Powder coating for wood surface and preparation method thereof

Abstract

The invention discloses a powder coating for wood surface and a preparation method thereof, belonging to the technical field of powder coatings. The powder coating comprises the following raw materials in parts by weight: 400-450 parts of a low-temperature curing polyester resin, 300 parts of an E12 epoxy resin, 2-3 parts of a leveling agent, 10-15 parts of a degassing agent, 3-4 parts of a charging agent, 10-15 parts of a hardening wax powder and 50-60 parts of a modified curing agent; the powder coating comprises the following steps of preparing the powder coating: adding the raw materials in parts by weight into a mixer for premixing, then transferring the raw materials into an extruder for melt extrusion at 80 DEG C, crushing the raw materials and passing through a 200-mesh sieve to prepare the powder coating for wood surface; the powder coating for wood surface prepared by the invention can be quickly cured at low temperature and has excellent storage stability; the coating obtained after coating exhibits excellent adhesion, hardness, water resistance and impact resistance, and has broad application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of powder coatings, and in particular relates to a powder coating for wood surface and a preparation method thereof. Background Art

[0002] Powder coating is an environmentally friendly coating that does not contain organic solvents. Its physical state is 100% solid powder. Powder coating is dispersed in the air and evenly coated on the surface of the workpiece by electrostatic adsorption. After high-temperature curing, it forms a film layer with specific uses. Most traditional powder coatings are high-temperature powder coatings, which need to be leveled and cured at high temperatures of 180-200°C. In addition to high energy consumption, the wood substrate is very prone to cracking, warping, and reduced mechanical strength at high temperatures. Therefore, traditional powder coatings cannot be directly electrostatically sprayed on the wood surface and leveled and cured at high temperatures, otherwise the high temperature will damage the wood.

[0003] Polyester resin and epoxy resin are the main raw materials of powder coatings. Powder coatings made of them have the advantages of good leveling performance, easy texture and color adjustment, and a wide range of material selection. At present, the market has higher and higher requirements for powder coatings for wood surfaces, which not only require low curing temperature (130-140℃), but also require fast curing speed (within 10 minutes). Therefore, it is necessary to improve the existing powder coatings for wood to meet the current market demand for low-temperature fast curing. Summary of the invention

[0004] The object of the present invention is to provide a powder coating for wood surface and a preparation method thereof, so as to solve the problems in the background technology.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A powder coating for wood surface, comprising the following raw materials in parts by weight:

[0007] 400-450 parts of low-temperature curing polyester resin, 300 parts of E12 epoxy resin, 2-3 parts of leveling agent, 10-15 parts of degassing agent, 3-4 parts of charging agent, 10-15 parts of hardening wax powder and 50-60 parts of modified curing agent; the leveling agent is any one of acrylate homopolymer, acrylate copolymer and polysiloxane; the degassing agent is benzoic acid ketone; the charging agent is quaternary ammonium salt;

[0008] The modified curing agent is prepared by the following steps:

[0009] After 1,4-butanediol and polytetrahydrofuran are stirred and mixed, the mixture is dehydrated at 160°C for 2 minutes, the temperature is lowered to 80°C, a polyisocyanate compound is added, the mixture is stirred and reacted for 3-5 hours, and the mixture is cooled to room temperature to obtain a polyurethane prepolymer; modified imidazole is added to the polyurethane prepolymer and stirred and mixed evenly, and then the mixture is added into deionized water and stirred and dispersed for 30 minutes, and then an aqueous solution of ethylenediamine is added dropwise, the mixture is reacted at room temperature for 1.5 hours, and a modified curing agent is obtained after freeze-drying.

[0010] Furthermore, the low temperature curing polyester resin is prepared by the following steps:

[0011] Neopentyl glycol, 1,4-cyclohexanedimethanol and monobutyltin oxide are stirred and mixed, and then terephthalic acid is added, and an esterification reaction is carried out at 240°C for 3-4 hours under nitrogen protection, and then a polycondensation reaction is carried out at -0.09MPa for 3 hours, and then dodecanedioic acid is added to continue the end-capping reaction at 240°C for 3 hours, and the material is discharged after cooling to room temperature to obtain a low-temperature curing polyester resin. Under the catalysis of monobutyltin oxide, neopentyl glycol and 1,4-cyclohexanedimethanol are subjected to an esterification polycondensation reaction with terephthalic acid, and then end-capped with dodecanedioic acid to obtain a low-temperature curing polyester resin containing a saturated six-membered ring structure and a terminal carboxyl group; the polyester resin containing a saturated six-membered ring structure can obtain a relatively compact coating structure, so that the prepared powder coating has a high degree of curing, excellent storage stability and water resistance.

