Aliphatic carbamate modified alkyd resin as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of resin, and discloses aliphatic urethane modified alkyd resin as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing oleic acid, polyol, anhydride, adipic acid and hypophosphorous acid, heating, and carrying out a first reaction to obtain a precursor; mixing the precursor with an aliphatic hexamethylene amine isocyanate tripolymer and triphenyl phosphite, and carrying out a second reaction, so as to obtain aliphatic carbamate modified alkyd resin; the polyhydric alcohol comprises ethylene glycol and trimethylolpropane. Specific raw materials and preparation steps are adopted to prepare the aliphatic carbamate modified alkyd resin, and the aliphatic carbamate modified alkyd resin is applied to the polyurethane coating, so that the polyurethane coating not only has a matte effect, but also has the advantages of low consumption of matting powder required to be added, low cost and low production cost. And the formed paint film also has good mechanical strength.

Description

Technical Field

[0001] The present invention belongs to the technical field of resins, and particularly relates to an aliphatic urethane-modified alkyd resin, a preparation method thereof, and an application thereof. Background Art

[0002] After years of development, the most commonly used oil-based wood coatings at present are PU polyurethane coatings and UV curable coatings. As a traditional coating, alkyd polyurethane coatings have a wide range of application fields and are mainly used for the coating of the surface of wooden furniture. At present, low-gloss and soft matte furniture are more favored by consumers.

[0003] Matte paint is mainly composed of varnish, added with an appropriate amount of matting agent and auxiliary materials. Due to the different dosages of the matting agent, the gloss of the paint film is also different. The paint film of matte paint has a soft, uniform, thin, smooth, temperature-resistant, water-resistant, acid- and alkali-resistant surface. Matte paint refers to paint with a lower gloss compared to bright paint. It is usually divided into two categories: semi-matte paint (gloss around 40-60) and full matte (gloss below 30), and can be further subdivided into various degrees of matte from three to eight points of matte, that is, the gloss ranges from 30 to 80.

[0004] The gloss of a coating is an optical characteristic of the coating film, indicating the reflection ability of the coating to light. When the specular reflection ability of the coating to the light irradiated on its surface is high, the coating has a bright surface, that is, the gloss of the coating is high. On the contrary, when the specular reflection ability of the coating to the light irradiated on the surface is weak and most of the light undergoes diffuse reflection, it is a softer low-gloss surface, that is, the gloss of the coating is low.

[0005] Different objects have different reflection abilities to light. Usually, glossiness is used to measure the reflection ability of an object to light, which is expressed by the ratio of the amount of specularly reflected light on the surface of the sample at a specified incident angle to the amount of specularly reflected light on the surface of an ideal standard plate under the same conditions. Glossiness is measured by a glossiness meter. According to the measured values, the coating gloss can be divided into high gloss (greater than 90), glossy (70-90), semi-gloss (30-70), matte (10-30), and dull (less than 10), corresponding to high-gloss coatings, semi-gloss coatings, matte coatings, etc.

[0006] Due to light environmental pollution and the change of people's aesthetic concepts, the market share of low-gloss coatings has been increasing year by year. Silicon dioxide has become a widely used matting agent in coatings due to its excellent matting performance and chemical stability. Fumed silica has good dispersibility and excellent matting effect in the coating matrix.

[0007] The main method for matting coatings is to make the originally smooth and flat coating surface uneven, forming a certain roughness, thereby increasing the scattering effect on light and reducing the gloss of the coating surface.

[0008] There are mainly the following three ways to reduce the gloss of the coating: One is to add inorganic particulate matting agents, which protrude from the coating surface during the volatilization of the solvent and the thinning of the coating, making the coating surface uneven, forming a certain roughness, and weakening the specular reflection of light; the second is to select polymers with matting properties as the film-forming substances of the coating or add them to other coating matrices, forming some spherical particles or matrices with different degrees of shrinkage on the surface. During the volatilization of the solvent, molecular curling occurs, resulting in the coating surface no longer being smooth, thereby changing the reflection of light and reducing the gloss of the coating; the third is to use methods such as sandblasting, etching, thermal processing, or solvent corrosion on the formed coating film to destroy the smooth surface of the coating film and obtain a certain rough surface. However, the increase in the preparation process will further increase the cost, and solvent corrosion will also have a certain negative impact on other properties of the coating.

