Application of epoxy resin on steel inner and outer coating

Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing

The epoxy resin refers to a polymer compound having two or more epoxy groups in its molecular structure. It is referred to as EPR (epoxy resin) resin, so some paint manufacturers refer to epoxy resin coatings as EPR (epoxy resin) coatings; some are called EPC (epoxy coating) coatings; some are called EPF (epoxy). Film) coating. Translated is epoxy resin coating (EPR); epoxy coating (EPC); epoxy coating (EPF), in fact, is one thing.

Due to the variety of epoxy resins, its chemical structure is usually expressed by its formula:

The invention of epoxy resin has been going on for quite a long time. As early as 1891, Lindmann of Germany reacted with hydroquinone by reacting hydroquinone with epichlorohydrin and solidifying it with an acid anhydride. But its use value has not been revealed. In 1930, Pierre Castan of Switzerland and SOGreenlee of the United States further studied the curing of the above resin with an organic polyamine, which showed high bonding strength, which attracted people's attention. Broadly speaking, epoxy resins can be synthesized from a parent compound containing an alkenyl group or a parent compound containing an active hydrogen atom. The epoxidation of olefins was first reported in the early 20th century, but it was not until the mid-1940s that Swern and his partners at the US Department of Agriculture began to study the epoxidation of polyunsaturated natural oils. Production of high molecular weight monoepoxides. It was applied to epoxy resin synthesis technology 10 years later. The reaction product of bisphenol A with epichlorohydrin was reported in the mid-1920s, and 15 years later, the production technology of unstable epoxidized fatty amine intermediates was pioneered. In 1933, Schlack [1] in Germany studied the separation technique of bisphenol A epoxy resin with bisphenol A. Although Schlack reported the reaction of diepoxides with organic acids, mineral acids, amines and thiols a year later, the industrial value of bisphenol A epoxy resins was determined by Castan of De Trey Freres, Switzerland and Devoe & Rayno1ds of the United States. Greenlee. In 1936, Castan produced an amber epichlorohydrin-bisphenol A resin and reacted with phthalic anhydride to produce industrially significant thermosets for casting and molding. In early 1939, Greenlee also produced a high molecular weight bisphenol A epichlorohydrin resin for advanced thermosetting coatings. From 1937 to 1939, Europe tried to fill the teeth with epoxy resin, but it was not successful. In addition, before the Second World War, epoxy resin technology was not fully developed. Shortly after the war, Devoe & Rayno1ds began trial production of coating resins, and CIBA was licensed by De Trey Freres to further develop liquid epoxy resins for liquid coatings, laminates and adhesives. However, the first industrially valuable epoxy resin was manufactured by Devoe-Raynolds in the United States in 1947. It opened up the technical history of epichlorohydrin-bisphenol A resin, and epoxy resin began industrial development. It is considered to be a technological advancement superior to older phenolic resins and polyester resins. This resin is comparable to most other thermoset plastics and performs better than phenolics and polyesters in some specialty applications. Soon CIBA (Ciba) in Switzerland, Shell (shell spleen) and Dow (Dow) in the United States began the industrial production and application development of epoxy resin. In the late 1950s, Ciba and Devoe-Raynolds continued to study glycidyl ether-type epoxy resins in the two major US companies. Shell Chemical Company only supplies epichlorohydrin. Union Carbide Plastics Co., Ltd. first manufactures phenolic resin and double Phenol A, European Ciba and Shell have developed epoxy resins.

In the summer of 1955, four basic epoxy resins were licensed for manufacturing in the United States, Dow Chemical co. Established an epoxy resin production line with Reichho1d Compound. At the same time as ordinary bisphenol A epoxy resin production applications, some new epoxy resins have been introduced. For example, in 1956, United Technologies Carbide began selling alicyclic epoxy resins. In 1959, Dow Chemical Company produced phenolic epoxy resins. Around 1960, Koppers, co. O-cresol novolac epoxy was produced. Between 1955 and 1965, the quality of epoxy resin was significantly improved, and bisphenol A epoxy resin had all the grades of average relative molecular quality grade. Phenolic epoxy establishes superior performance for significant high temperature applications. The patent for the synthesis of epoxy resin in 1957 was published by Shell Developmet co. Applied, the patent examines the application process of curing agents and additives, and discloses the application of cured epoxy resins.

