A bio-based lactic acid crotonate flame retardant plasticizer and its preparation method and application

Abstract

The present invention belongs to the field of polymer additives, and specifically relates to a bio-based lactic acid crotonate flame retardant plasticizer and its preparation method and application. The method of the present invention uses L-lactic acid, crotonic acid, diethylene glycol monobutyl ether, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide as raw materials, and through two-step esterification reaction and ring-opening reaction, a bio-based lactic acid crotonate flame retardant plasticizer is obtained. The bio-based lactic acid crotonate flame retardant plasticizer prepared by the present invention is characterized by being light yellow transparent in color, having moderate viscosity, good compatibility with polylactic acid, and compared with commercial plasticizers and tributyl acetylcitrate (ATBC), it has more excellent migration resistance, volatility resistance, transparency, plasticization performance, and at the same time can endow the polylactic acid material with excellent flame retardant performance. In addition, it is suitable for industrial production and is expected to replace traditional phthalate plasticizers.

Description

Technical Field

[0001] The present invention belongs to the field of bio-based flame retardant plasticizers, and particularly relates to a bio-based lactic acid crotonate flame retardant plasticizer, a preparation method thereof, and an application thereof. Background Art

[0002] As one of the three major important biodegradable materials developed globally, polylactic acid (PLA) is a commercial polymer synthesized from renewable resources and is a potential biodegradable alternative to polystyrene and polypropylene. PLA has good mechanical strength, high transparency, and excellent barrier properties. However, its high brittleness, slow crystallization rate, and flammability have become bottlenecks in expanding its application fields. By methods such as copolymerization, grafting, blending, and plasticization, PLA with excellent comprehensive properties can be obtained, and related research has attracted much attention. Among them, adding plasticizers is an effective solution to improve the comprehensive properties of PLA. However, existing plasticizers have disadvantages such as flammability, low plasticization efficiency, and easy migration. In addition, with the shortage of petroleum resources and the increasing awareness of environmental protection, people's requirements for the environmental protection performance and functionality of plasticizers are also continuously increasing. In response to the above problems, in recent years, domestic and foreign researchers have begun to develop bio-based flame retardant plasticizers with flame retardant and plasticization functions.

[0003] Patents CN105541912A and CN113372550A prepared flame retardant plasticizers for polyvinyl chloride resin. Although the prepared flame retardant plasticizers have good flame retardant effects, harmful substances such as dioxins will be released during combustion and heating, seriously threatening human health. At the same time, the raw materials used are all non-renewable resources, which does not conform to the concept of green environmental protection. Patents CN115677547A and CN112480565A prepared tung oil-based and castor oil-based flame retardant plasticizers for polyvinyl chloride and their preparation methods. The flexibility and flame retardant properties of the modified polyvinyl chloride resin have been improved to a certain extent. However, vegetable oils have poor compatibility with polylactic acid and cannot effectively plasticize polylactic acid. Therefore, for polylactic acid resin, it is urgent to prepare a bio-based flame retardant plasticizer with high flame retardancy and plasticization efficiency from renewable resources. Summary of the Invention

[0004] The object of the present invention is to provide a bio-based lactic acid crotonate flame retardant plasticizer, a preparation method thereof, and an application thereof for the modification requirements of polylactic acid materials with specific transparency, flame retardancy, and mechanical property requirements. The plasticizer provided by the present invention can overcome the deficiencies of traditional plasticizers, has excellent plasticization and flame retardant properties, and can simultaneously meet the transparency and mechanical property requirements of polylactic acid materials.

[0005] The technical solution of the present invention is a preparation method of a bio-based lactic acid crotonate flame retardant plasticizer. Using L-lactic acid, crotonic acid, diethylene glycol monobutyl ether and DOPO as main raw materials, a bio-based lactic acid crotonate flame retardant plasticizer is obtained through two-step esterification reactions and an addition reaction.

