Transparent polylactic acid film having a network crystal structure and method for manufacturing the same

Abstract

The application provides a transparent polylactic acid film with a network crystal structure and a preparation method thereof, wherein the preparation method is as follows: 95-99.5 parts of PLA and 0.5-5 parts of EBH nucleating agent are mixed as a mixture, then the mixture is sequentially subjected to screw melting extrusion granulation, hot pressing forming, and a stretched sheet is prepared, and finally the stretched sheet is subjected to unidirectional or bidirectional solid-phase stretching treatment, and a transparent polylactic acid film with a three-dimensional crystal network structure is prepared. The preparation method selects specific N, N-ethylene bis(12-hydroxystearamide) as a nucleating agent, and combines with a solid-phase stretching technology, and a PLA film product with high transparency is prepared. It is found that under a special solid-phase stretching ratio, a unique network crystal structure is formed between PLA and the EBH nucleating agent, and the film product is excellent in strength, heat resistance and toughness.

Description

Technical Field

[0001] The present invention belongs to the technical field of polylactic acid-based film materials, and relates to a transparent polylactic acid film with a network crystal structure and a preparation method thereof, and specifically to a method for preparing a polylactic acid film with super-strong, super-tough, heat-resistant and transparent properties by using polymer solid-phase stretching technology. Background Art

[0002] Biodegradable polymers are materials that undergo chemical, biological, or physical changes in their chemical structure in the natural environment, mediated by microorganisms (such as bacteria, fungi, and algae) or plant and animal tissue cells and enzymes. This leads to main chain scission, a decrease in molecular weight, and the subsequent formation of small molecules, ultimately transforming into water, carbon dioxide, methane, and other small molecules. These materials are non-toxic, harmless, and fully absorbed by the environment, resulting in no pollution. They have broad application prospects in the field of disposable plastic packaging products that require landfill disposal. Common synthetic biodegradable polymers include aliphatic polyesters, polyester ethers, polyphosphazenes, polyorthoesters, polycarbonates, polyanhydrides, and polyamino acids. Currently commercially produced in my country include polylactic acid (PLA), copolymers of polybutylene adipate and polybutylene terephthalate (PBAT), polycaprolactone (PCL), polypropylene carbonate (PPC), polyglycolic acid (PGA), polyhydroxyalkanoates (PHA), and polymers of dibasic acids and dibasic esters. Among them, PLA has excellent biodegradability, biocompatibility, and mechanical properties, and its raw material, lactic acid, is derived from biomaterials such as starch and cellulose, making it the mainstream biodegradable plastic product. However, PLA has only one methylene carbon atom between the ester groups in the main chain. The short and rigid repeating units give the molecular chain a helical structure, resulting in a slow crystallization rate and almost no crystallization during molding processes such as extrusion and injection molding. This results in long molding cycles and low heat deformation temperatures, limiting its application range.

[0003] At present, in order to improve the above-mentioned shortcomings of PLA materials, researchers have focused on the modification research of PLA itself, but most of the research aims to improve a specific performance alone, such as improving the heat resistance and toughness of PLA through modification; on the other hand, by adding fillers, through blending systems or reinforcement systems to improve the comprehensive performance of PLA-based composite materials, further broaden the application range of PLA materials.

[0004] However, the existing technologies and process methods for improving PLA materials mentioned above inevitably sacrifice the transparency of PLA products, especially PLA films, resulting in a significant deterioration in transparency after PLA modification. Furthermore, when adding fillers, the transparency of the final product depends greatly on the choice of filler. Furthermore, the processing aids required in the blending system or reinforcement system can also lead to a significant deterioration in product transparency.

[0005] Therefore, it is urgent to develop a polylactic acid film with excellent comprehensive performance and good transparency and a preparation process thereof, which is very important for widening the application field of PLA. SUMMARY

[0006] The present application provides a transparent polylactic acid film with network crystal structure and a preparation method thereof, which is prepared by selecting a specific N,N-ethylenebis-(12-hydroxystearamide) (EBH) as a nucleating agent and combining a solid-phase stretching technique, thereby obtaining a PLA film product with high transparency. It is found that a unique network crystal structure is formed between PLA and the EBH nucleating agent under a special solid-phase stretching ratio, which endows the film product with excellent strength, heat resistance and toughness.

[0007] To achieve the above-mentioned purpose, the present application is realized by adopting the technical scheme composed of the following technical measures.

