https://doi.org/10.1007/s13369-019-03724-x
R E S E A R C H A R T I C L E - M E C H A N I C A L E N G I N E E R I N G
Effect of Silica/Graphene Nanohybrid Particles on the Mechanical
Properties of Epoxy Coatings
Umit Esra Ozcan1· Fazliye Karabork2 · Sakir Yazman3· Ahmet Akdemir4
Received: 21 July 2018 / Accepted: 29 January 2019 / Published online: 7 February 2019 © King Fahd University of Petroleum & Minerals 2019
Abstract
Epoxy resins are used as coating materials, but the practical use of epoxy coatings in industries is limited due to their weak mechanical properties. In the present paper, different amounts of silica nanoparticles (SiO2) and graphene nanoplatelets (GNPs) were introduced separately and together into an epoxy coating matrix as reinforcements. Graphene, a newly discovered carbon allotrope, has been found to improve the mechanical properties of the polymer composites in which it is dispersed. Silica particles are also known to improve the mechanical properties of composites. In this study, mechanical, physical and thermal properties of the epoxy coatings are considered as multidimensional by the macro- and microanalyses. The experimental results showed that after the addition of GNPs into the epoxy matrix, the flexibility and impact resistance of the coatings increased by 8.3 and 157.1%, respectively, in relation to neat epoxy. The microhardness increased by 53.8% and penetration depth, which is indicative of the scratch resistance, decreased by 29.7%, with the addition of SiO2–GNPs nanohybrid. A remarkable synergistic effect was observed between the GNPs and SiO2, which improved the hardness and the scratch resistance of the epoxy coatings.
Keywords Coatings· Nanoparticles · Graphene · Mechanical properties
1 Introduction
Polymeric coating systems are widely used to protect steel structures against environmental and mechanical degrada-tion. Epoxy resins are extensively used as coating materials in various industrial fields because of their versatility and a wide range of accessible properties [1]. However, the prac-tical use of epoxy coatings in industries is seriously limited due to their weak tensile properties and brittle nature result-ing from their highly cross-linked structure [2,3]. In recent years, polymeric coatings have been reinforced with
differ-B
Fazliye Karaborkfazliyekarabork@hotmail.com
1 Graduate School of Natural and Applied Science, Aksaray University, 68100 Aksaray, Turkey 2 Department of Mechanical Engineering, Faculty of
Engineering, Aksaray University, 68100 Aksaray, Turkey 3 Ilgın Vocational School, Selcuk University, 42600 Konya,
Turkey
4 Department of Aircraft Engineering, Faculty of Aviation and Space Sciences, Necmettin Erbakan University,
42000 Konya, Turkey
ent types of nanoparticles to improve the thermal, physical, morphological and mechanical properties of the coatings [4]. The improvement in the properties of nanoparticle-reinforced polymeric coatings depends on many factors, such as chemical nature, particle size, and concentration of nanoparticles in the coating; in addition, the interaction between the nanoparticles and the polymer matrix is also an important factor [5,6]. It has been reported that the coating performance significantly depends on the homogeneity of the filler distribution and the microstructural characterization of fillers [7–9]. However, nanoparticles exhibit a strong trend for agglomeration because of their high surface energy and specific surface area. Therefore, ultrasonic mixing is utilized to reduce agglomeration and make them compatible with the polymer matrices. On the other hand, it has been shown that a remarkable improvement in the mechanical proper-ties, thermal stability, and adhesion of epoxy resin can be achieved with an appropriate amount of filler content [9–12]. Some researchers suggest that the concentration of nanopar-ticles should be low to achieve a good dispersion, since a higher percentage of nanoparticles leads to agglomeration. For example, Ramezanzadah et al. reported that a nanoparti-cle concentration of 3.5 % results in good dispersion [4,13].
Nanoparticles, such as TiO2, ZnO, ZrO2, CeO2, CaCO3, Fe2O3, SiO2, Al2O3[14,15], and nanocarbon forms [16–21] have been widely used in polymeric coatings to improve the various properties. Silica (SiO2) is one of the multi-purpose nanoparticles utilized to develop multifunctional composite coatings. It has high hardness and is low-priced compared to other nanoparticles, which makes SiO2an ideal material for the preparation of low-cost, scratch- and abrasion-resistant coatings [14].
