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Indian Journal of Engineering & Materials Sciences Vol. 22, April 2015, pp. 167-174

Influence of functionalized multi-walled carbon nanotubes on mechanical properties of glass fiber reinforced polyester composites D Selwyn Jebaduraia* & A Suresh Babub a

Department of Mechanical Engineering, SRM University, Kattankulathur, Chennai 603 203, India Department of Manufacturing Engineering, College of Engineering Guindy, Anna University, Chennai 600 025, India

b

Received 18 February 2014; accepted 29 September 2014 The effect of addition of carboxyl functionalized multi-walled carbon nanotubes (COOH MWCNT) to glass fiber reinforced polymer (GFRP) composites is studied. GFRP composite laminates are fabricated by vacuum resin infusion process (VRIP). Composite laminates are fabricated using neat polyester resin and nano-phased polyester resin (with 0.2, 0.5 wt% MWCNT). Composite laminates with MWCNT were divided into three sections based on the direction of resin flow namely entry, middle and exit. The ability of carbon nanotubes mixed with the resin to reinforce uniformly across the glass fiber mats is investigated by evaluating mechanical properties of specimens at different sections of the laminates. Specimens from entry, middle and exit sections were tested to determine tensile, flexural and inter-laminar properties. Composite laminate with 0.2 wt% MWCNT exhibited good mechanical properties compared to that with 0.5 wt% MWCNT. It could be observed that carbon nanotubes are able to spread across entire glass fiber mats as it resulted in enhancement of mechanical properties in all sections without much variation. It is also noticed that increase in wt% of MWCNT resulted in incomplete wetting of glass fiber mats. Keywords: Carbon nanotubes, Glass fiber, Inter-laminar shear strength, Polymer matrix composites, Vacuum resin infusion process

The use of polymer matrix composites (PMC) is diverse, reaching all industry sectors that include aerospace, automotive, construction, marine and wind energy1. They have high strength, simple manufacturing principles and are of low cost2. The most widely used, economical reinforcement in polymer-matrix composites is E-glass fiber. Glass fiber is by far the most widely used, because of the combination of low cost, corrosion resistance and in many cases efficient manufacturing potential3,4. Glass fiber reinforced polymer (GFRP) is widely used in the composite market since it offers good cost-performance advantage for many applications5. The superior resistance of glass fibers to environmental attack made GFRP more attractive for marine products and in the chemical industries. Unsaturated polyesters are commonly used as matrix for GFRP composite parts. They have the advantages of low viscosity, fast cure time and low cost. GFRP composites have been utilized in many applications including automotive, transportation, piping, chemical storage tanks and windmill components4,6. —————— *Corresponding author (E-mail: [email protected])

Fiber reinforced composite materials are known to have high in-plane tensile strength and stiffness properties. One of the major limitations of these materials is the out-of-plane/through thickness properties such as inter-laminar shear strength (ILSS) and the low stress or strain threshold resulting in early damage initiation5-7. Lack of fiber reinforcement in inter-laminar region may lead to failure through various modes including delamination and matrix cracking which may progress through the service life of the structure8-11. Generally, the characteristics of fiber determine inplane tensile properties of fiber reinforced composite materials, while compression and out-of-plane properties are defined by matrix resin12. In recent years, researchers have considered nano-scaled materials as filler for matrix resin to enhance mechanical properties and to produce high performance fiber reinforced structural composites4,9,12,13. Carbon nanotubes (CNT) have been the focus of research since their discovery by Iijima in 199114. CNTs have a combination of outstanding mechanical, electrical and thermal properties that make them suitable for numerous

