A phosphorus-nitrogen synergistic structural flame-retardant PET and its preparation method

Abstract

The present invention discloses a phosphorus-nitrogen synergistic structural flame-retardant PET and a preparation method thereof. In the present invention, melamine is combined with diphenyl phosphite via an Atherton-Todd reaction to prepare melamine diphenyl phosphite, which is then further polymerized with terephthalic acid and ethylene glycol prepolymer to prepare a structural flame-retardant PET. The phosphorus-nitrogen synergistic structural flame-retardant PET prepared by the present invention introduces phosphorus and nitrogen flame-retardant elements into the PET molecular chain via chemical bonds, resulting in good thermal stability and processing properties. Furthermore, phenyl phosphite has good antioxidant and chain-extending effects on PET, acting as an esterification accelerator to extend the PET molecular chain and generate a cross-linked structure, thereby improving its mechanical properties such as tensile strength and impact strength.

Description

Technical Field

[0001] The invention belongs to the technical field of polyester material preparation, and particularly relates to a phosphorus-nitrogen synergistic structural flame-retardant PET and a preparation method thereof. Background Art

[0002] The semi-crystalline thermoplastic polymer polyethylene terephthalate (PET) has been widely used in beverage bottles, fibers, films, and transportation due to its excellent mechanical properties, fatigue resistance, chemical resistance, spinnability, and low cost, meeting diverse daily needs. However, when considering fire safety requirements, PET's flammability and the severe flame dripping associated with combustion make it prone to fires and secondary injuries. This severely limits PET's application in medical, aviation, and household applications.

[0003] Currently, the primary method for improving the flame retardancy of PET is to add flame retardants to the formulation. Flame retardants are primarily divided into two categories: additive flame retardants and structural flame retardants. Compared to additive flame retardants, structural flame retardants primarily react during the polymerization or polycondensation process and bind to the polymer backbone or side chains, exerting a flame retardant effect. Because these flame retardants partially participate in the reaction and become incorporated into the polymer chain structure, they exhibit superior stability and are less likely to migrate. Among these, phosphorus-containing flame retardants are highly valued for their high flame retardant efficiency, excellent thermal stability, low toxicity, and environmental friendliness. Unfortunately, while most phosphorus-containing flame retardants can retard PET, they can also cause severe melting dripping when exposed to fire, leading to secondary fires and direct burns. Nitrogen-based flame retardants, on the other hand, primarily release non-combustible gases during decomposition, reducing the concentration of combustible gases and exerting their flame retardant effect. Therefore, there is a certain synergistic and synergistic effect between phosphorus and nitrogen flame retardant elements. The emerging phosphorus and nitrogen synergistic flame retardant is a type of environmentally friendly flame retardant. When its products are burned, a uniform carbon foam layer is formed on the surface of the product. The foam layer has the functions of heat insulation, oxygen isolation, smoke suppression and dripping prevention. It is in line with the current trend of low toxicity and low smoke of flame retardants. It is considered to be one of the future development directions of flame retardants and has become a hot spot in the flame retardant field in recent years. Summary of the Invention

[0004] The present invention aims to provide a phosphorus-nitrogen synergistic structural flame-retardant PET and a preparation method thereof. Melamine is combined with diphenyl phosphite via an Atherton-Todd reaction to prepare a phosphorus-nitrogen-containing flame-retardant chain extender (melamine diphenyl phosphite), which is then polymerized with terephthalic acid, dimethyl terephthalate, and an ethylene glycol prepolymer to prepare the structural flame-retardant PET.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The present invention first provides a phosphorus-nitrogen-containing flame retardant chain extender, namely melamine diphenyl phosphite, the preparation method of which is as follows:

[0007] (1) dissolving melamine and diphenyl phosphite in a molar ratio of 1:4 to 8 in a CCl4 solvent to obtain a melamine CCl4 solution and a diphenyl phosphite CCl4 solution, respectively;

[0008] (2) Place the melamine CCl4 solution in an ice bath, add 20% of the total mass of the solution to the NaOH solution and 1% of the total mass of the solution to the benzyltriethylammonium chloride solution, and mix well;

[0009] (3) Slowly add diphenyl phosphite CCl4 solution to the solution obtained in step (2) with continuous stirring for 1 hour;

[0010] (4) Heating to 15-30°C for 4 hours, washing with water, allowing to stand for stratification, removing the water layer, and distilling under reduced pressure to recover the CCl4 solvent to obtain melamine diphenyl phosphite, the structural formula of which is as follows:

[0011]

[0012] With the triazine ring of melamine as the center, three amino groups are respectively connected to a diphenyl phosphite group to form melamine diphenyl phosphite containing multiple nitrogen and phosphorus elements.

