A hydroxyl-functionalized soluble transparent polyimide resin, a method for its preparation, and an automotive body clearcoat composition

Abstract

This invention belongs to the field of automotive body clear coat technology, specifically disclosing a hydroxyl-functionalized soluble transparent polyimide resin, its preparation method, and an automotive body clear coat composition. The polyimide resin is polymerized from a hydroxyl-containing diamine monomer, a non-hydroxyl-containing diamine monomer, an amino-terminated polydimethylsiloxane, and a dianhydride monomer. The resin is prepared via chemical imidization. The clear coat composition comprises the polyimide resin, acrylic resin, polyester resin, additives, and solvents, and may optionally include an amino resin and / or a polyisocyanate curing agent, constituting a single-component or two-component system. This clear coat composition is specifically designed for original equipment manufacturer (OEM) automotive body clear coats and requires high-temperature baking and curing at 80-150°C. This invention improves the scratch resistance, weather resistance, chemical corrosion resistance, and stone chip resistance of clear coats at the molecular framework level, solving the problem of traditional clear coats' difficulty in simultaneously achieving high hardness and high toughness.

Description

Technical Field

[0001] This invention relates to the field of automotive body clear coat technology, and more particularly to a hydroxyl-functionalized soluble transparent polyimide resin, its preparation method, and an automotive body clear coat composition. Background Technology

[0002] As the outermost layer of the paint system, the performance of automotive body clearcoat directly determines the durability of a vehicle's appearance and its resale value. Currently, the original equipment manufacturer (OEM) clearcoats used by major domestic and international automakers mainly fall into two categories: one is a single-component acrylic-amino resin system, which relies on high-temperature baking to achieve cross-linking and curing; the other is a two-component acrylic-polyurethane or polyester-polyurethane system, which forms a cross-linked network through the reaction of polyisocyanates and hydroxyl groups. These two technologies have matured in terms of workability, gloss, and initial protective performance after decades of development. However, in recent years, with consumers' increasing demands for paint durability, their inherent defects have gradually become apparent.

[0003] The fundamental problem with existing clear coat systems lies in the chemical and physical limitations of the resin molecular skeleton. Firstly, they lack scratch resistance. Acrylic resins offer limited hardness, while polyurethane resins, although possessing some toughness, have poor surface scratch resistance, easily leaving fine scratches after washing or wiping, causing the paint surface to quickly lose its gloss. Secondly, they have poor chemical and hydrolytic resistance. Both the ester bonds in acrylic resins and the polyurethane bonds (-NH-COO-) are prone to hydrolytic degradation in corrosive media such as acid rain, bird droppings, and tree sap, as well as in high-temperature and high-humidity environments, leading to loss of gloss, pitting, and even chalking of the clear coat. Thirdly, their weather resistance is limited. Aromatic isocyanates yellow under ultraviolet light, and polyester resins themselves have weak UV resistance. Even with the addition of UV absorbers and hindered amine light stabilizers, these additives are consumed over time, failing to provide lasting protection throughout the paint film's lifespan.

[0004] Patent CN107057547B discloses an acrylic-polyurethane type clear varnish, which forms a main network through the reaction of isocyanate and hydroxyl groups, supplemented by secondary crosslinking with amino resin. However, its polyurethane bonds still have the risk of hydrolysis under humid and hot environments, and its tolerance to highly polar solvents is limited. Patent CN107057536B uses blocked isocyanate to prepare a single-package clear varnish, simplifying the construction process. However, when the sealant is desealed at high temperatures, it is prone to producing colored byproducts that cause yellowing, and the interlayer adhesion is poor, affecting the recoating and repair performance.

[0005] Polyimide materials are renowned for their excellent mechanical strength, thermal stability, and chemical inertness. However, traditional aromatic polyimides have long been excluded from automotive clear coat applications due to their deep yellow color, insolubility in common paint solvents, and curing temperatures far exceeding the limits allowed by automotive painting lines (typically not exceeding 140-160℃). Patent CN105764989B discloses a polyimide resin composition that prepares a semi-interpenetrating network structure by blending thermoplastic polyimide with end-modified imide oligomers. Primarily used in fiber-reinforced composites, this composition suffers from excessively high curing temperatures and a lack of active groups suitable for crosslinking, making it unsuitable for direct use in automotive clear coats. Therefore, how to retain the inherent advantages of polyimides—high strength, high toughness, and high weather resistance—while simultaneously endowing them with good low-temperature curing properties, solvent solubility, and optical transparency, and adapting them to traditional clear coat crosslinking systems, has become a pressing technical challenge in this field. Summary of the Invention

[0006] In view of the above-mentioned problems, the present invention provides a hydroxyl-functionalized soluble transparent polyimide resin, its preparation method, and an automotive body clear coat composition. The present invention aims to fundamentally innovate at the resin molecular skeleton level, resolving the technical contradictions in existing clear coat systems regarding scratch resistance, chemical resistance, weather resistance, and appearance, such as the difficulty in simultaneously achieving high hardness and high toughness, and the degradation of protective properties due to additive consumption, thereby achieving a systematic improvement in the overall performance of the clear coat.

[0007] To achieve the above objectives, the present invention first carried out a systematic design at the molecular structure level. Traditional polyimide cannot be used in automotive clear coats because it is insoluble in common paint solvents, has a curing temperature far exceeding the range allowed by automotive painting lines, and has a deep yellow color. This invention breaks through this technical barrier through the following three core designs: (1) Hydroxyl functionalization design - introducing a specific amount of hydroxyl groups into the polyimide molecular chain, controlling its hydroxyl value at 50-150 mg KOH / g. This hydroxyl group is not only a key active group for cross-linking reaction with polyisocyanates or amino resins, but also an important means to improve the solubility of the resin in polar solvents; (2) Soluble structure design - by selecting asymmetric monomers, monomers containing large-volume side groups, and introducing flexible siloxane segments, the tight packing of molecular chains is broken, enabling the resin to dissolve in ester, ketone, and alcohol ether mixed solvents commonly used in the automotive coating industry; (3) Transparency assurance - by selecting fluorinated monomers and / or alicyclic dianhydride monomers, the intramolecular / intermolecular charge transfer complexation is effectively suppressed, synthesizing a polyimide resin with a light color and high light transmittance, ensuring that the clear coat coating does not interfere with the display of the car body base color.

