Weather-resistant polyester resin for low-temperature curing and preparation method thereof

Abstract

The invention belongs to the field of polyester resins, and provides a weather-resistant polyester resin for low-temperature curing and a preparation method thereof. A three-step progressive design that a polyester prepolymer is terminated by silane and then is subjected to hydrolytic polycondensation with tetraethyl orthosilicate to construct an organic-inorganic covalent hybrid network is adopted; through cooperation of the condensation catalyst, the hindered amine light stabilizer and the ultraviolet absorbent, the mechanical properties that the pencil hardness reaches the HB level or above and the cross-cut test level is not higher than the first level under the low-temperature curing condition of not higher than 100 DEG C are achieved, and the long-term weather resistance that the color difference is not larger than 2.0 and the gloss retention rate is not lower than 80% after 500-hour artificial accelerated aging is achieved. The technical problems of poor storage stability and difficulty in consideration of high solid content and low viscosity caused by insufficient low-temperature curing capacity and non-uniform organic-inorganic hybridization of the traditional polyester resin are solved, and the polyester resin has wide application value in the fields of building coatings, automobile refinishing paints, wood coatings and the like with strict requirements on low-temperature curing and weather resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polyester resin materials, in particular to a weather-resistant polyester resin for low-temperature curing and a preparation method thereof. BACKGROUND

[0002] As the core film-forming material in the coating industry, polyester resin plays a key role in endowing products with excellent decorative and protective properties in the fields of architectural exterior wall coatings, automotive refinish coatings, high-end wood coatings, and industrial protective coatings. In practical application scenarios, the performance of polyester resin in these fields is required to meet multiple dimensions of stringent requirements: on the one hand, the construction environment is limited by season, region, and energy consumption, and it is urgent to complete the curing and crosslinking of polyester resin at a low temperature of not higher than 100℃ or even room temperature to 80℃ to form a dense coating film with sufficient hardness, adhesion, and chemical resistance, so as to meet the industrial demand for rapid construction cycle and energy saving; on the other hand, the end product is exposed to complex service environments such as ultraviolet radiation, temperature and humidity cycles, and acid rain erosion for a long time, and the coating film is required to maintain color stability, long-lasting gloss, and no obvious weathering performance failure such as chalking or cracking within a time span of several years or even tens of years. The synergistic realization of the improvement of low-temperature curing ability and the guarantee of long-term weather resistance has important technical and economic significance for widening the applicable temperature window of polyester resin, prolonging the service life of the coating, and reducing the maintenance cost in the whole life cycle, and is one of the core driving forces for promoting the development of the coating industry towards green and high performance.

[0003] Regarding the current state of development of polyester resins in terms of low-temperature curing and weather resistance, existing technologies generally suffer from the following shortcomings and their underlying causes. For example, Chinese patent CN110408013A discloses a polyester resin for powder coatings and its preparation method. While the use of bisphenol monomers in this method can increase the glass transition temperature of the polyester resin for powder coatings, improve its storage performance, and enhance its weather resistance and impact resistance, it requires relatively high temperatures for full crosslinking, resulting in insufficient low-temperature curing capability and failing to meet the requirements of energy-saving construction. Similarly, Chinese patent CN119931009A discloses a low-temperature curing polyester resin and powder coating. However, the isocyanate groups are highly sensitive to moisture, and pre-reactions can easily occur during storage, leading to increased viscosity or even gelation. Furthermore, the free isocyanates released during the curing process pose a potential health hazard to construction workers, highlighting issues of construction safety and storage stability. Furthermore, existing methods for improving weather resistance by physically modifying polyester resin with nano-inorganic particles often suffer from insufficient interfacial bonding between the inorganic and organic phases. This leads to phenomena such as inorganic particle agglomeration, phase separation, and sedimentation, resulting in poor coating uniformity, decreased mechanical properties, and accelerated weather resistance degradation due to interfacial peeling after long-term aging. Meanwhile, to achieve high solids content and low viscosity to improve workability, conventional practices involve reducing the resin molecular weight or increasing the solvent dosage. However, the former sacrifices coating crosslinking density and mechanical strength, while the latter increases VOC emissions, violating environmental regulations. Neither approach effectively balances workability and final performance. Summary of the Invention

[0004] The purpose of this invention is to provide a weather-resistant polyester resin for low-temperature curing and its preparation method, thereby solving the technical pain points of current weather-resistant polyester resins in three aspects: the coupling contradiction between low-temperature curing ability and long-term weather resistance, the balance between the uniformity of organic-inorganic covalent hybridization and processing and storage stability, and the balance between high solids content, low viscosity, workability and high crosslinking density mechanical weather resistance.

[0005] This invention addresses the aforementioned technical challenges through a three-step progressive synergistic design approach: precise acid value control of the polyester prepolymer combined with silane end-capping followed by in-situ organic-inorganic covalent hybridization. A polyester prepolymer with suitable molecular weight and end-carboxyl group density is constructed through melt polycondensation of neopentyl glycol, 1,6-hexanediol, and trimethylolpropane polyols with isophthalic acid and adipic acid. Hydrolyzable siloxane groups are introduced by chemically end-carboxylating the end-carboxyl groups with 3-glycidoxypropyltrimethoxysilane. Then, controlled hydrolysis and condensation with tetraethyl orthosilicate under mild pH and temperature conditions results in the in-situ growth of a nanoscale inorganic silicon-oxygen network at the polyester molecular chain end, forming Si-O-Si covalent bridges. This achieves molecular-level uniform composite and synergistic effect between the flexible organic polyester segments and the rigid inorganic silicon-oxygen domains. Combined with a multi-protective system of condensation catalysts, hindered amine light stabilizers, and UV absorbers, the crosslinking efficiency of the resin system under low-temperature curing conditions and the long-term weather resistance of the cured coating are significantly improved.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A weather-resistant polyester resin for low-temperature curing, comprising, by weight, the following components: (1) 50.00 to 99.00 parts by weight of organic-inorganic covalently hybrid polyester intermediate C; (2) 0.01 to 1.00 parts by weight of a condensation catalyst, wherein the condensation catalyst is selected from one or two of dibutyltin dilaurate and zinc 2-ethylhexanoate; (3) 0.05 to 0.80 parts by weight of hindered amine light stabilizer, wherein the hindered amine light stabilizer is selected from one or two of light stabilizer LS-770 and hindered amine light stabilizer HS-944; (4) 0.05 to 0.80 parts by weight of ultraviolet absorber, wherein the ultraviolet absorber is selected from one or two of ultraviolet absorber UV-328 and ultraviolet absorber UV-531; (5) 0 to 40.00 parts by weight of an organic solvent, wherein the organic solvent is selected from one or more of ethyl acetate, butyl acetate, and propylene glycol monomethyl ether acetate; The organic-inorganic covalently hybridized polyester intermediate C is obtained by silane-termined polyester intermediate B from polyester prepolymer A, and then organic-inorganic covalently hybridizing the silane-termined polyester intermediate B.

[0007] Furthermore, polyester prepolymer A is prepared through the following steps: A1. Raw material preparation: Neopentyl glycol 100-220 parts by weight, 1,6-hexanediol 10-120 parts by weight, trimethylolpropane 2-40 parts by weight, isophthalic acid 60-220 parts by weight, and adipic acid 20-180 parts by weight. A2. Melt esterification polycondensation: The reaction is carried out at 200-240℃ for 4-10 hours under an inert nitrogen atmosphere (nitrogen flow rate of 0.1-1.0 L / min), while continuously removing the reaction water; A3. Dehydration under reduced pressure and molecular weight control: The reaction was continued for 0.5 to 3.0 h under a vacuum of 0.2–2.0 kPa and a temperature of 200–240 °C; A4. Termination of reaction: Stop the reaction when the acid value drops to 15-40 mgKOH / g to obtain polyester prepolymer A; A5. Quality control: The hydroxyl value of the polyester prepolymer A is 20-80 mg KOH / g.

[0008] Furthermore, silane-terminated polyester intermediate B is prepared via the following steps: B1. Raw materials: The polyester prepolymer A is added to a reaction vessel, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane to the molar number of carboxyl groups calculated according to the acid value of the polyester prepolymer A is 0.60 to 1.20. Triethylamine is added, and the amount of triethylamine is 0.05 to 0.50% by mass of the polyester prepolymer A. B2. End-sealing reaction: react at 80–120℃ for 1.0–4.0 h; B3. Endpoint criterion: Stop the reaction when the acid value drops to no higher than 5.0 mg KOH / g; B4. Post-treatment: Remove volatiles at 50–90℃ and a vacuum of 0.5–5.0 kPa for 0.5–2.0 h to obtain silane-terminated polyester intermediate B.

