Preparation method and application of low-temperature fast-curing epoxy resin powder coating

Abstract

The invention discloses a preparation method and application of a low-temperature fast-curing epoxy resin powder coating. Epoxy resin is used as a film-forming substance, and a curing agent system composition matched with the epoxy resin is modified dicyandiamide and a substituted urea accelerant; the total amount of the modified dicyandiamide and the substituted urea accelerant is 100 parts, the modified dicyandiamide accounts for 83-97 parts, and the balance is the substituted urea accelerant; wherein the dicyandiamide is modified by using silicon dioxide and gamma-glycidoxy propyl trimethoxy silane; the preparation method comprises the following steps: premixing all the components, melting, mixing and extruding through a double-screw extruder, cold-pressing, crushing and sieving to obtain the powder coating. The curing film-forming process method comprises the following steps: electrostatic powder spraying; infrared radiation: transferring energy of infrared radiation to silicon dioxide through hydroxyl and carboxyl in the coating; heating and curing; the powder coating obtained by the invention is rapidly and sufficiently cured, and the film layer performance is good.

Description

Technical Field

[0001] The invention relates to the technical field of powder coatings, and in particular to a preparation method and application of low-temperature fast-curing epoxy resin powder coatings. Background Art

[0002] The mixer truck is a very important equipment in the engineering field, and the mixing drum can be called the soul structure of the mixer truck. The mixing drum is welded from thin plates. During use, it rolls along a circular track to complete the storage and transportation of premixed cement. In actual production, the mixing drum is very easy to deform. In order to meet the use and quality requirements, it is necessary to strictly control the deformation in the manufacturing process. Among these processes, the deformation caused by the spraying process accounts for about 20% of the total deformation. The main reason is the long spraying time and high temperature. It takes about 48 minutes for a workpiece to be cured from the spraying pretreatment to the curing, and the curing temperature is about 180-220℃. Therefore, it is imperative to study a low-temperature fast-curing epoxy resin powder coating.

[0003] For example, the invention patent with the authorization announcement number CN118085687B discloses an epoxy composite powder for energy storage power stations, a preparation method thereof, and a curing agent. The patent points out that the existing low-temperature powder coatings on the market can only achieve low-temperature curing, but not low-temperature rapid curing. It is not easy to achieve true low-temperature rapid curing. The first thing to solve is the storage stability problem. The low-temperature and rapid curing system has a high reaction activity. Micro-crosslinking reactions are prone to occur in the system at room temperature. It needs to be transported and stored at low temperatures, which is inconvenient to use. In addition, the resin used in the low-temperature rapid curing powder is a solid with a high softening point. When it is cured at low temperature, the melt viscosity is high and the coating film is difficult to level, which affects the surface effect. Although the use of a low-softening point resin can reduce the melt viscosity, the powder is easy to agglomerate and needs to be stored in a low-temperature refrigerator, which brings many difficulties to users. The third is the processing difficulty. Powder coatings are mostly processed by twin-screw melt extrusion. The hot melt temperature is close to the curing temperature, and the friction heat of the screw is easy to cure prematurely during the processing process, causing the screw to get stuck. Summary of the invention

[0004] The first purpose of the present invention is to provide a method for preparing a low-temperature fast-curing epoxy resin powder coating. The present invention reduces the surface activity of dicyandiamide by a silane coupling agent to form a low-energy interface layer. At the same time, the other side of the silane coupling agent forms a molecular bridge with silicon dioxide, thereby improving the dispersibility of the curing agent between resins, achieving low-temperature curing while improving the stability of the powder coating in storage and transportation.

