High heat resistant modified pet cast film composition and its co-extrusion process

Abstract

This invention relates to the field of plastic film technology, specifically to a high heat-resistant modified PET cast film composition and its co-extrusion preparation process. The composition includes an outer layer, a core layer, and an inner layer. The core layer consists of the following components: modified PET resin, poly(m-phenylene adipamide), and maleic anhydride-grafted POE. The inner layer consists of the following components: modified PET resin, PEN resin, ethylene-vinyl acetate copolymer, maleic anhydride-grafted POE, nano-silica, and glyceryl monostearate. This invention introduces dimethyl 2,6-naphthalenedicarboxylate copolymer units into the PET backbone and, through reactive extrusion with styrene-butyl acrylate-glycidyl methacrylate copolymer, achieves an FRR value of 4.0–6.0 for the modified PET resin. This significantly improves the melt strength after heat-resistant modification, ensures the stability of the cast film process, and effectively avoids necking and film breakage.

Description

Technical Field

[0001] This invention relates to the field of plastic film technology, specifically to a high heat-resistant modified PET cast film composition and its co-extrusion preparation process. Background Technology

[0002] Polyethylene terephthalate (PET) film is widely used in fields such as electronic and electrical insulation, food packaging, optical displays, and solar backsheets due to its excellent mechanical properties, optical transparency, chemical stability, and processing performance. Among them, PET film prepared by casting (CPET) occupies an important position in applications such as heat-sealing layers, composite substrates, and insulating materials because it has advantages such as no stretching process, high production efficiency, and good thickness uniformity.

[0003] However, the glass transition temperature of ordinary PET is only about 75°C, making it prone to dimensional shrinkage and softening failure in high-temperature environments, which is insufficient to meet the requirements of high heat resistance applications. To improve heat resistance, existing technologies typically involve introducing rigid copolymer units (such as naphthalene dicarboxylic acid units) into the PET backbone or blending it with polymers with high glass transition temperatures. However, while these modification methods improve heat resistance, they inevitably bring a prominent problem: the melt strength of the modified material is insufficient, failing to meet the requirements of casting molding processes.

[0004] Specifically, in the casting process, the melt adheres to the cooling rollers by gravity after being extruded from the die, requiring the material to have high melt elasticity (usually characterized by the Free Flow Rate (FRR) value; a higher FRR indicates more long branches and better melt elasticity). Ordinary linear PET has an FRR of only 2.0–2.5, already at the lower limit for casting. Introducing rigid copolymer units such as naphthalene rings increases molecular chain rigidity and reduces entanglement density, further decreasing the FRR. This leads to severe necking at the film edge, poor thickness uniformity, and even film breakage during casting. If the melt strength is compensated for by increasing the molecular weight, the melt viscosity rises sharply, resulting in excessive extrusion pressure, which also fails to achieve stable casting.

[0005] Therefore, how to achieve both high heat resistance and high melt elasticity at the molecular level, so that modified PET can be stably applied to the casting process, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a high heat-resistant modified PET cast film composition and its co-extrusion preparation process, so as to solve the technical problem that the melt strength of PET after heat-resistant modification is insufficient and cannot meet the requirements of the cast film molding process in the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a high heat-resistant modified PET cast film composition, the composition comprising an outer layer material, a core layer material, and an inner layer material in a thickness ratio of (1-3):(4-8):(1-3).

[0008] The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 10 to 20 parts of modified PET resin, 1 to 4 parts of nano-montmorillonite, 0.5 to 1.0 parts of hindered phenolic antioxidant, 0.5 to 1.0 parts of phosphite antioxidant, 0.1 to 0.8 parts of silane coupling agent, and 0.1 to 0.5 parts of ultraviolet absorber.

[0009] The core layer material is composed of the following components by weight: 100 parts modified PET resin, 15 to 25 parts poly(m-phenylene adipamide), and 2 to 5 parts maleic anhydride-grafted POE.

