Photocuring hub coating and preparation method thereof

Abstract

The invention relates to the technical field of coatings, in particular to a photocuring hub coating and a preparation method thereof. The invention overcomes the problems of low yield and poor aging resistance of photocureable coatings in the prior art. The preparation method comprises the following steps: preparing modified acrylic resin by taking isophorone diisocyanate, tricyclodecane dimethyl carbinol, trimethylolpropane, fluorine-containing alcohol and hydroxyethyl acrylate as raw materials; dispersing the silane modified nano aluminum oxide into a monomer to prepare a dispersed filler; the modified acrylic resin, the dispersed filler, the reactive diluent, the sulfhydryl compound, the initiator and the like are compounded to prepare an acrylic coating; and finally, carrying out infrared leveling and UV-LED and mercury lamp segmented curing film forming to obtain the photocuring wheel hub coating. According to the invention, a tricyclodecane rigid framework is introduced and is matched with a segmented curing process, so that the problem of low yield of the coating is solved; and meanwhile, by utilizing the surface migration characteristic of the fluorine-containing chain segment and nano inorganic enhancement, the aging resistance of the coating is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to a photocurable wheel hub coating and its preparation method. Background Technology

[0002] With the development of lightweighting and aesthetics in the automotive industry, aluminum alloy wheels have become mainstream. While traditional solvent-based thermosetting coatings are technologically mature, they suffer from drawbacks such as high VOC emissions, high energy consumption, and long curing times, making it difficult to meet increasingly stringent environmental regulations. UV-cured coatings, with their "5E" characteristics (high efficiency, energy saving, environmental friendliness, economy, and versatility), are gradually gaining widespread application in the field of wheel clear coats. They can complete cross-linking and curing within seconds, significantly shortening production line length and representing the core direction for the future of wheel coating.

[0003] Despite the significant advantages of UV-cured coatings, two major challenges remain in practical wheel applications: First, low yield rates. This is primarily due to the large volume shrinkage (up to 10%-15%) of acrylate monomers during free radical polymerization, leading to excessive internal stress in the coating and resulting in poor adhesion and microcracks. Simultaneously, the complex geometry of the wheel hub can cause uneven curing, and oxygen inhibition can lead to a sticky surface and uneven gloss, severely impacting the first-pass yield. Second, poor aging resistance. Wheel hubs are exposed to UV radiation, high temperatures, and acid rain for extended periods. Conventional aromatic polyurethane acrylic resins are prone to yellowing, while ordinary aliphatic acrylic resins, although slightly better in weather resistance, are susceptible to polymer chain breakage and degradation under prolonged UV radiation, resulting in coating loss of gloss, chalking, and even peeling, failing to meet the required weather resistance standards.

[0004] To address this, a photocurable wheel hub coating and its preparation method are proposed. Summary of the Invention

[0005] The purpose of this invention is to design a photocurable wheel hub coating and its preparation method. This invention uses isophorone diisocyanate, tricyclodecanediethanol, trimethylolpropane, fluorinated alcohols, and hydroxyethyl acrylate as raw materials to prepare a modified acrylic resin; silane-modified nano-alumina is dispersed in the monomer to prepare a dispersion filler; the modified acrylic resin, dispersion filler, reactive diluent, mercapto compound, and initiator are compounded to obtain an acrylic coating; finally, the coating is cured in stages using infrared leveling and UV-LED and mercury lamps to obtain a photocurable wheel hub coating. This invention solves the problem of low coating yield by introducing a rigid tricyclodecane framework and combining it with a staged curing process; at the same time, it significantly improves the aging resistance of the coating by utilizing the surface migration characteristics of fluorinated segments and nano-inorganic reinforcement.

[0006] To achieve the above objectives, the present invention provides the following technical solution: Unless otherwise specified, all the following parts are by weight.

[0007] This invention provides a method for preparing a photocurable wheel hub coating, comprising the following steps: Acrylic resin was modified to obtain modified acrylic resin; modified alumina and 1,6-hexanediol diacrylate were mixed and ground to obtain a dispersion filler; modified acrylic resin, isobornyl acrylate, dispersion filler, pentaerythritol tetra-3-mercaptopropionate, composite photoinitiator, ultraviolet absorber and leveling agent were mixed stepwise to obtain a coating; the coating was sprayed onto the surface of the wheel hub and subjected to segmented photocuring treatment to obtain a photocured wheel hub coating; Modified alumina is obtained by modifying nano-alumina with a silane coupling agent.

