Bibliographic review

In this section, we present different methods of processing techniques for obtaining polymeric nanocomposites of the epoxy type reinforced with graphene and graphene oxides available in literature. The focus is on the identification and quantification of the main increased tribochemical and mechanical properties, as well as the respective concentrations of reinforcement of graphene material in the composite polymeric coating applied in these studies.

Graphene and Graphene Oxide

Since its isolation in 2004, graphene has been the center of study and use in various applications in different industries, mainly as a reinforcement material in matrices. A layer of graphene is composed of carbon atoms arranged in a hexagonal network, where each atom is bound to three more neighboring carbon atoms in a hexagonal plane with sp² hybridization orbits ^3,10.

Before the revolutionary discovery of graphene, it was known as an exclusively theoretical material, being cited only for the explanation of the formation of the different allotropic forms of carbon, illustrated below in Figure 1, becausethe scientists considered its structure as unstable 11,12.

The International Union for Pure and Applied Chemistry (IUPAC) characterizes graphene by a unique layer of graphite, having its composition by aromatized hydrocarbons of quasi-infinite size. This definition is not always followed in literature since four statements differ from the IUPAC definition ¹³.

A particle that has more than one layer does not meet the definition of a single layer.

When the width is in the order of tens of nanometers, the quasi-infinite parameter defined by IUPAC is damaged.

When graphene originates from the graphene oxide reduction method, the material obtained will comprise a high amount of oxygen groups on its surface.

Finally, there are carbonaceous materials that should be called amorphous carbon, as they have a large number of sp³-type bonds, as well as defects and imperfections in their crystalline structure.

Graphene is one of the sheets that form the composition of graphite, as shown in Figure 2. In graphite, the interaction that occurs between graphene sheets is of the Wan der Waals type in 5.9 KJ.mol^-¹, this force being too high to easily separate them ¹⁴.

The reduced graphene oxide (rGO) or graphene is a material similar to graphene oxide but its structure resembles a “Christmas tree” that contains several groups of oxygenated functional compounds in their composition. Using oxidation and reduction reactions is possible to obtain graphene oxide from graphene. This graphene is also called reduced graphene oxide or rGOand can be obtained through graphene oxide. The generated structure can get some oxygenated and functional groups that have a large number of defects. This chemical oxidation method is similar to the other method used to functionalize carbon nanotubes, thus also producing several functional groups through the nanotube ¹⁴.

Patents and the use of Graphene and Graphene Oxide in nanocomposite coatings

The use of graphene and graphene oxide has been widely deliberated about the possible improvement of the anti-corrosion and anti-fouling properties of the substrates ^5,15 and its impact as an anti-corrosion protector of coated metal surfaces and as an anti-fouling protector used in boat hull paints.

It was addressed ^5,7 that to improve the dispersion of graphene sheets in nanocomposites it is the use of its form in graphene oxide GO, which has functional groups including hydroxyls, epoxides, and carboxylic in its structure, to alter interactions, Van der Waals, facilitating its dispersion in the polymeric matrix. In addition, these functional groups are more compatible with the polymeric structure. In this sense, the functional groups of GO play an important role in the dispersion of GO in the polymeric matrix and the transfer of interfacial stresses in the composite. In addition, the processing technique influences the level of dispersion of the nano foil in the epoxy matrix and the property of the nanocomposites because the strong hydrogen bonds can be formed between the GO sheets dispersed in the polymeric matrix during drying. The two-dimensional structure presented by graphene allows it to show excellent performance in barrier coatings ³.

Applied Graphene Materials (AGM) is an innovative and leading company in the production and application of graphene in coatings, in addition to having developed and patented a unique graphene synthesis process. In a survey carried out 16 in partnership with AGM, they carried out work with industries specialized in the development of new materials to finalize the evaluation of graphene platelets produced by AGM in an epoxy coating. The company's goal is to demonstrate the effectiveness of an improved graphene coating in preventing corrosion. AGM evaluated two graphene platelet grades developed by them, A-GNP10 and A-GNP35 (T). Graphene A-GNP10 has a medium density with a rigid platelet structure and built-in oxygen functionality, which provides excellent dispersibility. Graphene A-GNP35 (T), on the other hand, is an ultra-low density material with a high flexible surface area and crumpled sheet morphology. The selection of these two materials was made because each of them has properties that can be useful in preventing corrosion.

