WO2023057998A1 - A coating composition, process of obtaining the composition and methods thereof - Google Patents

Abstract

The present disclosure relates to a coating composition comprising modified hollow air-filled micron-sized spherical particles [HMS] optionally along with nano metal oxide (MO). It also relates to a process for obtaining the coating composition and methods for IR shielding or to produce heat-seal effect/cooling effect by said composition.

Description

A COATING COMPOSITION, PROCESS OF OBTAINING THE COMPOSITION AND METHODS THEREOF TECHNICAL FIELD The present disclosure relates to a coating composition comprising modified hollow air- filled micron-sized spherical particles [HMS] optionally along with nano metal oxide (MO). The HMS comprise alumina and silica and the nano metal oxide is TiO₂. It also relates to a process for obtaining the coating composition and methods for IR shielding or to produce heat- seal effect/cooling effect by said composition. BACKGROUND AND PRIOR ART Due to the climate change, temperature of cities is increasing rapidly. This dramatic change in temperature will produce a climate never before experienced by human civilization resulting in the increased demand for air conditioning and refrigeration, which in turn demand a significant portion of energy supply. The reduction of these energy demand and sustainability require proper design and materials of construction. The material of construction can be one that can give cooling effect, such as an external roof coating, for the buildings for better thermal management. Such coatings can be applied on steel surfaces for roofing or affordable housing applications. These coatings should be designed to reflect a significant amount of solar light, emit thermal radiations and reduce thermal transmission through it. It is advantageous to develop one with more than one of the aforementioned properties to maximize their efficacy. Researchers have made different approaches in order to prepare the heat seal/cool roof coatings. In EP1409596, Stephen et al. used the silica microsphere having a shell of silica- alumina filled with either CO₂, N₂ or vacuum along with rutile TiO₂, which is the solar reflective pigment in order to develop the radiative cooling surface coatings. They claim that the proposed coating system will provide decrease in temperature of about 6 to 7°C than the ambient temperature [1], upon exposure to direct sunlight. In CN104804568, Zhihai proposed an aqueous thermal insulation coating. The coating system consisted of a closed nano glass cenospheres, aluminosilicate fibers, titanium dioxide, talc, kaolin, brighteners, nano carbon tube as a principal raw material, with a special silicone acrylic emulsion and water as the fluorocarbon resin film former. It claimed that by using the specified coating system, under direct sunlight, about 20°C decrease in temperature can be achieved [2], but the coating thickness is not mentioned, since it is a very critical parameter in the final performance (thermal insulation) property of coating. In CN108753058, Xingfeng proposed a sunlight reflective thermal-insulation paint. The coating system comprised of polystyrene hollow microspheres with particle size of 0.2-0.5 μm along with several reflective material such as antimony doped tin oxide, aluminum-doped zinc oxide, gallium-doped zinc oxide, and potassium titanate. It also consists of nano metal powder, nano silica, rutile TiO₂ etc. By using the proposed system, a low UV light transmittance of about 5-15% and visible light transmittance of about 50-60% [3] was reported. In CN104985891A, inventors disclosed a multilayer energy saving, heat shielding and cooling anti-corrosive coating system. But it consists of a three layer composite material composed of an anticorrosive bottom layer, a heat insulating middle layer and a heat radiation surface layer. In addition to modified silica hollow microsphere and modified TiO₂, other heat insulating fillers for far-infrared radiation such as ceramic filler, glass beads, zinc oxide nano powder and other additives were used along with polyurethane resin. The coating thickness is very high, about 400 to 800 μm. Using the proposed system, the work achieved a temperature difference of 6 °C [4]. In JP2003201443A, Nobuhiro et al. proposed a coating system of low refractive index. The coating system consists of hollow micron particles and a matrix forming material which upon drying will form a porous layer with low refractive index. Perfluoro resin or a silica-based resin were used as the film forming material and the hollow fine particle with average particle size in the range of 5 nm to 2μm were used whose outer shell was made up of materials like SiO₂, SiOx, TiO₂, TiOx, SnO₂, CeO₂, Sb₂O₅, ITO, ATO, Al₂O₃, or combination of these. Curing catalyst, dispersing agents, silane coupling agent, levelling material and a viscosity