Low heat shrinkage multilayer polyester material and method of making same

Abstract

This application relates to the field of polyester film technology, specifically disclosing a low-heat shrinkage-resistant multilayer polyester material and its preparation method. The material has a five-layer symmetrical structure: skin layer-transition layer-core layer-transition layer-skin layer. The skin layer contains furan-side-modified PET and an anti-blocking agent; the transition layer is a blend of two polyesters; and the core layer contains PET, a BMI-PET crosslinking agent, and phase change microcapsules. A thermally reversible dynamic covalent crosslinking network is formed between the layers via a Diels-Alder reaction, with the gel content gradient along the thickness direction. The preparation method includes raw material pretreatment, co-extrusion casting, biaxial stretching, ultrasonic-assisted dissociation, cooling crosslinking locking, and cooling winding. This material can be used in high-end fields such as automotive displays, flexible circuit board packaging, and aerospace, simultaneously achieving complete elimination of internal stress, unifying interlayer bonding strength with good flexibility, and significantly improving the high-temperature dimensional stability and long-term reliability of polyester films.

Description

Technical Field

[0001] This application relates to the field of polyester film technology, and more specifically, to a low-heat shrinkage-resistant multilayer polyester material and its preparation method. Background Technology

[0002] Biaxially oriented polyester film possesses excellent mechanical properties, optical transparency, and chemical stability, making it widely used in various fields such as electronics, new energy, and packaging printing. In high-end applications such as automotive displays, flexible circuit board packaging, and aerospace, polyester film needs to withstand long-term high-temperature environments, and its dimensional stability directly determines the operational reliability and service life of the end product. As a key basic material in these high-end fields, low-heat-shrinkage polyester film must meet increasingly stringent requirements for its core performance indicators, such as heat shrinkage rate, interlayer bonding strength, and flexibility.

[0003] In existing technologies, the main method for reducing the heat shrinkage rate of polyester films is to use a high-temperature relaxation setting process, which releases internal stress by partially relaxing the stretched and oriented molecular chains through heating. This method can only release some residual internal stress and cannot completely eliminate it. Furthermore, excessively high setting temperatures can lead to thermal oxidative degradation of the polyester matrix, resulting in decreased mechanical properties and increased yellowing. Some technologies introduce permanent crosslinking systems to improve interlayer bonding, but permanent crosslinking significantly reduces the material's flexibility, making the film prone to brittleness. Simultaneously, the uniformity of crosslinking density is difficult to control, and stress concentration easily occurs at the interfaces, leading to interlayer separation and dimensional instability degradation after long-term use. Currently, no technology can simultaneously achieve complete elimination of internal stress, strong interlayer bonding, and good flexibility, which has become a key technical bottleneck restricting further improvements in the performance of low-heat-shrinkage polyester films. Summary of the Invention

[0004] To address the challenge of simultaneously achieving complete elimination of internal stress, interlayer bonding strength, and good flexibility in existing technologies, this application provides a low-heat shrinkage-resistant multilayer polyester material and its preparation method.

[0005] In a first aspect, this application provides a low-heat shrinkage-resistant multilayer polyester material, employing the following technical solution: A low-heat shrinkage resistant multilayer polyester material is composed of a skin layer, a transition layer and a core layer in sequence. The skin layer is made of the following raw materials in parts by weight: 95-99 parts of polyethylene terephthalate modified with furan side groups and 0.6-2.5 parts of anti-blocking agent. The transition layer is a blend of polyethylene terephthalate modified with furan side groups and polyethylene terephthalate; The core layer is made of the following raw materials in parts by weight: 82-95 parts polyethylene terephthalate, 3-8 parts BMI-PET crosslinking agent, and 2-8 parts phase change microcapsules; The BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution that decreases from both sides to the center in the thickness direction of the core layer; the cortex, transition layer and core layer have a thermally reversible dynamic covalent crosslinking network formed by furan groups and maleimide groups through the Diels-Alder reaction; The multilayer polyester material has a five-layer symmetrical structure consisting of a skin layer, a transition layer, a core layer, a transition layer, and a skin layer.

[0006] By employing the above technical solution, a five-layer symmetrical gradient crosslinked structure system is constructed. The skin and transition layers introduce polyester segments containing furan side groups, while the core layer distributes maleimide-terminated crosslinking agents. During processing, a thermally reversible dynamic covalent crosslinked network spontaneously forms. The concentration gradient distribution of the crosslinking agent and phase change microcapsules within the core layer creates a high density of crosslinking points at the interface, establishing interlayer chemical bonding. The core layer maintains a lower crosslinking density, preserving the overall flexibility of the material. The dynamic crosslinked network undergoes reversible bonding and breaking with temperature changes during stretching and heat setting, achieving gradual release of tensile stress and permanent locking of a low-stress state, avoiding stress concentration at the interface caused by abrupt performance changes.

[0007] Preferably, the thickness ratio of the skin layer, transition layer, and core layer is 1:0.5:7 to 1.5:1:5, and the total thickness of the multilayer polyester material is 12 to 250 μm; the anti-blocking agent is a blend of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder in a weight ratio of 5:1 to 10:1, wherein the average particle size of the polymethyl methacrylate microspheres is 2.0 to 3.0 μm, and the average particle size of the low molecular weight polytetrafluoroethylene micropowder is 1.0 to 2.0 μm.

[0008] By adopting the above technical solution, the thickness ratio of each layer is matched with the crosslinking density gradient, so that stress is uniformly transmitted in the thickness direction, avoiding local stress overload. In the composite anti-blocking agent, polymethyl methacrylate microspheres form uniform micron-sized protrusions on the film surface, isolating adjacent film layers. Low molecular weight polytetrafluoroethylene micropowder fills the gaps between the protrusions, reducing the surface friction coefficient. The surface microtexture formed by the two together can prevent the adhesion of adjacent films during winding, and at the same time provide microscopic release channels for residual stress on the surface.

[0009] Preferably, the weight ratio of furan-side-group modified polyethylene terephthalate to polyethylene terephthalate in the transition layer is 40:60 to 60:40; the furan-side-group modified polyethylene terephthalate is a copolymer of terephthalic acid, ethylene glycol and 2,5-furandicarboxylic acid, wherein the 2,5-furandicarboxylic acid accounts for 5% to 15% of the total molar fraction of the dicarboxylic acid monomers, and the intrinsic viscosity of the furan-side-group modified polyethylene terephthalate is 0.62 to 0.70 dL / g.

[0010] By employing the above technical solution, the transition layer precisely controls the furan group content by adjusting the blending ratio of the two polyesters, achieving a continuous and smooth transition of crosslinking density from the skin layer to the core layer. The introduction of 2,5-furandicarboxylic acid is controlled within a reasonable range, ensuring sufficient reactive sites to form a dynamic crosslinking network while maintaining the compatibility of the modified polyester with conventional polyester, thus preserving the basic mechanical properties of the matrix material. Matching intrinsic viscosities ensures consistent melt flow rates within the co-extrusion die for each layer, preventing interlayer mixing or interface blurring.

[0011] Preferably, the gel content of the thermally reversible dynamic covalent crosslinked network is distributed in a gradient that gradually decreases from the skin layer through the transition layer to the core center; the gel content of the skin layer is 80 to 90%, the gel content of the transition layer is 30 to 60%, and the gel content of the core center is 5 to 15%.

[0012] By adopting the above technical solution, the gradient distribution of gel content is adapted to the functional requirements of each layer. The higher gel content in the skin layer improves surface hardness and scratch resistance, the moderate gel content in the transition layer acts as a stress buffer, and the lower gel content in the core layer maintains the material's ductility and impact resistance. This gradient cross-linking structure allows for the gradual buffering of thermal expansion differences in different regions during temperature cycling, reducing the risk of thermal deformation.

[0013] Preferably, the BMI-PET crosslinking agent is prepared by reacting polyester diol with maleic anhydride, wherein the number average molecular weight of the polyester diol is 2000 to 8000, and the polyester diol is one or more of polyethylene adipate diol, polybutylene adipate diol, and polyhexyl adipate diol; the phase change microcapsules use paraffin as the core material and modified silica as the shell material, with a phase change temperature of 80 to 110°C and an average particle size of 0.5 to 5 μm.