[0013] The reaction equation of the preparation process of melamine diphenyl phosphite is:

[0014]

[0015] The reaction is achieved using the Atherton-Todd method. First, the phosphorus-hydrogen bond of the phosphite diester is broken under alkaline conditions. The resulting phosphite diester anion attacks carbon tetrachloride to yield a chlorine atom, forming an acyl chloride phosphite diester intermediate, converting the pH bond to a P-Cl bond. Due to the high reactivity of the acyl chloride phosphite diester, it then reacts with alcohols or amines to synthesize phosphates or phosphamides.

[0016]

[0017]

[0018] Since the triazine ring of melamine has three highly reactive amino groups, they can be combined with diphenyl phosphite by controlling the ratio of reactants and reaction conditions to obtain the above-mentioned melamine diphenyl phosphite.

[0019] The present invention also provides a method for preparing phosphorus-nitrogen synergistic structural flame-retardant PET, wherein the raw material ratio is as follows, calculated by mass:

[0020] 100 parts of terephthalic acid (or dimethyl terephthalate, or a mixture of the two in any proportion)

[0021] 50-500 parts of ethylene glycol

[0022] 2-5 parts of melamine diphenyl phosphite

[0023] 0.2-0.5 parts of catalyst

[0024] (a) stirring and dissolving melamine diphenyl phosphite and ethylene glycol (5 to 20 wt % of the total amount of ethylene glycol) at a temperature of 60 to 100° C., and then dispersing by ultrasonic oscillation to prepare a melamine diphenyl phosphite-ethylene glycol solution;

[0025] (b) mixing terephthalic acid, the remaining ethylene glycol, and the catalyst to carry out an esterification reaction at a temperature of 235 to 250° C., a pressure of 0.25 to 0.3 MPa, and a time of 2 to 4 hours;

[0026] (c) heating the product obtained in step (b) to 250-280° C. and reducing the pressure to 6000-10000 Pa to perform a pre-polycondensation reaction for 45-60 min;

[0027] (d) adding the melamine phenyl phosphite-ethylene glycol solution of step (a) to the product obtained in step (c), adjusting the vacuum degree to ≤70 Pa, and continuing the reaction for 1 to 4 hours to obtain a phosphorus-nitrogen synergistic structure flame-retardant PET.

[0028] Wherein, terephthalic acid can also be replaced by dimethyl terephthalate, or the two can be mixed in any proportion; the catalyst is antimony oxide or ethylene glycol antimony.

[0029]

[0030] The mechanism of melamine diphenyl phosphite participating in the PET polycondensation reaction is shown in the above formula. The present invention first utilizes the Atherton-Todd reaction to break the phosphorus-hydrogen bond in diphenyl phosphite, allowing it to react with the primary amino groups of melamine to synthesize melamine diphenyl phosphite. Melamine diphenyl phosphite is then added to the PET polycondensation reaction. The phosphite groups in the melamine diphenyl phosphite undergo an ester exchange reaction with the terminal hydroxyl groups of the PET prepolymer, linking them to the PET molecular chain and acting as a phosphorus-nitrogen synergistic flame retardant. Furthermore, due to its multiple phosphite groups, it acts as a chain extender during PET polycondensation, increasing the molecular weight and crystallinity of PET.

[0031] In summary, the present invention has the following beneficial effects:

[0032] 1. The present invention synthesizes a new nitrogen-phosphorus synergistic structural flame retardant from melamine and diphenyl phosphite, grafts melamine and phosphite onto the molecular chain of PET and connects them with covalent bonds, thereby achieving nitrogen-phosphorus synergistic flame retardancy at the molecular level and will not cause the flame retardancy of the product to decrease over time.

