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γ-aluminum oxide nanocomposites
Composites: Part A 40 (2009) 463–468
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Preparation and characterization of epoxy/c-aluminum oxide nanocomposites Cheng-Ho Chen a,b,*, Jian-Yuan Jian a, Fu-Su Yen c a b c
Department of Chemical and Materials Engineering, Southern Taiwan University, No. 1 Nan-Tai Street, Yong-Kang, Tainan 710, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan
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Article history: Received 28 August 2008 Received in revised form 8 January 2009 Accepted 17 January 2009
Keywords: A. Thermosetting resin Particle-reinforcement B. Thermal properties B. Mechanical properties B. Strength
a b s t r a c t Epoxy/c-Al2O3 nanocomposites were prepared with a homogenizer and followed by a stepwise thermal curing process in this study. The dispersion of c-Al2O3 nanoparticles was examined with a transmission electron microscopy (TEM). Meanwhile, the effects of c-Al2O3 nanoparticles on thermal, dynamic mechanical and tensile properties of epoxy/c-Al2O3 nanocomposites were also investigated and discussed. When the c-Al2O3 content was increased from 1phr to 5phr, results revealed that c-Al2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. Meanwhile, the maximum properties of glass transition temperature (Tg), Td5%, storage modulus, tensile modulus, and elongation at break were observed in the epoxy/5phr c-Al2O3 nanocomposite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Conventional polymer microcomposites generally contain with large amounts of inorganic fillers (more than 20 wt%) in order to achieve enhanced properties of stiffness and glass transition temperature (Tg). The fabrication of cheaper microcomposites is feasibly made by replacing the expensive resins with low cost inorganic fillers. However, these gains are usually offset by losses in ductility and toughness. Large amount of filler additions would also detrimentally affect the processability of the polymers and then increase the wear rate of processing facilities. In contrast, nanoparticles such as calcium carbonate and silica filled polymers possess significant improvements in both rigidity and toughness of the resulted composites. Therefore, polymer nanocomposites have been paid great attention because these materials often exhibit much better properties than polymers or polymers filled with micrometer-size inorganic fillers. The degree of dispersion and the type of nanoscale fillers are two key factors to manipulate the properties of nanocomposites. By only adding 2–5 wt% nanoscale fillers, the enormous enhancement in properties of a polymer nanocomposite can be observed. Furthermore, the weight of finished polymer nanocomposites can be obviously reduced by comparing with those of the conventional polymer microcomposites [1]. Various nanoscale fillers have been studied in order to improve
* Corresponding author. Address: Department of Chemical and Materials Engineering, Southern Taiwan University, No. 1 Nan-Tai Street, Yong-Kang, Tainan 710, Taiwan. Tel.: +886 6 3010001; fax: +886 6 2425741. E-mail address: [email protected] (C.-H. Chen). 1359-835X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2009.01.010
the mechanical and thermal properties of polymers, such as clay [2], silica [3], calcium carbonate (CaCO3) [4–6], and aluminum oxide (Al2O3) [7]. The improved properties include toughness, stiffness, damping, chemical resistance, heat resistance, thermal conductivity, coefficient of thermal expansion, and electrical properties. Epoxy resins have been widely used in many industrial applications, such as adhesives, construction materials, composites, laminates, and coatings due to their excellent mechanical properties, low cost, ease of processing, fine adhesion to many substrates, and good chemical resistance. However, the inherently brittle nature of epoxy has been an important issue and largely limited its application in some areas. Toughening epoxy with organic additives, such as rubber, has been proved to be an effective and widely used method [8,9]. However, the disadvantage of this approach is that it may sacrifice the stiffness of epoxy significantly. In addition, the incorporation of rubber would decrease the Tg of epoxy, which determines the upper limit of application temperature for epoxy resins. In order to increase the Tg and improve the mechanical properties of epoxy resin, epoxy nanocomposites have attracted considerable attention from both fundamental research and application points of view over the past 10 years [10–30]. Nanocomposite means that at least one solid phase dimension in the composite is in the nanometer scale. The inclusion of nanorigid particles with relatively huge surface area in an epoxy matrix leads to increase the particle–matrix interactions. This effort resulted in significant improvements in mechanical properties [10–17] and flame retardency of the composite [18]. Generally
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speaking, there are three types of nanofillers in terms of their shapes. These are spherical filler (three-dimension), sheet-like filler (two-dimension), and rod-like filler (one-dimension). In recent years, many researchers focused on the use of sheet-like nanoclays as fillers to modify epoxy [10–25]; however, only a few reports were related to the studies of spherical nanoparticles [30,31]. Incorporation of rigid spherical nanoparticles into epoxy brings less perturbing effects and this task does not decrease the crosslink density of epoxy significantly. Thus, using spherical fillers may be a promising approach to toughen epoxy without sacrificing the resulted product’s glass transition temperature and stiffness. Furthermore, investigation in nanocomposites with spherical nanoparticles will bring new insights in the study of polymer nanocomposites. Aluminum oxide (Al2O3) has good physical properties, such as abrasion resistance, corrosion resistance, thermal stability, electrical insulation, and high mechanical strength, etc. Therefore, in order to increase Tg and enhance mechanical properties of the epoxy resin, c-Al2O3 nanoparticles were added into the epoxy resin in this study. Meanwhile, in order to increase the toughness of epoxy resin, the rubber (carboxylterminated butadiene acrylonitrile (CTBN)) was used. The effects of various c-Al2O3 contents on the properties of epoxy/c-Al2O3 nanocomposites were examined and discussed. In addition, the dispersion of c-Al2O3 nanoparticle was examined with a transmission electron microscopy (TEM). Furthermore, the effects of c-Al2O3 nanoparticle on thermal, dynamic mechanical and tensile properties of epoxy/c-Al2O3 nanocomposites were also investigated.