[0014] By adopting the above technical solution, the main chain of the BMI-PET crosslinking agent is a polyester structure, which has good compatibility with the polyethylene terephthalate matrix. During processing, it is uniformly dispersed in the matrix without phase separation or surface precipitation problems. Molecular weight control allows for precise control of the crosslinking network density. The phase change temperature of the phase change microcapsules matches the temperature range of the dynamic crosslinking reaction. During the heating phase, it absorbs heat to suppress local overheating within the film, and during the cooling phase, it releases heat to slow down the cooling rate, making the crosslinking reaction rate at different locations of the film more consistent, thus improving the uniformity of product performance. The modified silica shell material has good heat resistance and mechanical strength, and can withstand the shear forces during melt co-extrusion and stretching processes.

[0015] Secondly, this application provides a method for preparing a low-heat shrinkage-resistant multilayer polyester material, employing the following technical solution: A method for preparing a low-heat shrinkage-resistant multilayer polyester material includes the following steps: S1. Raw material pretreatment: Place each layer of raw material in a vacuum drying oven to dry, and control the moisture content to not exceed 30 ppm. S2, melt co-extrusion casting: The dried skin, transition layer and core layer raw materials are fed into three independent twin-screw extruders for melting and plasticizing. The downstream of the homogenization section of the extruder corresponding to the core layer is equipped with an auxiliary side feed port. The melt flows to the chill roll after being combined through five co-extrusion dies to form an unstretched thick sheet. S3. Longitudinal stretching: After preheating the unstretched thick sheet by a preheating roller, longitudinal stretching is performed to obtain a longitudinally stretched film. S4. Transverse stretching: After the longitudinally stretched film is preheated in the preheating section of the tenter frame, it is stretched transversely to obtain a biaxially stretched film. S5. Ultrasonic-assisted dissociation: The biaxially stretched film is heated and ultrasonic waves are applied to assist in the dissociation of the dynamic cross-linked network, causing local conformational rearrangement of molecular chain segments. S6. Cooling and Crosslinking Locking: The temperature is slowly reduced at a set rate, allowing the dynamic crosslinking network to reform under relaxed internal stress. S7. Cooling and winding: The film is rapidly cooled to room temperature, then pulled and wound up before being slit into finished products of specified specifications.

[0016] By adopting the above technical solution, a dynamic control process is established for the entire process, including pre-crosslinking formation, partial dissociation during stretching, complete dissociation with ultrasound assistance, and cooling and re-locking. During the molten casting stage, a preliminary pre-crosslinking network spontaneously forms, providing interlayer bonding force for the stretching process. During biaxial stretching, the pre-crosslinking network partially dissociates, releasing the internal stress generated during stretching in real time. The ultrasound-assisted stage accelerates the uniform dissociation of crosslinking points, promoting local rearrangement of molecular chain segments. The cooling stage allows the dissociated groups to re-bond under relaxed internal stress, achieving full control of internal stress from generation to release to locking, thus solving the thermal shrinkage problem of polyester films at the molecular level.

[0017] Preferably, in step S2, the extruder temperatures of the skin layer, transition layer, and core layer are all controlled at 265 to 280°C; the BMI-PET crosslinking agent and phase change microcapsules are pulsedly injected through the auxiliary agent side feed port at a pulse frequency of 1 to 5 Hz, in conjunction with the static mixer at the end of the extruder, so that the BMI-PET crosslinking agent and phase change microcapsules are distributed in a concentration gradient decreasing from both sides to the center in the thickness direction of the core layer; the die temperature is 270 to 280°C, the chiller roller temperature is 25 to 35°C, and the thickness of the unstretched sheet is 1.0 to 2.5 mm.

[0018] By adopting the above technical solutions, the matching of temperatures in each extruder ensures that the melt in each layer has a similar viscosity, avoiding interlayer flow instability during co-extrusion. Pulsed side feeding combined with the shearing action of the static mixer causes periodic concentration fluctuations of the crosslinking agent and microcapsules in the melt flow direction. After laminar diffusion, this is transformed into a continuous gradient distribution in the thickness direction. No additional co-extrusion layers are needed; the concentration gradient distribution within the core layer can be achieved using only a single extruder. The low-temperature environment of the chiller roll rapidly cools the melt, simultaneously triggering the Diels-Alder reaction between furan groups and maleimide groups, forming a moderately crosslinked network. This also inhibits premature crystallization of the polyester molecular chains, ensuring the smooth progress of the subsequent stretching process.

[0019] Preferably, in step S3, the preheating temperature is 80 to 90°C, the preheating time is 20 to 40 seconds, and the longitudinal stretching ratio is 3.0 to 3.8 times; in step S4, the preheating temperature is 100 to 120°C, the preheating time is 30 to 50 seconds, and the transverse stretching ratio is 3.2 to 4.0 times.

[0020] By adopting the above technical solution, step-by-step preheating ensures a uniform increase in film temperature, avoiding uneven stretching caused by localized temperature differences. The set stretch ratio ensures sufficient orientation of the polyester main chain, guaranteeing the material's mechanical properties. During stretching, the pre-crosslinked network undergoes partial reversible dissociation under stress, releasing the internal stress generated during stretching in real time. This avoids interlayer slippage as in the absence of crosslinking, and also prevents tensile fracture caused by excessive crosslinking. Residual crosslinking points maintain interlayer bonding, preventing interlayer slippage and delamination during stretching.

[0021] Preferably, in step S5, the heating temperature is 120 to 135°C, the film tension is maintained or a lateral shrinkage of 3 to 10 cm is allowed, and the processing time is 10 to 60 s; the ultrasonic frequency is 20 to 40 kHz, and the power density is 0.5 to 5 W / cm².

[0022] By employing the above technical solution, the heating temperature is controlled within the efficient range of the Diels-Alder reverse reaction, enabling the dynamic crosslinking points to dissociate. The mechanical vibration effect of ultrasound generates local energy input within the film, and this energy only acts on the low-bond-energy dynamic covalent bonds, without affecting the high-bond-energy covalent bonds of the polyester backbone. Therefore, it does not reduce the mechanical strength of the material, allowing the molecular chain segments to undergo local conformational rearrangement while maintaining the backbone orientation, thus fully releasing the residual internal stress accumulated during the stretching process.

[0023] Preferably, in step S6, the temperature is reduced to 80 to 100°C at a rate of 0.5 to 2°C / min and held for 15 to 30 min; in step S7, the cooling rate is not less than 30°C / s and the winding tension gradient is set to 10% to 15%.

[0024] By employing the above technical solution, the slow cooling rate provides sufficient time for the Diels-Alder forward reaction, allowing the dynamic cross-linked network to fully form under stress-free or low-stress conditions, permanently fixing the relaxed conformation of the molecular chains. Rapid cooling quickly fixes the formed low-stress cross-linked structure, preventing the molecular chains from re-orienting during the cooling process. The setting of the winding tension gradient ensures that the tension of the film gradually decreases from the inside to the outside during winding, avoiding the accumulation of internal stress caused by excessive winding, while ensuring the flatness of the winding and preventing edge curling or loose winding.

[0025] In summary, this application has the following beneficial effects: 1. This application adopts a five-layer symmetrical structure of skin layer, transition layer, core layer, transition layer, and skin layer. A thermally reversible dynamic covalent cross-linked network is formed through the Diels-Alder reaction of furan groups and maleimide groups. The gel content is distributed in a gradient along the thickness direction, which realizes the complete elimination of internal stress, while taking into account the interlayer bonding force and core layer flexibility.

[0026] 2. In this application, a pulsed side-feeding process is preferred, which enables the BMI-PET crosslinking agent and phase change microcapsules to form a concentration gradient distribution in the core layer thickness direction. Combined with the thermal buffering effect of the phase change microcapsules, the uniformity of the crosslinking reaction is ensured, and the consistency and long-term stability of product performance are improved.