[0010] The inner layer material is composed of the following components by weight: 100 parts modified PET resin, 5 to 15 parts PEN resin, 3 to 8 parts ethylene-vinyl acetate copolymer, 1 to 3 parts maleic anhydride grafted POE, 0.3 to 1.0 parts nano silica, and 0.1 to 0.5 parts glyceryl monostearate.

[0011] The modified PET resin comprises a polyethylene terephthalate-co-2,6-naphthalenedicarboxylate copolymer matrix and a long-chain branched structure grafted onto the matrix. The long-chain branched structure is a branched segment formed by the reaction of an epoxy group with a terminal carboxyl group or a terminal hydroxyl group of the matrix in a styrene-butyl acrylate-glycidyl methacrylate copolymer. The modified PET resin has a melt index of 3 g / 10 min to 8 g / 10 min and a melt flow rate (FRR) of 4.0 to 6.0 under the condition of 270℃ / 2.16 kg. The FRR is defined as the ratio of the melt index at 270℃ / 5.0 kg load to that at 270℃ / 2.16 kg load.

[0012] Preferably, the preparation method of the modified PET resin includes the following steps: 88-97 parts by weight of dimethyl terephthalate, 3-12 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 74-108 parts by weight of ethylene glycol, and 0.02-0.08 parts by weight of zinc acetate are added to an esterification reactor, and reacted at 180°C-200°C for 1-1.5 hours under nitrogen protection, followed by further heating to 200°C-220°C and reacting for another 1-1.5 hours to obtain a copolymer oligomer; 0.03-0.06 parts by weight of antimony trioxide and 0.01-0.03 parts by weight of trimethyl phosphate are added to the copolymer oligomer, and then... Prepolymerization is carried out at 250℃~260℃ and vacuum degree 5kPa~10kPa for 30min~60min, followed by final polymerization at 275℃~285℃ and vacuum degree ≤50Pa for 1.5h~2.5h. The product is then discharged, granulated, and vacuum dried to a moisture content ≤50ppm to obtain copolymer PET chips. The copolymer PET chips are mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer at a mass ratio of 100:(0.3~1.5), and reactively extruded and granulated using a twin-screw extruder at 250℃~280℃ to obtain the modified PET resin.

[0013] Preferably, the styrene-butyl acrylate-glycidyl methacrylate copolymer has a weight-average molecular weight of 3000 g / mol to 8000 g / mol, an epoxy equivalent of 250 g / mol to 500 g / mol, and a glycidyl methacrylate unit mass content of 10% to 25%.

[0014] Preferably, the ultraviolet absorber is UV-326 or UV-328.

[0015] Preferably, the poly(m-phenylene adipamide) has an intrinsic viscosity of 0.8 dL / g to 1.2 dL / g, a glass transition temperature of 80 to 90°C, and a melting point of 230°C to 240°C.

[0016] Preferably, the grafting rate of the maleic anhydride-grafted POE is 0.5% to 1.5%, and the melt index at 2.16 kg / 190 °C is 1 g / 10 min to 10 g / 10 min.

[0017] Preferably, the vinyl acetate content in the ethylene-vinyl acetate copolymer is 5% to 28% by mass.

[0018] On the other hand, the present invention also provides the following technical solution: a method for preparing the above-mentioned high heat-resistant modified PET cast film composition, comprising the following steps: S1. Multilayer co-extrusion plasticization: The outer layer material, core layer material and inner layer material are melted and plasticized by three single-screw extruders. The extrusion temperature of the outer layer material is 260℃~275℃, the extrusion temperature of the core layer material is 255℃~270℃, and the extrusion temperature of the inner layer material is 240℃~260℃. The three layers of melt are combined and extruded through the die at 265℃~280℃, and then attached to the cooling roller at 20℃~40℃ for rapid cooling and casting to obtain a multilayer co-extruded sheet.