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applications. CNTs are allotropes of carbon with cylindrical nanostructure. Graphite is a 2-D sheet of carbon atoms arranged in a crystalline hexagonal structure held together by strong covalent bonds. The sheets are layered and very weakly held together by Van der Waals forces. CNTs are usually described as sheet of graphite rolled into a perfect tube. CNTs are crystalline carbon nanostructures consisting of single or multiple concentric graphene cylinders15. The unique mechanical properties of CNTs as well as their low density and enormous aspect ratio make them ideal candidates to act as reinforcement for polymer composites11,15-17. Functionalization of CNTs promote homogeneous distribution of CNTs in matrix and minimizes the risk of agglomeration when compared to pristine CNT18. From literatures, it is observed that the addition of very small quantity of CNT to matrix has resulted in significant enhancement of matrix dominated properties of traditional polymer composite materials17,19,20. However, it is not fully explored whether CNTs mixed in the resin spread till exit section without being entrapped in the entry and middle sections of glass fiber mats during vacuum resin infusion process (VRIP). It is realized that a study is required to examine whether CNTs could reinforce across all sections of GFRP composite laminates. Hence, the objective of this work is to determine if composite laminates with nano-phased polyester resin have uniform CNT reinforcement in the entry, middle and exit sections by evaluating their mechanical properties at these sections. This study was conducted by dividing the fabricated composite laminates with nano-phased resin into three sections namely entry, middle and exit as shown in Fig. 1. Mechanical properties of specimens from these

Fig. 1–Composite laminate with MWCNT divided into three sections namely entry, middle and exit

sections were determined and compared to analyse the uniformity of MWCNT reinforcement at these sections. This work also aims to determine the influence of nano-phased resin by comparing the mechanical properties of laminates fabricated using neat resin. Experimental Procedure Materials

Carboxyl functionalized multi-walled carbon nanotubes (COOH-MWCNT) were purchased from Quantum Materials Corporation, Bangalore, India. The CNTs were made by variable catalytic chemical vapour deposition process. The average outer and inner diameters of CNT were 12 nm and 8 nm, respectively. The length of nanotubes was between 4 and 5 microns and their specific surface area between 250 m2/g and 290 m2/g. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of MWCNT are shown in Figs 2(a) and 2(b), respectively.

Fig. 2–(a) SEM image of carboxyl functionalized multi-walled carbon nanotube and (b) TEM image of carboxyl functionalized multi-walled carbon nanotube

JEBADURAI & BABU: GLASS FIBER REINFORCED POLYSTER COMPOSITES

Chopped strand mat (CSM) and woven roving mat (WRM) were the types of glass fiber mats used in this study. CSM and WRM used in this study had surface area of 450 g/m2 and 600 g/m2, respectively. Unsaturated isophthalic polyester resin was used as the matrix. It has a density of 1.13 g/cm3 at 20°C and viscosity of 500-750 cPs. Glass fibers and polyester resin were purchased from Sakthi Fiber Glass, Chennai. Cobalt naphtanate and methyl ethyl ketone peroxide (MEKP) were used as accelerator and catalyst respectively. Fabrication of composite laminates

A flat glass plate of size 1.4 × 0.8 m and 8 mm thickness was used as the mould for fabricating composite laminates. Wax was coated over the glass plate as a release agent. Five layers of glass fiber mats of 0.3 m2 were stacked on it. These five layers comprised of alternate layers of CSM and WRM, with the top and bottom layers were of CSM type. Peel ply, distribution medium (infusion mesh), spiral tube and vacuum bag were arranged as shown in Fig. 1. Vacuum pressure of 95 kPa was created inside the vacuum bag using a vacuum pump connected to spiral tube. Polyester resin mixed with 2 wt% catalyst and 2 wt% accelerator was taken in the resin feed pot and the resin feed hose was unclamped to allow the resin to infuse the glass fiber mats. The distribution medium assisted the resin to flow across the glass fiber mats and impregnate them from entry to exit sections. Once the resin completely spread over the glass fiber mats, the resin feed hose was clamped and the vacuum pump was kept on for 90 min to maintain the vacuum. The composite laminate was allowed to cure at room temperature for 18 h and then released. To fabricate GFRP composite laminates with nano-phased polyester resin (with 0.2 wt% and 0.5 wt% MWCNT), polyester resin mixed with MWCNT and accelerator was kept in ultrasonicator for 1 h to obtain homogeneous dispersion. Then the resin was mixed with catalyst and infusion process was carried out. After curing, when the composite laminate was released from the mould, it was observed that at the bottom side of the composite laminate (near the exit section), the glass fiber mats were not fully impregnated by the resin. The same phenomenon was observed and reported by Fan et al.21 This might be due to inability of the nano-phased resin to penetrate through the layers of glass fiber mats and impregnate them. The addition of MWCNT to matrix might have increased the viscosity

