March 27, 2007
Clay-Epoxy Nanocomposites: Processing and Properties
By Gupta, Nikhil; Lin, Tien Chih; Shapiro, Michael
The work described in this paper is focused on evaluating the effect of the processing method and nanoclay (montmorillonite) content on the tensile, compressive, and impact properties of clay- epoxy nanocomposites. Nanocomposites are synthesized by two methods: mechanical mixing and shear mixing. Both these methods are capable of producing bulk quantities of clay-epoxy nanocomposites. X-ray diffraction analysis indicates that the nanoclay has exfoliated in the mechanically mixed specimens. Results show that as nanoclay content increases the tensile modulus increases for both mechanically and shear mixed specimens, while the compressive modulus remains largely unchanged. The total energy absorption under impact loading is found to be higher in mechanically mixed specimens.
The interest in clay-polymer nanocomposites has been increasing ever since Toyota demonstrated commercial applications of nylon 6/ clay nanocomposites.1 Other types of nanoparticles are also being incorporated into polymeric resins in order to fabricate materials with increased performance. Some common examples of the nanoparticulate reinforcements include carbon-based nanoparticles such as nanotubes, metals such as copper and aluminum, and ceramics such as alumina and silica. These nanoparticles are loaded intoepoxies, polymethyl methacrylate, nylon, and polystyrene, as well as other types of polymers. The nanoscale particles possess enormous surface area. Hence, the interfacial area between the two intermixed phases in a nanocomposite is substantially larger than traditional composites. This results in increased bonding between the particles and the matrix. Therefore, several mechanical, thermal, andelectrical properties of nanocomposites are observed to be better than those of conventional microcomposites or the neat matrix resin.2-9 In some cases the nanoparticle-reinforced resins have been used as matrix materials to fabricate conventional microcomposites.10-11
The main applications of nanocomposites are in the automotive, energy, and packaging sectors. The current global nanocomposite market size is around $300 million and is expected to exceed $1 billion within the next five years.12,13 Currently, nanoclay-filled composites account for almost 25% by volume of total nanocomposites usage and their market share is rapidly increasing. The relatively low cost of nanoclay and rising energy and polymer prices are contributing to the increased use of clays as fillers to achieve savings of matrix polymers.
Clay-epoxy nanocomposites have attracted considerable technological and scientific attention because these materials offer a wide array of property improvements at very low filler content. Montmorillonite clay has a layered structure in which individual layers of typically 1 nm thickness and 0.1-2 m length and width have interlayer spacing of 2-3 nm.14 These layers are bonded together by van der Waal's forces. The enhancement of mechanical properties in nanocomposites depends on the intercalated or exfoliated conlent (Figure 1) of nanoclay in the polymer matrix.15,16 In the exfoliated condition their surface area can be as high as 750 m2/g. However, the complete exfoliation of nanoclay remains a significant challenge in various types of polymers. The development of simple and cost- effective processing methods that lead to the complete exfoliation of nanoclay particles will result in the widespread applications of these composite materials.
For experimental details see the Experimental Procedures sidebar, and for more on nanoclay processing see the Processing Techniques sidebar.
RESULTS AND DISCUSSION
Among the methods of determining the dispersion characteristics of nanoclay in polymers, x-ray diffraction (XRD) and transmission- electron microscopy (TEM) are widely used. In the present study XRD is given preference over TEM because the composite material is processed in bulk quantities of 4 L in each batch. Transmission- electron microscopy observes the structure of the material in a very small area, which is less than 0.2 m in length and width at 10^sup 6^ magnification. The structure observed in such a small area may not be representative of the large batches. Therefore, XRD is used for characterization because it uses relatively large specimen size and sample selection will have a much smaller effect on the results.