[0012] Furthermore, the mass ratio of the neopentyl glycol, 1,4-cyclohexanedimethanol, monobutyltin oxide, terephthalic acid and dodecanedioic acid is 30-35g:4-5g:0.15g:50-54g:5g.

[0013] Furthermore, the usage ratio of the 1,4-butanediol, polytetrahydrofuran, polyisocyanate compound, modified imidazole, deionized water and ethylenediamine aqueous solution is 0.85g: 0.8-0.9g: 15-18g: 4-5g: 80-100g: 10mL; the concentration of the ethylenediamine aqueous solution is 20wt%.

[0014] Furthermore, the polyisocyanate compound is prepared by the following steps:

[0015] Step A1, adding 4,4'-dihydroxybenzophenone, dicyclohexylmethane-4,4'-diisocyanate and triethylenediamine to tetrahydrofuran, stirring and reacting at 45°C for 24 hours, cooling to room temperature and filtering, collecting the precipitate, and vacuum drying at 60°C for 1 hour to obtain an intermediate;

[0016] Step A2: Dissolve the intermediate in N,N-dimethylacetamide, then dropwise add isopropanol and acetic acid, and irradiate under nitrogen protection and ultraviolet light (395 nm, 6 mW / cm 3) for 5-7 days, then poured into n-hexane to generate a precipitate, which was collected by filtration to obtain a polyisocyanate compound.

[0017] Furthermore, the usage ratio of the 4,4'-dihydroxybenzophenone, dicyclohexylmethane-4,4'-diisocyanate, triethylenediamine and tetrahydrofuran is 10-10.5 g: 45-49 g: 0.01 g: 80-100 mL.

[0018] Furthermore, the usage ratio of the intermediate, N,N-dimethylacetamide, isopropanol and acetic acid is 20-25 g: 100 mL: 50 mL: 0.15 mL.

[0019] Furthermore, the modified imidazole is prepared by the following method:

[0020] Add 2-ethyl-4-methylimidazole to xylene, stir and mix, and heat to 65°C, then drop hexamethylene diisocyanate, keep warm and stir to react for 6 hours, and remove xylene by vacuum distillation to obtain modified imidazole. The isocyanate groups at both ends of hexamethylene diisocyanate react with the imino groups in 2-ethyl-4-methylimidazole to obtain a modified imidazole with low water solubility, which can be better coated.

[0021] Furthermore, the usage ratio of xylene, 2-ethyl-4-methylimidazole and hexamethylene diisocyanate is 10g:10-11g:7.5-8g.

[0022] A method for preparing a powder coating for a wood surface comprises the following steps:

[0023] Low-temperature curing polyester resin, E12 epoxy resin, leveling agent, degassing agent, charging agent, hardening wax powder and modified curing agent are added into a mixer for premixing, then transferred into an extruder for melt extrusion at 80° C., crushed and passed through a 200-mesh sieve to obtain a powder coating for wood surface.

[0024] Beneficial effects:

[0025] The present invention uses neopentyl glycol and 1,4-cyclohexanedimethanol in combination, thereby introducing a certain amount of saturated six-membered ring structure into the low-temperature curing polyester resin. On the one hand, the saturated six-membered ring has good thermal stability, can reduce the generation of oligomers, and make the prepared powder coating have better storage stability. On the other hand, the saturated six-membered ring structure can make the coating structure more compact, and can improve the water resistance and hardness of the coating. The terminal carboxyl group of the low-temperature curing polyester resin of the present invention has high reactivity, and it is used together with the epoxy resin as a resin raw material, which lays a basic condition for the low-temperature rapid curing of the powder coating.

