Layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant and preparation method thereof

Abstract

The present invention discloses a layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant and its preparation method and application. The method uses sodium tetraborate, chitosan, sodium hexametaphosphate, arginine or melamine and an aluminum-containing compound as raw materials, utilizes a hydrothermal one-pot method, and prepares the microspherical biomass-based intumescent flame retardant by electrostatic adsorption layer-by-layer self-assembly. The present invention combines a carbon source, an acid source, and a gas source into one to prepare a microspherical biomass-based flame retardant. The synthesis process is green and environmentally friendly, the reaction conditions are mild, and the operation is simple. The microspherical structure has good compatibility and dispersibility in a PLA matrix, so that the flame retardancy, smoke suppression, anti-melting and mechanical properties of polylactic acid can be effectively improved at a low addition amount, thereby realizing the multifunctional modification of PLA and having great significance for the promotion and use of PLA materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of biodegradable and flame-retardant composite materials, and particularly relates to a layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant, and a preparation method and application thereof. Background Art

[0002] Over the past few decades, renewable materials have attracted widespread attention due to the gradual depletion of fossil resources and growing awareness of environmental protection. As the first commercially available bio-based polymer, polylactic acid (PLA) offers excellent biodegradability and mechanical properties, holding the potential to replace traditional plastics. However, PLA is highly flammable, posing a potential fire hazard and limiting its application in many fields. Therefore, the development of a flame retardant for PLA is imperative to expand its practical industrial applications.

[0003] Traditional PLA flame retardants include halogen flame retardants, phosphorus-based flame retardants, nitrogen-based flame retardants, and metal compound flame retardants. With the increasing national requirements for environmental protection and green development, the use of environmentally harmful halogen flame retardants has been banned. Intumescent flame retardants composed of carbon, acid, and gas sources have replaced them. Biomass-based single-component intumescent flame retardants are widely favored due to their environmental friendliness, low dosage, and good compatibility with the PLA matrix. Summary of the Invention

[0004] The present invention aims to provide a microspherical biomass-based intumescent flame retardant formed by layer-by-layer self-assembly through electrostatic adsorption, as well as its preparation method and application. By combining a carbon source, an acid source, and a gas source, the biomass-based flame retardant produced has a unique microspherical structure and nanoscale dimensions. It exhibits excellent dispersibility and compatibility within a polylactic acid (PLA) matrix, effectively enhancing the flame retardancy, smoke suppression, and anti-drip properties of PLA at low addition levels. It also improves the mechanical properties of PLA composites, significantly contributing to the widespread use of PLA materials.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A self-assembled layer-by-layer microspherical biomass-based intumescent flame retardant, which is composed of sodium tetraborate cross-linked chitosan as the core, sodium hexametaphosphate as the shell, arginine or melamine and Al 3+ It is a microspherical structure composed of extra-shell grafted groups.

[0007] The preparation method of the microspherical biomass-based intumescent flame retardant comprises the following steps:

[0008] (1) Dissolving chitosan in a dilute acid solution and uniformly dispersing it by stirring and ultrasonic vibration to obtain a chitosan solution;

[0009] (2) Add an appropriate amount of sodium tetraborate solution to the chitosan solution obtained in step (1), and stir the mixture at 90°C under nitrogen for 2 hours;

[0010] (3) Add an appropriate amount of sodium hexametaphosphate solution to the reaction solution obtained in step (2), and stir the reaction at 70°C under nitrogen for 4 hours;

[0011] (4) Add an appropriate amount of arginine solution or melamine solution to the reaction solution obtained in step (3), and stir the reaction at 70°C under nitrogen for 1 hour;

[0012] (5) Add an appropriate amount of Al to the reaction solution obtained in step (4) 3+ The compound was stirred and reacted at 70 °C under nitrogen for 10 h;

[0013] (6) Adding sodium carbonate solution to the reaction solution obtained in step (5), adjusting the pH to neutral, and allowing to settle for 12 hours. Washing, filtering, vacuum drying, crushing, and sieving to obtain the layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant.

[0014] Furthermore, the amount of the dilute acid solution in step (1) is converted to 10-20 mL per 10 g of chitosan; the concentration of the dilute acid solution is 3 wt %, and the acid used is one of acetic acid, hydrochloric acid, and citric acid.

[0015] Furthermore, the frequency of the ultrasonic vibration in step (1) is 40 KHz.

[0016] Furthermore, the concentration of the sodium tetraborate solution in step (2) is 10 wt%.

[0017] Furthermore, the concentration of the sodium hexametaphosphate solution in step (3) is 10 wt%.

