Journal of
composites science
Article
Tuning Electrical and Thermal Properties in Epoxy/Glass Composites by Graphene-Based Interphase Haroon Mahmood 1 , Seraphin H. Unterberger 2,3 and Alessandro Pegoretti 1, * 1 2 3
*
ID
Department of Industrial Engineering, University of Trento, via Sommarive, 9-38123 Trento, Italy;
[email protected] Unit for Material Technology, University of Innsbruck, Technikerstraβe 13, 6020 Innsbruck, Austria;
[email protected] Christian Doppler Laboratory for Cement and Concrete Technology, Technikerstraße 13, 6020 Innsbruck, Austria Correspondence:
[email protected]; Tel.: +39-0461-282452
Received: 3 October 2017; Accepted: 25 October 2017; Published: 28 October 2017
Abstract: Multiscale epoxy/glass composites were fabricated by using E-glass fibers (GF) coated with different types of graphene nanosheets deposited by electrophoretic deposition. Graphene oxide (GO) was first synthesized using modified Hummer’s method and its subsequent ultrasonication in de-ionized water created a stable suspension of GO. GF were immersed in the water/GO suspension near a copper anode. The electrical potential applied between the electrodes caused GO to migrate towards the anode. Moreover, the GO coated yarns were exposed to hydrazine hydrate at 100 ◦ C to obtain reduced graphene oxide (rGO) coated yarns. Both GO and rGO coated GF yarns were used to create unidirectional epoxy-based multiscale composites by hand lay-up. The presence of a conductive rGO coating on GF improved both the electrical and thermal conductivities of composites. Moreover, enhanced permittivity was obtained by rGO based epoxy/glass composites, thus giving the option of using such structures for electromagnetic interference shielding. Keywords: multifunctional composites; graphene; fiber-matrix interphase; thermal conductivity; electrical resistivity
1. Introduction Due to their excellent mechanical properties and thermal and chemical stability, fiber reinforced polymer composites play an important role for high-end applications in various industrial sectors. Due to their prominent tensile mechanical properties and relatively low cost as compared to carbon fibers, E-glass fibers (GF) are often used to reinforce polymer matrices [1–3]. However, such composites have limitations for some applications due to a higher density and low electrical and thermal conductivities. Graphene has been in the spotlight of research since its discovery [4]. In fact, the extraordinary properties of this two-dimensional (2D) material, such as remarkably high electron mobility at room temperature (250,000 cm2 /V), high thermal conductivity (5000 Wm−1 ·K−1 ), and mechanical stiffness and strength (Young’s modulus = 1 TPa, strength 130 GPa) [5–7], make it an ideal candidate for the preparation of nanocomposites. Previous studies have confirmed that the dispersion of graphene in polymeric matrices resulted in not only improving mechanical properties [8], but also providing functional properties like electrical conductivity [9–11], thermal conductivity [12], dielectric properties [13–15], and electromagnetic interference shielding [15–17]. Moreover, recent studies have also indicated that an optimized amount of graphene nanoparticles dispersed in the polymer matrix can play a positive role in enhancing the fiber/matrix interfacial adhesion between both thermoplastic and thermosetting polymer matrices and glass fibers [18–20]. J. Compos. Sci. 2017, 1, 12; doi:10.3390/jcs1020012
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A substantial improvement of the properties of composites has been obtained by a very low nanofiller content, which has been attributed to the large surface area of the nanofiller and also to their high aspect ratio. Currently, the scale up to industrially relevant processes of improvement in polymer properties by nanofillers dispersion is still a big challenge since (i) achieving uniform dispersion of nanofiller in polymers is not easy; (ii) an adequate interfacial adhesion between matrix and nanofiller is a big challenge and most importantly; and, (iii) alignment of nanofillers cannot be achieved easily [21]. Both computational simulations and theoretical modelling have revealed significant advantages that could be realized by nano-scale fillers oriented in polymer matrices. Much work has been conducted to develop methods for aligning carbon nanofillers in polymer matrices. Looking mainly on carbon nanotubes (CNTs) and graphene nanosheets, remarkable improvements in mechanical and functional properties have been reported as compared to randomly-dispersed nanofillers. Various methods have been proposed to align fillers in polymer matrices like mechanical shearing [22], manufacturing under applied electric [23–25], and magnetic fields [25–27]. Alignment of nanofillers in an electrical field is considered as an effective method but the restraint of this technique is that it can only be applied to materials with very low electrical conductivity, since the field strength is usually restricted to avoid the dielectric breakdown of the polymer [28]. On the other hand, low magnetic susceptibility of fillers means that strong magnetic field (25 T or more) is required to align nanofillers like CNT and graphene, thus restricting the practical application of such methods [28]. Simultaneous dispersion and alignment can be obtained by using mixing equipment able to apply high shear forces. Unfortunately, these forces are often not large enough to break the nanofiller aggregates and disperse the nanofillers in the polymer matrix. Instead, high shear has the drawback to degrade both polymers and nanofillers [29]. The aim of this work is hence to create new multifunctional epoxy/glass hierarchical composites containing GO and rGO nanosheets. In particular, a method of utilizing GO and rGO is proposed in which electrophoretic deposition (EPD) technique is used to deposit GO nanosheets on E-glass fibers (GF); while, for rGO, the same GO coated GF were subjected to a thermochemical reduction process, hence yielding rGO coated GF. The mechanical properties of the composites obtained by using both GO and rGO coated GF fibers aligned in an epoxy matrix were extensively investigated in our previous works both from the micromechanical [30] and macromechanical [31] points of view. The peculiar electrical (resistivity and permittivity) and thermal (stability and conductivity) behaviour induced in the epoxy/glass composites by the presence of GO/rGO interphases are experimentally investigated in this manuscript. 