Bisphenol A-free polyester transparent oil and preparation method thereof

Abstract

The invention relates to the technical field of food packaging coatings, and particularly discloses bisphenol A-free polyester transparent oil and a preparation method thereof. The bisphenol A-free polyester transparent oil is prepared from the following raw materials: liquid polyester resin, a blocked polyisocyanate curing agent, an alkyl etherified melamine formaldehyde resin curing agent, polyester phosphate resin, a scratch-resistant agent, a catalyst, a flatting agent, a shrinkage-cavity-resistant auxiliary agent, a diluent and a solvent. The liquid polyester resin is linear polyester containing long-chain alkyl. The bisphenol-A-free polyester transparent oil can effectively reduce the curing temperature of a coating, and solves the problem of balance between low-temperature curing and high performance.

Description

Technical Field

[0001] This invention relates to the field of food packaging coating technology, and in particular to a bisphenol A-free polyester transparent oil and its preparation method. Background Technology

[0002] In the food packaging industry, three-piece cans are widely used for sealing and storing canned foods. Three-piece food cans typically use tin-plated or chrome-plated thin steel sheets as the base material, with a white PET film as the inner layer that comes into direct contact with the food. This film is laminated onto the metal substrate using a hot melt adhesive process, offering excellent barrier properties and environmental benefits. The other side of the base material needs to be coated with a baking-type transparent coating to protect the steel sheet.

[0003] In existing technologies, the transparent oil coating on the top and bottom covers of three-piece cans mostly adopts a polyester-based baking paint system, using bisphenol A-containing epoxy resin as the base resin. A typical example is a bisphenol A-based epoxy-polyester mixed resin coating. However, existing coatings usually need to be baked and cured at a high temperature of 180-200℃ to achieve sufficient crosslinking and performance. But due to its high curing temperature, the curing process is prone to local crystallization and thermal degradation of the PET film on the inner layer of the three-piece can, resulting in problems such as whitening, cracking, and decreased resistance to boiling. Summary of the Invention

[0004] To reduce the curing temperature of the coating and solve the problem of balancing low-temperature curing and high performance, this application provides a bisphenol A-free polyester transparent oil and its preparation method.

[0005] Firstly, this application provides a bisphenol A-free polyester transparent oil, which adopts the following technical solution: A bisphenol A-free polyester transparent oil, prepared by weight from the following raw materials: 65-80 parts liquid polyester resin, 8-12 parts blocked polyisocyanate curing agent, 3-6 parts alkyl etherified melamine formaldehyde resin curing agent, 1-3 parts polyester phosphate resin, 1-2 parts scratch-resistant agent, 0-1 part catalyst, 0-1 part leveling agent, 0-1 part anti-cratering agent, 5-10 parts diluent, and 6-8 parts solvent; The liquid polyester resin is a linear polyester containing long-chain alkyl groups.

[0006] By adopting the above technical solution, this application uses liquid polyester resin as the base resin. The liquid polyester resin and polyester phosphate resin have a synergistic effect. The phosphate group is introduced into the molecular chain of the liquid polyester resin through the polyester phosphate resin. The phosphorus-oxygen double bond on the phosphate group has strong electronegativity and can be strongly adsorbed on the surface of metal oxide to form coordination bonds, hydrogen bonds or ionic bonds. In addition, the phosphate group is a highly polar group, which makes the resin have better compatibility and wettability with the metal substrate. At the same time, the phosphorus hydroxyl group in the phosphate ester chelates with the metal surface to form a metal phosphate complex, which passivates the metal surface, thereby improving the coating adhesion, enhancing the coating's corrosion resistance and resistance to contents, and reducing the baking temperature.

[0007] The blocked polyisocyanate curing agent and the alkyl etherified melamine-formaldehyde resin curing agent work synergistically to interact with the active groups in the resin molecules at lower temperatures, promote the formation of cross-linked network structures, improve the density of the coating, make up for the deficiencies of the cross-linked network of linear polyester resin and polyester phosphate ester, and enhance the scratch resistance of the coating.

[0008] Preferably, the liquid polyester resin has a number-average molecular weight of 8000-10000 and a bubble viscosity of Z4-Z5.

[0009] By adopting the above technical solution, using polyester resin with a suitable molecular weight and long-chain alkyl hydrophobic groups, the ester bonds are shielded from water molecule attack through the hydrophobic effect, significantly improving the resin's resistance to boiling and hydrolysis. Furthermore, the long-chain alkyl groups are grafted onto the side chains of the polyester molecules, giving the resin good flexibility and crosslinking density.

