A degradable polylactic acid film with good toughness and a preparation method thereof

Abstract

In view of the fact that there is no biodegradable film which can keep high transparency and low haze and significantly improve tear strength and ductility in the existing plastic pyrolysis technology, the application specifically relates to a biodegradable polylactic acid film with good toughness and a preparation method thereof, and the prepared film meets the following requirements: the thickness is 15-30 microns, the residual CO2 content difference of the skin-core is 0.2-0.6wt%, the average pore size of the residual micropore is less than or equal to 0.5 microns, the Elmendorf tear strength is greater than or equal to 40kJ / m 2 , and the haze is less than or equal to 6%.

Description

Technical Field

[0001] This invention belongs to the field of waste plastic treatment technology, specifically relating to a biodegradable polylactic acid film with good toughness and its preparation method. Background Technology

[0002] Polylactic acid (PLA), as a biodegradable material, is considered the most promising candidate material for large-scale production to replace traditional petroleum-based flexible packaging due to its renewable raw material source, compostability, and increasingly mature synthesis chain. However, limited by its high glass transition temperature and rigid molecular structure, pure PLA has an elongation at break of only single-digit percentages, and its Elmendorf tear strength is typically below 10 kJ / m. 2 Its brittleness is significantly higher than that of general-purpose film materials such as polyethylene and polypropylene, making it difficult to meet the toughness requirements of film applications such as express delivery bags and modified atmosphere film.

[0003] The industry has attempted to improve toughness through methods such as blending flexible polyesters (e.g., PBAT), adding plasticizers, or using microcellular foaming and biaxial stretching. However, trade-offs often have to be made between transparency, barrier properties, and mechanical properties: adding a large proportion of flexible components such as PBAT significantly increases haze and reduces bio-based content; small molecule plasticizer systems pose risks of migration, aging, and food safety; while conventional microcellular foaming can effectively increase toughness, the uncontrollable pore size leads to loss of gloss on the film surface. Especially in the field of food flexible packaging with a thickness of less than 20μm and strict requirements for high transparency and low oxygen permeability, the market still lacks a biodegradable film that can maintain high transparency and low haze while significantly improving tear strength and ductility. Developing such materials will provide a practical material solution for addressing plastic pollution. Summary of the Invention

[0004] To address the lack of a biodegradable film in existing plastic pyrolysis technologies that can maintain high transparency and low haze while significantly improving tear strength and ductility, this invention provides a biodegradable polylactic acid film with good toughness. This film meets the following requirements: thickness 15–30 μm, skin-core residual CO2 content difference 0.2–0.6 wt%, average residual micropore diameter ≤0.5 μm, and Elmendorf tear strength ≥40 kJ / m². 2 Haze ≤ 6%;

[0005] This invention also provides a method for preparing a biodegradable polylactic acid film with good toughness, the specific technical solution of which is as follows:

[0006] (1) The raw materials, by mass fraction, include 60-85% polylactic acid (PLA), 10-35% polybutylene terephthalate-adipate (PBAT), 0.3-1.5% ionic liquid-epoxy bifunctional interface agent, 3-8% oligolactic acid (OLA) plasticizer and 0-0.5% talc.

[0007] (2) The above raw materials are melt-blended at 150-185°C: extruded from the T-shaped flat die, immediately pressed onto the cold roller for rapid cooling, to obtain the initial film;

[0008] (3) The above initial film is placed in a saturation chamber to allow the PBAT phase to absorb CO2 for supercritical CO2 selective plasticization. After rapid depressurization and nucleation, it is subjected to biaxial stretching and post-heat setting to obtain a biodegradable polylactic acid film with good toughness.

[0009] In step (2), the ionic liquid-epoxy bifunctional interface agent is one or more of the following: 1-(2,3-epoxypropyl)-3-dodecylimidazolium bis(trifluoromethanesulfonyl)imide, 1-(2,3-epoxypropyl)-3-hexylimidazolium, 1-(2,3-epoxypropyl)-3-octylimidazolium, 1-(2,3-epoxybutyl)-3-octylimidazolium, N,N,N-trimethyl-N-(2,3-epoxypropyl)ammonium, and bis(2,3-epoxypropyl)imidazolium.

