Preparation of a heavy-duty anti-corrosion epoxy powder coating

Qin Rui, Xiao Yuli
Lanzhou University of Technology Technical Engineering College
Gansu Ring Pass Engineering Testing Co., Ltd.
Abstract:
powder coating application prospects are broad, in order to improve its heavy corrosion resistance, developed a fast curing ring to meet the requirements of heavy corrosion protection Oxygen powder coating, discusses the choice of base resin, curing agent model and dosage on the performance of coating film, determines the pigment filler, leveling agent and curing promoter and other additives of the selected object, obtains the preferred formula of the coating, and tests the performance of the preferred formula coating. The results show that the coating is green and pollution-free, the coating has excellent chemical resistance, solvent resistance, excellent technical properties and good adhesion, the construction process is simple, no primer can be applied once thick, fast curing is conducive to pipeline operation, with a wide range of application prospects.
foreword
Epoxy powder corrosion-resistant coatings originated in the United States in the 1960s, in the transport of water, chemical and oil and gas pipelines have been widely used in

. China has only developed and applied this technology in the past ten years, with the continuous development of oil, natural gas and chemical industry, the requirements for pipeline corrosion prevention are getting higher and higher, some traditional solvent-based, high volatile organic compounds (VOC) content of anti-corrosion coatings have not adapted to the current anti-corrosion requirements, especially in heavy corrosion environment high safety level of buried oil and gas pipelines.
commonly used in heavy anti-corrosion coatings are epoxy resins, polyurethanes and chlorine sulfonide resins. The key to the development of heavy anti-corrosion coatings is high-performance synthetic resins, in order to achieve long-lasting objectives in harsh environments, the requirements for the main film-forming substances synthetic resins are: good adhesion and better physical and physical properties such as low shrinkage, high hardness, better toughness, wear resistance, Temperature resistance, etc., good corrosion resistance to various media (chemical gases, acids, alkalis, salts and other solvents), can effectively resist the penetration of coatings by various media, can be convenient construction under various conditions and meet the design requirements for coating thickness and coating structure.
epoxy resin has excellent adhesion and low shrinkage, good corrosion resistance and permeability and other properties, can be cured by more curing agents, with good film-forming properties, and gradually become the largest number of applications, the widest range of heavy anti-corrosion coating film-forming substances. Epoxy powder coating is a nearly 100% solid parts of the "green coating", no solvents, to meet the current requirements of environmental protection green. Powder coating has been popularized and applied rapidly in the field of heavy corrosion prevention because of its safe, efficient and pollution-free characteristics.
the "13th Five-Year Plan" issued by the Ministry of Environmental Protection, the "13th Five-Year Plan" for the prevention and control of volatile organic matter pollution requires all enterprises in the seven key industries to implement comprehensive VOC management, bringing new opportunities for powder coatings. Powder coating application direction for containers, ships, railways, energy, petrochemicals, power generation and other fields, heavy anti-corrosion coatings in these projects have been applied more than 3 million tons, the most promising prospects for development. In ships, bridges, steel structures and other heavy anti-corrosion coating applications, solvent-based coatings are still the mainstream, water-based coatings are difficult to completely replace solvent-based coatings, powder coatings are expected to occupy a larger market share under the trend of green coating. However, there are still some problems with powder coatings: dust and noise pollution in the production process, high temperature curing conditions of powder coatings put them at a disadvantage in energy conservation, less application on non-metallic substrates, difficulty in powder coating of very large metal parts (e.g. steel structures, containers, etc.) or metal parts of extremely complex shapes, and the heavy metal pigments such as lead-chromium yellow in the powder coating industry, which can cause injury or potential danger to the human body.
this work developed a rapid curing heavy corrosion-resistant coating of epoxy powder, which can be used for heavy corrosion protection under harsher corrosive environment conditions, and its chemical and chemical properties are indicated.
Test
1.1 Coating Preparation Process
The manufacturing method of heavy anti-corrosion epoxy powder coatings is completely different from that of conventional liquid coatings and must be made with special equipment. Figure 1 shows the powder coating production process.
preparation process for epoxy powder corrosion-resistant coatings includes the premix phase, the melt extrusion stage, the crushing phase and the screening phase. In the preparation process, the resin, curing agent, pigment and additives in the formulation are weighed accurately first, and the ingredients are fully mixed evenly. Premixed materials are transported through the adder to the melt extruder, so that the ingredients are melted and mixed under certain temperature conditions and evenly dispersed. After extruder extrusion, the cooling press is broken, graded and screened to make a product.
the main technical difficulties in the preparation of the coating: First, screening suitable resins, pigments and curing agents, etc., to meet the preparation of coatings to provide good corrosion resistance; Epoxy powder coating is mainly composed of base resin, curing agent, pigment filler and additive, according to the performance of the prepared coating, preferably the base resin type, curing agent type and dosage, as well as pigment and additive type and dosage, and the optimal coating performance characteristics.