[0026] The invention uses 4,4'-dihydroxybenzophenone and dicyclohexylmethane-4,4'-diisocyanate as raw materials, and reacts the hydroxyl group of 4,4'-dihydroxybenzophenone with the isocyanate group of dicyclohexylmethane-4,4'-diisocyanate under the catalysis of triethylenediamine to obtain an intermediate having isocyanate groups at both ends and a symmetrical structure, and then uses the intermediate as a raw material to capture hydrogen atoms from isopropanol to form hydroxyl groups under ultraviolet light irradiation and acetic acid catalysis, and then obtains a polyisocyanate compound containing a benzopinacol structure through dimerization; and then uses the polyisocyanate compound, polytetrahydrofuran, 1,4-butanediol and modified imidazole as raw materials, and adopts an interfacial polymerization method to prepare a modified curing agent with a waterborne polyurethane as a shell and a modified imidazole as a core, and through the coating of the waterborne polyurethane shell layer, not only can the modified curing agent be effectively The modified imidazole is isolated from the resin substrate, thereby greatly improving the storage stability of the powder coating, and the waterborne polyurethane shell layer has good compatibility with the resin substrate, so that the modified curing agent can be more evenly dispersed in the resin substrate; in addition, the benzopinacol structure contained in the modified curing agent shell layer is stable at room temperature or relatively low temperature, and can well wrap the modified imidazole therein without losing it, and can be thermally decomposed into a diphenylmethanol structure at above 100°C, thereby destroying the cross-linked network structure of the shell layer, and the modified imidazole therein can be quickly released, accelerating the low-temperature curing speed of the resin substrate, and in the cooling process after the coating is cured, the diphenylmethanol structures containing free radicals can be reorganized into the original benzopinacol structure to re-form the shell material, which plays a toughening role in the coating, thereby improving the impact resistance of the coating;

[0027] The invention uses the synergistic effect of the low-temperature curing polyester resin and the modified curing agent to enable the prepared powder coating to be quickly cured at a low temperature of 130°C, thereby being suitable for wood surfaces; in particular, the preparation of the modified curing agent solves the contradiction between the high reactivity and storage stability of the powder coating, and the finally prepared powder coating has excellent comprehensive properties such as adhesion, hardness, water resistance and impact resistance when used, and has broad application prospects. DETAILED DESCRIPTION

[0028] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] This embodiment provides a low temperature curing polyester resin, which is prepared by the following steps:

[0031] 30g of neopentyl glycol, 4g of 1,4-cyclohexanedimethanol and 0.15g of monobutyltin oxide were stirred and mixed, and then 50g of terephthalic acid was added, and an esterification reaction was carried out at 240°C for 3h under nitrogen protection, and then a condensation reaction was carried out at -0.09MPa for 3h, and then 5g of dodecanedioic acid was added and the end-capping reaction was continued at 240°C for 3h. After cooling to room temperature and discharging, a low-temperature curing polyester resin was obtained.

[0032] Example 2

[0033] This embodiment provides a low temperature curing polyester resin, which is prepared by the following steps:

[0034] 32g of neopentyl glycol, 4.5g of 1,4-cyclohexanedimethanol and 0.15g of monobutyltin oxide were stirred and mixed, and then 52g of terephthalic acid was added, and the esterification reaction was carried out at 240°C for 4h under nitrogen protection, and then the polycondensation reaction was carried out at -0.09MPa for 3h, and then 5g of dodecanedioic acid was added and the end-capping reaction was continued at 240°C for 3h. After cooling to room temperature and discharging, a low-temperature curing polyester resin was obtained.

[0035] Example 3

[0036] This embodiment provides a low temperature curing polyester resin, which is prepared by the following steps:

[0037] 35g of neopentyl glycol, 5g of 1,4-cyclohexanedimethanol and 0.15g of monobutyltin oxide were stirred and mixed, and then 54g of terephthalic acid was added, and the esterification reaction was carried out at 240°C under nitrogen protection for 4h, and then the polycondensation reaction was carried out at -0.09MPa for 3h, and then 5g of dodecanedioic acid was added and the end-capping reaction was continued at 240°C for 3h. After cooling to room temperature and discharging, a low-temperature curing polyester resin was obtained.

