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epoxy composites
Polymer 45 (2004) 8211–8219 www.elsevier.com/locate/polymer
Mechanical and thermal properties of graphite platelet/epoxy composites Asma Yasmin*, Isaac M. Daniel McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208, USA Received 21 April 2004; received in revised form 13 September 2004; accepted 21 September 2004
Abstract Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5–5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (Tg) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy. q 2004 Elsevier Ltd. All rights reserved. Keywords: Graphite platelets; Epoxy; Composites
1. Introduction There is an increasing demand for advanced materials with better properties to meet new requirements or to replace existing materials. The high performance of continuous fiber (e.g. carbon fiber, glass fiber) reinforced polymer matrix composites is well known and documented. However, these composites have some disadvantages related to the matrix dominated properties which often limit their wide application and create the need to develop new types of composite materials. In the plastics industry, the addition of filler materials to a polymer is a common practice. This improves not only stiffness, toughness, hardness, heat distortion temperature, and mold shrinkage, but also reduces the processing cost significantly. In fact, more than 50% of all produced polymers are in one way or another filled with inorganic fillers to achieve the desired * Corresponding author. Address: Center for Intelligent Processing of Composites, 2137 Tech Drive, Room 330, Evanston, IL 60208, USA. Tel.: C1 847 491 7961; fax: C1 847 491 5227. E-mail address: [email protected] (A. Yasmin). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.09.054
properties [1]. The most commonly used particles are CaCO3, clay, mica, alumina trihydrate, glass beads, and metal phosphates. The selection of filler is often made based on the desired properties in the final product. The improvement of mechanical and other properties of such composites depends strongly on the particle content, particle shape and size, surface characteristics and degree of dispersion [2–5]. Consequently, the toughening of these composites arises from a number of mechanisms such as crack-tip pinning, crack-surface bridging, debonding/microcracking, and crack deflection [2]. It is reported that the mechanical and thermomechanical properties of composites filled with micron-sized filler particles are inferior to those filled with nanoparticles of the same filler [4]. In addition, the improved physical properties, such as surface smoothness and barrier properties cannot be achieved by using conventional micron-sized particles. Therefore, in recent years nanoparticle based composites have drawn considerable attention. The most promising of these are polymer/clay nanocomposites [6– 19], polymer/graphite nanoplatelet composites [20–27] and polymer/carbon nanotube composites [28–31]. These
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nanocomposites contain a very low amount of filler (! 10 vol%) compared to conventional particulate composites where the usual filler content is in the range of 40–60 vol%. Furthermore, these nanocomposites are quasi-isotropic and can be processed by conventional means compared to the composites reinforced with continuous fibers. It is of interest to mention that both silicate clay (montmorillonite) and graphite particles show a layered structure by nature and possess high aspect ratios (O1000) once intercalated or exfoliated by chemical processes [7,21]. While nanocomposites with clay platelets show improved strength, modulus, heat distortion temperature and barrier properties, the nanocomposites with graphite platelets show additional excellent electrical and thermal conductivity. Carbon nanotubes also exhibit exceptional mechanical (modulusZ1 TPa, strengthZ10 times that of steel), thermal and electrical properties. Based on the above considerations, it can be suggested that the development of these nano-level particles will offer tailorability of desired properties in a material. It is also reported that graphite platelets, about 500 times less expensive than carbon nanotubes, can be exfoliated and compounded in a conventional way [25], whereas the nanotube-based composites require development of processing techniques with regard to dispersion, waviness and alignment of nanotubes. Therefore, graphite platelets are a potential alternative to carbon nanotubes with regard to cost and desired properties. However, the fundamental understanding of the nanoscale reinforcement mechanisms is still important and necessary. Since, graphite is well known for its high strength and high thermal conductivity, it offers the possibility of making truly multifunctional composites in a cost effective way. Polymers reinforced with such particles have many potential applications, e.g. CRTs and fuel cells, EMI shielding of electronic enclosures, radar absorbent coatings, and thermo-mechanically enhanced materials. The objective of the present study is to fabricate composite materials that contain reinforcing graphite platelets in an epoxy matrix and to evaluate their mechanical, thermal and viscoelastic properties as well as the failure mechanisms as a function of platelet concentration.
2. Experimental 2.1. Materials The matrix material was a three component epoxy system consisting of diglycidyl ether of bisphenol A (DGEBA) cured with an anhydride hardener, methyl tetrahydrophthalic anhydride (MTHPA) and an accelerator, 1-methylimidazole. This three-component system was obtained from Ciba-Geigy and was processed in proportions of 100:85:1 by weight. The chemical structure of these materials is shown in Fig. 1. The reinforcing particles were net crystalline graphite platelets developed by Cornerstone
Fig. 1. Chemical formula of (a) diglycidyl ether of bisphenol A, (b) methyl tetrahydrophthalic, and (c) 1-methylimidazole.
Technologies LLC (Wilkes-Barre, PA). The properties of the as-received graphite platelets are summarized in Table 1 [32]. The graphite platelets were of sizes ranging from 4 to 76 mm in diameter with a nominal average size of 22 mm. A SEM micrograph of the platelets is shown in Fig. 2. These platelets consist of thin hexagonal plates or distorted clusters of flaky plates. It was difficult to measure the thickness of as-received graphite platelets from SEM micrographs though an attempt showed a typical thickness of 150 nm. A simple relationship was developed to relate the surface area and the thickness of platelet materials, AZ
2 rt
(1)
where A, specific surface area per unit mass (m2/g); r, density (g/cc); t, platelet thickness (nm). The thickness of as-received graphite platelets was as high as 250 nm when calculated from Eq. (1) considering surface area and density of graphite platelets of 3.56 m2/g and 2.25 g/cc, respectively, as given in Table 1. This gives Table 1 Properties of graphite platelets [32] Composition Color Hardness Specific gravity (g/cc) Surface area (m2/g) Mean particle size (mm)
Carbon Dark gray to black 1–2 1.9–2.3 3.56 22
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2.3. Characterization techniques 2.3.1. Wide angle X-ray diffraction (WAXD) WAXD was used to verify the structure of the composite. WAXD was performed on a Rigaku diffractometer with ˚ ) operating at 40 kV and Cu Ka radiation (lZ1.541 A 20 mA. The scanning range was 4–608 with a scanning speed of 0.5 8/min.
Fig. 2. SEM micrograph of graphite platelets.
an aspect ratio of 15–300. Therefore, these graphite platelets are relatively thick compared to graphite nanoplatelets of less than 100 nm thick as reported elsewhere [21]. Further information on the synthesis of these graphite platelets is proprietary with Cornerstone Technologies, LLC. In general, natural graphite is exfoliated by a combination of chemical and thermal treatments [22,23]; graphite is intercalated by an acid treatment followed by exfoliation by a thermal shock at an elevated temperature. Such a process can generate exfoliated graphite platelets with a surface area of 40 m2/g and thickness of less than 50 nm as observed in a recent study [26]. However, with the aid of the right intercalant and process it is possible to produce graphite nanosheets or ‘graphene’ that correspond to surface areas of more than 2600 m2/g with a thickness of less than 1 nm. It is also reported that the acid treatment of natural graphite generates oxygen containing functional groups (–OH and –COOH) that facilitate both physical and chemical interactions between graphite and polymer [23,24]. 2.2. Composite fabrication The graphite platelet/epoxy composites were prepared following standard procedures. First, the epoxy resin (DGEBA) was mixed with 85 wt% hardener. Next, graphite platelets were added and mixed using a magnetic stirrer over a hot plate at approximately 60 8C for 1 h. Finally, an accelerator of 1 wt% was added to the mixture at ambient temperature with slow agitation. The accelerator was added at the end since it helps to start the curing process of the mixture. The mixture was then poured in an aluminum mold that was prepared following the ASTM standard D638-99. The mold was then placed in an oven and cured at 138 8C for 1 h. In this study, composites were prepared with graphite concentrations of 2.5 and 5 wt%. The reason for adding a higher concentration of graphite was to observe its effect on pure epoxy. However, above 5 wt%, the mixture became very viscous and difficult to process. Tensile specimens 165 mm long and 2.5 mm thick with a gage length of 50 mm and width of 13 mm were prepared.
