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Influence of seawater aging on mechanical properties of nano-Al2O3 embedded glass fiber reinforced polymer nanocomposites
Construction and Building Materials 221 (2019) 12–19
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of seawater aging on mechanical properties of nano-Al2O3 embedded glass fiber reinforced polymer nanocomposites Ramesh Kumar Nayak Department of Materials and Metallurgical Engineering, Maulana Azad National Institute of Technology (MANIT), Bhopal 462003, India
h i g h l i g h t s Nano-Al2O3 enhanced nanocomposites reduce the water diffusion coefficient by 17 %. It improves the interlaminar shear strength by11% and flexural strength by 13%. The glass transition temperature of the nanocomposites has not improved significantly. Nano-Al2O3 embedded nanocomposites create the opportunity to be used in seawater environment.
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Article history: Received 16 July 2018 Received in revised form 28 May 2019 Accepted 3 June 2019 Available online 12 June 2019 Keywords: Glass fiber Nanocomposites Nano-Al2O3 Flexural strength, Seawater aging
a b s t r a c t Retention of mechanical and thermal properties of nano-Al2O3 embedded glass fiber reinforced polymer (GFRP) composites in marine environment is necessary and need to be evaluated to realize the full potential of the nanocomposites. The plain and nanocomposites were seawater aged in a temperature controlled seawater bath at 70 °C for a duration of 40 days. The results revealed that seawater aged plain and nanocomposites deteriorate their flexural strength, interlaminar shear strength and glass transition temperature. However, with the addition of 0.1 wt% of nano-Al2O3 particles into the plain GFRP composite, the flexural strength increased by 12%, interlaminar shear strength by 11% in dry condition and seawater diffusion coefficient was reduced by 17% as compared to plain GFRP composites. Nevertheless, the glass transition temperature of the nanocomposites was not improved in dry and seawater aged conditions significantly. The fractured surfaces of tested samples were investigated through FESEM to support the new findings. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy polymer is used in different application such as adhesive, matrix of fiber reinforced polymer composites and coatings. It has good adhesion, chemical, and water resistance as compared to other thermosetting polymers. Fiber reinforced polymer (FRP) composites are used in some of the structural components of aircraft, automobile, and marine industries. The FRP composites are also used in the form of pipes in oil transport and chemical industries. The polymer matrix composites have the tendency to absorb moisture in the hydrothermal environment and resulting in deterioration of its physical and mechanical properties [1,2]. The absorbed moisture of the composites either bonded with the hydroxyl group of the epoxy or occupies the voids present in it or at the fiber/matrix interface. FRP composites degrade through plasticization, swelling, chain scission and hydrolysis of the polymeric chain. Overall, mois-
E-mail address: [email protected] https://doi.org/10.1016/j.conbuildmat.2019.06.043 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ture absorption leads to change in thermo-physical, mechanical and chemical properties of FRP composites [3], as a result the durability and reliability of FRP composite is uncertain. Therefore, improvement of its durability and reliability in a hydrothermal environment is necessary and need to be explored. In polymer matrix composites, the interface of matrix and fiber is the heart of the composites. This is because, at the interface, the matrix transfers the load to the fiber. A strong interface bond transfers the load from the matrix to the fiber fully and enables the composites more reliable and durable. However, in hydrothermal aging, the interface became weaker because of uneven thermal expansion of epoxy and fiber and entrapment of water at the interface. Therefore, in seawater environment, the mechanical properties of polymer matrix composites are deteriorated [4–6]. Nanoparticles enables to form good interface bond between polymer matrix and fiber [7]. Therefore, the addition of nanoparticles into the epoxy matrix is one of the methods to enhance the mechanical properties and durability in a hydrothermal environ-
R.K. Nayak / Construction and Building Materials 221 (2019) 12–19
ment [8,9]. Inorganic nanoparticles are basically Al2O3, TiO2, SiO2, and other metal and non-metallic oxides. Organic nanofillers are CNT, SWCNT, DWCNT and grapheme. Organic nanofillers require complex fabrication technique to blend with the epoxy matrix as compared to metal and non-metallic oxide nanoparticles. Therefore, non-metallic or metallic oxide nanofillers are popular in fiber reinforced polymer composites due to its low cost and easy fabrication method as compared to organic base nanofillers. Inorganic nanofillers are dispersed in the polymer matrix and form either physical or mechanical or chemical or combination of bonds with epoxy matrix [10]. The well dispersed nanoparticles in the epoxy matrix enhance the mechanical properties of the nanocomposites. This is because, during the deformation process, the microcrack may divert the path in front of the nanoparticles or blunt or pin on it. As a result, the micro crack needs more energy for the failure to occur [11]. Addition of nano-Al2O3 and TiO2 reduces water diffusivity and enhances the hydrothermal durability [12–15]. The addition of 2 wt% nanosilica particles into the epoxy polymer increases fracture toughness and wear resistance of the nanocomposites as compared to plain epoxy composites in a seawater environment [16]. Glass fiber reinforced polymer composites are used in the marine applications. The improvement of reliability and durability of these composites in seawater environment are necessary. The addition of nano-Al2O3 particles into glass fiber reinforced polymer matrix composites on mechanical and thermal properties of seawater aged nanocomposites were not evaluated and discussed in existing open literature. Therefore, the current investigation reports the effect of nano-Al2O3 content and seawater aging on mechanical and thermal properties of the nanocomposites. Furthermore, the mechanical properties were compared with plain GFRP composites. The new finding has been envisaged through the fractured surfaces analysis of FESEM images.