[0010] Preferably, the blocked polyisocyanate curing agent is selected from one or both of blocked HDI curing agents and blocked IPDI curing agents.

[0011] Preferably, the alkyl etherified melamine-formaldehyde resin curing agent is selected from one or two of isobutyl etherified melamine-formaldehyde resin curing agent and methyl etherified melamine-formaldehyde resin curing agent.

[0012] Preferably, the blocked polyisocyanate curing agent is a blocked HDI curing agent, and the alkyl etherified melamine-formaldehyde resin curing agent is an isobutyl etherified melamine-formaldehyde resin curing agent; the mass ratio of the blocked HDI curing agent to the isobutyl etherified melamine-formaldehyde resin curing agent is 1:(0.3-0.5).

[0013] By employing the above technical solution, the blocked polyisocyanate curing agent de-encapsulates upon heating, allowing its isocyanate groups to react with active groups such as hydroxyl groups in the liquid polyester resin to form a polyurethane structure, imparting excellent flexibility and abrasion resistance to the coating. The alkyl etherified melamine-formaldehyde resin curing agent, on the other hand, undergoes a cross-linking reaction with the liquid polyester resin, enhancing the coating's hardness and chemical resistance. The synergistic effect of these two agents optimizes the cross-linking network and enhances the overall performance of the coating. Specifically, the blocked HDI curing agent and the isobutyl etherified melamine-formaldehyde resin curing agent, when combined according to the aforementioned mass ratio, produce a superior synergistic effect.

[0014] Preferably, the scratch-resistant agent is polyethylene micron wax.

[0015] By adopting the above technical solution, polyethylene micron wax is precipitated during the film formation process, forming fine particles that float on the coating surface, which can effectively disperse friction and reduce local wear.

[0016] Preferably, the catalyst is an environmentally friendly organometallic catalyst.

[0017] By adopting the above technical solution, an environmentally friendly organometallic catalyst is used to provide catalytic action for the crosslinking reaction of the resin, thus ensuring the catalytic effect while meeting environmental protection requirements.

[0018] Preferably, the leveling agent is a polyacrylate leveling agent.

[0019] By adopting the above technical solution, the use of polyacrylate leveling agents can help improve the leveling properties of coatings, making the coating surface smoother and more even.

[0020] Preferably, the anti-cratering agent is an organosilicon anti-cratering agent.

[0021] By adopting the above technical solution, the use of organosilicon anti-cratering agent can reduce the occurrence of cratering in transparent oil during the coating process and ensure the appearance quality of the coating.

[0022] Preferably, the diluent is a low-naphthalenetetramethylbenzene diluent, and the solvent is diacetone alcohol.

[0023] By adopting the above technical solution, the diluent and solvent are used to adjust the viscosity of the transparent oil.

[0024] Secondly, this application provides a method for preparing a bisphenol A-free polyester transparent oil, which adopts the following technical solution: A method for preparing a bisphenol A-free polyester transparent oil includes the following steps: Liquid polyester resin, blocked polyisocyanate curing agent, alkyl etherified melamine formaldehyde resin curing agent, diluent, and solvent are added to a stirred tank and stirred to react. Then, polyester phosphate resin, scratch-resistant agent, and catalyst are added and stirred to react. Leveling agent and anti-cratering agent are added and stirring is continued. Samples are taken to monitor the viscosity and solid content of the reactants to obtain bisphenol A-free polyester transparent oil.

[0025] Preferably, the viscosity of the reactants is 100-120 seconds / 25°C, and the solid content is 38%-42%.

[0026] By adopting the above technical solution, the transparent oil prepared by the above method has a lower curing temperature, which can avoid the problem of local crystallization and thermal degradation of the PET film in the inner layer of the three-piece can caused by high-temperature curing, and solve the problem of balancing comprehensive performance under low-temperature flexible curing.

