Preparation method of high-transparency and high-flexibility modified polylactic acid

Abstract

The invention provides a preparation method of high-transparency and high-flexibility modified polylactic acid, and relates to the technical field of organic high-molecular compounds. The preparation method of the high-transparency and high-flexibility modified polylactic acid comprises the following steps: preparing a PLA-co-PVL copolymer solution, purifying a PLA-co-PVL copolymer, synthesizing an ATEC-HEA compound, and preparing a modified polylactic acid composite material. The preparation method of the PLA-co-PVL copolymer solution comprises the following steps: uniformly mixing L-lactide and delta-valerolactone to obtain a mixed solution, adding a catalyst and an initiator into the mixed solution, and reacting to obtain the PLA-co-PVL copolymer solution; the catalyst is Sn (Oct) 2, and the initiator is 1, 4-butanediol; the synthesis of the ATEC-HEA compound comprises the following raw materials: acetyl triethyl citrate, hydroxyethyl acrylate, p-toluenesulfonic acid and hydroquinone; the modified polylactic acid composite material is prepared from the following raw materials: a PLA-co-PVL solid copolymer, an ATEC-HEA compound and an antioxidant. The modified polylactic acid disclosed by the invention has relatively high light transmittance and excellent flexibility.

Description

Technical Field

[0001] The present invention relates to the technical field of organic polymer compounds, and particularly relates to a preparation method of a highly transparent and highly flexible modified polylactic acid. Background Art

[0002] Polylactic acid is a polymer obtained by polymerizing lactic acid as the main raw material. The raw material source is sufficient and can be recycled. The production process of polylactic acid is pollution-free, and the product can be biodegradable, realizing the cycle in nature. Therefore, it is an ideal green polymer material. In the market application of injection molding of polylactic acid (PLA), the more widely used is the modified resin. Pure PLA is hard and brittle, and its light transmittance is not high. The above disadvantages affect its application in injection molding products.

[0003] The existing patent document with the publication number CN117946506A discloses a degradable polylactic acid resin and its application in food packaging. This method significantly improves the flexibility of polylactic acid to a certain extent, while maintaining relatively high tensile strength and heat distortion temperature. However, its deficiency is that it does not mention the regulation of light transmittance and haze. The introduction of modified starch and crystallization nucleating agent will cause the light transmittance to decrease, thus limiting its use in some application scenarios with requirements for light transmittance. Another existing patent document with the publication number CN118562103B discloses a preparation method of a highly transparent polylactic acid for food packaging. By cationization, mercaptanization treatment and graft copolymerization of starch, PLA for highly transparent food packaging is prepared. This method effectively improves the light transmittance, excellent heat resistance and mechanical strength of polylactic acid by using the graft copolymerization of starch and lactic acid. However, the polylactic acid prepared by this method has obvious deficiencies in terms of flexibility, which limits its wide application in application scenarios such as flexible packaging and degradable films that require high deformation ability.

[0004] In summary, although the existing technical solutions have improved certain properties of polylactic acid to a certain extent, there are still the following technical problems: it is impossible to have both good flexibility and light transmittance. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a preparation method of a highly transparent and highly flexible modified polylactic acid, and achieves the following invention objectives: preparing a modified polylactic acid with high light transmittance and excellent flexibility.

[0006] To achieve the above object, the following technical solution is adopted: A preparation method of a highly transparent and highly flexible modified polylactic acid, comprising the steps of preparing a PLA-co-PVL copolymer solution, purifying the PLA-co-PVL copolymer, synthesizing an ATEC-HEA complex, and preparing a modified polylactic acid composite material; Preparation of the PLA-co-PVL copolymer solution: Mix L-lactide and δ-valerolactone evenly to obtain a mixed solution; add a catalyst and an initiator to the mixed solution, and react to obtain a PLA-co-PVL copolymer solution; the catalyst is Sn(Oct)2, and the initiator is 1,4-butanediol.

[0007] Purification of the PLA-co-PVL copolymer: Add the PLA-co-PVL copolymer solution to anhydrous methanol to precipitate a solid, wash the solid with methanol, and dry to obtain a solid PLA-co-PVL copolymer.

[0008] Synthesis of the ATEC-HEA composite: The raw materials used include: triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone. The mass ratio of the raw materials used for synthesizing the ATEC-HEA composite is: the mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone is 100:(38 - 40):(0.7 - 1):(0.3 - 0.5).

