An aluminum cap web coating and a method of making the same

Abstract

This invention relates to the field of coating technology, specifically to an aluminum cap coil coating and its preparation method. It achieves rapid curing at 245℃ / 1min by compounding saturated polyester resin A and saturated polyester resin B, combined with a dual crosslinking system of blocked isocyanate curing agent and n-butyl etherified melamine-formaldehyde resin, while maintaining good flexibility and resistance to boiling. Resin A provides moderate crosslinking activity, while resin B retains flexible segments; the two work synergistically to avoid brittleness caused by high crosslinking density. The melamine resin and isocyanate form a complementary crosslinking network, further improving the coating's resistance to damp heat. The addition of calcium sulfate whiskers significantly enhances the coating's flexibility and MEK resistance. This invention overcomes the inherent contradiction between rapid curing and high flexibility in coil coatings through a synergistic design of chemical crosslinking and physical toughening, while simultaneously meeting the stringent requirements of deep drawing and high-temperature boiling of aluminum caps.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to an aluminum cap coil coating and its preparation method. Background Technology

[0002] Aluminum bottle caps are widely used in packaging for food, beverages, and cosmetics due to their advantages such as light weight, ease of processing, and recyclability. The manufacturing process for aluminum caps has gradually shifted from sheet coating to roll coating. Roll coating involves pre-coating aluminum coils with a coating material, followed by uncoiling, stamping, and stretching to form the cap. This pre-coating and post-forming process places stringent requirements on the performance of the coating material.

[0003] First, aluminum caps undergo intense plastic deformation during stretching, requiring the coating to possess excellent flexibility and ductility to prevent cracking and peeling after deep drawing. Second, aluminum caps often require high-temperature sterilization during use, necessitating good adhesion of the coating in humid and hot environments, preventing loss of gloss or oil shedding. Furthermore, the formation of a dense oxide film on aluminum surfaces easily reduces coating adhesion, further complicating coating design. Additionally, on coil coating production lines, curing speed is a key factor determining production efficiency. Traditional coil coatings primarily use saturated polyester resin systems, which, while possessing good flexibility, often require long curing times to achieve sufficient crosslinking density for adequate resistance to humid and hot conditions. This limits the production line speed and reduces efficiency.

[0004] However, rapid curing requires increasing the crosslinking density, but if the crosslinking network is too dense, the coating becomes hard and brittle; high flexibility is the opposite, requiring the retention of uncrosslinked flexible segments. If there are too many flexible segments, the crosslinking density is insufficient, making it impossible to achieve rapid curing and resistance to boiling. This has become a technical challenge that is difficult to overcome in actual production.

[0005] Therefore, developing a coil coating that combines excellent flexibility and high-temperature cooking resistance with rapid curing and improved production line efficiency has significant application value. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing an aluminum cap roll coating with good flexibility, high-temperature cooking resistance, rapid curing capability, and improved production efficiency, as well as its preparation method, thereby solving the technical problems mentioned in the background art.

[0007] The objective of this invention is achieved through the following technical solution: an aluminum cap coil coating, which is composed of the following raw materials in parts by weight: 50-65 parts of white pulp; Saturated polyester resin B20-35 parts; 2-5 parts of blocked isocyanate curing agent; 2-6 parts of melamine-formaldehyde resin; 0-10 parts of calcium sulfate whiskers; 0.1-0.5 parts of defoamer; Surface additives: 0.05-0.3 parts; 2-8 parts of butyl acetate; 2-8 parts xylene; The white paste is composed of the following raw materials in parts by weight: 50-70 parts of saturated polyester resin A; 20-35 parts titanium dioxide; 5-15 parts of divalent ester; 3-8 parts of tetramethylbenzene.

[0008] Preferably, the saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

[0009] Preferably, the hydroxyl value of the saturated polyester resin B is 10-24 mgKOH / g, and the number-average molecular weight is 6000-6800.

[0010] Preferably, the blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

[0011] Preferably, the melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

[0012] Preferably, the defoamer is a compound defoamer of organosilicon and polymer.