[0008] The present application provides a preparation method of a transparent polylactic acid film with network crystal structure, which mainly comprises the following steps:

[0009] (1) The raw materials mainly comprising the following components are mixed by weight fraction to prepare materials, as a mixture:

[0010] PLA 95-99.5 parts,

[0011] EBH nucleating agent 0.5-5 parts,

[0012] wherein the total amount of polylactic acid and EBH nucleating agent is 100 parts;

[0013] (2) The mixture prepared in step (1) is sequentially subjected to screw melt extrusion granulation and hot pressing molding to prepare a stretching sheet for film product;

[0014] (3) The stretching sheet obtained in step (2) is subjected to unidirectional or bidirectional solid-phase stretching treatment to prepare a transparent polylactic acid film with network crystal structure;

[0015] The process parameters of unidirectional solid-phase stretching treatment are as follows: the stretching temperature is 70-120℃, the preheating time is 5-120s, the stretching ratio is 3-6 times, and the stretching rate is 5-30% / s;

[0016] The bidirectional solid-phase stretching treatment is synchronous stretching or asynchronous stretching. When the synchronous stretching is used, the process parameters are as follows: the stretching temperature is 70-120°C, the preheating time is 5-120s, the stretching ratio is 3x3-6x6 times, and the stretching rate is 5-30% / s. When the asynchronous stretching is used, the process parameters are as follows: the stretching temperature is 70-120°C, the preheating time is 5-120s, the stretching ratio of one stretching direction is 3-6 times, the stretching ratio of the other stretching direction is 1-3 times, and the stretching rate is 5-30% / s.

[0017] In the present application, the PLA in step (1) is a biodegradable material, and can be directly selected from conventional industrial PLA raw materials in the technical field. Those skilled in the art can select appropriate PLA grades according to specific needs and process requirements, or refer to conventional PLA grades in the application field of the final product, especially PLA raw materials suitable for film products in the prior art.

[0018] In one of the technical solutions, the PLA in step (1) is selected from any one of the following: NatureWorks 4032D, Dow L175, Haizheng Bio REVODE110, and Fengyuan FY801 / FY802.

[0019] In the present application, the EBH nucleating agent in step (1) is N,N-ethylenebis-(12-hydroxystearamide) (EBH), which can be directly obtained from the market or synthesized according to technical documents.

[0020] In one of the technical solutions, the mixture in step (1) can also be added with conventional additives used in PLA processing and molding, so as to further expand the functions of the product / process. Those skilled in the art can refer to the prior art or existing documents for the selection of specific additives, such as lubricants, flame retardants, anti-aging agents, thermal stabilizers, plasticizers, antibacterial agents, and other processing aids / function aids. It should be noted that the mixture in step (1) can or can not include conventional additives used in PLA processing and molding. In the preferred technical solutions and specific embodiments described below, no additives are added to the mixture in order to minimize the influencing factors in the comparative experiments. However, this does not mean that appropriate additives cannot be added to the mixture. It should be noted that the addition of the above-mentioned additives should not affect the transparency of the prepared PLA film. When selecting additives, additives that do not cause staining / color difference should be selected, and the total amount of the additives should not be higher than 30 parts.

[0021] In this article, the mixed material in step (2) is sequentially subjected to screw melt extrusion granulation and hot pressing to prepare a sheet to be stretched for film products, wherein screw melt extrusion granulation is a conventional process for making parts using PLA materials, and its specific process steps / process parameters can be directly referred to the existing technical records / existing process methods for screw melt extrusion granulation of PLA raw materials.

[0022] In one of the technical solutions, the mixture in step (2) is sequentially subjected to screw melt extrusion granulation and hot pressing molding, wherein the screw melt extrusion granulation is processed by a twin-screw extruder, and the specific process parameters are: the temperature from the feed port to the die is in the range of 140 to 200°C, the interval between each screw temperature zone does not exceed 20°C, and the screw speed is 30 to 100 r / min.

[0023] In this article, the mixed material in step (2) is sequentially subjected to screw melt extrusion granulation and hot pressing to prepare a sheet to be stretched for film products, wherein hot pressing is a conventional process for making parts using PLA materials. The specific process steps / process parameters can be directly referred to the existing technical records / existing process methods for hot pressing of PLA raw materials.

[0024] In one of the technical solutions, the mixture in step (2) is sequentially melt-extruded into granules and hot-pressed by a screw, wherein the hot-pressing is performed using a flat-plate vulcanizer, and the specific process parameters are: pressure of 2 to 10 MPa, upper plate temperature of 170 to 220°C, lower plate temperature of 170 to 220°C, preheating for 2 to 10 minutes, hot pressing for 2 to 7 minutes, and cold pressing for 2 to 6 minutes.

[0025] The main point of the present invention is that, based on the research results of the research group, the inventors of the present invention accidentally discovered that when EBH nucleating agent is selected, under a special solid-phase stretching ratio, the interaction between PLA and EBH nucleating agent can form a unique network crystal structure, giving the film product excellent strength, heat resistance and toughness.

[0026] In previous research results, the inventor's research group discovered and reported a new strategy for using EBH nucleating agents to effectively regulate the crystallization rate and crystal size of left-handed polylactic acid (PLLA) to obtain high-performance transparent polylactic acid. The results showed that by adding 0.4-0.8wt% EBH nucleating agent to PLLA, highly crystalline (crystallinity as high as 45%) PLLA products can be successfully prepared using a melt molding method. They have excellent heat resistance and optical transparency (transmittance can reach more than 70%).