[0006] The specific steps are as follows:

[0007] (1) The first-step esterification reaction, the preparation step of lactic acid crotonate: Add L-lactic acid, crotonic acid and p-toluenesulfonic acid monohydrate into a reactor, and heat for reaction; after the reaction is completed, purify to obtain lactic acid crotonate; the molar ratio of L-lactic acid to crotonic acid is 1-3:1; the dosage of p-toluenesulfonic acid monohydrate is 0.6%-1.0% (preferably 0.7-0.9%) of the total mass of L-lactic acid and crotonic acid, the reaction time is at least 3 h (preferably 3-5 hours), and the reaction temperature is 110-140 °C (preferably 120-140 °C). This method can ensure the high-efficiency acquisition of lactic acid crotonate on the one hand, and on the other hand, has better plasticizing performance and good stability in polylactic acid products.

[0008] Further, the purification method includes extracting the crude product with ethyl acetate, then washing with deionized water until neutral, and removing ethyl acetate and residual water by rotary evaporation. (2) The second-step esterification reaction, the preparation step of diethylene glycol monobutyl ether-lactic acid crotonate: Add the lactic acid crotonate, diethylene glycol monobutyl ether and p-toluenesulfonic acid monohydrate obtained in step (1) into a reactor, and heat for reaction; after the reaction is completed, purify to obtain diethylene glycol monobutyl ether-lactic acid crotonate.

[0009] Further, the purification method includes extracting the crude product with ethyl acetate, then washing with saturated sodium bicarbonate solution and deionized water until neutral, and removing ethyl acetate and residual water by rotary evaporation.

[0010] Further, in order to ensure a higher yield, in the second-step esterification reaction of step (2), the molar ratio of lactic acid crotonate to diethylene glycol monobutyl ether is 1:1-2 (more preferably 1:1-1.4).

[0011] Further, in order to ensure a higher yield, in the second-step esterification reaction of step (2), p-toluenesulfonic acid monohydrate is 0.6%-1.0% (more preferably 0.8%) of the total mass of lactic acid crotonate and diethylene glycol monobutyl ether.

[0012] Further, to ensure a higher yield, in the second esterification reaction of step (2), heat to 110 - 140 °C and react for 3 - 5 h. (3) The third addition reaction: Add DOPO and the diethylene glycol monobutyl ether-crotonic acid lactate obtained in step (2) to a reactor, heat and react, and purify after the reaction (specifically, the purification method includes: extracting the crude product with ethyl acetate, washing with deionized water until neutral, and removing ethyl acetate and residual water by rotary evaporation) to obtain a bio-based lactic acid crotonate flame retardant plasticizer;

[0013] Further, in the third addition reaction of step (3), the molar ratio of DOPO to diethylene glycol monobutyl ether-crotonic acid lactate is 0.1 - 1.0:1 (to better balance transparency and mechanical properties, more preferably, the molar ratio of DOPO to diethylene glycol monobutyl ether-crotonic acid lactate is 0.6:1).

[0014] Further, in the third addition reaction of step (3), heat to 140 - 180 °C and react for 4 - 6 h.

[0015] The present invention also provides a bio-based lactic acid crotonate flame retardant plasticizer prepared based on the above method.

[0016] Finally, the present invention provides the application of the bio-based lactic acid crotonate flame retardant plasticizer prepared by the above method in polylactic acid materials, aiming to obtain a polylactic acid material that meets the requirements of transparency, flame retardancy and mechanical properties. The mass ratio of the polylactic acid material to the bio-based lactic acid crotonate flame retardant plasticizer is 1:5.

[0017] Further, it includes the following steps: thermally plasticize and blend the bio-based lactic acid crotonate flame retardant plasticizer with polylactic acid, with a thermoplastic temperature of 150 - 180 °C, a rotation speed of 50 - 100 rpm, and a processing time of 6 min.

[0018] The dosage of the catalyst in the present invention is not strictly limited and can be adjusted according to the dosage of the reaction raw materials: the dosage of the catalyst is based on being able to catalyze the reaction and promote the reaction to proceed in the direction of the target product.

[0019] The bio-based lactic acid crotonate flame retardant plasticizer prepared by the present invention belongs to a bio-based environmental protection flame retardant plasticizer, has good compatibility with PLA, and at the same time has good plasticizing and flame retardant effects, and can be used as the main plasticizer for polylactic acid.

