An unsaturated polyester resin for open molding process and a method for preparing the same

Abstract

The application discloses an unsaturated polyester resin for an open molding process and a preparation method thereof, and relates to the technical field of high polymer materials.The resin comprises an unsaturated polyester base resin, a composite crosslinking monomer, a film forming inhibitor and a core-shell structure toughening agent; the base resin is formed by polycondensation of an unsaturated dibasic anhydride, a saturated dibasic acid and a dibasic alcohol according to a molar ratio of 0.5-0.7:0.3-0.5:1.05-1.15; the composite crosslinking monomer comprises styrene and a high-boiling-point crosslinking monomer, and the mass ratio of the two is 20-40:60-80; and the film forming inhibitor is a long-chain alkyl modified polysiloxane.The application solves the problem that low volatility and high toughness are difficult to be considered simultaneously by synergistically reducing volatilization of the high-boiling-point monomer and the surface barrier and synergistically toughening by flexible molecular chain design and the core-shell particle, and significantly improves the working environment and improves the durability of products.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to an unsaturated polyester resin for open molding processes and its preparation method. Background Technology

[0002] Unsaturated polyester resins are widely used in the manufacture of composite materials such as fiberglass and artificial stone due to their excellent mechanical properties, processing performance, and relatively low cost. In open molding processes (such as hand lay-up and spraying), the resin is typically in extensive contact with air, and its reactive diluent—styrene—easily volatilizes into the working environment during processing. Styrene, as a volatile organic compound, not only causes environmental pollution but also poses a potential threat to the health of operators. With increasingly stringent environmental regulations and rising workplace safety and health requirements, reducing the volatilization of styrene from resins has become an important direction for technological development in this field.

[0003] Currently, the main technical means in the industry to reduce styrene volatilization include partially replacing styrene with high-boiling-point monomers (such as vinyltoluene) or adding paraffinic film-forming inhibitors to the resin. However, these methods all have certain limitations. Simply using high-boiling-point monomers for replacement often fails to maintain the original reactivity of the resin system and the mechanical properties of the cured material while ensuring sufficiently low volatility, especially impact toughness. While traditional paraffinic inhibitors can migrate to the surface of the product in the later stages of curing to form a physical barrier and reduce styrene volatilization, this migration often leads to a decrease in interlayer adhesion of the composite material, affecting the integrity and durability of the product. In addition, traditional unsaturated polyester resins designed for low shrinkage or high strength often have relatively rigid molecular chains, exhibiting inherent brittleness and insufficient impact and crack resistance. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a low-volatility toughness unsaturated polyester resin with balanced comprehensive performance and its preparation method.

[0005] The technical solution adopted by the present invention to solve its technical problem is: a low-volatility toughness unsaturated polyester resin for open molding process, comprising the following components: unsaturated polyester base resin, composite crosslinking monomer, film-forming inhibitor and core-shell structure toughening agent;

[0006] The unsaturated polyester base resin is formed by polycondensation reaction of unsaturated diacid anhydride, saturated diacid and diol in a molar ratio of 0.5~0.7:0.3~0.5:1.05~1.15;

[0007] The composite crosslinking monomer comprises styrene and at least one high-boiling-point crosslinking monomer.

[0008] In this invention, the film-forming inhibitor is a surfactant that can form a monomolecular barrier on the resin surface. The unsaturated polyester base resin has flexible segments embedded in its molecular chain. The composite crosslinking monomer, film-forming inhibitor, and core-shell toughening agent work synergistically to give the unsaturated polyester resin both low volatility and high toughness. The low-volatility, high-toughness unsaturated polyester resin of this invention reduces the driving force of volatilization from the source through a high-boiling-point composite monomer system and forms a dense barrier on the surface using a highly efficient film-forming inhibitor. This dual synergistic effect significantly reduces styrene escape and significantly improves the open molding environment. Simultaneously, the resin matrix, through the flexible molecular chain design and the synergistic effect of core-shell microparticle toughening, effectively overcomes the brittleness of traditional resins while ensuring mechanical strength, giving the cured product excellent impact toughness and crack resistance. This solution successfully resolves the technical contradiction of balancing low volatility and high toughness, resulting in a resin with good process adaptability and reliable interlayer adhesion.