modifier were also added to the coating system [5]. One report by P.K. Sahu et al [6] used the hollow glass microspheres and cenosphere to study the cooling effect in coatings. However, the formulation contains several other components, such as emulsifier, wetting agent, cellulose, titania and talc. Therefore, the result of the work cannot specifically be due to one component, as so many other components were also present to give a cooling effect, such as cellulose and talc, which will also reflect solar radiations. Several foreign universities have worked on different HMS like glass microsphere, hollow ultra-low-density thermoplastic microspheres etc. to prepare the cool roof or IR shielding coatings. Nordin et al. proposed a coating system using TiO₂, ZnO, defoamers, and rheological modifiers along with CaCO₃ and hollow microspheres [7]. They studied the solar reflectance effect by replacing CaCO₃ with the hollow microspheres. They found that by using 37 vol% of hollow microsphere, they obtained total solar reflectance of about 87%, whereas with same quantity of CaCO₃, it was only about 82%. Sandin et al also made similar study. They suggest that the Hollow ultra-low-density thermoplastic microspheres can be used as filler in elastic waterproof coatings which help reflect solar radiation and reduce the temperature in cool roof coatings. They are using TiO₂ filler having crystal size of about 250 nm along with the hollow thermoplastic microspheres. They found that by using 20 μm sized microsphere, total solar reflectance of about 80% can be achieved [8]. Alden et al in his study demonstrates the white coatings consisting of silicone embedded with randomly distributed hollow microspheres (i.e. microbubbles) having outer diameter of about 1 μm provide highly efficient daytime radiative cooling with inexpensive materials and fabrication processes. The microbubble embedded silicon coating exhibits low solar light absorbance and provide the efficient heat insulation or radiative cooling effect. When applied over the test substrate, this system decrease the underlying substrate temperature well below the ambient temperature by as much as ~4°C [9]. Thus, there exists a need for materials/compositions, processes and methods for IR shielding or to produce heat-seal effect/cooling effect and the present disclosure achieves the same. SUMMARY OF THE DISCLOSURE The present disclosure relates to a coating composition comprising modified hollow air-filled micron-sized spherical particles [HMS] optionally along with nano metal oxide (MO). The HMS comprise alumina and silica and the nano metal oxide is TiO₂. In an embodiment of the present disclosure, the coating composition further comprise resin and hardener. In an embodiment of the present disclosure, the particle size of HMS is ranging from about 10- 100 μm and particle size of MO is ranging from about 10-200nm. The HMS is present at a concentration of about 5-20% with respect to resin and MO is present at a concentration of about 3-10% with respect to resin. The HMS and the MO is present together at a concentration of about 23-30% with respect to resin. The present disclosure also relates to a process for obtaining the coating composition, said process comprising steps of: - washing HMS particles with ethanol or ethanol water mixture followed by filtering and drying; - adding the washed HMS to resin and homogenizing to obtain HMS resin mixture; - -dding hardener to the HMS resin mixture to obtain the coating composition comprising modified HMS; or ¾ mixing MO with hardener followed by adding the same to the HMS resin mixture to obtain the coating composition comprising modified HMS and MO. In an embodiment of the present disclosure, the HMS undergoes surface modification due to ethanol wash or ethanol water mixture wash under high shear homogenization leading to modified HMS. The surface modification of the HMS enhances dispersion and interaction with the resin by creating surface functional groups. These surface functional groups are -OH groups which bonds with Si or Al in HMS or with the amine group in the hardener to form the coating. The present disclosure also relates to a method of coating a substrate/surface for IR shielding or a method of producing heat-seal effect/cooling effect, wherein the method comprise step of applying to the substrate/surface the coating composition mentioned above. In an embodiment of the present disclosure, the substrate is a metallic substrate selected from a group comprising steel, aluminium, tin, magnesium, copper, zinc alloys, galvanized steel and combinations thereof; and surface is selected from a roof, external surface of structures, battery cashing, pipelines, ship deck and any application requiring IR shielding. In an embodiment of the present disclosure, the coating provides cooling effect or temperature sustainability at a thickness range of about 60 - 100 μm. In an embodiment of the present disclosure, the method provides for improved solar reflection and heat insulation. In an embodiment of the present disclosure, the coating comprising only modified hollow air- filled micron-sized spherical particles [HMS] is S-layer and the coating comprising modified hollow air-filled micron-sized spherical particles [HMS] and nano metal oxide (MO) is D-layer. In an embodiment of the present disclosure, the S-layer and the D-layer provides up to 60-75% screening of temperature from an IR source. