A polylactic acid bio-composite plastic film and a method for regulating crystallization

Abstract

The present invention relates to the technical field of green natural materials, and particularly relates to a polylactic acid biocomposite plastic film and a method for regulating its crystallization. The method comprises the following steps: Step 1, by means of melt blending, first preheat an open mill to 170-200 °C, then add polylactic acid to the open mill and fully stir and melt it, and then add cottonseed protein and maleic anhydride to fully mix the materials. The weight ratio of polylactic acid, cottonseed protein and maleic anhydride is 20:0.5-1.2:0.1-0.3 to obtain a mixed material; Step 2, use a flat vulcanizing hot press, preheat it to 170-200 °C, place the mixed material into the template of the flat vulcanizing hot press for molding, and use a cooling device to control the flat vulcanizing hot press to cool the molded mixed material in a segmented cooling manner. The segmented cooling method is as follows: when the temperature drops to 120-130 °C, it is cooled to room temperature at 2 °C-10 °C / 5 min to obtain a polylactic acid biocomposite plastic film. This preparation method uses cottonseed protein as a nucleating agent, enabling polylactic acid to reach the crystallization point at a high temperature, effectively prolonging the crystallization process of polylactic acid, and improving the performance of the polylactic acid biocomposite film.

Description

Technical Field

[0001] The present invention relates to the technical field of green natural materials, and particularly relates to a method for regulating the crystallization of a polylactic acid biocomposite plastic film. Background Art

[0002] The environmental pollution caused by petroleum-based plastics is seriously affecting people's lives and health. As a green bio-based material that can replace petroleum-based plastics, polylactic acid (PLA) has been promoted in the fields of food packaging, drug delivery, tissue engineering, etc. due to its excellent transparency and mechanical properties. To improve the crystallinity of polylactic acid, at present, polylactic acid is combined with thermoplastic resins, polysaccharides or nano-compounds through means such as in-situ grafting and blending modification to provide effective nucleation sites for the polylactic acid matrix, promote the heterogeneous nucleation of polylactic acid, and improve the crystallinity of polylactic acid. Although this method can improve the crystallinity of polylactic acid, it has the problems of high cost and affecting the degradation of the polylactic acid film.

[0003] To solve this problem, it has been publicly reported to use amino acids to promote the nucleation of polylactic acid, but the requirement for the purity of amino acids is high, and there is still the problem of high cost. The prior art has publicly reported using proteins to replace amino acids as nucleating agents. For example, a starch-loaded polylactic acid crystallization nucleating agent and its preparation method disclosed in Chinese Patent CN201710000859.6 uses starch as the polylactic acid crystallization nucleating agent, and the crystallization temperature is 95-105°C. The application of natural protein powder as an aliphatic polyester nucleating agent disclosed in Chinese Patent CN201610097778.8 uses wool powder, cattle hair powder, feather powder, pig hair powder and silk powder as the nucleating agent for polylactic acid, and the highest temperature for nucleation is 70.4-108°C.

[0004] However, the above technologies still have deficiencies: only the promotion of the nucleation of polylactic acid by adding proteins is considered. However, the current proteins and polylactic acid only crystallize at relatively low temperatures. Therefore, the polylactic acid of the prior art needs to be cooled to a relatively low temperature before it can start to crystallize. On the one hand, the cooling time used is long, resulting in a slow overall crystallization process. On the other hand, the process in which proteins and polylactic acid can crystallize is relatively short, which will lead to insufficient crystallization and affect the mechanical properties or quality of the polylactic acid finished product. Summary of the Invention

[0005] One of the purposes of the present invention is to avoid the deficiencies in the prior art and provide a method for regulating the crystallinity of a polylactic acid biocomposite plastic film. This preparation method selects cottonseed protein as the nucleating agent, effectively improves the crystallization of polylactic acid at a relatively high temperature, enables polylactic acid to reach the crystallization point at a relatively high temperature, effectively extends the crystallization process of polylactic acid, and improves the performance of the polylactic acid biocomposite film.

