Primer-topcoat film-covering-free pre-coating plate finish paint as well as preparation and application thereof

Abstract

The invention relates to the technical field of color-coated steel plates, and provides primer-topcoat film-covering-free pre-coated plate finish paint and preparation and application thereof, and the primer-topcoat film-covering-free pre-coated plate finish paint comprises the following raw materials: first resin, second resin, a curing agent and a first auxiliary agent; the first resin is saturated polyester; the second resin is mainly modified epoxy resin obtained by reacting a-NCO-terminated polyurethane prepolymer with epoxy resin with hydroxyl; the first auxiliary agent comprises silsesquioxane; the molecular weight of the saturated polyester is the viscosity average molecular weight of 1.0 * 10 < 3 >-1.0 * 10 < 5 >, and the molecular weight distribution is less than or equal to 3.0; the mass ratio of the saturated polyester to the modified epoxy resin is (1-3): 1. According to the invention, the saturated polyester with specific molecular weight and molecular weight distribution is used as main resin, the specific second resin is used as auxiliary resin, and the specific auxiliary agent is used as a functional additive, so that the physical and mechanical properties and the chemical corrosion resistance can be synchronously improved on the basis of one-time coating and one-time drying.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of color coated steel plate, in particular to a bottom-surface-combined film-free pre-coated plate topcoat and its preparation and application. BACKGROUND

[0002] Color coated steel plate has been widely used in the field of household appliances due to its excellent decorative, formability and durability, and is commonly used to manufacture refrigerator door panels, washing machine housings, air conditioner outdoor units and other products. At present, the conventional color coated steel plate used in household appliances has a multi-coat structure. Its production process generally adopts a "two-coat two-bake" or "three-coat three-bake" construction method, that is, a primer with anti-corrosion and adhesion enhancement functions is first coated on the pretreated metal substrate, and after baking and curing, one or two topcoats are coated to provide the final color, texture and external protection functions, and baking and curing are performed again. This "multi-coat multi-bake" process has the following defects: first, the process steps are numerous, resulting in a complex production process and a long production cycle; second, the multiple baking processes cause huge energy consumption, directly increasing production costs; finally, the complex process also increases the difficulty of quality control. In addition, in order to protect the surface of the pre-coated plate from being scratched or damaged during subsequent transportation, storage, turnover and stamping forming processes, the existing technology usually needs to cover a layer of plastic protective film on the surface of the color coated plate. However, the film coating process itself also brings new problems: it not only increases the production process and material cost, but also requires manual or mechanical tearing of the protective film after the household appliance is finally formed. This process not only consumes manpower, but also generates a large amount of waste plastic film, causing solid waste pollution, which is contrary to the current green and environmentally friendly development trend of the manufacturing industry.

[0003] Although there have been attempts to combine the functions of primer and topcoat, the coatings prepared by the existing solutions often cannot simultaneously have excellent corrosion resistance, high hardness and good processability (such as T-bending performance), and cannot meet the stringent requirements of household appliance panels. Therefore, there is an urgent need in the art to develop a new type of pre-coated plate topcoat for household appliances, which can combine the functions of primer and topcoat into one, achieve efficient, low-cost and low-energy production of "one-coat one-bake", and also endow the paint film with sufficient physical and mechanical properties (such as high hardness, excellent T-bending performance and friction coefficient) and chemical corrosion resistance, to achieve the technical purposes of simplifying the process, reducing the cost and saving energy. SUMMARY

[0004] The present application aims to at least solve one of the technical problems existing in the related art. To this end, the present application provides a topcoat which can combine the functions of a primer and a topcoat, realize efficient, low-consumption and low-cost production of "one coating and one baking", and also endow the paint film with sufficient physical and mechanical properties (such as high hardness, excellent T-bending performance and friction coefficient) and chemical corrosion resistance, so as to achieve the technical purposes of simplifying the process, reducing the cost and saving energy. Based on this, the present application also provides a preparation method of the topcoat, a precoated plate and a household appliance.