[0038] Comparative Example 1

[0039] This comparative example is different from Example 3 in that an equal amount of neopentyl glycol is used to replace 1,4-cyclohexanedimethanol, and the other raw materials and steps are the same.

[0040] Example 4

[0041] This embodiment provides a modified curing agent, which is prepared by the following steps:

[0042] Step A1, add 10 g of 4,4'-dihydroxybenzophenone, 45 g of dicyclohexylmethane-4,4'-diisocyanate and 0.01 g of triethylenediamine to 80 mL of tetrahydrofuran, stir and react at 45°C for 24 hours, cool to room temperature and filter, collect the precipitate, and vacuum dry at 60°C for 1 hour to obtain an intermediate;

[0043] Step A2: 20 g of the intermediate was dissolved in 100 mL of N,N-dimethylacetamide, and then 50 mL of isopropanol and 0.15 mL of acetic acid were added dropwise. The mixture was stirred under nitrogen protection and ultraviolet light (395 nm, 6 mW / cm 3 ) was irradiated and reacted for 5 days, then poured into n-hexane to generate a precipitate, and the precipitate was collected by filtration to obtain a polyisocyanate compound;

[0044] Step A3, add 10 g of 2-ethyl-4-methylimidazole to 10 g of xylene, stir and mix, and heat to 65° C., then dropwise add 7.5 g of hexamethylene diisocyanate, stir and react for 6 hours, and remove xylene by vacuum distillation to obtain modified imidazole;

[0045] Step A4, 0.85g of 1,4-butanediol and 0.8g of polytetrahydrofuran were stirred and mixed, and then dehydrated at 160°C for 2min, cooled to 80°C, 15g of polyisocyanate compound was added, stirred and reacted for 3h, and cooled to room temperature to obtain a polyurethane prepolymer; 4g of modified imidazole was added to the polyurethane prepolymer and stirred and mixed evenly, and then added to 80g of deionized water and stirred and dispersed for 30min, and then 10mL of 20wt% ethylenediamine aqueous solution was added dropwise, and the mixture was reacted at room temperature for 1.5h. After freeze-drying, a modified curing agent was obtained.

[0046] Example 5

[0047] This embodiment provides a modified curing agent, which is prepared by the following steps:

[0048] Step A1, add 10.2 g of 4,4'-dihydroxybenzophenone, 47 g of dicyclohexylmethane-4,4'-diisocyanate and 0.01 g of triethylenediamine to 90 mL of tetrahydrofuran, stir and react at 45°C for 24 hours, cool to room temperature and filter, collect the precipitate, and vacuum dry at 60°C for 1 hour to obtain an intermediate;

[0049] Step A2: 23 g of the intermediate was dissolved in 100 mL of N,N-dimethylacetamide, and then 50 mL of isopropanol and 0.15 mL of acetic acid were added dropwise. The mixture was stirred under nitrogen protection and ultraviolet light (395 nm, 6 mW / cm 3 ) for 6 days under irradiation, then poured into n-hexane to generate a precipitate, and the precipitate was collected by filtration to obtain a polyisocyanate compound;

[0050] Step A3, add 10.5 g of 2-ethyl-4-methylimidazole to 10 g of xylene, stir and mix, and heat to 65° C., then dropwise add 7.8 g of hexamethylene diisocyanate, keep warm and stir to react for 6 hours, and remove xylene by vacuum distillation to obtain modified imidazole;

[0051] Step A4, 0.85g of 1,4-butanediol and 0.85g of polytetrahydrofuran were stirred and mixed, dehydrated at 160°C for 2min, cooled to 80°C, 17g of polyisocyanate compound was added, stirred and reacted for 4h, and cooled to room temperature to obtain a polyurethane prepolymer; 4.5g of modified imidazole was added to the polyurethane prepolymer and stirred and mixed evenly, then added to 90g of deionized water and stirred and dispersed for 30min, and then 10mL of 20wt% ethylenediamine aqueous solution was added dropwise, reacted at room temperature for 1.5h, and freeze-dried to obtain a modified curing agent.

