Modified polylactic acid film and preparation method thereof

Abstract

The invention relates to the technical field of packaging, and discloses a modified polylactic acid film and a preparation method thereof. The film comprises a polylactic acid base material layer and a modified layer, the modified layer contains a lactide copolymer and an end group functionalized lactide copolymer capable of forming hydrogen bond crosslinking. The invention also discloses a preparation method of the modified polylactic acid film. The preparation method comprises the following steps: S1, mixing the raw material components to obtain a modified material; the raw material components comprise a lactide copolymer and an end group functionalized lactide copolymer capable of forming hydrogen bond crosslinking; s2, polylactic acid and the modified material are prepared through a tape casting method, and the multi-layer film comprising a polylactic acid base material layer and a modified material layer is obtained. The modified polylactic acid film has the advantage of high heat sealing strength.

Description

Technical Field

[0001] The present invention relates to the technical field of packaging, and particularly to a modified polylactic acid film and a preparation method thereof. Background Art

[0002] As a bio-based material, polylactic acid not only has a wide range of sources but also has good mechanical, optical, and processability properties, making it a very promising biodegradable material. In recent years, with the development of biodegradable materials, the production capacity of polylactic acid has gradually increased, and the performance requirements for polylactic acid materials have become higher and higher, and the demand for developing high-end polylactic acid products has also increased day by day.

[0003] Polylactic acid film has characteristics such as high gloss and excellent mechanical properties, and is a new type of environmentally friendly film material, which has important uses in the packaging fields of food, medicine, cosmetics, etc. The polylactic acid film for packaging is usually a composite film with a multi-layer structure and requires a certain heat-sealing strength. Existing technologies mostly prepare polylactic acid films by conventional extrusion-biaxial stretching methods. The polylactic acid has a low crystallinity, poor heat resistance during the heat-sealing process, and low heat-sealing strength, which greatly limits the development and commercialization of polylactic acid films. Therefore, it is urgent to develop a polylactic acid film with high heat-sealing strength and anti-wrinkle properties. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problem of low heat-sealing strength existing in the prior art, and provide a modified polylactic acid film and a preparation method thereof. The modified polylactic acid film has the advantage of high heat-sealing strength.

[0005] To achieve the above purpose, in the first aspect of the present invention, a modified polylactic acid film is provided. The film comprises a polylactic acid base material layer and a modified layer; the modified layer contains a lactide copolymer and a terminal-functionalized lactide copolymer capable of forming hydrogen bond crosslinking.

[0006] Preferably, in the modified layer, the mass ratio of the lactide copolymer to the terminal-functionalized lactide copolymer is 1:0.4 - 9.

[0007] Preferably, the lactide polymer is selected from

[0008] at least one of; the terminal-functionalized lactide copolymer is selected from

[0009] at least one of, where m is an integer greater than 1; n is an integer greater than 1.

[0010] Preferably, the polylactic acid base material layer is L-polylactic acid and / or D-polylactic acid.

[0011] Preferably, two modified layers are provided, and the two modified layers are respectively provided on the upper and lower sides of the polylactic acid base material layer.

[0012] More preferably, the thickness of the modified layer is 2-3 μm, and the thickness of the polylactic acid base material layer is 20-22 μm.

[0013] More preferably, the thickness ratio of the modified layer to the polylactic acid base material layer is 1:7-10.

[0014] The second aspect of the present invention provides a method for preparing a modified polylactic acid film, comprising the following steps:

[0015] S1. Mix the raw material components to obtain a modified material; the raw material components contain a lactide copolymer and a terminal-functionalized lactide copolymer capable of forming hydrogen bond crosslinking;

[0016] S2. Prepare a multi-layer film including a polylactic acid base material layer and a modified material layer from polylactic acid and the modified material by a casting method.

[0017] Preferably, in the modified material, the mass ratio of the lactide copolymer to the terminal-functionalized lactide copolymer is 1:0.4-9.

[0018] More preferably, the molecular weights of the lactide copolymer and the terminal-functionalized lactide copolymer are 100-150 kg / mol.

[0019] More preferably, the mixing conditions of the modified material include at least: the temperature is 160-200 °C, the rotation speed is 50-100 rpm, and the time is 5-10 min.

[0020] Preferably, the polymerization monomers of the lactide copolymer are selected from at least one of L-lactide, ε-caprolactone, and D-lactide.

[0021] More preferably, the polymerization monomers of the lactide copolymer are a combination of L-lactide and ε-caprolactone or a combination of L-lactide and D-lactide.

[0022] More preferably, the molar ratio of L-lactide to ε-caprolactone is 1:0.1-0.5, and the molar ratio of L-lactide to D-lactide is 1:0.1-0.5.

[0023] Preferably, the raw material components further include a nucleating agent, and the nucleating agent is selected from at least one of TMC300, TMC306, TMC328, and TMB5.