[0019] S2. Heat setting: The multilayer co-extruded sheet is heat-set at 180℃~210℃ for 3s~10s, and then cooled to room temperature to obtain the composition.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention effectively improves the heat resistance of the matrix resin by introducing dimethyl 2,6-naphthalenedicarboxylate copolymer units into the PET main chain. Furthermore, by reactively extruding with a styrene-butyl acrylate-glycidyl methacrylate copolymer, a long branched structure is introduced into the molecular chain, enabling the modified PET resin to achieve an FRR value of 4.0–6.0. ​​This significantly improves the melt strength after heat-resistant modification, ensures the stability of the casting process, and effectively avoids necking and film breakage.

[0021] 2. This invention employs a three-layer co-extrusion functional partition design. The outer layer uses PEN as the matrix to provide a heat-resistant barrier and UV protection. The core layer introduces poly(m-phenylene adipamide) to synergistically enhance heat resistance and maleic anhydride-grafted POE to improve interlayer compatibility. The inner layer combines ethylene-vinyl acetate copolymer to impart heat-sealing performance. The peel force between the three layers is not less than 2.2N / 15mm. Each layer functions synergistically and does not interfere with each other, achieving high heat resistance, stable casting, and good interlayer bonding in a single process. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 This embodiment provides a high heat-resistant modified PET cast film composition, which includes an outer layer material, a core layer material, and an inner layer material with a thickness ratio of 2:6:2.

[0024] The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 15 parts of the above-mentioned modified PET resin, 2.5 parts of nano montmorillonite, 0.8 parts of hindered phenolic antioxidant (antioxidant 1010), 0.8 parts of phosphite antioxidant (antioxidant 168), 0.5 parts of silane coupling agent (KH-550), and 0.3 parts of ultraviolet absorber (UV-326).

[0025] The core layer material is composed of the following components by weight: 100 parts modified PET resin, 20 parts poly(m-phenylene adipamide) (intrinsic viscosity 1.0 dL / g, glass transition temperature 85℃, melting point 235℃), and 3.5 parts maleic anhydride-grafted POE (grafting rate 1.0%, melt index 5 g / 10 min at 2.16 kg / 190℃).

[0026] The inner layer material is composed of the following components by weight: 100 parts of the above modified PET resin, 10 parts of PEN resin, 5 parts of ethylene-vinyl acetate copolymer (vinyl acetate content 18%), 2 parts of maleic anhydride grafted POE (grafting rate 1.0%, melt index 5g / 10min at 2.16kg / 190℃), 0.6 parts of nano silica, and 0.3 parts of glyceryl monostearate.

[0027] The outer layer, core layer, and inner layer materials are melted and plasticized separately using three single-screw extruders. The extrusion temperature of the outer layer material is 268°C, the core layer material is 262°C, and the inner layer material is 250°C. The three melts are combined and extruded through a die at 272°C and attached to a 30°C cooling roller for rapid cooling to form a multilayer co-extruded sheet. The multilayer co-extruded sheet is heat-set at 195°C for 6 seconds and then cooled to room temperature to obtain the composite material.

[0028] The preparation method of modified PET resin includes the following steps: 92 parts by weight of dimethyl terephthalate, 8 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 90 parts by weight of ethylene glycol, and 0.05 parts by weight of zinc acetate are added to an esterification reactor. The reactor is reacted at 190℃ for 1.2 hours under nitrogen protection, and then heated to 210℃ for another 1.2 hours. When the methanol distillation reaches more than 95% of the theoretical value, the transesterification reaction ends, yielding a copolymer oligomer. 0.04 parts by weight of antimony trioxide and 0.02 parts by weight of trimethyl phosphate are added to the copolymer oligomer. The mixture is pre-polymerized at 255℃ and a vacuum of 8 kPa for 45 minutes, and then heated further. The copolymer PET chips were subjected to final polycondensation at 280℃ and a vacuum degree ≤50Pa for 2 hours, then discharged and granulated. The granules were then dried under vacuum at 140℃ for 5 hours until the moisture content was ≤50ppm to obtain copolymer PET chips. The copolymer PET chips were mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer (weight average molecular weight 5000g / mol, epoxy equivalent 380g / mol, mass content of glycidyl methacrylate unit 18%) at a mass ratio of 100:0.8. The mixture was then reactively extruded and granulated using a co-rotating twin-screw extruder at a screw speed of 200rpm and a barrel temperature of 265℃ to obtain modified PET resin.

