Biaxially oriented polylactic acid film with high heat sealing strength and preparation method thereof

Abstract

The invention relates to the technical field of packaging materials, in particular to a high-heat-sealing-strength two-way stretching polylactic acid film and a preparation method thereof. According to the polylactic acid film and the preparation method thereof, the polylactic acid film structurally comprises a copolymerized polylactic acid heat sealing layer and a polylactic acid base material layer from inside to outside in sequence. The copolymerized polylactic acid heat sealing layer mainly comprises the following raw materials: a polyester-polylactic acid block copolymer and a functional auxiliary agent, and the polylactic acid base material layer mainly comprises the following raw materials: polylactic acid, D-sorbitol and a multi-component epoxy chain extender. The heat-sealing strength of the high-heat-sealing-strength two-way stretching polylactic acid film provided by the invention is greater than 12N / 15mm, the gluing and compounding procedures are reduced on the premise of keeping good mechanical properties and heat shrinkage of the film, and the high-heat-sealing-strength two-way stretching polylactic acid film is particularly suitable for inflation protection food packaging application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of packaging materials, in particular to a high-heat-seal-strength biaxially oriented polylactic acid film and a preparation method thereof. BACKGROUND

[0002] Polylactic acid (PLA) is one of the most concerned biodegradable plastics. In addition to good biodegradability and compostability, it also has excellent mechanical strength, high transparency and good processability. These characteristics make PLA expected to replace traditional petroleum-based non-degradable materials in the packaging field (food packaging, clothing packaging, flower packaging, electronic product cushion packaging), preventing and mitigating plastic environmental pollution problems.

[0003] For food packaging, it is required to have high heat seal strength, especially in air-filled protective food packaging, the heat seal strength should be ≥12N / 15mm to prevent air leakage in different air pressure environments and during transportation. The heat seal strength of conventional biaxially oriented co-extruded films is relatively low.

[0004] A biaxially oriented polylactic acid film that can be directly heat sealed is disclosed in patent document No. 202010415595.2, and the maximum heat seal strength is 9.5N / 15mm. The commercially available biaxially oriented polylactic acid film, which is commercially available in model ESL or NTSS, has a heat seal strength ≤7N / 15mm.

[0005] To meet the requirement of high heat seal strength, BOPLA / CPLA (cast PLA) structure is usually adopted, and composite combined packaging film is used, which increases the composite cost and poses environmental risks due to the degradation of composite glue.

[0006] Therefore, it is still necessary to develop a high-heat-seal-strength biaxially oriented polylactic acid film to meet the requirement of high heat seal strength for food packaging. SUMMARY

[0007] To solve the problem of insufficient heat seal strength of ordinary biaxially oriented polylactic acid film, the present application provides a high-heat-seal-strength biaxially oriented polylactic acid film, the structure of which from inside to outside is a copolymerized polylactic acid heat seal layer, a polylactic acid base material layer and a polylactic acid surface layer. The raw material composition of the copolymerized polylactic acid heat seal layer includes 99-99.9% of polyester-poly-lactic acid block copolymer and 0.1-1% of functional additives, by mass percentage; The molar ratio of L-lactide and D-lactide in the polyester-poly-lactic acid block copolymer is 80-88: 12-20; The raw material composition of the polylactic acid substrate layer comprises 98-99.8% polylactic acid, 0.1-1% D-sorbitol and 0.1-1% multi-epoxy chain extender. The raw material composition of the polylactic acid surface layer comprises 99-99.9% polylactic acid and 0.1-1% anti-blocking agent.

[0008] In an embodiment, the polyester-polylactic acid block copolymer is one or a combination of polybutylene adipate terephthalate-polylactic acid copolymer, polycaprolactone-polylactic acid copolymer and polybutylene succinate-polylactic acid copolymer.

[0009] In an embodiment, the functional aid is one or a combination of anti-blocking agent, slip agent and antistatic agent.

[0010] In an embodiment, the anti-blocking agent is one or a combination of silicon dioxide, calcium carbonate, silicone and acrylic.

[0011] In an embodiment, the slip agent is one or a combination of PE wax, oleic acid amide, silicon wax, paraffin wax and erucic acid amide.

[0012] In an embodiment, the antistatic agent is one or a combination of monoglyceride fatty acid ester, sodium stearoyl lactylate and sodium alkyl sulfonate.

