Flame-retardant PET (Polyethylene Terephthalate) sheet as well as preparation method and application thereof

Abstract

The invention relates to a flame-retardant PET (Polyethylene Terephthalate) sheet as well as a preparation method and application thereof, and belongs to the technical field of PET sheets, the method comprises the following steps: carrying out condensation reaction on phytic acid and a siloxane prepolymer under an acidic condition to generate a phosphorus-containing siloxane compound; the preparation method comprises the following steps: dispersing nano cellulose into a phosphate solution for ultrasonic treatment, and grafting a phosphate group; the preparation method comprises the following steps: preheating a furan-maleimide type cross-linking agent at 100-120 DEG C for 10-20 minutes; the preparation method comprises the following steps: mixing a PET base material, a phosphorus-silicon flame retardant, modified nano cellulose, a dynamic cross-linking agent and polytetrafluoroethylene micro powder, and granulating at 220-260 DEG C by a double-screw extruder; and finally, carrying out sheet forming on the granules through a calender to realize the preparation of the flame-retardant PET sheet.

Description

Technical Field

[0001] The present invention relates to the technical field of PET sheets, and particularly relates to a flame-retardant PET sheet, a preparation method thereof, and an application thereof. Background Art

[0002] Flame retardants are classified into bromine-based, phosphorus-based, and nitrogen-based in terms of chemical properties, and are classified into copolymer modification and nanocomposite in terms of structural properties. Traditional bromine-based flame retardants such as decabromodiphenyl ether (DBDPO) were widely used due to their low cost and high flame retardancy efficiency, but they have been gradually replaced because their pyrolysis products may produce toxic substances (such as polybrominated dibenzofurans); organic phosphorus and inorganic phosphorus-based inhibit combustion through the condensed-phase flame retardancy mechanism, but the compatibility problem with PET needs to be solved; nitrogen-based flame retards through the dual actions of releasing inert gases and forming a carbon layer, such as melamine derivatives, but it is only suitable for fields with strict requirements on smoke density and toxicity. In terms of structural properties, monomers containing flame-retardant groups (such as phosphorus-containing or bromine-containing monomers) are introduced into the PET molecular chain through chemical reactions to avoid the problem of additive migration. For example, phosphorus-containing diol copolymerized PET; and the flame retardancy performance is enhanced by adding nanomaterials (such as nanoclay, carbon nanotubes, silica). For example, nanoclay can form a dense carbon layer to inhibit the spread of combustion. Summary of the Invention

[0003] To achieve the above object, the present invention provides a flame-retardant PET sheet, and the material parts composition includes:

[0004] PET substrate, 75 - 85 parts, and the viscosity of the PET substrate ≥ 0.8 dL / g;

[0005] Phosphorus-silicon flame retardant, 10 - 15 parts, and the phosphorus-silicon flame retardant contains a phosphorus-siloxane structure;

[0006] Nanocellulose, 3 - 5 parts;

[0007] Crosslinking agent, 1 - 2 parts, and the Diels-Alder reversible reaction is carried out by using the crosslinking agent;

[0008] Polytetrafluoroethylene micropowder, 0.5 - 1 part.

[0009] Further, the material parts composition includes:

[0010] PET substrate, 80 - 82 parts, and the viscosity of the PET substrate ≥ 0.8 dL / g;

[0011] Phosphorus-silicon flame retardant, 10 - 12 parts, and the phosphorus-silicon flame retardant contains a phosphorus-siloxane structure;

[0012] Nitrogen-based synergist, 3 - 5 parts, and the nitrogen-based synergist is melamine polyphosphate or chitosan derivative; wherein, the weight ratio of the nitrogen-based synergist to the phosphorus-silicon flame retardant is 1:2 to 1:4, and interfacial compatibility is achieved through a silane coupling agent to form a phosphorus-silicon-nitrogen triple synergistic flame retardant system;

[0013] Nanocellulose, 3 - 4 parts;

[0014] Crosslinking agent, 1 - 2 parts, and the Diels - Alder reversible reaction is carried out using the crosslinking agent;

[0015] Polytetrafluoroethylene micropowder, 0.5 - 1 part.