[0009] Preferably, the unsaturated polyester resin used in the open molding process comprises, by weight, the following components:

[0010] The formulation comprises: 40-60 parts unsaturated polyester base resin; 35-55 parts composite crosslinking monomer; 0.1-2.0 parts high-efficiency film-forming inhibitor; 1-10 parts core-shell toughening agent; 0.01-0.05 parts polymerization inhibitor; and 0.1-0.5 parts accelerator. This formulation achieves optimized performance through precise synergy among its components: the ratio of base resin to crosslinking monomer ensures good reactivity and suitable viscosity; the high-efficiency film-forming inhibitor, added in minute amounts, forms a dense surface barrier, significantly reducing styrene volatilization; and the appropriate introduction of the core-shell toughening agent effectively improves impact toughness, avoiding the increased brittleness caused by traditional volatilization reduction methods.

[0011] Preferably, in the unsaturated polyester resin used in the open molding process, the mass ratio of styrene to high-boiling-point crosslinking monomer in the composite crosslinking monomer is 20-40:60-80; the high-boiling-point crosslinking monomer is selected from at least one of vinyltoluene (VT), methyl methacrylate (MMA), divinylbenzene (DVB), and isobornyl acrylate (IBOA). The high proportion of high-boiling-point crosslinking monomer significantly reduces the overall volatility of the mixed monomers, effectively suppressing styrene escape from the source. Vinyltoluene balances reactivity and volatility, divinylbenzene increases crosslinking density, and isobornyl acrylate, through its rigid alicyclic structure, assists in reducing volatility and provides internal plasticizing.

[0012] More preferably, the high-boiling-point crosslinking monomer is composed of 85wt%~95wt% vinyltoluene and 5wt%~15wt% divinylbenzene. Synergistic effects are achieved through the functional complementarity of vinyltoluene and divinylbenzene. Vinyltoluene, as the main component, effectively reduces the overall volatility of the mixed monomers, laying the foundation for low volatility; while an appropriate amount of divinylbenzene, through its bifunctional characteristics, significantly enhances the crosslinking network density, improving the material's rigidity and heat resistance. The synergistic effect of these two components ensures a significant reduction in volatility while optimizing the network structure, resulting in a cured product with both excellent toughness and mechanical strength, successfully overcoming the shortcomings of unbalanced performance of single components.

[0013] Preferably, in the unsaturated polyester resin used in the open molding process, the film-forming inhibitor is a long-chain alkyl-modified polysiloxane surfactant. This preferred inhibitor forms a dense and stable monomolecular film by directional alignment on the resin surface, physically blocking styrene escape. Its molecular anchoring effect prevents complete migration similar to paraffin wax, thereby reliably maintaining the interlayer adhesion of the composite material while efficiently reducing volatility.

[0014] More preferably, the synthesis process of the long-chain alkyl-modified polysiloxane surfactant is as follows:

[0015] Feeding and inerting: Add metered amounts of hydrogen-containing silicone oil, long-chain terminal olefins, and solvents to the reactor; the molar ratio of the Si-H bonds in the long-chain terminal olefins to those in the hydrogen-containing silicone oil is 1.05~1.10:1; introduce nitrogen or argon gas to replace the air in the reaction system and maintain a slight positive pressure protection.

[0016] Heating and catalysis: Heat the reaction mixture to 80℃~90℃; add platinum catalyst while stirring, the amount of platinum catalyst added is 10ppm~50ppm of the total mass of the reactants (calculated as platinum metal).

[0017] Addition reaction: Maintain the temperature at 85℃~95℃ and stir continuously for 4h~8h; after the reaction is completed, cool the system to room temperature and remove the solvent and unreacted monomer by vacuum distillation to obtain the product.

[0018] This synthesis process ensures the regularity and batch stability of the inhibitor's molecular structure, enabling it to migrate efficiently to the resin surface and form a dense barrier. Through mild conditions and precise control, the process effectively avoids side reactions, ensuring that the inhibitor effectively suppresses styrene volatilization while maintaining excellent interlayer adhesion due to its stable anchoring effect. This significantly improves the overall performance and reliability of the resin product.

[0019] The hydrogen-containing silicone oil is preferably a polymethylhydrosiloxane with active Si-H bonds in its end groups or side chains, such as TH-202 from Shandong Taihe Chemical Co., Ltd. The long-chain terminal olefins include 1-dodecene (C12), 1-tetradecene (C14), or 1-hexadecene (C16). The catalyst is preferably a Karstedt catalyst (platinum-vinylsiloxane complex). The solvent is toluene or xylene. The use of a highly active Karstedt catalyst in combination with a suitable solvent (toluene / xylene) ensures the high efficiency, mildness, and completeness of the hydrosilylation reaction, effectively avoiding side reactions.