On the other hand, different nanocarbon forms, such as single or multiwall carbon nanotubes, graphene and its derivatives have been used as fillers in polymers, leading to improvements in the physical and mechanical proper-ties of polymer composites. Polymer coatings reinforced with graphene have recently been investigated by some researchers [16–21]. Graphene has attracted considerable attention for its unique mechanical, electrical, and ther-mal properties. In addition, in recent years, many papers have reported the use of nanoparticles as hybrid in epoxy coating applications [22–25]. However, there is insufficient data on polymer coatings reinforced with graphene and hybrid nanoparticles, especially regarding the effects on the mechanical properties of the coatings. Many researchers have investigated the corrosion resistance of epoxy-based polymer coating systems in the literature [17,20,22–28]. In addition, graphene-based nanocomposite coatings have been discussed in a review article by Tong et al. and a book chapter by Bastani and Darani [29,30]. Further research is required to understand the interaction between graphene and the poly-mer matrix for coating applications. Furthermore, the effects of graphene when used as a hybrid with other nanoparticles need to be investigated.
The objective of the current study is to investigate the effects of addition of nano SiO2, graphene nanoplatelets (GNPs), and SiO2-GNPs nanohybrid to the epoxy matrix to develop high-performance coatings for applications that are mechanically challenged. In this study, the graphene con-tent was added in especially small amount into the epoxy coating with the aim of investigating the reinforcing effect alone and in combination with a different low-cost additive; i.e., silica nanoparticles. Epoxy coatings reinforced with dif-ferent nanoparticle contents were prepared and applied to a steel substrate, and the mechanical performance of the nanoparticle-reinforced composite coatings were evaluated using cross-cut, conical mandrel, cupping, impact and Pen-dulum hardness tests. Microhardness and scratch tests were also used to measure the mechanical properties of coatings in which the modulus, hardness and penetration depth could be obtained. Furthermore, thermal properties and the interac-tion of the nanoparticles with the epoxy matrix were analyzed by thermogravimetric analysis (TGA) and Fourier-transform infrared (FTIR) spectroscopy analysis, respectively. In addi-tion, the nanoparticles dispersion in the epoxy matrix and the
role of the graphene nanoplatelets on crack propagation were examined by the SEM technique.
2 Material and Methods
2.1 Materials
The epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) and polyamine hardener (Cyclo aliphatic polyamide hard-ener) used in this study were obtained from Labkon Co. Ltd., Turkey. Nanosilica functionalized by 3-aminopropyltrime-thoxysilane (APTES; average particle size: 16 nm) and graphene nanoplatelets (thickness of 6 nm and diameter of 24µm) were obtained from Nanografi Co. Ltd., Turkey. Cold-rolled mild steel plates (160× 80 × 2 mm) were used as the metallic substrates. Prior to the application of the coating, the steel substrates were cleaned with acetone and completely dried.
2.2 Preparation of the Epoxy Coatings
The epoxy coatings containing three kinds of nanofillers, viz. nano-SiO2(1 wt%), GNPs (0.1 wt%), and SiO2–GNPs (1 and 0.1 wt%, respectively) hybrid were prepared by the follow-ing procedures. The nanoparticles were added to the epoxy resin, and probe ultrasonication was performed for 40 min (5 times for 8 min) in an ice bath to achieve a uniform dispersion of the nanoparticles in the epoxy matrix. A polyamide hard-ener (2.5:1 w/w) was added to the epoxy resin containing the nanoparticles, and the mixture was mechanically mixed for 5 min. Then, the mixture was degassed (for 10 min) in a vac-uum oven (at— 0.75 bar) to remove any air bubbles trapped within the epoxy resin. The final mixture was applied as a coating on the steel substrate using a film applicator. A neat epoxy coating, without nanoparticles, was also applied to the steel substrate as a reference sample. The coated steel plates were kept at room temperature for 24 h and then postcured in a vacuum oven at 80 ◦C for 15 h. The specimen prepa-ration steps are schematically shown in Fig.1. The average value of the thickness of the cured films was in the range of 60± 10 µm for all the samples.
