Lectura 1 (Effect of size and morphology on UV-blocking property of Nano ZnO).pdf

8/12/2019 Lectura 1 (Effect of size and morphology on UV-blocking property of Nano ZnO).pdf

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International Journal of Scientific and Research Publications, Volume 3, Issue 4, April 2013 1ISSN 2250-3153

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Effect of size and morphology on UV-blocking propertyof nanoZnO in epoxy coating

Narayani Rajagopalan * & A S Khanna *

*

Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, India

Abstract - Nano zinc oxide (ZnO) is extensively used as a UV-blocking material and finds application as a UV-blocker incosmetics, textiles and protective coatings. However, with varying sizes and morphologies in the nano scale length, theUV-blocking behavior of nanoZnO gets varied. In the present study a DGEBA based epoxy coating on MS substrate wasformulated and modified with nanoZnO with two different sizes and morphologies. Flake-like and spherical nanoZnOwere synthesized by chemical routes using different precursors and characterized for their structure, size and shape. ThenanoZnO modified epoxy coatings were exposed in UVB weathering conditions and the UV-blocking efficiency of thetwo types of nanoZnO particles were studied by evaluating the color change (dE) and yellowness index (YI) for the UVBexposed epoxy coatings with and without nanoZnO. It was observed that the flake-like nanoZnO was more efficient inlowering yellowing resistance of the epoxy coating on weathering compared to the spherical nanoZnO particles.

I ndex Terms - Nano ZnO, UV-blocking, yellowing, epoxy

I. I NTRODUCTION

In recent years, zinc oxide (ZnO) ultrafine particles are well known as UV blocking materials, which make it feasible to be widely used in polymers, fabrics and cosmetic materials [1]. Zinc oxide has a band-gap at around 3.37 eV correspondingto 376 nm and thus it absorbs light that matches or exceeds this band gap energy [1-2] . UV-range of solar spectrum lieswithin this range and thus UV light gets absorbed by zinc oxide particles. The UV-blocking property of zinc oxide is onlyenhanced when it comes to nano-sized ultra-fine ZnO particles. As they are inorganic and particulate, they have addedadvantages of being stable and non-migratory within a matrix and thus potentially impart better effectiveness and a longerservice life.

With the on-going development in nano-technology, different nano-structures can be synthesized via different synthesisroutes. An equally known fact is that the unique and fundamental properties of nano particles change with their size andshape [3-5] . The phase stability of nano particles depends significantly on its surface free energies and surface stress. Nano-

particles either can exist as isolated species or they can form aggregates or they can dissolve into ions in solutions. These processes result in different sizes and sizes regimes and with this variance in size, the properties also vary in nano scalelength [3]. The electronic properties for semiconductor nano-particles, localized surface plasmon resonance (LSPR) ofnoble nanoparticles are cited to be size dependent. LSPR for Ag nanoparticles shifts across the electromagnetic spectrumas the shape of the particle changes from a sphere to cylinder to cube to prism to pyramid [3]. Similarly, the opticalcharacteristics of zinc oxide materials have been found to depend on their micro structures, morphologies and particle size[6]. The UV-blocking property of nanoZnO also thus depends on its size and morphology i.e, nanoZnO particles withdifferent sizes and morphologies would have varied UV-blocking properties.

In the present study nanoZnO particles with two different sizes and morphologies were prepared via chemical route usingdifferent precursors. The synthesized nano particles were then incorporated in to a di-glycidylbisphenol A (DGEBA)epoxy coating based on micron-sized titanium dioxide (TiO 2) and then subjected to UVB weathering conditions for twocycles and the effect of synthesized nanoZnO particles in lowering yellowing of epoxy on weathering was evaluated.Itwas found that the nanoZnO particles with flake-like morphology compared to the spherical nanoZnOwere more efficientin imparting yellowing resistance to the formulated epoxy coating.


