A lignin-based ionic liquid, its preparation method and application

Abstract

A lignin-based ionic liquid, its preparation method, and its application are disclosed. The steps include: (1) preparation of lignin amine; (2) preparation of lignin quaternary ammonium salt; and (3) preparation of the lignin-based ionic liquid. This invention uses lignin as an initiator to prepare an ionic liquid flame retardant, opening a new path for the application of lignin in the field of flame retardants. The ionic liquid flame retardant is prepared by ion exchange, a simple, energy-efficient, and environmentally friendly method.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials, specifically to a lignin ionic liquid, its preparation method, and its application. The ionic liquid, when combined with APP, effectively improves the flame retardant properties of the composite material and reduces the amount of APP required. Background Technology

[0002] Plastic products are diverse and widely used in various industries, and my country has become a major global producer of plastic products. However, plastic products are flammable. For example, the oxygen index (OI) of polylactic acid (PLA) is only 19%, and that of polyethylene (PP) is about 18%. Generally, materials with an OI less than 22 are considered flammable, thus requiring flame-retardant modification. Among the many flame-retardant products, ammonium polyphosphate (APP) is widely used due to its rich phosphorus and nitrogen content, non-toxicity, low smoke, and good thermal stability. However, using APP alone does not significantly improve the flame-retardant properties of polymer composites, leading to extensive research on APP compound flame retardants. Liu Huiyuan et al. found that adding 20% ​​APP to PLA composites increased the OI to 25.3%. Compared to pure PLA, the dripping phenomenon during combustion was still severe, and it could still ignite degreased cotton. However, when the mass ratio of phosphated guar gum to APP was 9 / 11, the OI of the PLA composite reached its maximum value (33.4%), and no dripping phenomenon occurred (Liu Huiyuan et al., China Plastics, 2023). Studies by Sun Jinhao et al. found that when 20% APP was added to PLA alone, the LOI of the composite material increased to 27.3%, but the molten droplets generated after ignition ignited the degreased cotton. However, when 15% APP and 5% starch were added to PLA, the LOI value increased to 30.4% (Sun Jinhao et al., Acta Polymerica Sinica, 2022).

[0003] Ionic liquids are diverse and flexible in structural design. Current research mainly focuses on the dissolution and / or degradation of lignin to prepare plant polyphenols. Wang Yijing (Hebei University of Technology, 2019) designed 24 ionic liquids with 4 cations and 6 anions through enumeration to study the catalytic degradation of lignin. Zhao Wanting et al. (Chemical Engineering Journal, 2022) prepared a low-cost, halogen-free multifunctional [EOA][OAc] with dual functions of dissolving lignin and demethylating it, efficiently converting lignin into polyphenols. In addition, ionic liquids have advantages such as high solubility, low volatility, and high thermal stability, and possess flame retardant properties. When mixed with polymers, they exhibit characteristics of both plasticizers and flame retardants. Wu Kunlin et al. prepared the ionic liquid tetrabutyltetrafluoroborate, which, when compounded with APP, produced flame-retardant lacquer. The flame-retardant performance was superior to the combined effect of using ionic liquid and APP alone, indicating a synergistic flame-retardant effect between ionic liquid and APP. When the IL addition amount is 10 wt.% and the APP is 20 wt.%, the LOI value of the coating film is 28.6%, which meets the standard for flame-retardant materials (Wu Kunlin et al., Journal of Minjiang University, 2022). Summary of the Invention

[0004] Technical problem solved: This invention provides a lignin-based ionic liquid, its preparation method, and its application, mainly including the preparation of lignin amines, the preparation of lignin quaternary ammonium salts, and the preparation of lignin-based ionic liquids. This ionic liquid can assist APP in improving the flame retardancy of composite materials.

