Agricultural light conversion film with stable photoluminescence properties

Abstract

The application discloses an agricultural light conversion film with stable photoluminescence characteristics, and belongs to the technical field of polymer composites and agricultural covering materials. The film is prepared by compounding layered double hydroxides (LDHs) and low-density polyethylene (LDPE) and then performing heat treatment for a period of time. After the heat treatment, the film induces oxygen vacancies in the exfoliated LDHs to interact with carbonyl groups generated in the LDPE, reconstructs the energy level of the LDPE matrix through electron exchange, and generates spontaneous stable and uniform fluorescent-phosphorescent dual emission, so that ultraviolet light is efficiently converted into blue light. When the film is used as the agricultural light conversion film, the film has stable light conversion performance, excellent tensile strength, light transmittance and haze, can ensure long-term use, and can adjust the light environment and significantly promote the growth of crops that like blue light.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials and agricultural covering materials, specifically to a layered double hydroxide / low-density polyethylene composite material with fluorescence-phosphorescence stable photoluminescence properties, and its application as a light-conversion agricultural film. Background Technology

[0002] Low-density polyethylene (LDPE) is the most widely used covering film material in agricultural production, but its utilization efficiency for ultraviolet light is extremely low, failing to convert the high-energy ultraviolet light in sunlight into the blue or red light required for plant photosynthesis. Current technologies typically achieve spectral conversion by adding organic light-converting agents, rare-earth complexes, or quantum dots to LDPE. However, these added light-converting agents share the following common problems:

[0003] (1) The light conversion agent is prone to aggregation or agglomeration, which leads to a decrease in light conversion efficiency;

[0004] (2) Organic matrices are easily degraded under ultraviolet irradiation and thermo-oxidative aging conditions, resulting in decreased mechanical and optical properties and short service life;

[0005] Therefore, there is an urgent need to develop a polymer-based composite material that does not require the addition of an external light-converting agent and obtains stable photoluminescence properties by relying on the self-luminescence of the material matrix, and to verify its practical application value in agricultural films. Summary of the Invention

[0006] This invention aims to provide a layered double hydroxide / low-density polyethylene composite material. After heat treatment, the layered double hydroxide induces a structural transformation in the low-density polyethylene, enabling it to spontaneously produce stable and uniform fluorescence-phosphorescence dual emission, efficiently converting ultraviolet light into blue light. Using this material as an agricultural covering film can significantly promote the growth of blue-light-loving crops, while also exhibiting extremely high photostability and aging resistance.

[0007] An agricultural light-converting film with stable photoluminescence properties, characterized in that: the light-converting film is made by compositing layered double hydroxides (LDHs) and low-density polyethylene (LDPE) and then heat-treating for a certain period of time;

[0008] Furthermore, the mass fraction of the LDHs in the light conversion film is 1% to 5%, preferably 2%.

[0009] Furthermore, the LDHs are LDHs materials with different metal compositions and morphologies. The composition is preferably MgAl-LDHs, CoAl-LDHs, etc., and the morphology is preferably flake-like with a size range of 20 nm to 5 μm.

[0010] Furthermore, the heat treatment conditions are: treatment in an air atmosphere at 80–120°C for 48–200 h.

[0011] Furthermore, the LDHs gradually exfoliate during the heat treatment process, with an interlayer spacing greater than 0.230 nm, resulting in oxygen vacancy defects.

[0012] Furthermore, the carbonyl groups generated in the LDPE matrix during heat treatment interact with the defects generated by LDHs, reconstructing the energy levels of the LDPE matrix through electron transfer.

[0013] Furthermore, the light-converting film exhibits enhanced fluorescence emission in the range of 375–550 nm under 290 nm ultraviolet light excitation, with the emission peak located near 440 nm;

[0014] Furthermore, after UV excitation is stopped, the phosphorescence emission peak of the light-converting film is located at around 450 nm, and the phosphorescence intensity is more than 7.5 times higher than that of the pure LDPE film.

[0015] Furthermore, the stable photoluminescence properties of the light-converting film are due to the energy level reconstruction of the LDPE matrix caused by LDHs.

[0016] The light-converting film with stable photoluminescence properties obtained by this invention can be used as an agricultural light-converting film to convert ultraviolet light in sunlight into blue light, so as to promote the growth of blue light-loving crops.

[0017] Furthermore, when used as an agricultural covering film, the light-converting film exhibits stable mechanical and optical properties, with a tensile strength higher than that of pure LDPE film without added LDHs. Its light transmittance and haze are also improved after heat treatment.

[0018] Furthermore, the blue-light-loving crops include, but are not limited to, lettuce and garlic.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The preparation and use methods are simple: the film can be formed by conventional hot pressing process. Before use, it only needs to be treated under controlled heat conditions for a certain period of time to obtain the required luminescent properties, without changing the existing agricultural film production process.

