Lignin modified waterborne polyurethane strippable coating and preparation method thereof

Abstract

The invention discloses a lignin modified waterborne polyurethane peelable coating and a preparation method thereof. The peelable coating is obtained by sequentially reacting and mixing waterborne epoxy ester modified carbon dioxide-based polyurethane, functionalized lignin / silicon dioxide filler, color paste, a defoaming agent, an anti-sticking agent, a thickening agent and deionized water. According to the lignin-modified waterborne polyurethane strippable coating prepared by the preparation method disclosed by the invention, the matrix resin and the filler are subjected to relatively comprehensive molecular structure modification, and a cross-linked structure of a coating is obviously changed, so that on one hand, epoxy ester and carbon dioxide-based polyol are introduced, and the cross-linked density of a prepared coating film is obviously improved; on the other hand, the introduction of the functionalized lignin / silicon dioxide improves the outdoor aging resistance and corrosion resistance of the coating, and the strippable performance of a paint film exposed outdoors for two years is still very excellent.

Description

Technical Field

[0001] The invention belongs to the field of water-based coatings, and in particular relates to a lignin-modified water-based polyurethane strippable coating and a preparation method thereof. Background Art

[0002] Strippable coating is a special protective coating that is applied to the surface of the substrate to isolate it from external erosion and prevent it from being scratched, scratched, aged, polluted, dusty, oily, fingerprinted, etc. during production, transportation or storage. It is also called temporary protective coating, which can provide temporary protection for various substrate surfaces, such as metal, plastic, wood, ceramic, glass, rubber, pre-coated parts (such as pre-coated polyurethane topcoat), etc. The general protection period ranges from a few days to a few years, and after the protection is over, it can be easily peeled off from the substrate surface without negatively affecting the substrate surface. Strippable coating has a wide range of applications and can be used for indoor and outdoor, short-term and long-term, pre-coated and non-pre-coated, normal temperature and high temperature protection of metal and non-metal materials.

[0003] The resins used in strippable coatings mainly include solvent-based resins, hot-melt resins, water-based polyurethane, acrylic, epoxy and other systems, but different systems have different performance characteristics. There are currently three problems: First, the large amount of solvent VOC released by solvent-based systems has a greater negative impact on the environment, while the VOC of water-based systems is greatly reduced or even eliminated; second, many strippable coatings are only suitable for some substrates, and are not strippable for many substrates, and have poor broad-spectrum applicability; third, almost all strippable coating systems are difficult or impossible to strip after long-term protection, which will greatly reduce the application scope of strippable coatings. At present, most strippable coatings use the help of strippable additives to effectively reduce the adhesion between the paint film and the substrate. Common stripping additives include higher fatty acid esters, fatty acid salts, waxes, surfactants, silicones, etc. These additives have certain effects, but the side effects are relatively large, and they have a variety of adverse effects on the surface of the substrate.

[0004] Patent application number 201910851054.1 introduces a method for preparing a strippable waterborne polyurethane resin and its application in coatings. The strippable waterborne polyurethane is first prepared and then mixed with pullulan to obtain the coating. The waterborne strippable coating is biodegradable, does not pollute the environment, and can reduce manpower and financial investment. Compared with traditional films and other strippable resins, its environmental advantages are more obvious.

[0005] The patent with application number 201811435854.7 introduces a method for preparing a room-temperature self-repairing water-based polyurethane car cover with self-matting effect. The prepared water-based polyurethane car cover emulsion has good room-temperature self-repairing function, while providing different glossiness and excellent weather resistance. After the resin is compounded with additives, it is sprayed on the car surface, which can provide long-term protection for the car paint and can be peeled off at any time.

[0006] The water-based strippable coating synthesized by the above method fails to carry out a more comprehensive molecular structure modification of the base resin, the chemical medium resistance of the resin has not changed significantly, and the aging performance and strippability of the paint film after outdoor exposure for more than one year are not considered, and it cannot meet the market demand for strippable coatings under actual harsh environments. Summary of the invention

[0007] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a lignin-modified waterborne polyurethane strippable coating and a preparation method thereof.