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II. EXPERIMENTAL Synthesis of nanoZnO particles

1. Flake-like nanoZnO

MaterialsZinc sulfate heptahydrate (ZnSO 47H 2O), ammonia solution (NH 4OH), ammonium bicarbonate (NH 4HCO 3), anhydrousalcohol and deionized water

MethodologyThe first stage involved synthesis of the precursor, zinc carbonate hydroxide (ZCH) using a 1:10:1 (ratio by volume) ofaqueous ammonia (7 mol/L): ZnSO 47H 2O solution (0.5 mol/L): NH 4HCO 3 (3 mol/L). Aqueous ammonia followed by

NH 4HCO 3 solution, both, were drop-wise added to a continuously stirred solution of ZnSO47H2O at room temperature.The reaction mixture was then heated up to 60 0C and stirred at this temperature for 30 minutes. The ZCH precipitate wasthen filtered and washed with deionized water to ensure complete removal of sulphate ions (SO4 2-). The ZCH precursorwas air dried and calcined at 400 0C for two hours to obtain nano sized zinc oxide particles.

2. Spherical nanoZnO

MaterialsZinc acetate dihydrate, methanol, KOH

MethodologyThe following two solutions of appropriate concentrations were prepared which were mixed at room temperature withcontinuous stirring:Solution A - Zinc acetate dehydrate in methanolSolution B - KOH in methanolA bulky white precipitate arose immediately which became translucent after stirring for approximately 70 minutes. The

solution was then heated to boiling point, after which the heat source was switched off. The solution was then allowed tostand over-night. The sediment thus obtained was stirred and centrifuged off. The white gel like product obtained waswashed with de-ionized water and then dried to obtain the nano-sized zinc oxide particles.

The synthesized nano-powders were investigated by Transmission Electron Microscopy (TEM, Philips CM200 electronmicroscope), Scanning Electron Microscopy (SEM, Model no.S3400, Hitachi) and X-ray diff raction (XRD, XPert ProPhilip) for its shape, size and crystallinity. FTIR (JASCO FTIR 6100) studies of nano-powders were carried out to studythe structure of the oxides.

Preparation of epoxy coated samples

Materials

Di-glycidyl ether of bis-phenol A epoxy (EEW=185), Diethylene-tetramine (DETA), xylene, titanium dioxide (TiO 2)BYK additives

The mild steel (MS) samples to be coated were de-greased, cleaned and then roughened mechanically with abrasive paper(emery paper grade no.: 100). A TiO 2 based white coating was formulated with 35% pigment concentration, 50% epoxyresin concentration, ssssDETA as hardener and rest with the appropriate concentrations of additives and xylene as solvent.The nanoZnO modified epoxy coatings were prepared by addition of firstly the micron sized TiO 2 pigment along with therequired amount of nanoZnO at different loading levels, maintaining the 35% pigment concentration in the coatingfollowed by the additives. Nano zinc oxide was added to the epoxy resin using ultra-sonnication at 1%, 2% and 5% (byweight) of the total coating formulation. The only purpose to take TiO 2 as the pigment was to formulate a white coating


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and hence evaluate discoloration/ yellowness of the coating on weathering. The pigment concentration was alwaysmaintained at 35% with and without nanoZnO while formulating the coatings. The coating formulation for differentloading of nanoZnO in the coating is as tabulated as follows:

Table.1. Epoxy coating formulation with and without nanoZnO Component Neat Epoxy

(%)1% nanoZnO + Epoxy

(%)2% nanoZnO + Epoxy (%) 5% nanoZnO + Epoxy (%)

TiO 2 pigment 35.0 34 33 30

Nano ZnO 0 1 2 5

Resin 50.0 50.0 50.0 50.0

BYK 530 2.0 2.0 2.0 2.0

BYK 333 1.0 1.0 1.0 1.0

BYK 9076 1.5 1.5 1.5 1.5

BYK 320 0.5 0.5 0.5 0.5

Xylene 10 10 10 10

As mentioned in the table above, the resin content was kept constant throughout in the coatings, with different nanoZnOconcentrations . The resulting blend was ultrasonnicated for 20 minutes and the shear time via ultrasonication was keptconstant for all loadings of nanoZnO. After mixing TiO 2 pigment, nanoZnO and additives, at the end, hardener andsolvent were mixed to obtain the nanoZnO modified epoxy coatings. The formulated coating was applied using brush onthe surface treated MS panels so as to achieve uniformly coated panels with good finish and then allowed to hard cure.