[0009] Preparation of the modified polylactic acid composite material: The raw materials used include: solid PLA-co-PVL copolymer, ATEC-HEA composite, and antioxidant. The antioxidant is Irganox1010.

[0010] The mass ratio of the raw materials used for preparing the modified polylactic acid composite material is: the mass ratio of the solid PLA-co-PVL copolymer, ATEC-HEA composite, and antioxidant is (85 - 90):(10 - 15):0.2.

[0011] The molar ratio of L-lactide to δ-valerolactone is (4 - 5):1, the catalyst accounts for 0.05 - 0.1% of the total mass of L-lactide and δ-valerolactone, and the initiator accounts for 0.1 - 0.2% of the total mass of L-lactide and δ-valerolactone.

[0012] Further, for the preparation of the PLA-co-PVL copolymer solution: Mix L-lactide and δ-valerolactone evenly to obtain a mixed solution. Purge the air with nitrogen 3 - 4 times, then add the catalyst and the initiator to the mixed solution and stir evenly; heat up to 130 - 150 °C and react for 2 - 3 h, then heat up to 160 - 180 °C and react for 4 - 8 h; keep nitrogen flowing throughout the reaction, with a flow rate of 50 - 60 mL / min; after the reaction, cool to room temperature to obtain the PLA-co-PVL copolymer solution.

[0013] Further, for the purification of the PLA-co-PVL copolymer: Add the PLA-co-PVL copolymer solution to anhydrous methanol, and the mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol is 1:(8 - 10). Stir at 500 - 800 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol 2 - 3 times, and vacuum dry the precipitate at 50 - 60 °C for 22 - 24 h to obtain the solid PLA-co-PVL copolymer.

[0014] Further, for the synthesis of the ATEC-HEA composite: Stir triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone until completely dissolved, introduce nitrogen to displace air for 5 - 10 min, heat in an oil bath to 120 - 130 °C, and react for 3 - 4 h. After the reaction, cool to room temperature, perform rotary evaporation on the reaction solution, with the rotary evaporation temperature at 60 - 70 °C, the rotary evaporation vacuum degree at 0.08 - 0.09 MPa, and the rotary evaporation time at 7 - 8 h. After rotary evaporation, perform vacuum drying, with the temperature set at 40 - 50 °C, the pressure set at 0.090 - 0.095 MPa, and the drying time at 2 - 2.5 h. After drying, obtain the ATEC-HEA composite.

[0015] Further, for the preparation of the modified polylactic acid composite material: Mix the solid PLA-co-PVL copolymer, the ATEC-HEA composite, and an antioxidant evenly to obtain a mixed material. Then add the mixed material to a twin-screw extruder, and set the extruder parameters: the feeding section temperature is 160 - 165 °C, the melting section temperature is 170 - 175 °C, the mixing section temperature is 175 - 180 °C, the die head temperature is 170 - 175 °C, the screw speed is 80 - 100 rpm, and the feeding rate is 3 - 5 kg / h. After extrusion, cool and pelletize with water to obtain pellets with a diameter of 4 ± 0.5 mm, and then vacuum dry at 50 - 60 °C for 11 - 12 h to obtain the modified polylactic acid composite material.

[0016] The beneficial effects of the present invention are as follows: (1) The high-transparency and high-flexibility modified polylactic acid of the present invention has excellent optical properties. The light transmittance reaches 94.14 - 94.58%, and the haze reaches 3.87 - 4.46%. The relatively high light transmittance and relatively low haze mean that the prepared modified polylactic acid has better optical properties and higher light transmittance uniformity.

[0017] (2) The high-transparency and high-flexibility modified polylactic acid of the present invention has excellent flexibility. The notched impact strength reaches 7.8 - 8.6 kJ / m 2, the notched impact strength is higher, indicating a stronger ability to resist fracture; the elongation at break reaches 135 - 173%, and the higher elongation at break directly reflects better flexibility and ductility; the flexural modulus reaches 2.3 - 2.8 GPa, and the lower flexural modulus means a stronger ability to resist elastic bending deformation. Description of the Drawings

[0018] Appendix Figure 1 is the Fourier transform infrared spectrum of the PLA - co - PVL copolymer in the step of "Preparation of the PLA - co - PVL copolymer solution" in Example 1; Appendix Figure 2 is the Fourier transform infrared spectrum of the ATEC - HEA complex in the step of "Synthesis of the ATEC - HEA complex" in Example 1. Detailed Description of the Embodiments

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will further describe the embodiments of the present invention in detail. It should be understood that the specific embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention.

