Research Progress of Graphene/Epoxy Composite Coatings

epoxy resin (EP) can exhibit different properties because of its different molecular structures. And because it is easy to mix with different curing agents, thinners, additives and so on, the preparation of excellent mechanical, mechanical, thermal, adhesive, insulation and anti-corrosion properties of epoxy resin materials, and is widely used in anti-corrosion coatings. However, with the complexity of the application environment, the simple EP coating shows some shortcomings: First, due to the low thermal conductivity leading to poor heat resistance, most EPs are only applicable to environments below
100
degrees C; To make better use of the benefits of EPs, fillers are often added to improve performance.
graphene has great potential to improve the properties of resin-based materials by introducing flexible chain segments due to its unique crystal structure and excellent physical properties and the polymerization reaction caused by its derivatives. Because graphene is larger than the surface area and has a high surface energy, it is easy to reunite as a filler when added to epoxy resin, thus affecting coating performance. In order to distribute graphene evenly into the epoxy substate, scholars have done a lot of research. From the initial simple mixing to ultrasonic dispersion technology, the use of silane coupling agents to improve the bonding and compatibility between graphene and epoxy resins. The study found that the addition of graphene is good for improving coating performance, but when added to a certain amount, due to the accumulation of graphene will affect the further improvement of coating performance. In recent years, some scholars have prepared functional fossil ene by modifying the surface of graphene, and found that while retaining the basic properties of graphene, it can improve the bonding with the epoxy substate, which makes new progress in the research of graphene/epoxy composite coating.
1 Advances in the study of graphene/epoxy coatings
In terms of thermal properties, graphene is the material known to have the highest thermal conductivity (a single layer is approximately 5000W/mK) that can be added as a filler to improve the heat resistance of epoxy; From the mechanical and mechanical properties, graphene is composed of sp
2
hybridized flat carbon atoms, with high modality, high strength, and graphene layer with low shear and low coefficient of friction, easy to transfer to epoxy coating pairs The surface forms a transfer film, which, when used in combination with epoxy, improves the wear and impact resistance of the coating, and from the electrical performance point of view, the single-layer theoretical resistivity of graphene is about 10
-6
Ω.m. A small amount of graphene can be added to epoxy resin can have good conductivity, from the corrosion resistance, due to the small size effect of graphene and two-dimensional sheet layer structure, can improve the epoxy coating defects, so that it can form a dense insulation layer in the coating, thereby reducing corrosion.
1.1 Thermal Performance
Huang Kun and other graphene fillers added to the epoxy, epoxy modified silicone, vinyl resin 3 systems, through baking experiments and electrothermal aging experiments to test the effect of graphene on coating temperature resistance and electrothermal aging resistance. The results showed that the temperature resistance of all three was improved compared with the non-graphene, and after powering up 500h, epoxy appeared similar to the post-curing process, which made the curing crosslink denser, and the graphene shrinkage was more compact and the heat resistance was better. Yang et al. have found synergies between G and MWCNTs by studying graphene sheet (G)/multi-wall carbon nanotubes (MWCNTs)/EP) composites, which make their contact area with EP larger and avoid filler reunion. The thermal conductivity of the composite material was measured at 0.321W/mK, an increase of 146.9% over the pure EP (0.13W/mK).
1.2 Wear-resistant toughening performance
Wu Fang used graphene (G) and graphene oxide (GO) to improve the interface structure between silicon carbide and epoxy resins, and experiments measured that the coefficient of friction of G/EP composite coatings in dry and seawater frictions was lower than that of pure EP coatings. 14.5% and 33.7%, the wear rate decreased by 69.1% and 32.1%, the friction coefficient of the GO/EP composite coating decreased by 15.6% and 35.5% compared to the pure EP coating, and the wear rate decreased by 79% and 67.9%. Ren Xiaomeng and others prepared G, GO/EP composite materials, to investigate the two on the EP's toughening effect. The study showed that when the G and GO mass scores were 2%, the fracture toughness of the composites increased by 102% and 48.5%, respectively, and when the G and GO mass scores were 1%, the strength of the composites increased by 18% and 2%, respectively.
