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vinylester composites
Ocean Engineering 108 (2015) 393–401
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Investigation of seawater effects on the mechanical properties of untreated and treated MMT-based glass fiber/vinylester composites Garima Mittal a, Vivek Dhand a, Kyong Yop Rhee a,n, Soo Jin Park b, Hyeon-Ju Kim c, Dong Ho Jung c a
Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea Department of Chemistry, Inha University, 253 Nam-gu, Incheon 402-751, Republic of Korea c Maritime and Ocean Engineering Research Institute, Korea Institute of Ocean Science and Technology, Daejeon 305-600, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 March 2015 Accepted 14 August 2015 Available online 2 September 2015
The present investigation is focused on montmorillonite (Cloisites Na þ )-based vinylester/glass fiber composites. In the present study, two types of composites are processed by incorporating 1.0 wt% untreated and treated montmorillonite into vinylester/glass fiber composites and the effect of seawater absorption on their mechanical properties was studied for marine applications. The surface treatment of MMT was performed using 3-aminopropyltriethoxysilane. When montmorillonite clay comes into contact with water, swelling between layers occurs that consequently alters the mechanical properties of the composites. The tensile and bending strengths of the composites before and after seawater absorption were investigated using ASTM methods with a Universal Testing Machine. Chemical and physical changes were studied through X-ray diffraction and Fourier-transform infrared spectroscopy. Thermogravimetric analysis was performed to analyze the effect of seawater absorption on the thermal properties. The morphology of the fractured surfaces before and after seawater absorption was analyzed using FE-SEM. The results show that treated MMT-based composites show better mechanical properties in dry and seawater-absorbed conditions than the untreated MMT-based composites, but the mechanical properties of both composites decrease after seawater immersion. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Montmorillonite (MMT) Surface-treated montmorillonite Swelling Seawater absorption Mechanical properties
1. Introduction Multiscale composites in which both nano- and micro-sized reinforcing materials have been used to augment the mechanical and thermal properties of composites have recently attracted the interest of many researchers. Carbon nanotubes (CNTs), basalt fibers, glass fibers, carbon fibers, graphite, and montmorillonite (MMT) are examples of extensively-used reinforced materials. These multiscale composites are used in a broad range of applications (Azeez et al., 2013a, 2013b; Kim et al., 2011a, 2011b; Marroquin et al., 2013). The properties of these kinds of composites vary based on the reinforcing materials used and their interaction with the host matrix. The properties of the composite can usually be improved by enhancing the interaction between various reinforcements and the host matrix through surface modification of the reinforced material (Chen et al., 2014; Kim et al., 2011a, 2011b; Michelis and Vlachopoulos, 2013; Rhee et al., 2012; Wan et al., 2014). These multiscale composites with increased mechanical properties are
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Corresponding author. Tel.: þ 82 31 201 2565; fax: þ 82 31 202 6693. E-mail addresses: [email protected] (G. Mittal), [email protected] (V. Dhand), [email protected] (K.Y. Rhee), [email protected] (S.J. Park), [email protected] (H.-J. Kim), [email protected] (D.H. Jung). http://dx.doi.org/10.1016/j.oceaneng.2015.08.019 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
widely used for marine applications (Cui et al., 2013; Lee et al., 2011; Min et al., 2014; Rhee et al., 2004; Visco et al., 2008; Wang et al., 2014). Degradation of the mechanical properties during exposure to high humidity, pH, and temperature is the main drawback for polymer composites (Ha et al., 2010; Kim et al., 2014). Alkaline water absorption results in deterioration in the interfacial bond strength between the matrix and reinforced material, an increase in free volume, and reduction in the processing-generated internal stress in the composite (Mahesh et al., 2013). Over the past few decades, glass fiber-reinforced polymers (GFRPs) have been extensively used in certain marine applications. Glass fibers possess high strength, durability, and high stiffness. Moreover, glass fibers show high chemical resistance and are less brittle than carbon fibers; they are also easy to produce and are cost effective. For all of these reasons, GFRPs show potential for use in numerous applications (Correia et al., 2011; Sun et al., 2008; Viets et al., 2013). Nematollahi et al. (2010) studied the corrosion performance of epoxy coatings containing glass flakes as a reinforcing material. These results showed outstanding adhesion to the substrate and longer barrier spots for the nano-glass fiberfilled coating, which provided improvement in the corrosion resistance compared to neat epoxy material. Alternatively, vinylester is a broadly-used thermoset polymer for marine applications
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Fig. 1. Chemical structure of montmorillonite.