[0020] The beneficial effects of the present invention are:

[0021] The present invention uses renewable resources L-lactic acid and crotonic acid as the main raw materials, which not only avoids the dependence on petrochemical raw materials but also increases the use of bio-based raw materials. The bio-based lactic acid crotonate flame retardant plasticizer prepared by this method is light yellow and transparent in color, has low viscosity, and good compatibility with PLA. Compared with the commercial plasticizer ATBC applied in PLA, it has more excellent plasticization efficiency, migration resistance, and volatility resistance, and at the same time endows PLA with excellent flame retardant properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Infrared spectra of raw materials and products of Examples 1 to 5 of the present invention: 1. Lactic acid; 2. Lactic acid crotonate; 3. Diethylene glycol monobutyl ether-crotonic acid lactic acid ester; 4. Example 1; 5. Example 2; 6. Example 3; 7. Example 4; 8. Example 5.

[0023] Figure 2 Thermogravimetric analysis TGA curve of the product of the example of the present invention: 1. Diethylene glycol monobutyl ether-crotonic acid lactic acid ester; 2. Example 1; 3. Example 2; 4. Example 3; 5. Example 4; 6. Example 5.

[0024] Figure 3 Tensile stress-strain curve of polylactic acid products; 1. Sample 1; 2. Sample 2; 3. Sample 3; 4. Sample 4; 5. Sample 5; 6. Sample 6; 7. Sample 7; 8. Sample 8.

[0025] Figure 4 TGA curve of polylactic acid products: 1. Pure polylactic acid; 2. Comparative example 1; 3. Comparative example 2; 4. Sample 1; 5. Sample 2; 6. Sample 3; 7. Sample 4; 8. Sample 5.

[0026] Figure 5 Optical property diagram of polylactic acid products: 1. Pure polylactic acid; 2. Comparative example 1; 3. Comparative example 2; 4. Sample 1; 5. Sample 2; 6. Sample 3; 7. Sample 4; 8. Sample 5. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below with reference to specific examples. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several modifications and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0028] In the present invention, the plasticization method of the bio-based lactic acid crotonate flame retardant plasticizer preferably includes thermoplastic blending.

[0029] The synthesis route of the present invention is as follows:

[0030]

[0031] Example 1

[0032] The following is the screening of the reaction conditions of the present invention.

[0033] (1) The first-step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2:1 and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0034] (2) The second-step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2 and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0035] (3) The third-step addition reaction: Add DOPO into the reactor, heat up to 140 °C to completely dissolve it, and then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:0.2. Heat up to 160 °C and react for 5 h to obtain a crude product. After filtration, washing, and rotary evaporation under reduced pressure, obtain a bio-based lactic acid crotonate flame retardant plasticizer.

[0036] Example 2

[0037] (1) The first-step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2:1 and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0038] (2) The second-step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2 and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0039] (3) Third step addition reaction: Add DOPO into the reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:0.4, heat up to 160 °C, react for 5 h to obtain the crude product, and after filtration, washing, and rotary evaporation under reduced pressure, a bio-based lactic acid crotonate flame retardant plasticizer is obtained.

[0040] Example 3

[0041] (1) First step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2:1 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask, with a reaction temperature of 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, through rotary evaporation under reduced pressure, washing, and filtration, crotonic acid lactate is finally obtained.

[0042] (2) Second step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask, with a reaction temperature of 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, through rotary evaporation under reduced pressure, washing, and filtration, diethylene glycol monobutyl ether-crotonic acid lactate is finally obtained.

[0043] (3) Third step addition reaction: Add DOPO into the reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:0.6, heat up to 160 °C, react for 5 h to obtain the crude product, and after filtration, washing, and rotary evaporation under reduced pressure, a bio-based lactic acid crotonate flame retardant plasticizer is obtained.

[0044] Example 4

[0045] (1) First step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2:1 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask, with a reaction temperature of 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, through rotary evaporation under reduced pressure, washing, and filtration, crotonic acid lactate is finally obtained.

[0046] (2) Second step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask, with a reaction temperature of 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, through rotary evaporation under reduced pressure, washing, and filtration, diethylene glycol monobutyl ether-crotonic acid lactate is finally obtained.

[0047] (3) Third step addition reaction: Add DOPO into the reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:0.8. Heat up to 160 °C and react for 5 h to obtain the crude product. After filtration, washing, and rotary evaporation under reduced pressure, a bio-based lactic acid crotonate flame retardant plasticizer is obtained.

[0048] Example 5

[0049] (1) First step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2:1 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, obtain lactic acid crotonate through rotary evaporation under reduced pressure, washing, and filtration.

