Preparation method and application of composite halogen-free high-phosphorus-content flame-retardant polyester

Abstract

The present application relates to the field of high polymer materials, and aims at the problems of uneven dispersion of composite flame-retardant polyester components and poor anti-dripping effect, and provides a preparation method and application of composite halogen-free high-phosphorus-content flame-retardant polyester, the preparation method comprising: mixing dihydric alcohol, terephthalic acid, a catalyst, an esterification liquid of a reactive phosphorus flame retardant, and a reactive nitrogen flame retardant, and sequentially performing esterification reaction and polycondensation reaction to obtain a melt, and treating the melt to obtain the composite halogen-free high-phosphorus-content flame-retardant polyester. The present application also provides the application of the flame-retardant polyester. The reactive phosphorus flame retardant is involved in the polyester polymerization process, has little influence on the molecular regularity of the polyester itself, does not affect the crystallization behavior and post-processing performance of the polyester chip, the modified nitrogen flame retardant improves the apparent viscosity of the polyester, greatly shortens the polycondensation time, and the polyester obtained by compounding various halogen-free flame retardants has long-lasting flame-retardant effect and good anti-dripping effect.

Description

Technical Field

[0001] This invention relates to the field of polymer materials, and in particular to a method for preparing a composite halogen-free, high-phosphorus-content flame-retardant polyester and its application. Background Technology

[0002] PET is a widely used material in production and daily life, favored for its excellent mechanical properties and spinnability. However, its flammability poses a significant safety hazard. PET is a flammable material with a limiting oxygen index of 21-23%, meaning it is highly flammable under high temperatures and sufficient oxygen. PET burns rapidly, generating substantial heat and dense smoke, and may even drip molten material, all of which increase the risk of fire. Therefore, the research on flame-retardant modification of PET materials and flame retardants has become an important topic with significant practical implications and promising applications.

[0003] Early flame retardants typically used halogenated flame retardants, which offered good flame retardant effects; however, they released toxic and carcinogenic substances during combustion, posing a threat to human health and the natural environment. Therefore, halogen-free flame retardants have attracted considerable attention. Among them, phosphorus-containing flame retardants have broad application and development prospects due to their high flame retardant efficiency, low smoke toxicity, low heat release rate, and good char formation properties. However, single phosphorus-based flame retardants often fail to meet the required flame retardant effect, thus requiring the use of a combination of phosphorus and nitrogen-based retardants. Currently, the most widely used nitrogen-based flame retardants are melamine and its derivatives, dicyandiamide, biuret, and guanidine compounds.

[0004] Melamine has an endothermic enthalpy of sublimation of -963 kJ / kg. For a substrate with a specific heat capacity of 2.1 kJ / (kg / ℃), when it contains 20% melamine, the sublimation of melamine will lower its temperature by 115℃. This cooling effect can prevent the polymer from being ignited. Furthermore, sublimated melamine and its degradation product, nitrogen, can dilute combustibles and isolate the polymer from air. Simultaneously, sublimated melamine can capture free radicals in the flame zone, thus melamine exhibits high gas-phase flame retardant efficiency.

[0005] Patent CN116218153A discloses a flame-retardant polyester chip and its preparation method. While the flame-retardant chip prepared by the blending method achieves a certain flame-retardant effect, the flame-retardant components suffer from uneven dispersion and short-lasting flame-retardant effect due to the blending method. Patent CN116162343A discloses a melamine cyanurate-coated aluminum diethylphosphinate intumescent flame-retardant thermoplastic polyurethane and its preparation method. This method uses melamine cyanurate (MCA) and aluminum diethylphosphinate (ADP) to prepare flame-retardant chips, but the anti-drip effect is only moderate. Summary of the Invention

[0006] To overcome the problems of uneven dispersion and poor anti-dripping effect of composite flame-retardant polyester components, this invention provides a method for preparing composite halogen-free high-phosphorus flame-retardant polyester and its application. This method involves reactive phosphorus-based flame retardants participating in the polyester polymerization process, which have minimal impact on the molecular regularity of the polyester and do not affect the crystallization behavior and post-processing performance of the chips. It also includes modified nitrogen-based flame retardants, which can increase the apparent viscosity of the polyester and greatly shorten the polycondensation time. The combination of multiple halogen-free flame retardants yields a polyester with long-lasting flame retardant effect and good anti-dripping effect.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing a composite halogen-free, high-phosphorus-content flame-retardant polyester includes the following steps:

[0009] (1) A mixture of diol, carboxyl-containing reactive phosphorus flame retardant and anhydrous sodium acetate is heated to obtain an esterified solution of reactive phosphorus flame retardant.

