CN110804375A - Back paint composition for high-coating-weight coil, back paint for coil and application of back paint - Google Patents

Abstract

The invention discloses a back paint composition for a high-coating coil material, which comprises the following components in percentage by weight: polyester resin 30-45%; epoxy resin 12-16%; amino resin 8-11% ; Pigment 3‑53%; Solvent 10‑16%. The back paint composition has high coating amount and good processability, and can be well used as the back paint of coil steel. The invention also discloses the backcoat prepared from the backcoat composition and the application of the backcoat.

Description

A kind of back paint composition for high coating amount coil, back paint for coil and application thereof

technical field

The present invention relates to the technical field of coil coatings. More specifically, it relates to a backcoat composition for a high coating weight coil, a backcoat for a coil and its application.

Background technique

Coil coating, also known as pre-coated coil coating, is a kind of continuous coating on uninterrupted metal sheets through rapid automation (the sheet is derusted, degreasing, cleaned, passivated and surface treated before coating). A special coating with certain mechanical and physical properties that forms a film after high temperature curing. Pre-coated coil products are currently widely used in construction, home appliances, containers, automobile manufacturing and other fields. Pre-coated coil products (referred to as color-coated sheets), the color-coated sheet has a smooth surface, rich colors and a wide gloss range, which can achieve surface effects that cannot be obtained by ordinary spray coatings. The greater advantage is that it can be directly processed into various The product does not need to be repainted, which greatly simplifies the production process of metal sheet products, reduces paint loss during painting, and reduces environmental pollution.

Pre-coated coil products can be directly processed and formed, so the paint film is required to have good mechanical processing properties, and at the same time, it is required to have good weather resistance and various chemical resistance properties. , The back of the steel plate should also be protected by finishing, that is, the back paint of the coil. Considering the low requirements of its weather resistance and the general requirements of decorative protection, it is important to focus on its protection and post-processing. . Usually, while ensuring the protective post-processing requirements, in order to improve the scratch resistance of the coating, macromolecular epoxy resin main film-forming materials are used to enhance the mechanical strength, hardness and scratch resistance of the paint film. Moreover, epoxy coatings are also recognized as the best products in the market for the later foaming performance. However, with the increasingly fierce market competition, the customer's higher requirements for production line performance evaluation, the replacement of processing molds, the coating rate, environmental protection, and better processability of coatings have also become key factors for product competitiveness. Oxygen resin has low solid content, low coating rate, and the disadvantages of hard paint film and slightly poor workability gradually appear.

SUMMARY OF THE INVENTION

Based on the above facts, the first object of the present invention is to provide a backcoat composition for a high coating weight coil, which has high solid content, good fluidity, high coating weight, good processability, and can It is well used as the back paint of coil steel, and the obtained back paint has good mechanical and chemical properties including high hardness.

The second object of the present invention is to provide a backcoat for a high coating weight coil.

The third object of the present invention is to provide the application of the high coating weight coil backcoat in the coating of the coil steel substrate as described in the first object above.

For reaching above-mentioned first purpose, the present invention adopts following technical scheme:

A backcoat composition for a high coating weight coil, by weight percentage, comprises the following components:

Further, the solid content of the polyester resin is 55-65% by mass, the number average molecular weight is 13000-16000, and the hydroxyl value is 50-70 mg KOH/g.

Further, the solid content of the epoxy resin is 35-45% by mass, the number average molecular weight is 15000-20000, the hydroxyl value is 100-120 mg KOH/g, and the epoxy equivalent is 4000-6000.

Further, the amino resin is selected from one or more of urea formaldehyde resin, methyl etherified melamine resin, high imino methanol etherified melamine formaldehyde resin, and butyl etherified melamine resin.

Further, the amino resin is selected from one or a mixture of methyl etherified melamine resin and high imino methanol etherified melamine formaldehyde resin.

Further, the solvent is a mixed solvent of propylene glycol methyl ether acetate, DBE, solvent oil No. S-100, solvent oil No. S-150, and isobutanol.

Further, an auxiliary agent is also included in the back paint composition.

Further, the auxiliaries include one or more of wax auxiliaries, surfactants, defoaming agents, leveling agents, catalysts, adhesion promoters, and flexibility improvers.

Further, the wax auxiliary includes wax slurry and wax powder.

