Flame retardant, flame-retardant polyester film and preparation method of flame-retardant polyester film

Abstract

The invention relates to the technical field of high polymer materials, particularly provides a flame retardant, and also provides a flame-retardant polyester film based on the flame retardant and a preparation method of the flame-retardant polyester film. The flame retardant is prepared from the following raw materials in parts by weight: 63 to 68 parts of modified magnesium hydroxide, 10 to 15 parts of modified ammonium polyphosphate, 8 to 12 parts of pentaerythritol phosphate and 5 to 8 parts of melamine cyanurate. According to the invention, a synergistic flame-retardant system constructed by a main flame retardant and an intumescent synergist is combined with the enhanced synergist modified boehmite nanorods, so that multi-dimensional flame-retardant protection of heat absorption and cooling, oxygen isolation, catalytic char formation, intumescent heat resistance and physical barrier is realized, the raw materials are environment-friendly, and the preparation process is simple.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a flame retardant, as well as a flame-retardant polyester film based on the flame retardant and its preparation method. Background Technology

[0002] Polyester film (polyethylene terephthalate, PET) is widely used in electronic devices, packaging materials, photovoltaic backsheets, and other fields due to its excellent mechanical properties, transparency, and chemical stability. The limiting oxygen index (LOI) refers to the minimum oxygen concentration required for a material to burn completely under specific conditions. A higher LOI indicates better combustion performance and less flammability. However, traditional polyester films have a low LOI (typically <22%), making PET products highly flammable when exposed to an ignition source. Furthermore, the combustion process easily produces molten droplets, posing a serious fire hazard and a significant threat to life and property safety.

[0003] Currently, the main methods for improving the flame retardant properties of polyester films include adding traditional flame retardants, surface coating with flame retardant coatings, and copolymer modification. However, these methods all have significant drawbacks: among traditional flame retardants, halogenated flame retardants, while improving LOI (Liquidity Index), are not easily degraded in the environment, causing long-term impacts on the ecosystem and exhibiting poor environmental friendliness; inorganic flame retardants (such as aluminum hydroxide and nano-clay), while possessing environmental advantages, require large additions to achieve the V-1 flame retardant standard, and their poor dispersibility significantly weakens the mechanical properties of the polyester film. Surface coating with flame retardant coatings or copolymer modification processes can partially improve flame retardancy, but the former carries the risk of coating peeling and may affect the transparency and mechanical properties of the polyester film; the latter relies on specialized equipment and complex processes, resulting in high production costs. In summary, existing technical solutions generally face problems such as low flame retardant efficiency, insufficient environmental friendliness, deterioration of mechanical properties, or high process costs, necessitating the development of new polyester film materials that combine high flame retardancy, environmental friendliness, and process feasibility. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a flame retardant and a flame-retardant polyester film based on this flame retardant, as well as a method for its preparation. The flame-retardant polyester film provided by this invention employs a synergistic flame-retardant system constructed from a primary flame retardant and an intumescent synergist, further combined with boehmite nanorods modified by a reinforcing synergist. This achieves multi-dimensional flame-retardant protection through "heat absorption and cooling, oxygen isolation, catalytic char formation, expansion and heat insulation, and physical barrier," resulting in a polyester film with high flame-retardant performance.

[0005] To solve the above-mentioned technical problems, in a first aspect, the present invention provides a flame retardant comprising the following raw materials by weight: 63-68 parts of modified magnesium hydroxide, 10-15 parts of modified ammonium polyphosphate, 8-12 parts of pentaerythritol phosphate, and 5-8 parts of melamine cyanurate.

[0006] This invention employs a primary flame retardant and an intumescent synergist (acid source-carbon source-gas source) to synergistically construct a flame retardant system, achieving multi-dimensional flame retardant protection during polyester film combustion. Surface-modified magnesium hydroxide, as the primary flame retardant, first undergoes endothermic decomposition at approximately 340°C, absorbing 1300J of heat per gram. During decomposition, it generates a large amount of water vapor, which can dilute the oxygen concentration, lower the substrate temperature, and inhibit smoke generation.

