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Highly Reflective Nano Structured Titania Shell: A Sustainable Pigment for Cool Coatings Richa Sharma, Sangeeta Tiwari, and Sandeep Kumar Tiwari ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03423 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Sustainable Chemistry & Engineering

Highly Reflective Nano Structured Titania Shell: A Sustainable Pigment for Cool Coatings Richa Sharma[a], Sangeeta Tiwari*[a] and Sandeep Kumar Tiwari[b] a

Amity Institute of Applied Sciences, Amity University, Sector-125, Noida, Uttar Pradesh- 201303, India, E-mail: [email protected] b

Council of Scientific and Industrial Research New Delhi, India Keywords: Core-shell particles, Fly ash, Cool coatings, NIR reflectivity, Solar Reflective Index

Abstract: Core-shell materials are designed to bring synergy of two materials. We describe a greener approach for fabrication of fly ash (FA) based cool pigment having core-shell morphology, viz., FA as core and multilayers of nano TiO2 as shell- introducing high reflectivity in the NIR region. Core-shell structure was prepared in aqueous medium under sonication without use of organic solvent or surfactant. Dispersion of FA in aqueous medium is achieved by activating the surface of FA while increasing its surface roughness. High zeta potential (-42.7 mV) of FA, coupled with ultra-sonication, helps in achieving better suspension of FA. Well separated FA particles act as nuclei for the seeding of nano TiO2 resulting in uniform shell formation as seen in SEM and TEM studies. A significant improvement in reflectivity is evident by its high Solar Reflective Index of 94%. Thermal diffusivity of a 123±5 µ thick cool coating indicate a temperature difference of 10±0.50 C on front and back side of mild steel panel. Significant use of a waste material in making a value added product by adopting a green approach coupled with enhancing weight content of pigment in coating formulation is an attempt toward achieving sustainable material for energy conservation.

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Introduction: Cool coatings are suitable alternative for reducing the overall energy consumption and peak demand for cooling energy in addition to thermal discomfort1. Cool coatings are required to possess characteristics such as high solar reflectance and thermal emittance. The requirement for a good cool coating is that they are capable of reflecting majority of solar (thermal) radiation and at the same time show minimum adsorption of heat2. These characteristics allow the coatings to significantly decrease in accumulation of heat, thereby reducing heat transfer to the back side of substrate. Since these coatings do not permit heat transfer across the surface, their application helps in significant reduction in energy consumption on air conditioning.3 Other benefits offered by these coatings are high durability, longer life of substrate due to reduced polymer degradation and thermal expansion due to lower temperature. Cool coatings are made up of NIR (Near Infrared) reflective pigments capable of reflecting NIR radiation. Varieties of pigments which include colored as well as white pigments are being used for cool coating applications4-6. Conventionally available green pigments are chromium oxide based7 or alkaline earth metal doped rare earth phosphates8.A wide range of complex inorganic pigment systems based on Cr2O3 with NIR reflectance have been reported9. Use of Fe2O3 brown pigments have been reported to reduce the temperature of roof while improving their appearance10,11. However, many of these colored inorganic pigments consist of toxic metal ions and therefore their use is restricted12. Rare earth compounds based pigments have also been extensively studied13-17. However, high cost of the rare earth based pigments makes them economically unattractive. A detailed study of cool pigments has been reported on IR reflective pigments18 that are similar to the commercially available cool pigments by calcination of mixtures of several oxides of metals as Ti, Ni, Co and Mn in different atomic proportions. But


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none of their experiments resulted in significant IR reflectivity, depicting the complexity of the subject. Among white pigments, titanium dioxide in its rutile form continues to be the base pigment of choice in highly reflective coatings. A single-layer cool white coating is reported19 to show an initial reflective index of 0.879 and a temperature difference of 80C between coated and uncoated carbon iron plate. Nano crystallite titania is known to exhibit remarkable light scattering and infrared reflectance due to its high refractive index19. However, smaller particle size and difficulty in uniform dispersion in coating limits its use as NIR reflective pigment. The group has been working on developing a sustainable pigment material for cool coatings. Earlier, we have reported preparation of core-shell structures of nano titania on fly ash using surfactants in organic medium20. In the present work, a greener approach has been reported for coating nano titania on fly ash particles. The developed process is free of organic solvents or surfactants as it is carried out in aqueous medium. Growth of more uniform and thick titania shell is accomplished by suitably modifying the surface characteristics of fly ash. The developed process is more efficient for obtaining thick coating of titania on fly ash resulting in significant enhancement in NIR reflectivity. The synergy of a micron sized core with rutile nano titania shell comprising of nano crystallites enhances NIR reflectivity manifolds besides leveraging enhanced use of developed material in the cool coatings. The efficacy of material in cool coatings is studied and reported. Cool coatings containing the developed composite pigments were prepared using powder epoxy on metal substrates. The coatings were assessed for reflectivity, emissivity, and thermal diffusivity while determining the extent of pigment loading.