2.3.2. Mechanical testing Tensile tests were performed on an Instron 8500 servohydraulic testing machine at a crosshead rate of 0.25 mm/min which corresponds to a strain rate of 0.01% per second. The strains were recorded with strain gages. At least six tests were carried out for each case. 2.3.3. Microscopy Following tensile testing, the fracture surfaces were examined using a Hitachi S4500 FE Scanning Electron Microscope to identify the mode of failure. The fracture surfaces were gold coated prior to SEM investigation to avoid charging and were examined at 15 kV accelerating voltage. Since graphite is conductive, the particles could be examined at 3 kV accelerating voltage without gold coating. The distribution of graphite platelets in the composite was also examined using an optical microscope. 2.3.4. Dynamic mechanical analysis (DMA) DMA of both pure epoxy and graphite platelet/epoxy composites were performed on a model 2980 DMA Dynamic Mechanical Analyzer (TA Instruments, USA) to determine their thermomechanical properties, such as storage modulus E 0 , loss modulus E 00 damping factor tan d and glass transition temperature (Tg). The experiments were carried out on prismatic samples (30 mm!8 mm!2 mm) under single cantilever mode. A frequency of 1 Hz (corresponding to a strain rate of 0.05% per second) with a temperature ramp of 3 8C/min and a scanning temperature range from 30 to 200 8C were employed. The glass transition temperature, Tg, was determined from the peak of the tan d curve. At least three tests were carried out for each case. 2.3.5. Coefficient of thermal expansion The coefficient of thermal expansion (CTE) of pure epoxy and its composites was determined using a strain gage technique. The slope of the thermal strain versus temperature curve measures the CTE of the composite. The samples were heated in an oven from room temperature to 200 8C (well above the Tg) at a rate of 2 8C/min. The thermal strains were recorded using high temperature strain gages. The gages were bonded on the specimen surface following the Micro Measurements, Inc. standard procedures. Strain gage readings were calibrated using a titanium silicate sample and the error due to the purely thermal output of the strain gage was taken into account and corrected for.
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2.3.6. Thermogravimetric analysis (TGA) The thermal stability of the composites was investigated using a TGA-SDT 2960 Thermogravimetric Analyzer (TA Instrument, USA). The TGA scans were recorded at 20 8C/min under constant nitrogen flow of 100 ml/min from room temperature to 800 8C. At least three tests were carried out for each case. 3. Results and discussion
platelets and the composites. Both pure graphite platelets and the graphite platelet/epoxy composites exhibit an intense peak at a 2q value of 26.58, corresponding to a ˚ . This basal spacing is exactly the basal spacing of 3.36 A same as that observed for pure crystalline graphite [33]. This confirms that the graphite platelets, w250 nm in thickness (calculated), are still in order and multilayer, and thus, maintain their original d-spacing. This, further, indicates that the processing technique used in this study was unable to affect the order in the structure or exfoliate the graphite platelets in the epoxy matrix.
3.1. Microstructure Optical micrographs of the cross-section of graphite platelet/epoxy composites with concentrations of 2.5 and 5 wt% of graphite platelets are shown in Fig. 3(a) and (b), respectively. The distribution of graphite platelets is found to be nearly uniform without segregation after curing. The pure epoxy matrix is transparent but the addition of graphite platelets made the composites opaque. 3.2. WAXD measurements Fig. 4 shows the WAXD patterns for pure graphite
3.3. Density Epoxy materials are well known for their low density. However, the density of the filler is usually higher than that of pure epoxy. It is, therefore, important to know the change in density with filler addition. The variation of density with graphite platelet concentration was measured by conventional means. The density of the composite was also calculated using the rule of mixtures. The corresponding equations are: fZ
w=rp w=rp C ð1 K wÞ=rm
rc Z f rp C ð1 K f Þrm
(2) (3)
where f is the volume fraction of graphite platelets, w the weight fraction of graphite platelets, rp the graphite platelet density, rm the matrix density, and rc the composite density. It is reported that the density of graphite particles (rp) varies in the range of 1.9–2.3 g/cc (Table 1). Using rpZ 1.9 g/cc and rmZ1.2 g/cc (density of pure epoxy measured in this study), the density of a 2.5 wt% graphite platelet/ epoxy composite becomes 1.208 g/cc. Using rpZ2.3 g/cc, the density of a 2.5 wt% graphite platelet/epoxy composite becomes 1.211 g/cc. Both numbers are very close to the
Fig. 3. Optical micrographs of the cross-section of graphite platelet/epoxy composites. (a) 2.5 wt% graphite and (b) 5 wt% graphite.
Fig. 4. XRD patterns of pure graphite and its composites.
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measured value (1.21 g/cc). However, the density of the 5 wt% graphite platelet/epoxy system should range between 1.219 and 1.226 g/cc, which is lower than the measured density of 1.244 g/cc. This discrepancy may be due to the occasional presence of inclusions (e.g. clay) in the graphite particles, which becomes more pronounced as the graphite content increases. However, it can be suggested that the increase in density due to the addition of graphite platelets in pure epoxy is still negligible. 3.4. Mechanical properties The tensile properties of pure epoxy and graphite platelet/epoxy composites reinforced with 2.5 and 5 wt% graphite platelet concentrations are summarized in Table 2. From this table, it can be seen that both composites show higher elastic modulus and strength than the pure epoxy matrix. They also show failure at a higher strain than the pure epoxy. The variation of elastic modulus with graphite platelet concentration is shown in Fig. 5(a). The composite reinforced with 2.5 wt% graphite platelets showed about 10% increase in elastic modulus, whereas the composite reinforced with 5 wt% of graphite platelets showed about 25% increase in elastic modulus over the pure epoxy matrix. However, the variation in tensile strength with graphite concentration did not follow the same trend. In Fig. 5(b), the graphite platelet/epoxy composites showed about 21% higher tensile strength with the addition of 2.5 wt% but only 9% increase with the addition of 5 wt% of graphite platelets compared to the pure epoxy. Agag et al. [17] also reported that the addition of low clay content (1%) in polyimide matrix increased the tensile strength, however, the higher clay content (4%) decreased the tensile strength. It can be assumed that the increased concentration of graphite platelets made the composite more brittle and reduced its failure strain. The improvement in strength and modulus can be attributed to the high strength and high aspect ratio of graphite platelets as well as to the uniform distribution and good interfacial adhesion between the platelets and the epoxy matrix. All these characters provide good load transfer from the matrix to the platelets. However, the lower strength at higher graphite platelet concentration (R5 wt%) can be attributed to the inevitable aggregation of platelets at high concentration. The cluster of platelets shown in Fig. 3 also confirms this. Therefore, when the composites are under load, the platelets in the cluster may
Fig. 5. Effect of graphite platelet concentration on the tensile properties of graphite platelet/epoxy composites. (a) Elastic modulus and (b) tensile strength.