[0029] The modified PET resin was tested and found to have a melt index of 5.2 g / 10 min at 270℃ / 2.16 kg and a melt index of 24.4 g / 10 min at 270℃ / 5.0 kg, with an FRR of 4.7.

[0030] Example 2 This embodiment provides a high heat-resistant modified PET cast film composition, which includes an outer layer material, a core layer material, and an inner layer material with a thickness ratio of 1:8:1.

[0031] The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 10 parts of the above-mentioned modified PET resin, 1 part of nano montmorillonite, 0.5 parts of hindered phenolic antioxidant (antioxidant 1010), 0.5 parts of phosphite antioxidant (antioxidant 168), 0.1 parts of silane coupling agent (KH-550), and 0.1 parts of ultraviolet absorber (UV-326).

[0032] The core layer material consists of the following components by weight: 100 parts modified PET resin, 15 parts poly(m-phenylene adipamide) (intrinsic viscosity 0.8 dL / g, glass transition temperature 80℃, melting point 230℃), and 2 parts maleic anhydride-grafted POE (grafting rate 0.5%, melt index 1 g / 10 min at 2.16 kg / 190℃).

[0033] The inner layer material is composed of the following components by weight: 100 parts of the above modified PET resin, 5 parts of PEN resin, 3 parts of ethylene-vinyl acetate copolymer (vinyl acetate content 5%), 1 part of maleic anhydride grafted POE (grafting rate 0.5%, melt index 1g / 10min at 2.16kg / 190℃), 0.3 parts of nano silica, and 0.1 parts of glyceryl monostearate.

[0034] The outer layer, core layer, and inner layer materials are melted and plasticized separately using three single-screw extruders. The extrusion temperature of the outer layer material is 260℃, the core layer material is 255℃, and the inner layer material is 240℃. The three melts are combined and extruded through a die at 265℃ and attached to a 20℃ cooling roller for rapid cooling to form a multilayer co-extruded sheet. The multilayer co-extruded sheet is heat-set at 180℃ for 10 seconds and then cooled to room temperature to obtain the composite material.

[0035] The preparation method of modified PET resin includes the following steps: 95 parts by weight of dimethyl terephthalate, 5 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 82 parts by weight of ethylene glycol, and 0.03 parts by weight of zinc acetate are added to an esterification reactor. The reactor is reacted at 185°C for 1.5 hours under nitrogen protection, and then heated to 205°C for another 1.5 hours. When the methanol distillation reaches more than 95% of the theoretical value, the transesterification reaction ends, yielding a copolymer oligomer. 0.03 parts by weight of antimony trioxide and 0.01 parts by weight of trimethyl phosphate are added to the copolymer oligomer. The mixture is pre-polymerized at 250°C and a vacuum of 10 kPa for 60 minutes, and then heated to... The copolymer PET chips were obtained by final polycondensation at 275℃ and vacuum degree ≤50Pa for 2.5h, followed by pelleting and vacuum drying at 130℃ for 6h until the moisture content was ≤50ppm. The copolymer PET chips were then mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer (weight average molecular weight 3000g / mol, epoxy equivalent 250g / mol, glycidyl methacrylate unit mass content 10%) at a mass ratio of 100:0.5. The mixture was then reactively extruded and granulated using a co-rotating twin-screw extruder at a screw speed of 150rpm and a barrel temperature of 255℃ to obtain the modified PET resin.

[0036] The modified PET resin was tested and found to have a melt index of 7.5 g / 10 min at 270℃ / 2.16 kg and a melt index of 30.0 g / 10 min at 270℃ / 5.0 kg, with an FRR of 4.0.

[0037] Example 3 This embodiment provides a high heat-resistant modified PET cast film composition, which includes an outer layer material, a core layer material, and an inner layer material with a thickness ratio of 3:4:3.