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where: Figure 1: illustrates FTIR Spectrum of hollow air-filled micron-sized spherical particles (HMS) made largely of silica and alumina. Figure 2: illustrates FEG-SEM images of hollow air-filled micron-sized spherical particles (HMS) made largely of silica and alumina. Figure 3: Illustrates TEM images of nano metal oxide (MO). Figure 4: is a schematic of single- and dual-mechanism of cooling effect. Figure 5: is CLSM image of a coating with hollow air-filled micro-spheres particles (HMS). Figure 6: is a schematic of the mechanism of the in-situ surface modification by the method used for the preparation of formulation for dual mechanism (D-layer). Figure 7: is a FTIR spectrum of HMS, A) full spectrum before and after homogenisation; B) Zoomed spectrum in between the wavenumber 3600-3200 cm^-1 . Figure 8: is a schematic design of the set-up used to measure cooling effect of S- or D-layer on mild steel. Figure 9: illustrates A) Time vs Temperature just under the back surface of the steel surface with S- or D-layer measure inside the insulated box model. The under-surface temperature in steel with D-layer with modified HMS is 15°C lower than that under bare steel. B) Time vs Temperature about 15cm below the steel surface with S- or D-layer measure inside the insulated box model. The under-surface temperature in steel with D-layer is 18°C lower than that under bare steel. The temperature on the illuminated surface is 60 °C. Figure 10: illustrates percentage difference in temperature, as compared to the top surface temperature (i.e.60°C), A) at the bottom surface of steel with S- and D-layer; B) at 15 cm below the surface of steel with S- and D-layer and inside the model house. DETAILED DESCRIPTION The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent product/methods do not depart from the scope of the disclosure. Definitions: Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. As used herein, the singular forms ‘a’, ‘an’ and ‘the’ include both singular and plural referents unless the context clearly dictates otherwise. The term ‘comprising’, ‘comprises’ or ‘comprised of’ as used herein are synonymous with ‘including’, ‘includes’, ‘containing’ or ‘contains’ and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. Likewise, certain terms may be interchangeably used throughout the specification and thus have the same meaning even when they are referred interchangeably. For example, modified HMS may be interchangeably referred as HMS, metal oxide as MO, alumina and silica as inorganic shell material, etc. The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints. The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed. The present disclosure relates to a coating composition comprising modified hollow air-filled micron-sized spherical particles [HMS] optionally along with nano metal oxide (MO). The modified hollow air-filled HMS with inorganic shell material and nano metal oxide (MO) provides cooling effect without the use of any powder pigments. In an embodiment of the present disclosure, the coating composition comprise modified hollow air-filled micron-sized spherical particles [HMS] and nano metal oxide (MO). In an embodiment of the present disclosure, the HMS comprise alumina and silica. In an embodiment of the present disclosure, the nano metal oxide is TiO₂. In an embodiment of the present disclosure, the coating composition comprise modified hollow air-filled micron-sized spherical particles [HMS] and TiO2. In an embodiment of the present disclosure, the coating composition further comprise resin and hardener. In an embodiment of the present disclosure, the resin is selected from a group comprising epoxy resin, polyurethane, acrylic resin, polydimethylsiloxane [PDMS], alkyds, cellulose esters, polyvinyl acetate, polyesters and vinyl esters and combinations thereof. In an embodiment of the present disclosure, the hardener is selected from a group comprising amine-based hardener, anhydride-based hardener, polyamide, isocyanates, aliphatic compounds, cycloaliphatic compounds and combinations thereof. In an embodiment of the present disclosure, the resin to hardener ratio is taken according to the requirement. In one embodiment, the resin to hardener ratio is 1:1 by volume. In an embodiment of the present disclosure, particle size of HMS is ranging from about 