Heat-resistant polyester film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polyester films, in particular to a heat-resistant polyester film as well as a preparation method and application thereof. The thin heat-resistant polyester film comprises a base film and a heat-resistant functional coating coated on at least one surface of the base film, wherein the coating is formed by drying and heat-setting a heat-resistant composite coating liquid. The coating liquid comprises a waterborne polyurethane emulsion, an acrylic emulsion, a modified heat-resistant composite filler, a crosslinking curing agent, isobornyl acrylate, an antioxidant, a leveling agent and a defoaming agent. Wherein the modified heat-resistant composite filler is prepared by carrying out hydroxylation treatment on a heat-resistant filler and then carrying out grafting reaction on the heat-resistant filler and a modifier with a phosphorus-containing / silicon-containing double-bond copolymerization structure. Through filler surface modification and a synergistic film forming technology, the heat resistance, the dimensional stability, the flame retardance and the optical performance of the film are remarkably improved, and the film is suitable for the fields of flexible printed circuit boards, high-temperature-resistant labels, motor insulation, lithium battery packaging, solar battery backboards and the like.

Description

Technical Field

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

[0002] Polyester films, especially polyethylene terephthalate (PET) films, are widely used in flexible printed circuit boards (FPCs), motor insulation, high-temperature resistant labels, and new energy components due to their excellent mechanical properties, electrical insulation, and processing adaptability. However, with the continuous increase in power density and operating temperature of electronic devices, the heat resistance of traditional PET films is no longer sufficient to meet the requirements for long-term stable operation at 150°C or even higher temperatures. Their high thermal shrinkage rate can easily lead to dimensional instability and functional failure of devices. In existing technologies, polyethylene naphthalate (PEN) or copolymerized modified PET is usually used to increase the glass transition temperature, but these methods are often accompanied by problems such as a significant increase in raw material costs, a narrowing of the processing window, and increased film brittleness.

[0003] To improve the surface properties and heat resistance of polyester films, coating the base film surface with functional coatings is a common and effective technique. However, there are significant interfacial compatibility issues between the inorganic heat-resistant fillers (such as talc and silica) introduced into traditional coatings and the organic resin matrix. The fillers are unevenly dispersed and the interfacial bonding is weak. Under long-term thermal stress or thermal cycling, microcracks and peeling are prone to occur at the interface, leading to coating peeling and cracking. This not only fails to effectively suppress the thermal shrinkage of the film but may also introduce new failure points. At the same time, the glass transition temperature of the crosslinked network of conventional coatings is limited, and softening and deformation may still occur during subsequent high-temperature processing or use, making it difficult to form a solid skeletal support for the film as a whole.

[0004] Therefore, significantly improving the heat resistance, dimensional stability, and long-term reliability of polyester films without excessively increasing costs and process complexity, while simultaneously ensuring good interfacial adhesion, flame retardancy, and optical properties, has become a key technical challenge that urgently needs to be overcome in this field. Developing a heat-resistant polyester film that combines high performance with good process applicability is of great significance for promoting the development of high-end electronics and new energy industries. Summary of the Invention

[0005] The purpose of this invention is to address the technical problems of poor thermal stability, poor dimensional stability, and insufficient coating interface adhesion and compatibility of existing polyester films. This invention provides a heat-resistant polyester film, its preparation method, and its application.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A heat-resistant polyester film includes a base film and a heat-resistant functional coating disposed on at least one surface of the base film; the base film is a polyethylene terephthalate film; the heat-resistant functional coating is obtained by coating the base film surface with a heat-resistant composite coating liquid, followed by drying and heat setting; the heat-resistant composite coating liquid comprises an aqueous polyurethane emulsion, an acrylic emulsion, a modified heat-resistant composite filler, a crosslinking curing agent, isoborneol acrylate, an antioxidant, a leveling agent, and a defoamer; wherein the modified heat-resistant composite filler is obtained by grafting a heat-resistant filler with a phosphorus / silicon double bond copolymer modifier after hydroxylation treatment.

[0008] Traditional heat-resistant coatings applied to the base film surface often use inorganic fillers such as talc, silica, and unmodified hollow glass microspheres. These fillers have strong chemical inertness and poor interfacial bonding with resins such as waterborne polyurethane and acrylic emulsions. Under high temperature and thermal cycling, they are prone to micropores and cracks at the interface, leading to peeling, cracking, or even delamination from the base film. Secondly, the fillers in existing coatings have poor compatibility and dispersibility with the resin. The fillers are prone to agglomeration and sedimentation, making it impossible to form a continuous and uniform heat-resistant skeleton. This results in a limited glass transition temperature for the coating, which is still prone to softening and failure during heat setting. Therefore, it is difficult to simultaneously achieve high heat resistance, low thermal shrinkage, and long-term interfacial reliability.