[0005] To solve this technical problem, the technical solution of the present invention is: a method for preparing a low-temperature fast-curing epoxy resin powder coating, wherein the epoxy resin is used as a film-forming substance, and the curing agent system composition of the epoxy resin is a modified dicyandiamide and a substituted urea accelerator; Wherein, the ratio of the curing agent system composition and the epoxy resin by mass is (28 parts to 44 parts): 100 parts; The total amount of the modified dicyandiamide and the substituted urea accelerator is calculated as 100 parts, of which the modified dicyandiamide accounts for 83 to 97 parts, and the balance is the substituted urea accelerator; Among them, dicyandiamide was modified with silica and γ-glycidoxypropyltrimethoxysilane; The epoxy resin, curing agent system, leveling agent, brightener, barium sulfate, aluminum oxide and anti-yellowing agent are premixed and melt-mixed and extruded through a twin-screw extruder, and then cold-pressed on a flat vulcanizer, and then crushed with a pulverizer and sieved to obtain a powder coating.

[0006] Preferably, the modified preparation method of dicyandiamide comprises the following steps: S1, stirring and mixing silicon dioxide and anhydrous ethanol at room temperature, so that the silicon dioxide is evenly dispersed in the anhydrous ethanol to obtain a silicon dioxide / anhydrous ethanol mixed solution; Mixing γ-glycidyloxypropyltrimethoxysilane with water, stirring to fully hydrolyze the γ-glycidyloxypropyltrimethoxysilane, cooling to room temperature, and stopping stirring to obtain a γ-glycidyloxypropyltrimethoxysilane / water mixed solution; S2, mixing a mixture of γ-glycidyloxypropyltrimethoxysilane / water, a mixture of silicon dioxide / anhydrous ethanol and dicyandiamide, heating while stirring, and performing condensation reflux reaction at 70° C. for 3 hours; S3, after the reaction is completed, filter, wash repeatedly with anhydrous ethanol, place the filter cake in a vacuum drying oven after washing, dry at 70°C, grind into powder after drying to obtain modified dicyandiamide. The present invention utilizes the low surface energy of the silane coupling agent to form a low-energy interface layer on the surface of dicyandiamide, and at the same time, the other side of the silane coupling agent forms a molecular bridge with silicon dioxide, thereby improving the dispersibility of dicyandiamide between epoxy resins and promoting the interaction between dicyandiamide and epoxy resins during the curing process.

[0007] Preferably, the mass ratio of silicon dioxide, γ-glycidyloxypropyltrimethoxysilane and dicyandiamide is (1.5 to 1.8):(0.5 to 0.8):1.

[0008] Preferably, the particle size of the silicon dioxide is less than 50 μm. The present invention utilizes silicon dioxide particles with smaller particles to effectively promote the uniformity of the modified dicyandiamide dispersion in the powder coating. At the same time, because silicon dioxide has good photocatalytic properties and there are many gaps and defects inside the silicon dioxide crystal lattice, silicon dioxide can adsorb organic molecules. When silicon dioxide receives the energy of infrared radiation, silicon dioxide generates active groups, such as hydroxyl radicals as epoxy resin curing accelerators, thereby promoting the obtained powder coating to be fully cured under relatively low temperature conditions compared to the prior art.

[0009] Preferably the substituted urea accelerator is modified with silica and gamma-glycidoxypropyltrimethoxysilane.

[0010] Preferably, the mass ratio of silicon dioxide, γ-glycidyloxypropyltrimethoxysilane and substituted urea accelerator is (1.5 to 1.8): (0.5 to 0.8): 1.

[0011] The preferred substituted urea accelerator is N-(2-hydroxyphenyl)-N',N'-dimethylurea or N-(5-chloro-2-hydroxybenzene)-N',N'-dimethylurea. The present invention uses a substituted urea accelerator with a hydroxyl group to enhance the transfer of infrared radiation energy in the coating layer.

[0012] The second purpose of the present invention is to provide an application of low-temperature fast-curing epoxy resin powder coating. The present invention combines epoxy resin with modified dicyandiamide. During the curing process, infrared radiation is first used to effectively activate the coating layer. The silicon dioxide distributed in the powder coating effectively promotes the interaction between the epoxy resin and the curing agent to achieve low-temperature curing.

[0013] To solve this technical problem, the technical solution of the present invention is: the application of the powder coating prepared by the present invention, the process method of curing and forming a film comprises the following steps: S11, cleaning and drying the surface of the workpiece to be sprayed, and then electrostatically spraying powder; S12, infrared radiation for 2 to 3 minutes, transferring the energy of infrared radiation to silicon dioxide through hydroxyl and carboxyl groups in the coating; S13, heating and curing, the curing temperature is 120° C. to 160° C., and the curing time is 8 min to 16 min.

