High-barrier polylactic acid film and its preparation method

Abstract

The present invention discloses a high-barrier polylactic acid film and a preparation method thereof, relating to the technical field of polylactic acid films. The technical solution is as follows: It is composed of an inner layer, a middle layer and an outer layer. Each layer includes the following components in parts by weight: The inner layer includes 10-15 parts of PLLA, 68-77 parts of PBAT, 10-20 parts of calcium carbonate and 1-2 parts of compatibilizer; the middle layer includes 5-10 parts of PLLA, 5-10 parts of PDLA, 80.5-85 parts of PBAT and 1-2 parts of compatibilizer; the outer layer includes 10-15 parts of PLLA, 80-85 parts of PBAT, 1-3 parts of talc powder and 1-2 parts of compatibilizer; wherein, the number-average molecular weight of PLLA and PDLA is 4-6 kD. By adding PLLA and PDLA to the middle layer of the polylactic acid film and blending them, the present invention significantly improves the barrier performance of the polylactic acid film.

Description

Technical Field

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

[0002] As the foundation of modern scientific and technological development, the wide application of emerging materials plays a decisive role in solving environmental pollution and energy depletion. Polylactic acid (PLA) degradable plastics, as a type of environmentally friendly emerging material, have the advantages of wide raw material sources, green environmental protection, low pollution in the production process, low energy consumption, non-polluting decomposition end products, and recyclability.

[0003] Polylactic acid has good biocompatibility and biodegradability and can replace petroleum-based materials in applications such as food packaging, medical and health, and tissue engineering materials. However, its disadvantages such as poor mechanical properties and gas barrier properties make it difficult to meet the needs of actual production and life. To address this drawback, researchers have developed a series of technologies to improve the barrier properties of PLA films, such as nano-filler composite technology, surface coating modification, and biaxial stretching. Chinese invention patent CN115948037A discloses a high-barrier biodegradable material, its preparation method and application. Specifically, PLA, poly(methyl ethylene carbonate), cellulose powder, nano-crystalline cellulose masterbatch, and compatibilizer are melt-blown into a film. At the same time, poly(methyl ethylene carbonate) and nano-crystalline cellulose are used to modify PLA, which not only effectively improves the barrier effect of PLA against water vapor and oxygen but also effectively improves the mechanical properties of PLA, making it have high tensile strength, high tear strength, and high toughness. Different from the above patent, Chinese invention patent CN118085697A discloses a PLA barrier film with a multi-layer stereocrystalline layer and its preparation method. In this patent, L-polylactic acid (PLLA) and D-polylactic acid (PDLA) are alternately coated, and at the same time, cellulose nanocrystals are used for reinforcement. The PLA barrier film is obtained by layer-by-layer self-assembly, and the PLA barrier film is heat-treated to induce a multi-layer stereocrystalline layer, improving its barrier properties and making it have the characteristics of biosecurity, degradability, and high barrier property, which can be used as packaging materials in multiple fields. Although this method can improve the barrier properties of the film material, the operation is complex and the efficiency is low. Chinese invention patent CN117621499A provides a high-strength, tough, and high-barrier polylactic acid-based degradable film, its preparation method and application. Crystalline PLA and a flexible degradable polymer are laminated and compounded, and then subjected to biaxial stretching and heat setting treatments to obtain a high-strength, tough, and high-barrier polylactic acid-based degradable film.

[0004] Although the above patents have improved the barrier properties of PLA films through nano-filler composite technology, surface coating modification, and biaxial stretching, there are generally problems such as easy agglomeration of nano-fillers, high cost, complex processes, and excessive equipment requirements, resulting in low actual production efficiency of PLA high-barrier films. Therefore, exploring a new path to improve the barrier properties of PLA films with low cost and high efficiency has important development potential for enhancing benefits and expanding application fields. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: overcoming the deficiencies of the prior art, providing a high-barrier polylactic acid film and its preparation method. By adding PLLA and PDLA to the middle layer of the polylactic acid film and blending them, the barrier properties of the polylactic acid film are significantly improved.