[0009] To address the aforementioned deficiencies, this invention employs a modified heat-resistant composite filler in the heat-resistant functional coating introduced onto the polyester film surface. This filler is obtained by grafting a hydroxylated heat-resistant filler with a phosphorus / silicon double-bond copolymer modifier. The heat-resistant filler is treated with an alkaline aqueous solution to introduce hydroxyl groups onto its surface, giving the particle surface reactive functional groups. Titanium diboride and nano-boron nitride possess high modulus, high thermal decomposition temperature, and low coefficient of thermal expansion, while hollow glass microspheres have low density and low dielectric constant. These three components synergistically construct a lightweight, high-rigidity inorganic heat-resistant framework within the coating, which helps suppress thermal expansion and flow at high temperatures, reducing the film's thermal shrinkage rate and improving overall thermal stability from the source.

[0010] The heat-resistant composite coating liquid is composed of the following raw materials in parts by weight: 50-70 parts by weight of waterborne polyurethane emulsion, 8-15 parts by weight of acrylic emulsion, 12-20 parts by weight of the modified heat-resistant composite filler, 4-8 parts by weight of crosslinking curing agent, 5-9 parts by weight of isoborneol acrylate, 0.5-2 parts by weight of antioxidant, 0.5-1.5 parts by weight of leveling agent, and 1-3 parts by weight of defoamer.

[0011] The roles of each substance in the heat-resistant composite coating liquid: In this invention, the water-based polyurethane emulsion serves as the main film-forming resin of the coating. Its soft and hard segments can impart excellent flexibility, impact resistance, and adhesion to the coating, effectively buffering the thermal stress between the base film and the modified heat-resistant composite filler, and preventing cracking and delamination. At the same time, the polyurethane emulsion and acrylic emulsion have good compatibility and can jointly form a continuous and dense organic network structure, which plays a role in uniformly coating and fixing the filler, improving the mechanical strength, wear resistance, and water resistance of the coating, and providing a stable and reliable matrix for the heat-resistant functional coating.

[0012] Acrylic emulsions mainly function as a secondary film-forming resin and performance regulator: they can co-form films with phosphorus-containing waterborne polyurethane, improving the hardness, scratch resistance, and surface gloss of the coating; they can also improve the film-forming and network structure of the system and the wetting and adhesion of the base film, enhance the encapsulation and fixation ability of modified heat-resistant composite fillers, and take into account heat resistance, mechanical properties, and interfacial stability.

[0013] The modified heat-resistant composite filler is obtained by alkali treatment and synergistic modification of titanium diboride, nano-boron nitride, and hollow glass microspheres with organophosphorus / silane. TiB2 and h-BN have high modulus, high thermal decomposition temperature, and low coefficient of thermal expansion, forming a rigid heat-resistant skeleton within the coating, effectively suppressing thermal expansion and contraction. The hollow glass microspheres have low density and low dielectric constant, which can reduce the coating density and dielectric constant, while also reducing thermal conductivity. After hydroxylation and phosphorus / silicon-containing modification, the filler surface is coated with an organic shell layer compatible with waterborne polyurethane, which significantly improves dispersibility and interfacial compatibility in the coating, avoids agglomeration and interfacial defects, and thus maintains good mechanical properties while improving glass transition temperature and dimensional stability.

[0014] Isoborneol acrylate is an important organic compound with advantages such as low shrinkage, high adhesion, high hardness, and low irritation. It has a low shrinkage rate during crosslinking and is not prone to internal stress and cracking. It can improve the hardness of the coating while maintaining and improving its elasticity, and enhance the gloss and adhesion of the coating.

[0015] Antioxidants can capture free radicals or decompose peroxides, inhibiting thermo-oxidative degradation of resins during extrusion, drying, heat setting, and high-temperature use, thus reducing yellowing, cracking, and molecular weight degradation. The addition of antioxidants ensures the chemical stability of the coating and modified filler shell structure under high-temperature, long-term service conditions.

[0016] Leveling agents, through their surface-active segments oriented on the coating surface, significantly reduce surface tension differences, improve the wettability of the coating liquid on the PET base film and the coating itself, and promote the flow and self-leveling of the coating during drying and stretching. This effectively eliminates surface defects such as pinholes and fisheyes, resulting in a smooth, high-gloss, and uniformly thick coating interface, while not significantly affecting the crosslinking and heat resistance of the system.

[0017] Defoamers reduce local surface tension and disrupt the elasticity of the bubble film, causing the bubbles to burst and release rapidly during the coating and drying stages. This ensures that the coating is dense and free of pores, which is beneficial for improving heat and electrical resistance and mechanical strength.

[0018] The preparation method of the modified heat-resistant composite filler includes the following steps:

[0019] S1. The heat-resistant filler is hydroxylated in an alkaline aqueous solution to obtain hydroxylated heat-resistant filler;

[0020] S2. The organophosphorus monomer and the silane coupling agent are copolymerized in an organic solvent in the presence of an initiator to obtain the modifier with the phosphorus-containing / silicon-containing double bond copolymer structure.

