A polymer-based composite for passive day time radiative cooling (PDRC) and a process for the preparation thereof

Abstract

This present invention relates to a polymer-based composite for passive day time radiative cooling (PDRC), said composite composed of Ultra high molecular weight polyethylene (UHMWPE) and Titanium dioxide which can exhibit an average sub-ambient cooling of 2.6-6.1o C when exposed to direct sunlight.

Description

FIELD OF THE INVENTION:This invention relates to a polymer-based composite for passive day time radiative cooling (PDRC) and a process for the preparation thereof.BACKGROUND OF THE INVENTION:Space heating is a serious problem because of the global warming effect due to which the demand for effective cooling technologies are increasing. Passive daytime radiative cooling can achieve sub ambient temperatures without the use of electricity. Today, space cooling is achieved by active cooling and passive cooling methods. Active cooling technologies like compression-based air conditioning and heat exchangers tend to be very energy intensive which remains unaffordable to large sectors of population in an emerging country like India.There has been a rapid rise in the demand for active cooling systems like compression-based systems for space cooling in buildings, automobiles, and other sectors because of the global warming effect. It has been reported that the sale of air conditioners have almost tripled since 1990s. Not only that compression-based air conditioning is electricity-intensive, a rise in the wide usage requires significant investments in electricity generation and infrastructure implying a high carbon footprint (International Energy Agency. 2018. The Future of Cooling: Opportunities for EnergyEfficient Air Conditioning. International Energy Agency). In emerging economies like India, which has most of the regions experiencing hot and humid climates, a low-cost, scalable and environment friendly alternative to the existing cooling technologies are necessary.Within the passive cooling solutions, space cooling can be achieved by natural ventilation, green roofing, insulations etc., by reducing thermal gains to the interior space. However, these cooling solutions tend to be expensive and is restricted by design challenges (Panchabikesan, K., Vellaisamy, K., & Ramalingam, V. (2017). Passive cooling potential in buildings under various climatic conditions in India. Renewable and Sustainable Energy Reviews, 78, 1236-1252).Passive daytime radiative cooling (PDRC) is a technology with which one can achieve space cooling by reducing the interior temperature up to ~8°C below the ambient temperature passively. Such passive cooling technologies can drastically decrease the energy demand due to traditional active cooling compression-based air conditioners.Passive daytime radiative cooling (PDRC) is a phenomenon exhibited by objects that experience spontaneous cooling by simultaneous emission of thermal infrared radiation through the atmospheric window (8-13 μm) and efficient scattering of solar radiation (0.3-2.5 μm) to the cold universe (Zhao, D., Aili, A., Zhai, Y., Xu, S., Tan, G., Yin, X. and Yang, R., 2019. Radiative sky cooling: Fundamental principles, materials, and applications. Applied Physics Reviews, 6(2), p.021306). An ideal PDRC surface has a 0% absorptance in the wavelength of 0.3-2.5 μm indicating that the surface reflects 100% sunlight that falls on it and an emissivity of 1 in the infrared wavelength of atmosphere of 8 -13 μm. The combination of two properties allows the PDRC material to spontaneously cool below the ambient temperature because of the net heat loss to the cold outer space.The class of materials called passive daytime radiative coolers achieves a net thermal energy loss (because of efficient solar reflection and thermal emission) which leads to its spontaneous cooling when exposed to sunlight. There are several PDRC materials such as metamaterials, polymers and composites available today. For instance, metamaterials made of layered hafnium (IV) oxide, Alumina, Silicon Nitrate was shown to have up to ~97% solar reflection and a cooling of almost 8°C below the ambient temperature when exposed under direct sunlight (A.P. Raman, M.A. Anoma, L. Zhu, E. Rephaeli, S. Fan, Passive radiative cooling below ambient air temperature under direct sunlight, Nature 515 (7528) (2014) 540- 544; Lee, D., Go, M., Son, S., Kim, M., Badloe, T., Lee, H., Kim, J.K. and Rho, J., 2021. Sub-ambient daytime radiative cooling by silica-coated porous anodic aluminum oxide. Nano Energy, 79, p.105426). There has also been PDRC polymer-based materials such as polyesters (Angus R. Gentle, Geoff B. Smith, A., Sub-ambient open roof surface under the Mid-Summer sun, Adv. Sci. 2 (9) (2015) 150019), Polydimethylsiloxane (PDMS) (Jun-long Kou, Zoila Jurado, Zhen Chen, Shanhui Fan, Austin J. Minnich, Daytime Radiative Cooling Using Near-Black Infrared Emitters, ACS Photonics 4 (3) (2017) 626-630) that demonstrated 2~9°C daytime sub-ambient temperature reduction. Several composite structures have also been shown with compositions containing polymer matrix embedded with inorganic phases such as TPX polymers dispersed with microparticles of SiO2 (Zhai, Y., Ma, Y., David, S.N., Zhao, D., Lou, R., Tan, G., Yang, R. and Yin, X., 2017. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling. Science, 355(6329), pp.1062-1066) that have exhibited almost 93 W / m2 of cooling power. Porous Polyvinylidene difluoride (PVDF) films acting as PDRC, are prepared by solvent-based phase inversion techniques and can lead to sub-ambient daytime radiative cooling of upto 6°C (Mandal J, Fu Y, Overvig AC, et al. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling. Science. 2018;362(6412):315-319). Gamage S, et al. discloses reflective and transparent cellulose-based passive radiative coolers, prepared by solvent-based techniques followed by electrospinning and can provide cooling below ambient air temperature of up to 15°C. Zhou L, et al disclose multi-layered polydimethylsiloxane coating on aluminium surfaces, prepared by solvent based blade coating method and 2~9°C daytime sub-ambient temperature reduction ( Zhou L, Song H, Liang J, et al. A polydimethylsiloxane-coated metal structure for all day radiative cooling. Nat Sustain. 