The mechanism of rare earths in functional coatings and coatings

Abstract: The rare earth elements have special electronic structures and have important applications in the preparation of functional coatings. Some applications of rare earths in the preparation of functional organic coatings, corrosion-resistant coatings, high-temperature resistant coatings and other fields are described. The mechanism of rare earths in functional coatings and coatings is preliminarily explored.

Keywords: rare earth elements, organic coatings, corrosion resistance, high temperature resistance

0 Preface

Rare earths are collectively referred to as the 17 elements of the group IIIB of the Periodic Table of Elements, including yttrium Sc, yttrium Y, and lanthanum La, yttrium Ce, yttrium Pr, yttrium Nd, *Pm, yttrium Sm, and yttrium. Eu, 钆Gd, 铽Tb, 镝Dy, 钬Ho, 铒Er, 铥Tm, 镱Yb, 镥Lu, total 17 elements. Rare earth elements have the electronic configuration of [Xe]4f0-145d1-106S2. Due to the particularity of the 4f orbit and the presence of 5d orbitals, they have excellent properties such as light, electricity and magnetism. Rare-earth ions have abundant electronic energy levels, large ionic radius, high charge, and strong ability to complex, which provides more ways for chemically synthesizing new rare earth materials. The application of rare earth elements to functional coatings allows targeted protection of various materials.

1. Rinsing effect of rare earths in organic coatings

Organic coatings are used extensively in material protection, and organic paints often require the addition of driers during use. The traditional driers are mainly organic acid soaps such as cobalt, manganese, iron, lead, zinc, and calcium. However, they have obvious disadvantages: the price of cobalt soap is high, the color of manganese soap is deep, and the lead soap is toxic and polluted. . Rare earth metal soap driers as a new type of driers, not only has the advantages of low toxicity, light color, and suitable price, but also has the function of active driers and auxiliary driers, and can partially replace cobalt driers. All replace driers such as manganese, iron, lead, zinc and calcium, which will help reduce costs, eliminate lead poisoning and pollution, and improve the quality of the paint film. Because rare earth elements have a special outer layer electronic structure, the soap driers made of it can not only oxidize or bind the natural antioxidants in oils into complexes through their valence changes, but also eliminate antioxidants. Oxygen resistance accelerates the oxygen absorption rate of unsaturated fatty acids, promotes the surface oxidative polymerization and drying of unsaturated fatty acids in the oil, and can also pass through its empty orbits with hydroxyl groups in alkyd, phenolic, amino, epoxy resins, etc. The radicals such as radicals form coordination bonds, increase the degree of cross-linking of the molecular structure, and generate a coordination complex with a larger relative molecular mass, so that the middle and bottom coatings are coordinated and dried [1]. For example, rare earth elements Ce and isooctanic acid are used as the main raw materials, and an organic acid saponification method can be used to prepare high-efficiency rare earth paint driers. The driers can promote the generation of free radicals by changing the valence state of the cerium ions contained therein, and accelerate the organic coating. Oxidative polymerization drying, but also with organic molecules in the hydroxyl, methylol and other polar groups to form coordination bonds, so that organic coating polymerization drying [2].