[0014] Preferably, the infrared radiation in step S2 adopts medium-wave infrared radiation with a wavelength of 0.75 microns to 10 microns. In the present invention, the medium-wave spectrum frequency of the medium-wave infrared radiation corresponds to the absorption band frequency of the hydroxyl and carboxyl molecules contained in the epoxy resin and other substances in the powder coating. Therefore, the medium-wave infrared radiation acts on the carboxyl and hydroxyl groups in the coating to transfer energy to the silicon dioxide. With the photocatalytic properties of the silicon dioxide, there are many gaps and defects inside. When the silicon dioxide receives the energy of infrared radiation through the hydroxyl and carboxyl groups, active groups such as hydroxyl radicals are generated. This hydroxyl radical is exactly the accelerator required for the curing of epoxy resin. That is, this curing agent makes full use of the inherent characteristics of the existing production line, generates a promoting group for the curing reaction, and realizes low-temperature rapid curing.

[0015] By adopting the above technical solution, the beneficial effects of the present invention are: As a latent epoxy resin curing agent, dicyandiamide can be stably stored at room temperature, is insoluble in epoxy resin, and is dispersed in epoxy resin in the form of microparticles. When heated, dicyandiamide begins to dissolve and rapidly reacts to cure. The curing reaction temperature of dicyandiamide is generally between 160°C and 180°C, and the curing reaction time is about 20 minutes to 60 minutes. During the heating process, adding a small amount of accelerator can greatly reduce the curing temperature and time. The accelerator releases amine and imidazole alkaline substances to initiate anionic polymerization of epoxy functional groups. The anionic active center in the curing system reacts with dicyandiamide to activate the reaction activity of dicyandiamide. However, the existing technology has the problem that the use of accelerators will shorten the applicable period and affect the storage period. Therefore, the present invention proposes a low-temperature curing powder coating, which modifies dicyandiamide and a substituted urea accelerator, further improves the compatibility and interfacial reaction of silica and epoxy resin with the help of silica and a silane coupling agent, indirectly affects the decomposition behavior of the substituted urea at low temperature, and improves the low-temperature storage performance of the powder coating; on the other hand, the hydroxyl and carboxyl groups in the coating are used to transfer infrared radiation energy, and the photocatalytic effect of silica in the coating is used to generate active groups under infrared radiation to promote the curing reaction; the dispersibility of the powder coating is enhanced after modification with silica and a silane coupling agent, the product after decomposition of the modified curing agent contains terminal amine groups, which adjusts the curing reaction speed in the entire film layer, and is further decomposed into corresponding isocyanates and dimethylamine after heating with a substituted urea accelerator, and the dimethylamine promotes the reaction of dicyandiamide with epoxy groups in the epoxy resin at low temperature, thereby realizing low-temperature rapid curing of the epoxy resin powder coating, and having good film layer performance. DETAILED DESCRIPTION

[0016] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0017] Example 1 The present embodiment provides a method for preparing and applying a low-temperature fast-curing epoxy resin powder coating, the specific composition of which is shown in Table 1 by weight, wherein epoxy resin is used as a film-forming substance, and the curing agent system composition of the epoxy resin is a modified dicyandiamide and a substituted urea accelerator; Wherein, the ratio of the curing agent system composition to the epoxy resin by mass is 28 parts:100 parts; The total amount of the modified dicyandiamide and the substituted urea accelerator is calculated as 100 parts, of which the modified dicyandiamide accounts for 83 parts and the remainder is the substituted urea accelerator; Among them, dicyandiamide was modified with silica and γ-glycidoxypropyltrimethoxysilane; The epoxy resin, curing agent system, leveling agent, brightener, barium sulfate, aluminum oxide and anti-yellowing agent are premixed and melt-mixed and extruded through a twin-screw extruder, and then cold-pressed on a flat vulcanizer, and then crushed with a pulverizer and sieved to obtain a powder coating.