[0024] Further preferably, based on the total mass of the modified material and the polylactic acid, the addition amount of the nucleating agent is 0.1-0.5 wt%.

[0025] Further preferably, the conditions of the casting method at least include: the extrusion temperature of the modified layer is 160-200 °C, and the extrusion temperature of the polylactic acid base layer is 180-220 °C.

[0026] Preferably, the lactide copolymer is selected from

[0027] at least one of; the end-group functionalized lactide copolymer is selected from

[0028] at least one of, where m is an integer greater than 1; n is an integer greater than 1.

[0029] Through the above technical solutions, the modified polylactic acid film provided by the present invention comprises a polylactic acid base material layer and a modified layer; the modified layer contains a lactide copolymer and an end-group functionalized lactide copolymer capable of forming hydrogen bond crosslinking. Through the interaction between the end-group functionalized lactide copolymers, the combination between the modified layers is firmer when the polylactic acid film is heat-sealed, and thus the modified polylactic acid film has the advantage of high heat-sealing strength. Detailed Embodiments

[0030] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0031] In the first aspect of the present invention, a modified polylactic acid film is provided, which film comprises a polylactic acid base layer and a modified layer; the modified layer contains a lactide copolymer and a terminal-functionalized lactide copolymer capable of forming hydrogen bond crosslinking. During the research process of the modified polylactic acid film, the inventors found that when the modified layer uses a terminal-functionalized lactide copolymer capable of forming hydrogen bond crosslinking, the bonding strength between the modified layers of the obtained polylactic acid film is enhanced during heat sealing. The modified polylactic acid film provided by the present invention comprises a polylactic acid base layer and a modified layer; the modified layer contains a lactide copolymer and a terminal-functionalized lactide copolymer containing a large number of ureido and carbamate groups, which is prone to form multiple hydrogen bonds during the processing, making the bonding between the modified layers of the obtained polylactic acid film during heat sealing firmer, and thus the modified polylactic acid film has the advantage of high heat sealing strength.

[0032] According to the present invention, preferably, in the modified layer, the mass ratio of the lactide copolymer to the terminal-functionalized lactide copolymer is 1:0.4 - 9, specifically it can be 1:0.4, 1:0.5, 1:1, 1:3, 1:5, 1:7, 1:9, or any value between the above two ratios. The inventors found that when the above preferred embodiment is adopted, the heat sealing strength of the modified polylactic acid film can be further improved.

[0033] According to the present invention, preferably, the lactide copolymer is selected from

[0034] at least one of; the terminal-functionalized lactide copolymer is selected from

[0035]

[0036] at least one of, where m is an integer greater than 1; n is an integer greater than 1. The inventors found that when the above preferred embodiment is adopted, the heat sealing strength of the modified polylactic acid film can be further improved.

[0037] In the present invention, there is no particular limitation on the synthesis method of the lactide copolymer and the terminal-functionalized lactide copolymer. Those skilled in the art can synthesize the terminal-functionalized lactide polymer by ring-opening polymerization; exemplarily, the lactide copolymer and the terminal-functionalized lactide copolymer can be synthesized through the following routes and synthesis methods.

[0038]

[0039] Synthesis of UPy-NCO: 0.70 - 0.80 mol of 2-amino-4-hydroxy-6-methylpyrimidine was dissolved in 4.75 - 4.85 mol of hexamethylene diisocyanate solution, and the reaction was carried out at 100 - 105 °C for 16 - 18 hours. Subsequently, pentane was added, the precipitate was filtered and washed with pentane. The obtained white powder was dried under reduced pressure at a temperature of 50 - 55 °C to obtain UPy-NCO.

[0040]

[0041] Polymerization method of lactide copolymer A1: Dry L-lactide (0.21 - 0.22 mol, 30.0 - 31.43 g), ε-caprolactone (0.04 - 0.05 mol, 4.56 - 5.70 g), 1,6-HDO (0.50 - 0.60 mmol, 0.059 - 0.071 g) and Sn(Oct) 2 (0.22 - 0.23 mmol, 0.090 - 0.094 g) were added to a Schlenk flask and dried at a temperature of 65 - 70 °C under reduced pressure for 1 - 1.5 hours; then it was heated to 130 - 135 °C and reacted for 5 - 6 hours under argon protection. After the reaction, the crude product was dissolved in chloroform, and then the obtained solution was precipitated into ethanol to remove unreacted monomers. Finally, the polymer was vacuum dried at 80 - 85 °C for 6 - 7 hours to obtain lactide copolymer A1.