[0033] 2. The phosphorus-nitrogen synergistic structural flame-retardant PET prepared by the present invention introduces phosphorus-nitrogen flame-retardant elements into the PET molecular chain through chemical bonds, and has good thermal stability and processing performance. In addition, phenyl phosphite also has good antioxidant and chain-extending effects on PET. As an esterification accelerator, it causes the PET molecular chain to grow and produce a cross-linked structure, thereby improving its mechanical properties such as tensile strength and impact strength. DETAILED DESCRIPTION

[0034] The following examples may enable those skilled in the art to better understand the present invention, but are not intended to limit the present invention in any form.

[0035] Example 1

[0036] Preparation of melamine diphenyl phosphite

[0037] (1) dissolving melamine and diphenyl phosphite in a CCl4 solvent in a molar ratio of 1:6 to obtain a melamine CCl4 solution and a diphenyl phosphite CCl4 solution, respectively;

[0038] (2) Place the melamine CCl4 solution in an ice bath, add 20% of the total mass of the solution to the NaOH solution and 1% of the total mass of the solution to the benzyltriethylammonium chloride solution, and mix well;

[0039] (3) Slowly add diphenyl phosphite CCl4 solution to the solution obtained in step (2) with continuous stirring for 1 hour;

[0040] (4) heating to 20°C for 4 h, washing with water, allowing to stand for stratification, removing the water layer, and performing vacuum distillation to recover the CCl4 solvent to obtain melamine diphenyl phosphite.

[0041] Example 2

[0042] Preparation of phosphorus-nitrogen synergistic structural flame-retardant PET

[0043] The raw material ratio is as follows:

[0044]

[0045]

[0046] (a) at a temperature of 85° C., stirring and dissolving melamine diphenyl phosphite and ethylene glycol (10 wt % of the total amount of ethylene glycol), and then ultrasonically dispersing the mixture to prepare a melamine diphenyl phosphite-ethylene glycol solution;

[0047] (b) mixing the remaining ethylene glycol of terephthalic acid with the catalyst for esterification at a temperature of 240° C., a pressure of 0.3 MPa, and a time of 3 h;

[0048] (c) heating the product obtained in step (b) to 280° C. and reducing the pressure to 10,000 Pa to perform a pre-polycondensation reaction for 60 min;

[0049] (d) adding the melamine diphenyl phosphite-ethylene glycol solution obtained in step (a) to the product obtained in step (c), adjusting the vacuum degree to ≤70 Pa, and continuing the reaction for 2 hours to obtain a phosphorus-nitrogen synergistic structure flame-retardant PET.

[0050] Example 3

[0051] Preparation of phosphorus-nitrogen synergistic structural flame-retardant PET

[0052] The raw material ratio is as follows:

[0053]

[0054] (a) at a temperature of 80° C., stirring and dissolving melamine diphenyl phosphite and ethylene glycol (15 wt % of the total amount of ethylene glycol), and then ultrasonically dispersing the mixture to prepare a melamine diphenyl phosphite-ethylene glycol solution;

[0055] (b) esterification of the remaining ethylene glycol of terephthalic acid with the catalyst at a temperature of 245° C., a pressure of 0.28 MPa, and a time of 2.5 h;

[0056] (c) heating the product obtained in step (b) to 260° C. and reducing the pressure to 8000 Pa to perform a pre-polycondensation reaction for 50 min;

[0057] (d) adding the melamine diphenyl phosphite-ethylene glycol solution obtained in step (a) to the product obtained in step (c), adjusting the vacuum degree to ≤70 Pa, and continuing the reaction for 3 hours to obtain a phosphorus-nitrogen synergistic structure flame-retardant PET.

[0058] Example 4

[0059] Preparation of phosphorus-nitrogen synergistic structural flame-retardant PET

[0060] The raw material ratio is as follows:

[0061]

[0062] (a) at a temperature of 85° C., stirring and dissolving melamine diphenyl phosphite and ethylene glycol (12 wt % of the total amount of ethylene glycol), and then ultrasonically dispersing the mixture to prepare a melamine diphenyl phosphite-ethylene glycol solution;

[0063] (b) esterification of the remaining ethylene glycol of terephthalic acid with the catalyst at a temperature of 235° C., a pressure of 0.25 MPa, and a time of 3.5 h;

[0064] (c) heating the product obtained in step (b) to 280° C. and reducing the pressure to 6000 Pa to perform a pre-polycondensation reaction for 45 min;

[0065] (d) adding the melamine diphenyl phosphite-ethylene glycol solution obtained in step (a) to the product obtained in step (c), adjusting the vacuum degree to ≤70 Pa, and continuing the reaction for 3.5 hours to obtain a phosphorus-nitrogen synergistic structure flame-retardant PET.