2. Materials and Methods Graphite powder, potassium permanganate, sodium nitrate, sulfuric acid, and hydrogen peroxide were purchased from Sigma Aldrich (St. Louis, MO, USA) while hydrochloric acid was from Codec Chemical Co. Ltd. (Tokyo, Japan). All of the chemicals were of analytical grade and used without further purification. A bi-component epoxy resin, provided by Elantas Italia S.r.l. (Collecchio, Italy), consisting of an epoxy base (EC 252) and an aminic hardner (W 241), was selected as polymer matrix. E-glass fibers, with the trade name XG 2089, were kindly supplied by PPG Fiber Glass© (Pittsburgh, PA, USA), and were used as received. The fibers, with an average fiber diameter was 16.0 ± 0.1 µm, were supplied with an epoxy compatible silane-based sizing. GO was synthesized according to a modified Hummer’s method. In particular, 1 g of graphite powder was mixed in 46 mL of H2 SO4 cooled in an ice bath, followed by the addition of 1 g of NaNO3 and the mixture was stirred for 15 min. In the next step, 6 g of KMnO4 were gradually added, while keeping the temperature of the mixture under 20 ◦ C in order to avoid exothermic reactions. The mixture was stirred for at least 24 h at 35 ◦ C. Then, a surplus of deionized water was added while the temperature was kept between 60 ◦ C and 80 ◦ C. At the end, a solution of 30% H2 O2 with deionized water was added to the mixture to stop the reaction. The resulting suspension was thoroughly washed using an HCl solution and distilled water to remove Mn ions and acid respectively. The obtained brown solution was dried in a vacuum oven at 50 ◦ C for at least 36 h. The brown cake obtained was
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obtained brown solution was dried in a vacuum oven at 50 °C for at least 36 h. The brown cake obtained was then used create GO(1suspension (1 mg/mL) adding of a quantity of it in water deionized then used to create GO to suspension mg/mL) by adding aby quantity it in deionized and water and subjecting it to ultrasonication to create a stable suspension. subjecting it to ultrasonication to create a stable suspension. A A schematic schematic description description of of the the EPD EPDprocess process used used to to deposit deposit GO GOnanosheets nanosheetson onGFs GFsisisillustrated illustrated in Figure 1. The electrodes were copper sheets of 1 mm thickness and GF were placed in Figure 1. The electrodes were copper sheets of 1 mm thickness and GF were placedin infront frontof of an an electrode electrode using usingaametallic metallicwindow. window. This Thisapproach approachisisneeded neededdue duetotothe thenon-conducting non-conductingnature natureofofGF. GF. Therefore, Therefore, in in order order to to deposit deposit the the nanoparticles nanoparticleson onthe the GF, GF, the the anode anode was was placed placed behind behind the the fibers fibers during the deposition process. The electrodes along with a metallic frame holding GF bundles during the deposition process. The electrodes along with a metallic frame holding GF bundleswere were immersed in a GO dispersion in deionized water with a concentration of 1 mg/mL. When voltage is immersed in a GO dispersion in deionized water with a concentration of 1 mg/mL. When voltage applied between the electrodes, GO nanosheets move toward the anode, hit GFs and deposit on their is applied between the electrodes, GO nanosheets move toward the anode, hit GFs and deposit on surface. The deposition on oneonside the of fibers was carried out atout 10 V/cm for 5 min, the same their surface. The deposition oneofside the fibers was carried at 10 V/cm for 5and min, and the was repeated while reversing the metallic frame to expose the uncoated portion towards cathode same was repeated while reversing the metallic frame to expose the uncoated portion towards cathode while while coated coated side side of of the the fibers fibers towards towards the the anode. anode. After After the the whole whole coating coating process, process,the thefibers fibers were were dried under vacuum at 50 °C for at least 12 h. dried under vacuum at 50 ◦ C for at least 12 h.
Figure Figure1.1.Schematic Schematicdiagram diagramof ofthe theelectrophoretic electrophoreticdeposition deposition setup. setup.
Fibers coated were placed in a glass container and the reduction of the GO coating was carried Fibers coated were placed in a glass container and the reduction of the GO coating was carried out by placing a tissue paper soaked with hydrazine hydrate (N 2H4) in the container. The container out by placing a tissue paper soaked with hydrazine hydrate (N2 H4 ) in the container. The container was covered and then heated at 100 °C for 24 h. was covered and then heated at 100 ◦ C for 24 h. Composites were produced by a conventional hand lay-up method. Uncoated GF, GO coated Composites were produced by a conventional hand lay-up method. Uncoated GF, GO coated GF, and rGO coated GF bundles were wetted with the uncured epoxy resin by using a roller and GF, and rGO coated GF bundles were wetted with the uncured epoxy resin by using a roller and stacked in a metal mold. After laminating a number of laminas enough to reach the desired laminate stacked in a metal mold. After laminating a number of laminas enough to reach the desired laminate thickness (which in turns depends on the type of test performed), a constant pressure of thickness (which in turns depends on the type of test performed), a constant pressure of approximately approximately 1 kPa was applied on the mold, and curing was performed by pre-curing at room 1 kPa was applied on the mold, and curing was performed by pre-curing at room temperature for at temperature for at least 3 h and ◦then for 15 h at 60 °C. The obtained composites were termed as Epleast 3 h and then for 15 h at 60 C. The obtained composites were termed as Ep-GF, Ep-GO-GF, and GF, Ep-GO-GF, and EprGO-GF, respectively. EprGO-GF, respectively. X-ray diffraction (XRD) measurements were performed with a X-ray diffractometer (Rigaku III X-ray diffraction (XRD) measurements were performed with a X-ray diffractometer (Rigaku III Dmax, Tokyo, Japan) with a monochromatic radiation source (Cu-Kα, wavelength around 51.54056 Å ). Dmax , Tokyo, Japan) with a monochromatic radiation source (Cu-Kα, wavelength around 51.54056 Å). The measurements were carried out in a 2θ range of 5°–80° with a step size of 0.04°. The measurements were carried out in a 2θ range of 5◦ –80◦ with a step size of 0.04◦ . Field emission scanning electron microscopy (FESEM) microscopic observations were Field emission scanning electron microscopy (FESEM) microscopic observations were performed performed by a Zeiss Supra 40 microscope (Carl Zeiss AG, Oberkochen, Germany). Before by a Zeiss Supra 40 microscope (Carl Zeiss AG, Oberkochen, Germany). Before observations, the observations, the specimens were coated by a platinum/palladium alloy (80:20) layer of about 5 nm specimens were coated by a platinum/palladium alloy (80:20) layer of about 5 nm in thickness. in thickness. The cross-section of multiscale composites was observed by optical microscopy technique (using The cross-section of multiscale composites was observed by optical microscopy technique (using a Zeiss Axiophot optical microscope (Carl Zeiss AG, Oberkochen, Germany), connected to a Leica a Zeiss Axiophot optical microscope (Carl Zeiss AG, Oberkochen, Germany), connected to a Leica DC300 digital camera (Leica microsystems, Wetzlar, Germany) and by FESEM. The specimens were DC300 digital camera (Leica microsystems, Wetzlar, Germany) and by FESEM. The specimens were