[0006] The technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a high-barrier polylactic acid film, which consists of an inner layer, a middle layer, and an outer layer. Each layer includes the following components in parts by weight: the inner layer includes 10-15 parts of PLLA, 68-77 parts of PBAT, 10-20 parts of calcium carbonate, and 1-2 parts of a compatibilizer; the middle layer includes 5-10 parts of PLLA, 5-10 parts of PDLA, 80.5-85 parts of PBAT, and 1-2 parts of a compatibilizer; the outer layer includes 10-15 parts of PLLA, 80-85 parts of PBAT, 1-3 parts of talc powder, and 1-2 parts of a compatibilizer; wherein, the number-average molecular weight of PLLA and PDLA is 4-6 kD.

[0008] Preferably, the compatibilizer is maleic anhydride-modified PBAT.

[0009] Preferably, the thickness of the inner layer accounts for 20-30% of the thickness of the high-barrier polylactic acid film, the thickness of the middle layer accounts for 40-60% of the thickness of the high-barrier polylactic acid film, and the thickness of the outer layer accounts for 20-30% of the thickness of the high-barrier polylactic acid film

[0010] On the other hand, the present invention provides a preparation method of the above high-barrier polylactic acid film. The components in the inner layer, middle layer, and outer layer are granulated respectively by a twin-screw extruder water-cooling unit, and then blown into a film by a three-layer co-extrusion extruder to obtain a high-barrier polylactic acid film.

[0011] Preferably, the screw temperature of the three-layer co-extrusion extruder is 160-210 °C.

[0012] Preferably, the co-blending temperature of the components in the middle layer before granulation is ≥190 °C.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. In the present invention, the hydrogen bond interaction between two optical isomers of PLA (i.e., PLLA and PDLA) added in the middle layer forms a PLA stereocomplex (PLA-SC). PLA-SC not only has extremely strong intermolecular forces, making the chain segments more closely arranged, but also has an obvious nucleation effect, making the crystallization performance of the polylactic acid chain segments more excellent. Improving the crystallinity has a positive effect on enhancing the barrier performance of the polylactic acid film. Therefore, by blending PLLA and PDLA, the present invention significantly improves the barrier performance of the polylactic acid film.

[0015] 2. The number-average molecular weight of PLLA and PDLA used in the present invention is 4 - 6 kDa. The low-molecular-weight polylactic acid chain segments are more conducive to the formation of PLA-SC. And when the mass ratio of PLLA to PDLA is 1:1, the formation of PLA-SC dominates, and the barrier performance of the prepared polylactic acid film is better.

[0016] 3. The present invention adopts the physical blending method of PLA and PBAT. By utilizing the high strength and modulus of PLA and the excellent tensile properties and flexibility of PBAT, the mechanical properties of the polylactic acid film can be adjusted by simply controlling the addition amount. The present invention only adopts simple physical blending of materials, avoiding complex steps such as block or grafting, and does not require high requirements for equipment. While improving the barrier performance of the polylactic acid film, it also has the advantages of being simple and easy to implement and suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic structural diagram of the high-barrier polylactic acid film of the present invention.

[0018] In the figure, 1, inner layer; 2, middle layer; 3, outer layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention.

[0020] The raw materials used in the following examples and comparative examples are all conventional commercially available raw materials unless otherwise specified.

[0021] Some of the raw materials used in the following examples and comparative examples are as follows:

[0022] PLLA, Zhejiang Hisun Biomaterials Co., Ltd.;

[0023] PDLA, Suzhou Qihang Biotechnology Co., Ltd.;

[0024] PBAT, Huafeng Group Co., Ltd.;

[0025] Maleic anhydride modified PBAT, Nanjing Berton New Materials Co., Ltd.

