Halogen-free flame-retardant liquid epoxy resin, solid epoxy resin for powder coating and preparation method of solid epoxy resin

Abstract

The invention provides halogen-free flame-retardant liquid epoxy resin, solid epoxy resin for powder coating and a preparation method of the solid epoxy resin, and belongs to the technical field of powder coating. The liquid epoxy resin disclosed by the invention is prepared by polymerizing 2, 4-diamino-6-phenyl-1, 3, 5-triazine, formaldehyde, triethyl phosphite, epoxy chloropropane, sodium hydroxide, DMF (Dimethyl Formamide) and the like. The contents of halogen-free flame-retardant elements nitrogen and phosphorus in the product are high, a solid epoxy resin product obtained by carrying out chain extension on liquid epoxy resin serving as a synthetic monomer of solid epoxy resin of the powder coating and bisphenol F is good in flame retardance, the oxygen index reaches 37% or above, and the flame retardance is high. A coating film obtained after the powder coating and an accelerated dicyandiamide curing agent are subjected to low-temperature rapid curing (150 DEG C / 10 min) is high in gloss and leveling grade and good in impact resistance, hardness and the like, and the application requirements of the powder coating are met.

Description

Technical Field

[0001] This invention belongs to the field of powder coating technology, specifically relating to a halogen-free flame-retardant liquid epoxy resin, a solid epoxy resin for powder coating, and a method for preparing the same. Background Technology

[0002] Powder coatings, as a type of environmentally friendly coating, have seen rapid development in recent years. Powder coatings are mainly composed of film-forming resins, curing agents, and additives. The main film-forming resins include epoxy resins, polyester resins, and acrylic resins, with epoxy resins being the most important. With the increasing use of powder coatings in interior decoration materials, such as medium-density fiberboard for tabletops and cabinets, there is a growing demand for not only low-temperature curing coatings but also high requirements for environmentally friendly, halogen-free flame retardant coatings for fire safety. The oxygen index generally needs to reach a flame-retardant rating of 30% or higher.

[0003] Currently, the preparation of solid epoxy resins used in powder coating systems is mainly divided into one-step and two-step methods. The one-step method generates a large amount of low-concentration sodium chloride solution, making environmental treatment difficult. Therefore, the two-step method is currently the primary approach. The two-step method first prepares a small-molecule liquid epoxy resin, primarily represented by bisphenol A diglycidyl ether. The second step involves chain extension polymerization of solid bisphenol A and the liquid epoxy resin bisphenol A diglycidyl ether in a specific ratio. Currently, manufacturers using the two-step method typically purchase the liquid epoxy resin bisphenol A diglycidyl ether and then use bisphenol A for chain extension polymerization to obtain the product. However, solid epoxy resins prepared using bisphenol A diglycidyl ether as an epoxy monomer for chain extension lack flame retardant properties. Currently, there is a lack of liquid epoxy resin monomers with halogen-free flame retardant properties suitable for preparing solid epoxy resins for powder coatings, making it difficult to effectively commercialize the two-step method for preparing halogen-free flame-retardant epoxy resins. On the other hand, the one-step method for directly preparing flame-retardant epoxy resins involves a wide variety of raw materials, a complex process, and the difficulty in handling the large amount of low-concentration sodium chloride solution. The development of a liquid epoxy resin monomer with halogen-free flame retardant properties, and the solid epoxy resin prepared from it through chain extension reaction, which not only has excellent halogen-free flame retardant properties but also meets the conventional requirements of coatings, is of great significance. Summary of the Invention

[0004] One of the objectives of this invention is to provide a halogen-free flame-retardant liquid epoxy resin.

[0005] The second objective of this invention is to provide a method for preparing the halogen-free flame-retardant liquid epoxy resin.

[0006] A third objective of this invention is to provide a solid epoxy resin for powder coatings prepared from the halogen-free flame-retardant liquid epoxy resin.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a halogen-free flame-retardant liquid epoxy resin, wherein the halogen-free flame-retardant liquid epoxy resin comprises the following raw materials in molar amounts: 11-14 parts of 2,4-diamino-6-phenyl-1,3,5-triazine; Formaldehyde 22-29 parts; 3-4 parts acetonitrile; 8-11 copies of DMF; 23-30 parts of triethyl phosphite; 27-33 parts of epichlorohydrin; Sodium hydroxide 23-29 parts; The raw materials also include catalyst 1 and catalyst 2; Catalyst 1 is sodium methoxide, used at a rate of 0.1-0.2% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2 is benzyltrimethylammonium chloride, and the amount used is 0.1-0.15% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

[0008] Typical, but not limiting, molar amounts of 2,4-diamino-6-phenyl-1,3,5-triazine are, for example, 11, 12, 13, or 14 parts. Typical but non-limiting molar amounts of formaldehyde are, for example, 22, 23, 24, 25, 26, 27, 28, and 29 parts per molar. Formaldehyde was added in the form of an aqueous solution with a mass concentration of 37%.