[0013] In an embodiment, the multi-epoxy chain extender is multi-epoxy chain extender ADR.

[0014] In an embodiment, the mass ratio of polyester to polylactic acid in the polyester-polylactic acid block copolymer is 5-20:80-95.

[0015] In an embodiment, the polylactic acid has a light purity of ≥99% and a melt index of 2-5 g / 10 min at 190℃ / 2.16 kg; and the polyester-polylactic acid block copolymer has a melt index of 2-4 g / 10 min at 190℃ / 2.16 kg.

[0016] In an embodiment, the high-heat-seal-strength biaxially stretched polylactic acid film has a thickness of 30-60 μm, wherein the copolymerized polylactic acid heat-seal layer has a thickness of 8-10 μm.

[0017] The application also provides a preparation method of the high-heat-seal-strength biaxially stretched polylactic acid film as described above, comprising the following steps: S1: blending polyester-polylactic acid block copolymer and functional aid according to a proportion, melt blending, extruding and granulating through a double-screw extruder, drying the obtained high-concentration concentrated master batch and reserving it for use; S2: Poly (lactic acid), D-sorbitol, polybasic epoxy chain extender is mixed according to the proportion, melt blending, extrusion, granulation is carried out through double screw extruder, the high concentration concentrated master batch obtained is dried, and standby is used; S3: Poly (lactic acid), anti-knotting agent is mixed according to the proportion, melt blending, extrusion, granulation is carried out through double screw extruder, the high concentration concentrated master batch obtained is dried, and standby is used; S4: The high concentration concentrated master batch obtained in S1 is proportionally put into A auxiliary machine, and copolymerized poly (lactic acid) heat sealing layer is prepared;The high concentration concentrated master batch obtained in S2 is proportionally put into B main machine, and poly (lactic acid) base material layer is prepared;The high concentration concentrated master batch obtained in S3 is proportionally put into C auxiliary machine, and poly (lactic acid) surface layer is prepared; Wherein, the extruder temperature of the copolymerized poly (lactic acid) heat sealing layer is controlled between 110-200 DEG C, and the extruder temperature and the temperature of T-shaped die of poly (lactic acid) base material layer and poly (lactic acid) surface layer are controlled between 185-210 DEG C; S5: the LISIM synchronous stretching method is used, the layers in S3 are extruded, and the cast piece is rapidly cooled, then bidirectional stretching is carried out, the stretching temperature is 70-90 DEG C, the setting temperature is 80-100 DEG C, and the stretching ratio is 2.0*2.0-5.5*5.5; S6: the biaxially oriented poly (lactic acid) film obtained in S5 is rolled up, slitting is carried out, and packaging can be carried out.

[0018] Compared with the prior art, the biaxially oriented poly (lactic acid) film with high heat sealing strength provided by the application has the beneficial effects that: 1, the molar ratio of L-lactide and D-lactide in the copolymerized poly (lactic acid) heat sealing layer is 80-88: 12-20, so that the poly (lactic acid) is in an amorphous state, the polyester is introduced into the amorphous poly (lactic acid) to form a polyester-poly (lactic acid) block copolymer, which further disrupts the ordered state of poly (lactic acid) molecules, so that the glass transition temperature is reduced, and the heat sealing performance is further enhanced;The introduction of flexible polyester segments reduces the stress concentration of rigid poly (lactic acid) segments, and the heat sealing performance is enhanced; 2, the poly (lactic acid) base material layer uses poly (lactic acid), and the high crystallinity makes the biaxially oriented film have good heat resistance, and D-sorbitol can make the poly (lactic acid) have good nucleation effect at about 80-100 DEG C, so that the setting temperature of the biaxially oriented film can be set to 115 DEG C (the fastest crystallization temperature), thereby reducing the heat sealing property of the heat sealing layer;Adding chain extender can increase the melt strength stability of poly (lactic acid), improve the film thickness, and also solidify D-sorbitol to avoid migration to the heat sealing layer, thereby affecting the heat sealing property of the film. Specific embodiments

[0019] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0020] In order to better illustrate the beneficial effects of the present application, the following examples and comparative examples are provided.