[0026] Example 1

[0027] As Figure 1 shown, the high-barrier polylactic acid film of this example includes an inner layer 1, a middle layer 2, and an outer layer 3. The inner layer 1, middle layer 2, and outer layer 3 respectively include the following components in parts by weight:

[0028] The inner layer 1 includes 15 parts of 5 kDa PLLA, 69 parts of PBAT, 15 parts of calcium carbonate, and 1 part of maleic anhydride modified PBAT;

[0029] The middle layer 2 includes 10 parts of 5 kDa PLLA, 5 parts of 5 kDa PDLA, 83.5 parts of PBAT, and 1.5 parts of maleic anhydride modified PBAT;

[0030] The outer layer 3 includes 15 parts of 6 kDa PLLA, 80.5 parts of PBAT, 3 parts of talc powder, and 1.5 parts of maleic anhydride modified PBAT.

[0031] After mixing each layer of components evenly, they are granulated by co-blending with a twin-screw extruder. The co-blending temperature is 190 °C. Subsequently, they are blown into a film by a three-layer co-extrusion extruder. The screw temperatures of the outer layer 3 and the inner layer 1 are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperature of the middle layer 2 is 180 °C, 190 °C, 200 °C, 200 °C, 200 °C, and the die head temperature is 190 °C.

[0032] The thickness of the finally prepared high-barrier polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 16 μm, the thickness of the middle layer 2 is 48 μm, and the thickness of the outer layer 3 is 16 μm.

[0033] Example 2

[0034] As Figure 1 shown, the high-barrier polylactic acid film of this example includes an inner layer 1, a middle layer 2, and an outer layer 3. The inner layer 1, middle layer 2, and outer layer 3 respectively include the following components in parts by weight:

[0035] The inner layer 1 includes 15 parts of 4 kDa PLLA, 68.5 parts of PBAT, 15 parts of calcium carbonate, and 1.5 parts of maleic anhydride modified PBAT;

[0036] The middle layer 2 includes 7.5 parts of 4 kDa PLLA, 7.5 parts of 4 kDa PDLA, 83 parts of PBAT, and 2 parts of maleic anhydride modified PBAT;

[0037] The outer layer 3 includes 15 parts of 5 kDa PLLA, 81.5 parts of PBAT, 2 parts of talc powder, and 1.5 parts of maleic anhydride modified PBAT.

[0038] After mixing the components of each layer evenly, melt blending and pelletizing were carried out using a twin-screw extruder at a blending temperature of 195 °C. Subsequently, film blowing was carried out using a three-layer co-extrusion extruder. The screw temperatures of the outer layer 3 and the inner layer 1 were 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperatures of the middle layer 2 were 180 °C, 200 °C, 210 °C, 210 °C, 210 °C, and the die head temperature was 200 °C.

[0039] The thickness of the finally prepared high-barrier polylactic acid film was 80 μm, where the thickness of the inner layer 1 was 20 μm, the thickness of the middle layer 2 was 40 μm, and the thickness of the outer layer 3 was 20 μm.

[0040] Example 3

[0041] As Figure 1 shown, the high-barrier polylactic acid film of this example includes an inner layer 1, a middle layer 2, and an outer layer 3. The inner layer 1, the middle layer 2, and the outer layer 3 respectively include the following components in parts by weight:

[0042] The inner layer 1 includes 15 parts of 5 kDa PLLA, 68 parts of PBAT, 15 parts of calcium carbonate, and 2 parts of maleic anhydride-modified PBAT;

[0043] The middle layer 2 includes 5 parts of 6 kDa PLLA, 10 parts of 6 kDa PDLA, 83.5 parts of PBAT, and 1.5 parts of maleic anhydride-modified PBAT;

[0044] The outer layer 3 includes 15 parts of 5 kDa PLLA, 80 parts of PBAT, 3 parts of talcum powder, and 2 parts of maleic anhydride-modified PBAT.

