石墨片对环氧树脂的热学、力学和电学性能影响

Subhra Gantayat, Gyanaranjan Prusty, Dibya Ranjan Rout, Sarat K Swain

(1.Department of Chemistry,Veer Surendra Sai University of Technology,Burla,Sambalpur768018,India；2.School of Applied Science(Physics),KIIT University,Bhubaneswar751024,India)

石墨片对环氧树脂的热学、力学和电学性能影响

Subhra Gantayat1,2, Gyanaranjan Prusty1, Dibya Ranjan Rout2, Sarat K Swain1

(1.Department of Chemistry,Veer Surendra Sai University of Technology,Burla,Sambalpur768018,India；2.School of Applied Science(Physics),KIIT University,Bhubaneswar751024,India)

采用溶液技术制备出膨胀石墨增强环氧树脂复合材料。对石墨进行化学改性以提高与环氧树脂的相容性。采用XRD、FE-SEM和HR-TEM对环氧树脂/膨胀石墨复合材料进行表征。与环氧树脂相比,添加质量分数9%膨胀石墨后,该复合材料的热分解温度从340℃升高至480℃,抗张应力提高30%,导电率由10-15增加至10-5数量级。热学、力学和电学性能的显著提高,主要归因于膨胀石墨纳米片在环氧树脂基体中的良好分散性,从而可用于广泛的应用领域。

膨胀石墨；扫描电镜；透射电镜；导电率

1 Introduction

Polymer matrix composites are multi-phase materials produced by combining polymer resins with reinforcing fillers having improved properties in comparison with the matrix materials.Hence,different fillers are used to enhance the physical and mechanical properties of composites.Polymer matrix composites are of scientific and industrial interest because of their enhanced properties arising from the reinforceing function of nanofillers［1-4］.Different conducting fillers such as carbon nanotubes and graphite have been extensively studied because of their ability to increase the mechanical,thermal and electrical properties of the native polymers［5,6］.

Epoxy resins are a class of thermoset materials available in various forms from low viscosity liquid to high melting solids,which are widely used as polymer matrices in composites,owing to their high strength,low shrinkage,excellent adhesion to substrates,chemical resistance and low cost.Most of polymers are generally electrical insulators with very low concentrations of free charge carriers.Thus they are non-conductive and transparent to electromagnetic radiations.This property made them incapable for the use as enclosures for electronic equipments.Hence, these limitations are the causes of growing research activities for electrically conducting polymers.Conducting polymers can be either inherently conductive or insulating polymers composited with conductive fillers.Conductive composites are used in light emitting devices,batteries,electromagnetic shielding andother functional applications［7-9］.Conductive fillers such as carbon black,carbon nanotubes and graphite have been extensively investigated［10-16］.These fillers effectively improve the electrical conductivity of the polymers.The significant increase in electrical conductivity with the filler content has been observed for most composites,which could be explained by the percolation transition from the formation of the conductive network［17］.

In comparison to carbon nanotubes,graphite continues to attract considerable attentions because of their mechanical and electrical properties,low density,easy processing and low cost.Graphite exists as a layered material and the layers are packed closely by Van der Waals′force.For an efficient utilization of graphite as filler in a polymer composite,its layers must be partly separated to obtain expanded graphite that is dispersed throughout the polymeric matrix. Also in its natural form,little reactive groups exist on the graphite and as a result,it is difficult to intercalate monomers into the graphite interlayer to form a composite.If the raw graphite is used as reinforcement,it is not possible to disperse graphite layers in epoxy matrix.The EG is prepared when raw graphite exposed to strong oxidizers such as nitric acid(HNO3), sulphuric acid(H2SO4)or potassium permanganate (KMnO4).In comparison to raw graphite,the EG sheets are heavily oxygenated having hydroxyl and epoxide functional groups on their basal planes,in addition to carbonyl and carboxyl groups located at the sheet edges.The presence of these functional groups makesthem strongly hydrophilic.EG can be readily dispersed in water and incorporated into polymer matrices with a help of these functional groups for the preparation of composites.Chen et al.［18］measured the tensile strength of the EG/polystyrene composite and found that its tensile strength is a little higher that of the pure polystyrene.Kim et al.［19］compared the thermal property of virgin polylactic acid withthat of the EG/polylactic acidcomposites,and found that the thermal stability of the composites increased with the EG content.Xiao et al.［20］measured the thermal property of the polystyrene/graphite composite and reported a thermal degradation temperature of the composite 20℃ higher than that of pure polystyrene.

