Refrigerator door panel and manufacturing process thereof

Abstract

This invention discloses a refrigerator door panel and its manufacturing process. The refrigerator door panel comprises a cold-rolled steel sheet, the outer surface of which is sequentially coated with an alloy plating layer, an outer coating layer, and a topcoat layer; the inner surface of the cold-rolled steel sheet is sequentially coated with an alloy plating layer and an inner coating layer. This invention coats both the outer and inner surfaces of the door panel, providing dual optimization treatment for both surfaces. The outer surface significantly improves the adhesion of the topcoat, preventing paint film peeling and pinholes in ocean environments; the inner surface strengthens the adhesion of the foam layer, avoiding foam delamination and blistering. Combined with the self-healing properties of the zinc-aluminum-magnesium plating layer, the service life in ocean environments is extended by 3-5 years. This invention uses cold-rolled steel sheet as the substrate, reducing procurement costs by 50% compared to traditional MGSS stainless steel. It also eliminates the material costs of the 55μm epoxy water-based primer on the outer surface and the 15μm foam adhesive on the inner surface, as well as the costs of spraying and the energy consumption for primer drying.

Description

Technical Field

[0001] This invention relates to the field of refrigerator technology, and more specifically to a refrigerator door panel and its manufacturing process. Background Technology

[0002] Currently, refrigerated container doors are mainly made of MGSS stainless steel. The outer surface of the door is coated with a 55μm epoxy water-based primer and a 35μm water-based polyurethane topcoat, while the inner surface requires a separate 15μm expanding foam coating. The supporting door production process includes: sheet material cutting → bending and forming → welding and assembly → weld seam grinding → surface sanding → spraying 55μm epoxy water-based primer on the outer surface → primer drying → spraying 35μm water-based polyurethane topcoat on the outer surface → topcoat drying → spraying 15μm expanding foam coating on the inner surface, totaling 9 core processes.

[0003] The existing production process for refrigerated container doors has several technical defects: First, it involves numerous processes and a long production cycle. The outer surface requires epoxy water-based primer spraying, drying, and sandblasting, while the inner surface requires separate spraying of 15μm expanding foam. These multiple steps are time-consuming and reliant on manual operation, making them poorly adaptable to automated production. Second, controlling the roughness of the MGSS stainless steel surface after sandblasting is difficult, easily leading to surface oxidation and iron filings. This results in insufficient adhesion of the 55μm epoxy water-based primer on the outer surface, making it prone to missed areas and pinholes in weak areas such as door corners and welds. Furthermore, it is highly susceptible to corrosion in ocean salt spray environments. Third, the on-site application of the epoxy water-based primer and the inner surface expanding foam requires extremely high temperature, humidity, and cleanliness standards, easily leading to paint film defects and uneven foam adhesion, resulting in a low yield rate. Fourth, MGSS materials have high procurement costs, lack self-healing capabilities at the cut edges, and require separate coating of the inner and outer surfaces. This process is redundant, has a low tolerance for error, and incurs high maintenance costs. Summary of the Invention

[0004] The purpose of this invention is to provide a refrigerator door panel that at least partially solves the technical defects mentioned in the background art.

[0005] Therefore, the present invention provides a refrigerator door panel, the refrigerator door panel comprising a cold-rolled steel plate, wherein the outer side of the cold-rolled steel plate is provided with an alloy plating layer, an outer coating layer and a topcoat layer in sequence; and the inner side of the cold-rolled steel plate is provided with an alloy plating layer and an inner coating layer in sequence.

[0006] In some embodiments of the present invention, the coating weight of the alloy coating is 80-120 g / m². 2 .

[0007] In some embodiments of the present invention, the coating composition of the alloy coating, by weight, includes the following components: Al 9-11wt%, Mg 2.8-3.1wt%, Si 0.1-0.2wt%, with the balance being Zn.

[0008] In some embodiments of the present invention, the thickness of the outer coating is 5-10 μm, the thickness of the topcoat layer is 30-40 μm, and the thickness of the inner coating is 5-10 μm.