and combines the properties of epoxy and polyester resins to provide superior mechanical properties and easy processing. Moreover, vinylester possesses excellent barrier and thermal properties, chemical resistance, and corrosion resistance (Cook et al., 1997; Lima Sobrinho et al., 2009). Alternatively, layered silicates have attracted attention since prehistoric times due to their unique structural and barrier properties (De Bonis et al., 2014). MMT is one of the most widely used layered silicates from the smectite family with a 2:1 parallel arrangement of octahedral and tetrahedral sheets (Fig. 1). First group cations reside between negatively-charged MMT layers that can be separated and modified by ion exchange reactions (Wang et al., 2006). Many studies have been conducted to enhance the mechanical and thermal properties of polymers by incorporating clay particles into the polymer matrix. For instance, Lai et al. (2014) improved the abrasion resistance of poly(vinylidene fluoride) by incorporating nanoclay. Mohan and Kanny (2012) enhanced the mechanical properties of glass fiber (GF)-reinforced HDPE polymers with nanoclay reinforcements, while Mahesh et al. (2013) prepared MMT vinylester/glass nanocomposites to improve the fire retardation properties. Clay minerals are hydrophilic and swell upon exposure to water. A number of studies have been performed regarding the effect of moisture on clays (Bowders and Daniel, 1987; Ebrahimi et al., 2012; Kaya and Fang, 2000). When clay particles come into contact with moisture or high pH conditions, physiochemical changes occur, resulting in an increase in the interplanar distance between the clay platelets. Subsequently, the material performance decreases dramatically with increased swelling (Mahesh et al., 2013). The introduction of nanocomposites has created novel opportunities to design steadier, more durable, and more robust polymer composites. Composite properties are enhanced by the addition of a small weight fraction of reinforcing nanomaterials. Based on this, nanoclay is used in place of clay. MMT is hydrophilic in nature and contains Na þ and K þ ions in its interplanar region. Surface modification of MMT powder is achieved with silane coupling agents through an ion exchange reaction in which
quaternary or onium ions replace Na þ and K þ ions and increase the interplanar distance by functional group grafting, with each plate of the MMT powder behaving as a nanoplatelet (Ha et al., 2008; Lin et al., 2010; Shanmugharaj et al., 2006). Therefore, modified MMT shows enhanced interactions with the polymer matrix in the nanoform. Generally, alkylammonium ions are used to exfoliate the MMT filler into the matrix. Due to these ion exchange reactions, hydrophilic MMT changes into hydrophobic MMT, which repels water molecules and prevents water penetration into the composites. After silanization, modified hydrophobic MMT becomes more compatible with hydrophobic organic polymers. Hence, modified MMT-reinforced nanocomposites provide improved mechanical properties (Alamri and Low, 2012, Azeez et al., 2013a, 2013b, and Park et al., 2009). On the basis of the above discussion, we studied seawater effects on untreated and treated MMT-based glass fiber-reinforced polymer (GFRP) nanocomposites. Although many studies have been conducted on the effects of seawater on polymer composites, few comparative studies of seawater effects on the mechanical properties of multiscale GFRP vinylester composites have been performed. In this study, 3-aminopropyltriethoxysilane (3-APTES) was used for the surface treatment of MMT powder. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) were used to evaluate changes in the composite system after seawater absorption. A morphological investigation of the fractured surfaces of composites was performed using SEM. Furthermore, the effects of seawater absorption on the mechanical properties of the composites were analyzed through tensile and bending tests.
2. Experimental 2.1. Materials All chemicals were purchased from Sigma-Aldrich Co., USA. MMT clay powder (Cloisites Na þ ) with an interplanar distance of 11.74 Å and a cation exchange capacity of 92.6 meq/100 g clay was
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used. A surface coupling agent, 3-APTES, and sterile-filtered seawater were used. Vinylester resin SR 830 with a viscosity of 1.5–1.7 poise at room temperature and a 25–30 min pot life was used as the matrix. Benzoylperoxide (BPO) was used as a hardener. Chopped glass fibers mat of E-type glass (380 g/m2) was used. 2.2. Surface treatment of the MMT powder (treated/silanized MMT) As mentioned in previous studies, surface treatment of MMT was performed using 3-APTES (Shanmugharaj et al., 2006). An aqueous solution of as-received MMT (1 g in 10 ml) was mixed with 20 ml ethyl alcohol for 5 h at 50 1C. Separately, 4 g of 3-APTES was mixed with 12 ml ethyl alcohol for 2 h at 25 1C. These two solutions were then mixed together and stirred for 3 h at room temperature. After stirring, the solution was filtered, washed with distilled water, and dried for 48 h at 80 1C in vacuum. 2.3. Preparation of MMT-based composites The synthesis of MMT-based composites was performed using the following process (Fig. 2). First, 1.0 wt% untreated MMT powder was mixed into 100 g of vinylester resin with constant stirring at room temperature for 3 h, followed by the addition of 1.0 wt% hardener BPO. The mixture was poured over 5 plies of the chopped glass fibers mat (20 20 cm2) and pressed into a mold using a hot press at 110 1C and 20 MPa for 10 min. The prepared sample was maintained in a vacuum oven for 24 h. A similar procedure was followed for the synthesis of GFRP vinylester
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composites with treated MMT powder. Test specimens were prepared by molding and cutting. 2.4. Characterization XRD analysis of the MMT-based composites was performed using a copper cathode for X-ray radiation (λ ¼ 0.1542 nm) at a voltage of 40 kV and a current of 100 mA. Data was collected from 3 to 90 1C in 0.02 1C steps. XRD data provides the interlayer spacing of the MMTs after modification and in each polymer composite. FTIR spectroscopy was used to study the effect of silanization of MMT from 4000 to 400 cm 1. Thermal stabilities of the systems were also determined using thermogravimetric analysis (TGA, SQT 600 model) from 30 to 700 1C at a heating rate of 10 1C/min. SEM was performed to study the mechanical and interfacial properties of the fractured surfaces of MMT in the composites. Absorption tests were performed at room temperature by immersing the tensile specimen into a seawater bath for 65 days. Before immersion, the samples were weighed under dry conditions. After immersion, these samples were weighed regularly up to their saturation limit. The mechanical properties, i.e. tensile and bending strength, of the system were analyzed with a Universal Testing Machine (UTM, Instron 8830). Tensile tests were performed according to the ASTM D638 specimen (Fig. 3). Dog bone-shaped specimens were prepared using a cutting tool and the tests were performed at room temperature with a speed of 6 mm/min. The elastic modulus and tensile strength were obtained during the tensile test, and the data was collected by an automated computer program. Bending
Fig. 2. Schematic representation of preparation of untreated and treated MMT-based composites.
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Fig. 3. Tensile specimen, ASTM D638.
Fig. 4. X-ray diffraction spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions.
specimens were created by cutting the samples into 130 25 1.7 (70.5) mm samples. The flexural strength and flexural modulus were measured using a three-point bending method with a UTM (Instron 8830) at room temperature and a speed of 6 mm/min. The flexural strength and flexural modulus were measured during the bending tests. For both cases, five tests were performed to confirm the reproducibility of the results.