[0008] Preferably, the modified acrylic resin is prepared as follows: 40-50 parts of isophorone diisocyanate, 0.1 parts of dibutyltin dilaurate, and 0.1 parts of 2,6-di-tert-butyl-p-cresol are added to a four-necked flask (containing a reflux condenser). Nitrogen gas is introduced for protection, and the temperature is raised to 50°C. An acetone solution (1:1 ratio) dissolving 10-15 parts of tricyclodecanediethanol is slowly added dropwise, controlling the reaction temperature fluctuation to not exceed ±3°C. After the addition is complete, the temperature is raised to 70°C and the reaction is maintained for 1-2 hours. Then, 30 parts of acetone are added to lower the temperature. To determine the viscosity, dissolve 8-12 parts of trimethylolpropane in acetone (1:1 ratio) and slowly add it dropwise to a flask, controlling the temperature to not exceed 75°C. After the addition is complete, maintain the temperature for 2.5 hours. Then add 3-5 parts of 2-(perfluorohexyl)ethanol and continue the reaction at 75°C for 1.5 hours. After the reaction is complete, lower the temperature to 60°C and slowly add 30 parts of 2-hydroxyethyl acrylate, controlling the total addition time to 30 minutes. After the addition is complete, raise the temperature to 75°C and continue the reaction for 3.5 hours. Remove acetone by vacuum distillation to obtain the modified acrylic resin.

[0009] Preferably, the preparation method of the dispersing filler is as follows: 1-5 parts of modified alumina are added to 12 parts of 1,6-hexanediol diacrylate, and the mixture is ground using a sand mill (zirconia beads, diameter 0.5-0.8 mm), with the temperature controlled at <50℃, until the fineness is <5μm to obtain the dispersing filler.

[0010] Preferably, the preparation method of modified alumina is as follows: 95-105 parts of nano-alumina (average particle size of 20-30 nm) are added to 500 parts of ethanol aqueous solution (anhydrous ethanol: deionized water = 95:5), and dispersed for 30 min using an ultrasonic cell disruptor (power 400W) to obtain a dispersion; the pH value of the above dispersion is adjusted to about 4.5 with glacial acetic acid, and 5-9 parts of KH-570 are added dropwise under stirring, the temperature is raised to 80℃, and the reaction is refluxed for 5-7 h. After the reaction is completed, the mixture is centrifuged, washed 3 times with anhydrous ethanol, dried in a vacuum oven at 80℃ for 12 h, ground and sieved to obtain modified alumina.

[0011] The preferred stepwise mixing process is as follows: In a light-proof stainless steel mixing vessel, add 50-60 parts of modified acrylic resin, 18 parts of isobornyl acrylate, dispersing filler, and 6 parts of pentaerythritol tetra-3-mercaptopropionate in sequence. Turn on the high-speed disperser at 1200 rpm and stir for 15 minutes until the mixture is uniform. While stirring, add 3-5 parts of composite photoinitiator and Tinuvin R 796 (ultraviolet absorber), and stir for 20-30 minutes. Note that the material temperature should not exceed 50°C. Reduce the speed to 300 rpm, add 0.5 parts of BYK-333 leveling agent, stir for 10 minutes, and let stand for 30 minutes to degas. Finally, filter under pressure using a 1μm precision filter bag, fill and seal to obtain acrylic coating.

[0012] Preferably, the composite photoinitiator includes photoinitiator TPO and Omnipol 910, with a weight ratio of 3:2 for TPO and Omnipol 910.

[0013] Preferably, the specific process of segmented photocuring is as follows: Acrylic coating is sprayed onto the wheel hub surface using a high-speed rotary cup at a rotation speed of 30,000 rpm, a forming air pressure of 2.5-3.5 bar, an electrostatic voltage of -65 kV, and a dry film thickness of 35-45 μm; subsequently, infrared leveling is performed at a temperature of 60-70℃ for a dwell time of 4 minutes; followed by the first-stage pre-gelling: a UV-LED surface light source with a wavelength of 395 nm and an energy of 150 mJ / cm². 2 Second stage curing: High-pressure mercury lamp, full wavelength, energy 600-800 mJ / cm². 2 Finally, let it stand in the dark for 24 hours to form a light-cured wheel hub coating.