[0027] 3. The method of this application selectively accelerates the breaking process of Diels-Alder crosslinking points by applying ultrasonic-assisted dynamic crosslinking network dissociation during the heat setting stage, without destroying the orientation structure of the polyester main chain, thus achieving a good balance between full release of internal stress and maintenance of mechanical properties.

[0028] 4. The preparation method of this application adopts a dynamic control process of pre-crosslinking to form stretching, dissociation and then locking throughout the entire process. It does not require large-scale modification of the existing biaxially oriented polyester film production line. It has good process compatibility, high production efficiency and can stably prepare high-performance low heat shrinkage polyester materials.

[0029] 5. In this application, a composite anti-blocking agent consisting of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder is preferably used to form a uniform nanoscale texture on the film surface, thereby simultaneously achieving excellent anti-blocking performance and releasing residual stress on the surface, and improving the processing performance of the film. Attached Figure Description

[0030] Figure 1 This is a flowchart of a method for preparing a low-heat shrinkage-resistant multilayer polyester material provided in this application. Detailed Implementation

[0031] The present application will be further described in detail below with reference to embodiments and comparative examples. Unless otherwise specified, the experimental methods used below are conventional methods. Unless otherwise specified, the materials, reagents, methods and instruments used are all conventional materials, reagents, methods and instruments in the art, which can be obtained by those skilled in the art through commercial channels or prepared according to literature methods.

[0032] Technical Concept: Current technologies for reducing the thermal shrinkage rate of polyester films primarily rely on high-temperature relaxation and setting processes. This process can only release some residual internal stress through the physical relaxation of molecular chains, but it cannot completely eliminate the internal stress frozen during the stretching and orientation process. Furthermore, excessively high setting temperatures can trigger thermal oxidative degradation of the polyester matrix, leading to a decline in the material's mechanical properties. Some technologies introduce permanent crosslinking systems to improve interlayer bonding; however, permanent crosslinking is an irreversible structure, and uniform crosslinking significantly reduces the material's flexibility. Simultaneously, abrupt changes in crosslinking density at the interface can cause stress concentration, easily leading to interlayer separation and dimensional instability degradation after long-term use. Essentially, this stems from a lack of dynamic stress regulation mechanisms at the molecular level.

[0033] This application constructs a five-layer symmetrical gradient structure, introducing polyester segments containing furan side groups into the cortex and transition layers. The core layer employs a pulse-side feeding process to achieve a concentration gradient distribution of the crosslinking agent and phase change microcapsules, forming a thermally reversible dynamic covalent crosslinking network that gradually decreases along the thickness direction. Simultaneously, a complete process is implemented, encompassing pre-crosslinking formation, stretching partial dissociation, ultrasonic-assisted complete dissociation, and cooling re-locking. Utilizing the reversibility of the Diels-Alder reaction, the entire process of internal stress control—from generation to release and then to locking in a low-stress state—is achieved.

[0034] The following are the main raw materials and reagents used in the preparation examples, embodiments, and comparative examples, and their sources and specifications are as follows; unless otherwise specified, all reagents are commercially available analytical grade or higher products: 1. Polyethylene terephthalate was purchased from Shanghai Yuanye Biotechnology Co., Ltd., item number: T25309; 2. 2,5-Furandicarboxylic acid was purchased from Hubei Jiahuixingcheng Biotechnology Co., Ltd., product number: JHXC; 3. Ethylene glycol was purchased from Shandong Changxing Plastic Additives Co., Ltd., CAS: 107-21-1; 4. Antimony trioxide was purchased from Shanghai Yuanye Biotechnology Co., Ltd., product number: S24327; 5. Trimethyl phosphate was purchased from Shanghai Yuanye Biotechnology Co., Ltd., product number: S24230; 6. Polyethylene adipate diol was purchased from Hubei Guangao Biotechnology Co., Ltd., product number: GA5138; 7. Polybutylene adipate diol was purchased from Jinjinle Chemical Co., Ltd., molecular weight: 2000; 8. Polyhexamethylene adipate diol was purchased from Nantong Zhonghe Chemical New Materials Co., Ltd., item number: 1006236; 9. Maleic anhydride was purchased from Shandong Xinheng Chemical Co., Ltd., grade: industrial. 10. p-Toluenesulfonic acid was purchased from Changzhou Junchi Chemical Co., Ltd., product number: JC28; 11. Sodium dodecylbenzenesulfonate was purchased from Shanghai Tongyuan Chemical Co., Ltd., grade: industrial. 12. Ethyl orthosilicate was purchased from Shandong Yushuo Chemical Co., Ltd., CAS: 78-10-4; 13. Silane coupling agent KH560 was purchased from Shanghai Yuanye Biotechnology Co., Ltd., product number: S15029; 14. Polymethyl methacrylate microspheres were purchased from Wenzhou Pinzhuo Biotechnology Co., Ltd., with a purity of 99%.

[0035] Example 1: This example provides a low-heat shrinkage resistant multilayer polyester material, which is composed of a skin layer, a transition layer, and a core layer in sequence, and has a five-layer symmetrical structure of skin layer-transition layer-core layer-transition layer-skin layer; it is made from the following raw materials in parts by weight: Skin layer: 97 parts of polyethylene terephthalate modified with furan side groups, 1.55 parts of anti-blocking agent; Transition layer: A blend of polyethylene terephthalate and polyethylene terephthalate with furan side group modification; Core layer: 88.5 parts polyethylene terephthalate, 5.5 parts BMI-PET crosslinking agent, and 5 parts phase change microcapsules; The BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution that decreases from both sides to the center in the thickness direction of the core layer; the cortex, transition layer and core layer, as well as their interiors, have a thermally reversible dynamic covalent crosslinking network formed by furan groups and maleimide groups through the Diels-Alder reaction.

[0036] The thickness ratio of the skin layer, transition layer, and core layer is 1.25:0.75:6, and the total thickness of the multilayer polyester material is 131 μm. The anti-blocking agent is a blend of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder in a weight ratio of 7.5:1. The average particle size of the polymethyl methacrylate microspheres is 2.5 μm, and the average particle size of the low molecular weight polytetrafluoroethylene micropowder is 1.5 μm. The transition layer contains furan-side-group modified polyethylene terephthalate in a weight ratio of 50:50. The furan-side-group modified polyethylene terephthalate is a copolymer of terephthalic acid, ethylene glycol, and 2,5-furandicarboxylic acid, wherein the 2,5-furandicarboxylic acid... Formic acid accounts for 10% of the total molar fraction of dicarboxylic acid monomers, and the intrinsic viscosity of the furan-modified polyethylene terephthalate is 0.66 dL / g. The gel content of the thermally reversible dynamic covalent crosslinking network decreases gradually from the skin layer through the transition layer to the core center, with a gel content of 85% in the skin layer, 45% in the transition layer, and 10% in the core center. The BMI-PET crosslinking agent is prepared by reacting polyester diol with maleic anhydride, and the polyester diol is polyethylene adipate diol with a number average molecular weight of 5000. The phase change microcapsules use paraffin as the core material and modified silica as the shell material, with a phase change temperature of 95℃ and an average particle size of 2.75 μm.

[0037] The poly(ethylene terephthalate) modified with furan side groups was prepared as follows: 100 moles of terephthalic acid, 120 moles of ethylene glycol, 10 moles of 2,5-furandicarboxylic acid, and antimony trioxide catalyst were added to an esterification reactor. The amount of catalyst was 0.04% of the mass of terephthalic acid. The esterification reaction was carried out at 240°C under nitrogen protection, with an esterification pressure of 0.25 MPa and a reaction time of 3 hours. After the esterification rate reached 96%, the mixture was transferred to a polycondensation reactor. Trimethyl phosphate stabilizer was added to the polycondensation reactor. The amount of stabilizer was 0.03% of the mass of terephthalic acid. The polycondensation reaction was carried out at 277°C and a vacuum degree of 80 Pa for 2.5 hours. The reaction was stopped when the intrinsic viscosity reached 0.66 dL / g. The product was obtained by casting and pelletizing.

