Spectral selective greenhouse cooling water-saving film and preparation method thereof

Abstract

The invention discloses a spectral selective greenhouse cooling water-saving film and a preparation method thereof. The film sequentially comprises a polymer thermal radiation material layer, a metal coating and a polymer multilayer optical film from top to bottom. Wherein the polymer thermal radiation material layer on the top layer can dissipate heat in a thermal radiation form; the metal coating on the middle layer is used for reflecting near-infrared light with the wavelength larger than 800 nm; the polymer multilayer optical film on the bottom layer is used for reflecting green light of 500-600 nm. The spectrum separation technology and the radiation refrigeration technology are coupled, the polymer multilayer film and the metal coating are combined, the polymer multilayer film is used for replacing a dielectric layer in traditional coating, the technological process is effectively simplified, the cost is greatly reduced, and the machining speed is increased; in addition, the polymer multilayer film can serve as a protective layer of a metal coating, the oxidation speed of the metal coating is slowed down, and the overall durability of the film material is improved.

Description

Technical Field

[0001] The present invention relates to the fields of spectral selectivity, multi-layer composite materials, radiative cooling, etc., and particularly relates to a spectrally selective greenhouse cooling and water-saving film and a preparation method thereof. Background Art

[0002] With the intensification of global warming and climate change, extreme weather events occur frequently in summer, and the growth of greenhouse crops is severely threatened. Plants under high temperature are in a state of heat stress for a long time, and are extremely prone to wilting and even death, causing huge economic losses to agricultural production. In addition, agricultural water consumption accounts for 70% of the global fresh water consumption. Extreme high temperature weather accelerates the evaporation of surface water, further exacerbating the problem of water resource shortage. The existing strategies for alleviating high temperature in greenhouses are: using sunshade nets, wet curtain fan systems, bioengineering methods, etc. Among them, sunshade nets do not have spectral selectivity, affect the transmission of photosynthetically active radiation, and lead to the obstruction of the normal growth and development of plants. The wet curtain fan system is costly and consumes a large amount of electric energy and water during operation. Bioengineering technologies may cause ecological system hazards and social ethics problems. The optical properties of greenhouse covering materials determine the intensity of solar radiation entering the greenhouse interior, which is of great significance for regulating the temperature inside the greenhouse. Traditional greenhouse covering materials (such as: PE film, PO film, etc.) have an average transmittance of about 90% in the entire solar radiation band. In summer, the high transmittance results in too much solar radiation entering the greenhouse interior, causing too high air temperature inside the greenhouse, too fast evaporation of soil moisture and crop yield reduction. Therefore, designing new greenhouse covering materials to achieve efficient greenhouse cooling and water saving has been the research focus in the field of greenhouse heat regulation in recent years.

[0003] Spectral separation type radiative cooling materials are an important method to solve the above problems. Radiative cooling materials rely on the "atmospheric window" (8-13μm) to emit their own heat in the form of infrared radiation into the cold outer space, and can achieve the effect of reducing their own temperature without additional energy. At the same time, photosynthesis is an important way for plants to accumulate organic matter. Plant leaves absorb solar energy through plant pigments and convert it into biomass. Plant pigments mainly absorb blue-violet light of 400-500nm and red light of 600-700nm, and have a relatively low absorption rate for green light of 500-600nm and infrared light with a wavelength greater than 800nm. Separating the spectrum (transmitting the red and blue-violet light required for plant growth and reflecting the less utilized green light and near-infrared light) can effectively reduce the excess solar radiation entering the greenhouse interior. At the same time, radiative cooling technology can further enhance the ability of the greenhouse to output heat outward. Therefore, spectral separation type radiative cooling materials can effectively reduce the temperature of the greenhouse and reduce the water evaporation amount without affecting plant growth.