[0045] After mixing the components of each layer evenly, melt blending and pelletizing were carried out using a twin-screw extruder at a blending temperature of 190 °C. Subsequently, film blowing was carried out using a three-layer co-extrusion extruder. The screw temperatures of the outer layer 3 and the inner layer 1 were 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperatures of the middle layer 2 were 180 °C, 200 °C, 210 °C, 210 °C, 210 °C, and the die head temperature was 200 °C.

[0046] The thickness of the finally prepared high-barrier polylactic acid film was 80 μm, where the thickness of the inner layer 1 was 24 μm, the thickness of the middle layer 2 was 32 μm, and the thickness of the outer layer 3 was 24 μm.

[0047] Example 4

[0048] As Figure 1 shown, the high-barrier polylactic acid film of this example includes an inner layer 1, a middle layer 2, and an outer layer 3. The inner layer 1, the middle layer 2, and the outer layer 3 respectively include the following components in parts by weight:

[0049] The inner layer 1 comprises 12 parts of 5 kDa PLLA, 77 parts of PBAT, 10 parts of calcium carbonate, and 1 part of maleic anhydride modified PBAT;

[0050] The middle layer 2 comprises 5 parts of 5 kDa PLLA, 9 parts of 5 kDa PDLA, 85 parts of PBAT, and 1 part of maleic anhydride modified PBAT;

[0051] The outer layer 3 comprises 10 parts of 5 kDa PLLA, 85 parts of PBAT, 3 parts of talc powder, and 2 parts of maleic anhydride modified PBAT.

[0052] After mixing the components of each layer evenly, they are melt - blended and pelletized using a twin - screw extruder at a blending temperature of 200 °C, and then blown into a film using a three - layer co - extrusion extruder. Among them, the screw temperatures of the outer layer 3 and the inner layer 1 are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperatures of the middle layer 2 are 180 °C, 200 °C, 210 °C, 210 °C, 210 °C, and the die head temperature is 200 °C.

[0053] The thickness of the finally prepared high - barrier polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 24 μm, the thickness of the middle layer 2 is 32 μm, and the thickness of the outer layer 3 is 24 μm.

[0054] Example 5

[0055] As Figure 1 shown, the high - barrier polylactic acid film of this example comprises an inner layer 1, a middle layer 2, and an outer layer 3, and the inner layer 1, the middle layer 2, and the outer layer 3 respectively comprise the following components in parts by weight:

[0056] The inner layer 1 comprises 10 parts of 5 kDa PLLA, 69 parts of PBAT, 20 parts of calcium carbonate, and 1 part of maleic anhydride modified PBAT;

[0057] The middle layer 2 comprises 10 parts of 4 kDa PLLA, 8.5 parts of 4 kDa PDLA, 80.5 parts of PBAT, and 1 part of maleic anhydride modified PBAT;

[0058] The outer layer 3 comprises 13 parts of 5 kDa PLLA, 85 parts of PBAT, 1 part of talc powder, and 1 part of maleic anhydride modified PBAT.

[0059] After mixing the components of each layer evenly, they are melt - blended and pelletized using a twin - screw extruder at a blending temperature of 200 °C, and then blown into a film using a three - layer co - extrusion extruder. Among them, the screw temperatures of the outer layer 3 and the inner layer 1 are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperatures of the middle layer 2 are 180 °C, 200 °C, 210 °C, 210 °C, 210 °C, and the die head temperature is 200 °C.

[0060] The thickness of the finally prepared high-barrier polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 24 μm, the thickness of the middle layer 2 is 32 μm, and the thickness of the outer layer 3 is 24 μm.

[0061] Comparative Example 1

[0062] In the polylactic acid film of Comparative Example 1, the inner layer 1, the middle layer 2, and the outer layer 3 respectively include the following components in parts by weight:

[0063] The inner layer 1 includes 100 parts of 5 kDa PLLA;

[0064] The middle layer 2 includes 100 parts of 5 kDa PLLA;

[0065] The outer layer 3 includes 100 parts of 5 kDa PLLA.