[0050] (2) Second step esterification reaction: Add lactic acid crotonate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0051] (3) Third step addition reaction: Add DOPO into the reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:1. Heat up to 160 °C and react for 5 h to obtain the crude product. After filtration, washing, and rotary evaporation under reduced pressure, a bio-based lactic acid crotonate flame retardant plasticizer is obtained.

[0052] Example 6

[0053] (1) First step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 1.5:1 and 0.8% p-toluenesulfonic acid monohydrate into a round-bottomed three-necked flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottomed flask to collect the water generated during the reaction. After the reaction is completed, obtain lactic acid crotonate through rotary evaporation under reduced pressure, washing, and filtration.

[0054] (2) Second-step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2, and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0055] (3) Third-step addition reaction: Add DOPO into a reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:1. Heat up to 160 °C and react for 5 h to obtain a crude product. After filtration, washing, and rotary evaporation under reduced pressure, obtain a bio-based lactic acid crotonate flame retardant plasticizer.

[0056] Example 7

[0057] (1) First-step esterification reaction: Add L-lactic acid and crotonic acid with a molar ratio of 2.5:1, and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0058] (2) Second-step esterification reaction: Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1:1.2, and 0.8% p-toluenesulfonic acid monohydrate into a three-necked round-bottom flask. The reaction temperature is 130 °C, stir and reflux for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0059] (3) Third-step addition reaction: Add DOPO into a reactor, heat up to 140 °C to completely dissolve it, then add diethylene glycol monobutyl ether-crotonic acid lactate, where the molar ratio of diethylene glycol monobutyl ether-crotonic acid lactate to DOPO is 1:1. Heat up to 160 °C and react for 5 h to obtain a crude product. After filtration, washing, and rotary evaporation under reduced pressure, obtain a bio-based lactic acid crotonate flame retardant plasticizer.

[0060] In industrial production, the acid value of plasticizers has an important impact on the application performance and long-term stability of plasticizers in polymers. The larger the acid value, the more acidic components remain in the plasticizer, which easily causes resin decomposition. Therefore, the acid value of the reaction system is closely related to stability. Therefore, taking the acid value and yield of the plasticizer product as important indicators of the synthesis process, the control variable method was used to explore the optimal conditions for product synthesis through factors such as reaction time, reaction temperature, catalyst dosage, and monomer molar ratio. Therefore, based on Example 5, the reaction conditions were screened, and the results are as follows:

[0061] S1 Reaction condition screening:

[0062] Add L-lactic acid and crotonic acid with a molar ratio of 1 - 3:1 into a three-necked round-bottom flask, then add p-toluenesulfonic acid monohydrate accounting for 0.1% - 1% of the total mass of the reactants, heat up to 110 - 140 °C, stir the reaction and reflux it for 2 - 6 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain lactic acid crotonate through rotary evaporation under reduced pressure, washing, and filtration.

[0063] Table 1 S1 Condition screening

[0064] S1 Type Monomer molar ratio Catalyst dosage % Reaction conditions Acid value Yield % 1 L-Lactic acid: Crotonic acid 2:1 0.8 130℃，3h 78.11 89.37 2 L-Lactic acid: Crotonic acid 2:1 0.8 130℃，4h 70.12 93.50 3 L-Lactic acid: Crotonic acid 2:1 0.8 130℃，5h 70.13 93.50 4 L-Lactic acid: Crotonic acid 2:1 0.8 120℃，4h 75.76 90.63 5 L-Lactic acid: Crotonic acid 2:1 0.8 140℃，4h 70.40 93.52 6 L-Lactic acid: Crotonic acid 2:1 0.7 130℃，4h 74.51 91.30 7 L-Lactic acid: Crotonic acid 2:1 0.9 130℃，4h 70.36 93.51 8 L-Lactic acid: Crotonic acid 1:1 0.8 130℃，4h 83.10 81.60 9 L-Lactic acid: Crotonic acid 1.5:1 0.8 130℃，4h 71.36 90.21 10 L-Lactic acid: Crotonic acid 2.5:1 0.8 130℃，4h 71.94 92.30 11 L-Lactic acid: Crotonic acid 3:1 0.8 130℃，4h 73.31 91.20 12 L-Lactic acid: Fumaric acid 2:1 0.8 130℃，4h 89.80 86.30 13 L-Lactic acid: Maleic acid 2:1 0.8 130℃，4h 92.10 82.10 14 L-Lactic acid: Acrylic acid 2:1 0.8 130℃，4h / /

