Preparation method of a transparent polyurethane material with flame retardancy, luminescence and recyclability

Abstract

The present invention discloses a preparation method of a transparent polyurethane material with flame retardancy, luminescence and recyclability. After obtaining a luminescent flame retardant by reacting triethanolamine with diphenylphosphoryl chloride, the transparent polyurethane material is obtained by polymerizing with polyether polyol, isocyanate and chain extender. The preparation method of the present invention is simple, the process is easy to control, the prepared transparent polyurethane material can maintain a high tensile strength, and the limiting oxygen index can reach 30.3 and the UL-94 rating reaches V-0 level, having good flame retardant effect. In addition, due to its high transparency and fluorescence, it can be used as a light conversion film in flexible solar cells, thereby improving the photoelectric conversion efficiency of flexible solar cells.

Description

Technical Field

[0001] The present invention belongs to the field of polymer composites and relates to a preparation method of a transparent polyurethane material with flame retardancy, luminescence and recyclability. Background Art

[0002] Thermoplastic polyurethane elastomer is a block copolymer composed of soft segments and hard segments. It combines many advantages such as high elasticity, low temperature resistance, corrosion resistance, and wear resistance, and is widely used in fields such as transportation, electronic equipment, and architectural decoration. However, its application scenarios are often limited by its own defects. Pure polyurethane is a flammable material, which often cannot ensure its safety during actual application, posing a potential threat to people's lives and property. Therefore, the flame retardant modification of polyurethane materials has become particularly urgent. From the development process of polyurethane flame retardants, although halogen-containing flame retardants have excellent performance, they have been seriously inconsistent with the current concept of green environmental protection due to the generation of a large amount of corrosive and toxic gases during combustion, and have thus been abandoned by people.

[0003] At present, it has become an urgent task to develop more environmentally friendly and halogen-free flame retardant polyurethane materials. Among many flame retardants, nitrogen-phosphorus based synergistic flame retardants have attracted much attention due to their excellent flame retardant performance, high flame retardant efficiency, less smoke generation, and low toxicity, and are expected to provide more environmentally friendly, safe and efficient flame retardants for the polyurethane material industry. In addition, expanding the actual application scenarios of polyurethane materials and realizing multifunctional development is a major trend. The solar energy industry is a promising clean energy field, and among them, the encapsulation of solar cells is one of the ideal application scenarios for polyurethane elastomers. Flexible solar cells, as materials with broad prospects, have also been attracting attention. However, despite the many advantages of flexible solar cells, the problem of their low photoelectric conversion efficiency always exists. Therefore, improving the power generation efficiency of flexible solar cells has become the research focus of technicians in this field.

[0004] Based on this, the present invention aims to provide a polyurethane material with flame retardancy, luminescence, high tensile strength and recyclability, which can be applied to flexible solar cells and improve the photoelectric conversion efficiency of flexible solar cells. Summary of the Invention

[0005] In view of the above technical problems, the present invention aims to provide a preparation method of a flame-retardant, luminescent and recyclable transparent polyurethane material. After obtaining a luminescent flame retardant by reacting triethanolamine with diphenylphosphoryl chloride, it is then polymerized with polyether polyol, isocyanate and chain extender; the preparation method of the present invention is simple, the process is easy to control, the obtained transparent polyurethane material can maintain a high tensile strength, and the limiting oxygen index can reach 30.3 and the UL-94 rating reaches V-0 level, having a good flame retardant effect; in addition, due to its high transparency and fluorescence, it can be used as a light conversion film in flexible solar cells, thereby improving the photoelectric conversion efficiency of flexible solar cells.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A preparation method of a flame-retardant, luminescent and recyclable transparent polyurethane material is carried out in the following order of steps:

[0008] S1. Prepare a luminescent flame retardant

[0009] Add triethylamine to 50 mL of anhydrous tetrahydrofuran at 0 °C, then add triethanolamine and stir for 30 min, then dropwise add diphenylphosphoryl chloride and stir for 3 - 5 h, quench the reaction system with water, add 100 mL of water and 100 mL of ethyl acetate for extraction, the obtained organic phase is washed 2 - 4 times with 100 mL of deionized water and 100 mL of saturated brine respectively, add 5 g of anhydrous sodium sulfate for drying, filtration, and after concentration under reduced pressure, it is purified by column chromatography to obtain a luminescent flame retardant (NEDP);