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prepared surface using using abrasive abrasive grinding grinding papers papers with with grit grit size size 800, 800, prepared by by polishing polishing the the cross-sectional cross-sectional surface 1200, and 4000, sequentially. 1200, and 4000, sequentially. Two electrical behavior behavior of of Two different different resistivity resistivity measurement measurement methods methods were were utilized utilized based based on on the the electrical 66 Ω ·cm, the the fabricated composite materials. For specimens having resistivity levels more than 10 the fabricated composite materials. For specimens having resistivity levels more than 10 Ω·cm, the electrical using a Keithley 8009 8009 resistivity test chamber connected to a Keithley electrical resistivity resistivitywas wasevaluated evaluated using a Keithley resistivity test chamber connected to a 6517A high-resistance meter. However, forHowever, more conductive samples, a 6-1/2-digit electrometer/high Keithley 6517A high-resistance meter. for more conductive samples, a 6-1/2-digit resistance system (Keithley model 6517A) was used and a6517A) 2-points electrical measurement was chosen electrometer/high resistance system (Keithley model was used and a 2-points electrical as test configuration. Electrical resistivity was measured alongresistivity three orthogonal directionsalong as defined measurement was chosen as test configuration. Electrical was measured three in Figure 2. directions as defined in Figure 2. orthogonal
y
z x
Figure 2. 2. Description thermal Figure Description of of directions directions in in terms terms of of orientation orientation for for electrical electrical resistivity resistivity and and thermal conductivity measurements. conductivity measurements.
The measurement of relative permittivity and dielectric loss of the composites (uncoated and The measurement of relative permittivity and dielectric loss of the composites (uncoated and graphene reinforced composites) were performed by an Agilent 4284A impedance analyzer (Agilent graphene reinforced composites) were performed by an Agilent 4284A impedance analyzer (Agilent Technologies, Santa Clara, CA, USA) in the frequency range from 20 up to 106 Hz. The dimensions of Technologies, Santa Clara, CA, USA) in the frequency range from 20 up to 106 Hz. The dimensions the specimens were around 10 mm × 10 mm × 1.5 mm, and an aluminum foil was used as conductive of the specimens were around 10 mm × 10 mm × 1.5 mm, and an aluminum foil was used as electrode plate placed on the top and below of the specimen to create the parallel plate testing conductive electrode plate placed on the top and below of the specimen to create the parallel plate configuration. testing configuration. The thermal stability of the multiscale composites were investigated using thermogravimetric The thermal stability of the multiscale composites were investigated using thermogravimetric analysis (TGA) using a Mettler TG50 thermobalance (Mettler-Toledo, Columbus, OH, USA). Around analysis (TGA) using a Mettler TG50 thermobalance (Mettler-Toledo, Columbus, OH, USA). 40 mg of the specimens were selected of epoxy and composites, respectively. The tests were Around 40 mg of the specimens were selected of epoxy and composites, respectively. The tests conducted between 25 °C and 700 °C using a heating rate of 10 °C where the onset temperature were conducted between 25 ◦ C and 700 ◦ C using a heating rate of 10 ◦ C where the onset temperature (associated to a mass loss of 5%) and the residual mass at 700 °C were determined. The maximum (associated to a mass loss of 5%) and the residual mass at 700 ◦ C were determined. The maximum degradation temperature was evaluated from the main peak of mass loss rate curves. degradation temperature was evaluated from the main peak of mass loss rate curves. Thermal conductivity measurements were hence performed by a Netzsch Laser Flash Analysis Thermal conductivity measurements were hence performed by a Netzsch Laser Flash Analysis LFA 447 (Netzch, Selb, Germany). It consists of exposing an energy pulse from a light source (laser LFA 447 (Netzch, Selb, Germany). It consists of exposing an energy pulse from a light source (laser or xenon flash lamp) on one side of a specimen (10 mm × 10 mm × 2 mm) [32] and measuring the or xenon flash lamp) on one side of a specimen (10 mm × 10 mm × 2 mm) [32] and measuring the temperature history on the other side using a liquid nitrogen cooled infrared detector. The thermal temperature history on the other side using a liquid nitrogen cooled infrared detector. The thermal conductivity of the specimens was measured along three orthogonal directions (x-axis, y-axis and conductivity of the specimens was measured along three orthogonal directions (x-axis, y-axis and z-axis z-axis of Figure 2) and at three different temperatures i.e., 25 °C, 50 °C and 75 °C by performing three of Figure 2) and at three different temperatures i.e., 25 ◦ C, 50 ◦ C and 75 ◦ C by performing three shots, shots, respectively. The data were analyzed using the software Proteus (Netzch, Selb, Germany) respectively. The data were analyzed using the software Proteus (Netzch, Selb, Germany) whereas whereas Cowan method was used to calculate thermal diffusivity (α) with pulse correction. A Cowan method was used to calculate thermal diffusivity (α) with pulse correction. A standard Pyrex standard Pyrex 7740 reference material prepared according to ASTM-E 1461 was used to determine 7740 reference material prepared according to ASTM-E 1461 was used to determine the heat capacity the heat capacity (cp) and then was matched with the samples. Sample density () was determined by (cp ) and then was matched with the samples. Sample density (ρ) was determined by measuring the measuring the mass and volume of the specimen. Finally, the thermal conductivity (λ) was calculated mass and volume of the specimen. Finally, the thermal conductivity (λ) was calculated using the using the following equation: following equation: × cp× cp λ = αλ×= ρα×