2. Application of rare earths in functional organic coatings

The addition of a rare earth salt gel to the organic coating can increase the gloss of the coating. The light reflectance (light incidence angle 60°) of coatings made with different rare earth salts were: yttrium acetate 72%, yttrium chloride 69%, yttrium 74%, and yttrium acetate 71%. The yttrium acetate was 71%, the yttrium acetate was 71%, the yttrium acetate was 65%, and the yttrium was 71% [3]. Rare-earth waterborne gloss paints are generally composed of gels containing rare earth salts (2% to 20%), polymer latex resins (10% to 20%) and paste bases containing various additives (60% to 80%). Modulated from large components. This kind of coating can be used for the spraying of solid materials such as wood, metal, ceramics, paper, etc., which can increase the degree of smoothness and anti-corrosion. The addition of rare earth salts in the preparation of interior wall coatings can produce anions that are beneficial to humans [4]. This is because the outermost electronic structures of the rare earth elements are the same, they are all 2 electrons, the sub-epitaxial structures are similar, and the penultimate layer 3 has an unfilled 4f electronic layer structure. In the 4f orbit, there are unpaired electrons, and the outermost two electrons undergo electron transitions, resulting in a wide variety of electronic energy levels. A hole is formed on the surface of the atomic surface of the rare earth element to form a system with water and air. A catalytic reaction takes place, producing O2 and ·OH reactive oxygen radicals, ionizing molecules in the air, thereby increasing the concentration of negative ions in the air. Rare earth elements have special electronic structures and unique optical, electrical and magnetic properties. They are important elements in the formation of new functional materials such as light, electricity and magnetism. The rare earth-activated alkaline earth metal aluminate luminescent material refers to a type of luminescent material using a rare earth, in particular, Eu as an activating element and an alkaline earth metal aluminate as a matrix. The most representative and best performance aluminate-based long afterglow luminescent materials are rare-earth ion-doped MAl2O4:Eu2+, RE3+, where M is an alkaline-earth metal element, RE is a rare earth element, Eu2+ is a luminescent center, and RE can cause The formation of defect levels leads to long afterglow. Different M causes the crystal field intensity of Eu2+ to change significantly, resulting in different luminescence and persistence colors [5-6]. Pu Hongting, et al. [7] studied luminescence coatings based on lanthanum aluminate as the luminophore, with perchloroethylene, epoxy resin and acrylic resin as the base material. Studies have shown that the content of lanthanum aluminate is 20%. ～30% is the best. Luminous paint with acrylic resin as base material has good luminous intensity, adhesion, water resistance, weather resistance and chemical stability. U.S. Patent [8] describes a waterborne free-radical luminescent paint containing luminescent materials of MAl2O3 (M is one or more of Sr, Mg, Ca and Ba) co-activated by lanthanum and other rare earth ions. Polyurethane resin is a base material, is an environmentally friendly road paint, and can also be used for indoor and outdoor night indications. The rare earth heat preservation coating is made of silicate fiber as the main material, and is made of expanded pearl salt as the filler, doped with an appropriate amount of surfactant, rare earth and high-low temperature adhesive [9]. After the rare earth elements are added into the thermal insulation coating, rare earth oxides, rare earth silicates, rare earth compounds, and rare earth inclusions can be generated and certain gas substances can be released to improve the microporous structure of the thermal insulation coating. The X-ray fluorescence analyzer test confirmed that rare earth elements and its oxides, salts, etc. are a large number of multi-coexistence. The two types of insulation coatings were observed with an electronic scanning microscope. Insulating coatings of rare earth elements were added, and the fibers were arranged in a network. The insulation coating without the addition of rare earths had its fibers arranged in a mixed sheet. Since most of the "crystal habits" of rare earth elements are "close-packed hexagonal" bodies, some are coexistence of "close-packed hexagons" and "face-centered cubics", and the atom itself has a space-valence bond orbit, and thus its chemical properties. Lively, and with a large contact with other atoms, after the physical and chemical reaction to form a network structure centered on it, thus enhancing the cohesion between coating fibers. In addition, rare-earth additives have a trace amount of radioactivity and can emit gamma and beta rays. These rays can modify the binder in the coating and also help increase its adhesion [10]. The raw material component A is prepared by mixing silicon dioxide, carbonized silicon, boric acid, dihydrogen water and lead oxide in a certain proportion; and then the graphite powder and the rare earth oxide are mixed into a component B in a certain proportion; B components are mixed according to the proportion, adding solvent, can be made into viscous conductive paint [11], can effectively and safely heat and insulate the construction of winter concrete, ensure the normal hydration reaction of the concrete system in winter, and meet the strength design of concrete. Requirements to effectively prevent early frost damage of concrete.