[0014] Another aspect of the present invention provides a photocurable wheel hub coating, the raw materials of which include modified acrylic resin, modified alumina, isobornyl acrylate, pentaerythritol tetra-3-mercaptopropionate, composite photoinitiator, 1,6-hexanediol diacrylate, ultraviolet absorber and leveling agent.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention introduces tricyclic decanediethanol into the resin synthesis process. Utilizing its unique large-volume, rigid tricyclic alicyclic structure, a significant steric hindrance effect is introduced into the polymer chain. This rigid structure not only endows the coating with excellent hardness and heat resistance, but more importantly, it effectively hinders the volume shrinkage during the polymerization of acrylic double bonds, significantly reducing the internal stress during curing. This directly solves the problems of poor adhesion, microcracks, and even peeling caused by the large shrinkage rate of traditional high-functionality UV-cured coatings, thereby greatly improving the yield rate of wheel hub coatings.

[0016] This invention utilizes the low surface energy and thermal motion differences of fluorocarbon segments by grafting 2-(perfluorohexyl)ethanol onto the resin ends and employing a specific infrared (IR) leveling process. This induces the fluorinated segments to migrate and accumulate directionally on the coating surface before curing. After curing, a dense, hydrophobic, and oleophobic "fluorine shield" forms on the coating surface, effectively blocking moisture, acid rain, and ultraviolet radiation from eroding the internal organic framework of the coating. This fundamentally solves the problems of poor weather resistance and easy chalking and yellowing of ordinary acrylic coatings.

[0017] The formulation of this invention introduces pentaerythritol tetra-3-mercaptopropionate, a tetrafunctional compound, which undergoes a "mercapto-ene" click reaction with the carbon-carbon double bonds in the resin. This reaction mechanism is insensitive to oxygen, effectively overcoming the defect of traditional free radical photocuring where the surface easily becomes sticky in air (oxygen inhibition). At the same time, the mercapto-ene reaction has a more uniform gel network structure, further releasing the internal stress generated by crosslinking, and possesses dark reaction characteristics, allowing the coating to continue to improve the degree of crosslinking after removal from the curing line, ensuring the curing uniformity and long-term stability of complex wheel hub surfaces.

[0018] This invention employs a three-stage curing strategy: infrared leveling, LED pre-curing, and mercury lamp main curing. First, the long-wavelength penetration of UV-LEDs (395nm) is used for bottom-layer pre-curing, fixing the coating's volumetric framework and enhancing adhesion to the metal substrate. Then, the high energy across the entire wavelength range of a high-pressure mercury lamp is used for surface sealing and hardness enhancement. This inside-out curing sequence effectively avoids surface wrinkling (orange peel) and stress concentration caused by the surface layer drying before the bottom layer dries, significantly improving the coating's fullness and the consistency of the finished product.

[0019] This invention introduces reactive double bonds into the surface of nano-alumina using KH-570 silane coupling agent, enabling it to participate in the resin's cross-linking network through chemical bonding, rather than simple physical filling. This chemical anchoring effect not only prevents the aggregation of nanoparticles and ensures the transparency of the coating, but also utilizes the high hardness and wear resistance of alumina to enhance the coating's physical anti-aging capabilities (scratch resistance, stone chip resistance), extending the service life of the wheel hub coating under harsh road conditions. Attached Figure Description

[0020] Figure 1 This is a yield chart of Examples 1-5 and Comparative Examples 5-8 in this invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a photocurable wheel hub coating and its preparation method, the technical solution of which is as follows:

[0023] Example 1 100 parts of nano-alumina were added to 500 parts of an ethanol aqueous solution (anhydrous ethanol: deionized water = 95: 5) and dispersed for 30 min using an ultrasonic cell disruptor (power 400W) to obtain a dispersion. The pH of the dispersion was adjusted to about 4.5 with glacial acetic acid. 7 parts of KH-570 were added dropwise while stirring. The temperature was raised to 80℃ and refluxed for 6 h. After the reaction was completed, the mixture was centrifuged, washed 3 times with anhydrous ethanol, dried in a vacuum oven at 80℃ for 12 h, ground and sieved to obtain modified alumina.

