RESEARCH ARTICLE

Mechanical properties of epoxy reinforcedwith homogeneously dispersedcarbon nanofibre

Sohel Rana & Ramasamy Alagirusamy &

Mangala Joshi

Received: 6 July 2010 /Accepted: 3 January 2011 /Published online: 20 April 2011# Central Institute of Plastics Engineering & Technology 2011

Abstract This paper investigated the mechanical properties of epoxy reinforcedwith homogeneously dispersed vapor-grown carbon nanofibres (CNFs) at lowconcentrations (0.1–0.5 wt%). The homogeneous dispersion of CNF in epoxywas achieved using an optimized dispersion route which was selected among avariety of dispersion techniques. The state of CNF dispersion in the epoxy resinand within the nanocomposites was studied using optical and atomic forcemicroscopy. A detailed investigation on the CNF dispersion behaviour revealedthat an ultrasonic treatment assisted with high speed mechanical stirring at2,000 rpm was very effective in dispersing CNF uniformly in the epoxy resin.Dispersion of CNF up to 0.5 wt% using this route improved the tensile modulusand strength of epoxy by 35% and 63% respectively. Similarly, flexural andcompressive properties also improved significantly. Fracture surface of tensilefailed specimens was characterized by scanning electron microscopy in order toinvestigate the fracture mechanism.

Keywords Epoxy . Carbon nanofibre . Homogeneous dispersion .

Mechanical properties

Introduction

Epoxy resins are finding tremendous applications in the field of highperformance composites due to their excellent mechanical properties, resistanceto environmental degradation and chemicals, low shrinkage during curing andvery good thermal stability [1–3]. Use of epoxy based composites in several high

Int J Plast Technol (December 2010) 14(2):224–233DOI 10.1007/s12588-011-0020-z

S. Rana (*) : R. Alagirusamy :M. JoshiDepartment of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas,New Delhi 110016, Indiae-mail: soheliitd2005@gmail.com

end applications demand further improvement of matrix properties. Researchershave tried to modify epoxy resins either by adding rubber particles [4] or bydispersing rigid inorganic fillers such as silica, kaolin, alumina, barium titanite,dolomite, glass beads, aluminum hydroxide and CaCO3 [5–8]. Addition of theseagents improves the fracture toughness of epoxy but often reduces other importantproperties such as modulus, strength and thermo-mechanical properties [9].Recently attempts have been made to disperse carbon nanofibre (CNF) and carbonnanotube (CNT) in epoxy resin to improve fracture toughness as well as modulus,strength and other useful properties. These nanomaterials have been of significantimportance to the scientific community for many years because of theiroutstanding mechanical, thermal and electrical properties [10–14]. They providehuge interfacial area because of their nano scale diameters and lead to increasedmicro structural perfection of composites due to reduced size of potential defects[15]. However, the main problem associated with these nanomaterials is thedifficulty in dispersing them uniformly within the matrix due to agglomeration.CNT reinforced epoxy composites have been widely researched and severaltechniques have been proposed to overcome the dispersion problem [16–20]. CNFsare less expensive and relatively easy to disperse in epoxy resin due to their largerdiameters. Hence, efforts were made by some researchers to explore the potentialof CNF to enhance various properties of epoxy based composites. For example,Gauthier et al. reported significant improvements in tensile strength and strain(160% and 100% respectively) of a rubbery epoxy matrix by dispersing 5 wt%CNFs [21]. Ultrasonication was used to disperse CNFs in a hardener andsubsequently the dispersion was mixed with the epoxy resin and cured. With thisroute, however, modulus improvement with CNF content was low. Significantimprovement in flexural modulus and strength (97% and 37% respectively) of anepoxy matrix by dispersing 18.2 vol% CNF were reported by Patton et al. [22]. Intheir study, the solution of epoxy and acetone was infused through a mat of CNFunder vacuum to prepare CNF/epoxy composites. A combination of sonication andmechanical stirring (at 1,000 rpm) was used by Choi et al. to disperse CNFs intoboth high viscosity and low viscosity (obtained by diluting with acetone) epoxyresins [23]. For both types of matrices, tensile strength and Young’s modulusattained maximum values at 5 wt% CNF content and deteriorated with furtherloading of CNF.

However, all of these studies attempted to disperse higher CNF concentrations toachieve significant improvement in the properties of epoxy. Dispersion of higherCNF content is difficult and leads to agglomeration resulting in less transfer of CNFproperties in composites. To overcome this problem, the main focus of our researchwas to achieve a homogeneous CNF dispersion in order to maximize the reinforcingefficiency of CNF and thereby to achieve significant property improvements ofepoxy at low CNF concentrations. One of our recent studies showed that animprovement of 98% in Young’s modulus, 24% in breaking stress and 144% in workof rupture of a rubbery epoxy matrix was possible by homogeneously dispersingonly 0.07 vol% of vapour-grown CNF [24]. The present study, however, investigatedthe mechanical properties (tensile, flexural and compressive) of a glassy epoxymatrix reinforced with low concentrations (0.1–0.5 wt%) of homogeneouslydispersed CNF.