[0005] Technical solution: A method for preparing lignin ionic liquid, comprising the following steps: (1) Preparation of lignin amine: lignin or degraded lignin is synthesized into lignin amine by Mannich reaction. A solution of ferrous sulfate (K3Fe8(CN)6) is added to the lignin amine solution to completely precipitate the lignin amine. After filtration and washing with water, the lignin amine is dried. (2) Preparation of lignin quaternary ammonium salt: A mixture of isopropanol and water is used as a dispersant. The lignin amine and 2,3-epoxypropyltrimethylammonium chloride are refluxed. After the reaction is completed, the reaction mixture is washed with ethanol multiple times and filtered to obtain a solid substance. The solid substance is then dried under vacuum to obtain the lignin quaternary ammonium salt. (3) Preparation of lignin-based ionic liquid: The lignin quaternary ammonium salt is mechanically stirred with sodium trifluoromethanesulfonate or sodium fluoroborate in deionized water. After the ion exchange reaction is completed, the solid substance is repeatedly washed with deionized water and filtered under vacuum to obtain the lignin-based ionic liquid.

[0006] The lignin mentioned above is alkali lignin, lignin sulfonate, biobutanol lignin, or bioethanol lignin; the degradation method used to degrade the lignin is hydrogen peroxide oxidation degradation, ozone oxidation degradation, acid hydrolysis, alkaline hydrolysis, hydrogen reduction degradation, and high-temperature thermal decomposition; the amine used in the Mannich reaction is a straight-chain diamine, which is ethylenediamine, 1,3-propanediamine, triethylenetetramine, or tetraethylenepentamine.

[0007] One preferred method for preparing the above-mentioned lignin amine is as follows: 1-5g of lignin is added to 5-15mL of water and 2-8mL of 0.5mol / L NaOH solution according to the proportion; the mixture is stirred at 40-80℃ to fully dissolve the lignin; then 2-6mL of formaldehyde and 1-8mL of diamine are added, and the mixture is refluxed and stirred for 2-4h to synthesize lignin amine; after the reaction is completed, a sufficient amount of 5-15% ferrous sulfate (K3Fe8(CN)6) solution is added to the lignin amine solution to completely precipitate the lignin amine, which is then filtered, washed with water, and dried.

[0008] Secondly, the preparation method of the above-mentioned lignin amine is as follows: 3g of lignin is added to 8mL of water and 5mL of 0.5mol / L NaOH solution according to the ratio; the mixture is stirred at 75℃ to fully dissolve the lignin; then 3mL of formaldehyde and 4mL of diamine are added, and the mixture is refluxed and stirred for 3h to synthesize lignin amine; after the reaction is completed, a sufficient amount of 10% ferrous sulfate (K3Fe8(CN)6) solution is added to the lignin amine solution to completely precipitate the lignin amine, which is then filtered, washed with water, and dried.

[0009] Thirdly, the preparation method of the above-mentioned lignin quaternary ammonium salt is as follows: 0.5-2g of lignin amine, 1-4g of 2,3-epoxypropyltrimethylammonium chloride, and 100mL of water-isopropanol (mass ratio) = (1-3) / (2-6) are added as a dispersant. The mixture is stirred at 60-90℃ for 5-10h. After the reaction is completed, the reaction mixture is washed multiple times with ethanol and filtered to obtain a solid substance. Finally, the solid substance is dried under vacuum at 40℃ to obtain the lignin quaternary ammonium salt.

[0010] Fourthly, the preparation method of the above-mentioned lignin quaternary ammonium salt is as follows: according to the ratio, 1g of lignin amine, 3g of 2,3-epoxypropyltrimethylammonium chloride, and 100mL of water-isopropanol (mass ratio) = 2 / 5 are added as a dispersant. The mixture is stirred at 80℃ for 10h. After the reaction is completed, the reaction mixture is washed with ethanol multiple times and filtered to obtain a solid substance. Finally, the solid substance is dried under vacuum at 40℃ to obtain the lignin quaternary ammonium salt.