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2. Materials and experimental procedure The raw materials used to fabricate the plain GFRP composites was woven roving E-Glass fiber of 300gsm, procured from Owens Corning, India, epoxy of Diglycidyl ether of Bisphenol A (DGEBA) type and hardener of Triethylene tetra amine was procured from Atul Industries, India. The nano-Al2O3 (alpha) particles were procured from SRL Industries limited, India. The average particle sizes of the nanoparticles were <50 nm and molecular weight is 101.96. Fig. 1 shows the schematic diagram of fabrication process of nanocomposites. Nanoparticles were dried at 100 °C for two hours to ensure no moisture present in it before it gets mixed with epoxy matrix [17]. Nano-Al2O3 particles were blended with the epoxy matrix at different wt.% (0.1–0.7 wt%) through magnetic stirring followed by ultrasonication. During the mixing process, the temperature was maintained at 60 °C to reduce the viscosity of epoxy. The nanoAl2O3 blended epoxy was kept on hold for 1 h to ensure removal of entrapped air bubbles form it. The composite laminates contain 16 layers of glass fibers. The ratio of epoxy and glass fiber by weight was 40:60. The composite laminates were cured at 140 °C for 6 h in an oven. The composite laminates were cut as per the ASTM D 3171-99, ASTM D7264, and ASTM D2344 standard to evaluate different properties. Fig. 2 (a) and (b) shows the dispersion of nano-Al2O3 particles in the nanocomposites having 0.1 wt% and 0.7 wt% of nano-Al2O3 content respectively. It is observed that there is a reasonably good dispersion of nano- Al2O3 particles in the epoxy matrix of the nanocomposites having 0.1 wt% as compared to the composite having 0.7 wt% of nano-Al2O3. The physical and mechanical properties of the raw materials are reported in Table 1.
Fig. 1. Schematic view of the fabrication method of nano- Al2O3 content GFRP composites [13].
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Fig. 2. Dispersion of nano-Al2O3 particles in the epoxy matrix having (a) 0.1 (b) 0.7 wt% nano- Al2O3.
Table 1 Properties of the raw materials [15]. Properties
Epoxy
Al2O3(alpha)
Glass fiber
Density g/cm3 Tensile strength (MPa) Tensile modulus (GPa) Poisson’s ratio
1.15 70 3.6 0.30
3.90 260 370 0.21
2.58 3800 78 0.20
3. Results and discussions 3.1. Void content Seawater can penetrate into the polymer matrix composites through the fiber–matrix interface, voids and microcracks present in the composites resulting reduction of the fiber–matrix interface bond. In seawater, NaCl is present in the form of cations and anions. These ions would penetrate with the water molecules into the composites, causing damage to the matrix, fiber and their interface [18]. Therefore, determination of void content in plain and nano GFRP composites is essential. The voids content in the composites were determined as per ASTM D 3171–99 standard and using Eq. (1). As per the standard, six numbers of samples of each type having surface area 25 mm 25 mm were taken into account for this analysis. Initial weight and dimensions of the samples were measured through high accuracy (0.01 mg) weighing balance and digital vernier caliper respectively.
V v ¼ 1 qc
wf
qf
þ
wm
qm
! ð1Þ
where Vv is the volume fraction of void qc, qf and qm is the density of composites, fiber, and matrix respectively. The weight fraction of fiber and matrix is wf and wm respectively. Effect of nano-Al2O3 content on void percentage is reported in Fig. 3. The results revealed that with the increase in nano-Al2O3 content in the composites, void percentage increases. This may be due to the entrapped gas was unable to come out from the epoxy matrix during the fabrication and curing process. However, the presence of void in the composites facilitates seawater absorption into the composites which leads to degradation of its physical and mechanical properties.
Fig. 3. Void content versus nano-Al2O3 content (wt.%) in the composites.