[0018] Furthermore, the concentration of the arginine solution or melamine solution in step (4) is 20 wt%.

[0019] Furthermore, the Al-containing 3+ The compound is one of aluminum nitrate, aluminum sulfate, aluminum hydroxide and aluminum chloride.

[0020] Furthermore, the amount of each material used is 0.01-0.1 mol sodium tetraborate, 0.02-0.1 mol sodium hexametaphosphate, 0.06-0.3 mol arginine or melamine, 0.02-0.1 mol Al-containing 3+ Compound conversion.

[0021] Furthermore, the stirring speed in steps (1)-(5) is 300-500 rpm.

[0022] Furthermore, the concentration of the sodium carbonate solution in step (6) is 5 wt%.

[0023] The obtained layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant can be used to prepare flame-retardant polylactic acid. Specifically, the biomass-based intumescent flame retardant and polylactic acid are stirred and mixed in a mass ratio of 95:5, extruded through a twin-screw extruder and injection molded to obtain flame-retardant polylactic acid.

[0024] Chitosan, derived from the shells of crustaceans and the exoskeletons of arthropods, is a biodegradable and environmentally friendly material. Chitosan is an amino polysaccharide containing multiple hydroxyl groups and carries a positive charge in dilute acid solutions, which offers significant potential for modification. Furthermore, the carbon and nitrogen components of chitosan promote the formation of a char layer during combustion, making it a promising carbon source for intumescent flame retardants.

[0025] Sodium tetraborate is a high-boron compound derived from the dehydration of borax. Its molecular formula is Na₂B₄Oₐ. It is a white crystal at room temperature and is readily soluble in water. It is an important boron-containing compound widely used in cleaning, cosmetics, welding, and medicine. Sodium tetraborate has a high boron content. During combustion, boron participates in the dehydration process, forming a protective layer with a ceramic structure. This increases the density of the carbon layer and serves as a good acid source for intumescent flame retardants.

[0026] Sodium hexametaphosphate, with the molecular formula (NaPO₃)₆, is a white crystal at room temperature that is readily soluble in water. It is an important phosphorus-containing compound commonly used as a food additive, adhesive, expander, and metal chelator. Sodium hexametaphosphate has a high phosphorus content, and during combustion, phosphorus catalyzes the dehydration of carbon-containing organic matter, forming a dense char layer. This makes it a good acid source for intumescent flame retardants.

[0027] Arginine and melamine contain multiple amino groups in their structures. Amino groups produce a lot of inert gases during the combustion process, which can effectively reduce the concentration of combustible substances produced by the thermal decomposition of PLA. At the same time, they can also have a good smoke suppression effect. Therefore, they can be used as a good gas source for intumescent flame retardants.

[0028] The present invention utilizes the above materials and prepares a microspherical high-efficiency biomass-based flame retardant by rationally designing a reaction route.

[0029] The significant advantages of the present invention are:

[0030] (1) All the synthetic steps of the present invention are carried out in aqueous phase and a hydrothermal one-pot method is adopted. The desired product can be directly obtained without isolating the intermediate product. The preparation process is simple, the reaction conditions are mild, and industrial production can be easily realized.

[0031] (2) The raw materials used in the present invention are completely biodegradable materials, which are harmless and pollution-free to the environment and will not affect the degradation properties of the PLA matrix itself, so that the obtained material can have both flame retardant and degradable properties.

[0032] (3) The microspherical biomass-based intumescent flame retardant prepared by the present invention has good thermal stability. Its special microspherical structure makes it have better compatibility and dispersibility in the PLA matrix, and has little effect on the mechanical properties of PLA.

[0033] (4) The flame retardant efficiency of the present invention is high. When the addition amount is only 5wt% in PLA, the flame retardant grade can reach UL94 V-0. At the same time, it can also effectively improve the anti-melting dripping and smoke suppression effects of PLA. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a process flow chart for synthesizing a biomass-based intumescent flame retardant in Example 1.

[0035] Figure 2 This is the FT-IR image of the biomass-based intumescent flame retardant prepared in Example 1.

[0036] Figure 3 This is the SEM image of pure chitosan without modification.

[0037] Figure 4 This is the SEM image of the biomass-based intumescent flame retardant prepared in Example 1.

[0038] Figure 5 This is the SEM image of the biomass-based intumescent flame retardant prepared in Example 2.

[0039] Figure 6 This is the SEM image of the carbon layer of the specimen prepared in Application Example 1 after combustion.

[0040] Figure 7 This is the SEM image of the carbon layer of the specimen prepared in Application Example 2 after combustion.