Weathering TestThe coated MS panels were subjected to accelerated weathering in a UV weatherometer (QUV Weatherometer, Q-LabProducts & Services) equipped with UVB-313 nm lamps. The test cycle in UVB-weatherometer comprised of 4 hours

UVB simulation at 600

C followed by 4 h of condensation (UVB-lights off during condensation) at 500

C as per accordanceof ASTM G-154. The epoxy coated MS panels were exposed to two test cycles, i.e, 16 hours in the weatherometer.The coatings exposed in UVB weatherometer were characterized for color change (dE) and yellowness index (YI) using aspectrometer (BYK-Gardener Spectrometer) equipped with Color-Lab Quality Control software. The data were reportedon L, a, b scales and overall color difference was given in the following equation:

dE = [L 2+ a 2+ b 2]

where; L = L 2 - L 1, a = a 2 - a 1, and b = b 2 - b1

Respective numbers 1 and 2 are denoted to samples before and after the exposure test.The yellowness index was measured as per ASTM D 1925, formulated as:

YI = 100 (1.28 X CIE 1.06 Z CIE ) / Y CIE

As per ASTM D 1925, the conditions for measurement are as follows: Illuminant: C, Standard Observer function: 2 0

denoted as C/2 0. X, Y and Z are the tristimulus values and 1.28 and 1.06 are the coefficients as per C/2 0 conditions. The surface morphology of the coatings was studied using SEM microscopy (SEM, Model no.S3400, Hitachi) to look forthe distribution of nano zinc oxide at different loading levels in the coatings.

III. R ESULTS & DISCUSSION


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Characterization of nanoZnO

1. Flake-like nanoZnO The synthesis methodology involving zinc sulfate heptahydrate (ZnSO 4 7H 2O), ammonia solution (NH 4OH) andammonium bicarbonate (NH 4HCO 3) as starting materials, resulted in formation of zinc carbonated hydroxide (ZCH)which on calcination gave nanoZnO. The purity of nanoZnO was checked by FTIR and XRD analysis and their shape andsize were established by SEM, TEM and HRTEM analysis. Zinc oxide shows typical characteristic Zn-O peak at around470 and 480 cm -1[7-9] . This was confirmed by FTIR analysis as seen in figure.1 which showed Zn-O peaks at 468 and 480cm-1.

Figure.1. FTIR spectrum of flake-like nano zinc oxide

Figure.2 shows the XRD spectra of the synthesized nano zinc oxide. The XRD analysis identified well indexed diffraction peaks, in good agreement with those of a wurtzite hexagonal structure of ZnO (ICSD Reference code 01-075-0576) [10-12]

The flake-like nanoZnO particles showed the characteristic peaks at 320, 340 and 36 resulting from the 100, 002 and 101 planes respectively.


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Figure.2. XRD spectrum of flake-likenanoZnO

The SEM micrograph (figure.3), showed the nanoZnO particles to have flake-like morphology.

Figure.3. SEM micrograph of flake-like nanoZnO

The flake-like morphology was confirmed by TEM.As shown in figure.4, the flake-like morphology of ZnO composed ofsmall spherical particles with diameter ranging from 7 to 20 nm.


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Figure. 4. TEM image of flake-like nanoZnO

The lattice fringes of synthesized zinc oxide were clearly identified in the HR-TEM image as shown in figure.5. The inter- planar spacing i.e. d-spacing was obtained as 0.29 nm which closely matched to the d-spacing of 100 plane (0.281nm) ofzinc oxide [13-15] .

Figure.5. HR-TEM image of flake-like nano-ZnO

2. Spherical nanoZnO Nano ZnO was found to have spherical particle shape when prepared by chemical route using zinc acetate dehydrate andmethanol as starting materials. The FTI spectrum with characteristic Zn-O peaks at 468 cm -1 and 476 cm -1 is as shown infigure.4.6. iThe XRD spectrum of the nanoZnO particles is shown in figure.7. The XRD pattern observed was assigned tothe pure phase of zinc oxide (Joint Committee on Powder Diffraction Standards (JCPDS card number: 01-079-0205)which confirmed a wurtzite hexagonal structure.