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3.1. Graphene Oxide Synthesis and Its Reduction Figureand 3 shows the characterization results of synthesized graphene nanosheets. The X-ray 3. Results Discussion diffractograms of precursor graphite, synthesized GO and rGO products are exhibited in Figure 3a, where graphite exhibits an intense (0 0 2) at 26.4° which is typical of the crystalline nature of 3.1. Graphene Oxide Synthesis and Itspeak Reduction graphite powder. During the chemical reaction, the oxidation process of graphite powder causes a Figure the characterization results of synthesized graphene nanosheets. The J. Compos. 3 Sci.shows 2017, 1, 12 5 of 13X-ray replacement of the (0 0 2) peak with a (0 0 1) diffraction peak of GO. This is due to the insertion of diffractograms of precursor graphite, synthesized GO and rGO products are exhibited in Figure 3a, oxygen based functional groups in GO and water molecules, which increases the interlayer spacing 3. graphite Results and Discussion where exhibits an intense peak (0 0 2) at 26.4◦ which is typical of the crystalline nature of in the graphite layers including water molecules [33]. On the other hand, the rGO diffractogram graphite powder. During the chemical reaction, the oxidation process of graphite powder causes a 3.1. Oxide Synthesis Reduction reveals a Graphene peak relocated back toand theIts position of pristine graphite peak due to the removal of most of replacement of the (0 0 2) peak with a (0 0 1) diffraction peak of GO. This is due to the insertion of the oxygen groups of GO, thus decreasing the interlayer spacing.graphene It is interesting to note that both Figure 3 shows the characterization results of synthesized nanosheets. The X-ray oxygen based functional groups in GO and water molecules, which increases the interlayer spacing in GO and rGO peaks are broader and less intense hence confirming the exfoliation process. diffractograms of precursor graphite, synthesized GO and rGO products are exhibited in Figure 3a, the graphite layers including water molecules [33]. On the other hand, the rGO diffractogram reveals a Figure shows the exfoliation GO(0sheets observed from FESEM GOnature nanosheets where 3b,c graphite exhibits an intense of peak 0 2) atas 26.4° which is typical of theanalysis. crystalline of peak relocated back to the position of pristine graphite peak due to the removal of most of the oxygen graphite micrometers powder. During the in chemical oxidation process of graphite powder causes a the were several large lateralreaction, size butthe with different layered thickness. In addition, groups of GO, thus decreasing thewith interlayer spacing. It is interesting to note both GO and rGO replacement of the (0of0 nanosheets 2) peak a (0 be 0 1)observed diffraction of GO. This is duethat to the of and characteristic wrinkling can aspeak commonly encountered ininsertion thin films peaksoxygen are broader and less intense hence confirming the exfoliation process. membranes. based functional groups in GO and water molecules, which increases the interlayer spacing in the graphite layers including water molecules [33]. On the other hand, the rGO diffractogram reveals a peak relocated back to the position of pristine graphite peak due to the removal of most of the oxygen groups of GO, thus decreasing the interlayer spacing. It is interesting to note that both GO and rGO peaks are broader and less intense hence confirming the exfoliation process. Figure 3b,c shows the exfoliation of GO sheets as observed from FESEM analysis. GO nanosheets were several micrometers large in lateral size but with different layered thickness. In addition, the Graphite characteristic wrinkling GO of nanosheets can be observed as commonly encountered in thin films and rGO membranes. GO
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Figure 3.3. (a) (a)X-ray X-raydiffractograms diffractogramsofofpristine pristinegraphite, graphite,GO GOand andrGO rGO(the (theinternal internalbox boxshows shows the the Figure magnified picture of diffractograms of GO and rGO); (b,c) FESEM images of exfoliated GO nanosheets. magnified picture of diffractograms of GO and rGO); (b,c) FESEM images of exfoliated GO nanosheets. Graphite rGO
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3.2. Electrophoretic Deposition of GO andof Composite Fabrication Figure 3b,c shows the exfoliation GO sheets as observed from FESEM analysis. GO nanosheets 2(°) were several micrometers large in lateral size but with different layered thickness. In addition, Figure 4a shows the GF pasted on a metallic frames, whereas Figure 4b shows that was (c) the GF the characteristic (a) wrinkling of nanosheets can (b) be observed as commonly encountered in thin films obtained after the EPD of GO nanosheets. The slight color change of GF from white to beige suggests Figure 3. (a) X-ray diffractograms of pristine graphite, GO and rGO (the internal box shows the and membranes. the coating of GO on GF. In order to reduce the GO coating on GF laminate obtained in previous step, magnified picture of diffractograms of GO and rGO); (b,c) FESEM images of exfoliated GO nanosheets. the was subjected to thermochemical reduction, the resultant fibers appeared to be dark grey 3.2.same Electrophoretic Deposition of GO and Composite Fabrication in color which visually confirms the reduction of GO coating 3.2. Electrophoretic Deposition of GO and Composite Fabrication to rGO (Figure 4c). Figure 4a shows the GF pasted on a metallic frames, whereas Figure 4b shows the GF that was Figure 4a shows the GF pasted on a metallic frames, whereas Figure 4b shows the GF that was obtained after the EPD of GO nanosheets. The slight color change of GF from white to beige suggests obtained after the EPD of GO nanosheets. The slight color change of GF from white to beige suggests the coating of GO on GF. In order to reduce the GO coating on GF laminate obtained in previous step, the coating of GO on GF. In order to reduce the GO coating on GF laminate obtained in previous step, the same was subjected to thermochemical reduction, the resultant fibers appeared to be dark grey in the same was subjected to thermochemical reduction, the resultant fibers appeared to be dark grey colorin which confirms the reduction of GO coating totorGO color visually which visually confirms the reduction of GO coating rGO(Figure (Figure 4c). 4c). 10
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(b) (b)
(c) Figure 4. Metallic window frames with (a) uncoated E-glass fibers (GF) and (b) GO coated GF and
(c) E-glass fibers (GF) and (b) GO coated GF and Figure 4. Metallic window frames with (a) uncoated (c) rGO coated GF. (c) rGO coated GF. Figure 4. Metallic window frames with (a) uncoated E-glass fibers (GF) and (b) GO coated GF and (c) rGO coated GF. of the electrophoretically deposited GO and rGO coatings on GF can be The appearance visualized with FESEM observations. In Figure 5, a comparison the coatings FESEM pictures of uncoated, The appearance of the electrophoretically deposited GO andof rGO on GF can be visualized TherGO appearance of surfaces the electrophoretically GO rGO coatings on GF be GO and coated GF Thedeposited coating bothand GO and rGO appears to GO becan quite with FESEM observations. In Figureis5,presented. a comparison of the of FESEM pictures of uncoated, and rGO visualized with FESEM observations. In Figure 5, a comparison of the FESEM pictures of uncoated, compact and uniform in thickness a certain coated GF surfaces is presented. Theforcoating oflength. both GO and rGO appears to be quite compact and GO and rGO coated GF surfaces is presented. The coating of both GO and rGO appears to be quite uniform in thickness for a certain length. compact and uniform in thickness for a certain length.