[0008] The present invention is achieved through the following technical solutions:

[0009] A lignin-modified waterborne polyurethane strippable coating, wherein the strippable coating comprises the following components and the mass fractions of each component:

[0010]

[0011] In the above technical solution, the components and mass fractions of the components included in the waterborne epoxy ester modified carbon dioxide-based polyurethane are as follows:

[0012]

[0013]

[0014] In the above technical scheme, the isocyanate monomer is any one of isophorone diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate or pentamethylene diisocyanate; the carbon dioxide-based polyol is any one or more of PPCD-222, PCD-621 or PCD-632; the small molecule diol is any one or more of neopentyl glycol, 1,4-butanediol or 1,6-hexanediol; the hydrophilic monomer is any one or both of dihydroxymethylpropionic acid or dihydroxymethylbutyric acid; the dihydrazide small molecule chain extender is oxalic acid dihydrazide.

[0015] In the above technical solution, the color paste is any one of EC701, EW6102 or EI6200; the defoaming agent is any one of Airex 902W or BYK-025; the anti-sticking agent is any one of DC-51 or HY-51Z; the thickener is any one of ASE-60 or RM-8W.

[0016] A method for preparing a lignin-modified waterborne polyurethane strippable coating comprises the following steps:

[0017] (I) Preparation of waterborne epoxy ester modified carbon dioxide based polyurethane

[0018] Add 1,6-hexanediol diglycidyl ether, linoleic acid and tetrabutylammonium bromide in a reactor in proportion, react at 110° C. to 130° C. for 2 h to 3 h, then add carbon dioxide-based polyol, isocyanate monomer and dibutyltin dilaurate to react at 90° C. to 110° C. for 3 h to 5 h, then add small molecule diol and hydrophilic monomer, heat to 75° C. to 85° C. under a protective atmosphere and mix and react for 2 h to 6 h, then add triethylamine to neutralize the carboxyl group on the water-based chain extender, then add acetone to fully disperse, and fully disperse the obtained dispersion system into an aqueous solution containing a dihydrazide small molecule chain extender, after high-speed dispersion reaction, remove acetone by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane;

[0019] (II) Preparation of functionalized lignin / silica fillers

[0020] In the reactor, sodium lignin sulfonate and water are added, stirring is started, and when the temperature reaches 70°C to 90°C, 3-chloro-2-hydroxypropylammonium chloride is added dropwise, the reactants are kept warm for 3h to 5h, and then hexadecyltrimethylammonium bromide and corrosion inhibitor are added, and heating and stirring are performed at 70°C to 90°C for 0.5h to 1h, and then ethyl orthosilicate is added dropwise. After the addition is completed for 1h to 2h, the reaction is continued for 2 to 5h. In the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After the reaction is completed, the reaction is cooled to room temperature, filtered, washed with appropriate deionized water, and dried in a drying oven at 60°C for 24h to obtain a functionalized lignin / silica filler.

[0021] (III) Preparation of lignin-modified waterborne polyurethane strippable coating

[0022] The waterborne epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), the functionalized lignin / silica filler and the color paste prepared in step (II) are sequentially added into a mixing tank and stirred evenly at high speed. During the stirring process, the defoamer, the anti-sticking agent and the thickener are dispersed evenly into the mixing tank. Finally, deionized water is added and stirred evenly to obtain a lignin-modified waterborne polyurethane strippable coating.

[0023] In the above technical solution, the mass fractions of each component in the preparation of the functionalized lignin / silicon dioxide filler in step (II) are as follows:

[0024]

[0025] In the above technical solution, the corrosion inhibitor is any one or more of benzotriazole or 3-amino-5-mercapto-1,2,4-triazole.

[0026] The beneficial effects of the present invention are:

[0027] The invention provides a lignin-modified waterborne polyurethane strippable coating and a preparation method thereof. The prepared strippable coating has good outdoor aging resistance and strippability.

[0028] The lignin-modified waterborne polyurethane strippable coating provided by the present invention is modified by quaternization of lignin under alkaline conditions, and then ethyl orthosilicate is used to hydrolyze the lignin and corrosion inhibitor surface to form a mesoporous silica skeleton, so that lignin and corrosion inhibitor are directly loaded in the mesoporous structure of mesoporous silica, and the reaction of corrosion inhibitor and lignin with the coating resin component is successfully avoided, while ensuring that the corrosion inhibitor and lignin are effectively stored and controlled release. On the one hand, lignin contains a large amount of chromophores and co-chromophores (phenolic hydroxyl groups, ketones, sulfonates) and benzene ring structures, and continuously absorbs ultraviolet rays, which can prevent the degradation and aging of the coating. On the other hand, when metal corrosion occurs, the corrosion inhibitor can interact with the surface of the steel material to form a complex film layer, suppressing the aggravation of the corrosion reaction, and then giving the coating a certain self-repairing property.