[0027] On the basis of the above research results, the inventors of the present application accidentally found that, when preparing a transparent polylactic acid film product using an EBH nucleating agent, a unique network crystal structure is formed between PLA and the EBH nucleating agent through etching of the product and observation of the cross-section by electron microscopy at a special solid-phase stretching ratio, which is completely different from the nanofiber network structure formed by self-assembly of EBH nucleating agent small molecules in the matrix during the melting and cooling process in the above research results in principle and characterization, and which significantly improves the mechanical properties of the transparent polylactic acid film product. In order to better verify the above accidental discovery, in further comparative experiments, on the one hand, it is verified that the above unique network crystal structure can only be well formed at a special solid-phase stretching ratio (3-6 times), and below this stretching ratio, the orientation degree of the polylactic acid molecular chain is low, and the network crystal structure cannot be obtained, while above this stretching ratio, the network crystal structure is destroyed to form a unidirectional fiber structure; on the other hand, it is verified through comparative experiments that the above unique network crystal structure only exists in the interaction between PLA and the EBH nucleating agent, and when other nucleating agents are used, no network crystal structure similar to the above unique network crystal structure is formed.

[0028] Based on the above invention point, in order to further improve the performance enhancement brought by the network crystal structure, in a more preferred technical solution, the EBH nucleating agent in step (1) is 1-2 parts, and most preferably 1.4-1.8 parts.

[0029] It should be noted that when the addition amount of EBH exceeds 4 parts, the transparency of the prepared polylactic acid film will be greatly deteriorated.

[0030] The present application has the following beneficial effects:

[0031] 1. The present application provides a transparent polylactic acid film with a network crystal structure and a preparation method thereof. The preparation method uses a specific EBH as a nucleating agent and combines solid-phase stretching technology to prepare a PLA film product with high transparency. It is found that, at a special solid-phase stretching ratio, a unique network crystal structure is formed between PLA and the EBH nucleating agent, which gives the film product excellent strength, heat resistance and toughness.

[0032] 2. The technical solution of the present application has a simple preparation method, does not need to modify the production equipment, can be directly implemented in the production process of existing polylactic acid film products, can bring significant performance improvement, has considerable development and conversion prospects in the field of polymer-based film materials, and has good commercial value.

[0033] 3. The application provides a transparent polylactic acid film with a network crystal structure and a preparation method thereof, which uses polylactic acid as the main raw material and does not contain other fillers, has excellent biodegradability, can be directly used as a packaging material and meets the environmental protection requirements. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The longitudinal section electron microscope micrographs of the comparative sample prepared in Comparative Example 4 (left) and the sample prepared in Example 1 (right) after etching are shown in the figure. In the figure, (a) is the longitudinal section electron microscope micrograph of the comparative sample prepared in Comparative Example 4 after etching, and (b) is the longitudinal section electron microscope micrograph of the sample prepared in Example 1 after etching.

[0035] Figure 2 The mechanical property comparison column graphs of the comparative samples prepared in Comparative Examples 4-9 and the samples prepared in Examples 1 and 3 are shown in the figure. In the figure, in (a), the tensile ratio horizontal coordinate of 1 refers to the comparative sample prepared in Comparative Example 7, the tensile ratio horizontal coordinate of 2 refers to the comparative sample prepared in Comparative Example 6, the tensile ratio horizontal coordinate of 4 refers to the comparative sample prepared in Comparative Example 5, and the tensile ratio horizontal coordinate of 6 refers to the comparative sample prepared in Comparative Example 4; in (b), the tensile ratio horizontal coordinate of 1 refers to the comparative sample prepared in Comparative Example 9, the tensile ratio horizontal coordinate of 2 refers to the comparative sample prepared in Comparative Example 8, the tensile ratio horizontal coordinate of 4 refers to the sample prepared in Example 3, and the tensile ratio horizontal coordinate of 6 refers to the sample prepared in Example 1.

[0036] Figure 3The loss factor and storage modulus of the comparative sample prepared in Comparative Example 4 of the present application and the sample prepared in Example 1 were compared with respect to temperature. In the figure, PLA-1 in (a) refers to the comparative sample prepared in Comparative Example 7, PLA-2 refers to the comparative sample prepared in Comparative Example 6, PLA-4 refers to the comparative sample prepared in Comparative Example 5, and PLA-6 refers to the comparative sample prepared in Comparative Example 4; PLA-1 in (b) refers to the comparative sample prepared in Comparative Example 7, PLA-2 refers to the comparative sample prepared in Comparative Example 6, PLA-4 refers to the comparative sample prepared in Comparative Example 5, and PLA-6 refers to the comparative sample prepared in Comparative Example 4; PLA / EBH-1.5wt-1 in (c) refers to the comparative sample prepared in Comparative Example 9, PLA / EBH-1.5wt-2 refers to the comparative sample prepared in Comparative Example 8, PLA / EBH-1.5wt-4 refers to the sample prepared in Example 3, and PLA / EBH-1.5wt-6 refers to the sample prepared in Example 1; and PLA / EBH-1.5wt-1 in (d) refers to the comparative sample prepared in Comparative Example 9, PLA / EBH-1.5wt-2 refers to the comparative sample prepared in Comparative Example 8, PLA / EBH-1.5wt-4 refers to the sample prepared in Example 3, and PLA / EBH-1.5wt-6 refers to the sample prepared in Example 1.