10-100 μm and particle size of MO is ranging from about 10-200nm. In another embodiment of the present disclosure, particle size of HMS is about 10μm, about 20μm, about 30μm, about 40μm, about 50μm, about 60μm, about 70μm, about 80μm, about 90μm or about 100μm. g In another embodiment of the present disclosure, particle size of MO is about 10nm, about 20nm, about 30nm, about 40nm, about 50nm, about 60nm, about 70nm, about 80nm, about 90nm, about 100nm, about 110nm, about 120nm, about 130nm, about 140nm, about 150nm, about 160nm, about 170nm, about 180nm, about 190nm or about 200nm. In an embodiment of the present disclosure, particle size of HMS in the composition is ranging from about 20-60 μm and particle size of MO is ranging from about 10-50 nm. In an embodiment of the present disclosure, the HMS is present at a concentration of about 5- 20% with respect to resin and MO is present at a concentration of about 3-10% with respect to resin. In another embodiment of the present disclosure, the HMS is present at a concentration of about 5%, about 6%, about 7%, about 8%, about 9%, about 10%, about 11%, about 12%, about 13%, about 14%, about 15%, about 16%, about 17%, about 18%, about 19%, or about 20%. In another embodiment of the present disclosure, the MO is present at a concentration of about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, or about 10%. In an embodiment of the present disclosure, the HMS and the MO is present together at a concentration of about 23-30% with respect to resin. In another embodiment of the present disclosure, the HMS and the MO is present together at a concentration of about 23%, about 24%, about 25%, about 26%, about 27%, about 28%, about 29% or about 30%. In an embodiment of the present disclosure, the HMS is present at a concentration of about 15% with respect to resin and MO is present at a concentration of about 10% with respect to resin. In an embodiment of the present disclosure, the HMS:MO is 1.5. In an embodiment of the present disclosure, the HMS is present at a concentration of at least 15% with respect to resin. In an embodiment of the present disclosure, the MO is present at a concentration of 10% with respect to resin. The present disclosure also relates to a process for obtaining the coating composition, said process comprising steps of: - washing HMS particles with ethanol or ethanol water mixture followed by filtering and drying; - adding the washed HMS to resin and homogenizing to obtain HMS resin mixture; - adding hardener to the HMS resin mixture to obtain the coating composition comprising modified HMS; or - mixing MO with hardener followed by adding the same to the HMS resin mixture to obtain the coating composition comprising modified HMS and MO. In an embodiment of the present disclosure, the drying is carried out in an oven at about 30- 70°C for about 30-60 minutes. In an embodiment of the present disclosure, the drying is carried out in an oven at about 30- 50°C when HMS particles are washed with ethanol. In an embodiment of the present disclosure, the drying is carried out in an oven at about 40- 60°C when HMS particles are washed with ethanol water mixture. In an embodiment of the present disclosure, ratio of ethanol and water in ethanol water mixture is about 4:1. In an embodiment of the present disclosure, ethanol of about 80% and water of about 20% is present in ethanol water mixture. In an embodiment of the present disclosure, the homogenization is carried out in a high shear homogenizer at about 1500-3000 rpm for about 10-30 mins. In an embodiment of the present disclosure, the hardener is added to the HMS resin mixture and mixed by stirring for about 20-30 mins at about 500-1000rpm. In an embodiment of the present disclosure, the MO is mixed with hardener in a sonicator for about 10-20 minutes prior to the addition into the HMS resin mixture. In an embodiment of the present disclosure, the HMS undergoes surface modification due to ethanol wash or ethanol water mixture wash under high shear homogenization leading to modified HMS. In an embodiment of the present disclosure, the surface modification of the HMS enhances dispersion and interaction with the resin by creating surface functional groups. In an embodiment of the present disclosure, the surface functional groups are -OH groups which bonds with Si or Al in HMS or with the amine group in the hardener to form the coating. The present disclosure also relates to a method of coating a substrate/surface for IR shielding or a method of producing heat-seal effect/cooling effect, wherein the method comprise step of applying to the substrate/surface said coating composition as mentioned in the above embodiments. In an embodiment of the present disclosure, the substrate is a metallic substrate. In an embodiment of the present disclosure, the metallic substrate is selected from a group comprising but not limiting to