[0020] Preferably, in the unsaturated polyester resin used in the open molding process, the saturated diacid in the unsaturated polyester base resin is a flexible diacid, and the diol is a flexible diol. The flexible diacid includes adipic acid or azelaic acid, and the flexible diol includes diethylene glycol or 1,2-propanediol. By using flexible diacids such as adipic acid and azelaic acid, and flexible diols such as diethylene glycol and 1,2-propanediol as synthetic raw materials, flexible segments are successfully embedded into the main chain of the unsaturated polyester molecule. This fundamentally improves the molecular chain flexibility and internal plasticizing effect of the resin matrix, significantly enhances the intrinsic toughness, impact resistance, and crack resistance of the cured material, and effectively overcomes the problem of increased brittleness caused by the introduction of rigid cross-linking structures at the source.

[0021] Preferably, the core-shell toughening agent is MBS resin (methyl methacrylate-butadiene-styrene copolymer). MBS can be stably dispersed in composite crosslinking monomers such as styrene / vinyl toluene, and does not undergo phase separation during resin curing. Because its outer shell contains a styrene structure, MBS has excellent compatibility with the resin system of this invention. Alternatively, the core-shell toughening agent can be an ACR-type core-shell toughening agent, such as a core of acrylate rubber (e.g., butyl acrylate BA) and an outer shell of polymethyl methacrylate.

[0022] A method for preparing the above-mentioned unsaturated polyester resin for open molding process includes the following steps:

[0023] (1) Synthesis of unsaturated polyester base resin: Under a protective atmosphere, diol, unsaturated diacid anhydride and saturated diacid are mixed in proportion and subjected to a step-by-step esterification reaction until the acid value reaches 15mgKOH / g~30mgKOH / g, to obtain the unsaturated polyester base resin.

[0024] (2) Dilution and mixing: Cool the base resin obtained in step (1) to the first temperature, add the polymerization inhibitor and the composite crosslinking monomer, and stir to mix evenly;

[0025] (3) Additive dispersion: Cool the mixture obtained in step (2) to a second temperature, add a high-efficiency film-forming inhibitor, an accelerator and a core-shell structure toughening agent, stir and disperse evenly to obtain the low-volatility toughness unsaturated polyester resin.

[0026] The first temperature is 30°C to 40°C higher than the second temperature.

[0027] This invention achieves synergistic optimization of process and performance assurance by precisely controlling the key temperatures and the 30-40°C temperature difference between the dilution and additive dispersion steps. First, the base resin is cooled to a higher initial temperature for dilution, ensuring rapid and uniform dispersion of the composite crosslinking monomers and preventing premature polymerization of styrene. Then, the system is cooled to a lower second temperature before adding key additives such as inhibitors and accelerators, effectively preventing thermal decomposition or reaction inactivation and ensuring functional integrity. This specific stepwise cooling process improves the compatibility and stability of each component while providing a reliable foundation for the low volatility and high toughness of the final resin.

[0028] Preferably, the stepped heating esterification reaction includes: first, performing preliminary esterification of the reactants at 150℃~165℃, and then gradually heating to 190℃~210℃ for the main reaction until the acid value reaches a predetermined range. The initial gentle esterification at 150~165℃ ensures complete ring-opening and gradual dehydration of the anhydride, effectively preventing boiling over and material entrainment. The subsequent gradual heating to 190~210℃ for the main reaction ensures both the rate and completeness of the esterification reaction, while avoiding side reactions such as alcohol etherification and double bond isomerization caused by excessively rapid heating, thereby obtaining a base resin with a more uniform molecular weight distribution, lighter color, and more stable performance.

[0029] Preferably, in the above preparation method, the first temperature is 85℃~95℃, and the second temperature is 50℃~60℃. This preferred temperature range precisely matches the characteristics of each component: the first temperature of 85~95℃ ensures rapid and uniform mixing of the crosslinking monomer and the base resin, while effectively inhibiting the volatilization and polymerization of styrene; the second temperature of 50~60℃ provides a safe dispersion environment for heat-sensitive additives, avoiding their deactivation.

[0030] Preferably, in the above preparation method, the composite crosslinking monomer added in step (2) is a premix of styrene and a high-boiling-point crosslinking monomer. By premixing styrene with the high-boiling-point crosslinking monomer, it can be ensured that each component is uniformly dispersed at the molecular level before being added to the resin, effectively avoiding local uneven concentrations or compatibility problems that may occur due to sequential feeding, thereby ensuring the uniformity of the cured network structure, and ultimately making the low volatility and mechanical toughness of the resin stable and synergistic.