[0009] Typical, but not limiting, molar amounts of acetonitrile are, for example, 3, 3.5, or 4 parts; Typical, but not restrictive, molar numbers for DMF are, for example, 8, 9, 10, or 11 parts. Typical, but not limiting, molar amounts of triethyl phosphite are, for example, 23, 24, 25, 26, 27, 28, 29, or 30 parts. Typical, but not limiting, molar amounts of epichlorohydrin are, for example, 27, 28, 29, 30, 31, 32, or 33 parts. Typical, but not limiting, molar amounts of sodium hydroxide are, for example, 23, 24, 25, 26, 27, 28, or 29 parts.

[0010] Preferably, the molar ratio of formaldehyde to 2,4-diamino-6-phenyl-1,3,5-triazine is controlled at 2-2.1:1.

[0011] Secondly, the present invention provides a method for preparing the above-mentioned halogen-free flame-retardant liquid epoxy resin, comprising the following steps: A. Add the prescribed amount of formaldehyde aqueous solution and acetonitrile to the reaction vessel, start stirring, then adjust the pH value of the system to 7.5-8.0, then add the prescribed amount of 2,4-diamino-6-phenyl-1,3,5-triazine, and heat to carry out the hydroxymethylation reaction; B. When the conversion rate of 2,4-diamino-6-phenyl-1,3,5-triazine reaches 98% or more, stop the reaction, raise the temperature, and start the vacuum system to remove excess unreacted formaldehyde solution and solvent acetonitrile by vacuum distillation. C. When the volatile content is below 2%, release the vacuum system, add the formula amount of co-solvent DMF to the reaction vessel, and heat it to fully dissolve it. Then add the formula amount of triethyl phosphite and catalyst 1, keep it warm to carry out the transesterification reaction, and distill off the transesterification byproduct ethanol. D. Sampling and testing: When the hydroxyl value of the polymer is below 5 mg KOH / g, the temperature is increased, and the vacuum system is restarted to remove the co-solvent DMF and residual triethyl phosphite under reduced pressure. E. When the volatile content is less than 1%, release the vacuum system, cool down, and add the prescribed amount of epichlorohydrin and catalyst 2. After stirring to dissolve, continue the ring-opening reaction under heat preservation. F. When the polymer amine value is below 1 mg KOH / g, cool down, and then add sodium hydroxide in 3-4 batches in an equal amount to carry out the ring-closure reaction. After each addition, keep the temperature for the reaction. After all the addition is completed, continue to keep the temperature for the ring-closure reaction. G. Cool down, then filter to remove solid sodium chloride and excess unreacted sodium hydroxide solid generated in the polymerization system, and obtain filtrate. Then wash the filtrate with 5-7% water by weight to fully remove the residual inorganic salts. After standing and separating, remove the aqueous phase, heat the obtained organic phase, and use a high vacuum system to fully remove excess epichlorohydrin and water. When the volatile content is less than 1%, stop the vacuum system and discharge at high temperature to obtain liquid epoxy resin.