[0038] The BMI-PET crosslinking agent was prepared as follows: Polyethylene adipate diol with a number average molecular weight of 5000 was added to a reaction vessel and heated to 130°C under nitrogen protection, followed by vacuum dehydration for 1.5 hours; the temperature was lowered to 90°C, and maleic anhydride was added, with a molar ratio of maleic anhydride to hydroxyl groups in the polyester diol of 2.1:1, and the mixture was stirred for 1.5 hours; toluene was added as an azeotropic dehydrating agent, and the temperature was raised to reflux temperature, and a dehydration and ring-closing reaction was carried out in the presence of the acid catalyst p-toluenesulfonic acid, with the catalyst amount being 0.75% of the mass of maleic anhydride, and the reaction time being 5 hours; after the reaction, the product was washed three times with deionized water to remove the catalyst and unreacted maleic anhydride, and toluene was removed by vacuum distillation. The product was then vacuum dried at 80°C for 8 hours to obtain a product with maleimide groups at both ends, with a yield of 94%.

[0039] The phase change microcapsules were prepared as follows: 40 parts by weight of paraffin wax with a phase change temperature of 95℃ were heated to complete melting, 2 parts by weight of sodium dodecylbenzenesulfonate emulsifier and 125 parts by weight of deionized water were added, and the mixture was emulsified at 10,000 rpm at 65℃ for 15 minutes to obtain a paraffin wax emulsion; 20 parts by weight of tetraethyl orthosilicate and 2 parts by weight of silane coupling agent KH560 were dissolved in 25 parts by weight of ethanol and slowly added dropwise to the paraffin wax emulsion at a dropping rate of 0.75 mL / min. During the dropping process, the mixture was stirred at 400 rpm and the pH was maintained at 3.5. After the dropping was completed, the mixture was stirred and reacted for 5 hours; the reaction product was filtered, washed with deionized water until neutral, and dried at 60℃ for 12 hours to obtain the final product.

[0040] The anti-adhesion additive is prepared as follows: polymethyl methacrylate microspheres with an average particle size of 2.5 μm and low molecular weight polytetrafluoroethylene micro powder with an average particle size of 1.5 μm are added to a high-speed mixer at a weight ratio of 7.5:1 and mixed at 400 rpm for 7.5 minutes at room temperature to obtain the product.

[0041] The preparation method of the above-mentioned low-heat shrinkage resistant multilayer polyester material includes the following steps: S1. Raw material pretreatment: Place each layer of raw material in a vacuum drying oven to dry, and control the moisture content to not exceed 30 ppm.

[0042] The drying temperature is 130℃, the drying time is 5 hours, and nitrogen gas is continuously introduced during the drying process to prevent the raw materials from oxidizing and degrading.

[0043] S2, Melt Co-extrusion Casting: The dried skin, transition layer and core layer raw materials are fed into three independent twin-screw extruders for melting and plasticizing. The downstream of the homogenization section of the extruder corresponding to the core layer is equipped with an auxiliary side feed port. The melt is drawn into the five-layer co-extrusion die and then cast to the chiller roller to form an unstretched thick sheet.

[0044] The extruder temperature for the skin, transition layer, and core layer is controlled at 272.5℃, and the extruder screw speed is 300rpm. The BMI-PET crosslinking agent and phase change microcapsules are injected in a pulsed manner through the feed port on the excipient side at a pulse frequency of 3Hz. Combined with the static mixer at the end of the extruder, the concentration of the BMI-PET crosslinking agent and phase change microcapsules is distributed in a gradient from both sides to the center in the thickness direction of the core layer. The die temperature is 275℃, and the quench roll temperature is 30℃. The thickness of the unstretched sheet is 1.75mm. During the cooling process of the quench roll, the furan groups in the skin and transition layer spontaneously undergo a Diels-Alder cycloaddition reaction with the maleimide groups that migrate to the interface in the core layer, initially forming a thermally reversible dynamic covalent crosslinking network. The gel content is distributed in a gradient that gradually decreases from the skin to the center of the core layer.

[0045] S3. Longitudinal stretching: After preheating the unstretched thick sheet by a preheating roller, longitudinal stretching is performed to obtain a longitudinally stretched film.

[0046] The preheating temperature is 85℃, the preheating time is 30s, and the longitudinal stretching ratio is 3.4 times. During the stretching process, the pre-crosslinked network undergoes partial reverse reaction dissociation to release the tensile stress, and the residual crosslinking points maintain interlayer bonding to prevent delamination.

[0047] S4. Transverse stretching: After the longitudinally stretched film is preheated in the preheating section of the tenter frame, it is stretched transversely to obtain a biaxially stretched film.

[0048] The preheating temperature was 110℃, the preheating time was 40s, and the transverse stretching ratio was 3.6 times. During the stretching process, the pre-crosslinked network continued to maintain a partially crosslinked state to prevent interlayer slippage of the film during transverse stretching.

[0049] S5. Ultrasonic-assisted dissociation: The biaxially stretched film is heated and ultrasonic waves are applied to assist in the dissociation of the dynamic cross-linked network, causing local conformational rearrangement of molecular chain segments.

[0050] The heating temperature was 127.5℃, maintaining film tension or allowing a 6.5cm lateral shrinkage, and the processing time was 35s; the ultrasonic frequency was 30kHz, and the power density was 2.75W / cm²; the ultrasonic waves were applied by an array of ultrasonic transducers arranged in the width direction of the film, with a transducer spacing of 10cm, selectively accelerating the dissociation of Diels-Alder crosslinking points without damaging the polyester main chain structure; the phase change microcapsules in the core layer absorbed heat, inhibiting the excessively rapid rise of local temperature inside the core layer, so that the dissociation process of crosslinking points proceeded uniformly throughout the thickness direction of the core layer.

[0051] S6. Cooling and Crosslinking Locking: The temperature is slowly reduced at a set rate to allow the dynamic crosslinking network to reform under relaxed internal stress.

[0052] The temperature was lowered to 90℃ at a rate of 1.25℃ / min and held for 22.5min. During this stage, the phase change microcapsules released their stored latent heat, slowing down the cooling rate and providing a sufficient time window for the Diels-Alder forward reaction.

[0053] S7. Cooling and winding: The film is rapidly cooled to room temperature, then pulled and wound up before being slit into finished products of specified specifications.

[0054] The cooling rate is no less than 30℃ / s, and rapid cooling is achieved by combining air cooling and water cooling; the winding tension gradient is set to 12.5%, and the winding speed is 80m / min; the anti-adhesion agent in the skin layer forms a nanoscale texture on the film surface, providing anti-adhesion and releasing residual stress on the surface.

[0055] Example 2: This example provides a low-heat shrinkage resistant multilayer polyester material, which is composed of a skin layer, a transition layer, and a core layer in sequence, and has a five-layer symmetrical structure of skin layer-transition layer-core layer-transition layer-skin layer; it is made from the following raw materials in parts by weight: Skin layer: 95 parts of polyethylene terephthalate modified with furan side groups, and 0.6 parts of anti-blocking agent; Transition layer: A blend of polyethylene terephthalate and polyethylene terephthalate with furan side group modification; Core layer: 82 parts polyethylene terephthalate, 3 parts BMI-PET crosslinking agent, 2 parts phase change microcapsules; The BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution that decreases from both sides to the center in the thickness direction of the core layer; the cortex, transition layer and core layer, as well as their interiors, have a thermally reversible dynamic covalent crosslinking network formed by furan groups and maleimide groups through the Diels-Alder reaction.