[0041] Figure 8 This is the SEM image of the carbon layer of the specimen prepared in Comparative Example 1 after combustion. DETAILED DESCRIPTION

[0042] A microspherical biomass-based intumescent flame retardant, the preparation of which comprises the following steps:

[0043] (1) Dissolve 10 g of chitosan in 10-20 mL of a 3 wt% dilute acid solution, and uniformly disperse the solution by stirring at 300-500 rpm and ultrasonically vibrating at 40 kHz to obtain a chitosan solution.

[0044] (2) adding 10 wt% of a solution containing 0.01-0.1 mol of sodium tetraborate to the chitosan solution obtained in step (1), stirring and reacting at 90° C. under nitrogen for 2 h;

[0045] (3) Add 10 wt% of a solution containing 0.02-0.1 mol of sodium hexametaphosphate to the reaction solution obtained in step (2), and stir the reaction at 70°C under nitrogen for 4 h;

[0046] (4) Add 20 wt% of a solution containing 0.06-0.3 mol of arginine or melamine to the reaction solution obtained in step (3), and stir the reaction at 70°C under nitrogen for 1 h;

[0047] (5) Add 0.02-0.1 mol of Al to the reaction solution obtained in step (4) 3+ The compound was stirred and reacted at 70 °C under nitrogen for 10 h;

[0048] (6) Add 5% sodium carbonate solution to the reaction solution obtained in step (5), adjust the pH to neutral, let it stand for 12 hours, wash and filter, vacuum dry, crush and sieve to obtain a layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant.

[0049] Wherein, the acid used is one of acetic acid, hydrochloric acid and citric acid. 3+ The compound is one of aluminum nitrate, aluminum sulfate, aluminum hydroxide and aluminum chloride.

[0050] In order to make the contents of the present invention easier to understand, the technical solutions of the present invention are further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0051] Example 1:

[0052] 10 g of chitosan was weighed and dissolved in 20 mL of 3 wt% acetic acid solution, and uniformly dispersed by magnetic stirring and ultrasonic vibration to obtain a chitosan solution; then 5 g (0.025 mol) of sodium tetraborate was dissolved in 45 mL of deionized water to prepare a 10 wt% sodium tetraborate solution, and the sodium tetraborate solution was added dropwise to the chitosan solution, and stirred for 2 h at 90 ° C under nitrogen environment; then 12.2 g (0.02 mol) of sodium hexametaphosphate was weighed and dissolved in 109.8 mL of deionized water to prepare a 10 wt% sodium tetraborate solution. A 10 wt% sodium hexametaphosphate solution was prepared and added dropwise to the reaction solution. The mixture was stirred at 70°C under nitrogen for 4 hours. 10.45 g (0.06 mol) of arginine was dissolved in 41.8 mL of deionized water to prepare a 20 wt% arginine solution. This solution was added dropwise to the reaction solution and stirred at 70°C under nitrogen for 1 hour. Finally, 13.2 g (0.02 mol) of aluminum sulfate 18hydrate was added and stirred at 70°C under nitrogen for 10 hours. After the reaction, 50 mL of a 5 wt% sodium carbonate solution was added to adjust the pH to neutral. The mixture was then allowed to settle for 12 hours. The mixture was then washed, filtered, vacuum-dried, pulverized, and sieved to obtain a layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant.

[0053] Example 2:

[0054] Weigh 10g of chitosan and dissolve it in 20mL of 3 wt% hydrochloric acid solution, and disperse it evenly by magnetic stirring and ultrasonic vibration to obtain chitosan solution; then take 5g (0.025mol) of sodium tetraborate and dissolve it in 45mL of deionized water to prepare 10 wt% sodium tetraborate solution, and the sodium tetraborate solution was added dropwise to the chitosan solution, and stirred for reaction at 90°C under nitrogen for 2 hours; then 12.2g (0.02mol) of sodium hexametaphosphate was weighed and dissolved in 109.8mL of deionized water to prepare a 10wt% sodium hexametaphosphate solution, and the sodium hexametaphosphate solution was added dropwise to the above reaction solution, and stirred for reaction at 70°C under nitrogen for 4 hours; then 7.57g (0.06mol) of melamine was weighed and dissolved in 30.28mL of deionized water to prepare a 20wt% melamine solution, and the melamine solution was added dropwise to the above reaction solution, and stirred for reaction at 70°C under nitrogen for 1 hour; finally, 9g (0.042mol) of aluminum nitrate was added, and stirred for reaction at 70°C under nitrogen for 10 hours. After the reaction, 50 mL of 5 wt% sodium carbonate solution was added to adjust the pH to neutral, and the mixture was allowed to settle for 12 h. After washing, filtration, vacuum drying, crushing, and sieving, a layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant was obtained.