[0065] Conclusion: In the S1 reaction step, through monomer screening, changing the molar ratio of L-lactic acid and crotonic acid, catalyst dosage, reaction time, reaction temperature, and monomer selection, too high reaction temperature and too long reaction time will lead to side reactions, resulting in insignificant changes in yield. At the same time, too high reaction temperature and too long reaction time will also lead to an increase in production costs; while too low reaction temperature and too short reaction time will lead to incomplete reactions, reduced yield, and increased acid value; when the catalyst dosage is 0.8%, the yield and acid value of the product reach the best. When the catalyst dosage is 0.9%, it cannot effectively improve the product yield, and too high a catalyst will also lead to an increase in cost; at the same time, monomer type selection was carried out, and it was found that acrylic acid will self-polymerize at high temperatures, and the products prepared from maleic acid and fumaric acid have too high viscosity and too large acid value. Therefore, acrylic acid, maleic acid, and fumaric acid are not suitable as choices for plasticizers. Therefore, based on the above results, the following optimal reaction conditions can be preferably selected: the molar ratio of L-lactic acid:crotonic acid is 2:1; the catalyst dosage is 0.8%; the reaction temperature is 130 °C; the reaction time is 4 h.

[0066] S2 Reaction condition screening

[0067] Add crotonic acid lactate and diethylene glycol monobutyl ether with a molar ratio of 1-1.4:1 to a round-bottom three-necked flask, then add p-toluenesulfonic acid monohydrate accounting for 0.8% of the total mass of the reactants, heat up to 130 °C, stir the reaction and reflux it for 4 h. Connect a water separator to the round-bottom flask to collect the water generated during the reaction. After the reaction is completed, obtain diethylene glycol monobutyl ether-crotonic acid lactate through rotary evaporation under reduced pressure, washing, and filtration.

[0068] Table 2 Screening of S2 conditions

[0069] S2 S1 product: Diethylene glycol monobutyl ether molar ratio Catalyst dosage % Reaction conditions Acid value Yield % 1 1:1 0.8％ 130℃，4h 9.78 83.62 2 1:1.1 0.8％ 130℃，4h 5.06 90.13 3 1:1.2 0.8％ 130℃，4h 1.32 95.30 4 1:1.3 0.8％ 130℃，4h 1.31 95.32 5 1:1.4 0.8％ 130℃，4h 1.27 95.36

[0070] Conclusion: In the S2 reaction step, based on the S1 esterification reaction conditions, considering the influence of the ratio of S1 product: diethylene glycol monobutyl ether on the product yield, when the ratio of S1 product: diethylene glycol monobutyl ether is 1:1.2, the product yield reaches the maximum and the acid value is the lowest. When the ratio increases, it cannot effectively improve the product yield and acid value. At the same time, adding too much diethylene glycol monobutyl ether will make it difficult to purify the product. Therefore, the optimal reaction conditions are: S1 product: diethylene glycol monobutyl ether is 1:1.2, the dosage of the catalyst is 0.8%; the reaction temperature is 130 °C; the reaction time is 4 h.

[0071] S3 reaction condition screening

[0072] Add DOPO to the reactor, heat up to 140 °C to completely dissolve it, then add the S2 product, heat up to 150-180 °C, and the reaction time is 2-6 h to obtain the crude product. After filtration, washing, and rotary evaporation under reduced pressure, obtain the bio-based crotonic acid lactate flame retardant plasticizer.

[0073] Table 3 Screening of S3 conditions

[0074] S3 S2 product: DOPO molar ratio Reaction conditions Acid value Yield % 1 1：1 160℃，4h 1.32 89.22 2 1：1 160℃，5h 1.31 93.01 3 1：1 160℃，6h 1.36 93.13 4 1：1 150℃，5h 1.28 86.25 5 1：1 170℃，5h 1.29 93.19 6 1：0.2 160℃，5h 1.28 92.90 7 1：0.4 160℃，5h 1.33 92.60 8 1：0.6 160℃，5h 1.32 93.31 9 1：0.8 160℃，5h 1.36 93.13

[0075] Conclusion: In the S3 reaction step, screening the molar ratio of raw materials, reaction temperature, and reaction time, the result analysis shows that when the change of the molar ratio of S2 product: DOPO is 1:0.2-1.0, the yield and acid value do not change significantly; too high reaction temperature and too long reaction time will lead to side reactions; too low reaction temperature and too short reaction time will cause incomplete reactions; in summary, the optimal reaction conditions are: reaction temperature is 160 °C, reaction time is 5 h.