Due to the increase in the variety of epoxy resins and the development of application technology, epoxy resin has made breakthroughs in the fields of electrical insulation, anti-corrosion coatings, metal structure bonding, etc. Therefore, epoxy resin has developed vigorously as an industry; Variety and application development are still very active. Since 1960, hundreds of epoxy resins have been industrially developed. 40 to 50 different structures of epoxy can be commercialized or supplied by intermediate test plants. More than 100 kinds of industrial curing agents and a large number of modifiers and thinners are matched, which is just around the corner.

China's development of epoxy resin began in 1956, and it was the first success in Shanghai and Shenyang. In 1958, Shanghai and Wuxi began industrial production. In the mid-1960s, some new alicyclic epoxy resins were studied: novolac epoxy resin, polybutadiene epoxy resin, glycidyl ester epoxy resin, glycidylamine epoxy resin, etc.

Epoxy resin has excellent physical and mechanical properties, electrical insulation properties, chemical resistance and bonding properties. It can be used as a coating, casting material, molding material, adhesive, laminate in direct or indirect form to penetrate into daily life. Supplies to all aspects of the national economy in the high-tech sector. For example: aircraft, composite materials in spacecraft, packaging materials for large-scale integrated circuits, insulation materials for generators, coatings for steel and wood, adhesives for mechanical civil engineering, and even inner coatings for food cans and metal anti-corrosion coatings A large amount of epoxy resin is used in the installation. It has become an indispensable material in the development of the national economy. Its production and application level can also reflect the development of a country's industrial technology from one side.

The most suitable application of epoxy resin is the bisphenol A epoxy resin synthesized from epichlorohydrin and bisphenol A. Its synthesis reaction goes through four stages:

1, bisphenol A forms ions under the action of sodium hydroxide

2, epichlorohydrin ring opening and bisphenol A form an ether bond

3. The epoxy resin ion forming the ether bond forms a low molecular weight epoxy resin under the action of sodium hydroxide and excess epichlorohydrin:

4, further reaction to form a high molecular weight epoxy resin.

Each of its structural units imparts the following functions to the resin: the epoxy group and the hydroxyl group impart reactivity to the resin, and the resin cured product has strong cohesive force and adhesive force. Ether and hydroxyl groups are polar groups that help to improve wettability and adhesion. Ether and CC bonds give macromolecules flexibility. The benzene ring imparts heat resistance and rigidity to the polymer. The isopropyl group also imparts a certain rigidity to the macromolecule. The -CO- bond has a high bond energy, which improves alkali resistance. Figure:

Therefore, the molecular structure of bisphenol A epoxy resin determines its performance has the following characteristics:
1) It is a thermoplastic resin, but it has thermosetting properties, and can form a plurality of cured products excellent in performance with various curing agents, catalysts, and additives, and can satisfy almost various use requirements.
2) The chemical properties are very stable. Therefore, its chemical resistance and acid and alkali resistance are very good.
3) The cured product has high strength and bonding strength, and the adhesion of the finished coating to the metal is the best among all the coatings.
4) Combined with a variety of resins can meet food hygiene requirements, so it can be made into food coatings.
5) Since the epoxy group of the epoxy resin has a small internal rotation resistance at both ends of the molecule, its flexibility is good.

The active epoxy group and hydroxyl group in the epoxy resin structure can be cross-linked and cured with various organic compounds, such as: phenolic resin, amino resin, polythiol compound, isocyanate compound and hydroxyl-containing (or hydroxyl) resin, alkyd resin , polyester resin and acrylic resin. The phenolic resin, the amino resin and the carboxy-containing (hydroxyl)-based resin are polycondensation crosslinking curing agents; the isocyanate compound and the polythiol compound are additive curing agents.