[0013] Preferably, the surface additive is a polyether-modified acrylate.

[0014] Preferably, the calcium sulfate whiskers are hemihydrate calcium sulfate whiskers with an average diameter of 2-5 μm and an aspect ratio of 15-50, which have undergone surface modification treatment with a silane coupling agent.

[0015] More preferably, the calcium sulfate whiskers are prepared by: A) drying hemihydrate calcium sulfate whiskers at 60-80℃ for 1-2 hours; B) mixing KH550, ethanol, and deionized water at a volume ratio of 1:4:1 and stirring thoroughly at room temperature for 20-40 minutes to obtain a hydrolysate; C) pouring the dried whiskers into a high-speed mixer, spraying the prepared hydrolysate (1.5-2.0% of the whisker mass) onto the whiskers while stirring, and then subjecting the mixture to ultrasonic vibration for 10-15 minutes. The material is then allowed to stand at room temperature for 1-2 hours, and finally vacuum-dried at 60-70℃ for 1-2 hours. This invention selects hemihydrate calcium sulfate whiskers because they utilize their characteristic of undergoing in-situ dehydration during rapid curing at 245℃ / 1min, transforming into more dense anhydrous calcium sulfate whiskers. This process not only did not damage the whisker structure, but also generated beneficial compressive stress inside the coating through volume shrinkage, and enhanced the interfacial bonding strength between the whiskers and the polyester resin, thus achieving an in-situ reinforcement effect.

[0016] A method for preparing an aluminum cap roll coating includes the following process steps: Step (1) Preparing white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 20-40 minutes, and then grind to a fineness ≤10μm to obtain white paste; Step (2) Mixing paint: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, calcium sulfate whiskers, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 20-40 minutes to obtain an aluminum cap roll coating.

[0017] Preferably, in step (2), calcium sulfate whiskers are first mixed with butyl acetate accounting for 10%-20% of the total amount of butyl acetate, and stirred at low speed for 10-20 minutes at 500-1000 rpm to obtain a pre-dispersed whisker slurry, which is then mixed with other materials.

[0018] The beneficial effects of this invention are as follows: This invention achieves rapid curing at 245℃ / 1min by compounding saturated polyester resin A and saturated polyester resin B, combined with a dual crosslinking system of blocked isocyanate curing agent and n-butyl etherified melamine-formaldehyde resin, while maintaining good flexibility and resistance to boiling. Resin A provides moderate crosslinking activity, while resin B retains flexible segments; the two work synergistically to avoid brittleness caused by high crosslinking density. The melamine resin and isocyanate form a complementary crosslinking network, further improving the coating's resistance to damp heat. The addition of calcium sulfate whiskers significantly enhances the coating's flexibility and MEK resistance. This invention, through the synergistic design of chemical crosslinking and physical toughening, successfully overcomes the inherent contradiction between rapid curing and high flexibility in coil coatings, while simultaneously meeting the stringent requirements of deep drawing and high-temperature boiling of aluminum caps, demonstrating significant industrial application value. Detailed Implementation

[0019] The present invention will be further described in conjunction with the following embodiments.

[0020] In this embodiment of the invention, the blocked isocyanate curing agent is Covestro Desmodur 3175, the saturated polyester resin A is Evonik Dynapol LH818-02, the saturated polyester resin B is Evonik Dynapol LH818-08, the melamine-formaldehyde resin is CYMEL 688, and the defoamer is BYK088 from BYK Chemicals. The surface additive is BYK3560 from BYK Chemicals. Example 1

[0021] An aluminum cap coil coating, comprising the following raw materials in parts by weight: 50 parts white paste; 20 parts of saturated polyester resin B; Two parts of blocked isocyanate curing agent; Two parts of melamine-formaldehyde resin; Two parts of calcium sulfate whiskers; 0.1 parts of defoamer; 0.05 parts of surface additive; 2 parts butyl acetate; Two parts xylene; The white paste is composed of the following raw materials in parts by weight: 50 parts of saturated polyester resin A; 20 parts titanium dioxide; 5 parts of divalent ester; 3 parts of tetramethylbenzene.