[0010] S2. Prepare a transparent polyurethane material

[0011] Add polyether polyol to a three-necked flask and heat to 120 °C, then evacuate the glass container and fill it with argon, cool the system to 80 °C, add polyisocyanate and stir for 1 h, then lower the temperature to 55 °C, add the luminescent flame retardant and chain extender, stir for 1 - 2 min, add 0.6 mL of triethylamine to obtain a polymer emulsion, and pour it into a polytetrafluoroethylene mold, react and cure in an 80 °C oven for 48 h to obtain a transparent polyurethane material.

[0012] As a limitation of the preparation method of the present invention, in step S1, the molar ratio of the triethylamine, triethanolamine, and diphenylphosphoryl chloride is 5:1:3.

[0013] As another limitation of the preparation method of the present invention, in step S1, the dropping rate of the diphenylphosphoryl chloride is 0.5 - 0.6 mL / min.

[0014] The dropping rate of diphenylphosphoryl chloride in the present invention is crucial for the preparation of the luminescent flame retardant. When the dropping rate of diphenylphosphoryl chloride is 0.5 - 0.6 mL / min, it can make the reaction process more stable, thus making the reaction more complete. However, when the dropping rate is greater than 0.5 - 0.6 mL / min, the reaction rate will be too fast, resulting in an insufficient reaction. When the dropping rate is less than 0.5 - 0.6 mL / min, the test process will be prolonged, leading to a lower efficiency, which is not conducive to actual production and experiments.

[0015] As the third limitation of the preparation method of the present invention, in step S2, the molecular weight of the polyether polyol is 1000 - 3000.

[0016] As the fourth limitation of the preparation method of the present invention, in step S2, the isocyanate is any one of hexamethylene diisocyanate, isophorone diisocyanate, methylcyclohexyl diisocyanate, dicyclohexylmethylene diisocyanate, xylylene diisocyanate, 2,2,4 - trimethylhexane diisocyanate, and dimer acid diisocyanate.

[0017] As the fifth limitation of the preparation method of the present invention, in step S2, the molar ratio of the polyether polyol, isocyanate, luminescent flame retardant, and chain extender is 10:20:(3 - 12):10.

[0018] As the sixth limitation of the preparation method of the present invention, in step S2, the chain extender is obtained by adding 0.9 g of 1,4 - butanediol to 2 mL of N,N - dimethylformamide.

[0019] There is another limitation in the present invention. In step S1, the structural formula of the prepared luminescent flame retardant is:

[0020]

[0021] The transparent polyurethane material prepared by the present invention can achieve high - efficiency flame retardancy only through the luminescent flame retardant, which is mainly determined by the nitrogen - phosphorus synergistic mechanism of the luminescent flame retardant. First, the phosphorus - containing free radicals generated by the decomposition of the luminescent flame retardant can capture the free radical groups in the air, thus quenching the flame and achieving the purpose of terminating combustion. And the nitrogen - containing elements decompose to produce incombustible gases such as N2 and NH3, which dilute the concentration of combustible gases around the flame and also achieve a certain flame - retardant effect. Second, after capturing free radicals, the phosphorus - based groups form substances such as phosphates and metaphosphates, which cover the surface of the polyurethane matrix, repairing and thickening the carbon layer structure, making the carbon layer dense and crack - free, thus well isolating the heat transfer and further contact with combustible gases, and achieving high - efficiency flame retardancy. Therefore, the luminescent flame retardant improves the flame - retardant effect through both the gas phase and the condensed phase.

[0022] The present invention also provides an application of a transparent polyurethane material. The prepared transparent polyurethane material can be used as a light conversion film and applied to flexible solar cells.