2019).Current metamaterial-based PDR coolers achieve exceptional optical properties. However, the sophistication and cost factor in fabrication limits the mass production and large-scale applications (Zhai Y, Ma Y, David SN, et al. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling. Science. 2017;355(6329):1062- 1066). On the other hand, currently developed polymer-based PDRCs are made from a solvent-based fabrication route. Having a paint-like application strategy, they can be scaled up, however, it is not environmentally friendly majorly because of the usage of solvents. The other technique of polymer-based PDRC fabrication is melt extrusion technique which has a limited composition control and requires a high-energy input because of shear forces and temperature used in the process. For instance, a lab scale polymer extruder may contain multiple heater units for maintaining temperature zones and a DC motor which has a total power rating upwards of 40kW (Abeykoon, C., McMillan, A., & Nguyen, B. K. (2021). Energy efficiency in extrusion-related polymer processing: A review of state of the art and potential efficiency improvements. Renewable and Sustainable Energy Reviews, 147, 111219).Therefore, the need exists, to provide a PDRC material which is less energy intensive, having high composition control and highly environment-friendly compared to the existing PDRC materials and overcomes the drawbacks of the known materials.OBJECTS OF THE INVENTION:It is therefore an object of this invention to propose a polymer- based composite for passive day time radiative cooling (PDRC) and a process for the preparation thereof.It is a further object of this invention to propose a polymer-based composite for passive day time radiative cooling (PDRC), which is cost-effective.Another object of this invention is to propose a polymer-based composite for passive day time radiative cooling (PDRC), which can be prepared by simple means which is less energy intensive and has high composition control.Yet another object of this invention is to propose a polymer-based composite for passive day time radiative cooling (PDRC), which exhibits good sub-ambient cooling when exposed to direct sunlight.It is a still further object of this invention to propose a polymer-based composite for passive day time radiative cooling (PDRC), which is scalable.Another object of this invention is to propose a method to prepare polymer-based composite for passive day time radiative cooling (PDRC), which has limited or negligible CO2 / NOx emission and is highly environment- friendly.These and other objects and advantages of the invention will be apparent fromthe ensuing description when read in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:Figure 1: Illustration of the powder bed sintering route to fabricate UHMWPE-TiO2 composite films.Figure 2: FESEM images of the surface microstructure for different sintering conditions used to fabricate the UHMWPE+5% TiO2 PDRC films namely (a) 150°C 10 mins; (b) 150°C 20 mins; (c) 200°C 10 mins; (d) 200°C 20 mins.Figure 3: (a) The set-up used for the thermal measurements of PDRC material. The variation of the PDRC temperature compared to the ambient temperature over a duration of 1 hour when exposed to direct sunlight for the sintering condition corresponding to (b) 150°C 10 mins; (c) 150°C 20 mins; (d) 200°C 10 mins; (e) 200°C 20 mins.DETAILED DESCRIPTION OF THE INVENTION:Thus, according to this invention is provided a polymer-based composite for passive day time radiative cooling (PDRC) and a process for the preparation thereof.In accordance with this invention, a low-cost, scalable, and environment friendly polymer based passive daytime radiative cooler composed of ultra-high molecular weight polyethylene (UHMWPE) and titanium dioxide is developed which can exhibit an average sub-ambient cooling of 2.6-6.1°C when exposed to direct sunlight. The single layer UHMWPE-TiO2 film acts as reflector and emitter.A range of PDRC films are fabricated via powder bed sintering technique in which a uniform powder bed is sintered at various temperatures and time durations. The powder bed is composed of a composite powder containing UHMWPE satellited with Titania (TiO2). The UHMWPE-TiO2 composite powder is prepared with a planetary ball mill under dry conditions at a desired speed and for a predetermined period of time The powder bed is then sintered at the desired temperature and duration. Figure 1 illustrates the powder bed sintering route to fabricate the UHMWPE-TiO2 film.By way of an exemplary embodiment, a uniform powder bed is provided, having a dimension of 2X50X50 mm. The powder bed is prepared on a substrate such as for instance, a glass plate, glass slide and the like.The ultra-high molecular weight polyethylene (UHMWPE) used in the method according to the invention has an average molecular weight of 4.9 Mg / mol to 10.5 Mg / mol. The UHMWPE used is nascent reactor powders GUR 4170 obtained from Ticona (Oberhausen, Germany). The Titanium dioxide powder used is in the form of agglomerated nanoparticles with primary particle size <100nm. The TiO2 powder used was obtained from Thermofischer Scientific, India which contains a mixture of rutile and anatase phases.The powder bed is composed of a composite powder containing UHMWPE satellited with 5-10% Titania (TiO2) (by weight). The UHMWPE-TiO2 composite powder is prepared with a planetary ball mill under dry conditions at a speed of 100-200 rpm for 30-60 min. 2g of UHMWPE-TiO2 composite powder is dispersed on a glass slide of dimension 5X5 cm2 area to fabricate a freestanding powder bed of thickness 2mm. The glass slide with the powder bed is placed in a muffle furnace which was preheated to a temperature in the range of 100-250°C depending upon