Materials 2015, 8, 5526-5536; doi:10.3390/ma8085262
OPEN ACCESS
materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Thermal and Mechanical Behavior of Hybrid Polymer Nanocomposite Reinforced with Graphene Nanoplatelets Minh-Tai Le 1,2 and Shyh-Chour Huang 2, * 1
Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and Education, No.1, Vo Van Ngan, Thu Duc District, Ho Chi Minh City 70000, Vietnam; E-Mail: [email protected] 2 Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Sanmin District, Kaohsiung 80778, Taiwan * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-7-3814526 (ext. 5313); Fax: +886-7-3831373. Academic Editor: Teen-Hang Meen Received: 21 June 2015 / Accepted: 19 August 2015 / Published: 24 August 2015
Abstract: In the present investigation, we successfully fabricate a hybrid polymer nanocomposite containing epoxy/polyester blend resin and graphene nanoplatelets (GNPs) by a novel technique. A high intensity ultrasonicator is used to obtain a homogeneous mixture of epoxy/polyester resin and graphene nanoplatelets. This mixture is then mixed with a hardener using a high-speed mechanical stirrer. The trapped air and reaction volatiles are removed from the mixture using high vacuum. The hot press casting method is used to make the nanocomposite specimens. Tensile tests, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) are performed on neat, 0.2 wt %, 0.5 wt %, 1 wt %, 1.5 wt % and 2 wt % GNP-reinforced epoxy/polyester blend resin to investigate the reinforcement effect on the thermal and mechanical properties of the nanocomposites. The results of this research indicate that the tensile strength of the novel nanocomposite material increases to 86.8% with the addition of a ratio of graphene nanoplatelets as low as 0.2 wt %. DMA results indicate that the 1 wt % GNP-reinforced epoxy/polyester nanocomposite possesses the highest storage modulus and glass transition temperature (Tg ), as compared to neat epoxy/polyester or the other nanocomposite specimens. In addition, TGA results verify thethermal stability of the experimental specimens, regardless of the weight percentage of GNPs.
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Keywords: nanocomposite; graphene nanoplatelets; epoxy; unsaturated polyester; tensile strength; thermal conductivity; morphology
1. Introduction Graphene nanoplatelets (GNPs) are a type of filler in the new generation of reinforced materials. Producing high-purity and large amounts of GNPs has a highcost, and because of the issues related to their stability during the process of fabricating nanocomposite materials, researchers have recommended using them in a pristine state, i.e., not to be handled as a functional group [1]. Relating to the matrix for the construction of the composite, thermoset epoxy is the top choice because of its excellent thermomechanical properties, which contribute to the successful fabrication of advanced nanocomposite materials [2,3]. Another type of thermoset polymer used to reinforce as well as modify a material’s performance is polyester. With its high thermal and mechanical properties, it is both simpler and cheaper to use than epoxy, and polyester can be used as the second matrix phase mixed with epoxy to create a general matrix structure of composite materials [4–6]. There are studies in which GNPs were used as fillers to enhance the properties of the epoxy matrix [7,8]. The new generation of hybrid composite materials with versatile mechanical, thermal and electrical properties are of particular interest to researchers [9,10]. The thermal conductivity of epoxy nanocomposites having 0.5 and 1 wt % of silica-coated MWCNTs was found to be enhanced by 51% and 67%, respectively, by Cui et al. [11]. Viswanath et al. [12] presented a thermomechanical and electrical study of epoxy/hyperbranched polyester. Rahman et al. [13] optimized the mechanical properties of epoxy reinforced with E-glass and amino-functionalized MWCNTs. The MWCNTs were aligned in the epoxy resin to achieve the required properties by Park et al. [14]; the thermal conductivity of epoxy/MWCNT nanocomposites at room temperature (RT) was observed to be 55 W/mK; the stretched MWCNT-epoxy sheet showed a value of 100 W/mK, whereas pure epoxy was 0.11 W/mK at RT. In most previous studies, thermoset polymers did factor in the critical investigation of the performance of hybrid matrix materials made with epoxy and polyester. In this research, the added filler of GNPs was used to reinforce the tensile strength and thermal conductivity of the hybrid epoxy/polyester composite. The GNPs were mixed at different weight percentages with epoxy and polyester using high intensity ultrasonication. The resulting experimental specimens were obtained by means of the hot press casting method. The primary interest of this paper was to characterize the effect of graphene nanoplatelets on the thermal and mechanical properties of the epoxy/polyester hybrid composite. Tensile tests were performed to evaluate the mechanical performance, and thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) were carried out to evaluate the thermal properties. Additionally, the morphological characteristics of the surface structure of the nanocomposite samples in the tensile test were observed by a scanning electron microscope (SEM). The important contribution of this study is the successful fabrication of a hybrid matrix nanocomposite material of thermoset resins reinforced with the GNP filler. The nanocomposite specimens fabricated at a low cost evidenced high mechanical strength,
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good thermal conductivity and stability, making them suitable for applications in the manufacturing of good thermal and stability, making them suitable for applications in the manufacturing of components for conductivity electronic circuit boards and transportation. components for electronic circuit boards and transportation. 2. Materials and Methods 2. Materials and Methods 2.1. Material Preparation 2.1. Material Preparation The graphene nanoplatelets (GNPs) used in this study were Grade 4 (Figure 1), purchased from Cheap The graphene nanoplatelets (GNPs) used in this study were Grade 4 (Figure 1), purchased from Cheap Tubes Inc. (Cambridgeport, VT, USA), with an average plate diameter in the range of 1 to 2 µm and Tubes Inc. (Cambridgeport, VT, USA), with an average plate diameter in the range of 1 to 2 µm and over 99% pure. structureof ofthe thecomposite compositeconsisted consisted epoxy 6620, over 99% pure.The Thematerials materialsfor forcreating creating the the matrix matrix structure ofof epoxy 6620, hardener AH150 GC-0421,all allpurchased purchasedfrom from Golden Innovation hardener AH150and andunsaturated unsaturatedpolyester polyester resin resin (UPR) (UPR) GC-0421, Golden Innovation Business Co. by the the manufacturer, manufacturer,some somemodification modificationmaterials materials were Business Co.(Taibei, (Taibei,Taiwan). Taiwan). As As suggested suggested by were used: anan accelerator catalystof ofmethyl methylethyl ethylketone ketoneperoxide peroxide (MEKP). used: acceleratorofofcobalt cobaltnaphthenate naphthenate (6%) and aa catalyst (MEKP). ToTo easily was necessary necessary totouse usemethyl methylethyl ethylketone ketone (MEK) easilydisperse dispersethe theGNP GNPfiller fillerinin the the resin, resin, it was (MEK) a dissolution. as as a dissolution.