[0006] To achieve one of the above-mentioned invention purposes, the following technical solutions are provided:

[0007] A method for regulating the crystallinity of a polylactic acid biocomposite plastic film, comprising the following steps:

[0008] Step 1: By means of melt blending, first preheat an open mill to 170 - 200 °C, then add polylactic acid to the open mill and fully stir and melt it. Subsequently, add cottonseed protein and maleic anhydride to make the materials fully mixed to obtain a mixed material, wherein the weight ratio of the polylactic acid, the cottonseed protein, and the maleic anhydride is 20:0.5 - 1.2:0.1 - 0.3;

[0009] Step 2: Use a flat vulcanizing hot press and preheat it to 170 - 200 °C. Place the mixed material into the mold of the flat vulcanizing hot press for molding. Subsequently, use a cooling device to control the flat vulcanizing hot press to cool the molded mixed material in a stepwise cooling manner. The stepwise cooling method is: when cooling to 120 - 130 °C, cool it to room temperature at 2 °C - 10 °C / 5 min to obtain a polylactic acid biocomposite plastic film.

[0010] In some embodiments, before the polylactic acid is placed into the open mill, it is first placed in an oven for drying. The drying temperature is 40 - 55 °C, and the drying time is 30 - 48 h.

[0011] In some embodiments, in Step 1, the stirring and melting time of the polylactic acid in the open mill is 8 min - 12 min.

[0012] In some embodiments, in Step 1, the stirring time after adding the cottonseed protein and maleic anhydride is 4 min - 8 min.

[0013] In some embodiments, in Step 1, the open mill is preheated to 180 °C.

[0014] In some embodiments, in Step 1, the weight ratio of the polylactic acid, the cottonseed protein, and the maleic anhydride is 20:1:0.2.

[0015] In some embodiments, the stepwise cooling method is: cool it to room temperature at 6 °C / 5 min.

[0016] In some embodiments, in Step 1, the cottonseed protein is added to the open mill within 1 min.

[0017] The beneficial effects of a method for regulating the crystallinity of a polylactic acid biocomposite plastic film according to the present invention:

[0018] (1) The method for regulating the crystallinity of the polylactic acid bio-composite plastic film of the present invention uses cottonseed protein as a nucleating agent for polylactic acid. Cottonseed protein is an alkali-soluble plant protein with a high protein content. The alkaline property of cottonseed protein can effectively improve the compatibility between cottonseed protein and polylactic acid with high liposolubility, enabling cottonseed protein and polylactic acid to be compatible more quickly. At the same time, the high protein content of cottonseed protein also provides a more abundant number of crystal nuclei, further promoting the crystallization rate between cottonseed protein and polylactic acid. Therefore, during cooling crystallization, crystallization between cottonseed protein and polylactic acid starts at the high temperature point of 120 °C, enabling polylactic acid to crystallize in advance, effectively extending the crystallization time, and allowing cottonseed protein and polylactic acid to crystallize fully, and then the crystal growth is sufficient. Thus, the density of the entire polylactic acid bio-composite plastic film is good, and the mechanical properties of the film are better. The present invention cools and crystallizes the crystallization material in a stepwise cooling manner, enabling sufficient cooling crystallization in each temperature stage, further improving the crystallization effect. Especially when crystallization occurs at a high temperature, the more stages of stepwise cooling, the more sufficient the cooling crystallization.

[0019] (2) The method for regulating the crystallinity of the polylactic acid bio-composite plastic film of the present invention prepares the polylactic acid bio-composite plastic film by melt blending. This melt blending method can blend cottonseed protein and polylactic acid uniformly without using a solvent, having the advantage of environmental protection.

[0020] (3) The method for regulating the crystallinity of the polylactic acid bio-composite plastic film of the present invention uses cottonseed protein as a nucleating agent. Cottonseed protein has a wide source and low cost, effectively replacing the use of amino acids, and is suitable for large-scale production and application.