Experimental section: Materials and Characterizations:



The chemicals used are Titanium Isopropoxide (Puriss for Synthesis, Spectrochem), HCl (Emplura, 35% pure), Methylene blue dye (Sigma Aldrich), Escherichia coli and Staphylococcus aureus bacterial stain from NCIM, Pune, India. Sonicator with Hielscher Ultrasonic probe (output power of 50 W) was used for continuous agitation. Morphology of the composites was studied using FE-SEM (Hitachi S-4800) and HR-TEM (JEOL 2100 F). X-Ray Diffraction studies were carried out by using D-8 Advance (Bruker) with CuKα radiation (λ= 1.54 Ả). BET analysis was carried out using M/S Quantachrome, Model: Nova 2000e. The diffused reflectance spectra (DRS) of the samples were recorded using UV-VIS-NIR spectrophotometer (PerkinElmer Lambda-950). The NIR solar reflectance (R*) was calculated according to ASTM Standard No. E891-87 in the wavelength range 700-2500 nm. The emittance was measured by Emissometer (heated to 82°C). The emittance values were measured at four different positions, and the values reported are the average of four measurements. Whiteness Index of steel panels of standard dimensions was measured using Novo- Shade DUO, 450/0 Opacity/ Shade Meter. Thickness of panels was measured using Positest DFT, Model: DFT-C. For measuring thermal diffusivity, mild steel plates of standard dimensions were coated and exposed to Philips PRA 38 IR Red (150 W; 230 V reference E27 ES) infrared lamp (Figure S1). Methods: Preparation of core shell structures of aFA@TiO2: Purification of the flyash was carried out for the removal of unburnt carbon and all forms of iron by the method reported earlier20.Surface activation of fly ash was achieved by treating it with 0.08N NaOH solution at 900C followed by washing till neutral. Activated fly ash (1g) was then dispersed in mild HCl medium (pH=5.5) using ultrasonication followed with simultaneous addition (drop by drop) of titanium isopropoxide (31.6 mmol) for 1 hr. Seeding of nano-titania


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on the surface of fly ash is achieved by adopting this process. Multilayers of nano-titania were obtained by repeating the process. The prepared core shell particles (titania shell on activated fly ash core-aFA@TC) were then separated by centrifugation and washed till neutral pH. aFA@TC particles were then annealed at 1200, 6000 and 8000C temperatures for 2 hr. The designations assigned are aFA@TC-120 (treated at 1200C), aFA@TC-600 (annealed at 6000C) and aFA@TC800 (annealed at 8000C). Preparation and characterization of cool coatings based on aFA@TiO2: aFA@TC-800 pigment was chosen for studies in cool coating applications based on its highest Solar Reflective Index (SRI). Electrostatically sprayed epoxy coatings were prepared using aFA@TC-800. Powder epoxy and composite particles were uniformly mixed and grounded in a ball mill for 1 hr. This mixture was then electrostatically deposited by spray over mild steel (MS) panels of 6 x 4 inch dimensions. The coatings were then dried at 1000C in air atmosphere for 2 hr. for densification of network. NIR reflectivity and emissivity of the coatings was evaluated by standard methods20. Rubbing fastness of the coatings was determined to ascertain extent of pigment loading in the coating. The test comprised of rubbing the surface of coatings with black cloth and visually observing the cloth for removal of coating (Figure S7). Thermal diffusivity of coatings was measured using standard set up (Figure S1). Results and Discussions: X-Ray diffraction pattern (Figure 1) of aFA@TC powder samples annealed at 1200, 6000 and 8000C show characteristic diffraction peaks of TiO2 nanoparticles (JCPDS standard files #211272 and #21-1276) indicating formation of partially amorphous anatase (aFA@TC-120), crystalline anatase (aFA@TC-600) and crystalline rutile phase (aFA@TC-800).