Figure 1. Scanning electron microscope image of pristine graphene nanoplatelets. Figure 1. Scanning electron microscope image of pristine graphene nanoplatelets. Figure 2a shows the whole process of the experimental specimen preparation. The predetermined Figure the whole process the experimental specimen preparation. The predetermined amounts2a of shows GNPs, epoxy and UPR wereofmixed together in a suitable beaker. The stipulated amounts of amounts of GNPs, epoxy and UPR were mixed together in a suitable beaker. The stipulated GNP and epoxy/polyester blend were mixed thoroughly with a magnetic stirrer for about 1 hamounts at a of temperature GNP and epoxy/polyester blend were mixed thoroughly with a magnetic stirrer for about 1 of 60 °C. The beaker was then placed in an ultrasonicator at a high intensity for 1½ h seth atat a a pulse mode (9 ˝sC.on/9 off). Anwas external coolinginsystem was employed submerging temperature of 60 Thes beaker then placed an ultrasonicator at abyhigh intensity the for beaker 1½ h set the mixture in sanoff). ice bath avoid the temperature the sonication process. at containing a pulse mode (9 s on/9 An to external cooling systemrising was during employed by submerging theWhen beaker the process finished, agents, including thetemperature hardener, accelerator and the catalyst, with aprocess. weight ratio containing thewas mixture in anallice bath to avoid the rising during sonication When 2:1/1/1,was were added to previous epoxy/polyester mixture. An aluminum mold with (Figure 2) of the theofprocess finished, allthe agents, including the hardener, accelerator and catalyst, a weight ratio required dimensions was used for the making of samples on par with the American Society for Testing of 2:1/1/1, were added to the previous epoxy/polyester mixture. An aluminum mold (Figure 2) of the and Materials (ASTM) Themaking mold was covered on with mold-releasing agentSociety so the for samples required dimensions was standard. used for the of samples para with the American Testing and Materials (ASTM) standard. The mold was covered with a mold-releasing agent so the samples
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could Thecompleted completedmixture mixture was poured mold. The 2-part mold couldbebeeasily easilyremoved. removed. The was poured intointo the the mold. The 2-part castingcasting mold was could be easily removed. Theofcompleted poured into the mold. The 2-part casting was was maintained under a pressure 0.8 mixture MPa h attemperature. room temperature. To complete the curing maintained under a pressure 0.8ofMPa for 24for h was at24room To complete the curingmold process maintained under a pressure ofstresses 0.8 MPa for the 24 heat, hexperimental at room temperature. To complete the curing process process and eliminate residual due to the experimental samples (Figure 3b) cured were post and eliminate residual stresses due to heat, samples (Figure 3b) were post at and eliminate residual stresses due to heat, the experimental samples (Figure 3b) were post cured at ˝ cured at for 90 3Ch.for 3 h. 90 °C 90 °C for 3 h.
Figure2.2. (a) Schematic showing Figure (a) Schematic showing the the manufacturing manufacturing process processof ofthe thenanocomposite; nanocomposite; Figure 2. (a) Schematic showing the manufacturing process of the nanocomposite; (b) dimensions of specimen in ASTM standard Type V (mm). (b)(b) dimensions dimensionsofofspecimen specimenininASTM ASTMstandard standard Type Type V V (mm). (mm).
(a) (b) (a) (b) Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. 2.2. Mechanical Test 2.2. Mechanical Test 2.2. Mechanical Test set up for the tensile test was done on an Instron 5566-CN2081 machine The experimental The experimental set up for the tensile was done on an D638 Instron 5566-CN2081 machine (Instron Company, Hsinchu, Taiwan) with atest standard of ASTM (Type V). The following The experimental set up for the tensile test was done on an Instron 5566-CN2081 machine (Instron Company, Hsinchu, Taiwan) clamp with length a standard ofmm, ASTM D638 The following parameters were set for the experiment: of 26.3 tensile speed(Type of 0.5V). mm/min and room (Instron Company, Taiwan) with alength standard of ASTM D638 (Type V). The and following parameters were setHsinchu, for experiment: of 26.3 tensile speed mm/min room temperature of 30 °C. Forthe each content ofclamp filler, the tensile testmm, was carried out onofat0.5 least five specimens; parameters were for experiment: clampthe length 26.3 tensile speed of 0.5 and temperature ofthe 30set °C. Forthe each filler, tensileoftest wasmm, carried out on at least five mm/min specimens; the result was average valuecontent of the of five measurements. ˝ room temperature 30 C.value For of each of filler, the tensile test was carried out on at least five the result was theof average thecontent five measurements. specimens; the result was the average value of the five measurements.
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2.3. Thermal Analysis 2.3. Thermal Analysis
The thermal characteristics of the GNP/epoxy/polyester-blended nanocomposites were studied using dynamic mechanical analysis (TA Instruments Co., New Castle,were PA,studied USA)using and The thermal characteristics of the(DMA) GNP/epoxy/polyester-blended nanocomposites thermogravimetric analysis (TGA) (TA Instruments Co.,Co., New Castle, PA,PA, USA). dynamic mechanical analysis (DMA) (TA Instruments New Castle, USA) and thermogravimetric analysis (TA Instruments New Castle, PA, operated USA). in the three-point bending mode at a DMA was (TGA) performed using a TACo., Instruments 2980, DMA using a data TA Instruments 2980, operated in the three-point mode at a frequency of 1 was Hz. performed The experimental were obtained at room temperature to 160 ˝ Cbending at a scanning rate ˝ 1 Hz. The experimental data were obtained at nominal room temperature to 160 at a scanning rate of 10 frequency C/min. of The specimens for the bending test had the dimensions of °C 2 mm ˆ 15 mm ˆ of 10 °C/min. The specimens for the bending test had the nominal dimensions of 2 mm × 15 mm × 6 mm. 6 mm. TGA was used to investigate the thermal decomposition behavior of the nanocomposite blend. was used investigate the TGA2950 thermal decomposition behavior of the blend. Tests TGA were done with atoTA Instruments at a heat rate of 10 ˝ C/min in ananocomposite temperature range of Tests ˝were done with a TA Instruments TGA2950 at a heat rate of 10 °C/min in a temperature range of 30 to 600 C. A sample of 5 to 10 mg was used for each run. The weight change was recorded as a 30 to 600 °C. A sample of 5 to 10 mg was used for each run. The weight change was recorded as a function of temperature. function of temperature.
3. Results and Discussion
3. Results and Discussion
3.1. Mechnical Properties 3.1. Mechnical Properties The stress-strain responses of the nanocomposite specimens in Figure Figure4.4. The stress-strain responses of the nanocomposite specimensininthe thetensile tensiletests tests are are shown shown in The tensile properties of the at different weight The tensile properties of nanocomposites the nanocomposites at different weightpercentages percentagesof offiller filler were were transformed transformed in a similar way. way. Initially, the the tensile stress increased in in a linear in a similar Initially, tensile stress increased a linearstyle, style,showing showingan an elastic elastic deformation deformation of the material.Then, Then, the the the maximum valuevalue and decreased rapidly torapidly the complete stage stage of the material. thestress stressreached reached maximum and decreased to the failure state.state. The results of the teststests showed the the nanocomposite specimens to have an complete failure The results of tensile the tensile showed nanocomposite specimens to have elastoplasticityproperty propertyand andtotobe beslightly slightly brittle. brittle. an elastoplasticity
Figure 4. Stress-strain curves from tensile test.
Figure 4. Stress-strain curves from tensile test.
The analysis data of the 30 experiments, achieved using Nexygen Plus software with a high reliability
The data the 30inexperiments, achieved using Nexygen Plus software with a high reliability ofanalysis over 95%, areofshown Table 1. of over 95%, are shown in Table 1.