[0037] Figure 4 The actual photos and transparency verification photos of the comparative sample prepared in Comparative Example 4 of the present application and the sample prepared in Example 1 after being heated in an oven at 80℃ for 20 minutes were taken. In the figure, (a) is a photo of the comparative sample prepared in Comparative Example 4 and the sample prepared in Example 1 in the oven; (b) is an actual photo of the comparative sample prepared in Comparative Example 4 after being heated in an oven at 80℃ for 20 minutes; (c) is an actual photo of the sample prepared in Example 1 after being heated in an oven at 80℃ for 20 minutes; and (d) is a transparency verification photo of the sample prepared in Example 1 covering a pattern.

[0038] Figure 5 The longitudinal cross-sectional electron microscope micrograph and transparency verification photo of the comparative sample prepared in Comparative Example 1 after etching were taken. In the figure, (a) is a longitudinal cross-sectional electron microscope micrograph of the comparative sample prepared in Comparative Example 1 after etching, and (b) is a transparency verification photo of the comparative sample prepared in Comparative Example 1 covering a pattern.

[0039] Figure 6A mechanical property comparison column chart of the comparative sample prepared in Comparative Example 4 and the samples prepared in Examples 1-2 and a longitudinal section electron microscope micrograph of the sample prepared in Example 2 after etching are shown in the figure. In the figure, the left figure is the mechanical property comparison column chart of the comparative sample prepared in Comparative Example 4 and the samples prepared in Examples 1-2, PLA-6 refers to the comparative sample prepared in Comparative Example 4, PLA / EBH-1wt-6 refers to the sample prepared in Example 2, and PLA / EBH-1.5wt-6 refers to the sample prepared in Example 1; the right figure is the longitudinal section electron microscope micrograph of the sample prepared in Example 2 after etching. DETAILED DESCRIPTION

[0040] In order to further understand the present application, the preferred embodiments of the present application are described below in conjunction with the examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, and are not limitations to the claims of the present application. Those skilled in the art can modify the process parameters according to the content herein. It is particularly pointed out that all similar substitutions and modifications are obvious to those skilled in the art, and they are considered to be included in the present application. The methods and applications of the present application have been described by the preferred embodiments, and the related personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present application, to realize and apply the present application technology. Although it is believed that the following terms are well understood by those skilled in the art, the following definitions are stated to facilitate the explanation of the disclosed subject matter.

[0041] The present application provides a preparation method of a transparent polylactic acid film with a network crystal structure, mainly comprising the following steps:

[0042] (1) The raw materials mainly comprising the following components are mixed by weight fraction to prepare the materials, as the mixed materials:

[0043] PLA 95-99.5 parts,

[0044] EBH nucleating agent 0.5-5 parts,

[0045] wherein the total amount of polylactic acid and EBH nucleating agent is 100 parts;

[0046] (2) The mixed materials prepared in step (1) are sequentially subjected to screw melt extrusion granulation and hot pressing molding to prepare a stretching sheet for film products;

[0047] (3) The stretching sheet obtained in step (2) is subjected to unidirectional or bidirectional solid-phase stretching treatment to prepare a transparent polylactic acid film with a network crystal structure;

[0048] The process parameters of the unidirectional solid-phase stretching treatment are as follows: stretching temperature is 70-120°C, preheating time is 5-120s, stretching ratio is 3-6 times, and stretching rate is 5-30% / s.

[0049] The process parameters of the bidirectional solid-phase stretching treatment are as follows: when the stretching is synchronous, the stretching temperature is 70-120°C, the preheating time is 5-120s, the stretching ratio is 3x3-6x6 times, and the stretching rate is 5-30% / s; when the stretching is asynchronous, the stretching temperature is 70-120°C, the preheating time is 5-120s, the stretching ratio of one stretching direction is 3-6 times, the stretching ratio of the other stretching direction is 1-3 times, and the stretching rate is 5-30% / s.

[0050] In this article, the PLA in step (1) is named as polylactic acid, which is a biodegradable material. The PLA raw material can be directly selected according to the conventional industrial PLA raw material in the technical field. The skilled person in the art can select the appropriate PLA grade according to the specific needs and process requirements. The PLA grade can also be selected according to the conventional PLA grade in the application field of the final product, especially the PLA raw material suitable for the existing technology as a film product raw material.

[0051] In one embodiment, the PLA in step (1) is selected from any one of the following: NatureWorks 4032D, Dow L175, Haizheng Bio REVODE110, and Fengyuan FY801 / FY802.

[0052] In this article, the EBH nucleating agent in step (1) is named as N,N-ethylenebis-(12-hydroxystearamide) (EBH). The skilled person in the art can directly obtain it by market acquisition or self-synthesis according to technical literature.