[0016] Furthermore, the phosphorus-silicon flame retardant is a phosphorus-siloxane compound generated by reacting phytic acid as a phosphorus source with a siloxane prepolymer. The phosphorus-siloxane compound has a phosphorus-siloxane structure. Among them, phytic acid and the siloxane prepolymer are mixed at a molar ratio of 1:1 to 1:2 and reacted at 70 - 90 °C for 3 - 5 hours under the action of an acidic catalyst to generate a compound with a phosphorus-siloxane structure.

[0017] Furthermore, the nanocellulose is grafted to the PET substrate through phosphate groups on the CNF surface. Among them, the nanocellulose is dispersed in a phosphate solution and reacted at 60 - 80 °C for 1 - 3 hours under ultrasonic assistance to graft phosphate groups to the cellulose surface.

[0018] Furthermore, the crosslinking agent includes a Diels - Alder reaction type crosslinking agent containing furan / maleimide groups.

[0019] Furthermore, the nitrogen-based synergist uses a coupling agent to couple and compatibilize the phosphorus-silicon-nitrogen triple synergistic system, and the limiting oxygen index (LOI) is increased from 30% to more than 35%. The coupling agent includes a silane coupling agent.

[0020] The present invention also proposes a method for preparing a flame-retardant PET sheet, and the method includes:

[0021] Condensing and reacting phytic acid and a siloxane prepolymer under acidic conditions to generate a phosphorus-siloxane compound; dispersing nanocellulose in a phosphate solution and performing ultrasonic treatment to graft phosphate groups; preheating the furan-maleimide type crosslinking agent at 100 - 120 °C for 10 - 20 minutes;

[0022] Mixing the PET substrate, the phosphorus-silicon flame retardant, the modified nanocellulose, the dynamic crosslinking agent, and the polytetrafluoroethylene micropowder, and granulating through a twin-screw extruder at 220 - 260 °C;

[0023] Forming the granulated material into a sheet through a calender, wherein the cooling rate of the calender is controlled at 10 - 15 °C / min.

[0024] Furthermore, the molecular structure of the PET substrate with a viscosity ≥ 0.8 dL / g includes:

[0025]

[0026] Furthermore, the step of condensing phytic acid and a siloxane prepolymer under acidic conditions to produce a phosphorus-containing siloxane compound includes

[0027] Using phytic acid C6H 18 O 24 P6 with a purity ≥ 85% and mixing it with the siloxane prepolymer at a molar ratio of 1:1.5, where the siloxane prepolymer includes tetraethoxysilane;

[0028] Adding 0.5 wt% of p-toluenesulfonic acid for mixing, and stirring and reacting at 80 °C for 4 hours to produce a phosphorus-containing siloxane network (Si-O-P-O-Si);

[0029] After the reaction, wash it 3 times with ethanol and dry it under vacuum at 80 °C for 12 hours to obtain a powdered phosphorus-containing siloxane compound.

[0030] Furthermore, the step of dispersing nanocellulose in a phosphate ester solution and subjecting it to ultrasonic treatment to graft phosphate ester groups includes:

[0031] Disperse nanocellulose in trimethyl phosphate and add 0.1 wt% of concentrated sulfuric acid as a catalyst;

[0032] Perform ultrasonic treatment at 60 °C for 2 hours to graft the phosphate ester groups of the trimethyl phosphate onto the surface of the nanocellulose;

[0033] After centrifugal separation, wash it 3 times with acetone and dry it under vacuum at 60 °C to obtain a powdered material compatible with the PET substrate.