[0018] The modified preparation method of dicyandiamide described in the present embodiment comprises the following steps: S1, stirring and mixing silicon dioxide and anhydrous ethanol at room temperature, so that the silicon dioxide is evenly dispersed in the anhydrous ethanol to obtain a silicon dioxide / anhydrous ethanol mixed solution; Mixing γ-glycidyloxypropyltrimethoxysilane with water, stirring to fully hydrolyze the γ-glycidyloxypropyltrimethoxysilane, cooling to room temperature, and stopping stirring to obtain a γ-glycidyloxypropyltrimethoxysilane / water mixed solution; S2, mixing a mixture of γ-glycidyloxypropyltrimethoxysilane / water, a mixture of silicon dioxide / anhydrous ethanol and dicyandiamide, heating while stirring, and performing condensation reflux reaction at 70° C. for 3 hours; S3, after the reaction is completed, filter, wash repeatedly with anhydrous ethanol, place the filter cake in a vacuum drying oven after washing, dry at 70°C, grind into powder after drying to obtain modified dicyandiamide. The present invention utilizes the low surface energy of the silane coupling agent to form a low-energy interface layer on the surface of dicyandiamide, and at the same time, the other side of the silane coupling agent forms a molecular bridge with silicon dioxide, thereby improving the dispersibility of dicyandiamide between epoxy resins and promoting the interaction between dicyandiamide and epoxy resins during the curing process.

[0019] In this embodiment, the mass ratio of silicon dioxide, γ-glycidyloxypropyltrimethoxysilane and dicyandiamide is 50:20:30.

[0020] The particle size of the silicon dioxide described in this embodiment is less than 50 μm. The present invention utilizes silicon dioxide particles with smaller particles to effectively promote the uniformity of the modified dicyandiamide dispersion in the powder coating. At the same time, because silicon dioxide has good photocatalytic properties, there are many gaps and defects inside the silicon dioxide lattice, and silicon dioxide can adsorb organic molecules. When silicon dioxide receives the energy of infrared radiation, silicon dioxide generates active groups, such as hydroxyl radicals as epoxy resin curing accelerators, thereby promoting the obtained powder coating to be fully cured under relatively low temperature conditions relative to the prior art.

[0021] In this embodiment, the substituted urea accelerator is modified with silica and γ-glycidyloxypropyltrimethoxysilane.

[0022] In this embodiment, the mass ratio of silicon dioxide, γ-glycidyloxypropyltrimethoxysilane and substituted urea accelerator is 50:20:30, and the modification method of the substituted urea accelerator is the same as the modification method of dicyandiamide in this embodiment.

[0023] The substituted urea accelerator in this embodiment is N-(2-hydroxyphenyl)-N',N'-dimethylurea. The present invention uses a substituted urea accelerator with a hydroxyl group to enhance the transmission of infrared radiation energy in the coating layer.

[0024] The process for preparing a powder coating by curing and film-forming comprises the following steps: S11, cleaning and drying the surface of the workpiece to be sprayed, and then electrostatically spraying powder; S12, infrared radiation for 2 minutes, transferring the energy of infrared radiation to silica through the hydroxyl and carboxyl groups in the coating; S13, heating and curing, the curing temperature is 150°C, and the curing time is 15 minutes.

[0025] In the present embodiment, the infrared radiation in step S2 adopts medium-wave infrared radiation with a wavelength of 0.75 microns to 10 microns. The medium-wave spectrum frequency of the medium-wave infrared radiation used in the present invention corresponds to the absorption band frequency of the hydroxyl and carboxyl molecules contained in the epoxy resin and other substances in the powder coating, so the medium-wave infrared radiation acts on the carboxyl and hydroxyl groups in the coating to transfer energy to the silicon dioxide. With the photocatalytic properties of the silicon dioxide, there are many gaps and defects inside it. When the silicon dioxide receives the energy of infrared radiation through the hydroxyl and carboxyl groups, active groups such as hydroxyl radicals are generated. This hydroxyl radical is exactly the accelerator required for the curing of epoxy resin. That is, this curing agent makes full use of the inherent characteristics of the existing production line, generates a promoting group for the curing reaction, and realizes low-temperature rapid curing.