[0011] Fifthly, the preparation of the above-mentioned lignin ionic liquid is as follows: 1-5g of lignin quaternary ammonium salt, 2-10g of sodium trifluoromethanesulfonate or sodium fluoroborate and 100-300g of deionized water are mixed in proportion and mechanically stirred; after 24h, the mixture is repeatedly washed with deionized water and filtered, and then vacuum dried at 40℃ to obtain the lignin-based ionic liquid.

[0012] Sixth preferred method: Preparation of the above-mentioned lignin ionic liquid: Mix 2g of lignin quaternary ammonium salt, 6g of sodium trifluoromethanesulfonate or sodium fluoroborate and 200g of deionized water in a certain proportion and stir mechanically; after 24h, wash and filter repeatedly with deionized water and dry under vacuum at 40℃ to obtain lignin-based ionic liquid.

[0013] The lignin ionic liquid prepared by the above method.

[0014] The above-mentioned lignin ionic liquids are used in the preparation of flame-retardant plastics.

[0015] Beneficial effects: 1. High lignin char content. This invention uses industrial lignin as an initiator to prepare ionic liquid flame retardants, opening up a new path for the application of industrial lignin in the field of flame retardants.

[0016] 2. Ionic liquid flame retardants are prepared via ion exchange. This method is simple. During the preparation of lignin quaternary ammonium salts, a weakly acidic environment is established, which facilitates the ring-opening reaction of epoxy groups with amine groups. 2,3-epoxypropyltrimethylammonium chloride is water-soluble, and the two-phase reaction also facilitates easy separation of the products. Furthermore, existing techniques typically involve acid precipitation of lignin to pH 2-3 after the Mannich reaction to obtain lignin amines. However, in this case, the primary amines are essentially salted. In contrast, this invention uses ferrous sulfate to precipitate the lignin amines at pH 8-9, thus preventing amine salt formation and facilitating the subsequent reaction with epoxy groups. This method is energy-efficient and environmentally friendly. Attached Figure Description

[0017] Figure 1 Thermogravimetric plots of APP, lignin-APP, and lignin-based ionic liquid-AP are shown (under nitrogen atmosphere, taking alkali lignin degraded by hydrogen peroxide oxidation as an example); the three curves in the figure are the thermogravimetric plots of APP, lignin-APP, and lignin-based ionic liquid-APP under nitrogen atmosphere (where lignin is alkali lignin, and the anion of the ionic liquid is BF4). -1 The results show that at 800℃, the char residue of APP is 28%, that of lignin-APP is 41.06%, and that of lignin-based ionic liquid-APP is 48.79%. This demonstrates that lignin-based ionic liquid and APP have a synergistic heat resistance effect. Detailed Implementation

[0018] Schematic diagram of lignin-based ionic liquid preparation:

[0019] (1) Preparation of lignin amine

[0020]

[0021] (2) Preparation of lignin quaternary ammonium salts

[0022]

[0023] (3) Preparation of lignin-based ionic liquids

[0024]

[0025] Comparative Example 0-1

[0026] Preparation of flame-retardant PLA: Before use, PLA was dried in a vacuum drying oven at 80℃ for 12 hours, then added to a mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into shape at 190℃ to produce standard specimens. The critical oxygen index was then determined.

[0027] Comparative Examples 0-2

[0028] Preparation of flame-retardant PLA: Before use, both PLA and APP were dried in a vacuum drying oven at 80℃ for 12 hours. They were then mixed at a mass ratio of PLA / APP = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0029] Example 1

[0030] (1) Lignin amine: 2g of alkali lignin degraded by hydrogen peroxide oxidation, 6mL of water, and 3mL of 0.5mol / L NaOH solution were added to a 100mL three-necked flask. The mixture was stirred at 50℃ to fully dissolve the lignin. Then, 2.5mL of formaldehyde and 3mL of 1,3-propanediamine were added, and the mixture was refluxed and stirred for 2h to synthesize lignin amine. Sufficient 5% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0031] (2) Lignin quaternary ammonium salt: 1g of the above-mentioned lignin amine and 2g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250mL three-necked flask, followed by 100mL of water / isopropanol (mass ratio) = 1:4 as a dispersant. The mixture was refluxed at 70℃ and stirred for 6h. After the reaction was completed, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid substance. Finally, the solid substance was dried under vacuum at 40℃ to obtain the lignin quaternary ammonium salt.