[0021] Utilizing the self-luminescence of LDPE: The material achieves dual emission of fluorescence and phosphorescence through structural evolution during heat treatment, eliminating the need for adding a light-converting agent and completely avoiding problems such as agent aggregation and matrix degradation.

[0022] Extremely high luminescence stability: The photoluminescence performance originates from the energy level reconstruction of the LDPE matrix, which has extremely high stability and uniformity, and hardly undergoes migration or inactivation. Its service life far exceeds that of traditional agricultural films with added light-converting agents.

[0023] Highly efficient UV-to-blue light conversion performance: The emission peak is located at 440-460 nm, which is highly matched with the absorption peaks of chlorophyll a, b and phytochromes in plant photosynthesis, and is especially beneficial to the growth of blue light-loving crops such as lettuce and garlic;

[0024] Maintaining excellent mechanical and optical properties: Pure LDPE film has reduced mechanical and optical properties after heat treatment. In contrast, LDHs / LDPE composite film has higher tensile strength, better light transmittance and lower haze after heat treatment. As an agricultural film, it can ensure long-term use, reduce replacement frequency and effectively regulate the light environment. Attached Figure Description

[0025] Figure 1 The fluorescence properties of LDPE and 2% LDHs / LDPE composites after heat treatment are shown in Figure (A), which is a photograph of LDPE and 2% LDHs / LDPE composites under fluorescent lamp for different heat treatment times; Figure (B) is a photograph of LDPE and 2% LDHs / LDPE composites under ultraviolet lamp for different heat treatment times; Figure (C) is the fluorescence spectrum of LDPE and 2% LDH / LDPE composites after different heat treatment times; and Figure (D) is a comparison of the fluorescence intensity at 440 nm of LDPE and 2% LDH / LDPE composites after different heat treatment times.

[0026] Figure 2 The phosphorescence properties of LDPE and 2% LDHs / LDPE composites after heat treatment are shown in Figure (A). Figure (A) shows the afterglow image of LDPE and 2% LDHs / LDPE composites after heat treatment at 100℃ for 100 h after UV lamp is turned off. Figure (B) shows the phosphorescence spectrum of LDPE and 2% LDHs / LDPE composites after different heat treatment times. Figure (C) shows the comparison of phosphorescence intensity at 450 nm of LDPE and 2% LDHs / LDPE composites after different heat treatment times.

[0027] Figure 3 The structural changes of LDPE and 2% LDHs / LDPE composites before and after heat treatment are shown in Figure (A). Figure (B) is a TEM image of the 2% LDHs / LDPE composite; Figure (C) is a TEM image of the 2% LDHs / LDPE composite after heat treatment at 100 ℃ for 100 h; Figure (D) is an XRD pattern of the LDPE composite after heat treatment at 100 ℃ for different times; Figures (D) and (E) are XRD patterns of the 2% LDHs / LDPE composite after heat treatment at 100 ℃ for different times and magnified views in the 9.5°~13.5° region.

[0028] Figure 4To illustrate the stable photoluminescence mechanism of LDPE and 2% LDHs / LDPE composites, Figure (A) shows the FT-IR spectra of LDPE after heat treatment at 100℃ for different times; Figure (B) shows the FT-IR spectra of 2% LDHs / LDPE composites after heat treatment at 100℃ for different times; Figure (C) shows the XPS C1s spectra of LDHs before and after heat treatment at 100℃ for 400 h; and Figure (D) shows the ESR spectra of LDHs before and after heat treatment at 100℃ for 400 h.

[0029] Figure 5 The properties of LDPE and 2% LDHs / LDPE composites before and after heat treatment at 100℃ for 100 h are shown in Figure (A), which shows the tensile strength; Figure (B) shows the light transmittance; and Figure (C) shows the haze.

[0030] Figure 6 Image showing LDPE and 2% LDHs / LDPE composite material used as an agricultural light-converting film for lettuce cultivation. Detailed Implementation

[0031] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments.

[0032] Example 1:

[0033] (1) Synthesis of LDHs

[0034] 100 mL of a nitrate solution containing Mg(NO3)2·6H2O (0.3 M) and Al(NO3)3·9H2O (0.1 M) was mixed with 100 mL of a solution containing NaOH (0.48 M) and Na2CO3 (0.2 M) in a colloid mill for 1 min. The resulting mixture was then placed in a stainless steel autoclave lined with polytetrafluoroethylene and reacted at 100 °C for 24 h. The product was subsequently centrifuged and washed with deionized water until the pH reached 7. Finally, the LDH precipitate was dried at 60 °C and ground for later use.