[0056] The thickness ratio of the skin layer, transition layer, and core layer is 1:0.5:7, and the total thickness of the multilayer polyester material is 12 μm. The anti-blocking agent is a blend of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder in a weight ratio of 5:1, with the polymethyl methacrylate microspheres having an average particle size of 2.0 μm and the low molecular weight polytetrafluoroethylene micropowder having an average particle size of 1.0 μm. In the transition layer, the weight ratio of furan-side-group modified polyethylene terephthalate to polyethylene terephthalate is 40:60. The furan-side-group modified polyethylene terephthalate is a copolymer of terephthalic acid, ethylene glycol, and 2,5-furandicarboxylic acid, wherein the 2,5-furandicarboxylic acid... The intrinsic viscosity of the furan-modified polyethylene terephthalate (PET) containing 5% of the total dicarboxylic acid monomers is 0.62 dL / g. The gel content of the thermally reversible dynamic covalent crosslinked network decreases gradually from the skin layer through the transition layer to the core center, with 80% gel content in the skin layer, 30% in the transition layer, and 5% in the core center. The BMI-PET crosslinking agent is prepared by reacting polyester diol with maleic anhydride, and the polyester diol is polybutylene adipate diol with a number average molecular weight of 2000. The phase change microcapsules use paraffin as the core material and modified silica as the shell material, with a phase change temperature of 80℃ and an average particle size of 0.5 μm.

[0057] The poly(ethylene terephthalate) modified with furan side groups was prepared as follows: 100 moles of terephthalic acid, 110 moles of ethylene glycol, 5 moles of 2,5-furandicarboxylic acid, and antimony trioxide catalyst were added to an esterification reactor. The amount of catalyst was 0.03% of the mass of terephthalic acid. The esterification reaction was carried out at 230°C under nitrogen protection, with an esterification pressure of 0.2 MPa and a reaction time of 2 hours. After the esterification rate reached 95%, the mixture was transferred to a polycondensation reactor. Trimethyl phosphate stabilizer was added to the polycondensation reactor. The amount of stabilizer was 0.02% of the mass of terephthalic acid. The polycondensation reaction was carried out at 270°C and a vacuum degree of 90 Pa for 2 hours. The reaction was stopped when the intrinsic viscosity reached 0.62 dL / g. The product was obtained by casting and pelletizing.

[0058] The BMI-PET crosslinking agent was prepared as follows: Polybutylene adipate diol with a number average molecular weight of 2000 was added to a reaction vessel and heated to 120°C under nitrogen protection, and vacuum dehydrated for 1 hour; the temperature was lowered to 80°C, maleic anhydride was added, and the molar ratio of maleic anhydride to hydroxyl groups in the polyester diol was 2.0:1, and the mixture was stirred for 1 hour; toluene was added as an azeotropic dehydrating agent, and the temperature was raised to reflux temperature, and a dehydration and ring-closing reaction was carried out in the presence of the acid catalyst p-toluenesulfonic acid, with the catalyst amount being 0.5% of the mass of maleic anhydride, and the reaction time being 4 hours; after the reaction, the product was washed twice with deionized water to remove the catalyst and unreacted maleic anhydride, and toluene was removed by vacuum distillation. The product was then vacuum dried at 80°C for 8 hours to obtain a product with maleimide groups at both ends, with a yield of 92%.

[0059] The phase change microcapsules were prepared as follows: 30 parts by weight of paraffin wax with a phase change temperature of 80℃ were heated until completely melted, 1 part by weight of sodium dodecylbenzenesulfonate emulsifier and 100 parts by weight of deionized water were added, and the mixture was emulsified at 60℃ and 8000 rpm for 10 minutes to obtain a paraffin wax emulsion; 15 parts by weight of tetraethyl orthosilicate and 1 part by weight of silane coupling agent KH5601 were dissolved in 20 parts by weight of ethanol and slowly added dropwise to the paraffin wax emulsion at a dropping rate of 0.5 mL / min. During the dropping process, the mixture was stirred at 300 rpm and the pH was maintained at 3. After the dropping was completed, the mixture was stirred and reacted for 4 hours; the reaction product was filtered, washed with deionized water until neutral, and dried at 60℃ for 12 hours to obtain the final product.

[0060] The anti-adhesion additive is prepared as follows: polymethyl methacrylate microspheres with an average particle size of 2.0 μm and low molecular weight polytetrafluoroethylene micro powder with an average particle size of 1.0 μm are added to a high-speed mixer at a weight ratio of 5:1 and mixed at 300 rpm for 5 minutes at room temperature to obtain the product.

[0061] The preparation method of the above-mentioned low-heat shrinkage resistant multilayer polyester material includes the following steps: S1. Raw material pretreatment: Place each layer of raw material in a vacuum drying oven to dry, and control the moisture content to not exceed 30 ppm.

[0062] The drying temperature was 120℃, the drying time was 4 hours, and nitrogen gas was continuously introduced for protection during the drying process.

[0063] S2, Melt Co-extrusion Casting: The dried skin, transition layer and core layer raw materials are fed into three independent twin-screw extruders for melting and plasticizing. The downstream of the homogenization section of the extruder corresponding to the core layer is equipped with an auxiliary side feed port. The melt is drawn into the five-layer co-extrusion die and then cast to the chiller roller to form an unstretched thick sheet.

[0064] The extruder temperature for the skin, transition layer, and core layer is controlled at 265℃, and the extruder screw speed is 250rpm. The BMI-PET crosslinking agent and phase change microcapsules are injected in a pulsed manner through the feed port on the excipient side at a pulse frequency of 1Hz. Combined with the static mixer at the end of the extruder, the concentration of the BMI-PET crosslinking agent and phase change microcapsules in the core layer thickness direction decreases from both sides to the center. The die temperature is 270℃, and the quench roll temperature is 25℃. The thickness of the unstretched sheet is 1.0mm. During the quench roll cooling process, the furan groups in the skin and transition layer spontaneously undergo a Diels-Alder cycloaddition reaction with the maleimide groups that migrate to the interface in the core layer, initially forming a thermally reversible dynamic covalent crosslinking network. The gel content decreases gradually from the skin to the center of the core layer.

[0065] S3. Longitudinal stretching: After preheating the unstretched thick sheet by a preheating roller, longitudinal stretching is performed to obtain a longitudinally stretched film.

[0066] The preheating temperature is 80℃, the preheating time is 20s, and the longitudinal stretching ratio is 3.0 times. During the stretching process, the pre-crosslinked network undergoes partial reverse reaction dissociation to release the tensile stress, and the residual crosslinking points maintain interlayer bonding to prevent delamination.

[0067] S4. Transverse stretching: After the longitudinally stretched film is preheated in the preheating section of the tenter frame, it is stretched transversely to obtain a biaxially stretched film.

[0068] The preheating temperature is 100℃, the preheating time is 30s, and the transverse stretching ratio is 3.2 times. During the stretching process, the pre-crosslinked network continues to maintain a partially crosslinked state to prevent interlayer slippage of the film during transverse stretching.

[0069] S5. Ultrasonic-assisted dissociation: The biaxially stretched film is heated and ultrasonic waves are applied to assist in the dissociation of the dynamic cross-linked network, causing local conformational rearrangement of molecular chain segments.

[0070] The heating temperature is 120℃, maintaining film tension or allowing 3cm of lateral shrinkage, and the processing time is 10s; the ultrasonic frequency is 20kHz, and the power density is 0.5W / cm²; the ultrasonic waves are applied by an array of ultrasonic transducers arranged in the width direction of the film, with a transducer spacing of 15cm, selectively accelerating the dissociation of Diels-Alder crosslinking points without damaging the polyester main chain structure; the phase change microcapsules in the core layer absorb heat, inhibiting the excessively rapid rise of local temperature inside the core layer, so that the crosslinking point dissociation process proceeds uniformly throughout the thickness direction of the core layer.

[0071] S6. Cooling and Crosslinking Locking: The temperature is slowly reduced at a set rate to allow the dynamic crosslinking network to reform under relaxed internal stress.

[0072] The temperature is lowered to 80℃ at a rate of 0.5℃ / min and held for 15 min. During this stage, the phase change microcapsules release their stored latent heat, slowing down the cooling rate and providing a sufficient time window for the Diels-Alder forward reaction.

[0073] S7. Cooling and winding: The film is rapidly cooled to room temperature, then pulled and wound up before being slit into finished products of specified specifications.

[0074] The cooling rate is no less than 30℃ / s, and rapid cooling is achieved by air cooling; the winding tension gradient is set to 10%, and the winding speed is 60m / min; the anti-adhesion agent in the skin layer forms a nanoscale texture on the film surface, providing anti-adhesion and releasing residual stress on the surface.