[0066] Comparative Example 1

[0067] The raw material ratio is as follows:

[0068] 100 parts of terephthalic acid

[0069] 100 parts of ethylene glycol

[0070] Catalyst (Sb2O3) 0.5 parts

[0071] Terephthalic acid, ethylene glycol and a catalyst were mixed and esterified at 250°C and 0.3 MPa for 3 hours; a pre-polycondensation reaction was carried out at 260°C and 10,000 Pa for 1 hour; the polycondensation reaction was completed by vacuuming at 260°C until the pressure was less than 70 Pa and continuing the reaction for 3 hours to obtain PET material.

[0072] Table 1 Mechanical properties and flame retardant properties of PET materials

[0073]

[0074] As can be seen in Table 1, compared with Comparative Example 1, the limiting oxygen index and flame resistance of the PET prepared with the addition of melamine phenyl phosphite flame retardant in Examples 2-4 were significantly improved, reaching the UL-94 test V0 level, indicating that this phosphorus-nitrogen synergistic structural flame retardant can effectively improve the flame retardancy of PET materials. Furthermore, the PET material compounded with melamine diphenyl phosphite exhibited significant improvements in tensile strength and impact strength. This is because the polyphosphate groups in the structural flame retardant structure extend and crosslink the PET molecular segments, increasing molecular crystallinity and enhancing mechanical strength.

[0075] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art should understand that the above specific descriptions are illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

Claims

1. A method for preparing phosphorus-nitrogen synergistic structural flame-retardant PET, characterized in that: The flame retardant PET is prepared from the following raw materials in parts by mass: 100 parts of terephthalic acid and dimethyl terephthalate 50~500 parts of ethylene glycol 2-5 parts of melamine diphenyl phosphite 0.2~0.5 parts of catalyst The preparation method of described melamine diphenyl phosphite is as follows: (1) Melamine and diphenyl phosphite are dissolved in CCl4 solvent to obtain melamine CCl4 solution and diphenyl phosphite CCl4 solution respectively; (2) Place the melamine CCl4 solution in an ice bath, add NaOH solution and benzyltriethylammonium chloride, and mix well; (3) Slowly adding diphenyl phosphite CCl4 solution to the solution obtained in step (2) under stirring; (4) Heating to 15-30°C and reacting for 4 hours, washing with water, standing for stratification, removing the water layer, and distilling under reduced pressure to recover the CCl4 solvent to obtain melamine diphenyl phosphite; The preparation method of the flame retardant PET is as follows: (a) stirring and dissolving melamine diphenyl phosphite and a portion of ethylene glycol at a temperature of 60-100° C., and then dispersing the mixture by ultrasonic vibration to prepare a melamine diphenyl phosphite-ethylene glycol solution; (b) mixing terephthalic acid, the remaining ethylene glycol, and the catalyst to carry out an esterification reaction at a temperature of 235-250°C, a pressure of 0.25-0.3 MPa, and a time of 2-4 hours; (c) heating the product obtained in step (b) to 250-280° C. and reducing the pressure to 6000-10000 Pa to carry out a pre-polycondensation reaction for 45-60 min; (d) adding the melamine phenyl phosphite-ethylene glycol solution described in step (a) to the product obtained in step (c), and continuing the reaction for 1 to 4 hours to obtain a phosphorus-nitrogen synergistic structure flame-retardant PET.

2. The method for preparing a phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: In step (1), the molar ratio of melamine to diphenyl phosphite is 1:4-8.

3. The method for preparing a phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: In step (2), the amount of NaOH solution added is 20% of the total mass of the melamine CCl4 solution, and the amount of benzyltriethylammonium chloride added is 1% of the total mass of the melamine CCl4 solution.

4. The method for preparing a phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: The catalyst is antimony oxide or antimony glycol.

5. The method for preparing a phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: The terephthalic acid and dimethyl terephthalate are mixed in any proportion.

6. The method for preparing phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: In step (b), the amount of ethylene glycol added is 5-20 wt % of the total amount of ethylene glycol.

7. The method for preparing phosphorus-nitrogen synergistic structural flame-retardant PET according to claim 1, characterized in that: In step (d), the reaction vacuum is adjusted to ≤70Pa.

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