The influence of different epoxy systems on the an

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Analysis of the influence of different epoxy systems of powder coatings on coating corrosion prevention

with the rapid development of industrial technology, environmental protection has been paid more and more attention all over the world. In particular, in China, government agencies announced that they would start imposing VOC tax of up to 4% on solvent based paint manufacturers in early 2015. As the VOC of powder coating is almost zero, this is good news for powder coating manufacturers, which will provide more development opportunities and strong government support

powder coatings have been widely used in many fields, such as architecture, automobiles, pipelines, mobile communications and household appliances. In the powder coating industry, the work and research of coating anticorrosion have never been interrupted. Reports of corrosion damage to bridges, buildings, aircraft, cars and gas pipelines are common. Although an effective cathodic protection measure has been developed in the oil and gas pipeline industry, the coating is still damaged. There are many factors influencing coating damage [1], which may be affected by the coating system including film-forming substances, pigments and fillers, additives, substrate types, pretreatment processes, curing processes, coating thickness and coating adhesion, and may also be affected by the internal and external environment including temperature, humidity, bacteria and UV. The relationship between all corrosion influencing factors is shown in Figure 1

Figure 1 Relationship diagram of corrosion influencing factors

for powder coating manufacturers, the key to improving the anti-corrosion performance lies in the design of the chemical composition of the coating itself, which requires the selection of appropriate film-forming substances, pigments and fillers and anti-corrosion additives. Next, these influencing factors will be introduced one by one

1. Film forming material

the resins used in powder coatings are mainly polyester, acrylic acid, polyurethane and epoxy. There are ester groups in polyester structure, which are easy to hydrolyze in wet environment; Acrylic acid structure also contains ester group, which is proved to be easy to be destroyed in acid rain environment; Polyurethane can be used as anticorrosive coating, but the price is high; Epoxy resin with reasonable price and excellent anti-corrosion performance is a good choice, as shown in Figure 1, taking bisphenol A epoxy as an example

Figure 2 Chemical structure of bisphenol A epoxy

the determinants of the excellent corrosion resistance of bisphenol A epoxy are as follows:

1. The lateral hydroxyl group gives good adhesion to the metal

2. the ether bond in the skeleton gives excellent chemical resistance and alkali resistance

3. there is no ester group in the structure, which has good water resistance

4. High crosslinking density gives better shielding effect to the substrate

in addition to bisphenol A epoxy, linear phenolic modified epoxy and bisphenol F can also be used in anticorrosive powder coatings. The chemical structure diagrams of both are shown in Figure 3 and Figure 4

Figure 3 Chemical structure of linear phenolic modified epoxy

Figure 4 Chemical structure of bisphenol F epoxy

from the analysis of chemical structure, compared with bisphenol A epoxy and bisphenol F epoxy, linear phenolic modified epoxy contains more epoxy groups and aromatic rings per mole of material, and the crosslinking structure is denser after curing, with better corrosion resistance, but it also has the defect of brittleness, which is generally rarely used alone in powder coatings. In heavy-duty powder coatings, linear phenolic modified epoxy is often blended with bisphenol A epoxy to achieve excellent corrosion resistance and mechanical properties. Bisphenol F epoxy used for powder coating is mainly to improve the leveling property of the coating, and can also be blended with bisphenol A epoxy

in addition to chemical structure, molecular weight is also an important factor affecting the properties of epoxy, and its influence is shown in Figure 5. Compared with high molecular weight epoxy, because low molecular weight epoxy contains more epoxy groups, the crosslinking density of the cured coating increases, and then the hardness of the coating increases, while the flexibility and impact strength decrease. Low molecular weight epoxy contains fewer hydroxyl groups than high molecular weight epoxy, which will also reduce the wettability and adhesion between the coating and the substrate. Considering all the properties of epoxy, the molecular weight of epoxy must be within a certain range, and whether it is large or small will affect the corrosion resistance of the coating. In the field of heavy-duty corrosion protection, the molecular equivalent of epoxy is generally g/eq, and the molecular weight distribution is narrow