at 80 °C for 24 h and weighed again. The curing level was determined by the following equation:
Curing level ð%Þ ¼
AB 100 A
where A represents the weight of neat epoxy or epoxy/c-Al2O3 nanocomposite before immersed in MEK and B represents the weight of neat epoxy or epoxy/c-Al2O3 nanocomposites after immersed in MEK for 24 h and dried in an oven at 80 °C for 24 h. 2.4. TEM examination Ultrathin sections (about 80 nm) were cut from rectangular blocks of the epoxy and epoxy/c-Al2O3 nanocomposites by an ultramicrotome (RMC MT-X, RMC Instruments Corp., USA). The c-Al2O3 nanoparticle and its dispersion in the nanocomposites were directly observed by using a TEM (JEM-1230, JEOL Ltd., Japan) with an acceleration voltage of 100 kV. The filament type of TEM is LaB6. 2.5. TGA analysis A thermogravimetric analyzer (TGA; Perkin–Elmer, model: TGA 7, USA) was conducted to analyze the thermal degradation characteristics of the epoxy and epoxy/c-Al2O3 nanocomposites in the temperature ranged from 50 to 800 °C with a heating rate of 10 °C/min under a nitrogen stream. The thermal degradation onset temperature and the thermal degradation weight loss of the samples were recorded and analyzed, respectively.
2. Experimental 2.6. DMA analysis 2.1. Materials The epoxy used in this study was a widely used bifunctional epoxy resin, diglycidyl ether of bisphenol-A (DGEBA) with epoxy equivalent weight of 450–500 g/eq. The curing agent used in this study was 4,4-diaminodiphenyl sulfone (DDS). The rubber used in this study was CTBN with molecular weight of 3150 g/mol. DGEBA, DDS, and CTBN were all kindly provided by the Microcosm Technology Co., Ltd., Taiwan. Methyl ethyl ketone (MEK) was purchased from the Kanto Chemical Co., Inc, Japan. The c-Al2O3 nanoparticles were purchased from the Degussa Co., Germany. 2.2. Preparation of epoxy/c-Al2O3 nanocomposites DGEBA and CTBN were dissolved in 100 ml of MEK and then various c-Al2O3 contents were added into the DGEBA/CTBN solutions to form mixtures. In order to let the c-Al2O3 nanoparticle well dispersed in the solution, the rotor speed and stirring time were set as 5000 rpm and 15 min, respectively. Then, curing agent (DDS) was added into the mixture while stirring for additional 15 min in order to ensure that DDS was fully dissolved in the solution. The standard curing cycle for the resulted nanocomposite was indicated as following: heating to 45 °C and then holding for 1 h, a subsequent heating to 80 °C and then holding for 5 h, a subsequent heating to 140 °C and then holding for 6 h, a subsequent heating to 170 °C and then holding for 5 h, and then cooling the resulted material down to the ambient temperature. In order to remove residual MEK, all nanocomposites were vacuum dried at 50 °C for 24 h. Finally, the epoxy/c-Al2O3 nanocomposites were obtained. 2.3. Curing level analysis The weighed neat epoxy or epoxy/c-Al2O3 nanocomposite was immersed in MEK and stirred for 24 h. After 24 h, the undissolved neat epoxy or epoxy/c-Al2O3 nanocomposite was dried in an oven
The samples were cut into rectangular bars with dimensions of 6.0 mm 3.0 mm 0.7 mm. A dynamic mechanical analyzer (DMA; Perkin–Elmer, model: DMA 7e, USA) was conducted to measure the thermal dynamic mechanical properties of the neat epoxy and epoxy/c-Al2O3 nanocomposites. The sample was two-stage heated. First, the sample was heated from 0 to 100 °C with a heating rate 5 °C/min and then cooled to 0 °C in order to release the internal stress of the sample. Second, the sample was heated from 0 to 160 °C with operation conditions of heating rate 5 °C/min, amplitude 10 lm, and tension 105%. The storage modulus and tan d curves were recorded and analyzed. The peak position of tan d curve was regarded as the Tg of the sample. To ensure a consistency in the results, three samples from each specimen were taken for test. 2.7. Mechanical properties analysis Tensile strength and elongation at break were examined with a universal testing machine (Model: Shimadzu 10KN; AG-IS, Shimadzu Instruments Co., Japan) at a crosshead speed of 10 mm/min. Test specimens were rectangular bars with dimensions of 10.0 mm 50.0 mm 0.7 mm. Six different specimens were sampled from neat epoxy or each nanocomposite for measurement. The six testing results were averaged and then reported. 3. Results and discussion Fig. 1 showed the influence of various c-Al2O3 contents on the curing level of epoxy in epoxy/c-Al2O3 nanocomposites. For neat epoxy, the result indicated that the curing level was up to 99.99% after the curing process. As the amounts of c-Al2O3 nanoparticle were increased to 1phr, 3phr, and 5phr, the curing levels of epoxy in epoxy/c-Al2O3 nanocomposites were 99.9%, 99.8%, and 98.2%,
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100
Curing level (%)
90 80 70 60 50 40 0
2
4
6
8
γ-Al2 O3 content (phr) Fig. 1. Effect of the c-Al2O3 content on the curing level of epoxy/c-Al2O3 nanocomposites.
respectively. These values were close to the reported curing level of neat epoxy. However, as the amounts of c-Al2O3 nanoparticles were increased up to 7phr and 9phr, the curing levels of epoxy in the nanocomposites were decreased down to 93.1% and 92.4%, respectively. Thus, this observation revealed that adding too many c-Al2O3 nanoparticles during the formation of the nanocomposite may block the curing reaction between DGEBA and DDS. Therefore, the curing reaction cannot be executed completely [32]. Fig. 2 showed the morphology of spherical c-Al2O3 nanoparticles which were examined with TEM. It showed that the spherical c-Al2O3 nanoparticles strongly intended to aggregate together due to their huge specific surface area and high surface energy. It also revealed that the size of the c-Al2O3 nanoparticle was ranged from 10 to 30 nm and the size of c-Al2O3 clusters was ranged from 100 to 500 nm. Fig. 3(a) was the TEM images of neat epoxy. The dispersion of c-Al2O3 nanoparticles in epoxy was also examined through TEM. Fig. 3(b)–(d) showed the TEM images of epoxy/1phr c-Al2O3, epoxy/5phr c-Al2O3, and epoxy/9phr c-Al2O3, respectively.