[0023] The polyurethane material prepared by the present invention has high elasticity, high flame retardancy, strong photoluminescence characteristics, recyclability and transparency. The synthesis of the luminescent flame retardant (NEDP) during the synthesis of this material is crucial. It is composed of multiple rotatable benzene ring structures and contains nitrogen and phosphorus elements, which ensures that it can achieve effective flame retardancy through the synergy of nitrogen and phosphorus elements. Moreover, the rotation of the benzene rings is restricted due to aggregation, thereby triggering the photoluminescence effect. The luminescence principle of the luminescent flame retardant is due to the AIE mechanism. Most AIE fluorescent molecules have highly distorted propeller-like structures, indicating that the distorted structure plays a crucial role in the AIE phenomenon. When dispersed, the AIE molecules rotate and vibrate rapidly, resulting in a high non-radiative decay rate and thus fluorescence quenching. When the AIE molecules aggregate or are in a solid state, the molecular conformation is highly distorted, and the intermolecular π-π stacking and intramolecular motion are restricted, thereby suppressing the non-radiative decay rate. At this time, the radiative decay rate competes with the non-radiative decay rate, thus increasing the emission quantum yield and emitting a fluorescent effect. And after adding NEDP, it still does not affect the excellent physical properties of the polyurethane itself, such as high elasticity and high stretchability, and the recyclability of the thermoplastic polyurethane is also well maintained. This is because firstly, the added amount of NEDP introduced is small, so the influence on the polyurethane is small; secondly, NEDP is a synthesized small molecule structure and does not have groups such as hydroxyl and amino groups that react with the polyurethane, so it does not react with the polyurethane chain segments. However, other non-reactive flame retardants are inorganic additive types with large particle sizes, so they damage the structure of the polyurethane and cause a serious decline in mechanical properties. And because NEDP is a small molecule, the chain segment arrangement of the polyurethane itself remains complete. NEDP exists in the gaps of the polyurethane chain segment structure, so the polyurethane maintains good mechanical properties, thus well ensuring the requirements in practical applications. In addition, the excellent light transmittance of the polyurethane material is the key to enabling it to be applied to flexible solar cells. The high light transmittance ensures that it does not affect the normal visible light absorption of the solar cells. And due to the fluorescent characteristics of the polyurethane material, it can absorb ultraviolet light with extremely low utilization rate by the solar cells and emit visible blue fluorescence, which exactly converts the light that cannot be absorbed by the solar cells into light that can be absorbed, thereby further enhancing the intensity of the light absorbed by the solar cells, and thus realizing the improvement of the photoelectric conversion efficiency of the solar cells. Combined with the high elasticity and flexibility of this material, it is a clever application for flexible solar cells, and its excellent flame retardancy further ensures the fire safety of the battery during use.

[0024] As a whole, the above technical solutions of the present invention are closely related and interact with each other, jointly determining the morphological characteristics and properties of the product.

[0025] The above technical solution has the following advantages or beneficial effects:

[0026] 1. The transparent polyurethane material prepared by the present invention has high tensile strength, excellent flame retardancy, photoluminescence, few melt dripping characteristics, high transparency and recyclability, and can be used as a light conversion film for flexible solar cells, thereby improving the photoelectric conversion efficiency of flexible solar cells;

[0027] 2. The preparation method of the present invention is simple, the process is easy to control, the preparation period is short and the cost is low, which is suitable for large-scale industrial production;

[0028] 3. The tensile strength of the transparent polyurethane material prepared by the present invention can reach more than 25 MPa, the limiting oxygen index can reach 30.3, and the UL-94 rating reaches V-0 level, having good flame retardant effect.

[0029] The present invention is applicable to the preparation of transparent polyurethane materials with flame retardancy, luminescence and recyclability.

