Modified polyester type low-shrinkage additive and preparation method thereof

Abstract

The invention relates to a preparation method of a modified polyester low-shrinkage additive, which comprises the following steps: (1) mixing polyol and a lactone monomer, heating to 120-150 DEG C in an inert atmosphere, dropwise adding a catalyst, carrying out reflux reaction after dropwise adding, and cooling to obtain multi-branch saturated polyester; (2) changing a terminal functional group through acylation reaction, dissolving the multi-branched-chain saturated polyester obtained in the step (1), an acylating agent and a base catalyst in a solvent, reacting under stirring and room temperature, settling after the reaction is finished, and purifying to obtain modified multi-branched-chain saturated polyester; and (3) dissolving the modified multi-branched-chain saturated polyester obtained in the step (2) in a diluent, adding a polymerization inhibitor, and stirring to obtain the modified polyester low shrinkage agent. Compared with a conventional polyester type low shrinkage agent, the modified polyester type low shrinkage agent has the characteristics of small addition amount, good compatibility and small color difference.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of high molecular compound synthesis, and particularly relates to a modified polyester low shrinkage agent for polyester molding compound, a preparation method and application thereof. BACKGROUND

[0002] Sheet molding compound (SMC) is a molding compound formed by mixing unsaturated polyester resin, thickening agent, initiator, crosslinking agent, low shrinkage additive, mold release agent, filler, etc. into a resin paste, then impregnating the resin paste into chopped glass fiber coarse sand or glass fiber mat, and covering the two sides with polyethylene or polypropylene. The prepared sheet material is usually further shaped by molding to prepare products, i.e. a certain amount of molding compound is loaded into a mold, and the molding compound is cured and shaped into a plastic product under the action of temperature and pressure. The molded product has the advantages of high product precision, good repeatability, smooth surface, complex structure that can be shaped at one time, no need for secondary processing, easy realization of mechanization and automation, and high production efficiency.

[0003] Generally, the unsaturated polyester resin will produce a volume shrinkage of 6-10% during the curing process, which causes a large internal stress of the product and has a negative impact on the dimensional stability and surface smoothness of the product. The reasons for the curing shrinkage of the unsaturated polyester resin mainly include three aspects: (1) during the curing process of the unsaturated polyester resin, crosslinking reaction occurs, the unsaturated double bond is opened, a saturated single bond is generated, the bond length changes, and the occupied volume decreases due to chemical reaction; (2) during the curing process, the molecular chain of the unsaturated polyester resin is improved in order, so that the molecular arrangement is compact and the "free volume" is reduced; (3) thermal shrinkage is caused by temperature change.

[0004] The volume shrinkage problem in the curing process of the unsaturated polyester resin can be solved by adding a low shrinkage agent. For example, the commonly used non-polar low shrinkage agent PE and PS have excellent colorability and can be used in molding compound products of various colors, and have a certain volume shrinkage control ability, and the shrinkage performance can be controlled to 0.3-0.5‰, which can meet the requirements of general products. The non-polar low shrinkage agent includes PMMA and PVC, and the polar low shrinkage agent includes polyvinyl acetate and saturated polyester resin. Such products have poor colorability, are prone to color difference after curing, affect the appearance of the product, and are prone to phase separation due to poor compatibility of the low shrinkage agent with the unsaturated resin, which affects the use. The modification of polyvinyl acetate (PVAC) is relatively common, and the modification of saturated polyester resin is rarely involved.

[0005] CN108503756A discloses a vinyl monomer modified polyester resin, a low shrinkage agent and a preparation method. The preparation method of the vinyl monomer modified polyester resin is as follows: a polyol, a saturated polybasic acid or anhydride and an unsaturated polybasic acid or anhydride are subjected to a polymerization reaction under the action of a catalyst to obtain an unsaturated polyester resin; a vinyl monomer is reacted with the unsaturated polyester resin under the action of an initiator. The preparation method of the low shrinkage agent is that the vinyl monomer modified polyester resin, styrene and a polymerization inhibitor are mixed. The low shrinkage agent prepared from the vinyl monomer modified polyester resin has excellent low shrinkage, toughness, excellent mechanical strength and water resistance, can significantly improve the coloring property of a product, solves the bleeding problem of the product, makes the product achieve an A-level surface effect, and has excellent thickening capacity, and can be used for effectively thickening various resins to make the product achieve an SMC / BMC construction viscosity.