[0022] The saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

[0023] The saturated polyester resin B has a hydroxyl value of 10-24 mgKOH / g and a number-average molecular weight of 6000-6800.

[0024] The blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

[0025] The melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

[0026] The defoamer is a compound defoamer of organosilicon and polymer.

[0027] The surface additive is a polyether-modified acrylate.

[0028] The calcium sulfate whiskers are hemihydrate calcium sulfate whiskers with an average diameter of 2-5 μm and an aspect ratio of 15-50, which have undergone surface modification treatment with silane coupling agent.

[0029] A method for preparing an aluminum cap roll coating includes the following process steps: Step (1) Preparing white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 20 minutes, and then grind to a fineness ≤10μm to obtain white paste; Step (2) Adjusting paint: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, calcium sulfate whiskers, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 20 minutes to obtain an aluminum cap roll coating.

[0030] In step (2), calcium sulfate whiskers are first mixed with butyl acetate accounting for 10% of the total amount of butyl acetate, and stirred at low speed for 10 minutes at 500 rpm to obtain a pre-dispersed whisker slurry, which is then mixed with other materials.

[0031] The calcium sulfate whiskers are prepared as follows: A. Hemihydrate calcium sulfate whiskers are dried at 60°C for 2 hours; B. KH550, ethanol, and deionized water are mixed in a volume ratio of 1:4:1 and stirred thoroughly at room temperature for 20 minutes to obtain a hydrolysate; C. The dried whiskers are poured into a high-speed mixer, and while stirring, the prepared hydrolysate (accounting for 1.5% of the whisker mass) is sprayed onto the whiskers, and ultrasonic vibration is applied. The mixture is mixed for 10 minutes, and the material is allowed to stand at room temperature for 1 hour. Finally, it is vacuum dried at 60°C for 2 hours. Example 2

[0032] An aluminum cap coil coating, comprising the following raw materials in parts by weight: 58 portions of white pulp; 28 parts of saturated polyester resin B; 3 parts of blocked isocyanate curing agent; 4 parts of melamine-formaldehyde resin; 6 parts of calcium sulfate whiskers; 0.3 parts of defoamer; 0.1 parts of surface additive; 5 parts of butyl acetate; 5 parts xylene; The white paste is composed of the following raw materials in parts by weight: 60 parts of saturated polyester resin A; 28 parts of titanium dioxide; 10 parts of divalent ester; 5 parts of tetramethylbenzene.

[0033] The saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

[0034] The saturated polyester resin B has a hydroxyl value of 10-24 mgKOH / g and a number-average molecular weight of 6000-6800.

[0035] The blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

[0036] The melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

[0037] The defoamer is a compound defoamer of organosilicon and polymer.

[0038] The surface additive is a polyether-modified acrylate.

[0039] The calcium sulfate whiskers are hemihydrate calcium sulfate whiskers with an average diameter of 2-5 μm and an aspect ratio of 15-50, which have undergone surface modification treatment with silane coupling agent.

[0040] A method for preparing an aluminum cap roll coating includes the following process steps: Step (1) Preparing white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 30 minutes, and then grind to a fineness ≤10μm to obtain white paste; Step (2) Mixing paint: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, calcium sulfate whiskers, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 30 minutes to obtain an aluminum cap roll coating.

[0041] In step (2), calcium sulfate whiskers are first mixed with butyl acetate accounting for 15% of the total amount of butyl acetate, and stirred at low speed at 800 rpm for 15 minutes to obtain a pre-dispersed whisker slurry, which is then mixed with other materials.

