A polylactic acid multilayer composite active liner and a method of making the same

Abstract

This invention provides a polylactic acid (PLA) multilayer composite active liner and its preparation method, comprising: synthesizing LDHs@CDs in situ using a one-pot method with Spartina alterniflora extract, metal salts, and urea; and constructing the outer layer of the active liner (LDHs@CDs / PVA) with a PVA substrate via electrospinning; simultaneously, constructing the intermediate active layer (MPN / PVA) of the active liner using MPN particles and a PVA substrate via electrospinning; and finally forming a PLA multilayer active composite liner based on a porous PLA base film. LDHs@CDs combine the structural stability of LDHs with the surface activity advantages of SA-CDs, which helps improve the material's dispersibility, interfacial interaction ability, and functional component loading efficiency; while MPN has a continuous and dense network structure, exhibiting good environmental responsiveness, bioactivity, and interfacial synergistic potential. Therefore, introducing LDHs@CDs and MPN into the preservation liner system endows the material with multiple functions such as moisture regulation, antibacterial, antioxidant, and responsive regulation, while enhancing its practical application value in fresh food preservation.

Description

Technical Field

[0001] This invention relates to the field of active packaging materials technology, and particularly to a polylactic acid multilayer composite active liner and its preparation method. Background Technology

[0002] Fresh food is highly susceptible to quality deterioration during post-harvest storage and distribution, including microbial growth, oxidative browning, and moisture loss. This not only reduces food quality and commercial value but also wastes resources and burdens the environment. Strawberries are beloved by consumers for their unique flavor and rich nutrition, but their delicate flesh, high water content, and weak epidermal barrier make them highly vulnerable to mechanical damage and microbial contamination after harvest, leading to problems such as dehydration, softening, mold, and discoloration. While traditional packaging methods can provide some physical protection and isolation, their functions in antibacterial, antioxidant, and quality monitoring are limited, and they cannot meet the current practical needs for green, safe, and efficient preservation. Therefore, developing preservation materials that combine environmental friendliness and multifunctionality has become an important direction for research in high-value fresh food packaging.

[0003] In recent years, moisture-absorbing pads have been commonly used in fruit and vegetable packaging boxes to regulate local humidity. However, existing pads generally suffer from problems such as limited functionality and insufficient adaptability, and lack the ability to directly intervene in the proliferation of pathogens and quality deterioration during storage. Summary of the Invention

[0004] The purpose of this invention is to provide a polylactic acid (PLA) multilayer composite active liner, which has a good hierarchical structure and functional synergy, thereby achieving the synergistic unity of multiple functions such as ultraviolet shielding, structural stability, active release and continuous preservation.

[0005] Another objective of this invention is to provide a method for preparing a polylactic acid (PLA) multilayer composite active liner. An LDHs@CDs / PVA outer layer and an MPN / PVA intermediate active layer are constructed using an electrospinning process, and a PLA multilayer composite active liner is successfully prepared by combining it with a porous PLA base film. This method offers controllable parameters and is suitable for large-scale industrial production.

[0006] The technical problem solved by this invention is achieved by the following technical solution.

[0007] This invention proposes a method for preparing a polylactic acid multilayer composite active liner, comprising the following steps:

[0008] S1. Urea, AlCl3·H2O, MgCl2·H2O and Spartina alterniflora extract (SA extract) were dissolved in deionized water in sequence, ultrasonicated and then placed in a vacuum drying oven for reaction. After cooling, centrifugation and freeze drying, LDHs@CDs powder was obtained. S2. Quercetin (QUE) was added to anhydrous ethanol, sonicated, and then CuCl2 aqueous solution was added. The pH of the resulting mixed solution was adjusted to 6.8-7.6, and the mixture was reacted. After centrifugation, washing, and freeze-drying, MPN was obtained. S3. Dissolve the MPN in deionized water, add the obtained MPN dispersion to polyvinyl alcohol (PVA) and stir at high temperature. After cooling and standing, P-MP spinning solution is obtained. S4. Dissolve the LDHs@CDs powder in deionized water, add the resulting LDHs@CDs dispersion to polyvinyl alcohol and stir at high temperature, then cool and let stand to obtain P-MPL spinning solution; S5. Using an electrospinning process, the P-MP spinning solution and the P-MPL spinning solution are sequentially electrospun onto a polylactic acid microporous base membrane to obtain a polylactic acid multilayer composite active liner.

[0009] This invention proposes a polylactic acid multilayer composite active liner, which is prepared according to the aforementioned preparation method.

[0010] The beneficial effects of the polylactic acid multilayer composite active liner and its preparation method in this invention are as follows: This invention utilizes Spartina alterniflora extract, metal salts, and urea to synthesize SA-CDs-loaded LDHs (LDHs@CDs) in situ via a one-pot method. These LDHs are then electrospun with a PVA substrate to form the outer layer of an active liner (LDHs@CDs / PVA). Simultaneously, MPN particles are electrospun with a PVA substrate to form the intermediate active layer of the active liner (MPN / PVA). Finally, a multilayer PLA active composite liner is formed based on a porous PLA film. LDHs@CDs combine the structural stability of LDHs with the surface activity advantages of SA-CDs, contributing to improved material dispersibility, interfacial interaction capabilities, and functional component loading efficiency. MPN, with its continuous and dense network structure, exhibits excellent environmental responsiveness, bioactivity, and interfacial synergistic potential. Therefore, combining LDHs@CDs with MPN in a preservation liner system endows the material with multiple functions such as moisture regulation, antibacterial activity, antioxidant properties, and responsiveness regulation, thereby enhancing its practical application value in fresh food preservation.