[0021] S3. The hydroxylated heat-resistant filler and nonionic surfactant are dispersed in an ethanol aqueous solution, and the modifier containing the phosphorus / silicon double bond copolymer structure is added to carry out a grafting reaction. After post-treatment, the modified heat-resistant composite filler is obtained.

[0022] This invention prepares a modifier with a phosphorus / silicon double bond copolymer structure by copolymerizing a double-bond organophosphorus monomer with a silane coupling agent in an organic solvent in the presence of an initiator. The modifier is then grafted onto a hydroxylated heat-resistant filler to coat the surface of the inorganic filler with an organic shell containing both phosphate ester groups and alkoxysilane groups. On the one hand, the alkoxysilanes on the surface undergo hydrolysis and condensation in the ethanol / water system and during subsequent heat setting, forming stable covalent bonds with the hydroxylated filler surface. This firmly anchors the inorganic particles within the organic network, significantly improving the interfacial bonding between the filler and the coating resin, and preventing interfacial microcracks and delamination under high temperature and thermal cycling. On the other hand, the phosphate ester groups and acrylic double bonds in the modifier segments exhibit good polar compatibility and copolymerization / crosslinking ability with waterborne polyurethane emulsions and acrylic emulsions. At high temperatures, they possess both a high glass transition temperature and a low coefficient of linear expansion, effectively limiting the heat flow and thermal strain transfer of the coating. This allows the modified heat-resistant composite filler to be uniformly and stably dispersed in the coating, forming a continuous and stable structure of organic and inorganic heat-resistant fillers. Furthermore, titanium diboride and nano-boron nitride themselves possess high modulus and low coefficient of thermal expansion. During heating, this heat-resistant filler preferentially bears and disperses thermal stress from the base film, preventing stress concentration at the interface and the generation of microcracks and warping, thereby significantly improving the smoothness of the film. Hollow glass microspheres are uniformly dispersed in an organic / inorganic network, which can reduce the local refractive index gradient and thermal conductivity, and further reduce the overall density and thermal shrinkage deformation of the coating without significantly sacrificing light transmittance.

[0023] Preferably, the modified heat-resistant composite filler is prepared as follows:

[0024] S1. Take 16-28 parts by weight of heat-resistant filler and add it to 150-300 parts by weight of 5-12wt% sodium hydroxide aqueous solution. Sonicate for 0.5-2h; stir at 60-80℃ and 300-600rpm for 1-4h; after the reaction is completed, filter, wash and dry to obtain hydroxylated heat-resistant filler.

[0025] S2. Under nitrogen atmosphere, mix 2-6 parts by weight of organophosphorus monomer, 4-8 parts by weight of silane coupling agent and 80-160 parts by weight of N,N-dimethylformamide, and then add 0.2-0.8 parts by weight of benzoyl peroxide; react at 65-85℃ and 300-600 rpm for 2-6 hours, remove solvent by vacuum distillation to obtain a modifier with a phosphorus / silicon double bond copolymer structure;

[0026] S3. Take 15-24 parts by weight of the above hydroxylated heat-resistant filler, add 150-300 parts by weight of 50-80 wt% ethanol aqueous solution, add 0.5-2 parts by weight of nonionic surfactant, and ultrasonically disperse for 20-60 min; add 2-6 parts by weight of a modifier containing phosphorus / silicon double bond copolymer structure, and react at 60-80℃ and 300-500 rpm for 2-8 h, filter, wash, and spray dry to obtain the modified heat-resistant composite filler.

[0027] The heat-resistant filler is selected from at least two of titanium diboride, nano boron nitride, hollow glass microspheres, silicon dioxide, alumina, and silicon carbide; preferably, the heat-resistant filler includes titanium diboride, nano boron nitride, and hollow glass microspheres, wherein the mass ratio of titanium diboride, nano boron nitride, and hollow glass microspheres is (1-3):(1-3):(2-6).

[0028] In step S2, the organophosphorus monomer is selected from at least one of 2-methacryloyloxyethyl phosphoric acid choline, 2-methyl-2-acrylate-2-hydroxyethyl phosphate, and (meth)acrylate phosphate; preferably, the organophosphorus monomer includes 2-methacryloyloxyethyl phosphoric acid choline and 2-methyl-2-acrylate-2-hydroxyethyl phosphate, with a mass ratio of 1:(1-3); further, the organophosphorus monomer is composed of 2-methacryloyloxyethyl phosphoric acid choline and 2-methyl-2-acrylate-2-hydroxyethyl phosphate in a mass ratio of 1:2.