[0007] Modified ammonium polyphosphate, as an acid source, decomposes during heating to form phosphoric acid substances. On the one hand, it promotes the dehydration of the PET matrix to form a coke layer covering the surface of the PET matrix, which isolates the PET matrix from the combustion-supporting gases in the external environment and blocks heat transfer, which is the core of the formation of the expanded carbon layer. On the other hand, modified ammonium polyphosphate can react with ammonia gas released from melamine cyanurate gas source to generate nitrogen-containing pyrophosphate with higher thermal stability, which accelerates the formation of a dense carbon layer.

[0008] Pentaerythritol phosphate (PEPA) is used as a carbon source and cross-linked and carbonized under acid catalysis to construct the framework structure of the expanded carbon layer.

[0009] Melamine cyanurate (MCA), used as a gas source, releases a large amount of non-flammable gases, such as ammonia and nitrogen, when it decomposes upon heating. This causes the molten char layer to expand and foam, while simultaneously reducing the concentration of flammable gases and slowing down the combustion rate. Furthermore, the cyanuric acid produced by the decomposition of MCA can react with the ammonia produced by the decomposition of modified ammonium polyphosphate to form a more stable nitrogen-containing heterocyclic structure with better thermal insulation, further enhancing the strength of the char layer.

[0010] The flame retardant provided by this invention uses modified magnesium hydroxide, modified ammonium polyphosphate, pentaerythritol phosphate, and melamine cyanurate as its core components. It employs a multiple mechanism of "endothermic cooling - oxygen isolation - catalytic char formation - expansion and heat insulation" to achieve highly efficient flame retardant protection for polyester films. Furthermore, the raw materials used in this flame retardant are environmentally friendly, achieving both high efficiency and environmental friendliness. It significantly reduces the burning rate of polyester films and suppresses smoke release, aligning with green environmental protection principles and possessing promising application prospects and market value.

[0011] Preferably, the modified magnesium hydroxide is silane coupling agent modified magnesium hydroxide; the silane coupling agent is selected from at least one of γ-methacryloyloxypropyltrimethoxysilane (KH-570), vinyl-tris(2-methoxyethoxy)silane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, and γ-aminopropyltriethoxysilane.

[0012] The present invention coats magnesium hydroxide with a silane coupling agent, which can improve its compatibility with the PET matrix, enhance its dispersibility, and reduce its negative impact on the mechanical properties of the material.

[0013] Preferably, the method for preparing the modified magnesium hydroxide includes the following steps: Magnesium hydroxide slurry is prepared by mixing magnesium hydroxide, deionized water, and an alkali. Separately, a silane coupling agent is dissolved in an organic solvent, the pH is adjusted to 11-13, and the solution is added to the magnesium hydroxide slurry. The mixture is stirred and reacted at 40-80°C for 30-120 minutes, filtered, and dried to obtain the modified magnesium hydroxide. The alkali is selected from sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. The amount of silane coupling agent is 0.1%-0.5% of the mass of magnesium hydroxide. The organic solvent is selected from methanol, ethanol, isopropanol, etc. The particle size D50 of the magnesium hydroxide is <2 μm.

[0014] Preferably, the modified ammonium polyphosphate is melamine-modified ammonium polyphosphate; the amount of melamine used is 5%-15% of the mass of the ammonium polyphosphate.

[0015] This invention uses melamine to modify ammonium polyphosphate to form a partially "pre-expanded" structure, which effectively improves the initial reaction activity of ammonium polyphosphate with carbon and gas sources, and promotes the formation of a faster and denser carbon layer.