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Experimental

Materials

Pyrograf III (PR24 AGLD) grade CNFs were obtained from Applied Sciences, Inc(USA). Properties of these CNFs are listed in Table 1. An epoxy resin based onbisphenol A diglycidyl ether (DGEBA) with epoxide equivalent weight of 172–176was supplied by Sigma Aldrich (India). A triethylene tetramine type hardener (HY951) supplied by Huntsman India Ltd. was used to cure the resin. Polyoxyethylene 4lauryl ether was used as a nonionic surfactant and obtained from Acros Organics(Belgium). AR grade acetone supplied by Qualigens Fine Chemicals (India) wasused in the dispersion studies.

Dispersion of CNF

CNF dispersions in the epoxy resin were prepared using various dispersiontechniques such as ultrasonication, high speed mechanical stirring, use of surfactant,solvent and higher temperature and analyzed with the help of an optical microscope(LEICA DMLP). Table 2 provides a list various dispersion routes used along withtheir codes. The effect of these techniques on the dispersion behaviour of CNF wasthoroughly investigated and the route which led to a homogeneous dispersion of0.1–0.5% CNF was selected for the preparation of nanocomposites. Ultrasonicationwas carried out using a bath sonicator (T490DH Transsonic Digital S from Elma)operated at 40 kHz with temperature maintained at 40 °C. Mechanical stirring wascarried out at 2,000 rpm using a high speed stirrer equipped with a digital speedcontroller. Acetone was used as the solvent due to its low boiling point to dilute theepoxy resin and the nonionic surfactant (Polyoxyethylene 4 lauryl ether) was used ata concentration of 0.2 wt% of resin.

Preparation of nanocomposites

The prepared CNF/epoxy dispersion was then kept under vacuum at 40 °C for 12 hto remove acetone and air bubbles introduced during the dispersion process. 15%

Table 1 Properties of pyrograf III CNF

Properties Valuesa Units

Diameter 60–150 nm

Length 30–100 μm

Density 1.8 gm/cm3

Tensile strength 2.7 GPa

Tensile modulus 400 GPa

Electrical resistivity 1×10−3 Ω cm

Thermal conductivity 20 W/m K

a Applied Sciences, Inc

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(by weight of resin) of HY 951 hardener was mixed properly with the CNFdispersed resin by mechanical stirring and the mixture was then kept under vacuumfor 10 min. The nanocomposites were cast in specially designed silicon rubbermoulds with dimensions as per ASTM test standards. The nanocomposites werecured at room temperature for 12 h and then post cured at 80 °C for another 2 h.

Characterization of nanocomposites

Dispersion of CNF within the nanocomposites prepared using the optimized dispersionroute was studied using the phase imaging technique of Atomic Force Microscopy(AFM, Digital Instruments NanoScope). Pure epoxy and CNF/epoxy composites weretested for tensile properties in Zwick Z020 Universal Testing Machine according toASTM D638-03 standard at 1 mm/min crosshead speed. Flexural and compressiveproperties were characterized according to ASTM D790-03 and ASTM 695-02a teststandards at 5 mm/min and 1.3 mm/min crosshead speeds respectively. Five samplesfrom each category were tested and the average value was calculated and reported.Fracture surfaces of the samples after tensile test were studied by a Scanning ElectronMicroscope (SEM, ZEISS, Model: Evo 50, type of filament: Tungsten) using a voltageof 20 kV.

Results and discussion

Effect of ultrasonication on CNF dispersion

Figure 1 shows the optical micrographs of epoxy resin dispersed with 0.1% CNFusing different durations of ultrasonic treatment. Large CNF agglomerates or clusterswere present in the 30 min sonicated dispersion as indicated by arrows in Fig. 1a.With increased ultrasonication time, the CNF agglomerates were broken down in tosmaller sizes and gradually dispersed in the resin. After 8 h of ultrasonication, CNFclusters were hardly visible in the micrographs at this level of magnification. Hence,a homogeneous dispersion of 0.1 wt% CNF free from agglomerates was obtainedafter 8 h of ultrasonication. However, some previous studies showed that theexposure to ultrasonic energy for longer durations leads to considerable CNF

Table 2 Coding of various dispersion routes

Dispersion codes CNF wt.% Dispersion route

1CNF2So/5CNF2So 0.1/0.5 2 h sonication

1CNF4So/5CNF4So 0.1/0.5 4 h sonication

1CNF2SoAc/5CNF2SoAc 0.1/0.5 2 h sonication+acetone

1CNF2SoMs/5CNF2SoMs 0.1/0.5 2 h sonication+1 h stirring

1CNF2SoSf/5CNF2SoSf 0.1/0.5 2 h sonication+surfactant

1CNFMs/5CNFMs 0.1/0.5 1 h stirring

1CNF2Ms/5CNF2Ms 0.1/0.5 2 h stirring

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breakage [24]. Therefore the duration of ultrasonic treatment was restricted to 2 hand combined with other dispersion techniques to achieve a homogeneousdispersion.