[0011] This active composite liner exhibits excellent UV shielding (nearly 100% UVA and UVB shielding); the introduction of the LDHs@CDs / PVA outer layer improves the surface hydrophilicity of PVA fibers, preventing the porous fiber structure from absorbing water and collapsing; moreover, LDHs@CDs, as a secondary active substance, can supplement the long-term sustained release of active ingredients in the intermediate MPN / PVA layer. Compared with double-layer PVA-PLA (PP), double-layer (P-MP) and triple-layer active liners (P-MPL) offer superior performance. E. coliThe antibacterial rates increased by 36.2% and 39.6%, respectively; S.aureus The antibacterial rate reached 99.6%; the DPPH free radical scavenging rate increased by 49.26% and 68.06%, respectively. Strawberry preservation experiments showed that the active liner could effectively and significantly delay the color decay and shrinkage of strawberries, and could play a sustained preservation role.

[0012] The PLA multilayer composite active liner constructed in this invention achieves a synergistic unity of multiple functions, including UV shielding, structural stability, active release, and continuous preservation, by integrating an outer LDHs@CDs / PVA layer with an intermediate MPN / PVA active layer. This composite active liner not only possesses good structural integrity and functional activity but also demonstrates significant application effects in strawberry preservation experiments, showing promising prospects for active packaging of fresh fruits and vegetables. Attached Figure Description

[0013] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of the PLA microporous base membrane of Embodiment 1 of the present invention; Figure 2 The chemical structure analysis diagrams are of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the bilayer composite active liner of Comparative Examples 2-3. Figure 3 Microscopic morphology images of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the bilayer composite active liner of Comparative Examples 2-3. Figure 4 Comparative digital photographs of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2-3. Figure 5 The UV-Vis transmittance curves are for the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base film of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2-3. Figure 6 Comparison of water contact angles of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the double-layer composite active liners of Comparative Examples 2-3. Figure 7Comparison of ABTS and DPPH free radical scavenging activities of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the bilayer composite active liner of Comparative Examples 2-3. Figure 8 The antibacterial rate comparison chart shows the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2-3. Figure 9 Comparison of digital photographs of strawberries at different storage times; Figure 10 This is a comparison chart of the weight loss rate of strawberries at different storage periods. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0016] The following is a detailed description of the polylactic acid multilayer composite active liner and its preparation method according to embodiments of the present invention.

[0017] This invention provides a method for preparing a polylactic acid multilayer composite active liner, comprising the following steps: S1. Urea, AlCl3·H2O, MgCl2·H2O, and Spartina alterniflora extract were sequentially dissolved in deionized water. After ultrasonic treatment, the mixture was placed in a vacuum drying oven for reaction. After cooling, centrifugation, and freeze-drying, LDHs@CDs powder was obtained. Specifically, urea, AlCl3·H2O, MgCl2·H2O, and Spartina alterniflora extract were sequentially dissolved in deionized water and ultrasonically dispersed to ensure uniform dispersion. The mixed solution was then transferred to a 50 mL polytetrafluoroethylene-lined stainless steel autoclave and placed in a vacuum drying oven for reaction. After the reaction was completed, the stainless steel autoclave was cooled to room temperature and centrifuged. The supernatant was discarded, and the centrifuged product was freeze-dried to obtain LDHs@CDs powder.

[0018] LDHs@CDs retains typical layered structure characteristics and flower-like microstructure. EDS analysis shows that elements such as C, N, O, and Al are uniformly distributed, indicating that SA-CDs can be uniformly loaded on the LDHs surface. This composite structure, while maintaining the original layered structure and stability of LDHs, further improves the surface activity, dispersibility, and interfacial interaction ability with the polymer matrix. Furthermore, the CDs in LDHs@CDs exhibit excellent antibacterial activity, achieving antibacterial action through the combined effects of surface functional groups, surface charge, and reactive oxygen species.

[0019] Furthermore, in a preferred embodiment of the present invention, the mass-to-volume ratio of the urea, the AlCl3·H2O, the MgCl2·H2O, the Spartina alterniflora extract, and the deionized water is 1.6~1.7:1.2~1.3:3~3.3:1:100~110 (g / g / g / g / mL).

[0020] Further, in a preferred embodiment of the present invention, the reaction temperature is 155~165℃, the reaction time is 5.5~6.5h, and the freeze-drying time is 46~50h. Preferably, the reaction temperature is 160℃, the reaction time is 6h, and the freeze-drying time is 48h.

[0021] S2. Quercetin was added to anhydrous ethanol, sonicated, and then CuCl2 aqueous solution was added. The pH of the resulting mixed solution was adjusted to 6.8-7.6, and the reaction was carried out. After centrifugation, washing, and freeze-drying, MPN was obtained. Specifically, QUE was added to anhydrous ethanol and sonicated for 30 min until QUE was completely dispersed. Then, the QUE dispersion was transferred to a magnetic stirrer, and CuCl2 aqueous solution was added to the QUE dispersion. The pH of the mixed solution was adjusted to 6.8-7.6 with HCl and NaOH solutions. The reaction was carried out at room temperature for 3 h (observing the color change). After the reaction was completed, the mixture was washed three times with water by centrifugation and then freeze-dried.

[0022] The microstructure of MPN indicates that a continuous and relatively dense network covering layer has formed on its surface. MPN exhibits good environmental responsiveness and bioactivity, providing functional support for the subsequent construction of multilayer active pads.

[0023] Further, in a preferred embodiment of the present invention, the mass-to-volume ratio of quercetin to anhydrous ethanol is 1:190~210 (g / mL), and the preparation step of the CuCl2 aqueous solution is as follows: CuCl2·2H2O is added to water to obtain the CuCl2 aqueous solution, wherein the mass-to-volume ratio of quercetin, CuCl2·2H2O and water is 1.6~1.8:1:170~180 (g / g / mL).