[0027] This application has the following beneficial effects: This application uses liquid polyester resin as the base resin. The liquid polyester resin and polyester phosphate resin have a synergistic effect. The phosphate group is introduced into the molecular chain of the liquid polyester resin through the polyester phosphate resin. The phosphorus-oxygen double bond on the phosphate group has strong electronegativity and can be strongly adsorbed on the surface of metal oxide to form coordination bonds, hydrogen bonds or ionic bonds. In addition, the phosphate group is a highly polar group, which makes the resin have better compatibility and wettability with the metal substrate. At the same time, the phosphorus hydroxyl group in the phosphate ester chelates with the metal surface to form a metal phosphate complex, which passivates the metal surface, thereby improving the coating adhesion, enhancing the coating's corrosion resistance and resistance to contents, and reducing the baking temperature.

[0028] The blocked polyisocyanate curing agent and the alkyl etherified melamine-formaldehyde resin curing agent work synergistically to interact with the active groups in the resin molecules at lower temperatures, promote the formation of cross-linked network structures, improve the density of the coating, make up for the deficiencies of the cross-linked network of linear polyester resin and polyester phosphate ester, and enhance the scratch resistance of the coating. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the embodiments.

[0030] Under single-layer coating conditions, the coating itself lacks sufficient hardness, resistance to pointer scratches, resistance to boiling, and adhesion. This makes the coating susceptible to mold scratches and loss of gloss during cap making and transportation. The molecular chains are prone to hydrolysis and lack flexibility, making it difficult to balance energy saving, cost control, and performance requirements.

[0031] Therefore, this application provides a method that can reduce the coating curing temperature and solve the problem of balancing low-temperature curing and high performance. Example

[0032] Example 1 A bisphenol A-free polyester transparent oil, comprising: 650g liquid polyester resin, 80g blocked HDI curing agent, 30g isobutyl etherified melamine-formaldehyde resin curing agent, 10g polyester phosphate resin, 10g polyethylene micron wax, 3g environmentally friendly organometallic catalyst, 3g polyacrylate leveling agent, 3g organosilicon anti-cracking agent, 50g low naphthalene tetramethylbenzene diluent, 60g diacetone alcohol.

[0033] The liquid polyester resin is a linear polyester with a number-average molecular weight of 8000, a bubble viscosity of Z4, and a solid content of 50%. The linear polyester resin's molecular structure contains long-chain alkyl hydrophobic groups. The liquid polyester resin is a resin solution with a solid polyester resin content of 50% obtained by dissolving isophorone and tetramethylbenzene at a mass ratio of 1:1. The solid polyester resin can be selected from Suzhou Hanhai New Material Co., Ltd.'s CL275 solid saturated polyester resin.

[0034] Among them, the closed-type HDI curing agent can be DH 3175 from Dihe New Materials (Foshan) Co., Ltd.; The curing agent for isobutyl etherified melamine-formaldehyde resin can be 586 amino resin from Dongguan Juncheng Chemical Co., Ltd. Polyester phosphate resin can be selected from Jiangxi Longhai Chemical Co., Ltd.; Polyethylene micronized wax can be selected from Tianshi New Material Technology Co., Ltd.'s PEW-0256; For environmentally friendly organometallic catalysts, Shanghai Deyin Chemical Co., Ltd.'s DY-20 organobismuth catalyst can be selected; BYK-355 can be used as a polyacrylate leveling agent; For silicone anti-cratering additives, BYK-333 can be selected.

[0035] The preparation method of the bisphenol A-free polyester transparent oil in this embodiment includes the following steps: Liquid polyester resin, blocked HDI curing agent, isobutyl etherified melamine-formaldehyde resin curing agent, low-naphthalene tetramethylbenzene diluent, and diacetone alcohol were added to a stirred tank and stirred at 1000 r / min for 15 min. Then, polyester phosphate resin, polyethylene micron wax, and environmentally friendly organometallic catalyst were added and stirred at 1000 r / min for 8 min. Finally, polyacrylate leveling agent and organosilicon anti-cratering agent were added and stirred at 1000 r / min for another 8 min. The viscosity and solid content of the reactants were sampled and monitored. When the viscosity of the reactants was 100 seconds / 25℃ and the solid content was 38%, a bisphenol A-free transparent polyester oil was obtained.

[0036] Example 2 A bisphenol A-free polyester transparent oil, comprising: 700g liquid polyester resin, 100g blocked HDI curing agent, 45g isobutyl etherified melamine-formaldehyde resin curing agent, 20g polyester phosphate resin, 15g polyethylene micron wax, 5g environmentally friendly organometallic catalyst, 5g polyacrylate leveling agent, 5g organosilicon anti-cratering agent, 80g low naphthalene tetramethylbenzene diluent, 70g diacetone alcohol.