[0029] The lignin-modified waterborne polyurethane strippable coating provided by the present invention comprises a resin molecular structure system containing double bonds of fatty acids, epoxy esters of long carbon chains and carbon dioxide-based polyols, both of which are secondary hydroxyls and have low reaction activity. By reacting together at a relatively high temperature, the reaction activity is improved, and the two are successfully introduced into the molecular skeleton together. On the one hand, the double bonds of fatty acids can undergo oxidative crosslinking when solidified into a film, thereby improving the water resistance and solvent resistance of the film. On the other hand, the carbon dioxide-based polyol is an aliphatic polycarbonate diol with excellent high mechanical properties and high hydrolysis resistance. The carbon dioxide-based polyol is added together with the epoxy ester in the early stage to balance the hardness and flexibility of the system.

[0030] The lignin-modified waterborne polyurethane strippable coating provided by the present invention has good dispersibility and interface compatibility of the functionalized lignin / silicon dioxide filler due to the amphiphilic molecular structure of lignin, thus providing good physical barrier properties for the coating. DETAILED DESCRIPTION

[0031] In order to enable persons skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention is further described below through specific implementation methods.

[0032] Example 1

[0033] A method for preparing a lignin-modified waterborne polyurethane strippable coating comprises the following steps:

[0034] (I) In a four-necked flask equipped with a spherical condenser, a nitrogen inlet tube, a stirrer and a thermometer, 12 g of 1,6-hexanediol diglycidyl ether, 14 g of linoleic acid and 0.3 g of tetrabutylammonium bromide were added, and the mixture was reacted at 110° C. for 3 h. 120 g of carbon dioxide-based polyol PPCD-222, 60 g of isophorone diisocyanate and 0.2 g of dibutyltin dilaurate were added and reacted at 90° C. for 3 h. Then 4 g of neopentyl glycol and 12 g of dimethylolpropionic acid were added. The mixture was heated to 75° C. under a protective atmosphere and mixed for reaction for 3 h. Then 9 g of triethylamine was added for neutralization. Then 50 g of acetone was added for full dispersion. The obtained dispersion system was fully dispersed in 480 g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 5 g. After high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane.

[0035] (II) Add 5g of sodium lignin sulfonate and 4000g of water to a four-necked flask equipped with a spherical condenser, a stirrer and a thermometer, start stirring, and when the temperature reaches 70°C, add 2g of 3-chloro-2-hydroxypropylammonium chloride dropwise, and keep the reactants warm for 3 hours. Then add 4g of hexadecyltrimethylammonium bromide and 3g of benzotriazole, and heat and stir at 70°C for 0.5h. Add 40g of ethyl orthosilicate dropwise, and continue the reaction for 2h after the addition is completed for 1h. During the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After cooling to room temperature, filter and wash with appropriate deionized water, place in a drying oven at 60 degrees for 24h, and finally prepare functionalized lignin / silica filler.

[0036] (III) 85 g of the water-based epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), 2 g of the functionalized lignin / silica filler prepared in step (II), and 4 g of color paste EC701 are added to a mixing tank in sequence and stirred evenly at high speed. During the stirring process, 0.2 g of defoamer Airex902W, 0.2 g of anti-sticking agent DC-51, and 0.2 g of thickener ASE-60 are dispersed evenly in the above mixing tank. Finally, 3 g of deionized water is added and stirred evenly to obtain a lignin-modified water-based polyurethane strippable coating.

[0037] Example 2

[0038] A method for preparing a lignin-modified waterborne polyurethane strippable coating comprises the following steps:

[0039] (I) In a four-necked flask equipped with a spherical condenser, a nitrogen inlet tube, a stirrer and a thermometer, 12 g of 1,6-hexanediol diglycidyl ether, 20 g of linoleic acid and 0.3 g of tetrabutylammonium bromide were added, and the mixture was reacted at 120° C. for 3 h. 140 g of carbon dioxide-based polyol PCD-621, 70 g of dicyclohexylmethane diisocyanate and 0.2 g of dibutyltin dilaurate were added and the mixture was reacted at 100° C. for 4 h. Then, 6 g of 1,4-butanediol and 13 g of dihydroxymethylbutyric acid were added. The mixture was heated to 80° C. under a protective atmosphere and mixed for reaction for 4 h. Then, 9.5 g of triethylamine was added for neutralization. Then, 70 g of acetone was added for full dispersion. The obtained dispersion system was fully dispersed in 500 g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 5 g. After high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane.