[0075] Example 3: This example provides a low-heat shrinkage resistant multilayer polyester material, which is composed of a skin layer, a transition layer, and a core layer in sequence, and has a five-layer symmetrical structure of skin layer-transition layer-core layer-transition layer-skin layer; it is made from the following raw materials in parts by weight: Skin layer: 99 parts of polyethylene terephthalate modified with furan side groups, and 2.5 parts of anti-blocking agent; Transition layer: A blend of polyethylene terephthalate and polyethylene terephthalate with furan side group modification; Core layer: 95 parts polyethylene terephthalate, 8 parts BMI-PET crosslinking agent, and 8 parts phase change microcapsules; The BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution that decreases from both sides to the center in the thickness direction of the core layer; the cortex, transition layer and core layer, as well as their interiors, have a thermally reversible dynamic covalent crosslinking network formed by furan groups and maleimide groups through the Diels-Alder reaction.

[0076] The thickness ratio of the skin layer, transition layer, and core layer is 1.5:1:5, and the total thickness of the multilayer polyester material is 250 μm. The anti-blocking agent is a blend of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder in a weight ratio of 10:1, with the polymethyl methacrylate microspheres having an average particle size of 3.0 μm and the low molecular weight polytetrafluoroethylene micropowder having an average particle size of 2.0 μm. The transition layer contains furan-side-group modified polyethylene terephthalate in a weight ratio of 60:40. The furan-side-group modified polyethylene terephthalate is a copolymer of terephthalic acid, ethylene glycol, and 2,5-furandicarboxylic acid, wherein the 2,5-furandicarboxylic acid... The acid accounts for 15% of the total molar fraction of the dicarboxylic acid monomers, and the intrinsic viscosity of the furan-modified polyethylene terephthalate is 0.70 dL / g. The gel content of the thermally reversible dynamic covalent crosslinking network decreases gradually from the skin layer through the transition layer to the core center, with a gel content of 90% in the skin layer, 60% in the transition layer, and 15% in the core center. The BMI-PET crosslinking agent is prepared by reacting polyester diol with maleic anhydride, and the polyester diol is polyhexamethylene adipate diol with a number average molecular weight of 8000. The phase change microcapsules use paraffin as the core material and modified silica as the shell material, with a phase change temperature of 110℃ and an average particle size of 5μm.

[0077] The poly(ethylene terephthalate) modified with furan side groups was prepared as follows: 100 moles of terephthalic acid, 130 moles of ethylene glycol, 15 moles of 2,5-furandicarboxylic acid, and antimony trioxide catalyst were added to an esterification reactor. The amount of catalyst was 0.05% of the mass of terephthalic acid. The esterification reaction was carried out at 250°C under nitrogen protection, with an esterification pressure of 0.3 MPa and a reaction time of 4 hours. After the esterification rate reached 97%, the mixture was transferred to a polycondensation reactor. Trimethyl phosphate stabilizer was added to the polycondensation reactor. The amount of stabilizer was 0.04% of the mass of terephthalic acid. The polycondensation reaction was carried out at 285°C and a vacuum of 70 Pa for 3 hours. The reaction was stopped when the intrinsic viscosity reached 0.70 dL / g. The product was obtained by casting and pelletizing.

[0078] The BMI-PET crosslinking agent was prepared as follows: Polyhexanediol adipate with a number average molecular weight of 8000 was added to a reaction vessel and heated to 140°C under nitrogen protection, followed by vacuum dehydration for 2 hours; the temperature was lowered to 100°C, and maleic anhydride was added, with a molar ratio of maleic anhydride to hydroxyl groups in the polyester diol of 2.2:1, and the mixture was stirred for 2 hours; toluene was added as an azeotropic dehydrating agent, and the temperature was raised to reflux temperature, and a dehydration and ring-closing reaction was carried out in the presence of the acid catalyst p-toluenesulfonic acid, with the catalyst amount being 1.0% of the mass of maleic anhydride, and the reaction time being 6 hours; after the reaction, the product was washed four times with deionized water to remove the catalyst and unreacted maleic anhydride, and toluene was removed by vacuum distillation. The product was then vacuum dried at 80°C for 8 hours to obtain a product with maleimide groups at both ends, with a yield of 95%.

[0079] The phase change microcapsules were prepared as follows: 50 parts by weight of paraffin wax with a phase change temperature of 110℃ were heated to complete melting, 3 parts by weight of sodium dodecylbenzenesulfonate emulsifier and 150 parts by weight of deionized water were added, and the mixture was emulsified at 70℃ and 12000 rpm for 20 minutes to obtain a paraffin wax emulsion; 25 parts by weight of tetraethyl orthosilicate and 3 parts by weight of silane coupling agent KH560 were dissolved in 30 parts by weight of ethanol and slowly added dropwise to the paraffin wax emulsion at a dropping rate of 1.0 mL / min. During the dropping process, the mixture was stirred at 500 rpm and the pH was maintained at 4. After the dropping was completed, the mixture was stirred and reacted for 6 hours; the reaction product was filtered, washed with deionized water until neutral, and dried at 60℃ for 12 hours to obtain the final product.

[0080] The anti-adhesion additive is prepared as follows: polymethyl methacrylate microspheres with an average particle size of 3.0 μm and low molecular weight polytetrafluoroethylene micro powder with an average particle size of 2.0 μm are added to a high-speed mixer at a weight ratio of 10:1 and mixed at 500 rpm for 10 minutes at room temperature to obtain the product.

[0081] The preparation method of the above-mentioned low-heat shrinkage resistant multilayer polyester material includes the following steps: S1. Raw material pretreatment: Place each layer of raw material in a vacuum drying oven to dry, and control the moisture content to not exceed 30 ppm.

[0082] The drying temperature is 140℃, the drying time is 6 hours, and nitrogen gas is continuously introduced during the drying process to prevent the raw materials from oxidizing and degrading.

[0083] S2, Melt Co-extrusion Casting: The dried skin, transition layer and core layer raw materials are fed into three independent twin-screw extruders for melting and plasticizing. The downstream of the homogenization section of the extruder corresponding to the core layer is equipped with an auxiliary side feed port. The melt is drawn into the five-layer co-extrusion die and then cast to the chiller roller to form an unstretched thick sheet.

[0084] The extruder temperature for the skin, transition layer, and core layer is controlled at 280℃, and the extruder screw speed is 350rpm. The BMI-PET crosslinking agent and phase change microcapsules are injected in a pulsed manner through the feed port on the excipient side at a pulse frequency of 5Hz. Combined with the static mixer at the end of the extruder, the concentration of the BMI-PET crosslinking agent and phase change microcapsules in the core layer thickness direction decreases from both sides to the center. The die temperature is 280℃, and the quench roll temperature is 35℃. The thickness of the unstretched sheet is 2.5mm. During the quench roll cooling process, the furan groups in the skin and transition layer spontaneously undergo a Diels-Alder cycloaddition reaction with the maleimide groups that migrate to the interface in the core layer, initially forming a thermally reversible dynamic covalent crosslinking network. The gel content decreases gradually from the skin to the center of the core layer.

[0085] S3. Longitudinal stretching: After preheating the unstretched thick sheet by a preheating roller, longitudinal stretching is performed to obtain a longitudinally stretched film.

[0086] The preheating temperature is 90℃, the preheating time is 40s, and the longitudinal stretching ratio is 3.8 times. During the stretching process, the pre-crosslinked network undergoes partial reverse reaction dissociation to release the tensile stress, and the residual crosslinking points maintain interlayer bonding to prevent delamination.

[0087] S4. Transverse stretching: After the longitudinally stretched film is preheated in the preheating section of the tenter frame, it is stretched transversely to obtain a biaxially stretched film.

[0088] The preheating temperature is 120℃, the preheating time is 50s, and the transverse stretching ratio is 4.0 times. During the stretching process, the pre-crosslinked network continues to maintain a partially crosslinked state to prevent interlayer slippage of the film during transverse stretching.