[0035] (2) Preparation of LDHs / LDPE light conversion film

[0036] LDHs and LDPE were physically mixed at a weight ratio of 2% to obtain a uniform 2% LDHs / LDPE composite film with a certain thickness (e.g., 0.2 mm, 0.5 mm). The LDPE and 2% LDHs / LDPE composite film were then heat-treated in a forced-air drying oven at 100℃, with samples taken at different time intervals to obtain samples with varying heat treatment times.

[0037] (3) Fluorescence properties of LDHs / LDPE composite film

[0038] Under sunlight, the LDPE film gradually turns milky white throughout the heat treatment process, while the 2% LDHs / LDPE composite film exhibits good transparency. Figure 1 A). After heat treatment for 0–100 h, the LDPE film showed almost no luminescence under 365 nm ultraviolet irradiation, while the 2% LDHs / LDPE composite film showed bright blue fluorescence under 365 nm ultraviolet irradiation after heat treatment with prolonged heat treatment time. Figure 1 B). Fluorescence spectroscopy showed that in the absence of LDHs, the LDPE film exhibited no fluorescence in the 400–780 nm range, while the 2% LDHs / LDPE composite film showed an increase in fluorescence from ~1400 to ~7000 at 440 nm. Figure 1 C and Figure 1 D).

[0039] (4) Phosphorescence properties of LDHs / LDPE composite film

[0040] The LDHs / LDPE composite film emits afterglow after UV exposure has ceased, such as Figure 2 As shown in Figure A, afterglow detection using an ultra-low light detection chip revealed that the afterglow of the LDPE film increased from 20 s to 40 s after 100 h of heat treatment; while the afterglow of the 2% LDHs / LDPE composite film was significantly enhanced and prolonged to 160 s. Figure 2 As shown in Figure B, the phosphorescence spectrum reveals that the presence of LDHs significantly affects the phosphorescence emission characteristics of LDPE. The phosphorescence emission at 450 nm is greatly enhanced in the 2% LDHs / LDPE composite film after heat treatment for different times. The phosphorescence emission intensity shows that... Figure 2 As shown in Figure C, the phosphorescence intensity of the LDPE film increases slightly after 100 h of treatment, while the phosphorescence emission intensity of the 2% LDHs / LDPE composite film increases dramatically, with the phosphorescence intensity after 100 h of heat treatment being approximately 7.5 times that of LDPE.

[0041] (5) Structural evolution of LDHs / LDPE composite membranes

[0042] During heat treatment, LDHs in the LDHs / LDPE composite film gradually exfoliated. High-resolution transmission electron microscopy (HRTEM) revealed that the lattice fringes of LDHs in the untreated 2% LDHs / LDPE composite film were approximately 0.204 nm. Figure 3 As shown in Figure A; after 100 h of heat treatment, the dispersibility of LDHs in the composite membrane improved, as shown in Figure A. Figure 3 As shown in Figure B, the layered structure of LDHs is extended in the vertical direction, and the lattice fringes are broadened to approximately 0.239 nm.

[0043] The characteristic peaks of the (003) crystal plane in the X-ray diffraction (XRD) pattern also confirmed the gradual delamination of LDHs during the heat treatment process. Figure 3 As shown in Figure C, the diffraction peaks of LDPE did not change significantly with increasing processing time; as... Figure 3 As shown in Figure D, the diffraction peak of MgAl-LDHs at 11.4 ° decreases with increasing heat treatment time; from Figure 3 The magnified image of D in the 9.5°~13.5° region shows that when the heat treatment time reaches 100 h, the (003) crystal plane diffraction peak at 11.4° almost disappears.

[0044] (6) Stable photoluminescence mechanism of LDHs / LDPE composite film

[0045] The reasons for photoluminescence caused by structural changes were explored in depth, and Fourier transform infrared spectroscopy (FT-IR) was used to examine the changes in carbonyl groups after different heat treatment times. Figure 4 As shown in Figure A, LDPE exhibits a gradual increase in carbonyl groups during the first 48 hours; after 48 hours, the carbonyl content increases rapidly. In contrast, the carbonyl content of the 2% LDHs / LDPE composite film increases only slightly after heat treatment at different times, such as... Figure 4 As shown in B, this is attributed to the interaction between the carbonyl groups generated in the LDPE matrix during the processing and the exfoliated LDHs.

[0046] The exfoliated LDHs generated more defects, providing oxygen vacancies for interaction with carbonyl groups, which was confirmed by the increase in the number of oxygen vacancies in the XPS O 1s spectra of LDHs after heat treatment for 400 h. Figure 4 C). Furthermore, the presence of a single peak with a g-factor of 1.998 in the ESR spectrum also confirms the presence of oxygen vacancies in LDHs, and the stronger ESR signal after 400 hours of heat treatment indicates a further increase in oxygen vacancies in LDHs. Figure 4 D). Therefore, the carbonyl groups generated in the LDPE matrix during heat treatment interact with the defects generated by LDHs, reconstructing the S1 and T1 energy levels of the LDPE matrix through electron transfer, thus resulting in a stable enhancement of photoluminescence properties.