[0021] Also provided is a polylactic acid bio-composite plastic film prepared by the above method for regulating the crystallinity of the polylactic acid bio-composite plastic film. Description of the Drawings

[0022] Figure 1 are the infrared spectra of the PLA / CPC polylactic acid bio-composite films of Examples 1 - 4 of the present invention, the polylactic acid film PLA of Comparative Example 1, and CPC, a) full wavelength range, b) 3500 - 2500 cm-1 range, c) 2000 - 1000 cm-1 range, d) 1000 - 500 cm-1 range.

[0023] Figure 2 are the DSC diagrams of the PLA / CPC polylactic acid bio-composite films of Examples 1 - 4 of the present invention and the polylactic acid film PLA of Comparative Example 1, a) first heating stage, b) first cooling stage, c) second heating stage.

[0024] Figure 3 are the isothermal crystallization diagrams of the polylactic acid film PLA of Comparative Example 1 taken at 0 min, 5 min, 10 min, and 15 min in sequence.

[0025] Figure 4 This is a schematic diagram of the isothermal crystallization of the PLA / CPC5 polylactic acid biocomposite film of Example 1 taken at 0 min, 5 min, 10 min and 15 min in sequence.

[0026] Figure 5 This is a schematic diagram of the isothermal crystallization of PLA / CPC10 in Example 2 taken at 0 min, 5 min, 10 min and 15 min respectively.

[0027] Figure 6 This is a schematic diagram of the isothermal crystallization of PLA / CPC20 in Example 4 taken at 0 min, 5 min, 10 min and 15 min respectively.

[0028] Figure 7 These are scanning electron microscope images of the polylactic acid film PLA of comparative example 1 and the PLA / CPC10 of example 2, wherein a) is the polylactic acid film PLA of comparative example 1; and b) is the PLA / CPC10 of example 2. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0030] The method for controlling the crystallinity of a polylactic acid biocomposite plastic film disclosed in this embodiment comprises the following steps:

[0031] Step 1: Using melt blending method, first preheat the open mill to 170-200° C., preferably 180° C., then add polylactic acid into the open mill and stir it thoroughly to melt, and the stirring and melting time is preferably 8-12 minutes.

[0032] The open mill is heated to 170-200°C to melt and mix the cottonseed protein and polylactic acid.

[0033] Then, cottonseed protein and maleic anhydride are added to fully mix the materials for 4 to 8 minutes. Maleic anhydride is used as a solubilizer to allow cottonseed protein and polylactic acid to fully form a uniform phase. The weight ratio of the polylactic acid, the cottonseed protein and the maleic anhydride is 20:0.5 to 1.2:0.1 to 0.3, preferably 20:1:0.2, to obtain a mixed material.

[0034] Wherein, the cottonseed protein is added into the mixing mill within 1 minute to prevent the cottonseed protein from flying.

[0035] Step 2: Use a flat vulcanizing hot press, preheat it to 170 - 200 °C, place the mixed material into the mold of the flat vulcanizing hot press for molding, and use a cooling device to control the flat vulcanizing hot press to cool the molded mixed material in a stepwise cooling manner. The stepwise cooling manner is as follows: when the temperature drops to 120 - 130 °C, it drops to room temperature at 2 °C - 10 °C / 5 min to obtain a polylactic acid bio-composite plastic film. Preferably, the stepwise cooling manner is: it drops to room temperature at 6 °C / 5 min.

[0036] The flat vulcanizing hot press is preheated to the temperature of the open mill to prevent the phase change of the mixed material. Since the protein used in the present invention is cottonseed protein and the crystallization point of cottonseed protein is around 120 °C, after the temperature drops to 120 - 130 °C near the crystallization point, a cooling device is used to gradually cool the temperature, and the cooling temperature is maintained for a certain period of time, so that sufficient crystallization can be ensured in each cooling stage and the crystallization effect can be improved.

[0037] The flat vulcanizing hot press gradually generates crystallization of the mixed material by gradually increasing the temperature, effectively improving the crystallinity.

[0038] In this embodiment, before the polylactic acid is placed into the open mill, it is first placed in an oven for drying. The drying temperature is 40 °C - 55 °C, and the drying time is 30 h - 48 h.