[0004] Spectral separation type radiative cooling materials must meet the following spectral requirements: 1. Maintain high transmittance in two wavelength bands of 400 - 500 nm and 600 - 700 nm; 2. Maintain high reflectance in the wavelength bands of 500 - 600 nm and greater than 800 nm; 3. Maintain high thermal radiation emissivity in the infrared radiation transparent window of 8 - 13 μm. However, achieving the above strict spectral characteristics is a huge challenge. Currently, the most commonly used is the optical coating technology, that is, by sequentially evaporating metal layers and inorganic dielectric layers on an optical-grade polymer substrate to achieve spectral separation, and using composite polymer materials to achieve high thermal radiation emissivity. Among them, the metal layer can achieve broadband reflection, generally used to achieve the initial spectral separation process, while the inorganic dielectric layer can achieve narrowband reflection, generally used for the secondary separation of the spectrum. In the actual coating processing, the metal coating has a relatively thin thickness (generally about 5 - 20 nm), fewer layers, and the metal target can be processed using a DC power supply, and its process is relatively simple and the processing speed is relatively fast. While the inorganic dielectric layer has a relatively large thickness (generally about 40 - 200 nm), more layers, and the inorganic dielectric target can only be processed through a radio frequency power supply, and its processing speed is extremely slow. This also makes the traditional coated optical cooling film costly and slow in processing speed, and it cannot be popularized and applied in the field of greenhouse cultivation.

[0005] In summary, spectral separation type radiative cooling materials can effectively regulate the greenhouse climate, but the current optical coating technology cannot be applied to the field of greenhouse cultivation due to high costs and insufficient production capacity. Therefore, designing and preparing a low-cost and high-processing-speed spectral selective greenhouse cooling and water-saving material is of great significance for effectively alleviating plant heat stress in summer, reducing greenhouse water consumption, and increasing summer crop yields. Summary of the Invention

[0006] The purpose of the present invention is to provide a spectral selective greenhouse cooling and water-saving thin film and its preparation method. This thin film uses a polymer multi-layer film to replace the inorganic dielectric coating in traditional coating materials, can achieve precise spectral separation, effectively simplifies the structure of the traditional coated optical film, reduces the processing cost, greatly improves the processing speed, and at the same time combines radiative cooling technology to efficiently regulate the temperature of the greenhouse without affecting plant growth.

[0007] The purposes of the present invention are as follows:

[0008] The first purpose of the present invention is to provide a spectral selective greenhouse cooling and water-saving thin film, which sequentially includes a polymer thermal radiation material layer, a metal coating, and a polymer multi-layer optical film from top to bottom. Among them:

[0009] The polymer thermal radiation material layer on the top has a high mid-infrared thermal radiation emissivity, which can dissipate heat in the form of thermal radiation, further enhancing the cooling effect of the cooling film. At the same time, this material is also used as a flexible substrate, which has extremely high optical transmittance (higher than 90%), does not affect the transmission of photosynthetically active radiation, and also provides excellent mechanical properties and environmental stability. The candidate materials for the polymer thermal radiation material layer described in the present invention are PET, PC, and PMMA. Among them, PET has a high mid-infrared emissivity in the "atmospheric window", and also has many advantages such as flexibility, low cost, high transparency, and mature process, and is the preferred material for polymer thermal radiation materials.

[0010] The metal coating layer in the middle is used to achieve a high degree of reflection of near-infrared light with a wavelength greater than 800 nm in the solar spectrum. The energy in the near-infrared band of the solar spectrum accounts for about 50%, which is an important reason for the excessive temperature inside the greenhouse. Highly reflecting this part of the solar spectrum can effectively reduce the air temperature and soil temperature inside the greenhouse and effectively reduce the water evaporation rate. The metal coating material described in the present invention is silver. The preferred thickness of the silver metal coating is 8-12 nm. The metal coating shows a broadband reflection effect and can only achieve a preliminary separation of the solar spectrum.