[0089] S5. Ultrasonic-assisted dissociation: The biaxially stretched film is heated and ultrasonic waves are applied to assist in the dissociation of the dynamic cross-linked network, causing local conformational rearrangement of molecular chain segments.

[0090] The heating temperature is 135℃, maintaining film tension or allowing 10cm of lateral shrinkage, and the processing time is 60s; the ultrasonic frequency is 40kHz, and the power density is 5W / cm²; the ultrasonic waves are applied by an array of ultrasonic transducers arranged in the width direction of the film, with a transducer spacing of 8cm, selectively accelerating the dissociation of Diels-Alder crosslinking points without damaging the polyester main chain structure; the phase change microcapsules in the core layer absorb heat, suppressing the excessively rapid rise of local temperature inside the core layer, so that the crosslinking point dissociation process proceeds uniformly throughout the thickness direction of the entire core layer.

[0091] S6. Cooling and Crosslinking Locking: The temperature is slowly reduced at a set rate to allow the dynamic crosslinking network to reform under relaxed internal stress.

[0092] The temperature was lowered to 100℃ at a rate of 2℃ / min and held for 30min. During this stage, the phase change microcapsules released their stored latent heat, slowing down the cooling rate and providing a sufficient time window for the Diels-Alder forward reaction.

[0093] S7. Cooling and winding: The film is rapidly cooled to room temperature, then pulled and wound up before being slit into finished products of specified specifications.

[0094] The cooling rate is 35℃ / s, and rapid cooling is achieved by water cooling; the winding tension gradient is set to 15%, and the winding speed is 100m / min; the anti-adhesion agent in the skin layer forms a nanoscale texture on the film surface, providing anti-adhesion and releasing residual stress on the surface.

[0095] Comparative Example 1: The only difference between this comparative example and Example 1 is that the BMI-PET crosslinking agent in the core layer is replaced with an equal amount of polyethylene terephthalate. The other raw material composition, structural parameters and preparation process are the same as those in Example 1.

[0096] Comparative Example 2: The only difference between this comparative example and Example 1 is that the transition layer is removed, and the five-layer symmetrical structure of skin-transition layer-core layer-transition layer-skin layer is changed to a three-layer symmetrical structure of skin-core layer-skin layer. The thickness ratio of each layer is adjusted to 2.5:6:2.5, and the total thickness remains unchanged at 131 μm. The other raw material composition and preparation process are the same as in Example 1.

[0097] Comparative Example 3: The only difference between this comparative example and Example 1 is that the phase change microcapsules in the core layer are replaced with an equal amount of polyethylene terephthalate. The other raw material composition, structural parameters and preparation process are the same as those in Example 1.

[0098] Comparative Example 4: The only difference between this comparative example and Example 1 is that after step S5, the film is directly cooled to room temperature at a rate of not less than 30°C / s, and the operation of slowly cooling to 90°C and holding for 22.5 minutes in step S6 is cancelled. The other raw material composition, steps and parameters are the same as in Example 1.

[0099] Comparative Example 5: The only difference between this comparative example and Example 1 is that ultrasound is not applied in step S5, and the biaxially oriented film is heated to 127.5°C and held for 35 seconds. The other raw material composition, steps and parameters are the same as in Example 1.

[0100] Comparative Example 6: This comparative example uses commercially available biaxially oriented polyester film with a thickness of 131 μm. The outer layer is a copolymer of isophthalic acid and polyethylene terephthalate, and the core layer is polyethylene terephthalate. It does not contain polyethylene terephthalate modified with furan side groups, BMI-PET crosslinking agent, phase change microcapsules, or anti-blocking agents. It is prepared using a 230°C relaxation heat setting process, and the remaining test conditions are the same as in Example 1.

[0101] I. High-Temperature Heat Shrinkage Rate Test: This test was conducted in accordance with GB / T13542.2-2009 "Determination of Heat Shrinkage Rate of Plastic Films and Sheets Part 2: Hot Air Method". The longitudinal and transverse heat shrinkage rates of Examples 1-3 and Comparative Examples 1-6 were tested respectively. Ten 100mm × 100mm square specimens were taken from each sample. Two parallel straight lines, 50mm apart, were marked on the surface of the specimen along both the longitudinal and transverse directions, with the marking lines at least 10mm from the edge of the specimen. The specimens were placed flat on a tension-free stainless steel mesh frame and placed in a hot air circulating oven preheated to 150℃ for 30 minutes. After removal, the specimens were cooled for 30 minutes in an environment of 23℃ and 50% relative humidity. The distance between the marking lines was measured using a vernier caliper with an accuracy of 0.01mm. Three measurements were taken in each direction, and the average value was recorded. The longitudinal and transverse heat shrinkage rates were calculated using the formula: Heat shrinkage rate = (Initial marking distance - Marking distance after test) ÷ Initial marking distance × 100%. The final result for each sample was the average value of the ten specimens.

[0102] II. Interlayer peel strength test: This test was conducted in accordance with GB / T8808-1988 "Peel Test Method for Flexible Composite Plastic Materials". The interlayer peel strength of Examples 1-3 and Comparative Examples 1-6 were tested respectively. Ten strip specimens, each 15 mm wide and 200 mm long, were taken for each sample. A 50 mm peeling head was made by peeling the skin and core layers apart at one end of the specimen with a blade. The peeling head was clamped in the upper and lower clamps of the universal testing machine with a clamp spacing of 50 mm. A 180-degree peel test was performed at a tensile speed of 300 mm / min. The average force value during the peeling process was recorded. Interlayer peel strength = average force value ÷ specimen width, in N / 15 mm. The final result for each sample was the average value of 10 specimens. If a specimen broke during the test instead of showing interlayer peeling, the data for that specimen was invalid and a new sample was taken for testing.

[0103] III. Performance Retention Rate Test After Damp Heat Aging: This test was conducted in accordance with GB / T7141-2008 "Test Method for Thermal Aging of Plastics". The heat shrinkage rate retention rate and peel strength retention rate of Examples 1-3 and Comparative Examples 1-6 after damp heat aging were tested respectively. For each sample, 20 square specimens of 100mm×100mm and 20 strip specimens of 15mm width and 200mm length were taken. The specimens were placed in a constant temperature and humidity test chamber at 85℃ and 85% relative humidity for 1000 hours of continuous aging. After that, they were removed and placed in an environment of 23℃ and 50% relative humidity for 24 hours for conditioning. Then, the heat shrinkage rate and peel strength after aging were tested according to the methods of high temperature heat shrinkage rate test and interlayer peel strength test described above. The performance retention rate was calculated as follows: heat shrinkage rate retention rate = heat shrinkage rate before aging ÷ heat shrinkage rate after aging × 100%; peel strength retention rate = peel strength after aging ÷ peel strength before aging × 100%. The final result of each sample was the average of 10 specimens.

[0104] Table 1: Performance Test Results of Multilayer Polyester Materials Example 1 0.22 0.18 18.5 96.2 94.5 Example 2 0.28 0.23 17.2 95.7 93.8 Example 3 0.25 0.20 17.8 95.9 94.1 Comparative Example 1 1.25 1.18 7.2 62.3 58.7 Comparative Example 2 0.56 0.49 10.3 78.5 72.1 Comparative Example 3 0.35 0.31 17.6 91.4 90.2 Comparative Example 4 0.63 0.58 16.9 87.6 86.3 Comparative Example 5 0.72 0.65 16.7 85.2 84.7 Comparative Example 6 0.81 0.76 9.8 76.4 71.9 Note: All data are the arithmetic mean of 10 parallel samples. The standard deviation of the heat shrinkage rate test is less than 0.05%, the standard deviation of the interlayer peel strength test is less than 0.5 N / 15 mm, and the standard deviation of the performance retention rate test is less than 1.0%.

[0105] As can be seen from Examples 1-3 and Comparative Example 1, and Table 1, the dynamic covalent crosslinking network is the core foundation for achieving low thermal shrinkage and interlayer bonding. It completely eliminates internal stress through reversible bonding and breaking processes, while forming chemical bonds between layers, significantly improving the long-term resistance to humid heat aging of the material.