[0066] After mixing the components of each layer evenly, they are melt-blended and pelletized using a twin-screw extruder, and the blending temperature is 190 °C. The components of the inner layer 1, the middle layer 2, and the outer layer 3 are blown into a film using a three-layer co-extrusion extruder, where the screw temperatures are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C, and the die head temperature is 175 °C.

[0067] The thickness of the finally prepared polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 16 μm, the thickness of the middle layer 2 is 48 μm, and the thickness of the outer layer 3 is 16 μm.

[0068] Comparative Example 2

[0069] In the polylactic acid film of Comparative Example 2, the inner layer 1, the middle layer 2, and the outer layer 3 respectively include the following components in parts by weight:

[0070] The inner layer 1 includes 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT;

[0071] The middle layer 2 includes 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT;

[0072] The outer layer 3 includes 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT.

[0073] After mixing the components of each layer evenly, they are melt-blended and pelletized using a twin-screw extruder, and the blending temperature is 190 °C. The components of the inner layer 1, the middle layer 2, and the outer layer 3 are blown into a film using a three-layer co-extrusion extruder, where the screw temperatures are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C, and the die head temperature is 175 °C.

[0074] The thickness of the finally prepared polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 16 μm, the thickness of the middle layer 2 is 48 μm, and the thickness of the outer layer 3 is 16 μm.

[0075] Comparative Example 3

[0076] In the polylactic acid film of Comparative Example 3, the inner layer 1, the middle layer 2, and the outer layer 3 respectively comprise the following components in parts by weight:

[0077] The inner layer 1 comprises 15 parts of 5 kDa PLLA, 68.5 parts of PBAT, 15 parts of calcium carbonate, and 1.5 parts of maleic anhydride-modified PBAT;

[0078] The middle layer 2 comprises 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT;

[0079] The outer layer 3 comprises 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT.

[0080] After mixing the components of each layer evenly, they are melt blended and pelletized using a twin-screw extruder at a blending temperature of 190 °C. The components of the inner layer 1, the middle layer 2, and the outer layer 3 are blown into a film using a three-layer co-extrusion extruder, where the screw temperatures are 160 °C, 170 °C, 180 °C, 180 °C, 180 °C, and the die head temperature is 175 °C.

[0081] The thickness of the finally prepared polylactic acid film is 80 μm, where the thickness of the inner layer 1 is 16 μm, the thickness of the middle layer 2 is 48 μm, and the thickness of the outer layer 3 is 16 μm.

[0082] Comparative Example 4

[0083] In the polylactic acid film of Comparative Example 4, the inner layer 1, the middle layer 2, and the outer layer 3 respectively comprise the following components in parts by weight:

[0084] The inner layer 1 comprises 15 parts of 5 kDa PLLA, 68.5 parts of PBAT, 15 parts of calcium carbonate, and 1.5 parts of maleic anhydride-modified PBAT;

[0085] The middle layer 2 comprises 15 parts of 5 kDa PLLA, 83 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT;

[0086] The outer layer 3 comprises 15 parts of 6 kDa PLLA, 80 parts of PBAT, 3 parts of talc powder, and 2 parts of maleic anhydride-modified PBAT.

[0087] After mixing the components of each layer evenly, melt blending and pelletizing were carried out using a twin-screw extruder at a blending temperature of 190 °C. Subsequently, film blowing was performed using a three-layer co-extrusion extruder. The screw temperatures of the outer layer 3 and the inner layer 1 were 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperature of the middle layer 2 was 180 °C, 190 °C, 200 °C, 200 °C, 200 °C, and the die head temperature was 190 °C.

[0088] The thickness of the finally prepared polylactic acid film was 80 μm, among which the thickness of the inner layer 1 was 16 μm, the thickness of the middle layer 2 was 48 μm, and the thickness of the outer layer 3 was 16 μm.