Epoxy resins are widely applied as anti-corrosive coatings. They also can be used as an agglutination component in several industrial sectors, as they are classified as thermostable polymers that provide good adhesion on metal substrates, chemical, and dimensional stability, besides having easy applicability. Polymeric matrices of the thermo-rigid type are utilized in composite materials containing allotropic variations of carbon, one of the main matrices being epoxy resin. They have excellent mechanical properties combined with corrosion resistance, in addition to having a low cost. Its use in coatings is due to the factor of its low uncured molar mass that ensures high mobility of molecules during its application and in this way makes the resin to be easily spread on the surface of fibers used as reinforcement ^5,17.

The graphene particles added in a polymeric matrix have the function of structural reinforcement, thus improving the mechanical, thermal, and electrical properties of the composite. For homogeneous dispersion of the graphene particles in the polymeric matrix, it is of utmost importance to optimize the means of adhesion and interaction between the reinforcement and its matrix ^5,6,18.

Graphene particles will behave as a physical barrier between the metallic substrate and the corrosive environment, thus protecting the metal by filling micropores and possible flaws in the coating polymer matrix, in addition to generating increased adhesion to the substrate ^5,19,20

The dispersion of nanoparticles in polymeric matrices implies the modification of certain properties of this matrix because the polymer and the charges interact chemically ²¹. These charges have a nanometric dimensioning on the scale of 1-500 nanometers and have a large surface area, thus contributing to an excellent dispersion in the polymer and the material to present a physical improvement of its characteristics, there must be homogeneity in the material. The addition of inorganic fillers in the polymeric matrices develops materials with high mechanical resistance, improves thermal stability, as well as good optical, electrical or magnetic characteristics. In the case of epoxy-based coatings used as solvents and filled with graphene oxide (GO) nanosheets, studies ^5,7 demonstrated their improvement in the protection against corrosion of carbon steel substrates. The results show that the dispersion of GO in the polymeric matrix is ​​the main parameter that influences the execution of the coating. The viscosity of the polymeric matrix and the percentage of GO weight (% by weight) are principal characteristics for the quality of the GO dispersion.

For a coating to present a good performanceis necessary a good adhesion to the substrate and also good resistance to friction to avoid a reduction in its protection 22. As illustrated in Figure 3, there are four types of wear that can culminate in the deterioration of the metal: Adhesive wear, abrasive wear, fatigue wear, and lastly the erosion wear that occurs in corrosive media and together with the contact between the two surfaces generating a synergistic effect that results in corrosion faster than the individual action of these two factors: Corrosive environment and Contact. In general, this phenomenon is called tribochemical reactions. Being the factors that influence the protection of the substrate: roughness of the substrate, thickness, and the hardness ratio of the substrate and coating.

Electrochemical and mechanical properties

2.4.1. Corrosion resistance

Many studies have shown that the addition of graphene nanoparticles and graphene oxide to coatings can significantly increase corrosion resistance in metallic materials when compared to coatings without addition.

Nanocomposites of polymeric matrix of the epoxy type were prepared with different percentages in weight of OG (0.05, 0.1, 0.3 and 0.5), these were submitted to the salt spray test (salt spray test) for the analysis of the corrosion protection performance of the nanocomposite coating. Figure 4 shows images of the samples after 300 h of exposure to accelerated conditions of the salt spray chamber ⁷.

According to the results obtained ⁷, drastic corrosion points, rust, and bubbles were observed in the pure epoxy coating. The corrosion extends the risk and means that the pure epoxy coating has low resistance to corrosion (Figure 5). Nanocomposite coatings with different percentages by weight of graphene oxide have divergent behavior, although there is also rust (to a lesser extent) in coatings with 0.05% by weight of Oxide Graphene. There is less expansion of rust and corrosion products on substrates coated with traces of 0.5% by weight of Graphene Oxide. In contrast, nanocomposite coatings, including 0.1% and 0.3% by weight of Graphene Oxide, do not exhibit severe corrosion except for a few rust spots observed on them and corrosion around the scratch is not extended. The Salt Spray Test confirms that a percentage of Graphene Oxide by weight has a great effect on the corrosion resistance of nanocomposite coatings. And a nanocomposite with Graphene Oxide with 0.1% by weight shows the best corrosion protection performance in a NaCl electrolyte.