[0008] To address the above problems, the present invention provides an automotive body clear coat composition comprising a hydroxyl-functionalized soluble transparent polyimide resin; The hydroxyl-functionalized soluble transparent polyimide resin is polymerized from the following monomer components: hydroxyl-containing diamine monomer, non-hydroxyl diamine monomer, amino-terminated polydimethylsiloxane, and dianhydride monomer; The hydroxyl-functionalized soluble transparent polyimide resin has a hydroxyl value of 50-150 mg KOH / g.

[0009] Preferably, the hydroxyl-functionalized soluble transparent polyimide resin has a number-average molecular weight of 5000-50000 g / mol and a glass transition temperature of 120-200℃. It should be noted that an appropriate molecular weight ensures good solubility and workability of the resin, while a higher glass transition temperature imparts excellent heat resistance and indentation resistance to the coating.

[0010] Preferably, based on 100% of the total mass of the composition, the composition comprises: The solids content of the hydroxyl-functionalized soluble transparent polyimide resin is 10%~50%; The solids content of acrylic resin is 5% to 30%; The solids content of polyester resin is 5%~30%; The solid content of amino resins is 0%~30%; Additives 0.5%~4%; Polyisocyanate curing agent 0%~30%; Solvent, wherein the solvent is in the balance; Wherein, when the content of the polyisocyanate curing agent is 0%, the content of the amino resin is greater than 0%. It should be noted that the above components are not simply superimposed, but rather work synergistically to achieve breakthroughs in coating performance: the polyimide resin, as the core film-forming component, provides extremely high hardness and scratch resistance with its rigid aromatic heterocyclic backbone, while the introduced flexible siloxane segments endow the coating with excellent stone impact resistance and flexibility, solving the problem of easy cracking in traditional high-hardness varnishes; acrylic resin and polyester resin, as auxiliary film-forming resins, are used to adjust the application performance and optimize costs of the composition. The combination of these three can improve the working performance and economic benefits of the formulation while maintaining the high-performance advantages of polyimide. The amino resin (for the 1K system) and the polyisocyanate curing agent (for the 2K system) undergo cross-linking reactions with the hydroxyl groups on the polyimide resin, forming a dense three-dimensional network structure.

[0011] Preferably, the composition is a two-component varnish composition comprising component A and component B; Component A comprises the hydroxyl-functionalized soluble transparent polyimide resin, the acrylic resin, the polyester resin, the additives, and the solvent; Component B contains the polyisocyanate curing agent; In this composition, the molar ratio of isocyanate groups to hydroxyl groups is 0.9:1 to 1.2:1. Within this ratio range, the crosslinking reaction is most complete, and the overall performance of the coating film is optimal.

[0012] Preferably, the composition is a one-component varnish composition comprising the hydroxyl-functionalized soluble transparent polyimide resin, the acrylic resin, the polyester resin, the amino resin, the additives, and the solvent, and does not contain the polyisocyanate curing agent.

[0013] It is worth emphasizing that the polyimide resin of this invention contains hydroxyl groups in its molecular structure that can crosslink with both amino resins and polyisocyanates, thus it can be adapted to both of the aforementioned distinct curing systems, exhibiting excellent versatility. This characteristic provides great flexibility in coating formulation design, allowing for the selection of single-component or two-component solutions based on actual production conditions without changing the core resin material.

[0014] Based on the same inventive concept, the present invention also provides a hydroxyl-functionalized soluble transparent polyimide resin of any of the above-described automotive body clear coat compositions, said resin being polymerized from a component comprising the following monomers: Diamine monomers containing hydroxyl groups; Non-hydroxydiamine monomers; Amino-terminated polydimethylsiloxane; Dihydride monomer; The resin has a hydroxyl value of 50-150 mg KOH / g and a number-average molecular weight of 5000-50000 g / mol; its glass transition temperature is 120-200℃; and its solubility in a mixed solvent of butyl acetate, N-methylpyrrolidone, xylene, and propylene glycol methyl ether acetate at 25℃ is not less than 10 g / 100 mL. This solubility ensures that the resin can be formulated into a varnish suitable for spray application. In the synthetic example of this invention, the actual solubility of the resin can reach more than 50 g / 100 mL. Exemplarily, the mass ratio of butyl acetate, N-methylpyrrolidone, xylene, and propylene glycol methyl ether acetate in the mixed solvent is 3:1:1:1.

[0015] For example, hydroxyl-containing diamine monomers include, but are not limited to, 2,4-diaminophenol, 3,5-diaminophenol, 2,5-diaminophenol, 3,3'-dihydroxybenzidine, 3,3'-dihydroxy-4,4'-biphenyldiamine, 2,2'-dihydroxy-5,5'-diaminobiphenyl, 3,3'-diamino-4,4'-dihydroxydiphenyl sulfone, 2,6-diamino-4-methylphenol, 2,4-diamino-6-hydroxypyrimidine, 4,6- Diamino-1,3-benzenediol, methyl 2,4-diamino-5-hydroxybenzoate, 2,4-diamino-6-hydroxy-s-triazine, bis(3-amino-4-hydroxyphenyl)methane, bis(3-amino-4-hydroxyphenyl) ether, bis(3-amino-4-hydroxyphenyl) ketone, bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,2-bis(3-amino-4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane, 1,4- Diamino-2,3-dihydroxynaphthalene, 2,7-diamino-4,5-dihydroxynaphthalene, 1,5-diamino-4,8-dihydroxyanthraquinone, 9,9-bis(3-amino-4-hydroxyphenyl)fluorene, 2,7-diamino-9,9-di(hydroxyalkyl)fluorene, 3,6-diamino-9-(2-hydroxyethyl)carbazole, 2,2-bis(4-hydroxy-3-aminophenyl)propane, 1,3-bis(3-hydroxy-4-aminophenoxy)benzene, 4,4'- At least one of bis(3-amino-4-hydroxyphenoxy)biphenyl, 4,4'-bis(3-amino-4-hydroxyphenoxy)diphenyl sulfone, α,ω-bis(3-amino-4-hydroxyphenoxy)polyethylene glycol, 1,4-bis(3-amino-4-hydroxyphenoxymethyl)cyclohexane, bis(3-amino-4-hydroxyphenyl)dimethylsilane, bis(3-amino-4-hydroxyphenyl)tetramethyldisiloxane, and tris(4-amino-3-hydroxyphenyl)methane.