[0021] Example 1 A high heat seal strength biaxially oriented polylactic acid film, comprising a three-layer structure, from inside to outside, a copolymer polylactic acid heat seal layer, a polylactic acid base material layer and a polylactic acid surface layer. The copolymer polylactic acid heat seal layer is composed of the following raw materials in mass percentage: polybutylene adipate terephthalate-poly-lactic acid block copolymer 99.5%, silicon dioxide 0.1%, erucic acid amide 0.2%, alkyl sulfonate 0.2%; the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 15:85; the molar ratio of L-lactide to D-lactide in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 86:14. The polylactic acid base material layer is composed of the following raw materials: polylactic acid 99%, D-sorbitol 0.5%, multi-epoxy chain extender ADR 0.5%. The polylactic acid surface layer is composed of the following raw materials: polylactic acid 99.5%, silicon dioxide 0.5%.

[0022] Example 2 A high heat seal strength biaxially oriented polylactic acid film, comprising a three-layer structure, from inside to outside, a copolymer polylactic acid heat seal layer, a polylactic acid base material layer and a polylactic acid surface layer. The copolymer polylactic acid heat seal layer is composed of the following raw materials in mass percentage: polybutylene adipate terephthalate-poly-lactic acid block copolymer 99.9%, silicon dioxide 0.025%, erucic acid amide 0.05%, alkyl sulfonate 0.025%; the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 5:95; the molar ratio of L-lactide to D-lactide in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 80:20.

[0023] The polylactic acid base material layer is composed of the following raw materials: polylactic acid 99.8%, D-sorbitol 0.1%, multi-epoxy chain extender ADR 0.1%.

[0024] The polylactic acid surface layer component is composed of the following raw materials: polylactic acid 99.9%, silicon dioxide 0.1%.

[0025] Example 3 A high heat seal strength biaxially oriented polylactic acid film, comprising a three-layer structure, from inside to outside, a copolymer polylactic acid heat seal layer, a polylactic acid base layer and a polylactic acid surface layer; The copolymer polylactic acid heat seal layer component is composed of the following raw materials by mass fraction: polybutylene adipate terephthalate-poly-lactic acid block copolymer 99%, silicon dioxide 0.2%, erucic acid amide 0.4%, alkyl sulfonate 0.4%; the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 20:80; the molar ratio of L-lactide and D-lactide in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 80:20.

[0026] The polylactic acid base layer component is composed of the following raw materials: polylactic acid 98%, D-sorbitol 1%, multi-epoxy chain extender ADR 1%.

[0027] The polylactic acid surface layer component is composed of the following raw materials: polylactic acid 99%, silicon dioxide 1%.

[0028] Example 4 A high heat seal strength biaxially oriented polylactic acid film, comprising a three-layer structure, from inside to outside, a copolymer polylactic acid heat seal layer, a polylactic acid base layer and a polylactic acid surface layer; The copolymer polylactic acid heat seal layer component is composed of the following raw materials by mass fraction: polybutylene adipate terephthalate-poly-lactic acid block copolymer 99%, silicon dioxide 0.2%, erucic acid amide 0.4%, alkyl sulfonate 0.4%; the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 20:80; the molar ratio of L-lactide and D-lactide in the polybutylene adipate terephthalate-poly-lactic acid block copolymer is 80:20.

[0029] The polylactic acid base layer component is composed of the following raw materials by mass fraction: polylactic acid 99%, D-sorbitol 0.5%, multi-epoxy chain extender ADR 0.5%.

[0030] The polylactic acid surface layer component is composed of the following raw materials by mass fraction: polylactic acid 99.5%, silicon dioxide 0.5%.