3. Results and discussion XRD was performed to investigate the changes in the composites, exfoliation of the MMT layers, and intercalation of the polymer between the MMT platelets. Fig. 4 shows the XRD patterns of the untreated and treated MMT powders (a) and their composites (b) in dry and seawater-exposed conditions. Information regarding the dispersion of the MMT layers and d-spacing was obtained using Bragg's equation. Peaks and peak shifts illustrate the changes in the interplanar spacing between silicon platelets (Cook et al., 1997). As shown in Fig. 4(a), the untreated MMT
powder shows a characteristic (001) peak at 7.24, which corresponds to the d-value of 12.20 Å, while in the case of the treated MMT powder, the peak shifts to a lower theta value of 4.28 with a 20.60 Å d-value. The shift in the peak (approximately 2θ ¼2.96) indicates increased interlayer spacing between the MMT layers because of the silane treatment (Shanmugharaj et al., 2006). However, there is one more peak near 2θ ¼8.401, which may be due to some layers remaining in the original, unexfoliated form (Madaleno et al., 2010). Fig. 4(b) compares the XRD patterns of the untreated and treated MMT (1.0 wt%)-based composites in dry and seawater-exposed conditions. The diffractograms show that the peaks were shifted to lower theta values in both cases, indicating intercalation of the polymer between the MMT layers. In the dry case with the untreated MMTbased composites, the peak value shifted (2θ ¼ 6.66; d¼13.26 Å) due to intercalation, i.e. intercalation of the vinylester chains into the MMT layers without interrupting the morphology. Alternatively, for treated MMT-based composites, the peak shifted to a lower value (2θ ¼3.90; d¼22.58 Å) because of exfoliation, i.e., individual dispersion of MMT layers maintaining the average distance, caused by maximized
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Fig. 5. FTIR spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions.
interaction between the polymer and MMT layers. The diffractograms of seawater-exposed untreated and treated MMT-based composites do not indicate specific peaks that may be due to seawater absorption into the clay layers, resulting in swelling that increases the interlayer spacing, causing the peaks to disappear (Shanmugharaj et al., 2006). Fig. 5 provides the FTIR spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions. FTIR was conducted to investigate the changes in the chemical structure because of the surface treatment by 3-APTES. As shown in Fig. 5(a), significant features of the MMT were observed, including a band corresponding to –OH stretching bound to Al þ or Mg þ at 3634 cm 1, a strong absorption band of Si–O bond stretching between 1200 and 1000 cm 1 with a 1035 cm 1 maximum, a band at 916 cm 1 for Al(Al)OH and a band at 800 cm 1 for Mg(Mg)OH bending vibrations. For treated MMT, additional characteristic bands were revealed corresponding to the symmetric and asymmetric stretching of –CH and –CH2 near 2930 and 2850 cm 1, respectively, which signifies the successful attachment of silane groups to the surfaces of the MMT layers, accompanied by bands representing –NH2 and –CH2 bending vibrations at 1566, 1496 and 690 cm 1. In Fig. 5(b), MMT-based composites in dry and wet conditions exhibited very similar FTIR spectra, which is evidence of the lack of chemical changes in the composite after seawater immersion. Thermograms of 1.0 wt% untreated and treated MMT-based composites in dry and wet conditions are presented in Fig. 6. All of the samples displayed one-step degradation, but demonstrated a significant difference in the weight residue. For the dry case, the weight decomposition % for treated MMT-based composites was 32.08, which is less than the untreated MMT-based composites (33.77%) and, therefore, the mass residue is greater. This significant difference in thermal stability illustrates the incorporation of silane groups to the MMT layers (Cai et al., 2008). MMT layers provide thermal stability during decomposition by acting as a heat barrier. Therefore, when MMT is treated by 3-APTES, silane groups are attached to the MMT layers and the silane groups exfoliate them by increasing the interplanar distance. Consequently, each layer behaves as a nanoentity and uniform distribution of MMT layers occurs. Hence, because of the enhanced interactions and bonding, thermal stability of the treated MMT-based composite is better (5% more stable) than that of the untreated MMT composites. Alternatively, seawater-absorbed composites show the same pattern, but with decreased thermal stability as compared to the dry case. Seawater-absorbed untreated MMTbased composites show 3% less stability than the dry untreated
Fig. 6. Thermograms of untreated and treated MMT-based composites in dry and wet conditions.
MMT-based composites, while for the treated MMT-based composites its 8% less. Furthermore, weight loss for the seawater-absorbed untreated MMT composite (34.74) is slightly greater than that of the seawater-absorbed treated MMT composite (34.67) because of the evaporation of absorbed seawater into the system. These results indicate that the thermal stability of the composites decreases after seawater absorption. However, treated MMT-based composites are more stable than untreated MMT-based composites in both cases. DSC analysis of the samples was performed to determine the effect of the composition on the glass transition temperature of the system. Fig. 7 shows the DSC thermograms of untreated and treated MMT-based composites before and after 65 days of seawater exposure. All of the thermograms show the same pattern, indicating that it was an endothermic process. For the dry case, the onset degradation temperatures for untreated and treated MMTbased composites were 420.90 and 433.34 1C, respectively. The degradation temperature of the treated MMT-based composites increased after surface treatment, which may be due to the physio-chemical interactions between the matrix and reinforced material. After surface treatment, homogenous dispersion of the nano-sized MMT layers may occur and restrict the mobility of
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segments that enhance the degradation temperature (Mahesh et al., 2013). DSC analysis of samples with absorbed seawater showed the same thermogram pattern as the dry case. However, there was a gradual decrease in degradation temperature after seawater immersion, which might be because of plasticization of the matrix. Therefore, when water molecules penetrate the system through voids or destruction of weak zones, MMT swells, which undermines the adhesion by breaking the cross-links. Hence, the mobility of these segments in the system increases, which results in the decreased degradation temperature. Compared to the untreated MMT-based composites, the thermal stability of the treated MMT-based composites was better. Fig. 8 presents SEM images of untreated (a) and treated (b) MMT-based composites in dry conditions. In the case of the untreated MMT-based composites, there were fewer interactions between the matrix and fillers because of the hydrophobic nature and poor distribution of the MMTs in the polymer matrix. The smooth surface of the glass fibers confirmed the poor adhesion of the matrix to the fillers, which resulted in an increase in the number of voids. A large degree of fiber breakage was also observed. Due to the weak interfacial strength and poor load transfer, fiber pull-out and failure occur. Alternatively, when MMT
Fig. 7. DSC curves of untreated and treated MMT-based composites in dry and wet conditions.