produce a high stress concentration and cause premature failure [14,15]. Attempts were also made to calculate the elastic modulus of the composites as a function of particle content using existing micromechanical models. The constituent materials properties used in the analysis are: matrix Young’s modulusZ3 GPa, matrix Poisson’s ratioZ0.34, graphite in-plane modulusZ600 GPa, graphite out-of-plane modulusZ10.5 GPa, graphite Poisson’s ratioZ0.25, graphite platelet widthZ22 mm, and thicknessZ250 nm. The Reuss (series) model predicted moduli of 3.03 and 3.07 GPa for 2.5 and 5 wt% graphite/epoxy composites, respectively, whereas, the Mori–Tanaka model [34]
Table 2 Mechanical properties of pure epoxy and graphite platelet/epoxy composites Properties
Epoxy
Epoxy—2.5 wt% graphite
Epoxy—5 wt% graphite
Elastic modulus, E (GPa) Tensile strength, sult (MPa) Ultimate tensile strain, 3ut (%) Poisson’s ratio, n
3.0G0.1 34G3 1.24G0.2 0.34
3.3G0.1 41G4.6 1.5G0.2 0.35
3.75G0.2 37G3 1.3G0.3 0.33
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predicted moduli of 3.78 and 4.61 GPa for 2.5 and 5 wt%, respectively. The Voigt model predicted even higher values. The experimental results are not in agreement with these models. The most obvious reasons would be the random distribution of platelets and numerous aggregates instead of good exfoliation, and orientation of graphite platelets. Therefore, a processing technique that provides better dispersion, exfoliation and orientation of platelets is highly desirable. In addition, the values of Young’s modulus and the thickness of graphite platelets used in the calculations were assumed. It is, therefore, important to measure the Young’s modulus as well as the geometry of the graphite platelets to predict the modulus of composites more accurately. 3.5. Viscoelastic properties A dynamic mechanical analyzer is often used to study the viscoelastic properties of polymers under stress and elevated temperature. Fig. 6 shows the variation of storage modulus and tan d with temperature for pure epoxy and its composites. For the purpose of clarity, only one representative curve for each case is shown. At 30 8C, the 2.5 and 5 wt% graphite platelet/epoxy composites showed about 8 and 18% higher storage modulus than the pure epoxy matrix. As the temperature increased, both pure epoxy and its composites showed a gradual drop in storage modulus followed by a sudden drop at the glass transition temperature (Tg). The drop in modulus is related to the material transition from a glassy state to a rubbery state. Fig. 6 shows that at the onset of glass transition temperature (w125 8C), both 2.5 and 5 wt% graphite platelet/epoxy composites showed similar improvements in storage modulus as observed at 30 8C. A higher storage modulus with the higher graphite content was also observed above Tg, or in the rubbery region. At 150 8C, the 2.5 and 5 wt% graphite platelet/epoxy composites showed about 34 and
Fig. 6. (a) Dynamic mechanical properties of pure epoxy and its composites.
53% higher storage modulus, respectively, than the pure epoxy, a much higher increase than observed at room temperature. In the present study, the graphite platelet/ epoxy nanocomposites showed consistently higher storage modulus over pure epoxy, increasing with the concentration of graphite particles. The DMA analysis also revealed that the effect of graphite is more pronounced on the viscoelastic behavior. From Fig. 6, it can also be seen that the glass transition temperature of pure epoxy increased slightly with the incorporation of graphite platelets. The Tgs were 143, 145 and 146 8C for graphite contents of 0, 2.5 and 5 wt%, respectively. These results are more promising compared to clay/epoxy nanocomposites, where a decrease in Tg with increased clay content is a common problem [10,12,16]. In general, the increase in Tg is attributed to the good adhesion between the polymer and the reinforced particles so that the nanometer size particles can restrict the segmental motion of cross-links under loading [16,17]. It is reported that the acid treatment of natural graphite generates oxygen containing functional groups that facilitate both physical and chemical interactions between graphite and polymer [23,24]. The other important factors that can affect Tg are degree of particle dispersion and curing conditions. The degree of particle dispersion includes size, homogeneity, orientation and spacing between particles, whereas curing conditions include curing speed and degree of cross-linking. The small improvement in Tg observed in this study for graphite platelet/epoxy composites may arise from some of these factors. 3.6. Thermal expansion coefficient The thermal expansion coefficient is an important issue for polymers in engineering applications. A low thermal expansion coefficient is often desirable to achieve dimensional stability and can be achieved by incorporation of a rigid and low CTE filler material. In the present study, the graphite platelets were added to pure epoxy to study the CTE of the resultant composites. It was found that both pure epoxy and the graphite platelet/epoxy composites showed an initial linear slope followed by a sudden discontinuity in thermal expansion. However, in contrast to pure epoxy and 5 wt% graphite platelet/epoxy composite, the 2.5 wt% graphite platelet/epoxy composite showed a bilinear response with an initial slope of 36!10K6/8C. This was slightly smaller than the final value of 41!10K6/8C; the transition in slope occurred at 77 8C. Tandon et al. [35] also reported a bilinear behavior when they measured the CTE of graphite–epoxy composites using a strain gage technique. In the present study, the CTE of pure epoxy was found to be 60!10K6/8C, and the addition of 2.5 wt% graphite platelets reduced it to 36–41!10K6/8C, therefore, about 30–40% lower than the CTE of pure epoxy. The lowering of CTE can be attributed to the fine dispersion and rigidity of graphite platelets in the epoxy matrix, which can inhibit the
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expansion of polymer chains as the temperature is raised. However, further addition of graphite platelets, w5 wt%, increased the CTE to 50!10K6/8C. This can be explained with regard to the inevitable aggregation of graphite platelets at higher concentration. It is expected that the lack of uniform distribution or graphite platelets in cluster form should not provide enough obstacles to the expansion of polymer chains. A similar behavior is also reported elsewhere [17], where the addition of low clay content (2%) in polyimide matrix decreased the CTE, but the higher clay content (4%) increased the CTE. Therefore, it can be suggested that the graphite content influences the thermal behavior of the composites significantly. The effects of processing techniques as well as the orientation and aspect ratio of platelet materials can also influence the thermal expansion behavior of composites significantly as reported by Yoon et al. [36] for a nylon 6/clay composite system. A similar study for graphite platelet/epoxy composites is now under investigation. 3.7. Thermal stability The thermal stability of composites was determined using a thermo-gravimetric analyzer. The TGA curves of pure epoxy and the composites with different graphite content are shown in Fig. 7. It is observed that pure graphite exhibits very high thermal stability with only 1.6% total weight loss up to 800 8C. On the other hand, both pure epoxy and the composites show thermal degradation at much lower temperatures and significant weight loss with temperature. The onset and the end set of thermal degradation temperature were determined from the intersection of two tangents. The TGA values of pure epoxy and its composites (onset; end set; degradation temperatures at 5, 10 and 50% weight loss; non-volatile part or char content) are given in Table 3, which indicate that the thermal stability of the pure epoxy was enhanced by the incorporation of graphite particles. For pure epoxy, the onset temperature is 360 8C, while for the composites it increases to 366 8C for 2.5 wt% graphite platelet/epoxy and 368 8C for the 5 wt% graphite platelet/epoxy. In all cases, the thermal degradation occurs in one step, which is mainly due to the degradation of cross-linking. Further, the incorporation of 2.5 and 5 wt% graphite particles in pure epoxy matrix increases the 5% decomposition temperature of pure epoxy by 13 and 25 8C, respectively. The composites also show higher char content or reduced weight loss at 800 8C as the graphite content increases. While pure epoxy shows a
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Fig. 7. TGA of pure graphite, pure epoxy and the composites.