[0009] Therefore, the first two ways are the main coating matting methods.

[0010] Currently, the commonly used matting agents are organic matting agents and inorganic matting agents. Organic matting agents mainly include metal soaps, waxes, matting resins, etc. Inorganic matting agents are mainly some functional extender pigments, commonly used ones are diatomaceous earth, talc powder, light calcium carbonate, kaolin, synthetic silica, etc. Adding these matting agents to the coating will form a certain uneven rough structure on the coating surface after drying, converting the reflection of incident light into diffuse reflection and reducing the gloss of the coating.

[0011] Commercial silica matting agents are mainly fumed silica. Fumed silica has good dispersibility and excellent matting effect in the coating matrix. However, its preparation conditions are harsh and the cost is high, and it is only used in some expensive high-grade coatings.

[0012] Existing polyurethane coatings need to add a large amount of matting powder to obtain a matte effect, which greatly increases the preparation cost. In addition, existing polyurethane coatings also have problems with mechanical strength, such as insufficient hardness.

[0013] Therefore, there is an urgent need to provide a new polyurethane coating, which not only has a matte effect, but also requires less matting powder to be added, and the formed paint film also has good mechanical strength. Summary of the Invention

[0014] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. For this purpose, the present invention provides an aliphatic urethane-modified alkyd resin, its preparation method and application. The aliphatic urethane-modified alkyd resin of the present invention can be applied to polyurethane coatings, making the polyurethane coatings not only have a matte effect, but also require less matting powder to be added, and the formed paint film also has good mechanical strength.

[0015] The first aspect of the present invention provides a method for preparing an aliphatic urethane-modified alkyd resin.

[0016] Specifically, a method for preparing an aliphatic urethane-modified alkyd resin includes the following steps:

[0017] Mix oleic acid, polyol, acid anhydride, adipic acid, and hypophosphorous acid, heat up, and carry out the first reaction to obtain a precursor.

[0018] Mix the precursor with an aliphatic type hexamethylene diisocyanate trimer and triphenyl phosphite, and carry out the second reaction to obtain the aliphatic urethane-modified alkyd resin.

[0019] The polyol includes ethylene glycol and trimethylolpropane.

[0020] Preferably, the acid anhydride includes maleic anhydride and phthalic anhydride.

[0021] Preferably, during the preparation of the precursor, a phenol substance is also added, and the phenol substance includes 2,6-di-tert-butyl-4-methylphenol.

[0022] Preferably, after the first reaction, butyl acetate is also added. Its function is to obtain a precursor with a specific solid content and viscosity, which can improve the compatibility of the precursor with the subsequent reaction raw materials and is beneficial to the progress of the second reaction.

[0023] Preferably, the temperature of the first reaction is 150 - 156 °C, and the reaction time is 2 - 4 hours; more preferably, the temperature of the first reaction is 152 - 156 °C, and the reaction time is 3 - 4 hours.

[0024] Preferably, after the first reaction, the temperature is further raised to 200 - 215 °C, esterified until the acid value ≤ 15 mgKOH / g, then cooled to below 180 °C, and then butyl acetate is added. Butyl acetate is also called n-butyl acetate.

[0025] Preferably, xylene is also added before the first reaction.

[0026] Preferably, the weight ratio of oleic acid, polyol, acid anhydride, adipic acid, and hypophosphorous acid is 15 - 22:(22 - 30):(20 - 42):(3 - 8):(0.12 - 0.22), and more preferably 17.11 - 20.11:(24.84 - 27.64):(22.82 - 40.83):(3.86 - 6.73):(0.12 - 0.22).

[0027] Preferably, in the preparation process of the precursor, the weight ratio of oleic acid, polyol, anhydride, adipic acid, hypophosphorous acid, phenol substance, xylene, and butyl acetate is 17.11 - 20.11:(24.84 - 27.64):(22.82 - 40.83):(3.86 - 6.73):(0.12 - 0.22):0.1:(1.5 - 1.85):(25 - 26.5).

[0028] Preferably, the solid content of the precursor is 65 - 72%, and the viscosity at 25°C is 2300 - 2500 mPa·S.