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of matrix17. The reduced space in between layers of fiber mats owing to high compaction achieved by the vacuum process would have also made it difficult for the viscous matrix to wet the glass fiber mats completely21. The phenomenon of incomplete wetting of glass fiber mats was also observed in composite laminate with 0.5 wt% of MWCNT. The area of incomplete wetting observed in this case was more compared to that observed in composite with 0.2 wt% MWCNT as shown in Fig. 3. The incomplete wetting and exposure of glass fibers was not observed in the composite laminate fabricated using neat resin.

Fig. 3–Images of fabricated composite laminates (a) composite without MWCNT – no incomplete wetting, (b) laminate with 0.2 wt% MWCNT – area of incomplete wetting 30 × 30 mm2 and (c) Laminate with 0.5 wt% MWCNTs – area of incomplete wetting 30×60 mm2

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Tensile properties

The resin entry and exit points were marked in the GFRP composite laminates with nano-phased polyester resin and were divided into three sections namely entry, middle and exit based on the resin flow direction. Specimens cut from these sections were evaluated for tensile properties namely, tensile strength and tensile modulus. The tensile properties were also evaluated for the composite laminates with neat polyester resin. The tensile test was conducted using a universal testing machine incorporating a tension/compression load cell operated at a cross head speed of 2.5 mm/min. The tensile tests were carried out according to ASTM standard D3039. Flexural properties

Flexural strength was measured by three-point bending test using universal testing machine according to ASTM D790 standard. The size of specimens prepared for testing was 100 × 12.7 × 3 mm with a support span of 50.8 mm in length. Flexural strength (σUF) and flexural modulus (EF) were calculated using the following equations3. σUF = 3PmaxL / 2bh2

… (1)

EF = mL3/4bh3

… (2)

where Pmax is the maximum load at failure, m is the initial slope of the load-deflection curve, b and h are width and thickness of the specimen respectively and L is the specimen length between two support points.

glass fiber mats and the failure occurred due to the delamination of glass fiber mats. ILSS was calculated using the relation3, ILSS = 0.75 x Pmax / bh

… (3)

Results and Discussion Tensile properties

The comparison of average tensile strength of composites with neat polyester and nano-phased polyester (with 0.2 wt% and 0.5 wt % MWCNT) is shown in Fig. 6. Composite laminate containing 0.2 wt% MWCNT showed 58% increase in tensile strength compared to composite without MWCNT, while laminate with 0.5 wt% MWCNT exhibited 18% increase in tensile strength. The comparison of

Fig. 4–Inter-laminar shear strength test set-up

Inter-laminar shear strength

ILSS refers to shear strength parallel to the plane of lamination. It is the maximum shear strength existing between layers of laminated material. When this maximum shear strength is exceeded, failure occurs due to delamination between layers of reinforcing fibers21. ILSS of fiber reinforced composites is determined by generating a pattern of pure shear stress which results in an inter-laminar shear failure21. ILSS is measured in a short-beam shear (SBS) test in accordance with ASTM D234433,11,17,21. The experimental setup is shown in Fig. 4. Specimen was placed on two fixed cylindrical supports and a cylindrical head was moved down to apply a force at the centre and generated an increasing transverse load until either of these conditions – (i) a load drop-off of 30%, (ii) two-piece specimen failure, (iii) head travel exceeds the specimen nominal thickness. As shown in Fig. 5, the crack initiated in-between the layers of

