A heat-shrinkable polylactic acid preservative film and a method for preparing the same

Abstract

The application provides a heat-shrinkable polylactic acid preservative film and a preparation method thereof, and belongs to the technical field of fruit and vegetable preservative packaging materials. The film comprises an outer layer, a middle layer and an inner layer which are stacked in sequence. The outer layer raw material comprises polylactic acid, polyurethane synthesized by taking carbonate polyol as raw material and a compatilizer. The middle layer raw material comprises polylactic acid, polyurethane synthesized by taking polycaprolactone polyol as raw material and a compatilizer. The inner layer raw material comprises polylactic acid, polybutylene succinate and a compatilizer. The middle layer of the preservative film is prepared by blending polylactic acid and polyurethane synthesized by taking polycaprolactone polyol as raw material, has good oxygen resistance and carbon dioxide permeability, and by adjusting the blending ratio of polylactic acid and polyurethane synthesized by taking polycaprolactone polyol as raw material, preservative films with different CO2 / O2 permeation ratios can be prepared, which can meet the preservative requirements of different respiration intensities and different fruit and vegetable varieties, and can realize more stable gas regulation performance and improve the stability of the packaging effect.

Description

Technical Field

[0001] This invention belongs to the technical field of fruit and vegetable preservation packaging materials, and more specifically, it relates to a heat-shrinkable polylactic acid preservation film and its preparation method. Background Technology

[0002] Fresh fruits and vegetables continue to respire after being harvested, undergoing oxidation of nutrients, releasing heat, and producing metabolic byproducts such as water and carbon dioxide. At room temperature and when left untreated, the respiration rate of fruits and vegetables is high, causing them to age quickly and lose their edible value. In the sealed environment of ordinary food storage bags, the carbon dioxide produced by respiration cannot escape in time, or the bags do not effectively prevent oxygen from entering, both of which accelerate the aging process of fruits and vegetables.

[0003] Therefore, reducing the respiration rate of fruits and vegetables and establishing a suitable low-oxygen, low-carbon dioxide atmosphere are key to solving the problem of fruit and vegetable preservation. To this end, many methods and technologies for fruit and vegetable preservation have been developed, such as the commonly used modified atmosphere packaging (MAP). MAP involves placing food or other perishable items in sealed packaging bags and controlling the proportions of gases such as oxygen, carbon dioxide, and humidity within the packaging bag to extend their shelf life. MAP can slow down the oxidation and microbial growth of food, thereby maintaining its freshness, nutritional value, and taste. It is commonly used for preserving meat, seafood, fruits, and vegetables. Although MAP has been widely adopted, some problems and pain points still exist, mainly including the following aspects:

[0004] High packaging material costs: Modified atmosphere packaging requires the use of special materials with selective gas permeability, which are expensive and increase packaging costs.

[0005] The packaging process has high technical requirements: Modified atmosphere packaging requires precise gas control, and the packaging process has high technical requirements, requiring professional operators and equipment, which increases packaging costs and production difficulty.

[0006] Unstable packaging effect: The effect of modified atmosphere packaging is affected by a variety of factors, including packaging materials, packaging process, storage temperature, etc. The packaging effect will vary under different conditions, and the preservation effect is prone to instability.

[0007] Limited packaging size: Modified atmosphere packaging requires specialized packaging equipment and has limited packaging size, making it difficult to adapt to food and medicine of different shapes and sizes.

[0008] Environmental issues: Modified atmosphere packaging uses mostly plastic products, which cause some pollution to the environment and need to be recycled and disposed of.

[0009] In addition, different varieties of fruits and vegetables have very different levels of respiration. Currently, conventional plastic wrap generally has a fixed permeability to oxygen / carbon dioxide, which can be well suited to the preservation needs of a certain type of fruit or vegetable, but it is difficult to meet the preservation needs of different varieties of fruits and vegetables. Summary of the Invention

[0010] The purpose of this invention is to provide a heat-shrinkable polylactic acid (PLA) food preservation film and its preparation method, so as to solve at least one of the shortcomings of existing food preservation films or food preservation bags.