char content of 10%, the composites show about 13 and 15% for 2.5 and 5 wt% graphite platelet/epoxy composites, respectively. Therefore, the incorporation of the graphite platelet resulted in pronounced improvement in thermal stability. This can be attributed to the homogeneous distribution of graphite particles as well as the tortuous path in the composites that hinders diffusion of the volatile decomposition products in the composites compared to that in pure epoxy. The improvement of thermal stability due to the addition of nanoparticles has also been reported for other composites [17–19]. 3.8. Fractography Fig. 8(a) shows the fracture surface of pure epoxy. The crack usually initiates from surface defects or high stress regions. The mirror-like region in Fig. 8(a) represents the slow growth of crack-like defects. The lines next to the mirror-like region are shear cusps developed by nucleation and propagation of micro shear zones within the matrix ligament. These shear cusps are also found to be oriented at 458 with the loading direction with almost equal widths. This observation is consistent with the brittle failure mechanisms of epoxy materials as mentioned elsewhere [37]. Fig. 8(b) shows the shear cusp at higher magnification. Fig. 9(a) shows the fracture surface of a composite reinforced with 5 wt% of graphite platelets. As in the case of pure epoxy, it also shows shear cusps but only at the beginning when the crack growth rate is low. Further, the shear cusps appear to be shorter and smaller compared to
Table 3 TGA values for pure epoxy and its composites Sample
Onset (8C)
Endset (8C)
T (8C) for 5% loss
T (8C) for 10% loss
T (8C) for 50% loss
Char (%)
Pure epoxy 2.5 wt% graphite/epoxy 5 wt% graphite/epoxy
360 366 368
442 448 450
305G1.0 318G5.3 330G2.1
348G2.0 355G5.0 357G0.7
407G1.5 412G2.0 414G2.8
10G2.0 13G4.0 15G3.5
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Fig. 8. SEM fractographs of pure epoxy. (a) Low magnification and (b) high magnification.
those in pure epoxy. This may be due to the interference of graphite platelets to the flow stress, where the graphite platelets restrict the free movement of polymer chains by crack tip pinning. Fig. 9(b) shows a combination of shear cusps and coarse features which indicate the transition from slow to fast crack growth. Finally, Fig. 9(c) shows only coarse features due to fast fracture. A similar type of fracture surface is also observed for the composite with 2.5 wt% graphite concentration. Further investigation of the fracture surface of the 5 wt% graphite platelet/epoxy composite at higher magnification shows debonding between matrix and graphite platelets at different locations, as shown in Fig. 10. It can be assumed that the platelets debonded from the epoxy matrix under loading and generated microcracks. Upon further loading, these microcracks joined together to form a single dominant crack, and the final failure of the specimen occurred by microvoid coalescence. The 5 wt% graphite platelet/epoxy composite also failed at both lower stress and lower strain than the 2.5 wt% graphite platelet/ epoxy composite. This can be attributed to the lower particle spacing in the former case and as a result the microvoids coalesced earlier.
Fig. 9. SEM fractographs of 5 wt% graphite/epoxy composite. (a) Slow crack growth region, (b) transition from slow to fast fracture region and (c) fast fracture region.
4. Summary The excellent combination of lightweight and superb thermal, mechanical, electrical, and flame retardant properties of graphite has drawn attention to the development of multifunctional polymer based nanocomposites in recent years. In the present study, graphite platelets of around
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References
Fig. 10. Fracture surface of 5 wt% graphite/epoxy composite showing debonding at different locations (arrows).
250 nm thickness were used to prepare 2.5–5 wt% graphite platelet/epoxy composites. XRD studies of composites revealed that there was no change in d-spacing of graphite platelets. This indicates that the graphite platelets were in order and multilayer, and the processing technique used in this study was unable to affect the order in the structure or exfoliate the graphite platelets in the epoxy matrix. However, the addition of graphite platelets increased both elastic modulus and tensile strength of the composites. The DMA also showed increased storage modulus and Tg with increased graphite concentration. The CTE of pure epoxy (60!10K6/8C) decreased to almost half of its original value with the addition of 2.5 wt% of graphite platelets (34–41! 10K6/8C). The composites also showed higher thermal stability than pure epoxy. Finally, the fractographic analysis of graphite platelet/epoxy composites showed failure mechanisms consisting of crack tip pinning, debonding and void coalescence, which were absent in the case of pure epoxy. It can be suggested that the 2.5 wt% graphite platelet/epoxy showed the best balance of mechanical (modulus, strength), viscoelastic (storage modulus, Tg) and thermal (CTE, thermal stability, decomposition temperature) properties due to the homogeneous dispersion of stiff and rigid graphite platelets. The influence of expanded or exfoliated graphite nanoplatelets of less than 50 nm thick on the mechanical and thermal properties of pure epoxy is now under investigation and will be communicated later.
Acknowledgements This research was sponsored by the Office of Naval Research (ONR). We are grateful to Dr Y. D. S. Rajapakse of ONR for his encouragement and cooperation. We are also thankful to Dr Peter A. Kanjorski of Cornerstone Technologies LLC, PA for supplying the graphite materials for our experiments. Thanks are also due to Dr Jyi-Jiin Luo for his help in modeling the composites.