[0005] Specifically, in a first aspect, the present application provides a topcoat, raw materials of which include: a first resin, a second resin, a curing agent, a first additive; The first resin is a saturated polyester; The second resin is mainly a modified epoxy resin obtained by reacting a polyurethane prepolymer terminated with -NCO with an epoxy resin with hydroxyl groups; The first additive includes a silsesquioxane; The saturated polyester has a viscosity-average molecular weight of 1.0x10 3 ~1.0x10 5 , and a molecular weight distribution of ≤3.0; The mass ratio of the saturated polyester to the modified epoxy resin is 1-3:1.

[0006] According to the topcoat provided by the present application, the silsesquioxane is selected from one or a combination of two or more of random silsesquioxane, ladder silsesquioxane, half-cage silsesquioxane and cage polyhedral oligomeric silsesquioxane.

[0007] According to the topcoat provided by the present application, the mass ratio of the saturated polyester to the silsesquioxane is 15-35:0.5-3.

[0008] According to the topcoat provided by the present application, the first additive further includes an organic wax; and the mass ratio of the saturated polyester to the organic wax is 15-35:0.01-5.

[0009] According to the topcoat provided by the present application, the polyurethane prepolymer terminated with -NCO is obtained by reacting -OH in a polyether diol with -NCO in an aliphatic isocyanate; and the molecular weight of the polyether diol is 100-3000.

[0010] According to the topcoat provided by the present application, the epoxy resin with hydroxyl groups has an epoxy value of 0.4-0.65.

[0011] According to the topcoat provided by the present application, the topcoat further includes a second additive, and the second additive includes a leveling agent and / or a dispersant.

[0012] According to the topcoat provided by the present invention, the raw materials, by weight, include:

[0013] Secondly, the present invention provides a method for preparing the topcoat as described above, wherein a coating with a viscosity of 50~200s / 25℃ prepared with the raw materials is applied to the surface of a substrate using a one-time coating process, and a one-stage baking process is employed.

[0014] According to the method for preparing the topcoat provided by the present invention, the preparation of the coating includes: Mix 15-25% of the total amount of the first resin, 15-25% of the total amount of the second resin, filler, and 15-35% of the total amount of solvent for the first time, and grind them into a mixture with a fineness ≤15μm. The mixture is then mixed a second time with the remaining first resin, the remaining second resin, all of the curing agent, all of the silsesquioxane, and the remaining solvent to obtain the coating.

[0015] Thirdly, the present invention also provides a pre-coated panel, comprising the topcoat as described above or the topcoat prepared by the above preparation method.

[0016] Fourthly, the present invention also provides a household appliance product, including the pre-coated plate as described above.

[0017] The pre-coated board topcoat provided by this invention, along with its preparation and application, achieves simultaneous improvement in physical and mechanical properties and chemical corrosion resistance on the basis of "one coat and one bake" by using saturated polyester with a specific molecular weight and molecular weight distribution as the main resin, the aforementioned specific second resin as the auxiliary resin, and the aforementioned specific additives as functional additives, thus meeting the application requirements of no-coating.

[0018] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0019] The embodiments of the present invention will be described in further detail below with reference to examples. These examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0020] In the description of the embodiments of the present invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the embodiments of the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] In the description of the embodiments of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention based on the specific circumstances.

[0022] In embodiments of the present invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0023] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0024] In a specific embodiment of the present invention, a topcoat is first provided, the raw materials of which include: a first resin, a second resin, a curing agent and a first additive; The first resin is a saturated polyester; The second resin is mainly a modified epoxy resin obtained by reacting a polyurethane prepolymer with -NCO end caps with an epoxy resin containing hydroxyl groups. The first auxiliary agent includes silsesquioxane; The saturated polyester has a viscosity-average molecular weight of 1.0 × 10⁻⁶. 3 ~1.0×10 5 Molecular weight distribution ≤ 3.0; The mass ratio of the saturated polyester to the modified epoxy resin is 1~3:1.