[0009] Furthermore, the organic-inorganic covalent hybrid polyester intermediate C is prepared via the following steps: C1. Raw materials: The silane-terminated polyester intermediate B is heated to 25-60°C and stirred mechanically, and tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added is 0.50-8.00% by mass of the silane-terminated polyester intermediate B. C2. Hydrolysis and condensation: Add deionized water to make the molar ratio of deionized water to tetraethyl orthosilicate 0.50-2.00, and add glacial acetic acid to adjust the pH of the aqueous phase formed by the deionized water to 4.0-6.0. React at 25-60℃ for 0.5-3.0h. C3. Removal of free small molecules: Remove free small molecules at 40–80℃ and a vacuum of 0.5–5.0 kPa for 0.5–3.0 h; C4. Endpoint criterion: The water content of the organic-inorganic covalent hybrid polyester intermediate C after the reaction is not higher than 0.20% by mass; C5. Organic-inorganic covalent hybrid polyester intermediate C is obtained.

[0010] 5. The weather-resistant polyester resin for low-temperature curing according to claim 1, characterized in that the acid value of the organic-inorganic covalent hybrid polyester intermediate C is 0.5 to 8.0 mg KOH / g.

[0011] Furthermore, the low-temperature curing refers to: after the weather-resistant polyester resin is prepared into a coating film, it is cured at a temperature not exceeding 100°C for 2 to 24 hours, and the cured coating film has a pencil hardness of HB or above as measured by GB / T6739-2022, and a cross-cut test grade not exceeding level 1 as measured by GB / T9286-2021.

[0012] As a concept of this invention, the design of constructing an organic-inorganic covalent hybrid network by end-capping polyester prepolymer A with 3-glycidoxypropyltrimethoxysilane followed by in-situ hydrolysis and condensation with tetraethyl orthosilicate is mainly used to enhance the synergistic improvement of low-temperature curing ability and long-term weather resistance. Polyester prepolymer A, through the steric hindrance of the neopentyl side groups introduced by neopentyl glycol and the synergistic effect of the flexible chain segments provided by 1,6-hexanediol, maintains the flexibility of the main chain, lowers the glass transition temperature to facilitate low-temperature curing, and increases the density of terminal hydroxyl groups and the number of crosslinking sites through the branched structure constructed by trimethylolpropane. Precise control of the acid value within the range of 15–40 mg KOH / g through melt condensation ensures the stoichiometric accuracy of the subsequent silane end-capping reaction. The chemical capping reaction of the prepolymer carboxyl group by 3-glycidoxypropyltrimethoxysilane forms a stable ester bond through the ring-opening addition of the epoxy group to the carboxyl group, while introducing a hydrolyzable trimethoxysilane group at the end of the molecular chain. This design not only eliminates the interference of the terminal carboxyl group on the subsequent hydrolysis reaction, but also provides a covalent bonding site for organic-inorganic hybridization. Controlled hydrolysis and condensation of tetraethyl orthosilicate under weakly acidic conditions (pH 4.0–6.0) and mild temperatures (25–60°C) leads to the co-condensation reaction of the generated silanol groups with the trimethoxysilyl groups at the ends of silane-terminated polyester intermediate B. This results in the in-situ growth of a nanoscale inorganic silicon-oxygen network at the ends of the polyester molecular chains, which is then covalently bridged by Si-O-Si to form an organic-inorganic covalent hybrid structure. This molecular-level homogeneous composite avoids the phase separation and aggregation problems caused by physical blending, significantly improving the uniformity and storage stability of the hybrid system. At the same time, the introduction of the inorganic silicon-oxygen network can serve as additional crosslinking sites during low-temperature curing, accelerating the curing reaction and increasing the crosslinking density. Furthermore, the shielding effect of its rigid structural domains on ultraviolet light, combined with the synergistic protection of hindered amine light stabilizers and ultraviolet absorbers, significantly enhances the long-term weather resistance of the coating film.

[0013] This invention also discloses a method for preparing a weather-resistant polyester resin for low-temperature curing as described above, comprising the following steps: S1. Neopentyl glycol, 1,6-hexanediol, trimethylolpropane, isophthalic acid and adipic acid are subjected to melt polycondensation under an inert atmosphere to obtain polyester prepolymer A with an acid value of 15-40 mgKOH / g; S2. The polyester prepolymer A is subjected to an end-capping reaction with 3-glycidoxypropyltrimethoxysilane to obtain silane-capped polyester intermediate B with an acid value not higher than 5.0 mgKOH / g; S3. Add tetraethyl orthosilicate and deionized water to the silane-terminated polyester intermediate B, and perform hydrolysis, condensation and removal of free small molecules under the conditions of pH 4.0-6.0 and temperature 25-60℃ to obtain organic-inorganic covalent hybrid polyester intermediate C. S4. The organic-inorganic covalent hybrid polyester intermediate C is mixed with a condensation catalyst, a hindered amine light stabilizer, an ultraviolet absorber and an organic solvent to obtain a weather-resistant polyester resin for low-temperature curing.

[0014] Furthermore, step S1 includes the following sub-steps: S1-1. Raw material preparation: Neopentyl glycol 100-220 parts by weight, 1,6-hexanediol 10-120 parts by weight, trimethylolpropane 2-40 parts by weight, isophthalic acid 60-220 parts by weight, and adipic acid 20-180 parts by weight. S1-2. Melt esterification polycondensation: The reaction is carried out at 200-240℃ for 4-10 hours under an inert nitrogen atmosphere (nitrogen flow rate of 0.1-1.0 L / min), while continuously removing the reaction water; S1-3. Dehydration under reduced pressure and molecular weight control: Continue the reaction for 0.5-3.0 h under vacuum of 0.2-2.0 kPa and temperature of 200-240 °C; S1-4. Termination of reaction: Stop the reaction when the acid value drops to 15-40 mgKOH / g to obtain polyester prepolymer A; S1-5. Quality control: The hydroxyl value of the polyester prepolymer A is 20-80 mg KOH / g.

[0015] Furthermore, step S2 includes the following sub-steps: S2-1. Raw materials: Add the polyester prepolymer A to the reactor, add 3-glycidoxypropyltrimethoxysilane, and make the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane to the molar number of carboxyl groups calculated according to the acid value of the polyester prepolymer A 0.60 to 1.20, and add triethylamine, the amount of which is 0.05 to 0.50% by mass of the polyester prepolymer A; S2-2. End-sealing reaction: react at 80–120℃ for 1.0–4.0 h; S2-3. Endpoint criterion: Stop the reaction when the acid value drops to no higher than 5.0 mg KOH / g; S2-4. Post-treatment: Remove volatiles at 50-90℃ and vacuum of 0.5-5.0 kPa for 0.5-2.0 h to obtain silane-terminated polyester intermediate B.

[0016] Furthermore, step S3 includes the following sub-steps: S3-1. Raw materials: The silane-terminated polyester intermediate B is heated to 25-60°C and stirred mechanically, and tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added is 0.50-8.00% by mass of the silane-terminated polyester intermediate B. S3-2. Hydrolysis and condensation: Add deionized water to make the molar ratio of deionized water to tetraethyl orthosilicate 0.50 to 2.00, and add glacial acetic acid to adjust the pH of the aqueous phase formed by the deionized water to 4.0 to 6.0. React at 25 to 60°C for 0.5 to 3.0 h. S3-3. Removal of free small molecules: Remove free small molecules at 40–80℃ and a vacuum of 0.5–5.0 kPa for 0.5–3.0 h; S3-4. Endpoint criterion: The water content of the organic-inorganic covalent hybrid polyester intermediate C after the reaction is not higher than 0.20% by mass; S3-5. Organic-inorganic covalent hybrid polyester intermediate C is obtained.

[0017] Furthermore, the low-temperature curing is preferably carried out at a temperature of 60–80°C, and the curing time is 4–12 hours.

[0018] Furthermore, the low-temperature cured coating film has a pencil hardness of HB or higher as measured by GB / T6739-2022, and a cross-cut test grade of no higher than 1 as measured by GB / T9286-2021.

[0019] Furthermore, in the weather-resistant polyester resin by weight, the weight of each component is based on component (1) organic-inorganic covalent hybrid polyester intermediate C, and the other components are the amount added relative to component (1).

[0020] Furthermore, the amount of glacial acetic acid added is 0.10 to 1.50% by mass of the silane-terminated polyester intermediate B.

[0021] Furthermore, the glacial acetic acid is added dropwise and monitored in real time with a pH meter until the pH of the aqueous phase formed by the deionized water reaches 4.0 to 6.0.

[0022] Furthermore, the mechanical stirring speed is 100-500 rpm.