[0011] To achieve the above objectives, a first aspect of the present invention provides a heat-shrinkable polylactic acid (PLA) food preservation film, comprising an outer layer, a middle layer, and an inner layer stacked sequentially; the outer layer comprises PLA, polyurethane synthesized from carbonate polyol, and a compatibilizer; the middle layer comprises PLA, polyurethane synthesized from polycaprolactone polyol, and a compatibilizer; and the inner layer comprises PLA, polybutylene succinate, and a compatibilizer.

[0012] Furthermore, the mass ratio of the outer layer, the middle layer, and the inner layer is 0.5-1:7.5-8.5:0.5-1.

[0013] Furthermore, the mass ratio of polylactic acid to polyurethane synthesized from carbonate polyol in the outer layer is 40-70:30-60.

[0014] Furthermore, the mass ratio of polylactic acid to polyurethane synthesized from polycaprolactone polyol in the intermediate layer is 20-90:10-80.

[0015] Furthermore, the mass ratio of polylactic acid to polybutylene succinate in the inner layer is 95-97:3-5.

[0016] Furthermore, the polyurethane synthesized from carbonate polyol has a melt index of 10-15 g / 10 min.

[0017] Furthermore, the polyurethane synthesized from carbonate polyol is PPC-TPU T2.

[0018] Furthermore, the polyurethane synthesized from polycaprolactone polyol has a melt index of 15-20 g / 10 min.

[0019] Furthermore, the polyurethane synthesized from polycaprolactone polyol is CX-5190.

[0020] Furthermore, the melting point of the polybutylene succinate in the inner layer is 80-85°C.

[0021] Furthermore, an anti-fogging agent is added to the inner layer.

[0022] Furthermore, the antifogging agent is sorbitol palmitate, and the amount of the antifogging agent added to the inner layer is 0.3-0.5 wt%.

[0023] Furthermore, the compatibilizer in the outer layer, the middle layer and the inner layer is BASF ADR4468, and the addition ratio of the compatibilizer in each layer is 0.2-0.5wt%.

[0024] A second aspect of the present invention provides a method for preparing a heat-shrinkable polylactic acid (PLA) food preservation film, comprising the following steps:

[0025] The outer, middle, and inner layer raw materials are fed into three twin-screw extruders, melted at a temperature of 200-220℃, and extruded through a co-extrusion die to form a film with a three-layer co-extrusion structure.

[0026] The three-layer co-extruded film is heated to 60-90°C and stretched longitudinally and laterally with a stretching ratio of 3-5 times.

[0027] Compared with the prior art, the present invention has the following technical effects:

[0028] The present invention discloses a heat-shrinkable polylactic acid (PLA) food preservation film. The intermediate layer is made of a blend of PLA and polyurethane synthesized from polycaprolactone polyol. This intermediate layer exhibits excellent oxygen barrier and carbon dioxide permeability properties. By adjusting the blending ratio of PLA and polyurethane, different CO2 / O2 permeability ratios can be prepared to meet the preservation needs of different respiration intensities and different fruit and vegetable varieties. The outer layer is made of a blend of PLA and polyurethane synthesized from carbonate polyol. The addition of polyurethane synthesized from carbonate polyol as a water-blocking material to the outer layer gives the food preservation film excellent waterproof properties, preventing PLA hydrolysis and protecting the intermediate preservation layer, thus enabling the food preservation film to better perform its preservation function. The inner layer contains polybutylene succinate as a low-temperature heat-sealing material, allowing the food preservation film to be heat-sealed at temperatures as low as 90°C while maintaining a heat-sealing strength of over 5 N / 15 m.

[0029] The heat-shrinkable polylactic acid (PLA) preservation film of the present invention can adjust the blending ratio of the intermediate layer raw materials according to the preservation needs of fruits and vegetables, thereby adjusting the CO2 / O2 permeability ratio, achieving more stable gas conditioning performance, and improving the stability of packaging effect.

[0030] The present invention discloses a heat-shrinkable polylactic acid (PLA) preservation film, which uses PLA as the main material. The prepared preservation film is a bio-based biodegradable material, which meets the requirements of low-carbon, green and environmentally friendly development. Moreover, the production cost is low, it can meet the preservation needs of different fruit and vegetable varieties, and has better preservation performance.