[0006] CN112920334A discloses a preparation method of a low-shrinkage unsaturated polyester resin: a saturated diol is reacted with a saturated dibasic acid (anhydride) to obtain a double-end hydroxyl linear polyester; the double-end hydroxyl linear polyester is reacted with a diisocyanate to obtain a double-end isocyanate block copolymer; the double-end isocyanate block copolymer is reacted with a hydroxyl acrylate to obtain a resin base; and the resin base is mixed with a crosslinking monomer to obtain the low-shrinkage unsaturated polyester resin. The molar ratio of the saturated diol to the saturated dibasic acid (anhydride) is 2:1; the molar ratio of the diisocyanate to the saturated dibasic acid (anhydride) is 2:1; and the molar ratio of the hydroxyl acrylate to the saturated dibasic acid (anhydride) is (2-5):1. The application also provides a preparation method of the low-shrinkage unsaturated polyester resin, which can efficiently and simply obtain the low-shrinkage unsaturated polyester resin. SUMMARY

[0007] The application discloses a modified polyester low-shrinkage additive. The preparation process is that a multi-branched saturated polyester is synthesized through ring-opening polymerization of a saturated polyester, and the multi-branched polyester is end-capped through an acylation reaction to change end group functions and surface polarity. Compared with a common saturated polyester low-shrinkage additive, the modified polyester low-shrinkage additive has the characteristics of improved compatibility and uniform color distribution.

[0008] In one aspect of the application, a preparation method of a modified polyester low-shrinkage additive is disclosed, which comprises the following steps: (1) a polyol and an internal ester monomer are mixed, a catalyst is added dropwise under an inert atmosphere, the reaction is carried out under reflux after the dropwise addition is completed, and then the product is purified, dried and cooled to obtain a multi-branched saturated polyester; (2) by acylation reaction to change the end group functional group, the branched saturated polyester obtained in step (1), acylation agent, base catalyst are dissolved in a solvent, and the reaction is carried out under stirring and room temperature, after the reaction is completed, the sedimentation treatment is carried out, and the modified branched saturated polyester is obtained after purification; (3) the modified branched saturated polyester obtained in step (2) is dissolved in a diluent, and a polymerization inhibitor is added, and the modified polyester low shrinkage agent is obtained by stirring.

[0009] In step (1), the polyol is one or two of pentaerythritol, dipentaerythritol, glycerol, and ethylene glycol; and the lactone monomer is one of caprolactone and valerolactone.

[0010] Preferably, in step (1), the molar ratio of the polyol to the lactone is 1:50-1:400, and the reaction time is 6-24 h.

[0011] In step (1), the catalyst is one of stannous octoate, tin oxide, and aluminum isopropyl alcohol.

[0012] In step (2), the acylation agent is one of acryloyl chloride, benzoyl chloride, acetyl chloride, succinic anhydride, and carbonamide; the base catalyst is one of 4-dimethylaminopyridine (DMAP), triethylamine (TEA), and potassium carbonate (K2CO3); the molar ratio of the saturated polyester end group functional group (hydroxyl), the acylation agent, and the base catalyst is 1:1-1.5:0.1-2.0; the solvent is one of dichloromethane, trichloromethane, and dichloroethane; the reaction temperature is 20-50°C, and the reaction time is 6-24 h; and the sedimentation is carried out in one of isopropyl alcohol, methanol, ethanol, and petroleum ether.

[0013] In step (3), the diluent is one of styrene, benzyl methacrylate, and methyl methacrylate, the polymerization inhibitor is one of naphthaquinone, p-benzoquinone, catechol, and p-dihydroxybenzene; and the low shrinkage agent is prepared under stirring; the preparation temperature is 20-60°C, and the stirring time is 2-12 h.

[0014] In another aspect of the present application, a modified polyester low shrinkage additive prepared by the method of the present application is provided, and the amount of the modified polyester low shrinkage additive in the polyester molding compound is 5-20 wt%.

[0015] In another aspect of the present application, a BMC batch containing the modified polyester low shrinkage additive of the present application is disclosed, which comprises the following components: 50-100 parts of unsaturated polyester resin, 20-50 parts of modified polyester low shrinkage additive, 40-100 parts of glass fiber, 100-250 parts of calcium carbonate, 0.05-0.1 parts of polymerization inhibitor, 1-3 parts of initiator, 3-5 parts of internal release agent, 3-5 parts of thickening agent, and 4-15 parts of color paste.