[0023] Furthermore, the inert atmosphere is a nitrogen atmosphere, and the nitrogen flow rate is 0.1 to 1.0 L / min.

[0024] Furthermore, the continuous removal of reaction water is carried out using a water separator.

[0025] Furthermore, the acid value was determined in accordance with GB / T6743-2008.

[0026] Furthermore, the determination of the hydroxyl value was performed in accordance with GB / T31412-2015.

[0027] Furthermore, the water content was determined using the Karl Fischer method, in accordance with GB / T6283-2008.

[0028] Furthermore, the weather resistance was tested according to GB / T1865-2009. After 500 hours of artificial accelerated aging, the appearance of the coating film showed no significant change, the color difference ΔE was not greater than 2.0, and the gloss retention rate was not less than 80%.

[0029] As another aspect of this invention, the three-step progressive preparation process is designed primarily to enhance the uniformity of the organic-inorganic hybridization, process controllability, and product quality stability. Step S1 involves continuous removal of reaction water via melt polycondensation under a high-temperature, inert atmosphere to drive the esterification reaction. The staged heating and dehydration strategies precisely control the molecular weight distribution and end-group structure of the polyester prepolymer A. Maintaining the acid value within a narrow range of 15–40 mg KOH / g ensures the stoichiometric accuracy of the subsequent silane end-capping reaction, preventing incomplete end-capping due to low acid values ​​or increased side reactions due to high acid values. Step S2 involves an epoxy ring-opening addition reaction of 3-glycidoxypropyltrimethoxysilane with a terminal carboxyl group under triethylamine catalysis. By controlling the molar ratio of the silanizing agent to the carboxyl group within the range of 0.60–1.20, efficient chemical end-capping of the terminal group can be achieved. Triethylamine, as a tertiary amine catalyst, promotes the activation of the epoxy group while avoiding the interference of residual metal catalyst on the subsequent hydrolysis and polycondensation reaction. The combination of reaction temperature of 80–120℃ and reaction time of 1.0–4.0 h ensures the completeness of the reaction while preventing the degradation of the polyester backbone. The final acid value is controlled to be no higher than 5.0 mg KOH / g to ensure the integrity of the silane end-capping. Step S3 involves the controlled hydrolysis and condensation of tetraethyl orthosilicate under weakly acidic pH and mild temperature conditions. The degree of hydrolysis is controlled by the precise molar ratio of deionized water to tetraethyl orthosilicate. Adjusting the pH to 4.0–6.0 with glacial acetic acid ensures that the hydrolysis rate of tetraethyl orthosilicate matches the polycondensation rate of the silane-terminated siloxane groups in the polyester, avoiding the independent growth and phase separation of the inorganic silicon-oxygen network caused by excessively rapid hydrolysis. Mechanical stirring combined with mild temperature promotes uniform mixing of organic and inorganic components at the molecular scale. The subsequent vacuum removal of free small molecules effectively removes unreacted volatiles such as ethanol and water, keeping the water content below 0.20% by mass to ensure the product's storage stability and subsequent performance.

[0030] The synergistic mechanism of the organic-inorganic covalent hybrid polyester intermediate C with the condensation catalyst, hindered amine light stabilizer, and UV absorber in this invention is manifested in the following aspects. The organic-inorganic covalent hybrid polyester intermediate C provides additional crosslinking sites during the curing process through the silicon-oxygen network structure at the end of its molecular chain. The condensation catalyst (dibutyltin dilaurate or zinc 2-ethylhexanoate), as a Lewis acid, can significantly reduce the activation energy of silicon-oxygen bond formation and polyester hydroxyl polycondensation, accelerating the curing reaction process and increasing the crosslinking density under low temperature conditions. The synergistic effect of the two achieves a significant improvement in the hardness and adhesion of the coating film under low temperature curing conditions not exceeding 100°C. In terms of long-term weather resistance, the inorganic silicon-oxygen network in the organic-inorganic covalent hybrid structure absorbs and scatters ultraviolet light, reducing the ultraviolet light flux reaching the organic polyester segments. Hindered amine light stabilizers inhibit photodegradation chain reactions by capturing free radicals generated by polyester photooxidation, while ultraviolet absorbers prevent photochemical reactions by dissipating the absorbed ultraviolet light energy as heat. These three components form a multi-layered synergistic protection system at the levels of ultraviolet light shielding, free radical scavenging, and light energy dissipation. Furthermore, the rigid structural domains of the inorganic silicon-oxygen network restrict the thermal motion of polyester molecular segments, improving the glass transition temperature and dimensional stability of the coating film, while the flexible polyester segments impart good flexibility and crack resistance to the coating film. The covalent bonding of organic and inorganic components at the molecular scale avoids phase interface defects caused by physical blending, significantly improving the durability of the coating film under temperature and humidity cycling and mechanical stress.

[0031] Beneficial technical effects 1. Significantly improved low-temperature curing capability and mechanical properties of cured coating: By reducing the glass transition temperature through the flexible segment design of polyester prepolymer A, combined with the additional crosslinking sites provided by the hydrolyzable siloxane groups introduced by silane end-capping and the inorganic silicon-oxygen network grown in situ from tetraethyl orthosilicate, and the low-temperature activation effect of the condensation catalyst, full curing and crosslinking of the coating is achieved at a low temperature of no more than 100℃ (preferably 60-80℃). After curing, the coating achieves a pencil hardness of HB or higher and a cross-cut test grade of no more than 1. Compared with the traditional polyester resin technology route that requires high-temperature curing above 120℃, the curing temperature is reduced by more than 20℃, energy consumption is significantly reduced, and construction adaptability is significantly improved.

[0032] 2. Achieving multiple synergistic protections and long-term stability for long-term weather resistance: The inorganic silicon-oxygen network in the organic-inorganic covalent hybrid structure forms a triple synergistic protection system, which shields against ultraviolet light, efficiently captures photo-oxidative free radicals by hindered amine light stabilizers, and dissipates ultraviolet light energy through the heat dissipation mechanism of ultraviolet absorbers. This ensures that after 500 hours of artificial accelerated aging, the color difference ΔE of the cured coating is no greater than 2.0, the gloss retention rate is no less than 80%, and there is no obvious chalking or cracking. The long-term weather resistance is significantly better than the control system without silane end-capping and organic-inorganic covalent hybrid treatment, significantly extending the service life of the coating.

[0033] 3. Effectively solves the problem of balancing the uniformity of organic-inorganic hybridization and storage stability: Hydrolyzable siloxane groups are introduced by chemically sealing the end carboxyl groups of the polyester prepolymer with 3-glycidoxypropyltrimethoxysilane. Then, controlled hydrolysis and polycondensation of tetraethyl orthosilicate under mild conditions of pH 4.0–6.0 (weakly acidic) and 25–60℃ allows the inorganic siloxane network to grow in situ at the ends of the polyester molecular chains and bond with organic segments via Si-O-Si covalent bonds. This achieves uniform composite at the molecular scale, avoiding the problems of inorganic particle agglomeration, phase separation, and sedimentation caused by physical blending. The product's water content is controlled below 0.20% by mass, exhibiting excellent storage stability. Under 90 days of sealed storage at room temperature, the viscosity increase rate can be controlled within the range of approximately 6%–12%.

[0034] 4. Balancing high solids content, low viscosity, and workability with high crosslinking density and mechanical and weather resistance: The molecular structure design of the organic-inorganic covalent hybrid polyester intermediate C balances the flexibility of the polyester backbone with the crosslinking functionality of the silica network. While maintaining a suitable molecular weight, the crosslinking density is increased by providing additional crosslinking sites through the inorganic silica network. This allows the resin to maintain a suitable workability viscosity even under high solids content conditions. For example, at 70% solids content and 25°C, the viscosity can be controlled at approximately 3200–4200 mPa·s (see Table 1 for example data). It can meet the requirements of various construction processes such as spraying and brushing without the need for excessive addition of organic solvents, reducing VOC emissions and meeting environmental protection requirements. At the same time, the cured coating film has high crosslinking density and excellent mechanical and weather resistance properties.

[0035] 5. Simple and controllable process route, high product quality stability: The three-step progressive preparation process (melt polycondensation → silane end-capping → organic-inorganic hybridization) has mild reaction conditions in each step and clear endpoint criteria (acid value, water content, etc.), which is easy to scale up for industrial production. The quality of key intermediates is controllable, and the product has excellent batch-to-batch performance stability. It is suitable for large-scale application in fields with strict requirements for low-temperature curing and weather resistance, such as architectural coatings, automotive refinish paints, wood coatings, and industrial protective coatings. Attached Figure Description

[0036] Figure 1The Fourier transform infrared transmittance-wavenumber superposition spectra of Example 1 and Comparative Example 7 are shown.