[0016] Preferably, the preparation method of the melamine-modified ammonium polyphosphate includes the following steps: Melamine was dissolved in water, and ammonium polyphosphate was added. The mixture was stirred at 100-180°C for 1-2 hours to obtain the modified ammonium polyphosphate.

[0017] Secondly, the present invention provides a flame-retardant polyester film, the flame-retardant polyester film comprising a substrate layer and flame-retardant layers located on both sides of the substrate layer, forming a superimposed structure of flame-retardant layer-substrate layer-flame-retardant layer, wherein the raw materials for preparing the flame-retardant layer include the flame retardant agent described in the present invention.

[0018] Preferably, the flame retardant layer comprises the following raw materials by weight: 86-100 parts polyester chips, 7-16 parts flame retardant, 2-9 parts modified boehmite nanorods, and 1-3 parts compatibilizer; the matrix layer is prepared from polyester chips.

[0019] Preferably, the compatibilizer includes at least one of maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and glycidyl methacrylate-grafted ethylene-octene copolymer.

[0020] The selected compatibilizer can effectively improve the compatibility between the flame retardant and the PET matrix, reduce interfacial tension, prevent the dispersion of the dispersed phase, and form a stable blend structure.

[0021] The modified boehmite nanorods are silane coupling agent-modified boehmite nanorods; the silane coupling agent has amino or epoxy functional groups.

[0022] This invention employs modified boehmite nanorods (chemical formula γ-AlO(OH)) as a reinforcing synergist. Their surface is modified with amino or epoxy-functionalized silane coupling agents to ensure good compatibility and dispersion stability with the PET matrix and flame retardant, and to enable them to participate in cross-linking network formation. During the flame retardant process, the modified boehmite nanorods can form a labyrinth effect in the PET matrix, delaying gas and heat transfer and acting as a physical barrier. Furthermore, the modified boehmite nanorods, upon thermal decomposition, release water vapor, absorbing heat and simultaneously generating high-specific-surface-area active Al2O3, catalyzing the dehydration of the PET matrix into char. In addition, the nanorod-like structure can effectively penetrate the char layer, acting as a "steel bar" reinforcement, significantly improving the mechanical strength, continuity, and thermal shock resistance of the char layer, and preventing char layer cracking and failure.

[0023] Preferably, the silane coupling agent is selected from γ-aminopropyltriethoxysilane (KH-550), N-aminoethyl-γ-aminopropyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560), 3-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, etc.

[0024] Preferably, the preparation method of the modified boehmite nanorods includes the following steps: dispersing boehmite nanorods and a silane coupling agent in anhydrous ethanol, refluxing at 83-87°C for 18-24 hours, and then precipitating and centrifuging the reaction solution to obtain the modified boehmite nanorods. The amount of the silane coupling agent used is 0.1%-0.5% of the mass of the boehmite nanorods. Preferably, the thickness ratio of the base layer to the flame retardant layer on either side is (8-10):1; the thickness ratio of the flame retardant layers located on both sides of the base layer is 1:(1-1.1).

[0025] Preferably, the thickness of the flame-retardant polyester film is 10-100 μm.

[0026] Thirdly, the present invention provides a method for preparing the above-mentioned flame-retardant polyester film, comprising the following steps: Flame retardant, modified boehmite nanorods, and compatibilizer are dispersed in a solvent and then ultrasonically treated to obtain a flame retardant slurry. The flame-retardant slurry and polyester chips are added to a first extruder and melt-extruded to obtain a first melt; polyester chips are added to a second extruder and melt-extruded to obtain a second melt; the first melt and the second melt are used to produce a cast sheet through a three-layer co-extrusion process; the cast sheet is then stretched longitudinally, stretched laterally, shaped, and wound to obtain the flame-retardant polyester film.

[0027] Preferably, the solvent is selected from N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), ethyl acetate, butyl acetate, etc.