[0042] Polymerization method of end-functionalized lactide copolymer B1: Lactide copolymer A1 (3.0 - 3.1 g) and UPy-NCO with a molar amount 10 - 11 times that of the terminal hydroxyl groups were added to a dry Schlenk flask and dried at a temperature of 65 - 70 °C under reduced pressure for 1 - 2 hours. The flask was purged with dry argon, and then toluene (150 - 200 mL) and Sn(Oct) 2 (0.22 - 0.23 mmol, 0.090 - 0.094 g) were added. The reaction mixture was stirred at a temperature of 110 - 120 °C for 12 - 13 hours, and the solvent was removed by a rotary evaporator after the reaction. The obtained product was dissolved in chloroform (200 - 300 mL), treated with silica (5.0 - 5.5 g) and dibutyltin dilaurate (20.0 - 21.0 mg) at a temperature of 60 - 65 °C for 1 - 2 hours, then the silica-bound UPy was removed by filtration, and the filtrate was concentrated under reduced pressure. Finally, the residual solvent in the product was removed by vacuum drying at a temperature of 80 - 85 °C for 6 - 7 hours to obtain end-functionalized lactide copolymer B1.

[0043] The polymerization methods of lactide copolymer A2 and end - group functionalized lactide copolymer B2 are carried out by the similar methods as above, and the specific synthesis route is as follows:

[0044]

[0045] According to the present invention, preferably, the polylactic acid base material layer is poly - L - lactic acid and / or poly - D - lactic acid. Specifically, the polylactic acid base material layer can be single poly - L - lactic acid, or single poly - D - lactic acid, or a mixture of poly - L - lactic acid and poly - D - lactic acid. The inventors found that when the above - mentioned preferred implementation mode is adopted, the heat - sealing strength of the modified polylactic acid film can be further improved.

[0046] According to the present invention, preferably, there are two modified layers, and the two modified layers are respectively arranged on the upper and lower sides of the polylactic acid base material layer. The inventors found that when the above - mentioned preferred implementation mode is adopted, the heat - sealing strength of the modified polylactic acid film can be further improved.

[0047] According to the present invention, further preferably, the thickness of the modified layer is 2 - 3 μm, specifically it can be 2 μm, 2.5 μm, 3 μm, or any value between the above two values; the thickness of the polylactic acid base material layer is 20 - 22 μm, specifically it can be 20 μm, 21 μm, 22 μm, or any value between the above two values. The inventors found that when the above - mentioned preferred implementation mode is adopted, the heat - sealing strength of the modified polylactic acid film can be further improved.

[0048] According to the present invention, further preferably, the thickness ratio of the modified layer to the polylactic acid base material layer is 1:7 - 10, specifically it can be 1:7, 1:8, 1:9, 1:10, or any value between the above two values. The inventors found that when the above - mentioned preferred implementation mode is adopted, the heat - sealing temperature stability and heat - sealing strength of the modified polylactic acid film can be further improved.

[0049] The second aspect of the present invention provides a preparation method of a modified polylactic acid film, which includes the following steps:

[0050] S1. Mix the raw material components to obtain a modified material; the raw material components contain a lactide copolymer and an end - group functionalized lactide copolymer capable of forming hydrogen - bond cross - linking;

[0051] S2. Prepare a multi - layer film including a polylactic acid base material layer and a modified material layer by casting the polylactic acid and the modified material. The inventors found that when the above - mentioned preparation method is adopted, the polylactic acid base material layer and the modified layer can be better compounded, and a large number of hydrogen - bond cross - links are formed between the modified layers, which can further improve the heat - sealing strength of the modified polylactic acid film.

[0052] According to the present invention, preferably, in the modified material, the mass ratio of the lactide copolymer to the end-group functionalized lactide copolymer is 1:0.4 - 9, specifically, it can be 1:0.4, 1:0.5, 1:1, 1:3, 1:5, 1:7, 1:9, or any value between the above two values. The inventors have found that when the above preferred implementation mode is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0053] According to the present invention, more preferably, the molecular weights of the lactide copolymer and the end-group functionalized lactide copolymer are 100 - 150 kg / mol, specifically, it can be 100 kg / mol, 110 kg / mol, 120 kg / mol, 130 kg / mol, 140 kg / mol, 150 kg / mol, or any value between the above two values. The inventors have found that when the above preferred implementation mode is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0054] According to the present invention, more preferably, the mixing conditions of the modified material include at least: the temperature is 160 - 200 °C, specifically, it can be 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, or any value between the above two values; the rotation speed is 50 - 100 rpm, specifically, it can be 50 rpm, 60 rpm, 70 rpm, 80 rpm, 90 rpm, 100 rpm, or any value between the above two values; the time is 5 - 10 min, specifically, it can be 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, or any value between the above two values. The inventors have found that when the above preferred implementation mode is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0055] According to the present invention, preferably, the polymerization monomers of the lactide copolymer are selected from at least one of L-lactide, ε-caprolactone, and D-lactide. Specifically, it can be a combination of L-lactide and ε-caprolactone, a combination of L-lactide and D-lactide, or ε-caprolactone and D-lactide. The inventors have found that when the above preferred implementation mode is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0056] According to the present invention, more preferably, the polymerization monomers of the lactide copolymer are a combination of L-lactide and ε-caprolactone or a combination of L-lactide and D-lactide. The inventors have found that when the above preferred implementation mode is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0057] According to the present invention, further preferably, the molar ratio of the L-lactide and the ε-caprolactone is 1:0.1 - 0.5, specifically it can be 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, or any value between the above two ratios; the molar ratio of the L-lactide and the D-lactide is 1:0.1 - 0.5, specifically it can be 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, or any value between the above two ratios. The inventors found that when the above preferred embodiments are adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0058] According to the present invention, preferably, the raw material components further include a nucleating agent, and the nucleating agent is selected from at least one of TMC300, TMC306, TMC328, and TMB5. The inventors found that when the above preferred embodiments are adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0059] According to the present invention, further preferably, based on the total mass of the modified material and the polylactic acid, the addition amount of the nucleating agent is 0.1 - 0.5 wt%, specifically it can be 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, or any value between the above two values. The inventors found that when the above preferred embodiments are adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0060] According to the present invention, further preferably, the conditions of the casting method include: the extrusion temperature of the modified layer is 160 - 200 °C, specifically it can be 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, or any value between the above two values; the extrusion temperature of the polylactic acid substrate layer is 180 - 220 °C, specifically it can be 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, or any value between the above two values. The inventors found that when the above preferred embodiments are adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0061] According to the present invention, preferably, the lactide copolymer is selected from