[0039] The polylactic acid removes moisture through the drying effect of the oven.

[0040] The above method for regulating the crystallinity of the polylactic acid biocomposite plastic film uses cottonseed protein as a nucleating agent for polylactic acid. Cottonseed protein is an alkali-soluble plant protein with a high protein content. The alkaline property of cottonseed protein can effectively improve the compatibility of cottonseed protein with polylactic acid, which has high liposolubility, enabling cottonseed protein and polylactic acid to be compatible more quickly. At the same time, the high protein content of cottonseed protein also provides a more abundant number of crystal nuclei, further promoting the crystallization rate between cottonseed protein and polylactic acid. Therefore, during cooling crystallization, crystallization begins between cottonseed protein and polylactic acid at the high temperature point of 120 °C, enabling the entire polylactic acid to crystallize in advance, effectively extending the crystallization time, allowing cottonseed protein and polylactic acid to crystallize fully, and then enabling the crystal growth to be sufficient. Thus, the density of the entire polylactic acid biocomposite plastic film is good, and the mechanical properties of the film are better. The present invention cools and crystallizes the crystallization material in a stepwise cooling manner, enabling sufficient cooling crystallization in each temperature stage, further improving the crystallization effect. Especially when crystallization occurs at a high temperature, the more stages of stepwise cooling, the more sufficient the cooling crystallization. The polylactic acid biocomposite plastic film is prepared by a melt blending method. This melt blending method can uniformly blend cottonseed protein and polylactic acid without using a solvent, having the advantage of environmental protection. Using cottonseed protein as a nucleating agent, cottonseed protein has a wide source and low cost, effectively replacing the use of amino acids, and is suitable for large-scale production and application.

[0041] Example 1

[0042] This example discloses a method for regulating the crystallinity of a polylactic acid biocomposite plastic film.

[0043] Step 1: Dry polylactic acid (PLA) in an oven at 50 °C for 48 h for later use.

[0044] Step 2: Adopt a melt blending processing method. Preheat an open mill to 180 °C, then place a certain amount of polylactic acid (20 g), cottonseed protein (CPC) (1 g) on the open mill and stir and melt for about 10 min, along with a small amount of maleic anhydride (0.2 g) as a compatibilizer, and at the same time use a stirring blade to stir to make the materials fully stirred and uniform for 5 min.

[0045] Step 3: Preheat a flat vulcanizing hot press to 180 °C, and quickly transfer all the mixed materials to the required template while it is hot. Use a cooling device to control the flat vulcanizing hot press to cool and form the mixed materials in a stepwise cooling manner. The stepwise cooling method is as follows: When the temperature drops to 120 - 130 °C, it drops to room temperature at a rate of 6 °C / 5 min to obtain a polylactic acid biocomposite plastic film.

[0046] Step 4: After sufficient cooling, release the pressure and take out the product to obtain polylactic acid / cottonseed protein plastic, marked as PLA / CPC5.

[0047] Example 2

[0048] This embodiment discloses a method for regulating the crystallinity of a polylactic acid biocomposite plastic film.

[0049] Step 1: Dry polylactic acid (PLA) in an oven at 50 °C for 48 h for later use.

[0050] Step 2: Adopt a melt blending processing method. Preheat an open mill to 180 °C, then place a certain amount of polylactic acid (20 g), cottonseed protein (CPC) (2 g) on the open mill, stir and melt them for about 10 min, and use a trace amount of maleic anhydride (0.2 g) as a compatibilizer. At the same time, use a stirring knife to stir to make the materials fully stirred and uniform for 5 min.

[0051] Step 3: Preheat a flat vulcanizing hot press to 180 °C. While it is hot, transfer all the mixed materials to the required template, and use a cooling device to control the flat vulcanizing hot press to cool and form the mixed materials in a segmented cooling manner. The segmented cooling method is: when the temperature drops to 120 - 130 °C, it drops to room temperature at a rate of 6 °C / 5 min to obtain a polylactic acid biocomposite plastic film.