[0038] The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 20 parts of the above-mentioned modified PET resin, 4 parts of nano montmorillonite, 1.0 part of hindered phenolic antioxidant (antioxidant 1010), 1.0 part of phosphite antioxidant (antioxidant 168), 0.8 parts of silane coupling agent (KH-550), and 0.5 parts of ultraviolet absorber (UV-328).

[0039] The core layer material is composed of the following components by weight: 100 parts modified PET resin, 25 parts poly(m-phenylene adipamide) (intrinsic viscosity 1.2 dL / g, glass transition temperature 90℃, melting point 240℃), and 5 parts maleic anhydride-grafted POE (grafting rate 1.5%, melt index 10 g / 10 min at 2.16 kg / 190℃).

[0040] The inner layer material is composed of the following components by weight: 100 parts of the above modified PET resin, 15 parts of PEN resin, 8 parts of ethylene-vinyl acetate copolymer (vinyl acetate content 28%), 3 parts of maleic anhydride grafted POE (grafting rate 1.5%, melt index 10g / 10min at 2.16kg / 190℃), 1.0 part of nano silica, and 0.5 parts of glyceryl monostearate.

[0041] The outer layer, core layer, and inner layer materials are melted and plasticized separately using three single-screw extruders. The extrusion temperature of the outer layer material is 275℃, the core layer material is 270℃, and the inner layer material is 260℃. The three melts are combined and extruded through a die at 280℃ and attached to a 40℃ cooling roller for rapid cooling to form a multilayer co-extruded sheet. The multilayer co-extruded sheet is heat-set at 210℃ for 3 seconds and then cooled to room temperature to obtain the composite material.

[0042] The preparation method of modified PET resin includes the following steps: 90 parts by weight of dimethyl terephthalate, 10 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 98 parts by weight of ethylene glycol, and 0.06 parts by weight of zinc acetate are added to an esterification reactor. The reactor is reacted at 195°C for 1 hour under nitrogen protection, and then heated to 215°C for another hour. When the methanol distillation reaches more than 95% of the theoretical value, the transesterification reaction ends, yielding a copolymer oligomer. 0.05 parts by weight of antimony trioxide and 0.03 parts by weight of trimethyl phosphate are added to the copolymer oligomer. The mixture is pre-polymerized at 258°C and a vacuum of 5 kPa for 30 minutes, and then heated to 28°C. The copolymer PET chips were obtained by final polycondensation at 3℃ and vacuum degree ≤50Pa for 1.5h, followed by pelleting and vacuum drying at 150℃ for 4h until the moisture content was ≤50ppm. The copolymer PET chips were then mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer (weight average molecular weight 8000g / mol, epoxy equivalent 500g / mol, mass content of glycidyl methacrylate unit 25%) at a mass ratio of 100:1.2. The mixture was then reactively extruded and granulated using a co-rotating twin-screw extruder at a screw speed of 280rpm and a barrel temperature of 275℃ to obtain the modified PET resin.

[0043] The modified PET resin was tested and found to have a melt index of 3.5 g / 10 min at 270℃ / 2.16 kg and a melt index of 18.2 g / 10 min at 270℃ / 5.0 kg, with an FRR of 5.2.

[0044] Example 4 This embodiment provides a high heat-resistant modified PET cast film composition, which includes an outer layer material, a core layer material, and an inner layer material with a thickness ratio of 2:5:2.

[0045] The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 18 parts of the above-mentioned modified PET resin, 3 parts of nano montmorillonite, 0.8 parts of hindered phenolic antioxidant (antioxidant 1010), 0.7 parts of phosphite antioxidant (antioxidant 168), 0.6 parts of silane coupling agent (KH-550), and 0.4 parts of ultraviolet absorber (UV-328).

[0046] The core layer material consists of the following components by weight: 100 parts modified PET resin, 22 parts poly(m-phenylene adipamide) (intrinsic viscosity 1.1 dL / g, glass transition temperature 88℃, melting point 238℃), and 4 parts maleic anhydride-grafted POE (grafting rate 1.2%, melt index 8 g / 10 min at 2.16 kg / 190℃).