The XRD spectra for the synthesized nanocomposites are shown in Figure 2, which are arbitrarily separated on the y-axis for various nanoclay contents. The spectra for pure nanoclay and neat epoxy resin are also included in the same figure for comparison. The neat epoxy resin specimens contain resin, diluent, and hardener. It can be observed that the nanocomposites synthesized using mechanical mixing (Figure 2a) do not exhibit any peak in their spectra, which indicates that the nanoclay has exfoliated. Most of the specimens synthesized using shear mixing show a small bump in the spectra in the 2θ range of 4.1-4.2 as shown in Figure 2b, which is a shift from the 2θ value of 4.6 for the pure nanoclay. Hence, the nanoclay has intercalated in these composites but not exfoliated. The specimens tested contain up to 2 vol.% (approximately 4 wt.%) of nanoclay, which can result in low intensity of peaks in the XRD data. Comparing Figure 2a and 2b, peaks can be clearly identified in the shear mixed specimens, which also contain nanoclay in the same volume fractions. Hence, the absence of peaks in mechanically mixed specimens is a result of exfoliation.
Tensile and Compression Tests
The stress-strain curves obtained from the tensile and compressive testing of the neat matrix resin and mechanically mixed nanocomposites are presented in Figure 3. Similar trends were observed for specimens prepared by shear mixing. The calculated tensile and compressive modulii and strengths are presented in Figure 4a and b, respectively. The tangent modulus is presented as the tensile modulus because the tensile stress-strain curves did not show a significantly large linear elastic region. The fracture strength is presented as the tensile strength because the curvature in the graphs is small. The compressive modulus is calculated as the slope of the linear elastic regions of the stress-strain curves. The 0.2% yield strength is presented as the compressive strength.
It can be observed from Figure 4 that the tensile modulus increases with increasing nanoclay content. The mechanically mixed specimens with exfoliated nanoclay exhibit a higher modulus than the shear mixed specimens, which contain intercalated nanoclay. The modulus shows an increase of about 50% with the addition of only 2 vol.% nanoclay. Similar observations have been reported by other researchers.20 The added stiffness achieved through the incorporation of nanoclay particles results in no discernable optimum value for the elastic modulus. However, the maximum strength was exhibited by the 0.25 vol.% nanoclay specimens. The tensile strength decreased with further increase in the nanoclay content. An increase in tensile modulus and decrease in strength have also been observed in other published studies.21
The effect of nanoclay on the compressive properties of nanocomposites is similar. The compressive modulus increased by about 30% and the maximum strength was observed for 0.25 vol.% of exfoliated nanoclay composites. However, in this case the shear mixed specimens showed higher strength and modulus as opposed to the mechanically mixed compositions.
A better understanding of the compressive properties of composites can be obtained by observing the stress-strain curves in Figure 3b. The curve for the neat resin shows an approximate 20% decrease in strength past the yield strength. However, the nanocomposites do not show asimilar decrease. The main compressive failure mode in epoxies is the initiation of cracks in the direction of compression due to the lateral expansion under the Poisson's ratio effect. In the case of nanocomposites the increased modulus leads to smaller lateral expansion, leading to delayed crack initiation. Hence, the decrease in strength after the yield point is not observed. An increase in the nanoclay content leads to higher compressive modulus resulting in the final failure at lower strain values.
The load-deflection and energy-deflection curves for mechanically mixed specimens are presented in Figure 5. Curves for shear mixed specimens also showed similar trends. The total fracture process takes less than 1.5 ms for most nanocomposites. Figure 5a and b shows that most specimens attain the peak energy absorption in the range of 2-2.5 mm deflection. The presence of second-phase particles leads to the deflection and branching of the crack tip in composite materials and slows down the fracture process, leading to lower slope in the energy-deflection curves. The results presented in Figure 6 show that the energy absorption in the neat epoxy resin is higher than that of nanocomposites. Reduced tensile and compressive failure strains lead to a fracture of nanocomposites at lower deflection leading to lower total energy absorption compared to the neat epoxy resi\n. It is observed that the specimens with exfoliated nanoclay show higher fracture load and total energy absorption than specimens containing intercalated nanoclay. The dominant effect of crack initiation and propagation on the properties of clay-epoxy nanocomposites had been established previously through fracture toughness studies.22 For specimens with a notch or a blunt crack the Mode I stress intensity factors (K^sub IF^) were observed to be lower than the neat epoxy in previously published studies. The K^sub IF^ was found to be higher only in the nanocomposites that used a sharp starter crack.