[0016] For example, non-hydroxydiamine monomers include, but are not limited to, 4,4'-diaminodiphenyl ether, 1,2-diaminoethane, 1,3-diaminopropane, 1,2-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, 1,8-diaminooctane, 1,10-diaminodecane, 1,12-diaminododecane, 2-methyl-1,5-diaminopentane, 1,2-diaminocyclohexane, 1,3-diaminocyclohexane, 1,4-diaminocyclohexane, 4,4'-methylenebis(cyclohexylamine), bis(4-aminocyclohexyl)methane, 3(4),8(9)-bis(aminomethyl)tricyclo[5.2.1.0², 6Decane, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, 2,2(4),4-trimethyl-1,6-diaminocyclohexane, 1,3-diaminoadamantane, 1,2-diaminobenzene, 1,3-diaminobenzene, 1,4-diaminobenzene, 2,4-diaminotoluene, 2,6-diaminotoluene, 4,4'-diaminobiphenyl, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl ketone, 4,4'-diaminoazobenzene, 1,5-diaminonaphthalene At least one of the following: 1,8-diaminonaphthalene, 2,7-diaminofluorene, 9,9-bis(4-aminophenyl)fluorene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis(3-aminophenyl)hexafluoropropane, bis[4-(4-aminophenoxy)phenyl]sulfone, 2,3,5,6-tetramethyl-1,4-diaminobenzene, and N-(4-aminophenyl)-4-aminobenzamide.

[0017] Preferably, the hydroxyl-containing diamine monomer is selected from one or more of 2,4-diaminophenol, 3,5-diaminophenol, 3,3'-dihydroxybenzidine, and 3,3'-dihydroxy-4,4'-benzidine. And / or, The non-hydroxyl diamine monomer is selected from at least one of aromatic diamines, alicyclic diamines, and fluorinated diamines. It should be noted that aromatic diamines (such as 4,4'-diaminodiphenyl ether) can provide a rigid structure, alicyclic diamines (such as 1,3-bis(aminomethyl)cyclohexane) can improve transparency and solubility, and fluorinated diamines (such as 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl) can further reduce color and improve hydrophobicity. And / or, The amino-terminated polydimethylsiloxane has a number average molecular weight of 2,500-27,000. This component is key to imparting flexibility and solubility to the resin. If its molecular weight is too low, the flexibility will be insufficient, and if it is too high, it may affect the mechanical strength of the resin. And / or, The dianhydride monomer is selected from at least one of fluorinated dianhydrides, alicyclic dianhydrides, and aromatic dianhydrides containing flexible units, large volume, non-planar and / or twisted main chain structures. It should be noted that fluorinated dianhydrides (such as 4,4'-(hexafluoroisopropene)phthalic anhydride) can improve transparency and weather resistance, alicyclic dianhydrides can impart low color and high light transmittance to the resin, while aromatic dianhydrides with non-planar structures (such as 3,3',4,4'-benzophenone tetracarboxylic dianhydride) can help disrupt the regularity of molecular chains and improve solubility.

[0018] For example, fluorinated dianhydrides include, but are not limited to, 4,4'-(hexafluoroisopropene)phthalic anhydride, 3-(4-trifluoromethylphenyl)pyromellitic anhydride, 3-[3,5-bis(trifluoromethyl)phenyl]pyromellitic anhydride, 3,6-bis(3-trifluoromethylphenyl)pyromellitic anhydride, 3,6-bis(4-trifluoromethylphenyl)pyromellitic anhydride, 3,6-bis[3',5'-bis(trifluoromethylphenyl)]pyromellitic anhydride, and 2,2'-bis(trifluoromethyl)-4,4',5 5'-Biphenyl dianhydride, 2,2'-bis(2''-trifluoromethylphenyl)-4,4',5,5'-biphenyltetracarboxylic dianhydride, 2,2'-bis(3''-trifluoromethylphenyl)-4,4',5,5'-biphenyltetracarboxylic dianhydride, 2,2'-bis(4-trifluoromethylphenyl)-4,4',5,5'-biphenyltetracarboxylic dianhydride, 2,2'-bis[3'',5”-bis(trifluoromethyl)phenyl]-4,4',5,5'-biphenyltetracarboxylic dianhydride Acetic dianhydride, 4,4'-[2,2,2-trifluoro-1-(3-trifluoromethylphenyl)ethoxy]phthalic anhydride, 4,4'-[2,2,2-trifluoro-1-(3,5-ditrifluoromethylphenyl)ethoxy]phthalic anhydride, 2,7-bis-(3,4-dicarboxyphenyl)-2,7-dimethyloctane dianhydride, 4,4'-(1,3-hexafluoropropylene)phthalic anhydride, 4,4'-(1,4-octafluorobutane)phthalic anhydride, 4,4'-( At least one of the following: 1,5-decafluoropentane)diphthalic anhydride, 4,4'-(1,6-dodecanohexane)diphthalic anhydride, 4,4'-(1,7-tetradecanofluoroheptane)diphthalic anhydride, 4,4'-(1,8-hexadecanofluorooctane)diphthalic anhydride, 4,4'-(perfluoroalkyl)diphthalic anhydride, 4,4'-(3-fluorodi(o-phenylene)diether dianhydride), 4,4'-(m-trifluorotolyl-p-phenylene)diether dianhydride, and 4,4'-[3,5-di(trifluorotolyl)p-phenylene]diether dianhydride.