[0030] The technical solution of the present invention will be further described in detail below in conjunction with the specification drawings and specific embodiments. Description of the Drawings

[0031] Figure 1 It is the structural characterization diagram of the luminescent flame retardant prepared in step S1 of Example 1 of the present invention, wherein: (a) is the nuclear magnetic resonance hydrogen spectrum diagram, and (b) is the Fourier transform infrared spectrum diagram;

[0032] Figure 2 It is the infrared spectrum diagram of the transparent polyurethane materials prepared in Examples 1-4 of the present invention;

[0033] Figure 3 It is the flame retardant performance characterization diagram of the transparent polyurethane materials prepared in Examples 1-4 of the present invention, wherein: (a) is the LOI test diagram, (b) is the UL-94 test diagram, and (c) is the LOI and UL-94 evaluation diagram;

[0034] Figure 4 It is the mechanical property diagram of the transparent polyurethane materials prepared in Examples 1-4 of the present invention;

[0035] Figure 5 It is the ultraviolet transmittance diagram of the transparent polyurethane materials prepared in Examples 1-4 of the present invention;

[0036] Figure 6SEM images of the carbon layers after combustion of the transparent polyurethane material and the pure polyurethane material prepared in Example 1 of the present invention, where: (a1) is the 50-μm SEM image of the carbon layer after combustion of the pure polyurethane, (a2) is the 50-μm SEM image of the carbon layer after combustion of the transparent polyurethane prepared in Example 1, (b1) is the 2-μm SEM image of the carbon layer after combustion of the pure polyurethane, and (b2) is the 2-μm SEM image of the carbon layer after combustion of the transparent polyurethane prepared in Example 1;

[0037] Figure 7 Characterization diagram of the recyclability of the transparent polyurethane material prepared in Example 1 of the present invention, where: (a) is the recycling process diagram, and (b) is the mechanical property diagram;

[0038] Figure 8 Luminescence property characterization diagram of the polyurethane materials prepared in Examples 1-4 of the present invention. Detailed implementation manners

[0039] The following examples are only a part of the examples of the present invention, rather than all examples. Therefore, the detailed descriptions in the examples of the present invention provided below are not intended to limit the scope of the claimed invention, but merely represent the selected examples of the present invention. All other examples obtained by those skilled in the art based on the examples of the present invention without creative efforts belong to the protection scope of the present invention.

[0040] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in this industry. The methods in the following examples are all conventional methods in the art unless otherwise specified. Example 1

[0041] In this example, a transparent polyurethane material is prepared, and the preparation process and steps are as follows:

[0042] S1. Preparation of the luminescent flame retardant

[0043] At 0 °C, 5 mol of triethylamine is added to 50 mL of anhydrous tetrahydrofuran, then 1 mol of triethanolamine is added and stirred for 30 min. Subsequently, 3 mol of diphenylphosphoryl chloride is added dropwise (the dropping rate of diphenylphosphoryl chloride is 0.5 mL / min), and the mixture is stirred for 3 h. The reaction system is quenched with water, 100 mL of water and 100 mL of ethyl acetate are added for extraction. The obtained organic phase is washed twice with 100 mL of deionized water and 100 mL of saturated brine, 5 g of anhydrous sodium sulfate is added for drying and filtration, and after concentration under reduced pressure, it is purified by column chromatography to obtain the luminescent flame retardant;

[0044] S2. Preparation of the transparent polyurethane material

[0045] Add 10 mol of polyether polyol into a glass container and heat it to 120 °C. Then evacuate the glass container and fill it with argon. Cool the system to 80 °C, add 20 mol of hexamethylene diisocyanate and stir for 1 h. Then lower the temperature to 55 °C, add 9 mol of luminescent flame retardant and 10 mol of chain extender (prepared by adding 0.9 g of 1,4-butanediol into 2 mL of N,N-dimethylformamide), stir for 1 min, add 0.6 mL of triethylamine to obtain a polymer emulsion, and pour it into a polytetrafluoroethylene mold. React and cure it in an oven at 80 °C for 48 h to obtain a transparent polyurethane material. Example 2

[0046] In this example, a transparent polyurethane material is prepared, and its preparation process and steps are as follows:

[0047] S1. Prepare the luminescent flame retardant

[0048] Add 5 mol of triethylamine into 50 mL of anhydrous tetrahydrofuran at 0 °C, then add 1 mol of triethanolamine and stir for 30 min. Subsequently, dropwise add 3 mol of diphenylphosphoryl chloride (the dropping rate of diphenylphosphoryl chloride is 0.55 mL / min), and stir for 4 h. Quench the reaction system with water, add 100 mL of water and 100 mL of ethyl acetate for extraction. The obtained organic phase is washed 3 times with 100 mL of deionized water and 100 mL of saturated brine respectively, add 5 g of anhydrous sodium sulfate for drying and filtration, concentrate under reduced pressure and purify by column chromatography to obtain the luminescent flame retardant;