[0034] Furthermore, the step of mixing the PET substrate, phosphorus-silicon flame retardant, modified nanocellulose, dynamic crosslinking agent, and polytetrafluoroethylene micropowder and granulating them with a twin-screw extruder at 220 - 260 °C includes:

[0035] Pre-mix each component for 3 - 7 minutes using a high-speed mixer with a rotation speed of 1000 rpm;

[0036] Perform temperature zoning on the twin-screw extruder, where the temperature zoning is 220 °C for the feeding section, 240 °C for the melting section, 250 °C for the mixing section, and 260 °C for the die head, and output the mixed pellets.

[0037] Furthermore, the step of forming the pellets into sheets through a calender includes:

[0038] Preheat the pellets extruded from the twin-screw extruder to 230 °C and then feed them into the calender;

[0039] Set the roll spacing of the calender to 0.5 - 1.0 mm, adjust the pellets to the target thickness, and obtain the first flame-retardant PET sheet, where the target thickness includes 0.8 - 1.6 mm;

[0040] Perform gradient cooling on the first flame-retardant PET sheet to obtain the second flame-retardant PET sheet. The gradient cooling performs a 20 °C water bath on the sheet at a cooling rate of 10 °C / s for final shaping.

[0041] The present invention sets application requirements for the flame-retardant PET sheet, including:

[0042] Applied to the field of electronic appliances;

[0043] Applied to the construction field;

[0044] Applied to the field of new energy vehicles

[0045] The flame-retardant PET sheet provided by the present invention, its preparation method and application have the following beneficial effects:

[0046] (1) A dense silicon-phosphorus-carbon composite carbon layer is formed through the phosphorus-silicon flame retardant to isolate the transfer of oxygen and heat, and no halogen is released, and the degradation rate of the bio-based raw material is increased by 30%;

[0047] (2) The mechanical enhancement and flame retardancy are synergistically achieved by grafting and modifying nano-cellulose with phosphate groups. The nano-fibers are dispersed to form a framework structure, and the impact strength is increased by 20% to reach 6 kJ / m 2 ;

[0048] (3) The balance between processability and heat resistance is achieved through the Diels-Alder reversible reaction of the dynamic cross-linking agent;

[0049] (4) The limit oxygen index (LOI) is increased through the phosphorus-silicon-nitrogen triple synergistic system. The LOI is increased from 30% to over 35%, and the smoke density is reduced by 40%;

[0050] (5) The anti-melting drop and stability are improved through the uniform dispersion of polytetrafluoroethylene micropowder, the generation of melting drops during combustion is inhibited, and the shrinkage rate of the material is reduced by 50%; Description of the Drawings

[0051] Figure 1 It is a schematic diagram of a method for preparing a flame-retardant PET sheet in an embodiment of the present invention;

[0052] The realization, functional characteristics and advantages of the object of the present invention will be further described with reference to the embodiments and the accompanying drawings. Detailed Embodiments

[0053] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0054] A flame-retardant PET sheet, the material composition in parts by weight includes:

[0055] PET substrate, 75 - 85 parts, the viscosity of the PET substrate ≥ 0.8 dL / g;

[0056] Phosphorus-silicon flame retardant, 10 - 15 parts, the phosphorus-silicon flame retardant contains a phosphorus-siloxane structure;

[0057] Nanocellulose, 3 - 5 parts;

[0058] Crosslinking agent, 1 - 2 parts, using the crosslinking agent to carry out Diels - Alder reversible reaction;

[0059] Polytetrafluoroethylene micropowder, 0.5 - 1 part.

[0060] Table 1 presents three embodiments of the material composition in parts by weight:

[0061] Examples 1 - 3

[0062]

[0063]

[0064] Table 2 is the performance test data table of Examples 1 - 3

[0065]

[0066] By interpreting the data in Table 2, the formulation ratio of Example 2 is the best, specifically divided into three items:

[0067] Flame retardancy efficiency

[0068] (12 parts of phosphorus-silicon flame retardant + 1.5 parts of dynamic crosslinking agent) is the best overall: UL94V - 0, LOI 35%, pHRR 138 kW / m 2 . The comparative example (without flame retardant) completely fails to pass the UL94V level, and the LOI is only 21%.