[0055] Figure 2This is the FI-IR spectrum of the microspherical biomass-based intumescent flame retardant prepared in Example 1. As can be seen from the figure, compared with chitosan (CS), its -1 、1390 cm -1 、1180 cm -1 , 953 cm -1 and 519 cm -1 Three new peaks appeared at + , BOC on sodium tetraborate, POO and -PO on sodium hexametaphosphate and Al-O bond, which indicates that chitosan has a strong affinity with sodium tetraborate, arginine, sodium hexametaphosphate and Al 3+ A reaction occurred, proving that the layer-by-layer self-assembled biomass-based intumescent flame retardant was successfully prepared.

[0056] Figure 3-5 The following are SEM images of chitosan and the flame retardants prepared in Examples 1 and 2, respectively. As can be seen, the unmodified chitosan has a flaky morphology and micron-scale particles, resulting in poor dispersibility in the PLA matrix and a significant impact on the mechanical properties of the PLA composite. In contrast, the prepared flame retardant has a microspherical morphology and nanometer-scale particles, resulting in good dispersibility in the PLA matrix and a significant improvement in the mechanical properties of the PLA composite.

[0057] Application Example 1:

[0058] Weigh 5 parts (mass parts) of the biomass-based intumescent flame retardant prepared in Example 1, stir and mix with 95 parts (mass parts) of PLA, extrude and injection mold through a twin-screw extruder to produce flame-retardant polylactic acid composite material standard vertical burning (UL-94), limiting oxygen index (LOI) specimens and standard tensile, standard impact, and standard bending specimens for testing.

[0059] Application Example 2

[0060] Weigh 5 parts (mass parts) of the biomass-based intumescent flame retardant prepared in Example 2, stir and mix with 95 parts (mass parts) of PLA, extrude and injection mold through a twin-screw extruder to produce flame-retardant polylactic acid composite material standard vertical burning (UL-94), limiting oxygen index (LOI) specimens and standard tensile, standard impact, and standard bending specimens for testing.

[0061] Comparative Example 1 (pure sample)

[0062] Polylactic acid pellets were injection molded to produce standard vertical burning (UL-94), limiting oxygen index (LOI) specimens and standard tensile, standard impact, and standard bending specimens for testing.

[0063] Comparative Example 2 (Chitosan)

[0064] 5 parts (by mass) of chitosan were weighed and mixed with 95 parts (by mass) of PLA. The mixture was extruded through a twin-screw extruder and injection molded to produce standard vertical burning (UL-94) and limiting oxygen index (LOI) specimens as well as standard tensile, standard impact, and standard bending specimens for testing.

[0065] Comparative Example 3 (Flame Retardant on the Market)

[0066] 10 parts by mass of commercially available ammonium polyphosphate (phosphorus content: 30%-32%, nitrogen content: 14%-16%) flame retardant were weighed and mixed with 90 parts by mass of PLA. The mixture was then extruded and injection molded using a twin-screw extruder to produce standard vertical burning (UL-94) and limiting oxygen index (LOI) test bars as well as standard tensile, standard impact, and standard bending test bars for testing.

[0067] The vertical combustion specimens, limiting oxygen index specimens, standard tensile specimens, standard impact specimens, and standard bending specimens prepared above were subjected to combustion performance and mechanical property tests in accordance with ASTM D3801, ASTM D2863-97, GB / T 1843-2008, GB / T 1843-2008, and GB / T9341-2008. The test results are shown in Table 1.

[0068] Table 1 Combustion performance and mechanical properties

[0069]

[0070] As shown in Table 1, compared with Comparative Example 1, the flame retardant properties of Application Example 1 and Application Example 2 have been significantly improved. The limiting oxygen index has increased from 19.5% to over 31.2%, and the UL-94 vertical burning grade has also increased from NR to V-0. In addition, the impact strength and elongation at break have been improved compared with Comparative Example 1.

[0071] Application Example 1 and Application Example 2 show better flame retardant effect than Comparative Example 2 and Comparative Example 3, and their mechanical properties are much higher than those of the composite materials prepared by adding chitosan and conventional flame retardants.

[0072] Figure 6-8 The following are SEM images of the char layers after combustion in the materials prepared in Application Examples 1 and 2, and Comparative Example 1. The images further demonstrate that the PLA composites prepared with the flame retardant exhibit a dense and compact char layer after combustion, effectively isolating heat and mass transfer and providing excellent flame retardancy. In contrast, the PLA composites prepared without the flame retardant exhibit a porous char layer that fails to effectively block heat, thus achieving an ideal flame retardant effect.