[0049] S2. Prepare the transparent polyurethane material

[0050] Add 10 mol of polyether polyol into a glass container and heat it to 120 °C. Then evacuate the glass container and fill it with argon. Cool the system to 80 °C, add 20 mol of hexamethylene diisocyanate and stir for 1 h. Then lower the temperature to 55 °C, add 3 mol of luminescent flame retardant and 10 mol of chain extender (prepared by adding 0.9 g of 1,4-butanediol into 2 mL of N,N-dimethylformamide), stir for 1.5 min, add 0.6 mL of triethylamine to obtain a polymer emulsion, and pour it into a polytetrafluoroethylene mold. React and cure it in an oven at 80 °C for 48 h to obtain a transparent polyurethane material. Example 3

[0051] In this example, a transparent polyurethane material is prepared, and its preparation process and steps are as follows:

[0052] S1. Prepare the luminescent flame retardant

[0053] At 0 °C, 5 mol of triethylamine was added to 50 mL of anhydrous tetrahydrofuran, then 1 mol of triethanolamine was added and stirred for 30 min. Subsequently, 3 mol of diphenylphosphoryl chloride was added dropwise (the dropping rate of diphenylphosphoryl chloride was 0.6 mL / min), and stirred for 5 h. The reaction system was quenched with water, 100 mL of water and 100 mL of ethyl acetate were added for extraction. The obtained organic phase was washed 4 times with 100 mL of deionized water and 100 mL of saturated brine respectively, 5 g of anhydrous sodium sulfate was added for drying, filtration, and after concentration under reduced pressure, it was purified by column chromatography to obtain the luminescent flame retardant;

[0054] S2. Preparation of transparent polyurethane material

[0055] 10 mol of polyether polyol was added to a glass container and heated to 120 °C. Then the glass container was evacuated and filled with argon. The system was cooled to 80 °C, 20 mol of hexamethylene diisocyanate was added and stirred for 1 h. Then the temperature was lowered to 55 °C, 6 mol of the luminescent flame retardant and 10 mol of chain extender (prepared by adding 0.9 g of 1,4-butanediol to 2 mL of N,N-dimethylformamide) were added, stirred for 2 min, 0.6 mL of triethylamine was added to obtain a polymer emulsion, and it was poured into a polytetrafluoroethylene mold and reacted and cured in an oven at 80 °C for 48 h to obtain the transparent polyurethane material. Example 4

[0056] In this example, the transparent polyurethane material was prepared, and the preparation process and steps were as follows:

[0057] S1. Preparation of luminescent flame retardant

[0058] At 0 °C, 5 mol of triethylamine was added to 50 mL of anhydrous tetrahydrofuran, then 1 mol of triethanolamine was added and stirred for 30 min. Subsequently, 3 mol of diphenylphosphoryl chloride was added dropwise (the dropping rate of diphenylphosphoryl chloride was 0.5 mL / min), and stirred for 3 h. The reaction system was quenched with water, 100 mL of water and 100 mL of ethyl acetate were added for extraction. The obtained organic phase was washed 2 times with 100 mL of deionized water and 100 mL of saturated brine respectively, 5 g of anhydrous sodium sulfate was added for drying, filtration, and after concentration under reduced pressure, it was purified by column chromatography to obtain the luminescent flame retardant;

[0059] S2. Preparation of transparent polyurethane material

[0060] 10 mol of polyether polyol was added to a glass container and heated to 120 °C. Then, the glass container was evacuated and filled with argon. The system was cooled to 80 °C, 20 mol of hexamethylene diisocyanate was added and stirred for 1 h. Then the temperature was lowered to 55 °C, 12 mol of luminescent flame retardant and 10 mol of chain extender (prepared by adding 0.9 g of 1,4-butanediol to 2 mL of N,N-dimethylformamide) were added, stirred for 1 min, and 0.6 mL of triethylamine was added to obtain a polymer emulsion, which was then poured into a polytetrafluoroethylene mold and reacted and cured in an oven at 80 °C for 48 h to obtain a transparent polyurethane material. Comparative Example