[0026] Example 2 The main differences between this embodiment and embodiment 1 are shown in Table 1 and Table 2.

[0027] In this embodiment, the substituted urea accelerator is N-(5-chloro-2-hydroxybenzene)-N', N'-dimethylurea.

[0028] Example 3 The main differences between this embodiment and embodiment 1 are shown in Table 1 and Table 2.

[0029] In this embodiment, the substituted urea accelerator is N-(5-chloro-2-hydroxybenzene)-N', N'-dimethylurea.

[0030] Example 4 The main differences between this embodiment and embodiment 1 are shown in Table 1 and Table 2.

[0031] In this embodiment, the substituted urea accelerator is N-(2-hydroxyphenyl)-N',N'-dimethylurea.

[0032] Example 5 The main differences between this embodiment and embodiment 1 are shown in Table 1 and Table 2.

[0033] In this embodiment, the substituted urea accelerator is N-(2-hydroxyphenyl)-N',N'-dimethylurea.

[0034] Table 1 Composition of powder coatings in Examples 1 to 5 (parts by mass)

[0035] Table 2 Curing agent system composition (percentage)

[0036] Comparative Example This comparative example discloses a powder coating, which is the same as Example 3 in terms of mass fractions, wherein the curing agent system includes unmodified dicyandiamide and unmodified substituted urea accelerator, and the specific substance used in the substituted urea accelerator in this comparative example is the same as Example 3, and the mass proportions of the two are 90% and 10% respectively, and the film-forming and curing process is the same as Example 3.

[0037] The storage performance of the powder coatings of Test Examples 1 to 5 and the Comparative Example is shown in Table 3. The specific storage conditions are an indoor environment with a temperature of 30° C. and a humidity of 60%.

[0038] Table 3 Storage time of powder coatings obtained in Examples 1 to 5 and Comparative Example until slight agglomeration occurs (days)

[0039] Combining Table 1, Table 2 and Table 3, it can be seen that the storage time of the powder coating obtained by the present invention when slight agglomeration begins to appear increases significantly. When the modified substituted urea accelerator accounts for 6% to 17% by mass of the curing system, the storage time of the obtained powder coating is more than 138 days. When the proportion of the modified accelerator is further reduced to 3%, the storage stability is significantly reduced to 122 days, but it is still much higher than the performance of the powder coating obtained in the comparative example.

[0040] The performance indicators of the film layers obtained in Examples 1 to 5 and the comparative example were further tested, and the specific data are shown in Table 4.

[0041] Table 4 Performance index data of the film layers obtained in Examples 1 to 5 and Comparative Examples

[0042] From Table 4, it can be seen that when the weight ratio of the curing agent system to the epoxy resin of the powder coating proposed by the present invention is (31 to 39): 100, the performance of the obtained film layer is relatively more excellent and stable.

[0043] From the above data, it can be seen that the present invention has obvious advantages in hardness and impact resistance, and meets the use requirements. It reflects from the side that the powder coating proposed by the present invention is completely cured, and the curing reaction of the comparative example is insufficient. Through the storage test, it can be seen that the present invention has obvious advantages in preventing the agglomeration of the powder coating. The modified curing system significantly improves the stability of the powder coating. It is recommended that the storage temperature of the powder coating be between 5-30°C and the humidity be below 60%. High temperature and high humidity may cause the powder coating to agglomerate or deteriorate due to moisture absorption.

[0044] The powder coating obtained in Example 4 was cured according to the process parameters of film formation recorded in Table 5 to obtain a film layer. The specific performance indicators are shown in Table 6.

[0045] Table 5 Film formation and curing of the powder coating obtained in Example 4 using different process parameters

[0046] The performance indicators of the film layers obtained according to the different film forming and curing parameters in Table 5 are shown in Table 6.