(a) (a)
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Figure 5. Cont.
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(c) (c) Figure 5. Field emission scanning electron microscopy (FESEM) pictures of GF (a) coated with GO using Figure applied 10V/cm (b) andelectron rGO after chemical reduction (c). of GF (a) coated with GO 5. field Field of emission scanning microscopy (FESEM) pictures Figure 5. Field emission scanning electron microscopy (FESEM) pictures of GF (a) coated with GO using applied field of 10V/cm (b) and rGO after chemical reduction (c). using applied field of 10V/cm (b) and rGO after chemical reduction (c).
The quantitative analysis of the depositions of GO and rGO interphase was evaluated, in which quantitative of thewere depositions of GO andfiber rGO interphase whichwere the fibers The before and afteranalysis deposition weighed. Four laminates was (100evaluated, mm × 100inmm) the fibers before and after deposition were weighed. Four fiber laminates (100 mm × 100 mm) were The quantitative analysis of the depositions of GO and rGO interphase was evaluated, in which weighed first and then the deposition of GO was carried out as described in this work previously. weighed firstand andafter then deposition the deposition of GO was carried out as described in(100 thismm work×previously. the fibers before were weighed. Four fiber laminates 100 mm) were After drying of the fibers under vacuum at 50 °C, the fibers were again weighed to find the difference Afterfirst drying of then the fibers under vacuum at 50 °C, the fibers were again weighed in to find the difference weighed and the deposition of GO was carried out as described this work previously. of the weight due to deposition. Afterwards, epoxy resin was infused and cured, and the final weight of the weight due to deposition. Afterwards, resin waswere infused andweighed cured, and final ◦ C, the After drying of the fibers under vacuum at 50 epoxy fibers again tothe find theweight difference of theof composite was measured. ByBy taking deposition andcomposite composite weight, the GO the composite was measured. takingthe theratio ratio of of deposition and weight, the GO of the weight due to deposition. Afterwards, epoxy resin was infused and cured, and the final weight deposition weight content in Ep-GO-GF composites was calculated to be around 0.31% ± 0.03% deposition weight content in Ep-GO-GF composites was calculated to be around 0.31% ± 0.03% overover of the composite was measured. By taking thetoratio of deposition and composite weight, the GO the total weight of the composite. the deposition deposition content of rGO in the the total weight of the composite.However, However, to evaluate evaluate the content of rGO in the deposition weight content in Ep-GO-GF composites was calculated to be around 0.31% ± 0.03% over composite, the GO coated fibers were weighedfirst first and and were to to reduction process. composite, the GO coated fibers were weighed were then thensubjected subjected reduction process. the total the evaluated composite. However, to evaluate the deposition content of rGO inGO the composite, rGOweight weight with respectto tothe the percentage percentage decrease ininweight from due to to rGO weight wasofwas evaluated with respect decrease weight from GO due the GO coated fibers were weighed first and were then subjected to reduction process. rGO weight reduction which was 0.11%, hence the final weight content of rGO in Ep-rGO-GF was practically the reduction which was 0.11%, hence the final weight content of rGO in Ep-rGO-GF was practically the same. was evaluated with respect to the percentage decrease in weight from GO due to reduction which was same. The the fiberfinal volume fraction of theofcomposites was determined quantitatively considering the 0.11%, hence rGO in Ep-rGO-GF was practically theby same. The fiber volume weight fractioncontent of the composites was determined quantitatively by considering the weight of the fibers before composite fabrication and then measuring the weight the whole the The fraction of the composites was determined quantitatively byofconsidering weight offiber the volume fibers before composite fabrication and then measuring the weight of the whole composite after curing. The weight of the matrix was then calculated by subtracting the weight of the weight of the fibers before composite fabrication and then measuring the weight of thethe whole composite composite after curing. The weight of the matrix was then calculated by subtracting weight of the composite by the weight of the fiber and by considering the densities of the fiber and the matrix, the after curing. The weight of the matrix was then calculated by subtracting the weight of the composite composite byof the weight thevolume fiber and by composite considering densitiesInofthis theway, fiberthe andfiber the volume matrix, the volume fiber used of and of the wasthe calculated. by the weight ofused the fiber by considering the densities ofvolume. the fiberInand matrix, of volume of fiber and and volume of the to composite calculated. thisthe way, the the fibervolume volume fraction of the composite was estimated be around was 50% by fiber used and volume of the composite was calculated. In this way, the fiber volume fraction of the views of the composites observed OM and FESEM are shown in fraction ofThe the cross-sectional composite was estimated to be around 50% bythrough volume. composite was estimated to be around 50% by volume. Figure 6. The images reveal a highly compact fiber arrangement embedded in epoxy matrix along in The cross-sectional views of the composites observed through OM and FESEM are shown The cross-sectional views of the composites observed through OM and FESEM are shown in with some resin rich areas and voids which is typical of composites fabricated by hand lay-up Figure 6. The images reveal a highly compact fiber arrangement embedded in epoxy matrix along method. Figure 6. The images reveal a highly compact fiber arrangement embedded in epoxy matrix along with with some resin rich areas and voids which is typical of composites fabricated by hand lay-up some resin rich areas and voids which is typical of composites fabricated by hand lay-up method. method. Optical microscopy
Optical microscopy
Scanning electron microscopy
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Figure 6. Cont.