[0029] By compounding choline-type organophosphorus monomers with hydroxyethyl phosphate-type organophosphorus monomers in a certain proportion, on the one hand, the strong polarity of choline-type monomers and their strong adsorption to heat-resistant fillers are utilized to form a dense and strongly anchored organic shell layer on the filler surface; on the other hand, the excellent compatibility and cross-linking properties of hydroxyethyl phosphates are relied upon to firmly integrate this shell layer with the waterborne polyurethane / acrylic network. Combined with silane, it can inhibit interface debonding and coating flow at high temperatures, reduce thermal shrinkage, improve flatness and dimensional stability, and promote the formation of a dense phosphorus-rich carbon layer during combustion, which works synergistically with ceramics to improve the limiting oxygen index, while maintaining high light transmittance and low haze.

[0030] The silane coupling agent is selected from silane coupling agents containing double bonds, and the double bond silane coupling agent is selected from at least one of N-(3-acryloyloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-(methacryloyloxy)propyltrimethoxysilane, and vinyltri(2-methoxyethoxy)silane.

[0031] The nonionic surfactant includes at least one of alkylphenol polyoxyethylene ethers, polysorbate esters, and polyoxyethylene fatty acid esters.

[0032] The crosslinking curing agent is selected from at least one of acid anhydride curing agents and isocyanate curing agents; the defoamer is selected from at least one of phosphate ester, organosilicon, and polyether defoamers; the antioxidant is selected from any one of hindered phenolic or phosphite antioxidants; and the leveling agent is selected from any one of polyether-modified polysiloxane or fluorinated acrylate leveling agents.

[0033] The method for preparing the heat-resistant polyester film includes the following steps:

[0034] (1) Place the base film slices in an extruder for melt extrusion at a temperature of 260-300℃ to obtain polyester melt; after the polyester melt flows through the die head, it forms a cast plastic sheet on the cooling roller; after the cast plastic sheet is preheated at 70-110℃, it is stretched longitudinally by 3-5 times to obtain a longitudinally stretched film.

[0035] (2) The surface of the longitudinally stretched diaphragm is subjected to corona treatment to increase its surface tension to 50 mN / m;

[0036] (3) Apply the heat-resistant composite coating liquid to one side of the longitudinally stretched film to obtain a film coated with the heat-resistant composite coating liquid; introduce it into the preheating zone for transverse stretching and stretch it transversely 3-6 times to obtain a film with a heat-resistant interface coating.

[0037] (4) Heat set the above film at a heat setting temperature of 200-240℃ for 3-15s, cool it and then roll it up to obtain a heat-resistant polyester film.

[0038] The heat-resistant composite coating liquid has a wet coating thickness of 5-20 μm after coating. After coating, it is pre-dried at 80-120℃ before stretching.

[0039] The heat-resistant polyester film is used in flexible printed circuit boards, high-temperature resistant labels, motor insulation, lithium battery packaging, or solar cell backsheets.

[0040] The beneficial effects of this invention are:

[0041] 1. This invention provides a heat-resistant polyester film, its preparation method, and its application. The heat-resistant composite filler is obtained by compounding titanium diboride, nano-boron nitride, and hollow glass microspheres in a specific ratio, followed by hydroxylation treatment and grafting with a phosphorus / silicon double-bond copolymer modifier. This forms a continuously and uniformly dispersed heat-resistant filler in an aqueous polyurethane / acrylic resin matrix. The high modulus and low thermal expansion characteristics of titanium diboride and nano-boron nitride significantly suppress the thermal expansion and thermal strain of the coating. After synergistic modification with P and Si, a dense chemical bond interface is formed between the filler and the resin, effectively avoiding interfacial debonding and micropores that are prone to occur in traditional unmodified filler systems. This significantly reduces the thermal shrinkage and warpage of the film during high-temperature heat setting and use, and greatly improves the limiting oxygen index of the film. Simultaneously, the modification improves interfacial compatibility, reducing the interfacial refractive index difference and the number of light scattering centers while maintaining the density and heat-resistant framework structure of the coating. This allows the film of this invention to maintain high light transmittance and low haze while improving flame retardancy and heat resistance, and the film flatness is significantly better than that of the unmodified system.

[0042] 2. This invention employs a synergistic film-forming process using waterborne polyurethane emulsion and acrylic emulsion, combined with a blocked isocyanate crosslinking curing agent and low-shrinkage comonomers such as isoborneol acrylate. During the coating curing process, a crosslinking network with a reasonable balance of flexible and hard segments is formed, ensuring both adhesion and flexibility of the coating to the PET base film while avoiding the brittleness issues common in high-crosslink density systems. Through the synergistic effect of antioxidants, leveling agents, and defoamers, the coating further ensures resistance to yellowing, surface smoothness, and internal density under high-temperature heat setting and long-term service conditions, exhibiting an overall excellent balance of heat resistance, flame retardancy, optical performance, and smoothness. The film of this invention has significantly improved light transmittance, making it suitable for applications requiring high optical performance, such as flexible printed circuit boards and photovoltaic backsheets. Detailed Implementation

[0043] The invention will now be described in further detail with reference to specific embodiments, but it should not be construed as limiting the scope of the invention to the following embodiments.

