CN115071240A

CN115071240A - Biodegradable bidirectional synchronous stretching polylactic acid film and preparation method thereof

Info

Publication number: CN115071240A
Application number: CN202210606413.9A
Authority: CN
Inventors: 王畅; 张彪; 罗定彬; 常大锴; 李钉钉; 刘辉
Original assignee: Zhejiang Nanyang Technology Co ltd
Current assignee: Zhejiang Nanyang Technology Co ltd
Priority date: 2022-05-31
Filing date: 2022-05-31
Publication date: 2022-09-20

Abstract

The invention discloses a film, aiming at providing a biodegradable two-way synchronous stretching polylactic acid film and a preparation method thereof, and the key point of the technical scheme is that the biodegradable two-way synchronous stretching polylactic acid film comprises an upper surface layer, a core layer and a lower surface layer: the upper surface layer comprises the following components in percentage by weight: 60-90 wt% of crystalline PLA-1 chips and 10-40 wt% of PLA-2 opening agent master batch; the core layer is: slicing the crystalline PLA-1; the lower surface layer comprises the following components in percentage by weight: 60-90 wt% of PLA slice and 10-40 wt% of PLA-2 opening agent master batch, and the process flow is as follows: the method comprises the steps of equipment cleaning preparation → preparation of crystalline PLA-1 slicing raw materials, non-crystalline PLA-3 slicing raw materials and PLA-2 opening agent master batch raw materials → drying of the raw materials → extrusion → melting → filtration → die head distribution → tape casting into sheets → functional surface coating → bidirectional stretching → sizing → traction → trimming → rolling → slitting → packaging.

Description

Biodegradable bidirectional synchronous stretching polylactic acid film and preparation method thereof

Technical Field

The invention belongs to the technical field of film materials, and particularly relates to a biodegradable bidirectional synchronous stretching polylactic acid film and a preparation method thereof.

Background

With the acceleration of modernization pace, social life is developing in the direction of convenience and hygienization. Disposable film packaging materials have come into existence and bring great convenience to life, but a great deal of white pollution is caused, and serious harm is caused to the environment and economy of China. Therefore, there is a strong need to develop biodegradable film materials. Polylactic acid (PLA) derived from biomass raw materials is thermoplastic aliphatic polyester, has the advantages of biodegradability, high transparency, high glossiness, good printability, high water vapor permeability and good bonding performance, can completely replace the application fields of traditional films such as BOPP, BOPET and BOPA, is known as a novel packaging material with the most development prospect in the new century, and is widely applied to the fields of envelope window films, household appliances, food outer packaging, fresh-keeping packaging, oil-resistant packaging and the like.

For example, chinese patent application No. (CN201910681690.4) discloses a fully biodegradable high-barrier vacuum evaporation film and a method for preparing the same, wherein the film sequentially comprises, from top to bottom, a high-barrier coating layer and a biaxially oriented polylactic acid (BOPLA) film substrate layer, the substrate layer comprises an upper surface layer, a core layer, and a lower surface layer; the preparation steps are divided into two steps, firstly, a polylactic acid film (BOPLA) base material meeting the coating requirement is prepared by using a flat drawing method, then a high-barrier coating is formed on the surface of the film base material by using an online coating process or an offline coating process, and the optimal oxygen transmission rate of an aluminized film and an aluminized film produced by using the high-barrier evaporation film base material prepared by the method can be less than 0.02cc/m2.24h, and the optimal water vapor transmission rate can reach 0.05g/m 2.24h; the high-barrier evaporation film base material not only has excellent barrier effect, but also is biodegradable, can effectively solve the environmental problem caused by waste packaging films, and has high industrial utilization value, but the PLA material has extremely high brittleness, poor heat resistance and difficult stretch forming, and needs to be improved to improve the processing performance of the polylactic acid film.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a biodegradable bidirectional synchronous stretching polylactic acid film with high tensile strength, good toughness, good isotropic equilibrium, good heat resistance, high production speed, good transparency, good thickness uniformity and stable film forming property and a preparation method thereof.