Further, by weight percentage, the back paint composition comprises the following components:

In order to achieve the above-mentioned second object, the present invention provides a backcoat for a high-coating volume coil prepared from the backcoat composition as described in the first object above.

In order to achieve the above-mentioned third object, the present invention provides the application of the high-coating-amount coil backcoat described in the second object above in the coating of coil steel substrates.

Further, the application includes: coating the high coating weight coil material on the back of the coil steel substrate with a back paint.

The beneficial effects of the present invention are as follows:

The back paint composition for a high coating amount coil provided by the present invention has high coating amount and good processability, and is used to prepare a back paint for a coil material, and the obtained back paint has good adhesion, processing High gloss, good surface smoothness, good scratch resistance and excellent polyurethane foaming characteristics, suitable for the back of various coated steel plates. In addition, the coating amount of the back paint for a high coating amount coil provided by the present invention can reach more than 350 m ² /kg.μ, the coating amount of the traditional epoxy back paint is 280 m ² /kg. μ, and the coating amount is increased by 25 %.

Detailed ways

In order to illustrate the present invention more clearly, the present invention will be further described below with reference to the preferred embodiments. Those skilled in the art should understand that the content specifically described below is illustrative rather than restrictive, and should not limit the protection scope of the present invention.

A specific embodiment of the present invention provides a backcoat composition for a high-coating volume coil, by weight percentage, comprising the following components:

In the backcoat composition, the combination of a certain amount of polyester resin, epoxy resin and amino resin enables the backcoat coating composition to have high coating rate and processability when used for preparing the backcoat for coils, resulting in The back paint has good adhesion, processability, high gloss, good surface slip and scratch resistance. It overcomes the problems of low coating rate and high hardness of the paint film of epoxy resin back paint.

In a preferred example, the polyester resin is a saturated polyester resin with a number average molecular weight of 13000-16000, a solid content of 55-65% by mass, and a hydroxyl value of 50-70 mg KOH/g. Its molecular structure is specially designed. , increases the molecular weight, provides high hydroxyl value and highly branched dendritic structure, achieves the adhesion performance to the substrate equivalent to epoxy, and also retains the good balance of hardness and toughness of saturated polyester resin. At the same time, the combination of the polyester resin and other components enables the solid content of the back paint composition obtained in this embodiment to reach more than 60%, and the coating amount can reach 350 m ² /kg.μ, which is higher than the traditional epoxy back paint 280 m ² /kg.μ.

The polyester resin suitable for this embodiment can be provided and sold by some resin manufacturers currently on the market, or can be self-produced by the applicant. Saturated polyester resins that can be used in this embodiment include saturated polyester resins produced by, for example, Evonik, Changxing, Zhongchu, etc. Specific models include DYNAPOL LH 826-05/A, ETERKYD5084-R-60-6, 3272, etc. , including NIPPON's own resin NSY 9983, and other similar saturated polyester resins that are commercially available.

The polyester resin of the present invention is preferably 30-45 wt %, more preferably 30-40 wt %, based on the total weight of the coating composition.

Epoxy resin refers to a class of polymers containing more than two epoxy groups in the molecule. It is the polycondensation product of epichlorohydrin and bisphenol A or polyol. In a preferred example, the solid content of the epoxy resin is 35-45% by mass, the number average molecular weight is 15000-20000, the hydroxyl value is 100-120 mg KOH/g, and the epoxy equivalent is 4000-6000. Not only has good compatibility with the polyester resin, but also the paint film cured by the cross-linking of high molecular weight epoxy resin and isocyanate, amino resin and other resins contains abundant hydroxyl and ether bonds, which gives the back paint and the base. The material provides good adhesion, and has good mechanical properties and certain corrosion resistance.

The epoxy resin suitable for this embodiment can provide epoxy resins for steel coils for some manufacturers in the market, including but not limited to epoxy resins produced by Baobo, Sanmu, Nanya, Momentive, etc. The specific models include 609-40, 609A, 1009, etc., and other similar epoxy resins that are commercially available. The epoxy resin is preferably 12-16 wt %, more preferably 13-15 wt %, based on the total weight of the coating composition.