[0011] The underlying polymer multi-layer optical film is used to achieve a narrow-band spectral separation effect. The polymer multi-layer optical film has a flexible design. According to the thickness change of the unit film layer, it can be divided into the equal-thickness type and the thickness-graded type. When the thickness of the unit film layer remains unchanged, the polymer multi-layer film group can achieve a narrow-band and high-efficiency reflection effect. When the thickness of the unit film layer shows a gradient change, the polymer multi-layer module can achieve a wide-band but low-efficiency reflection effect. The polymer multi-layer optical film is generally prepared by a multi-layer co-extrusion technique. In the actual preparation process, since the separation ratio of the multiplier is generally fixed, the process of preparing a polymer multi-layer film group with a thickness gradient change is much more complex than that of the equal-thickness type. Therefore, the present invention uses an equal-thickness type polymer multi-layer film to achieve the reflection effect in the 500-600 nm band. Specifically, the polymer multi-layer optical film described in the present invention is composed of a plurality of polymer unit film layers with the same thickness. The polymer unit film is composed of two polymer material layers with different refractive indexes, specifically a high refractive index material layer and a low refractive index material layer. The candidate materials with high refractive indexes include: polyethylene naphthalate (PEN), polycarbonate (PC), polystyrene (PS), and polyethylene terephthalate (PET), while the low refractive index material is usually polymethyl methacrylate (PMMA). The polymer multi-layer film is a kind of photonic crystal and can achieve strong reflection in a certain wavelength band. The thickness of the multi-layer film is directly related to the position of the reflection band. Generally speaking, as the thickness increases, the position of the reflection peak will move towards the direction of increasing wavelength. On the contrary, as the thickness decreases, the position of the reflection peak will move towards the direction of decreasing wavelength. The thickness of the polymer multi-layer film described in the present invention needs to be 10.5 μm (±1 μm), otherwise it is impossible to achieve an efficient reflection effect in the 500-600 nm range. The reflectivity of the polymer multi-layer film is related to the number of layers. The higher the number of layers, the higher the reflectivity. When the number of layers reaches 128 layers, the reflectivity no longer increases. Therefore, the number of layers of the polymer multi-layer film described in the present invention is preferably 128 layers. The polymer multi-layer optical film in the present invention can achieve the purpose of replacing the inorganic dielectric coating.

[0012] The second object of the present invention is to provide a preparation method of the spectral selective greenhouse cooling and water-saving thin film described in the first object, including the following steps:

[0013] Step 1: Preparation of the polymer multi-layer optical film. The preparation of the multi-layer film system relies on the polymer multi-layer co-extrusion technology. Before the start of multi-layer co-extrusion, the raw materials are added to a vacuum dryer for sufficient drying to eliminate the moisture contained in the raw materials. At the same time, the extruder and the lamination device are preheated. During the multi-layer co-extrusion process, the polymer fluids are pre-stacked to form the initial two layers, and then their number of layers is increased by a multiplier. The multiplier divides the polymer fluid into two parts and forces them through the upper and lower "fish-tail" channels. During the extrusion process, the polymer fluid is stretched and compressed, and a four-layer structure is formed after leaving the multiplier. This process is repeated continuously until the required number of layers is reached. After the multiplication is completed, it is extruded and formed by an extrusion die head, and then rolled into a film through cooling. The film thickness is controlled by the speed of the extrusion screw.

[0014] Step 2: Preparation of the metal coating. The metal coating is prepared using the magnetron sputtering process. The candidate material for the coating metal is silver. The preparation process of the metal coating requires multiple steps such as substrate cleaning, magnetron sputtering, and post-treatment.

[0015] Step 3: The polymer heat radiation emission material can be used as a flexible substrate. After a metal coating is evenly evaporated on its surface, the polymer multi-layer optical film is bonded to it, and a common optical adhesive can be used for bonding.

[0016] The innovations of the present invention are as follows:

[0017] The present invention proposes a new type of spectral selective greenhouse cooling and water-saving film. This film realizes excellent spectral selectivity by using a polymer multi-layer film to replace the dielectric layer in the traditional optical coating, effectively simplifies the process flow, reduces the cost, and improves the processing speed. Combining the radiative cooling technology further enhances the cooling effect of the cooling film. In addition, since the metal coating is prone to oxidation and yellowing, especially the reflectivity of the silver coating will decrease significantly after oxidation and yellowing, a protective layer is used in the traditional optical coating to prevent its aging, while the polymer multi-layer film can be used as a coating protective film to slow down the oxidation rate of the metal coating and improve the overall durability of the film material.