[0037] Figure 2 The graph shows the temperature-storage modulus E′ and loss factor tanδ superimposed curves of the dynamic mechanical analysis of Example 1 and Comparative Example 4.

[0038] Figure 3 The image shows the superimposed ultraviolet-visible wavelength-absorbance spectra of Example 1, Comparative Example 2, and Comparative Example 3. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0040] Example 1 I. Preparation of Polyester Prepolymer A Raw material preparation: Take 160 parts by weight of neopentyl glycol, 65 parts by weight of 1,6-hexanediol, 21 parts by weight of trimethylolpropane, 140 parts by weight of isophthalic acid, and 100 parts by weight of adipic acid, and add them to a reaction vessel equipped with a stirring device, a temperature control system, a water separator, and a nitrogen protection system.

[0041] Melt esterification polycondensation: Under nitrogen atmosphere protection (nitrogen flow rate 0.55 L / min), start stirring, heat to 220℃, react for 7 hours, and continuously remove reaction water through a water separator.

[0042] Dehydration under reduced pressure and molecular weight control: The reaction was continued for 1.8 hours under a vacuum of 1.1 kPa and a temperature of 220 °C to further remove the reaction water.

[0043] Termination of reaction: The reaction was stopped when the acid value, as determined by GB / T6743-2008, dropped to 28 mg KOH / g, yielding polyester prepolymer A.

[0044] Quality control: According to GB / T31412-2015, the hydroxyl value of polyester prepolymer A in this example is 50 mg KOH / g.

[0045] II. Preparation of Silane-Terminated Polyester Intermediate B Raw materials: The above-mentioned polyester prepolymer A is added to the reactor, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane in this embodiment to the molar number of carboxyl groups calculated according to the acid value of polyester prepolymer A in this embodiment is 0.90. Triethylamine is added, and the amount of triethylamine used in this embodiment is 0.28% by mass of polyester prepolymer A in this embodiment.

[0046] End-sealing reaction: react at 100°C for 2.5 hours.

[0047] Endpoint criterion: The reaction is stopped when the acid value, as determined by GB / T6743-2008, drops to 3.0 mg KOH / g.

[0048] Post-treatment: volatiles were removed at 70℃ and 2.8 kPa for 1.3 hours to obtain silane-terminated polyester intermediate B.

[0049] III. Preparation of Organic-Inorganic Covalent Hybrid Polyester Intermediate C Raw materials: The silane-terminated polyester intermediate B of this embodiment is heated to 43°C and stirred mechanically (300 rpm). Tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added in this embodiment is 4.25% by mass of the silane-terminated polyester intermediate B of this embodiment.

[0050] Hydrolysis and condensation: Deionized water was added to make the molar ratio of deionized water to tetraethyl orthosilicate 1.25, and glacial acetic acid was added dropwise (the amount added was 0.80% of the mass of silane-terminated polyester intermediate B in this embodiment). The pH of the aqueous phase formed by the deionized water in this embodiment was adjusted to 5.0 in real time by using a pH meter, and the reaction was carried out at 43°C for 1.8 hours.

[0051] Removal of free small molecules: Free small molecules were removed at 60℃ and a vacuum of 2.8 kPa for 1.8 hours.

[0052] Endpoint criteria: The water content of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by the Karl Fischer method according to GB / T6283-2008. After the reaction, the water content was 0.10% by mass. The acid value of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by GB / T6743-2008 to be 5.3 mg KOH / g.

[0053] IV. Preparation of Weather-Resistant Polyester Resin for Low-Temperature Curing The organic-inorganic covalent hybrid polyester intermediate C, 0.50 parts by weight of condensation catalyst dibutyltin dilaurate, 0.40 parts by weight of hindered amine light stabilizer LS-770, 0.40 parts by weight of ultraviolet absorber UV-328, and 20 parts by weight of organic solvent butyl acetate were mixed evenly to obtain a weather-resistant polyester resin for low-temperature curing.

[0054] V. Curing Performance Test The weather-resistant polyester resin of this embodiment was prepared into a coating film and cured at 70°C for 8 hours. After curing, the coating film was tested according to GB / T6739-2022 and found to be H grade in pencil hardness and 0 grade in cross-cut test according to GB / T9286-2021. Weather resistance was tested according to GB / T1865-2009. After 500 hours of accelerated aging, the coating film showed no significant change in appearance, a color difference ΔE of 1.2, and a gloss retention rate of 88%.

[0055] Features of this embodiment: This embodiment uses moderate formulation parameters and process conditions, with balanced proportions of raw materials. The amount of organic-inorganic covalent hybrid polyester intermediate C is 75 parts by weight, with appropriate amounts of catalyst, light stabilizer, and UV absorber. The amount of organic solvent is 20 parts by weight. The ratio of polyol to diacid in polyester prepolymer A is moderate. The process parameters such as temperature, time, and pH value of silane end-capping reaction and organic-inorganic covalent hybrid reaction are all at moderate levels, ensuring the stability and repeatability of the resin system. It has excellent curing performance, with a film hardness of H grade, an adhesion grade of 0 grade, and good weather resistance. It is suitable for conventional outdoor coating applications with high requirements for product quality stability, and is particularly suitable for applications requiring long-term outdoor exposure, such as building exterior wall coatings and metal protective coatings.

[0056] Example 2 I. Preparation of Polyester Prepolymer A Raw material preparation: Take 135 parts by weight of neopentyl glycol, 45 parts by weight of 1,6-hexanediol, 14 parts by weight of trimethylolpropane, 102 parts by weight of isophthalic acid, and 68 parts by weight of adipic acid, and add them to a reaction vessel equipped with a stirring device, a temperature control system, a water separator, and a nitrogen protection system.

[0057] Melt esterification polycondensation: Under nitrogen atmosphere protection (nitrogen flow rate 0.40 L / min), start stirring, heat to 215℃, react for 6 hours, and continuously remove reaction water through a water separator.

[0058] Dehydration under reduced pressure and molecular weight control: The reaction was continued for 1.2 hours under a vacuum of 0.8 kPa and a temperature of 215 °C to further remove the reaction water.

[0059] Termination of reaction: The reaction was stopped when the acid value, as determined by GB / T6743-2008, dropped to 22 mg KOH / g, yielding polyester prepolymer A.

[0060] Quality control: According to GB / T31412-2015, the hydroxyl value of polyester prepolymer A in this example is 38 mgKOH / g.

[0061] II. Preparation of Silane-Terminated Polyester Intermediate B Raw materials: The above-mentioned polyester prepolymer A is added to the reactor, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane in this embodiment to the molar number of carboxyl groups calculated according to the acid value of polyester prepolymer A in this embodiment is 0.80. Triethylamine is added, and the amount of triethylamine in this embodiment is 0.18% by mass of polyester prepolymer A in this embodiment.

[0062] End-sealing reaction: reacted at 92°C for 2.0 hours.

[0063] Endpoint criterion: The reaction is stopped when the acid value, as determined by GB / T6743-2008, drops to 2.5 mg KOH / g.

[0064] Post-treatment: Remove volatiles at 65℃ and 2.0 kPa for 1.0 hour to obtain silane-terminated polyester intermediate B.

[0065] III. Preparation of Organic-Inorganic Covalent Hybrid Polyester Intermediate C Raw materials: The silane-terminated polyester intermediate B of this embodiment is heated to 35°C and stirred mechanically (220 rpm). Tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added in this embodiment is 2.80% by mass of the silane-terminated polyester intermediate B of this embodiment.

[0066] Hydrolysis and condensation: Deionized water was added to make the molar ratio of deionized water to tetraethyl orthosilicate 1.00, and glacial acetic acid was added dropwise (the amount added was 0.50% of the mass of silane-terminated polyester intermediate B in this embodiment). The pH of the aqueous phase formed by the deionized water in this embodiment was adjusted to 4.8 in real time by using a pH meter, and the reaction was carried out at 35°C for 1.3 hours.

[0067] Removal of free small molecules: Remove free small molecules at 55℃ and a vacuum of 2.2 kPa for 1.2 hours.

[0068] Endpoint criteria: The water content of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by the Karl Fischer method according to GB / T6283-2008. After the reaction, the water content was 0.08% by mass. The acid value of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by GB / T6743-2008 to be 3.8 mg KOH / g.

[0069] IV. Preparation of Weather-Resistant Polyester Resin for Low-Temperature Curing The organic-inorganic covalent hybrid polyester intermediate C, 0.35 parts by weight of condensation catalyst zinc 2-ethylhexanoate, 0.65 parts by weight of hindered amine light stabilizer HS-944, 0.70 parts by weight of ultraviolet absorber UV-531, and 28 parts by weight of organic solvent propylene glycol monomethyl ether acetate were mixed evenly to obtain a weather-resistant polyester resin for low-temperature curing.