[0069] Mechanical properties and thermal stability

[0070] The dynamic crosslinking agent improves the HDT: Example 2 is 23 °C higher than the comparative example.

[0071] The reinforcing effect of the modified CNF: The impact strength of Example 2 is increased by 87% compared with the comparative example.

[0072] Environmental protection advantages

[0073] Bromine / halogen was not detected in all examples, and the smoke density was significantly lower than that of the comparative examples (decrease of 58 - 61%).

[0074] In one example, the material composition of the flame - retardant PET sheet includes:

[0075] PET substrate, 80 - 82 parts, and the viscosity of the PET substrate ≥ 0.8 dL / g;

[0076] Phosphorus - silicon flame retardant, 10 - 12 parts, and the phosphorus - silicon flame retardant contains a phosphorus - silicon - oxygen structure;

[0077] Nitrogen - based synergist, 3 - 5 parts, and the nitrogen - based synergist is melamine polyphosphate or chitosan derivative; wherein, the weight ratio of the nitrogen - based synergist to the phosphorus - silicon flame retardant is 1:2 - 1:4, and the interfacial compatibility is achieved through a silane coupling agent to form a phosphorus - silicon - nitrogen triple - synergistic flame - retardant system;

[0078] Nanocellulose, 3 - 4 parts;

[0079] Cross - linker, 1 - 2 parts, and the Diels - Alder reversible reaction is carried out using the cross - linker;

[0080] Polytetrafluoroethylene micropowder, 0.5 - 1 part.

[0081] Table 3 presents three examples of the material composition in another example

[0082] Examples 4 - 6, and Table 3 is as follows:

[0083]

[0084] Table 4 is the performance test data table of Examples 4 - 6, and Table 4 is as follows:

[0085]

[0086]

[0087] Through the interpretation of the data in Table 4, the formulation of Example 5 is the best, specifically divided into three items

[0088] Improved flame - retardant efficiency

[0089] LOI jump: The LOI of Example 5 (4 parts of nitrogen - based synergist) reaches 38%, which is 6% higher than that of Comparative Example 1 (without nitrogen - based) and 10% higher than that of Comparative Example 2 (bromine - based). pHRR reduction: The pHRR of Example 5 is only 105 kW / m 2 , which is 24% lower than that of Comparative Example 1, and the smoke density is reduced by 26%.

[0090] Mechanical properties maintained

[0091] Synergistic effect of dynamic crosslinking agent and silane coupling agent: The impact strength in Example 5 is 6.1 kJ / m 2 , which is 74% higher than that of Comparative Example 2 (brominated system).

[0092] Environmental protection advantages

[0093] Halogen-free risk: The bromine content was not detected in all examples, while the bromine residue in Comparative Example 2 reached 1200 ppm (violating RoHS). Low toxicity: The HCN release amount ≤ 15 ppm (lower than 18 ppm in Comparative Example 1), and there is no risk of brominated dioxins.

[0094] The phosphorus-silicon flame retardant is a phosphorus-silicon-oxygen-containing compound formed by reacting phytic acid as a phosphorus source with a silicone prepolymer. The phosphorus-silicon-oxygen-containing compound has a phosphorus-silicon-oxygen structure. Among them, phytic acid and the silicone prepolymer are mixed at a molar ratio of 1:1 to 1:2 and reacted at 70 - 90 °C for 3 - 5 hours under the action of an acidic catalyst to form a compound with a phosphorus-silicon-oxygen structure.

[0095] The nanocellulose is grafted to the PET substrate through phosphate groups on the CNF surface. Among them, the nanocellulose is dispersed in a phosphate solution and reacted at 60 - 80 °C for 1 - 3 hours under ultrasonic assistance to graft the phosphate groups to the cellulose surface.