[0018] The beneficial effects of the present invention are as follows:

[0019] Spectral separation type radiative cooling materials can effectively alleviate the high temperature in greenhouses in summer, regulate the greenhouse microclimate environment, reduce the greenhouse water evaporation rate and promote plant growth. Traditional spectral selective radiative cooling films rely on vacuum coating technology, which is costly and has a slow processing speed, and cannot be applied to the field of greenhouse planting. Specifically, the introduction of inorganic dielectric layers in traditional materials is the main reason for the difficulty in popularizing traditional vacuum coating technology. The present invention proposes a method of using polymer multi-layer optical films to replace inorganic dielectric layers, and designs a new type of spectral selective greenhouse cooling and water-saving film. This film has excellent spectral selectivity, can effectively reduce the solar radiation intensity entering the greenhouse interior, and at the same time further enhances the cooling effect of the cooling film through radiative cooling technology. Moreover, this film has low cost and simple process, and has great application potential in the field of greenhouse planting. Brief Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the structure and principle of the spectral selective greenhouse cooling and water-saving film;

[0021] Figure 2 It is a schematic diagram of the principle of the spectral selective greenhouse cooling and water-saving film to achieve greenhouse cooling;

[0022] Figure 3 It is a schematic diagram of the preparation process flow of the spectral selective greenhouse cooling and water-saving film;

[0023] Figure 4 It is a theoretical spectral diagram of the spectral selective greenhouse cooling and water-saving film;

[0024] Figure 5 It is an infrared emissivity diagram of the polymer thermal radiation emission material;

[0025] Figure 6 It is a theoretical thermal power analysis of the spectral selective greenhouse cooling and water-saving film;

[0026] Reference numerals: 1 - polymer thermal radiation emission material layer, 2 - metal coating, 3 - polymer multi-layer optical film, 301 - high refractive index material layer, 302 - low refractive index material layer. Detailed Embodiments

[0027] The following further illustrates the present invention in conjunction with embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the examples given are not intended to limit the present invention.

[0028] In addition, in the preparation processes in the following embodiments, if not otherwise specified, they are all conventional means in the prior art in this field. Therefore, they will not be described in detail. All raw materials used in the present invention are commercially available products and can be obtained by purchasing in the market.

[0029] Example 1

[0030] The present invention provides a spectrally selective greenhouse cooling and water-saving film, which is composed of three parts, and its structure is shown by Figure 1 . The upper layer is a polymer thermal radiation material layer 1, which is used to achieve efficient radiative cooling effect and maintain the mechanical strength and stability of the film as a flexible substrate; the middle layer is a metal coating 2, which is used to reflect near-infrared light with a wavelength greater than 800 nm; the bottom layer is a polymer multi-layer optical film 3, which is used to highly reflect green light in the 500-600 nm band; wherein: the polymer multi-layer optical film 3 is composed of multiple polymer unit films with the same thickness, and the polymer unit film is composed of a high refractive index material layer 301 and a low refractive index material layer 302 stacked together.

[0031] The principle of the film to achieve greenhouse cooling refers to Figure 1 and Figure 2 . In actual use, the polymer thermal radiation emission material faces the space to obtain the maximum radiative cooling effect, and at the same time can avoid dust pollution of the metal coating and the polymer multi-layer film. When solar radiation reaches the film surface, the polymer thermal radiation emission material on the top layer transmits the solar radiation, and the metal coating and the polymer multi-layer film in the middle and bottom layers reflect near-infrared light and green light respectively. Finally, only the photosynthetically active radiation in the 400-500 nm and 600-700 nm bands can reach the greenhouse interior and be used for plant growth. At the same time, the polymer thermal radiation material on the top layer dissipates heat in the form of thermal radiation, further reducing the temperature of the greenhouse. In addition, since the absorption rate of solar radiation by the soil can reach more than 50%, the spectral selectivity enables only the photosynthetically active radiation to enter the greenhouse interior, which greatly reduces the absorption of solar radiation energy by the soil and slows down the rate of soil temperature increase and soil water evaporation. Therefore, the cooling film described in the present invention has the effect of saving water.