[0070] V. Curing Performance Test The weather-resistant polyester resin of this embodiment was prepared into a coating film and cured at 65°C for 10 hours. After curing, the coating film was tested according to GB / T6739-2022 and found to be HB grade in pencil hardness and 1 grade in cross-cut test according to GB / T9286-2021. Weather resistance was tested according to GB / T1865-2009. After 500 hours of accelerated aging, the coating film showed no significant change in appearance, a color difference ΔE of 0.8, and a gloss retention rate of 92%.

[0071] Features of this embodiment: This embodiment enhances weather resistance by optimizing the formulation composition. The amount of organic-inorganic covalent hybrid polyester intermediate C is 68 parts by weight. The amount of hindered amine light stabilizer is significantly increased to 0.65 parts by weight and the amount of UV absorber is increased to 0.70 parts by weight. The amount of organic solvent is increased to 28 parts by weight to improve coating performance. The ratio of polyol to diacid in polyester prepolymer A is biased towards lower molecular weight. The silane end-capping reaction adopts a lower silane / carboxyl molar ratio of 0.80 and a lower reaction temperature of 92°C. The amount of tetraethyl orthosilicate in the organic-inorganic covalent hybrid reaction is 2.80% and the reaction temperature is lower at 35°C. The overall process parameters are biased towards mild conditions. After 500 hours of artificial accelerated aging, the color difference of the coating film is only 0.8 and the gloss retention rate is as high as 92%, showing excellent weather resistance. It is particularly suitable for applications with high weather resistance requirements, such as automotive topcoats, high-end outdoor furniture coatings, and marine engineering protective coatings, which require long-term resistance to ultraviolet rays and harsh climatic conditions.

[0072] Example 3 I. Preparation of Polyester Prepolymer A Raw material preparation: Take 190 parts by weight of neopentyl glycol, 85 parts by weight of 1,6-hexanediol, 28 parts by weight of trimethylolpropane, 180 parts by weight of isophthalic acid, and 130 parts by weight of adipic acid, and add them to a reaction vessel equipped with a stirring device, a temperature control system, a water separator, and a nitrogen protection system.

[0073] Melt esterification polycondensation: Under nitrogen atmosphere protection (nitrogen flow rate 0.72 L / min), stir and heat to 230℃, react for 8.5 hours, and continuously remove reaction water through a water separator.

[0074] Dehydration under reduced pressure and molecular weight control: The reaction was continued for 2.3 hours under a vacuum of 1.5 kPa and a temperature of 230 °C to further remove the reaction water.

[0075] Termination of reaction: The reaction was stopped when the acid value, as determined by GB / T6743-2008, dropped to 33 mg KOH / g, yielding polyester prepolymer A.

[0076] Quality control: According to GB / T31412-2015, the hydroxyl value of polyester prepolymer A in this example is 62 mgKOH / g.

[0077] II. Preparation of Silane-Terminated Polyester Intermediate B Raw materials: The above-mentioned polyester prepolymer A is added to the reactor, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane in this embodiment to the molar number of carboxyl groups calculated according to the acid value of polyester prepolymer A in this embodiment is 1.05. Triethylamine is added, and the amount of triethylamine used in this embodiment is 0.38% by mass of polyester prepolymer A in this embodiment.

[0078] End-sealing reaction: react at 110°C for 3.2 hours.

[0079] Endpoint criterion: The reaction is stopped when the acid value, as determined by GB / T6743-2008, drops to 4.0 mg KOH / g.

[0080] Post-treatment: volatiles were removed at 78℃ and 3.5 kPa for 1.6 hours to obtain silane-terminated polyester intermediate B.

[0081] III. Preparation of Organic-Inorganic Covalent Hybrid Polyester Intermediate C Raw materials: The silane-terminated polyester intermediate B of this embodiment is heated to 52°C and stirred mechanically (380 rpm). Tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added in this embodiment is 5.80% by mass of the silane-terminated polyester intermediate B of this embodiment.

[0082] Hydrolysis and condensation: Deionized water was added to make the molar ratio of deionized water to tetraethyl orthosilicate 1.60, and glacial acetic acid was added dropwise (the amount added was 1.10% of the mass of silane-terminated polyester intermediate B in this embodiment). The pH of the aqueous phase formed by the deionized water in this embodiment was adjusted to 5.5 in real time by using a pH meter, and the reaction was carried out at 52°C for 2.3 hours.

[0083] Removal of free small molecules: Free small molecules were removed at 68℃ and a vacuum of 3.5 kPa for 2.2 hours.

[0084] Endpoint criteria: The water content of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by the Karl Fischer method according to GB / T6283-2008. After the reaction, the water content was 0.15% by mass. The acid value of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by GB / T6743-2008 to be 7.5 mg KOH / g.

[0085] IV. Preparation of Weather-Resistant Polyester Resin for Low-Temperature Curing The organic-inorganic covalent hybrid polyester intermediate C of this embodiment, 85 parts by weight, condensation catalyst (0.45 parts by weight of dibutyltin dilaurate and 0.30 parts by weight of zinc 2-ethylhexanoate, totaling 0.75 parts by weight), hindered amine light stabilizer LS-770, UV absorber UV-328, and organic solvent (7 parts by weight of ethyl acetate and 5 parts by weight of butyl acetate, totaling 12 parts by weight) are mixed evenly to obtain a weather-resistant polyester resin for low-temperature curing.

[0086] V. Curing Performance Test The weather-resistant polyester resin of this embodiment was prepared into a coating film and cured at 75°C for 6 hours. After curing, the coating film was tested according to GB / T6739-2022 and found to be 2H in pencil hardness and 0 in cross-cut test according to GB / T9286-2021. Weather resistance was tested according to GB / T1865-2009. After 500 hours of accelerated aging, the coating film showed no significant change in appearance, a color difference ΔE of 1.5, and a gloss retention rate of 85%.

[0087] Features of this embodiment: This embodiment enhances the rapid curing performance through formulation design. The amount of organic-inorganic covalent hybrid polyester intermediate C is increased to 85 parts by weight, the amount of condensation catalyst reaches 0.75 parts by weight and a compound system of dibutyltin dilaurate and zinc 2-ethylhexanoate is adopted, the amount of organic solvent is reduced to 12 parts by weight and a compound system of ethyl acetate and butyl acetate is adopted to balance the evaporation rate, the polyol to diacid ratio of polyester prepolymer A is biased towards higher molecular weight and a higher hydroxyl value of 62 mgKOH / g is adopted, and the silane end-capping reaction adopts a higher silane / carboxyl molar ratio of 1.05 and a higher [missing information]. The reaction temperature is 110℃ to promote complete end-capping. The amount of tetraethyl orthosilicate used in the organic-inorganic covalent hybridization reaction is 5.80%, and the reaction temperature is relatively high at 52℃ with a longer reaction time of 2.3 hours to form a denser organic-inorganic network structure. The overall process parameters are biased towards strengthening the reaction conditions. The coating can reach a pencil hardness of 2H after curing at 75℃ for 6 hours. The curing speed is fast and the coating hardness is high, making it particularly suitable for applications that require rapid curing and high hardness, such as rapid coating on industrial production lines, high-hardness scratch-resistant coatings, and coatings for electronic product casings, which have high requirements for curing efficiency and surface hardness.

[0088] Example 4 I. Preparation of Polyester Prepolymer A Raw material preparation: Take 115 parts by weight of neopentyl glycol, 25 parts by weight of 1,6-hexanediol, 5 parts by weight of trimethylolpropane, 195 parts by weight of isophthalic acid, and 155 parts by weight of adipic acid, and add them to a reaction vessel equipped with a stirring device, a temperature control system, a water separator, and a nitrogen protection system.

[0089] Melt esterification polycondensation: Under nitrogen atmosphere protection (nitrogen flow rate 0.25 L / min), start stirring, heat to 208℃, react for 5.5 hours, and continuously remove reaction water through a water separator.

[0090] Dehydration under reduced pressure and molecular weight control: The reaction was continued for 1.0 hour under a vacuum of 0.4 kPa and a temperature of 208 °C to further remove the reaction water.

[0091] Termination of reaction: The reaction was stopped when the acid value, as determined by GB / T6743-2008, dropped to 18 mg KOH / g, yielding polyester prepolymer A.

[0092] Quality control: According to GB / T31412-2015, the hydroxyl value of polyester prepolymer A in this example is 32 mg KOH / g.

