Development and application of epoxy resin coating

Introduction:
epoxy resin was synthesized in 1930 by Pierre Castan of Switzerland and SO Greenlee of the United States. In 1947, Devoe and Reynolds of the United States, and then Shell obtained the patent production rights of swiss auto-bus company, and successively achieved industrial production. Epoxy resins are widely used in coatings, adhesives and composites in various fields
Chined China
with their excellent properties of adhesion, adhesion, stability, chemical resistance, insulation and mechanical strength. China began to develop epoxy resins in 1956 and successfully tested them in 1958. Epoxy coating is widely used as one of the varieties of protective coatings, epoxy resin coating formed coating not only on the corrosion media has a shielding effect, but also can passivate the protected metal, play an electrochemical role, epoxy resin coating formed by the coating film with excellent "wet state" bonding force, so that other coatings are out of reach. Therefore, epoxy coatings have been a hot topic in the field of coatings.
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1
Overview of epoxy coatings
Coatings with epoxy resin as the main film-forming substance are called epoxy coatings, and compounds containing two or more epoxy groups belong to epoxy resins. Epoxy group is a ring composed of one oxygen atom and two carbon atoms, which is highly active, so that epoxy resin can cross-link with a variety of types of curing agents to form a three-dimensional mesh structure of polymer.
1.1
Classification of epoxy coatings
Epoxy coatings are one of the main products of synthetic resin coatings, and there are generally five classification methods (Table 1).
Table 1 Epoxy resin coating type
classification mode
types
construction mode
spray epoxy resin coating, electrostation construction epoxy resin coating, electrophoretic construction epoxy resin coating, powder construction Epoxy coatings, immersion epoxy coatings
purpose use
building epoxy coatings, industrial epoxy resin coatings, special epoxy resin coatings
cationing method
self-drying epoxy resin coatings Drying epoxy coatings and radiation-cured epoxy resin coatings
sorting agent names
acid-cured epoxy coatings, amine-cured epoxy coatings, ion-triggered cured epoxy coatings and synthetic resins Cured epoxy coatings
coating status
solvent-based epoxy coatings, no (less) solvent-based epoxy coatings, powder epoxy coatings and water-based epoxy coatings
The use and status of epoxy resin coatings are mostly classified.
1.2
Composition of epoxy coatings
Epoxy coatings are made up of substrates (including epoxy resins, epoxy esters and modified synthetic resins), curing agents, coloring pigments and body pigments (or pigments), solvents (including water), various functional additives and other materials.
1.3
The properties of epoxy coatings
1.3.1
good adhesion
epoxy coatings have excellent adhesion mainly determined by the molecular structure of epoxy resins and their curing reaction characteristics. Epoxy resin molecular structure has epoxy end, hydroxyl and ether bonds and other polar groups, the presence of these groups so that epoxy resin molecules and adjacent interface to produce electromagnetic adsorption or chemical bonds, so epoxy resin coating film and metal, wood, concrete and other substrate surface can produce a strong bonding force. The internal stress produced by epoxy coating in cross-curing film is low, and many factors in the coating process will cause the resulting stress in the coating film and cause the final defect of the coating film, especially the decrease of the coating film's adhesion to the substrate.
1.3.2
Good corrosion resistance
after the epoxy coating cured into a film, because the molecule contains stable benzene ring and ether bonds, so the stability of chemical media is better, such as can adapt to medium concentrations of acids, alkalis and salts and other media. Also because epoxy resin coating cured into a three-dimensional mesh structure, but also resistant to oil impregnation, can also be used in large numbers of tankers, aircraft and other overall fuel tank wall lining and other anti-corrosion.
1.3.3
Good toughness
compared with thermosolytic phenolic resin coating, epoxy coating is hard because of its aromatic ring structure, because it contains ether bonds to facilitate the rotation of the molecular chain, has a certain toughness, and unlike phenolic resin is very brittle (because its cross-link spacing is shorter than epoxy resin). Epoxy cross-linking spacing is long, easy to rotate inside, and because of the large molecular weight of epoxy resin, so cross-link spacing is long.