[0010] (2) Melamine salt and formaldehyde are mixed, pH is adjusted to alkaline, and the reaction is heated to obtain a prepolymer of reactive nitrogen flame retardant. The prepolymer is filtered, washed, dried and ground to obtain a micron-sized reactive nitrogen flame retardant.

[0011] (3) Mix the diol, terephthalic acid, catalyst, esterification liquid of the reactive phosphorus flame retardant in step (1) and the reactive nitrogen flame retardant in step (2), and carry out esterification reaction and polycondensation reaction in sequence to obtain a melt. The melt is then processed to obtain a composite halogen-free high phosphorus content flame retardant polyester.

[0012] Preferably, the heating reaction temperature in step (1) is 165-200℃. To obtain a phosphorus-based reactive flame retardant with a phosphorus content of approximately 50,000 ppm, the esterification temperature must be controlled at 165-200℃, and the esterification rate must be controlled above 90%. Too low an esterification temperature will lead to a lower esterification rate, affecting subsequent masterbatch preparation; too high a temperature will cause the diol to self-polymerize, increasing the diethylene glycol content in the system and affecting polyester quality. Both excessively low and excessively high esterification temperatures are detrimental to polyester quality.

[0013] Preferably, the reactive phosphorus flame retardant in step (1) is one or both of 2-carboxyethylphenyl hypophosphite (CEPPA) and [(6-oxo-6H-dibenzo[1,2]oxophosphoric acid-6-yl)methyl]succinic acid (DDP).

[0014] Preferably, the diol in step (1) is one or a mixture of ethylene glycol, propylene glycol, butanediol, pentanediol, and 1,4-cyclohexanediethanol.

[0015] Preferably, in step (1), the alkyd molar ratio of the diol and the reactive phosphorus-based flame retardant is 3:(6-8). Anhydrous sodium acetate accounts for 0.5% of the total mass of the reaction system.

[0016] A more preferred method is to use CEPPA and ethylene glycol in a mass ratio of 3:7, and the reaction temperature is 175-185℃.

[0017] Preferably, step (1) is carried out in a reactor. When the amount of water produced from the reaction reaches 95-98% of the theoretical value, heating is stopped, and diol is added to the reaction system for dilution. When the temperature inside the reactor drops below 120°C, the material is discharged to obtain the esterified liquid of the reactive phosphorus flame retardant.

[0018] Preferably, the melamine salt in step (2) is one or both of melamine cyanurate and melamine polyphosphate. More preferably, it is melamine cyanurate.

[0019] Preferably, in step (2), the pH is adjusted to 9-10, the temperature of the heating reaction is 75-95℃, and the reaction time is 2-3h.

[0020] Preferably, the grinding time in step (2) is 25-35 minutes, and the process of filtering, washing, drying, and grinding the prepolymer is repeated 1-2 times. The micron-sized particles are 1-5 μm in diameter. This yields a micron-sized nitrogen-based flame retardant with more uniform size and more stable properties.

[0021] Preferably, in step (3), the molar ratio of diol to terephthalic acid is 1:(1.2-1.5), the amount of catalyst added is 100-1000 ppm of the total mass of the copolyester, the amount of esterification liquid of the reactive phosphorus flame retardant is 0.5-2% of the total mass of the copolyester, and the amount of reactive nitrogen flame retardant is 1-3% of the total mass of the copolyester.

[0022] Preferably, the phosphorus content in the esterification solution of the reactive phosphorus-based flame retardant is 15,000-25,000 ppm of the total mass of the copolyester. High phosphorus content generally refers to phosphorus content exceeding 10,000 ppm of the total mass of the copolyester. Because reactive phosphorus-based flame retardants contain large molecular structures such as benzene rings, their steric hindrance is significant, making it difficult to prepare high-phosphorus-content flame-retardant polyesters. This invention pre-esterifies the carboxyl-containing reactive phosphorus-based flame retardant with ethylene glycol, greatly reducing its steric hindrance with the polyester and increasing the degree of polymerization between the flame retardant and the matrix, thus enabling the preparation of high-phosphorus-content flame-retardant polyesters.