The morphological studies by SEM (Figure 2) indicate the formation of core-shell structure having a thick nano-titania shell on fly ash. The challenge of growth of nano titania layers on an inert core of fly ash has been addressed by making the fly ash surface rough and charged (Zeta potential- 42.7 mV) thus enabling better dispersion of fly ash particles. The dispersion is further maintained by ultrasonic agitation. As there is no agglomeration, each fly ash particle acts as a substrate for seeding of nano titania leading to more uniform and thick coating covering entire surface of fly ash particle. Coating of titania in multilayers is followed by repeating the process to obtain a layered nanostructured titania shell which when heat treated to 8000 C forms a porous crystalline shell giving enhanced reflection, discussed later in this paper. The presence of titania on fly ash is observed by energy dispersive spectroscopy (EDS) (Figure 3). Core shell formation is clearly visible in TEM images (Figure 4a-h). The size of the nano titania crystallite in the shell is ̴ 5nm for sample aFA@TC-120 treated at 1200C. (Figure 4e). The sample aFA@TC-600 annealed at 6000C shows partial crystallinity with a lattice constant value of 0.35 nm with (101) plane of tetragonal anatase. The lattice constant of 0.34 nm in Figure 4g matches with (110) plane of TiO221 indicates formation of rutile phase in aFA@TC-800 which has crystallite size ranging 20-25 nm. The corresponding SAED pattern in Figure 4h shows some distinctive diffraction spots rather than obvious rings. This confirms the conversion of polycrystalline anatase phase to crystalline rutile phase. The rings change from continuous form to dots as the size of the grains increases for aFA-TC@800. The diffraction rings are indexed as (110), (101) and (211) planes confirming the formation of rutile phase which is in agreement with XRD results. Since it is difficult to measure the exact shell thickness, a rough idea of shell thickness is gathered from the TEM image (Figure 4d) indicating the average shell thickness to be ̴ 50 nm.


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The particle size and crystallite size of different phases of titania shown in Table 1, have been calculated using SBET method22and Scherrer’s equation,23 respectively. Equation 1 is used to calculate the particle size by SBET method. D= 6000/ (SBET x ρ)………………….. Eq. 1 where, D is particle size, SBET is BET- specific surface area and ρ is true density (ρ for titania is 4.2 g/ml). It was observed that with increase in the annealing temperature, particle size as well as crystallite size increases which is due to fusion of particles at high temperature and is followed by crystal growth. The favorable morphology and crystallinity of aFA@TC core shell structure established thus far indicate enhancement in the solar reflectivity. To ascertain enhancement in solar reflectivity the Solar Reflective Index (SRI) (Table 2) was determined (ASTM standard E1980-01) using emittance and reflectance values obtained from DRS and NIR solar reflectance graphs (Figure 5a & b). aFA@TC-800 showed high SRI of 94% which is significantly higher than that reported earlier16 (SRI- 77%). The increase in solar reflectance can be mainly attributed to the formation of more uniform and thick layers of nano TiO2 on fly ash core. Formation of uniform nano titania shell in synergy with core, effectively scatter NIR radiations throughout its surface. The explanation of enhanced reflection by aFA@TC-800 particles can be based on the structural morphology of these particles consisting of micron sized fly ash core (poly dispersed particles ranging from 0.5-5µ), having a refractive index (RI) of 1.6. The meso porous shell, formed by multilayers nano crystalline rutile titania particles (crystallite size- 20-25 nm), having high RI (2.67). The nano titania particles are appropriately spaced as indicated by BET analysis (Figure



S2) which allows passage of the incident radiations to inner layers of the shell. The phenomenon is complex consisting of reflection by the top layer, refraction and diffraction of the radiation by titania particles inside the multilayered shell and backscattering by the fly ash core. The reflection by the nanocrystalline titania layer is high as nanoparticles provide more crystalline grain boundaries for reflecting the NIR radiation. The multilayered structure plays an important role in increasing the reflection of NIR radiations.24 It is further aided by the difference in the RI of materials used in core and the shell, which gives rise to constructive and destructive interferences of the radiation leading to backscattering24. Because of reasonably good RI of fly ash, the radiations reaching its surface are backscattered- thus aiding the reflectivity. The favorable morphology and physical characteristics of core-shell particles therefore enhances the NIR reflectivity as illustrated in Scheme 1. The suitability of developed composite pigment for cool coating applications is established for NIR reflectivity. A trade of between rutile and anatase can be established while annealing to impart additional benefits of self-cleaning and anti-bacterial characteristics (Figure S5 & S6).