[0009] In some embodiments of the present invention, the coating of the outer coating comprises a modified epoxy ester resin, an adhesion promoter, and a salt spray resistant additive, wherein the mass ratio of the modified epoxy ester resin, the adhesion promoter, and the salt spray resistant additive is (85-95): (2-8): (3-7).

[0010] In some embodiments of the present invention, the adhesion promoter includes organophosphorus coupling agents and / or silane coupling agents; the salt spray resistance aid includes flake shielding agent, hydrotalcite ion scavenger and chromium-free corrosion inhibitor, wherein the mass ratio of flake shielding agent, hydrotalcite ion scavenger and chromium-free corrosion inhibitor is (50-60): (25-35): (10-20).

[0011] In some embodiments of the present invention, the coating composition of the topcoat layer includes waterborne polyurethane.

[0012] In some embodiments of the present invention, the coating of the inner coating comprises polyurethane modified acrylic resin and foaming interface compatibilizer, wherein the mass ratio of polyurethane modified acrylic resin to foaming interface compatibilizer is (95-99): (1-5).

[0013] In some embodiments of the present invention, the foaming interface compatibilizer includes a hydroxyl-terminated polyurethane prepolymer, a polyurethane-grafted acrylate copolymer, a modified polyether polyol, and a polyether-modified organosilicon, wherein the mass ratio of the hydroxyl-terminated polyurethane prepolymer, the polyurethane-grafted acrylate copolymer, the modified polyether polyol, and the polyether-modified organosilicon is (50-60):(25-35):(8-15):(2-5).

[0014] The present invention also provides a manufacturing process for the aforementioned refrigerator door panel, comprising: (1) Hot-dip alloy coating is applied to the outer side of the cold-rolled steel sheet, and an outer coating is applied to the surface of the alloy coating; hot-dip alloy coating is applied to the inner side of the cold-rolled steel sheet, and an inner coating is applied to the surface of the alloy coating. (2) The cold-rolled steel sheet is bent into a door panel; (3) Weld the corner seams of the door panel. After welding, only clean the welding slag and spatter. No overall grinding is required. (4) Locally repair the damaged areas of the coating at the weld seams and bends of the door panel. After repair, the dry film thickness is ≥80μm. After the surface is dry at room temperature, apply weather-resistant polyurethane sealant to the edge of the door panel to achieve full sealing and corrosion protection. (5) Apply a topcoat layer to the outer surface of the door panel, dry and cure to obtain the refrigerator door panel.

[0015] Compared with the prior art, the advantages and positive effects of the present invention are: This invention eliminates four redundant processes: surface sanding, epoxy water-based primer spraying, primer drying, and 15μm foaming adhesive spraying on the inner surface. It simplifies nine processes into five, saving primer, foaming adhesive materials, labor, and drying energy consumption. The production cycle is shortened by 50%, making it suitable for automated mass production. It is more environmentally friendly with no dust pollution, and the process is greatly simplified, doubling the efficiency.

[0016] This invention is fully compatible with existing systems, requires zero modification costs, retains the 35μm waterborne polyurethane topcoat, and allows for seamless production switching without changing the coating line, topcoat formula, or curing parameters.

[0017] This invention applies a coating to both the outer and inner surfaces of the door panel, providing dual optimization for both surfaces. The outer surface significantly improves paint adhesion, preventing paint film peeling and pinholes in ocean environments; the inner surface strengthens the adhesion of the foam layer, preventing foam delamination and blistering. Combined with the self-healing properties of the zinc-aluminum-magnesium plating, this extends the service life in ocean environments by 3-5 years.

[0018] The invention significantly reduces overall costs and yields substantial cumulative benefits. Using cold-rolled steel sheet as the substrate, the procurement cost is 50% lower than that of traditional MGSS stainless steel. Simultaneously, it completely eliminates the material costs of the 55μm epoxy water-based primer on the outer surface and the 15μm foam adhesive on the inner surface, as well as the costs of spraying, primer drying energy consumption, and surface sandblasting pretreatment and environmental protection. This multi-layered cost reduction results in a 50% reduction in the overall manufacturing cost of a single door panel compared to traditional processes, offering a significantly higher cost-performance ratio than existing technologies, with particularly outstanding benefits in mass production.