[0020] Example 1 A Preparation Method of High - Transparency and High - Flexibility Modified Polylactic Acid A preparation method of high - transparency and high - flexibility modified polylactic acid includes the following steps: Step 1. Preparation of the PLA - co - PVL Copolymer Solution Mix L - lactide and δ - valerolactone evenly, with the molar ratio of L - lactide to δ - valerolactone being 4:1, to obtain a mixed solution. Purge the air with nitrogen 3 times, then add the catalyst and initiator to the mixed solution and stir evenly. The catalyst accounts for 0.05% of the total mass of the mixed solution, and the initiator accounts for 0.1% of the total mass of the mixed solution; heat up to 130 °C and react for 3 h, then heat up to 160 °C and react for 8 h; keep nitrogen flowing continuously during the reaction, with a flow rate of 50 mL / min; after the reaction, cool to room temperature to obtain the PLA - co - PVL copolymer solution. The catalyst is Sn(Oct)2; the initiator is 1,4 - butanediol.

[0021] Characterize by Fourier transform infrared spectrum. A characteristic peak of the C - C backbone structure appears at 883 cm -1 a characteristic peak of the ester bond appears at 1216 cm -1 a characteristic peak of the ester carbonyl appears at 1745 cm -1 which proves that the reaction has obtained the PLA - co - PVL copolymer; specifically see Appendix Figure 1 .

[0022] Step 2. Purification of the PLA - co - PVL Copolymer The PLA-co-PVL copolymer solution was added to anhydrous methanol, and the mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol was 1:8. Stir at 500 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol twice, and vacuum dry the precipitate at 50 °C for 24 h to obtain the solid PLA-co-PVL copolymer.

[0023] Step 3: Synthesize the ATEC-HEA composite Stir triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone until completely dissolved. The mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone is 100:38:0.7:0.3. Pass nitrogen to displace air for 5 min, heat in an oil bath to 120 °C, and react for 4 h. After the reaction, cool to room temperature, perform rotary evaporation on the reaction solution. The rotary evaporation temperature is 60 °C, the rotary evaporation vacuum degree is 0.09 MPa, and the rotary evaporation time is 8 h. After rotary evaporation, perform vacuum drying. The temperature is set at 40 °C, the pressure is set at 0.095 MPa, and the drying time is 2.5 h. After drying, the ATEC-HEA composite is obtained.

[0024] Characterization was carried out by Fourier transform infrared spectroscopy. Characteristic peaks of the HEA double bond structure appeared at 964 cm -1 , 1636 cm -1 , and 3040 cm -1 , and a characteristic peak of the ester carbonyl appeared at 1710 cm -1 , proving that the ATEC-HEA composite was obtained; see the attachment for details Figure 2 .

[0025] Step 4: Prepare the modified polylactic acid composite material Mix the solid PLA-co-PVL copolymer, the ATEC-HEA composite, and the antioxidant evenly to obtain a mixed material. The mass ratio of the solid PLA-co-PVL copolymer, the ATEC-HEA composite, and the antioxidant is 85:15:0.2. Then add the mixed material to a twin-screw extruder. The parameters of the extruder are set as follows: the temperature of the feeding section is 160 °C, the temperature of the melting section is 170 °C, the temperature of the mixing section is 175 °C, the temperature of the die head is 170 °C, the screw speed is 80 rpm, and the feeding rate is 3 kg / h. After extrusion, it is cooled and pelletized by water to obtain pellets with a diameter of 4 ± 0.5 mm, and then vacuum dried at 50 °C for 12 h to obtain the modified polylactic acid composite material.