[0029] Preferably, the acid value of the precursor is 10 - 12 mgKOH / g, and the hydroxyl value is 115 - 162 mgKOH / g.

[0030] Preferably, the weight ratio of the precursor, aliphatic type hexamethylene diisocyanate trimer, and triphenyl phosphite is 80.92 - 88.20:(4.28 - 9.28):(0.10 - 0.20).

[0031] Preferably, n-butyl acetate is added before the second reaction.

[0032] Preferably, the weight ratio of n-butyl acetate to the precursor, aliphatic type hexamethylene diisocyanate trimer, and triphenyl phosphite is 9.6 - 9.7:(80.92 - 85.92):(4.28 - 9.28):(0.10 - 0.20).

[0033] Preferably, the temperature of the second reaction is 120 - 130°C, and the time is 1.2 - 6 hours.

[0034] The molar ratio of the hydroxyl group of the precursor to the -NCO of the aliphatic type hexamethylene diisocyanate trimer can be 1:1 to 100:1. Preferably 1.5 - 10:1.

[0035] The second aspect of the present invention provides an aliphatic urethane-modified alkyd resin.

[0036] Specifically, an aliphatic urethane-modified alkyd resin is prepared by the above preparation method.

[0037] Preferably, the solid content of the aliphatic urethane-modified alkyd resin is 64.5 - 66.1%, and the hydroxyl value is 98 - 131 mgKOH / g.

[0038] Preferably, the viscosity of the aliphatic urethane-modified alkyd resin at 25°C is 3000 - 6500 mPa·S.

[0039] The aliphatic carbamate-modified alkyd resin is a slightly yellowish transparent liquid.

[0040] The third aspect of the present invention provides an application of a preparation method of an aliphatic carbamate-modified alkyd resin.

[0041] A polyurethane coating, comprising the aliphatic carbamate-modified alkyd resin prepared by the above preparation method, a matting powder, an auxiliary agent, and a solvent, and the weight percentage content of the matting powder in the polyurethane coating does not exceed 3.8%.

[0042] Preferably, the weight percentage content of the matting powder in the polyurethane coating is 1-3.8%.

[0043] Preferably, the auxiliary agent includes at least one of a dispersant, an anti-settling agent, an anti-foaming agent, and a leveling agent. The auxiliary agent is a conventional substance in the art.

[0044] Preferably, the solvent includes propylene glycol methyl ether acetate and n-butyl acetate.

[0045] Preferably, the polyurethane coating is a two-component matte coating.

[0046] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0047] The present invention uses specific raw materials and preparation steps to prepare an aliphatic carbamate-modified alkyd resin. When the aliphatic carbamate-modified alkyd resin is applied to a polyurethane coating, the polyurethane coating not only has a matte effect, but also requires less matting powder to be added, and the formed paint film also has good mechanical strength. Detailed Embodiments

[0048] In order to make those skilled in the art more clearly understand the technical solutions of the present invention, the following examples are listed for illustration. It should be noted that the following examples do not limit the protection scope required by the present invention.

[0049] Unless otherwise specified, the raw materials, reagents or devices used in the following examples can be obtained from conventional commercial channels or can be obtained by existing known methods.

[0050] The aliphatic type hexamethylene diisocyanate trimer used in the following examples is provided by Wanhua Company in Yantai, Shandong, and the product model is HT-100.

[0051] Preparation of precursor a:

[0052] The weight parts of the raw materials used in the process of preparing precursor a are as follows:

[0053] 17.11 parts of oleic acid, 0.71 part of ethylene glycol, 26.93 parts of trimethylolpropane, 1.68 parts of maleic anhydride, 3.86 parts of adipic acid, 21.14 parts of phthalic anhydride, 0.12 part of hypophosphorous acid, 1.85 parts of xylene, 26.50 parts of n-butyl acetate, 0.10 part of 2,6-di-tert-butyl-4-methylphenol;

[0054] Charge oleic acid, ethylene glycol, trimethylolpropane, maleic anhydride, adipic acid, phthalic anhydride, hypophosphorous acid, and xylene into a reaction kettle equipped with a fractionating column, a condenser, and a water separator. Pass nitrogen, heat up to reflux, about 155 °C. After reflux occurs, react for 4 hours, then slowly heat up to 215 °C, esterify until the acid value ≤ 15 mg KOH / g, cool down to below 180 °C, add n-butyl acetate and 2,6-di-tert-butyl-4-methylphenol to obtain precursor a.