[0023] Preferably, the diol in step (3) is one or more of ethylene glycol, propylene glycol, butanediol, pentanediol, and 1,4-cyclohexanediethanol; the catalyst is selected from one or more of antimony trioxide, antimony glycol, antimony acetate, calcium acetate, zinc acetate, zinc oxide, sodium acetate, tetrabutyl titanate, tetraisopropyl titanate, tetraisooctyl titanate, and organic amine compounds. More preferably, the diol is ethylene glycol and the catalyst is antimony glycol.

[0024] Preferably, the esterification reaction conditions in step (3) are a temperature of 180-250℃ and a pressure of 0-0.40MPa. More preferably, the temperature is 210-245℃.

[0025] Preferably, after the esterification reaction in step (3), an oxide flame retardant synergist is added to the reaction system. The oxide flame retardant synergist is one or more of silicon dioxide, titanium dioxide, zinc oxide, calcium oxide, magnesium oxide, titanium pentoxide, and aluminum oxide. Silicon dioxide is more preferably preferred.

[0026] Preferably, the polycondensation reaction in step (3) includes a pre-condensation reaction and a final condensation reaction. The temperature of the pre-condensation reaction is 240-280℃ and the pressure is 0.1-3 kPa; the temperature of the final condensation reaction is 270-285℃ and the pressure is 30-250 Pa. More preferably, the temperature of the pre-condensation reaction is 255-270℃ and the pressure is 0.2-2 kPa; the temperature of the final condensation reaction is 275-285℃ and the pressure is 50-250 Pa.

[0027] Preferably, the melt in step (3) is allowed to stand, discharged, and pelletized to obtain the composite halogen-free high-phosphorus flame-retardant polyester.

[0028] The present invention also discloses the application of the obtained composite halogen-free high phosphorus content flame retardant polyester, which is dried in vacuum at 70-120°C and then spun to obtain composite halogen-free high phosphorus content flame retardant polyester fiber.

[0029] As a preferred method, the drying steps are as follows: first, heat the temperature to 70℃ and dry for 2 hours, then keep the temperature at 70℃ for 2 hours, then heat the temperature to 110℃ and dry for 2 hours, and continue drying at 110℃ for 8-10 hours, and the drying is completed.

[0030] Therefore, the beneficial effects of the present invention are as follows:

[0031] (1) Reactive phosphorus flame retardants containing carboxyl groups can significantly reduce the steric hindrance between the flame retardant and polyester by pre-esterification with ethylene glycol, thereby increasing the degree of polymerization between the flame retardant and the polyester matrix. Moreover, this process does not adversely affect the subsequent processing performance.

[0032] (2) Reactive phosphorus flame retardants directly participate in the polyester polymerization process and are grafted onto the polyester molecular chain as intrinsic flame retardant functional components of polyester, reducing the migration and precipitation of flame retardants.

[0033] (3) Melamine salts and reactive phosphorus flame retardants in the polyester system generate hydrogen bonds or chemical bonds, which can form a flame-retardant interconnection network, increasing the intermolecular adsorption force and reducing the generation of droplets during combustion.

[0034] (4) The present invention improves flame retardant performance in the following aspects: First, the carboxyl-containing phosphorus reactive flame retardant used in this method is the main flame retardant component. During the combustion process, phosphoric acid is formed as a dehydrating agent and promotes char formation. Phosphoric acid can absorb heat and prevent CO from being oxidized to CO2. A thin glassy or liquid protective layer is formed on the surface of the condensed phase, which reduces oxygen diffusion and heat and mass transfer between the gas phase and the solid phase, inhibits the carbon oxidation process, and reduces the thermal decomposition of the phosphorus-containing flame retardant. The following changes occur: phosphorus-containing flame retardant → metaphosphoric acid → phosphoric acid → polymetaphosphoric acid. Polymetaphosphoric acid is a stable compound that is not easily volatile and has strong dehydration properties. It forms a dense carbon layer that isolates the polymer surface from the air. At the same time, when the polymer burns, the phosphorus-containing compound produces PO· free radicals, which capture the active H· or HO· in the flame area and reduce the concentration of H· or HO· in the flame, i.e., PO· + H· = HPO, so that the combustion chain reaction cannot continue. Therefore, the phosphorus-based reactive flame retardant used in this method can serve as both an acid source and a gas source in the intumescent flame retardant. Secondly, the nitrogen-based flame retardant used in this method, on the one hand, dilutes the concentration of combustible gases and lowers the surface temperature of the material by generating non-combustible gases such as N2, CO2, and H2O through thermal decomposition during combustion, thus inhibiting the chain reaction; on the other hand, when heated, nitrogen-based flame retardants (such as melamine salts) can cross-link with each other, forming a char layer on the material surface, which isolates heat and oxygen and suppresses smoke, thereby achieving anti-dripping effect. Detailed Implementation