When an epoxy resin is combined with these organic compounds to form a coating, the epoxy resin is crosslinked with these organic compounds to form a stable and stable three-dimensional network structure film, which has outstanding chemical resistance and good mechanical properties. As well as excellent adhesion to metals, epoxy resin is used as a steel drum inside and outside coatings, and is the oldest and most widely used coating in the world.

Since both epichlorohydrin and bisphenol A are difunctional compounds, epoxy resins of this type are linear structures. This epoxy resin contains a hydroxyl group, an ether group and a very reactive epoxy group. Among them, the hydroxyl group and the ether group have a strong polarity, so that the epoxy resin generates a strong force with the adjacent interface. Therefore, the application of this epoxy resin to steel drum coatings has a strong adhesion to the surface of the steel drum, while the active epoxy group imparts a high strength bonding force to the surface of the coating and the steel drum.

Moreover, the molecular chain of the epoxy resin is carbon-carbon bond and ether bond structure, and the chemical property is very stable. Therefore, its chemical resistance and acid and alkali resistance are very good. In addition, since the epoxy group of the epoxy resin has a small internal rotation resistance at both ends of the molecule, its flexibility is good. This is very suitable for the inner coating process of the steel drum industry (coating of the barrel and the bottom of the bucket, top coating, baking and curing, and then re-winding).

According to the reactivity of the epoxy resin, various organic compounds are reacted as a cross-linking curing agent to form a network structure film, which can exert an excellent effect on the inner and outer coatings of the steel drum and have various characteristics.

First, the epoxy resin reacts with the amino resin in the inner and outer coatings of the steel drum _620 coating:

The amino resin is used as a curing agent for the epoxy resin, and the curing reaction is a methylol group of the amino resin, and a group such as an alkoxymethyl group and an imino group is crosslinked and cured with an epoxy group and a hydroxyl group of the epoxy resin.

Different structures of amino resins have different curing activities for epoxy resins, and the resulting coating film network structure is also very different. For example, methanol-etherified amino resins have higher hardness than coating films formed from butanol etherified amino resins. The completely etherified amino resin is more elastic than the partially etherified amino resin; the amino resin having a high degree of etherification is more likely to generate pinholes or the like than the coating film formed by the amino resin having a high degree of polymerization. So what kind of amino resin is chosen is very important.

620 coating is an internal and external coating for steel drums combined with epoxy resin and special amino resin. This kind of paint is bright and clean when applied to steel drum in transparent and colorless form. It has excellent adhesion to chemical film and good food resistance. It can be used for filling beer, juice, edible oil and other foods. Liquid, can also be filled with a variety of chemical corrosives; when applied to steel drums in various colors, good weather resistance, bright color, and also anti-pollution is not easy to dirty, the most important thing is tolerate Splashes or dripping of a variety of chemically corrosive liquids can be corroded and intact on the surface of the steel drum. Therefore, 620 coatings are used in both interior and exterior coatings in many barrel plants.

Second, the epoxy resin and the soluble phenolic resin reacted in the steel barrel coating _621 coating:

The resol phenolic resin mainly reacts with a hydroxyl group and an alkoxy group and a hydroxyl group of an epoxy resin to form a network structure film. This phenolic resin is produced by a polycondensation reaction of phenol and formaldehyde catalyzed by ammonia. When choosing this phenolic resin, pay attention to the content of methylol group. If the content of methylol group is small, the coating film formed by combining with epoxy resin is too soft. Otherwise, the coating film formed by combining with epoxy resin has high crosslink density. , good chemical resistance. Coatings made from these two resins are also fully compliant with food hygiene requirements.

621 paint is baked and solidified, it is very bright golden transparent color. It is golden like gold, very beautiful, and the color of the coating can be adjusted by the baking temperature or the length of baking time. The higher the temperature, the longer the temperature. The darker the color, the lighter the color. The steel drum coated with this kind of coating is most suitable for containing all kinds of sulfur-containing foods and acidic foods, and can also hold most chemical liquids. In fact, this kind of coating is to enhance the anti-corrosion on the basis of most of the current food can coatings. Improved performance. The coating was sent to Europe by a Taiwan-funded company in Shanghai, which fully complies with European food packaging and coating standards, so they have applied this coating to the pure water packaging for export to Europe.

