix (2 wt% halloysite), (b) crack bridging effects (2 wt% halloysite) and (c) agglomerate (5 wt% halloysite).
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