[0042] The calcium sulfate whiskers are prepared as follows: A. Hemihydrate calcium sulfate whiskers are dried at 70°C for 2 hours; B. KH550, ethanol, and deionized water are mixed in a volume ratio of 1:4:1 and stirred thoroughly at room temperature for 30 minutes to obtain a hydrolysate; C. The dried whiskers are poured into a high-speed mixer, and while stirring, the prepared hydrolysate (accounting for 1.8% of the whisker mass) is sprayed onto the whiskers, and ultrasonic vibration is applied. The mixture is mixed for 12 minutes, and the material is allowed to stand at room temperature for 2 hours. Finally, it is vacuum dried at 65°C for 2 hours. Example 3

[0043] An aluminum cap coil coating, comprising the following raw materials in parts by weight: 65 parts of white pulp; 35 parts of saturated polyester resin B; 5 parts of blocked isocyanate curing agent; 6 parts of melamine-formaldehyde resin; 10 parts of calcium sulfate whiskers; 0.5 parts of defoamer; 0.3 parts of surface additive; 8 parts of butyl acetate; 8 parts xylene; The white paste is composed of the following raw materials in parts by weight: 70 parts of saturated polyester resin A; 35 parts titanium dioxide; 15 parts of divalent ester; 8 parts of tetramethylbenzene.

[0044] The saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

[0045] The saturated polyester resin B has a hydroxyl value of 10-24 mgKOH / g and a number-average molecular weight of 6000-6800.

[0046] The blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

[0047] The melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

[0048] The defoamer is a compound defoamer of organosilicon and polymer.

[0049] The surface additive is a polyether-modified acrylate.

[0050] The calcium sulfate whiskers are hemihydrate calcium sulfate whiskers with an average diameter of 2-5 μm and an aspect ratio of 15-50, which have undergone surface modification treatment with silane coupling agent.

[0051] A method for preparing an aluminum cap roll coating includes the following process steps: Step (1) Preparing white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 40 minutes, and then grind to a fineness ≤10μm to obtain white paste; Step (2) Adjusting paint: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, calcium sulfate whiskers, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 40 minutes to obtain an aluminum cap roll coating.

[0052] In step (2), calcium sulfate whiskers are first mixed with butyl acetate accounting for 20% of the total amount of butyl acetate, and stirred at low speed at 1000 rpm for 20 minutes to obtain a pre-dispersed whisker slurry, which is then mixed with other materials.

[0053] The calcium sulfate whiskers are prepared as follows: A. Dry the hemihydrate calcium sulfate whiskers at 80°C for 1 hour; B. Mix KH550, ethanol, and deionized water at a volume ratio of 1:4:1 and stir thoroughly at room temperature for 40 minutes to obtain a hydrolysate; C. Pour the dried whiskers into a high-speed mixer, and while stirring, spray the prepared hydrolysate (accounting for 2.0% of the whisker mass) onto the whiskers, and supplement with ultrasonic vibration treatment, mix for 15 minutes, let the material stand at room temperature for 2 hours, and finally vacuum dry at 70°C for 1 hour. Example 4

[0054] An aluminum cap coil coating, comprising the following raw materials in parts by weight: 58 portions of white pulp; 28 parts of saturated polyester resin B; 3 parts of blocked isocyanate curing agent; 4 parts of melamine-formaldehyde resin; 0.3 parts of defoamer; 0.1 parts of surface additive; 5 parts of butyl acetate; 5 parts xylene; The white paste is composed of the following raw materials in parts by weight: 60 parts of saturated polyester resin A; 28 parts of titanium dioxide; 10 parts of divalent ester; 5 parts of tetramethylbenzene.

[0055] The saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

[0056] The saturated polyester resin B has a hydroxyl value of 10-24 mgKOH / g and a number-average molecular weight of 6000-6800.

[0057] The blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

[0058] The melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

[0059] The defoamer is a compound defoamer of organosilicon and polymer.

[0060] The surface additive is a polyether-modified acrylate.

[0061] A method for preparing an aluminum cap roll coating includes the following process steps: Step (1) Preparing white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 30 minutes, and then grind to a fineness ≤10μm to obtain white paste; Step (2) Adjusting paint: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 30 minutes to obtain an aluminum cap roll coating.

[0062] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that no unblocked isocyanate curing agent is used.

[0063] Comparative Example 2 The difference between Comparative Example 1 and Example 2 is that neither the unblocked isocyanate curing agent nor calcium sulfate whiskers are used.