[0062]

[0063] at least one of; the end-group functionalized lactide copolymer is selected from

[0064]

[0065] at least one of them, where m is an integer greater than 1; n is an integer greater than 1. The inventor found that when the above preferred embodiment is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0066] In the present invention, during the preparation process of the modified polylactic acid film, additives well-known in the art can be used. The heat-sealing film additives are selected from at least one of antioxidants, antistatic agents, stiffening agents, slip agents, and heat stabilizers. There is no strict limit on the dosage of the additives, and the conventional addition amounts in the art can be adopted. Exemplarily, the antioxidant can be antioxidant 1010, antioxidant 1035. The inventor found that when the above preferred embodiment is adopted, the heat-sealing strength of the modified polylactic acid film can be further improved.

[0067] In the present invention, the casting method can be a multi-layer co-extrusion casting method. There are no special restrictions on the equipment used to prepare the heat-sealing film and the draw ratio during the preparation process, and the equipment and methods well-known in the art can be adopted for preparation. As a relatively preferred embodiment of the present invention, taking the preparation of a multi-layer modified polylactic acid film by a three-layer co-extrusion casting method as an example, it specifically includes the following steps:

[0068] S1. Mix lactide copolymer, end-functionalized lactide copolymer, antioxidant, and nucleating agent I and put them into a mixer (model RM-200C), and mix for 5-10 min at a temperature of 160-200 °C and a rotation speed of 50-100 rpm to obtain a blend; among them, the mass ratio of lactide copolymer A to end-functionalized lactide copolymer B is 1:0.4-9; the polymerization monomers of the lactide copolymer are L-lactide and ε-caprolactone, and the molar ratio is 1:0.1-0.5. Based on the total mass of the modified material and the polylactic acid, the addition amount of the antioxidant is 0.1-0.5 wt%, and the addition amount of the nucleating agent is 0.1-0.5 wt%; extrude and pelletize the blend to obtain the modified material.

[0069] S2. Prepare a modified polylactic acid film by a three-layer co-extrusion casting method, where there are two modified layers with the modified material as the raw material, and the two modified layers are respectively arranged on the upper and lower sides of the polylactic acid substrate layer; the polylactic acid substrate layer uses left-handed polylactic acid as the raw material; the extrusion temperature of the modified layer is 160-200 °C, and the thickness is 2-3 μm; the extrusion temperature of the polylactic acid substrate layer is 180-220 °C, and the thickness is 20-22 μm; the thickness ratio of the modified layer to the polylactic acid substrate layer is 1:7-10.

[0070] The present invention will be described in detail below with reference to examples. In the following examples, the haze was measured according to the method in "Determination of Transmittance and Haze of Transparent Plastics (GB / T 2410-2008)"; the glossiness was measured according to the method in "Test Method for Specular Gloss of Plastics (GB / T 8807-1988)"; the heat seal strength was measured according to the method in "Biaxially Oriented Polypropylene (BOPP) Films for General Purposes (GB / T 10003-2008)". Poly-L-lactic acid and poly-D-lactic acid were both purchased from NatureWorks, USA, with a melt index of 2.6 - 3.2 g / 10 min; L-lactide and D-lactide were purchased from Hisun Biomaterials Co., Ltd., with a purity of 99.0%; 1,6-hexanediol (1,6-HDO), trimethylolpropane as initiator (TMP), pentane, ε-caprolactone, chloroform, and ethanol were all purchased from Merck (Aldrich-Sigma); Sn(Oct) 2 , silica, dibutyltin dilaurate, and hexamethylene diisocyanate were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; nucleating agents TMC300, TMC306, TMC328, and TMB5 were all purchased from Shanxi Research Institute of Chemical Industry; the internal mixer was purchased from Harp Electric Technology Co., Ltd., with the model of RM-200C.