[0024] S3. Dissolve the MPN in deionized water, add the resulting MPN dispersion to polyvinyl alcohol and stir at high temperature, then cool and let stand to obtain P-MP spinning solution.

[0025] Furthermore, in a preferred embodiment of the present invention, the mass ratio of MPN to polyvinyl alcohol is 1:19~21, and the temperature of the high-temperature stirring is 95~100℃.

[0026] S4. Dissolve the LDHs@CDs powder in deionized water, add the resulting LDHs@CDs dispersion to polyvinyl alcohol and stir at high temperature. After cooling and standing, obtain P-MPL spinning solution.

[0027] Further, in a preferred embodiment of the present invention, the mass ratio of the LDHs@CDs powder to the polyvinyl alcohol is 1:33~34, and the high-temperature stirring temperature is 95~100°C. Preferably, the mass of MPN is based on 5% of polyvinyl alcohol, the mass of LDHs@CDs powder is based on 3% of polyvinyl alcohol, and the high-temperature stirring temperature is 98°C.

[0028] S5. The P-MP spinning solution and the P-MPL spinning solution are sequentially electrospun onto a polylactic acid microporous substrate membrane using an electrospinning process to obtain a polylactic acid multilayer composite active liner. The electrospinning machine used in this invention is model ES-BS200Pro, manufactured by Sukon.

[0029] This invention employs an electrospinning process to progressively construct different functional fiber layers onto the surface of a porous PLA base membrane, successfully preparing a multilayer PLA composite active liner. LDHs@CDs combine the structural stability of LDHs with the surface activity advantages of SA-CDs (carbon dots derived from Spartina alterniflora), contributing to improved material dispersibility, interfacial interaction capabilities, and functional component loading efficiency. Meanwhile, MPN possesses a continuous and dense network structure, exhibiting excellent environmental responsiveness, bioactivity, and interfacial synergistic potential. Therefore, combining LDHs@CDs with MPN and introducing them into the preservation liner system endows the material with multiple functions such as moisture regulation, antibacterial activity, antioxidant properties, and responsive control, while enhancing its practical application value in fresh food preservation. Further, in a preferred embodiment of the present invention, the step of electrospinning the P-MP spinning solution and the P-MPL spinning solution layer by layer onto the polylactic acid microporous base membrane using an electrospinning process is as follows: the P-MPL spinning solution and the P-MP spinning solution are electrospinned layer by layer onto the polylactic acid microporous base membrane at a feed rate of 2 mL / h and a positive voltage of 25 kV. During the electrospinning process, the temperature is controlled at 25°C and the humidity is maintained at 65%.

[0030] Furthermore, in a preferred embodiment of the present invention, the electrospinning process uses a 20 G needle, the distance from the needle to the collector is 15 cm, and the roller speed is 150 rpm.

[0031] Furthermore, in a preferred embodiment of the present invention, the pore size of the polylactic acid microporous membrane is 300 nm.

[0032] This invention also provides a polylactic acid (PLA) multilayer composite active liner, which is prepared according to the above-described method. This invention employs electrospinning to construct an LDHs@CDs / PVA outer layer and an MPN / PVA intermediate active layer, and successfully prepares a PLA multilayer composite active liner by combining it with a porous PLA base film. This invention utilizes SA-CDs and QUE-Cu... 2+ Using coordination compound (MPN) as the active ingredient and PVA as the spinning substrate, a PLA multilayer composite active liner constructed on a porous polylactic acid (PLA) base film exhibits excellent hierarchical structure and functional synergy, achieving a synergistic unity of multiple functions such as UV shielding, structural stability, active release, and continuous freshness preservation. It can be applied to the preservation of strawberries, promoting the application of this composite liner in the field of active packaging.

[0033] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0034] Example 1 This embodiment provides a polylactic acid multilayer composite active liner, which is prepared according to the following steps: (1) Preparation of LDHs@CDs Urea (0.50675 g), AlCl3·H2O (0.37725 g), MgCl2·H2O (0.953 g), and SA extract (0.3 g) were dissolved sequentially in 32 mL of deionized water and sonicated to ensure uniform dispersion. The mixture was then transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene and placed in a vacuum drying oven at 160 °C for 6 h. After the reaction was complete, the autoclave was cooled to room temperature and centrifuged. The supernatant was discarded, and the centrifuged product was freeze-dried for 48 h to obtain LDHs@CDs powder.

[0035] (2) Preparation of MPN Add QUE (0.500 g) to 100 mL of anhydrous ethanol and sonicate for 30 min until QUE is completely dispersed. Then, transfer the QUE dispersion to a magnetic stirrer. Dissolve CuCl2·2H2O (0.282 g) in 50 mL of H2O and add it to the QUE dispersion. Adjust the pH of the mixed solution to 6.8-7.6 with HCl and NaOH solutions. React at room temperature for 3 h (observe the color change). After the reaction is complete, wash three times with H2O by centrifugation and freeze-dry.

[0036] (3) Preparation of PLA composite active liner.

[0037] PLA microporous films with a pore size of 300 nm were prepared using laser drilling equipment, such as... Figure 1The diagram shows a PLA microporous membrane. Figure 1 (a) is a schematic diagram of the distribution of PLA microporous base film; Figure 1 (b) is a physical image of the PLA microporous base membrane.