[0064] The experimental data of Examples 1-4 and Comparative Examples 1-2 of this invention are shown in Table 1. Fineness was determined using a scraper fineness meter according to GB / T1724-2019, requiring ≤10μm; viscosity was measured using a Forecast-4 cup at 25℃ (winter) or 30℃ (summer) according to GB / T1723-1993, and the flow time (seconds) was recorded; solid content was calculated by drying in an oven at 105±2℃ to constant weight according to GB / T1725-2007; curing time was simulated under roll coating conditions in an oven at 245±5℃, and the shortest time for complete curing of the coating (non-sticky, no mark when pressed with a fingernail, and MEK wiping meets the standard) was measured; MEK wiping was performed by wiping repeatedly with a 1kg weight on a cotton ball soaked in MEK according to ASTM D5402. Wipe and record the number of times the coating breaks; impact resistance is tested according to GB / T1732-2020, using a 1kg hammer and an 8mm steel ball to drop from a height of 50cm, the paint film is considered qualified if it does not crack; T-bending is tested according to GB / T30791-2014, folding 180° and tearing with tape, the minimum T value is evaluated; adhesion is tested according to GB / T9286-2021, using a 1mm spacing cross-cutting tool to draw a 6×6 grid, tearing with 3M tape, grade 0 or 1 is considered 100%; high temperature boiling is tested according to GB / T1733-1993, the boiling water method, immersing 2 / 3 of the test panel in boiling water at 100±1℃ for 1 hour, after taking it out, visually inspect for loss of gloss, oil peeling, blistering, and discoloration, and retest the adhesion retention rate.

[0065] Table 1 Test Project Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Fineness (μm) ≤10 ≤10 ≤10 ≤10 ≤10 ≤10 Viscosity (seconds / 25°C) 142 148 155 145 138 135 Solid content (%) 62.5 63.5 64.8 63.0 61.2 60.5 Curing time (245℃) 1min 1min 1min 1min >2min >2min MEK wipes (times) 42 50 55 32 18 12 Impact resistance (50cm / kg) No cracking No cracking No cracking No cracking cracking cracking T-bend 2T 1T 1T 2T 4T 5T Adhesion (cross-cut test) 100% 100% 100% 100% 95% 90% Adhesion after high-temperature cooking 100% 100% 100% 100% 60% 50% Appearance after high-temperature steaming No loss of shine, no oil residue No loss of shine, no oil residue No loss of shine, no oil residue No loss of shine, no oil residue Significant loss of shine, localized oil shedding Severe loss of gloss and extensive oil shedding Analysis of experimental data, as shown in Table 1, reveals that Examples 1-3 of this invention exhibit excellent performance across various aspects. Although Example 4 lacks calcium sulfate whiskers, it forms a complementary cross-linked network due to the simultaneous presence of a blocked isocyanate curing agent and melamine-formaldehyde resin. The blocked isocyanate reacts rapidly with the polyester resin after unblocking at 245°C, achieving rapid curing within 1 minute; the melamine resin further fills the network pores, increasing the cross-linking density. Therefore, it achieves 32 MEK wiping cycles, maintains 100% adhesion after high-temperature boiling, and exhibits impact resistance without cracking. However, lacking the physical toughening effect of calcium sulfate whiskers, its T-bending is only 2T, indicating lower flexibility than Example 2 (1T) containing whiskers. This demonstrates that chemical cross-linking alone is insufficient to achieve optimal deep-drawing performance.

[0066] In Comparative Example 1, the lack of blocked isocyanate prevented rapid curing. The crosslinking reaction of the n-butylated melamine-formaldehyde resin was extremely incomplete under conditions of 245℃ / 1min without a strong acid catalyst, resulting in an under-cured coating that was only surface-dry but not internally dry. Therefore, the MEK test was only 18 cycles, with impact cracking resistance and a T-bending test of 4T (brittle fracture). Although calcium sulfate whiskers were added, the insufficient crosslinking of the matrix resin led to insufficient cohesive strength in the coating, preventing the whiskers from functioning effectively. Instead, the weak interfacial bonding caused stress concentration points, exacerbating brittleness. During high-temperature cooking, water molecules easily penetrated the under-cured coating, causing adhesion to drop to 60%, resulting in significant loss of gloss and oil shedding.