[0035] The technical solution of the present invention will be further described below through specific embodiments.

[0036] In this invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the field. The methods in the embodiments, unless otherwise specified, are conventional methods in the field.

[0037] General Implementation Examples

[0038] A method for preparing a composite halogen-free, high-phosphorus-content flame-retardant polyester includes the following steps:

[0039] (1) Preparation of esterification liquid for reactive phosphorus flame retardants

[0040] Diol and carboxyl-containing reactive phosphorus flame retardant (selected from CEPPA and DDP) are mixed at an alcohol-acid molar ratio of 3:(6-8). Anhydrous sodium acetate is added at 0.5% of the total mass of the reaction system. The mixture is reacted in a reactor at 165-200℃. When the amount of water produced reaches 95-98% of the theoretical value, heating is stopped. Diol is added to the reaction system to dilute it. The mixture is discharged when the temperature inside the reactor drops below 120℃ to obtain the esterified liquid of the reactive phosphorus flame retardant.

[0041] (2) Preparation of reactive nitrogen-based flame retardants

[0042] Melamine salt (selected from melamine cyanurate and melamine polyphosphate) and formaldehyde are mixed, and the pH is adjusted to 9-10 with sodium hydroxide solution. The mixture is reacted at 75-95℃ for 2-3 hours to obtain a prepolymer of reactive nitrogen-based flame retardant. The prepolymer is then filtered, washed, dried, and ground for 25-35 minutes. The above-mentioned filtration, washing, drying, and grinding process is repeated 1-3 times to obtain a reactive nitrogen-based flame retardant with a particle size of 1-5 μm.

[0043] (3) Preparation of composite halogen-free high phosphorus content flame-retardant polyester

[0044] Diol, terephthalic acid, catalyst, esterification solution of reactive phosphorus flame retardant from step (1), and reactive nitrogen flame retardant from step (2) are mixed. The molar ratio of alcohol to terephthalic acid is 1:(1.2-1.5). The amount of catalyst added is 100-1000 ppm of the total mass of the copolyester. The amount of esterification solution of reactive phosphorus flame retardant is 0.5-2% of the total mass of the copolyester. The phosphorus element in the esterification solution of reactive phosphorus flame retardant accounts for 15000-25000 ppm of the total mass of the copolyester. The amount of reactive nitrogen flame retardant is 1-3% of the total mass of the copolyester.

[0045] The above raw materials are first subjected to esterification at 180-250℃ and 0-0.40MPa, then an oxide flame retardant synergist is added, followed by polycondensation. The polycondensation reaction is divided into a pre-condensation reaction and a final condensation reaction. The pre-condensation reaction is carried out at a temperature of 240-280℃ and a pressure of 0.1-3KPa; the final condensation reaction is carried out at a temperature of 270-285℃ and a pressure of 30-250Pa.

[0046] Polycondensation reaction yields a melt, which is then allowed to stand, discharged, and pelletized to obtain a composite halogen-free, high-phosphorus flame-retardant polyester.

[0047] Example 1

[0048] A method for preparing a composite halogen-free, high-phosphorus-content flame-retardant polyester, comprising the following steps:

[0049] (1) Add ethylene glycol and 2-carboxyethylphenyl hypophosphite with an alcohol-acid molar ratio of 3:7 to the reactor, and then add anhydrous sodium acetate accounting for 0.5% of the total mass of the reaction system; turn on the stirring motor and control the motor frequency at 50Hz; the temperature inside the reactor gradually rises. When the temperature inside the reactor reaches 180℃, adjust the water discharge rate and control the water discharge speed. After the water discharge reaches 98% of the theoretical value, stop heating, add diol for dilution, and discharge the material after the internal temperature cools down to 120℃ to obtain the esterification liquid of the reactive phosphorus flame retardant.

