Preparation and Application of Self-adaptive Porous Polyethylene for Mulch Film

Porous polyethylene has emerged as a consequential constituent within the realm of infrared thermal management, owing to its commendable transmittance in the thermal infrared domain and noteworthy high scattering capabilities within the visible spectrum. In this study, we have fabricated porous polyethylene and conducted a comprehensive investigation into its adaptable emissivity alterations within the infrared range, while concurrently upholding a pronounced scattering proficiency in the visible band. Furthermore, we have proffered the prospective application of the prepared adaptive porous polyethylene in the context of mulch film. Empirical has unequivocally demonstrated that the utilization of adaptive porous polyethylene can effectively engender adaptive cooling and insulation modulation in the realm of subterranean plant growth, by harnessing its efficient performance across both the visible and thermal infrared spectra. Simultaneously, the material continually scatters visible light, thereby providing supplemental illumination to plant leaves and fruits in a retrograde manner. This breakthrough affords an efficacious and cost-effective energy-conserving apparatus in the domain of agricultural production.


Introduction
Porous polyethylene has exhibited remarkable efficacy in enhancing energy utilization across various domains, including individual and building insulation as well as cooling applications [1] [2].Radiation-based sky insulation and cooling capitalize on the 8-14μm atmospheric window to exchange energy with outer space [3] [4].In agricultural production, infrared low transmittance mulch films are commonly employed to isolate the soil and ambient air, thereby facilitating insulation [5] [6].However, conventional mulch film materials struggle to achieve efficient switching between insulation and cooling.Even under scorching noon temperatures, they retain a high insulation effect, impeding the maintenance of optimal temperature conditions.
With the advancements in materials science and the growing emphasis on energy utilization, porous polyethylene materials are progressively finding applications in the realms of radiation insulation and thermal regulation [7].Liu et al. harnessed commercially available porous polyethylene to achieve modulations in transmittance across the visible and thermal infrared spectra.Nonetheless, the commercial porous polyethylene, characterized by a pore size of only 50nm, exhibits increased visible transmittance upon water immersion, which proves unsuitable for mulch film usage.An ideal mulch film material should ensure low visible band transmittance to provide supplemental backlighting for plants and impede weed growth.In this context, we employ a simple thermal scraping technique to fabricate micrometer-pore-sized porous polyethylene films, capable of maintaining high visible scattering even when exposed to airborne moisture [9][10][11].In this study, we successfully achieved the manipulation of infrared emissivity while ensuring high visible light scattering by developing a porous polyethylene film as an alternative to commercially available materials.Overall, our findings present a novel thin film material that offers an effective and cost-efficient energy-saving solution in the realm of agricultural production.

Material and sample preparation
High-density polyethylene (HDPE) was procured from Maoming Petrochemicals, while solid paraffin and chloroform were obtained from Aladdin.All reagents were used as received without any additional purification steps.
The experimental procedure is as follows: Firstly, a specific amount of solid paraffin is placed in a magnetic stirrer and heated at 160°C.Subsequently, 10% (mass ratio) of HDPE is slowly added to the molten paraffin while maintaining a temperature of 160°C.The mixture is stirred for 3 hours to ensure thorough blending of the polyethylene with the paraffin.
Next, a flat glass bottom plate is positioned on a heating table and heated to 160°C.The 400μm scraper is preheated in advance.The molten mixture is swiftly transferred onto the glass plate, and using the scraper, it is rapidly spread into a thin wet film.The glass plate and wet film are then submerged in a water bath for 10 minutes, facilitating Thermally Induced Phase Separation (TIPS).
Once the film has fully cured, it is delicately peeled off the glass substrate.
The peeled polyethylene film is soaked in chloroform for 10 minutes, repeating this process thrice to ensure complete dissolution of the paraffin within the film.To prevent the volatilization of chloroform into the environment, anhydrous ethanol is employed to dissolve and remove chloroform from the porous membrane.However, due to the rapid evaporation of anhydrous ethanol, caution must be exercised to avoid cracking and curling of the porous membrane during the drying process.Therefore, deionized water is used as a solvent to dissolve and remove anhydrous ethanol from the porous membrane.Finally, the porous film is placed in a vacuum drying oven and subjected to a 2hour drying process to remove any remaining moisture, resulting in the production of the final porous film product.

Mie scattering
When the size of particles within a medium is comparable to the wavelength of light, these particles are recognized as scattering elements [12].As light interacts with these scattering elements during propagation, it gives rise to secondary waves that disperse in multiple directions throughout free space.Consequently, when light traverses a scattering medium, its transmittance IOP Publishing doi:10.1088/1742-6596/2706/1/0120263 Among these variables, let us denote N as the number of scattering elements per unit volume, μ as the scattering cross-section of particles, and dx as the transmission distance.The scattering crosssection can be mathematically represented as Where r is the radius of the scattering element and Q is the scattering factor.A scale factor is usually used in scattering