[0052] Example 6

[0053] This embodiment provides a modified curing agent, which is prepared by the following steps:

[0054] Step A1, add 10.5 g of 4,4'-dihydroxybenzophenone, 49 g of dicyclohexylmethane-4,4'-diisocyanate and 0.01 g of triethylenediamine to 100 mL of tetrahydrofuran, stir and react at 45°C for 24 hours, cool to room temperature and filter, collect the precipitate, and vacuum dry at 60°C for 1 hour to obtain an intermediate;

[0055] Step A2: 25 g of the intermediate was dissolved in 100 mL of N,N-dimethylacetamide, and then 50 mL of isopropanol and 0.15 mL of acetic acid were added dropwise. The mixture was stirred under nitrogen protection and ultraviolet light (395 nm, 6 mW / cm 3 ) was irradiated and reacted for 7 days, then poured into n-hexane to generate a precipitate, and the precipitate was collected by filtration to obtain a polyisocyanate compound;

[0056] Step A3, add 11 g of 2-ethyl-4-methylimidazole to 10 g of xylene, stir and mix, and heat to 65° C., then dropwise add 8 g of hexamethylene diisocyanate, keep warm and stir to react for 6 hours, and remove xylene by vacuum distillation to obtain modified imidazole;

[0057] Step A4, 0.85g of 1,4-butanediol and 0.9g of polytetrahydrofuran were stirred and mixed, and then dehydrated at 160°C for 2min, cooled to 80°C, 18g of polyisocyanate compound was added, stirred and reacted for 5h, and then cooled to room temperature to obtain a polyurethane prepolymer; 5g of modified imidazole was added to the polyurethane prepolymer and stirred and mixed evenly, and then added to 100g of deionized water and stirred and dispersed for 30min, and then 10mL of 20wt% ethylenediamine aqueous solution was added dropwise, and the mixture was reacted at room temperature for 1.5h. After freeze-drying, a modified curing agent was obtained.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 6 is that the operations of steps A1 and A2 are not performed, and in step A4, the polyisocyanate compound is replaced by isophorone diisocyanate in an equal amount, and the remaining raw materials and steps are the same.

[0060] Comparative Example 3

[0061] The difference between this comparative example and Example 6 is that the operation of step A3 is not performed, and in step A4, an equal amount of modified imidazole is replaced by 2-ethyl-4-methylimidazole, and the remaining raw materials and steps are the same.

[0062] Example 7

[0063] This embodiment provides a powder coating for wood surface, which is prepared by the following raw materials in parts by weight and steps:

[0064] 400 parts of the low-temperature curing polyester resin prepared in Example 1, 300 parts of E12 epoxy resin, 2 parts of acrylate homopolymer, acrylate copolymer and polysiloxane, 10 parts of dibenzoyl ketone, 3 parts of quaternary ammonium salt charging agent, 10 parts of hardening wax powder and 50 parts of the modified curing agent prepared in Example 4 were added into a mixer for premixing, then transferred into an extruder for melt extrusion at 80° C., crushed and sieved through a 200-mesh sieve to obtain a powder coating for wood surface.

[0065] Example 8

[0066] This embodiment provides a powder coating for wood surface, which is prepared by the following raw materials in parts by weight and steps:

[0067] 420 parts of the low-temperature curing polyester resin prepared in Example 2, 300 parts of E12 epoxy resin, 2.5 parts of acrylate homopolymer, acrylate copolymer and polysiloxane, 13 parts of dibenzoyl ketone, 3.5 parts of quaternary ammonium salt charging agent, 12 parts of hardening wax powder and 55 parts of the modified curing agent prepared in Example 5 were added into a mixer for premixing, then transferred into an extruder for melt extrusion at 80° C., crushed and sieved through a 200-mesh sieve to obtain a powder coating for wood surface.

[0068] Example 9

[0069] This embodiment provides a powder coating for wood surface, which is prepared by the following raw materials in parts by weight and steps:

[0070] 450 parts of the low-temperature curing polyester resin prepared in Example 3, 300 parts of E12 epoxy resin, 3 parts of acrylate homopolymer, acrylate copolymer and polysiloxane, 15 parts of dibenzoyl ketone, 4 parts of quaternary ammonium salt charging agent, 15 parts of hardening wax powder and 60 parts of the modified curing agent prepared in Example 6 were added into a mixer for premixing, then transferred into an extruder for melt extrusion at 80° C., crushed and sieved through a 200-mesh sieve to obtain a powder coating for wood surface.