Fig. 2. TEM image of c-Al2O3 nanoparticles.
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Results revealed that the aggregation phenomenon of c-Al2O3 nanoparticles was significantly increased as the amount of cAl2O3 nanoparticles was increased. For epoxy/1phr c-Al2O3 and epoxy/5phr c-Al2O3 nanocomposites, the aggregation phenomenon of c-Al2O3 nanoparticles was not significant. Moreover, some c-Al2O3 nanoparticles can be well dispersed in the epoxy matrix. This observation was due to the high stirring speed, i.e. 5000 rpm, resulting in high shear force during mixing process. Thus, the agglomerates can be destroyed or reduced in sizes. This phenomenon is illustrated in Scheme 1. For epoxy/9phr c-Al2O3 nanocomposite, the aggregation phenomenon of c-Al2O3 nanoparticles was much more significant than those of epoxy/1phr c-Al2O3 and epoxy/5phr c-Al2O3 nanocomposites. As results indicated in Fig. 3, the size of incompact c-Al2O3 clusters was approximately ranged from 100 to 600 nm. The aggregation of c-Al2O3 nanoparticles might result from their huge specific surface area and high surface energy. Fig. 4 showed TGA curves of neat epoxy, epoxy/5phr c-Al2O3, and epoxy/9phr c-Al2O3 nanocomposites. Fig. 5 illustrated the influence of c-Al2O3 contents on the Td5% of the epoxy/c-Al2O3 nanocomposites. It was found that the Td5% of neat epoxy was 379 °C. Further, increasing the amount of c-Al2O3 nanoparticles up to 5phr in the epoxy resin, the Td5% of the nanocomposite could be increased up to 392 °C, which was about 13 °C higher than that of neat epoxy. These phenomena were resulted from the addition of c-Al2O3 nanoparticles which might act as the thermal stabilizer in the epoxy nanocomposite. However, when adding c-Al2O3 nanoparticles up to 7phr and 9phr into the epoxy matrix, the curing levels of epoxy nanocomposites were decreased to 93.1% and 92.4%, respectively. Thus, the Td5% of epoxy/7phr c-Al2O3 and epoxy/ 9phr c-Al2O3 nanocomposites was dramatically down to 215 and 201 °C, respectively. DMA was employed to evaluate the effects of c-Al2O3 nanoparticles on dynamic response of epoxy under a given set of conditions. The applied displacement and the resulting load were measured in both amplitude and relative phase, which gave both an elastic storage and a viscous loss modulus. Storage modulus directly associated with elastic response of epoxy nanocomposites. Fig. 6 showed the storage modulus curves of neat epoxy and epoxy/c-Al2O3 nanocomposites. For epoxy/5phr c-Al2O3 nanocomposite, the storage modulus of epoxy nanocomposite was significantly improved at the temperature ranged from 0 to 80 °C as compared with those of the neat epoxy and other epoxy/c-Al2O3 nanocomposites. Fig. 7 showed the storage modulus and tan d curves of the epoxy/5phr c-Al2O3 nanocomposite. The peak position of tan d curve can be regarded as the Tg of epoxy/5phr c-Al2O3 nanocomposite. Fig. 8 illustrated the effect of various c-Al2O3 amounts on the Tg of the epoxy/c-Al2O3 nanocomposites. For neat epoxy, the Tg was about 83 °C. A maximum Tg was observed at 5phr c-Al2O3 content. The Tg of the epoxy/5phr c-Al2O3 nanocomposite was 96 °C, which was about 13 °C higher than that of neat epoxy. Result indicated that the Tg of the epoxy nanocomposite can be increased by increasing the amount of c-Al2O3 nanoparticles in the resulted product. This observation was due to the increasing inorganic filler resulted in decreasing the free spaces among molecules and further increasing the difficulty for epoxy molecules to either rotate or move. However, when the c-Al2O3 content was increased up to 9phr, Tg of the resulted product was decreased down to 80 °C. This phenomenon might be resulted from uncompleted curing reaction between DGEBA and DDS since too many c-Al2O3 nanoparticles were added into the epoxy resin during the formation of the nanocomposite [33]. It can be concluded that the Tg of epoxy/c-Al2O3 nanocomposite was affected by two factors. The first factor was the curing level of epoxy. The second factor was the c-Al2O3 content. For epoxy/5phr
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Fig. 3. TEM images of epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/1phr c-Al2O3, (c) epoxy/5phr c-Al2O3, and (d) epoxy/9phr c-Al2O3.