[0038] Next, the P-MP spinning solution was electrospun onto the PLA microporous substrate membrane at a feed rate of 2 mL / h and a positive voltage of 25 kV to obtain an intermediate active layer. Then, the P-MPL spinning solution was electrospun onto this intermediate active layer to form the outer layer of the polylactic acid multilayer composite active liner. During the electrospinning process, the temperature was controlled at 25°C and the humidity was maintained at 65%. A 20 G needle was used, with a needle-to-collector distance of 15 cm and a roller speed of 150 rpm, to prepare the polylactic acid multilayer composite active liner PLA-MPN / PVA-LDHs@CDs / PVA (P-MPL). The preparation methods of the P-MP spinning solution and the P-MPL spinning solution are as follows: Weigh 0.1 g of MPN (5% based on PVA) into 18 mL of deionized water and sonicate it to disperse it completely and uniformly. The mass of PVA is 2 g. Place PVA in a three-necked flask, pour in the MPN dispersion, and mechanically stir at high temperature (98℃) to disperse it completely and uniformly. Cool to room temperature and let stand for 2 h to prepare P-MP spinning solution.

[0039] Weigh 0.06 g of LDHs@CDs (3% based on PVA) into 18 mL of deionized water and sonicate to disperse it completely and uniformly. With 2 g of PVA, place the PVA in a three-necked flask, pour in the LDHs@CDs dispersion, and mechanically stir at high temperature (98℃) to disperse it completely and uniformly. Cool to room temperature and let stand for 2 h to prepare the P-MPL spinning solution.

[0040] Comparative Example 1 This comparative example provides a PLA microporous base film, which is a PLA microporous base film with a pore size of 300nm prepared by laser drilling equipment.

[0041] Comparative Example 2 This comparative example provides a double-layer composite active liner, which is prepared according to the following steps: A PLA microporous membrane with a pore size of 300 nm was prepared using a laser drilling device. Then, a PP spinning solution was electrospun onto the PLA microporous membrane at a feed rate of 2 mL / h and a positive voltage of 25 kV. During electrospinning, the temperature was controlled at 25℃ and the humidity was maintained at 65%. A 20 G needle was used, with a needle-to-collector distance of 15 cm and a roller speed of 150 rpm, to prepare a double-layer composite active liner PLA-PVA (PP). The preparation method of the PP spinning solution is as follows: Weigh 2 g of PVA and place it in a three-necked flask. Pour in 18 mL of deionized water and continuously stir mechanically at high temperature (98℃) to make it completely and evenly dispersed. Cool to room temperature and let stand for 2 h to prepare the PP spinning solution.

[0042] Comparative Example 3 This comparative example provides a double-layer composite active liner, which is prepared according to the following steps: A PLA microporous membrane with a pore size of 300 nm was prepared using a laser drilling device. Then, a P-MP spinning solution was electrospun onto the PLA microporous membrane at a feed rate of 2 mL / h and a positive voltage of 25 kV. During electrospinning, the temperature was controlled at 25℃ and the humidity was maintained at 65%. A 20 G needle was used, with a needle-to-collector distance of 15 cm and a roller speed of 150 rpm, to prepare the double-layer composite active liner PLA-MPN / PVA (P-MP). The preparation method of the P-MP spinning solution is as follows: Weigh 0.1 g of MPN (5% based on PVA) into 18 mL of deionized water and sonicate it to disperse it completely and uniformly. The mass of PVA is 2 g. Place PVA in a three-necked flask, pour in the MPN dispersion, and mechanically stir at high temperature (98℃) to disperse it completely and uniformly. Cool to room temperature and let stand for 2 h to prepare P-MP spinning solution.

[0043] Experimental Example 1 This embodiment analyzes the chemical structures of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2-3. Figure 2 The diagram shows the chemical structure analysis of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2-3. Among them, Figure 2 (a) is the infrared spectrum; Figure 2 (b) is the XRD pattern.

[0044] from Figure 2 (a) It can be seen that PLA retains the most typical ester group characteristics of polylactic acid, approximately 1750 cm. -1 The strong absorption is due to the C=O stretching vibration of the ester group, 1180~1080 cm⁻¹. -1 Multiple absorption bands within the range correspond to COC / CO stretching vibrations. P-MP and P-MPL exhibit broader weak absorption bands in the high wavenumber region and more complex peaks in the low wavenumber region. Due to the doping of MPN and LDHs@CDs, the metal-phenol network typically introduces OH, aromatic ring C=C, and CO vibrations, while LDHs@CDs exhibit signals such as lamellar -OH and interlayer anions. Figure 2(b) It appears that the PLA base film has only one very strong main peak, which is the most common characteristic diffraction region of PLA, belonging to the diffraction near the (200) / (110) crystal plane. The diffraction peaks of LDHs@CDs exist at approximately 11°, 23°, 35°, 39°, and 47°, corresponding to (003), (006), (012), (015), and (018) of LDHs. Figure 2 The peaks at 34.7° and 39° in (b) may be weak diffraction signals left over from LDHs@CDs. Since the amount of LDHs@CDs added is relatively low compared to the PLA matrix, the signal is covered by a large amount of polymer.

[0045] Experimental Example 2 This experiment analyzed the microstructure of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2 and 3. Figure 3 The images show the microstructures of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2 and 3. Figure 3 (a) ~ Figure 3 (d) are, in order, surface SEM images of the PLA microporous base membrane of Comparative Example 1, the double-layer composite active pads of Comparative Examples 2-3, and the polylactic acid multilayer composite active pad of Example 1. Figure 3 (e)~ Figure 3 (h) are cross-sectional SEM images of the PLA microporous base membrane of Comparative Example 1, the double-layer composite active liner of Comparative Examples 2-3 and the polylactic acid multilayer composite active liner of Example 1, in sequence.