[0044] The raw materials described in this application are partially described; all other raw materials not described are commercially available.

[0045] The base film used is commercially available PET film, purchased from Shijiazhuang Dajia New Material Technology Co., Ltd., model PETL5.

[0046] The waterborne polyurethane emulsion was purchased from Shanghai Bolino New Material Technology Co., Ltd., grade: 1352C.

[0047] The acrylic emulsion was purchased from Shanghai Zhenlishi Network Technology Co., Ltd., item number: ST-954.

[0048] The crosslinking curing agent was purchased from Shanghai Mingxu Electronic Technology Co., Ltd., item number: MX-8140.

[0049] 2-Methacryloxyethylphosphocholine, CAS: 67881-98-5.

[0050] 2-Methyl-2-acrylate-2-hydroxyethyl phosphate, CAS: 52628-03-2.

[0051] The polyether-modified polysiloxane was purchased from Jining Fangyu Chemical Co., Ltd., model: SR-260.

[0052] Titanium diboride was purchased from Hebei Tengbimetallic Materials Co., Ltd., grade: TiB2-1

[0053] The boron nitride nanoparticles were purchased from Shanghai Maoguo Nanotechnology Co., Ltd., grade: MG-BN-100.

[0054] The hollow glass microspheres were purchased from Lingshou County Haibin Mineral Products Trading Co., Ltd., with a particle size of 100 mesh.

[0055] Example 1

[0056] A heat-resistant polyester film includes a base film and a heat-resistant functional coating disposed on the surface of the base film, wherein the base film is a PET film; the heat-resistant functional coating is obtained by coating the base film surface with a heat-resistant composite coating liquid and then drying and heat-setting.

[0057] The heat-resistant composite coating liquid is composed of the following raw materials in parts by weight: 55 parts by weight of waterborne polyurethane emulsion, 12 parts by weight of acrylic emulsion, 16 parts by weight of modified heat-resistant composite filler, 6 parts by weight of crosslinking curing agent, 7 parts by weight of isoborneol acrylate, 1 part by weight of antioxidant, 1 part by weight of leveling agent, and 2 parts by weight of defoamer.

[0058] The crosslinking curing agent is a water-based blocked isocyanate curing agent.

[0059] The antioxidant is antioxidant 1010.

[0060] The leveling agent is a polyether-modified polysiloxane.

[0061] The defoamer is tributyl phosphate.

[0062] The modified heat-resistant composite filler is prepared as follows:

[0063] S1. Take 20 parts by weight of heat-resistant filler and add it to 180 parts by weight of 8wt% sodium hydroxide aqueous solution, and sonicate for 1 hour; stir at 70℃ and 400 rpm for 2 hours; after the reaction is completed, filter, wash, and dry to obtain hydroxylated heat-resistant filler; the heat-resistant filler includes titanium diboride, nano boron nitride and hollow glass microspheres, wherein the mass ratio of titanium diboride, nano boron nitride and hollow glass microspheres is 1:2:2;

[0064] S2. Under nitrogen atmosphere, 4 parts by weight of organophosphorus monomer, 6 parts by weight of silane coupling agent, and 100 parts by weight of N,N-dimethylformamide are mixed, and 0.5 parts by weight of benzoyl peroxide is added; the mixture is reacted at 78°C and 400 rpm for 4 hours, and the solvent is removed by vacuum distillation to obtain a modifier with a phosphorus / silicon double bond copolymer structure; the silane coupling agent is N-(3-acryloyloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane; the organophosphorus monomer is composed of 2-methacryloyloxyethyl phosphate choline and 2-methyl-2-acrylate-2-hydroxyethyl phosphate in a mass ratio of 1:2.

[0065] S3. Take 18 parts by weight of the above hydroxylated heat-resistant filler, add 180 parts by weight of 60wt% ethanol aqueous solution, add 1 part by weight of nonionic surfactant, wherein the nonionic surfactant is Tween 20, and ultrasonically disperse for 30 min; add 4 parts by weight of a modifier containing phosphorus / silicon double bond copolymer structure, react at 70℃ and 400 rpm for 4 h, filter, wash, and spray dry to obtain the modified heat-resistant composite filler.

[0066] The method for preparing the heat-resistant polyester film includes the following steps:

[0067] (1) The base film slices are placed in an extruder for melt extrusion at a temperature of 280°C to obtain polyester melt; the polyester melt flows through the die head and forms a cast plastic sheet on the cooling roller; the cast plastic sheet is preheated at 90°C and then stretched longitudinally by 4 times to obtain a longitudinally stretched film.