[0012] In some embodiments, the temperature of the hydroxymethylation reaction in step A is 57-62°C; In some embodiments, in step B, the temperature is raised to 100-105°C; the vacuum degree is controlled at -0.095 to -0.098 MPa. In some implementations, in step C, the temperature is raised to 120-125°C; In some embodiments, in step D, the temperature is raised to 130-135°C; the vacuum degree is controlled at -0.095 to -0.098 MPa. In some implementations, in step E, the temperature is lowered to 90-95°C; In some embodiments, in step F, the temperature is lowered to 35-40°C; after each addition, the reaction is maintained at this temperature for 10-15 minutes; after all additions are completed, the reaction is continued at this temperature for 0.5-1 hour in a closed-loop manner. In some embodiments, in step G, the temperature is lowered to 5-10°C; the filtrate is washed for 20-30 minutes; the organic phase is heated to 90-95°C; and the vacuum degree is controlled at -0.097 MPa to -0.099 MPa. In one specific embodiment, the preparation method of halogen-free flame-retardant liquid epoxy resin includes the following steps: A. Add the prescribed amount of formaldehyde aqueous solution and acetonitrile to the reaction vessel, start stirring, then add sodium carbonate to adjust the pH of the system to 7.5-8.0, then add the prescribed amount of 2,4-diamino-6-phenyl-1,3,5-triazine, and heat to 57-62℃ to carry out the hydroxymethylation reaction; B. The conversion of 2,4-diamino-6-phenyl-1,3,5-triazine was detected by liquid chromatography. When the conversion rate of 2,4-diamino-6-phenyl-1,3,5-triazine reached 98% or more, the reaction was stopped, the temperature was raised to 100-105℃, and the vacuum system was started to remove excess unreacted formaldehyde solution and solvent acetonitrile by vacuum distillation; the vacuum degree was controlled at -0.095 to -0.098 MPa. C. When the volatile content is less than 2%, release the vacuum system, add the formula amount of co-solvent DMF into the reaction vessel, and heat it to 120-125℃ to fully dissolve it. Then add the formula amount of triethyl phosphite and catalyst 1, keep it warm to carry out the transesterification reaction, and distill off the transesterification byproduct ethanol. D. When the hydroxyl value of the sampled polymer is less than 5 mg KOH / g, it indicates that the transesterification reaction is basically complete. Raise the temperature to 130-135℃ and restart the vacuum system to remove the co-solvent DMF and residual triethyl phosphite under reduced pressure. The vacuum degree is controlled at -0.095 to -0.098 MPa. E. When the volatile content is less than 1%, release the vacuum system, cool down to 90-95℃, and add the prescribed amount of epichlorohydrin and catalyst 2. After stirring to dissolve, continue the ring-opening reaction under heat preservation. F. Take samples to test the amine value of the polymer. When the amine value is lower than 1 mg KOH / g, it indicates that the ring-opening reaction of the amino group has been completed. Cool down to 35-40℃, and then add sodium hydroxide in 3-4 batches in an equal amount to carry out the ring-closing reaction. After each addition, keep the reaction temperature for 10-15 minutes. After all the addition is completed, continue to keep the reaction temperature for 0.5-1 hour to carry out the ring-closing reaction. G. Cool to 5-10℃, then filter to remove solid sodium chloride and excess unreacted sodium hydroxide solid generated in the polymerization system, obtaining filtrate. Wash the filtrate with 5-7% water by weight for 20-30 minutes to fully remove residual inorganic salts. After standing and separating, remove the aqueous phase, heat the obtained organic phase to 90-95℃, and use a high vacuum system to fully remove excess epichlorohydrin and water, controlling the vacuum degree at -0.097Mpa to -0.099Mpa. When the volatile content is less than 1%, stop the vacuum system and discharge at high temperature to obtain liquid epoxy resin.

[0013] The resulting liquid epoxy resin is a colorless, transparent, viscous liquid with an epoxy equivalent of 310-330 g / mol and a viscosity (25℃) of 9700-9860 mPa·s. The structural diagram of liquid epoxy resin is as follows: .

[0014] Thirdly, the present invention provides a solid epoxy resin for powder coating, which is obtained by chain extension polymerization of the above-mentioned halogen-free flame-retardant liquid epoxy resin and bisphenol F at a molar ratio of 2:1.05-1.15.

[0015] Preferably, the chain extension polymerization is carried out in the presence of catalyst 3, which is triphenylphosphine, and the amount of catalyst 3 is 0.05-0.07% of the mass of bisphenol F.

[0016] The preparation method of the solid epoxy resin for powder coating includes the following steps: The halogen-free flame-retardant liquid epoxy resin, catalyst 3, and bisphenol F are added to a polymerization reactor. Stirring is started and the temperature is raised to 127-130℃ for heat preservation and chain extension polymerization until the reaction polymer becomes completely transparent. After heat preservation for another 20-30 minutes, samples are taken every 10 minutes to detect the content of free bisphenol F using liquid chromatography. When the content of free bisphenol F is lower than 0.5%, the reaction is stopped, and the material is melted and discharged at the current temperature. The material is cooled and crushed by steel belt to obtain a solid epoxy resin product for powder coating.

