Coating formulation: formulation design of heavy-duty anticorrosive powder coating

Core tips: 1 Overview The heavy-duty anticorrosive coating is a kind of coating which can be used effectively for a long time under the severe corrosive environment In order to achieve the purpose of heavy-duty anticorrosion, on the one hand, the thick film of the coating is needed; on the other hand, the selection of coating matrix resin and auxiliary agent, the surface treatment of the base material, the coating construction and maintenance requirements are also very strict This kind of coating used to be liquid coating, which usually requires multiple coats to meet the requirements In recent years, powder coating has become more and more important in the field of heavy-duty corrosion protection, especially in the field of heavy-duty corrosion protection Application scope and advantages of roller ceramic coating Recommended introduction: the surface of ceramic rubber plate used for roller ceramic coating is cast into durable rubber plate by hundreds of independent small ceramic pieces Each ceramic piece has raised characteristics Under general conveyor belt pressure, thousands of unique cast bumps can produce positive traction, prevent slipping and extend the service life of conveyor belt roller At the same time, the bottom layer adopts high-quality rubber, which has strong elasticity and can play a good impact resistance role The scope of application of roller ceramic coating is one three six 83857180 (the same as V) 1 Roller coating in extremely harsh working environment 2 The belt joint is not suitable for fixing with iron buckle; 3 It is suitable for leather Coating home news: first, the overview of heavy-duty anticorrosive coating is a kind of coating that can be used effectively for a long time under the harsh corrosive environment In order to achieve the purpose of heavy-duty anticorrosion of coating, on the one hand, the coating needs to be thick film; on the other hand, the requirements for the selection of coating matrix resin and auxiliaries, surface treatment and coating construction and maintenance of substrate are also very strict This kind of coating used to be liquid coating, which usually requires multiple coats to meet the requirements In recent years, powder coating, with its characteristics of safety, high efficiency and pollution-free, has played a more and more important role in the field of heavy corrosion protection, especially in the field of pipeline heavy corrosion protection From the initial oil and natural gas pipeline corrosion protection, it has been extended to the urban underground sewage pipe network corrosion protection, and even the small caliber self-made water pipe coating In addition to the pipeline, the anti-corrosion coating of building steel bars and cables is also emerging; the coating methods include high-voltage electrostatic spraying, fluid bed coating and vacuum suction coating At present, the industrial chain from the production of raw materials of heavy-duty anticorrosive powder coating to powder manufacturing and spraying application is forming The development of heavy-duty anticorrosive powder coating and the expansion of its application field also put forward many new requirements for the production of raw materials and the design of powder formula A single powder coating formula has been unable to meet the needs of different applications Therefore, new materials and formula technology must be developed continuously Although the output of powder coating in China is very large, it is mainly decorative powder coating There are few powder coating formulations that can truly meet the requirements of heavy-duty anticorrosion performance From the production of epoxy resin and curing agent as the main raw material to the development of formulation technology, there is little communication, which seriously hinders the development and lifting of heavy-duty anticorrosion powder coating In the process of development and production of epoxy resin and curing agent for heavy-duty anticorrosive powder coating, through repeated tests and comparison, and through analysis and discussion with relevant technical personnel of powder coating factory, the author obtains the influence relationship of various raw materials of powder coating on the performance of powder coating, and gets some experience, although it is not mature, I hope to introduce it here We hope to attract the attention of people in the industry and discuss together, so as to promote the production and application technology of heavy anti-corrosion powder coating in China II Selection of epoxy resin and curing agent for heavy-duty anti-corrosion powder coating epoxy resin has excellent adhesion to base material, especially metal base material due to a large number of benzene ring, ether bond and hydroxyl structure in molecular structure Meanwhile, the coating has excellent mechanical strength, such as hardness, cold and hot impact resistance and mechanical impact resistance In addition, a variety of epoxy resin and curing agent can be combined to form a variety of powder coating formulation system, which provides a variety of options to meet various requirements and complex use occasions The above advantages make epoxy resin become the preferred resin for heavy-duty anti-corrosion powder coating There are many kinds of epoxy resin, but due to the limitations of the production process requirements of heavy-duty anticorrosive powder coating, the selection of the variety is not as wide as that of liquid coating In addition to the above chemical performance requirements, the manufacturing process of powder coating should also be considered, which not only requires the mechanical and chemical resistance of the curing substance, but also requires that the epoxy resin is a stable solid at room temperature, so as to facilitate the production of powder coating Storage stability is not easy to agglomerate With the development of powder coatings toward low temperature and fast curing, on the one hand, epoxy resin and curing agent should be reacted with high reactivity However, on the other hand, resin and curing agents are basically inert or low degree of reaction under the extrusion temperature of powder coating, otherwise, if gel particles are produced during extrusion, it will inevitably happen Affect the leveling and curing of powder coating These requirements make the choice of epoxy resin and curing agent for heavy-duty anticorrosive powder coating much less than that for liquid coating At present, the epoxy resin used in heavy-duty anticorrosive powder coating is mainly bisphenol A epoxy resin and phenolic epoxy resin with medium molecular weight The typical product of bisphenol-A epoxy resin with medium molecular weight is E-12, which has the characteristics of low price and good toughness However, only two ends of the molecular structure of this kind of epoxy resin have epoxy group In order to obtain good toughness and high softening point, the selected epoxy resin has low epoxy value and low crosslinking density of the cured product, resulting in poor chemical resistance and heat resistance, The hardness, wear resistance and adhesion