[0028] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention employs a synergistic flame retardant system constructed from the main flame retardant modified magnesium hydroxide and an intumescent synergist (modified ammonium polyphosphate, pentaerythritol phosphate, and melamine cyanurate), and further combines it with a reinforcing synergist modified boehmite nanorods to achieve multi-dimensional flame retardant protection through "heat absorption and cooling - oxygen isolation - catalytic char formation - expansion and heat insulation - physical barrier", resulting in a polyester film with high flame retardant performance. Compared to traditional flame retardants that require large amounts to achieve a certain flame retardant standard, the flame retardant of this invention can significantly improve the limiting oxygen index (LOI>30%) of the polyester film with a smaller amount of addition, achieving a UL-94 vertical burning VTM-0 rating, effectively reducing the burning rate and suppressing smoke release.

[0029] (2) The present invention uses a three-layer co-extrusion process to prepare flame-retardant polyester film. This process is compatible with traditional production equipment, is easy to operate, does not require complex processes and special equipment, effectively reduces production costs, and ensures that the flame-retardant layer and the substrate layer are firmly bonded without coating peeling. It combines process feasibility and economic practicality.

[0030] (3) By modifying the surface of magnesium hydroxide, ammonium polyphosphate and boehmite nanorods and adding compatibilizer, the present invention greatly improves the compatibility and dispersibility of flame retardant with PET matrix, effectively avoiding the problem of material mechanical properties and light transmittance deterioration caused by the addition of flame retardant components. The tensile strength retention rate is >90% and the light transmittance is >85%.

[0031] (4) The raw materials selected in this invention have good environmental protection properties and no halogens are added. While achieving high-efficiency flame retardancy, they are in line with the concept of green environmental protection and can avoid long-term impact on the ecosystem, thus meeting the current demand for green materials. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a schematic diagram of the structure of the three-layer co-extrusion die used in Embodiment 1 of the present invention; Figure 2 This is a cross-sectional schematic diagram of the flame-retardant polyester film prepared in Example 1 of the present invention. Detailed Implementation

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the specific embodiments will be briefly introduced below. Obviously, the embodiments described below are some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these embodiments without creative effort.

[0034] Where specific experimental steps or conditions are not specified in the embodiments, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. All raw materials or instruments used are commercially available conventional products, including but not limited to those used in the embodiments of this application.

[0035] In the embodiments and comparative examples of this invention, the modified magnesium hydroxide used was prepared by the following method: Mix 200g of magnesium hydroxide (D50<2μm), 1000g of deionized water, and 4g of sodium hydroxide evenly to obtain magnesium hydroxide slurry; Separately, 0.6 g of silane coupling agent KH-570 was dissolved in 100 mL of ethanol, the pH was adjusted to 11, and the prepared magnesium hydroxide slurry was added. The mixture was stirred and reacted at 60 °C for 80 min, filtered, and dried to obtain modified magnesium hydroxide.

[0036] In the embodiments and comparative examples of this invention, the modified ammonium polyphosphate used was prepared using the following method: Mix 100g of ammonium polyphosphate with 120mL of an aqueous solution containing 12g of melamine (10wt%), stir at 120℃ and 1200rpm for 90 minutes, and dry to obtain modified ammonium polyphosphate.

[0037] In the embodiments and comparative examples of this invention, the modified boehmite nanorods used were all prepared using the following method: Add 5g of boehmite nanorods and 0.5g of silane coupling agent KH-560 to 250ml of anhydrous ethanol, and sonicate for 30min to form a uniform dispersion. The dispersion was transferred to a round-bottom flask and refluxed at 85°C for 20 h. After the reaction was completed, the product was precipitated with petroleum ether, centrifuged at 5000 rpm for 4 min to remove unreacted silane coupling agent, and dried under vacuum to obtain modified boehmite nanorods.

[0038] Example 1 This embodiment provides a flame retardant comprising the following raw materials by weight: 65 parts modified magnesium hydroxide, 12 parts modified ammonium polyphosphate, 10 parts pentaerythritol phosphate, and 7 parts melamine cyanurate.