1.3 Electrical Properties
Wang Jian, etc. through home-made graphene and commercial-grade carbon nanotubes, Fullerene and graphite as nano-conductive materials into the EP preparation composite materials, to study its electrical properties. The study shows that G is a conductive filler better than carbon nanotubes, fullene and graphite×, and when G's volume fraction is 0.25%, the conductivity of the composite material has a seepage mutation, indicating that
G has formed a conductive network channel in the
EP at this time; Serena and others compared the electrical properties of the two with homemade diamond and graphene/epoxy composites. The results show that the threshold of graphene is much lower than that of synthetic diamond, and when graphene is added by 0.5% (volume fraction), the resistivity of the composite material drops from 7.14×10
7
Ω m to 1.02×10
3
Ω.m, because graphene is an excellent electrical conductor.
1.4 anti-corrosion performance
Zhou Nan and other bio-based non-food acid (GA) and epoxy chloropropane (ECP) as raw materials, the synthesis of non-food-based acid epoxy resin (GEP), and as a graphene dispersant, prepared a GEP-G/EP composite coating. The corrosion resistance was indicated by the use of coating water absorption, Tafel polarization curve and neutral salt spray testing. The results show that compared with pure EP coating, the polarization resistance and self-corrosion current density of the coating increased by 1 order of magnitude, and the water absorption rate decreased by 0.22%, and the salt spray resistance was also effectively improved. Wang Yuqiong and so on with sodium polyacrylic acid as a dispersant, through high-speed centrifuge dispersion 2h, and then ultrasonic dispersion of 30min, obtained graphene water-based dispersion, and prepared a G/water-based epoxy resin E44 composite coating with a G content of 0.5% (mass fraction). The results show that the addition of graphene improves the water isolation effect of water-based epoxy and reduces the Fick diffusion coefficient of pure E44 coating by 2 orders of magnitude, while the self-corrosion current density of pure E44 coating is 0.13μA/cm
2
, while the self-corrosion current density of G/E44 composite coating is only 0.038μA/cm
2
.
2 Problems
Because graphene is larger than the surface area (theoretical value is about 2630m
2
/g), the surface energy is high, in the epoxy resin when the addition of large amounts will occur reunion and tangle, resulting in its dispersion in the substation, poor stability. For thermal and electrical properties, when a small amount of graphene is added, the seepage threshold can be reached, the graphene content can continue to be increased, and the degree of further improvement in heat resistance and conductivity is reduced. However, for mechanical and mechanical properties, anti-corrosion properties, although a small amount of graphene can improve performance, but to a certain amount of time due to its reunification in the epoxy coating, will cause cracks in the coating, stress concentration points and defects, resulting in performance decline.
Wufang measured the coefficient of friction between dry and seawater frictions of different content g/EP coatings by means of a coefficient of friction meter, and found that when G is 1% (mass fraction), the coefficient of friction and wear rate of the coating increases. It is also pointed out that this is due to the G content is too high, will occur in the coating reunion caused by cracks, resulting in the coating in the friction process easy to peel off, resulting in abrasives increase the friction coefficient and wear rate of the coating.
Zhi and others used ultrasonic dispersion technology to prepare G/EP composite coatings, and after the coating curing, three-point bending test, and then use the field emission scanning electron microscope (FE-ESM) to observe the fracture surface of the coating. The study found that when the graphene content was 1% (mass fraction), the coating was dispersed more evenly, and the toughness of the coating increased significantly when the content was less than 1%. However, when the content reaches 2%, it will be reunited in the coating, resulting in defects that form stress concentration points, resulting in reduced toughness of the coating.
Liu and others added G as corrosion inhibitor to the epoxy E44 system to prepare the G/EP composite coating, and after placing 48h in 3.5% of the NaCl solution, the dynamic power polarization curve was measured. The study showed that the self-corrosiveity of 0.5% (mass fraction) G/E44 and 1% (mass fraction) G/E44 coatings was significantly lower than that of E44 coatings, and the corrosion current density of 0.5% G/E44 (0.0551μA/cm
2
) Well below the 1% G/E44 (0.934 s/cm
2
) and E44 (0.121 sa/cm
2
) coatings, the addition of graphene improves the water isolation performance of the epoxy coating and reduces the penetration of the corrosion medium. However, adding excess graphene will create a reunion on the surface of the coating, reducing the water isolation performance of the coating.