[0024] Add 45 parts isophorone diisocyanate, 0.1 parts dibutyltin dilaurate, and 0.1 parts 2,6-di-tert-butyl-p-cresol to a four-necked flask. Purge with nitrogen for protection and heat to 50°C. Slowly add a 1:1 solution of acetone containing 12 parts tricyclodecanediethanol, controlling the temperature fluctuation to no more than ±3°C. After the addition is complete, raise the temperature to 70°C and maintain the reaction for 1.5 hours. Add 30 parts acetone to reduce viscosity and dissolve 10 parts trimethylolpropane. Dissolve the acetone (1:1 ratio) and slowly add it dropwise to a flask, controlling the temperature to not exceed 75°C. After the addition is complete, maintain the temperature for 2.5 h. Then add 4 parts of 2-(perfluorohexyl)ethanol and continue the reaction at 75°C for 1.5 h. After the reaction is complete, cool down to 60°C and slowly add 30 parts of 2-hydroxyethyl acrylate, controlling the total addition time to 30 min. After the addition is complete, raise the temperature to 75°C and continue the reaction for 3.5 h. Remove the acetone by vacuum distillation to obtain the modified acrylic resin. Three parts of modified alumina were added to 12 parts of 1,6-hexanediol diacrylate and ground using a sand mill (zirconia beads) at a controlled temperature of <50℃ until the fineness was <5μm to obtain a dispersed filler. In a light-proof stainless steel mixing vessel, add 55 parts modified acrylic resin, 18 parts isobornyl acrylate, dispersing filler, and 6 parts pentaerythritol tetra-3-mercaptopropionate in sequence. Turn on the high-speed disperser at 1200 rpm and stir for 15 minutes until uniformly mixed. While stirring, add 4 parts composite photoinitiator and Tinuvin R 796 and stir for 25 minutes, paying attention to monitoring the material temperature to ensure it does not exceed 50°C. Reduce the speed to 300 rpm, add 0.5 parts BYK-333 leveling agent, stir for 10 minutes, and let stand for 30 minutes to degas. Finally, filter under pressure using a 1μm precision filter bag, fill and seal to obtain acrylic coating. Acrylic coating was sprayed onto the wheel hub surface using a high-speed rotary cup at 30,000 rpm, with a forming air pressure of 3 bar, an electrostatic voltage of -65 kV, and a dry film thickness of 35-45 μm. This was followed by infrared leveling at 65°C for 4 minutes, and then the first stage of pre-gelling: a UV-LED surface light source with a wavelength of 395 nm and an energy of 150 mJ / cm². 2 Second stage curing: High-pressure mercury lamp, full wavelength, energy 700 mJ / cm². 2 Finally, let it stand in the dark for 24 hours to form a light-cured wheel hub coating.

[0025] Examples 2-5 refer to the parameter conditions in Example 1, with specific differences shown in Table 1.

[0026] Table 1 Parameters and conditions for Examples 1-5 Example Example 1 Example 2 Example 3 Example 4 Example 5 Dosage / parts of isophorone diisocyanate 45 40 42 48 50 Volume / parts of tricyclodecanediethanol in acetone 12 10 11 14 15 Incubation reaction time / h 1.5 1 2 1 2 Dosage of trimethylolpropane / part 10 8 9 11 12 Dosage / parts of 2-(perfluorohexyl)ethanol 4 3 3 5 5 Dosage of modified alumina / parts 3 1 2 4 5 Dosage of nano-alumina / parts 100 95 98 102 105 Dosage of KH-570 / serving 7 9 8 6 5 Reflux reaction time / h 6 5 5.5 6.5 7 Dosage of modified acrylic resin (parts) 55 60 60 50 50 Dosage / parts of composite photoinitiator 4 3 3 5 5 Stirring time / min 25 20 30 20 30 Formed air pressure / bar 3 2.5 3.5 3.5 2.5 Temperature of infrared leveling / °C 65 70 60 70 60 <![CDATA[Energy of the second - stage curing / mJ / cm 2 > 700 650 600 750 800 Comparative Example 1 follows the same parameters and conditions as in Example 1, except that tricyclodecanediethanol is removed during the synthesis of the modified acrylic resin and replaced with an equimolar amount of 1,4-butanediol, while the other steps remain unchanged.

[0027] Comparative Example 2 follows the same parameters and conditions as in Example 1, except that 2-(perfluorohexyl)ethanol is removed during the synthesis of the modified acrylic resin, and the corresponding NCO groups are capped with n-butanol, while the other steps remain unchanged.

[0028] Comparative Example 3 follows the same parameters and conditions as in Example 1, except that isophorone diisocyanate was replaced with toluene diisocyanate in the synthesis of the modified acrylic resin, while the other steps remained unchanged.