[0026] Example 2 A preparation method of a highly transparent and highly flexible modified polylactic acid A preparation method of a highly transparent and highly flexible modified polylactic acid, comprising the following steps: Step 1: Preparation of the PLA-co-PVL copolymer solution Mix L-lactide and δ-valerolactone evenly. The molar ratio of L-lactide to δ-valerolactone is 5:1 to obtain a mixed solution. Replace the air with nitrogen three times, and then add the catalyst and initiator to the mixed solution and stir evenly. The catalyst accounts for 0.05% of the total mass of the mixed solution, and the initiator accounts for 0.1% of the total mass of the mixed solution; heat up to 140 °C and react for 2.5 h, then heat up to 170 °C and react for 6 h; keep nitrogen flowing in during the reaction, with a flow rate of 50 mL / min; after the reaction, cool to room temperature to obtain a PLA-co-PVL copolymer solution. The catalyst is Sn(Oct)2; the initiator is 1,4-butanediol.

[0027] Step 2. Purification of the PLA-co-PVL copolymer Add the PLA-co-PVL copolymer solution to anhydrous methanol. The mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol is 1:10. Stir at 800 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol three times, and vacuum dry the precipitate at 60 °C for 24 h to obtain a solid PLA-co-PVL copolymer.

[0028] Step 3. Synthesis of the ATEC-HEA complex Stir triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone until completely dissolved. The mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone is 100:38:0.8:0.4. Replace the air with nitrogen for 5 min, heat in an oil bath to 120 °C, and react for 3 h. After the reaction, cool to room temperature, and perform rotary evaporation on the reaction solution. The rotary evaporation temperature is 60 °C, the rotary evaporation vacuum degree is 0.09 MPa, and the rotary evaporation time is 8 h. After rotary evaporation, perform vacuum drying, with the temperature set at 50 °C, the pressure set at 0.095 MPa, and the drying time is 2 h. After drying, obtain the ATEC-HEA complex.

[0029] Step 4. Prepare the modified polylactic acid composite material Mix the solid PLA-co-PVL copolymer, the ATEC-HEA complex, and the antioxidant evenly to obtain a mixed material. The mass ratio of the solid PLA-co-PVL copolymer, the ATEC-HEA complex, and the antioxidant is 88:12:0.2. Then add the mixed material to a twin-screw extruder. The parameters of the extruder are set as follows: the temperature of the feeding section is 160 °C, the temperature of the melting section is 170 °C, the temperature of the mixing section is 175 °C, the temperature of the die head is 170 °C, the screw speed is 100 rpm, and the feeding rate is 3 kg / h. After extrusion, cool and pelletize with water to obtain pellets with a diameter of 4 ± 0.5 mm, and then vacuum dry at 60 °C for 12 h to obtain the modified polylactic acid composite material.

[0030] Example 3 Preparation Method of High-Transparency and High-Flexibility Modified Polylactic Acid A preparation method of high-transparency and high-flexibility modified polylactic acid includes the following steps: Step 1: Preparation of PLA-co-PVL copolymer solution Mix L-lactide and δ-valerolactone evenly, with the molar ratio of L-lactide to δ-valerolactone being 5:1, to obtain a mixed solution. Replace the air with nitrogen 4 times, then add the catalyst and initiator to the mixed solution and stir evenly. The catalyst accounts for 0.1% of the total mass of the mixed solution, and the initiator accounts for 0.2% of the total mass of the mixed solution; heat up to 150°C and react for 2 h, then heat up to 180°C and react for 4 h; keep nitrogen flowing through during the reaction, with a flow rate of 60 mL / min; after the reaction, cool to room temperature to obtain the PLA-co-PVL copolymer solution. The catalyst is Sn(Oct)2; the initiator is 1,4-butanediol.

[0031] Step 2: Purification of PLA-co-PVL copolymer Add the PLA-co-PVL copolymer solution to anhydrous methanol, with the mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol being 1:10. Stir at 800 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol 3 times, and vacuum dry the precipitate at 60°C for 22 h to obtain the solid PLA-co-PVL copolymer.

[0032] Step 3: Synthesis of ATEC-HEA complex Stir triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone until completely dissolved, with the mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone being 100:40:1:0.5. Replace the air with nitrogen for 10 min, heat in an oil bath to 130°C, and react for 3 h. After the reaction, cool to room temperature, and perform rotary evaporation on the reaction solution. The rotary evaporation temperature is 70°C, the rotary evaporation vacuum degree is 0.08 MPa, and the rotary evaporation time is 7 h. After rotary evaporation, perform vacuum drying, with the temperature set at 50°C, the pressure set at 0.090 MPa, and the drying time being 2 h. After drying, obtain the ATEC-HEA complex.