3.2. Weight gain (Seawater Aging) Plain and nanocomposites were dried in an oven at 100 °C for five hours and furnace cooled. The rate of water absorption increases with increase in temperature [15]. Therefore, the samples were dipped into a temperature controlled seawater bath which was maintained at 70 °C throughout the aging process. The seawater was collected from sea at Puri, Odisha, India. The pH of the seawater was measured and found to be 6.7. The edges of specimen were not sealed and the diffusion of water took place from the sides as well as thickness direction. Accelerated aging has been done to realize the maximum degradation of the nanocomposites. The samples were weighed before it gets dipped into the seawater bath. As per the standard, six numbers of samples of each type were dipped into the seawater for this analysis. The weight of the samples was measured at a different interval of time. The amount of seawater absorbed by the composites was calculated using Eq. (2).
Mt ¼
m mo 100% mo
ð2Þ
where Mt is the amount of seawater absorbed in percentage at time t, mo and m is the weight of the samples in dry and after time t respectively.
R.K. Nayak / Construction and Building Materials 221 (2019) 12–19
Fig. 4. (a) Absorbed seawater wt.% as a function of square root of aging time and (b) diffusion coefficient versus nana-Al2O3 content (wt.%) of the composites.
Fig. 5. Comparison of (a) flexural strength (b) strain and (c) modulus versus nano-Al2O3 content in dry and seawater aged condition.
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respectively. Dz indicates diffusion coefficient in thickness direction only. However, in reality, the diffusion takes place in all direction. Therefore the corrected diffusion coefficient(D) was calculated by using the Eq. (4) [19].
D¼
Fig. 6. Interlaminar shear strength of dry and seawater aged composite samples versus nano-Al2O3 content.
The Fickian diffusion coefficient was calculated using the Eq. (3).
Dz ¼ p
h 4 ð%M m Þ
2
2 ð%M2 %M1 Þ pffiffiffiffi pffiffiffiffi t2 t1
ð3Þ
where Dz is the diffusion coefficient in the thickness direction and h is the thickness of the sample. M1, M2 and Mm is the percentage of water gain in t1, t2 and in saturation condition of the aging process
Dz 2 1 þ hl þ wh
ð4Þ
where l and w is the length and width of the sample respectively. Fig. 4 (a) shows seawater gain (wt.%) as a function of square root of time. It is observed that with the increase in nano-Al2O3 content in the composites water gain wt.% increases. Nano-Al2O3 particles enhance the interface bond between fiber and matrix and at the same time it increases the void content in the nano-composites. As a result, the reduction of water absorption with the increase in nano-Al2O3 content is not realized. Fig. 4 (b) shows the effect of nano-Al2O3 content on the diffusion coefficient of seawater. It is observed that the nanocomposites having 0.1 wt% nano-Al2O3 has reduced the diffusion coefficient by 17% as compared to plain GFRP composites. This is because of uneven thermal expansion of epoxy (6.2 105 K1) [20], fiber (5–12 106 K1) [21] and nano-Al2O3 particles (8.1 106 K1) [22] leading to matrix swelling and microcrack formation at the interface. Therefore, it is expected that with the increase in moisture content of the nanocomposites mechanical and thermal properties would be degraded [23,24]. 3.3. Flexural strength The flexural strength of the composites was determined as per ASTM D7264 and using Universal Testing Machine (UTM) of INSTRON 5967 equipped with a 5KN load cell. The span length
Fig. 7. FESEM images of fracture surfaces of the composites having (a) 0.0 (b) 0.1 (c) 0.3 (d) 0.7 wt% of nano-Al2O3 content hybrid composites in dry condition.
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Fig. 8. FESEM images of fracture surfaces of the composites having (a) 0.0 (b) 0.1 (c) 0.3 (d) 0.7 wt% of nano-Al2O3 composites in seawater aged composites.
and crosshead speed of the test was 72 mm and 1 mm/min respectively. Fig. 5 shows the effect of nano-Al2O3 content on (a) flexural strength (b) strain and (c) modulus of dry and seawater aged composites. The results revealed that with the addition of 0.1 wt% of nano-Al2O3 into the epoxy matrix, flexural strength increased by 11% in dry condition. In sweater aged condition, the flexural strength of the 0.1 wt% of nano-Al2O3 content nanocomposites was improved by 12% as compared to plain epoxy composites. With the increase in nano-Al2O3 content, flexural strain reduces, resulting, flexural modulus increases in the nanocomposites [25]. This is due to degradation of polymer matrix in seawater increases its brittleness of the nanocomposites. The enhancement of mechanical properties may be attributed to better interface bond between epoxy and glass fiber at 0.1 wt% nano-Al2O3 content, resulting reduction in water diffusion into the nanocomposites. However, with further increases in nano-Al2O3 content, flexural strength reduces. This is because with the increase in nano-Al2O3 content void content and agglomeration of nano-Al2O3 particle increases, leads to matrix swelling and microcrack formation at the interface [25]. 3.4. Interlaminar shear strength (ILSS) Interface is the heart of the fiber reinforced polymer composites. Interlaminar shear strength (ILSS) is the resistance to shear failure. As the composites were fabricated layer by layer, it is necessary to evaluate the adhesive bond between two layers through this test. ILSS test has been carried out as per the ASTM D2344
standard, where the span length was considered 27 mm and crosshead speed of 1 mm/min. Fig. 6 shows the ILSS of plain and nanocomposites. It is observed that with the addition of 0.1 wt% nano-Al2O3 into the epoxy matrix, ILSS increases by 11% in seawater aged samples as compared to plain GFRP composites. The improvement in mechanical properties may be due to good interface between matrix and fiber and less water absorption in the nanocomposites as compared to plain GFRP composites. However, with further increase in nano-Al2O3 content in the nano-composites, ILSS reduces. This is because of the higher amount of void formation, more water absorption and agglomeration leads to matrix swelling and microcrack formation at the interface resulting decrease in the interface bond between matrix and fiber [26]. 3.5. Analysis of fractured surfaces (FESEM) Fracture surfaces of ILSS and flexural test samples were investigated through field emission scanning electron microscope (NOVA NANOSEM450). Fig. 7 shows the fracture surface of plain and Table 2 Glass transition temperature of dry and sea water aged GFRP composites. S No.