[0067] Comparative Example 2 lacked both a fast-curing primary crosslinking agent and a physical toughening filler. The coating could barely form effective crosslinks within 1 minute, achieving only 12 MEK cycles, severe cracking at 5T bending, and only 50% adhesion after high-temperature cooking, resulting in large-area oil peeling. This comparative example demonstrates that both the dual crosslinking system and whiskers are indispensable; melamine resin alone cannot meet the requirements for rapid curing of coil coatings; flexibility is limited without whiskers; and complete failure occurs without both.

[0068] In summary, this invention, through the synergistic design of chemical cross-linking and physical toughening, successfully overcomes the inherent contradiction between rapid curing and high flexibility of coil coatings, while simultaneously meeting the stringent requirements of deep drawing and high-temperature steaming of aluminum caps.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A coating for aluminum cap rolls, characterized in that: It consists of the following raw materials in parts by weight: 50-65 parts of white pulp; Saturated polyester resin B20-35 parts; 2-5 parts of blocked isocyanate curing agent; 2-6 parts of melamine-formaldehyde resin; 0-10 parts of calcium sulfate whiskers; 0.1-0.5 parts of defoamer; Surface additives: 0.05-0.3 parts; 2-8 parts of butyl acetate; 2-8 parts xylene; The white paste is composed of the following raw materials in parts by weight: 50-70 parts of saturated polyester resin A; 20-35 parts titanium dioxide; 5-15 parts of divalent ester; 3-8 parts of tetramethylbenzene.

2. The coating for aluminum cap coils according to claim 1, characterized in that: The saturated polyester resin A has a hydroxyl value of 25-40 mgKOH / g and a number-average molecular weight of 5000-5800.

3. The coating for aluminum cap rolls according to claim 1, characterized in that: The saturated polyester resin B has a hydroxyl value of 10-24 mgKOH / g and a number-average molecular weight of 6000-6800.

4. The coating for aluminum cap rolls according to claim 1, characterized in that: The blocked isocyanate curing agent is a blocked aliphatic polyisocyanate based on hexamethylene diisocyanate.

5. The coating for aluminum cap coils according to claim 1, characterized in that: The melamine-formaldehyde resin is n-butyl etherified melamine-formaldehyde resin.

6. The coating for aluminum cap coils according to claim 1, characterized in that: The defoamer is a compound defoamer of organosilicon and polymer.

7. The coating for aluminum cap coils according to claim 1, characterized in that: The surface additive is a polyether-modified acrylate.

8. The coating for aluminum cap coils according to claim 1, characterized in that: The calcium sulfate whiskers are hemihydrate calcium sulfate whiskers with an average diameter of 2-5 μm and an aspect ratio of 15-50, which have undergone surface modification treatment with silane coupling agent.

9. A method for preparing an aluminum cap coil coating as described in any one of claims 1-8, characterized in that: It includes the following process steps: Step (1) Preparation of white paste: Mix saturated polyester resin A, titanium dioxide, divalent ester and tetramethylbenzene according to the formula amount, disperse for 20-40 minutes, and then grind to fineness ≤10μm to obtain white paste; Step (2) Paint preparation: Mix the white paste obtained in step (1) with saturated polyester resin B, blocked isocyanate curing agent, melamine formaldehyde resin, calcium sulfate whiskers, defoamer, surface additive, butyl acetate and xylene according to the formula amount, disperse for 20-40 minutes to obtain an aluminum cover coil coating.

10. The method for preparing an aluminum cap coil coating according to claim 9, characterized in that: In step (2), calcium sulfate whiskers are first mixed with butyl acetate accounting for 10%-20% of the total amount of butyl acetate, and stirred at low speed for 10-20 minutes at 500-1000 rpm to obtain a pre-dispersed whisker slurry, which is then mixed with other materials.