[0071] Comparative Example 4

[0072] The difference between this comparative example and Example 9 is that the low-temperature curing polyester resin prepared in Example 3 is replaced by an equal amount of the polyester resin prepared in Comparative Example 1, and the other raw materials and steps are the same.

[0073] Comparative Example 5

[0074] The difference between this comparative example and Example 9 is that the modified curing agent prepared in Example 6 is replaced by the modified curing agent prepared in Comparative Example 2 in equal amount, and the other raw materials and steps are the same.

[0075] Comparative Example 6

[0076] The difference between this comparative example and Example 9 is that the modified curing agent prepared in Example 6 is replaced by the modified curing agent prepared in Comparative Example 3 in equal amount, and the other raw materials and steps are the same.

[0077] First, the storage stability of the powder coatings prepared in Examples 7 to 9 and Comparative Examples 4 to 6 was tested in accordance with GB / T 21782.8-2008 to observe whether compaction or agglomeration occurred; then, the powder coatings prepared in Examples 7 to 9 and Comparative Examples 4 to 6 were respectively coated on the same wood, and all were cured for 3 min at an infrared controlled temperature of 130° C., and the degree of curing of the coating was calculated according to the formula degree of curing a=(ΔH0-ΔHR) / ΔH0, wherein ΔH0 (J / g) is the total heat released when the uncured powder coating is completely cured, and ΔHR (J / g) is the remaining reaction heat when it is not completely cured, and ΔH0 and ΔHR are both obtained by DSC test; the adhesion of the coating was tested in accordance with GB / T 4893.4-2013; the pencil hardness of the coating was tested in accordance with GB / T6739-2006; the water resistance of the coating was tested in accordance with GB / T 17657-2013; and the water resistance of the coating was tested in accordance with GB / T 1732-1993 Test the impact resistance of the coating; the results are shown in Table 1:

[0078] Table 1

[0079]

[0080] It can be seen from the data in Table 1 that the powder coatings prepared in Examples 7-9 of the present invention have high storage stability and high degree of curing. After being coated on the wood surface, the adhesion, hardness, water resistance and impact resistance of the formed coatings are significantly better than those of the powder coatings prepared in Comparative Examples 4-6, and have more excellent comprehensive performance; the modified curing agent used in Comparative Example 5 cannot completely release the modified imidazole therein at low temperatures, thereby affecting the degree of curing; the modified curing agent used in Comparative Example 6 does not modify 2-ethyl-4-methylimidazole, resulting in a large amount of 2-ethyl-4-methylimidazole dissolved in deionized water, and cannot be effectively coated in the waterborne polyurethane shell layer, ultimately resulting in too low an imidazole content in the modified curing agent, which cannot play a corresponding role.

[0081] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0082] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A powder coating for wood surface, characterized in that: It includes the following raw materials in parts by weight: 400-450 parts of low temperature curing polyester resin, 300 parts of E12 epoxy resin, 2-3 parts of leveling agent, 10-15 parts of degassing agent, 3-4 parts of charging agent, 10-15 parts of hardening wax powder and 50-60 parts of modified curing agent; The low-temperature curing polyester resin is obtained by esterification polycondensation reaction of neopentyl glycol and 1,4-cyclohexanedimethanol with terephthalic acid under the catalysis of monobutyltin oxide, and then end-capping with dodecanedioic acid to obtain a low-temperature curing polyester resin containing a saturated six-membered ring structure and a terminal carboxyl group; The modified curing agent is prepared by the following steps: 1,4-butanediol and polytetrahydrofuran were stirred and mixed, and then dehydrated at 160°C for 2 minutes. After cooling to 80°C, a polyisocyanate compound was added and stirred for reaction for 3-5 hours. After cooling to room temperature, modified imidazole was added and stirred for mixing. Then, deionized water was added and stirred for emulsification. Then, an ethylenediamine aqueous solution was added dropwise and reacted at room temperature for 1.5 hours. After freeze-drying, a modified curing agent was obtained. The polyisocyanate compound is prepared by reacting the hydroxyl group of 4,4'-dihydroxybenzophenone with the isocyanate group of dicyclohexylmethane-4,4'-diisocyanate under the catalysis of triethylenediamine to obtain an intermediate having isocyanate groups at both ends and a symmetrical structure, and then using the intermediate as a raw material to capture hydrogen atoms from isopropanol under ultraviolet light irradiation and acetic acid catalysis to form a hydroxyl group, and then dimerizing to obtain a polyisocyanate compound containing a benzopinacol structure; The modified imidazole is a low-water-soluble modified imidazole prepared by reacting the isocyanate groups at both ends of hexamethylene diisocyanate with the imino groups in 2-ethyl 4-methylimidazole.