[0037] The liquid polyester resin is a linear polyester with a number-average molecular weight of 9000, a bubble viscosity of Z4, and a solid content of 50%. The linear polyester resin's molecular structure contains long-chain alkyl hydrophobic groups. The liquid polyester resin is a resin solution with a solid polyester resin content of 50% obtained by dissolving isophorone and tetramethylbenzene at a mass ratio of 1:1. The solid polyester resin can be selected from Suzhou Hanhai New Material Co., Ltd.'s CL275 solid saturated polyester resin.

[0038] Among them, the closed-type HDI curing agent can be DH 3175 from Dihe New Materials (Foshan) Co., Ltd.; The curing agent for isobutyl etherified melamine-formaldehyde resin can be 586 amino resin from Dongguan Juncheng Chemical Co., Ltd. Polyester phosphate resin can be selected from Jiangxi Longhai Chemical Co., Ltd.; Polyethylene micronized wax can be selected from Tianshi New Material Technology Co., Ltd.'s PEW-0256; For environmentally friendly organometallic catalysts, Shanghai Deyin Chemical Co., Ltd.'s DY-20 organobismuth catalyst can be selected; BYK-355 can be used as a polyacrylate leveling agent; For silicone anti-cratering additives, BYK-333 can be selected.

[0039] The preparation method of the bisphenol A-free polyester transparent oil in this embodiment includes the following steps: Liquid polyester resin, blocked HDI curing agent, isobutyl etherified melamine-formaldehyde resin curing agent, low-naphthalenetetramethylbenzene diluent, and diacetone alcohol were added to a stirred tank and stirred at 1200 rpm for 18 min. Then, polyester phosphate resin, polyethylene micron wax, and environmentally friendly organometallic catalyst were added and stirred at 1000 rpm for 10 min. Finally, polyacrylate leveling agent and organosilicon anti-cratering agent were added and stirred at 1000 rpm for another 10 min. The viscosity and solid content of the reactants were sampled and monitored. When the viscosity of the reactants was 110 seconds / 25℃ and the solid content was 40%, a bisphenol A-free transparent polyester oil was obtained.

[0040] Example 3 A bisphenol A-free polyester transparent oil, comprising: 800g liquid polyester resin, 120g blocked HDI curing agent, 60g isobutyl etherified melamine-formaldehyde resin curing agent, 30g polyester phosphate resin, 20g polyethylene micron wax, 8g environmentally friendly organometallic catalyst, 8g polyacrylate leveling agent, 8g organosilicon anti-cracking agent, 100g low naphthalene tetramethylbenzene diluent, 80g diacetone alcohol.

[0041] The liquid polyester resin is a linear polyester with a number average molecular weight of 10,000, a bubble viscosity of Z5, and a solid content of 50%. The linear polyester resin's molecular structure contains long-chain alkyl hydrophobic groups. The liquid polyester resin is a resin solution with a solid polyester resin content of 50% obtained by dissolving isophorone and tetramethylbenzene at a mass ratio of 1:1. The solid polyester resin can be selected from Suzhou Hanhai New Material Co., Ltd.'s CL275 solid saturated polyester resin.

[0042] Among them, the closed-type HDI curing agent can be DH 3175 from Dihe New Materials (Foshan) Co., Ltd.; The curing agent for isobutyl etherified melamine-formaldehyde resin can be 586 amino resin from Dongguan Juncheng Chemical Co., Ltd. Polyester phosphate resin can be selected from Jiangxi Longhai Chemical Co., Ltd.; Polyethylene micronized wax can be selected from Tianshi New Material Technology Co., Ltd.'s PEW-0256; For environmentally friendly organometallic catalysts, Shanghai Deyin Chemical Co., Ltd.'s DY-20 organobismuth catalyst can be selected; BYK-355 can be used as a polyacrylate leveling agent; For silicone anti-cratering additives, BYK-333 can be selected.