[0047] (7) Performance of LDHs / LDPE composite material as agricultural film

[0048] LDHs / LDPE composite membranes have stronger resistance to environmental stress, such as Figure 5As shown in Figure A, the tensile strength was determined according to GB / T1040.3-2006. Using a single specimen with a width of 10 mm and a thickness of 0.2 mm, the tensile strength of LDPE was observed to be 34.9 MPa, decreasing to 27.9 MPa after heat treatment. In contrast, the tensile strength of 2% LDHs / LDPE increased to 51.7 MPa, decreasing by only 8.40% after 100 h of heat treatment. LDHs / LDPE, as an agricultural film, can ensure long-term use and reduce replacement frequency. Furthermore, the LDHs / LDPE composite film exhibits good optical properties, effectively regulating the light environment to promote optimal plant growth, such as… Figure 5 As shown in Figure B, the light transmittance of the 2% LDHs / LDPE composite film is 86.5% (0.2mm thickness, corresponding to white light), significantly better than that of LDPE (82.5%). After heat treatment, the light transmittance of LDPE increased to 86.4%, while the 2% LDHs / LDPE composite film achieved an even higher transmittance (89.5%). Finally, as... Figure 5 As shown in Figure C, the haze of the 2% LDHs / LDPE composite film decreased to 27.5% after heat treatment for 100 h. The low haze effectively prevented light scattering and ensured sufficient light during plant growth.

[0049] LDHs / LDPE composite film was used as an agricultural light-converting film to promote the growth of lettuce, a plant that prefers blue light. LDPE and 2% LDHs / LDPE composite film were respectively used to cover the top of the lettuce, as shown in the image. Figure 6 As shown in the figure. After 3 days, lettuce began to germinate under both types of agricultural films. After 6 days, lettuce seedlings under the LDHs / LDPE composite film grew more vigorously compared to those under the LDPE film. After 12 days, lettuce under the LDHs / LDPE composite film was twice as tall as that under the LDPE film, and also had more leaves. These results indicate that the LDHs / LDPE composite film, as an agricultural light-converting film, has the advantage of converting simulated sunlight into blue light to promote lettuce growth.

Claims

1. An agricultural light-conversion film with stable photoluminescence properties, characterized in that: The light-converting film is made by combining layered double hydroxides (LDHs) with low-density polyethylene (LDPE) and undergoing a period of heat treatment.

2. The agricultural light-converting film according to claim 1, characterized in that, The mass fraction of LDHs in the light conversion film is 1% to 5%, preferably 2%.

3. The agricultural light-converting film according to claim 1, characterized in that, The LDHs are LDHs materials with different metal compositions and morphologies. The preferred compositions are MgAl-LDHs, CoAl-LDHs, etc., and the preferred morphology is flake-like with a size range of 20 nm to 5 μm.

4. The agricultural light-converting film according to claim 1, characterized in that, The heat treatment conditions are: treatment in an air atmosphere at 80–120°C for 48–200 h.

5. The agricultural light-converting film according to claim 1, characterized in that, During the heat treatment process, the LDHs gradually exfoliate, with an interlayer spacing greater than 0.230 nm, generating oxygen vacancy defects. The carbonyl groups generated in the LDPE matrix during the heat treatment process interact with the defects generated by the LDHs, reconstructing the energy levels of the LDPE matrix through electron transfer.

6. The agricultural light-converting film according to claim 1, characterized in that, The light-converting film exhibits enhanced fluorescence emission in the range of 375–550 nm under 290 nm ultraviolet light excitation, with the emission peak located near 440 nm. After UV excitation is stopped, the phosphorescence emission peak of the light-converting film is located at around 450 nm, and the phosphorescence intensity is more than 7.5 times higher than that of pure LDPE film.

7. The agricultural light-converting film according to claim 1, characterized in that, The stable photoluminescence properties of the light-converting film are due to the energy level reconstruction of the LDPE matrix caused by LDHs.

8. The agricultural light-converting film according to any one of claims 1-7, as an agricultural light-converting film, can be used to convert ultraviolet light in sunlight into blue light to promote the growth of blue light-loving crops.

9. According to the application described in claim 8, the agricultural light-converting film has stable mechanical and optical properties, and its tensile strength is higher than that of pure LDPE film without added LDHs. Its light transmittance and haze are improved after heat treatment.

10. The application according to claim 8, wherein the blue-light-loving crops include, but are not limited to, lettuce and garlic.

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