The path of diffusion of water and oxygen from the coating to the substrate is obstructed due to the barrier effect of graphene oxide nano-foils that act as anticorrosive solutes as shown schematically in Figure 5, therefore, a smaller amount of corrosion products, rust and bubbles are formed. These properties are achieved if the Oxide Graphene nano-foils are well dispersed in the polymeric coating, as they increase the resistance of the coating against the diffusion of corrosive species (H₂O, O₂, and Cl^-), extending the life of the coating 7. The results show that the GO nanoparticles disperse well in polymers with low viscosity. The Oxide Graphene sheets tend to agglomerate by increasing the percentage of Graphene Oxide by weight in a coating system. This effect occurs due to the tendency of oxygen functionalities to agglomerate and it also forms highly oxidized agglomerates surrounded by areas of crystalline graphene.

In recent researches 8, authors investigated the barrier protection mechanism of graphene functionalized in steel panels using a hybrid film (APTES) composed of 3-trimethoxysilyl (aminopropyl), purchased from Alfa Aesar, which was hydrolyzed with acidified demineralized water with pH = 5 and used to chemically modify (functionalized) different concentrations of graphene. The authors carried out electrochemical tests in which the coated panels were exposed to the test solution composed of 3.5% by weight of NaCl freely exposed to air, pH 5.5 at 25 ° C for long durations.

The results 8 illustrate the condition of 5% functionalized and hydrolyzed graphene surfaces, APTES coated systems, before and after the test is completed, are shown in Figure 6. The surface is severely damaged in the case of hydrolyzed APTES coated mild steel after 288 h of exposure. The damage is much less in the case of a 5% functionalized graphene-coated system, even after 480 h of exposure. The result indicates that even if the functionalized graphene coating delaminates at a certain point, most of the surface remains unchanged.

The company AGM, in partnership with other companies specialized in Paints and Coatings, such as the Paints Research Association, tests were carried out to evaluate the anticorrosive performance of epoxy coatings with the incorporation of graphene 16. A two-component epoxy system, Epikote 828 with Epikure 3234 hardener, was used, which is widely used as epoxy primer systems to protect steel and aluminum structures. Graphene was dispersed directly in the resin, at concentration levels ranging from 0.1% by weight to 5% by weight for A-GNP10. A-GNP35 (T) was limited to a maximum concentration of only 1.0% by weight due to the very high surface area of ​​this graphene type. The coatings were applied to steel panels for testing using an extraction method, and the thickness of the coatings was monitored before testing.

The steel panels submitted to the Salt Spray Test, where the samples were exposed using a repeated 60-minute cycle of diluted electrolytic fog (0.35% ammonium sulfate, 0% sodium chloride, 0.5%) at (24 ± 3) °C, followed by 60 minutes dry with the temperature rising to 35 ° C for a total of 1000 hours. The coated steel panels were named (A, B, C, D, E, and F) according to Table 1, and checked regularly to monitor the progression of corrosion 16.

According to the analysis of the results of the salt spray test presented in Figure 7, the addition of the lowest amount of graphene significantly increased the protection against corrosion of the epoxy coating. It was found that the control sample containing epoxy coating without graphene quickly showed rust spots and clear evidence of corrosion after just a few days of exposure, and the rust became quite severe during the test period 16. The epoxy-coated samples loaded with graphene remained free of corrosion for up to 12 days and then showed only small localized stains. This localized corrosion appeared to be limited to regions where there were defects or corrosion in the coating. There was also an increase in the time it took for corrosion to start (appearance of black rust spots) with an increase in the graphene load. The best performance was achieved with the composite containing 5% A-GNP10 and 0.5% A-GNP35 (T).

According to the study carried out 16, AGM also carried out the immersion test in which the steel panels were prepared similarly and then subjected to total immersion in synthetic seawater, prepared to ASTM D1141 - “Standard practice for the preparation of substitute ocean water”, at a temperature of 20 to 30 ° C for 30 days. After the immersion test was completed, the samples were cut and photographed by SEM (Table 2).

To complement the analysis of the results obtained in the immersion test, the authors¹⁶ performed micrographs of the epoxy-coated panels in the SEM as shown in figure ⁸, where it is possible to observe the before and after the 30 days of immersion in synthetic seawater. Again the control epoxy without graphene suffers severe corrosion and rust, while the epoxy samples loaded with graphene are practically free from the corrosion process. Graphene significantly increased the corrosion mitigation of the epoxy coating, even at load levels as low as 0.1% by weight. Corrosion mitigation has improved as the level of graphene loading has been increased. Both A-GNP10 and A-GNP35 (T) offered better protection against corrosion compared to pure epoxy.