[0050] (2) Melamine cyanurate was mixed with formaldehyde, and sodium hydroxide solution was added dropwise to adjust the pH to 9. The temperature was raised to 80°C and the reaction time was 2h to obtain a prepolymer of reactive nitrogen flame retardant. After the prepolymer was filtered, washed and dried, it was placed in a grinder and 1 / 6 of the mass of melamine dispersion deionized water was added. The mixture was ground in the grinder at 20°C for 35min until the particle size of the nitrogen flame retardant was about 200μm, the thermal stability was qualified, and no obvious sedimentation was observed after 12h. A nitrogen reactive flame retardant dispersion was obtained. After filtration, drying and grinding, a modified reactive nitrogen flame retardant with a particle size of 5μm was obtained.

[0051] (3) Add ethylene glycol and terephthalic acid with an alkyd molar ratio of 1:1.3 to the reactor, and then add the esterification liquid of the reactive phosphorus flame retardant prepared in step (1). Control the phosphorus content in the overall polyester to 7000 ppm (mass percentage). Add the reactive nitrogen flame retardant prepared in step (2). Control the mass of melamine salt in the overall polyester to 2%. Add antimony glycol catalyst at a mass of 200 ppm of the overall polyester mass. Perform esterification reaction at 235℃ and 0.3 MPa. After esterification, add 0.3% of nano-sized silica by mass of the overall polyester. Perform low vacuum pre-shrinking reaction at 270℃ and 0.5 KPa to remove excess diol. After pre-shrinking, perform high vacuum final shrinking reaction at 275℃ and 100 Pa to obtain the product melt. Then, let the melt stand, discharge, and granulate to obtain composite halogen-free high phosphorus content flame retardant polyester.

[0052] Example 2

[0053] The difference from Example 1 is that the amount of modified phosphorus-based reactive flame retardant esterification liquid used in step (3) is different, and the phosphorus content in the overall polyester is controlled at 10,000 ppm (mass percentage).

[0054] Example 3

[0055] The difference from Example 1 is that the amount of modified phosphorus-based reactive flame retardant esterification liquid used in step (3) is different, and the phosphorus content in the overall polyester is controlled at 15,000 ppm (mass percentage).

[0056] Example 4

[0057] The difference from Example 1 is that the amount of modified phosphorus-based reactive flame retardant esterification liquid used in step (3) is different, and the phosphorus content in the overall polyester is controlled at 19,000 ppm (mass percentage).

[0058] Comparative Example 1

[0059] The difference from Example 1 is that no flame retardant components are added, that is, pure polyester is prepared.

[0060] Comparative Example 2

[0061] The difference from Example 1 is that no reactive nitrogen-based flame retardant is added, that is, a polyester containing only phosphorus flame retardant is prepared, and the overall phosphorus content in the polyester is still controlled at 7000 ppm (by mass).

[0062] Comparative Example 3

[0063] The difference from Example 1 is that no reactive phosphorus-based flame retardant is added, that is, a polyester containing only nitrogen flame retardant is prepared, and the mass of melamine salt in the overall polyester is still controlled at 2%.

[0064] Comparative Example 4

[0065] The difference from Example 1 is that no nano-sized silica is added after esterification in step (3).

[0066] Comparative Example 5

[0067] The difference from Example 1 is that the reaction temperature in step (1) is controlled at 205°C to prepare a phosphorus-based flame retardant.

[0068] Comparative Example 6

[0069] The difference from Example 1 is that the reaction temperature in step (1) is controlled at 145°C to prepare a phosphorus-based flame retardant.

[0070] The difference between Comparative Example 7 and Example 1 is that 2-carboxyethylphenyl hypophosphite was not subjected to the pre-esterification treatment in step (1) and was directly added to the polyester to participate in the reaction.