[0029] Comparative Example 4 follows the same parameters and conditions as in Example 1, except that trimethylolpropane is removed during the synthesis of the modified acrylic resin, while the other steps remain unchanged.

[0030] Comparative Example 5 follows the same parameters and conditions as in Example 1, except that pentaerythritol tetra-3-mercaptopropionate is not added, and the amount of modified acrylic resin is increased to make up the total amount.

[0031] Comparative Example 6 follows the same parameters and conditions as in Example 1, except that the nano-alumina is not modified.

[0032] Comparative Example 7 follows the same parameters and conditions as in Example 1, except that modified alumina is not added.

[0033] Comparative Example 8 follows the same parameters and conditions as in Example 1, except that isobornyl acrylate is replaced with 1,6-hexanediol diacrylate.

[0034] Comparative Example 9 follows the same parameters and conditions as in Example 1, except that no composite photoinitiator is added.

[0035] Comparative Example 10 follows the same parameters and conditions as in Example 1, except that infrared leveling is not performed.

[0036] Comparative Example 11 follows the same parameters and conditions as in Example 1, except that the first-stage pregelation treatment is not performed.

[0037] Comparative Example 12 follows the same parameters and conditions as in Example 1, except that a second-stage pregelation treatment is not performed.

[0038] Comparative Example 13 follows the same parameters and conditions as in Example 1, except that no dark resting process is performed.

[0039] Experimental Example 1: Hardness, Weather Resistance and Water Resistance Tests The hardness of Examples 1-5 and Comparative Examples 1-4 was tested according to GB / T 6739-2006 standard; the weather resistance of Examples 1-5 and Comparative Examples 1-4 was tested according to GB / T 14522-2008 standard, with a color difference ΔE < 3; Examples 1-5 and Comparative Examples 1-4 were immersed in pure water at 40℃ for 120 hours, and the changes in appearance were observed. The results are shown in Table 2.

[0040] Table 2 Hardness, weather resistance and water resistance of Examples 1-5 and Comparative Examples 1-4 Example hardness Initial values ​​(L / a / b) Color difference ΔE after 400 hours Powdering grade Water resistance Example 1 3H 68.32 / -2.19 / -1.05 1.8 0 No change in appearance Example 2 3H 68.25 / -2.15 / -1.02 1.9 0 No change in appearance Example 3 3H 68.41 / -2.21 / -1.08 1.7 0 No change in appearance Example 4 3H 68.30 / -2.18 / -1.04 2.0 0 No change in appearance Example 5 3H 68.35 / -2.20 / -1.06 1.8 0 No change in appearance Comparative Example 1 HB 68.45 / -2.10 / -0.98 2.8 1 Slight bubbling Comparative Example 2 3H 68.28 / -2.16 / -1.01 6.5 2 Severe whitening and blistering Comparative Example 3 3H 68.15 / -1.85 / -0.50 12.2 0 Severe yellowing Comparative Example 4 H 68.38 / -2.17 / -1.03 2.5 1 Slight loss of light Table 2 shows that Comparative Example 1 used a linear 1,4-butanediol to replace the rigid alicyclic tricyclic decanediol. The test results show that the pencil hardness of Comparative Example 1 decreased significantly from 3H in the example to HB. This indicates that the large tricyclic rigid skeleton of tricyclic decanediol is the core source of the high hardness of the coating. In addition, Comparative Example 1 showed slight blistering in the water resistance test and slightly poor weather resistance (ΔE close to 3). This is because linear diols lack steric hindrance and have a large volume shrinkage rate during curing, which increases the internal stress of the coating and reduces the density between the coating and the substrate, making it easier for moisture to penetrate. Comparative Example 2 removed 2-(perfluorohexyl)ethanol and used only ordinary n-butanol for end capping. Although its hardness remained at 3H (indicating that the rigid skeleton did not change), its weather resistance and water resistance were severely reduced; the color difference ΔE was as high as 6.5, accompanied by grade 2 chalking; severe whitening and blistering occurred in the water resistance test. This fully demonstrates that the use of infrared leveling to induce the migration of fluorinated segments to the surface in the example is crucial. Without this low surface energy fluorine shield, the hydrophilicity of the coating surface increases, and it cannot block the erosion of moisture and oxygen, resulting in the rapid degradation of the internal resin skeleton under ultraviolet light and hot and humid environments. Comparative Example 3 used aromatic toluene diisocyanate instead of alicyclic isophorone diisocyanate. The results showed that although the hardness remained excellent, the color difference ΔE after 400 hours of aging reached 12.2, and the coating visibly yellowed severely. This is because the toluene diisocyanate structure contains benzene ring conjugated double bonds, which readily absorb energy and oxidize to form chromophores (such as quinone imine structures) under ultraviolet irradiation. Conversely, the isophorone diisocyanate used in the examples is an aliphatic isocyanate, does not contain benzene rings, and is transparent to ultraviolet light, thus ensuring the excellent gloss and color retention of the wheel coating. Comparative Example 4 removed trimethylolpropane. Test results showed that its hardness decreased to H, and its water resistance exhibited slight loss of gloss. This is because the crosslinking density of linear molecules is much lower than that of hyperbranched structures, resulting in a less dense network after curing and weakened resistance to external scratches. Simultaneously, the hyperbranched structure provides better intermolecular forces and shielding effects through its highly branched peripheral functional groups; its removal leads to a reduction in the overall physical protective performance of the coating.