[0039] This embodiment also provides a flame-retardant polyester film, including a substrate layer and flame-retardant layers located on both sides of the substrate layer, in a superimposed structure of flame-retardant layer-substrate layer-flame-retardant layer, with a thickness ratio of 1:8:1 (flame-retardant layer: substrate layer: flame-retardant layer).

[0040] The flame-retardant layer comprises the following raw materials by weight: 100 parts polyester chips, 7 parts flame retardant, 2 parts modified boehmite nanorods, and 1.4 parts maleic anhydride-grafted polyethylene. The base layer is made of polyester chips.

[0041] The method for preparing the flame-retardant polyester film provided in this embodiment includes the following steps: Seven parts of flame retardant, two parts of modified boehmite nanorods, and 1.4 parts of maleic anhydride-grafted polyethylene were dispersed in 52 parts of N,N-dimethylformamide. The mixture was ultrasonically treated for 30 minutes at a power of 500W to obtain a flame retardant slurry. The flame retardant slurry and 100 parts of polyester chips were added to a first extruder for melt extrusion at a temperature of 270℃ to obtain the first melt. 100 parts of polyester chips were added to the second extruder for melt extrusion at an extrusion temperature of 265°C to obtain the second melt. The first melt and the second melt are simultaneously fed into a three-layer co-extrusion die (structure as follows). Figure 1 As shown in the diagram, the first melt is evenly distributed to both sides of the second melt. Following a three-layer structure of flame-retardant layer-substrate layer-flame-retardant layer, the two melts are stacked and merged, flowing onto a rotating casting cooling drum with a surface temperature of 20°C to obtain a casting sheet. The casting sheet is then longitudinally stretched at 85°C with a stretch ratio of 3.5 times, followed by transverse stretching at 120°C with a stretch ratio of 3.6 times. Next, it is heat-treated at 210-240°C for 15 seconds for setting, gradually cooled, and then drawn and wound to obtain a flame-retardant polyester film. A cross-sectional schematic diagram is shown in the diagram. Figure 2 As shown.

[0042] The flame-retardant polyester film has a thickness of 50 μm, of which the flame-retardant layer has a thickness of 5 μm and the substrate layer has a thickness of 40 μm.

[0043] Example 2 This embodiment provides a flame retardant comprising the following raw materials by weight: 65 parts modified magnesium hydroxide, 12 parts modified ammonium polyphosphate, 10 parts pentaerythritol phosphate, and 7 parts melamine cyanurate.

[0044] This embodiment also provides a flame-retardant polyester film, including a substrate layer and flame-retardant layers located on both sides of the substrate layer, in a superimposed structure of flame-retardant layer-substrate layer-flame-retardant layer, with a thickness ratio of 1:8:1 (flame-retardant layer: substrate layer: flame-retardant layer).

[0045] The flame-retardant layer comprises the following raw materials by weight: 94 parts polyester chips, 12 parts flame retardant, 6 parts modified boehmite nanorods, and 2.2 parts maleic anhydride-grafted polyethylene. The matrix layer is prepared from polyester chips.

[0046] The method for preparing the flame-retardant polyester film provided in this embodiment includes the following steps: 12 parts of flame retardant, 6 parts of modified boehmite nanorods, and 2.2 parts of maleic anhydride-grafted polyethylene were dispersed in 52 parts of N,N-dimethylformamide. The mixture was ultrasonically treated for 30 minutes at a power of 500W to obtain a flame retardant slurry. The flame retardant slurry and 94 parts of polyester chips were added to a first extruder for melt extrusion at a temperature of 270℃ to obtain a first melt. 100 parts of polyester chips were added to the second extruder for melt extrusion at an extrusion temperature of 265°C to obtain the second melt. The first and second melts are simultaneously fed into a three-layer co-extrusion die. The first melt is evenly distributed to both sides of the second melt. Following a three-layer structure of flame-retardant layer-substrate layer-flame-retardant layer, the two sets of melts are stacked and merged, flowing onto a rotating casting cooling drum with a surface temperature of 20°C to obtain a casting sheet. The casting sheet is then longitudinally stretched at 85°C with a stretch ratio of 3.5 times, followed by transverse stretching at 120°C with a stretch ratio of 3.6 times. It is then heat-treated at 210-240°C for 15 seconds for setting, gradually cooled, and then drawn and wound to obtain a flame-retardant polyester film.