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Figure.6. FTIR spectrum of spherical nanoZnO


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Figure.7. XRD spectrum of spherical nanoZnO

The SEM micrograph of the nano-ZnO particles showed single phase primary particles with spherical shape. TEMmonograph showed that the product consisted of particles with diameter ranging from 20-25 nm.

Figure.8. SEM micrograph of spherical nanoZnO


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Figure.9. TEM micrograph of spherical nanoZnOWeathering Study of nanoZnO modified Epoxy CoatingsThe effect of nano-ZnO addition to epoxy was studied by formulating epoxy coating with different concentrations ofnanoZnO. A TiO 2 based coating with DGEBA epoxy resin and DETA hardener, as mentioned in the previous chapter,was formulated with different concentrations of nanoZnO particles to optimize the concentration of nano-ZnO in thecoating. Nano ZnO, both flake-like and spherical shaped, was added at 1%, 2% and 5% by weight of total coatingformulation. The pigment concentration was maintained as 35% during the loading of nanoZnO particles in the coating.

Evaluation of color change and yellowing of weathered nanoZnO modified epoxy coatingsThe neat epoxy and both flake-like and spherical shaped nanoZnO modified epoxy coatings formulated as per the abovetabulated values were exposed to accelerated UVB exposure for two cycles (16 hours). The color change (dE) andyellowness index (YI) values obtained by spectrophotometric analysis are as shown in figure.10-11

Figure.10. dE and YI plot for flake-like nanoZnO + epoxy coating


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Figure.11. dE and YI plot for spherical nanoZnO + epoxy coatingThe epoxy coatings formulated with and without nano-ZnO, were applied on 0.7 mm thick MS panels by brush. Thecoating thickness was determined using a thickness measuring gauge (Elcometer DFT Gauge) and the coating thicknessfor the coating systems were found to be in the range of 85-100 microns. The color change (dE) and yellowness index(YI) results were evaluated after 16 hours of UVB exposure. i.e, two cycles of UV (B) weathering were each cycle of 8hours comprises of 4 hour UVB light followed by 4 hour condensation.The discoloration observed in epoxies onweathering is attributed to the formation of a quinone- methide structure and has been confirmed in many cited studies by

both infrared and UV-Vis analysis [16] . As observed from the dE and YI plots, both flake-like and spherical shaped nanoZnO exhibited lower dE and YI values atall loading levels compared to the neat epoxy coating system. The optimized loading concentration of both flake-like andspherical nanoZnO was observed to be 2 wt% of the total coating formulation. Flake-like nanoZnO lowered yellowingand color change by 12 and 8 units respectively while the spherical nanoZnO lowered the same by only 5 and 3 unitsrespectively. The increase in dE and YI values for both flake-like and spherical nanoZnO beyond the optimized 2 wt%loading is attributed to the agglomeration of nano particles in the coating matrix. Agglomeration of nano-particles is un-desirable as it makes nano-particles lose their high surface area and thus the desired functional properties [17] . The result ofagglomeration is that the nanoZnO would not be evenly distributed and so there will be regions in the coating that would

be starved of zinc oxide and not protected, by zinc oxide in the coating.The extent of weathering resistance, offered by the nanoZnO modified epoxy coating systems were determined byevaluating the percentage reductions in color change (dE) and yellowing (YI) during the exposure. The percentagereduction values were calculated as follows:Suppose after a test cycle,

Neat Epoxy = AModified Epoxy = BA and B are dE/YI values after the weathering cycle, then;

% Reduction = [(A-B) / A] x 100The percentage reduction values in dE and YI, thus calculated for the nanoZnO modified epoxy coatings are as shown in

figure.12 13.


