Application of copper citrate in preparation of polylactic acid film

Abstract

The invention discloses an application of copper citrate in preparation of a polylactic acid film, and a preparation method of the copper citrate comprises the following steps: S1, dissolving citric acid in water, and adjusting the pH value to 6.7-7.1; s2, adding a copper chloride solution, and heating to 60-80 DEG C for reaction; and S3, after the reaction is finished, collecting solids, and drying the solids at 45-70 DEG C to obtain the copper citrate. The copper citrate prepared by the specific preparation method is used as the nucleating agent, so that the polylactic acid film has proper crystallinity, and the polylactic acid film has good mechanical property, barrier property and optical property.

Description

Technical Field

[0001] This invention belongs to the field of polylactic acid blend materials technology, specifically relating to the application of copper citrate in the preparation of polylactic acid films. Background Technology

[0002] In the context of increasingly prominent "white pollution" and energy issues, the use of non-degradable polymer materials has been severely limited. Therefore, exploring ways to replace traditional polymer materials with biodegradable ones is an important direction for research. In recent years, in addition to common natural biodegradable polymers such as starch, lignin, and cellulose, a large number of polymeric biodegradable polymers have emerged, such as polybutylene terephthalate (PBAT), polylactic acid (PLA), polycaprolactone (PCL), and polybutylene succinate (PBS). Due to the massive consumption and wide application of traditional plastic packaging, it has caused increasingly serious environmental pollution. Therefore, developing biodegradable plastic packaging films has become an important direction for current environmental protection and sustainable development.

[0003] Polylactic acid (PLA) is a biodegradable natural polymer and is considered one of the most promising alternatives to synthetic polymers. However, PLA's low crystallinity results in poor mechanical, barrier, and optical properties, limiting its applications. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide an application of copper citrate in the preparation of polylactic acid (PLA) films, so that the PLA films possess suitable crystallinity, thereby enabling the PLA films to have good mechanical properties, barrier properties, and optical properties.

[0005] This invention provides an application of copper citrate in the preparation of polylactic acid films, wherein the preparation method of copper citrate includes the following steps:

[0006] S1. Citric acid is dissolved in water, and the pH is adjusted to 6.7–7.1;

[0007] S2. Add copper chloride solution and heat to 60-80℃ to react;

[0008] S3. After the reaction is complete, collect the solid and dry it at a temperature of 45-70°C to obtain copper citrate.

[0009] This invention provides an application of copper citrate in the preparation of polylactic acid (PLA) films. Using copper citrate prepared by a specific method of this invention as a nucleating agent, the PLA film can possess suitable crystallinity, thereby enabling the PLA film to exhibit excellent mechanical, barrier, and optical properties. Specifically, the preparation method of copper citrate in this invention employs a combination of specific raw materials and specific process conditions to form copper citrate with a specific structure, which helps to achieve suitable crystallinity in the PLA film and improves the overall performance of the PLA film.

[0010] In step S1 of this invention, the pH is adjusted to 6.7 to 7.1. The pH can be, for example, 6.7, 6.8, 6.9, 7.0, or 7.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0011] In step S2 of this invention, the reaction is heated to 60-80°C. The temperature of the reaction can be, for example, 60°C, 65°C, 70°C, 75°C, or 80°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0012] In step S2 of this invention, the drying temperature is 45-70°C. The drying temperature can be, for example, 45°C, 50°C, 55°C, 60°C, 65°C, or 70°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0013] Preferably, in step S1, the ratio of the amount of citric acid to the volume of water is 1 mol:(1.8 to 2.2 L), for example, it can be 1 mol:1.8 L, 1 mol:1.9 L, 1 mol:2.0 L, 1 mol:2.1 L, 1 mol:2.2 L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0014] Preferably, in step S2, the concentration of the copper chloride solution is 1–1.5 mol / L, for example, it can be 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, or 1.5 mol / L, but is not limited to the listed values; other unlisted values ​​within the range are also applicable. The copper chloride solution is added dropwise, and the dropwise rate is 0.1–3 mL / min, for example, it can be 1 mL / min, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0015] In S2 of this scheme, the concentration of the copper chloride solution is 1-1.5 mol / L, and the copper chloride solution is added dropwise at a rate of 1 mL / min. By controlling the concentration of the copper chloride solution and the dropwise rate, the prepared copper citrate can have a specific structure. This specific structure can further and effectively improve the crystallinity of the polylactic acid film, ensuring that the polylactic acid film has good impact strength and light transmittance while further improving its tensile strength and barrier properties.