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Table 1. Dependence of the tensile strength on the filler ratio of graphene nanoplatelets (GNPs). % of GNPs
Tensile strength (MPa)
0% 0.2% 0.5% 1% 1.5% 2%
17.009 31.775 28.489 19.914 18.900 18.492
The dependence of the tensile strength on the filler content of GNPs is shown in Table 1. When the weight percentage of GNPs was in the range of 0% to 0.2%, the tensile strength of the nanocomposite samples increased rapidly. When the content of GNPs increased to a range of 0.2% to 1%, the tensile strength decreased sharply. In a weight percentage range of 1% to 2%, the change in tensile strength was slight. The maximum value of tensile strength was achieved with a GNP content of only 0.2%. This value showed an increase of approximately 86.8% over the tensile strength of the specimens without added filler. The enhancement of the mechanical properties of the nanocomposites was due to the homogeneous dispersion of the reinforcement material and the good interaction between the reinforcement and the matrix, which was also confirmed by Gojny and Schulte [15]. 3.2. Thermal Properties Figure 5 presents the DMA results of the relationship between the storage modulus and temperature with different weight percentages of GNP filler. The plots show the storage modulus as a function of the filler content. With GNP filler increases from 0.2% to 1%, the storage modulus tended to increase. When the GNP content exceeded 1%, the storage modulus tended to decrease markedly. Here, epoxy/polyester nanocomposite reinforced with 1 wt % of GNP content achieved an increase of up to 3100 MPa of storage modulus at a temperature of 30 ˝ C. The loss factor curves (tanδ) of all the nanocomposites were obtained by DMA, as shown in Figure 6. The maximum value of the glass transition temperature (Tg ) was identified in the peak position of tanδ. This value was observed in the nanocomposite with filler content of 1.5%. As can be seen in Figure 6, the peak height of tanδ decreased with an increase in the GNP weight percentage, while the width of the loss factor showed little variation with the changes in nano-filler content. The peak factor, Γ, defined as the full width at half maximum of the tanδ peak divided by its height, can be qualitatively used to assess the homogeneity of the epoxy/polyester network. The low peak factor of the neat epoxy/polyester blend indicated that the crosslink density and homogeneity of the resin network were high. Figure 7 shows the thermal conductivity of the epoxy and its nanocomposites prepared by different processing conditions. The thermal conductivity of the 0.2 wt % GNP sample and 0.5 wt % GNP was found to be 0.09 W/mK and 0.104 W/mK, respectively, lower than that of the neat epoxy/polyester (0.12 W/mK) due to the debonding between the reinforcement and the matrix. It was also observed that the thermal conductivity of the 1 wt % GNP sample increased with the reinforcement of GNPs,
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2015, andMaterials the enhancement Materials 2015, 88 was significant as compared to that of the other samples. This enhancement77of thermal conductivity was due to the homogeneous dispersion of GNPs in the epoxy/polyester resin and them. The enhancement of conductivity was to for % thebonding bondingbetween between them. The enhancement of thermal conductivity was found to be 33.3% wt % bonding between them. The enhancement of thermal thermal conductivity was found found to be be 33.3% 33.3% for 11 wt wtfor % 1GNP GNP sample compared to the neat epoxy/polyester. sample compared to the neatneat epoxy/polyester. GNP sample compared to the epoxy/polyester. 3500 3500
StorageModulus Modulus(MPa) (MPa) Storage
3000 3000 2500 2500 2000 2000
0.2wt% 0.2wt% of of GNPs GNPs 0.5wt% of GNPs 0.5wt% of GNPs
1500 1500
Neat Neat epoxy/polyester epoxy/polyester 1wt% 1wt% of of GNPs GNPs
1000 1000
1.5wt% 1.5wt% of of GNPs GNPs
500 500 00
00
50 50
100 100
150 150
200 200
250 250
-500 -500
Temperature Temperature (°C) (°C)
Figure modulus vs. temperature plots of Figure 5. Storage Storage modulusvs. vs.temperature temperatureplots plots of of GNP/epoxy/polyester GNP/epoxy/polyester nanocomposite. nanocomposite. Figure 5. 5. Storage modulus GNP/epoxy/polyester nanocomposite. 1.2 1.2
TanDelta Delta Tan
11 0.2 0.2 wt% wt% of of GNPs GNPs
0.8 0.8
11 wt% wt% of of GNPs GNPs
0.6 0.6
0.5 0.5 wt% wt% of of GNPs GNPs 0.4 0.4
Neat Neat epoxy/polyester epoxy/polyester blend blend
0.2 0.2 00
1.5 1.5 wt% wt% of of GNPs GNPs
00
50 50
100 100
150 150
200 200
Temperature Temperature (°C) (°C)
Figure 6. Relationship between the loss factor (tanδ) of GNP-reinforced Figure Relationshipbetween betweenthe theloss lossfactor factor (tanδ) (tanδ) and and temperature temperature Figure 6. 6. Relationship and temperature of ofGNP-reinforced GNP-reinforced hybrid composite specimens. hybrid composite specimens. hybrid composite specimens.
Thermal Conductivity (W/mK) Thermal Conductivity (W/mK)
Materials 2015, 88 Materials 2015, Materials 2015, 8 0.18 0.18 0.16 0.16 0.14 0.14 0.12 0.12 0.1 0.1 0.08 0.08 0.06 0.06 0.04 0.04 0.02 0.02 0 0
5533 8 8
Neat epoxy/polyester Neat epoxy/polyester
0.2 wt% of 0.2GNPs wt% of GNPs
0.5 wt% of 0.5GNPs wt% of GNPs
1 wt% of GNPs 1 wt% of GNPs
1.5 wt% of 1.5GNPs wt% of GNPs
Figure Figure7.7.Thermal Thermalconductivity conductivity of of nanocomposites. nanocomposites. Figure 7. Thermal conductivity of nanocomposites. Thermogravimetric analysis (TGA) was also performed to evaluate the thermal stability of all the Thermogravimetric analysis (TGA) was also performed to evaluate the thermal stability of all the Thermogravimetric analysis (TGA) was also performed to evaluate thermal stability all the fabricated nanocomposite samples, as shown in Figure 8. These samplesthewere confirmed as of having a fabricated nanocomposite samples, as shown in Figure 8. These samples were confirmed as having a fabricated nanocomposite samples, as shown in Figure 8. These samples were confirmed as having 48% weight loss, evidence of the destruction in composite structure when affected by heat. Moreover,a 48% weight loss, evidence ofof thethedestruction inincomposite structure when affected by heat. heat. Moreover, Moreover, 48% evidence destruction composite structure by as thisweight figure loss, shows, the decomposition temperatures were almost thewhen sameaffected for all the nanocomposites as this figure shows, thethe decomposition temperatures were almost the all the the nanocomposites nanocomposites asthe this figure shows, decomposition were almost the same same for for nanocomposites all in experiment, which proved that the temperatures heat destruction caused to the resulting in this in the experiment, which proved that the heat destruction caused to the resulting nanocompositesininthis this in the experiment, which proved that the heat destruction caused to the resulting nanocomposites study was not dependent on the content of fillers. study waswas notnot dependent onon thethe content study dependent contentofoffillers. fillers.