Scattered results can be found published on other mechanical and physical properties of a number of nanoclaypolymer systems. The Vickers hardness was found to increase by 16% in clayepoxy nanocomposites compared to the neat resin.23 In dynamic mechanical analysis the storage modulus was found to increase by 31% due to the addition of about 1.6 vol.% nanoclay.24 In this study the glass transition temperature was observed to be the maximum at 0.5 vol.% nanoclay. The tensile modulus showed a trend similar to that observed in the present study, but the strength increased up to the addition of 1 vol.% nanoclay and decreased after that. The wear rates of bentonite clay-polyester nanocomposites were found to improve whereas for organoclays-poly ester these properties were found to deteriorate with an increase in the clay content.25 A similar trend was exhibited by the nylon 6/clay nanocomposites.26 Water uptake was found to be lower in some clay-epoxy nanocomposites than the neat resin.27 The data on mechanical properties of a large number of clay-reinforced nylon, polyester, and polypropylene-based nanocomposites is available in a recent review article.28
Various mechanical properties exhibited mixed trends with increased nanoclay content in lhe composite. In most cases, exfoliated specimens showed better properties than intercalated specimens. Hence, applications of these materials should be selected in such a way that enhanced performance can be obtained by using the improved properties.
The authors acknowledge the Othmer Institute of Interdisciplinary Studies at Polytechnic University for partial funding for this study. The help of Alessandro Belli in specimen preparation and Sandeep Gupta in various stages of experimental work is greatly appreciated.
The nanoclay selected tor the study is Cloisile 30B, supplied by Southern Clay Products, Gonzales, Texas. This clay is a natural montmorillonite mineral modified with a quaternary ammonium salt. The nanoclay particles were surface modified in order to facilitate their dispersion and exfoliation within the epoxy matrix, Epoxy resin D.E.R. 332. manufactured by Dow Chemical Company, Midland, Michigan, was used as the matrix material. Amine-based hardener D.E.H. 24 is used with the selected epoxy resin. A diluent C^sub 12^- C^sub 14^ aliphaticglycidylether is mixed with the epoxy resin to reduce its viscosity. The ratio of epoxy. diluent, and hardener is taken as 83.5:4.4:12.1 in all specimens.
Five compositions of nanoconiposiies were synthesized in the study, containing nanoclay in 0.125 vol.%. 0.25 vol.%, 0.50 vol.%, 1 vol.%, and 2 vol.%. These composites were synthesized by mechanical and shear mixing methods providing ten types of nanocomposite specimens.
Obtaining the complete exfoliation of nanoclay in the synthesized composites is a significant challenge. This process was first optimized for mechanical mixing. The viscosity of the resin plays an important role in determining the shear forces exerted on nanoclay particles during the mixing process. Therefore, resin and diluent were mixed to reduce the viscosity of the mixture. Then, the desired quantity of nanoclay was added to the resin and hand stirred until all the clay was immersed. The mixture was stirred at 650 rpm for 2 h using a variable speed drill press (JDP-17FSE) fitted with a high shear impeller. The mixing speed and time were selected based on preliminary investigations. It was found that the best exfoliation results were obtained when the mixing was carried out at 50C. The mixture was then degassed at 45C. First a small quantity of the material was cured for x-ray diffraction (XRD) analysis. If the exfoliation was not complete it was stirred for an additional 30 min, and then the hardener was mixed. The composite slabs were cast in aluminum molds, cured at room temperature for 24 h, then post- cured at 100C for 3 h. The processing was carried out with 4 L of resin in each batch.