[0017] The resulting solid epoxy resin for powder coating is colorless or pale yellow resin particles with an epoxy equivalent of 840-890 g / mol and a softening point of 85-91℃.

[0018] Schematic diagram of solid epoxy resin structure: .

[0019] Beneficial effects: This invention relates to a liquid epoxy resin that can be used as a raw material for the two-step preparation of solid epoxy resin for powder coatings. It is polymerized from 2,4-diamino-6-phenyl-1,3,5-triazine, formaldehyde, triethyl phosphite, epichlorohydrin, sodium hydroxide, and DMF. First, 2,4-diamino-6-phenyl-1,3,5-triazine is hydroxymethylated with formaldehyde, followed by phosphite esterification and chain extension epoxidation. The liquid epoxy resin has relatively short molecular chains, with the main chain containing both a high-nitrogen triazine ring and a high-phosphite ester group. This results in good flame retardant properties. Furthermore, the use of specific phenyl groups as side chains improves the rigidity and water resistance of the final coating film. The introduction of phosphite also enhances the lubricity of the epoxy resin chain segments, which is beneficial for achieving better coating leveling under low-temperature curing conditions. Liquid epoxy resin, as a monomer for synthesizing solid epoxy resins for powder coatings, yields solid epoxy resin products after chain extension with bisphenol F. These products exhibit good flame retardant properties, with oxygen indices exceeding 37%. When cured at low temperature (150℃ / 10min) with an accelerator dicyandiamide curing agent, the resulting coating demonstrates high gloss and leveling, as well as good impact resistance and water resistance, meeting the application requirements of powder coatings. The liquid epoxy resin monomer obtained in this invention can be used alone as a raw material with bisphenol F to prepare solid epoxy resins for powder coatings, or it can be used in combination with E-51 liquid epoxy resin (such as bisphenol A diglycidyl ether).

[0020] The present invention has been described in detail above; however, the above embodiments are merely illustrative in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments. Detailed Implementation

[0021] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0022] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0023] The hydroxyl value was tested according to Method C, Part 7 of HG / T 2709-2022 "Determination of Hydroxyl Value of Polyester Polyols Used in the Production of Polyurethane in Plastics". The amine value was tested according to the method in ZBG71005-1989 "Octadecylamine"; The epoxy equivalent was tested according to GB / T 4612-2008 "Determination of Epoxy Equivalent in Epoxy Compounds for Plastics"; The test conditions for volatile matter are 130℃ / 30min. Under these conditions, the weight lost / the original weight of the sample × 100% is the volatile matter.

[0024] Example 1 A halogen-free flame-retardant liquid epoxy resin comprises the following raw materials in molar amounts: 12 parts of 2,4-diamino-6-phenyl-1,3,5-triazine; 24 parts of formaldehyde; Acetonitrile 3.5 parts; 9 copies of DMF; 26 parts of triethyl phosphite; 29 parts of epichlorohydrin; 26 parts of sodium hydroxide; Catalyst 1: Sodium methoxide, used at 0.1% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2: Benzyltrimethylammonium chloride, in an amount of 0.15% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