[0031] The present invention provides a heat-shrinkable polylactic acid (PLA) food preservation film with low-temperature heat shrinkage properties, which can be used in a simple heat-sealing packaging process. It can be developed into modified atmosphere packaging for food of different shapes and sizes, suitable for pillow-type shrink packaging or household food preservation bags, to meet market demand and improve packaging adaptability, while reducing packaging difficulty and cost.

[0032] The present invention discloses a method for preparing a heat-shrinkable polylactic acid (PLA) food preservation film using a three-layer co-extrusion and biaxial stretching process. The preparation method is simple, and the prepared food preservation film has good and stable preservation and mechanical properties, meeting market demand. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a graph showing the change in CO2 content inside a preservation bag over storage time when the preservation film provided in Embodiment 10 of the present invention is used to preserve cherry tomatoes.

[0035] Figure 2 The graph shows the change in O2 content inside the preservation bag over storage time when the preservation film provided in Embodiment 10 of the present invention is used to preserve cherry tomatoes. Detailed Implementation

[0036] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0037] In this invention, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0038] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0039] In a first aspect, the present invention provides a heat-shrinkable polylactic acid (PLA) food preservation film, comprising an outer layer, a middle layer, and an inner layer stacked sequentially; the outer layer comprises polylactic acid (PLA), polyurethane (TPU) synthesized from carbonate polyol, and a compatibilizer; the middle layer comprises polylactic acid, polyurethane (TPU) synthesized from polycaprolactone polyol, and a compatibilizer; and the inner layer comprises polylactic acid, polybutylene succinate (PBS), and a compatibilizer.

[0040] In this embodiment of the invention, the middle layer serves as the key layer for preservation, the outer layer as the water-blocking layer, and the inner layer as the low-temperature heat-sealing layer. Preferably, the mass ratio of the outer, middle, and inner layers of the preservation film in this embodiment is 0.5-1:7.5-8.5:0.5-1 to ensure good preservation effect. In one specific embodiment, the mass ratio of the outer, middle, and inner layers of the preservation film is 1:8:1.

[0041] PLA readily absorbs water and hydrolyzes. In this embodiment of the invention, the water absorption of a pure PLA membrane (30 μm thick) was measured to be 180 g / (m²). 2 *d) To prevent PLA from absorbing water and hydrolyzing, leading to performance degradation, the outer layer of the food preservation film in this embodiment of the invention is modified by blending PLA with the water-blocking material TPU. Preferably, the mass ratio of PLA to TPU in the outer layer is 40-70:30-60 to ensure that the outer layer can play a good water-blocking role and give the food preservation film excellent water-blocking performance. After modification with the water-blocking material TPU, the water absorption of the outer film (30 μm thick) can be significantly reduced to 60 g / (m²). 2 Approximately *d). In addition, BASF ADR4468 is added to the outer layer as a compatibilizer, with the compatibilizer added at a ratio of 0.2-0.5 wt%.

[0042] The polyurethane synthesized from carbonate polyols in this embodiment of the invention has a melt index of 10-15 g / 10 min (2.16 kg, 180 °C). The polyurethane synthesized from carbonate polyols can be obtained from commercially available raw materials or synthesized directly. The polyurethane synthesized from carbonate polyols in this embodiment of the invention can be any one of the following: polyurethane synthesized from carbonate polyols and isocyanate MDI, polyurethane synthesized from carbonate polyols and isocyanate TDI, etc. Specifically, this embodiment of the invention selects polypropylene carbonate resin (product brand PPC-TPU T2, produced by Jiangsu Zhongke Jinlong Environmental Protection New Materials Co., Ltd.) as the water-blocking TPU modifier.

[0043] In this embodiment of the invention, the middle layer of the food preservation film is modified by blending PLA with a carbon dioxide permeable material. The carbon dioxide permeable material is a polyurethane synthesized from polycaprolactone polyol. The inherent oxygen barrier properties of the PLA in the middle layer of this invention endow the food preservation film with excellent oxygen barrier performance, while the introduction of the carbon dioxide permeable material (i.e., polyurethane synthesized from polycaprolactone polyol) endows the food preservation film with excellent carbon dioxide permeability, allowing the carbon dioxide generated inside the packaging due to the respiration of fruits and vegetables to be released in a timely manner.