Al2O3 (Wt. %)
1 2 3 4
0.0 0.1 0.3 0.7
Glass Transition Temperature(Tg),0C Dry Condition
Seawater Aged
122 123 125 124
95 95 96 97
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R.K. Nayak / Construction and Building Materials 221 (2019) 12–19
Fig. 9. (a) Heat flow versus temperature in dry and (b) seawater aged condition of plain and nanocomposites.
nanocomposites having different wt. % of Nano–Al2O3. It is observed that matrix drainage and interfacial debonding was the cause of failure in plain GFRP composites. Highly oriented shear cups, good interface bonding, and tough matrix were observed in nano-GFRP composites leads to improvement of strength and toughness of the nano composites. However, with increase in wt.% of nano-Al2O3 content, flexural and interlaminar shear strength reduces. This is due to agglomeration of nanoparticles at higher wt.% of nano-Al2O3 content reduces the interface bond between fiber and matrix shown in Fig. 2 (b). Fig. 8 shows the fracture surfaces of seawater aged plain and nanocomposites. Fig. 8 (a) shows smooth fiber imprints of seawater aged plain GFRP composites which are due to leaching out of silane from glass fiber leads to decrease in interface bond strength between matrix and glass fiber. However, in Fig. 8 (b), (c) and (d) matrix toughening, shear cusps and good interface bond was observed which lead to the enhancement of mechanical properties of the nanocomposites in seawater aging.
3.6. Thermal properties (glass transition temperature) Composites are used in different environment such as water, seawater, and chemical industries. The temperature of the working environment will be different in different during the service. Therefore, the thermal properties of the composites are very critical in terms of durability and reliability of the composites. The glass transition temperature was measured for dry and seawater aged composites using Differential Scanning Calorimetry (DSC822 of Mettler Toledo) and reported in Table 2. Fig. 9(a) and 9(b) show heat flow versus temperature for dry and seawater aged composites respectively. It is observed that the glass transition temperature has been reduced after seawater aging in all types’ composites. However, the effect of nano-Al2O3 does not have significant effect on it. The reduction in glass transition temperature is attributed to plasticization, oxidation and hydrolysis of epoxy matrix [27]. Nayak et al. [25,28,29] found that in hydrothermal aging, with the addition of nano-TiO2 or nano-Al2O3 into the epoxy matrix has not changed the glass transition temperature of the nanocomposites significantly. Therefore, the recommended temperature at which the nanocomposites can be used is around 120 °C in dry condition and 95 °C in hydrothermal environment.
4. Conclusion The addition of nano-Al2O3 particles into the epoxy matrix at different wt.% and seawater aging on mechanical and thermal properties was investigated for GFRP nanocomposites. The following conclusions may be drawn. 1. With the addition of 0.1 wt% of nano-Al2O3 particles into the epoxy matrix reduces the water diffusion coefficient by 17%, improved interlaminar shear strength by11% and flexural strength by 13% as compared to plain GFRP composites. 2. However, with the addition of nano-Al2O3 particles into the GFRP composites, glass transition temperature has not improved significantly. 3. The mode of failure of the composites was basically interface de-bonding, matrix cracking and deformation. 4. The enhancement of mechanical properties of nano-Al2O3 embedded nanocomposites creates possible opportunity to be used in seawater environment as compared to plain GFRP composites. 5. However, other design parameters need to be evaluated to understand the full potential of the nanocomposites.