[0061] In order to explore the influence of different parameters in the preparation process of the present invention or on the properties of the products of the present invention, the following comparative experiments were specifically carried out. The following comparative examples respectively prepared different polyurethane materials, specifically as follows:

[0062] Comparative Example 1

[0063] In this comparative example, a polyurethane material was prepared. The preparation process was similar to that of Example 1, except that in step S1, the molar ratio of triethylamine, triethanolamine, and diphenylphosphoryl chloride was 5:2:3.

[0064] Comparative Example 2

[0065] In this comparative example, a polyurethane material was prepared. The preparation process was similar to that of Example 1, except that in step S1, the dropping rate of diphenylphosphoryl chloride was 1 mL / min.

[0066] Comparative Example 3

[0067] In this comparative example, a polyurethane material was prepared. The preparation process was similar to that of Example 1, except that in step S2, the molar ratio of polyether polyol to hexamethylene diisocyanate, luminescent flame retardant, and chain extender was 1:4:3:3.

[0068] Performance Test

[0069] The polyurethane materials prepared in the examples of the present invention and the comparative examples were subjected to performance characterization, specifically as follows:

[0070] As Figure 1, which is the structural characterization diagram of the luminescent flame retardant prepared in step S1 of Example 1 of the present invention. Among them, (a) is the nuclear magnetic resonance hydrogen spectrum. From the figure (a), it can be seen that the chemical shift at 7.79 - 7.47 ppm belongs to the H on the benzene ring, and the peaks at 3.47 and 2.65 ppm belong to the H on the oxygen and nitrogen of triethanolamine respectively, indicating that the target luminescent flame retardant NEDP was successfully synthesized; (b) is the Fourier transform infrared spectrum of the luminescent flame retardant (NEDP), triethanolamine (TEOA) and diphenylphosphoryl chloride (DPC). From the Fourier transform infrared spectrum of triethanolamine, the absorption peak at 2885 cm -1 belongs to C-H, and the absorption peak at 3321 cm -1 belongs to the stretching vibrations of N-H and -OH. For diphenylphosphoryl chloride, the sharp peaks at 3060 -1 , 1438 -1 , 1231 -1 and 616 cm -1 are respectively attributed to the stretching vibrations of C-H, P-Ph, P=O and P-Cl. By comparing the Fourier transform infrared spectra of -OH and P-Cl, it can be concluded that the peaks of -OH and P-Cl disappear, and a new P-O peak appears at 934 cm -1 . Therefore, it can be proved that triethanolamine and diphenylphosphoryl chloride were successfully synthesized into the target product, the luminescent flame retardant (NEDP).

[0071] As Figure 2 , which is the infrared spectrum of the transparent polyurethane material prepared in Examples 1 - 4 of the present invention. It can be seen from the figure that the synthesized luminescent flame retardant successfully entered the polyurethane matrix. Among them, the -NCO absorption peak near 2260 cm -1 in the sample disappeared, indicating that the -NCO group reacted fully with the -OH group. The stretching vibration peaks of saturated CH2 in the molecular chain appear near 2939 -1 , 2924 -1 and 2856 cm -1 respectively. Under the influence of N-H, the characteristic absorption peak of C-O in polyurethane shifted slightly to 1728 cm -1 with a lower wavelength. Compared with pure polyurethane (TPU), the characteristic peaks of NEDP in the polyurethane prepared in Examples 1 - 4 of the present invention are located at 1222 cm -1 (P=O), 955 cm -1 (P-O) and 3343 cm -1 (N-H) respectively. Among them, at 1222 cm -1 (P=O) and 955 cm -1The characteristic peak at (P-O) is the most obvious. The results show that the luminescent flame retardant NEDP has been successfully introduced into the polyurethane structure, and polyurethane materials with different NEDP contents have been successfully prepared.