[0031] The present application also provides a preparation method of the above-mentioned examples 1-4: S1: The polyester-poly-lactic acid block copolymer and functional additives are mixed in proportion, and then melt blended, extruded and granulated by a double screw extruder, and the obtained high concentration concentrate master batch is dried for standby; S2: mixing polylactic acid, D-sorbitol and polybasic epoxy chain extender according to the proportion, melt blending, extruding and granulating through a double screw extruder, drying the obtained high concentration concentrate master batch for standby; S3: mixing polylactic acid and anti-blocking agent according to the proportion, melt blending, extruding and granulating through a double screw extruder, drying the obtained high concentration concentrate master batch for standby; S4: putting the high concentration concentrate master batch obtained in S1 and copolymerized polylactic acid into A auxiliary machine according to the proportion to make copolymerized polylactic acid heat sealing layer; putting the high concentration concentrate master batch obtained in S2 and polylactic acid into B main machine according to the proportion to make polylactic acid base material layer; putting the high concentration concentrate master batch obtained in S3 and polylactic acid into C auxiliary machine according to the proportion to make polylactic acid surface layer; Among them, the temperature of the extruder of the copolymerized polylactic acid heat sealing layer is controlled between 110-200℃, and the temperature of the extruder and the temperature of the T-shaped die of the polylactic acid base material layer and the polylactic acid surface layer are controlled between 185-210℃; S5: using LISIM synchronous stretching method, extruding, quenching and casting the layers in S3, then bidirectional stretching, the stretching temperature is 87℃, the setting temperature is 90℃, and the stretching ratio is 3.2x3.2; S6: winding, slitting and packaging the bidirectional stretched polylactic acid film obtained in S5.

[0032] Comparative Example 1 Comparative Example 1 is different from Example 1 in that: polyester-polyactic acid block copolymer is not used, only polylactic acid copolymer is used, the rest of the raw material components and the ratio are the same as Example 1, and the preparation method is the same as Example.

[0033] Comparative Example 2 Comparative Example 2 is different from Example 1 in that: the molar ratio of L-lactide and D-lactide in the polybutylene adipate terephthalate-polyactic acid block copolymer is 99:1; The rest of the raw material components and the ratio are the same as Example 1, and the preparation method is the same as Example.

[0034] Comparative Example 3 Comparative Example 3 is different from Example 1 in that: the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-polyactic acid block copolymer is 3:97; The rest of the raw material components and the ratio are the same as Example 1, and the preparation method is the same as Example.

[0035] Comparative Example 4 Comparative Example 4 is different from Example 1 in that: the mass ratio of polyester to polylactic acid in the polybutylene adipate terephthalate-polyactic acid block copolymer is 30:70; The other raw material components and proportions are the same as in Example 1, and the preparation method is the same as in the example.

[0036] Comparative Example 5 Comparative Example 5 differs from Example 1 in that the polylactic acid base material layer component consists of the following raw materials by mass percentage: polylactic acid 100%; The other raw material components and proportions are the same as in Example 1, and the preparation method is the same as in the example.

[0037] Comparative Example 6 Comparative Example 6 differs from Example 1 in that the polylactic acid base material layer component consists of the following raw materials by mass percentage: polylactic acid 99.5%, D-sorbitol 0.5%; The other raw material components and proportions are the same as in Example 1, and the preparation method is the same as in the example.

[0038] Comparative Example 7 Comparative Example 7 differs from Example 1 in that the preparation method has a setting temperature of 110°C. The raw material components and proportions are the same as in Example 1.

[0039] Performance Testing Each example and comparative example was tested for various performance tests, and the test standards are as follows: Thickness: tested according to GB / T20220-2006 "Plastic Film and Sheet Sample Average Thickness, Roll Average Thickness, and Unit Mass Surface"; The test method for film heat sealing strength is to test the film heat sealing strength according to QB / T 2358-98, the heat sealing pressure is 135kpa, the heat sealing temperature is 85°C, and the heat sealing time is 2s; Haze and light transmittance: tested according to GB / T2410-2008 "Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics"; Friction coefficient: tested according to GB / T10006-1988 "Test Method for Friction Coefficient of Plastic Film and Sheet"; Tensile strength and elongation at break: tested according to GB / T 1040-3 "Determination of the Tensile Properties of Plastics - Part 3: Test Conditions for Films and Sheeting"; Heat shrinkage rate: GB / T12027-2004 Test Method for Dimensional Change on Heating of Plastic Film and Sheet, test temperature is 100°C, and test time is 10min.