Declaration of Competing Interest None. Acknowledgement I would like to thank KIIT deemed to be University, Bhubaneswar and NIT, Rourkela for their support to carry out the experiment successfully. I would like thank to that the Director, MANIT, Bhopal to his support and encouragement to write the research article successfully. References [1] P. Gonon, A. Sylvestre, J. Teysseyre, C. Prior, Combined effects of humidity and thermal stress on the dielectric properties of epoxy-silica composites, Mater. Sci. Eng. B. 83 (2001) 158–164, https://doi.org/10.1016/S0921-5107(01) 00521-9.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of seawater aging on mechanical properties of nano-Al2O3 embedded glass fiber reinforced polymer nanocomposites Ramesh Kumar Nayak Department of Materials and Metallurgical Engineering, Maulana Azad National Institute of Technology (MANIT), Bhopal 462003, India
h i g h l i g h t s Nano-Al2O3 enhanced nanocomposites reduce the water diffusion coefficient by 17 %. It improves the interlaminar shear strength by11% and flexural strength by 13%. The glass transition temperature of the nanocomposites has not improved significantly. Nano-Al2O3 embedded nanocomposites create the opportunity to be used in seawater environment.
a r t i c l e
i n f o
Article history: Received 16 July 2018 Received in revised form 28 May 2019 Accepted 3 June 2019 Available online 12 June 2019 Keywords: Glass fiber Nanocomposites Nano-Al2O3 Flexural strength, Seawater aging
a b s t r a c t Retention of mechanical and thermal properties of nano-Al2O3 embedded glass fiber reinforced polymer (GFRP) composites in marine environment is necessary and need to be evaluated to realize the full potential of the nanocomposites. The plain and nanocomposites were seawater aged in a temperature controlled seawater bath at 70 °C for a duration of 40 days. The results revealed that seawater aged plain and nanocomposites deteriorate their flexural strength, interlaminar shear strength and glass transition temperature. However, with the addition of 0.1 wt% of nano-Al2O3 particles into the plain GFRP composite, the flexural strength increased by 12%, interlaminar shear strength by 11% in dry condition and seawater diffusion coefficient was reduced by 17% as compared to plain GFRP composites. Nevertheless, the glass transition temperature of the nanocomposites was not improved in dry and seawater aged conditions significantly. The fractured surfaces of tested samples were investigated through FESEM to support the new findings. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy polymer is used in different application such as adhesive, matrix of fiber reinforced polymer composites and coatings. It has good adhesion, chemical, and water resistance as compared to other thermosetting polymers. Fiber reinforced polymer (FRP) composites are used in some of the structural components of aircraft, automobile, and marine industries. The FRP composites are also used in the form of pipes in oil transport and chemical industries. The polymer matrix composites have the tendency to absorb moisture in the hydrothermal environment and resulting in deterioration of its physical and mechanical properties [1,2]. The absorbed moisture of the composites either bonded with the hydroxyl group of the epoxy or occupies the voids present in it or at the fiber/matrix interface. FRP composites degrade through plasticization, swelling, chain scission and hydrolysis of the polymeric chain. Overall, mois-
E-mail address: [email protected] https://doi.org/10.1016/j.conbuildmat.2019.06.043 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ture absorption leads to change in thermo-physical, mechanical and chemical properties of FRP composites [3], as a result the durability and reliability of FRP composite is uncertain. Therefore, improvement of its durability and reliability in a hydrothermal environment is necessary and need to be explored. In polymer matrix composites, the interface of matrix and fiber is the heart of the composites. This is because, at the interface, the matrix transfers the load to the fiber. A strong interface bond transfers the load from the matrix to the fiber fully and enables the composites more reliable and durable. However, in hydrothermal aging, the interface became weaker because of uneven thermal expansion of epoxy and fiber and entrapment of water at the interface. Therefore, in seawater environment, the mechanical properties of polymer matrix composites are deteriorated [4–6]. Nanoparticles enables to form good interface bond between polymer matrix and fiber [7]. Therefore, the addition of nanoparticles into the epoxy matrix is one of the methods to enhance the mechanical properties and durability in a hydrothermal environ-
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ment [8,9]. Inorganic nanoparticles are basically Al2O3, TiO2, SiO2, and other metal and non-metallic oxides. Organic nanofillers are CNT, SWCNT, DWCNT and grapheme. Organic nanofillers require complex fabrication technique to blend with the epoxy matrix as compared to metal and non-metallic oxide nanoparticles. Therefore, non-metallic or metallic oxide nanofillers are popular in fiber reinforced polymer composites due to its low cost and easy fabrication method as compared to organic base nanofillers. Inorganic nanofillers are dispersed in the polymer matrix and form either physical or mechanical or chemical or combination of bonds with epoxy matrix [10]. The well dispersed nanoparticles in the epoxy matrix enhance the mechanical properties of the nanocomposites. This is because, during the deformation process, the microcrack may divert the path in front of the nanoparticles or blunt or pin on it. As a result, the micro crack needs more energy for the failure to occur [11]. Addition of nano-Al2O3 and TiO2 reduces water diffusivity and enhances the hydrothermal durability [12–15]. The addition of 2 wt% nanosilica particles into the epoxy polymer increases fracture toughness and wear resistance of the nanocomposites as compared to plain epoxy composites in a seawater environment [16]. Glass fiber reinforced polymer composites are used in the marine applications. The improvement of reliability and durability of these composites in seawater environment are necessary. The addition of nano-Al2O3 particles into glass fiber reinforced polymer matrix composites on mechanical and thermal properties of seawater aged nanocomposites were not evaluated and discussed in existing open literature. Therefore, the current investigation reports the effect of nano-Al2O3 content and seawater aging on mechanical and thermal properties of the nanocomposites. Furthermore, the mechanical properties were compared with plain GFRP composites. The new finding has been envisaged through the fractured surfaces analysis of FESEM images.