100
b
a
Shear force
c
Scheme 1. Break up of large agglomerate into each individual or finer ones due to the shear force.
c-Al2O3 nanocomposite, the curing level was 98.2% which was close to that of neat epoxy. Therefore, its Tg was higher than that of neat epoxy. For epoxy/9phr c-Al2O3 nanocomposite, the curing level was 92.4%. Therefore, its Tg was slightly lower than that of neat epoxy, although its c-Al2O3 content was 9phr. Fig. 9 revealed the effects of amount of c-Al2O3 nanoparticles on storage moduli of epoxy/c-Al2O3 nanocomposites at various temperatures, i.e. 0, 20, 40, 60, and 80 °C. Fig. 9 was plotted for epoxy/c-Al2O3 nanocomposites with various c-Al2O3 contents as indicated on Fig. 6. From data shown on Fig. 9, a maximum storage modulus appeared at 5phr c-Al2O3 content. At 0, 20, 40, and 60 °C, the storage modulus of neat epoxy was the lowest as compared with those of other samples with c-Al2O3 nanoparticles. However, at 80 °C, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was the lowest as compared with those of other samples with c-Al2O3 nanoparticles. From the observed results, it can be concluded that there were two major factors to affect the storage mod-
Weight %
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Temperature (°C) Fig. 4. TGA curves of the epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/ 5phr c-Al2O3, and (c) epoxy/9phr c-Al2O3.
uli of the neat epoxy and epoxy/c-Al2O3 nanocomposites. The first factor was c-Al2O3 content in epoxy/c-Al2O3 nanocomposites. The second factor was Tg of epoxy/c-Al2O3 nanocomposites. At lower temperature, i.e. 0, 20, 40, and 60 °C, which were lower than the Tg s of neat epoxy and epoxy/c-Al2O3 nanocomposites, both cAl2O3 content and Tg can simultaneously affect the storage moduli
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Fig. 8. Effect of various c-Al2O3 contents on Tg of epoxy/c-Al2O3 nanocomposites.
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Fig. 6. DMA curves of the epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/1phr c-Al2O3, (c) epoxy/3phr c-Al2O3, (d) epoxy/5phr c-Al2O3, (e) epoxy/ 7phrc-Al2O3, and (f) epoxy/9phr c-Al2O3.
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Temperature (°C) Fig. 7. Effect of temperature on storage modulus and tan d of the epoxy/5phr c-Al2O3 nanocomposite.
Fig. 9. Effects of various c-Al2O3 contents on storage moduli of epoxy/c-Al2O3 nanocomposites at various temperatures: (a) 0 °C, (b) 20 °C, (c) 40 °C, (d) 60 °C, and (e) 80 °C.
of the epoxy/c-Al2O3 nanocomposites. Therefore, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was higher than those of neat epoxy, epoxy/1phr c-Al2O3, and epoxy/3phr c-Al2O3 nanocomposites, although the Tg of epoxy/9phr c-Al2O3 nanocomposite was lower than those of neat epoxy, epoxy/1phr c-Al2O3, and epoxy/3phr c-Al2O3 nanocomposites. At higher temperature, i.e. 80 °C, which were close to the Tg of neat epoxy or epoxy/c-Al2O3 nanocomposites, the second factor dominated the storage moduli of the epoxy/c-Al2O3 nanocomposites. Therefore, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was the lowest as compared with neat epoxy and other epoxy/c-Al2O3 nanocomposites. It can be concluded that the influence of Tg on the storage modulus of epoxy/c-Al2O3 nanocomposite was more important than that of c-Al2O3 content at the temperature close to Tg. Fig. 10 illustrated the effects of the c-Al2O3 content on tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites. It was observed that both tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites were increased as the cAl2O3 contents were increased from 0phr to 5phr in the epoxy nanocomposites. At 5phr c-Al2O3 content, the tensile modulus and elongation at break of epoxy nanocomposite can be increased
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found that both tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites were increased with the c-Al2O3 content increased from 0phr to 5phr. It also revealed that cAl2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. Among various contents of cAl2O3 in the nanocomposites, epoxy/5phr c-Al2O3 nanocomposite had the maximum Tg, Td5%, storage modulus, tensile modulus, and elongation at break. When the c-Al2O3 content in the nanocomposite was increased up to 7phr or 9phr, not only the coagulation phenomena of c-Al2O3 nanoparticles were significantly observed, but also resulted in decreasing of thermal stability of the epoxy nanocomposites.
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up to about 32% and 39%, respectively. It revealed that c-Al2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. The enhanced toughness was attributed to the c-Al2O3 nanoparticles which can induce cavitation and shear yielding of the polymer resins during mechanical deformation [34]. The presence of inorganic nanoparticles with large surface areas would alter the local stress state of surrounding matrix. Thus, the polymer chain dynamics in the vicinity of particles were substantially different from that observed in the bulk due to specific polymer–nanoparticle interaction. It was well established that the addition of lower modulus elastomer particles into polymer materials leaded to enhancement of both ductility and toughness at the expenses of tensile strength and stiffness. It is believed that cavitation of elastomer particles or debonding at the interface between particles and matrix is responsible for such an improvement. The cavities relieved the triaxial stress state presented in the matrix, inhibited bulk polymer void formation and subsequent craze, and promoted shear yielding of the polymer matrix. Rigid c-Al2O3 nanoparticles were considered to be more effective than elastomers as they increased both stiffness and toughness of the polymers. It was considered that cavitation of nanoparticles and its successive shear yielding contributed to an improvement in the toughness of nanocomposites [35,36]. 4. Conclusions In this study, the epoxy/c-Al2O3 nanocomposites were prepared by a solution blending process with a homogenizer and followed by a step thermal curing process. Results revealed that adding too many c-Al2O3 nanoparticles (i.e. 7phr and 9phr) could block the curing reaction between DGEBA and DDS. Therefore, the curing reaction cannot be executed completely. From TEM results, the aggregation phenomena of c-Al2O3 nanoparticles became more significant as the c-Al2O3 content in the nanocomposites was increased. The results of TEM revealed that the sizes of incompact c-Al2O3 clusters were approximately ranged from 100 to 600 nm for epoxy/9phr c-Al2O3 nanocomposite. It was