the experimental conditions. Inside the muffle furnace, the powder bed is let to be sintered at the desired temperature for a period in the range of 10 to 20 minutes. The glass slide containing the sintered layer is taken out of furnace and is let to cool down to room temperature. The sintered layer is then peeled off from the glass slide. The ball mill and the muffle furnace used has a combined power rating of ~5 kW which is much less energy intensive compared to a polymer extruder.The films were subjected to different sintering conditions, e.g., 150°C for 10 minutes, 150°C for 20 minutes, 200°C for 10 minutes and 200°C for 20 minutes. Table 1 shows the different sintering conditions used to fabricate the PDRC films and the corresponding average sub-ambient cooling (Tambient-TPDRC) achieved in 1 hour of exposure to direct sunlight. The Figure 2 shows the FESEM images of the surface microstructure for different sintering conditions used to fabricate the PDRC films.Table 1: The different sintering conditions used to fabricate the PDRC films.The invention will now be explained in greater details with the help of the following non-limiting examples.EXAMPLE:The ultra-high molecular weight polyethylene (UHMWPE) used in the process is obtained from Ticona (Oberhausen, Germany). The TiO2 powder used in the process was obtained from Thermofischer Scientific, India.Example 1:UHWMPE powder was mixed with 5% TiO2 by weight and ball milled to prepare UHMWPE-TiO2 composites. The UHMWPE-TiO2 composite powder is prepared with a planetary ball mill under dry conditions at a speed of 200 rpm for 60 mins. UHWMPE-TiO2 composite powder was dispersed on a glass slide of dimension 5X5cm2 to prepare a freestanding powder bed of thickness 2mm. The powder bed is then placed in a preheated muffle furnace at a temperature of 200°C for a period of 20 minutes. During the exposure of the powder in the muffle furnace, the powder particles in the powder bed sinter together. The glass slide containing the sintered layer is taken out of furnace and is let to cool down to room temperature. The sintered layer of 5X5 cm2 area and 2mm of thickness is then peeled off from the glass slide. The sintered layer of thickness 2mm when exposed to direct sunlight for a period of 1 hour duration exhibited a final sub-ambient cooling of 9°C.Example 2UHWMPE powder was mixed with 5% TiO2 by weight and ball milled to prepare UHMWPE-TiO2 composites. The UHMWPE-TiO2 composite powder is prepared with a planetary ball mill under dry conditions at a speed of 200 rpm for 60 mins. UHWMPE-TiO2 composite powder was dispersed on a glass slide of dimension 5X5cm2 to prepare a freestanding powder bed of thickness 2mm. The powder bed is then placed in a preheated muffle furnace at a temperature of 150°C for a period of 10 minutes. During the exposure of the powder in the muffle furnace, the powder particles in the powder bed sinter together. The glass slide containing the sintered layer is taken out of furnace and is let to cool down to room temperature. The sintered layer of 5X5 cm2 area and 2mm of thickness is then peeled off from the glass slide. The sintered layer of thickness 2mm when exposed to direct sunlight for a period of 1 hour duration exhibited a final sub-ambient cooling of 4°C.The fabricated UHMWPE-TiO2 PDRC films were subjected to thermal measurements to check for their cooling efficiency. Thermal measurements were performed by placing the PDRC sample exposed to direct sunlight on a rooftop area. Two thermocouples were used to simultaneously measure the temperature of the ambient environment and that of PDRC material respectively. The ambient temperature was measured by placing the thermocouple in direct sunlight and the temperature of the PDRC was measured by keeping the thermocouple beneath the PDRC. The thermal measurement set-up is shown in figure 3 (a). Thermal measurements were performed by UHMWPE+TiO2 PDRC film fabricated via 4 differentsintering conditions. Table 2 provides the different sintering conditions used to fabricate the PDRC films and the corresponding average sub-ambient cooling (Tambient-TPDRC) achieved in 1 hour of exposure to direct sunlight. The figure 3 (b), (c), (d) and (e) show the sub-ambient variation of the PDRC temperature compared to the ambient temperature over a duration of 1 hour when exposed to direct sunlight.The present invention discloses a unique method for the fabrication of a polymer-based PDRC, and the product obtained thereby. The following points below describe the novel and inventive features of the technique:i. The energy consumption in the powder bed sintering method described hereinbefore is during the ball milling and a single heating operation for which a cumulative electric power consumption ~5kW is required. The power consumption is therefore much lower than a polymer extrusion method.ii. The powder bed sintering is a solventless method of fabricating the polymer film. Hence it is an eco-friendly method compared to a solvent-based preparation of polymer-based PDRCs. Further, powder sintering is carried out at temperature below its degradation temperature. Hence, the process has a limited or negligible CO2 / NOx emission.iii. The polymer product fabricated from powder sintering method is a single layer solar reflector and IR emitter unlike other PDRCs which are multilayered material with dedicated solar reflector layers and IR emitter layer.iv. Further, the film described herein has a thickness of as low as about 2mm which can be simply applied over virtually any surface / geometry.The passive day time radiative cooler was fabricated to reduce the problem of space heating in the world today. The space heating is a serious problem because of the global warming effect due to which the demand for effective cooling technologies are increasing. Passive daytime radiative cooling can achieve sub ambient temperatures without the use of electricity. Considering the unique advantages associated with the powder bed sintering route, polymer-based PDRC was fabricated.