[0044] Preferably, the mass ratio of PLA to TPU blend in the intermediate layer is 20-90:10-80. Different PLA to TPU blend ratios can yield preservation films with different CO2 / O2 permeability ratios. Therefore, by adjusting the mass ratio of PLA to TPU blend in the intermediate layer, preservation films with different CO2 / O2 permeability ratios can be prepared to better meet the preservation needs of different fruit and vegetable varieties. Furthermore, BASF ADR4468 is added to the intermediate layer as a compatibilizer, with an addition ratio of 0.2-0.5 wt%.

[0045] The polyurethane synthesized from polycaprolactone polyol in this embodiment of the invention has a melt index of 15-20 g / 10 min (2.16 kg, 180 °C). The polyurethane synthesized from polycaprolactone polyol can be obtained from commercially available raw materials or synthesized directly. The polyurethane synthesized from polycaprolactone polyol used in this embodiment can be any one of the following: polyurethane synthesized from polycaprolactone polyol and isocyanate MDI, or polyurethane synthesized from polycaprolactone polyol and isocyanate TDI. Specifically, this embodiment of the invention uses CX-5190 (produced by Hubei Chuangxin Polyurethane Materials Co., Ltd.) as the carbon dioxide-permeable TPU modification material.

[0046] The intermediate layer in this embodiment of the invention can block oxygen and enhance the permeability of carbon dioxide, preventing food from spoiling due to the accumulation of carbon dioxide, thereby extending the shelf life of food.

[0047] In this embodiment of the invention, the inner layer of the cling film is modified with a blend of PLA and PBS to give the cling film good low-temperature heat-sealing performance and ensure a tight seal during packaging. PBS is added to the inner layer as a low-temperature heat-sealing modifier; however, the amount of PBS added should not be too high to avoid degrading the optical properties of the cling film and affecting its performance. Therefore, preferably, the mass ratio of PLA to PBS in the inner layer is 95-97:3-5. The PBS used in this embodiment of the invention has a melting point of 80-85℃. By adding only a small amount (3-5 wt%) of PBS to the inner layer, the initial sealing temperature of the cling film can be as low as 90℃, while the heat-sealing strength still reaches above 5 N / 15m.

[0048] This invention improves the heat-sealing effect of cling film at lower temperatures by adding a low-temperature heat-sealing material PBS to the inner layer, enabling the cling film to meet the rapid sealing requirements of household food packaging. Furthermore, BASF ADR4468 is added to the inner layer as a compatibilizer at a ratio of 0.2-0.5 wt%.

[0049] Preferably, an anti-fogging agent is also added to the inner layer of this embodiment of the invention, giving the cling film excellent anti-fogging properties. The anti-fogging agent effectively prevents fogging on the surface of refrigerated food, keeping the cling film clear and transparent, and maintaining the visibility and appearance of the food inside the packaging. Specifically, the anti-fogging agent can be sorbitol palmitate, and the amount of anti-fogging agent added to the inner layer is 0.3-0.5 wt%.

[0050] The middle layer of a heat-shrinkable polylactic acid (PLA) food preservation film of this invention is made of a blend of PLA and polyurethane synthesized from polycaprolactone polyol. This middle layer has good oxygen barrier and carbon dioxide permeability properties. By adjusting the blending ratio of PLA and polyurethane synthesized from polycaprolactone polyol, food preservation films with different CO2 / O2 permeability ratios can be prepared to meet the preservation needs of different respiration intensities and different fruit and vegetable varieties. The outer layer is made of a blend of PLA and polyurethane synthesized from carbonate polyol. The addition of polyurethane synthesized from carbonate polyol as a water-blocking material to the outer layer gives the food preservation film good waterproof performance, prevents PLA hydrolysis, and protects the middle preservation layer, allowing the food preservation film to better perform its preservation function. Polybutylene succinate is added to the inner layer as a low-temperature heat-sealing material, so that the heat-sealing temperature of the food preservation film can be as low as 90°C, while the heat-sealing strength still reaches more than 5N / 15m.