[0025] The method for preparing the above-mentioned halogen-free flame-retardant liquid epoxy resin includes the following steps: A. Add the prescribed amount of formaldehyde aqueous solution and acetonitrile to the reaction vessel, start stirring, then add sodium carbonate to adjust the pH of the system to 7.5-8.0, then add the prescribed amount of 2,4-diamino-6-phenyl-1,3,5-triazine, and heat to 60℃ to carry out the hydroxymethylation reaction; B. The conversion of 2,4-diamino-6-phenyl-1,3,5-triazine was detected by liquid chromatography. When the conversion rate of 2,4-diamino-6-phenyl-1,3,5-triazine reached more than 98%, the reaction was stopped, the temperature was raised to 100℃, and the vacuum system was started to remove excess unreacted formaldehyde solution and solvent acetonitrile by vacuum distillation; the vacuum degree was controlled at -0.098 MPa. C. When the volatile content is less than 2%, release the vacuum system, add the formula amount of co-solvent DMF into the reaction vessel, and heat it to 125℃ to fully dissolve it. Then add the formula amount of triethyl phosphite and catalyst 1, keep it warm to carry out the transesterification reaction, and distill off the transesterification byproduct ethanol. D. When the hydroxyl value of the sampled polymer is less than 5 mg KOH / g, it indicates that the transesterification reaction is basically complete. The temperature is raised to 135℃, and the vacuum system is restarted to remove the co-solvent DMF and residual triethyl phosphite under reduced pressure. The vacuum degree is controlled at -0.098 MPa. E. When the volatile content is less than 1%, release the vacuum system, cool down to 90°C, and add the prescribed amount of epichlorohydrin and catalyst 2. After stirring to dissolve, continue the ring-opening reaction under heat preservation. F. Take samples to test the amine value of the polymer. When the amine value is lower than 1 mg KOH / g, it indicates that the ring-opening reaction of the amino group has been completed. Cool down to 40℃, and then add sodium hydroxide in 3-4 batches in an equal amount to carry out the ring-closing reaction. After each addition, keep the reaction temperature for 15 min. After all the addition is completed, continue to keep the reaction temperature for 1 h to carry out the ring-closing reaction. G. Cool to 10℃, then filter to remove solid sodium chloride and excess unreacted sodium hydroxide solid generated in the polymerization system, obtaining filtrate. Wash the filtrate with 5-7% water by weight for 30 minutes to fully remove residual inorganic salts. After standing and separating, remove the aqueous phase, heat the obtained organic phase to 90℃, and use a high vacuum system to fully remove excess epichlorohydrin and water, controlling the vacuum degree at -0.098 MPa. When the volatile content is less than 1%, stop the vacuum system and discharge at high temperature to obtain liquid epoxy resin.

[0026] Liquid epoxy resin epoxy equivalent: 317 g / mol, viscosity (25℃): 9781 mPa·s; The preparation method of solid epoxy resin for powder coating includes the following steps: The above-mentioned liquid epoxy resin, catalyst 3, and bisphenol F were added to the polymerization reactor. The molar ratio of liquid epoxy resin to bisphenol F was 2:1.1. The catalyst 3 was triphenylphosphine, and the amount used was 0.06% of the mass of bisphenol F. The stirring was started and the temperature was raised to 130°C for heat preservation and chain extension polymerization until the reaction polymer became completely transparent. After heat preservation for another 30 minutes, samples were taken every 10 minutes. The content of free bisphenol F was detected by liquid chromatography. When the content of free bisphenol F was lower than 0.5%, the reaction was stopped, and the material was melted and discharged at the current temperature. The material was cooled and crushed by steel belt to obtain solid epoxy resin product for powder coating.

[0027] Solid epoxy resin epoxy equivalent: 859 g / mol, softening point: 87℃.

[0028] Example 2 A halogen-free flame-retardant liquid epoxy resin comprises the following raw materials in molar amounts: 11 parts of 2,4-diamino-6-phenyl-1,3,5-triazine; 23 parts of formaldehyde; 3 parts acetonitrile; 8 copies of DMF; 23 parts of triethyl phosphite; 28 parts of epichlorohydrin; 24 parts of sodium hydroxide; Catalyst 1: Sodium methoxide, used at 0.15% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2: Benzyltrimethylammonium chloride, in an amount of 0.15% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

[0029] The preparation methods for liquid epoxy resin and solid epoxy resin are the same as in Example 1.

[0030] Liquid epoxy resin epoxy equivalent: 311 g / mol, viscosity (25℃): 9844 mPa·s; Solid epoxy resin epoxy equivalent: 844 g / mol, softening point: 90℃.

[0031] Example 3 A halogen-free flame-retardant liquid epoxy resin comprises the following raw materials in molar amounts: 14 parts of 2,4-diamino-6-phenyl-1,3,5-triazine; 29 portions of formaldehyde; 4 parts acetonitrile; 11 copies of DMF; 30 parts of triethyl phosphite; 33 parts of epichlorohydrin; 29 parts of sodium hydroxide; Catalyst 1: Sodium methoxide, used at 0.2% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2: Benzyltrimethylammonium chloride, in an amount of 0.1% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

[0032] The preparation methods for liquid epoxy resin and solid epoxy resin are the same as in Example 1.

[0033] Liquid epoxy resin epoxy equivalent: 328 g / mol, viscosity (25℃): 9812 mPa·s; Solid epoxy resin epoxy equivalent: 883 g / mol, softening point: 89℃.