[0025] To obtain a topcoat that combines the functions of primer and topcoat, achieving efficient, low-consumption, and low-cost production with a "one-coat-one-bake" process, while also imparting sufficient physical and mechanical properties (such as high hardness, excellent T-bending performance, and coefficient of friction) and chemical corrosion resistance to the paint film for use on pre-coated panels (metal sheets with pre-coated surfaces, such as color steel sheets and aluminum composite panels), this invention has discovered in experiments that by using saturated polyester with a specific molecular weight and molecular weight distribution as the main resin, the aforementioned specific secondary resin as the auxiliary resin, and the aforementioned specific additives as functional additives, and further optimizing the proportions of each raw material, simultaneous improvement in physical and mechanical properties and chemical corrosion resistance can be achieved on the basis of a "one-coat-one-bake" process. If the saturated polyester content decreases, the bending performance decreases; if the saturated polyester resin content increases, the alkali resistance is insufficient. If the molecular weight of the saturated polyester is too low and the molecular weight distribution is greater than 3.0, the product has many curing crosslinking points, high film shrinkage, high internal stress, and reduced bending performance and corrosion resistance; if the molecular weight is too high, the viscosity is high, which is not conducive to application.

[0026] Specifically, in the second resin used in this invention, epoxy resin is used to increase the crosslinking density, while polyurethane is used to increase the toughening effect and improve the brittleness of epoxy resin. This second resin with a specific structure has a dense crosslinked structure to improve the corrosion resistance (such as acid and alkali resistance, salt spray resistance) and surface scratch resistance of the coating. Introducing silsesquioxane into the above-mentioned resin system of this invention, silsesquioxane has good compatibility with the resin, can be well dispersed in the matrix resin, and can be tightly bound to its chain segments through van der Waals forces, hydrogen bonding, and dipole interactions. The inorganic specific morphological skeleton structure (cage-shaped / semi-cage-shaped / trapezoidal, etc.) can effectively restrict chain segment movement, strengthen the establishment of the crosslinking network of the system, effectively terminate the development of microcrack tips in the paint film; optimize the stress-strain characteristics of the paint film, promote the improvement of toughness; increase the surface hardness of the paint film, reduce the coefficient of friction, improve scratch resistance, and achieve the performance requirements of no-coating.

[0027] In addition, the direct introduction of silsesquioxanes into the system can reduce the system viscosity, improve the solid content and pigment content of the system, and expand the curing process options.

[0028] In some specific embodiments of the present invention, the silsesquioxane is selected from one or more combinations of random, trapezoidal, semi-cage, cage-shaped polyhedral oligomeric silsesquioxanes and polysilsesquioxanes; in order to obtain better wear resistance and hardness, cage-shaped silsesquioxanes are preferred, and octaoctyl cage-shaped polysilsesquioxane EcotionPOSS107 is even more preferred.

[0029] In some specific embodiments of the present invention, the mass ratio of the saturated polyester to the silsesquioxane is 15~35:0.5~3.

[0030] In some specific embodiments of the present invention, the first auxiliary agent further includes an organic wax; the mass ratio of the saturated polyester to the organic wax is 15~35:0.01~5.

[0031] Preferably, the organic wax is selected from polyethylene wax or PTFE modified wax.

[0032] Experiments have shown that by adding the above-mentioned amount of organic wax, the hardness of the topcoat can be further improved and the coefficient of friction reduced, thereby achieving scratch resistance without the need for a protective film.

[0033] In some specific embodiments of the present invention, the polyurethane prepolymer with -NCO end capping is obtained by reacting -OH in a polyether diol with -NCO in an aliphatic isocyanate, wherein the molecular weight of the polyether diol is 100 to 3000.

[0034] Preferably, the aliphatic isocyanate is selected from one or more combinations of dicyclohexylmethane diisocyanate H12MDI, isophorone diisocyanate IPDI, and TMDI.