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Preparation and characterization of epoxy/c-aluminum oxide nanocomposites Cheng-Ho Chen a,b,*, Jian-Yuan Jian a, Fu-Su Yen c a b c
Department of Chemical and Materials Engineering, Southern Taiwan University, No. 1 Nan-Tai Street, Yong-Kang, Tainan 710, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 28 August 2008 Received in revised form 8 January 2009 Accepted 17 January 2009
Keywords: A. Thermosetting resin Particle-reinforcement B. Thermal properties B. Mechanical properties B. Strength
a b s t r a c t Epoxy/c-Al2O3 nanocomposites were prepared with a homogenizer and followed by a stepwise thermal curing process in this study. The dispersion of c-Al2O3 nanoparticles was examined with a transmission electron microscopy (TEM). Meanwhile, the effects of c-Al2O3 nanoparticles on thermal, dynamic mechanical and tensile properties of epoxy/c-Al2O3 nanocomposites were also investigated and discussed. When the c-Al2O3 content was increased from 1phr to 5phr, results revealed that c-Al2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. Meanwhile, the maximum properties of glass transition temperature (Tg), Td5%, storage modulus, tensile modulus, and elongation at break were observed in the epoxy/5phr c-Al2O3 nanocomposite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Conventional polymer microcomposites generally contain with large amounts of inorganic fillers (more than 20 wt%) in order to achieve enhanced properties of stiffness and glass transition temperature (Tg). The fabrication of cheaper microcomposites is feasibly made by replacing the expensive resins with low cost inorganic fillers. However, these gains are usually offset by losses in ductility and toughness. Large amount of filler additions would also detrimentally affect the processability of the polymers and then increase the wear rate of processing facilities. In contrast, nanoparticles such as calcium carbonate and silica filled polymers possess significant improvements in both rigidity and toughness of the resulted composites. Therefore, polymer nanocomposites have been paid great attention because these materials often exhibit much better properties than polymers or polymers filled with micrometer-size inorganic fillers. The degree of dispersion and the type of nanoscale fillers are two key factors to manipulate the properties of nanocomposites. By only adding 2–5 wt% nanoscale fillers, the enormous enhancement in properties of a polymer nanocomposite can be observed. Furthermore, the weight of finished polymer nanocomposites can be obviously reduced by comparing with those of the conventional polymer microcomposites [1]. Various nanoscale fillers have been studied in order to improve
* Corresponding author. Address: Department of Chemical and Materials Engineering, Southern Taiwan University, No. 1 Nan-Tai Street, Yong-Kang, Tainan 710, Taiwan. Tel.: +886 6 3010001; fax: +886 6 2425741. E-mail address: [email protected] (C.-H. Chen). 1359-835X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2009.01.010
the mechanical and thermal properties of polymers, such as clay [2], silica [3], calcium carbonate (CaCO3) [4–6], and aluminum oxide (Al2O3) [7]. The improved properties include toughness, stiffness, damping, chemical resistance, heat resistance, thermal conductivity, coefficient of thermal expansion, and electrical properties. Epoxy resins have been widely used in many industrial applications, such as adhesives, construction materials, composites, laminates, and coatings due to their excellent mechanical properties, low cost, ease of processing, fine adhesion to many substrates, and good chemical resistance. However, the inherently brittle nature of epoxy has been an important issue and largely limited its application in some areas. Toughening epoxy with organic additives, such as rubber, has been proved to be an effective and widely used method [8,9]. However, the disadvantage of this approach is that it may sacrifice the stiffness of epoxy significantly. In addition, the incorporation of rubber would decrease the Tg of epoxy, which determines the upper limit of application temperature for epoxy resins. In order to increase the Tg and improve the mechanical properties of epoxy resin, epoxy nanocomposites have attracted considerable attention from both fundamental research and application points of view over the past 10 years [10–30]. Nanocomposite means that at least one solid phase dimension in the composite is in the nanometer scale. The inclusion of nanorigid particles with relatively huge surface area in an epoxy matrix leads to increase the particle–matrix interactions. This effort resulted in significant improvements in mechanical properties [10–17] and flame retardency of the composite [18]. Generally
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speaking, there are three types of nanofillers in terms of their shapes. These are spherical filler (three-dimension), sheet-like filler (two-dimension), and rod-like filler (one-dimension). In recent years, many researchers focused on the use of sheet-like nanoclays as fillers to modify epoxy [10–25]; however, only a few reports were related to the studies of spherical nanoparticles [30,31]. Incorporation of rigid spherical nanoparticles into epoxy brings less perturbing effects and this task does not decrease the crosslink density of epoxy significantly. Thus, using spherical fillers may be a promising approach to toughen epoxy without sacrificing the resulted product’s glass transition temperature and stiffness. Furthermore, investigation in nanocomposites with spherical nanoparticles will bring new insights in the study of polymer nanocomposites. Aluminum oxide (Al2O3) has good physical properties, such as abrasion resistance, corrosion resistance, thermal stability, electrical insulation, and high mechanical strength, etc. Therefore, in order to increase Tg and enhance mechanical properties of the epoxy resin, c-Al2O3 nanoparticles were added into the epoxy resin in this study. Meanwhile, in order to increase the toughness of epoxy resin, the rubber (carboxylterminated butadiene acrylonitrile (CTBN)) was used. The effects of various c-Al2O3 contents on the properties of epoxy/c-Al2O3 nanocomposites were examined and discussed. In addition, the dispersion of c-Al2O3 nanoparticle was examined with a transmission electron microscopy (TEM). Furthermore, the effects of c-Al2O3 nanoparticle on thermal, dynamic mechanical and tensile properties of epoxy/c-Al2O3 nanocomposites were also investigated.