[0051] According to an embodiment of the present invention, a heat-shrinkable polylactic acid (PLA) preservation film can adjust the blending ratio of the intermediate layer raw materials according to the preservation needs of fruits and vegetables, thereby adjusting the CO2 / O2 permeability ratio, achieving more stable gas conditioning performance, and improving the stability of packaging effect.

[0052] The heat-shrinkable polylactic acid (PLA) preservation film of this invention uses PLA as the main material. The prepared preservation film is a bio-based biodegradable material, which meets the development requirements of low carbon, green and environmental protection. Moreover, the production cost is low, it can meet the preservation needs of different fruit and vegetable varieties, and has better preservation performance.

[0053] The heat-shrinkable polylactic acid (PLA) food preservation film of this invention has low-temperature heat shrinkage properties and can be produced using a simple heat-sealing packaging process. It can be used to develop modified atmosphere packaging for food of different shapes and sizes, and is suitable for pillow-type shrink packaging or household food preservation bags. This meets market demand, improves packaging adaptability, and reduces packaging difficulty and cost.

[0054] A second aspect of the present invention also provides a method for preparing a heat-shrinkable polylactic acid (PLA) food preservation film, comprising the following steps:

[0055] (1) The outer, middle and inner layers of raw materials are fed into three twin-screw extruders and melted at a temperature of 200-220°C. They are then extruded through a co-extrusion die to form a film with a three-layer co-extrusion structure.

[0056] (2) Heat the three-layer co-extruded film to 60-90℃ and stretch it longitudinally and laterally with a stretching ratio of 3-5 times.

[0057] This invention provides a polylactic acid shrink film prepared through three-layer co-extrusion and biaxial stretching, enabling the film to maintain good shrinkage properties even at low temperatures, making it particularly suitable for the tight packaging of frozen foods. The preparation method of this invention is simple, and the resulting film possesses good and stable preservation and mechanical properties, meeting market demands.

[0058] The following examples illustrate a heat-shrinkable polylactic acid (PLA) food preservation film and its preparation method according to embodiments of the present invention.

[0059] Example 1

[0060] Example 1 of this invention provides a heat-shrinkable polylactic acid (PLA) food preservation film and its preparation method:

[0061] This heat-shrinkable polylactic acid (PLA) food preservation film comprises an outer layer, a middle layer, and an inner layer stacked sequentially, with the following mass percentage content for each layer:

[0062] Outer layer formulation: PLA 70%, water-blocking TPU 30%, compatibilizer 0.3%, opening agent 0.05%. Among them, the water-blocking TPU is polypropylene carbonate resin (product brand PPC-TPU T2, produced by Jiangsu Zhongke Jinlong Environmental Protection New Materials Co., Ltd.).

[0063] Intermediate layer formulation: PLA 70%, CO2-permeable TPU 30%, compatibilizer 0.4%. Among them, the CO2-permeable TPU is CX-5190 (produced by Hubei Chuangxin Polyurethane Materials Co., Ltd.).

[0064] Inner layer formulation: PLA 95%, PBS 5%, antifogging agent 0.5%, compatibilizer 0.3%, opening agent 0.05%.

[0065] This heat-shrinkable polylactic acid (PLA) food preservation film is prepared according to the following method:

[0066] (1) The above three-layer formulation raw materials are respectively put into three twin-screw extruders, melted at a temperature of 200-220℃, and extruded through a co-extrusion die to form a film with a three-layer co-extrusion structure;

[0067] (2) The co-extruded film is heated to 60°C and stretched longitudinally by 3 times, and then stretched transversely by 4 times at 75°C to obtain a plastic wrap with a thickness of 30 μm.

[0068] Example 2

[0069] The difference between this example and Example 1 is that the transverse stretching temperature is 80°C, while other process conditions are the same.

[0070] Example 3

[0071] The difference between this example and Example 1 is that the transverse stretching temperature is 85°C, while other process conditions are the same.