steel, aluminium, tin, magnesium, copper, zinc alloys, galvanized steel and combination thereof. In an embodiment of the present disclosure, the surface is selected from a group but not limiting to a roof, external surface of structures, battery cashing, pipelines, ship deck and other external parts/surface, or any application requiring IR shielding. In an embodiment of the present disclosure, the coating composition is applied by using brush, roll coater or spray technique to metal substrate or surfaces. In an embodiment of the present disclosure, the coating provides cooling effect or temperature sustainability at a thickness range of about 60 - 100 μm. In an embodiment of the present disclosure, the coating provides cooling effect or temperature sustainability at a thickness of about 60μm, about 70μm, about 80μm, about 90μm or about 100μm. In an embodiment of the present disclosure, the method provides for improved solar reflection and heat insulation. In an embodiment of the present disclosure, the coating comprising only modified hollow air- filled micron-sized spherical particles [HMS] is S-layer and the coating comprising modified hollow air-filled micron-sized spherical particles [HMS] and nano metal oxide (MO) is D-layer. In an embodiment of the present disclosure, the S-layer and the D-layer provides up to 60-75% screening of temperature from an IR source. In an embodiment of the present disclosure, the coating composition provides for temperature reduction of up to 60-75%. In an embodiment of the present disclosure, the HMS has thermal conductivity of lower than or equal to 0.08 W m^-1 K^-1 and density of about 0.35 g/cm³. In an embodiment, the present disclosure uses cheap and economical hollow microsphere material [HMS] in the composition, without using other insulating material such as cellulose and Talc. The HMS is a light weight, inert, hollow sphere made of silica and alumina and filled with air or inert gas. The HMS like cenosphere are typically produced as a by-product of coal combustion at thermal power plants. It has low density, low thermal conductivity, high thermal resistance and stability. There are several advantages of using the HMS in the formulation. Because of their low density, low thermal conductivity, high thermal resistance and stability provide better cooling effect than the powdered pigments. Low thermal conductivity and low density of hollow spheres is due to a large amount of stagnant air. Stagnant air inside hollow spheres act as an insulator for heat because it is a bad conductor of heat. In addition, method for incorporation of the HMS into the coating system as per the present disclosure results in the surface modification and better dispersion of HMS in the matrix material which enhances the final performance of the coating composition. The performance is further enhanced when the coating composition comprise modified HMS along with metal oxides such as TiO_2. The coating composition of the present disclosure doesn't require any pretreatment layer or primer to achieve the results. It can be applied by using brush, roll coater or spray technique to metal substrate or surfaces. It can produce 10 times higher shielding effect with a single layer. After 3hrs of IR light illumination, by using modified HMS in the composition, about 62% thermal screening is achieved. After 3hrs of IR light illumination, by using modified HMS and TiO₂ in the composition, about 75% thermal screening is achieved. It is to be understood that the foregoing description is illustrative not a limitation. While considerable emphasis has been placed herein on particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein. Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure, certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments may be practiced and to further enable those of skill in the art to practice the embodiments. Accordingly, following examples should not be construed as limiting the scope of the embodiments herein. Examples: Example 1: Method for the preparation of IR shielding coatings Formulation-1: The desired weighed quantity of the resin and hardener were mixed together in a prescribed standard ratio. The prepared formulation was coated on mild steel panels using standard application procedure. The dry film thickness (DFT) of the layer was set in an optimized range. Formulation-2: The desired weighed quantity of HMS was washed with ethanol vigorously, dried and added to a resin and mixed in homogeniser for an optimized rpm and time. This process results in the surface modification of HMS and improve the resin-filler interaction and hence the dispersion with the resin. Then the hardener was added to the mix in a prescribed ratio. The prepared formulation was coated on mild steel panels using standard application procedure. The dry film thickness (DFT) of the layer was set in an optimized range. The use of only HMS particles