[0035] More preferably, the second resin is prepared by the following method: After the polyether diol and solvent are mixed evenly, the aliphatic isocyanate is slowly added dropwise over 1 hour, and then a catalyst is added to carry out the first reaction to obtain the -NCO-terminated polyurethane prepolymer; the -NCO-terminated polyurethane prepolymer, the hydroxyl-containing epoxy resin and the catalyst carry out the second reaction, vacuum degassing, cooling, to obtain the modified epoxy resin.

[0036] Preferably, the first reaction is carried out under nitrogen protection at a temperature of 60-65°C for 1-3 hours.

[0037] Preferably, the second reaction is stirred evenly at 65~80℃ and the reaction is continued for 0.5~3h.

[0038] Preferably, the mass-volume concentration of the polyether diol in the mixture obtained by mixing the polyether diol and the solvent is 3% to 20%.

[0039] Preferably, the ratio of n (aliphatic isocyanate):n (polyether diol) is (2.0~2.5):1.

[0040] Preferably, the ratio of n (polyurethane prepolymer with -NCO end caps) to n (epoxy resin with hydroxyl groups) is (0.8~1.2):(1.5~2.5).

[0041] More preferably, the polyether diol is used after being vacuumed at 110℃±2℃ for 2~5 hours using a rotary evaporator.

[0042] In some specific embodiments of the present invention, the epoxy value of the hydroxyl-containing epoxy resin is between 0.4 and 0.65. When it is less than 0.4, it is easy to cause insufficient film density and poor corrosion resistance; when it is greater than 0.65, it has insufficient flexibility and poor bending performance.

[0043] Preferably, the epoxy resin containing hydroxyl groups is selected from bisphenol A epoxy resin (such as E-44) and / or bisphenol F epoxy resin (such as F-51).

[0044] More preferably, the epoxy resin and solvent are dried in a vacuum drying oven at 80℃~100℃ for 4h~12h, and then sealed and stored for later use.

[0045] In some specific embodiments of the present invention, the topcoat further includes a second additive, which includes a leveling agent and / or a dispersant.

[0046] Preferably, the leveling agent is selected from polyester-modified silicone, such as BYK370 or Tech-27515.

[0047] Preferably, the dispersant is selected from organic bentonite type (such as FHGEL-203) or kaolin type (polycarboxylate).

[0048] In some specific embodiments of the present invention, the raw materials, by weight, include:

[0049] Preferably, the curing catalyst is selected from BYK-450, BYK-3560, BYK-3565, and Desmodur N3600.

[0050] Preferably, the saturated polyester is a hydroxyl-terminated polyester.

[0051] Preferably, the curing agent is an amino resin, and more preferably a butylated amino resin.

[0052] In a specific embodiment of the present invention, a method for preparing the topcoat as described above is first provided, wherein a coating with a viscosity of 50~200s / 25℃ prepared with the raw materials is applied to the substrate surface using a one-time coating process, and a one-stage baking process is adopted.

[0053] Preferably, the roll coating line speed in the single coating process is 35~80m / min; Preferably, the baking temperature is 215~240℃ and the baking time is 40~60s.

[0054] In some specific embodiments of the present invention, the preparation of the coating includes: Mix 15-25% of the total amount of the first resin, 15-25% of the total amount of the second resin, filler, and 15-35% of the total amount of solvent for the first time, and grind them into a mixture with a fineness ≤15μm. The mixture is then mixed a second time with the remaining first resin, the remaining second resin, all of the curing agent, all of the first additive, and the remaining solvent to obtain the coating.

[0055] Preferably, the solid content of the coating is 30-70%.

[0056] Preferably, all of the solidification catalyst is added during the second mixing.

[0057] Preferably, all of the organic waxes are added during the second mixing.

[0058] Preferably, the second auxiliary agent is added at 40-50% of the total amount during the first mixing, and the remaining second auxiliary agent is added during the second mixing.

[0059] Preferably, the solvent used to formulate the coating is an organic solvent.

[0060] The first mixing mainly controls the particle size of the filler; ensuring the fineness of the coating slurry and guaranteeing the gloss and leveling of the product surface during roller coating. The second mixing is mainly to integrate the first additive, organic wax, and resin system, maximizing the binding effect between the resin and the first additive, ensuring good dispersion, and reducing the system viscosity to 50~200s / 25℃.