[0010] In step (2), the temperature of the T-shaped flat die head is 170-180℃.

[0011] Preferably, the temperature of the T-shaped flat die head in step (2) is 175°C.

[0012] In step (2), the initial film thickness is 80–120 μm.

[0013] In step (3), the pressure of the saturation chamber is 6.5-8 MPa and the temperature is 38-45℃.

[0014] Preferably, in step (3), the pressure of the saturation cavity is 8 MPa and the temperature is 42 °C.

[0015] In step (3), the selective plasticization time of supercritical CO2 is 35 to 120 seconds.

[0016] Preferably, the supercritical CO2 selective plasticization time in step (3) is 90 seconds.

[0017] The rapid decompression nucleation in step (3) specifically involves reducing the pressure of the saturated cavity to ≤1MPa within 50ms at a rate of ≥120MPa / s.

[0018] In step (3), the bidirectional stretching is specifically performed as follows: stretching in the machine direction by 2.3 to 3.0 times at 60 to 65°C, and then stretching in the transverse direction by 2.5 to 3.2 times at 70 to 75°C, with linear speeds of 100 to 150 m / min and 250 to 300 m / min, respectively.

[0019] Preferably, the bidirectional stretching in step (3) specifically involves stretching the machine direction by 2.3 times at 65°C and then stretching it laterally by 3.2 times at 75°C, with linear speeds of 120m / min and 250m / min respectively.

[0020] In step (3), heat setting is performed at 95-105°C for 15-40 seconds.

[0021] Preferably, in step (3), the heat setting is performed at 100°C for 25 seconds.

[0022] The beneficial effects of this invention are as follows: This invention uses polylactic acid as the main component, supplemented with no more than 35% PBAT, and employs a biodegradable ionic liquid interface agent, eliminating the need for petroleum-based plasticizers or chlorinated solvents; it has a high bio-based content and can be completely degraded through industrial composting, making it more environmentally friendly than blend membranes containing a large number of non-degradable blocks. This invention achieves mass production by connecting a supercritical CO2 saturation chamber and a biaxially stretched wire in series, eliminating the need for additional solvent recovery and high-temperature crosslinking steps; this reduces equipment modification costs and avoids the high foaming agent dosage and venting problems of traditional microporous foaming. This invention achieves an Elmendorf tear strength ≥40 kJ / m in biodegradable films with a thickness of only 15–30 μm. 2 Furthermore, its excellent balance of haze ≤6% solves the problem of the traditional polylactic acid film's trade-off between toughness and transparency. The average pore size of residual micropores ≤0.5μm ensures high light transmittance below the wavelength of scattered light. During the biaxial stretching and heat setting stages, the transdermal directional crystallization and the layered micropore bending effect help to prevent oxygen from passing through, meeting the high barrier requirements for food preservation films and improved modified atmosphere packaging, thus balancing the needs for high barrier and high transparency. Attached Figure Description

[0023] Figure 1 This is a scanning electron microscope image of Example 4.

[0024] Figure 2 This is a scanning electron microscope image of Example 7.

[0025] Figure 3 This is a scanning electron microscope image of Example 8.

[0026] Figure 4 This is a scanning electron microscope image of Example 9.

[0027] Figure 5 The Fourier transform infrared spectra of Example 4 and Comparative Example 1 are shown. Detailed Implementation

[0028] The present invention will be described in more detail below through embodiments, but these are not intended to limit the scope of the invention.

[0029] Example 1

[0030] (1) The raw materials, by mass fraction, include 60% polylactic acid (PLA), 34% polybutylene terephthalate adipate (PBAT), 0.8% 1-(2,3-epoxypropyl)-3-dodecylimidazolium bis(trifluoromethanesulfonyl)imide, 5% oligolactic acid (OLA) plasticizer and 0.2% talc;

[0031] (2) After the above raw materials are melt-blended at 170°C, they are extruded from a T-shaped flat die at 175°C and immediately pressed onto a room temperature cold roller for rapid cooling to obtain an initial film;

[0032] (3) The above initial film is placed in a saturation chamber with a pressure of 8 MPa and a temperature of 42 °C, and the PBAT phase absorbs CO2 to selectively plasticize it with supercritical CO2 for 90 seconds. Then, the pressure of the saturation chamber is reduced to 0.5 MPa within 50 ms at a rate of 125 MPa / s for rapid depressurization nucleation. Then, it is stretched 2.3 times in the machine direction at 65 °C and then stretched 3.2 times in the transverse direction at 75 °C. A biodegradable polylactic acid film with good toughness can be obtained at linear speeds of 120 m / min and 250 m / min respectively.