[0016] Preferably, in step S2, the reaction is carried out at a temperature of 60–70°C.

[0017] In the above scheme, in step S2, the reaction is heated to 60-70°C. By further controlling the temperature of the heating reaction, the prepared copper citrate can have a specific structure. This specific structure can further and effectively improve the crystallinity of the polylactic acid film. While ensuring that the polylactic acid film has good impact strength and light transmittance, it can further improve its tensile strength and barrier properties.

[0018] Preferably, in step S3, the drying temperature is 50–65°C.

[0019] In the above method, in step S3, the drying temperature is 50-65°C. By further controlling the drying temperature, the prepared copper citrate can have a specific structure. This specific structure can further and effectively improve the crystallinity of the polylactic acid film. While ensuring that the polylactic acid film has good impact strength and light transmittance, it can further improve its tensile strength and barrier properties.

[0020] Preferably, the method for preparing polylactic acid film includes the following steps:

[0021] S1. After mixing polylactic acid and copper citrate evenly, melt extrusion is performed, followed by cooling, granulation, and drying to obtain a premix.

[0022] S2. The premixed material is melted, extruded, and cast to obtain a cast sheet;

[0023] S3. The cast sheet is stretched longitudinally, stretched laterally, and heat-set, and then wound up to obtain the polylactic acid film.

[0024] This solution provides an application of copper citrate in the preparation of polylactic acid (PLA) films. By using copper citrate as a nucleating agent and combining it with the specific preparation method of PLA films, PLA films can have suitable crystallinity, thereby enabling PLA films to possess good mechanical properties, barrier properties, and optical properties.

[0025] More preferably, the mass ratio of copper citrate to polylactic acid is (0.3-0.8):99, for example, it can be 0.3:99, 0.4:99, 0.5:99, 0.6:99, 0.7:99, 0.8:99, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] The mass ratio of copper citrate to polylactic acid in this scheme is (0.3-0.8):99. Within this range, the copper citrate to polylactic acid mass ratio can provide sufficient nucleation sites to accelerate the crystallization process and ensure that the obtained polylactic acid film has sufficient crystallinity. At the same time, it can avoid excessive nucleating agent causing the polylactic acid film to become too crystallized, which would increase the brittleness of the prepared polylactic acid film, reduce its impact strength, and decrease its transparency.

[0027] More preferably, the mass ratio of copper citrate to polylactic acid is (0.4-0.6):99.

[0028] More preferably, in S1, the temperature of the melt extrusion is 175-190°C; in S2, the temperature of the melt extrusion is 185-200°C.

[0029] In S1, the temperature of the melt extrusion is 175–190°C; in S2, the temperature of the melt extrusion is 185–200°C. The nucleating agent copper citrate in this solution can be effectively dispersed in the polylactic acid matrix within the above-mentioned melt extrusion temperature range, providing nucleation sites for crystallization, thereby accelerating the crystallization process and improving the crystallinity.

[0030] More preferably, in S3, the stretching ratio of the longitudinal stretching to the transverse stretching is 1 to 4, for example, it can be 1, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] In the above scheme, in S3, the stretching ratio of the longitudinal stretching to the transverse stretching is 1 to 4. Within this stretching ratio range, the orientation of the polylactic acid molecular chains and the stress-induced effect further promote the formation of the crystalline structure, giving the polylactic acid film a more suitable degree of crystallinity, while avoiding excessive crystallinity of the polylactic acid film.

[0032] More preferably, in S3, the stretching ratio of the longitudinal stretching to the transverse stretching is 2 to 3.

[0033] More preferably, in S3, the stretching rate of the longitudinal stretching and the transverse stretching is 40% / s to 600% / s, for example, it can be 40% / s, 100% / s, 200% / s, 300% / s, 400% / s, 500% / s, or 600% / s, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] In S3 of this scheme, the stretching rate of the longitudinal stretching and the transverse stretching is 40% / s to 600% / s. Within this stretching rate range, the orientation of the polylactic acid molecular chains and the stress-induced effect further promote the formation of the crystalline structure, giving the polylactic acid film a more suitable degree of crystallinity, while avoiding excessive crystallinity of the polylactic acid film.