[0071] Performance testing

[0072] Viscosity tests were conducted on the polyesters used in the above embodiments and comparative examples. The obtained composite halogen-free, high-phosphorus flame-retardant polyester was then prepared into strips and fibers for flame-retardant performance testing. The strips were prepared by drying the polyester under vacuum at 80-120℃ for 8 hours, followed by injection molding. The oxygen index was tested according to the national standard GB / T2406-2009. The fibers were prepared by drying the polyester under vacuum, specifically by first heating to 70℃ and drying for 2 hours, holding at 70℃ for 2 hours, then heating to 110℃ and drying for 2 hours, and continuing to dry at 110℃ for 8-10 hours. After drying, the fibers were spun according to conventional polyester processes. Testing methods were performed according to GB / T14189-2008, GBT 2408-2008, GBT 5454-1997, and FZ / T50021.

[0073]

[0074] As can be seen from the table above:

[0075] ① Compared to Comparative Examples 2 and 3, the combination of CEPPA and MCA in Example 1 showed significant advantages. This advantage is mainly reflected in its excellent flame-retardant effect. CEPPA provides abundant acid sources during combustion, which can combine with silica to work synergistically, thereby significantly enhancing the char formation effect. This enhanced char formation effect, in turn, leads to a reduction in the number of droplets, which undoubtedly plays a crucial role in improving flame-retardant efficiency.

[0076] On the other hand, MCA releases a large amount of gases such as NO, NO2, and NH3 during combustion. The release of these gases not only further enhances the charring effect but also causes the substrate to expand, forming an expansion layer. This expansion layer effectively blocks oxygen, thus achieving excellent flame retardant properties. This dual effect makes the combination of CEPPA and MCA far superior in flame retardant performance to either CEPPA or MCA alone.

[0077] In summary, the synergistic effect of the CEPPA and MCA blend during combustion gives it a significant advantage in flame retardancy. This advantage is mainly reflected in its strong char-forming ability, reduced droplet count, and effective oxygen barrier properties of the expansion layer. These factors combined make the CEPPA and MCA blend a highly efficient and environmentally friendly flame retardant.

[0078] ② In the comparison between Example 1 and Comparative Example 4, the esterification reaction with added flame retardant synergist silica showed a significant advantage in char formation while reducing the number of droplets. This result indicates that the addition of flame retardant synergist silica has a positive promoting effect on improving the char formation performance of polyester. This advantage can be attributed to the multiple effects of silica, including promoting the formation of the char layer and improving its stability and quality. In addition, silica may also inhibit the dripping phenomenon during polyester combustion, thereby further improving combustion safety. This discovery has important practical value for polyester production and flame retardant development, and also provides new ideas for research in related fields.

[0079] ③ Comparing Examples 1-4, it can be found that the increase in phosphorus content in the polyester shows a significant positive correlation with its flame retardant effect. That is, as the phosphorus content in the polyester increases, its flame retardant effect becomes better and better. However, it should be noted that in the prior art, because phosphorus-based flame retardants contain large molecular structures such as benzene rings, their steric hindrance is relatively large, which brings great difficulties to the preparation of high-phosphorus-content flame-retardant polyesters.

[0080] In existing technologies, if flame retardants are directly added to the polyester reaction system, the polymerization reaction becomes increasingly difficult when the phosphorus content exceeds 6000 ppm. Furthermore, due to the low viscosity of polyester, its melt flow index becomes extremely poor, resulting in a lack of spinnability.

[0081] To address the shortcomings of existing technologies, this invention proposes a novel method for preparing high-phosphorus-content flame-retardant polyesters. The core of this method involves pre-esterifying a carboxyl-containing reactive phosphorus-based flame retardant with ethylene glycol. This pre-esterification reaction significantly reduces the steric hindrance between the carboxyl-containing phosphorus-based flame retardant and the polyester, thereby increasing the degree of polymerization between the flame retardant and the polyester matrix. Through this method, we have successfully achieved the preparation of high-phosphorus-content flame-retardant polyesters.

[0082] ④ The difference between Comparative Example 5 and Example 1 is that the esterification temperature of the phosphorus-based reactive flame retardant in Comparative Example 5 is higher, leading to a self-polymerization reaction of ethylene glycol in the reaction system. This self-polymerization reaction causes the diethylene glycol content in the system to exceed the control range, directly affecting the color value of the polyester product. Substandard color values ​​not only affect the quality of the polyester product but also limit its subsequent use.