Infrared radiation
The thermal interaction between objects and their surroundings predominantly transpires via three fundamental pathways: heat conduction, heat convection, and heat radiation.In the context of employing plastic film, a notable occurrence transpires wherein a layer of air becomes entrapped between the film and the underlying terrain.Owing to the intrinsic low thermal conductivity of air and the effective hindrance of convective heat transfer from the ambient atmosphere, thermal radiation becomes the primary mode of heat exchange.Every entity, encompassing celestial bodies such as the sun and terrestrial surfaces, emits energy outwardly as electromagnetic waves, in accordance with the tenets of the black-body radiation principle ( )* ( )* ( , ) In this context, a critical relationship between various parameters can be observed, wherein the transmittance (denoted as "t"), the emissivity (represented by "ε") of the object, and the radiant energy (symbolized as "E") emitted by a blackbody at the identical temperature as the object play pivotal roles.The wavelength-dependent behavior of these quantities adheres to the fundamental principles delineated by Planck's law Hence, to effectively accomplish infrared radiation cooling, a two-fold approach is required: maximizing the reflection of incident solar radiation while concurrently augmenting the outward radiation emanating from the terrestrial surface.
In light of the above discussion, it becomes evident that the peak radiation emitted by the land at a temperature of 30°C occurs around 10 microns, while the solar radiation peak lies around 470nm.Consequently, to achieve effective insulation, it is imperative to minimize the radiation emitted by the land.Conversely, for cooling purposes, maximizing the radiation emitted by the land becomes crucial (with a transmittance, t, between 8-14μm equal to 1), while simultaneously minimizing the transmittance of solar radiation (t between 380-780nm equal to 0).

MSEE-2023
Exploiting the principles of Mie scattering, it is observed that particles with similar sizes to the incident wavelength exhibit substantial backscattering.To demonstrate the superior performance of the porous polyethylene developed in this study compared to traditional commercial porous polyethylene in mulch film applications, Python simulations were employed to assess the visible scattering efficiency under dry and wet conditions.Figure 2 illustrates the scattering cross-sections in the visible range for the prepared porous polyethylene (with a pore size of approximately 1000 nanometers) and the traditional commercial porous polyethylene (with a pore size of approximately 50 nanometers).The black line represents the dry condition, while the red line represents the wet condition (i.e., the porous polyethylene after absorbing water).It is evident that when commercial polyethylene materials absorb water, their scattering cross-section significantly decreases due to the similarity in refractive indices between water and polyethylene.This transition results in a shift from a high-scattering material to one with high transmittance.In the context of mulch film, this phenomenon allows sunlight to transmit to the ground during early morning and evening hours, which is not conducive to inhibiting weed growth.Conversely, the porous polyethylene developed in this study, with a pore size of approximately 1000 nanometers, maintains a consistent scattering cross-section even when absorbing water, owing to the similarity between the pore size and the visible wavelength.This observation suggests that it remains an efficient scattering material.
Based on these findings, the mulch film material devised in our study can preserve dryness during intense midday sunlight, effectively scatter sunlight within the visible range to provide backlighting for plants, and exhibit transparency in the infrared range to dissipate excess heat.During early morning or evening periods when the air is humid, the material absorbs moisture from the atmosphere, yet still maintains high scattering in the visible range, thereby offering continued backlighting for plants and inhibiting weed growth.Furthermore, due to moisture absorption, the material transitions from high transmittance to low transmittance in the infrared range, reducing the transmission of infrared heat and achieving insulation effects.a b Figure 2. illustrates the outcomes of simulations conducted to examine the scattering cross sections of porous polyethylene films with varying pore sizes.

Experiment and Results Discussion
To demonstrate the performance of the developed material in the context of mulch film applications, we designed the following experiments.Firstly, we placed black tape, which serves as a high radiance material and mimics the radiance of the land in the infrared band, on a heated platform to represent the land surface.Subsequently, we placed dry and wet porous polyethylene films on top of the black tape to evaluate the effect of the mulch film.Visible scattering characteristics were captured and analyzed using a visible camera.Upon analysing the visible images, it is evident that both materials exhibit effective scattering properties.The fact that the black tape is not visible beneath the porous polyethylene indicates that sunlight is unable to penetrate the film and reach the underlying soil.This demonstrates that the prepared material achieves desirable backlighting and effectively inhibits weed growth throughout the day.

Conclusion
This paper presents a novel approach to achieving self-adaptive adjustment of infrared emissivity throughout the day by employing porous polyethylene film instead of traditional commercial materials.By maintaining a consistently high infrared scattering effect, the porous polyethylene film enables effective backlighting of plant fruits and leaves.Moreover, the film's ability to block sunlight from reaching the ground and inhibit weed seed germination further contributes to its weed inhibition effects.Through the use of an infrared camera, it is observed that the radiation temperature of the porous polyethylene film significantly decreases when it absorbs water, compared to dry conditions.Simultaneously, the film retains its opaque characteristics, as observed with a visible camera.In conclusion, the proposed thin film material offers an efficient and cost-effective energy-saving solution in agricultural production.

Acknowledgment
This work is supported by the National Natural Science Foundation of China (61975181).

Figure 1 .
Figure 1.(a) illustrates the application of a self-adaptive mulch film.(b) illustrates the underlying principle of the self-adaptive mulch film.

Figure 3 .
Figure 3. presents a visual characterization of the experimental setup involving the black tape and porous polyethylene films placed on the black tape.The images captured aim to showcase the performance of the materials in different conditions.