[0019] The present invention has a higher yield rate. The pre-base coating is applied by standardized roller coating in steel mills, resulting in uniform film thickness, no missed coating, and no pinholes. This avoids the defects of on-site water-based primer application and improves the finished product qualification rate.

[0020] Other features and advantages of the present invention will become clearer after reading the detailed embodiments of the invention in conjunction with the accompanying drawings. Attached Figure Description

[0021] Figure 1 This is a structural cross-sectional schematic diagram of one embodiment of the refrigerator door panel of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0023] like Figure 1As shown, the present invention discloses a refrigerator door panel, which includes a cold-rolled steel plate 10. The outer and inner sides of the cold-rolled steel plate 10 are respectively provided with an alloy coating 11. The surface of the alloy coating 11 on the outer side of the cold-rolled steel plate 10 is provided with an outer coating layer 12, and the surface of the outer coating layer 12 is provided with a topcoat layer 14. The surface of the alloy coating 11 on the inner side of the cold-rolled steel plate is provided with an inner coating layer 13.

[0024] The coating weight of alloy coating 11 is 80-120 g / m². 2 This can reduce production costs while ensuring that the salt spray test is passed.

[0025] The alloy coating 11 has the following components by weight: Al 9-11wt%, Mg 2.8-3.1wt%, Si 0.1-0.2wt%, with the balance being Zn.

[0026] Alloy coating 11 has self-healing properties at the cut, and its resistance to marine salt spray is superior to MGSS stainless iron. The self-healing principle of zinc-aluminum-magnesium alloy is as follows: after the coating is damaged, Mg and Al ions dissolve and react with moisture and oxygen in the environment to form a dense alkaline protective film at the damaged site, preventing the spread of rust. This process requires the coating to provide sufficient Mg, Al, and Zn ions as "repair materials".

[0027] The thickness of the outer coating 12 is 5-10μm. If the thickness of the outer coating 12 is too low, the adhesion cannot be guaranteed, and if the thickness of the outer coating 12 is too high, it will result in a waste of cost.

[0028] The outer coating 12 comprises the following components: modified epoxy ester resin, adhesion promoter, and salt spray resistant additive. The mass ratio of modified epoxy ester resin, adhesion promoter, and salt spray resistant additive is (85-95): (2-8): (3-7).

[0029] Adhesion promoters include organophosphorus coupling agents and / or silane coupling agents.

[0030] Salt spray resistant additives include flake shielding agent, hydrotalcite ion scavenger and chromium-free corrosion inhibitor, with a mass ratio of (50-60): (25-35): (10-20).

[0031] The coating composition of the topcoat layer 14 includes water-based polyurethane, and the thickness of the topcoat layer is 30-40μm, which meets the anti-corrosion requirements of the topcoat layer while taking into account the manufacturing cost.

[0032] The outer coating 12 is mainly composed of modified epoxy ester resin, combined with an organophosphorus / silane composite adhesion promoter. The active functional groups on its molecular chain undergo an irreversible chemical cross-linking reaction with the functional groups of the waterborne polyurethane topcoat, forming covalent bonds / coordination bonds, which transforms the two coatings from "physical adhesion" to "molecular-level fusion". This is the core reason why the interlayer bonding force is far superior to that of traditional processes.

[0033] The core function of the outer coating 12 is to strengthen the interlayer adhesion with the water-based polyurethane topcoat, ensuring that the topcoat does not peel off or bubble.

[0034] The thickness of the inner coating 13 is 5-10μm. If the thickness of the inner coating is too low, the adhesion cannot be guaranteed, and if the thickness of the inner coating is too high, it will result in a waste of cost.

[0035] The inner coating 13 comprises the following components: polyurethane modified acrylic resin and foaming interface compatibilizer.