[0047] The inner layer material is composed of the following components by weight: 100 parts of the above modified PET resin, 12 parts of PEN resin, 6 parts of ethylene-vinyl acetate copolymer (vinyl acetate content 22%), 2.5 parts of maleic anhydride grafted POE (grafting rate 1.2%, melt index 8g / 10min at 2.16kg / 190℃), 0.8 parts of nano silica, and 0.4 parts of glyceryl monostearate.

[0048] The outer layer, core layer, and inner layer materials were melted and plasticized separately using three single-screw extruders. The extrusion temperature of the outer layer material was 272°C, the core layer material was 268°C, and the inner layer material was 255°C. The three melts were combined and extruded through a die at 276°C and attached to a 35°C cooling roller for rapid cooling to form a multilayer co-extruded sheet. The multilayer co-extruded sheet was heat-set at 200°C for 5 seconds and then cooled to room temperature to obtain the composite material.

[0049] The preparation method of modified PET resin includes the following steps: 88 parts by weight of dimethyl terephthalate, 12 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 105 parts by weight of ethylene glycol, and 0.07 parts by weight of zinc acetate are added to an esterification reactor. The reactor is reacted at 200℃ for 1 hour under nitrogen protection, and then heated to 220℃ for another hour. When the methanol distillation reaches more than 95% of the theoretical value, the transesterification reaction ends, yielding a copolymer oligomer. 0.06 parts by weight of antimony trioxide and 0.02 parts by weight of trimethyl phosphate are added to the copolymer oligomer. The mixture is pre-polymerized at 260℃ and a vacuum of 6 kPa for 40 minutes, and then heated to... The copolymer PET chips were obtained by final polycondensation at 285℃ and vacuum degree ≤50Pa for 2 hours, followed by pelleting and vacuum drying at 145℃ for 5 hours until the moisture content was ≤50ppm. The copolymer PET chips were then mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer (weight average molecular weight 6000g / mol, epoxy equivalent 420g / mol, mass content of glycidyl methacrylate unit 22%) at a mass ratio of 100:1.5. The mixture was then reactively extruded and granulated using a co-rotating twin-screw extruder at a screw speed of 300rpm and a barrel temperature of 280℃ to obtain the modified PET resin.

[0050] The modified PET resin was tested and found to have a melt index of 3.0 g / 10 min at 270℃ / 2.16 kg and a melt index of 18.0 g / 10 min at 270℃ / 5.0 kg, with an FRR of 6.0.

[0051] Comparative Example 1 The difference from Example 1 is that reactive extrusion is not performed during the preparation of the modified PET resin, that is, it is not mixed and extruded with styrene-butyl acrylate-glycidyl methacrylate copolymer. The resulting copolymer PET chips are used directly as modified PET resin; the rest of the formulation and process conditions are the same as in Example 1.

[0052] The resin was tested and found to have a melt index of 4.8 g / 10 min at 270℃ / 2.16 kg and a melt index of 11.5 g / 10 min at 270℃ / 5.0 kg, with an FRR of 2.4.

[0053] Comparative Example 2 The difference from Example 1 is that no dimethyl 2,6-naphthalenedicarboxylate copolymer unit is introduced in the preparation of the modified PET resin. Instead, 100 parts of dimethyl terephthalate are directly mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer and extruded. The remaining formulation and process conditions are the same as in Example 1.

[0054] Experimental Example 1: Thin Film Performance Testing The compositions obtained in Examples 1-4 and Comparative Examples 1-4 were used as samples, and the following performance tests were performed respectively: Heat distortion temperature: According to GB / T 1634.2-2019, a load of 0.45MPa was used for testing. Sample preparation: Cut 80mm×10mm strips from the film, stack 12 layers, and hot-press them at 260℃ and 5MPa pressure for 30s to make a standard sample with a thickness of 4mm. After cooling to room temperature, cut and trim the sample, and record the temperature at which the deflection of the sample reaches 0.34mm. Take the average value of 3 samples.