[0016] In another aspect of the present application, a processing method of BMC material is disclosed, comprising the following steps: using a 200T press and a test flat mold to press the BMC material into a test flat with a thickness of 1-10 mm, a molding pressure of 100-150 T, an upper mold temperature of 145-150 DEG C, a lower mold temperature of 140-145 DEG C, and a pressure maintaining time of 120-360 s.

[0017] The prepared flat is placed at room temperature of 23±5 DEG C for 12-48 hours to ensure that the sample reaches a stable size state before testing, a vernier caliper with an accuracy of 0.02 mm is used to measure the centerline size of the mold cavity and the centerline size of the sample respectively, and the vernier caliper is perpendicular to the sample during measurement. According to the measured cavity size (L0) and sample size (L1), the product shrinkage (MS) is calculated, MS=(L0-L1) / L0*100%, each sample is tested at least three times, and the arithmetic mean is taken as the final result.

[0018] The prepared flat is tested for color difference using a color difference meter (Konica Minolta), and the compatibility of the low shrinkage agent with the resin and the color paste can be quantitatively judged by the color difference at different positions of the flat. The better the compatibility of the low shrinkage agent, the smaller the color difference value.

[0019] The prepared flat is processed into a tensile specimen, and a universal tensile testing machine is used to test the tensile strength of the product.

[0020] The main chemical reaction formula involved in the present application is as follows: The present application has the following beneficial effects: By selecting different types of polyol initiators, the number of branched chains of saturated polyester can be controlled, and when used as a low shrinkage agent, less polybranched polyester can be added to achieve equivalent anti-shrinkage performance compared to linear polyester.

[0021] By controlling the type and amount of initiator, the number of branched chains and the relative molecular weight can be adjusted, and saturated polyesters with different molecular weights and branched chain numbers exhibit different rheological properties, so that the formula with good rheological properties in the SMC molding process can be selected.

[0022] The terminal functional group of the conventional saturated polyester is hydroxyl, and the terminal functional group of the polyester can be capped by acylation reaction, and the terminal functional group is converted into methyl, carboxyl, amino, double bond and other functional groups, and the compatibility between the saturated polyester and the unsaturated polyester and the color paste is further optimized, so that the color of the SMC product is more uniform. DETAILED DESCRIPTION

[0023] ​In order to better understand the technical solutions of the present application, the present application includes but is not limited to the specific embodiments described below, and similar techniques and methods should be considered as falling within the scope of the present application. In order to make the technical problems, technical solutions and advantages of the present application clearer, the following will be described in detail in conjunction with specific embodiments.

[0024] Example 1 10 g of dipentaerythritol and 984 g of caprolactone were added to a three-necked flask, the bottom of which was heated by electricity, and the heating temperature was set to 120°C. 0.49 g of stannous octoate was added dropwise to the three-necked flask, which was sealed after nitrogen was introduced. Condensing equipment was connected, and the reaction was carried out for 12 h. After purification and rotary evaporation, poly-caprolactone powder was obtained. 100 g of the obtained poly-caprolactone powder, 1.88 g of acetyl chloride and 3.31 g of potassium carbonate (molar ratio 1:7.2:7.2) were dissolved in dichloromethane. Magnetic stirring was used, and the reaction was carried out at room temperature for 12 h. After the reaction was completed, petroleum ether was used for sedimentation treatment, and after purification and rotary evaporation, a modified multi-branched saturated polyester was obtained. 20 mg of the modified multi-branched saturated polyester sample was dissolved in 10 ml of tetrahydrofuran for GPC testing. The modified multi-branched saturated polyester was subjected to thermal analysis using DSC. The synthesized saturated polyester was dissolved in dichloromethane for NMR hydrogen spectrum testing. The polyester powder was dissolved in styrene, and p-benzoquinone was added as a polymerization inhibitor to prepare a saturated polyester low shrinkage agent. 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste and 4 parts of thickening agent were weighed and prepared into a BMC mass and subjected to curing. The cured BMC mass was pressed into a test sample plate using a flat plate mold and a 200T press, and the shrinkage and color difference were tested. Tensile specimens were machined from the sample plate, and the tensile strength was tested.