[1] Rothon RN. Mineral fillers in thermoplastics: filler manufacture and characterization. In: Jancar J, editor. Adv Polym Sci, 139, 1999. p. 67. [2] DiBerardino MF, Pearson RA. Mater Res Soc Symp Proc 1998;515:239. [3] Chan C-M, Wu J, Li J-X, Cheung Y-K. Polymer 2002;43:2981. [4] Sumita M, Tsukurmo T, Miyasaka K, Ishikawa K. J Mater Sci 1983; 18:1758. [5] Komarneni S. J Mater Chem 1992;2:1219. [6] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, Kamigaito O. J Mater Res 1993;8:1179. [7] Giannelis EP. Adv Mater 1996;8(1):29. [8] Chen C, Curliss D. SAMPE J 2001;37(5):11. [9] Wang Z, Massam J, Pinnavaia TJ. Epoxy–clay nanocomposites. In: Pinnavaia TJ, Beall GW, editors. Polymer–clay nanocomposites. Wiley, 2000. p. 127–48. [10] Yasmin A, Abot JL, Daniel IM. Mater Res Soc Symp Proc 2002;740:75. [11] Abot JL, Yasmin A, Daniel IM. Mater Res Soc Symp Proc 2002;740: 167. [12] Yasmin A, Abot JL, Daniel IM. Scripta Mater 2003;49(1):81. [13] Yasmin A, Abot JL, Daniel IM. Characterization of structure and mechanical behavior of clay/epoxy nanocomposites, 14th international conference on composite materials, San Diego, CA; 2003. [14] Wu CL, Zhang MQ, Rong MZ, Friedrich K. Compos Sci Technol 2002;62:1327. [15] Liu X, Wu Q. Polymer 2001;42:10013. [16] Becker O, Varley R, Simon G. Polymer 2002;43:4373. [17] Agag T, Koga T, Takeichi T. Polymer 2001;42:3399. [18] Zhang W, Chen D, Zhao Q, Fang Y. Polymer 2003;44:7953. [19] Zheng Z, Wilkie CA. Polym Degrad Stab 2003;82:441. [20] Pan YX, Yu ZZ, Ou YC, Hu GH. J Polym Sci, Part B Polym Phys 2000;38:1626. [21] Chen GH, Wu DJ, Weng WG, He B, Yan WL. J Appl Polym Sci 2001; 82:2506. [22] Chen G, Weng W, Wu D, Wu C. Eur Polym J 2003;39:2329. [23] Zheng W, Wong S-C. Compos Sci Technol 2003;63:225. [24] Shen J-W, Chen X-M, Huang W-Y. J Appl Polym Sci 2003;88:1864. [25] Fukushima H, Drzal LT. Graphite nanoplatelets as reinforcements for polymers: structural and electrical properties, 17th international conference on American society for composites, Purdue University; 2002. [26] Yasmin A, Luo JJ, Daniel IM. Processing of graphite nanosheet reinforced polymer nanocomposites, 19th international conference on American society for composites, Georgia Institute of Technology; 2004. [27] Yasmin A, Luo JJ, Daniel IM. Structure and mechanical properties of expanded graphite reinforced polymer nanocomposites, in preparation. [28] Schadler LS, Giannaris SC, Ajayan PM. Appl Phys Lett 1998;73: 3842. [29] Thostenson ET, Ren ZF, Chou TW. Comp Sci Tech 2001;61(13): 1899. [30] Thostenson ET, Li WZ, Wang DZ, Ren ZF, Chou TW. J Appl Phys 2002;91(9):6034. [31] Fisher FT, Bradshaw RD, Brinson LC. Appl Phys Lett 2002;80:4647. [32] Personal communications with Cornerstone Technologies LLC. [33] Smith WF. Principles of materials science and engineering. New York: McGraw-Hill, Inc.; 1996. [34] Mori T, Tanaka K. Acta Metall Mater 1973;21(5):571. [35] Tandon GP, Kim RY, Rice BP. Influence of Vapor-grown carbon nanofibers on thermomechanical properties of graphite–epoxy composites, 17th international conference on American society for composites, Purdue University; 2002. [36] Yoon PJ, Fornes TD, Paul DR. Polymer 2002;43:6727. [37] Powell PC. Engineering with polymers. London: Chapman & Hall Ltd; 1983.
Mechanical and thermal properties of graphite platelet/epoxy composites Asma Yasmin*, Isaac M. Daniel McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208, USA Received 21 April 2004; received in revised form 13 September 2004; accepted 21 September 2004
Abstract Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5–5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (Tg) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy. q 2004 Elsevier Ltd. All rights reserved. Keywords: Graphite platelets; Epoxy; Composites
1. Introduction There is an increasing demand for advanced materials with better properties to meet new requirements or to replace existing materials. The high performance of continuous fiber (e.g. carbon fiber, glass fiber) reinforced polymer matrix composites is well known and documented. However, these composites have some disadvantages related to the matrix dominated properties which often limit their wide application and create the need to develop new types of composite materials. In the plastics industry, the addition of filler materials to a polymer is a common practice. This improves not only stiffness, toughness, hardness, heat distortion temperature, and mold shrinkage, but also reduces the processing cost significantly. In fact, more than 50% of all produced polymers are in one way or another filled with inorganic fillers to achieve the desired * Corresponding author. Address: Center for Intelligent Processing of Composites, 2137 Tech Drive, Room 330, Evanston, IL 60208, USA. Tel.: C1 847 491 7961; fax: C1 847 491 5227. E-mail address: [email protected] (A. Yasmin). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.09.054
properties [1]. The most commonly used particles are CaCO3, clay, mica, alumina trihydrate, glass beads, and metal phosphates. The selection of filler is often made based on the desired properties in the final product. The improvement of mechanical and other properties of such composites depends strongly on the particle content, particle shape and size, surface characteristics and degree of dispersion [2–5]. Consequently, the toughening of these composites arises from a number of mechanisms such as crack-tip pinning, crack-surface bridging, debonding/microcracking, and crack deflection [2]. It is reported that the mechanical and thermomechanical properties of composites filled with micron-sized filler particles are inferior to those filled with nanoparticles of the same filler [4]. In addition, the improved physical properties, such as surface smoothness and barrier properties cannot be achieved by using conventional micron-sized particles. Therefore, in recent years nanoparticle based composites have drawn considerable attention. The most promising of these are polymer/clay nanocomposites [6– 19], polymer/graphite nanoplatelet composites [20–27] and polymer/carbon nanotube composites [28–31]. These
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nanocomposites contain a very low amount of filler (! 10 vol%) compared to conventional particulate composites where the usual filler content is in the range of 40–60 vol%. Furthermore, these nanocomposites are quasi-isotropic and can be processed by conventional means compared to the composites reinforced with continuous fibers. It is of interest to mention that both silicate clay (montmorillonite) and graphite particles show a layered structure by nature and possess high aspect ratios (O1000) once intercalated or exfoliated by chemical processes [7,21]. While nanocomposites with clay platelets show improved strength, modulus, heat distortion temperature and barrier properties, the nanocomposites with graphite platelets show additional excellent electrical and thermal conductivity. Carbon nanotubes also exhibit exceptional mechanical (modulusZ1 TPa, strengthZ10 times that of steel), thermal and electrical properties. Based on the above considerations, it can be suggested that the development of these nano-level particles will offer tailorability of desired properties in a material. It is also reported that graphite platelets, about 500 times less expensive than carbon nanotubes, can be exfoliated and compounded in a conventional way [25], whereas the nanotube-based composites require development of processing techniques with regard to dispersion, waviness and alignment of nanotubes. Therefore, graphite platelets are a potential alternative to carbon nanotubes with regard to cost and desired properties. However, the fundamental understanding of the nanoscale reinforcement mechanisms is still important and necessary. Since, graphite is well known for its high strength and high thermal conductivity, it offers the possibility of making truly multifunctional composites in a cost effective way. Polymers reinforced with such particles have many potential applications, e.g. CRTs and fuel cells, EMI shielding of electronic enclosures, radar absorbent coatings, and thermo-mechanically enhanced materials. The objective of the present study is to fabricate composite materials that contain reinforcing graphite platelets in an epoxy matrix and to evaluate their mechanical, thermal and viscoelastic properties as well as the failure mechanisms as a function of platelet concentration.
2. Experimental 2.1. Materials The matrix material was a three component epoxy system consisting of diglycidyl ether of bisphenol A (DGEBA) cured with an anhydride hardener, methyl tetrahydrophthalic anhydride (MTHPA) and an accelerator, 1-methylimidazole. This three-component system was obtained from Ciba-Geigy and was processed in proportions of 100:85:1 by weight. The chemical structure of these materials is shown in Fig. 1. The reinforcing particles were net crystalline graphite platelets developed by Cornerstone
Fig. 1. Chemical formula of (a) diglycidyl ether of bisphenol A, (b) methyl tetrahydrophthalic, and (c) 1-methylimidazole.
