Low cost, high hardness epoxy resin for powder coating and method for its preparation

Abstract

The present application relates to a kind of low cost, high hardness epoxy resin for powder coating, which is obtained by reaction of alkali lignin, ethylene glycol monobutyl ether, resorcinol, 3-ethyl-3-methyl glutaric acid, epichlorohydrin, sodium hydroxide, bisphenol A and other raw materials. The present application not only obtains high hardness, excellent comprehensive performance pure epoxy powder coating epoxy resin product, but also provides an industrialization scheme for the comprehensive application of alkali lignin, with better market prospect and application value.

Description

Technical Field

[0001] This invention relates to the field of epoxy resin technology, specifically to a low-cost, high-hardness epoxy resin for powder coatings and its preparation method. Background Technology

[0002] Currently, lignin, as a byproduct of processing various wood and cellulose materials such as timber, bagasse, and corn stalk residue, is produced in large quantities. Alkaline lignin, commonly known as xylose powder, is a major component of wastewater from alkaline pulping and papermaking, produced in large quantities. Its alkalinity makes it difficult to use, offering almost no added value compared to ordinary lignin or high-grade enzymatically hydrolyzed lignin. How to comprehensively utilize it is a challenge for the industry. Powder coatings, as a new type of environmentally friendly coating, have a series of advantages including no VOC emissions, recyclability, excellent corrosion resistance, and good decorative properties, and have developed rapidly in recent years. Epoxy powder coatings have become a major type of powder coating due to their excellent comprehensive performance. However, compared to polyester powder coatings or hybrid powder coatings, epoxy powder coatings have a higher formulation cost due to the higher cost of E-12 epoxy resin and its larger usage in the formulation, resulting in a disadvantage in market promotion. Therefore, developing a low-cost epoxy resin suitable for pure epoxy powder coating systems is of great significance. Summary of the Invention

[0003] To address the issues of excess and difficult application of alkali lignin, and the high cost of E-12 epoxy resin in conventional pure epoxy powder coatings, this invention combines the inherent alkalinity of alkali lignin with the requirement of a strongly alkaline environment in epoxy resin synthesis, demonstrating good compatibility between the two. This invention utilizes alkali lignin, ethylene glycol monobutyl ether, resorcinol, 3-ethyl-3-methylglutaric acid, epichlorohydrin, sodium hydroxide, and bisphenol A as raw materials through chain extension polymerization and other reactions. First, alkali lignin, ethylene glycol monobutyl ether, and resorcinol undergo high-temperature phenolation treatment to obtain a phenolized solution. After separation and purification, small-molecule alkali lignin components with phenolic depolymerization are obtained. These components undergo a preliminary epoxidation reaction with epichlorohydrin and sodium hydroxide. Then, 3-ethyl-3-methylglutaric acid, which adjusts the impact toughness and water resistance of the molecular chain, participates in the epoxidation polymerization reaction. Finally, a secondary chain extension reaction with bisphenol A yields the final epoxy resin product for powder coatings. This invention not only yields epoxy resin products for pure epoxy powder coatings with high hardness and excellent comprehensive performance, but also provides an industrializable solution for the comprehensive application of alkali lignin, with superior market prospects and application value.

[0004] A low-cost, high-hardness epoxy resin for powder coatings, comprising the following raw materials in parts by weight:

[0005]

[0006] The raw materials also include catalysts.

[0007] Preferably, the catalyst is triphenylphosphine, and the amount used is 0.5-0.8% of the mass of bisphenol A. Sodium hydroxide needs to be dissolved first to form an aqueous solution with a mass concentration of 37-40% before use.

[0008] The preparation method of a low-cost, high-hardness epoxy resin for powder coatings, as described above, includes the following steps:

[0009] A. Add the prescribed amount of ethylene glycol monobutyl ether and resorcinol to the reaction vessel, start stirring, and after mixing evenly, raise the temperature to 130-135℃. Then add the prescribed amount of alkali lignin and continue to keep warm to carry out full high-temperature phenolization treatment.

[0010] B. When the alkali lignin in the system no longer dissolves, lower the temperature to 50-60℃, and then perform vacuum filtration to obtain the filtered and purified phenolized liquid and the alkali lignin residue that cannot be phenolized.

[0011] C. Pump the phenolic filtrate into the reaction vessel, heat it to 130-135℃, and at the same time start the vacuum system to remove the ethylene glycol monobutyl ether solvent under reduced pressure.