[0089] Comparative Example 5

[0090] In the polylactic acid film of Comparative Example 5, the inner layer 1, the middle layer 2, and the outer layer 3 respectively included the following components in parts by weight:

[0091] The inner layer 1 included 10 parts of 5 kDa PLLA, 73.5 parts of PBAT, 15 parts of calcium carbonate, and 1.5 parts of maleic anhydride-modified PBAT;

[0092] The middle layer 2 included 10 parts of 5 kDa PLLA, 88 parts of PBAT, and 2 parts of maleic anhydride-modified PBAT;

[0093] The outer layer 3 included 10 parts of 5 kDa PLLA, 85 parts of PBAT, 3 parts of talc powder, and 2 parts of maleic anhydride-modified PBAT.

[0094] After mixing the components of each layer evenly, melt blending and pelletizing were carried out using a twin-screw extruder at a blending temperature of 190 °C. Subsequently, film blowing was performed using a three-layer co-extrusion extruder. The screw temperatures of the outer layer 3 and the inner layer 1 were 160 °C, 170 °C, 180 °C, 180 °C, 180 °C; the screw temperature of the middle layer 2 was 180 °C, 190 °C, 200 °C, 200 °C, 200 °C, and the die head temperature was 190 °C.

[0095] The thickness of the finally prepared polylactic acid film was 80 μm, among which the thickness of the inner layer 1 was 16 μm, the thickness of the middle layer 2 was 48 μm, and the thickness of the outer layer 3 was 16 μm.

[0096] Comparative Example 6

[0097] The difference from Example 1 was that the number-average molecular weight of PLLA was 3 kDa and the number-average molecular weight of PDLA was 3 kDa.

[0098] Comparative Example 7

[0099] The difference from Example 1 was that the number-average molecular weight of PLLA was 8 kDa and the number-average molecular weight of PDLA was 8 kDa.

[0100] Comparative Example 8

[0101] The difference from Example 1 is that after uniformly mixing the components of each layer, melt blending and pelletizing are carried out using a twin-screw extruder at a melt blending temperature of 175°C, and then blown into a film using a three-layer coextrusion extruder.

[0102] Comparative Example 9

[0103] The difference from Example 1 is that the middle layer 2 includes 64 parts of PLLA, 34.5 parts of PDLA, and 1.5 parts of maleic anhydride-modified PBAT.

[0104] Comparative Example 10

[0105] The difference from Example 1 is that the inner layer 1 includes 83.5 parts of PLLA, 15 parts of calcium carbonate, and 1.5 parts of maleic anhydride-modified PBAT; the outer layer 3 includes 95.5 parts of PLLA, 3 parts of talc powder, and 1.5 parts of maleic anhydride-modified PBAT.

[0106] Due to the high brittleness of PLLA, the film material of Comparative Example 10 cannot be stably blown out.

[0107] The mechanical properties and barrier properties of the polylactic acid films prepared in Examples 1-5 and Comparative Examples 1-10 were tested. The test results are shown in Tables 1-2. Among them, the tensile strength and tensile elongation were tested with reference to "GB / T 1040.3-2006 Plastics - Determination of tensile properties", and the oxygen permeability and water vapor permeability were tested with reference to "GB / T 1038-2000 Plastics - Film and sheeting - Gas transmission rate - Manometric method".