[0083] Furthermore, the self-polymerization reaction of ethylene glycol not only affects the color value of polyester but also interferes with subsequent polymerization reactions. Because incompletely esterified flame retardants are present in the reaction system, these unesterified flame retardants are extracted from the system along with small-molecule byproducts, leading to a significant decrease in the phosphorus content of the system. This situation not only affects the quality of polyester products but also greatly reduces the effectiveness of the flame retardants.

[0084] ⑤ Compared to Example 1, Comparative Example 6 showed a lower esterification temperature for the phosphorus-based reactive flame retardant, which resulted in the esterification rate failing to meet the expected standard. Since the unesterified flame retardant could not successfully attach to the PET molecular chain, these unreacted flame retardants were removed along with the oligomers during polymerization. This phenomenon caused the overall phosphorus content of the system to fall short of the theoretically expected value, thus significantly reducing the flame retardant effect.

[0085] ⑥ Compared to Example 1, Comparative Example 7, due to the addition of an unesterified phosphorus-based flame retardant, caused interference with the polyester polymerization process. This phenomenon was reflected in the decreased viscosity of the polyester chips after the discharge test, and the phosphorus content was far from the theoretical expectation.

[0086] In summary, better results can only be achieved within the scope of the steps, reagents, and preferred parameters of this invention.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a composite halogen-free, high-phosphorus-content flame-retardant polyester, characterized in that, Includes the following steps: (1) Mix diol, reactive phosphorus flame retardant and anhydrous sodium acetate. The reactive phosphorus flame retardant is one or both of CEPPA and DDP. React at 165-200℃ to obtain an esterified solution of reactive phosphorus flame retardant. (2) Melamine salt and formaldehyde are mixed, pH is adjusted to alkaline, and the reaction is heated to obtain a prepolymer of reactive nitrogen flame retardant. The prepolymer is filtered, washed, dried and ground to obtain a micron-sized reactive nitrogen flame retardant. (3) Mix diol, terephthalic acid, catalyst, esterification liquid of reactive phosphorus flame retardant in step (1), and reactive nitrogen flame retardant in step (2) and carry out esterification reaction. After esterification reaction, add oxide flame retardant synergist to the reaction system. The oxide flame retardant synergist is one or more of silicon dioxide, titanium dioxide, zinc oxide, calcium oxide, magnesium oxide, titanium pentoxide, and aluminum oxide. Carry out polycondensation reaction to obtain melt. The melt is treated to obtain composite halogen-free high phosphorus content flame retardant polyester.

2. The preparation method according to claim 1, characterized in that, The diol mentioned in step (1) is one or a mixture of ethylene glycol, propylene glycol, butanediol, pentanediol, and 1,4-cyclohexanediethanol.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the alkyd molar ratio of the diol and the reactive phosphorus flame retardant is 3:(6-8).

4. The preparation method according to claim 1, characterized in that, The melamine salt mentioned in step (2) is one or both of melamine cyanurate and melamine polyphosphate.

5. The preparation method according to claim 1, characterized in that, In step (3), the molar ratio of diol and terephthalic acid is 1:(1.2-1.5), the amount of catalyst added is 100-1000 ppm of the total mass of copolyester, the amount of esterification liquid of reactive phosphorus flame retardant is 0.5-2% of the total mass of copolyester, and the amount of reactive nitrogen flame retardant is 1-3% of the total mass of copolyester.

6. The preparation method according to claim 1 or 5, characterized in that, In step (3), the phosphorus element in the esterification liquid of the reactive phosphorus flame retardant accounts for 15,000-25,000 ppm of the total mass of the copolyester.

7. The preparation method according to claim 1, characterized in that, The esterification reaction conditions in step (3) are a temperature of 180-250 °C and a pressure of 0-0.40 MPa.

8. The preparation method according to claim 1 or 7, characterized in that, The polycondensation reaction in step (3) includes a pre-condensation reaction and a final condensation reaction. The temperature of the pre-condensation reaction is 240-280 ℃ and the pressure is 0.1-3 kPa. The temperature of the final condensation reaction is 270-285 ℃ and the pressure is 30-250 Pa.

9. The application of the composite halogen-free high-phosphorus flame-retardant polyester prepared by any one of claims 1-8, wherein the composite halogen-free high-phosphorus flame-retardant polyester is dried in a vacuum at 70-120 °C and then spun to obtain composite halogen-free high-phosphorus flame-retardant polyester fiber.

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