1.2 paint performance detection
(1) appearance visual epoxy powder coating, to observe the color of the coating is uniform, whether the lumps and so on.
(2) gel time, curing time, low temperature impact (-30 degrees C), bending resistance, wear resistance, adhesion, chemical corrosion resistance reference SY/T0315-2013 determination.
(3) solid content (100 degrees C) % by reference to GB/T6554.
(4) volume resistivity is determined by reference to GB/T1410.
(5) flexibility is determined by reference to GB/T1731-1993.
(6) salt spray resistance was determined by reference to GB/T1771-1991.
results and discussion
2.1 coating formulation preferably
2.1.1 base resin selection
epoxy resin including BPA type and lipid ring family and other types. Bisphenol A epoxy resin has a high polarity of epoxy and hydroxyl, once added curing agent cross-linking, the formation of insoluble, non-melting three-dimensional mesh structure, the formation of the coating film with high cross-linking, high hardness, good wear resistance, good corrosion-resistant effect and so on. Therefore, in the design of coating formulation, more bisphenol A-type epoxy resin is selected as the main film-forming substance, the structure of which is shown in Figure 2. Because the molecule contains stable benzene ring and ether bond, the molecular structure is relatively tight, so the chemical resistance is better, basically can meet the needs of most corrosive environment in the long run. In addition, the corrosion resistance of epoxy coatings can be improved by various curing crosslinking methods.
table 1 is a technical test index for powder coatings with different models of Bisphenol A epoxy resins. Table 2 is a corrosion-resistant performance test for powder coatings made from different types of epoxy resins. According to the softening point in Table 1, the range of epoxy values, etc., it is finally determined that epoxy resin 3 is the base resin, and its appearance is obviously better than that of ordinary epoxy resin (colorless transparent). Table 2 shows that powder coatings made of epoxy resin 3 have the best anti-corrosion and chemical resistance. Because the coating is thick coating type, requires a spray curing molding, so the coating film thick film anti-flow hanging requirements, which requires the coating has a lower melting fluidity. Epoxy resin 3 has a higher softening point and a narrow molecular weight distribution, the molecular structure is more regulated, which makes the resin at high temperature curing with a lower melting fluidity, not easy to produce flow hanging phenomenon, vertical plate surface is more flat. Therefore, epoxy resin 3 is selected as the base resin.
2.1.2 Curing agent Selection
The performance of epoxy powder coatings depends not only on their chemical stability and adhesion in the medium, but also on the mesh crosslinking structure obtained by their curing. Only by obtaining a dense cross-linking structure can the coating be effectively prevented from swelling and penetration in the external medium. Therefore, the selection of a suitable curing agent is of great significance to control the quality of the coating film.
epoxy powder coatings are commonly used in a variety of curing agents, such as amines, acidocarbons, methane, omega, cyclomy, phenolic hydroxyl resins, phenolic resins, polyester resins and acrylic resins. Among them, phenolic hydroxyl resin is an important curing agent commonly used in epoxy powder coatings, which is mainly used in the field of heavy corrosion prevention and electrical insulation. This curing agent is a low molecular weight epoxy resin and biphenol A in alkali catalysis, after the addition of the reaction to obtain the phenolic hydroxyl products, no small molecules produced. The curing agent is non-toxic, production and use of safety and hygiene, after curing coating gel time is short, curing time is fast, the formation of a coating film hardness is high, toughness is good, wear resistance, bending resistance, heat and cold impact performance is good, adhesion is good. The use of this type of curing agent has a conventional epoxy resin with double cyanide curing agent can not be compared with the superior performance. Its structure contains a large number of hydroxyl groups, so that the overall coating film after curing has excellent adhesion;
table 3 is the selected technical indicator for a specific curing agent for heavy corrosion-resistant epoxy powder coatings. Table 4 is a coating performance index for epoxy powders made from different models of phenolic hydroxyl resin curing agents and epoxy resins (base resin selection table 1 EOX 3). After testing and demonstration, the curing agent 2 was selected with epoxy resin as the main film-forming substance of the coating. Table 5 is the effect of different curing agent dosage on coating performance (base resin selection table 1 central epoxy resin 3).
can be seen from Table 5: different curing agent dosages have a greater impact on coating performance. Curing agent dosage is too small, resin curing is not complete (or even not curing), coating crosslink density is low, will affect the appearance and mechanical properties of the coating, so that the coating strength is reduced; A high-performance coating can only be obtained with the appropriate proportions. As can be seen from Table 5, when the quality ratio of curing agent and resin is 1:5, the coating performance is the best and the best performance.