[0108] Table 1 Test results of the mechanical properties of the polylactic acid films prepared in Examples 1-5 and Comparative Examples 1-10

[0109]

[0110] Table 2 Test results of the barrier properties of the polylactic acid films prepared in Examples 1-5 and Comparative Examples 1-10

[0111]

[0112] As can be seen from Table 1, compared with Comparative Example 1, the transverse and longitudinal tensile strengths of the polylactic acid film of Comparative Example 2 both decreased significantly, from 48.43MPa and 56.76MPa to 21.13MPa and 37.27MPa, respectively; while the tensile elongation was significantly improved. The transverse and longitudinal tensile elongations of the polylactic acid film of Comparative Example 1 were only 3.5% and 2.5%, respectively, but the transverse and longitudinal tensile elongations of the polylactic acid film of Comparative Example 2 could reach 604.6% and 345.9%, respectively, which were increased by about 172 and 137 times. This is because after adding PBAT to Comparative Example 2, PLA and PBAT are physically blended, and the higher strength and modulus of PLA, the excellent tensile properties and flexibility of PBAT are utilized to enhance the elongation properties of the polylactic acid film through the entanglement of the chain segments, and the decrease in tensile strength is caused by the decrease in PLLA content.

[0113] Compared with Comparative Example 2, although the transverse and longitudinal tensile elongation of the polylactic acid film of Comparative Example 3 decreased slightly, especially the longitudinal tensile elongation decreased significantly, from 345.9% to 339.3%; but its longitudinal tensile strength increased slightly, from 37.27MPa to 37.63MPa. This shows that the addition of inorganic powder calcium carbonate affects the movement of PLLA and PBAT chain segments during stretching, thereby reducing the flexibility of the polylactic acid film. However, compared with Comparative Example 2, the smooth effect between the polylactic acid films of Comparative Example 3 is greatly improved, which significantly improves the use effect of the polylactic acid film. Compared with Comparative Example 3, Comparative Example 4 introduces talcum powder in the outer layer 3, which enhances the smooth effect of the outer layer 3 of the polylactic acid film, but hardly affects the mechanical properties of the polylactic acid film.

[0114] The above solves the problems of poor mechanical properties and poor practical use effect of the polylactic acid film in comparative example 1 by adding PBAT, calcium carbonate and talcum powder. Unlike comparative example 4, Example 1 introduces PDLA in the middle layer 2, so that the transverse and longitudinal tensile strength and tensile elongation of the polylactic acid film are significantly improved, and the water vapor permeability and oxygen permeability of the polylactic acid film are significantly reduced. This is mainly because when PLLA and PDLA are blended, PLLA and PDLA form PLA-SC through hydrogen bond interaction. The extremely strong intermolecular chain interaction between PLA-SC makes the chain segments more closely arranged, and has an obvious nucleation effect, making the crystallization performance of the polylactic acid segment more excellent, and improving the crystallinity has a positive effect on enhancing the barrier performance of the polylactic acid film. Therefore, the barrier performance of the polylactic acid film can be significantly improved by blending PLLA and PDLA.

[0115] In addition, compared with Comparative Example 4, the tensile strength and barrier properties of the polylactic acid film in Example 2 were further improved. Specifically, the transverse and longitudinal tensile strengths increased from 22.35 MPa and 37.84 MPa to 25.77 MPa and 40.37 MPa respectively, and the transverse and longitudinal tensile elongation rates increased from 606.1% and 335.3% to 621.3% and 360.3% respectively; in particular, the water vapor transmission rate and oxygen transmission rate of the polylactic acid film decreased significantly, from 207.5087 g / (m 2 ·day) and 1171.896 cm 3 / (m 2 ·24 h·0.1 MPa) to 176.8128 g / (m 2 ·day) and 821.734 cm 3 / (m 2 ·24 h·0.1 MPa). This is attributed to the fact that the low-molecular-weight polylactic acid chain segments in Example 2 are more conducive to the formation of PLA-SC. When the mass ratio of PDLA to PLLA in Middle Layer 2 is 1:1, the formation of PLA-SC dominates, which provides strong support for improving the barrier properties of the polylactic acid film. This is also demonstrated in Example 3. When the mass ratio of PLLA to PDLA exceeds 1:1 and reaches 1:2, both the tensile strength and tensile elongation rate, as well as the barrier properties, show varying degrees of decline. It can thus be proven that the mass ratio of PLLA to PDLA has a decisive effect on the barrier properties of the polylactic acid film. Especially when the mass ratio of the two is 1:1, the polylactic acid film has the optimal barrier properties.