In order to achieve the purpose, the invention provides the following technical scheme: a biodegradable two-way synchronous stretching polylactic acid film comprises an upper surface layer, a core layer and a lower surface layer:

the upper surface layer comprises the following components in percentage by weight: 60-90 wt% of crystalline PLA-1 chips and 10-40 wt% of PLA-2 opening agent master batch;

the core layer is: slicing the crystalline PLA-1;

the lower surface layer comprises the following components in percentage by weight: 60-90 wt% of PLA slices and 10-40 wt% of PLA-2 opening agent master batch;

wherein the total thickness of the biodegradable bidirectional synchronous stretching polylactic acid film is 4-50 μm, the upper surface layer accounts for 8-30% of the total thickness, the core layer accounts for 40-84% of the total thickness, and the lower surface layer accounts for 8-30% of the total thickness.

In the technical scheme, the PLA-2 opening agent master batch is added into the components of the upper surface layer and the lower surface layer to roughen the surface of the film, so that the anti-adhesion performance of the upper surface layer and the lower surface layer is effectively improved, the rolling of the film is ensured, the tensile strength and toughness of the film are improved, meanwhile, the transparency of the film is not influenced by the addition of the PLA-2 opening agent master batch, the better transparency of the film is ensured, meanwhile, a synchronous stretching method is adopted, compared with the step-by-step stretching, the crystallization process is accelerated, the internal stress of the film is eliminated, the crystallinity is improved, the alert structure tends to be perfect, the thermal shrinkage rate of the film is greatly reduced, the obtained polylactic acid film is not shrunk and curled, the film forming property is ensured to be stable, and meanwhile, the polylactic acid film does not contact with any roller during synchronous stretching, is not influenced by the quality of the roller surface, and can ensure high transparency and no scratch, and the slice receives the limiting action of anchor clamps, and the necking can not be produced in the slice, improves film thickness homogeneity greatly.

The invention is further set up as characterized in that:

the PLA slices in the lower surface layer are crystalline PLA-1 slices;

wherein the melting point of the crystalline PLA-1 is 155-165 ℃, and the melt index is as follows: MFI (190 ℃/2.16kg) is 2-4 g/10min, and the optical purity is more than or equal to 96% L;

the melting point of the open-ended master batch PLA-2 is 155-165 ℃, and the melt index is as follows: MFI (190 ℃/2.16kg) is 4-6 g/10min, and the optical purity is more than or equal to 96% L.

In this technical scheme, the PLA raw materials relative melting point of crystallization type is higher, and the heat resistance is good, is fit for drawing the membrane, consequently is unanimous with the upper surface layer through the constitution with lower top layer, guarantees that two surfaces of this application film all have better anti-adhesion property, and then improves the tensile strength and the toughness of film, the stretch forming of being convenient for.

The invention is further set up as characterized in that:

the PLA slices in the lower surface layer are non-crystalline PLA-3 slices;

the melting point of the open-ended master batch PLA-2 is 155-165 ℃, and the melt index is as follows: MFI (190 ℃/2.16kg) is 4-6 g/10min, and the optical purity is more than or equal to 96% L;

the melting point of the amorphous PLA-3 is 80-140 ℃; melt index: the MFI (190 ℃/2.16kg) is 4-6 g/10min, and the optical purity is more than or equal to 86% L.

In the technical scheme, because the crystallization of the non-crystallization type PLA-3 slice raw material is poor, the relative melting point is lower, and the melting point is at least lower than that of the crystallization type by more than 20 ℃, the composition of the lower surface layer is set into 60-90 wt% of PLA-3 slice and 10-40 wt% of PLA-2 opening agent master batch, so that a heat sealing layer is formed on the lower surface layer, the heat sealing property and the heat sealing effect of the film are improved, and the film has a better promoting effect when being applied to the packaging industry

The invention is further characterized by further comprising:

and the coating layer is aqueous coating liquid and is positioned on the surfaces of the upper surface layer and the lower surface layer.

In the technical scheme, the coating layer is adopted, and surface coating functional treatment and regulation and control can be carried out according to the requirements of different industries, so that the film can be applied to various industries such as food packaging, gift packaging, building material architectural decoration, degradable adhesive tapes, direct heat-seal packaging and the like, and the practical effect is effectively improved.