The efficacy of the cool pigment was evaluated by preparing epoxy based powder coatings containing aFA@TC-800. The electrostatically deposited coatings on mild steel (MS) substrates show white appearance with a whiteness index (WI) of 80.8 and a thickness of 123±5µ (Table 3). The epoxy powder coatings on MS were also prepared using Ticom for comparative assessment. Figure 6a & b show DRS and NIR solar reflectance spectra of the developed cool coatings using aFA@TC-800 and Ticom. The SRI of aFA@TC-800 based coating, obtained by using reflectance and emittance values, is 89%. It is comparable to SRI (90%) of Ticom based coating (Table 3).


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The reflectivity of coatings is combination of reflectivity of the pigment and the binder used in it. Relative refractive index of pigment and the binder is another important factor affecting reflectivity of coatings. The higher the difference between RI of pigment and the binder resin, the higher is the backscattering, leading to high reflectivity24. In this particular case the RI of titania particles is 2.725,26 which is much higher than the RI of epoxy resin (1.6)27. Other factors contributing to high reflectivity are uniform dispersion of core-shell pigment within the binder and high pigment to binder weight ratio24. The micron size fly ash core facilitates better dispersion of pigment particles within coating matrix without any agglomeration unlike nano sized pigments. Rubbing fastness test (Figure S7) is indicative of the extent to which pigment can be loaded in the coating. The maximum pigment to binder ratio, measured in terms of percentage of pigment incorporated was 40% for aFA@TC-800 coating. Upto 40%, coating retains good adhesion to the substrate as well as rubbing fastness as the inter-particle adhesion between pigment and binder particles is retained. However, for Ticom based coating a maximum of 20% pigment could be incorporated. Beyond this, the coating is observed to loose adhesion and exhibit poor rubbing fastness. Thermal emittance, another critical parameter of cool coatings, refers to the ability of a surface to radiate thermal energy28. The measured thermal emittance value of aFA@TC-800 coatings was 0.91 (Table 3), which indicates that the cool coating can strongly radiate absorbed thermal energy to reduce the accumulation of heat. As the emissivity of smooth and shiny surfaces is observed to be lower than that of structured, rough and weathered surfaces29, the emittance of developed nano structured pigment was found to be better than that of Ticom.



Thermal diffusivity (Table 4) was studied by measuring the transfer of heat from one side of the MS panel to the other (back) side. The temperature of the mild steel panel was raised from ~250C to 600C ± 0.5 (30 min. of irradiation by IR lamp) using thermostat control. A difference in temperature of about 10.50C ± 0.5 was recorded for coatings based on aFA@TC-800 which is higher than that of Ticom based coating which is showing a difference of 8.60C ± 0.5. Presence of fly ash also appears to contribute in imparting thermal insulation characteristics to the coating.

Conclusion: A greener approach has been developed for preparation of cool pigment coating based on a waste material. A thick and more uniform growth of nano titania layers has been achieved on fly ash used as the core. Surface activation of fly ash helps in obtaining well dispersed fly ash particles. It enables effective seeding of nano titania on fly ash surface during insitu precipitation process. A significant NIR reflectivity is achieved in composite particles and their coatings by designing particles with favorable morphology and crystallinity. The developed core-shell particles possess high thermal emittance and Solar Reflective Index. The cool coatings prepared using core-shell particles displayed low thermal diffusivity in comparison to the coatings based on commercial titania. The use of aFA@TC-800 in cool coating applications would ensure reduction in requirement of costly nano titania. It would be a cost effective material since it utilizes fly ash which is an industrial waste. Use of aFA@TC-800 would also ensure higher loadings of pigment in cool coating formulation in comparison to commercial titania thus conserving use of polymeric binder. This would further result in cost reduction besides enhancing sustainability aspect.


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Figures:

Figure 1. XRD images of fly ash (FA) and triple coating of TiO2 on Fly ash (aFA@TC) treated at 1200, 6000 and 8000C.