[0035] More preferably, in S3, the stretching rate of the longitudinal stretching and the transverse stretching is 300% / s to 600% / s.

[0036] More preferably, in S3, the stretching temperature for longitudinal stretching and transverse stretching is 60 to 110°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, or 110°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] More preferably, in step S3, the heat setting temperature is 60 to 110°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, or 110°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable. Attached Figure Description

[0038] Figure 1 The Fourier transform infrared (FTIR) spectrum of copper citrate synthesized in Example 1 of this invention is shown. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0040] Example 1

[0041] Synthesis of copper citrate

[0042] S1. Dissolve 0.09 mol of citric acid in 180 mL of distilled water, disperse by ultrasonication for 30 min, and then add NaOH solution (1 mol / L) dropwise to adjust the pH to 7;

[0043] S2. Add 90 mL of copper chloride solution (1 mol / L) dropwise at a dropping rate of ... mL / min, heat to 70 °C and stir for 2.5 h;

[0044] S3. After the reaction is complete, let the solution stand and then filter it to collect the solid. Place the solid reaction product in a vacuum drying oven at 60°C and dry it for 6 hours to obtain copper citrate.

[0045] The chemical structure of the synthesized copper citrate was characterized using Fourier transform infrared spectroscopy (FTIR). At 3428 cm⁻¹... -1 The absorption peaks appearing on the left and right correspond to the stretching vibration peaks of the hydroxyl group (-OH) in the water of crystallization; at 1568 cm⁻¹... -1 The absorption peak appearing nearby is the stretching peak of the carbonyl group (C=O); while the peak at 1354 cm⁻¹ is... -1 1232cm -1 The absorption peak corresponds to the stretching vibration peak of the carboxyl group (-COOH). Furthermore, at 462 cm⁻¹... -1 The absorption peaks indicate that citric acid and copper chloride underwent a coordination reaction, demonstrating the successful synthesis of copper citrate.

[0046] Preparation of polylactic acid films

[0047] S1. Preparation of premix:

[0048] 99 parts by weight of dried polylactic acid, 0.5 parts by weight of copper citrate, 0.2 parts by weight of antioxidant (composed of antioxidant 168 and antioxidant 1010 in a mass ratio of 1:1), and 0.3 parts by weight of ethylene bis-stearamide (EBS) slip agent were added to a mixer and stirred evenly at 25°C. The mixer was first mixed at a low speed of 100 rpm for 7 minutes, and then mixed at a high speed of 200 rpm for 8 minutes.

[0049] The above-mentioned uniformly stirred mixture is fed into a parallel twin-screw extruder for melt blending. The twin-screw speed is 240 r / min, and the temperatures of zones 1-10 of the twin-screw extruder are set to 160℃, 165℃, 170℃, 180℃, 180℃, 170℃, 160℃, 155℃, 155℃, and 170℃, respectively. The die head temperature is 180℃. After extrusion molding, the mixture is water-cooled, air-dried, pelletized, and dried to obtain the premix.

[0050] S2. Preparation of cast sheets:

[0051] The premixed material obtained in step S1 is added to the extruder of the extrusion casting equipment for melt extrusion and casting by casting rollers to obtain a cast sheet. The extruder temperature (melt extrusion temperature) is 185-200℃, the screen changing zone temperature is 180℃, the die head temperature is 190℃, and the casting roller temperature is 40℃.

[0052] S3. Biaxial stretching forming:

[0053] The cast sheet obtained in step S2 is subjected to biaxial stretching using a biaxial stretching machine. The stretching method used is to first perform longitudinal (MD) stretching followed by transverse (TD) stretching, wherein the stretching ratio of MD stretching to TD stretching is 3, the stretching temperature is 80℃, the heat setting temperature is 80℃, and the stretching rate is 600% / s, to obtain a polylactic acid film.

[0054] Example 2

[0055] The method for synthesizing copper citrate in this embodiment is the same as in Example 1, except that in step S2, 45 mL of copper chloride solution (2 mol / L) is added dropwise.