The invention is further characterized in that the coating layer comprises:

acrylic acid adhesive, silicon dioxide particles, polyethyleneimine, ethyl acrylate, polyethylene oxide, melamine and ethylene-vinyl acetate copolymer;

wherein the particle size of the silicon dioxide particles is 50-100 nm.

In the technical scheme, the coating layer composed of the materials has good printability, and the adhesion of ink is good and the ink does not fall off when the film is used at the later stage.

The invention is further characterized in that the coating layer comprises the following components:

and mixing the pure water, the degradable waterborne polyurethane, the defoaming agent, the viscosity reducing agent and the antistatic agent to obtain the antistatic coating.

Wherein the antistatic agent comprises 30 parts of polylactic acid butanediol ester, 6-12 parts of polybutylene succinate, 22-26 parts of isophorone diisocyanate, 9-11 parts of hexamethylene diisocyanate, 5.1-5.4 parts of hydrophilic chain extender, 7.1-7.5 parts of cross-linking agent, 3-3.6 parts of hydroxyl acrylate, 20-30 parts of acetone, 3-3.6 parts of neutralizer, 70-90 parts of deionized water and 1.2-2.1 parts of diamine

In the technical scheme, the coating layer composed of the materials enables the film to have good antistatic property.

The invention is further characterized in that the coating layer comprises:

polyurethane adhesive, alumina particles, polyethyleneimine, polyethylene oxide and ethylene-vinyl acetate copolymer;

wherein the particle size of the alumina particles is 2-3 μm.

In the technical scheme, the coating layer formed by the materials has good aluminized adhesive force, so that the film is more suitable for the food packaging industry.

The invention is further characterized in that the PLA-2 opening agent master batch comprises:

inorganic particles, wherein the inorganic particles are one or more compounds of aluminum oxide, magnesium oxide, barium sulfate, silicon dioxide, calcium carbonate, zinc oxide and aluminum nitride;

wherein, the diameter of the inorganic particles is 200nm-5um, and the content is 1% -5%.

In the technical scheme, the inorganic particles are added into the PLA-2 opening agent master batch, so that the transparency and the strength of the film can be ensured, the film is not easy to crack, white spots can not appear, the printing performance of the film is ensured, less precipitates are precipitated on the surface of the film, the pollution to packaged materials is avoided, the film is better applied to industries such as food packaging, liquid packaging and medicine packaging, unevenness is generated on the surface of the film, the negative pressure in the film is reduced, the film is convenient to separate, and the anti-adhesion performance of the film is improved.

The invention is further characterized in that the preparation process of the PLA-2 opening agent master batch comprises the following steps:

step 1: dehumidifying, stirring and drying the polylactic acid particles at the temperature of 80-120 ℃, wherein the drying time is 2-6 h, and the final water content of the drying is controlled to be not more than 500 ppm;

step 2: stirring inorganic particles, a coupling agent and a dispersing agent at a high speed to obtain mixed powder;

and step 3: and (3) the materials obtained in the step (1) and the step (2) are subjected to precise weightlessness weighing and burdening according to a set proportion, blending extrusion is carried out on a double-screw extruder, and vacuum volatiles are removed, wherein the processing temperature of the double-screw extruder is 150-220 ℃.

And 4, step 4: and then extruding the mixture through a mouth die, performing air cooling drying and dicing to obtain PLA-2 opening agent master batch.

In the technical scheme, because polylactic acid is a degradable material, hydrolysis is required in the processing process, and the viscosity of the obtained opening master batch is less than 0.05dl/g, the dispersibility is good, and the water content is lower by adopting the steps, so that the subsequent film drawing process is ensured, and the adverse effect on the film drawing process is avoided.

A preparation method of a biodegradable bidirectional synchronous stretching polylactic acid film comprises the following steps:

s1: preparing a PLA-2 opener concentrate according to the preparation steps of the PLA-2 opener concentrate of claim 6;

s2: drying the crystalline PLA-1 slices, the non-crystalline PLA-3 slices and the PLA-2 opening agent master batches obtained in S1, and then extruding, melting, casting and cooling the dried product on a three-layer extruder to form sheets;

s3: performing surface precoating treatment on the sheet obtained in the step S2 to form a precoating layer;

s4: performing bidirectional synchronous stretching on the sheet subjected to the S3 pre-coating treatment to obtain a polylactic acid film;

s5: carrying out heat treatment setting on the polylactic acid film obtained in the step S4, cooling, and rolling to obtain a biodegradable two-way synchronous stretched polylactic acid film;

wherein the precoat layer is subjected to moisture evaporation during stretching and setting to form a coating layer on the surface.