[0032] (3) Lignin-based ionic liquid: 2g of lignin quaternary ammonium salt, 8g of sodium fluoroborate and 150g of deionized water were added to a 500mL three-necked flask and mechanically stirred. After 24h, the mixture was repeatedly washed with deionized water and filtered, and then dried under vacuum at 40℃ to obtain the lignin-based ionic liquid.

[0033] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were mixed according to the mass ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 and added to a mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The mixed sample was then placed in a flat vulcanizing machine and pressed into shape at 190℃ to make standard specimens. The critical oxygen index was determined.

[0034] Comparative Example 1-1

[0035] Preparation of flame-retardant PLA: PLA and the degraded lignin from Example 1 were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / degraded lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was determined.

[0036] Comparative Examples 1-2

[0037] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 1 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0038] Example 2

[0039] (1) Lignin amine: 3g of lignin sulfonate degraded by ozone oxidation, 7mL of water, and 4mL of 0.5mol / L NaOH solution were added to a 100mL three-necked flask. The mixture was stirred at 60℃ to fully dissolve the lignin. Then, 4mL of formaldehyde and 4mL of ethylenediamine were added, and the mixture was refluxed and stirred for 3h to synthesize lignin amine. Sufficient 7% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0040] (2) Lignin quaternary ammonium salt: 1.5 g of lignin amine and 2.5 g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250 mL three-necked flask, followed by 100 mL of water / isopropanol (mass ratio) = 2:3 as a dispersant. The mixture was refluxed at 75 °C and stirred for 7 h. After the reaction was complete, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid substance. Finally, the solid substance was dried under vacuum at 40 °C to obtain the lignin quaternary ammonium salt.

[0041] (3) Lignin-based ionic liquid: Add 3g of lignin quaternary ammonium salt, 6g of sodium fluoroborate and 200g of deionized water to a 500mL three-necked flask and stir mechanically. After 24h, wash repeatedly with deionized water and filter, then dry under vacuum at 40℃ to obtain lignin-based ionic liquid.

[0042] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed in a ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0043] Comparative Example 2-1

[0044] Preparation of flame-retardant PLA: PLA and the degraded lignin from Example 2 were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / degraded lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was determined.

[0045] Comparative Example 2-2

[0046] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 2 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0047] Example 3

[0048] (1) Lignin amine: 4g of acid-hydrolyzed biobutanol lignin, 8mL of water, and 8mL of 0.5mol / L NaOH solution were added to a 100mL three-necked flask. The mixture was stirred at 70℃ to fully dissolve the lignin. Then, 4.5mL of formaldehyde and 5mL of triethylenetetramine were added, and the mixture was refluxed and stirred for 3h to synthesize lignin amine. Sufficient 8% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0049] (2) Lignin quaternary ammonium salt: 2g of lignin amine and 4g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250mL three-necked flask, followed by 100mL of water / isopropanol (mass ratio) = 2:5 as a dispersant. The mixture was refluxed at 70℃ and stirred for 8h. After the reaction was complete, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid. Finally, the solid was dried under vacuum at 40℃ to obtain the lignin quaternary ammonium salt.

[0050] (3) Lignin-based ionic liquid: 3g of lignin quaternary ammonium salt, 10g of sodium trifluoromethanesulfonate and 2000g of deionized water were added to a 500mL three-necked flask and mechanically stirred. After 24h, the mixture was repeatedly washed with deionized water and filtered, and then vacuum dried at 40℃ to obtain the lignin-based ionic liquid.