[0071] Preparation Example

[0072] Synthesis of UPy-NCO:

[0073]

[0074] 0.70 mol of 2-amino-4-hydroxy-6-methylpyrimidine was dissolved in 4.75 mol of hexamethylene diisocyanate solution and reacted at 100 °C for 16 hours. Subsequently, pentane was added, the precipitate was filtered and washed with pentane, and the obtained white powder was dried under reduced pressure at 50 °C to obtain UPy-NCO.

[0075] Synthesis of lactide copolymer A1 and end-functionalized lactide copolymer B1:

[0076]

[0077] Polymerization method of lactide copolymer A1: The dried L-lactide (0.21 mol, 30.0 g), ε-caprolactone (0.04 mol, 4.56 g), 1,6-HDO (0.50 mmol, 0.059 g), and Sn(Oct) 2(0.22 mmol, 0.09 g) was added to a Schlenk flask and dried at 65 °C under reduced pressure for 1 hour; then it was heated to 130 °C and reacted for 5 hours under argon protection. After the reaction, the crude product was dissolved in chloroform, and then the resulting solution was precipitated into ethanol to remove unreacted monomers. Finally, the polymer was dried in vacuo at 80 °C for 6 hours to obtain lactide copolymer A1 with a molecular weight of about 100 kg / mol.

[0078] Polymerization method of end-functionalized lactide copolymer B1: Lactide copolymer A1 (3.0 g) and UPy-NCO in a molar amount 10 times that of the terminal hydroxyl group were added to a dry Schlenk flask and dried at 65 °C under reduced pressure for 1 hour. The flask was purged with dry argon, and then toluene (150 mL) and Sn(Oct) 2 (0.22 mmol, 0.09 g) were added. The reaction mixture was stirred at 110 °C for 12 hours, and the solvent was removed by rotary evaporation after the reaction. The resulting product was dissolved in chloroform (200 mL), treated with silica (5.0 g) and dibutyltin dilaurate (20.0 mg) at 60 °C for 1 hour, and then the silica-bound UPy was removed by filtration and the filtrate was concentrated under reduced pressure. Finally, the residual solvent in the product was removed by drying in vacuo at 80 °C for 6 hours to obtain end-functionalized lactide copolymer B1 with a molecular weight of about 120 kg / mol.

[0079] The polymerization methods of lactide copolymer A3 and end-functionalized lactide copolymer B3 are similar to those of A1 and B1, and D-lactide can be used instead of ε-caprolactone; the structure of lactide copolymer A3 is with a molecular weight of about 130 kg / mol, and the structure of end-functionalized lactide copolymer B3 is with a molecular weight of about 140 kg / mol.

[0080] Synthesis of lactide copolymer A2 and end-functionalized lactide copolymer B2:

[0081]

[0082] Polymerization method of lactide copolymer A2: Dry L-lactide (0.21 mol, 30.0 g), ε-caprolactone (0.04 mol, 4.56 g), TMP (0.50 mmol, 0.059 g) and Sn(Oct) 2(0.22 mmol, 0.09 g) was added to a Schlenk flask and dried at 65 °C under reduced pressure for 1 hour; then it was heated to 130 °C and reacted for 5 hours under argon protection. After the reaction, the crude product was dissolved in chloroform, and then the resulting solution was precipitated into ethanol to remove unreacted monomers. Finally, the polymer was dried in vacuo at 80 °C for 6 hours to obtain the end-functionalized lactide copolymer A2 with a molecular weight of about 100 kg / mol.

[0083] Polymerization method of the end-functionalized lactide copolymer B2: The lactide copolymer A2 (3.0 g) and UPy-NCO with a molar amount 10 times that of the terminal hydroxyl groups were added to a dry Schlenk flask and dried at 65 °C under reduced pressure for 1 hour. The flask was purged with dry argon, and then toluene (150 mL) and Sn(Oct) 2 (0.22 mmol, 0.09 g) were added. The reaction mixture was stirred at 110 °C for 12 hours, and the solvent was removed by a rotary evaporator after the reaction. The resulting product was dissolved in chloroform (200 mL), treated with silica (5.0 g) and dibutyltin dilaurate (20.0 mg) at 60 °C for 1 hour, then the silica-bound UPy was removed by filtration, and the filtrate was concentrated under reduced pressure. Finally, the residual solvent in the product was removed by drying in vacuo at 80 °C for 6 hours to obtain the end-functionalized lactide copolymer B2 with a molecular weight of about 120 kg / mol.