3 Advances in the research of functional fossil ink/epoxy coatings
3.1 functional fossil inks
Due to the large π bond structure of the graphene surface, hydrophobic and chemical inertness, they are easy to stack and gather in epoxy coatings, making it difficult for graphene to perform fully in epoxy substations. In order to solve this problem, domestic and foreign scholars have formed a new type of functional fossil ink by adding other components and structures on the basis of graphene. While maintaining its original basic properties, this graphene also gives a new identity and can be optimized according to the need for coating performance.
according to the chemical structure, the functionalization of graphene is divided into co-price combination and non-co-price combination. Co-priced binding is by destroying the π-bond structure of graphene surface, making it surface-active, but the destruction of this stable structure, will lead to functional fossil graphene than the natural graphene in conductivity, thermal conductivity and other performance decreased. Non-co-priced binding refers to the use of graphene has an oversized surface area, through the surface adsorption method, with other particles with excellent performance composite. Although this method does not damage the basic structure of graphene, maintaining the inherent performance characteristics of graphene, but the dispersion effect is slightly less than co-price combination, generally need to add stabilizers or ultrasonic dispersion.
although the study of functional fossil ink is still in its preliminary stage, the research on its application to epoxy resin antiseptic coatings is still relatively small. However, some scholars have modified the surface of graphene through some functional groups, added to the epoxy system, and proved that functional fossil ink is better than simple graphene addition effect.
3.2 Functional fossil inks are used in epoxy coatings
Ghaleb and others analyzed the glass conversion temperature Tg of G/EP coating and ch-G/EP (chloroform functional fossil ink/epoxy resin) coating by differential scanning thermometer, and found that Tg in G/EP was higher than pure EP only when graphene volume content was 0.1%, while all samples in ch-G/EP were higher than pure EP Tg. This is due to the fact that pure graphene, when added to a certain amount, forms a reunion in the coating that affects coating performance, while chloroform-functionalized graphene is well dispersed in the coating.
Martin-GALLEGO, etc., is chemically reduced by Au
3 plus
to functionally modify the graphene surface with self-depositing gold nanoparticles that occur on the surface of the gold particles and disperse Au/G in the photo-cured epoxy coating through ultrasonic dispersion. The study found that au-G/EP had a conductivity about four orders of magnitude higher than G/EP at the same amount of addition. Chen Yu used hydrothermal method, with formaldehyde resin and graphene oxide as raw materials, prepared phenolic resin modified graphene aerogel (p-GA), and as a conductive filler and EP to form a composite material. The study found that because the addition of methyl phenolic resin makes the three-dimensional network structure of p-GA more perfect and solid, adding a small amount of p-GA can obtain excellent conductivity and electromagnetic shielding performance. When the filler content is 0.33% (mass fraction), the conductivity is 73S/m and the electromagnetic shielding performance is 35dB.
Qi and so on in the graphene oxide surface branched silane, obtained silane functional fossil inkene (g-GO), and liquid crystal epoxy (LCE) as a mixed filler added to the epoxy substate, prepared epoxy resin composite coating. The study showed that when the mixed filler was 3%, the impact resistance of the composite coating was increased by 132.6%, and the pull strength and bend strength were increased by 27.6% and 37.5%, respectively, compared to the pure epoxy coating. The performance of less functional graphene has been further improved.
Ramezanzadeh and others modified graphene oxide through gel-based silane, prepared a functional graphene oxide/epoxy resin coating, and studied the effect of silane functional graphene oxide on coating properties through electrochemical impedance spectrum, salt spray method and cathode stripping tests. The results show that by scanning the electron microscope, graphene oxide modified by silane was observed to be evenly dispersed in the epoxy substate, and the corrosion resistance of the coating was effectively improved, and cathode stripping was reduced.although the research of
functional fossil ink epoxy resin coating has made progress to varying degrees, but because the reaction conditions are not easy to control, the formulation design of composite coating is not convenient, not suitable for large-scale preparation, but also need to further seek simple and efficient preparation routes.
4 Outlook
With the development of modern science and technology, people's requirements for epoxy-based composite coating performance are getting higher and higher, but due to the current technical research on graphene/ epoxy composite coating preparation is not yet mature, but also need to carry out research in the following areas.
(1) Cannot be limited to considering the comprehensive properties of graphene/epoxy coatings, graphene should be targeted for functional modification for special circumstances or the search for targeted high-efficiency dispersants to enhance a specific performance of the coating.
(2) graphene oxygen-containing energy group content and type is the choice of suitable change