[0034] Example 4 A halogen-free flame-retardant liquid epoxy resin comprises the following raw materials in molar amounts: 13 parts of 2,4-diamino-6-phenyl-1,3,5-triazine; 27 parts of formaldehyde; 4 parts acetonitrile; 10 copies of DMF; 28 parts of triethyl phosphite; 31 parts of epichlorohydrin; 27 parts of sodium hydroxide; Catalyst 1: Sodium methoxide, used at 0.2% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2: Benzyltrimethylammonium chloride, in an amount of 0.1% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

[0035] The preparation methods for liquid epoxy resin and solid epoxy resin are the same as in Example 1.

[0036] Liquid epoxy resin epoxy equivalent: 324 g / mol, viscosity (25℃): 9726 mPa·s; Solid epoxy resin epoxy equivalent: 875 g / mol, softening point: 86℃.

[0037] Comparative Example 1 The process is the same as in Example 1, except that triethyl phosphite is not used for transesterification.

[0038] Liquid epoxy resin epoxy equivalent: 177 g / mol, viscosity (25℃): 8358 mPa·s; In the synthesis of solid epoxy resin, only the molar ratio of the prepared liquid epoxy resin to bisphenol F is changed to 1.4:1, while everything else remains the same.

[0039] Solid epoxy resin epoxy equivalent: 809 g / mol, softening point: 97℃.

[0040] Comparative Example 2 The process is the same as in Example 1, except that an equal amount of bisphenol A is used instead of bisphenol F to prepare the solid epoxy resin.

[0041] Solid epoxy resin epoxy equivalent: 783 g / mol, softening point: 93℃.

[0042] Comparative Example 3 The rest is the same as in Example 1, except that in the preparation of solid epoxy resin, commercially available general-purpose liquid epoxy resin bisphenol A diglycidyl ether is mixed with the prepared liquid epoxy resin at a molar ratio of 3:7, and then chain-extended polymerization is carried out with bisphenol F to obtain the solid epoxy resin product.

[0043] Solid epoxy resin epoxy equivalent: 751 g / mol, softening point: 89℃.

[0044] In the performance test, the powder coating formulation was only modified by increasing the amount of dicyandiamide that accelerates curing from 24 parts to 26 parts, while all other components remained unchanged.

[0045] Comparative Example 4 Commercially available general-purpose E-12 epoxy resin was used as comparative example 4. The resin was model 604, which appeared as light yellow transparent resin granules with an epoxy equivalent of 814 g / mol and a softening point of 95℃. It was sourced from Anhui Xinyuan Technology Co., Ltd.

[0046] Performance testing The application test of the solid epoxy resin for powder coating of the present invention is based on a low-temperature curing powder coating formulation, which is as follows by weight: 560 parts of solid epoxy resin; 24 parts of accelerated dicyandiamide curing agent; 180 parts of titanium dioxide; 150 parts of barium sulfate; 5 parts leveling agent; 10 parts brightening agent; Accelerating dicyandiamide curing agent, model K7108; leveling agent, model SA88, both selected from Liuan Jietongda New Materials Co., Ltd.; gloss enhancer, model 701, selected from Huangshan Jinfeng Industrial Co., Ltd.; other raw materials are all commercially available ordinary raw materials.

[0047] Coating preparation: The solid epoxy resins of Examples 1-4 and Comparative Examples 1-4 were mixed according to the above powder coating formulation. The mixture was then extruded, pressed into sheets, and crushed using a twin-screw extruder. The sheets were then pulverized and sieved (160-180 mesh) to produce a powder coating. The powder coating was applied to a surface-treated tinplate substrate using an electrostatic spray gun, with a film thickness of approximately 80 μm. After rapid curing at 150°C for 10 min, the coating was obtained, and its performance was tested.

[0048] The coating performance testing standards are as follows: Coating performance testing was conducted according to GB / T 21776-2008 "Guideline for Testing Standards of Powder Coatings and Their Coatings". Flame retardancy testing was conducted according to GB / T2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics", using directly prepared solid epoxy resin to prepare test samples. The sample size was Type I, and the specific method was 8.2.3 Partial B-Diffusion Ignition Method. Pencil hardness testing was conducted according to GB / T6739-2006 "Determination of Hardness of Paints and Varnishes by Pencil Method". Leveling grade testing was conducted according to the PCI leveling grade, from 1 to 10, with higher numbers indicating better leveling.

[0049] The coating performance test results are shown in Table 1 below.