[0019] For example, alicyclic dianhydrides include, but are not limited to, cyclobutanetetracarboxylic dianhydride, hydrogenated pyromellitic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, adamantane-1,3,5,7-tetracarboxylic dianhydride, norbornene-2,3,5,6-tetracarboxylic dianhydride, 3,3',4,4'-dicyclohexyltetracarboxylic dianhydride, decahydronaphthalenetetracarboxylic dianhydride (including its various stereoisomers), isomeric dicyclohexyltetracarboxylic dianhydride, and at least one tetracarboxylic dianhydride based on bridged cycloalkanes, spirocycloalkanes, norbornene skeletons, or tricyclodecane skeletons.

[0020] For example, aromatic dianhydrides comprising flexible units, large volume, non-planar and / or twisted main chain structures include, but are not limited to, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3'-biphenyl dianhydride, 4,4'-oxophthalic anhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, 4,4'-methylene phthalic anhydride, 4,4'-thiophthalic anhydride, 1,3-bis(3,4-dicarboxyphenoxy)phthalic anhydride, and 4,4'-bis(3,4-dicarboxyphenoxy)phthalic anhydride. At least one of the following: diphenyl sulfone dianhydride, 4,4'-ethylene bisphthalic anhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 3,3',4,4'-terphenyltetracarboxylic anhydride, 2,3,3',4'-biphenyltetracarboxylic anhydride, bisphenol A type diether dianhydride, spiro[fluorene-9,9'-oxanthracene]-2,3,6,7-tetracarboxylic anhydride, 3,3',5,5'-tetraphenyl-4,4'-biphenyltetracarboxylic anhydride, and 4,4'-methylenebis(3-cyclohexylphthalic anhydride).

[0021] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned hydroxyl-functionalized soluble transparent polyimide resin, comprising the following steps: (1) In a solvent, the diamine component reacts with the dianhydride monomer to generate a polyamic acid prepolymer solution; wherein the diamine component includes a hydroxyl-containing diamine monomer, a non-hydroxyl diamine monomer and an amino-terminated polydimethylsiloxane; in order to ensure the smooth progress of the polymerization reaction and the controllability of the product molecular weight, the reaction temperature should be controlled below 10°C, and the dianhydride monomer should be added in batches.

[0022] (2) Add a dehydrating agent to the reaction solution in step (1), heat and stir to react, so that polyamic acid is closed-ringed to form polyimide; this step is a chemical imidization process, which is milder than thermal imidization and can obtain polyimide products with better solubility.

[0023] (3) Precipitate the reaction solution from step (2) in a non-solvent, collect the precipitate, wash and dry it to obtain the hydroxyl-functionalized soluble transparent polyimide resin. To further improve the purity of the product, a two-stage dissolution-precipitation method can be used for purification.

[0024] Preferably, in step (1), the solvent is a strongly polar aprotic solvent selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; and / or, in step (2), the dehydrating agent is a mixture of acetic anhydride and pyridine.

[0025] It should be noted that the automotive body clear coat composition of this invention is specifically designed for original equipment manufacturer (OEM) automotive body painting lines and is not suitable for automotive repair paint. This is because the clear coat composition of this invention requires high-temperature baking at 80-150°C to achieve full cross-linking and curing (e.g., 1K system requires 150°C / 35 minutes, 2K system requires two-stage baking at 80°C / 10 minutes + 140°C / 35 minutes) to fully utilize the high hardness and weather resistance of polyimide resin. However, in automotive repair paint application scenarios, the car body is already assembled and contains multiple layers of existing paint film, plastics, rubber, electronic components, and other heat-sensitive parts, which cannot withstand prolonged baking above 60°C. Typically, only room temperature air drying or low-temperature (≤60°C) forced drying can be used. If the clear coat composition of this invention is used in repair paint processes, insufficient curing temperature will lead to incomplete cross-linking, preventing the formation of a dense network in the paint film. This results in problems such as low hardness, poor scratch resistance, and insufficient chemical resistance, failing to achieve the technical effects claimed by this invention. Therefore, the application scope of this invention is clearly limited to original equipment manufacturer (OEM) automotive body clear coats.

[0026] Based on the same inventive concept, this invention also provides the application of any of the above-described automotive body clear coat compositions or hydroxyl-functionalized soluble transparent polyimide resins in original automotive body clear coats. The curing process (baking at 80-150℃) in this application scenario is highly compatible with the resin system of this invention, ensuring sufficient cross-linking of the coating while adapting to the production of original automotive painting lines.

