Synergistic effect of (3-Aminopropyl)Trimethoxysilane treated ZnO and corundum nanoparticles under UV-irradiation on UV-cutoff and IR-absorption spectra of acrylic polyurethane based nanocomposite coating
Introduction
Radiative sky coolers are one of best invention that has been developed by several researchers and academics since 1972 in-order to preserve the energy consumption as they expected to reduce in near future. Radiative sky coolers absorb or reflect the ultra-violate (UV) and the infrared near- and mid-range portion of the solar spectrum due to the properties of their functional materials. Solar illumination brings 91.7% of the infrared radiation energy to the ground. This energy is concentrated in the wavelength range from 700 to 2500 nm while 8.3% is other electromagnetic radiation [1]. The objects convert 42.3% of the visible light and 49.4% of the near-infrared radiation energy of the solar radiation into heat, causing the temperature of the surface to increase [1]. Consequently, researchers and academics have developed materials to absorb and emit heat rays via different mechanisms.
Since 1972, there are number of materials have been used to satisfy the radiative cooling principle including; heat insulation materials such as hollow glass microspheres and/or hollow ceramic beads [2]; the IR-absorber or reflective material including SiO2 [3], titanium dioxide, tin oxide, antimony trioxide, magnesium oxide, zinc oxide, aluminum oxide and generally, antimony doped tin oxide (ATO), indium tin oxide (ITO), lanthanum hexaboride (LaB6) and nanophotonic devices [[4], [5], [6], [7], [8], [9], [10]]; low heat absorbing transparent polymer resin called polymethyl-pentene (TPX) [2]. An inorganic metal oxide based effective multilayer nanophotonic device and a bilayer randomized glass-polymer hybrid metamaterial using TPX polymer matrix have been developed recently [2,10]. A silver layer was deposited on one side of the hybrid metamaterial using an electron beam evaporator. To resist UV-radiation where they observed that the IR-absorption has depend upon the diameter of the microsphere [2]. On one hand, nanoscale materials have shown significant cooling ability while on the other hand microscale materials were used for the same. Both have cost effective and environmental issues. Consequently, it is utmost importance to study the ability to cutoff UV and IR-radiation using low cost and environmentally friendly materials. Nanomaterials and microscale materials provide a novel way to fabricate green composite materials for radiative cooling applications.
UV radiation is an utmost important factor which affect living organisms with having a wavelength shorter than 380 nm and commonly subdivided into UVA, UVB and UVC. Inorganic metal oxides such as TiO2, ZnO, and CeO2 are commonly used for this purpose rather than other organic UV absorbers due to their gradual embrittlement and undergoes rapid yellowing properties. However, photocatalytic activity of these metal oxides has degraded the polymer matrix of their nanocomposites coating films when they are exposed to UV-irradiations. Besides, UV-resistance materials itself have no ability to act as radiative cooling (below or above ambient temperature) materials. Consequently, IR-active material is required to make a complete radiative cooler.
Aluminum oxide (Al2O3) has been reported as radiative cooler below-ambient temperature [11]. Al2O3 is one of the metal oxides which can give different structural phases including γ-Al2O3, δ-Al2O3 and α-Al2O3. Al2O3 is a ceramic material showing great importance in many chemical and technological applications including optics, heat insulation, heat transfer, absorbent, antibacterial, biomedical etc. Thus beside other transition phases, corundum (α-phase of Al2O3) is the thermodynamically stable form of the Al2O3. The formation of the corundum can be described as a hexagonal close packed sub-lattice of oxygen ions and an ordered array of Al3+ cations occupying 2/3 of the octahedral interstices [12]. Although, an intrinsic absorption of the corundum is expected to alternate the IR-absorption ability of the nanocomposite coating in the presence of UV-irradiation via charge transfer activity among their counter ions in the crystalline lattice. Furthermore, to the extent of our knowledge corundum has not yet been experienced with their IR-absorption activity as radiative cooler in the literature.