[0084] The polymerization methods of the lactide copolymer A4 and the end-functionalized lactide copolymer B4 are similar to those of A2 and B2, and D-lactide can be used instead of ε-caprolactone; the structure of the lactide copolymer A4 is with a molecular weight of about 150 kg / mol, and the structure of the end-functionalized lactide copolymer B4 is with a molecular weight of about 150 kg / mol.

[0085] Example 1

[0086] The preparation method of the multilayer modified polylactic acid film comprises the following steps:

[0087] S1. Mix 7 g of lactide copolymer A1, 35 g of end - functionalized lactide copolymer B1, antioxidant 1010, and nucleating agent TMC306, and put the mixture I into a Banbury mixer (model RM - 200C). Mix at a temperature of 170 °C and a rotational speed of 50 rpm for 5 min to obtain a blend. Among them, the polymerization monomers of lactide copolymer A1 are L - lactide and ε - caprolactone, and the molar ratio is 1:0.3. The mass ratio of lactide copolymer A1 to end - functionalized lactide copolymer B1 is 1:5. Based on the total mass of the modified material and the polylactic acid, the addition amount of antioxidant 1010 is 0.1 wt%, and the addition amount of nucleating agent TMC306 is 0.1 wt%. Extrude and pelletize the blend to obtain the modified material.

[0088] S2. Prepare a modified polylactic acid film by the three - layer co - extrusion casting method. Among them, there are two layers of modified layers made of the modified material, and the two modified layers are respectively arranged on the upper and lower sides of the polylactic acid substrate layer. The polylactic acid substrate layer is made of L - polylactic acid. The extrusion temperature of the modified layer is 180 °C, and the thickness is 2 μm. The extrusion temperature of the polylactic acid substrate layer is 190 °C, and the thickness is 20 μm. The thickness ratio of the modified layer to the polylactic acid substrate layer is 1:10.

[0089] Example 2

[0090] Prepare a multi - layer modified polylactic acid film according to the method of Example 1, except that the mass ratio of lactide copolymer A1 to end - functionalized lactide copolymer B1 is 1:1.

[0091] Example 3

[0092] Prepare a multi - layer modified polylactic acid film according to the method of Example 1, except that the mass ratio of lactide copolymer A1 to end - functionalized lactide copolymer B1 is 1:2.3.

[0093] Example 4

[0094] The preparation method of the multi - layer modified polylactic acid film includes the following steps:

[0095] S1. Mix 7 g of lactide copolymer A2, 2.8 g of end-functionalized lactide copolymer B2, antioxidant 1035 and nucleating agent TMC300, put them into an internal mixer (model: RM-200C), and mix them for 5 minutes at a temperature of 180° C. and a rotation speed of 50 rpm to obtain a blend; wherein the polymerization monomers of the lactide copolymer A2 are L-lactide and ε-caprolactone, the molar ratio is 1:0.4, the mass ratio of the lactide copolymer A2 to the end-functionalized lactide copolymer B2 is 1:0.5, and based on the total mass of the modified material and the polylactic acid, the added amount of the antioxidant 1035 is 0.1 wt%, and the added amount of the nucleating agent TMC300 is 0.1 wt%; the blend is extruded and granulated to obtain a modified material.

[0096] S2. A modified polylactic acid film is prepared by a three-layer co-extrusion casting method, wherein the modified layer is provided with two layers using the modified material as the raw material, and the two modified layers are respectively provided on the upper and lower sides of the polylactic acid substrate layer; the polylactic acid substrate layer is made of left-handed polylactic acid as the raw material; the extrusion temperature of the modified layer is 190°C, and the thickness is 2.5μm; the extrusion temperature of the polylactic acid substrate layer is 200°C, and the thickness is 20μm; the thickness ratio of the modified layer to the polylactic acid substrate layer is 1:8.

[0097] Example 5

[0098] A multilayer modified polylactic acid film was prepared according to the method of Example 4, except that the mass ratio of the lactide copolymer A2 to the end-group functionalized lactide copolymer B2 was 1:1.

[0099] Example 6

[0100] A multilayer modified polylactic acid film was prepared according to the method of Example 4, except that the mass ratio of the lactide copolymer A2 to the end-group functionalized lactide copolymer B2 was 1:2.3.

[0101] Example 7

[0102] A multilayer modified polylactic acid film was prepared according to the method of Example 4, except that the lactide copolymer A2 and the terminal functionalized lactide copolymer B2 were replaced by the lactide copolymer A1 and the terminal functionalized lactide copolymer B1.

[0103] Example 8

[0104] A multilayer modified polylactic acid film was prepared according to the method of Example 1, except that the polymerization monomers of the lactide copolymer A1 were L-lactide and ε-caprolactone, and the molar ratio of L-lactide to ε-caprolactone was 1:0.1.