3. The application of rare earth in anti-corrosion coating

Corrosion of metal materials not only brings huge economic losses, but also leads to the destruction of the ecological environment. The steel equipment that is scrapped every year in the world due to corrosion is equivalent to 30% of steel output. The indirect losses caused by corrosion, such as production stoppages, reduced efficiency, higher costs, product pollution and personal accidents, are even more striking. It can be seen that the corrosion problem has become an urgent problem to be solved. problem. Arc spray long-term anti-corrosion is one of the most important thermal spraying technologies currently in use and has important application fields. Japan announced the results of field tests on erosion and corrosion under uninterrupted seawater conditions conducted on the coast of Japan. The protective performance of arc sprayed aluminum coatings has reached Class A [12-13]. At present, aluminum coating and zinc coating are one of the main materials for corrosion protection of metal components. As a sacrificial anode, it plays a role of cathodic protection, and can also isolate metal products from external corrosion media and prevent corrosion from occurring. The corrosion resistance of arc spraying layers of several aluminum and aluminum alloy materials containing rare earth elements was tested, indicating that the addition of rare earth can effectively improve the performance of the coating. This is mainly due to the rare earth can not only refine the grains, improve the corrosion resistance of the alloy, but also can improve the bonding strength of the coating, reduce the porosity, and make the pores smaller. The decrease of coating porosity plays an important role in improving the corrosion resistance of coatings [14]. In practical applications, the presence of the coating pores weakens the isolation of the coating, and the corrosive medium passes through the coating from the pores to the substrate, causing corrosion under the coating. When the steel components are all covered with a coating, the empirical formula of the coating life is T = 0.64d/S (T is the design life, year; d is the coating thickness, μm; S is the percentage of bare steel area). S depends on the coating. The porosity of the layer, the porosity of the coating is reduced, the S value is reduced, and the coating life is extended. Yu Xingwen, et al. [15] reported the corrosion resistance of rare earth elements on aluminum alloy surface conversion coatings. The formation of a rare earth conversion film on the surface of the aluminum alloy suppresses the diffusion and migration of oxygen and electrons on the surface of the aluminum alloy and the solution, so that the power of corrosion disappears, and a better passivation protection effect can be achieved. Wen Jiuba, et al. [16] conducted an experimental study on the hot-dip galvanizing process of rich Ce-rich rare earth aluminum alloys and their corrosion resistance after aluminizing. The results show that: rare earths have good leaching effect on hot dip aluminizing. After the hot-dip impregnated rare earth aluminum on the steel surface, it has good corrosion resistance, among which the aluminum alloy containing 013% of Ce-rich mixed rare earth has better corrosion resistance, and its corrosion resistance is 2 to 3 times that of pure aluminum. Epoxy resin has excellent adhesion, flexibility, and good corrosion resistance, but its poor acid resistance and organic solvent resistance, high water absorption rate limit its application, can add phenolic resin modified epoxy resin matrix . The addition of phenolic resin results in a cross-linking reaction between the reactive functional groups of the two resins. The resulting modified coating has both the advantages of strong adhesion, flexibility, and alkali resistance, as well as water resistance of the phenolic resin. Excellent solvent resistance and acid resistance. Based on the basic formula of epoxy resin powder coating, rare earth elements are added. Because rare earth elements are generally easy to lose three electrons, they are positively trivalent, and their reaction activity is extremely high. They are high-activity agents that participate in the reaction, and are also self-catalysts. After the reaction of the resin, the bond strength of the compound is extremely strong, so the heat resistance, wear resistance, and corrosion resistance of the resin can be further improved [17]. This type of paint consists of corrosion inhibitors (rare earth compounds) and fillers that produce neutral to slightly acidic. Combining the corrosion inhibition component with other components (such as fillers, amino acid and amino acid derivatives, gel and gel derivatives, organic exchange resins, and combinations thereof) can improve the corrosion resistance of the resulting coating film on the substrate (e.g., Metals, including aluminum and aluminum alloys, have good adhesion, such as those prepared with epoxy polyamides, dispersants, 2-butanol, kaolin, and enamel, and have good corrosion inhibition.

The improvement of the material's high temperature oxidation resistance is not always considered only from the material itself. Practice has proved that it is very difficult for the high temperature material itself to have good high temperature strength, but also to have excellent resistance to oxidation and corrosion, and to develop and use it. High-temperature coating, its funding is much lower than high-temperature materials. In recent years, people have studied a variety of high-temperature coatings and have achieved great progress, from traditional aluminide coatings to thermal barrier coatings, from single-layer coatings to multilayer coatings [19]. There are many coating methods for high temperature resistant coatings, and different types of high temperature resistant coatings have different methods of preparation.