at 80 °C for 24 h and weighed again. The curing level was determined by the following equation:
Curing level ð%Þ ¼
AB 100 A
where A represents the weight of neat epoxy or epoxy/c-Al2O3 nanocomposite before immersed in MEK and B represents the weight of neat epoxy or epoxy/c-Al2O3 nanocomposites after immersed in MEK for 24 h and dried in an oven at 80 °C for 24 h. 2.4. TEM examination Ultrathin sections (about 80 nm) were cut from rectangular blocks of the epoxy and epoxy/c-Al2O3 nanocomposites by an ultramicrotome (RMC MT-X, RMC Instruments Corp., USA). The c-Al2O3 nanoparticle and its dispersion in the nanocomposites were directly observed by using a TEM (JEM-1230, JEOL Ltd., Japan) with an acceleration voltage of 100 kV. The filament type of TEM is LaB6. 2.5. TGA analysis A thermogravimetric analyzer (TGA; Perkin–Elmer, model: TGA 7, USA) was conducted to analyze the thermal degradation characteristics of the epoxy and epoxy/c-Al2O3 nanocomposites in the temperature ranged from 50 to 800 °C with a heating rate of 10 °C/min under a nitrogen stream. The thermal degradation onset temperature and the thermal degradation weight loss of the samples were recorded and analyzed, respectively.
2. Experimental 2.6. DMA analysis 2.1. Materials The epoxy used in this study was a widely used bifunctional epoxy resin, diglycidyl ether of bisphenol-A (DGEBA) with epoxy equivalent weight of 450–500 g/eq. The curing agent used in this study was 4,4-diaminodiphenyl sulfone (DDS). The rubber used in this study was CTBN with molecular weight of 3150 g/mol. DGEBA, DDS, and CTBN were all kindly provided by the Microcosm Technology Co., Ltd., Taiwan. Methyl ethyl ketone (MEK) was purchased from the Kanto Chemical Co., Inc, Japan. The c-Al2O3 nanoparticles were purchased from the Degussa Co., Germany. 2.2. Preparation of epoxy/c-Al2O3 nanocomposites DGEBA and CTBN were dissolved in 100 ml of MEK and then various c-Al2O3 contents were added into the DGEBA/CTBN solutions to form mixtures. In order to let the c-Al2O3 nanoparticle well dispersed in the solution, the rotor speed and stirring time were set as 5000 rpm and 15 min, respectively. Then, curing agent (DDS) was added into the mixture while stirring for additional 15 min in order to ensure that DDS was fully dissolved in the solution. The standard curing cycle for the resulted nanocomposite was indicated as following: heating to 45 °C and then holding for 1 h, a subsequent heating to 80 °C and then holding for 5 h, a subsequent heating to 140 °C and then holding for 6 h, a subsequent heating to 170 °C and then holding for 5 h, and then cooling the resulted material down to the ambient temperature. In order to remove residual MEK, all nanocomposites were vacuum dried at 50 °C for 24 h. Finally, the epoxy/c-Al2O3 nanocomposites were obtained. 2.3. Curing level analysis The weighed neat epoxy or epoxy/c-Al2O3 nanocomposite was immersed in MEK and stirred for 24 h. After 24 h, the undissolved neat epoxy or epoxy/c-Al2O3 nanocomposite was dried in an oven
The samples were cut into rectangular bars with dimensions of 6.0 mm 3.0 mm 0.7 mm. A dynamic mechanical analyzer (DMA; Perkin–Elmer, model: DMA 7e, USA) was conducted to measure the thermal dynamic mechanical properties of the neat epoxy and epoxy/c-Al2O3 nanocomposites. The sample was two-stage heated. First, the sample was heated from 0 to 100 °C with a heating rate 5 °C/min and then cooled to 0 °C in order to release the internal stress of the sample. Second, the sample was heated from 0 to 160 °C with operation conditions of heating rate 5 °C/min, amplitude 10 lm, and tension 105%. The storage modulus and tan d curves were recorded and analyzed. The peak position of tan d curve was regarded as the Tg of the sample. To ensure a consistency in the results, three samples from each specimen were taken for test. 2.7. Mechanical properties analysis Tensile strength and elongation at break were examined with a universal testing machine (Model: Shimadzu 10KN; AG-IS, Shimadzu Instruments Co., Japan) at a crosshead speed of 10 mm/min. Test specimens were rectangular bars with dimensions of 10.0 mm 50.0 mm 0.7 mm. Six different specimens were sampled from neat epoxy or each nanocomposite for measurement. The six testing results were averaged and then reported. 3. Results and discussion Fig. 1 showed the influence of various c-Al2O3 contents on the curing level of epoxy in epoxy/c-Al2O3 nanocomposites. For neat epoxy, the result indicated that the curing level was up to 99.99% after the curing process. As the amounts of c-Al2O3 nanoparticle were increased to 1phr, 3phr, and 5phr, the curing levels of epoxy in epoxy/c-Al2O3 nanocomposites were 99.9%, 99.8%, and 98.2%,
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100
Curing level (%)
90 80 70 60 50 40 0
2
4
6
8
γ-Al2 O3 content (phr) Fig. 1. Effect of the c-Al2O3 content on the curing level of epoxy/c-Al2O3 nanocomposites.
respectively. These values were close to the reported curing level of neat epoxy. However, as the amounts of c-Al2O3 nanoparticles were increased up to 7phr and 9phr, the curing levels of epoxy in the nanocomposites were decreased down to 93.1% and 92.4%, respectively. Thus, this observation revealed that adding too many c-Al2O3 nanoparticles during the formation of the nanocomposite may block the curing reaction between DGEBA and DDS. Therefore, the curing reaction cannot be executed completely [32]. Fig. 2 showed the morphology of spherical c-Al2O3 nanoparticles which were examined with TEM. It showed that the spherical c-Al2O3 nanoparticles strongly intended to aggregate together due to their huge specific surface area and high surface energy. It also revealed that the size of the c-Al2O3 nanoparticle was ranged from 10 to 30 nm and the size of c-Al2O3 clusters was ranged from 100 to 500 nm. Fig. 3(a) was the TEM images of neat epoxy. The dispersion of c-Al2O3 nanoparticles in epoxy was also examined through TEM. Fig. 3(b)–(d) showed the TEM images of epoxy/1phr c-Al2O3, epoxy/5phr c-Al2O3, and epoxy/9phr c-Al2O3, respectively.
