PET (Polyethylene Terephthalate) film-coated color-coated sheet and preparation method thereof

Abstract

The invention relates to the technical field of color-coated sheets, and discloses a PET film-coated color-coated sheet and a preparation method thereof.The color-coated sheet comprises a hot-dip galvanized substrate, a polyurethane primer layer and a PET film layer are sequentially arranged on the front face of the hot-dip galvanized substrate, and a pure epoxy back paint layer is arranged on the back face of the hot-dip galvanized substrate. According to the invention, the hot-dip galvanized plate is used as a base, the front side of the hot-dip galvanized plate and the PET film are subjected to non-adhesive hot coating through active functional groups of polyurethane primer, the aging risk of an adhesive layer is eradicated, and the hot-dip galvanized plate is strong in adhesive force, scratch-resistant and aging-resistant; compact pure epoxy back paint is adopted on the back face, excellent shielding and corrosion prevention are provided, and high durability and attractiveness are achieved through the overall structure collaboratively. The film-coated color-coated sheet has good processing performance, salt mist resistance and aging resistance, and meets the production requirements of household appliances such as refrigerators and washing machines.

Description

Technical Field

[0001] This invention relates to the field of color-coated steel sheet technology, and in particular to a PET-coated color-coated steel sheet and its preparation method. Background Technology

[0002] Color-coated steel sheets are made from hot-dip galvanized steel sheets and other substrates, and are produced through processes such as degreasing, cleaning, passivation, coating with primer and topcoat, drying, and cooling. They are rust-proof, weather-resistant, and decorative, and are widely used in home appliances, construction, and other fields.

[0003] Although existing color-coated steel sheets have certain corrosion resistance and decorative properties, they are still insufficient in terms of aging resistance, scratch resistance, and surface gloss durability. Especially when used for the appearance parts of household appliances such as refrigerators and washing machines, the requirements for surface aesthetics and durability are even higher. PET film has been introduced into the surface treatment of color-coated steel sheets due to its excellent transparency, heat resistance, mechanical strength and aging resistance. However, the existing PET composite sheets have the following problems: (1) The process is complicated and the cost is high. For example, some methods require multiple chemical conversion film treatment processes such as setting a phosphating layer and a chromium-free passivation layer between the substrate and the primer layer. This not only increases the complexity and time cost of the production process, but also brings more chemical consumption and wastewater treatment problems. (2) It depends on adhesives and has potential defects: When using glue to bond PET film, firstly, the adhesive itself may have aging risks, affecting the long-term durability of the product; secondly, the presence of the adhesive layer may affect the final surface flatness and gloss; thirdly, the adhesive bonding process has high requirements for environmental and process control, and is prone to quality problems such as uneven adhesive layer, bubbles or delamination.

[0004] Therefore, there is a need for improvements to PET coated steel sheets in the existing technology. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a PET coated sheet and its preparation method. The coated sheet of this invention has good processing, salt spray resistance and aging resistance, which meets the production needs of household appliances such as refrigerators and washing machines.

[0006] To achieve the above objectives, this invention provides a PET coated sheet, characterized in that it includes a hot-dip galvanized substrate, with a polyurethane primer layer and a PET film layer sequentially disposed on the front side of the hot-dip galvanized substrate, and a pure epoxy back coating layer disposed on the back side of the hot-dip galvanized substrate.

[0007] In some embodiments, the Zn content in the coating of the hot-dip galvanized substrate is ≥99%, and the coating weight is 80~120g / m².

[0008] In some embodiments, the thickness of the polyurethane primer layer is 7~10μm, and the thickness of the PET film layer is 12~15μm.

[0009] In some implementations, the thickness of the pure epoxy back coating is 5~7μm.

[0010] In some embodiments, the hot-dip galvanized substrate undergoes a finishing process, with the finishing roller having a roughness Ra of 3.0~4.0μm and a finishing elongation ≥0.8%.

[0011] Another aspect of the present invention provides a method for preparing PET coated steel sheets, for preparing the PET coated steel sheets as described above, comprising: The hot-dip galvanized substrate is degreased, cleaned, chemically coated, and dried. A polyurethane primer is coated on the front side of the hot-dip galvanized substrate, and a pure epoxy back coat is coated on the back side. The board is cured, with the temperature controlled at 214~232℃ and the drying time at 15~40s. A PET film is thermally laminated onto a polyurethane primer layer, bonded through an adhesive chain reaction, and then rolled up after cooling to obtain a PET-coated color-coated sheet.

[0012] In some embodiments, the coating method is either convex or contra-convex roller coating.

[0013] In some embodiments, the PET film lamination is performed on a hot laminator at a lamination temperature of 180~220°C.