[0053] In one embodiment, the EBH nucleating agent in step (1) is 0.5-5 parts, for example, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1.0 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2.0 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, or any range or point value between them.

[0054] In one embodiment, the mixture in step (1) can also be added with the auxiliary agents commonly used in PLA processing and molding to achieve further functional expansion of the product / process assistance. The specific selection of auxiliary agents can be made by referring to the existing technology or existing literature, such as lubricants, flame retardants, anti-aging agents, thermal stabilizers, plasticizers, antibacterial agents, and other processing aids / function aids. It is noted that the mixture in step (1) can or can not include the auxiliary agents commonly used in PLA processing and molding. In the following preferred technical solutions and specific embodiments, in order to minimize the influencing factors in the comparative experiments, no auxiliary agents are added to the mixture, but this does not mean that the mixture cannot add appropriate auxiliary agents. However, it is important to note that the addition of the above-mentioned auxiliary agents is subject to the transparency of the prepared polylactic acid film, and when selecting auxiliary agents, auxiliary agents that do not cause staining / color difference should be selected, and the total amount of auxiliary agents should not exceed 30 parts.

[0055] In this paper, the mixture in step (2) is sequentially prepared into a stretching sheet for film products by screw melting extrusion granulation and hot pressing. The screw melting extrusion granulation is a conventional process for PLA material products, and the specific process steps / process parameters can be directly referred to the existing technical records / existing process methods of PLA raw materials during screw melting extrusion granulation.

[0056] In one embodiment, the mixture in step (2) is sequentially prepared into a stretching sheet for film products by screw melting extrusion granulation and hot pressing. The screw melting extrusion granulation is processed by a double-screw extruder, and the specific process parameters are as follows: the temperature from the feeding port to the die is in the range of 140-200°C, the temperature interval of each screw section is not more than 20°C, and the screw rotation speed is 30-100 r / min.

[0057] In this paper, the mixture in step (2) is sequentially prepared into a stretching sheet for film products by screw melting extrusion granulation and hot pressing. The hot pressing is a conventional process for PLA material products, and the specific process steps / process parameters can be directly referred to the existing technical records / existing process methods of PLA raw materials during hot pressing.

[0058] In one embodiment, the mixture in step (2) is sequentially prepared into a stretching sheet for film products by screw melting extrusion granulation and hot pressing. The hot pressing is processed by a flat vulcanizing machine, and the specific process parameters are as follows: the pressure is 2-10 MPa, the upper plate temperature is 170-220°C, the lower plate temperature is 170-220°C, the preheating time is 2-10 min, the hot pressing time is 2-7 min, and the cold pressing time is 2-6 min.

[0059] In one embodiment, the stretching temperature in step (3) is 70-120°C, such as 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, or any range or point value between them; the preheating time is 5-120s, such as 5s, 10s, 15s, 20s, 25s, 30s, 35s, 40s, 50s, 60s, 70s, 80s, 90s, 100s, 110s, 120s, or any range or point value between them; the stretching ratio is 3-6 times, such as 3 times, 3.5 times, 4 times, 4.5 times, 5 times, 5.5 times, 6 times, or any range or point value between them; and the stretching rate is 5-30% / s, such as 5% / s, 6% / s, 7% / s, 8% / s, 9% / s, 10% / s, 11% / s, 12% / s, 13% / s, 14% / s, 15% / s, 16% / s, 17% / s, 18% / s, 19% / s, 20% / s, 21% / s, 22% / s, 23% / s, 24% / s, 25% / s, 26% / s, 27% / s, 28% / s, 29% / s, 30% / s, or any range or point value between them.

[0060] The main inventive point of the present application is that the inventors of the present application have found, by chance, that when the EBH nucleating agent is selected, a unique network crystal structure can be formed between PLA and the EBH nucleating agent at a special solid-phase stretching ratio, which endows the film product with excellent strength, heat resistance, and toughness.

[0061] In the previous research results, the research group of the inventors found and reported a new strategy for effectively regulating the crystallization rate and crystal size of poly-L-lactic acid (PLLA) and obtaining high-performance transparent poly-L-lactic acid by using an EBH nucleating agent. The results showed that by adding 0.4-0.8 wt% of the EBH nucleating agent to PLLA, highly crystalline (crystallinity of 45%) PLLA products can be successfully prepared by a melt forming process, which has excellent heat resistance and optical transparency (transmittance of more than 70%).

[0062] On the basis of the above research results, the inventors of the present application accidentally found that, when preparing a transparent polylactic acid film product using an EBH nucleating agent, a unique network crystal structure is formed between PLA and the EBH nucleating agent through etching of the product, which is completely different from the nanofiber network structure formed by self-assembly of EBH nucleating agent small molecules in the matrix during the melting and cooling process in the above research results in principle and characterization, and which significantly improves the mechanical properties of the transparent polylactic acid film product. In order to better verify the above accidental discovery, in further comparative experiments, on the one hand, it is verified that the above unique network crystal structure can only be well formed at a special solid-phase stretching ratio (3-6 times), and below this stretching ratio, the orientation degree of the polylactic acid molecular chain is low, and the network crystal structure cannot be obtained, while above this stretching ratio, the network crystal structure is destroyed to form a unidirectional fiber structure; on the other hand, it is verified through comparative experiments that the above unique network crystal structure only exists in the interaction between PLA and the EBH nucleating agent, and when other nucleating agents are used, no network crystal structure similar to the above unique network crystal structure is formed.