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Scanning Scanning electron electron microscopy microscopy (High (High magnifcation) magnifcation)
Ep-GF Ep-GF
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Ep-rGO-GF Ep-rGO-GF
Figure 6. Cross-section of composite observed through optical and electron microscopy at different Figure 6.6.Cross-section of composite observed through opticaloptical and electron microscopy at different Figure Cross-section of composite observed through and electron microscopy at magnifications. magnifications. different magnifications.
3.3. Electrical Resistivity 3.3. 3.3. Electrical Electrical Resistivity Resistivity The effect of GO and rGO coatings on the volume electrical resistivity values along the x axis of The GO coatings on volume electrical resistivity values along thethe x axis of The effect effect of ofcomposites GOand andrGO rGO onthe the resistivity along x axis the glass-epoxy is coatings summarized in volume Table 1.electrical As expected, the values electrical resistivity of the glass-epoxy composites is summarized in Table 1. As expected, the electrical resistivity of of the glass-epoxy composites is summarized in Table 1. As expected, the electrical resistivity of epoxy-glass composites is not modified by the presence of insulating GO coating. On the other hand, epoxy-glass composites is modified by the of insulating GO On other hand, epoxy-glass is not not on modified byfibers the presence presence insulating GO coating. coating. On the theby other hand, the presencecomposites of a rGO coating the glass lead to aofdrop of the electrical resistivity 12 orders the presence of a rGO coating on the glass fibers lead to a drop of the electrical resistivity by 12 orders the presence ofthus a rGO coating the glassconductive fibers leadbehaviour. to a drop of the electrical resistivity by 12 orders of magnitude yielding anon electrical of conductive behaviour. behaviour. of magnitude magnitude thus thus yielding yielding an an electrical electrical conductive Table 1. Electrical resistivity values of composites with uncoated and coated fibers. Table Table 1. Electrical resistivity values of composites with uncoated and coated fibers.
Property Property Property Volume resistivity (Ω cm) Volume resistivity (Ω cm)
Volume resistivity (Ω cm)
Ep-GF Ep-GF14 Ep-GF 3.7 × 1014 3.7 × 10 14
3.7 × 10
Ep-GO-GF Ep-GO-GF Ep-GO-GF 6.9 × 1013 6.9 × 1013 13 6.9 × 10
Ep-rGO-GF Ep-rGO-GF Ep-rGO-GF 4.5 × 102 4.5 × 102 2 4.5 × 10
In order to investigate the effect of having a continuous rGO interphase oriented along the fibers In order to investigate the effect of having a continuous rGO interphase oriented along the fibers direction, the electrical resistivity of the composites was tested along three mutually orthogonal In order investigate the effect of having a continuous interphase orientedorthogonal along the direction, the to electrical resistivity of the composites was testedrGO along three mutually directions i.e., x-axis, y-axis and z-axis. In Figure 7, the volume resistivity values along the three fibers direction, the electrical resistivity composites tested along three mutually directions i.e., x-axis, y-axis and z-axis.ofIntheFigure 7, the was volume resistivity values alongorthogonal the three directions of rGO coated composite are compared. The composites showed a very low resistivity directions of i.e.,rGO x-axis, y-axis and z-axis. In Figure 7,The the composites volume resistivity the three directions coated composite are compared. showedvalues a veryalong low resistivity along the x-axis which contains theare continuous path for electrons showed to travelathrough the structure. On directions of rGO coated composite compared. The composites very low resistivity along along the x-axis which contains the continuous path for electrons to travel through the structure. On the x-axis other hand, the y-axis andcontinuous z-axis showed higher resistivity because of the alternating conductive the whichthe contains pathaafor electrons to travel through the alternating structure. On the other the other hand, y-axis the and z-axis showed higher resistivity because of the conductive (graphene) and non-conductive (epoxy) layers. Between these, y-axis had less resistivity as compared hand, the y-axis and z-axis showed a higher resistivity because of the alternating conductive (graphene) (graphene) and non-conductive (epoxy) layers. Between these, y-axis had less resistivity as compared to z-axis as the load was applied on this direction during the compositeasmanufacturing hence and non-conductive layers. Between these, y-axis had less compared to z-axis as to z-axis as the load(epoxy) was applied on this direction during the resistivity composite manufacturing hence compressing the fibers and providing better tunneling effect or possibly direct contact between the the load was applied on this direction during the composite manufacturing hence compressing the compressing the fibers and providing better tunneling effect or possibly direct contact between the fibers.and providing better tunneling effect or possibly direct contact between the fibers. fibers fibers.
Direction Direction
z-axis z-axis
y-axis y-axis
x-axis x-axis 0 0
1x103 1x103
2x103 2x103
3x103 3x103
Volume resistivity (.cm) Volume resistivity (.cm) Figure 7. Volume measured along three different directions of the Volumeresistivity resistivityofofEp-rGO-GF Ep-rGO-GFcomposites composites measured along three different directions of Figure 7. Volume resistivity of Ep-rGO-GF composites measured along three different directions of sample withwith respect to fiber orientation. the sample respect to fiber orientation. the sample with respect to fiber orientation.