[0093] II. Preparation of Silane-Terminated Polyester Intermediate B Raw materials: The above-mentioned polyester prepolymer A is added to the reactor, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane in this embodiment to the molar number of carboxyl groups calculated according to the acid value of polyester prepolymer A in this embodiment is 1.10. Triethylamine is added, and the amount of triethylamine used in this embodiment is 0.12% by mass of polyester prepolymer A in this embodiment.

[0094] End-sealing reaction: react at 88°C for 1.5 hours.

[0095] Endpoint criterion: The reaction was stopped when the acid value, as determined by GB / T6743-2008, dropped to 1.8 mg KOH / g.

[0096] Post-treatment: volatiles were removed at 58℃ and 1.5 kPa for 0.8 hours to obtain silane-terminated polyester intermediate B.

[0097] III. Preparation of Organic-Inorganic Covalent Hybrid Polyester Intermediate C Raw materials: The silane-terminated polyester intermediate B of this embodiment is heated to 30°C and stirred mechanically (150 rpm). Tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added in this embodiment is 6.80% by mass of the silane-terminated polyester intermediate B of this embodiment.

[0098] Hydrolysis and condensation: Deionized water was added to make the molar ratio of deionized water to tetraethyl orthosilicate 0.65, and glacial acetic acid was added dropwise (the amount added was 0.30% of the mass of silane-terminated polyester intermediate B in this embodiment). The pH of the aqueous phase formed by the deionized water in this embodiment was adjusted to 4.3 in real time by using a pH meter, and the reaction was carried out at 30°C for 1.0 hour.

[0099] Removal of free small molecules: Remove free small molecules at 50℃ and 1.8kPa vacuum for 1.0 hour.

[0100] Endpoint criteria: The water content of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by the Karl Fischer method according to GB / T6283-2008. After the reaction, the water content was 0.06% by mass. The acid value of the organic-inorganic covalent hybrid polyester intermediate C in this embodiment was determined by GB / T6743-2008 to be 2.2 mg KOH / g.

[0101] IV. Preparation of Weather-Resistant Polyester Resin for Low-Temperature Curing The organic-inorganic covalent hybrid polyester intermediate C of this embodiment, 0.08 parts by weight of condensation catalyst dibutyltin dilaurate, 0.12 parts by weight of hindered amine light stabilizer HS-944, 0.10 parts by weight of ultraviolet absorber UV-328, and 5 parts by weight of organic solvent ethyl acetate were mixed evenly to obtain a weather-resistant polyester resin for low-temperature curing.

[0102] V. Curing Performance Test The weather-resistant polyester resin of this embodiment was prepared into a coating film and cured at 68°C for 9 hours. After curing, the coating film was tested according to GB / T6739-2022 and found to be HB grade in pencil hardness and 1 grade in cross-cut test according to GB / T9286-2021. Weather resistance was tested according to GB / T1865-2009. After 500 hours of accelerated aging, the coating film showed no significant change in appearance, a color difference ΔE of 1.8, and a gloss retention rate of 83%.

[0103] Features of this embodiment: This embodiment employs a specific formulation to explore the feasibility boundaries of the parameter space of the technical solution. The amount of organic-inorganic covalent hybrid polyester intermediate C reaches 93 parts by weight, while the amounts of condensation catalyst, hindered amine light stabilizer, and UV absorber are reduced to 0.08, 0.12, and 0.10 parts by weight, respectively. The amount of organic solvent is only 5 parts by weight to form a high solids content system. The amounts of neopentyl glycol, 1,6-hexanediol, and trimethylolpropane in polyester prepolymer A are 115, 25, and 5 parts by weight, respectively, while the amounts of isophthalic acid and adipic acid are increased to 195 and 155 parts by weight, respectively. The silane / carboxyl molar ratio in the silane-capping reaction reaches 1.10 to ensure sufficient capping. The amount of tetraethyl orthosilicate in the organic-inorganic covalent hybridization reaction is 6.80% to form a rich inorganic network structure. The overall formulation demonstrates the flexibility and adaptability of the technical solution while ensuring basic performance. The coating film can achieve HB hardness and Grade 1 cross-cut adhesion after curing at 68℃ for 9 hours. The weather resistance meets the basic requirements. It is particularly suitable for cost-sensitive applications with moderate performance requirements, such as general industrial coatings, interior wall coatings, and general metal protective coatings, where cost-effectiveness is a high priority.

[0104] Comparative Example 1: Basically the same as Example 1, except that the amount of organic-inorganic covalent hybrid polyester intermediate C is 45 parts by weight, while the amounts of other components and preparation conditions remain unchanged.

[0105] Comparative Example 2: Basically the same as Example 1, except that no hindered amine light stabilizer was added, while the amounts of other components and preparation conditions remained unchanged.

[0106] Comparative Example 3: Basically the same as Example 1, except that no ultraviolet absorber was added, while the amounts of other components and preparation conditions remained unchanged.

[0107] Comparative Example 4: Basically the same as Example 1, except that the ratio of the number of moles of 3-glycidoxypropyltrimethoxysilane to the number of moles of carboxyl groups calculated based on the acid value of polyester prepolymer A is 0.50 when preparing silane-terminated polyester intermediate B. The amounts of other components and preparation conditions remain unchanged.

[0108] Comparative Example 5: Basically the same as Example 1, except that when preparing organic-inorganic covalent hybrid polyester intermediate C, the amount of tetraethyl orthosilicate added is 0.30% of the mass of silane-terminated polyester intermediate B, while the amounts of other components and preparation conditions remain unchanged.

[0109] Comparative Example 6: Basically the same as Example 1, except that the pH of the aqueous phase formed by deionized water during the preparation of organic-inorganic covalent hybrid polyester intermediate C is 3.5, while the amounts of other components and preparation conditions remain unchanged.

[0110] Comparative Example 7: Basically the same as Example 1, except that after the polyester prepolymer A was prepared, it was directly mixed with the condensation catalyst, hindered amine light stabilizer, ultraviolet absorber and organic solvent, without silane end-capping and organic-inorganic covalent hybridization treatment, and the amount of other components and preparation conditions remained unchanged.

[0111] Comparative Example 8: Basically the same as Example 1, except that the amount of organic solvent used is 45 parts by weight, while the amounts of other components and preparation conditions remain unchanged.

[0112] Performance testing: Low-temperature curing performance test Test Object: Weather-resistant polyester resin coating for low-temperature curing. Test Objective: To evaluate the resin's curing ability under low-temperature conditions and verify the coupling effect of low-temperature curing and high crosslinking density. Test Principle: By controlling the curing temperature and time, the surface hardness and adhesion of the coating are detected, characterizing the degree of crosslinking network formation under low-temperature conditions. Experimental Method: The resin is coated onto a standard tinplate with a film thickness controlled at 30-40 μm. It is cured in a constant temperature oven at 60-80℃ for 4-12 hours. Performance testing is conducted after the coating cools to room temperature. Key Parameters: Curing temperature 70℃±2℃, curing time 8 hours±0.5 hours, ambient temperature 23℃±2℃, relative humidity 50%±5%. Data Processing: Pencil hardness is the lowest value from 5 tests; cross-cut test grade is rated according to the standard; each sample is tested in parallel 3 times, and the average value is taken.

[0113] Weather resistance test Test Object: Low-temperature curing weather-resistant polyester resin coating after curing. Test Objective: To evaluate the long-term weather resistance of the resin coating under simulated natural environmental conditions and verify the resistance of the organic-inorganic covalent hybrid structure to UV aging. Test Principle: An accelerated aging test simulates the cycles of light, temperature, and humidity in a natural environment, detecting changes in the coating's appearance, color difference, and gloss retention. Experimental Method: The coating sample was placed in a xenon lamp aging test chamber with a light intensity of 0.55 W / m² (340 nm), a blackboard temperature of 63℃±3℃, and cycling conditions of 102 minutes of light exposure + 18 minutes of light exposure + water spray, for a continuous aging period of 500 hours. Key Parameters: Aging time 500 hours, xenon lamp irradiance 0.55 W / m², test points before aging, 250 hours of aging, and 500 hours of aging. Data processing: Color difference ΔE was measured using a spectrophotometer. Gloss retention rate was the percentage of gloss value after aging to the initial gloss value. Each group of samples was tested in parallel for 3 times, and the average value ± standard deviation was taken.