[0063] Based on the above invention point, in order to further improve the performance enhancement brought by the network crystal structure, in a more preferred technical solution, the EBH nucleating agent in step (1) is 1-2 parts, and most preferably 1.4-1.8 parts.

[0064] It should be noted that when the addition amount of EBH exceeds 4 parts, the transparency of the prepared polylactic acid film will be greatly deteriorated.

[0065] The present application will be further explained in detail with reference to the following examples. However, those skilled in the art will understand that the examples are provided only for illustrative purposes, and are not intended to limit the scope of the present application.

[0066] Examples

[0067] The embodiments of the present application will be described in detail below with reference to the examples, but those skilled in the art will understand that the following examples are only for the purpose of illustration, and should not be regarded as limiting the scope of the present application. If no specific conditions are specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If no manufacturer of the reagent or instrument is specified, it is a conventional product that can be obtained by purchase. The present application should not be interpreted as being limited to the specific examples described.

[0068] 1. Raw materials

[0069] PLA: Fengyuan FY801 pellets;

[0070] EBH nucleating agent: Jiangxi Dongyuan Technology Co., Ltd.

[0071] TMC nucleating agent, TMC-300.

[0072] 2. Test method

[0073] 1) Tensile property test

[0074] The elongated section of the sample after solid phase stretching is cut to an appropriate length, and then both ends are clamped by the tensile test fixture of the electronic universal material testing machine, with 25 mm reserved in the middle as the test section. Then the tensile property test is carried out at a speed of 5 mm / min.

[0075] 2) Scanning electron microscope test (SEM):

[0076] The sample to be observed is cut along the stretching direction to obtain a longitudinal section slice, which is then placed in a methanol / water solution containing sodium hydroxide for etching to remove amorphous phases. Finally, it is placed on a sample stage and observed and photographed by scanning electron microscope after gold spraying treatment to obtain the longitudinal section electron microscope micrograph.

[0077] 3) Thermal property test (DMA):

[0078] The elongated section of the sample after solid phase stretching is cut to an appropriate length, and then both ends are clamped by the tensile test fixture of the electronic universal material testing machine, with 25 mm reserved in the middle as the test section. Then the tensile property test is carried out at a speed of 5 mm / min.

[0079] 3. Preparation method

[0080] (1) The raw materials mainly including the following components are mixed by weight fraction to prepare the material, which is the mixed material:

[0081] PLA 95-99.5 parts,

[0082] EBH nucleating agent 0.5-5 parts,

[0083] wherein the total amount of polylactic acid and EBH nucleating agent is 100 parts;

[0084] (2) The mixed material prepared in step (1) is sequentially subjected to screw melting extrusion granulation and hot pressing to prepare a stretching sheet for film products;

[0085] The screw melting extrusion granulation is processed by a twin-screw extruder, and the specific process parameters are as follows: the temperatures from the feeding port to the die are 140℃, 160℃, 170℃, 180℃, 180℃, 180℃, 185℃, 185℃, 185℃, 180℃, and 180℃, and the screw rotation speed is 60 r / min;

[0086] Hot press molding is performed using a flat plate vulcanizing machine, and the specific process parameters are as follows: the pressure is 10 MPa, the upper plate temperature is 190°C, the lower plate temperature is 190°C, the preheating time is 6 min, the hot pressing time is 5 min, and the cold pressing time is 4 min.

[0087] (3) The obtained to-be-stretched sheet in step (2) is subjected to a unidirectional solid-phase stretching treatment to prepare a transparent polylactic acid film with a network crystal structure and a thickness of 200 μm;

[0088] The process parameters of the unidirectional solid-phase stretching treatment are as follows: the stretching temperature is 90°C, the preheating time is 20 s, the stretching ratio is 4-6 times, and the stretching rate is 15% / s.

[0089] Example 1

[0090] Example 1 is a transparent polylactic acid film with a network crystal structure prepared according to the above-mentioned “3. Preparation method” as a sample, wherein the addition amount of EBH nucleating agent is 1.5 parts, and the stretching ratio is 6 times.

[0091] Example 2

[0092] Example 2 is a transparent polylactic acid film with a network crystal structure prepared according to the above-mentioned “3. Preparation method” as a sample, wherein the addition amount of EBH nucleating agent is 1 part, and the stretching ratio is 6 times.

[0093] Example 3

[0094] Example 3 is a transparent polylactic acid film with a network crystal structure prepared according to the above-mentioned “3. Preparation method” as a sample, wherein the addition amount of EBH nucleating agent is 1.5 parts, and the stretching ratio is 4 times.