Though graphite was extensively investigated as filler in polymer matrix composites,EG was paid less attention.In the present study,the dispersion of EG in epoxy matrix to prepare EG/epoxy composites was investigated to reveal its influence on the mechanical, thermal and electrical properties of EG/epoxy composites.

2 Experimental

2.1 Materials

Epoxy resin was purchased from Merck,India. Concentrated H2SO4and HNO3were analytical grade chemicals and used directly without any further purification.Graphite fine powder with an average diameter of 500 μm was purchased from Loba chemical Pvt.Ltd.,India for preparing the EG.

2.2 Preparation of EG

Raw graphite was first dried in a vacuum oven for 24 h at 100℃.Then a mixture of concentrated H2SO4and HNO3with a volume ratio of 4∶1 was added slowly to a glass flask containing graphite powder with vigorous stirring.After 24 h of reaction,the acid treated graphite powder was filtered and washed with deionised water until the pH value of the filtrate reached 6.4.After drying at 100℃ for 24 h,the resulting graphite intercalation compound was subjected to a thermal shock at 900℃ for one minute in a furnace to form the EG.

2.3 Synthesis of EG/epoxy composites

EG/epoxy composites were synthesized to have different contents of EG(3,6,and 9 wt%based on epoxy weight)by a solution mixing method.Calculated amount of epoxy and EG were separately dispersed in deionised water at ambient temperature via stirring for 0.5 h.The EG suspension was added to the epoxy solution and stirring was continued for 3 h. The resulting solution was centrifuged for 15 min and the resulting sample was dried in an oven at 50℃. The detail synthetic process is illustrated in Fig.1.

Fig.1 Schematic representation for the preparation of EG/epoxy composite.

2.4 Characterization

X-ray diffraction(XRD)of the composites was carried out by a Rigaku X-ray diffractometer(Model No.P.DD966)with Cu Kα radiation at 40 kV and 150 mA.The morphology and dispersion of the EG in epoxy resin were investigated by using a field emission scanning electron microscope (JEOL-JSM-5800).A high resolution transmission electron microscope(Tec-nai 12,Philips)operating at 120 kV was used to study the dispersion of EG in epoxy matrix.Mechanical properties of the EG/epoxy composites were measured with ASTM-D-638-00 using an Instron testing machine(Model-5 567)and the test was performed at a rate of 50 mm/min with a load of 0.5 ton.The five specimens for each composition were used for measurement and average values are reported.The TGA analysis was carried out by taking the sample in the pan(8-10 mg)and the temperature was increased by 10℃ per minute and heated up to 800℃.Conducting measurment was carried out using LCR-Hi Tester,HOIKI after the sample being processed into petlet form.

3 Results and discussion

3.1 Structural Analysis

The XRD patterns of raw graphite(RG),epoxy and the EG/epoxy composites are shown in Fig.2. The raw graphite exhibits a sharp diffraction peak at 2θ value of 26.36°.The peaks at 2θ values of 77°, 54°and 44°belong to epoxy resin and the peak at 2θ values of 26.36°is ascribed to graphite.All the above peaks of epoxy and graphite are present in the EG/epoxy composites confirm the formation of composites.Similarly,the FE-SEM image of EG is shown in Fig.3a.It is found that the EG changes into sheets with thickness about 60-70 nm.Fig.3b,c and d show the FE-SEM images of the EG/epoxy composites at 3%,6%and 9%of EG respectively. In all these micrographs,the white spots indicate the epoxy matrix,whereas,the dark spots represent the EG sheets.The good dispersion of EG sheet in epoxy matrix directly correlates with its effectiveness for improving mechanical,thermal and electrical properties, which is another indirect evidence for a better interfacial adhesion between epoxy resin and EG.The similar results have been reported in the earlier literatures［21,22］.Due to the delamination nature of EG layers,epoxy molecules easily enter into the graphite layers to form an exfoliated structure.Fig.4a shows the HR-TEM image of the EG.The dispersion state of EG sheets in the HR-TEM of epoxy resin is shown in Fig.4b.Further,it is noticed that the EG sheets are distributed in the epoxy resin with some local agglomerations.