Fig. 5–Microscopic view of delamination during SBS test

JEBADURAI & BABU: GLASS FIBER REINFORCED POLYSTER COMPOSITES

average tensile modulus of composite laminates without MWCNT and with 0.2, 0.5 wt% of MWCNT is shown in Fig. 7. Composite laminate with 0.2 wt% MWCNT exhibited tensile modulus which is nearly 3 times higher than laminate without MWCNT, while laminate with 0.5 wt% MWCNT resulted in 9% increase of tensile modulus. The better strength and stiffness exhibited by laminate with 0.2 wt% MWCNT could be due to homogeneous dispersion of exfoliated MWCNTs in the polyester resin at lower concentration compared to higher concentration of MWCNT. The exfoliation of MWCNT could be attributed to the use of functionalized MWCNT17. Generally, the tensile properties are fiber dominating. The presence of carbon nanotubes along the in-plane direction served as additional nano-reinforcement to glass fibers. When composite laminates were pulled along the longitudinal direction during tensile testing, MWCNTs having high tensile strength might have contributed to the increase in tensile properties of composite laminates with MWCNT. The MWCNTs in the matrix strengthened the fiber matrix interface, resulting in the enhancement of tensile properties of

Fig. 6–Tensile strength of GFRP composites with different wt% of MWCNT

Fig. 7–Tensile modulus of GFRP composites with different wt% of MWCNT

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the composite laminate9. The values of tensile strength at different sections of GFRP composite laminates with nano-phased polyester resin are shown in Fig. 8. Specimens from each section were tested and the average value was taken. The tensile modulus at different sections of nano-phased is shown in Fig. 9. There is no significant variation of tensile strengths at entry, middle and exit sections of laminate with 0.5 wt% MWCNT. It indicates that MWCNT mixed with polyester resin are able to reach till the exit section of GFRP laminate with 0.5 wt% MWCNT, without getting entrapped in between the glass fibers. Flexural properties

The comparison of average flexural strength of GFRP composites with neat polyester and nano-phased polyester (0.2 wt% and 0.5 wt% MWCNT) is shown in Fig. 10. Composite laminate with 0.2 wt% MWCNT showed 23% increase in flexural strength compared to composite without MWCNT, while laminate with 0.5 wt% MWCNT resulted in around 18% drop in flexural strength. The

Fig 8–Tensile strength at different sections of GFRP composites with different wt% of MWCNT

Fig. 9–Tensile modulus at different sections of GFRP composites with different wt% of MWCNT

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comparison of average flexural modulus of composite laminates without MWCNT and with 0.2, 0.5 wt% of MWCNT is shown in Fig. 11. Composite laminate with 0.2 wt% MWCNT exhibited 13% increase in flexural modulus compared to laminate without MWCNT. The composite laminate with 0.5 wt% MWCNT resulted in 12% decrease in flexural modulus. The properties of matrix contribute significantly to the bending strength of the laminate. From the chart, it is observed that matrix with lower concentration of MWCNT resulted in better flexural strength compared to that with higher wt% of MWCNT. Hossain et al.4 has also observed the enhancement in flexural properties of laminates containing up to 0.2 wt% carbon nanofiber and beyond that there was a decreasing trend. The high aspect ratio of MWCNTs could have prevented crack formation and contributed to the improvement in flexural properties of laminates with 0.2 wt% MWCNT compared to that with neat polyester resin4. Incomplete wetting of glass fibers by resin containing 0.5 wt% MWCNT due to increase in viscosity and