Numerous experimental methods have reported the preparation method of corundum. These include hydrolysis of aluminum alkoxides [13], chemical vapor deposition [14], thermal decomposition of aluminum alum [15], and thermal decomposition of inorganic aluminum salts [16,17]. However, the heat treatment method such as calcination and sintering which are followed with dry or wet milling will give maximum purity of 99.6–99.9% [18,19]. Further, when the temperature exceeds 1200 °C, the primary corundum particles will grow together to form three-dimensional aggregates. These pyrogenic corundum powders are generally ground by various grinders such as ball mill, jet mill and vibration mill to break their joints which form during sintering and to obtain monodispersed fine particles.
In this study (3-aminopropyl)trimethoxysilane (APTMS) has been used to coat the surface of the ZnO nanoparticles. The prepared corundum and commercially available surface modified SiO2 (M − SiO2) nanoparticles have been used to investigate the impact of mixed nanoparticles on UV and IR-absorptions properties of the nanocomposite coating compared with their individual performance. Commercial poly-macrynal-510n coating resin has been used as polymer matrix to investigate the stability of the nanoparticles after exposure to UV-irradiation. To assess the functional stability of the nanoparticles, the coated polyurethane (PU) films were exposed to UV-irradiations in a weatherometer for three consecutive cycles of UV-irradiations. The IR-active property of the corundum and M − SiO2 nanoparticles become weaken, fade, and may lose their functions with respect to exposure of UV-irradiations. Consequently, different stoichiometric ratio of the respective APTMS-ZnO, corundum and M − SiO2 nanoparticles have been embedded into the polymer matrix to coat PU film with different coating thickness. The synthesized nanoparticles are characterized by XRD, and FTIR. The effect of UV-irradiations on IR-active property of the corundum and M − SiO2 embedded nanocomposite with and without APTMS-ZnO nanoparticle are investigated via UV–visible and FTIR spectroscopy. Results have been compared before and after exposure of UV-irradiations using MRD-UV accelerated weathering chamber.
Section snippets
Materials
Synerburg Materail Co., Ltd provided polyurethane film (PU), Allnex Holding S.à r l. supplied poly-macrynal® sm 510n/60xmpac coating resin. Nippon Aerosil® Co., Ltd supplied surface modified (hydrophobic) crystalline free pyrogenic amorphous SiO2 nanoparticles with a 12–14 nm particle size. (3-aminopropyl)trimethoxysilane, Al(NO3)3.9H2O, Zn(NO3)2. 4H2O, Na2CO3, NaOH, 95% absolute ethanol were purchased from Sigma-Aldrich (Kuala Lumpur, Malaysia). All materials were used as received without any
Results and discussion
FTIR spectroscopy and XRD methods were used to confirm the surface modification of ZnO nanoparticles and phase transformation of the corundum nanoparticles. The formation of corundum and the APTMS-treatment of ZnO nanoparticles have investigated by FTIR. Crystalline nature of the prepared nanoparticles was confirmed by XRD. In order to understand the UV and IR-absorption properties of the APUC, the nanoparticles embedded coated PU films were investigated using FTIR and UV spectroscopies via
Conclusion
The nanosized of ZnO and corundum particles were successfully synthesized using sophisticated two steps routines with high yield. The obtained ZnO powders were coated with APTMS and its effect on the UV and IR spectrum of the APUC coating containing corundum and M − SiO2 nanoparticle were studied. The following conclusions can be drawn from the investigation and spectral analysis in this study.
- 1)
Surface coating of ZnO nanoparticles via APTMS is depend upon the reaction time where the coating can
Acknowledgment
This research was funded by Ministry of Education Malaysia: FRGS grant no FP053-2015A and PR005-2017A; University Malaya research grants: PPP grant no PG159-2016A, RU grant no GPF033A-2018 and RU018H-2016 and ST012-2017.
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