Claims

1. A polymer-based composite for passive day time radiative cooling (PDRC), said composite composed of Ultra high molecular weight polyethylene (UHMWPE) and Titanium dioxide which can exhibit an average sub-ambient cooling of 2.6-6.1°C when exposed to direct sunlight.

2. The polymer-based composite for passive day time radiative cooling (PDRC), said composite being a single layer solar reflector and IR emitter.

3. A process for the preparation of the polymer-based composite for passive day time radiative cooling (PDRC) as claimed in claim 1, comprising the steps of providing a mixture of ultra-high molecular weight polyethylene (UHMWPE) and Titania (TiO2), subjecting the UHMWPE-TiO2 mixture to ball milling to produce a composite powder containing UHMWPE satellited with Titania (TiO2), preparing a uniform powder bed therewith on a substrate and subjecting the same to sintering to obtain the polymer-based composite.

4. The process as claimed in claim 3, wherein the powder bed is composed of UHMWPE satellited with 5-10 % Titania (TiO2) (by weight).

5. The process as claimed in claim 3, wherein UHMWPE-TiO2 composite powder is prepared by milling under dry conditions at a speed of 100-200 rpm for 30-60 min.

6. The process as claimed in claim 3, wherein the powder bed is sintered at a temperature in the range of 100 °C to 250 °C for a period in the range of 10 to 20 minutes.

7. The process as claimed in claim 3, wherein the milling and sintering has a combined power rating of about 5 kW.

8. The process as claimed in claim 3, wherein said ultra-high molecular weight polyethylene (UHMWPE) has an average molecular weight of 4.9 Mg / mol to 10.5 Mg / mol