[0027] The hydroxyl-functionalized soluble transparent polyimide resin of this invention is synthesized using a classic two-step polycondensation process, with the chemical reaction comprising two stages: the formation of polyamic acid and imidization. Step 1: Synthesis of polyamic acid; Step 2: Chemical imidization; (1) Synthesis of polyamic acid: Under nitrogen protection and anhydrous conditions, a hydroxyl-containing diamine monomer, a non-hydroxyl-containing diamine monomer, and an amino-terminated polydimethylsiloxane were dissolved in a strongly polar aprotic solvent. The dianhydride monomer was added in batches at low temperature (below 10°C). The amino group of the diamine monomer acted as a nucleophile, attacking the carbonyl carbon of the dianhydride monomer, resulting in a nucleophilic addition-ring-opening reaction to generate a linear prepolymer containing an amic acid bond—polyamic acid. This reaction was a forward reaction with an equilibrium constant as high as 10. 5 The rapid, exothermic reaction at L / mol yields high-molecular-weight polyamic acid, and low-temperature conditions help suppress the reverse reaction. Strict control of the feeding sequence and reaction temperature effectively avoids localized overheating that could broaden the molecular weight distribution, ensuring the uniformity of the product structure. (2) Chemical imidization: A dehydrating agent is added to a polyamic acid solution, and the mixture is heated to approximately 80°C with stirring to induce intramolecular dehydration and ring-closure of the amic acid groups on the polyamic acid molecular chain, generating an imide five-membered ring structure. Compared with traditional thermal imidization (which typically requires a high temperature of 200-300°C), chemical imidization can complete the ring-closure reaction at a lower temperature, and the reaction conditions are mild and easy to control, making it particularly suitable for preparing soluble polyimides. After the reaction is complete, the reaction solution is precipitated in deionized water, the precipitate is collected and repeatedly washed and purified, and finally vacuum dried to obtain a white to light yellow hydroxyl-functionalized soluble transparent polyimide resin solid.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The aromatic heterocyclic backbone of the polyimide of the present invention gives it extremely high chain rigidity, which makes the pencil hardness of the varnish ≥ HB, and the scratch resistance (gloss retention rate) ≥ 87% in the 2K system and ≥ 89% in the 1K system, far exceeding the mainstream products on the market. At the same time, the flexible siloxane segments introduced into the rigid backbone make the material both rigid and flexible, and the stone impact resistance can reach level 2 or below, solving the problem of traditional high hardness varnishes being brittle and easy to crack; (2) The heterocyclic structure of the polyimide in this invention has extremely high UV stability, which greatly enhances the anti-yellowing and gloss retention capabilities of the varnish based on it. After 3000 hours of QUV aging, the gloss retention rate is still ≥91%, and the color difference Δb value is only ≤+2.8, which is significantly better than the traditional acrylic-polyurethane and acrylic-amino systems. The varnish can resist environmental erosion such as acid rain and ultraviolet rays for a long time, significantly extending the life of the paint surface; (3) The chemical inertness of the polyimide of the present invention enables it to effectively resist the erosion of corrosive media such as acids, alkalis, and oxidants. The acid resistance test (0.1N H2SO4, 24h) and water resistance test (360h) results were both qualified, while the traditional comparative example showed slight blistering and other defects under the same conditions; (4) Through the monomer structure design (introducing fluorine-containing or alicyclic monomers), the polyimide resin of the present invention has a light color and high light transmittance, and its clear coating has higher gloss and vividness (DOI value ≥ 92.8), which can form a smoother paint film surface and obtain better vividness and deeper visual effect. (5) The polyimide resin of the present invention can be adapted to both 1K (amino resin crosslinking) and 2K (polyisocyanate crosslinking) curing systems, exhibiting superior mechanical properties and weather resistance in both systems. Data from Examples 1-6 show that both high polyimide content (40%) and low polyimide content (15%) formulations can outperform traditional comparative formulations in their respective systems, fully demonstrating the potential of the resin of the present invention as a general high-performance platform; (6) Compared with existing technologies that rely on adding additives such as ultraviolet absorbers and light stabilizers to delay aging, but additives will be consumed and become ineffective over time. The polyimide resin of the present invention provides intrinsic weather resistance, chemical resistance and mechanical properties at the molecular skeleton level, and can provide long-lasting protection throughout the life cycle of the coating film without excessive reliance on additives, fundamentally solving the industry problem of "protection decay caused by additive consumption".

[0029] In summary, this invention designs and synthesizes a hydroxyl-functionalized colorless / light-colored transparent soluble polyimide resin and applies it to automotive body clear coats. This fundamentally solves several long-standing technical contradictions in existing clear coat systems at the molecular level, achieving a systematic breakthrough in the overall performance of automotive clear coats. It has significant technological advancements and industrial application value. Detailed Implementation

[0030] To make the present invention easier to understand, specific embodiments are described below to further illustrate the invention. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art; unless otherwise specified, the raw materials and reagents involved herein can be purchased commercially or obtained by known methods.

[0031] The performance testing methods involved in the following embodiments and comparative examples are specifically described in Table 1 below: Table 1:

[0032] Example of synthesis of hydroxyl-functionalized soluble transparent polyimide resin: Synthesis Example 1: Preparation of Hydroxyl-Functionalized Soluble Transparent Polyimide Resins (KPIs) First, add 1000 mL of dry N-methylpyrrolidone solvent to the flask. Under continuous nitrogen atmosphere protection and ice-water bath cooling, slowly add 41.086 g (0.190 mol) of 3,3'-dihydroxy-4,4'-biphenyldiamine (containing hydroxydiamine monomer), keeping the reaction system temperature below 10 °C until the solid is completely dissolved.

[0033] Next, 25.000 g (0.010 mol) of amino-terminated polydimethylsiloxane (Mn=2500) (flexible diamine monomer) was added dropwise while maintaining a low temperature, and the mixture was stirred until homogeneous to obtain a mixed diamine solution.

[0034] A pre-mixed mixture of 42.189 g (0.095 mol) of 4,4'-(hexafluoroisopropene) phthalic anhydride (a fluorinated dianhydride) and 30.611 g (0.095 mol) of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (a non-planar dianhydride containing flexible ketone groups) was slowly added to the reaction system in five batches, with the reaction temperature strictly controlled not to exceed 10°C. After each batch was added, the mixture was stirred for 10-15 minutes, and the next batch was added only after the system temperature had stabilized.

[0035] After all the dianhydrides have been added, remove the ice-water bath and allow the reaction mixture to be stirred continuously at room temperature (25°C) for 12 hours. At this time, the viscosity of the solution increases significantly, and a pale yellow transparent polyamic acid prepolymer solution is obtained.

[0036] Subsequently, a mixture of acetic anhydride and pyridine (87.3 g) was added to the viscous solution as a dehydrating agent. The reaction system was heated to 80°C and stirred at this temperature with a reflux condenser for 4 hours to promote the ring-closure of polyamic acid to form the target polyimide. During the reaction, the system gradually lightened in color, eventually yielding a slightly yellow transparent solution.

[0037] After the reaction was complete, the resulting reaction solution was slowly poured into 2 L of deionized water under vigorous stirring, immediately forming a white, filamentous precipitate. The precipitate was collected by filtration through a Buchner funnel, and the filter cake was thoroughly washed three times with fresh deionized water.

[0038] The moistened polymer product was dissolved again in an appropriate amount of N-methylpyrrolidone for secondary purification, and the same precipitation, filtration and washing operations were repeated.

[0039] Finally, the obtained pure polymer was placed in a vacuum drying oven at 80°C and dried for 24 hours to produce a solid hydroxyl-functionalized polyimide resin product, which was named KPIs.

