Research progress of graphene/epoxy composite coatings

Epoxy resins (EP) can exhibit different properties due to their different molecular structures. And because it is easy to mix with different curing agents, thinners, additives, etc., it is widely used in anticorrosive coatings by preparing epoxy resin materials with excellent mechanical, mechanical, thermal, cohesive, insulating and anticorrosive properties. . However, with the complexity of the application environment, pure EP coatings show some shortcomings: First, due to the low thermal conductivity, the heat resistance is poor. Most EPs are only suitable for environments below 100 °C. Second, the crosslinking density is high after curing. So that the friction coefficient is high, wear resistance and impact resistance are poor; third, the high resistivity is easy to produce electrostatic effects; fourth, it is easy to produce defects after curing, affecting anti-corrosion performance. In order to make better use of the advantages of EP, fillers are often added to improve performance.

Graphene has great potential in improving the performance of resin-based materials due to its unique crystal structure and excellent physical properties and its derivatives can initiate polymerization. Since graphene has a large specific surface area and a high surface energy, it is easily agglomerated when added as a filler to an epoxy resin, thereby affecting coating properties. In order to uniformly disperse graphene into epoxy groups, scholars have conducted extensive research. From the initial simple mixing to the ultrasonic dispersion technology, silane coupling agents are used to improve the bondability and compatibility between graphene and epoxy resin. The study found that the addition of graphene is beneficial to improve the performance of the coating, but when added to a certain amount, the accumulation of graphene will affect the further improvement of coating performance. In recent years, some scholars have prepared functionalized graphene by functional group modification on the surface of graphene. It has been found that it can improve the bondability with epoxy groups while retaining the properties of graphene, so that graphene/epoxy New progress has been made in the research of resin composite coatings.

1. Research progress of graphene/epoxy resin coatings

In terms of thermal properties, graphene is currently known as the material with the highest thermal conductivity (single layer is about 5000W/mK). Adding as a filler can improve the heat resistance of epoxy; from the viewpoint of mechanical properties and mechanical properties, Graphene is composed of sp2 hybridized planar carbon atoms, has high modulus and high strength, and has low shear force and low friction coefficient between graphene layers, and is easily transferred to the epoxy coating to form a transfer film on the dual surface. When used in combination with epoxy, the wear resistance and impact resistance of the coating can be improved; from the viewpoint of electrical properties, the theoretical resistivity of the graphene single layer is about 10-6 Ω·m, and due to its low bulk density, in the epoxy When a small amount of graphene is added to the resin, it has good electrical conductivity; from the viewpoint of corrosion resistance, due to the small size effect of the graphene and the two-dimensional sheet structure, defects in the epoxy coating can be improved, so that it can be coated. A dense barrier layer is formed in the layer to reduce corrosion.

1.1 Thermal performance

Huang Kun et al. added graphene as a filler to three systems of epoxy, epoxy-modified silicone and vinyl resin. The graphene tested the temperature resistance and electrothermal aging resistance of the coating through baking experiments and electrothermal aging experiments. Sexual influence. The results show that compared with no graphene, the temperature resistance of the three is improved, and after 500 hours of electrification, the epoxy has a similar post-cure process, which makes the cross-linking more compact after curing, and the graphene shrinks. Compact and heat resistant. Yang et al. found that there is a synergistic effect between G and MWCNTs by studying graphene sheet (G)/multi-walled carbon nanotubes (MWCNTs)/epoxy resin (EP) composites. Due to this bridging effect, it is compatible with EP. The contact area becomes large to avoid agglomeration of the filler. The thermal conductivity of the composite was measured to be 0.321 W/mK, which was 146.9% higher than that of pure EP (0.13 W/mK).

1.2 wear resistance and toughness

Wu Fang used graphene (G) and graphene oxide (GO) to improve the interface structure between silicon carbide and epoxy resin. The friction coefficient of G/EP composite coating in dry friction and seawater friction was measured experimentally. The pure EP coating was reduced by 14.5% and 33.7%, and the wear rate was reduced by 69.1% and 32.1%. The friction coefficient of the GO/EP composite coating was 15.6% and 35.5% lower than that of the pure EP coating, and the wear rate was reduced by 79%. And 67.9%. Ren Xiaomeng et al. prepared G, GO/EP composite materials to investigate the toughening and strengthening effects of the two on EP. The results show that when the mass fraction of G and GO is 2%, the fracture toughness of the composite increases by 102% and 48.5%, respectively. When the mass fraction of G and GO is 1%, the strength of the composite increases by 18% and 2%, respectively.