[0114] Organic-inorganic hybrid structure characterization Test Object: Organic-inorganic covalently hybridized polyester intermediate C. Test Objective: To characterize the formation of the siloxane network structure and its covalent connection with polyester segments, and to verify the uniformity of the organic-inorganic covalent hybridization. Test Principle: Changes in characteristic peaks of Si-O-Si and Si-OC are detected by Fourier transform infrared spectroscopy, combined with solid-state nuclear magnetic resonance to characterize the degree of condensation of the siloxane network. Experimental Method: Intermediate C samples are prepared as thin films or powders, and ATR-FTIR is used at 4000-4000 nm. Scan within range, resolution 4 32 scans; using Solid-state NMR, magic angle rotation frequency 5 kHz, cumulative scans 2000. Key parameters: FTIR characteristic peaks 1100-1000. (Si-O-Si), 800-750 (Si-C),² 9 T and Q structure signals in the SiNMR chemical shift range of -40 to -120 ppm. Data processing: Peak intensity normalization, calculation of siloxane network condensation degree. In total, each group of samples was tested twice.

[0115] Storage stability test Test Object: Weather-resistant polyester resin for low-temperature curing. Test Objective: To evaluate the stability of the resin system under room temperature storage conditions and verify the influence of organic-inorganic covalent hybridization on processing and storage stability. Test Principle: By monitoring changes in viscosity, acid value, and curing properties of the resin during storage, the chemical stability and service life of the system are assessed. Experimental Method: The resin was placed in a sealed container and stored at 23℃±2℃ and 50%±10% relative humidity. Samples were taken at 0, 30, 60, and 90 days to test viscosity, acid value, and curing properties. Key Parameters: Storage temperature 23℃±2℃, viscosity test temperature 25℃±0.1℃, rotor speed 60 rpm, acid value titration accuracy ±0.5 mgKOH / g. Data Processing: Viscosity is expressed in mPa·s, acid value in mgKOH / g, and curing properties were characterized by changes in pencil hardness. Each sample was tested in triplicate, and the average value ± standard deviation was taken.

[0116] High solids content and low viscosity performance test Test Object: Weather-resistant polyester resin for low-temperature curing. Test Objective: To evaluate the viscosity characteristics of the resin system under high solids content conditions, and to verify the ability to balance high solids content, low viscosity, workability, and high crosslinking density. Test Principle: By measuring the viscosity curves of the resin at different solids contents, the rheological properties and workability of the system are evaluated. Experimental Method: Using a rotational viscometer, at 25℃±0.1℃, the resin solids content was adjusted to 50%, 60%, 70%, and 80%, and different shear rates (10-100) were measured. Viscosity at ( ). Key parameters: Test temperature 25℃±0.1℃, solid content determination conditions: oven drying at 105℃±2℃ for 3 hours, shear rate range 10-100 Step interval 10 Data processing: Viscosity is expressed in mPa·s, and solid content is expressed as mass percentage. Viscosity-shear rate curves and viscosity-solid content curves were plotted. Each sample was tested in triplicate, and the average value ± standard deviation was taken.

[0117] Crosslinking density characterization test Test Object: Weather-resistant polyester resin coating for low-temperature curing. Test Objective: To quantitatively characterize the crosslinking network density of the coating and verify the degree of achievement of high crosslinking density under low-temperature curing conditions. Test Principle: The crosslinking density is determined by the swelling method, and the average molecular weight between crosslinking points is calculated based on the Flory-Rehner theory. Experimental Method: The cured coating is prepared into 5mm × 5mm × 0.1mm samples, swollen in toluene at 25℃ for 24 hours until equilibrium is reached. The mass before and after swelling is weighed, and the degree of swelling and gel content are calculated. The crosslinking density is calculated using the Flory-Rehner equation. Where φ is the polymer volume fraction, χ is the Flory-Huggins action parameter, and V1 is the solvent molar volume. Key parameters: swelling temperature 25℃±1℃, swelling time 24 hours, toluene purity ≥99.5%, sample thickness 0.10±0.02mm. Data processing: crosslinking density is expressed in mol / cm³, gel content is expressed as mass percentage, and each sample was tested in parallel 5 times, with the average value ± standard deviation taken.

[0118] Figure 1 This is a transmittance-wavenumber superposition spectrum of Fourier transform infrared spectroscopy, with fixed parameters that ensure the sample film preparation and testing conditions are consistent and the scanning range covers 1800 to 650. And focus on comparing 1200 to 700 The fingerprint region, with the parameter change being the sample type switching from Example 1 to Comparative Example 7 without silane end-capping and organic-inorganic covalent hybridization treatment. Example 1 was measured at 1100 to 1000. A more significant Si-O-Si correlated absorption band appears in the region and at 800 to 750. The region exhibits Si-C related absorption characteristics, and the carbonyl peak position shows a identifiable shift and spectral shape difference relative to Comparative Example 7. This indicates that the siloxane network and polyester segments have undergone chemical connection rather than simple physical mixing, proving that an organic-inorganic covalent hybrid structure has indeed been formed and that the control sample lacks this structure.

[0119] Figure 2 The temperature-storage modulus E′ and loss factor tanδ dual Y-axis superposition curves are used for dynamic mechanical analysis. The parameters were fixed: a temperature scan range of -50 to 150°C, consistent DMA conditions including test frequency and strain amplitude, and the same curing regime for coating preparation. The parameter variation was changing the sample type from Example 1 to Comparative Example 4 with a silane / carboxyl molar ratio of 0.50. Example 1 exhibited a higher E′ plateau value and a tanδ peak position closer to the high-temperature side, showing more restricted chain segment movement characteristics, indicating a denser crosslinked network, a higher glass transition temperature Tg, and more controlled energy dissipation peaks. Comparative Example 4 showed a decrease in E′ and a downward shift in Tg due to insufficient silane end-capping, demonstrating that silane end-capping and subsequent organic-inorganic network construction are crucial for forming a high crosslinked density network under low-temperature curing conditions, thus supporting the rationality of achieving mechanical properties at lower temperatures in this scheme.

[0120] Figure 3 The wavelength-absorbance superposition spectrum of the UV-Vis spectrum was used. The parameters were fixed: coating thickness and optical path length were consistent, and the spectral scanning range covered 200 to 800 nm, with a focus on the 200 to 400 nm UV region. The results compared the states before and after 500 hours of aging. The changing parameters were: sample formulation was changed from Example 1 to Comparative Example 2 lacking the hindered amine light stabilizer, and Comparative Example 3 lacking the UV absorber. Example 1 maintained stronger and more stable absorption characteristics in the UV region, and the peak intensity decayed less after aging, indicating that the UV energy absorption and free radical inhibition mechanism worked synergistically. Comparative Example 2 showed more significant attenuation of the UV absorption peak and baseline change in the visible region after aging, indicating that the photo-oxidation chain reaction was more difficult to suppress. Comparative Example 3 showed insufficient initial UV absorption capacity and more pronounced spectral deterioration after aging, indicating that the lack of UV shielding accelerates photo-induced degradation. This demonstrates that the synergistic configuration of the hindered amine light stabilizer and the UV absorber can significantly improve the weather resistance stability of the system, consistent with the design goals of this scheme.

[0121] As can be seen from the performance of the examples and comparative examples in Table 1, Examples 1-4 all exhibited excellent comprehensive performance under low-temperature curing conditions, with pencil hardness reaching HB-2H level and cross-cut test grade of 0-1, indicating that the coating film has good hardness and adhesion. After 500 hours of artificial accelerated aging, the color difference ΔE of the examples was controlled within the range of 0.8-1.8, and the gloss retention rate reached 83%-92%, significantly better than the comparative examples, proving that the synergistic effect of the organic-inorganic covalent hybrid structure and the light stabilizer system effectively improved the weather resistance. In terms of storage stability, the viscosity increase rate of the examples after 90 days was only 6%-12%, while... The viscosity increase rate of Comparative Example 7, lacking silane end-capping or organic-inorganic hybridization, reached as high as 35%, indicating that the covalent hybridization structure significantly improved the storage stability of the system. Under high solids content conditions, the viscosity of the examples was controlled at 3200-4200 mPa·s at 70% solids content, while the viscosity of Comparative Examples 1, 5, and 7, lacking organic-inorganic hybridization, significantly increased to 5200-6800 mPa·s, verifying that the present invention achieves the goal of high solids content and low viscosity through silane end-capping and the construction of an appropriate amount of inorganic network. The crosslinking density and gel content data further confirm that the crosslinking density of the examples reached 4.9-6.5 × 10⁻⁶ mPa·s under low-temperature curing conditions. With a gel content of 92%-96% and a mol / cm³ content, this invention successfully resolves the coupling contradiction between low-temperature curing capability and high cross-linking density mechanical properties. In contrast, the performance of comparative models lacking key components or processes is significantly reduced, fully demonstrating the superiority of the technical solution of this invention.