[0041] Experimental Example 2: Acid and Alkali Resistance, Yield, and Hardness Tests Examples 1-5 and Comparative Examples 5-9 were immersed in a 10% sulfuric acid solution for 24 hours to test acid resistance, and the changes in appearance were observed. Examples 1-5 and Comparative Examples 5-9 were immersed in a 10% sodium hydroxide solution for 4 hours to test alkali resistance, and the changes in appearance were observed. The number of good products (no shrinkage cavities, no orange peel, no whitening) was counted out of 100 processed pieces, and the yield rate was calculated. The hardness of Examples 1-5 and Comparative Examples 5-9 was tested according to the method of Experimental Example 1. The results are shown in Table 3. The yield rates of Examples 1-5 and Comparative Examples 5-8 are as follows: Figure 1 As shown.

[0042] Table 3. Acid and alkali resistance, yield, and hardness of Examples 1-5 and Comparative Examples 5-9 Example acid resistance Alkali resistance Yield / % hardness Example 1 No change in appearance No change in appearance 98 3H Example 2 No change in appearance No change in appearance 96 3H Example 3 No change in appearance No change in appearance 96 3H Example 4 No change in appearance No change in appearance 98 3H Example 5 No change in appearance No change in appearance 97 3H Comparative Example 5 Slight fogging No change in appearance 78 2H Comparative Example 6 foaming foaming 82 2H Comparative Example 7 No change in appearance No change in appearance 97 HB Comparative Example 8 No change in appearance No change in appearance 85 H Comparative Example 9 Dissolution Dissolution 0 ＜H From Table 3 and Figure 1 It can be observed that Comparative Example 5, by removing pentaerythritol tetra-3-mercaptopropionate from the formulation, showed a significant drop in yield from 98% to 78%, with the hardness decreasing to 2H and slight hazing observed in acid resistance. This is because mercapto compounds play a key role as antioxidants and polymerization inhibitors in the photocuring system. Without mercapto groups, the coating surface is affected by oxygen in the air, resulting in incomplete curing (leading to sticky or slightly wrinkled surfaces) and a sharp drop in yield. Furthermore, the mercapto-alkene reaction can significantly release the internal stress generated by curing shrinkage. Removing it increases the internal stress of the coating, reduces its density, and makes the coating surface more susceptible to acid corrosion. Comparative Example 6 used ordinary nano-alumina without KH-570 modification. The results showed that the yield rate was only 82%, and blistering occurred in both acid and alkali resistance tests. This is because the unmodified nanoparticles have high surface energy and are prone to aggregation, forming visible particle spots or dirt spots in the coating, directly leading to poor appearance. At the same time, the poor interfacial bonding between the aggregated inorganic particles and the organic resin matrix forms microscopic leakage channels. Acid and alkali solutions can quickly penetrate into the substrate through these interfacial defects, causing the coating to blister and fail. Comparative Example 7 did not add any modified alumina dispersion filler. Although its yield rate was high (97%, because there were no particles interfering with leveling), the drawback was that the hardness was only HB, far lower than the 3H of the example. This fully demonstrates that although the modified acrylic resin in this invention has rigidity, it still needs to be combined with the skeletal support of nano-alumina to achieve the high hardness and scratch resistance standard required for wheel hub coatings. Comparative Example 8 replaced isobornyl acrylate with a cyclocyclic structure using linear 1,6-hexanediol diacrylate. The results showed a decrease in hardness to H and a yield rate of 85%. This is because the large isobornyl groups on the side chains of isobornyl acrylate provide rigidity, while 1,6-hexanediol diacrylate, being a flexible chain, cannot maintain the high hardness of the coating. More importantly, as a bifunctional monomer, 1,6-hexanediol diacrylate has a much higher curing shrinkage rate than the monofunctional isobornyl acrylate, resulting in excessive internal stress after coating curing. This easily leads to microcracks or decreased adhesion at the wheel hub edges, thus reducing the final yield rate. Comparative Example 9, without the addition of a composite photoinitiator (as a reverse control), showed that the coating could not form a film, exhibiting severe dissolution and peeling, extremely low hardness, and a yield rate of 0%. Although this was the expected result, it indirectly confirms that for the system of this invention, a highly efficient initiator system must be used to initiate the polymerization reaction, which is fundamental to ensuring complete reaction of the coating from bottom to top and the formation of a dense, chemically resistant protective layer.