[0076] Figure 1 For the infrared spectra of raw material lactic acid, intermediate crotonic acid lactate, intermediate diethylene glycol monobutyl ether-crotonic acid lactate, and final products Examples 1-5. Observe all the curves. At 1740 cm -1 is the stretching vibration peak of the ester carbonyl C=O; compared with the curve of lactic acid, in the curve of crotonic acid lactate, at 1690 cm-1 Since the absorption peak of C═C appeared, it proved that the esterification reaction of crotonic acid and lactic acid was successfully synthesized; on the curve of diethylene glycol monobutyl ether-crotonic acid lactate, at 1178 and 853 cm -1 the characteristic absorption peak of the fatty ether bond appeared, and at the same time, at 1740 cm -1 the intensity of the ester group peak at this position became higher, indicating that the esterification reaction occurred between diethylene glycol monobutyl ether and crotonic acid lactate. In the infrared spectra of Examples 1 to 5, it was found that as the dosage of DOPO increased, the characteristic absorption peak of C═C gradually disappeared, and at the same time, at 1249 and 928 cm -1 the characteristic absorption peaks of P═O and P-O-C appeared respectively; between 1667 and 1561 cm -1 four sharp absorption bands appeared, which were the aromatic ring skeletal vibration peaks of DOPO; the above proved that the bio-based lactic acid crotonate flame retardant plasticizer was successfully synthesized.

[0077] Figure 2 Figure 13 is the TGA curve of the final product bio-based lactic acid crotonate flame retardant plasticizer and the intermediate product diethylene glycol monobutyl ether-crotonic acid lactate in Examples 1 to 5. It can be seen from the figure that as the dosage of DOPO increased, the thermal stability of the bio-based lactic acid crotonate flame retardant plasticizer gradually increased, indicating that the bio-based lactic acid crotonate flame retardant plasticizer had good thermal stability.

[0078] Application Example 1

[0079] The application of the above bio-based lactic acid crotonate flame retardant plasticizer for modifying polylactic acid in the preparation of polylactic acid materials. 20 parts of the bio-based lactic acid crotonate flame retardant plasticizer prepared in Examples 1 to 7 were thermally blended with 100 parts of polylactic acid. The thermoplastic temperature was 150 to 180 °C, the rotation speed was 50 to 100 rpm, the processing time was 6 min, and the bio-based lactic acid crotonate flame retardant plasticizer-modified polylactic acid material was obtained after discharging. The composite materials were named Samples 1 to 7. Then, dumbbell-shaped samples for tensile testing, oxygen index testing, and vertical burning strip samples were prepared using an injection molding machine. The tensile testing was carried out with reference to ASTM D638-2003, the oxygen index was carried out according to the ASTM D2863 standard, and the UL-94 vertical burning performance was carried out according to the ASTM D 3801 standard. The test results are shown in Table 1.

[0080] Application Comparative Example 1

[0081] In this comparative example, 100 parts of PLA resin and 20 parts of commercially available plasticizer ATBC were subjected to thermoplastic blending. The thermoplastic temperature was 150 - 180°C, the rotation speed was 50 - 100 rpm, the processing time was 6 min, and the modified polylactic acid material was obtained after discharging. Then, dumbbell-shaped samples for tensile testing, strip samples for oxygen index testing and vertical burning were prepared using an injection molding machine. The tensile testing was carried out with reference to ASTM D638 - 2003, the oxygen index was determined according to the ASTM D2863 standard, and the UL-94 vertical burning performance was determined according to the ASTM D3801 standard. The results are shown in Table 1.