[0033] Step 4: Obtain the modified polylactic acid composite material Mix the PLA-co-PVL solid copolymer, ATEC-HEA complex, and antioxidant evenly to obtain a mixed material. The mass ratio of the PLA-co-PVL solid copolymer, ATEC-HEA complex, and antioxidant is 90:10:0.2. Then add the mixed material to a twin-screw extruder. The parameters of the extruder are set as follows: the temperature of the feeding section is 165 °C, the temperature of the melting section is 175 °C, the temperature of the mixing section is 180 °C, the temperature of the die head is 175 °C, the screw speed is 100 rpm, and the feeding rate is 5 kg / h. After extrusion, it is granulated by water cooling to obtain pellets with a diameter of 4 ± 0.5 mm, and then vacuum dried at 60 °C for 11 h to obtain the modified polylactic acid composite material.

[0034] Comparative Example 1 A preparation method of polylactic acid includes the following steps: Step 1: Preparation of the PLA-co-PVL copolymer solution Mix L-lactide and δ-valerolactone evenly. The molar ratio of L-lactide to δ-valerolactone is 5:1 to obtain a mixed solution. Pass nitrogen to displace air 3 times, and then add the catalyst and initiator to the mixed solution and stir evenly. The catalyst accounts for 0.05% of the total mass of the mixed solution, and the initiator accounts for 0.1% of the total mass of the mixed solution; heat up to 140 °C and react for 2.5 h, then heat up to 170 °C and react for 6 h; keep nitrogen flowing in during the reaction, and the flow rate is 50 mL / min; after the reaction, cool to room temperature to obtain the PLA-co-PVL copolymer solution. The catalyst is Sn(Oct)2; the initiator is 1,4-butanediol.

[0035] Step 2: Purification of the PLA-co-PVL copolymer Add the PLA-co-PVL copolymer solution to anhydrous methanol. The mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol is 1:9. Stir at 800 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol 3 times, and vacuum dry the precipitate at 60 °C for 24 h to obtain the PLA-co-PVL solid copolymer.

[0036] Step 3: Prepare the modified polylactic acid composite material The PLA-co-PVL solid copolymer, triethyl acetylcitrate, and antioxidant were mixed evenly to obtain a mixed material. The mass ratio of the PLA-co-PVL solid copolymer, triethyl acetylcitrate, and antioxidant was 88:12:0.2. Then, the mixed material was added to a twin-screw extruder. The parameters of the extruder were set as follows: the temperature of the feeding section was 160°C, the temperature of the melting section was 170°C, the temperature of the mixing section was 175°C, the temperature of the die head was 170°C, the screw speed was 100 rpm, and the feeding rate was 3 kg / h. After extrusion, it was cooled and pelletized by water, and pellets with a diameter of 4 ± 0.5 mm were obtained. Then, they were vacuum dried at 60°C for 12 h to obtain the modified polylactic acid composite material. The antioxidant was Irganox1010.

[0037] Comparative Example 2 A preparation method of polylactic acid includes the following steps: Step 1. Preparation of the PLA solution L-lactide was stirred evenly, and the air was displaced with nitrogen 3 times. Then, the catalyst and initiator were added to L-lactide and stirred evenly. The catalyst accounted for 0.05% of the mass of L-lactide, and the initiator accounted for 0.1% of the mass of L-lactide. The temperature was raised to 140°C and reacted for 2.5 h, then the temperature was raised to 170°C and reacted for 6 h. During the reaction, nitrogen was continuously introduced with a flow rate of 50 mL / min. After the reaction ended, it was cooled to room temperature to obtain the PLA solution. The catalyst was Sn(Oct)2; the initiator was 1,4-butanediol.

[0038] Step 2. Purification of PLA The PLA solution was added to anhydrous methanol, and the mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol was 1:9. It was stirred at 800 rpm to precipitate the copolymer, and the precipitate was collected by filtration. The precipitate was washed with methanol 3 times, and the precipitate was vacuum dried at 60°C for 24 h to obtain the PLA solid.