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Figure.12. % Reduction in dE and YI plot for flake-like nanoZnO + epoxy coating

Figure.4.13. % Reduction in dE and YI plot for spherical nanoZnO + epoxy

Table.2. dE, YI, % Reduction in dE and YI for nanoZnO modified epoxy coatings

Systems Flake-like nanoZnO + Epoxy Spherical nanoZnO + Epoxy

dE YI %

Reduction

dE

%

Reduction

YI

dE YI %

Reduction

dE

%

Reduction

YI

0% nanoZnO

+Epoxy

18.25 29.99 18.25 29.99

1% nanoZnO +

Epoxy

14.07 23.51 23 22 15.86 25.75 13 14


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2% nanoZnO

+ Epoxy

10.74 18.24 41 40 15.2 24.69 17 18

5% nanoZnO +

Epoxy

11.75 19.33 36 35 16.69 27.7 8.5 8

High percentage reduction values in dE and YI indicates more resistance to color change and yellowing. The highest percentage reduction values were observed at 2% loading level respectively for flake-like and spherical nanoZnOmodified epoxy coatings compared to 1% and 5%. Flake-like nanoZnO reduced color change and yellowing by 40%while spherical nanoZnO reduced the same by 17%, at 2% loading in the coating formulation.

The enhanced weathering resistance for the nano-ZnO incorporated epoxy coatings is due to UV-absorption property ofZnO owing to its large band gap energy. Zinc oxide has a band-gap at around 3.37 eV corresponding to 376 nm and thus itabsorbs light that matches or exceeds this band gap energy [1, 2] . UV-range of solar spectrum lies within this range and thusUV light gets absorbed by zinc oxide particles. The UV-blocking property of zinc oxide is only enhanced when it comesto nano-sized ultra-fine ZnO particles.

Though, both flake-like and spherical nanoZnO offered resistance to color change and yellowing in the nanoZnOmodified epoxy coatings, the flake-like nanoZnO outperformed spherical nanoZnO. Nano ZnO with flake-likemorphology acted as a better UV-blocker than the spherical nano-ZnO and hence imparted more efficient weatheringresistance when incorporated in the epoxy coatings. It was thus inferred that nanoZnO with different shape and size

performed differently when incorporated in epoxy coatings and subjected to UVB exposures. The better performance ofthe flake-like nano-ZnO is attributed to its morphology and size . Nano particles possess unique properties owing to theirnano size dimensions however these properties change with their shape and size and the causes are different for differentmaterials [3, 4] . RoshidahRusdi et al. (2011) have reported in their work that ZnOnano-tubes have a wider band gapcompared to that of spherical nanoZnO particles and have explained this by the characteristics of materials at nanodimensions [6]. At nano scale, increase in pressure results in strong forces in the interiors of crystallites and hence thelattice spacing of ZnO nanotube decreases. It was proposed then, that due to the strong internal forces, the electronstaking part in transition from the valence band to the conduction band need greater energy, accounting for the observedwider band gap [3, 4] . All these changes eventually change the properties of the nano particles. It was also seen that theabsorption profile for nanoZnO with nanotube and nano rod structures were a little different from that of the spherical

particles. Also, from thermodynamic considerations, the total free energy is a sum of free energy of the bulk and thesurface of nano particle.

G nanoparticle = G bulk + G surfaceFor nanoparticles, G surface is no longer a minor component and hence all fundamental properties of nanoparticles are sizedependent as well as shape dependent [5].

The flake like nanoZnO particles diameter ranged from 7 20 nm while the spherical shaped nano particles had diametersize of 20 25 nm .The different behavior of flake-like and spherical nanoZnO in imparting weathering resistance toepoxy coating can be thusexplained on the basis of size and morphology difference of the nanoZnO particles. As the nanosize decreases, the space in which charge carriers move decreases and additional quantum confinement is imposed ontheir motion. This leads to increase in the band gap energy, electron-hole kinetic energy and the density of the chargecarriers within and at the nanoparticle surface.The enhanced reduction in color change and yellowing for the flake-likenanoZnO compared to spherical nanoZnO in the epoxy coating at all loadings in the epoxy coating is thus attributed to thedifference in their size, shape and morphology.