[0068] (2) The surface of the longitudinally stretched diaphragm is subjected to corona treatment to increase its surface tension to 50 mN / m;

[0069] (3) The heat-resistant composite coating liquid is coated on one side of the longitudinally stretched film to obtain a film coated with the composite coating liquid. The thickness of the wet coating is controlled at 10 μm. After coating, it is pre-dried at 100°C. The film coated with the heat-resistant composite coating liquid is held by clamps and introduced into the preheating zone for transverse stretching. It is stretched transversely by 4.8 times at 110°C to obtain a film with a heat-resistant interface coating.

[0070] (4) The above film is heat-set at a heat-setting temperature of 220°C for 8 seconds, cooled and then rolled up to obtain a heat-resistant polyester film.

[0071] Example 2

[0072] The method is basically the same as that in Example 1, except that the organophosphorus monomer in the preparation method of the modified heat-resistant composite filler is 2-methacryloyloxyethyl phosphocholine.

[0073] Example 3

[0074] The method is basically the same as that in Example 1, except that the organophosphorus monomer in the preparation method of the modified heat-resistant composite filler is 2-methyl-2-acrylate-2-hydroxyethyl phosphate.

[0075] Example 4

[0076] The method is basically the same as that in Example 1, except that the silane coupling agent in the preparation method of the modified heat-resistant composite filler is 3-(methacryloyloxy)propyltrimethoxysilane.

[0077] Comparative Example 1

[0078] The process is basically the same as in Example 1, except that the modified heat-resistant composite filler in the heat-resistant composite coating liquid is replaced with a heat-resistant composite filler in the following parts by weight of raw materials.

[0079] The heat-resistant filler includes titanium diboride, nano-boron nitride, and hollow glass microspheres, wherein the mass ratio of titanium diboride, nano-boron nitride, and hollow glass microspheres is 1:2:2.

[0080] Comparative Example 2

[0081] The method is basically the same as in Example 1, except that the modified heat-resistant composite filler is prepared as follows: S1, 20 parts by weight of heat-resistant filler are added to 180 parts by weight of 8wt% sodium hydroxide aqueous solution and sonicated for 1 hour; the mixture is stirred at 70℃ and 400 rpm for 2 hours; after the reaction is completed, the mixture is filtered, washed, and dried to obtain hydroxylated heat-resistant filler; the heat-resistant filler includes titanium diboride, boron nitride nanoparticles and hollow glass microspheres, wherein the mass ratio of titanium diboride, boron nitride nanoparticles and hollow glass microspheres is 1:2:2;

[0082] S2 Take 18 parts by weight of the above hydroxylated heat-resistant filler, add 180 parts by weight of 60wt% ethanol aqueous solution, add 1 part by weight of nonionic surfactant, the nonionic surfactant being Tween 20, and ultrasonically disperse for 30 min; add 4 parts by weight of silane coupling agent, react at 70℃ and 400 rpm for 4 h, filter, wash, and spray dry to obtain the modified heat-resistant composite filler.

[0083] The silane coupling agent is N-(3-acryloyloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane.

[0084] Comparative Example 3

[0085] The method is basically the same as in Example 1, except that the modified heat-resistant composite filler is prepared as follows: S1, 20 parts by weight of heat-resistant filler are added to 180 parts by weight of 8wt% sodium hydroxide aqueous solution and sonicated for 1 hour; the mixture is stirred at 70℃ and 400 rpm for 2 hours; after the reaction is completed, the mixture is filtered, washed, and dried to obtain hydroxylated heat-resistant filler; the heat-resistant filler includes titanium diboride, boron nitride nanoparticles and hollow glass microspheres, wherein the mass ratio of titanium diboride, boron nitride nanoparticles and hollow glass microspheres is 1:2:2;

[0086] S2. Take 18 parts by weight of the above hydroxylated heat-resistant filler, add 180 parts by weight of 60wt% ethanol aqueous solution, add 1 part by weight of nonionic surfactant, wherein the nonionic surfactant is Tween 20, and ultrasonically disperse for 30 min; add 4 parts by weight of 2-methacryloyloxyethyl phosphocholine, react at 70℃ and 400 rpm for 4 h, filter, wash, and spray dry to obtain the modified heat-resistant composite filler.

[0087] Comparative Example 4

[0088] It is basically the same as Example 1, except that the organophosphorus monomer is tributyl phosphate without double bonds.

[0089] Comparative Example 5

[0090] It is basically the same as Example 1, except that the organophosphorus monomer is dimethyl methylphosphonate without double bonds.

[0091] Comparative Example 6

[0092] The method is basically the same as in Example 1, except that the organophosphorus monomer is composed of 2-methacryloyloxyethyl phosphocholine and tributyl phosphate in a mass ratio of 1:2.

[0093] Comparative Example 7

[0094] The method is basically the same as that in Example 1, except that the silane coupling agent in S2 of the preparation method of the modified heat-resistant composite filler is replaced with a silane coupling agent without double bonds, and the silane coupling agent without double bonds is propyltrimethoxysilane.