[0036] The foaming interface compatibilizer includes hydroxyl-terminated polyurethane prepolymer, polyurethane-grafted acrylate copolymer, modified polyether polyol, and polyether-modified organosilicon. The mass ratio of hydroxyl-terminated polyurethane prepolymer, polyurethane-grafted acrylate copolymer, modified polyether polyol, and polyether-modified organosilicon is (50-60): (25-35): (8-15): (2-5).

[0037] The core function of the inner coating 13 is to enhance the bonding strength with the rigid polyurethane foam inside the refrigerator and prevent delamination and blistering. The inner coating 13 relies on the structural homology of the resin body for basic bonding, the directional interface regulation and strengthening of the compatibilizer, the physical micro-interlocking for dense bonding, and the long-term guarantee of environmental weather resistance. These four functions work together to eliminate the core causes of delamination and blistering, ultimately achieving high-strength and long-lasting bonding with the rigid polyurethane foam.

[0038] This invention also provides a manufacturing process for refrigerator door panels, including: (1) Hot-dip galvanize an alloy coating 11 on the outer side of the cold-rolled steel plate 10, and coat an outer coating 12 on the surface of the alloy coating 11; hot-dip galvanize an alloy coating 11 on the inner side of the cold-rolled steel plate 10, and coat an inner coating 13 on the surface of the alloy coating 11.

[0039] The coating weight of alloy coating 10 is 80-120 g / m². The coating composition of alloy coating 10, by mass, includes the following components: Al 9-11 wt%, Mg 2.8-3.1 wt%, Si 0.1-0.2 wt%, with the balance being Zn.

[0040] An outer coating 12 is applied to the surface of the alloy plating layer 11 on the outer side of the cold-rolled steel sheet 10 using a precision roller coating and high-temperature curing process.

[0041] The outer coating 12 is a pre-primer coating that is directly bonded to the topcoat, and the thickness of the outer coating 12 is 5-10μm.

[0042] An inner coating 13 is applied to the surface of the alloy plating layer 11 on the inner side of the cold-rolled steel sheet 10 using a precision roller coating and high-temperature curing process.

[0043] The inner coating 13 is a foamed direct-bonding pre-base coating with a thickness of 5-10 μm.

[0044] No surface sandblasting, epoxy water-based primer spraying, primer drying, or 15μm foaming of the inner surface are carried out at the container production site. Instead, the process proceeds directly to the processing and topcoat coating stage.

[0045] (2) Cut the material according to the design size of the door panel using CNC cutting, and then bend it into a door panel. Control the bending force to ensure that the pre-coating and base coating is free from cracks and peeling.

[0046] (3) Weld the corner seams of the molded door panel. After welding, only the welding slag and spatter need to be cleaned. No overall grinding is required.

[0047] (4) For the damaged areas of the coating at the weld seams and bends of the door panel, apply epoxy repair paint of the same system as the pre-primer coating locally. The dry film thickness after repair is ≥80μm. After surface drying at room temperature, apply weather-resistant polyurethane sealant to the edges of the door panel to achieve full sealing and corrosion protection.

[0048] (5) Spray a 35μm water-based polyurethane topcoat on the outer coating 13 surface, dry and cure at low temperature, and obtain the finished refrigerator door panel after curing; the coating can greatly improve the adhesion of the topcoat and ensure that the paint film is stable and does not fall off for a long time.

[0049] This invention eliminates four redundant processes: surface sanding, epoxy water-based primer spraying, primer drying, and 15μm foaming adhesive spraying on the inner surface. It simplifies nine processes into five, saving primer, foaming adhesive materials, labor, and drying energy consumption. The production cycle is shortened by 50%, making it suitable for automated mass production. It is more environmentally friendly with no dust pollution, and the process is greatly simplified, doubling the efficiency.

[0050] This invention is fully compatible with existing systems, requires zero modification costs, retains the 35μm waterborne polyurethane topcoat, and allows for seamless production switching without changing the coating line, topcoat formula, or curing parameters.