is treated with silane groups, fewer voids and less fiber breakage occur. After the incorporation of silane, MMT becomes hydrophilic and strengthens the interactions between the matrix and filler. With the treatment of silane groups, the interplanar distance within MMT increases, allowing MMT layers to behave as nanoclay. Therefore, dispersion and adhesion also increase, resulting in enhanced mechanical properties (Mahesh et al., 2013; Kornmann et al., 2001). Fig. 8 shows SEM images of the specimens after seawater immersion. As shown in Fig. 9(a), there is more breakage and fiber pull-out and there is very weak adhesion between the matrix and fibers, while treated MMT-based composites experience less pull-out and fewer voids. When MMT comes into contact with water molecules, it swells, resulting in the generation of more gaps. Furthermore, water molecule insertion generates forces that cause fibers to be displaced from their initial positions, which results in failure. Water molecules enter by the destruction of weak zones on the surface, thereby creating voids. Due to the immense force caused by the water, the void sizes increase and consequently, the volume of the water within the composite also increases. Additionally, a small amount of MMT may be lost due to void creation, and a crack may form, prompting subsequent failure. Since MMT becomes hydrophobic after surface treatment, it resists water molecule insertion, while untreated MMT cases favor water absorption. Fig. 10 shows the weight increment in untreated and treated MMT composites due to seawater absorption. The composite containing untreated MMT showed a greater uptake of seawater compared to the composite with treated MMT. Initially, there was a sudden increase in absorption, which gradually increased in both composites. The number of voids, interfacial bonding strength, and properties of the reinforced material affected the water uptake of the composites. Poor adhesion and weak interaction between the matrix and untreated MMT and glass fibers caused an increase in the number of voids, which in addition to the weak zones, acted as entry points for water. In treated MMT-based composites, the whole system is hydrophobic in nature because surface treatment changes the MMT from hydrophilic to hydrophobic (and vinylester is already hydrophobic), which limits the absorption of water into the system compared to the untreated MMT-based composites. Due to long-term seawater absorption, the polymer matrix age and the mechanical properties become weak. In order to analyze the effect of long-term seawater absorption on the mechanical properties of the composites, tensile and bending tests were performed. The (a) tensile strength and
Fig. 8. SEM image of a fractured surface of (a) untreated MMT-based composites and (b) treated MMT-based composites, without seawater absorption.
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Fig. 9. SEM image of a fractured surface of (a) untreated MMT-based composites and (b) treated MMT-based composites after seawater absorption.
Fig. 10. Effect of seawater absorption on the weight of untreated and treated MMTbased composites.
(b) elastic modulus were compared between untreated and treated MMT-based composites in dry and seawater-absorbed conditions in Fig. 11. The results showed that the tensile strength of untreated and treated MMT-based composites decreased 7% and 8%, respectively, after seawater absorption. However, as compared to the untreated MMT-based composites, treated MMTbased composites displayed a 7% greater tensile strength in the dry case and a 6% greater higher tensile strength in seawaterabsorbed conditions. Similarly, a slight decrease ( 4%) was observed in the elastic modulus (Fig. 11(b)) of both the composites after seawater absorption and the treated MMT-based composites had a 4.5% greater elastic modulus in both the dry and seawaterabsorbed conditions. The treated MMT-based composites may exhibit better interactions and adhesion between the matrix and fillers due to the homogenous dispersion of MMT layers after silanization. Therefore, when the load is applied, crack propagation is confined by optimal distribution and the strong adhesion of MMT layers with the matrix, which leads to improved tensile strength. However, when these specimens are immersed in seawater, the tensile strength is diminished because of water molecule penetration into the system. Since MMT swells when it comes into contact with water molecules due to interactions with interlayer cations, the d-spacing between the layers increases.
Because of poor adhesion, weak interactions, and an increased number of voids, untreated MMT-based composites absorb more water than treated composites. Furthermore, MMT is hydrophilic in nature, but becomes hydrophobic after silanization and restricts water molecule penetration into the system. When force is applied to the specimen, the tensile strength is limited due to poor fiber adhesion or MMT pull-out from the matrix. Fig. 12 demonstrates the (a) maximum flexural strength and (b) flexural modulus of MMT-based composites in both dry and wet conditions. As shown in Fig. 12(a), the flexural strength of treated MMT-based composites is 9% greater in dry conditions and 11.5% greater in seawater-absorbed conditions than that of the untreated MMT-based composites. However, the flexural strength of both composites decreases 7% and 4.5%, respectively, after seawater absorption. Alternatively, Fig. 12(b) shows very marginal changes in flexural modulus of both untreated and treated MMT-based composites, with both exhibiting a very slight decrease ( 1–3%) after seawater absorption. Similar to the tensile tests, the moduli increase after silanization because of optimized dispersion and better adhesion that inhibits the crack propagation. After seawater absorption, the mechanical strength decreases due to the plasticity of the polymer matrix. Since water absorption was considerably greater in untreated MMT, the untreated composites showed a more substantial weakening of the mechanical properties.
4. Conclusion The above experiments were performed to examine the effects of seawater absorption on the mechanical properties of untreated and treated MMT-based vinylester/glass fiber composites. Surface treatment of MMT was performed with 3-APTES. The results indicate that after silanization, the interfacial interactions between MMT, glass fibers and vinylester increase and optimal distribution of MMT layers occurs, resulting in improved mechanical properties such as tensile and bending strength. However, after seawater absorption, the mechanical properties of both untreated and treated MMT-based composites decreased. The factor behind the weakening of these properties was the swelling of MMT after seawater absorption, which resulted in deterioration of the adhesion and interactions between the reinforced material and matrix. Treated MMT-based composites showed less substantial weakening than untreated MMT-based composites.
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Fig. 11. Comparison of the (a) maximum tensile strength and (b) elastic modulus of tensile specimens of untreated and treated MMT-based composites in dry and seawaterabsorbed conditions.
Fig. 12. Comparison of the (a) maximum flexural strength and (b) flexural modulus of bending specimens of untreated and treated MMT-based composites in dry and seawater-absorbed conditions.