[0055] The viscosity of precursor a is 3000 mPa·S / 25 °C, the acid value is 12 mg KOH / g, the solid content is 70.0%, and the hydroxyl value is 162 mg KOH / g.

[0056] Preparation of precursor b:

[0057] The weight parts of the raw materials used in the preparation of precursor b are as follows:

[0058] 20.11 parts of oleic acid, 2.51 parts of ethylene glycol, 22.33 parts of trimethylolpropane, 1.50 parts of maleic anhydride, 6.73 parts of adipic acid, 18.50 parts of phthalic anhydride, 0.22 part of hypophosphorous acid, 1.50 parts of xylene, 26.50 parts of n-butyl acetate, 0.10 part of 2,6-di-tert-butyl-4-methylphenol;

[0059] Charge oleic acid, ethylene glycol, trimethylolpropane, maleic anhydride, adipic acid, phthalic anhydride, hypophosphorous acid, and xylene into a reaction kettle equipped with a fractionating column, a condenser, and a water separator. Pass nitrogen, heat up to reflux, about 155 °C. After reflux occurs, react for 4 hours, then slowly heat up to 215 °C, esterify until the acid value ≤ 15 mg KOH / g, cool down to below 180 °C, add n-butyl acetate and 2,6-di-tert-butyl-4-methylphenol to obtain precursor b.

[0060] The viscosity of precursor b is 2500 mPa·S / 25 °C, the acid value is 10 mg KOH / g, the solid content is 70.04%, and the hydroxyl value is 115 mg KOH / g.

[0061] Example 1

[0062] A preparation method of an aliphatic urethane-modified alkyd resin, comprising the following steps:

[0063] Take 85.92 parts by weight of precursor a, with a hydroxyl molar number of 0.1736, 4.28 parts by weight of aliphatic hexamethylene diisocyanate trimer, with an NCO molar number of 0.0224, 0.10 parts by weight of triphenyl phosphite, 9.70 parts by weight of n-butyl acetate. The molar ratio of precursor hydroxyl -OH to aliphatic isocyanate -NCO is 7.8:1.

[0064] Put precursor a, aliphatic hexamethylene diisocyanate trimer, triphenyl phosphite, and n-butyl acetate into a reaction kettle equipped with a condenser and a water separator, pass nitrogen, and react at 125 °C for 1.5 hours. After the NCO groups have basically reacted, an aliphatic urethane-modified alkyd resin is obtained.

[0065] The solid content of the aliphatic urethane-modified alkyd resin is 64.5%, the viscosity is 3000 mPa·S / 25 °C, the solid hydroxyl value is 131 mgKOH / g, and it is a yellow viscous transparent liquid.

[0066] Example 2

[0067] A preparation method of an aliphatic urethane-modified alkyd resin, comprising the following steps:

[0068] Take 80.92 parts by weight of precursor a, with a hydroxyl molar number of 0.1635, 9.28 parts by weight of aliphatic hexamethylene diisocyanate trimer, with an NCO molar number of 0.0481, 0.20 parts by weight of triphenyl phosphite, 9.60 parts by weight of n-butyl acetate. The molar ratio of precursor hydroxyl -OH to aliphatic isocyanate -NCO is 3.4:1.

[0069] Put precursor a, aliphatic hexamethylene diisocyanate trimer, triphenyl phosphite, and n-butyl acetate into a reaction kettle equipped with a condenser and a water separator, pass nitrogen, and react at 130 °C for 6 hours. After the NCO groups have basically reacted, an aliphatic urethane-modified alkyd resin is obtained.

[0070] The solid content of the aliphatic urethane-modified alkyd resin is 66.1%, the viscosity is 6500 mPa·S / 25 °C, the solid hydroxyl value is 98 mgKOH / g, and it is a yellow viscous transparent liquid.