[0012] D. After no obvious solvent has been removed, cool the temperature to 100-105℃, add the prescribed amount of epichlorohydrin, and let it dissolve and disperse evenly. Then add the prescribed amount of sodium hydroxide aqueous solution to carry out the epoxidation polymerization reaction.

[0013] E. Take a sample and use gas chromatography to detect the content of epichlorohydrin. When the conversion rate of epichlorohydrin reaches 52-60%, add the amount of 3-ethyl-3-methylglutaric acid in the formula and continue the epoxy polymerization reaction at 100-105℃.

[0014] F. Sampling and gas chromatography detection: When the conversion rate of epichlorohydrin reaches 95% or more, it indicates that the epoxy resin prepolymer has been basically synthesized. Cool down to 90-95℃, separate the aqueous phase by standing, and then add the formulated amount of bisphenol A and the catalyst triphenylphosphine to the organic phase prepolymer. Heat up to 120-125℃ to carry out the prepolymer chain extension polymerization reaction.

[0015] G. Take samples to test the epoxy equivalent of the epoxy resin. When the epoxy equivalent of the epoxy resin reaches 790-850 g / mol, stop the reaction, cool down to 90-95℃, and then wash with boiling water.

[0016] H. When the pH value of the epoxy resin reaches 7-7.5, raise the temperature to 115-120℃ and start the vacuum system to fully remove small molecule compounds. When the volatile content is less than 1%, release the vacuum system, discharge the material while it is hot, cool the epoxy resin, and then crush and granulate it to obtain epoxy resin.

[0017] Preferably, in step C, the vacuum degree is controlled between -0.097 MPa and -0.099 MPa; in step H, the vacuum degree is controlled between -0.098 MPa and -0.099 MPa; in step G, boiling water is added for washing 3-4 times, with the amount of boiling water added each time being one-fifth of the total mass of the material, in order to remove water-soluble impurities including sodium chloride and sodium hydroxide; in step H, the epoxy resin is cooled with a steel belt containing condensate.

[0018] The method described above for preparing a low-cost, high-hardness epoxy resin for powder coatings yields pale yellow transparent granules of epoxy resin with an epoxy equivalent of 790-850 g / mol and a softening point of 87-95℃.

[0019] For example, a method for preparing a low-cost, high-hardness epoxy resin for powder coatings includes the following steps:

[0020] A. Add the prescribed amount of ethylene glycol monobutyl ether and resorcinol to the reaction vessel, start stirring, and after mixing evenly, raise the temperature to 130-135℃. Then add the prescribed amount of alkali lignin and continue to keep warm to carry out full high-temperature phenolization treatment.

[0021] B. When the alkali lignin in the system no longer dissolves, it indicates that the phenolation is basically complete. At this time, reduce the temperature to 50-60℃ and then perform vacuum filtration to obtain the filtered and purified phenolized liquid and the alkali lignin residue that cannot be phenolized.

[0022] C. Pump the phenolic filtrate into the reactor, heat it to 130-135℃, and at the same time start the vacuum system to remove the ethylene glycol monobutyl ether solvent under reduced pressure. The vacuum degree is controlled at -0.097Mpa to -0.099Mpa.

[0023] D. After no obvious solvent has been removed (less than 1 drop within 20 seconds), cool down to 100-105℃, add the prescribed amount of epichlorohydrin, and let it dissolve and disperse evenly. Then add the prescribed amount of sodium hydroxide aqueous solution to carry out the epoxidation polymerization reaction.

[0024] E. Take a sample and use gas chromatography to detect the content of epichlorohydrin. When the conversion rate of epichlorohydrin reaches 52-60%, add the amount of 3-ethyl-3-methylglutaric acid in the formula and continue the epoxy polymerization reaction at 100-105℃.

[0025] F. Sampling and gas chromatography detection: When the conversion rate of epichlorohydrin reaches 95% or more, it indicates that the epoxy resin prepolymer has been basically synthesized. Cool down to 90-95℃, separate the aqueous phase by standing, and then add the formulated amount of bisphenol A and the catalyst triphenylphosphine to the organic phase prepolymer. Heat up to 120-125℃ to carry out the prepolymer chain extension polymerization reaction.

[0026] G. Take samples to test the epoxy equivalent of the epoxy resin. When the epoxy equivalent of the epoxy resin reaches 790-850 g / mol, stop the reaction, cool down to 90-95℃, and then add boiling water for 3-4 washes. Each time, the amount of boiling water added is one-fifth of the total mass of the material to remove water-soluble impurities such as sodium chloride and sodium hydroxide.