[0061] In a specific embodiment of the present invention, a pre-coated plate is also provided, comprising the topcoat as described above or the topcoat prepared by the preparation method described above.

[0062] Preferably, the substrate of the pre-coated plate is a hot-dip galvanized steel plate.

[0063] Preferably, the zinc coating weight (both sides) of the selected hot-dip galvanized steel sheet is 50~100g / m². 2 .

[0064] Preferably, a chromium-free passivation solution is uniformly coated onto the surface of the substrate using a roller coating method to form a passivation film. The passivated substrate is then coated with the topcoat within 7 days.

[0065] In a specific embodiment of the present invention, a household appliance product is also provided, including the pre-coated plate as described above.

[0066] Unless otherwise specified, the structural and model correspondences of some of the raw materials involved in this invention are as follows: Table 1

[0067] Some of the raw materials involved in this invention are obtained in-house, as detailed below: Preparation Example 1 A method for preparing a modified epoxy resin, comprising the following steps: (1) Raw material pretreatment: Polyether diol PTMG with a viscosity-average molecular weight of 500 was vacuumed at 110°C for 3 hours using a rotary evaporator, and then sealed and stored for later use.

[0068] E-44 (bisphenol A type epoxy resin) was dried in a vacuum drying oven at 90℃ for 4 hours and then sealed and stored for later use.

[0069] Dimethyl nylonate (DBE) was dried in a vacuum drying oven at 90°C for 4 hours and then sealed for storage.

[0070] (2) Add pretreated PTMG and DBE (mass ratio of PTMG to DBE is 90:10) to a three-necked flask, mix well, and then slowly add dicyclohexylmethane diisocyanate H12MDI (n(H12MDI):n(PTMG) = 2.0:1) dropwise over 1 hour. Then add dibutyltin dilaurate DBTDL (the amount added is 2% of the total reactant mass in a single reaction), and react at 65°C under nitrogen protection for 2 hours to obtain polyurethane prepolymer.

[0071] (3) Add the polyurethane prepolymer, E-44, and dibutyltin dilaurate (DBTDL) obtained in step (2) to a three-necked flask (n(polyurethane prepolymer):n(E-44) = 1:2, and the amount of DBTDL added is 2% of the total reactant mass in a single reaction). Stir at 75°C until homogeneous, and continue the reaction for 1 hour. Degas under vacuum for 30 minutes, cool to room temperature, and discharge the product, which is the polyurethane-modified epoxy resin.

[0072] Preparation Example 2 A method for preparing a modified epoxy resin, comprising the following steps: (1) Raw material pretreatment: Polyether diol PTMG with a viscosity-average molecular weight of 1500 was vacuumed at 110°C for 3 hours using a rotary evaporator and then sealed for storage.

[0073] F-51 (bisphenol F type epoxy resin) was dried in a vacuum drying oven at 90℃ for 4 hours and then sealed and stored for later use.

[0074] Dimethyl nylonate (DBE) was dried in a vacuum drying oven at 90°C for 4 hours and then sealed for storage.

[0075] (2) Add pretreated PTMG and DBE (mass ratio of PTMG to DBE is 90:10) to a three-necked flask, mix well, and then slowly add isophorone diisocyanate (IPDI) (n(IPDI):n(PTMG) = 2.0:1) over 1 hour. Then add dibutyltin dilaurate (DBTDL) (the amount added is 2% of the total reactant mass in a single reaction), and continue the reaction at 65°C under nitrogen protection for 2 hours to obtain polyurethane prepolymer.

[0076] (3) Add the polyurethane prepolymer, F-51, and dibutyltin dilaurate (DBTDL) obtained in step (2) to a three-necked flask (n(polyurethane prepolymer):n(F-51) = 1:2, and the amount of DBTDL added is 2% of the total reactant mass in a single reaction). Stir at 75°C until homogeneous, and continue the reaction for 1 hour. Degas under vacuum for 30 minutes, cool to room temperature, and discharge the product, which is the polyurethane-modified epoxy resin.