[0116] Compared with Comparative Example 4, the transverse and longitudinal tensile strengths of the polylactic acid film in Comparative Example 5 decreased from 22.35 MPa and 37.84 MPa to 19.85 MPa and 35.48 MPa respectively, while the transverse and longitudinal tensile elongation rates increased from 606.1% and 335.5% to 605.8% and 325.6% respectively. This shows that by regulating the mass ratio of PLLA to PBAT, the mechanical properties of the polylactic acid film can be controlled, and the fundamental reason is the complementarity between the rigid segments of PLLA and the flexible segments of PBAT. On this basis, by comparing Example 1 and Example 2, it can be clearly found that when the mass ratio of PLLA to PDLA in the middle layer 2 is 1:1, both the tensile elongation rate and the barrier performance of the polylactic acid film are improved. This also proves again that when the mass ratio of PDLA to PLLA is 1:1, the formation of PLA-SC dominates, and the formation of PLA-SC plays an important role in improving the tensile elongation rate and barrier performance of the polylactic acid film. Most importantly, the preparation of the above polylactic acid film can be easily achieved only by changing the ratio during component blending, without special equipment, and can be industrially produced, which has a positive promoting effect on expanding the application range of the polylactic acid film.

[0117] Compared with Example 1, the barrier performance of the polylactic acid film in Comparative Examples 6-7 decreased significantly. The main reason is that when the molecular weights of PLLA and PDLA are too low or too high, it is not conducive to the formation of PLA-SC, resulting in a decrease in the barrier performance of the film material. At the same time, in Comparative Example 8, due to the relatively low blending temperature of the components in the middle layer 2, the formation of PLA-SC between molecular chains was affected, resulting in a decrease in the barrier performance of the film material to water vapor and oxygen. In addition, it can be seen from Comparative Examples 9-10 that PBAT must be added to all three layers of the polylactic acid film to reduce the problem of premature rupture caused by the large difference in the mechanical properties of the interlayer film materials and achieve the purpose of effectively regulating the mechanical strength of the film material.

Claims

1. High barrier polylactic acid film, characterized in that: The invention comprises an inner layer (1), a middle layer (2) and an outer layer (3), wherein each layer comprises the following components in parts by weight: the inner layer (1) comprises 10-15 parts of PLLA, 68-77 parts of PBAT, 10-20 parts of calcium carbonate and 1-2 parts of a compatibilizer; the middle layer (2) comprises 7.5 parts of PLLA, 7.5 parts of PDLA, 83 parts of PBAT and 2 parts of a compatibilizer; the outer layer (3) comprises 10-15 parts of PLLA, 80-85 parts of PBAT, 1-3 parts of talc and 1-2 parts of a compatibilizer; wherein the number average molecular weight of PLLA and PDLA is 4-6 kDa; The method for preparing a high-barrier polylactic acid film comprises the following steps: granulating the components of an inner layer (1), a middle layer (2) and an outer layer (3) respectively by a twin-screw extruder water cooling unit, and then blowing the components into a film by a three-layer co-extruder to obtain a high-barrier polylactic acid film; the blending temperature of the components of the middle layer (2) before granulation is ≥190°C.

2. The high barrier polylactic acid film according to claim 1, characterized in that: The compatibilizer is maleic anhydride modified PBAT.

3. The high barrier polylactic acid film according to claim 1, characterized in that: The thickness of the inner layer (1) accounts for 20-30% of the thickness of the high barrier polylactic acid film, the thickness of the middle layer (2) accounts for 40-60% of the thickness of the high barrier polylactic acid film, and the thickness of the outer layer (3) accounts for 20-30% of the thickness of the high barrier polylactic acid film.

4. The high barrier polylactic acid film according to claim 1, characterized in that: The screw temperature of the three-layer co-extrusion extruder is 160-210°C.

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