2.3 Characterization
The adhesion strength of the coating on the steel substrate was examined by cross-cut tests according to ASTM D 3359. In this test method, the surface of the coated plate was cut using a six-blade cross-cut tool; then, the test plate was rotated by 90◦to create a grid on the surface. Standard pressure sensitive tape was adhered to the surface and then removed. The plate surface was analyzed, and adhesion was graded with respect to the amount of the removed coating.
Fig. 1 Specimen preparation steps
The flexibility of the coating, which is a criterion for evalu-ation of crack resistance, was measured by a standard conical mandrel test using coated panels (ASTM D 522). In this test, the coated plate was bent on a conical mandrel. It was examined whether a crack had occurred in the coating in the bending area. The Erichsen cupping test (EN ISO 1520) was performed to measure the changes in the deformation of the epoxy coatings at different loadings of nanoparticles (Byk Mallinckrodt Cupping Tester). The test was undertaken after the coated panels were placed in the test device, the outer coating being pushed by a hemispherical indicator at a constant speed to the test piece to form a dome shape. The deformation was increased until the first crack appeared on the coating surface and the depth at which the crack was observed was recorded.
Impact tests were carried out on the coated panels using the standard test method EN ISO 6272 (Byk Mallinckrodt Impact Tester). In this test, a spherical indenter of a 0.9 kg weight was dropped onto the sample on the coated side, and the drop height was recorded when the coating was dam-aged. The hardness was investigated using the Pendulum hardness test (ASTM D 4366). This test is based on the damp-ing time of a pendulum oscillatdamp-ing on a specimen, which gives a value that is taken as the hardness of the coating. The nanohardness of the coated samples was tested using Berkovich nanoindentation (CSM Nanoindentation Tester). The maximum load was fixed at 50 mN. The microscratch resistance was measured under a normal load ranging from
0.05 to 5 N using a Rockwell indenter (Diamond, 100µm radius).
The interaction of the nanoparticles with the epoxy matrix was characterized using FTIR analysis in the wavelength range of 500–4000 cm−1 (Perkin Elmer FT-IR spectrom-eter). TGA was performed in a nitrogen atmosphere at a heating rate of 10 ◦C/min and a temperature range of 25– 800◦C. The morphology of the nanoparticles dispersion in the epoxy matrix was examined using a ZEISS EVO LS 10 scanning electron microscope (SEM).
3 Results and Discussion
3.1 Mechanical and Physical Properties
The physical and the mechanical properties of the coated steel panels were examined, and the results are given in Table1. The cross-cut test method was used for determin-ing the level of adhesion strength of the epoxy coatdetermin-ing on steel. The adhesion strength was determined according to the following the criteria: 0B (>65% coating removal), 1B (35–65%), 2B (15–35%), 3B (5–15%), 4B (<5%), and 5B (0%, no coating removal). Thus, a value of 0 corresponds to very poor adhesion and a value of 5 indicates very good adhesion. The adhesion test results shown in Table1indicate that the coatings exhibited very good adhesion.
Table 1 Ph ysical and m echanical properties o f the coatings Sample code Cross-cut test C onical mandrel test Erichsen cupping test (mm) Impact test (weight: 0.9 kg; height at which damage occurs) (cm) Pendulum hardness (s) Microhardness (GP a) Modulus (GP a) Neat epoxy 5B Crack ed (+++) 4.8 3 5 325 0.26 4.94 1w t% S iO 2 5B Crack ed (++) 4.5 5 0 347 0.27 4.30 0.1 w t% GNPs 5 B C rack ed (trace amounts) 5.2 9 0 345 0.28 6.80 1w t% S iO 2 –0.1 w t% GNPs 5 B C rack ed (+) 2 .9 50 331 0.40 6.81
The results of the impact resistance test of the coated steel panels are also given in Table 1. It can be seen that the epoxy coatings reinforced with GNPs were damaged at a height of 90 cm, while neat epoxy coatings and epoxy coatings reinforced with both silica and hybrid nanoparti-cles were damaged at a height of 35 and 50 cm, respectively. The improved impact resistance can be attributed to the high aspect ratio of GNPs and to better dispersion and interaction of GNPs with the epoxy resin [31]. On the other hand, as seen on the SEM images (Fig.6), it is thought that the GNPs fillers clearly deflected the crack propagation and caused the cracks to branch out, which led to the higher impact strength of the epoxy coatings reinforced with GNPs [31,32]. Further-more, the addition of nanoparticles has a positive effect on the impact resistance and improves the delamination resistance of the coatings.