[0051] This invention applies a coating to both the outer and inner surfaces of the door panel, providing dual optimization for both surfaces. The outer surface significantly improves paint adhesion, preventing paint film peeling and pinholes in ocean environments; the inner surface strengthens the adhesion of the foam layer, preventing foam delamination and blistering. Combined with the self-healing properties of the zinc-aluminum-magnesium plating, this extends the service life in ocean environments by 3-5 years.

[0052] The invention significantly reduces overall costs and yields substantial cumulative benefits. Using cold-rolled steel sheet as the substrate, the procurement cost is 50% lower than that of traditional MGSS stainless steel. Simultaneously, it completely eliminates the material costs of the 55μm epoxy water-based primer on the outer surface and the 15μm foam adhesive on the inner surface, as well as the costs of spraying, primer drying energy consumption, and surface sandblasting pretreatment and environmental protection. This multi-layered cost reduction results in a 50% reduction in the overall manufacturing cost of a single door panel compared to traditional processes, offering a significantly higher cost-performance ratio than existing technologies, with particularly outstanding benefits in mass production.

[0053] The present invention has a higher yield rate. The pre-base coating is applied by standardized roller coating in steel mills, resulting in uniform film thickness, no missed coating, and no pinholes. This avoids the defects of on-site water-based primer application and improves the finished product qualification rate. Example

[0054] The refrigerator door panel of this embodiment 1 includes a cold-rolled steel plate. The outer side of the cold-rolled steel plate is provided with an alloy plating layer, an outer coating layer and a topcoat layer in sequence; the inner side of the cold-rolled steel plate is provided with an alloy plating layer and an inner coating layer in sequence.

[0055] The cold-rolled steel sheet is 1.5mm thick, with a total alloy coating of 100g / m² on both sides. The coating composition includes: Al 10wt%, Mg 3.0wt%, Si 0.15wt%, and the balance is Zn.

[0056] The outer coating is a topcoat direct-adhesion coating, and the inner coating is a foamed direct-adhesion coating.

[0057] Steel mill roller coating and curing, curing plate temperature 180-200℃, heat preservation for 30s, cross-cut adhesion grade 0; Topcoat system: Industry standard 35μm water-based polyurethane topcoat, which is directly sprayed onto the outer coating surface without primer; the inner surface does not require foaming adhesive and is directly adapted for foaming.

[0058] The manufacturing process of the refrigerator door panel in this embodiment 1 includes: (1) Hot-dip alloy coating is applied to the outer side of the cold-rolled steel sheet, and an outer coating is applied to the surface of the alloy coating; hot-dip alloy coating is applied to the inner side of the cold-rolled steel sheet, and an inner coating is applied to the surface of the alloy coating. (2) The cold-rolled steel sheet is bent into a door panel; (3) Weld the corner seams of the door panel. After welding, only clean the welding slag and spatter. No overall grinding is required. (4) Locally repair the damaged coating areas of the door panel welds and bends, with a dry film thickness of ≥80μm. After surface drying at room temperature, apply weather-resistant polyurethane sealant to the edge of the door panel to achieve full sealing and corrosion protection. (5) Apply a 35μm water-based polyurethane topcoat layer to the outer surface of the door panel, dry and cure to obtain the refrigerator door panel.

[0059] Topcoat system: Industry standard 35μm water-based polyurethane topcoat, which is directly sprayed onto the outer coating surface without primer; the inner surface does not require foaming adhesive and is directly adapted for foaming.

[0060] The door panel of the refrigerated container in Example 1 has a cross-cut adhesion rating of 0, no red rust or blistering after 3000 hours of neutral salt spray testing, and no coating peeling after high and low temperature cycling from -40℃ to 70℃. The production cycle is shortened by 50% compared to the traditional MGSS process, and the overall cost is reduced by 50%, which fully meets the usage requirements of ocean-going refrigerated containers.

