Plasticheskie Massy, No. 3, 2011, pp. 56 60 Rheological and mechanical properties of epoxy composites modified with montmorillonite nanoparticles S.O. Il in, 1 I.Yu. Gorbunova, 2 E.P. Plotnikova, 1 and M.L. Kerber 2 1 A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow 2 D.I. Mendeleev Russian Chemico-Technological University, Moscow Selected from International Polymer Science and Technology, 38, No. 7, 2011, reference PM 11/03/56; transl. serial no. 16449 Translated by P. Curtis Summary An examination is made of the effect of modifying montmorillonite with organic compounds and its ultrasound treatment on the rheological properties of filled epoxy oligomers and the structure and physicomechanical properties of cured composites. To improve the physicomechanical properties of filled epoxy composites, it is recommended that use be made of modified montmorillonite of grade Cloisite 30B, which has the best compatibility with the epoxy matrix. improve these characteristics, provided a high degree of dispersion of the filler particles in the oligomer is achieved. Therefore, the development of methods for producing nanocomposites with a high homogeneity of distribution of the nanoparticles in the polymer matrix is an urgent problem. In the present work an examination was made of the effect of modifying natural aluminosilicates with organic compounds and ultrasound treatment of the filler on the structure and certain properties of the composite obtained. One of the most promising and rapidly developing directions in the science and technology of polymer composites in recent years has been the development of nanocomposites. Practical interest in this direction is due to the considerable improvement in a number of mechanical, thermophysical, and other properties of polymeric materials when a small amount of nanoparticles is introduced. Among the fillers widely used in polymer nanocomposites, lamellar silicates stand out. These are noted for readily available raw material, and, on their basis, it is possible to produce homogeneous nanoparticles of flaky form that ensure improved mechanical properties of the nanocomposites obtained. At present, epoxy oligomers are among the most widely used thermosetting binders. Composite materials based on them are widely used in aviation and rocket technology. One of the shortcomings of epoxy plastics is their low impact strength and limited heat resistance. The introduction of nanoparticles makes it possible to EXPERIMENTAL The investigation was conducted on an epoxy amine composite based on epoxy oligomer ED-20 and curing agent 4,4 -diaminodiphenylsulphone (DADPS). The content of curing agent corresponded to a stoichiometric ratio and amounted to 30 parts per 70 parts epoxy oligomer. The fillers used were commercial Cloisite lamellar silicates supplied by Southern Clay Products (USA): Cloisite Na +, a natural unmodified montmorillonite (MMT); Cloisite 15A, 20A, 93A, and 30B modified with different quaternary ammonium salts (Table 1). The components were mixed by hand, with subsequent homogenisation by ultrasound treatment ( Sapphire ultrasonic bath, working frequency 35 khz, generator power 50 W). The composite was cured at a temperature of 180 C. The viscosity of the composites was studied by rotation viscometry at room temperature on a PIRSP-2 rheometer 2012 Smithers Rapra Technology T/57
Table 1. Characteristics of layered silicates (Southern Clay Products) Clay grade, Cloisite Na + 30B 93A 20A 15A Modifier used a Modifier concentration (mequ/100 g clay) 90 90 95 125 Interlayer distance (Å) 11.7 18.5 23.6 24.2 31.5 a T residue of fatty acids; HT hydrogenised residue of fatty acids (~65% C18; ~30% C16; ~5% C14) [1] manufactured at the Institute of Petrochemical Synthesis. The morphology of specimens of the composites in the form of films of 10 µm thickness, placed between cover glasses, was studied by optical microscopy on an MIN-8 microscope. Micrographs were subjected to statistical processing, from the results of which the particle size distribution was calculated. The glass transition temperature was determined using an MK-3 torsion pendulum by the method of freely attenuating vibrations during heating of the specimen. The impact strength of the cured specimens was assessed according to GOST 14235-69 (DIN 53453) on a Dinstat instrument. The compressive strength was determined according to GOST 4651-63 (ASTM D695). RESULTS AND DISCUSSION The structure of nanocomposite materials largely determines their properties. The main problem in producing nanocomposites is the aggregation of filler particles. Therefore, to produce nanocomposites it is necessary to ensure dispersion of the aggregates to the nanosize level [2]. One of the methods