[0051] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were mixed according to the mass ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 and added to a Banbury mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The mixed sample was then placed in a flat vulcanizing machine and pressed into shape at 190℃ to make standard specimens. The critical oxygen index was determined.

[0052] Comparative Example 3-1

[0053] Preparation of flame-retardant PLA: PLA and the degraded lignin from Example 3 were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / degraded lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0054] Comparative Example 3-2

[0055] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 3 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0056] Example 4

[0057] (1) Lignin amine: 2.5 g of alkaline hydrolyzed bioethanol lignin, 10 mL of water, and 5 mL of 0.5 mol / L NaOH solution were added to a 100 mL three-necked flask. The mixture was stirred at 75 °C to fully dissolve the lignin. Then, 5 mL of formaldehyde and 7 mL of tetraethylenepentamine were added, and the mixture was refluxed and stirred for 4 h to synthesize lignin amine. Sufficient 10% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0058] (2) Lignin quaternary ammonium salt: 1 g of lignin amine and 2 g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250 mL three-necked flask, followed by 100 mL of water / isopropanol (mass ratio) = 2:6 as a dispersant. The mixture was refluxed at 90 °C and stirred for 4 h. After the reaction was complete, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid. Finally, the solid was dried under vacuum at 40 °C to obtain the lignin quaternary ammonium salt.

[0059] (3) Lignin-based ionic liquid: 5g of lignin quaternary ammonium salt, 9g of sodium trifluoromethanesulfonate and 220g of deionized water were added to a 500mL three-necked flask and mechanically stirred. After 24h, the mixture was repeatedly washed with deionized water and filtered, and then dried under vacuum at 40℃ to obtain the lignin-based ionic liquid.

[0060] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were mixed according to the mass ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 and added to a Banbury mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The mixed sample was then placed in a flat vulcanizing machine and pressed into shape at 190℃ to make standard specimens. The critical oxygen index was determined.

[0061] Comparative Example 4-1

[0062] Preparation of flame-retardant PLA: PLA and the degraded lignin from Example 4 were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / degraded lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0063] Comparative Example 4-2

[0064] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 4 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0065] Example 5

[0066] (1) Lignin amine: 3.2 g of hydrogenated and reduced alkali lignin, 9 mL of water, and 6 mL of 0.5 mol / L NaOH solution were added to a 100 mL three-necked flask. The mixture was stirred at 40 °C to fully dissolve the lignin. Then, 6 mL of formaldehyde and 8 mL of 1,3-propanediamine were added, and the mixture was refluxed and stirred for 4 h to synthesize lignin amine. Sufficient 12% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0067] (2) Lignin quaternary ammonium salt: 1 g of lignin amine and 3 g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250 mL three-necked flask, followed by 100 mL of water / isopropanol (mass ratio) = 1:2 as a dispersant. The mixture was refluxed at 60 °C and stirred for 8 h. After the reaction was complete, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid. Finally, the solid was dried under vacuum at 40 °C to obtain the lignin quaternary ammonium salt.

[0068] (3) Lignin-based ionic liquid: 4g of lignin quaternary ammonium salt, 6g of sodium fluoroborate and 300g of deionized water were added to a 500mL three-necked flask and mechanically stirred. After 24h, the mixture was repeatedly washed with deionized water and filtered, and then vacuum dried at 40℃ to obtain the lignin-based ionic liquid.

[0069] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed in a ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0070] Comparative Example 5-1

[0071] Preparation of flame-retardant PLA: PLA and the degraded lignin from Example 5 were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / degraded lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was determined.