[0040] The film test evaluation results of the examples and comparative examples are shown in Tables 1 and 2: Table 1

[0041] Table 2

[0042] From the test results of Examples 1-4 in Table 1, the heat seal strength of Examples 1-4 all significantly exceeds the ≥12N / 15mm standard required for air-filled protective food packaging, fully verifying that the specific L-lactide, D-lactide molar ratio of the polyester-polylactic acid block copolymer can effectively improve the heat seal bonding force; in terms of transparency, the low haze and high light transmittance characteristics enable the film to clearly display the food form inside the package, meeting the appearance display needs of food packaging; in terms of mechanical properties, the tensile strength MD direction (longitudinal direction) is 122-126 MPa, and the TD direction (transverse direction) is 119-124 MPa, and the high strength performance can cope with external force impact during transportation and handling of food packaging, avoiding film rupture; the heat shrinkage rate MD direction is 1.3%-3.0%, and the TD direction is 1.0%-2.4%, and the low shrinkage rate can prevent air-filled packaging from causing the seal to crack or the package to deform due to size shrinkage during subsequent heat treatment or storage environment.

[0043] From the performance comparison of Comparative Examples 1-7 and Examples in Table 2, each comparative example deviates from the design parameters of the examples due to a single variable, and all have different degrees of performance defects, which inversely verifies the rationality of the formula and process design of the examples. Comparative Example 1 does not use polyester-polylactic acid block copolymer in the heat seal layer, but only uses polylactic acid copolymer, and the heat seal strength is reduced, which shows that the lack of polyester segments cannot effectively disrupt the ordered arrangement of polylactic acid molecules, reduce the glass transition temperature, and also cannot relieve the stress concentration of the polylactic acid segment through flexible polyester segments, directly leading to insufficient heat seal performance; Comparative Example 2 adjusts the molar ratio of L-lactide to D-lactide to 99:1, which is far beyond the range of 80-88:12-20 of the examples, and the heat seal strength is directly reduced to 0, which is because the high proportion of L-lactide makes the polylactic acid form a stable crystal structure, losing the heat seal ability required for the amorphous state, proving that a specific proportion of lactide composition is a prerequisite for the heat seal layer to have heat sealability; In Comparative Example 3, the mass ratio of polyester to polylactic acid is 3:97, which is lower than the lower limit of 5-20:95-80 of the examples, and the heat seal strength is only 9.4N / 15mm, indicating that when the amount of polyester is insufficient, its effect on improving the molecular structure and enhancing the heat seal flexibility is limited, and it cannot meet the high heat seal strength standard; Comparative Example 4 increases the ratio to 30:70, which exceeds the upper limit, and the heat seal strength is only 10.8N / 15mm, which is speculated to be that the high proportion of polyester destroys the continuity of the polylactic acid matrix, leading to a decrease in intermolecular bonding force during heat sealing. It can be seen from this that the mass ratio range of polyester to polylactic acid in the examples is an optimized balance interval; The substrate layer of Comparative Example 5 is not added with D-sorbitol and multi-epoxy chain extender, and the thermal shrinkage is increased, which indicates that the nucleation effect of D-sorbitol can help the substrate layer to form a stable structure at a lower setting temperature, avoiding the influence of high temperature setting on the heat sealing layer, and the multi-epoxy chain extender can enhance the melt strength and control the dimensional stability, and the lack of both will lead to the out-of-control of the film thermal shrinkage, which cannot meet the requirements of the dimensional stability of the inflatable packaging; The heat sealing strength of the substrate layer of Comparative Example 6 only lacks multi-epoxy chain extender, and does not reach the standard of 12N / 15mm; Comparative Example 7 increases the setting temperature to 110℃, and the heat sealing strength is reduced to 7.3N / 15mm, because the too high setting temperature makes the amorphous polylactic acid in the heat sealing layer partially crystallize, which destroys the amorphous structure required for heat sealing, resulting in a significant decrease in heat sealing performance, which proves that the control of the setting temperature in the embodiment is the key process link to retain the performance of the heat sealing layer, and avoids the failure of the advantages of the raw material design due to the deviation of the process parameters.

[0044] In summary, the high heat sealing strength biaxially oriented polylactic acid film of the application has high heat sealing strength and low thermal shrinkage, which meets the requirements of non-deformation and air-tightness of the inflatable food packaging under heat sealing conditions.

[0045] Although the terms such as copolymerized polylactic acid heat sealing layer, polylactic acid substrate layer and polylactic acid surface layer are used more in this paper, but the possibility of using other terms is not excluded. The use of these terms is only to facilitate the description and explanation of the essence of the application; any additional limitation is contrary to the spirit of the application.