[0027] H. When the pH value of the epoxy resin reaches 7-7.5, the washing is complete. At this time, the temperature is raised to 115-120℃, and the vacuum system is started to fully remove small molecule compounds, including epichlorohydrin and water. The vacuum degree is controlled at -0.098Mpa to -0.099Mpa. When the volatile content is less than 1%, the vacuum system is released, and the material is discharged while hot. The epoxy resin is cooled with a steel belt with condensate, and then crushed and granulated to obtain epoxy resin.

[0028] The resulting product is a pale yellow transparent granule with an epoxy equivalent of 790-850 g / mol and a softening point of 87-95℃.

[0029] The epoxy resin described above or the epoxy resin obtained by the preparation method described above is used as a pure epoxy resin coating.

[0030] For example, a pure epoxy powder coating formulation includes the epoxy resin described in this invention, dicyandiamide curing agent, titanium dioxide, barium sulfate, leveling agent, gloss enhancer, benzoin, etc. The materials are mixed according to the pure epoxy powder coating formulation, extruded using a twin-screw extruder, pressed into sheets, crushed, and then the sheets are pulverized and sieved to produce a powder coating. The powder coating is sprayed onto a surface-treated tinplate substrate using an electrostatic spray gun, and cured to obtain the coating layer.

[0031] Beneficial effects:

[0032] This invention relates to a low-cost, high-hardness epoxy resin for powder coatings and its preparation method. First, alkali lignin, ethylene glycol monobutyl ether, and resorcinol are subjected to high-temperature phenolation treatment to obtain a phenolic liquid. After separation and purification, small-molecule alkali lignin components that have undergone phenol depolymerization are obtained. This alkali lignin undergoes a preliminary epoxidation reaction with epichlorohydrin and sodium hydroxide. Then, 3-ethyl-3-methylglutaric acid, which adjusts the impact toughness and water resistance of the molecular chain, participates in the epoxidation polymerization reaction. Finally, a secondary chain extension reaction is performed with bisphenol A to obtain the final epoxy resin product for powder coatings. This invention employs resorcinol to phenolize and purify alkali lignin, removing some insoluble substances that cannot be phenolized. This lays the foundation for using alkali lignin to prepare epoxy resins for powder coatings. The introduction of 3-ethyl-3-methylglutaric acid not only effectively regulates the excessively high phenolic hydroxyl functionality in alkali lignin, enabling the coating film to achieve the required impact toughness, but also improves the water resistance of the coating film by introducing ethyl and methyl branches, making its water resistance meet the application requirements of powder coatings. This invention not only yields a pure epoxy resin product for powder coatings with high hardness and excellent comprehensive performance, but also provides an industrially feasible solution for the comprehensive application of alkali lignin, possessing superior market prospects and application value. Detailed Implementation

[0033] The present invention will be further described below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0034] All raw materials described in this invention are commercially available.

[0035] Preparation Example 1

[0036] A low-cost, high-hardness epoxy resin for powder coatings, comprising the following raw materials in parts by weight:

[0037]

[0038]

[0039] The catalyst is triphenylphosphine, and the amount used is 0.8% of the mass of bisphenol A.

[0040] The method for preparing the low-cost, high-hardness epoxy resin for powder coatings includes the following steps:

[0041] A. Add the prescribed amount of ethylene glycol monobutyl ether and resorcinol to the reaction vessel, start stirring, and after mixing evenly, raise the temperature to 135℃, then add the prescribed amount of alkali lignin, and continue to keep warm to carry out full high-temperature phenolization treatment.

[0042] B. When the alkali lignin in the system no longer dissolves, it indicates that the phenolation is basically complete. At this time, the temperature is reduced to 60°C, and then the filter is performed to obtain the filtered and purified phenolized liquid and the alkali lignin residue that cannot be phenolized.

[0043] C. Pump the phenolic filtrate into the reactor, heat it to 135°C, and simultaneously start the vacuum system to remove the ethylene glycol monobutyl ether solvent under reduced pressure, with the vacuum degree controlled at -0.099 MPa.

[0044] D. After no obvious solvent has been removed (less than 1 drop within 20 seconds), cool down to 105°C, add the prescribed amount of epichlorohydrin, and let it dissolve and disperse evenly. Then add the prescribed amount of sodium hydroxide aqueous solution to carry out the epoxidation polymerization reaction.

[0045] E. Take a sample and use gas chromatography to detect the content of epichlorohydrin. When the conversion rate of epichlorohydrin reaches 60%, add the amount of 3-ethyl-3-methylglutaric acid in the formula and continue the epoxy polymerization reaction at 105℃.