The results of the bending test of the epoxy coatings in Table 1 show that crack traces were observed at the bend angle of all the coatings with and without reinforcement. However, the crack trace was very narrow in the coat-ings reinforced with GNPs, and the crack width increased gradually for coatings with hybrid nanoparticles, silica, and without nanoparticles, respectively. In addition, linear crack formation was observed at the bend angle of the neat epoxy coatings, while zigzag crack formation (nonlinear) was observed at the bend angle of the coatings reinforced with GNPs. This is attributed to the fact that the nanopar-ticles dispersed in coatings cause crack bridging and crack deflection because of its 2D structure. As a result, coatings reinforced with GNPs exhibited improved bending behavior. The effect of different nanoparticles on the cupping prop-erty of the nanoparticles reinforced epoxy coatings is listed in Table1. The cupping test was performed at a slow rate, and the cupping action was stopped when cracks in the coating were observed. Similar to the bending test results, the epoxy coatings reinforced with GNPs exhibited improved cupping properties. The improvement in the mechanical properties of GNP-reinforced epoxy coatings is attributed to the structure of GNPs, because graphene exhibits a 2D structure with a high surface/volume ratio. Thus, a higher average number of connections between the filler particles and the resin matrix can be achieved using GNPs compared to the use of spheri-cal silica nanoparticles. This maximizes the area where stress can be transferred to the filler [33].
In this study, the surface hardness was evaluated by two different methods, namely pendulum and indentation tests, and the results were compared. As expected, different meth-ods in the determination of surface hardness produce differ-ent results, depending on the mechanism of forces causing the surface deformation. In the indentation test, an increas-ing normal force is applied usincreas-ing a sharp Berkovich indenter that penetrates the coating surface, and depending on the penetration depth versus load per unit surface area, the
hard-ness is determined [34]. However, the pendulum hardhard-ness test consists of measuring the damping time of a pendulum oscillating on the surface of the specimen. In this method, the surface hardness, roughness, and coating thickness affect the results, and the shorter the damping time means lower hardness. As shown in Table 1, the hardness of the coat-ing surface was affected by the presence of nanoparticles on the surface, with the addition of different nanoparticles, both the pendulum hardness and the microhardness of the coat-ings increased. It is evident that the microhardness increases significantly with the loading of hybrid nanoparticles, but the values of the other samples are similar. It is expected that by increasing the nanoparticle content (SiO2and GNPs), the indenter interacts with more nanoparticles. It is expected that by increasing the nanoparticle content (adding hybrid nanoparticles; SiO2 and GNPs), the indenter interacts with more nanoparticles. The hardness increase can also be attributed to the limiting of molecular chain mobility induced by the hybrid nanoparticles dispersed in the epoxy matrix [35]. Therefore, the material resists more against the defor-mation caused by a load [36]. Similar results were obtained in the pendulum test, but the maximum hardness was obtained in the presence of nanosilica, while it was obtained in the presence of hybrid nanoparticles in the indentation method. The presence of hard silica nanoparticles that can probably in part migrate toward the surface of the coatings may have resulted in increased pendulum hardness results [37,38]. In addition, the area and the volume of the deformed region vary greatly depending on the scale of the applied methods. Depending on the method, there may be more contact with the matrix or the nanoparticle because of the difference in the size of the area where the hardness measurement is made. Therefore, this may have caused the results to differ.