Technologies LLC (Wilkes-Barre, PA). The properties of the as-received graphite platelets are summarized in Table 1 [32]. The graphite platelets were of sizes ranging from 4 to 76 mm in diameter with a nominal average size of 22 mm. A SEM micrograph of the platelets is shown in Fig. 2. These platelets consist of thin hexagonal plates or distorted clusters of flaky plates. It was difficult to measure the thickness of as-received graphite platelets from SEM micrographs though an attempt showed a typical thickness of 150 nm. A simple relationship was developed to relate the surface area and the thickness of platelet materials, AZ
2 rt
(1)
where A, specific surface area per unit mass (m2/g); r, density (g/cc); t, platelet thickness (nm). The thickness of as-received graphite platelets was as high as 250 nm when calculated from Eq. (1) considering surface area and density of graphite platelets of 3.56 m2/g and 2.25 g/cc, respectively, as given in Table 1. This gives Table 1 Properties of graphite platelets [32] Composition Color Hardness Specific gravity (g/cc) Surface area (m2/g) Mean particle size (mm)
Carbon Dark gray to black 1–2 1.9–2.3 3.56 22
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2.3. Characterization techniques 2.3.1. Wide angle X-ray diffraction (WAXD) WAXD was used to verify the structure of the composite. WAXD was performed on a Rigaku diffractometer with ˚ ) operating at 40 kV and Cu Ka radiation (lZ1.541 A 20 mA. The scanning range was 4–608 with a scanning speed of 0.5 8/min.
Fig. 2. SEM micrograph of graphite platelets.
an aspect ratio of 15–300. Therefore, these graphite platelets are relatively thick compared to graphite nanoplatelets of less than 100 nm thick as reported elsewhere [21]. Further information on the synthesis of these graphite platelets is proprietary with Cornerstone Technologies, LLC. In general, natural graphite is exfoliated by a combination of chemical and thermal treatments [22,23]; graphite is intercalated by an acid treatment followed by exfoliation by a thermal shock at an elevated temperature. Such a process can generate exfoliated graphite platelets with a surface area of 40 m2/g and thickness of less than 50 nm as observed in a recent study [26]. However, with the aid of the right intercalant and process it is possible to produce graphite nanosheets or ‘graphene’ that correspond to surface areas of more than 2600 m2/g with a thickness of less than 1 nm. It is also reported that the acid treatment of natural graphite generates oxygen containing functional groups (–OH and –COOH) that facilitate both physical and chemical interactions between graphite and polymer [23,24]. 2.2. Composite fabrication The graphite platelet/epoxy composites were prepared following standard procedures. First, the epoxy resin (DGEBA) was mixed with 85 wt% hardener. Next, graphite platelets were added and mixed using a magnetic stirrer over a hot plate at approximately 60 8C for 1 h. Finally, an accelerator of 1 wt% was added to the mixture at ambient temperature with slow agitation. The accelerator was added at the end since it helps to start the curing process of the mixture. The mixture was then poured in an aluminum mold that was prepared following the ASTM standard D638-99. The mold was then placed in an oven and cured at 138 8C for 1 h. In this study, composites were prepared with graphite concentrations of 2.5 and 5 wt%. The reason for adding a higher concentration of graphite was to observe its effect on pure epoxy. However, above 5 wt%, the mixture became very viscous and difficult to process. Tensile specimens 165 mm long and 2.5 mm thick with a gage length of 50 mm and width of 13 mm were prepared.
2.3.2. Mechanical testing Tensile tests were performed on an Instron 8500 servohydraulic testing machine at a crosshead rate of 0.25 mm/min which corresponds to a strain rate of 0.01% per second. The strains were recorded with strain gages. At least six tests were carried out for each case. 2.3.3. Microscopy Following tensile testing, the fracture surfaces were examined using a Hitachi S4500 FE Scanning Electron Microscope to identify the mode of failure. The fracture surfaces were gold coated prior to SEM investigation to avoid charging and were examined at 15 kV accelerating voltage. Since graphite is conductive, the particles could be examined at 3 kV accelerating voltage without gold coating. The distribution of graphite platelets in the composite was also examined using an optical microscope. 2.3.4. Dynamic mechanical analysis (DMA) DMA of both pure epoxy and graphite platelet/epoxy composites were performed on a model 2980 DMA Dynamic Mechanical Analyzer (TA Instruments, USA) to determine their thermomechanical properties, such as storage modulus E 0 , loss modulus E 00 damping factor tan d and glass transition temperature (Tg). The experiments were carried out on prismatic samples (30 mm!8 mm!2 mm) under single cantilever mode. A frequency of 1 Hz (corresponding to a strain rate of 0.05% per second) with a temperature ramp of 3 8C/min and a scanning temperature range from 30 to 200 8C were employed. The glass transition temperature, Tg, was determined from the peak of the tan d curve. At least three tests were carried out for each case. 2.3.5. Coefficient of thermal expansion The coefficient of thermal expansion (CTE) of pure epoxy and its composites was determined using a strain gage technique. The slope of the thermal strain versus temperature curve measures the CTE of the composite. The samples were heated in an oven from room temperature to 200 8C (well above the Tg) at a rate of 2 8C/min. The thermal strains were recorded using high temperature strain gages. The gages were bonded on the specimen surface following the Micro Measurements, Inc. standard procedures. Strain gage readings were calibrated using a titanium silicate sample and the error due to the purely thermal output of the strain gage was taken into account and corrected for.
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2.3.6. Thermogravimetric analysis (TGA) The thermal stability of the composites was investigated using a TGA-SDT 2960 Thermogravimetric Analyzer (TA Instrument, USA). The TGA scans were recorded at 20 8C/min under constant nitrogen flow of 100 ml/min from room temperature to 800 8C. At least three tests were carried out for each case. 3. Results and discussion
platelets and the composites. Both pure graphite platelets and the graphite platelet/epoxy composites exhibit an intense peak at a 2q value of 26.58, corresponding to a ˚ . This basal spacing is exactly the basal spacing of 3.36 A same as that observed for pure crystalline graphite [33]. This confirms that the graphite platelets, w250 nm in thickness (calculated), are still in order and multilayer, and thus, maintain their original d-spacing. This, further, indicates that the processing technique used in this study was unable to affect the order in the structure or exfoliate the graphite platelets in the epoxy matrix.
3.1. Microstructure Optical micrographs of the cross-section of graphite platelet/epoxy composites with concentrations of 2.5 and 5 wt% of graphite platelets are shown in Fig. 3(a) and (b), respectively. The distribution of graphite platelets is found to be nearly uniform without segregation after curing. The pure epoxy matrix is transparent but the addition of graphite platelets made the composites opaque. 3.2. WAXD measurements Fig. 4 shows the WAXD patterns for pure graphite
3.3. Density Epoxy materials are well known for their low density. However, the density of the filler is usually higher than that of pure epoxy. It is, therefore, important to know the change in density with filler addition. The variation of density with graphite platelet concentration was measured by conventional means. The density of the composite was also calculated using the rule of mixtures. The corresponding equations are: fZ
w=rp w=rp C ð1 K wÞ=rm
rc Z f rp C ð1 K f Þrm
(2) (3)
where f is the volume fraction of graphite platelets, w the weight fraction of graphite platelets, rp the graphite platelet density, rm the matrix density, and rc the composite density. It is reported that the density of graphite particles (rp) varies in the range of 1.9–2.3 g/cc (Table 1). Using rpZ 1.9 g/cc and rmZ1.2 g/cc (density of pure epoxy measured in this study), the density of a 2.5 wt% graphite platelet/ epoxy composite becomes 1.208 g/cc. Using rpZ2.3 g/cc, the density of a 2.5 wt% graphite platelet/epoxy composite becomes 1.211 g/cc. Both numbers are very close to the
Fig. 3. Optical micrographs of the cross-section of graphite platelet/epoxy composites. (a) 2.5 wt% graphite and (b) 5 wt% graphite.
Fig. 4. XRD patterns of pure graphite and its composites.