Third, the epoxy resin and the butanol etherified phenolic resin reaction applied to the steel barrel coating _622 coating:

The 622 steel drum inner coating is the most widely used and the oldest steel drum inner coating in China. It is also the steel drum inner coating with the most export, the most applied country and the best reputation. Just as 362 phenolic paint was successfully replaced by foreign barrel mills in foreign well-known RDL-50 steel drum coatings, 622 epoxy coatings were replaced by well-known RDL-44 and 2088 steel drum coatings by many foreign barrel factories.

The 622 coating utilizes an active epoxy group in the epoxy resin structure to crosslink and solidify with various organic compounds to form a strong three-dimensional network structure film, which is crosslinked with a butanol etherified phenolic resin at a high temperature. The mechanical properties and chemical resistance of the coatings used in the X-622 steel drum are greatly enhanced. The cross-linking curing reaction at this high temperature only releases three parts of water, no other by-products are released during the process, and the shrinkage is small, and the cross-linking curing has a very strong and dense network structure film, thereby further enhancing Adhesion, mechanical properties, corrosion resistance and chemical resistance of the coating.

622 coating has the following characteristics:

l. The bisphenol A glycidyl ether (DGEBPA) epoxy resin used in the coating of 622 steel drum is treated with low viscosity softening by â€œX-6 technologyâ€, so its viscosity is better under the same resin content. Under the condition of low resin content, its viscosity is low, which makes it easy to form a thick coating film at one time, which is beneficial to the coating process of steel drums.

2. No softener is added to the paint used in the 622 steel drum. Softeners are softened by increasing the molecular gap of the epoxy resin, but while achieving softness, the anticorrosive properties of the coating are affected by the increase in the molecular gap of the epoxy resin. The epoxy resin used in the X-622 steel drum is softened by low viscosity, so even if no softener is added, it can be curled freely after being applied to the steel drum.

3, 622 steel barrel internal coating does not add tackifiers, leveling agents, defoamers and other additives. Because these additives improve the coating performance of epoxy coatings, they have a certain influence on the crosslinking of epoxy resin and butanol etherified phenolic resin, which easily causes defects of three-dimensional network structure film, so ultimately The corrosion resistance of the coating is affected.

4. The inner coating of 622 steel drum completely adopts the imported rutile titanium dioxide powder which is surface-treated by particles as the pigment and filler, and eliminates the use of cheap pigments and fillers such as talc powder, calcium carbonate, barium sulfate, zinc barium white. Therefore, the surface of the coating film is bright and smooth, and the coating film has excellent corrosion resistance.

5. Because the bisphenol A diglycidyl ether (DGEBPA) type epoxy resin used in the coating of 622 copper barrel contains hydroxyl and ether bonds, it can form a chemical bond with the free bond on the metal surface, so the steel drum It is the best coating for adhesion.

6. After the cross-linking and solidification of the coating in the 622 steel drum, the structural stability of the carbon-carbon and ether bonds in the molecular chain is greatly enhanced, and the coating film structure is more dense, so the chemical resistance of the coating film is enhanced ( In particular, alkali resistance), the coating further enhances the ability to resist corrosion of the integrated medium.

After the film is solidified into a film in the 622 steel drum, the coating film forms a three-dimensional three-dimensional structure, so the mechanical strength of the coating film is high, stability and insulation performance are also good.

8. After curing the film in the 622 steel drum, the adhesion between the film and the other resin film is not very good due to the compactness and stability of the film itself. Therefore, it is not suitable to apply other coatings on the film. If two coats are required for the inner coating of the barrel, the first coating should be baked at a lower temperature for a shorter period of time.

9. The common problem of bisphenol A glycidyl ether epoxy resin is poor weather resistance, easy to be powdered by ultraviolet light irradiation, so the paint used in 622 steel drum can not be used as steel drum outer coating.