Figure 2. SEM image of (a & b) Fly ash (c & d) Activated Fly ash showing surface roughness at different magnifications (e) Coating of TiO2 on Activated Fly ash treated at 1200C (f) Coating of TiO2 on Activated Fly ash treated at 8000C

Figure 3. EDS image showing uniform distribution of TiO2 on fly ash core and corresponding EDS Spectrum of aFA@TiO2.


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Fly ash (a)

Activated fly ash (b)

aFA@TC-120 (surface magnification) (e)

aFA@TC-120 (SAED pattern) (f)

aFA@TC-120 (core-shell) (c)

aFA@TC-800 (surface magnification) (g)

aFA@TC-120 (core-shell interface) (d)

aFA@TC-800 (SAED pattern) (h)

Figure 4. HRTEM images of (a) Fly ash (b) Activated fly ash (c) aFA@TC-120 (core-shell) (d) aFA@TC-120 (core-shell interface) (e) aFA@TC-120 (surface magnification) (f) aFA@TC-120 (SAED pattern) (g) aFA@TC-800 (surface magnification) (h) aFA@TC-800 (SAED pattern) (a)

(b)

Figure 5. (a) DRS of triple coating of TiO2 on activated fly ash annealed at 1200C, 8000C temperature and commercial TiO2(Ticom). (b) NIR solar reflectance triple coating of TiO2 on activated fly ash annealed at 1200C, 8000C temperature and commercial TiO2(Ticom).



(a)

(b)

Figure 6. a) Diffuse Reflectance Spectra of epoxy coatings based on aFA@TC-800, aFA@TC120 and Ticom (b) NIR solar reflectance spectra of epoxy coatings based on Ticom, aFA@TC-800 and aFA@TC-120.

Scheme 1: Schematic illustration of scattering of light by rutile phase of TiO2.


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Table 1: Particle size and crystallite size of aFA@TC samples annealed at different temperatures: Sample

Particle size (SBET method)

Crystallite size (XRD)

aFA@TC-120 (Amorphous TiO2)

36.9 nm

-

aFA@TC-600 (Anatase)

57.2 nm

12.04 nm (at 25.40)

aFA@TC-800 (Rutile)

126.8 nm

22.41 nm (at 27.50)

Table 2: NIR Solar Reflectance as per ASTM standard E891-87 and Solar Reflective Index of pigments as per ASTM standard E 1980-01 Sample

NIR Solar Reflectance (R*) (%) [ASTM standard E891-87]

Emittance

SRI value at R* [ASTM standard E1980-01]

aFA@TC-120

51.20

0.93

61

aFA@TC-800

75.32

0.94

94

Ticom

76.5

0.92

95

Table 3: Physical parameters and solar reflective index of coatings on steel panels (ASTM Standard E 1980-01) S.no.

Sample

Thickness

Whiteness

(µ)

Index

Reflectance

Emittance

SRI

1.

aFA@TC-120

100±5

70.2

0.68

0.91

83

2.

aFA@TC-800

123±5

80.8

0.71

0.91

89

3.

Ticom

112±5

80.67

0.73

0.90

90



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Table 4: Thermal diffusivity (0C) showing difference of temperature between two sides of painted MS plates. Sample

Front side (coated) (0C)

Back side (uncoated)

Difference in temp.

(0C)

(0C)

Uncoated MS plates

38.9

35.2

3.7

aFA@ TC- 800(20%)

40.5

31.7

8.8

aFA@ TC-800 (40%)

42.9

32.4

10.5

Ticom (20%)

42.6

34

8.6

AUTHOR INFORMATION Corresponding Author *Sangeeta Tiwari E-mail: [email protected] Amity Institute of Applied Sciences, Amity University Noida, Uttar Pradesh- 201301, India

ACKNOWLEDGMENT The authors are thankful to Ministry of Environment & Forest, Govt. of India Project No. 13/2010-CT for their financial assistance and support. The authors are also thankful to Dr. S.K.Dhawan, Senior Principle Scientist, CSIR-National Physical Laboratory, New Delhi (India) for providing facilities for fabrication of cool coatings on mild steel substrate. References: 1.

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Synopsis: A sustainable material developed from an industrial waste (fly ash) by greener approach having core-shell morphology for cool coating applications.


Highly Reflective Nanostructured Titania Shell: A ... - ACS Publications

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