In a preferred example, the amino resin is selected from one or more of urea-formaldehyde resin, methyl-etherified melamine resin, high imino-methanol etherified melamine-formaldehyde resin, and butylated melamine resin, more preferably methyl-etherified melamine resin A mixture of one or two of melamine resin and high imino methanol etherified melamine formaldehyde resin. The amino resin can cross-link the film-forming material molecules into a network structure through thermal cross-linking, so that the paint film has good gloss, hardness, water resistance and other properties. At present, there are many resin manufacturers on the market that provide and sell amino resins. Amino resins that can be used in this embodiment include amino resins such as those produced by Cytec, Changxing, Changchun, Yuanbang, etc. The specific models include CYMEL 303, CYMEL325, NP 115-60, YP 5617, YP5627B, etc. Other similar amino resins are commercially available. The amino resin is preferably 8-11 wt %, more preferably 8-10 wt %, based on the total weight of the coating composition.

Further, the pigment comprises a color pigment and an extender pigment, wherein in the back paint composition, the content of the color pigment is 3-35% by weight; the content of the extender pigment is 0-18% by weight.

In a preferred example, the coloring pigments include organic and inorganic pigments such as titanium dioxide, carbon black, and iron yellow commonly used in the art, so that the coating has certain functions such as covering, coloring, and protection. Pigments suitable for use with the backcoat compositions are known to those skilled in the art and can vary depending on the desired color or appearance of the coating. If pigments are used, preferably, the colored pigments are present in the backcoat composition in an amount of 3-53% by weight, more preferably 3-35%, based on the total weight of the coating composition.

In a preferred example, the extender pigment is usually a white or colorless filler, mainly to increase the volume of the paint, play a filling role, and reduce the cost of the paint. Some fillers can also improve the properties of the paint film, such as improving the leveling of the paint film and improving the hardness of the paint film. Fillers suitable for use with the backcoat composition are calcium carbonate, kaolin, precipitated barium sulfate, talc, and the like known to those skilled in the art. If fillers are used, preferably, precipitated barium sulfate is present in the coating composition in an amount of 3-53 wt%, or 0-18 wt%, more preferably 3-18 wt%, based on the total weight of the backcoat composition.

In yet another preferred example, the solvent is a mixture of propylene glycol methyl ether acetate, DBE, S-100 solvent oil, S-150 solvent oil, and isobutanol.

In this embodiment, the back paint composition further includes an auxiliary agent. The adjuvants may contain one or more of wax adjuvants, surfactants, defoaming agents, leveling agents, catalysts, adhesion promoters, and flexibility improvers. Further, the wax auxiliary includes wax slurry and wax powder.

The wax slurry is a wax slurry formed by dispersing modified polyolefin and polyethylene wax in naphtha and propylene glycol methyl ether solvent. The main function is to improve the abrasion resistance, smoothness and scratch resistance of the paint film. Preferably, the content of the wax slurry is 0.5-2 wt % based on the total weight of the back paint composition, more preferably 0.5-1 %.

The main function of the wax powder is to improve the abrasion resistance, smoothness and scratch resistance of the paint film. Suitable wax powders for use with the compositions described herein are micronized PTFE wax powders, tetrafluoroethylene wax powders, polyester based wax powders, and the like known to those skilled in the art. Preferably, the content of the wax powder is 0.1-0.5 wt % based on the total weight of the back paint composition, more preferably 0.1-0.3 wt %.

The surface activity improving assistant is a polyester mixture that enhances the adhesion with the PU component, and can improve the leveling performance of the paint film at the same time. Preferably, 0-2 wt%, more preferably 0.5-1.5 wt%, based on the total weight of the coating composition.

The flexibility improver is a high molecular weight polymer that improves the flexibility of the paint film. Preferably, the content of the flexibility improving agent is 0-5 wt %, more preferably 1-5 wt %, based on the total weight of the coating composition. An exemplary flexibility improver is preferably NP 005AAP.

The defoamer is a non-silicon system defoamer, and defoamers suitable for use with the backcoat composition are known to those skilled in the art and may vary depending on the desired appearance of the coating. At least one of BYK-057, BYK-354 and LevelerT is preferred. Preferably, the antifoaming agent is present in an amount of at least 0.01 wt %, and more preferably at least 0.1 wt %, based on the total weight of the coating composition.

The leveling agent is a leveling agent for a non-silicon system, and defoaming agents suitable for use with the backcoat composition are known to those skilled in the art and may vary depending on the appearance of the desired coating. At least one of BYK's TEGO FLOW300, BYK-392, AFCONA-3777 and Leveler C is preferred. Preferably, the leveling agent is present in an amount of at least 0.01 wt %, and more preferably at least 0.1 wt %, based on the total weight of the coating composition.