constitutes a single mechanism of cooling effect (henceforth called S-layer). Formulation-3: The nano metal oxide (MO) was used in a required proportion along with required weight percentage of HMS. First the HMS was mixed with resin as mentioned in formulation-2. At the same time MO is mixed with hardener in a sonicator for an optimized time duration to get better dispersion. The addition of MO and HMS constitute a dual mechanism of cooling effect (henceforth called D-layer). Both mechanisms are illustrated in Figure 4. Formulation-4 (Unmodified): The desired weighed quantity of HMS and MO are added to the resin and hardener is also added to the mix in prescribed ratio and are mixed with standard magnetic stirrer. This procedure did not result in in-situ surface modification of HMS. The prepared formulation was coated on mild steel panels using standard application procedure. This formulation is termed as unmodified system, since no surface modification of HMS occurred. In one system, only HMS was used (Formulation – 4A) and in other both HMS and MO are used (Formulation – 4B). The average particle size of the HMS in the coated panel were analysed using the Confocal Laser Scanning Microscopy (CLSM) and it was found about 25 + 5 μm. The HMS particles filled with air with average size 25 μm (±5), were characterised by Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Figure-1 represents the FTIR spectrum of the HMS particles, which contains two major bands characteristic of the internal vibrations in TO₄ tetrahedra (T = Al, Si). One, at around 1081 cm- ¹, is associated with the T–O bond asymmetric stretching vibrations, while the other, centred at 457 cm^-1, corresponds to the T-O bond internal deformation vibrations. The absorption peak at 3409 cm^-1 is caused by the flexible vibration of the hydroxyl group (–OH). The SEM images of HMS (Figure-2) confirms the hollow structure with broad distribution of size, with an average size of 25 (±5) Pm. The TEM study of the MO is performed to find the average particle size and TEM images are shown in Figure 3. Average particle size of MO is found about 20+ 5 nm. Example 2: Mechanism of the surface modification by the set method for coating preparation The coating systems are prepared by using a set method of mixing. The steps in the set method of mixing includes, first the HMS particles are washed with ethanol using vigorously, filtered and dried in an oven at 60°C for 1 hour. Desired weighed quantity of this HMS is then added to the resin and are subjected to homogenisation in a high shear homogeniser (at 3000 rpm) for an optimized time duration. Then the hardener was added to the mix in a prescribed ratio and mixed in the magnetic stirrer for an optimised time duration. In the case of dual mechanism systems, the MO is mixed with hardener in a sonicator for an optimized time duration [for about 10-20 mins] prior to the addition into the HMS resin mixture. By the use of this high shear homogenisation, the surface modification of the HMS enhances dispersion and interaction with the resin by creating large number of surface functional groups. The schematics of the mechanism of modifications of HMS with surface functional groups and its interaction with the resin, by the procedure used, are presented in Figure 6. During the washing of the HMS with ethanol (in homogeniser), the –OH from the ethanol is get bonded to Si or Al present in the HMS (which is confirmed by FTIR given in Figure 7). When this HMS is added to the resin and homogenised, the HMS particle gets distributed in the matrix (with the help of high shear force and hydrogen bonding) and upon addition of the hardener the –OH bond present in the HMS reacts with the amine group in the hardener in addition to resin-hardener crosslinking reaction to from the final coating film. The ultra-sonication process after the MO addition to hardener will prevent the agglomeration of nano-sized MO particles and provides uniform distribution of the particles in the system. The insitu surface modification by this mechanism is confirmed by FTIR study shown in Figure 7. From the FTIR spectrum, it is confirmed that the surface modification of the HMS is caused by the use of high shear homogenisation, by forming a modified surface layer without collapsing the hollow structure. The –OH peak intensity (at 3409 cm⁻¹) has been increased ~6 times (~480%) after the homogenisation. In order to check the reproducibility, two different batches were prepared. The increase in the -OH peak intensity indicates the increase in absorbance. The higher absorbance in a particular wavenumber indicate the presence of more number of specific functional group or bonds in a sample corresponding to that wavenumber. Hence, after homogenisation, the –OH (at 3409 cm⁻¹) intensity in the HMS has increased by about 400%. These –OH groups interact with the resin to provide better dispersion and