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3.4. Dielectric Properties 3.4. Dielectric Properties a control test, the permittivity of uncoated fiber composite was measured room As a As control test, the permittivity of uncoated fiber composite was measured at roomattemperature. AsFigure it can 8a, be seen in Figure 8a,level the permittivity levelasdid changefrequency as the applied As it temperature. can be seen in the permittivity did not change thenot applied increased, frequency increased, which is in accordance with the behavior commonly observed for insulating which is in accordance with the behavior commonly observed for insulating materials such as epoxy materials such as epoxy and GF [34]. Due to the insulating nature of GO, coating of GF with GO did and GF [34]. Due to the insulating nature of GO, coating of GF with GO did not provide any capacitive not provide any capacitive properties to Ep-GO-GF composite. In the case of rGO coated fibers, properties to Ep-GO-GF composite. In the case of rGO coated fibers, however, the composite showed an however, the composite showed an improvement over the entire frequency range. At 100 Hz, the improvement over the entire frequency range. At 100 Hz, the permittivity value increased by a factor permittivity value increased by a factor of 3.6 when compared to the value measured on the of 3.6composite when compared to the fibers. value The measured onof the compositeinwith uncoated fibers. The of with uncoated induction permittivity glass/epoxy composites wasinduction due to permittivity in glass/epoxy composites was due to the presence of rGO interphase, which possesses a the presence of rGO interphase, which possesses a higher electrical conductive. A similar trend was higher electrical similar trend obtainedwhile in the casetested of dissipation factor of the also obtainedconductive. in the case of A dissipation factorwas of thealso composites, being for their capacitive properties, as shown Figure 8b.their capacitive properties, as shown in Figure 8b. composites, while being in tested for
Ep-GF Ep-GO-GF Ep-rGO-GF
Dielectric constant
25 20 15 10 5 0 102
103
104
105
106
Frequency (Hz)
(a) Ep-GF Ep-GO-GF Ep-rGO-GF
Dissipation factor
0.6
0.4
0.2
0.0 102
103
104
105
106
Frequency (Hz)
(b) Figure 8. Dielectric properties of the investigated composites: (a) permittivity (or dielectric constant)
Figure 8. Dielectric properties of the investigated composites: (a) permittivity (or dielectric constant) related to the applied frequency; and, (b) dissipation factor (or dielectric loss). related to the applied frequency; and, (b) dissipation factor (or dielectric loss). 3.5. Thermal Stability
3.5. Thermal Stability
Figure 9 shows the TGA curves of the composites containing uncoated, GO and rGO coated
Figure 9 shows the TGA the composites containing uncoated, GOofand rGO coated fibers, and a summary of the curves obtainedofresults is given in Table 2. The thermal stability epoxy/glass increases for both GO and rGO coated where rGO stability interphase a fibers,fiber andcomposites a summary of the obtained results is given in glass Tablefibers 2. The thermal ofimpart epoxy/glass fiber composites increases for both GO and rGO coated glass fibers where rGO interphase impart a better thermal stability (onset temperature for thermal degradation of 354.4 ◦ C) as compared to composites containing GO interphase (340.3 ◦ C). In the case of composites containing GO coated
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better thermal stability (onset temperature for thermal degradation of 354.4 °C) as compared to composites containing GO interphase (340.3 °C). In the case of composites containing GO coated fibers, the relatively low thermal stability could be attributed to the decomposition of the oxygen fibers, the relatively low thermal stability could be attributed to the decomposition of the oxygen functional moieties such as epoxy and hydroxyl which takes place around 250 ◦ C. However, composites functional moieties such as epoxy and hydroxyl which takes place around 250 °C. However, containing rGOcontaining coated fibers offer better thermal stability others todue difficult pathpath effect of composites rGO coated fibers offer better thermal than stability than due others to difficult graphene nanosheets (non-oxidized) which delays the escape of volatile degradation products effect of graphene nanosheets (non-oxidized) which delays the escape of volatile degradation thus favoring the char formation Similar trendSimilar was also observed the residual mass of the products thus favoring theprocess. char formation process. trend was also in observed in the residual ◦ mass ofat the composites °C which after thesupports test, which the above results. composites 700 C after at the700 test, thesupports above results.
Residual mass (%)
100
Ep-GF Ep-GO-GF Ep-rGO-GF
95 90 85
95.4 95.2
80
95.0 94.8
75
94.6 320
70
330
340
350
360
65 60 0
100
200
300
400
500
600
700
Temperature (°C)
Derivative mass loss (wt%.°C-1)
(a) Ep-GF Ep-GO-GF Ep-rGO-GF
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
100
200
300
400
500
600
700
Temperature (°C)
(b) Figure 9. Thermogravimetric analysis (TGA) thermograms of uncoated and graphene coated fiber
Figure 9. Thermogravimetric analysis (TGA) thermograms of uncoated and graphene coated fiber reinforced composites. (a) Residual mass as a function of temperature; (b) derivative of the mass loss. reinforced composites. (a) Residual mass as a function of temperature; (b) derivative of the mass loss. Table 2. Results of TGA on composites with uncoated and coated fibers.