[0043] The preparation method of the bisphenol A-free polyester transparent oil in this embodiment includes the following steps: Liquid polyester resin, blocked HDI curing agent, isobutyl etherified melamine-formaldehyde resin curing agent, low-naphthalenetetramethylbenzene diluent, and diacetone alcohol were added to a stirred tank and stirred at 1100 rpm for 20 min. Then, polyester phosphate resin, polyethylene micron wax, and environmentally friendly organometallic catalyst were added and stirred at 1000 rpm for 12 min. Finally, polyacrylate leveling agent and organosilicon anti-cratering agent were added and stirred at 1000 rpm for another 12 min. The viscosity and solid content of the reactants were sampled and monitored. When the viscosity of the reactants was 120 seconds / 25℃ and the solid content was 42%, a bisphenol A-free transparent polyester oil was obtained.

[0044] Example 4 The difference between this embodiment and Embodiment 2 is that the same amount of closed-type HDI curing agent is replaced with closed-type IPDI curing agent.

[0045] Among them, the closed-type IPDI curing agent can be selected from DH 5078 of Dihe New Materials (Foshan) Co., Ltd.

[0046] Example 5 The difference between this embodiment and Embodiment 2 is that the isobutyl etherified melamine-formaldehyde resin curing agent is replaced with methyl etherified melamine-formaldehyde resin curing agent.

[0047] Among them, the curing agent for methylated melamine-formaldehyde resin can be 5767 methylated amino resin from Zhejiang Xinhua New Material Technology Co., Ltd.

[0048] Example 6 The difference between this embodiment and Embodiment 2 is that the amount of the blocked HDI curing agent is 110g and the amount of the isobutyl etherified melamine-formaldehyde resin curing agent is 55g, that is, the mass ratio of the blocked HDI curing agent to the isobutyl etherified melamine-formaldehyde resin curing agent is 1:0.5.

[0049] Among them, the closed-type HDI curing agent can be DH 3175 from Dihe New Materials (Foshan) Co., Ltd.; The curing agent for isobutyl etherified melamine-formaldehyde resin can be 586 amino resin from Dongguan Juncheng Chemical Co., Ltd.

[0050] Comparative Example Comparative Example 1 A bisphenol A-free polyester transparent oil differs from Example 2 in that the liquid polyester resin is replaced by an equal mass of type A epoxy resin E44.

[0051] Comparative Example 2 A bisphenol A-free polyester transparent oil differs from Example 2 in that the liquid polyester resin is replaced by a hyperbranched polyester resin by an equal mass.

[0052] The hyperbranched polyester resin is a resin solution with a solid polyester resin content of 50% obtained by dissolving isophorone and tetramethylbenzene in a 1:1 mass ratio. The solid polyester resin can be selected from Suzhou Hanhai New Material Co., Ltd.'s CL151 solid saturated polyester resin.

[0053] Comparative Example 3 A bisphenol A-free polyester transparent oil differs from Example 2 in that the liquid polyester resin is replaced by an equal mass of polyester resin that does not contain hydrophobic groups.

[0054] Among them, the polyester resin that does not contain hydrophobic groups can be selected from Evonik Specialty Chemicals (Shanghai) Co., Ltd.'s DYNAPOL L651.

[0055] Comparative Example 4 A bisphenol A-free polyester transparent oil differs from Example 2 in that the polyester phosphate resin is replaced by the liquid polyester resin in Example 2.

[0056] Comparative Example 5 A bisphenol A-free polyester transparent oil differs from Example 2 in that the isobutyl etherified melamine-formaldehyde resin curing agent is replaced by a closed-type HDI curing agent.

[0057] Comparative Example 6 A bisphenol A-free polyester transparent oil differs from Example 2 in that the same mass of the blocked HDI curing agent is replaced with isobutyl etherified melamine-formaldehyde resin curing agent.

[0058] Comparative Example 7 A bisphenol A-free polyester transparent oil differs from Example 2 in that the isobutyl etherified melamine-formaldehyde resin curing agent is replaced by n-butanol etherified melamine-formaldehyde resin curing agent.

[0059] Among them, the curing agent for n-butanol etherified melamine-formaldehyde resin can be Dongguan Juncheng Chemical Co., Ltd.'s 582-2A amino resin.

[0060] Comparative Example 8 A bisphenol A-free polyester transparent oil differs from Example 2 in that the isobutyl etherified melamine-formaldehyde resin curing agent is replaced by a urea-formaldehyde resin curing agent.

[0061] Among them, the urea-formaldehyde resin curing agent can be ETERMINO 9115-60-2 n-butyl etherified urea amino resin from Changxing Materials Industry Co., Ltd.