In the technical scheme, by adopting the steps, after the film is extruded from the raw materials on the three-layer extruder, melted and cooled into the sheet by tape casting, the sheet is subjected to pre-coating treatment (pre-coating layers made of different materials are adopted according to different industry requirements), so that the surface of the sheet forms a pre-coating layer, and after subsequent bidirectional synchronous stretching and heat treatment setting, the obtained surface of the film has a functional coating layer, so that the requirements of different industries are met, and the practical effect is improved.

The invention has the beneficial effects that: the stable casting sheet effect is obtained through selection and requirements of polylactic acid slices (PLA-1, PLA-2 and PLA-3) with different grades, meanwhile, few degradation products are generated in the extrusion process, a synchronous bidirectional stretching method is adopted, the film forming property is good, the uniformity of various properties is high, the thickness is uniform and controllable, the obtained film is high in strength and not crisp, and meanwhile, the surface coating functional treatment and regulation and control are carried out according to the requirements of different industries, so that the polylactic acid film applicable to various industries such as food packaging, gift packaging, building material architectural decoration, degradable adhesive tapes and direct heat-sealing packaging is obtained.

Drawings

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is a process flow diagram of the present invention;

FIG. 3 is a table of processing parameters for various embodiments of the present invention;

FIG. 4 is a table of product performance data for various embodiments of the present invention;

FIG. 5 is a graph of the average percent biodegrading versus time for the present invention;

FIG. 6 is a test photograph of the present invention;

in the drawings: 1. an upper surface layer; 2. a core layer; 3. and (4) a lower surface layer.

Detailed Description

The invention is further described in the following with reference to fig. 1 to 2 in a specific embodiment:

example 1:

the embodiment provides a biodegradable two-way synchronous stretching polylactic acid film, which comprises an upper surface layer, a core layer and a lower surface layer, wherein,

the upper surface layer comprises the following components in percentage by weight: 75 wt% of crystalline PLA-1 chips and 25 wt% of PLA-2 opening agent master batch;

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

the lower surface layer comprises the following components in percentage by weight: 80 wt% of crystalline PLA-3 slices and 20 wt% of PLA-2 opening agent master batch;

wherein the extrusion temperature is 185 ℃, the CR cooling temperature is 35 ℃, the stretching ratio is 3.6 multiplied by 3.5, the setting temperature is 130 ℃, and no coating layer is formed.

The biodegradable bidirectional synchronous stretching polylactic acid film produced by the components has the following data:

thickness 12 μm, tensile strength MD: 167Mpa, tensile strength TD: 158Mpa, transmittance 93.2%, haze 2.2%, heat shrinkage MD 100 ℃/10 min: 3.8%, heat shrinkage TD 100 ℃/10 min: 1.45 percent.

Example 2:

the core layer is: 100 wt% of crystalline PLA-1 chips;

wherein the extrusion temperature is 187 ℃, the CR cooling temperature is 40 ℃, the draw ratio is 3.5 multiplied by 3.5, the setting temperature is 135 ℃, and no coating layer is formed.

thickness 9 μm, tensile strength MD: 165Mpa, tensile strength TD: 145Mpa, light transmittance 93.2%, haze 2.2%, heat shrinkage MD 100 ℃/10 min: 3.97%, thermal shrinkage TD 100 ℃/10 min: 1.42 percent.

Example 3:

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

wherein the extrusion temperature is 184 ℃, the CR cooling temperature is 40 ℃, the stretching ratio is 3.8 multiplied by 3.8, the setting temperature is 130 ℃, and no coating layer is formed.

The following data are measured by the biodegradable bidirectional synchronous stretching polylactic acid film produced according to the components:

thickness 25 μm, tensile strength MD: 182Mpa, tensile strength TD: 166MPa, light transmittance 93.5%, haze 2%, heat shrinkage MD 100 ℃/10 min: 3.96%, heat shrinkage TD 100 ℃/10 min: 1.54 percent.