[0095] Example 4

[0096] Example 4 is a transparent polylactic acid film with a network crystal structure prepared according to the above-mentioned “3. Preparation method” as a sample, wherein the addition amount of EBH nucleating agent is 1 part, and the stretching ratio is 4 times.

[0097] Comparative Example 1

[0098] Comparative Example 1 is a polylactic acid film prepared according to the above-mentioned “3. Preparation method” as a comparative sample, but the EBH nucleating agent is replaced by TMC nucleating agent (octamethylene dicarboxylic acid diphenylhydrazide, TMC-300), and the addition amount is 1.5 parts, and the stretching ratio is 6 times.

[0099] Comparative Example 2

[0100] Comparative Example 2 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but the EBH nucleating agent is replaced by carbon nanotubes, and the addition amount is 1.5 parts, and the stretching ratio is 6 times.

[0101] Comparative Example 3

[0102] Comparative Example 3 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but the EBH nucleating agent is replaced by talc, and the addition amount is 1.5 parts, and the stretching ratio is 6 times.

[0103] Comparative Example 4

[0104] Comparative Example 4 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but no EBH nucleating agent is added, that is, the addition amount of polylactic acid is 100 parts, and the stretching ratio is 6 times.

[0105] Comparative Example 5

[0106] Comparative Example 5 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but no EBH nucleating agent is added, that is, the addition amount of polylactic acid is 100 parts, and the stretching ratio is 4 times.

[0107] Comparative Example 6

[0108] Comparative Example 6 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but no EBH nucleating agent is added, that is, the addition amount of polylactic acid is 100 parts, and the stretching ratio is 2 times.

[0109] Comparative Example 7

[0110] Comparative Example 7 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, but no EBH nucleating agent is added, that is, the addition amount of polylactic acid is 100 parts, and no unidirectional solid-phase stretching treatment in step (3) is performed, and the stretched sheet obtained in step (2) is directly used as a comparative sample for testing.

[0111] Comparative Example 8

[0112] Comparative Example 8 is a polylactic acid film prepared according to the above "3. Preparation method" as a comparative sample, wherein the addition amount of EBH nucleating agent is 1.5 parts, but the stretching ratio is 2 times.

[0113] Comparative Example 9

[0114] Comparative Example 9 is a polylactic acid film prepared according to the above "3. Preparation method", wherein the addition amount of EBH nucleating agent is 1.5 parts, but no unidirectional solid-phase stretching treatment in step (3) is performed, and the stretched sheet obtained in step (2) is directly used as a comparative sample for testing.

[0115] 4. Test Results

[0116] The test results are as follows Figures 1-6 As shown:

[0117] from Figure 1 It is obvious that the crystals of the comparison sample composed of pure PLA are clearly oriented along the stretching direction to form a fibrous crystal structure. After the addition of EBH nucleating agent, the crystal structure changes significantly under the joint action of PLA and EBH nucleating agent, forming a network crystal structure, which plays a role of physical cross-linking.

[0118] from Figure 2 It is obvious that at the same stretching ratio, the mechanical strength and elongation at break of the sample with the addition of EBH nucleating agent are higher than those of pure PLA, especially when the stretching ratio is 6 times, the tensile strength and elongation at break of the sample show a significant increase, reaching 112 MPa and 63% respectively. Compared with the control sample with the same stretching ratio, the increase is 53.4% ​​and 117% respectively.

[0119] exist Figure 3 In the figure, (a) is a comparison curve of the loss factor of the comparative sample at different stretching ratios as a function of temperature, and the glass transition temperature (T g ) is about 66.2℃; and as the stretching ratio increases, T g When the stretching ratio is 6, the T g The temperature increased to 79.9℃, an increase of 13.7℃. (b) The figure shows the storage modulus of the comparative samples at different stretching ratios as a function of temperature. The storage modulus of the comparative samples without solid-phase stretching treatment and with a stretching ratio of 2 decreases rapidly around 60℃. At 80℃, the storage modulus is only about 4MPa and 10MPa, respectively, indicating that the comparative samples have undergone severe thermal deformation at this temperature. As the temperature further increases, the comparative samples without solid-phase stretching treatment and with a stretching ratio of 2 undergo cold crystallization, and the storage modulus increases instead. However, the comparative samples become opaque due to cold crystallization. When the stretching ratio is increased to 4x, the thermal deformation of the sample at 80℃ is suppressed, the storage modulus increases to 300MPa, and no cold crystallization occurs. The comparative samples still maintain good transparency. When the stretching ratio is further increased to 6x, the storage modulus of the comparative samples at 80℃ increases significantly to 1500MPa. Even at 110℃, the storage modulus of the comparative samples is still 220MPa. Figure (c) shows the comparison curve of the loss factor of the sample with EBH nucleating agent and the control sample at different stretching ratios. It is obvious that when no solid phase stretching treatment is performed, the control sample undergoes severe thermal deformation at 80°C and also undergoes cold crystallization as the temperature rises. When the stretching ratio is increased to 6 times, the T gThe temperature is increased to 84.4℃, which is 4.5℃ higher than the pure PLA sample with the same draw ratio; and as shown in (d), the storage modulus of the sample at 110℃ is 350MPa, which is much higher than that of the pure PLA sample with the same draw ratio (220MPa), indicating that the heat resistance of the film product is further improved after adding the EBH nucleating agent.