[0082] Comparative Example 2 for Application

[0083] In this comparative example, the intermediate product diethylene glycol monobutyl ether-crotonic acid lactate synthesized in Example 1 was used to modify polylactic acid. 100 parts of PLA resin and 20 parts of diethylene glycol monobutyl ether-crotonic acid lactate were subjected to thermoplastic blending. The thermoplastic temperature was 150 - 180°C, the rotation speed was 50 - 100 rpm, the processing time was 6 min, and the modified polylactic acid material was obtained after discharging. Then, dumbbell-shaped samples for tensile testing, strip samples for oxygen index testing and vertical burning were prepared using an injection molding machine. The tensile testing was carried out with reference to ASTM D638 - 2003, the oxygen index was determined according to the ASTM D2863 standard, and the UL-94 vertical burning performance was determined according to the ASTM D3801 standard. The results are shown in Table 1.

[0084] Table 1 Properties of Modified Polylactic Acid Products

[0085] Name Tensile strength (MPa) Elongation at break (%) Oxygen index (%) UL-94 Pure polylactic acid 50.68 3.7 20 NR Sample 1 15.92 433.72 27 V-0 Sample 2 18.48 394.82 30.5 V-0 Sample 3 22.90 177.63 34.5 V-0 Sample 4 25.79 127.91 37.0 V-0 Sample 5 28.61 72.39 39.5 V-0 Sample 6 29.31 53.20 39.0 V-0 Sample 7 26.61 58.31 39.5 V-0 Control example 1 22.12 188.31 19.5 NR Control example 2 14.37 479.96 19 NR

[0086] As can be seen from Table 1 and Figure 3 It can be seen that the bio-based lactic acid crotonic acid ester flame retardant plasticizer modified polylactic acid products prepared in Examples 1 - 7 of the present invention all have good plasticizing properties and flame retardant properties. The oxygen index is between 27 and 40, and the flame retardant grade is V-0. At the same time, as the DOPO content increases, the elongation at break decreases, while the tensile strength increases. Therefore, the plasticizing efficiency order of the six flame retardant plasticizers prepared in the examples should be: Example 1 > Example 2 > Example 3 > Example 4 > Example 5. Examples 5 - 7 show the influence of different lactic acid and crotonic acid ratios on the mechanical properties of the modified polylactic acid material. When the ratio of lactic acid to crotonic acid is 2:1, the product has the best mechanical properties. Through comprehensive comparison, it can be known that the flame retardant plasticizer prepared in this application has both flame retardant and plasticizing effects. The bio-based lactic acid crotonic acid ester flame retardant plasticizer can be used as a flame retardant plasticizer for polylactic acid to obtain polylactic acid products with excellent mechanical properties and flame retardant properties.

[0087] Compared with the comparative examples, the polylactic acid products modified with the bio-based lactic acid crotonate flame retardant plasticizer provided by the present invention simultaneously possess excellent plasticizing properties and flame retardant properties. Among them, compared with the commercially available plasticizer ATBC in Comparative Example 1, the polylactic acid products modified in Examples 1 to 3 have a high elongation at break and excellent flame retardant properties; the polylactic acid products modified with diethylene glycol monobutyl ether-crotonic acid lactate in Comparative Example 2 have a relatively high elongation at break, reaching 479.96%, but in the flame retardancy test, the oxygen index is only 19 and the UL-94 test grade is NR, indicating that the modified polylactic acid products are extremely flammable. Through comprehensive comparison, it can be seen that diethylene glycol monobutyl ether-crotonic acid lactate is effectively endowed with flame retardant function through the bonding action of DOPO.

[0088] As Figure 4 shown, it can be seen from the thermogravimetric curve that the polylactic acid products modified with the bio-based lactic acid crotonate flame retardant plasticizer have better thermal stability than the polylactic acid products modified with the ATBC plasticizer and the polylactic acid products modified with diethylene glycol monobutyl ether-crotonic acid lactate, indicating that the polylactic acid samples plasticized with the bio-based lactic acid crotonate flame retardant plasticizer exhibit good thermal stability.

[0089] The polylactic acid products prepared from the products of the present invention were subjected to optical property tests, and the specific optical property results are as Figure 5 shown. The transmittance of the polylactic acid samples plasticized with the bio-based lactic acid crotonate flame retardant plasticizer can reach about 90%, which is almost the same as that of the commercially available polylactic acid products plasticized with ATBC, indicating that the polylactic acid samples plasticized with the bio-based lactic acid crotonate flame retardant plasticizer have excellent transparency.