Figure 5 Relationship between epoxy molecular weight and various properties

in the formula of thermosetting powder coating, the film-forming material is mainly composed of resin and curing agent. Common curing agents for epoxy used in powder coatings include imidazole, amines (dicyandiamide and aromatic amines), anhydride and phenolic curing agents. Among them, imidazole is rarely used alone because of its low activation energy and intense curing reaction, high internal stress and poor mechanical properties of the cured coating, but often used as a catalyst for epoxy powder coatings. Dicyandiamide has good comprehensive performance and cost performance, and is a common curing agent for powder coatings. Several aromatic amines can also be used as curing agents for powder coatings. For example, Ng et al. Studied the corrosion resistance of three aromatic amine curing agents (diaminobenzene sulfone, m-phenylenediamine and phenolic amine) curing coatings in hot sulfuric acid solution with a concentration of 10%, and found that diaminobenzene sulfone curing coatings have better corrosion resistance. The application of anhydrides in powder coatings is limited to a certain extent because they are prone to produce irritant gases and have poor storage stability. Phenolic curing agents have become the first choice in the field of heavily corrosive environments such as oil and gas pipelines because of their advantages of low baking temperature, rapid curing and excellent corrosion resistance

2. Pigments and fillers used in anti-corrosion coatings

as an indispensable component of powder coatings, the anti-corrosion pigments and fillers are three times higher than that at the end of "1015" in terms of anti-corrosion mechanism, and achieve the purpose of anti-corrosion and anti-corrosion through physical barrier, electrochemical reaction and corrosion inhibition reaction

physical barrier protection is realized by the coating thickening and seepage prevention function of layered filler. Widely used in anti-corrosion powder coatings, in April 2015, the Ministry of finance, the Ministry of science and technology, the Ministry of industry and information technology, and the national development and Reform Commission issued the notice on the implementation of financial support policies for the utilization of new energy vehicles in 2016 (2) 020. The layered fillers are mainly micaceous iron oxide, sericite, and glass flakes. The corrosive medium tends to migrate and diffuse linearly in the coating containing spherical filler, but in the coating containing flake filler, because the filler layers are separated from each other, it can only migrate and diffuse curvilinearly, which greatly delays the corrosion progress of the coating. In addition, it should be pointed out that it may be difficult to maintain the original sheet shape of sheet filler in the process of powder coating processing and extrusion, which limits its application in powder coating

in principle, metals that are more active than the electrochemical properties of the substrate can be used as pigments and fillers for anti-corrosion coatings. However, at present, metallic zinc particles are the most widely used. It is reported that the "king of new materials" said that the average diameter of spherical zinc particles with the best corrosion resistance is 2 μ m。 The anti-corrosion mechanism of metallic zinc is that zinc participates in the corrosion reaction to produce insoluble substances such as ZnFe2O4 and basic zinc carbonate. In particular, when the sheet metal zinc developed by eckave Werke company replaced the traditional spherical zinc, the anti-corrosion performance of the coating was further improved due to its unique parallel lapping and shielding functions

slow release pigments and fillers can be divided into cathode type and anode type according to the type of reaction. Cathodic corrosion inhibitors, such as inorganic salts of magnesium and aluminum, inhibit the corrosion of coatings by reacting with hydroxide ions in a neutral environment to form insoluble substances. Anodic corrosion inhibitors, such as phosphate, silicate or hydroxide, can form an oxidation protective layer on the metal surface. If the slow-release pigments and fillers are insufficient, it will form a poor electrode area, which will accelerate the corrosion progress. At present, the most widely used slow-release pigments and fillers are phosphate containing pigments and fillers, such as zinc phosphate and magnesium phosphate. In addition, there is a class of slow-release pigments and fillers with less toxicity, which are spinel pigments and fillers based on metal oxide mixtures

3. Additives

theoretically, all additives that help to improve the adhesion between the coating and the metal substrate can be used to improve the corrosion resistance of the coating. Silane or modified silane is a common kind, which can improve the adhesion between the coating and the metal substrate. In addition to silane, there are other types of adhesion promoters. For example, chatwell international has developed a series of adhesion promoters for powder coatings, such as chartsil B 515.2h/1h and chartsil C containing high concentrations of amino, carboxyl or hydroxyl groups, which can be introduced into the coating before extrusion processing. In addition, it is reported that adding polyaniline to epoxy powder coating can improve its corrosion resistance

4. Outlook

with more and more attention to environmental protection, powder coatings will have greater application prospects in the near future. However, so far, due to various conditions, powder coatings have not been applied to the C5 and C6 corrosion grades, which still has a lot of work and research to do for powder coating technicians, equipment suppliers and professionals who deal with substrates before the samples are not damaged. Among many anti-corrosion additives, low toxicity green corrosion inhibitors are the development trend of anti-corrosion coatings. Some green corrosion inhibitors such as biodegradable polymers and plant extracts have been mentioned in the field of liquid coatings, which show outstanding anti-corrosion performance in many corrosive environments. Although these green corrosion inhibitors have the advantages of low toxicity and biodegradability, there are still various technical bottlenecks. How to widely use these green corrosion inhibitors at the industrial level, especially in powder coatings, needs further research and exploration

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