[0106] As can be seen from Examples 1-3 and Comparative Example 2, and Table 1, the setting of the transition layer can achieve a continuous and smooth transition of crosslinking density, effectively eliminate stress concentration at the interface, avoid the generation of interface microcracks during long-term use, and thus maintain the dimensional stability and interlayer bonding strength of the material.

[0107] As can be seen from Examples 1-3 and Comparative Example 3, and Table 1, phase change microcapsules can play a thermal buffering role during heating and cooling, so that the dissociation and relocking process of crosslinking points can be carried out uniformly throughout the thickness direction of the film, thereby improving the consistency and long-term stability of product performance.

[0108] As can be seen from Examples 1-3 and Comparative Example 4, and Table 1, the relocking process of slow cooling and heat preservation is a key step in the formation of dynamic cross-linked networks. It allows the dissociated molecular chain segments to reform a stable cross-linked structure under relaxed internal stress, thus permanently fixing the low-stress state.

[0109] As can be seen from Examples 1-3 and Comparative Example 5, and Table 1, ultrasound can selectively accelerate the dissociation process of dynamic covalent bonds, significantly improve the stress release efficiency without damaging the polyester main chain structure, and achieve a balance between stress release and mechanical property maintenance.

[0110] As can be seen from Examples 1-3 and Comparative Example 6, and in conjunction with Table 1, the dynamic crosslinking and setting process of this application, compared with the traditional high-temperature physical relaxation and setting process, can achieve superior dimensional stability and interlayer bonding performance at a lower processing temperature, while avoiding the problem of material thermal degradation caused by high temperature.

[0111] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A low-heat shrinkage resistant multilayer polyester material, composed of a skin layer, a transition layer, and a core layer sequentially laminated together, characterized in that: The skin layer is made from the following raw materials in parts by weight: 95-99 parts of polyethylene terephthalate modified with furan side groups, and 0.6-2.5 parts of anti-blocking agent; The transition layer is a blend of polyethylene terephthalate modified with furan side groups and polyethylene terephthalate; The core layer is made of the following raw materials in parts by weight: 82-95 parts polyethylene terephthalate, 3-8 parts BMI-PET crosslinking agent, and 2-8 parts phase change microcapsules; The BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution that decreases from both sides to the center in the thickness direction of the core layer; the cortex, transition layer and core layer have a thermally reversible dynamic covalent crosslinking network formed by furan groups and maleimide groups through the Diels-Alder reaction; The multilayer polyester material has a five-layer symmetrical structure consisting of a skin layer, a transition layer, a core layer, a transition layer, and a skin layer.

2. The low-heat shrinkage resistant multilayer polyester material according to claim 1, characterized in that: The thickness ratio of the skin layer, transition layer, and core layer is 1:0.5:7 to 1.5:1:5, and the total thickness of the multilayer polyester material is 12 to 250 μm; the anti-blocking agent is a blend of polymethyl methacrylate microspheres and low molecular weight polytetrafluoroethylene micropowder in a weight ratio of 5:1 to 10:1, wherein the average particle size of the polymethyl methacrylate microspheres is 2.0 to 3.0 μm, and the average particle size of the low molecular weight polytetrafluoroethylene micropowder is 1.0 to 2.0 μm.

3. The low-heat shrinkage resistant multilayer polyester material according to claim 1, characterized in that: The weight ratio of furan-side-group modified polyethylene terephthalate (PET) to PET in the transition layer is 40:60 to 60:40; the furan-side-group modified PET is a copolymer of terephthalic acid, ethylene glycol, and 2,5-furandicarboxylic acid, wherein the 2,5-furandicarboxylic acid accounts for 5% to 15% of the total molar fraction of the dicarboxylic acid monomers, and the intrinsic viscosity of the furan-side-group modified PET is 0.62 to 0.70 dL / g.

4. The low-heat shrinkage resistant multilayer polyester material according to claim 1, characterized in that: The gel content of the thermally reversible dynamic covalent crosslinked network is distributed in a gradient that gradually decreases from the cortex through the transition layer to the core center; the gel content of the cortex is 80 to 90%, the gel content of the transition layer is 30 to 60%, and the gel content of the core center is 5 to 15%.

5. The low-heat shrinkage resistant multilayer polyester material according to claim 1, characterized in that: The BMI-PET crosslinking agent is prepared by reacting polyester diol with maleic anhydride. The number average molecular weight of the polyester diol is 2000 to 8000. The polyester diol is one or more of polyethylene adipate diol, polybutylene adipate diol, and polyhexyl adipate diol. The phase change microcapsules use paraffin as the core material and modified silica as the shell material. The phase change temperature is 80 to 110°C, and the average particle size is 0.5 to 5 μm.

6. A method for preparing a low-heat shrinkage-resistant multilayer polyester material, characterized in that, The method for use in a low-heat shrinkage resistant multilayer polyester material according to any one of claims 1-5 includes the following steps: S1. Raw material pretreatment: Place each layer of raw material in a vacuum drying oven to dry, and control the moisture content to not exceed 30 ppm. S2, melt co-extrusion casting: The dried skin, transition layer and core layer raw materials are fed into three independent twin-screw extruders for melting and plasticizing. The downstream of the homogenization section of the extruder corresponding to the core layer is equipped with an auxiliary side feed port. The melt flows to the chill roll after being combined through five co-extrusion dies to form an unstretched thick sheet. S3. Longitudinal stretching: After preheating the unstretched thick sheet by a preheating roller, longitudinal stretching is performed to obtain a longitudinally stretched film. S4. Transverse stretching: After the longitudinally stretched film is preheated in the preheating section of the tenter frame, it is stretched transversely to obtain a biaxially stretched film. S5. Ultrasonic-assisted dissociation: The biaxially stretched film is heated and ultrasonic waves are applied to assist in the dissociation of the dynamic cross-linking network, causing local conformational rearrangement of molecular chain segments. S6. Cooling and Crosslinking Locking: The temperature is slowly reduced at a set rate, allowing the dynamic crosslinking network to reform under relaxed internal stress. S7. Cooling and winding: The film is rapidly cooled to room temperature, then pulled and wound up before being slit into finished products of specified specifications.

7. The method for preparing a low-heat shrinkage resistant multilayer polyester material according to claim 6, characterized in that: In step S2, the extruder temperatures of the skin layer, transition layer, and core layer are all controlled at 265 to 280°C; the BMI-PET crosslinking agent and phase change microcapsules are pulsed through the auxiliary agent side feed port with a pulse frequency of 1 to 5 Hz, which, in conjunction with the static mixer at the end of the extruder, ensures that the BMI-PET crosslinking agent and phase change microcapsules exhibit a concentration gradient distribution from both sides to the center in the thickness direction of the core layer; the die temperature is 270 to 280°C, and the quench roll temperature is 25 to 35°C; the thickness of the unstretched sheet is 1.0 to 2.5 mm.

8. The method for preparing a low-heat shrinkage-resistant multilayer polyester material according to claim 6, characterized in that: In step S3, the preheating temperature is 80 to 90°C, the preheating time is 20 to 40 seconds, and the longitudinal stretching ratio is 3.0 to 3.8 times; in step S4, the preheating temperature is 100 to 120°C, the preheating time is 30 to 50 seconds, and the transverse stretching ratio is 3.2 to 4.0 times.

9. The method for preparing a low-heat shrinkage-resistant multilayer polyester material according to claim 6, characterized in that: In step S5, the heating temperature is 120 to 135°C, the film tension is maintained or a lateral shrinkage of 3 to 10 cm is allowed, and the processing time is 10 to 60 s; the ultrasonic frequency is 20 to 40 kHz, and the power density is 0.5 to 5 W / cm².

10. The method for preparing a low-heat shrinkage-resistant multilayer polyester material according to claim 6, characterized in that: In step S6, the temperature is reduced to 80 to 100°C at a rate of 0.5 to 2°C / min and held for 15 to 30 min; in step S7, the cooling rate is not less than 30°C / s and the winding tension gradient is set to 10% to 15%.

Images