The SEM image of the panels characterizes the corrosion of the substrate under the epoxy coating, which is shown in Figure 9. The image on the left (A-D) is a cross-section of the panel coated with epoxy without graphene after 30 days of immersion. It can be seen that the seawater sneaked into the surface of the steel substrate causing it to corrode and show rust 16.

2.4.2 Wear Analysis

To perform the wear analysis by ball-on-plate test on substrates coated with composite film, the friction coefficient is measured in relation to the distance covered by the alumina sphere in the coating, thus, the measurement is interrupted when this friction coefficient on a quick change, that change is linked to the fracture of the lining. Using AA2024-T3 aluminum alloy substrates coated through the Sol-Gel deposition process by a hybrid film formed according to the composition shown in Table 3 with and without the addition of graphene oxide particles, performed the ball-on-test plate to assess its wear behavior 22.

Varying concentrations of graphene oxide were added to the Si-1, Si-2, Si-3 and Si-u samples, which correspond to 1 g.L^-1, 0.5 g.L^-1, 0.25 g.L^-1and 0 g.L^-1 respectively on Figure 10.

The result of the wear analysis pointing to the graph suggests that the increase in GO particle concentration in the hybrid film causes an increase in wear resistance 22, as shown in Figure 10.

2.4.3 Mechanical Resistance

Studies conducted by our research group at CEFET-MG 23,demonstrated that epoxy composite materials reinforced by carbon fibers (carbon allotropes) have shown much higher tensile and flexural strength values when compared to low-density metals widely used in the steel industry in special applications, reaching values higher than the order of 7 times for aluminum alloys and 1.5 times for titanium alloys. With promising results obtained by the usage of epoxy composite materials reinforced by carbon fibers, researches for improvement of its properties and application in various sectors have been subject of studies in a large scale.

Using epoxy resin composites with the addition of graphene and carbon nanotubes (CNT), our research group at CEFET-MG evaluated the behavior of specimens subjected to mechanical tests. Table 4 shows the analysis of the results obtained 24 and allows evaluating how the addition of nanomaterials (carbon nanotubes and graphene) can modify the structural behavior of the epoxy resin. In total, 15 substrates were produced for each composite, with 6 substrates submitted to the tensile test, another 6 submitted to the microhardness test, and 3 submitted to the flexion test. Analyzing the figure above, the author pointed out that composite 4 was superior to composite 7 in Flexion, reaching a value of 23%, and also superior to composite 1 (pure resin) by 10%. Regarding traction and the Elasticity Module, the difference between composites 4 and 7 was low, as shown in Table 4. It is worth noting that at low levels of nanoparticles by mass, composites 8 (0.1% CNT), 2 (0.1% graphene I), and 5 (0.1% graphene II) obtained very similar values ​​in tensile, in the order of 70% above composite 1.

It points out that composites 4 and 7, epoxy resin with graphene I at 0.5% and graphene II at 0.5% by weight, respectively, stood out compared to the other composites from the structural point of view 24. Both obtained an increase in the Tensile Strength Limit by more than 120% in relation to composite 1, as shown in Figure 11.

The tests also showed a considerable increase in toughness, reaching more than 100%, to the point of modifying the performance of the material under stress, changing it from brittle (pure resin) to ductile 24, as shown in Figure 12.

Using sizings of epoxy and carbon nanotubes (CNT) that consist of allotropic carbon material, it was found that when evaluating the trend line of the flexural modulus for surface treatment of the carbon fibers (CF) (Figure 13), the oxidation of the fibers causes a fall in the elastic modulus, while the deposition of 0.10% and 0.30% of CNT promoted a high increase in the modulus 23. The deposition of CNT at 0.50% indicated an intermediate value of modulus, being higher than carbon fibers and oxidized carbon fiber (FCoxi) and lower than depositions of CNT at low levels. The results of mechanical resistance to flexion carried out in his study indicated that the oxidative surface treatment of the fibers caused a decrease in strength, while the sizing of CNT promotes an expressive gain in resistance to flexion.