[0046] Finally, it should be pointed out that: the above embodiments are only used to illustrate the technical solutions of the application, but not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A high heat-sealing strength biaxially oriented polylactic acid film, characterized in that: The structure of the high heat-sealing strength biaxially oriented polylactic acid film, from the inside out, consists of a copolymer polylactic acid heat-sealing layer, a polylactic acid substrate layer, and a polylactic acid surface layer. The raw material composition of the copolymer polylactic acid heat-sealing layer, by mass percentage, includes 99-99.9% polyester-polylactic acid block copolymer and 0.1-1% functional additives; The molar ratio of L-lactide to D-lactide in the polyester-polylactic acid block copolymer is 80-88: 12-20; The raw material composition of the polylactic acid substrate layer includes 98-99.8% polylactic acid, 0.1-1% D-sorbitol, and 0.1-1% multi-element epoxy chain extender; The raw material composition of the polylactic acid surface layer includes 99-99.9% polylactic acid and 0.1-1% anti-caking agent.

2. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 1, characterized in that: The polyester-polylactic acid block copolymer is one or a combination of several of the following: polybutylene adipate-polylactic acid copolymer, polycaprolactone-polylactic acid copolymer, and polybutylene succinate-polylactic acid copolymer.

3. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 1, characterized in that: The functional additives are one or more of the following: anti-caking agents, slip agents, and antistatic agents.

4. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 3, characterized in that: The anti-caking agent is composed of one or more of silicon dioxide, calcium carbonate, organosilicon, and acrylic.

5. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 3, characterized in that: The slip agent is composed of one or more of PE wax, oleamide, silicone wax, paraffin wax, and erucamide.

6. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 3, characterized in that: The antistatic agent is composed of one or more of monoglyceride fatty acid esters, sodium stearoyl lactylate, and sodium alkyl sulfonate; the multi-component epoxy chain extender is ADR multi-component epoxy chain extender.

7. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 1, characterized in that: The mass ratio of polyester to polylactic acid in the polyester-polylactic acid block copolymer is 5-20:80-95.

8. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 1, characterized in that: The polylactic acid has a light purity of ≥99% and a melt index of 2-5 g / 10 min at 190℃ / 2.16 kg; the polyester-polylactic acid block copolymer has a melt index of 2-4 g / 10 min at 190℃ / 2.16 kg.

9. The high heat-sealing strength biaxially oriented polylactic acid film according to claim 1, characterized in that: The thickness of the high heat-sealing strength biaxially oriented polylactic acid film is 30-60 μm, wherein the thickness of the copolymer polylactic acid heat-sealing layer is 8-10 μm.

10. A method for preparing a high heat-sealing strength biaxially oriented polylactic acid film as described in any one of claims 1-9, characterized in that, Includes the following steps: S1: Polyester-polylactic acid block copolymer and functional additives are blended in proportion, melt-blended, extruded and granulated by twin-screw extruder, and the resulting high-concentration concentrated masterbatch is dried for later use; S2: Polylactic acid, D-sorbitol, and multi-element epoxy chain extender are blended in proportion, melt-blended, extruded, and granulated using a twin-screw extruder, and the resulting high-concentration concentrated masterbatch is dried for later use. S3: Polylactic acid and anti-caking agent are mixed in proportion, melt-blended, extruded and granulated by twin-screw extruder, and the resulting high-concentration concentrated masterbatch is dried and set aside for later use; S4: The high-concentration masterbatch obtained in S1 is fed into auxiliary machine A in a certain proportion to produce the polylactic acid copolymer heat-sealing layer; the high-concentration masterbatch obtained in S2 is fed into main machine B in a certain proportion to produce the polylactic acid substrate layer; the high-concentration masterbatch obtained in S3 is fed into auxiliary machine C in a certain proportion to produce the polylactic acid surface layer. Among them, the extruder temperature of the copolymer polylactic acid heat-sealing layer is controlled between 110 and 200°C, and the extruder temperature of the polylactic acid substrate layer and the polylactic acid surface layer and the temperature of their T-die are controlled between 185 and 210°C. S5: Using the LISIM synchronous stretching method, each layer of S3 is extruded, rapidly cooled and cast, and then biaxially stretched. The stretching temperature is 70-90℃, the setting temperature is 80-100℃, and the stretching ratio is 2.0×2.0-5.5×5.

5. S6: The biaxially oriented polylactic acid film obtained in S5 is wound up, slit, and packaged.

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