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measured value (1.21 g/cc). However, the density of the 5 wt% graphite platelet/epoxy system should range between 1.219 and 1.226 g/cc, which is lower than the measured density of 1.244 g/cc. This discrepancy may be due to the occasional presence of inclusions (e.g. clay) in the graphite particles, which becomes more pronounced as the graphite content increases. However, it can be suggested that the increase in density due to the addition of graphite platelets in pure epoxy is still negligible. 3.4. Mechanical properties The tensile properties of pure epoxy and graphite platelet/epoxy composites reinforced with 2.5 and 5 wt% graphite platelet concentrations are summarized in Table 2. From this table, it can be seen that both composites show higher elastic modulus and strength than the pure epoxy matrix. They also show failure at a higher strain than the pure epoxy. The variation of elastic modulus with graphite platelet concentration is shown in Fig. 5(a). The composite reinforced with 2.5 wt% graphite platelets showed about 10% increase in elastic modulus, whereas the composite reinforced with 5 wt% of graphite platelets showed about 25% increase in elastic modulus over the pure epoxy matrix. However, the variation in tensile strength with graphite concentration did not follow the same trend. In Fig. 5(b), the graphite platelet/epoxy composites showed about 21% higher tensile strength with the addition of 2.5 wt% but only 9% increase with the addition of 5 wt% of graphite platelets compared to the pure epoxy. Agag et al. [17] also reported that the addition of low clay content (1%) in polyimide matrix increased the tensile strength, however, the higher clay content (4%) decreased the tensile strength. It can be assumed that the increased concentration of graphite platelets made the composite more brittle and reduced its failure strain. The improvement in strength and modulus can be attributed to the high strength and high aspect ratio of graphite platelets as well as to the uniform distribution and good interfacial adhesion between the platelets and the epoxy matrix. All these characters provide good load transfer from the matrix to the platelets. However, the lower strength at higher graphite platelet concentration (R5 wt%) can be attributed to the inevitable aggregation of platelets at high concentration. The cluster of platelets shown in Fig. 3 also confirms this. Therefore, when the composites are under load, the platelets in the cluster may
Fig. 5. Effect of graphite platelet concentration on the tensile properties of graphite platelet/epoxy composites. (a) Elastic modulus and (b) tensile strength.
produce a high stress concentration and cause premature failure [14,15]. Attempts were also made to calculate the elastic modulus of the composites as a function of particle content using existing micromechanical models. The constituent materials properties used in the analysis are: matrix Young’s modulusZ3 GPa, matrix Poisson’s ratioZ0.34, graphite in-plane modulusZ600 GPa, graphite out-of-plane modulusZ10.5 GPa, graphite Poisson’s ratioZ0.25, graphite platelet widthZ22 mm, and thicknessZ250 nm. The Reuss (series) model predicted moduli of 3.03 and 3.07 GPa for 2.5 and 5 wt% graphite/epoxy composites, respectively, whereas, the Mori–Tanaka model [34]
Table 2 Mechanical properties of pure epoxy and graphite platelet/epoxy composites Properties
Epoxy
Epoxy—2.5 wt% graphite
Epoxy—5 wt% graphite
Elastic modulus, E (GPa) Tensile strength, sult (MPa) Ultimate tensile strain, 3ut (%) Poisson’s ratio, n
3.0G0.1 34G3 1.24G0.2 0.34
3.3G0.1 41G4.6 1.5G0.2 0.35
3.75G0.2 37G3 1.3G0.3 0.33
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predicted moduli of 3.78 and 4.61 GPa for 2.5 and 5 wt%, respectively. The Voigt model predicted even higher values. The experimental results are not in agreement with these models. The most obvious reasons would be the random distribution of platelets and numerous aggregates instead of good exfoliation, and orientation of graphite platelets. Therefore, a processing technique that provides better dispersion, exfoliation and orientation of platelets is highly desirable. In addition, the values of Young’s modulus and the thickness of graphite platelets used in the calculations were assumed. It is, therefore, important to measure the Young’s modulus as well as the geometry of the graphite platelets to predict the modulus of composites more accurately. 3.5. Viscoelastic properties A dynamic mechanical analyzer is often used to study the viscoelastic properties of polymers under stress and elevated temperature. Fig. 6 shows the variation of storage modulus and tan d with temperature for pure epoxy and its composites. For the purpose of clarity, only one representative curve for each case is shown. At 30 8C, the 2.5 and 5 wt% graphite platelet/epoxy composites showed about 8 and 18% higher storage modulus than the pure epoxy matrix. As the temperature increased, both pure epoxy and its composites showed a gradual drop in storage modulus followed by a sudden drop at the glass transition temperature (Tg). The drop in modulus is related to the material transition from a glassy state to a rubbery state. Fig. 6 shows that at the onset of glass transition temperature (w125 8C), both 2.5 and 5 wt% graphite platelet/epoxy composites showed similar improvements in storage modulus as observed at 30 8C. A higher storage modulus with the higher graphite content was also observed above Tg, or in the rubbery region. At 150 8C, the 2.5 and 5 wt% graphite platelet/epoxy composites showed about 34 and
Fig. 6. (a) Dynamic mechanical properties of pure epoxy and its composites.
53% higher storage modulus, respectively, than the pure epoxy, a much higher increase than observed at room temperature. In the present study, the graphite platelet/ epoxy nanocomposites showed consistently higher storage modulus over pure epoxy, increasing with the concentration of graphite particles. The DMA analysis also revealed that the effect of graphite is more pronounced on the viscoelastic behavior. From Fig. 6, it can also be seen that the glass transition temperature of pure epoxy increased slightly with the incorporation of graphite platelets. The Tgs were 143, 145 and 146 8C for graphite contents of 0, 2.5 and 5 wt%, respectively. These results are more promising compared to clay/epoxy nanocomposites, where a decrease in Tg with increased clay content is a common problem [10,12,16]. In general, the increase in Tg is attributed to the good adhesion between the polymer and the reinforced particles so that the nanometer size particles can restrict the segmental motion of cross-links under loading [16,17]. It is reported that the acid treatment of natural graphite generates oxygen containing functional groups that facilitate both physical and chemical interactions between graphite and polymer [23,24]. The other important factors that can affect Tg are degree of particle dispersion and curing conditions. The degree of particle dispersion includes size, homogeneity, orientation and spacing between particles, whereas curing conditions include curing speed and degree of cross-linking. The small improvement in Tg observed in this study for graphite platelet/epoxy composites may arise from some of these factors. 3.6. Thermal expansion coefficient The thermal expansion coefficient is an important issue for polymers in engineering applications. A low thermal expansion coefficient is often desirable to achieve dimensional stability and can be achieved by incorporation of a rigid and low CTE filler material. In the present study, the graphite platelets were added to pure epoxy to study the CTE of the resultant composites. It was found that both pure epoxy and the graphite platelet/epoxy composites showed an initial linear slope followed by a sudden discontinuity in thermal expansion. However, in contrast to pure epoxy and 5 wt% graphite platelet/epoxy composite, the 2.5 wt% graphite platelet/epoxy composite showed a bilinear response with an initial slope of 36!10K6/8C. This was slightly smaller than the final value of 41!10K6/8C; the transition in slope occurred at 77 8C. Tandon et al. [35] also reported a bilinear behavior when they measured the CTE of graphite–epoxy composites using a strain gage technique. In the present study, the CTE of pure epoxy was found to be 60!10K6/8C, and the addition of 2.5 wt% graphite platelets reduced it to 36–41!10K6/8C, therefore, about 30–40% lower than the CTE of pure epoxy. The lowering of CTE can be attributed to the fine dispersion and rigidity of graphite platelets in the epoxy matrix, which can inhibit the