[0062] Performance testing Preparation of coating: The bisphenol A-free polyester transparent oil prepared in Examples 1-6 and Comparative Examples 1-8 was coated onto a tin-plated thin steel plate in one layer, with a coating amount of 5 g / m². 2 Cured at 140℃, and the following tests were performed after curing.

[0063] 1. Adhesion test: The test is conducted according to GB / T 9286-2021 "Paints and Varnishes Cross-cut Test". 100 small squares are cut on a tin-plated thin steel plate coated with BPA-free polyester transparent oil using a sharp blade. Then, special tape is applied and quickly peeled off. The extent of coating peeling within the squares is observed to evaluate the adhesion. The adhesion level is divided into 0-5, with 0 being the best and 5 being the worst.

[0064] 2. Hardness test: The hardness of the coatings in the examples and comparative examples were tested according to GB / T6739-2006 "Determination of Hardness of Paints and Varnishes by Pencil Method".

[0065] 3. Pointer Scratch Resistance Test: Based on the test principle of ASTM D 5178-98, scratch resistance is determined by pushing an arc-shaped (ring or circular) scriber onto the test plate. The scriber is installed at a 45-degree angle and pressed down onto the surface of the test plate. The load on the test plate is gradually increased until the coating is scratched.

[0066] 4. Bending Test: A tin-plated steel sheet coated with a BPA-free polyester transparent oil was cut into five pieces approximately 70mm x 40mm. The tin-plated steel sheets were bent along the 70mm axis using a metal rod (coated side outwards). The bent tin-plated steel sheets were placed in the grooves of a bending tester. The impact block was raised to its highest point and dropped to impact the bent tin-plated steel sheets. After immersing the bent sheets in the mixed solution for 2 minutes, the length (mm) of the red dot at the fold was recorded for each of the five sheets. The average value was calculated as the bending performance data of the transparent oil. The mixed solution was prepared by mixing CuSO4·5H2O: concentrated HCl: H2O in a mass ratio of approximately 2:1:7.

[0067] 5. Deep-drawing test: Cut a tin-plated thin steel sheet coated with BPA-free polyester transparent oil into approximately 120mm × 120mm squares. Apply a thin layer of lubricating oil to both sides of the tin-plated thin steel sheet, and use a punch to press the coated surface to form a square can. Classify the coating peeling at different corners (R1-R4) of the can according to Table 1. Find the corresponding score for each corner in Table 1 according to the grade, and take the worst score from corners R1 to R4 as the deep-drawing performance evaluation result for this transparent oil.

[0068] Table 1 6. High-pressure boiling resistance test: Cut two pieces of tin-plated thin steel sheet coated with BPA-free polyester transparent oil, one as a standard sample and the other as a test sample. Place the test sample in a sterilizer filled with distilled water, start timing when the temperature reaches 120℃, and maintain it at (120±5)℃ for 90 minutes. After that, take it out and compare the test sample and the standard sample. According to Table 2, score each test item and take the worst score as the comprehensive score of the high-pressure boiling of the transparent oil.

[0069] Table 2 The test results are shown in Table 3 below: Table 3 Based on the comparison between Example 2 and Comparative Example 1 and the data in Table 3, it can be seen that: Comparative Example 1 uses bisphenol A type resin. Bisphenol A type resin has high rigidity and poor flexibility. Fish scales will fall off during deep drawing of the R-angle. Due to insufficient resin extensibility, stress concentration during stamping leads to coating peeling. The coating cracks when bending, resulting in bending deviation.

[0070] Based on the comparison between Example 2 and Comparative Example 2, and the data in Table 3, it can be seen that: since nonlinear polyester resin requires a higher curing temperature, the nonlinear polyester resin in Comparative Example 2 may have incomplete cross-linking and curing at a lower temperature, resulting in poor coating hardness, poor scratch resistance, easy scratching of the coating surface, and easy coating peeling off during stamping.

[0071] Based on the comparison between Example 2 and Comparative Example 3 and the data in Table 3, it can be seen that: Comparative Example 3 uses polyester resin without hydrophobic groups. Ester bonds without hydrophobic group protection are easily hydrolyzed. After cooking, the coating whitens and loses its gloss. Hydrolysis causes water to enter the micropores. The adhesion of the coating will be weakened due to the hydrolysis of ester bonds, thereby weakening the interfacial bonding force between the resin and the substrate. At the same time, the coating becomes more brittle after hydrolysis, cracks when bent, and its flexibility deteriorates.