[0043] Experimental Example 3: Adhesion, Acid and Alkali Resistance and Weather Resistance Tests The adhesion of Examples 1-5 and Comparative Examples 10-13 was tested according to GB / T 9286-2021 standard; the acid and alkali resistance and weather resistance of Examples 1-5 and Comparative Examples 10-13 were tested according to the test method of Experimental Examples 1-2. The results are shown in Table 4.

[0044] Table 4 Adhesion, acid and alkali resistance and weather resistance of Examples 1-5 and Comparative Examples 10-13 Example Adhesion / Grade acid resistance Alkali resistance Initial values ​​(L / a / b) Color difference ΔE after 400 hours Example 1 0-1 No change in appearance No change in appearance 68.32 / -2.19 / -1.05 1.8 Example 2 0-1 No change in appearance No change in appearance 68.25 / -2.15 / -1.02 1.9 Example 3 0-1 No change in appearance No change in appearance 68.41 / -2.21 / -1.08 1.7 Example 4 0-1 No change in appearance No change in appearance 68.30 / -2.18 / -1.04 2.0 Example 5 0-1 No change in appearance No change in appearance 68.35 / -2.20 / -1.06 1.8 Comparative Example 10 1 foaming white 68.20 / -2.12 / -1.00 6.2 Comparative Example 11 4-5 Coating peeling Coating peeling 68.35 / -2.18 / -1.05 2.8 Comparative Example 12 5 Swelling Swelling 68.30 / -2.15 / -1.03 14.3 Comparative Example 13 2 Slight fogging Slight fogging 68.33 / -2.19 / -1.04 2.6 As can be seen from Table 4, Comparative Example 10 omitted the infrared leveling step and went directly to photocuring. The test results showed that although the adhesion was acceptable (level 1), the weather resistance deteriorated sharply (ΔE as high as 6.2), and blistering and whitening occurred in the acid and alkali resistance test. This is because the fluorinated resin of the present invention relies on the thermodynamic window provided by IR heating to reduce the viscosity of the system and induce the low surface energy fluorinated segments to migrate from the inside of the coating to the surface to form a dense hydrophobic protective layer. Omitting this step caused the fluorinated segments to be instantly locked inside the coating, and the surface lacked fluorine shield protection, resulting in a significant decrease in impermeability and UV resistance. Comparative Example 11 omitted the long-wave pre-curing with 395nm UV-LED and directly used a high-pressure mercury lamp for full-band curing. The results showed a significant reduction in adhesion, and the coating peeled off during the chemical resistance test. This is because the high-pressure mercury lamp has high short-wave energy but weak penetration, instantly curing the coating surface and blocking light transmission to the bottom, leaving the bottom coating in a liquid or semi-cured state, unable to form effective chemical bonds with the wheel hub substrate. The long-wave pre-gelling step with UV-LED is crucial to ensuring the thick film coating is completely dry at the bottom and establishes strong adhesion. Comparative Example 12 omitted the second-stage high-pressure mercury lamp curing and relied solely on LED pre-curing. Test results showed a comprehensive reduction in coating performance, and the coating swelled during the acid and alkali resistance test. This is because the LED can only provide long-wave energy, mainly for deep initiation, and cannot excite all photoinitiators in the system (especially short-wave absorbers), resulting in a severely insufficient cross-linking density of the coating, essentially remaining in a semi-gel state and unable to form a chemically resistant polymer network. Comparative Example 13 was tested immediately after photocuring without a 24-hour dark rest period. The results showed that the adhesion dropped to level 2 and the acid and alkali resistance showed slight haze. This confirmed the dark reaction characteristics of the "thiol-ene" click chemistry in this system. Even after the light source was removed, the residual thiol and double bond would still slowly undergo an addition reaction, further increasing the crosslinking density and consuming the residual monomer. Omitting this process not only resulted in insufficient final conversion rate, but also the internal stress generated in the coating during the rapid photocuring process was not dissipated in time, resulting in a less stable adhesion than in the examples.