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2 Development of epoxy resin coatings
epoxy resin coatings can be divided into four categories in status: solvent-based epoxy coatings, no (less) solvent-based epoxy coatings, powdered epoxy coatings and water-based epoxy coatings. Among the four epoxy coatings, the development prospects of water-based epoxy coatings and powder epoxy coatings are the most promising, so this paper mainly introduces the development of water-based epoxy coatings and powder epoxy resin coatings.
2.1
Development of water-based epoxy resin coatings
water-based epoxy resin refers to a stable dispersion system that, by physical or chemical means, allows epoxy resin to be dispersed in the form of particles or droplets in a dispersing medium with water as a continuous phase. Water-based epoxy coatings can only be made by introducing hydroxyl, carbide, amino, ether bonds and acetaminophen in curing agents or substations. It not only has the solvent-based epoxy coating good chemical resistance, adhesion, physical mechanicality, attachment, electrical insulation, but also low pollution, cheap, simple construction and so on, so the rapid development of water-based epoxy coatings to all walks of life. According to the different preparation methods, epoxy hydration can be divided into mechanical method, chemical modified method and reverse conversion method.
2.1.1
Michine method
mechanical method is direct emulsification method, is the use of ball mill, collomic grinding, errant and other equipment to pre-grind solid epoxy resin into micron-grade epoxy resin powder, under the conditions of heating to add emulsifiers, mechanical stirring can obtain water-based epoxy emulsion. The emulsification agents used are polyoxyethylene alkyl esters (HLB 9.0-16.5), polyoxyethylene alkyl ether (HLB 10.8-16.5) and polyoxyethylene alkyl ether (HLB 10.9-19.5). The advantage of epoxy emulsion produced by this method is that the production process is simple, the amount of emulsion required is small, but the particle size of epoxy dispersion phase particles in emulsion is larger (up to 50 m), the particle shape is irregular, the particle size distribution is wide, the resulting emulsion stability is poor, and the particles are reunited due to the easy collision with each other, and the film-forming of the emulsion is not good.
2.1.2
Chemical modified method
chemical modified method also known as self-emulsification method, that is, hydro-based group insertion or branching into the epoxy resin molecular chain, so that epoxy resin to obtain the nature of self-emulsification, when the modified polymer water emulsification, dredging Water-based polymer molecular chain will gather into particles, polar groups or ion groups distributed on the surface of these particles, because these groups with the same kind of charge and mutual exclusion, as long as certain dynamic conditions are met, you can form a stable water-based epoxy emulsion, which is the basic principle of chemically modified legal water-based epoxy resin. According to the introduction of a water-based group with surfactant properties, chemically modified hydro-epoxy emulsions can be divided into etherized, esterified and reforested.
2.1.3
Contrary conversion method
the reverse conversion method is a more effective method for the preparation of high molecular weight epoxy emulsion, type II water-based epoxy resin coating system selected emulsion is usually prepared by the opposite conversion method. The more commonly used preparation method is to mix emulsion with epoxy resin evenly under the condition of high shear force, and then slowly add water to the system under certain shear conditions, with the gradual increase of water added, the whole system is gradually changed from oil-packed water type to water-packed oil type, forming a uniform and stable system. The emulsification process is usually carried out at room temperature, and for solid epoxy resins, a small amount of solvent and heating are often required to reduce the viscosity of the epoxy resin before emulsifying.
2.2
The development of powder-type epoxy resin coating
epoxy resin powder coating with bisphenol A-type epoxy resin, adipose epoxy resin and line phenolic resin as the main body, with curing agents, colorants and other additives, at a certain temperature after mixing, cooling crushing, graded over-sieve and produced. Epoxy powder coating is pollution-free, the coating film is strong, the melting viscosity is low, the leveling is good, does not need the primer coating film, so it has been widely used.
current production of epoxy coatings is mainly using one-step and two-step production processes. One-step synthesis method can be divided into two kinds of washing method and solvent extraction method, and two-step synthesis method can be divided into ontogenal polyruptification legal and catalytic polyruptification legal two kinds. In one-step synthesis, the washing method and solvent extraction method are basically the same, but after the resin synthesis treatment method is different. The treatment method of washing is to wash the resin directly with water until it is negative and chlorine-free ions, which results in a very large amount of water consumption, often washed more than a dozen times to achieve the goal.