The modulus of the epoxy coatings reinforced with both GNPs and hybrid nanoparticles increased significantly. This increase in the modulus may be attributed to the following mechanism. Well-dispersed nanoparticles tend to enter cav-ities in the coating and act as bridges to the interconnected matrix, causing a reduced total free volume and enhanced cross-linking density of the cured epoxy. Therefore, the cured coatings have reduced segmental chain motions and improved stiffness [39,40].
Although the lowest value of the penetration depth was obtained for the epoxy coating reinforced with hybrid nanoparticles, the penetration depth was significantly lim-ited during the scratch tests with the addition of nanoparticles to the epoxy resin. In addition, the residual depths after the scratch tests were reduced with the addition of nanoparticles (Table2). This result indicated that the presence of SiO2, GNPs, or hybrid nanoparticles enabled the epoxy resin to deform in the elastic field, with most of the imposed deforma-tion occurring during the scratch and readily recovering after the release of the load. The low penetration depth obtained is
Table 2 Scratch test results (at an applied load of 5 N) Sample code Penetration
depth (µm) Residual penetra-tion depth (µm) Elastic recovery (%) neat epoxy 38.12 15.05 60.52 1 wt% SiO2 34.29 10.72 68.74 0.1 wt% GNPs 33.5 10 70.15 1 wt% SiO2–0.1 wt% GNPs 26.78 10.33 61.43
an indication of the enhancement in the scratch resistance of the material, and the increase in hardness observed in Table1 also supports this result.
3.2 FTIR Analysis
Figure2shows the FTIR spectra of the epoxy coatings rein-forced with several nanoparticles. There is a slight shift in the FTIR band frequencies after the incorporation of nanoparti-cles. The shifts in the peaks correspond to interaction between the epoxy matrix and the nanoparticles [41], but this inter-action is more a physical interinter-action than a chemical one because of the absence of new peaks in the spectrum.
3.3 Thermogravimetric Analysis
The effect of GNPs, SiO2, and SiO2-GNPs nanohybrid on the thermal stability of the epoxy coating was investigated by TGA analysis, and the test results are given in Fig.3and Table3. The incorporated nanoparticles did not significantly affect the thermal behavior of the epoxy coating, but slightly increased the temperature values at 50% weight loss. The reason for this slight increase in temperature is that the pres-ence of nanoparticles physically inhibits the degradation of the epoxy network around them [42,43]. Khun et al. reported that the thermal stability and the frictional properties of epoxy composite coatings were correlated. They explained that the improved thermal stability of the nanoparticle-reinforced epoxy coatings can reduce the friction by preventing the soft-ening of the coatings by dissipation of frictional heat [44].
The char yield value at 800 ◦C for neat epoxy resin, and epoxy coatings reinforced with SiO2, GNPs and hybrid nanoparticles were 3.08, 5.65, 7.04, and 4.46, respectively. The coating degradation temperature not only depends on the chemical structure of the network, but also on the cross-linking density of the network. The cross-cross-linking density of the coating increases and this highly cross-linked network has high thermal stability [45]. Well-dispersed graphene nanoparticles have been reported to increase the cross-linking density of coatings [46,47]. A higher cross-linking density of the coating may result in enhanced thermal resistance prop-erties [46,47].
Fig. 2 FTIR spectra of the epoxy matrix before and after the addition of nanoparticles
Fig. 3 Thermogravimetric (TG) curves of the coatings
Table 3 Temperature of 50% weight loss and residual mass obtained by TGA
Sample Weight loss
tempera-ture 50 % (◦C) Residues at 800◦C (%) Neat epoxy 327 3.08 1 wt% SiO2 329 5.65 0.1 wt% GNPs 330 7.04 1 wt% SiO2–0.1 wt% GNPs 328 4.46
3.4 Microstructure and Dispersion Analysis of
Coatings
The dispersion level of the nanoparticles in the epoxy matrix and the interfacial interaction are two important factors that affect the performance of the nanoparticle-reinforced epoxy coatings. A strong interface and improved dispersion have significant effects on the mechanical properties of composite coatings [48]. As shown in Fig.4a, the fracture surfaces of the neat epoxy resin exhibit randomly distributed cracks, indicat-ing brittle nature and poor impact strength of the neat epoxy. However, the coatings reinforced with nanoparticles exhib-ited rough surfaces, as shown in Fig.4b, suggesting that crack
propagation could be delayed so that more energy would be required for fracture. This typical morphology was also considered responsible for the enhancement in the impact strength [31].