[0072] Comparative Example 5-2

[0073] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 5 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0074] Example 6

[0075] (1) Lignin amine: 3.8 g of enzymatically hydrolyzed lignin obtained by high-temperature thermal pyrolysis, 15 mL of water, and 6 mL of 0.5 mol / L NaOH solution were added to a 100 mL three-necked flask. The mixture was stirred at 80 °C to fully dissolve the lignin. Then, 3 mL of formaldehyde and 5 mL of ethylenediamine were added, and the mixture was refluxed and stirred for 4 h to synthesize lignin amine. Sufficient 15% ferrous sulfate (K3Fe8(CN)6) was added to the lignin amine solution to completely precipitate the lignin amine. The precipitate was then filtered and dried.

[0076] (2) Lignin quaternary ammonium salt: 0.5 g of lignin amine and 2 g of 2,3-epoxypropyltrimethylammonium chloride were added to a 250 mL three-necked flask, followed by 100 mL of water / isopropanol (mass ratio) = 1:1 as a dispersant. The mixture was refluxed at 70 °C and stirred for 10 h. After the reaction was completed, the reaction mixture was washed multiple times with ethanol and filtered to obtain a solid substance. Finally, the solid substance was dried under vacuum at 40 °C to obtain the lignin quaternary ammonium salt.

[0077] (3) Lignin-based ionic liquid: 2g of lignin quaternary ammonium salt, 8g of sodium fluoroborate and 150g of deionized water were added to a 500mL three-necked flask and mechanically stirred. After 24h, the mixture was repeatedly washed with deionized water and filtered, and then dried under vacuum at 40℃ to obtain the lignin-based ionic liquid.

[0078] (4) Preparation of flame-retardant PLA: PLA, APP, and lignin-based ionic liquid were all dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed in a ratio of PLA / APP / lignin-based ionic liquid = 95 / 2.5 / 2.5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃ and the rotation speed was 50 r / min. The mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0079] Comparative Example 6-1

[0080] Preparation of flame-retardant PLA: PLA and lignin were dried in a vacuum drying oven at 80℃ for 12 hours before use. They were then mixed at a PLA / lignin ratio of 95 / 5 (mass ratio) and added to an internal mixer. The temperature was controlled at 180-190℃, the rotation speed at 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190℃. The critical oxygen index was then determined.

[0081] Comparative Example 6-2

[0082] Preparation of flame-retardant PLA: PLA and the lignin-based ionic liquid from Example 6 were dried in a vacuum drying oven at 80°C for 12 hours before use. They were then mixed at a mass ratio of PLA / lignin-based ionic liquid = 95 / 5 and added to an internal mixer. The temperature was controlled at 180-190°C, the rotation speed was 50 r / min, and the mixture was melt-blended for 10 minutes. The resulting sample was then placed in a flat vulcanizing machine and pressed into standard specimens at 190°C. The critical oxygen index was then determined.

[0083] Example 7

[0084] Preparation of ordinary phenolic foam: 300g commercially available foaming phenolic resin (Hangmo Technology New Materials (Fuyang) Co., Ltd.), 4.5g phosphate binder...

[0085] Mix 4.5g of DC-193 and 24g of n-pentane in a plastic cup at high speed until homogeneous. Then add 75g of curing agent (phosphoric acid, p-toluenesulfonic acid and water in a mass ratio of 30:95:20) and mix at high speed. Quickly pour the mixture into a preheated stainless steel mold and bake in an oven at 70-75℃ for about 15-30 minutes. After curing, cool to room temperature and cut the sample to test the oxygen index, compressive strength and thermal conductivity.

[0086] Comparative Example 7-1

[0087] Preparation of lignin-based flame-retardant phenolic foam: 285g of commercially available foaming phenolic resin (Hangmo Technology New Materials (Fuyang) Co., Ltd.), 4.5g of Tween-80 and 4.5g of DC-193, 24g of n-pentane, and 15g of lignin degraded in Example 5 (5% percentage) were mixed evenly at high speed in a plastic cup. Then, 75g of curing agent (phosphoric acid, p-toluenesulfonic acid, and water in a mass ratio of 30:95:20) was added and mixed at high speed. The mixture was then quickly poured into a preheated stainless steel mold and foamed in an oven at 70-75℃ for about 15-30 minutes. After curing, the mixture was cooled to room temperature, and the samples were cut to test the oxygen index, compressive strength, and thermal conductivity.