1.3 Electrical performance

Wang Guojian and others prepared their composites by adding self-made graphene and commercial grade carbon nanotubes, fullerenes and graphite as nano-conductive materials, respectively, to study their electrical properties. Studies have shown that G is a conductive filler superior to carbon nanotubes, fullerenes and graphite. When the volume fraction of G is 0.25%, the conductivity of the composite undergoes a percolation mutation, indicating that G has formed in EP at this time. The conductive network channel; when the volume fraction exceeds 0.5%, the conductivity tends to be stable to 2.02×10-7S/m. Serena et al. compared their electrical properties with self-made diamond and graphene/epoxy composites. The results show that the threshold of graphene is much lower than that of synthetic diamond. When the amount of graphene added is 0.5% (volume fraction), the resistivity of the composite decreases from 7.14×107 Ω·m to 1.02×103 Ω·m. Alkene is an excellent electrical conductor.

1.4 anti-corrosion performance

Zhou Nan et al. synthesized bio-gallic acid (GA) and epichlorohydrin (ECP) as raw materials to synthesize gallic acid-based epoxy resin (GEP) and used it as a graphene dispersant to prepare GEP-G/EP. Composite coating. The corrosion resistance was characterized by using the coating water absorption, Tafel polarization curve and neutral salt spray test. Studies have shown that compared with pure EP coating, the polarization resistance and self-corrosion current density of the coating are increased by one order of magnitude, and the water absorption rate is decreased by 0.22%, and the salt spray resistance is also effectively improved. Wang Yuqiong et al. used sodium polyacrylate as dispersant, dispersed by high-speed centrifuge for 2h, and then ultrasonically dispersed for 30min, then obtained graphene aqueous dispersion, and prepared G/waterborne epoxy resin with G content of 0.5% (mass fraction). E44 composite coating. Studies have shown that the addition of graphene improves the water-blocking effect of waterborne epoxy, and the Fick diffusion coefficient of pure E44 coating is reduced by two orders of magnitude; the pure E44 coating has a self-corrosion current density of 0.13 μA/cm2, and G/ The self-corrosion current density of the E44 composite coating is only 0.038 μA/cm 2 .

2, the problem

Due to the large specific surface area of ​​the graphene (theoretical value is about 2630 m2/g) and high surface energy, agglomeration and entanglement occur when the amount of epoxy resin is large, resulting in poor dispersion and stability in the matrix. . For thermal and electrical properties, when a small amount of graphene is added, the percolation threshold can be reached, and the graphene content is continuously increased, and the extent of further improvement in heat resistance and conductivity is small. However, for mechanical and mechanical properties, corrosion resistance, a small amount of graphene can improve the performance, and when it reaches a certain amount, it will cause cracks, stress concentration points and defects in the coating due to its agglomeration in the epoxy coating. Causes a decline in performance.

Wu Fang measured the friction coefficient of dry friction and seawater friction of different G/EP coatings by friction coefficient measuring instrument. It was found that when G is 1% (mass fraction), the friction coefficient and wear rate of the coating will increase. It is pointed out that this is because when the G content is too high, agglomeration will occur in the coating to cause cracks, which will cause the coating to peel off during the friction process, and the generated abrasive grains increase the friction coefficient and wear rate of the coating.

Zhi et al. prepared the G/EP composite coating by ultrasonic dispersion technique, and performed a three-point bending test after the coating was cured, and then observed the fracture surface of the coating by field emission scanning electron microscopy (FE-ESM). It was found that when the graphene content is 1% (mass fraction), the dispersion in the coating is relatively uniform, and when the content is less than 1%, the toughness of the coating is remarkably increased. However, when the content reaches 2%, agglomeration occurs in the coating, thereby causing defects to form stress concentration points, resulting in a decrease in the toughness of the coating.

Liu et al. added G as a corrosion inhibitor to epoxy resin E44 system to prepare G/EP composite coating, and measured the potentiodynamic polarization curve after standing for 48h in 3.5% NaCl solution.

The study shows that the self-corrosion potential of 0.5% (mass fraction) G/E44 and 1% (mass fraction) G/E44 coating is significantly lower than that of E44 coating, and the corrosion current density of 0.5% G/E44 (0.0551μA/cm2). ) far less than 1% G/E44 (0.934μA/cm2) and E44 (0.121μA/cm2) coatings, indicating that the addition of graphene improves the water barrier properties of the epoxy coating and reduces the penetration of corrosive media. . However, the addition of excess graphene will agglomerate on the surface of the coating, reducing the water barrier properties of the coating.

3. Research progress of functionalized graphene/epoxy coatings

3.1 Functionalized graphene

Since the large π bond structure of the intrinsic graphene surface is hydrophobic and chemically inert, it tends to be stacked and aggregated in the epoxy coating, which makes it difficult for graphene to fully exert its performance in the epoxy group. In order to solve this problem, domestic and foreign scholars have formed a new type of functionalized graphene by adding other components and structures on the basis of graphene. This graphene retains its original properties while imparting a new property, and can also be specifically optimized for graphene based on the performance requirements of the coating.