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Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Investigation of seawater effects on the mechanical properties of untreated and treated MMT-based glass fiber/vinylester composites Garima Mittal a, Vivek Dhand a, Kyong Yop Rhee a,n, Soo Jin Park b, Hyeon-Ju Kim c, Dong Ho Jung c a
Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea Department of Chemistry, Inha University, 253 Nam-gu, Incheon 402-751, Republic of Korea c Maritime and Ocean Engineering Research Institute, Korea Institute of Ocean Science and Technology, Daejeon 305-600, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 March 2015 Accepted 14 August 2015 Available online 2 September 2015
The present investigation is focused on montmorillonite (Cloisites Na þ )-based vinylester/glass fiber composites. In the present study, two types of composites are processed by incorporating 1.0 wt% untreated and treated montmorillonite into vinylester/glass fiber composites and the effect of seawater absorption on their mechanical properties was studied for marine applications. The surface treatment of MMT was performed using 3-aminopropyltriethoxysilane. When montmorillonite clay comes into contact with water, swelling between layers occurs that consequently alters the mechanical properties of the composites. The tensile and bending strengths of the composites before and after seawater absorption were investigated using ASTM methods with a Universal Testing Machine. Chemical and physical changes were studied through X-ray diffraction and Fourier-transform infrared spectroscopy. Thermogravimetric analysis was performed to analyze the effect of seawater absorption on the thermal properties. The morphology of the fractured surfaces before and after seawater absorption was analyzed using FE-SEM. The results show that treated MMT-based composites show better mechanical properties in dry and seawater-absorbed conditions than the untreated MMT-based composites, but the mechanical properties of both composites decrease after seawater immersion. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Montmorillonite (MMT) Surface-treated montmorillonite Swelling Seawater absorption Mechanical properties
1. Introduction Multiscale composites in which both nano- and micro-sized reinforcing materials have been used to augment the mechanical and thermal properties of composites have recently attracted the interest of many researchers. Carbon nanotubes (CNTs), basalt fibers, glass fibers, carbon fibers, graphite, and montmorillonite (MMT) are examples of extensively-used reinforced materials. These multiscale composites are used in a broad range of applications (Azeez et al., 2013a, 2013b; Kim et al., 2011a, 2011b; Marroquin et al., 2013). The properties of these kinds of composites vary based on the reinforcing materials used and their interaction with the host matrix. The properties of the composite can usually be improved by enhancing the interaction between various reinforcements and the host matrix through surface modification of the reinforced material (Chen et al., 2014; Kim et al., 2011a, 2011b; Michelis and Vlachopoulos, 2013; Rhee et al., 2012; Wan et al., 2014). These multiscale composites with increased mechanical properties are
n
Corresponding author. Tel.: þ 82 31 201 2565; fax: þ 82 31 202 6693. E-mail addresses: [email protected] (G. Mittal), [email protected] (V. Dhand), [email protected] (K.Y. Rhee), [email protected] (S.J. Park), [email protected] (H.-J. Kim), [email protected] (D.H. Jung). http://dx.doi.org/10.1016/j.oceaneng.2015.08.019 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
widely used for marine applications (Cui et al., 2013; Lee et al., 2011; Min et al., 2014; Rhee et al., 2004; Visco et al., 2008; Wang et al., 2014). Degradation of the mechanical properties during exposure to high humidity, pH, and temperature is the main drawback for polymer composites (Ha et al., 2010; Kim et al., 2014). Alkaline water absorption results in deterioration in the interfacial bond strength between the matrix and reinforced material, an increase in free volume, and reduction in the processing-generated internal stress in the composite (Mahesh et al., 2013). Over the past few decades, glass fiber-reinforced polymers (GFRPs) have been extensively used in certain marine applications. Glass fibers possess high strength, durability, and high stiffness. Moreover, glass fibers show high chemical resistance and are less brittle than carbon fibers; they are also easy to produce and are cost effective. For all of these reasons, GFRPs show potential for use in numerous applications (Correia et al., 2011; Sun et al., 2008; Viets et al., 2013). Nematollahi et al. (2010) studied the corrosion performance of epoxy coatings containing glass flakes as a reinforcing material. These results showed outstanding adhesion to the substrate and longer barrier spots for the nano-glass fiberfilled coating, which provided improvement in the corrosion resistance compared to neat epoxy material. Alternatively, vinylester is a broadly-used thermoset polymer for marine applications
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Fig. 1. Chemical structure of montmorillonite.