Effect of surfactant on CNF dispersion

The purpose of using nonionic surfactant during ultrasonication was to improve thewetting of CNF by epoxy resulting in an easier dispersion process and thereby toreduce the duration of ultrasonic treatment required. The effect of surfactant on thedispersion of 0.1% CNF is shown in Fig. 2a and b. The size of the CNF

Fig. 1 Effect of ultrasonication time on CNF dispersion as observed by optical microscopy at a 30 min b2 h c 4 h and d 8 h

Fig. 2 Effect of surfactant on CNF dispersion: optical micrograph of a 1CNF2So b 1CNF2SoSf c5CNF2So d & e 5CNF2SoSf and f fracture surface of cured 5CNF2SoSf as observed by SEM

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agglomerates were reduced considerably when ultrasonication was carried out in thepresence of surfactant (Fig. 2b) and a homogeneous dispersion was obtained. Hence,0.1% CNF was successfully dispersed using 2 h of ultrasonication assisted with0.2% nonionic surfactant. However, it was not possible to disperse 0.5% CNFhomogeneously either using ultrasonication or surfactant assisted ultrasonication.The dispersion of 0.5% CNF prepared using 2 h ultrasonication showed lots of CNFagglomerates (Fig. 2c). The use of surfactant during ultrasonication has improvedthe dispersion but could not remove large CNF clusters. Hence, the surfactantassisted dispersion showed homogeneously dispersed CNFs (Fig. 2d) as well aslarge CNF agglomerates at some places (indicated by arrows in Fig. 2e). Theseagglomerates can also be observed in the fracture surface of the nanocompositeprepared using this dispersion (Fig. 2f). Increasing surfactant concentration could behelpful to disperse 0.5% CNF. However, concentrations of more than 0.2 wt% werenot used in our study since the presence of surfactant in the final composites maydeteriorate the composite properties [16].

Effect of solvent and temperature on CNF dispersion

For dispersing 0.5% CNF, ultrasonication was carried out in the presence of asolvent (acetone) or at higher temperatures to assist the dispersion process byreducing resin viscosity. However, this idea did not prove much helpful due toconsiderable re-agglomeration of dispersed CNFs with time in the low viscositymedium. The quality of CNF dispersion deteriorated significantly because of thisre-agglomeration. Figure 3a–d shows the extent of CNF re-agglomeration after10 min due to acetone and different temperatures.

Fig. 3 Re-agglomeration of CNF as observed by optical microscopy a due to acetone b at 40 °C c at 60 °Cand d at 80 °C

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Effect of high speed stirring on CNF dispersion

High speed mechanical stirring at 2,000 rpm was previously used to homogeneouslydisperse single-walled CNTs in the epoxy resin [18]. Hence, this technique has beentried in our study for dispersing 0.5% CNF for which none of the other dispersiontechniques was found suitable. It can be seen from Fig. 4 that the dispersion of 0.5%CNF was greatly improved with the high speed stirring treatment and a very gooddispersion containing only a few small CNF agglomerates was obtained after 2 h oftreatment (Fig. 4b). Hence, this technique was combined with the ultrasonictreatment to disperse 0.5% or higher concentrations of CNF. Figure 4c and d showthe dispersion of 0.1% and 0.5% CNF respectively prepared using a combination of2 h ultrasonication and 1 h mechanical stirring. In both the cases, a homogeneous

Fig. 4 Effect of high speed stirring on CNF dispersion as observed by optical microscopy a 5CNFMs b5CNF2Ms c 1CNF2SoMs and d 5CNF2SoMs

Fig. 5 AFM phase image of 0.1% CNF/epoxy nanocomposite (a) and TEM micrograph of CNF (b)

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CNF dispersion free from large agglomerates was obtained. The AFM phase imageof 0.1% CNF/epoxy nanocomposite prepared with this dispersion route is shown inFig. 5a. The CNFs are indicated by arrows. The black spot inside CNF indicates thehollow core which can also be observed by Transmission Electron Microscopy(TEM) as shown in Fig. 5b. It is clear from the AFM image that CNFs arehomogeneously dispersed within the matrix. Hence, the combination of 2 hultrasonication and 1 h high speed stirring was used for the preparation of epoxynanocomposites containing CNF concentration of 0.1–0.5%.