[0120] From Figure 4 It is obvious that the comparative sample prepared in Comparative Example 4 is severely deformed under the same weight of the weight, while the sample prepared in Example 1 is hardly deformed. In addition, the sample prepared in Example 1 has excellent transparency.

[0121] As Figure 5 shown, after replacing the EBH nucleating agent with the TMC nucleating agent, the comparative sample prepared according to the same embodiment does not form a network crystal structure, but forms fibers along the stretching direction. In addition, the light transmittance of the product is poor. The same characterization phenomenon also occurs in the comparative samples prepared in Comparative Example 2 and Comparative Example 3, fully proving the necessity of adding the EBH nucleating agent.

[0122] From Figure 6 it is obvious that when 1wt% of the EBH nucleating agent is added in Example 2, the tensile strength is only increased from 73MPa of the pure PLA in Comparative Example 4 to 84MPa, which is much lower than that of the sample in Example 1 (112MPa) with the same 6 times draw ratio. By observing the etching electron microscope image, it is found that although the sample in Example 2 forms a certain network crystal structure, the network density is much lower than that of the sample in Example 1 with the same 6 times draw ratio, and the mechanical properties exhibited are also much lower than those of the sample in Example 1. Figure 1 it is obvious that when 1wt% of the EBH nucleating agent is added in Example 2, the tensile strength is only increased from 73MPa of the pure PLA in Comparative Example 4 to 84MPa, which is much lower than that of the sample in Example 1 (112MPa) with the same 6 times draw ratio. By observing the etching electron microscope image, it is found that although the sample in Example 2 forms a certain network crystal structure, the network density is much lower than that of the sample in Example 1 with the same 6 times draw ratio, and the mechanical properties exhibited are also much lower than those of the sample in Example 1.

[0123] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods, which are all included in the protection scope of the present application.

Claims

1. A method for preparing a transparent polylactic acid film having a network crystal structure, characterized in that The main steps include: (1) The raw materials mainly including the following components are mixed and prepared as a mixed material in parts by weight: PLA 95~99.5 parts, EBH nucleating agent 0.5~5 parts, The total amount of polylactic acid and EBH nucleating agent is 100 parts; (2) The mixed material prepared in step (1) is sequentially subjected to screw melt extrusion granulation and hot pressing to prepare a sheet to be stretched for film products; (3) subjecting the sheet to be stretched obtained in step (2) to uniaxial or biaxial solid phase stretching to prepare a transparent polylactic acid film having a network crystal structure; The process parameters of the unidirectional solid phase stretching treatment are as follows: stretching temperature 70-120°C, preheating time 5-120s, stretching ratio 3-6 times, stretching rate 5-30% / s; The biaxial solid-phase stretching treatment is synchronous stretching or asynchronous stretching. When it is synchronous stretching, the process parameters are: stretching temperature of 70~120℃, preheating time of 5~120s, stretching ratio of 3×3~6×6 times, and stretching rate of 5~30% / s; when it is asynchronous stretching, the process parameters are: stretching temperature of 70~120℃, preheating time of 5~120s, stretching ratio of one stretching direction is 3~6 times, stretching ratio of the other stretching direction is 1~3 times, and stretching rate is 5~30% / s.

2. The preparation method according to claim 1, wherein: The mixture in step (1) also includes additives used in PLA processing and molding.

3. The preparation method according to claim 2, characterized in that: The total amount of additives added is not more than 30 parts.

4. The preparation method according to claim 1, wherein: The mixed material in step (2) is sequentially subjected to screw melt extrusion granulation and hot pressing molding, wherein the screw melt extrusion granulation is processed by a twin-screw extruder, and the specific process parameters are: the temperature from the feed port to the die is in the range of 140~220℃, and the screw speed is 30~100r / min.

5. The preparation method according to claim 1, characterized in that: The mixed material in step (2) is sequentially melt-extruded into granules and hot-pressed by a screw, wherein the hot-pressing is performed using a flat-plate vulcanizing machine, and the specific process parameters are: pressure of 2-10 MPa, upper plate temperature of 170-220°C, lower plate temperature of 170-220°C, preheating for 2-10 min, hot pressing for 2-7 min, and cold pressing for 2-6 min.

6. The preparation method according to claim 1, characterized in that: The EBH nucleating agent in step (1) is 1 to 2 parts.

7. The transparent polylactic acid film prepared by the method for preparing a transparent polylactic acid film having a network crystal structure as claimed in claim 1.

8. Use of the transparent polylactic acid film as claimed in claim 7 as a packaging material.

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