Example 4:

the core layer is: 100 wt% of crystalline PLA-1 chips;

wherein the extrusion temperature is 182 ℃, the CR cooling temperature is 45 ℃, the stretching ratio is 3.5 multiplied by 3.5, the setting temperature is 125 ℃, and no coating layer is formed.

thickness 36 μm, tensile strength MD: 176Mpa, tensile strength TD: 163Mpa, light transmittance 93.2%, haze 1.8%, heat shrink MD 100 ℃/10 min: 3.52%, heat shrinkage TD 100 ℃/10 min: 1.65 percent.

Example 5:

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

wherein the extrusion temperature is 186 ℃, the CR cooling temperature is 35 ℃, the stretching ratio is 3 multiplied by 3, the setting temperature is 136 ℃, and no coating layer is formed.

thickness 38 μm, tensile strength MD: 165Mpa, tensile strength TD: 147MPa, light transmittance of 93.2%, haze of 2.2%, heat shrinkage MD 100 ℃/10 min: 3.2%, heat shrinkage TD 100 ℃/10 min: 1.89 percent.

Example 6:

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

wherein the extrusion temperature is 188 ℃, the CR cooling temperature is 25 ℃, the stretching ratio is 3.9 multiplied by 3.9, the setting temperature is 125 ℃, and the aluminum plating reinforced coating layer is arranged.

thickness 6 μm, tensile strength MD: 120Mpa, tensile strength TD: 116Mpa, light transmittance 93.1%, haze 2%, heat shrinkage MD 100 ℃/10 min: 4.2%, heat shrinkage TD 100 ℃/10 min: 1.52 percent.

Example 7:

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

the lower surface layer comprises the following components in percentage by weight: 75 wt% of crystalline PLA-3 slices and 25 wt% of PLA-2 opening agent master batch;

wherein the extrusion temperature is 186 ℃, the CR cooling temperature is 38 ℃, the draw ratio is 3.5 multiplied by 3.5, the setting temperature is 125 ℃ and the printing coating layer is arranged.

The biodegradable two-way synchronous stretching polylactic acid film produced by the components has the following data:

thickness 50 μm, tensile strength MD: 156Mpa, tensile strength TD: 145Mpa, transmittance 93.5%, haze 2%, heat shrinkage MD 100 ℃/10 min: 3.87%, thermal shrinkage TD 100 ℃/10 min: 1.65 percent.

Example 8:

the core layer is: 100 wt% of a crystalline PLA-1 pellet;

wherein the extrusion temperature is 178 ℃, the CR cooling temperature is 45 ℃, the stretching ratio is 3.5 multiplied by 3.5, the setting temperature is 125 ℃, and the coating layer has antistatic property.

thickness 45 μm, tensile strength MD: 149Mpa, tensile strength TD: 145Mpa, light transmittance 93.2%, haze 2.4%, heat shrinkage MD 100 ℃/10 min: 3.98%, heat shrinkage TD 100 ℃/10 min: 1.36 percent.

In the above embodiments 1-8, the preparation of the PLA-2 opener master batch includes the following steps:

step 1: dehumidifying, stirring and drying the polylactic acid particles at the temperature of 80-120 ℃, wherein the drying time is 2-6 h, preferably 4-6h, and the final water content of drying is controlled to be not more than 500ppm as a drying end point, preferably not more than 200ppm as a drying end point;

and step 3: and (3) the materials obtained in the step (1) and the step (2) are subjected to precise weightlessness weighing and burdening according to a set proportion, blending extrusion is carried out on a double-screw extruder, and vacuum volatiles are pumped out, wherein the processing temperature of the double-screw extruder is 150-220 ℃.

And 4, step 4: and then extruding out through an oral die, performing air cooling drying and dicing to obtain PLA-2 opening agent master batches.

The preparation method of the biodegradable bidirectional synchronous stretched polylactic acid film comprises the following steps:

s1: preparing PLA-2 opening agent master batch according to the preparation steps of the PLA-2 opening agent master batch;

s5: carrying out heat treatment setting on the polylactic acid film obtained in the step S4, cooling and rolling to obtain a biodegradable bidirectional synchronous stretched polylactic acid film;

The process flow of the invention is as follows:

the method comprises the steps of equipment cleaning preparation → preparation of crystalline PLA-1 slicing raw materials, non-crystalline PLA-3 slicing raw materials and PLA-2 opening agent master batch raw materials → drying of the raw materials → extrusion → melting → filtration → die head distribution → flow casting into sheets → functional surface coating → bidirectional stretching → sizing → traction → trimming → rolling → slitting → packaging.