[0047] The flame-retardant polyester film has a thickness of 50 μm, of which the flame-retardant layer has a thickness of 5 μm and the substrate layer has a thickness of 40 μm.

[0048] Example 3 This embodiment provides a flame retardant comprising the following raw materials by weight: 68 parts modified magnesium hydroxide, 15 parts modified ammonium polyphosphate, 12 parts pentaerythritol phosphate, and 8 parts melamine cyanurate.

[0049] This embodiment also provides a flame-retardant polyester film, including a substrate layer and flame-retardant layers located on both sides of the substrate layer, in a superimposed structure of flame-retardant layer-substrate layer-flame-retardant layer, with a thickness ratio of 1:8:1 (flame-retardant layer: substrate layer: flame-retardant layer).

[0050] The flame-retardant layer comprises the following raw materials by weight: 86 parts polyester chips, 16 parts flame retardant, 9 parts modified boehmite nanorods, and 3 parts maleic anhydride-grafted polyethylene. The matrix layer is prepared from polyester chips.

[0051] The method for preparing the flame-retardant polyester film provided in this embodiment includes the following steps: 16 parts of flame retardant, 9 parts of modified boehmite nanorods, and 3 parts of maleic anhydride-grafted polyethylene were dispersed in 52 parts of N,N-dimethylformamide. The mixture was ultrasonically treated for 30 minutes at a power of 500W to obtain a flame retardant slurry. The flame retardant slurry and 86 parts of polyester chips were added to a first extruder for melt extrusion at a temperature of 270℃ to obtain a first melt. 100 parts of polyester chips were added to the second extruder for melt extrusion at an extrusion temperature of 265°C to obtain the second melt. The first and second melts are simultaneously fed into a three-layer co-extrusion die. The first melt is evenly distributed to both sides of the second melt. Following a three-layer structure of flame-retardant layer-substrate layer-flame-retardant layer, the two sets of melts are stacked and merged, flowing onto a rotating casting cooling drum with a surface temperature of 20°C to obtain a casting sheet. The casting sheet is then longitudinally stretched at 85°C with a stretch ratio of 3.5 times, followed by transverse stretching at 120°C with a stretch ratio of 3.6 times. It is then heat-treated at 210-240°C for 15 seconds for setting, gradually cooled, and then drawn and wound to obtain a flame-retardant polyester film.

[0052] The flame-retardant polyester film has a thickness of 50 μm, of which the flame-retardant layer has a thickness of 5 μm and the substrate layer has a thickness of 40 μm.

[0053] Comparative Example 1 The polyester film provided in this comparative example is made from pure polyester chips, which do not contain flame retardant components and have a thickness of 50 μm.

[0054] Comparative Example 2 This comparative example provides a flame-retardant polyester film, including a matrix layer and flame-retardant layers located on both sides of the matrix layer, in a superimposed structure of flame-retardant layer-matrix layer-flame-retardant layer, with a thickness ratio of 1:8:1 (flame-retardant layer: matrix layer: flame-retardant layer).

[0055] The flame-retardant layer comprises the following raw materials by weight: 100 parts polyester chips, 7 parts flame retardant, 2 parts modified boehmite nanorods, and 1.4 parts maleic anhydride-grafted polyethylene. The base layer is prepared from polyester chips. The flame retardant is modified magnesium hydroxide.