IV. CONCLUSION Nano zinc oxide (ZnO) particles were synthesized with two different morphologies namely, flake-like and spherical withdifferent sizes. Flake-like nanoZnO had diameter ranging from 7-20nm while spherical nanoZnO had diameter in the


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range of 20-25nm. Both flake-like and spherical nanoZnO when incorporated in a DGEBA based epoxy coating loweredyellowing of the epoxy coating on weathering. The optimized loading concentration was 2 wt% of the total coatingformulation. The efficiency of the nanoZnO particles were found to be size and morphology dependent as the flake-likenanoZnO lowered yellowing and color change by 40% while the spherical nanoZnO particles lowered the same by 18%.

REFERNCES

[1]Cao, Z., et al., Synthesis and UV shielding properties of zinc oxide ultrafine particles modified with silica andtrimethylsiloxane . Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2009. 340 (1 3): p. 161-167.[2]GeunjaeKwak, et. Al.,Multifunctional transparent ZnOnanorod films.Journal of Nanotechnology, 2011. 22 .[3]ClememsBurda., et al.,Chemistry and Properties of Nanocrystals of Different Shapes .Chemical Review, 2005.105 : p. 1025 - 1102.[4] El-Sayed, M.A., Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different ShapesAccounts of Chemical Research, 2001. 34 (4): p. 257-264.[5]Grassian, V.H., When Size Really Matters: Size-Dependent Properties and Surface Chemistry of Metal and MetalOxide Nanoparticles in Gas and Liquid Phase Environments.Journal of Physical Chemistry C, 2008. 112 (47): p.18303 18313[6] R. Rusdi, et al., Preparation and band gap energies of ZnO nanotubes, nanorods and spherical nanostructures.Journal of Powder Technology, 2011, 210 (1), 18-22[7] Cao, Z., et al., Synthesis and UV shielding properties of zinc oxide ultrafine particles modified with silica andtrimethylsiloxane. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2009. 340 (1 3): p. 161167.[8] Geunjae Kwak, S.J.a.K.Y., Multifunctional transparent ZnO nanorod lms. NANOTECHNOLOGY, 2011. 22 .[9] Xiong, M., et al., Preparation and characterization of poly(styrene butylacrylate) latex/nano-ZnO nanocomposites.Journal ofApplied Polymer Science, 2003. 90 (7): p. 1923-1931.[10] Allen, N.S., et al., Behaviour of nanoparticle (ultrafine) titanium dioxide pigments and stabilisers on the

photooxidative stability of water based acrylic and isocyanate based acrylic coatings. Polymer Degradation and

Stability, 2002. 78 (3): p. 467 - 478.[11] Amirudin, A. and D. Thieny, Application of electrochemical impedance spectroscopy to study the degradation of

polymercoated metals. Progress in Organic Coatings, 1995. 26 (1): p. 1-28.[12] Ariosa, D., et al., Texture vs morphology in ZnO nano-rods: On the x-ray diffraction characterization ofelectrochemicallygrown samples. Journal of Applied Physics, 2011. 110 (12): p. 124901.[13] Armstrong, R.D., A.T.A. Jenkins, and B.W. Johnson, An investigation into the uv breakdown of thermoset polyester

coatingsusing impedance spectroscopy. Corrosion Science, 1995. 37 (10): p. 1615-1625.[14] Xu, X., et al., Size Dependence of Defect-Induced Room Temperature Ferromagnetism in Undoped ZnO

Nanoparticles. TheJournal of Physical Chemistry C, 2012. 116 (15): p. 8813-8818.[15] R. Sreej., et al., Linear and nonlinear optical properties of luminescent ZnOnanoparticles embedded in PMMAmatrix Optics Communications, 2010. 283 : p. 2908 2913.[16] Down, J.L., The Yellowing of Epoxy Resin Adhesives: Report on High-Intensity Light Aging. Studies in

Conservation, 1986. 31 (4): p. 159-170.[17] Raymond, H.F., Nanocomposite and Nanostructured Coatings: Recent Advancements, in Nanotechnology

Applications in Coatings2009, American Chemical Society. p. 2-21.

AUTHORS

First Author NarayaniRajagopalan, Research Scholar, IIT Bombay, [email protected] Second Author Anand S Khanna, PhD, IIT Bombay, [email protected] Correspondence Author NarayaniRajagopalan ,[email protected] , [email protected]

Contact number - 9833357520
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