[0071] Example 3

[0072] A preparation method of an aliphatic urethane-modified alkyd resin, comprising the following steps:

[0073] Take 88.20 parts by weight of precursor b with a hydroxyl molar number of 0.1265, 3.14 parts by weight of aliphatic hexamethylene diisocyanate trimer with an NCO molar number of 0.0163, 0.20 parts by weight of triphenyl phosphite, 8.46 parts by weight of n-butyl acetate. The molar ratio of precursor hydroxyl - OH to aliphatic isocyanate - NCO is 7.8:1.

[0074] Put precursor b, aliphatic hexamethylene diisocyanate trimer, triphenyl phosphite, and n-butyl acetate into a reaction kettle equipped with a condenser and a water separator, introduce nitrogen, and react at 120 °C for 2 hours. After the NCO groups have basically reacted, an aliphatic urethane-modified alkyd resin is obtained.

[0075] The solid content of the aliphatic urethane-modified alkyd resin is 65.1%, the viscosity is 2700 mPa·S / 25 °C, and it is a yellow viscous transparent liquid.

[0076] Example 4

[0077] A preparation method of an aliphatic urethane-modified alkyd resin, comprising the following steps:

[0078] Take 85.20 parts by weight of precursor b with a hydroxyl molar number of 0.1222, 6.14 parts by weight of aliphatic hexamethylene diisocyanate trimer with an NCO molar number of 0.0318, 0.11 parts by weight of triphenyl phosphite, 8.55 parts by weight of n-butyl acetate. The molar ratio of precursor hydroxyl - OH to aliphatic isocyanate - NCO is 3.8:1.

[0079] Put precursor b, aliphatic hexamethylene diisocyanate trimer, triphenyl phosphite, and n-butyl acetate into a reaction kettle equipped with a condenser and a water separator, introduce nitrogen, and react at 130 °C for 6 hours. After the NCO groups have basically reacted, an aliphatic urethane-modified alkyd resin is obtained.

[0080] The solid content of the aliphatic urethane-modified alkyd resin is 65.9%, the viscosity is 5700 mPa·S / 25 °C, the solid hydroxyl value is 98 mgKOH / g, and it is a yellow viscous transparent liquid.

[0081] Comparative Example 1

[0082] Use precursor a as Comparative Example 1.

[0083] Comparative Example 2

[0084] Use precursor b as Comparative Example 2.

[0085] Comparative Example 3

[0086] The parts by weight of the raw materials for Comparative Example 3 are as follows:

[0087] 23.73 parts of oleic acid, 5.42 parts of ethylene glycol, 2.24 parts of 99.5% glycerol, 12.75 parts of pentaerythritol, 27.12 parts of phthalic anhydride, 0.18 parts of hypophosphorous acid, 2.96 parts of xylene, 25.55 parts of n-butyl acetate, 0.05 parts of 2,6-di-tert-butyl-4-methylphenol;

[0088] Charge oleic acid, ethylene glycol, 99.5% glycerol, pentaerythritol, phthalic anhydride, hypophosphorous acid, and xylene into a reaction kettle equipped with a fractionating column, a condenser, and a water separator. Pass nitrogen, heat up to reflux, about 155 °C. After reflux occurs, react for 4 hours, then slowly heat up to 215 °C. When the acid value is ≤ 15 mg KOH / g after esterification, cool down to below 180 °C, and add n-butyl acetate and 2,6-di-tert-butyl-4-methylphenol to obtain the product of Comparative Example 3.

[0089] The viscosity of the product of Comparative Example 3 is 3000 mPa·S / 25 °C, the acid value is 10.9 mg KOH / g, the solid content is 69.8%, and the hydroxyl value is 132 mg KOH / g.

[0090] Take the resins prepared in the above examples and comparative examples to prepare polyurethane coatings.

[0091] The polyurethane coating includes component A, a curing agent containing a diluent, and a diluent. The weight ratio of component A, the curing agent containing a diluent, and the diluent is 1:0.5:0.5.

[0092] The composition of component A is shown in Table 1 (the numerical unit in Table 1 is parts by weight).

[0093] Table 1

[0094]

[0095]

[0096] In the table, "-" indicates no addition.