[0040] (II) In a four-necked flask equipped with a spherical condenser, a stirrer and a thermometer, add 7g of sodium lignin sulfonate and 5000g of water, start stirring, and when the temperature reaches 80°C, add 3g of 3-chloro-2-hydroxypropyl ammonium chloride dropwise, and keep the reactants warm for 4 hours. Then add 4g of hexadecyltrimethylammonium bromide and 3g of 3-amino-5-mercapto-1,2,4-triazole, heat and stir at 80°C for 0.5h. Add 40g of ethyl orthosilicate dropwise, and continue the reaction for 3h after the addition is completed in 1h. During the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After cooling to room temperature, filter and wash with appropriate deionized water, place in a drying oven at 60 degrees for 24h, and finally prepare functionalized lignin / silica filler.

[0041] (III) 90 g of the waterborne epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), 4 g of the functionalized lignin / silica filler prepared in step (II), and 4 g of color paste EW6102 were added to a mixing tank in sequence and stirred evenly at high speed. During the stirring process, 0.3 g of defoamer BYK-025, 0.4 g of anti-sticking agent DC-51, and 0.2 g of thickener RM-8W were added to the above mixing tank and dispersed evenly. Finally, 3 g of deionized water was added and stirred evenly to obtain a lignin-modified waterborne polyurethane strippable coating.

[0042] Example 3

[0043] A method for preparing a lignin-modified waterborne polyurethane strippable coating comprises the following steps:

[0044] (I) In a four-necked flask equipped with a spherical condenser, a nitrogen inlet tube, a stirrer and a thermometer, 12 g of 1,6-hexanediol diglycidyl ether, 23 g of linoleic acid and 0.3 g of tetrabutylammonium bromide were added and reacted at 130°C for 3 h. Then, 160 g of carbon dioxide-based polyol PCD-632, 70 g of dicyclohexylmethane diisocyanate, 20 g of hexamethylene diisocyanate and 0.2 g of dibutyltin dilaurate were added and reacted at 100°C for 4 h. Then, 6 g of 1,6-hexanediol and 13g of dihydroxymethylpropionic acid were heated to 80°C under a protective atmosphere and mixed for reaction for 4h, then 9.5g of triethylamine was added for neutralization, and then 100g of acetone was added for full dispersion. The obtained dispersion system was fully dispersed in 600g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 10g. After high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane.

[0045] (II) Add 9g of sodium lignin sulfonate and 5000g of water to a four-necked flask equipped with a spherical condenser, a stirrer and a thermometer, start stirring, and when the temperature reaches 90°C, add 4g of 3-chloro-2-hydroxypropylammonium chloride dropwise, and keep the reactants warm for 5 hours. Then add 5g of hexadecyltrimethylammonium bromide and 6g of 3-amino-5-mercapto-1,2,4-triazole, and heat and stir at 80°C for 0.5h. Add 60g of ethyl orthosilicate dropwise, and continue the reaction for 5h after the addition is completed in 2h. During the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After cooling to room temperature, filter and wash with appropriate deionized water, place in a drying oven at 60 degrees for 24h, and finally prepare functionalized lignin / silica filler.

[0046] (III) Add 95g of the water-based epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), 9g of the functionalized lignin / silica filler prepared in step (II), and 4g of color paste EI6200 into a mixing tank in sequence, and stir evenly at high speed. During the stirring process, add 0.3g of defoamer Airex902W, 0.2g of anti-sticking agent HY-51Z, 0.2g of anti-sticking agent DC-51, and 0.2g of thickener RM-8W into the above mixing tank and disperse evenly. Finally, add 5g of deionized water and stir evenly to obtain a lignin-modified water-based polyurethane strippable coating.