Figure 8. Thermal stability of nanocomposites. Figure8.8.Thermal Thermalstability stabilityofofnanocomposites. nanocomposites. Figure
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3.3. Morphological Figure 9 shows theCharacteristics results of the morphological structure analysis of the fracture surface using SEM. The observed results were consistent with the obtained data of previous tensile tests. A relatively Figure 9 shows the results of the morphological structure analysis of the fracture surface using SEM. smooth surface can be seen in Figure 9a for the neat matrix material of epoxy and polyester mix without The observed results were consistent with the obtained data of previous tensile tests. A relatively smooth fillers, while in Figure 9b the rather rough fracture surface of the nanocomposites reinforced by 0.2% surface can be seen in Figure 9a for the neat matrix material of epoxy and polyester mix without fillers, GNP filler is evident in the brittle fracture-style development of this material. The agglomeration of while in Figure 9b the rather rough fracture surface of the nanocomposites reinforced by 0.2% GNP filler fillers negatively affected the mechanical properties of material. the nanocomposite. This phenomenon usually is evident in the brittle fracture-style development of this The agglomeration of fillers negatively occurs when there is a large increase in the filler percentage. Figure 9c shows an SEM the affected the mechanical properties of the nanocomposite. This phenomenon usually occursimage when of there 0.2% GNP-reinforced nanocomposite at a Figure magnification upantoSEM 30,000. It can be0.2% seen GNP-reinforced from this image is a large increase in the filler percentage. 9c shows image of the that the GNP filler completelyup covered by the matrix phase, indicating a good adhesion between nanocomposite at awas magnification to 30,000. It can be seen from this image that the GNP filler was the two structural phases, which contributed to the enhancement of the mechanical properties of the completely covered by the matrix phase, indicating a good adhesion between the two structural phases, produced nanocomposites. which contributed to the enhancement of the mechanical properties of the produced nanocomposites.
(a)
(b)
(c) Figure9.9.SEM SEMgraphs graphsofof(a) (a)neat neatepoxy/polyester; epoxy/polyester;(b) (b)epoxy/polyester epoxy/polyester composite composite reinforced Figure reinforced with 0.2% GNPs at a magnification of 1,000; (c) epoxy/polyester composite reinforced with with 0.2% GNPs at a magnification of 1000; (c) epoxy/polyester composite reinforced with 0.2% GNPs at a magnification of 30,000. 0.2% GNPs at a magnification of 30,000.
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4. Conclusions This research used a nano-filler of GNPs to reinforce an epoxy/polyester blend. The tensile strength of the hybrid matrix nanocomposite reaches the maximum value with a very small amount of GNP filler added. When the weight percentage of the filler exceeded this determined value, the tensile strength of the nanocomposite decreased markedly and the material became brittle. The tensile strength of the GNP-reinforced epoxy/polyester blend nanocomposite reached approximately 86.8% with an addition of only 0.2% GNP content, as compared to neat epoxy/polyester. The DMA results indicated an improvement of up to 3100 MPa in the storage modulus for the epoxy/polyester nanocomposite reinforced with 1 wt % GNPs. The TGA results confirmed the thermal stability of the resulting nanocomposite specimens, regardless of the weight percentage of the GNPs. Additionally, as observed in the SEM images, there was good adhesion between the fillers and the resin. Therefore, the novel materials created in this study were of high mechanical strength, good thermal conductivity and stability, and can be used to produce high-performance components which are light weight, high quality and low cost for applications in electronic circuit boards and transportation. Acknowledgments The authors acknowledge and thank the Ministry of Science and Technology of the Republic of China for their financial support of this study under Contract Number: MOST 103-2221-E-151-035. Author Contributions All the authors contributed significantly for the completions of this manuscript. Shyh-Chour Huang is the leader of the research group and responsible for supervising the experiments, writing and editing the article. Minh-Tai Le is mainly responsible for design and implementing of experiments, collecting, processing and analyzing the data as well as writing the main parts of the article. Conflicts of Interest The authors declare no conflict of interest. References 1. Potts, J.R.; Dreyer, D.R.; Bielawski, C.W.; Ruoff, R.S. Graphene-based polymer nanocomposites. Polymer 2011, 52, 5–25. [CrossRef] 2. Li, J.; Guo, Z.J. The influence of polyethylene-polyamine surface treatment of carbon nanotube on the TPB and friction and wear behavior of thermoplastic polyimide composite. Polym. Plast. Technol. Eng. 2011, 50, 996–999. [CrossRef] 3. Chinnakkannu, K.C.; Muthukaruppan, A.; Josephine, S.R.; Periyannan, G. Thermo mechnical behaviour of unsaturated polyester toughened epoxy-clay hybrid nanocomposites. J. Polym. Res. 2007, 14, 319–328. 4. Shahryar, P.; Siddaramaiah, A.S.; Rajulu, A.V.; Kumar, S.V.; Rao, G.B. Tensile properties of glass roving’s/hydroxyl terminated polyester toughened epoxy composites. Polym. Plast. Technol. Eng. 2011, 50, 973–982.
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5. Cherian, A.B.; Varghese, L.A.; Thachil, E.T. Epoxy-modified, unsaturated polyester hybrid networks. Eur. Polym. J. 2007, 43, 1460–1469. [CrossRef] 6. Tan, S.G.; Chow, W.S. Thermal properties, fracture toughness and water absorption of epoxy-palm oil blends. Polym. Plast. Technol. Eng. 2010, 49, 900–907. [CrossRef] 7. Zandiatashbar, A.; Picu, R.C.; Koratkar, N. Mechanical behavior of epoxy-graphene platelets nanocomposites. J. Eng. Mater. Technol. 2012, 134, 031011–031016. [CrossRef] 8. Yang, S.Y.; Lin, W.N.; Huang, Y.L.; Tien, H.W.; Wang, J.Y.; Ma, C.C.; Li, S.M. Synergetic effects of graphene platelets and carbon nanotubes on themechanical and thermal properties of epoxy composites. Carbon 2011, 49, 793–803. [CrossRef] 9. Ahmadi-Moghadam, B.; Taheri, F. Effect of processing parameters on the structure and multi-functional performance of epoxy/GNP-nanocomposite. J. Mater. Sci. 2014, 49, 6180–6190. [CrossRef] 10. Li, W.; Dichiara, A.; Bai, J. Carbon nanotube-graphene nanoplatelet hybrids as high-perfprmance multifunctional reinforcements in epoxy composites. Compos. Sci. Technol. 2013, 74, 221–227. [CrossRef] 11. Cui, W.; Du, F.; Zhao, J. Improving thermal conductivity while retaining high electrical resistivity of epoxy composites by incorporating silica-coatedmulti-walled carbon nanotubes. Carbon 2011, 49, 495–500. [CrossRef] 12. Viswanath, G.R.; Thangaraj, R.; Guhanathan, S. Thermomechanical and electrical studies on epoxy/hyperbranched polyester blends. Int. J. Polym. Anal. Charact. 2009, 14, 493–507. [CrossRef] 13. Rahman, M.M.; Zainuddin, S.; Hosur, M.V. Improvements in mechanical and thermo-mechanical properties of eglass/epoxy composites using amino functionalizedMWCNTs. Compos. Struct. 2012, 94, 2397–2406. [CrossRef] 14. Park, J.G.; Cheng, Q.; Lu, J. Thermal conductivity of MWCNT/epoxy composites: the effects of length, alignment and functionalization. Carbon 2012, 50, 2083–2090. [CrossRef] 15. Gojny, F.H.; Schulte, K. Functionalisation effect on the thermo-mechanical behaviour of multi-wall carbon nanotube/epoxy-composites. Compos. Sci. Technol. 2004, 64, 2303–2308. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Thermal and Mechanical Behavior of Hybrid Polymer Nanocomposite Reinforced with Graphene Nanoplatelets Minh-Tai Le 1,2 and Shyh-Chour Huang 2, * 1
Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and Education, No.1, Vo Van Ngan, Thu Duc District, Ho Chi Minh City 70000, Vietnam; E-Mail: [email protected] 2 Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Sanmin District, Kaohsiung 80778, Taiwan * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-7-3814526 (ext. 5313); Fax: +886-7-3831373. Academic Editor: Teen-Hang Meen Received: 21 June 2015 / Accepted: 19 August 2015 / Published: 24 August 2015
Abstract: In the present investigation, we successfully fabricate a hybrid polymer nanocomposite containing epoxy/polyester blend resin and graphene nanoplatelets (GNPs) by a novel technique. A high intensity ultrasonicator is used to obtain a homogeneous mixture of epoxy/polyester resin and graphene nanoplatelets. This mixture is then mixed with a hardener using a high-speed mechanical stirrer. The trapped air and reaction volatiles are removed from the mixture using high vacuum. The hot press casting method is used to make the nanocomposite specimens. Tensile tests, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) are performed on neat, 0.2 wt %, 0.5 wt %, 1 wt %, 1.5 wt % and 2 wt % GNP-reinforced epoxy/polyester blend resin to investigate the reinforcement effect on the thermal and mechanical properties of the nanocomposites. The results of this research indicate that the tensile strength of the novel nanocomposite material increases to 86.8% with the addition of a ratio of graphene nanoplatelets as low as 0.2 wt %. DMA results indicate that the 1 wt % GNP-reinforced epoxy/polyester nanocomposite possesses the highest storage modulus and glass transition temperature (Tg ), as compared to neat epoxy/polyester or the other nanocomposite specimens. In addition, TGA results verify thethermal stability of the experimental specimens, regardless of the weight percentage of GNPs.