for solving this problem is to modify MMT in order to give it greater hydrophobic properties, and to achieve exfoliation (separation into layers) or intercalation (increase in the interplanar distance in layers of the silicate by means of a modifier) [3, 4]. The use of ultrasound is also an effective method of particle dispersion [5]. Traditional methods for investigating the structure of composites, such as electron transmission microscopy, X-ray diffraction analysis, and so on, are fairly complex and laborious. It is complex by these methods to determine the structures of composites throughout the material and features of its behaviour during processing. At the same time, study of the rheological characteristics of composites makes it possible to assess the relationship of the structure and morphology of nanocomposites with features of their processing, which was used in the present work [6, 7]. At the first stage, a study was made of the rheological properties of mixtures of epoxy oligomer containing 5 parts different types of filler (Figure 1). Epoxy oligomer ED-20 is a Newtonian fluid, and its viscosity is not dependent on the shear rate (curve 1, Figures 1 and 2). The introduction of unmodified clay (curve 2, Figure 1) leads to increased viscosity. For systems containing modified MMT (curves 3 6), the manifestation of non-newtonian behaviour is marked, i.e. its viscosity decreases with increasing shear rate. It was shown experimentally that the action of ultrasound (sonication) causes no marked change in viscosity of unfilled epoxy oligomer (curve 1, Figure 2). For composites with different types of filler, as a rule a reduction in viscosity was observed, apart from the case of using Cloisite 30B (curve 4, Figure 2). The degree of dispersion of filler particles and the uniformity of distribution of particles in the material have an influence on the viscosity of the system. In the present case, filler particles differ in the magnitude of the interlayer distance and in the modifier used (Table 1), which in turn possesses a certain affinity for the epoxy matrix. Thus, Cloisite 30B is characterised by Figure 1. Dependence of viscosity on shear rate: 1 ED-20; blends of ED-20 with 5 parts filler: 2 Na + ; 3 15A; 4 30B; 5 20A; 6 93A T/58 International Polymer Science and Technology, Vol. 39, No. 7, 2012
the smallest interlayer distance among all the modified MMT examined, and therefore, with mixing by hand, the viscosity of this system increases little. However, with sonication there is an improvement in the dispersion of particles on account of an increase in the affinity of the modifier for the epoxy oligomer owing to the presence of polar OH groups in the modifier, on account of which the viscosity of this system also increases. In the other systems, however, aggregation of the filler particles occurs during sonication, which is confirmed by micrographs of both uncured and crosslinked systems (Figures 3 and 4). As can be seen from the given micrographs, the degree of aggregation and structure formation in crosslinked specimens is higher. This can be attributed to the fact (a) (b) Figure 2. Dependence of viscosity (lg = 1.06 (s -1 )) on the sonication time: 1 ED-20; blends of ED-20 with 5 parts filler: 2 Na + ; 3 15A; 4 30B; 5 20A; 6 93A (c) (a) (d) (b) Figure 3. Micrographs of uncured systems containing 5 parts MMT: (a) 30B; (b) 15A Figure 4. Micrographs of crosslinked systems containing 1 part MMT: (a) 30B; (b) 15A; (c) 60 min ultrasound, 30B; (d) 60 min ultrasound, 15A 2012 Smithers Rapra Technology T/59
that the process of crosslinking proceeded at high temperatures, which led to a considerable reduction in viscosity and to acceleration of agglomeration processes. Figure 5 shows the size distribution of filler particles in an uncured system containing 5 parts MMT. From the figure it can be seen that the composite with Cloisite 30B is characterised by a lower particle size with a Figure 5. Particle size distribution in a system containing 5 parts MMT: 1 Na + ; 2 15A; 3 30B Figure 6. Concentration dependence of the properties of a composite filled with 30B: 1 glass transition temperature; 2 impact strength; 3 compressive strength; 4 shear modulus at 80 C; 5 shear modulus at 200 C narrower distribution peak, and also by the absence of large aggregates, which is a positive factor for increasing the effectiveness of filling. The particle distribution in the matrix should also influence the final properties of the composite. Whereas to assess the change in viscous properties it is expedient to use an increased degree of filling, to establish the strength and thermomechanical characteristics it is necessary to lower the number of nanoparticles introduced, as the