The fabrication of shear mixed nanocomposites was carried out using a three-step process. Hirst, the desired volume fraction of nanoclay was added to the epoxy-diluent mixture and hand stirred until all the nanoclay was immersed. Then, the mixture was placed under the drill machine and mixed at 650 rpm for 30 min. to obtain homogeneous distribution of nanoclay within the resin. The mixture was then shear mixed using a three roll mill (EXAKT 50) at 180 rpm. The shear mixing was carried out twice on each batch. Following the mixing process, degassing and curing were done in a manner similar to that described previously.
The nanocomposites were characterized by Rigaku Miniflex x-ray diffractometer using Cu Kα radiation, measured at 30 kV/15 mA. The data was recorded in the range of 2θ = 2-10,6,7 at the step size of 0.01 and the counting speed of 0.5/min. These parameters were selected based on preliminary studies to give sufficient resolution in the acquired XRD data.
The dimensions of mechanical test specimens are presented in Figure A. Tensile and compression tests were performed on an Instron 4467 mechanical test system at a deformation rate of 0.5 mm/min. ASTM D63K-02(17) and D695-02(18) were adopted to determine the test parameters for these tests, respectively. The tensile test specimen had straight sides, as shown in Figure A, and had dimensions suggested for the type IV specimens in the selected standard. An extensomeler with gauge length of 25.4 mm was used to obtain the strain data in tensile tests. The displacement was measured for calculating strain in compression tests.
Izod impact testing was performed using a Dynatup POE 2000 instrumented impact tester, which specializes in testing polymers and composites. The unnotched test specimens were prepared in accordance with the ASTM standard D4812-05.19 Load, displacement, and energy data were obtained for impact testing. At least five specimens of each type were tested and the average values are reported.
Clay-Epoxy Nanocomposite Synthesis
The advantage of using a low-cost tiller can only be realized if the synthesis process does not add significantly in the cost of the raw materials. It is desired that the existing composites synthesis methods he used to obtain high-quality nanocomposiles so that the infrastructure establishment cost can be minimized. Numerous techniques, based on chemical or mechanical processing methods, are available for dispersing nanoparticles in polymers. Some of the mechanical processing methods used for clay-thermoset nanneomposites are mechanical mixing, shear mixing, and ultrasonic mixing.
Mechanical or stir mixing is a widely used technique for dispersing microparticles in pohmers. Use of this technique for synthesis of nanocomposiles is highly desired because it can save the cost of establishing new infrastructure. The parameters affecting the mixing process are temperature, mixing speed, and impeller design. The processing temperature affects viscosity and plays an important role in achieving exfoliation. The vortices formed by the impeller generate shear forces that can break van der Waal's bonds between nanoclay platelets and lead to exfoliation. Hence, the impeller design is also an important parameter in this process.
Shear mixing is commonly used as a method for dispersing nanoclay in thermosets. Three-roll mills are widely used for the purpose of mixing. The material is led between the feed roll and the center roll and is collected from the apron roll. The exfoliation of nanoclay depends on the shear force applied by the rolls on the material, which is controlled by the separation between rolls and the viscosity of the resin. Two shear mechanisms are applicable in such processing systems, which include the direct shear forces applied by the rolls and the shear caused in the vonices formed in the material coming out of the rolls. The reagglomeration of dispersed particles can he a problem in this method if the processing parameters are not optimized.
In several recent studies ultrasonic mixing has been used as a means to exfoliate nanoclay in polymers. This technique is based on imparting high energy vibrations in a localized region causing separation of nanoclay platelets. However, such vibrations lead to significant localized healing. Polymeric resins such as epoxies are thermal insulators. Hence, the heat generated in the mixing process is not dissipated effectively and can initiate the self- polymerization reaction in the mixing region. Recent process modifications include using ultrasonic mixing in conjunction with mechanical mixing and the use of an external cooling bath. Ultrasonic frequency, power, and mixing time are the variables in this process.
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Nikhil Gupta and Tien Chih Linare with the Polytechnic University, Composite Materials and Mechanics Laboratory, Mechanical, Aerospace and Manufacturing Engineering Department, Brooklyn, NY 11201. Michael Shapiro is with Yeshiva Univesirty, New York. Dr. Gupta can be reached at (718) 260-3080; e-mail [email protected]
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