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Keywords: nanocomposite; graphene nanoplatelets; epoxy; unsaturated polyester; tensile strength; thermal conductivity; morphology
1. Introduction Graphene nanoplatelets (GNPs) are a type of filler in the new generation of reinforced materials. Producing high-purity and large amounts of GNPs has a highcost, and because of the issues related to their stability during the process of fabricating nanocomposite materials, researchers have recommended using them in a pristine state, i.e., not to be handled as a functional group [1]. Relating to the matrix for the construction of the composite, thermoset epoxy is the top choice because of its excellent thermomechanical properties, which contribute to the successful fabrication of advanced nanocomposite materials [2,3]. Another type of thermoset polymer used to reinforce as well as modify a material’s performance is polyester. With its high thermal and mechanical properties, it is both simpler and cheaper to use than epoxy, and polyester can be used as the second matrix phase mixed with epoxy to create a general matrix structure of composite materials [4–6]. There are studies in which GNPs were used as fillers to enhance the properties of the epoxy matrix [7,8]. The new generation of hybrid composite materials with versatile mechanical, thermal and electrical properties are of particular interest to researchers [9,10]. The thermal conductivity of epoxy nanocomposites having 0.5 and 1 wt % of silica-coated MWCNTs was found to be enhanced by 51% and 67%, respectively, by Cui et al. [11]. Viswanath et al. [12] presented a thermomechanical and electrical study of epoxy/hyperbranched polyester. Rahman et al. [13] optimized the mechanical properties of epoxy reinforced with E-glass and amino-functionalized MWCNTs. The MWCNTs were aligned in the epoxy resin to achieve the required properties by Park et al. [14]; the thermal conductivity of epoxy/MWCNT nanocomposites at room temperature (RT) was observed to be 55 W/mK; the stretched MWCNT-epoxy sheet showed a value of 100 W/mK, whereas pure epoxy was 0.11 W/mK at RT. In most previous studies, thermoset polymers did factor in the critical investigation of the performance of hybrid matrix materials made with epoxy and polyester. In this research, the added filler of GNPs was used to reinforce the tensile strength and thermal conductivity of the hybrid epoxy/polyester composite. The GNPs were mixed at different weight percentages with epoxy and polyester using high intensity ultrasonication. The resulting experimental specimens were obtained by means of the hot press casting method. The primary interest of this paper was to characterize the effect of graphene nanoplatelets on the thermal and mechanical properties of the epoxy/polyester hybrid composite. Tensile tests were performed to evaluate the mechanical performance, and thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) were carried out to evaluate the thermal properties. Additionally, the morphological characteristics of the surface structure of the nanocomposite samples in the tensile test were observed by a scanning electron microscope (SEM). The important contribution of this study is the successful fabrication of a hybrid matrix nanocomposite material of thermoset resins reinforced with the GNP filler. The nanocomposite specimens fabricated at a low cost evidenced high mechanical strength,
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good thermal conductivity and stability, making them suitable for applications in the manufacturing of good thermal and stability, making them suitable for applications in the manufacturing of components for conductivity electronic circuit boards and transportation. components for electronic circuit boards and transportation. 2. Materials and Methods 2. Materials and Methods 2.1. Material Preparation 2.1. Material Preparation The graphene nanoplatelets (GNPs) used in this study were Grade 4 (Figure 1), purchased from Cheap The graphene nanoplatelets (GNPs) used in this study were Grade 4 (Figure 1), purchased from Cheap Tubes Inc. (Cambridgeport, VT, USA), with an average plate diameter in the range of 1 to 2 µm and Tubes Inc. (Cambridgeport, VT, USA), with an average plate diameter in the range of 1 to 2 µm and over 99% pure. structureof ofthe thecomposite compositeconsisted consisted epoxy 6620, over 99% pure.The Thematerials materialsfor forcreating creating the the matrix matrix structure ofof epoxy 6620, hardener AH150 GC-0421,all allpurchased purchasedfrom from Golden Innovation hardener AH150and andunsaturated unsaturatedpolyester polyester resin resin (UPR) (UPR) GC-0421, Golden Innovation Business Co. by the the manufacturer, manufacturer,some somemodification modificationmaterials materials were Business Co.(Taibei, (Taibei,Taiwan). Taiwan). As As suggested suggested by were used: anan accelerator catalystof ofmethyl methylethyl ethylketone ketoneperoxide peroxide (MEKP). used: acceleratorofofcobalt cobaltnaphthenate naphthenate (6%) and aa catalyst (MEKP). ToTo easily was necessary necessary totouse usemethyl methylethyl ethylketone ketone (MEK) easilydisperse dispersethe theGNP GNPfiller fillerinin the the resin, resin, it was (MEK) a dissolution. as as a dissolution.