[0040] Testing revealed that the KPIs had a hydroxyl value of 93 mg KOH / g, a number-average molecular weight (Mn) of 15600 g / mol, a molecular weight distribution index (PDI) of 2.3 (a narrow molecular weight distribution (PDI in the range of 2.0-2.5), indicating a uniform product chain structure and a stable and controllable reaction process), and a glass transition temperature (Tg) of 156℃. At 25℃, it is completely soluble in common coating solvents such as butyl acetate, N-methylpyrrolidone, and xylene, and can be stably formulated into high-solids solutions with a concentration of not less than 50 wt%, exhibiting excellent solubility and processability.

[0041] Before applying it to coating formulations, the KPIs resin is dissolved in a mixed solvent of butyl acetate / N-methylpyrrolidone / xylene / propylene glycol methyl ether acetate (mass ratio 3:1:1:1) to prepare a 50% stock solution for later use.

[0042] Synthesis Example 2: Preparation of polyimide resin KPI-70 with different hydroxyl contents The difference between this synthesis example and Synthesis Example 1 is that the molar ratio of the hydroxyl-containing diamine monomer to the non-hydroxyl-containing diamine monomer is different (specifically, the amount of 3,3'-dihydroxy-4,4'-biphenyl diamine is adjusted to 0.140 mol, and 0.050 mol of 4,4'-diaminodiphenyl ether is added as a non-hydroxyl-containing diamine monomer (rigid aromatic diamine)). The other raw materials and steps are the same as in Synthesis Example 1. The obtained product is named KPI-70, with a hydroxyl value of 71 mg KOH / g, a Mn value of 13700 g / mol, and a Tg value of 168 °C.

[0043] Synthesis Example 3: Preparation of High Transparency Polyimide Resin KPI-H The difference between this synthesis example and Synthesis Example 1 is that the dianhydride monomer was replaced entirely with 4,4'-(hexafluoroisopropene) diaphthalic anhydride (0.190 mol), excluding 3,3',4,4'-benzophenone tetracarboxylic dianhydride. The remaining raw materials and steps were the same as in Synthesis Example 1. The resulting product, named KPI-H, exhibits higher optical transparency, with a hydroxyl value of 85 mg KOH / g, a Mn value of 17800 g / mol, and a Tg of 141 °C.

[0044] Example: The soluble polyimide resin KPIs used in Examples 1-6 below are all stock solutions with a concentration of 50% prepared in Synthesis Example 1.

[0045] Example 1: 2K varnish (high polyimide content) This embodiment represents the optimal implementation of a two-component varnish.

[0046] Component A formulation (based on 100% total mass of Component A): 40% soluble polyimide resin KPIs (solids), 5% acrylic resin (solids), 5% polyester resin (solids), 0.3% leveling agent (modified polysiloxane, BYK-306), 0.05% defoamer (acrylic polymer, DISPARLON OX-60), 0.5% wetting agent (polyether-modified siloxane, Tego 270), 1.0% mixture of UV absorber (OMNISTAB Tinuvin 1130) and hindered amine light stabilizer (OMNISTAB Tinuvin 292) (mass ratio 1:1), and 1.0% composite catalyst (dibutyltin dilaurate BYK-420 and triethylenediamine Evonik). DABCO® 33-LV (mass ratio 2:1) 0.5%, mixed solvent (butyl acetate / N-methylpyrrolidone / xylene / propylene glycol methyl ether acetate (mass ratio 3:1:1:1)) is the balance (i.e., make up to 100%). Component B: HDI trimer (purchased from Covestro, brand name Desmodur N3300), the amount used is 33 parts (based on 100 parts of component A), at this time the molar ratio of NCO / OH in the system is 1.05:1.

[0047] Preparation method: Under light-protected conditions, add all raw materials of component A to a dispersion tank in sequence and stir at 600 rpm for 30 minutes at room temperature until uniform and transparent. Component B is packaged separately. Before use, mix component A and component B evenly according to the above ratio, let stand for 10 minutes to defoam, and then apply. The curing process adopts a two-stage curing procedure: pre-curing at 80℃ for 10 minutes, followed by complete curing at 140℃ for 35 minutes.

[0048] Example 2: 2K varnish (medium polyimide content) This example demonstrates a moderate alternative.

[0049] In component A, the solid content of soluble polyimide resin KPIs is adjusted to 30%, the solid content of acrylic resin is maintained at 5%, and the solid content of polyester resin is adjusted to 15%. The remaining components (leveling agent, defoamer, wetting agent, UV absorber, composite catalyst, and mixed solvent) are the same as in Example 1, and the solvent dosage is adjusted proportionally. Component B and the curing process are the same as in Example 1.

[0050] Example 3: 2K varnish (low polyimide content) This example demonstrates a low-cost alternative for the initial replacement.

[0051] In component A, the solid content of soluble polyimide resin KPIs was adjusted to 20%, the solid content of acrylic resin remained at 5%, and the solid content of polyester resin was adjusted to 25%. The remaining components were the same as in Example 1, and the solvent dosage was adjusted proportionally. Component B and the curing process were the same as in Example 1.

[0052] Example 4: 1K varnish (high polyimide content) This embodiment represents the optimal implementation for a single-component varnish.

[0053] Formulation (based on 100% of the total mass of the composition): 35% soluble polyimide resin KPIs (solids), 5% amino resin (solids), 5% acrylic resin (solids), 5% polyester resin (solids), 0.3% leveling agent (BYK-306), 0.05% defoamer (DISPARLON OX-60), 0.5% wetting agent (Tego 270), 1.0% mixture of UV absorber and hindered amine light stabilizer (same as Example 1), 0.5% composite catalyst (dibutyltin dilaurate BYK-420 and triethylenediamine Evonik DABCO® 33-LV, mass ratio 2:1), and the balance being mixed solvent (same as Example 1). The preparation method is the same as component A of Example 1. Curing process: Baking at 150°C for 35 minutes.

[0054] Example 5: 1K varnish (medium polyimide content) This example demonstrates the moderate substitution effect of the 1K system.

[0055] In the formulation, the soluble polyimide resin KPIs (solids) was adjusted to 25%, the amino resin (solids) to 10%, the acrylic resin (solids) to 5%, and the polyester resin (solids) to 10%. The remaining components were the same as in Example 4, and the solvent dosage was adjusted proportionally. The curing process was the same as in Example 4.