and combines the properties of epoxy and polyester resins to provide superior mechanical properties and easy processing. Moreover, vinylester possesses excellent barrier and thermal properties, chemical resistance, and corrosion resistance (Cook et al., 1997; Lima Sobrinho et al., 2009). Alternatively, layered silicates have attracted attention since prehistoric times due to their unique structural and barrier properties (De Bonis et al., 2014). MMT is one of the most widely used layered silicates from the smectite family with a 2:1 parallel arrangement of octahedral and tetrahedral sheets (Fig. 1). First group cations reside between negatively-charged MMT layers that can be separated and modified by ion exchange reactions (Wang et al., 2006). Many studies have been conducted to enhance the mechanical and thermal properties of polymers by incorporating clay particles into the polymer matrix. For instance, Lai et al. (2014) improved the abrasion resistance of poly(vinylidene fluoride) by incorporating nanoclay. Mohan and Kanny (2012) enhanced the mechanical properties of glass fiber (GF)-reinforced HDPE polymers with nanoclay reinforcements, while Mahesh et al. (2013) prepared MMT vinylester/glass nanocomposites to improve the fire retardation properties. Clay minerals are hydrophilic and swell upon exposure to water. A number of studies have been performed regarding the effect of moisture on clays (Bowders and Daniel, 1987; Ebrahimi et al., 2012; Kaya and Fang, 2000). When clay particles come into contact with moisture or high pH conditions, physiochemical changes occur, resulting in an increase in the interplanar distance between the clay platelets. Subsequently, the material performance decreases dramatically with increased swelling (Mahesh et al., 2013). The introduction of nanocomposites has created novel opportunities to design steadier, more durable, and more robust polymer composites. Composite properties are enhanced by the addition of a small weight fraction of reinforcing nanomaterials. Based on this, nanoclay is used in place of clay. MMT is hydrophilic in nature and contains Na þ and K þ ions in its interplanar region. Surface modification of MMT powder is achieved with silane coupling agents through an ion exchange reaction in which
quaternary or onium ions replace Na þ and K þ ions and increase the interplanar distance by functional group grafting, with each plate of the MMT powder behaving as a nanoplatelet (Ha et al., 2008; Lin et al., 2010; Shanmugharaj et al., 2006). Therefore, modified MMT shows enhanced interactions with the polymer matrix in the nanoform. Generally, alkylammonium ions are used to exfoliate the MMT filler into the matrix. Due to these ion exchange reactions, hydrophilic MMT changes into hydrophobic MMT, which repels water molecules and prevents water penetration into the composites. After silanization, modified hydrophobic MMT becomes more compatible with hydrophobic organic polymers. Hence, modified MMT-reinforced nanocomposites provide improved mechanical properties (Alamri and Low, 2012, Azeez et al., 2013a, 2013b, and Park et al., 2009). On the basis of the above discussion, we studied seawater effects on untreated and treated MMT-based glass fiber-reinforced polymer (GFRP) nanocomposites. Although many studies have been conducted on the effects of seawater on polymer composites, few comparative studies of seawater effects on the mechanical properties of multiscale GFRP vinylester composites have been performed. In this study, 3-aminopropyltriethoxysilane (3-APTES) was used for the surface treatment of MMT powder. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) were used to evaluate changes in the composite system after seawater absorption. A morphological investigation of the fractured surfaces of composites was performed using SEM. Furthermore, the effects of seawater absorption on the mechanical properties of the composites were analyzed through tensile and bending tests.
2. Experimental 2.1. Materials All chemicals were purchased from Sigma-Aldrich Co., USA. MMT clay powder (Cloisites Na þ ) with an interplanar distance of 11.74 Å and a cation exchange capacity of 92.6 meq/100 g clay was
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used. A surface coupling agent, 3-APTES, and sterile-filtered seawater were used. Vinylester resin SR 830 with a viscosity of 1.5–1.7 poise at room temperature and a 25–30 min pot life was used as the matrix. Benzoylperoxide (BPO) was used as a hardener. Chopped glass fibers mat of E-type glass (380 g/m2) was used. 2.2. Surface treatment of the MMT powder (treated/silanized MMT) As mentioned in previous studies, surface treatment of MMT was performed using 3-APTES (Shanmugharaj et al., 2006). An aqueous solution of as-received MMT (1 g in 10 ml) was mixed with 20 ml ethyl alcohol for 5 h at 50 1C. Separately, 4 g of 3-APTES was mixed with 12 ml ethyl alcohol for 2 h at 25 1C. These two solutions were then mixed together and stirred for 3 h at room temperature. After stirring, the solution was filtered, washed with distilled water, and dried for 48 h at 80 1C in vacuum. 2.3. Preparation of MMT-based composites The synthesis of MMT-based composites was performed using the following process (Fig. 2). First, 1.0 wt% untreated MMT powder was mixed into 100 g of vinylester resin with constant stirring at room temperature for 3 h, followed by the addition of 1.0 wt% hardener BPO. The mixture was poured over 5 plies of the chopped glass fibers mat (20 20 cm2) and pressed into a mold using a hot press at 110 1C and 20 MPa for 10 min. The prepared sample was maintained in a vacuum oven for 24 h. A similar procedure was followed for the synthesis of GFRP vinylester
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composites with treated MMT powder. Test specimens were prepared by molding and cutting. 2.4. Characterization XRD analysis of the MMT-based composites was performed using a copper cathode for X-ray radiation (λ ¼ 0.1542 nm) at a voltage of 40 kV and a current of 100 mA. Data was collected from 3 to 90 1C in 0.02 1C steps. XRD data provides the interlayer spacing of the MMTs after modification and in each polymer composite. FTIR spectroscopy was used to study the effect of silanization of MMT from 4000 to 400 cm 1. Thermal stabilities of the systems were also determined using thermogravimetric analysis (TGA, SQT 600 model) from 30 to 700 1C at a heating rate of 10 1C/min. SEM was performed to study the mechanical and interfacial properties of the fractured surfaces of MMT in the composites. Absorption tests were performed at room temperature by immersing the tensile specimen into a seawater bath for 65 days. Before immersion, the samples were weighed under dry conditions. After immersion, these samples were weighed regularly up to their saturation limit. The mechanical properties, i.e. tensile and bending strength, of the system were analyzed with a Universal Testing Machine (UTM, Instron 8830). Tensile tests were performed according to the ASTM D638 specimen (Fig. 3). Dog bone-shaped specimens were prepared using a cutting tool and the tests were performed at room temperature with a speed of 6 mm/min. The elastic modulus and tensile strength were obtained during the tensile test, and the data was collected by an automated computer program. Bending
Fig. 2. Schematic representation of preparation of untreated and treated MMT-based composites.
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Fig. 3. Tensile specimen, ASTM D638.
Fig. 4. X-ray diffraction spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions.
specimens were created by cutting the samples into 130 25 1.7 (70.5) mm samples. The flexural strength and flexural modulus were measured using a three-point bending method with a UTM (Instron 8830) at room temperature and a speed of 6 mm/min. The flexural strength and flexural modulus were measured during the bending tests. For both cases, five tests were performed to confirm the reproducibility of the results.