[0045] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a light-cured wheel hub coating, characterized in that, Includes the following steps: The acrylic resin was modified by using tricyclodecanediethanol and 2-(perfluorohexyl)ethanol to obtain the modified acrylic resin. Modified alumina and 1,6-hexanediol diacrylate are mixed and ground to obtain a dispersion filler; the modified acrylic resin, isobornyl acrylate, the dispersion filler, pentaerythritol tetra-3-mercaptopropionate, composite photoinitiator, ultraviolet absorber and leveling agent are mixed stepwise to obtain an acrylic coating; the acrylic coating is sprayed onto the surface of the wheel hub and subjected to segmented photocuring treatment to obtain the photocured wheel hub coating; the modified alumina is obtained by modifying nano-alumina with a silane coupling agent.

2. The method for preparing a photocurable wheel hub coating according to claim 1, characterized in that, The modified acrylic resin is prepared as follows: isophorone diisocyanate, dibutyltin dilaurate, and 2,6-di-tert-butyl-p-cresol are added to a flask under nitrogen protection. An acetone solution containing the tricyclodecanediethanol is added dropwise, and the reaction is maintained at a constant temperature after the addition. Trimethylolpropane is dissolved in acetone and added dropwise to the flask. After the addition is complete, the reaction is maintained at a constant temperature. Then, 2-(perfluorohexyl)ethanol is added and reacted. After the reaction is complete, 2-hydroxyethyl acrylate is added dropwise. After the reaction, the mixture is distilled under reduced pressure to obtain the modified acrylic resin.

3. The method for preparing a photocurable wheel hub coating according to claim 1, characterized in that, The method for preparing the dispersion filler is as follows: the modified alumina is added to the 1,6-hexanediol diacrylate, and the mixture is ground using a sand mill to obtain the dispersion filler.

4. The method for preparing a photocurable wheel hub coating according to claim 1, characterized in that, The modified alumina is prepared by adding the nano-alumina into an ethanol aqueous solution and ultrasonically dispersing it to obtain a dispersion; adjusting the pH of the dispersion to acidity with glacial acetic acid, adding KH-570 dropwise under stirring, refluxing the reaction, centrifuging after the reaction is completed, washing, vacuum drying, grinding and sieving to obtain the modified alumina.

5. The method for preparing a photocurable wheel hub coating according to claim 1, characterized in that, The stepwise mixing process is as follows: the modified acrylic resin, isobornyl acrylate, the dispersing filler, and the pentaerythritol tetra-3-mercaptopropionate are added sequentially to a light-proof stirring tank and stirred until homogeneous; the composite photoinitiator and the ultraviolet absorber are added and stirred; finally, the leveling agent is added, stirred, allowed to stand to degas, filtered, and then filled and sealed to obtain the acrylic coating.

6. The method for preparing a photocurable wheel hub coating according to claim 1, characterized in that, The specific process of the segmented photocuring treatment is as follows: the acrylic coating is sprayed onto the surface of the wheel hub with a dry film thickness of 35-45μm; then it enters infrared leveling at a temperature of 60-70℃; then it enters the first stage of pre-gelling with a UV-LED light source. After a second stage of curing, the light source is a mercury lamp with an energy of 600-800 mJ / cm³. 2 Finally, it is left to stand in the dark to form the light-cured wheel hub coating.

7. A light-cured wheel hub coating, characterized in that, The raw materials for synthesizing the photocurable wheel hub coating include modified acrylic resin, modified alumina, isobornyl acrylate, pentaerythritol tetra-3-mercaptopropionate, composite photoinitiator, 1,6-hexanediol diacrylate, ultraviolet absorber, and leveling agent; the photocurable wheel hub coating is prepared by the preparation method according to any one of claims 1-6.

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