[0056] The preparation method of the flame-retardant polyester film provided in this comparative example is the same as that in Example 1. The flame-retardant polyester film has a thickness of 50 μm, wherein the thickness of the flame-retardant layer is 5 μm and the thickness of the substrate layer is 40 μm.

[0057] Comparative Example 3 This comparative example provides a flame-retardant polyester film, including a matrix layer and flame-retardant layers located on both sides of the matrix layer, in a superimposed structure of flame-retardant layer-matrix layer-flame-retardant layer, with a thickness ratio of 1:8:1 (flame-retardant layer: matrix layer: flame-retardant layer).

[0058] The flame-retardant layer comprises, by weight, the following raw materials: 94 parts polyester chips, 12 parts flame retardant, 6 parts modified boehmite nanorods, and 2.2 parts maleic anhydride-grafted polyethylene. The matrix layer is prepared from polyester chips. The flame retardant comprises, by weight, the following raw materials: 36 parts modified ammonium polyphosphate and 30 parts pentaerythritol phosphate.

[0059] The preparation method of the flame-retardant polyester film provided in this comparative example is the same as that in Example 1. The flame-retardant polyester film has a thickness of 50 μm, wherein the thickness of the flame-retardant layer is 5 μm and the thickness of the substrate layer is 40 μm.

[0060] Performance testing The limiting oxygen index (LOI) refers to the volume fraction of oxygen required to sustain combustion of a polymer in an oxygen-nitrogen mixture. Test method: Cut a PET film to a specified size, 100 mm long and 10 mm wide. Vertically fix the sample inside a combustion chamber, ensuring the top of the sample is at least 100 mm below the top of the combustion chamber. Ignite the sample from the top using an igniter and observe the combustion. Adjust the oxygen concentration based on the combustion time or length of the sample, and repeat the test until the minimum oxygen concentration required to sustain combustion is found.

[0061] Flame retardancy rating test method: The VTM flame retardancy standard belongs to the vertical burning test system for thin film materials in the US UL94 standard, which includes three levels: VTM-0, VTM-1, and VTM-2. Test method: The sample is made into a 200mm × 50mm rectangle and rolled into a cylinder, which is then vertically fixed on the test device. A specified ignition source is applied twice (10 seconds each time) to ignite the lower end of the sample. The burning time and whether the dripping material ignites the absorbent cotton are recorded.

[0062] Grading and Judgment Criteria : VTM-0: Burning time ≤ 10 seconds, no dripping or residue, fastest self-extinguishing, and highest flame retardant performance.

[0063] VTM-1: Burning time ≤30 seconds, no dripping or residue, flame retardancy is slightly lower.

[0064] VTM-2: Burning time ≤ 60 seconds, allows brief dripping but cannot ignite absorbent cotton, lowest flame retardancy.

[0065] Tensile strength and elongation at break test methods: The test shall be conducted in accordance with the standard GB / T 1040.3-2006. Cut the PET film into strips of the specified size, 15 mm wide and 200 mm long. Use an electronic tensile testing machine, setting parameters such as test speed and specimen width. Clamp both ends of the specimen in the upper and lower fixtures of the testing machine, ensuring that the long axis of the specimen coincides with the center of the fixtures. Start the testing machine to begin the test, stretching the specimen at the set speed until it breaks. The instrument automatically records the maximum tensile force and the elongation at break, calculating the tensile strength and elongation at break.

[0066] Transmittance Test Method: The test shall be conducted in accordance with ISO 13468-1 standard. Cut the PET film to the specified size, typically 100*100mm, ensuring a smooth and contaminated surface. Use a spectrophotometer or transmittance haze meter, calibrating the instrument before measurement to ensure accuracy. Place the PET film sample on the instrument's sample stage and adjust the position of the light source and detector to align them with the sample. Record the transmitted light intensity at different wavelengths and the corresponding incident light intensity. Calculate the transmittance of the PET film based on the transmitted and incident light intensities.