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2. Materials and experimental procedure The raw materials used to fabricate the plain GFRP composites was woven roving E-Glass fiber of 300gsm, procured from Owens Corning, India, epoxy of Diglycidyl ether of Bisphenol A (DGEBA) type and hardener of Triethylene tetra amine was procured from Atul Industries, India. The nano-Al2O3 (alpha) particles were procured from SRL Industries limited, India. The average particle sizes of the nanoparticles were <50 nm and molecular weight is 101.96. Fig. 1 shows the schematic diagram of fabrication process of nanocomposites. Nanoparticles were dried at 100 °C for two hours to ensure no moisture present in it before it gets mixed with epoxy matrix [17]. Nano-Al2O3 particles were blended with the epoxy matrix at different wt.% (0.1–0.7 wt%) through magnetic stirring followed by ultrasonication. During the mixing process, the temperature was maintained at 60 °C to reduce the viscosity of epoxy. The nanoAl2O3 blended epoxy was kept on hold for 1 h to ensure removal of entrapped air bubbles form it. The composite laminates contain 16 layers of glass fibers. The ratio of epoxy and glass fiber by weight was 40:60. The composite laminates were cured at 140 °C for 6 h in an oven. The composite laminates were cut as per the ASTM D 3171-99, ASTM D7264, and ASTM D2344 standard to evaluate different properties. Fig. 2 (a) and (b) shows the dispersion of nano-Al2O3 particles in the nanocomposites having 0.1 wt% and 0.7 wt% of nano-Al2O3 content respectively. It is observed that there is a reasonably good dispersion of nano- Al2O3 particles in the epoxy matrix of the nanocomposites having 0.1 wt% as compared to the composite having 0.7 wt% of nano-Al2O3. The physical and mechanical properties of the raw materials are reported in Table 1.
Fig. 1. Schematic view of the fabrication method of nano- Al2O3 content GFRP composites [13].
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Fig. 2. Dispersion of nano-Al2O3 particles in the epoxy matrix having (a) 0.1 (b) 0.7 wt% nano- Al2O3.
Table 1 Properties of the raw materials [15]. Properties
Epoxy
Al2O3(alpha)
Glass fiber
Density g/cm3 Tensile strength (MPa) Tensile modulus (GPa) Poisson’s ratio
1.15 70 3.6 0.30
3.90 260 370 0.21
2.58 3800 78 0.20
3. Results and discussions 3.1. Void content Seawater can penetrate into the polymer matrix composites through the fiber–matrix interface, voids and microcracks present in the composites resulting reduction of the fiber–matrix interface bond. In seawater, NaCl is present in the form of cations and anions. These ions would penetrate with the water molecules into the composites, causing damage to the matrix, fiber and their interface [18]. Therefore, determination of void content in plain and nano GFRP composites is essential. The voids content in the composites were determined as per ASTM D 3171–99 standard and using Eq. (1). As per the standard, six numbers of samples of each type having surface area 25 mm 25 mm were taken into account for this analysis. Initial weight and dimensions of the samples were measured through high accuracy (0.01 mg) weighing balance and digital vernier caliper respectively.
V v ¼ 1 qc
wf
qf
þ
wm
qm
! ð1Þ
where Vv is the volume fraction of void qc, qf and qm is the density of composites, fiber, and matrix respectively. The weight fraction of fiber and matrix is wf and wm respectively. Effect of nano-Al2O3 content on void percentage is reported in Fig. 3. The results revealed that with the increase in nano-Al2O3 content in the composites, void percentage increases. This may be due to the entrapped gas was unable to come out from the epoxy matrix during the fabrication and curing process. However, the presence of void in the composites facilitates seawater absorption into the composites which leads to degradation of its physical and mechanical properties.
Fig. 3. Void content versus nano-Al2O3 content (wt.%) in the composites.
3.2. Weight gain (Seawater Aging) Plain and nanocomposites were dried in an oven at 100 °C for five hours and furnace cooled. The rate of water absorption increases with increase in temperature [15]. Therefore, the samples were dipped into a temperature controlled seawater bath which was maintained at 70 °C throughout the aging process. The seawater was collected from sea at Puri, Odisha, India. The pH of the seawater was measured and found to be 6.7. The edges of specimen were not sealed and the diffusion of water took place from the sides as well as thickness direction. Accelerated aging has been done to realize the maximum degradation of the nanocomposites. The samples were weighed before it gets dipped into the seawater bath. As per the standard, six numbers of samples of each type were dipped into the seawater for this analysis. The weight of the samples was measured at a different interval of time. The amount of seawater absorbed by the composites was calculated using Eq. (2).