dependence of these properties on the filler concentration is often extremal. Table 2 gives the properties of composites containing 1 part filler, the dispersion of which was carried out both by hand and by means of ultrasound. As can be seen from the table, during the sonication of composites containing filler with a poor affinity for the polymer matrix, deterioration in the strength properties is observed on account of particle aggregation. Specimens of composites with a greater viscosity and consequently a more uniform filler distribution in the system exhibit higher mechanical properties. The lower glass transition temperature after sonication may be attributed to the extremal nature of the dependence of T g on filler content. The highest mechanical properties are observed for composites filled with Cloisite 30B, evidently on account of more effective mixing of the components of the system. Figure 6 gives the concentration dependence of the mechanical properties of this system. As can be seen from Figure 6, with increase in the filler concentration there is a reduction in a number of strength characteristics. This effect is possibly due to the presence of defects (gas bubbles) at the boundary between the epoxy polymer and the surface of the clay [8], which are formed during mixing of the components. The number of these defects is proportional to the filler content. Another reason may be the formation of a heterogeneous network in specimens, caused by an increase in viscosity of the system during curing of the filled oligomer. The dependence of the glass transition temperature on the clay concentration passes through a maximum (curve 1, Figure 6). Increase in T g may be due to a reduction in segmental mobility on account of the adsorption of Table 2. Properties of composites containing 1 part layered silicate Properties Type of modifier a 15A b 20A 93A 30B Na + Viscosity c (Pa s) 24 39 17 50 42 56 46 42 57 32 20 Glass transition temperature ( C) 180 184 180 197 173 184 180 199 192 197 186 Impact strength (kj/m 2 ) 3.3 3.7 3.1 3.3 3.3 3.6 3.1 3.4 3.9 3.5 2.6 Shear strength (MPa) 125 101 90 107 103 121 125 107 115 93 98 a Properties of the unfilled system b First column mixing by hand; second column 60 min of ultrasound treatment c Viscosity of the uncured system containing 5 parts filler at lg g = 1.06 s -1 T/60 International Polymer Science and Technology, Vol. 39, No. 7, 2012
polymer chains on the filler, and further reduction may occur on account of the plasticising action of the clay modifier [9]. The considerable increase in impact strength with increase in the proportion of introduced MMT above 1 part (curve 2, Figure 6) should be noted. This effect can be attributed to slowing down of the growth of microcracks by filler particles and to the consumption of additional energy on the separation of the epoxy matrix from the clay particles [10, 11]. CONCLUSIONS It was shown that the change in viscosity of the epoxy oligomer when filler is introduced may serve as an indicator of the degree of dispersion of the nanofiller. It was established that the action of ultrasound when a number of modifiers are used may lead both to dispersion and to aggregation of montmorillonite particles. To achieve high mechanical properties of filled epoxy systems, more preferable is the use of clays modified with polar compounds ensuring better compatibility of the filler with the epoxy matrix (in the present case, MMT of grade Cloisite 30B). The optimum filler concentration should be determined by the service requirements of the end product. The increase in heat resistance and physicomechanical properties of composite materials when nanoparticles are introduced makes it possible to recommend them as filler for epoxy binders. ACKNOWLEDGEMENTS This work was supported financially by the Federal Target Programme Research and Scientific-Pedagogical Cadres of Innovative Russia for 2009 2013 (State Contract No. 02.740.11.5180). REFERENCES 1. G.V. Vinogradov et al., Vys. Soed., A20(1):226 (1978). 2. P.M. Ajayan et al., Nanocomposite Science and Technology. Wiley-VCH Verlag, Weinheim, Germany (2003). 3. V. Nigam et al., J. Appl. Polym. Sci., 93(5):2201 (2004). 4. T. Lan and T.J. Pinnavaia, Chem. Mater., (6):2216 (1994). 5. A.L. Poli et al., J. Colloid Interface Sci., (325):386 (2008). 6. K.M. Lee and C.D. Han, Macromolecules, 36(19):7165 (2003). 7. C.M. Koo et al., J. Appl. Polym. Sci., 88(6):1526 (2003). 8. O. Ishai and L.J. Coheno, J. Compos. Mater., 2(3):302 (1968). 9. J. Park and S.C. Jana, Macromolecules, 36(22):8391 (2003). 10. A.S. Zerda and A.J. Lesser, J. Polym. Sci. B Polym. Phys., 39(11):1137 (2001). 11. K. Wang et al., Macromolecules, 38(3):788 (2005). 2012 Smithers Rapra Technology T/61