[0039] Step 3. Synthesis of the ATEC-HEA complex Triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone were stirred until completely dissolved. The mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone was 100:38:0.8:0.4. The air was displaced with nitrogen for 5 min, and it was heated in an oil bath to 120°C and reacted for 3 h. After the reaction ended, it was cooled to room temperature, and the reaction solution was rotary evaporated. The rotary evaporation temperature was 60°C, the rotary evaporation vacuum degree was 0.09 MPa, and the rotary evaporation time was 8 h. After rotary evaporation, it was dried under reduced pressure. The temperature was set at 50°C, the pressure was set at 0.095 MPa, and the drying time was 2 h. After drying ended, the ATEC-HEA complex was obtained.

[0040] Step 4. Preparation of the polylactic acid composite material The PLA solid, ATEC-HEA complex, and antioxidant were mixed evenly to obtain a mixture. The mass ratio of the PLA solid, ATEC-HEA complex, and antioxidant was 90:10:0.2. Then the mixture was added to a twin-screw extruder, and the extruder parameters were set as follows: the temperature of the feeding section was 160 °C, the temperature of the melting section was 170 °C, the temperature of the mixing section was 175 °C, the temperature of the die head was 170 °C, the screw speed was 100 rpm, and the feeding rate was 3 kg / h. After extrusion, it was cooled and pelletized by water to obtain pellets with a diameter of 4 ± 0.5 mm, and then vacuum dried at 60 °C for 12 h to obtain the polylactic acid composite material. The antioxidant was Irganox1010.

[0041] Example 4 Performance Test (1) The light transmittance and haze performance of the polylactic acid prepared in Examples 1-3 and Comparative Examples 1-2 were tested according to the test method in GB / T2410-2008. The specific test results are shown in Table 1.

[0042] Table 1

[0043] It can be seen from the test results in Table 1 that the polylactic acid prepared in Examples 1-3 has a high light transmittance, reaching 94.14-94.58%; compared with the comparative examples, the haze decreases significantly, reaching 3.87-4.46%. The high light transmittance and low haze mean that the modified polylactic acid prepared by the present invention has better optical properties and higher light transmittance uniformity.

[0044] (2) The tensile strength and notched impact strength performance of the polylactic acid prepared in Examples 1-3 and Comparative Examples 1-2 were tested according to the test method in GB / T29284-2012; the elongation at break performance was tested according to the test method in GB / T1040.1-2018; the flexural strength and flexural modulus performance were tested according to the test method in GB / T9341-2008. The specific test results are shown in Table 2.

[0045] Table 2

[0046] It can be seen from the test results in Table 2 that the notched impact strength and elongation at break of the polylactic acid prepared in Examples 1-3 are significantly improved, and the notched impact strength reaches 7.8-8.6 kJ / m 2, the notched impact strength is higher, indicating a stronger ability to resist fracture. The elongation at break reaches 135-173%, and the higher elongation at break directly reflects better flexibility and ductility. The tensile strength and flexural strength of the polylactic acid prepared in Examples 1-3 are significantly lower than those of the polylactic acid prepared in Comparative Example 1 and significantly higher than those of the polylactic acid prepared in Comparative Example 2. The tensile strength is 43.25-48.95 MPa, and the flexural strength is 72.5-81.4 MPa. The flexural modulus of the polylactic acid prepared in Examples 1-3 is significantly reduced to 2.3-2.8 GPa. A lower flexural modulus means a stronger ability to resist elastic bending deformation. In summary, the modified polylactic acid prepared by the present invention has excellent flexibility.

[0047] Obviously, there are many specific implementation methods that can be changed under the concept of the present invention. Here, it should be stated that any changes made under the inventive concept of the present invention will fall within the protection scope of the present invention.

Claims

1. A preparation method of a highly transparent and highly flexible modified polylactic acid, characterized in that: It includes the steps of preparing a PLA-co-PVL copolymer solution, purifying the PLA-co-PVL copolymer, synthesizing an ATEC-HEA complex, and obtaining a modified polylactic acid composite material; The preparation of the PLA-co-PVL copolymer solution: Mix L-lactide and δ-valerolactone evenly to obtain a mixed solution. Add a catalyst and an initiator to the mixed solution and react to obtain a PLA-co-PVL copolymer solution; the catalyst is Sn(Oct)2, and the initiator is 1,4-butanediol; The synthesis of the ATEC-HEA complex: The raw materials used include: triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, hydroquinone; The preparation of the modified polylactic acid composite material: The raw materials used include: a solid PLA-co-PVL copolymer, an ATEC-HEA complex, and an antioxidant.