increased filler-matrix interaction. Example 3: Method for the measurement of cooling effect (Temperature measurement) The heat insulation effect of the S- and D- layers was studied by using self-designed model made of insulating materials. The mild steel panels with S- or D- layer materials was placed at the roof opening of the box model, as shown in Figure 8, which was illuminated by an infrared heat lamp having power of 150 W. The position of the IR lamp with respect to the mild steel surface with S- or D-layers, were varied to generate a spectrum of variations in incidence of heat, as shown in Figure 8. Temperatures were recorded using thermal sensors placed at various opportune locations from the back surface of the mild steel with S- or D-layer to the temperature and hence cooling effect. Temperature readings were recorded at fixed intervals of time. The temperature just below the IR-illuminated steel panel having S- or D-layer (following single or dual-mechanism of cooling, respectively) are presented vs the time of exposure in Figure 9. In Figure 9A, temperature just at the back surface of the steel surface with S- or D-layer is presented. The temperature in steel with D-layer with modified HMS is 15°C lower compared to that under the bare steel, indicating huge cooling effect in steel with D-layer consisting modified HMS. Whereas in the case of D-layer with “unmodified HMS”, the temperature decrease is no better than the simple resin layer of the same thickness. The cooling was even larger when the temperature was measured some distance (15 cm) below the steel panel with D-layer, as shown in figure 9B. Figure 9B shows at 15 cm below the steel surface with D-layer with modified HMS, the temperature was unchanged for whole duration of measurement indicating excellent heat buffering effect by the D-layer and excellent temperature control. However, in case of a bare steel, temperature keeps increasing (reaching 47°C from room temperature of 29°C) at the same distance below it, indicating neither temperature control nor cooling effect. In the case of the layer with “unmodified HMS”, the temperature kept increasing from 29°C to 34°C [Figure 9B]. From this, it is clear that the in-situ surface modification of HMS during the set preparation process, gives huge cooling effect of ~ 18°C. In all cases, the surface temperature of the illuminated surface (surface exposed to the light source) was 60°C, which is typically the temperature realized in a tropical summer season. Therefore, the D- and S-layer on steel demonstrate high degree of temperature control which is desired for sustainability. With respect to source temperature (i.e.60°C at the illuminated surface), the decrease in temperature below the steel surface with these layers i.e. cooling effect by S- and D-layer, was very high, as presented in Figure 10. Using this, a percentage difference temperature was calculated using following formula, Percen

Where T1 is the top surface temperature and T₂ is the temperature below the steel panel. After 3 hours of exposure to shield the simulated light, the percentage difference temperature at the bottom surface of steel with S-layer is 37% and 46% with D-layer consisting of modified HMS (Figure 10A). At the same time interval, at 15 cm below the surface of steel, the percentage difference in temperature with S-layer is 62% and with D-layer consisting of modified HMS is about 75% inside the model house. Whereas in case of bare steel, the percentage difference is merely between 10 to 25% (Figure 10B). With unmodified HMS, the maximum percentage difference obtained (at 15 cm below the surface) is 55%. References [1] Wojtysiak Conrad Stephen, Radiative cooling surface coatings, EP1409596, 2009 [2] Zhao Zhihai, Sun-proof thermal insulation coating, CN104804568 [3] Wu Xingfeng, Sunlight reflective thermal-insulation paint, and preparation method thereof, CN108753058 [4] Debang New Material Co., Ltd. Coating material with characteristics of efficiency, energy saving, heat shield, cooling and corrosion resistance, CN104985891A [5] Nobuhiro Ide, Nobuhiro Ito, Yasuhisa Kishigami, Coating material composition and article bearing coating film formed thereof, JP2003201443A [6] P.K. Sahu and P.A. Mahanwar, Effect of hollow glass microspheres and cenospheres on insulation properties of coatings, Pigment & Resin Technology, Volume 42, Number 4, 2013, 223–230 [7] Jan Nordin, Olof Sandin, and Peter Greenwood, Hollow Thermoplastic Microspheres in Elastomeric Cool Roof Coatings, American coatings journal, Volume 16, 2019, 20-26 [8] Olof Sandin, Jan Nordin, Magnus Jonsson, Reflective properties of hollow microspheres in cool roof coatings, J. Coat. Technol. Res., 14 (4) 817–821, 2017. DOI 10.1007/s11998-017-9973-y [9] Joseph D. Alden, Sarun Atiganyanun, Robert Vanderburg, Seung Ho Lee, John B. Plumley, Omar K. Abudayyeh, Sang M. Han, Sang Eon Han, Radiative cooling by silicone-based coating with randomly distributed microbubble inclusions, J. Photon. Energy 9(3), 032705 (2019), doi: 10.1117/1.JPE.9.032705