[0033] Example 2

[0034] In step (1), PLA 65%, PBAT 29%, ionic liquid 0.8%, OLA 5%, talc 0.2%, and the rest of the preparation process is basically the same as in Example 1 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0035] Example 3

[0036] In step (1), PLA 70%, PBAT 24%, ionic liquid 0.8%, OLA 4%, talc 0.2%, and the rest of the preparation process is basically the same as in Example 1 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0037] Example 4

[0038] In step (1), PLA 75%, PBAT 19%, ionic liquid 0.8%, OLA 4%, talc 0.2%, and the rest of the preparation process is basically the same as in Example 1 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0039] Example 5

[0040] In step (3), the pressure of the saturation chamber is 6.5 MPa and the temperature is 45 °C. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0041] Example 6

[0042] In step (3), the pressure of the saturation chamber is 8 MPa and the temperature is 38°C. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0043] Example 7

[0044] In step (3), the supercritical CO2 selective plasticization is performed for 120 seconds. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0045] Example 8

[0046] In step (3), the supercritical CO2 selective plasticization lasts for 40 seconds. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0047] Example 9

[0048] In step (3), the biaxial stretching is specifically performed as follows: the machine stretching is performed at 65°C for 3.0 times, and then the transverse stretching is performed at 70°C for 3.2 times, with linear speeds of 150m / min and 300m / min respectively. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0049] Scanning electron microscope images of Examples 4, 7, 8, and 9 are shown below. Figures 1-4 It can be observed that the residual micropores on its surface have a pore size in the range of 0.05 to 0.5 μm.

[0050] Example 10

[0051] In step (1), PLA 80%, PBAT 14%, ionic liquid 0.8%, OLA 4%, talc powder 0.2%, and the rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0052] Example 11

[0053] In step (1), PLA 85%, PBAT 11%, ionic liquid 0.8%, OLA 3%, talc 0.2%, and the rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, a biodegradable polylactic acid film with good toughness can be obtained.

[0054] Comparative Example 1

[0055] Step (3) The initial film from step (2) is directly stretched 2.3 times in the machine direction at 65°C, and then stretched 3.2 times in the transverse direction at 75°C, with linear speeds of 120 m / min and 250 m / min respectively. The rest of the preparation process is basically the same as in Example 4 above, thus obtaining Comparative Example 1.

[0056] The Fourier transform infrared spectra of Comparative Example 1 and Example 4 are shown below. Figure 5 2995cm -1 and 2945cm -1 An asymmetric stretching vibration peak of –CH3 / –CH2 appears at the position, corresponding to the methyl and methylene groups at the ends of the PLA main chain and the butanediol segment of PBAT. The strong absorption region of the ester carbonyl group is divided into two segments: 1758 cm⁻¹. -1 The peak is a characteristic peak of polylactic acid carbon ester bonds. The peak position shifts slightly with stereoregular PLA, while the peak position is 1716 cm⁻¹. -1 The C=O stretching of the PBAT aromatic ester (terephthalic acid unit) results in a slightly lower concentration than PLA due to conjugation with the benzene ring. (1505cm) -1 and 1410cm -1 The vibration of the para-benzene ring skeleton is an important marker for identifying the aromatic segment of PBAT. At 1268 cm⁻¹ -1 Up to 1225cm -1 In one area, the C–O–C stretching shoulder of PBAT aromatic esters is visible; and at 1180cm... -1 1125cm -1 and 1085cm -1 This refers to the PLA main chain C–O–C and C–O scaling.

[0057] Comparative Example 2

[0058] In step (3), the pressure of the saturation cavity is 8 MPa and the temperature is 50 °C. The rest of the preparation process is basically the same as in Example 4 above, so Comparative Example 2 can be obtained.