[0096] The crosslinking agent includes a Diels-Alder reaction type crosslinking agent containing furan / maleimide groups.

[0097] The nitrogen-based synergist uses a coupling agent to couple and compatibilize the triple synergistic coupling of phosphorus-silicon-nitrogen, and the limiting oxygen index (LOI) is increased from 30% to more than 35%. The coupling agent includes a silane coupling agent.

[0098] Reference appendix Figure 1 A method for preparing a flame-retardant PET sheet proposed by the present invention, the method includes:

[0099] Condensing and reacting phytic acid and a silicone prepolymer under acidic conditions to form a phosphorus-silicon-oxygen-containing compound; dispersing nanocellulose in a phosphate solution and performing ultrasonic treatment to graft phosphate groups; preheating the furan-maleimide type crosslinking agent at 100 - 120 °C for 10 - 20 minutes;

[0100] Mixing the PET substrate, the phosphorus-silicon flame retardant, the modified nanocellulose, the dynamic crosslinking agent and polytetrafluoroethylene micropowder, and granulating through a twin-screw extruder at 220 - 260 °C;

[0101] Forming the granulated material into a sheet through a calender, wherein the cooling rate of the calender is controlled to be 10 - 15 °C / min.

[0102] The molecular structure of the PET substrate with a viscosity ≥ 0.8 dL / g includes:

[0103]

[0104] The step of condensing phytic acid and a siloxane prepolymer under acidic conditions to produce a phosphorus-containing siloxane compound includes

[0105] Using phytic acid C6H 18 O 24 P6 with a purity ≥ 85% and mixing it with the siloxane prepolymer at a molar ratio of 1:1.5, where the siloxane prepolymer includes tetraethoxysilane;

[0106] Adding 0.5 wt% of p-toluenesulfonic acid for mixing and stirring at 80 °C for 4 hours to form a phosphorus-containing siloxane network (Si-O-P-O-Si);

[0107] After the reaction, wash it 3 times with ethanol and dry it in vacuum at 80 °C for 12 hours to obtain a powdery phosphorus-containing siloxane compound.

[0108] The step of dispersing nanocellulose in a phosphate ester solution and subjecting it to ultrasonic treatment to graft phosphate ester groups includes:

[0109] Disperse nanocellulose in trimethyl phosphate and add 0.1 wt% of concentrated sulfuric acid as a catalyst;

[0110] Perform ultrasonic treatment at 60 °C for 2 hours to graft the phosphate ester groups of the trimethyl phosphate onto the surface of the nanocellulose;

[0111] After centrifugal separation, wash it 3 times with acetone and dry it in vacuum at 60 °C to obtain a powdery material compatible with the PET substrate.

[0112] The step of mixing the PET substrate, a phosphorus-silicon flame retardant, modified nanocellulose, a dynamic crosslinking agent, and polytetrafluoroethylene micropowder and granulating them with a twin-screw extruder at 220 - 260 °C includes:

[0113] Pre-mix each component for 3 - 7 minutes using a high-speed mixer with a rotation speed of 1000 rpm;

[0114] Perform temperature zoning on the twin-screw extruder, where the temperature zoning is 220 °C for the feeding section, 240 °C for the melting section, 250 °C for the kneading section, and 260 °C for the die head, and output the mixed pellets.

[0115] The step of forming a sheet from the pellets through a calender includes:

[0116] Preheat the pellets extruded from the twin-screw extruder to 230 °C and then feed them into the calender;

[0117] Set the roll spacing of the calender to 0.5 - 1.0 mm, adjust the pellet to the target thickness, and obtain the first flame-retardant PET sheet, where the target thickness includes 0.8 - 1.6 mm;

[0118] Perform gradient cooling on the first flame-retardant PET sheet to obtain the second flame-retardant PET sheet. The gradient cooling is to perform a 20°C water bath on the sheet at a cooling rate of 10°C / s for final shaping.