[0088] Comparative Example 7-2

[0089] Preparation of flame-retardant phenolic foam using lignin ionic liquid: 285g of commercially available foaming phenolic resin (Hangmo Technology New Materials (Fuyang) Co., Ltd.), 4.5g of Tween-80 and 4.5g of DC-193, 24g of n-pentane, and 15g (5%) of the lignin ionic liquid prepared in Example 5 were stirred at high speed in a plastic cup until homogeneous. Then, 76g of curing agent (phosphoric acid, p-toluenesulfonic acid, and water in a mass ratio of 30:95:20) was added and stirred at high speed to mix. The mixture was then quickly poured into a preheated stainless steel mold and foamed in an oven at 70-75℃ for about 15-30 minutes. After curing, the mixture was cooled to room temperature, and the samples were cut to test the oxygen index, compressive strength, and thermal conductivity.

[0090] Comparative Example 7-3

[0091] Preparation of flame-retardant phenolic foam synergistically formulated with lignin ionic liquid and APP: 285g of commercially available foaming phenolic resin (Hangmo Technology New Materials (Fuyang) Co., Ltd.), 4.5g of Tween-80 and 4.5g of DC-193, 24g of n-pentane, 7.5g of lignin ionic liquid prepared in Example 5, and 7.5g of APP were mixed evenly at high speed in a plastic cup. Then, 78g of curing agent (phosphoric acid, p-toluenesulfonic acid, and water in a mass ratio of 30:95:20) was added and mixed at high speed. The mixture was quickly poured into a preheated stainless steel mold and foamed in an oven at 70-75℃ for about 15-30 minutes. After curing, the mixture was cooled to room temperature, and the samples were cut to test the oxygen index, compressive strength, and thermal conductivity.

[0092] Performance testing

[0093] Oxygen index determination: ASTM D2863-2017

[0094] Table 1. Comparison of Critical Oxygen Index of Flame-Retardant PLA (%)

[0095] Example Oxygen Index Example Oxygen Index Example Oxygen Index Example Oxygen Index 0-1 19.05 2 39.08 3-2 26.15 5-1 24.12 0-2 21.47 2-1 24.15 4 40.72 5-2 25.60 1 38.45 2-2 26.31 4-1 22.83 6 43.12 1-1 23.42 3 41.56 4-2 25.70 6-1 23.78 1-2 25.63 3-1 23.71 5 46.22 6-2 26.04

[0096] As can be seen from the table, the critical oxygen index of flame-retardant PLA is consistent with the heat resistance of flame-retardant raw materials.

[0097] Table 2 Comparison of Main Performance Ratios of Flame-Retardant Phenolic Foam

[0098] Example Oxygen Index / % Thermal conductivity / W / m·K Compressive strength / MPa 7 37.26 0.023 0.10 7-1 40.13 0.022 0.11 7-2 44.45 0.021 0.15 7-3 58.37 0.022 0.21

[0099] As can be seen from the table, lignin-based ionic liquids can improve the flame retardancy of foam. However, when compounded with APP, they not only significantly improve the flame retardancy of foam, but also improve the mechanical properties of foam without sacrificing the thermal insulation properties of foam.