[0046] from Figure 3 (a) ~ Figure 3 (d) It can be seen that the PLA base film surface is smooth and dense, without obvious pores and cracks, indicating that it can serve as a stable support substrate for the subsequent electrospun active layer. After electrospinning the PVA film, a continuous fiber network was formed on the surface of the PP sample, with relatively uniform fiber distribution and no obvious beaded structure was observed. After electrospinning the MPN / PVA solution, the P-MP fibers still maintained a continuous network structure, but the fiber morphology was more regular than that of PP, and the fiber stacking tended to be more compact, indicating that the introduction of MPN enhanced the intermolecular interaction of the system. Further electrospinning the P-MP surface with LDHs@CDs / PVA solution, the P-MPL sample still retained a complete fiber network, but the surface roughness increased significantly, and granular or nodular protrusions appeared locally, which is related to the partial aggregation of LDHs@CDs. Figure 3 (e)~ Figure 3The cross-sectional view in (h) further demonstrates that the PLA multilayer composite active pads all formed a clear layered structure, with the PLA base film serving as a dense support layer; the upper layers of P-MP and P-MPL were denser than PP, and there was no obvious delamination between the layers. Overall, these results indicate that the layer-by-layer electrospinning strategy successfully achieved the construction of multilayer functionalized active pads from a single-layer support film, and provided a morphological basis for subsequent active material loading, interfacial stability, and controlled-release performance.

[0047] Experimental Example 3 This experiment analyzes the optical properties of the polylactic acid multilayer composite active pad of Example 1, the PLA microporous base film of Comparative Example 1, and the bilayer composite active pads of Comparative Examples 2-3.

[0048] like Figure 4 The images shown are digital photographs comparing the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2 and 3. Figure 4 As can be seen, PLA has the smoothest surface, appears transparent to the naked eye, and has uniform pores spaced 1 cm apart; PP has a slightly more obvious surface texture; P-MP is noticeably yellow, and relatively regular local protrusions can be seen on the surface; P-MPL has more obvious wrinkles and undulations on the surface, and is generally light grayish-white with a yellowish tinge.

[0049] Therefore, after electrospinning P-MP solution onto PLA base film, the overall color tone and surface morphology of the material changed significantly. After further electrospinning P-MPL solution, the color became lighter and the surface roughness increased. It can be clearly seen that nanofibers will gather in the perforated area during the electrospinning process in all three types of PLA multilayer composite active pads.

[0050] Table 1 shows the color parameters of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2-3. As can be seen from Table 1, the brightness of PLA and PP... L The values ​​were 98.34 and 98.20 respectively, showing almost no difference, while P-MP's... L The value dropped to 77.49, and the material darkened significantly; P-MPL's... L It rebounded to 87.56, brighter than P-MP, but still lower than PLA and PP. PLA and PP red-green saturation. a Both values ​​are close to 0, indicating that the PLA multilayer composite active pad has virtually no significant red-green shift. b Reflecting the yellow-blue direction, PLA and PP are only 0.65 and 0.79 respectively, showing almost no yellowing, while P-MP...b An increase to 15.16 indicates a significant increase in yellowness, while P-MPL's... b It dropped slightly to 7.51, and the yellow tint weakened.

[0051] Table 1 Color parameters of PLA multilayer composite active pad

[0052] Test Example 4 This experimental example studies the ultraviolet light shielding ability of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base film of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2-3. The specific steps are as follows: The UV transmittance of polylactic acid (PLA) multilayer composite active liner was characterized using a UV-Vis spectrophotometer, with a test range of 190–1100 nm. UV protection performance was determined according to GB / T 18830-2009, "Evaluation of UV Protection Performance of Textiles," and this method includes calculating UVA and UVB transmittance. The specific calculation formula is as follows: (1) (2) (3) In the formula A 600 T represents the absorbance of the composite active film at 600 nm, and t represents the thickness of the composite active film. i (λ) is the spectral transmittance of sample i at wavelength λ; m and k are the number of measurements between 315 nm and 400 nm and between 290 nm and 315 nm, respectively. Formulas (2) and (3) are only applicable to the case where the measurement wavelength interval Δλ is a constant (5 nm).

[0053] like Figure 5 The image shows the UV-Vis transmittance curves of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2-3. Figure 5It can be seen that the PLA base film has high light transmittance, while the electrospun PP, P-MP, and P-MPL all exhibit extremely strong ultraviolet light shielding capabilities, with P-MP and P-MPL achieving near-complete ultraviolet blocking. The PLA base film's transmittance rises rapidly after 300 nm, stabilizing at approximately 80%–90% or higher in the visible and near-infrared regions. PP, P-MP, and P-MPL remain close to the baseline throughout the 200–1100 nm range, with consistently low transmittance. Although PP's transmittance is slightly higher than the other two groups, it still fluctuates at a very low level overall. Therefore, the PLA multilayer composite active liner not only blocks ultraviolet light but also has a strong shielding effect on visible light.

[0054] Table 2 shows the polylactic acid multilayer composite active liner of Example 1, the PLA microporous base membrane of Comparative Example 1, and the TUVA and TUVB of the double-layer composite active liner of Comparative Examples 2-3.

[0055] The quantitative results in Table 2 further confirm this trend: T of PLA-based films UVA and T UVB The UV transmittance of P-MP and P-MPL was 86.23% and 83.16% respectively, indicating virtually no UV protection capability, with PP dropping sharply to 2.46% and 2.24%. This demonstrates that the UV transmittance of the material was significantly reduced after the formation of the electrospun fiber membrane; while the UV transmittance of P-MP and P-MPL was much lower. UVA T UVB All values ​​are 0, indicating almost complete shielding.