[0014] In some implementations, the cooling method is air cooling, cooling to room temperature.

[0015] In some embodiments, the PET film is a biaxially oriented PET film, and the surface of the PET film is corona-treated to enhance adhesion.

[0016] The present invention has at least the following beneficial technical effects: This invention utilizes a hot-dip galvanized substrate as its core, providing an excellent anti-corrosion base. The front side employs a polyurethane primer, whose active functional groups in its molecular structure provide a chemical basis for the subsequent adhesive-free thermal lamination of the PET film, fundamentally avoiding the risks of aging and delamination associated with traditional adhesives—a key factor in achieving high adhesion. Simultaneously, the outer PET film imparts excellent scratch resistance, UV resistance, and aesthetic appeal to the product. The back side uses a more dense pure epoxy back coating, which, in synergy with the hot-dip galvanized substrate, provides strong shielding and anti-corrosion properties, effectively resisting moisture and chemical corrosion in the operating environment. The coated color-coated sheet of this invention possesses excellent processing, salt spray resistance, and aging resistance, meeting the production requirements of household appliances such as refrigerators and washing machines. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of an embodiment of the method for preparing PET coated color-coated sheets provided by the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. For example, terms such as “length,” “width,” “upper,” “lower,” “left,” “right,” “front,” “rear,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” and “outer” indicate orientations or positions based on the orientations or positions shown in the accompanying drawings and are for ease of description only, and should not be construed as limiting the technical solution.

[0021] The terms "comprising" and "having," and any variations thereof, used in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion; the terms "first," "second," etc., used in the specification, claims, and accompanying drawings are used to distinguish different objects, not to describe a particular order. "A plurality of" means two or more, unless otherwise explicitly specified.

[0022] In the description and claims of this invention and the foregoing drawings, when an element is referred to as "fixed to," "mounted to," "disposed on," or "connected to" another element, it can be located directly or indirectly on that other element. For example, when an element is referred to as "connected to" another element, it can be directly or indirectly connected to that other element.

[0023] Furthermore, the reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] The present invention provides a PET coated sheet, including a hot-dip galvanized substrate, wherein a polyurethane primer layer and a PET film layer are sequentially disposed on the front side of the hot-dip galvanized substrate, and a pure epoxy back coating layer is disposed on the back side of the hot-dip galvanized substrate.

[0025] Furthermore, the zinc content in the coating of the hot-dip galvanized substrate is ≥99%, and the coating weight is 80~120 g / m². By limiting the zinc content of the coating of the hot-dip galvanized substrate to ≥99% and controlling the coating weight within the range of 80~120 g / m², a uniform, dense, and sufficient zinc layer protection is ensured for the substrate. This not only provides excellent cathodic protection, effectively preventing corrosion propagation at substrate cut edges and scratches, but also provides a stable and consistent surface for subsequent coating processes, avoiding the problem of decreased coating adhesion caused by fluctuations in coating quality.

[0026] Furthermore, the polyurethane primer layer has a thickness of 7-10 μm, and the PET film layer has a thickness of 12-15 μm. This thickness control achieves an optimal balance between performance and cost. The polyurethane primer layer of this thickness is sufficient to form a continuous, dense film layer, providing a strong anchoring foundation for the PET film, while avoiding the reduced flexibility and increased cost caused by excessive thickness. The PET film of this thickness fully utilizes its weather-resistant and scratch-resistant protective properties, maintains a good appearance, and ensures that the entire sheet has good processability.

[0027] Furthermore, the thickness of the pure epoxy back coating is 5~7μm. This fully utilizes the excellent chemical resistance, barrier properties, and adhesion of epoxy resin. This thickness of coating forms an effective barrier, significantly improving the corrosion resistance of the back of the board, especially against humid environments and various chemicals (such as household cleaners), thus ensuring the overall service life of the product. At the same time, this thickness avoids material waste and the risk of cracking that may result from excessively thick coatings.

[0028] Furthermore, the hot-dip galvanized substrate undergoes a finishing process, with a finishing roller roughness Ra of 3.0~4.0μm and a finishing elongation ≥0.8%. This operation effectively eliminates minute defects on the substrate surface, forming a uniform and suitable micro-roughness, greatly increasing the contact area and mechanical interlocking effect between the substrate and the coating, thereby fundamentally improving the coating adhesion. In addition, the finishing process also improves the flatness and yield strength of the substrate, which is beneficial for subsequent precision machining.

[0029] In another aspect, the present invention provides a method for preparing a PET coated steel sheet, which includes the following steps: The hot-dip galvanized substrate is degreased, cleaned, chemically coated, and dried. A polyurethane primer is coated on the front side of the hot-dip galvanized substrate, and a pure epoxy back coat is coated on the back side. The board is cured, with the temperature controlled at 214~232℃ and the drying time at 15~40s. A PET film is thermally laminated onto a polyurethane primer layer, bonded through an adhesive chain reaction, and then rolled up after cooling to obtain a PET-coated color-coated sheet.