[0119] An application of a flame-retardant PET sheet proposed by the present invention is specifically applied in:

[0120] Applied in the field of electronic appliances;

[0121] Applied in the field of construction;

[0122] Applied in the field of new energy vehicles.

[0123] In summary, the present invention relates to a flame-retardant PET sheet, its preparation method and application, belonging to the technical field of PET sheets. The method includes condensing phytic acid and a siloxane prepolymer under acidic conditions to generate a phosphorus-containing siloxane compound; dispersing nanocellulose in a phosphate solution and performing ultrasonic treatment to graft phosphate groups; preheating a furan-maleimide type crosslinker at 100 - 120°C for 10 - 20 minutes; mixing a PET substrate, a phosphorus-silicon flame retardant, modified nanocellulose, a dynamic crosslinker and polytetrafluoroethylene micropowder, and granulating through a twin-screw extruder at 220 - 260°C; finally, forming the pellets into sheets through a calender to realize the preparation of the flame-retardant PET sheet.

[0124] It should be noted that in this article, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, device, article or method including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such a process, device, article or method. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, device, article or method including that element.

[0125] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, are equally included in the patent protection scope of the present invention.

Claims

1. A flame-retardant PET sheet, characterized in that, The composition of the material by parts includes: PET substrate, 75 - 85 parts, with the viscosity of the PET substrate ≥ 0.8 dL / g; Phosphorus-silicon flame retardant, 10 - 15 parts, the phosphorus-silicon flame retardant contains a phosphorus-siloxane structure; Nanocellulose, 3 - 5 parts; Crosslinking agent, 1 - 2 parts, using the crosslinking agent for Diels - Alder reversible reaction; Polytetrafluoroethylene micropowder, 0.5 - 1 part.

2. The flame-retardant PET sheet according to claim 1, wherein, The composition of the material by parts includes: PET substrate, 80 - 82 parts, with the viscosity of the PET substrate ≥ 0.8 dL / g; Phosphorus-silicon flame retardant, 10 - 12 parts, the phosphorus-silicon flame retardant contains a phosphorus-siloxane structure; Nitrogen-based synergist, 3 - 5 parts, the nitrogen-based synergist is melamine polyphosphate or chitosan derivative; wherein, the weight ratio of the nitrogen-based synergist to the phosphorus-silicon flame retardant is 1:2 to 1:4, and the interfacial compatibility is achieved through a silane coupling agent to form a phosphorus-silicon-nitrogen triple synergistic flame retardant system; Nanocellulose, 3 - 4 parts; Crosslinking agent, 1 - 2 parts, using the crosslinking agent for Diels - Alder reversible reaction; Polytetrafluoroethylene micropowder, 0.5 - 1 part.

3. The flame-retardant PET sheet according to claim 1 or 2, characterized in that, The phosphorus-silicon flame retardant is a phosphorus-siloxane compound generated by reacting phytic acid as a phosphorus source with a siloxane prepolymer. The phosphorus-siloxane compound has a phosphorus-siloxane structure. Among them, phytic acid and the siloxane prepolymer are mixed at a molar ratio of 1:1 to 1:2 and reacted at 70 - 90 °C for 3 - 5 hours under the action of an acidic catalyst to generate a compound with a phosphorus-siloxane structure.

4. The flame-retardant PET sheet according to claim 1 or 2, wherein The nanocellulose is grafted to the PET substrate through phosphate groups on the surface of CNF. Among them, the nanocellulose is dispersed in a phosphate solution and reacted at 60 - 80 °C for 1 - 3 hours under ultrasonic assistance to graft the phosphate groups to the cellulose surface.

5. The flame-retardant PET sheet according to claim 1 or 2, characterized in that The crosslinking agent includes a Diels - Alder reaction type crosslinking agent containing furan / maleimide groups.