[0056] Table 2. T of PLA multilayer composite active liner UVA and T UVB

[0057] Note: T UVA T represents the spectral transmittance of the sample in the 315-400 nm range. UVB The spectral transmittance of the sample in the 290-315 nm range. Experimental Example 5 This experiment analyzed the surface hydrophilicity of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2 and 3. The specific steps were as follows: The water contact angle of the samples was tested using an optical contact angle meter (OCA15EC, Dataphysics, Germany). The injection speed and water droplet volume were 1 μL / s and 1 μL, respectively. All samples were tested three times and the average value was taken. The test surface was the coated surface.

[0058] like Figure 6The image shows a comparison of the water contact angles of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2 and 3. Figure 6 It can be seen that the water contact angles of PLA base film, PP, P-PM, and P-PML are 61.77°, 42.47°, 75.73°, and 86.17°, respectively. Among them, PP has the lowest angle, indicating that water droplets spread the fastest on its surface and that it has the strongest hydrophilicity, while P-PML has the highest angle, indicating that its surface is the least likely to be rapidly wetted by water. Electrospinning a layer of PVA film on the PLA base film makes it easier for the surface to absorb water and spread due to the hydrophilic nature of PVA. The contact angles of P-PM and P-PML increase again, especially P-PML, which reaches 86.17°. This indicates that the apparent wettability of the PLA multilayer composite active liner surface is weakened after the addition of LDHs@CDs, because P-PML is thicker and has more obvious surface undulations. In summary, the water contact angles of all PLA multilayer composite active liners are less than 90°, indicating overall hydrophilicity. Among them, PP has the lowest water contact angle, indicating its strongest wettability. The increased contact angles of P-PM and P-PML indicate that the introduction of active components alters the chemical composition and fiber structure of the membrane surface, thereby reducing apparent wettability.

[0059] Experimental Example 6 This experiment analyzed the antioxidant properties of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liner of Comparative Examples 2 and 3. The specific steps were as follows: The antioxidant performance of the samples was evaluated using an ABTS free radical scavenging capacity test. A 20 mm × 20 mm square sample was prepared and placed in a beaker containing 20 mL of deionized water, stirred in the dark for 24 h. A 7 mmol / L ABTS solution was then prepared. The specific ratio was: 10 mL deionized water, 0.0384 g ABTS, and 0.0066 g potassium persulfate. The mixture was stirred at room temperature in the dark for 16 h. The ABTS solution was diluted, and the absorbance at 734 nm was adjusted to 0.70 ± 0.02. Then, 0.3 mL of the prepared solution was taken and mixed with 4 mL of the diluted ABTS solution. The mixture was shaken well and reacted for 6 min. After the reaction, the absorbance of the mixture at 734 nm was measured, and the ABTS free radical scavenging rate (SA) was calculated using the formula. ABTS (Scavenging Activity) The results were averaged and the standard deviation was calculated. Deionized water was used as the blank control in the experiment. The specific calculation formula is as follows: (4) In the formula A S and A CThe absorbance values ​​at 734 nm are for the sample and blank sample, respectively.

[0060] The antioxidant properties of the samples were evaluated using a DPPH free radical scavenging capacity test.

[0061] 3 mg of DPPH was added to 100 mL of anhydrous methanol to obtain a DPPH solution with a concentration of 75 μmol / L. A composite active film sample with dimensions of 20 mm × 20 mm was then placed in 20 mL of deionized water and stirred in the dark for 24 h to obtain a sample mixture. Deionized water was used as a blank control in this experiment. 1 mL of the sample solution was mixed with 4 mL of the DPPH solution and shaken well, then allowed to stand for 1 h in the dark. After the reaction was completed, the absorbance of DPPH free radicals at the characteristic peak at 520 nm was measured, and the DPPH free radical scavenging rate SA was calculated according to the formula. DPPH The results are averaged and the standard deviation is calculated. The specific calculation formula is as follows: (5) In the formula A S and A C The absorbance values ​​at 520 nm are for the sample and blank sample, respectively.

[0062] like Figure 7 The diagram shows a comparison of the ABTS and DPPH free radical scavenging activities of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liners of Comparative Examples 2-3. Figure 7 (a) is a comparison chart of ABTS; Figure 7 (b) is a comparison chart of DPPH free radical scavenging activity. From Figure 7 (a) It can be seen that the scavenging rate of PLA base film in ABTS is only 10.94%, with no obvious antioxidant activity; when MPN is added, the antioxidant activity of P-MP increases to 50.54%, while that of P-MPL is 53.00%. Figure 7In (b), the free radical scavenging activity of PLA remained very low, at only 2.43%; PP was only 5.71%, but P-MP and P-MPL reached 54.97% and 73.77% respectively, showing significantly enhanced activity. The antioxidant activity of P-MPL in ABTS was significantly lower than its free radical scavenging activity in DPPH because DPPH is a methanol solution, which performs better in hydrophobic / organic free radical systems. In summary, the PLA base film itself has weak antioxidant capacity in the PLA multilayer composite active liner. After further introducing MPN, the ABTS and DPPH scavenging capabilities of the PLA multilayer composite active liner were significantly enhanced, indicating that MPN is the main source of antioxidant function. P-MPL has a rougher surface and more distinct layers than P-MP, and the upper layer on the cross-section is also thicker, adding a diffusion path to the PLA multilayer composite active liner and reducing the instantaneous diffusion and rapid release of active ingredients.