[0031] Preferably, in the above preparation method, the stirring and dispersion time in step (3) is 1h to 2h. This stirring time ensures that the film-forming inhibitor and key additives such as the core-shell toughening agent can be fully dispersed to a molecular-level uniform state, thereby ensuring the density of the film formed on the surface of the inhibitor and the uniformity of the distribution of the toughening agent in the matrix, ultimately achieving a stable synergy between the low volatility and high toughness of the resin.

[0032] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention reduces the driving force of volatilization from the chemical source by using high-boiling-point crosslinking monomers, and innovatively uses long-chain alkyl-modified polysiloxane film-forming inhibitors to form a stable and dense physical barrier on the resin surface. The synergistic effect of these two mechanisms significantly reduces styrene volatilization and greatly improves the open molding environment. Simultaneously, by embedding flexible segments into the unsaturated polyester molecular backbone and combining them with core-shell toughening agents for multi-scale synergistic toughening, the intrinsic toughness and impact resistance of the material are fundamentally improved, successfully solving the technical challenge of balancing low volatilization and high toughness. Furthermore, the precisely controlled step-heat esterification and two-step cooling dilution dispersion preparation process ensures the activity and dispersion uniformity of each functional component, resulting in a final resin product with excellent mechanical strength, reliable interlayer adhesion, and good process adaptability, exhibiting significantly superior overall performance compared to traditional products. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments, wherein Embodiment 1 is the best implementation.

[0034] Example 1

[0035] Accurately weigh 50 parts by weight of unsaturated polyester base resin, 45 parts by composite crosslinking monomer, 0.8 parts by high-efficiency film-forming inhibitor, 5 parts by weight of MBS, 0.03 parts by weight of polymerization inhibitor, and 0.3 parts by weight of accelerator. The unsaturated polyester base resin is formed by polycondensation reaction of maleic anhydride, adipic acid, and diethylene glycol in a molar ratio of 0.60:0.40:1.10.

[0036] The specific synthesis process is as follows: The reactants are first initially esterified at 160℃ for 2 hours, then the temperature is gradually increased to 200℃ for the main reaction until the acid value drops to 25 mg KOH / g. The composite crosslinking monomer is a premixed solution, wherein the mass ratio of styrene to high-boiling-point crosslinking monomer is 30:70; the high-boiling-point crosslinking monomer is composed of 90 wt% vinyltoluene and 10 wt% divinylbenzene. The film-forming inhibitor is a self-made long-chain alkyl (C14) modified polysiloxane according to the aforementioned synthesis process. During preparation, the synthesized base resin is first cooled to 90℃, and the polymerization inhibitor and composite crosslinking monomer premixed solution are added, and stirred at high speed for 0.5 hours until homogeneous; then the mixture is cooled to 55℃, and the high-efficiency film-forming inhibitor, accelerator, and MBS are added, and stirring continues for 1.5 hours until completely dispersed and homogeneous, before discharging.

[0037] Example 2

[0038] By weight, 40 parts of unsaturated polyester base resin, 55 parts of composite crosslinking monomer, 0.1 parts of high-efficiency film-forming inhibitor, 1 part of MBS, 0.01 parts of polymerization inhibitor, and 0.1 parts of accelerator were weighed. The molar ratio of alkyd to acid in the base resin was 1.05:1, and maleic anhydride accounted for 70% of the total acid. The mass ratio of styrene to high-boiling-point monomers (85wt% vinyltoluene and 15wt% divinylbenzene) in the composite crosslinking monomer was 20:80. The preparation process was the same as in Example 1, with a first temperature of 85℃, a second temperature of 55℃, and a stirring and dispersion time of 1 hour.

[0039] Example 3

[0040] By weight, 60 parts of unsaturated polyester base resin, 35 parts of composite crosslinking monomer, 2.0 parts of high-efficiency film-forming inhibitor, 10 parts of MBS, 0.05 parts of polymerization inhibitor, and 0.5 parts of accelerator were weighed. The alkyd-acid molar ratio in the base resin was 1.15:1, maleic anhydride accounted for 50% of the total acid, and all saturated acids were adipic acid. The mass ratio of styrene to high-boiling-point monomers (95wt% vinyltoluene and 5wt% divinylbenzene) in the composite crosslinking monomer was 40:60. The preparation process was the same as in Example 1, with a first temperature of 95℃, a second temperature of 60℃, and a stirring and dispersion time of 2 hours.