[0072] As Figure 3 and Figure 4 can be seen, the polyurethane materials prepared in Examples 1-4 of the present invention have excellent flame retardant properties and mechanical properties. As can be seen from Figure 3 Figure (a), compared with the pure polyurethane without adding the luminescent flame retardant, the combustion situation and dripping phenomenon of the polyurethane materials prepared in the present invention have been greatly improved. The mass loss of the polyurethane materials has decreased, and the limiting oxygen index LOI has increased from 17.8 to 30.3.

[0073] Figure 3 Figure (b) is the vertical burning test of pure polyurethane and different samples of Example 1. It can be seen that the pure polyurethane sample has obvious combustion phenomenon and serious dripping phenomenon in the vertical burning test. The dripping droplets ignite the absorbent cotton below, and the secondary ignition not only ignites the absorbent cotton again, but also the sample bar is almost completely burned out; after adding the luminescent flame retardant, the combustion phenomenon of the sample bar has been greatly alleviated, and the dripping phenomenon has also been greatly improved. From the easy ignition of the absorbent cotton below by the pure polyurethane elastomer to the gradual extension of the ignition time of the absorbent cotton, until after adding 9 mol of the luminescent flame retardant, the absorbent cotton cannot be ignited at all, and it is found that the slight dripping still cannot ignite the absorbent cotton below after the secondary ignition, and the best flame retardant grade is obtained. The UL-94 flame retardant grade has been improved from no grade to V-0 grade. As can be seen from Figure 3 Figure (c), it can also be seen that after introducing NEDP, there is a greater improvement in flame retardancy compared to pure polyurethane. The high-performance flame retardant performance of the polyurethane material is mainly due to the good synergistic flame retardant effect of the gas phase and condensed phase of the luminescent flame retardant, thus obtaining better flame retardant properties.

[0074] In addition, as Figure 4 , compared with the pure polyurethane material without adding the luminescent flame retardant, the tensile strength of the polyurethane materials prepared in Examples 1-4 of the present invention still remains above 25 MPa. Specifically, the stresses of Examples 1-4 are 27.9 MPa, 28 MPa, 27.81 MPa, and 26.8 MPa respectively, and the strains are 1022%, 1002%, 997%, and 1466% respectively. This shows that the polyurethane material after adding the luminescent flame retardant still maintains good mechanical properties, thus ensuring the requirements in practical applications.

[0075] As Figure 5, as can be seen from the figure, the light transmittance of the polyurethane materials prepared in Examples 1-4 of the present invention all remains above 80%, while the light transmittance of the polyurethane materials prepared in Comparative Examples 1-3 fluctuates between 60-78%, showing a significant gap compared with the light transmittance of the polyurethane materials prepared in the examples. This indicates that the polyurethane materials prepared in Examples 1-4 of the present invention have high light transmittance and can be used as light conversion films in flexible solar cells, thereby improving the photoelectric conversion efficiency of flexible solar cells.

[0076] As Figure 6 , it is the scanning electron microscope characterization diagram of the carbon layer after combustion of the polyurethane material prepared in Example 1 of the present invention and the pure polyurethane material. From Figure 6 (a1) and (b1), it can be seen that the carbon layer of pure polyurethane is thin and has pore defects, which are extremely prone to collapse, and the defects will also cause continuous contact of combustible gases and continuous heat transfer, resulting in continuous combustion; while from Figure 6 (a2) and (b2), it can be seen that after adding the luminescent flame retardant, nitrides and phosphate products promote the formation of ammonium phosphate salt, which coats the surface of the polyurethane matrix and forms a dense and smooth carbon layer, making the carbon layer thicker, achieving the effect of physically blocking air and combustible gases and blocking heat transfer, thereby achieving a better flame retardant effect.

[0077] Figure 7 It is the recyclability test of the polyurethane material prepared in Example 1 of the present invention. During the recycling process, the polyurethane material is first cut into small pieces, and these pieces are then reshaped into dumbbell-shaped specimens, hot-pressed at 120 °C and 5 MPa for 1 h, and then gradually cooled to 25-30 °C. The cooled specimens are kept at this temperature for 24 h to achieve structural stability, and effective re-integration of the pieces can be observed during this period. The performance comparison evaluation through stress-strain curve analysis shows that the mechanical properties of the original polyurethane specimen and the reshaped polyurethane specimen are almost exactly the same. This remarkable performance retention can prove that the polyurethane material prepared by the present invention is a recyclable material and can be reused, meeting the requirements of contemporary ecological protection and low-carbon goals.