[0063] Experimental Example 7 This experiment analyzed the antibacterial properties of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2 and 3. The specific steps were as follows: The antibacterial performance was tested using the plate count method, with Gram-negative bacteria Escherichia coli (E. coli) as the bacterial strain. E. coli ) and Gram-positive bacteria Staphylococcus aureus ( S.aureus First, E. coli and S.aureus Incubate with broth until the logarithmic growth phase, then wash with shaking and dilute to a concentration of 10. 6 CFU / mL. Cut out circular samples with a diameter of 1 cm using a cutter, immerse them in 3 mL of bacterial suspension, and incubate at 37 ℃ for 24 h. Take 100 μL of culture medium and dilute it to 10⁻⁶. 2 Subsequently, 100 μL of the diluted solution was spread onto LB agar plates and incubated at 37 ℃ for 24 h. The colony count (C) was counted to calculate the antibacterial rate (AR), and the results were digitally photographed. The specific calculation formula is as follows: (6) In the formula C S and C C The colony counts are for the sample group and the blank group, respectively.

[0064] like Figure 8 The image shows a comparison of the antibacterial rates of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the bilayer composite active liner of Comparative Examples 2-3. Figure 8 (a) is E. coli Antibacterial rate comparison chart; Figure 8 (b) is S.aureusAntibacterial rate comparison chart. From Figure 8 It can be seen that PLA, PP, P-MP, and P-MPL have different effects on... E. coli The antibacterial rates were 17.1%, 30.5%, 66.7%, and 70.1%, respectively. S.aureus The antibacterial rates were 15.5%, 56.0%, 99.6%, and 99.6%, respectively. Among them, P-MP showed better antibacterial activity than PP. E. coli and S.aureus The antibacterial rates increased by 36.2% and 43.6%, respectively; P-MPL compared to PP... E. coli and S.aureus The antibacterial rates increased by 39.6% and 43.6%, respectively. The introduction of MPN significantly enhanced the antibacterial performance of the PLA multilayer composite active liner, indicating that MPN is the most crucial antibacterial substance in the PLA multilayer composite active liner. Further compounding with LDHs@CDs maintained the significant antibacterial effect of the PLA multilayer composite active liner, suggesting that the top layer of LDHs@CDs acts more like an active layer and a controlled-release layer, allowing the active components to be released more continuously rather than instantaneously.

[0065] Experimental Example 8 This experiment conducted a strawberry preservation test to analyze the preservation performance of the polylactic acid multilayer composite active liner of Example 1, the PLA microporous membrane of Comparative Example 1, and the double-layer composite active liner of Comparative Examples 2-3. The specific steps are as follows: Strawberries were stored in sealed polypropylene boxes measuring 17 cm × 11 cm × 5 cm, with an average temperature of 23℃ and an average humidity of 80%. The experiment included a blank group, a PLA group, a PP group, a P-MP group, and a P-MPL group, with each pad measuring 13 cm × 11 cm. Changes in the strawberries were recorded every 24 hours using a digital camera, an electronic balance, and a colorimeter.

[0066] like Figure 9 The image shown is a comparison of digital photographs of strawberries stored at different times. Figure 9 It can be seen that the blank sample and the PLA base film group showed the earliest and most obvious deterioration: by the second day, obvious mold spots had appeared, and by the third day, the white mycelium had further expanded, the fruit surface lost its luster, and browning intensified. The PP group maintained a relatively intact appearance for the first two days, but obvious mold appeared by the third day. P-MP and P-MPL did not show obvious mycelium until the third day, and the fruit fullness and red appearance remained good.

[0067] Table 3 shows the color parameters of strawberries at different storage periods. The color parameters in Table 3 show a consistent pattern of change with the photographs. From day 0 to day 3, the brightness of the blank sample... L The PLA base film decreased from 35.85 to 21.68, while the P-MP and P-MPL values ​​remained at 30.99 and 32.49, respectively. a The values ​​decreased to 22.37 and 23.08 in the blank sample and PLA-based film, respectively, while remaining at 29.28 in P-MPL. Due to... L A decrease usually indicates that the browning of the fruit is worsening. a A decrease in color indicates a weakening of the red hue, therefore, the P-MP and P-MPL multilayer composite active pads significantly delay the color decay of strawberries.

[0068] Table 3. Color parameters of strawberries at different storage periods

[0069] Figure 10 This chart compares the weight loss rates of strawberries at different storage periods. The weight loss rate further demonstrates that the PLA multilayer composite active liner can significantly delay water loss in strawberries. Figure 10 It can be seen that on day 3, the weight loss rates of the blank sample, PLA base film, PP, P-MP, and P-MPL were 16.2%, 13.9%, 5.8%, 2.4%, and 3.1%, respectively. Compared with the blank sample, the weight loss rates of P-MP and P-MPL decreased by approximately 85.2% and 80.9%, respectively.

[0070] Previous studies have shown that the acceptable weight loss rate for strawberries is approximately 6%, exceeding which often leads to significant shrinkage, browning, and textural deterioration. Therefore, the blank sample and PLA-based film significantly exceeded the acceptable range for marketability by day 2, PP approached the critical value by day 3, while P-MP and P-MPL remained well below 6% even by day 3. This indicates that the PLA multilayer composite active liner significantly reduced moisture loss, with P-MP exhibiting the strongest water retention capacity.

[0071] This invention employs electrospinning to construct an LDHs@CDs / PVA outer layer and an MPN / PVA intermediate active layer, and successfully prepares a PLA multilayer composite active liner by combining it with a porous PLA base film. The resulting liner exhibits excellent hierarchical structure and functional synergy. The LDHs@CDs / PVA outer layer imparts excellent UV shielding performance, achieving near-complete blocking of both TUVA and TUVB. Simultaneously, the introduction of this outer layer improves the hydrophilicity regulation of the PVA fiber surface, to some extent preventing the porous fiber structure from collapsing due to water absorption, thus contributing to maintaining the stability and performance of the liner structure. Furthermore, LDHs@CDs, as the active component of the outer layer, complements the intermediate MPN / PVA active layer, thereby prolonging the continuous release of active substances, demonstrating the advantages of multilayer structure design in terms of functional continuity.