[0122] Table 1. Performance comparison data of the embodiments and comparative examples. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A weather-resistant polyester resin for low-temperature curing, characterized in that, By weight, the low-temperature curing weather-resistant polyester resin comprises the following components: (1) 50.00 to 99.00 parts by weight of organic-inorganic covalently hybrid polyester intermediate C; (2) 0.01 to 1.00 parts by weight of a condensation catalyst, wherein the condensation catalyst is selected from one or two of dibutyltin dilaurate and zinc 2-ethylhexanoate; (3) 0.05 to 0.80 parts by weight of hindered amine light stabilizer, wherein the hindered amine light stabilizer is selected from one or two of light stabilizer LS-770 and hindered amine light stabilizer HS-944; (4) 0.05 to 0.80 parts by weight of ultraviolet absorber, wherein the ultraviolet absorber is selected from one or two of ultraviolet absorber UV-328 and ultraviolet absorber UV-531; (5) 0 to 40.00 parts by weight of an organic solvent, wherein the organic solvent is selected from one or more of ethyl acetate, butyl acetate, and propylene glycol monomethyl ether acetate; The organic-inorganic covalently hybridized polyester intermediate C is obtained by silane-termined polyester intermediate B from polyester prepolymer A, and then organic-inorganic covalently hybridizing the silane-termined polyester intermediate B.

2. The weather-resistant polyester resin for low-temperature curing according to claim 1, characterized in that, Polyester prepolymer A is prepared by the following steps: A1. Raw material preparation: Neopentyl glycol 100-220 parts by weight, 1,6-hexanediol 10-120 parts by weight, trimethylolpropane 2-40 parts by weight, isophthalic acid 60-220 parts by weight, and adipic acid 20-180 parts by weight. A2. Melt esterification polycondensation: The reaction is carried out at 200-240℃ for 4-10 hours under an inert atmosphere, and the reaction water is continuously removed; A3. Dehydration under reduced pressure and molecular weight control: The reaction was continued for 0.5 to 3.0 h under a vacuum of 0.2–2.0 kPa and a temperature of 200–240 °C; A4. Termination of reaction: Stop the reaction when the acid value drops to 15-40 mgKOH / g to obtain polyester prepolymer A; A5. Quality control: The hydroxyl value of the polyester prepolymer A is 20-80 mg KOH / g.

3. The weather-resistant polyester resin for low-temperature curing according to claim 2, characterized in that, Silane-terminated polyester intermediate B is prepared by the following steps: B1. Raw materials: The polyester prepolymer A is added to a reaction vessel, and 3-glycidoxypropyltrimethoxysilane is added so that the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane to the molar number of carboxyl groups calculated according to the acid value of the polyester prepolymer A is 0.60 to 1.

20. Triethylamine is added, and the amount of triethylamine is 0.05 to 0.50% by mass of the polyester prepolymer A. B2. End-sealing reaction: react at 80–120℃ for 1.0–4.0 h; B3. Endpoint criterion: Stop the reaction when the acid value drops to no higher than 5.0 mg KOH / g; B4. Post-treatment: Remove volatiles at 50–90℃ and a vacuum of 0.5–5.0 kPa for 0.5–2.0 h to obtain silane-terminated polyester intermediate B.

4. The weather-resistant polyester resin for low-temperature curing according to claim 3, characterized in that, Organic-inorganic covalently hybrid polyester intermediate C is prepared by the following steps: C1. Raw materials: The silane-terminated polyester intermediate B is heated to 25-60°C and stirred mechanically, and tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added is 0.50-8.00% by mass of the silane-terminated polyester intermediate B. C2. Hydrolysis and condensation: Add deionized water to make the molar ratio of deionized water to tetraethyl orthosilicate 0.50-2.00, and add glacial acetic acid to adjust the pH of the aqueous phase formed by the deionized water to 4.0-6.

0. React at 25-60℃ for 0.5-3.0h. C3. Removal of free small molecules: Remove free small molecules at 40–80℃ and a vacuum of 0.5–5.0 kPa for 0.5–3.0 h; C4. Endpoint criterion: The water content of the organic-inorganic covalent hybrid polyester intermediate C after the reaction is not higher than 0.20% by mass; C5. Organic-inorganic covalent hybrid polyester intermediate C is obtained.

5. The weather-resistant polyester resin for low-temperature curing according to claim 1, characterized in that, The acid value of the organic-inorganic covalent hybrid polyester intermediate C is 0.5–8.0 mg KOH / g.

6. The weather-resistant polyester resin for low-temperature curing according to claim 1, characterized in that, The low-temperature curing refers to: after the weather-resistant polyester resin is prepared into a coating film, it is cured at a temperature not exceeding 100°C for 2 to 24 hours, and the cured coating film has a pencil hardness of HB or above as measured by GB / T6739-2022, and a cross-cut test grade not exceeding level 1 as measured by GB / T9286-2021.

7. A method for preparing a low-temperature curing weather-resistant polyester resin as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Neopentyl glycol, 1,6-hexanediol, trimethylolpropane, isophthalic acid and adipic acid are subjected to melt polycondensation under an inert atmosphere to obtain polyester prepolymer A with an acid value of 15-40 mgKOH / g; S2. The polyester prepolymer A is subjected to an end-capping reaction with 3-glycidoxypropyltrimethoxysilane to obtain silane-capped polyester intermediate B with an acid value not higher than 5.0 mgKOH / g; S3. Add tetraethyl orthosilicate and deionized water to the silane-terminated polyester intermediate B, and perform hydrolysis, condensation and removal of free small molecules under the conditions of pH 4.0-6.0 and temperature 25-60℃ to obtain organic-inorganic covalent hybrid polyester intermediate C. S4. The organic-inorganic covalent hybrid polyester intermediate C is mixed with a condensation catalyst, a hindered amine light stabilizer, an ultraviolet absorber and an organic solvent to obtain a weather-resistant polyester resin for low-temperature curing.

8. The method according to claim 7, characterized in that, Step S1 includes the following sub-steps: S1-1. Raw material preparation: Neopentyl glycol 100-220 parts by weight, 1,6-hexanediol 10-120 parts by weight, trimethylolpropane 2-40 parts by weight, isophthalic acid 60-220 parts by weight, and adipic acid 20-180 parts by weight. S1-2. Melt esterification polycondensation: The reaction is carried out at 200-240℃ for 4-10 hours under an inert atmosphere, and the reaction water is continuously removed; S1-3. Dehydration under reduced pressure and molecular weight control: Continue the reaction for 0.5-3.0 h under vacuum of 0.2-2.0 kPa and temperature of 200-240 °C; S1-4. Termination of reaction: Stop the reaction when the acid value drops to 15-40 mgKOH / g to obtain polyester prepolymer A; S1-5. Quality control: The hydroxyl value of the polyester prepolymer A is 20-80 mg KOH / g.

9. The method according to claim 7, characterized in that, Step S2 includes the following sub-steps: S2-1. Raw materials: Add the polyester prepolymer A to the reactor, add 3-glycidoxypropyltrimethoxysilane, and make the ratio of the molar number of 3-glycidoxypropyltrimethoxysilane to the molar number of carboxyl groups calculated according to the acid value of the polyester prepolymer A 0.60 to 1.20, and add triethylamine, the amount of which is 0.05 to 0.50% by mass of the polyester prepolymer A; S2-2. End-sealing reaction: react at 80–120℃ for 1.0–4.0 h; S2-3. Endpoint criterion: Stop the reaction when the acid value drops to no higher than 5.0 mg KOH / g; S2-4. Post-treatment: Remove volatiles at 50-90℃ and vacuum of 0.5-5.0 kPa for 0.5-2.0 h to obtain silane-terminated polyester intermediate B.

10. The method according to claim 7, characterized in that, Step S3 includes the following sub-steps: S3-1. Raw materials: The silane-terminated polyester intermediate B is heated to 25-60°C and stirred mechanically, and tetraethyl orthosilicate is added. The amount of tetraethyl orthosilicate added is 0.50-8.00% by mass of the silane-terminated polyester intermediate B. S3-2. Hydrolysis and condensation: Add deionized water to make the molar ratio of deionized water to tetraethyl orthosilicate 0.50 to 2.00, and add glacial acetic acid to adjust the pH of the aqueous phase formed by the deionized water to 4.0 to 6.

0. React at 25 to 60°C for 0.5 to 3.0 h. S3-3. Removal of free small molecules: Remove free small molecules at 40–80℃ and a vacuum of 0.5–5.0 kPa for 0.5–3.0 h; S3-4. Endpoint criterion: The water content of the organic-inorganic covalent hybrid polyester intermediate C after the reaction is not higher than 0.20% by mass; S3-5. Organic-inorganic covalent hybrid polyester intermediate C is obtained.

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