The addition of trace rare earth elements in the surface modification layer of the material can improve the compactness of the modified layer and the bonding force with the substrate, reduce the oxidation rate, and improve the anti-stripping performance of the oxide film, thereby significantly improving the high temperature oxidation resistance of the modified layer. . The role played by trace rare earth elements is called reactive element effects, abbreviated as REE [20]. In the preparation of the high-temperature resistant coating, rare earths may be added in different ways. For example, rare earth compounds are generally added in chemical heat treatment, laser cladding or thermal spraying, and rare earth elements may be added in ion implantation or plasma coating. To the target. The influence of rare earths on the properties of the surface modification layer is firstly related to the trace solid solution and alloying. Theoretical analysis and test results have shown that "solid solution rare earths" are mainly concentrated on the grain boundaries or other crystal defects (such as dislocations, vacancies, etc.), through the interaction with defects or other elements, causing grain boundary physics, Changes in the chemical environment or interface energy affect the behavior of other elements and lead to the evolution of new phases, ultimately leading to changes in the microstructure and properties of the modified layer. Secondly, rare earth elements can be used to control the second phase or inclusions in the modified layer, thereby improving the performance of the modified layer and refining the structure and structure. Rare earth can make the infiltrated layer or coating structure fine and dense, which is one of the important reasons for improving the mechanical properties and oxidation resistance and corrosion resistance of the modified layer. In chemical heat treatment, it is generally believed that rare earth elements have strong affinity with oxygen, hydrogen, and other impurity elements, and can inhibit the role of these impurity elements in promoting the loosening of the structure, thereby making the microstructure of the permeation layer dense, and the rare earth can make the nucleation rate of the new phase possible. The increase is conducive to the refinement of the permeation layer [21-22]. Wang Yinzhen, et al. [23] studied the effect of CeO2 on the thermal shock resistance of plasma sprayed Cr2O3 coatings. It was found that proper amount of CeO2 allowed the micro cracks to be distributed in the coating layer in a mesh shape, which had the effect of releasing the internal stress of the coating and could delay The cracks are generated and expanded, and the penetration holes in the coating are reduced, thereby improving the thermal shock resistance of the coating. Thermal barrier coatings are widely used to protect aeroengine high-temperature components due to their excellent thermal insulation properties. In the thermal barrier coating ceramic material, pure ZrO2 can not be directly used as a thermal barrier coating due to its own phase transformation problem, and the stabilized ZrO2 becomes the first choice for thermal barrier coating ceramic layer due to its good comprehensive performance. Material [24]. The main phase composition of the rare earth oxide coating is generally La2O3, CeO2, Pr2O3, and Nb2O5. Hanshin Choi, et al. [25] studied plasma-sprayed CeO2-Y2O3-ZrO2 (CYSZ) thermal barrier coatings. As Ce4+-transformed Ce3+ is reoxidized to Ce4+ during plasma spraying, oxygen vacancies in coatings are reduced. This reduces the driving force of the cubic monoclinic phase transformation, making the CYSZ coating have better phase stability, lower thermal conductivity and thermal fatigue life than the YSZ coating. Rare earth elements have a very good effect on the high temperature performance of ceramic coatings. He Zhongyi, et al.[26] discussed the application of rare earth high temperature structural ceramics. The working temperature of Si3N4 ceramics doped with La and Y can reach 1650°C. It is mainly used in high temperature bearings and high temperature gas turbines. La and Y mainly serve as fluxes. Improve grain boundary effect. Doped rare earth ZrO2 toughened ceramics can be used as high temperature wear resistance materials, materials Y2O3 or CeO2 as a stabilizer. Yang Liu, et al. [27] showed that when Yb2O3 and CeO2 were added to Si3N4 ceramics, a large amount of Yb2Si2O7 crystals precipitated between the crystals, which increased the strength of the grain junctions at high temperatures. Yoshimura M and Kim Young-Wook et al [28-29] found that the addition of Y2O3 in SiC ceramics increases the high temperature strength of the material to 630-750 MPa. For Al2O3 ceramics, Yoshikowa A, et al. [30] showed that adding appropriate amount of Y2O3 can improve its high temperature strength, and Mitsuoka, et al. [31] showed that adding 0.105% (molar fraction) of Yb2O3 can make its strength reach 560 MPa.

5. Application of rare earth in other functional coatings

Rare-earth-modified carbon nanotube broad-band wave-absorbing material is made of rare-earth doped carbon nanotubes as a radar wave absorber, and is fully mixed with an epoxy resin to form a composite wave-absorbing coating and coated on an aluminum plate to form a wave-absorbing coating [32]. The wave-absorbing performance of carbon nanotubes was measured using a reflectance sweep measurement system. The results showed that the wave-absorbing properties of carbon nanotubes were greatly improved after modification with appropriate amounts of rare earth oxides. When rare earth oxides are added to far-infrared ceramic powders, the far-infrared ceramic powder contains a large amount of TiO2. TiO2 is a photocatalyst semiconductor. A valence electron band filled with electrons is composed of a conduction band capable of conducting electrons and a band gap where electrons cannot exist. . Because the valence electron band exists in the outer layer of rare earth elements, when a certain amount of energy is irradiated to the far-infrared ceramic powder, the valence electron band of the rare earth element captures the photocatalytic electrons, so most of the electrons generated by TiO2 are exposed to the outer layer of rare earth elements. Electron bands (positive trivalent) are trapped, so that more holes are generated. Therefore, far-infrared ceramic powders containing rare earth oxides have much higher electron and hole concentrations than far-infrared ceramics without rare earth oxides. Powder, because most ceramic materials are polycrystalline dielectric materials, and the infrared radiation characteristics of dielectric crystal materials are mainly related to electrons or electron holes in the far-infrared short-wave range, so the increase of electron-hole concentration will make the material infrared radiation. Strengthen [33].

6. Conclusion

The addition of rare earth elements can effectively improve various properties of coatings and coatings, especially high temperature oxidation resistance and corrosion resistance, and have good application prospects for workpieces and parts that require working under severe conditions. However, the mechanism of action of rare earths in coatings or coatings is still far from clear. So far, no complete explanation has been given. The application of rare earth elements in coatings or coatings has not been studied and reported so far. All these require materials work. For further experimental analysis and research, from the crystallization chemistry, thermodynamics, dynamics, etc. to explain the role of rare earth in the coating or coating, so that the rare earth can play a greater role in the coating or coating modification.