3.2 Thermal properties

Thermogravimetric analysis(TGA)is used to study the thermal properties of EG,epoxy and the EG/epoxy composites as shown in Fig.5.It is found that the thermal decomposition temperatures of the composites in all samples shift towards high temperatures as compared with that of virgin epoxy.The thermal degradation temperature for epoxy resin is 340℃ while those of the EG/epoxy composites are 360,440 and 480℃ at 3%,6%and 9%of EG respectively. So the addition of EG lowers the thermal degradation rate of epoxy matrix.The residual weight of the EG/ epoxy composites is higher than that of epoxy resin. The residual weight of the EG/epoxy composites are 8%,24%and 30%for the composites containing 3%,6%and 9%of EG respectively,whereas,in case of epoxy no residue is left.The high residual mass of the composites is due to strong compatibility and interaction of EG with epoxy resin.Thus,the increased thermal degradation temperature for the EG/ epoxy composites indicate the enhancement of thermal stability of epoxy resin by EG.Otherwise,EG is acting as a thermal stabilizer［23-25］for epoxy resin,which could have a wide range of potential applications.

Fig.2 XRD patterns of Epoxy and EG/epoxy composite at different percentage of EG concentration and XRD of raw graphite(Inset).

3.3 Mechanical properties

Mechanicalpropertiesincluding extension at break,load at break,tensile stress and tensile strain of epoxy resin and its composites with different EG percentages are compared in Fig.6.The extension at break of the EG/epoxy composites decreases with the EG percentages(Fig.6(a)).It is interesting to note that extension at break is reduced by 3 times with an addition of 3%of EG.The load at break of the composites increases monotonically with the EG percentages.From Fig.6(b),load at break of the composites increases by approximately 6 times as compared with that of raw epoxy resin.It may be due to strong interfacial adhesion between EG and epoxy matrix. Further,it is observed that tensile stress increases with ithe EG percentages(Fig.6(c)).However,the tensile strain of the EG/epoxy composite at 9%of EG is reduced by 9 times in comparison with that of epoxy resin.A sudden fall of tensile strain of the composites is noticed by an addition of 3%of EG(Fig.6(d)). It may be due to the uniform dispersion of EG sheets within the epoxy matrix.Hence due to the rigidity of epoxy,EG sheets cannot be deformed by external stressin the composite specimen but act as stress concentration during the deformation process of the composites.From the results of mechanical properties,it is remarked that the dispersion state of EG sheetsin epoxy matrix played a vital role in decreasing the strain at break and increasing the tensile strength of the composites.

Fig.3 FE-SEM images of(a)expanded graphite and EG/epoxy composite at graphite concentration of(b)3%,(c)6%,(d)9%.

Fig.4 HR-TEM images of(a)expanded graphite(b)EG/epoxy composite at 9%of EG concentration.

Fig.5 TGA curves of(a)epoxy,(b)EG/epoxy,3% (c)EG/epoxy,6%(d)EG/epoxy,9%(e)expanded graphite.

3.4 Electrical conductivity

Electrical conductivity of a composite generally depends upon the particle size,extent of dispersion and structure of conducting nanofillers as well as the properties of host polymers.The addition of conductive nanofillers to an insulating polymer can result in an electrically conductive composite,if the filler concentration exceeds the percolation threshold,which is defined as the minimum amount of filler required for the formation of a three dimensional conductive network within the polymer matrix.The EG/polymer composites exhibit a very low percolation threshold for electrical conductivity because of a large aspectratio and the nanoscale dimension of the EG in polymer matrix.