[0055] Heat shrinkage rate: According to GB / T 12027-2004, cut 100mm×100mm samples from the film, mark the longitudinal and transverse reference lines within the gauge length, lay them freely on a stainless steel mesh frame, place them in a forced-air oven at 200℃±2℃ for 30min, take them out and cool them to room temperature under standard environment (23℃±2℃, 50%±5%RH) to measure the dimensional changes, calculate the heat shrinkage rate in the MD and TD directions, and take the average value of 5 samples.

[0056] Tensile strength: According to GB / T 1040.3-2006, long strip specimens (15 mm wide, 100 mm gauge length) were used, and the tensile strength in the MD direction was tested at a tensile speed of 200 mm / min. The average value of 5 specimens was taken.

[0057] Interlayer peel strength: According to GB / T 8808-2016, the T-type peel method (180° peel) was used, with a sample width of 15 mm and a peel speed of 300 mm / min. A 20 mm peel opening was pre-made at the interface between the outer layer and the core layer, and the peel force was tested. The average value of the stable section was taken. Five samples were taken for each sample.

[0058] Casting stability: During the continuous casting process, the following quantitative indicators were recorded for 60 minutes: maximum necking at the film edge (mm); number of film breaks; coefficient of variation (CV%) of transverse film thickness (using an online thickness gauge, data were collected over a continuous length of 10m). Evaluation criteria: Excellent: CV% ≤ 3% with no film breaks; Good: CV% 3%–5% with ≤ 1 film break; Poor: CV% > 5% or > 1 film break.

[0059] The test results are summarized in Table 1.

[0060] Table 1: Thin Film Performance Test Results As shown in Table 1, the heat distortion temperature of all examples reached above 125℃, the heat shrinkage rate at 200℃ did not exceed 1.2%, the peel strength was not less than 2.2 N / 15 mm, and the casting stability was excellent. This indicates that the present invention successfully achieved a balance between high heat resistance and casting processability through the synergistic combination of 2,6-naphthalenedicarboxylate copolymer modified PET resin and the three-layer co-extrusion functional partition design. Compared with Example 1, Comparative Example 1 did not undergo reactive extrusion, and the resulting resin had an FRR of only 2.4. The necking amount during casting reached 18 mm, and the film broke 3 times within 60 minutes. This indicates that the long branched structure introduced by reactive extrusion of styrene-butyl acrylate-glycidyl methacrylate copolymer significantly improved melt elasticity, which is the key to ensuring the stability of casting molding. Compared to Example 1, Comparative Example 2 did not introduce dimethyl 2,6-naphthalenedicarboxylate copolymer units. Although its casting stability was normal, its heat distortion temperature was only 96°C, and its MD heat shrinkage rate at 200°C was as high as 5.8%, which was much lower than that of Example 1 (138°C and 0.8%). This indicates that the naphthalene ring copolymer unit is the core structure that imparts high heat resistance and high-temperature dimensional stability to the film.

[0061] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high heat-resistant modified PET cast film composition, characterized in that, The composition comprises an outer layer material, a core layer material, and an inner layer material in a thickness ratio of (1-3):(4-8):(1-3); The outer layer material is composed of the following components by weight: 100 parts of polyethylene naphthalate, 10 to 20 parts of modified PET resin, 1 to 4 parts of nano-montmorillonite, 0.5 to 1.0 parts of hindered phenolic antioxidant, 0.5 to 1.0 parts of phosphite antioxidant, 0.1 to 0.8 parts of silane coupling agent, and 0.1 to 0.5 parts of ultraviolet absorber; The core layer material is composed of the following components by weight: 100 parts modified PET resin, 15 to 25 parts poly(m-phenylene adipamide), and 2 to 5 parts maleic anhydride-grafted POE. The inner layer material is composed of the following components by weight: 100 parts modified PET resin, 5 to 15 parts PEN resin, 3 to 8 parts ethylene-vinyl acetate copolymer, 1 to 3 parts maleic anhydride grafted POE, 0.3 to 1.0 parts nano silica, and 0.1 to 0.5 parts glyceryl monostearate. The modified PET resin comprises a polyethylene terephthalate-co-2,6-naphthalenedicarboxylate copolymer matrix and a long-chain branched structure grafted onto the matrix. The long-chain branched structure is a branched segment formed by the reaction of an epoxy group with a terminal carboxyl group or a terminal hydroxyl group of the matrix in a styrene-butyl acrylate-glycidyl methacrylate copolymer. The modified PET resin has a melt index of 3 g / 10 min to 8 g / 10 min and a melt flow rate (FRR) of 4.0 to 6.0 under the condition of 270℃ / 2.16 kg. The FRR is defined as the ratio of the melt index at 270℃ / 5.0 kg load to that at 270℃ / 2.16 kg load.