[0052] Step 4: After sufficient cooling, release the pressure and take out to obtain polylactic acid / cottonseed protein plastic, marked as PLA / CPC10.

[0053] Example 3

[0054] Step 1: Dry polylactic acid (PLA) in an oven at 50 °C for 48 h for later use.

[0055] Step 2: Adopt a melt blending processing method. Preheat an open mill to 180 °C, then place a certain amount of polylactic acid (20 g), cottonseed protein (CPC) (3 g) on the open mill, stir and melt them for about 10 min, and use a trace amount of maleic anhydride (0.2 g) as a compatibilizer. At the same time, use a stirring knife to stir to make the materials fully stirred and uniform for 5 min.

[0056] Step 3: Preheat a flat vulcanizing hot press to 180 °C. While it is hot, transfer all the mixed materials to the required template, and use a cooling device to control the flat vulcanizing hot press to cool and form the mixed materials in a segmented cooling manner. The segmented cooling method is: when the temperature drops to 120 - 130 °C, it drops to room temperature at a rate of 6 °C / 5 min to obtain a polylactic acid biocomposite plastic film.

[0057] Step 4: After sufficient cooling, release the pressure and take out to obtain polylactic acid / cottonseed protein plastic, marked as PLA / CPC15.

[0058] Example 4

[0059] Step 1: Dry polylactic acid (PLA) in an oven at 50 °C for 48 h for later use.

[0060] Step 2: Adopt the processing method of melt blending. Preheat the open mill to 180 °C in advance, and then fully stir and melt a certain amount of polylactic acid (20 g) and cottonseed protein (CPC) (4 g) on the open mill for about 10 min, and a trace amount of maleic anhydride (0.2 g) as a compatibilizer. At the same time, use a stirring knife to stir to make the materials fully stirred and evenly mixed for 5 min.

[0061] Step 3: Preheat the flat vulcanizing hot press to 180 °C, and transfer all the mixed materials to the required template while it is hot. Use a cooling device to control the flat vulcanizing hot press to cool and form the mixed materials in a stepwise cooling manner. The stepwise cooling method is: when the temperature drops to 120 - 130 °C, it drops to room temperature at 6 °C / 5 min to obtain the polylactic acid biocomposite plastic film.

[0062] Step 4: After sufficient cooling, release the pressure and take out to obtain polylactic acid / cottonseed protein plastic, marked as PLA / CPC20.

[0063] Comparative Example 1

[0064] This example discloses a method for regulating the crystallinity of a polylactic acid biocomposite plastic film.

[0065] Step 1: Dry polylactic acid (PLA) in an oven at 50 °C for 48 h for later use.

[0066] Step 2: Adopt the processing method of melt blending. Preheat the open mill to 180 °C in advance, and then fully stir and melt a certain amount of polylactic acid (20 g) on the open mill for about 10 min, and a trace amount of maleic anhydride (0.2 g) as a compatibilizer. At the same time, use a stirring knife to stir to make the materials fully stirred and evenly mixed for 5 min.

[0067] Step 3: Preheat the flat vulcanizing hot press to 180 °C, and transfer all the mixed materials to the required template while it is hot. Use a cooling device to control the flat vulcanizing hot press to cool and form the mixed materials in a stepwise cooling manner. The stepwise cooling method is: when the temperature drops to 120 - 130 °C, it drops to room temperature at 6 °C / 5 min to obtain the polylactic acid biocomposite plastic film.

[0068] Step 4: After sufficient cooling, release the pressure and take out to obtain polylactic acid plastic.