According to the chemical structure, the functionalization of graphene is divided into covalent bonding and non-covalent bonding. Covalent bonding is to activate the surface by destroying the π bond structure on the surface of graphene. However, the destruction of this stable structure leads to a decrease in the properties of functionalized graphene over intrinsic graphene in terms of conduction and heat conduction. Non-covalent bonding refers to the use of the superabsorbent surface area of ​​graphene, which is combined with other particles with excellent properties by surface adsorption. Although this method does not damage the basic structure of graphene, and maintains the inherent performance characteristics of graphene, the dispersion effect is slightly inferior to the covalent bond, and generally requires the addition of a stabilizer or ultrasonic dispersion.

Although the research on functionalized graphene is still in its preliminary stage, there are few studies on its application in epoxy resin anticorrosive coatings. However, some scholars have modified the surface of graphene by certain functional groups and added it to the epoxy system, and proved that the functionalized graphene is superior to the simple graphene.

3.2 Application of Functionalized Graphene in Epoxy Coatings

Ghaleb et al. analyzed the glass transition temperature Tg of G/EP coating and ch-G/EP (chloroform functionalized graphene/epoxy resin) coating by differential scanning calorimeter, and found that only Gene in G/EP The Tg at a volume content of 0.1% was higher than that of pure EP, and all samples in ch-G/EP were higher than the Tg of pure EP. This is because the simple graphene will form agglomeration in the coating when it is added to a certain amount to affect the coating performance, and the graphene functionalized by chloroform can be well dispersed in the coating.

Martin-GALLEGO and other chemically reduced Au3+, functionalized modification of graphene surface by gold nanoparticles generated by self-deposition on the surface of gold particles, and dispersion of Au/G in photocurable epoxy coating by ultrasonic dispersion in. The study found that the conductivity of Au-G/EP is about 4 orders of magnitude higher than G/EP at the same amount of addition. Chen Yu used hydrothermal method to prepare phenolic resin modified graphene aerogel (p-GA) with resole phenolic resin and graphene oxide as raw materials, and used it as a conductive filler to form a composite with EP. It is found that the three-dimensional network structure of p-GA is more perfect and firm due to the addition of resol phenolic resin, and excellent conductivity and electromagnetic shielding performance can be obtained by adding a small amount of p-GA. When the filler content is 0.33% (mass fraction), the conductivity is 73 S/m, and the electromagnetic shielding performance reaches 35 dB.

Qi et al. grafted silane onto the surface of graphene oxide to obtain silane functionalized graphene (g-GO), and added liquid crystal epoxy (LCE) as a mixed filler to the epoxy group to prepare epoxy resin composite coating. . Studies have shown that when the mixed filler is 3% [2% (mass fraction) g-GO and 1% LCE], the impact resistance of the composite coating is improved by 132.6% compared with the pure epoxy coating, tensile strength And the flexural strength increased by 27.6% and 37.5%, respectively. The performance of unfunctionalized graphene has been further improved.

Ramezanzadeh et al. modified silane functionalized graphene oxide/epoxy resin coating by gel-based silane modification, and studied silane functionalized graphene oxide by electrochemical impedance spectroscopy, salt spray method and cathodic disbondment test. The effect on the properties of the coating. Studies have shown that the silane-modified graphene oxide is uniformly dispersed in the epoxy group by scanning electron microscopy, and the anticorrosion performance of the coating is effectively improved, and the cathodic disbonding phenomenon is reduced.

Although the research on functionalized graphene epoxy resin coatings has made different degrees of progress, the formulation of composite coatings is inconvenient and difficult to control, and it is not suitable for large-scale preparation. Further, a simple and efficient preparation route is needed.

4. Outlook

With the development of modern science and technology, people have higher and higher requirements on the performance of epoxy resin-based composite coatings. However, due to the current research on the preparation of graphene/epoxy composite coatings, the research needs to be carried out in the following aspects. the study.

(1) It should not be limited to the comprehensive performance of graphene/epoxy coatings. Targeted functional modification of graphene should be targeted for specific environments or targeted high-efficiency dispersants should be sought to improve a particular performance of the coating.

(2) The content and type of oxygen-containing functional groups in graphene are the basis for selecting suitable modified molecules and modification methods. The macroscopic preparation of functionalized graphene with controllable structure and properties should be the focus of future research.

(3) With the improvement of environmental protection requirements, the process of water-based anti-corrosion coatings is accelerating. Waterborne graphene epoxy coatings have broad prospects. The problem to be solved is the dispersibility of graphene in waterborne epoxy resins and the good electrical and thermal conductivity of the coatings.

(4) The performance testing and application research of functionalized graphene and epoxy resin composite coatings need further research. As an interdisciplinary, graphene-based composite coatings involve many fields, such as flame retardancy and resistance of graphene-based epoxy coatings. Hanging flow, etc., is waiting for further research and exploration by scientists.

(5) Introducing the quantity control and performance characterization of functionalized functional groups on the surface of graphene, and accurately selecting functionalized sites on the graphene surface, and refining the chemical structure of graphene/epoxy resin to suit Different applications of coatings require further research.

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