[0095] Test Example 1

[0096] Heat resistance test: The heat-resistant polyester films in the above examples and comparative examples were tested for heat shrinkage in accordance with the national standard QB / T 5077-2017 "Biaxially oriented polyethylene terephthalate film for heat transfer ribbon". Four tests were conducted for each group and the average value was taken. The results are shown in Table 1.

[0097] Flatness test: Take 10 samples from each group on the heat-resistant polyester film of the obtained product. The sample size is 100cm×100cm. Lay the sample flat on a flat black marble slab. Use a feeler gauge to measure the gap between the angle and side length of the sample and the marble slab. Rating is done according to the following method: after heat treatment at 150℃ for 30min, the number of protrusions with a height ≥1mm and a depth L ≥100mm, and the number of protrusions with a height ≥0.5mm and a depth L ≥10mm.

[0098] Acceptable: The number of protrusions with a height ≥ 1 mm and a depth L ≥ 100 mm is ≤ 2, and the number of protrusions with a height ≥ 0.5 mm and a depth L ≥ 10 mm is ≤ 4.

[0099] Unacceptable: The number of protrusions with a height ≥ 1 mm and a depth L ≥ 100 mm > 2 or the number of protrusions with a height ≥ 0.5 mm and a depth L ≥ 10 mm > 5.

[0100] Table 1 Results of heat resistance and smoothness tests

[0101] Heat shrinkage (%) flatness Example 1 0.02 qualified Example 2 0.06 qualified Example 3 0.04 qualified Example 4 0.05 qualified Comparative Example 1 0.63 Unqualified Comparative Example 2 0.35 Unqualified Comparative Example 3 0.51 Unqualified Comparative Example 4 0.43 Unqualified Comparative Example 5 0.48 Unqualified Comparative Example 6 0.37 Unqualified Comparative Example 7 0.47 Unqualified

[0102] Test Example 2

[0103] Flame retardant performance test: The flame retardant performance of the heat-resistant polyester films in the above examples and comparative examples was tested in accordance with the national standard GB / T2406.2-2009 "Determination of burning behavior of plastics by oxygen index method - Part 2: Room temperature test". Four tests were conducted for each group, and the average value was taken. The results are shown in Table 2.

[0104] Transmittance test: The transmittance of the heat-resistant polyester films in the above examples and comparative examples was tested by method B spectrophotometry in the national standard GB / T2410-2008 "Determination of transmittance and haze of transparent plastics". The films were processed into discs with a diameter of 50 mm and a thickness of 2 mm and conditioned at a temperature of 23℃ and a relative humidity of 50% for 48 h. Four sets of tests were conducted for each group, and the average value was taken. The results are shown in Table 2.

[0105] Table 2. Test results of light transmittance and flame retardant performance.

[0106] Light transmittance (%) Limiting oxygen index (%) Example 1 91.7 35.8 Example 2 89.3 33.7 Example 3 90.1 34.2 Example 4 90.8 34.6 Comparative Example 1 68.2 23.7 Comparative Example 2 81.3 26.3 Comparative Example 3 74.6 31.8 Comparative Example 4 78.3 26.3 Comparative Example 5 75.1 27.8 Comparative Example 6 80.9 31.6 Comparative Example 7 76.1 30.3

[0107] The results above show that the heat-resistant polyester film prepared by this invention has good heat resistance, as well as good light transmittance and flame retardant properties. The specific reasons are as follows: The examples use heat-resistant fillers synergistically modified with modifiers containing phosphorus / silicon double bonds in a copolymer structure, and combined with a waterborne polyurethane / acrylic crosslinking coating to form a continuous and dense organic and inorganic heat-resistant skeleton structure. The thermal shrinkage rate of the films in the examples is controlled below 0.06%, the flatness is qualified, the light transmittance remains above approximately 90%, and the limiting oxygen index reaches 33-36%. Further comparison of Examples 1-3 shows that the modified filler prepared in Example 1 using a specific ratio of compounded organophosphorus monomers exhibits more balanced and superior performance in terms of thermal shrinkage rate, light transmittance, and limiting oxygen index compared to Examples 2 and 3 which use a single organophosphorus monomer.

[0108] In contrast, in systems where the filler was either not surface-modified, or only silanes, single organophosphorus monomers, or only non-reactive low-molecular-weight phosphorus plasticizers were introduced, or systems that were unmodified or only partially modified, the filler was prone to agglomeration, had poor interfacial compatibility, and suffered from severe interfacial debonding and stress concentration at high temperatures. This resulted in a thermal shrinkage rate generally greater than 0.28% and unsatisfactory flatness. Simultaneously, interfacial defects and agglomerated particles significantly increased light scattering, with light transmittance only 68-82%. Regarding flame retardancy, the lack of phosphorus-containing cross-linked structures and the synergistic effect of silicon made it difficult to form a dense char layer barrier during combustion, thus reducing flame retardant performance.