[0105] Example 9

[0106] The preparation method of the multilayer modified polylactic acid film comprises the following steps:

[0107] S1. Mix 7 g of lactide copolymer A3, 63 g of end - group functionalized lactide copolymer B3, antioxidant 1010, and nucleating agent TMB5, and put the mixture I into a Banbury mixer (model RM - 200C). Mix for 5 min at a temperature of 190 °C and a rotation speed of 50 rpm to obtain a blend. Among them, the polymerization monomers of lactide copolymer A3 are L - lactide and D - lactide, with a molar ratio of 1:0.1. The mass ratio of lactide copolymer A3 to end - group functionalized lactide copolymer B3 is 1:9. Based on the total mass of the modified material and the polylactic acid, the addition amount of antioxidant 1010 is 0.1 wt%, and the addition amount of nucleating agent TMB5 is 0.1 wt%. Extrude and pelletize the blend to obtain the modified material.

[0108] S2. Prepare the modified polylactic acid film by the three - layer co - extrusion casting method. Among them, there are two layers of modified layers with the modified material as the raw material, and the two modified layers are respectively arranged on the upper and lower sides of the polylactic acid substrate layer. The polylactic acid substrate layer uses left - handed polylactic acid as the raw material. The extrusion temperature of the modified layer is 200 °C, and the thickness is 3 μm. The extrusion temperature of the polylactic acid substrate layer is 210 °C, and the thickness is 21 μm. The thickness ratio of the modified layer to the polylactic acid substrate layer is 1:7.

[0109] Example 10

[0110] Prepare the multi - layer modified polylactic acid film according to the method of Example 9, except that the mass ratio of lactide copolymer A3 to end - group functionalized lactide copolymer B3 is 1:2.3.

[0111] Example 11

[0112] Prepare the multi - layer modified polylactic acid film according to the method of Example 9, except that lactide copolymer A3 and end - group functionalized lactide copolymer B3 are replaced by lactide copolymer A4 and end - group functionalized lactide copolymer B4.

[0113] Example 12

[0114] Prepare the multi - layer modified polylactic acid film according to the method of Example 1, except that lactide copolymer A1 and end - group functionalized lactide copolymer B1 are replaced by lactide copolymer A4 and end - group functionalized lactide copolymer B4.

[0115] Example 13

[0116] Prepare the multi - layer modified polylactic acid film according to the method of Example 12, except that the mass ratio of lactide copolymer A4 to end - group functionalized lactide copolymer B4 is 1:1.

[0117] Example 14

[0118] Prepare a multilayer modified polylactic acid film according to the method of Example 12, except that the mass ratio of lactide copolymer A4 to end-functionalized lactide copolymer B4 is 1:2.3.

[0119] Example 15

[0120] Prepare a multilayer modified polylactic acid film according to the method of Example 1, except that the mass ratio of lactide copolymer to end-functionalized lactide copolymer is changed from 1:5 to 1:0.1.

[0121] Example 16

[0122] Prepare a multilayer modified polylactic acid film according to the method of Example 1, except that the molar ratio of L-lactide to the ε-caprolactone is 1:0.05.

[0123] Example 17

[0124] Prepare a multilayer modified polylactic acid film according to the method of Example 1, except that the thickness of the modified layer is 1 μm.

[0125] Example 18

[0126] Prepare a multilayer modified polylactic acid film according to the method of Example 1, except that the thickness of the polylactic acid substrate layer is 28 μm.

[0127] Comparative Example 1

[0128] Prepare a multilayer modified polylactic acid film according to the method of Example 1, except that the mass ratio of lactide copolymer A1 to end-functionalized lactide copolymer B1 is 1:0.

[0129] Comparative Example 2

[0130] Prepare a multilayer modified polylactic acid film according to the method of Example 4, with the mass ratio of lactide copolymer A2 to end-functionalized lactide copolymer B2 being 1:0.

[0131] Test Example

[0132] The performance tests of haze, gloss and heat-sealing strength were carried out on the modified polylactic acid films prepared in the above examples and comparative examples, and the test results are shown in Table 1. The haze test was determined according to the method in "Determination of Transmittance and Haze of Transparent Plastics (GB / T 2410-2008)"; the gloss test was determined according to the method in "Test Method for Specular Gloss of Plastics (GB / T 8807-1988)"; the heat-sealing strength test was determined according to the method in "Biaxially Oriented Polypropylene (BOPP) Films for General Purposes (GB / T 10003-2008)". Samples with a length of 15 cm and a width of 1.5 cm were randomly cut in the width direction of the modified polylactic acid film, and then the samples were heat-sealed. The heat-sealing temperature was set at 135 °C, the heat-sealing pressure was 0.18 MPa, and the heat-sealing time was 1 s. The two ends of the sample were clamped by clamps with a spacing of 100 mm on a tensile machine and stretched at a speed of 100 mm / min, and the maximum load when the sample was broken was recorded. The average value of the maximum loads of 5 samples was used as the heat-sealing strength of the material at 135 °C. Judgment of the initial heat-sealing temperature: If the heat-sealing strength of the sample at a certain temperature is greater than or equal to 3 N / 15 mm, and the heat-sealing strength of the sample is less than 3 N / 15 mm at a temperature lower than this temperature, then this temperature is the initial heat-sealing temperature of the sample.