Claims

1. The application of a lignin-based ionic liquid in the preparation of flame-retardant plastics, characterized in that, The flame-retardant plastic contains ammonium polyphosphate, and the lignin-based ionic liquid is prepared by the following methods: (1) Preparation of lignin amine: lignin raw material is synthesized into lignin amine by Mannich reaction, and red blood salt solution is added to the lignin amine solution to make the lignin amine completely precipitate, and then it is dried after filtration and washing with water; the lignin raw material is alkali lignin, lignin sulfonate, biobutanol lignin, bioethanol lignin or degraded lignin; (2) Preparation of lignin quaternary ammonium salt: a mixture of isopropanol and water is used as a dispersant, lignin amine and 2,3-epoxypropyltrimethylammonium chloride are refluxed and reacted. After the reaction is completed, the reaction mixture is washed and filtered with ethanol multiple times to obtain a solid substance, and then dried under vacuum to obtain lignin quaternary ammonium salt; (3) Preparation of lignin-based ionic liquid: lignin quaternary ammonium salt is mechanically stirred with sodium trifluoromethanesulfonate or sodium fluoroborate in deionized water. After the ion exchange reaction is completed, the mixture is repeatedly washed and filtered with deionized water, and then dried under vacuum to obtain lignin-based ionic liquid.

2. The application according to claim 1, characterized in that, The degradation methods used to degrade lignin include hydrogen peroxide oxidation degradation, ozone oxidation degradation, acid hydrolysis, alkaline hydrolysis, hydrogenation reduction degradation, and high-temperature thermal decomposition; the amine used in the Mannich reaction is a straight-chain diamine, which is ethylenediamine or 1,3-propanediamine.

3. The application according to claim 2, characterized in that, The preparation method of the lignin amine is as follows: 1-5g of lignin is added to 5-15mL of water and 2-8mL of 0.5mol / L NaOH solution according to the proportion; the mixture is stirred at 40-80℃ to fully dissolve the lignin; then 2-6mL of formaldehyde and 1-8mL of diamine are added, and the mixture is refluxed and stirred for 2-4h to synthesize lignin amine; after the reaction is completed, a sufficient amount of 5-15% ferrous sulfate solution is added to the lignin amine solution to completely precipitate the lignin amine, which is then filtered, washed with water, and dried.

4. The application according to claim 3, characterized in that, The preparation method of the lignin amine is as follows: 3 g of lignin is added to 8 mL of water and 5 mL of 0.5 mol / L NaOH solution according to the ratio; the mixture is stirred at 75℃ to fully dissolve the lignin; then 3 mL of formaldehyde and 4 mL of diamine are added, and the mixture is refluxed and stirred for 3 h to synthesize lignin amine; after the reaction is completed, a sufficient amount of 10% ferrous sulfate solution is added to the lignin amine solution to completely precipitate the lignin amine, which is then filtered, washed with water, and dried.

5. The application according to claim 1, characterized in that, The preparation method of the lignin quaternary ammonium salt is as follows: 1g of lignin amine, 3g of 2,3-epoxypropyltrimethylammonium chloride, and 100 mL of water-isopropanol with a water / isopropanol mass ratio of 2 / 5 are added as a dispersant. The mixture is stirred at 80℃ for 10h. After the reaction is completed, the reaction mixture is washed with ethanol multiple times and filtered to obtain a solid substance. Finally, the solid substance is dried under vacuum at 40℃ to obtain the lignin quaternary ammonium salt.

6. The application according to claim 1, characterized in that, Preparation of the lignin-based ionic liquid: 1-5g of lignin quaternary ammonium salt, 2-10g of sodium trifluoromethanesulfonate or sodium fluoroborate and 100-300g of deionized water are mixed in proportion and mechanically stirred; after 24 h, the mixture is repeatedly washed with deionized water and filtered, and then vacuum dried at 40 °C to obtain the lignin-based ionic liquid.

7. The application according to claim 6, characterized in that, Preparation of the lignin-based ionic liquid: 2g of lignin quaternary ammonium salt, 6g of sodium trifluoromethanesulfonate or sodium fluoroborate and 200g of deionized water were mixed in proportion and mechanically stirred; after 24 h, the mixture was repeatedly washed and filtered with deionized water and vacuum dried at 40 °C to obtain the lignin-based ionic liquid.

Images