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Selection of epoxy resin and curing agent for heavy-duty powder coatings

Date of release:2018-07-27 Author: Click:

Liaoning epoxy resin has excellent mechanical strength, such as hardness, resistance to cold and heat shock and mechanical impact, because of the large number of phenyl ring, ether bond and hydroxyl groups in the molecular structure of epoxy resin, which have excellent adhesion to the substrate, especially metal substrate, heat resistance and chemical corrosion resistance. In addition, a variety of epoxy resins and curing agents can be combined into a variety of powder coatings formulation system, to meet a variety of complex requirements for the use of a variety of occasions to provide a variety of options, the above advantages make epoxy resin to become the preferred resin for heavy-duty anti-corrosion coatings.

There are many kinds of epoxy resins, but due to the limitation of the production process requirements of heavy-duty powder coatings, the selection of varieties is not as extensive as that of liquid coatings. In addition to the above-mentioned chemical properties, the manufacturability of powder coatings should be considered, which requires both mechanical and chemical properties of cured materials and epoxy resins at room temperature. It is a stable solid to facilitate the storage stability of powder coatings and is not easy to agglomerate; with the development of powder coatings turning to low temperature and rapid curing, on the one hand, it is required that epoxy resin and curing agent should have high reactivity, but on the other hand, resin and curing agent should be chemical inert or chemical inert under the condition of powder extrusion temperature. The degree of reaction is very low, otherwise, if gel particles are produced during extrusion, it will affect the leveling and curing of powder coatings. These requirements make the choice of epoxy resin and curing agent for heavy duty powder coatings much less than that for liquid coatings. At present, epoxy resins used in heavy-duty powder coatings are mainly medium molecular weight bisphenol A epoxy resin and phenolic epoxy resin. The typical product of bisphenol A epoxy resin with middle molecular weight is E-12, which has the characteristics of low price and good toughness. But this kind of epoxy resin only has epoxy group at both ends of the molecular structure. In order to obtain good toughness and high softening point, the epoxy value of the resin selected is low, and the crosslinking density of the cured product is low. Chemical resistance and heat resistance are not good enough, coating hardness, wear resistance and adhesion is not good enough. There are many epoxy groups in the molecular structure of phenolic epoxy resin. The crosslinking density and aromatic density of cured products are higher. The hardness and heat resistance, wear resistance, chemical corrosion resistance and adhesion to the substrate of the film are better. Therefore, phenolic epoxy resin or phenolic epoxy resin are often used in the design of heavy-duty powder coatings abroad. Modified phenolic epoxy resin. However, all phenolic epoxy resins, especially when the epoxy value of phenolic epoxy resin is high, may lead to the brittleness of cured products, low-temperature bending and impact performance is poor. Therefore, adding part of bisphenol A epoxy resin into phenolic epoxy resin is helpful to improve its low-temperature resistance. The mixing ratio is phenolic epoxy/bisphenol A ring. Oxygen 80/20-20/80. The influence of Phenolic Epoxy Content on the coating properties is as follows: with the increase of phenolic epoxy content, the adhesion, hardness and chemical corrosion resistance of the film increase, but the flexibility and flexural impact resistance decrease. The specific proportion is also related to the selected accelerator system, the type of pigments and fillers and the dosage and the final performance required by the film.

In addition to epoxy resin, the choice of curing agent has great influence on the properties and process of powder coatings. Like epoxy resin, curing agents used in heavy-duty powder coatings are much less than those used in liquid coatings. Polyester resin curing agents commonly used in decorative powder coatings can not meet the rapid curing requirements because of poor chemical corrosion resistance of ester bonds and low reactivity of macromolecular aromatic carboxyl groups. Dicyanamide, a curing agent for early heavy-duty powder coatings, is seldom used because of its small molecular weight, high heat release, high internal stress and brittleness. High melting point aromatic amine curing agent such as DDSDDM can be used as a curing agent for powder coatings theoretically, but it is seldom used in practice. The reason may be that the molecular weight is too small, the functionality is too high, and the toughness of cured products is not enough. Imidazole curing agent curing speed, but the formation of the curing substance is too brittle, so generally only used as an accelerator. At present, the main curing agents for heavy-duty powder coatings at home and abroad are hydroxyl-terminated macromolecular polymer curing agents, such as adducts of phenols and epoxy compounds, linear phenolic resins, etc. This kind of macromolecule curing agent has the similar structure with epoxy resin and has good compatibility with epoxy resin. The macromolecule structure makes the cured powder coatings have better flexibility, but its disadvantage is that the reactivity is low, especially in the later period of reaction, when the viscosity of the system increases, it is difficult to participate in the reaction because of the curling of the reactive end group. In order to improve its reactivity and meet the requirements of rapid curing, some small molecular hydroxyl compounds and catalysts are often added to the structure design of curing agent to improve the reaction rate. The selection of hydroxyl compounds, molecular weight, types and dosage of catalysts are kept secret by different curing agent manufacturers, and the curing effect of different curing agents is also different. In the course of development, we have developed a series of curing agents to meet different requirements according to different painting operations and coating performance requirements.

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Key word:Selectionofepoxyresinandcuringagentforheavy-dutypowdercoatings,Crystallineepoxyresinprice,Epoxyresinplant

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