[0077] Examples 1-5 Examples 1-5 provide a pre-coated board topcoat that integrates the base and topcoat without film. The raw material composition is shown in Tables 2-2 to 2-2 below, where Table 2-2 is a continuation of Table 2-1: Table 2-1

[0078] Table 2-2

[0079] Examples 1-5 also provide a method for preparing the pre-coated board topcoat that integrates the base and top layers without film coating, the steps of which are as follows: (1) Mix 20% of the total amount of saturated polyester, 20% of the total amount of modified epoxy resin, 45% of the total amount of dispersant, pigments and fillers, and 30% of the total amount of solvent, and grind them into a mixture with a fineness of ≤15μm.

[0080] (2) The mixture obtained by grinding in step (1) is mixed with the remaining saturated polyester, the remaining modified epoxy resin, all the amino resin, the remaining dispersant, all the curing catalyst and the remaining solvent in the above formula content; after stirring at 300 r / min for 30 min, silsesquioxane is added dropwise, and after stirring for another 30 min, leveling agent is added dropwise, and stirring is continued for 1.5 h to obtain a topcoat composition.

[0081] Examples 1-5 also provide the application of the above-mentioned topcoat-coated, film-free pre-coated sheet paint in color-coated steel sheets, the specific process of which is as follows: (1) Select hot-dip galvanized steel coils as the base material and uncoil them. The zinc coating weight (double-sided) of the selected hot-dip galvanized steel sheet is 80g / m². 2 .

[0082] (2) Chemical conversion treatment: The chromium-free passivation solution is uniformly coated on the surface of the hot-dip galvanized substrate by roller coating to form a passivation film.

[0083] (3) The chemically converted substrate is sent to a pretreatment drying oven for drying. To ensure the activity of the passivation film and the best adhesion to subsequent coatings, the passivated substrate is coated on the 6th day.

[0084] The specific coating method is as follows: a one-time coating and one-stage baking process is used to coat the topcoat composition obtained in step (2) on the substrate after the above treatment, and the thickness of the topcoat on the substrate is 20±3μm. The roller coating line speed is 60m / min, the baking temperature is 230℃, the baking time is 60s, and the substrate is cooled and wound up.

[0085] Comparative Example 1 It is basically the same as Example 1, except that: Ecotion POSS107 does not contain octyl-cage polysilsesquioxane.

[0086] Comparative Example 2 It is basically the same as Comparative Example 1, except that: The mass ratio of saturated polyester to modified epoxy resin was 20:30, and the total amount of both was the same as in Comparative Example 1.

[0087] Comparative Example 3 It is basically the same as Comparative Example 1, except that: The mass ratio of saturated polyester to modified epoxy resin was 42:8, and the total amount of both was the same as in Comparative Example 1.

[0088] Comparative Example 4 It is basically the same as Example 1, except that: Saturated polyester was replaced by polyester oligomers (number average molecular weight on the order of 10) by mass.2 (Molecular weight distribution 3.5).

[0089] Test case The color-coated steel sheets obtained in the above embodiments and comparative examples, as well as the topcoat formed thereon, were tested using the following methods: T-bend: According to GB / T 30791-2014 T-bend test for paints and varnishes.

[0090] L-value (L-value in color difference test): directly measured using a Konica Minolta CM-17d spectrophotometer.

[0091] Resistance to 5% dilute hydrochloric acid solution (24h immersion): The edge sealing of the paint film sample was immersed in a 5% hydrochloric acid solution for 24 hours, then removed and dried. No defects such as blistering, wrinkling, or peeling were observed on the coating surface.

[0092] Resistance to 5% sodium hydroxide solution (24h immersion): The edge sealing of the paint film sample was immersed in a 5% sodium hydroxide solution for 24 hours, then removed and dried. No defects such as blistering, wrinkling, or peeling were observed on the coating surface.

[0093] Resistance to neutral salt spray (96h): Environmental test performed according to GB / T 2423.17-2024, with a test cycle of 96h.