Figure 1. Scanning electron microscope image of pristine graphene nanoplatelets. Figure 1. Scanning electron microscope image of pristine graphene nanoplatelets. Figure 2a shows the whole process of the experimental specimen preparation. The predetermined Figure the whole process the experimental specimen preparation. The predetermined amounts2a of shows GNPs, epoxy and UPR wereofmixed together in a suitable beaker. The stipulated amounts of amounts of GNPs, epoxy and UPR were mixed together in a suitable beaker. The stipulated GNP and epoxy/polyester blend were mixed thoroughly with a magnetic stirrer for about 1 hamounts at a of temperature GNP and epoxy/polyester blend were mixed thoroughly with a magnetic stirrer for about 1 of 60 °C. The beaker was then placed in an ultrasonicator at a high intensity for 1½ h seth atat a a pulse mode (9 ˝sC.on/9 off). Anwas external coolinginsystem was employed submerging temperature of 60 Thes beaker then placed an ultrasonicator at abyhigh intensity the for beaker 1½ h set the mixture in sanoff). ice bath avoid the temperature the sonication process. at containing a pulse mode (9 s on/9 An to external cooling systemrising was during employed by submerging theWhen beaker the process finished, agents, including thetemperature hardener, accelerator and the catalyst, with aprocess. weight ratio containing thewas mixture in anallice bath to avoid the rising during sonication When 2:1/1/1,was were added to previous epoxy/polyester mixture. An aluminum mold with (Figure 2) of the theofprocess finished, allthe agents, including the hardener, accelerator and catalyst, a weight ratio required dimensions was used for the making of samples on par with the American Society for Testing of 2:1/1/1, were added to the previous epoxy/polyester mixture. An aluminum mold (Figure 2) of the and Materials (ASTM) Themaking mold was covered on with mold-releasing agentSociety so the for samples required dimensions was standard. used for the of samples para with the American Testing and Materials (ASTM) standard. The mold was covered with a mold-releasing agent so the samples
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could Thecompleted completedmixture mixture was poured mold. The 2-part mold couldbebeeasily easilyremoved. removed. The was poured intointo the the mold. The 2-part castingcasting mold was could be easily removed. Theofcompleted poured into the mold. The 2-part casting was was maintained under a pressure 0.8 mixture MPa h attemperature. room temperature. To complete the curing maintained under a pressure 0.8ofMPa for 24for h was at24room To complete the curingmold process maintained under a pressure ofstresses 0.8 MPa for the 24 heat, hexperimental at room temperature. To complete the curing process process and eliminate residual due to the experimental samples (Figure 3b) cured were post and eliminate residual stresses due to heat, samples (Figure 3b) were post at and eliminate residual stresses due to heat, the experimental samples (Figure 3b) were post cured at ˝ cured at for 90 3Ch.for 3 h. 90 °C 90 °C for 3 h.
Figure2.2. (a) Schematic showing Figure (a) Schematic showing the the manufacturing manufacturing process processof ofthe thenanocomposite; nanocomposite; Figure 2. (a) Schematic showing the manufacturing process of the nanocomposite; (b) dimensions of specimen in ASTM standard Type V (mm). (b)(b) dimensions dimensionsofofspecimen specimenininASTM ASTMstandard standard Type Type V V (mm). (mm).
(a) (b) (a) (b) Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. Figure 3. (a) Aluminum mold; (b) nanocomposite specimens. 2.2. Mechanical Test 2.2. Mechanical Test 2.2. Mechanical Test set up for the tensile test was done on an Instron 5566-CN2081 machine The experimental The experimental set up for the tensile was done on an D638 Instron 5566-CN2081 machine (Instron Company, Hsinchu, Taiwan) with atest standard of ASTM (Type V). The following The experimental set up for the tensile test was done on an Instron 5566-CN2081 machine (Instron Company, Hsinchu, Taiwan) clamp with length a standard ofmm, ASTM D638 The following parameters were set for the experiment: of 26.3 tensile speed(Type of 0.5V). mm/min and room (Instron Company, Taiwan) with alength standard of ASTM D638 (Type V). The and following parameters were setHsinchu, for experiment: of 26.3 tensile speed mm/min room temperature of 30 °C. Forthe each content ofclamp filler, the tensile testmm, was carried out onofat0.5 least five specimens; parameters were for experiment: clampthe length 26.3 tensile speed of 0.5 and temperature ofthe 30set °C. Forthe each filler, tensileoftest wasmm, carried out on at least five mm/min specimens; the result was average valuecontent of the of five measurements. ˝ room temperature 30 C.value For of each of filler, the tensile test was carried out on at least five the result was theof average thecontent five measurements. specimens; the result was the average value of the five measurements.
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2.3. Thermal Analysis 2.3. Thermal Analysis
The thermal characteristics of the GNP/epoxy/polyester-blended nanocomposites were studied using dynamic mechanical analysis (TA Instruments Co., New Castle,were PA,studied USA)using and The thermal characteristics of the(DMA) GNP/epoxy/polyester-blended nanocomposites thermogravimetric analysis (TGA) (TA Instruments Co.,Co., New Castle, PA,PA, USA). dynamic mechanical analysis (DMA) (TA Instruments New Castle, USA) and thermogravimetric analysis (TA Instruments New Castle, PA, operated USA). in the three-point bending mode at a DMA was (TGA) performed using a TACo., Instruments 2980, DMA using a data TA Instruments 2980, operated in the three-point mode at a frequency of 1 was Hz. performed The experimental were obtained at room temperature to 160 ˝ Cbending at a scanning rate ˝ 1 Hz. The experimental data were obtained at nominal room temperature to 160 at a scanning rate of 10 frequency C/min. of The specimens for the bending test had the dimensions of °C 2 mm ˆ 15 mm ˆ of 10 °C/min. The specimens for the bending test had the nominal dimensions of 2 mm × 15 mm × 6 mm. 6 mm. TGA was used to investigate the thermal decomposition behavior of the nanocomposite blend. was used investigate the TGA2950 thermal decomposition behavior of the blend. Tests TGA were done with atoTA Instruments at a heat rate of 10 ˝ C/min in ananocomposite temperature range of Tests ˝were done with a TA Instruments TGA2950 at a heat rate of 10 °C/min in a temperature range of 30 to 600 C. A sample of 5 to 10 mg was used for each run. The weight change was recorded as a 30 to 600 °C. A sample of 5 to 10 mg was used for each run. The weight change was recorded as a function of temperature. function of temperature.
3. Results and Discussion
3. Results and Discussion
3.1. Mechnical Properties 3.1. Mechnical Properties The stress-strain responses of the nanocomposite specimens in Figure Figure4.4. The stress-strain responses of the nanocomposite specimensininthe thetensile tensiletests tests are are shown shown in The tensile properties of the at different weight The tensile properties of nanocomposites the nanocomposites at different weightpercentages percentagesof offiller filler were were transformed transformed in a similar way. way. Initially, the the tensile stress increased in in a linear in a similar Initially, tensile stress increased a linearstyle, style,showing showingan an elastic elastic deformation deformation of the material.Then, Then, the the the maximum valuevalue and decreased rapidly torapidly the complete stage stage of the material. thestress stressreached reached maximum and decreased to the failure state.state. The results of the teststests showed the the nanocomposite specimens to have an complete failure The results of tensile the tensile showed nanocomposite specimens to have elastoplasticityproperty propertyand andtotobe beslightly slightly brittle. brittle. an elastoplasticity
Figure 4. Stress-strain curves from tensile test.
Figure 4. Stress-strain curves from tensile test.
The analysis data of the 30 experiments, achieved using Nexygen Plus software with a high reliability
The data the 30inexperiments, achieved using Nexygen Plus software with a high reliability ofanalysis over 95%, areofshown Table 1. of over 95%, are shown in Table 1.