[0056] Example 6: 1K varnish (low polyimide content) This example demonstrates a low-cost solution for the 1K system.

[0057] In the formulation, the soluble polyimide resin KPIs (solids) was adjusted to 15%, the amino resin (solids) was adjusted to 15%, the acrylic resin (solids) remained at 5%, the polyester resin (solids) was adjusted to 15%, and the remaining components were the same as in Example 4, with the solvent dosage adjusted proportionally. The curing process was the same as in Example 4.

[0058] Comparative example: Comparative Example 1: Traditional 2K Acrylic-Polyurethane Clear Coating 2K acrylic-polyurethane varnish was prepared according to the method disclosed in patent CN107057547B. The specific operation is as follows: Under nitrogen protection, add 20 parts of high-solids thermosetting hydroxyl acrylic resin (using the high-solids thermosetting hydroxyl acrylic resin described in the December 2001 issue 4 of "Acrylic Acid Chemical Industry and Application" recommended by this patent), 5 parts of polyester resin SETAL 1606 BA-80, 10 parts of butylated amino resin SETAMINE US-146 BB-72, 15 parts of diurea compound modified polyacrylic resin SETALUX 91757 VX-60, 1 part of benzotriazole UV absorber Tinuvin 928, 0.5 parts of sterically hindered amine light stabilizer Tinuvin 292, 0.1 parts of polyester modified polydimethylsiloxane leveling agent BYK-331, 0.5 parts of acrylate leveling agent BYK-358N, and 10 parts of diluent butyl acetate to a container, and stir with a mixer for 30 minutes to obtain component A. Under nitrogen protection, 30 parts of butyl acetate diluent were added to another container, followed by 70 parts of Desmodur 3300 isocyanate resin while stirring. The mixture was stirred for another 10 minutes to obtain component B. Component A and component B were mixed at a weight ratio of 100:30 to obtain the acrylic-polyurethane type clear varnish composition. The curing process was the same as in Example 1 (80℃ / 10min + 140℃ / 35min).

[0059] Comparative Example 2: Traditional 1K Acrylic-Amino Varnish 1K acrylic-amino varnish was prepared according to the method disclosed in patent CN111019469B. The specific operation is as follows: Ethylene glycol butyl ether acetate (1 part) was added sequentially to main cylinder a, along with a dicarboxylic acid diester (DBE, 1 part), ethyl 3-ethoxypropionate (EEP, 4.5 parts), and butyl acetate (3.9 parts). The mixture was stirred at 200-400 rpm for 10 minutes. Then, at 300-500 rpm, TINUVIN 292 (0.5 parts), TINUVIN 384-2 (0.75 parts), BYK358N (0.4 parts), and BYK 310 (0.2 parts) were added sequentially to main cylinder a. After the addition was complete, stirring was continued for 30 minutes. n-Butanol (3 parts) was added to auxiliary cylinder b, followed by the addition of catalyst AAC under uniform stirring at 200-400 rpm. 2500 (1.05 parts), continue stirring for 15 minutes; then add the mixture obtained in auxiliary cylinder b to main cylinder a while stirring at 300-500 rpm, and continue stirring for 20 minutes after addition; at 300-500 rpm, add ETERAC 7393-S-80 acrylic resin (26 parts) to main cylinder a, and continue stirring for 20 minutes after addition; at 300-500 rpm, add SETALUX91796SS-69 (20 parts) and SETALUX 91772SS-60 (4 parts) to main cylinder a in sequence, and continue stirring for 30 minutes after addition; at 300-500 rpm, add SETAMINE US-138BB-70 (27.24 parts) and CYMEL F in sequence to main cylinder a. Add 2000A (6 parts), and continue stirring for 30 minutes after completion; add DURANOLTMT5650E (6 parts) to main cylinder a at a speed of 300-500 rpm, and continue stirring for 30 minutes after completion; use an appropriate amount of n-butanol (1.4 parts) and DBE (2.7 parts) to dilute the varnish in main cylinder a to a suitable viscosity to obtain the 1K varnish. The curing process is the same as in Example 4 (150℃ / 35min).

[0060] Performance testing and results analysis: The clear coat compositions prepared in Examples 1-6 and Comparative Examples 1-2 were applied to galvanized steel sheets (150mm × 70mm × 0.8mm) of automobile bodies after intermediate coating treatment using a standard spraying process, with the dry film thickness controlled at 40-50μm. After baking and curing according to the curing processes corresponding to each example and comparative example, the samples were placed in a constant temperature and humidity chamber (25±2℃, relative humidity 50±5%) for 24 hours, and the performance tests shown in Table 1 were performed. The results are shown in Table 2 below.

[0061] Table 2:

[0062] As shown in Examples 1-3 of Table 2, as the polyimide resin content decreased from 40% (Example 1) to 30% (Example 2) and then to 20% (Example 3), the scratch resistance (gloss retention rate) decreased from 93% to 91% and then to 87%, the stone chip resistance changed from grade 0.5 to grade 0.5 and then to grade 1, and the color difference Δb value increased from +0.8 to +1.5 and then to +2.5. This indicates that in the 2K varnish system of the present invention, the content of polyimide resin is positively correlated with the scratch resistance, stone chip resistance and yellowing resistance of the paint film. That is, within a certain range, the higher the polyimide resin content, the better the mechanical properties and weather resistance of the paint film.

[0063] As can be seen from Examples 4-6 in Table 2, as the polyimide resin content decreased from 35% (Example 4) to 25% (Example 5) and then to 15% (Example 6), the scratch resistance (gloss retention rate) decreased from 95% to 92% and then to 89%, the pencil hardness decreased from 2H to H and then to H, and the color difference Δb value increased from +1.0 to +1.8 and then to +2.8. This indicates that in the 1K clear varnish system of the present invention, the polyimide resin also plays a core reinforcing role in the hardness, scratch resistance and anti-yellowing properties of the varnish film, and also shows a content-dependent trend.