Claims

We Claim: 1. A coating composition comprising modified hollow air-filled micron-sized spherical particles [HMS] optionally along with nano metal oxide (MO). 2. The coating composition as claimed in claim 1, wherein the HMS comprise alumina and silica. 3. The coating composition as claimed in claim 1, wherein the nano metal oxide is TiO₂. 4. The coating composition as claimed in claim 1, wherein the coating composition further comprise resin and hardener. 5. The coating composition as claimed in claim 1, wherein the resin is selected from a group comprising epoxy resin, polyurethane, acrylic resin, polydimethylsiloxane [PDMS], alkyds, cellulose esters, polyvinyl acetate, polyesters and vinyl esters and combinations thereof; and the hardener is selected from a group comprising amine-based hardener, anhydride-based hardener, polyamide, isocyanates, aliphatic compounds, cycloaliphatic compounds and combinations thereof. 6. The coating composition as claimed in claim 1, wherein particle size of HMS is ranging from about 10-100 μm and particle size of MO is ranging from about 10-200nm. 7. The coating composition as claimed in claim 6, wherein particle size of HMS in the composition is ranging from about 20-60 μm and particle size of MO is ranging from about 10-50 nm. 8. The coating composition as claimed in claim 1, wherein the HMS is present at a concentration of about 5-20% with respect to resin and MO is present at a concentration of about 3-10% with respect to resin. 9. The coating composition as claimed in claim 8, wherein the HMS and the MO is present together at a concentration of about 23-30% with respect to resin. 10. The coating composition as claimed in claim 8, wherein the HMS is present at a concentration of about 15% with respect to resin and MO is present at a concentration of about 10% with respect to resin. 11. A process for obtaining coating composition as claimed in claims 1-10, said process comprising steps of: ¾ washing HMS particles with ethanol or ethanol water mixture followed by filtering and drying; ¾ adding the washed HMS to resin and homogenizing to obtain HMS resin mixture; ¾ adding hardener to the HMS resin mixture to obtain the coating composition comprising modified HMS; or ¾ mixing MO with hardener followed by adding the same to the HMS resin mixture to obtain the coating composition comprising modified HMS and MO. 12. The process as claimed in claim 11, wherein the drying is carried out in an oven at about 30-70°C for about 30-60 mins; and homogenization is carried out in a high shear homogenizer at about 1500-3000 rpm for about 10-30 mins. 13. The process as claimed in claim 11, wherein the hardener is added to the HMS resin mixture and mixed by stirring for about 20-30 mins at about 500-1000rpm. 14. The process as claimed in claim 11, wherein the MO is mixed with hardener in a sonicator for about 10-20 mins prior to the addition into the HMS resin mixture. 15. The process as claimed in claim 11, wherein the HMS undergoes surface modification due to ethanol wash or ethanol water mixture wash under high shear homogenization leading to modified HMS. 16. The process as claimed in claim 15, wherein the surface modification of the HMS enhances dispersion and interaction with the resin by creating surface functional groups. 17. The process as claimed in claim 16, wherein the surface functional groups are -OH groups which bonds with Si or Al in HMS or with the amine group in the hardener to form the coating. 18. A method of coating a substrate/surface for IR shielding or a method of producing heat- seal effect/cooling effect, wherein the method comprise step of applying to the substrate/surface a coating composition claimed in claims 1-10. 19. The method as claimed in claim 18, wherein the substrate is a metallic substrate selected from a group comprising steel, aluminium, tin, magnesium, copper, zinc alloys, galvanized steel and combinations thereof; and surface is selected from a roof, external surface of structures, battery cashing, pipelines, ship deck and any application requiring IR shielding 20. The method as claimed in claims 18, wherein the coating provides cooling effect or temperature sustainability at a thickness range of about 60 - 100 μm. 21. The method as claimed in claim 18, wherein the method provides for improved solar reflection and heat insulation. 22. The method as claimed in claim 18, wherein the coating comprising only modified hollow air-filled micron-sized spherical particles [HMS] is S-layer and the coating comprising modified hollow air-filled micron-sized spherical particles [HMS] and nano metal oxide (MO) is D-layer. 23. The method as claimed in claim 22, wherein the S-layer and the D-layer provides up to 60-75% screening of temperature from an IR source.