[0090] From the above application examples and comparative examples, it can be seen that the bio-based lactic acid crotonate flame retardant plasticizer provided by the present invention simultaneously possesses excellent plasticizing properties and flame retardant properties. In addition, the bio-based lactic acid crotonate flame retardant plasticizer provided by the present invention does not contain toxic substances, and the plasticizing effect can effectively replace ATBC.

[0091] The above are only several preferred and feasible embodiments of the present invention. Those skilled in the art, according to the spirit of the present invention, any modifications or changes made should reasonably be included within the scope of the technical solution of the present invention.

[0092] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. Preparation method of a bio-based lactic acid crotonate flame retardant plasticizer, characterized in that, it is characterized by: including the following steps: (1) The first esterification reaction, the preparation step of lactic acid crotonate: Add L-lactic acid, crotonic acid and p-toluenesulfonic acid monohydrate to a reactor, and heat for reaction; after the reaction is completed, purify to obtain lactic acid crotonate; the molar ratio of L-lactic acid to crotonic acid is 1-3:1; the dosage of p-toluenesulfonic acid monohydrate is 0.6%-1.0% of the total mass of L-lactic acid and crotonic acid, the reaction time is at least 3 h, and the reaction temperature is 110-140 °C; (2) The second esterification reaction, the preparation step of diethylene glycol monobutyl ether-lactic acid crotonate: Add the lactic acid crotonate, diethylene glycol monobutyl ether and p-toluenesulfonic acid monohydrate obtained in step (1) to a reactor, and heat for reaction; after the reaction is completed, purify to obtain diethylene glycol monobutyl ether-lactic acid crotonate; (3) The third addition reaction: Add DOPO and the diethylene glycol monobutyl ether-lactic acid crotonate obtained in step (3) to a reactor, heat for reaction, and after the reaction is completed, purify to obtain the lactic acid crotonate flame retardant plasticizer; 。 2. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: in step (1), the dosage of p-toluenesulfonic acid monohydrate is 0.7%-0.9% of the total mass of L-lactic acid and crotonic acid; and / or, in step (1), the reaction temperature is 120-140 °C; and / or, in step (1), the reaction time is 3-5 hours.

3. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: the purification method in step (1) includes extracting the crude product with ethyl acetate, then washing with deionized water until neutral, and removing ethyl acetate and residual water by rotary evaporation.

4. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: the purification method in step (2) includes extracting the crude product with ethyl acetate, then washing with saturated sodium bicarbonate solution and deionized water until neutral, and removing ethyl acetate and residual water by rotary evaporation.

5. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: in the second esterification reaction of step (2), the molar ratio of lactic acid crotonate to diethylene glycol monobutyl ether is 1:1-2; and / or, in the second esterification reaction of step (2), p-toluenesulfonic acid monohydrate is 0.6%-1.0% of the total mass of lactic acid crotonate and diethylene glycol monobutyl ether; and / or, in the second esterification reaction of step (2), heat to 110-140 °C and react for 3-5 h.

6. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: in the second esterification reaction of step (2), the molar ratio of lactic acid crotonate to diethylene glycol monobutyl ether is 1:1-1.4; and / or, in the second esterification reaction of step (2), p-toluenesulfonic acid monohydrate is 0.8% of the total mass of lactic acid crotonate and diethylene glycol monobutyl ether.

7. The preparation method of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 1, characterized by: In step (3), the purification method includes extracting the crude product with ethyl acetate, washing it with deionized water until neutral, and removing the ethyl acetate and residual water by rotary evaporation.

8. A bio-based lactic acid crotonate flame retardant plasticizer prepared by the method according to any one of claims 1-7.

9. Use of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 8 in the preparation of a flexible polylactic acid material, characterized in that: The mass ratio of the polylactic acid material to the bio-based lactic acid crotonate flame retardant plasticizer is 1:

5.

10. Use of the bio-based lactic acid crotonate flame retardant plasticizer according to claim 9 in the preparation of a flexible polylactic acid material, characterized in that: It includes the following steps: thermally blending the bio-based lactic acid crotonate flame retardant plasticizer with polylactic acid, with a thermal plasticization temperature of 150-180°C, a rotation speed of 50-100 rpm, and a processing time of 6 min.

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