[0025] Example 2 Take 10 g dipentaerythritol, 984 g caprolactone into a three-necked flask, use electric heating at the bottom, set the heating temperature to 120 °C, add 0.49 g stannous octoate dropwise into the three-necked flask, seal after nitrogen is introduced. Connect the condensing equipment, react for 12 h, and obtain poly caprolactone powder after purification and rotary evaporation. Dissolve 200 g of the obtained poly caprolactone powder, 4.8 g succinic anhydride and 0.98 g 4-dimethylaminopyridine (molar ratio 1:7.2:1.2) in dichloromethane. Stir using a magnetic stirrer, and react for 12 h at room temperature. After the reaction is completed, use petroleum ether to perform sedimentation treatment, and obtain modified multi-branched saturated polyester after purification and rotary evaporation. Dissolve 20 mg of the modified multi-branched saturated polyester sample in 10 ml of tetrahydrofuran, and perform GPC testing. Perform thermal analysis on the synthesized modified multi-branched saturated polyester using DSC. Dissolve the synthesized saturated polyester in dichloromethane to perform NMR hydrogen spectrum testing. Dissolve the polyester powder in styrene, add p-benzoquinone as a polymerization inhibitor, and prepare a saturated polyester-based low shrinkage agent. Weigh 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent to prepare a BMC batch and perform curing. Use a flat mold and a 200T press to press the cured BMC batch into a test sample, test the shrinkage and color difference, and process the sample into a tensile specimen to test the tensile strength.

[0026] Example 3 Dissolve 100 g of the modified multi-branched saturated polyester obtained in Example 2 in dichloromethane, slowly add 3.3 g of triethanolamine (molar ratio of saturated polyester to triethanolamine is 1:6.6) dropwise under ice water bath, stir at room temperature for 30 min, and obtain poly caprolactone powder with an ammonium salt end group after rotary evaporation. Dissolve 20 mg of the sample in 10 ml of tetrahydrofuran, and perform GPC testing. Dissolve the polyester powder in styrene, add p-benzoquinone as a polymerization inhibitor, and prepare a saturated polyester-based low shrinkage agent. Weigh 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent to prepare a BMC batch and perform curing. Use a flat mold and a 200T press to press the cured BMC batch into a test sample, test the shrinkage and color difference, and process the sample into a tensile specimen to test the tensile strength.

[0027] Example 4 Into a three-necked flask, 5 g of ethylene glycol and 2016 g of caprolactone were added, and the bottom was heated by electricity, with the heating temperature set at 120°C. 1.0 g of stannous octoate was added dropwise into the flask, which was sealed after nitrogen was introduced. Condensing equipment was connected, and the reaction was carried out for 12 h. After purification and rotary evaporation, poly(caprolactone) powder was obtained. 100 g of the obtained poly(caprolactone) powder, 0.55 g of carbonamide, and 0.93 g of triethylamine (molar ratio 1:2.4:2.4) were dissolved in dichloromethane. The mixture was stirred using a magnetic stirrer, and the reaction was carried out at room temperature for 12 h. After the reaction was completed, petroleum ether was used for sedimentation treatment. After purification and rotary evaporation, a modified multi-branched saturated polyester was obtained. 20 mg of the modified multi-branched saturated polyester was dissolved in 10 ml of tetrahydrofuran for GPC testing. The synthesized saturated polyester was subjected to thermal analysis using DSC. The synthesized saturated polyester was dissolved in dichloromethane for NMR hydrogen spectrum testing. The polyester powder was dissolved in styrene, and p-benzoquinone was added as a polymerization inhibitor to prepare a saturated polyester low shrinkage agent. 70 parts of unsaturated polyester resin, 30 parts of the low shrinkage agent, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent were weighed and mixed to prepare a BMC batch, which was then aged. The aged BMC batch was pressed into a test sample plate using a flat mold and a 200T press, and the shrinkage and color difference were tested. The sample plate was processed into a tensile specimen for testing the tensile strength.