The catalyst is an acid catalyst, preferably at least one of dinonylnaphthalene disulfonic acid (DNNDSA), dinonylnaphthalene sulfonic acid (DNNSA) and dodecylbenzene sulfonic acid (DDBSA). Preferably, the catalyst is present in an amount of at least 0.1-2 wt%, and more preferably at least 0.5-1.0 wt%, based on the total weight of the paint composition.

The adhesion promoter is epoxy, silanol, polyester, phosphate modified polymer, preferably phosphate modified polymer, such as SN7063, TZ8805, ADP, PAE 206, Lubrizol 2063. at least one. Preferably, the content of the adhesion promoter is 0.1-4 wt %, more preferably 0.3-2 wt %, based on the total weight of the backcoat composition.

Yet another embodiment of the present invention provides a backcoat for a high-coating coil material prepared from the above-mentioned backcoat composition. Specifically, the preparation includes the following steps:

The pigment is dispersed in the polyester resin, epoxy resin and solvent, optionally the auxiliary agent is added, and the pigment is dispersed uniformly to obtain the back paint.

Yet another embodiment of the present invention provides the application of the above-mentioned high coating weight coil backcoat in the coating of coil steel substrates.

Further, the application includes coating the high coating weight coil with a back paint on the back of the coil equipment.

Further, the application includes: coating the back paint on the back of the coiled steel substrate, and baking and curing to obtain a back paint layer formed on the substrate.

Further, the baking and curing conditions are: baking in an oven at 250°C-350°C for 30-60s, the substrate temperature is controlled at PMT: 210-249°C, preferably PMT 216-232°C, and the dry film thickness is usually 4 -10 μm range, preferably 5-8 μm.

Further, the coil steel base material is a base material formed of cold-rolled steel, zinc-phosphate treated steel, hot-dip galvanized steel, zinc-aluminum-magnesium alloy, electro-galvanized steel sheet, and the like.

Below in conjunction with some specific embodiments, the technical scheme of the present invention will be described:

Example 1

The backcoat compositions of the coating systems of the present invention were prepared according to the formulations set forth in Table 1 below:

The formula of table 1 embodiment 1 back paint composition

Add polyester resin, epoxy resin, mixed solvent (PMA/DBE/S100/S150 solvent) in sequence to a suitable container, mix and stir evenly, then add titanium dioxide and precipitated barium sulfate in sequence under stirring, stir for 15 minutes, Then grind it with a grinder to a fineness of not more than 15μ, and then add amino resin, wax slurry, wax powder, surface activity improvement auxiliary, flexibility improvement auxiliary, defoamer, leveling agent, catalyst and auxiliary agent under stirring state. Stress accelerator, stir evenly, adjust color, gloss and viscosity as required, adjust the viscosity of the mixture to be 100-140 seconds/T-4 cups/25°C, filter and pack to obtain a high-coating back paint composition.

Coating and plate making: the obtained high-coating amount back paint for coil steel base material is rolled on the 0.5mm galvanized steel sheet of the pretreatment coating with a wire rod, and baked in an oven at 320 ° C for 30s , to obtain a back paint coating with a film thickness of 5-7 μm.

Example 2

The backcoat compositions of the coating systems of the present invention were prepared according to the formulations described in Table 2 below:

The formula of table 2 embodiment 2 back paint composition

component name Content (wt%) polyester resin NP 013RJZ 41% epoxy resin 609-40 15% solvent PMA/DBE/S100/S150 10% coloring pigment Titanium dioxide R2196 8.5% extender pigment Precipitated barium sulfate 8.5% Amino resin Yp5627b 10% wax slurry HI-DISPER 6413(C) 0.5% wax powder LINEPLUS PF 251-RB Wax Powder 0.1% Surface Active Improvement Auxiliary NP 003AAF 0.5% Flexibility Improver NP 005AAP 2.9% defoamer LEVELER T(NPZ) 0.5% leveling agent LEVELER C(NPZ) 0.5% catalyst NACURE 1051 1.0% adhesion promoter AAC 9999 1.0%

Add polyester resin, epoxy resin, mixed solvent (PMA/DBE/S100/S150 solvent) in sequence to a suitable container, mix and stir evenly, then add titanium dioxide and precipitated barium sulfate in sequence under stirring, stir for 15 minutes, Then grind it with a grinder to a fineness of not more than 15μ, and then add amino resin, wax slurry, wax powder, surface activity improvement auxiliary, flexibility improvement auxiliary, defoamer, leveling agent, catalyst and auxiliary agent under stirring state. Stress accelerator, stir evenly, adjust color, gloss and viscosity as required, adjust the viscosity of the mixture to be 100-140 seconds/T-4 cups/25°C, filter and pack to obtain a high-coating back paint composition.