[0050] Table 1 Performance of Powder Coating Film

[0051] As shown in Table 1, the halogen-free flame-retardant liquid epoxy resin prepared in this invention, through a two-step chain extension reaction with bisphenol F, produces a solid epoxy resin product with good flame retardant properties, chain segment hardness, and water resistance due to the special rigid structure of the benzotriazine ring. The introduction of phosphite, while increasing the phosphorus content, not only enhances the flame retardant properties but also strengthens the chain segment lubrication and improves its leveling properties under molten conditions. The coating film, after low-temperature rapid curing (150℃ / 10min) with an accelerator dicyandiamide curing agent, exhibits a smooth and even surface with a gloss level above 94%. After passing through a 50cm positive impact test, the leveling grade is consistently 6. The leveling properties are excellent; after boiling in water for 2 hours, the coating films show no significant changes, and the hardness of all films reaches 2H or higher.

[0052] Comparative Example 1 did not use triethyl phosphite for transesterification, which resulted in a decrease in the lubricity and flame retardancy of the epoxy resin, a poor coating appearance, and a higher content of active hydroxyl groups in the final product, leading to a significant decrease in its water resistance.

[0053] Comparative Example 2 directly used bisphenol A, which has a higher rigidity, instead of bisphenol F for chain extension polymerization, resulting in excessively high rigidity of the solid epoxy resin chain segments. In terms of impact resistance, slight cracking occurred during backflush, indicating that bisphenol F is better matched with the liquid epoxy resin of this invention.

[0054] Comparative Example 3 involved mixing commercially available general-purpose liquid epoxy resin bisphenol A diglycidyl ether with the liquid epoxy resin prepared in this invention, followed by a chain extension reaction with bisphenol F. The resulting solid epoxy resin, after curing, exhibited good overall performance in the coating film. Only the gloss, flame retardancy, and hardness of the coating film showed a slight decrease, but these remained within the range of typical coating applications. This demonstrates that the halogen-free flame-retardant liquid epoxy resin prepared in this invention has excellent compatibility and can be used as a functional liquid epoxy resin monomer in the preparation of two-step solid epoxy resins, based on market customer requirements for flame retardancy and hardness. This provides significant application flexibility and market value.

[0055] Comparative Example 4 uses commercially available E-12 epoxy resin, and the coating film prepared by it has lower flame retardancy, gloss and hardness than the product prepared in this invention.

[0056] Therefore, the halogen-free flame-retardant liquid epoxy resin prepared by this invention can be used alone as an epoxy resin monomer, or it can be mixed with conventional bisphenol A type liquid epoxy resin to prepare solid epoxy resin for powder coatings, and has better market prospects and value.

[0057] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and substance defined by the claims of the present invention; and such modifications or substitutions are still within the scope defined by the claims of the present invention.

Claims

1. A halogen-free flame-retardant liquid epoxy resin, characterized by, The halogen-free flame-retardant liquid epoxy resin comprises the following raw materials by mole fraction: 2,4-diamino-6-phenyl-1,3,5-triazine 11-14 parts; Formaldehyde 22-29 parts; Acetonitrile 3-4 parts; DMF 8-11 parts; Triethyl phosphite 23-30 parts; Epichlorohydrin 27-33 parts; Sodium hydroxide 23-29 parts; The raw materials further comprise catalyst 1 and catalyst 2; Catalyst 1 is sodium methoxide, and the amount is 0.1-0.2% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine; Catalyst 2 is benzyltrimethylammonium chloride, and the amount is 0.1-0.15% of the mass of 2,4-diamino-6-phenyl-1,3,5-triazine.

2. The halogen-free flame-retardant liquid epoxy resin according to claim 1, characterized by, The formaldehyde is added in the form of formaldehyde aqueous solution, and the mass concentration of the formaldehyde aqueous solution is 37%.

3. The halogen-free flame-retardant liquid epoxy resin according to claim 1, characterized by, The mole ratio of formaldehyde to 2,4-diamino-6-phenyl-1,3,5-triazine is controlled to be 2-2.1:

1.