[0047] Example 4

[0048] A method for preparing a lignin-modified waterborne polyurethane strippable coating comprises the following steps:

[0049] (I) Add 23 g of 1,6-hexanediol diglycidyl ether, 28g linoleic acid and 0.3g tetrabutylammonium bromide were reacted at 130°C for 3h, 180g carbon dioxide-based polyol PCD-621, 100g dicyclohexylmethane diisocyanate, 20g isophorone diisocyanate, and 0.4g dibutyltin dilaurate were added and reacted at 110°C for 5h, then 6g neopentyl glycol and 15g dihydroxymethylpropionic acid were added, and the mixture was heated to 80°C under a protective atmosphere and mixed for 6h, then 11g triethylamine was added for neutralization, and 150g acetone was added for full dispersion, and the obtained dispersion system was fully dispersed in 660g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 15g, and after high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane.

[0050] (II) Add 9g of sodium lignin sulfonate and 6000g of water to a four-necked flask equipped with a spherical condenser, a stirrer and a thermometer, start stirring, and when the temperature reaches 85°C, add 4g of 3-chloro-2-hydroxypropylammonium chloride dropwise, and keep the reactants warm for 5 hours. Then add 12g of hexadecyltrimethylammonium bromide and 7g of 3-amino-5-mercapto-1,2,4-triazole, heat and stir at 90°C for 1h. Add 70g of ethyl orthosilicate dropwise, and continue the reaction for 5h after the addition is completed in 2h. During the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After cooling to room temperature, filter and wash with appropriate deionized water, place in a drying oven at 60 degrees for 24h, and finally prepare functionalized lignin / silica filler.

[0051] (III) Add 95g of the water-based epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), 5g of the functionalized lignin / silica filler prepared in step (II), and 5g of color paste EC701 into a mixing tank in sequence, and stir evenly at high speed. During the stirring process, add 0.3g of defoamer Airex902W, 0.6g of anti-sticking agent DC-51, and 0.2g of thickener ASE-60 into the above mixing tank and disperse evenly. Finally, add 3g of deionized water and stir evenly to obtain a lignin-modified water-based polyurethane strippable coating.

[0052] Comparative Example 1

[0053] The addition of functionalized lignin / silicon dioxide filler was not involved, and the remaining operations were the same as those in Example 4:

[0054] (I) Add 23 g of 1,6-hexanediol diglycidyl ether, 28g linoleic acid and 0.3g tetrabutylammonium bromide were reacted at 130°C for 3h, 180g carbon dioxide-based polyol PPCD-222, 100g dicyclohexylmethane diisocyanate, 20g isophorone diisocyanate, and 0.4g dibutyltin dilaurate were added and reacted at 110°C for 5h, then 6g neopentyl glycol and 15g dihydroxymethylpropionic acid were added, and the mixture was heated to 80°C under a protective atmosphere and mixed for 4h, then 11g triethylamine was added for neutralization, and 150g acetone was added for full dispersion, and the obtained dispersion system was fully dispersed in 660g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 15g, and after high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane.

[0055] (II) 95 g of the water-based epoxy ester modified carbon dioxide-based polyurethane prepared in step (I) and 5 g of color paste EC701 were added to a mixing tank in sequence and stirred evenly at high speed. During the stirring process, 0.3 g of defoamer Airex 902W, 0.6 g of anti-sticking agent DC-51 and 0.2 g of thickener ASE-60 were evenly dispersed in the above mixing tank. Finally, 3 g of deionized water was added and stirred evenly to obtain a water-based polyurethane strippable coating.

[0056] Comparative Example 2

[0057] The addition of 1,6-hexanediol diglycidyl ether and 28 g of linoleic acid was not involved, and the remaining operations were the same as those in Example 4:

[0058] (I) In a four-necked flask equipped with a spherical condenser, a nitrogen inlet tube, a stirrer and a thermometer, 180 g of carbon dioxide-based polyol PPCD-222, 100 g of dicyclohexylmethane diisocyanate, 20 g of isophorone diisocyanate, and 0.4 g of dibutyltin dilaurate were added and reacted at 110° C. for 5 h, and then 6 g of neopentyl glycol and 15 g of dimethylolbutyric acid were added. The mixture was heated to 80° C. under a protective atmosphere and mixed for reaction for 4 h, and then 11 g of triethylamine was added for neutralization, and then 150 g of acetone was added for full dispersion. The obtained dispersion system was fully dispersed in 660 g of an aqueous solution containing oxalic acid dihydrazide, wherein the amount of oxalic acid dihydrazide added was 15 g. After high-speed dispersion reaction, acetone was removed by rotary evaporation to obtain an aqueous carbon dioxide-based polyurethane.