[0028] Example 5 Into a three-necked flask, 10 g of dipentaerythritol and 984 g of caprolactone were added, and the bottom was heated by electricity, with the heating temperature set at 120°C. 0.49 g of stannous octoate was added dropwise into the flask, which was sealed after nitrogen was introduced. Condensing equipment was connected, and the reaction was carried out for 12 h. After purification and rotary evaporation, poly(caprolactone) powder was obtained. 100 g of the obtained poly(caprolactone) powder, 0.6 g of acryloyl chloride, and 3.31 g of potassium carbonate (molar ratio 1:2:7.2) were dissolved in dichloromethane. The mixture was stirred using a magnetic stirrer, and the reaction was carried out at room temperature for 12 h. After the reaction was completed, petroleum ether was used for sedimentation treatment. After purification and rotary evaporation, a modified multi-branched saturated polyester was obtained. 20 mg of the modified multi-branched saturated polyester sample was dissolved in 10 ml of tetrahydrofuran for GPC testing. The modified multi-branched saturated polyester was subjected to thermal analysis using DSC. The synthesized saturated polyester was dissolved in dichloromethane for NMR hydrogen spectrum testing. The polyester powder was dissolved in styrene, and p-benzoquinone was added as a polymerization inhibitor to prepare a saturated polyester low shrinkage agent. 70 parts of unsaturated polyester resin, 30 parts of the low shrinkage agent, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent were weighed and mixed to prepare a BMC batch, which was then aged. The aged BMC batch was pressed into a test sample plate using a flat mold and a 200T press, and the shrinkage and color difference were tested. The sample plate was processed into a tensile specimen for testing the tensile strength.

[0029] Comparative Example 1 Into a three-necked flask, 10 g of pentaerythritol and 1838 g of caprolactone were added, and the bottom was electrically heated with the temperature set at 120°C. 0.92 g of stannous octoate was added dropwise into the flask, and the flask was sealed after nitrogen was introduced. Condensing equipment was connected, and the reaction was carried out for 12 hours. After purification and rotary evaporation, poly caprolactone powder was obtained. 20 mg of the sample was dissolved in 10 ml of tetrahydrofuran for GPC test. The synthesized saturated polyester was subjected to thermal analysis using DSC. The synthesized saturated polyester was dissolved in dichloromethane for NMR hydrogen spectrum test. The polyester powder was dissolved in styrene, and p-benzoquinone was added as a polymerization inhibitor to prepare a saturated polyester low shrinkage agent. A BMC batch was prepared from 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent, and was aged. The aged BMC batch was pressed into a test sample plate using a flat mold and a 200T press, and the shrinkage and color difference were tested. The sample plate was processed into a tensile specimen for testing the tensile strength.

[0030] Comparative Example 2 Into a three-necked flask, 10 g of dipentaerythritol and 984 g of caprolactone were added, and the bottom was electrically heated with the temperature set at 120°C. 0.49 g of stannous octoate was added dropwise into the flask, and the flask was sealed after nitrogen was introduced. Condensing equipment was connected, and the reaction was carried out for 12 hours. After purification and rotary evaporation, poly caprolactone powder was obtained. 20 mg of the sample was dissolved in 10 ml of tetrahydrofuran for GPC test. The synthesized saturated polyester was subjected to thermal analysis using DSC. The synthesized saturated polyester was dissolved in dichloromethane for NMR hydrogen spectrum test. The polyester powder was dissolved in styrene, and p-benzoquinone was added as a polymerization inhibitor to prepare a saturated polyester low shrinkage agent. A BMC batch was prepared from 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6 mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent, and was aged. The aged BMC batch was pressed into a test sample plate using a flat mold and a 200T press, and the shrinkage and color difference were tested. The sample plate was processed into a tensile specimen for testing the tensile strength.

[0031] Comparative Example 3 Put 10g dipentaerythritol, 394g caprolactone into a three-necked flask, use electric heating at the bottom, set the heating temperature to 120℃, add 0.16g stannous octoate dropwise into the three-necked flask, seal after nitrogen is bubbled in. Connect the condensing equipment, react for 12h, get the poly caprolactone powder after purification and rotary evaporation. Dissolve 20mg sample in 10ml tetrahydrofuran, and test by GPC. Use DSC to analyze the thermal properties of the synthesized saturated polyester. Dissolve the synthesized saturated polyester in dichloromethane for NMR hydrogen spectrum test. Dissolve the polyester powder in styrene, add p-benzoquinone as a polymerization inhibitor to make the saturated polyester low shrinkage agent. Weigh 70 parts of unsaturated polyester resin, 30 parts of the above low shrinkage resin, 50 parts of 6mm glass fiber, 200 parts of calcium carbonate, 3 parts of crosslinking agent, 0.08 parts of polymerization inhibitor, 1.5 parts of initiator, 4 parts of internal release agent, 5 parts of 7035 color paste, and 4 parts of thickening agent to prepare BMC mass and perform curing. Use a flat mold and a 200T press to press the cured BMC mass into test samples, test the shrinkage and color difference, and process the samples into tensile specimens to test the tensile strength.