[0061] Comparative Example 1 The refrigerator door panel of Comparative Example 1 is made of 1.5mm thick SUS409L MGSS stainless steel and follows the standard process: material cutting → bending → welding → grinding → surface sanding → spraying 55μm epoxy water-based primer on the outer surface → primer drying → spraying 35μm water-based polyurethane topcoat on the outer surface → topcoat drying → spraying 15μm foam adhesive on the inner surface.

[0062] According to industry standards, the neutral salt spray test can reach 1000 hours, but the welds and weak corners are prone to premature corrosion, the foam on the inner surface is prone to uneven bonding, the production cycle is long, the overall cost is nearly twice that of Example 1, and the overall economic benefits are low.

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A refrigerator door panel, characterized in that, The refrigerator door panel is made of cold-rolled steel sheet. The outer surface of the cold-rolled steel sheet is sequentially provided with an alloy coating, an outer coating layer, and a topcoat layer; The inner surface of the cold-rolled steel sheet is provided with an alloy plating layer and an inner coating layer in sequence.

2. The refrigerator door panel as described in claim 1, characterized in that, The coating weight of the alloy coating is 80-120 g / m². 2 .

3. The refrigerator door panel as described in claim 1, characterized in that, The alloy coating composition, by weight, includes the following components: Al 9-11wt%, Mg 2.8-3.1wt%, Si 0.1-0.2wt%, with the balance being Zn.

4. The refrigerator door panel as described in claim 1, characterized in that, The outer coating has a thickness of 5-10 μm, the topcoat has a thickness of 30-40 μm, and the inner coating has a thickness of 5-10 μm.

5. The refrigerator door panel as described in claim 1, characterized in that, The coating for the outer layer includes a modified epoxy ester resin, an adhesion promoter, and a salt spray resistant additive. The mass ratio of modified epoxy ester resin, adhesion promoter and salt spray resistant additive is (85-95): (2-8): (3-7).

6. The refrigerator door panel as described in claim 5, characterized in that, Adhesion promoters include organophosphorus coupling agents and / or silane coupling agents; Salt spray resistant additives include flake-shaped shielding agents, hydrotalcite ion scavengers, and chromium-free corrosion inhibitors. The mass ratio of the sheet-like shielding agent, the hydrotalcite ion scavenger, and the chromium-free corrosion inhibitor is (50-60): (25-35): (10-20).

7. The refrigerator door panel as described in claim 1, characterized in that, The coating composition of the topcoat layer includes water-based polyurethane.

8. The refrigerator door panel as described in claim 1, characterized in that, The coating for the inner side includes polyurethane-modified acrylic resin and a foaming interface compatibilizer. The mass ratio of polyurethane modified acrylic resin to foaming interface compatibilizer is (95-99): (1-5).

9. The refrigerator door panel as described in claim 8, characterized in that, Foaming interface compatibilizers include hydroxyl-terminated polyurethane prepolymers, polyurethane-grafted acrylate copolymers, modified polyether polyols, and polyether-modified silicones. The mass ratio of hydroxyl-terminated polyurethane prepolymer, polyurethane-grafted acrylate copolymer, modified polyether polyol and polyether-modified organosilicon is (50-60): (25-35): (8-15): (2-5).

10. A manufacturing process for a refrigerator door panel as described in any one of claims 1-9, characterized in that, include: (1) Hot-dip galvanize an alloy coating on the outer side of the cold-rolled steel sheet, and apply an outer coating to the surface of the alloy coating; An alloy coating is hot-dip galvanized on the inner side of a cold-rolled steel sheet, and an inner coating is applied to the surface of the alloy coating. (2) The cold-rolled steel sheet is bent into a door panel; (3) Weld the corner seams of the door panel. After welding, only clean the welding slag and spatter. No overall grinding is required. (4) Locally repair the damaged areas of the coating at the weld seams and bends of the door panel. After repair, the dry film thickness is ≥80μm. After the surface is dry at room temperature, apply weather-resistant polyurethane sealant to the edge of the door panel to achieve full sealing and corrosion protection. (5) Apply a topcoat layer to the outer surface of the door panel, dry and cure to obtain the refrigerator door panel.

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