[0056] The preparation method of the polylactic acid film in this embodiment is the same as that in Example 1.

[0057] Example 3

[0058] The synthesis method of copper citrate in this embodiment is the same as in Example 1, except that in step S2, the reaction is carried out by heating to 80°C and stirring.

[0059] The preparation method of the polylactic acid film in this embodiment is the same as that in Example 1.

[0060] Example 4

[0061] The synthesis method of copper citrate in this embodiment is the same as in Example 1, except that in step S3, the solid reaction product is placed inside and outside a vacuum drying oven at 70°C.

[0062] The preparation method of the polylactic acid film in this embodiment is the same as that in Example 1.

[0063] Example 5

[0064] The method for synthesizing copper citrate in this embodiment is the same as in Example 1.

[0065] The preparation method of polylactic acid film in this embodiment is the same as in Example 1, except that copper citrate in S1 is 0.8 parts by mass.

[0066] Example 6

[0067] The method for synthesizing copper citrate in this embodiment is the same as in Example 1.

[0068] The preparation method of polylactic acid film in this embodiment is the same as in Example 1, except that the stretching ratio in S3 is 4.

[0069] Example 7

[0070] The method for synthesizing copper citrate in this embodiment is the same as in Example 1.

[0071] The preparation method of polylactic acid film in this embodiment is the same as in Example 1, except that the stretching rate in S3 is 200% / s.

[0072] Comparative Example 1

[0073] The synthesis method of copper citrate in this comparative example is the same as that in Example 1, except that in step S2, the reaction is carried out by heating to 90°C and stirring.

[0074] The preparation method of the polylactic acid film in this comparative example is the same as that in Example 1.

[0075] Comparative Example 2

[0076] The synthesis method of copper citrate in this comparative example is the same as in Example 1, except that in step S3 the solid reaction product is placed inside and outside a vacuum drying oven at 80°C.

[0077] The preparation method of the polylactic acid film in this comparative example is the same as that in Example 1.

[0078] Comparative Example 3

[0079] This comparative example uses commercially available copper citrate (purchased from Shanghai Maclean Biochemical Technology Co., Ltd.), and is otherwise identical to Example 1.

[0080] The preparation method of the polylactic acid film in this comparative example is the same as that in Example 1.

[0081] Performance testing

[0082] (1) Tensile strength

[0083] The test method is as follows: Before the test, the film sample is placed under constant temperature and humidity conditions of 23℃ and 50% RH (relative humidity) for 48 hours. The treated film sample is tested at a tensile speed of 50mm / min. Five sample data are measured for each group and the average value is taken.

[0084] (2) Light transmittance

[0085] The test method is as follows: The absorbance is measured using a UV spectrophotometer. A polylactic acid film is cut into a rectangular strip of 4cm×1cm and attached to the inner surface of one side of the cuvette of the spectrophotometer. The absorbance at a wavelength of 660nm is measured. This is repeated three times to reflect the opacity of the film.

[0086] (3) Oxygen permeability coefficient

[0087] The testing method is as follows: the test is conducted in accordance with the national standard GB1038-2000.

[0088] The polylactic acid films prepared in the above examples and comparative examples were subjected to the above performance tests, and the test results are shown in Table 1.

[0089] Table 1

[0090]

[0091] As shown in Table 1:

[0092] In Examples 1-7, the application of copper citrate in the preparation of polylactic acid (PLA) films falls within the scope of protection of this invention. The films exhibited good performance in tests of tensile strength, light transmittance, and oxygen transmittance, indicating that the PLA films prepared using the copper citrate application method of this invention possess good mechanical, barrier, and optical properties. However, the preparation methods of Comparative Examples 1-3 do not fall within the scope of protection of this invention. They showed significant deterioration in at least one of the performance tests for tensile strength, light transmittance, and oxygen transmittance, meaning that the PLA films prepared using the methods of Comparative Examples 1-3 do not possess good mechanical, barrier, and optical properties.