2. The preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The mass ratio of the raw materials used for the preparation of the modified polylactic acid composite material is: the mass ratio of the solid PLA-co-PVL copolymer, the ATEC-HEA complex, and the antioxidant is (85 - 90):(10 - 15):0.

2.

3. The preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The molar ratio of L-lactide to δ-valerolactone is (4 - 5):1; the catalyst accounts for 0.05 - 0.1% of the total mass of L-lactide and δ-valerolactone, and the initiator accounts for 0.1 - 0.2% of the total mass of L-lactide and δ-valerolactone.

4. A preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The mass ratio of the raw materials used for the synthesis of the ATEC-HEA complex is: the mass ratio of triethyl acetylcitrate, 2-hydroxyethyl acrylate, p-toluenesulfonic acid, and hydroquinone is 100:(38 - 40):(0.7 - 1):(0.3 - 0.5).

5. A preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The preparation of the PLA-co-PVL copolymer solution: Mix L-lactide and δ-valerolactone evenly to obtain a mixed solution; introduce nitrogen to displace air 3 - 4 times, then add the catalyst and the initiator to the mixed solution and stir evenly; heat up to 130 - 150 °C and react for 2 - 3 h, then heat up to 160 - 180 °C and react for 4 - 8 h; after the reaction, cool to room temperature to obtain a PLA-co-PVL copolymer solution.

6. The preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 5, characterized in that: In the preparation step of the PLA-co-PVL copolymer solution, keep nitrogen flowing continuously during the reaction, and the flow rate is 50 - 60 mL / min.

7. A method for preparing a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The purification of the PLA-co-PVL copolymer: Add the PLA-co-PVL copolymer solution to anhydrous methanol, and the mass ratio of the PLA-co-PVL copolymer solution to anhydrous methanol is 1:(8 - 10); stir at 500 - 800 rpm to precipitate the copolymer, filter and collect the precipitate, wash the precipitate with methanol 2 - 3 times, and vacuum dry the precipitate at 50 - 60 °C for 22 - 24 h to obtain a solid PLA-co-PVL copolymer.

8. A method for preparing a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The synthesis of the ATEC-HEA composite: Triethyl acetylcitrate, 2-Hydroxyethyl acrylate, p-Toluenesulfonic acid, and Hydroquinone were stirred until completely dissolved. Nitrogen was introduced to displace air for 5 - 10 min, and then the mixture was heated in an oil bath to 120 - 130 °C and reacted for 3 - 4 h. After the reaction, it was cooled to room temperature, and the reaction solution was rotary evaporated at a temperature of 60 - 70 °C, a vacuum degree of 0.08 - 0.09 MPa, and a rotary evaporation time of 7 - 8 h. After rotary evaporation, vacuum drying was carried out at a temperature of 40 - 50 °C, a pressure of 0.090 - 0.095 MPa, and a drying time of 2 - 2.5 h. After drying, the ATEC-HEA composite was obtained.

9. A preparation method of a highly transparent and highly flexible modified polylactic acid according to claim 1, characterized in that: The preparation of the modified polylactic acid composite material: The PLA-co-PVL solid copolymer, the ATEC-HEA composite, and an antioxidant were mixed evenly to obtain a mixed material. Then the mixed material was added to a twin-screw extruder. After extrusion by the twin-screw extruder, it was cooled and pelletized by water to obtain pellets with a diameter of 4 ± 0.5 mm. Then it was vacuum dried at 50 - 60 °C for 11 - 12 h to obtain the modified polylactic acid composite material; the antioxidant was Irganox1010.

10. A method for preparing a highly transparent and highly flexible modified polylactic acid according to claim 9, characterized in that: In the step of preparing the modified polylactic acid composite material, the parameters of the twin-screw extruder were set as follows: the temperature of the feeding section was 160 - 165 °C, the temperature of the melting section was 170 - 175 °C, the temperature of the mixing section was 175 - 180 °C, the temperature of the die head was 170 - 175 °C, the screw speed was 80 - 100 rpm, and the feeding rate was 3 - 5 kg / h.

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