[0032] Example 2

[0033] The preparation method of the spectrally selective greenhouse cooling film is as follows:

[0034] The preparation method of this material is the polymer multi-layer co-extrusion method and the vacuum evaporation method. The process flow chart is shown in Figure 3 and is mainly divided into the following steps:

[0035] Step 1: Preparation of the polymer multi-layer optical film (taking PC and PMMA as examples). The raw materials selected are PC and PMMA, where the refractive index of PC is 1.58 and the refractive index of PMMA is 1.48. The total number of film layers is designed to be 128 layers, and the film structure is designed as Air∣(HL)^ 64∣Air, the reflection bandwidth of the film is 500 - 550 nm, covering the main band of green light in solar radiation. The total thickness of the designed multi-layer optical film is 10 μm. The polymer multi-layer co-extrusion process is selected as the preparation method, and a one-fourth multiplier divided into three equal parts is chosen. The specific preparation steps are as follows: All raw materials are first dried in a vacuum drying oven at 90 °C for 9 hours, and then added to the extruder. The melting temperatures are as follows: PC extruder: diameter 30 mm, rotation speed 16 rpm, temperature curve 270 / 260 / 255 / 250 °C; PMMA extruder: diameter 30 mm, speed 20 rpm, temperature distribution 270 / 260 / 255 / 250 °C, and the total polymer flow rate is maintained at 10 kg h -1 , to prevent a large number of bubbles caused by humidity. After layer multiplication, the fluid is extruded through an extrusion die and stretched at 120 °C. The extrusion screw speed is designed to be 20 rpm.

[0036] Step 2: Using silver as the raw material, the coating thickness is 10 nm, and PET is selected as the flexible substrate. The specific process of vacuum evaporation is as follows: 1. Cleaning and activation of the PET substrate: The PET substrate is ultrasonically cleaned with deionized water and ethanol to remove surface organic substances and particles, and the PET surface is activated by low-pressure oxygen plasma (power 80 W, time 2 minutes) to improve its hydrophilicity and adhesion (treatment gas: mixed gas of Ar and O2 (ratio 4:1), air pressure 15 Pa); 2. Magnetron sputtering: The raw material uses a high-purity silver target (99.99%), the distance between the PET substrate and the silver target is set to 8 cm, the air pressure in the working chamber is maintained below 5×10 -4 Pa, the working gas uses high-purity argon (99.999%), the flow rate is set to 25 sccm, the sputtering air pressure is set to 0.5 Pa, the DC power supply is selected for the evaporation process, its power is set to 80 W, and the deposition rate is about 10 nm / min; 3. Post-treatment: Low-temperature annealing (70 °C, 30 minutes) is carried out in an inert gas (N2 or Ar) to relieve the internal stress in the coating and enhance the stability of the coating.

[0037] Step 3: After uniformly evaporating a silver coating on the PET surface, the polymer multi-layer optical film is bonded to it. A common optical adhesive can be used for bonding to obtain the final product.

[0038] Performance Analysis

[0039] The spectral selective greenhouse cooling film prepared in Example 2 was subjected to performance analysis, and the results are as follows:

[0040] Theoretical spectral performance. The theoretical spectrum of the spectral selective greenhouse cooling film in the range of 0.3 - 2.5 μm is as Figure 4As shown, its average transmittance in the 400 - 500 nm band is 81.6%, in the 600 - 700 nm band is 75.1%, in the 500 - 600 nm band is 49.8%, and in the near - infrared band greater than 800 nm is 25.3%. The theoretical spectrum shows that the film has significant spectral selectivity and meets the spectral requirements described above.

[0041] Infrared spectral performance. The radiative cooling performance of the material is related to its chemical properties. In Example 2, PET is selected as the radiative cooling material. PET contains a large number of specific chemical bonds (such as C - O - C bonds), etc. These chemical bonds strongly absorb infrared radiation in the 8 - 13 μm band. According to Kirchhoff's law, for an object in thermal equilibrium, its infrared absorptivity is equal to its infrared emissivity. Figure 5 is the infrared emissivity diagram of PET. The average infrared emissivity of PET in the "atmospheric window" is 91%, significantly higher than that of traditional PE film (about 26%).