2. The high heat-resistant modified PET cast film composition according to claim 1, characterized in that, The preparation method of the modified PET resin includes the following steps: 88-97 parts by weight of dimethyl terephthalate, 3-12 parts by weight of dimethyl 2,6-naphthalenedicarboxylate, 74-108 parts by weight of ethylene glycol, and 0.02-0.08 parts by weight of zinc acetate are added to an esterification reactor. The reaction is carried out at 180-200°C for 1-1.5 hours under nitrogen protection, followed by further heating to 200-220°C and reacting for another 1-1.5 hours to obtain a copolymer oligomer. 0.03-0.06 parts by weight of antimony trioxide and 0.01-0.03 parts by weight of trimethyl phosphate are added to the copolymer oligomer, and the reaction is carried out at 25°C. Prepolymerization is carried out at 0℃~260℃ and vacuum degree 5kPa~10kPa for 30min~60min, followed by final polymerization at 275℃~285℃ and vacuum degree ≤50Pa for 1.5h~2.5h. The product is then discharged, granulated, and vacuum dried to a moisture content ≤50ppm to obtain copolymer PET chips. The copolymer PET chips are mixed with styrene-butyl acrylate-glycidyl methacrylate copolymer at a mass ratio of 100:(0.3~1.5), and reactively extruded and granulated using a twin-screw extruder at 250℃~280℃ to obtain the modified PET resin.

3. The high heat-resistant modified PET cast film composition according to claim 2, characterized in that, The styrene-butyl acrylate-glycidyl methacrylate copolymer has a weight-average molecular weight of 3000 g / mol to 8000 g / mol, an epoxy equivalent of 250 g / mol to 500 g / mol, and a glycidyl methacrylate unit mass content of 10% to 25%.

4. The high heat-resistant modified PET cast film composition according to claim 1, characterized in that, The ultraviolet absorber is UV-326 or UV-328.

5. The high heat-resistant modified PET cast film composition according to claim 1, characterized in that, The intrinsic viscosity of the poly(m-phenylene adipamide) is 0.8 dL / g to 1.2 dL / g, the glass transition temperature is 80 to 90 °C, and the melting point is 230 °C to 240 °C.

6. The high heat-resistant modified PET cast film composition according to claim 1, characterized in that, The grafting rate of the maleic anhydride-grafted POE is 0.5% to 1.5%, and the melt index at 2.16 kg / 190 °C is 1 g / 10 min to 10 g / 10 min.

7. The high heat-resistant modified PET cast film composition according to claim 1, characterized in that, The vinyl acetate content in the ethylene-vinyl acetate copolymer is 5% to 28% by mass.

8. A method for preparing a high heat-resistant modified PET cast film composition as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Multilayer Co-extrusion Plasticization: The outer layer material, core layer material, and inner layer material are melted and plasticized separately through three single-screw extruders. The extrusion temperature of the outer layer material is 260℃~275℃, the extrusion temperature of the core layer material is 255℃~270℃, and the extrusion temperature of the inner layer material is 240℃~260℃. The three layers of melt are combined and extruded through a die at 265℃~280℃, and then attached to a cooling roller at 20℃~40℃ for rapid cooling and casting to obtain a multilayer co-extruded sheet. S2. Heat setting: The multilayer co-extruded sheet is heat-set at 180℃~210℃ for 3s~10s, and then cooled to room temperature to obtain the composition.

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