[0059] (II) Add 9g of sodium lignin sulfonate and 6000g of water to a four-necked flask equipped with a spherical condenser, a stirrer and a thermometer, start stirring, and when the temperature reaches 85°C, add 4g of 3-chloro-2-hydroxypropylammonium chloride dropwise, and keep the reactants warm for 5 hours. Then add 12g of hexadecyltrimethylammonium bromide and 7g of 3-amino-5-mercapto-1,2,4-triazole, heat and stir at 90°C for 1h. Add 70g of ethyl orthosilicate dropwise, and continue the reaction for 5h after the addition is completed in 2h. During the above reaction process, sodium hydroxide solution is added to keep the pH of the reaction system above 11. After cooling to room temperature, filter and wash with appropriate deionized water, place in a drying oven at 60 degrees for 24h, and finally prepare functionalized lignin / silica filler.

[0060] (III) 95 g of the aqueous carbon dioxide-based polyurethane prepared in step (I), 5 g of the functionalized lignin / silica filler prepared in step (II), and 5 g of color paste EC701 were added to a mixing tank in sequence and stirred evenly at high speed. During the stirring process, 0.3 g of defoamer Airex 902W, 0.6 g of anti-sticking agent DC-51, and 0.2 g of thickener RM-8W were added to the above mixing tank and dispersed evenly. Finally, 3 g of deionized water was added and stirred evenly to obtain a waterborne polyurethane strippable coating.

[0061] Coating performance measurement:

[0062] The coatings of Examples 1 to 4 and Comparative Examples 1 to 2 were coated on the corresponding substrates, respectively, with a thickness of 70 to 100 μm, and then cured for 7 days at room temperature to prepare the coating films of Examples 1 to 4 and Comparative Examples 1 to 2. The obtained coating films were subjected to peelability test, water resistance, outdoor exposure weather resistance, tensile strength, elongation, and peel strength tests, and the results are shown in Table 1.

[0063] The coating film properties of the coatings prepared in Examples 1 to 4 and the coatings of Comparative Examples 1 to 2 were compared, and the results are shown in Table 1.

[0064] Table 1: Test results of coating properties of Examples 1 to 4 and Comparative Examples 1 to 2

[0065]

[0066] It can be seen from Table 1 that, compared with the coating films of Comparative Examples 1 to 2, the strippability, water resistance, fastest stripping time and anti-corrosion performance of the lignin-modified waterborne polyurethane strippable coatings prepared in Examples 1 to 4 of the present invention are far superior to those of Comparative Examples 1 to 2. In contrast, due to the lack of corrosion inhibitor loaded in the filler, Comparative Example 1 failed to inhibit the aggravation of the corrosion reaction when metal corrosion occurred, and the coating film had poor salt water resistance; at the same time, the lack of chemical groups that continuously absorb ultraviolet rays, especially when attached to tinplate, the coating structure of the paint film changed after 2 years of outdoor exposure, and the strippability was poor.

[0067] Compared with Example 4, although both Comparative Example 2 contain functionalized lignin / silica fillers, Comparative Example 2 only relies on the reaction of carbon dioxide-based polyols with isocyanates, and does not contain double bonds of fatty acids in the molecular structure, and cannot be oxidatively cross-linked. The cross-linking density is relatively low, and especially the fastest peeling time is relatively long, which makes it difficult to meet certain quick-drying application requirements. At the same time, the water resistance and salt water resistance are slightly worse.

[0068] Therefore, the water-based strippable coating prepared by the present invention carries out a relatively comprehensive molecular structure modification on the base resin and the filler, and the anti-corrosion performance is significantly improved. At the same time, the strippable performance of the paint film is still relatively good after two years of outdoor exposure, which fully meets the market demand for strippable coatings under actual harsh environments.

[0069] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A lignin-modified waterborne polyurethane strippable coating, characterized in that: The components included in the strippable coating and the mass fractions of each component are as follows:

2. The lignin-modified waterborne polyurethane strippable coating according to claim 1, characterized in that: The components and mass fractions of the waterborne epoxy ester modified carbon dioxide-based polyurethane are as follows:

3. The lignin-modified waterborne polyurethane strippable coating according to claim 2, characterized in that: The isocyanate monomer is any one of isophorone diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate or pentamethylene diisocyanate; the carbon dioxide-based polyol is any one or more of PPCD-222, PCD-621 or PCD-632; the small molecule diol is any one or more of neopentyl glycol, 1,4-butanediol or 1,6-hexanediol; the hydrophilic monomer is any one or two of dihydroxymethylpropionic acid or dihydroxymethylbutyric acid; the dihydrazide small molecule chain extender is oxalic acid dihydrazide.