The biodegradability of the invention is tested as follows:

the biodegradability of the obtained film is evaluated by a composting degradation method in International GB/T19277.1-2001, the biodegradability of the film obtained in example 3 is tested, and the 180-day biodegradation data is as follows:

the average biological decomposition rate is 91.6 percent, the reference material is cellulose, the average biological decomposition rate is 85.4 percent, the biological decomposition rate is higher than that of the reference material, and the relative biological decomposition rate of the polylactic acid film obtained by the invention is 107 percent, so the biaxially stretched polylactic acid film obtained by the invention has good biological degradability, and the table is as follows:

while the present embodiments have been described with reference to the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiments described above, which are meant to be illustrative and not restrictive, and that various changes may be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A biodegradable two-way synchronous stretching polylactic acid film comprises an upper surface layer (1), a core layer (2) and a lower surface layer (3), and is characterized in that:

the upper surface layer (1) comprises the following components in percentage by weight: 60-90 wt% of crystalline PLA-1 chips and 10-40 wt% of PLA-2 opening agent master batch;

the core layer (2) is: slicing the crystalline PLA-1;

the lower surface layer (3) comprises the following components in percentage by weight: 60-90 wt% of PLA slices and 10-40 wt% of PLA-2 opening agent master batch.

2. The biodegradable biaxially oriented polylactic acid film according to claim 1, wherein:

the PLA slice in the lower surface layer (3) is a crystalline PLA-1 slice.

3. The biodegradable biaxially oriented synchronously oriented polylactic acid film according to claim 1, wherein:

the PLA slices in the lower surface layer (3) are non-crystalline PLA-3 slices.

4. The biodegradable biaxially oriented polylactic acid film according to claim 1, further comprising:

and the coating layer is an aqueous coating liquid and is positioned on the surfaces of the upper surface layer (1) and the lower surface layer (3).

5. The biodegradable biaxially oriented polylactic acid film according to claim 4, wherein the coating layer comprises:

wherein the particle size of the silicon dioxide particles is 50-100 nm.

6. The biodegradable biaxially oriented simultaneously oriented polylactic acid film according to claim 4, wherein said coating layer comprises:

30 parts of polylactic acid butanediol ester, 6-12 parts of polybutylene succinate, 22-26 parts of isophorone diisocyanate, 9-11 parts of hexamethylene diisocyanate, 5.1-5.4 parts of hydrophilic chain extender, 7.1-7.5 parts of cross-linking agent, 3-3.6 parts of hydroxy acrylate, 20-30 parts of acetone, 3-3.6 parts of neutralizer, 70-90 parts of deionized water and 1.2-2.1 parts of diamine.

7. The biodegradable biaxially oriented polylactic acid film according to claim 4, wherein the coating layer comprises:

wherein the alumina particles have a particle size of 2-3 μm.

8. The biodegradable biaxially oriented polylactic acid film according to claim 1, wherein the PLA-2 opening agent masterbatch comprises:

9. The biodegradable biaxially oriented and simultaneously stretched polylactic acid film according to claim 8, wherein the preparation process of the PLA-2 opening agent masterbatch comprises the following steps:

10. The preparation method of the biodegradable biaxially oriented synchronously oriented polylactic acid film applied to the film of claim 9 is characterized by comprising the following steps:

s1: preparing a PLA-2 opening agent masterbatch according to the preparation steps of the PLA-2 opening agent masterbatch of claim 6;

s2: drying the crystalline PLA-1 slices, the non-crystalline PLA-3 slices and the PLA-2 opening agent master batch obtained in S1, and then extruding, melting, casting and cooling on a three-layer extruder to form sheets;

s5: carrying out heat treatment setting on the polylactic acid film obtained in the step S4, cooling and rolling to obtain the biodegradable bidirectional synchronous stretched polylactic acid film;