[0067] The results are shown in Table 1 below.

[0068] Table 1 As can be seen from the above data, the intumescent flame retardant system composed of modified magnesium hydroxide, modified ammonium polyphosphate, pentaerythritol phosphate, and melamine cyanurate in this invention effectively blocks flames by absorbing heat, cooling down, catalyzing char formation, and releasing non-flammable gases to promote the expansion and foaming of the char layer when heated. The modified boehmite nanorods embedded in the char layer further block heat penetration, ultimately constructing an "expansion-reinforcement" synergistic flame retardant system. The components exhibit significant synergistic effects, resulting in a LOI value >30% for the prepared polypolyester film. Although the addition of flame retardants inevitably affects the interaction between components, thanks to the modification of each raw material and the rational use of compatibilizers, the mechanical properties and light transmittance of the flame-retardant polyester film remain at a good level. Its tensile strength retention rate is greater than 90%, and its light transmittance is still higher than 85%. While meeting the requirements for high-efficiency flame retardancy, it also takes into account the mechanical and optical properties of the material, demonstrating excellent comprehensive performance.

[0069] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A flame retardant, characterized in that, The raw materials included by weight are as follows: 63-68 parts of modified magnesium hydroxide, 10-15 parts of modified ammonium polyphosphate, 8-12 parts of pentaerythritol phosphate, and 5-8 parts of melamine cyanurate.

2. The flame retardant as described in claim 1, characterized in that, The modified magnesium hydroxide is silane coupling agent modified magnesium hydroxide; the silane coupling agent is selected from at least one of γ-methacryloxypropyltrimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, and γ-aminopropyltriethoxysilane.

3. The flame retardant as described in claim 1, characterized in that, The modified ammonium polyphosphate is melamine-modified ammonium polyphosphate.

4. A flame-retardant polyester film, characterized in that, The flame-retardant polyester film includes a base layer and flame-retardant layers located on both sides of the base layer, forming a superimposed structure of flame-retardant layer-base layer-flame-retardant layer, wherein the raw materials for preparing the flame-retardant layer include the flame retardant agent as described in any one of claims 1-3.

5. The flame-retardant polyester film as described in claim 4, characterized in that, The flame-retardant layer comprises the following raw materials by weight: 86-100 parts polyester chips, 7-16 parts flame retardant, 2-9 parts modified boehmite nanorods, and 1-3 parts compatibilizer; the base layer is prepared from polyester chips.

6. The flame-retardant polyester film as described in claim 5, characterized in that, The compatibilizer includes at least one of maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and glycidyl methacrylate-grafted ethylene-octene copolymer.

7. The flame-retardant polyester film as described in claim 5, characterized in that, The modified boehmite nanorods are silane coupling agent modified boehmite nanorods; the silane coupling agent has amino or epoxy functional groups.

8. The flame-retardant polyester film as described in claim 4, characterized in that, The thickness ratio of the base layer to the flame retardant layer on any side is (8-10):1; the thickness ratio of the flame retardant layers on both sides of the base layer is 1:(1-1.1).

9. The flame-retardant polyester film as described in claim 8, characterized in that, The thickness of the flame-retardant polyester film is 10-100 μm.

10. A method for preparing a flame-retardant polyester film according to any one of claims 4-9, characterized in that, Includes the following steps: Flame retardant, modified boehmite nanorods, and compatibilizer are dispersed in a solvent and then ultrasonically treated to obtain a flame retardant slurry. The flame-retardant slurry and polyester chips are added to a first extruder and melt-extruded to obtain a first melt. Polyester chips are added to a second extruder for melt extrusion to obtain a second melt; The first melt and the second melt are used to produce a cast sheet through a three-layer co-extrusion process. The cast sheet is then stretched longitudinally, stretched laterally, shaped, and wound up to obtain the flame-retardant polyester film.

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