[0069] Effect verification:

[0070] 1. Infrared spectrum test:

[0071] The polylactic acid biocomposite films of Examples 1 to 4, the polylactic acid film of Comparative Example 1, and cottonseed protein (CPC) were respectively subjected to infrared detection in the full wavelength range, the 3500 - 2500 cm -1 section, the 2000 - 1000 cm -1 section, and the 1000 - 500 cm -1 section. The detection results are as shown in Figure 1 :

[0072] Figure 1 (c) The stretching of the ester group (C=O) carbon skeleton of PLA can be observed at 1755 cm -1 and 1078 cm -1 , and the enhancement effect is obvious, indicating that the increase in ester groups may be related to the content of CPC amide bonds, etc. In addition, 1458 cm -1 and 1358 cm -1 are assigned to the stretching vibration of C-H, and 1084 cm -1 is attributed to the stretching vibration of the C-O single bond. As the protein content increases, the peak gradually strengthens, indicating that cottonseed protein and polylactic acid are uniformly mixed. At the same time, some high molecular polymers will show absorption of the polymer main chain in the low-frequency region of infrared. In Figure 1 (d), 870 cm -1 and 754 cm -1 are assigned to the crystalline region and amorphous region of polylactic acid, and the peak gradually becomes stronger, which is closely related to the crystallinity, crystallization rate, etc. of PLA. Figure 1 (b) The absorption of CPC at 3295 cm -1 can be attributed to the vibrations of N-H bonds and O-H bonds, but it is not obvious in the composite material, indicating that after cottonseed protein and polylactic acid are fully mixed, the physical interaction between cottonseed protein and polylactic acid is enhanced, and polylactic acid / cottonseed protein plastic is obtained.

[0073] 2. Differential Scanning Calorimetry (DSC) Analysis

[0074] The differential scanning calorimetry (DSC) analysis detection results of the polylactic acid biocomposite films of Examples 1 to 4 and the polylactic acid film of Comparative Example 1 are as shown in Figure 2 and Table 1:

[0075] Table 1

[0076]

[0077] During the first heating process, no glass transition and cold crystallization peaks were detected. Only a melting peak was observed near 170 °C, indicating that the material melted near 170 °C. Here, the crystalline region completely disappeared and the material entered the molten state. At a cooling rate of 10 °C / min, no cold crystallization peak was detected during the cooling process either. After eliminating the thermal history and performing the second heating, a "platform" appeared near 60 °C, which is a sign of the glass transition. The glass transition means a process in which the material transforms from the normal state to the high elastic state. As the content of cottonseed protein increases, the glass transition temperature decreases, which is considered to be related to the toughness of the material. A cold crystallization peak appeared near 120 °C, and a melting peak appeared near 164 °C. Moreover, as the cottonseed protein increased, both the cold crystallization temperature and the melting temperature gradually decreased. From this differential scanning calorimetry (DSC) analysis, it can be seen that the crystallization temperature of PLA / CPC is near 120 °C, with a relatively high crystallization temperature.

[0078] 3. Analysis of Crystallization Effect

[0079] The polylactic acid biocomposite films of Examples 1-4 and the polylactic acid film in Comparative Example 1 were subjected to crystallization analysis.

[0080] Sample preparation: Obtain the polylactic acid biocomposite films of Examples 1-4 and the polylactic acid film of Comparative Example 1, and use them as samples respectively. The following treatments were carried out respectively: dissolve them in 10 ml of chloroform solution and disperse them evenly by ultrasonic treatment, and then observe the crystallization and morphology under a polarized light microscope. Specifically, take 1 ml of the sample solution, drop it onto a heating stage and heat it up to 200 °C, keep it for 3 min, and then cool it down to 120 °C at a rate of 10 °C / min. Keep it at this temperature for 15 min, and at the same time, use a polarized light microscope to capture the growth behavior of the crystals by taking pictures.

[0081] As Figure 4 shown, after adding cottonseed protein as a filling matrix, crystallization behavior was observed near the isothermal crystallization point. At 5 min, large crystal growth regions could be seen on the screen. After 10 min, the crystals grew slowly and gradually occupied the entire screen. Within 15 min, the crystal growth was basically completed and the shape remained unchanged. The crystal shapes were mostly "rhombic" or "cross-shaped", and they were regularly arranged and interlocked with each other. Compared with pure polylactic acid (PLA), the PLA / CPC5 of the composite material with added cottonseed protein in Example 1 showed a faster crystallization rate, indicating that after adding cottonseed protein, cottonseed protein can start to crystallize rapidly at high temperatures, which is beneficial for full crystal growth.

