2.Problems that exist
Since graphene has a large specific surface area (a theoretical value of about 2630 m2/g) and a high surface energy, agglomerates and tangles occur when the amount of graphene is large, resulting in poor dispersion and stability in the matrix. . For the thermal and electrical properties, when a small amount of graphene is added, the percolation threshold can be reached, and the graphene content is further increased, and the magnitude of the further improvement in heat resistance and electrical conductivity becomes smaller. However, for mechanical and mechanical properties, anti-corrosion properties, although a small amount of graphene can improve performance, due to its agglomeration in the epoxy coating at a certain amount, it will cause cracks, stress concentration points and defects in the coating. Causes a drop in performance.
Wu Fang measured the friction coefficient in the dry friction and seawater friction of different G/EP coatings with a friction coefficient meter and found that when G is 1% (mass fraction), the friction coefficient and wear rate of the coating will increase. And pointed out that this is due to G content is too high, it will occur in the coating caused by the agglomeration of cracks, resulting in the coating is easy to peel off in the friction process, the resulting wear debris increases the coating's friction coefficient and wear rate.
Zhi et al. used ultrasonic dispersion technology to prepare a G/EP composite coating, and performed three-point bending test after the coating was cured, and then observed the fracture surface of the coating using a field emission scanning electron microscope (FE-ESM). It was found that when the content of graphene 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 significantly increases. However, when the content reaches 2%, agglomeration will occur in the coating, which will cause defects to form stress concentration points, resulting in a decrease in the toughness of the coating.
Liu et al. applied G as a corrosion inhibitor to the epoxy resin E44 system to prepare a G/EP composite coating, and measured the potentiodynamic polarization curve after placing it in 3.5% NaCl solution for 48 hours.
The results show 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) ) is much lower than 1% G/E44 (0.934μA/cm2) and E44 (0.121μA/cm2) coatings, indicating that the addition of graphene improves the water-repellent performance of epoxy coatings and reduces the penetration of corrosive media. . However, adding excess graphene will agglomeration on the surface of the coating and reduce the water-repellent properties of the coating.
3. Research progress of functionalized graphene/epoxy coatings
3.1 Functionalized graphene
Due to the hydrophobicity and chemical inertness of the large π-bonded structure on the surface of intrinsic graphene, it is easy to stack and aggregate in the epoxy coating, and it is difficult for the graphene to fully exert its performance in the epoxy matrix. In order to solve this problem, domestic and foreign scholars form a new type of functionalized graphene by adding other components and structures on the basis of graphene. This graphene, while maintaining its basic properties, will also impart a new property, and can also be targeted to optimize graphene based on the need for coating properties.
According to the chemical structure, the functionalization of graphene is divided into covalent bonding and non-covalent bonding. Covalent bonding destroys the π-bonded structure on the surface of graphene, making its surface active. However, the destruction of this stable structure will lead to a decrease in the electrical and thermal conductivity of functionalized graphene than intrinsic graphene. Non-covalent bonding refers to the use of the characteristic of super-large specific surface area of graphene, which is compounded with other particles with excellent properties through surface adsorption. Although this method does not destroy the basic structure of the graphene, and retains the inherent performance characteristics of the graphene, the dispersion effect is slightly inferior to the covalent bonding. Generally, it is necessary to add a stabilizer or ultrasonic dispersion.
Although the research on functionalized graphene is still in the preliminary stage, there are few studies on its application in epoxy resin anti-corrosion coatings. However, some scholars have modified the surface of graphene through certain functional groups and added epoxy resin, and proved that functionalized graphene is superior to pure graphene.
3.2 Application of Functionalized Graphene in Epoxy Coatings
Ghaleb et al. analyzed the glass transition temperature Tg of G/EP coatings and ch-G/EP (chloroform-functionalized graphene/epoxy resin) coatings by differential scanning calorimetry. It was found that G/EP has only graphene. The Tg at a volume content of 0.1% is higher than that of pure EP, while all samples in ch-G/EP are higher than the Tg of pure EP. This is because pure graphene will form agglomerates in the coating when it is added to a certain amount, which affects coating performance, and graphene functionalized with chloroform can be well dispersed in the coating.
The chemical reduction of Au3+ by Martin-GALLEGO et al. functionally modified the surface of graphene with gold nanoparticles generated by autodeposition on the surface of the gold particles, and dispersed the Au/G in the light-cured epoxy coating by ultrasonic dispersion. in. It was found that the electrical conductivity of Au-G/EP is about 4 orders of magnitude higher than that of G/EP at the same addition amount. Chen Yu used hydrothermal method, using resole phenolic resin and graphene oxide as raw materials, prepared phenolic resin modified graphene aerogel (p-GA), and used it as a conductive filler to form a composite material with EP. The study found that: due to the addition of resole phenolic resin to make the three-dimensional network structure of p-GA more perfect, a small amount of p-GA can get excellent conductivity and electromagnetic shielding performance. When the filler content is 0.33% (mass fraction), the electrical conductivity is 73 S/m, and the electromagnetic shielding performance reaches 35 dB.
Qi et al. grafted silane on the surface of graphene oxide to obtain silane-functionalized graphene (g-GO) and added it to the epoxy matrix with liquid crystal epoxy (LCE) as a mixed filler to prepare an epoxy resin composite coating. . The study shows that when the mixed filler is 3% [2% (mass fraction) g-GO and 1% LCE], compared with the pure epoxy coating, the impact resistance of the composite coating increases by 132.6%, and the tensile strength And bending strength increased by 27.6% and 37.5%, respectively. The performance of unfunctionalized graphene has been further improved.
Ramezanzadeh et al. modified graphene oxide by gel-based silane, prepared silane functionalized graphene oxide/epoxy resin coating, and studied silane functionalized graphene oxide by electrochemical impedance spectroscopy, salt spray method and cathodic disbondment test. Effect on paint performance. The results showed that the silane-modified graphene oxide was uniformly dispersed in the epoxy matrix, and the corrosion resistance of the coating was effectively improved and the cathodic disbondment was reduced.
Although the study of functionalized graphene epoxy resin coatings has achieved varying degrees of progress, because the reaction conditions are not easy to control, the formulation design of composite coatings is inconvenient, and it is not suitable for large-scale production. It is still necessary to further seek simple and efficient preparation routes.
With the development of modern science and technology, people are increasingly demanding the performance of epoxy-based composite coatings. However, due to the fact that the technology for the preparation of graphene/epoxy resin composite coatings is not yet mature, it needs to be developed in the following areas. the study.
(1) It is not confined to considering the overall performance of graphene/epoxy coatings. Targeted functional modifications of graphene should be targeted for specific environments or targeted high-efficiency dispersants should be used to enhance a particular property of the coating.
(2) The content and species of oxygen-containing functional groups in graphene are the basis for selecting suitable modified molecules and modification methods. Macro-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. Water-based graphene epoxy coatings have broad prospects. The problem to be solved is the dispersion of graphene in aqueous epoxy resins and the guarantee of good conductive and thermal conductivity of the coatings.
(4) Performance testing and application of functionalized graphene and epoxy resin composite coatings need to be further studied. As a cross-disciplinary, graphene-based composite coatings are involved in many fields, such as flame retardancy and resistance of graphene-based epoxy coatings. Continuity, etc., need to be further studied and explored by scientists.
(5) Introduction of quantitative control and performance characterization of functionalized functional groups on the surface of graphene, as well as accurate selection of functionalized sites on the graphene surface, and design of graphene/epoxy resin for refinement of chemical structures to accommodate Different applications of paints need further study.