Claims

1. A heat-resistant polyester film, characterized in that, The invention includes a base film and a heat-resistant functional coating disposed on at least one surface of the base film; the base film is a polyethylene terephthalate film; the heat-resistant functional coating is obtained by coating the base film surface with a heat-resistant composite coating liquid and then drying and heat-setting; the heat-resistant composite coating liquid contains an aqueous polyurethane emulsion, an acrylic emulsion, a modified heat-resistant composite filler, a crosslinking curing agent, isoborneol acrylate, an antioxidant, a leveling agent, and a defoamer; wherein the modified heat-resistant composite filler is prepared by grafting a heat-resistant filler with a phosphorus / silicon double bond copolymer modifier after hydroxylation treatment.

2. The heat-resistant polyester film according to claim 1, characterized in that, The heat-resistant composite coating liquid is composed of the following raw materials in parts by weight: 50-70 parts by weight of waterborne polyurethane emulsion, 8-15 parts by weight of acrylic emulsion, 12-20 parts by weight of the modified heat-resistant composite filler, 4-8 parts by weight of crosslinking curing agent, 5-9 parts by weight of isoborneol acrylate, 0.5-2 parts by weight of antioxidant, 0.5-1.5 parts by weight of leveling agent, and 1-3 parts by weight of defoamer.

3. The heat-resistant polyester film according to claim 1, characterized in that, The preparation method of the modified heat-resistant composite filler includes the following steps: S1. The heat-resistant filler is hydroxylated in an alkaline aqueous solution to obtain hydroxylated heat-resistant filler; S2. The organophosphorus monomer and the silane coupling agent are copolymerized in an organic solvent in the presence of an initiator to obtain the modifier with the phosphorus-containing / silicon-containing double bond copolymer structure. S3. The hydroxylated heat-resistant filler and nonionic surfactant are dispersed in an ethanol aqueous solution, and the modifier containing the phosphorus / silicon double bond copolymer structure is added to carry out a grafting reaction. After post-treatment, the modified heat-resistant composite filler is obtained.

4. The heat-resistant polyester film according to claim 3, characterized in that, The heat-resistant filler is selected from at least two of titanium diboride, nano boron nitride, hollow glass microspheres, silicon dioxide, alumina, and silicon carbide.

5. The heat-resistant polyester film according to claim 3, characterized in that, In step S2, the organophosphorus monomer is selected from at least one of 2-methacryloyloxyethyl phosphocholine, 2-methyl-2-acrylate-2-hydroxyethyl phosphate, and (meth)acrylate phosphate; the silane coupling agent is selected from silane coupling agents containing double bonds, and the double-bonded silane coupling agent is selected from at least one of N-(3-acryloyloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-(methacryloyloxy)propyltrimethoxysilane, and vinyltris(2-methoxyethoxy)silane.

6. The heat-resistant polyester film according to any one of claims 1-5, characterized in that, The crosslinking curing agent is selected from at least one of acid anhydride curing agents and isocyanate curing agents; the defoamer is selected from at least one of phosphate ester, organosilicon, and polyether defoamers; the antioxidant is selected from any one of hindered phenolic or phosphite antioxidants; and the leveling agent is selected from any one of polyether-modified polysiloxane or fluorinated acrylate leveling agents.

7. A method for preparing a heat-resistant polyester film as described in any one of claims 1-6, characterized in that, Includes the following steps: (1) Place the base film slices in an extruder for melt extrusion at a temperature of 260-300℃ to obtain polyester melt; after the polyester melt flows through the die head, it forms a cast plastic sheet on the cooling roller; after the cast plastic sheet is preheated at 70-110℃, it is stretched longitudinally by 3-5 times to obtain a longitudinally stretched film. (2) The surface of the longitudinally stretched diaphragm is subjected to corona treatment to increase its surface tension to 50 mN / m; (3) Apply the heat-resistant composite coating liquid to one side of the longitudinally stretched film to obtain a film coated with the heat-resistant composite coating liquid; introduce it into the preheating zone for transverse stretching and stretch it transversely 3-6 times to obtain a film with a heat-resistant interface coating. (4) Heat set the above film at a heat setting temperature of 200-240℃ for 3-15s, cool it and then roll it up to obtain a heat-resistant polyester film.

8. The method for preparing the heat-resistant polyester film according to claim 7, characterized in that, The heat-resistant composite coating liquid has a wet coating thickness of 5-20 μm after coating. After coating, it is pre-dried at 80-120℃ before stretching.

9. The application of a heat-resistant polyester film as described in any one of claims 1-6 in flexible printed circuit boards, high-temperature resistant labels, motor insulation, lithium battery encapsulation, or solar cell backsheets.