[0047] Table 6 Performance indicators of the film obtained by the powder coating obtained in Example 4 under different film-forming and curing processes

[0048] As shown in Table 6, the powder coating obtained in Example 4 can meet the coating quality requirements under medium-wave infrared radiation for 2 minutes, a curing temperature of 130°C, and a curing time of 10 minutes. From the completion of workpiece pretreatment to the completion of curing, the cycle time is reduced from 48 minutes to 25 minutes, which greatly improves production efficiency and reduces energy consumption. Due to the reduction in curing time and curing temperature, the deformation caused by the spraying process is also greatly reduced, from about 20% to 5%.

Claims

1. A method for preparing a low-temperature fast-curing epoxy resin powder coating, using epoxy resin as a film-forming substance, characterized in that: The curing agent system composition for the epoxy resin is a modified dicyandiamide and a substituted urea accelerator; Wherein, the ratio of the curing agent system composition to the epoxy resin by mass is (28 parts to 44 parts): 100 parts; The total amount of the modified dicyandiamide and the substituted urea accelerator is calculated as 100 parts, of which the modified dicyandiamide accounts for 83 to 97 parts, and the balance is the substituted urea accelerator; Among them, dicyandiamide was modified with silica and γ-glycidoxypropyltrimethoxysilane; The epoxy resin, curing agent system, leveling agent, brightener, barium sulfate, aluminum oxide and anti-yellowing agent are premixed and melt-mixed and extruded through a twin-screw extruder, and then cold-pressed on a flat vulcanizer, and then crushed with a pulverizer and sieved to obtain a powder coating.

2. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 1, characterized in that: The modified preparation method of dicyandiamide comprises the following steps: S1, stirring and mixing silicon dioxide and anhydrous ethanol at room temperature, so that the silicon dioxide is evenly dispersed in the anhydrous ethanol to obtain a silicon dioxide / anhydrous ethanol mixed solution; Mixing γ-glycidyloxypropyltrimethoxysilane with water, stirring to fully hydrolyze the γ-glycidyloxypropyltrimethoxysilane, cooling to room temperature, and stopping stirring to obtain a γ-glycidyloxypropyltrimethoxysilane / water mixed solution; S2, mixing a mixture of γ-glycidyloxypropyltrimethoxysilane / water, a mixture of silicon dioxide / anhydrous ethanol and dicyandiamide, heating while stirring, and performing condensation reflux reaction at 70° C. for 3 hours; S3. After the reaction is completed, filter and wash repeatedly with anhydrous ethanol. After washing, place the filter cake in a vacuum drying oven and dry it at 70° C. After drying, grind it into powder to obtain modified dicyandiamide.

3. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 2, characterized in that: The mass ratio of silicon dioxide, γ-glycidyloxypropyltrimethoxysilane and dicyandiamide is (1.5 to 1.8):(0.5 to 0.8):

1.

4. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 2, characterized in that: The particle size of the silicon dioxide is less than 50 μm.

5. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 1, characterized in that: The substituted urea accelerator is modified with silica and γ-glycidoxypropyltrimethoxysilane.

6. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 5, characterized in that: The mass ratio of silica, γ-glycidoxypropyltrimethoxysilane and substituted urea accelerator is (1.5 to 1.8): (0.5 to 0.8):

1.

7. The method for preparing a low-temperature fast-curing epoxy resin powder coating according to claim 5, characterized in that: The substituted urea accelerator is N-(2-hydroxyphenyl)-N',N'-dimethylurea or N-(5-chloro-2-hydroxybenzene)-N',N'-dimethylurea.

8. An application of a powder coating obtained by the preparation method according to any one of claims 1 to 7, characterized in that: The process of curing film includes the following steps: S11, cleaning and drying the surface of the workpiece to be sprayed, and then electrostatically spraying powder; S12, infrared radiation for 2 to 3 minutes, transferring the energy of infrared radiation to silicon dioxide through hydroxyl and carboxyl groups in the coating; S13, heating and curing, the curing temperature is 120° C. to 160° C., and the curing time is 8 min to 16 min.

9. The use according to claim 8, characterized in that: In step S2 , the infrared radiation uses medium-wave infrared radiation with a wavelength of 0.75 microns to 10 microns.

Images