[0046] F. Sampling and gas chromatography detection. When the conversion rate of epichlorohydrin reaches 99%, it indicates that the prepolymer of epoxy resin has been basically synthesized. Cool down to 93°C, separate the aqueous phase by standing, and then add the formulated amount of bisphenol A and the catalyst triphenylphosphine to the organic phase prepolymer. Heat up to 124°C to carry out the chain extension polymerization reaction of the prepolymer.

[0047] G. Take samples to test the epoxy equivalent of the epoxy resin. When the epoxy equivalent of the epoxy resin reaches the expected value, stop the reaction, cool down to 90-95℃, and then add boiling water for 3-4 washes. Each time, add boiling water in an amount of one-fifth of the total mass of the material to remove water-soluble impurities such as sodium chloride and sodium hydroxide.

[0048] H. When the pH value of the epoxy resin reaches 7.5, the washing is complete. At this time, the temperature is raised to 120℃, and the vacuum system is started to fully remove small molecule compounds, including epichlorohydrin and water. The vacuum degree is controlled at -0.099Mpa. When the volatile content is less than 1%, the vacuum system is released, and the material is discharged while hot. The epoxy resin is cooled with a steel belt with condensate, and then crushed and granulated to obtain epoxy resin.

[0049] The resulting product is a pale yellow transparent granule with an epoxy equivalent of 793 g / mol and a softening point of 87℃.

[0050] Preparation Example 2

[0051] A low-cost, high-hardness epoxy resin for powder coatings, comprising the following raw materials in parts by weight:

[0052]

[0053] The catalyst is triphenylphosphine, and the amount used is 0.8% of the mass of bisphenol A.

[0054] The preparation method is the same as in Preparation Example 1.

[0055] The resulting product is a pale yellow transparent granule with an epoxy equivalent of 848 g / mol and a softening point of 94℃.

[0056] Preparation Example 3

[0057] A low-cost, high-hardness epoxy resin for powder coatings, comprising the following raw materials in parts by weight:

[0058]

[0059] The catalyst is triphenylphosphine, and the amount used is 0.7% of the mass of bisphenol A.

[0060] The preparation method is the same as in Preparation Example 1.

[0061] The resulting product is a pale yellow transparent granule with an epoxy equivalent of 824 g / mol and a softening point of 90℃.

[0062] Preparation Example 4

[0063] A low-cost, high-hardness epoxy resin for powder coatings, comprising the following raw materials in parts by weight:

[0064]

[0065] The catalyst is triphenylphosphine, and the amount used is 0.8% of the mass of bisphenol A.

[0066] The preparation method is the same as in Preparation Example 1.

[0067] The resulting product is a pale yellow transparent granule with an epoxy equivalent of 828 g / mol and a softening point of 92℃.

[0068] Comparative Preparation Example 1

[0069] Example 1 uses commercially available E-12 epoxy resin as a comparative preparation.

[0070] Examples 1-4 and Comparative Example 1 (obtained from the epoxy resins prepared in Examples 1-4 and Comparative Example 1, respectively)

[0071] The pure epoxy powder coating formulation, by weight, is as follows:

[0072]

[0073] The dicyandiamide curing agent used is a product of Liuan Jietongda New Materials Co., Ltd., model SA2830.

[0074] Coating preparation: All materials are mixed according to the pure epoxy powder coating formula, extruded, sheeted, and crushed using a twin-screw extruder. The sheet material is then pulverized and sieved (160-180 mesh) to produce powder coating. The powder coating is then sprayed onto a surface-treated tinplate substrate using an electrostatic spray gun, with a film thickness of approximately 80 μm. After curing at 180℃ for 15 min, the coating is obtained.

[0075] Performance Comparison

[0076] The coating index testing is based on GB / T 21776-2008 "Guideline for Testing Standards of Powder Coatings and Their Coatings". Pencil hardness testing is conducted according to GB / T 6739-2006 "Determination of Hardness of Paints and Varnishes by Pencil Method". The pencil hardness parameters are 2B-B-HB-FH-2H-3H-4H-5H, with the hardness grade gradually increasing from left to right.

[0077] The epoxy resins prepared in the above embodiments and comparative examples were used to prepare coatings according to the coating formulations provided by the present invention. The coating performance results are shown in Table 1 below.