Mt ¼
m mo 100% mo
ð2Þ
where Mt is the amount of seawater absorbed in percentage at time t, mo and m is the weight of the samples in dry and after time t respectively.
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Fig. 4. (a) Absorbed seawater wt.% as a function of square root of aging time and (b) diffusion coefficient versus nana-Al2O3 content (wt.%) of the composites.
Fig. 5. Comparison of (a) flexural strength (b) strain and (c) modulus versus nano-Al2O3 content in dry and seawater aged condition.
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respectively. Dz indicates diffusion coefficient in thickness direction only. However, in reality, the diffusion takes place in all direction. Therefore the corrected diffusion coefficient(D) was calculated by using the Eq. (4) [19].
D¼
Fig. 6. Interlaminar shear strength of dry and seawater aged composite samples versus nano-Al2O3 content.
The Fickian diffusion coefficient was calculated using the Eq. (3).
Dz ¼ p
h 4 ð%M m Þ
2
2 ð%M2 %M1 Þ pffiffiffiffi pffiffiffiffi t2 t1
ð3Þ
where Dz is the diffusion coefficient in the thickness direction and h is the thickness of the sample. M1, M2 and Mm is the percentage of water gain in t1, t2 and in saturation condition of the aging process
Dz 2 1 þ hl þ wh
ð4Þ
where l and w is the length and width of the sample respectively. Fig. 4 (a) shows seawater gain (wt.%) as a function of square root of time. It is observed that with the increase in nano-Al2O3 content in the composites water gain wt.% increases. Nano-Al2O3 particles enhance the interface bond between fiber and matrix and at the same time it increases the void content in the nano-composites. As a result, the reduction of water absorption with the increase in nano-Al2O3 content is not realized. Fig. 4 (b) shows the effect of nano-Al2O3 content on the diffusion coefficient of seawater. It is observed that the nanocomposites having 0.1 wt% nano-Al2O3 has reduced the diffusion coefficient by 17% as compared to plain GFRP composites. This is because of uneven thermal expansion of epoxy (6.2 105 K1) [20], fiber (5–12 106 K1) [21] and nano-Al2O3 particles (8.1 106 K1) [22] leading to matrix swelling and microcrack formation at the interface. Therefore, it is expected that with the increase in moisture content of the nanocomposites mechanical and thermal properties would be degraded [23,24]. 3.3. Flexural strength The flexural strength of the composites was determined as per ASTM D7264 and using Universal Testing Machine (UTM) of INSTRON 5967 equipped with a 5KN load cell. The span length
Fig. 7. FESEM images of fracture surfaces of the composites having (a) 0.0 (b) 0.1 (c) 0.3 (d) 0.7 wt% of nano-Al2O3 content hybrid composites in dry condition.
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Fig. 8. FESEM images of fracture surfaces of the composites having (a) 0.0 (b) 0.1 (c) 0.3 (d) 0.7 wt% of nano-Al2O3 composites in seawater aged composites.
and crosshead speed of the test was 72 mm and 1 mm/min respectively. Fig. 5 shows the effect of nano-Al2O3 content on (a) flexural strength (b) strain and (c) modulus of dry and seawater aged composites. The results revealed that with the addition of 0.1 wt% of nano-Al2O3 into the epoxy matrix, flexural strength increased by 11% in dry condition. In sweater aged condition, the flexural strength of the 0.1 wt% of nano-Al2O3 content nanocomposites was improved by 12% as compared to plain epoxy composites. With the increase in nano-Al2O3 content, flexural strain reduces, resulting, flexural modulus increases in the nanocomposites [25]. This is due to degradation of polymer matrix in seawater increases its brittleness of the nanocomposites. The enhancement of mechanical properties may be attributed to better interface bond between epoxy and glass fiber at 0.1 wt% nano-Al2O3 content, resulting reduction in water diffusion into the nanocomposites. However, with further increases in nano-Al2O3 content, flexural strength reduces. This is because with the increase in nano-Al2O3 content void content and agglomeration of nano-Al2O3 particle increases, leads to matrix swelling and microcrack formation at the interface [25]. 3.4. Interlaminar shear strength (ILSS) Interface is the heart of the fiber reinforced polymer composites. Interlaminar shear strength (ILSS) is the resistance to shear failure. As the composites were fabricated layer by layer, it is necessary to evaluate the adhesive bond between two layers through this test. ILSS test has been carried out as per the ASTM D2344
standard, where the span length was considered 27 mm and crosshead speed of 1 mm/min. Fig. 6 shows the ILSS of plain and nanocomposites. It is observed that with the addition of 0.1 wt% nano-Al2O3 into the epoxy matrix, ILSS increases by 11% in seawater aged samples as compared to plain GFRP composites. The improvement in mechanical properties may be due to good interface between matrix and fiber and less water absorption in the nanocomposites as compared to plain GFRP composites. However, with further increase in nano-Al2O3 content in the nano-composites, ILSS reduces. This is because of the higher amount of void formation, more water absorption and agglomeration leads to matrix swelling and microcrack formation at the interface resulting decrease in the interface bond between matrix and fiber [26]. 3.5. Analysis of fractured surfaces (FESEM) Fracture surfaces of ILSS and flexural test samples were investigated through field emission scanning electron microscope (NOVA NANOSEM450). Fig. 7 shows the fracture surface of plain and Table 2 Glass transition temperature of dry and sea water aged GFRP composites. S No.