agglomeration of MWCNTs might have contributed to the drop in the flexural properties of composite laminate with 0.5 wt% MWCNT. The higher concentration of MWCNT (0.5 wt%) in polyester resin might have resulted in the formation of agglomerates due to inter-molecular interactions such as Van der Waals forces and these agglomerates act as stress concentrators which would have caused a drop in flexural strength of composite with 0.5 wt% MWCNT. The better flexural properties exhibited by laminate with 0.2 wt% MWCNT might be due to the homogeneously dispersed MWCNT in the polyester resin. These MWCNTs in the matrix act as interfaces for stress transfer and hence serve as additional nano-reinforcement to micro-sized glass fibers. The flexural strengths of specimens from different sections of GFRP composite laminate with nanophased polyester resin are shown in Fig. 12. The flexural modulus at different sections of composite laminates with 0.2 and 0.5 wt% of MWCNT is shown in Fig. 13. No significant difference was observed in the flexural strength between different sections (entry, middle and exit) of laminate with 0.2 wt% of MWCNT. Some agglomerates of MWCNT could be observed as shown in Fig. 14, from the images obtained when fracture surfaces of composite laminate with 0.5 wt% MWCNT were examined under scanning electron microscope. Inter-laminar shear strength

Fig. 10–Comparison of flexural strength of GFRP composites with different wt% of MWCNT

Fig. 11–Comparison of flexural modulus of GFRP composites with different wt% of MWCNT

ILSS at different sections of GFRP composite laminate with nano-phased polyester (0.2 wt% and 0.5 wt% MWCNT) and the comparison of ILSS of GFRP composite laminate with neat polyester and nano-phased polyester are depicted in Figs 15 and 16, respectively. ILSS is higher at the resin entry portion of the composite laminate. Composite with

Fig. 12–Flexural strength at different sections of GFRP composites with different wt% of MWCNT

JEBADURAI & BABU: GLASS FIBER REINFORCED POLYSTER COMPOSITES

Fig 13–Flexural modulus at different sections of GFRP composites with different wt% of MWCNT

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Fig. 15–ILSS at different sections of GFRP composites with different wt% of MWCNT

Fig. 16–Comparison of ILSS of GFRP composites with different wt% of MWCNT

resin is devoid of fibers in the inter-laminar region. Composite laminate with nano-phased resin contains MWCNTs dispersed in the inter-laminar region. These MWCNTs act as nano reinforcement to the resin increasing the ILSS of composite laminate with 0.2 wt% MWCNT. However, the increase in wt% of MWCNT (0.5) has not improved the ILSS and this might be due to the formation of MWCNT aggregates in the polyester resin. The interactions between MWCNTs in the aggregates are very weak and could fail at lower shear stress resulting in decrease of ILSS of laminate with 0.5 wt% MWCNT. Fig. 14–SEM images of composite with 0.5 wt% of MWCNT showing CNT agglomerates

0.5 wt% MWCNTs showed a lesser ILSS compared to that with 0.2 wt% MWCNTs. Laminate with 0.2 wt% MWCNTs exhibited 12% increase in ILSS compared to composite laminate with neat polyester. Similar enhancement in ILSS of polymer composites using CNT was observed by Wicks et al.10 and Fan et al.21 Composite laminate with neat polyester

Conclusions The potential of using COOH-MWCNT as additional nano-phase reinforcement to conventional glass fiber composites was explored. GFRP composite laminates with nano-phased polyester matrix were fabricated successfully by vacuum resin infusion process. Multi-walled carbon nanotubes mixed with polyester resin were able to reinforce all sections of laminates without getting trapped in the entry or

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middle sections as it resulted in enhancement of mechanical properties across all sections without much variation. However, it could be observed that polyester resin containing higher wt% of MWCNT (0.5) was not able to completely impregnate till the last layer of glass fiber mat due to increase in viscosity. Hence, polyester resin with lower concentration of MWCNT (< 0.5 wt%) is preferred for achieving good mechanical properties. Composite laminate with 0.2 wt% MWCNT exhibited better mechanical properties than composite with 0.5 wt% MWCNT. However, a heated vacuum resin transfer molding might enhance the dispersion of 0.5 wt% MWCNT and could result in better mechanical properties22.

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Acknowledgements The authors gratefully acknowledge the Department of Manufacturing Engineering, Anna University, Chennai for creating vacuum resin infusion facility in the composite laboratory for the fabrication of GFRP composite laminates for this study.

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