[0097] The preparation process of component A is as follows: Mix the resin prepared in the example or comparative example with a polymer dispersant at a stirring speed of 500 revolutions per minute for 5 minutes, then add polyethylene wax powder and matte powder, stir at a speed of 2000 revolutions per minute for 15 minutes, and then add the remaining components and stir at 500 revolutions per minute for 5 minutes to obtain component A.

[0098] The preparation process of the curing agent containing a diluent is as follows: Dilute the commercially available L-75 curing agent with n-butyl acetate to a curing agent with a solid content of 40%.

[0099] The preparation process of the diluent is as follows: 75 parts by weight of butyl acetate, 22 parts by weight of PMA (propylene glycol methyl ether acetate), and 3 parts by weight of DBE (a product name of DuPont Company in the United States, which is a mixture of dimethyl succinate, dimethyl glutarate, and dimethyl adipate) are formulated into the diluent.

[0100] The polyurethane coating, including component A, the curing agent containing the diluent, and the diluent, is mixed and stirred evenly and then applied by spraying (the ambient temperature during spraying is 25 °C, the relative humidity is 60%, and a No. 4 gun is used for spraying) on the medium density fiberboard. After curing, a paint film is formed, and the performance of the paint film is tested. The results are shown in Table 2.

[0101] Refer to the national standard GBT 23997-2009 Solvent-based polyurethane wood coatings for interior decoration and renovation to test the performance of the paint film. Among them, refer to GB / T 6739-2006 Paints and varnishes - Pencil method for the determination of film hardness.

[0102] Table 2

[0103]

[0104]

[0105] Note: The transparency is based on a standard score of 5 and varies in the range of 1-9. 1 represents the worst performance, and 9 represents the best performance.

[0106] The results in Table 2 show that the alkyd resins modified by aliphatic urethane in Example 1, Example 2, Example 3, and Example 4 have more excellent matting properties under the same addition amount of matting powder.

Claims

1. A method for preparing an aliphatic urethane-modified alkyd resin, characterized in that: The following steps are involved: Mix oleic acid, polyol, acid anhydride, adipic acid and hypophosphorous acid, raise the temperature, and conduct a first reaction to obtain a precursor; The precursor is mixed with aliphatic hexamethylene aminoisocyanate trimer and triphenyl phosphite to carry out a second reaction to obtain the aliphatic urethane-modified alkyd resin; The polyols include ethylene glycol and trimethylolpropane.

2. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: The acid anhydrides include maleic anhydride and phthalic anhydride.

3. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: In the process of preparing the precursor, phenolic substances are also added, and the phenolic substances include 2,6-di-tert-butyl-4-methylphenol.

4. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: After the first reaction is completed, butyl acetate is added; and / or the temperature of the first reaction is 150-156° C., and the reaction time is 2-4 hours; and / or xylene is added before the first reaction.

5. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: The weight ratio of oleic acid, polyol, acid anhydride, adipic acid and hypophosphorous acid is 15-22: (22-30): (20-42): (3-8): (0.12-0.22).

6. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: The solid content of the precursor is 65-72%, and the viscosity at 25° C. is 2300-2500 mPa·S; and / or the acid value of the precursor is 10-12 mgKOH / g, and the hydroxyl value is 115-162 mgKOH / g.

7. The method for preparing an aliphatic urethane-modified alkyd resin according to claim 1, characterized in that: The weight ratio of the precursor, aliphatic hexamethylene aminoisocyanate trimer and triphenyl phosphite is 80.92-88.20: (4.28-9.28): (0.10-0.20).

8. An aliphatic urethane-modified alkyd resin, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 7.

9. The aliphatic urethane-modified alkyd resin according to claim 8, characterized in that: The aliphatic urethane modified alkyd resin has a solid content of 64.5-66.1% and a hydroxyl value of 98-131 mgKOH / g; and / or the aliphatic urethane modified alkyd resin has a viscosity of 3000-6500 mPa·S at 25° C.

10. A polyurethane coating, characterized in that: The polyurethane coating comprises an aliphatic urethane modified alkyd resin prepared by the preparation method according to any one of claims 1 to 7, a matting agent, an auxiliary agent, and a solvent, wherein the weight percentage of the matting agent in the polyurethane coating does not exceed 3.8%.

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