2.2.1
solvent process
a certain proportion of bisphenol A, epoxy chloropropane and organic solvents are first put into the reactor for stirring, heated and dissolved, between 50 degrees C-75 degrees C drip NaOH aqueous solution to make it react. After the NaOH water solution droplets are added, the reaction temperature is raised to 85 degrees C-95 degrees C, the temperature is continued to be 8h-9h, and then a large number of solvents are added for extraction, and then washed, filtered, and removed solvent is the finished product. This method reacts with easy temperature control, good transparency of finished resin, fewer mechanical impurities and gel particles, and high product yield.
2.2
The choice of solvents
the effects of different solvents on epoxy resin synthesis, when xylene, toluene and chlorobenzene are used as solvents, the products obtained are not qualified. This may be due to the high dissolving temperature of BPA in these solvents, resulting in a high reaction temperature, which causes BPA to react too quickly with epoxy propane and the resulting epoxy resin molecular weight is distributed too wide. The use of mixed solvents significantly reduces the dissolved temperature of BPA, so the polymerization temperature can be controlled within the lower temperature range (85 degrees C-90 degrees C). In this way, the polymerization velocity of BPA and epoxy chloropropane can be well controlled, resulting in qualified products.
effect of
concentration of
.2.3
NaOH solution on epoxy resin synthesis, and the epoxy resin obtained when the concentration of NaOH solution is too high cannot be a powdered solid. This may be due to the high concentration of alkalis, the speed of reaction, the resin's molecular weight distribution is too wide.
effect of

alkali dosing on epoxy resin synthesis, when bisphenol A: epoxy chloropropane: NaOH has a molar ratio of 1:1.218:1.350 0.0001 equivalent/100g is significantly lower than HG2-741-72 (<0.02 equivalent/100g), and its epoxy value and softening point meet HG2-741-72. This may be due to the presence of excessive alkalis, which can further hydrolysis of organochlorine in epoxy resins, thereby reducing the level of organochlorine (high levels of organochlorine, which have a significant impact on the film-forming properties of epoxy resins).
2.2.5
Effect of reaction time
Effect of reaction time on epoxy resin synthesis, from different reaction time to get different epoxy values can be seen, when the reaction time is less than 8h, its epoxy value is greater than 0.13, indicating that epoxy resin molecular weight is low, polymerization is less than 4. When the reaction time is greater than 11h, its epoxy value is less than 0.12, indicating that the molecular weight of epoxy resin is too high and the polymerization is greater than 4. The reaction time is therefore 8h-9h best.
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The application status of epoxy coatings
3.1
anti-corrosion coatings
people to prevent corrosion coatings based on the specific requirements, design solvent-based, solvent-free (including powder) and high solid components of epoxy corrosion-resistant coatings, applied to steel surfaces, drinking water systems, electrical equipment, oil tankers, ballast chamber, aluminum and aluminum alloy surfaces and special media corrosion-resistant, obtained excellent results.
3.2
Ship coatings
sea moisture, salt spray, strong ultraviolet light and micro-alkaline sea water immersion and other harsh environments, paint is a severe test. Epoxy coatings have strong adhesion, excellent rust and water resistance, good mechanical strength and chemical resistance, and play an important role in ship protection. Epoxy coatings are used in hulls, waterlines and decks, and are resistant to wear, water, oil and adhesion. Epoxy water tank coating has been widely used.
3.3
Electrical insulation coating
epoxy coating with large resistance coefficient, high dielectric strength, small media loss and three defenses (wet heat, mold resistance, salt spray resistance) performance, etc. Advantages, widely used in impregnated motors and electrical appliances and other equipment coils, winding and a variety of insulation fiber materials, as well as a variety of combinations of accessories surface coating, but also can be used to bond a variety of insulation materials and nude wire coating coating and the production of a variety of pouring materials.
3.4
Food canned inner wall coatings
can use the corrosion resistance and excellent bonding of epoxy coatings, to make acid- and sulfur-resistant media of food canned coatings;
3.5
water-based
with water-based electrophoresis coatings made from epoxy lipids have unique properties. The coating not only has good corrosion resistance, but also has a certain decorative and color preservation. In addition to its application in the automotive industry, electrophoresis coatings are also used in medical devices, electrical appliances and light industrial products. The two-part epoxy coating protects the nuclear reactor equipment and easily removes radioactive contamination.
04
development trend of epoxy resin coatings
with the rise of the four pillar industries,