Coating and plate making: the obtained high-coating amount back paint for coil steel base material is rolled on the 0.5mm galvanized steel sheet of the pretreatment coating with a wire rod, and baked in an oven at 320 ° C for 30s , to obtain a back paint coating with a film thickness of 5-7 μm.

Example 3

The backcoat compositions of the coating systems of the present invention were prepared according to the formulations shown in Table 3 below:

The formula of table 3 embodiment 3 back paint composition

component name Content (wt%) polyester resin NP 013RJZ 36% epoxy resin 609-40 13% solvent PMA/DBE/S100/S150 9% coloring pigment Titanium dioxide R2196 10% extender pigment Precipitated barium sulfate 17% Amino resin Yp5627b 8% wax slurry HI-DISPER 6413(C) 0.5% wax powder LINEPLUS PF 251-RB Wax Powder 0.1% Surface Active Improvement Auxiliary NP 003AAF 0.5% Flexibility Improver NP 005AAP 2.9% defoamer LEVELER T(NPZ) 0.5% leveling agent LEVELER C(NPZ) 0.5% catalyst NACURE 1051 1.0% adhesion promoter AAC 9999 1.0%

Add polyester resin, epoxy resin, mixed solvent (PMA/DBE/S100/S150 solvent) in sequence to a suitable container, mix and stir evenly, then add titanium dioxide and precipitated barium sulfate in sequence under stirring, stir for 15 minutes, Then grind it with a grinder to a fineness of not more than 15μ, and then add amino resin, wax slurry, wax powder, surface activity improvement auxiliary, flexibility improvement auxiliary, defoamer, leveling agent, catalyst and auxiliary agent under stirring state. Stress accelerator, stir evenly, adjust color, gloss and viscosity as required, adjust the viscosity of the mixture to be 100-140 seconds/T-4 cups/25°C, filter and pack to obtain a high-coating back paint composition.

Coating and plate making: the obtained high-coating amount back paint for coil steel base material is rolled on the 0.5mm galvanized steel sheet of the pretreatment coating with a wire rod, and baked in an oven at 320 ° C for 30s , to obtain a back paint coating with a film thickness of 5-7 μm.

Example 4

The backcoat compositions of the coating systems of the present invention were prepared according to the formulations shown in Table 4 below:

The formula of table 4 embodiment 4 back paint composition

Add polyester resin, epoxy resin, mixed solvent (PMA/DBE/S100/S150 solvent) in sequence to a suitable container, mix and stir evenly, then add titanium dioxide under stirring, stir for 15 minutes, and then grind with a grinder To the fineness of not more than 15μ, then add amino resin, wax slurry, wax powder, surface activity improving aid, flexibility improving aid, defoamer, leveling agent, catalyst and adhesion promoter under stirring, stirring Uniform, adjust color, gloss and viscosity as required, adjust the viscosity of the mixture to be 100-140 seconds/T-4 cups/25°C, filter and package to obtain a high-coating amount of back paint coating composition.

Coating and plate making: the obtained high-coating amount back paint for coil steel base material is rolled on the 0.5mm galvanized steel sheet of the pretreatment coating with a wire rod, and baked in an oven at 320 ° C for 30s , to obtain a back paint coating with a film thickness of 5-7 μm.