4. A process for the preparation of a halogen-free flame-retardant liquid epoxy resin according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: A. The formaldehyde aqueous solution, acetonitrile and 2,4-diamino-6-phenyl-1,3,5-triazine of the formula amount are added into a reaction kettle, stirring is started, the pH value of the system is adjusted to 7.5-8.0, and then the hydroxymethylation reaction is carried out by heating; B. When the conversion rate of 2,4-diamino-6-phenyl-1,3,5-triazine reaches more than 98%, the reaction is stopped, heating is carried out, and the vacuum system is started to reduce the pressure to remove the excess unreacted formaldehyde solution and solvent acetonitrile by distillation; C. When the volatile matter is less than 2%, the vacuum system is released, the formula amount of the cosolvent DMF is added into the reaction kettle, and heating is carried out to fully dissolve it, then the formula amount of triethyl phosphite and catalyst 1 are added, and the ester exchange reaction is carried out while the by-product ethanol of the ester exchange is distilled out; D. Sampling detection is carried out, when the hydroxyl value of the polymer is less than 5 mgKOH / g, heating is carried out, and the vacuum system is started again to remove the cosolvent DMF and residual triethyl phosphite by reducing the pressure; E. When the volatile matter is less than 1%, the vacuum system is released, cooling is carried out, and the formula amount of epichlorohydrin and catalyst 2 are added, after stirring and dissolving, the ring-opening reaction is continuously carried out by heating; F. When the amine value of the polymer is less than 1 mgKOH / g, cooling is carried out, then the sodium hydroxide is added in an equal amount in 3-4 batches to carry out the ring-closing reaction, after each addition, the reaction is carried out by heating, after all the sodium hydroxide is added, the ring-closing reaction is continuously carried out by heating; G. Cooling is carried out, then the generated solid sodium chloride and the excess unreacted sodium hydroxide in the polymerization system are removed by filtration, the filtrate is obtained, and the filtrate is washed with water of 5-7% of the mass of the filtrate to fully remove the residual inorganic salt, then after the static stratification, the water phase is removed, the obtained organic phase is heated, and the excess epichlorohydrin and water are removed by means of the high vacuum system, when the volatile matter is less than 1%, the vacuum system is stopped, the material is discharged at high temperature, and the liquid epoxy resin is obtained.

5. The production method according to claim 4, characterized by, In step A, the temperature of the hydroxymethylation reaction is 57-62℃; In step B, the temperature is increased to 100-105℃, and the vacuum degree is controlled to be-0.095 to-0.098 Mpa; In step C, the temperature is increased to 120-125℃; In Step D, the temperature is raised to 130-135℃; the vacuum degree is controlled at -0.095 to -0.098 Mpa.

6. The preparation method according to claim 4, characterized in that, In Step E, the temperature is lowered to 90-95℃; In Step F, the temperature is lowered to 35-40℃; after each addition, the reaction is kept for 10-15 min; after all the additions, the reaction is kept for 0.5-1 h for the ring closure reaction. In Step G, the temperature is lowered to 5-10℃; the filtrate is washed for 20-30 min; the organic phase is heated to 90-95℃; the vacuum degree is controlled at -0.097 Mpa to -0.099 Mpa.

7. A solid epoxy resin for powder coating, characterized by, The halogen-free flame-retardant liquid epoxy resin of any one of claims 1-3 or the halogen-free flame-retardant liquid epoxy resin prepared by the preparation method of any one of claims 4-6 is subjected to chain extension polymerization with bisphenol F.

8. The solid epoxy resin for powder coatings according to claim 7, characterized in that, The molar ratio of the halogen-free flame-retardant liquid epoxy resin to bisphenol F is 2:1.05-1.

15.

9. The solid epoxy resin for powder coatings according to claim 7, characterized in that, The chain extension polymerization is carried out in the presence of catalyst 3, which is triphenylphosphine, and the amount of catalyst 3 is 0.05-0.07% of the mass of bisphenol F.

10. The solid epoxy resin for powder coatings according to claim 7, characterized in that, The preparation method of the powder coating solid epoxy resin comprises the following steps: The halogen-free flame-retardant liquid epoxy resin, catalyst 3 and bisphenol F are added into a polymerization reactor, stirring is started and the temperature is raised to 127-130℃ for chain extension polymerization, until the reaction polymer becomes completely transparent, then the reaction is kept for another 20-30 min, and every 10 min, a sample is taken for free bisphenol F content detection by liquid chromatography, when the free bisphenol F content is less than 0.5%, the reaction is stopped, and the material is discharged at the current temperature, and the material is cooled by a steel belt and crushed to obtain a powder coating solid epoxy resin product.

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