[0030] Furthermore, the coating method can be either directional or reverse roller coating. The choice between directional and reverse roller coating provides great flexibility in the production process. The most suitable coating method can be selected based on the rheological properties of different coatings and specific requirements for coating thickness and surface condition, ensuring a high-quality coating with uniform thickness and no defects, thus meeting the production needs of diverse products.

[0031] Furthermore, the PET film lamination is performed on a hot laminator at a temperature of 180-220℃. Maintaining the lamination temperature within this range is crucial for achieving adhesive-free bonding through a "chain reaction." Within this temperature range, the molecular chains of the polyurethane primer layer are fully activated, enabling efficient chemical bonding with the PET film surface and forming a strong interface far exceeding physical adsorption strength. This temperature range ensures the reaction proceeds fully while preventing excessively high temperatures from deteriorating the PET film or coating.

[0032] Furthermore, the cooling method is air cooling, cooling to room temperature.

[0033] Furthermore, the PET film is a biaxially oriented PET film, and its surface is corona-treated to enhance adhesion. Using a biaxially oriented PET film and corona-treating its bonding surfaces further optimizes the final performance. The biaxial stretching process gives the PET film higher strength, stiffness, and dimensional stability. Corona treatment effectively increases the polar functional groups and roughness of the PET film surface, significantly improving its surface energy, allowing for a more complete and robust adhesive reaction with the polyurethane primer layer, thus ensuring optimal adhesion.

[0034] This invention controls parameters such as polyurethane coating and PET film lamination processes to tightly bond polyurethane-coated steel sheets with PET films, enabling laminated color-coated sheets produced using this process to have excellent processing, salt spray resistance, and aging resistance, meeting the production needs of household appliances such as refrigerators and washing machines.

[0035] The present invention will be further explained below with reference to specific embodiments. In the preparation method of PET coated color-coated sheet of the present invention, the PET film coating process on the front side needs to be completed on a hot laminating machine. After the polyurethane primer on the front side and the pure epoxy backing paint on the back side are cured, the PET film with adhesive is applied to the polyurethane primer surface on the front side by a hot laminating machine. The polyurethane primer adheres to the PET film through a glue chain reaction. After complete curing, it is cooled to room temperature by air cooling and then rolled up.

[0036] Example 1 In this embodiment, we aim to verify the feasibility of the product under the lower limit of process parameters.

[0037] On a certain home appliance color coating production line, hot-dip galvanized steel sheets with a coating weight of 80g / m² are used as substrates. First, the substrates undergo degreasing, cleaning, chemical coating, and drying pretreatment. Then, a 7μm thick polyurethane primer is applied to the front side using a unidirectional coating method, and a 5μm thick pure epoxy back coat is applied to the back side. After coating, the substrate enters a curing oven, where the temperature is controlled at 214℃ and the drying time is 15 seconds.

[0038] After curing, the substrate is fed into a thermal laminator, where a 12μm thick PET film is applied to the surface of the polyurethane primer at 180°C, achieving a strong bond through a chain reaction. After lamination, the substrate is cooled to 20°C using an air-cooling system and finally rolled into a roll.

[0039] Tests have shown that PET-coated color-coated sheets produced under these process parameters still have good adhesion and corrosion resistance, making them particularly suitable for household appliance components where material cost control is more critical and the operating environment is relatively mild.

[0040] Example 2 On a certain home appliance color coating production line, hot-dip galvanized steel sheets with a coating weight of 100g / m² are used as substrates. First, the substrates undergo degreasing, cleaning, chemical coating, and drying pretreatment. Then, a reverse coating method is used to apply an 8μm thick polyurethane primer to the front side and a 6μm thick pure epoxy back coat to the back side. After coating, the substrate enters a curing oven, with the temperature controlled at 225℃ and the drying time at 30 seconds.

[0041] After curing, the substrate is fed into a thermal laminator, where a 14μm thick PET film is applied to the polyurethane primer surface at 220°C, achieving a strong bond through a chain reaction. After lamination, the substrate is cooled to 25°C using an air-cooling system and finally rolled into a roll.

[0042] Example 3 On a certain home appliance color coating production line, hot-dip galvanized steel sheets with a coating weight of 100g / m² are used as substrates. First, the substrates undergo degreasing, cleaning, chemical coating, and drying pretreatment. Then, a reverse coating method is used to apply an 8.5μm thick polyurethane primer to the front side and a 6μm thick pure epoxy back coat to the back side. After coating, the substrate enters a curing oven, where the temperature is controlled at 223℃ and the drying time is 28 seconds.