[0093] Compared to the method in Example 1 where 90 mL of copper chloride solution (1 mol / L) was added, the method in Example 2 where 45 mL of copper chloride solution (2 mol / L) was added. That is, the amount of copper chloride used in Examples 1 and 2 was the same. The method in Example 2 did not further optimize the concentration of the copper chloride solution to be in the range of 1–1.5 mol / L. The polylactic acid film prepared using the copper citrate synthesized in Example 2 had a lower tensile strength and a higher oxygen permeability coefficient than that in Example 1. This indicates that a copper chloride solution concentration of 1–1.5 mol / L during the synthesis of copper citrate can further improve the tensile strength and barrier properties of the polylactic acid film.

[0094] Compared to the method of synthesizing copper citrate in Example 1, which involves heating to 70°C and stirring, the method of synthesizing copper citrate in Example 3 involves heating to 80°C and stirring. That is, the temperature of the heating reaction in the method of synthesizing copper citrate in Example 3 is not in the further preferred range of 60-70°C. The polylactic acid film made from the copper citrate synthesized in Example 3 has a lower tensile strength and a higher oxygen permeability coefficient than that in Example 1. This indicates that heating to 60-70°C during the synthesis of copper citrate can further improve the tensile strength and barrier properties of polylactic acid film.

[0095] Compared to the method for synthesizing copper citrate in Example 1, where the solid reaction product was placed in a vacuum drying oven at 60°C, the method for synthesizing copper citrate in Example 4 placed the solid reaction product in a vacuum drying oven at 70°C. That is, the drying temperature in the method for synthesizing copper citrate in Example 4 is not in the further preferred range of 50-65°C. The polylactic acid film made from the copper citrate synthesized in Example 4 has a lower tensile strength and a higher oxygen permeability coefficient than that in Example 1. This indicates that when synthesizing copper citrate, a drying temperature of 50-65°C can further improve the tensile strength and barrier properties of the polylactic acid film.

[0096] Compared to Example 1, which used 0.5 parts by mass of copper citrate as a nucleating agent in the preparation of polylactic acid (PLA) films, Example 5 used 0.8 parts by mass of copper citrate as a nucleating agent in the preparation of PLA films. That is, the mass ratio of copper citrate to PLA in Example 5 is not further optimized within the range of (0.4-0.6):99. The PLA film prepared by the method of Example 5 showed worse light transmittance performance than that of Example 1. This indicates that when preparing PLA films, a mass ratio of copper citrate to PLA of (0.4-0.6):99 can further improve the light transmittance of PLA films.

[0097] Compared to the stretching ratio of 3 for longitudinal and transverse stretching in the preparation of polylactic acid film in Example 1, the stretching ratio of longitudinal and transverse stretching in the preparation of polylactic acid film in Example 6 is 4. That is, the stretching ratio of longitudinal and transverse stretching in Example 6 is not in the further preferred range of 2 to 3. The polylactic acid film prepared by the preparation method of Example 6 has a worse light transmittance test effect than that of Example 1. This shows that when preparing polylactic acid film, a stretching ratio of 2 to 3 for longitudinal and transverse stretching can further improve the light transmittance of polylactic acid film.

[0098] Compared to the stretching rate of 600% / s in the longitudinal and transverse stretching of the polylactic acid film prepared in Example 1, the stretching rate of 40% / s in the longitudinal and transverse stretching of the polylactic acid film prepared in Example 7 is not within the further preferred range of 300% / s to 600% / s. The polylactic acid film prepared by the method of Example 7 has a worse light transmittance test result than that of Example 1. This indicates that when preparing polylactic acid films, a stretching rate of 300% / s to 600% / s in the longitudinal and transverse stretching can further improve the light transmittance of the polylactic acid film.

[0099] Compared to the method of synthesizing copper citrate in Example 1, which involves heating to 70°C and stirring, the method of synthesizing copper citrate in Comparative Example 1 involves heating to 90°C and stirring. This means that the heating temperature in Comparative Example 1 is outside the 60-80°C range of the present invention. Consequently, the polylactic acid film prepared using the copper citrate synthesized in Comparative Example 1 exhibits significantly lower tensile strength and oxygen permeability than that in Example 1. This indicates that when the heating temperature exceeds 60-80°C during the synthesis of copper citrate, the polylactic acid film shows significantly deteriorated performance in terms of tensile strength and oxygen permeability. Controlling the heating temperature to 60-80°C allows the prepared copper citrate to possess a specific structure. This specific structure effectively improves the crystallinity of the polylactic acid film, ensuring good light transmittance while enhancing its tensile strength and barrier properties. This results in a polylactic acid film possessing excellent mechanical, barrier, and optical properties.