[0078] As Figure 8, which is the luminescence performance characterization diagram of the polyurethane materials prepared in Examples 1-4 of the present invention. It can be seen from the figure that each material exhibits obvious fluorescence intensity under the excitation wavelength of 365 nm, and it continuously increases with the addition of the luminescent flame retardant. The polyurethane material after adding the luminescent flame retardant absorbs the incident ultraviolet light of 365 nm and converts it into visible blue light above 400 nm, and the fluorescence intensity is relatively strong. Since the flexible solar panel has a very low absorption and utilization rate of ultraviolet light below 400 nm, while the power generation efficiency of visible light is high, the photoelectric conversion efficiency becomes higher after being converted into visible light, so that its application on flexible solar cells increases the photoelectric conversion efficiency of solar cells.

[0079] In addition, the polyurethane materials prepared in Examples 1-4 and Comparative Examples 1-3 of the present invention and pure polyurethane are used as light conversion films on flexible solar panels. By inputting the same intensity of light power on the flexible solar panels, their photoelectric conversion efficiencies are tested. The specific data are shown in the following table:

[0080]

[0081] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a flame retardant, luminous and recyclable transparent polyurethane material, characterized in that: Follow the steps below in order: S1. Preparation of luminescent flame retardant Triethylamine was added to 50 mL of anhydrous tetrahydrofuran at 0°C, and triethanolamine was added and stirred for 30 min, followed by diphenylphosphonyl chloride being added dropwise at a rate of 0.5-0.6 mL / min and stirred for 3-5 h, the reaction system was quenched with water, 100 mL of water and 100 mL of ethyl acetate were added for extraction, the obtained organic phase was washed 2-4 times with 100 mL of deionized water and 100 mL of saturated brine, 5 g of anhydrous sodium sulfate was added for drying, filtration, concentration under reduced pressure and purification by column chromatography to obtain a luminescent flame retardant; The molar ratio of triethylamine, triethanolamine and diphenylphosphonyl chloride is 5:1:3; S2. Preparation of transparent polyurethane material Add polyether polyol into a three-necked flask and heat it to 120°C, then evacuate the glass container and fill it with argon, cool the system to 80°C, add isocyanate and stir for 1 h, then lower the temperature to 55°C, add luminescent flame retardant and chain extender, stir for 1-2 min, add 0.6 mL triethylamine to obtain a polymer emulsion, pour it into a polytetrafluoroethylene mold, react and cure in an oven at 80°C for 48 h to obtain a transparent polyurethane material; The molar ratio of the polyether polyol, isocyanate, luminescent flame retardant and chain extender is 10:20:(3-12):

10.

2. The method for preparing a flame retardant, luminescent and recyclable transparent polyurethane material according to claim 1, characterized in that: In step S2, the molecular weight of the polyether polyol is 1000-3000.

3. The method for preparing a flame retardant, luminescent and recyclable transparent polyurethane material according to claim 1, characterized in that: In step S2, the isocyanate is any one of hexamethylene diisocyanate, isophorone diisocyanate, methylcyclohexyl diisocyanate, dicyclohexyl methylene diisocyanate, xylylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and dimer acid diisocyanate.

4. The method for preparing a flame retardant, luminescent and recyclable transparent polyurethane material according to claim 1, characterized in that: In step S2, the chain extender is prepared by adding 0.9 g of 1,4-butanediol to 2 mL of N,N-dimethylformamide and stirring the mixture evenly.

5. The method for preparing a flame retardant, luminous and recyclable transparent polyurethane material according to claim 1, characterized in that: In step S1, the structural formula of the luminescent flame retardant prepared is: 。 6. Application of the transparent polyurethane material prepared by the method for preparing a flame-retardant, luminescent and recyclable transparent polyurethane material according to any one of claims 1 to 5 as a light transfer film on a flexible solar cell.

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