2. A powder coating for wood surface according to claim 1, characterized in that: The low temperature curing polyester resin is prepared by the following steps: Neopentyl glycol, 1,4-cyclohexanedimethanol and monobutyltin oxide were stirred and mixed, and then terephthalic acid was added, and the reaction was carried out at 240°C for 3-4 hours under nitrogen protection, and then polycondensation reaction was carried out at -0.09MPa for 3 hours, and then dodecanedioic acid was added and reacted at 240°C for 3 hours. After cooling to room temperature and discharging, a low-temperature curing polyester resin was obtained.

3. A powder coating for wood surface according to claim 2, characterized in that: The mass ratio of the neopentyl glycol, 1,4-cyclohexanedimethanol, monobutyltin oxide, terephthalic acid and dodecanedioic acid is 30-35g: 4-5g: 0.15g: 50-54g: 5g.

4. The powder coating for wood surface according to claim 1, characterized in that: The usage ratio of the 1,4-butanediol, polytetrahydrofuran, polyisocyanate compound, modified imidazole, deionized water and ethylenediamine aqueous solution is 0.85g: 0.8-0.9g: 15-18g: 4-5g: 80-100g: 10mL; the concentration of the ethylenediamine aqueous solution is 20wt%.

5. The powder coating for wood surface according to claim 1, characterized in that: The polyisocyanate compound is prepared by the following steps: Step A1, adding 4,4'-dihydroxybenzophenone, dicyclohexylmethane-4,4'-diisocyanate and triethylenediamine to tetrahydrofuran, stirring and reacting at 45°C for 24 hours, cooling to room temperature and filtering, collecting the precipitate, and vacuum drying at 60°C for 1 hour to obtain an intermediate; Step A2: dissolving the intermediate in N,N-dimethylacetamide, then dropping isopropanol and acetic acid, reacting for 5-7 days under nitrogen protection and ultraviolet light irradiation, then pouring into n-hexane to generate a precipitate, collecting the precipitate by filtration, and obtaining a polyisocyanate compound.

6. A powder coating for wood surface according to claim 5, characterized in that: The usage ratio of the 4,4'-dihydroxybenzophenone, dicyclohexylmethane-4,4'-diisocyanate, triethylenediamine and tetrahydrofuran is 10-10.5 g: 45-49 g: 0.01 g: 80-100 mL.

7. The powder coating for wood surface according to claim 5, characterized in that: The usage ratio of the intermediate, N,N-dimethylacetamide, isopropanol and acetic acid is 20-25 g: 100 mL: 50 mL: 0.15 mL.

8. The powder coating for wood surface according to claim 1, characterized in that: The modified imidazole is prepared by the following method: Add 2-ethyl-4-methylimidazole to xylene, stir and mix, and heat to 65°C. Then, add hexamethylene diisocyanate dropwise, keep warm and stir to react for 6 hours, and distill under reduced pressure to obtain modified imidazole.

9. The powder coating for wood surface according to claim 8, characterized in that: The usage ratio of xylene, 2-ethyl-4-methylimidazole and hexamethylene diisocyanate is 10g:10-11g:7.5-8g.

10. The method for preparing a powder coating for wood surface according to claim 1, characterized in that: The following steps are involved: Low-temperature curing polyester resin, E12 epoxy resin, leveling agent, degassing agent, charging agent, hardening wax powder and modified curing agent are added into a mixer for premixing, then transferred into an extruder for melt extrusion at 80° C., crushed and passed through a 200-mesh sieve to obtain a powder coating for wood surface.

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