[0041] Example 4

[0042] By weight, 55 parts of unsaturated polyester base resin, 40 parts of composite crosslinking monomer, 0.5 parts of high-efficiency film-forming inhibitor, 8 parts of MBS, 0.03 parts of polymerization inhibitor, and 0.3 parts of accelerator were weighed. In the synthesis of the base resin, propylene glycol, maleic anhydride, and adipic acid were specifically used in a molar ratio of 1.12:0.50:0.50. In the composite crosslinking monomer, the mass ratio of styrene to the high-boiling-point monomer was 35:65. The high-boiling-point monomer was composed of 88 wt% vinyltoluene and 12 wt% isobornyl acrylate, utilizing the internal plasticizing effect of IBOA to further improve toughness. The preparation process was the same as in Example 1.

[0043] Example 5

[0044] By weight, 45 parts of unsaturated polyester base resin, 50 parts of composite crosslinking monomer, 1.5 parts of high-efficiency film-forming inhibitor, 3 parts of MBS, 0.02 parts of polymerization inhibitor, and 0.2 parts of accelerator were weighed. The base resin used conventional raw materials, with a molar ratio of ethylene glycol, maleic anhydride, and phthalic anhydride of 1.08:0.65:0.35. In the composite crosslinking monomer, the mass ratio of styrene to the high-boiling-point monomer was 25:75, and the high-boiling-point monomer was a single vinyltoluene. During preparation, the first temperature was adjusted to 88℃, the second temperature was adjusted to 58℃, and the stirring time was shortened to 1 hour.

[0045] Example 6

[0046] By weight, 48 parts of unsaturated polyester base resin, 47 parts of composite crosslinking monomer, 1.2 parts of high-efficiency film-forming inhibitor, 4 parts of MBS, 0.04 parts of polymerization inhibitor, and 0.4 parts of accelerator were weighed. In the synthesis of the base resin, propylene glycol, maleic anhydride, and isophthalic acid were used in a molar ratio of 1.07:0.68:0.32 to obtain high reactivity. In the composite crosslinking monomer, the mass ratio of styrene to high-boiling-point monomer was 28:72. The high-boiling-point monomer was composed of 82 wt% vinyltoluene and 18 wt% divinylbenzene; increasing the DVB content accelerated the crosslinking rate. During the preparation process, special attention was paid to the complete dispersion of the film-forming inhibitor, and the stirring time was extended to 2 hours to ensure the formation of a perfect, defect-free surface.

[0047] Comparative Example 1

[0048] Except for the absence of a highly efficient film-forming inhibitor, the other components, proportions, and preparation process are exactly the same as in Example 1.

[0049] Comparative Example 2

[0050] Except for the absence of MBS, the other components, proportions, and preparation process are exactly the same as in Example 1.

[0051] To verify the actual effect of the present invention, key performance tests were conducted on the unsaturated polyester resins obtained in Examples 1-6 and Comparative Examples 1-2. The test results are shown in Table 1. The test methods are as follows:

[0052] Styrene volatilization reduction rate (%): Referring to industry standards, under the same temperature, humidity and surface area conditions, the amount of styrene volatilization of the resin of this invention and the conventional general-purpose unsaturated polyester resin with 35% styrene content were measured per unit time, and the reduction percentage was calculated.

[0053] Impact strength (kJ / m) 2 According to GB / T 1043.1-2008 standard, the impact strength of the cured specimen was tested using a cantilever beam impact testing machine.

[0054] Interlaminar shear strength (MPa): The interlaminar shear strength of fiberglass composites was tested by the short beam shear method according to JC / T 773-2010 standard.

[0055] Barcol Hardness: The hardness of the cured resin surface is determined using a Barcol hardness tester according to GB / T 3854-2017 standard.

[0056] Gel time (min, 25℃): The time from the addition of the accelerator-initiator system to the appearance of gel in the resin is determined according to GB / T 7193.6-1987 standard.