[0042] Theoretical thermal analysis. To illustrate the heat insulation and cooling effect of the spectral - selective greenhouse cooling film, theoretical thermal analysis was carried out, and the results are as Figure 6 shown. The solar radiation intensity is set to 1000 W / m 2 , and a traditional PE film (average transmittance of 90% and average infrared thermal emissivity of 26%) is set as a control. The average transmittance of the spectral - selective greenhouse cooling film in the 0.3 - 2.5 μm band is 48.9%, and the average infrared emissivity in the "atmospheric window" is 91%. After using the film described in the present invention as the covering material, compared with using a PE film covering, the solar radiation heat input power inside the greenhouse has decreased by 411 W / m 2 , and the heat output power of the greenhouse to the outside has increased by 83.9 W / m 2 . Therefore, the total heat input power of the greenhouse covered with the spectral - selective cooling film is 369.5 W / m 2 , while under the same conditions, the total heat input power of the greenhouse covered with a PE film is 864.4 W / m 2 . This shows that the spectral - selective cooling film can effectively reduce the heat input power of the greenhouse and effectively slow down the heat accumulation rate inside the greenhouse. At the same time, the average transmittance of the spectral - selective cooling film in the 400 - 500 nm and 600 - 700 nm bands has little difference from that of the PE film, which means that the film will not have a significant impact on the normal growth of plants. In summary, the spectral - selective greenhouse cooling and water - saving film can effectively reduce the heat entering the greenhouse interior without affecting the normal growth of plants, which will significantly reduce the air temperature, soil temperature, and soil moisture evaporation rate, and achieve an increase in crop yield.

[0043] Obviously, the described embodiments are some embodiments and application examples of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without any creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A spectral selective greenhouse cooling and water-saving film, characterized in that: From top to bottom, it successively includes a polymer heat radiation material layer, a metal coating layer, and a polymer multi-layer optical film; wherein: The polymer heat radiation material layer is used to dissipate heat in the form of thermal radiation; The metal coating layer is used to reflect near-infrared light with a wavelength greater than 800 nm; The polymer multi-layer optical film is composed of multiple polymer unit films with the same thickness stacked together. The polymer unit film is composed of a high refractive index material layer and a low refractive index material layer stacked together; the polymer multi-layer optical film is used to reflect green light with a wavelength of 500 - 600 nm.

2. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, wherein: The material of the polymer heat radiation material layer is PET, PC, or PMMA.

3. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, wherein: The material of the metal coating layer is silver.

4. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, wherein: The thickness of the metal coating layer is 8 - 12 nm.

5. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, characterized in that: The thickness of the polymer multi-layer optical film is 10.5 ± 1 μm.

6. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, wherein: The material of the high refractive index material layer is polyethylene naphthalate, polycarbonate, polystyrene, or polyethylene terephthalate.

7. The spectral selectivity greenhouse cooling and water-saving film according to claim 1, characterized in that: The material of the low refractive index material layer is polymethyl methacrylate.

8. The preparation method of the spectral selective greenhouse cooling and water-saving film according to any one of claims 1 to 7, characterized in that: It includes the following steps: Prepare the polymer multi-layer optical film by polymer multi-layer co-extrusion technology; Prepare a metal coating layer on the surface of the polymer heat radiation material layer, and then laminate the polymer multi-layer optical film on the surface of the metal coating layer to obtain the spectral selective greenhouse cooling and water-saving film.

9. The preparation method of the spectrally selective greenhouse cooling and water-saving film according to claim 8, characterized in that: The method for preparing the metal coating layer on the surface of the polymer heat radiation material layer is: using the polymer heat radiation material layer as a substrate, and depositing the metal coating layer on its surface by magnetron sputtering technology.

10. The preparation method of the spectrally selective greenhouse cooling and water-saving film according to claim 8, characterized in that: The polymer multi-layer optical film is laminated on the surface of the metal coating layer through optical glue.

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