3. Results and discussion XRD was performed to investigate the changes in the composites, exfoliation of the MMT layers, and intercalation of the polymer between the MMT platelets. Fig. 4 shows the XRD patterns of the untreated and treated MMT powders (a) and their composites (b) in dry and seawater-exposed conditions. Information regarding the dispersion of the MMT layers and d-spacing was obtained using Bragg's equation. Peaks and peak shifts illustrate the changes in the interplanar spacing between silicon platelets (Cook et al., 1997). As shown in Fig. 4(a), the untreated MMT
powder shows a characteristic (001) peak at 7.24, which corresponds to the d-value of 12.20 Å, while in the case of the treated MMT powder, the peak shifts to a lower theta value of 4.28 with a 20.60 Å d-value. The shift in the peak (approximately 2θ ¼2.96) indicates increased interlayer spacing between the MMT layers because of the silane treatment (Shanmugharaj et al., 2006). However, there is one more peak near 2θ ¼8.401, which may be due to some layers remaining in the original, unexfoliated form (Madaleno et al., 2010). Fig. 4(b) compares the XRD patterns of the untreated and treated MMT (1.0 wt%)-based composites in dry and seawater-exposed conditions. The diffractograms show that the peaks were shifted to lower theta values in both cases, indicating intercalation of the polymer between the MMT layers. In the dry case with the untreated MMTbased composites, the peak value shifted (2θ ¼ 6.66; d¼13.26 Å) due to intercalation, i.e. intercalation of the vinylester chains into the MMT layers without interrupting the morphology. Alternatively, for treated MMT-based composites, the peak shifted to a lower value (2θ ¼3.90; d¼22.58 Å) because of exfoliation, i.e., individual dispersion of MMT layers maintaining the average distance, caused by maximized
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Fig. 5. FTIR spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions.
interaction between the polymer and MMT layers. The diffractograms of seawater-exposed untreated and treated MMT-based composites do not indicate specific peaks that may be due to seawater absorption into the clay layers, resulting in swelling that increases the interlayer spacing, causing the peaks to disappear (Shanmugharaj et al., 2006). Fig. 5 provides the FTIR spectra of untreated and treated (a) MMT powder and (b) MMT-based composites in dry and wet conditions. FTIR was conducted to investigate the changes in the chemical structure because of the surface treatment by 3-APTES. As shown in Fig. 5(a), significant features of the MMT were observed, including a band corresponding to –OH stretching bound to Al þ or Mg þ at 3634 cm 1, a strong absorption band of Si–O bond stretching between 1200 and 1000 cm 1 with a 1035 cm 1 maximum, a band at 916 cm 1 for Al(Al)OH and a band at 800 cm 1 for Mg(Mg)OH bending vibrations. For treated MMT, additional characteristic bands were revealed corresponding to the symmetric and asymmetric stretching of –CH and –CH2 near 2930 and 2850 cm 1, respectively, which signifies the successful attachment of silane groups to the surfaces of the MMT layers, accompanied by bands representing –NH2 and –CH2 bending vibrations at 1566, 1496 and 690 cm 1. In Fig. 5(b), MMT-based composites in dry and wet conditions exhibited very similar FTIR spectra, which is evidence of the lack of chemical changes in the composite after seawater immersion. Thermograms of 1.0 wt% untreated and treated MMT-based composites in dry and wet conditions are presented in Fig. 6. All of the samples displayed one-step degradation, but demonstrated a significant difference in the weight residue. For the dry case, the weight decomposition % for treated MMT-based composites was 32.08, which is less than the untreated MMT-based composites (33.77%) and, therefore, the mass residue is greater. This significant difference in thermal stability illustrates the incorporation of silane groups to the MMT layers (Cai et al., 2008). MMT layers provide thermal stability during decomposition by acting as a heat barrier. Therefore, when MMT is treated by 3-APTES, silane groups are attached to the MMT layers and the silane groups exfoliate them by increasing the interplanar distance. Consequently, each layer behaves as a nanoentity and uniform distribution of MMT layers occurs. Hence, because of the enhanced interactions and bonding, thermal stability of the treated MMT-based composite is better (5% more stable) than that of the untreated MMT composites. Alternatively, seawater-absorbed composites show the same pattern, but with decreased thermal stability as compared to the dry case. Seawater-absorbed untreated MMTbased composites show 3% less stability than the dry untreated
Fig. 6. Thermograms of untreated and treated MMT-based composites in dry and wet conditions.
MMT-based composites, while for the treated MMT-based composites its 8% less. Furthermore, weight loss for the seawater-absorbed untreated MMT composite (34.74) is slightly greater than that of the seawater-absorbed treated MMT composite (34.67) because of the evaporation of absorbed seawater into the system. These results indicate that the thermal stability of the composites decreases after seawater absorption. However, treated MMT-based composites are more stable than untreated MMT-based composites in both cases. DSC analysis of the samples was performed to determine the effect of the composition on the glass transition temperature of the system. Fig. 7 shows the DSC thermograms of untreated and treated MMT-based composites before and after 65 days of seawater exposure. All of the thermograms show the same pattern, indicating that it was an endothermic process. For the dry case, the onset degradation temperatures for untreated and treated MMTbased composites were 420.90 and 433.34 1C, respectively. The degradation temperature of the treated MMT-based composites increased after surface treatment, which may be due to the physio-chemical interactions between the matrix and reinforced material. After surface treatment, homogenous dispersion of the nano-sized MMT layers may occur and restrict the mobility of
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segments that enhance the degradation temperature (Mahesh et al., 2013). DSC analysis of samples with absorbed seawater showed the same thermogram pattern as the dry case. However, there was a gradual decrease in degradation temperature after seawater immersion, which might be because of plasticization of the matrix. Therefore, when water molecules penetrate the system through voids or destruction of weak zones, MMT swells, which undermines the adhesion by breaking the cross-links. Hence, the mobility of these segments in the system increases, which results in the decreased degradation temperature. Compared to the untreated MMT-based composites, the thermal stability of the treated MMT-based composites was better. Fig. 8 presents SEM images of untreated (a) and treated (b) MMT-based composites in dry conditions. In the case of the untreated MMT-based composites, there were fewer interactions between the matrix and fillers because of the hydrophobic nature and poor distribution of the MMTs in the polymer matrix. The smooth surface of the glass fibers confirmed the poor adhesion of the matrix to the fillers, which resulted in an increase in the number of voids. A large degree of fiber breakage was also observed. Due to the weak interfacial strength and poor load transfer, fiber pull-out and failure occur. Alternatively, when MMT
Fig. 7. DSC curves of untreated and treated MMT-based composites in dry and wet conditions.