[0072] Example 4

[0073] The difference between this example and Example 1 is that the transverse stretching temperature is 90°C, while other process conditions are the same.

[0074] Examples 1-4 of this invention investigated the effect of different transverse stretching temperatures on the shrinkage rate of the prepared food preservation films. The shrinkage rate of the food preservation films prepared in Examples 1-4 was tested according to GB / T 13519. The test method included: immersing the film sample in a constant temperature medium at 120°C for 20 seconds, then removing it and cooling it, and measuring the dimensional change after shrinkage. The test results are shown in Table 1 below. Table 1 shows that by adjusting the transverse stretching temperature, polylactic acid food preservation films with a shrinkage rate of 10-70% can be prepared.

[0075] Table 1

[0076] Transverse tensile temperature / ℃ Longitudinal shrinkage rate / % Lateral shrinkage rate / % Example 1 75 63 69 Example 2 80 53 57 Example 3 85 32 40 Example 4 90 14 21

[0077] Example 5

[0078] The difference between this and Example 2 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 90:10, while other process conditions are the same.

[0079] Example 6

[0080] The difference between this and Example 2 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 80:20, while other process conditions are the same.

[0081] Examples 2, 5, and 6 of this invention investigated the effect of the blending ratio of PLA and carbon dioxide-permeable TPU in the intermediate layer on the shrinkage rate of the cling film. The results are shown in Table 2 below.

[0082] Table 2

[0083] Blending ratio Longitudinal shrinkage rate / % Lateral shrinkage rate / % Example 2 70:30 53 57 Example 5 90:10 63 68 Example 6 80:20 62 65

[0084] Example 7

[0085] The difference between this and Example 1 is that the PBS added to the inner layer is 3wt%, while other process conditions are the same.

[0086] Example 8

[0087] The difference between this and Example 1 is that the PBS addition ratio in the inner layer is 4.5 wt%, while other process conditions are the same.

[0088] Example 9

[0089] The difference between this and Example 1 is that the PBS added to the inner layer is 6 wt%, while other process conditions are the same.

[0090] The heat seal strength of the film is one of the key indicators for measuring the quality of packaging bags. The heat seal strength performance of the food preservation films prepared in Examples 7-9 of this invention was tested according to the Chinese Light Industry Standard QB / T2358-2002. The heat seal strength test method includes setting parameters such as heat sealing temperature, time and pressure, and testing is carried out by a professional heat sealing tester.

[0091] Examples 7, 8, and 9 investigated the effect of different PBS addition amounts in the inner layer on the heat-sealing performance of the plastic wrap. The test results are shown in Table 3 below. By adjusting the PBS addition ratio in the inner layer, plastic wraps with different heat-sealing strengths can be obtained to meet the sealing requirements of different scenarios. In particular, the plastic wrap can be heat-sealed at a low temperature of 90℃, preventing phenomena such as stringing or shrinkage and wrinkling at high temperatures, thus ensuring the aesthetics of the seal line.

[0092] Table 3

[0093] PBS / % 120℃ heat seal strength N / 15mm 90℃ heat seal strength N / 15mm Example 7 3 6.9 5.7 Example 8 4.5 7.5 6.5 Example 9 6 8.1 7.3

[0094] Example 10

[0095] The difference between this and Example 1 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 75:25, while other process conditions are the same.

[0096] Example 11

[0097] The difference between this and Example 1 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 50:50, while other process conditions are the same.

[0098] Example 12

[0099] The difference between this and Example 1 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 25:75, while other process conditions are the same.

[0100] Comparative Example 1

[0101] The difference between this and Example 1 is that the mass ratio of PLA to carbon dioxide permeable TPU in the intermediate layer formulation is 100:0, that is, no carbon dioxide permeable TPU is added to the intermediate layer, and other process conditions are the same.

[0102] Comparative Example 2

[0103] The difference between this and Example 1 is that the mass ratio of PLA to carbon dioxide-permeable TPU in the intermediate layer formulation is 0:100, that is, no PLA is added to the intermediate layer, and other process conditions are the same.