[0078] Table 1 Performance of Pure Epoxy Powder Coating Film

[0079]

[0080] As shown in Table 1, the epoxy resin product prepared using a special raw material formulation and process can replace commercially available ordinary E-12 epoxy resin in pure epoxy powder coatings. The resulting coating film has a smooth and even surface, and meets application requirements in both forward and reverse impact tests at 50cm. The gloss level is generally above 93%. Due to the use of alkali lignin as a synthetic raw material, the hardness of the coating film is generally 2H or higher, exceeding that of powder coatings prepared with ordinary E-12 epoxy resin. Its boiling water resistance also meets application requirements. This invention utilizes a special raw material formulation and preparation process, achieving comprehensive utilization of alkali lignin and reducing the cost of epoxy resin for powder coatings, which is beneficial for further expanding the application market of pure epoxy powder coatings.

[0081] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above description is illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a low-cost, high-hardness epoxy resin for powder coatings, characterized in that, Including the following raw materials in parts by weight: 10-13 parts of alkali lignin; 15-20 parts of ethylene glycol monobutyl ether; 8-11 parts of resorcinol; 13-17 parts of 3-ethyl-3-methylglutaric acid; 25-30 parts of epichlorohydrin; 13-16 parts sodium hydroxide; Bisphenol A 4-5 parts; The catalyst is triphenylphosphine, and its dosage is 0.5-0.8% of the mass of bisphenol A. Sodium hydroxide is dissolved into a sodium hydroxide aqueous solution with a mass concentration of 37-40% for use. The preparation method includes the following steps: A. Add the prescribed amount of ethylene glycol monobutyl ether and resorcinol to the reaction vessel, start stirring, and after mixing evenly, raise the temperature to 130-135℃. Then add the prescribed amount of alkali lignin and continue to keep warm to carry out full high-temperature phenolization treatment. B. When the alkali lignin in the system no longer dissolves, the temperature is lowered to 50-60℃, and then the system is filtered to obtain the purified phenolic liquid and the alkali lignin residue that cannot be phenolized. C. Pump the phenolic filtrate into the reaction vessel, heat it to 130-135℃, and at the same time start the vacuum system to remove the ethylene glycol monobutyl ether solvent under reduced pressure. D. After no obvious solvent has been removed, cool the temperature to 100-105℃, add the prescribed amount of epichlorohydrin, and let it dissolve and disperse evenly. Then add the prescribed amount of sodium hydroxide aqueous solution to carry out the epoxidation polymerization reaction. E. Take a sample and use gas chromatography to detect the content of epichlorohydrin. When the conversion rate of epichlorohydrin reaches 52-60%, add the amount of 3-ethyl-3-methylglutaric acid in the formula and continue the epoxy polymerization reaction at 100-105℃. F. Sampling and gas chromatography detection. When the conversion rate of epichlorohydrin reaches more than 95%, the temperature is lowered to 90-95℃, the aqueous phase is separated by static separation, and then the formula amount of bisphenol A and the catalyst triphenylphosphine are added to the organic phase prepolymer. The temperature is raised to 120-125℃ to carry out the chain extension polymerization reaction of the prepolymer. G. Take samples to test the epoxy equivalent of the epoxy resin. When the epoxy equivalent of the epoxy resin reaches 790-850 g / mol, stop the reaction, cool down to 90-95℃, and then wash with boiling water. H. When the pH value of the epoxy resin reaches 7-7.5, the temperature is raised to 115-120℃, and the vacuum system is started at the same time to fully remove small molecule compounds. When the volatile content is less than 1%, the vacuum system is released, the material is discharged while hot, and the epoxy resin is cooled. Then it is crushed and granulated to obtain epoxy resin.

2. The method for preparing a low-cost, high-hardness epoxy resin for powder coatings as described in claim 1, characterized in that, In step C, the vacuum level is controlled between -0.097 MPa and -0.099 MPa; in step H, the vacuum level is controlled between -0.098 MPa and -0.099 MPa.

3. The method for preparing a low-cost, high-hardness epoxy resin for powder coatings as described in claim 1, characterized in that, In step G, boiling water is added for washing 3-4 times, with the amount of boiling water added each time being one-fifth of the total mass of the material, in order to remove water-soluble impurities, including sodium chloride and sodium hydroxide.

4. The method for preparing a low-cost, high-hardness epoxy resin for powder coatings as described in claim 1, characterized in that, In step H, the epoxy resin is cooled with a steel strip containing condensate.

5. The method for preparing a low-cost, high-hardness epoxy resin for powder coatings as described in claim 1, characterized in that, The resulting epoxy resin consists of pale yellow transparent granules with an epoxy equivalent of 790-850 g / mol and a softening point of 87-95℃.

6. The low-cost, high-hardness epoxy resin for powder coatings obtained by the preparation method according to any one of claims 1-5.

7. Use of the epoxy resin as described in claim 6 as a pure epoxy resin coating.

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