Comparative Example 1

The backcoat compositions of conventional epoxy backcoat coating systems were prepared according to the formulations shown in Table 5 below:

The formula of table 5 comparative example 1 back paint composition

component name Content (wt%) epoxy resin 609-40 53% solvent Ethylene glycol ether acetate/BC 7% coloring pigment Titanium dioxide R2196 twenty three% extender pigment Precipitated barium sulfate 5% Amino resin NP 115-60 8% wax slurry HI-DISPER 6413(C) 1% wax powder LINEPLUS PF 251-RB Wax Powder 0.1% defoamer LEVELER T(NPZ) 0.4% leveling agent LEVELER C(NPZ) 0.4% catalyst NACURE 1051 0.2% adhesion promoter SN-7063 2%

Add polyester resin, epoxy resin, mixed solvent (PMA/DBE/S100/S150 solvent) in sequence to a suitable container, mix and stir evenly, then add titanium dioxide under stirring, stir for 15 minutes, and then grind with a grinder To the fineness of not more than 15μ, then add amino resin, wax slurry, wax powder, defoamer, leveling agent, catalyst and adhesion promoter under stirring state, stir evenly, adjust color, gloss, viscosity as required, adjust The viscosity of the mixture was measured at 100-140 seconds/T-4 cups/25°C, filtered and packaged to obtain a high coat weight backcoat coating composition.

Coating and plate making: the obtained high-coating amount back paint for coil steel base material is rolled on the 0.5mm galvanized steel sheet of the pretreatment coating with a wire rod, and baked in an oven at 320 ° C for 30s , to obtain a back paint coating with a film thickness of 5-7 μm.

Comparative Example 2

Example 1 was repeated, except that the content of polyester resin was 38 wt%, the content of epoxy resin was 10 wt%, and the content of amino resin was 8 wt%, and other conditions remained unchanged to obtain a back paint coating.

Comparative Example 3

Example 1 was repeated, except that the content of polyester resin was 30wt%, the content of epoxy resin was 18wt%, the content of amino resin was 8wt%, and other conditions remained unchanged to obtain a back paint coating.

The test properties of the coil coatings of the above examples and comparative examples are shown in Table 6.

Table 6 Properties of Coil Coatings

The foaming affinity of the present invention is tested according to Baosteel Coil Coating Test Method BSYE-01: Coating Back Adhesion-Tension Machine Method;

In addition to this, other test standards in the above examples and comparative examples are tested according to the national standard GB/T 13448-2006, and the results meet the requirements of the national standard GB/T 13448-2006.

The above test results show that:

1. Compared with Comparative Example 1, the coating amount of the back paint coating of the present invention is significantly improved, and the average is increased by more than 25%.

2. The coating formed by the back paint coating of the present invention has good adhesion to the metal substrate, good MEK wiping performance and surface hardness, better bending performance, better scratch resistance, and at the same time The formed coating can also meet the bonding requirements with PU foaming agents and conventional adhesives. Fully meet the requirements of color-coated panels for ordinary buildings and color-coated panels for home appliances

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Changes or changes in other different forms cannot be exhausted here, and all obvious changes or changes derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims (9)

1. The back paint composition for the high-coating-weight coil is characterized by comprising the following components in percentage by weight:

2. the back paint composition as claimed in claim 1, wherein the polyester resin has a solid content of 55-65 mass%, a number average molecular weight of 13000-16000, and a hydroxyl value of 50-70mg KOH/g.

3. The back paint composition as claimed in claim 1, wherein the epoxy resin has a solid content of 35-45 mass%, a number average molecular weight of 15000-20000, a hydroxyl value of 100-120mg KOH/g, and an epoxy equivalent of 4000-6000.

4. The back paint composition as claimed in claim 1, wherein the amino resin is selected from one or more of urea formaldehyde resin, methylated melamine resin, high imino carbinol etherified melamine formaldehyde resin and butylated melamine resin; preferably, the amino resin is selected from one or a mixture of two of methylated melamine resin and high imino carbinol etherified melamine formaldehyde resin.

5. The back paint composition as claimed in claim 1, wherein the solvent is a mixed solvent of propylene glycol methyl ether acetate, DBE, S-100 # solvent oil, S-150 # solvent oil and isobutanol.

6. The back paint composition according to claim 1, further comprising an auxiliary agent;

preferably, the auxiliary agent comprises one or more of a wax auxiliary agent, a surfactant, a defoaming agent, a leveling agent, a catalyst, an adhesion promoter and a flexibility improver;

preferably, the wax adjuvant comprises a wax slurry and a wax powder;

preferably, the back paint composition comprises the following auxiliary agents in percentage by weight:

7. a high coat weight coil back-coat prepared from the back-coat composition as claimed in any one of claims 1 to 6.

8. Use of the high coating weight coil backing paint according to claim 7 in the coating of coil steel substrates.

9. The application according to claim 8, wherein the application comprises: and coating the high-coating-weight coil on the back surface of the coil steel substrate by using a back paint.