Fig. 2. TEM image of c-Al2O3 nanoparticles.
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Results revealed that the aggregation phenomenon of c-Al2O3 nanoparticles was significantly increased as the amount of cAl2O3 nanoparticles was increased. For epoxy/1phr c-Al2O3 and epoxy/5phr c-Al2O3 nanocomposites, the aggregation phenomenon of c-Al2O3 nanoparticles was not significant. Moreover, some c-Al2O3 nanoparticles can be well dispersed in the epoxy matrix. This observation was due to the high stirring speed, i.e. 5000 rpm, resulting in high shear force during mixing process. Thus, the agglomerates can be destroyed or reduced in sizes. This phenomenon is illustrated in Scheme 1. For epoxy/9phr c-Al2O3 nanocomposite, the aggregation phenomenon of c-Al2O3 nanoparticles was much more significant than those of epoxy/1phr c-Al2O3 and epoxy/5phr c-Al2O3 nanocomposites. As results indicated in Fig. 3, the size of incompact c-Al2O3 clusters was approximately ranged from 100 to 600 nm. The aggregation of c-Al2O3 nanoparticles might result from their huge specific surface area and high surface energy. Fig. 4 showed TGA curves of neat epoxy, epoxy/5phr c-Al2O3, and epoxy/9phr c-Al2O3 nanocomposites. Fig. 5 illustrated the influence of c-Al2O3 contents on the Td5% of the epoxy/c-Al2O3 nanocomposites. It was found that the Td5% of neat epoxy was 379 °C. Further, increasing the amount of c-Al2O3 nanoparticles up to 5phr in the epoxy resin, the Td5% of the nanocomposite could be increased up to 392 °C, which was about 13 °C higher than that of neat epoxy. These phenomena were resulted from the addition of c-Al2O3 nanoparticles which might act as the thermal stabilizer in the epoxy nanocomposite. However, when adding c-Al2O3 nanoparticles up to 7phr and 9phr into the epoxy matrix, the curing levels of epoxy nanocomposites were decreased to 93.1% and 92.4%, respectively. Thus, the Td5% of epoxy/7phr c-Al2O3 and epoxy/ 9phr c-Al2O3 nanocomposites was dramatically down to 215 and 201 °C, respectively. DMA was employed to evaluate the effects of c-Al2O3 nanoparticles on dynamic response of epoxy under a given set of conditions. The applied displacement and the resulting load were measured in both amplitude and relative phase, which gave both an elastic storage and a viscous loss modulus. Storage modulus directly associated with elastic response of epoxy nanocomposites. Fig. 6 showed the storage modulus curves of neat epoxy and epoxy/c-Al2O3 nanocomposites. For epoxy/5phr c-Al2O3 nanocomposite, the storage modulus of epoxy nanocomposite was significantly improved at the temperature ranged from 0 to 80 °C as compared with those of the neat epoxy and other epoxy/c-Al2O3 nanocomposites. Fig. 7 showed the storage modulus and tan d curves of the epoxy/5phr c-Al2O3 nanocomposite. The peak position of tan d curve can be regarded as the Tg of epoxy/5phr c-Al2O3 nanocomposite. Fig. 8 illustrated the effect of various c-Al2O3 amounts on the Tg of the epoxy/c-Al2O3 nanocomposites. For neat epoxy, the Tg was about 83 °C. A maximum Tg was observed at 5phr c-Al2O3 content. The Tg of the epoxy/5phr c-Al2O3 nanocomposite was 96 °C, which was about 13 °C higher than that of neat epoxy. Result indicated that the Tg of the epoxy nanocomposite can be increased by increasing the amount of c-Al2O3 nanoparticles in the resulted product. This observation was due to the increasing inorganic filler resulted in decreasing the free spaces among molecules and further increasing the difficulty for epoxy molecules to either rotate or move. However, when the c-Al2O3 content was increased up to 9phr, Tg of the resulted product was decreased down to 80 °C. This phenomenon might be resulted from uncompleted curing reaction between DGEBA and DDS since too many c-Al2O3 nanoparticles were added into the epoxy resin during the formation of the nanocomposite [33]. It can be concluded that the Tg of epoxy/c-Al2O3 nanocomposite was affected by two factors. The first factor was the curing level of epoxy. The second factor was the c-Al2O3 content. For epoxy/5phr
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Fig. 3. TEM images of epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/1phr c-Al2O3, (c) epoxy/5phr c-Al2O3, and (d) epoxy/9phr c-Al2O3.
100
b
a
Shear force
c
Scheme 1. Break up of large agglomerate into each individual or finer ones due to the shear force.
c-Al2O3 nanocomposite, the curing level was 98.2% which was close to that of neat epoxy. Therefore, its Tg was higher than that of neat epoxy. For epoxy/9phr c-Al2O3 nanocomposite, the curing level was 92.4%. Therefore, its Tg was slightly lower than that of neat epoxy, although its c-Al2O3 content was 9phr. Fig. 9 revealed the effects of amount of c-Al2O3 nanoparticles on storage moduli of epoxy/c-Al2O3 nanocomposites at various temperatures, i.e. 0, 20, 40, 60, and 80 °C. Fig. 9 was plotted for epoxy/c-Al2O3 nanocomposites with various c-Al2O3 contents as indicated on Fig. 6. From data shown on Fig. 9, a maximum storage modulus appeared at 5phr c-Al2O3 content. At 0, 20, 40, and 60 °C, the storage modulus of neat epoxy was the lowest as compared with those of other samples with c-Al2O3 nanoparticles. However, at 80 °C, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was the lowest as compared with those of other samples with c-Al2O3 nanoparticles. From the observed results, it can be concluded that there were two major factors to affect the storage mod-
Weight %
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Temperature (°C) Fig. 4. TGA curves of the epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/ 5phr c-Al2O3, and (c) epoxy/9phr c-Al2O3.