Table 2. Results of TGA on composites with uncoated and coated fibers. Characteristic Ep-GF Ep-GO-GF Ep-rGO-GF Onset temperature (°C) 340.3 Characteristic Ep-GF 331.8 Ep-GO-GF 354.4 Ep-rGO-GF Residual mass at 700 °C (%) 64.2 71.6 77.8 Onset temperature (◦ C) 331.8 380.3 340.3 354.4 Peak temperature (°C) 380.3 380.3 Residual mass at 700 ◦ C (%) 64.2 71.6 77.8 Peak temperature (◦ C) 380.3 380.3 380.3
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3.6. Thermal Conductivity 3.6. Thermal Conductivity The influence of GO and rGO interphase on the thermal conductivity of samples was evaluated The influence of GO and rGO interphase on the thermal conductivity of samples was evaluated by testing the composite samples without and with graphene interphase (GO and rGO). Thermal by testing the composite samples without and with graphene interphase (GO and rGO). Thermal conductivity was tested along three different mutual orthogonal directions, respectively, along (x) conductivity was tested along three different mutual orthogonal directions, respectively, along (x) and transversally (y and z) to fiber direction and at three different temperatures i.e., 25 °C, 50 °C and and transversally (y and z) to fiber direction and at three different temperatures i.e., 25 ◦ C, 50 ◦ C 75 °C. Figure 10 reveals the effect of the presence of an interphase along all of the directions (i.e., and 75 ◦ C. Figure 10 reveals the effect of the presence of an interphase along all of the directions x-axis, y-axis, and z-axis of the composite) in the fiber reinforced composites produced. In case of the (i.e., x-axis, y-axis, and z-axis of the composite) in the fiber reinforced composites produced. In case x-axis (Figure 10a), the thermal conductivity of composites containing rGO coated fibers was of the x-axis (Figure 10a), the thermal conductivity of composites containing rGO coated fibers was significantly higher than that of composites with uncoated fibers or GO coated fibers (around +20%). significantly higher than that of composites with uncoated fibers or GO coated fibers (around +20%). This increase of thermal conductivity is also a confirmation of the reduction of GO during chemical This increase of thermal conductivity is also a confirmation of the reduction of GO during chemical treatment with hydrazine hydrate. Moreover, the advantage of aligning (even a very low content of) treatment with hydrazine hydrate. Moreover, the advantage of aligning (even a very low content of) rGO nanosheets as a continuous interphase between the matrix and fiber results in a 20% increase in rGO nanosheets as a continuous interphase between the matrix and fiber results in a 20% increase in the thermal conductivity. Along the y-axis and z-axis of the composites, the thermal conductivity did the thermal conductivity. Along the y-axis and z-axis of the composites, the thermal conductivity did not showed any significant change, as shown in Figure 10b,c. In these two directions, the GO or rGO not showed any significant change, as shown in Figure 10b,c. In these two directions, the GO or rGO interphase did not create a continuous network making it impossible to create a percolation threshold interphase did not create a continuous network making it impossible to create a percolation threshold enough to improve the thermal conductivity. enough to improve the thermal conductivity.
Figure y-axis; and, and, (c) (c) z-axis z-axis directions. directions. Figure10. 10.Thermal Thermalconductivity conductivity of of three three composites composites along along (a) (a) x-axis; x-axis; (b) (b) y-axis;
4. 4. Conclusions Conclusions In In this this study, study,epoxy/glass epoxy/glass composites were prepared in which the glass fibers (GF) were coated coated with with either either GO GO or or rGO. rGO. The The coating coating of of GO GO on on GF GF was was performed performed using using electrophoretic electrophoretic deposition deposition with with deposition deposition parameters parameters being being optimized optimized to to obtain obtain aa homogenous homogenous coating. coating. rGO rGO coated coated GF GF were were obtained obtained by by subsequent subsequent chemical chemical reduction reduction of of the the GO GO coating. coating. Multiscale composites were produced produced by method, which produced high high fiber volume fraction composites containing around by the thehand handlay-up lay-up method, which produced fiber volume fraction composites containing 50% of fiber volume content with only 0.3% by weight of graphene nanosheets. The interphase of around 50% of fiber volume content with only 0.3% by weight of graphene nanosheets. The graphene was effective in improving theimproving thermal stability of thestability composites. interphasenanosheets of graphene nanosheets was effective in the thermal of the composites. Electrical Electrical resistivity resistivity measurements measurements revealed that the orientation orientation of of rGO rGO nanosheets nanosheets along along the the length resistivity as as compared to other orientations, hence confirming the lengthof ofthe thefibers fibersoffered offeredthe thelowest lowest resistivity compared to other orientations, hence confirming advantage of oriented and aligned rGO interphase for tailored properties. properties. The conductive the advantage of oriented and aligned rGO interphase forfunctional tailored functional The behavior of epoxy/glass composite containing rGO interphase also induced the property ofthe permittivity conductive behavior of epoxy/glass composite containing rGO interphase also induced property in composites. was verifiedThis along with the other composites uncoated and GO of the permittivity in This the composites. was verified along with the containing other composites containing coated fibers. Thiscoated functionality offers the possibility to the use possibility such composites for electromagnetic uncoated and GO fibers. This functionality offers to use such composites for interference shielding in advanced applications. Other than electrical functionalities, aligned rGO electromagnetic interference shielding in advanced applications. Other than electricalthe functionalities, interphase with the fibers epoxy/glass composites offered a better thermal conductivity. the alignedalong rGO interphase alongin with the fibers in epoxy/glass composites offered a better thermal This was verified comparing conductivity values along othervalues orientations the conductivity. Thisbywas verifiedthe by thermal comparing the thermal conductivity along ofother composite based either uncoated, GO, or rGO coatedGO, GF.or This result supports theresult advantage of orientations of theoncomposite based on either uncoated, rGO coated GF. This supports aligning graphene interphase in epoxy/glass composites for improved functional properties. the advantage of aligning graphene interphase in epoxy/glass composites for improved functional properties. Acknowledgments: The authors greatly acknowledge the help of Gian-Franco Dalla Betta and Andrea Ficorella in carrying out the dielectric properties measurement in their laboratory. Acknowledgements: The authors greatly acknowledge the help of Prof. Gian-Franco Dalla Betta and Mr. Andrea Ficorella in carrying out the dielectric properties measurement in their laboratory.
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Author Contributions: Haroon Mahmood and Alessandro Pegoretti conceived and designed the experiments; Haroon Mahmood performed the experiments; Seraphin H. Unterberger performed the thermal analysis tests; Haroon Mahmood, Alessandro Pegoretti and Seraphin H. Unterberger analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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