6. The flame-retardant PET sheet according to claim 2, wherein The nitrogen-based synergist uses a coupling agent to couple and compatibilize the phosphorus-silicon-nitrogen triple synergism, and the limiting oxygen index (LOI) is increased from 30% to more than 35%. The coupling agent includes a silane coupling agent.

7. A method for preparing a flame-retardant PET sheet, characterized in that, The method includes: Condensing and reacting phytic acid with a siloxane prepolymer under acidic conditions to generate a phosphorus-siloxane compound; Dispersing nanocellulose in a phosphate solution and performing ultrasonic treatment to graft phosphate groups; Preheating the furan - maleimide type crosslinking agent at 100 - 120 °C for 10 - 20 minutes; Mixing the PET substrate, phosphorus-silicon flame retardant, modified nanocellulose, dynamic crosslinking agent and polytetrafluoroethylene micropowder, and granulating through a twin-screw extruder at 220 - 260 °C; Forming the granulated material into a sheet through a calender, wherein the cooling rate of the calender is controlled at 10 - 15 °C / min.

8. The method for preparing a flame-retardant PET sheet according to claim 7, wherein The molecular structure of the PET substrate with viscosity ≥ 0.8 dL / g includes:

9. The method for preparing a flame-retardant PET sheet according to claim 7, wherein, The step of condensing and reacting phytic acid with a siloxane prepolymer under acidic conditions to generate a phosphorus-siloxane compound includes Phytic acid C6H with a purity of ≥85% is used 18 O 24 is mixed with a siloxane prepolymer in a molar ratio of 1:1.5, where the siloxane prepolymer includes tetraethoxysilane; Adding 0.5 wt% p-toluenesulfonic acid for mixing, and stirring and reacting at 80 °C for 4 hours to generate a phosphorus-siloxane network (Si - O - P - O - Si); After the reaction was completed, it was washed three times with ethanol and dried in vacuo at 80 °C for 12 hours to obtain a phosphosiloxane compound in powder form.

10. The method for preparing a flame-retardant PET sheet according to claim 7, wherein The step of grafting phosphate groups by dispersing nanocellulose in a phosphate ester solution and performing ultrasonic treatment includes: Disperse the nanocellulose in trimethyl phosphate and add 0.1 wt% concentrated sulfuric acid as a catalyst; Perform ultrasonic treatment at 60 °C for 2 hours to graft the phosphate ester groups of the trimethyl phosphate onto the surface of the nanocellulose; After centrifugal separation, it was washed three times with acetone and dried in vacuo at 60 °C to obtain a powder material compatible with the PET substrate.

11. The method for preparing a flame-retardant PET sheet according to claim 7, wherein, The step of mixing the PET substrate, the phosphorus-silicon flame retardant, the modified nanocellulose, the dynamic crosslinking agent and the polytetrafluoroethylene micropowder and granulating them at 220 - 260 °C by a twin-screw extruder includes: Pre-mix the components for 3 - 7 minutes using a high-speed mixer with a rotation speed of 1000 rpm; Perform temperature zoning on the twin-screw extruder, where the temperature zoning is 220 °C for the feeding section, 240 °C for the melting section, 250 °C for the kneading section and 260 °C for the die head, and output the mixed pellets.

12. The method for preparing a flame-retardant PET sheet according to claim 7, characterized in that, The step of forming a sheet from the pellets by a calender includes: Preheat the pellets extruded from the twin-screw extruder to 230 °C and then feed them into the calender; Set the roll spacing of the calender to 0.5 - 1.0 mm and adjust the pellets to the target thickness, obtaining a first flame-retardant PET sheet, where the target thickness includes 0.8 - 1.6 mm; Perform gradient cooling on the first flame-retardant PET sheet to obtain a second flame-retardant PET sheet. The gradient cooling is to perform a 20 °C water bath on the sheet at a cooling rate of 10 °C / s for final shaping.

13. Application of a flame-retardant PET sheet, characterized in that, Include: Applied to the field of electronic appliances; Applied to the construction field; Applied to the field of new energy vehicles.

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