Mechanical properties

Mechanical properties of epoxy and nanocomposites are listed in Tables 3, 4 and 5.It can be seen from Table 3 that the tensile properties of epoxy improvedsignificantly with the addition of vapour-grown CNFs. An improvement of 35% inYoung’s modulus and 63% in tensile strength was obtained by uniformly dispersing0.5% CNF using the optimized dispersion route. The fracture surface of neat epoxysample is smooth and featureless which is indicative of brittle fracture mechanism.Figure 6b shows the cleavage plane of epoxy which is smooth and flat. On thecontrary, the nanocomposites showed very rough fracture morphology with lots ofridges and wavy lines (Fig. 6c and d) which indicate ductile fracture mechanism andimproved mechanical properties. Dispersion of CNFs also improved the flexural andcompressive properties of epoxy significantly (see Tables 4 and 5). An improvementup to 31% in flexural modulus, 44% in flexural strength, 30% in compressivemodulus and 10% in compressive strength was achieved by dispersing CNF up to0.5%. However, it is interesting to note that the improvement in mechanicalproperties slowed down with increase in CNF%. This is due to the fact that at higherconcentration it is extremely difficult to disperse CNF due to agglomeration and theefficiency of the selected dispersion route becomes less. Hence, for dispersing CNF

Table 3 Tensile properties of epoxy and nanocomposites

Materials Young’s modulus (GPa) Tensile strength (MPa)

Epoxy 2.35±0.15 38.5±3.5

0.1% CNF/Epoxy 2.92±0.02 61.4±3.2

0.3% CNF/Epoxy 2.96±0.16 59.3±3.6

0.5% CNF/Epoxy 3.18±0.05 62.8±3.8

Table 4 Flexural properties of epoxy and nanocomposites

Materials Flexural modulus (GPa) Flexural strength (MPa)

Epoxy 2.04±0.14 51.9±5.4

0.1% CNF/Epoxy 2.66±0.03 66.5±6.2

0.3% CNF/Epoxy 2.68±0.05 74.5±0.7

0.5% CNF/Epoxy 2.58±0.10 75.0±0.1

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concentration more than 0.5%, it is recommended to increase the duration of bothultrasonic and mechanical stirring treatment to achieve a homogeneous dispersion.

Conclusions

This paper investigated the method of dispersing vapour-grown CNFs homogeneouslyin an epoxy resin and the potential of well dispersed CNFs to improve mechanicalproperties of epoxy nanocomposites. Ultrasonication was found to be an effectivetechnique for dispersing CNFs. However, it was necessary to carry out ultrasonicationfor longer duration to achieve a homogeneous dispersion. Reduction in the treatmenttime was possible by using a non-ionic surfactant during ultrasonication and a very gooddispersion of 0.1% CNF was obtained using only 2 h of ultrasonication assisted with0.2% non-ionic surfactant. However, surfactant assisted ultrasonication could notdisperse large CNF agglomerates in case of higher CNF concentration (0.5% or higher).A combination of ultrasonication and high speed mechanical stirring at 2,000 rpm wasfound to be the best dispersion route for dispersing vapour-grown CNFs in epoxy resin.This route was found suitable for dispersing low as well as high concentrations of CNF.

Table 5 Compressive properties of epoxy and nanocomposites

Materials Compressive modulus (GPa) Compressive strength (MPa)

Epoxy 2.30±0.17 100.1±1.2

0.1% CNF/Epoxy 2.93±0.09 110.1±1.6

0.3% CNF/Epoxy 2.89±0.10 105.0±3.1

0.5% CNF/Epoxy 3.00±0.01 109.2±2.4

Fig. 6 Fracture surface of neat epoxy and nanocomposites as observed by SEM a neat epoxy at 500 ×magnification b neat epoxy at 30,000× magnification c cured 1CNF2SoMs and d cured 5CNF2SoMs

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Nanocomposites prepared using this route showed very good distribution of CNFswithin the matrix as detected by AFM phase imaging technique. Dispersion of CNF upto 0.5% using this dispersion technique led to an improvement of 35% in Young’smodulus, 63% in tensile strength, 31% in flexural modulus, 44% in flexural strength,30% in compressive modulus and 10% in compressive strength of epoxy.

Acknowledgements The authors would like to thank Prof. Tibor Czigany (Budapest University ofTechnology and Economics, Department of Polymer Engineering, Hungary) and his team for his kindsupport in mechanical testing of CNF/epoxy composites.

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