[0104] Examples 10-12 and Comparative Examples 1 and 2 of this invention investigated the effect of the blending ratio of PLA and TPU permeable to carbon dioxide in the intermediate layer on the CO2 / O2 permeability of the preservation film. The gas permeability of the preservation films prepared in Examples 10-12 and Comparative Examples 1 and 2 was tested according to GB / T 1038-2000, and the test results are shown in Table 4 below. Table 4 shows that by adjusting the blending ratio of PLA and TPU permeable to carbon dioxide in the intermediate layer, preservation films with different CO2 permeability, O2 permeability, and CO2 / O2 permeability ratios can be prepared, which can meet the preservation needs of different respiration intensities and different fruit and vegetable varieties.

[0105] Table 4

[0106]

[0107] Packaging and preservation tests were conducted on cherry tomatoes using the preservation films prepared in Examples 10-12 and Comparative Examples 1 and 2 of this invention, respectively. Five portions of freshly picked cherry tomatoes of the same weight and from the same batch were taken. The preservation films prepared in Examples 10-12 and Comparative Examples 1 and 2 were used to make pillow-shaped preservation bags of the same size, which were then sealed and stored at 0-5℃. The tests showed that the preservation film prepared in Example 10 of this invention is more suitable for preserving cherry tomatoes, and its preservation effect on cherry tomatoes is as follows: Figure 1 , Figure 2 As shown. Figure 1 This is a graph showing the change in CO2 content inside the food storage bag over storage time. Figure 2 This is a graph showing the change in O2 content inside the food storage bag over storage time.

[0108] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A heat-shrinkable polylactic acid (PLA) food preservation film, characterized in that, It comprises an outer layer, a middle layer, and an inner layer stacked sequentially; the outer layer material includes polylactic acid, polyurethane synthesized from carbonate polyol, and a compatibilizer; the middle layer material includes polylactic acid, polyurethane synthesized from polycaprolactone polyol, and a compatibilizer; the inner layer material includes polylactic acid, polybutylene succinate, and a compatibilizer. The mass ratio of the outer layer, the middle layer, and the inner layer is 0.5-1:7.5-8.5:0.5-1; The mass ratio of polylactic acid to polyurethane synthesized from carbonate polyol in the outer layer is 40-70:30-60; and / or, The mass ratio of polylactic acid to polyurethane synthesized from polycaprolactone polyol in the intermediate layer is 20-90:10-80; and / or, The mass ratio of polylactic acid to polybutylene succinate in the inner layer is 95-97:3-5. The compatibilizer is added at a ratio of 0.2-0.5 wt% in each layer.

2. The heat-shrinkable polylactic acid food preservation film as described in claim 1, characterized in that, The polyurethane synthesized from carbonate polyol has a melt index of 10-15 g / 10 min.

3. The heat-shrinkable polylactic acid food preservation film as described in claim 2, characterized in that, The polyurethane synthesized from carbonate polyols is PPC-TPU T2.

4. The heat-shrinkable polylactic acid food preservation film as described in claim 1, characterized in that, The polyurethane synthesized from polycaprolactone polyol has a melt index of 15-20 g / 10 min.

5. The heat-shrinkable polylactic acid food preservation film as described in claim 4, characterized in that, The polyurethane synthesized from polycaprolactone polyol is CX-5190.

6. The heat-shrinkable polylactic acid food preservation film as described in claim 1, characterized in that, The inner layer contains polybutylene succinate with a melting point of 80-85°C; and / or the inner layer also contains an anti-fogging agent.

7. A heat-shrinkable polylactic acid food preservation film as described in any one of claims 1-6, characterized in that, The compatibilizer in the outer layer, the middle layer and the inner layer is BASF ADR4468.

8. A method for preparing a heat-shrinkable polylactic acid food preservation film according to any one of claims 1-7, characterized in that, Includes the following steps: The outer, middle, and inner layer raw materials are fed into three twin-screw extruders, melted at a temperature of 200-220℃, and extruded through a co-extrusion die to form a film with a three-layer co-extrusion structure. The three-layer co-extruded film is heated to 60-90°C and stretched longitudinally and laterally with a stretching ratio of 3-5 times.

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