[0072] PLA multilayer composite active liners also exhibited significantly enhanced antibacterial and antioxidant activity. Compared to PP, P-MP and P-MPL showed significantly enhanced antibacterial and antioxidant activity. E. coli The antibacterial rates increased by 36.2% and 39.6% respectively, and the antibacterial rate was improved. S.aureus The antibacterial rate reached 99.6%. The DPPH free radical scavenging rate increased by 49.26% and 68.06%, respectively. The synergistic introduction of multilayer active components can significantly improve the bioactivity of the liner, among which P-MPL performs better in antibacterial and antioxidant aspects, indicating that a good synergistic effect is formed between the outer LDHs@CDs and the middle MPN.

[0073] Strawberry preservation experiments further validated the practical application effect of the multi-layer composite active liner. During storage, the PLA multi-layer composite active liner effectively delayed water loss, shrinkage, mold growth, and color deterioration in strawberries, indicating its positive role in maintaining fruit appearance quality and delaying quality decline. Specifically, P-MP showed better performance in inhibiting strawberry water loss, while P-MPL was more stable in maintaining overall color and appearance integrity. This demonstrates that the introduction of different functional layers allows for a certain degree of division of labor and synergy in moisture regulation, preservation, activity protection, and appearance maintenance. This multi-layer structure can continuously exert its preservation effect during storage, thereby improving the shelf life of strawberries.

[0074] In summary, the PLA multilayer composite active liner constructed in this invention achieves a synergistic unity of multiple functions, including UV shielding, structural stability, active release, and continuous preservation, by integrating the LDHs@CDs / PVA outer layer with the MPN / PVA intermediate active layer. This composite active liner not only possesses good structural integrity and functional activity but also demonstrated significant application effects in strawberry preservation experiments, showing promising prospects for active packaging of fresh fruits and vegetables.

[0075] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a polylactic acid multilayer composite active liner, characterized in that, Includes the following steps: S1. Urea, AlCl3·H2O, MgCl2·H2O and Spartina alterniflora extract were dissolved in deionized water in sequence, ultrasonicated and then placed in a vacuum drying oven for reaction. After cooling, centrifugation and freeze drying, LDHs@CDs powder was obtained. S2. Quercetin was added to anhydrous ethanol, sonicated, and then CuCl2 aqueous solution was added. The pH of the resulting mixed solution was adjusted to 6.8-7.6, and the mixture was reacted. After centrifugation, washing, and freeze-drying, MPN was obtained. S3. Dissolve the MPN in deionized water, add the resulting MPN dispersion to polyvinyl alcohol and stir at high temperature, then cool and let stand to obtain P-MP spinning solution; S4. Dissolve the LDHs@CDs powder in deionized water, add the resulting LDHs@CDs dispersion to polyvinyl alcohol and stir at high temperature, then cool and let stand to obtain P-MPL spinning solution; S5. Using an electrospinning process, the P-MP spinning solution and the P-MPL spinning solution are sequentially electrospun onto a polylactic acid microporous base membrane to obtain a polylactic acid multilayer composite active liner.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass-to-volume ratio of urea, AlCl3·H2O, MgCl2·H2O, Spartina alterniflora extract and deionized water is 1.6~1.7:1.2~1.3:3~3.3:1:100~110 (g / g / g / g / mL).

3. The preparation method according to claim 1, characterized in that, In step S1, the reaction temperature is 155~165℃, the reaction time is 5.5~6.5h, and the freeze-drying time is 46~50h.

4. The preparation method according to claim 1, characterized in that, In step S2, the mass-to-volume ratio of quercetin to anhydrous ethanol is 1:190~210 (g / mL), and the preparation step of the CuCl2 aqueous solution is as follows: CuCl2·2H2O is added to water to obtain the CuCl2 aqueous solution, wherein the mass-to-volume ratio of quercetin, CuCl2·2H2O and water is 1.6~1.8:1:170~180 (g / g / mL).

5. The preparation method according to claim 1, characterized in that, In step S3, the mass ratio of MPN to polyvinyl alcohol is 1:19~21, and the temperature of the high-temperature stirring is 95~100℃.

6. The preparation method according to claim 1, characterized in that, In step S4, the mass ratio of the LDHs@CDs powder to the polyvinyl alcohol is 1:33~34, and the temperature of the high-temperature stirring is 95~100℃.

7. The preparation method according to claim 1, characterized in that, In step S5, the step of electrospinning the P-MP spinning solution and the P-MPL spinning solution onto the polylactic acid microporous base membrane layer by layer using electrospinning technology is as follows: the P-MPL spinning solution and the P-MP spinning solution are electrospinned onto the polylactic acid microporous base membrane layer by layer at a feed rate of 2 mL / h and a positive voltage of 25 kV. During the electrospinning process, the temperature is controlled at 25°C and the humidity is maintained at 65%.

8. The preparation method according to claim 1, characterized in that, In step S5, the electrospinning process uses a 20G needle, the distance from the needle to the collector is 15 cm, and the roller speed is 150 rpm.

9. The preparation method according to claim 1, characterized in that, The polylactic acid microporous membrane has a pore size of 300 nm.

10. A polylactic acid multilayer composite active liner, characterized in that, Prepared by the preparation method according to any one of claims 1 to 9.

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