[0073] Through the above example analysis, we can know that:

[0074] (1) The raw materials used in the self-assembled microsphere-shaped biomass-based intumescent flame retardant prepared by the present invention are mostly bio-based materials, which can be completely degraded into non-toxic and harmless small molecules in the natural environment. The addition of the biomass-based intumescent flame retardant to the polylactic acid matrix does not affect its degradation performance, and can even increase the natural degradation rate of polylactic acid to a certain extent. This is because arginine and chitosan have good hydrophilicity, which can improve the water absorption performance of polylactic acid and more easily catalyze the cleavage of ester bonds in polylactic acid in the natural environment. At the same time, chitosan, arginine, etc. can be decomposed and utilized by microorganisms, further accelerating the degradation of the polylactic acid matrix.

[0075] (2) The biomass-based intumescent flame retardant prepared by the present invention transforms the chitosan from flakes to microspheres in terms of microstructure, and also changes its scale from micrometer level to nanometer level, which is conducive to enhancing its compatibility with the polylactic acid matrix. At the same time, the unreacted groups on chitosan and arginine also form intramolecular hydrogen bonds with the polylactic acid matrix, which has a certain effect on improving the mechanical properties of polylactic acid.

[0076] (3) The present invention prepares a self-assembled layer-by-layer microspherical biomass-based intumescent flame retardant, which can achieve excellent flame retardant properties when added in an amount of only 5wt%. 3+ It can effectively inhibit the serious melting drop phenomenon caused by the addition of chitosan to polylactic acid, and at the same time quickly catalyze and efficiently form carbon to form a dense carbon layer, thereby blocking the transfer of heat and the exchange of oxygen with the outside world, thereby achieving the purpose of high flame retardant performance at a low addition amount.

[0077] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant, characterized by: The flame retardant is composed of sodium tetraborate cross-linked chitosan as the core, sodium hexametaphosphate as the shell, arginine or melamine and Al 3+ It is a microspherical structure composed of extra-shell grafted groups.

2. A method for preparing the microspherical biomass-based intumescent flame retardant according to claim 1, characterized in that: The following steps are involved: (1) Dissolving chitosan in a dilute acid solution and uniformly dispersing it by stirring and ultrasonic vibration to obtain a chitosan solution; (2) adding sodium tetraborate solution to the chitosan solution obtained in step (1), stirring and reacting at 90°C under nitrogen for 2 hours; (3) Adding sodium hexametaphosphate solution to the reaction solution obtained in step (2), stirring and reacting at 70°C under nitrogen for 4 hours; (4) Add arginine solution or melamine solution to the reaction solution obtained in step (3), and stir the reaction at 70°C under nitrogen for 1 hour; (5) Add Al to the reaction solution obtained in step (4) 3+ The compound was stirred and reacted at 70 °C under nitrogen for 10 h; (6) Adding sodium carbonate solution to the reaction solution obtained in step (5), adjusting the pH to neutral, and allowing to settle for 12 hours. Washing, filtering, vacuum drying, crushing, and sieving to obtain the layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant.

3. The preparation method according to claim 2, wherein: The amount of the dilute acid solution in step (1) is converted to 10-20 mL per 10 g of chitosan; the concentration of the dilute acid solution is 3 wt %, and the acid used is one of acetic acid, hydrochloric acid, and citric acid.

4. The preparation method according to claim 2, wherein: The concentration of the sodium tetraborate solution in step (2) is 10 wt%.

5. The preparation method according to claim 2, wherein: The concentration of the sodium hexametaphosphate solution in step (3) is 10 wt%.

6. The preparation method according to claim 2, wherein: The concentration of the arginine solution or melamine solution in step (4) is 20 wt%.

7. The preparation method according to claim 2, characterized in that: The Al-containing 3+ The compound is one of aluminum nitrate, aluminum sulfate, aluminum hydroxide and aluminum chloride.

8. The preparation method according to claim 2, wherein: The amount of each material used is 0.01-0.1 mol sodium tetraborate, 0.02-0.1 mol sodium hexametaphosphate, 0.06-0.3 mol arginine or melamine, 0.02-0.1 mol Al-containing 3+ Compound conversion.

9. The preparation method according to claim 2, wherein: The concentration of the sodium carbonate solution in step (6) is 5 wt%.

10. Use of the layer-by-layer self-assembled microspherical biomass-based intumescent flame retardant according to claim 1 in the preparation of flame-retardant polylactic acid, characterized in that: The biomass-based intumescent flame retardant and polylactic acid are stirred and mixed at a mass ratio of 95:5, and the mixture is extruded and injection molded to obtain flame-retardant polylactic acid.

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