Fig.6 Mechanical properties of EG/epoxy composites as a function of EG concentration for study of(a)extension at break(b)load at break(c)tensile stress at break(d)tensile strain at break.

Fig.7 shows the variation of the electrical conductivity of the EG/epoxy composites as a function of EG content.The addition of EG within epoxy improves its conductivity significantly with a sharp transition from an electrical insulator to an electrical conductor.The conductivity as a function of EG content is plotted at constant frequency and it is found that the conductivity increases with the EG content from 3% to 9%.The increase in conductivity with EG content from 3%to 6%is used to determine the percolation threshold,a critical value at which a three dimensional conductive network is formed.The conductivity of epoxy is about 2.3×10-15S/cm in the initial stage, which is regarded as a typical insulator.The conductivity of the composites is about 2.1×10-5S/cm at 9% of EG,which is nearly a typical conductor. Hence,an incorporation of EG into epoxy resin increases the electrical conductivity significantly due to a good dispersion.The observations in this paper are in good agreement with those of our earlier reports［26,27］.The epoxy resin composites reinforced by EG are good antistatic materials(conductivity～10-5S·cm-1).

Fig.7 Electrical conductivities of the EG/epoxy composites as a function of EG content.

4 Conclusions

A series of EG/epoxy composites were prepared by a solution mixing method.The interaction of EG with epoxy matrix was investigated.The structure and morphology of the composites were studied by XRD and electron microscopy.The thermal,mechanical and electrical properties of epoxy resin are improved with increasing EG contents.In the EG/epoxy composites,EG sheets plays a vital role in decreasing the strain at break and increasing the tensile strength of the composites as compared with those of epoxy res-in.The thermal stability of epoxy resin is enhanced with increasing the EG percentages.The mechanical and thermal properties of epoxy are improved due to the strong interfacial adhesion of the EG with epoxy matrix.Moreover,the epoxy resin is converted into electrically conductive materials by dispersing EG sheets into epoxy matrix.

Acknowledgements

The authors are thankful to Department of Atomic Energy,BRNS,and Government of India for providing financial support under Grant OM#2008/20/ 37/5/BRNS/1936.Authors are also thankful to Dr. D.Das of Inter University Consortium,Kolkata,India for analysis of XRD.

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Expanded graphite as a filler for epoxy matrix composites to improve their thermal,mechanical and electrical properties

Subhra Gantayat1,2, Gyanaranjan Prusty1, Dibya Ranjan Rout2, Sarat K Swain1
(1.Department of Chemistry,Veer Surendra Sai University of Technology,Burla,Sambalpur768018,India; 2.School of Applied Science(Physics),KIIT University,Bhubaneswar751024,India)

Expanded graphite(EG)-reinforced epoxy composites were prepared by a solution mixing method.The structure and morphology of the EG/epoxy composites were investigated by XRD,FE-SEM and HR-TEM.The EG prepared by acid oxidation and thermal expansion shows good compatibility with the epoxy resin that enters the EG layers to decrease their thickness to 60-70 nm,owing to its abundant oxygen-containing functional groups.With the addition of 9 wt%EG,the thermal decomposition temperature of the composite increases from 340 to 480℃,the electrical conductivity from 10-15 to 10-5 S/cm and the tensile stress is increased by more than 30%.These improvements are attributed to the good dispersion of EG sheets in the epoxy matrix.

Expanded graphite；FE-SEM；HR-TEM；Conductivity

Sarat K Swain.E-mail:swainsk2@yahoo.co.in

TB332

A

Sarat K Swain.E-mail:swainsk2@yahoo.co.in

1007-8827(2015)05-0432-06

10.1016/S1872-5805(15)60200-1

Received date:2015-03-05； Revised date:2015-10-08

English edition available online ScienceDirect(http://www.sciencedirect.com/science/journal/18725805).