[0059] Comparative Example 3

[0060] In step (3), the supercritical CO2 selective plasticization is performed for 150 seconds. The rest of the preparation process is basically the same as in Example 4 above. After the reaction is completed, Comparative Example 3 can be obtained.

[0061] Experimental Example

[0062] Gas chromatography-dispersion and detection were performed using a coupled system (8890, Agilent Technologies, USA). Headspace-GC quantification was performed at 80°C for 10 min → headspace gas was then introduced into GC-TCD.

[0063] Determination of residual CO2 difference between skin and core: The film was divided into skin and core layers of 20–30 mg each using a -40℃ cryogenic ultramicrotome (CM3050S, Leica) and immediately sealed in 40 mL headspace vials. After being kept at 80℃ for 10 min, the film was separated and detected by gas chromatography (8890, Agilent). The CO2 mass fraction of each layer was calculated according to the standard curve, and the difference was taken as ΔCO2.

[0064] Elmendorf tear strength determination: The film needs to be cut into rectangular specimens with a standard size of 63mm × 76mm, and a 20mm slit should be pre-cut at the center of the short side of the specimen as the tear starting point. The instrument setup requires an Elmendorf tear strength tester (including pendulum, clamps, and energy scale). Ten specimens should be tested at 23℃ and 50% relative humidity. The tear energy should be recorded, and the tear strength should be calculated as: tear energy ÷ (sample thickness × 0.043). The results are expressed in kJ / m³. 2 express.

[0065] Haze determination: After zeroing the instrument using an integrating sphere haze meter (HunterLab Vista), first measure the total transmitted light flux of the film in the visible light range, then block the diffuser and measure only the parallel transmitted light flux; divide the difference between the two readings by the total transmitted light flux and convert it to a percentage to obtain the haze of the film.

[0066] All measurement results are shown in Table 1.

[0067] Table 1 shows the hydrogen production from the catalytic degradation of high-density polyethylene powder in different embodiments and comparative examples.

[0068]

[0069] The above embodiments only illustrate several implementation methods of the present invention, and their descriptions are relatively simple.

[0070] While detailed and specific, this should not be construed as limiting the scope of the invention. It should be noted that...

[0071] It should be noted that, for those skilled in the art, without departing from the inventive concept...

[0072] It should be noted that several modifications and improvements can be made, all of which fall within the scope of protection of this invention.

Claims

1. A method for preparing a biodegradable polylactic acid film with good toughness, characterized in that, The prepared films meet the following requirements: thickness 15~30 µm, skin-core residual CO2 content difference 0.2~0.6 wt%, average residual micropore diameter ≤0.5 µm, and Elmendorf tear strength ≥40 kJ / m. 2 Haze ≤6%; The specific preparation method includes the following steps: (1) The raw materials for preparation, by mass fraction, include 60-85% polylactic acid (PLA), 10-35% polybutylene terephthalate (PBAT), 0.3-1.5% 1-(2,3-epoxypropyl)-3-dodecylimidazolium bis(trifluoromethanesulfonyl)imide, 3-8% oligolactic acid (OLA) plasticizer and 0-0.5% talc; (2) The above raw materials are melt-blended at 150~185℃: extruded from the T-shaped flat die, immediately pressed onto the cold roller for rapid cooling, to obtain the initial film; (3) The above-mentioned initial film is placed in a saturation chamber and subjected to supercritical CO2 selective plasticization for 35 to 120 seconds at 6.5 to 8 MPa and 38 to 45°C, so that the polybutylene terephthalate phase absorbs CO2; then the pressure of the saturation chamber is reduced to ≤1 MPa at a rate of ≥120 MPa / s within 50 ms for rapid depressurization nucleation; then the machine direction is stretched by 2.3 to 3.0 times at 60 to 65°C, and the transverse stretching is performed by 2.5 to 3.2 times at 70 to 75°C, with linear speeds of 100 to 150 m / min and 250 to 300 m / min, respectively; finally, it is heat-set at 95 to 105°C for 15 to 40 seconds.

2. The method for preparing a biodegradable polylactic acid film with good toughness according to claim 1, characterized in that, The temperature of the T-shaped flat die head in step (2) is 170~180℃.

3. The method for preparing a biodegradable polylactic acid film with good toughness according to claim 1, characterized in that, The initial film thickness in step (2) is 80~120 µm.

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