is treated with silane groups, fewer voids and less fiber breakage occur. After the incorporation of silane, MMT becomes hydrophilic and strengthens the interactions between the matrix and filler. With the treatment of silane groups, the interplanar distance within MMT increases, allowing MMT layers to behave as nanoclay. Therefore, dispersion and adhesion also increase, resulting in enhanced mechanical properties (Mahesh et al., 2013; Kornmann et al., 2001). Fig. 8 shows SEM images of the specimens after seawater immersion. As shown in Fig. 9(a), there is more breakage and fiber pull-out and there is very weak adhesion between the matrix and fibers, while treated MMT-based composites experience less pull-out and fewer voids. When MMT comes into contact with water molecules, it swells, resulting in the generation of more gaps. Furthermore, water molecule insertion generates forces that cause fibers to be displaced from their initial positions, which results in failure. Water molecules enter by the destruction of weak zones on the surface, thereby creating voids. Due to the immense force caused by the water, the void sizes increase and consequently, the volume of the water within the composite also increases. Additionally, a small amount of MMT may be lost due to void creation, and a crack may form, prompting subsequent failure. Since MMT becomes hydrophobic after surface treatment, it resists water molecule insertion, while untreated MMT cases favor water absorption. Fig. 10 shows the weight increment in untreated and treated MMT composites due to seawater absorption. The composite containing untreated MMT showed a greater uptake of seawater compared to the composite with treated MMT. Initially, there was a sudden increase in absorption, which gradually increased in both composites. The number of voids, interfacial bonding strength, and properties of the reinforced material affected the water uptake of the composites. Poor adhesion and weak interaction between the matrix and untreated MMT and glass fibers caused an increase in the number of voids, which in addition to the weak zones, acted as entry points for water. In treated MMT-based composites, the whole system is hydrophobic in nature because surface treatment changes the MMT from hydrophilic to hydrophobic (and vinylester is already hydrophobic), which limits the absorption of water into the system compared to the untreated MMT-based composites. Due to long-term seawater absorption, the polymer matrix age and the mechanical properties become weak. In order to analyze the effect of long-term seawater absorption on the mechanical properties of the composites, tensile and bending tests were performed. The (a) tensile strength and
Fig. 8. SEM image of a fractured surface of (a) untreated MMT-based composites and (b) treated MMT-based composites, without seawater absorption.
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Fig. 9. SEM image of a fractured surface of (a) untreated MMT-based composites and (b) treated MMT-based composites after seawater absorption.
Fig. 10. Effect of seawater absorption on the weight of untreated and treated MMTbased composites.
(b) elastic modulus were compared between untreated and treated MMT-based composites in dry and seawater-absorbed conditions in Fig. 11. The results showed that the tensile strength of untreated and treated MMT-based composites decreased 7% and 8%, respectively, after seawater absorption. However, as compared to the untreated MMT-based composites, treated MMTbased composites displayed a 7% greater tensile strength in the dry case and a 6% greater higher tensile strength in seawaterabsorbed conditions. Similarly, a slight decrease ( 4%) was observed in the elastic modulus (Fig. 11(b)) of both the composites after seawater absorption and the treated MMT-based composites had a 4.5% greater elastic modulus in both the dry and seawaterabsorbed conditions. The treated MMT-based composites may exhibit better interactions and adhesion between the matrix and fillers due to the homogenous dispersion of MMT layers after silanization. Therefore, when the load is applied, crack propagation is confined by optimal distribution and the strong adhesion of MMT layers with the matrix, which leads to improved tensile strength. However, when these specimens are immersed in seawater, the tensile strength is diminished because of water molecule penetration into the system. Since MMT swells when it comes into contact with water molecules due to interactions with interlayer cations, the d-spacing between the layers increases.
Because of poor adhesion, weak interactions, and an increased number of voids, untreated MMT-based composites absorb more water than treated composites. Furthermore, MMT is hydrophilic in nature, but becomes hydrophobic after silanization and restricts water molecule penetration into the system. When force is applied to the specimen, the tensile strength is limited due to poor fiber adhesion or MMT pull-out from the matrix. Fig. 12 demonstrates the (a) maximum flexural strength and (b) flexural modulus of MMT-based composites in both dry and wet conditions. As shown in Fig. 12(a), the flexural strength of treated MMT-based composites is 9% greater in dry conditions and 11.5% greater in seawater-absorbed conditions than that of the untreated MMT-based composites. However, the flexural strength of both composites decreases 7% and 4.5%, respectively, after seawater absorption. Alternatively, Fig. 12(b) shows very marginal changes in flexural modulus of both untreated and treated MMT-based composites, with both exhibiting a very slight decrease ( 1–3%) after seawater absorption. Similar to the tensile tests, the moduli increase after silanization because of optimized dispersion and better adhesion that inhibits the crack propagation. After seawater absorption, the mechanical strength decreases due to the plasticity of the polymer matrix. Since water absorption was considerably greater in untreated MMT, the untreated composites showed a more substantial weakening of the mechanical properties.
4. Conclusion The above experiments were performed to examine the effects of seawater absorption on the mechanical properties of untreated and treated MMT-based vinylester/glass fiber composites. Surface treatment of MMT was performed with 3-APTES. The results indicate that after silanization, the interfacial interactions between MMT, glass fibers and vinylester increase and optimal distribution of MMT layers occurs, resulting in improved mechanical properties such as tensile and bending strength. However, after seawater absorption, the mechanical properties of both untreated and treated MMT-based composites decreased. The factor behind the weakening of these properties was the swelling of MMT after seawater absorption, which resulted in deterioration of the adhesion and interactions between the reinforced material and matrix. Treated MMT-based composites showed less substantial weakening than untreated MMT-based composites.
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Fig. 11. Comparison of the (a) maximum tensile strength and (b) elastic modulus of tensile specimens of untreated and treated MMT-based composites in dry and seawaterabsorbed conditions.
Fig. 12. Comparison of the (a) maximum flexural strength and (b) flexural modulus of bending specimens of untreated and treated MMT-based composites in dry and seawater-absorbed conditions.
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