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expansion of polymer chains as the temperature is raised. However, further addition of graphite platelets, w5 wt%, increased the CTE to 50!10K6/8C. This can be explained with regard to the inevitable aggregation of graphite platelets at higher concentration. It is expected that the lack of uniform distribution or graphite platelets in cluster form should not provide enough obstacles to the expansion of polymer chains. A similar behavior is also reported elsewhere [17], where the addition of low clay content (2%) in polyimide matrix decreased the CTE, but the higher clay content (4%) increased the CTE. Therefore, it can be suggested that the graphite content influences the thermal behavior of the composites significantly. The effects of processing techniques as well as the orientation and aspect ratio of platelet materials can also influence the thermal expansion behavior of composites significantly as reported by Yoon et al. [36] for a nylon 6/clay composite system. A similar study for graphite platelet/epoxy composites is now under investigation. 3.7. Thermal stability The thermal stability of composites was determined using a thermo-gravimetric analyzer. The TGA curves of pure epoxy and the composites with different graphite content are shown in Fig. 7. It is observed that pure graphite exhibits very high thermal stability with only 1.6% total weight loss up to 800 8C. On the other hand, both pure epoxy and the composites show thermal degradation at much lower temperatures and significant weight loss with temperature. The onset and the end set of thermal degradation temperature were determined from the intersection of two tangents. The TGA values of pure epoxy and its composites (onset; end set; degradation temperatures at 5, 10 and 50% weight loss; non-volatile part or char content) are given in Table 3, which indicate that the thermal stability of the pure epoxy was enhanced by the incorporation of graphite particles. For pure epoxy, the onset temperature is 360 8C, while for the composites it increases to 366 8C for 2.5 wt% graphite platelet/epoxy and 368 8C for the 5 wt% graphite platelet/epoxy. In all cases, the thermal degradation occurs in one step, which is mainly due to the degradation of cross-linking. Further, the incorporation of 2.5 and 5 wt% graphite particles in pure epoxy matrix increases the 5% decomposition temperature of pure epoxy by 13 and 25 8C, respectively. The composites also show higher char content or reduced weight loss at 800 8C as the graphite content increases. While pure epoxy shows a
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Fig. 7. TGA of pure graphite, pure epoxy and the composites.
char content of 10%, the composites show about 13 and 15% for 2.5 and 5 wt% graphite platelet/epoxy composites, respectively. Therefore, the incorporation of the graphite platelet resulted in pronounced improvement in thermal stability. This can be attributed to the homogeneous distribution of graphite particles as well as the tortuous path in the composites that hinders diffusion of the volatile decomposition products in the composites compared to that in pure epoxy. The improvement of thermal stability due to the addition of nanoparticles has also been reported for other composites [17–19]. 3.8. Fractography Fig. 8(a) shows the fracture surface of pure epoxy. The crack usually initiates from surface defects or high stress regions. The mirror-like region in Fig. 8(a) represents the slow growth of crack-like defects. The lines next to the mirror-like region are shear cusps developed by nucleation and propagation of micro shear zones within the matrix ligament. These shear cusps are also found to be oriented at 458 with the loading direction with almost equal widths. This observation is consistent with the brittle failure mechanisms of epoxy materials as mentioned elsewhere [37]. Fig. 8(b) shows the shear cusp at higher magnification. Fig. 9(a) shows the fracture surface of a composite reinforced with 5 wt% of graphite platelets. As in the case of pure epoxy, it also shows shear cusps but only at the beginning when the crack growth rate is low. Further, the shear cusps appear to be shorter and smaller compared to
Table 3 TGA values for pure epoxy and its composites Sample
Onset (8C)
Endset (8C)
T (8C) for 5% loss
T (8C) for 10% loss
T (8C) for 50% loss
Char (%)
Pure epoxy 2.5 wt% graphite/epoxy 5 wt% graphite/epoxy
360 366 368
442 448 450
305G1.0 318G5.3 330G2.1
348G2.0 355G5.0 357G0.7
407G1.5 412G2.0 414G2.8
10G2.0 13G4.0 15G3.5
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Fig. 8. SEM fractographs of pure epoxy. (a) Low magnification and (b) high magnification.
those in pure epoxy. This may be due to the interference of graphite platelets to the flow stress, where the graphite platelets restrict the free movement of polymer chains by crack tip pinning. Fig. 9(b) shows a combination of shear cusps and coarse features which indicate the transition from slow to fast crack growth. Finally, Fig. 9(c) shows only coarse features due to fast fracture. A similar type of fracture surface is also observed for the composite with 2.5 wt% graphite concentration. Further investigation of the fracture surface of the 5 wt% graphite platelet/epoxy composite at higher magnification shows debonding between matrix and graphite platelets at different locations, as shown in Fig. 10. It can be assumed that the platelets debonded from the epoxy matrix under loading and generated microcracks. Upon further loading, these microcracks joined together to form a single dominant crack, and the final failure of the specimen occurred by microvoid coalescence. The 5 wt% graphite platelet/epoxy composite also failed at both lower stress and lower strain than the 2.5 wt% graphite platelet/ epoxy composite. This can be attributed to the lower particle spacing in the former case and as a result the microvoids coalesced earlier.
Fig. 9. SEM fractographs of 5 wt% graphite/epoxy composite. (a) Slow crack growth region, (b) transition from slow to fast fracture region and (c) fast fracture region.
4. Summary The excellent combination of lightweight and superb thermal, mechanical, electrical, and flame retardant properties of graphite has drawn attention to the development of multifunctional polymer based nanocomposites in recent years. In the present study, graphite platelets of around
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References
Fig. 10. Fracture surface of 5 wt% graphite/epoxy composite showing debonding at different locations (arrows).
250 nm thickness were used to prepare 2.5–5 wt% graphite platelet/epoxy composites. XRD studies of composites revealed that there was no change in d-spacing of graphite platelets. This indicates that the graphite platelets were in order and multilayer, and the processing technique used in this study was unable to affect the order in the structure or exfoliate the graphite platelets in the epoxy matrix. However, the addition of graphite platelets increased both elastic modulus and tensile strength of the composites. The DMA also showed increased storage modulus and Tg with increased graphite concentration. The CTE of pure epoxy (60!10K6/8C) decreased to almost half of its original value with the addition of 2.5 wt% of graphite platelets (34–41! 10K6/8C). The composites also showed higher thermal stability than pure epoxy. Finally, the fractographic analysis of graphite platelet/epoxy composites showed failure mechanisms consisting of crack tip pinning, debonding and void coalescence, which were absent in the case of pure epoxy. It can be suggested that the 2.5 wt% graphite platelet/epoxy showed the best balance of mechanical (modulus, strength), viscoelastic (storage modulus, Tg) and thermal (CTE, thermal stability, decomposition temperature) properties due to the homogeneous dispersion of stiff and rigid graphite platelets. The influence of expanded or exfoliated graphite nanoplatelets of less than 50 nm thick on the mechanical and thermal properties of pure epoxy is now under investigation and will be communicated later.
Acknowledgements This research was sponsored by the Office of Naval Research (ONR). We are grateful to Dr Y. D. S. Rajapakse of ONR for his encouragement and cooperation. We are also thankful to Dr Peter A. Kanjorski of Cornerstone Technologies LLC, PA for supplying the graphite materials for our experiments. Thanks are also due to Dr Jyi-Jiin Luo for his help in modeling the composites.
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