[0082] And, as Figure 7As shown in the figure, after adding cottonseed protein as a filler, the improvement of compatibility behavior was observed under a scanning electron microscope. In particular, the PLA / CPC10 sample of Example 2 clearly showed fewer cracks and a smoother and more uniform interface compared to pure PLA, which again shows that cottonseed protein can effectively promote the nucleation of polylactic acid.

[0083] like Figure 5 As shown, after 10 minutes, the crystals slowly grow and gradually occupy the entire screen, the nucleation density increases, the grain size is slightly smaller, and the number of grains is large. Within 15 minutes, the crystal growth is basically completed and the shape remains unchanged. The crystal shape is mostly "diamond" or "cross", and the arrangement is regular and embedded in each other. The POM of PLA / CPC10 in Example 2 shows that the 10% CPC content provides a large number of nucleation sites, which is conducive to the heterogeneous nucleation of PLA. At this time, the nucleation barrier is minimized, and the PLA macromolecular chain is more likely to be orderly arranged near the CPC to form a crystalline phase.

[0084] like Figure 6 As shown in the figure, excessive CPC addition will lead to uneven stacking and dispersion, and PLA will begin to agglomerate to reduce the surface energy. At this time, the nucleation sites will be reduced, which will intuitively cause the crystal morphology to become larger and more "loose".

[0085] like Figure 3 As shown in the figure, the crystallization speed of single PLA material is relatively slow, and no crystals appear from 0min to 15min. This phenomenon can be attributed to the fact that single-component PLA does not have many nucleation sites. According to the nucleation theory, the nucleation barrier of PLA is very high at this time, which makes the activation energy required for nucleation growth very large, objectively resulting in slow crystallization speed and low crystallinity.

[0086] Through the calculation of crystallinity, it is found that the PLA / CPC10 composite material has the highest crystallinity. When the cottonseed protein content continues to increase, the crystallinity will decrease. This is because the excess cottonseed protein leads to a decrease in the proportion of polylactic acid, and the excess cottonseed protein is prone to aggregation effect, reducing nucleation sites, thus affecting the crystallinity.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for regulating the crystallinity of a polylactic acid biocomposite plastic film, characterized in that: The following steps are involved: Step 1: preheating the open mill to 170-200° C. by melt blending, adding polylactic acid to the open mill and stirring to melt, adding cottonseed protein and maleic anhydride, and mixing the materials to obtain a mixed material, wherein the weight ratio of the polylactic acid, the cottonseed protein and the maleic anhydride is 20:2:0.2; Step 2: Use a flat plate vulcanization hot press and preheat it to 180°C, place the mixed material into the flat plate vulcanization hot press The mixed material is formed in a template of a press, and then a cooling device is used to control the flat plate vulcanization hot press to cool the formed mixed material in a segmented cooling manner. The segmented cooling manner is: when the temperature drops to 120°C, it drops to room temperature at 6°C / 5min to obtain a polylactic acid biocomposite plastic film.

2. The method for controlling the crystallinity of a polylactic acid biocomposite plastic film according to claim 1, characterized in that: Before the polylactic acid is placed in the open mill, it is first placed in an oven for drying at a temperature of 40 to 55° C. for a drying time of 30 to 48 hours.

3. The method for controlling the crystallinity of a polylactic acid biocomposite plastic film according to claim 1, characterized in that: In step 1, the polylactic acid is stirred and melted in an open mixer for 8 to 12 minutes.

4. The method for controlling the crystallinity of a polylactic acid biocomposite plastic film according to claim 1, characterized in that: In step 1, the stirring time after adding cottonseed protein and maleic anhydride is 4 minutes to 8 minutes.

5. The method for controlling the crystallinity of a polylactic acid biocomposite plastic film according to claim 1, characterized in that: In step 1, the cottonseed protein is added into the mixing mill within 1 minute.

6. A polylactic acid biocomposite plastic film, characterized in that: The polylactic acid biocomposite plastic film is prepared by the method for controlling the crystallinity of the polylactic acid biocomposite plastic film according to any one of claims 1 to 5.

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