4. The lignin-modified waterborne polyurethane strippable coating according to claim 1, characterized in that: The color paste is any one of EC701, EW6102 or EI6200; the defoamer is any one of Airex 902W or BYK-025; the anti-sticking agent is any one of DC-51 or HY-51Z; the thickener is any one of ASE-60 or RM-8W.

5. A method for preparing a lignin-modified waterborne polyurethane strippable coating according to any one of claims 1 to 4, characterized in that: The following steps are involved: (I) Preparation of waterborne epoxy ester modified carbon dioxide based polyurethane In a reactor, 1,6-hexanediol diglycidyl ether, linolenic acid and tetrabutylammonium bromide are added in proportion for reaction, and then a carbon dioxide-based polyol, an isocyanate monomer and dibutyltin dilaurate are added for reaction, and then a small molecule diol and a hydrophilic monomer are added for mixed reaction, and triethylamine and acetone are added in sequence after the mixed reaction to fully disperse, and the obtained dispersion system is fully dispersed in an aqueous solution containing a dihydrazide small molecule chain extender, and after a high-speed dispersion reaction, the acetone is removed by rotary evaporation to obtain a water-based epoxy ester-modified carbon dioxide-based polyurethane; (II) Preparation of functionalized lignin / silica fillers In a reactor, sodium lignin sulfonate and water are added, 3-chloro-2-hydroxypropylammonium chloride is added dropwise under stirring, then hexadecyltrimethylammonium bromide and a corrosion inhibitor are added, and then ethyl orthosilicate is added dropwise, and the reaction is continued. After the reaction is completed, the mixture is cooled to room temperature, filtered, washed, and dried to obtain a functionalized lignin / silica filler; (III) Preparation of lignin-modified waterborne polyurethane strippable coating The waterborne epoxy ester modified carbon dioxide-based polyurethane prepared in step (I), the functionalized lignin / silica filler and the color paste prepared in step (II) are sequentially added into a mixing tank and stirred evenly at high speed. During the stirring process, a defoamer, an anti-sticking agent and a thickener are added into the mixing tank and dispersed evenly. Finally, deionized water is added and stirred evenly to obtain a lignin-modified waterborne polyurethane strippable coating.

6. The method for preparing the lignin-modified waterborne polyurethane strippable coating according to claim 5, characterized in that: In the step (I), the reaction temperature and duration of 1,6-hexanediol diglycidyl ether, linolenic acid and tetrabutylammonium bromide are 110°C to 130°C for 2h to 3h; the reaction temperature and duration after adding carbon dioxide-based polyol, isocyanate monomer and dibutyltin dilaurate are 90°C to 110°C for 3h to 5h; the reaction temperature and duration after adding small molecule diol and hydrophilic monomer are: heating to 75°C to 85°C under a protective atmosphere and mixing for 2h to 6h.

7. The method for preparing the lignin-modified waterborne polyurethane strippable coating according to claim 5, characterized in that: The mass fractions of each component in the preparation of the functionalized lignin / silicon dioxide filler in step (II) are as follows:

8. The lignin-modified waterborne polyurethane strippable coating according to claim 7, characterized in that: The corrosion inhibitor is any one or more of benzotriazole or 3-amino-5-mercapto-1,2,4-triazole.

9. The method for preparing the lignin-modified waterborne polyurethane strippable coating according to claim 5, characterized in that: After 3-chloro-2-hydroxypropylammonium chloride is added dropwise in step (II), the reactants are kept warm for 3h to 5h; after adding hexadecyltrimethylammonium bromide and corrosion inhibitor, the reaction temperature and duration are heating and stirring at 70°C to 90°C for 0.5h to 1h; the duration of adding tetraethyl orthosilicate is 1h to 2h; the duration of continuing the reaction after the addition of tetraethyl orthosilicate is 2h to 5h; in the preparation process of the functionalized lignin / silica filler (II), sodium hydroxide solution is added to keep the pH of the reaction system above 11; the drying condition is to place it in a drying oven at 60°C for 24h.

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