10. According to China GB4805-1994 "Sanitary standard for epoxy phenolic coating on inner wall of food cans", the content of free phenol and free formaldehyde in the coating after baking epoxy phenolic paint should be less than 0.1 mg per gram, but 622 coatings pass through Shanghai The Institute of Preventive Medicine has not detected the test. The safety and health indicators of other indicators such as evaporation residue and potassium permanganate consumption are also more than three times higher than the above national standards. Therefore, 622 coatings can pass SGS international certification, in line with the US FDA standards for food and pharmaceutical coatings.

Fourth, the epoxy resin and polyisocyanate adduct reaction is applied to steel drum inside and outside coating _623/624 coating

The isocyanate (-NCO) in the isocyanate compound easily reacts with the epoxy group and the hydroxy compound. Since the epoxy resin used in the 623/624 coating contains a large amount of hydroxyl groups, the 623/624 coating is applied even at normal temperature. Can be cured.

623 and 624 are actually the same product, but one is transparent and the one is white. Both coatings are two-component and, when used, the two components are mixed together and cured at room temperature. However, although it can be dried in more than ten minutes at room temperature, it can be dried in about half an hour, and it looks like it is completely cured, but it is only dry, not cured. Full curing at room temperature can be as long as seven days (in fact, the inventory of barrels in barrels is much more than seven days); however, if it can be baked for more than ten minutes at low temperature, it can be fully cured.

These two coatings have excellent water resistance, solvent resistance and chemical resistance after being fully cured, so they are very suitable for the production of inner coated steel drums in barrel factories without baking conditions and low production requirements.

The performance of X-623 and X-624 coatings is close to that of X-620 coatings. In particular, X-623 has almost the same visual effect as X-620. Some barrel mills use its colorless, transparent, corrosion-resistant and room-temperature curing characteristics. A thin layer of X-623 paint is sprayed on the top of the barrel coated with the amino baking paint, so that the steel barrel is not afraid of damaging the paint when it is filled with corrosive chemical liquid. After the X-624 white paint is coated with steel drum, the inner wall is white as snow, smooth as porcelain, and bright as a mirror, just like a layer of white ceramic tile.

5. Epoxy resin reacts with polyester resin in steel drum inside and outside coating _625 coating

Epoxy resin reacts with polyester resin. Most of the coatings used inside and outside the steel drum are powder coatings. In epoxy polyester powder coatings, both epoxy resin and polyester resin can be regarded as both curing agents and Think of each other as a film-forming substance. This coating was first invented and industrialized in Europe. The principle is that the epoxy group in the epoxy resin reacts with the carboxyl group in the polyester resin in an equivalent amount. When the epoxy resin is increased, the corrosion resistance and adhesion of the coating film formed by the reaction are enhanced. When the polyester resin is increased, The coating film produced by the reaction has leveling property, glossiness and weather resistance. If the ratio of the two is improper, the cross-linking density of the coating film formed by the reaction is lowered, resulting in a decrease in the mechanical properties and corrosion resistance of the coating film. Therefore, the ring should generally be calculated according to the theoretical ratio: G=56100E/A. The amount of oxygen resin and polyester resin (in the formula, G--100g epoxy resin needs to be blended with polyester resin; A--polyester resin acid value; E-epoxy resin epoxy value).

In this coating, the main technical indicators are controlled at a softening point of 88~93 Â°C, and the epoxy value is 0.11~0.13. The polyester resin herein is generally produced by polycondensation and addition reaction of a polyvalent carboxylic acid, an acid anhydride and a polyhydric alcohol. The currently used polyester resin has an acid value ranging from 20 to 85 mg KOH/g.

625 coating has the dual superior performance of epoxy resin coating and polyester resin coating, which has excellent adhesion and anti-corrosion performance of epoxy resin coating, and has the beauty and aging resistance of polyester resin coating, so it is in the steel barrel industry. It can be used for both internal coating and exterior coating.

There are many resins that can react with epoxy resins, and most of them can be made into coatings with excellent properties. It is hoped that more such coatings will be developed for use in steel drums in the near future.