[0032] Test the properties of the prepared examples 1-3 and comparative examples 1-3, the test items, test equipment and test standards are shown in Table 1, and the test results of each test item are shown in Table 2.

[0033] Table 1 Table 2 From the test results, it can be seen that the modified polyester low shrinkage agent prepared by the method described in the application has good shrinkage improvement performance, and the example samples have better color uniformity compared to the comparative examples.

[0034] Examples 2, 3 and 5 have good shrinkage and color uniformity, and have higher tensile strength compared to the comparative examples.

Claims

1. A method for preparing a modified polyester low-shrinkage additive, comprising the following steps: (1) Mix the polyol and the lactone monomer, heat to 120-150°C under an inert atmosphere, add the catalyst dropwise, reflux reaction after the addition is complete, and cool to obtain a multi-branched saturated polyester; (2) changing the terminal functional group by acylation reaction, dissolving the multi-branched saturated polyester obtained in step (1), an acylating agent, and an alkali catalyst in a solvent, reacting the product under stirring at room temperature, and subjecting the product to sedimentation treatment after the reaction is completed, and obtaining the modified multi-branched saturated polyester after purification; (3) The modified multi-branched saturated polyester obtained in step (2) is dissolved in a diluent, a polymerization inhibitor is added, and the mixture is stirred to obtain a modified polyester low shrinkage agent.

2. The method according to claim 1, characterized in that The polyol in step (1) is one or two of pentaerythritol, dipentaerythritol, propylene glycol, and ethylene glycol; and the lactone monomer is one of caprolactone and valerolactone.

3. The method according to claim 1, characterized in that The molar ratio of the polyol to the lactone in step (1) is 1:50-1:400, and the reaction time is 6-24 hours.

4. The method according to claim 1, characterized in that The catalyst in step (1) is one of stannous octoate, tin oxide, and aluminum isopropoxide.

5. The method according to claim 1, characterized in that The acylating agent in step (2) is one of acryloyl chloride, benzoyl chloride, acetyl chloride, succinic anhydride, and carbonamide; the base catalyst is one of 4-dimethylaminopyridine (DMAP), triethylamine (TEA), and potassium carbonate (K2CO3); the molar ratio of the saturated polyester functional end group (hydroxyl group), the acylating agent, and the base catalyst in step (2) is 1:1~1.5:0.1~2.0; and the solvent is one of dichloromethane, chloroform, and dichloroethane.

6. The method according to claim 1, characterized in that The reaction temperature in step (2) is 20-50° C., and the reaction time is 6-24 h. The precipitation is carried out in one of isopropanol, methanol, ethanol, and petroleum ether.

7. The method according to claim 1, characterized in that The diluent in step (3) is one of styrene, benzyl methacrylate, and methyl methacrylate, and the polymerization inhibitor is one of naphthoquinone, para-benzoquinone, catechol, and hydroquinone; the low shrinkage agent is prepared under strong stirring conditions; the preparation temperature is 20-60°C, and the stirring time is 2-12h.

8. A modified polyester low shrinkage additive prepared according to the method according to any one of claims 1 to 7, characterized in that: The amount of the modified polyester low shrinkage agent additive in the polyester molding compound is 5-20 wt %.

9. A BMC pellet comprising the following components: 50-100 parts of an unsaturated polyester resin, 20-50 parts of a modified polyester low-shrinkage additive, 40-100 parts of glass fiber, 100-250 parts of calcium carbonate, 0.05-0.1 parts of a polymerization inhibitor, 1-3 parts of an initiator, 3-5 parts of an internal mold release agent, 3-5 parts of a thickener, and 4-15 parts of a color paste; the modified polyester low-shrinkage resin is prepared by the method according to any one of claims 1 to 7.

10. A method for processing a BMC mass according to claim 9, comprising the following steps: The BMC mass is pressed into a test plate with a thickness of 1-10 mm using a 200T press and a test plate mold. The molding pressure is 100-150T, the upper mold temperature is 145-150℃, the lower mold temperature is 140-145℃, and the holding time is 120-360s.

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