[0072] Based on the comparison between Example 2 and Comparative Example 4, and the data in Table 3, it can be seen that liquid polyester resin and polyester phosphate resin have a synergistic effect. Comparative Example 4 did not add polyester phosphate resin, and the resin molecules did not contain phosphate groups, resulting in a significant decrease in coating adhesion.

[0073] Based on the comparison between Example 2 and Comparative Examples 5-6, and the data in Table 3, it can be seen that the blocked polyisocyanate curing agent and the alkyl etherified melamine-formaldehyde resin curing agent have a synergistic effect. Comparative Example 5 used only the blocked HDI curing agent, and Comparative Example 6 used only the isobutyl etherified melamine-formaldehyde resin curing agent. Because the single curing agent could not form a dense network at 140°C, the surface crosslinking degree was low, the coating hardness was insufficient, the scratch resistance was poor, the coating surface was easily scratched, and the insufficiently crosslinked coating peeled off completely during stamping, resulting in poor deep drawing performance.

[0074] Based on the comparison between Example 2 and Comparative Example 7, and the data in Table 3, it can be seen that: during low-temperature curing, the isobutyl etherified melamine-formaldehyde resin curing agent has higher reactivity and a higher degree of cross-linking with the resin, and can produce a better synergistic effect when combined with the blocked HDI curing agent.

[0075] Based on the comparison between Example 2 and Comparative Example 8, and the data in Table 3, it can be seen that urea-formaldehyde resin may retain residual free hydrophilic groups such as hydroxymethyl groups. These hydrophilic groups will continuously absorb or release moisture, causing the adhesive layer to repeatedly expand and shrink, and even detach. Furthermore, urea-formaldehyde resin may have a continuous release problem due to unreacted free formaldehyde present during the synthesis process. In contrast, the isobutyl etherified melamine-formaldehyde resin curing agent in this application has a lower formaldehyde release rate. Its synergistic effect with the blocked HDI curing agent can optimize the crosslinking network and enhance the overall performance of the coating.

[0076] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this specific embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A bisphenol A-free polyester transparent oil, characterized in that, It is prepared from the following raw materials in parts by weight: 65-80 parts liquid polyester resin, 8-12 parts blocked polyisocyanate curing agent, 3-6 parts alkyl etherified melamine formaldehyde resin curing agent, 1-3 parts polyester phosphate resin, 1-2 parts scratch-resistant agent, 0-1 part catalyst, 0-1 part leveling agent, 0-1 part anti-cratering agent, 5-10 parts diluent, and 6-8 parts solvent; The liquid polyester resin is a linear polyester containing long-chain alkyl groups.

2. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The liquid polyester resin has a number average molecular weight of 8000-10000 and a bubble viscosity of Z4-Z5.

3. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The blocked polyisocyanate curing agent is selected from one or both of blocked HDI curing agents and blocked IPDI curing agents.

4. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The alkyl etherified melamine-formaldehyde resin curing agent is selected from one or two of isobutyl etherified melamine-formaldehyde resin curing agents and methyl etherified melamine-formaldehyde resin curing agents.

5. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The blocked polyisocyanate curing agent is selected from the blocked HDI curing agent, and the alkyl etherified melamine-formaldehyde resin curing agent is selected from the isobutyl etherified melamine-formaldehyde resin curing agent; the mass ratio of the blocked HDI curing agent to the isobutyl etherified melamine-formaldehyde resin curing agent is 1:(0.3-0.5).

6. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The scratch-resistant agent is polyethylene micron wax.

7. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The catalyst is an environmentally friendly organometallic catalyst.

8. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The leveling agent is a polyacrylate leveling agent.

9. The bisphenol A-free polyester transparent oil according to claim 1, characterized in that, The anti-shrinkage agent is an organosilicon anti-shrinkage agent.

10. A method for preparing a bisphenol A-free polyester transparent oil according to any one of claims 1-9, characterized in that, Includes the following steps: Liquid polyester resin, blocked polyisocyanate curing agent, alkyl etherified melamine-formaldehyde resin curing agent, diluent, and solvent are added to a stirred tank and stirred to react. Then, polyester phosphate resin, scratch-resistant agent, and catalyst are added and stirred to react. Leveling agent and anti-cratering agent are added and stirring is continued. The viscosity and solid content of the reactants are sampled and monitored to obtain a bisphenol A-free polyester transparent oil.

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