[0057] The detailed performance test results of each embodiment and comparative example are summarized in the table below:

[0058] Table 1 Summary of performance test results for each embodiment and comparative example

[0059]

[0060] As shown in Table 1, the embodiments of the present invention exhibited improvements in styrene volatilization reduction rate (≥65%) and impact strength (≥14kJ / m²). 2 Both core performance indicators were excellent, successfully achieving a balance between low volatility and high toughness. In contrast, the volatility reduction rate of Comparative Example 1 (lacking film-forming inhibitor) dropped sharply to 40%, demonstrating the crucial role of the inhibitor in building the surface barrier; the impact strength of Comparative Example 2 (lacking core-shell toughening agent) decreased significantly to 10 kJ / m. 2 This indicates that the toughening agent is key to achieving high toughness. All embodiments of the present invention maintained good processability (appropriate gel time) and basic mechanical properties (hardness and interlayer strength), fully verifying the effectiveness and superiority of the technical solution of the present invention.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An unsaturated polyester resin for open-mold manufacturing processes, characterized in that, It includes the following components: unsaturated polyester base resin, composite crosslinking monomer, film-forming inhibitor and core-shell toughening agent; The unsaturated polyester base resin is formed by polycondensation reaction of unsaturated diacid anhydride, flexible diacid and flexible diol in a molar ratio of 0.5~0.7:0.3~0.5:1.05~1.15; wherein the flexible diacid is selected from adipic acid or azelaic acid, and the flexible diol is selected from diethylene glycol or 1,2-propanediol. The composite crosslinking monomer comprises styrene and at least one high-boiling-point crosslinking monomer, wherein the mass ratio of styrene to the high-boiling-point crosslinking monomer is 20~40:60~80, and the high-boiling-point crosslinking monomer is selected from at least one of vinyltoluene, divinylbenzene, and isobornyl acrylate. The film-forming inhibitor is a long-chain alkyl-modified polysiloxane surfactant; The core-shell toughening agent is MBS resin or ACR type core-shell toughening agent.

2. The unsaturated polyester resin for open-mold manufacturing process according to claim 1, characterized in that, The unsaturated polyester resin comprises the following components in parts by weight: 40-60 parts of unsaturated polyester base resin; 35-55 parts of composite crosslinking monomer; 0.1-2.0 parts of film-forming inhibitor; 1-10 parts of core-shell toughening agent; 0.01-0.05 parts of polymerization inhibitor; Accelerator 0.1~0.5 parts.

3. The unsaturated polyester resin for open molding process according to claim 2, characterized in that, The high-boiling-point crosslinking monomer consists of 85wt% to 95wt% vinyltoluene and 5wt% to 15wt% divinylbenzene.

4. The unsaturated polyester resin for open molding process according to claim 1, characterized in that, The synthesis process of the long-chain alkyl-modified polysiloxane surfactant is as follows: Feeding and inerting: Metered amounts of hydrogen-containing silicone oil, long-chain terminal olefins, and solvents are added to the reactor, wherein the molar ratio of the long-chain terminal olefins to the Si-H bonds in the hydrogen-containing silicone oil is 1.05~1.10:1; Heating and catalysis: Heat the reaction mixture to 80℃~90℃, and add a platinum catalyst while stirring. The amount of platinum catalyst added, calculated as platinum metal, is 10ppm~50ppm of the total mass of the reactants. Addition reaction: Keep the temperature at 85℃~95℃ and stir continuously for 4h~8h. After the reaction is completed, cool the system to room temperature and remove the solvent and unreacted monomer by vacuum distillation to obtain the product.

5. A method for preparing an unsaturated polyester resin for an open molding process according to any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Synthesis of unsaturated polyester base resin: Under a protective atmosphere, the flexible diol, unsaturated diacid anhydride and the flexible diacid are mixed in proportion and subjected to a step-heating esterification reaction until the acid value reaches 15mgKOH / g~30mgKOH / g to obtain the unsaturated polyester base resin. (2) Dilution and mixing: Cool the base resin obtained in step (1) to the first temperature, add the polymerization inhibitor and the composite crosslinking monomer, and stir to mix evenly; (3) Additive dispersion: Cool the mixture obtained in step (2) to a second temperature, add the film-forming inhibitor, accelerator and core-shell toughening agent, stir and disperse evenly to obtain the unsaturated polyester resin; The first temperature is 30°C to 40°C higher than the second temperature.

6. The preparation method according to claim 5, characterized in that, The first temperature is 85℃~95℃, and the second temperature is 50℃~60℃.

7. The preparation method according to claim 5, characterized in that, The composite crosslinking monomer added in step (2) is a premix of styrene and a high-boiling-point crosslinking monomer.

8. The preparation method according to claim 5, characterized in that, The stirring and dispersion time in step (3) is 1h to 2h.

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