The SEM analysis also shows that the dispersion of graphene and silica nanoparticles in the epoxy matrix is homogeneous (Fig. 5). Although the particle size of silica is very small, there is no agglomeration owing to the use of modified nanosilica.
The SEM images of the GNPs/epoxy composite coatings show the presence of dimple-like structures (Fig.6a), and the GNPs exist in the center of the dimples as indicated by the yellow arrow.
During the failure process, GNPs can generate several microcracks to expend the fracture energy. The SEM micro-graphs of the epoxy matrix reinforced with the GNPs in Fig.6show the different effects on crack propagation, namely crack branching along several layers of graphene (Fig.6b), increased crack deflection due to the large specific surface area of GNPs (Fig.6c), crack stopping due to the action of GNPs (Fig.6c), and crack bridging when fracture energy is effectively distributed by GNPs (Fig.6d). Similar phenom-ena have been reported by other researchers [49,50].
4 Conclusions
In this work, the properties of epoxy coatings reinforced with nano SiO2, GNPs, and SiO2-GNPs nanohybrid were investi-gated. The mechanical properties of the coating were affected by the presence of nanoparticles, and the following conclu-sions were reached:
• The addition of graphene nanoplatelets to the epoxy matrix resulted in an increase in flexibility and impact resistance by 8.3 and 157.1%, respectively, when com-pared to neat epoxy and other reinforced coatings. The
Fig. 4 SEM images of the fracture surfaces of a neat epoxy and b epoxy + 0.1 wt% GNPs nanoparticles (1.00 KX)
10 µm 10 µm
(a) (b)
Fig. 5 SEM images revealing the distribution of a silica (40.00 KX), b graphene (20.00 KX) and c silica and graphene (40.00 KX)
bending test results, which are another indicator of flex-ibility, showed that the narrowest crack trace was in the coatings reinforced with GNPs.
• The hardness of the coating surface was affected by the presence of hybrid nanoparticles, but the highest increase (6.8%) in the pendulum hardness was achieved with the addition of nanosilica to epoxy, while the highest increase (53.8%) in the microhardness was attained by adding hybrid nanoparticles (GNPs/SiO2) to the epoxy matrix. The different results are attributed to the different types and mechanisms of external forces causing the surface deformation. Additionally, the results are also affected by contact with the matrix or nanoparticle and the extent of variation is determined by the size of the area where the hardness is measured. In addition, the penetration
depth, an indicator of the scratch resistance, decreased by 29.7%, with the addition of hybrid nanoparticles. The improvement in the microhardness and scratch resistance of the SiO2/GNPs hybrid system can be explained the synergistic effect of SiO2and GNPs in the epoxy matrix. This is related to the unique two-dimensional structure of the GNP and its interaction with functionalized SiO2. • The thermal stability improved slightly due to the inter-action between the organic and the inorganic networks with the addition of nanoparticles, but, in general, the thermal properties did not change significantly.
• Consequently, if interfacial interaction with the polymer matrix is achieved, the addition of nanoparticles to the epoxy resin may be an effective method for improving the mechanical properties of epoxy coatings. It has been
Fig. 6 SEM images indicating the different crack propagation mechanisms: a microcracks (10.00 KX), b crack branching (10.00 KX), c crack deflection and crack stopping (5.00 KX), and d crack bridging (10.00 KX)
(a) (b)
(c) (d)
Crack stopping and crack deflection Graphene Graphene bridging of crack Crack branching along the interfaces
of graphenes
2 µm
revealed that the epoxy coatings reinforced with GNPs have better elasticity and impact resistance, and the epoxy coatings reinforced with silica and hybrid nanoparticles have better hardness and scratch resistance. Therefore, it is envisaged that the coatings can be used in applications exposed to such mechanical challenges.
Acknowledgements This work was supported by the Research Fund of Aksaray University. Project Number: 2016-007.
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