[0064] As shown in Table 2, comparing Example 1 and Comparative Example 1, Example 1 (containing 40% polyimide resin) exhibits a scratch resistance of 93%, higher than the 86% of Comparative Example 1 (traditional 2K acrylic-polyurethane clear varnish); its gloss retention rate after 3000 hours of QUV aging is 95%, higher than the 91% of Comparative Example 1; its stone chip resistance is grade 0.5, better than the grade 1 of Comparative Example 1; and its acid resistance is satisfactory, while Comparative Example 1 shows slight bubbling. This demonstrates that the 2K clear varnish system constructed by replacing traditional acrylic resin with hydroxyl-functionalized soluble transparent polyimide resin in this invention significantly outperforms traditional 2K acrylic-polyurethane clear varnish in terms of scratch resistance, weather resistance, stone chip resistance, and acid resistance.

[0065] As shown in Table 2, comparing Example 4 with Comparative Example 2, Example 4 (containing 35% polyimide resin) exhibits a scratch resistance of 95%, higher than the 89% of Comparative Example 2 (traditional 1K acrylic-amino varnish); its gloss retention rate after 3000 hours of QUV aging is 93%, higher than the 88% of Comparative Example 2; its stone chip resistance is Grade 1, superior to Grade 2 of Comparative Example 2; and both water resistance and acid resistance of Example 4 are satisfactory, while Comparative Example 2 shows slight bubbling. This demonstrates that the 1K varnish system constructed by replacing traditional acrylic resin with hydroxyl-functionalized soluble transparent polyimide resin in this invention significantly outperforms traditional 1K acrylic-amino varnish in terms of scratch resistance, weather resistance, stone chip resistance, and water and acid resistance.

[0066] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A clear coat composition for automotive bodies, characterized in that, The composition comprises a hydroxyl-functionalized soluble transparent polyimide resin; The hydroxyl-functionalized soluble transparent polyimide resin is polymerized from the following monomer components: hydroxyl-containing diamine monomer, non-hydroxyl diamine monomer, amino-terminated polydimethylsiloxane, and dianhydride monomer; The hydroxyl-functionalized soluble transparent polyimide resin has a hydroxyl value of 50-150 mg KOH / g.

2. The automotive body clear coat composition according to claim 1, characterized in that, The hydroxyl-functionalized soluble transparent polyimide resin has a number average molecular weight of 5000-50000 g / mol and a glass transition temperature of 120-200℃.

3. The automotive body clear coat composition according to claim 1, characterized in that, Based on the total mass of the composition (100%), the composition comprises: The solids content of the hydroxyl-functionalized soluble transparent polyimide resin is 10%~50%; The solids content of acrylic resin is 5% to 30%; The solids content of polyester resin is 5%~30%; The solid content of amino resins is 0%~30%; Additives 0.5%~4%; Polyisocyanate curing agent 0%~30%; Solvent, wherein the solvent is in the balance; Wherein, when the content of the polyisocyanate curing agent is 0%, the content of the amino resin is greater than 0%.

4. The automotive body clear coat composition according to claim 3, characterized in that, The composition is a two-component varnish composition, comprising component A and component B; Component A comprises the hydroxyl-functionalized soluble transparent polyimide resin, the acrylic resin, the polyester resin, the additives, and the solvent; Component B contains the polyisocyanate curing agent; The molar ratio of isocyanate groups to hydroxyl groups in the composition is 0.9:1 to 1.2:

1.

5. The automotive body clear coat composition according to claim 3, characterized in that, The composition is a one-component varnish composition comprising the hydroxyl-functionalized soluble transparent polyimide resin, the acrylic resin, the polyester resin, the amino resin, the additives, and the solvent, but does not contain the polyisocyanate curing agent.

6. A hydroxyl-functionalized soluble transparent polyimide resin for use in the automotive body clear coat composition according to any one of claims 1-5, characterized in that, The resin is polymerized from a component comprising the following monomers: Diamine monomers containing hydroxyl groups; Non-hydroxydiamine monomers; Amino-terminated polydimethylsiloxane; Dihydride monomer; The resin has a hydroxyl value of 50-150 mg KOH / g and a number-average molecular weight of 5000-50000 g / mol; a glass transition temperature of 120-200℃; and a solubility of not less than 10 g / 100 mL in a mixed solvent of butyl acetate, N-methylpyrrolidone, xylene, and propylene glycol methyl ether acetate at 25℃.

7. The hydroxyl-functionalized soluble transparent polyimide resin according to claim 6, characterized in that, The hydroxyl-containing diamine monomer is selected from one or more of 2,4-diaminophenol, 3,5-diaminophenol, 3,3'-dihydroxybenzidine, and 3,3'-dihydroxy-4,4'-benzidine. And / or, The non-hydroxy diamine monomer is selected from at least one of aromatic diamines, alicyclic diamines, and fluorinated diamines; And / or, The number-average molecular weight of the amino-terminated polydimethylsiloxane is 2500-27000. And / or, The dianhydride monomer is selected from at least one of fluorinated dianhydrides, alicyclic dianhydrides, and aromatic dianhydrides containing flexible units, large volume, non-planar and / or twisted main chain structures.

8. A method for preparing a hydroxyl-functionalized soluble transparent polyimide resin according to claim 6 or 7, characterized in that, Includes the following steps: (1) In a solvent, the diamine component is reacted with the dianhydride monomer to generate a polyamic acid prepolymer solution; wherein the diamine component comprises a hydroxyl-containing diamine monomer, a non-hydroxyl-containing diamine monomer and an amino-terminated polydimethylsiloxane; (2) Add a dehydrating agent to the reaction solution of step (1), heat and stir to carry out the reaction, so that the polyamic acid ring is closed to form polyimide; (3) The reaction solution from step (2) is precipitated in a non-solvent, the precipitate is collected, washed and dried to obtain the hydroxyl-functionalized soluble transparent polyimide resin.

9. The preparation method according to claim 8, characterized in that: In step (1), the solvent is a strongly polar aprotic solvent selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; and / or, in step (2), the dehydrating agent is a mixture of acetic anhydride and pyridine.

10. The use of the automotive body clear coat composition according to any one of claims 1-5 or the hydroxyl-functionalized soluble transparent polyimide resin according to claim 6 or 7 in original automotive body clear coats.

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