of the coating are not good enough There are many epoxy groups in the molecular structure of phenolic epoxy resin The crosslinking density and aromatic density of the cured products are relatively high The hardness and heat resistance, wear resistance, chemical corrosion resistance of the coating film and the adhesion to the substrate are relatively good Therefore, phenolic epoxy resin or modified phenolic epoxy resin are mostly selected in the design of heavy-duty anti-corrosion powder coating formula in foreign countries However, if phenolic epoxy resin is used completely, especially when the epoxy value of phenolic epoxy resin is high, it may lead to high brittleness of cured products and poor low-temperature bending and impact performance Therefore, adding part of bisphenol-A epoxy resin to phenolic epoxy resin will help to improve its low-temperature resistance The mixing ratio is 80 / 20-20 / 80 of phenolic epoxy resin / bisphenol-A epoxy resin With the increase of the amount of phenolic epoxy, the adhesion, hardness and chemical corrosion resistance of the coating increased, but the flexibility and bending impact resistance decreased The specific proportion was also related to the accelerant system selected, the type and amount of pigments and fillers, and the final performance required by the coating In addition to epoxy resin, the choice of curing agent has a very important impact on the performance and processability of powder coating As with epoxy resin, the curing agent which can be used in heavy-duty anticorrosive powder coating is much less than that of liquid coating Polyester resin curing agent which is widely used in decorative powder coating can not meet the requirements of rapid curing due to its poor chemical corrosion resistance of ester bond and low reaction activity of macromolecular aromatic carboxyl group Dicyandiamide, the curing agent of early heavy-duty anticorrosive powder coating, is rarely used because of its small molecular weight, large curing heat release, large internal stress and brittleness of the coating film Aromatic amine curing agent with high melting point, such as DDS / DDM, can be used as curing agent for powder coating in theory, but it is rarely used in practice The reason may be that the molecular weight is too small, the functionality is too high, and the toughness of the cured product is not enough The curing speed of imidazole curing agent is fast, but the curing product is too brittle, so it is generally used only as accelerator At present, the curing agents of heavy-duty anti-corrosive powder coating are mainly hydroxyl terminated polymer curing agents, such as addition products of phenols and epoxides, linear phenolic resin, etc This kind of macromolecular curing agent has similar structure with epoxy resin, and has good compatibility with epoxy resin Macromolecular structure makes the cured powder coating have better flexibility, but its disadvantage is that the reaction activity is low, especially in the later stage of the reaction, when the viscosity of the system increases, it is difficult to participate in the reaction due to the curly wrapping of the reactive active end group In order to improve its reaction activity and adapt to rapid fixation In order to meet the requirements of curing agent structure design, some small molecules of hydroxyl compounds and catalysts are often added to improve the reaction speed Different curing agent manufacturers keep secret in the selection of hydroxyl compounds, molecular weight, type and dosage of catalyst, and different curing effects are brought by different selection In the development process, we developed a series of curing agents according to different coating methods and coating performance requirements, which can meet different requirements 3 The ratio of epoxy resin to curing agent and the effect of curing conditions on the performance of powder coatings There are two kinds of hydroxyl groups in the molecular structure of hydroxyl terminated polymer curing agent, one is phenolic hydroxyl group, the other is alcohol light group formed by ring opening of epoxide In the determination of hydroxyl group in the solid, the hydroxyl value measured is the sum of the two hydroxyl values As the active group of curing epoxy resin, the activity of phenolic hydroxyl is much higher than that of alcohol hydroxyl The former can react with epoxy group smoothly in weak alkaline or even non catalytic conditions, while alcohol light group can react only in strong alkaline or strong acid such as Lewis acid At present, the powder coating uses the weak basic anionic polymerization catalyst Under the fast curing condition required by the powder coating, the alcohol hydroxyl is difficult to participate in the reaction Therefore, only the phenol hydroxyl reaction needs to be considered when calculating the catalyst dosage In theory, the molar ratio of phenol hydroxyl group to epoxy group is 1, while in the actual formula design, the amount of curing agent is less than the molar amount of unit phenol hydroxyl group, and the molar ratio of phenol light group to epoxy group is 0.6 -9 The reason why the amount of phenolic hydroxyl is less than that of epoxy group is that in addition to the addition reaction between phenolic hydroxyl compound and epoxy group, there are also anionic polymerization catalysts added to make the ring opening polymerization of epoxy group At present, in order to speed up the curing speed, the amount of such catalysts is still large, so part of epoxy group has been consumed by anionic polymerization and has not participated in the addition reaction That is to say, the actual phenol hydroxyl consumption? 182? Will be less than the theoretical value The excess of phenolic hydroxyl may bring some disadvantages, such as the decrease of solvent resistance and chemical resistance In the curing process of heavy-duty anticorrosive powder coating, there are actually two kinds of reaction competition Increasing the amount of anionic polymerization catalyst accelerates the curing speed, but the proportion of anionic polymerization increases, the number of its own flexible groups is small, and the coating film is easy to be brittle However, the addition reaction speed of phenolic hydroxyl compounds is significantly lower than that of anionic polymerization Therefore, to speed up the reaction speed, it is necessary to increase the reaction temperature, unilaterally reduce the reaction temperature and shorten the curing time, which will be detrimental to the final performance of the film The determination of curing system can not only get the best formula by testing the relationship between different curing conditions and coating performance, but also by DSC curing curve of specific powder The starting temperature, the maximum exothermic temperature, the end temperature and the time of curing are obtained from the isokinetic heating curing curve of powder coatings, and the enthalpy value of curing exothermic is determined The reaction activation energy and the chemical reaction kinetic constant are calculated from the kinetic analysis, and the different temperature conditions are calculated according to the kinetic formula