Al2O3 (Wt. %)
1 2 3 4
0.0 0.1 0.3 0.7
Glass Transition Temperature(Tg),0C Dry Condition
Seawater Aged
122 123 125 124
95 95 96 97
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Fig. 9. (a) Heat flow versus temperature in dry and (b) seawater aged condition of plain and nanocomposites.
nanocomposites having different wt. % of Nano–Al2O3. It is observed that matrix drainage and interfacial debonding was the cause of failure in plain GFRP composites. Highly oriented shear cups, good interface bonding, and tough matrix were observed in nano-GFRP composites leads to improvement of strength and toughness of the nano composites. However, with increase in wt.% of nano-Al2O3 content, flexural and interlaminar shear strength reduces. This is due to agglomeration of nanoparticles at higher wt.% of nano-Al2O3 content reduces the interface bond between fiber and matrix shown in Fig. 2 (b). Fig. 8 shows the fracture surfaces of seawater aged plain and nanocomposites. Fig. 8 (a) shows smooth fiber imprints of seawater aged plain GFRP composites which are due to leaching out of silane from glass fiber leads to decrease in interface bond strength between matrix and glass fiber. However, in Fig. 8 (b), (c) and (d) matrix toughening, shear cusps and good interface bond was observed which lead to the enhancement of mechanical properties of the nanocomposites in seawater aging.
3.6. Thermal properties (glass transition temperature) Composites are used in different environment such as water, seawater, and chemical industries. The temperature of the working environment will be different in different during the service. Therefore, the thermal properties of the composites are very critical in terms of durability and reliability of the composites. The glass transition temperature was measured for dry and seawater aged composites using Differential Scanning Calorimetry (DSC822 of Mettler Toledo) and reported in Table 2. Fig. 9(a) and 9(b) show heat flow versus temperature for dry and seawater aged composites respectively. It is observed that the glass transition temperature has been reduced after seawater aging in all types’ composites. However, the effect of nano-Al2O3 does not have significant effect on it. The reduction in glass transition temperature is attributed to plasticization, oxidation and hydrolysis of epoxy matrix [27]. Nayak et al. [25,28,29] found that in hydrothermal aging, with the addition of nano-TiO2 or nano-Al2O3 into the epoxy matrix has not changed the glass transition temperature of the nanocomposites significantly. Therefore, the recommended temperature at which the nanocomposites can be used is around 120 °C in dry condition and 95 °C in hydrothermal environment.
4. Conclusion The addition of nano-Al2O3 particles into the epoxy matrix at different wt.% and seawater aging on mechanical and thermal properties was investigated for GFRP nanocomposites. The following conclusions may be drawn. 1. With the addition of 0.1 wt% of nano-Al2O3 particles into the epoxy matrix reduces the water diffusion coefficient by 17%, improved interlaminar shear strength by11% and flexural strength by 13% as compared to plain GFRP composites. 2. However, with the addition of nano-Al2O3 particles into the GFRP composites, glass transition temperature has not improved significantly. 3. The mode of failure of the composites was basically interface de-bonding, matrix cracking and deformation. 4. The enhancement of mechanical properties of nano-Al2O3 embedded nanocomposites creates possible opportunity to be used in seawater environment as compared to plain GFRP composites. 5. However, other design parameters need to be evaluated to understand the full potential of the nanocomposites.
Declaration of Competing Interest None. Acknowledgement I would like to thank KIIT deemed to be University, Bhubaneswar and NIT, Rourkela for their support to carry out the experiment successfully. I would like thank to that the Director, MANIT, Bhopal to his support and encouragement to write the research article successfully. References [1] P. Gonon, A. Sylvestre, J. Teysseyre, C. Prior, Combined effects of humidity and thermal stress on the dielectric properties of epoxy-silica composites, Mater. Sci. Eng. B. 83 (2001) 158–164, https://doi.org/10.1016/S0921-5107(01) 00521-9.
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