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Table 1. Dependence of the tensile strength on the filler ratio of graphene nanoplatelets (GNPs). % of GNPs
Tensile strength (MPa)
0% 0.2% 0.5% 1% 1.5% 2%
17.009 31.775 28.489 19.914 18.900 18.492
The dependence of the tensile strength on the filler content of GNPs is shown in Table 1. When the weight percentage of GNPs was in the range of 0% to 0.2%, the tensile strength of the nanocomposite samples increased rapidly. When the content of GNPs increased to a range of 0.2% to 1%, the tensile strength decreased sharply. In a weight percentage range of 1% to 2%, the change in tensile strength was slight. The maximum value of tensile strength was achieved with a GNP content of only 0.2%. This value showed an increase of approximately 86.8% over the tensile strength of the specimens without added filler. The enhancement of the mechanical properties of the nanocomposites was due to the homogeneous dispersion of the reinforcement material and the good interaction between the reinforcement and the matrix, which was also confirmed by Gojny and Schulte [15]. 3.2. Thermal Properties Figure 5 presents the DMA results of the relationship between the storage modulus and temperature with different weight percentages of GNP filler. The plots show the storage modulus as a function of the filler content. With GNP filler increases from 0.2% to 1%, the storage modulus tended to increase. When the GNP content exceeded 1%, the storage modulus tended to decrease markedly. Here, epoxy/polyester nanocomposite reinforced with 1 wt % of GNP content achieved an increase of up to 3100 MPa of storage modulus at a temperature of 30 ˝ C. The loss factor curves (tanδ) of all the nanocomposites were obtained by DMA, as shown in Figure 6. The maximum value of the glass transition temperature (Tg ) was identified in the peak position of tanδ. This value was observed in the nanocomposite with filler content of 1.5%. As can be seen in Figure 6, the peak height of tanδ decreased with an increase in the GNP weight percentage, while the width of the loss factor showed little variation with the changes in nano-filler content. The peak factor, Γ, defined as the full width at half maximum of the tanδ peak divided by its height, can be qualitatively used to assess the homogeneity of the epoxy/polyester network. The low peak factor of the neat epoxy/polyester blend indicated that the crosslink density and homogeneity of the resin network were high. Figure 7 shows the thermal conductivity of the epoxy and its nanocomposites prepared by different processing conditions. The thermal conductivity of the 0.2 wt % GNP sample and 0.5 wt % GNP was found to be 0.09 W/mK and 0.104 W/mK, respectively, lower than that of the neat epoxy/polyester (0.12 W/mK) due to the debonding between the reinforcement and the matrix. It was also observed that the thermal conductivity of the 1 wt % GNP sample increased with the reinforcement of GNPs,
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2015, andMaterials the enhancement Materials 2015, 88 was significant as compared to that of the other samples. This enhancement77of thermal conductivity was due to the homogeneous dispersion of GNPs in the epoxy/polyester resin and them. The enhancement of conductivity was to for % thebonding bondingbetween between them. The enhancement of thermal conductivity was found to be 33.3% wt % bonding between them. The enhancement of thermal thermal conductivity was found found to be be 33.3% 33.3% for 11 wt wtfor % 1GNP GNP sample compared to the neat epoxy/polyester. sample compared to the neatneat epoxy/polyester. GNP sample compared to the epoxy/polyester. 3500 3500
StorageModulus Modulus(MPa) (MPa) Storage
3000 3000 2500 2500 2000 2000
0.2wt% 0.2wt% of of GNPs GNPs 0.5wt% of GNPs 0.5wt% of GNPs
1500 1500
Neat Neat epoxy/polyester epoxy/polyester 1wt% 1wt% of of GNPs GNPs
1000 1000
1.5wt% 1.5wt% of of GNPs GNPs
500 500 00
00
50 50
100 100
150 150
200 200
250 250
-500 -500
Temperature Temperature (°C) (°C)
Figure modulus vs. temperature plots of Figure 5. Storage Storage modulusvs. vs.temperature temperatureplots plots of of GNP/epoxy/polyester GNP/epoxy/polyester nanocomposite. nanocomposite. Figure 5. 5. Storage modulus GNP/epoxy/polyester nanocomposite. 1.2 1.2
TanDelta Delta Tan
11 0.2 0.2 wt% wt% of of GNPs GNPs
0.8 0.8
11 wt% wt% of of GNPs GNPs
0.6 0.6
0.5 0.5 wt% wt% of of GNPs GNPs 0.4 0.4
Neat Neat epoxy/polyester epoxy/polyester blend blend
0.2 0.2 00
1.5 1.5 wt% wt% of of GNPs GNPs
00
50 50
100 100
150 150
200 200
Temperature Temperature (°C) (°C)
Figure 6. Relationship between the loss factor (tanδ) of GNP-reinforced Figure Relationshipbetween betweenthe theloss lossfactor factor (tanδ) (tanδ) and and temperature temperature Figure 6. 6. Relationship and temperature of ofGNP-reinforced GNP-reinforced hybrid composite specimens. hybrid composite specimens. hybrid composite specimens.
Thermal Conductivity (W/mK) Thermal Conductivity (W/mK)
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Neat epoxy/polyester Neat epoxy/polyester
0.2 wt% of 0.2GNPs wt% of GNPs
0.5 wt% of 0.5GNPs wt% of GNPs
1 wt% of GNPs 1 wt% of GNPs
1.5 wt% of 1.5GNPs wt% of GNPs
Figure Figure7.7.Thermal Thermalconductivity conductivity of of nanocomposites. nanocomposites. Figure 7. Thermal conductivity of nanocomposites. Thermogravimetric analysis (TGA) was also performed to evaluate the thermal stability of all the Thermogravimetric analysis (TGA) was also performed to evaluate the thermal stability of all the Thermogravimetric analysis (TGA) was also performed to evaluate thermal stability all the fabricated nanocomposite samples, as shown in Figure 8. These samplesthewere confirmed as of having a fabricated nanocomposite samples, as shown in Figure 8. These samples were confirmed as having a fabricated nanocomposite samples, as shown in Figure 8. These samples were confirmed as having 48% weight loss, evidence of the destruction in composite structure when affected by heat. Moreover,a 48% weight loss, evidence ofof thethedestruction inincomposite structure when affected by heat. heat. Moreover, Moreover, 48% evidence destruction composite structure by as thisweight figure loss, shows, the decomposition temperatures were almost thewhen sameaffected for all the nanocomposites as this figure shows, thethe decomposition temperatures were almost the all the the nanocomposites nanocomposites asthe this figure shows, decomposition were almost the same same for for nanocomposites all in experiment, which proved that the temperatures heat destruction caused to the resulting in this in the experiment, which proved that the heat destruction caused to the resulting nanocompositesininthis this in the experiment, which proved that the heat destruction caused to the resulting nanocomposites study was not dependent on the content of fillers. study waswas notnot dependent onon thethe content study dependent contentofoffillers. fillers.