[0043] After curing, the substrate is fed into a thermal laminator, where a 13.5μm thick PET film is applied to the surface of the polyurethane primer at 200°C, achieving a strong bond through a chain reaction. After lamination, the substrate is cooled to 25°C using an air-cooling system and finally rolled into a roll.

[0044] Example 4 In this embodiment, we aim to verify the performance of the product under the upper limit of process parameters.

[0045] On a certain home appliance color coating production line, hot-dip galvanized steel sheets with a coating weight of 120g / m² are used as substrates. First, the substrates undergo degreasing, cleaning, chemical coating, and drying pretreatment. Then, a 10μm thick polyurethane primer is applied to the front side using a reverse coating method, and a 7μm thick pure epoxy back coat is applied to the back side. After coating, the substrate enters a curing oven, where the temperature is controlled at 232℃ and the drying time is 40 seconds.

[0046] After curing, the substrate is fed into a thermal laminator, where a 15μm thick PET film is applied to the surface of the polyurethane primer at 220°C, achieving a strong bond through a chain reaction. After lamination, the substrate is cooled to 30°C using an air-cooling system and finally rolled into a roll.

[0047] Table 1 Comparison of Test Results In the above embodiments, Examples 1 and 4 demonstrate that even when parameters such as coating weight, coating thickness, temperature, and time are configured with lower or upper limits, the performance of the prepared color-coated plates still far exceeds that of traditional adhesive-coated products.

[0048] The products in Examples 2 and 3 exhibit extremely high adhesion levels, excellent water and flexural resistance, and good processability, fully meeting the stringent requirements for durability and aesthetics in high-end home appliance exterior parts, demonstrating the mature and reliable industrialization level of this invention.

[0049] Testing revealed that the PET-coated color-coated sheets produced under these process parameters exhibited extremely high coating crosslinking density, adhesion, and durability, making them particularly suitable for high-end home appliances or applications in harsh environments that require the highest level of protection and long-term weather resistance.

[0050] Tests have shown that the adhesion of the sheet material provided by the embodiments of the present invention is improved by about 10-15%, and the scratch resistance is significantly enhanced compared with the prior art.

[0051] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0052] It should be understood that, as used herein, the singular form “a” is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, “and / or” refers to any and all possible combinations of one or more of the associated listed items.

[0053] The embodiment numbers disclosed in the above embodiments of the present invention are merely for description and do not represent the superiority or inferiority of the embodiments.

[0054] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A PET coated sheet, characterized in that, The substrate includes a hot-dip galvanized substrate, wherein a polyurethane primer layer and a PET film layer are sequentially disposed on the front side of the hot-dip galvanized substrate, and a pure epoxy back coating layer is disposed on the back side of the hot-dip galvanized substrate.

2. The PET coated steel sheet according to claim 1, characterized in that, The hot-dip galvanized substrate has a Zn content ≥99% in its coating and a coating weight of 80~120g / m².

3. The PET coated sheet according to claim 1, characterized in that, The thickness of the polyurethane primer layer is 7~10μm, and the thickness of the PET film layer is 12~15μm.

4. The PET coated steel sheet according to claim 1, characterized in that, The thickness of the pure epoxy back coating is 5~7μm.

5. The PET coated sheet according to claim 1, characterized in that, The hot-dip galvanized substrate undergoes a finishing process, with a finishing roller roughness Ra of 3.0~4.0μm and a finishing elongation ≥0.8%.

6. A method for preparing PET coated color-coated sheets, characterized in that, For preparing PET coated color-coated sheets as described in any one of claims 1 to 5, comprising: The hot-dip galvanized substrate is degreased, cleaned, chemically coated, and dried. A polyurethane primer is coated on the front side of the hot-dip galvanized substrate, and a pure epoxy back coat is coated on the back side. The board is cured, with the temperature controlled at 214~232℃ and the drying time at 15~40s. A PET film is thermally coated onto the polyurethane primer layer, bonded through an adhesive chain reaction, and then rolled up after cooling to obtain a PET-coated color-coated sheet.

7. The method for preparing PET coated color-coated sheets according to claim 6, characterized in that, The coating method is either forward or reverse roller coating.

8. The method for preparing PET coated color-coated sheets according to claim 6, characterized in that, The PET film lamination is performed on a hot laminator at a temperature of 180~220℃.

9. The method for preparing PET coated color-coated sheets according to claim 6, characterized in that, The cooling method is air cooling, cooling to room temperature.

10. The method for preparing PET coated color-coated sheet according to claim 6, characterized in that, The PET film is a biaxially oriented PET film, and the surface of the PET film is corona-treated to enhance adhesion.

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