[0100] Compared to the method for synthesizing copper citrate in Example 1, where the solid reaction product was placed in a vacuum drying oven at 60°C, the method for synthesizing copper citrate in Comparative Example 2 placed the solid reaction product in a vacuum drying oven at 80°C. That is, the drying temperature in the method for synthesizing copper citrate in Comparative Example 2 is outside the 45-70°C range of the present invention. The polylactic acid film prepared using the copper citrate synthesized in Comparative Example 2 showed significantly lower tensile strength and oxygen permeability than that in Example 1. This indicates that when the drying temperature exceeds the 45-70°C range during the synthesis of copper citrate, the polylactic acid film exhibits significantly deteriorated performance in terms of tensile strength and oxygen permeability. Controlling the drying temperature to 45-70°C allows the prepared copper citrate to possess a specific structure. This specific structure effectively improves the crystallinity of the polylactic acid film, ensuring good light transmittance while enhancing its tensile strength and barrier properties, thus giving the polylactic acid film excellent mechanical, barrier, and optical properties.

[0101] Compared with the specific copper citrate prepared by the method of the present invention in Example 1, Comparative Example 3 uses commercially available copper citrate to prepare polylactic acid film. The polylactic acid film prepared in Comparative Example 3 showed significantly deteriorated test results in terms of tensile strength and oxygen permeability coefficient. This indicates that commercially available copper citrate does not have the specific structure of the copper citrate prepared by the present invention, and therefore cannot enable the prepared polylactic acid film to have good mechanical properties, barrier properties and optical properties.

[0102] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An application of copper citrate in the preparation of polylactic acid films, characterized in that, The method for preparing copper citrate includes the following steps: S1. Citric acid is dissolved in water, and the pH is adjusted to 6.7–7.1; S2. Add copper chloride solution and heat to 60-80℃ to react; S3. After the reaction is complete, collect the solid and dry it at a temperature of 45-70°C to obtain copper citrate.

2. The application of copper citrate according to claim 1 in the preparation of polylactic acid films, characterized in that, In S1, the ratio of the amount of citric acid to the volume of water is 1 mol:(1.8-2.2 L).

3. The application of copper citrate according to claim 1 or 2 in the preparation of polylactic acid films, characterized in that, In step S2, the concentration of the copper chloride solution is 1–1.5 mol / L, and the copper chloride solution is added by dropping, with a dropping rate of 0.5–1.5 mL / min.

4. The application of copper citrate according to claim 1 in the preparation of polylactic acid films, characterized in that, In step S2, the reaction is carried out at a temperature of 60–70°C.

5. The application of copper citrate according to claim 1 in the preparation of polylactic acid films, characterized in that, In step S3, the drying temperature is 50–65°C.

6. The application of copper citrate according to claim 1 in the preparation of polylactic acid films, characterized in that, The method for preparing polylactic acid film includes the following steps: S1. After the polylactic acid and copper citrate are mixed evenly, the mixture is melt-extruded, cooled, granulated, and dried to obtain a premix. S2. The premixed material is melted, extruded, and cast to obtain a cast sheet; S3. The cast sheet is stretched longitudinally, stretched laterally, and heat-set, and then wound up to obtain the polylactic acid film.

7. The application of copper citrate according to claim 6 in the preparation of polylactic acid films, characterized in that, In S1, the mass ratio of copper citrate to polylactic acid is (0.3-0.8):

99.

8. The application of copper citrate according to claim 6 in the preparation of polylactic acid films, characterized in that, In S1, the temperature of the melt extrusion is 175–190°C, and / or, in S2, the temperature of the melt extrusion is 185–200°C.

9. The application of copper citrate according to claim 6 in the preparation of polylactic acid films, characterized in that, In S3, the stretching ratio of the longitudinal stretching to the transverse stretching is 1 to 4.

10. The application of copper citrate according to claim 6 in the preparation of polylactic acid films, characterized in that, In S3, the stretching rate of the longitudinal stretching and the transverse stretching is 40% / s to 600% / s.

Images