Figure 8. Thermal stability of nanocomposites. Figure8.8.Thermal Thermalstability stabilityofofnanocomposites. nanocomposites. Figure
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3.3. Morphological Figure 9 shows theCharacteristics results of the morphological structure analysis of the fracture surface using SEM. The observed results were consistent with the obtained data of previous tensile tests. A relatively Figure 9 shows the results of the morphological structure analysis of the fracture surface using SEM. smooth surface can be seen in Figure 9a for the neat matrix material of epoxy and polyester mix without The observed results were consistent with the obtained data of previous tensile tests. A relatively smooth fillers, while in Figure 9b the rather rough fracture surface of the nanocomposites reinforced by 0.2% surface can be seen in Figure 9a for the neat matrix material of epoxy and polyester mix without fillers, GNP filler is evident in the brittle fracture-style development of this material. The agglomeration of while in Figure 9b the rather rough fracture surface of the nanocomposites reinforced by 0.2% GNP filler fillers negatively affected the mechanical properties of material. the nanocomposite. This phenomenon usually is evident in the brittle fracture-style development of this The agglomeration of fillers negatively occurs when there is a large increase in the filler percentage. Figure 9c shows an SEM the affected the mechanical properties of the nanocomposite. This phenomenon usually occursimage when of there 0.2% GNP-reinforced nanocomposite at a Figure magnification upantoSEM 30,000. It can be0.2% seen GNP-reinforced from this image is a large increase in the filler percentage. 9c shows image of the that the GNP filler completelyup covered by the matrix phase, indicating a good adhesion between nanocomposite at awas magnification to 30,000. It can be seen from this image that the GNP filler was the two structural phases, which contributed to the enhancement of the mechanical properties of the completely covered by the matrix phase, indicating a good adhesion between the two structural phases, produced nanocomposites. which contributed to the enhancement of the mechanical properties of the produced nanocomposites.
(a)
(b)
(c) Figure9.9.SEM SEMgraphs graphsofof(a) (a)neat neatepoxy/polyester; epoxy/polyester;(b) (b)epoxy/polyester epoxy/polyester composite composite reinforced Figure reinforced with 0.2% GNPs at a magnification of 1,000; (c) epoxy/polyester composite reinforced with with 0.2% GNPs at a magnification of 1000; (c) epoxy/polyester composite reinforced with 0.2% GNPs at a magnification of 30,000. 0.2% GNPs at a magnification of 30,000.
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4. Conclusions This research used a nano-filler of GNPs to reinforce an epoxy/polyester blend. The tensile strength of the hybrid matrix nanocomposite reaches the maximum value with a very small amount of GNP filler added. When the weight percentage of the filler exceeded this determined value, the tensile strength of the nanocomposite decreased markedly and the material became brittle. The tensile strength of the GNP-reinforced epoxy/polyester blend nanocomposite reached approximately 86.8% with an addition of only 0.2% GNP content, as compared to neat epoxy/polyester. The DMA results indicated an improvement of up to 3100 MPa in the storage modulus for the epoxy/polyester nanocomposite reinforced with 1 wt % GNPs. The TGA results confirmed the thermal stability of the resulting nanocomposite specimens, regardless of the weight percentage of the GNPs. Additionally, as observed in the SEM images, there was good adhesion between the fillers and the resin. Therefore, the novel materials created in this study were of high mechanical strength, good thermal conductivity and stability, and can be used to produce high-performance components which are light weight, high quality and low cost for applications in electronic circuit boards and transportation. Acknowledgments The authors acknowledge and thank the Ministry of Science and Technology of the Republic of China for their financial support of this study under Contract Number: MOST 103-2221-E-151-035. Author Contributions All the authors contributed significantly for the completions of this manuscript. Shyh-Chour Huang is the leader of the research group and responsible for supervising the experiments, writing and editing the article. Minh-Tai Le is mainly responsible for design and implementing of experiments, collecting, processing and analyzing the data as well as writing the main parts of the article. Conflicts of Interest The authors declare no conflict of interest. References 1. Potts, J.R.; Dreyer, D.R.; Bielawski, C.W.; Ruoff, R.S. Graphene-based polymer nanocomposites. Polymer 2011, 52, 5–25. [CrossRef] 2. Li, J.; Guo, Z.J. The influence of polyethylene-polyamine surface treatment of carbon nanotube on the TPB and friction and wear behavior of thermoplastic polyimide composite. Polym. Plast. Technol. Eng. 2011, 50, 996–999. [CrossRef] 3. Chinnakkannu, K.C.; Muthukaruppan, A.; Josephine, S.R.; Periyannan, G. Thermo mechnical behaviour of unsaturated polyester toughened epoxy-clay hybrid nanocomposites. J. Polym. Res. 2007, 14, 319–328. 4. Shahryar, P.; Siddaramaiah, A.S.; Rajulu, A.V.; Kumar, S.V.; Rao, G.B. Tensile properties of glass roving’s/hydroxyl terminated polyester toughened epoxy composites. Polym. Plast. Technol. Eng. 2011, 50, 973–982.
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5. Cherian, A.B.; Varghese, L.A.; Thachil, E.T. Epoxy-modified, unsaturated polyester hybrid networks. Eur. Polym. J. 2007, 43, 1460–1469. [CrossRef] 6. Tan, S.G.; Chow, W.S. Thermal properties, fracture toughness and water absorption of epoxy-palm oil blends. Polym. Plast. Technol. Eng. 2010, 49, 900–907. [CrossRef] 7. Zandiatashbar, A.; Picu, R.C.; Koratkar, N. Mechanical behavior of epoxy-graphene platelets nanocomposites. J. Eng. Mater. Technol. 2012, 134, 031011–031016. [CrossRef] 8. Yang, S.Y.; Lin, W.N.; Huang, Y.L.; Tien, H.W.; Wang, J.Y.; Ma, C.C.; Li, S.M. Synergetic effects of graphene platelets and carbon nanotubes on themechanical and thermal properties of epoxy composites. Carbon 2011, 49, 793–803. [CrossRef] 9. Ahmadi-Moghadam, B.; Taheri, F. Effect of processing parameters on the structure and multi-functional performance of epoxy/GNP-nanocomposite. J. Mater. Sci. 2014, 49, 6180–6190. [CrossRef] 10. Li, W.; Dichiara, A.; Bai, J. Carbon nanotube-graphene nanoplatelet hybrids as high-perfprmance multifunctional reinforcements in epoxy composites. Compos. Sci. Technol. 2013, 74, 221–227. [CrossRef] 11. Cui, W.; Du, F.; Zhao, J. Improving thermal conductivity while retaining high electrical resistivity of epoxy composites by incorporating silica-coatedmulti-walled carbon nanotubes. Carbon 2011, 49, 495–500. [CrossRef] 12. Viswanath, G.R.; Thangaraj, R.; Guhanathan, S. Thermomechanical and electrical studies on epoxy/hyperbranched polyester blends. Int. J. Polym. Anal. Charact. 2009, 14, 493–507. [CrossRef] 13. Rahman, M.M.; Zainuddin, S.; Hosur, M.V. Improvements in mechanical and thermo-mechanical properties of eglass/epoxy composites using amino functionalizedMWCNTs. Compos. Struct. 2012, 94, 2397–2406. [CrossRef] 14. Park, J.G.; Cheng, Q.; Lu, J. Thermal conductivity of MWCNT/epoxy composites: the effects of length, alignment and functionalization. Carbon 2012, 50, 2083–2090. [CrossRef] 15. Gojny, F.H.; Schulte, K. Functionalisation effect on the thermo-mechanical behaviour of multi-wall carbon nanotube/epoxy-composites. Compos. Sci. Technol. 2004, 64, 2303–2308. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