2.1.3 Pigment and additive selection
(1) pigment selection because of the special nature of powder coating heating curing into film, only some pigments are suitable for powder coatings. The pigments used in titanium white, carbon black, indistinate green, iron red, chromium yellow, etc. were selected as heavy anti-corrosion epoxy powder coatings. Among them, titanium white cover force is good, and the price is moderate, carbon black, indigo green has a good high temperature resistance, 200 degrees C curing, there will be no loss of light, color change, iron red has a strong cover, high fastness characteristics. At the same time can also add some anti-rust pigments, such as zinc chromium yellow, aluminum tripolyphosphate, etc. can also play a better anti-corrosion effect.
(2) The role of filler selection is to improve the hardness, rigidity, scratch resistance, impact resistance and other physical properties of the coating film, while improving the looseness of the powder coating and improve the glass temperature of the coating and other properties. The amount of filler is too high, the coating is brittle, the impact resistance is not good, the filler volume is too low, and it will affect the rigidity of the coating film, scratch resistance and other physical and physical properties. After many tests, the paper finally selected the cost-effective precipitation of niobium sulfate, silicon powder and so on as a heavy anti-corrosion epoxy powder coating filler components. Table 6 shows the effect of different filler dosing on the physical properties of the coating.
from Table 6, IV. formula is the best, the selection of a certain precipitation of nicosal sulfate, silicon micro-powder and gas-phase silica ratio can make the coating film with better comprehensive performance.
(3) additive selection due to the particularity of powder coating status, such as the requirements of epoxy resin and curing agent is easy to crush and disperse solid powder or flakes; Additives include leveling agents, curing promoters, brighteners, degassing agents, anti-blocking agents, anti-lighting agents, etc. Among them, leveling agent and curing promoter are the most important.
leveling agent can effectively improve the gloss and smoothness of the coating film, in the powder coating without the addition of leveling agent, the coating has serious "hemp pit" and internal pores and other phenomena, as well as poor gloss and so on. The curing promoter can effectively reduce the activity energy of cross-linking reaction between resin and curing agent, improve the reaction speed of coating curing, and greatly improve the production efficiency. Experiments show that no curing promoter is added, resin and curing agent curing reaction cross-linking is incomplete, the preparation of the coating film brittleness is very high, adhesion is very poor.
the epoxy powder coating requires a thick coating to be sprayed and molded at once, and in order to prevent hanging, the coating is required to have low melting fluidity, which can easily lead to uneven surfaces of the coating. The leveling of powder coatings is restricted by leveling aid, the structure of film resin and the curing mechanism. Suitable resins and curing agents are selected, with higher softening points and narrow molecular weight distribution, and the molecular structure is more regulated. Finally, epoxy resin and curing agent in high temperature curing has a lower melting fluidity, not easy to produce hanging phenomenon, and the plate surface is relatively flat. By changing the amount of additives used, the level of paint mobility can be reduced, can play a role in anti-flow hanging. Table 7 shows the effect of different curing promoter dosages on curing time. Table 8 is the type, dosage and use of the relevant additives. Because the epoxy powder coating is more used in acid-alkali heavy corrosion environment anti-corrosion, so in the pigment and additive selection should first consider its chemical stability, good acid resistance, alkali resistance, temperature-resistant pigment fillers and additives are the first choice.
2.2 Basic formulation and coating performance of heavy corrosion-resistant epoxy powder coatings
As can be seen from Table 10, the coating has good comprehensive performance and good anti-chemical abrasiveness, which can provide reference for the study of epoxy powder coating.
Conclusion
Developed a protective epoxy powder coating for anti-corrosion protection in the heavy corrosion environment of acid-alkali salts, the coating is green and pollution-free, through the co-conjugent reaction of resin and curing agent, the selection of suitable pigments and additives, etc., increases the crosslink density of the system, improves the media penetration ability of coating film resistance, alkali resistance and other performance improvements. And the construction process is simple, do not need primer can be coated at a time, fast curing is conducive to pipeline operation, easy to detect and repair, coating quality is easy to control, with a wide range of applications.
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