uli of the neat epoxy and epoxy/c-Al2O3 nanocomposites. The first factor was c-Al2O3 content in epoxy/c-Al2O3 nanocomposites. The second factor was Tg of epoxy/c-Al2O3 nanocomposites. At lower temperature, i.e. 0, 20, 40, and 60 °C, which were lower than the Tg s of neat epoxy and epoxy/c-Al2O3 nanocomposites, both cAl2O3 content and Tg can simultaneously affect the storage moduli
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γ-Al2O3 content (phr) Fig. 5. Effect of the c-Al2O3 content on Td5% of epoxy/c-Al2O3 nanocomposites (Td5% stands for the temperature which the sample remains 95 wt% during the TGA test).
Fig. 8. Effect of various c-Al2O3 contents on Tg of epoxy/c-Al2O3 nanocomposites.
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Fig. 6. DMA curves of the epoxy/c-Al2O3 nanocomposites: (a) neat epoxy, (b) epoxy/1phr c-Al2O3, (c) epoxy/3phr c-Al2O3, (d) epoxy/5phr c-Al2O3, (e) epoxy/ 7phrc-Al2O3, and (f) epoxy/9phr c-Al2O3.
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Temperature (°C) Fig. 7. Effect of temperature on storage modulus and tan d of the epoxy/5phr c-Al2O3 nanocomposite.
Fig. 9. Effects of various c-Al2O3 contents on storage moduli of epoxy/c-Al2O3 nanocomposites at various temperatures: (a) 0 °C, (b) 20 °C, (c) 40 °C, (d) 60 °C, and (e) 80 °C.
of the epoxy/c-Al2O3 nanocomposites. Therefore, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was higher than those of neat epoxy, epoxy/1phr c-Al2O3, and epoxy/3phr c-Al2O3 nanocomposites, although the Tg of epoxy/9phr c-Al2O3 nanocomposite was lower than those of neat epoxy, epoxy/1phr c-Al2O3, and epoxy/3phr c-Al2O3 nanocomposites. At higher temperature, i.e. 80 °C, which were close to the Tg of neat epoxy or epoxy/c-Al2O3 nanocomposites, the second factor dominated the storage moduli of the epoxy/c-Al2O3 nanocomposites. Therefore, the storage modulus of epoxy/9phr c-Al2O3 nanocomposite was the lowest as compared with neat epoxy and other epoxy/c-Al2O3 nanocomposites. It can be concluded that the influence of Tg on the storage modulus of epoxy/c-Al2O3 nanocomposite was more important than that of c-Al2O3 content at the temperature close to Tg. Fig. 10 illustrated the effects of the c-Al2O3 content on tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites. It was observed that both tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites were increased as the cAl2O3 contents were increased from 0phr to 5phr in the epoxy nanocomposites. At 5phr c-Al2O3 content, the tensile modulus and elongation at break of epoxy nanocomposite can be increased
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found that both tensile modulus and elongation at break of epoxy/c-Al2O3 nanocomposites were increased with the c-Al2O3 content increased from 0phr to 5phr. It also revealed that cAl2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. Among various contents of cAl2O3 in the nanocomposites, epoxy/5phr c-Al2O3 nanocomposite had the maximum Tg, Td5%, storage modulus, tensile modulus, and elongation at break. When the c-Al2O3 content in the nanocomposite was increased up to 7phr or 9phr, not only the coagulation phenomena of c-Al2O3 nanoparticles were significantly observed, but also resulted in decreasing of thermal stability of the epoxy nanocomposites.
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up to about 32% and 39%, respectively. It revealed that c-Al2O3 nanoparticles were effective to enhance both the stiffness and toughness of epoxy resin. The enhanced toughness was attributed to the c-Al2O3 nanoparticles which can induce cavitation and shear yielding of the polymer resins during mechanical deformation [34]. The presence of inorganic nanoparticles with large surface areas would alter the local stress state of surrounding matrix. Thus, the polymer chain dynamics in the vicinity of particles were substantially different from that observed in the bulk due to specific polymer–nanoparticle interaction. It was well established that the addition of lower modulus elastomer particles into polymer materials leaded to enhancement of both ductility and toughness at the expenses of tensile strength and stiffness. It is believed that cavitation of elastomer particles or debonding at the interface between particles and matrix is responsible for such an improvement. The cavities relieved the triaxial stress state presented in the matrix, inhibited bulk polymer void formation and subsequent craze, and promoted shear yielding of the polymer matrix. Rigid c-Al2O3 nanoparticles were considered to be more effective than elastomers as they increased both stiffness and toughness of the polymers. It was considered that cavitation of nanoparticles and its successive shear yielding contributed to an improvement in the toughness of nanocomposites [35,36]. 4. Conclusions In this study, the epoxy/c-Al2O3 nanocomposites were prepared by a solution blending process with a homogenizer and followed by a step thermal curing process. Results revealed that adding too many c-Al2O3 nanoparticles (i.e. 7phr and 9phr) could block the curing reaction between DGEBA and DDS. Therefore, the curing reaction cannot be executed completely. From TEM results, the aggregation phenomena of c-Al2O3 nanoparticles became more significant as the c-Al2O3 content in the nanocomposites was increased. The results of TEM revealed that the sizes of incompact c-Al2O3 clusters were approximately ranged from 100 to 600 nm for epoxy/9phr c-Al2O3 nanocomposite. It was
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