[0094] Pencil hardness (500g force): Performed according to GB / T 6739-2022 "Determination of Hardness of Paints and Varnishes by Pencil Method".

[0095] Friction coefficient: The static friction coefficient was tested in accordance with GB / T 10006-2021 "Determination of friction coefficient of plastic films and sheets".

[0096] Viscosity: Performed in accordance with GB / T 22314-2008 Determination of viscosity of epoxy resins for plastics.

[0097] The test results are as follows: Table 3

[0098] In Table 3, "ok" indicates pass, and "NG" indicates fail.

[0099] Chemical corrosion resistance (acid and alkali resistance) and salt spray resistance are core indicators for evaluating the long-term service capability of coatings in harsh environments. A good balance between high hardness and excellent flexibility reflects mechanical properties, while the coefficient of friction is an important parameter affecting the surface texture, wear resistance, and subsequent processing properties of the topcoat. As can be seen from the comparison of Comparative Examples 1-4, the technical solutions of this invention, particularly the structure and dosage of the saturated polyester and the use of silsesquioxane, are interconnected. This results in the technical solutions of the embodiments achieving a comprehensive performance improvement in multiple indicators compared to the comparative examples, yielding a color-coated steel sheet product with balanced and excellent overall performance.

[0100] Finally, it should be noted that the above embodiments are only for illustrating the present invention and not for limiting the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and should be covered within the scope of the claims of the present invention.

Claims

1. A topcoat, characterized in that, Its raw materials include: a first resin, a second resin, a curing agent, and a first additive; The first resin is a saturated polyester; The second resin is mainly a modified epoxy resin obtained by reacting a polyurethane prepolymer with -NCO end caps with an epoxy resin containing hydroxyl groups. The first auxiliary agent includes silsesquioxane; The saturated polyester has a viscosity-average molecular weight of 1.0 × 10⁻⁶. 3 ~1.0×10 5 Molecular weight distribution ≤ 3.0; The mass ratio of the saturated polyester to the modified epoxy resin is 1~3:

1.

2. The topcoat according to claim 1, characterized in that, The silsesquioxane is selected from one or more combinations of atactic silsesquioxane, ladder-shaped silsesquioxane, semi-cage-shaped silsesquioxane, and cage-shaped polyhedral oligomeric silsesquioxane.

3. The topcoat according to claim 1, characterized in that, The first additive also includes organic wax; the mass ratio of the saturated polyester to the organic wax is 15~35:0.01~5.

4. The topcoat according to claim 1, characterized in that, The -NCO-terminated polyurethane prepolymer is obtained by reacting -OH in a polyether diol with -NCO in an aliphatic isocyanate, wherein the molecular weight of the polyether diol is 100~3000. And / or, the epoxy value of the hydroxyl-containing epoxy resin is between 0.4 and 0.

65.

5. The topcoat according to claim 1, characterized in that, The topcoat also includes a second additive, which includes a leveling agent and / or a dispersant.

6. The topcoat according to any one of claims 1 to 5, characterized in that, By weight, the raw materials include:

7. A method for preparing the topcoat according to any one of claims 1 to 6, characterized in that, A coating with a viscosity of 50~200s / 25℃ prepared from the raw materials is applied to the substrate surface using a one-time coating process, followed by a one-stage baking process.

8. The method for preparing the topcoat according to claim 7, characterized in that, The preparation of the coating includes: Mix 15-25% of the total amount of the first resin, 15-25% of the total amount of the second resin, filler, and 15-35% of the total amount of solvent for the first time, and grind them into a mixture with a fineness ≤15μm. The mixture is then mixed a second time with the remaining first resin, the remaining second resin, all of the curing agent, all of the silsesquioxane, and the remaining solvent to obtain the coating.

9. A pre-coated panel, characterized in that, The topcoat includes the topcoat described in any one of claims 1 to 6 or the topcoat prepared by the preparation method described in claim 7 or 8.

10. A household appliance, characterized in that, Includes the pre-coated plate as described in claim 9.

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