[0133] Table 1

[0134]

[0135]

[0136] It can be seen from the results in Table 1 that the examples of the present invention have significantly better performance than the comparative examples. Comparing Examples 1-3 and 4-6, increasing the content of the end-functionalized lactide copolymer, the heat-sealing strength of the polylactic acid film increases. Comparing Example 4 and Example 7, Example 9 and Example 11, when the polylactic acid film contains a star-shaped lactide copolymer, its heat-sealing strength is higher.

[0137] The modified polylactic acid film provided by the present invention comprises a polylactic acid substrate layer and a modified layer; the modified layer contains a lactide copolymer and an end-functionalized lactide copolymer capable of forming hydrogen bond crosslinking. Through the interaction between the end-functionalized lactide copolymers, the combination between the modified layers is firmer when the prepared polylactic acid film is heat-sealed, and thus the modified polylactic acid film has the advantage of high heat-sealing strength. Moreover, increasing the content of the end-functionalized lactide, the heat-sealing strength of the polylactic acid film increases. Comparing Example 4 and Example 7, Example 9 and Example 11, when the heat-sealing layer contains the same mass fraction of the end-functionalized lactide copolymer, the heat-sealing strength of the star-shaped lactide copolymer is higher because the hydrogen bond density in the star-shaped lactide copolymer is higher.

[0138] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A modified polylactic acid film, characterized in that: The film comprises a polylactic acid substrate layer and a modified layer; the modified layer contains a lactide copolymer and an end-group functionalized lactide copolymer capable of forming hydrogen bond crosslinking.

2. The modified polylactic acid film according to claim 1, characterized in that: In the modified layer, the mass ratio of the lactide copolymer to the terminal functionalized lactide copolymer is 1:0.4-9.

3. The modified polylactic acid film according to claim 1, characterized in that: The lactide copolymer is selected from At least one of; the end-functionalized lactide copolymer is selected from At least one of , wherein m is an integer greater than 1; n is an integer greater than 1.

4. The modified polylactic acid film according to claim 3, characterized in that: The polylactic acid substrate layer is left-handed polylactic acid and / or right-handed polylactic acid.

5. The modified polylactic acid film according to any one of claims 1 to 3, characterized in that: The modified layer is provided with two layers, and the two modified layers are respectively provided on the upper and lower sides of the polylactic acid substrate layer; Preferably, the thickness of the modified layer is 2-3 μm, and the thickness of the polylactic acid substrate layer is 20-22 μm; Preferably, the thickness ratio of the modified layer to the polylactic acid substrate layer is 1:7-10.

6. A method for preparing a modified polylactic acid film, characterized in that: The steps include: S1. Mixing raw material components to obtain a modified material; the raw material components contain a lactide copolymer and an end-functionalized lactide copolymer capable of forming hydrogen bond crosslinks; S2. Prepare a multilayer film including a polylactic acid substrate layer and a modified material layer by a casting method using polylactic acid and the modified material.

7. The preparation method according to claim 6, characterized in that: In the modified material, the mass ratio of the lactide copolymer to the terminal functionalized lactide copolymer is 1:0.4-9; Preferably, the molecular weight of the lactide copolymer and the end-functionalized lactide copolymer is 100-150 kg / mol; Preferably, the mixing conditions of the modified material at least include: temperature of 160-200° C., rotation speed of 50-100 rpm, and time of 5-10 min.

8. The preparation method according to claim 6 or 7, characterized in that: The polymerizable monomer of the lactide copolymer is selected from at least one of L-lactide, ε-caprolactone and D-lactide; Preferably, the polymerized monomer of the lactide copolymer is a combination of L-lactide and ε-caprolactone or a combination of L-lactide and D-lactide; Preferably, the molar ratio of the L-lactide to the ε-caprolactone is 1:0.1-0.5, and the molar ratio of the L-lactide to the D-lactide is 1:0.1-0.

5.

9. The preparation method according to claim 6 or 7, characterized in that: The raw material components also include a nucleating agent, and the nucleating agent is selected from at least one of TMC300, TMC306, TMC328 and TMB5; Preferably, the amount of the nucleating agent added is 0.1-0.5wt% based on the total mass of the modified material and the polylactic acid; Preferably, the conditions of the casting method at least include: the extrusion temperature of the modified layer is 160-200°C, and the extrusion temperature of the polylactic acid based layer is 180-220°C.

10. The preparation method according to claim 6 or 7, characterized in that: The lactide copolymer is selected from At least one of; the end-functionalized lactide copolymer is selected from At least one of , wherein m is an integer greater than 1; n is an integer greater than 1.

Images